High energy storage efficiency and electrocaloric effect in Pb-free Ba0.85Ca0.15Hf0.10Ti0.90O3 ceramics for sustainable green energy applications

Abstract

The growth of the economy and culture, as well as the fossil-fuel problem, global warming, and environmental pollution, have all driven huge efforts to build clean and sustainable energy alternatives in recent years. Compact refrigeration and energy conversion devices are in high demand these days. Dielectric/ferroelectric capacitors exhibit a high power density and fast charging/discharging speed than other energy storage devices, but their low energy storage density has limited their applications for commercialization. Also, electrocaloric effect-based solid-state cooling devices are having the potential to replace present vapor-compression-based refrigeration. However small adiabatic temperature change (ΔT) and narrow temperature span of ΔT of ferroelectric ceramics at room temperature limited their application for practical cooling devices. So, the research community is focusing to develop new ferroelectric materials that can have high ΔT, energy storage density, and efficiency at room temperature [1-4]. Pb-free BaTiO₃-based ceramics can achieve high energy storage density and large ΔT because their properties can be easily tailored by doping and fabrication conditions, and thus have the potential for clean energy applications [4, 5]. Porosity has great effect on the dielectric, energy storage, and electrocaloric properties of ferroelectric ceramics [6, 7]. Here in this work, we have successfully synthesized Pb-free Ba_0.85Ca_0.15Hf_0.10Ti_0.90O₃ (BCHT) ceramics by sol-gel method (fig.1A) and their structural, dielectric, ferroelectric, electrocaloric, and energy storage properties are presented in fig.1B. The room temperature XRD patterns show pseudo cubic type perovskite polycrystalline structure without any secondary phase. Fig.1(a) shows Rietveld refinement of two-phase co-existence model tetragonal (P4mm) and cubic (Pm3m) phase, with each phase contribution of 58 % and 42 % respectively. Fig.1(b) shows the FESEM micrograph of porous BCHT ceramic. The average grain size and pores diameter of around 2.5 µm and 1 µm are observed. The volumetric porosity of BCHT ceramic is found to be around 11 %. Fig.1(c) shows Raman spectra of BCHT ceramics recorded at room temperature. The spectra exhibit widened vibrational peaks, which indicate a phase transition from tetragonal to cubic phase. Temperature-dependent dielectric properties are shown in fig.1(d) at different frequencies 1kHz, 10kHz, 100kHz, and 1MHz. The dielectric constant characteristic curves show slight variation in the lower temperature region and very broad dielectric peaks are observed in temperature range 25 to 120 ⁰C at various frequencies, this is due to porosity [8]. Fig.1(e) and fig.1(f) show temperature-dependent P-E loops and P-T curves at the various electric fields from 0 to 20 kV/cm respectively. The adiabatic temperature change ΔT and entropy change ΔS have been calculated by an indirect method using Maxwell relations.  The ΔS and ΔT are calculated at a comparatively low electric field of 20 kV/cm as shown in fig.1(h) and fig.1(i). The room temperature value of ΔS and ΔT are -0.207 and 0.157 respectively, with a wide temperature span of 50 K. Temperature-dependent energy storage properties of BCHT ceramics are investigated by using temperature-dependent P-E loops.  Energy density (W_tot), recoverable energy density (W_rec), and energy storage efficiency (η) are estimated by using equations W_tot, W_rec, and η % [1]. Fig.1(g) shows temperature-dependent W_tot, W_rec, and η of BCHT ceramics.  The observed W_tot, W_rec, and η values at Room temperature are 84.43 mJ/cm³, 64.38 mJ/cm³ and 76.25 % respectively. The W_tot and W_rec values decreases and η increase with increasing temperature, the maximum value of η ~ 85.03 % has been observed at temperature 80 ⁰C. In conclusion, the high energy storage and electrocaloric properties of BT-based ceramics can be achieved by optimizing porosity level within the material and high applied electric field (high dielectric breakdown strength), So as to meet the desired performance for possible device applications in the field of sustainable green energy.