Double flipping behavior of domains under unidirectional electric field and giant electrostrain by defect regulation

Ferroelectric materials find extensive applications in brake systems due to their capability to convert electrical energy into mechanical energy. Recent research has focused on lead-free materials for their environmentally friendly characteristics. However, they exhibit several challenges such as significant negative strain, limited strain values, and large driving field. In this work, novel preparation techniques (electrospinning) were utilized for BaTiO₃ to introduce oxygen vacancies and barium defects, facilitating the creation of oriented defect dipoles coupled with an intrinsic electric field (E_i) after poling and aging. Due to the existence of E_i, two minimum points in the strain hysteresis loop were shifted to the same quadrant in the Strain-Electric field space. Thus, when applying an electric field along the E_i direction, negative strain is eliminated. Additionally, the actual electric field is the sum of the applied electric field and E_i, thereby reducing the required driving field of the piezoelectric. The stretching of defect dipoles under the electric field further amplified the total strain. Through the proposed mechanisms, this work achieved a substantial unipolar electrostrain of 1.04 % under a relatively low electric field (30 kV/cm) in BaTiO₃. This work successfully addressed the challenges of high-driving electric fields, limited strain values, and negative strain, providing a comprehensive approach for improving field-induced strain performance through point defect engineering in ferroelectric materials.