Dielectric ceramic composition and process for its production containing MgTiO.sub.3 and Pb.sub.3 O.sub.4 having a quantitative relationship

A dielectric ceramic composition consists essentially of strontium titanate, magnesium titanate, bismuth oxide, titanium oxide, and lead oxide, these components being present in the following compositional proportion when expressed by the formulas, SrTiO.sub.3, MgTiO.sub.3, Bi.sub.2 O.sub.3, TiO.sub.2 and Pb.sub.3 O.sub.4, respectively: ______________________________________ SrTiO.sub.3 30.0-60.0 wt % MgTiO.sub.3 2.0-32.0 wt % Bi.sub.2 O.sub.3 10.0-34.0 wt % TiO.sub.2 3.0-15.0 wt % Pb.sub.3 O.sub.4 2.0-20.0 wt % Pb.sub.3 O.sub.4 /MgTiO.sub.3 0.625-10.0 (weight ratio) ______________________________________ The composition exhibits high permittivity, small dielectric loss, small dependency of permittivity on temperature, and high dielectric breakdown voltage in combination, so that it makes it possible to produce physically small capacitors with large capacity and high rating voltage. The process for production of the composition is characterized by the use of MgTiO.sub.3 and Pb.sub.3 O.sub.4 as respective sources of magnesium and lead.

BACKGROUND OF THE INVENTION 
The present invention relates to dielectric ceramic compositions which 
exhibit high permittivity, small dependency of permittivity on 
temperature, small dielectric loss and, high dielectric breakdown voltage 
and a process for its production. 
Ceramic capacitors with large capacity having a high rating DC voltage of 
500 V and above are conventionally made from dielectric ceramic 
compositions containing a large percentage of barium titanate. However, 
such ceramic materials have serious disadvantages in that they exhibit 
large dielectric loss and large dependency of permittivity on voltage, and 
causes noises due to the piezoelectricity and the generation of large 
distortion of wave forms. In order to overcome these disadvantages, it has 
been proposed to use strontium titanate-based dielectric ceramic 
compositions, such as those containing bismuth titanate and lead titanate 
as additives, or those containing bismuth titanate and magnesium oxide as 
additives. However, these strontium titanate-based dielectric ceramic 
compositions, although exhibiting either small dielectric loss, small 
temperature change rate of permittivity or large permittivity, do not 
exhibit all these properties in combination. For example, the former 
exhibits large permittivity, but has disadvantages that it has large 
dielectric loss and large temperature change rate of permittivity. The 
latter may exhibit small dielectric loss of about 0.02% and small 
temperature change rate of permittivity of .+-.0%, but has the 
disadvantage that it exhibits low permittivity of about 1000. 
SUMMARY OF THE INVENTION 
It is therefore a main object of the present invention to provide 
dielectric ceramic compositions exhibiting high permittivity, small 
dielectric loss, small change in permittivity with temperature and high 
dielectric breakdown voltage characteristic in combination. 
Another object of the present invention is to provide a dielectric ceramic 
composition which makes it possible to produce physically small ceramic 
capacitors with large capacity and high rating voltage of 1000 V and 
above. 
Still another object of the present invention is to provide a process for 
producing dielectric ceramic compositions which makes it possible to 
produce physically small ceramic capacitors with large capacity a high 
rating voltage of 1000 V and above. 
According to the present invention, there is provided a dielectric ceramic 
composition consisting essentially of strontium titanate, magnesium 
titanate, bismuth oxide, titanium oxide and lead oxide, these components 
being present in the following compositional proportions when expressed by 
the formulas SrTiO.sub.3, MgTiO.sub.3, Bi.sub.2 O.sub.3, TiO.sub.2 and 
Pb.sub.3 O.sub.4 : 
The percentage and weight ratio of the components have been limited as 
being within the above respective ranges for the following reasons. If 
SrTiO.sub.3 is less than 30.0 wt%, the permittivity becomes less than 1000 
and the dielectric loss becomes great. Further, if SrTiO.sub.3 is more 
than 60.0 wt%, the change rate of permittivity goes up to over -20%, which 
is the maximum allowable change rate of the permittivity, and the 
permittivity becomes less than 1500. If MgTiO.sub.3 is less than 2.0 wt%, 
the permittivity becomes low and the change rate of permittivity goes up 
to over -20%. If MgTiO.sub.3 is more than 32.0 wt%, it causes considerable 
decrease of the permittivity. If Bi.sub.2 O.sub.3 is less than 10 wt% or 
more than 34.0 wt%, it causes the increase of dielectiric loss. If 
TiO.sub.2 is less than 3.0 wt%, the dielectric loss becomes great, and if 
more than 15 wt%, the change rate of permittivity becomes more than -20%. 
If Pb.sub.3 O.sub.4 is less than 2.0 wt%, the permittivity becomes low 
and, if more than 20.0 wt%, the dielectric loss goes up to over 1%. If the 
weight ratio of Pb.sub.3 O.sub.4 to MgTiO.sub.3 is less than 0.625 or more 
than 10.0, the permittivity does not go up to 1500. Finally, when lead 
oxide is expressed by the formula: PbO, lead oxide should be present in 
the amount from 1.95 to 19.53 wt% and, the weight ratio of PbO to 
MgTiO.sub.3 is from 0.61 to 9.76. 
The dielectric ceramic compositions of the present invention may be 
produced in the known manner, for example, by techniques conventionally 
employed for the production of dielectric ceramics. However, a preferred 
method, hereinafter described, consists in the use of magnesium titanate 
and trilead tetroxide or lead (II) oxide as respective sources of 
magnesium and lead. The use of these sources contributes to improvement in 
the dielectric breakdown voltage characteristic of the dielectric 
ceramics. 
The dielectric ceramic compositions of the present invention exhibit high 
permittivity, small dielectric loss, high dielectric breakdown voltage 
characteristic, and small dependency of permittivity on temperature in 
combination. Thus, the compositions of the present invention make it 
possible to produce physically small ceramic capacitors with large 
capacity and high voltage rating. The noises due to piezoelectricity and 
distortion of wave form are considerably lowered, compared with those of 
the conventional ceramic capacitors. 
Other objects and further scope of applicability of the present invention 
will become apparent from the detailed description given above and 
hereinafter with reference to the examples. It should be understood, 
however, that the detailed description and specific examples, while 
indicating preferred embodiments of the invention are given by way of 
illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description.