Ceramic capacitors and dielectric compositions

A dielectric composition, particular suitable for the manufacture of ceramic capacitors, comprising non-stoichiometric lead magnesium niobate, non-stoichiometric lead iron niobate and one or more oxide additives, which may be chosen from the group comprising silica, managanese dioxide, ceric oxide, lanthanum oxide, zinc oxide, alumina, tungsten oxide, nickel oxide, cobalt oxide and cuprous oxide. The compositions can be fired at temperatures between 980.degree. and 1075.degree. C., so that in the case of multilayer ceramic capacitors high silver content internal electrodes can be used and, in cases where three or more of the oxide additives are employed, higher dielectric constants, for example 10600 to 16800, of the fired ceramics than conventional ceramics can be achieved, thus permitting capacitor device size reduction. Additions of lead titanate may also be made to the dielectric compositions.

BACKGROUND OF THE INVENTION 
This invention relates to ceramic capacitors and in particular, but not 
exclusively, to multilayer ceramic capacitors and dielectric compositions 
for use therein. 
A multilayer ceramic capacitor basically comprises a stack consisting of a 
plurality of dielectric members formed of a ceramic material, with 
electrodes positioned between the members. The electrodes may be 
screen-printed onto the ceramic material, in the unfired state thereof, 
using conductive inks. A stack of screen-printed dielectric members is 
assembled, pressed together, cut into individual components, if 
appropriate, and fired until sintering occurs, in order to ensure 
non-porosity. The internal electrodes may be of rectangular form and cover 
the whole or part of the area of the adjacent dielectric layers. The 
internal electrodes in successive layers may be sideways stepped relative 
to one another or have elongate portions which cross one another, as 
described in our British Patent Application No. 7841677 (Ser. No. 
2032689A) (A. L. Oliver--G. Mills 1-1). 
With conventionally employed dielectrics the capacitors must be fired at 
temperatures of the order of 1200.degree.-1400.degree. C., which means 
that the internal electrodes must be of a suitable material to withstand 
such temperatures and that expensive noble metals, such as platinum and 
palladium must be used. However, if the firing temperature can be reduced, 
by a suitable choice of dielectric, then internal electrodes with a high 
silver content (50-100% silver) could be used, thus reducing costs for 
materials and manufacture. In our British Patent Application No. 8120605 
(Ser. No. 2107300A) (J. M. Wheeler 1) there is disclosed a dielectric 
composition which can be fired at a temperature between 950.degree. C. and 
1100.degree. C. and can thus be used with high silver content internal 
electrodes. These low firing temperature dielectrics comprise lead 
magnesium niobate (PbMg.sub.1/2 Nb.sub.1/2 O.sub.3) with one or more of 
the following, namely lead titanate, lead stannate, lead zirconate, and 
some of these dielectric compositions have dielectric constants in the 
range 7500-10000, which make them particularly suitable for multilayer 
ceramic capacitors. The conventionally employed ceramics (U.S. coding Z5U) 
are not compatible with high silver content electrodes and usually have 
dielectric constants lower than 7500-10000. The electronics industry, 
generally, requires smaller components, and smaller and cheaper capacitors 
can be obtained by producing dielectrics which are compatible with high 
silver content electrodes and have even higher dielectric constants than 
those proposed in British Patent Application No. 8120605. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention there is provided a 
dielectric composition comprising non-stoichiometric lead magnesium 
niobate, non-stoichiometric lead iron niobate and one or more oxide 
additives. 
According to another aspect of the present invention there is provided a 
ceramic capacitor including dielectric comprising non-stoichiometric lead 
magnesium niobate, non-stoichiometric lead iron niobate and one or more 
oxide additives. 
According to a further aspect of the present invention there is provided a 
multilayer ceramic capacitor including a plurality of layers of a 
dielectric and a plurality of high silver content internal electrodes 
arranged between the dielectric layers, which dielectric layers are formed 
of non-stoichiometric lead magnesium niobate, non-stoichiometric lead iron 
niobate and one or more oxide additives. 
According to yet another aspect of the present invention there is provided 
method of manufacturing a multilayer ceramic capacitor including the steps 
of screen printing a plurality of electrodes onto each of a plurality of 
dielectric members, assembling a stack of the resultant screen-printed 
members, pressing the stack together, dividing the pressed stack into 
individual capacitor components and firing the individual components at a 
temperature between 980.degree. C. and 1075.degree. C., and wherein the 
dielectric comprises non-stoichiometric lead magnesium niobate, 
non-stoichiometric lead iron niobate and one or more oxide additives. 
The oxide additives may be chosen from silica, manganese dioxide, ceric 
oxide, lanthanum oxide, zinc oxide, alumina, tungsten oxide, nickel oxide, 
cobalt oxide and cuprous oxide. Preferably three or more oxide additives 
are chosen from the first eight of the ten mentioned above and after 
firing at temperatures between 980.degree. C. and 1075.degree. C. have 
dielectric constants in the range 10600 to 16800. In addition, 
particularly in dependence on the processing employed, additions of lead 
titanate (PbTiO.sub.3), may be employed.