Enhanced electrocaloric effect in lead-free NBT-based ceramics

Direct and indirect determination of electrocaloric effect in Na0.5Bi0.5TiO3

“…The obtained results of direct ECE measurements in NBT are in deep contrast to the published results obtained using the indirect ECE determining method [5,7,8,14]. The negative T of NBT, obtained by measurements of bipolar polarization hysteresis loops in the ferroelectric state and by applying of the Maxwell relation, can be explained by the increased resistance of domain reorientation to the change of the electric field if the temperature is lowered [9,25,47].…”

“…(3)) can be used in the interpretation of P(T) E=const , obtained from polarization hysteresis loops. This contribution is remarkable in case if E max does not exceed E coerc much [7,8,14] or in case of inclined polarization hysteresis loops with weakly expressed saturation [5]. In both cases a completely poled state is not achieved and P rem depends on E max .…”

“…Na 0.5 Bi 0.5 TiO 3 (NBT) -based compositions in the concentration region of rhombohedral phase and the morphotropic phase boundary (MPB), which are widely studied due to promising piezoelectric properties, have been recently considered also with respect to ECE [5][6][7][8][9][10][11][12][13][14][15][16][17]. Except for [11,17], all studies based on the indirect measurements of ECE revealed temperature regions with remarkable so-called negative ECE -decreasing of temperature if the electric field is applied and increasing of temperature if the electric field is removed.…”

Electrocaloric enhancement near the morphotropic phase boundary in lead-free NBT-KBT ceramics

“…23 Only when fully saturated hysteresis loops are observed (above 130 C), the trend of the indirect ECE starts to match that obtained by direct measurements. Indirect ECE measurements performed on NBT-18KBT were reported by Cao et al 20 Although they applied electric fields high enough to saturate the ceramics and observed a positive ECE over the studied range of temperature, they found a maximum ECE of 1.06 K at 80 C under 50 kV/cm, which correspond to an ECE responsivity of only 0.22 Â 10 À6 K m/V and is much lower than what we measured directly. The significant discrepancies between the direct and indirect measurements highlights the limitations of indirect ECE measurements and emphasises the need for a systematic direct characterisation of the ECE properties, especially for complex relaxor ferroelectric systems, as previously reported by Lu et al 24 Fig .…”

“…19 NBT-KBT shows its highest reported electromechanical properties, including polarization and piezoelectric coefficient near this composition. 19 NBT-18KBT has been considered for electrocaloric cooling and its ECE has been reported; 20 however, only indirect evaluations of the ECE have been published and actual ECE values remain to be confirmed by direct measurements.…”