Electrostrictive Characteristics of PMN Ceramics as Raw Materials of Micro-displacement Actuators

Some actuators made from piezoelectric ceramics are recently used in high precision X-Y tables for manufacturing semiconductors. However, it is very difficult to control the table movement with the high accuracy and high reproducibility of sub-micron order, because the piezoelectric ceramics such as PZT have a peculiar hysteresis. For the materials of micro-displacement actuators, one of the electrostrictive ceramics, PMN (Pb(Mg_1/3Nb_2/3)O₃-based ceramics), is more preferable than the other piezoelectric ceramics, because it generates large strains up to 10-³, has the small hysteresis and Young's modulus equal to 1/2 times the steel one. However, the PMN ceramics has a defect that the induced electrostrictive strain becomes very small if an impurity-pyrochlore structure appears in the sintered PMN. So the sintering methods have been researched to obtain the pure PMN material. The authors tried some sintering methods to reduce the hysteresis of the PMN material as much as possible. As the results, the followings are obtained : (1) There are the interesting relations among the impurity-pyrochlore structure, the electrostrictive strain and the hysteresis. (2) It is found a sintering method by which the hysteresis is much reduced, though the electrostrictive strain also becomes only a little.