US3360761A

US3360761A - Resistor substrate having integral metal terminations

Info

Publication number: US3360761A
Application number: US451917A
Authority: US
Inventors: Earl W Stapleton; Irvin J Mckeand
Original assignee: Air Reduction Co Inc
Current assignee: Airco Inc
Priority date: 1965-04-29
Filing date: 1965-04-29
Publication date: 1967-12-26
Anticipated expiration: 1984-12-26

Description

1967 E. w. STAPLETON ETAL 3,

RESISTOR SUBSTRATE HAVING INTEGRAL METAL TERMINATIONS Filed April 29, 1965 FIG. 4

28 FIG 2 2/ 24 I I 2m 23 23 FIG. 3 /2- /3 /4 CE 221, C WET ORV/N6 GRIND/N6 MATERML BALL A/vo A/v0 BATCH/N6 M/X/NG CALC/N/NG M/Xl/VG /7 CERAMIC AND J5 METAL POWDER M/X/A/G 5pm y I SCREENING DR y/NG M/l/EA/TORS PRESS/N6 IRVIN J. McKEAND FIR/N6 70 FORM EARL W. STAPLETON SUBSTRATE By WM AGENT United States Patent Ofilice 3,360,761 RESISTOR SUBSTRATE HAVING INTEGRAL METAL TERMINATIONS Earl W. Staplcton, Syracuse, and Irvin J. McKeand,

Fayetteville, N.Y., assignors to Air Reduction Company Incorporated, New York, N.Y., a corporation of New York Filed Apr. 29, 1965, Ser. No. 451,917

4 Claims. (Cl. 338-330) ABSTRACT OF THE DISCLOSURE A mechanically durable and electrically stable electrical resistor is formed by coating a fully vitrified ceramic substrate having integrally formed metallic terminals with a resistive composition of glass in which inorganic conductive particles are dispersed. The resistive composition is fired in situ on the prefired substrate to form a completely vitrified resistor unaifected by temperature, moisture or surrounding atmosphere.

The present invention relates to electrical resistors and the method of making the same. More particularly, the invention relates to resistors which comprise a substrate having an electrically non-conductive ceramic portion and integral conductive metal or metal-ceramic mixture (cermet) terminal portions, and a resistive coating deposited on the non-conductive portion of the substrate and electrically connecting the terminal portions. Wire leads are preferably attached to the conductive terminal portions.

Prior art film resistors generally consist of a solid ceramic substrate, a resistive coating applied thereto, and metal end-caps slidably mounted over the end portions of the resistive coating with the lead wires soldered or otherwise suitably aflixed to the end-caps. At this point, it should be noted that any reference herein to a film resistor is intended to include resistors which consist of a substrate which is coated with metal, metal oxide, or carbon thin films, or with a thicker coating such as an inorganic composition comprising a glass and metal or metal oxide mixture. The coating may completely enclose the substrate or it may be deposited on the substrate in a spiral or other desired configuration. United States Patent No. 1,814,583 illustrates a typical prior art resistor with metal end-caps. The necessity of such end-caps presents many disadvantages in both the use and manufacture of film resistors. The use of end-caps automatically creates an additional step in the assembly of a resistor. The endcaps are generally of thin, delicate construction, particularly when used with resistors of small physical size, and presents serious difiiculties in attaching a wire lead thereto. In the use of the resistor, the thin flanges are easily damaged if the leads are accidentally pulled during circuit installation, and the leads may often completely detach from the end caps. Further, in modern compact electronic equipment where size is of extreme importance, the overlapping flange of the end-caps results in a large and cumbersome resistor which is undesirable. Additionally, end caps are expensive and substantially increase the cost of the resistor.

It is an object of the present invention to provide improved film resistors for use in electronic circuits. It is another object of the present invention to provide film resistors which are extremely reliable. It is another object of the invention to provide film resistors which eliminate the necessity of metal end-caps. It is still another object of the invention to provide film resistors which are small in size and which may be manufactured economically. It is a further object of the invention to provide a resistor comprising a substrate having a ceramic portion and integral metal or cermet terminal portions. It is a further object of the invention to provide a resistor comprising a substrate having a ceramic portion and integral metal or cermet terminal portions fired in situ with the center portion. It is yet another object of the invention to provide a film resistor comprising a substrate having a ceramic portion and integral metal or cermet terminal portions in which wire leads are securely imbedded. It is a further object of the invention to provide a film resistor comprising a substrate having a ceramic portion with integral metal or cermet terminal portions and wire leads welded, brazed, or otherwise securely fastened to the terminal portions. It is an additional object of the invention to provide a film resistor comprising a substrate having a ceramic portion with integral metal or cermet terminal portions fired in situ therewith, with the ends of the conductive film overlapping and electrically connecting the terminal portions. It is another object of the invention to provide a method for making the new and improved resistors of the present invention. Other objects, features, and advantages of this invention will be readily apparent from consideration of the following specification relating to the annexed drawings in which:

FIG. 1 is a sectional longitudinal view of one embodiment of the invention;

FIG. 2 is a partly elevational and partly sectional longitudinal view of a second embodiment of the invention;

FIG. 3 is a flow diagram showing the general steps by which the resistor of our invention is manufactured;

FIG. 4 represents a vertical section through a mold showing the method of assembling the powdered end por tions and the powdered center portion of the substrate prior to the substrate being formed in the molding operation; and

FIG. 5 represents a vertical section through the mold similar to FIG. 4, but showing the assembled materials after they have been pressed.

Referring to the drawings, FIG. 1 is a view in section of one form of the resistance unit. The body of the resistor is indicated at 1 and is preferably cylindrical in form as shown, although other configurations are possible and within the scope of this invention. The body of the resistor is formed by a substrate having an electrically nonconductive center portion 2 and conductive metal or cermet terminal end portions 3, the substrate having a substantially uniform diameter along its length. It is obvious, however, that the substrate may take other forms without departing from the scope of this invention. A conductive material 4 is applied to the center portion of the substrate and overlaps the end portions 3 as shown at 8. Wire leads 5 are embedded in the terminations 3 as shown at 6. Finally, the resistor is coated with a suitable protective coating 7.

The resistor shown in FIG. 2 is identical to the resistor of FIG. 1 with the exception of the manner in which the wire leads 5 are electrically connected to the end portions 3. Consequently, like parts of FIGS. 1 and 2 are designated by the same reference numerals. In FIG. 2, leads 5 are welded, brazed, soldered, or the like, to end portions 3 as shown at 9.

In manufacturing the resistor, with reference to FIG. 3, raw materials necessary for the preparation of a ceramic formulation are weighed and brought together in a raw material batching operation as at 11. The materials are then mixed in some convenient manner such as by wet ball milling indicated at 12. An example of a satisfactory formulation for the ceramic is 78% by weight prefused forsterite, 5% barium carbonate, 5% calcium carbonate, and 12% silica. Other ceramic formulations may obviously be advantageously employed if selected so as to be compatible with the terminal portions and the resistive film. The mixed ceramic is then dried and calcined at 13 by heating to some temperature at which vitrification is imminent, i.e. in the range of from 2100 F. to 2400" F. The calcined ceramic is ground at 14 by any well known method such as in a dry ball mill and prepared for pressing by some standard technique. The ground material may be mixed with an organic binder in the ball mill before pressing if desired, but it is not necessary to the procedure. In the event a binder is used, a satisfactory composition is a mixture of water, 0.2% calcium stearate, and 1% methocel, which is mixed with the calcined ceramic in the ball mill. At the conclusion of mixing, the ceramic is spray dried as at 15 to from to 1% moisture by weight. The mix is then screened at 16 to produce the proper powder consistency. To provide cermet terminal portions, a portion of this ceramic is mixed at 17 with a metal powder such as steel, nickel, iron, etc., in a suitable proportion to provide both the desired electrical conductivity and the desired physical properties. The metal used in the mixture is determined by the firing temperature of the ceramic and the proportions are dependent upon mixing efficiency and relative particle size of the ceramic and metal. In the preferred mixture, the metal and ceramic are in the range of 2 to 50 microns in size. The cermet mixture and pure ceramic mixture are then brought together and pressed at 18 to form the unitary structure as shown in FIG. 1, or as shown in FIG. 2, depending upon the manner in which the lead wires are to be attached to the cermet ends. The pressing operation is fully disclosed below. If pure metal or metal alloy terminal portions are to be used, then the ceramic and metal powder mixing at step 17 in FIG. 3 is obviously unnecessary. Metal powder alone is used in the pressing operation 18 for the terminal portions.

As previously noted, pure metal or metal alloys may be used for the terminal portions. Whether cermet or metal end portions are used, the criteria in selecting the particular terminal portion is the same in that terminal portion material must be matched to certain physical characteristics of the ceramic center portion so that a suificiently strong bond between the center portion and terminal portions may be obtained. The terminal portion material must also be highly conductive (since it forms electrical terminals) and it should be capable of forming a good weld joint when lead Wires are welded thereto. I have found that, in the case of the cermet material, the amount of metal must exceed 50%, and preferably exceeds 65%, of the total weight of the cermet mix in order to obtain the necessary conductive and welding characteristics. Nickel, iron, and steel have been found satisfactory for use in the cermet terminal material. In the case of metal terminal portions, an alloy consisting of 45% to 55% by weight iron, 55% to 45% by weight nickel, and up to by weight molybdenum, has been found to produce satisfactory bonding strengths with ceramic when processed according to the procedures set forth in FIG. 3 of the drawing.

Following the pressing operation, the unit is placed upon or in a supporting refractory as at 19 and fired t0 the vitrification point in a reducing atmosphere. For example, the unit is set in a V-groove vat and fired in disassociated ammonia in the temperature range of 2300 F. to 2600" F. The firing produces a rod having a vitrified ceramic center portion with end portions of sintered metal-ceramic when a cermet mixture is used. The metal particles in the cermet end portions provide a continuous path for thermal and electrical conductivity and the ceramic present being vitrified holds the metal together, bonds the disc to the ceramic rod, and provides compatible coelficients of thermal expansion to the ceramic rod. When metal alloy end portions are formed, it is found that the metal is sintered to the ceramic center portion in such a manner that extremely high bonding strength is obtained. The unit is then cleaned and a metal flash such as gold or silver is placed over the metal ceramic terminal portions, if desired, by plating, painting, or other convenient techniques to prevent oxidation of the terminal portion during application of the resistive coating to the substrate. If the lead wires were not previously inserted in the end portions, they are now welded, brazed, soldered, or inserted into or against the end portions.

If the film resistor of FIGS. 1 and 2 is to be made, a layer of resistive material is now paint sprayed, vacuum deposited, or otherwise placed over the unit and given subsequent treatment as necessary to meet the resistor requirements for production and electrical properties. A preferred example of a resistive material which has been found to be particularly adapted to the disclosed substrate is fully disclosed in the pending application of Kee Hyong Kim, Ser. No. 286,202, filed June 7, 1963, entitled Resistor Manufacture, and assigned to the assignee of the present application. The preferred resistive coating disclosed in the above-identified application is an inorganic composition containing thallium oxide as the major conductive component dispersed in a glassy matrix. The preferred proportions of thallium oxide to matrix are thallium oxide 95% to 20% by weight and glassy matrix 5% to by weight. Other resistive coatings that may be used on the disclosed substrate are mixtures of glass and any one or any combination of the metal oxides, such as ruthenium oxide, silver oxide, tantalum oxide, titanium oxide, palladium oxide, and the above-mentioned thallium oxide. Any one of the compositions disclosed in United States Patent No. 3,052,573 may also be used as the resistive coating. It is obvious that many other resistive compositions will be equally useful as a coating for the disclosed substrate. If desired, the resistance of the component may be increased by spiraling or otherwise removing selected areas of the conductive coating to thereby increase the length of the electrical path between the conductive terminals.

The resistor is made into a finished product by applica tion of a plastic or other non-conductive jacket by any well-known molding process, such as by hot molding of epoxy resins, alkyd resin, or diallyl phthal'ate resin. In addition, epoxy resin or epoxy-silicon resin may be applied by conformal coating techniques.

The pressing apparatus necessary to carry out the pressing operation 18 of FIG. 3 and to produce the resistor of FIGS. 1 and 2 is illustrated in FIGS. 4 and 5. The apparatus consists of a die block 21 having a vertical die opening 14 therethrough in which the resistor will be molded. In the drawings, a single die opening is shown but in commercial practice the die block properly would be provided with a large number of symmetrically arranged die openings. The bottom plate 23 is provided with a stem 24 which fits into the die opening 22 with a close sliding fit. The apparatus also includes a top plate 21 and a pin 20 which also fits into the die opening 22 with a close sliding fit. In the molding operation with the stem 16 inserted part way into the die opening 14 as shown in FIG. 4, a measured quantity of metal or cermet powder 25 is poured into the die opening. On top of this powder is poured the correct amount of ceramic 26 and on top of the ceramic is placed an additional quantity of the metal or cermet powder 19. In accordance with conventional practice, the die block will be rapped or vibrated following the pouring in of each of these materials to insure settlement in uniform layers.

Next the pin 28 projecting from the top plate 29 is inserted in the die opening and hydraulic pressure is applied to the plates 23 to 29 by a press in a known manner. The pressure employed will be sutficient to provide a high density in relation to the fired density with the preferred range of pressure being 5 to 10 tons per square inch. Following the molding operation, the resistor will be ejected from the mold, for example, by the stem 16 which is long enough to push the molded resistor out of the die block. The relative position of the parts of the pressing apparatus are illustrated in FIG. 5, following the moment of press and just prior to the ejection of the resistor substrate.

If lead wires are to be embedded in the cermet portions of the substrate the wires may be inserted at the moment of press by any well-known method. For example, the pins of FIGS. 4 and 5 may be hollow to hold the lead wires in a manner as fully explained in United States Patent No. 3,013,240.

It will be understood that the invention herein disclosed may be variously mod'fied and embodied within the scope of the following claims.

We claim:

1. A fully vitrified electrical resistor comprising a substrate having a non-conductive ceramic center portion and conductive end portions formed from powders pressed and fired together into a vitrified unitary structure, and a resistive composition supported thereby to form an electrically continuous path from one conductive end portion of the substrate to the other, the said resistive composition comprising glass fired to vitrification after application to the prefired substrate and having finely divided conductive particles dispersed therein.

2. An electrical resistor according to claim 1 in which wire leads are welded to the metal containing conductive end portions of the substrate.

3. An electrical resistor according to claim 1 in which the finely divided conductive particles dispersed in the 25 glass are conductive metal oxide particles.

conductive particles and firing the so coated unit to vitrify the coating material and form an electrically conductive resistive coating comprising conductive particles in a glass matrix in an electrically continuous path from one conductive end portion of the said substrate to the other and to simultaneously bond the said coating to the said substrate.

References Cited UNITED STATES PATENTS 3,192,497 6/1965 Bender et al 338-331 3,220,097 11/1965 'Griest 338237 X 3,238,151 3/1966 Kim 2525l8 3,295,090 12/1966 Hay 338270 X RICHARD M. WOOD, Primary Examiner. I. G. SMITH, Assistant Examiner.