CN217789917U

CN217789917U - Thick film heating element

Info

Publication number: CN217789917U
Application number: CN202221743456.3U
Authority: CN
Inventors: W·J·P·佩特斯; 格雷德斯·约翰尼斯·克罗普斯
Original assignee: Otter Controls Ltd
Current assignee: Otter Controls Ltd
Priority date: 2021-07-06
Filing date: 2022-07-06
Publication date: 2022-11-11
Anticipated expiration: 2032-07-06
Also published as: GB2608618A; GB202109730D0

Abstract

A thick film heating element comprising a substrate, an insulating layer formed on the substrate, one or more heater traces formed on the insulating layer, and one or more connection pads connected to the one or more heater traces, wherein at least one of the insulating layer, the one or more heater traces, and the one or more connection pads comprise glass or ceramic fibers.

Description

Thick film heating element

Technical Field

The utility model relates to a thick film heating element and manufacturing method.

Background

Thick film heating elements typically include one or more heating traces that are screen printed (screen printed) as an ink or paste onto a dielectric substrate and fired to form high resistivity traces. The connection traces or pads may be printed in separate layers with 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 to 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 a more conventional glass enamel (vitreou enameling) process. 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. In addition, protective ceramic or glass coatings may be added. This can also be applied by spraying or screen printing and subsequently fired.

More details of Thick film technology are described, for example, in White n. (2017) Thick Films, in Kasap s., capper p. (eds) Springer Handbook of Electronics and Photonic Materials, pages 707 to 709 and 712. The thick film paste may include an active material, a 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 a characteristic structure and properties, not merely a comparative term or designation for the product when manufactured by a particular process.

The heater traces and connection pads comprise metal particles, typically silver, platinum or palladium, or a mixture of two or more of these metals, and glass. They are applied to the insulating layer in the form of a paste by screen printing and then dried and fired as described above.

Since the thick film material comprises glass or ceramic, the thick film material has low tensile strength and care must be taken during design and processing and use to ensure that the material is subjected to compressive forces rather than tension. This is typically accomplished by selecting a material for the substrate that has a coefficient of thermal expansion that is greater than the coefficients of thermal expansion of the thick film material and the insulating layer. As a result, the thick film material and insulating layer are subjected to compressive stress when the thick film heater cools down after the firing process. Sometimes the substrate is bent before or after the firing process to place the thick film material under compressive stress. Even so, a common failure mode is cracking of the insulation layer due to thermal shock.

Another problem is the connection of the heater trace to the power supply. It is common practice to provide connection pads of low resistance material at the ends of the heater traces. The connection pads comprise a material similar to the heater trace but with a very low resistance and therefore no significant heating in the connection area. The connection pads overlap the ends of the heater traces to provide a good electrical connection. A resilient (spring) contact may be used to connect the pad to the power supply. Typically these are copper alloys and provide a low resistance silver face by electroplating or by attaching silver contacts of the type typically seen in switches.

However, cheaper and more compact connections can be made by soldering the wires directly to the connection pads. The soldering process may cause failure of the heating element, for example due to volumetric shrinkage of the solder as it solidifies. This is particularly prevalent in the case of lead-free Sn/Ag/Cu and Sn/Cu/Ni alloys. As the solder solidifies and contracts, the solder exerts a stress on the connection pad that exceeds the strength of the bond between the pad and the heater trace, or the strength of the pad, heater trace, or insulating layer in the region of the connection pad.

Thick film elements having a steel substrate require an electrically insulating layer between the substrate and the heater trace. Glass or ceramic materials, like most materials that are electrical insulators, are poor thermal conductors. Although the insulating layer is relatively thin, on the order of 100 μm, there can still be a significant temperature gradient across the layer, increasing the temperature of the heater trace. The operating temperature of the traces limits the power that can be dissipated by the heater. If the thermal conductivity of the insulating layer is increased, the power density or heat flux can be increased. As a result, a heating element of a given power can be made smaller, or more power can be delivered through a given size element.

It is also desirable to be able to measure the temperature on the surface of the thick film heater. This can be done by printing the sensor with a material that has a relatively large change in resistance with temperature. Most of these materials have a negative temperature coefficient of resistance. The resistance of the heater trace may also be used as a sensor. The choice of materials with suitable resistivity is limited.

SUMMERY OF THE UTILITY MODEL

The aspects of the invention are defined by the appended claims.

At least some embodiments of the present invention relate to solutions to the above-mentioned problems of increasing the strength of thick film materials and insulating layers. Mechanical failure of glass and ceramics is typically caused by brittle fracture initiated by cracks that propagate at defects in the material such as scratches. In at least some embodiments, certain properties of the insulation layer are improved by using a Ceramic Matrix Composite (CMC). These composites have ceramic fibers embedded in a ceramic matrix. The ceramic fibers increase the stress required for crack propagation through the matrix, thereby increasing the energy consumed during crack propagation. When through-thickness cracks begin to form through the matrix, the fibers bridge these cracks without breaking, thereby increasing the tensile strength of the material. Ceramic fiber reinforcement increases the initial resistance of the composite to crack propagation and avoids sudden brittle failure as compared to monolithic ceramics.

Drawings

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

fig. 1 is a perspective view of a thick film heating element in an embodiment of the invention;

FIG. 2 is an exploded perspective view of a thick film heating element in an embodiment; and

figure 3 is a cross-sectional view of a thick film heating element in an embodiment.

Detailed Description

Fig. 1 to 3 show a thick film heating element 1, the thick film heating element 1 comprising a substrate 2 with an insulating layer 3 of enamel. The substrate 2 may be made of steel such as ferritic stainless steel. In the method of manufacturing the thick film heating element 1, an insulating layer 3 is formed on the substrate 2 and connection pads 4 and one or more heating tracks 5 are formed on the insulating layer 3 using, for example, the thick film printing and firing process described above. One or more glaze layers (not shown) may be formed over the heating traces 5 leaving the connection pads 4 exposed.

In this embodiment, one or more of the insulating layer 3, connection pads 4, heater trace 5, and glaze layer(s) comprise glass or ceramic fibers, for example as a composite in a matrix of glass or ceramic material. There are many conventional methods of introducing ceramic matrices into the spaces between the fibers, including sintering, deposition of the matrix by a gas mixture, chemical reactions, and pyrolysis. None of these methods is compatible with two typical methods of applying the insulating layer 3 or heater trace 5 to the substrate 2: these methods are spray coating such as, but not limited to, electrophoretic spray coating, as well as flame spray coating and screen printing.

In one method according to an embodiment, ceramic fibers are introduced into a sprayed enamel or screen-printed paste to form the insulating layer 3 after firing. In an alternative method according to this embodiment, the enamel and ceramic fibers are sprayed separately onto the substrate 2 and then fired to form the insulating layer.

The size of the fibers, and the amount of fibers in the matrix, as well as the materials of the matrix and fibers, determine the properties of the composite. The matrix may be or have the same glass or ceramic material as the glass or ceramic material of the fibers.

The length of the fibres used in the CMC material may be up to 4mm or 5mm, and in practice the fibres are at least 0.5mm long. The diameter may vary from 1 μm to 50 μm. However, the size of the fibers is limited by the applied construction method and thickness of the insulating layer 3 and heater trace 5. The size of the fibers in the heater traces or connector pads is limited by the size of the holes in the printing screen.

In the insulating layer 3, the fibers may be long enough to break through the surface of the insulating layer 3. This may cause problems when the heater trace 5 is subsequently screen printed, but the fibres may also provide a good bond between the heater trace 5 and the insulating layer 3.

The fibers may comprise one or more of a range of materials. The insulating layer 3 mainly comprises glass and/or ceramic fibres may be used to reinforce the insulating layer 3. A ceramic such as boron nitride or alumina may be used as the fiber to add strength to the insulating layer 3, the heater trace(s) 5, or the connection pad 4. The thermal conductivity of the insulating layer 3, the heater trace(s) 5 or the connection pad 4 may be increased by adding fibers of a ceramic having a high thermal conductivity, such as silicon carbide or silicon nitride. The fibers may comprise ceramic oxides, nitrides or carbides of aluminum, boron or silicon.

In a variation of this embodiment, the temperature sensor trace may be deposited on the insulating layer, alongside the heater trace(s) 5 and separate from the heater trace(s) 5, using thick film technology, in order to sense the temperature of the heater trace(s) 5. The addition of ceramic fibers with low electrical resistance provides the material with a temperature coefficient of resistance that makes it suitable for use in such temperature sensor traces. The addition of fibers allows the temperature coefficient of resistance to be adjusted so that the properties of the sensor are matched to the intended application. Fibers of silicon carbide are among those suitable for this application.

Alternative embodiments

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

Claims

1. A thick film heating element comprising a substrate, an insulating layer formed on said substrate, one or more heater traces formed on said insulating layer, and one or more connection pads connected to said one or more heater traces, wherein at least one of said insulating layer, said one or more heater traces and said one or more connection pads comprise glass or ceramic fibers.

2. The thick film heating element of claim 1 further comprising a cover layer formed over the one or more heater traces, the cover layer comprising glass or ceramic fibers.

3. The thick film heating element of claim 1 further comprising one or more temperature sensor traces comprising glass or ceramic fibers.

4. The thick film heating element of claim 3 wherein said one or more temperature sensor traces comprise fibers of silicon carbide.

5. A thick film heating element as claimed in claim 1 wherein the fibres are in a matrix of glass or ceramic material different to that of the fibres.

6. A thick film heating element as claimed in claim 1 wherein the fibres are in a matrix of the same glass or ceramic material as that of the fibres.

7. A thick film heating element as claimed in claim 1 wherein the length of the fibres is between 0.5mm and 5 mm.

8. The thick film heating element of claim 1 wherein the diameter of the fibers is between 1 and 50 μ ι η.

9. The thick film heating element of claim 1 wherein said fibers comprise glass.

10. A thick film heating element as claimed in claim 1 wherein the fibres comprise a ceramic oxide, nitride or carbide of aluminium, boron or silicon.

11. A thick film heating element comprising in part a ceramic matrix composite, the ceramic matrix composite comprising glass or ceramic fibres.

12. A thick film heating element as claimed in claim 11 wherein the length of the fibres is between 0.5mm and 5 mm.

13. A thick film heating element as claimed in claim 11 wherein the diameter of the fibres is between 1 and 50 μm.

14. The thick film heating element of claim 11 wherein said fibers comprise glass.

15. A thick film heating element as claimed in claim 11 wherein the fibres comprise a ceramic oxide, nitride or carbide of aluminium, boron or silicon.