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BACKGROUND
This invention relates to a low-temperature sintered, lead-free
dielectric-ceramic body of barium titanate and a small amount of cadmium
silicate flux, the ceramic body having a high dielectric constant and a
Y5V temperature coefficient of dielectric constant, suitable for use in
high quality multilayer ceramic capacitors.
Multilayer ceramic capacitors (MLC's) having a Y5V dielectric temperature
coefficient and low dielectric losses (low DF) are becoming an increasing
portion of the large ceramic capacitor market. Progress toward better Y5V
ceramic bodies having higher dielectric constants (K) has been impeded by
the conflicting performance demands for higher K in a ceramic material
capable of being sintered at a low temperature to permit use of
high-silver content buried electrodes. Multilayer ceramic capacitors are
otherwise known as monolithic capacitors.
The capability of a MLC capacitor body for being sintered to maturity and
high density at low firing temperatures makes it possible to include
electrodes of mostly silver and of low precious metal content (e.g. Pd)
buried in the ceramic during firing. Sintering cost is consequently
reduced by lowering the required sintering temperature and the electrode
material costs are even more significantly reduced.
Typically a sintering aid or flux melts at sintering providing a medium in
which the process of simultaneous dissolution and recrystallization of the
ceramic materials ensue. For example, see U.S. Pat. No. 4,898,844 (Feb. 6,
1990); U.S. Pat. No. 4,120,677 (Oct. 17, 1978); U.S. Pat. No. 4,066,426
(Jan. 3, 1978); and U.S. Pat. No. 3,885,941 (May 27, 1975).
The cadmium silicates make an exceptional flux that does not melt at low
sintering temperatures, i.e. below about 1200.degree. C., as is explained
in patent U.S. Pat. No. 4,266,265 (May 5, 1981). Ceramic precursors, such
as the alkaline-earth-metal titanates, and a cadmium flux will combine at
low sintering temperatures, around 1100.degree. C., through a solid
diffusion process to form a low melting eutectic cadmium-bearing
alkaline-earth metal silicate. This silicate then melts to promote liquid
phase sintering. Cadmium fluxes are thus uniquely stable at such low
sintering temperatures and unlike other cadmium fluxes do not loose much
free cadmium and cadmium oxide to the atmosphere. A more complete
description of this process is provided in the above noted U.S. Pat. No.
4,266,265 and this patent is therefore incorporated by reference herein.
Lead zirconate is used in combination with the barium zirconate in most of
the examples in the above-noted patent U.S. Pat. No. 4,266,265, whereby
the large cation lead reduces the required sintering temperature to the
effect that smaller quantities of the sintering flux is required to enable
sintering at a low temperature. Typically, double the amount of flux is
required when the lead is omitted to obtain the same low sintering
temperature, and this greater amount of flux results in a decrease of the
dielectric constant. All but two of these ceramic compositions include a
very large quantity of lead. The lead zirconate amounts to as much as a
quarter of the total composition by weight. This may present a potential
environmental threat of lead poisoning both in manufacturing and even in
use of the capacitor product, raising the cost and limiting the use of the
product. Lead containing relaxor MLC bodies are also much weaker and break
easily during surface mounting.
Fine-grained bodies must be made by sintering very fine particle (e.g. 0.2
to 0.7 micron) powders having a high surface energy and such ceramic
bodies advantageously sinter at a somewhat lower temperature. In the paper
by Hennings et al, entitled Temperature-Stable Dielectric Based on
Chemically Inhomogeneous BaTiO.sub.3, Journal of the American Ceramics
Society, Vol. 67, No. 4, 1984, pages 249-254, it is shown that using 21/2
wt % of an additive composed of NbO.sub.2.5 .multidot.CoO with a pure fine
grained barium titanate, there was provided an X7R ceramic body with a K
of 3000 but required a sintering temperature of 1300.degree. C.
Fine-grained bodies of pure barium titanate, having been hot pressed and
sintered, have a K of about 3700 and are far from meeting the above Y5V
and high K.
To meet the Y5V standard, the K over the operating temperature range of
from -30.degree. C. to +85.degree. C. may vary only between -82% and +22%
relative to the K at 25.degree. C., and many users require a DF in Y5V
bodies that is no greater than 3.5%.
There are described in patents U.S. Pat. No. 5,010,443 (Apr. 23, 1991) and
U.S. Pat. No. 5,258,338 (Nov. 2, 1993) low-firing fine grained X7R barium
titanate ceramic bodies, i.e. a bodies wherein the average grain size has
increased only slightly from that of the start powders and the TCC is very
flat (+/-5%) over a wide temperature range (-55.degree. to +125.degree.
C.). The bodies include a cadmium silicate sintering aid, or flux. The
sintering caused little grain growth because of the intentional inclusion
of a grain-growth inhibitor, e.g. niobium, in the start powders. These
small grain bodies, in which during sintering there was essentially no
reaction of the grain growth inhibitor with the barium titanate, provides
a MLC capacitor having a smooth temperature coefficient of capacitance
(TCC) meeting the X7R standard and having bodies with a dielectric
constant of several thousands.
The above-mentioned patents show the issue dates in parentheses and these
patent are assigned to the same assignee as is the present invention.
It is an object of this invention to provide an improved ceramic powder
mixture for use in manufacturing an essentially lead-free, high dielectric
constant, ceramic body for use in multilayer capacitors capable of being
sintered at a the low temperature of 1100.degree. C., the resulting
ceramic bodies meeting the Y5V temperature coefficient standard and having
a dielectric constant at the Curie temperature of ten thousand or greater.
SUMMARY
A method for making a ceramic powder mixture initially includes preparing a
powder mixture having an average particle size of about 1 micron by
combining from 96 to 98 mole percent of precursors of a stoichiometric
barium zirconate titanate (BZT) wherein zirconium amounts to from 13.5 to
15.0 mole percent, and wherein up to 4 mole % of the barium is replaced by
strontium.
There is added to this combination a compound containing from 0.5 to 1.5
mole percent of Curie-point shifter cations, from 1.2 to 2.6 weight
percent of a cadmium silicate sintering flux wherein the molar ratio of
CdO to SiO.sub.2 ranges from 1:1 to 3:1 inclusive, from zero to no more
than 0.01 mole percent of Pb, and from zero to 2 mole % of an alkaline
earth metal selected from barium, calcium, strontium and combinations
thereof to effect in the powder mixture a ratio (A/B) of the large cations
(A) to the small cations (B) in the range from 1.024 to 1.035.
This homogenous mixture is then mildly calcined at approximately
700.degree. C. to obtain a powder comprised of agglomerates of the powder
mixture wherein each of the agglomerates has essentially the same
composition.
Each of the ingredient powders in the mixture have been adjusted within the
corresponding ranges stated above to provide a powder mixture capable of
being formed into a body that can be fired to maturity at 1100.degree. C.,
and subsequently annealed at a lower temperature to produce a mature
dielectric ceramic body meeting the Y5V standard and having a dielectric
constant greater than 10,000 at the Curie temperature.
The ceramic body is thus manufactured by the additional method steps of
forming a body of the calcined homogeneous mixture, applying at least two
separate electrodes to the body, sintering the body at about 1100.degree.
C. in a closed container, and annealing the body at about 1050.degree. C.
in an open air atmosphere to provide a mature dielectric ceramic body
having a dielectric constant greater than 10,000 at the Curie temperature
and meeting the Y5V standard. It is preferred that the fore-mentioned
adjusting is additionally to provide in the mature dielectric ceramic body
a Curie temperature lying between 0.degree. C. and 20.degree. C., and even
better between 5.degree. C. and 15.degree. C.
This invention recognizes that during the low temperature liquid phase
sintering facilitated by the cadmium silicate flux, the crystal formation
process itself favors both a balance of large cations to small cations in
the crystals of the sintered ceramic body and a charge balance in the
crystals of allovalent large cations to allovalent small cations. In this
BZT formulation, allovalent large cations are those that do not have
charge of +2 as do Ba and Zr, and allovalent small cations are those that
do not have a charge of +4 as does Ti, Zr, etc.
Extensive reaction and grain growth is made possible here by providing a
slight surplus of all large cations to all small cations in the start
powder mixture, whereby the most of the excess large cations will be left
in the grain boundaries after sintering along with the modified cadmium
silicate flux.
The extent to which the additive cations (to BZT) are incorporated in the
BZT grains will be determined by those that pair up to achieve balance,
i.e. both large to small cation balance and charge balance, the grains
will continue to incorporate them and to grow until the available
balancing cations are consumed. After sintering, the cations that were
initially dissolved in the flux during sintering which could not mate up
in a combination that has large-small cation balance and charge balance is
left in the grain boundaries. On the other hand, unreacted small cations
left in the grain boundaries tend to act as grain growth inhibitors. For
example, some of the cadmium from the flux is incorporated in the grains
taking with it some niobium and/or displacing some of the barium of the
start BZT. The silicate in the grain boundaries is then likely to contain
more barium and less cadmium than before sintering. Cadmium when
incorporated in the grains has a pronounced downward shifting influence on
the Tc in barium titanates, having promoted the incorporation of niobium
in the grains. This large-cation surplus also insures that the Curie-point
shifter compound (niobium), does not behave as a sintering inhibiter agent
as in the X7R ceramic bodies described in the above-mentioned patents U.S.
Pat. No. 5,258,338 and U.S. Pat. No. 5,010,443 but rather reacts fully
with the zirconate titanate of the start powders in this invention to
contribute to a downward shift of the Curie temperature. Thus capacitors
made from the prior art powders of the later two mentioned patents exhibit
comparatively little shifts in Curie temperature of the barium titanate
during the sintering step, and in the process of making them there was
little grain growth.
This invention provides a method for making a very high dielectric constant
ceramic dielectric with a Curie temperature in the range from about
5.degree. C. to 20.degree. C. employing a dominantly BZT composition
having at once a small amount of additives including a cadmium silicate
flux and essentially no lead. The BZT with high zirconate content and the
carefully balanced additives provide a strongly down-shifted Curie
temperature and otherwise make it possible to manufacture such dielectric
ceramic bodies without lead that meet the EIA TCC standard Y5V.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows in side sectional view a wafer or disc type ceramic capacitor
of this invention.
FIG. 2 shows in side sectional view a monolithic or multilayer ceramic
capacitor (MLC) of this invention.
Table A shows the effect on dielectric properties of varying the amount of
zirconium in the BZT start powders.
Table B shows the effect on dielectric properties of varying the types of
compounds in the start powders, which compounds will during subsequent
sintering form the BZT component of the ceramic.
Table C shows the effect on dielectric properties of varying the
composition of the sintering flux in the ceramic composition.
Table D shows the effect on dielectric properties of varying the amount of
flux in the ceramic composition.
Table E shows the effect on dielectric properties of varying the amount of
barium in the form of barium carbonate or other oxide equivalents in the
ceramic composition.
Table F shows the effect on dielectric properties of varying the amount of
niobium in the form of niobium oxide or other oxide equivalents in the
ceramic composition.
Table G shows the effect on dielectric properties of varying the amount of
manganese in the form of manganese oxide or other oxide equivalents in the
ceramic composition.
Table H shows the effect on dielectric properties of varying the amount of
free silica in the form of silicon dioxide or other oxide equivalents in
the ceramic composition.
Table I shows the effect on dielectric properties of substituting lanthanum
titanate for niobium oxide in the ceramic composition.
Table J shows the effect on dielectric properties of substituting calcium
oxide for barium oxide in the ceramic composition.
Table K shows the effect on dielectric properties of adding strontium
titanate to the ceramic composition.
Table L shows the effect on dielectric properties of adding calcium
titanate to the ceramic composition.
Table M shows the effect on dielectric properties of varying the amount of
additives to the BZT component in the start powders of the ceramic
composition.
Table N shows examples of compositions and dielectric properties of
multilayer ceramic bodies having been sintered with buried electrodes of
30%/70% palladium/silver.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A number of related experiments, namely Examples 1 through 46, were carried
out for producing test capacitors of the disc type as shown in FIG. 1
having a dielectric ceramic body 10 and conductive electrodes on opposite
faces of the body 10. The procedure employed in each case entailed forming
a mature barium zirconate titanate disc or chip of about 30 mils (0.76 mm)
thick and forming electrodes on the two opposite major surfaces of the
chip.
The start barium zirconate titanate powder (BZT) in Examples 4, 9-31, 36-37
and 44-48 was a fully co-reacted pure stoichiometric barium zirconate
titanate powder. The start barium titanate powder in Examples 1-3, 5-8,
32-35, 38-43 and 49-52 was a mixture, e.g. a mixture of barium titanate
(BT) and barium zirconate (BZ), or a mixture of barium titanate (BT) and
barium zirconate titanate (BZT), or a mixture of BZT's containing
different molar amounts of zirconium, or a mixture of the oxides of
barium, zirconium and titanium. A BZT powder having 10 mole percent
zirconium is indicated in the data tables as either 10Z*BT or simply as
BZT with the zirconium content spelled out below in mole %. The proportion
in a mixture of two BZT powders, e.g. 10Z*BZT+20Z*BZT as in Examples
38-40, is indicated by the stated mole % zirconium in the mixture. These
start powders in all examples herein were produced by conventional
commutation and have an average particle size of about 1 micron.
A start cadmium silicate powder was made by ball milling cadmium oxide
(CdO) and silica (SiO.sub.2) with an isopropyl alcohol wetting agent and
using high density yttria-stabilized zirconia balls in a polyethylene
bottle. The milled powder was then dried and calcined at about 950.degree.
C., and the resulting calcine was remilled to obtain a calcined powder.
A slip suspension was prepared by mixing 100 parts by weight of the start
barium zirconium titanate powder, or powder mixture, with about 1 weight
percent of niobium oxide powder Nb.sub.2 O.sub.5, 1.31 weight percent
barium carbonate (BaCo.sub.3), 0.1 weight percent manganese carbonate
(MnCo.sub.3), and 2.02 weight percent of a powdered cadmium silicate
sintering flux in an organic vehicle.
The start barium titanate zirconate powder or powder mixture with additives
had an average particle size of about 1 micron. These start powders were
combined and mixed by ball milling with yttria stabilized zircona balls
for 12 hours in a 250 cc polyethylene container.
The resulting slip was cast in a thin layer, dried and granulated to
produce a powder, which was then pressed at about 50,000 p.s.i. to form
disks having diameters of 0.5 inches (12.7 mm) and a thickness of 30 mils
(0.76 mm). The discs were baked to drive off all organic materials.
Sintering was accomplished in a closed alumina container at 1100.degree.
C. After cooling the discs, a silver electroding paste was applied to the
opposite surfaces of the sintered discs which were subsequently heated to
800.degree. C. to cure the electrodes.
The ratio A/B of large cations A and small cations B in the total ceramic
start composition is shown in the tables. The dielectric constant K and
percent dissipation factor DF were measured at 25.degree. C. and at 1 volt
and 1 KHz. The symbol K.sub.R is used to mean K at 25.degree. C. The Curie
temperature, Tc, is in centigrade degrees. The temperature coefficient of
capacitance (TCC) of a ceramic dielectric is essentially the same as the
temperature coefficient of the finished capacitor. The TCC data given in
the tables are the percent above and below (-) the value of the dielectric
constant at 25.degree. C. at temperatures respectively of -30.degree. C.,
T.sub.c, and 85.degree. C., respectively. Also given are %K @85.degree.
C., % (K.sub.C -K.sub.R)/K.sub.R and %K @-30.degree. C., where the symbols
K.sub.R and K.sub.C mean respectively K at 25.degree. C. and K at Tc. The
Electronic Industries Standard designated Y5V requires the K of a
dielectric ceramic be no greater than 22% and not below -82% of the K at
25.degree. C. (K.sub.R) over this range of temperatures. The highest K for
a BZT dielectric is always at the Curie point, and is thus K.sub.C.
In Examples 1, 2 and 3 it can be seen that with increasing zirconium the
Curie temperature Tc decreases significantly. Also as shown in Table A,
although the dielectric constant K is very high in all of the Examples 1,
2 and 3, the peak K (at Tc) is greater than 22 percent higher than the K
at 25.degree. C. and does not meet the Y5V standard. The capacitor of
Example 1, having 14 mole % zirconium in the BZT start powders, is however
much the closest.
It is estimated that a start BZT ceramic dielectric containing 13.5 mole %
zirconium could be made to meet the Y5V standard if it included a slightly
greater amount of niobium, or other a downward Tc shifting element for
compensating the smaller shift in Tc effected by reduction in the
zirconium amount. But such compositions can only be marginally good for
Y5V applications.
Five groups of experimental disc capacitors designated respectfully as
Examples 4, 5, 6, 7 and 8, each included a dozen capacitors for which data
is provided in Table B. These capacitors were made in accordance with the
above described method that was employed to make the capacitors of three
groups designated Examples 1, 2 and 3, except the zirconium is held at 14
mole percent in the barium zirconate titanate start powder for all five
examples and the procedure for making the barium zirconate titanate start
powder was varied.
In Example 4 of Table B, a fully calcined and co-reacted barium zirconate
titanate powder was used. The method for making disc capacitors in Example
5 is exactly as for the capacitors of Example 1 and serves as a control in
the Examples 4, 5, 6, 7, and 8 with respect to Examples 1, 2 and 3. The
barium zirconate titanate start powder of Example 6 is a mixture of pure
barium titanate (BT) and a barium zirconate titanate having 20 mole %
zirconium (20Z*BZT) proportioned to contain the desired 14 mole percent
zirconium. The barium zirconate titanate start powder of Example 7 is a
mixture of a pure barium titanate (BT) and a pure barium zirconate (BZ).
The start barium zirconate powder in Example 8 began with a mixture of the
oxides of barium, zirconium and titanium. In all cases the average
particle size of the mixture was about 1 micron.
In the start powders for making the capacitors of Table B, the additives to
the barium zirconate titanate start mixture are all essentially the same
in the compositions in this family of five examples, except the niobium
content in Examples 4 and 7 is about 5% greater. As will be explained
below with respect to Examples 23 to 27, a change in niobium content of 5%
is not nearly enough to explain the differences in Examples 6 and 7
regarding Tc, TCC and K. The differences in performance between examples
here is dominantly attributable to the different combinations of start
compounds for introducing zirconium and titanium to the start mixture, and
their very small resulting differences in the ceramic of degree of
fullness of the reaction of the components of the start materials after
sintering.
Thus in all the Examples 4 through 8 it can be concluded that all of the
experimental start materials for forming the BZT heart of the ceramic are
viable for making Y5V ceramic dielectric bodies. Specifically, the disc
capacitors of Examples 7 and 8 meet the Y5V standard and those of Example
4, within experimental error in the data for (K.sub.C -K.sub.R)/.sub.R,
are seen capable of meeting the standard also. The ceramic of Example 4 is
seen to be most fully reacted and provides the best performance, though
not much different from the other four.
The disc capacitors of Examples 9, 10, 11, 12 and 13 for which compositions
and electrical performance data are given in Table C, provide a means for
assessing the effect of variations in the compositions of the cadmium
sintering flux. (Note that Example 11 is the same as Example 2 in Table
A.) The trend seen in the series of Examples 9, 10 and 11 is that as the
amount of cadmium in the cadmium silicate flux is increased, the Curie
temperature Tc is reduced, which is attributable to the fact that for each
Cd.sup.+1 (large cadmium cation of charge +1) that is taken into the
grains, there must be for charge balance an equal molar amount of a small
cation of +5, and niobium meets that description and is available. The
niobium that accompanies the cadmium into the grains acts as a strong
shifter of the Curie temperature, in the downward direction. Further
increases in cadmium content as in Example 12 has little effect, which
indicates that the BZT grains have taken in all the cadmium that they can,
i.e. exceeding the saturation limit.
The borate flux of Example 13 produces a dramatic reduction in K because,
explained in the above-mentioned patent U.S. Pat. No. 4,266,265, boron
flux tends to totally envelope each ceramic grain during sintering, which
when cooled leaves a low K film around each grain and so separates the
grains and reduces the dielectric of the ceramic body. On the other hand,
the inter-granular cadmium flux tends to ball up at only the largest
points between the grains.
Although the performance of only Example 9 in Table C meets the Y5V
standard, the ceramic dielectric bodies of Examples 10 through 12 have a
high K, a much reduced Tc and thus a lower DF. Their failure to have a low
enough (K.sub.C -K.sub.R)/K.sub.R causes them to fall outside the Y5V
standard. However, using the more cadmium rich fluxes of Examples 10, 11
and 12, and making only small changes in these examples in the amounts of
zirconium, niobium and the total amount of flux toward those used in
Example 14 of Table D, the Y5V standard can be met.
These cadmium richer fluxes are thus preferred, and it is concluded from
the data provided herein and from other experience that no less than 1.2
weight percent of the flux will be necessary for sintering these bodies to
maturity at 1100.degree. C.
It is also concluded that it will not be possible in this BZT system to
achieve Y5V performance with high dielectric constants when the cadmium
flux content exceeds about 2.6 weight percent. Higher flux content tends
to contribute more low K intergranular material to the detriment of high
dielectric constant and at the same time begins to degrade the life test
performance, particularly insulation resistance.
The theme of charge balance and cation ratio is pursued further in the
examples of Tables D and E. In Table D the effects of the amount of flux
are shown, and in Table E the amount of BaCO3 additive is changed, both of
which alter the charge balance and the cation ratio.
Referring to Table D, Examples 14, 15, 16 and 17, only the amount of flux
is changed. As the amount of the cadmium silicate flux is increased the
Curie temperature decreases and A/B increases. The higher A/B implies a
larger amount of intergranular material which is consistent with the trend
toward lower dielectric constant at Curie temperature. But these data
indicate that amounts of flux ranging from 1.2 to 2.6 weight percent can
be included for making Y5V ceramic bodies with K.sub.C greater than
10,000.
In examples 18, 19, 20, 21 and 22 for which data is shown in Table E, the
amount of the barium carbonate is varied while all other components of the
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