Voltage non-linear resistor and fabricating method

Abstract: A non-linear resistor having a high impulse withstanding ability is obtained by directly applying a crystallised glass to the side surface of a sintered body of a ZnO element, and then bake-attaching the crystallised glass to the side surface of the ZnO element by heat treatment. Since impulse withstanding ability is improved by applying the crystallised glass as the side surface high-resistivity layer of the voltage non-linear resistor, the performance and the reliability of an arrester can be improved.

Description: BACKGROUND OF THE INVENTION The present invention relates to a voltage non-linear resistor and a fabricating method of voltage non-linear resistor made of ZnO as the main component mainly used in the electric power field such as a transmission/transforming system. Since the voltage non-linear resistor made of ZnO as the.major constituent (hereinafter referred to as "ZnO element") has an excellent non-linear current/voltage characteristics, it has been widely used as an arrester element in a transmission/transforming system. The voltage non-linear resistor is formed of the main component of ZnO containing Bi oxide as a main additive and small amounts of oxides of Sb, Mn, Co, Cr, Si, Ni, Al, B as sub-additives through a common ceramic fabrication technology. The common ceramic fabricating technology here mean processes of mixing, calcining and granulating of raw material powder, compacting the powder to form the powder in a proper shape such as disk, plate, cylinder or torus, baking and heat-treating the compacted body to form a sintered body, then forming electrodes. The voltage non-linear resistor for electric power use fabricated through the above precesses is required to have various important characteristics such as high non-linear coefficient (α-value), optimization of limiting voltage (varistor voltage), increase of impulse withstanding ability, improvement of loading life time and so on. The most important characteristic among them is that current does not short-circuit to flow along the side surface of the ZnO element when an impulsive high voltage such as thunder serge, switching surge or the like is applied to the ZnO element (prevention of creeping short-circuit). In order to cope with this requirement, there are proposed some method for preventing the creeping short-circuit current flow along the surface of ZnO element by forming an inorganic high resistance layer having a resistivity higher than that of ZnO element itself on the side surface of the ZnO element through applying and bake-attaching processes. The typical examples of the inorganic high resistance layers are made of boron silicate zinc glass and aluminum silicate glass as disclosed in Japanese Patent Publication No.54-26710 (1979) and Japanese Patent Publication No.58-27643 (1983) or of boron silicate zinc glass containing lead oxide and vanadium oxide, as known from JP-A-05 275 211. Prevention of creepage short-circuit of a ZnO element keeps the stability of an arrester using the ZnO element, which leads to improvement of reliability and safety of the transmission/transforming system itself. The voltage non-linear resistors of the prior art described above have the following disadvantages from view point of prevention of creepage short-circuit. In a case of forming a boron silicate zinc glass layer, the non-linear coefficient for the ZrO element is decreased. Further, since the acid-resistivity of the glass is low, there is a disadvantage in that the creepage short-circuit resistivity is decreased due to corrosion of the glass by nitric acid gas produced by corona discharge when the ZnO element is used by being contained in a nitrogen atmosphere as in an arrester. Furthermore, in a case of aluminum silicate glass which is proposed to eliminate the above disadvantages, according to the inventors' experiment result using the glass having the same chemical composition and the same component ratios as disclosed, wetness between the ZnO element and the glass itself is worse. Thereby, there is a problem of decrease in the creepage short-circuit resistivity because micro-cracks occur from the interface between the element and the glass layer during fabricating process and during use as an arrester to cause separation of the glass layer as a result. SUMMARY OF THE INVENTION In order to prevent creepage short-circuit of a ZnO element to keep the stability and reliability of an arrester, a better side surface high-resistivity layer and its fabricating method are required. An object of the present invention is, in regard to creepage short-circuit resistivity of an arrester, to provide a voltage non-linear resistor preventing creepage short-circuit of ZnO element and a method of fabricating the voltage non-linear resistor. The factors required for the side surface high-resistivity layer to prevent creepage short-circuit of ZnO element are as follows: (1) tight attaching ability with the ZnO element, (2) low non-uniformity in resistivity distribution inside the material, and (3) without impairment of the characteristics of ZnO element by the heat treatment process for forming the side surface high-resistivity layer. These are considered to be important. The inventors have selected crystallized glass for the side surface high-resistivity layer as the result of study in considering thermal expansion characteristic, acid resistant ability and so on from the view point of the above items. Further, as the result of study on attaching ability with ZnO element, it has been found that wetness with ZnO element is improved by adding ZnO and alkaline earth metals together to the glass and a reaction layer is formed in the interface. As the result of a detailed study on the components of glass, it has been clarified that a crystallized glass composed of ZnO, Al 2 O 3 , SiO 2 , ZrO 2 , BaO, CaO as major components is suitable for the side surface high-resistivity layer. Further, study on condition of heat treatment based on the above results has led to the present invention. The voltage non-linear resistor of the present invention is defined in claim 1. A method of fabricating such resistor is set forth in claim 3. As described above, a crystallized glass without impairment of the non-linearity of ZnO element itself and with better acid resistant ability is basically used for the side surface high-resistivity layer. The major components of the crystallized glass are ZnO, BaO, SiO 2 , Al 2 O 3 , ZrO 2 , CaO. The wetness and the attaching ability between the ZnO element and the glass are improved with ZnO and BaO in the glass. Improvement of the effect does not appear when only ZnO is added, or when alkaline earth oxide metal other than BaO is added. By adding ZnO and BaO together, a reaction layer with the ZnO element is easily formed, and the effect of improvement in attaching ability appears. Since CaO reacts with ZnO element inside more deeply than BaO, SiO 2 , Al 2 O 3 , ZrO 2 , there is an effect to lessen a step in resistivity distribution between the glass reaction layer of a high resistivity layer and the ZnO element. As a result, electric field does not concentrate to cracks or voids in the interface, and non-uniform resistance distribution in the ZnO element is lowered to decrease occurrence of the creepage short-circuit. The glass used for the side surface high-resistivity layer according to the present invention is turned into a crystallized glass by performing heat treatment. The compositions of the glass are 10∼20 wt% ZnO, 10∼30 wt% Al 2 O 3 , 20∼40 wt% SiO 2 , 20∼30 wt% BaO, 1.5∼5 wt% ZrO 2 , 0.5∼1.0 wt% CaO. When SiO 2 is more than 40 wt%, it is unfavorable because the softening temperature or temperature for working becomes so high that the baking temperature of the glass is higher than the sintering temperature of the ZnO element. On the contrary, when SiO 2 is less than 20 wt% or Al 2 O 3 is more than 30 wt%, it is unfavorable because a lot of cracks occur inside the glass layer and accordingly the glass cannot play a role as the high resistively layer. When Al 2 O 3 is less than 10 wt%, it is unfavorable because the softening temperature of the glass becomes high. When ZnO is less than 10 wt%, the thermal expansion coefficient of the glass does not match with that of ZnO element (ZnO element: 50∼70×10 7 /°C) and accordingly a problem is caused in that the glass layer is separated in fabricating process. On the contrary, when ZnO is more than 20 wt%, it is unfavorable because the acid resistant ability and the baking temperature of the glass are decreased. When BaO is less than 20 wt%, there is no effect of improving wetness with ZnO element. When BaO exceeds 30 wt%, it is unfavorable because non-uniform chemical reaction occurs during heat treating process to cause non-uniform resistance distribution inside the glass reaction layer. When ZrO 2 is less than 1.5 wt% or more than 5 wt%, it is unfavorable because the thermal expansion coefficient does not match with that of ZnO element. When CaO is less than 0.5 wt% or more than 1.0 wt%, it is unfavorable because non-uniform resistivity distribution occurs between the glass layer and the ZnO element. The glass composition according to the present invention may contain SrO, MgO, CoO, B 2 O 3 , CuO, Y 2 O 3 , MnO 2 , Na 2 O, Li 2 O as impurity. However, total amount of these components is preferably less than 1 wt% since the characteristic of the glass is changed when the containing amount is too large. When the added Al 2 O 3 is a filler, it is possible to lower the softening temperature, to improve strengthen of glass and to obtain a glass having better crystallization, which meets with the object of the present invention. The voltage non-linear resistor according to the present invention can be obtained by applying the aforementioned glass powder formed in a paste state by adding a proper organic material to the side surface of a disk-shaped, cylindrical or torus ZnO element fabricated through a common ceramic fabrication technology with spray method, dip method or mechanical transfer method, and after drying heating up the sintered body to 800∼950°C in the atmosphere and keeping the state for longer than one hour. Finally, Al electrodes are formed on the upper and lower end surfaces of the sintered body through melt spray method or bake-attaching method. The reason to limit the heat treating temperature is as follows. When the heat treating temperature is lower than 800°C, the glass does not melt. When the heat treating temperature is higher than 950°C, it is unfavorable because thermal strain is apt to remain in the ZnO element and micro-cracks occur in the interface of the reaction layer and in the glass due to change in the quantity of the glass reaction layer and excessive crystallization. It is preferable to keep the sintered body at the baking temperature for more than 1 hour. When the keeping time is shorter than 1 hour, it is unfavorable from the view point of attaching ability because the reaction does not progress sufficiently. In this fabricating process, it is possible to apply such a heat treatment condition as disclosed by the inventors (Japanese Patent Application No.6-16080) to improve the characteristic of ZnO element itself (performing heat treatment twice). This does not degrade the effect of the present invention. It is also possible to provide a high-resistivity ceramic layer (for example, complex oxide material of Bi 2 O 3 , SiO 2 , Sb 2 O 3 and the like)in the interface between the ZnO element and the glass layer. This does not degrade the effect of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a cross-sectional view explaining a ZnO element in accordance with the present invention. FIG.2 is a schematic chart of characteristic X-ray intensity identifying metal elements near the glass reaction layer of a ZnO element in accordance with the present invention. FIG.3 is a view showing the structure of an arrester using voltage non-linear resistors in accordance with the present invention. DESCRIPTION OF EMBODIMENTS <Embodiment 1> A starting raw material is prepared by weighing specified amounts of powders as to become the ratio of ZnO having a purity above 99.9% of 94.39 mol%, Bi 2 O 3 of 1.0 mol%, Sb 2 O 3 of 1.0 mol%, MnCO 3 of 0.5 mol%, Co 2 O 3 of 1.0 mol%, Cr 2 O 3 of 1.0 mol%, NiO of 1.0 mol%, B 2 O 3 of 0.1 mol% and Al(NO 3 ) 3 of 0.01 mol%, mixing the powders excluding ZnO using a pearl-mill, after drying calcining the mixed powder in air at 850°C for 2 hours, then crushing the calcined material to produce a complex oxide material, adding a proper amount of polyvinyl alcohol to the specified amounts of the complex oxide material and the ZnO powder, and mixing the powders using a ball-mill to produce a granulated powder. After press-compacting the granulated powder, the compacted body is sintered in air at 1190°C for approximately 4 hours. The rising and falling rates of temperature at that time are approximately 70°C/h. The dimension of the ZnO element after sintering is 50×25t. On the other hand, powder of glass (softening temperature: 850°C, composition: ZnO=15 wt%, BaO=27 wt%, Al 2 O 3 filler=25 wt%, SiO 2 =29.2 wt%, ZrO 2 =3 wt%, CaO=0.8 wt%) is suspended in carbithol solution of ethyl cellulose to form in a paste state, and the paste-state material is applied to the side surface of the above sintered body through spray method so that the thickness becomes 100∼200µm and then dried. The sintered body is heated up to 850°C and kept for 2 hours, and then cooled down to room temperature at cooling rate of approximately 75°C/h. Electrodes are formed by melt-spray Al on the top and bottom end surfaces of the sintered body obtained to fabricate a ZnO element. It is confirmed that the bake-attached glass is crystallized. FIG.1 is a schematic cross-sectional view showing the fabricated ZnO element, wherein reference number 1, 2 and 3 represent the ZnO element, glass layers and Al electrods, respectively. Table 1 shows a result of the non-linear coefficient (α-value) and the impulse withstanding ability of the fabricated ZnO element. NON-LINEAR COEFFICIENT(α) IMPULSE WITHSTANDING ABILITY 40kA 60kA 80kA 100kA PRESENT INVENTION 25∼30 ○ ○ ○ ○ CONVENTIONAL 1 (BORON SILICATE ZINC GLASS) 5∼10 ○ × × × CONVENTIONAL 2 (ALUMINUM SILICATE GLASS) 15∼20 ○ × × × The non-linear coefficient (α-value) is obtained by Equation (1) using V 1 and V 2 which are voltage between the ZnO element when DC 10µA(I 1 ) and 1mA(I 2 ) current flow to the ZnO element. α={log(I 1 /I 2 )}/´log(V 1 /V 2 )} The impulse withstanding ability is evaluated by presence or absence of damage (creepage short-circuit) of the ZnO element when impulse current of 8×20µs (four kinds of current) is conducted twice. In the table, the mark ○ indicates a normal case and the mark × indicates a damaged case. The non-linear coefficient of the ZnO element according to the present invention is nearly twice as large as that of the conventional element the side surface of which boron silicate zinc glass (conventional 1 in the table) or aluminum silicate glass (conventional 2) is bake-attached on. As for the impulse withstanding ability, the conventional elements are damaged at 40 kA. On the other hand, the element according to the present invention is in normal condition up to 100 kA. <Embodiment 2> Similar to the embodiment 1, a starting raw material is prepared by weighing specified amounts of powders as to become the ratio of ZnO having a purity above 99.9% of 94.39 mol%, Bi 2 O 3 of 1.0 mol%, Sb 2 O 3 of 1.0 mol%, MnCO 3 of 0.5 mol%, Co 2 O 3 of 1.0 mol%, Cr 2 O 3 of 1.0 mol%, NiO of 1.0 mol%, B 2 O 3 of 0.1 mol% and Al(NO 3 ) 3 of 0.01 mol%, mixing the powders excluding ZnO using a pearl-mill, after drying calcining the mixed powder in air at 850°C for 2 hours, then crushing the calcined material to produce a complex oxide material, adding a proper amount of polyvinyl alcohol to the specified amounts of the complex oxide material and the ZnO powder, and mixing the powders using a ball-mill to produce a granulated powder. After press-compacting the granulated powder, the compacted body is sintered in air at 1190°C for approximately 4 hours. The dimension of the ZnO element after sintering is 50×25t. On the other hand, each of twenty-nine (29) kinds of powder of glass shown in table 2 (combination of individual metal oxides consist of ZnO : 5, 10, 13, 14, 15, 17, 20, and 25 wt%, SiO 2: 15, 20, 26.2, 27.7, 28, 28.2, 29.2, 30, 40, and 44.2 wt%, BaO=15, 20, 23, 24.2, 24.5, 25, 25.9, 26, 26.2, 26.5, 26.6, 27, 29.2, 30, and 35 wt%, ZrO 2 =1.0, 1.5, 3, 4,5, 5.5 wt%, Al 2 O 3 filler=7, 10, 15, 22, 23, 25, 28, and30 wt%, CaO=0.4, 0.5, 0.8, 1.0, and 1.1 wt%) is suspended in carbithol solution of ethyl cellulose to form in a paste state, and the paste-state material is applied to the side surface of the above sintered body through spray method so that the thickness becomes 100∼200µm and then dried. In the above table 2, T.E.C., S.T., W.T., N.L.C., and I.W.A. represent thermal expansion coefficient, softening temperature, temperature of working, non-linear coefficient, and impulse withstanding ability, respectively. The sintered body is heated up to 850°C and kept for 2 hours, and then cooled down to room temperature at cooling rate of approximately 75°C/h. Electrodes are formed by melt-spray Al on the top and bottom end surfaces of the sintered body obtained to fabricate a ZnO element. Table 2 shows twenty-nine (29) kinds of composition, thermal expansion coefficient, softening temperature, temperature of working, and non-linear coefficient and impulse withstanding ability of ZnO element bake-attached with each of twenty-nine kinds of glass on the side surface by heat treatment. The impulse withstanding ability is evaluated by presence or absence of damage (creepage short-circuit) of the ZnO element when impulse current of 100kA (8×20µs) is conducted twice. In the table, the mark ○ indicates a normal case and the mark × indicates a damaged case. The non-linear coefficients of the elements bake-attached with twenty-nine kinds of glass pastes are nearly 27 to 30 and not largely different. However, the elements bake-attached with the glass pastes No.1, 5, 6, 11, 16, 17, 21, 22, 24, 25, and 29 are damaged by the impulse withstanding test of 100kA. The main reasons of damage of the elements can be considered are that in the glass No.6, 11, 22, separation occurs in the interface between the ZnO element and the glass and cracks occur in the glass because the thermal expansion coefficient of the glass does not match with the thermal expansion coefficient of the ZnO element (50 to 70×10 7 /°C); in the glass No.21, the glass is not bake-attached to the ZnO element because the softening temperature is too high; in the glass No.1, 5, 7, 24, cracks occur in the glass because non-uniform layer is produced in the glass. On the other hand, the main reasons of damage of elements can be considered are that in the glass No.12, separation occurs in the interface between the ZnO element and the glass because wetness between the ZnO element and the glass is bad; in the glass No.16, a low-resistivity portion is produced because the glass non-uniformly reacts with the ZnO element; in the glasses No.25 and No.29, the resistivity distribution between the glass layer and the ZnO element is non-uniform. From the above results, the optimum composition of the glass is preferably 10∼20 wt% ZnO, 20∼40 wt% SiO 2 , 20∼30 wt% BaO, 1.5∼5 wt% ZrO 2 , 10∼30 wt% Al 2 O 3 , 0.5∼1.0 wt% CaO. From scanning electron microscope observation of compositions in a cross-section near the side surface of a ZnO element bake-attached with the glass by heat treatment and the schematic chart of characteristic X-ray intensity (measuring apparatus: X-ray micro-analyzer) identifying metallic element near the glass reaction layer shown in FIG.2 in which reference numbers 4, 5 and 6 represent glass layer, glass raction layer and ZnO element, respectively, it can be understood that the ZnO element 6 and the glass 4 are closely attached to each other through a glass reaction layer 5, and Ca deeply enters into and reacts with the ZnO element 6 from the glass layer 4 through the glass reaction layer 5 comparing with Ba, Si, Zr, Al. It is considered that this lessens the resistivity step between the glass reaction layer and the ZnO element. The balance of the resistances accompanied by the glass reaction layer substantially contributes the improvement of impulse withstanding ability. <Embodiment 3> The glass paste No.3 shown in Table 2 is applied to the side surface of the ZnO element fabricated in the embodiment 2 and dried, and heated up to 850°C and kept for 2 hours, and then cooled down to room temperature at cooling speed of near 70°C/h. The ZnO element obtained through this manner is ground, cleaned, dried, and then dipped in an etching solution (ratio of nitric acid to water is 1:9) for 2 minutes. The index of acid resistance of glass is determined as the weight decrease before and after dipping. At that time, an element bake-attached with the conventionally used boron silicate zinc glass is also dipped in the etching solution for 2 minutes in order to test its acid resistivity for comparison. The test result is shown in Table 3. The glass according to the present invention has a glass dissolving rate (weight decreasing rate) of nearly one-third as small as that of the conventional one, and accordingly has better acid resistivity. GLASS ACCORDING TO THE PRESENT INVENTION CONVENTIONAL GLASS (BORON SILICATE ZINC GLASS) ACID RESISTIVITY (WEIGHT DECREASE µg/cm 2 ) 6000∼7000 30000∼40000 <Embodiment 4> The glass paste (No.3 shown in Table 2) is applied to the side surface of the ZnO element fabricated in the embodiment 2 and dried, and heat-treated by changing heating temperature in heat treating process to 750, 800, 900, 950, 1000°C, and electrodes are formed in the element after heat treatment. The relationships between the temperature of heat treatment of ZnO element and the attaching ability of glass to the ZnO element, and the impulse withstanding ability are tested. The condition of impulse is the same as in the embodiment 2. In the table, the mark ○ indicates a normal case and the mark × indicates a damaged case. The result is shown in Table 4. When heating temperature in the heat treatment process is 750 and 1000°C, the attaching ability is bad and voids and cracks occur in the interface. And the impulse withstanding ability is bad. On the other hand, when the heat treating temperature is 800 ∼ 950°C, the attaching ability is good and the impulse withstanding ability is good. Therefore, the heating temperature in the heat treatment process is preferably 800 ∼ 950°C. <Embodiment 5> The glass paste (No.3 shown in Table 2) is applied to the side surface of the ZnO element fabricated in the embodiment 2 and dried, and heat-treated at 850°C for 2 hours. The voltage non-linear elements are contained in an insulator pipe to fabricate an insulator type arrester shown in FIG.3 in which reference number 7, 8, 9 and 10 represent a voltage non-linear resistor, insulator, shield and an isulator base, respectively. The same impulse withstanding ability test as in the embodiment 4 has been conducted using the arrester. After the test, presence and absence of penetrating damage of the ZnO elements in the insulator pipe has inspected. No penetrating damage has been found. According to the present invention, it is possible to obtain a voltage non-linear resistor having better impulse withstanding ability than the conventional one. As a result, the reliability and the stability of electric power transmission/transforming system using the voltage non-linear resistor are improved. Therefore, the effect is very large.

Claims: A voltage non-linear resistor comprising a sintered body (1) having ZnO as the major constituent and containing Bi oxide as an additive, wherein the side surface of said sintered body is coated with a high melting-point crystallised glass (3) containing SiO 2 , Al 2 O 3 , ZnO, ZrO 2 , BaO, CaO as essential components, and electrodes (2) are formed on the both ends of said sintered body, and wherein the ranges of composition for the individual components of the coating glass (3) in oxide base are 10∼20 wt% ZnO, 20∼40 wt% SiO 2 , 10∼30 wt% Al 2 O 3 , 20∼30 wt% BaO, 1.5∼5 wt% ZrO 2 , 0.5∼1.0 wt% CaO. The voltage non-linear resistor of claim 1, wherein the Al 2 O 3 contained in the coating glass (3) is a filler. A method of fabricating the voltage non-linear resistor of claim 1 or 2, comprising the steps of sintering a powder having ZnO as a main component and containing Bi oxide as a main additive at a temperature of 1150∼1300°C, cooling down the sintered body below 300°C, applying a powder of the coating glass in a pasty state to the side surface of the sintered body, then heating up the sintered body to 800∼950°C, keeping the state for longer than one hour to bake, and forming electrodes on the ends of said sintered body. An arrester containing in an insulator pipe or a tank the voltage non-linear resistor according to claim 1 or 2.

Inventors: MAEDA KUNIHIRO, MOTOWAKI SHIGEHISA, SHIRAKAWA SHINGO, SYOUJI MORITAKA, TAKAHASHI KEN, TANAKA SHIGERU, YAMADA SEIICH, MAEDA, KUNIHIRO, MOTOWAKI, SHIGEHISA, SHIRAKAWA, SHINGO, SYOUJI, MORITAKA, TAKAHASHI, KEN, TANAKA, SHIGERU, YAMADA, SEIICH
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1995
Main classification: H01C7
IPC codes: H01C7, H01B1