SEMICONDUCTIVE CERAMIC COMPOSITION

Abstract: A semiconductive ceramic composition for a boundary insulation type semiconductive ceramic capacitor capable of causing the capacitor to exhibit increased dielectric constant, satisfactory frequency and temperature characteristics, and decreased dielectric loss. The composition includes a SrTiO₃ base component and a minor component consisting of CaTiO₃, Y and Nb. The composition may further contain Mn and SiO₂, which contribute to an increase in insulation resistance of the capacitor and enlargement of an appropriate range of a (SrO + CaO)/TiO₂, respectively. Also, a composition includes a SrTiO₃ base component and a minor component consisting of CaTiO₃, BaTiO₃, Y and Nb.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a semiconductive ceramic composition for a semiconductive ceramic capacitor, and more particularly to a semiconductive ceramic composition suitable for use for manufacture of a boundary insulation type semiconductive ceramic capacitor. Description of the Prior Art A semiconductive ceramic capacitor serving as a passive electronic circuit element is generally classified into two types or a boundary-layer type and a surface-layer type. The boundary-layer type semiconductive ceramic capacitor includes a boundary insulation type one and the surface-layer type semiconductive ceramic capacitor includes a barrier-layer type one and a reduction reoxidation type one. However, it was found that the conventional semiconductive capacitors have the following disadvantages. The boundary insulation type semiconductive capacitor is relatively decreased in capacity and dielectric strength. The barrier-layer type one is deteriorated in frequency characteristics, increased in dielectric loss (tan δ) and decreased in dielectric strength. The reduction reoxidation type one is deteriorated in frequency characteristics and increased in temperature characteristics and dielectric loss. Such disadvantages are caused due to the fact that a semiconductive ceramic composition which is capable of providing a semiconductive ceramic capacitor free of the above-noted disadvantages is not still developed. SUMMARY OF THE INVENTION The present invention has been made in view of the foregoing disadvantages of the prior art. Accordingly, it is an object of the present invention to provide a semiconductive ceramic composition for a semiconductive ceramic capacitor which is capable of causing the capacitor to exhibit not only increased dielectric constant and satisfactory frequency characteristics and temperature characteristics but decreased dielectric loss. It is another object of the present invention to provide a semiconductive ceramic composition for a semiconductive ceramic capacitor which is capable of improving insulation resistance of the capacitor. It is a further object of the present invention to provide a semiconductive ceramic composition for a semiconductive ceramic capacitor which is capable of being readily manufactured while exhibiting the above-described advantages. It is still another object of the present invention to provide a semiconductive ceramic composition for a boundary insulation type semiconductive ceramic capacitor which is capable of causing the capacitor to exhibit not only increased dielectric constant and satisfactory frequency characteristics and temperature characteristics but decreased dielectric loss. It is yet another object of the present invention to provide a semiconductive ceramic composition for a boundary insulation type semiconductive ceramic capacitor which is capable of improving insulation resistance of the capacitor. It is still a further object of the present invention to provide a semiconductive ceramic composition for a boundary insulation type semiconductive ceramic capacitor which is capable of being readily manufactured while exhibiting the above-described advantages. It is yet a further object of the present invention to provide a semiconductive ceramic composition for a boundary insulation type semiconductive ceramic capacitor which is capable of improving insulation resistance and D.C. breakdown voltage of the capacitor. In accordance with the present invention, a semiconductive ceramic composition for a semiconductive ceramic capacitor is provided. The composition comprises a SrTiO₃ base component and a minor component comprising CaTiO₃, Y and Nb. CaTiO₃ is present at 0.1 to 2.0 mol% based on the composition, and Y and Nb are present at 0.1 to 0.4 mol% on Y₂O₃ and Nb₂O₅ bases based on the composition, respectively. Also, in accordance with the present invention, a semiconductive ceramic composition for a semiconductive ceramic capacitor is provided which comprises a SrTiO₃ base component, and a minor component comprising CaTiO₃, BaTiO₃, Y and Nb. SrTiO₃ which constitutes the base component comprises SrO and TiO₂ in a molar ratio of SrO to TiO₂ of between 0.970 and 1.000 SrO to 1.000 TiO₂. CaTiO₃ which constitutes a part of the minor component is present in amounts respectively corresponding to 0.1 to 20.0, 1.0 to 20.0, 5.0 to 20.0 and 10.0 to 20.0 mol% in relation to the molar ratio of SrO to TiO₂ in SrTiO₃ of 1.000, 0.990, 0.980 and 0.970 which are representative molar ratios of SrO. BaTiO₃ which also constitutes a part of the minor component is present in an amount of 0.05 to 3.0 mol% based on the composition. Y and Nb which constitute the remaining of the minor component are present in amounts of 0.1 to 1.0 mol% and 0.1 to 0.4 mol% on Y₂O₃ and Nb₂O₅ bases based on the composition, respectively. Japanese patent documents JP-A-56 54 024, JP-A-56 54 025 and JP-A-56 54 026 list a range of ceramic compositions but never those claimed in the present invention. In particular, it is never suggested that the specific quantities of the present invention should be used, nor that both Nb and Y in combination produce the desired, advantageous result of increased dielectric constant and D.C. breakdown voltage shown in the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to a semiconductive ceramic composition for a semiconductive ceramic capacitor, particularly, a boundary insulation type semiconductive ceramic capacitor which comprises a SrTiO₃ base component and a minor component comprising CaTiO₃, Y and Nb. The term "minor component" used herein indicates an essential component less in quantity than the base component which is also essential. CaTiO₃ which constitutes a part of the minor component is present in an amount of 0.1 to 2.0 mol% based on the composition, and Y and Nb which constitute the remaining of the minor component are present in amounts of 0.1 to 0.4 mol% on Y₂O₃ and Nb₂O₅ bases based on the composition, respectively. It was found that use of the composition of such construction for a semiconductive ceramic capacitor, particularly, a boundary insulation type one causes dielectric constant and D.C. breakdown voltage of the capacitor to be significantly increased, resulting in the capacitor being substantially small-sized. For example, such advantages of the present invention permit a diameter of a capacitor element having a capacity of 10⁵PF to be decreased from 10mm to 8mm. Also, use of the composition significantly improves frequency characteristics, temperature characteristics and dielectric loss of the capacitor. The composition may contain Mn, which may be present in an amount of 0.02 to 0.2 mol% on a MnO base based on the composition. Addition of Mn in such an amount increases insulation resistance of the capacitor. Also, the composition may contain SiO₂ in an amount of 0.01 to 0.1 mol%. Such addition of SiO₂ substantially enlarges an appropriate range of a (SrO + CaO)/TiO₂ ratio to lead to facilitation of manufacturing of the composition and mass production of the composition, resulting in manufacturing costs of the composition and capacitor being effectively decreased. The present invention is also directed to a semiconductive ceramic composition for a semiconductive ceramic capacitor, particularly, a boundary insulation type semiconductive ceramic capacitor which comprises a SrTiO₃ base component, and a minor component comprising CaTiO₃, BaTiO₃, Y and Nb. SrTiO₃ which constitutes the base component comprises SrO and TiO₂ in a molar ratio of SrO to TiO₂ of 0.970 to 1.000. CaTiO₃ which constitutes a part of the minor component may be present in amounts respectively corresponding to 0.1 to 20.0, 1.0 to 20.0, 5.0 to 20.0 and 10.0 to 20.0 mol% in relation to the molar ratio of SrO to TiO₂ in SrTiO₃ of 1.000, 0.990, 0.980 and 0.970 which are representative molar ratios of SrO. BaTiO₃ which also constitutes a part of the minor component is present in an amount of 0.05 to 3.0 mol% based on the composition. Y and Nb which constitute the remaining of the minor component are present in amounts of 0.1 to 1.0 mol% and 0.1 to 0.4 mol% on Y₂O₃ and Nb₂O₅ bases based on the composition, respectively. Use of the so-formulated composition for a semiconductive ceramic capacitor, particularly, a boundary layer type one likewise causes the capacitor to be not only increased in dielectric constant capacity and decreased in dielectric loss but increased in insulation resistance and D.C. breakdown voltage, so that the capacitor likewise may be small-sized. Also, the composition permits an appropriate range of a SrO/TiO₂ ratio to be enlarged to a degree sufficient to lead to facilitation of manufacturing of the composition and mass production of the composition, resulting in manufacturing costs of the composition and capacitor being lowered. The composition may include Mn and SiO₂. Mn may be present at 0.02 to 0.20 mol% on a MnO base based on the composition and SiO₂ may be present at 0.01 to 0.6 mol% based on the composition. Such addition of both MnO and SiO₂ to the composition further improves insulation resistance and D.C. breakdown voltage of the capacitor. Each of the so-formulated semiconductive ceramic compositions of the present invention may be compacted into a semiconductive ceramic element and sintered to cause Bi deposited on the element to be present at a grain boundary thereof. The invention will be understood more readily with reference to the following examples, however, these examples are intended to illustrate the invention and are not to be construed to limit the scope of the invention. Example 1 SrCO₃, CaCO₃, TiO₂, MnCO₃, and SiO₂ each were used as a starting material. Also, Y₂O₃ and Nb₂O₅ were uses as an additive for semiconductivity. The materials were weighed so that each composition may be obtained which has a composition ratio as shown in Tables 1 and 2. The materials were subjected to wet blending in a ball mill of synthetic resin in which water and pebbles were placed for 20 hours while stirring, to thereby prepare each composition mixture. Then, the so-obtained mixture was dewatered and dried, and provisionally burned at 1200°C for 2 hours, wherein rise and fall in temperature of the mixture were carried out at a rate of 200°C/hr. This resulted in a chemical reaction of the mixture. Subsequently, the mixture was powdered and blended for 20 hours by means of a ball mill in which water and pebbles were placed and then dewatered and dried, to which polyvinyl alcohol (PVA) in an amount of 2 wt% was added as an organic binder to carry out granulation and grading to prepare granulated powder of the composition. The powder was then formed into a disc-like compact or semiconductive ceramic element of 10mm in diameter and 0.5mm in thickness at compacting pressure of about 294 MPa (3 tons/cm²). The element was heat-treated at 800°C for 1 hour to remove the binder therefrom and then subjected to burning at about 1450°C for about 2 hours in a stream of a reducing atmosphere (H₂ + N₂), to thereby be provided with a semiconductive property. This resulted in a semiconductive ceramic element of 8.5mm in diameter and 0.4mm in thickness being obtained. Then, a Bi₂O₃-CuO system frit paste was applied in an amount of 3mg onto both surfaces of the so-obtained semiconductive ceramic element by printing and then subjected to a heat treatment at 1150°C for 2 hours, resulting in the semiconductive ceramic element being formed at a grain boundary thereof with an insulation layer. Thereafter, a silver paste was applied onto both surfaces of the element by screen printing, which was then baked at about 800°C to obtain a boundary insulation type semiconductive ceramic capacitor specimen. A test was then carried out to consider electrical characteristics of each of the so-prepared capacitor specimens. The results were as shown in Tables 1 and 2, wherein dielectric constant and dielectric loss were measured at frequency of 1kHz and voltage of 1V, and insulation resistance was measured at a room temperature of 20°C while applying D.C. voltage of 25V to the specimen. As can be seen from Tables 1 and 2, specimens included in the scope of the present invention each exhibited dielectric constant increased to about 70,000 or more, dielectric loss decreased to a degree as low as 0.30 to 0.71%, and high insulation resistance. More particularly, Table 1 indicates that addition of only any one of Y (Y₂O₃) and Nb (Nb₂O₅) failed to provide a boundary insulation type semiconductive ceramic capacitor which is increased in dielectric constant (Specimen Nos. 1, 2, 6, 20 and 26). Also, addition of both Y₂O₃ and Nb₂O₅ each in an amount below 0.1 mol% failed to significantly increase dielectric constant and D.C. breakdown voltage of the capacitor (Specimen Nos. 3, 4, 5, 21 and 40). Y₂O₃ in an amount above 0.4 mol% caused the capacitor to exhibit decreased dielectric constant (Specimen Nos. 40, 41 and 42). Nb₂O₅ above 0.4 mol% disadvantageously decreased D.C. breakdown voltage of the capacitor (Specimen Nos. 5, 25 and 42). When the composition contained CaTiO₃ in an amount below 0.1 mol%, insulation resistance of the boundary insulation type semiconductive ceramic capacitor was not significantly increased; whereas the amount of CaTiO₃ exceeding 2.0 mol% led to a decrease in dielectric constant (Specimen Nos. 7, 8, 12, 14, 15, 19, 27, 28, 32, 34, 35 and 39). Further, Table 1 indicates that addition of MnO in an amount below 0.02 mol% failed in a significant increase in insulation resistance of the boundary insulation type semiconductive ceramic capacitor, whereas the addition above 0.2 mol% caused an increase in dielectric loss and a decrease in dielectric constant (Specimen Nos. 23, 43 and 48). Table 2 indicates that addition of SiO₂ in an amount below 0.01 mol% narrowed an appropriate range of a (SrO + CaO)/TiO₂ ratio (0.002) (Specimen Nos. 50 and 51), whereas the amount of SiO₂ above 0.1 mol% caused a decrease in dielectric constant of the capacitor (Specimen Nos. 65 and 66). Addition of SiO₂ in an amount between 0.01 mol% and 0.1 mol% significantly enlarged a (SrO + CaO)/TiO₂ ratio (0.004, 0.006) (Specimen Nos. 54, 55, 58, 59, 62 and 63). Example 2 Example 1 was substantially repeated except that SrCO₃, CaCO₃ BaCO₃, TiO₂ MnCO₃ and SiO₂ in such amounts as shown in Tables 3 and 4 were used as a starting material. The results were as shown in Tables 3 and 4. As can be seen from Tables 3 and 4, specimens included in the scope of the present invention each exhibited dielectric constant increased to about 50,000 or more, dielectric loss decreased to 1.0% or less, and high insulation resistance. More particularly, Table 3 indicates that when the amount of CaTiO₃ which constitutes a part of the minor component was below 0.05 mol%, insulation resistance and D.C. breakdown voltage of the boundary insulation type semiconductive ceramic capacitor were disadvantageously decreased (Specimen Nos. 4 and 5), whereas addition of CaTiO₃ in an amount between 0.10 mol% and 20.0 mol% significantly enlarged an appropriate range of a SrO/TiO₂ ratio (0.030) (Specimen Nos. 6, 7, 8, 11, 12, 13, 15, 16, 18 and 19). However, a combination of CaTiO₃ below 0.10 mol% and a SrO/TiO₂ molar ratio in SrTiO₃ of 0.990, that of CaTiO₃ below 1.0 mol% and the ratio of 0.980, and that of CaTiO₃ below 5.0 mol% and the ratio of 0.970 each decreased insulation resistance and D.C. breakdown voltage of the capacitor (Specimen Nos. 10, 14 and 17). The amount of CaTiO₃ exceeding 20.0 mol% led to a decrease in dielectric constant and an increase in dielectric loss (Specimen Nos. 9 and 20). Addition of BaTiO₃ in an amount below 0.05 mol% decreased insulation resistance and D.C. breakdown voltage of the capacitor (Specimen No. 23), whereas the addition above 3.0 mol% caused a decrease in dielectric constant and an increase in dielectric loss (Specimen No. 28). Table 3 also indicates that addition of only any one of Y (Y₂O₃) and Nb (Nb₂O₅) failed to provide a boundary insulation type semiconductive ceramic capacitor which is increased in dielectric constant (Specimen Nos. 29 and 30). Also, addition of both Y₂O₃ and Nb₂O₅ each in an amount below 0.1 mol% failed to significantly increase dielectric constant and D.C. breakdown voltage of the capacitor (Specimen Nos. 30, 31, 32 and 40). Y₂O₃ in an amount above 1.0 mol% caused the capacitor to exhibit decreased dielectric constant (Specimen Nos. 44 and 45). Nb₂O₅ above 0.4 mol% disadvantageously decreased D.C. breakdown voltage of the capacitor (Specimen Nos. 35 and 43). Table 4 shows results of addition of Mn (MnO) and SiO₂, wherein Specimen No. 46 indicates a reference. Addition of only any one of Mn and SiO₂ failed in a significant increase in insulation resistance of the capacitor (Specimen Nos. 47 to 53). Addition both Mn and SiO₂ increased dielectric loss of the capacitor when the amount of MnO exceeded 0.2 mol% (Specimen Nos. 61 to 63). SiO₂ above 0.6 mol% caused a decrease in dielectric constant (Specimen Nos. 56, 60 and 63). While the invention has been described with a certain degree of particularity with reference to the examples, obvious modifications and variations are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims: A semiconductive ceramic composition comprising: a base component comprising SrTiO₃; and a minor component comprising CaTiO₃, Y and Nb; said CaTiO₃ being present in an amount of 0.1 to 2.0 mol% and said Y and Nb being present in amounts of 0.1 to 0.4 mol% on Y₂O₃ and Nb₂O₅ bases, respectively. A semiconductive ceramic composition as defined in Claim 1 further comprising Mn, said Mn being present in an amount of 0.02 to 0.2 mol% on a MnO base based on said composition. A semiconductive ceramic composition as defined in Claim 2 further comprising SiO₂, said SiO₂ being present in an amount of 0.01 to 0.1 mol% based on said composition. A semiconductor ceramic composition as defined in any one of Claims 1 to 3, wherein said composition is compacted to a semiconductive ceramic element and sintered to cause Bi deposited on the element to be present at a grain boundary thereof. A semiconductive ceramic composition comprising: a base component comprising SrTiO₃; and a minor component comprising CaTiO₃, BaTiO₃, Y and Nb; said SrTiO₃ comprising SrO and TiO₂ in a molar ratio of said SrO to said TiO₂ of 0.970 to 1.000; said CaTiO₃ being present in amounts respectively corresponding to 0.1 to 20.0, 1.0 to 20.0, 5.0 to 20.0 and 10.0 to 20.0 mol % in relation to the molar ratio of said SrO to said TiO₂ in said SrTiO₃ of 1.000, 0.990, 0.980 and 0.970 which are representative molar ratios of said SrO; said BaTiO₃ being present in an amount of 0.05 to 3.0 mol % and said Y and Nb being present in amounts of 0.1 to 1.0 mol % and 0.1 to 0.4 mol % on Y₂O₃ and Nb₂O₅ bases, respectively. A semiconductive ceramic composition as defined in Claim 5 further comprising Mn and SiO₂, said Mn being present in an amount of 0.02 to 0.20 mol% on a MnO base based on said composition and said SiO₂ being present in an amount of 0.01 to 0.6 mol% based on said composition. A semiconductive ceramic composition as defined in Claim 5 or 6, wherein said composition is compacted to a semiconductive ceramic element and sintered to cause Bi deposited on the element to be present at a grain boundary thereof.

Inventors: ITAGAKI, SHUICHI, ONO, SHUICHI, YAHAGI, MASAHIRO
Applicants: TDK CORPORATION
Application year: 1987
Main classification: C04B35
IPC codes: H01G4, C04B35