Compositions – Piezoelectric – Lead – zirconium – titanium or compound thereof containing
Reexamination Certificate
2000-04-06
2001-08-28
Koslow, C. Melissa (Department: 1755)
Compositions
Piezoelectric
Lead, zirconium, titanium or compound thereof containing
C501S134000, C310S328000, C310S334000, C340S384600
Reexamination Certificate
active
06280650
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition and a buzzer and a piezoelectric actuator using the piezoelectric ceramic composition.
2. Description of the Related Art
There have been known various different piezoelectric devices formed by using a piezoelectric ceramic composition. Several such known piezoelectric devices are, for example, a piezoelectric buzzer, a piezoelectric actuator, a piezoelectric sensor and a piezoelectric ceramic filter.
In a process for attaching a piezoelectric device to a circuit board, the piezoelectric ceramic forming the piezoelectric device will be exposed to a high temperature of 200° C. or higher in a reflow furnace. For this reason, it is required that the piezoelectric ceramic should have a sufficient heat resistance.
In order to increase heat resistance of a piezoelectric ceramic formed by using lead zirconate titanate (hereinafter referred to as PZT), it has been known that some effective treatments are to increase the Curie temperature of the piezoelectric ceramic and to reduce the electrical resistivity of the piezoelectric ceramic. The reason for reducing the electrical resistivity of a piezoelectric ceramic may be explained as follows. If the electrical resistivity of a piezoelectric ceramic is reduced, it may be made possible to inhibit generation of pyroelectric charges at a temperature which is lower than Curie temperature, the pyroelectric charges causing a field reversing phenomenon that will in turn cause a reversed polarization, hence resulting in a deterioration in its piezoelectric characteristics such as the piezoelectric d constant and electromechanical coupling coefficient.
Further, in order to reduce the electric resistivity without greatly reducing the Curie temperature of a PZT ceramic, there is known another method in which an extremely small amount of Cr, Mn or the like is added to the ceramic. However, if Cr, Mn or the like is added in the above ceramic, its piezoelectric characteristics will be deteriorated. In order to avoid such a deterioration, further added was Nb or Sb, or the Pb displaced with Ca, Sr, Ba, La or the like, thereby prohibiting the deterioration of the piezoelectric characteristics while at the same time keeping its electric resistivity at a relatively low value although its Curie temperature will become a little lower. As such a piezoelectric ceramic composition, there has been, for example, an improved material formed by the PZT containing La, Cr and Sb as disclosed in Japanese laid-open patent applications (Tokkai) No. 10-7460 and No. 10-95666.
However, it was found that the above discussed conventional piezoelectric ceramic composition suffers a considerable deterioration in its piezoelectric characteristics due to the presence of Cr or Mn, and it was also found that even if Nb or Sb was added or Pb was displaced by Ca, Sr, Ba, La or the like in order to inhibit deterioration in its piezoelectric characteristics, the piezoelectric characteristics were still very low, resulting in a problem that it was impossible to obtain high piezoelectric characteristics and high heat resistance, both of which are however required when forming a piezoelectric buzzer or a piezoelectric actuator.
In general, larger piezoelectric characteristics such as a piezoelectric d constant and electromechanical coupling coefficient will ensure a higher performance for a piezoelectric device such as a piezoelectric buzzer and a piezoelectric actuator. On the other hand, since the performance of a piezoelectric device is likely to be deteriorated during the attachment process carried out in a reflow furnace, and since it is very important to ensure a stable drive at a high temperature, it is required that a piezoelectric ceramic should have a high heat resistance.
SUMMARY OF THE INVENTION
It is one object of the present invention to provide a piezoelectric ceramic composition suitable for use in forming a piezoelectric ceramic having both a higher heat resistance and a higher piezoelectric characteristics than those of a conventional product.
Further, the present invention is to provide a piezoelectric buzzer formed by using a piezoelectric ceramic having both higher heat resistance and higher piezoelectric characteristics than those of a conventional product.
Moreover, the present invention is to provide a piezoelectric actuator formed by using a piezoelectric ceramic having both a higher heat resistance and a higher piezoelectric characteristics than those of a conventional product.
A piezoelectric ceramic composition of the present invention is characterized in that about 0.2 to 2 mol % of the Pb of the lead zirconate titanate (PZT) represented by a composition formula Pb
A
[(Ti
x
Zr
1-x
)
a
(Sn
y
Cr
1-y
)
b
Sb
c
]O
3
has been displaced by La, wherein A, x, y, a, b, c in the above composition formula satisfy the following equations:
0.94≦A≦1.02, 0.46≦x≦0.51, 0.01≦y≦0.5, 0.01≦b≦0.03, 0.01≦c≦0.04,
a+b+c=
1.
A piezoelectric buzzer of the present invention is formed by using a piezoelectric ceramic, characterized in that the piezoelectric ceramic is formed using the piezoelectric ceramic composition mentioned above.
A piezoelectric actuator of the present invention is formed by using a piezoelectric ceramic, characterized in that the piezoelectric ceramic is formed by using the piezoelectric ceramic composition mentioned above.
With the piezoelectric ceramic composition prepared according to the present invention, since a part of the Pb atoms of the PZT has been displaced by La, and since Ti and Zr site (B site) have been partially displaced by Sb, it is possible to obtain a piezoelectric ceramic having high piezoelectric characteristics such as piezoelectric d constant and electromechanical coupling coefficient.
Further, with the piezoelectric ceramic composition prepared according to the present invention, since the B site is partially displaced by Cr, it is possible to practically maintain the piezoelectric characteristics of the piezoelectric ceramic, to reduce its electrical resistivity and to increase its heat resistance.
Moreover, with the piezoelectric ceramic composition prepared according to the present invention, since Cr is partially displaced by Sn, it is possible to inhibit possible deterioration of its piezoelectric characteristics which is otherwise caused due to the presence of Cr, while at the same time allowing the piezoelectric ceramic to maintain a good heat resistance. The reason for this fact may be explained as follows. Since the Sn will be dissolved into composition particles more readily than Cr, the process for Sb to be dissolved will be promoted. Since the precipitating amount of Sb onto the particle interfaces is reduced, it is possible to inhibit the formation of a different phase compound consisting of Pb and Sb, thereby improving the piezoelectric characteristics of the piezoelectric ceramic.
The above main object and other objects of the present invention, as well as the features and advantages of the invention, will become more readily understandable through a detailed description of preferred embodiments of the present invention.
REFERENCES:
patent: 4210546 (1980-07-01), Ouchi et al.
patent: 4255272 (1981-03-01), Ogawa
Horikawa Katsuhiro
Ogawa Tomoyuki
Koslow C. Melissa
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
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