US3479216A - Cermet resistance element - Google Patents

Description

Nair. 18,1969 w. E. COUNTS ET AL 3,479,216

CERMET RESISTANCE ELEMENT Filed Nov. 4, 1964 IN VEN TORS DONALD A. BRUHL' JR.

WILLIAM E. COUNTS ATTORNEY sistance elements constructed therefrom.

.. In brief, this a cermet mixture formedof glass and an alloy of iridium United States Patent U.S. Cl.'117227 1 Claims ABSTRACT OF THE DISCLOSURE A cermet resistance element formed of a non-conductive base member having deposited thereon a single layer of resistance material formed of 50 to 95% by weight of solidified glass and 5 to 50% by weight of a conductive constituent comprising iridium and atleast one of the metals selected from the group consisting of platinum,

silver, ruthenium, rhodium and palladium, the conductive metal constituent being in finely divided particulate form and uniformly dispersed in electrically conductive relationship throughout the solidified glass,

' The present invention relates to an improvedelectrical resistance material formed of a glass-metal mixture of the type commonly called cermet material, and to re- Cermet resistance materials and elements presently known in the art are exemplified by US. Patents 2,950,-

995 of Thomas M. Place,Sr., et-al., entitled Electrical Resistance Element and 2,950,996 of Thomas M. Place,

by a layer of resistance material comprising a heterogeneous mixture of non-conducting-material and conducting metalsfired toja non-conducting base. The non-con- ,du-cting material is a ceramic type material such as glass and the layer is formed by heating the'metal-glass mixture at least'to the'melting point'of' the glass, so as to create a: smooth," glassy phase. Additional 'prior art directed toward resistors 'formed of a glass-metal" composition are U.S.' Patent No. 2,837,487of Daniel E; Huttar, en-

titled Resistor Enamel and: Resistor Made Therefrom, and U. S. Patent No. 2,924,540 of James B. DAndrea, entitled Ceramic Composition and Article.

It is an object. of thisinvention vto..provide a.,.cermet resistance materialforproducing resistance elements :hav-

sing increasedresistivities.in proportion. to the relatively --large amount of metal, contained therein as compared to knowntypes of flmetal-glass resistance materials.

It is another objectof this invention to provide an improved cermet material and an improved resistance element rn'ade, therefrom having a high power rating, a low noise output, and a low temperature coefiicient of resistivity. p I

A further object of this invention is to provide an im- -sproved cerme t resistance element which has extremely :stable electrical characteristics, e.g., its resistivity and temperature coefiicient of resistivity do not vary substantially over long periods'of time, either when the element is connected .in a circuit or when it is stored in a non-operative state.

' Other and further objects, features and advantages of the invention will become apparent as the description proceeds.

invention relates to the discovery that and at least one of the other noble metals selected from the group consisting of platinum, silver, ruthenium, rho dium and palladium provides a resistance material having a high resistivity. The amount of resistance obtained is extremely high for the relatively large amounts of iridium alloy metal employed in the mixture thereby producing resistance elements of high metal content capable of performance at high power ratings as compared to known types of glass-metal resistance materials.

A more thorough understanding of the invention may be obtained by a study of the following detailed description taken in connection with the accompanying drawing in which:

FIG. 1 is an isometric view of an embodiment of the invention which is suitable for use in rotary potentiometers; and

FIG. 2 is an isometric view of another embodiment of the invention which is suitable for use in linear potentiometers as well as for fixed resistors.

' In the structure of FIG. 1, a layer 10 of resistance material is fired to a base 11, the electrodes 12, 13 being provided at each end of the layer 10 for connecting the fired element into an electrical circuit. This resistance element may be used as a fixed resistor or may be combined with a rotating contact arm for use as a rotary rheostat or potentiometer. The base 11 may be of any suitable electrically non-conducting material which will withstand the elevated temperatures normally used to fire the resistance material. Various ceramic materials are suitable for this use, those having a smooth, fine textured surface and being impervious to moisture and other liquids being preferred. Steatite, Forsterite, sintered or fused aluminas and zircon porcelains are examples of preferred materials for forming the base 11.

The electrically conductive electrodes 12, 13 are conventional and may be formed by applying any of the well-known conducting silver or other metal pastes over or under the layer of resistance material and firing the unit to convert the paste to a layer of metal which is firmly attached to the layer of resistance material. Alternatively, terminal structures such as are shown in the US. Patent 3,134,085 of Kenneth F. Miller et al., entitled Variable Resistor With Terminal Structure, also assigned to Beckman Instruments, Inc., assignee of the present invention, may be employed for making electrical contact with the resistance layer 10.

FIG. 2 illustrates another form of the resistance element of the invention in which a layer 15 of resistance material is applied to a rectangular base 16 and electrodes 17, 18 are then added at-the ends of the layer 15.

It will be understood that the elements'illustrated in FIGS. land 2 are enlarged and that, tn practice,the resistance layers formed on the substrate members are approximately .0005 to .005 inch in thickness-The particular configurations of the resistanc layer and the substrate members can vary and are not limited to the arrangements disclosed in FIGS. 1 and 2. The form of the invention illustrated are, however, particularly suitable for use in ltnear variable'resistors and potentiometers.

The aforementioned US. Patents Nos. 2,950,995 and 2,950,996 teach methods of preparing the cermet resistance layer 10. A preferred method taught therein comprises mixing the resinates or organic compounds of one or more noble metals. The glass binder, in the form of very small glass particles, is mixed or milled with the resinate solution so that each glass particle is thoroughly wetted with the metal solution. This mixture is gradually heated and constantly stirred to remove the volatiles and organic materials from the mixture and to decompose the noble metal compounds. The resulting dry material is ground to a fine powder and calcined for a short period to assure removal of all organic materials. The resulting calcine is ground to a fine powder, producing a dr material consisting of very small glass particles mixed with extremely small particles of metal.

The mixtures formed by the method described above may be stored indefinitely and may be used in small portions to produce unlimited numbers of resistance elements. When it is desired to make resistance elements using the material, the dry powder is mixed with a suitable liquid carrier to form a fluid composition which can be applied to the base material. The base with the layer applied thereto is then fired to drive off the volatiles and fuse the mixture into a continuous phase of solidified glass.

It has been discovered that high resistance values can be obtained in cermet resistance elements formed of mixtures of finely divided particles of glass and alloy of iridium combined with one or more of the noble metals perferably selected from the group consisting of palladium, platinum, silver rhodium, and ruthenium. In this material, the metal content in the form of an iridium alloy comprises from about 5 to 50% by weight of the mixture and glass or ceramic material comprises about 50% to 95% by weight of the mixture. The following are examples of iridium alloys in cermet mixtures formed with iridium and metals taken from the group consisting of silver, palladium, platinum, ruthenium and rhodium.

EXAMPLE A Iridium-silver, 5% metals by weight Percent Glass 95 Iridium 2.5 Silver 2.5

The resistivity of a resistance layer of approximately .001 inch thickness formed from this material was 140,000 ohms/square and its temperature coeffcient of resistivity was +195 p.p.m./ C.

EXAMPLE B Iridium-silver, metals by weight Percent Glass 90 Iridium 5.0 Silver 5.0

The resistivity of a resistance layer of approximately .001 inch in thickness formed from this material was 50,000 ohms/square and its temperatur coefiicient of resistivity was +86 p.p.m./ C.

EXAMPLE C Iridium-silver, 20% metals by weight Glass percent 80 Iridium do 18 Silver do 2 Resistance ohms/square" 3920 Tempco p.p.m./ C 160 EXAMPLE D Iridium-silver, 30% metals by weight Glass percent 70 Iridium do Silver do 15 Resistance ohms/square" 3000 Tempco p.p.m./ C +58.7

EXAMPLE E Iridium-silver, 40% metals by weight Glass percent 60. Iridium do 20 Silver do 20 Resistance ohms/square 1500 4 EXAMPLE F Iridium-silver, 50% metals by weight Glass percent 50 Iridium do 25 Silver do 25 Resistance ohms./square 600 Tempco p.p.m./ C +227 EXAMPLE G Iridium-palladium, 5% metals by weight Glass percent 95 Iridium d0 4.17 Palladium do .83 Resistance ohms./square 300,000 Tempco p.p.m./ C 382 EXAMPLE H Iridium-palladium, 10% metals by weight Glass percent 90 Iridium do 2.5 Palladium do 7.5 Resistance ohms/square 168,600 Tempco p.p.m./ C 426 EXAMPLE I Iridium-palladium, 20% metals by weight Glass percent-.. Iridium do 5 Palladium d0 15 Resistance ohms./square 25,400 Tempco p.p.m./ C 42.3

EXAMPLE I Iridium-palladium, 50% metals by weight Glass percent 50 Iridium do 46.15 Palladium d0 3.85 Resistance "ohms/square" 130 Tempco p.p.m./C +330 EXAMPLE K Iridium-platinum, 10% metals by weight Glass percent Iridium do 6.67 Platinum d0 3.33 Resistance ohms/square 77,300 Tempco p.p.m./ C 386 EXAMPLE L Iridium-platinum, 20% metals by weight Glass percent 80 Iridium do 16 Platinum do 4 Resistance ohms/square 1,590 Tempco p.p.m./C +102 EXAMPLE M Iridium-platinum, 50% metals by weight Glass "percent" 50 Iridium do 46.15 Platinum do 3.85 Resistance "ohms/square" 200 Tempco p.p.m./C- +303 EXAMPLE N Iridium-ruthenium, 5% metals by weight Glass percent Iridium do 4.5 Ruthenium do .5 Resistance ohms/square 17,000

Tempco p.p.m./ C +147 75 Tempco p.p.m./C 256 EXAMPLE 0 Iridium-ruthenium, 30% metals by weight Glass Q percent 70 Iridium do a Ruthenium do Resistance ohms/square 38.4 Tempco Y p.p.m./C +286 1' EXAMPLE P Iridium-ruthenium, 40% metals by'weight Glass percent 60 Iridium do 10 Ruthenium do Resistance ..ohms/square 12.5 Tempco p.p.m./-C +464 EXAMPLE Q Iridium-ruthenium, 50% metals by weight Glass percent; 50 Iridium dO 46.15 Ruthenium do 3.85 Resistance "ohms/square 100 'Tempco L p.p.m./C +229 EXAMPLE R Iridiumrhodium, 5% metals by weight Glass percent 95 Iridium do 4.5 Rhodium do 0.5 Resistance L ohms/square 160,000 Tempco p.p.m./C 400 EXAMPLE s I Iridium-rhodium, 25% metals by weight Glass percent-.. 75 Iridium do 9.51 Rhodium do 'v 15.59 Resistance ohms/square" 400 Tempco p.p.m./C +77 EXAMPLE T Iridium rhodium, 50% metals by weight Glass i percent 50 -Iridium- I do 16.30 Rhodium do 33.70 (Resistance ohms/square 102 Tempco p.p.m./C -105 The particular composition of the glass utilized is not critical to the practice of the invention except that it must have a melting temperature below that of the metal constituents of the mixture, Four illustrative examples are as followsz.

. The glass may be produced by any conventional process; it is preferred, however, that it be as homogeneous as possible. One method of making a glass includes thoroughly mixing a batch of raw materials together while 1 dry, melting thebatch in ceramic crucibles to produce a clear fluid glass, quenching the molten glass by pouring into cold water, drying the resulting shattered glass and then grinding it to a very fine powder.

' Combinations of these glasses and other resistance materials using an alloy of iridium with more than one of the other specified metals have been employed. Examples of these are:

EXAMPLE U Iridium-ruthenium-rhodium, 5% metals by weight Glass No. 1 percent. 47.5 ,Glass- No. 2 do 47.5 Iridium d0 1 Ruthenium do 3 Rhodium 1 Resistance ohms/square 14,000 Tempco p.p.m./C 400 EXAMPLE V Iridium-ruthenium-rhodium, 30% metals by weight Glass No. 1 percent 35 Glass No. 2 ..do 35 Iridium do 2 Ruthenium I do"-.. 16 'Rhodium do 12 Resistance ohm s/square Tempco p.p.m./C +43.7 EXAMPLEW Iridium-ruthenium-rhodium, 50% metals by weight "Glass No. 1 "percent" 25 Glass No. 2 do 25 "Iridium do 10 :Rutheniurn do 30 Rhodium do 10 Resistance ohms/square.. 10

.Te mpco, approximately 300 p.p.m./ C.

Although it is not completely understood why th presence of. iridium, in an alloy with the metals silver,

platinum, palladium, ruthenium and rhodium, inhibits the tendency for such materials to agglomerate or form globules in the fused glass binder, it appears that iridium contributes to the interaction between the respective surfaces of the finely divided metal and glass particles and prevents agglomeration of the metal particles even though they are present in relatively large quantities. This apparently causes the mixture to retain its homogeneity during firing: and results in a more stable resistance element having substantially larger quantities of metal. It is known, for example,,that an all-0y of gold, palladium and silver, when utilized in a glass-metal (cermet) resistance mixture,, in which the gold, palladium, silver content is about 11.5% of the mixture, produces a resistance element having a resistance of approximately 55 ohms per square for a layer approximately .001 inch in thickness. Compare this with an iridium-silver alloy using 20% metal content. Such a cermet material produces a resistance element (of like thickness) having a resistance of 3920 ohms per square, which is over 71 times more resistance using almost twice as much metal in the resistance element. A cermet using iridium-silver alloy at 40% metal content produces a resistance of 1500 ohms per square and at 50% metal content the iridiumsilver alloy still produces a resistance vof over 600 ohms per square. It will be noted that a cermet mixture using the alloy iridium-ruthenium-rhodium at 50% metal content producesa resistance of 10 ohms per square, and at 30% metal content (which is approximately three times that of the above-mentioned cermet using an alloy of gold, palladium and silver) still produces a resistance of 80 ohms per square which are extremely high resistances for the large quantities of metal utilized in the cermet element.

The significance of the above becomes more apparent when it is understood that the ability of a resistor to dissipate heat and thereby to withstand higher power levels depends to a large extent upon the metal content of the device. The greater the metal content of'the resistor, the more heat it can dissipate, permitting application of much greater power to the device. The metal content of cermet mixtures containing alloys of iridium and the'abovementioned metals is in the order of 5 to times that of glass-metal resistors heretofore employed in the art for comparable resistances, thereby permitting an extremely broad range of power applications.

The high metal content of cermet resistance elements using these iridium alloys is extremely-important in another respect. One of the major problems encountered when using cermet resistors as variable resistors or poten-tiometers is the high electrical noise characteristics of such devices due to the contact made between the movable wiper of such devices and the resistance element. It is believed that this electrical noise is at least in part due to the somewhat sporadic contact made by the wiper and the metal particles on the surface of the element. Due to the vast increase in the metal content of cermet elements containing iridium alloys for high resistance elements, it will be understood that there are a substantially greater number of metal particles available for contact with the wiper. This greatly reduces the electrical noise" of such elements and makes their application to variable resistance devices and potentiometers extremely advantageous.

Another extremely advantageous improvement attained with cermet resistance materials containing these iridium alloys is in the manner in which they can be fired to a nonconductive base member. With cermet materials formed of the above specified iridium metal alloys in the ranges 5% to 50% metal content, it is possible to deposit the material on a base member, place it in a furnace which is at the fusion temperature of the glass, and, after a short firing period, remove the elements and cool them at room temperature. This quick fire method makes it possible to produce resistance elements in a period of a few minutes, which contrasts with the much longer cycles of previous films.

In most instances the iridium alloy cermet resistance elements have a very smooth surface, which is important when such elements are used in variable resistance devices and Potentiometers. The iridium alloy materials do not agglomerate or produce blisters on the surface of the resistance elements during the firing operation. These materials produce extremely stable resistance elements that can be produced on a batch to batch basis with uniform resistance values and with temperature coefficients of resistance that are well within the range i500 p.p.m./ C. that is considered essential for application of such resistance elements to otentiometers variableresistance devices.

These high metal conten-telements have been found to have a substantially increased stability. That is, elements so constructed may be stored or used for very long periods of time, either in or out of a circuit, without substantially changing their resistivity, temperature coefiicient of resistivity, noise and resolution characteristics. These resistance materials have also been found to have very improved thermal stability, i.e., they may be heated and subsequently cooled without effecting a permanent change in their electrical characteristics.

What is claimed is:

1. A cermet resistance element characterized by a resistivity in the range of 10 to 700,000 ohms/square and a temperature coefficient of resistivity of less than i500 p.p.m./ C. comprising a high-temperature resistant, electrically non-conductive base having fired thereto a single layer of cermet resistance material having a thickness of .0005 to .005 inch comprising about 50 to by weight of solidified glass and over 5 to 50% by weight of an alloy formed of iridium and one or more of the metals selected from the group consisting of silver, ruthenium, rhodium, platinum and palladium, said alloy being in finely divided form and uniformly dispersed in electrically conductive relationship throughout said solidified glass.

2. A cermet resistance element comprising a high temperature resistant, electrically non-conductive base having fired thereto asingle layer of glassmetal mixture having a thickness of .0005 to .005 inch characterized by a resistivity of greater than 2000 ohms/square when the ratio of glass to metal content by weight is 20 to 1 and a resistivity of greater than 10 ohms/square when the ratio of glass to metal content by weight is 1 to 1 comprising about 50 to 95 by weight of solidified glass and over 5 to 50% by Weight of an alloy formed of iridium and one or more of the metals selected from the group consisting of silver, ruthenium, rhodium, platinum and palladium, said alloy being in finely divided form and uniformly dispersed in electrically conductive relationship throughout said solidified glass.

3. A cermet resistance element comprising a high temperature resistant, electrically non-conductive base having fired thereto a single layer of glass metal mixture having a thickness of .0005 to .005 inch characterized by a resistivity of greater than 2000 ohms/square when the ratio of glass to metal content by weight is 20 to 1 and a resistivity of greater than 10 ohms/square when the ratio of glass to metal content by weight is l to 1 and a tempera-. ture coefiicient of resistivity of less than :500 p.p.m./ C. comprising about 50 to 95 by weight of solidified glass and over 5 to 50% by Weight of an alloy consisting essentially of iridium and silver, said alloy being in finely divided form and uniformly dispersed in electrically conductive relationship throughout said solidified glass.

4. A cermet resistance element comprising r a high temperature resistant, electrically non-conductive base having fired thereto a single layer of glassmetal mixture having a thickness of .0005 to .005 inch and characterized by a resistivity of greater than 2000 ohms/square when the ratio of glass tometal content by weight is 20 to 1 and a resistivity-of greater than 10 ohms/square when the ratioofglass -to metal content by weight is 1 to 1 and a temperature coefficient of resistivity of less than $500 p.p.m./ C. comprising l about 50 to 95% by weight of solidified glass and over 5 to 50% by weight of an alloy consisting essen- 10 content by welght 1s 20 to 1 and a resistivity of greater than 10 ohms/ square when the ratio of glass to metal content by weight is l to 1 and a temperature coefficient of resistivity of less than 1-500 p.p.m./ C. comprising about 50 to 95% by weight of solidified glass and over 5 to 50% by weight of an alloy consisting essentially of iridium and palladium, said alloy being in finely divided form and uniformly dispersed in electrically conductive relationship throughout said solidified glass.

6. A cermet resistance element comprising a high temperature resistant, electrically non-conductive base having fired thereto a single layer of glassmetal mixture having a thickness of .0005 to .005 inch characterized by a resistivity of greater than 2000 ohms/square when the ratio of glass to metal content by weight is 20 to 1 and a resistivity of greater than ohms/ square when the ratio of glass to metal content 'by weight is l to 1 and a temperature coefiicient of resistivity of less than :500 p.p.m./ C. comprising about 50 to 95 by weight of solidified glass and over 5 to 50% by weight of an alloy consisting essentially of iridium, ruthenium and rhodium, said alloy being in finely divided form and uniformly dispersed in electrically conductive relationship throughout said solidified glass.

7. A cermet resistance element characterized by a resistivity in the range of 10 to 700,000 ohms/square and a temperature coeflicient of resistivity of less than 1500 p.p.m. C. comprising a high-temperature resistant, electrically non-conductive base having fired thereto a single layer of cermet resistance material having a thickness of .0005 to .005 inch and comprising about 5-0 to 95% by weight of solidified glass and over 5 to by weight of an alloy consisting essentially of iridium and platinum, said alloy being in finely divided form and uniformly dispersed in electrically conductive relationship throughout said solidified glass.

References Cited UNITED STATES PATENTS 2,950,996 8/1960 Place et a1 1l7-227 3,052,573 r 9/ 1962 'Dumesnil 117-227 XR 3,079,282 2/1963 Haller et al. l17227 3,207,706 9/1965 Hoffman 117-227 X 3,271,193 9/1966 'Boykin 252514 XR 3,326,720 6/1967 Bruhl et al. l17160 XR 3,329,526 7/1967 Daily et al. 117227 WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R.

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