Large direct and inverse electrocaloric effects in lead-free Dy doped 0.975KNN-0.025NBT ceramics
Résumé
Undoped and Dy3+ doped 0.975 K1/2Na1/2NbO3-0.025Na1/2Bi1/2TiO3 (0.975KNN-0.025NBT) ceramics were synthesized using the solid-state synthesis method. X-ray diffraction analysis revealed pure perovskite structure without any secondary phases. The dielectric investigations of both ceramics revealed low dielectric losses in the dysprosium doped ceramic, particularly for temperatures higher than 100 degrees C. The dielectric loss anomaly around room temperature corroborates the polymorphic phase boundary (PPB) between orthorhombic and tetragonal phases detected by XRD investigations. The electrocaloric effect (ECE) was investigated using the indirect method based on the Maxwell equation and thermal P-E hysteresis loops. The maximum ECE value was found equal to 0.63 K at 83 degrees C for KNNBT0.025. However, in Dy doped KNNBT0.025 ceramics, both direct and inverse ECE responses were found with values of 1.01 K at 118 degrees C and -1.79 K at 80 degrees C, respectively. Besides, yellow light emission was detected under an excitation wavelength of 350 nm. The simultaneous existence of high ECEs, good dielectric performance, and photoluminescence properties makes KNNBT-Dy a promising candidate for solid-state refrigeration and optoelectronic devices.