Electrical resistor material, resistor made therefrom and method of making the same

A vitreous enamel resistor material comprising a mixture of a glass frit, and fine particles of tin oxide (SnO.sub.2), a primary additive of particles of oxides of manganese, nickel, cobalt or zinc, and a supplemental additive of oxides of tantalum, niobium, tungsten or nickel. An electrical resistor is made from the resistor material by applying the material to a substrate and firing the coated substrate to a temperature at which the glass melts. The tin oxide may be heat treated prior to mixing the glass frit. Upon cooling, the substrate has on the surface thereof, a film of the glass having the particles of the mixture embedded therein and dispersed therethroughout. The resistor material provides a resistor having a wide range of resistivities and a low temperature coefficient of resistance.

The present invention relates to a resistor material, resistors made from 
the material, and a method of making the material. More particularly, the 
present invention relates to a vitreous enamel resistor material which 
provides resistors over a wide range of resistivities and with relatively 
low temperature coefficients of resistance, and which are made from 
relatively inexpensive materials. 
A type of electrical resistor material which has recently come into 
commercial use is a vitreous enamel resistor material which comprises a 
mixture of a glass frit and finely divided particles of an electrical 
conductive material. The vitreous enamel resistor 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. 
Since there are requirements for electrical resistors having a wide range 
of resistance values, it is desirable to have vitreous enamel resistor 
materials with respective properties which will allow the making of 
resistors over a wide range of resistance values. However, a problem has 
arisen with regard to providing a vitreous enamel resistor material which 
will provide resistors having a wide range of resistivity values and which 
are also relatively stable with changes in temperature, i.e., has a low 
temperature coefficient of resistance. The resistor materials which 
provide both a wide range of resistivities and low temperature 
coefficients of resistance generally utilize the noble metals as the 
conductive particles and are therefore relatively expensive. As described 
in the article by J. Dearden entitled "High Value, High Voltage 
Resistors," ELECTRONIC COMPONENTS, March 1967, pages 259-261, a vitreous 
enamel resistor material using tin oxide doped with antimony has been 
found to provide high resistivities and is of a less expensive material. 
However, this material also has a high negative temperature coefficient of 
resistance. 
It is therefore an object of the present invention to provide a novel 
resistor material and resistor made therefrom. 
It is another object of the present invention to provide a novel vitreous 
enamel resistor material and a resistor made therefrom. 
It is still a further object of the present invention to provide a vitreous 
enamel resistor material which provides a resistor having a wide range of 
resistivities and a relatively low temperature coefficient of resistance. 
It is another object of the present invention to provide a vitreous enamel 
resistor material including tin oxide particles which provides a resistor 
having a lower resistivity than is attainable with a tin oxide glaze 
resistor, a relatively low temperature coefficient of resistance, and the 
high stability of such glaze resistors but without using expensive 
material. 
It is yet another object of the present invention to provide a vitreous 
enamel resistor material which provides resistors having a high 
compatibility with inexpensive nickel terminations. 
Other objects will appear hereinafter. 
These objects are achieved by a resistor material comprising a mixture of a 
glass frit and finely divided particles of tin oxide, a primary additive 
of MnO.sub.2, NiO, Co.sub.3 O.sub.4, or ZnO, and a supplemental additive 
of Ta.sub.2 O.sub.5, NiO, Nb.sub.2 O.sub.5 or WO.sub.3. The tin oxide may 
be heat treated prior to mixing with the glass frit. 
The invention accordingly comprises a composition of matter possessing the 
characteristics, properties, and the relation of components which are 
exemplified in the compositions hereinafter described, and the scope of 
the invention is indicated in the claims.

In general the vitreous enamel resistor material of the present invention 
comprises a mixture of a vitreous glass frit and fine particles of tin 
oxide (SnO.sub.2). The glass frit is present in the resistor material in 
the amount of 10% to 80% by volume, and preferably in the amount of 35% to 
60% by volume. A primary additive of MnO.sub.2, NiO, Co.sub.3 O.sub.4 or 
ZnO of between 0.07 to 18.5% by volume, and preferably between 1 to 10% by 
volume is included in the mixture, while a supplemental additive, when 
used, provides by volume of the mixture up to about 1% tantalum oxide, 
0.4% niobium oxide, 7% tungsten trioxide, or 5% nickel oxide. 
The glass frit used must have a softening point below the melting point of 
the oxide particles of the conductive phase. It has been found that the 
use of a borosilicate frit is preferable, and particularly an alkaline 
earth borosilicate frit, such as a barium or calcium borosilicate frit. 
The preparation of such frits is well known and consists, for example, of 
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 compound that 
will yield the desired oxides under the usual conditions of frit 
production. For example, boric oxide will be obtained from boric acid, 
silicon dioxide will be produced from flint, barium oxide will be produced 
from barium carbonate, etc. 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. 
The resistor material of the present invention may be made by thoroughly 
mixing together the glass frit, and the tin oxide and additive particles 
in the appropriate amounts. The mixing is preferably carried out by ball 
milling the ingredients 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 resistor material to a substrate by either adding or removing 
the liquid medium of the mixture. For screen stencil application, the 
liquid may be evaporated and the mixture blended with a screening vehicle 
such as manufactured by L. Reusche and Company, Newark, N.J. 
Another method of making the resistor material which provides a wider 
resistance range and better control of temperature coefficient of 
resistivity, is to first heat treat the tin oxide. The heat treated tin 
oxide is then mixed with the additives and glass frit to form the resistor 
material. The tin oxide powder was heat treated as follows: A boat 
containing the tin oxide is placed on the belt of a continuous furnace. 
The boat is fired at a peak temperature of 575.degree. C. over a one-half 
hour cycle in a forming gas atmosphere (95% N.sub.2 and 5% H.sub.2). 
To make a resistor with the resistor material of the present invention, the 
resistor 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 resistor material. The substrate is 
generally a body of a ceramic or glass, such as porcelain, steatite, 
barium titanate, alumina, or the like. The resistor material may be 
applied on the substrate by brushing, dipping, spraying, or screen stencil 
application. The resistor material is then dried, such as by heating at a 
low temperature, e.g., 150.degree. C. for 15 minutes. The vehicle mixed 
with the tin oxide may be burned off by heating at a slightly higher 
temperature prior to the firing of the resistor. 
The substrate with the resistor material coating is then fired in a 
conventional furnace at a temperature at which the glass frit becomes 
molten. The resistor material is fired in an inert atmosphere, such as 
argon, helium or nitrogen. The resistance and temperature coefficient of 
resistance varies with the firing temperature used. The firing temperature 
can be selected to provide a desired resistance value with an optimum 
temperature coefficient of resistance. The minimum firing temperature, 
however, is determined by the melting characteristics of the glass frit 
used. When the substrate and the resistor material are cooled, the 
vitreous enamel hardens to bond the resistance material to the substrate. 
As shown in the FIGURE of the drawing, a resultant resistor of the present 
invention is generally designated as 10. Resistor 10 comprises a ceramic 
substrate 12 having a layer 14 of the resistor material of the present 
invention coated and fired thereon. The resistor material layer 14 
comprises the glass 16 containing the finely divided tin oxide and 
additive oxide particles 18. The tin oxide and additive oxide particles 18 
are embedded in and dispersed throughout the glass 16. 
The following examples are given to illustrate certain preferred details of 
the invention, it being understood that the details of the examples are 
not to be taken as in any way limiting the invention thereto. 
EXAMPLE I 
A resistance material was made by mixing together 55% by volume of tin 
oxide particles (SnO.sub.2) which were heat treated as described above and 
additive particles, and 45% by volume of particles of a glass of the 
composition, by weight, of 50% barium oxide (BaO), 20% boron oxide 
(B.sub.2 O.sub.3) and 30% silicon dioxide (SiO.sub.2). The tin oxide, 
additives and glass mixture was ball milled in butyl carbitol acetate for 
one day. The butyl carbitol acetate was then evaporated and the dry 
mixture was then blended with a Ruesche screening vehicle on a three roll 
mill. 
The resistance material was made into resistors by screening the material 
onto alumina substrates containing thick film nickel termination pads. The 
resistance material layers were dried for 15 minutes at 150.degree. C. 
Various ones of the resistors were then fired at a temperature of 
1000.degree. C. over a one-half hour cycle in a nitrogen atmosphere in a 
continuous belt furnace. The resistors formed on the substrates each had a 
length of one and a half times their width, each providing 1.5 square 
resistor patterns. 
Table I shows the resistance values and temperature coefficients of 
resistance of the various resistors made in accordance with Example I for 
the volume % of the additives shown. 
TABLE I 
______________________________________ 
Temperature Coefficient 
Resistance K 
of Resistance (ppm/.degree. C.) 
Additive 
Volume % ohms/square 
-81.degree. C. 
150.degree. C. 
______________________________________ 
None 0 54.0 42 136 
MnO.sub.2 
0.10 48.6 198 186 
1.1 25.8 43 206 
8.4 24.6 -1334 -589 
NiO 0.07 45.1 246 201 
0.73 13.7 .+-.44 315 
5.0 13.7 -493 -328 
Co.sub.3 O.sub.4 
0.08 44.2 207 193 
5.3 10.3 182 505 
10.5 50.8 -130 -108 
ZnO 0.33 44.4 56 122 
9.4 4.97 187 704 
18.5 31.2 -2576 -2704 
______________________________________ 
EXAMPLE II 
A resistance material was made in the same manner as in Example I, except 
that the tin oxide particles were not heat treated, the additive being 
9.44% by volume of zinc oxide (ZnO). The resistance material was made into 
resistors in the same manner as described in Example I. Table II shows the 
resistance values and temperature coefficients of resistance of the 
resistors made without and with heat treated tin oxide particles 
(SnO.sub.2). 
TABLE II 
______________________________________ 
Heat Volume % Resistance 
Temperature Coefficient 
Treatment 
ZnO K ohms of Resistance (ppm/.degree. C.) 
of SnO.sub.2 
Additive per square 
-81.degree. C. 
+150.degree. C. 
______________________________________ 
575.degree. C. 1/2 
9.44 4.97 187 704 
Hr in 95% 
N.sub.2 /5% H.sub.2 
None 9.44 5.70 103 638 
______________________________________ 
EXAMPLE III 
A resistance material was made in the same manner as in Example I, except 
that composition "A" of the glass particles contained, by weight, 48% 
barium oxide (BaO), 8% calcium oxide (CaO), 23% boron oxide (B.sub.2 
O.sub.3), and 21% silicon dioxide (SiO.sub.2), and composition "B" 
contained, by weight, 42% barium oxide (BaO), 23% boron oxide (B.sub.2 
O.sub.3), and 29% silicon dioxide (SiO.sub.2). The resistance materials 
were made into resistors in the same manner as described in Example I. 
Table III shows the resistance values and temperature coefficients of 
resistance of the resistors. 
TABLE III 
______________________________________ 
Glass Resistance 
Temperature Coefficient 
Composi- 
Volume % K ohms of Resistance (ppm/.degree. C.) 
tion Additive per square 
-81.degree. C. 
+150.degree. C. 
______________________________________ 
A 9.44 5.83 -214 323 
ZnO 
B 0.89 7.87 -440 .+-.38 
Co.sub.3 O.sub.4 
______________________________________ 
EXAMPLE IV 
A resistance material was made in the same manner as in Example I, and the 
resistance material was made into resistors in the same manner as 
described in Example I. Table IV shows the resistance values and 
temperature coefficients of resistance of the resistors which were fired 
at different temperatures. 
TABLE IV 
______________________________________ 
Temperature 
of Coefficient 
Peak Resistance 
Resistanct 
Addi- Volume Firing K ohms (ppm/.degree. C.) 
tive % Temp. .degree. C. 
per square 
-81 +150 
______________________________________ 
MnO.sub.2 
1.1 950.degree. C. 
79.0 -36 40 
1050.degree. C. 
11.8 175 226 
NiO 0.73 950.degree. C. 
37.5 .+-.21 91 
1050.degree. C. 
5.2 196 443 
Co.sub.3 O.sub.4 
5.3 950.degree. C. 
18.2 166 337 
1000.degree. C.* 
6.8 68 541 
1050.degree. C. 
5.0 224 541 
ZnO 9.4 950.degree. C. 
6.5 432 714 
1000.degree. C.* 
18.9 448 124 
______________________________________ 
*Fired for 1 hour 
EXAMPLE V 
Resistance materials were made in the same manner as Example I, using 
various primary and supplemental additives, and the materials were used to 
make resistors in the same manner as described in Example I. Table V shows 
the resistance values and temperature coefficients of resistance of the 
resistors for the various compositions. 
TABLE V 
______________________________________ 
Resis- 
tance Temp. Coeff. 
Supple. Vol- K ohms of Resistance 
Primary 
Volume Addi- ume per (ppm/.degree. C.) 
Additive 
% tive % square -81 +150 
______________________________________ 
MnO.sub.2 
1.1 None -- 25.8 43 206 
1.07 Ta.sub.2 O.sub.5 
0.33 40.7 -142 -97 
1.4 NiO 1.9 9.99 -120 16 
NiO 0.73 None -- 13.7 .+-.44 
315 
0.73 Ta.sub.2 O.sub.5 
0.33 9.09 -147 -139 
Co.sub.3 O.sub.4 
5.32 None -- 10.3 182 505 
5.32 Ta.sub.2 O.sub.5 
0.23 6.88 -65 -63 
1.78 NiO 1.91 7.93 268 43 
ZnO 9.4 None -- 4.97 187 704 
9.45 Ta.sub.2 O.sub.5 
0.16 1.86 54 57 
9.44 Nb.sub.2 O.sub.5 
0.07 2.23 -131 42 
9.45 WO.sub.3 
3.7 2.86 96 164 
MnO.sub.2 / 
1.07/ NiO 1.43 6.57 143 40 
Co.sub.3 O.sub.4 
1.33 
______________________________________ 
EXAMPLE VI 
Resistance materials were made in the same manner as in Example I with the 
glass content varying from 10 to 80 volume percent, and tin oxide and 
additive particles, as shown in Table VI. The resistance materials were 
made into resistors in the same manner as described in Example I. Table VI 
shows the resistance values of the resistors. 
TABLE VI 
______________________________________ 
Tin Oxide Resistance 
Glass SnO.sub.2 Additive 
K ohms 
Volume % 
Volume % Additive Volume % 
per square 
______________________________________ 
80.0 20.0 None -- 356 
19.55 NiO 0.45 7066 
19.25 MnO.sub.2 0.75 3917 
18.0 Co.sub.3 O.sub.4 
2.00 255 
17.0 ZnO 3.00 2255 
60.0 40.0 None -- 470 
39.25 MnO.sub.2 0.75 479 
38.0 Co.sub.3 O.sub.4 
2.00 120.4 
37.0 ZnO 3.00 122.4 
59.23 40.32 NiO 0.45 369 
45.0 55.0 None -- 54.9 
54.27 NiO 0.73 13.7 
53.9 MnO.sub.2 1.10 28.5 
49.68 Co.sub.3 O.sub.4 
5.32 10.3 
45.6 ZnO 9.40 4.97 
35.0 65.0 None -- 11.8 
64.55 NiO 0.45 5.70 
64.25 MnO.sub.2 0.75 8.26 
63.0 Co.sub.3 O.sub.4 
2.00 2.88 
62.0 ZnO 3.00 2.39 
30.0 70.0 None -- 9.31 
69.55 NiO 0.45 5.67 
69.25 MnO.sub.2 0.75 5.92 
68.0 Co.sub.3 O.sub.4 
2.00 2.42 
67.0 ZnO 3.00 2.07 
20.0 80.0 None -- 10.3 
79.55 NiO 0.45 5.07 
79.25 MnO.sub. 2 
0.75 2.54 
78.0 Co.sub.3 O.sub.4 
2.00 1.41 
77.0 ZnO 3.00 2.69 
15 85.0 None -- 10.6 
84.55 NiO 0.45 5.71 
84.25 MnO.sub.2 0.75 2.97 
83.00 Co.sub.3 O.sub.4 
2.00 1.49 
82.00 ZnO 3.00 10.5 
10 90.0 None -- 19.9 
89.55 NiO 0.45 11.2 
89.25 MnO.sub.2 0.75 7.7 
88.00 Co.sub.3 O.sub.4 
2.00 1.63 
87.00 ZnO 3.00 27.7 
______________________________________ 
From the above examples there can be seen the effects, on the electrical 
characteristics of the resistor of the present invention, of variations in 
the composition of the resistance material and the method of making the 
resistance material. Examples I, III, V and VI show the effects of varying 
the composition and ratio of the oxide particles. Example II shows the 
effect of heat treating the tin oxide particles, while Example III shows 
the effects of varying the composition of the glass frit. Example IV shows 
the effect of varying the firing temperature of the resistors, and Example 
VI shows the effect of varying the composition, and proportion of the 
glass particles to the tin oxide and additive particles. Thus, there is 
provided by the present invention a vitreous enamel resistor using tin 
oxide and additives which is relatively stable with regard to temperature 
and is made of materials which are relatively inexpensive. 
The resistors of the invention were terminated with thick film nickel glaze 
terminations to obtain the test results. Resistor glazes based on noble 
metals are typically terminated with expensive precious metal materials 
such as platinum, palladium, and gold. This resistor, however, is 
compatible with terminations made on non-noble metals such as copper and 
nickel. This has the advantage of both reducing the cost of the resistor, 
and providing a more solderable termination. 
It will thus be seen that the objects set forth above, among those made 
apparent from the preceding description, are efficiently attained and, 
since certain changes may be made in the above composition of matter 
without departing from the scope of the invention, it is intended that all 
matter contained in the above description shall be interpreted as 
illustrative and not in a limiting sense.