US 3304199 A
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1967 w. M. FABER, SR., ETAL ELECTRICAL RESISTANCE ELEMENT Filed Nov. l2, 1963 RUO AND/OR FINELY DIVIDED VEHICLE IRO GLASS PARTICLES RESISTANCE COMPOSITION APPLICATION ONTO PREPARATION OF SUBSTRATE FIXED RESISTANCE ELEMENT FIRING FIRING FIGURE FIGURE 2.
INVENTORS WILLIAM M. FABER SR.
GAYLORD L. FRANCIS CURTIS HOLMES- OTIS F. BOYKIN ATTORNEY United States Patent 3,304,199 ELECTRICAL RESISTANCE ELEMENT William M. Faber, Sr., Elkhart, Gaylord L. Francis,
Berne, and Curtis L. Holmes, South Bend, Ind., and Otis F. Boykin, Chicago, Ill., assignors to CTS Corporation,
Elkhart, Ind., a corporation of Indiana Filed Nov. 12, 1963, Ser. No. 322,702 11 Claims. (Cl. 117-201) This invention is a continuation-in-part of our copending application entitled Electrical Resistor and Method of Making the Same, Serial No. 169,355, filed on January 29, 1962, and now abandoned.
The present invention relates to electrical resistance elements and to resistance compositions and, more particularly, to an improved resistance composition for producing electrical resistance element of the ceramic type such as a fixed volumetric resistance element or as a thin film resistance element fired onto a surface of a base or substrate of high-temperature-resistant electrically nonconductive material.
It had generally been believed that the material employed to produce the conductive fraction of a ceramic resistance element should be metal in its chemically pure state, especially a metal highly resistant to oxidation. For example, Place et a1. (Patents Nos. 2,959,995 and 2,959,996) combined a selected metal or metals with a powdered glass frit, either by admixing the same in finely divided form, i.e., in a powdered state, or as larger particles, and grinding the mixture in a ball mill. The metal or metals employed in the mixture were referred to as being nonreactive and nonoxidizable. In an alternative procedure disclosed by Place et al., the powdered glass frit was mixed with the selected metal or metals in the form of soluble metal compounds which are decomposable by heat.
DAndrea Patent No. 2,924,540 also discloses a ceramic resistor composition and electrical resistors produced therefrom, the resistor composition comprising finely divided palladium metal, with or without the addition of silver, in combination with a vitreous enamel frit. Dumesnil Patent No. 3,052,573, however, points out that the DAndrea ceramic resistor composition was not altogether satisfactory because the resistance obtained was too critically tied to the maturing temperature of the enamel frit. According to Dumesnil, changing from one enamel frit to another produced widely divergent sheet resistances, inasmuch as different enamel frits have different maturing temperatures. To overcome this deficiency, Dumesnil substituted a mixture of palladium oxide or rhodium oxide for the finely divided pure metal used by DAndrea as the principal part of the conductive fraction of the composition.
Dumesnils use of palladium or rhodium oxide as a component of a resistor composition for a ceramic resistor in a sense contradicts certain deep-rooted and universally held convictions of the prior artwhich we too shared until our discovery that by using an oxide or ruthenium or iridium to produce the conductive fraction of the composition, we could produce, and reliably reproduce, ceramic resistance elements that are far superior in many respects-and especially in the higher resistance ranges to anything heretofore available, including those obtainable from the teachings of Dumesnil.
It is to be understood that the term sheet resistance, unless otherwise defined, shall denote the resistance in ohms per square of a film of material having a thickness of approximately .001 inch.
3,304,199 Patented Feb. 14, 1967 Although some manufacturers of ceramic resistance elements have previously represented that they could supply acceptable resistance elements in the very high resistance ranges by using a material having a very high sheet resistance, we have, however, never been able, following the teachings of the prior art, to produce uniformly and economically ceramic resistance elements in ranges above 30,000 ohms per square. Only by sufliciently elongating the resistance path formed with a material having a low resistance per square could ceramic resistance elements with high sheet resistance values be attained.
Recognizing this limitation in the prior art sheet resistance it is, therefore, an object of the present invention to provide a ceramic resistance element produced from ruthenium oxide and/ or iridium oxide which can be reliably and economically manufactured with a sheet resistance in ohms per square to at least 600% higher than the sheet resistance heretofore attainable.
Another disadvantage we observed in ceramic resistance elements heretofore available was a difiiculty in maintaining the temperature coefiicient of resistance at a low value, i.e., at approximately 01% per degree centigrade or less, throughout the entire resistance range and especially so in the resistance ranges below ohms per square and above 30,000 ohms per square. Where the conductive fraction employed to produce the resistance element was kept low in an efiort to increase the sheet resistance, the element exhibited a relatively high negative temperature coetficient, and where the conductive fraction was higher so as to produce a sheet resistance of less than 100 ohms per square, a relatively high positive temperature coefiicient resulted.
The temperature coefficient of resistance of a ceramic resistance element is an extremely important consideration in present day electronics. If the TCR (temperature coefiicient of resistance) is too high, inevitable changes in ambient temperature in many modern applications of electronic circuits could lead to serious consequences. Thus, if a resistance element with a TCR of 900 parts per million per degree centigrade were used in a circuit subjected to a 100 C. change in ambient temperature, the resistance thereof would change by a factor of 9 percent or 0.09% per degree centigrade. In some circuits such a TCR could not be tolerated. On the other hand, if a resistance element with a TCR of only one hundred parts per million per degree centigrade were used, the change in resistance for the same change in temperature would be only one percent or 0.01% per degree centigrade.
Heretofore, in order to maintain a low TCR for ceramic sheet resistance elements having a resistance above 100 ohms and below 30,000 ohms per square it was necessary to control several variables, e.g., the fineness of division of the metal oxide, the time and temperature of subsequent firing or heat treating periods, and the proportion of a metal oxide to a pure metal. When prior known materials were used for preparing sheet resistance elements having a resistance above 30,000 ohms per square, the TCR would be too high and generally uncontrollable. It would, therefore, be desirable to prepare a resistance composition for producing a ceramic resistance element having a low TCR, i.e., approximately 100 parts per million or 0.01% per degree centigrade, for ohmic values throughout the entire range of less than 100 ohms per square to at least 180,000 ohms per square.
Accordingly, another object of the present invention is to. provide a ceramic resistance element having a TCR of approximately 0.01% per degree centigrade for ohmic ranges below 100 ohms per square and up to and at least as high as 180,000 ohms per square. We have discovered that this can be done reliably and predictably by using a specified percentage of an oxide of ruthenium or iridium in composition for producing a ceramic resistance element.
Extensive tests have shown that there is something unique about the oxide of ruthenium or iridium for when another metal oxide is employed to produce ceramic resistance elements of the film or the fixed volumetric type the sheet resistance cannot be properly controlled. When a metal oxide other than ruthenium or iridium oxide is used, a reduction in the percentage of the metal oxide present in the vitreous material below a specified value in an effort to increase its sheet resistance, brings about a change in resistance so abrupt and unpredictable that reliable consistency of the end product is well nigh impossible, and the resistance elements produced therewith have a high negative TCR.
It has, therefore, been necessary that at least 8 to of a metal oxide be admixed with the vitreous material; otherwise there is very little control over the ohmic resistance of the finished ceramic resistance element. In accord with the prior art, whenever less than 8 to 10% of the metal oxide is employed, it is necessary to add a small percentage of a nonoxidizable metal, e.g., gold. Thus, combinations of certain metal oxides and metals do alleviate this objectionable situation to some extent, possibly because of the increased specific resistance which the combination possesses; but nothing heretofore accomplished in the ceramic resistance element art can compare with the results we have achieved by the use of an oxide of ruthenium or iridium to produce such resistance elements. For example, when iridium dioxide is used to produce ceramic resistance elements, we have found that by altering the percentage of iridium dioxide in the composition, sheet resistances at least as high as 180,000
ohms per square have been uniformly and economically reproduced. Since the previous resistance range did not exceed 30,000 ohms per square for ceramic resistance elements having a low TCR, we have been able to increase the resistivity by 600%. V
The only metal oxide, other than that of iridium which we have found at all suitable for high ohmic values, i.e., values in excess of 30,000 ohms per square, is an oxide of ruthenium. The similarity in crystalline structure of these two metal oxides, that is, both oxides have a rutile lattice, and the marked difference of the crystalline structure from that of the other metal oxides, e.g., rhodium or palladium oxide, heretofore used in the production of ceramic resistance elements to obtain precision resistance elements having a low TCR throughout a wide range no doubt explains why we have been able to achieve the superior results we have attained with oxides of these two metals. Since the oxides of ruthenium and iridium have substantially the same characteristics as concerns the purposes and objects of the present invention, it is to be understood that the oxide of ruthenium can be replaced wholly or in part by the oxide of iridium throughout the specification and claims.
In general, the present invention relates to a resistance composition and to a ceramic resistance element wherein an oxide of ruthenium and/or iridium oxide is used to produce a resistance path within the ohmic range of less than 100 ohms per square and up to and at least as high as 180,000 ohms per square having a TCR of generally .01% per degree centrigrade throughout the entire range.
For a better understanding 'of the present invention, reference may be had to the accompanying drawing wherein like reference numerals designate like parts and wherein:
FIGURE 1 is an operational diagram of one form of the method of this invention, employed for producing improved ceramic resistance elements;
FIGURE 2 is a grossly enlarged sectional view of a 4 ceramic resistance element made in accord with the present invention; and
"FIGURE 3 is a grossly enlarged sectional view of another embodiment of a ceramic resistance element of the present invention.
Referring to FIGURE 1, the resistance composition of the present invention for producing ceramic resistance elements comprises the selected metal oxide or oxides, and finely divided glass particles suspended in a vehicle, e.g., an organic screening agent, to form a paste which is applied onto a surface of a high-temperature-resistant electrically nonconductive substrate or base 11. Preferably, the surface of the base should be lapped and polished to make it as smooth as possible before the composition is applied, for as the surface becomes smoother the reproducibility of the electrical characteristics improves. Lapping and polishing the surface of the substrate is especially advantageous in the case of resistance elements for variable resistors and, for this purpose, it may also be desirable to polish the surface of the fired-on film.
Whenever a fixed volumetric resistance element is desired, such element is produced by coating a substrate, e.-g., a cylindrical substrate 21, or by adding an electrically nonconductive refractory material to the mixture for forming the resistance element into a desired shape as thoroughly disclosed in our above-menitoned co-pending applicationand connecting not shown terminals to opposite ends of the element in a suitable manner.
The formula for the glass frit used in the practice of the present invention may be any one of several ordinarily used in this art, an example thereof having a softening temperature of approximately 750 C. is as follows:
Percent PbO 63 B 0 25 SiO 12 The finely divided oxide of ruthenium or iridium oxide is mixed with the finely divided glass particles in the following proportions (percentages by weight):
The metal oxide-glass mixture is combined with a vehicle, such as ethyl cellulose dissolved in acetone-toluene if the mixture is being prepared for application onto a flat or cylindrical surface. Varying amounts of the vehicle may be used, depending upon the consistency desired. A ratio of about one part of the metal oxide-glass mixture to three or four parts of the vehicle forms a suitable resistance composition having the proper viscosity. The resistance composition is then deposited onto the surface of a substrate and fired at suitable temperatures ranging from 500 C. to 1000 C. to fuse the glass particles into a glass matrix, the temperature depending upon the melting point of the glass employed in the composition.
The desired resistance for the finished unit can be obtained by simply controlling the percent of the oxide of ruthenium and/ or iridium oxide and of the vehicle in the resistance com-position without bringing about an abrupt and unpredictable change in the sheet resistance, the percent of the vehicle generally determining the thickness of the fired ceramic resistance element unless the thickness of the unfired film is altered. I
The following table presents pertinent data of a number of different ceramic resistance elements made in accord with this invention, each element having a thickness in the range of about .0008 to .0011 which is approximately 5 equal to .001 inch. The permissible thickness of the film can range from about .0002 to about .003 inch.
Percent Sheet Re- TC R. Per- Example Percent Metal Oxide Glass sistance centl" C.
ohms/square G 65 0.01 82 81 0. 007 74 24.8 0.007 80 1. 300 0. 007 80 2, 860 0. 002 2 s0 5. 700 0. 006 1. -RuOz/4.1-IrO 94.3 6,1 0 (.008 91 i9, 000 O. 003 93. 2 36, 000 0. 0 90 49. 200 0. 004 90 72. 400 0. 007 95. 5 128. 000 0. 005 95. 5 181, 000 0. (it
When 110 is substituted for RuO ceramic sheet resistance elements having a higher resistance are produced, for instance, Examples 6 and 13 have the same percent of metal oxide as Examples 5 and 12 respectively, however, the resistance per square is substantially higher. Other values of sheet resistance can be obtained by altering the percent of the vehicle in the composition, i.e., if the per cent of the vehicle is increased without changing the thickness of the unfired film, the thickness of the fired film is decreased. Such changes account for the increase in sheet resistance for Examples 5 and 11 when compared to Examples 4 and respectively. Other sheet resistance values may be obtained by combining various percentages of R110 and IrO as well as altering the percent of the vehicle as in Examples 7 and 9.
In order to control the stability of the ceramic resistance elements, small percentages of cupric oxide and/ or manganese oxide are preferably dissolved in the glass as thoroughly disclosed in the Holmes c-opending application entitled Precision Resistance Element, Serial No. 276,064, filed on April 26, 1963, and now abandoned, and assigned to the same assignee as the present invention.
An example of the production of a complete ceramic resistance element is described below. Twenty grams of RuO (particle size less than 325 mesh) and 80 grams of powdered glass frit (particle size less than 325 mesh, lead borosilicate frit 63% PhD, 25% B 0 and 12% SlOg) are mixed with a liquid, e.g., water, to form a slurry which is ground in a ball mill for two hours. The slurry after ball milling is of a homogeneous consistency. The liquid is evaporated and the dry mixture of powdered glass frit and RuO is combined with a vehicle, for example, a solution of ethyl cellulose in acetone-toluene. About 400 grams of the vehicle are mixed with 100 grams of the dry mixture of powdered glass frit and RuO to form a resistance composition which is mixed by suitable means until a homogeneous suspension is obtained. The composition is applied onto a surface of a ceramic substrate forming a layer about 0.003 inch in thickness on the ceramic substrate. The substrate with the layer of powdered RuO powdered glass frit, and screening agent is then fired in an oxidizing atmosphere causing the orga-nic materials to be driven off. As a result, the glass matrix containing the conductive particles is fused to the surface of the substrate.
The powdered glass frit forming a component of the ceramic resistance element can be made of any suitable glass or vitreous material having a softening point below the temperature at which the base or substrate deforms. For instance, it can be a borosilicate frit, lead borosilicate frit, cadmium, barium, calcium, or other frit having the proper fusion temperature and expansion coetficient. The preparation of such frits is well known and consists, e.g., of melting together boric oxide, silicon dioxide and lead oxide, cadmium oxide, or barium oxide and pouring such molten composition into water to form the glass frit. The batch ingredients may, of course, be any compound that will yield the desired oxides under the fusing conditions of frit production, i.e., boric oxide will be obtained from boric acid, silicon dioxide will be produced from fiint, lead oxide will be produced from red lead or white lead, barium oxide will be produced from barium carbonate, etc. The coarse glass frit is preferably milled for 2 to 20 hours, e.g., in a ball mill with water.
Instead of homogenizing a mixture of glass frit, an oxide of ruthenium and/or iridium oxide, and a vehicle to produce a suspension which is suitable for forming a fixed volumetric or thin film ceramic resistance element, organo metallic compounds of ho RuO or mixtures of the two may be employed in conjunction with organo metallic compounds of frit forming materials to produce ceramic resistance elements.
The vehicle can consist of most any of the wellknown organic compounds which are capable of being completely volatilized or decomposed by heat. Preferably the vehicle should be able to keep the finely divided glass and metal oxide particles in suspension after the mixture has been deposited onto the base. One such mixture of organic compounds which will maintain the particles in suspension is ethyl cellulose dissolved in a trichloroethylenefenchone solution.
While there has been illustrated and described what is at present considered to be a preferred embodiment of the present invention and a method of making the same and a single modification thereof, it will be appreciated that numerous changes and modifications are likely to occur to those skilled in the art, and it is intended in the appended claims to cover all those changes and the modifications which fall within the true spirit and scope of the present invention.
The invention claimed is:
1. A composition adapted to be applied onto a hightemperature-resistant, electrically nonconductive substrate and fired to form an electrical resistance element comprising 2-70 percent by weight of a finely divided metal oxide selected from the group consisting of RnO and IrO and 98-30 percent by weight powdered glass frit.
2. A composition adapted to be applied to and fired on a high-temperature-resistant, electrically nonconductive substrate to form an electrical resistance element comprising 4-50 percent by weight of a finely divided metal oxide selected from the group consisting of RuO and IrO and 96-50 percent by weight powdered glass frit.
3. An electrical resistance element of the type wherein the resistance path is a film of glass having a conductor in a fine state of subdivision dispersed therein as an ingredient, fired onto a base of high-temperature-resistant, electrically non-conductive material, wherein the conductor is an oxide of a metal selected from the group consisting of Ru and Ir.
4. A resistance element com-prising a high-temperatureresistant electrically nonconductive substrate having fired thereon a film of resistance material comprising a solid-ified glass, and a finely divided metal oxide taken from the group consisting of R and IrO dispersed as an ingredient throughout the solidified glass in electrically conductive relationship.
5. A resistance element comprising a high-temperatureresistant, electrically nonconductive base having fired thereon a film of resistance material comprising 98-30 percent by weight of solidified glass, and 2-70 percent by weight of at least one oxide of a metal selected from the group consisting of Ru and Ir in finely divided form dispersed throughout the solidified glass in electrically conductive relationship.
6. A resistance element comprising an electrically nonconductive base having disposed on the surface thereof a composition comprising between 2-70 percent by weight of at least one oxide of a metal selected from the group consisting of Ru and Ir in finely divided form dispersed in a glass matrix.
7. A resistance element comprising an electrically nonconductive base having disposed on the surface thereof, a film comprising between 2-70 percent by weight of at least one oxide of a metal selected from the group consisting of Ru and Ir in finely divided form dispersed in a glass matrix, the thickness of the film being in the range of .0O0'2-.003 inch..
8. A resistance elementcomprising a high-temperatureresistant, electrically nonconductive base having fired thereon a film of resistance material comprising 9830 percent by weight of solidified glass, and 2-70 percent by weight of an oxide of Ru in finely divided form dispersed throughout the solidified glass.
9. A resistance element comprising a high-temperatureresistant, electrically nonconducting base having fired thereon a film of resistance material comprising 98-30 percent by weight of solidified glass, and 2-70 percent by weight of an oxide of Ir in finely divided form dispersed throughout the solidified glass.
10. A resistance element comprising a high-temperature-resistant, electrically nonconductive base having fired thereon a film of resistance material comprising 98-30 percent by Weight of solidified glass, and 27() percent by weight of an oxide of Ru in finely divided form dispersed throughout the solidified glass, the film having a thickness in the range of 0002-003 inch.
11. A resistance element comprising a high-temperature-resistant, electrically nonconductive base having fired thereon a film of resistance material comprising 98-30 percent by weight of solidified glass, and 270 percent by Weight of an oxide of Ir in finely divided form dispersed throughout the solidified glass, the film having a thickness in the range of 0002-.003 inch.
References Cited by the Examiner UNITED STATES PATENTS 3,052,573 9/1962 Dumesnil 117-221 3,149,002 9/1964 Place et al ll7-227 ALFRED L. LEAVITT, Primary Examiner.
RICHARD D. NEVIUS, Examiner.
W. L. JARVIS, Assistant Examiner.