Electrocaloric Effect And Energy Storage Characteristics Of Barium Titanate Based Ceramic Composites

Over the last few decades,lead-based materials are universally used in electronic devices and people's daily life because of their excellent electronic properties,such as PT,PZT,PLZT and so on.But the lead-based ceramics are a detrimental source for our living environment and personal health because its toxicity.Consequently,World Health Organization has legislation to reduce the use of electronic products containing Pb and PbO.In view of the above,the eco-friendly BaTiO₃-based materials have been extensively studied in recent years,because of their widespread applications in various microelectronic devices.In this paper,The structure,dielectric,ferroelectric property and ECE were systematically studied for the various BaTiO₃-based ferroelectric ceramics.First of all,the microstructure,dielectric property and electrocaloric effect were investigated in the lead-free?1-x?BaTiO₃–x(BiMg_1/2Ti_1/2)O₃??1-x?BT–x BMT,x=0.0–0.05?ferroelectric ceramics.Under the low electric field(E?20 kV cm^-1),the coexistence of a positive and negative electrocaloric effect was obtained in?1-x?BT–x BMT?x?0.04?ceramics system using the Maxwell relation.Negative electrocaloric behavior disappears and the positive electrocaloric effect was only observed at high electric field(E?25 kV cm^-1).Firstly,the values of both negative electrocaloric effect under low electric field of E?20 kV cm^-1?NECE?and positive electrocaloric effect in the high electric field of E?25 kV cm^-1?PECE?increase,then decrease with increasing BMT content in the ceramics.The0.98BT–0.02BMT ceramic shows the excellent PECE and NECE capabilities.The corresponding giant electrocaloric effect values of-0.37 K?42°C?at 20 kV cm^-1 and 1.21 K?143°C?at 55 kV cm^-1 were observed.These results show great potential in achieving large cooling power as refrigerants.And then,the the high-temperature dielectric relaxation behaviours?HTRBs?and energy storage properties were investigated in?1-x?BT–x BMT?x=0.0–0.07?.HTRBs were observed in all the samples.The activation energy calculated from impedance?E_a?and conductivity(E_cond),which revealed that the relaxation behaviours were linked with the migration of OVAs.The values of E_a were almost equivalent to E_cond.It was concluded that the short-range hopping of oxygen vacancy contributed to the dielectric relaxation and long-distance movement of doubly ionized oxygen vacancies contributed to the conduction.On the other hand,the energy-storage density W=0.62 J cm^-3 and the associated efficiency?=88%were determined from the P–E loops data of 0.94BT–0.06BMT ceramic.What's more,The structure,dielectric,ferroelectric and electrocaloric properties were studied in Ba(Hf_xTi_1-x)O₃?BHT?ferroelectric ceramics.Large electrocaloric effect values of1.64 K?117°C?and 1.21 K?76°C?were observed for BHT with x=0.05 compositions.And corresponding electrocaloric coefficients respectively were 0.33 and 0.24 K·mm·kV^-1.In addition,compared with the second-order phase transition region,larger electrocaloric coefficient(?T_f/?E?0.19 K·mm·kV^-1)were obtained at first-order phase transition region for all compositions.The maximum values of electrocaloric coefficients?T_f/?E=0.38K·mm·kV^-1 was achieved in a smaller electric field for Ba(Hf_0.03Ti_0.97)O₃ compositions.On the other hand,pyroelectric energy harvesting properties based on the Olsen cycle and HTRBs were systematically studied for the first time in the BHT ferroelectric ceramics.Mechanisms of HTRBs were studied from complex impedance and ac conductivity,exposing that it was concerned with the drifting of oxygen vacancies?OVAs?.Based on the value of theoretical calculation for activation energy,it was concluded that the HTRBs were attributed to the short distance hopping of the OVAs and the conduction was ascribed to the long-range move of doubly-ionized OVAs in BHT system.Meanwhile,electron paramagnetic resonance spectra were measured to detect the defects in BHT ceramics.The results showed that the OVAs-related defect signals were observed at g=1.942.In addition,the dielectric constant peak of relaxation could be significantly restrained by O₂ annealing.This further proved that the HTRBs being associated with the defect of oxygen.In addition,The maximum pyroelectric energy harvesting density N_D=491.30 kJ/m³(?T=120°C,E_H=50 kV cm^-1)was achieved in Ba(Hf_0.05Ti_0.95)O₃ ceramic.Finally,lead-free ferroelectric ceramics?1-x?BaTiO₃–x Bi(Mg_1/2Hf_1/2)O₃?x=0.0–0.12?were synthesized by conventional solid state reaction method and the correlation of structure,dielectric,ferroelectric and energy storage properties were investigated.It was found that T_m and?_m showed decreasing trend with the increase of BMH content.On the other hand,P–E loops were found to become slimmer and slimmer with increasing BMH contents,A significantly enhanced energy-storage property was observed at room temperature,the energy-storage density W=2.4 J cm^-3 and the associated efficiency?=87%were determined from the P–E loops data of 0.88BT–0.12BMT ceramic.