Metallisation layer for a PTC semiconductor ceramic, method for applying a metallisation layer, PTC semiconductor ceramic having a metallisation layer, and use in a heating device

Abstract

The invention relates to a metallisation layer for a PTC semiconductor ceramic for at least partly coating the surface of the PTC semiconductor ceramic, which metallisation layer contains at least one functional layer, wherein the at least one functional layer serves for current distribution, wherein the porosity of the at least one functional layer is less than 5%. The invention also relates to a method for applying the metallisation layer to a PTC semiconductor ceramic, to a PTC thermistor which has this metallisation layer, and to the use.

Description

[0001] Metallization layer for a PTC semiconductor ceramic, method for applying a metallization layer and PTC semiconductor ceramic with a metallization layer and use in a heating device

[0002] The invention relates to a metallization layer for a PTC semiconductor ceramic, a method for applying a metallization layer and a PTC semiconductor ceramic with a metallization layer and the use in a heating device according to the preamble of the independent patent claims.

[0003] Semiconductor ceramics, whose electrical conductivity decreases with increasing temperature within a certain temperature range due to increasing resistance, exhibit thermal self-regulation, which can be exploited when used as heating elements. This effect is also known as the PTC effect (Positive Temperature Coefficient). Since the waste heat generated in the combustion engine, which has been the mainstay of motor vehicles to date, can be used to heat the interior, PTC heating elements were previously used only as additional heating for rapid heating or in cold outside temperatures. However, in electric vehicles, no waste heat from a combustion engine is available. Nevertheless, heat is required both to regulate the temperature of the vehicle interior and to regulate the temperature of the battery.For contacting a PTC semiconductor ceramic, metallization is required on two opposite sides of the semiconductor ceramic. For example, DE 10202213863 A1 relates to an ohmic electrode layer containing at least one element selected from the group consisting of Al, Au, Ag, and In as a base metal.

[0004] It is an object of the present invention to demonstrate new ways of improving the electrical properties of metallization layers for PTC semiconductor ceramics and the corresponding method for applying such layers. This object is achieved according to the invention by the subject matter of the independent patent claims. Advantageous embodiments are the subject matter of the dependent patent claims.

[0005] According to the invention, it is provided to provide a metallization layer for a PTC semiconductor ceramic for at least partially coating the surface of the PTC semiconductor ceramic, wherein the metallization layer contains at least one functional layer and wherein the at least one functional layer serves to distribute the current and wherein the porosity of the at least one functional layer is less than 5%.

[0006] The metallization layer according to the invention with the features of independent claim 1 has the significant advantage over the prior art that it has lower porosity, which leads to a reduction in sheet resistance at the same thickness. This avoids problems with current dissipation, even at high electrical power levels.

[0007] A further significant advantage of a lower sheet resistance is that a heating element with the inventive metallization can easily be operated directly from the vehicle's on-board voltage, even if a higher on-board voltage than the nominal one is used. This avoids costs and sources of error, as no complex protection and control technology is required, for example, for conversion to a lower voltage range.

[0008] Another significant advantage is that, due to the good properties of the metallization layer, relatively thin layers can be used, which in turn leads to low material consumption and cost savings. Furthermore, the metallization layer according to the invention has the significant advantage that crack formation at the transition between the PTC substrate and the metallization layer can be largely avoided. Furthermore, the method according to the invention allows the entire metallization to be produced using a single coating process, thus avoiding the need to combine multiple coating processes to produce a single metallization.

[0009] In addition, the use of a heating element with the innovative metallization can enable optimal battery temperature control during charging and driving. Another significant advantage of this is that the battery life and range of the corresponding vehicles can be significantly increased.

[0010] This is achieved in particular by specifying a metallization that has low porosity and the lowest possible sheet resistance, which enables good current dissipation even at high electrical power levels.

[0011] The metallization layer according to the invention for a PTC semiconductor ceramic, the method according to the invention for applying a metallization layer and the corresponding PTC semiconductor ceramic with a metallization layer and the use in a heating device are explained in more detail below, wherein the figures used in the context of the explanation of the metallization layer according to the invention and in this context also apply mutatis mutandis to the method and the use and the figures used in this context and vice versa.

[0012] The present invention is based on the general idea of ​​providing a metallization for a PTC semiconductor ceramic for at least partially coating the surface of the PTC semiconductor ceramic, which metallization has low porosity and thus a low sheet resistance. This makes the metallization suitable for use at high electrical power levels.

[0013] The metallization layer according to the invention contains at least one functional layer that serves to distribute the current. This means that the at least one functional layer must be made of a material with good electrical conductivity. A metal or alloy with good electrical conductivity is particularly suitable for this purpose. In principle, the metallization layer can be applied to the PTC semiconductor ceramic in any conceivable arrangement that allows contact with two electrodes.

[0014] Advantageously, the coating is applied to two opposite sides of the PTC semiconductor ceramic. According to the invention, the porosity of the at least one functional layer is less than 5%. This ensures that the coating has a low surface resistance.

[0015] Without wishing to be bound by this explanation, the inventors assume that the comparatively low porosity according to the present invention is advantageously influenced by the proposed deposition processes. If layers are applied and fired using particle-containing pastes, as is frequently encountered in the prior art for this application, the organic compounds often contained in the pastes, such as dispersants and stabilizers, lead to pore formation during firing, among other things because the organic residues partially enter the gas phase at the high temperatures or react to form gaseous compounds and, for example, remain at least partially in pores in the layer, and, for example, at least partially escape from the layer.Furthermore, during firing, the metallic particles grow / sinter together where they touch, but not completely, leaving pores. While it would be possible to reduce the pores by applying pressure and / or using nanoparticles, this is complex and expensive. This can be avoided with chemical or electroplating deposition or deposition using physical vapor deposition. Here, the deposition can be controlled so that only the desired metals and / or alloys are incorporated into the layers, and not large amounts of other compounds present in the solution, for example, during chemical or electroplating deposition.

[0016] In principle, any semiconductor ceramic that exhibits a PTC effect within a temperature range relevant to the respective application is suitable as a PTC semiconductor ceramic. PTC semiconductor ceramic based on BaTiO and / or related compounds is advantageously used.

[0017] The porosity of a layer can be determined by analyzing a metallographic section. Porosity can be determined by evaluating the section using image processing. For example, porosity assessment can be performed using cross-section and section testing and quantitative image analysis according to BDG Guideline P202 (BDG: Federal Association of the German Foundry Industry), Section 5.

[0018] In principle, it is conceivable to use all coating methods known to the person skilled in the art for applying the metallization that can be used without a baking step, such as galvanic / electrolytic deposition, electroless / chemical deposition, vapor deposition or deposition from ionic liquids.

[0019] Preferably, the at least one functional layer comprises at least one element and / or at least one alloy selected from the group consisting of Ag, Al, Cu, AgZn alloys, and CuAl bronzes. More preferably, the at least one functional layer comprises at least one element selected from the group consisting of Ag, Al, and Cu. This enables particularly good current distribution.

[0020] This means that the at least one functional layer can comprise at least one element selected from the group consisting of Ag, Al, and Cu, but the at least one functional layer can also comprise at least one alloy selected from the group consisting of AgZn alloys and CuAl bronzes. It is also conceivable for the at least one functional layer to comprise both at least one element selected from the group consisting of Ag, Al, and Cu and at least one alloy selected from the group consisting of AgZn alloys and CuAl bronzes.

[0021] Alternatively, the at least one functional layer consists of at least one element and / or at least one alloy selected from the group consisting of Ag, Al, Cu, AgZn alloys and CuAl bronzes, more preferably the at least one functional layer consists of at least one element selected from the group consisting of Ag, Al and Cu.

[0022] This means that the at least one functional layer can consist of at least one element selected from the group consisting of Ag, Al, and Cu, but the at least one functional layer can instead also consist of at least one alloy selected from the group consisting of AgZn alloys and CuAl bronzes. It is also conceivable for the at least one functional layer to consist of at least one element selected from the group consisting of Ag, Al, and Cu, as well as at least one alloy selected from the group consisting of AgZn alloys and CuAl bronzes.

[0023] In this context, the term AgZn alloy refers to a composition with Ag and Zn as the main components, with the Zn content being < 25 at.% Zn. Furthermore, it is conceivable that the alloy contains small amounts of other elements, with the additional elements preferably being present in amounts of less than approximately 5 at.%. It goes without saying that the sum of all components contained must equal 100 at.%.

[0024] In this context, the CuAl alloy is further understood to mean a bronze containing copper and aluminum as its main components, with the bronze containing a minimum of approximately 4 at-% to a maximum of approximately 14 at-% aluminum and a minimum of approximately 86 at-% to a maximum of approximately 96 at-% copper. Furthermore, it is conceivable that the bronze contains small amounts of other elements, with the additional elements preferably being present in amounts of less than 0.5 at-%. It goes without saying that the sum of all components contained must equal 100 at-%.

[0025] The chemical composition can be determined by chemical analysis methods such as ICP-OES (inductively coupled plasma optical emission spectrometry).

[0026] In an advantageous development of the metallization layer, the thickness of the at least one functional layer is approximately a minimum of 1 pm to a maximum of approximately 10 pm, preferably the thickness of the at least one functional layer is approximately a minimum of 1 pm to a maximum of approximately 8 pm, more preferably the thickness of the at least one functional layer is approximately a minimum of 2 pm to a maximum of approximately 6 pm. A thickness of the functional layer in the range from 2 pm to a value that is less than 5 pm is advantageous, in particular from 2 pm to 4.8 pm. The thickness is measured perpendicular to the surface of the PTC semiconductor ceramic to which the metallization layer is applied. The thickness measurement can be carried out using a metallographic section, wherein the thickness for layers down to a minimum of approximately 1 pm can be determined using a light microscope.

[0027] It is advantageous for the metallization layer to further comprise at least one adhesion layer, wherein the at least one adhesion layer is arranged between the surface of the PTC semiconductor ceramic and the at least one functional layer. The at least one adhesion layer serves, on the one hand, to improve the adhesion of the at least one functional layer. On the other hand, the at least one adhesion layer can additionally serve as a diffusion barrier between the PTC semiconductor ceramic and the at least one functional layer. In particular, it has proven advantageous for the metallization layer to comprise precisely one adhesion layer in order to achieve good dielectric strength of the coated PTC semiconductor ceramic.

[0028] Dielectric strength is familiar to those skilled in the art. It is a characteristic value that indicates the voltage at which a component can be operated. Dielectric strength refers to the ability to prevent the flashover of an arc between two points up to a certain voltage. The dielectric strength of a component is typically tested using a high-voltage test with alternating current.

[0029] In an advantageous development of the metallization layer, the thickness of the at least one adhesion layer is a minimum of approximately 0.5 μm to a maximum of approximately 1 μm. As already mentioned above in connection with the functional layer, the thickness measurement can be performed using a metallographic section, whereby the thickness for layers down to a minimum of approximately 1 μm can be determined using a light microscope. In the case of smaller layer thicknesses, the analysis can be performed using a scanning electron microscope, or possibly a field emission scanning electron microscope.

[0030] It has also proven advantageous that the at least one adhesive layer comprises at least one element or compound selected from the group consisting of Ni, NiP, Cr and Cu.

[0031] Alternatively, it has proven advantageous that the at least one adhesion layer consists of at least one element or compound selected from the group consisting of Ni, NiP, Cr and Cu.

[0032] In particular, in order to achieve good dielectric strength of the coated PTC semiconductor ceramic, it has proven advantageous that the metallization layer has exactly one adhesion layer and that exactly one adhesion layer has nickel.

[0033] Alternatively, in order to achieve a good dielectric strength of the coated PTC semiconductor ceramic, it has proven particularly advantageous for the metallization layer to have exactly one adhesion layer and for the exactly one adhesion layer to consist of nickel.

[0034] With regard to the aforementioned precisely one bonding layer containing or consisting of nickel, tests have shown that it is advantageous if the precisely one bonding layer containing or consisting of nickel was applied using a wet-chemical coating process. Electrolytic deposition is particularly advantageous. Alternatively, chemical deposition is particularly advantageous.

[0035] Furthermore, it has proven advantageous for the at least one adhesive layer to have a nucleation layer applied to the surface of the PTC semiconductor ceramic; the nucleation layer preferably consists of colloidal Pd. In this context, nucleation of the surface of the PTC semiconductor ceramic with, for example, Pd is understood to mean that Pd nuclei are applied to the surface of the PTC semiconductor ceramic in order to generate a first attachment point for the subsequent deposition of the following layer.

[0036] It is therefore conceivable, for example, that nucleation with Pd is first carried out and then a layer comprising at least one element or compound selected from the group consisting of NiP and Cu is chemically applied.

[0037] It has proven particularly advantageous for the at least one adhesion layer to have a nucleation layer that is applied to the surface of the PTC semiconductor ceramic with plasma support. Advantageously, the nucleation layer consists of colloidal Pd. Plasma-assisted nucleation has the advantage that the nucleation layer can be applied in a structured manner. In this context, it is preferred for the adhesion layer to have a structured nucleation layer with colloidal Pd, which is applied to the surface of the PTC semiconductor ceramic with plasma support.

[0038] For example, nucleated stripes can alternate with non-nucleated stripes. This allows you to control where on the surface the adhesion layer should grow, thus achieving an overall patterned metallization layer.

[0039] This is achieved by first roughening the surface of the PTC semiconductor ceramic using plasma pretreatment. During the plasma pretreatment, the areas of the surface to be roughened can be precisely controlled. Subsequently, the roughened surface can be selectively nucleated with colloidal Pd. This nucleation can be achieved wet-chemically by deposition from a solution containing palladium salt. Alternatively, it can be performed using atmospheric-pressure plasma.

[0040] It is conceivable that the plasma treatment and the nucleation with colloidal Pd are followed by a chemical deposition of an adhesion layer containing nickel.

[0041] Furthermore, for some functional layers, such as functional layers comprising Cu or Cu alloys, it has proven advantageous for the metallization layer to additionally comprise at least one cover layer, wherein the at least one cover layer is arranged above the functional layer. The at least one cover layer serves to protect the functional layer from corrosion. This means that it is conceivable for the metallization layer to comprise at least one adhesion layer, at least one functional layer located thereon, and at least one cover layer located on the functional layer. Alternatively, the metallization layer can also comprise only at least one functional layer and at least one cover layer located on the functional layer.

[0042] Alternatively, the metallization layer may also comprise only at least one functional layer. As a further alternative, the metallization layer may also comprise only at least one adhesion layer and at least one functional layer.

[0043] In an advantageous development of the metallization layer, the thickness of the at least one cover layer is at least approximately 0.05 pm and at most less than approximately 1 pm, preferably at least approximately 0.05 pm and at most less than approximately 0.5 pm.

[0044] It has also proven advantageous for the at least one cover layer to contain at least one element or compound selected from the group consisting of Ni and NiCr. This results in particularly good protection against corrosion of the at least one functional layer. Alternatively, it has proven advantageous for the at least one cover layer to contain, for example, at least one element selected from the group consisting of Ag or Au.

[0045] In a further alternative advantageous embodiment, the at least one cover layer consists of at least one element or compound selected from the group consisting of Ni and NiCr. Alternatively, it has proven advantageous for the at least one cover layer to consist, for example, of at least one element selected from the group consisting of Ag or Au.

[0046] In a further advantageous development of the invention, the porosity of the at least one adhesive layer and the at least one cover layer is less than 5%. If only at least one adhesive layer or only at least one cover layer is contained in the metallization layer in addition to the at least one functional layer, then in an alternative advantageous development of the invention, the porosity of the at least one adhesive layer or the at least one cover layer is correspondingly less than 5%.

[0047] Furthermore, it has proven advantageous for the porosity of the functional layer and optionally the adhesive layer and / or optionally the cover layer to be less than 3%, preferably less than 1%. If only at least one adhesive layer or only at least one cover layer is contained in the metallization layer in addition to the at least one functional layer, this means that the porosity of the at least one adhesive layer or the at least one cover layer is less than 3% and preferably less than 1%. This leads to a particularly low sheet resistance.

[0048] The invention further relates to a method for applying a metallization layer as explained above by means of a wet-chemical coating method, comprising the steps: a1) applying a primer to at least part of the surface of the PTC semiconductor ceramic, b1) optionally nucleating in an activator step and reducing the nucleation, preferably the nucleation is carried out with Pd, c1) optionally applying at least one adhesive layer, d1) applying at least one functional layer, e1) optionally applying at least one cover layer.

[0049] Wet chemical coating processes are understood to mean two possible processes: i) electrolytic deposition and ii) chemical deposition.

[0050] In this context, the term "galvanic" or "electrolytic" deposition / coating refers to a deposition reaction on the surface to be coated, induced by an applied current. The term "electroless" or "chemical" deposition refers to the deposition of a layer without external current, meaning that the layer formation is based on an autocatalytic process that is largely determined by the electrochemical potentials of the reactants involved. The reduction of a metal salt therefore occurs due to the reducing agent available in the solution.

[0051] In step a1), a primer is first applied to at least part of the surface of a PTC semiconductor ceramic. It goes without saying that at least the part of the surface to which the metallization layer is subsequently applied must be treated. This is a chemical etching process. This is carried out, for example, in sulfuric acid solutions with the addition of H2O2 or peroxodisulfates as an oxidizing agent. Alternatively, the primer consists, for example, of a fluoride-containing medium, which leads to a roughening of the ceramic surface.

[0052] Subsequently, in step b1), the previously roughened surface is optionally nucleated, for example with Pd. Highly reactive metal nuclei are deposited, for example colloidal palladium, which enables a uniform start to growth of the subsequent layer during a subsequent chemical coating. In this case, palladium is deposited, for example, from a solution containing palladium salt onto at least part of the surface of the PTC semiconductor ceramic. The electrons required for the reduction originate from a reducing agent available in the solution. This is an autocatalytic deposition reaction, since palladium, for example, catalyzes the reduction of the palladium salt contained in the solution.This creates nuclei of palladium on at least part of the surface of the PTC semiconductor ceramic, which serve as the first starting points for the subsequent deposition of at least one further layer.

[0053] Following step a1) or step b1), at least one adhesion layer is optionally applied in step c1). The thickness of the at least one adhesion layer is advantageously a minimum of approximately 0.5 pm to a maximum of approximately e pm. For example, it is conceivable that first a layer with a minimum of approximately 1 pm to a maximum of approximately 2 pm thick and containing NiP as the main component is deposited without external current. For example, sodium hypophosphite can act as the reducing agent for this. Subsequently, for example, a further layer, also with a minimum of approximately 1 pm to a maximum of approximately 2 pm thick and containing Ni as the main component is deposited by electrodeposition. The main component is understood to be the predominantly present element. It goes without saying that it is conceivable that small amounts of impurities with other elements and / or compounds may also be present.For example, it is conceivable that such a layer may contain small amounts, in particular amounts of less than 1 wt.%, preferably less than 0.1 wt.%, based on the total composition, of impurities with another element or compound. In addition to facilitating adhesion of the at least one functional layer located above it, the at least one adhesive layer has the further function of minimizing diffusion between the PTC semiconductor ceramic and the other layers as much as possible.

[0054] Following step a1) or optionally following step b1) or optionally following step c1) in step d1) at least one functional layer is applied. Advantageously the thickness of the at least one functional layer is approximately a minimum of 1 pm to a maximum of approximately 10 pm, preferably a minimum of approximately 1 pm to a maximum of approximately 8 pm, more preferably a minimum of approximately 2 pm to a maximum of approximately 8 pm. A thickness of the functional layer is advantageously in the range from 2 pm to a value that is less than 5 pm, in particular from 2 pm to 4.8 pm. For example, it is conceivable to first apply a layer, referred to here as pre-silver layer, containing silver as the main component using one of the usual suitable electrolyte solutions known to the person skilled in the art, such as ARGUNA® 3430, an alkaline, cyanide-free silver electrolyte. The pre-silver layer is used to carry out a deposition, i.e.spontaneous formation of unusable silver layers during immersion. This would otherwise lead to defective and non-adherent electrolytic silver layers. For this reason, pre-silvering is carried out. Subsequently, a further approximately 4 pm thick layer containing silver as the main component is applied. It goes without saying that it is conceivable that small amounts of impurities with other elements and / or compounds may also be present. For example, it is conceivable that such a layer may contain small amounts, in particular amounts of less than 1 wt.% in each case, preferably less than 0.1 wt.%, based on the total composition, of impurities with another element or compound.

[0055] Subsequently, in step e1 ), at least one cover layer can optionally be applied, which protects the underlying at least one functional layer from corrosion.

[0056] The metallization layers obtained by the above process have a low porosity, for example, compared to prior art layers obtained by firing.

[0057] The invention further relates to a method for applying a metallization layer as explained above by means of physical vapor deposition (PVD), comprising the steps: a2) cleaning the surface of the PTC semiconductor ceramic, b2) optionally applying at least one adhesion layer, c2) applying at least one functional layer, d2) optionally applying at least one cover layer.

[0058] All physical vapor deposition (PVD) methods, in which atoms are released from a solid by bombardment with high-energy ions and transferred into the gas phase by cathodic sputtering, are generally suitable. For example, it is conceivable to apply the layers using high-frequency sputtering.

[0059] In step a1), at least a portion of the surface of a PTC semiconductor ceramic is first cleaned. It goes without saying that at least the portion of the surface to which the metallization layer is subsequently applied must be treated. This can be done, for example, by bombardment with argon ions.

[0060] Subsequently, in step b2), at least one adhesion layer is optionally applied. The thickness of the at least one adhesion layer is advantageously at least approximately 0.5 pm to a maximum of approximately 6 pm. For example, it is conceivable that first an approximately 0.1 pm thick layer containing chromium as the main component is sputtered on, followed by an approximately 0.4 pm thick layer containing nickel as the main component. In this case, for example, this results in at least one adhesion layer with a thickness of approximately 0.5 pm. It goes without saying that it is conceivable that small amounts of impurities with other elements and / or compounds may also be present.For example, it is conceivable that such a layer may contain small amounts, in particular amounts of less than 1% by weight, preferably less than 0.1% by weight, based on the total composition, of impurities with another element or compound.

[0061] In step c1) at least one functional layer is then applied. Advantageously the thickness of the at least one functional layer is approximately a minimum of 1 pm to a maximum of approximately 10 pm, preferably the thickness of the at least one functional layer is approximately a minimum of 1 pm to a maximum of approximately 8 pm, more preferably the thickness of the at least one functional layer is approximately a minimum of 2 pm to a maximum of approximately 8 pm. A thickness of the functional layer in the range from 2 pm to a value which is less than 5 pm is advantageous, in particular from 2 pm to 4.8 pm. For example, it is conceivable that a functional layer approximately 4 pm thick is deposited containing copper as the main component. It goes without saying that it is conceivable that small amounts of impurities with other elements and / or compounds may also be present.For example, it is conceivable that such a layer may contain small amounts, in particular amounts of less than 1% by weight, preferably less than 0.1% by weight, based on the total composition, of impurities with another element or compound.

[0062] The porosity of the functional layer is less than 5%, preferably less than 3% and more preferably less than 1%.

[0063] Furthermore, in step d1 ) at least one cover layer is optionally applied, which protects the at least one functional layer from corrosion.

[0064] For example, it is conceivable that a cover layer approximately 0.15 μm thick, containing NiCr20 as the main component, is deposited. It is also conceivable, for example, that an alternative cover layer approximately 0.15 μm thick containing silver as the main component is applied. It goes without saying that it is conceivable that small amounts of impurities with other elements and / or compounds may also be present. For example, it is conceivable that such a layer may contain small amounts, in particular amounts of less than 1 wt.% in each case, preferably less than 0.1 wt.%, based on the total composition, of impurities with another element or compound.Furthermore, the invention relates to a PTC thermistor which has a sintered body consisting of a PTC semiconductor ceramic and a metallization layer as explained above, wherein the metallization layer is applied to two opposite sides of the PTC semiconductor ceramic on the surface of the PTC semiconductor ceramic.

[0065] A sintered body is defined by experts as a workpiece, such as a ceramic, that is created during sintering. This involves heating precisely prepared, fine-grained ceramic starting materials.

[0066] A thermistor is understood by those skilled in the art to be a variable and heat-sensitive electrical resistor with a nonlinear temperature characteristic whose value changes reproducibly with temperature. In this case, the sintered body, which consists of a PTC semiconductor ceramic, has a metallization on two opposite surfaces. This serves for electrical contact.

[0067] Furthermore, the invention relates to a PTC thermistor which has a sintered body consisting of a PTC semiconductor ceramic and a metallization layer as explained above, wherein the metallization layer is applied to two opposite sides of the PTC semiconductor ceramic on the surface of the PTC semiconductor ceramic and the PTC semiconductor ceramic is joined to current-carrying contact rails by means of a rigid metallic joining connection.

[0068] Such a metallic joint can advantageously be obtained by means of a process selected from the group of Ag sintering, Cu sintering, and soft soldering with Sn-based solders.

[0069] Furthermore, the invention relates to the use of a metallization layer as explained above together with a PTC semiconductor ceramic in an electrical heating application such as an air heater or a coolant heater in an automotive application, a switching element, a measuring element or a temperature control element, preferably the metallization layer is used in a high-voltage heating application.

[0070] In this context, a high-voltage application is particularly the use as a PTC heating element for a vehicle with a high on-board voltage of at least approximately 350 V and preferably > 800 V.

[0071] Advantageously, the metallization layer as explained above is used together with a PTC semiconductor ceramic in an electrical heating application, wherein the metallization layer is applied to two opposite sides of the PTC semiconductor ceramic on the surface of the PTC semiconductor ceramic and the PTC semiconductor ceramic is joined to current-carrying contact rails by means of a rigid metallic joining connection.

[0072] Furthermore, it can be used as overvoltage protection, as a switch with delay option, as overtemperature protection or as inrush current limiter.

[0073] Further important features and advantages of the invention emerge from the subclaims, from the drawings and the associated description of the figures as well as from the examples.

[0074] It goes without saying that the features mentioned above and those to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

[0075] Fig. 1: Schematic representation of an example of a semiconductor ceramic with metallization on two opposite sides. Fig. 2: Schematic representation of a metallization layer on a PTC semiconductor ceramic.

[0076] Fig. 3: Schematic representation of the sequence of a wet-chemical process for applying a metallization layer on a PTC semiconductor ceramic.

[0077] Fig. 4: Schematic representation of the process of a PVD process for applying a metallization layer on a PTC semiconductor ceramic.

[0078] Fig. 5: Scanning electron microscopy (SEM) image of a section of an advantageous metallization layer obtained using wet-chemical processes. Fig. 6: SEM image of a section of an advantageous metallization layer obtained using PVD processes.

[0079] Fig. 7: SEM image of a section of an Ag-based metallization layer obtained according to the prior art.

[0080] Fig. 8: SEM image of a section of an Al-based metallization layer obtained according to the prior art.

[0081] Fig. 9: SEM image of a section of an Ag-based metallization layer obtained according to the prior art.

[0082] Fig. 10: Schematic representation of a metallization layer on a PTC semiconductor ceramic and a current-carrying contact rail contacted by means of a rigid metallic joint.

[0083] Figure 1 a) shows, by way of example, a PTC semiconductor ceramic with a sintered body 101. The semiconductor ceramic 101 is coated on two opposite sides with a metal layer 102, as described in the present application, which can be used for electrical contact. As can be seen from Figures 1 b) and 1 c), it is conceivable that this is a cylindrical arrangement (tablet), but it is also conceivable that the arrangement is box-shaped (cuboid). In this context, it should be noted that in Figure 1 c, a metal layer is also present on the two opposite sides, one side being omitted from the perspective view. A cuboid shape is advantageously used. Figure 2 schematically shows a preferred embodiment of the metallization layer.Here, at least on a portion of the surface of the semiconductor ceramic, a nucleation layer 204 comprising Pd is first applied, and on top of this, in turn, at least one adhesion layer 203 is applied. The thickness of the adhesion layer d203 is preferably at least approximately 0.5 pm to a maximum of approximately e pm. However, it is also conceivable for the at least one adhesion layer 203 and / or the nucleation layer 204 not to be present. For example, it is conceivable for the adhesion layer 203 to consist of a chemically deposited NiP layer and an electroplated Ni layer, each with the aforementioned thicknesses (not shown in Fig. 2). At least one functional layer 202 is then applied to this at least one adhesion layer 203. The thickness of the functional layer d202 is preferably at least approximately 1 pm to a maximum of approximately 10 pm. Optionally, at least one cover layer 205 is applied on top of this.The thickness of the cover layer d205 is preferably at least approximately 0.05 pm and at most less than approximately 1 pm.

[0084] Figure 3 schematically shows the sequence of a wet-chemical process for applying a metallization layer to a PTC semiconductor ceramic. In step a1), a primer is first applied 300, followed in an optional step b1) by nucleation 301 with Pd. In a further optional step c1), at least one adhesion layer 302 is applied by means of a wet-chemical process. Either following step a1), b1) or following step c1), at least one functional layer 303 is then applied by means of a wet-chemical process in a step d1). Optionally, this is followed in a step e1) by the application of at least one cover layer 304 by means of a wet-chemical process. For all process steps a1) to e1), unless otherwise stated, what has already been stated above in connection with the process steps applies.Figure 4 schematically shows the sequence of a PVD process for applying a metallization layer to a PTC semiconductor ceramic. First, in step a2), the surface of the semiconductor ceramic 400 is cleaned, preferably by bombardment with argon ions. In a subsequent optional step b2), at least one adhesion layer 401 is applied using a PVD process. Either following step a2) or following step b2), at least one functional layer 402 is applied using a PVD process in a step c2). Optionally, this is followed by the application of at least one cover layer 403 using a PVD process in a step d2). For all process steps a2) to d2), unless otherwise stated, what has already been stated above in connection with the process steps applies.

[0085] Figures 5 to 9, which are described in more detail below, show images taken with scanning electron microscopes. A tungsten cathode device and a field emission SEM were used. The accelerating voltages were typically approximately 20 kV, and the magnifications ranged from 1,000 to 10,000. The images were taken in secondary electron mode.

[0086] Figure 5 shows an SEM image of the metallization layer obtained in Example IE1 using a wet-chemical process. It can be seen that, first, a layer containing NiP as the main component 500 is applied to the surface of the PTC semiconductor ceramic 101 according to the process described in IE1. A layer containing Ni as the main component 501 is deposited thereon according to the process described in IE1. The two aforementioned layers form the adhesion layer. On layer 501, a layer containing Ag as the main component 502 can be seen as a functional layer according to the process described in IE1. For further explanations, reference is made to the inventive example IE1. An optional cover layer was not applied here.

[0087] Figure 6 shows an SEM image of the metallization layer obtained in Example IE2 using the PVD process. It can be seen that, first, an approximately 0.1 μm thick layer containing Cr as the main component is deposited on the surface of the PTC semiconductor ceramic 101, followed by an approximately 0.4 μm thick layer containing Ni as the main component as the main component according to the process described in IE2, as at least one adhesion layer 600. On top of this, a layer containing Cu as the main component 601 is deposited as the functional layer according to the process described in IE2, and on top of this, a layer containing NiCr2O as the main component 602 is deposited as the cover layer according to the process described in IE2. For further explanations, reference is made to the inventive example IE2.

[0088] Figure 7 shows an SEM image of the metallization layer obtained in Comparative Example CE1. It shows a section of the PTC semiconductor ceramic 101, an adhesion layer containing Cr and Ni as the main components 700, and an overlying functional layer containing Ag as the main component 701. The pores 702 in the functional layer are particularly noticeable. For further explanations, please refer to Comparative Example CE1.

[0089] Figure 8 shows an SEM image of the metallization layer obtained in Comparative Example CE2. It shows a section of the PTC semiconductor ceramic 101, as well as an overlying functional layer containing Al as the main component 800. Here, too, the pores 702 in the functional layer are particularly noticeable. For further explanations, please refer to Comparative Example CE2.

[0090] Figure 9 further shows an SEM image of the metallization layer obtained in Comparative Example CE3. A section of the PTC semiconductor ceramic 101 can be seen, as well as an overlying functional layer containing Ag as the main component 901. In addition to the pores 702 in the functional layer, the clearly visible cracks 900 on the surface of the PTC semiconductor ceramic are particularly noticeable. Figure 10 shows an example of a PTC semiconductor ceramic with a sintered body 101. The PTC semiconductor ceramic 101 is advantageously coated on two opposite sides with a metal layer 1001, whereby only one side of the coating with a metal layer 1001 is shown in Figure 10.The metal layer 1001 is advantageously obtained by means of a wet-chemical process, wherein a layer containing NiP as the main component is advantageously first applied to the surface of the PTC semiconductor ceramic 101 according to the method described in IE1. A layer containing Ni as the main component is advantageously deposited thereon according to the method described in IE1. In this described embodiment, the two aforementioned layers form the adhesion layer. A layer containing Ag as the main component is advantageously applied as a functional layer to the layer containing Ni as the main component according to the method described in IE1.The layer containing NiP as the main component according to the method described in IE1, the layer containing Ni as the main component according to the method described in IE1, and the layer containing Ag as the main component according to the method described in IE1 form the metal layer 1001 shown in Figure 10 in the described embodiment. The two Ni-containing layers can alternatively advantageously be replaced by a Ni-containing layer. For further explanations, reference is made to the inventive example IE1. A current-carrying contact bar 1003 is applied to the metal layer 1001 by means of a rigid joining seam 1002. The rigid joining seam is advantageously obtained by Ag sintering. It goes without saying that it is conceivable that on the opposite side, which is not shown in detail in Figure 10, a metallization layer is also connected to a second current-carrying contact bar via a rigid joining seam.

[0091] In the following, the invention is described in detail using examples, without limiting the scope of the invention thereto:

[0092] Inventive Example IE1 According to inventive example IE1, a primer was first applied to a portion of the surface of the semiconductor ceramic to roughen the surface. In the present example, the primer is a sulfuric acid solution containing peroxodisulfates as an oxidizing agent. This was followed by nucleation with Pd, whereby Pd was deposited from an acidic colloidal PdSn solution. Sodium hypophosphite was used for reduction. Of course, other reducing agents are also conceivable. Subsequently, a layer approximately 0.2 μm to 3 μm thick, containing NiP as the main component, was chemically deposited. Subsequently, a layer approximately 0.3 μm to 3 μm thick, containing Ni as the main component, was electrodeposited.In the next step, a first layer containing Ag was electrodeposited from Arguna® 3430 electrolyte solution, followed by a further approximately 4 pm thick layer containing Ag as the main component. Figure 5 shows an SEM image of the resulting metallization layer. It is clearly visible that the functional layer containing Ag as the main component is virtually pore-free, allowing the use of a significantly thinner layer thickness compared to the prior art.

[0093] Tests have shown that coatings according to the inventive example IE1 exhibit very good adhesion to BaTiOs PTC substrates. This allows a current-carrying contact rail to be contacted using a rigid metallic joint with good thermal conductivity. This is shown schematically in Fig. 10. Joining methods such as Ag sintering, Cu sintering, and soft soldering with Sn-based solders are possible. Such a joint requires very good adhesion of the coating to the substrate and has the advantage over the known joining by adhesive bonding that a low thermal resistance is achieved due to the good thermal conductivity of the rigid metallic joint.

[0094] Inventive Example IE2 According to inventive example IE2, the metallization layer was applied using PVD high-frequency sputtering. The surface of the PTC semiconductor ceramic was first cleaned by bombardment with argon ions. Subsequently, an approximately 0.1 pm thick layer containing Cr as the main component was deposited, followed by an approximately 0.4 pm thick layer containing Ni as the main component. On top of this layer, an approximately 4 pm thick layer containing Cu as the main component was deposited as a functional layer, and for corrosion protection, a further approximately 0.15 pm thick layer containing NiCr2O as the main component was deposited. Figure 6 shows an SEM image of the resulting metallization layer. It can be clearly seen that the functional layer containing Cu as the main component is virtually pore-free.

[0095] Comparison example CE1

[0096] For comparison, a metallization layer with an adhesion layer containing Cr and Ni as the main components was produced using a PVD process and an overlying functional layer containing Ag as the main component by screen printing and subsequent firing. Figure 7 shows an SEM image of the resulting metallization layer. The resulting coating is approximately 10 pm thick and exhibits clearly visible porosity.

[0097] Comparison example CE2

[0098] For further comparison, a metallization layer consisting of a functional layer containing Al as the main component was applied by screen printing and fired. An Al paste was used for this purpose. Figure 8 shows an SEM image of the resulting metallization layer. The resulting coating is approximately 22 pm thick and exhibits clearly visible porosity. Comparative Example CE3

[0099] In addition, a metallization layer consisting of a functional layer containing Ag as the main component was applied by screen printing and fired. Figure 9 shows an SEM image of the resulting metallization layer. In addition to the large number of pores in the functional layer, it is clearly visible that a large number of small cracks have formed on the surface of the PTC semiconductor ceramic.

[0100] The comparison of the SEM images of the coatings obtained according to the methods described here (Figure 5 and Figure 6) and according to the methods known in the prior art (Figure 7 to Figure 9) clearly shows that the porosity and crack formation are significantly reduced in the proposed methods.

[0101] List of reference symbols:

[0102] 101 : Semiconductor Ceramics

[0103] 102: Coating with electrodes

[0104] 202: Functional layer

[0105] 203: Adhesive layer

[0106] 204: Germination

[0107] 205: Cover layer d202: Thickness of the functional layer d203: Thickness of the adhesive layer d2os: Thickness of the cover layer

[0108] 300: Applying a primer

[0109] 301 : Germination

[0110] 302: Applying at least one adhesive layer by means of an advantageous wet-chemical process

[0111] 303: Applying at least one functional layer by means of an advantageous wet-chemical process

[0112] 304: Applying at least one covering layer by means of an advantageous wet-chemical process

[0113] 400: Cleaning the surface of the PTC semiconductor ceramic

[0114] 401 : Applying at least one adhesive layer using an advantageous PVD process

[0115] 402: Applying at least one functional layer by means of an advantageous PVD process

[0116] 403: Applying at least one cover layer using an advantageous PVD process

[0117] 500: Layer containing NiP as main component, applied according to an advantageous wet chemical process

[0118] 501 : Layer containing Ni as main component, applied according to an advantageous wet chemical process

[0119] 502: Layer containing Ag as main component, applied according to an advantageous wet-chemical process 600: At least one adhesion layer containing a layer containing Cr as

[0120] Main component and on this a layer containing Ni as

[0121] Main component, applied according to an advantageous PVD process

[0122] 601 : Layer containing Cu as the main component, applied according to an advantageous PVD process

[0123] 602: Layer containing NiCr20 as main component, applied according to an advantageous PVD process

[0124] 700: Layer containing Cr and Ni as main components, applied according to a prior art method

[0125] 701 : Layer containing Ag as the main component, applied according to a prior art method

[0126] 702: large and small pores

[0127] 800: Layer containing Al as main component, applied according to a prior art method

[0128] 900: Cracks

[0129] 901 : further layer containing Ag as the main component, applied according to a prior art method

[0130] 1001 : Metal layer

[0131] 1002: Joining seam

[0132] 1003: current-carrying contact rail

Claims

Claims 1 . Metallization layer for a PTC semiconductor ceramic for at least partially coating the surface of the PTC semiconductor ceramic, comprising at least one functional layer, i. wherein the at least one functional layer serves for current distribution, ii. wherein the porosity of the at least one functional layer is less than 5%.

2. Metallization layer according to claim 1, characterized in that the at least one functional layer comprises at least one element and / or at least one alloy selected from the group consisting of Ag, Al, Cu, AgZn alloys and CuAl bronzes, preferably the at least one functional layer comprises at least one element selected from the group consisting of Ag, Al and Cu.

3. Metallization layer according to one of claims 1 or 2, characterized in that the thickness of the at least one functional layer is from approximately a minimum of 1 pm to a maximum of approximately 10 pm, preferably the thickness of the at least one functional layer is from a minimum of approximately 1 pm to a maximum of approximately 8 pm, more preferably the thickness of the at least one functional layer is from a minimum of approximately 2 pm to a maximum of approximately 8 pm, in particular the thickness of the functional layer is a value of 2 pm to a value which is less than 5 pm, in particular from 2 pm to 4.8 pm.

4. Metallization layer according to one of the preceding claims, characterized in that the metallization layer further comprises at least one adhesive layer, wherein the at least one adhesive layer is arranged between the surface of the PTC semiconductor ceramic and the at least one functional layer.

5. Metallization layer according to claim 4, characterized in that the at least one adhesion layer comprises at least one element or compound selected from the group consisting of Ni, NiP, Cr and Cu.

6. Metallization layer according to claim 4, characterized in that the metallization layer has exactly one adhesive layer, preferably the exactly one adhesive layer comprises nickel.

7. Metallization layer according to one of the preceding claims, characterized in that the metallization layer further comprises a nucleation, wherein the nucleation is applied to the surface of the PTC semiconductor ceramic, preferably the nucleation consists of colloidal Pd.

8. Metallization layer according to one of the preceding claims, characterized in that the metallization layer further comprises a nucleation, wherein the nucleation is applied to the surface of the PTC semiconductor ceramic in a plasma-assisted manner, preferably the nucleation consists of colloidal Pd, more preferably the nucleation consists of colloidal Pd and is applied in a structured manner.

9. Metallization layer according to one of the preceding claims, characterized in that the metallization layer further comprises at least one cover layer, wherein the at least one cover layer is arranged above the at least one functional layer.

10. Metallization layer according to one of the preceding claims, characterized in that the at least one cover layer comprises at least one element or compound selected from the group consisting of Ni and NiCr.

11. Metallization according to one of the preceding claims, characterized in that the porosity of the functional layer and optionally the Adhesive layer and / or optionally the cover layer is less than 3%, preferably the porosity is less than 1%.

12. A method for applying a metallization layer according to one of claims 1 to 11 by means of a wet-chemical coating method, comprising the steps: a1) applying a primer to at least part of the surface of the PTC semiconductor ceramic, b1) optionally nucleating in an activator step and reducing the nucleation, preferably the nucleation is carried out with Pd, c1) optionally applying at least one adhesive layer, d1) applying at least one functional layer, e1) optionally applying at least one cover layer.

13. A method for applying a metallization layer according to one of claims 1 to 11 by means of physical vapor deposition (PVD), comprising the steps: a2) cleaning the surface of the PTC semiconductor ceramic, b2) optionally applying at least one adhesion layer, c2) applying at least one functional layer, d2) optionally applying at least one cover layer.

14. PTC thermistor comprising: a sintered body consisting of a PTC semiconductor ceramic, and a metallization layer according to one of claims 1 to 11, which is applied to two opposite sides of the PTC semiconductor ceramic on the surface of the PTC semiconductor ceramic.

15. PTC thermistor according to claim 14, characterized in that the PTC semiconductor ceramic is joined to current-carrying contact rails by means of a rigid metallic joining connection.

16. Use of a metallization layer according to one of claims 1 to 11 together with a PTC semiconductor ceramic in an electrical heating application, a switching element, a measuring element or a temperature control element, preferably the metallization layer is used in a high-voltage heating application.

17. Use of a metallization layer according to one of claims 1 to 11 together with a PTC semiconductor ceramic in an electrical heating application, a switching element, a measuring element or a temperature control element, wherein the metallization layer is applied to two opposite sides of the PTC semiconductor ceramic on the surface of the PTC semiconductor ceramic and the PTC semiconductor ceramic is joined to current-carrying contact rails by means of a rigid metallic joining connection.

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