Composition for Dielectric Porcelain, Dielectric Porcelain and Capacitor by Use of Said Composition

1. Field of the Invention

This invention relates to a composition for dielectric porcelain, and a dielectric porcelain and a capacitor by use of the composition.

2. Related Background Art

Heretofore, there have been employed as the dielectric member to be used for capacitors, etc. porcelains comprising BaTiO.sub.3, BaSnO.sub.3, CaTiO.sub.3 or a solid solution having other components added to these double oxides as the main component, or these materials modified with various additives. However, these porcelains are required to be fired at a high temperature of 1300.degree. to 1400.degree. C. and therefore, for example, when applied for a laminated capacitor, there ensues the problem that no silver type electrode can be used as the internal electrode, and it has been sought to develop a porcelain which can be fired at lower temperatures.

Also, in the dielectric porcelain of the prior art, if the dielectric constant at around normal temperature is made greater, the temperature dependent rate of change in dielectric constant becomes also greater; on the contrary, if the temperature dependent rate of change in dielectric constant at around normal temperature is made smaller, the dielectric constant also becomes smaller. Thus, it has been difficult to obtain a porcelain with a great dielectric constant and a small temperature dependent rate of change in dielectric constant.

Further, in the case when the porcelain has high dielectric property, the Curie point becomes around normal temperature due to modification with an additive, whereby mechanical strength will be lowered due to phase transition at this temperature, and there is also involved the problem that it can be used as a functional element with difficulty.

An object of the present invention is to provide a composition for dielectric porcelain which can give a dielectric porcelain by firing at a relatively lower temperature, and a dielectric porcelain and a capacitor by use of the composition.

Another object of the present invention is to provide a composition for dielectric porcelain which can give a dielectric porcelain which is great in dielectric constant at around room temperature and yet relatively smaller in dielectric loss and temperature dependent rate of change in dielectric constant, and a dielectric porcelain and a capacitor by use of the composition.

Still another object of the present invention is to provide a composition for dielectric porcelain caspable of constituting a dielectric porcelain which is free from such a problem of lowering in mechanical strength due to phase transition at around normal temperature and can be used advantageously as a functional element, and a dielectric porcelain and a capacitor by use of the composition.

Still another object of the present invention is to provide a composition for dielectric porcelain comprising Bi.sub.2 O.sub.3, TiO.sub.2 and RuO.sub.2.

Still another object of the present invention is to provide a dielectric porcelain prepared by use of the above composition for dielectric porcelain.

Still another object of the present invention is to provide a capacitor having a dielectric porcelain prepared by use of the above composition for dielectric porcelain and electrodes.

FIG. 1 is a schematic perspective view of a capacitor.

The composition for dielectric porcelain of the present invention consists of Bi.sub.2 O.sub.3, TiO.sub.2 and RuO.sub.2, or comprises the basic components of Bi.sub.2 O.sub.3, TiO.sub.2 and RuO.sub.2. As the mode of the composition, there may be included, for example, powdery or pressed powdery mixture of the above components, and powdery or pressed powdery mixture containing a compound oxide (including solid solution) obtained by subjecting a part or all of the above components to firing, or molded products obtained by subjecting these mixtures to the steps of the film preparation method such as CVD method or the single crystal synthetic method, etc.

Also, the dielectric porcelain of the present invention is prepared by use of the composition for dielectric porcelain as described above.

Further, the capacitor of the present invention is prepared by use of the composition for dielectric porcelain as described above.

In the present invention, the composition of particularly high utilization value as the composition for dielectric porcelain may include a composition consisting of Bi.sub.2 O.sub.3, TiO.sub.2 and RuO.sub.2 (hereinafter referred to as composition "A"), a composition comprising Bi.sub.2 O.sub.3, TiO.sub.2 and RuO.sub.2 as the basic components and further containing at least one of the respective oxides of Ce, La, Pr, Sm, Sc, Yb, Ho, Tb, Er, Pb, Zr and Hf, for example, a composition consisting of the above three components of Bi.sub.2 O.sub.3, TiO.sub.2 and RuO.sub.2 in which a part of Bi.sub.2 O.sub.3 is substituted with at least one of respective oxides of Ce, La, Pr, Sm, Sc, Yb, Ho, Tb and Er [the term "substituted" means that, in the case of, for example, the above powdery or pressed powdery mixture, the substituting oxide is used in place of Bi.sub.2 O.sub.3, TiO.sub.2 or RuO.sub.2 which are components to be substituted, or in the case of the above dielectric porcelain, it is used as the solid solution component (substituent component) for Bi.sub.2 O.sub.3, TiO.sub.2 or RuO.sub.2 which are components to be substituted, or used in place of the components to be substituted as the component of a compound comprising two or more kinds of the components to be substituted] (hereinafter referred to as composition "B"), a composition consisting of the above three components in which a part of TiO.sub.2 and/or a part of RuO.sub.2 are substituted with at least one of respective oxides of Pb, Zr, Tb and Hf (hereinafter referred to as composition "C"), and a composition consisting of the above three components, in which a part of Bi.sub.2 O.sub.3 is substituted with at least one of respective oxides Ce, La, Pr, Sm, Sc, Yb, Ho, Tb and Er, and a part of TiO.sub.2 and/or a part of RuO.sub.2 are substituted with at least one of respective oxides of Pb, Zr, Tb and Hf (hereinafter referred to as composition "D").

In the above composition, in the case where the compound to be substituted is Bi.sub.2 O.sub.3, the ratio of substitution should preferably be 37 mole % or less of the compound to be substituted, while in the case where the compound to be substituted is TiO.sub.2 or RuO.sub.2, it should preferably be 30 mole % or less of the compound to be substituted.

Further, a more preferable compositional ratio of the above composition A comprises 25.0 mole % to 43.0 mole % of Bi.sub.2 O.sub.3, 37.0 mole % to 66.0 mole % of TiO.sub.2 and 5.0 mole % to 20.0 mole % of RuO.sub.2.

Also, in the composition B, the composition C and the composition D, more preferable compositions are those in which the ratios of the number of moles of Bi.sub.2 O.sub.3 added with the number of moles of the substituting oxides, the number of moles of TiO.sub.2 added with the number of moles of the substituting oxides and the number of moles of RuO.sub.2 added with the number of moles of the substituting oxides, when adding the number of moles of the substituting oxides to the number of moles of Bi.sub.2 O.sub.3, TiO.sub.2 and RuO.sub.2 which are components to be substituted, are 25.0 mole % to 43.0 mole %, 37.0 mole % to 66.0 mole % and 5.0 mole % to 20.0 mole %, respectively, relative to the sum of these numbers of moles.

In the above compositions A, B, C and D, the ratios of the number of moles of Bi.sub.2 O.sub.3 or the number of moles of Bi.sub.2 O.sub.3 added with the number of moles of the substituting oxides, number of the moles of TiO.sub.2 added with the number of moles of the substituting oxides and the number of moles of RuO.sub.2 or the number of moles of RuO.sub.2 added with the number of moles of the substituting oxides are particularly preferred within the numerical ranges as specified above, respectively, for the following reasons.

If the number of moles of Bi.sub.2 O.sub.3 or the number of moles of Bi.sub.2 O.sub.3 added with the number of moles of the substituting oxides are less than 25.0 mole %, sintering characteristics become worsened to be lowered in mechanical strength as the firing temperature is elevated.

If the number of moles of Bi.sub.2 O.sub.3 or the number of moles of Bi.sub.2 O.sub.3 added with the number of moles of the substituting oxides exceed 43.0 mole %, dielectric constant will be lowered.

If the number of moles of TiO.sub.2 or the number of moles of TiO.sub.2 added with the number of moles of the substituting oxides are less than 37.0 mole %, dielectric constant will be lowered and the temperature dependent rate of change in dielectric constant becomes greater.

If the number of moles of TiO.sub.2 or the number of moles of TiO.sub.2 added with the number of moles of the substituting oxides exceed 66.0 mole %, the firing temperature is elevated and also sintering characteristics becomes worsened.

If the number of moles of RuO.sub.2 or the number of moles of RuO.sub.2 added with the number of moles of the substituting oxides are less than 5.0 mole %, dielectric constant will be lowered.

If the number of moles of RuO.sub.2 or the number of moles of RuO.sub.2 added with the number of moles of the substituting oxides exceed 20.0 mole %, dielectric strength will be lowered.

The ratio of the number of moles of the substituting oxides for BiO.sub.2 O.sub.3 and the ratio of the number of moles of the substituting oxides for TiO.sub.2 and/or RuO.sub.2 are preferably within the numerical ranges as specified above for the following reasons.

If the ratio of the moles of the substituting oxides for Bi.sub.2 O.sub.3 exceed 37 mole %, the firing temperature will be elevated, while if the ratio of the moles of the substituting oxides for TiO.sub.2 and/or RuO.sub.2 exceed 30 mole %, dielectric constant will be lowered.

The dielectric porcelain and the capacitor in the present invention are prepared by use of the composition for dielectric porcelain as described above.

The respective starting materials were weighed and mixed in a wet system so that the dielectric porcelains with the compositional ratios shown in Table 1 could be obtained. After drying of the mixture, the reaction temperature was determined according to the differential thermal analysis. After the above mixture was sintered at the temperature higher by 50.degree. C. than the reaction temperature for 2 hours, it was groun in a wet system in a ball mill. The ground product was mixed with polyvinyl alcohol as the binder, pressure molded into discs of 11 mm in diameter and 5 mm in thickness, which were further sintered at the firing temperature indicated in Table 1 higher by 100.degree. C. than the prefiring temperature for 5 hours. The sintered products were attached with silver as electrodes on both surfaces by the vacuum vapor deposition method to form capacitors as shown in FIG. 1, and the electrical characteristics of respective capacitors were measured. FIG. 1 shows a schematic perspective view of a capacitor, in which 11 show electrodes and 12 is a dielectric porcelain sample. First, dielectric constant and dielectric loss (tan .delta.) of the dielectric porcelain samples were measured at a frequency of 1 MHz and 25.degree. C.

Also, the temperature dependent rate of change in dielectric constant at 50.degree. C., 80.degree. C. at a frequency of 1 MHz was also measured for each sample with the value at 25.degree. C. as the standard. The results are shown in Table 1.

As is apparent from Table 1, the samples No. 3 to No. 21 which are dielectric porcelains according to the present invention can be varied largely in dielectric constant in practical range depending on the composition, and the firing temperature was also lower as compared with the porcelains of the prior art. Also, it was found that the dielectric loss characteristic (tan .delta.) could be improved, with the temperature dependent rate of change of dielectric constant being small.

As can be clearly seen from the above detailed description, particularly Examples, according to the composition for dielectric porcelain of the present invention, dielectric porcelains can be obtained by firing at a relatively lower temperature. Therefore, for example, when applied for a laminated ceramic capacitor, silver type electrodes can be used as the internal electrode, which can contribute much to lowering of cost. Besides, since the dielectric constant at around normal temperature at which dielectric members are used is great, it can contribute to minituarization of capacitors to be of great utilization value in industry. At the same time, due to small temperature dependent rate of change in dielectric constant at around normal temperature, stable performance can be exhibited with high reliability, and yet there is no problem of lowering in mechanical strength due to phase transition at around normal temperature, and therefore it can be utilized effectively as a functional element.