Voltage non-linear resistor

Abstract: A voltage non-linear resistor consisting essentially of zinc oxide and containing at least bismuth oxide as an additive, comprising at least two phases of the alpha type and gamma-type crystal phases of bismuth oxide, the quantity ratio alpha/gamma being 0.1-0.8.

Description: The present invention relates to a voltage non-linear resistor consisting essentially of zinc oxide. Heretofore, resistors consisting essentially of zinc oxide and containing a small amount of an additive, such as Bi₂O₃, Sb₂O₃, SiO₂, Co₂O₃, or MnO₂, etc., have been widely known as superior voltage non-linear resistors, and have been used as arrestors or the like using such characteristic property. See, for example, EP-A-241 150. Among such additives, bismuth oxide has α, β, γ and δ type crystal phase, but a bismuth oxide in conventional zinc oxide element is usually only β phase, γ phase or β+γ phase. The present inventors have found the following problems. Crystal phases of bismuth oxide in the zinc oxide element have a large influence on the characteristics of the varistor, so that optimum crystal phases have to be used. If only β phase is present, the life performance against applied voltage becomes short and discharge current withstanding capability is decreased. While, if only γ phase is present, current leakage becomes large, the index α of voltage non-linearity becomes small, and electrical insulation resistance becomes also low. If only β+γ phase is adopted, the ratio of β and γ relative to each other is unstable and constant characteristic properties can not be obtained. An object of the present invention is to obviate the above previously unknown disadvantages. Another object of the present invention is to provide a voltage non-linear resistor having an improved discharge current withstanding capability, improved varistors characteristics, and small variations of various characteristic properties. The present invention is a voltage non-linear resistor as set out in claim 1. The ratio of the α/γ phases of bismuth oxide is by weight. Because the resistor of the invention contains at least a desired amount ratio of α type crystal phase and γ type crystal phase as the crystal phases of bismuth oxide in the resistor, a voltage non-linear resistor can be obtained having an improved discharge current withstanding capability, and improved varistor characteristics, and not having variation of various characteristic properties. The reason for limiting the amount ratio of α/γ to 0.1-0.8 is because, if α/γ is less than 0.1, the characteristic property of the varistor at a low current region is poor and the electrical insulation resistance is much decreased. While, if α/γ exceeds 0.8, the lightning discharge current withstanding capability is decreased and the life performance against applied voltage also becomes bad. From a viewpoint of the lightning discharge current withstanding capability, α/γ is preferably 0.2-0.5 For incorporating at least the desired amount ratio of α type and γ type crystal phases of bismuth oxide, preferably silicon oxide in the form of amorphous silicon is added in an amount of 7-11 mol% calculated as SiO₂ relative to zinc oxide, the sintering is effected at a relatively low temperature of 1,050-1,200°C, and insulative covering of the side glass of the resistor is heat-treated at a temperature of 450-550°C. More preferably, a portion or the whole of the components of the additives including SiO₂ is calcined to 700-1,000°C in advance, adjusted as predetermined, mixed with zinc oxide, and then sintered. If the silica component is crystalline, the reactivity thereof with zinc oxide becomes bad, the zinc silicates formed are not distributed uniformly, and the discharge current withstanding capability tends to decrease, so that the use of amorphous silica is preferable. If the added amount of SiO₂ is less than 7 mol%, the desired γ phase of bismuth oxide is difficult to obtain. While, if this amount exceeds 11 mol%, a crystal phase of zinc silicate (Zn₂SiO₄) increases too much and the discharge current withstanding capability is likely to deteriorate. If the sintering temperature is less than 1,050°C, a sufficiently dense sintered body is hard to obtain. While, if it exceeds 1,200°C, the pores are increased so much that a good sintered body is difficult to obtain. If the heat-treating temperature of the side glass is less than 450°C, the desired γ phase is hard to obtain. While, if it exceeds 550°C, all α phase is transformed into γ phase. The components of the additives including SiO₂ are preferably calcined at 700-1,000°C, because such calcination prevents gelation of a slurry of mixed raw materials of the resistor, and affords a uniform distribution of the small amounts of the additives in the resistor. In producing the present voltage non-linear resistor consisting essentially of zinc oxide, at first, a raw material of zinc oxide adjusted as predetermined, and a raw material of an additive selected from the group consisting of bismuth oxide, cobalt oxide, manganese oxide, antimony oxide, chromium oxide, silicon oxide, nickel oxide, boron oxide, silver oxide, or mixtures thereof, etc., and adjusted to a desired fineness, are mixed in desired amounts. In this case, instead of silver oxide or boron oxide, silver nitrate or boric acid may be used, preferably bismuth borosilicate glass containing silver may be used. In this case, preferably SiO₂ is amorphous silica, and used in an amount of 7-11 mol% relative to zinc oxide. Preferably, an additive including the amorphous silica is calcined at 700-1000°C, adjusted as predetermined, and mixed with zinc oxide in desired amounts. Then, the powders of these raw materials are added and mixed with a desired amount of an aqueous solution of polyvinyl alcohol, etc., as a binder, and preferably with a desired amount of a solution of aluminum nitrate as a source of aluminum oxide. The mixing operation is effected preferably in a disper mill to obtain a mixed slurry. The mixed slurry thus obtained is granulated preferably by a spray dryer to obtain granulates. After the granulation, the granulates are shaped into a desired form under a forming pressure of 800-1,000 kg/cm². The formed body is calcined up to 800-1,000°C, at a temperature heating and cooling rate of 50-70°C/hr, for 1-5 hrs to flow away and remove the binder. Next, an insulative covering layer is formed on the calcined body at the side surface thereof. In an embodiment of the present invention, a paste of desired amounts of oxides, such as Bi₂O₃, Sb₂O₃, ZnO, SiO₂, or the mixtures thereof, etc., added and mixed with an organic binder, such as ethyl cellulose, butyl carbitol, n-butyl acetate, or the mixtures thereof, etc., is applied on side surface of the calcined body to a thickness of 60-300 µm. In this case also, preferably amorphous silica is used as the silica component. The calcined body applied with the paste is sintered up to 1,000-1,300°C, preferably 1,050-1,200°C, at a temperature heating and cooling rate of 40-60°C/hr, for 3-7 hrs to form a glassy layer. In a preferred embodiment, a glass paste of a glass powder in an organic binder, such as ethyl cellulose, butyl carbitol, n-butyl acetate, etc., is applied on the insulative covering layer to a thickness of 100-300 µm, and heat treated in air up to 450-550°C, at a temperature heating and cooling rate of 100-200°C/hr, for 0.5-2 hrs to form a glass layer. Afterwards, both the top and bottom flat surfaces of the disklike voltage non-linear resistor thus obtained is polished by SiC, Al₂O₃, diamond or the like polishing agent corresponding to #400-2,000, using water or preferably an oil as a polishing liquid. Then, the polished surfaces are rinsed, and provided with an electrode material, such as aluminum, etc., over the entire polished end surfaces by metallizing, for example, so as to form electrodes at the polished end surfaces thereby to obtain a voltage non-linear resistor. The electrodes are preferably formed on the end surfaces about 0.5-1.5 mm in from the circumferential edge thereof. In the aforementioned method, the preferred ranges of components in the raw materials are 0.1-2.0 mol% of Bi₂O₃, Co₃O₄, MnO₂, Sb₂O₃, Cr₂O₃ and NiO, 0.001-0.01 mol% of Al(NO₃)₃·9H₂O, 0.01-0.5 mol% of bismuth borosilicate glass containing silver, 0.5-15 mol% of amorphous SiO₂ and the rest ZnO. These materials were used in compositions and sintering and glass heat-treating conditions as set out in Tables 1 and 2. to produce voltage non-linear resistors of a diameter of 47 mm and a thickness of 20 mm. In order to examine crystal phases of bismuth oxide and quantity ratio thereof, a voltage of 400 V is used for a variation V ImA/mm after application of a lightning discharge current, and specimen Nos. 1-16 having crystal phase of Bi₂O₃ and quantity ratio within the scope of the present invention, and comparative specimen Nos. 1-12 having either the crystal phases or the quantity ratio outside the scope of the present invention, are prepared. The specimen Nos. 1-6 which are within the scope of the present invention were prepared by adding 7-11 mol% of amorphous silica, sintering at a temperature of 1,050-1,200°C, and a glass heat-treating at a temperature of 450-550°C. The specimen Nos. 7-16 which are also within the scope of the present invention were prepared by adding 7-8 mol% of amorphous silica, calcining the raw materials other than ZnO and Al(NO₃)₃·9H₂O at 700-1,000°C for 2-8 hrs for preparing the raw materials, sintering at a temperature of 1,050-1,200°C, and glass heat-treating at a temperature of 450-550°C. The comparative specimen Nos. 1-3 were prepared at a glass heat-treating temperature different from the above glass heat-treating temperatures. The comparative specimen Nos. 4-12 were prepared with an added amount of silica different from the above amounts of silica. These specimens of the present invention and the comparative specimens are measured for voltage non-linearity index α and lightning discharge current withstanding capability. The results are shown in Table 3 below. Crystal phases of bismuth oxide and the quantity ratio of the crystal phase are measured by an inner standard method using X-ray diffraction. In the inner standard method, the peak of 2ϑ=23.0° (102) of CaCO₃ is used, and quantitative analysis is effected using 2ϑ=26.9° (113) for α-Bi₂O₃, and 2ϑ=30.4° (222) for γ-Bi₂O₃. Voltage non-linearity index α is based on an equation I=KV α (wherein, I is an electric current, V is a voltage, and K is a proportional constant), and measured from a ratio of V ImA and V 100 µA . Lightning discharge current withstanding capability test is effected by applying twice an electric current of 60 KA, 65 KA, 70 KA, or 80 KA of a waveform of 4/10 µs, and an element destroyed by the test is denoted by a symbol X, and the element not destroyed by a symbol ○. Specimen No. Bi₂O₃ crystal phase α phase: γ phase (α/γ) Sintering temp. (°C) Heat-treating temp. (°C) Present Invention 1 α+γ 0.12 1050 550 2 " 0.18 " 500 3 " 0.21 " 550 4 " 0.25 1100 " 5 " 0.33 " 500 6 " 0.37 1200 " 7 α+γ+δ 0.41 1050 530 8 " 0.44 " 500 9 " 0.49 " 500 10 " 0.53 1100 450 11 " 0.59 " 500 12 " 0.63 " 450 13 " 0.66 1150 550 14 " 0.73 " 500 15 " 0.77 1200 " 16 " 0.80 " 450 Comparative 1 α 0 1100 400 2 β 0 " - 3 γ 0 " 650 4 α+γ 0.08 " 550 5 α+γ+δ 0.82 " 450 6 α+β+γ 0.98 " 500 7 " 1.06 " " 8 " 1.25 " " 9 " 1.63 " " 10 " 2.42 " " 11 " 3.01 " " 12 " 5.78 " " As seen clearly from the above Table 3, the specimen Nos. 1-16 which are the voltage non-linear resistor of the present invention have improved voltage non-linearity index α and good lightning discharge current withstanding capability as compared with the comparative specimen Nos. 1-12. As explained above in detail in the foregoing the voltage non-linear resistor containing a desired quantity ratio of α type and γ type crystal phases as crystal phases of bismuth oxide in the resistor can provide various superior characteristics of resistor, particularly voltage non-linearity index and lightning discharge current withstanding capability of varistor. Stable characteristics of resistors are also obtained on switching impulse discharge current withstanding capability, life performance against applied voltage, and V ImA variation after application of lightening discharge current, and limit voltage characteristic property. Although the present invention has been explained with specific examples and numerical values, it is of course apparent to those skilled in the art that various changes and modifications thereof are possible.

Claims: A voltage non-linear resistor consisting primarily of zinc oxide and Containing at least bismuth oxide as an additive, characterized in that at least α type and γ type crystal phases of bismuth oxide are present, and the quantity ratio α/γ of the amount of the α type crystal phase and the amount of the γ type crystal phase is in the range 0.1-0.8. A resistor according to claim 1 also containing as additive at least one of antimony oxide and silicon oxide.

Inventors: IMAI OSAMU, SATO RITSU, IMAI, OSAMU, SATO, RITSU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1989
Main classification: H01C7
IPC codes: H01C7, C04B35