US 3326645 A
Description (OCR text may contain errors)
June 20, 1967 Wl E, COUNTS ETAL 3,326,645
CERMET RESISTANCE ELEMENT AND MATERIAL Filed Sept. 22, 1965 FIG. 4
INVENTORS WILLUAM E. COUNTS WILLHAM T. KELLY FIG. e 3%1/ Y United States Patent O 3,326,645 CERMET RESIISTANCE ELEMENT AND MATERIAL William Edward Counts, Anaheim, and William Thomas Kelly, Garden Grove, Calif., assignors to Beckman lnstruments, liuc., a corporation of California Filed Sept. 22, 1965, Ser. No. 489,337 4 Claims. (Ci. 29--l82.5)
The present invention relates to an improved electrical resistance material formed of a mixture of finely divided particles of glass and metal, of the type commonly called cermet material, and to resistance elements constructed therefrom.
Cermet resistance elements presently known in the art are exemplified by U.S. Patent 2,950,995 of Thomas M. Place, Sr., et al., entitled Electrical Resistance Element and 2,950,996 of Thomas M. Place, Sr., et al., entitled Electrical Resistance Material and Method of Making Same, both of which are assigned to Beckman Instruments, Inc., assignee of the present invention. These patents describe a resistance element formed of a layer of resistance material comprising a heterogeneous mixture of particles of non-conducting material and conducting metals xed to a non-conducting base. The non-conducting material is a ceramic-type material such as glass or vitreous enamel and the layer is formed by heating the metal-glass mixture at least to the melting point of the glass or enamel, so as to create a smooth, glassy phase.
In brief, the present invention resides in the discovery that a cermet mixture, formed of finely divided particles of glass and an alloy of about to 50% total weight of ruthenium and rhodium, in which the metal alloy is so correlated to the glass material that the minimum proportions of ruthenium metal is at least by weight of the metal alloy for all proportions of the alloy weight, can be deposited on a non-conductive base member and red to form a cermet resistance element having a high resistivity and a suitable temperature coefficient of resistance. Such resistance elements have a high resistance for the relatively large amounts of metal material employed and are capable of performance at high power ratings as compared to glass-metal resistance elements employed in the past.
A more thorough understanding of the invention may be obtained by a study of the following detailed description taken in connection with the accompanying drawings in which:
FIG. 1 is an isometric view of an embodiment of the invention which is suitable for use in rotary potentiometers;
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;
FIG. 3 is an isometric view of an embodiment of the invention particularly adapted for fixed resistance elements of a micro-miniaturized electrical circuit;
FIGS. 4 and 5 are sectional views taken along lines l-i and 5 5 respectively of FIG. 3, and
FIG. 6 is an underside view of the embodiment of PIG. 3.
The cermet material of the present invention is particularly applicable to the manufacture of miniature or micro-miniature resistance elements of all types, representative ones being shown in the gures. Thus, in the structure of FIG. l, a layer l0 of resistance material is fired to a base lll, the electrodes l2, 13 being provided at each end of the layer for connecting 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 lll may 'be of any suitable electrically non-conducting roce 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, tine-textured surface and being impervious to moisture and other liquids being preferred. Steatite, fosterite, sintered or fused aluminas and zircon porcelains are examples of preferred materials for forming the base 1l.
The electrically conductive electrodes i2, 13 are conventional and may be formed by applying any of the Wellknown conducting silver or other metal pastes over 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 Patent No. 3,134,085 of Kenneth F. Miller et al., entitled Variable Resistor With Terminal Structure may be employed for making electrical contact with the resistance layer l0.
FIG. 2 illustrates another form of the resistance element of the invention in which a layer l5 of resistance material is applied to -a rectangular base 16 and electrodes 17, 18 are then added at the ends of the layer l5. This form of the invention is particularly suitable in fixed resistors and linear potentiometers.
FIGS. 3, 4, 5 and 6 illustrate substantially enlarged views of a micro-miniature circuit element 20 advantageously formed of alumina and supporting a pair of fixed cermet resistance elements 21. The respective ends of these resistance elements engage conductive electrodes 22 as shown.
The invention contemplates a cermet composition for use in the above-described resistor elements comprising about 50% to about 95% glass and the balance an al-loy of the metals ruthenium and rhodium. The particular amounts of ruthenium :and rhodium necessary in the metal alloy to produce a resistance layer having a commercially applicable temperature coefficient of resistivity depends on the total metal content employed in the cermet composition. For relatively high resistance elements, in the order of 10,000 ohms/square, the range of proportions of ruthenium content may be about 95% by Weight of the total metal content of the cermet compositions. In general, the maximum preferred allowable proportion of ruthenium decreases as the total metal content of the metal-glass cermet is increased. Preferably, the rutheniumrhodium containing alloy is so related to the ceramic material that when the total content of metal alloy in the cermet composition is 540% the maximum proportion of ruthenium metal is about 95% by weight of the alloy and the `maximum proportion of rhodium is about 40% lby weight of the alloy; when the total content of the metal alloy in the cermet composition is between lil-20% of the composition, the maximum proportion of ruthenium metal is about 90% by Weight of the alloy and the maximum proportion of rhodium is :about 55% by Weight of the alloy; when the total metal content of the cermet composition is between 20-35 of the composition, the maximum proportion of ruthenium metal is about 70% by weight of the alloy and the maximum proportion of rhodium is about 60% by weight of the alloy; when the total metal content is between S55-50%, the maximum proportion of ruthenium metal is about 65% by weight of the alloy and the maximum proportion of rhodium is about by weight of the alloy.
In a preferred embodiment of the invention, the cermet compositions of this invention may be prepared by mixing the resinates of ruthenium together with the resinates of rhodium. The glass binder, in the form of finely divided 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 to approximately 700 F. and constantly stirred to remove the volatiles and organic materials from the mixture and t decompose the metal compounds. The resulting dry material is ground to a line powder and calcined at about 850 F. The resulting calcine is ground to a fine powder, producing a dry material consisting of very small glass particles coated with or mixed with extremely small particles of the metal alloy.
The particular range of proportions of glass to metal by weight in the iinal resistance material may Ibe varied in the procedure just described by varying the amount of glass added to a given resinate solution. Each of the individual resinate solutions contains a predetermined quantity by weight of ruthenium-rhodium alloy. After heating the glass and metal resinate solution, only the glass and metal remain, the total weight or amount of metal particles coating the glass particles Ibeing that amount of metal which was originally in the resinate solutions.
The mixtures formed by the method described above may be stored indefinitely and may be used to produce stable resistance elements. When it is desired to make resistance elements using the material, the dry powder is mixed with a suitable liquid carrier, eg., 7% ethyl hydroxyethyl cellulose-93% octyl alcohol to form a fluid composition which can be applied -to the basev member by any suitable process such as silk screening, spraying or stenciling. The base with the layer or layers applied thereto is then tired to drive off the volatiles and fuse the glass material into a continuous phase of solidiiied glass with the metal alloy particles uniformly distributed therein. One tiring procedure comprises inserting the base with applied resistance layer or layers into a cold furnace. The furnace is slowly heated, eg., over a period of four hours to a temperature lof the order of 1450' to 1550 F. This temperature is maintained a short time period, eg., 20 to 30 minutes, after which the furnace is slowly cooled, eg., over a period of four hours before withdrawal of the cermet elements. This procedure is referred to below as the slow firing technique. Another tiring procedure which may be used (denoted hereinafter as the fast ring method) involves inserting the base of they tired resistance layer or layers into a furnace which has been pre-heated to a temperature in the range of 1400-2000" F. The `fired element is then removed -after a relatively short time period, eg., to 30 minutes.
For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative examples are given for various cermet resistance elements using ruthenium-rhodium alloys tired onto a steatite base member to form a thin resistance layer approximately .001 inch in thickness. In these examples, it will be understood that the time and temperature of ring and the type of substrate material employed may cause some minor variations `in the resistivity and temperature coeiiicient of the resulting resistance elements.
Example A (5% metals by weight) z Percent `Glass 95 Ruthenium 3.33 Rhodium 1.67
The resistivity of a resistance l-ayer of approximately .001 inch thickness formed of the material was approximately 10,000 ohms/ square and its temperature coeflicient of resistivity (TEMPCO) was 338 p.p.m./ C.
Example B (10% metals by weight): Percent Glass 90 Ruthenium 6.67
The resistivity of a resistance layer of approximately .001 inch thickness formed of this material was approximately 1900 ohms/square and its TEMPCO was 160 p.p.m./ C.
4 Example C (10% metals by weight):
Glass, percent 90 Ruthenium, percent 5 Rhodium, percent 5 Resist-ance, ohms/ squ-are 8000 TEMPCO, p.p.m./ C. -738 This composition produced a resistance layer having a TEMPCO outside the more desired commercially applicable range of i500 p.p.m./ C. In order to produce a resistance element and within the more commercially applicable TEMPCO range, it is desirable to limit the maximum proportions of rhodium in those resistance materials having a metal content of between 5-10% to about 40% of the total metal content.
Example D (l5 metals thy weight) TEMPCO, p.p.m./ C 420 Example F (20% metals by weight):
Glass, percent Ruthenium, percent 12 Rhodium, percent 8 Resistance, ohms/ square 166 TEMPCO, p.p.II1./ C -885 Example G (20% metals by weight):
Glass, percent 80 Ruthenium, percent l0 Rhodium, percent l0 Resistance, ohms/square 272 `TEMPCO, p.p.m./ C -458 Example H (20% metals by weight):
Glass, percent 80 Ruthenium, percent 6.67 Rhodium, percent 13.33 Resistance, ohms/square 1055 TEMPCO, p.p.m./C. 681
This composition produced a resistance element having a TEMPCO outside the more desired commercially applicable range of i500 p.p.m./ C. In order to produce a resistance element within the more commercially aplicable TEMPCO range, it is desirable to limit the maximum proportions of rhodium in those resistance materials having a metal content of between 20-35% by Weight to about 60% of the total metal content.
Example I (20% metals by weight):
Glass, percent 80 Ruthenium, percent 17.14 Rhodium, percent 2.86 Resistance, ohms/square TEMPCO, p.p.m./ C '+245 Example J (25% metals by weight):
Glass, percent 75 Ruthenium, percent 12.50 Rhodium, percent 12.50 Resistance, ohms/square TEMCO, p.p.m./ C. -98
Example K (25% metals by weight):
Glass, percent 75 Ruthenium, percent 11.18 Rhodium, percent 13.82 Resistance, Ohms/ square TEMPCO, p.p.m./ C. 428
3,326,645 e? Y Example L (30% metals by weight): TEMPCO within the commercially applicable desired Glass, percent 70 range of i500 p.p.m./ C. This is over 34 times more Ruthenium, percent resistance using approximately the same quantity of metal Rhodium, percent l5 in the cermet composition. The resistance of the cermet Resistance, ohms/square 68 5 composition utilizing 50% ruthenium-rhodium alloy is TEMPCO, p.p.m./ C. -208 still approximately 30 ohms per square which is extremely Example M (35% metals by Weight): high when one considers that the total metal content is Glass, percent 65 approximately five times that of the above-described gold, Ruthenum, percent palladium, silver cermet composition. Rhodium percent 15 10 While .it is not completely understood why the rutheni- Resistance ohms/square 3 5 tim-rhodium metal alloy can be employed in such large TEMPCO, ppm/o C +101 quantities to form a resistance material, it is believed that the presence of ruthenium inhibits the tendency for the Example N (35% metals by Welght): metal to agglomerate and form strings or globules in the Glass Percent e5 15 fused glass binder. It appears that ruthenium contributes Ruthemum percent e- 15'55 to the interaction between the respective surfaces of the Rhedlum Percent 19'45 finely divided metal alloy and glass particles and prevents Reslstance ohms/segnare 83 the agglomeration thereof even when they are present in TEMPCO PP'm'/ C' *295 relatively large quantities. This apparently causes the mix- Example 0 (40% metals by weight): 20 ture to retain its homogeneity during firing and results in a Glass, percent 60 much more stable resistance element having relatively Rutlienium, percent `20 large quantities of metal for the resistance values ob- Rhodium, percent 20 tained. Resistance, ohms/square 32 The homogeneity of resistance elements formed of TEMPCO, p.p.m./ C. 42.8 25 cermet composition using ruthenium-rhodium alloys is Example P (50% metals by Weight): believed to be one reason why these elements are capable Glass, percent 50 of withstanding higher power levels. Because the materials Ruthenium percent do not agglomerate when the glass material is fused and,
Rhodium percent 25 therefore, do not produce strings or globules of metal llo the current liow throu h the resistance element is Resistance, ohms/square 32 a .y g TEMPCO, ppm/n C. l +124 believed to he more uniformly applied .across the entire a volume of the resistance element. That is, the metal par- Vaflatmns 111 the glass COmPOSlUOIl UtlllZed may Change ticles are uniformly distributed throughout the element me resistance and TEMPCO Values achieved le 'a mmof and there are no areas or regions through which electrical extent. It is believed that almost any glass composition 35 current 50W is confined to a Single or narrow path may be employed S0 long ae me glass eenslmems d0 not through any portion of the element. This, in addition to reeel Wltll the metal eenstltuems at the lmg tempera' the greater metal content of the resistance element, creates tures employed and S0 long 'as the glass has a melting tem' a more even distribution and dissipation of the heat perature below that of the metal constituents of the mixproduced. ture" Tlle palllcular eemposltlens of the glass Were em" e0 The relatively high metal content of cermet elements Pleyed m Venous quanmles m the above examples' formed of ruthenium-rhodium cermet material and the extremely uniform distribution of the metal alloy through- Glass formulae Glass #l Glass #2 Glass #3 out the element is extremely important in another respect. One of the major problems encountered when using cermet Pmgfs Pemfgl 10 Fermi?) 20 45 resistors as variable resistance devices, such as precision 5:41 5:41 potentiometers and circuit trimmers, is the high electrical Mgg ------lnoise characteristics associated with such devices due to i65i 245o 2000 the contact made between the movable wiper of such 240 1-20 devices and the resistance element. It is believed that this 50 electrical noise is at least in part due to the somewhat sporadic contact made by the wiper and the metal par- The above formulae are the fritted percentages. The ticlcs on the surface of the element. Due to the high metal glass may be produced by any conventional process. lIt is content -of cermet elements employing ruthenium-rhopreferred, however, that it be as homogeneous as possidium alloys for relatively high resistance elements, and Ible. One method of making glass includes thoroughly miX- because the metal particles do not agglomerate but are ing a batch of the raw materials together while dry, meltevenly distributed throughout, it can be understood that ing the batch in ceramic crucibles to produce a clear fluid there are substantially greater numbers of metal particles glass, quenching the molten glass by pouring it into cold available for contact with the wiper. This greatly reduces Waef, drying the resulting Shattered glass and then gfldthe electrical noise associated with such elements. ing it to a very line powderg Resistance elements formed of cermet compositions As will be noted from the examples, relatively large quantities of ruthenium-rhodium alloys can be utilized in these cermet compositions to form `resistance elements having relatively high resistance values as compared to cermet resistance elements produced in the past. It is known, for example, that an alloy of gold, palladium and silver, when utilized in a glass-metal (cermet) resistance mixture of finely divided particles in which the gold, pal ladium, silver content is about 11.5% of the total mixture, produces a resistance element having approximately 55 ohms per square for a layer approximately .001 inch in thickness. As will be seen in Example B hereinabove, a cermet composition using 10% ruthenium-rhodium alloy produces a resistance element (of like thickness) having a resistance of 1900 ohms per square and having a using ruthenium-rhodium cermet material also have an extremely smooth surface. This is considered extremely important when such elements are employed in variable resistance devices, such as potentiometers. These ruthenium-rhodium cermet materials do not produce blisters or other irregularities on the surface of the element when the minimum percentage of ruthenium in the 4composition is at least 10% Iof the total metal alloy content in the composition. For example, in order to produce smooth surfaced resistance elements, the ruthenium content should not be less than .5% of the total composition when the total metal content is 5% of the composition and should not be less than 5% of the composition when the total metal alloy content is 50% of the composition. Of course, it will be understood that the above minimum percentage of ruthenium does not necessarily produce a resistance element having a temperature coecient of resistivity falling within the desired commercially applicable range of i500 p.p.m./ C. and, for the more desired range, greater percentages of ruthenium must be employed.
Although exemplary embodiments ofthe invention have been disclosed and discussed, it will be understood that other applications of the invention are possible and that the embodiments disclosed may be subjected to various changes, modifications and substitutions without necessarily departing from the spirit of the invention.
What is claimed is:
1. A cermet composition adapted to be red Aonto a non-conducting base member to form a thin resistance layer consisting essentially of:
about 50 to 95% by Weight of glass material; and
to 50% by weight of an alloy of ruthenium and rhodium in which the metal alloy is so correlated to the glass material that the minimum proportion of ruthenium metal is at least 10% by weight of the metal alloy for all proportions of the alloy content, said glass material having a melting point lower than that of said alloy of ruthenium and rhodium.
2. A cermet composition adapted to be tired onto a nonconducting base member to form a thin resistance layer consisting essentially of:
about to 95 by weight glass material; and
about 5 to 50% by weight of an alloy of ruthenium and rhodium, said glass material having a melting point lower than that of said alloy of ruthenium and rhodium, said alloy constituents being so co-related to said glass material that when the total metal content of said composition is between 5-l0%, the maximum proportion of ruthenium is abou-t 95% by weight of the alloy and the maximum proportion of rhodium is about 40% by Weight of the alloy; when the total metal content is between 10-20% of the composition, the maximum proportion of ruthenium is about 90% by weight of the alloy and the maximum proportion of rhodium is about by weight of the alloy; when the total metal content is between 1Z0-35% of the composition, the maximum proportion of ruthenium is about 70% by weight of the alloy and the maximum proportion of rhodium is about by weight of the alloy; and when the total metal content is between 3550% of the composition, the maximum proportion of ruthenium is about by weight of the lalloy yand the maximum proportion of rhodium is about 75% by weight of the alloy.
3. A cermet composition adapted to be tired onto a nonconducting base member to form a thin resistance layer consisting essentially of:
Iabout 50 to 95 by weight glass material; and
about 5 to 50% by weight of an alloy of the metals ruthenium and rhodium in which the maximum proportions of ruthenium metal t-o total metal content is inversely proportional to the total metal content and the maximum proportion of ruthenium is about 95% by Weight of the alloy when the metal content is 5% and the maximum proportion of ruthenium in the alloy is about 65% when the total content is 50% by weight of the cermet composition, said glass material having a melting point lower than that of said alloy of ruthenium and rhodium.
A cermet resistance element comprising high temperature resistant, electrically non-conducting base;
a thin layer of fused glass-metal mixture tired to said base, said layer consisting essentially of 50 to 95% by weight of solidified glass and 5 to 50% by weight of an alloy of the metals ruthenium and rhodium in nely divided form homogeneously mixed throughout said fused glass, said glass having -a melting point lower than that of said alloy of ruthenium and rhodium, said metal alloy being so co-related to said glass constituents that when the total metal content of said composition is between 5-10%, the maximum proportion of ruthenium is about 95 by weight of the alloy an-d the maximum proportion of rhodium is about 40% of the alloy; when the total metal -content is between itl-20% of the composition, the maximum proportion of ruthenium is about 90% by weight of the alloy and the maximum proportion of rhodium is about 55% by weight of the alloy; when the total metal content is between 20-35% of the composition, the maximum proportion of ruthenium is about by weight of the alloy and the maximum proportion of rhodium is about 60% by weight of the alloy; and when the total metal content is between 35-50% of the composition, the maximum proportion of ruthenium is about 65% by weight of the alloy and the maximum proportion of rhodium is about by weight of the alloy thereby providing a resistance element having a resistivity of 30 to 10,- 000 ohms per square when said layer is approximately .001 inch in thickness and a temperature coecient of resistivity of less than i500 p.p.m./ C.
)References Cited UNITED STATES PATENTS FOREIGN PATENTS 8/ 1965 Great Britain.
CARL D. QUARFORTH, Primary Examiner.
55 BENJAMIN R. PADGETT, Examiner.
A. l. STEINER, Assistant Examiner.
` ent requiring correction and that t UNITED STATES PATENT oEFICE CERTIFICATE OF CORRECTION Patent No 3 ,326 ,645 June 20, 1967 William Edward Counts et al.
It is hereby certified that error appears in the above numbered pathe said Letters Patent should read as corrected below.
Column 8, line 5, after "total" insert alloy Signed and sealed this 2nd day of July 1968.
EDWARD J. BRENNER Edward M. Fletcher, Jr.
Commissioner of Patents Attesting Officer