Note: Descriptions are shown in the official language in which they were submitted.
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This invelltion relates to polycrystalline semi-
conducting ceramic compositions having internal insulated
grain boundaries, to methods for producing such compositions,
and to capacitors utilizing such compositions as a dielectric.
Ceramic compositions having internal insulated
grain boundarles are known in the art. For example, composi-
tions based on barium titanate having a perovskite lattice
doped with Sb2O3 to provide n-type conductivity and with CuO
to provide p-type conductivity are disclosed in Brauer, et al.,
10 U.S. Patent No. 3,569,802. Such compositions have grain
sizes in the range of about 20-300 microns and have a high
dielectric constant value; however, because they are based on
a ferroelectric material, they exhibit the usual Curie-Weiss
temperature dependence with as high as +80% change in
capacitance over a temperature range of -30 to 85C, and a
saturation of dielectric constant with voltage.
A somewhat more stable polycrystalline semiconduct-
; ing ceramic composition can be produced by sintering strontium
titanate with a small amount of Nb2O5 or Ta2O5 and a small
amount of GeO2 or ZnO. The internal grain boundaries are
insulated by diffusing therein Bi2O3 or a mixture of si2O3,
PbO, and B2O3 from the surface of a pellet. Such compositions
are disclosed in Takahashi, et al., U.S. Patent No. 3,933,668.
These compositions typically exhibit high dielectric constant
values~ temperature coefficients of capacitance in the
neighbourhood of +15~, and grain sizes greater than 25
ml crons .
Although the above-described semiconducting ceramic
compositions having internal insulating boundaries provide
high dielectric constant values, they are limited in their
application because of their relatively high temperature
and voltage coefficients of capacitance, and their large
grain sizes; further they cannot be successfully fabricated
into thin-layer, multi-layer devices.
Accordingly, it is an object of this invention
to provide polycrystalline semi conducting ceramic composi-
tions having internal insulated grain boundaries that exhibit
high dielectric constant values, low dissipation factors,
and low temperature and voltage coefficients of capacitance.
: Another object of this invention is to produce
fine-grained ceramics suitable for the fabrication of thin-
layer, multi-layer capacitors.
A further object of this invention is to provide
: a process in which the internal insulating grain boundaries
are produced by liquid phase sintering with lead germanate
at temperatures well below those required in prior art diffu-
sion processes.
: These and other objects will become apparent as
description of the invention proceeds.
BRIEF DESCRIPTION OF THE DRAWING
;~ ~ Fig. 1 is a phase diagram of the PbO-GeO2 system
showing the melting points of various lead germanate composi-
tions.
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;~ SUMMARY OF THE INVENTION
The semiconducting ceramic compositions of this
invention comprise a major amount of a polycrystalline alkaline
:~ earth metal titanate doped with a small amount of a metal
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oxide capable of providing n-type semiconductivity and a
minor amount of a lead germanate composition. The term
"lead germanate" in this specification is meant to include
lead and germanium containing compounds such as Pb5Ge3Oll,
and lead, germanium, and silicon containing compounds defined
by the formula
(PbO)x(GeO2)y_z(SiO2)z
where x ranges from 1 to 6, y ranges from 1 to 3, and z
ranges from 0.0 to 0.75y. Mixtures of two or more composi-
tions can also be used.
The ceramic compositions are characteri2ed in
that the lead germanate composition is incorporated therein
by liquid phase sintering which provides an insulating boundary
layer about the grains of the polycrystalline semiconducting
alkaline earth metal titanate.
Suitable alkaline earth metal titanates possess
a perovskite lattice structure and comprise strontium titanate,
; 20 barium titanate, calcium titana~e, and mixtures thereof.
Strontium titanate is the preferred material and can be
~ obtained from mixtures of SrO and TiO2, SrCO3 and TiO2f
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and from other sources well known in the art.
The metal oxide providing n-type semiconductivity
is suitably a hexavalent metal oxide having the formula
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IMO3)(Bi2~3)X
; where M is tungsten (W~ or molybdenum tMo), and x ranges
from 0 to 7. Materials that afford Wo3 or MoO3 during calcina-
tion can be used to advan-tage; examples of such materials
are the acids H2WO4 and H2MoO4 and salts thereof.
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Lead germanate compositions suitable for the liquid
phase sintering step can be prepared by heating various
mixture of PbO and GeO2 up to temperatures of about 1000C.
The phase diagram in Figure 1 shows the melting points for
various molar ratios of PbO and GeO2. For the purpose of
this invention, lead germanate compositions having melting
points below about 850C are preferred. Because of the
relatively high cost of GeO2 a portion of it can be replaced
advantageously with SiO2.
The lead germanate composition having the formula
Pb5Ge3Oll is a preEerred composition because it melts con-
gruently (i.e., solid and liquid phases have identical composi-
tions) to provide a low viscosity melt which permits rapid
and complete liquid phase sintering at temperatures as low
: as 850C. The composition is prepared by heating a mixture
of 5 moles of PbO and 3 moles of GeO2 in a platinum crucible
at 800C. The cooled product, having the formula Pb5Ge3Oll,
is crushed and powdered in a mor~ar and pestle, screened
through a 325 mesh screen, and further reduced by ball milling.
The final powder has a particle size ranging between about
0.5 and 30 microns with the greatest concentration at about
~ 5 microns.
: DESCRIPTION OF THE PREFERRED EMBODIMENTS
The ceramic compositions of this invention are
produced by the steps of calcining fine particle size alkaline
earth metal titanate, or a mixture of alkaline earth oxide
or carbonate with TiO2, with a small amount o~ the hexavalent
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metal oxide disclosed above to produce n-type semiconductivity;
milling and blending the powder with the lead germanate
composition disclosed above; and pressing and firing the
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mixture in air or a reducing atmosphere to produce internal
insulating boundary layers about the grains of the semiconduct-
ing titanate by liquid phase sintering.
Generally from about 90 to about 99.99~ preferably
from about 95 to about 99, percent by weight of the alkaline
earth metal titanate is calcined with generally from about
0.01 to about 10, preEerably from about 1 to about 5, percent
by weight of the hexavalent metal oxide in air or a reducing
atmosphere to produce n-type semiconductivity~ Calcination
is conveniently carried out at a temperature of about 1000
to about 1400C for about 1 to about 4 hours in a weakly
reducing atmosphere, for example a forming gas containing
about 10% of hydroyen. The resulting powder is milled and
blended with generally from about 0.01 to about 35, preferably
from about 1 to about 10, percent by weight of the lead
germanate composition, and is then pressed and fired to
provide, by liquid phase sintering, internal insulating
~ boundary layers about the grains of the metal titanate.
; The liquid phase sintering step is conveniently carried
out a temperature in the range of from about 850 to about
1300C for from about 1 to about 60 minutes in air or a
weakly reducing atmosphereO
The method of the invention is particularly advan-
tageous in that it yields a ceramic composition having a
grain size which is not appreciably larger than the particle
; size of the alkaline earth metal titanate used in making
;
the ceramicl in contrast to methods of the prior art which
caused a substantial increase in grain size. Accordingly,
by using appropriately fine titanate as starting ~aterial,
the invention provides ceramic compositions having a grain
size less than 10 microns, suitably about 2 microns or less,
which are particularly useful for the production of thin-
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layer, multi-layer capacitors having excellent dielectric
properties. Since the thickness of the layers in a multi-
layer capacitor is limited by the grain size of the ceramic,
the ceramic compositions of the invention can be used to
make multi-layer capacitors in which each layer is much
thinner than was previously possible using the ceramic composi-
tions heretofore known.
The invention is further illustrated by reference
to the following procedures and examples.
EXAMPLE 1
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Fine particle size strontium titanate (1 micron)
was reduced by calcining with 2% by weight of WO3 in a 10%
hydrogen atmosphere at 1380C for 4 hours to produce a n-
type semiconductor. To the resulting powder was added 5%
by weight of Pb5Ge3Oll, the mixture was milled and blended,
and then pressed into discs (1 cm in diameter, 0.5 mm thick)
and Eired at 1200C for 30 minutes to produce internal insulat-
ing boundary layers about the strontium titanate grainsby liquid phase sintering. The discs were electroded with
fritted silver paste and fired in air at 800C for 5 minutes.
;~ Dielectric constant (K~ and dissipation factor
DF) were recorded at 100 KHz and 0.5 volts AC. The voltage
coefficient of capacitance ~VC) was measured at 200 volts
DC. The temperature coefficient of capacitance (TC) was
measured over the temperature range of -55 to +125C.
Typical values found for the capacitors prepared
in this example were: k = 5,000-10,000; Df = 3% max; VC = -5
max: TC = ~5% max.
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EXAMPLE 2
Fine particle size strontium titanate (1 micron)
was reduced by calcining with 2% by weight of Bi2W06 in
a lO~ hydrogen atmosphere at 1200C for 2 hours to produce
an n-type semiconductor. To the resulting powder was added
5% by weight of Pb5Ge3Oll, the mixture was milled and blended,
and then pressed into discs (l cm in diameter, 0~5 mm thick)
and fired at 1200C for 30 minutes to produce internal insulat-
ing boundary layers by liquid phase sintering. The discswere electroded with fritted silver paste and fired in air
at 800C for 5 minutes.
Typical values found for capacitors prepared in
this example were: K = 5,000-10,000; DF = 3% max; VC = -5%
max; TC = +5% max.
It is clear that a stabilized high dielectric
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constant ceramic composition having internal insulated boundary
layers can be produced from the herein disclosed components
by a liquid phase sintering technique using the herein dis-
closed lead germanate composition. Electrical propertiesof the capacitors fabricated from the inventive ceramic
composition such as dielectric constant, dissipation factor,
voltage and temperature coefficients of capacitance, appear
exceedingly attractive for various applications including
high capacitance, low voltage markets and thin-layer, multi-
layer devices.
Although this invention has been disclosed with
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; particular reference to certain preEerred embodiments thereof,
it is understood that variations and modifications can be
effected within the spirit and scope of the appended claims.
It is intended tha~ all matter contained in the above descrip-
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tion or shown in the accompanying drawing shall be interpreted
in an illus~rative and not in a limiting sense.
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