Piezoelectric porcelain compositions are utilized in sound generators, vibrators, actuators, etc., as electrical-mechanical conversion elements utilizing the inverse piezoelectric effect, and also in sensors, generators, etc., as mechanical-electrical conversion elements utilizing the inverse piezoelectric effect. They are also utilized as electrical-mechanical-electrical circuit elements and also for mechanical-electrical-mechanical vibration control. Conventional piezoelectric porcelain compositions, normally, are constituted with Pb being the main ingredient. Conventional piezoelectric porcelain compositions are made, for example, of PZT constituted by two ingredients of PbZrO3 and PbTiO3, or of materials obtained by modifying PZT by Pb(Mg⅓Nb⅔)O3, Pb(Zn⅓Nb⅔)O3 or other third ingredient. Since Pb is harmful to the human body, however, efforts are underway to develop Pb-free piezoelectric materials.
Ferroelectrics, Vol. 160, P265-276 (1994) (Non-patent Literature 1) reports a Pb-free piezoelectric material made of a composite oxide of tungsten bronze type expressed by the formula Sr2-xCaxNaNb5O15 and, in particular, it reports that the composite oxide of tungsten bronze type has single-crystal piezoelectric characteristics. Also, Japanese Patent Laid-open No. Hei 11-240759 (Patent Literature 1) discloses, as piezoelectric porcelain for actuators, a composite oxide of tungsten bronze type expressed by the formula Sr2NaNb5O15. In Patent Literature 1, compositions that are obtained by partially substituting Nb of the composite oxide of tungsten bronze type with V and Ta, as well as those obtained by partially substituting Sr with at least one of Mg, B and Ca and also partially substituting Na with K, are proposed. Similarly, Japanese Patent Laid-open No. Hei 11-278932 (Patent Literature 2) discloses a composite oxide of tungsten bronze type expressed by the formula Sr2NaNb5O15, where compositions are proposed that are obtained by partially substituting Sr of this oxide with at least one of (Bi1/2Li1/2), (Bi1/2Na1/2) and (Bi1/2Na1/2) or with at least one of Mg, Ba and Ca. Furthermore, Japanese Patent Laid-open No. 2000-169229 (Patent Literature 3) discloses a piezoelectric material produced by adding 0.5 to 3 percent by weight of rare earth oxide Re2O3, as a characteristic improvement component, to a polycrystal piezoelectric compound expressed by the formula Sr2-xCaxNaNb5O15 (x=0.05 to 0.35). Particularly in Patent Literature 3, it is proposed that A in the composition expressed by the formula Sr2-xAxNaNb5O15 (in the formula, x=0.075 to 0.25) represent at least two types of elements selected from Ca, B and Mg. Japanese Patent Laid-open No. Hei 10-297969 (Patent Literature 4) discloses a piezoelectric ceramic material expressed by the formula (1−y)(Ba1-xSrx)Nb2O6-yNaNbO3, whose main ingredient is a porcelain component whose x is in the range of 0≦x≦1, while y is in the range of 0.15≦y<⅓.