Title of Invention	ELECTRIC RESISTANCE ELEMENT EXHIBITING VOLTAGE NONLINEARITY CHARACTERISTIC AND METHOD OF MANUFACTURING THE SAME
Abstract	1. A composition for an electric material, containing as a primary component zinc oxide and additionally containing bismuth oxide, antimony oxide, chromium oxide, nickel oxide, cobalt oxide, manganese oxide, silicon oxide and boron oxide, said composition further containing at least one of rare-earth elements in a range of 0.01 mol% to 3.0 mol% in terms of oxide thereof given by R203 where R represents generally said rare-earth elements; and aluminum in a range of 0.0005 mol% to 0.005 mol % in terms of aluminum oxide given by A1203.

Title of Invention

ELECTRIC RESISTANCE ELEMENT EXHIBITING VOLTAGE NONLINEARITY CHARACTERISTIC AND METHOD OF MANUFACTURING THE SAME

Abstract

1. A composition for an electric material, containing as a primary component zinc oxide and additionally containing bismuth oxide, antimony oxide, chromium oxide, nickel oxide, cobalt oxide, manganese oxide, silicon oxide and boron oxide, said composition further containing at least one of rare-earth elements in a range of 0.01 mol% to 3.0 mol% in terms of oxide thereof given by R203 where R represents generally said rare-earth elements; and aluminum in a range of 0.0005 mol% to 0.005 mol % in terms of aluminum oxide given by A1203.

Full Text	a composition tor The present invention relates to/an electric resistance element which is made of a sintered material containing zinc oxide as a primary component and which exhibits a nonlinear voltage characteristic (also referred to as the voltage nonlinearity characteristic or simply as voltage nonlinearity). The invention is also concerned with composition of the electric resistance element mentioned above and a method of manufacturing the same. **** Description of Related Art ^ Heretofore, it is well known in the art that a sintered material containing » zinc oxide as a primary component and added with bismuth oxide, cobalt oxide ° and/or other oxides exhibits a nonlinear voltage characteristic or voltage nonlinearity. The resistance element formed of such sintered material is widely employed in v. practical applications, as typified by a surge absorber for protecting circuit elements by absorbing a surge current (steep current rise), an arrester for protecting P4 electric/electronic apparatuses or equipment against an abnormal voltage brought Pi about by lightning and others. For having better understanding of the present invention, description will first be directed to the background techniques of the invention in some detail. Figure 10 is a schematic diagram showing a structure of a typical one of the sintered materials known heretofore from which the nonlinear voltage resistance element is made. Referring to the figure, some of spinel grc ii i; 1 each consisting of antimony compound and having a grain size in a rangii; of c ni? to several microns exist within zinc oxide grains while the other spinel grains I exist Internally of or adjacent to inter-grain boundary regions which contain bithmus oxide 3 as a primary component existing in the vicinity of triple points (multiple points) of zinc oxide grains. It is observed that some of bithmus oxide grains 3 not only exist at the multiple points but also penetrate deeply between the zinc oxide grains 2. Parenthetically, reference numeral 4 in Fig. 10 denotes a twin crystal boundary. It has experimentally been established by a measurement conducted by using point.electrodes that the grain containing primarily zinc oxide acts by itself simply as electric resistor while the boundary regions between the zinc oxide grains 2 exhibit voltage nonlinearity (see G.D. Median; L.M. Levinson and H.R. Phillip: "THEORY OF CONDUCTION IN ZnO VAHISTORS", J. Appl. Phys. 50(4)2799 (1979)). Furthermore, it has also experimentally been demonstrated that the number of boundaries between the zinc oxide grains 2 (also referred to as the inter-grain boundaries) determines a varistor voltage. The sintered material having such fine or microscopic structure as mentioned above and containing zinc oxide as the primary component usually exhibits such a voltage-versus-current characteristic (hereinafter also referred to as the V-l characteristic) as illustrated in Fig. 1 I. This V-l curve may be divided into three sections or regions in view of physical mechnniorns mentioned below. (1) A region in which a leakage current ru mains small when compared with an applied voltage due to a current limiting function of a Schottoky barrier presented by the grain boundaries (a region including ; i:oin ! shdwn in Fig. 11) in which the typical current value on the order of 10 u.A a ordi iari\|y selected for the resistance element having a diameter of about 100 mm). (2) A region in which resistance value decreases steeply as the applied voltage is increased, which causes a tunnel current flowing through the grain boundaries to increase for thereby decreasing steeply the resistance for the voltage as applied (i.e., a region including a transition point S shown in Fig. 11 at which transition or changing point from the region (1) to the region (2) occurs) and in which a current of a value typically in a range of 1 to 3 mA is generally selected for a resistance element having a diameter $ of about 100 mm. (3) A (V-l) region which is determined by the electric resistance of zinc oxide grains themselves (a region cover I rig a point H shown in Fig. 11 in which a current value typically of 10 kA is generally selected for the resistance element having a diameter on the order of 100 mm $). In the case of a sintered material containing zinc oxide grains which material is an n-type semiconductor material, it Is observed that when oxygen adheres or exists in excess in the crystal grain boundaries, an electron capturing level is formed at the interfaces, as a result of which depletion layers in which no electron exists are formed along the grain interludes, orming eventually generating the electron barriers (i.e., Schottoky barrieiiii/ a.1 o along the grain boundaries. Consequently, as the barrier height or levtii of thi:i £i;:hottoky barriers increases, the leakage current decreases. Thus, there can .:■•) obtained an.- electric resistance element which is excellent in respect to flatness t! the V-l characteristic curve in the small-current region. In this conjunction, it is noted that the electric characteristic at the grain boundary exerts a great influence to the flatness of the V-l characteristic curve in the small-current region, while resistance of the zinc oxide grains themselves affects remarkably the flatness of the V-l characteristic curve in a large-current region. More specifically, because increasing in the electric resistance of zinc oxide grains degrades the flatness of the V-l characteristic curve in the aforementioned region, it is preferred that the electric resistance of the zinc oxide grains should be as low as possible. Now, for convenience of description, a phrase "flatness ratio" used herein will be defined. A ratio between a voltage Vn in the large-current region H shown in Fig. 11 and a voltage VL in the small-current region L, i.e., VH/VL, is defined as the flatness ratio, as can be seen in Fk\| 11. Furthermore, a ratio between the varistor voltage Vs and the voltage \A. !n Ihe iirhill current region, i.e., VS/VL, is referred to as the flatness ratio in the small currc vl regldn, while a ratio between the varistor voltage Vs and the voltage VH in the large • current region, i.e., the ratio VH/VS, is referred to as the flatness ratio in the \|arge-ci ineiil region. ! In the case of the resistance: element exhibiting the nonlinear voltage characteristic, the varistor voltage Vs shown in Fl(:\|, 1 I represents a threshold voltage. In this conjunction, it is important to set the varistor voltage Vs in dependence on a voltage of a power transmission system or line to which an arrester constituted by the resistance element having the nonlinear voltage characteristic is' to be actually applied. Parenthetically, in many practical cases, the varistor voltage Vs is typically represented by an inter-electrode voltage (or terminal voltage) appearing across the resistance element upon flowing of a current of 1 mA therethrough. This terminal voltage which will hereinafter be represented by VimA is in proportion to a thickness of the resistance element. In recent years, there arises a great demand for an arrester of higher performance for the purpose of protectiny various apparatuses and instruments employed in power transmission systems in which electric power transmission tends to be realized with higher and higher voltage. In order to satisfy such demand, it becomes necessary to obtain such a resistance element which is capable of exhibiting excellent nonlinear voltage characteristic such as, for example, a resistance element having a smaller flatness ratio which plays an important role in realizing desired characteristics of the arrester. In this connection, in order to reduce the flatness ratio, there is required a manufacturing technique which is capable of Increasing the barrier height of the Schottoky barriers existing in the grain boundaries between the zinc oxide grains. However, the attempt for improving the flatness ratio in the large-current region will usually be accompanied with degradation oi tho flatness ratio in the small-current region. On the other hand, approach for improving the flatness ratio in the small-current region will involve degradation of the flatnoes ratio in the large-current region. On the other hand, in the caso of thi; mrester which is used in an ultra¬high voltage power transmission systep' iate I d example, on the order of 100 million volts, a number of elements haying :i .substantially same geometrical configuration and the varistor voltage value Vs CMUI silent to that of the resistance elements known heretofore are stacked with the ir cmirji„ial elemerits being electrically connected in series to one another. In I hat c,:iM! (he number of the electrical resistance elements as stacked necessarily lencje to increase, involving not only a bulky or large structure of the arrester aei a whole but also complication in the techniques required for realizing the serial connection, thus giving rise to many problems in respect to the arrester designs not only from the electrical view point but also from the thermal as well as mechanical standpoint. Such being the circumstances, it is believed that if the electrical resistance elements each capable of exhibiting a high varistor voltage Vs per unit iBfiylh (e.g. 1 mm) are available, the problems mentioned above can be solved, because then the voltage to be born by each of the electrical resistance elements can be increased, which in turn allows the number of the resistance elements stacked for realizing the series connection to be decreased. In summarization, it can be said that in the case of the electrical resistance element exhibiting the voltage nonllnearity as well as the method of manufacturing the same, the flatness ratio is often degraded when the varistor voltage is increased by varying the composition of the electrical resistance material by changing correspondingly the ratios or proportions of the additives. In particular, in the case of the resistance elements destined for application to a high or ultra-high power transmission system, a large leakage current tends to be generated upon application of a high voltage. Thus, there ie encountered a great difficulty in realizing compatibility between increasing of the varistor voltage and the flatness ratio in the small-current region which is a very important factor, as mentioned hereinbefore. SUMMARY OF THE INVENTION In the light of the state of the art described above, it is an object of the present invention to provide a electrical resistance element exhibiting a nonlinear voltage characteristic which element can evade the problems mentioned above. Another object of the present invention is to provide a method of manufacturing the electrical resistance element mentioned above. In view of the above and other object which will become apparent as the description proceeds, there is provided according an aspect of the present invention an electric resistance element exhibiting a nonlinear voltage characteristic, which element contains as a primary component zinc oxide and additionally contains bismuth oxide, antimony oxide, chromium oxide, nickel oxide, cobalt oxide, manganese oxide, silicon oxide and boron oxide. The resistance element further contains at least one of rare-earth elements in a range of 0.01 mol% to 3.0 mol% in terms of oxide thereof given by R2O3 where R represents generally the rare-earth elements, and aluminum in a range of 0.0006 mol% to 0.005 mol% in terms of aluminum oxide given by AC2O3. In a preferred mode for carrying out the invention, the rear-earth elements may.include yttrium (Y), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu). With the above-mentioned composition of the resistance element of voltage nonlinearity, the varistor voltage can be increased over the whole current range from small to large current levels without being accompanied with any appreciable degradation in the flatness ratio of the V-l characteristic curve. Furthermore, according to another aspect of the present invention, there is provided a method of manufacturing an electric) resistance element exhibiting a nonlinear voltage characteristic, starting, from E\| mixture containing as a primary component zinc oxide and additionally f;;l:!;rnut ■= :nide, antimony oxide, chromium oxide, nickel oxide, cobalt oxide, manganese 0 ido silicon oxide and boion oxide, and further containing at least one of rare-Mirth E:\| iinei'its in a range of 0.01 mol% to 3.0 mol% in terms of oxide thereof givefl by R2O3 where R represents generally the 1 rare-earth elements, and aluminum in a mhgje o\| 11 0005 mol% to 0.005 mol% in terms of aluminum oxide given by AC2O3. The method includes a step of preparing current region. Further, oxygen concentration of the oxidizing atmosphere in the second firing step may preferably be so selected as to fall within a range of 21 to 30% during the temperature lowering phase from the maximum firing temperature to the temperature corresponding to changing point of the temperature lowering rate in the second firing step. With the method described above, there can be manufactured a nonlinear voltage resistance element (or varistor element) having the varistor voltage increased significantly with a small flatness ratio over a substantially whole current range from large to small current region. Accordingly, the present invention provides a composition for an electric material, containing as a primary component zinc oxide and additionally containing bismuth oxide, antimony oxide, chromium oxide, nickel oxide, cobalt oxide, manganese oxide, silicon oxide and boron oxide, said composition further containing at least one of rare-earth elements in a range of 0.01 mol% to 3.0 mol% in terms of oxide thereof given by R203 where R represents generally said rare-earth elements; and aluminum in a range of 0.0005 mol% to 0.005 mol % in terms of aluminum oxide given by A1203. I The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the course of the description which follows, reference is made to the drawings, in which: Fig. 1 is a view for illustrating sintering processes together with the atmosphere and the temperatures therefor in the resistance element manufacturing method according to first and second exemplary embodiments of the present invention; Fig. 2 is a view for illustrating a pattern of firing temperature adopted in the sintering process; Fig. 3 is a view illustrating varistor voltages of resistance elements manufactured, being added with rare-earth elements; Fig. 4 is a view for illustrating varistor voltages and flatness ratios of V-l characteristic curve in nonlinear-voltage resistance elements manufactured, being added with A203 and rare-earth elements; Fig. 5 is a view showing a relation between amounts of addition of rare-earth elements and varistor voltage; Fig. 6 is a view illustrating varMor voltages and flatness ratios in resistance elements subjected to gradual cooling In a temperature lowering process in an oxidizing atmosphere in a second firlnc) st;p; Fig. 7 is a view illustrating eel. (ions "ivl-^;:en the varistor voltage and the V-l characteristic flatness ratio of reoi^ancc element as manufactured and concentration of oxygen in an oxidizing at mo:-p he r;i employed in a second firing step of the sintering process; 1 Fig. 8 is a view for illustrating firing patterns in the second firing step of the sintering process; Fig. 9 is a view showing varistor voltagee and V-l characteristic flatness ratios of resistance elements manufactured by firing in accordance with firing patterns shown in Fig. 8; Fig. 10 is a schematic diagram showing a structure of a voltage-nonlinear resistance element made of a sintered material and known heretofore; and Fig. 11 is a view for illustrating a voltage-versus-current (V-l) characteristic of the same. DESCRIPTION OF THE PREFERRED EMBODIMENTS In General At fist, the basic concept underlying the present invention will be described. In general, the resistance element exhibiting tne nonlinear vonage characteristic is formed by shaping a mixture containing as a primary component zinc oxide and additives of metals or compounds and by sintering a preform thus formed at a high temperature in an oxidizing atmosphere. With a view to improving the nonlinear voltage characteristic of the resistance element, enhancing durability or withstanding capabilities, extending the use life and realizing other desirabie properties of the resistance element, the composition of the raw material or starting mixture should preferably be prepared such that the content of zinc oxide or oxides is of 90 to 97 mol% and more preferably in a range of 92 to 96 mol% in terms of ZnO. Usually, bismuth oxide having a grain size of 1 to 5 p.m is used as an additive. In that case, content of bismuth oxide or oxides in the starting composition should preferably be so selected as to be of 0.1 to 5 mol% and more preferably 0.2 to 2 mol% in terms of E^Os in view of the fact that the content of bismuth oxide or oxides higher than 5 mol% exerts adverse in1luehi.:e to the effect of suppressing the grain growth of zinc oxide owing to the addition of n iieeeuth element or elements and that the contents of bismuth oxide or oxides loss th ::iu 0.1 mol% tends to increase the leakage current. Antimony oxide having a grain -size it :i range of 0.5 to 5 \|jm ib used as an additive. In this conjunction, it is to \|:>6 nnmifoned that antimony oxide(s) contributes to increasing the varistor voltage d thin resietance elerfient exhibiting the voltage nonlinearity characteristic. When (he cptifrvit of antimony oxide or oxides exceeds 5 mol%, there will exist in the resistance! element as manufactured lots of the spinel grains (serving for insulation) which ate reaction products of antimony oxide(s) and zinc oxide(s), as a result of which limitation imposed to current flow paths becomes remarkable although the varistor voltage can be increased. This in turn means that impulse withstanding capability or energy accommodating capability of the resistance element is degraded, giving i'lwu to a problem that the resistance element is likely to suffer destruction. On the other hand, when the content of antimony oxide(s) is less than 0.5 mol%, the siijjjiession effect of the grain growth of zinc oxide(s) can not be achieved suffln inntly frr ihese reasons, composition of the raw or starting material or mixture should hf* no prepared that the content of antimony oxide(s) lies within a range of 0.6 b 5 r,:.:)!% .Jind more preferably in a range 1 of 0.75 to 2 mol% in terms of Sb203. Furthermore, with a view to Impro^/lny the voltage nonlinearity of the resistance element, the starting material on colnpo&u/jn is added with chromium oxide(s), nickel oxide(s), cobalt oxide(s), manganese oxide(s) and silicon oxide(s). In this conjunction, it is desirable that each of these oxides should have grain size not greater than 10 urn on an average. The contents of these components in the starting or raw material should preferably be so selected as to be greater than 0.1 mol% and more preferably greater than 0.2 mol% Inclusive, in terms of O2O4, NiO, CQ3O4, Mn304 and Si02, respectively. However, when each of the contents of these components exceed 5 mol%, then proportions of the spinel phase, pyrochrore phase (intermediate reaction product making appearance in the spinel producing reaction) and zinc silicate increase, which tends to involve lowering of the energy accommodating (or impulse withstanding) capability as well as deterioration of the voltage nonlinearity because the current flow paths will then be bent complicatively, as described hereinbefore in conjunction with the addition of antimony oxide(s) (Sb2C>3). Thus, composition of the raw material should preferably be so adjusted that the contents of chromium oxide(s), nickel oxide(s), cobalt oxide(s), manganese oxide(s) and silicon oxide(s) are smaller than 3 mol% and more preferably less than 2 mol% in terms of Cr204, NiO, C03O4, Mn304 and SI02, respectively. Additionally, in order to decrease effectively pores possibly existing between the zinc oxide grains by increasing fluidity of bismuth oxide(s) in a high temperature sintering process by lowering a melting point thereof while improving the voltage nonlinearity by reducing resistance presented by the zinc oxide grains, the raw or starting mixture should contain 0.0006 to 0.005 mol% of aluminum in terms of A02O3 and 0.001 to 0.1 mol% of boron oxide(s) in terms of B2O3. Besides, in order to improve the voltage nonlinearity by enhancing the flatness of the V-l (voltage-versus-current) characteristic curve of the resistance element in the large current region while increasing the varistor voltage thereof, the starting composition should contain at least one of rare-earth elements (represented collectively by R) at a ratio of 0.01 to 3 mol% In tofcitl in lorms of oxide given by R2O3. Oxides of these rare-earth elements (R) should pmferably have a size usually less than 5 u.m on an average. Next, description will be dirtu led t<: n method of manufacturing the resistance element voltage-nonlinear clui nctei adjusting appropriately ih3 gialn sizes on an average components starting mixture by means a ikili liiill or like slurry is formed adding for example aqunous solution polyvinyl alcohol aqueous such as piratic field which resolving trace additive boron oxide into walor driud using spray drier and then granulated.> The granulated mixture material thus obtained is then pressurized in uniaxial direction under a pressure, for example, of 200 to 500 kgf/cm2, to thereby The conditions mentioned above are required to obtain a sintered product exhibiting highly excellent characteristics by allowing a solid phase reaction to take place sufficiently with sintering reaction being adequately promoted. In this conjunction, it is noted that the crystallization temperature range of bismuth oxide(s), starting from which the temperature lowering rate is caused to change, tends to vary finely or subtly in dependence on the composition. Accordingly, the temperature setting to this end should be performed by resorting to the use of a suitable tool, e.g. with the aid of a TMA (ThermoMechanical Analysis) apparatus or the like. Now, the present invention will he described In detail in conjunction with what is presently considered as preferred or typical embodiments thereof by reference to the drawings, being understood, however, that the invention is never restricted to them but can be carried in oth • yar ;u!> modes conceivable for those skilled in the art. Exemplary embodiment 1 A starting composition or mixture is fye\|:iarea! such that the contents of bismuth oxide, chromium oxide, nickel oxkje cot", ah oxjcje, manganese oxide and silicon oxide are each of 0.5 mol%, and that of antimony oxide is 1.2 mol% with aluminum oxide being contained in 0.002 mol% in terms of AC203 while boron oxide, which is a trace amount of additive, is contained in 0.04 mol%, respectively. Starting from the basic composition mentioned above, specimens 1 to 16 enumerated in the following table 1 are prepared by adding rare-earth elements, i.e., yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu) (which will be generally represented by "R") each in 0.5 mol% in terms of R.?03 (where R designates representatively each of the rare-earth element** mentioned above). The remaining part of the content is that of zinc oxide (ZpP). TdtilG 1 Each of the starting materials prepared as mentioned above is mixed with an aqueous solution of polyvinyl alcohol serving as binder and an aqueous solution of such as, for example, boracic acid or the like which is formed by resolving a trace additive of boron oxide into water, by using a ball mill or disperse mill to thereby form a slurry, which is then dried by nieaxis of a spray drier and then granulated. The granulated material is shaped into a preform by applying a uniaxial pressure in a range of 200 to 500 kgf/cm2. Parenthetically, each of the specimen preforms thus obtained has a nominal diameter ( A sintering process is then caried oral for the specimens mentioned Reference symbol Vc designates a temperqtuie rising rate up to a maximum temperature in the second firing step, Ta designates the maximum temperature in the second firing step, Vd designates a temperature lowering rate from the maximum temperature Ta to a changing point of the temperature lowering rate in the second firing step. Further, Tb designates the changing point of the temperature lowering rate in the second firing step, and Ve designates a temperature lowering rate after passing through the changing point Tb in the second firing step. After polishing and cleaning the elements as obtained, aluminum electrodes are attached to measure the varistor voltage, the results of which are illustrated in Fig. 3. Let's compare the specimen No. 1 which contains no rare-earth element with the specimens Nos. 2 to 16. As can be seen from Fig. 3, the varistor voltage increases when rare-earth element is added. However, in the case of the specimens Nos. 3 and 5 which is added with lanthanum (La) and praseodymium (Pr), respectively, increasing of the varistor voltage is not significant. Accordingly, with a view to realizing the resistance element of nonlinear voltage characteristic which exhibits a large varistor voltage while suppressing dispersions of the electric characteristic among the specimens to a minimum, it Is preferred from the practical viewpoint to exclude those rare-earth elements whoso addition does not contribute to increasing the varistor voltage more than 10 % of the highest varistor voltage which indicated by a firing pattern No. 1 shown in Fly. 1 in two firing steps, wherein sintering temperature is controlled in such a manner as illustrated graphically in Fig. 2. After polishing and cleaning the elements as obtained, aluminum electrodes are attached to measure the varistor voltage (VWrnm), the results of which are illustrated in Fig. 4. In Fig. 4, all the measurement values represent the means values for all the specimens added with eleven different rare-earth elements. Comparison of the specimehs NOB. 17 to 22 shows that the varistor voltage becomes higher as the amount of addition of rear-earth element increases, as can be seen from Fig. 5. The specimen No. 17 containing none of rare-earth element corresponds to the conventional resistance element known heretofore. The specimen No. 18 added with 0.001 mol% of rare-earth element certainly shows that the varistor voltage is increased, the extent of which la however only to be negligible. By contrast, in the case of the specimei : Mo:; 19 lo 22, the mean values of the varistor voltage are all higher than 350 V/inrn, in:: taking improvement by 50 to 100 % when compared with that of the conventional resistance element. On the other hand, in the case of the specimen No. 22, the varistor voltage certainly assumes a high value. However, the flatness ratio of the V characteristic curve in the small current region is degraded more than 10 % when compared with that of the specimen No. 17. Thus, it is safe to say that the resistance element corresponding to the specimen No. 22 should be excluded from practical use because of possibility of intolerably high leakage current. For the reasons mentioned above, the optimal amount of addition of rare-earth element should preferably be so selected as to fall within a range of 0.01 to 3 mol% in terms of the FI2O3. Further, comparison of the specimens Nos. 23 to 27 shows that the flatness ratio of the V-l characteristic curve decreases in the small current region as the amount of aluminum (Ac) as added is decreased while the flatness ratio increases silicon oxide are each of 0.5 mol%, that ol filirrK iy G licie is 1.2 mol%, with that of aluminum, a trace additive, being contained in 0.002 mol%, while boron oxide is contained in 0.04 mol%. Starting from the basic composition mentioned above, rare-earth elements, i.e., yttrium (Y), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu) (collectively represented by "R") are added in 0.1 mol% in terms of oxides (R203) of rare-earth elements, respectively. The remaining part is the content of zinc oxide (ZnO). Each of the starting materials prepared as mentioned above is mixed with an aqueous solution of polyvinyl alcohol serving as a binder and an aqueous solution of such as, for example, boracic acid or the like which is formed by resolving a trace additive of boron oxide into water, by Using a ball mill or disperse mill to thereby form a slurry, which is then dried by means of a spray drier and then granulated. The granulated material is shaped Into a preform by applying a uniaxial pressure in a range of 200 to 500 kgf/crrr Partinthotically, each of the specimen preforms thus obtained has a nominal diametei (4>) of 125 mm and a thickness of 30 mm. The granulated preforms or specimens undergo preheating for five hours at a temperature of 600 °C to thereby rerriq i: the ; r d^r. A second firing step is carried out on the conditions indicated by a firing pattern No. 1 shown in Fig. 1 in two sintering firing steps, wherein the firing temperature is controlled in such a manner as illustrated graphically in Fig. 2. After polishing and cleaning the elements as obtained, aluminum electrodes are attached to measure the varistor voltage (VWrnrn) nnd ihe flatness ratio of the V-l characteristic, the results of which are Illustrated in Fig. 6. In Fig. 6, all the measurement values represent the means; values for all the specimens added with eleven different rare-earth elements. Additionally, comparison of the specimens Nos. 28, 30 and 31 shows that although the temperature-lowering rate in the first firing step carried out in the atmosphere of air contributes to improvement more or less of the flatness ratio in the large current region of the V-l characteristic curve, the temperature-lowering rate is not a factor affecting remarkably the V-l characteristic of the resistance element. Accordingly, the temperature-lowering rate now under discussion should be as high as possible so long as the manufacturing conditions allow, in consideration of the firing in the second step. i Additionally, comparison of the spocimens Nos. 28, 32 and 33 shows that any appreciable variation can not be observed in the flatness ratio of the V-l characteristic curve in the second firing step at the temperature rising rate range of 50 to 200 °C/hr within the oxidizing atmosphere (e.g. at the oxygen partial pressure of 100 % by volume in the case of the instant example). The temperature rising rate higher than 500 °C/hr brings about cracking in the resistance element as manufactured. Thus, in view of the manufacturing efficiency as well as from the economical standpoint, the temperature rising rate should preferably be selected to the first firing step has been completed. [0045] Further, comparison of the specimens Nos. 28, 38 and 39 shows that when the maximum temperature in the second firing step is higher than that in the first firing step, the flatness ratio of the V-l characteristic in the small current region can remarkably be improved. However, in that case, the porosity is increased, giving rise to a problem that moisture absorbing capability is degraded. On the other hand, when the maximum firing temperature in the Beconcl firing step is lower than the maximum temperature (1300 °C) in the first firing step by 300 °C or more, the flatness ratio of the V-l characteristic curve la degraded, rendering the intended effect of the second firing step ineffective. Accordingly, the maximum temperature in the second firing step should be set equal (o that of the first firing step or at a temperature within a range lower than that of the first firing step by 300 °C at the most. Furthermore, comparison of the specimens Nos. 28, 34 and 40 shows that the temperature-lowering rate from the maximum point to the changing point (or trasition point) of the temperature-lowering rate in the second firing step contributes to reducing the flatness ratio of the V-l chamoterlutio curve in the large current region as the temperature-lowering rate is higher.forever, when the temperature-lowering rate exceeds the rate of 200 °C/hr, the flatness the of the V-l characteristic curve is degraded in the small current region Si :h bejng the circumstances, the The temperature-lowering rale changing point in the second firing step plays a very important role in carrying out the present invention. More specifically, for the purpose of reducing oxygen defect of zirib oxide grains and supply oxygen in excess to the inter-grain boundaries of zinc oxide during the temperature-lowering process, the temperature-lowering rate is changed within a range around the crystallization temperature of bismuth oxide whlih ie good conductor for oxygen ions. Comparison of the specimen Nos. 28, 35 and 4i! with one another shows that when the point at which the temperature-lowering rate le changed in the second firing step is set lower, the flatness ratio of the V-l i.ive , lin: small current region becomes degraded, causing the aimed effects of trio two the sintering process to disappear. On the other hand, even when the changing! of the temperature-lowering rate is set high, any significant change can scarcely to observed Since thfe flatness ratio of the V-l characteristic curve in the small current: region can not be improved unless the temperature-lowering rate following the changing point mentioned above is made lower than that preceding to the changing point, the changing point of concern should preferably be set at a temperature as low as possible within a range where the aimed effect can be realized, from the standpoint of manufacturing efficiency or productivity. More specifically, changing point of the temperature-lowering rate in the second firing step should preferably be set in a temperature range of 450 to 900 °C and more preferably in a range of 500 to 800 °C although it depends on the composition of the starting material as well as the conditions for the sintering process. In this conjunction, setting of the changing point of the lumperature-lowering rate should be performed with the aid of an appropriate tool such as a TMA (ThermoMechanical Analysis apparatus) or the like in consideration of the fact that crystallization temperature of bismuth oxide varies delicately or subtly in dependence on the undergoing the firing process in the oxidizing atmosphere in the second firing step, whereby a sufficient amount of oxygen is supplied to the inter-grain boundaries between the zinc-oxide crystal grains. Thus, there can be obtained a voltage-nonlinear resistance element which is excellent in the flatness ratio of the V-l characteristic. Exemplary Embodiment 4 A starting composition or mixture is prepared such that the contents of bismuth oxide, chromium oxide, nickel oxide, oobalt oxide, manganese oxide and silicon oxide are each of 0.5 mol%, and that of antimony oxide is 1.2 mol% with boron oxide, which is a trace amount of additive, is contained in 0.04 mol%. Starting from the basic composition mentioned above, aluminum and rare-earth elements, i.e., yttrium (Y), samarium (Sm), europium (En), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu) (collectively represented by "IT) am gelded in the amounts illustrated in Fig. 7 in terms of AP2O3 and R2O3, respecli - ;ly.the remaining part is the content of zinc oxide (ZnO). Each of the starting materials prepared as mentioned above is mixed with an aqueous solution of polyvinyl alcohol living as binder and an aqueous solution of such as, for example, boracic acid orthe like which is formed by resolving a trace additive of boron oxide into water, by using a disperse mill to thereby form a slurry, which is then dried by means of a spray drier and granulated subsequently. The granulated material is shaped into a preform by applying a uniaxial pressure in a range of 200 to 500 kgf/cm2. Parenthetically, each of the specimen preforms thus obtained has a nominal diameter (cj)) of 125 mm and a thickness of 30 pirn. The i granulated preforms or specimens undergo preheating for five hours at a temperature of 600 °C to thereby remove the binder. A The first firing step (at 1,150 ' C x B hrj of the two-step sintering process is carried out in accordance with a sintering or firing pattern No. 1 shown in Fig. 1 (i.e., in the atmosphere of air with Va = 30 °C/hr and Vb = 50 °C/hr). Parenthetically, oxygen concentrations of the oxidizing atmosphere employed in the second sintering or firing step are shown in Fig. 7. After polishing and cleaning the elements as obtained, aluminum electrodes are attached to measure the electric characteristics, the results of which are illustrated in Fig. 7. In Fig. 7, the values ol the flatness ratio enumerated in the table represent the flatness ratio (VioK/vVionroA) over the whole region inclusive of the large current, region and the small current region, all the measurement values representing the means values for all the specimens added with oxides of eleven different rare-earth elements. These results show the facts which will be described below. As can be seen from comparison of the specimens Nos. 44 to 58, the flatness ratio substantially comparable to that obtained by the firing process carried in the oxidizing atmosphere containing oxygen at a concentration of 100 % can be realized with the oxygen concentration ol 80 %. On the other hand, in the cases where the oxygen concentration is 60 % or less the flatness ratio becomes degraded in all the specimens. Thus, in order to supply a sufficient amount of oxygen to the inter-grain boundary regions between the zinc oxide grains by employing the oxidizing atmosphere in the second sintering or , it is desirable to set the oxygen concentration at least at 80 %. In that case, voly excellent flatness ratio can be obtained. I i As will now be understood from the above, according to the? invention incarnated in the fourth exemplary embodiment. there can be obtained a voltage- nonlinear resistance element ensuring a large visitor voltage which has a small flatness ratio over the whole current regioin from a laudge current to a small current by setting the oxygen concentration of the axlidizing atmosphere at 80% at 80 % or more in the second firing step. Exemplary Embodiment 5 A starting composition or mixture la prepared such that the contents of bismuth oxide, chromium oxide, nickel oxide, cobalt oxide, manganese oxide and silicon oxide are each of 0.5 mol%, and that of antimony oxide is 1.2 mol% with a trace additive of boron oxide being contained in 0.04 mol%. Starting from the basic composition mentioned above, rare-earth elements, i.e., yttrium (Y), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutotlum (Lu) (collectively represented by "R") are added in 0.5 mol% in terms of oxides (R2O3) of rare-earth elements, respectively. The remaining part is the content of zinc oxide (ZnO). Each of the starting materials prepared as mentioned above is mixed with an aqueous solution of polyvinyl alcohol serving as a binder and an aqueous solution of such as, for example, boracic acid or the like which is formed by resolving a trace additive of boron oxide into water, by using 9 disperse mill to form a slurry, which is then dried by means of a spray drier and then granulated. The granulated material is shaped into a preform by applying a uniaxial pressure in a range of 200 to 500 kgf/cm2. Parenthetically, each of the specimen preforms thus obtained has a nominal diameter ($) of 125 mm and a thickness of 30 mm. The granulated preforms or specimens undergo preheating for five hours at a temperature of 600 °C to thereby remove the binder. j The first sintering or firing step (s.l I \|i!i0 °C x 5 hr) of the two-step sintering process is carried out in accordance with the firing pattern No. 1 shown in Fig. 1. Thereafter, the second firing step is carried out in accordance with a firing pattern No. 1 shown in Fig. 8. After polishing and cleaning the elements as obtained, aluminum electrodes are attached to measure the electric characteristics, the results of which are illustrated in Fig. 9. These results show the facts which will be described below. As can be seen from comparisons of the data of the specimens Nos. 59 to 73, the flatness ratio of the resistance element becomes smaller, as ijhe oxygen concentration of the firing atmosphere employed during the temperature-lowering period from the maximum temperature (Ta) to the changing point (Tb) of the temperature-lowering rate in the second firing process is lower. Substantially same tendency can be observed when the atmosphere (oxygen concentration) is changed from 100 to 80 % and then to 30 % during the whole second firing period. Such phenomenon may be explained by the fact thai when the resistance material or composition is placed in the atmosphere locking excessively in oxygen in the high- temperature phase of the firing or sintering proccess lots of oxygen defects will take place in zinc oxide crystal grains which .:: element, involving thus low resistance vi-jue a the zinc oxide grains themselves. Accordingly, the oxygen concentration of the alniutiiphere employed in the second firing step from the maximum temperature Ic the chapcjing poin^ of the temperature- lowering rate should be set as low aci j:)os» l\|:i\|,:?. In practical applications, in consideration of the workability (i.e., process rrwilpulatability), it is preferred to set the oxygen concentration of concern at a value equivalent to that of the ambient air (20 %) or less. j As will now be understood from the above, according to the present invention incarnated in the fifth exemplary embodiment, by setting the oxygen concentration in the temperature-lowering phase of the second firing step from the maximum temperature to the changing point of the temperature- lowering rate at 30 % or less, there can be obtained a voltage-nonlinear resistance element which can exhibit a large varistor voltage while ensuring a small flatness ratio over the whole region from the large current to the small current region, because lots of oxygen defects take place within the region containing zinc oxide as a primary component, to thereby lower the resistance of;: i;:; oxide itself. Many modifications and var ■;ion:;i of the present invention are possible in the light of the above techniques. It is there!) am to be understood that within the scope of the appended claims, the invention i!i% tie practiced otherwise than as ! specifically described. WE CLAIM: 1. A composition for an electric material, containing as a primary component zinc oxide and additionally containing bismuth oxide, antimony oxide, chromium oxide, nickel oxide, cobalt oxide, manganese oxide, silicon oxide and boron oxide, said composition further containing at least one of rare-earth elements in a range of 0.01 mol% to 3.0 mol% in terms of oxide thereof given by R203 where R represents generally said rare-earth elements; and aluminum in a range of 0.0005 mol% to 0.005 mol % in terms of aluminum oxide given by A1203. 2. A composition for an electric material substantially as herein described with reference to the accompanying drawings.

Full Text

a composition tor The present invention relates to/an electric resistance element which
is made of a sintered material containing zinc oxide as a primary component and
which exhibits a nonlinear voltage characteristic (also referred to as the voltage
nonlinearity characteristic or simply as voltage nonlinearity). The invention is also
concerned with composition of the electric resistance element mentioned above and
a method of manufacturing the same.
**** Description of Related Art
^ Heretofore, it is well known in the art that a sintered material containing
» zinc oxide as a primary component and added with bismuth oxide, cobalt oxide
*° and/or other oxides exhibits a nonlinear voltage characteristic or voltage nonlinearity.
The resistance element formed of such sintered material is widely employed in
v*. practical applications, as typified by a surge absorber for protecting circuit elements
by absorbing a surge current (steep current rise), an arrester for protecting
P4 electric/electronic apparatuses or equipment against an abnormal voltage brought
Pi about by lightning and others.
For having better understanding of the present invention, description
will first be directed to the background techniques of the invention in some detail.
Figure 10 is a schematic diagram showing a structure of a typical one of the sintered
materials known heretofore from which the nonlinear voltage resistance element is

made. Referring to the figure, some of spinel grc ii i; 1 each consisting of antimony compound and having a grain size in a rangii; of c ni? to several microns exist within zinc oxide grains while the other spinel grains I exist Internally of or adjacent to inter-grain boundary regions which contain bithmus oxide 3 as a primary component existing in the vicinity of triple points (multiple points) of zinc oxide grains. It is observed that some of bithmus oxide grains 3 not only exist at the multiple points but also penetrate deeply between the zinc oxide grains 2. Parenthetically, reference numeral 4 in Fig. 10 denotes a twin crystal boundary.
It has experimentally been established by a measurement conducted by using point.electrodes that the grain containing primarily zinc oxide acts by itself simply as electric resistor while the boundary regions between the zinc oxide grains 2 exhibit voltage nonlinearity (see G.D. Median; L.M. Levinson and H.R. Phillip: "THEORY OF CONDUCTION IN ZnO VAHISTORS", J. Appl. Phys. 50(4)2799 (1979)). Furthermore, it has also experimentally been demonstrated that the number of boundaries between the zinc oxide grains 2 (also referred to as the inter-grain boundaries) determines a varistor voltage.
The sintered material having such fine or microscopic structure as
mentioned above and containing zinc oxide as the primary component usually
exhibits such a voltage-versus-current characteristic (hereinafter also referred to as
the V-l characteristic) as illustrated in Fig. 1 I. This V-l curve may be divided into
three sections or regions in view of physical mechnniorns mentioned below.
(1) A region in which a leakage current ru mains small when compared with
an applied voltage due to a current limiting function of a Schottoky barrier presented by the grain boundaries (a region including ; i:oin ! shdwn in Fig. 11) in which the typical current value on the order of 10 u.A a ordi iari|y selected for the resistance

element having a diameter of about 100 mm).
(2) A region in which resistance value decreases steeply as the applied voltage is increased, which causes a tunnel current flowing through the grain boundaries to increase for thereby decreasing steeply the resistance for the voltage as applied (i.e., a region including a transition point S shown in Fig. 11 at which transition or changing point from the region (1) to the region (2) occurs) and in which a current of a value typically in a range of 1 to 3 mA is generally selected for a resistance element having a diameter $ of about 100 mm.
(3) A (V-l) region which is determined by the electric resistance of zinc oxide grains themselves (a region cover I rig a point H shown in Fig. 11 in which a current value typically of 10 kA is generally selected for the resistance element having a diameter on the order of 100 mm $).
In the case of a sintered material containing zinc oxide grains which material is an n-type semiconductor material, it Is observed that when oxygen adheres or exists in excess in the crystal grain boundaries, an electron capturing level is formed at the interfaces, as a result of which depletion layers in which no electron exists are formed along the grain interludes, orming eventually generating the electron barriers (i.e., Schottoky barrieiiii/ a.1 o along the grain boundaries. Consequently, as the barrier height or levtii of thi:i £i;:hottoky barriers increases, the leakage current decreases. Thus, there can .:■•) obtained an.- electric resistance element which is excellent in respect to flatness t! the V-l characteristic curve in the small-current region.
In this conjunction, it is noted that the electric characteristic at the grain boundary exerts a great influence to the flatness of the V-l characteristic curve in the small-current region, while resistance of the zinc oxide grains themselves affects

remarkably the flatness of the V-l characteristic curve in a large-current region. More specifically, because increasing in the electric resistance of zinc oxide grains degrades the flatness of the V-l characteristic curve in the aforementioned region, it is preferred that the electric resistance of the zinc oxide grains should be as low as possible.
Now, for convenience of description, a phrase "flatness ratio" used herein will be defined. A ratio between a voltage Vn in the large-current region H shown in Fig. 11 and a voltage VL in the small-current region L, i.e., VH/VL, is defined as the flatness ratio, as can be seen in Fk| 11. Furthermore, a ratio between the varistor voltage Vs and the voltage \A. !n Ihe iirhill current region, i.e., VS/VL, is referred to as the flatness ratio in the small currc vl regldn, while a ratio between the varistor voltage Vs and the voltage VH in the large • current region, i.e., the ratio VH/VS, is referred to as the flatness ratio in the |arge-ci ineiil region. !
In the case of the resistance: element exhibiting the nonlinear voltage characteristic, the varistor voltage Vs shown in Fl(:|, 1 I represents a threshold voltage. In this conjunction, it is important to set the varistor voltage Vs in dependence on a voltage of a power transmission system or line to which an arrester constituted by the resistance element having the nonlinear voltage characteristic is' to be actually applied. Parenthetically, in many practical cases, the varistor voltage Vs is typically represented by an inter-electrode voltage (or terminal voltage) appearing across the resistance element upon flowing of a current of 1 mA therethrough. This terminal voltage which will hereinafter be represented by VimA is in proportion to a thickness of the resistance element.
In recent years, there arises a great demand for an arrester of higher performance for the purpose of protectiny various apparatuses and instruments

employed in power transmission systems in which electric power transmission tends to be realized with higher and higher voltage. In order to satisfy such demand, it becomes necessary to obtain such a resistance element which is capable of exhibiting excellent nonlinear voltage characteristic such as, for example, a resistance element having a smaller flatness ratio which plays an important role in realizing desired characteristics of the arrester.
In this connection, in order to reduce the flatness ratio, there is required a manufacturing technique which is capable of Increasing the barrier height of the Schottoky barriers existing in the grain boundaries between the zinc oxide grains. However, the attempt for improving the flatness ratio in the large-current region will usually be accompanied with degradation oi tho flatness ratio in the small-current region. On the other hand, approach for improving the flatness ratio in the small-current region will involve degradation of the flatnoes ratio in the large-current region.
On the other hand, in the caso of thi; mrester which is used in an ultra¬high voltage power transmission systep' iate I d example, on the order of 100 million volts, a number of elements haying :i .substantially same geometrical configuration and the varistor voltage value Vs CMUI silent to that of the resistance elements known heretofore are stacked with the ir cmirji„ial elemerits being electrically connected in series to one another. In I hat c,:iM! (he number of the electrical resistance elements as stacked necessarily lencje to increase, involving not only a bulky or large structure of the arrester aei a whole but also complication in the techniques required for realizing the serial connection, thus giving rise to many problems in respect to the arrester designs not only from the electrical view point but also from the thermal as well as mechanical standpoint. Such being the circumstances, it is believed that if the electrical resistance elements each capable

of exhibiting a high varistor voltage Vs per unit iBfiylh (e.g. 1 mm) are available, the problems mentioned above can be solved, because then the voltage to be born by each of the electrical resistance elements can be increased, which in turn allows the number of the resistance elements stacked for realizing the series connection to be decreased.
In summarization, it can be said that in the case of the electrical resistance element exhibiting the voltage nonllnearity as well as the method of manufacturing the same, the flatness ratio is often degraded when the varistor voltage is increased by varying the composition of the electrical resistance material by changing correspondingly the ratios or proportions of the additives. In particular, in the case of the resistance elements destined for application to a high or ultra-high power transmission system, a large leakage current tends to be generated upon application of a high voltage. Thus, there ie encountered a great difficulty in realizing compatibility between increasing of the varistor voltage and the flatness ratio in the small-current region which is a very important factor, as mentioned hereinbefore.
SUMMARY OF THE INVENTION
In the light of the state of the art described above, it is an object of the present invention to provide a electrical resistance element exhibiting a nonlinear voltage characteristic which element can evade the problems mentioned above.
Another object of the present invention is to provide a method of manufacturing the electrical resistance element mentioned above.
In view of the above and other object which will become apparent as the description proceeds, there is provided according an aspect of the present

invention an electric resistance element exhibiting a nonlinear voltage characteristic, which element contains as a primary component zinc oxide and additionally contains bismuth oxide, antimony oxide, chromium oxide, nickel oxide, cobalt oxide, manganese oxide, silicon oxide and boron oxide. The resistance element further contains at least one of rare-earth elements in a range of 0.01 mol% to 3.0 mol% in terms of oxide thereof given by R2O3 where R represents generally the rare-earth elements, and aluminum in a range of 0.0006 mol% to 0.005 mol% in terms of aluminum oxide given by AC2O3.
In a preferred mode for carrying out the invention, the rear-earth elements may.include yttrium (Y), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu).
With the above-mentioned composition of the resistance element of voltage nonlinearity, the varistor voltage can be increased over the whole current range from small to large current levels without being accompanied with any appreciable degradation in the flatness ratio of the V-l characteristic curve.
Furthermore, according to another aspect of the present invention, there is provided a method of manufacturing an electric) resistance element exhibiting a nonlinear voltage characteristic, starting, from E| mixture containing as a primary component zinc oxide and additionally f;;*l:!;rnut ■= :nide, antimony oxide, chromium oxide, nickel oxide, cobalt oxide, manganese 0 ido silicon oxide and boion oxide, and further containing at least one of rare-Mirth E:| iinei'its in a range of 0.01 mol% to 3.0 mol% in terms of oxide thereof givefl by R2O3 where R represents generally the
1
rare-earth elements, and aluminum in a mhgje o| 11 0005 mol% to 0.005 mol% in terms of aluminum oxide given by AC2O3. The method includes a step of preparing

current region.

Further, oxygen concentration of the oxidizing atmosphere in the second firing step may preferably be so selected as to fall within a range of 21 to 30% during the temperature lowering phase from the maximum firing temperature to the temperature corresponding to changing point of the temperature lowering rate in the second firing step.
With the method described above, there can be manufactured a nonlinear voltage resistance element (or varistor element) having the varistor voltage increased significantly with a small flatness ratio over a substantially whole current range from large to small current region.
Accordingly, the present invention provides a composition for an electric
material, containing as a primary component zinc oxide and additionally containing
bismuth oxide, antimony oxide, chromium oxide, nickel oxide, cobalt oxide,
manganese oxide, silicon oxide and boron oxide, said composition further containing

at least one of rare-earth elements in a range of 0.01 mol% to 3.0 mol% in terms of
oxide thereof given by R203 where R represents generally said rare-earth elements;
and aluminum in a range of 0.0005 mol% to 0.005 mol % in terms of aluminum oxide
given by A1203.
I

The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the course of the description which follows, reference is made to the drawings, in which:
Fig. 1 is a view for illustrating sintering processes together with the atmosphere and the temperatures therefor in the resistance element manufacturing method according to first and second exemplary embodiments of the present invention;
Fig. 2 is a view for illustrating a pattern of firing temperature adopted in the sintering process;
Fig. 3 is a view illustrating varistor voltages of resistance elements manufactured, being added with rare-earth elements;

Fig. 4 is a view for illustrating varistor voltages and flatness ratios of V-l characteristic curve in nonlinear-voltage resistance elements manufactured, being added with A203 and rare-earth elements;
Fig. 5 is a view showing a relation between amounts of addition of rare-earth elements and varistor voltage;
Fig. 6 is a view illustrating varMor voltages and flatness ratios in resistance elements subjected to gradual cooling In a temperature lowering process in an oxidizing atmosphere in a second firlnc) st*;p;
Fig. 7 is a view illustrating eel. (ions "ivl-^;:en the varistor voltage and the
V-l characteristic flatness ratio of reoi^ancc element as manufactured and
concentration of oxygen in an oxidizing at mo:-p he r;i employed in a second firing step
of the sintering process; 1
Fig. 8 is a view for illustrating firing patterns in the second firing step of the sintering process;
Fig. 9 is a view showing varistor voltagee and V-l characteristic flatness ratios of resistance elements manufactured by firing in accordance with firing patterns shown in Fig. 8;
Fig. 10 is a schematic diagram showing a structure of a voltage-nonlinear resistance element made of a sintered material and known heretofore; and
Fig. 11 is a view for illustrating a voltage-versus-current (V-l) characteristic of the same.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In General
At fist, the basic concept underlying the present invention will be

described. In general, the resistance element exhibiting tne nonlinear vonage characteristic is formed by shaping a mixture containing as a primary component zinc oxide and additives of metals or compounds and by sintering a preform thus formed at a high temperature in an oxidizing atmosphere.
With a view to improving the nonlinear voltage characteristic of the resistance element, enhancing durability or withstanding capabilities, extending the use life and realizing other desirabie properties of the resistance element, the composition of the raw material or starting mixture should preferably be prepared such that the content of zinc oxide or oxides is of 90 to 97 mol% and more preferably in a range of 92 to 96 mol% in terms of ZnO.
Usually, bismuth oxide having a grain size of 1 to 5 p.m is used as an additive. In that case, content of bismuth oxide or oxides in the starting composition should preferably be so selected as to be of 0.1 to 5 mol% and more preferably 0.2 to 2 mol% in terms of E^Os in view of the fact that the content of bismuth oxide or oxides higher than 5 mol% exerts adverse in1luehi.:e to the effect of suppressing the grain growth of zinc oxide owing to the addition of n iieeeuth element or elements and that the contents of bismuth oxide or oxides loss th ::iu 0.1 mol% tends to increase the leakage current.
Antimony oxide having a grain -size it :i range of 0.5 to 5 |jm ib used as an additive. In this conjunction, it is to |:>6 nnmifoned that antimony oxide(s) contributes to increasing the varistor voltage d thin resietance elerfient exhibiting the voltage nonlinearity characteristic. When (he cptifrvit of antimony oxide or oxides exceeds 5 mol%, there will exist in the resistance! element as manufactured lots of the spinel grains (serving for insulation) which ate reaction products of antimony oxide(s) and zinc oxide(s), as a result of which limitation imposed to current flow

paths becomes remarkable although the varistor voltage can be increased. This in
turn means that impulse withstanding capability or energy accommodating capability
of the resistance element is degraded, giving i'lwu to a problem that the resistance
element is likely to suffer destruction. On the other hand, when the content of
antimony oxide(s) is less than 0.5 mol%, the siijjjiession effect of the grain growth
of zinc oxide(s) can not be achieved suffln inntly frr ihese reasons, composition of
the raw or starting material or mixture should hf* no prepared that the content of
antimony oxide(s) lies within a range of 0.6 b 5 r,:.:)!% .Jind more preferably in a range
1 of 0.75 to 2 mol% in terms of Sb203.
Furthermore, with a view to Impro^/lny the voltage nonlinearity of the
resistance element, the starting material on colnpo&u/jn is added with chromium
oxide(s), nickel oxide(s), cobalt oxide(s), manganese oxide(s) and silicon oxide(s).
In this conjunction, it is desirable that each of these oxides should have grain size not
greater than 10 urn on an average. The contents of these components in the starting
or raw material should preferably be so selected as to be greater than 0.1 mol% and
more preferably greater than 0.2 mol% Inclusive, in terms of O2O4, NiO, CQ3O4,
Mn304 and Si02, respectively. However, when each of the contents of these
components exceed 5 mol%, then proportions of the spinel phase, pyrochrore phase
(intermediate reaction product making appearance in the spinel producing reaction)
and zinc silicate increase, which tends to involve lowering of the energy
accommodating (or impulse withstanding) capability as well as deterioration of the
voltage nonlinearity because the current flow paths will then be bent complicatively,
as described hereinbefore in conjunction with the addition of antimony oxide(s)
(Sb2C>3). Thus, composition of the raw material should preferably be so adjusted that
the contents of chromium oxide(s), nickel oxide(s), cobalt oxide(s), manganese

oxide(s) and silicon oxide(s) are smaller than 3 mol% and more preferably less than 2 mol% in terms of Cr204, NiO, C03O4, Mn304 and SI02, respectively.
Additionally, in order to decrease effectively pores possibly existing between the zinc oxide grains by increasing fluidity of bismuth oxide(s) in a high temperature sintering process by lowering a melting point thereof while improving the voltage nonlinearity by reducing resistance presented by the zinc oxide grains, the raw or starting mixture should contain 0.0006 to 0.005 mol% of aluminum in terms of A02O3 and 0.001 to 0.1 mol% of boron oxide(s) in terms of B2O3.
Besides, in order to improve the voltage nonlinearity by enhancing the flatness of the V-l (voltage-versus-current) characteristic curve of the resistance element in the large current region while increasing the varistor voltage thereof, the starting composition should contain at least one of rare-earth elements (represented collectively by R) at a ratio of 0.01 to 3 mol% In tofcitl in lorms of oxide given by R2O3. Oxides of these rare-earth elements (R) should pmferably have a size usually less than 5 u.m on an average.
Next, description will be dirtu led t<: n method of manufacturing the resistance element voltage-nonlinear clui nctei adjusting appropriately ih3 gialn sizes on an average components starting mixture by means a ikili liiill or like slurry is formed adding for example aqunous solution polyvinyl alcohol aqueous such as piratic field which resolving trace additive boron oxide into walor driud using spray drier and then granulated.> The granulated mixture material thus obtained is then pressurized in uniaxial direction under a pressure, for example, of 200 to 500 kgf/cm2, to thereby

The conditions mentioned above are required to obtain a sintered

product exhibiting highly excellent characteristics by allowing a solid phase reaction to take place sufficiently with sintering reaction being adequately promoted. In this conjunction, it is noted that the crystallization temperature range of bismuth oxide(s), starting from which the temperature lowering rate is caused to change, tends to vary finely or subtly in dependence on the composition. Accordingly, the temperature setting to this end should be performed by resorting to the use of a suitable tool, e.g. with the aid of a TMA (ThermoMechanical Analysis) apparatus or the like.
Now, the present invention will he described In detail in conjunction with what is presently considered as preferred or typical embodiments thereof by reference to the drawings, being understood, however, that the invention is never restricted to them but can be carried in oth • yar ;u!> modes conceivable for those skilled in the art. Exemplary embodiment 1
A starting composition or mixture is fye|:iarea! such that the contents of bismuth oxide, chromium oxide, nickel oxkje cot", ah oxjcje, manganese oxide and silicon oxide are each of 0.5 mol%, and that of antimony oxide is 1.2 mol% with aluminum oxide being contained in 0.002 mol% in terms of AC203 while boron oxide, which is a trace amount of additive, is contained in 0.04 mol%, respectively. Starting from the basic composition mentioned above, specimens 1 to 16 enumerated in the following table 1 are prepared by adding rare-earth elements, i.e., yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu) (which will be generally represented by "R") each in 0.5 mol% in terms of R.?03 (where R designates representatively each of the rare-earth element** mentioned above). The remaining

part of the content is that of zinc oxide (ZpP).
TdtilG 1

Each of the starting materials prepared as mentioned above is mixed with an aqueous solution of polyvinyl alcohol serving as binder and an aqueous solution of such as, for example, boracic acid or the like which is formed by resolving a trace additive of boron oxide into water, by using a ball mill or disperse mill to thereby form a slurry, which is then dried by nieaxis of a spray drier and then granulated. The granulated material is shaped into a preform by applying a uniaxial pressure in a range of 200 to 500 kgf/cm2. Parenthetically, each of the specimen preforms thus obtained has a nominal diameter (

A sintering process is then caried oral for the specimens mentioned

Reference symbol Vc designates a temperqtuie rising rate up to a maximum temperature in the second firing step, Ta designates the maximum temperature in the second firing step, Vd designates a temperature lowering rate from the maximum temperature Ta to a changing point of the temperature lowering rate in the second firing step. Further, Tb designates the changing point of the temperature lowering rate in the second firing step, and Ve designates a temperature lowering rate after passing through the changing point Tb in the second firing step.
After polishing and cleaning the elements as obtained, aluminum electrodes are attached to measure the varistor voltage, the results of which are illustrated in Fig. 3.
Let's compare the specimen No. 1 which contains no rare-earth element with the specimens Nos. 2 to 16. As can be seen from Fig. 3, the varistor voltage increases when rare-earth element is added. However, in the case of the specimens Nos. 3 and 5 which is added with lanthanum (La) and praseodymium (Pr), respectively, increasing of the varistor voltage is not significant. Accordingly, with a view to realizing the resistance element of nonlinear voltage characteristic which exhibits a large varistor voltage while suppressing dispersions of the electric characteristic among the specimens to a minimum, it Is preferred from the practical viewpoint to exclude those rare-earth elements whoso addition does not contribute to increasing the varistor voltage more than 10 % of the highest varistor voltage which

indicated by a firing pattern No. 1 shown in Fly. 1 in two firing steps, wherein sintering

temperature is controlled in such a manner as illustrated graphically in Fig. 2. After polishing and cleaning the elements as obtained, aluminum electrodes are attached to measure the varistor voltage (VWrnm), the results of which are illustrated in Fig. 4. In Fig. 4, all the measurement values represent the means values for all the specimens added with eleven different rare-earth elements.
Comparison of the specimehs NOB. 17 to 22 shows that the varistor voltage becomes higher as the amount of addition of rear-earth element increases, as can be seen from Fig. 5. The specimen No. 17 containing none of rare-earth element corresponds to the conventional resistance element known heretofore. The specimen No. 18 added with 0.001 mol% of rare-earth element certainly shows that the varistor voltage is increased, the extent of which la however only to be negligible. By contrast, in the case of the specimei : Mo:; 19 lo 22, the mean values of the varistor voltage are all higher than 350 V/inrn, in:: taking improvement by 50 to 100 % when compared with that of the conventional resistance element. On the other hand, in the case of the specimen No. 22, the varistor voltage certainly assumes a high value. However, the flatness ratio of the V characteristic curve in the small current region is degraded more than 10 % when compared with that of the specimen No. 17. Thus, it is safe to say that the resistance element corresponding to the specimen No. 22 should be excluded from practical use because of possibility of intolerably high leakage current. For the reasons mentioned above, the optimal amount of addition of rare-earth element should preferably be so selected as to fall within a range of 0.01 to 3 mol% in terms of the FI2O3.
Further, comparison of the specimens Nos. 23 to 27 shows that the flatness ratio of the V-l characteristic curve decreases in the small current region as the amount of aluminum (Ac) as added is decreased while the flatness ratio increases

silicon oxide are each of 0.5 mol%, that ol filirrK iy G licie is 1.2 mol%, with that of

aluminum, a trace additive, being contained in 0.002 mol%, while boron oxide is contained in 0.04 mol%. Starting from the basic composition mentioned above, rare-earth elements, i.e., yttrium (Y), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu) (collectively represented by "R") are added in 0.1 mol% in terms of oxides (R203) of rare-earth elements, respectively. The remaining part is the content of zinc oxide (ZnO).
Each of the starting materials prepared as mentioned above is mixed with an aqueous solution of polyvinyl alcohol serving as a binder and an aqueous solution of such as, for example, boracic acid or the like which is formed by resolving a trace additive of boron oxide into water, by Using a ball mill or disperse mill to thereby form a slurry, which is then dried by means of a spray drier and then granulated. The granulated material is shaped Into a preform by applying a uniaxial pressure in a range of 200 to 500 kgf/crrr Partinthotically, each of the specimen preforms thus obtained has a nominal diametei (4>) of 125 mm and a thickness of 30 mm. The granulated preforms or specimens undergo preheating for five hours at a temperature of 600 °C to thereby rerriq i: the ; r d^r.
A second firing step is carried out on the conditions indicated by a firing pattern No. 1 shown in Fig. 1 in two sintering firing steps, wherein the firing temperature is controlled in such a manner as illustrated graphically in Fig. 2. After polishing and cleaning the elements as obtained, aluminum electrodes are attached to measure the varistor voltage (VWrnrn) nnd ihe flatness ratio of the V-l characteristic, the results of which are Illustrated in Fig. 6. In Fig. 6, all the measurement values represent the means; values for all the specimens added with eleven different rare-earth elements.

Additionally, comparison of the specimens Nos. 28, 30 and 31 shows that although the temperature-lowering rate in the first firing step carried out in the atmosphere of air contributes to improvement more or less of the flatness ratio in the large current region of the V-l characteristic curve, the temperature-lowering rate is not a factor affecting remarkably the V-l characteristic of the resistance element. Accordingly, the temperature-lowering rate now under discussion should be as high as possible so long as the manufacturing conditions allow, in consideration of the
firing in the second step.
i Additionally, comparison of the spocimens Nos. 28, 32 and 33 shows
that any appreciable variation can not be observed in the flatness ratio of the V-l
characteristic curve in the second firing step at the temperature rising rate range of
50 to 200 °C/hr within the oxidizing atmosphere (e.g. at the oxygen partial pressure
of 100 % by volume in the case of the instant example). The temperature rising rate
higher than 500 °C/hr brings about cracking in the resistance element as
manufactured. Thus, in view of the manufacturing efficiency as well as from the
economical standpoint, the temperature rising rate should preferably be selected to

the first firing step has been completed. [0045]
Further, comparison of the specimens Nos. 28, 38 and 39 shows that when the maximum temperature in the second firing step is higher than that in the first firing step, the flatness ratio of the V-l characteristic in the small current region can remarkably be improved. However, in that case, the porosity is increased, giving rise to a problem that moisture absorbing capability is degraded. On the other hand, when the maximum firing temperature in the Beconcl firing step is lower than the maximum temperature (1300 °C) in the first firing step by 300 °C or more, the flatness ratio of the V-l characteristic curve la degraded, rendering the intended effect of the second firing step ineffective. Accordingly, the maximum temperature in the second firing step should be set equal (o that of the first firing step or at a temperature within a range lower than that of the first firing step by 300 °C at the most.
Furthermore, comparison of the specimens Nos. 28, 34 and 40 shows that the temperature-lowering rate from the maximum point to the changing point (or trasition point) of the temperature-lowering rate in the second firing step contributes to reducing the flatness ratio of the V-l chamoterlutio curve in the large current region as the temperature-lowering rate is higher.forever, when the temperature-lowering rate exceeds the rate of 200 °C/hr, the flatness the of the V-l characteristic curve is degraded in the small current region Si :h bejng the circumstances, the

The temperature-lowering rale changing point in the second firing step plays a very important role in carrying out the present invention. More specifically, for the purpose of reducing oxygen defect of zirib oxide grains and supply oxygen in excess to the inter-grain boundaries of zinc oxide during the temperature-lowering process, the temperature-lowering rate is changed within a range around the crystallization temperature of bismuth oxide whlih ie good conductor for oxygen ions. Comparison of the specimen Nos. 28, 35 and 4i! with one another shows that when the point at which the temperature-lowering rate le changed in the second firing step is set lower, the flatness ratio of the V-l i.ive , lin: small current region becomes degraded, causing the aimed effects of trio two the sintering process to disappear. On the other hand, even when the changing! of the temperature-lowering rate is set high, any significant change can scarcely to observed Since thfe flatness ratio of the V-l characteristic curve in the small current: region can not be improved unless the temperature-lowering rate following the changing point mentioned above is made lower than that preceding to the changing point, the changing point of concern should preferably be set at a temperature as low as possible within a range where the aimed effect can be realized, from the standpoint of manufacturing efficiency or productivity. More specifically, changing point of the temperature-lowering rate in the second firing step should preferably be set in a temperature range of 450 to 900 °C and more preferably in a range of 500 to 800 °C although it depends on the composition of the starting material as well as the conditions for the sintering process. In this conjunction, setting of the changing point of the lumperature-lowering rate should be performed with the aid of an appropriate tool such as a TMA (ThermoMechanical Analysis apparatus) or the like in consideration of the fact that crystallization temperature of bismuth oxide varies delicately or subtly in dependence on the

undergoing the firing process in the oxidizing atmosphere in the second firing step,

whereby a sufficient amount of oxygen is supplied to the inter-grain boundaries between the zinc-oxide crystal grains. Thus, there can be obtained a voltage-nonlinear resistance element which is excellent in the flatness ratio of the V-l characteristic. Exemplary Embodiment 4
A starting composition or mixture is prepared such that the contents of bismuth oxide, chromium oxide, nickel oxide, oobalt oxide, manganese oxide and silicon oxide are each of 0.5 mol%, and that of antimony oxide is 1.2 mol% with boron oxide, which is a trace amount of additive, is contained in 0.04 mol%. Starting from the basic composition mentioned above, aluminum and rare-earth elements, i.e., yttrium (Y), samarium (Sm), europium (En), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutetium (Lu) (collectively represented by "IT) am gelded in the amounts illustrated in Fig. 7 in terms of AP2O3 and R2O3, respecli - ;ly.the remaining part is the content of zinc oxide (ZnO).
Each of the starting materials prepared as mentioned above is mixed with an aqueous solution of polyvinyl alcohol living as binder and an aqueous solution of such as, for example, boracic acid orthe like which is formed by resolving a trace additive of boron oxide into water, by using a disperse mill to thereby form a slurry, which is then dried by means of a spray drier and granulated subsequently. The granulated material is shaped into a preform by applying a uniaxial pressure in a range of 200 to 500 kgf/cm2. Parenthetically, each of the specimen preforms thus
obtained has a nominal diameter (cj)) of 125 mm and a thickness of 30 pirn. The
i granulated preforms or specimens undergo preheating for five hours at a temperature
of 600 °C to thereby remove the binder.
A

The first firing step (at 1,150 ' C x B hrj of the two-step sintering process is carried out in accordance with a sintering or firing pattern No. 1 shown in Fig. 1 (i.e., in the atmosphere of air with Va = 30 °C/hr and Vb = 50 °C/hr). Parenthetically, oxygen concentrations of the oxidizing atmosphere employed in the second sintering or firing step are shown in Fig. 7.
After polishing and cleaning the elements as obtained, aluminum electrodes are attached to measure the electric characteristics, the results of which are illustrated in Fig. 7. In Fig. 7, the values ol the flatness ratio enumerated in the table represent the flatness ratio (VioK/vVion*roA) over the whole region inclusive of the large current, region and the small current region, all the measurement values representing the means values for all the specimens added with oxides of eleven different rare-earth elements. These results show the facts which will be described below.
As can be seen from comparison of the specimens Nos. 44 to 58, the flatness ratio substantially comparable to that obtained by the firing process carried in the oxidizing atmosphere containing oxygen at a concentration of 100 % can be realized with the oxygen concentration ol 80 %. On the other hand, in the cases where the oxygen concentration is 60 % or less the flatness ratio becomes degraded in all the specimens. Thus, in order to supply a sufficient amount of oxygen to the inter-grain boundary regions between the zinc oxide grains by employing the oxidizing atmosphere in the second sintering or , it is desirable to set the oxygen concentration at least at 80 %. In that case, voly excellent flatness ratio can be
obtained.
I i
As will now be understood from the above, according to the? invention
incarnated in the fourth exemplary embodiment. there can be obtained a voltage-

nonlinear resistance element ensuring a large visitor voltage which has a small flatness ratio over the whole current regioin from a laudge current to a small current by setting the oxygen concentration of the axlidizing atmosphere at 80% at 80 % or more in the second firing step. Exemplary Embodiment 5
A starting composition or mixture la prepared such that the contents of bismuth oxide, chromium oxide, nickel oxide, cobalt oxide, manganese oxide and silicon oxide are each of 0.5 mol%, and that of antimony oxide is 1.2 mol% with a trace additive of boron oxide being contained in 0.04 mol%. Starting from the basic composition mentioned above, rare-earth elements, i.e., yttrium (Y), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) and lutotlum (Lu) (collectively represented by "R") are added in 0.5 mol% in terms of oxides (R2O3) of rare-earth elements, respectively. The remaining part is the content of zinc oxide (ZnO).
Each of the starting materials prepared as mentioned above is mixed
with an aqueous solution of polyvinyl alcohol serving as a binder and an aqueous
solution of such as, for example, boracic acid or the like which is formed by resolving
a trace additive of boron oxide into water, by using 9 disperse mill to form a slurry,
which is then dried by means of a spray drier and then granulated. The granulated
material is shaped into a preform by applying a uniaxial pressure in a range of 200
to 500 kgf/cm2. Parenthetically, each of the specimen preforms thus obtained has
a nominal diameter ($) of 125 mm and a thickness of 30 mm. The granulated
preforms or specimens undergo preheating for five hours at a temperature of 600 °C
to thereby remove the binder. j
The first sintering or firing step (s.l I |i!i0 °C x 5 hr) of the two-step

sintering process is carried out in accordance with the firing pattern No. 1 shown in Fig. 1. Thereafter, the second firing step is carried out in accordance with a firing pattern No. 1 shown in Fig. 8.
After polishing and cleaning the elements as obtained, aluminum electrodes are attached to measure the electric characteristics, the results of which are illustrated in Fig. 9. These results show the facts which will be described below.
As can be seen from comparisons of the data of the specimens Nos. 59
to 73, the flatness ratio of the resistance element becomes smaller, as ijhe oxygen
concentration of the firing atmosphere employed during the temperature-lowering
period from the maximum temperature (Ta) to the changing point (Tb) of the
temperature-lowering rate in the second firing process is lower. Substantially same
tendency can be observed when the atmosphere (oxygen concentration) is changed
from 100 to 80 % and then to 30 % during the whole second firing period. Such
phenomenon may be explained by the fact thai when the resistance material or
composition is placed in the atmosphere locking excessively in oxygen in the high-
temperature phase of the firing or sintering proccess lots of oxygen defects will take
place in zinc oxide crystal grains which .:: element, involving thus low resistance vi-jue a the zinc oxide grains themselves.
Accordingly, the oxygen concentration of the alniutiiphere employed in the second
firing step from the maximum temperature* Ic the chapcjing poin^ of the temperature-
lowering rate should be set as low aci j:)os» l|:i|,:?. In practical applications, in
consideration of the workability (i.e., process rrwilpulatability), it is preferred to set
the oxygen concentration of concern at a value equivalent to that of the ambient air
(20 %) or less. j
As will now be understood from the above, according to the present

invention incarnated in the fifth exemplary embodiment, by setting the oxygen concentration in the temperature-lowering phase of the second firing step from the maximum temperature to the changing point of the temperature- lowering rate at 30 % or less, there can be obtained a voltage-nonlinear resistance element which can exhibit a large varistor voltage while ensuring a small flatness ratio over the whole region from the large current to the small current region, because lots of oxygen defects take place within the region containing zinc oxide as a primary component, to thereby lower the resistance of;: i;:; oxide itself.
Many modifications and var ■;ion:;i of the present invention are possible in the light of the above techniques. It is there!) am to be understood that within the
scope of the appended claims, the invention i!i% tie practiced otherwise than as
! specifically described.

WE CLAIM:
1. A composition for an electric material, containing as a primary component zinc
oxide and additionally containing bismuth oxide, antimony oxide, chromium
oxide, nickel oxide, cobalt oxide, manganese oxide, silicon oxide and boron
oxide, said composition further containing at least one of rare-earth elements in
a range of 0.01 mol% to 3.0 mol% in terms of oxide thereof given by R203
where R represents generally said rare-earth elements; and aluminum in a range
of 0.0005 mol% to 0.005 mol % in terms of aluminum oxide given by A1203.
2. A composition for an electric material substantially as herein described with
reference to the accompanying drawings.

Documents:

1291-mas-1996 claims.pdf

1291-mas-1996 correspondence-others.pdf

1291-mas-1996 correspondence-po.pdf

1291-mas-1996 description (complete).pdf

1291-mas-1996 drawings.pdf

1291-mas-1996 form-1.pdf

1291-mas-1996 form-13.pdf

1291-mas-1996 form-26.pdf

1291-mas-1996 form-4.pdf

1291-mas-1996 form-6.pdf

1291-mas-1996 others.pdf

1291-mas-1996 petition.pdf

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Patent Number

194876

Indian Patent Application Number

1291/MAS/1996

PG Journal Number

08/2007

Publication Date

23-Feb-2007

Grant Date

02-Jan-2006

Date of Filing

22-Jul-1996

Name of Patentee

M/S. MITSUBISHI DENKI KABUSHIKI KAISHA

Applicant Address

2-3, MARUNOUCHI 2-CHOME, CHITODA-KU, TOKYO 100

Inventors:

#	Inventor's Name	Inventor's Address
1	NAOMI FURUSE,	C/O M/S. MITSUBISHI DENKI KABUSHIKI KAISHA 2-3, MARUNOUCHI 2-CHOME, CHITODA-KU, TOKYO 100
2	MASAHIRO KOBAYASHI;	C/O M/S. MITSUBISHI DENKI KABUSHIKI KAISHA 2-3, MARUNOUCHI 2-CHOME, CHITODA-KU, TOKYO 100
3	TOSHIHIRO SUZUKI	C/O M/S. MITSUBISHI DENKI KABUSHIKI KAISHA 2-3, MARUNOUCHI 2-CHOME, CHITODA-KU, TOKYO 100
4	JUNICHI SHIMIZU;	C/O M/S. MITSUBISHI DENKI KABUSHIKI KAISHA 2-3, MARUNOUCHI 2-CHOME, CHITODA-KU, TOKYO 100
5	YOSHIO TAKADA;	C/O M/S. MITSUBISHI DENKI KABUSHIKI KAISHA 2-3, MARUNOUCHI 2-CHOME, CHITODA-KU, TOKYO 100
6	HIROSHI NAKAJOH;	C/O M/S. MITSUBISHI DENKI KABUSHIKI KAISHA 2-3, MARUNOUCHI 2-CHOME, CHITODA-KU, TOKYO 100

PCT International Classification Number

H01C17/14

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	7-230169	1995-09-07	Japan