US3408311A - Thermistor compositions and thermistors made therefrom - Google Patents

Description

United States Patent 3,408,311 THERMISTOR COMPOSITIONS AND THERMIS- TORS MADE THEREFROM Oliver A. Short, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, DeL, a corporation of Delaware No Drawing. Filed Sept. 29, 1966, Ser. No. 583,068 25 Claims. (Cl. 252-514) ABSTRACT OF THE DISCLOSURE The invention relates to thermistor compositions which can be printed and fired to produce thermistors possessing a desired range of electrical resistance and a negative temperature coetficient of resistance, comprising (a) a finely divided metal powder consisting essentially of palladium and up to 60% silver, (b) a finely divided metal oxide powder mixture consisting essentially of C0 0 and MnO and (c) finely divided inorganic binder powder. Additionally, an inert vehicle may also be included to disperse the powders.

Background of the invention Thermistors are electrical resistors made of materials whose resistance varies sharply in a known manner with the temperature. Thermistors whose resistances decrease with an increase in temperature are referred to as having a negative temperature coefiicient of resistance (-TCR), while thermistors whose resistances increase with an increase in temperature are referred to as having a positive temperature coefficient of resistance (+TCR). Many interesting commercial and industrial applications for thermistors have been developed in the last few years, and thermistors are available in a large variety of types covering a broad range of electrical characteristics. Thermistors are ideal for temperature controls and temperature sensing devices and can also be used to produce temperature compensating propetries in various electrical circuits.

Resistor compositions comprising mixtures of one or more finely divided metals with glass are well known. More particularly, resistor compositions comprising mixtures of palladium and glass, with or without silver, have been produced heretofore, for example, as shown by DAudrea, US. Patent No. 2,924,540. By making routine adjustments in the metal: glass ratio, resistor compositions can be printed and fired to produce resistors having the desired resistance values (R). These resistors having a small positive or negative temperature coefficient of resistance (:TCR), and small TCRs are not suitable for present purposes.

Thermistor compositions comprising one or more metal oxides have been utilized heretofore. These thermistor compositions are compressed into the desired shape and sintered to produce thermistors. These thermistors usually have negative temperature coeflicients of resistance (TCR) and may have semiconductive properties. Some of the more commonly used oxides are of cobalt, nickel, iron, chromium, zinc, manganese, vanadium, zirconium and cerium. When such previously known thermistor compositions have been made from mixtures of C0 0 and MnO and then printed and fired, they have yielded thermistors that have exceptionally high resistance values (R). The high resistance values make such thermistor compositions impractical and unsuitable for use in "ice thermistors which are printed and fired. Also, these metal oxides have not heretofore been used in conjunction with metals in printed and fired metalizing compositions of the DAudrea type (supra) due to the unduly high resistance values produced by the metal oxides. Additionally, such previously known thermistors made with mixtures of oxides are very difficult to reproduce or duplicate. In the first place, the obtainment of thermistors having specific desired properties required achieving a specific degree of oxidation of the oxides present; in general, the lower the degree of oxidation, the lower the resistance. It is exceedingly difiicult to prepare resistor materials having a uniform resistance by using oxides in their lower states of oxidation. The degree of oxidation of such formerly used oxide mixtures was dependent upon the furnace temperature and upon the oxidation or reduction conditions surrounding the oxides during firing. Slight changes or variations in the temperature or in the oxidation or reduction conditions resulted in wide variations in the specific resistivity.

Thus, a definite need exists for thermistors of the printed and fired on type which are characterized by a wide range of resistances (R) and also substantial negative coeificients of resistance (-TCR.)

Description of the invention This invention relates to highly useful thermistor compositions which can be printed and fired onto a substrate to produce thermistors having desirably low or moderate resistances (R) and substantial negative temperature coeflicients of resistance (-TCR). Briefly, this is accomplished by modifying the known palladium or palladium/ silver resistor compositions with small amounts of a particular metal oxide mixture.

Accordingly, the thermistor compositions of the present invention comprise (a) a finely divided metal powder consisting essentially of palladium and up to 60% silver by weight based on the total weight of palladium and silver, (b) a finely divided metal oxide mixture consisting essentially of C0 0 and MnO in which the C0 0 Mn0 ratio is from 85:15 to 40:60 and (c) a finely divided inorganic binder powder, said components (a), (b) and (0) being present in amounts of 30 to 75%, 1 to 15% and 15 to respectively, based on the total combined Weight of (a), (b) and (0).

Moreover, such thermistor composition may be dispersed in a liquid vehicle, preferably inert, to provide a thermistor paint or paste that can be applied to the surface of a ceramic dielectric and fired to form a fixed stable thermistor.

The metal and binder-containing thermistor compositions of this invention which may 'be readily fired to yield easily reproducible thermistors of good stability contain a particular combination of metal oxides. These thermistors have the desirable low or moderate resistances and substantial negative temperature coefficients of resistance, i.e. TCRs greater than about --1,000 p.p.m./ C. By properly selecting the metals, metal oxides, binder and relative amounts thereof in the present thermistor compositions, printed and fired thermistors having the desirable low or moderate resistances and substantial negative temperatures coefiicients or resistance can be tailormade in accordance with the teachings described hereafter.

As indicated above, the palladium silver, C0 0 MnO and inorganic binder should be in a finely divided or powder form, i.e., a powder sufficiently finely divided to pass through a 325 mesh (U.S. Standard Sieve Scale) stencil screen. Generally, the powders will have an average particle size not exceeding 20 microns. Desirably, the average particle size of the metals and metal oxides will range from 0.1 to microns, while an average particle size range of 1 to microns for the inorganic binder is preferred. The powders may be obtained through conventional methods. For example, the metals can be produced by chemical precipitation or by mechanical comminution.

The palladium:silver ratio is not critical and can vary from 100:0 to 40:60. Thus, 100% palladium is operable for purposes of this invention, while up to 60% silver in admixture with the palladium is also operable. A preferred embodiment involves the use of about a 50:50 mixture of palladium and silver.

The weight ratio of C0 0 to MnO has been found to be important in obtaining the desired negative temperature coefiicient of resistance yet only slightly altering the desired resistance of the Pd/binder or Pd-Ag/binder mixture. Generally, an acceptable range has been found to be from about 85 to about 40:60 of C0 0 to MnO Weight ratios outside this range impart high resistances which are not acceptable for purposes of this invention because such high resistances cannot be lowered to the desired resistances by merely adjusting the metal:binder ratio. The preferred range is 82:18 to 78:22 and a preferred composition is an 80:20 Co O /MnO mixture. It has been found that within the above ranges, increasing the proportion of Mn0 in the mixture increases the resistance of the printed and fired thermistors. Conversely, as the proportion of C0 0 increases, the resistance decreases. However, the critical factor concerning the ratio of C0 0 to MnO is that the proportion of metal oxides remain within the range of from 85:15 to 40:60. Within this range the proportion of metal oxides can be varied to produce the desired resistances for thermistors which are printed and fired. This is, of course, in addition to adjusting the metal:binder ratios to obtain desired resistances.

Any inorganic material which serves to bind the metal and metal oxide to the substrate can be used as the inorganic binder component. The inorganic binder can be any of the glass frits employed in resistor compositions of this general type. Such frits are generally prepared by melting a glass batch composed of the desired metal oxides, or compounds which will produce the glass during melting, and pouring the melt into water. The coarse :frit is then milled to a powder of the desired fineness. The patents to Larsen and Short, U.S. Patent No. 2,822,- 279 and Hoffman, U.S. Patent No. 3,207,706 described some frit compositions which can be employed either alone or in combination with glass wetting agents such as bismuth oxide. Typical frits usable as binders in the compositions of this invention include lead borate, lead silicate, lead borosilicate, cadmium borate, lead-cadmium borosilicate, zinc borosilicate and sodium-cadmium borosilicate.

The thermistor compositions of the invention will usually, although not necessarily, be dispersed in an inert vehicle to form a paint or paste for application to various substrates. The proportion of vehicle to thermistor composition may vary considerably depending upon the manner in which the paint or paste is to be applied and the kind of vehicle used. Generally, from 2 to 20 parts by weight of the thermistor composition (metals, metal oxides and inorganic binder) per part by weight of vehicle will be used to produce a paste or paint of the desired consistency. Preferably, 4 to 10 parts per part of vehicle will be used.

Any liquid, preferably inert, may be employed as the vehicle. Water or various organic liquids, with or without thickening and/or stabilizing agents, and other common additives may be utilized as the vehicle. Examples of organic liquids that can be used are the higher alcohols such as decanol; esters of such alcohols-for example, the acetates and propionates; the terpenes such as pine oil, alphaand beta-terpineol and the like; and solutions of resins such a polymethacrylates of lower alcohols, or solutions of ethyl cellulose, in solvents such as pine oil and the monobutyl ether of ethylene glycol monoacetate (butyl-O--CH CH -OOCH The vehicle may contain or be composed of volatile liquids to promote fast setting after application; or it may contain waxes, thermoplastic resins or the like materials which are thermofiuid so that the vehicle-containing composition may be applied at an elevated temperature to a relatively cold ceramic body upon which the composition sets immediately.

The thermistor compositions are conventionally made by admixing the metals(s), metal oxides and inorganic binder solids in the proportions of 30 to 1 to 15% and 15 to 70%, respectively, based on their total combined weights. Additionally, one part of an inert vehicle for every 2 to 20 ports of solids listed above may be admixed. Then the thermistor composition is applied to a ceramic body and fired to form a stable thermistor.

In a preferred embodiment the C0 0 and MnO miture are calcined, for example, at 1000 C. for two hours, cooled and then ball milled to a finely divided powder, prior to admixing with the other components (i.e., metals, binder and vehicle).

Application of the thermistor composition in paint or paste form to the substrate may be effected in any desired manner. It will generally be desired, however, to effect the application in precise pattern form, which can be readily done employing well-known screen stencil techniques or methods. The resulting print or pattern will then be fired in the usual manner at a temperature of from about 650 to 820 C. (1200 to 1500 F.) in an air atmosphere employing the usual firing lehr.

The invention is illustrated by the following examples. In the examples and elsewhere in the specification, all parts, ratios, and percentages of materials or components are by weight.

Various thermistor compositions were prepared employing finely divided palladium silver, C0 0 Mn0 and inorganic binders in varying proportions. All were suspended in an inert vehicle consisting of 8% ethyl cellulose and 92% beta-terpineol. The metals, metal oxides and inorganic binders had particle sizes ranging from 0.1 to 20 microns, which are sufficiently finely divided to pass through a 325 mesh (U.S. Standard Sieve Scale) stencil screen. The weight ratio of solid thermistor composition to vehicle was the same in all cases, i.e. 4:1 to insure paints having a desirable consistency. The ratio of Co O :MnO 'was :20. The paints were printed onto alumina substrates and fired to 760 C. (1400 F.) for 45 minutes. The resistance (R) and temperature coefiicient of resistance (TOR) were determined for each example.

TABLE I Example No 1 2 3 4 5 6 7 CozOa/MIlOz, percent 30 10 10 10 6. 21 6. 88 7. 31 Palladium, percent. 60 32 31 30 22. 22. 69 36. 83 Silver, percent.-- 0 18 19 20 22. 85 22.69 36. 83 Inorganic Binder, 10 40 4 40 48. 09 47. 74 19. 03 R (ohms/square) 40, 000 35, 000 5, 000 15, 000 13, 000 2, l, 100 'ICR (p.p.m./ C.) NJ). -10, 000 -9, 000 -8, 000 3, 100 -3, 500 3, 500

1 The zinc borosllicate frlt composition set forth in Example 1 of U.S. Patent No. 3,207,700 was utilized.

6 11. The composition of claim 1 which is dispersed in an inert vehicle, said composition being present in an amount of from 2 to 20 parts by weight per part by weight 5 In the following tabulated Examples 8 to 14, inclusive, the C0 0 and Mn0 were pre-mixed, calcined at 1000" C. for two hours, cooled and then ball milled to a finely divided powder of particle size of from 0.1 to 20 microns.

of inert vehicle.

TABLE II Example No 8 9 10 11 12 13 14 CozOs/MnOz, percent... 11 10 10 10 10 10 13.1 Palladium, percent... 35 38 44 36 35 40 32.1 Silver, percent 14 12 11 14 13 1O 14. 9 Inorganic Binder, percent 40 40 35 40 42 40 39. 9 R (ohms/square) 1, 000 9, 000 7, 000 1, 000 l, 000 50, 000 5, 000 TOR (p.p.m./ 0.) 6, 000 6, 000 4, 000 4, 000 -4, 000 7, 000 6, 000

l A lead-cadmium borosilicate irit of the composition which consisted of 70.11% PbO, 13.44% B203, 9.55% SiOz and 6.90% 0110 was utilized.

Thus, the descriptive portion of the specification in conjunction with the tabulated data demonstrate that a proper balance must be maintained between the amounts of pal-' ladium, silver, Co O /MnO mixture and inorganic binder utilized so as to obtain the desired negative temperature coefiicient of resistance (T CR) and resistance (R).

By using the teachings of this invention, thermistor compositions having various TCRs and resistances can be tailor-made to suit the needs of those skilled in the art by proper adjustment of the type and amount of the metals, CO203/M11O2 and inorganic binder.

Since it is obvious that many changes and modifications can -be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

I claim:

1. A thermistor composition comprising (a) a finely divided metal powder consisting essentially of palladium and up to 60% silver by weight based on the total combined weight of palladium and silver, (b) a finely divided metal oxide powder mixture consisting essentially of C0 0 and MnO in the Co O :MnO weight ratio of from 85:15 to 45:60, and (c) a finely divided inorganic binder powder, said components (a), (b) and (c) being present in the amounts of 30 to 75%, 1 to and 15 to 70%, respectively, based on the total combined weights of (a), (b) and (c).

2. The composition of claim 1 wherein the co,o :Mno, weight ratio is from 82:18 to 78:22.

3. The composition of claim 1 wherein the Co O :MnO weight ratio is 80:20.

4. The composition of claim 1 wherein the average particle size of said finely divided components (a), (b) and (c) does not exceed microns.

5. The composition of claim 1 wherein the inorganic binder is a zinc borosilicate glass.

6. The composition of claim 1 wherein the inorganic binder is a lead-cadmium borosilicate glass.

7. The composition of claim 1 wherein the inorganic binder is a lead silicate glass.

8. The composition of claim 1 wherein the inorganic binder is a lead borosilicate glass.

9. The composition of claim 1 wherein the PdzAg weight ratio is about 50:50.

10. The composition of claim 1 wherein component (b) is a finely divided calcined mixture of C0 0 and MnO 12. The composition of claim 11 wherein the CO2O3:MHO2

weight ratio is from 82:18 to 78:22.

13. The composition of claim 11 wherein the weight ratio is 80:20.

14. The composition of claim 11 wherein the average particle size of said finely divided components (a), (b) and (0) does not exceed 20 microns.

15. The composition of claim 11 wherein the inorganic binder is a zinc borosilicate glass.

16. The composition of claim 11 wherein the inorganic binder is a lead-cadtium borosilicate glass.

17. The composition of claim 11 wherein the inorganic binder is a lead silicate glass.

18. The composition of claim 11 wherein the inorganic binder is a lead borosilicate glass.

19. The composition of claim 11 wherein the PdzAg weight ratio is about 50:50.

20. The composition of claim 11 wherein component (b) is a finely divided calcined mixture of C0 0 and M 21. A glaze thermistor comprising a ceramic dielectric substrate having fired thereon a thermistor material of the composition of claim 1.

22. A glaze thermistor comprising a ceramic dielectric substrate having fired thereon a thermistor material of the composition of claim 3.

23. A glaze thermistor comprising a ceramic dielectric substrate having fired thereon a thermistor material of the composition of claim 5.

24. A glaze thermistor comprising a ceramic dielectric substrate having fired thereon a thermistor material of the composition of claim 6.

25. A glaze thermistor comprising a ceramic dielectric substrate having fired thereon a thermistor material of the composition of claim 10.

References Cited UNITED STATES PATENTS 2,720,573 10/ 1955 Lundquist 252-519 XR 2,924,540 2/1960 DAndrea 252514 XR LEON D. ROSDOL, Primary Examinner.

J. D. WELSH, Assistant Examiner.