Method for making porous bottom-layer dielectric composite structure

DETAILED DESCRIPTION OF THE INVENTION A.
Ceramic Solids The invention is applicable to virtually any high melting inorganic solid material.
However, it is particularly suitable for making dispersions of dielectric solids such as alumina, titanates, zirconates and stannates.
It is also applicable to precursors of such materials, i.
e.
, solid materials which upon firing are converted to such dielectric solids, and to mixtures of any of these.
Among the many dielectric solids which are likely to be used in the invention are BaTiO.
sub.
3, CaTiO.
sub.
3, SrTiO.
sub.
3, PbTiO.
sub.
3, CaZrO.
sub.
3, BaZrO.
sub.
3, CaSnO.
sub.
3, BaSnO.
sub.
3, and Al.
sub.
2 O.
sub.
3.
As will be apparent to those skilled in the ceramic arts, the exact chemical composition of the ceramic solids to be used in the composition of the invention is not ordinarily critical in the rheological sense.
However, it is preferred that the ceramic solids not have swelling characteristics in the organic dispersion since the rheological properties of the dispersion may be substantially changed thereby.
It has been found that the dispersions of the invention must contain no significant amount of solids having a particle size of less than 0.
3 .
mu.
m in order to obtain adequately complete burnout of the organic medium when the films or layers thereof are fired to remove the organic medium and to effect sintering of the inorganic binder.
However, none of the ceramic solids may exceed 20 .
mu.
m and, furthermore, at least 75 wt.
% of the ceramic solids must have a size of 1-10 .
mu.
m.
When the dispersions are used to make thick film pastes, which are usually applied by screen printing, the maximum particle size must not exceed the thickness of the screen, and when the dispersion is used to make dry film, the maximum particle size must not exceed the thickness of the film.
In addition, it is preferred that surface area/weight ratio of the ceramic particles not exceed 10 m.
sup.
2 /g for the reason that such particles tend to affect adversely the sintering characteristics of the accompanying inorganic binder