Piezoelectric ceramic composition

A lead-oxide-free piezoelectric ceramic composition having an electromechanical coupling coefficient greater than a predetermined value and having a Curie point of at least 500.degree. C., whose main component is a two-component-based solid solution of the formula of (1-x)Bi.sub.a Ti.sub.b Nb.sub.c O.sub.9 -xNa.sub.p Bi.sub.q Nb.sub.r O.sub.9, and which has a bismuth layer structure in the range of 0<x<1, PA1 the solid solution of the above formula satisfying 2.4<(1-x)a+xq<3.1, PA1 0<b<1.1, PA1 0.9<(1-x)c+xr<2.1, and PA1 0<p<0.6.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a piezoelectric ceramic composition for 
use in a pressure sensor for use at high temperatures, a resonator, and 
the like. 
2. Background Art 
At present, piezoelectric ceramics are widely used not only in the fields 
of conventional electronic machines and devices such as resonators and 
filters but also in products using charges and displacements such as 
sensors and actuators. Generally, the material therefor has been selected 
from ferroelectric materials having a perovskite structure such as lead 
zirconate titanate ("PZT" hereinafter) or lead titanate ("PT" 
hereinafter). These materials give excellent piezoelectric characteristics 
when a third component or additive is incorporated. Since, however, most 
of them as a practical composition have a Curie point of approximately 
200.degree. C. to 400.degree. C., they become ordinary dielectric 
materials at temperatures higher than the above and lose piezoelectric 
characteristics, so that they can no longer be used at ultrahigh 
temperatures for a nuclear reactor and the like. Further, since the above 
materials contain approximately 60 to 70% by weight of lead oxide (PbO), 
they are undesirable in the ecological point of view and in view of the 
prevention of environmental pollution. 
As a piezoelectric ceramic containing no lead oxide, BaTiO.sub.3 having a 
perovskite structure similarly to the above materials is well known. 
Further, JP-A-9-100156 discloses a composition of (BiNa)TiO.sub.3 
-NaNbO.sub.3 which has a perovskite structure similarly to the above. 
Of piezoelectric ceramic materials having the above compositions: 
1 BaTiO.sub.3 has piezoelectric characteristic (electromechanical coupling 
coefficient) which is good to a great extent, while it has a Curie point 
of as low as 120.degree. C., so that it is environmentally extremely 
limited in use. 
2 (BiNa)TiO.sub.3 -NaNbO.sub.3 disclosed in JP-A-9-100156 has a large and 
excellent electromechanical coupling coefficient, while its Curie 
temperature is 370.degree. C. or lower, so that it cannot be used at a 
temperature higher than the above. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a 
piezoelectric ceramic composition which contains no lead oxide, has an 
electromechanical coupling coefficient greater than a predetermined value 
and shows a Curie temperature of 500.degree. C. or higher. 
According to the present invention, the above object of the present 
invention is achieved by a piezoelectric ceramic composition whose main 
component is a two-component-based solid solution of the formula of 
(1-x)Bi.sub.a Ti.sub.b Nb.sub.c O.sub.9 -xNa.sub.p Bi.sub.q Nb.sub.r 
O.sub.9, and which has a bismuth layer structure in the range of 0&lt;x&lt;1, 
the solid solution of the above formula satisfying 2.4&lt;(1-x)a+xq&lt;3.1, 
0&lt;b&lt;1.1, 
0.9&lt;(1-x)c+xr&lt;2.1, and 
0&lt;p&lt;0.6. 
The piezoelectric ceramic composition of the present invention contains no 
PbO and is therefore effective from the ecological point of view and in 
view of the prevention of environmental pollution. 
Further, the piezoelectric ceramic composition of the present invention has 
a high Curie point as compared with any one of the conventional materials 
(PZT and PT) containing PbO, BaTiO.sub.3 and the composition disclosed in 
JP-A-9-100156 and can be used at higher temperatures.

DETAILED DESCRIPTION OF THE INVENTION 
The specific constitution of the present invention will be explained 
hereinafter. 
The piezoelectric ceramic composition of the present invention is a solid 
solution whose main component is an entirely lead-free two-component-based 
solid solution of the formula of (1-x)Bi.sub.a Ti.sub.b Nb.sub.c O.sub.9 
-xNa.sub.p Bi.sub.q Nb.sub.r O.sub.9, and which has a bismuth layer 
structure in the range of 0&lt;x&lt;1. 
In the above formula, 
2.4&lt;(1-x)a+xq&lt;3.1, 
0&lt;b&lt;1.1, 
0.9&lt;(1-x)c+xr&lt;2.1, and 
0&lt;p&lt;0.6 
are satisfied. The oxygen content may deviate from its stoichiometric 
compositional amount. 
When the content of each component, particularly the Bi content, of the 
piezoelectric ceramic composition of the present invention is within the 
above range, the piezoelectric ceramic composition has a bismuth layer 
structure and has a high Curie point, specifically a Curie point of 
500.degree. C. or higher. As a ususal composition, the two-component-based 
piezoelectric ceramic composition of (1-x)Bi.sub.a Ti.sub.b Nb.sub.c 
O.sub.9 -xNa.sub.p Bi.sub.q Nb.sub.r O.sub.9, provided by the present 
invention, has the formula in which a=3, b=1, c=1, p=0.5, q=2.5 and r=2. 
That is, it is a composition of 
EQU (1-x)Bi.sub.3 TiNbO.sub.9 -xNa.sub.0.5 Bi.sub.2.5 Nb.sub.2 O.sub.9. 
Further, the above piezoelectric ceramic composition also contains 
additives such as Mn, and the like. When these additives are added, the 
piezoelectric ceramic composition has a remarkably improved mechanical 
quality coefficient Qm. The amount of these additives based on the above 
composition is preferably 0.005 to 1% by weight, more preferably 0.1 to 
0.8% by weight, particularly preferably 0.2 to 0.5% by weight. The 
additives such as Mn are generally added in the form of MnCO.sub.3 or 
MnO.sub.2. 
The piezoelectric ceramic composition of the present invention has an 
acicular or plate-like crystal form, characteristic of a bismuth layer 
structure. 
The method of producing the piezoelectric ceramic composition of the 
present invention will be explained hereinafter. 
As starting materials, there are used powdery materials of bismuth oxide 
(Bi.sub.2 O.sub.3), sodium carbonate (Na.sub.2 CO.sub.3), titanium oxide 
(TiO.sub.2), niobium oxide (Nb.sub.2 O.sub.5), and the like. Preferably, 
the powdery materials which have been fully dried at 100.degree. C. or 
higher are weighed so as to have a final composition in the above range 
specified in the present invention, and the powdery materials are mixed in 
a ball mill in the presence of an organic solvent (e.g., acetone) for 5 to 
20 hours, to obtain fully mixed powdery materials. The mixed powdery 
materials are fully dried and press-shaped, and the press-shaped mixture 
is calcined at approximately 750 to 900.degree. C. for about 1 to 3 hours. 
Further, the calcined mixture is milled in a ball mill or the like for 10 
to 30 hours and dried, a binder is added, and the mixture is granulated. 
Then, the granulated powder is press-shaped to form, e.g., pellets, and 
the press-shaped material is heat-treated at 400 to 600.degree. C. for 2 
to 4 hours, to volatilize the binder. Then, the resultant material is 
fired at approximately 900 to 1,350.degree. C. for 2 to 4 hours. The 
firing condition is that both the temperature elevation rate and the 
temperature decrease rate are 80 to 150.degree. C. per minute. A 
piezoelectric ceramic composition obtained by the firing is polished as 
required, and an electrode is provided. 
EXAMPLES 
The present invention will be explained more in detail with reference to 
specific Examples of the present invention hereinafter. 
As starting materials, powdery materials of bismuth oxide (Bi.sub.2 
O.sub.3), sodium carbonate (Na.sub.2 CO.sub.3), titanium oxide (TiO.sub.2) 
and niobium oxide (Nb.sub.2 O.sub.5) were provided, and weighed such that 
a final two-component-based piezoelectric ceramic composition had a 
composition of (1-x)Bi.sub.3 TiNbO.sub.9 -xNa.sub.0.5 Bi.sub.2.5 Nb.sub.2 
O.sub.9 in which x was as shown in Table 1. Then, these powdery materials 
were mixed in a ball mill using zirconia balls in the presence of acetone 
as a solvent for approximately 10 hours. The mixed powdery materials were 
fully dried, press-shaped and calcined at 800.degree. C. for 2 hours. The 
press-shaped mixture was again milled in the ball mill and then dried, PVA 
(polyvinyl alcohol) was added as a binder, and the mixture was granulated. 
The granulated powder was shaped into disc-shaped pellets having a 
diameter of 20 mm and a thickness of 1.5 mm with a single-axis 
press-shaping machine under a load of 196 MPa. The pellets were 
heat-treated at 500.degree. C. for 3 hours to volatilize the binder and 
then fired at a firing temperature of 1,100.degree. C. for 2 hours. The 
firing condition was that both the temperature elevation rate and the 
temperature decrease rate were 100.degree. C./minute. The sample obtained 
by the firing was polished to form a parallel flat plate having a 
thickness of approximately 0.5 mm, and a silver paste was baked at 
550.degree. C. to form electrodes. Then, an electric field of about 7 
MV/mm was applied to the sample in a silicone oil bath at 200.degree. C. 
for 30 minutes. Then, the sample was measured for mechanical quality 
coefficient of a radial mode, Qm, with an impedance analyzer. Then, the 
sample was cut with a dicing saw to give piezoelectric samples Nos. 1 to 5 
of Examples. 
The above-obtained piezoelectric samples Nos. 1 to 5 as a piezoelectric 
ceramic compositions were measured for X-ray diffractions. FIG. 1 shows 
the results. Then, lattice constants were calculated with regard to the 
above piezoelectric samples Nos. 1 to 5 as piezoelectric ceramic 
compositions. Table 1 shows the results. In the lattice constants shown in 
Table 1, Miller indices and c axis alone are very large (about five times 
as large as a axis and b axis), so that it is seen that the piezoelectric 
ceramic compositions of this Example had a bismuth layer structure. 
Further, each piezoelectric sample was measured for a Curie point and an 
electromechanical coupling coefficient of length-extension mode (k33). 
Table 1 shows the results. In addition, the electromechanical coupling 
coefficient of length-extensional (k33) and the mechanical quality 
coefficient (Qm) were calculated on the basis of a resonance and 
anti-resonance method using an impedance analyzer according to EMAS6000 
series (Standards of Electronic Manufactures Association of Japan). 
Comparative Examples 
Comparative piezoelectric samples Nos. 6 and 7 were obtained in the same 
manner as in Example 1 except that the raw materials were replaced with 
barium titanate (BaTiO.sub.3) or 0.97(Bi.sub.0.5 Na.sub.0.5)TiO.sub.3 
-0.03NaNbO.sub.3. The obtained samples Nos. 6 and 7 were measured for 
Curie points, electromechanical coupling coefficients of length-extension 
mode (k33) and mechanical quality coefficients (Qm) in the same manner as 
in Example. Table 1 also shows the results. 
TABLE 1 
__________________________________________________________________________ 
List of Characteristics of (1-x)Bi3TiNbO9-xNa0.5B12.5Nb2O9 
Lattice constant 
Sample No. 
Composition x 
a(.ANG.) 
b(.ANG.) 
c(.ANG.) 
Curie point (.degree. C.) 
k33(%) 
Qm 
__________________________________________________________________________ 
1 0.05 5.399 
5.446 
25.09 
905 6.45 
682 
2 0.10 5.408 
5.442 
25.09 
896 15.3 
606 
3 0.25 5.419 
5.451 
25.07 
880 11.1 
569 
4 0.50 5.438 
5.466 
25.05 
840 8.54 
1576 
5 0.75 5.446 
5.465 
25.02 
806 7.11 
507 
6* BaTiO.sub.3 120 49.3 
430 
7* 0.97(Bi.sub.0.5 Na.sub.0.5)TiO.sub.3 - 
341 43.3 
238 
0.03NaNbO.sub.3 
__________________________________________________________________________ 
*comparison 
Table 1 shows that since the piezoelectric ceramic composition of the 
present invention has a remarkably high Curie point as compared with the 
piezoelectric ceramics completely free of lead oxide, such as BaTiO.sub.3 
and (BiNa)TiO.sub.3 -NaNbO.sub.3, it retains piezoelectric characteristics 
at high temperatures at which the conventional piezoelectric ceramics 
cannot be used. Although the samples of Examples of the present invention 
showed lower electromechanical coupling coefficients of length-extension 
mode than the samples of Comparative Examples, the samples of Examples 
showed length-extensional electromechanical coupling coefficients of more 
than 6%, which are values to cause no particular problems on a sensor, a 
resonator, etc., for use at high temperatures.