LAMINATED AND GRAIN BOUNDARY INSULATED TYPE SEMICONDUCTIVE CERAMIC CAPACITOR AND METHOD OF PRODUCING THE SAME

Abstract: A laminated and grain boundary insulated type semiconductor ceramic capacitor which normally works as a capacitor absorbing noises of low voltages and noises of high frequencies and works as a varistor when high voltage pulses or static electricity noises are applied thereto. The material of the ceramic capacitor is chiefly Sr (1-x) Ca x TiO₃ by which the material and the internal electrode material can be baked simultaneously in the process of production. A method of producing the same is also disclosed. The reason why the both materials can be simultaneously baked is as follows. A component for changing a material into a semiconductor is added to a Ti-excess of Sr (1-x) Ca x TiO₃; Mn and Si components are further added which turn into MnO₂ and SiO₂ respectively during the step of baking; and accordingly grain boundary insulated semiconductor for capacitors is easily obtained only through re-oxidation without the conventional step of surface diffusion of metal oxides. Further, according to the invention, the compact after lamination is calcined in advance in the air to obtain a laminated ceramic capacitor with a varistor function. Problems such as breakage in the internal electrode, delamination, cracks, low density after sintering, and nonhomogeneity of structure of the sintered body are suppressed as far as possible. The capacitor exhibits remarkable improvement in electric characteristics such as capacity, voltage non-linear exponential α, varistor voltage and in reliability. The invention exhibits the greatest features in the material composition and process in regard to the above two points.

Description: SPECIFICATION Technical field The present invention relates to a semiconductor-type laminated ceramic capacitor with a grain boundary-insulated structure and a method for producing the same, and more particularly to a ceramic capacitor which absorbs both low voltage noises and high frequency noises under the normal operational conditions as a capacitor, works as a varistor against invading high voltage loads such as pulses and electrostatic charges, thereby protecting built-in semiconductors and electronic equipment from being damaged by abnormal voltage loads such as noises, pulses and electrostatic charges arising from surrounding electronic equipment, wherein characteristics of said ceramic capacitor being stable to temperature changes. Background art Recently, semiconductor elements such as IC and LSI are widely used in electronic devices and equipment to realize multifunctional applications of the equipment and to make the equipment light, small and handy. However, the use of many semiconductor elements makes the electronic device less resistant to disturbance by electric noises. The conventional method to protect the electronic devices from invading electric noises is to integrate a by-pass capacitors such as film capacitor, laminated ceramic capacitor and semiconductor ceramic capacitor into the power line of IC and LSI. These capacitors display excellent performances in absorbing low voltage noises and high frequency noises. They are, however, so impotent to high voltage pulses and electrostatic charges that mulfunctionings of the equipment, break-down of semiconductors and/or capacitors themselves occur frequently when the electronic equipment are invaded by high voltage pulses or electrostatic charges. Therefore, these technical problems in the conventional capacitors should be improved. A new type capacitor, which has sufficient resistance and excellent absorbing ability to pulses as well as good noise absorbing ability and stability to temperature and frequency changes, was developed and disclosed in Japanese Laid-Open Patent Publication No. 57-27001 and Japanese Laid-Open Patent Publication No. 57-35303 etc., wherein a varistor function was added to a ceramic capacitor made of SrTiO₃ series of ceramic materials and the capacitor was defined as "a semiconductor-type ceramic capacitor with a grain boundary-insulated structure having a varistor function" (hereinafter referred to as a ceramic capacitor with varistor function). This ceramic capacitor with varistor function, works as a varistor when high voltage pulses and electrostatic charges invade it and it absorbs low voltage noises and high frequency noises as a conventional capacitor, thereby protecting the electronic equipment and built-in semiconductors from abnormal high voltage noises, pulses and electrostatic charges generated by surrounding electronic equipment and devices, and providing wide application fields for the capacitor. Since the electronic parts are made more sophisticated, lighter, smaller, and more handy, ceramic capacitors with varistor function are also required to be smaller but higher in performance. The effective electrode area of the conventional single-plate type ceramic capacitor with varistor function will be, however, decreased when the capacitor is miniaturized, resulting in the reduction of electric capacitance and hence inferior reliability of the capacitor. A capacitor having laminated structure of the electrode is devised to solve the aforementioned problem, wherein the effective electrode area is expected to be increased. However, the ceramic capacitor with varistor function is conventionally manufactured by the process comprising a step coating the surface of SrTiO₃ type semiconductor element with oxides, followed by a thermal diffusion process to form an electric insulating layer in the grain boundaries. It is recognized to be very difficult to manufacture "a laminate type ceramic capacitor having varistor function" (hereinafter referred to as a laminated ceramic capacitor with varistor function) by sintering the material of the ceramic capacitor simultaneously with the material of an inner electrode, though conventional laminated ceramic capacitors made of BaTiO₃ series of materials are manufactured by this process. As a method to overcome the problem of simultaneous formation of a material of laminated ceramic capacitor with varistor function by sintering the material of the ceramic capacitor together with the material of inner electrode, the following method for manufacturing a laminated ceramic capacitor with varistor function is developed and provided, using methods as is disclosed in Japanese Laid-Open Patent Publication No. 54-53248 and Japanese Laid-Open Patent Publication No. 54-53250, comprising the steps of: printing a pattern corresponding to the inner electrode by using ceramic paste enriched in organic binder on the surface of the ceramic substrate; forming a porous sheet corresponding to the pattern of the inner electrode by sintering; and impregnating the porous sheet with electrically conducting metals under the appropriate pressure, or alternatively a step of forming a pattern of inner electrodes by gilding or fusion method. These processes, however, involve many difficulties and they are yet far from practical applications. Japanese Laid-Open Patent Publication No. 59-215701 provides a method comprising the manufacturing steps of: forming a raw sheet made of ceramic powder calcinated in the non-oxidizing atmosphere; printing a pattern of the inner electrode by using an electric conductive paste mixed with a thermal diffusion material on the surface of the row sheet, said thermal diffusion material having the ability to form an electric insulating layers in the grain boundaries; and sintering said raw sheet in the oxidizing atmosphere. Another method disclosed in Japanese Laid-Open Patent Publication No. 63-219115 comprises the manufacturing steps of: forming a raw sheet made of semi-conductive ceramic powder as a main component, said main component being mixed with oxide to form an insulating layer and/or diffusion material containing a glass material; laminating said raw sheets with a plurality of inner electrode sheets mutually one by one; and sintering said laminated sheets in air or in the oxidizing atmosphere. In the above-discussed two manufacturing methods, however, sintering is performed at relatively low temperatures of 1000 - 1200°C. Therefore, the capacitor elements thus formed are not a perfect sintered body of ceramic powder wherein grain boundaries are scarcely contacted with their faces. The ceramic capacitors obtained entail the shortcomings comprising: relatively small electric capacitance; small value of voltage non-linearity coefficient a which is a representative characteristics of a capacitor having varistor function; instability of varistor voltage; and inferior reliability as a capacitor. The manufacturing method disclosed in the latter invention, Japanese Laid-Open Patent Publication No. 63-219115, involves the step of adding a glass material to the ceramic powder which entails a problem in that glassy phase deposits in the grain boundaries, whereby said electric characteristics tend to be poor and the capacitor element is not recognized as reliable. The manufacturing process is considered to be far from practical. Concerning the laminated capacitors having varistor function, voltage non-linear laminated capacitor elements made of ZnO, Fe₂O₃ and TiO₂ series of ceramic material have been provided in Japanese Laid-Open Patent Publication No. 58-23921. Capacitance of the capacitor elements of this type are, however, very small and they involve deficiencies in that they display little effect to the electric noises with voltage of below the varistor region or to high frequency noises, while they have excellent ability to absorb pulses and electrostatic charges of relatively high voltage. DE 34 35 806 discloses a capacitor composition comprising (Sr 1-x M II x )O.Z(Ti 1-y M IV y )O₂, wherein M II is Ca, Ba and/or Mg, and M IV is Si, Sn or Zr. Z is in the range of 0,97 ≦ Z ≦ 1,05. The composition additionally comprises Nb, La, Yt or Bi in an amount of 0.05 to 2.0 %. Disclosure of the invention Although many attempts have been made and the technical results are provided in the prior art concerning the compositions and manufacturing methods of the various kinds of laminated ceramic capacitors with varistor function, they involve deficiencies and shortcomings with regards to the manufacturing process itself or to the performances of the capacitor elements manufactured. None of them are considered to be practical yet, and novelties in compositions and manufacturing methods of the laminated ceramic capacitors with varistor function are expected to be developed. The present invention with respect to the above has an objective to provide a semiconductor-type laminated ceramic capacitor with a grain boundary-insulated structure made of the materials of Sr (1-x) Ca x TiO₃ type ceramics as a main component and amethod for producing the same, providing the functions of: a conventional capacitor to absorb low voltage noises and high frequency noises; a varistor when high voltage pulses and electrostatic charges invade the circuit; and a capacitor having the characteristics of being stable to temperature variations, wherein the capacitor element is manufactured by simultaneous sintering of the materials of ceramic capacitor together with the inner electrode materials. The present invention, which overcomes the deficiencies and shortcomings of the prior art, provides a semiconductor-type laminated ceramic capacitor with a grain boundary-insulated structure according to claim 1 and a method for producing the same according to claim 6. Generally speaking, ceramic material of the composition of Sr (1-x) Ca x TiO₃ is made semiconductive by a forced reduction process or, after adding an accelerating agent to make the material semiconductive, by a sintering process in the reducing atmosphere. The process to make the material semiconductive, however, may remain incomplete according to the nature of the agent used. When the amount of Sr (1-x) Ca x or Ti is made slightly in excess out of the stoichiometric ratio of Sr (1-x) Ca x TiO₃, lattice defect in the crystal of the ceramics will increase, thereby accelerating the material to be semiconductive. Moreover, growth of grains is inhibited by Ca ion replacing the site of Sr ions in the crystal lattice. In addition, the ceramic material is made semiconductive advantageously by adding Nb₂O₅, Ta₂O₅, V₂O₅, W₂O₅, Dy₂O₃, Nd₂O₃, Y₂O₃, La₂O₃ and CeO₂ (hereinafter referred to as component c)) since these compounds control the atomic valency. Next, Mn and Si (hereinafter referred to as component a)) are the essential compounds to form a laminated structure and absence of one of the two compounds will results in the incomplete formation of the lamellar structure. As mentioned above, manufacturing of laminated ceramic capacitors with varistor function was considered to be difficult in the prior art. The first reason depends on the fact that the material for the ceramic capacitor with varistor function such as SrTiO₃ series of ceramics displays a different nature and behavior with the material of inner electrode during the sintering and re-oxidation processes. The former materials require a reducing atmosphere in the sintering process while the latter materials made of metals expand in volume by occluding hydrogen gas in the reducing atmosphere. Furthermore, the latter material is oxidized into metal oxide and tends to block the re-oxidation of the former material during the re-oxidation process in the air. Formation of a ceramic capacitor element with varistor function by using the former material, which is the second reason why the process is difficult, involves a process of surface diffusion comprising the steps of: making the material semiconductive by sintering in the reducing atmosphere; covering the surface of the ceramic sheets with metal oxides with high electric resistance (i.e. MnO₂, CuO₂, Bi₂O₃, Co₂O₃) followed by re-oxidation in the air; and insulating grain boundaries by selective diffusion of the metal oxides. However, in the structure of the capacitor element laminated by the ceramic plate and inner electrode material mutually one by one, diffusion of metal oxides into grain boundaries is technically difficult. The inventors of the present invention discovered the following facts through their ardent studies. First, a ceramic capacitor with varistor function was readily manufactured by adding the components a), c) and d) into the ceramic material of Sr (1-x) Ca x TiO₃ with excess Ti, and by sintering the material in the reducing atmosphere followed by re-oxidation process. Spreading of metal oxide paste with high electric resistance over the surface of the ceramic plates was found to be needless to form insulating grain boundaries. The experimental facts is interpreted as follows; the component a) added, together with excess Ti, forms a liquid phase comprising of Mn, Si and Ti ternary oxide system at relatively low temperatures during sintering process. The liquid phase enhances sintering of grains while the oxides melt and segregate in the grain boundaries. Re-oxidation of the capacitor element in the boundaries of which a liquid phase comprising of Mn, Si and Ti ternary oxide system is formed in the air completes the formation of an insulating metal oxide film comprising segregated ternary oxides of Mn, Si and Ti in the grain boundaries, thereby readily producing a ceramic capacitor with varistor function having the structure of a grain boundary insulation layer. Excess Ti was found to prevent oxidation of inner electrode as well as diffusion of metal oxides into the inner electrode. For this reason the ceramic material of Sr (1-x) Ca x TiO₃ with excess Ti was used in the present invention. Second, the material of Sr (1-x) Ca x TiO₃, with excess Ti, to which the components a), c) and d) were added, was made semiconductive by sintering in the nitrogen atmosphere as well as in the reducing atmosphere. The finding can be partly interpreted by the same reason as is described in the discussions of the first experiment, in that the material is made into a liquid phase at relatively low temperatures. The Mn added not only forms a liquid phase but also works as an atomic valence control agent. In working as an atomic valence control agent, the ionic valence of Mn ion is +2 or +4 and its electronic state is unstable and liable to be excited, thereby increasing the sinterability. This is the reason the ceramic material is readily made semiconductive through the sintering process in the nitrogen atmosphere by adding Mn ion as component a). Third, the raw materials formed into the laminated capacitor sheets was calcinated in the air before subjecting to the sintering process. This treatment prevented the product, laminated ceramic capacitor with varistor function, from breaking electric connections in the inner electrodes, de-lamination of the ceramic sheets, cracking in the ceramic sheets, decrease in sintering density and nonuniformity in the sintered body etc. Furthermore, electric characteristics and reliability in performance of the capacitor such as capacitance, voltage non-linearity coefficient a and varistor voltage was also largely improved. According to the present invention, with respect to the above, a laminated ceramic capacitor with varistor function, comprising the manufacturing steps of sintering the material of ceramic capacitor with varistor function together with the materials of inner electrode simultaneously, is readily manufactured. Brief descriptions of the drawings Figure 1 is an exploded perspective view to explain the examples of the present invention, showing laminated raw sheets and the pattern of the inner electrode paste printed on said raw sheet. Figure 2 is a partially cutaway view of the laminated ceramic capacitor with varistor function obtained from the examples of the present invention. Figure 3 is a flow chart showing the process to manufacture the laminated ceramic capacitor with varistor function to explain the examples of the present invention. Best mode for carrying out the invention Details of the present invention will be described hereinafter with reference to the examples. Preliminary considerations to the examples Powders of CaCO₃ and TiO₂ were added to the powder material of SrTiO₃ of mean particle size of 0.5 µm or less and purity of 98% or more, adjusting the molar ratio of Sr (1-x) Ca x / Ti. Component c), Nb₂O₅, and component a), MnO₂ and SiO₂ (the relative molar content of the two compounds are made equal, respectively), were weighed and mixed to the material described above. The mixed powder was ground and mixed in a wet state by using a ball mill and, after drying, the ground powder was subjected to a calcination process by heating at 600 - 1200°C in the air. The calcinated powder was ground again to make a mean particle size of 0.5 µm or less, to provide the powder for the starting material of a laminated ceramic capacitor with varistor function. The finely powdered starting material was dispersed in an organic solvent together with an organic binder such as butyral resin compound to form a slurry. The slurry was next subjected to the formation of raw sheet with its thickness of about 50 µm by using the method of Dr. Blade, and the sheet was cut into a predetermined size. A pattern of inner electrode paste 2 containing Pd was printed by a screen printing technique corresponding to the predetermined size, as is shown in Figure 1, on the raw sheet 1 formed by the above-described method. As is apparent from Figure 1, inner electrode pastes 2 are not printed on the uppermost and lowermost part of the raw sheet 1a in the laminated capacitor material. The pattern of inner electrode paste 2 on the intermediate part of the laminated raw sheets 1 reaches to the one edge of the sheet, and the edges are distributed alternatively one by one to the opposite(different) directions in the laminates. The raw sheets la were disposed on the uppermost and lowermost parts and raw sheets 1 printed with the above inner electrode pastes thereon were laminated therebetween, a plurality of this sheet pairs were pressed and compacted with a heat treatment. The pressed sheets were then calcinated and degreased by heating at 600 - 1250°C in the air. The calcinated sheets were next subjected to sintering by heating at 1200 - 1350°C in a reducing atmosphere, followed by re-oxidation by heating at 900 - 1250°C in the air. As shown in Figure 2, the outer electrode paste containing Ag was painted on the edges of the sheets where terminals of inner electrodes 2a were exposed alternatively and the silver paste was baked by heating at 800°C for 15 minutes in the air, thereby completing a ceramic capacitor 4 with varistor function, comprising: a plurality of inner electrode 2a on the surface of the grain boundary insulated semiconductor type ceramic sheets, the terminals of said inner electrodes 2a reaching to each of opposite edge of the sheet alternatively one by one; and outer electrodes 3 at opposite edges of the semiconductor type ceramic sheets, said outer electrodes being connected electrically to the terminals of said inner electrodes 2a exposed to the edges of the sheets alternatively. Size of the laminated ceramic capacitor of this example, abbreviated as Type 5.5, is 5.70 mm in width, 5.00 mm in length and 2.00 mm in thickness. The capacitor is composed of 10 effective sheets on which a pattern of the inner electrode is printed, and non-effective sheets on which the pattern of inner electrode is not printed, wherein both types of sheets are laminated alternatively, one by one. Figure 3 shows a manufacturing process flow sheet of the present invention. According to the laminated ceramic capacitor with varistor function thus manufactured, electrical characteristics such as capacitance, tan δ, varistor voltage, voltage non-linearity coefficient α, equivalent series resistance (ESR), capacitance-temperature rate of change and temperature coefficient of varistor voltage are listed in the tables below. The experimental conditions for sample preparation were; 1200°C, 2 hours for calcination and degreasing in the air; 1300°C, 2 hours for sintering in the reducing atmosphere of N₂ : H₂ = 99 : 1; and 1100°C, 1 hour for re-oxidation. Each electrical characteristic was obtained by the following experimental conditions. Capacitance was measured at 1.0 V, 1.0 KHz. Varistor voltage V 0.1mA was measured at 0.1 mA. Voltage non-linearity coefficient a was calculated from the values of varistor voltage measured at 0.1 mA and 1.0 mA, respectively, using the following equation α = 1 / log (V 1.0mA / V 0.1mA ) Equivalent series resistance (ESR) is defined as resistance at resonance frequency measured at 1.0 V. Capacitance-temperature rate of change (ΔC/C) was calculated from the values measured at -25°C and 85°C, respectively. Temperature coefficient of varistor voltage (ΔV/V) was calculated from the values measured at 25°C and 50°C, respectively. The samples marked by the symbol * in the tables below are listed for comparison and they are not included in the range of claims in the present invention. The sintered bodies marked by the symbol * possess small capacitance, small value of voltage non-linearity coefficient α, which is a representative characteristics of varistors, and a large value of equivalent series resistance (ESR). The capacitor having the above-discussed characteristics cannot display the performances both of a normal capacitor, which absorbs low voltage noises and high frequency noises, and of a varistor which absorbs high voltage pulses and electrostatic charges, at the same time. The capacitor has relatively large capacitance-temperature rate of variation and varistor voltage temperature coefficient, whereby making reliability and electric characteristics of the capacitor liable to be affected by temperature change. Therefore, these samples are not suited for the use of a ceramic capacitor with varistor function which protects electronic devices and equipment from being damaged by abnormal voltage such as noises, pulses and electrostatic charge generated by surrounding electronic equipment. Other samples which are not marked in the tables, possess large capacitance, large value of voltage non-linearity coefficient a and small equivalent series resistance (ESR). The capacitor with these characteristics displays the performances both of a normal capacitor, which absorbs low voltage noises and high frequency noises, and of a varistor which absorbs high voltage pulses and electrostatic charges. The capacitor has relatively small capacitance-temperature rate of variation and varistor voltage temperature coefficient, thereby making reliability and electric characteristics of the capacitor hardly liable to be affected by temperature change. Therefore, these samples are well suited for the use of a ceramic capacitor with varistor function which protects electronic devices and equipment from being damaged by abnormal voltage such as noises, pulses and electrostatic charge generated by surrounding electronic equipment. In the present invention, the molar ratio of Sr (1-x) Ca x / Ti in the ceramic material of Sr (1-x) Ca x TiO₃ is controlled because, when the ratio is greater than 1.00, the amount of Sr (1-x) Ca x will be in excess relative to the amount of Ti, thereby preventing the formation of the liquid phase composed of Mn, Si and Ti ternary oxide system. Grain boundary insulated structure of the ceramics is difficult to be formed with above-mentioned composition and, moreover, oxidation and diffusion of the materials of the inner electrode occur, resulting in poor electric characteristics and reliability. On the other hand, when the molar ratio of Sr (1-x) Ca x / Ti is less than 0.95, the sintered body obtained will be porous and thesintering density decreases. Powder materials of its mean particle size of 0.5 µm or less was used for the starting material of laminated ceramic capacitor with varistor function because, if the powder of mean particle size of more than 0.5 µm is used, the particles tend to coagulate in the slurry, thereby making the surface of raw sheets formed by using such non-uniform slurry gritty and not smooth. Sintering density and packing density of the sintered body obtained are small and the sintered body is difficult to make semiconductive, thereby electric characteristics of the ceramic capacitor tends to be unstable. Next, total amount of component a) added, MnO₂ and SiO₂, are also controlled because, when the combined relative molar content of this component is less than 0.2%, the effect of the added material is hardly reflected in the formation of the liquid phase composed of Mn, Si and Ti ternary oxide system. Grain boundary insulated structure of the ceramics is difficult to form, thereby sintering density of the ceramic capacitor obtained is decreased and electric characteristics tend to be poor. On the other hand, when the relative molar content of this component added exceeds 5.0%, the amount of metal oxides with high electric resistance segregating in the grain boundaries increases and electric characteristics of the ceramic capacitor also degrades. The amounts of the component c) added are controlled because, when the relative molar contents of component c) is less than 0.05%, the effects of the additives are not enough to make the material semiconductive. When combined relative molar contents of this component exceeds 2.0%, the material is depressed so as to be semiconductive, thereby preventing the capacitor from having predetermined electric characteristics and decreasing sintering density. The addition of Nb₂O₅ or Ta₂O₅ as component c) seemed to result in better electric characteristics of the capacitor than the cases when V₂O₅, W₂O₅, Dy₂O₅, Nd₂O₃, Y₂O₃, La₂O₃ or CeO₂ was added. The effects of mixed composition of component c) to the electric characteristics of the capacitor were studied in several cases. The addition of two or more kinds of component c) to the ceramic material displayed slight appreciable effect to the electric characteristics of the capacitor compared to the cases when one component was added to the material. The addition of Nb₂O₅ and Ta₂O₅ were, however, seemed to be more effective in manufacturing a capacitor of better electric characteristics than the addition of other components. The process of calcination and degreasing by heating the raw sheet at 600 - 1250°C in the air is most important in the method of formation of a laminated ceramic capacitor with varistor function in the present invention. Most of electric characteristics and reliability of the ceramic capacitor with varistor function manufactured are determined during this process. The purposes of the process are to enhance adhesive force between the material of ceramic capacitor with varistor function and material of the inner electrode, and to adjust sintering density, uniformity in the texture of sintered body, and mean grain size of the crystal particles in the grain boundary insulated, of the semiconductor type ceramics of laminated ceramic capacitor with varistor function manufactured. The rate of temperature increase was adjusted to 200°C/hr or less in the degreasing and calcination process because the slower the temperature increase was, the more desirable results were obtained after the degreasing and calcination process. The temperature for degreasing and calcination in the air is limited between 600 and 1250°C. When the temperature is lower than 600°C, the process is not effective enough and when the temperature for calcination exceeds 1250°C, electric characteristics and reliability of the capacitor will be largely decreased because of the following reasons; (1) The ceramic capacitor with varistor function is sintered instead of being calcinated by heating at this high temperature range. When the capacitor sintered previously in the air is subjected to sintering in the reducing atmosphere or in the nitrogen atmosphere, the sintered body shrinks rapidly and suffers concentrated stress, thereby producing de-lamination and cracks in the laminated ceramic capacitor with varistor function. (2) Oxidation of Ni as well as sintering of ceramic capacitor with varistor function will proceed at the same time when Ni is used as a material for inner electrode. The sintered body reacts with Ni followed by the diffusion of Ni, resulting in breaking of inner electrode, de-lamination and formation of cracks in the laminated ceramic capacitor with varistor function. (3) Sintering in the liquid phase of Mn, Si and Ti ternary oxide system proceed rapidly under the sintering condition at high temperature over 1250°C and growth of grains is accelerated, thereby largely decreasing sintering density and packing density. (4) The sintered body treated by above-mentioned manner is difficult to make semiconductive by the sintering process in reducing or nitrogen atmosphere afterward. The laminated ceramic capacitor with varistor function thus manufactured has larger capacitance and displays much better temperature and frequency characteristics than the capacitor disclosed in aforementioned Japanese Laid-Open Patent Publication No. 58-23921. The ceramic capacitor in the present invention is manufactured by laminating ceramic capacitor materials with varistor function which possesses both functions of a normal capacitor, and of a varistor, while the above-mentioned capacitor in the prior art is simply made of piled varistor material which shows prominent absorbing ability for surge current. The laminated ceramic capacitor with varistor function in the present invention is distinct from that in the prior art in both its functions and uses. When Ca is not added to the material, growth of crystal grains is not controlled and grain size distributes in a wide range, thereby entailing large tan δ value and inferior temperature characteristics. The addition of large amount of Ca progresses oxidation, resulting in decreased capacitance and hence poor varistor characteristics. The relative amount of replacement X of Ca with part of Sr in A-site is preferably within the range of 0.001≦X≦0.2. The sintering process was performed in a reducing atmosphere of the composition of N₂:H₂=99:1. When H₂ concentration in the atmosphere is increased, the following phenomena are observed with regard to both the material of the inner electrode and the material of the ceramic capacitor with varistor function; (1) Electrode material is expanded by occluding H₂ gas. (2) The material of the ceramic capacitor with varistor function is facilitated to be semiconductive. These phenomena entail the problems in laminated ceramic capacitor with varistor function with respect to electric and mechanical characteristics, comprising; breaking of electrical connections in the inner electrodes, de-lamination of the ceramic sheet, occurrence of cracking in ceramic sheets and incomplete re-oxidation of ceramic materials. Consequently, when sintering is performed in an atmosphere in which H₂ concentration is increased, it is preferable for manufacturing a ceramic capacitor with desirable characteristics that sintering temperature range is made a little lower (1200-1300°C). On the contrary, ceramic capacitor with varistor function is rather difficult to be semiconductive when it is sintered in N₂ atmosphere or in an atmosphere containing a smaller amount of H₂ gas. Therefore, the material is preferably sintered in a little higher temperature range (1300°C-1450°C). H₂ concentration relative to N₂ is most preferably in the range of 99.5:0.5≦N₂:H₂≦95: 5. The mixed powder was calcinated in the air. Calcination process performed in N₂ atmosphere was confirmed, however, to be advantageous also. With regard to the re-oxidation process, the material was re-oxidized at a predetermined temperature of 1100°C. The temperature range of 900-1250°C is also advantageous for re-oxidation to manufacture a capacitor element with desirable characteristics. When re-oxidation is performed at the highest temperature of 1200°C or more, the duration of high temperature should be as short as possible. Otherwise, not only the materials in grain boundaries but also crystal grains are oxidized and made insulating. When Ni is used for the inner electrode material, prolonged heating at 1200°C or more for reoxidation will oxidize metallic Ni itself. Therefore, duration of heating time at higher temperature for reoxidation should be carefully kept as short as possible. Ag was used for the material of the outer electrode in the examples. However, Pd, Ni, Cu and Zn was confirmed to be used advantageously for the same purpose. The material for the outer electrode comprises at least one or more kinds of Pd, Ag, Ni, Cu and Zn, and alloys or compositions of them. When Pd or Ag is used for the material of the outer electrode, an appreciable polarization is observed in the varistor voltage because the metals are rather difficult to make ohmic contact with the capacitor element. However, the basic characteristics of the capacitor is not influenced at all by polarization. Mean particle size of the powder material of laminated ceramic capacitor with varistor function manufactured by the disclosed method was approximately in the range of 2.0-3.0 µm. When calcination is achieved by heating at 1300°C or higher in the air, sintering of liquid phase of Mn, Si and Ti ternary oxide system is accelerated and consequently growth of grains is enhanced, thereby increasing mean grain size up to twice or more. Increased mean grain size entails many problems comprising; decreasing in sintering density and packing density, decreasing in voltage non-linearity coefficient α, decreasing in equivalent series resistance (ESR) and dispersing in electric characteristics. The ceramic capacitor thus manufactured would be poor in electric characteristics and reliability, thereby making the capacitor inappropriate to practical uses. (Example 1) Na₂SiO₃ is added as component d) and the effects will be described in further detail with reference to the example. A laminated ceramic capacitor with varistor function was manufactured by using similar methods described and the amount of the added component d), Na₂SiO₃, was adjusted and changed variously. A/B ratio of Sr (1-x) Ca x TiO₃, X = 0.10, was fixed to 0.97; the relative content of component c) added, Nb₂O₅, was fixed to 1.0% while those of component a) were fixed to 1.0% for MnO₂ and SiO₂, respectively, the total amount of which was fixed to 2.0%. The results are listed in Table 1. As described in Table 1, capacitance-temperature rate of variation and varistor voltage temperature coefficient are improved by adding component d), Na₂SiO₃. Na₂SiO₃ works as a carrier to diffuse liquid phase of Mn, Si and Ti ternary oxide system uniformly in the grain boundaries, thereby forming distinct interfaces among the semiconductor crystal region and a high resistance grain boundary region. When the relative molar content of added Na₂SiO₃, is less than 0.05%, the effects of the additive is not clear and few improvements in capacitance-temperature rate of variation and varistor voltage temperature coefficient are observed. When relative molar content of this component is over 2.0%, Na₂SiO₃, which serves as a carrier, will be in excess in grain boundaries, whereby decreasing capacitance and voltage non-linearity coefficient α, increasing equivalent series resistance (ESR), decreasing sintering density and degrading mechanical strength. A mixture of Na₂O and SiO₂ can also be used as component d) instead of Na₂SiO₃. Since Na₂O is a very unstable compound and decomposes readily during the sintering process, it is liable to scatter or diffuse into the atmosphere, leaving few Na atoms in the manufactured sintered body. Moreover, it was confirmed that migration of partially ionized Na⁺¹ ion under the load of high temperature and high voltage entails degradation of capacitor characteristics. The effects of added Na ion is provided advantageously by adding the ion in the form of a compound with SiO₂. It was confirmed that Na ion should be added to the material in the form of a compound of Na₂SiO₃. (Example 2) Next, an example where NaAlO₂ is added as component d) will be described. A laminated ceramic capacitor with varistor function was manufactured by using similar methods described previously. The amount of the added component d), NaAlO₂, was adjusted and changed variously as is described in Example 1. A/B ratio of Sr (1-x) Ca x TiO₃, X = 0.10, was fixed to 0.97; the relative content of the component c) added, Nb₂O₅, was fixed to 1.0% while those of component a) was fixed to 1.0% for MnO₂ and SiO₂, respectively, the total amount of which was fixed to 2.0%. The results are listed in Table 2. As described in Table 2, the first advantage of adding NaAlO₂, to the ceramic material is, just like the addition of Na₂SiO₃ described in Example 1, in that it improves capacitance-temperature rate of change and varistor voltage temperature coefficient. Na atoms in added NaAlO₃ work as carriers to diffuse liquid phase of Mn, Si and Ti ternary oxide system homogeneously into grain boundaries, thereby forming distinct interfaces among semiconductive crystal grains and grain boundaries of high electric resistance. Second, voltage non-linearity coefficient α increases and equivalent series resistance (ESR) decreases by adding NaAlO₂, because Al atoms in NaAlO₂ is solubilized in a solid state into the crystal lattice, thereby decreasing electric resistance of crystal grains. When relative molar content of added NaAlO₂ is less than 0.05%, characteristics of the capacitor are not influenced desirably by the additive; giving no improvements in capacitance-temperature rate of change and varistor temperature coefficient, nor increasing voltage non-linearity coefficient α, and not decreasing equivalent series resistance (ESR). On the other hand, when relative molar content of the additive exceeds 4.0%, which means it exceeds the saturated solubility in the crystal lattice, excess NaAlO₂ segregates in the grain boundaries, thereby increasing electric resistance of the grain boundaries and hence decreasing capacitance and voltage non-linearity coefficient α rapidly and decreasing mechanical strength of the capacitor. Mixture of Na₂O and Al₂O₃ can also be used as component d) instead of NaAlO₂. Since Na₂O is a very unstable compound and decomposes readily during the sintering process, it is liable to scatter or diffuse into the atmosphere, leaving few Na atoms in the manufactured sintered body. Moreover, it was confirmed that migration of partially ionized Na⁺¹ ion under the load of high temperature and high voltage entails degradation of capacitor characteristics. The effects of added Na ion is provided advantageously by adding the ion in the form of a compound of NaAlO₂. It was confirmed that Na ion should be added to the material in the form of a compound of NaAlO₂. (Example 3) An example where Li₂SiO₃ is added as component d) will be described. A laminated ceramic capacitor with varistor function was manufactured by using similar methods described above and the amount of the added Li₂SiO₃, was adjusted and changed variously as is described in Examples 1 and 2. A/B ratio of Sr (1-x) Ca x TiO₃, X = 0.10, was fixed to 0.97; the relative content of component c) added, Nb₂O₅, was fixed to 1.0% while those of combined components a) were fixed to 1.0% for MnO₂ and SiO₂, respectively, the total amount of which was fixed to 2.0%. The results are listed in Table 3. As described in Table 3, the first advantage of adding component d), Li₂SiO₃, to the ceramic material is, just like Examples 1 and 2, it improves the capacitance-temperature rate of change and varistor voltage temperature coefficient. Added Li₂SiO₃ works as carriers to diffuse the liquid phase of Mn, Si and Ti ternary oxide system homogeneously into grain boundaries, thereby forming distinct interfaces among semiconductive crystal grains and grain boundaries of high electric resistance. When relative molar content of added Li₂SiO₃ is less than 0.05%, characteristics of the capacitor are not influenced desirably by the additive, giving no improvements in capacitance-temperature rate of change and varistor temperature coefficient. On the other hand, when relative molar content of the additive exceeds 2.0%, the amount of carrier Li₂SiO₃ increases, thereby decreasing capacitance and voltage non-linearity coefficient a, increasing equivalent series resistance (ESR), and decreasing sintering density and mechanical strength of the capacitor. A mixture of Li₂O and SiO₂ can also be used as component d) instead of Li₂SiO₃. Since Li₂O is a very unstable compound and decomposes readily during the sintering process, it is liable to scatter or diffuse into the atmosphere, leaving few Li atoms in the manufactured sintered body. Moreover, it was confirmed that migration of partially ionized Li⁺¹ ion under the load of high temperature and high voltage entails degradation of capacitor characteristics. The effects of added Li ion is provided advantageously by adding the ion in the form of a compound of Li and SiO₂. Thus, Li ion should be added to the material in the form of a compound of Li₂SiO₃. (Example 4) Next, an example where LiAlO₂ is added as component d) will be described. A laminated ceramic capacitor with varistor function was manufactured by using similar methods described in Examples 1 to 3, except that the amount of the added component d), LiAlO₂, was adjusted and changed variously as described in Examples 1 to 3. A/B ratio of Sr (1-x) Ca x TiO₃, X = 0.10, was fixed to 0.97; the relative content of component c) added, Nb₂O₅, was fixed to 1.0% while component a) was fixed to 1.0% for MnO₂ and SiO₂, respectively, the total amount of which was fixed to 2.0%. The results are listed in Table 4. As described in Table 4, the first advantage of adding LiAlO₂, to the ceramic material is, just like the addition of component d) described in Examples 1 to 3 it improves the capacitance-temperature rate of change and the varistor voltage temperature coefficient. Li atoms in added LiAlO₂ works as carriers to diffuse liquid phase of Mn, Si and Ti ternary oxide system homogeneously into grain boundaries, thereby forming distinct interfaces among semiconductive crystal grains and grain boundaries of high electric resistance. Second, voltage non-linearity coefficient a increases and equivalent series resistance (ESR) decreases by adding LiAlO₂, because Li atoms in added LiAlO₂ is solubilized in a solid state into the crystal lattice, thereby decreasing electric resistance of the crystal grains. When relative molar content of added NaAlO₂ as a third component is less than 0.05%, characteristics of the capacitor are not influenced desirably by the additive; giving no improvements in capacitance-temperature rate of change and varistor temperature coefficient, nor increasing voltage non-linearity coefficient α, and not decreasing equivalent series resistance (ESR). On the other hand, when the relative molar content of the additive exceeds 4.0%, which means it exceeds the saturated solubility in the crystal lattice, excess LiAlO₂ segregates in the grain boundaries, thereby decreasing electric resistance of grain boundaries and hence decreasing capacitance and voltage non-linearity coefficient α rapidly and decreasing mechanical strength of the capacitor. A mixture of Li₂O and AlO₂ can also be used as component d) instead of LiAlO₂. However, when a mixture of Li₂O and Al₂O₃ is used, since Li₂O is a very unstable compound and decomposes readily during sintering process, it is liable to scatter or diffuse into the atmosphere, leaving few Li atoms in the manufactured sintered body. Moreover, it was confirmed that migration of partially ionized Li⁺¹ ion under the load of high temperature and high voltage entails degradation of capacitor characteristics. The effects of added Li ion is provided advantageously by adding the ion in the form of a compound of LiAlO₂. It was confirmed that Li ion should be added to the material in the form of a compound of LiAlO₂. (Example 5) Na₂SiO₃ and Al₂O₃ are added as in this example. A laminated ceramic capacitor with varistor function was manufactured by using similar methods described in Examples 1 to 4, except that the amount of the added Al₂O₃, was adjusted and changed variously. A/B ratio of Sr (1-x) Ca x TiO₃, X = 0.10, was fixed to 0.97; the relative molar content of component c) added, Nb₂O₅, was fixed to 1.0% while the one of component a) was fixed to 1.0% for MnO₂ and SiO₂, respectively, the total amount of which was fixed to 2.0%. Relative molar content of third material, Na₂SiO₃, was also fixed to 0.5%. The results are listed in Table 5. As described in Table 5, the addition of Al₂O₃, increases voltage non-linearity coefficient α and decreases equivalent series resistance (ESR), because Al₂O₃ added is solubilized in solid state into the crystal lattice and decreases electric resistance of the crystal grains. When relative molar content of the added Al₂O₃, is less than 0.05%, characteristics of the capacitor are not influenced desirably by the additive; giving no improvements in capacitance-temperature rate of change and varistor temperature coefficient, nor increasing voltage non-linearity coefficient α and not decreasing equivalent series resistance (ESR). On the other hand, when relative molar content of the additive exceeds 2.0%, which means it exceeds the saturated solubility in the crystal lattice, excess Al₂O₃ segregate in the grain boundaries, thereby decreasing electric resistance of grain boundaries and hence decreasing capacitance and voltage non-linearity coefficient α rapidly. (Example 6) Li₂SiO₃ and Al₂O₃ are added in this example. A laminated ceramic capacitor with varistor function was manufactured by using similar methods described in Examples 1 to 4, except that the amount of added Al₂O₃, was adjusted and changed variously. A/B ratio of Sr (1-x) Ca x TiO₃, X = 0.10, was fixed to 0.97; the relative molar content of component c) added, Nb₂O₅, was fixed to 1.0% while those of combined components a) were fixed to 1.0% for MnO₂ and SiO₂, respectively, the total amount of which was fixed to 2.0%. Relative molar content of the third material, Li₂SiO₃, was also fixed to 0.5%. The results are listed in Table 6. As described in Table 6, addition of Al₂O₃, increases voltage non-linearity coefficient α and decreases equivalent series resistance (ESR), because Al₂O₃ added is solubilized in solid state into the crystal lattice and decreases electric resistance of crystal grains. When relative molar content of added, Al₂O₃, is less than 0.05%, characteristics of the capacitor are not influenced desirably by the additive; giving no improvements in capacitance-temperature rate of change and varistor temperature coefficient, nor increasing voltage non-linearity coefficient α and not decreasing equivalent series resistance (ESR). On the other hand, when relative molar content of the additive exceeds 2.0%, which means the saturated solubility in the crystal lattice, excess Al₂O₃ segregates in the grain boundaries, thereby decreasing electric resistance of grain boundaries and hence decreasing capacitance and voltage non-linearity coefficient α rapidly. The effects of adding one of components d), Na₂SiO₃, NaAlO₂, Li₂SiO₃ and LiAlO₂, to the ceramic material were described in Examples 1 to 4. In Examples 5 and 6, two combinations of the additives, Na₂SiO₃ and Al₂O₃; and Li₂SiO₃ and Al₂O₃ were described. However, various combinations other than those described in Examples 6 and 7, for example, combination of Na₂SiO₃ and Li₂SiO₃ or NaAlO₂ and LiAlO₂, are also devised. Improvements in characteristics of the capacitors by adding Na₂SiO₃, Li₂SiO₃ and Al₂O₃ are considered to be possible. In summary, any combinations of the aforementioned compounds are possible as component d) in order to obtain a capacitor of desired characteristics. Industrial applicability As described above, according to the present invention, a laminated ceramic capacitor with varistor function is obtained comprising both functions of a capacitor and a varistor, and the capacitor is capable of mounting on the circuit board. The capacitor works as an ordinary capacitor which absorbs low voltage noises and high frequency noises while it functions a varistor when high voltage pulses and electrostatic charges invade, thereby protecting semiconductors and electronic instruments from abnormal voltages such as noises, pulses and electrostatic charges generated by surrounding electronic instruments. The characteristics of the capacitor is always stable to temperature changes. Practical applications of the above capacitor are; (1) The capacitor can replace conventional film, laminated ceramic and semiconductor type capacitors which are used for by-pass capacitors for protecting IC and LSI mounted on the circuit cards of electronic equipment. (2) The capacitor can replace ZnO type capacitors which are used for preventing malfunctioning of electronic equipment and absorbing ON-OFF surges arising from inductive loads. The capacitor possesses both functions described in above-discussed terms of (1) and (2) and a wide range of applications are to be expected. Manufacturing of a laminated ceramic capacitor with varistor function of the present invention is made easy because simultaneous sintering of the materials of ceramic capacitor with varistor function with the materials of inner electrode has became possible for the following reasons. Besides materials to be semiconductive are added to Sr (1-x) Ca x TiO₃ excess in Ti, Mn-Si components composed of MnO₂ and SiO₂ is also added in the sintering process. Grain boundary insulated structure of semiconductor type ceramic capacitor is formed readily by a re-oxidation process by using the material of the above-described composition, whereas surface diffusion process of metal oxides is necessary in the manufacturing process of conventional ceramic capacitor. Many troubles of breaking electrical connections in inner electrodes, de-lamination, cracks in ceramic sheets, decrease in sintering density and non-uniformity of the texture in the sintered body in the laminated ceramic capacitor with varistor function manufactured are prevented from occurring by sintering the raw laminated body in the air. Electric characteristics such as capacitance, voltage non-linearity coefficient α and varistor voltage, and reliability of the products are largely improved. The present invention provides two major advantages in the aforementioned composition of the starting materials and in the manufacturing process of the capacitors. Since the laminated ceramic capacitor with varistor function of the present invention is smaller in size and possesses larger capacitance and higher performances compared to conventional single plate type ceramic capacitor with varistor function, the former capacitor is expected to have a wide range of applications as highly integrated mounting elements for the electronic equipment such as video cameras and communication apparatus. Therefore, according to the present invention capacitor elements comprising the functions of protecting semiconductors and electronic equipment from invading abnormal voltages such as noises, pulses and electrostatic charges can be obtained. Characteristics of the capacitor is so stable to temperature changes that effects of the present invention are greatly improved from the point of practical applications.

Claims: A semiconductor-type laminated ceramic capacitor (4) with a grain boundary-insulated structure comprising a plurality of inner electrodes (2a) on the surfaces of grain boundary insulated, semiconductor type ceramic sheets, the terminal of each of said inner electrodes (2a) being extended to one edge of the corresponding ceramic sheet and said ceramic sheets (1) being laminated so that said terminals of said inner electrodes (2a) extend to the corresponding opposite edges of said ceramic sheets alternatly; outer electrodes (3) are electrically connected to the terminals of said inner electrodes (2a) at opposite edges of each of said grain boundary insulated, semiconductor type ceramic sheets (1), said ceramic sheets (1) comprise a material of the composition of Sr (1-x) Ca x TiO₃; comprising the following features : a) Mn and Si are also included in said ceramic material to the amount of their combined relative molar content, converted into MnO₂ and SiO₂ respectively, in a range of 0.2 to 5.0 % ; b) said composition of Sr (1-x) Ca x TiO₃ contains excess Ti to bring the final molecular ratio of Ti to Sr (1-x) Ca x in the range of 0.95≦Sr (1-x) Ca x Ti<1.00 (where x is in the range of 0.001≦x ≦0.2); c) at least one or more of the compounds selected from Nb₂O₅, Ta₂O₅, V₂O₅, W₂O₅, Dy₂O₃, Nd₂O₃, Y₂O₃, La₂O₃ or CeO₂ are added to said ceramic material to bring their relative molar content in the range from 0.05 to 2.0 %; and d) at least one compound selected from Na₂SiO₃, Li₂SiO₃, NaAlO₂ and LiAlO₂ to obtain a relative molar content of 0.05 - 2.0% for the silicates and 0.05 - 4.0 for the aluminates, respectively. A capacitor according to claim 1, characterized in that said inner electrode (2a) is made of at least one or more kinds of metals selected from Au, Pt, Rh, Pd or Ni, or alloys or compositions thereof. A capacitor according to claim 1 or claim 2, characterized in that said outer electrode (3) is made of at least one or more kinds of metals selected from Pd, Ag, Ni, Cu or Zn, or alloys or compositions thereof. A capacitor according to one of claims 1 to 3, characterized in that Na₂SiO₃ and Al₂O₃ are further included in said ceramic material to make their relative molar contents in the range of 0.05 - 2.0 % and 0.05 - 2.0 %, respectively. A capacitor according to one of claims 1 to 3, characterized in that Li₂SiO₃ and Al₂O₃ are further included in said ceramic material to make their relative molar contents in the range of 0.05 - 2.0 % and 0.05 - 2.0 %, respectively. A method for manufacturing a semiconductor-type laminated ceramic capacitor (4) with a grain boundary-insulated structure comprising the steps of calcinating starting material of mixed powder in air or in a nitrogen atmosphere after grinding, mixing and drying said mixed powder; forming raw sheets (1) by dispersing said calcinated powder in a solvent with organic binder and molding said dispersed powder, said calcinated powder being re-ground before dispersing and after calcinating; printing a pattern (2) of an inner electrode paste on the surfaces of said raw sheets (1), the terminals of said inner electrodes (2a) being extended to each of the corresponding opposite edges of said raw sheets (1) alternatly wherein patterns of inner electrodes (2a) are not printed on uppermost and lowermost parts of raw sheets (1a); forming a lamainated body by laminating and compressing said raw sheets (1) on which surfaces patterns of said inner electrode paste have been printed; calcinating said laminated body in air; sintering said laminated raw sheets in a reducing atmosphere or in a nitrogren atmosphere after calcination; re-oxidizing in air after sintering; and covering said edges of said sintered ceramic sheets with an outer electrode paste and baking after re-oxidation, the terminals of said inner electrodes being exposed to said edges, wherein: said staring material of mixed powder comprises a material of the composition of a) Sr (1-x) Ca x TiO₃ containing excess Ti to make a final molecular ratio of Ti to Sr (1-x) Ca x in the range of 0.95 ≦ Sr (1-x) Ca x /Ti<1.00 (where x is in the range of 0.001 ≦ x ≦ 0.2); b) at least one or more kinds of the compounds selected from Nb₂O₅, Ta₂O₅, V₂O₅, W₂O₅, Dy₂O₃, Nd₂O₃, Y₂O₃, La₂O₃, or CeO₂ are added to said starting material to make their relative molar content in the range from 0.05 to 2.0 %; c) Mn and Si in an amount of their combined relative molar content, converting into MnO₂ and SiO₂ respectively, in the range of 0.2 to 5.0 %; and d) at least one compound selected from the group consisting of Na₂SiO₃, Li₂SiO₃, NaAlO₂, and LiAlO₂ to obtain a relative molar content of 0.05 - 2,0 for the silicates and 0.05 - 4,0 for the aluminates, respectively. A method according to claim 6, wherein said inner electrode (2a) is made of at least one or more kinds of metals selected from Au, Pt, Rh, Pd or Ni, or alloys or compositions thereof. A method according to claim 6 or 7, wherein said outer electrode (3) is made of at least one or more kinds of metals selected from Pd, Ag, Ni, Cu or Zn, or alloys or compositions thereof. A method according to one of claims 6 to 8, wherein Na₂SiO₃ and Al₂O₃ are further included in said ceramic material to make their relative molar contents in the range of 0.05 - 2.0 % and 0.05 - 2.0%, respectively. A method according to one of claims 6 to 9, wherein Li₂SiO₃ and Al₂O₃ are further included in said ceramic material to make their relative molar contents in the range of 0.05 - 2.0 % and 0.05 - 2.0 %, respectively. A method according to one of claims 6 to 10, wherein Al₂O₃ is further included in said ceramic meterial to make a relative molar content in the range of 0.01 -2.0 %.

Inventors: OKAMOTO KAORI - JYOHOKU-CHO -C, UENO IWAO, WAKAHATA YASUO - MYOKENZAKA -C, OKAMOTO, KAORI 4-14, JYOHOKU-CHO 2-CHOME, UENO, IWAO, WAKAHATA, YASUO 7-11, MYOKENZAKA 6-CHOME
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: H01G4
IPC codes: H01G4, H01C7