Note: Descriptions are shown in the official language in which they were submitted.
110()750
13r/\CKGRC)UND OF Tl~ INVENTION
Field of the Invention
Tllis invention is in the field of piezoelectric ceramic
compositions of perovskite crystalline structure, said compositions
being suitable for use as surface acoustic wave filters or the like.
DESCRIPTION OF THE PRIOR ART
There is a piezoelectric ceramic composition already known
having the formula:
Pb(N i 1/3Nb2/3)xTiyzrz 3
where x + y + z = l~ Examples of similar compositions are
PbTixZryO3, and Pb(Mg] /3Nb2/3)xTiyZrzO3. These materials, however,
have very high sintering temperatures so that they cannot be conveniently
produced despite their excellent piezoelectric properties.
Prior art perovskite piezoelectric ceramic materials which
contain lead oxide (PbO) as their main component are low in price and
relatively easy to produce as compared with single crystal piezoelectric
materials, In addition, there are various characteristics, for example7
such as electro-mechanical coupllng factor (Kp) dielectric constant (~ )
and the like can be adjusted by proper selection of the composition.
These materials are widely utilized for ignition elements, filters, pick-
ups, and the like. Such commercial materials, however, contain a
substantial quantity of PbO in their compositions. Consequently, when
the material is fired at a sintering temperature in the range of about
1250 to 1350~C, PbO is violently vaporized to cause the generation of
pores and a variation in the composition with the result that a uniform,
fine ceramic composi~ion cannot be obtained. If the firing temperature
is lowered in order to suppress the vaporization of PbO, a gas remains
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in the cer~llnic comlx)sition clue to the imperfect reaction, to produce a
number of la rge pores. ~ccordingly, the ceramic cornpositions being
produced in this area have a density which is substantially lower than
the theoretical density. The commercial materials average about 95
to 967~ of the theoretical density.
As will be described later, as tbe density becomes lower
than the theoretical density, the piezoelectric modulus becomes more
irregular. Consequentlywhen such a ceramic composition is used as a
high frequency vibrating source of more than 10 MHz, its characteristics
are very irregular and its propagation loss is great due to the presence
of pores each having a diameter of several tens of microns. When this
ceramic composition is attached to a comb-like electrode, each tooth of
which is less than 50 microns wide, to provide a surface acoustic wave
-Eilter, ~iscontinuous portions appear in the electrode resulting in the
deterioration of the element in its performance.
In the prior art, PbO, Bi203 or the like have been added
in substantial amounts in order to lower the sintering temperature. In
such case, however, these materials deposit in the grain boundaries and
in forming a surface acoustic wave filter device, the PbO or Bi203
in such boundaries is dissolved away when the sureace is chemically
washed. This etching of the grain boundaries by the cleansing liquid
spoils the desired polished surface of,the material.
SUMMARY OF T~!E Il~IVENTION
The present invention provides a piezoelectric ceramic
composition which is free from the above-described defects of the prior
art. It provides a piezoelectric ceramic composition which has a high
sintering density and has stabLe characteristics.
--2--
11()(~750
According to the present invention, the lead in the
prior art materials is partially replaced by cadmium. The
piezoelectric composition of the present invention has a
perovskite structure having the formula:
Pbl ACdA (Nil/3Nb2/3) xTiyZrz3
where x is in the range from 0.05 to 0.25
y is in the range from 0.30 to 0.95
z is in the range from 0 to 0.65
and x ~ y + z = 1
in which the Pb is partially replaced by Cd in the crystal such
that A is in the range from 0.005 to 0.02 and includes at least
one additional element, the additional element being cadmium,
manganese or tungsten. In the case of cadmium, I can add an
amount of from 0.7 to 1.5 weight %, calculated as CdCO3 of the
perovskite structure and including the amount specified in A.
The amount of manganese, calculated as MnO2 is in the range from
0 to 1.5 weight % of the perovskite structure. The amount of
tungsten, calculated as WO3 is in the range from 0 to 1.0 weight
% of the perovskite structure.
BRIEF DESC~IPTION OF THE DRAWINGS
Figure 1 is a ternary system diagram of the system
l-A A ( 1/3Nb2/3) - (Pbl_ACdA) TiO3 - (Pb Cd ) ZrO
-3-
11~(J750
I~`igllre 2 is a graph showing the relationship between the
cellter frequency of an acoustic surface wave filter manufactured by
using a ceramic composition as its substrate and sintering density
thereof; and
Figure 3 is a chart showing the center frequency distribu-
tion s~f the acoustic surface wave filter using the ceramic composition
produced according to the present invention as its s ubstrate .
DESCRIPTION O~ THE PREFERRED EMBODIMENTS
A description will first be given on the preparation of
a ceramic composition according to this invention, In the first step,
the materials are mixed and ground in the same manner as used in
the preparation of prior art piezoelectric ceramic materials. That is,
predetermined amounts of PbO, ~rO2, WO3, NiO, Nb2O5, TiO2,
CdCO3, and MnO2 are weighed out to form the composition and are
mixed together by either the wet or dry process. The calcination is
then carried out at a temperature of 800~C to 850C according to the
nature of the composition. Then, grinding is carried out either by wet
or dry process. The thus obtained calcined powder is molded in a
press at a pressure of 1 metric ton/cm2 and then fired for 1 to 3
hours at a predetermined firing temperature while oxygen gas is
supplied at a rate of 1 to 5 litres per minute. This rate of oxygen
flow is used when the molded element is located in a cover having a
cavity of 1 litre. As the capacity of the cover is increased, the
flow rate must accordingly be increased.
Table 1 sets forth a series of measurements of sintering
temperatures, sintering density and porosity relative to variations in
composition in the ceramic element according to this invention.
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In I able 1, X is P,b,~dfl(Nil/3 Nb2/3)03, Y is (PbCdA) '~q 6
TiO3, and Z; is (PbCd~ZrO3.
Aswillbe noted from Table 1,dependin~on the composi-
tion, the sintering temperature is lowered by 200 to 300C as com-
pared witll prior art sintering temperatures which occurred in the
range of 1250C to 1350C. Accordingly, a high sintering density is
obtained. The theoretical density is about 7.9 to 8. 1 which differs
depending on the composition, but the sintering density o~ the composi-
tions produced according to the present invention is very close to the
theoretical density.
Figure 1 illustrates a ternary system diagram of
(Pbl-A C~dA) (Nil/3Nb2/3)3 - (Pbl A CdA) -rio3 - (Pbl A CdA) ZrO3,
in which the compositions of samples 1 to 19 of Table 1 are repre-
sented by re~erence numerals 1 to l9, respectively. The composition
according to the present invention is circumscribed by the lines a b c
d a o~ Figure 1. The composition in the area above the line ab of
F igure 1 has a very low Curie point so that its temperature charac-
teristics deteriorate and also does not have the proper perovskite
structure. The composition in the area at the ri~ht of the line bc
has a low Curie point which, as mentioned, causes a deterioration in
its temperature characteristic, and a composition in the area below the
A~ ~ q ~ 3
line cd has low ~:rys-t~lfi~abi=~ '
Table 2 shows a series of measured results o~ sintering
tempera~ure, sintering density and porosity relative to adding various
amounts o~ CdC03, MnO2 and W03, to the composition
Pb1 -A CdA (Ni1 /3Nh2 /3)xTiyZrz 3 .
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In I able 2, ~ is PhCd(Nil~3N~2~3)03, Y is
(PbC,d)TiO3 and ~ is (PbCd)7rO3.
It will be noted from Table 2 that where the manganese
dioxide is constant, and tungsten oxide is not added7 if the total amount
of cadmium, including the amount used in replacing the lead as well as
the additional ~f~a~ amounts is less than 0. 7 weight ~
calculated as CdCO3, or higher than 1. 5 weight ~, the sintering density
is lowered and the porosity is substantially increased.
In the case where the amount of CdCO3 was constant
and WO3 was not added, if the additional amount of MnO2 was 1~ 5
weight % or less, good sintering occurred but if the additional amount
of MnO2 exceeded 1. 5 weight ~, the sintering was deteriorated.
When MnO2 was adcled in more than 1 5 weight ~, its accumulation at
the grain boundaries becomes great enough to increase the instability.
In the case where the amount of CdCO3 was constant and MnO2 was
not added, sintering proceeded well when the added amount of WO3 was
1 0 weight % or less, but the sintering deteriorated and the porosity
increased when the amount of WO3 exceeded 1.0 weight 7~. For these
reasons, the amount of Cd calculated as CdCO3 should be confined to
a range from 0.7 to 1.5 weight 7~, the amount of Mn, calculated as
MnO2 should be in the range from 0 to 1. 5 weight 7~, and the weight
of W, calculated as WO3 should be in the range from 0 to 1.0 weight ~.
Table 3 sets forth a series of measured values of di-
electric constant (~), dielectric loss (tan~), electro-mechanical
coupling factor (Kp), frequency constant (fR) and mechanical Q value
1~0~50
(QM) in the SitU~tiOIl where the ~mounts of CdC~3, MnC)2 and W03 are
va r ied .
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From Table 3 it will be understood that the piezoelectric
characteristics of Kp,~ and QM can be controlled by composite
addition of CdCO3 and MnO2; CdCO3 and WO3; and CdCO3; MnO2 and
WO3. Sample numbers in Table 3 correspond to the points
illustrated in Figure 1 of the drawings.
In Table 4 there is shown a comparison between a piezo-
electric ceramic element whose composition is Pb(Nil/3Nb2/3)0 1-
Tio 45ZrO 453 and that of Sample No. 13-3 of Table 1 according
to the present invention, with respect to pore size and porosity.
Table 4
Size of pore, Number of pores
(microhs) Element on the market Sample No. 13-3
0 - 5 1~4 105
5 - 10 50 0
10 - 20 26 0
20 - 30 7 0
30 - 40 1 0
-40 - 50 1 0
Porosity (~) 15.1 0.58
From Table 4, it will be evident that in the ceramic
element according to the present invention there are no large ~-
pores present, and the porosity is decreased by a factor of
about 30. In the above Table, the pores are measured by a
microscope with respect to a ceramic element having an area of
300 microns by 300 microns, and the surface being polished to
a mirror finish.
-17-
11~)(~750
l~ig~lr~ 2 shows the relationship between the sintering
density and the center frequency fO with respect to surface acoustic
wave filters made of materials of the same composition but with
various sintering density. According to Figure 2, assuming that the
sintering density of a ceramic element is 7.72 g/cc and varied + 0.2%
at the minimum, the center frequency fO would vary + 60 kHz as
indicated by ~ f2 in Figure 2. Since the ceramic composition of this
invention has a sintering density more than 8.00 g/cc, however, the
center frequency fO varies only about + 10 kHz as indicated by ~ f
even when the density is varied + 0.2%~ ~hus, with the ceramic
compositions according to the present invention, their piezoelectric
constants are less dependent on sintering density.
A surface acoustic wave filter of 10. 7 MHz was formed
from Sample No. 14 according to the present invention and the dis-
tribution of the center frequency fO was displayed by the dots shown in
l~igure 3. Samples of two lots, each having 3 blocks, were used to
form 54 elements. In this case, only 2 elements were out of the range
of + 0 27~, and 96.3% of the total elements were included in the range
of + ~%, 77.8% of the elements being in the range of + 0.15% In this
connection, the catalog value of a filter on the market using a prior
art ceramic material is 10.7 MHz + 0.13 MHz which represents a
scattering of about + 1.2%. With such a large scattering, it will cost
a considerable amount of labor to sort the filter elements, thus creating
a higher price. In addition, several types of specifications are necessary
for tuners to be used and the like. According to the present invention,
however, the scattering can be controlled to about + 0.2% and this
defect can be avoided.
-18-
.,
110~7so
In accordance with the present invention, the sintering
temperature of the material is lowered and the sintering density is
improved. The scattering of the center frequency is decreased, and
the fineness of the product is increased. Consequently, the charac~
teristics of the ceramic material are standardized, the reproducilibity
is improved, and the propagation efficiency is enhanced. In addition,
the discontinuous portions of electrodes can be prevented from being
formed, thereby increasing the yield. In producing a ceramic element
according to this invention, if the sintering is carried out in an oxygen
containing gas, the vaporization of PbO can be effectively suppressed.
Further, according to the invention, since a portion of
the Pb is replaced by Cd, the ceramic composition of this invention is
chemically stable even when the ceramic surface is washed or an
acid etchant is used with photoetching for selective formation of an
electrode on the ceramic surface.
It will be evident that various modifications can be made
to the described embodiments without departing from the scope of the
present invention,
-1 9 -
