1. Field of the Invention
The present invention relates to piezoelectric materials, and, particularly, to a lead-free piezoelectric material. The present invention also relates to a piezoelectric element, a liquid ejection head, an ultrasonic motor and a dust removing device that use the piezoelectric material.
2. Description of the Related Art
Piezoelectric materials are broadly used in, for example, actuators, ultrasonic oscillators, micro power sources, high-voltage generators. Many of the piezoelectric materials used in these devices are materials called PZT (lead zirconate titanate), which is an oxide containing lead (Pb), zirconium (Zr) and titanium (Ti). On the other hand, from the viewpoint of environmental protection, lead-free piezoelectric materials are being developed.
For example, a Ba-based perovskite oxide expressed by the general formula BaM′O3 is an example of the lead-free piezoelectric material. In this general formula, M′ represents a single element or a mixed crystal containing two or more elements in a certain composition ratio, and the charge of BaM′O3 is neutral. BaTiO3, which has a tetragonal crystal structure at room temperature, is an example of the piezoelectric materials expressed by BaM′O3.
In addition to oxides, oxynitride is another example of the perovskite compounds. BaTaO2N is a lead-free perovskite oxynitride used as a photocatalyst, as described in M. Higashi et al., Chemistry of Materials, vol. 21, pp. 1543-1549 (2009).
In Japanese Patent Laid-Open No. 2010-143788, a lead-free alkaline-earth metal-Bi perovskite oxynitride such as Ba0.6Bi0.4Ti0.6Si0.4O2.6N0.4 is used as a piezoelectric material. This material has a tetragonal crystal structure because of the presence of Bi atoms, and particularly when N atoms are arranged in the c axis direction, the tetragonal crystal structure has a large c/a (ratio of the c axis length to the a axis length in a unit cell). Accordingly, a piezoelectric material having a high Curie temperature (TC) can be expected, where the Curie temperature (TC) is defined as the temperature at which structural phase transition occurs from a tetragonal form into a cubic form and the piezoelectric characteristics of the material are disappeared. Also, the lead-free perovskite oxynitride shows a piezoelectric constant d33 half that of BaTiO3. The piezoelectric constant d33 is defined as a small strain in the c axis direction at a constant stress when a very small electric field is applied in the c axis direction.
For example, Japanese Patent Laid-Open No. 2010-143789 discloses an alkaline-earth metal-based perovskite oxynitride such as SrNbO2N as a perovskite oxynitride piezoelectric material. This is a piezoelectric material having a tetragonal crystal structure, and when N atoms are arranged in the c axis direction, the absolute value of the product of the piezoelectric constant d31 multiplied by the Young's modulus Y11 is large. The piezoelectric constant d31 refers to a small strain in the a axis direction at a constant stress when a very small electric field is applied in the c axis direction.
However, the perovskite oxynitride piezoelectric materials disclosed in the above-cited patent documents feature the structure in which N atoms are arranged in the c axis direction, and the synthesis process is limited. Accordingly, the industrial production of such a perovskite oxynitride piezoelectric material is difficult.
In addition, there have been reported very few lead-free perovskite compounds having a larger piezoelectric constant than BaTiO3.