Heretofore, as a dielectric ceramic composition which exhibits a small voltage dependence, high ceramic strength, and flat dielectric constant-temperature characteristics there has been widely known a ceramic composition mainly composed of BaTiO.sub.3 comprising a bismuth compound and a rare earth element incorporated therein as subsidiary components.
Besides the dielectric ceramic composition having the foregoing formulation, a dielectric ceramic composition mainly composed of BaTiO.sub.3 comprising Nb.sub.2 O.sub.5, rare earth oxides, and oxide of transition metals such as Cr, Mn, Fe, Co and Ni incorporated therein as subsidiary components is reported to exhibit flat dielectric constant-temperature characteristics irrespective of its dielectric constant as high as not less than 3,000.
The temperature characteristics of these dielectric ceramic compositions satisfy X7R characteristics of EIA Standard stipulating that the percent capacitance change from the capacitance at +25.degree. C. as reference be within .+-.15% in the temperature range of from -55.degree. C. to +125.degree. C.
However, the dielectric ceramic composition mainly composed of BaTiO.sub.3 comprising a bismuth compound incorporated therein exhibits a dielectric constant as low as about 1,000. If the dielectric constant is raised, the capacitance change with temperature becomes large. In addition, when a green composition is calcined at an elevated temperature, such a dielectric ceramic composition is subject to evaporation of Bi.sub.2 O.sub.3, causing the ceramic to be deformed or change in the percent composition, and hence making it impossible to obtained required electrical characteristics or mechanical strength or causing these characteristics to be scattered.
The dielectric ceramic composition mainly composed of BaTiO.sub.3 comprising Nb.sub.2 O.sub.5, rare earth oxides, and oxide of transition metals such as Cr, Mn, Fe, Co and Ni incorporated therein as subsidiary components exhibits a dielectric constant of not less than 3,000 and flat temperature characteristics. However, this dielectric ceramic composition must be fired at a temperature as high as not lower than 1,200.degree. C.
In recent years, ceramic capacitors tend to have a smaller size. In particular, laminated ceramic capacitors tend to be composed of thinner dielectric ceramic layers having a thickness of from 5 to 15 .mu.m for miniaturization and larger capacitance. A dielectric ceramic composition having a small voltage dependence is thus desired.
However, since the foregoing dielectric ceramic composition having a high dielectric constant exhibits a large voltage dependence, its thickness cannot be reduced as required by the recent market, making it impossible to prepare a small-sized large capacitance laminated ceramic capacitor. Further, since the foregoing dielectric ceramic composition exhibits a low ceramic mechanical strength, it may be destroyed during using.
USPTO Backgrounds
USPTO Backgrounds