DOI: 10.20350/digitalcsic/15626
title: Electrocaloric effect near room temperature in lead-free Aurivillius phase Sr2Bi4Ti5O18 upon La and Nb codoping

description: [Description of methods used for collection/generation of data] Crystal structures were characterized by powder X-ray diffraction (XRD) at room temperature in a Rigaku D-Max system using Cu K??1,2 wavelengths. Rietveld analysis of the X-ray patterns was performed using the Fullprof package program. Electrical and electrocaloric properties measurements were conducted on sintered disc-shaped samples, with typical dimensions of 8 mm diameter and thickness between 0.5 and 1 mm. Electrodes were made with silver paste in sandwich geometry. The dielectric constant was derived from measurements with an impedance analyzer (Wayne Kerr Electronics 6500B), applying sinusoidal excitations of 1 V amplitude at several frequencies between 100 Hz and 4 MHz. The polarization versus electric field P(E) curves were recorded using a commercial polarization analyzer (aixACCT Easy Check 300) jointly with a high voltage amplifier (Trek 610E), applying sinusoidal excitations with typical frequency of 10 Hz and E field amplitudes up to 70 kV/cm. Direct measurements of the electrocaloric effect (ECE) were performed using a dedicated in-house-developed quasi-adiabatic calorimeter. The adiabatic temperature change was measured as a function of temperature every 1 K and at different applied E field variations. Samples were measured in the ???contactless??? mode to minimize heat losses, held only by the high voltage coaxial cable and K-type thermocouple for the direct measurement of the temperature variation. Voltages up to either 2 kV or 5 kV were applied using two different homemade high-voltage converters. Heat capacity measurements were also performed, using a commercial adiabatic calorimeter (from Termis Ltd.).
description: [Methods for processing the data] For the indirect estimation of the electrocaloric effect, well-known thermodynamic formulations based on Maxwell relations have been applied to the polarization data recorded at several temperatures, to derive entropy variations as a function of temperature.
description: La and Nb codoping of the five-layer Aurivillius phase Sr2Bi4Ti5O18 induces compositional disorder and diffuse relaxor transitions at much lower temperatures than the undoped ferroelectric compound (560 K), while preserving the orthorhombic structure. For moderate La and Nb concentrations, the ferroelectric properties are enhanced in the vicinity of room temperature, and so is their electrocaloric effect measured directly with a homemade quasi-adiabatic calorimeter. The adiabatic temperature change reaches, e.g., ??T ??? 0.3 K at 355 K with a temperature span ??? 15 K, under ??E = 37 kV/cm. The large field dispersion of the effect is ascribed to field-induced transitions from short-range-ordered relaxor to long-range-ordered ferroelectric states. Indirect estimations of the electrocaloric effect from electric polarization data are not able to quantitatively predict the direct results and emphasize the need of applying direct measurement methods, especially for relaxor ferroelectrics.
description: SBLTN_Fig2_XRay.dat SBLTN_Fig3a_DielectricConstant.dat SBLTN_Fig3b_DielectricConstant.dat SBLTN_Fig4a_DielectricConstant.dat SBLTN_Fig4b_DielectricConstant.dat SBLTN_Fig5_Polarization.dat SBLTN_Fig6a_Polarization.dat SBLTN_Fig6b_Polarization.dat SBLTN_Fig6c_Polarization.dat SBLTN_Fig6d_Polarization.dat SBLTN_Fig7a_DeltaT.dat SBLTN_Fig7b_DeltaT.dat SBLTN_Fig7c_DeltaT.dat SBLTN_Fig7d_DeltaT.dat SBLTN_FigS1_XRay.dat SBLTN_FigS2_XRay.dat SBLTN_FigS3_XRay.dat SBLTN_FigS4_XRay.dat SBLTN_FigS5a_Polarization.dat SBLTN_FigS5b_Polarization.dat SBLTN_FigS6a_Polarization.dat SBLTN_FigS6b_Polarization.dat SBLTN_FigS6c_Polarization.dat SBLTN_FigS6d_Polarization.dat SBLTN_FigS7a_Polarization.dat SBLTN_FigS7b_Polarization.dat SBLTN_FigS7c_Polarization.dat SBLTN_FigS7d_Polarization.dat SBLTN_FigS8_DirectECE.dat SBLTN_FigS9_DirectECE.dat SBLTN_FigS10_HeatCapacity.dat