US 3207706 A
Description (OCR text may contain errors)
United States Patent M 3,207,706 RESISTOR COMPOSITIONS Lewis Charles Hofiman, Wyclifie, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed Sept. 20, 1962, Ser. No. 225,121 3 Claims. (Cl. 252-514) This invention relates to electrical resistors; and more particularly, it relates to new and improved resistor compositions.
Precious metal resistor compositions containing a vitreous enamel binder have recently become highly desirable for use in the production of fired electrical resistors. Such resistor compositions and resistors prepared therefrom are, for example, shown in DAndrea US. Patent No. 2,924,540 and Dumesnil US. Patent 3,052,573.
Electrical resistors made with these and other prior art compositions have had an undesirably high temperature coefiicient of resistance and an objectionable high percentage of drift.
Temperature coefiicient of resistance, (TCR), generally expressed in parts per million per degree centigrade, is an important characteristic of resistors since changes in temperature will create relatively large changes in resistance when the TCR is high. TCR is generally measured by measuring (1) Resistance at room temperature (2) Resistance at -75 C. (3) Resistance at 105 C.
great care being taken to achieve thermal equilibrium at each temperature. The change in resistance is expressed as a function of the room temperature resistance, divided by the temperature increment to give the coefiicient.
Drift is referred to as an irreversible change of resistance upon heating. This effect may be noticeable at temperatures as low as 100 C. This was a severe limitation on the aforementioned prior art resistor-s. One Way of testing for this effect is to measure the resistance of a soldered, completed resistor and place it in an oven, at atmospheric conditions, at 150 C. for 16 hours. After this high temperature exposure, the resistance is remeasured and the change expressed as a percentage of the original value. The prior art compositions averaged about 5% under these test conditions.
It is an object of this invention to produce resistor compositions composed of finely divided silver and palladium and finely divided glass frit which when incorporated in a fired resistor will have a reduced TCR and lower percentage of drift.
Other objects will appear hereinafter.
These objects may be accomplished by mixing together 35 to 85 weight percent of a finely divided frit of a specific composition with to 65 weight percent of finely divided palladium and finely divided silver in a weight ratio of Pd:Ag of 3:2 to 2:3.
The finely divided dry mixture may be made into a paste or liquid by the addition of a vehicle in a known manner. Fifteen percent to sixty percent by weight of an inert liquid, for example, Water, methyl, ethyl, propyl, butyl or higher alcohols, the corresponding esters such as acetates, propionates, the terpenes and liquid resins such as pine oil or alpha or beta terpineol and the like may be used to prepare a suitable consistency for application purposes. The vehicles may contain or be composed of volatile liquids to promote fast setting after application, or they may contain waxes, thermoplastic resins, or wax-like materials which are thermofiuid by nature whereby the composition may be applied to a 3,207,706 Patented Sept. 21, 1965 ceramic insulator while at elevated temperature to set upon contact with a cold ceramic base.
The essential combination of the present invention is the specific composition of the glass frit together with a ratio of Pd:Ag of 3:2 to 2:3. The glass frit must have a linear thermal expansion between about 3% and 7% lower than the linear expansion of the substrate on which it is fired to produce the resistor. It is common to fabricate resistor substrates, or bases, from ceramic bodies having a thermal expansion of around 10 C. Such bodies are usually composed of about 94 to 99% A1 0 Temperature coefficients of resistance of less than about 200 ppm./ C. may be obtained on such ceramic bodies with a frit having a thermal expansion of about 73 to 77 lO-"". Other ceramic bodies, e.g., steatite, titanates, mullite, glass and zircon porcelains may, however, be used as substrates for resistors of this invention.
The glass frit for use in accordance With this invention should contain, in weight percent, 2434% ZnO, 18-25% SiO 22-30% B 0 3-7% A1 0 4-10% Na O, 4 6% ZrO 0-4% CaO, 0-4% P 0 less than 1% each, or less than 2% total amount, of PbO, Sb O' TiO NiO or Fe O and less than 0.1% of other alkali metal oxides. The above percentages of the constituents are essential to obtain the low TCR and drift in resistors produced from the resistor compositions of this invention.The easily reducible oxides, PbO, Sb O TiO NiO and Fe O must either be entirely absent or in very small amounts, as indicated, since these can react with the silver and palladium metal powders and give rise to unstable alloy phases or semi-conducting oxides with deleterious effect. The presence of A1 0 in the compositions is essential for low drift and the minimizing of time consuming elastic recovery processes in the glass enamel. Zr0 is essential in my compositions for low voltage coefiicient and the prevention of dissolution of sodium ions in resistor surface moisture films with resulting ionic surface conductivity.
The following table discloses eight examples of frit compositions suitable for use in producing the low TCR and drift resistors of this invention.
TABLE I Frit compositions, weight percent The frit is prepared in the conventional manner by melting the constituents of the examples at about 1200 C. in a fire clay crucible for just sufficient time to dissolve all the constituents. It is then quenched in water and ball milled to a particle size of 0.1 to 50 microns and then dewatered and dried.
The resistor compositions are prepared by mixing with about 35-85% of the frit 65% to 15% of a mixture of finely divided silver and palladium in which the silver and palladium have a weight ratio of 3:2 to 2:3. The silver and palladium may be obtained by chemical precipitation techniques or in any other known manner and may consist of relatively pure silver and palladium or to a large extent of silver oxide or palladium oxide. The
3 silver and palladium or their oxides should have a particle size of 0.1 to 50 microns.
These materials may be dry mixed or they may be mixed with a liquid or pasty vehicle to produce liquid or pasty resistor compositions for easy application to a ceramic base. The most usual manner of forming resistors is to apply a paste form of the resistor composition by a screen stencil operation on a ceramic base and then subject the same to a firing operation to form a glaze resistor film in which the metal particles are imbedded. During the firing operation any silver oxide present is usually reduced to relatively pure silver and palladium or palladium oxide ends in a composition containing palladium and a minor amount of palladium oxide.
The firing of the resistor composition is commonly carried out at a temperature of 750 C. to 780 C. for about minutes.
The fired resistor pattern on the ceramic substrate should be provided with a solderable terminal connection. This may be done by applying a relatively heavy coating of the silver-palladium-frit resistor composition at each end of the fired resistor and refiring at a sufiicient temperature to form a glaze film. Alternatively, a solderable silver composition or a solderable platinum-gold composition can be applied over each end of the fired resistor pattern, and the whole refined at 750 C. to 780 C. If the solderable terminal compositions are applied under the resistor pattern at the ends thereof, only one firing operation will be necessary.
The new resistor compositions of this invention as illustrated in the above examples when tested for drift and TCR gave the following average results as compared to average results of these characteristics of prior art compositions as illustrated in the aforementioned DAndrea and Dumesnil patents.
Throughout the specification and claims, any reference to parts, proportions and percentages refers to parts, proportions and percentages by weight unless otherwise specified.
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.
1. A resistor composition consisting essentially of 35 to 85 percent of a frit having 2434% ZnO, 18-25% SiO 22-30% B 0 3-7% A1 0 4-10% Na O, 46% ZrO 04% CaO, 04% P 0 less than 1% each and less than 2% total amount of PbO, Sb O TiO NiO and Fe O and less than 0.1% of other alkali metal oxides, and to 15% of silver and a substance of the group consisting of palladium and palladium oxide, in which the ratio of Ag:the Pd content of said substance is between 3:2 and 2:3.
2. A resistor composition consisting essentially of 35 to percent of a frit having 24-34% ZnO, 18-25% SiO 22-30% B 0 3-7% A1 0 4-10% Na O, 46% ZrO 0-4% CaO, O-4% P 0 less than 1% each and less than 2% total amount of PbO, Sb O TiO NiO and F3203 and less than 0.1% of other alkali metal oxides, and 65% to 15 of silver and a substance of the group consisting of palladium and palladium oxide, in which the ratio of Agzthe Pd content of said sustance is between 3:2 and 2:3, in admixture with 15 to 60 percent by weight of the aforesaid composition of an inert vehicle.
3. A glass enamel frit suitable for use in a resistor composition consisting essentially of 24-34% ZnO, 18- 25% SiO 22-30% B 0 3-7% A1 0 4l0% Na O, 46% ZrO 0-4% CaO, 04% P 0 less than 1% each andless than 2% total amount of PbO, Sb O TiO NiO and Fe O and less than 0.1% of other alkali metal oxides.
References Cited by the Examiner UNITED STATES PATENTS 2,688,560 9/54 Armistead 106--54 XR 2.726,965 12/55 Cressman et al. 106-48 2,924,540 2/60 DAndrea 252514 XR 3,052,573 9/62 Dumesnil 252520 XR JULIUS GREENWALD, Primary Examiner.