GB1559523A - Resistor material and resistor made thereform - Google Patents

Description

(54) RESISTOR MATERIAL AND RESISTOR MADE THEREFROM (71) We, TRW INC., a Corporation organized and existing under the laws of the State of Ohio, United States of America, of 10880 Wilshire Boulevard Los Angeles, California 20024, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a method of making resistors and to resistors when made by the method.

A type of electrical resistance material which has recently come into commercial use is a vitreous resistance material which comprises a mixture of a glass frit and finely divided particles of an electrical conductive material. The vitreous resistance material is coated on the surface of a substrate of an electrical insulating material, usually a ceramic, and fired to melt the glass frit. When cooled, there is provided a film of glass having the conductive particles dispersed therein. Terminations are connected to the film to permit the resultant resistor to be connected in the desired circuit.

The materials which have been generally used for the conductive particles are the noble metals. Although the noble metals provide vitreous resistance materials which have satisfactory electrical characteristics they have the disadvantage that they are expensive.

Thus, the resistors made from the vitreous resistance materials containing the noble metals are expensive to manufacture. Therefore, it would be desirable to have a vitreous electrical resistance material which utilises a relatively inexpensive conductive material so as to provide an electrical resistor which is relatively inexpensive to manufacture. In addition, the conductive material used must be capable of providing a resistance material having a wide range of resistance values and which is relatively stable over the entire range of the resistance values. By being stable it is meant that the resistance value of the resistance material does not change or changes only a small amount under operating conditions, particularly when subjected to changes in temperature. The change in resistance value of an electrical resistor per degree change in temperature is referred to as the"temperature coefficient of resistance"of the resistor. The closer the temperature coefficient of resistance is to zero, the more stable is the resistor with respect to changes in temperature.

According to the present invention there is provided a method of making an electrical resistor comprising: preparing a vitreous resistor composition consisting essentially of 25% to 90% by weight of a borosilicate glass frit and 75% to 10% by weight of finely divided conductive particles of a metal silicide selected from tungsten disilicide, molybdenum disilicide, vanadium disilicide, titanium disilicide, zirconium disilicide, chromium disilicide and tantalum disilicide ; Applying a coating of the composition to an insulating substrate; Firing the coated substrate at a temperature in the range 970 C to 1150 C and below the melting temperature of the conductive particles so that the glass frit becomes molten; Cooling the coated substrate, to form thereon a glass matrix having the conductive particles dispersed therein, and connecting terminations to the vitreous resistor composition.

The drawing is a cross-sectional view, on a highly exaggerated scale, of a resistor produced in accordance with the present invention.

In general, the vitreous resistance material of the present invention comprises a mixture of a glass frit and fine particles of a metal silicide of certain ones of the transistion elements of Groups IV, V and VI of the periodic chart. The metal silicide canbe molybdenum disilicide (MoSi2), tungsten disilicide (WSi2), vanadium disilicide (VSi2), titanium disilicide TiSi2), zirconium disilicide (ZrSi2), chromium disilicide (CrSi2) or tantalum disilicide (TaSi2). More particularly, the vitreous resistance material of the present invention comprises a mixture of a glass frit and a metal silicide of the above-stated group in the proportion of by weight, 25% to 90% glass frit and 75% to 10% metal silicide.

The glass frit used in the resistance material of the present invention has a melting temperature below that of the refractory metal silicide and is a borosilicate frit, such as lead, bismith, cadmium, barium, calcium or other alkaline earth metal borosilicate frit. The preparation of such glass frits is well known and consists, for example, in melting together the constituents of the glass in the form of the oxides of the constituents, and pouring such molten composition into water to form the frit. The batch ingredients may, of course, be any compounds that will yield the desired oxides under the usual conditions of frit production. For example, boric oxide may be obtained from boric acid, silicon dioxide may be produced from flint and, barium oxide may be produced from barium carbonate. The coarse frit is preferably milled in a ball-mill with water to reduce the particle size of the frit and to obtain a frit of substantially uniform size.

To make the resistance material of the present invention, the glass frit and, refractory metal silicide are broken down, such as by ball-milling, to a substantially uniform particle size. An average particle size of 1 to 2 microns has been found to be preferable. The glass frit and, refractory metal silicide powder are thoroughly mixed together, such as by ball-milling in water or an organic medium, such as butyl carbitol acetate or a mixture of butyl carbitol acetate and toluol. The mixture is then adjusted to the proper viscosity for the desired manner of applying the resistance material to a substrate by either adding or removing the liquid medium of the material.

To make a resistor with the resistance material the resistance material is applied to a uniform thickness on the surface of a substrate. The substrate may be a body of any material which can withstand the firing temperature of the resistance material composition.

The substrate is generally a body of a ceramic, glass, porcelain, refractory or, barium titanate. The resistance material may be applied on the substrate by brushing, dipping spraying or screen stencil application. The substrate with the resistance material coating is then fired in a conventional furnace at a temperature at which the glass frit becomes molten. For resistance materials containing any of the above-stated metal silicides except molybdenum disilicide, it has been found preferable to fire the coated substrate in an inert atmosphere, such as argon, helium, nitrogen or a mixture of nitrogen and hydrogen, to achieve a resistor of better stability. However, for a resistance material in which the metal silicide is molybdenum disilicide, it has been found that firing the resistor in air provides a more stable resistor. When the coated substrate is cooled, the glass hardens to bond the resistance material to the substrate.

As shown in the drawing, the resultant resistor of the present invention is generally designated as 10. Resistor 10 comprises a ceramic substrate 12 having a layer 14 of the resistance material of the present invention coated and fixed thereon. The resistance material layer 14 comprises the glass 16 and the finely divided particles 18 of the metal silicide embedded within and dispersed throughout the glass 16.

EXAMPLE I A plurality of resistance materials of the present invention were made in which the conductive material was molybdenum disilicide in the various amounts shown in Table I and the glass frit was a barium, titanium, aluminium borosilicate glass. Each of the resistance materials was made by mixing together the glass frit and molybdenum disilicide particles in a ball-mill in butyl carbitol acetate. Resistors were made with each of the resistance materials by coating cylindrical ceramic bodies with the resistance material and firing the coated ceramic bodies in a conveyor furnace for approximately a thirty minute cycle, at a temperature and in an atmosphere as indicated in Table I. A number of resistors of each of the compositions were made, and the average resistance values and temperature coefficients of resistance of the resulting resistors of each group are shown in Table I.

TABLE I Temperature Molybdenum And Temperature Coef. of Resistance Disilicide Firing Resistance (% per C) (% by wt.) Atmosphere (ohms/g) +25 C to 150 C +25 C to-55 C 15 1020 C-Air 1,900 +. 0080 +. 0053 20 1020 C-Air 490 . 0109 +. 0094 25 1020 C-Air 70 +. 0217 +. 0222 50 970 C-Air 6 +. 1420 +. 1465 60 970 C-Air 25 +. 1038 +. 1038 10 1050 CN2 8,900-. 0214-. 0346 15 1100 C-N2* 1,300 . 0055-. 0119 25 1020 C-N2 500 +. 0120 +. 006650 970 CN2 5 +. 1117 +. 1156 60 970 C-N2 4.3 +. 1196 +. 1222 * Fired on a 20 minute cycle.

EXAMPLE II A plurality of resistance materials of the present invention were made in which the conductive material was tungsten disilicide in the various amounts shown in Table II, and the glass frit was a barium, titanium, aluminium borosilicate glass. Each of the resistance materials was made in the same manner as the resistance materials of Example I, and resistors were made with each of the resistance materials in the same manner as described in Example 1. The resistors were fired at 1030 C in the type of atmosphere indicated in Table II and the average resistance values and temperature coefficients of resistance for each group of the resultant resistors are indicated in Table II.

TABLE II Tungsten Temperature Coef. of Resistance Disilicide Firing Resistance (% per C) (% by wt.) Atmosphere (ohms/7) +25 C to 150 C +25 C to-55 C 11 Air 5,000 +. 1346 +. 0984 15 Air 2,300 +. 0547 +. 0810 20 Air 600 +. 0570 +. 0957 25 Air 219 +. 1073 +. 1074 30 Air 75 +. 1307 +. 1286 11 N2 875,000-. IOI ()-. 1 : 158 15 N2 2,500-. 0063-. 0077 20 N2 5,000-. 0025-.0()n9 25 N2 2, 000 . 0055--0039 . 30 N2 1 500 +. 0162 +. 0123 50 N2 36 +. 0638 +. 0670 60 N2 21 +. 0683-. 0688 EXAMPLE III A plurality of resistance materials of the present invention were made in which the conductive material was zirconium disilicide in the various amounts shown in Table III and the glass frit was a barium, titanium, aluminium borosilicate glass. Each of the resistance materials was made in the same manner as the resistance materials of Example I, and resistors were made with each of the resistance materials in the same manner as described in Example I. The resistors were fired at 970 C in this type of atmosphere indicated in Table III, and the average resistance values and temperature coefficients of resistance for each group of th resultant resistors are indicated in Table III.

TABLE III Zirconium Temperature Coef. of Resistance Disilicide Firing Resistance (% per C) (% by wt.) Atmosphere (ohms/2) +25 C to 150 C +25 C to-55 C 15 N2 6,300 . 0021 . 0035 20 N2 475 +. 0225 +. 0232 25 N2 104 +. 0252 +. 0278 30 N2 44 +. 0265 +. 0277 15 Air 3,000 +. 0130 +. 0127 20 Air 610 +. 0184 +. 0178 25 Air 238 +. 0285 +. 0257 30 Air 112'+. 0334 +. 0344 EXAMPLE IV Table IV shows the resistance values and temperature coefficiants of resistance of a number of resistors of the present invention using resistance materials made from the various metal silicides indicated in Table IV in the indicated amounts with a barium, titanium borosilicate glass frit. The resistance materials were made in the same manner as the resistance materials of Example I and resistors were made with the resistance material in the same manner as described in Example 1. The resistors were fired at approximately 1000 C in a Nitrogen atmosphere.

TABLE IV Temperature Coef. of Resistance Conducting % by Resistance (% per C) Material Weight (ohms/El) +25 C to 150 C +25 C to-55 C TiSi2 15 124 +. 0163 +. 0161 TiSi2 25 63 +. 0166 +. 0161 TiSi2 30 41 +. 0143 +. 0154 VSi2 20 1,300 +. 0222 . 0108 VSi2 25 275 +. 0298 . 0355 VSi2 30 42 . 0411 . 0495 CrSi2 20 275 +. 0184 +. 0235 CrSi2 30 99 +. 0588 +. 0780 TaSi2 50 81 +. 0319 +. 0303 EXAMPLE V A plurality of resistance materials of the present invention were made in which the conductive material was a metal silicide shown in Table V, and the glass frit was a barium, titanium borosilicate glass. Each of the resistance materials was made in the same manner as the resistance material of Example I, and the resistors were made with each of the resistance materials in the same manner as described in Example I. The resistors were fired in a nitrogen atmosphere on a 30 minute cycle at a temperature as indicated in Table V and the average resistance values and temperature coefficients of resistance for the resultant resistors are shown in Table V.

TABLE V Conducting Temperature Coef. of Resistance Material Firing Resistance (% per C) (% by vol.) Temperature (ohms/g) +25 C to 150 C +25 C to-55 C WSi2 5% 1150 C 9K-. 0148-. 0220 MoSi2 6% 1100 C 925 +. 0257 65.0215 MOSi2 8% 1100 C 560 +. 0327 +. 0304 MoSi2 10% 1100 C 413 +. 0372 +. 0360 WSi2 12% 1100 C 269 +. 0268 +. 0297 WSi2 15% 1100 C 179 +. 0294 +. 0294 EXAMPLE VI A plurality of resistance materials of the present invention were made in which 30% by weight of a silicide shown in Table VI, and 70% by weight of a barium, titanium, aluminium borosilicate frit were used. Each of the resistance materials was made in the same manner as the resistance materials of Example I, and resistors were made with each of the resistance materials in the same manner as described in Example I. The resistors were fired in a nitrogen atmosphere on a 30 minute cycle at a temperature indicated in Table VI. The average resistance values, temperature coefficients of resistance of the resistors, and the reaction products for the resultant resistor glazes are shown in Table VI. The reaction products for the resistor glazes were determined by analysis of detected X-ray diffraction patterns. The detected products are given in the order of decreasing strength of their diffraction pattern lines.

TABLE VI Temperature Coef. of Metal Firing Resistance Resistance (% per C) Reaction Silicide Temperature (ohms/El) +25 C to +25 C to Products 130 C-55 C WSi2 1100 C 1K +. 0206 +. 0209 oWB. WSiz MoSi2 1100 C 13 +. 1092 +. 1010 MoSi2, Mo2B5 VSi2 1100 C 33 +. 0931 +. 1042 VSi2, BaSi205 CrSi2 1100 C 21 +. 0960 +. 1266 CrSi2, CrB2, BaSi205 TaSi2 1100 C Non---TaSi, Cond. TaB2, yTaB TaSi2 1150 C 80 +. 0340 +. 0137 TaSi., TaB2, yTaB TiSi2 1100 C 9 +. 0464 . 0303 TiSi2, TiB2, BaSi203, TiC2 ZnSi2 1100 C 9 +. 0526 +. 0485 ZrSi2, ZrB2 * 50% by weight of TaSi2 fired in nitrogen on a 20 minute cycle.

Analysis of the diffraction pattern data of the resistor glazes in Table VI, indicates that during the firing of the resistance material, the silicon of the metal silicide has a strong tendancy to react with the glass. The remaining metal of the silicide then combines with boron from the glass to form a boride or with barium to form a mixed oxide. The conductors which are formed by firing the resistance materials, thus, include both the metal silicides and their borides.

Claims (6)

WHAT WE CLAIM IS :-

1. A method of making an electrical resistor comprising : preparing a vitreous resistor composition consistmg essentially of 25% to 90% by weight of a borosilicate glass frit and 75% to 10% by weight of finely divided conductive particles of a metal silicide selected from tungsten disilicide, molybdenum disilicide, vanadium disilicide, titanium disilicide, zirconium disilicide, chromium disilicide and tantalum disilicide ; applying a coating of the composition to an insulating substrate; firing the coated substrate at a temperature in the range 970 C to 1150 C and below the melting temperature of the conductive particles so that the glass frit becomes molten; cooling the coated substrate, for form thereon a glass matrix having the conductive particles dispersed therein, and connecting terminations to the vitreous resistor composition.

2. The method as claimed in claim 1, wherein the coated substrate is fired in air.

3. The method as claimed in claim 1, wherein the coated substrate is fired in a non-oxidising atmosphere.

4. The method as claimed in claim 3, wherein the non-oxidising atmosphere comprises nitrogen.

5. A method of making an electrical resistor as claimed in claim 1 and substantially as hereinbefore described with reference to any one of the Examples.

6. An electrical resistor when made by the method claimed in any preceding claims.