Study On Fabrication And Field-induced Strain Property Of Bi_0.5Na_0.5TiO₃-based Lead-free Piezoceramics

Piezoceramics are important functional materials in modern society. They are widely used in sensors, drivers, ultrasonic transducers, filters and other electron devices. Bi0.5Na0.5TiO3(BNT) is considered as one of the most promising lead-free piezoelectric materials because of its excellent ferroelectric property and high electromechanical coupling factor. In 2007, researchers found giant strain in BNT-based system. However, the electric field required to trigger giant strain is often higher than that in actual working conditions, making them unpractical. In the search for ways to address this issue, researchers found that relaxor/ferroelectric composite ceramics require lower electric field to generate giant strain. Above all, we will firstly construct BNT-based binary solid solutions, in the intention of achieving giant strain. Using the binary solid solutions as a base, we will construct relaxor/ferroelectric composite ceramics and study their strain property under low electric field. The structure, dielectric, ferroelectric and piezoelectric properties of the piezoceramics were also investigated.(1) We introduced SrTiO3(ST) into BNT and constructed(1-x)BNT-xST(0.2≤ x≤0.28) lead-free piezoceramics. Through the XRD analysis, we found that all groups of (1-x)BNT-xST(0.2≤x≤0.28) formed pure perovskite structure, showing quasi-cubic phase. Dielectric property revealed that the ceramics exhibited typical relaxor behavior when x≥0.22. Ferroelectric property indicated that ST reduced the ferroelectricity and pinched hysteresis loops were observed when x≥0.22. This suggested ST disrupted the dominant ferroelectric ordering in BNT and induced non-polar ordering. Field-induced strain property showed that the system obtained the largest unipolar strain at x=0.26 when the electric field is higher than 50 kV/cm. When x=0.26, d33* was found to be 259,412,521,488 and 463 pm/V at 30,40,50, 60 and 70 kV/cm, respectively.(2) Based on the study of BNT-ST, we constructed (1-y)(0.74BNT-0.26ST)-yBT (0.1≤y≤0.4) relaxor/ferroelectric composite lead-free piezoceramics. Two different types of grains were identified by microstructure characterization. Dielectric property revealed that relaxor feature still existed in these ceramics. Ferroelectric property suggested that ferroelectricity was improved with introduction of BT. Field-induced strain property revealed that the largest unipolar strain at 30 kV/cm was achieved at y=0.2(-0.10%), with d33*=344 pm/V. The giant strain under low electric field is due to the altered field distribution in relaxor/ferroelectric composite. The internal electric field of relaxor phase is enhanced, facilitating giant strain to be realized under low electric field.