Strain Actuation Behavior of Barium Titanate Single Crystal Loaded Electromechanically in Non-Variant [110] Direction

Strain hysteresis evolution of a (001)-oriented 5 × 5 × 2 mm³ cuboidal barium titanate (BaTiO₃) single crystal during a combined electromechanical loading sequence in the non-variant [110] direction is investigated. The goal is to compare the strain behaviors of the BaTiO₃ single crystal subjected to loading in the variant [001] and non-variant [110] directions. The simultaneous application of compressive stress and electric field in the [110] direction was achieved by machining the square cuboid crystal into a hexagonal prism and applying the loads parallel to the hexagonal side faces of the prism (i.e., perpendicular to the [001] and 45° to the [100] and [010] directions). The room temperature strain hystereses show that the maximum total electro strains produced from loading and measuring in the [110] and [001] directions (denoted as ε_[110],max,RT and ε_[001],max,RT, respectively, where the last term of the subscript describes the testing temperature) are 0.20% at 3.0 MPa and 0.45% at 2.7 MPa, respectively. The ratio between ε_[110],max,RT and ε_[001],max,RT is 0.44, which is in good agreement with the ratio, predicated by the analytical calculations. Factors which may influence the strain behavior, such as the bias stress level, depolarization field and switching coercivities, are examined by repeating the loading experiment at 55 °C. The strain hystereses measured at 55 °C show that ε_[110],max,55 is 0.19% at 11.9 MPa — this maximum [110] strain is similar to the one obtained at room temperature, but is only achieved with a much larger bias stress. When the out-of-plane depolarization field and the in-plane switching coercivities are reduced at 55 °C, more domains are randomized in the in-plane variant directions during electric field unloading by the depolarization fields. Therefore, a much larger bias stress is required at 55 °C to switch a sufficient number of domains to the out-of-plane variant directions at small electric fields, which can then be switched back to the in-plane variant directions at high electric fields, producing strain in the [110] direction. The strain hysteresis study has revealed that the combined effect of the depolarization field and switching coercivity is a critical factor governing the strain behavior of the BaTiO₃ single crystal.