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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a method for forming a
glass-reacted-ceramic dielectric material and more particularly to such a
method wherein cadmium from the glass reacts with the ceramic during the
firing to maturity of the dielectric.
In the preparation of such low firing two phase (glass and ceramic)
dielectric materials, it is conventional to pre-react the basic ceramic
materials, such as barium titanate and calcium zirconate by prefiring or
calcining at a high temperature, namely above 2200.degree. F., to form a
solid solution thereof and thus to form a single phase ceramic such as an
alkaline-earth titanate-zirconate. This single phase ceramic is then
conventionally pulverized, mixed with a glass powder, usually in a
suspension medium, deposited on a substrate or in a multi-layer stack and
fired to maturity at a temperature less than about 2050.degree. F. Such a
method is described in the patent to G. Maher U.S. Pat. No. 3,885,941
issued May 27, 1975 and assigned to the same assignee.
Although it is known to react two different alkaline-earth titanates in a
glass flux at temperatures lower than 2050.degree. F., it has hitherto
been believed that the above noted high temperature calcining step was
necessary to achieve a solid solution of two basic but distinctly
different ceramic materials such as titanates and zirconates.
It is an object of the present invention to provide a method for forming a
glass-reacted-ceramic dielectric material wherein a mixture of two
different basic ceramic materials and glass is fired to maturity achieving
a solid solution of said ceramic materials that is intermixed with glass.
It is a further object of this invention to provide a method for forming a
glass-reacted-ceramic dielectric material from at least two distinctly
different ceramic materials and a glass, without calcining said distinctly
different ceramic materials prior to mixing and firing to maturity with
glass.
It is a further object of this invention to provide a low cost method for
making a glass-reacted-ceramic dielectric material having a high
dielectric constant at the Curie temperature.
It is a further object of this invention to provide a low cost method for
making a glass-reacted-ceramic dielectric material having a Curie
temperature of about 25.degree. C.
SUMMARY OF THE INVENTION
A method for making a cadmium containing glass-reacted-ceramic dielectric
body comprises mixing in an organic suspension medium the finely ground
prefired powders of barium titanate, an alkaline-earth-zirconate and a
cadmium containing glass. A body is formed of the slip mixture, which is
dried and then fired at between 1800.degree. F. and 2050.degree. F. to
produce a mature glass-reacted-ceramic body. The low temperature firing
surprisingly causes the zirconate to form a solid solution with the
titanate and also causes some of the cadmium ions from the glass to
diffuse into and react with the titanate-zirconate. The effect of the
cadmium reaction is to enhance grain growth of the titanate-zirconate
phase and to shift the Curie temperature downwardly. This produces a
glass-reacted-ceramic body having a ceramic phase consisting in a cadmium
containing barium-titanate-zirconate that is intermixed with glass, which
exhibits a high dielectric constant at room temperature and provides other
excellent dielectric properties. The term high dielectric constant is used
herein to mean values greater than 3000 at 25.degree. C. unless otherwise
noted.
The mechanism by which the zirconate and titanate components form a solid
solution at the low temperature firing is believed to derive from the
strong enhancement of grain growth by cadmium in the glass which in turn
enables the zirconate to be continuously incorporated in the growing
titanate crystal.
The dielectric materials as produced by the method of this invention are
particularly useful for making monolithic capacitors having low cost
electrodes such as silver palladium alloys as is explained in U.S. Pat.
No. 3,619,220 issued Nov. 9, 1971 and assigned to the same assignee. The
method of the present invention makes it possible to omit the conventional
calcining or prefiring step by which the basic ceramic components of
heretofore known glass-ceramic dielectric materials are caused to form a
solid solution and single phase ceramic at high temperatures. In barium
titanate ceramics the diffusion of large cations such as calcium and
strontium occurs readily. This is because large cation vacancies are quite
common in the titanate lattice. Diffusion of small cations is generally
considered much more difficult as in the example of replacing some of the
titanium with zirconium. It has been found, however, that a sufficient
solid solution can indeed be obtained without the conventional
precalcining step by the method of this invention to achieve the good
electrical properties and high dielectric constants above mentioned.
Omission of the calcining step leads to a substantial reduction in the
cost of making the glass-reacted-ceramic dielectric material. It also
advantageously reduces the total energy requirements of the process.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE shows a graph of the percent change in dielectric constant (K)
as a function of operating temperature for four experimental glass ceramic
bodies.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A slip suspension was prepared by adding particles of barium titanate and
particles of a cadmium containing glass in an organic binder. The glass
was composed of 35.5% CdO, 24.5% Bi.sub.2 O.sub.3, 24.8% PbO, 4% ZnO, 5.2%
B.sub.2 O.sub.3, 4.9% SiO.sub.2, and 1% A1.sub.2 O.sub.3 by weight. The
glass amounted to 11.5% by weight of the solids in the slip mixture. After
milling for 12 hours in a 200 cc porcelain mill to thoroughly mix and
reduce the solids to particles on the order of 1 micron diameter, the slip
was cast on a glass plate using a standard doctor blade technique, and
when dry was cut into small squares. These squares were fired at about
2025.degree. F. after which silver electrodes were applied to the opposite
major faces of each square to form disc type capacitors and to permit the
evaluation of the glass-ceramic dielectric material.
These capacitors, being designated Example 1, were made as a reference and
control sample against which the evaluation of modifications in the
ceramic composition might be made. The Curie temperature (Tc) of the
material is seen in Table I to be quite high and the room temperature
dielectric constant is quite low, namely about 1000.
The remainder of the examples for which composition and Curie temperature
are shown in Table I, each have 11.5 weight percent of the glass
composition used in Example I, but have varying amounts of basic ceramic
modifiers added. The quantity of the basic ceramic modifier added in each
of the Examples 2 through 9 is shown both as the weight percent of the
total weight of the glass-ceramic dielectric material and also as mole
percent taken only relative to the ceramic components.
The method for making these modified dielectric materials includes the
steps used in making the control material of Example 1. However, to the
slip containing particles of barium titanate, and the cadmium containing
glass there were added particles of the modifier composition, all three of
which were then milled, cast and fired at 2025.degree. F. for about 2
hours.
TABLE I
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Ex- Type of BaTiO.sub.2
Modifier Tc
ample Modifier (wt.%) (Wt. %/mole %)
.degree. C
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1 -- 88.5 0/0 144
2 CaZrO.sub.3
85.8 2.7/3.9 100
3 CaZrO.sub.3
83.2 5.3/7.6 28
4 CaZrO.sub.3
77.0 11.5/16.3 -55
5 BaZrO.sub.3
82.5 6.0/5.7 63
6 BaZrO.sub.3
80.6 7.9/7.6 0
7 SrZrO.sub.3
83.6 4.9/5.6 38
8 SrZrO.sub.3
83.0 5.5/6.4 22
9 ZrO.sub.2 84.8 3.7/7.6 *
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*double peak
Surprisingly it was found that in all but one of these examples having
modifiers added to the ceramic and glass slip, a solid solution of the
barium titanate and the modifier was achieved at the low firing
temperature of 2025.degree. C. In each of the examples 2 through 8, an
alkaline-earth titanate-zirconate ceramic phase was formed having a single
dominant peak in the characteristic curve of dielectric constant versus
operating temperature.
From other experiments and from the evidence presented in the above-noted
U.S. Pat. No. 3,885,941, the glass composition employed in Examples 2
through 8 is about optimum for achieving a high dielectric constant at
room temperature, although an increase of any one of the constituent glass
oxides (except bismuth) of as much as 100% and a decrease in any one of
these constituents (except cadmium) by as much as about 50% would
represent suitable glass compositions for use in this invention. The
bismuth oxide should not exceed about 30 wt.% and the cadmium oxide is
preferably not less than about 30 wt.% of the glass to obtain a strong
synergism at firing between the cadmium and the reacting titanate and
zirconate, as described earlier. The total amount of the glass used in the
glass ceramic mixture should not be less than about 10.5 wt.% because the
glass-ceramic would not densify properly at the low firing temperatures
used. Larger amounts of glass than about 15 wt.% would prevent the
achievement of dielectric constants as high as 3000. It is also apparent
from the data in Table I that the zirconate modifier may vary over the
range of from about 4 to 16 mole percent of the titanate-zirconate mixture
to provide a range of useful dielectrics having a wide spectrum of Curie
temperatures.
In the drawing the characteristic curve X9 of dielectric constant versus
temperature for the material of Example 9, having the zirconia modifier,
shows two peaks, one at 30.degree. C. and the other at 125.degree. C.
Clearly the zirconia remained essentially a separate and distinct phase
within this dielectric material. Zirconia is thus an unsuitable modifier
for use in the method of this invention although the rather flat
temperature coefficient of capacitance suggests that this composition may
be useful for applications calling for low TCC and lower dielectric
constants.
Small molar amounts of the alkaline-earth modifiers are shown to have a
substantial depressing influence on the Curie temperature in the
dielectric material produced by the method of this invention as is shown
in Table II. This effect is similar to that achieved by the method taught
by G. Maher in patent application Ser. No. 690,225, filed May 26, 1976 and
assigned to the same assignee. For example, the material of this patent
containing 4.5 mole percent barium zirconate has a Curie temperature of
about +5.degree. C., while the material of the present invention (Example
6) having 7.6 mole percent barium zirconate exhibits about the same Curie
temperature, namely 0.degree. C. Thus, a little more of the zirconate
modifier is required in the method of the present invention to achieve the
same depressing effect on the Curie temperature. However, it is disclosed
in the patent application that a barium titanate without the cadmium
containing glass and with about 7.6 mole percent of a barium zirconate
modifier will exhibit a Curie temperature as high as 60.degree. C. It is
apparent that in the method of the present invention the zirconate
modifier and cadmium from the glass together will effectively react with
the barium titanate during the firing to maturity of the glass-ceramic
body to cause the substantial lowering of the Curie temperature.
TABLE II
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DF at
.5 Volt
IR at
at
Ex- Type of
BaTiO.sub.2
Modifier per mil
25.degree. C
125.degree. C
ample
Modifier
(Wt. %)
Wt. %/mole %)
K (%) F F Tc
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10 CaZrO.sub.3
83.2 5.3/7.6 3850
3.9 3850
245 28
11 BaZrO.sub.3
80.6 7.9/7.6 3150
1.4 10 120 0
12 SrZrO.sub.3
83.0 5.5/6.4 4500
2.6 4450
85 22
13* SrZrO.sub.3
83.0 5.5/6.4 4150
2.0 8400
200 25
14**
SrZrO.sub.3
83.0 5.5/6.4 4300
1.4 10800
370 25
15***
SrZrO.sub.3
83.0 5.5/6.4 -- -- -- -- --
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*ceramic with 0.2 UO.sub.3 by weight (0.2 mole %)
**ceramic with 0.1 MnCO.sub.3 by weight (0.3 mole %)
***ceramic with 0.2 NbO.sub.2.5 by weight (0.4 mole %)
The most promising of the dielectric materials from Examples 2 through 8
for use as high K bodies (K>3000 at 25.degree. C.) were those of Examples
3, 6 and 8. These same materials were used to make the hand screened
monolithic capacitors of Examples 10, 11 and 12, respectively.
These monolithic capacitors were made by first mixing the finely ground
powders of barium titanate, the alkaline-earth zirconate modifier and the
cadmium containing glass in an organic binder. A first layer of this slip
was hand screened on a glass substrate and dried. A 70% Ag-30% Pd
electrode was screen deposited on the dried slip layer. A second layer of
the glass-ceramic slip was hand screened over the electrode, another
electrode film deposited in the same manner as the first and a third slip
layer was applied over the second electrode. This stack was removed from
the glass substrate and fired at 2025.degree. F. for about 2 hours. Silver
terminations were provided at either end of the fired stack making contact
with each of the two buried electrodes, respectively.
The Curie temperature and characteristic temperature curves of the hand
screened monolithic capacitors were the same as for the disc capacitors
having the same start materials. The characteristic temperature curves for
Examples 10, 11 and 12 are shown in FIG. 2. Other electrical
characteristics for these examples are given in Table II.
The material of Example 12 is preferred for the above noted high K
applications but it was seen that the DF was higher and the high
temperature insulation resistance (IR) was lower than desired. Small
quantities of dopants were added to the slip used in Example 12, namely
uranium oxide in Example 13, manganese carbonate in Example 14, and
niobium pentoxide in Example 15. The manganese dopant caused no change in
the characteristic temperature curve (X-12 in FIG. 2) and no change in
dielectric constant at 25.degree. C., but was most effective and is
preferred for reducing the DF and increasing the high temperature IR as is
seen from the data in Table II. On the other hand, the niobium additive
prevented complete sintering at the low firing temperature of 2025.degree.
C. and no useful dielectric material resulted in Example 15. It has thus
been concluded that no more than about 0.2 mole percent of the niobium
compound should be added to the titanate, zirconate and glass mixture.
Therefore, in the method of this invention, to obtain any significant
quantity of niobium in the ceramic phase, it would be necessary to calcine
the niobium with at least one of the ceramic start materials prior to
mixing with the glass and firing the body to maturity; and this, of
course, defeats a major purpose of the invention, namely to eliminate the
usual calcining step and reduce processing costs.
The grains of the ceramic phase were found to have grown to greater than 20
microns diameter as determined by a standard sectioning and electron
microscopy technique. The grains were further analyzed by a standard
electron probe method and found to contain about 0.5 weight percent
cadmium.
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