Title:
Process for making a voltage dependent resistor
United States Patent 3872582
Abstract:
A process for making a voltage dependent resistor which has a zinc oxide sintered body which itself has voltage dependent properties. The process is made up of the steps of: (1) providing a formed body of a powder mixture having as a major part zinc oxide and additive; (2) coating on the side surfaces of the formed body a paste having as the solid ingredient composition at least one member selected from the group of a) more than 50 mole % of silicon dioxide (SiO2), and less than 50 mole % of bismuth oxide (Bi2 O3), b) the same composition as that of said additive, c) more than 30 mole percent of antimony oxide (Sb2 O3) and less than 70 mole percent of bismuth oxide (Bi2 O3), and d) more than 50 mole percent of indium oxide (In2 O3) and less than 50 mole percent of bismuth oxide (Bi2 O3); (3) sintering the coated body; and (4) applying electrodes to opposite surfaces of the sintered body.
US Patent References:
NON-LINEAR RESISTORS
Matsuoka et al. - February 1970 - 3496512
NON-LINEAR RESISTOR
Matsuoka - March 1970 - 3503029
/3632528.html
Matsuoka et al. - January 1972 - 3632528
/3663458.html
Masuyama et al. - May 1972 - 3663458
PROCESS FOR MAKING A VOLTAGE DEPENDENT RESISTOR
Masuyama et al. - September 1973 - 3760318
NON-LINEAR RESISTORS
Matsuoka et al. - February 1970 - 3496512
NON-LINEAR RESISTOR
Matsuoka - March 1970 - 3503029
/3632528.html
Matsuoka et al. - January 1972 - 3632528
/3663458.html
Masuyama et al. - May 1972 - 3663458
PROCESS FOR MAKING A VOLTAGE DEPENDENT RESISTOR
Masuyama et al. - September 1973 - 3760318
Inventors:
Matsuoka, Michio (Osaka, JA)
Itakura, Gen (Osaka, JA)
Iga, Atsushi (Osaka, JA)
Masuyama, Takeshi (Osaka, JA)
Itakura, Gen (Osaka, JA)
Iga, Atsushi (Osaka, JA)
Masuyama, Takeshi (Osaka, JA)
Application Number:
05/428737
Publication Date:
03/25/1975
Filing Date:
12/27/1973
View Patent Images:
Export Citation:
Assignee:
Matsushita Electric Industrial Co., Ltd. (Osaka, JA)
Primary Class:
Other Classes:
252/519.540, 29/610.100, 338/20
International Classes:
H01B1/08; H01C7/102; H01C17/02; H01C17/00; H01C1/14; H01C17/00
Field of Search:
29/610,621 338/20,21,22R 252/518,521
US Patent References:
Primary Examiner:
Herbst, Richard J.
Assistant Examiner:
Di Palma, Victor A.
Attorney, Agent or Firm:
Wenderoth, Lind & Ponack
Claims:
What we claim is
1. A process for making a voltage dependent resistor comprised of a zinc oxide sintered body which itself has voltage dependent properties, said process comprising: (1) providing a formed body of powder mixture comprising, as a major part, zinc oxide and the remainder being an additive; (2) coating on the side surface of said body a paste having a solid ingredient composition of at least one member selected from the group consisting of a) more than 50 mole percent of silicon dioxide (SiO2) and less than 50 mole percent of bismuth oxide (Bi2 O3), b) the same composition as that of said additive, c) more than 30 mole percent of antimony oxide (Sb2 O3) and less than 70 mole percent of bismuth oxide (Bi2 O3), and d) more than 50 mole percent of indium oxide (In2 O3) and less than 50 mole percent of bismuth oxide (Bi2 O3); (3) sintering said coated body; and (4) applying two electrodes to the opposite end surfaces of said sintered body.
2. A process according to claim 1, in which the coating paste has a solid ingredient composition of 70 to 95 mole percent of silicon dioxide (SiO2) and 30 to 5 mole percent of bismuth oxide (Bi2 O3).
3. A process according to claim 1, in which the coating paste has a solid ingredient composition of 70 to 95 mole percent of antimony oxide (Sb2 O3) and 30 to 5 mole percent of bismuth oxide (Bi2 O3).
4. A process according to claim 1, in which the coating paste has a solid ingredient composition of 50 to 95 mole percent of silicon dioxide (SiO2), 2 to 45 mole percent of antimony oxide (Sb2 O3) and 2 to 20 mole percent of bismuth oxide (Bi2 O3).
5. A process according to claim 1 which said powder mixture consists essentially of, as a major part, 99.9 to 80.0 mole percent of zinc oxide (ZnO) and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi2 O3) and 0.05 to 10.0 mole percent of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO) antimony oxide (Sb2 O3), barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).
1. A process for making a voltage dependent resistor comprised of a zinc oxide sintered body which itself has voltage dependent properties, said process comprising: (1) providing a formed body of powder mixture comprising, as a major part, zinc oxide and the remainder being an additive; (2) coating on the side surface of said body a paste having a solid ingredient composition of at least one member selected from the group consisting of a) more than 50 mole percent of silicon dioxide (SiO2) and less than 50 mole percent of bismuth oxide (Bi2 O3), b) the same composition as that of said additive, c) more than 30 mole percent of antimony oxide (Sb2 O3) and less than 70 mole percent of bismuth oxide (Bi2 O3), and d) more than 50 mole percent of indium oxide (In2 O3) and less than 50 mole percent of bismuth oxide (Bi2 O3); (3) sintering said coated body; and (4) applying two electrodes to the opposite end surfaces of said sintered body.
2. A process according to claim 1, in which the coating paste has a solid ingredient composition of 70 to 95 mole percent of silicon dioxide (SiO2) and 30 to 5 mole percent of bismuth oxide (Bi2 O3).
3. A process according to claim 1, in which the coating paste has a solid ingredient composition of 70 to 95 mole percent of antimony oxide (Sb2 O3) and 30 to 5 mole percent of bismuth oxide (Bi2 O3).
4. A process according to claim 1, in which the coating paste has a solid ingredient composition of 50 to 95 mole percent of silicon dioxide (SiO2), 2 to 45 mole percent of antimony oxide (Sb2 O3) and 2 to 20 mole percent of bismuth oxide (Bi2 O3).
5. A process according to claim 1 which said powder mixture consists essentially of, as a major part, 99.9 to 80.0 mole percent of zinc oxide (ZnO) and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi2 O3) and 0.05 to 10.0 mole percent of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO) antimony oxide (Sb2 O3), barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).
Description:
This invention relates to the preparation of a voltage dependent resistor the properties of which are due to the bulk thereof, and more particularly to a varistor comprising a zinc oxide sintered body having a high resistance layer of a composition such as silicon dioxide, antimony oxide or indium oxide on the side surface of the sintered body.
Various voltage dependent resistors such as silicon carbide varistors, selenium rectifiers and germanium or silicon p-n junction diodes have been widely used for stabilization of voltage or current of electrical circuits. The electrical characteristics of such a voltage dependent resistor are expressed by the relation:
I =(v/c) n
Where V is the voltage across the resistor, I is the current flowing through the resistor, C is a constant corresponding to the voltage at a given current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation:
n = log 10 (I 2 /I 1 )/log 10 (V 2 /V 1 )
where V 1 and V 2 are voltages at a given currents I 1 and I 2 , respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the degree to which the resistors depart from ohmic characteristics.
There have been known voltage dependent resistors comprising sintered bodies of zinc oxide with or without additives and having silver paint electrodes applied thereto, as disclosed in the U.S. Pat. No. 3,496,512. The non-linearity of such voltage dependent resistors is attributed to the interface between the sintered body of zinc oxide with or without additives and the silver paint electrode and is controlled mainly by changing the composition of said sintered body and said silver paint electrode. Therefore, it is not easy to control the C-value over a wide range after the sintered body is prepared. Similarly, in the voltage dependent resistors comprising germanium or silicon p-n junction diodes it is difficult to control the C-value over a wide range because the non-linearity of these voltage dependent resistors is not attributed to the bulk thereof but to the p-n junction. On the other hand, silicon carbide varistors have non-linearity due to the contacts among individual grains of silicon carbide bonded together by a ceramic binding material i.e. to the bulk and are controlled with respect to the C-value by changing the dimension in the direction in which the current flows through the varistors. The silicon carbide varistors, however, have a relatively low n-value ranging from 3 to 6 and are prepared by firing in a non-oxidizing atmosphere, especially for the purpose of obtaining a lower C-value. In U.S. Pat. Nos. 3,663,458, 3,669,058, 3,637,529, 3,632,528, 3,634,337 and 3,598,763, there have been disclosed voltage dependent resistors comprising sintered bodies of zinc oxide with additives such as bismuth oxide, uranium oxide, strontium oxide, lead oxide, barium oxide, cobalt oxide and manganese oxide. The non-linearity of such voltage dependent resistors is attributable to the bulk thereof and is independent of the interface between the sintered bodies and the electrodes. Therefore, it is easy to control the C-value over a wide range by changing the thickness of the sintered body itself. Such voltage dependent resistors of the bulk type have more excellent properties with respect to the n-value, transient power dissipation and AC power dissipation than do SiC varistors.
A disadvantage of the zinc oxide voltage-dependent resistors is their poor stability in an electric load life test in a high ambient humidity. When D.C. power is applied to the zinc oxide sintered body in a high ambient humidity, the sintered body shows a decrease in the surface electrical resistance. This decrease causes in particular an increase in the leakage current in the zinc oxide voltage-dependent resistor of the bulk type and results in a poor non-linear property. The deterioration of the non-linear property of the voltage-dependent resistor occurs even at a load of low power such as a load lower than 0.01 watt in a high ambient humidity, for example 90 percent R.H at 70°C. Therefore, it is necessary that the sintered body is completely protected against outside moisture by a protective coating.
Another disadvantage of the zinc oxide voltage dependent resistors aforesaid exists in their poor ability to withstand impulse current. When an impulse wave is applied to the zinc oxide sintered body, the sintered body suffers a flashover along its side surface at an impulse voltage above 500V/mm, and despite no deterioration in the interior of sintered body the side surface of the sintered body is heavily damaged. The poor ability to withstand impulse current is unfavorable particularly for application of the varistor as a lightning arrester.
There is other prior art that relates to a voltage dependent resistor comprising a sintered body comprising zinc oxide and other additives and being characterized by a high C-value, high n-value, high stability with respect to temperature, humidity and electric load, and good ability to withstand impulse current. Such a resistor is disclosed in U.S. Pat. No. 3,760,318. More specifically, a zinc oxide sintered body according to said U.S. Pat. No. 3,760,318 has Li ions or Na ions diffused into said sintered body from the side surface thereof at a temperature of 600°C to 1000°C. This diffusing process inevitably results in lowering the n-value of the resultant resistor in the current region lower than 10 μA. The low n-value in such low current region is undesirable for an application requiring low leakage current.
An object of the present invention is to provide a method for making a voltage dependent resistor characterized by a high stability with respect to a d.c. load in high humidity and a good ability to withstand impulse current.
Another object of the present invention is to provide a method for making a voltage dependent resistor characterized by a high n-value even in a low current region and a high stability with respect to a d.c. load in high humidity and a good ability to withstand impulse current.
These and other objects of the invention will become apparent upon consideration of the following description taken together with the accompanying drawing in which the single FIGURE is a partly cross-sectional view of a voltage-dependent resistor in accordance with the invention.
Before proceeding with a detailed description of the manufacturing process for the voltage-dependent resistor contemplated by the invention, the construction of the resultant resistor will be described with reference to the aforesaid FIGURE wherein reference character 10 designates, as a whole, a voltage-dependent resistor comprising, as its active element, a sintered body having surfaces consisting of a side surface 2 and opposite end surfaces 3 and 4 to which a pair of electrodes 5 and 6 are applied. Said sintered body 1 is prepared in a manner hereinafter set forth and has a high resistance layer 11 at said side surface 2 and can have any cross-sectional form such as circular, square or rectangular.
The process for making a voltage dependent resistor of a bulk type characterized by a high humidity resistance and a good ability to withstand current surges according to the invention comprises: (1) providing a formed body of a powder mixture comprising, as a major part, zinc oxide, and an additive including BiO 2 ; (2) coating on the side surfaces of said body a paste comprising, solid ingredient composition, at least one member selected from the group consisting of a more than 50 mole percent of silicon dioxide (SiO 2 ) and less than 50 mole percent of bismuth oxide (Bi 2 O 3 ), b) the same composition as that of said addition, c) more than 30 mole percent of antimony oxide (Sb 2 O 3 ) and less than 70 mole percent of bismuth oxide (Bi 2 O 3 ), and d) more than 50 mole percent of indium oxide (In 2 O 3 ) and less than 50 mole percent of bismuth oxide (Bi 2 O 3 ); (3) sintering said coated body; and (4) applying two electrodes to the opposite end surfaces of said sintered body.
Said zinc oxide sintered body which itself has voltage dependent properties can be prepared by using a composition described in U.S. Pat. Nos. 3,663,458, 3,669,058, 3,636,529, 3,632,528, 3,634,337 and 3,598,763. Among various compositions, a better result can be obtained with a composition consisting essentially of, as a major part, 80.0 to 99.9 mole percent of zinc oxide and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi 2 O 3 ) and 0.05 to 10.0 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO), antimony oxide (Sb 2 O 3 ), barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).
According to the present invention, the resultant resistor has an excellent ability to withstand current surges in an impulse current test, when said coating paste comprises, as the solid composition, 70 to 95 mole percent of silicon dioxide (SiO 2 ) and 30 to 5 mole percent of bismuth oxide (Bi 2 O 3 ). Similarly, the ability to withstand surge current can be improved greatly by using coating paste comprising, as the solid ingredient composition, 70 to 95 mole percent of antimony oxide (Sb 2 O 3 ) and 30 to 5 mole percent of bismuth oxide (Bi 2 O 3 ).
According to the present invention, the ability to withstand surge current can be further improved by using coating paste comprising, as the solid ingredient composition, 50 to 95 mole percent of silicon dioxide (SiO 2 ), 2 to 45 mole percent of antimony oxide (Sb 2 O 3 ) and 2 to 20 mole percent of bismuth oxide (Bi 2 O 3 ).
It has been discovered according to the invention that the D.C. stability in high humidity and the ability to withstand surge current of the resultant resistor is improved when said powder mixture consists essentially of, as a major part, 99.9 to 80.0 mole percent of zinc oxide (ZnO) and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi 2 O 3 ) and 0.05 to 10.0 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO) antimony oxide (Sb 2 O 3 ), barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).
The sintered body 1 can be prepared by a per se well known ceramic technique. The starting materials comprising zinc oxide powder and additives such as bismuth oxide, cobalt oxide, manganese oxide, antimony oxide, barium oxide, strontium oxide, lead oxide, uranium oxide and tin oxide are mixed in a wet mill so as to produce a homogeneous mixture. The mixtures are dried and pressed in a mold into desired shapes at a pressure from 100 kg/cm 2 to 1000 kg/cm 2 . When a rod-shaped resistor is desired, the mixed slurry can be fabricated into the desired shape by extruding and then dried. The mixtures may be preliminarily calcined at a temprature of 700° to 1000°C and pulverized for easy fabrication in the subsequent pressing step. The mixtures may be admixed with a suitable binder such as water, polyvinyl alcohol, etc.
After the mixtures are formed into the desired shapes, the formed bodies are coated, on the side surfaces, with a paste including powder having the same composition as said additive, or a combination of bismuth oxide with silicon dioxide, antimony oxide or indium oxide, so as to form a high resistance layer at the side surfaces after sintering. Said paste comprises, as the solid ingredient composition, at least one member selected from the group consisting of a more than 50 mole percent of silicon dioxide (SiO 2 ) and less than 50 mole percent of bismuth oxide (Bi 2 O 3 ), b) the same composition as that of said additive, c) more than 30 mole percent of antimony oxide (Sb 2 O 3 ) and less than 70 mole percent of bismuth oxide (Bi 2 O 3 ), and d) more than 50 mole percent of indium oxide (In 2 O 3 ) and less than 50 mole percent of bismuth oxide (Bi 2 O 3 ), and, as a binding material, an organic resin such as epoxy, vinyl or phenol resin in an organic solvent such as butyl acetate, toluene or the like. Said silicon dioxide, bismuth oxide, antimony oxide and indium oxide can be replaced, respectively, with any silicon compound, bismuth compound, antimony compound and indium compound such as an oxalate, carbonate, nitrate, sulfate, iodide, fluoride or hydroxide which is converted into the corresponding oxide at the sintering temperature.
After being coated with said paste, the formed bodies are sintered in air at a temperature of 1000° to 1450°C for 1 to 5 hours, and then furnace-cooled to room temperature. The sintering temperature is determined based on the desired electrical resistivity, nonlinearity stability and the thickness of the high resistance layer formed at the side surface of the sintered body. Also, the electrical resistivity can be reduced by air-quenching from the sintering temperature to room temperature. The sintered body has non-ohmic resistance due to the bulk itself. Therefore, its C-value can be changed without impairing the n-value by changing the distance between said opposite end surfaces. A shorter distance results in a lower C-value. The coating paste forms a high resistance layer, as can be proved by measurement of the resistance distribution in the cross-section of the sintered body, which will show a high resistance at the side surface of the sintered body. The high resistance layer is controlled so as to have a thickness more than 10μ. Particularly, it can be shown from an x-ray analysis of the cross-sectional portion of sintered body, that the paste comprising a combination of silicon dioxide and bismuth oxide, or antimony oxide and bismuth oxide forms a layer having a thickness of more than 3μ and that said layer comprises, in a region to a 3μ depth from the side surface, more than 70 mole percent of at least zinc silicate (Zn 2 SiO 4 ) and/or zinc antimonate (Zn 7 Sb 2 O 12 ).
After sintering, the sintered body has electrodes applied to the opposite end surfaces of the sintered body. Said electrodes can be made by any available method such as heating of noble metal paint, electroless or electrolytic plating of Ag, Cu, Ni, Sn etc. vacuum evaporating of Al, Zn, Sn etc. and flame spraying of Cu, Sn, Al, Zn etc. in accordance with the prior well known techniques.
Lead wires can be attached to the electrodes in a per se conventional manner by using conventional solder. It is convenient to employ a conductive adhesive comprising silver powder and resin in an organic solvent in order to connect the lead wires to the silver electrodes. The n-value of a voltage-dependent resistor according to this invention does not deteriorate even in a low current region due to the introduction of the covering layer at the side surface of the sintered body, and it has a high stability with respect to temperature and humidity and in the load life test, which is carried out at 70°C, 90 percent RH at a rating power for 500 hours. The n-value and C-value do not change appreciably after the load life test. From a surge test, which is carried out by applying a 4× 10 μsec impulse current twice, it is shown that this voltage-dependent resistor has the ability to withstand surges of more than 2000A/cm 2 .
EXAMPLE 1.
Starting materials listed in Table 1 were mixed in a wet mill for 5 hours. Each mixture was dried and pressed in a mold into a disc of 40 mm in diameter and 25 mm in thickness at a pressure of 340 kg/cm 2 . The pressed bodies had the side surface covered by coating paste including solid ingredients listed in Table 1, and were dried. Then, the bodies were sintered in air for 5 hours at 1200°C and furnace-cooled. The sintered bodies were lapped to the thickness listed in table 1 by lapping the opposite end surfaces thereof with silicon carbide abrasive having a particle size of 600 mesh. The opposite end surfaces of the sintered discs were provided with a spray metallized film of aluminum by a per se well known technique. The electric characteristics of the resultant resistors are shown in Table 1. It will be readily understood that the C-value changes in proportion to the thickness of the sintered body.
Size of disc: 32 mm in dia.
Thickness of high resistive layer: 30μ
Table 1 ____________________________________________________________ ______________ Composition Solid Ingredient Thickness C(V) n of Sintered of Paste of Sintered Body Body (mol. %) (mol. %) (mm) (at 1mA) (0.1-1mA) ____________________________________________________________ ______________ SiO 2 (50) 5 150 15 10 302 14 Bi 2 O 3 (50) 20 605 15 SiO 2 (90) 5 153 15 10 310 16 Bi 2 O 3 (10) 20 605 16 SiO 2 (100) 5 155 14 ZnO (99.0) 10 310 15 Bi 2 O 3 ( 0.5) Bi 2 O 3 ( 0) 20 615 15 CoO ( 0.5) Sb 2 O 3 (90) 5 150 15 10 300 15 Bi 2 O 3 (10) 20 603 15 In 2 O 3 (90) 5 145 14 10 300 14 Bi 2 O 3 (10) 20 600 15 SiO 2 (72) 5 160 16 Sb 2 O 3 (20) 10 315 16 Bi 2 O 3 ( 8) 20 615 16 SiO 2 (90) 5 510 44 10 1025 45 Bi 2 O 3 (10) 20 2040 45 ZnO (97.5) Sb 2 O 3 (90) 5 500 45 Bi 2 O 3 ( 0.5) 10 1010 45 CoO ( 0.5) Bi 2 O 3 (10) 20 2010 46 MnO ( 0.5) In 2 O 3 (90) 5 505 45 Sb 2 O 3 ( 1.0) 10 1010 44 Bi 2 O 3 (10) 20 2015 46 SiO 2 (72) 5 515 46 Sb 2 O 3 (20) 10 1025 46 Bi 2 O 3 ( 8) 20 2040 46 ZnO (99.0) Sb 2 O 3 (90) 5 250 22 Bi 2 O 3 ( 0.5) 10 505 22 MnO (0.5) Bi 2 O 3 (10) 20 1000 23 ZnO (99.0) SiO 2 (90) 5 240 8.2 Bi 2 O 3 (0.5) 10 490 8.4 Sb 2 O 3 (0.5) Bi 2 O 3 (10) 20 985 8.4 ZnO (99.0) In 2 O 3 (90) 5 200 10 Bi 2 O 3 ( 0.5) 10 410 10 BaO ( 0.5) Bi 2 O 3 (10) 20 815 10 ZnO (99.0) SiO 2 (72) 5 205 11 Bi 2 O 3 ( 0.5) Sb 2 O 3 (20) 10 400 11 SrO ( 0.5) Bi 2 O 3 ( 8) 20 810 12 ____________________________________________________________ ______________
EXAMPLE 2
Starting materials of Table 2 were fabricated into voltage dependent resistors by the same process as that of Example 1. An impulse test was carried out by applying a 4×10μs impulse and the ability to withstand current surges was thus determined. A humidity test was carried out by boiling the disc in the pure water for 24 hours. The electric characteristics of the resultant resistors are shown in Table 2.
Size of disc: 32 mm in dia. and 20 mm in thickness
Sintering: 1200°C for 5 hours
Thickness of high resistance layer: 30μ
TABLE 2 ____________________________________________________________ ______________ Composition Electric Characteristics of Resultant of Sintered Solid Ingredient Resistor Body of Paste C(V) n Impulse Boiling (mol. %) (mol %) (at 1mA) 0.1-1mA Withstand, Test (KA) ΔC ____________________________________________________________ ______________ (%) SiO 2 (50) Bi 2 O 3 (50) 605 15 20 -5.0 SiO 2 (60) Bi 2 O 3 (40) 605 15 20 -4.7 SiO 2 (70) Bi 2 O 3 (30) 600 15 25 -4.7 SiO 2 (80) Bi 2 O 3 (20) 600 16 30 -3.8 SiO 2 (90) Bi 2 O 3 (10) 605 16 35 -2.9 SiO 2 (95) Bi 2 O 3 ( 5) 610 16 30 -3.2 SiO 2 (100) Bi 2 O 3 ( 0) 615 15 30 -3.5 Sb 2 O 3 (30) Bi 2 O 3 (70) 600 14 20 -5.3 ZnO (99.0) Sb 2 O 3 (50) Bi 2 O 3 (50) 600 14 25 -4.5 Bi 2 O 3 (0.5) Sb 2 O 3 (70) Bi 2 O 3 (30) 600 15 25 -3.5 CoO (0.5) Sb 2 O 3 (90) Bi 2 O 3 (10) 603 15 35 -2.7 Sb 2 O 3 (95) Bi 2 O 3 (5) 605 15 30 -3.0 Sb 2 O 3 (100) Bi 2 O 3 (0) 610 14 25 -3.3 In 2 O 3 (50) Bi 2 O 3 (50) 595 14 20 -5.7 In 2 O 3 (70) Bi 2 O 3 (30) 600 14 25 -4.3 In 2 O 3 (90) Bi 2 O 3 (10) 600 15 35 -3.1 In 2 O 3 (95) Bi 2 O 3 (5) 600 15 30 -3.4 In 2 3 (100) Bi 2 O 3 ( 0) 610 14 30 -3.5 SiO 2 (50) Bi 2 O 3 (50) 1960 42 25 -5.5 SiO 2 (60) Bi 2 O 3 (40) 1980 42 30 -4.8 SiO 2 (70) Bi 2 O 3 (30) 2000 44 35 -3.9 SiO 2 (80) Bi 2 O 3 (20) 2100 44 40 - 3.2 SiO 2 (90) Fi 2 O 3 (10) 2040 45 40 -1.5 SiO 2 (95) Bi 2 O 3 ( 5) 2040 45 35 -2.1 SnO (97.5) SiO 2 (100) Bi 2 O 3 ( 0) 2030 44 30 -2.3 Bi 2 O 3 ( 0.5) Sb 2 O 3 (30) Bi 2 O 3 (70) 1980 44 25 -5.1 CoO ( 0.5) Sb 2 O 3 (50) Bi 2 O 3 (50) 2000 44 30 -4.9 MnO (0.5) Sb 2 O 3 (70) Bi 2 O 3 (30) 2000 45 35 -3.8 Sb 2 O 3 (1.0) Sb 2 O 3 (90) Bi 2 O 3 (10) 2010 46 40 -2.5 Sb 2 O 3 (95) Bi 2 O 3 ( 5) 2015 45 40 -3.1 Sb 2 O 3 (100) Bi 2 O 3 ( 0) 2020 45 30 -3.5 In 2 O 3 (50) Bi 2 O 3 (50) 1990 44 25 -5.3 In 2 O 3 (70) Bi 2 O 3 (30) 2005 44 30 -4.9 In 2 O 3 (90) Bi 2 O 3 (10) 2015 46 40 -3.1 In 2 O 3 (95) Bi 2 O 3 ( 5) 2015 45 40 -3.4 In 2 O 3 (100) Bi 2 O 3 ( 0) 2000 45 25 -3.4 SiO 2 Sb 2 O 3 Bi 2 O 3 50 45 5 600 15 30 -4.4 ZnO (99.0) 50 30 20 600 15 30 -4.8 Bi 2 O 3 (0.5) 95 3 2 615 16 35 -3.2 CoO (0.5) 95 2 3 615 16 40 -3.4 58 40 2 610 15 35 -3.0 78 2 20 610 15 40 -2.5 72 20 8 620 17 45 -1.7 50 45 5 2050 44 40 -3.4 ZnO (97.5) 50 30 20 2065 45 45 -2.8 Bi 2 O 3 (0.5) 95 3 2 2045 45 50 -2.7 CoO (0.5) 95 2 3 2075 46 50 -2.7 MnO (0.5) 58 40 2 2060 44 50 -2.0 Sb 2 O 3 (1.0) 78 2 20 2080 46 55 -1.2 72 20 8 2100 48 60 -0.5 ____________________________________________________________ ______________
EXAMPLE 3
Starting materials of Table 3 were fabricated into voltage dependent resistors by the same process as that of Example 1. Then the tests were carried out by the same methods as those of Example 2. The electric characteristics of the resultant resistors are shown in Table 3.
Size of disc: 32 mm in dia. and 20 mm in thickness
Sintering: 1200°C for 5 hours
Thickness of high resistance layer: 30μ
TABLE 3 ____________________________________________________________ ______________ Composition of Sintered Electric Characteristics of Body Solid Ingredient Resultant Resistor (mol %) of Paste C(V) n Impulse Boiling Further (mol %) with- Test ZnO Bi 2 O 3 Additives (at 1mA) 0.1-1mA stand. ΔC(%) (KA) ____________________________________________________________ ______________ 99.90 0.05 CoO 0.05 350 10 15 -6.2 89.95 0.05 CoO 10 SiO 2 (90) 420 12 18 -6.2 89.95 10 CoO 0.05 Bi 2 O 3 (10) 420 13 20 -3.9 80.00 10 CoO 10 750 14 20 -4.0 99.0 0.5 CoO 0.5 605 16 35 -6.3 99.90 0.05 MnO 0.05 500 13 15 -6.3 89.95 0.05 MnO 10 600 14 15 -5.9 89.95 10 MnO 0.05 Sb 2 O 3 (90) 900 18 25 -3.3 80.00 10 MnO 10 Bi 2 O 3 (10) 1250 17 25 -3.5 99.0 0.5 MnO 0.5 1000 23 35 -2.8 99.90 0.05 Sb 2 O 3 0.05 300 7.9 15 -7.0 89.95 0.05 Sb 2 O 3 10 800 7.2 15 -5.5 89.95 10 Sb 2 O 3 0.05 In 2 O 3 (90) 720 8.2 17 -3.9 80.00 10 Sb 2 O 3 10 Bi 2 O 3 (10) 1300 8.6 18 -4.2 99.0 0.5 Sb 2 O 3 0.5 990 8.4 25 -2.0 99.90 0.05 BaO 0.05 320 7.2 18 -5.3 89.95 0.05 BaO 10 470 8.0 15 -4.9 89.95 10 BaO 0.05 SiO 2 (90) 510 9.4 20 -2.9 80.00 10 BaO 10 Bi 2 O 3 (10) 1200 0.5 20 -3.4 99.0 0.5 BaO 0.5 815 10 25 -2.5 99.90 0.05 SrO 0.05 300 9.2 12 -7.2 89.95 0.05 SrO 10 Sb 2 O 3 (90) 1150 8.1 14 - 5.7 89.95 10 SrO 0.05 Bi 2 O 3 (10) 1200 11 17 -4.3 80.00 10 SrO 10 1400 11 18 -4.5 99.0 0.5 SrO 0.5 810 12 20 -3.3 CoO 0.5 98.5 0.5 MnO 0.5 850 27 45 -3.5 CoO 0.5 98.5 0.5 Sb 2 O 3 0.5 1700 40 50 -4.2 CoO 0.5 98.5 0.5 BaO 0.5 1000 22 35 -4.5 CoO 0.5 98.5 0.5 SrO 0.5 SiO 2 (72) 950 25 40 -5.3 MnO 0.5 Sb 2 O 3 (20) 98.5 0.5 Sb 2 O 3 0.5 Bi 2 O 3 ( 8) 1800 40 50 -4.7 MnO 0.5 98.5 0.5 BaO 0.5 1300 32 40 -3.8 MnO 0.5 98.5 0.5 SrO 0.5 1250 30 40 -3.8 Sb 2 O 3 0.5 98.5 0.5 BaO 0.5 1300 20 30 -4.7 Sb 2 O 3 0.5 98.5 0.5 SrO 0.5 1220 20 30 -5.2 BaO 0.5 98.5 0.5 SrO 0.5 750 17 25 -7.0 CoO 0.5 98.0 0.5 MnO 0.5 1800 40 50 -1.5 Sb 2 O 3 0.5 CoO 0.5 98.0 0.5 MnO 0.5 800 29 35 -2.5 BaO 0.5 CoO 0.5 98.0 0.5 MnO 0.5 770 26 35 -3.0 SrO 0.5 CoO 0.5 98.0 0.5 Sb 2 O 3 0.5 1500 33 40 -2.7 BaO 0.5 CoO 0.5 98.0 0.5 Sb 2 O 3 0.5 1450 31 35 -2.2 SrO 0.5 SiO 2 (72) CoO 0.5 Sb 2 O 3 (20) 98.0 0.5 BaO 0.5 Bi 2 O 3 ( 8) 880 18 25 -3.3 SrO 0.5 MnO 0.5 98.0 0.5 Sb 2 O 3 0.5 1650 35 40 -3.1 BaO 0.5 MnO 0.5 98.0 0.5 Sb 2 O 3 0.5 1600 33 40 -2.4 SrO 0.5 MnO 0.5 98.0 05 BaO 0.5 1000 21 35 -2.5 SrO 0.5 Sb 2 O 3 0.5 98.0 0.5 BaO 0.5 1050 18 30 -3.0 SrO 0.5 ____________________________________________________________ ______________
EXAMPLE 4
The fabrication process and testing method were the same as those of Example 2 and the thickness of the high resistance layer was varied with the results as shown in Table 4. It is easily understood that the ability to withstand impulses increases with an increase in the thickness of the high resistance layer and the rate of change of the C-value caused by the boiling test decreases with an increase of thickness of the high resistance layer.
Size of disc: 32 mm in dia. and 20 mm in thickness
Sintering: 1200°C for 5 hours
TABLE 4 ____________________________________________________________ ______________ Composition Solid Ingredient Thickness Electric Characteristics of of Sintered of Paste of High- Resultant Resistor Body (mol %) Resistive C(V) n Impulse Boiling (mol %) layer (at 1 mA) 0.1-1 mA Withstand Test (μ) (KA) ΔC ____________________________________________________________ ______________ (%) 10 600 16 30 -4.3 SiO 2 (90) 30 605 16 35 -2.9 Bi 2 O 3 (10) 100 605 16 40 -3.2 300 615 16 50 -1.2 10 600 14 30 -4.2 Sb 2 O 3 (90) 30 603 15 35 -2.7 ZnO (99.0) Bi 2 O 3 (10) 100 605 15 40 -2.2 Bi 2 O 3 ( 0.5) 300 610 15 45 -1.7 CoO ( 0.5 10 590 15 25 -4.8 In 2 O 3 (90) 30 600 15 35 -3.1 Bi 2 O 3 (10) 100 605 15 40 -3.3 300 610 16 45 -2.7 10 605 17 35 -3.3 SiO 2 (72) 30 620 17 45 -1.7 Sb 2 O 3 (20) 100 620 17 50 -1.2 Bi 2 O 3 ( 8) 300 630 18 60 -1.0 10 200 43 30 -2.1 SiO 2 (90) 30 2040 45 40 -1.5 Bi 2 O 3 (10) 100 2070 45 45 -1.1 300 2100 46 45 -0.5 10 1950 43 30 -3.3 ZnO (97.5) Sb 2 O 3 (90) 30 2010 46 40 -2.5 Bi 2 O 3 (0.5) Bi 2 O 3 (10) 100 2030 46 40 -2.0 CoO (0.5) 300 2050 46 50 -1.6 MnO (0.5) 10 2000 44 30 -4.7 Sb 2 O 3 (1.0) In 2 O 3 (90) 30 2015 46 40 -3.1 Bi 2 O 3 (10) 100 2050 46 55 -2.2 300 2100 47 60 -1.8 10 2050 46 50 -1.2 SiO 2 (72) 30 2100 48 60 -0.5 Sb 2 O 3 (20) 100 2120 50 70 -0.5 Bi 2 O 3 (8) 300 2150 50 80 -0.4 ____________________________________________________________ ______________
EXAMPLE 5
Starting materials of Table 5 were fabricated into voltage dependent resistors by the same process as in Example 1. The pressed bodies were sintered at a temperature between 1000°C to 1450°C for 5 hours after covering the side surface with coating pastes as listed in Table 5. The test conditions were the same as those of Example 2. The electric characteristics of resulting resistors are shown in Table 5.
Size of disc: 32 mm in dia. and 20mm
Thickness of high resistive layer; 30μ.
TABLE 5 ____________________________________________________________ ______________ Composition Solid Ingredient Sintering Electric Characteristics of of Sintered Resultant Resistor Body of Paste Temp. C (V) n Impulse Boiling (mol. %) (mol. %) (°C) (at 1 mA) 0.1-1mA Withstand Test ΔC(%) ____________________________________________________________ ______________ 1000 1200 11 15 -9.5 1100 850 14 17 -7.2 SiO 2 (50) 1200 605 15 20 -5.0 Bi 2 O 3 (50) 1300 420 13 18 -5.1 1450 280 11 18 -5.3 1000 1220 13 20 -7.7 1100 870 14 25 -4.1 SiO 2 (90) 1200 605 16 35 -2.9 Bi 2 O 3 (10) 1300 450 16 35 -2.9 1450 300 15 30 -3.5 1000 1250 12 20 -7.0 1100 900 14 25 -5.1 SiO 2 (100) 1200 615 15 30 -3.5 Bi 2 O 3 ( 0) 1300 470 14 23 -3.7 1450 330 14 20 -4.0 1000 1200 11 15 -8.1 ZnO (99.0) Sb 2 O 3 (30) 1200 600 14 20 -5.3 Bi 2 O 3 (0.5) Bi 2 O 3 (70) 1450 300 13 18 -5.7 CoO (0.5) 1000 1190 13 28 -4.1 Sb 2 O 3 (90) 1200 603 15 35 -2.7 Bi 2 O 3 (10) 1450 285 14 30 -3.3 1000 1220 12 20 -5.0 Sb 2 O 3 (100) 1200 610 14 25 -3.5 Bi 2 O 3 ( 0) 1450 310 13 20 -4.0 1000 1200 12 15 -7.5 In 2 O 3 (50) 1200 595 14 20 -5.7 Bi 2 O 3 (50) 1450 320 12 18 -6.0 1000 1230 13 25 -4.7 In 2 O 3 (90) 1200 600 15 35 -3.1 Bi 2 O 3 (10) 1450 295 15 25 -3.6 1000 1200 14 25 -5.1 In 2 O 3 (100) 1200 610 14 30 -3.5 Bi 2 O 3 ( 0) 1450 305 14 30 -4.0 1000 1250 14 35 -3.6 SiO 2 (72) 1100 910 15 40 -2.1 Sb 2 O 3 (20) 1200 620 17 45 -1.7 Bi 2 O 3 ( 8) 1300 430 16 40 -1.8 1450 300 15 40 -2.3 1000 3800 38 30 -2.9 SiO 2 (90) 1200 2040 45 40 -1.5 Bi 2 O 3 (10) 1450 1200 42 35 -2.0 1000 3900 41 35 -3.5 ZnO (97.5) Sb 2 O 3 (90) 1200 2010 46 40 -2.5 Bi 2 O 3 (0.5) Bi 2 O 3 (10) 1450 1250 43 35 -2.7 CoO (0.5) 1000 4000 42 35 -4.7 MnO (0.5) In 2 O 3 (90) 1200 2015 46 40 -3.1 Sb 2 O 3 (1.0) Bi 2 O 3 (10) 1450 1300 40 40 -3.5 1000 4050 40 40 -1.3 SiO 2 (72) 1100 3200 44 550 -0.9 Sb 2 O 3 (20) 1200 2100 48 60 -0.5 Bi 2 O 3 ( 8) 1300 1550 44 50 -1.1 1450 1300 40 45 -1.5 ____________________________________________________________ ______________
EXAMPLE 6
The mixtures of Table 6 were pressed and covered by coating paste comprising the same oxides as the additives in the body. Then the bodies were sintered in air for 5 hours. The test conditions were the same as those of Example 2. The electric characteristics of the resultant resistors are shown in Table 6. The excellent ability to withstand impulses and the small change in the C-value were obtained by coating paste containing the same materials as the additives in the sintered body.
Size of disc; 32 mm in ida. and 20 mm in thickness
Thickness of high resistive layer; 30μ
Table 6 ____________________________________________________________ ______________ Composition of Sintered body Solid Ingredient Sintering Electric Characteristics of (mol. %) of Paste Temp. Resultant Resistor Impulse Boiling Further C (V) n Withstand Test ZnO Bi 2 O 3 Additives (mol. %) (°C) (at 1mA) 0.1-1mA (KA) ΔC ____________________________________________________________ ______________ (%) 99.5 0.5 -- Bi 2 O 3 100 1200 4000 4.1 10 -7.5 99.5 -- CoO 0.5 CoO 100 1200 2200 3.9 10 -6.2 99.5 -- MnO 0.5 MnO 100 1200 2600 3.4 10 -5.3 99.5 -- Sb 2 O 3 0.5 Sb 2 O 3 100 1200 3000 3.7 12 -6.2 99.5 -- BaO 0.5 BaO 100 1200 1600 9.0 15 -7.0 99.5 -- SrO 0.5 SrO 100 1200 1500 7.8 12 -8.3 99.5 -- UO 2 0.5 UO 2 100 1200 2000 4.1 10 -7.9 99.5 -- PbO 0.5 PbO 100 1200 4000 4.3 12 -7.1 99.0 0.5 CoO 0.5 Bi 2 O 3 (50) CoO (50) 1200 600 15 22 -3.5 99.0 0.5 MnO 0.5 Bi 2 O 3 (50) MnO(50) 1200 1000 23 25 -3.7 99.0 0.5 Sb 2 O 3 0.5 Bi 2 O 3 (50) Sb 2 O 3 (50) 1200 985 8.3 18 -4.2 99.0 0.5 BaO 0.5 Bi 2 O 3 (50) BaO(50) 1200 820 11 20 -3.3 99.0 0.5 SrO 0.5 Bi 2 O 3 (50) SrO(50) 1200 800 12 20 -3.7 CoO 0.5 CoO 50 99.0 -- SnO 0.5 SrO 50 1200 4000 30 40 -5.0 MnO 0.5 MnO 50 99.0 -- BaO 0.5 BaO 50 1300 3500 30 35 -4.7 BaO 0.5 BaO 50 99.0 -- SrO 0.5 SrO 50 1100 2000 20 30 -3.3 Bi 2 O 3 50 98.0 1.0 CoO 0.5 CoO 25 1200 1800 15 25 -2.7 MnO 0.5 MnO 25 Bi 2 O 3 50 98.0 1.0 BaO 0.5 BaO 25 1200 1650 14 20 -3.5 SrO 0.5 SrO 25 Bi 2 O 3 20 97.5 0.5 CoO 0.5 CoO 20 1200 2000 46 55 -1.7 MnO 0.5 MnO 20 Sb 2 O 3 1.0 Sb 2 O 3 60 Bi 2 O 3 10 CoO 0.5 CoO 10 97.0 0.5 MnO 0.5 MnO 10 1200 2600 50 60 -0.5 Sb 2 O 3 1.0 Sb 2 O 3 40 SnO 2 0.5 SnO 2 30 Bi 2 O 3 10 CoO 0.5 CoO 10 97.0 0.5 MnO 0.5 MnO 10 1200 2800 50 60 -0.5 Sb 2 O 3 1.0 Sb 2 O 3 60 Cr 2 O 3 0.5 Cr 2 O 3 10 Bi 2 O 3 10 CoO 0.5 CoO 5 96.5 0.5 MnO 0.5 MnO 5 1200 4400 55 70 -0.3 Sb 2 O 3 1.0 Sb 2 O 3 25 Cr 2 O 3 0.5 Cr 2 O 3 5 SiO 2 0.5 SrO 2 50 Bi 2 O 3 5 CoO 0.5 CoO 5 MnO 0.5 MnO 5 94.0 0.5 Sb 2 O 3 1.0 Sb 2 O 3 20 1200 5600 60 70 -0.3 Cr 2 O 3 0.5 Cr 2 O 3 3 SiO 2 2.0 SiO 2 60 NiO 1.0 NiO 2 Bi 2 O 3 25 1000 3800 35 35 -1.2 CoO 0.5 CoO 25 98.0 0.5 MnO 0.5 MnO 25 1200 1800 40 50 -0.8 Sb 2 O 3 0.5 Sb 2 O 3 25 1450 850 41 40 -1.3 ____________________________________________________________ ______________
Various voltage dependent resistors such as silicon carbide varistors, selenium rectifiers and germanium or silicon p-n junction diodes have been widely used for stabilization of voltage or current of electrical circuits. The electrical characteristics of such a voltage dependent resistor are expressed by the relation:
I =(v/c) n
Where V is the voltage across the resistor, I is the current flowing through the resistor, C is a constant corresponding to the voltage at a given current and exponent n is a numerical value greater than 1. The value of n is calculated by the following equation:
n = log 10 (I 2 /I 1 )/log 10 (V 2 /V 1 )
where V 1 and V 2 are voltages at a given currents I 1 and I 2 , respectively. The desired value of C depends upon the kind of application to which the resistor is to be put. It is ordinarily desirable that the value of n be as large as possible since this exponent determines the degree to which the resistors depart from ohmic characteristics.
There have been known voltage dependent resistors comprising sintered bodies of zinc oxide with or without additives and having silver paint electrodes applied thereto, as disclosed in the U.S. Pat. No. 3,496,512. The non-linearity of such voltage dependent resistors is attributed to the interface between the sintered body of zinc oxide with or without additives and the silver paint electrode and is controlled mainly by changing the composition of said sintered body and said silver paint electrode. Therefore, it is not easy to control the C-value over a wide range after the sintered body is prepared. Similarly, in the voltage dependent resistors comprising germanium or silicon p-n junction diodes it is difficult to control the C-value over a wide range because the non-linearity of these voltage dependent resistors is not attributed to the bulk thereof but to the p-n junction. On the other hand, silicon carbide varistors have non-linearity due to the contacts among individual grains of silicon carbide bonded together by a ceramic binding material i.e. to the bulk and are controlled with respect to the C-value by changing the dimension in the direction in which the current flows through the varistors. The silicon carbide varistors, however, have a relatively low n-value ranging from 3 to 6 and are prepared by firing in a non-oxidizing atmosphere, especially for the purpose of obtaining a lower C-value. In U.S. Pat. Nos. 3,663,458, 3,669,058, 3,637,529, 3,632,528, 3,634,337 and 3,598,763, there have been disclosed voltage dependent resistors comprising sintered bodies of zinc oxide with additives such as bismuth oxide, uranium oxide, strontium oxide, lead oxide, barium oxide, cobalt oxide and manganese oxide. The non-linearity of such voltage dependent resistors is attributable to the bulk thereof and is independent of the interface between the sintered bodies and the electrodes. Therefore, it is easy to control the C-value over a wide range by changing the thickness of the sintered body itself. Such voltage dependent resistors of the bulk type have more excellent properties with respect to the n-value, transient power dissipation and AC power dissipation than do SiC varistors.
A disadvantage of the zinc oxide voltage-dependent resistors is their poor stability in an electric load life test in a high ambient humidity. When D.C. power is applied to the zinc oxide sintered body in a high ambient humidity, the sintered body shows a decrease in the surface electrical resistance. This decrease causes in particular an increase in the leakage current in the zinc oxide voltage-dependent resistor of the bulk type and results in a poor non-linear property. The deterioration of the non-linear property of the voltage-dependent resistor occurs even at a load of low power such as a load lower than 0.01 watt in a high ambient humidity, for example 90 percent R.H at 70°C. Therefore, it is necessary that the sintered body is completely protected against outside moisture by a protective coating.
Another disadvantage of the zinc oxide voltage dependent resistors aforesaid exists in their poor ability to withstand impulse current. When an impulse wave is applied to the zinc oxide sintered body, the sintered body suffers a flashover along its side surface at an impulse voltage above 500V/mm, and despite no deterioration in the interior of sintered body the side surface of the sintered body is heavily damaged. The poor ability to withstand impulse current is unfavorable particularly for application of the varistor as a lightning arrester.
There is other prior art that relates to a voltage dependent resistor comprising a sintered body comprising zinc oxide and other additives and being characterized by a high C-value, high n-value, high stability with respect to temperature, humidity and electric load, and good ability to withstand impulse current. Such a resistor is disclosed in U.S. Pat. No. 3,760,318. More specifically, a zinc oxide sintered body according to said U.S. Pat. No. 3,760,318 has Li ions or Na ions diffused into said sintered body from the side surface thereof at a temperature of 600°C to 1000°C. This diffusing process inevitably results in lowering the n-value of the resultant resistor in the current region lower than 10 μA. The low n-value in such low current region is undesirable for an application requiring low leakage current.
An object of the present invention is to provide a method for making a voltage dependent resistor characterized by a high stability with respect to a d.c. load in high humidity and a good ability to withstand impulse current.
Another object of the present invention is to provide a method for making a voltage dependent resistor characterized by a high n-value even in a low current region and a high stability with respect to a d.c. load in high humidity and a good ability to withstand impulse current.
These and other objects of the invention will become apparent upon consideration of the following description taken together with the accompanying drawing in which the single FIGURE is a partly cross-sectional view of a voltage-dependent resistor in accordance with the invention.
Before proceeding with a detailed description of the manufacturing process for the voltage-dependent resistor contemplated by the invention, the construction of the resultant resistor will be described with reference to the aforesaid FIGURE wherein reference character 10 designates, as a whole, a voltage-dependent resistor comprising, as its active element, a sintered body having surfaces consisting of a side surface 2 and opposite end surfaces 3 and 4 to which a pair of electrodes 5 and 6 are applied. Said sintered body 1 is prepared in a manner hereinafter set forth and has a high resistance layer 11 at said side surface 2 and can have any cross-sectional form such as circular, square or rectangular.
The process for making a voltage dependent resistor of a bulk type characterized by a high humidity resistance and a good ability to withstand current surges according to the invention comprises: (1) providing a formed body of a powder mixture comprising, as a major part, zinc oxide, and an additive including BiO 2 ; (2) coating on the side surfaces of said body a paste comprising, solid ingredient composition, at least one member selected from the group consisting of a more than 50 mole percent of silicon dioxide (SiO 2 ) and less than 50 mole percent of bismuth oxide (Bi 2 O 3 ), b) the same composition as that of said addition, c) more than 30 mole percent of antimony oxide (Sb 2 O 3 ) and less than 70 mole percent of bismuth oxide (Bi 2 O 3 ), and d) more than 50 mole percent of indium oxide (In 2 O 3 ) and less than 50 mole percent of bismuth oxide (Bi 2 O 3 ); (3) sintering said coated body; and (4) applying two electrodes to the opposite end surfaces of said sintered body.
Said zinc oxide sintered body which itself has voltage dependent properties can be prepared by using a composition described in U.S. Pat. Nos. 3,663,458, 3,669,058, 3,636,529, 3,632,528, 3,634,337 and 3,598,763. Among various compositions, a better result can be obtained with a composition consisting essentially of, as a major part, 80.0 to 99.9 mole percent of zinc oxide and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi 2 O 3 ) and 0.05 to 10.0 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO), antimony oxide (Sb 2 O 3 ), barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).
According to the present invention, the resultant resistor has an excellent ability to withstand current surges in an impulse current test, when said coating paste comprises, as the solid composition, 70 to 95 mole percent of silicon dioxide (SiO 2 ) and 30 to 5 mole percent of bismuth oxide (Bi 2 O 3 ). Similarly, the ability to withstand surge current can be improved greatly by using coating paste comprising, as the solid ingredient composition, 70 to 95 mole percent of antimony oxide (Sb 2 O 3 ) and 30 to 5 mole percent of bismuth oxide (Bi 2 O 3 ).
According to the present invention, the ability to withstand surge current can be further improved by using coating paste comprising, as the solid ingredient composition, 50 to 95 mole percent of silicon dioxide (SiO 2 ), 2 to 45 mole percent of antimony oxide (Sb 2 O 3 ) and 2 to 20 mole percent of bismuth oxide (Bi 2 O 3 ).
It has been discovered according to the invention that the D.C. stability in high humidity and the ability to withstand surge current of the resultant resistor is improved when said powder mixture consists essentially of, as a major part, 99.9 to 80.0 mole percent of zinc oxide (ZnO) and, as an additive, 0.05 to 10.0 mole percent of bismuth oxide (Bi 2 O 3 ) and 0.05 to 10.0 mole percent, in total, of at least one member selected from the group consisting of cobalt oxide (CoO), manganese oxide (MnO) antimony oxide (Sb 2 O 3 ), barium oxide (BaO), strontium oxide (SrO) and lead oxide (PbO).
The sintered body 1 can be prepared by a per se well known ceramic technique. The starting materials comprising zinc oxide powder and additives such as bismuth oxide, cobalt oxide, manganese oxide, antimony oxide, barium oxide, strontium oxide, lead oxide, uranium oxide and tin oxide are mixed in a wet mill so as to produce a homogeneous mixture. The mixtures are dried and pressed in a mold into desired shapes at a pressure from 100 kg/cm 2 to 1000 kg/cm 2 . When a rod-shaped resistor is desired, the mixed slurry can be fabricated into the desired shape by extruding and then dried. The mixtures may be preliminarily calcined at a temprature of 700° to 1000°C and pulverized for easy fabrication in the subsequent pressing step. The mixtures may be admixed with a suitable binder such as water, polyvinyl alcohol, etc.
After the mixtures are formed into the desired shapes, the formed bodies are coated, on the side surfaces, with a paste including powder having the same composition as said additive, or a combination of bismuth oxide with silicon dioxide, antimony oxide or indium oxide, so as to form a high resistance layer at the side surfaces after sintering. Said paste comprises, as the solid ingredient composition, at least one member selected from the group consisting of a more than 50 mole percent of silicon dioxide (SiO 2 ) and less than 50 mole percent of bismuth oxide (Bi 2 O 3 ), b) the same composition as that of said additive, c) more than 30 mole percent of antimony oxide (Sb 2 O 3 ) and less than 70 mole percent of bismuth oxide (Bi 2 O 3 ), and d) more than 50 mole percent of indium oxide (In 2 O 3 ) and less than 50 mole percent of bismuth oxide (Bi 2 O 3 ), and, as a binding material, an organic resin such as epoxy, vinyl or phenol resin in an organic solvent such as butyl acetate, toluene or the like. Said silicon dioxide, bismuth oxide, antimony oxide and indium oxide can be replaced, respectively, with any silicon compound, bismuth compound, antimony compound and indium compound such as an oxalate, carbonate, nitrate, sulfate, iodide, fluoride or hydroxide which is converted into the corresponding oxide at the sintering temperature.
After being coated with said paste, the formed bodies are sintered in air at a temperature of 1000° to 1450°C for 1 to 5 hours, and then furnace-cooled to room temperature. The sintering temperature is determined based on the desired electrical resistivity, nonlinearity stability and the thickness of the high resistance layer formed at the side surface of the sintered body. Also, the electrical resistivity can be reduced by air-quenching from the sintering temperature to room temperature. The sintered body has non-ohmic resistance due to the bulk itself. Therefore, its C-value can be changed without impairing the n-value by changing the distance between said opposite end surfaces. A shorter distance results in a lower C-value. The coating paste forms a high resistance layer, as can be proved by measurement of the resistance distribution in the cross-section of the sintered body, which will show a high resistance at the side surface of the sintered body. The high resistance layer is controlled so as to have a thickness more than 10μ. Particularly, it can be shown from an x-ray analysis of the cross-sectional portion of sintered body, that the paste comprising a combination of silicon dioxide and bismuth oxide, or antimony oxide and bismuth oxide forms a layer having a thickness of more than 3μ and that said layer comprises, in a region to a 3μ depth from the side surface, more than 70 mole percent of at least zinc silicate (Zn 2 SiO 4 ) and/or zinc antimonate (Zn 7 Sb 2 O 12 ).
After sintering, the sintered body has electrodes applied to the opposite end surfaces of the sintered body. Said electrodes can be made by any available method such as heating of noble metal paint, electroless or electrolytic plating of Ag, Cu, Ni, Sn etc. vacuum evaporating of Al, Zn, Sn etc. and flame spraying of Cu, Sn, Al, Zn etc. in accordance with the prior well known techniques.
Lead wires can be attached to the electrodes in a per se conventional manner by using conventional solder. It is convenient to employ a conductive adhesive comprising silver powder and resin in an organic solvent in order to connect the lead wires to the silver electrodes. The n-value of a voltage-dependent resistor according to this invention does not deteriorate even in a low current region due to the introduction of the covering layer at the side surface of the sintered body, and it has a high stability with respect to temperature and humidity and in the load life test, which is carried out at 70°C, 90 percent RH at a rating power for 500 hours. The n-value and C-value do not change appreciably after the load life test. From a surge test, which is carried out by applying a 4× 10 μsec impulse current twice, it is shown that this voltage-dependent resistor has the ability to withstand surges of more than 2000A/cm 2 .
EXAMPLE 1.
Starting materials listed in Table 1 were mixed in a wet mill for 5 hours. Each mixture was dried and pressed in a mold into a disc of 40 mm in diameter and 25 mm in thickness at a pressure of 340 kg/cm 2 . The pressed bodies had the side surface covered by coating paste including solid ingredients listed in Table 1, and were dried. Then, the bodies were sintered in air for 5 hours at 1200°C and furnace-cooled. The sintered bodies were lapped to the thickness listed in table 1 by lapping the opposite end surfaces thereof with silicon carbide abrasive having a particle size of 600 mesh. The opposite end surfaces of the sintered discs were provided with a spray metallized film of aluminum by a per se well known technique. The electric characteristics of the resultant resistors are shown in Table 1. It will be readily understood that the C-value changes in proportion to the thickness of the sintered body.
Size of disc: 32 mm in dia.
Thickness of high resistive layer: 30μ
Table 1 ____________________________________________________________ ______________ Composition Solid Ingredient Thickness C(V) n of Sintered of Paste of Sintered Body Body (mol. %) (mol. %) (mm) (at 1mA) (0.1-1mA) ____________________________________________________________ ______________ SiO 2 (50) 5 150 15 10 302 14 Bi 2 O 3 (50) 20 605 15 SiO 2 (90) 5 153 15 10 310 16 Bi 2 O 3 (10) 20 605 16 SiO 2 (100) 5 155 14 ZnO (99.0) 10 310 15 Bi 2 O 3 ( 0.5) Bi 2 O 3 ( 0) 20 615 15 CoO ( 0.5) Sb 2 O 3 (90) 5 150 15 10 300 15 Bi 2 O 3 (10) 20 603 15 In 2 O 3 (90) 5 145 14 10 300 14 Bi 2 O 3 (10) 20 600 15 SiO 2 (72) 5 160 16 Sb 2 O 3 (20) 10 315 16 Bi 2 O 3 ( 8) 20 615 16 SiO 2 (90) 5 510 44 10 1025 45 Bi 2 O 3 (10) 20 2040 45 ZnO (97.5) Sb 2 O 3 (90) 5 500 45 Bi 2 O 3 ( 0.5) 10 1010 45 CoO ( 0.5) Bi 2 O 3 (10) 20 2010 46 MnO ( 0.5) In 2 O 3 (90) 5 505 45 Sb 2 O 3 ( 1.0) 10 1010 44 Bi 2 O 3 (10) 20 2015 46 SiO 2 (72) 5 515 46 Sb 2 O 3 (20) 10 1025 46 Bi 2 O 3 ( 8) 20 2040 46 ZnO (99.0) Sb 2 O 3 (90) 5 250 22 Bi 2 O 3 ( 0.5) 10 505 22 MnO (0.5) Bi 2 O 3 (10) 20 1000 23 ZnO (99.0) SiO 2 (90) 5 240 8.2 Bi 2 O 3 (0.5) 10 490 8.4 Sb 2 O 3 (0.5) Bi 2 O 3 (10) 20 985 8.4 ZnO (99.0) In 2 O 3 (90) 5 200 10 Bi 2 O 3 ( 0.5) 10 410 10 BaO ( 0.5) Bi 2 O 3 (10) 20 815 10 ZnO (99.0) SiO 2 (72) 5 205 11 Bi 2 O 3 ( 0.5) Sb 2 O 3 (20) 10 400 11 SrO ( 0.5) Bi 2 O 3 ( 8) 20 810 12 ____________________________________________________________ ______________
EXAMPLE 2
Starting materials of Table 2 were fabricated into voltage dependent resistors by the same process as that of Example 1. An impulse test was carried out by applying a 4×10μs impulse and the ability to withstand current surges was thus determined. A humidity test was carried out by boiling the disc in the pure water for 24 hours. The electric characteristics of the resultant resistors are shown in Table 2.
Size of disc: 32 mm in dia. and 20 mm in thickness
Sintering: 1200°C for 5 hours
Thickness of high resistance layer: 30μ
TABLE 2 ____________________________________________________________ ______________ Composition Electric Characteristics of Resultant of Sintered Solid Ingredient Resistor Body of Paste C(V) n Impulse Boiling (mol. %) (mol %) (at 1mA) 0.1-1mA Withstand, Test (KA) ΔC ____________________________________________________________ ______________ (%) SiO 2 (50) Bi 2 O 3 (50) 605 15 20 -5.0 SiO 2 (60) Bi 2 O 3 (40) 605 15 20 -4.7 SiO 2 (70) Bi 2 O 3 (30) 600 15 25 -4.7 SiO 2 (80) Bi 2 O 3 (20) 600 16 30 -3.8 SiO 2 (90) Bi 2 O 3 (10) 605 16 35 -2.9 SiO 2 (95) Bi 2 O 3 ( 5) 610 16 30 -3.2 SiO 2 (100) Bi 2 O 3 ( 0) 615 15 30 -3.5 Sb 2 O 3 (30) Bi 2 O 3 (70) 600 14 20 -5.3 ZnO (99.0) Sb 2 O 3 (50) Bi 2 O 3 (50) 600 14 25 -4.5 Bi 2 O 3 (0.5) Sb 2 O 3 (70) Bi 2 O 3 (30) 600 15 25 -3.5 CoO (0.5) Sb 2 O 3 (90) Bi 2 O 3 (10) 603 15 35 -2.7 Sb 2 O 3 (95) Bi 2 O 3 (5) 605 15 30 -3.0 Sb 2 O 3 (100) Bi 2 O 3 (0) 610 14 25 -3.3 In 2 O 3 (50) Bi 2 O 3 (50) 595 14 20 -5.7 In 2 O 3 (70) Bi 2 O 3 (30) 600 14 25 -4.3 In 2 O 3 (90) Bi 2 O 3 (10) 600 15 35 -3.1 In 2 O 3 (95) Bi 2 O 3 (5) 600 15 30 -3.4 In 2 3 (100) Bi 2 O 3 ( 0) 610 14 30 -3.5 SiO 2 (50) Bi 2 O 3 (50) 1960 42 25 -5.5 SiO 2 (60) Bi 2 O 3 (40) 1980 42 30 -4.8 SiO 2 (70) Bi 2 O 3 (30) 2000 44 35 -3.9 SiO 2 (80) Bi 2 O 3 (20) 2100 44 40 - 3.2 SiO 2 (90) Fi 2 O 3 (10) 2040 45 40 -1.5 SiO 2 (95) Bi 2 O 3 ( 5) 2040 45 35 -2.1 SnO (97.5) SiO 2 (100) Bi 2 O 3 ( 0) 2030 44 30 -2.3 Bi 2 O 3 ( 0.5) Sb 2 O 3 (30) Bi 2 O 3 (70) 1980 44 25 -5.1 CoO ( 0.5) Sb 2 O 3 (50) Bi 2 O 3 (50) 2000 44 30 -4.9 MnO (0.5) Sb 2 O 3 (70) Bi 2 O 3 (30) 2000 45 35 -3.8 Sb 2 O 3 (1.0) Sb 2 O 3 (90) Bi 2 O 3 (10) 2010 46 40 -2.5 Sb 2 O 3 (95) Bi 2 O 3 ( 5) 2015 45 40 -3.1 Sb 2 O 3 (100) Bi 2 O 3 ( 0) 2020 45 30 -3.5 In 2 O 3 (50) Bi 2 O 3 (50) 1990 44 25 -5.3 In 2 O 3 (70) Bi 2 O 3 (30) 2005 44 30 -4.9 In 2 O 3 (90) Bi 2 O 3 (10) 2015 46 40 -3.1 In 2 O 3 (95) Bi 2 O 3 ( 5) 2015 45 40 -3.4 In 2 O 3 (100) Bi 2 O 3 ( 0) 2000 45 25 -3.4 SiO 2 Sb 2 O 3 Bi 2 O 3 50 45 5 600 15 30 -4.4 ZnO (99.0) 50 30 20 600 15 30 -4.8 Bi 2 O 3 (0.5) 95 3 2 615 16 35 -3.2 CoO (0.5) 95 2 3 615 16 40 -3.4 58 40 2 610 15 35 -3.0 78 2 20 610 15 40 -2.5 72 20 8 620 17 45 -1.7 50 45 5 2050 44 40 -3.4 ZnO (97.5) 50 30 20 2065 45 45 -2.8 Bi 2 O 3 (0.5) 95 3 2 2045 45 50 -2.7 CoO (0.5) 95 2 3 2075 46 50 -2.7 MnO (0.5) 58 40 2 2060 44 50 -2.0 Sb 2 O 3 (1.0) 78 2 20 2080 46 55 -1.2 72 20 8 2100 48 60 -0.5 ____________________________________________________________ ______________
EXAMPLE 3
Starting materials of Table 3 were fabricated into voltage dependent resistors by the same process as that of Example 1. Then the tests were carried out by the same methods as those of Example 2. The electric characteristics of the resultant resistors are shown in Table 3.
Size of disc: 32 mm in dia. and 20 mm in thickness
Sintering: 1200°C for 5 hours
Thickness of high resistance layer: 30μ
TABLE 3 ____________________________________________________________ ______________ Composition of Sintered Electric Characteristics of Body Solid Ingredient Resultant Resistor (mol %) of Paste C(V) n Impulse Boiling Further (mol %) with- Test ZnO Bi 2 O 3 Additives (at 1mA) 0.1-1mA stand. ΔC(%) (KA) ____________________________________________________________ ______________ 99.90 0.05 CoO 0.05 350 10 15 -6.2 89.95 0.05 CoO 10 SiO 2 (90) 420 12 18 -6.2 89.95 10 CoO 0.05 Bi 2 O 3 (10) 420 13 20 -3.9 80.00 10 CoO 10 750 14 20 -4.0 99.0 0.5 CoO 0.5 605 16 35 -6.3 99.90 0.05 MnO 0.05 500 13 15 -6.3 89.95 0.05 MnO 10 600 14 15 -5.9 89.95 10 MnO 0.05 Sb 2 O 3 (90) 900 18 25 -3.3 80.00 10 MnO 10 Bi 2 O 3 (10) 1250 17 25 -3.5 99.0 0.5 MnO 0.5 1000 23 35 -2.8 99.90 0.05 Sb 2 O 3 0.05 300 7.9 15 -7.0 89.95 0.05 Sb 2 O 3 10 800 7.2 15 -5.5 89.95 10 Sb 2 O 3 0.05 In 2 O 3 (90) 720 8.2 17 -3.9 80.00 10 Sb 2 O 3 10 Bi 2 O 3 (10) 1300 8.6 18 -4.2 99.0 0.5 Sb 2 O 3 0.5 990 8.4 25 -2.0 99.90 0.05 BaO 0.05 320 7.2 18 -5.3 89.95 0.05 BaO 10 470 8.0 15 -4.9 89.95 10 BaO 0.05 SiO 2 (90) 510 9.4 20 -2.9 80.00 10 BaO 10 Bi 2 O 3 (10) 1200 0.5 20 -3.4 99.0 0.5 BaO 0.5 815 10 25 -2.5 99.90 0.05 SrO 0.05 300 9.2 12 -7.2 89.95 0.05 SrO 10 Sb 2 O 3 (90) 1150 8.1 14 - 5.7 89.95 10 SrO 0.05 Bi 2 O 3 (10) 1200 11 17 -4.3 80.00 10 SrO 10 1400 11 18 -4.5 99.0 0.5 SrO 0.5 810 12 20 -3.3 CoO 0.5 98.5 0.5 MnO 0.5 850 27 45 -3.5 CoO 0.5 98.5 0.5 Sb 2 O 3 0.5 1700 40 50 -4.2 CoO 0.5 98.5 0.5 BaO 0.5 1000 22 35 -4.5 CoO 0.5 98.5 0.5 SrO 0.5 SiO 2 (72) 950 25 40 -5.3 MnO 0.5 Sb 2 O 3 (20) 98.5 0.5 Sb 2 O 3 0.5 Bi 2 O 3 ( 8) 1800 40 50 -4.7 MnO 0.5 98.5 0.5 BaO 0.5 1300 32 40 -3.8 MnO 0.5 98.5 0.5 SrO 0.5 1250 30 40 -3.8 Sb 2 O 3 0.5 98.5 0.5 BaO 0.5 1300 20 30 -4.7 Sb 2 O 3 0.5 98.5 0.5 SrO 0.5 1220 20 30 -5.2 BaO 0.5 98.5 0.5 SrO 0.5 750 17 25 -7.0 CoO 0.5 98.0 0.5 MnO 0.5 1800 40 50 -1.5 Sb 2 O 3 0.5 CoO 0.5 98.0 0.5 MnO 0.5 800 29 35 -2.5 BaO 0.5 CoO 0.5 98.0 0.5 MnO 0.5 770 26 35 -3.0 SrO 0.5 CoO 0.5 98.0 0.5 Sb 2 O 3 0.5 1500 33 40 -2.7 BaO 0.5 CoO 0.5 98.0 0.5 Sb 2 O 3 0.5 1450 31 35 -2.2 SrO 0.5 SiO 2 (72) CoO 0.5 Sb 2 O 3 (20) 98.0 0.5 BaO 0.5 Bi 2 O 3 ( 8) 880 18 25 -3.3 SrO 0.5 MnO 0.5 98.0 0.5 Sb 2 O 3 0.5 1650 35 40 -3.1 BaO 0.5 MnO 0.5 98.0 0.5 Sb 2 O 3 0.5 1600 33 40 -2.4 SrO 0.5 MnO 0.5 98.0 05 BaO 0.5 1000 21 35 -2.5 SrO 0.5 Sb 2 O 3 0.5 98.0 0.5 BaO 0.5 1050 18 30 -3.0 SrO 0.5 ____________________________________________________________ ______________
EXAMPLE 4
The fabrication process and testing method were the same as those of Example 2 and the thickness of the high resistance layer was varied with the results as shown in Table 4. It is easily understood that the ability to withstand impulses increases with an increase in the thickness of the high resistance layer and the rate of change of the C-value caused by the boiling test decreases with an increase of thickness of the high resistance layer.
Size of disc: 32 mm in dia. and 20 mm in thickness
Sintering: 1200°C for 5 hours
TABLE 4 ____________________________________________________________ ______________ Composition Solid Ingredient Thickness Electric Characteristics of of Sintered of Paste of High- Resultant Resistor Body (mol %) Resistive C(V) n Impulse Boiling (mol %) layer (at 1 mA) 0.1-1 mA Withstand Test (μ) (KA) ΔC ____________________________________________________________ ______________ (%) 10 600 16 30 -4.3 SiO 2 (90) 30 605 16 35 -2.9 Bi 2 O 3 (10) 100 605 16 40 -3.2 300 615 16 50 -1.2 10 600 14 30 -4.2 Sb 2 O 3 (90) 30 603 15 35 -2.7 ZnO (99.0) Bi 2 O 3 (10) 100 605 15 40 -2.2 Bi 2 O 3 ( 0.5) 300 610 15 45 -1.7 CoO ( 0.5 10 590 15 25 -4.8 In 2 O 3 (90) 30 600 15 35 -3.1 Bi 2 O 3 (10) 100 605 15 40 -3.3 300 610 16 45 -2.7 10 605 17 35 -3.3 SiO 2 (72) 30 620 17 45 -1.7 Sb 2 O 3 (20) 100 620 17 50 -1.2 Bi 2 O 3 ( 8) 300 630 18 60 -1.0 10 200 43 30 -2.1 SiO 2 (90) 30 2040 45 40 -1.5 Bi 2 O 3 (10) 100 2070 45 45 -1.1 300 2100 46 45 -0.5 10 1950 43 30 -3.3 ZnO (97.5) Sb 2 O 3 (90) 30 2010 46 40 -2.5 Bi 2 O 3 (0.5) Bi 2 O 3 (10) 100 2030 46 40 -2.0 CoO (0.5) 300 2050 46 50 -1.6 MnO (0.5) 10 2000 44 30 -4.7 Sb 2 O 3 (1.0) In 2 O 3 (90) 30 2015 46 40 -3.1 Bi 2 O 3 (10) 100 2050 46 55 -2.2 300 2100 47 60 -1.8 10 2050 46 50 -1.2 SiO 2 (72) 30 2100 48 60 -0.5 Sb 2 O 3 (20) 100 2120 50 70 -0.5 Bi 2 O 3 (8) 300 2150 50 80 -0.4 ____________________________________________________________ ______________
EXAMPLE 5
Starting materials of Table 5 were fabricated into voltage dependent resistors by the same process as in Example 1. The pressed bodies were sintered at a temperature between 1000°C to 1450°C for 5 hours after covering the side surface with coating pastes as listed in Table 5. The test conditions were the same as those of Example 2. The electric characteristics of resulting resistors are shown in Table 5.
Size of disc: 32 mm in dia. and 20mm
Thickness of high resistive layer; 30μ.
TABLE 5 ____________________________________________________________ ______________ Composition Solid Ingredient Sintering Electric Characteristics of of Sintered Resultant Resistor Body of Paste Temp. C (V) n Impulse Boiling (mol. %) (mol. %) (°C) (at 1 mA) 0.1-1mA Withstand Test ΔC(%) ____________________________________________________________ ______________ 1000 1200 11 15 -9.5 1100 850 14 17 -7.2 SiO 2 (50) 1200 605 15 20 -5.0 Bi 2 O 3 (50) 1300 420 13 18 -5.1 1450 280 11 18 -5.3 1000 1220 13 20 -7.7 1100 870 14 25 -4.1 SiO 2 (90) 1200 605 16 35 -2.9 Bi 2 O 3 (10) 1300 450 16 35 -2.9 1450 300 15 30 -3.5 1000 1250 12 20 -7.0 1100 900 14 25 -5.1 SiO 2 (100) 1200 615 15 30 -3.5 Bi 2 O 3 ( 0) 1300 470 14 23 -3.7 1450 330 14 20 -4.0 1000 1200 11 15 -8.1 ZnO (99.0) Sb 2 O 3 (30) 1200 600 14 20 -5.3 Bi 2 O 3 (0.5) Bi 2 O 3 (70) 1450 300 13 18 -5.7 CoO (0.5) 1000 1190 13 28 -4.1 Sb 2 O 3 (90) 1200 603 15 35 -2.7 Bi 2 O 3 (10) 1450 285 14 30 -3.3 1000 1220 12 20 -5.0 Sb 2 O 3 (100) 1200 610 14 25 -3.5 Bi 2 O 3 ( 0) 1450 310 13 20 -4.0 1000 1200 12 15 -7.5 In 2 O 3 (50) 1200 595 14 20 -5.7 Bi 2 O 3 (50) 1450 320 12 18 -6.0 1000 1230 13 25 -4.7 In 2 O 3 (90) 1200 600 15 35 -3.1 Bi 2 O 3 (10) 1450 295 15 25 -3.6 1000 1200 14 25 -5.1 In 2 O 3 (100) 1200 610 14 30 -3.5 Bi 2 O 3 ( 0) 1450 305 14 30 -4.0 1000 1250 14 35 -3.6 SiO 2 (72) 1100 910 15 40 -2.1 Sb 2 O 3 (20) 1200 620 17 45 -1.7 Bi 2 O 3 ( 8) 1300 430 16 40 -1.8 1450 300 15 40 -2.3 1000 3800 38 30 -2.9 SiO 2 (90) 1200 2040 45 40 -1.5 Bi 2 O 3 (10) 1450 1200 42 35 -2.0 1000 3900 41 35 -3.5 ZnO (97.5) Sb 2 O 3 (90) 1200 2010 46 40 -2.5 Bi 2 O 3 (0.5) Bi 2 O 3 (10) 1450 1250 43 35 -2.7 CoO (0.5) 1000 4000 42 35 -4.7 MnO (0.5) In 2 O 3 (90) 1200 2015 46 40 -3.1 Sb 2 O 3 (1.0) Bi 2 O 3 (10) 1450 1300 40 40 -3.5 1000 4050 40 40 -1.3 SiO 2 (72) 1100 3200 44 550 -0.9 Sb 2 O 3 (20) 1200 2100 48 60 -0.5 Bi 2 O 3 ( 8) 1300 1550 44 50 -1.1 1450 1300 40 45 -1.5 ____________________________________________________________ ______________
EXAMPLE 6
The mixtures of Table 6 were pressed and covered by coating paste comprising the same oxides as the additives in the body. Then the bodies were sintered in air for 5 hours. The test conditions were the same as those of Example 2. The electric characteristics of the resultant resistors are shown in Table 6. The excellent ability to withstand impulses and the small change in the C-value were obtained by coating paste containing the same materials as the additives in the sintered body.
Size of disc; 32 mm in ida. and 20 mm in thickness
Thickness of high resistive layer; 30μ
Table 6 ____________________________________________________________ ______________ Composition of Sintered body Solid Ingredient Sintering Electric Characteristics of (mol. %) of Paste Temp. Resultant Resistor Impulse Boiling Further C (V) n Withstand Test ZnO Bi 2 O 3 Additives (mol. %) (°C) (at 1mA) 0.1-1mA (KA) ΔC ____________________________________________________________ ______________ (%) 99.5 0.5 -- Bi 2 O 3 100 1200 4000 4.1 10 -7.5 99.5 -- CoO 0.5 CoO 100 1200 2200 3.9 10 -6.2 99.5 -- MnO 0.5 MnO 100 1200 2600 3.4 10 -5.3 99.5 -- Sb 2 O 3 0.5 Sb 2 O 3 100 1200 3000 3.7 12 -6.2 99.5 -- BaO 0.5 BaO 100 1200 1600 9.0 15 -7.0 99.5 -- SrO 0.5 SrO 100 1200 1500 7.8 12 -8.3 99.5 -- UO 2 0.5 UO 2 100 1200 2000 4.1 10 -7.9 99.5 -- PbO 0.5 PbO 100 1200 4000 4.3 12 -7.1 99.0 0.5 CoO 0.5 Bi 2 O 3 (50) CoO (50) 1200 600 15 22 -3.5 99.0 0.5 MnO 0.5 Bi 2 O 3 (50) MnO(50) 1200 1000 23 25 -3.7 99.0 0.5 Sb 2 O 3 0.5 Bi 2 O 3 (50) Sb 2 O 3 (50) 1200 985 8.3 18 -4.2 99.0 0.5 BaO 0.5 Bi 2 O 3 (50) BaO(50) 1200 820 11 20 -3.3 99.0 0.5 SrO 0.5 Bi 2 O 3 (50) SrO(50) 1200 800 12 20 -3.7 CoO 0.5 CoO 50 99.0 -- SnO 0.5 SrO 50 1200 4000 30 40 -5.0 MnO 0.5 MnO 50 99.0 -- BaO 0.5 BaO 50 1300 3500 30 35 -4.7 BaO 0.5 BaO 50 99.0 -- SrO 0.5 SrO 50 1100 2000 20 30 -3.3 Bi 2 O 3 50 98.0 1.0 CoO 0.5 CoO 25 1200 1800 15 25 -2.7 MnO 0.5 MnO 25 Bi 2 O 3 50 98.0 1.0 BaO 0.5 BaO 25 1200 1650 14 20 -3.5 SrO 0.5 SrO 25 Bi 2 O 3 20 97.5 0.5 CoO 0.5 CoO 20 1200 2000 46 55 -1.7 MnO 0.5 MnO 20 Sb 2 O 3 1.0 Sb 2 O 3 60 Bi 2 O 3 10 CoO 0.5 CoO 10 97.0 0.5 MnO 0.5 MnO 10 1200 2600 50 60 -0.5 Sb 2 O 3 1.0 Sb 2 O 3 40 SnO 2 0.5 SnO 2 30 Bi 2 O 3 10 CoO 0.5 CoO 10 97.0 0.5 MnO 0.5 MnO 10 1200 2800 50 60 -0.5 Sb 2 O 3 1.0 Sb 2 O 3 60 Cr 2 O 3 0.5 Cr 2 O 3 10 Bi 2 O 3 10 CoO 0.5 CoO 5 96.5 0.5 MnO 0.5 MnO 5 1200 4400 55 70 -0.3 Sb 2 O 3 1.0 Sb 2 O 3 25 Cr 2 O 3 0.5 Cr 2 O 3 5 SiO 2 0.5 SrO 2 50 Bi 2 O 3 5 CoO 0.5 CoO 5 MnO 0.5 MnO 5 94.0 0.5 Sb 2 O 3 1.0 Sb 2 O 3 20 1200 5600 60 70 -0.3 Cr 2 O 3 0.5 Cr 2 O 3 3 SiO 2 2.0 SiO 2 60 NiO 1.0 NiO 2 Bi 2 O 3 25 1000 3800 35 35 -1.2 CoO 0.5 CoO 25 98.0 0.5 MnO 0.5 MnO 25 1200 1800 40 50 -0.8 Sb 2 O 3 0.5 Sb 2 O 3 25 1450 850 41 40 -1.3 ____________________________________________________________ ______________