Oxygen Vacancies Related Dielectric Relaxation,Ferroelectric Energy-storage And Electrocaloric Effects In Lead Based Ferroelectric Materials

Defects are common states in ferroelectric materials.The most representative defects in ferroelectric materials are oxygen vacancies(OVs).OVs show an important influence on the electrical characteristics of materials as a kind of self-doping(donor or acceptor).In perovskite ferroelectric ceramic materials,the generation of OVs is unavoidable during the preparation of lead ceramics,OVs play important roles in electronic properties(such as:ferromagnetic,dielectric relaxation characteristics,resistive switching characteristics,photovoltaic effect,and etc.).In high temperature region,OVs induced dielectric relaxation is one of the most remarkable properties of ferroelectric materials.At the same time,the presence of OVs also influences the ferroelectric characteristics of the materials(such as:ferroelectric pinning phenomenon),which also shows important influence on the energy storage characteristics and the electrocaloric effects of ferroelectric materials.(1)(Pb,Ba)ZrO3(PBZ)and(Pb,Ba)(Zr,Ti)O3(PBZT)ceramics are prepared by high temperature solid atate fabrication method.The phase transition behaviours,high temperature dielectric relaxation are analyzed.By introducing impedance technologies,we draw the conclusion:Ba doping can reduce FE-PE phase transition temperatures of PBZ and PBZT ceramics.The vaporation of lead cannot be inevitable during the high temperature sintering process,which will lead to oxygen vacancies(OVs)and related defects.In high temperature region,OVs will gain enough energy to make transportation behaviours,which lead to the high temperature dielectric relaxation phenomenon.The results show that the short-range transition of OVs can lead to high temperature dielectric relaxation behavior,while the long range transition behavior of double ionization OVs will increase the conductivity of the material.(2)We also studied the relaxation phase transition,ferroelectric,energy storage and other electrical properties of Pb(Mg1/3Nb2/3)O3-PbTiO3(PMN-PT),PbZr03-SrTi03(PSZT)ceramic and PbZr03(PZ)films.Both PMN-PT and PSZT ceramics showed obvious dispersion phase transition behavior.The maximum energy-storage density at room temperature is about 0.47 and 0.46 J/cm3 reectively for PMN-PT and PSZT ceramics,energy storage density and energy loss density increase sharply with increasing electric field.However,with increasing temperatures,the energy storage density and loss density showed a decreasing trend,but the energy storage efficiency remained basically unchanged for PSZT ceramics.For PZ film,we studied the ferroelectric and dielectric tuning characteristic,3 layers PbZrO3 film shows double hysteresis loop curves indicating the AFE nature,the energy storage density of about 25.25 J/cm3,energy storage efficiency of about 62%.(3)We studied the dielectric,ferroelectric characteristics and electricaloric characteristics of 0.76Pb(Mg1/3Nb2/3)O3-24PbTiO3(PMN-24PT)single crystal and(Pb,La)TiO3(PLT)ceramics.We studied the ferroelectric properties of PMN-24PT single crystal at different temperatures,and obtained the maximum electricaloric effect of 0.8 K by theoretical calculation.For PLT ceramics,we adopt the means of the combination of theoretical calculation and direct measurements to study the electrocaloric effects,the maximum electricaloric effect of 1.67 K by theoretical calculation shows highly consistent to direct measurements results.(4)We studied the electrical characteristics of Nb-doped SrTiO3(Nb:STO)single crystals.Results revealed that device shows resistive switching behaviours when choosing top electrodes Au and Ag as measured electrodes,and the device shows good retention characterictics.Device shows diode charecteristics while choosing top electrode Au as measured electrodes,we come up with an energy band diagram to explain this.Symmetry selector characteristic become dominant when choosing bottom electrode Ag and top siliver as test electrode,and asymmetry selector characteristic become domiant when choosing bottom Ag and top Au as test electrode.(5)We prepared high frequency ultrasound transducers and study the performances,we select LiNbO3(LNO)single crystal as piezoelectric materials,due to higher center frequency,we make one matching layer,the designed depth is 6.5 mm,the impedance and pulse-echo results is similar to designed parameters,the center frequency is 75 MHz,bandwidth is 92%.Finally,we scan a pig eye to detect the imaging ability of the transducer,and the results show that the probe has good performance.