US3163841A

US3163841A - Electric resistance heater

Info

Publication number: US3163841A
Application number: US163494A
Authority: US
Inventors: Frank C Willett
Original assignee: Corning Glass Works
Current assignee: Corning Glass Works
Priority date: 1962-01-02
Filing date: 1962-01-02
Publication date: 1964-12-29
Anticipated expiration: 1981-12-29

Description

Dec. 29, 1964 F. c. wlLLET'r 3,163,841

ELECTRIC RESISTANCE HEATER Filed Jan. 2, 1952 Arme/evs United States Patent O 3,163,841 ELECTRIC RESISTANCE HEATER Frank C. Willett, Painted Post, NY., lassignor to Corning Glam Works, Corning, Nfl/, a corporation of New York Filed Ian. 2',` 1962, Ser. No. 163,494 7 Claims. (Cl. 338-292) This invention relates to electrical resistance heaters.

It is an' object of the present invention to provide an electrical resistance heater which is characterized by a large surface to weight ratio, and which is largely a ceramic structure yet is rugged and simple to manufacture.

This and other objects are attained in accordance with this invention in which a heater is provided in the form of a` sinteredcer-amic thin walled honeycomb structure having unobstructed gas passages extending through it andan electrically conducting film on the walls of the honeycomb.

The structure of the invention is best understood by f reference to the attached drawing in which:

F-IG. l shows a heater in accordance with the invention;

FIG. 2 is an end view of the heater of FIG. 1;

FIG. 3 is-a perspective view of a heating element and its supporting base;

FIG. 4 is a section along line IV-TV of FIG. 3;

FIG. 5 is a partial sectional View, along line V-V of FIG. 2; and

FIG. 6 is a View similar to FIG. 5 showing but one end of a heater and an additional end element whereby power leads are more easily attached to the heater.

Referring now to the drawing, a heater 10 is shown enclosed in an outer? member 12' that is made of insulating material such as fiberglass or the like or is conducting but is insulated from the Working heater portion to protect against electrical shock. Leads 14 and 16, for connection to a power source, are shown extending from the ends of the heater.

The active or working portion of the heater is a sintered ceramic honeycomb structure 18 (see especially FIG. 3) having a plurality of openchannels 20 extending through it from its first major end face 22 to the other end face 24. These channels are separated from one another by thin walls 26 of sintered ceramic material provided in a manner hereinafter indicated. Walls 26 support electrically conductive films 28 (see FIGS. 4 and 5) that extend between

faces

22 and 24. Second electrically conductive films 30 (FIG. 5) are placed on the edges of walls26 at the

faces

22 and 24 and are in contact'with the internal films 2S at each end of the unit. The leads 14 and 16 are then joined to the structure as by soldering to film 30.

The sintered ceramic honeycomb structure for this invention suitably is provided by depositing pulverized ceramic material and a binder on a flexible carrier, corrugating the coated carrier, forming the desired shape from such coated carriers and thereafter firing the shape to sinter the ceramic particles to produce a unitary structure.

Any known sinterable ceramic material is suitable for making a honeycomb for the present invention. By sinterable ceramic material, I mean an inorganic substance in the crystalline or amorphous state which can be compacted or agglomerated by heating to a temperature near, but below, the temperature at which it melts or has low enough viscosity to deform. Thus glasses, such as borosilicates, soda-lime-silicates, lead-silicates, alumino-silicates, alkaline earth silicates, etc.; refractory compositions such as sillimanite, magnesium silicates, magnesia,

Zircon, zirconia, petalite, spodumene', cordierite, corundum, aluminosilicates, etc.; the glass-ceramics (crystalline materials made from glass); or combinations of any of the foregoing materials,.are all suitable for the present purpose. Refractory compositions consisting of weight percent of petalite and 5 weight percent of talc, or 75 weight percent of petalite and 25 weight percent of glass ceramic, are particularly suitable for low thermal expansion and high thermal shock resistance. A refractory composition comprising corundum (alumina) is particularly suitable for high-temperature stability. However, the invention is not dependent upon the sinterable ceramic material selected' and hence the material which has the most suitable properties for the conditions of its use may be selected.

The particle size of the ceramic material can be of any convenient dimension as dictated by the size of the desired final article and the wall thickness required.

In forming the honeycomb structure, a binder isused to bond the unired ceramic particles to a carrier, to impart green strength to the coated carrier and to retain the formed, unred article in the desired shape prior to firing. Any material having the requisite well-known characteristics ofceramic binders can be used, and these can be of temporary or permanent character. Typical materials that can be used include such natural materials as gum arabic, colophony and shellac, such synthetic organic resins as acrylate resins, methacrylate resins, alkyd resins, cellulose derivatives, cournarone-indene resins, epoxyresins, furan resins, polyisobutylene, isocyanate resins,fplienolic resins, ypolyarnides, polyesters, resorcinol resins, styrene resins, terpene resins, ureaI resins, vinyl resins, chlorinated parafiins and melamine resins, and inorganic binders such as sodium silicate and silicone resins. Preferably, heat curable organic binders that can be removed by decomposition and volatilization are used. The characteristics of suitable binders are well known to those in the ceramic arts and the curing conditions for each generally are published in magazines, journals and brochures of the tradey that are universally available.

The carrier provides support for the unfired coating to allow it to be formed to the desiredshape prior to sintering the ceramic coating. kAmong the carriers suitable are inorganic materials including aluminum foil, aluminum silicate paper, and asbestos cloth and organic materials such as cellulose acetate paper, onion skin paper, tea bag paper, nylon cloth, rayon cloth, and polyethylene film. The organic film materials are preferred as they substantially decompose upon tiring the formed article and thus result in a base that is substantially all ceramic.

As noted, the base of the heater is termed a honeycomb structure. By this is meant a unitary body having a multitude of open channels extending from one major surface throughy to the other major surface, of predetermined size and shape, each channel being defined by ceramic walls 26Y `Thus, a multiude of unobstructed gas paths through the structure is provided. This structure is achieved by assembling corrugated coated carriers with or without plain coated carriers. For example, corrugated coatedlilms can be assembled with alternate layers laterally disposed a distance equal to half the width of the individual pattern sothat layers do not nest with each other. Another method of forming ya suitable structure is to lay alternate corrugated and uncorrugated coated carriers on one another until the desired gross dimensions are obtained. Then after tiring and applying the conductive films, the honeycomb is enclosed within a ceramic enclosure having the desired cross section or within a metal or other material suitably insulated from the electrical portions in the usual manner. Other ways of producing the base structure, as by using extruded ribbons of a ceramic-binder mixture, will be apparent to those skilled in the art.

The ring of the heater base structure is accomplished in the manner normal in the ceramic arts by placing it in a furnace and heating at a rate slow enough to prevent breakage due to thermal shock to a temperature high enough to cause the ceramic particles to sinter. While the heating rates and sintering temperatures are dependent upon both the ceramic materials utilized and the shape of the article formed, they are not critical conditions and suitable conditions are readily determinable by one skilled in the art of firing ceramic articles.

Once the honeycomb substrate or heater structure has been produced, its internal surfaces are then provided with an electrically conductive film 28 of any desired predetermined electrical resistance. Such films are produced by conventional processes. Vacuum evaporation of metal or decomposition of a metal carbonyl are useful in producing a suitable lm. In addition, the film can be formed by chemical deposition of the metal from a solution containing a salt of it in which the honeycomb is immersed. Common materials that are useful for forming the film include copper, nickel, aluminum, silver and the like. Electroconductive films can be provided from metal oxides as set forth in the patent of Davis, Number 2,85 7,294. Obviously, the choice of electrically conductive material and its manner of application are Well Within the skill of the art.

After the internal surfaces have been coated, a terminal film 30 is applied to each of the two

major faces

22 and 24 of the structure where the air channels terminate. There are many methods by which such terminals can be applied. For example, low resistance conductive lms can be produced on the edge surfaces of each channel by spraying, dipping, or painting. Du Pont silver paste No. 4535 is suitable for this purpose. In addition, one can apply a fine Wire mesh or metal screen 34 (see FIG. 6),

`for example of copper, over the terminal contact, by

soldering if desired. Thereafter, the leads can'then be attached to the screen 34 by soldering or any other conventional procedure. The structure is then completed by mounting it in the housing designed to protect the structure but which permits free air movement through the channels.

The invention will be described further in conjunction with the following example.

A ceramic composition is made consisting of 75 parts by Weight of petalite and parts by weight of a glass ceramic having the followingT composition in Weight percent: 70 percent SiOg, 18 percent A1203, 5 percent TiOz, 3 percent LiOg, 3 percent MgG and 1 percent ZnG. The composition is ball-milled until a particle size of minus 200 mesh (Tyler) is obtained. A solution of the following composition is then added to the ball mill for each 2160 grams of ceramic material:

Isopropanol 640 Epoxy resin Hysol 6111 480 Ethyl-acetate a- 860 Versarnid 115 180 The term Hysol is the trade name for epoxy resins supplied by Houghton Laboratories, lnc. The Hysol 6111 is an epoxy resin solution containing 57 percent by Weight of epoxy resin having a viscosity of about 2.5-4.0 poises at 25 C. The resin has an epoxide equivalent (grams of resin containing 1 g. chemical equivalent of epoxy) of 595i50, and a melting range of 73-85 C. Such resins may be obtained readily on the open market as there are a number of producers supplying such resins. Versamid 115 is the tradename of a thermoplastic polymer supplied by General Mills, Inc. It is prepared by condensation of polymerized unsaturated fatty acids, such as dilinoleic acid, with aliphatic amines such as ethylene diamine.

The ceramic material and the binder are further ballmilled for about three hours to produce a uniform suspenison. A porous, natural cellulose paper, commonly known as 31/2 pound tea bag paper, cut to a Width of 4 inches is then dipped into the suspension and dried by heating to 120 C. for 2 minutes. The dried, coated paper is then heated to 180 C. and crimped to produce a pattern, taken in cross-section, in the shape of an isosceles triangle with legs about 0.07 inch long and an open base about 0.1 inch wide. Crimped, unfired, coated papers of that type are assembled simultaneously with alternate sheets of coated tea bag paper of the same width that is not crimped; until the desired size is obtained,

The unfired stack is then placed in a furnace and heated at about 350 C. per hour to 700 C. and held at this temperature for about 1 hour. The stack is then further heated up to 1220 C. at the furnace rate and held for 30 minutes. After cooling to room temperature at the furnace rate, it is retired to 1240 C. by heating at a rate of 300 C. per hour and held for seven hours. The sintered article is then cooled to about 100 C. at furnace rate in about 16 hours and removed from the furnace.

The resulting article has about 400 channels per square inch of cross-section, a total-available-surface of about 80() ft2/fhg, a density of about 30 pounds per cubic foot, a thermal expansion coefiicient of minus 2.0 1O7/ C.

Vover the range of 25 to 300 C., and 70 percent free air space measured in a plane perpendicular to the axis to the stack.

The resulting honeycomb is then provided with an electroconductive coating on its walls upon heating it to a temperature of about 800 C. and then immersing it momentarily in a 37 percent aqueous HC1 solution containing zinc chloride and antimony chloride in a ratio of 30 parts of -the Zinc chloride to seventy parts of antimony chloride. Then the low resistance terminal films are applied by painting the ends of the honeycomb with Du Pont silver paste No. 4535. The electrical leads are attached by soldering them against the silver terminal films after the latter have been fired to firmly bond them to the honeycomb.

ln the foregoing manner a highly efficient, compact and rugged heater is provided. The very high surface area per unit of heater makes it evident that rapid heating of substantial volumes of air are possible. It is, of course, evident that larger devices for larger loads can be made readily simply by joining several of the honeycomb bases or by using larger carriers, without loss of anyy of the advantages of the structure.

ln accordance With the provisions of the patent statutes, the principle of the invention has been explained and there has been illustrated and described what is now considered to be its best embodiment. However, it should be understood that, within the scope of the appended claims, the invention can be practiced otherwise than as specifically illustrated and described.

What is claimed is:

l. A heater comprising a sintered ceramic thin walled honeycomb shape with opposed major surfaces and having a plurality of open channels extending therethrough from one of said surfaces to the other, said channels being defined and separated from one another by sintered ceramic walls, an electrically conductive film on the surfaces of said Walls in each channel extending to the said major surfaces, a low resistivity electrically conductive lm over the edges of said ceramic walls at each of said major surfaces, said low resistivity films being in electrical contact sassari a plurality of open channels extending through said shape from one or said surfaces to the other, an electrically conductive hlm on the surfaces of said Walls in each channel extending to the said major surfaces, a low resistivity electrically conductive lin over the edges of said ceramic walls at cach of said major surfaces, said low resistivity films being in electrical contact at each surface With said films in said channels, and an electrical lead from each of said loW resistivity films.

4. A heater in accordance With claim 3 including a conductive mesh on each of said low resistivity films, said electrical leads being joined to said mesh.

5. A heater comprising a ceramic thin Walled honeycomb shape having opposed major surfaces, said shape being formed by sintering a honeycomb assembly con1- prising a crimped ceramic coated carrier, thereby delining a plurality of open channels extending through said shape from one or said surfaces to the other, an electrically conductive lm on the Walls in each channel extending to the said major surfaces, a low resistivity electrically conductive lm over the edges of said ceramic Walls at each of said major surfaces, said low resistivity films being in electrical Contact at cach surface with said films in the channels, and an electrical lead from each of said low resistivity iilms.

6. An electric heater comprising a ceramic honeycomb structure provided with a multitude of open-end channels extending therethrough, electrical resistance films coating the Walls of said channels, and electrical conducting means joined to said Iilms for connecting them with an electric power source.

7. An electric heater comprising a ceramic honeycomb structure provided with a multitude of open-end channels extending therethrough, electrical resistance films coating the Walls of said channels, a pair of electrical leads adapted to be connected to an electric power source, and low resistivity electrically conductive iilms on said honeycomb structure electrically connecting said resistance lilins to said leads.

References Qited in the tile of this patent UNITED STATES PATENTS

Claims

1. A HEATER COMPRISING A SINTERED CERAMIC THIN WALLED HONEYCOMB SHAPE WITH OPPOSED MAJOR SURFACES AND HAVING A PLURALITY OF OPEN CHANNELS EXTENDING THERETHROUGH FROM ONE OF SAID SURFACES TO THE OTHER, SAID CHANNELS BEING DEFINED AND SEPARATED FROM ONE ANOTHER BY SINTERED CERAMIC WALLS, AN ELECTRICALLY CONDUCTIVE FILM ON THE SURFACES OF SAID WALLS IN EACH CHANNEL EXTENDING TO THE SAID MAJOR SURFACES, A LOW RESISTIVITY ELECTRICALLY CONDUCTIVE FILM OVER THE EDGES OF SAID CERAMIC WALLS AT EACH OF SAID MAJOR SURFACES, SAID LOW RESISTIVITY FILMS BEING IN ELECTRICAL CONTACT AT EACH SURFACE WITH SAID FILMS IN SAID CHANNELS, AND AN ELECTRICAL LEAD FROM EACH OF SAID LOW RESISTIVITY FILMS.