Electrocaloric Effect And Energy Storage Performance In Bismuth-based Perovskite-type Lead-free Ferroelectric Relaxor Ceramics

The lead-free ceramics with high electrocaloric effect and energy storage performance play a vital role in the application of power electronics and are gradually to be the research frontier in functional ceramics.Bismuth-based perovskite-type ferroelectric relaxor ceramics,such as Bi_0.5Na_0.5TiO₃-based?BNT?and Bi_0.5K_0.5TiO₃-based?BKT?,are considered as the most promising lead-free ferroelectric relaxor ceramics.Pristine BNT and BKT materials,due to poor sintering abilities and the inferior electrocaloric effect?ECE?and energy storage performances,are strictly limited in the applications of refrigeration and capacitors.By adding external components,phase structure and dielectric relaxation properties of the matrix are modulated,thus improving the ECE and energy storage performances.By refining the microstructure of the BKT-based material,energy storage performances are improved as well.This dissertation focuses on the scientifically important issues and develops the ECE and energy storage performance in bismuth based lead-free relaxor ferroelectric ceramics and deeply discusses the influence of phase structure transformations,dielectric relaxation properties,ferroelectric/piezoelectric properties on ECE and energy storage performances.The main contents and conclusions are listed below:?1?ECE in?1-x?Bi_0.5Na_0.5TiO₃-xSrTiO₃[?1-x?BNT-xST]systems are measured directly.Results show that optimal directly-measured ECE is obtained when the freezing temperature is tailored to around room temperature for BNT-0.25ST ceramics with coexistence of nonergodic-ergodic relaxor phase.It possesses?T=0.51 K under electric field of 6 kV/mm and an excellent thermal stability of ECE?the instability less than 20%over 30¹20°C?.The in-situ Raman spectra are performed to determine the phase structure of studied samples and its influence on the ECE.The underlying mechanism of ECE is rationalized by in-situ PFM,thermodynamic and statistical analysis.?2?The substitution of NaNbO₃ into the 0.8BNT-0.2ST matrix significantly induces an evolution of dielectric relaxation behavior from nonergodic phase to ergodic phase at room temperature.A moderate energy storage density of W_R=0.73J/cm³ at 7 kV/mm is obtained for 0.05NN composition.The x=0.05 sample is found to exhibit a good temperature stability?25¹60 ^oC?and frequency?0.1²0Hz?insensitivity,and the achievement of above characteristics for 0.05NN is mainly attributed to the rapid response of electric field induced ferroelectric phase back to its original ergodic phase together with a dielectric diffusion process.Pulsed discharge current measurement shows that the stored energy can be released within 1?s and indicates the prospective of BNT-based relaxor ferroelectric ceramics for pulse power capacitors.?3?The ECE in BNT–0.06BT is reported by direct measurements and the maximum?T_max=0.86 K is achieved in the vicinity of T_FR with the pseudo-first-order phase transition?PFOFT?.Furthermore,the responsivity?has a maximum value of0.22 K·mm/kV at critical point E_CP.?T remains considerable??0.65 K?in a wide temperature range higher than T_FR,which can be ascribed to the contributions of the polar nanoregions?PNRs?.Finally,a modified schematic Landau free-energy curve is delineated to give an insight into the underlying mechanism of the ECE evolution for BNT-0.06BT ceramics.?4?ECE in lead–free BNT–BT–xSBT?x=0⁰.24?ceramics are measured directly.A maximum adiabatic temperature change?T=0.4 K is found at x=0.08 under an electric field of 6 kV/mm at room temperature.The coexistence of nonergodic and ergodic relaxor phase leads to a maximal change in entropy for the field–induced phase transformation between the ergodic and long–range ferroelectric phase.The shift of the ECE peak at higher electric field is mainly due to the difference in the number of polar states?in different states.The crossover from nonergodic to ergodic relaxor phase,resulted from an increasing SBT content,determines an increased temperature stability of the ECE,with a variation lower than 20%in the temperature range from room temperature to 100°C for x=0.08.?5?We have studied the charge-discharge performance for the ergodic relaxor BNT–BT–0.32SBT ceramics.The energy storage density of the studied ceramic maintains?0.5 J/cm³ up to 140°C with high efficiency more than 90%.What's more,large power density 18.2 MW/cm³ and large current density 606 A/cm² can be obtained.The increasing amplitude of current as temperature increases can be ascribed to the thermal evolution of PNRs.?6?Mn doping is beneficial to enhance breakdown field strength and W_R=1.07J/cm³,?=119 J/?kV·m²?and?>90%are simultaneously obtained in 1Mn ceramics.The high performance of the ceramic is originated from the quasi-hysteresis-free loops with high polarization as result of the appearance of defect dipoles,which is confirmed by synergistic measurements,including the XPS,complex impedance and EPR analysis.?7?The influence of crystal structure,dielectric,ferroelectric,piezoelectric properties on electrocaloric effect of the BKT-xLMT binary solid solutions are discussed in detail.The introduction of LMT decreases the tetragonality of the BKT ceramic and an MPB region is formed at x=0.01⁰.02.The 0.99BKT-0.01LMT ceramic shows an enhanced piezoelectric coefficient(d₃₃¹03 pC/N)and ECE??T=1.19 K?at room temperature.The?T value of the x=0.01 ceramic exhibits a good temperature stability and the temperature instability is lower than 10%over25¹20°C,which is clarified by in-situ XRD patterns,Raman spectra and TEM images.The good thermal stability is attributed to the diffuse phase transition with high T_d and the existence of polar nanoregions.?8?The energy storage performance of Bi_0.5K_0.5TiO₃-0.06La(Mg_0.5Ti_0.5)O₃ceramic is improved by hot-press technique.Hot-pressed ceramic possesses ultrahigh stored energy density?W_s=3.07 J/cm³?and recoverable energy storage density?W_R=2.08 J/cm³?and excellent thermal stability?variation of W_R is less than 8%in the temperature range of 25¹40 ^oC?.The stored energy can be released in a very short time?⁰.2?s?regardless of variation of temperature.