Embedding functional electronic devices into fiber reinforced polymers by simultaneous sintering and solidification

Abstract

A method of embedding an electronic device in a composite structure, comprising: a) placing an electronic device on one or more prepreg layers of a composite structure; b) printing a trace of conductive ink on the prepreg layers, wherein the electronic device and the trace are in contact with each other after the placing step and the printing step; and c) simultaneously curing the composite structure, sintering the ink, and forming an electrical connection between the sintered ink and the electronic device.

Description

[0001] Invention Field This application relates to the field of materials engineering, and more specifically, but not exclusively, to a method for embedding functional electronic devices into a composite structure by simultaneously sintering conductive ink and a layer of cured composite structure. Background of the Invention Composite structures are multilayer structures made by combining different materials together. A common composite structure is a fiber-reinforced polymer, which is made of a polymer matrix reinforced by fibers. Typically, the polymer is an epoxy or phenolic resin, and the fibers are made of carbon or glass fibers. One method of manufacturing a fiber-reinforced polymer involves forming one or more prepreg layers comprising fibers and a partially cured polymer, shaping the prepreg layers, and curing the polymer by applying energy such as UV radiation or heat. When curing with heat, typical curing temperatures range from about 60°C to 180°C. Fiber-reinforced polymers are attractive for a variety of industrial functions due to their strength, durability, and moldability. These include aircraft wings, automobile bodies, and bicycle frames.

[0003] Composite structures can be enhanced to include capabilities beyond simply bearing mechanical loads. In particular, it is advantageous to embed materials for sensing, communication, and other functions between the layers of the composite structure. For example, in the case of aircraft parts, making the aircraft skin conductive helps protect internal components from lightning strikes. Strain gauges or capacitive sensors can be used to assess whether a composite structure has begun to deteriorate. Similarly, embedding sensors within the composite structure allows for the collection of information about the function of devices forming part of the composite. Other possible functions include, but are not limited to, de-icing, heating, energy harvesting, or actuating mechanical structures within aircraft wings.

[0004] Despite these potential benefits, fabricating electronics within composite structures presents significant practical challenges. In particular, introducing wiring and sensors between the layers of a composite structure while maintaining its structural integrity is challenging. The thickness of the wiring can potentially lead to separation between the layers during the curing process. Furthermore, and more importantly, soldering sensors to an external power source introduces foreign matter (i.e., solder material) into the polymer matrix. Consequently, the strength of the composite structure is sacrificed.

[0005] One method for effectively introducing conductive wiring into fiber-reinforced polymers is to introduce the conductive wiring as a conductive ink. The conductive ink is spread onto the surface of the polymer resin. A curing process also sinterstallates the ink. To date, this solution has been applied to create circuit trace patterns for providing lightning protection on aircraft fuselages. Invention Overview This invention proposes a method for producing functional electronic devices within a composite structure. The method includes patterning conductive ink onto a prepreg laminate, placing the electronic device on the prepreg adjacent to the conductive ink, and simultaneously sintering the ink and curing the polymer to form the composite structure. Particularly advantageously, an electrical connection is formed between the ink and the sensor without sacrificing the structural integrity of the composite structure.

[0007] According to a first aspect, a method for embedding an electronic device in a composite structure is disclosed. The method includes: placing the electronic device on one or more prepreg layers of the composite structure; printing traces of conductive ink on the prepreg layers, wherein after the placement step and the printing step, the electronic device and the traces are in contact with each other; simultaneously curing the composite structure, sintering the ink, and forming an electrical connection between the sintered ink and the electronic device.

[0008] In another embodiment of the first aspect, the placement step includes forming the electronic device in an additive manufacturing process.

[0009] In another embodiment according to the first aspect, the method further includes placing one or more additional prepreg layers over the conductive ink and electronic device. The curing step includes curing the additional layers together with the first prepreg layer.

[0010] In another embodiment according to the first aspect, the curing step includes applying heat at a temperature between 130°C and 180°C.

[0011] In another embodiment according to the first aspect, the conductive ink is a copper-based ink.

[0012] In another embodiment according to the first aspect, the functional electronic device is a strain gauge, a capacitive sensor, or a piezoelectric sensor.

[0013] In another embodiment according to the first aspect, the steps of simultaneous curing, sintering and forming further include setting values ​​for temperature, pressure, vacuum and residence time to control the integrity of ink sintering and the extent to which volatiles are released from the ink and prepreg into the composite structure. Brief description of the attached diagram Figure 1 The figure illustrates the steps in a method for generating a composite structure with embedded functional electronic devices according to an embodiment of the present disclosure; Figure 2 The schematic diagram illustrates the steps of forming a composite structure with embedded electronic devices according to an embodiment of the present disclosure; Figures 3A-3D The figure illustrates the traces of electromagnetic ink at multiple stages of sintering according to an embodiment of the present disclosure; Figure 3E The diagram illustrates an implementation scheme according to this disclosure. Figures 3A-3D The different absorbance of the traces on the electromagnetic spectrum; Figure 4 The figure illustrates the bubbles formed between the layers of the composite structure; Figures 5A-5D The figure illustrates the production stages of fiber-reinforced polymers connected to flexible printed circuit boards; Figures 6A-6B The figure illustrates the production stages of a fiber-reinforced polymer containing electronic devices; Figure 7 The figure illustrates an example of an additively manufactured sensor according to an embodiment of the present disclosure; Figure 8 The figure illustrates a frequency-selective surface that can be embedded within a composite structure according to an embodiment of the present disclosure; and Figures 9A-9B The figure illustrates a prior art sintered connection between electronic devices embedded in a composite structure and external wiring. Invention Details This application relates to the field of materials engineering, and more specifically, but not exclusively, to a method for embedding functional electronic devices into a composite structure by simultaneously sintering conductive ink and a polymer of a cured composite structure.

[0016] Before explaining at least one embodiment of the invention in detail, it should be understood that the invention is not necessarily limited in its application to the details of the construction and arrangement of the components and / or methods set forth in the following description and / or shown in the drawings and / or examples. The invention is capable of other embodiments, or can be practiced or carried out in various ways.

[0017] In particular, in the examples described herein, fiber-reinforced polymers are chosen as examples of composite structures. Fiber-reinforced polymers can be made of carbon fibers or glass fibers and can be incorporated into any suitable polymer or resin. However, electronic devices can be realized on any suitable composite structure using the methods described herein, provided that the composite structure meets the requirements for structural stability under the temperature and pressure conditions required for sintering the ink.

[0018] Figure 1 The figure illustrates the steps in method 100 for embedding electronic devices in a composite structure. Figure 2 A- Figure 2 The schematic diagram illustrates the steps involved in performing this method on a composite material. For simplicity, the diagram depicts a single prepreg layer, but those skilled in the art will readily understand that the same process can be performed on more than one such layer.

[0019] refer to Figure 2 A. The process begins with the preparation of the prepreg layer 12a. The prepreg layer is made of a reinforcing material pre-impregnated with epoxy resin or other resin. In the example below, the reinforcing material is carbon fiber. Other reinforcing materials, such as glass fiber, glass cloth, aramid fiber, or basalt fiber, can be optionally used. Furthermore, although in Figure 2 The illustrations in A and the following illustrations show a single prepreg layer, but this is merely exemplary, and it should be understood that many prepreg layers may be stacked on top of each other as needed before any electronic components are placed.

[0020] refer to Figure 2 B and Figure 1 In step 101 of method 100, one or more traces of conductive nanoparticle ink 14 (also referred to herein as "nanoink") are printed onto the surface of prepreg layer 12a. The conductive ink can be deposited, for example, by inkjet printing. Any other suitable method, such as screen printing, electroplating, spin coating, or sputtering, can also be used.

[0021] Conductive inks comprise nanoparticles of a conductive metal. Preferably, the metal is copper or silver. These nanoparticles are suspended in a carrier solution, which may include, for example, grease, epoxy resin, resin, and / or solvent. The ink is not conductive when applied. After a sintering process, the nanoparticles fuse together and become conductive.

[0022] In a particularly advantageous embodiment, the conductive ink has a sintering temperature below about 180°C. This sintering temperature is significantly lower than the sintering temperature of most conductive inks, which is about 300°C. In experiments conducted by the inventors of this disclosure, a proprietary conductive ink containing copper nanoparticles was used.

[0023] Now for reference Figure 2 In step C and 102, the electronic device 16 is placed on the prepreg layer 12a. The electronic device 16 can be any suitable sensor configured to transmit an electronic output, including, for example, a strain gauge, piezoresistive sensor, capacitive sensor, or temperature sensor. The electronic device 16 can also be, for example, an actuator, LED, semiconductor device, or external power line. More than one electronic device can be laid near the trace—for example, multiple actuators, multiple sensors, or one sensor and one power line.

[0024] Optionally, the electronic device 16 can be additively manufactured directly on top of the prepreg layer 12a. Alternatively, the electronic device 16 can be manufactured separately and then placed on the prepreg layer.

[0025] Electronic device 16 and conductive traces are positioned such that electronic device 16 and at least one trace 14 are in contact with each other. For the purpose of forming this contact, traces 14 are typically stacked before electronic device 16 is placed to ensure uniform printing of traces 14. However, it is also possible to place electronic device 16 first and then stack traces 14 on top of it.

[0026] Now for reference Figure 2 In steps D and 103, optionally, additional prepreg layers 12b and 12c are laminated on top of the first prepreg layer 12a, the conductive ink 14, and the electronic device 16. When the additional prepreg layers are added, the traces and electronic device become embedded within the final composite structure. This embedding may be preferred to protect the electronic components from environmental influences and to enable sensing data, such as temperature, mechanical motion, and pressure, from the depth of the structure, which is not readily available under existing technology, which is primarily limited to data available at or near the surface. However, the process described herein is equally effective even without the application of additional prepreg material layers, such as for electronic devices designed to attach to the outer surface of a composite structure.

[0027] refer to Figure 2 In steps E and 104, the entire stacked device is cured under suitable temperature and pressure conditions. For example, curing can be carried out in an autoclave. In a preferred embodiment, heating is performed at a temperature between 130°C and 180°C. This temperature is sufficient to cure the polymer in the composite structure without damaging it, and also sufficient to sinter the conductive nanoparticles. Therefore, curing can be characterized as a “sintering-curing” process, or simply “sin-curing”. The result of this process is a cured fiber-reinforced polymer in which conductive wiring and electronic devices, such as… Figure 2 As shown in F.

[0028] Advantageously, the sin-cur process is not only sufficient to cure and sinter the conductive ink onto the composite structure, but also to form an electronic connection between the wire and the electronic device 16. This electronic connection is formed as long as there is physical contact between the ink and the electronic device 16. This contact can be minimal. Experimental results show that a working electronic connection can be formed using a circular contact pad with a diameter of 2 mm. However, it is assumed that even smaller contact areas are sufficient. Furthermore, electronic connections are formed regardless of the type of conductive metal present in the wire and the electronic device 16. For example, working connections can be formed using copper and silver.

[0029] It is noteworthy that no additional welding process is required to form this electronic connection. This is particularly advantageous because existing techniques for providing electronic connections to sensors embedded in composite structures require welding. Welding processes destabilize composite structures. An illustration of the instability inherent in welding is provided in... Figures 9A-9B The image shows... In Figure 9A The diagram shows a composite structure 900, in which an outer conductor 902 is soldered to an inner conductor 901 at a solder point 903. As can be seen, the composite structure is structurally weakened at solder point 903. Figure 9B The image shows a soldered connection between sensor 904 and wire 901. Solder constitutes foreign matter debris (FOD) within the composite structure, leading to delamination between the layers. In contrast, the method of this disclosure enables electrical connectivity to be achieved without soldering during standard curing processes of the composite matrix. Furthermore, after the sin-cur process described herein, the cured fiber-reinforced polymer remains as robust and strain-resistant as the polymer without embedded wiring and sensors.

[0030] Achieving all these results simultaneously—sintering, curing, forming electrical connections, and maintaining structural integrity—requires precise control of process parameters. For both composite structures and conductive inks, these parameters may have to vary depending on the materials used. Process parameters include: temperature, vacuum, pressure on the prepreg layer, residence time, and the use of specific auxiliary materials during the prepreg lamination process.

[0031] Regarding temperature, the temperature conditions must be set based on both the desired curing value of the composite matrix and the simultaneous sintering of the ink. When exposed to elevated temperatures, the ink releases volatiles. These volatile compounds can introduce unstable bubbles into the composite material. Therefore, it is crucial to select an ideal temperature that balances the conditions for a successful Sin-Cur process with the avoidance of volatile release.

[0032] Regarding vacuum, the sin-cur process is preferably performed under vacuum conditions. These vacuum conditions allow for the effective evacuation of sintered ink volatiles, as well as "standard" volatiles released from the prepreg layer during the matrix curing reaction. These vacuum conditions are synchronized with the curing scheme, thereby minimizing the viscosity of the matrix and ensuring that the time window for evacuation is sufficient for all volatiles to escape. Otherwise, volatiles may become trapped within the solidified matrix. Viscosity control is further achieved by controlling the process temperature distribution, as will be discussed further below.

[0033] "Pressure" refers to the direct application of positive pressure to the prepreg layer during the sin-cur process. The pressure value is set to allow for compaction of the prepreg layer while also facilitating ink sintering. Generally, pressure promotes the aggregation and bonding of ink particles. Furthermore, applying pressure during the sin-cur process allows for the use of lower sintering temperatures. However, excessively high pressure values ​​lead to the dissipation of ink particles within the epoxy matrix and hinder the continuous and successful sin-cur process. Therefore, similar to temperature, the pressure parameter exhibits a complex dynamic behavior for achieving local optima in the desired sin-cur process.

[0034] Regarding the duration or residence time, it is necessary to maintain the temperature, pressure, and vacuum conditions for a sufficient period of time to allow the desired effect or chemical reaction to occur. Therefore, it is desirable to plan the duration (residence time) at a specific temperature during the sin-cur process to allow all three of the following processes to occur: 1) matrix curing, 2) ink sintering, and 3) the release of volatiles during curing and sintering (made possible by matrix viscosity kinetics, as discussed above). A successful sin-cur process with the correct temperature and residence time, and of course, appropriate pressure and vacuum values, enables the phenomena mentioned above to occur during the process and to the desired extent.

[0035] Auxiliary materials include a 'peel-ply' fabric, a Teflon layer, and a release agent for manufacturing composite parts. Regarding the sin-cur process, these auxiliary materials ensure that the final composite part has the desired fiber-to-matrix ratio and prevent any ink leakage or adhesion to unwanted surfaces (such as tooling materials in part molds or any mechanical connectors or joints present in the part near the composite layer), which could cause ink to peel off from the composite surface.

[0036] Typically, curing processes can take anywhere from minutes to hours and can be performed at temperatures ranging from 130°C to 180°C. Furthermore, the typical (but not limiting) ranges for pressure and vacuum are 1 bar to 7 bar and as low as -1 bar, respectively. Moreover, different prepreg materials require different recommended matrix curing cycles regarding temperature, temperature residence time, vacuum, and pressure values. Therefore, each different prepreg matrix requires a “tailor-made” sin-cur process that provides the desired composite structural properties as well as the desired electrical properties of the embedded conductors and devices.

[0037] To illustrate a challenge that may arise from insufficient control of the sintering process, refer to Figures 3A-3E . Figures 3A-3D The figure shows the traces of conductive ink after sintering to various success levels. Figure 3D The image shows trace 301 before sintering. Figure 3C In the process, trace 302 is sintered to a minimum. Figure 3B In the middle, trace 303 is sintered to a moderate degree, and in Figure 3A In this case, trace 304 is fully sintered. Traces with incomplete sintering undergo sintering processes under insufficient temperature, pressure, or residence time conditions, as discussed above. The difference in sintering is related to the difference in electrical conductivity. Figure 3E The figure shows Figures 3A-3D The conductivity of conductive inks sintered differently varies at different wavenumbers. Although all four graphs have the same wavenumber scale, the absorbance scale differs for each graph. As can be seen, the ink shown in graph 311 before sintering exhibits almost no absorbance. Graphs 312 and 313, corresponding to traces 302 and 303, show additional absorbance, although still insufficient for the function of electronic devices. Graph 314 corresponds to the fully sintered trace 304 and demonstrates sufficient conductivity for the devices within the composite structure to function properly.

[0038] Figure 4 The figure illustrates another challenge of the sin-cur process that can lead to suboptimal performance of the composite structure. The composite structure 400 comprises a fiber-reinforced composite material 401 surrounding a conductive trace 402. Bubbles 403 form between the conductive trace 402 and the upper layer of the composite structure due to volatiles being trapped during the sintering-curing process. This volatile trapping can result from a lack of vacuum, a short time window for minimum matrix viscosity, exposure to temperatures above the necessary level, and any combination of these conditions. As a result, the layers of prepreg are delaminated, and the integrity of the entire structure is compromised. As stated above, optimizing the parameters of the sin-cur process for the specific materials used helps ensure that these challenges do not occur in practice.

[0039] As stated above, the principles of this disclosure can be applied to create a variety of “intelligent” composite structures. Figures 5A-5D and Figures 6A-6B The diagram illustrates two such examples. (Reference) Figure 5A A base prepreg layer 501a is laid on the surface, and conductive ink 502 is printed thereon. Another prepreg layer 501b is laid on top of the conductive ink 502. A flexible printed circuit board 503 is laid on the prepreg layer 501a and at least partially on the conductive ink 502. In the illustrated embodiment, the flexible printed circuit board includes electronic traces embedded in Kapton® (polyimide film). Optionally, in Figure 5BIn this embodiment, the junction between the flexible PCB and the conductive ink is covered with an additional prepreg layer 501c. Although a portion of the conductive ink 502 remains uncovered in the illustrated embodiment, this is merely to delineate the different layers of the composite structure. After the additional prepreg laminates are stacked, the entire device is heated in an autoclave to produce the composite structure 504, as shown. Figure 5C As shown in the image. Figure 5C As further illustrated, the composite structure 504 includes conductive traces of a flexible printed circuit board 503 and conductive lines 505 from sintered conductive ink. The flexible printed circuit board 503 and the sintered conductive ink are electrically connected at location 506. Figure 5D In this structure, the flexible printed circuit board 503 may also include a control panel 507 to which the flexible printed circuit board 503 has previously been attached. Therefore, the composite structure includes a connection to the external world for the purpose of supplying power to and receiving data from the sensor.

[0040] Figures 6A-6B The figure illustrates another example of a smart composite structure manufactured according to the principles of this disclosure. Figure 6A In this process, conductive ink 602 is printed onto a prepreg layer 601. A strain gauge 603 is placed on the prepreg layer 601, and a lead 604 extends from the strain gauge 603 over the conductive ink 602. After a sin-cur process, a composite structure is created. This composite structure includes the strain gauge 603 embedded therein. An electrical connection is formed between the lead 604 and the sintered trace 602 without soldering.

[0041] As is readily understood by those skilled in the art, it can be Figures 5A-5D and Figures 6A-6B The proposed implementation scheme combines various methods. The result of this combination is a composite structure with internal conductive traces that include internal connections to both the internal sensors and the external control panel. All electrical connections in the composite structure are achieved without soldering.

[0042] Figure 7 The figure illustrates a strain gauge that can be incorporated into a composite structure according to an embodiment of this disclosure. In the illustrated embodiment, the strain gauge is formed by an additive manufacturing process. More specifically, the strain gauge is additively manufactured onto a prepreg layer prior to curing.

[0043] Various modifications can be made to the methods described herein without departing from the principles described herein. For example, in the embodiments described above, conductive ink and electronic devices are deposited onto the substrate while the substrate is still in prepreg form, and then an additional prepreg layer is laminated on top of the conductive ink and electronic devices. However, it is also possible to deposit the ink and devices onto a fully cured composite material, subsequently laminating a prepreg laminate on top of the ink and devices, and sintering and curing all components in the manner described.

[0044] In another example, instead of placing electrical conductors and electronics between the layers of a composite structure, or as an alternative, a frequency-selective surface (FSS) can be placed between the layers. A FSS is a repeating printed pattern of conductive material that acts as a filter for certain electromagnetic wavelengths. Conductive ink can be printed onto a prepreg surface in the pattern of the FSS and can be sintered during the curing process. After the sin-cur process, the FSS functions as intended, filtering out unwanted electromagnetic wavelengths.

Claims

1. A method for embedding electronic devices in a composite structure, comprising: The electronic device is placed on one or more prepreg layers of the composite structure; Traces of conductive ink are printed on the prepreg layer, wherein after the placement step and the printing step, the electronic device and the traces come into contact with each other; Simultaneously, the composite structure is cured, the ink is sintered, and an electrical connection is formed between the sintered ink and the electronic device.

2. The method of claim 1, wherein the placement step comprises forming the electronic device in an additive manufacturing process.

3. The method of claim 1, further comprising placing one or more additional prepreg layers over the conductive ink and the electronic device, wherein the curing step comprises curing the additional layers together with the first prepreg layer.

4. The method of claim 1, wherein the curing step comprises applying heat at a temperature between 130°C and 180°C.

5. The method according to claim 1, wherein the conductive ink is a copper-based ink.

6. The method according to claim 1, wherein the electronic device is a strain gauge, a capacitive sensor, or a piezoelectric sensor.

7. The method of claim 1, wherein the steps of simultaneous curing, sintering, and forming further include: Values ​​for temperature, pressure, vacuum, and residence time are set to control the integrity of the ink sintering and the extent to which volatiles are released from the ink and the prepreg into the composite structure.

Images