Improved aluminum-based resistive heater.

Abstract

The present invention relates to an aluminum resistive heater comprising a thermally conductive substrate having an electrically insulating surface. An aluminum resistive structure is disposed on the electrically insulating surface, the aluminum resistive structure comprising at least one aluminum conductive layer covering at least a portion of the electrically insulating surface, the aluminum conductive layer comprising aluminum and at least a first glass. The aluminum resistive structure further comprises at least two terminal contact pads contacting the aluminum conductive layer, the terminal contact pads comprising silver. An overglaze comprising glass covers at least a portion of the aluminum resistive structure. The present invention also relates to a method of manufacturing an aluminum resistive heater.

Description

[Technical Field]

[0001] The present invention is directed to an improved aluminum-based resistive heater that is easy to manufacture, and a method for manufacturing such an aluminum-based resistive heater.

[0002] Introduction Typically, thick film heaters are based on silver, ruthenium, palladium, or some combination of these conductive materials. Advantages of these heaters are that these materials are easy to handle during manufacturing and the resulting structures exhibit good corrosion resistance. However, due to the high and fluctuating prices of precious metals, it is desirable to replace silver and palladium with cheaper materials that have comparable electrical conductivity.

[0003] For this purpose, aluminum-based resistive heaters have been developed, since aluminum is known to be a very suitable conductor and is also corrosion-resistant due to its self-passivating properties. Aluminum-based resistive heaters are known, for example, from U.S. Pat. No. 6,531,181 (B1). Prior art heaters are manufactured via thick-film technology by printing aluminum thick-film pastes. The aluminum thick-film pastes contain aluminum powder, glass frit, and a vehicle. These thick-film pastes can be applied in any desired layout. Typically, heater structures are printed by thick-film screen printing. The printed thick-film structures are subsequently fired.

[0004] To ensure proper heat distribution throughout the device, the resistive heater may be applied directly to a ceramic substrate (i.e., alumina) that acts as an insulating layer, or to a metal substrate (aluminum or steel) that is covered with a dielectric insulating layer. The dielectric layer may be fabricated via a thick film coating, such as a ceramic thick film paste, which is then fired to obtain a dielectric surface onto which the resistive heater structure can be applied without any short circuits.

[0005] One problem encountered during the manufacture of aluminum thick film heaters is that aluminum, particularly aluminum thick film paste, tends to ball when fired at temperatures above the melting point of aluminum, which reduces the quality of the film formation. Nevertheless, high firing temperatures for aluminum thick film pastes are desirable to ensure good adhesion and stable resistance, and such high-temperature pastes can be processed under the same firing conditions and in the same firing oven, e.g., a conveyor belt oven, as the dielectric material placed below the resistance heater.

[0006] Another problem with aluminum resistive heaters is that they are more difficult to contact because aluminum cannot be directly soldered. The aluminum resistive heater structure of U.S. Patent No. 6,531,181 B1 can be contacted, for example, by clamping.

[0007] It was an object of the present invention to overcome at least one of the problems of the prior art.

[0008] In particular, it was an object to provide an aluminum resistance heater that is easy and quick to manufacture and easy to handle.

[0009] Another object of the present invention was to provide a method for providing an aluminum resistive heater with performance comparable to silver-based heaters.

[0010] In particular, it was another object to provide an aluminum resistance heater having high thermal stability.

[0011] In particular, it was an object to provide an aluminum resistance heater that is free of precious metals.

[0012] Another object of the present invention was to reduce spheroidization of aluminum during the manufacture of aluminum conductive structures. Less spheroidization can result in fewer defects in the aluminum conductive structures.

[0013] At least one of the above problems presented in the prior art is solved by the subject matter of the independent claims.

[0014] In a first aspect, the present invention provides a method for producing a medicament for the treatment of a pulmonary arthritis, comprising: a. a thermally conductive substrate including an electrically insulating surface; b. An aluminum resistor structure disposed on an electrically insulating surface, i. at least one aluminum conductive layer covering at least a portion of the electrically insulating surface, the aluminum conductive layer comprising aluminum and at least a first glass; ii. an aluminum resistor structure comprising at least two terminal contact pads contacting the aluminum conductive layer, the contact pads comprising silver; and c. an overglaze comprising glass covering at least a portion of the aluminum resistive structure.

[0015] A resistive heater may be understood as an electrically conductive structure that generates heat when an electric current is passed through the structure. In the context of the present invention, the resistive heater is preferably provided as a conductive layer on an electrically insulating substrate.

[0016] The aluminum resistance heater includes a conductive substrate including an electrically insulating surface. Preferably, the conductive substrate is a metal sheet, metal foil, or metal body. The metal may include elemental metals and metal alloys. The elemental metal is preferably selected from the group consisting of aluminum, copper, and steel. An advantage of a metal substrate is that metals exhibit good thermal conductivity, thus providing good heat transfer to the object being heated.

[0017] Alternative thermally conductive substrates include ceramics, such as, for example, oxide ceramics or nitride ceramics.

[0018] The thermally conductive substrate includes an electrically insulating surface. Preferably, the electrically insulating surface comprises or consists of aluminum oxide.

[0019] In the case of thermally conductive substrate materials that are inherently electrically insulating, such as oxide or nitride ceramic materials, the surfaces of these substrates are also electrically insulating, and therefore it is optional for such thermally conductive structures to not include additionally added layers.

[0020] On the other hand, when the thermally conductive substrate material includes a metal, the thermally conductive substrate preferably includes a dielectric layer that presents an electrically insulating surface. The dielectric layer may include a material selected from the group consisting of ceramic, glass, or a mixture thereof. The ceramic may include a material selected from the group consisting of oxide ceramic, nitride ceramic, and carbide ceramic. The oxide ceramic may include, for example, aluminum oxide, silicon oxide, or titanium oxide. The nitride ceramic may include, for example, silicon nitride or titanium nitride. The carbide ceramic may include, for example, silicon carbide.

[0021] 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 blade coating. Thin film coating includes methods such as sputtering, plasma coating, CVD and PVD. It is particularly preferred that the dielectric layer is produced by screen printing.

[0022] The aluminum resistive heater further comprises an aluminum resistive structure disposed on the electrically insulating surface, the aluminum resistive structure being capable of radiating heat when a sufficient current flows through it.

[0023] The aluminum resistor structure includes at least one aluminum conductive layer covering at least a portion of the electrically insulating surface, the aluminum conductive layer including aluminum and at least a first glass.

[0024] The aluminum conductive layer may have any layout that one skilled in the art of resistive heaters may deem useful for a desired application. Preferably, the aluminum conductive layer exhibits a serpentine portion. Optionally, the aluminum conductive layer may have a trimming section. The trimming section may include a structure that allows for adjusting the resistance of the aluminum conductive layer.

[0025] The aluminum conductive layer preferably contains at least 40 wt. %, particularly at least 50 wt. %, of aluminum, based on the total weight of the aluminum conductive layer. The aluminum conductive layer preferably contains up to 90 wt. %, particularly up to 80 wt. %, of aluminum, based on the total weight of the aluminum conductive layer. Optionally, at least one aluminum conductive layer further contains copper. Preferably, at least one aluminum conductive layer contains at least 2 wt. %, more preferably at least 3.5 wt. %, of copper. At the same time, at least one aluminum conductive layer optionally contains up to 10 wt. %, preferably up to 8 wt. %, of copper, based on the total weight of the at least one aluminum conductive layer. Preferably, the aluminum conductive layer does not contain any additional metals other than aluminum and copper in an amount greater than 0.5 wt. %, particularly greater than 0.1 wt. Optionally, the aluminum conductive layer does not contain any intentionally added metals other than aluminum or copper.

[0026] The at least one aluminum conductive layer preferably comprises at least 5% by weight, preferably at least 10% by weight, of glass. Optionally, the aluminum conductive layer comprises up to 25% by weight of glass. The weight percentages are calculated based on the total weight of the at least one aluminum conductive layer. The glass comprises a first glass and optionally a second glass.

[0027] The first glass of the aluminum conductive layer preferably has a glass transition temperature in the range of 400°C to 500°C. Glasses with glass transition temperatures in this range can reduce aluminum spheroidization during firing of aluminum-containing thick film pastes, even when fired at temperatures above 750°C. The glass transition temperature in the context of the present invention can be measured in accordance with 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. The softening point in the context of the present invention can be measured in accordance with ASTM C338-93(2019).

[0028] In a preferred embodiment, the at least one aluminum conductive layer comprises two or more glasses. In particular, the at least one aluminum conductive layer may comprise two glasses exhibiting different glass transition temperatures, softening temperatures, or melting temperatures. The at least one conductive layer preferably comprises at least a second glass exhibiting a glass transition temperature in the range of 600°C to 650°C.

[0029] At least one or both of the first and second glasses may be selected from the group of borosilicate glasses, preferably lead-free borosilicate glasses. Preferably, the first and second glasses are selected from the group of borosilicate glasses, such as aluminoborosilicate glasses.

[0030] The first glass, particularly 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 to 15% by weight of an alkali metal oxide, such as lithium oxide, sodium oxide, or potassium oxide. Furthermore, the first glass may optionally contain 20 to 35% by weight of bismuth. The weight percentages are calculated from the total weight of the first glass.

[0031] The second glass, particularly 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% to 15% by weight of alumina and 10% to 30% by weight of zinc oxide. The weight percentages are calculated from the total weight of the second glass.

[0032] Optionally, the aluminum resistor structure includes two or more aluminum conductive layers. In this case, the amounts given for the above components preferably also apply to these further aluminum conductive layers. Preferably, the first and any further conductive layers may be present as multiple traces adjacent to each other on the same surface. Optionally, the first and any further aluminum conductive layers are contacted by the same terminal contact pad.

[0033] The sheet resistance of the aluminum conductive layer is preferably in the range of 25 milliohms / square to 500 milliohms / square.

[0034] The aluminum resistor structure further comprises at least two terminal contact pads in contact with the conductive layer. Terminal contact pads in the context of the present invention preferably mean that the contact pads are located at opposite ends of the aluminum conductive layer so that current can be injected through one contact pad, flow through at least one aluminum conductive layer, and subsequently extracted at a second contact pad. Optionally, the aluminum resistor structure may comprise multiple terminal contact pads.

[0035] The contact pads can improve the contactability of at least one conductive layer. Preferably, the contact pads allow contact to the aluminum-containing conductive layer via soldering or brazing. The terminal contact pads can be advantageously used to facilitate connection and protect the aluminum conductive layer from mechanical damage during contact. The terminal contact pads preferably comprise silver, particularly sintered silver particles. Optionally, the terminal contact pads comprise at least one additional metal. In particular, the additional metal may be selected from palladium and platinum. When at least one additional metal is included in the terminal contact pad, the silver and the at least one additional metal can be present as a mixture or 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 may be present in a weight ratio of 9:1 to 25:1. [DJ1]

[0036] The aluminum resistive heater further includes an overglaze containing or consisting of glass. The overglaze covers at least a portion of the aluminum resistive structure. Preferably, the overglaze completely covers the aluminum resistive structure except for at least a portion of the terminal contact pads, ensuring electrical connection of the aluminum resistive heater, for example, by wire. The overglaze can protect the aluminum resistive structure from mechanical damage, chemical corrosion, and electrical shorts. The glass of the overglaze 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. Additionally, preferred overglaze glasses have a softening point in the range of 500°C to 570°C.

[0037] In one embodiment, the overglaze comprises a glass selected from the group of bismuth borosilicate glasses. Optionally, the bismuth borosilicate glass may contain 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% by weight, particularly at least 85%. Optionally, the amount of silicon oxide is in the range of 25% to 60% by weight.

[0038] In a second aspect, the present invention provides a method of making an aluminum resistance heater, comprising: a) applying at least one layer of a dielectric paste onto a thermally conductive substrate; b) firing at least one layer of the dielectric paste at a first firing temperature to obtain a thermally conductive substrate having an electrically insulating surface; c) applying a layer of aluminum thick film paste onto the electrically insulating surface, the thick film paste comprising aluminum, at least a first glass, and a vehicle; d) firing the layer of aluminum thick film paste at a second firing temperature to obtain an aluminum conductive layer; e) applying a thick film conductor paste containing silver onto the aluminum conductive layer to obtain at least two terminal contact pads; f) firing the terminal contact pads at a third firing temperature to obtain an aluminum resistor structure; g) applying an overglaze paste overlying at least the aluminum resistor structure; h) firing the overglaze paste at a fourth firing temperature to obtain an aluminum resistance heater.

[0039] Preferably, the method of the present invention can be used to make an aluminium resistance heater according to the first aspect of the present invention.

[0040] In step a), at least one layer of the dielectric paste is applied onto a conductive substrate. The conductive substrate is thermally conductive and may optionally be electrically conductive, such as a metal foil or metal sheet. Preferably, the conductive substrate is a metal sheet.

[0041] Preferably, the dielectric paste contains ceramic or glass as a dielectric. The dielectric is preferably provided as a powder consisting of particles having a particle size distribution. The dielectric paste preferably contains at least ceramic powder, such as alumina powder, glass powder, or a mixture thereof.

[0042] The dielectric paste further comprises at least one vehicle. The vehicle may be an organic vehicle or an inorganic vehicle. In particular, the vehicle comprises at least one solvent. Preferably, the vehicle is an organic vehicle. The solvent may be selected from the group consisting of, for example, terpene alcohols, such as alpha terpineol, terpene hydrocarbons, glycols and diglycols, glycol ethers and glycol esters. Optionally, the dielectric paste may comprise additional components, such as rheology modifiers, stabilizers, dispersants, surfactants, etc. The dielectric paste is preferably applied by printing, in particular by screen printing or blade coating. After application, the paste may optionally be dried.

[0043] Preferably, the dielectric paste completely covers the surface of the conductive substrate to which at least one aluminum resistor structure is applied. Alternatively, the dielectric layer may be applied only over areas of the conductive substrate to prevent direct electrical contact between the conductive substrate and the aluminum resistor structure.

[0044] In a preferred embodiment, two or more layers of the dielectric paste are applied to the surface of the conductive substrate, and in particular, three or more layers of the dielectric paste are applied. When two or more layers of the dielectric paste are applied, optionally, at least one layer of the dielectric paste is dried after application to the conductive substrate. In one embodiment, each layer of the dielectric paste is dried after its application.

[0045] In the next step, at least one layer of the dielectric paste is fired at a first firing temperature to obtain a conductive substrate having an electrically insulating surface. Firing partially or completely removes the vehicle and preferably sinters the dielectric particles in the paste. The dielectric layer produced by firing the dielectric paste has an electrically insulating surface.

[0046] 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. Firing in the context of the present invention is preferably carried out in an oven, such as a conveyor belt oven or a batch oven. The temperatures given in this disclosure are preferably measured at the substrate.

[0047] Preferably, each layer of dielectric paste is dried after application and subsequently fired, or alternatively, drying occurs after each application step and firing occurs only after the final drying step.

[0048] Preferably, the thickness of each individual dielectric layer 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 Gigaohm (Gohm) at 100 V as measured with an Omnia II Series Electrical Analyzer.

[0049] In a next step, a layer of aluminum thick film paste is applied onto the electrically insulating surface, the aluminum thick film paste comprising aluminum, at least a first glass, and a vehicle. Preferably, the thick film paste is applied by screen printing, blade coating, or stencil printing.

[0050] Preferably, the aluminum and at least one first glass are contained in the thick film paste as powders. 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 It has.

[0051] The vehicle is preferably contained in the aluminum thick film paste in an amount of 15% to 30% by weight.

[0052] The aluminum thick film paste preferably contains at least 40 wt. % aluminum, in particular at least 50 wt. % aluminum, based on the total weight of the aluminum thick film paste. Furthermore, the aluminum thick film paste may contain up to 80 wt. % aluminum, preferably up to 70 wt. % aluminum.

[0053] Optionally, the aluminum thick film paste further contains copper. Copper in the paste can reduce the tendency of aluminum to spheroidize during firing. The copper in the thick film paste can be present as particles separate from the aluminum, as part of the aluminum particles, for example, 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. % copper. At the same time, the aluminum thick film paste optionally contains up to 7 wt. %, preferably up to 5 wt. % copper, based on the total weight of the aluminum thick film paste. Preferably, the aluminum thick film paste does not contain any metal other than aluminum and copper in an amount greater than 0.5 wt. %, particularly greater than 0.1 wt. Optionally, the aluminum thick film paste does not contain any intentionally added metals other than aluminum or copper.

[0054] The aluminum thick film paste preferably contains at least 5% by weight, preferably at least 10% by weight, of glass. Optionally, the aluminum thick film paste contains up to 35% by weight of glass. The weight percentages are calculated based on at least the total weight of the aluminum thick film paste.

[0055] The first glass of the aluminum thick film paste preferably has a glass transition temperature in the range of 400°C to 500°C. Glasses with glass transition temperatures in this range can reduce spheroidization of aluminum during firing of aluminum-containing thick film pastes, even when fired at temperatures above 750°C. The glass transition temperature in the context of the present invention can be measured in accordance with 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. The softening point in the context of the present invention can be measured in accordance with ASTM C338-93(2019).

[0056] In a preferred embodiment, the at least one aluminum thick film paste comprises two or more glasses. In particular, the at least one aluminum thick film paste may comprise two glasses exhibiting different glass transition temperatures, softening temperatures, or melting temperatures. The aluminum thick film paste preferably comprises at least a second glass exhibiting a glass transition temperature in the range of 600°C to 650°C. This may be useful for adjusting the electrical properties of the aluminum conductive layer.

[0057] At least one or both of the first and second glasses may be selected from the group of borosilicate glasses, preferably lead-free borosilicate glasses. Preferably, the first glass and any further glasses are selected from the group of borosilicate glasses.

[0058] The first glass, particularly a 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% to 15% by weight of an alkali metal oxide, such as lithium oxide, sodium oxide, or potassium oxide. Furthermore, the first glass may optionally contain 20% to 35% by weight of bismuth.

[0059] The second glass, particularly borosilicate glass, preferably comprises 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 comprises 1% to 15% by weight of alumina and optionally 10% to 30% by weight of zinc oxide.

[0060] In a subsequent step, the layer of 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, more preferably in the range of 800°C to 900°C. Preferably, the vehicle is an organic vehicle that can be removed by firing at the second temperature. Firing the aluminum thick film paste at a temperature at least 150°C higher than the melting temperature of aluminum has the advantage that better adhesion to the underlying substrate can be achieved.

[0061] In a preferred embodiment, the first and second baking temperatures differ by at most 50°C, preferably at most 20°C. In a particularly preferred embodiment, the first and second baking temperatures are the same. This has the advantage that the first and second baking steps can be performed with the same oven settings. If the temperature is not changed between the first and second baking, the oven does not need to be equilibrated at the new temperature, which shortens the overall process time.

[0062] The fired aluminum conductive layer preferably has a sheet resistance in the range of 25 milliohms / square to 500 milliohms / square using a standard four-probe measurement.

[0063] The layer thickness of the fired aluminum conductive layer can optionally be in the range of 20 μm to 50 μm, preferably in the range of 30 μm to 40 μm.

[0064] The fired aluminum conductive layer 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, when measured according to ISO 25178-2:2021 in conjunction with ISO 25178-602:2010 using an optical profilometer Cyberscan Vantage 2 with software Scan Suite from Cybertechnologies. a ) is preferred.

[0065] In a 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 carried out by screen-printing the thick-film conductor paste. The thick-film conductor paste preferably contains a metal and an organic vehicle. Preferably, the amount of metal is at least 70 wt. % based on the weight of the paste. The metal may, for example, contain 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 sinter paste. Preferably, the terminal contact pads at least partially or completely overlap the aluminum conductive layer.

[0066] In the next step, the terminal contact pads are fired at a third firing temperature to obtain an aluminum resistor 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 preferably sinter together. Sintering typically involves a surface diffusion mechanism. It should be noted that the firing temperature must be below the melting point of aluminum throughout the entire process to avoid degrading the quality of the aluminum conductive layer. When sintered below the melting point of aluminum, terminal contact pads made from silver sintering paste advantageously have very good adhesion to the aluminum conductive layer, while simultaneously providing excellent electrical conductivity and mechanical protection.

[0067] The aluminum resistor structure preferably has a temperature coefficient of resistance in the range of 2000 ppm / K to 3000 ppm / K.

[0068] After the firing step at the third temperature, an overglaze paste is applied to cover at least the aluminum resistor structure. The overglaze paste includes a glass powder and a vehicle. Preferably, the vehicle is an organic vehicle. Optionally, the overglaze paste may include additional components. For example, the overglaze paste may contain components that facilitate printing or improve storage time.

[0069] Preferably, the overglaze is applied, in particular by screen printing or blade coating. Optionally, the overglaze covers the complete aluminum conductive layer and the underlying electrically insulating surface. It is particularly preferred that the terminal contact pads are at least partially free from the overglaze paste.

[0070] The overglaze 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.

[0071] In a preferred embodiment, the third and fourth bake temperatures differ by at most 50° C., preferably at most 20° C. This has the advantage that the third and fourth bake steps can be performed with the same oven settings. If the temperature is not changed between the third and fourth bake steps, the oven does not need to be equilibrated at the new temperature, which shortens the overall process time.

[0072] The thickness of the overglaze layer after firing is preferably in the range of 5 μm to 20 μm.

[0073] The aluminum resistive heaters obtained by the process according to the present invention preferably exhibit the same or similar performance as silver-based resistive heaters. [Example]

[0074] A 1 cm x 1 cm stainless steel foil was prepared as a conductive substrate. Subsequently, three layers of alumina thick film paste (SD 1010A, manufactured by Heraeus GmbH, Germany) were applied to the surface of the conductive substrate by screen printing (280 mesh to 325 mesh stainless steel screen). Each layer was dried for 10 minutes and individually fired at a first temperature of 850 °C to obtain an electrically insulating surface on the conductive substrate. In the next step, an aluminum thick film paste containing glass was printed in a serpentine structure onto the previously prepared electrically insulating surface. The glass was a borosilicate glass containing 25 wt. % bismuth oxide and a total of 13 wt. % lithium oxide, sodium oxide, and potassium oxide. The glass had a glass transition temperature of approximately 480 °C and a softening temperature of approximately 560 °C. The aluminum powder had an average particle size d of approximately 1 μm. 50 The glass has a grain 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 fired film thickness was about 40 μm.

[0075] Next, two terminal contact pads were screen-printed onto each terminal of the serpentine structure of the aluminum conductive structure using lead-free silver conductor paste (available commercially as C8829D from Heraeus Deutschland GmbH & Co. KG, Germany). The contact pads were fired at a third temperature of 550°C to a fired film thickness of 15 μm, resulting in an aluminum resistor structure. Finally, a Pb-, Cd-, and Ni-free overglaze (IP9038A from Heraeus Deutschland GmbH & Co. KG, Germany) was screen-printed onto the surface of the substrate, except for the openings at the locations of the terminal contact pads. This overglaze was allowed to dry for 10 minutes and then fired at a fourth temperature of 550°C for 10 minutes to obtain a fired film thickness of 15 μm. Firing was performed in a conveyor belt oven. Due to the fact that the first and second firing temperatures were the same, and the third and fourth firing temperatures were the same, the process was very rapid, requiring only one temperature change within the oven. [Explanation of symbols]

[0076] 110 Thermally conductive substrate 120 dielectric layer 130 Overglaze 140 terminal contact pads 150 Aluminum conductive layer 210 Thermally conductive substrate 220 Dielectric Layer 230 Overglaze 240 terminal contact pads 250 Aluminum conductive layer [Brief explanation of the drawings]

[0077] In the following, the invention will be explained by means of figures showing preferred embodiments. [Figure 1] 1 shows a cross section of an aluminum resistance heater according to the present invention. [Figure 2] FIG. 2 shows a top view of the same aluminum resistance heater as shown in FIG. 1.

[0078] FIG. 1 shows an aluminum resistive heater including a thermally conductive substrate (110). The thermally conductive substrate is preferably a metal substrate, but alternatively, the material may include or consist of ceramic. A dielectric layer (120) is disposed on the surface of the thermally conductive substrate (110). The dielectric layer (120) has an electrically insulating surface. An aluminum conductive layer (150) is disposed on the electrically insulating surface of the substrate created by the dielectric layer (120). Preferably, the aluminum conductive layer (150) has a serpentine section. A terminal contact pad (140) is disposed at the end of the aluminum conductive layer (150). The aluminum conductive layer (150) and the terminal contact pad (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.

[0079] FIG. 2 shows a top view of the same aluminum resistive heater shown in FIG. 1. A thermally conductive substrate (210) includes a dielectric layer (220). Because the dielectric covers the thermally conductive substrate, the reference numerals (210 / 220) are used together in this perspective top view. An aluminum conductive layer (250) is formed on the electrically insulating surface. The aluminum conductive layer may have a serpentine structure. Terminal contact pads (240) are located at the ends of the aluminum resistive layer (250). The electrically insulating surface of the thermally conductive substrate (210) and the aluminum conductive layer are covered by an overglaze (230).

Claims

1. a. a thermally conductive substrate including an electrically insulating surface; b. an aluminum resistor structure disposed on said electrically insulating surface, i. at least one aluminum conductive layer covering at least a portion of the electrically insulating surface, the aluminum conductive layer comprising aluminum and at least a first glass; ii. an aluminum resistor structure comprising: at least two terminal contact pads contacting said aluminum conductive layer, said terminal contact pads comprising silver; and c) an overglaze comprising glass covering at least a portion of said aluminum resistive structure.

2. 10. The aluminum resistive heater of claim 1, wherein said at least one aluminum conductive layer further comprises copper.

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

4. 10. 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. 10. The aluminum resistance heater of claim 1, wherein the electrically insulating surface comprises aluminum oxide.

6. 1. A method for making an aluminum resistance heater, comprising: a) applying at least one layer of a dielectric paste onto a thermally conductive substrate; b) firing at least one layer of the dielectric paste at a first firing temperature to obtain a thermally conductive substrate having an electrically insulating surface; c) applying a layer of aluminum thick film paste onto the electrically insulating surface, the thick film paste comprising aluminum, at least a first glass, and a vehicle; d) firing the layer of aluminum thick film paste at a second firing temperature to obtain an aluminum conductive layer; e) applying a thick film conductor paste containing silver onto the aluminum conductive layer to obtain at least two terminal contact pads; f) firing the terminal contact pads at a third firing temperature to obtain an aluminum resistor structure; g) applying an overglaze paste overlying at least the aluminum resistor structure; h) firing the overglaze paste at a fourth firing temperature to obtain an aluminum resistance heater.

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

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

10. 7. 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. 9. 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. 7. 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.

13. 7. The method of claim 6, wherein the first and second baking temperatures differ by at most 50°C, preferably at most 20°C.

14. 7. The method of 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.

15. 7. The method of claim 6, wherein the third and fourth baking temperatures differ by at most 50°C, preferably at most 20°C.

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