Resistance Heater And Method For Preparation Thereof

Abstract

A resistance heater comprising a sintered mass of refractory particles, each particle comprising an insulating core coated with a thin film of an electrically conducting material, is obtained by a processing sequence involving coating the particles of interest, compacting the coated particles to form a pellet and sintering the pellet. Devices produced in accordance with the described technique manifest enhanced reliability and uniformity as compared with prior art heaters, and permit a new degree of freedom in the design of heating elements.

Description

This invention relates to a technique for the fabrication of a resistance heater and to the heaters so produced.

Heretofore, it has been widely recognized by those skilled in the art that the total exploitation of the resistance heat concept in electron device technology has been limited by certain inherent drawbacks. Among the most severe of such drawbacks is the fact that the resistive element used to carry current and develop heat in consequence is restricted in geometry by the fundamental relationship W I.sup.2 R, where W represents power or heat output, I represents current and R resistance. Accordingly, if the resistance, R, of the element is low, the current, I, flowing in the element must be proportionately high to maintain a given output. Unfortunately, power sources capable of sustaining high currents are intrinsically expensive and undesirable currents in excess of about 10 amperes being generally avoided. Thus, in order to fabricate a useful resistive heating element from a metal, its aspect ratio must be increased to the point which we recognize as a wire.

Although wire configurations are useful as resistance heaters in numerous applications, they cannot be efficiently employed for the purpose of heating objects, whose dimensions are large, with respect to typical wire diameters, uniformly by direct contact with the object surface. This end may only be obtained by interposing a thermal conductor between the object and the wire, such thermal conductor acting as a thermal diffuser. Additionally, metal heating elements in which at least one dimension is very small are prone to mechanical damage, local variations in resistivity drastically reducing the useful life and reliability of the element.

Recognizing these limitations, workers in the art focused their attention upon thin sheets and the concept of passing current therethrough. Unfortunately, it was found that the only technique for limiting current levels involved causing current flow along a major dimension of a very thin sheet, the limits on thickness being such as to make fabrication of reproducible, high reliability elements almost impossible. This obviates the likelihood of using the configuration for heating systems other than those which are linear and which function at low temperature.

More recent investigations of the resistance heater technology have concentrated upon the synthesis of composite materials manifesting resistivities intermediate that of a metal and an insulator. This concept has typically taken the form of a mechanical mixture of metal and insulator but it too has been handicapped by practical limitations such as the establishment of complete conducting paths, the likelihood of creating reproducible resistances, etc.

In accordance with the present invention these prior-art difficulties are successfully overcome by a novel processing sequence which results in the formation of a resistance heater comprising a sintered mass of electrically insulating refractory particles individually coated with a thin film of an electrically conductive material. The described structure includes a continuous chain of such metal coated particles which are held together by the sintering action of the metal films at the points of contact, such chain manifesting a resistance determined by the thickness of the applied metal film and the diameter of the conductive necks formed between each particle, that is, the conductive region formed by migration of metal from the surface of the films on adjacent particles into the contact region under the influence of the cohesive force induced between metal surfaces in contact with each other at elevated temperatures. In light of the fact that these variables are controllable, the technique permits the formation of composites evidencing resistivities suitable for a wide range of applications. Studies of the characteristics of the resultant devices have revealed that not only have all the prior-art limitations been overcome but also that there is obtained a structure manifesting enhanced reliability and uniformity as compared with the prior art structures.

The invention will be more readily understood from the following detailed description taken in conjunction with the accompanying drawing, wherein:

FIG. 1 is a cross-sectional view of a resistance heater of the present invention;

FIG. 2 is a front elevational view of a sphere bearing a resistance heater of the present invention; and

FIG. 3 is a front elevational view in cross section of a typical device heated by the resistance heater of the invention.

A general outline of the procedure employed in fabricating the novel structures described herein will now be given.

The first step in the practice of the present invention involves coating a plurality of independent particles of an electrically insulating refractory material with a thin film of an electrically conducting material. Typically, the insulating material is selected from among ceramic materials, for example, alumina, beryllia, magnesia, zirconia, etc., the choice of a particular insulating material being dependent upon the intended use of the resultant structure. More specifically, the insulating material must be capable of withstanding the temperatures to which the desired structure will be heated without chemically reacting with the conducting material. Thus, a suitable choice of materials for one desiring to fabricate a structure capable of heating uniformly at 1,800.degree. C. might be tungsten and alumina, such materials not reacting at an appreciable rate until a temperature of 2,000.degree. C. is reached. Accordingly, the only limitation on the insulating material is that it not react with the conducting material at the desired temperature of operation.

Particle size of the insulating material is of no criticality. However, a preference exists for the use of crushed polycrystalline material having particle size ranging from 80 mesh to Fischer subsieve size average 7.0.

The electrically conducting materials found suitable for coating in accordance with the present invention may be selected from among the transition metal elements of groups 6B and 8 of the Periodic Table of the Elements (see Handbook of Chemistry and Physics, 45the Edition, 45th by the Chemical Rubber Company). Materials found to be particularly useful for this Chemical include iron, nickel, tungsten, molybdenum, platinum, iridium, etc.

Coating of the insulating particles may be effected by any conventional coating or plating technique, for example, fluidization by dry or wet methods, electroless plating, etc. A particularly useful method for effecting this end when particle diameters less than 10.mu. are desired, the wet fluidization procedure, is described by D. W. Maurer et al. in U.S. Pat. No. 3,404,034 which issued on Oct. 1, 1968. It will be appreciated by those skilled in the art that the thickness of the metal film so deposited is not critical and may vary from a few monolayers to thousandths of an inch, such range being dictated by considerations relating to end use and the resistivity of the conductive element. The particles so coated are not in the form of a powder and are ready for the next stage of processing.

Following, the coated particles, either alone or in combination with a binder, are compacted by any well-known compacting procedure such as pressing, electrophoresis, etc. The most convenient procedure, pressing, involves insertion of the coated particles in a suitable die followed by pressing at pressures up to 150,000 p.s.i.g., thereby resulting in the formation of a pellet comprising a plurality of individually metal-coated electrically insulating refractory particles wherein metal contacts are formed between metal layers of adjacent particles.

The compacted pellet is then placed in a suitable boat which is inserted in a furnace maintained at room temperature. Then, the furnace is purged with an inert gas such as purified dry nitrogen, argon or helium for several minutes and the inert gas replaced by a reducing gas such as hydrogen. The furnace is then put into operation and is heated to a temperature such to effect sintering of the powders contained within the pellet. For the purposes of the present invention, it will be understood that the sintering temperature varies with the metal of the coating but typically occurs at temperatures above one-half the melting point of the metal. A typical sintering temperature with molybdenum involves heating to a temperature within the range of 1,300-1,600.degree. C. for a time period ranging from 1 to 180 minutes, the shorter time period corresponding with the higher temperature. The sintering operation, as described, results in the growth of the metal contacts alluded to above into electrically conductive necks between adjacent compacted particles. The binder, if used, will be volatilized at low temperature during the warmup to the sintering temperature.

With reference now to FIG. 1, there is shown a cross-sectional view of the resistance heater of the present invention. Shown in the Figure is a sintered body 11 comprising a plurality of electrically insulating particles 12, each of which is coated with a thin film of an electrically conductive material 13, adjacent particles being in contact with each other by means of conductive necks 14.

The unique application of the instant invention will be more fully appreciated by reference to FIGS. 2 and 3. In FIG. 2, there is shown an elevational view of a sphere 21 having a pair of electrodes 22 and 23 wound around the circumference thereof and intermediate said electrode pair a laminated layer 24 of a sintered mass of electrically insulating coated particles coated with a thin layer of an electrically conductive material. It is evident by reference to the drawing that uniform resistive heating of the sphere will be obtained by passage of current through the electrodes. The principle embodied can clearly be extended to irregularly shaped bodies of any kind, the only limitation being that the geometric magnitude of the irregularity should be large compared to the particle size of the cermet powder employed.

FIG. 3 is a front elevational view in cross section of another embodiment of the present invention wherein an electronic device 31, such as a semiconductor device, is heated by resistance heater 32 laminated between metal film electrodes 33 and 34.

An example of the present invention is set forth below. It is intended merely as an illustration and it is to be appreciated that the process described may be varied by one skilled in the art without departing from the spirit and scope of the invention.

EXAMPLE

A plurality of particles of crushed polycrystalline aluminum oxide obtained from commercial sources and ranging in particle size from 1 to 10 microns was suspended in silicone oil and charged to a fluidization column which was immersed in a constant temperature oil bath. Fluidization was initiated by admitting a stream of hydrogen containing molybdenum carbonyl vapor into the column and coating attained by refluxing for 6 hours, thereby causing decomposition of the carbonyl and coating of the aluminum oxide with a thin film of molybdenum. The coated particles were then cooled, separated from the oil by filtration, washed with acetone and dried in air.

The coated particles in powder form were then intimately mixed with 1 milligram of stearic acid per gram of powder and inserted in a conventional hydraulic press. Then, 85,000 p.s.i.g. of pressure were applied by actuating the press, so resulting in the formation of a pellet. The pellet was then positioned in a furnace and, with hydrogen flowing, was fired for 30 minutes at 1,600.degree. C. Upon cooling, the resultant pellet evidenced a resistivity of approximately 5 ohm-centimeter and upon the application of evaporated molybdenum electrodes thereon was available for use as a resistance heater in a configuration of the type disclosed in FIG. 3. During operation of such a structure, it was determined that the device of interest could be heated at a temperature of 1,050.degree. C. over a time period of 9,000 hours without significant change in uniformity or electrical characteristics.

Claims

We claim:

1. A method for the fabrication of a resistance heater comprising the steps of (a) coating the major faces and periphery of a plurality of individual particles of an electrically insulating refractory material with a thin layer of an electrically conducting metal material, (b) compacting the resultant powder comprising the coated particles to form a pellet in which a plurality of contacts are formed between the conductive material of adjacent particles, and (c) sintering said pellet at elevated temperatures in a reducing ambient, so resulting in the growth of said contacts into electrically conductive necks between adjacent particles.

2. Method in accordance with claim 1 wherein sintering is conducted at a temperature greater than one-half the melting point of the said conductive material.

3. Method in accordance with claim 1 wherein compacting is effected by pressing techniques.

4. Method in accordance with claim 1 wherein said insulating material is aluminum oxide and said conductive material is molybdenum.