Monolithic functional ceramic element and method for establishing a contact-connection for a functional ceramic

Abstract

The present invention relates to a method for establishing a contact-connection for a functional ceramic (3), comprising the steps of: providing a functional ceramic (2, 3), applying metal paste (4) to two opposite surfaces of the functional ceramic (2, 3), laminating ceramic substrate green films (9) on the two opposite surfaces of the functional ceramic (2, 3) on the metal paste (4), jointly sintering the functional ceramic (2, 3), the ceramic substrate green films (9) for forming electrically insulating ceramic layers (10), and the metal paste (4) for forming electrically conductive metal structures (5). The present invention also relates to a monolithic functional ceramic element, in particular produced in accordance with the described method.

Description

[0001] Description

[0002] Monolithic functional ceramic element and method for producing a contact for a functional ceramic

[0003] The present invention relates to a functional ceramic element, a method for producing a contact of a functional ceramic and the use of the element in a heating module.

[0004] The use of functional ceramic elements in heating modules, particularly the use of PTC (Positive Temperature Coefficient) thermistor elements, has the advantage that, due to their property as a temperature-dependent resistor, the power consumption is automatically limited when a certain temperature is reached. This property, in particular, prevents overloading of the heating module.

[0005] Such heating modules are increasingly being used as heating registers in electric vehicles. Such applications require the register to be operated directly with the high-voltage battery (typically 200-800 V). Therefore, the insulation strength must be designed accordingly.

[0006] Typically, PTC elements are electrically connected on two opposite sides by a conductor. The conductor is supported by a substrate, which couples the generated heat to the other side.

[0007] The heat output that can be extracted depends heavily on the thermal path through the layer structure described above. Heat must travel from the point of origin (the PTC) via the contact and through the substrate to the extraction surface. Here, thermal and electrical considerations for optimizing the heating element are often subject to conflicting arguments, meaning that state-of-the-art designs are compromise solutions between power density, thermal agility, and insulation capacity, or robustness and reliability.

[0008] The PTC element itself acts as a heat source when Joule heating is generated by current. However, this is not generated homogeneously within the material; rather, depending on the geometry and possible material inhomogeneities, the electric field distribution within the component can cause a temperature gradient. The heat must first reach the surface of the elements from hot spots before it can be transported further. This can be very slow and sluggish due to the relatively poor thermal conductivity of the PTC ceramic (typically ~ 5 W / mK).

[0009] The document DE 11 2017 006 124 T5 describes a corresponding electrical heating device according to the state of the art with insulation layers between conductor tracks and cooling fins.

[0010] Document EP 1 182 908 A1 describes a similar PTC heating device with at least one PTC element and two contact plates that contact the PTC element. A metal foil coated with adhesive on both sides is provided to connect the surface of the PTC element to the contact plates. Insulation of the contact plates is not provided here.

[0011] Furthermore, a PTC heater with reduced inrush current is known from document DE 2017 101 946 A1. Document DE 10 2016 108 604 A1 further describes how a similar functional ceramic can be embedded in a ceramic substrate.

[0012] An object of the present invention is to provide an improved functional ceramic element which can also be used in a heating module.

[0013] The present invention relates to a method for producing a contact for a functional ceramic, in particular a PTC ceramic, or a method for producing a functional ceramic element, in particular a, preferably monolithic, thermistor element. The method comprises at least the steps described below.

[0014] In a first step, a functional ceramic is provided. The ceramic, referred to below as functional ceramic, is preferably a thermistor ceramic and even more preferably a PTC ceramic.

[0015] The functional or PTC ceramic can be provided in the green state or in the sintered state. The functional ceramic is preferably provided as a film in the green state, wherein the film has a small film thickness compared to the film surface. The functional ceramic only exhibits its desired functionality in the sintered state. In the present application, a green ceramic is also referred to as a functional ceramic, which only exhibits functionality in the sintered state. The same applies to a PTC ceramic.

[0016] The functional ceramic can be any suitable

[0017] Contain ceramic material. Possible ceramic materials include, for example, barium titanate ceramics. Furthermore, the ceramic may also comprise, for example, lead and / or strontium. The ceramic may further be doped with suitable dopants such as, for example, yttrium or manganese in suitable amounts in order to provide a desired functionality, in particular a thermistor functionality.

[0018] In a further step, metal paste is applied to two opposite surfaces of the functional ceramic.

[0019] The metal paste is preferably applied with a thickness of a few micrometers or a few 100 nanometers.

[0020] The metal paste preferably comprises an electrically conductive metal such as nickel, cobalt, copper, silver, another precious metal, or a metal alloy in powder form. Furthermore, the metal paste comprises, for example, suitable suspension agents.

[0021] In one embodiment, the metal paste can in particular not be applied over a large area, but in a form that can be converted into comb-shaped metal structures by a sintering step. For this purpose, the metal paste can in particular be applied in a comb structure. The two comb structures are preferably applied to the opposite surfaces in such a way that the comb structures do not lie on top of one another, but are arranged offset. The comb structures each comprise a continuous section as the main strand and sections branching off from it as secondary strands. The metal paste can be applied, for example, by (screen) printing or by sputtering.

[0022] In a further step, ceramic substrate green films are applied and laminated onto the two opposite surfaces of the functional ceramic.

[0023] The ceramic substrate green sheets may comprise a similar ceramic material to the functional ceramic or a different ceramic material. The ceramic material formed from the ceramic substrate green sheets after sintering is preferably electrically insulating and has good thermal conductivity.

[0024] The ceramic substrate green sheets are applied in such a way that they preferably cover the entire surface of the functional ceramic and the metal paste applied to it. The ceramic substrate green sheets are applied directly to the surface of the functional ceramic or to the metal paste applied to it.

[0025] Through the steps described above, which are preferably carried out in the specified order, a layer stack is provided comprising the two ceramic substrate green sheets that enclose the functional ceramic in a sandwich-like structure. Furthermore, metal paste structures are arranged between the functional ceramic and the ceramic substrate green sheets, which serve to establish electrical contact with the functional ceramic.

[0026] In a further step, the layer stack is sintered together to form the functional ceramic element. In a preferred embodiment, the

[0027] Monolithic functional ceramic element. "Monolithic" means that the functional ceramic element is not made up of various individual elements, but of a single element. Therefore, no (sub-)elements need to be mechanically connected or bonded.

[0028] In one embodiment, the functional ceramic element is a monolithic thermistor element.

[0029] The term "sintering" refers here only to the so-called temperature treatment step. The term "sintering" does not imply that everything being sintered was previously in the green state. Already sintered structures can also be subjected to a corresponding temperature treatment step, which then has little or no effect on the already sintered structures.

[0030] The functional ceramic element is preferably a thermistor element with thermistor functionality. The functional ceramic is then a ceramic with thermistor properties, in particular an NTC or preferably a PTC ceramic.

[0031] The monolithic functional ceramic element is formed by the joint sintering of the functional ceramic to form a functional ceramic layer, the ceramic substrate green sheets to form electrically insulating ceramic layers, and the metal paste to form electrically conductive metal structures. The functional ceramic element thus formed comprises the functional ceramic layer, the electrically insulating ceramic layers, and the electrically conductive metal structures.

[0032] By forming a monolithic functional ceramic element, defects that arise when assembling a functional ceramic element from various sub-elements can be avoided. For example, voids that can arise during bonding can be avoided. Furthermore, leakage or smearing of the adhesive can be prevented. Furthermore, an incomplete connection between separate functional ceramic elements and insulating ceramic elements can be avoided.

[0033] In addition, the process is simplified because there is no need to assemble various sub-elements.

[0034] By forming a functional ceramic layer, electrically conductive structures and insulating ceramic layers in a monolithic element, the stability and durability of the functional ceramic element can be increased.

[0035] Furthermore, the thermal coupling between the individual layers is improved so that when a functional ceramic layer designed as a thermistor layer is heated by applying an electrical voltage, the resulting heat can be easily dissipated to the outside via the electrically insulating ceramic layers. The ceramic layers are preferably thin for this purpose. By using film technology in the production of the functional ceramic element, a large-area and very thin functional ceramic element can be provided. The thickness of the functional ceramic element can therefore be reduced. Furthermore, one functional ceramic film can replace several conventional functional ceramic blocks, e.g. PTC blocks, each with significantly smaller surface dimensions.

[0036] The thin films also allow the formation of a homogeneous electric field even without a surface application of the electrically conductive structures and thus, in the case of a thermistor element, a homogeneous heating of the thermistor element.

[0037] The functional ceramic element can still be easily produced using existing automated processes for the production of multilayer ceramic elements.

[0038] By producing a monolithic functional ceramic element, it is also possible to dispense with the provision of individual components such as conventional functional ceramic stones or insulating ceramic components and electrically conductive metal foils, which have to be manufactured and subsequently assembled at different locations.

[0039] The electrically conductive metal structures can be electrically contacted to the outside. For this purpose, for example, recesses are provided in the electrically insulating ceramic layers. The recesses can be formed, for example, by incompletely covering the metal structures with ceramic foils or by later removal of ceramic material. In the region of the recesses, the metal structures can then be electrically contacted, for example, with wires. The wires are, for example, soldered to the metal structures. Alternatively, the metal structures can be electrically contacted, for example, by means of clamp contacts. Other suitable contacting methods are also possible.

[0040] Preferably, the metal structures do not extend to the edge of the ceramic layers in order to form an area at the edges of the functional ceramic element which is not subjected to an electrical voltage even during operation.

[0041] In one embodiment, the functional ceramic is provided as a functional ceramic film in the green state. In one embodiment, the functional ceramic is in particular provided as a film in the green state, and the metal paste and the ceramic substrate green films are applied directly to the functional ceramic film in the green state. Preferably, no further processing steps are carried out between the aforementioned steps. In an alternative embodiment, the functional ceramic is provided as a film in the green state, and the green functional ceramic is first sintered to form a functional ceramic layer. The metal paste and the ceramic substrate green films are then applied to the functional ceramic layer in the sintered state.

[0042] In one embodiment, the functional ceramic is provided as a green film and sintered at high temperatures above 1000°C, preferably above 1300°C, prior to application of the metal paste and the ceramic substrate green films to form the functional ceramic layer. The functional ceramic then preferably comprises an HTCC (high-temperature cofired ceramics) ceramic material.

[0043] Preferably, the metal paste is dried at elevated temperature before sintering in order to evaporate a suspension or solvent.

[0044] In an alternative embodiment, the functional ceramic is provided in the sintered state.

[0045] The ceramic substrate green sheets preferably comprise an LTCC (low temperature cofired ceramics) ceramic material. The subsequent co-sintering is preferably carried out at a lower temperature below 1000 °C, preferably below 800 °C.

[0046] Sintering at low temperatures, for example, preserves the thermistor functionality of the functional ceramic. In particular, subsequent sintering at low temperatures prevents undesirable oxidation of the functional ceramic.

[0047] In an alternative embodiment, the metal paste and the ceramic substrate green films are applied to the functional ceramic provided as a film in the green state. The layer stack thus formed is then sintered together. The joint sintering is preferably carried out at a high temperature above 1000°C, preferably above 1300°C. In the present embodiment, both the functional ceramic and the additional ceramic layers preferably comprise an HTCC ceramic.

[0048] This allows the ceramics to be sintered at the same temperature. By selecting ceramics that are as similar as possible, the development of mechanical stresses during sintering can be reduced or avoided.

[0049] In a preferred embodiment, the functional ceramic film and the ceramic substrate green films have essentially the same composition.

[0050] Preferably, the composition of the functional ceramic film and the ceramic substrate green films differs only in the proportion of dopants in the composition. Such dopants can be, for example, yttrium or manganese. In particular, a higher proportion of the aforementioned dopants can increase the electrical resistance of the ceramic material, thus providing an electrically insulating ceramic material.

[0051] Generally speaking, it may be preferred that the functional ceramic film and the ceramic substrate green films have the same ceramic base material, wherein the PTC functionality or the substrate function is adjusted or defined by the choice of dopants and / or the concentration of the dopants.

[0052] By selecting ceramics that are as similar as possible for the various ceramic layers, the formation of mechanical stresses during sintering can be reduced or avoided. This prevents the formation of defects in the functional ceramic element, such as the formation of cavities between the layers. Furthermore, the selection of similar materials improves the thermal coupling between individual layers.

[0053] In a preferred method, several functional ceramic foils are separated from a larger functional ceramic foil. This enables simple serial production of the functional ceramic elements. By isolating the functional ceramic from the foil, a large-area, yet very thin functional ceramic can be easily produced.

[0054] The functional ceramic foils are separated, for example, by cutting or punching.

[0055] Each individual functional ceramic film preferably has a rectangular shape with dimensions of at least 3 cm x 10 cm. The film thickness is preferably a maximum of 150 μm. The dimensions of the functional ceramic layer are then smaller, corresponding to the normal sintering shrinkage.

[0056] The invention further relates to a monolithic functional ceramic element, in particular a monolithic thermistor element. The functional ceramic element is preferably manufactured according to the method described above. All features and embodiments described with reference to the method can also apply to the functional ceramic element. In particular, the functional ceramic element can be a thermistor element in all embodiments and exemplary embodiments. In particular, the invention also relates to a monolithic functional ceramic element, preferably a monolithic thermistor element, which comprises at least the following layers, which are laminated in a stacking direction perpendicular to an outer surface of the monolithic functional ceramic element.

[0057] All features and embodiments described with respect to the method may also apply to the monolithic functional ceramic element.

[0058] The monolithic functional ceramic element comprises, on the one hand, a functional ceramic layer, preferably a PTC ceramic layer, with two opposite surfaces.

[0059] Furthermore, the monolithic functional ceramic element comprises two electrically conductive metal structures with different polarities in the operating state, which are arranged in direct contact on each of the opposing surfaces of the functional ceramic layer. Direct contact here means that the electrically conductive metal structures rest directly on the surfaces of the functional ceramic layer, and no further intermediate structures are formed.

[0060] The electrically conductive metal structures are also electrically connected to the electrically conductive functional ceramic layer. This means that an electrical field can be applied to the functional ceramic layer or an electrical voltage can be applied via the electrically conductive metal structures. Furthermore, the functional ceramic element comprises two electrically insulating ceramic layers, each arranged on one of the opposite surfaces of the functional ceramic layer and the metal structures arranged thereon. The electrically insulating ceramic layers lie directly on the surface of the functional ceramic layer or the metal structures.

[0061] In one embodiment, the functional ceramic layer comprises or consists of an HTCC ceramic and the electrically insulating ceramic layers comprise or consist of an LTCC ceramic.

[0062] Preferably, the electrically insulating ceramic layers in this embodiment comprise an aluminum oxide ceramic.

[0063] The electrically insulating ceramic layers should also be good thermal conductors and be made of a material with high thermal conductivity.

[0064] In an alternative embodiment, the functional ceramic layer and the electrically insulating ceramic layers each comprise or consist of an HTCC ceramic.

[0065] Preferably, the functional ceramic layer and the electrically insulating ceramic layers then have essentially the same ceramic composition.

[0066] In one design, the ceramic

[0067] Composition of the functional ceramic layer and the electrically insulating ceramic layers only by the proportion of dopants in the ceramic composition.

[0068] Such a functional ceramic element has a particularly high thermal coupling between the individual layers, which is advantageous, for example, for use as a thermistor element in a heating module.

[0069] In any embodiment, the functional ceramic layer comprises a barium titanate ceramic, which may further contain, for example, a strontium compound such as strontium oxide and / or a lead compound such as lead oxide and a dopant such as yttrium or manganese.

[0070] In one embodiment, the functional ceramic layer has a maximum layer thickness of 150 pm. Preferably, the functional ceramic layer has a smaller layer thickness of a maximum of 100 pm or a maximum of 50 pm. The minimum layer thickness should be 40 pm.

[0071] A homogeneous electric field can be easily generated in such a thin layer. Even with non-planar metal structures, such as comb-shaped structures, it is possible to apply an electric field that is uniformly distributed across the entire area of ​​the functional ceramic layer covered by the metal structures.

[0072] In one embodiment, the electrically insulating ceramic layers have a maximum layer thickness of 200 pm.

[0073] The insulating ceramic layers preferably cover the entire functional ceramic layer along the two opposite surfaces. Due to the thin design of the layers, the dimensions of the entire

[0074] functional ceramic element can be reduced.

[0075] The thin insulating ceramic layers also enable good heat conduction to the outer sides of the functional ceramic element.

[0076] According to one embodiment, the functional ceramic element has a maximum thickness of 800 pm, preferably 500 pm, more preferably 400 pm in a stacking direction of the said layers.

[0077] In one embodiment, the electrically conductive metal structures are formed in a comb structure.

[0078] The comb structures each comprise a continuous section and a plurality of sections branching off from the continuous section. Preferably, the electrically conductive metal structures are not arranged one above the other in the stacking direction, so that during operation, all conduction paths in the functional ceramic layer, via which electrical current is conducted through the functional ceramic layer, run diagonally. Thus, despite the small thickness of the functional ceramic layer, a minimum conduction path through the layer of preferably at least 4 mm can be provided.

[0079] The minimum conduction path, i.e., the shortest path along which current can flow between two metal structures with different polarity during operation, is preferably defined in the functional ceramic layer between two branching sections of one of the electrically conductive metal structures. The formation of the comb structure also allows for savings in metallic material.

[0080] The minimal conduction path thus guaranteed allows for compliance with specified creepage distances, thus achieving the desired insulation strength while simultaneously further reducing the thickness of the functional ceramic layer. Despite the low film thickness, electrical voltages of preferably 450 to 800 volts, and even more preferably up to 1000 volts, can be applied.

[0081] The guaranteed minimum conduction path also reduces the maximum current flow when a specific electrical voltage is applied to the ceramic layer. This can reduce the energy consumption of a connected battery, for example. Furthermore, inrush current peaks, which place a significant strain on the battery or connected switching electronics, can be reduced.

[0082] The present invention further relates to a heating module comprising one or more of the monolithic thermistor elements described above.

[0083] The described improved thermal coupling, thermal conduction and heat transfer properties of the monolithic thermistor element can increase the efficiency of the heating module.

[0084] The heating module is, for example, a fin heating module comprising several of the described monolithic thermistor elements, on whose surfaces fins are applied, through which a thermal fluid flows. The thermal fluid is heated by the thermistor elements during operation. A corresponding heating module can be used, for example, in the automotive sector and should preferably have a heat output of at least 5 kilowatts.

[0085] In the following, the invention is described in more detail with reference to exemplary embodiments and associated figures.

[0086] The invention is not limited to the examples shown in the figures.

[0087] Similar or apparently identical elements in the figures are designated by the same reference numerals. The figures and their proportions are not necessarily to scale.

[0088] The figures show:

[0089] Figure 1: Schematic representation of the manufacturing process of a first embodiment of a monolithic thermistor element.

[0090] Figure 2: Cross section through a first embodiment of the monolithic thermistor element with the minimum conduction path indicated.

[0091] Figure 3: Microscopic image of a section in the edge region of the first exemplary embodiment of the monolithic thermistor element. Figure 4: Microscopic image of a cross-section through a second exemplary embodiment of a monolithic thermistor element.

[0092] Figure 5: Top view of an embodiment of the monolithic thermistor element with external contact by wires.

[0093] Figure 6: Change in the cold resistance of a PTC ceramic layer of an exemplary thermistor element according to the invention as a function of the number of cycles. In each cycle, 450 volts DC is applied to the thermistor element for 5 seconds and then cooled for 30 seconds.

[0094] Figure 7: Inrush current curve of the exemplary thermistor element according to the invention. It shows the current flow I through a PTC ceramic layer when a DC voltage U of 450 volts is applied as a function of time t from switching on.

[0095] Figure 8 : Photograph of a heating module comprising monolithic thermistor elements.

[0096] Figure 1 shows the production of a first exemplary embodiment of a functional ceramic element according to the invention. In the present example, it is, in particular, a monolithic thermistor element 100.

[0097] In a first step, a PTC ceramic film 1 is used as

[0098] A functional ceramic foil with a large areal dimension and a small thickness is provided. The dimension of the large PTC ceramic foil 1 is, for example, 4 inches x 4 inches. Alternatively, the dimension can be any other, preferably larger, dimension. The thickness of the PTC ceramic foil 1 is between 40 and 250 micrometers, preferably between 50 and 150 micrometers, even more preferably less than 100 micrometers.

[0099] Any number of PTC ceramic films 2 with a smaller expansion can be separated from the provided large PTC ceramic film 1. The individual PTC ceramic films 2 are, for example, punched or cut out of the large PTC ceramic film 1.

[0100] For example, three PTC ceramic films 2 are separated from the exemplary large PTC ceramic film 1 with an area of ​​4 inches x 4 inches. The separated PTC ceramic films 2 preferably have a rectangular shape with an area of ​​approximately 3 cm x 10 cm each. The PTC ceramic films 2 can also have dimensions larger than 3 cm x 10 cm.

[0101] The PTC ceramic foils 2 produced in this way have significantly larger surface areas and a smaller thickness compared to conventionally used PTC ceramic blocks. Thus, a monolithic thermistor element comprising a single PTC ceramic foil 2 can be produced, whereas conventional processes use a large number of PTC ceramic blocks. Furthermore, the thickness of the thermistor element can be reduced by using the thin PTC ceramic foil 2.

[0102] The separated PTC ceramic foils 2 are sintered in a subsequent step. The PTC ceramic foils 2 are preferably used to produce a desired

[0103] Thermistor functionality sintered at a high temperature, for example between 1240 ° C and 1320 ° C.

[0104] During sintering, the expansion of the PTC ceramic film 2 is reduced by an amount typical of sintering shrinkage. Sintering converts the green PTC ceramic film 2 into a sintered functional ceramic layer, namely a PTC ceramic layer 3. The planar expansion of the PTC ceramic layer 3 is, for example, 26 mm x 78 mm and preferably no more than 3 mm x 9 mm.

[0105] Electrically conductive metal structures 5 are then applied to the sintered PTC ceramic layer 3. For this purpose, a metal paste 4 is, for example, printed or sputtered onto the two opposite surfaces of the PTC ceramic layer 3. The metal paste 4 is preferably applied in the form of a comb.

[0106] The metal paste 4 comprises, for example, nickel, copper, aluminum, a precious metal or an alloy of individual ones of the mentioned metals.

[0107] As shown in the figures, the comb comprises a continuous section 6, which is essentially the main strand of the comb, from which several sections 7 branch off, preferably at a right angle, which are essentially the secondary strands of the comb.

[0108] The metal paste 4 is therefore not applied evenly to the surfaces.

[0109] Although the metal paste 4 is not applied over the entire surface, the advantageous thin layer thickness of the PTC ceramic layer 3 according to the invention enables the formation of a uniform electric field in the PTC ceramic layer 3 during operation. This leads, in particular, to the fact that electrical current is uniformly converted into thermal energy in the PTC ceramic layer 3 during operation.

[0110] The thermal energy is dissipated to the environment via the additional ceramic layers 10, which preferably have good thermal conductivity. Heat dissipation to the environment is further promoted by the good thermal coupling between the individual, jointly sintered layers of the monolithic thermistor element 100.

[0111] The two combs on the two surfaces of the PTC ceramic layer 3 are structured in such a way that they do not lie on top of one another in a direction perpendicular to the surface of the PTC ceramic layer 3. This means that, in the theoretical case of a transparent PTC ceramic layer 3, when viewed from a direction starting from one of the surfaces of the PTC ceramic layer 3, both comb structures would be visible next to one another. The main strands 6 of the combs are applied to different sides of the respective surfaces. The branching sections 7 are each applied next to one another with recesses in between in such a way that the sections 7 of the two combs do not lie on top of one another, but each point in the direction of the other comb structure.

[0112] Through this structuring of the metal paste 4 and thus also of the electrically conductive metal structures 5 subsequently formed therefrom, the conduction path 8 in the PTC ceramic layer 3 is maximized as shown in Figure 2. The conduction path 8 is the distance that an electrical current would travel in the PTC ceramic layer 3 in the operating state. The shortest conduction path 8 in the PTC ceramic layer 3 between two metal structures 5 should preferably be a minimum of 4 mm. This shortest conduction path 8 is preferably formed between two adjacent branching sections 7 of one of the two electrically conductive metal structures 5.

[0113] The described minimal conduction path 8 enables the application of high electrical voltages, for example in the range between 400 and 1000 volts, preferably in the range above 800 volts, despite the small ceramic thicknesses.

[0114] The applied metal paste 4 is then dried at a temperature of, for example, at least 180 ° C for a period of, for example, at least 30 minutes.

[0115] Subsequently, as shown in Figure 3, a ceramic substrate green film 9 is applied to both surfaces of the PTC ceramic layer 3, each covering the entire surface of the PTC ceramic layer 3 and the metal paste 4 applied thereto. The thickness of the metal paste 4 structure is negligible compared to the thickness of the ceramic layers or films and is in the micrometer or sub-micrometer range.

[0116] While the PTC ceramic layer 3 preferably comprises a high-temperature sintered HTCC ceramic, the further ceramic layers 10, which are formed from the ceramic substrate green sheets 9, preferably comprise an LTCC ceramic material that is sintered at comparatively lower temperatures. The material of the PTC ceramic layer 3 is, for example, a barium titanate ceramic or a similar material, which may also comprise other metals, such as lead or strontium. However, it is preferably a lead-free ceramic. To produce the thermistor functionality, the ceramic of the PTC ceramic layer 3 is preferably doped with other elements, such as yttrium and / or manganese.

[0117] The LTCC ceramic of the further ceramic layers 10 is, for example, an aluminum oxide ceramic or a similar material which is preferably a good thermal conductor but electrically insulating.

[0118] The ceramic substrate green films 9 preferably have a film thickness between 50 and 200 micrometers.

[0119] After laminating the ceramic substrate green sheets 9, the entire layer stack is pressed and sintered together. Sintering is preferably carried out at low temperatures, for example, between 850 and 950 °C in an air atmosphere, so that the ceramic substrate green sheets 9 are converted into electrically insulating ceramic layers 10, and the metal paste 4 is converted into electrically conductive metal structures 5.

[0120] The lower sintering temperature during joint sintering ensures that the PTC ceramic layer 3 is not or hardly oxidized, so that the desired thermistor functionality is maintained.

[0121] For an alternative embodiment, the method can be slightly modified. In the modified method, all steps that are not described in detail again are carried out analogously to the previous method. In contrast to the previously described method, in the modified method, the PTC ceramic film 2 is not sintered before the application of the metal paste 4 and the ceramic substrate green films 9. Rather, the metal paste 4 and the ceramic substrate green films 9 are applied to the unsintered, green PTC ceramic film 2.

[0122] In contrast to the previously described method, it is necessary that the ceramic substrate green sheets 9 comprise a similar material to the PTC ceramic sheet 2. The ceramic substrate green sheets 9 therefore comprise an HTCC ceramic, like the PTC ceramic sheet 2.

[0123] Preferably, the PTC ceramic film 2 and the ceramic substrate green films 9 comprise essentially the same ceramic material, differing only in the amount of added doping elements. A suitable material would be, for example, a barium titanate ceramic provided with a boron nitride sintering additive. The thermistor functionality of the PTC ceramic layer 3 or the electrically insulating properties of the additional ceramic layers 10 are adjusted by the amount of doping with additional elements such as yttrium and / or manganese.

[0124] Alternatively, two different HTCC ceramics can be selected for the PTC ceramic film 2 and the ceramic substrate green films 9.

[0125] The entire stack, comprising the foils 2 and 9 and the metal paste 4, is sintered together at a high temperature. An exemplary sintering temperature is between 1000 and 1300 °C. For example, the stack is sintered at 1150 °C.

[0126] In a subsequent step, the formed monolithic thermistor element 100 can be reoxidized by heating to 600 to 800 ° C under an air atmosphere in order to produce the thermistor functionality of the PTC ceramic layer 2 .

[0127] A scanning electron microscope image of a cross section through a correspondingly manufactured monolithic thermistor element 100 is shown in Figure 4.

[0128] For external electrical contacting, wires 11, for example, can then be connected to the electrically conductive structures 5, as shown in Figure 5.

[0129] For example, the wires 11 are soldered onto a surface of the electrically conductive structures 5. For this purpose, recesses 12 can be provided in the electrically insulating ceramic layers 10 or subsequently formed at appropriate locations by removing the ceramic material. These recesses 12 are preferably formed at or near the corners of the monolithic thermistor element 100.

[0130] The monolithic thermistor element 100 manufactured using the described method can be made significantly thinner than previously known thermistor elements. Due to the described layer structure and the joint sintering of the entire layer stack to form a monolithic element, additional assembly steps such as pressing and bonding individual components are eliminated. Eliminating these steps also avoids or minimizes potential assembly errors such as the formation of gaps or cavities between the individual elements.

[0131] The reliability of the thermistor element 100 in operation and the stability of its functionality over time can thus be increased.

[0132] Furthermore, the described method enables a flexible production of thermistor elements 100 of different dimensions and with different desired electrical properties by means of established automated manufacturing processes using multilayer ceramic technology.

[0133] Figure 6 shows an example diagram of the cold resistance of the PTC ceramic layer 3 as a function of the number of switching cycles. In each switching cycle, a DC voltage of 450 volts is applied to the PTC ceramic layer for 5 seconds. The current is then switched off and the thermistor element 100 is cooled for 30 seconds. The cold resistance is measured in the cooled state. The next switching cycle then begins.

[0134] The diagram shows that the cold resistance depends scarcely on the number of switching cycles, meaning that the properties of the thermistor element 100 do not change, for example, due to delamination of the layers. The fluctuations shown are due to the short cycle times, which prevent the establishment of thermal equilibrium.

[0135] Figure 7 shows another diagram illustrating the inrush current curve for a monolithic thermistor element 100 according to the invention. The thermistor element, with a room-temperature resistance of approximately 25 kΩ, reaches the maximum inrush current or minimum electrical resistance after approximately 50 ms (milliseconds) at an applied DC voltage of 450 volts. Based on the resistance-temperature data of the PTC ceramic used, this would correspond to a temperature of approximately 170 °C. The voltage curve is also shown in steps in the diagram.

[0136] Due to the comparatively long conduction path 8 created by the diagonal arrangement of the electrically conductive structures 5 on the PTC ceramic layer 3, the current peak after the current is switched on, which can be seen in the diagram at approximately 50 ms, can be reduced. This reduces power consumption and protects the stressed material.

[0137] The monolithic thermistor element 100 according to the invention is preferably used in a heating module 200. The heating module 200, which is shown in Figure 8, comprises several, for example six, thermistor elements 100.

[0138] Lamellar structures 201 are then applied to the surface of the electrically insulating but highly thermally conductive ceramic layers 10, through which a fluid heat medium is passed.

[0139] The heat medium is heated as it flows through the lamellar structures 201 and can then release the heat to the areas to be heated.

[0140] Such heating modules are used, for example, in the automotive sector to heat the passenger compartment or in the electro-automotive sector to heat the battery to a uniform, desired temperature, for example, 40 °C. The heating output of such a heating module 200 should preferably be at least 5 kilowatts.

[0141] Due to the monolithic structure of the thermistor element 100, no special requirements such as a high mechanical driving force are required when assembling the heating module 200.

[0142] Reference symbol list

[0143] 1 large PTC ceramic foil

[0144] 2 PTC ceramic foils

[0145] 3 PTC ceramic layer

[0146] 4 Metal paste

[0147] 5 electrically conductive structure

[0148] 6 connected ridge section

[0149] 7 branching ridge sections

[0150] 8 Line route

[0151] 9 ceramic substrate green films

[0152] 10 electrically insulating ceramic layers

[0153] 11 wires

[0154] 12 recesses in the ceramic layers

[0155] 100 monolithic thermistor element

[0156] 200 heating module

[0157] 201 lamellar structures

Claims

Patent claims 1. A method for producing a contact for a functional ceramic (3), comprising the steps of: providing a functional ceramic (2,3), Applying metal paste (4) to two opposite surfaces of the functional ceramic (2,3), laminating ceramic substrate green films (9) on the two opposite surfaces of the functional ceramic (2,3) on the metal paste (4), jointly sintering the functional ceramic (2,3), the ceramic substrate green films (9) to form electrically insulating ceramic layers (10) and the metal paste (4) to form electrically conductive metal structures (5).

2. The method according to claim 1, wherein the functional ceramic is provided as a functional ceramic film (2) in the green state.

3. Method according to claim 2, wherein several functional ceramic films (2) are separated from a functional ceramic film (1) of larger dimensions.

4. The method according to claim 3, wherein each functional ceramic film (2) has a rectangular shape with a dimension of at least 3 cm x 10 cm.

5. Method according to one of claims 1 to 4, wherein the functional ceramic (3) is a thermistor ceramic.

6. The method according to claim 5, wherein the functional ceramic (2,3) is a PTC ceramic (2,3).

7. Method according to one of claims 1 to 6, wherein a monolithic functional ceramic element (100) is formed.

8. Method according to one of claims 1 to 7, wherein the functional ceramic (3) is provided in the sintered state.

9. Method according to one of claims 1 to 7, wherein the functional ceramic is provided as a film (2) in the green state and before applying the metal paste (4) and the ceramic substrate green sheets (9) are sintered at high temperatures above 1000 °C to form a functional ceramic layer (3), and wherein the subsequent joint sintering to form the functional ceramic element (100) is carried out at a lower temperature below 1000 °C.

10. The method according to any one of claims 1 to 7, wherein the functional ceramic is provided as a film (2) in the green state, and wherein the metal paste (4) and the ceramic substrate green films (9) are applied to the functional ceramic (2) in the green state, and wherein the subsequent co-sintering to form the functional ceramic element (100) is carried out at a high temperature above 1000 °C.

11. The method according to claim 10, wherein the functional ceramic film (2) and the ceramic substrate green films (9) have substantially the same composition and the composition of the functional ceramic film (2) and the ceramic substrate green films (9) differ only in the proportion of dopants in the composition.

12. Method according to one of claims 1 to 11, wherein the functional ceramic film (2) has a rectangular shape with dimensions of at least 3 cm x 10 cm.

13. Method according to one of claims 1 to 12, wherein the metal paste (4) is applied in a structure which is converted into comb-shaped metal structures (5) by the joint sintering.

14. The method according to claim 2, wherein, in addition to the functional ceramic film (2), ceramic substrate green films (9) are also applied in a green state and are converted into a non-green, sintered state by a common sintering step.

15. Monolithic functional ceramic element (100) comprising at least the following layers, which are laminated in a stacking direction perpendicular to an outer surface of the functional ceramic element (100): a functional ceramic layer (3) with two opposite surfaces, two electrically conductive metal structures (5) which have different polarities during operation and are arranged in direct contact on one of the opposite surfaces of the functional ceramic layer (3), two electrically insulating ceramic layers (10), which are each arranged on one of the opposite surfaces of the functional ceramic layer (3) and the metal structures (5) arranged thereon.

16. Monolithic functional ceramic element (100) according to claim 15, wherein the functional ceramic layer (3) comprises or consists of an HTCC ceramic and the electrically insulating ceramic layers (10) comprise or consist of an LTCC ceramic.

17. Monolithic functional ceramic element (100) according to claim 16, wherein the electrically insulating ceramic layers (10) comprise an aluminum oxide ceramic.

18. Monolithic functional ceramic element (100) according to claim 15, wherein the functional ceramic layer (3) and the electrically insulating ceramic layers (10) each comprise or consist of an HTCC ceramic.

19. Monolithic functional ceramic element (100) according to claim 18, wherein the functional ceramic layer (3) and the electrically insulating ceramic layers (10) have substantially the same ceramic composition and the ceramic composition of the functional ceramic layer (3) and the electrically insulating ceramic layers (10) differ only in the proportion of dopants in the ceramic composition.

20. Monolithic functional ceramic element (100) according to one of claims 15 to 19, wherein the functional ceramic layer (3) comprises a barium titanate ceramic.

21. Monolithic functional ceramic element (100) according to one of claims 15 to 20, wherein the electrically insulating ceramic layers (10) have a high thermal conductivity.

22. Monolithic functional ceramic element (100) according to one of claims 15 to 21, wherein the functional ceramic layer (3) has a layer thickness of at most 150 pm.

23. Monolithic functional ceramic element (100) according to one of claims 15 to 22, wherein the electrically insulating ceramic layers (10) have a layer thickness of at most 200 pm.

24. Monolithic functional ceramic element (100) according to one of claims 15 to 23, which has a maximum thickness of 500 pm in a stacking direction of said layers.

25. Monolithic functional ceramic element (100) according to one of claims 15 to 24, wherein the electrically conductive metal structures (5) are formed in a comb structure, each comprising a continuous section (6) and a plurality of sections (7) branching off from the continuous section.

26. Monolithic functional ceramic element (100) according to claim 25, wherein the electrically conductive metal structures (5) are not arranged one above the other in the stacking direction, so that during operation all conduction paths (8) in the functional ceramic layer (3) via which electrical current is conducted through the functional ceramic layer (3) run diagonally.

27. Monolithic functional ceramic element (100) according to claim 26, wherein a minimum conduction path (8) in the functional ceramic layer (3) is formed between two branching sections (7) of each of the electrically conductive metal structures (5) and is at least 4 mm.

28. Monolithic functional ceramic element (100) according to one of claims 15 to 27, wherein the monolithic Functional ceramic element (100) is a monolithic thermistor element (100).

29. Monolithic functional ceramic element (100) according to claim 28, wherein the functional ceramic (2,3) is a PTC ceramic (2,3).

30. Monolithic functional ceramic element (100) according to one of claims 15 to 29, wherein the functional ceramic layer (3) is derived from a functional ceramic foil having a dimension of at least 3 cm x 10 cm.

31. A heating module (200) comprising the monolithic thermistor element (100) according to any one of claims 28 or 29.

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