Plastic and nonmetallic article shaping or treating: processes – Disparate treatment of article subsequent to working,... – Effecting temperature change
Reexamination Certificate
2000-08-16
2001-10-09
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Disparate treatment of article subsequent to working,...
Effecting temperature change
C264S332000
Reexamination Certificate
active
06299815
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing piezoelectric ceramics. Particularly, the invention relates to a process for producing piezoelectric ceramics which are useful as materials for, for example, piezoelectric ceramic filters, piezoelectric ceramic resonators and other piezoelectric ceramic devices.
2. Description of the Related Art
Piezoelectric ceramics mainly containing lead titanate zirconate (PbTi
x
Zr
1−x
O
3
) or lead titanate (PbTiO
3
) have been widely used as piezoelectric ceramics for piezoelectric ceramic filters, piezoelectric ceramic resonators and other piezoelectric ceramic devices.
Piezoelectric ceramics having a layer perovskite structure, such as CaBi
4
Ti
4
O
15
or PbBi
4
Ti
4
O
15
have higher heat resistance and lower loss at high frequency than the piezoelectric ceramics mainly containing lead titanate zirconate or lead titanate. The former piezoelectric ceramics are expected to be materials for piezoelectric resonators for use at high temperatures or at high frequency.
However, piezoelectric ceramics having a layer perovskite structure include highly anisotropic crystals and cannot yield a high electromechanical coefficient if they are produced by conventional processes for producing piezoelectric ceramics.
As possible solutions to this problem, some processes have been proposed. In these processes, the crystallographic axis of a piezoelectric ceramic mainly containing a compound having a layered perovskite crystal structure is preferably oriented in a uniaxial direction by using the crystal anisotropy of the compound to yield a high electromechanical coefficient. For example, T. TAKENAKA et al. reported that a c-axis oriented ceramic made of PbBi
4
Ti
4
O
15
is prepared by hot forging technique and that the resulting ceramic has an electromechanical coefficient k
33
about 1.6 times higher than a sample prepared by a conventional process for producing piezoelectric ceramics (J. Appl. Phys., Vol. 55, No. 4 (1984) P.1092). H. WATANABE et al. reported that a powered Bi
4
Ti
3
O
12
having anisotropic dimensions is prepared by the flux method and the powder having anisotropic dimensions is tape cast to orient the powder, and the grain oriented powder is fired to yield a c-axis oriented ceramic made of Bi
4
Ti
3
O
12
(J. Am. Ceram. Soc., Vol. 72, No. 2 (1989) P.289). T. TANI et al. reported that a c-axis oriented ceramic of CaBi
4
Ti
4
O
15
is prepared by adding powdered TiO
2
, Bi
2
O
3
and CaCo
3
to a powdered Bi
4
Ti
3
O
12
having anisotropic dimensions, subjecting the resulting mixture to tape casting to orient the powder having anisotropic dimensions, and firing the grain oriented powder, and that the resulting c-axis oriented ceramic has an electromechanical coefficient k
33
about three times higher than a sample prepared by a conventional process for producing piezoelectric ceramics (Proceedings of Presentations, The 16th Ferroelectric Application Conference, (1999) p.35).
The hot forging technique, however, requires firing with uniaxial pressing in the production of such a piezoelectric ceramic, requires expensive firing furnaces and kilns and exhibits a lower productivity as compared with conventional processes for producing piezoelectric ceramics. The hot forging technique has therefore not yet been in wide practical use.
In the processes for producing oriented ceramics using powders having anisotropic dimensions, impurities are liable to contaminate material powders having anisotropy in the production step. In addition, these processes require a preparation step of an anisotropic powder and thus require complicated production steps as compared with conventional processes for producing piezoelectric ceramics. These processes have therefore not yet been in wide practical use.
SUMMARY OF THE INVENTION
Accordingly, it is a main object of the invention to provide a process for producing piezoelectric ceramics, which can produce grain oriented ceramics from ceramic compositions mainly containing a compound having a layered perovskite crystal structure, and can improve the electromechanical coefficient of piezoelectric ceramics mainly containing a compound having a layered perovskite crystal structure.
The invention provides a process for producing piezoelectric ceramics. This process includes the steps of heating a ceramic composition at a temperature higher than the melting point of the ceramic composition to make the ceramic composition molten or semi-molten, which ceramic composition mainly contains a compound having a layered perovskite crystal structure exhibiting ferroelectricity at ordinary temperature, and annealing and solidifying the molten or semi-molten ceramic composition to form a grain oriented ceramic.
In the invented process, the ceramic composition main component preferably includes bismuth.
The ceramic composition in the invented process preferably includes at least one of, for example, Bi
2
WO
6
, CaBi
2
Nb
2
O
9
, SrBi
2
Nb
2
O
9
, BaBi
2
Nb
2
O
9
, PbBi
2
Nb
2
O
9
, CaBi
2
Ta
2
O
9
, SrBi
2
Ta
2
O
9
, BaBi
2
Ta
2
O
9
, PbBi
2
Ta
2
O
9
, Bi
3
TiNbO
9
, Bi
3
TiTaO
9
, Bi
4
Ti
3
O
12
, SrBi
3
Ti
2
NbO
12
, BaBi
3
Ti
2
NbO
12
, PbBi
3
Ti
2
NbO
12
, CaBi
4
Ti
4
O
15
, SrBi
4
Ti
4
O
15
, BaBi
4
Ti
4
O
15
, PbBi
4
Ti
4
O
15
, Na
0.5
Bi
4.5
Ti
4
O
15
, K
0.5
Bi
4.5
Ti
4
O
15
, Ca
2
Bi
4
Ti
5
O
18
, Sr
2
Bi
4
Ti
5
O
18
, Ba
2
Bi
4
Ti
5
O
18
, Pb
2
Bi
4
Ti
5
O
18
, Bi
6
Ti
3
WO
18
, Bi
7
Ti
4
NbO
21
and Bi
10
Ti
3
W
3
O
30
.
According to the invented process for producing piezoelectric ceramics, a compound having a layered perovskite crystal structure exhibiting ferroelectricity at ordinary temperature is oriented to yield a piezoelectric ceramic having a high electromechanical coefficient.
The new process can be performed in conventionally employed firing furnaces and kilns for the production of piezoelectric ceramics, and requires no extra facilities. In addition, the new process can employ conventional materials for use in the production of conventional piezoelectric ceramics. The invented process can more easily produce grain oriented ceramics at lower cost than the hot forging technique or the processes using a powder having anisotropic dimensions.
According to the invented process for producing piezoelectric ceramics, the temperature for making a material ceramic composition molten or semi-molten can be set at a temperature slightly higher than the melting point of the ceramic composition. By this procedure, even if processing is required after annealing, the processing operation can be easily performed. At a temperature slightly higher than the melting point of the ceramic composition, crystal grains of the ceramic composition can be moved or deformed to orient, the overall dimensions of the ceramic composition are not significantly changed as compared with those prior to heating, and the ceramic composition does not firmly adhere to a sagger or to a refractory powder. The sagger serves to house or hold the ceramic composition, and the refractory powder is placed between the sagger and the ceramic composition (sample) to prevent seizure.
Further objects, features, and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.
REFERENCES:
patent: 62-091458 (1987-04-01), None
Ando Akira
Kimura Masahiko
Fiorilla Christopher A.
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
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