Energy Storage Mechanism And Properties Of Sr_0.7Bi_0.2TiO₃ Based Relaxor Ferroelectric Ceramics

Dielectric energy storage materials as the key material of the pulsed energy storage capacitor have become one of the research focuses in this field.With the development of pulsed energy storage capacitors towards miniaturization and chip type,the research on new high-energy and high-voltage dielectric materials has become inevitable.Sr_0.7Bi_0.2TiO₃(SBT)is a kind of linear-like relaxor ferroelectric,and it is very suitable for applying in pulsed dielectric energy storage materials due to its low remanent polarization(P_r),moderate dielectric breakdown strength(BDS)and high energy storage efficiency(?).However,the maximum polarization(P_max)of SBT is relatively low,which limits the recoverable energy storage density(W_rec).This study started by improving the P_max of SBT,and many strategies were used to improve the energy storage performance of SBT.The enhancement mechanism of P_max,BDS and relaxation ferroelectricity was deeply analyzed using various testing techniques,first-principles calculation and finite element simulation calculation.Meanwhile,the influence of these mechanisms on energy storage performance was investigated,and energy storage performance parameters are comprehensively regulated.Finally,SBT-based ceramics with high W_rec and?were obtained under the moderate electric field.The main research content and main conclutions of this dissertation are as follows:1.KBT was introduced in SBT((1-x)SBT-xKBT).The introduction of KBT induces the off-center displacement of defect ions and the formation of K⁺-Bi³⁺defect dipole pair.This results in the local polarization and destroys the paraelectric phase of SBT.The size and number of domain structures are increased,significantly increasing the P_max under the same electric field.The relaxation enhances due to the enhanced ionic disorder.However,the BDS deteriorates due to the increased oxygen vacancy concentration caused by the volatilization of the A-site element.When x?0.48,the BDS deteriorates significantly(?170 kV/cm),and the P_r increases significantly due to the enhanced ferroelectricity,resulting in the deteriorated energy storage performance(W_rec?1.76 J/cm~3,??90.2%).At last,the x=0.38 sample exhibits a W_rec of 2.02 J/cm~3,a high?of 91.4%,a W_d of 1.81 J/cm~3 and a P_D of 49.5 MW/cm~3 under 220 kV/cm~3.2.To save the deterioration of BDS in the above study,0.5SBT-0.5KBT(SBKT)with high P_max was selected as the base materials,and Er₂O₃ was introduced to improve the BDS(SBKT-x wt.%Er₂O₃).After introducing Er₂O₃,most of Er did not enter the main phase lattice,but a second phase was detected,forming the composite ceramics with the main phase.The moderate introduction of Er₂O₃ results in the reduced average grain size,oxygen vacancy concentration,electrical conductivity and leakage current density and increased conductivity activation energy and bandgap(E_g),inhibiting oxygen vacancy transport and improving BDS.When x>2,the BDS deteriorates due to forming many regions with a high local electric field(LEF).At last,the x=2 sample exhibits a high W_recof 2.17 J/cm~3 and an?of 83.9%at 245 kV/cm~3,and its P_max under the same electric field,BDS and W_rec are higher than SBT.In addition,the energy storage performance of this component exhibits excellent temperature stability(the change rate of the W_rec and?is less than 1%in-25?150°C),frequency stability(the rate of change of W_rec is about 1.6%,and the rate of change of?is about 3.6%in 10?500 Hz)and fatigue resistance(almost unchanged W_rec and?within 10~5 cycles).Meanwhile,this component ceramic possesses stable and good charge-discharge performance.3.In the above study,although W_rec increases,?is low.To obtain high W_rec and?simultaneously,NaNbO₃(NN)was introduced into SBKT((1-x)SBKT-xNN)to optimize the domain structure and improve the relaxation and BDS.Moderate NN doping results in reduced grain size and oxygen vacancy concentration,increased conductivity activation energy and reduced electrical conductivity.Meanwhile,the thermal conductivity increases,and the LEF decreases and becomes uniform.These factors improve the BDS.Nb⁵⁺in the B site weakens the B-O bond strength,enhancing the ferroelectric instability and relaxation state.The introduction of Na⁺and Nb⁵⁺further destroys the long-range ordered structure and forms more nanodomains or polar nano regions with smaller sizes.This improves the relaxation performance and weakens the dielectric nonlinearity,P_r and polarization saturation.Meanwhile,the broadened dielectric peak enables the x=0.15 sample to meet the X7R standard.At last,the x=0.05sample exhibits high W_rec of 2.28 J/cm~3,?of 93.1%,W_d of 2.1 J/cm~3 and P_D of 54.1MW/cm~3 at 220 kV/cm~3.The P_max under the same electric field and W_rec of this sample are higher than those of SBT,and the BDS and?are similar to those of SBT.Meanwhile,the energy storage performance and charge-discharge performance show excellent stability.Oxygen sintering can further reduce the concentration of oxygen vacancies and the content of the second phase,thereby improving the BDS and P_max.This component sintered in oxygen exhibits higher W_rec(2.6 J/cm~3)and high?(89%)at 250 kV/cm~3.4.Li⁺can compensate for the A-site vacancies in the perovskite lattice caused by the volatilization of Na,K,and Bi during the sintering process,it is beneficial to reduce the grain size and oxygen vacancy concentration.Therefore,according to the previous study,to prevent BDS from deteriorating while increasing P_max and simultaneously increasing P_max and BDS by one-step doping,(Li_0.5Bi_0.5)²⁺was introduced in SBT((1-x)SBT-xLi_0.5Bi_0.5TiO₃).After proper doping(x?0.3),the BDS is enhanced and the electrical insulation performance is improved.Meanwhile,this is also associated with decreased electrical conductivity,increased conductivity activation energy and E_g,high thermal conductivity,and low leakage current density.When x>0.4,the increased second phase content,the increased and uneven LEF,and the decreased electrical insulation performance and thermal conductivity result in the deterioration of the BDS.Furthermore,(Li_0.5Bi_0.5)²⁺mainly occupies the A site,which induces the off-center displacement and the formation of Li⁺-Bi³⁺defect dipole pair.This destroys the paraelectric phase,and the larger lattice distortion induces a unique triple-modulated superlattice structure,indicating a formation of the locally ordered phase and an increase in the size and number of the domain structure.Finally,the x=0.3 sample exhibits excellent and stable energy storage performance and charge-discharge performance.O₂ annealing further reduces the concentration of oxygen vacancies and improves the BDS,and the x=0.3 sample after O₂ annealing exhibits a high W_rec of 2.51 J/cm~3 and a high?of 97.03%at a high critical electric field of 260 kV/cm~3.Its P_max under the same electric field,BDS,W_rec and?are higher than those of SBT.