VOLTAGE NON-LINEAR RESISTOR

Abstract: A voltage non-linear resistor having a voltage non-linear resistance element consisting mainly of zinc oxides and a high resistance layer including a zinc silicate phase consisting mainly of Zn₂SiO₄ and a spinel phase consisting mainly of Zn₇Sb₂O₁₂. The layer has a continuous zinc silicate phase in which zinc silicate particles are arranged continuously. In this voltage non-linear resistor the risk of flashover is prevented or reduced, and thus stable electric characteristics especially lightning discharge current withstanding capability can be obtained.

Description: The present invention relates to a voltage non-linear resistor comprising, as its main ingredient, zinc oxides and more particularly a voltage non-linear resistor which has stable electric characteristics such as a lightning discharge current withstanding capability. A voltage non-linear resistor comprising zinc oxides as its main ingredient and a small amount of Bi₂O₃, Sb₂O₃, SiO₂, Co₂O₃, MnO₂ etc. as its additive ingredient is known as showing an excellent voltage non-linearity. Therefore, the voltage non-linear resistor is widely utilized in arrestors etc. Since the voltage non-linear resistor has the characteristic of acting normally as an insulator but as a conductor when an overcurrent flows, a line accident due to a thunderbolt can be effectively prevented even when the thunderbolt strikes the arrester utilizing the voltage non-linear resistor. In a voltage non-linear resistance element of the voltage non-linear resistor mentioned above, when a surge current such as a thunderbolt etc. is applied to the element, a lightning discharge mainly along a peripheral side surface of the element i.e. flashover occurs and the resistor is liable to be broken. Therefore, it is necessary to arrange a high resistance layer on a peripheral side surface of the element. However, flashover cannot be effectively prevented even though the high resistance layer is present, because the occurrence of flashover is largely dependent upon the structural state of the high resistance layer. That is to say, in the high resistance layer comprising a zinc silicate phase consisting mainly of Zn₂SiO₄ and a spinel phase consisting mainly of Zn₇Sb₂O₁₂, particle states of respective phases especially zinc silicate phase is very significant for the prevention of flashover, so that, as the case may be, flashover cannot be effectively prevented. The object of the present invention is to reduce or eliminate the drawbacks mentioned above and to provide a voltage non-linear resistor which has stable electric characteristics, especially an excellent lightning discharge current withstanding capability. According to the invention, there is provided a voltage non-linear resistor as set out in claim 1. In the high resistance layer structure used in the invention, zinc silicate particles are continuous in the zinc silicate phase constituting the high resistance layer. Consequently the resistivity of the high resistance layer becomes better as compared with the high resistance layer having discontinuous zinc silicate particles, and thus flashover can be effectively prevented. Therefore, according to the invention, it is possible to obtain a voltage non-linear resistor which has excellent electric characteristics, especially an excellent lightning discharge current withstanding capability. Fig. 1a and Fig. 1b are cross sectional views (Scanning Electron Microscope Images) showing particle structures of a voltage non-linear resistor according to the present invention and the comparision one, respectively. Typically to obtain a voltage non-linear resistor comprising zinc oxides as a main ingredient, a zinc oxides material having a particle size adjusted as predetermined is mixed, for 50 hours in a ball mill, with a predetermined amount of an additive comprising respective oxides of Bi, Co, Mn, Sb, Cr, Si, Ni, Al, B, Ag, etc. having a particle size adjusted as predetermined. The thus prepared starting powder is mixed with a predetermined amount of polyvinylalcohol aqueous solution as a binder and, after granulation, formed into a predetermined shape, preferably a disc, under a forming pressure of 800 to 1,000 kg/cm². The formed body is provisionally calcined under conditions of heating and cooling rates of 50° to 70° C./hr. and a retention time at 800° to 1,000°C. of 1 to 5 hours, to expel and remove the binder. Next, the insulating covering layer is formed on the peripheral side surface of the provisional calcined disc-like body. For example, an oxide paste comprising bismuth oxides, antimony oxides, zinc oxides and silicon oxides etc. admixed with ethylcellulose, butyl carbitol, n-butylacetate or the like as an organic binder, is applied to form layers 60 to 300 µm thick on the peripheral side surface of the provisional calcined disc-like body. Then, this is subjected to a main sintering under conditions of heating and cooling rate of 40° to 60°C./hr. and a retention time at 1,000° to 1,300°C., preferably at 1,100° to 1,250° C., of 3 to 7 hours, and a voltage non-linear resistor comprising a disc-like element and an insulating covering layer with a thickness of about 30 to 100 µm is obtained. It is preferred that a glass paste comprising glass powder admixed with ethylcellulose, butyl carbitol, n-butylacetate or the like as an organic binder, is applied with a thickness of 100 to 300 µm onto the aforementioned insulating covering layer and then heat-treated in air under conditions of heating and cooling rates of 100° to 200° C./hr. and a temperature retention time at 400° to 600°C. of 0.5 to 2 hours, to superimpose a glassy layer with a thickness of about 50 to 100 µm. Finally, both the top and bottom flat surfaces of the disc-like voltage non-linear resistor are polished to smooth by means of SiC, Al₂O₃ or diamonds and provided with aluminum electrodes by means of metallizing. In this case, the voltage non-linear resistor having a suitable high resistance layer with a continuous zinc silicate phase can be obtained by suitably combining various factors such as oxide paste compositions, methods of applying the oxide paste and sintering conditions. That is to say, it is preferable to use an oxide paste comprising the mixture for the insulating covering layer including 50 to 95 mol% silicon compounds calculated as SiO₂, 1 to 10 mol% bismuth compounds calculated as Bi₂O₃ and less than 30 mol% antimony compounds calculated as Sb₂O₃, and an organic binder such as ethylcellulose, buthyl carbitol, n-buthylacetate or the like, whose weight ratio is 1 (amount of mixture for insulating covering layer): 1 to 3 (amount of organic binder). In this case, for the composition of the mixture for insulating covering layer other than silicon compounds, bismuth compounds and antimony compounds, use may be made of zinc compounds or the like which can be changed into oxides under 1,000°C preferably under 800°C. That is to say, use may be made of carbonates, nitrates, hydroxides or the like, but it is preferable to use oxides. In this case, for the silicon oxides, it is most preferable to use amorphous silicon oxides. Moreover, for the composition of the mixture for the insulating covering layer, it is preferable to use SiO₂-Sb₂O₃-Bi₂O₃ system or SiO₂-Sb₂O₃-Bi₂O₃-ZnO system. Further, for the method of applying oxide paste, use is made of the method wherein the above oxide paste is applied on the peripheral side surface of the provisional calcined body a plurality of times to form layers 60 to 300 µm thick, by means of a dipping method or methods utilizing roller or brush. In this case, it is preferable to effect vacuum degassing operation for the oxide paste under 200 mmHg to eliminate pores in the oxide paste. Moreover, it is preferable to sinter the calcined body with the oxide paste layer under conditions of heating and cooling rates of 40 to 60°C/hr. and a retention time at 1,000 to 1,300°C, preferably at 1,100 to 1,250°C of 3 to 7 hours. With respect to voltage non-linear resistors inside and outside the scope of the invention, results of measurement on various characteristics will be explained hereinafter. In the examples, silicon oxides, zinc oxides, bismuth oxides and antimony oxides are contained as an oxide paste, but an equivalent effect will be realized with carbonates, hydroxides, etc. which can be converted to oxides during the firing. Apart from the silicon, zinc, antimony and bismuth compounds, any materials not impairing the effects of these compounds may be added to the paste in accordance with the intended use of the voltage non-linear resistor. With respect to the composition of the element also the same can be said. EXAMPLE 1 Specimens of disc-like voltage non-linear resistors 47 mm in diameter and 20 mm in thickness were prepared in accordance with the above-described process under the conditions of the following table 1, which had continuous or discontinuous zinc silicate phase, either inside or outside the scope of the invention, as shown in Table 1 below. With respect to each specimen, a lightning discharge current withstanding capability was evaluated. Moreover, in this example, other than the continuity of the zinc silicate phase, whether or not a mixture layer of zinc silicate and spinel arranged between the zinc silicate phase and the element is present and whether or not the spinel phase on the zinc silicate phase is continuous were observed. Further, the lightning discharge current withstanding capability means withstandability against impulse current having a waveform of 4×10 µs under various currents such as 100 KA, 120 KA, 140 KA, and the mark ○ denotes no flashover occurred upon two applications and the mark × denotes flashover occurred. In the above embodiments in the scape at the invention, use is made of amorphous SiO₂. The result is shown in Table 1. As is clear from the result shown in Table 1, the specimens Nos. 1 to 5 according to the invention each having the continuous zinc silicate phase are good and stable in the lightning discharge current withstanding capability as compared with the comparison specimens Nos. 1 and 2. Fig. 1a and Fig. 1b are cross sectional views showing particle structures of the voltage non-linear resistor according to the present invention and the conventional one, respectively. In the embodiment according to the invention shown in Fig. 1a, the continuous zinc silicate phase of dark grey having a thickness of about 60 to 70 µm is located substantially at a center of Fig. 1a. Moreover, a mixture layer composed of the zinc silicate of dark grey and the spinel of light grey is located between the continuous zinc silicate phase and the element. Further, the spinel phase of light grey is located on the continuous zinc silicate phase. In contrast to this, in the embodiment according to the conventional one shown in Fig. 1b, the zinc silicate phase of dark grey located at a center of Fig. 1b is discontinuous, and the bismuth oxide phase of white and the spinel phase of light grey are present in the discontinuous portion of the zinc silicate phase. Moreover, in the embodiment according to the invention, it is preferable, for the increase of the cohering strength between the resistance element and the high resistance layer and the insulating characteristics, that the thickness of the continuous zinc silicate phase is set within a range of 20 to 100 µm and an average particle size of zinc silicate is set within a range of 5 to 40 µm. Further, it is preferable that a thickness of the mixture layer of zinc silicate and spinel located between the continuous zinc silicate phase and the resistance element is set within a range of 5 to 70 µm and average particle sizes of zinc silicate and spinel are each set within a range of 1 to 10 µm, respectively. Furthermore, it is preferable that the spinel phase located on the continuous zinc silicate phase is discontinuous and an average particle size of spinel therein is set within a range of 10 to 30 µm. As is clear from the description above, according to the invention, since the zinc silicate phase is formed continuously in the high resistance layer, flashover can be effectively prevented, so that the stable electric characteristics especially the lightning discharge current withstanding capacity can be obtained. Furthermore, according to the invention, good life performances and good surge characteristics such as switching surge etc. can be obtained.

Claims: 1. A voltage non-linear resistor having a voltage non-linear resistance element consisting mainly of zinc oxides, and a high resistance layer provided on a peripheral side surface of said voltage non-linear resistance element and including a zinc silicate phase and a Zn₇Sb₂O₁₂-spinel phase, characterised in that said zinc silicate phase is a continuous phase. 2. A voltage non-linear resistor according to claim 1, wherein the thickness of said continuous zinc silicate phase is within the range of 20 to 100 µm and the average particle size of zinc silicate particles therein is within the range of 5 to 40 µm. 3. A voltage non-linear resistor according to claim 1 or claim 2, wherein a mixture layer of zinc silicate and spinel having a thickness of 5 to 70 µm is present between said continuous zinc silicate phase and said resistance element, and the average particle sizes of zinc silicate and spinel therein are within the range of 1 to 10 µm, respectively. 4. A voltage non-linear resistor according to any one of claims 1 to 3, wherein located on said continuous zinc silicate phase is a discontinuous region of said spinel phase and the average particle size of the spinel phase in said region is within the range of 10 to 30 µm.

Inventors: IMAI, OSAMU, NAKATA, MASAMI 1-16 AZA-MUKAIYAMASHITA
Applicants: NGK INSULATORS, LTD.
Application year: 1988
Main classification: H01C7
IPC codes: H01C7