1. Field of the Invention
The present invention relates to a bismuth layer compound which exhibits a piezoelectric property and which is applicable to pressure sensors, high-frequency filters, or the like. Description of the Related Art
The following have been known as ceramics which exhibit piezoelectric properties: barium titanate (BaTiO.sub.3), lead titanate (PbTiO.sub.3), lead zirconate titanate solid solution (hereinafter simply referred to as "PZT"), bismuth layer compound (SrBi.sub.4 Ti.sub.4 O.sub.15), and the like.
For example, since the PZT exhibits an exceptionally high sensitivity, it has been used as piezoelectric actuators.
The bismuth layer compound does not exhibit a sensitivity so high as that of the PZT. However, it exhibits a great strength and a high Curie temperature falling in a range of from 500.degree. to 800.degree. C. approximately. Accordingly, it can be used in a wider temperature range, and it is expected to be applied to pressure sensors.
Japanese Examined Patent Publication (KOKOKU) No. 55-16,380 discloses a piezoelectric porcelain composition which includes a bismuth layer compound containing manganese. The composition exhibits a high resistivity and a low permitivity. Further, it exhibits a large electromechanical coupling coefficient (k.sub.t) and a large mechanical quality coefficient (Q.sub.m). Furthermore, it exhibits a small capacitance temperature dependency coefficient (C.sub.x TC) and a small resonant frequency temperature dependency coefficient (f.sub.r TC). All in all, it is appropriate for making high-frequency filters.
However, the conventional bismuth layer compound has been found to exhibit considerably fluctuating sensitivities in the piezoelectric property in a temperature range of from 30.degree. to 150.degree. C. Even when manganese is added to the conventional bismuth layer compound in accordance with the aforementioned publication, the resulting bismuth layer compound has been also found to keep exhibiting the fluctuation in the sensitivity. As illustrated in FIG. 1, the sensitivity in the piezoelectric property herein means an electric charge per a unit force, i.e., E/F (in a unit of pC/N), when a force "F" is applied to a test specimen to generate an electric charge in an amount of "E."
Hence, in order to use the conventional bismuth layer compound as pressure sensors which should operate in a wide temperature range, it is indispensable to employ a temperature compensation circuit. Further, in order to use such pressure sensors at places where temperatures vary violently, there arises the problem in that the temperature compensation circuit tends to be complicated considerably. Thus, the bismuth layer compound has been inhibited from being applied to pressure sensors.