US3551355A - Resistor composition and element - Google Patents

Description

United States Patent 3,551,355 RESISTOR COMPOSITION AND ELEMENT Jason D. Provance, Glendora, Califi, assignor to Bourns, Inc., a corporation of California No Drawing. Filed July 12, 1967, Ser. No. 652,674 Int. Cl. H01b 1/06; C03c 3/04 US. Cl. 252514 12 Claims ABSTRACT OF THE DISCLOSURE An electrical resistance composition and element of the cermet type for a potentiometer or variable resistor, characterized by low resistivity and low metal content and low contact-resistance variation, the composition comprising as a glassy matrix the product of a glassmaking operation in which a nucleating agent is included with constituents of a lead borosilicate glass, the product thus comprising numerous crystallites, the composition further comprising within the glassy matrix a dispersion of metal selected from among the platinum family, the composition being adapted to form a film characterized by low resistivity of the order of 100 ohms per square (sheet resistivity, e.g., a film one inch square and one mil thick) and a low metal content of the order of less than 7% by weight, and low contact resistance variation when fired.

Herein for convenience and in the interest of brevity, the term glass is used to denote a product of a glassmaking operation wherein there is included with glassmaking constituents a nucleating agent or component whereby the product contains numerous crystallites and hence is not amorphous or vitreous and thus in a strict sense is not a true glass.

BACKGROUND OF THE INVENTION In the field of potentiometer resistance elements not using wire as a resistive material, it is widely known to use an insulative ceramic base to support a wear-resistant glassy composition film or layer of electrically resistive material and which film or layer is formed from an applied coat of liquid carrier carrying finely-divided glass comprising conductive metal, the film or layer being fired in situ to provide a hard resistive layer firmly bonded to the insulative base. In all types of potentiometers and variable resistors, it is desirable that the contact resistance (CR), that is, the electrical resistance exhibited between the movable contact and the element or film, be a minimum and, more importantly, that the contact resistance variation (CRV) be a minimum as the contact is moved along the film or element. In general in the prior art, low-resistance elements could be obtained concurrently characterized by low values of CRV, only when the metal content of the layer or film was increased to above about 7% (by weight) of the film to provide the lower resistivity necessary in the lower-resistance ranges. However, the large amount of metal thus introduced into the film invariably caused the surfaces of such lower-resistance range elements to be highly abrasive, with resultant excessive wear of the wiper or contact. The present invention provides resistor elements whose resistivities are Within the lower-resistance ranges desired, using low-metal-content material, whereby there are retained the desirable low contact-resistance variation and non-abrasive surface characteristics of the low-metalcontent higher-resistivity range film-elements previously attainable. This the invention accomplishes by utilizing a low-metal-content mixture of a finely-divided glassy material formed by a glassmaking procedure in which a nucleating agent or agents is included with the oxides "ice used in the glassmaking process, and metal-containing material, the two components being very finely divided and thoroughly intermixed and applied as a suspended component of a liquid-mix (by such procedures as silkscreening, roller coating, dipping, or the like), followed by subsequent firing to remove organic and other volatile materials and to form the adhering film with a desired matte surface. The glassy material produced by nucleation of the glass forming oxides into a crystallitecontaining glass is no longer a glass in the strict sense, since it contains innumerable crystallites, and the crystallite structure is at least to a considerable extent responsible for the aforementioned lower resistance obtained with metal contents of less than about 7% and is retained in the finally produced film as a matrix in and on which metal is incorporated.

Prior art procedures that probably most closely approach those used in the production of the presentlyclaimed resistance elements are delineated in U.S. patents to Place, Sr., et al., No. 3,149,002, and to Hofiman, No. 3,207,706. In the former, use is made of nonconducting material such as glass interspersed with metal to provide the high resistance values that are otherwise attainable only with metal films that are too thin to successfully withstand the wearing effect of the wiper during repetitive operation of the latter. Thus means and methods are therein disclosed for making satisfactory small-size resistance elements having total resistance values so high as to be not attainable with wire-wound elements. However use of the techniques and materials therein taught, for making elements of low values of total resistance (TR), results in the aforenoted highly abrasive surface which causes excessive wiper wear and which surface is characterized by excessive CRV. In the process set out in Pat. No. 3,207,706, glass frit containing aluminum oxide (A1 0 is mixed with metal particles of silver or palladium to provide a resistor which has a lower value of temperature coeflicient of resistivity (TCR) than the previously known vitreous resistance elements. In that process as well, the low values of resistivity required for resistors of low ohmic value are obtained only by increase of the metal content to those high values (in excess of about 7%) that result in the noted highly abrasive surface characterized by undesirably high CRV.

In accord with the present invention, an insulative base such as one of alumina or the like is used, with metalfilm terminals at the ends to receive or connect with the resistive film or layer. The film is formed as the product of a fired composition or mixture applied in paste-like form by one of the previously mentioned procedures such as silk screening, the applied mixture being one comprising a finely-divided powder of the product produced by fritting the product produced by nucleating the components of a lead borosilicate glass, and the other a metallic salt, both dispersed together in a liquid vehicle. The object of the procedure is to secure glassy films of low resistivity with low metal content and characterized by low CRV. By glassy is meant having appearance and some other characteristics of glass, but not being an amorphous structure. That object is attained by producing by chemical reduction in situ enough molecules of metal from one of the glass-forming materials (as, for example, by reduction of a fractional portion of PhD molecules to metallic lead) along edges and/ or interfaces of the crystallites, whereby a multiplicity of conductive paths are formed, together with reduction of the added metallic salt to conductive metal along crystallite boundaries and interspersed throughout the remaining glassy material or crystallites, to provide multiplicities of conductive paths through the length of the film, The noted desired result may in most instances be attained by a combination of the two phenomena, the conductive paths being both through the crystallites and via the intercrystallite material. Since the percentage of metal content in the improved film is at least as low as that in the somewhat similar resistance elements of far higher resistivity disclosed in the aforemention d prior art which are characterized by low CRV and non-abrasive surface, the elements thus made according to the invention are characterized by low resisitivity, low metal content and low CRV, and are similarly non-abrasive.

The preceding brief general description of the invention makes it evident that it is a principal object of the invention to provide improvements in resistive compositions for film type resistance elements.

Another object of the invention is to provide improvements in processes for producing electrical resistance devices.

Another object of the invention is to provide improvements in the manufacture of glassy crystallite materials.

Another object of the invention is to provide an improved material for use in the manufacture of resistors.

Another object of the invention is to provide a cermet resistance element characterized by a resistive film of very low resistivity and metal content less than five percent by weight of the film.

Another object of the invention is to provide a cermet resistance element characterized by very low film resistivity of less than a maximum value of about 500 ohms per square and low CRV of the order of 1% or less.

Other objects and advantages of the invention are set out or made evident in the appended claims and the following description of a preferred mode and products of the invention.

The frit component of the mixture is prepared by admixing and thoroughly blending high-purity glassmaking materials, including one such constituent as zinc oxide common in the art of glassmaking but which in an alumina, lead, borosilicate glass system reacts in such a manner as to form a devitrified glass, in carefully measured amounts, melting at about 900 C. the admixed materials in an alumina-silica refractory crucible, and fritting the homogeneous molten product by pouring it into distilled water. The frit is thoroughly dried, then ground in a mechanical grinder of the mortar and pestle type. Typical mesh sizes of the ground nucleated product thus produced, by weight percentages, are:

Retained on 100-mesh screen 0.0 Retained on 250-mesh screen 35.2 Retained on 270-mesh screen 9.2 Retained on 325-mesh screen 5.0 Passed by 325-mesh screen 50.6

The thus produced divided glass or product is further reduced as to particle size in a vibratory mill, using an organic solvent such as methanol or isopropyl alcohol as a carrier. The reduction is carried to the point where average particle size is between 2 and 5 microns.

As is made evident by consideration of exemplary data hereinafter tabulated, the frit according to the invention contains, by mole percentage, of the order of from 3% to 21% of SiO from 50% to 66% of PhD, from 4.5% to 22% of B 0 from 6% to 15.5% of ZnO and from 6% to 15.5% of A1 0 but in relative proportions set out in the tabulation, it being noted that ZnO is essential to promote crystallization and improve the TCR (temperature coefiicient of resistivity). In the latter matter, it should be noted that the absence of ZnO from the glass (as indicated in composition No. V) results in much higher CRV and poor TCR. B 0 is essential to low resistivity in that the solubility and homogenization of the metal particles is increased. Boric oxide also contributes to improved resistance to the effects of moisture and elevated temperatures. The A1 0 component acts as a stabilizer against deterioration of the glass by water and acids, and promotes low TCR. The PhD component contributes to low total resistance. SiO contributes to improved moisture resistance, thermal sability and low TCR.

The following tabulation portrays the chemical components or compositions and the physical properties of. the glass components of the resistive compositions, and also the glass/ metal weight ratio of the resistive composition of the fired film elements, the thermal stability characteristics, moisture stability characteristics, and CRVs, of elevent representative compositions labeled I to XI, inclusive. The tabulation is for reasons of clarity and space requirement divided into two sections, namely Table I and Table Ia.

TABLE I I II III IV V VI S102, mole percent. 14. 10. 65 12.27 13. 28 14.48 20. 18 PhD, mole percent- 63.18 65.46 52. 76 57.28 62.06 52.66 B203, mole percent 4 90 11.17 12. 73 13.76 14.98 12. 75 Z110, mole percent 8. 61 6. 36 15. 03 7. 84 7. 28 1 03 8.61 6.36 7.21 7.84 8.48 7.13 Glass properties:

I.S.P. C) 347 318 333 346 366 364 Expansion, Tce. 105.0 127. 6 105.1 103.0 98. 0 98. 0 Density, g./cc 6. 82 6. 76 6. 36 6.32 6. 17 6.08 Resistance material:

Glass, percent 97. 22 97. 19 97.03 97.00 96. 93 96. 89 Metal, percent 2. 78 2. 81 2. 97 3.00 3.07 3. 11 Resistor properties:

Resistivity range,

kilo-Ohms Thermal stability,

percent 0.2 -0. 7 1.4 1. 7 0.4 1. 0 Moisture stability,

percent 0.2 -0. 1 0.9 0.08 0.2 0.2 C RV, percent 0.5 1.0 0.5 1. 0 1. 5 1.0

1 From 1.50 (650 C.) to 7.00 (850 C.) 2 From .25 (650 C.) to 2.20 (850 0.). 3 From .13 (700 C.) to 2.8) (500 0.). 4 From .13 (700 C.) to .55 (500 0.). 6 From .09 (850 C.) to .25 (650 .C.) 4 From .30 (700 C.) to .65 (500 C.).

TABLE Ia VII VIII IX X XI S102, mole percent 15. 50 12.23 14.47 5. 56 12. 1O PbO, mole percent. 50.07 52. 63 62. 01 62. 40 51. 89 B203, mole percent. 16. 10 12. 69 14. 97 15.01 21. 79 ZnO, mole percent..- 9.24 7. 28 8. 55 8.60 7. 11 A1203, mole percent 9.09 15. 17 8. 43 7. 11 Glass properties:

I.S.P. C.) 306 373 366 338 366 Expansion, 'Ice. 95. 2 89. 5 100.0 103. 9 91. 2 Density, g./cc 5. 97 5. 93 5. 78 6. 34 5. 98 Resistance material:

Glass, percent 96.83 96. 81 96. 73 97.01 90. 84 Metal, percent 3. 17 3. 19 3. 27 2. 99 3. 16 Resistor properties: Resistivity range,

kilo-ohms Thermal stability, percent. 0. 7 0.8 0. 8 2. 1 0.3 Moisture stability, pcreent 0 5 0. 9 0.9 0.2 0. 5 CRV, percent 0.5 0.5 1.0 1.0 1.0

1 From .30 (700 C.) to .55 (500 C.). 2 From .25 (600 C.) to .35 (700 C.). 3 From .08 (700 C.) to .40 (500 C.). 4 From .15 (700 C.) to .50 (600 0.). 5 From .20 (750 C.) to 1.00 (850 0.).

In the preceding tables, the glass-cOmpOsitiOn data are in terms of mole percentages of the several oxides, and the interferometer softening points of the crystallite-containing glassy materials (glasses) are denoted I.S.P., and indicate the interferometer softening point on the temperature scale. The softening point values given are, as indicated, in degrees centigrade. The interferometer softening point is a relatively precisely measurable temperature and thus provides a basis for accurate comparisons among glassy materials. It is related to the less accurately determinable melting temperature of the material in a well understood relationship. In the tables, the resistance composition glass-to-metals data are in terms of weight percentages. For example, the weight percentages of glass and metal in Example VII (Table Ia) are glass 96.83% and metal 3.17%. From that set of data it is forcibly evident that the resistive films according to the invention comprise exceptionally high percentages of glass and unusually low percentages of metal (the maximum metal percentage among the eleven examples being 3.27%

Further in the tables, the resistivity ranges data are in terms of kilo-ohms per square. It will be noted that the resistivity of a film material having a given glass to metal ratio (97.22% to 2.78% in the case of Example I), may be made to fall at a determinable value within a fairly wide range of resistivity values (from 1.50 kilo-ohms per square to 7.00 kilo-ohms per square in the cited Example I), depending upon the temperature to which the film is heated during the firing operation. Firing temperatures range from about 500 C. to about 850 C., and firing durations are of the order of ten minutes. As indicated in the resistivity range data section of the tabulations, resistivity values may increase with increase of firing temperature (for example from 1.50 kilo-ohms per square with a firing temperature of 650 C. to 7.00 kiloohms per square with a firing temperature of 850 C. in the case of the material of Example I) or the resistivity of some resistance materials may decrease with increasing firing temperatures, as indicated in the data for Example III. Thus resistivity may be regulated, and a desired particular value within the indicated limits attained by appropriate selection of the firing temperature in relation to the exposure time during firing.

In the tabulations, further, the linear-thermal coefficient of expansion (Tee) of the glassy product of the glassmaking operation, is given in terms of numbers which must be multiplied in each case by to give the unit expansion per unit of length per degree Centigrade. The data are valid within the temperature range from 22 C. to 300 C.

In the tabulations the thermal stabilities and moisture stabilities are recorded in terms of percentage shift of total resistance of the fired film after 24 hours at 175 C. in the case of thermal stability, and in terms of percentage shift of total resistance after 24 hours in water in the case of moisture stability.

The contact resistance variation (CRV) of the finished exemplary resistive films is in each example given in terms of percentage of the total resistance variation of the element between the terminations. It will be noted from the tabulations that the CRV values range between 0.5% and 1.5%, in contrast to the usual CRV values of the order of from 5% to in the case of cermet resistive films of comparably low resistivity values.

The resistive compositions or mixes are made as previously indicated, those of Examples III, V and VIII being typical; additional examples are shown following, the quantities shown being by weight in any convenient unit:

Following preparation of the micronized frit, a resistor composition or mix is prepared by mixing thoroughly frit and a salt of the metal to be incorporated in the final resistive film (for example, Rh C1 Ru C1 in the weight proportions approximately 80% frit and metal salt and vehicle, the salt being a dispersion in triethylene glycol or other similar liquid as a vehicle, in a ratio of about 1:2. A suflicient amount of the vehicle liquid is added to bring the solids to liquid ratio to 2:1 by weight, for example, whereby to facilitate spreading, silk screening, or other procedure used in forming a film of the suspension or mix on a base of alumina or other suitable fired ceramic. As will be evident to those skilled in the art, the vehicle may be glycerol, octyl alcohol, mineral thinner, ethylene glycol, triethylene glycol, or combinations of two or more such known carriers. The liquid, the metal salt, and the frit are thoroughly mixed together to form a paste for application to the base.

6 Resistance film mixtures are made up as indicated in the following, those of Examples III, V and VIII of the previous table being cited as exemplary:

All of the exemplary values are in arbitrary weight units, e.g., grams, ounces, pounds, etc.

Consideration of the data comprised in Tables I and Ia clearly indicates that the glassy component of the mixture is characterized in each instance by a low softening temperature, e.g., 318 C. for Example II to 366 C. for several others. The advantages of the low softening points are known to those skilled in the art, it being evident that manufacture of cermet resistance elements requiring low fusing or maturity temperatures to attain a particular resistance can be effected with less expensive apparatus, less expenditure of time and heat energy, and less corrosive deterioration of the apparatus. The glassy materials from which the resistance compositions are derived are particularly adaptable to manufacturing methods employing in expensive electric or gas-fired furnaces; the melting and refining temperatures (less than 900 C.) of the glassy compositions are well below those temperatures required for conventional or more common types of glasses, or substrate. The viscosity of the thus prepared mix or paste may be adjusted by variation of the amount of liquid added, or by otherwise changing the liquid to solids ratio in order to facilitate application by one method or another.

The prepared mix or paste is applied to the substrate by any suitable means and process, as by a silk screen apparatus and procedure to form a layer or film on the base. The film is then dried at a temperature of the order of 175 C., either separately or in a continuous firing operation, whereby the organic material is driven out or removed. Thereafter the base or substrate with the applied film is subjected in either a separate or continuous firing operation to a temperature of from 500 C. to 850 C., as indicated in the tables, for a period of the order of ten minutes. The metal particles remaining from decomposition of the metal salt remain dispersed in and/or on the particles or crystallities and throughout the glassy matrix of the devitrified glass. The exact submicroscopic details of the structure are unknown but are such that with a very low content of metal, very low final film resistivity is attained (e.g., ohms per square) while retaining the very low CRV previously associated only with much higher metal content and without the aforementioned abrasive surfaces caused by the high metal content. Further, the structure is such that TCR values are exceptionally low, being in the range of from 80 parts per million to parts per million. Thus Where in the prior art resistance elements having a sufliciently high metal content to produce low resistivities the CRV was of the order of from 5% to 15% of total element resistance (TR), elements made as outlined above are characterized by CRV values of the order of only 0.5% to 1.0% of TR.

In those instances wherein the TCR is deemed to be higher than desirable, a trade-off of a slight increase in resistivity for a relatively large reduction in TCR may be etfected by substituting for a portion of the ruthenium a small percentage (1%10%) of Rhodium, the substitution being by a decrease of Ru salt and corresponding addition of Rh salt.

The glasses in Tables I and Ia are derived from combinations of oxides and compounds as indicated, that of Example VIII being exemplary. That combination (Example VIII) is obtained as indicated by the following:

The preceding disclosure of the principles of the invcntion and a range of typical preferred compositions all within the specific general composition and all having the same general novel properties, indicates full attainment of the aforementioned objectives. As is made evident by the examples covering the range of the invention, changes within the boundaries of the invention and within the true spirit and scope of the invention will occur to those skilled in the art and hence it is not desired to restrict the invention to exact proportions specified in the several examples other than as is required by the appended claims.

I claim:

1. A resistive film for a potentiometer or like variable resistor having a contact arranged to brush on the film,

said resistive film consisting essentially of the fused mixture of a crystallite-containing frit and interspersed conductive particles of which particles the major part is ruthenium, the said frit comprising the fused product of glassmaking oxides and a nucleating agent, the glassmaking oxides including PbO, SiO and B and the said nucleating agent including ZnO, the percentage of said conductive particles in said resistive film being in the range of from 1% to 7% by weight,

said resistive film further being characterized by contact-resistance variation not in excess of 1.5% of the total resistance of said resistive film between the terminals thereof, and further being characterized by a matte-type non-abrasive surface and by thinfilm electrical resistivity within the range from 80 ohms/ square and 7000 ohms/square.

2. A resistive film according to claim 1, in which said film comprises ruthenium in the range from 2.78% to 3.2% by weight of said film and the thin-film resistivity of said film is of the order of 80 ohms/square.

3. A resistive film according to claim 1, in which said film comprises among said conductive particles rhodium to the extent of from about 0.1% to about 10% of the total metal in the film, by weight.

4. A resistive film according to claim 1, in which the mole percentage of the glassmaking oxides and nucleating agent are within the ranges of from 50% to 66% PbO, from 4.5% to 22% of B 0 and from 6% to 15.5% ZnO,

from 3% to 21% of SiO and from 6% to 15.5% A1 0 respectively.

5. A resistive film according to claim 1, in which the mole percentages of said glassmaking oxides and nucleating agent are: PbO, 63.18%; SiO 14.70%; B 0 4.90%; and ZnO, 8.61% and A1 0 8.6 1%, respectively.

6. A resistive film according to claim 1, in which the mole percentages of said glassmaking oxides and nucleating agent are: PbO, 51.89%; SiO 12.10%; B 0 21.79%; and ZnO, 7.1 1% and A1 0 7.11%, respectively.

7. A resistive film according to claim 1, in which the mole percentages of said glassmaking oxides and nucleating agent are: PbO, 62.01%; SiO 14.47%; B 0 14.97%, and ZnO, 8.55%, respectively.

8. A resistive film according to claim 1, in which the mole percentages of said glassmaking oxides and nucleating agent are: PbO, 52.76%; SiO 12.27%;B O 12.73%; and ZnO, 15.03%; and A1 0 7.21%, respectively.

9. A resistive film according to claim 1, in which the mole percentages of said glassmaking oxides and nucleating agent are: PbO, 52.63; S10 12.23%; B 0 12.69%; and ZnO, 7.28%; and A1 0 15.17%, respectively.

10. A resistive film according to claim 1, in which the mole percentages of said glassmaking oxides and nucleating agent are: PbO, 62.40%; SiO 5.56%; B 0 15.01; and ZnO, 8.60%; and A1 0 8.43%, respectively.

11. A resistive film according to claim 1, in which the mole percentages of said glassmaking oxides and nucleating agent are: PbO, 65.46%; SiO 10.65%; B 0 11.17%; and ZnO, 6.36%; and A1 0 6.36%, respectively.

12. A resistive film according to claim 1, in which the mole percentages of said glassmaking oxides and nucleating agent are: PbO, 57.28%;SiO 13.28%; B 0 13.76%; and ZnO, 7.84%; and A1 0 7.84%, respectively.

References Cited UNITED STATES PATENTS 2,739,901 3/1956 Herold et al. 10653 3,425,817 2/1969 Ikeda et al. 10653 3,441,422 4/1969 Graft 106-53 3,462,252 8/1969 Veres 106-53 2,924,540 2/1960' DAndrea 2525l4 3,149,002 9/1964 Place 252-5 14 3,154,503 10/1964 Janakirama-Rao 2525l4 3,207,706 9/1965 Hoffman 2525l4 3,252,831 5/1966 Ragan 2525l4 3,271,193 9/1966 Boykin 2525l4 3,329,526 4/1967 Daily et a1. 2525l4 3,352,797 11/1967 Kim 2525l4 DOUGLAS J. DRUMMOND, Primary Examiner US. Cl. X.R. 252518; 10653 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.. 3,551,355 December 29, 197

Jason D. Provance It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, lines 29 and 30, "However" should read How "i Column 4, line 10, "elevent" should read eleven Column 5, line 75, "base." should read base Column 6, 31, glasses," should read glasses. Column 5, line 45 beginning with The resistive compositions" cancel all to am including "for application to the base" in line 75, same c011 5, and insert the same after common types of glasses." in

column 6, line 31. Column 6, line 28, "in expensive" should 1 inexpensive Column 8, line 19, "PbO, 52.63" should re: PbO, 52.63% 3 lines 25 and 26, "B 0 15.01;" should re:

Signed and sealed this 25th day of May 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLEI 'Attesting Officer Commissioner of Pa-