Stable thin film heater based on transparent conductive coating, structure formed by the stable thin film heater, and its application

Abstract

a transparent substrate; a transparent resistive heat-generating element mounted on the substrate; metal traces forming electrodes positioned around a perimeter of a heated region that define a circuit for electrical current through the transparent resistive heat-generating element, the electrodes being disposed in electrical contact with the transparent heat-generating element; and a power source connected to the electrodes, the power source being capable of delivering at least 1 volt to the electrodes, wherein the transparent resistive heat-generating element has a sheet resistance of about 1 ohm / sq. to about 300 ohm / sq. and a surface area of ​​at least about 0.25 cm 2 A transparent structure comprising a rough metal conductive layer comprising noble metal coated silver nanowire segments having unpatterned areas of 0.1 to 1.0 μm.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 441,656, filed January 27, 2023, to Chen et al., entitled "Stable Thin Film Heaters Based on Noble Metal Coated Silver Nanowires and Applications Thereof," and Chinese Patent Application No. 202311352632.X, filed October 18, 2023, to Chen et al., entitled "Stable Thin Film Heaters Based on Transparent Conductive Coatings, Structures Formed With the Heaters and Applications Thereof," both of which are incorporated herein by reference.

[0002] The present invention relates to thin film heaters that can be effective for heating surfaces of transparent structures. The present invention further relates to methods for applying thin film heaters to various surfaces, including but not limited to, forming heating elements on flexible polymer films for lamination to heated surfaces. The present invention also relates to heater constructions in the context of various heater geometries. [Background technology]

[0003] Various situations suggest the desire to have a heated surface. In some situations, the surface is transparent, so heating may be used to help remove ice, snow, or moisture from the surface in general to improve visualization. For example, windows on automobiles and other vehicles (such as trucks, airplanes, etc.) may have window heaters. With the increasing use of machine vision, useful wavelengths for visualization may extend from visible wavelengths to infrared. Autonomous vehicles generally rely on the transmission and reception of electromagnetic radiation to provide the construction of images to guide movement. Summary of the Invention [Means for solving the problem]

[0004] In a first aspect, the present invention relates to a heater structure including a transparent substrate; a transparent resistive heat-generating element mounted on the substrate; electrodes positioned in electrical contact with the transparent resistive heat-generating element, the electrodes being positioned along boundaries of a heated area that define a current path through the transparent resistive heat-generating element, thereby forming the heated area; and a power source connected from electrode to electrode, wherein the transparent resistive heat-generating element includes a roughened metal conductive layer including nanowire segments and having a sheet resistance of about 0.5 ohms / sq. to about 300 ohms / sq. In some embodiments, the power source can deliver at least 1 volt to the electrodes. A vehicle window can include the transparent heater structure as described above. An infrared-based imaging system can include the transparent heater structure as described above.

[0005] In another aspect, the present invention provides a method for heating a surface of a structure, the method comprising heating a surface of a structure for at least about 30 seconds at a rate of at least about 0.05 W / cm 2 and applying a voltage of at least 1 volt to a heating element to generate a surface power density of about 0.5 ohms / sq. to about 300 ohms / sq., wherein the heating element includes a transparent conductive film including nanowire segments within a rough metal conductive layer and having a sheet resistance of about 0.5 ohms / sq. to about 300 ohms / sq.

[0006] In another aspect, the invention relates to a method of fabricating a transparent conductive heater on a surface, the method comprising: 1) forming a coating of metal nanowires from a solution; 2) drying the nanowire coating to form a transparent conductive film; and 3) forming conductive electrodes by establishing electrical connections to the transparent conductive film by circuit paths along the transparent conductive film between the two electrodes, which forms a heater surface having a significant surface power density generated across the heater surface from an applied voltage between the two electrodes.

[0007] In another aspect, the present invention relates to a vehicle including a visualization device having a surface exposed to an ambient environment, the visualization device transmitting and / or receiving infrared light over a specific range having wavelengths from 750 nm to 3 micrometers; a transparent resistive heat-generating element interfaced with the surface of the visualization device; and a control element. The transparent resistive heat-generating element can include a rough metal conductive layer including nanowire segments having a sheet resistance of about 1 ohm / sq. to about 300 ohm / sq. and a transmittance of at least about 70% for infrared light over the specific range.

[0008] Additionally, the present invention relates to a heater structure having a transparent non-rectangular heater surface and including a conductive element, the conductive element having a non-uniform resistance and the conductive element being between one or more pairs of electrodes such that power dissipation over the surface of the stable thin film heater is more uniform relative to power dissipation in an equivalent structure having a uniform transparent conductive coating.

[0009] Additionally, the present invention relates to a heater structure comprising a transparent conductive layer, electrodes connected to a voltage source, and a polymer overcoat on at least a portion of the transparent conductive layer, wherein the transparent conductive coating provides electrical conduction between the electrodes to provide heating, and the polymer overcoat comprises nanoparticles that provide improved thermal conductivity.

[0010] In another aspect, the invention relates to a method for fabricating a transparent conductive heater on a non-planar surface, the method comprising: attaching a flexible polymer substrate having a nanowire-based transparent conductive heater surface onto the non-planar transparent surface. In some embodiments, the heater surface is oriented relative to the transparent non-planar surface. In some embodiments, the resulting structure is a heated transparent structure. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a plot of transmittance as a function of wavelength for transparent silver nanowire-based films formed with sheet resistances of 30-50 ohms / sq. on either polyethylene terephthalate or polycarbonate optical films. [Figure 2A]FIG. 1 shows a top view of the general layout of a heater and controller with transparent nanowire-based heating elements supporting bus bars. [Figure 2B] FIG. 1 is a schematic top-down view of a trapezoidal heater having a conductive layer between non-parallel metal traces showing the effect of non-uniform distribution of current. [Figure 2C] FIG. 10 is a schematic top-down view of a portion of a heater showing segmented metal traces electrically configured to each be driven with the same or different voltages V1-V3. [Figure 2D] FIG. 1 is a schematic top-down view of a heater divided into several segments, each heater segment being electrically configured with a set of metal traces and a resistor to modify the current flowing through the corresponding heater segment. [Figure 3A] FIG. 1 is a schematic top-down view of a trapezoidal heater having a conductive layer between non-parallel metal traces with notional horizontal lines running across the conductive layer dividing the layer into several zones. [Figure 3B] FIG. 3B is a schematic top-down view of the trapezoidal heater shown in FIG. 3A with etched spots distributed throughout the conductive layer. [Figure 4] FIG. 1 is a schematic diagram of a windshield configured with transparent heaters. [Figure 5A-5B] 1 is a schematic cross-sectional view of a transparent heater with an additional layer or substrate. [Figure 6] FIG. 1 shows a top view of the trace design used to evaluate the heating performance of transparent heater elements made with platinum-coated and uncoated silver nanowires. [Figure 7A] 7 is a plot of the change in temperature as a function of power density for heater elements made from platinum-coated and uncoated silver nanowires and configured for heating by using the trace design shown in FIG. 6. [Figure 7B] 7 is a plot of the change in temperature as a function of voltage for heater elements made from platinum-coated and uncoated silver nanowires and configured for heating by using the trace design shown in FIG. 6. [Figure 8A-8B]FIG. 1 shows a top view of the trace design used to evaluate the heating performance of a transparent heater element made of platinum-coated silver nanowires. [Figures 9A-9C] 8C is a plot of the change in temperature as a function of power density for a heater element made of platinum-coated silver nanowires and configured for heating by using the trace design shown in FIGS. 8A and 8B. [Figures 10A-10C] 8B shows a thermal image of a heater element made of platinum coated silver nanowires and configured for heating by using the trace design shown in FIG. 8A. [Figures 11A-11B] FIG. 1 shows a top view of the trace design used to evaluate the heating performance of a transparent heater element made of platinum-coated silver nanowires. [Figure 12A] 11C is a plot of the change in temperature as a function of power density for a heater element made of platinum-coated silver nanowires and configured for heating by using the trace design shown in FIGS. 11A and 11B. FIG. [Figure 12B] 11C is a plot of the change in temperature as a function of voltage for a heater element made of platinum-coated silver nanowires and configured for heating by using the trace design shown in FIGS. 11A and 11B. [Figures 13A-13B] 11B shows a thermal image of a heater element made of platinum coated silver nanowires and configured for heating by using the trace design shown in FIG. 11A. [Figure 14] FIG. 10 is a top view of the trace design used to evaluate the heating performance of a transparent heater element made of platinum-coated silver nanowires. [Figure 15A] 15 is a plot of the change in temperature as a function of power density for a heater element made of platinum-coated silver nanowires and configured for heating by using the trace design shown in FIG. 14. [Figure 15B] 15 is a plot of the change in temperature as a function of voltage for a heater element made of platinum coated silver nanowires and configured for heating by using the trace design shown in FIG. 14. [Figures 16A-16B] 15 shows a thermal image of a heater element made of platinum coated silver nanowires and configured for heating by using the trace design shown in FIG. 14. [Figures 17A-17B] 15 is a plot of temperature as a function of time for a heater element made from platinum coated silver nanowires and configured for heating by using the trace design shown in FIG. 14. [Figure 18] FIG. 1 is a schematic top-down view of a trapezoidal heater having a conductive layer between non-parallel metal traces, with approximate dimensions indicating the size and shape of the conductive layer. [Figure 19A] FIG. 1 is a schematic top-down view of a trapezoidal heater having a conductive layer between non-parallel metal traces with positive and negative terminals located on opposing metal traces along the short sides of the stable thin film heater. [Figure 19B] FIG. 1 is a schematic top-down view of a trapezoidal heater having a conductive layer between non-parallel metal traces with positive and negative terminals located on opposing metal traces along the long sides of the stable thin film heater. [Figure 20A] 10 is a thermal image of a coated sample configured as a trapezoidal heater with the positive and negative terminals located on opposing metal traces along the short sides of the stable thin film heater. [Figure 20B] 10 is a thermal image of a coated sample configured as a trapezoidal heater with the positive and negative terminals located on opposing metal traces along the long sides of the stable thin film heater. [Figure 21] FIG. 1 is a schematic top-down view of a trapezoidal heater having a conductive layer between non-parallel metal traces with notional horizontal lines running across the conductive layer dividing the conductive layer into ten zones. [Figure 22A] 10 is a thermal image of a coated sample configured as a trapezoidal heater with the positive and negative terminals located on opposing metal traces along the long sides of a stable thin film heater, and with the coating not etched. [Figure 22B]22B is a thermal image of a coated sample constructed as described with respect to FIG. 22A, with the coating etched horizontally in several lines. [Figure 22C] FIG. 22C is a thermal image of a coated sample constructed as described with respect to FIG. 22B, which is horizontally etched with fewer lines compared to the coating of FIG. 22B. [Figure 23A] 22B is a thermal image of a second transparent heater constructed and configured as described with respect to FIG. 22A. [Figure 23B] 22C is a thermal image of a second transparent heater constructed and configured as described with respect to FIG. 22B. [Figure 23C] 22D is a thermal image of a second transparent heater constructed and configured as described with respect to FIG. 22C. DETAILED DESCRIPTION OF THE INVENTION

[0012] Thin films formed from metal nanowires, such as noble metal-coated silver nanowires, can be used to form heater elements that can carry sufficient current to provide a high enough surface power density to heat a surface while not degrading the conductive element. In some embodiments, the heater film is transparent to visible and / or infrared light so that the heater can be used to help remove ice or frost that can obscure visibility through structures generally involving a transparent substrate supporting the heater. Conductive coatings can also function as heaters in a wide variety of other applications where the surface, component, or element is heated and transparent. Heater characteristics can be balanced with desired optical properties to define design parameters. Heater applications can set a range of available voltages for the heater so that the electrical resistance of the heater element can be selected to generate a surface power density compatible with the voltage. Silver nanowires have been developed for use in transparent conductive films for use in transparent touch sensors. Silver nanowire-based inks can be coated onto the surfaces of a wide range of materials, allowing heaters to be placed on the surfaces of a range of structures. In some embodiments, a noble metal coating on silver nanowires allows for the generation of higher surface power densities without damaging the heater to provide useful heating for a wide range of applications. The heater's conductive film can be flexible and bendable, as well as conformable to irregular shapes, which can be a significant advantage over some alternative technologies. To the extent that the heater element itself has an irregular (non-rectangular) shape, heaters can be designed to provide more uniform heating over irregularly shaped structures, and these heater designs can be effectively employed with a variety of transparent resistive materials, including: noble metal-based materials; silver nanowires without a noble metal coating; other metal nanowires with or without a noble metal coating; transparent metal oxide films such as indium tin oxide; nanocarbon-based resistive materials, and so on. Without design modifications, irregularly shaped heater elements can generate more non-uniform surface power densities across the heater element, provided that the resistivity is uniform across the heater surface and the voltage is uniform along the edges of the heated surface.

[0013] Forming a heater generally involves coating and treating a transparent conductive film. Based on the transparent conductive film, electrodes can be applied to provide electrical connections for supplying power to the heater. In the art, such electrodes may alternatively be referred to as bus bars or metal traces, or perhaps similar terms, as the context dictates. Metal wires can be attached to the electrodes, and the metal wires can be connected to a power source to complete the circuit. The formed heater structure can be laminated or molded to the surface to be heated, or the heater structure can be formed on a surface, and a protective hard coat or other protective material, such as a polymer and / or glass, can be placed over the heater to protect the surface. Conductive films can be formed from solutions with suitable binders as processing aids or to improve film properties (e.g., adhesion). Thus, in some cases, a transparent heating layer can be produced by direct deposition of a nanowire (such as a platinum-coated nanowire) ink onto another component (e.g., a camera lens or component of a Light Detection and Ranging (LiDAR) system), eliminating the need for a separate transparent (e.g., polymer) substrate.

[0014] A polymer overcoat can be provided to protect the heater from damage. Generally, a polymer overcoat can be useful for any transparent conductive film. To provide suitable protection against potential abrasion or other damage, the overcoat and / or additional protective coating, possibly with multiple layers, can include a hard coat polymer to provide scratch resistance and with a total thickness sufficient to adequately protect the heating element. To reduce the thermal insulation caused by a thicker protective coating, the coating or portions thereof can be loaded with thermally conductive nanoparticle fillers. Improving thermal conductivity through the protective coating can improve the effectiveness of the heater. Through the use of suitable nanoparticles to improve thermal conductivity, optical transmittance and other optical properties can be modified within acceptable limits. Additionally, a surface coating of photoactive material can provide self-cleaning capabilities through exposure to ambient UV light (e.g., from sunlight) to oxidize dirt and other debris so that they can be easily washed away. Improved protective coatings can even be effective with other transparent conductive film materials, such as those formed from transparent conductive oxides such as indium tin oxide or carbon nanotubes.

[0015] To form the device, a conductive film can be cast directly onto the structure with the heated surface. In additional or alternative embodiments, the conductive film can be formed on a polymer sheet as a substrate and then laminated to the structure with the heated surface; this is a possible processing approach due to the flexibility and formability of nanowire-based coated polymer sheets and films. The sheets and films can be formed in a roll-to-roll process. Processing can occur at lower temperatures, and in some embodiments, at or near room temperature. For practical devices, the heater would have bus bars or electrodes (which could be placed within the bezel area of ​​the transparent structure) for connecting the heater element to a power source and control system. Adaptation of the heater element in embodiments without rectangularly arranged electrodes / bus bars is described to provide a reasonably uniform power density generated throughout the heater.

[0016] It has been discovered that noble metal-coated silver nanowires offer significantly improved stability in heater configurations relative to corresponding heaters with uncoated silver nanowires, providing effective thin-film heating elements that may be useful in a range of applications. Silver nanowire synthesis has progressed to the point of commercial-scale synthesis with very good nanowire properties. Effective coating processes can be used to apply suitable noble metal coatings onto commercial-scale silver nanowires. Noble metal-coated silver nanowires provide cost-effective structural production and commercial-scale components. Due to improved thermal stability and corrosion resistance, the resulting heater can now provide effective amounts of heat without sacrificing processability or desired coating properties.

[0017] The use of metal nanowires for heating elements offers the possibility of very thin layers for the heating element. The heating element can be transparent with a range of transparency from highly transparent to semi-transparent. Generally, a more transparent and less hazy element has a higher electrical resistance, which must be considered in the overall design. Transparency can be in the visible wavelengths for displays and windows and / or in the infrared for machine vision systems such as LIDAR. Low voltage constraints, overall heater design and size, and other factors can provide a trade-off between the level of transparency and surface power density.

[0018] Transparent conductive heaters based on transparent electrodes formed from conductive ceramic oxides (such as indium tin oxide (ITO)) are known; see, for example, U.S. Pat. No. 5,354,966 to Sperbeck, entitled "Window Defogging System With Optically Clear Overlay Having Multi-Layer Silver Bus Bars and Electrically Isolating Peripheral Grooves," and PCT Patent Application Publication No. 2022 / 136102, entitled "Glazing Having an Electrically Heatable Communication Window for Sensors and Camera Systems," both of which are incorporated herein by reference. Because ITO is a ceramic material, it is not flexible or bendable, but it can be somewhat flexible if it is thin enough. Metal mesh can be formed into relatively thin and potentially somewhat flexible heaters, but these materials are not very flexible, thin, or low-temperature processable. Like ITO, processing with metal mesh can require significant heating, which limits the material, and can impose other significant processing constraints. Furthermore, metal mesh has large gaps between metal lines or conductors, which may be undesirable for many applications. There are also often limitations to producing metal meshes and metal oxide films on a wide variety of related substrates, such as polycarbonate. Additionally, these conductors typically present the challenge of being deposited directly onto curved surfaces, 2.5-dimensional (2.5D) surfaces such as assembly stacks of 2D materials, or three-dimensional (3D) textured surfaces.

[0019] Nanowire-based heaters can be formed using several possible techniques and on a wide range of materials. Because the system is solution-based, the layers forming the heating element can be coated directly onto a substrate to support the heater. Direct coating techniques can be applied to reasonable surface geometries. Slot-die coating is a well-established technique for nanowire coating. See, for example, U.S. Patent Application Publication No. 2020 / 0245457 to Chen et al., entitled "Thin Flexible Structures with Surfaces With Transparent Conductive Films and Processes for Forming the Structures," which is incorporated herein by reference. In another embodiment, the nanowire solution can be spray coated onto a target surface with a nozzle opening large enough to avoid clogging. Dip coating using a noble metal-coated silver nanowire ink may also be suitable for some substrates. Direct coating of nanowire ink can be performed on flat or curved substrates made of wood, paper, fabric, polymers (polycarbonate (PC) and polymethyl methacrylate (PMMA)), etc. In additional or alternative embodiments, heating elements can be coated onto flexible polymer sheets. Due to the excellent flexibility of silver (and metal-coated silver) nanowire-based conductive films, heaters can function successfully on flexible polymer sheets that maintain their flexible form. As further described below, it is generally desirable to provide a melt of nanowires into a molten metal nanostructured network using applicant's proprietary technology.

[0020] Slot-coating-type coating processes for metal nanowires, such as noble-metal-coated silver nanowires, can be readily applied to coating on polymer sheets. Such roll-to-roll processes are commonly used to form transparent conductive electrodes for touch sensor formation in portable electronic devices, whether the final device is fixed, bendable, or flexible. Roll-to-roll formation of heater elements on polymer sheets and films can adapt established roll-to-roll deposition techniques to metal nanowires to form heater structures for various devices. The use of roll-to-roll coating on flexible polymer sheets can be useful for applying heating elements to surfaces of various shapes, as the polymer sheet bearing the heating elements can be conformed to the target surface. Mounting techniques are further described below. Other deposition techniques include spray coating, flow coating, slit coating, inkjet printing, and dip coating, and these methods can be particularly useful for direct application to a range of surfaces. Based on these processing options, transparent heating elements can be formed on a range of devices.

[0021] Silver nanowires can be synthesized as described in U.S. Pat. No. 10,714,230 to Hu et al. (hereinafter the '230 patent), entitled "Thin and Uniform Silver Nanowires, Method of Synthesis and Transparent Conductive Films Formed From the Nanowires," which is incorporated herein by reference. The '230 patent involves an improved implementation of a synthesis method based on a glycol solvent / reducing agent and a polyvinylpyrrolidone capping agent. Commercially available nanowires have been synthesized based on the teachings of the '230 patent. Generally, thicker nanowires can be formed more economically, while thinner nanowires can be useful for improving some of the optical properties of transparent structures. The formation of a noble metal coating on silver nanowires is described in U.S. Pat. No. 9,530,534 to Hu et al. (hereinafter the '534 patent), entitled "Transparent Conductive Film," which is incorporated herein by reference. The '534 patent described methods based on either galvanic exchange or direct vapor deposition to apply the noble metal coating. The '534 patent exemplifies the coating of gold and platinum and discusses the applicability of the precious metals iridium, rhodium, palladium, and osmium. The amount of precious metal coating is generally driven by the amount of coating precursor added and the specific reaction conditions selected.

[0022] Silver nanowire-based heaters can be formed by using applicant's proprietary fusing technique to convert silver nanowires into a single conductive structure to provide improved electrical conductivity without compromising optical properties (including upon bending or stretching the conductive film), as well as mechanical stabilization. While fusing may not be used to form metal nanowire-based heaters, fusing can be useful for achieving desired performance levels or reduced metal usage. It is believed that fusing occurs at least primarily during ink drying as the solvent evaporates. The evaporated solvent concentrates the species that drive the rejection reaction. It has recently been discovered that fusing can be driven to occur at or near room temperature, such that no heat needs to be applied to the molten metal nanostructured network during fusing. Room-temperature fusing is described in U.S. Patent Application Publication No. 2023 / 0416552 (hereinafter the '552 application) to Yang et al., entitled "Formation of Electrically Conductive Layers at Room Temperature Using Silver Nanoparticulate Processing and Inks for Forming the Layers," which is incorporated herein by reference.

[0023] Heaters are typically incorporated into transparent structures. Within the range of structures in which heaters can be effectively used, the nature of transparency may differ from the optical properties associated with, for example, displays. For example, the structure may be transparent in only a portion of the spectrum. Machine vision devices may require transparency in the infrared but not necessarily in the visible. As another example, automobile windows may be tinted to limit glare. Some systems are designed to block UV light. In any case, structures incorporating heaters typically have a transmittance that is at least about 70% across the entire useful spectral range from infrared to visible light. For imaging systems operating in the infrared, the relevant spectral region may be 750 nm to 1750 nm, or a selected portion thereof. For optical systems used for human vision, such as vehicle windows, transmittance across all or a significant portion of the visible spectrum (which may be set to 400 nm to 750 nm, or a different range as desired) is at least about 70%, but in some embodiments may be significantly more transparent (e.g., at least about 80% transmittance). Transmittance measurements of a representative set of transparent silver nanowire films formed with sheet resistances of 30-50 ohms / sq. on either polyethylene terephthalate or polycarbonate optical films are presented in Figure 1. Over most of the relevant wavelength range, the transmittance through the substrate is >90%, and corresponding values ​​with the substrate transmittance removed would yield transmittance values ​​of the conductive layer alone of >95%.

[0024] Heater Structure The overall heater structure generally includes electrodes, bus bars or other drive electrodes that frame and connect the thin-film heating elements to a power source, and a controller via the drive electrodes. As used herein and as a convention in the art, a bus bar refers to a highly conductive, non-transparent electrode or metal trace that provides the application of an electrical potential to generate heat, resulting in current flow across the transparent conductive film. The electrodes can be formed using materials available in the art (such as silver paste or other convenient products). Because the electrodes do not form a patterned grid with significant areas of conductive and non-conductive material, current typically flows from one side of the heater element to the other to generate the desired power output density. Typically, the electrodes are placed at or near the outer edges of the transparent conductive film so that significant portions of the transparent conductive film do not reside outside the circuit formed by the electrodes. The electrode / bus bar comprises at least two opposite polarity elements, and additional electrode elements (two or more additional electrode elements of at least one polarity) can be used to provide more uniform current flow through the heating element. Each member of a pair of electrodes may or may not have the same length as the other. If multiple pairs of electrodes are provided, each pair of electrodes can have a different voltage set between them to provide more uniform heating throughout the non-rectangular heating element. The busbars can be formed from metal traces, conductive metal paste, or similar metal structures that can be formed at a suitable point in the assembly process. Busbars generally cover a significantly smaller area than thin-film heating elements and are expected to have a much lower resistance contribution to the overall circuit. The power source generally depends on the specific application and whether it is a fixed location. For example, a suitable power source can be a battery with or without a transformer to control the voltage, a transformer connected to a line voltage, or any other suitable power source. The control device can be as simple as a switch that turns a fixed voltage on or off, a switch with an adjustable voltage, a digital processor with an associated power supply, or any other reasonable controller.

[0025] FIG. 2A shows a general layout (top view) of a heater and controller with a transparent nanowire-based heating element and supporting busbars. In the depicted embodiment, the heater element is generally rectangular. Heater 10 includes a coating sample 11, which includes a coating of nanowire-based ink on a substrate, such as a transparent substrate. In some embodiments, the heater is placed on a non-transparent substrate, in which case the heater's transparency allows for a barrier-free view of the substrate surface, which may be decorative. Metal traces (electrodes) 12a and 12b are formed on the nanowire-based coating to provide heating throughout the coating when power is applied. Trace 12a is connected to wire 14a via area 13a. Trace 12b is connected to wire 14b via area 13b. The specific design (i.e., placement, width, and number of traces) can vary, and several designs were evaluated, as described below with respect to several examples. Trace 12a is parallel to and coextensive with trace 12b to form a substantially rectangular heater surface. The dimensions of the coated sample 11 can be any size depending on the particular application. Specific sizes of the coated sample 11 as a test sample are described below with reference to some examples. The controller 15, including the power supply 16 and switch 17, is configured to provide power as desired and may also include additional elements such as a digital processor or analog circuitry.

[0026] Heater performance is simplest for structures in which the electrodes / bus bars, or substantial portions thereof, are arranged parallel to one another to form a generally rectangular heating element. For these configurations, ignoring edge effects and other relatively minor potential design asymmetries for connecting the electrodes, the heat generated is typically generally uniform across the heater. If the substantial cross sections of the electrodes / bus bars are not parallel to one another or have significantly different lengths, the current across the heater element will not be uniform if the sheet resistance of the conductive film forming the heater element is uniform. This is shown schematically in FIG. 2B of a trapezoidal heater 20, which includes conductive film 21 and supports bus bars 22a and 22b. If the heater material is uniform across the heater surface and the opposing polarity electrodes are not parallel, the power density generated across the heater surface will not be uniform across the heater surface. As discussed herein, the properties of the transparent conductive film forming the heater element can be modified to improve the uniformity of the surface power density, such that the heat generated across the structure adjacent to the heater element may be more uniform so that more effective heating functionality may be observed. As explained further below, because the minimum length between adjacent points of opposing polarity electrodes varies along the length of the electrodes, the resistance at relevant locations of the heater element can be adjusted along the heater surface to improve the uniformity of the surface power density.

[0027] In principle, the electrode shape could be altered to attempt to direct a more uniform current through the transparent heating element. To make such modifications to the current flow, the electrodes could be altered to introduce electrical resistance at appropriate locations within the electrode / busbar, resulting in some parts of the electrode being at a different voltage than other parts of the electrode. Having a voltage drop across the electrode itself would result in significant heat generation within the electrode. This would be undesirable as the heat at the electrode would not be efficient at heating the desired transparent portion of the device, resulting in a waste of power. The design described herein avoids these complications.

[0028] In additional or alternative embodiments, the electrode may be segmented so that different voltages can be applied to elements of the segmented electrode to provide more uniform heating, although this approach required several power supplies to be modified to provide the multiple voltages. This embodiment is shown in FIG. 2C, where heater portion 30 includes conductive film 31 and segmented metal traces 32a-32c, each electrically configured to be driven with the same or different voltages V1-V3. The use of series resistors allows the total resistance to be measured at a fixed voltage V=IR=I(R h +R r )=IR h +IR r provides a convenient way to effectively adjust the effective current across the heater segment by increasing h is the resistance of the heater segment, and R r is the resistance of the series resistor, where V1 is the effective voltage across the heater, reduced by V2 as a result of the series resistor.

[0029] It may also be desirable to form thin etch lines along the transparent conductive film to form isolated conductive stripes. Etching the transparent conductive film can be thin lines that limit any electrical conduction between adjacent stripes, while thermal conduction effectively produces a uniform heated surface without gaps, but with a more uniform surface power density.

[0030] In additional or alternative embodiments, the insulated conductive stripes (having thin etched lines to electrically isolate the stripes of heater material from one another) can be electrically configured so that different voltages can be applied to each stripe to provide more uniform heating. This embodiment is shown in FIG. 2D , where a heater 33 including three insulated conductive stripes 34 a-34 c is electrically configured in series with pairs of metal traces (electrodes) 35 a-35 c, respectively. Resistors R1-R3 and wires 36 a and 36 b connect the pair of electrodes to a controller 37 including a power supply 39 and switch 38, configured to supply power as desired. The heater 33 can include additional elements, such as a digital processor or analog circuitry. Resistors R1-R3 have varying resistances and are arranged so that the voltages sent to each insulated conductive stripe are different and improve heating uniformity.

[0031] Although heaters are typically supported on a substrate once formed, transparent conductive heating layers can also be deposited directly onto another layer, component, or element (e.g., by spray coating or flow coating). The conductive nanowire-based layer may have a polymer overcoat for stability during processing, but a polymer overcoat is not required. In some cases, other layers, encapsulation, anti-reflective coatings, adhesives, and sealants may also provide improved stability and reliability, thereby making the use of an overcoat unnecessary. Additionally, other conductive materials, or layers and components that distribute or modify heat and its dissipation, may be applied directly to the conductive coating or onto the polymer overcoat. The optional polymer overcoat is typically thin enough to allow electrical conduction through the overcoat. Naturally, in the final structure, the nanowire-based conductive layer would be expected to have some protective cover (which may be applied once electrical connections are completed to provide connectivity) to prevent damage during use or so that electrical connections can be made through windows in the insulating cover. Details of potential overcoats are discussed further below in the context of device structures.

[0032] As noted above, various coating methods can be used to apply the metal nanowire-based conductive precursor ink. For heater applications, the final product may not have a flat or rectangular shape. Depending on the production protocol of the device itself, it may or may not be appropriate to coat the nanowire precursor solution directly onto the surface of a transparent structure that forms the core of the actual device, such as a glass surface. A protective cover may then be applied, with or without an initial thin overcoat.

[0033] If a conductive layer is applied directly onto the structure to incorporate the heater, further processing can be performed on the conductive layer. A protective polymer or glass layer can be laminated or applied with an adhesive onto the conductive layer. In some embodiments, a protective polymer coating can be applied over the transparent heater surface with or without an overcoat layer over the transparent heater surface. The top layer can be a scratch-resistant hard coat or other suitable material. Such a protective cover can include an optically transparent polymer substrate laminated with an optically transparent adhesive. The optically transparent substrate material can have a hard coat surface.

[0034] Suitable commercially available polycarbonate substrates include, for example, MAKROFOL SR243 1-1 CG available from Bayer Material Science; TAP® Plastic available from TAPPlastics; and LEXAN™ 8010CDE available from SABIC Innovative Plastics. Optical-quality PET substrates are available from, for example, DuPont-Teijin and Toray Films (Lumirror™). Polyimide substrates are available from Kolon, and polysulfone substrates are available from Solvay. Cyclic polyolefins (COP) are available from Zeon Corporation. Optically clear adhesives are commercially available as liquid adhesives or as adhesive tapes (double-sided). Suitable transparent adhesive tapes are commercially available, for example, from Lintec Corporation (MO series); Saint Gobain Performance Plastics (DF713 series); Nitto Americas (Nitto Denko) (LUCIACS CS9621T and LUCIAS CS9622T); LG Hausys OCA (OC9102D, OC9052D); DIC Corporation (DAITAC LT series OCA, DAITAC WS series OCA and DAITAC ZB series); PANAC Plastic Film Company (PANACLEAN series); Tesa SE (Germany) (AF61, 694 series, 696 series and 697 series); Minnesota Mining and Manufacturing (3M, Minnesota, USA - product numbers 8146, 8171, 8172, 8173, 1414-1, 9894, and similar products) and Adhesive Research (e.g., product 8932). These companies may also sell optically clear liquid adhesives.

[0035] In some embodiments, a conductive nanowire layer is applied to a polymer substrate to facilitate heater formation. In these embodiments, a thin polymer overcoat may be desirable to protect the conductive heater layer during or prior to further processing. This processing approach has the advantage of providing a product with a transparent conductive layer and polymer for forming a transparent heater that can be adapted for a range of applications. Fused metal nanostructured networks have been shown to be conformable to formats that may involve bending and / or stretching. See, for example, U.S. Pat. No. 11,343,911 to Kambe et al., entitled "Formable Transparent Conductive Films with Metal Nanowires," incorporated herein by reference.

[0036] Once the conductive nanowire layer is applied to a polymer sheet that will have a heated surface and be attached to the final structure, the polymer film with the conductive layer can be attached with an adhesive (such as a contact adhesive or a transparent adhesive) selected to suit the particular structure. Additional layers can be applied as needed. In particular, in the context of roll-to-roll processing, as well as other embodiments, a thin polymer overcoat can be placed over the transparent conductive layer, typically thin enough to avoid significant electrical resistance through the overcoat. The polymer overcoat can provide protection for the conductive layer during processing; a desirable uniform surface for the placement of additional layers during processing; a refractive index to reduce undesirable changes in indices through the optical stack; and possibly other benefits. The conductive heating element should have some protective coating, but a suitable coating should not be overly thermally insulating, since the heating element is generally intended to heat the surface of the final structure. Furthermore, we can apply a thin protective coating, then a Bisel material to ensure good electrical contact, and then a thicker protective coating to ensure both good electrical connectivity and good reliability and durability.

[0037] In another embodiment, the flexible layer with the transparent heating elements can be secured to the device surface to be heated using an insert molding or overmolding process. The structures are aligned and a polymer is injected to complete the structure, where the injected polymer forms a protective coating along the outer surface. The injected polymer can be polycarbonate, polymethyl methacrylate, acrylonitile butadiene styrene (ABS) polymer, or other suitable polymer. Additional discussion of device structures is provided below. Transparent conductive films are discussed in detail in the following section.

[0038] Transparent conductive heater film The formation of metal nanowire-based transparent conductive films involves forming a thin coating of metal nanowires at a density that provides nanoscale gaps between the nanowires, dispersed in a random pattern. Because the diameter of the nanowires is typically significantly smaller than the wavelength of visible light, the layer has a uniform transparent film with optical properties (such as haze and transmittance) determined by the characteristics of the nanowires and the amount of metal deposited. Fusing the nanowire junctions by chemical melting can increase electrical conductivity without degrading the optical properties, while also improving the mechanical resilience of the conductive structure.

[0039] The design of the thin film heater can depend on the desired use. For use, the amount of metal deposition and the characteristics of the metal nanowires typically determine the properties of the heating element, balancing electrical resistance and transparency. In particular, to maintain transparency through the film, the resistance cannot be arbitrarily adjusted independently of the transmittance or optical clarity and haze. If the voltage cannot be adjusted, it can be used to set the surface power density based on the resistance of the heating element. Since the primary purpose of the heating element will be to heat, these parameters are discussed next.

[0040] For heaters, a meaningful performance measure can be the power output per unit area, which can also be referred to as surface power density. While the environment as well as other design parameters such as the heat capacity of other materials can affect the resulting surface temperature, the temperature is typically a function (nearly linear) of the power output of the heating element for a given structure. The power output depends on the electrical resistance of the heating element and the applied voltage. In some embodiments of particular interest, heating elements described herein incorporating noble metal coated silver nanowires can withstand significantly greater power outputs without failure compared to comparable heating elements based on silver nanowires without the noble metal coating. Surface power density is a device-level parameter because it depends on the current across the heater element and, in turn, on the electrical resistance. The area of ​​the device can be measured and its resistance evaluated by using a multimeter. Assuming a uniform structure with a rectangular configuration, the surface power density (P d ) is P d =V 2 / (RA), where V is the voltage across the heater, R is the resistance, and A is the area of ​​the heater. If V is in volts and R is in ohms, then P d is in watts divided by the units of A. Sheet resistance (R s ) is the resistance x (width of the resistor element divided by the length of the resistor element): R s =Rw / L, where w is the width of the heater structure perpendicular to the current flow and L is the length of the heater structure along the current flow. scan be conveniently measured for the conductive film. In the present application, the sheet resistance of the transparent conductive film can be from about 0.5 ohm / sq. to about 250 ohm / sq., and in further embodiments from about 1 ohm / sq. to about 250 ohm / sq., and in additional embodiments from about 2 ohm / sq. to about 150 ohm / sq. The formation of fused metal nanostructured networks and coarse metal conductive films with lower values ​​of sheet resistance and corresponding transmittance is described in U.S. Provisional Patent Application No. 18 / 212,297 to Yang et al., entitled "Formation of Electrically Conductive Layers at Room Temperature Using Silver Nanoparticulate Processing and Inks for Forming the Layers," which is incorporated herein by reference. Noble metal coated silver nanowires have a sheet resistance of about 0.5 W / cm 2 (5000W / m 2 ) greater than P d , in some embodiments, a value of about 0.6 W / cm 2 Larger P d and in some embodiments at least about 1 W / cm 2 P d In addition to being able to withstand higher surface power densities without failure, the noble metal coated silver nanowires are suitable for use over the expected product life of the device at useful power outputs. For extended use, the surface power density should be at least about 0.05 W / cm. 2 , about 0.2W / cm 2 ~Approx. 1.5W / cm 2 , and in other embodiments, about 0.1 W / cm 2 ~Approx. 1.25W / cm 2 A person of ordinary skill in the art will recognize that additional ranges of power per unit area and sheet resistance within the explicit ranges above are contemplated and are within the present disclosure.

[0041] While the evaluation of surface power density is relatively straightforward for a rectangular heater configuration, a simple formulation based on an assumed constant resistivity and current over the area of ​​interest does not consider potential variations in the local surface power density. Assuming a current flows between the electrodes, the surface power density is a function of the voltage, V, and sheet resistance, R. s and can be expressed in terms of length L as: P d =V 2 / (R s L 2 ), where L is the length between the electrodes. Sheet resistance R s can be a function of position along the heater surface. There are many potential ways to address the non-uniformity due to the non-rectangular shape of the heater surface, and the degree of complexity should be balanced with the practical reality that heat also diffuses and is therefore not a closed local variable, so a reasonable approximation should work well. To this end, an alternative approach is provided for modifying the resistivity along the heater surface to improve the uniformity of heat generation.

[0042] Resistivity is typically expressed as mg / cm 2 The electrical conductivity depends on the properties of the heater coating, including the metal surface loading, which can be expressed as: With enough metal deposited, the electrical conductivity can approach that of the bulk metal, but this process does not involve melting into a single mass to eliminate its electrical resistance. For transparent heaters, the metal loading and corresponding amount of metal are accordingly limited. The deposited metal is a form of what may be called a rough metal conductive layer, where the metal nanowires are sufficiently rough to have adequate visibility through the conductive layer. Improved optical performance can typically be achieved with equivalent electrical performance by using a melting process to form a fused metal nanostructured network, as further explained below. The melting process can be achieved at low processing temperatures and has been effectively achieved in room-temperature processing. This dependence of resistance on metal surface loading provides an approach for modifying the heat generated at various locations along the heater surface by correspondingly altering the metal deposition density and / or selective removal to achieve greater uniformity in heat generation.

[0043] In the context of some loaded polymer systems, noble-metal coated silver nanowires have been found to provide surprisingly improved conductive properties. In the context of bulk metal nanowire-loaded polymers, platinum coated silver nanowires exhibit improved conductivity and lower percolation thresholds relative to metal loading compared to silver nanowires in some polymer systems. These properties are described in co-pending U.S. patent application Ser. No. 18 / 376,952 to Virkar et al., entitled "Silver Nanowire and Noble-Metal Coated Silver Nanowire Conductive Polymer Composites With Low Loading Percolation Conduction," which is incorporated herein by reference. When formed into transparent conductive layers with fused metal nanostructured networks, noble-metal coated silver nanowires have been found to exhibit a corresponding drop in sheet resistance; see the above-cited '534 patent.

[0044] For transparent conductive layers used as heating elements, there is a trade-off between metal loading and transmittance. With higher metal loading, transmittance generally decreases, and electrical resistance generally decreases as well. For a fixed voltage, lower resistance generates more power output in the form of heat. To the extent that higher transmittance is desired, lower metal loadings can be used at the expense of increased sheet resistance. The use of thinner nanowires can allow for increased transmittance and lower haze at a fixed metal loading, but thinner nanowires are generally more expensive to manufacture. The nanowire loading level can provide a useful parameter of the network that can be easily evaluated, and the loading value provides a surrogate parameter related to thickness. Thus, as used herein, the loading level of nanowires on a substrate is typically presented as milligrams of nanowires per square meter of substrate. Typically, nanowire networks have a density of approximately 1 milligram (mg) / m². 2 ~about 2000mg / m 2 and in another embodiment about 2.5 mg / m 2 ~about 400mg / m 2 , and in other embodiments, about 5 mg / m2 ~about 300mg / m 2 A person of ordinary skill in the art will recognize that additional ranges of thickness and loading within the explicit ranges above are contemplated and are within the present disclosure.

[0045] Components used in various applications may have several voltages available for use to drive heaters. Voltages may also be used to provide a desired surface energy density, with adjustable voltages being an option. It may or may not be desirable to provide voltage conversion, which involves space considerations, energy losses, and possibly some cost considerations. While handheld portable devices operate at 5V, stationary devices may utilize 120V or 240V line voltages, which may depend on the location. In some applications, such as electric vehicles, power sources up to 1000V may be available. Therefore, for most applications, heaters may be designed for operation between 1V and 1000V, between 2V and 20V in some embodiments, and between 5V and 120V in other embodiments. Generally, voltages may be provided by direct current (DC) or alternating current (AC). While automobile batteries may provide 12V, some trucks have battery systems with 24V outputs. Generally, heaters may be designed for operation within the appropriate voltage window. As noted, in some embodiments, the bus bar may be segmented such that segments of the bus bar can be set to different voltages. These embodiments are further described below. A person of ordinary skill in the art will recognize that additional ranges within the explicit ranges above are contemplated and are within the present disclosure.

[0046] Transparent conductive films are typically rated in terms of sheet resistance in ohms / sq., which can be evaluated without measuring layer thickness. For touch sensor applications, sheet resistance is an effective assessment of the performance of such devices. Sheet resistance can be evaluated directly along the surface of the material and is more commonly used to characterize materials with effectively uniform thickness. Whether a transparent conductive film is uniform in the microscopic sense is not directly relevant, but transparent conductive films are sufficiently uniform in the operational sense that this use of sheet resistance makes sense. For heaters operating at significant power outputs, the resistance of the electrical circuit is significant and can therefore be evaluated using conventional circuit measurements by using Ohm's Law, V = IR. Thus, current is measured at an applied voltage, for example, using an ohmmeter (multimeter) or the like to obtain R = V / I.

[0047] Generally, resistance depends on the sheet resistance and configuration of the heater structure (including electrode placement). With uniform layer thickness, resistance and sheet resistance can be easily interchanged. The target resistance value can depend on the desired transmittance, available voltage, and desired power output. Of course, these are not individually adjustable parameters. Nanowire loading can be used to adjust the sheet resistance, which changes the transmittance accordingly. With effectively symmetric electrodes across a rectangular shape, the heater element can have a nearly uniform nanowire loading while providing nearly uniform heat generation. Adjustments to other electrode geometries are discussed further below. To the extent that the voltage is adjustable for a specific application, it can provide an additional adjustable parameter. Similarly, to the extent that the busbar / drive electrodes can have flexible placement during the design phase, having a shorter path length between the busbar poles will typically result in a lower resistance value, since resistance is expressed as R = ρL / A, where ρ is resistivity, L is length, and A is area. To the extent that the busbar placement provides a reduction in L or an increase in A, the resistance can be reduced. In some embodiments, the resistance can be from about 1 ohm to about 300 ohms, in other embodiments from about 2 ohms to about 200 ohms, and in further embodiments from about 5 ohms to about 100 ohms. A person of ordinary skill in the art will recognize that additional ranges of resistance within the explicit ranges above are contemplated and are within the present disclosure.

[0048] To obtain optimal optical properties of nanowire-based transparent conductive films, applicants have relied on their melting techniques to form nanowires into a fused metal nanostructured network. The melting process can be performed to allow thermodynamically controlled metal deposition to preferentially provide metal deposited at the nanowire junctions. If the heater design allows for higher resistance or reduced optical properties, it may be possible to use unfused nanowires to provide sufficient conductivity for the heater. Thus, while forming a fused metal nanostructured network may be desirable from the perspective of improved performance, the heater may or may not contribute to forming a fused metal nanostructured network that is a single metal structure extending throughout the entire conductive element. The fused metal nanostructured network may also provide desired mechanical and / or wear resistance, which may be desirable in addition to improved electrical conductivity.

[0049] In general, various rough metal conductive layers can be formed from metal nanowires, such as noble metal-coated nanowires. Films formed from metal nanowires that are treated to planarize the nanowires at junctions and improve conductivity are described in U.S. Patent No. 8,049,333 to Alden et al., entitled "Transparent Conductors Comprising Metal Nanowires," which is incorporated herein by reference. Structures including surface-embedded metal nanowires to increase metal conductivity are described in U.S. Patent No. 8,748,749 to Srinivas et al., entitled "Patterned Transparent Conductors and Related Manufacturing Methods," which is incorporated herein by reference. However, improved properties have been discovered for fused metal nanostructured networks, both in terms of high electrical conductivity and desirable optical properties such as transparency and low haze. The fusion of adjacent metal nanowires can be achieved through chemical processes under commercially reasonable processing conditions.

[0050] In particular, a significant advance in achieving conductive films based on metal nanowires was the discovery of a sufficiently controllable process for forming fused metal networks, in which adjacent sections of metal nanowires fuse into a single structure without separate nanowires within the conductive network. In particular, it was initially discovered that halide ions could drive the melting of metal nanowires to form fused metal nanostructures. Melting agents containing halide anions have been introduced in various ways to successfully achieve melting with a corresponding significant drop in electrical resistance. It should be noted that halide ions in this processing context should not be confused with the halide ions used in nanowire synthesis reactions. Specifically, melting of metal nanowires with halide anions has been achieved not only with vapors and / or solutions of acid halides, but also with solutions of halide salts. The fusing of metal nanowires with halide sources is further described in U.S. Pat. No. 10,029,916 to Virkar et al., entitled "Metal Nanowire Networks and Transparent Conductive Material," and U.S. Pat. No. 920,207 to Virkar et al. (the '207 patent), entitled "Metal Nanostructured Networks and Transparent Conductive Material," both of which are incorporated herein by reference.

[0051] An extension of the process to form fused metal nanowire networks was based on a reduction / oxidation (redox) reaction that can be applied to produce fused nanowires without compromising the optical properties of the resulting film. Metal for deposition at the junction can be effectively added as a fused metal composition or dissolved from the metal nanowire itself. The effective use of redox chemistry to fuse metal nanowires into nanostructured networks is further described in U.S. Pat. No. 10,020,807 to Virkar et al. (the '807 patent), entitled "Fused Metal Nanostructured Networks, Fusing Solutions with Reducing Agents and Methods for Forming Metal Networks," which is incorporated herein by reference. The '807 patent also described a single-solution approach for the formation of fused metal nanostructured networks. The single solution approach for the formation of fused metal nanostructured layers is further described in U.S. Patent No. 9,183,968 B1 to Li et al. (hereinafter the '968 patent), entitled "Metal Nanowire Inks for the Formation of Transparent Conductive Films with Fused Networks," which is incorporated herein by reference, and a single solution or ink process for forming fused metal nanostructured networks is used in the examples below. The effective fusing of noble metal coated silver nanowires into fused metal nanostructured networks is described in the above-cited '534 patent.

[0052] Silver is the metal with the highest electrical conductivity. Therefore, silver salts have typically been used to fuse nanowire junctions. Silver salts for fusing are exemplified below. However, the present applicant has successfully used a range of metals for fusing with silver nanowires by using corresponding salts. See the '807 patent. In view of the present objective of forming a heater with desired durability, the use of a more highly fusible metal for fusing may be desirable, since platinum-coated nanowires, even with very thin platinum coatings, have been found to stabilize the heater. Thus, for example, it may be desirable to perform fusing using palladium salts (e.g., Pd(NO3)2 or K2PdCl4), which were successfully used in the '807 patent. Palladium has a melting point significantly higher than silver and electrical conductivity comparable to that of platinum, though not significantly higher. Of course, other metals with high melting points and good electrical conductivity (e.g., nickel or rhodium) may also be used for fusing if desired.

[0053] To achieve an effective single-deposition ink that hardens into a fused nanostructured metal network, the desired ink contains a desired amount of metal nanowires to achieve appropriate loading of the metal within the resulting transparent film. In a suitable solution, the ink is stable prior to ink deposition and drying. The ink may contain a reasonable amount of polymer binder that contributes to the formation of a stable conductive film for further processing. To achieve good fusing results with a single ink system, hydrophilic polymers such as fibers or chitosan-based polymers have been found to be effective. Metal ions as the source of metal for the fusing process can be provided as a dissolvable metal composition.

[0054] A single ink formulation simultaneously provides the deposition of the desired metal loading as a film on the substrate surface and provides components in the ink that drive the melting process as the ink dries under appropriate conditions. These inks may be conveniently referred to as fused metal nanowire inks, with the understanding that melting typically does not occur until drying. In some embodiments, the ink typically comprises an aqueous solvent that may further include alcohol and / or other organic solvents, although other solvents can be effectively used if desired, as long as soluble metal salts are present. The ink may also include a dissolved metal composition as a metal source for the melting process. While not wishing to be limited by theory, it is believed that components of the ink (e.g., alcohol or other organic compositions, such as polymeric polyhydroxyl compositions) reduce metal ions from solution to drive the melting process. Previous experience with melting processes in these systems suggests that metal deposits preferentially at the junctions between adjacent metal nanowires. A polymer binder may be provided to stabilize the film and influence ink properties. The specific ink formulation may be adjusted to select ink properties suitable for a particular deposition technique and specific coating properties on the substrate surface. Drying conditions can be selected to effectively carry out the melting process.

[0055] Nanoparticle inks for forming fused metal nanostructured networks typically include an aqueous solvent, metal nanowires, metal ions, a polyhydroxyl polymer binder, and optional wetting agents or other processing aids. In some embodiments, the aqueous solvent can further include an alcohol, such as ethanol or isopropyl alcohol and / or other organic solvents. Metal ions as a source of metal for the fusion process can be provided as soluble metal salts. Metal nanowire inks can include about 0.01 wt% to about 1 wt% metal nanowires, about 0.02 wt% to about 5 wt% polymer binder, about 0.001 wt% to about 1 wt% wetting agent / surfactant and / or other processing aids, and silver ions or other metal ions for fusion at a concentration of about 0.01 mg / mL to about 2.0 mg / mL. Those skilled in the art will recognize that component ranges within the explicit ranges above are contemplated and within the present disclosure. Suitable silver salts for achieving sufficient solubility include, for example, silver tetrafluoroborate (AgBF), silver hexafluorophosphate (AgPF), silver perchlorate (AgClO), silver hexafluoroantimonate (AgSbF), silver trifluoroacetate (AgCFCOO), silver heptafluorobutyrate (AgCFO), silver methylsulfonate (AgCHSO), silver tolylsulfonate (AgCCHSO), or mixtures thereof. Suitable palladium salts are mentioned above. Other metal salts include, for example, salts that are soluble in aqueous solvents and reducible under the process conditions. A variety of surfactants can be used in principle, and fluorosurfactants have proven popular for a variety of practical reasons. Alcohols can serve as both solvents and wetting agents to form good coatings at high concentrations.

[0056] Within reasonable limits, the amount of solvent in nanowire inks can be adjusted to modify the ink's solids concentration and associated rheology. To fabricate transparent conductive films for high-optical-quality applications, such as touch sensors in smart devices, inks are typically applied using slot-die coating techniques (which can be performed in a roll-to-roll format) onto polymer films. A similar approach can be used to fabricate heater elements. As with touch sensor formation, the resulting coating can then be laminated to another layer structure or applied with an adhesive film. Slot coating can also be applied directly to specific pieces for assembly into heater structures. For larger area coverage (e.g., greater than a square millimeter), spray coating can also be used. In principle, inkjet printing can be used with nanowire inks from a practical standpoint, but typical inkjet resolution is difficult to achieve due to clogging if the nozzle is too narrow. However, for larger area coverage, spray or inkjet coating can be achieved by using larger nozzles, which may not be as vulnerable to clogging due to the nanowire morphology. Screen printing has been described for metal nanowire inks, but the resolution was typically not desirable for most applications. See Li et al., "Screen printing of silver nanowires; balancing conductivity with transparency while maintaining flexibility and stretchability," Nature Flexible Electronics (2019) 3:13; https: / / doi.org / 10.1038 / s41528-019-0057-1, incorporated herein by reference.

[0057] After coating, the solvent is removed by evaporation. Fusing occurs during the drying process. Heat can be applied to facilitate solvent removal and help drive the fusing process. As noted above, room temperature fusing can be used with proper selection of ink and process conditions. See the above-cited '552 application. Heat can be applied as needed for the particular process framework; blowing warm air, passing through an oven, using heat lamps, combinations thereof, etc. may be appropriate. Once the conductive film is dry, additional processing can occur.

[0058] Patterning of transparent conductive films for touch sensor applications is typically done using laser ablation or photolithography in conjunction with etching. In some embodiments, the transparent structures are at least about 0.25 cm 2 100%. For heater applications, patterning is usually not required or can be done with low resolution consistent with macrostructures. For heater applications, patterning may not be used because heating is desired across the entire heater surface for heat generation. However, some etching of the conductive material may be desirable to improve heat generation uniformity. This adjustment of the heater surface can be particularly useful for non-rectangular heater shapes.

[0059] A method of fabricating a transparent conductive heater on a precursor surface may include forming a coating of metal nanowires onto the surface from a solution; drying the nanowire coating to form a transparent conductive film including a rough metal conductive layer; and forming a conductive electrode that establishes an electrical connection to the transparent conductive film by a circuit path along the transparent conductive film between two electrodes that form the heater surface, wherein a significant surface power density is generated across the heater surface from an applied voltage between the conductive electrodes.

[0060] The heater surface can be a non-planar transparent surface designed for exposure to the ambient environment and further includes applying a protective overcoat over the heater surface. The protective overcoat can be laminated in place with an optically transparent adhesive, or the protective overcoat can be formed by applying a liquid that is then cured. A transparent polymer barrier layer can be applied as a liquid to form the protective covering. Insert molding or overmolding can be used to form the protective polymer cover, either by first applying a transparent conductive layer to the device surface, coating it directly, or by forming a transparent conductive layer on a polymer substrate that is then affixed to the surface during the molding process. Insert molding of transparent materials is generally described in PCT Application WO 2022 / 138052 to Okuda et al., entitled "Decorative Film for Insert Formation, Method of Decorative Film for Insert Formation, and Manufacture Method for Resin Molded Product," which is incorporated herein by reference. The transparent conductive surface can be on a coated polymer sheet in a roll-to-roll process, and further includes laminating a section of the coated polymer sheet after forming the electrodes to the transparent device surface to be heated.

[0061] A method for fabricating a transparent conductive heater on a non-planar precursor surface can include laminating a flexible polymer substrate with a rough metal conductive layer and forming a nanowire-based transparent conductive heater surface on the non-planar transparent surface with the heater surface oriented in the direction of the transparent non-planar surface to form a heated transparent structure. The rough metal conductive layer can be formed on the flexible polymer substrate using solution coating of a precursor ink in a roll-to-roll process. The precursor ink can include metal nanowires and metal ions, and drying of the coated precursor solution results in the formation of a fused metal nanostructured network.

[0062] The method may further include printing electrodes at selected locations of the heater structure by using a silver conductive paste and / or cutting the flexible polymer substrate to obtain a heater structure having a heater surface framed by electrodes. Conductive silver paste is commercially available from DuPont, Shanghai Daejao Electronic Materials, Hunan National Silver New Materials, BLT, NanoTOP, Eisho, Shanghai Silver paste, Junyng Electronic, Nanometal Technology, Resink, Soltrium, Shanghai Sunsen Electronic Materials, and Shanren New Materials. The heater structure may be laminated to the device surface, optionally using an optically transparent adhesive.

[0063] Electrical Properties and Heat Generation For heater applications, electrical conduction / resistance is used to generate heat, so current flow is selected to provide the desired heat generation in response to an applied voltage. In principle, resistivity could be considered with mathematical precision, but this would be illusory because it ignores atomic-level structure and, more importantly, the collective nature of material properties, which are inherently coarser scale. Nevertheless, Maxwell's equations can be numerically solved for the heater element with some resolution, with boundary conditions set by the voltage at the electrodes and by the resistivity along the heater element. This will provide a value for current flow throughout the heater, which can then be used to estimate power density. While this modeling can be done, reasonable approximations, such as the use of reasonable grid sizes, can be used to significantly simplify calculations. Approximations can be made for any heater shape and electrode configuration. From a practical standpoint, the heat generated as a result of resistivity is further obscured in practical terms, since heat also flows and does not remain localized. To guide the patterning of the heating surface, some reasonable use of grids and approximations can provide sufficient practical guidance.

[0064] For heaters with the appropriate configuration, one reasonable approximation is to have current flow directly between the electrodes. For rectangularly oriented electrodes, this current flow reflects the geometry when edge effects are ignored. For electrodes arranged in a trapezoidal shape with equal-length electrodes, a shorter length connecting one end of the electrode and a longer length connecting the other end of the electrode, the stripes can be thought of as conceptually dividing the heater surface into zones, an example of which is depicted in FIG. 3A. A trapezoidal heater 40 includes a conductive layer 41 between nonparallel metal traces 42a and 42b, with conceptually or actually etched horizontal lines 46 extending across the conductive layer, dividing the layer into zones 44, such as zones 44a-44c. An approximation will be assumed that the current in each stripe (a zone) is constant and transverse between the electrodes. In the limit of thinner stripes, this will be accurate, subject to the approximation of the current flow direction and the ambiguity required to define related parameters that are not atomic-scale values. Within each stripe, the length L between the electrodes can be thought of as an average, so it can essentially be the length at the center of the stripe. With these approximations, the power density can be approximated for each stripe.

[0065] By the approximation specified in the previous section, the powder density of the stripe is P d (i)=V 2 / (R s (i)L(i) 2 ), where "i" is a designator for a particular stripe or zone. With a constant voltage V along the electrode, as the length changes, the resistivity should change inversely to maintain a nearly constant surface power density. Alternatively, the voltage can optionally be varied while maintaining a nearly constant sheet resistance, as explained further below. It should be noted that the parameters can be considered with reasonable granularity (which is appropriate due to heat conduction and other reasonable factors) and that greater mathematical precision can be achieved if numerical modeling is deemed desirable.

[0066] With regard to modifying the resistivity along the heater surface, this can be done by modifying the conductive elements in one or more of three ways: 1) non-uniform deposition, 2) etching of some conductive material, and / or 3) selectively damaging the conductive material.

[0067] With non-uniform deposition, the deposition of additional silver nanowires correlates with lower resistivity, and conversely, the deposition of fewer silver nanowires correlates with higher resistivity. Therefore, non-uniform deposition can be designed to have reduced loading of silver nanowires in regions of small L to increase resistivity in these regions toward a more constant value of the product of ρ(i) and L(i). The dispersion pattern can depend on the deposition technique. While coating methods are usually designed for uniform deposition, modifications are possible, especially for lower resolution variations. Although nanowire printing techniques are complicated due to the aspect ratio, spray deposition can be performed through a larger nozzle, and the spray deposition can be modified as the spray nozzle is moved across the heater area.

[0068] Etching has been used to pattern transparent conductive films for forming touch sensors and the like. This heater etch is different because its purpose is to increase sheet resistance and not necessarily to control the direction of current flow. However, etching can create areas of virtually infinite resistivity, altering the overall resistivity by carefully considering the resistivity between opposite-polarity electrodes. Due to these differences, the amount of material etched is generally smaller for heater applications. For clarity, in this context, "removal" refers not only to physical removal from the conductive path but also to electrical isolation, if not complete physical removal. Etching can be performed with a laser or other focused radiation, which ablates the conductive film from the substrate surface (and may also electrically isolate some domains to remove them from the conductive path). Additionally or alternatively, etching can be performed with lithography and chemical, plasma, or vapor etching through a temporary mask. The etching should substantially maintain current flow between the electrodes for improved uniformity of surface power density.

[0069] Laser etching along a thin line between electrodes is exemplified as described below. In alternative embodiments, etching can be performed using lithography and etching materials, fluids, vapors, plasmas, or combinations thereof, and such techniques have been used to form sensors. The primary effect of this etching is to reduce the current-carrying area so as not to create any small zones of power generation. The goal is to fine-tune power generation, which can be zones of more uniform power generation, removed in areas of greater power output so that the observed surface power density is more uniform at the granular level. The etch design can be selected to achieve a more uniform surface power density. Focusing on this goal, any etched zones should be small in size relative to the length of heat conduction so that the lack of power generation from the etched area does not significantly affect the observed surface power density. With these constraints in mind, other etch patterns can be used. For example, a pattern of spots, which can be formed using laser etching or lithography, can effectively increase resistivity at the granular level by introducing small local areas of effectively infinite resistivity. More etched points can be introduced into areas where surface power density reduction is desired. This patterning of etched spots may, in principle, be effective in improving surface power density uniformity with a properly distributed etch pattern. A schematic diagram of patterning with etched spots is shown in FIG. 3B. FIG. 3B is a schematic top-down view of a trapezoidal heater 50 including a conductive film 51 between nonparallel metal traces 52a and 52b, with a conceptual horizontal line 56 extending across the entire conductive layer by dividing the layer into zones 54. The conductive film 51 is etched with spots 58. With the goal of a more uniform surface power density, the minimum dimension of the etched features, such as the diameter of the circular zone or the width of the line, should generally be small relative to the length of the heat conduction so that any thermal gradients are small. While suitable dimensions are generally material-dependent, these dimensions may generally be less than 1 millimeter and, in some embodiments, may be between 5 micrometers and 500 micrometers.A person of ordinary skill in the art will recognize additional ranges within these explicit ranges within the present disclosure, given the dimensions taken into account.

[0070] One model for patterning etched spots can be based on the above idea of ​​a conceptual stripe with approximately uniform current. As a rough approximation, we can assume that the change in sheet resistance along the stripe is approximately R s 0SA n / SA, where R s where 0 is the sheet resistance of the unetched material, SA is the surface area of ​​the stripe, and SA n is the adjusted surface area for the surface area lost due to etching. This assumes a linear relationship of the resistive material and its conductivity properties to surface area. Empirical corrections can be made to correct for errors in this approximation. This suggestion should provide a reasonable starting point. Each of the conceptual stripes can be adjusted with etched spots to achieve a nearly constant surface power density based on the percentage of conductor removed. This simple model does not consider any pattern of spots, but spacing the sparse etched spots from each other may be appropriate to avoid the etched spots interfering with each other. While etched stripes and etched spots would appear to present a relatively simple process approach, other etch shapes can be used as desired.

[0071] A third approach, directed at damaging the transparent conductive layer to modify its resistivity, can be applied over most or all of the area of ​​the transparent conductive film, or in smaller patterns similar to etching, and the degree of damage can be adjusted accordingly to achieve the goal of a more uniform surface power density. For example, a strong acid can be used to damage the conductive layer. At higher concentrations, the strong acid can be severely damaging, so it can be applied in a pattern, and the damage can be qualitatively similar to the etching process described above. At lower concentrations, the strong acid can be less damaging, so the concentration can be varied at various locations along the heater surface to provide a more consistent surface power density. The degree of damage can also affect the sheet resistance R along the stripe to compensate for changes in length along the stripe to produce a less variable surface power density. s It can be empirically adjusted to yield (i).

[0072] As noted above, the busbars / electrodes may be divided to accommodate the application of varying voltages along the heater surface to provide a more uniform surface power density. In this situation, the surface power density is P d =V(i) 2 / (R s L 2), where V(i) is the voltage along the stripe. It may be desirable to etch gaps between stripes at varying voltages to provide current flow across the heater with less current flow in the vertical direction, as depicted in FIG. 3B. The gaps between electrode / busbar segments may be 5 micrometers to 500 micrometers, and similarly, the etched gaps in the conductive layer between adjacent stripes may be 5 micrometers to 500 micrometers. Such a design would combine features from FIGS. 2C and 3B. The change in voltage may roughly correspond to the change in length (e.g., average per stripe), so that for smaller lengths, the voltage may be adjusted to be smaller by the same percentage. Generally, the number of divided busbar segments may be at least 3, and in some embodiments may be 4, 5, 6, 7, 8, 9, 10, 15, 20, or more than 20, or from 3 to 20. A person of ordinary skill in the art will recognize that additional ranges of segment numbers within the explicit ranges above are contemplated and are within the present disclosure. The power supply supplies the selected voltage to the busbar / electrode segments accordingly by using either an appropriate voltage converter such as a transformer for ac power, a DC-DC converter or other suitable circuitry.

[0073] As noted above, current flow can be numerically modeled, but approximate structures should adequately guide device design. The temperature dependence of resistivity is assumed to be negligible over the entire temperature range of interest. Empirical measurements can be useful for adjusting device design to achieve desired thermal properties of the heater for practical use. Desired optical properties and visual appearance can also influence device design. Increasing resistivity typically involves increasing transmittance (all else being equal) due to either less metal deposition or etching. If more uniform transmittance is desired, there may be ways to increase transmittance from some regions by dyeing or in various other routine ways, but increasing transmittance in some regions due to relatively low transmittance resulting from lower resistivity is less straightforward. Also, pattern visibility may provide aesthetic motivation for selecting some embodiments to improve surface power density over other approaches.

[0074] Device Structure Heaters are generally desirably incorporated into transparent structures. Within the range of structures in which heaters can be effectively used, the nature of transparency may differ from optical properties associated with, for example, displays. For example, the structure may be transparent in only a portion of the spectrum. Machine vision devices may require transparency in the infrared (but not necessarily in the visible). As another example, automobile windows may be tinted to limit glare. Some systems are designed to block UV light. In any event, structures incorporating heaters typically have a transmittance that is at least about 70% across the useful spectral range from infrared to visible light. For imaging systems operating in the infrared, the relevant spectral region may be 750 nm to 1750 nm, or a selected portion thereof. For optical systems used for human vision, such as vehicle windows, transmittance across all or a significant portion of the visible spectrum (which may be set from 400 nm to 750 nm or a different range, as desired) is at least about 70%, but in some embodiments may be significantly more transparent (e.g., at least about 80% transmittance), and in further embodiments, at least about 90% across the entire desired region of the spectrum. Transmittance measurements of a representative set of transparent silver nanowire films formed with sheet resistances of 30-50 ohms / sq. on either polyethylene terephthalate or polycarbonate optical films are presented in Figure 1. Over most of the relevant wavelength range, transmittance through the substrate is >90%, and corresponding values ​​with the substrate transmittance removed would yield transmittance values ​​of the conductive layer alone of >95%. Figure 4 is a schematic diagram of a vehicle portion 60 having a windshield 62 constructed with a transparent conductive film 61 and supporting bus bars 64a and 64b.

[0075] The transmittance through the structure incorporating the heater typically includes transparent structures on either side of the heater element itself, and the total transmittance accounts for the cumulative effect of the structural elements, including the heater element itself. The transmittance of the heater element correlates with the electrical resistance, with increased silver loading generally reducing transmittance and decreasing resistance. Thus, if lower transmittance is required, the electrical resistance can be reduced as desired. Generally, the heater element layer can have a sheet resistance of about 250 ohms / sq. to about 1 ohm / sq. and a visible or infrared transmittance of about 80% to about 99.5%. A person of ordinary skill in the art will recognize that additional ranges of sheet resistance and transmittance within the explicit ranges above are contemplated and are within the present disclosure.

[0076] To the extent that the device is a window that is part of another structure (such as a vehicle and / or sensing device), there is still generally a concept of an inner surface and an outer surface. If the heater structure is mounted directly onto a device component, the mounting surface may be integral to the device, with the inner surface being another layer of the device. In either case, however, the heater is typically mounted on a surface that is oriented in some way towards the outer surface (the surface that is generally expected to experience environmental challenges (such as low temperatures and / or precipitation, such as fog, rain, snow, ice, combinations thereof, etc.)). The heater structure requires protection from environmental attack, but accordingly needs to provide heat to the exterior of the protective layer that directly experiences the environmental attack. The various purposes of these protective materials should be balanced accordingly.

[0077] Depending on the process, the structure may be constructed sequentially, or may be assembled from separately assembled parts for assembly, or may be assembled from some combination of these techniques. A layer of optically clear adhesive may be used if necessary. If the transparent conductive film is formed on a sheet or roll of substrate for assembly onto the device, specific assembly instructions are followed. Regardless of the procedure, the final structure typically has a transparent inner layer, a transparent conductive film forming the heater structure, optional overcoats and / or undercoats, a transparent protective outer layer, and one or more optically clear adhesives.

[0078] 5A and 5B present schematic cross-sectional views of exemplary embodiments showing layer structures. FIG. 5A shows layer structure 70 including a conductive film 72 disposed between a substrate 74 and an optional overcoat 75. An optical adhesive layer 76 bonds a protective layer 77 to the optional overcoat 75 or to the conductive film 72 if no overcoat is present. FIG. 5B shows layer structure 78 including a conductive film 72 disposed between the optical adhesive layer 76 and the protective layer 77. The optional overcoat 75 is disposed between the conductive film 72 and the optical adhesive layer 76. The optical adhesive layer 76 bonds the optional overcoat 75 to the substrate 74, or bonds the conductive film 72 to the substrate 74 if no overcoat is present. Embodiments including layer structures are not limited to those shown in FIGS. 5A and 5B. In general, coatings and / or layers (such as undercoats, hardcoats, adhesive layers, protective films, and substrates) can be used in various configurations to form the layer structure, and in some configurations, not all of the layers may be used.

[0079] In some embodiments, the rough metal conductive layer may be covered with a polymer overcoat to provide mechanical protection to the conductive layer (especially during the manufacturing process). If useful for improved adhesion, a similar undercoat may be used to form a surface for deposition of a transparent conductive film that forms the heater structure. The terms overcoat and undercoat refer to layers relevant to the deposition process, but not to their orientation in the final device. The thickness of the overcoat or undercoat is typically about 250 nm or less (e.g., about 5 nm to about 200 nm). Suitable hardcoat polymers for use in forming the overcoat are typically highly crosslinked polymers having crosslinked polyacrylates, which may be combined with other crosslinked moieties, such as polyurethanes, epoxy polymers, polysiloxanes, and / or other crosslinked polymers. In some embodiments, it may be possible to select the overcoat so that haze is significantly reduced after application of the overcoat without significantly degrading other properties. Additionally, the thickness and composition of the overcoat may be selected so that sheet resistance measurements through the overcoat are not significantly altered compared to measurements without the overcoat. The incorporation of additional stabilizers into coatings is further described below and in U.S. Patent Application Publication No. 2018 / 0105704 to Yang et al. (hereinafter the '704 application), entitled "Stabilized Sparse Metal Conductive Films and Solutions for Delivery of Stabilizing Compounds," which is incorporated herein by reference. The incorporation of silver ions or other noble ions to provide stabilization or improved melting of molten metal nanostructured networks is described in U.S. Patent Application Publication No. 2021 / 0151216 to Yang et al., entitled "Coatings and Processing of Transparent Conductive Films for Stabilization of Sparse Metal Conductive Layers," which is incorporated herein by reference.

[0080] Depending on the use of the overcoat or undercoat, more generally, suitable overcoat polymers include, for example, polyethylene terephthalate (PET), polyethylene nephthalate (PEN), polyacrylates, poly(methyl methacrylate), polyolefins, epoxies, polyvinyl chloride, fluoropolymers, polyamides, polyimides, polysulfones, polysiloxanes, polyetheretherketones, polyethersulfones, polynorbornenes, polyesters, polystyrenes, polyurethanes, polyvinyl alcohols, polyvinyl acetates, acrylonitrile-butadiene-styrene copolymers, cyclic olefin polymers, cyclic olefin copolymers, polycarbonates, copolymers thereof, or mixtures thereof. The overcoat polymer can be applied by solution coating (e.g., by UV light exposure) with optional subsequent crosslinking. The overcoat and undercoat polymers can be applied using the same techniques as the nanowire ink. More generally, the overcoat can have an average thickness of about 5 nm to about 2 micrometers, in another embodiment about 7 nm to about 1 micrometer, and in other embodiments about 8 nm to about 250 nm. A person of ordinary skill in the art will recognize that additional ranges of overcoat thickness within the explicit ranges above are contemplated and are within the present disclosure.

[0081] The overcoat and / or undercoat may contain stabilizing compounds that can help maintain good electrical conductivity even when exposed to environmental attack. Previous research has found that vanadium(+5) compounds can be effective in providing the desired stability. See U.S. Patent Application Publication No. 2018 / 0105704 (hereinafter the '704 application) to Yang et al., entitled "Stabilized Sparse Metal Conductive Films and Solutions for Delivery of Stabilizing Compounds," incorporated herein by reference. Other research has found that iron(+2) and other metal salts can be effective stabilizers. See U.S. Patent Application Publication No. 2015 / 0270024A1 to Allemand, entitled "Light Stability of Nanowire-Based Transparent Conductors," incorporated herein by reference. Cobalt(+2) ions complexed with ligands have also been found to provide stabilization within fused metal nanostructured network layers. The performance of these stabilizing compositions alone or in combination may be enhanced through the incorporation of precious metal ions, particularly silver ions within the coating (overcoat and / or undercoat) to further enhance stability, likely due to further melting of the structure through migration of the metal ions. The benefits of precious metal ions in the coating may be exploited similarly to pentavalent vanadium during actual use of the product structure, but alternatively or additionally, it may be beneficial to have precious metal ions in the coating during post-deposition heat / humidity treatment prior to assembly into the final product.

[0082] Suitable vanadium+5 compounds include compounds with vanadium as a cation as well as compounds with vanadium as part of a multiatomic anion (metavanadates (VO3 - ) or orthovanadate (VO4 -3), etc. Corresponding salt compounds with the pentavalent vanadium anion in the oxometal salts include, for example, ammonium metavanadate (NH4VO3), potassium metavanadate (KVO3), tetrabutylammonium vanadate (NBu4VO3), sodium metavanadate (NaVO3), sodium orthovanadate (Na3VO4), other metal salts, etc., or mixtures thereof. Suitable pentavalent vanadium cation compounds include, for example, vanadium oxytrialkoxides (VO(OR)3, where R is an alkyl group (e.g., n-propyl, isopropyl, ethyl, n-butyl, etc., or combinations thereof), vanadium oxytrihalides (VOX3, where X is Cl, F, Br, or combinations thereof), vanadium complexes such as VO2Z1Z2 (where Z1 and Z2 are independently ligands (such as those described below for Co+2 complexes), or combinations thereof). In the coating, pentavalent vanadium may be present, for example, from about 0.01 wt% to about 9 wt%, in another embodiment from about 0.02 wt% to about 8 wt%, and in additional embodiments from about 0.05 wt% to about 7.5 wt%. In a coating solution, the solution typically contains solids, primarily comprising the curable polymer, along with some solvent. Generally, the corresponding coating solution may have a concentration of pentavalent vanadium compound from about 0.0001 wt% to about 1 wt%. One of ordinary skill in the art will recognize that additional ranges of concentrations within the explicit ranges above are contemplated and are within the present disclosure. In additional or alternative embodiments, iron(+2) or other metal ions may be included in addition to or in place of the pentavalent vanadium ions.

[0083] Although the overcoat is thin, it may still be desirable to promote thermal conduction through the overcoat. Nanoparticles of a high thermal conductivity material can be added to the overcoat to improve thermal conductivity while maintaining transparency. Suitable high thermal conductivity materials can be selected from the group consisting of diamond, graphene, silicon nitride, boron nitride, aluminum nitride, gallium arsenide, indium phosphide, or mixtures thereof. The high thermal conductivity material can have a thermal conductivity of at least about 30 W / (mK). Some high thermal conductivity nanoparticles can also provide wear resistance as an added benefit. In some embodiments, the nanoparticles can have an average primary particle size of about 100 nm or less. Transparent coatings formed from nanoparticle-loaded polymers can contain between about 0.01 weight percent (wt%) and 70 wt% property-enhancing nanoparticles, between about 0.05 wt% and about 60 wt% in other embodiments, between about 0.1 wt% and about 50 wt% in other embodiments, and between about 0.2 wt% and about 40 wt% property-enhancing nanoparticles in additional embodiments. The transparent coating may further include a polymeric binder, optional property modifiers such as a crosslinker, a wetting agent, a viscosity modifier, and / or a stabilizer, such as an antioxidant and / or a UV stabilizer. A person of ordinary skill in the art will recognize that additional ranges of average particle size and nanoparticle concentrations in the loaded polymer within the explicit ranges above are contemplated and are within the present disclosure. The use of property-enhancing nanoparticles in optically clear overcoats is further described in U.S. Pat. No. 10,738,212 to Virkar et al., entitled "Property Enhancing Fillers for Transparent Coatings, and Transparent Conductive Films," incorporated herein by reference.

[0084] Depending on the particular application, the base structure supporting the heater can take a variety of forms, but the base structure typically provides a solid, if not flat, foundation. Thus, for example, the base structure could be an automobile window. The base structure could have multiple layers, each with or without patterning. In either case, a protective layer covers the transparent conductive film forming the heater surface to protect the heater. The protective layer should provide its protective function, but the specific selection can be influenced by the properties of the base structure (e.g., due to shape and / or rigidity, etc.).

[0085] In some embodiments, the final structure includes a transparent protective layer comprising a polymer and / or glass in one or more layers. Suitable polymers may include hardcoat polymers, such as those described above, or may include a layer of material with a hardcoat top surface for abrasion resistance. It may be desirable for the transparent protective layer and overcoat layer to provide good thermal conductivity to provide the desired heater performance in providing heat to the surface of the structure. As noted above, applicants have described the incorporation of desirable nanostructures to modify the properties of the transparent coating. In particular, highly thermally conductive nanoparticle additives, such as nanodiamond, graphene, silicon nitride, boron nitride, aluminum nitride, gallium arsenide, indium phosphide, or mixtures thereof, may be incorporated. To provide good transparency, the additive particles typically have an average particle size of about 100 nm or less. These thermally conductive nanoparticles may be incorporated into other protective layers of the device structure in addition to the overcoat layer. The loading concentration of the thermally conductive additive may be selected to balance the mechanical, optical, and thermal properties of the protective material. Heat conductive glass is also known; see, for example, Atsuo et al., JP 2012111665A, entitled "Heat Conductive Glass, and Method for Manufacturing Same," which is incorporated herein by reference.

[0086] While the structure is transparent, transmittance may not need to be extremely high, depending on the specific application. Thus, transmittance of 75% or greater may be preferred, and for some applications, transmittance of less than 75% may be preferred. The hardcoat protective layer may be adapted for other automotive uses, such as hardcoats for headlights. The hardcoat polymer is typically at least about 2 micrometers thick, in some embodiments at least about 4 micrometers thick, and in further embodiments, from about 5 micrometers to about 250 micrometers thick. An example of a hardcoat for an automotive headlight is described in U.S. Patent Application Publication No. 2011 / 0003142 to Asuka et al. (hereinafter the '142 Application), entitled "Nanoparticle Sol-Gel Composite Hybrid Transparent Coating Materials," which is incorporated herein by reference. The '142 Application discusses the inclusion of nanoparticles, including silicon nitride, and may include other thermally conductive nanoparticles. The material in the '142 Application is also a silicate, which should provide good thermal conductivity. Processing of this structure should not involve excessive heat that could damage the heating element. The acceptable temperature may depend on how long the temperature is applied. The '142 application discusses an embodiment that uses a catalyst to allow processing of the coating below 100°C, which would be suitable for the heating element for a reasonable period of time.

[0087] As noted above, device resistance depends on the sheet resistance of the heating element and the configuration of the heater electrodes. In particular, current flows between the two electrode poles. Bus bars are typically not transparent, even if the heater element is transparent. While it may be desirable to place the bus bars within the device frame to hide them from view, as noted above, this may tend to result in the electrodes formed by the bus bars being farther apart than in other configurations, and longer distances tend to result in higher resistance. If the resistance is nevertheless low enough and / or the voltage can be adjusted high enough, the desired power output may be achieved with bus bars placed within the device frame. In alternative or additional embodiments, the bus bars may be placed within the field of view without unduly obstructing the field of view to produce lower resistance and higher power output for available voltages. Based on the above discussion and as shown in some examples, adjacent placement of multiple electrode pairs in an effectively parallel configuration may not significantly alter performance, assuming that the bus bars can be assumed to have negligible resistance. The distance between bus bars of opposite polarity may affect resistance and therefore help reduce it. Additional electrode pairs can then be used as desired to cover larger areas. Some examples also describe non-rectangular heater surfaces, such as electrode placement to form trapezoidal shapes of heater areas.

[0088] example Several examples illustrate the heating performance of a transparent thin layer of conductive coating based on platinum-coated silver nanowires on a polymer substrate. Failure limits as well as parameters contributing to a specific power output from the heater are discussed. A comparison of the heating performance of platinum-coated silver nanowires versus uncoated silver nanowires is also described.

[0089] General Materials and Methods The silver nanowires (Ag NWs) were available in the form of ActiveGrid® GEN5 ink obtained from the assignee of the present invention, C3Nano, Inc. The silver nanowires had an average diameter of about 20-22 nm and were prepared as described in the above-cited '230 patent. Platinum-coated silver nanowires (Ag@Pt) were prepared by coating Ag NWs with platinum using the direct deposition method described in the above-cited '534 patent. The platinum coating may include approximately one or several monolayers of platinum on the silver nanowires.

[0090] Inks containing platinum-coated silver nanowires were made by combining nanowires at levels of 0.1 wt% to 1.0 wt% in water and a small amount of alcohol with 0.01 wt% to 1 wt% fiber-based binder, 0.002 wt% to 0.01 wt% fluorosurfactant, and 0.005 wt% to 0.1 wt% silver salt. The inks were slot-coated at various thicknesses onto 125-micrometer-thick polyethylene terephthalate (PET) substrates, with some coatings being performed on 175-micrometer polycarbonate substrates. The films were dried by heating in an oven at 100°C for 10 minutes. The average sheet resistance was measured using a non-contact measurement device from SURAGUS GmbH.

[0091] Each coated sample was cut into several sections, and silver paste obtained from BTL CHME LLC was applied to the coated side of the specimen to define a multi-dimensional rectangle, followed by annealing at approximately 130°C for 30 minutes to cure and dry the silver paste. The trace designs and their evaluation are discussed in the context of the following examples. Wire leads were connected to the silver paste as indicated in each of the trace designs, and the circuit resistance based on the trace designs and transparent conductor conduction was measured using a commercially available multimeter.

[0092] To assess the heating uniformity across the transparent conductor, thermal images were acquired using a FLIR thermal imaging camera from MoviTHERM.

[0093] Example 1 This example was performed to compare the relative stability of films formed from platinum-coated silver nanowires with films formed from equivalent silver nanowires without a metal coating. The nanowire-based inks were coated separately onto 125 micrometer PET, and the coating thickness was selected accordingly. Three coated films of platinum-coated nanowires and two coated films of silver nanowires without a metal coating were fabricated to provide a range of sheet resistances. The results are shown in Table 1.

[0094] [Table 1]

[0095] Each coating sample shown in Table 1 was constructed for testing by using the trace design shown in Figure 6. Trace design 80 shows silver paste traces 84a and 84b, each having a width of 2 mm and defining an area 82 of the coating sample. Each coating sample had a length L1 = 12.5 cm and a width W = 3.5 cm. The neck area of ​​the design had a length L2 = 1 cm. Surface cuts 86 were made between the traces, as shown by the dashed lines, and leads were connected to define the positive and negative terminals (as shown in Figure 6).

[0096] The heating performance of each sample in the constructed samples was approximately 0.1 W / cm 2 ~about 0.7W / cm 2 The temperature was evaluated by measuring the change in temperature as a function of power density in the range of P d is the formula P d =(V 2 The thermal stability of the platinum-coated silver nanowires can be calculated based on the power density (V / R) / A, where V is the applied voltage, R is the measured circuit resistance, and A is the area of ​​the heater element in selected units. A plot showing the change in temperature as a function of power density is shown in FIG. 7A. The data indicate that platinum-coated silver nanowires exhibit improved thermal stability compared to uncoated silver nanowires. A plot showing the change in temperature as a function of voltage is shown in FIG. 7B.

[0097] Example 2 This example was performed to evaluate a test design configuration, including terminal insulation and trace thickness, for the evaluation of the heating performance of transparent conductive coatings formed from nanowire inks. This example also measured temperature as a function of power density.

[0098] Each of the coated samples, Ag@Pt-100, Ag@Pt-130, and Ag@Pt-200, was constructed for testing by using the trace designs shown in Figures 8A and 8B (referred to as Method A and Method B, respectively). Trace design 90 shows silver paste traces 94a and 94b, which define area 92 of the coated sample. Surface cuts 96 were made between the traces, as shown by the dashed lines, and leads were connected to define the positive and negative terminals, as shown in Figure 8A. Trace design 100 shows silver paste traces 104a and 104b, which define area 102 of the coated sample. Area 106 (3.5 cm x 12.5 cm with a 1 cm x 1 cm neck area) was cut between the traces, and leads were connected to define the positive and negative terminals, as shown in Figure 8B. Methods A and B were each evaluated by using two different widths of traces formed by the silver paste. One set included traces each having a width of 2 mm, and the other set included traces each having a width of 3 mm.

[0099] Methods A and B with two trace widths were evaluated for each test sample by measuring the circuit resistance. The results are shown in Table 2. The data suggests that Methods A and B provide similar isolation between the positive and negative terminals with only minor differences due to the width of the trace.

[0100] [Table 2]

[0101] The heating performance of the test samples Ag@Pt-100, Ag@Pt-130, and Ag@Pt-200 was approximately 0.1 W / cm 2 ~approx. 0.8W / cm2 The temperature was measured as a function of the power density in the range of 1000 Hz to 1000 Hz. In the experiment, the voltage was gradually increased after the temperature was stabilized until the sample could no longer provide a value for the power density, and the temperature was measured near the center of the sample at each voltage. d can be calculated as described above with respect to Example 1. Plots showing the change in temperature as a function of power density are shown in Figures 9A-9C. The data show little to no difference between Methods A and B, and little to no difference due to the change in silver paste width for each of the methods.

[0102] Thermal images are shown in Figures 10A-10C. These images suggest that heating is uniform throughout the coating when heat is generated from a single side of the test sample with a 2 mm wide trace, as shown in Figures 8A and 8B (Methods A and B, respectively). Temperatures were taken at approximately the center of the heated area, and the results are shown in Table 3.

[0103] [Table 3]

[0104] The breakdown voltages are shown in Table 4. The data suggests that the higher the resistance, the better the voltage withstand capability.

[0105] [Table 4]

[0106] Example 3 This example was performed to evaluate a test design configuration: here, the coating is supplied with current from opposite sides of the test sample (compared to providing current from one side as described for Example 2).

[0107] Coated samples Ag@Pt-130 and Ag@Pt-200 were constructed for testing by using the trace designs shown in FIGS. 11A and 11B (referred to as Method C and Method D, respectively). Trace design 110 shows silver paste traces 114a and 114b that define area 112 of the coated sample. Surface cuts 116a and 116b were made between traces on opposite sides as shown, and leads were connected to define two sets of positive and negative terminals as shown in FIG. 11A. Trace design 120 shows silver paste traces 124a, 124b, 125a, and 125b that define area 122 of the coated sample. Gap 128a separates traces 124a and 125a, and gap 128b separates traces 124b and 125b. Gap G between gaps 128a and 128b was approximately 1 mm. Leads were connected to the traces to define the positive and negative terminals as shown in FIG. 11B. The width of the silver paste traces was 2 mm.

[0108] Methods C and D were evaluated for Ag@Pt-130 and Ag@Pt-200 by measuring the circuit resistance, and the results are shown in Table 5. The data suggest that the heating elements exhibit a greater circuit resistance when evaluated using Method D with a 1 mm gap compared to Method C.

[0109] [Table 5]

[0110] The heating performance of the test samples Ag@Pt-130 and Ag@Pt-200 was approximately 0.1 W / cm 2 ~approx. 0.8W / cm 2 The temperature was evaluated by measuring the change in temperature as a function of power density in the range of P dcan be calculated as described above with respect to Example 1. Plots showing the change in temperature as a function of power density are shown in Figures 12A and 12B. As described above, the power density was varied by changing the voltage accordingly. The data suggest that Ag@Pt-130 and Ag@Pt-200 exhibit relatively stable performance using both Methods C and D.

[0111] Thermal images of test sample Ag@Pt-200 evaluated using Methods C and D are shown in Figures 13A and 13B, respectively. The images suggest that heating is uniform throughout the coating as heat is generated from both sides of each sample. Temperatures were taken at approximately the center of the heated area, and the results are shown in Table 6. When heated using Method D, the temperature of Ag@Pt-200 is slightly lower compared to Method C.

[0112] [Table 6]

[0113] Example 4 This example was performed to test the design configuration: here the coating is supplied with a diagonal current across the sample.

[0114] Coated samples Ag@Pt-130 and Ag@Pt-200 were constructed for testing by using the trace design (referred to as Method E) shown in FIG. 14 (Ag@Pt-100 was not evaluated because its dielectric strength was less than 40 V). Trace design 130 shows silver paste traces 134a and 134b that define the area 132 of the coated sample. Leads were connected to the positive and negative terminals as shown in FIG. 14. Trace design 130 shows silver paste traces 134a and 134b that define the area 132 of the coated sample. The width of the silver paste traces was 2 mm.

[0115] The heating performance of the test samples Ag@Pt-130 and Ag@Pt-200 was approximately 0.1 W / cm 2~approx. 0.8W / cm 2 The temperature was evaluated by measuring the change in temperature as a function of power density in the range of P d can be calculated as described above with respect to Example 1. A plot showing the change in temperature as a function of power density is shown in FIG. 15A. As described above, the voltage was adjusted to achieve the power density. The data suggests that Ag@Pt-130 and Ag@Pt-200 exhibit relatively stable performance using Method E. A plot showing the change in temperature as a function of voltage is shown in FIG. 15B. As can be seen in FIG. 15A, the temperature increase depends on the sheet power density, and as can be seen in FIG. 15B, for a particular voltage, a larger temperature increase is observed for the lower sheet resistance heater sample. The circuit resistance for each sample was 50-70 ohms.

[0116] Thermal images of the test samples Ag@Pt-130 and Ag@Pt-200 are shown in Figures 16A and 16B, respectively. The images suggest that heating throughout the coating is uniform and stable when heat is generated diagonally. Temperatures were taken at approximately the center of the heated area, and the results are shown in Table 7. The temperature of Ag@Pt-200 is lower compared to that of Ag@Pt-130.

[0117] [Table 7]

[0118] These samples were also used to evaluate temperature as a function of time. Plots of temperature versus time at constant voltages of 25 V and 35 V are shown in Figures 17A and 17B, respectively. The data show similar changes in temperature over time at both voltages, but as expected, the temperature is higher for samples tested at a constant voltage of 35 V compared to 25 V. Overall, the data suggest a gradual increase in temperature that reaches a plateau.

[0119] Example 5 This example was carried out to explore the design configuration: here, the coating sample is irregularly shaped as a trapezoid, and the coating is etched according to lines so that the resistance distribution of the coating varies.

[0120] 18 is a top-down view of a trapezoid 140 with approximate dimensions to indicate the size and shape of the coated sample used in this example. Area 142 is defined by parallel sides 144a and 144b and sides 146a and 146b that are at approximately 60° to side 144b as indicated by angles 148a and 148b, respectively. Side 144a has a length L1 that is approximately 80 mm, and side 144b has a length L2 that is approximately 160 mm. Trapezoid 140 has a height H of approximately 69 mm.

[0121] A coated sample of Ag@Pt-130 on a PET substrate (Rs ≈130 ohms / sq.) was prepared and cut into trapezoidal samples as shown in Figure 18. Approximately 5 mm of silver paste traces were formed on opposing sides 146a and 146b. Leads were connected to define positive and negative terminals on either the short side as shown in Figure 19A or the long side as shown in Figure 19B.

[0122] Figures 20A and 20B show thermal images of coated samples when configured as shown in Figures 19A and 19B, respectively. The images show non-uniform heating for both configurations, with opposite corners of the base having the lowest temperatures. The non-uniform heating is visible.

[0123] Additional coated samples of Ag@Pt-130 on PET substrate were prepared to test for improved heating uniformity, and the coated substrate was cut into trapezoidal samples as shown in Figure 21. Heater 160 includes coated sample 161 with approximately 5 mm silver paste traces 162a and 162b formed on both sides. Zones 1-10 are conceptually designated by dotted lines 166, dividing coated sample 161 into ten zones 164, designated Zones 1-10. Samples 1 and 2, each with Zones 1-10, were prepared, with each zone either unetched or etched with a different percentage of etched area, as shown in Table 8 (Sample 1) and Table 9 (Sample 2). The number of etched lines was greater for Sample 1 compared to Sample 2. Laser ablation was used to etch lines that were approximately 50 micrometers thick. Tables 8 and 9 show the correlation between each zone and the etched line length, percent etched area, and number of etched lines for Samples 1 and 2, respectively.

[0124] [Table 8]

[0125] [Table 9]

[0126] The leads were connected to define the positive and negative terminals on the long sides as shown in Figure 19B. The total resistance (circuit or wire resistance) of each heater was measured. Temperature measurements were taken for each zone by applying a voltage of 24 V and reading the temperature after 60 seconds. The results are shown in Table 10. Figures 22A, 22B, and 22C show thermal images of the coated samples: from top to bottom: unetched sample, Sample 1, and Sample 2.

[0127] [Table 10]

[0128] This experiment was repeated except that the leads were connected to define the positive and negative terminals on the short sides as shown in Figure 19A. The results are shown in Table 11. Figures 23A, 23B, and 23C show thermal images of the coated samples: from top to bottom: unetched sample, Sample 1, and Sample 2. While the reproducibility of the samples is reasonable, careful examination of the samples revealed some imperfections in the laser etching that should be addressed by process adjustments.

[0129] [Table 11]

[0130] The above embodiments are intended to be illustrative and not limiting. Additional embodiments are within the scope of the following claims. In addition, while the present invention has been described with reference to specific embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the present invention. Any incorporation by reference of the above patent documents is limited to not incorporating any subject matter contrary to the explicit disclosure herein. To the extent that a particular structure, composition, and / or process is described herein in terms of a component, element, composition, or other barrier, it will be understood that the disclosure not only covers embodiments that include the particular embodiment, particular component, element, composition, other barrier, or combination thereof, as suggested in the discussion, but also covers embodiments that consist essentially of such particular component, composition, or other barrier, or combinations thereof, which may include additional features that do not alter the basic nature of the subject matter. The use of the term "about" herein refers to the predictive uncertainty of the associated value as would be understood in a particular context by a person of ordinary skill in the art.

Claims

1. 1. A heater structure comprising: a substrate; a transparent resistive heat generating element mounted on the substrate; an electrode disposed in electrical contact with the transparent resistive heat generating element, the electrode being positioned along a boundary of a heated region that defines a path of current through the transparent resistive heat generating element, thereby forming the heated region; and a power source connected to the electrode, wherein the transparent resistive heat generating element comprises a roughened metal conductive layer including nanowire segments and having a sheet resistance of about 0.5 ohms / sq. to about 300 ohms / sq.

2. The heater structure of claim 1 , wherein the nanowire segments comprise noble metal coated silver.

3. At least about 0.25 cm ２ 3. The heater structure of claim 1, having an unpatterned area of:

4. 4. The heater structure of claim 1, wherein the circuit includes a segmented electrode having a plurality of segments configured to receive a series of voltages across the segments to generate a more uniform surface power density.

5. 5. The heater structure of claim 4, wherein the transparent conductive film is etched to form conductive domains that isolate the segmented electrode from a counter electrode to have an electrically insulating stripe as part of the heater.

6. The heater structure of any one of claims 1 to 5, wherein the metal traces do not form a grid.

7. A heater structure according to any one of claims 1 to 6, wherein said electrodes of opposite polarity are generally parallel to form a rectangular heating surface.

8. A heater structure according to any one of claims 1 to 6, wherein the electrodes of opposite polarity are angled to form a trapezoidal heater surface.

9. The power source may deliver at least about 1 volt to the electrodes, and the transparent resistive heat-generating element may deliver at least about 0.05 W / cm ２ A heater structure according to any one of claims 1 to 8, capable of producing a sustained surface power density of

10. A heater structure according to any one of claims 1 to 9, wherein the transparent resistive heater element is capable of producing a sustained surface temperature of at least 220°C.

11. A heater structure according to any preceding claim, wherein the rough metal conductive layer comprises a fused metal nanostructured network.

12. The heater structure of any one of claims 1 to 11, wherein the rough metal conductive layer comprises a plurality of conductive stripes.

13. 13. The heater structure of any one of claims 1 to 12, having a non-rectangular heater surface and an etched transparent conductive layer having no more than about 25% of the transparent conductive layer area removed and / or excluded from conductive paths.

14. 14. The heater structure of claim 13, wherein the rough metal conductive layer is etched with a plurality of lines while maintaining a conductive path between electrodes of opposite polarity, and the surface power density is more uniform than that with a corresponding unetched transparent heater structure.

15. 14. The heater structure of claim 13, wherein said rough metal conductive layer is etched with a plurality of spots, and said surface power density is more uniform than that with a corresponding unetched transparent heater structure.

16. The heater structure of any one of claims 1 to 15, wherein the transparent heater structure has a transmittance of at least about 70% over the entire wavelength range of about 400 nm to about 750 nm.

17. The heater structure of any one of claims 1 to 16, wherein the transparent heater structure has a transmittance of at least about 80% over the entire wavelength range of about 400 nm to about 750 nm.

18. The heater structure of any one of claims 1 to 17, wherein the transparent heater structure has a transmittance of at least about 70% over the entire wavelength range of about 750 nm to about 1750 nm.

19. 19. The heater structure of any one of claims 1 to 18, wherein the rough metal conductive layer has a sheet resistance of about 0.5 ohm / sq to about 250 ohm / sq.

20. The heater structure of any one of claims 1 to 19, wherein the substrate comprises a transparent polymer substrate.

21. The heater structure of any one of claims 1 to 19, wherein the substrate comprises glass.

22. 22. The heater structure of claim 1, wherein the substrate comprises a first transparent substrate, and the transparent heater structure further comprises a second transparent substrate mounted on the transparent resistive heat generating element opposite the first transparent substrate.

23. A heater structure according to any preceding claim, wherein the structure is bendable and flexible.

24. A vehicle window comprising a heater arrangement according to any one of claims 1 to 23.

25. An infrared-based imaging system comprising a heater structure according to any one of claims 1 to 23.

26. 1. A method of heating a surface of a structure, the method comprising heating a surface of a structure at a rate of at least about 0.05 W / cm for at least about 30 seconds. ２ delivering a voltage of at least 1 volt to a heating element to generate a surface power density of about 0.5 ohms / sq. to about 300 ohms / sq., wherein the heating element comprises a transparent conductive film comprising nanowire segments within a rough metal conductive layer and having a sheet resistance of about 0.5 ohms / sq. to about 300 ohms / sq.

27. 27. The method of claim 26, wherein the nanowires comprise noble metal coated silver.

28. The transparent structure is at least about 0.25 cm ２ 28. The method of claim 26 or claim 27, having unpatterned areas of

29. 29. The method of any one of claims 26 to 28, wherein the voltage is delivered by a circuit in electrical contact with the transparent structure, the circuit comprising a segmented electrode having a plurality of segments configured to receive a series of voltages across the plurality of segments to generate a more uniform surface power density.

30. 30. The method of claim 29, wherein the structure includes a conductive domain that has an electrically insulating stripe as part of the heater to isolate the segmented electrode from a counter electrode.

31. A method according to any one of claims 26 to 30, wherein the voltage is carried by metal traces which are not arranged to form a grid.

32. A method according to any one of claims 26 to 31, wherein the voltage is delivered by electrodes of opposite polarity that are generally parallel to form rectangular faces of the transparent conductive element.

33. A method according to any one of claims 26 to 31, wherein the voltages are delivered by electrodes of opposite polarity that are angled to form trapezoidal surfaces of the transparent conductive element.

34. The heating element has a resistance of at least about 0.05 W / cm ２ 34. The method of any one of claims 26 to 33, capable of generating a sustained surface power density of

35. A method according to any one of claims 26 to 34, wherein the heating element is capable of producing a sustained surface temperature of at least about 220°C.

36. The method of any one of claims 26 to 35, wherein the rough metal conductive layer comprises a fused metal nanostructured network.

37. The method of any one of claims 26 to 36, wherein the rough metal conductive layer comprises a plurality of conductive stripes.

38. 38. The method of any one of claims 26 to 37, wherein the structure has a non-rectangular heater surface and the rough metal conductive layer is etched with no more than about 25% of the layer area removed and / or excluded from conductive paths.

39. 39. The method of claim 38, wherein the rough metal conductive layer is etched with a plurality of lines while maintaining a conductive path between electrodes of opposite polarity, and the surface power density is more uniform than with a corresponding unetched transparent heater structure.

40. 39. The method of claim 38, wherein the rough metal conductive layer is etched with a plurality of spots, and the surface power density is more uniform than that with a corresponding unetched transparent heater structure.

41. The method of any one of claims 26 to 40, wherein the structure has a transmittance of at least about 70% over the wavelength range of about 400 nm to about 750 nm.

42. 42. The method of any one of claims 26 to 41, wherein the structure has a transmittance of at least about 80% over the wavelength range of about 400 nm to about 750 nm.

43. 42. The method of any one of claims 26 to 41, wherein the structure has a transmittance of at least about 70% over the wavelength range of about 750 nm to about 1750 nm.

44. 44. The method of any one of claims 26 to 43, wherein the transparent conductive film has a sheet resistance of about 0.5 ohm / sq to about 250 ohm / sq.

45. The method of any one of claims 26 to 44, wherein the transparent conductive film is mounted on a transparent polymer substrate.

46. The method of any one of claims 26 to 44, wherein the transparent conductive film is mounted on glass.

47. The method of any one of claims 26 to 44, wherein the transparent conductive film is mounted between a first transparent substrate and a second transparent substrate.

48. A method of fabricating a transparent conductive heater on a surface, the method comprising: 1) forming a coating of metal nanowires from a solution; 2) drying the nanowire coating to form a transparent conductive film; and 3) forming conductive electrodes by establishing electrical connections to the transparent conductive film by circuit paths along the transparent conductive film between the electrodes, forming a heater surface having a significant surface power density generated across the heater surface from an applied voltage between two electrodes.

49. 49. The method of claim 48, wherein the surface is a non-planar transparent surface designed for exposure to the ambient environment, and further comprising applying a protective overcoat over the heater surface.

50. 50. The method of claim 49, wherein the protective overcoat is laminated in place with an optically clear adhesive.

51. 50. The method of claim 49, wherein the protective overcoat is applied as a liquid and then cured.

52. 52. The method of any one of claims 48 to 51, wherein the surface is on a polymer sheet coated in a roll-to-roll process, the method further comprising attaching a section of the coated polymer sheet after forming an electrode to a transparent device surface to be heated.

53. 53. The method of claim 52, wherein the device surface is a vehicle window.

54. 53. The method of claim 52, wherein the device surface is a window of an imaging device mounted on a vehicle.

55. 55. The method of any one of claims 48 to 54, wherein the precursor solution further comprises metal ions, and wherein said drying forms a fused metal nanostructured network, and wherein said metal nanowires comprise noble metal coated silver nanowires.

56. The electrode has a resistance of at least about 0.05 W / cm ２ 56. The method of any one of claims 48 to 55, wherein the surface of the substrate is connected to a power source capable of providing a voltage sufficient to generate a surface power density of

57. 57. The method of claim 56, wherein the electrodes are comprised of generally parallel, opposite polarity electrodes, and the sheet resistance of the heater surface is between about 0.5 ohms / sq. and about 250 ohms / sq.

58. 57. The method of claim 56, wherein the electrodes are comprised of opposite polarity electrodes that are angled relative to one another to form a trapezoidal shaped heating surface, and the resistivity of the transparent conductive film is non-uniform across the heater surface to achieve a more uniform surface power density compared to a corresponding heater surface having a uniform resistivity.

59. 59. The method of claim 58, wherein the non-uniform resistivity is formed by etching the transparent conductive film.

60. 59. The method of claim 58, wherein the electrode is divided into segments having connectors for a power source configured to provide different voltages to different segments of the electrode.

61. 1. A vehicle comprising: a visualization device having a surface exposed to an ambient environment, the visualization device transmitting and / or receiving infrared light over a specific range having wavelengths from 750 nm to 3 micrometers; a transparent resistive heat-generating element interfaced with the surface of the visualization device; and a control element, the transparent resistive heat-generating element comprises a roughened metal conductive layer comprising nanowire segments having a sheet resistance of about 1 ohm / sq. to about 300 ohm / sq. and a transmittance of infrared light over a specified range of at least about 70%.

62. 62. The vehicle of claim 61, wherein the nanowire segments comprise precious metal coated silver.

63. 63. The vehicle of claim 61 or claim 62, wherein the transparent resistive heat-generating element has a visible light transmittance of at least about 80%.

64. 64. A vehicle as claimed in any one of claims 61 to 63, wherein the transparent resistive heat-generating element is configured to heat a window.

65. 65. The vehicle of any one of claims 61 to 64, wherein the transparent resistive heat-generating element is non-rectangular.

66. A vehicle as claimed in any one of claims 61 to 64, wherein the transparent resistive heat-generating element is a heating structure as claimed in any one of claims 1 to 23.

67. A heater structure having a non-rectangular transparent heater surface and including a conductive element, the conductive element having a non-uniform resistance, the conductive element being between one or more pairs of electrodes such that power dissipation over the surface of the heater is more uniform relative to power dissipation in an equalization structure having a uniform transparent conductive coating.

68. 68. A heater structure as claimed in claim 67, wherein the electrodes are connected to a power source to provide a voltage between each member of a pair of electrodes.

69. 69. The heater structure of claim 68, wherein the one or more pairs of electrodes include at least two pairs of electrodes having respective members of a pair of aligned adjacent corresponding members of adjacent pairs, the pairs of electrodes being connected to a power source that provides different voltages between respective members of the different pairs.

70. 69. The heater structure of claim 68, wherein the one or more pairs of electrodes include at least two pairs of electrodes having respective members of a pair of aligned adjacent corresponding members of adjacent pairs, and wherein a series electrical resistor between the power source and the electrodes provides an effective voltage drop between the corresponding electrode pairs.

71. A heater structure according to any one of claims 67 to 70, wherein the rough metal conductive layer is etched.

72. 72. The heater structure of claim 71, wherein said rough metal conductive layer is etched in selected locations.

73. 72. The heater structure of claim 71, wherein the rough metal conductive layer is etched in lines across the heater surface.

74. 74. The heater structure of any one of claims 67 to 73, wherein the rough metal conductive layer has different metal loadings at different locations along the heater surface.

75. 74. A heater structure according to any one of claims 67 to 73, wherein the coarse metal conductive layer comprises a fused metal nanostructured network forming metal nanowire segments, the nanowire segments comprising noble metal coated silver.

76. 76. The heater structure of any one of claims 67 to 75, wherein said transparent conductive film has a sheet resistance at any selected point of between about 0.5 ohm / sq. and about 250 ohm / sq.

77. The electrode has a resistance of at least about 0.05 W / cm ２ 77. The conductive structure of claim 76, capable of being connected to a power source to provide a surface power density of

78. 1. A heater structure comprising: a transparent conductive layer; electrodes connected to a voltage source; and a polymer overcoat on at least a portion of the transparent conductive layer, wherein the transparent conductive coating provides electrical conduction between the electrodes to provide heating, and the polymer overcoat comprises nanoparticles that provide improved thermal conductivity.

79. 79. The heater structure of claim 78, wherein said thermally conductive nanoparticles comprise diamond, graphene, silicon nitride, boron nitride, aluminum nitride, gallium arsenide, indium phosphide, or mixtures thereof.

80. A heater structure according to claim 78 or claim 79, comprising one or more of the features of claims 2 to 23.

81. A method for fabricating a transparent conductive heater on a non-planar surface, the method comprising attaching a flexible polymer substrate having a nanowire-based transparent conductive heater surface onto the non-planar surface.

82. 82. The method of claim 81, wherein the rough metal conductive layer is formed onto the flexible polymer substrate by using solution coating of a precursor ink in a roll-to-roll process.

83. 83. The method of claim 82, wherein the precursor ink comprises metal nanowires and metal ions, and drying of the coated precursor solution results in the formation of a fused metal nanostructured network.

84. 84. The method of claim 82 or claim 83, wherein the flexible polymer substrate comprises polycarbonate.

85. 85. The method of any one of claims 82 to 84, further comprising forming electrodes at selected locations of the heater structure by using a silver conductive paste.

86. 86. The method of any one of claims 82 to 85, further comprising forming electrodes at selected locations of a heater structure; and cutting the flexible polymer substrate to obtain a heater structure having a heater surface framed by the electrodes.

87. 87. The method of claim 86, wherein the heater surface is oriented toward the transparent non-planar surface to form a heated transparent structure.

88. 88. The method of any one of claims 81 to 87, wherein said attaching is performed by laminating with an optically clear adhesive.

89. 88. The method of any one of claims 81 to 87, wherein said attaching is performed by insert molding or overmounting.

90. The method of any one of claims 81 to 89, wherein the device surface is non-flat.

91. 91. The method of any one of claims 81 to 90, wherein the heater structure is non-rectangular and the electrodes have non-parallel pairs of opposite polarities.

92. 92. The method of claim 91, wherein sheet resistance is non-uniform across the heater surface to induce a more uniform surface power density from the heater.

93. 92. The method of claim 91, wherein the electrodes are segmented to form separate electrode pairs of opposite polarity, the transparent conductive layer is etched to form insulating zones corresponding to the electrode segments, and the electrode segments are connected to specific voltages to provide a more uniform surface power density.

94. 94. The method of claim 93, wherein the different voltages are provided by using series resistors.

95. The method of any one of claims 81 to 94, wherein the flexible polymer substrate comprises polycarbonate.

96. 96. The method of any one of claims 81 to 95, further comprising forming electrodes at selected locations of the heater structure by using a silver conductive paste.

97. 97. The method of any one of claims 81 to 96, further comprising forming electrodes at selected locations of a heater structure, wherein the heater structure is non-rectangular and the electrodes have non-parallel pairs of opposite polarities.

98. 98. A method according to any one of claims 81 to 97, wherein the sheet resistance is non-uniform across the heater surface to induce a more uniform surface power density from the heater.

99. 99. The method of claim 98, wherein the electrodes are segmented to form separate electrode pairs of opposite polarity, the transparent conductive layer is etched to form insulating zones corresponding to the electrode segments, and the electrode segments are connected to specific voltages to provide a more uniform surface power density.

100. 99. The method of claim 98, wherein the electrodes are segmented to form separate electrode pairs of opposite polarity, the transparent conductive layer is etched to form insulating zones corresponding to the electrode segments, and series resistors in the separate electrode pairs adjust the current in the resulting heater segments to provide more uniform heat generation.

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