Thick-film element and method for producing a thick-film element

Abstract

The invention relates to a thick-film element (1) comprising: - a substrate (110) consisting of a titanium alloy and - an insulating layer (120) which at least partly covers a surface of the substrate (110). The insulating layer (120) is made of a glass paste which is applied to the surface of the substrate (110) and is baked at a temperature ranging from 600°C to 700°C, said glass paste in the original state having a glass frit and an organic carrier, wherein the glass frit contains a Bi2O3 - B2O3 - BaO - ZnO - SrO - CaO - Al2O3 glass system. The invention also relates to a method for producing such a thick-film element (1).

Description

[0001] Thick-film element and method for producing a thick-film element

[0002] The invention relates to a thick-film element. Furthermore, the invention relates to a method for producing such a thick-film element.

[0003] Printed heating structures on stainless steel (e.g., type "316L") are widely used as heating elements in both industrial applications such as plastic injection molding machines and everyday consumer goods such as kettles or coffee machines. The cost-effective production of these heating elements is typically achieved using thick-film techniques such as screen printing.

[0004] To insulate the current-carrying heating structure from the conductive stainless steel, an insulating glass layer is printed onto the stainless steel component. Glass and screen printing pastes made from it with adapted thermal expansion coefficients, glass transition, crystallization, and firing temperatures are commercially available and represent the current state of the art.

[0005] However, stainless steel as a substrate for printed heating structures has several disadvantages in applications that differ from those mentioned above. These include temperatures above 230°C, applications in aggressive or easily contaminated media (such as acids or blood), and in-vivo applications. These disadvantages can be overcome by using other substrate materials such as ceramics (e.g., aluminum oxide) or metals or alloys.

[0006] However, inherent disadvantages of ceramics, especially low fracture toughness, cannot be accepted in some applications. Therefore, in these cases, a ductile material, i.e., a metal or alloy, must be used. Among these alloys, an alloy of titanium, aluminum, and vanadium (Ti-6AI-4V, Ti64, TitanGradeö, etc.) occupies a prominent position. It is characterized by low thermal conductivity, high chemical resistance, biological compatibility, high elasticity, and low density.

[0007] No insulating glass paste is commercially available for such alloys, which means that they cannot currently be processed using cost-effective thick-film processes.

[0008] Based on the problems listed, the invention is based on the object of specifying a thick-film element that can be used both at temperatures above 230 °C and in aggressive or easily contamination-prone media. This object is achieved by a thick-film element comprising: a substrate consisting of a titanium alloy, and an insulation layer that at least partially covers a surface of the substrate, wherein the glass layer is formed from a glass paste applied to the surface of the substrate and fired at a temperature between 600 °C and 700 °C, which glass paste, in its original state, comprises a glass frit and an organic carrier, wherein the glass frit contains a Bi2O3-B2O3-BaO-ZnO-SrO-CaO-Al2O3 system.

[0009] The essence of the invention is the discovery that in the Bi2O3 - B2O3 - BaO - ZnO - SrO - CaO - Al2O3 system, it is possible to find a recrystallizing glass that is well compatible with the Ti6AI4V titanium substrate, while its Tg is less than 500°C. The layer formed by thick-film technology and firing at 600-700°C has good insulating properties, a good thermal expansion match with the titanium substrate (bending test), and sufficient stability at the operating temperature of 300°C.

[0010] The substrate can be, for example, a planar substrate or an element of any shape with a suitable surface for applying the glass paste, such as the outside or inside of a tube. The thick-film element according to the invention can therefore be used in a wide variety of applications, for example, as a carrier material for sensors, and in areas of application, such as in-vivo diagnostics or aviation.

[0011] The term “mass fraction” is to be considered equivalent to the term “weight percentage” (wt%).

[0012] An insulation layer is an electrical insulation layer. This layer can also be chemically and / or mechanically insulating or protective.

[0013] According to an advantageous embodiment of the thick-film element according to the invention, the Bi2O3-B2O3-BaO-ZnO-SrO-CaO-Al2O3 system comprises Bi2O3 with a mass fraction of 30 to 50%, B2O3 with a mass fraction of 15 to 25%, BaO with a mass fraction of 15 to 25%, and ZnO with a mass fraction of 5 to 10%. The mass fractions of the other components of the system are selected accordingly.

[0014] A system consisting of Bi2O3 with a mass fraction of 40%, B2O3 with a mass fraction of 23%, BaO with a mass fraction of 20%, ZnO with a mass fraction of 6%, SrO with a mass fraction of 6%, CaO with a mass fraction of 4% and AI2O3 with a mass fraction of 1% has proven particularly advantageous.

[0015] According to an advantageous embodiment of the thick-film element according to the invention, it is provided that the titanium alloy is Ti-6AI-4V (6% AI, 4% vanadium), Ti3AI-2.5V or Ti-6AI-7Nb.

[0016] According to an advantageous embodiment of the thick-film element according to the invention, the thick-film element comprises a functional layer, in particular a structured one. The functional layer is formed from a metal-containing paste applied and fired onto at least a portion of the insulation layer. This metal-containing paste contains the glass frit of the glass paste as a component with a mass fraction of approximately 5 to 10%. By incorporating the glass frit into the metal-containing paste, the functional layer is firmly bonded to the insulation layer.

[0017] Alternatively, the metal-containing paste can also be designed without glass frit. In this case, the adhesion of the metal particles is achieved by a capillary effect generated by the glass of the insulation layer.

[0018] According to an advantageous embodiment of the thick-film element according to the invention, the metal-containing paste contains precious metal particles. These advantageously consist of platinum, gold, silver, palladium, or an alloy of two or more of the aforementioned metals.

[0019] According to an advantageous embodiment of the thick-film element according to the invention, it is provided that the functional metal layer is designed and structured in such a way that it can be operated as a sensor element and / or as a heating element.

[0020] For example, the sensor element acts as a temperature sensor. For this purpose, the functional metal layer is designed as a resistance structure and, in particular, has a meander shape. Alternatively, the functional metal layer can also be structured in such a way that the thick-film element can be operated as one of the following sensor elements: a thermal flow sensor, a conductivity sensor, a humidity sensor, or similar.

[0021] Such a heating element was designed for an operating temperature of 300 °C and proved stable in experiments for at least 1000 hours. According to an advantageous embodiment of the thick-film element according to the invention, the thick-film element comprises a passivation layer, wherein the passivation layer is formed from the glass layer, which is applied and fired onto at least a portion of the functional layer. The passivation layer serves to mechanically and chemically protect the functional layer. By incorporating the glass frit into the metal-containing paste, the passivation layer is firmly bonded to the functional layer.

[0022] Furthermore, the object is achieved by a method for producing a thick-film element, which method comprises the following method steps:

[0023] Providing a substrate consisting of a titanium alloy;

[0024] Applying a glass paste to a surface of the substrate, which glass paste comprises a glass frit and an organic carrier, wherein the glass frit contains a bismuth compound with a mass fraction of 40% to 60%, preferably 50%; and

[0025] Firing the glass paste to form an insulating layer.

[0026] The combination of the titanium alloy substrate and the special design of the glass paste results in the advantages listed above with regard to the thick-film element.

[0027] According to an advantageous embodiment, the method according to the invention further comprises:

[0028] Applying a metal paste to at least a portion of the insulation layer; and

[0029] Firing the glass paste to form a functional layer, which metal-containing paste contains the glass frit of the glass paste as a component with a mass fraction of approximately 10%.

[0030] By incorporating the glass frit into the metal-containing paste, the functional layer is firmly bonded to the insulation layer.

[0031] An advantageous embodiment of the method according to the invention provides that the functional metal layer is structured before or after annealing. In particular, the functional metal layer is structured in such a way that it is designed as a resistance structure. Before annealing, the functional metal layer is structured, for example, using a screen printing process. It can also be provided that the metal-containing paste is applied as a flat structure and then parts of the paste are mechanically removed. After annealing, the functional metal layer can be structured, for example, using a laser or an etching process.

[0032] According to an advantageous embodiment, the method according to the invention further comprises:

[0033] Applying the glass paste to at least a portion of the functional metal layer; and

[0034] Firing the glass paste to form a passivation layer.

[0035] By introducing the glass frit into the metal-containing paste, the passivation layer is firmly bonded to the functional layer.

[0036] According to an advantageous embodiment, a thick-film process, in particular a screen printing process, is used for the respective steps of applying the glass paste and / or the metal paste. This allows the thick-film element to be produced cost-effectively in large quantities.

[0037] An advantageous embodiment of the method according to the invention provides that a temperature of less than 700 °C is used for the respective baking steps. Baking takes place in an oxygen-containing atmosphere.

[0038] According to the invention, one or more of the thick-film elements according to the invention are used in a thermal flow sensor, wherein the thick-film element(s) are operated as temperature sensors and / or heating elements. Such a thermal flow sensor serves to determine a flow rate or the flow velocity of a measuring medium or fluid, for example, a gas, gas mixture, or liquid. In their simplest form, thermal flow sensors contain a single thick-film element that is operated alternately as a heating element and as a temperature sensor. Alternatively, thermal flow sensors are constructed with multiple heating elements and one or more temperature sensors.

[0039] Calorimetric thermal flow sensors determine the flow rate of the fluid in a channel by measuring a temperature difference between two temperature sensors located downstream and upstream of a heating element. This takes advantage of the fact that the temperature difference is linear with the flow rate up to a certain point. This method is extensively described in the relevant literature.

[0040] Anemometric thermal flow sensors consist of at least one heating element, which is heated during flow measurement. As the measured medium flows around the heating element, heat is transferred into the measured medium, which changes with the flow velocity. By measuring the electrical parameters of the heating element, the flow velocity of the measured medium can be determined.

[0041] Thick-film elements according to the invention can be used in thermal flow sensors of both measuring principles.

[0042] The invention is explained in more detail with reference to the following figures.

[0043] Fig. 1: an embodiment of a thick-film element according to the invention; and

[0044] Fig. 2: a flow diagram of an embodiment of the method according to the invention.

[0045] Fig. 1 shows schematically the structure of an embodiment of a thick-film element 1 according to the invention as a cross-section through the thick-film element 1. Such a thick-film element 1 can be used, for example, as a temperature sensor or as a heating element.

[0046] The thick-film element 1 consists of a substrate 110, which essentially consists of a titanium alloy. With regard to chemical resistance and thermal conductivity, an alloy consisting of titanium, aluminum, and vanadium has proven advantageous. In particular, the titanium alloy is Ti-6Al-4V, Ti3Al-2.5V, or Ti-6Al-7Nb.

[0047] The substrate 110 is coated with an insulating layer 120, also referred to as an “underglaze,” in particular by means of a thick-film technology process, for example by screen printing. One or more functional layers 130 are applied to the insulating layer 120. This functional layer 130, or functional layers, is electrically conductive and contains platinum, gold, silver, palladium, or an alloy of two or more of the aforementioned metals. The functional layer 130 is designed such that it can be used as a heating element or temperature sensor. The functional layer 130 can also be used as conductor tracks, contact pads, or the like. For this purpose, the functional layer 130 is structured, for example as a meander structure or a similarly suitable shape, so that it has a defined electrical resistance. By applying an electrical current, orWhen an electrical voltage is applied, the functional layer 130 heats up and emits the heat into the environment. By measuring the change in electrical resistance, the temperature of the immediate surroundings of the functional layer 130 can be determined.

[0048] To protect the functional layer 130 or the functional layers against mechanical and chemical influences, a final passivation layer 140 is applied to at least a partial area of ​​this functional layer(s) 130.

[0049] The insulation layer 120 and the passivation layer 130 are made of an identical glass paste. Before firing, this glass paste consists of a glass frit and an organic carrier. The composition of the glass paste has a solid content of approximately 60 to 80 wt.% (mass fraction) of the glass frit. The glass frit comprises Bi2O3-B2O3-BaO-ZnO-SrO-CaO-Al2O3-glass system Bi2O3 with a mass fraction of 30 to 50%, B2O3, and is fully compatible with the substrate 110 due to the same thermal expansion coefficient. In an exemplary embodiment, the glass frit contains (each as a mass fraction) Bi2O3 at 40%, B2O3 at 23%, BaO at 20%, ZnO at 6%, SrO at 6%, CaO at 4%, and Al2O3 at 1%.

[0050] A thick-film technology process is also to be used for the production of the functional layer(s) 130. This requires the use of a metal-containing paste. For better adhesion of the electrically conductive precious metal particles described above to the insulation layer 120, the same glass frit that was already used in the glass paste for the insulation layer 120 is used in the paste of the functional layer(s) 130. The composition of this paste includes the glass frit of the glass paste as a component with a mass fraction of approximately 5 to 10%.

[0051] The manufacturing steps of such a thick-film element 1 are shown in Fig. 2.

[0052] In a first process step a), the glass paste is applied to the substrate 110. Application is performed using a screen printing process or a similar thick-film technology process. In the subsequent process step b), the glass paste is fired onto the substrate 110 at a temperature of less than 700°C in an oxygen-containing atmosphere. This burns out the organic carrier and melts the glass frit, bonding to the substrate 110 as a glass coating and forming the insulating layer 120. The low glass transition temperature (Tg <500°C) of the lead-free glass composition enables the paste to be fired at temperatures below 700°C and in an oxygen-containing atmosphere, with no significant oxidation of the substrate material being observed – this would otherwise be a problem at higher temperatures.

[0053] In a process step c), the paste containing metal particles is applied to the insulation layer 120. The application is again carried out using a screen printing process or a similar thick-film technology process. In the subsequent process step d), the paste containing metal particles is baked onto the insulation layer 120 at a temperature of less than 700°C in an oxygen-containing atmosphere. This burns out the organic carrier of the glass paste containing metal particles and melts the glass frit, which bonds the metal particles to the insulation layer, thus forming the functional layer 130. Process steps c) and d) can be repeated to form additional functional layers. It can also be provided to apply additional insulation layers.

[0054] By using the screen printing process, which uses stencils or masks, the applied functional layer 130 has already been structured. In an optional process step e), the functional layer can be further structured, for example, using a laser process. It can also be provided to swap the chronological sequence of process steps d) and e), so that the "wet" paste containing metal particles is structured before firing.

[0055] In a process step f), the glass paste is applied to the substrate 110 in such a way that at least a portion of the functional layer 130 and optionally additionally at least a portion of the insulation layer 120 is covered by the glass paste. The application is again carried out using the screen printing process or a similar thick-film technology process. In a final process step g), the glass paste is fired at a temperature in the range of 600°C to 700°C in an oxygen-containing atmosphere. This burns out the organic carrier and melts the glass frit, which bonds as a glass coating to the functional layer 130 and the insulation layer 120 and forms the passivation layer 140.

[0056] As a functional test of the paste system, the conductivity of the metal particle-containing paste was experimentally tested. The newly developed glass paste was applied as both the insulation layer 120 and the passivation layer 140. The average sheet resistance was determined to be 0.0088 ohms. The resistance measurements shown in Table 1 indicate good compatibility of the glass system with both the precious metal particles and the material of the substrate 110. This was achieved by applying the novel glass frit, which was used in all parts of the thick-film element, namely the insulation layer 120, the functional layer 130, and the passivation layer 140.

[0057] For the experiments, gold was used as the precious metal particle for the metal particle-containing paste. The firing temperature was 650 °C for all firing steps. Four thick-film elements 1, each with a functional layer 130 (meander-shaped resistance element) and with an applied passivation layer 140, and three thick-film elements 1, each with a functional layer 130 (meander-shaped resistance element) and without such a passivation layer 140, were manufactured. The electrical resistance of the respective functional layer 130, as well as its sheet resistance (equivalent to the sheet resistance of the metal particle-containing paste itself), were subsequently determined. The second row of Table 1 contains the measured values ​​of the four thick-film elements 1 with the applied passivation layer 140; the third row of Table 1 contains the measured values ​​of the four thick-film elements 1 without the passivation layer 140.

[0058] Table 1 :

[0059] List of reference symbols

[0060] 1 thick-film element

[0061] 110 Substrat

[0062] 120 insulation layer

[0063] 130 functional layer

[0064] 140 passivation layer

Claims

Patent claims 1. Thick-film element (1), comprising: a substrate (110) consisting of a titanium alloy, and an insulation layer (120) which at least partially covers a surface of the substrate (110), wherein the insulation layer (120) is formed from a glass paste applied to the surface of the substrate (110) and fired at a temperature between 600 °C and 700 °C, which glass paste in its original state comprises a glass frit and an organic carrier, wherein the glass frit contains a Bi2O3- B2O3- BaO - ZnO - SrO - CaO - Al2O3 glass system.

2. Thick-film element according to claim 1, wherein the Bi2O3-B2O3-BaO-ZnO-SrO-CaO-Al2O3 glass system comprises Bi2O3 with a mass fraction of 30 to 50%, B2O3 with a mass fraction of 15 to 25%, BaO with a mass fraction of 15 to 25% and ZnO with a mass fraction of 5 to 10%.

3. Thick-film element (1) according to claim 1, wherein the titanium alloy is Ti-6AI-4V, Ti3AI-2.5V or Ti-6AI-7Nb.

4. Thick-film element (1) according to claim 1 or 2, comprising a, in particular structured, functional layer (130), wherein the functional layer (130) is formed from a metal-containing paste applied to at least a partial area of ​​the insulation layer (120) and fired, which metal-containing paste contains the glass frit of the glass paste as a component with a mass fraction of approximately 5 to 10%.

5. Thick-film element (1) according to claim 3, wherein the metal-containing paste contains noble metal particles.

6. Thick-film element (1) according to claim 4, wherein the noble metal particles consist of platinum, gold, silver, palladium or an alloy of two or more of the aforementioned metals.

7. Thick-film element (1) according to one of claims 3 to 5, wherein the functional layer (130) is designed and structured such that it can be operated as a sensor element and / or as a heating element.

8. Thick-film element (1) according to claim 6, wherein the functional layer (130) is a resistance structure, in particular of meandering design.

9. Thick-film element (1) according to one of claims 3 to 7, comprising a passivation layer (140), wherein the passivation layer (140) is formed from the glass paste which is applied to at least a partial area of ​​the functional layer (130) and fired.

10. A method for producing a thick-film element (1), comprising: Providing a substrate (110) consisting of a titanium alloy; Applying a glass paste to a surface of the substrate (110), which glass paste comprises a glass frit and an organic carrier, wherein the glass frit contains a Bi2O3-B2O3-BaO-ZnO-SrO-CaO-Al2O3 system; and Firing the glass paste to form an insulating layer (120) at a temperature between 600 °C and 700 °C.

11. The method of claim 9, further comprising: Applying a metal paste to at least a portion of the insulation layer (120); and Firing the glass paste to form a functional layer (130), which metal-containing paste contains the glass frit of the glass paste as a component with a mass fraction of approximately 10%.

12. The method according to claim 10, wherein the functional layer (130) is structured before or after firing.

13. The method according to claim 10 or 11, further comprising: Applying the glass paste to at least a portion of the functional layer (130); and Firing the glass paste to form a passivation layer (140).

14. Method according to one of claims 9 to 12, wherein a thick-film method, in particular a screen printing method, is used for the respective steps of applying the glass paste and / or the metal paste.

15. A method according to any one of claims 9 to 13, wherein a temperature of less than 700°C is used for the respective baking steps.

16. Use of one or more thick-film elements (1) according to one of claims 1 to 8 in a thermal flow sensor, wherein the thick-film element (1) or the thick-film elements are operated as temperature sensors and / or heating elements.

Images