| Sodium bismuth titanate lead-free piezoelectric ceramic (Na0.5Bi0.5TiO3, BNT) is one of perovskite-type ferroelectrics, in which both Bi3+ and Na+ occupy the A-site. During sintering process at high temperature, Bi3+ and Na+ would volatilize and introduce point defects in the ceramics, which can affect electrical properties. Here, (Bi0.5+xNa0.5+x)0.94Ba0.06TiO3+? lead-free piezoelectric ceramics were prepared via the conventional solid state reaction method. Microstructure, crystallite structure, piezoelectric, dielectric and ferroelectric properties of the ceramics were studied systemically.(1) 0.94(Na0.5Bi0.5+x)Ti03-0.06BaTi03(x=-0.04,0,0.02; named NB0.46T-6BT, NB0.50T-6BT, NB0.52T-6BT, respectively) lead-free piezoelectric ceramics were prepared. Effects of Bi3+ nonstoichiometry on structure and electrical properties were studied. All ceramics show pure perovskite structure. The lattice parameters increase with the increase in the Bi3+ content. The electron probe microanalysis demonstrates that the excess Bi3+ can compensate the Bi3+ loss induced during sample processing and facilitate improving the density of the ceramcis. The size and shape of grains are close related to the Bi3+ content. For the unpoled NB0.50T-6BT and NB0.52T-6BT, there are two dielectric anomalies in the dielectric constant-temperature curves. After poling, a new dielectric anomaly appears around depolarization temperature (Td) for all ceramics and the Td values increase with the Bi3+ amount decreasing from excess to deficiency. The diffuse phase transition character was observed for all samples. The activation energy values obtained via the impedance analysis are 0.69,1.05 and 1.16 eV for NB0.46T-6BT, NB0.50T-6BT and NB0.52T-6BT, respectively. The changes in structure and electrical properties of the ceramics with the change in the Bi3+amount are close related to the variation of the concentration of point defects.(2) The lead-free piezoelectric ceramics (Bi0.5Na0.5)0.94+xBa0.06TiO3 with x=-0.01, 0,0.01 and 0.02 were prepared. Effect of (Bi0.5Na0.4)2+ nonstoichiometry on structure and electrical properties were studied. The higher amount of (Na0.5Bi0.5)2+, the higher values of bulk densities. The mean grain size are 1.8,1.6,1.2 and 0.5 ?m for the ceramics x=-0.01,0,0.01 and 0.02, respectively. All ceramics show pure perovskite structure. The lattice parameters increase with the increase in x. The activation energy values obtained via the impedance analysis are 0.99,1.05,1.16 and 1.52 eV for the ceramics with x=-0.01,0,0.01 and 0.02, respectively. With the increase in x, Curie temperature of the ceramics decreases; depolarization temperature decreases first and then increases; piezoelectric constant increases first and then decreases. The ceramic with x= 0.01 shows the highest remnant polarization (Pr= 37.2?C/cm2) and highest piezoelectric constant (d33= 137 pC/N).(3) (N30.5Bi0.5)0.94Ba0.06TiO3 lead-free ceramics were prepared. Effect of heating rate at the sintering process on structure and electric properties of the ceramics were studied. All ceramics have pure perovskite structure. The mean grain sizes are 1.9,1.8, 1.4 and 1.3 ?m for the heating rates of 0.5,1,3 and 5?/min, respectively. Curie temperature and maximum dielectric constant decrease with the increase in the heating rate. The depolarization temperature are 117,111,110 and 108? for the heating rates of 0.5,1,3 and 5?/min, respectively. All samples show a diffuse character of the phase transition. The increase in the heating rates results in an increase in the ferroelectric properties and piezoelectric properties. |
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