Improved aluminum-based resistance heater

Abstract

The present invention relates to an aluminum resistance heater comprising a thermally conductive substrate, the thermally conductive substrate comprising an electrically insulating surface. An aluminum resistive structure is disposed on the electrically insulating surface, where the aluminum resistive structure includes at least one aluminum conductive layer covering at least a portion of the electrically insulating surface, where the aluminum conductive layer includes aluminum and at least a first glass. The aluminum resistive structure also includes at least two terminal contact pads that contact the aluminum conductive layer, where the terminal contact pads include silver. Comprising glass covers at least a portion of the aluminum resistive structure. The invention further relates to a method for producing an aluminum resistance heater.

Description

[0001] The present invention relates to an improved aluminum-based resistive heater that is easy to produce and a method for producing such an aluminum-based resistive heater.

[0002] Introduction

[0003] Typically, thick film heaters are based on silver, ruthenium, palladium, or some combination of these conductive materials. The advantage of those heaters is that these materials are easy to handle during production and exhibit good corrosion resistance of the resulting structure. On the other hand, due to the high and fluctuating prices of precious metals, there is a desire to replace silver and palladium with cheaper materials having comparable conductivity.

[0004] For this purpose, aluminum-based resistive heaters have been developed because aluminum is known to be very suitable as a conductor and, due to its self-passivating nature, it is also corrosion-resistant. Aluminum-based resistive heaters are known, for example, from US6531181B1. The heaters of the prior art are produced via thick film technology by printing an aluminum thick film paste. The aluminum thick film paste contains aluminum powder, frit, and a carrier. Those thick film pastes can be applied in any desired layout. Typically, the heater structure is printed via thick film screen printing. Subsequently, the printed thick film structure is fired.

[0005] To ensure proper heat distribution over the entire device, the resistive heater can be applied directly to a ceramic substrate (i.e., alumina) that serves as an insulating layer or to a metal substrate (aluminum or steel) covered with a dielectric insulating layer. The dielectric layer can be produced via thick film coating (e.g., a ceramic thick film paste) and subsequent firing of the ceramic paste to obtain a dielectric surface on which the resistive heater structure can be applied without any short circuits.

[0006] One problem that occurs during the production of aluminum thick film heaters is that when fired at a temperature above the melting point of aluminum, the aluminum and specifically the aluminum thick film paste tend to ball up, which deteriorates the film formation. However, a high firing temperature of the aluminum thick film paste is desired, which is to ensure good adhesion, stable resistance, and also because such high temperature pastes can be processed under the same firing conditions and in the same firing furnace (e.g., a conveyor belt furnace) as the dielectric material located under the resistive heater.

[0007] Another problem with aluminum resistive heaters is that they are more difficult to contact because aluminum cannot be directly welded. The aluminum resistive heater structure of US6531181B1 can be contacted, for example, by clamping.

[0008] The object of the present invention is to overcome at least one of the problems of the prior art.

[0009] Specifically, an object of the present invention is to provide an aluminum resistive heater that is easy and quick to produce and easy to handle.

[0010] Another object of the present invention is to provide a method that provides an aluminum resistance heater having performance comparable to that of a silver-based heater.

[0011] Specifically, another object is to provide an aluminum resistance heater having high thermal stability.

[0012] Specifically, one object is to provide an aluminum resistance heater that does not contain precious metals.

[0013] Another object of the present invention is to reduce the balling of aluminum during the production of an aluminum conduction structure. Less balling can result in fewer defects in the aluminum conduction structure.

[0014] At least one of the above problems pointed out in the prior art is solved by the subject matter of the independent claims.

[0015] In a first aspect, the present invention relates to an aluminum resistance heater comprising:

[0016] a. a thermally conductive substrate comprising an electrically insulating surface,

[0017] b. an aluminum resistance structure disposed on the electrically insulating surface, wherein the aluminum resistance structure comprises:

[0018] i. at least one aluminum conduction layer covering at least a portion of the electrically insulating surface, wherein the aluminum conduction layer comprises aluminum and at least a first glass,

[0019] ii. at least two terminal contact pads contacting the aluminum conduction layer, wherein the terminal contact pads comprise silver,

[0020] c. a cover glaze comprising glass, the cover glaze covering at least a portion of the aluminum resistance structure.

[0021] A resistance heater can be understood as a conductive structure that generates heat when an electric current flows through the structure. In the context of the present invention, the resistance heater is preferably provided as a conduction layer on an electrically insulating substrate.

[0022] The aluminum resistance heater comprises an electrically conductive substrate having an electrically insulating surface. Preferably, the conductive substrate is a metal sheet, metal foil or metal body. The metal can include elemental metals and metal alloys. The elemental metal is preferably selected from at least one of the group consisting of: aluminum, copper, steel. The advantage of the metal substrate is that the metal exhibits good thermal conductivity and thus provides good heat transfer to the object to be heated.

[0023] Alternative thermally conductive substrates include ceramics such as, for example, oxide or nitride ceramics.

[0024] The heat conduction substrate includes an electrically insulating surface. Preferably, the electrically insulating surface comprises or consists of alumina.

[0025] In the case of inherently electrically insulating heat conduction substrate materials (e.g., oxide or nitride ceramic materials), the surfaces of those substrates are also electrically insulating. Thus, optionally, such heat conduction structures do not include any additionally added layers.

[0026] On the other hand, if the heat conduction substrate material contains a metal, the heat conduction substrate preferably includes a dielectric layer exhibiting an electrically insulating surface. The dielectric layer may include materials selected from the group consisting of ceramics, glasses, or mixtures thereof. The ceramics may include materials selected from the group consisting of oxide ceramics, nitride ceramics, and carbide ceramics. The oxide ceramics may include, for example, alumina, silica, and titanium oxide. The nitride ceramics may include, for example, silicon nitride or titanium nitride. The carbide ceramics may include, for example, silicon carbide.

[0027] The dielectric layer is preferably applied by a method selected from thick film coating or thin film coating. Thick film coating includes printing, such as screen printing or knife coating. Thin film coating includes methods such as sputtering, plasma coating, CVD, and PVD. Particularly preferably, the dielectric layer is prepared by screen printing.

[0028] The aluminum resistance heater further includes an aluminum resistance structure disposed on the electrically insulating surface. If sufficient current flows through the aluminum resistance structure, the aluminum resistance structure can radiate heat.

[0029] The aluminum resistance structure includes at least one aluminum conduction layer that covers at least a portion of the electrically insulating surface, wherein the aluminum conduction layer contains aluminum and at least a first glass.

[0030] The aluminum conduction layer may have any layout of a resistance heater that those skilled in the art may consider useful for the desired application. Preferably, the aluminum conductive layer presents a meandering portion. Optionally, the aluminum conduction layer may include a trimming section. The trimming section may include a structure that allows adjustment of the resistance of the aluminum conduction layer.

[0031] Based on the total weight of the aluminum conductive layer, the aluminum conductive layer preferably contains at least 40% by weight, specifically at least 50% by weight, of aluminum. Based on the total weight of the aluminum conductive layer, the aluminum conductive layer preferably contains at most 90% by weight, specifically at most 80% by weight, of aluminum. Optionally, at least one aluminum conductive layer further contains copper. Preferably, at least one aluminum conductive layer contains at least 2% by weight, more preferably at least 3.5% by weight, of copper. At the same time, based on the total weight of at least one aluminum conductive layer, at least one aluminum conductive layer optionally contains at most 10% by weight, preferably at most 8% by weight, of copper. Preferably, any additional metals contained in the aluminum conductive layer other than aluminum and copper do not exceed 0.5% by weight, specifically exceed 0.1% by weight. Optionally, the aluminum conductive layer does not contain any intentionally added metals other than aluminum or copper.

[0032] At least one aluminum conductive layer preferably contains at least 5% by weight, preferably at least 10% by weight, of glass. Optionally, the aluminum conductive layer includes at most 25% by weight of glass. The weight percentages are calculated based on the total weight of at least one aluminum conductive layer. The glass includes a first glass and optionally a second glass.

[0033] The first glass of the aluminum conductive layer preferably has a glass transition temperature in the range of 400 °C to 500 °C. Glass having a glass transition temperature in this range can reduce the balling of aluminum during the firing of the aluminum-containing thick film paste, even when fired at a temperature higher than 750 °C. In the context of the present invention, the glass transition temperature can be measured according to ASTM E1356-03 (the version in effect on the filing date). The first glass of the aluminum conductive layer preferably has a softening point in the range of 470 °C to 550 °C. In the context of the present invention, the softening point can be measured according to ASTM C338-93 (2019).

[0034] In a preferred embodiment, at least one aluminum conductive layer contains two or more glasses. Specifically, at least one aluminum conductive layer may include two glasses that exhibit different glass transition temperatures, softening temperatures, or melting temperatures. At least one conductive layer preferably contains at least a second glass that exhibits a glass transition temperature in the range of 600 °C to 650 °C.

[0035] At least one or both of the first glass and the second glass may be selected from borosilicate glasses, preferably lead-free borosilicate glasses. Preferably, the first glass and the second glass are selected from borosilicate glasses, such as aluminoborosilicate glasses.

[0036] The first glass, specifically borosilicate glass, preferably contains at least one component selected from the group consisting of vanadium oxide, lithium oxide, potassium oxide, sodium oxide, zinc oxide, calcium oxide, barium oxide, magnesium oxide, titanium oxide, zirconium oxide, and bismuth oxide. Optionally, other components may be added to the glass to adjust the glass properties in a desired manner. In a preferred embodiment, the first glass is a borosilicate glass containing 7 wt% to 15 wt% of alkali metal oxides (such as, for example, lithium oxide, sodium oxide, and potassium oxide). Additionally, the first glass may optionally contain 20 wt% to 35 wt% of bismuth. The weight percentages are calculated based on the total weight of the first glass.

[0037] The second glass, specifically borosilicate glass, preferably contains at least one component selected from the group consisting of zinc oxide, calcium oxide, titanium oxide, zirconium oxide, barium oxide, magnesium oxide, and molybdenum oxide. In one example, the second glass contains 1 wt% to 15 wt% of alumina and 10 wt% to 30 wt% of zinc oxide. The weight percentages are calculated based on the total weight of the second glass.

[0038] Optionally, the aluminum resistance structure includes two or more aluminum conductive layers. In this case, the given amounts of the above components are also preferably applicable to those additional aluminum conductive layers. Preferably, the first conductive layer and any additional conductive layers may exist as multiple adjacent traces on the same surface. Optionally, the first conductive layer and any additional conductive layers are in contact with the terminal contact pads through the same terminal.

[0039] The sheet resistance of the aluminum conductive layer is preferably in the range of 25 mΩ / square to 500 mΩ / square.

[0040] The aluminum resistance structure further includes at least two terminal contact pads that contact the conductive layer. In the context of the present invention, the terminal contact pads preferably mean those contact pads located at opposite ends of the aluminum conductive layer such that current can be injected via one contact pad, flow through at least one aluminum conductive layer, and then be drawn at the second contact pad. Optionally, the aluminum resistance structure may include multiple terminal contact pads.

[0041] The contact pads can improve the contactability of at least one conductive layer. Preferably, the contact pads allow contact with the aluminum-containing conductive layer via soldering or brazing. The terminal contact pads can advantageously be used to facilitate the connection and protect the aluminum conductive layer from mechanical damage during contact. The terminal contact pads preferably comprise silver, specifically sintered silver particles. Optionally, the terminal contact pads comprise at least one additional metal. Specifically, the additional metal can be selected from palladium and platinum. In the case where at least one additional metal is included in the terminal contact pads, the silver and the at least one additional metal can be present as a mixture or as an alloy. Optionally, the terminal contact pads consist of silver, a silver mixture or a silver alloy. In the mixture or alloy, the silver and the at least one additional metal can be present in a weight ratio of 9:1 to 25:1. [DJ1]

[0042] The aluminum resistance heater further comprises a cover glaze, which comprises glass or consists of glass. The cover glaze covers at least a part of the aluminum resistance structure. Preferably, the cover glaze completely covers the aluminum resistance structure, except for at least a part of the terminal contact pads, to ensure the electrical connection of the aluminum resistance heater, for example, via a wire. The cover glaze can protect the aluminum resistance structure from mechanical damage, chemical corrosion and electrical short circuits. The glass of the cover glaze preferably has a glass transition temperature in the range of 400 °C to 500 °C, preferably in the range of 450 °C to 480 °C. Moreover, preferably, the glass of the cover glaze has a softening point in the range of 500 °C to 570 °C.

[0043] In one embodiment, the cover glaze comprises a glass selected from bismuth borosilicate glass. Optionally, the bismuth borosilicate glass can comprise at least one element selected from the group consisting of zinc, aluminum and titanium or a combination thereof. Preferably, the total concentration of bismuth, boron and silicon contained in the glass as oxides is at least 80 wt%, specifically at least 85%. Optionally, the amount of silica is in the range of 25 wt% to 60 wt%.

[0044] In a second aspect, the present invention relates to a method for preparing an aluminum resistance heater, the method comprising the following steps:

[0045] a) applying at least one layer of a dielectric paste on a thermally conductive substrate,

[0046] b) firing at least one layer of the dielectric paste at a first firing temperature to obtain a thermally conductive substrate comprising an electrically insulating surface,

[0047] c) applying a layer of an aluminum thick film paste on the electrically insulating surface, wherein the thick film paste comprises aluminum, at least a first glass and a carrier,

[0048] d) firing the layer of the aluminum thick film paste at a second firing temperature to obtain an aluminum conductive layer,

[0049] e) Apply a silver-containing thick film conductor paste onto the aluminum conductive layer to obtain at least two terminal contact pads.

[0050] f) Fire the terminal contact pads at a third firing temperature to obtain an aluminum resistance structure.

[0051] g) Apply a cover glaze paste over at least the aluminum resistance structure, and

[0052] h) Fire the cover glaze paste at a fourth firing temperature to obtain an aluminum resistance heater.

[0053] Preferably, the method of the present invention can be used to prepare an aluminum resistance heater according to the first aspect of the present invention.

[0054] In step a), at least one layer of dielectric paste is applied onto a conductive substrate. The conductive substrate is thermally conductive and optionally can be electrically conductive, such as a metal foil or sheet. Preferably, the conductive substrate is a metal sheet.

[0055] Preferably, the dielectric paste contains a ceramic or glass as the dielectric. The dielectric is preferably provided as a powder, which consists of particles having a particle size distribution. The dielectric paste preferably contains at least one ceramic powder, such as alumina powder, glass powder, or a mixture thereof.

[0056] The dielectric paste further contains at least one carrier. The carrier can be an organic or inorganic carrier. Specifically, the carrier includes at least one solvent. Preferably, the carrier is an organic carrier. The solvent can be selected from at least one of the group consisting of, for example, terpene alcohols (such as α-terpineol), terpenes, diols and diglycols, glycol ethers, and glycol esters. Optionally, the dielectric paste can contain additional components, such as, for example, rheology modifiers, stabilizers, dispersants, surfactants, etc. The dielectric paste is preferably applied by printing, specifically by screen printing or doctor blading. After application, the paste can be optionally dried.

[0057] Preferably, the dielectric paste completely covers the surface of the conductive substrate on which at least one aluminum resistance structure should be applied. Alternatively, the dielectric layer can be applied only on the area of the conductive substrate to prevent direct electrical contact between the conductive substrate and the aluminum resistance structure.

[0058] In a preferred embodiment, more than one layer of dielectric paste is applied to the surface of the conductive substrate, specifically three or more layers of dielectric paste are applied. In the case of applying more than one layer of dielectric paste, optionally, at least one layer of the dielectric paste is dried after being applied to the electrically conductive substrate. In one embodiment, each layer of the dielectric paste is dried after its application.

[0059] In the next step, at least one layer of the dielectric paste is fired at a first firing temperature to obtain a conductive substrate including an electrically insulating surface. Firing partially or completely removes the carrier and preferably sinters the dielectric particles in the paste. The dielectric layer produced by firing the dielectric paste has an electrically insulating surface.

[0060] The first firing temperature is preferably in the range of 750 °C to 900 °C, preferably in the range of 800 °C to 900 °C. In the context of the present invention, firing is preferably done in a furnace (such as, for example, a conveyor belt furnace or a batch oven). The temperatures given in the present disclosure are preferably measured at the substrate.

[0061] Preferably, each layer of the dielectric paste is dried and then fired after application. Alternatively, drying is carried out after each application step and firing is carried out only after the last drying step.

[0062] Preferably, the layer thickness of the individual dielectric layers obtained after firing is in the range of 15 μm to 35 μm. In a preferred embodiment, the insulation resistance of the dielectric layer is at least 1 Gohm at 100 V, as measured with an Omnia II Series Electric Analyzer.

[0063] In the next step, a layer of an aluminum thick film paste is applied on the electrically insulating surface, wherein the aluminum thick film paste contains aluminum, at least a first glass, and a carrier. Preferably, the thick film paste is applied by screen printing, doctor blading, or stencil printing.

[0064] Preferably, aluminum and at least one first glass are included as powders in the thick film paste. Preferably, the aluminum powder has an average particle size d in the range of 1 μm to 5 μm 50 . The glass powder preferably has an average particle size d in the range of 2 μm to 5 μm 50 .

[0065] The carrier is preferably included in the aluminum thick film paste in an amount of 15 wt% to 30 wt%.

[0066] Based on the total weight of the aluminum thick film paste, the aluminum thick film paste preferably contains at least 40 wt%, specifically at least 50 wt% of aluminum. Additionally, the aluminum thick film paste may contain up to 80 wt%, preferably up to 70 wt% of aluminum.

[0067] Optionally, the aluminum thick film paste further contains copper. The copper in the paste can reduce the tendency of aluminum to ball up during firing. The copper in the thick film paste can exist as particles, separately from aluminum, as part of the aluminum particles (e.g., as a coating), or in the form of aluminum-copper alloy particles. Preferably, the aluminum thick film paste contains at least 1 wt%, more preferably at least 2.5 wt% of copper. At the same time, based on the total weight of the aluminum thick film paste, the aluminum thick film paste optionally contains at most 7 wt%, preferably at most 5 wt% of copper. Preferably, any metal other than aluminum and copper contained in the aluminum thick film paste does not exceed 0.5 wt%, specifically more than 0.1 wt%. Optionally, the aluminum thick film paste does not contain any intentionally added metal other than aluminum or copper.

[0068] The aluminum thick film paste preferably contains at least 5 wt%, preferably at least 10 wt% of glass. Optionally, the aluminum thick film paste contains at most 35 wt% of glass. The weight percentages are calculated based on the total weight of at least one aluminum thick film paste.

[0069] The first glass of the aluminum thick film paste preferably has a glass transition temperature in the range of 400 °C to 500 °C. The glass with a glass transition temperature in this range can reduce the balling of aluminum during the firing of the aluminum-containing thick film paste, even when fired at a temperature above 750 °C. In the context of the present invention, the glass transition temperature can be measured according to ASTM E1356-03 (the version in effect on the filing date). The first glass of the aluminum thick film paste preferably has a softening point in the range of 470 °C to 550 °C. In the context of the present invention, the softening point can be measured according to ASTM C338-93 (2019).

[0070] In a preferred embodiment, at least one aluminum thick film paste contains two or more glasses. Specifically, at least one aluminum thick film paste can include two glasses that exhibit different glass transition temperatures, softening temperatures, or melting temperatures. The aluminum thick film paste preferably contains at least a second glass that exhibits a glass transition temperature in the range of 600 °C to 650 °C. This can be used to adjust the electrical properties of the aluminum conductive layer.

[0071] At least one or both of the first glass and the second glass can be selected from borosilicate glasses, preferably lead-free borosilicate glasses. Preferably, the first glass and any other glass are selected from borosilicate glasses.

[0072] The first glass, specifically borosilicate glass, preferably contains at least one component selected from the group consisting of vanadium oxide, lithium oxide, potassium oxide, sodium oxide, zinc oxide, calcium oxide, barium oxide, magnesium oxide, titanium oxide, zirconium oxide, and bismuth oxide. Optionally, other components may be added to the glass to adjust the glass properties in a desired manner. In a preferred embodiment, the first glass is a borosilicate glass containing 7 wt% to 15 wt% of alkali metal oxides (such as, for example, lithium oxide, sodium oxide, and potassium oxide). Additionally, the first glass may optionally contain 20 wt% to 35 wt% of bismuth.

[0073] The second glass, specifically borosilicate glass, preferably contains at least one component selected from the group consisting of zinc oxide, calcium oxide, aluminum oxide, titanium oxide, zirconium oxide, barium oxide, magnesium oxide, and molybdenum oxide. In one example, the second glass contains 1 wt% to 15 wt% of aluminum oxide and optionally 10 wt% to 30 wt% of zinc oxide.

[0074] In a subsequent step, the layer of the aluminum thick film paste is fired at a second firing temperature to obtain an aluminum conductive layer. The second firing temperature is preferably in the range of 750 °C to 900 °C, preferably in the range of 800 °C to 900 °C. Preferably, the carrier is an organic carrier, which can be removed by firing at the second temperature. The advantage of firing the aluminum thick film paste at a temperature at least 150 °C higher than the melting temperature of aluminum is that better adhesion to the underlying substrate can be achieved.

[0075] In a preferred embodiment, the first firing temperature and the second firing temperature differ by at most 50 °C, preferably at most 20 °C. In a particularly preferred embodiment, the first firing temperature and the second firing temperature are the same. The advantage of doing so is that the first firing step and the second firing step can be completed with the same furnace settings. If the temperature does not change between the first firing and the second firing, the furnace does not need to be equilibrated at the new temperature, which reduces the overall processing time.

[0076] When using standard four-probe measurement, the aluminum conductive layer after firing preferably has a sheet resistance in the range of 25 mΩ / square to 500 mΩ / square.

[0077] The layer thickness of the aluminum conductive layer after firing may optionally be in the range of 20 μm to 50 μm, preferably in the range of 30 μm to 40 μm.

[0078] The aluminum conductive layer after firing preferably has a roughness (R) in the range of 0.5 μm to 3 μm, preferably in the range of 1.0 μm to 1.5 μm. a), such as measured in accordance with ISO 25178-2:2021 in combination with ISO 25178-602:2010 using an optical profiler Cyberscan Vantage 2 from Cybertechnologies and software Scan Suite.

[0079] In the next step, a thick film conductor paste is applied onto the aluminum conductive layer to obtain at least two terminal contact pads. The application of the terminal contact pads is preferably accomplished by screen printing the thick film conductor paste. The thick film conductor paste preferably contains a metal and an organic vehicle. Preferably, based on the weight of the paste, the amount of the metal is at least 70 wt%. The metal may for example comprise silver or a silver alloy or consist of silver or a silver alloy. In a particularly preferred embodiment, the thick film conductor paste is a silver thick film paste, optionally a glass-free silver thick film paste. The thick film conductor paste is preferably a silver sintering paste. Preferably, the terminal contact pads at least partially or completely overlap with the aluminum conductive layer.

[0080] In the next step, the terminal contact pads are fired at a third firing temperature to obtain an aluminum resistance structure. Preferably, the third firing temperature is in the range of 500 °C to 600 °C. In this temperature range, the particles of the thick film conductor paste for the contact pads are preferably sintered together. Sintering generally involves a surface diffusion mechanism. During the whole process, in order not to deteriorate the aluminum conductive layer, the firing temperature should be lower than the melting temperature of aluminum. The terminal contact pads prepared from a silver sintering paste when sintered below the aluminum melting temperature advantageously have very good adhesion to the aluminum conductive layer and at the same time provide excellent conductivity and mechanical protection.

[0081] The aluminum resistance structure preferably has a resistance temperature coefficient in the range of 2000 ppm / K to 3000 ppm / K.

[0082] After the firing step at the third temperature, a cover glaze paste is applied at least over the aluminum resistance structure. The cover glaze paste contains glass powder and a vehicle. Preferably, the vehicle is an organic vehicle. Optionally, the cover glaze paste may contain additional components. For example, the cover glaze paste may contain components that promote printing or improve the storage time.

[0083] Preferably, the cover glaze is specifically printed by screen printing or doctor blading. Optionally, the cover glaze covers the entire aluminum conductive layer and the underlying electrically insulating surface. Particularly preferably, the terminal contact pads are not at least partially covered by the cover glaze paste.

[0084] The cover glaze paste is fired at a fourth firing temperature to obtain an aluminum resistance heater. Preferably, the fourth firing temperature is in the range of 500 °C to 600 °C.

[0085] In a preferred embodiment, the third firing temperature and the fourth firing temperature differ by at most 50 °C, preferably at most 20 °C. This has the advantage that the third firing step and the fourth firing step can be completed with the same furnace settings. If the temperature does not change between the third firing step and the fourth firing step, the furnace does not need to be equilibrated at the new temperature, which reduces the overall processing time.

[0086] The layer thickness of the covering glaze after firing is preferably in the range of 5 μm to 20 μm.

[0087] The aluminum resistance heater obtained by the process according to the invention preferably exhibits the same or similar properties as a silver-based resistance heater. Examples

[0088] A 1 cm × 1 cm stainless steel foil was provided as the conductive substrate. Subsequently, three layers of an alumina thick film paste (SD 1010A, obtained from Heraeus GmbH, Germany) were applied to the surface of the conductive substrate by screen printing (280 to 325 mesh stainless steel screen), and each layer was dried for 10 minutes and fired individually at a first temperature of 850 °C to obtain an electrically insulating surface on the electrically conductive substrate. In the next step, an aluminum thick film paste containing glass was printed in a meandering structure on the previously prepared electrically insulating surface. The glass is a borosilicate glass which contains 25 wt% bismuth oxide and a total of 13 wt% lithium oxide, sodium oxide and potassium oxide, and the glass has a glass transition temperature of about 480 °C and a softening temperature of about 560 °C. The aluminum powder has an average particle size d of about 1 μm 50 , and the glass has a particle size d of about 2 μm 50 . The printed aluminum thick film paste was dried for about 10 minutes and then fired at a second temperature of 850 °C for about 10 minutes to form an aluminum conductive structure. The film thickness after firing was about 40 μm.

[0089] Subsequently, two terminal contact pads are screen-printed at each terminal end of the meandering structure of the aluminum conduction structure using a lead-free silver conductor paste (available as C8829D from Heraeus Deutschland GmbH & Co. KG, Germany). The contact pads are fired at a third temperature of 550 °C to a fired film thickness of 15 μm to obtain an aluminum resistance structure. Finally, a cover glaze free of Pb, Cd, and Ni (IP9038A from Heraeus Deutschland GmbH & Co. KG) is screen-printed on the surface of the substrate, except for the openings at the positions of the terminal contact pads. The cover glaze is dried for 10 minutes and fired at a fourth temperature of 550 °C for 10 minutes to obtain a fired film thickness of 15 μm. The firing is completed in a conveyor belt furnace. Due to the fact that the first firing temperature and the second firing temperature are the same and the third firing temperature and the fourth firing temperature are the same, the process is very rapid because only one temperature change has to be made in the furnace.

[0090] List of Reference Numerals in the Drawings

[0091] 110 Thermal conduction substrate

[0092] 120 Dielectric layer

[0093] 130 Cover glaze

[0094] 140 Terminal contact pad

[0095] 150 Aluminum conduction layer

[0096] 210 Thermal conduction substrate

[0097] 220 Dielectric layer

[0098] 230 Cover glaze

[0099] 240 Terminal contact pad

[0100] 250 Aluminum conduction layer Description of the drawings

[0101] Hereinafter, the present invention will be illustrated by drawings showing preferred embodiments.

[0102] Figure 1 A cross-section of an aluminum resistance heater according to the present invention is shown.

[0103] Figure 2 Shows the same as Figure 1 The top view of the aluminum resistance heater depicted.

[0104] Figure 1An aluminum resistive heater is shown, which includes a thermally conductive substrate (110). The thermally conductive substrate is preferably a metal substrate, but alternatively, the material may include or consist of ceramics. On the surface of the thermally conductive substrate (110), a dielectric layer (120) is disposed. The dielectric layer (120) has an electrically insulating surface. On the electrically insulating surface of the substrate created by the dielectric layer (120), an aluminum conductive layer (150) is disposed. Preferably, the aluminum conductive layer (150) has a meandering section. At the terminal ends of the aluminum conductive layer (150), terminal contact pads (140) are disposed. The aluminum conductive layer (150) and the terminal contact pads (140) form an electrical contact. The aluminum conductive layer (150) and the electrically insulating surface of the thermally conductive substrate (110 / 120) are at least partially covered by an overglaze (130). The overglaze preferably contains or consists of glass.

[0105] Figure 2 A top view of the same aluminum resistive heater as depicted in Figure 1 is shown. The thermally conductive substrate (210) includes a dielectric layer (220). Since the dielectric covers the thermally conductive substrate, the reference numerals (210 / 220) are used together in this top perspective view. On the electrically insulating surface, an aluminum conductive layer (250) is formed. The aluminum conductive layer may have a meandering structure. At the terminal ends of the aluminum resistive layer (250), terminal contact pads (240) are disposed. The electrically insulating surface of the thermally conductive substrate (210) and the aluminum conductive layer are covered by an overglaze (230).

Claims

1. An aluminum resistance heater, comprising: a. a thermally conductive substrate, the thermally conductive substrate comprising an electrically insulating surface, b. an aluminum resistor structure, the aluminum resistor structure being disposed on the electrically insulating surface, wherein the aluminum resistor structure comprises: i. at least one aluminum conductive layer covering at least a portion of the electrically insulating surface, wherein the aluminum conductive layer comprises aluminum and at least a first glass, ii. at least two terminal contact pads, the at least two terminal contact pads contacting the aluminum conductive layer, wherein the terminal contact pads comprise silver, c. A covering glaze, the covering glaze comprising glass, the covering glaze covering at least a portion of the aluminum resistor structure.

2. The aluminum resistance heater of claim 1, wherein the at least one aluminum conductive layer further comprises copper.

3. The aluminum resistance heater of claim 1, wherein the terminal contact pads comprise sintered metal particles.

4. The aluminum resistance heater of claim 1, wherein the conductive substrate comprises at least one metal selected from the group consisting of: steel, copper, aluminum, and stainless steel.

5. The aluminum resistance heater of claim 1 wherein the electrically insulating surface comprises aluminum oxide.

6. A method for preparing an aluminum resistance heater, comprising the following steps: a) applying at least one layer of a dielectric slurry on a thermally conductive substrate, b) firing the at least one layer of dielectric slurry at a first firing temperature to obtain a thermally conductive substrate comprising an electrically insulating surface, c) applying a layer of an aluminum thick film paste on the electrically insulating surface, wherein the thick film paste comprises aluminum, at least a first glass and a vehicle, d) firing said layer of said aluminum thick film paste at a second firing temperature to obtain an aluminum conductive layer, e) applying a thick film conductor paste comprising silver on said aluminum conductive layer to obtain at least two terminal contact pads, f) firing the terminal contact pad at a third firing temperature to obtain an aluminum resistor structure, g) applying a capping glaze slurry over at least the aluminum resistor structure, and h) firing the covering glaze slurry at a fourth firing temperature to obtain an aluminum resistance heater.

7. The method of claim 6, wherein the first glass of the aluminum thick film paste has a glass transition temperature in the range of 400°C to 500°C.

8. The method of claim 6, wherein the aluminum thick film paste comprises at least a second glass, the second glass exhibiting a glass transition temperature in the range of 600°C to 650°C.

9. The method according to claim 6 or 8, wherein at least one of the first glass and the second glass is selected from borosilicate glass, preferably lead-free borosilicate glass.

10. The method of claim 6, wherein the first glass comprises at least one component selected from the group consisting of vanadium oxide, lithium oxide, potassium oxide, sodium oxide, zinc oxide, calcium oxide, barium oxide, magnesium oxide, titanium oxide, zirconium oxide, and bismuth oxide.

11. The method of claim 8, wherein the second glass comprises at least one component selected from the group consisting of zinc oxide, aluminum oxide, calcium oxide, titanium oxide, zirconium oxide, barium oxide, magnesium oxide, and molybdenum oxide.

12. The method of claim 6, wherein the first firing temperature is in the range of 800°C to 900°C, or the second firing temperature is in the range of 800°C to 900°C, or both the first firing temperature and the second firing temperature are in the range of 800°C to 900°C.

13. The method according to claim 6, wherein the first firing temperature and the second firing temperature differ by at most 50°C, preferably at most 20°C.

14. The method according to claim 6, wherein the third firing temperature is in the range of 500°C to 600°C, or the fourth firing temperature is in the range of 500°C to 600°C, or both the third firing temperature and the fourth firing temperature are in the range of 500°C to 600°C.

15. The method according to claim 6, wherein the third firing temperature and the fourth firing temperature differ by at most 50°C, preferably at most 20°C.

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