Heating Member

Abstract

A heating plate comprises a generally refractory substrate of, for example, glass upon which is deposited by flame spraying, a contact member occupying a limited portion of a surface of the substrate for connecting the heating element in an electric circuit. The contact member is preferably copper or a copper-based alloy (e.g. 90 - 99 percent by weight copper, 0 - 9 percent by weight titanium and 0 - 2 percent by weight chromium). A protective metal (conductive) layer is provided on the contact member which is directly formed on the substrate and a heating element is constituted of a further conductive layer applied over the remainder of the substrate surface and on the protective layer covering the contact member.

Description

FIELD OF THE INVENTION

The present invention relates to heating elements and, more particularly, to a method of making a heating element and to an improved heating element made by this method.

BACKGROUND OF THE INVENTION

Heating elements, disks, plates and the like have been provided heretofore for many purposes, e.g., for furances, ovens and kilns, for domestic and industrial heating tasks and as a substitute for heating coils in appliances of varying sorts. For example, a heating element of the plate-like type may be used for the baking or reflowing of paints when mounted in a suitable array adjacent the transport path of mass-produced articles such as automobiles. Smaller heating elements of the same general type may be used for the infrared heating and thawing of foods in toasters, broilers, roasters and the like. Between these two extremes, there is a variety of applications for these heating elements which depend upon the generally planar configuration thereof.

Prior-art heating elements have been provided heretofore for these purposes in a variety of configurations. For example, a refractory plate (of circular or rectangular outline) has been formed with a coil of heating wire (e.g., nichrome) so that the wire lies substantially in the plane of the surface. Other plate-like substrates have been formed with recesses or channels receiving strips or coils of heating wire. Still another construction known to the art provides a refractory plate upon which a heating element is deposited or to which a heating element is bonded by an adhesive or the like so that, upon firing, a ceramic-metal bond is formed. The heating element in this case may be a metallic conductor or a semiconductor and the passage of an electric current through the heating element, having spaced-apart contact members which are applied to the material forming the heating element, results in resistive generation of heat. Heating bodies of the latter type have proved to be especially desirable because the danger of damage to a heating coil can be excluded and the assembly made more compact than arrangements using wires, coils and the like. However, heating bodies (in which the heating element is deposited upon a refractory substrate and contact members are applied to the heating element to afford facilities for electrical connection of the device in a circuit) are characterized by a high production cost, poor solderability of the contact members to other elements of the circuit, and poor mechanical properties of the contact members. Thus the contact members may be dislodged from the heating elements to which they are bonded and cannot be soldered readily into a circuit.

OBJECTS OF THE INVENTION

It is, therefore, the principal object of the present invention to provide an improved heating body whereby the aforementioned disadvantages are obviated.

It is another object of the invention to provide an improved method of making a heating vody of the character described.

Still another object of the invention is to provide a heating device in which the contact members have excellent solderability and are bonded to the structures with such adhesion that damage to the connection is substantially completely excluded.

SUMMARY OF THE INVENTION

These objects and others which will become apparent hereinafter are attained, in accordance with the present invention, in a heating plate (of disk or circular configuration or of rectangular or any other desirable outline) which comprises a refractory substrate (e.g., of glass or a ceramic consisting of alumina, magnesia, silica, titania, zirconia and mixtures and combinations thereof or of other minerals containing same) at least one but preferably two or more metallic electrical-contact members extending over only a limited portion of the substrate and bonded thereto directly by flame-spraying a metal protective layer overlying this contact member or each of the contact members and, in turn, an electrically conductive layer overlies the protective layer in the region of the contact member and directly is formed upon the remainder of the substrate surface. The electrically conductive layer is thus in electrically conducting relationship with the contact member (through the protective layer) and forms the heating element.

In other words, the invention comprises a generally flat support upon which is deposited a heating element of electrically conductive material in the form of a layer and conductive members electrically connected to the heating element. The electrically conductive layer forming the heating element can be composed of transparent or nontransparent metals or semiconductors in the form of a thin film or layer, especially of gold, platinum, silver, copper or aluminum. Suitable semiconductor heating-element layers are composed, for example, of zinc oxide, doped with antimony. This example merely represents one of a wide variety of semiconductive heating elements which may be used, these substances being generally described as metal oxide doped with a substance from Group III or V of the Periodic Table. The electrically conductive layer forming the heating element may be deposited in vacuo upon the substrate (i.e., vapor-deposited) or can be formed on the substrate by wet-chemical process. The term "wet-chemical process" is used herein to describe electroplating or electrodeposition and so-called chemical plating or electroless plating or deposition.

The invention resides in applying the contact members directly to the support by flame-spraying from a copper alloy and thereafter coating the copper-alloy contact member with a metallic protective layer, and only thereafter applying the electrically conductive layer forming the heating element.

A heating plate of this type is of course of much simpler manufacture than earlier heating elements because complications which have faced prior-art systems in applying the contact members to the heating element are eliminated. The contact members are mechanically locked in the structure and are both solderable and free from adhesion loss. In effect, therefore, the contact members are imbedded in a body forming the heating element, thereby ensuring excellent electrical connection between the heating element and the contact member.

According to a more specific feature of the invention, the contact member is composed of an alloy containing copper and at least 1 percent by weight of alloying metals selected from the group which consists of titanium, chromium or combinations thereof. Best results have been found with 90 to 99 percent by weight copper, 0 - 9 percent by weight titanium and 0 - 2 percent by weight chromium. For reasons it has been unable to fully ascertain, this alloy bonds directly to glass or ceramic substrates significantly better than other alloys and is particularly suitable for flame-spraying without causing difficulties during the process. In addition, it has high solderability.

According to another feature of this invention, the protective layer is applied in a thickness ranging between molecular thickness and 1 micron, i.e., in a thickness up to 1 micron, and consists of gold or a gold alloy, or tin or a tin alloy. The protective layer should be resistant to oxidation and should be solderable or capable of forming electrical connections of low contact resistance. Gold, tin and their alloys fulfill these requirements. The protective layer may be applied by chemical plating, galvanic (electroplating) flame-spraying or vapor deposition, the latter being preferred.

Advantageously, the diffusion barrier layer of nickel, to a thickness of 3 to 5 microns, is applied between the copper-alloy contact member and the protective layer, e.g., by galvanic methods. This barrier prevents diffusion of the contact material into the protective layer and also prevents diffusion of material from the protective layer into the contact member. The electrically conductive layer which preferably covers the remainder of the surface of the substrate, is applied by vapor deposition.

The system described above has some significant advantages over the prior art, aside from the not unimportant advantage of substantially reduced cost. The bond between the contact member and the substrate, especially when the latter is composed of glass, is specially strong and ablation of the contact member need not be feared. The contact member can be soldered with ease to other circuit elements without the danger of dislodging because mechanical and thermal conditions during the soldering operation. While it is preferred to use a glass or ceramic support as described above, it has been found that even synthetic resin (e.g., phenolic supports) may be used.

DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a plan view of a portion of a heating plate according to the present invention;

FIG. 2 is a section taken along the line II -- II of FIG. 1;

FIG. 3 is a view similar to FIG. 2 but illustrating another embodiment of the invention; and

FIG. 4 is a flow diagram illustrating the method of the invention.

SPECIFIC DESCRIPTION

In FIGS. 1 and 2, it can be seen that the heating plate according to the present invention comprises a glass substrate 1, which can be of rectangular or any other configuration, e.g., circular, that may be desired. An electrically conductive layer 2, (e.g. of the order of thickness of 1 to 50 microns) is applied to this substrate over substantially its entire surface and directly adheres thereto. The strip-like contact member 3, to which other circuit elements are connected by soldering, is applied adjacent an edge of the heating plate and represents a plurality (at least 2) contact members. The contact members 3 are applied directly to the glass plate 1 by flame-spraying and consist of a copper-alloy body 4 which is coated with a layer 5 of gold, tin, or an alloy thereof. In the region of the contact members, the layer 2 overlies the protective layer 5.

FIG. 3 shows a modification wherein the substrate 1 carries directly the flame-sprayed strip 4 of copper alloy but is coated, in turn, with 3 to 5 microns of a nickel layer 10. The gold or tin protective layer 5 is thereupon applied to the nickel layer and the heating element layer 2 formed thereover.

SPECIFIC EXAMPLE

As diagrammed in FIG. 4, the substrate, e.g., glass, is degreased and, if desirable, roughened by abrasion, in the region of the contact strip or slightly edged over its entire surface with hydrofluoric acid. The glass plate had a thickness of 10 mm and was composed of quartz glass. At the first stage (I) of the process, a copper alloy consisting of 97.3 percent by weight copper, 2.3 percent by weight titanium and 0.4 percent by weight chromium, was deposited on the glass in the form of a strip 4 at a temperature slightly above the melting point of the alloy. In the second stage (II), a 4-micron layer of nickel was electro-plated at a temperature of about 40.degree.C with a cathode-density of 25 amp,./square foot and an anode current density of 8 amp./square foot from a nickel sulfamate plating bath. After the layer was thoroughly dried, 0.9 microns of gold was vapor-deposited followed by the heating element layer 2 to a thickness of 25 microns. The heating element layer was composed of copper. The contact member was found to be firmly adherent to the glass substrate and even during soldering was not released therefrom.

Claims

1. A heating plate comprising: an insulating substrate; at least one metallic electrical contact member of a copper alloy containing at least 1 percent by weight titanium, chromium or a combination thereof extending over only a portion of the surface of said substrate and bonded directly thereto; a metallic protective layer overlying said member; and an electrically conductive layer overlying said surface and said protective layer while being in electrically conductive relationship with said contact member through said protective layer, said conductive layer forming a heating element and being directly bonded to said surface over

2. The heating plate defined in claim 1 wherein said copper alloy consists of 90 to 99 percent by weight copper, 0 to 9 percent by weight titanium

3. The heating plate defined in claim 2 wherein said copper alloy consists

4. The heating plate defined in claim 3 wherein said protective layer has a

5. The heating plate defined in claim 4 wherein said protective layer is a material selected from the group consisting of gold, a gold alloy, tin and

6. The heating plate defined in claim 5, further comprising a diffusion-barrier layer of nickel between said contact member and said protective layer, said barrier layer having a thickness of 3 to 5 microns.

7. A method of making a heating plate, comprising the steps of:

Flame-spraying an electrical contact member comprising a copper alloy containing at least 1 percent by weight of titanium, chromium or a combination thereof onto a portion of the surface of an insulating substrate;

coating said contact member with a protective layer of a material selected from the group consisting of gold, a gold alloy, tin and a tin alloy; and thereafter applying to said surface and said protective layer an

8. The method defined in claim 7, further comprising the step of electroplating a nickel layer upon said contact member prior to the

9. The method defined in claim 7 wherein said protective layer is

10. The method defined in claim 7 wherein said conductive layer is

11. The method defined in claim 7 wherein said conductive layer is deposited by wet-chemical process upon said surface and said protective layer.