CN115515260A

CN115515260A - Thick film heating element and method of manufacture

Info

Publication number: CN115515260A
Application number: CN202210714843.2A
Authority: CN
Inventors: 亚历克斯·赖尼尔·尼霍夫; 瑞纳杜斯·赫曼努斯·伯纳杜斯·迪嫩; 罗布·马特坎普; 安德鲁·亨特
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 2021-06-22
Filing date: 2022-06-22
Publication date: 2022-12-23
Also published as: GB202209036D0; GB202108938D0; CN217116424U; GB2610037A

Abstract

The thick film heating element comprises a metal substrate (2), an insulating layer (3) and conductive tracks (5) and contact pads (4) formed on the insulating layer using a thick film manufacturing process. The substrate (2) may comprise a plurality of layers (2 a, 2b, 2 c) of different metallic materials, for example a copper layer (2 b) between two layers (2 a, 2 c) made of steel. This may provide a substrate with good thermal conductivity and a thermal expansion coefficient compatible with the material of the insulating layer (3). The layers are bonded together using a rolling process such as cold rolling bonding.

Description

Thick film heating element and method of manufacture

Technical Field

The present invention relates to a thick film heating element and a method of manufacturing the same.

Background

Thick film heating elements typically include one or more heating traces that are screen printed (fired) as an ink or paste onto an insulating substrate and fired (fired) to form high resistivity traces. The connection traces or pads may be printed in separate layers using different types of inks or pastes and fired to form low resistivity connection traces and pads.

The insulating substrate may be an electrically insulating material such as a ceramic, or may be a metal with an insulating surface layer. Thick film heating elements with metal substrates are typically manufactured by applying an electrically insulating layer onto a metal substrate and then forming heater traces onto the surface of the insulating layer. The insulating layer may be a glass or ceramic material applied using screen printing techniques or more conventional glass enamel processes. The metal substrate is most commonly stainless steel. The firing temperature and other characteristics of the insulating material, heater traces and pads must be compatible with the characteristics of the metal.

More details of Thick film technology are described, for example, in White n. (2017) Thick Films on pages 707-709 and 712 in Kasap s., capper p. (eds) spring Handbook of Electronics and photonics Materials. The thick film paste may include an active material, a glass frit (glass frit), and an organic carrier or vehicle. The glass frit remains after firing and forms part of the structure of the thick film resistor. Thus, "thick film" refers to a particular type of resistor having characteristic structures and properties, not merely a comparative term or reference to a product when manufactured by a particular process.

Thick film heating elements have many applications including kettles and cooking devices in which it is desirable to increase the power density of the heating element to reduce the size of the heating element and potentially reduce cost, or to reduce the noise generated by the appliance, or to provide very uniform heat. In some applications, such as cooking machines, it is desirable that the temperature of the heating surface is uniform. Hot spots may cause overheating of the material being heated. One of the features that limit the ability to achieve these goals is the material on which the substrate of many thick film heating elements is fabricated. Stainless steel has a relatively low coefficient of thermal conductivity; for 440 steel, the thermal conductivity is 24.2W/mK. A substrate with low thermal conductivity will not conduct heat in a significant amount in the lateral direction, resulting in high temperatures at the trace locations and low temperatures between the traces. ase:Sub>A solution to this problem is to provide ase:Sub>A diffuser plate attached to ase:Sub>A steel substrate by brazing (brazing), as described in GB-ase:Sub>A-2547148. However, brazing two different metals is not easy and the brazing temperature must be compatible with the firing temperature of the insulating layer and the traces.

Stainless steel is used because stainless steel provides a durable, corrosion resistant surface that can be polished or textured depending on the application. Stainless steel is very suitable for heating elements used in kettles, hot water heaters, cookers, and irons.

The coefficient of thermal expansion of the substrate material is also important. It is desirable that the coefficient of expansion of the substrate be slightly greater than the coefficient of expansion of the insulating material and the heater trace material so that, when the heater cools after firing, the insulating material and trace are subjected to a compressive stress, different from the tensile stress, that the insulating material and trace can withstand. The temperature at which the material will be subjected to tensile stress due to thermal expansion is above the normal operating temperature of the heating element.

Stainless steels, and in particular ferritic or martensitic steels (ferritic steels), have a thickness of about 10X 10 ^-6 Expansion coefficient of/K. The coefficient of expansion of the glass is about 8.5X 10 ^-6 and/K is used. Coefficient of copper 17X 10 ^-6 K, which makes copper incompatible with the heating element material.

Disclosure of Invention

According to one aspect of the present invention there is provided a composite or laminated metal substrate for a thick film heating element, the substrate comprising two or more layers bonded or secured together by a rolling process.

One or more of the layers may provide desired characteristics of the outer surface, and one or more layers may provide increased thermal conduction. This aspect of the invention can provide a substrate having good thermal conductivity and a coefficient of thermal expansion compatible with insulating materials.

Differences in the coefficients of thermal expansion between the various layers making up the heating element (particularly the insulating layer and the laminated substrate) can cause distortion or bowing of the substrate as the element cools after the firing process. The insulating layer will be on the convex side of the component. This effect may be reduced or eliminated by providing the two outer layers of the substrate with unequal thicknesses and/or by providing the outer layers of different materials. The thickness and/or material may be selected such that the deformation or bend is reversed such that the insulating layer is located on the concave side of the heating element.

When the substrate is thin, the need to provide a layer with good thermal conduction increases. For example, it may be desirable to manufacture a flexible thick film heating element. One such method of manufacturing ase:Sub>A thin flexible heating element is described in GB-ase:Sub>A-2576895, in which ase:Sub>A dielectric layer is formed on opposite surfaces of ase:Sub>A substrate to balance the compressive forces on the surfaces of the substrate. The method may also be applied to embodiments of the present invention, particularly those having a thin substrate, for example, having a thickness below 0.5 mm.

Alloys with very low coefficients of expansion may be used in the construction of the metal substrate. These materials include iron/nickel alloys and iron/nickel/cobalt alloys. It should be noted that these alloys may be used as the substrate itself, rather than as part of the composite, and that these alloys may be used to achieve a finished heating element with little or no distortion, especially when the substrate is thin, e.g., below 0.5 mm.

The use of the above alloys in metal substrates for thick film heating elements is considered independently inventive. Thus, according to another aspect of the present invention there is provided a metal substrate for a thick film heating element, the metal substrate comprising an iron/nickel alloy or an iron/nickel/cobalt alloy.

Drawings

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

figure 1 is a perspective view of a thick film heating element in a first embodiment of the invention;

figure 2 is an exploded perspective view of a thick film heating element in a first embodiment;

figure 3 is a cross-section of a thick film heating element in a first embodiment;

figure 4 shows a bonding process for a thick film heating element in a first embodiment;

FIG. 5 is a flow chart of the bonding process; and

figure 6 is a cross section of a thick film heating element in a second embodiment.

Detailed Description

Fig. 1-3 show a thick film heating element 1 comprising a substrate 2 composed of three

layers

2a, 2b, 2c, the substrate 2 having an insulating layer 3 with an enamel (enamel). The

outer layers

2a, 2c may be made of steel such as ferritic stainless steel (ferritic stainless steel). The intermediate layer 2b may be made of copper.

The

outer layers

2a, 2c may have a thickness in the range between about half the thickness of the intermediate layer 2b and equal to the thickness of the intermediate layer 2 b. For some applications the thickness of the intermediate layer 2b may be between 1mm and 2mm, but where flexibility of the thick film heating element 1 is required, the total thickness of the

layers

2a, 2b, 2c may be in the range 0.1mm-0.2mm, for example 0.15mm, with each layer being equal in thickness, for example 0.05mm.

In the method of manufacturing the substrate 2, the

layers

2a, 2b, 2c may be bonded together by any of a variety of methods (e.g., welding, riveting or brazing). Hot or cold rolling methods such as those used to manufacture thermostatic bimetals are well suited for such applications and may be used instead.

The method of bonding the

layers

2a, 2b, 2c together by rolling, e.g. cold roll bonding, is schematically illustrated in fig. 4 and 5. Cold roll bonding is a solid phase cold welding process in which the

layers

2a, 2b, 2c are forced together under sufficient pressure to reduce the overall thickness of the

layers

2a, 2b, 2 c. This severe plastic deformation creates metallurgical bonds (metallurgical bonds) as the atomic lattices of the different metals fuse into a common structure. Bond strength between dissimilar metals (bond Strength) can be increased by heating the material to induce diffusion at the interface. The rolling process may include the following steps.

Cleaning (step S1) -before the bonding is produced, the surface of the

layers

2a, 2b, 2c is cleaned. Preferably, substantially all traces of contaminants (in particular, oil or grease) are removed. Suitable cleaning processes include solvent cleaning, detergent, alkaline cleaning; including electrical cleaning or anode cleaning.

Mechanical abrasion (step S2) -in addition to one or more cleaning processes, mechanical abrasion of the surface (typically by scratch brushing) is used to remove metal oxides.

Heating (step S3) -although the process is described as cold-roll bonding, sometimes one or more of the materials to be bonded are heated above ambient temperature to increase the ductility of the materials. The temperature tends to be lower than the temperature that would cause diffusion (diffusion) of the material at the interface and therefore the process is still classified as a cold rolling process.

Rolling (step S4) -the

layers

2a, 2b, 2c are then put together and run through a rolling mill. The rollers 10 in the rolling mill are arranged such that the gap distance between the rollers is smaller than the total thickness of the

layers

2a, 2b, 2c, so that the

layers

2a, 2b, 2c are plastically deformed. The friction at the surface interface between the

layers

2a, 2b, 2c is sufficient to break any remaining oxide layer and the surface being forced to one place will have sufficient force to weld the

layers

2a, 2b, 2c together.

Heat treatment (step S5) -after rolling, the material properties of the substrate 2 may be enhanced by a heat treatment, such as an annealing process, to relieve the stresses induced during rolling, and also to increase the weld strength by promoting diffusion of the respective materials at the interfaces between the

layers

2a, 2b, 2 c.

Further processing (step S6) -if necessary, the substrate 2 may then be processed by conventional metal processing procedures. For example, the substrate 2 may be cut to a desired shape, straightened by a tension leveling process, and/or pressed or blanked/blanked in a press tool.

Other methods of forming the substrate include electrochemical or electroless plating, sputtering or thin film techniques, flame spraying or screen printing.

After fabrication, the substrate may be used as a component in the fabrication of a thick film heating element. In the manufacture of a thick film heating element, using the thick film printing and firing process described above, an insulating layer 3 is formed on a substrate and contact pads 4 and one or more heating tracks 5 are formed on the insulating layer 3. One or more overglaze layers (not shown) may be formed on the heating traces 5 so that the contact pads 4 are exposed.

In embodiments where the flat substrate 2 is made using two layers of similar steel sandwiching a different metal (e.g., copper), the resulting substrate may remain flat when the temperature changes (e.g., during a thick film firing process). The lateral expansion coefficient of the substrate will depend on the expansion coefficients of the two materials (steel and different metals), the young's modulus of the materials and the relative thickness of the materials. In the examples of stainless steel and copper, the young's modulus of the material is:

copper 121GPa

Steel 444 220Gpa

The total thermal expansion may be proportional to the product of the expansion coefficient, thickness and young's modulus of the respective layers. For example, in a substrate where the copper layer has a thickness equal to the thickness of each of the steel layers, the coefficient of expansion will be:

Ce＝(2×220×10 ^-6 )+(121×17×10 ^-6 )/(2×220+121)

Ce＝11.51×10 ^-6 /K

if the stainless steel is half the thickness of the copper, the coefficients will be:

Ce＝12.50×10 ^-6 /K

12.50×10 ^-6 the coefficient of/K is compatible with the insulation and trace materials typically used for steel substrates.

Fig. 6 shows a second embodiment, which differs from the first embodiment in that: the substrate 2 comprises a sheet having a very low coefficient of thermal expansionLayer metals, such as iron/nickel alloys or iron/nickel/cobalt alloys. For example, alloy 1.3981 (which is an iron/nickel/cobalt alloy sold under the trademark Dilver) has a 7.9X 10 temperature range between 30 ℃ and 600 ℃ ^-6 The average coefficient of expansion of (a). The coefficient of the nickel/iron alloy with 36% nickel is 1-3X 10 ^-6 Although the coefficient increases at around 200 ℃ or higher due to phase transition.

The thick film heating element 1 may be fitted within a domestic appliance such as a milk frother (milk frother), a kettle, a cooking machine or an iron. The power supply is connected to the contact pads 4 and is controlled to control the heat provided by the thick film heating element. The diameter of the base 2 may be between 70mm and 130mm, and may for example be square, rectangular or circular in plan, depending on the type and size of the appliance. For the flexible element 1, the dimensions may be, for example, 5mm x 250mm.

In some appliances, the surface of the substrate 2 opposite the heating trace 5 provides a heating surface for contacting a liquid or other material to be heated; this may be referred to as the wet side of the thick film heating element. The surface of the substrate 2 closest to the heating trace 5 is referred to as the dry side.

Alternative embodiments

In an alternative embodiment, the substrate 2 may comprise only two

layers

2a, 2b, for example provided by a copper layer 2b in case a heated copper surface is required. The insulating layer 3 will then be formed on the single steel layer 2 a.

Alternatively or additionally, the substrate 2 may have an insulating or dielectric layer formed on the opposite side to the insulating layer 3 (e.g., on the outer surface of the outer layer 2c (or 2b in the case of only two layers)) to balance the compression applied by the insulating layer 3 to the surface of the substrate 2. This may allow the substrate 2 to be thin (e.g. < 0.5mm thickness) and/or flexible.

Alternative embodiments that may be apparent to those of ordinary skill in the art upon reading the foregoing disclosure may still fall within the scope of the appended claims.

Claims

1. A method of manufacturing a thick film heating element having a metal substrate comprising two or more layers of metal or metal alloy, each of which is different, the method comprising:

bonding the layers together to form the metal substrate by using a rolling process;

forming a dielectric layer or an insulating layer on at least one surface of the substrate; and

one or more thick film heating traces are formed on the dielectric or insulating layer.

2. The method of claim 1, wherein the rolling process comprises cold rolling bonding.

3. The method of claim 1, wherein at least one of the layers has a different thermal conductivity coefficient than another of the layers.

4. The method of claim 1, wherein the substrate comprises at least three of the layers.

5. The method of claim 4, wherein an intermediate one of the layers has a higher thermal conductivity coefficient than an outer one of the layers.

6. The method of claim 4, wherein outer ones of the layers have equal thicknesses.

7. The method of claim 4, wherein outer ones of the layers have unequal thicknesses.

8. The method of claim 1, wherein the substrate remains substantially flat after the thick film heating trace is formed.

9. The method of claim 4, wherein the outer layers are composed of mutually different materials having different coefficients of thermal expansion.

10. The method of claim 4, wherein at least one of the layers comprises copper.

11. The method of claim 10, wherein an intermediate one of the layers comprises copper.

12. The method of claim 4, wherein at least one of the layers comprises steel.

13. The method of claim 12, wherein an outer one of the layers comprises steel.

14. The method of claim 1, wherein the substrate has a thickness of less than 0.5 mm.