March 10, 1970 s mo u FUJUNARA ETAL 3,499,772
CERAMIC DIELECTRICS Filed Dec. 29, 1967 2 Sheets-Sheet 1 Bi O -2TiO CqTiOa INVENTOIS SHINOBU FUJIWARA K IYOTO KAWAKAMI ATTORNEYS March 10, 1970 SHINQBU FUJIWARA ETAL 3,499,772
CERAMIC DIELECTRICS Filed Dec. 29, 1967 a 2 Sheets-Sheet 2 SrTi O 3 Bi O -2TiO K 20 CqTiO -|2oox 10/ aizogzrioa a0 60 40 2o cQ i KIYOTO KAWAKAMI FIG.3
ATTORNEYS United States Patent C) US. Cl. 10639 1 Claim ABSTRACT OF THE DISCLOSURE Ceramic dielectric materials are provided which have high dielectric constants, low temperature coeflicients and high Q values. The materials consist essentially of a sintered mixture of 6 to 80% by weight of SrTiO to 60% by weight of CaTiO and 15 to 34% by weight of Bi O -2TiO This is a continuation-in-part application of copending application, Ser. No. 359,131, filed Apr. 13, 1964, now abandoned.
This invention relates to improvements in ceramic dielectrics.
An object of the invention is to provide ceramic dielectrics which have relatively high dielectric constants, lower temperature coefficients and high Q-values, and which show excellent characteristics when they are applied in the various communication equipment.
As a ferroelectric material, barium titanate (BaTiOg) has hitherto been well known to be of practical use over a wide electrical field, but its application to the electrical devices has greatly been limited by the existence of the maximum of its dielectric constant at about 120 C., and by the existence of the transition point at about 0 C.
Since BaTiO has the property of easily forming a solid solution with isomorphic compounds, the addition of calcium titanate (OaTiO magnesium titanate (MgTi0 and other titanates to BaTiO has been tried to obtain a small temperature coefiicient. Although the temperature dependence of the dielectric constant of BaTiO is improved by the addition of these titanates, the dielectric constant of the body is much lowered resulting in the loss of its practical use.
Other high dielectric materials have also had the similar shortcomings of large temperature coefiicients and low Q-values.
The object of the present invention is to provide ceramic dielectrics which are free of the above mentioned shortcomings of other ceramic dielectrics, and which are composed mainly of strontium titanate, calcium titanate, and a mixture of bismuth oxide and titanium oxide, (S1TiO CaTiO and Bi O -2TiO to obtain a high dielectric constant and a small temperature coefficient.
In this invention, in order to obtain the most successful results, the composition of the above three ingredients, according to the detailed experimental discussion, should be limited to the ranges given below:
Percent by wt.
When the content of strontium titanate SrTiO exceeds by weight, the temperature coefiicient of dielectric constant of the body becomes too large for practical use and firing temperature is required to be higher than 1380 C. When the content of strontium titanate SrTiO is less than 6% by weight, the dielectric constant of the body is lowered to give predominance to the characteristics of calcium titanate CaTiO itself, and the body could not be used where high dielectric constant is required. When the content of calcium titanate CaTiO is less than 5% by weight, the Q-value of the body is lowered and its dielectric constant depends greatly on temperature. When the content of Bi O -2TiO exceeds 34% by weight, both the dielectric constant and the Q-value .are lowered and 'the body has high porosity and vitreous surface. When the content of Bi O -2TiO is less than 15% by weight, dielectric constant is low and firing temperature should be as high as 1360 C.
The relations between the three ingredients of the ceramic dielectrics of the invention, i.e. strontium titanate, calcium titanate and mixture of bismuth oxide and titanium oxide (SrTiO CaTiO and Bi O -2TiO and the dielectric constants, the temperature coefficients, and Q-values are given by the triaxial diagrams of FIGURES 1 to 3, where the values indicated refer to the dielectric constants, the temperature coefiicients, and the Q-values respectively, and all the values are taken from the measurements at 1 mc./s.
In preparing the ceramic dielectrics of the invention, SrTiO and CaTiO are first finished by pre-sintering at about 1300 C. and by grinding, and then they are mixed with Bi O and TiO After the mixture is finished by compression molding, the mixture is made up into the dielectrics by sintering at above 1000 C.
In this manner the main ingredients, SrTiO and are prefired so that loss of usefulness of the raw materials by moisture absorption during storage can be completely eliminated, and the obtained dielectrics live to expectations sufliciently. Examples of the ceramic dielectrics produced according to the invention are described below in some detail.
EXAMPLE 1 Powdered SrTiO and CaTiO each of which has been pre-sintered at 1300 C., are mixed with Bi O -2TiO- at the following ratio:
Parts by wt. 60 10 30 EXAMPLE 2 Powered SrTiO and CaTiO which have been presintered in the same way as in Example 2, are mixed with Bi 0 -2TiO at the following ratio:
srrio CaTiO Parts by Wt. 40 40 20 SrTiO CaTiO Bi O After molding, the mixture is sintered at 1350 C. to make a dielectric. The body shows the following characteristics: the dielectric constant=357,
the temperature coefficient=1826 10- deg? and QESSOO.
As described above, the application provides ceramic dielectrics which have higher dielectric constant and smaller temperature coefficient in comparison with those hitherto used. Ceramic dielectrics of the application have been applied for practical uses, e.g., as capacitor elements for television tuning circuits and as rotor parts of variable ceramic capacitors, and were found to function excellently. Moreover, dielectrics of the application possess low porosity and dense structure, and this enables them to be used reliably as capacitors under conditions of severe temperature variations or high humidity. Such excellent characteristics were first exhibited by the body of the application.
What is claimed is:
1. A ceramic dielectric consisting essentially of a sintered mixture of 40 parts by weight of SrTiO 40 parts by Weight of CaTiO and 20 parts by weight of Bl203 References Cited UNITED STATES PATENTS 12/1962 Domanski l0639 3,179,525 4/1965 Welsby et a]. l0639 HELEN M. MCCARTHY, Primary Examiner US. Cl. X.R.