Study On The Preparation And Properties Of The High-Curie Temperature PMN-PH-PT Piezoelectric Ceramics

With the development of science and technology,searching for high performance ferroelectrics with high-Curie temperature?T_C?has been a continuous task all over the world due to the urgent demands for the high-temperature and high-power-density piezoelectric materials in the fields of geothermal exploration,automotive,aerospace applications and so on.Recently,a novel piezoelectric perovskite material Pb(Mg_1/3Nb_2/3)O₃-PbHfO₃-PbTiO₃?PMN-PH-PT?with relatively high T_C was developed,exhibiting excellent dielectric,ferroelectric and piezoelectric properties,which is promising to be applied in the field of transducers at elevated temperature.However,there are few reports concerning the PMN-PH-PT ceramics up to now.Therefore,the PMN-PH-PT ceramics were deeply and systematically studied from four aspects in this thesis.In part one,the Pb(Mg_1/3Nb_2/3)O₃-PbHfO₃-PbTiO₃?PMN-PH-PT?ceramics were prepared by the conventional ceramic processing via the columbite precursor method.The effects of sintering processes and compositions on the structure and electrical properties of the PMN-PH-PT ceramics were systematically investigated.XRD analysis showed that the sintered PMN-PH-PT ceramics exhibit pure perovskite structure,in which the rhombohedral and tetragonal phases coexist.Their phase structure can be changed by the sintering processes and compositions,which induce the structural phase transition.Temperature dependent dielectric performance indicated that the PMN-PH-PT ceramics exhibit first-order ferroelectric phase transition from rhombohedral/tetragonal ferroelectric phase to cubic paraelectric phase at T_C,from which the Curie-Weiss law and the quadratic law fittings suggested that the PMN-PH-PT ceramics belong to the normal ferroelectrics with diffuse phase transition?DPT?characteristic.The0.15PMN-0.38PH-0.47PT ceramics sintered under the optimum sintering process?1260?C-2 h?exhibit the best electrical properties,in which?_m=22598,T_C=291?C,d^*₃₃=716.7 pm/V,d₃₃=367.4 pC/N,K_p=53.42%,Q_m=170.5,and N_p=208.24 kHz?cm,respectively.In part two,the 0.15PMN-0.38PH-0.47PT ceramics were fabricated by the citrate method and the partial oxalate route to improve the electrical properties.The influences of preparation processing of precursors,calcining temperature and sintering temperature on the crystal structure,micro-morphology and electrical properties were systematically investigated.The enhanced calcining and sintering activity of the obtained uniform PMN-PH-PT precursor powders prepared by the wet chemical synthesis methods declines the calcining and sintering temperatures.With the increase of the calcining temperature and the sintering temperature,the electrical properties of the PMN-PH-PT ceramics prepared the two wet chemical synthesis methods both get better first and then worse,and both the 750°C-calcined and 1245°C-sintered samples exhibit the best piezoelectric properties,far superior to the counterparts synthesized by the columbite precursor method,which correlates with the MPB effect and densified homogenous microstructure brought by the two wet chemical synthesis methods.Among the three methods,the partial oxalate route prepared 0.15PMN-0.38PH-0.47PT ceramics exhibit the best electrical properties with T_C=291?C,?_m=28937,P_r=32.89?C/cm²,E_c=9.35 kV/cm,d^*₃₃=845.3 pm/V,d₃₃=456.2 pC/N,and K_p=67.20%,respectively,and excellent thermal stability of the piezoactivity properties under T_C.In part three,the microscopic mechanisms of the piezoelectric response in the PMN-PH-PT ceramics were discussed.The interplay between macroscopic electrical properties and microscopic structures in the PMN-PH-PT ceramics with different compositions and the0.15PMN-0.38PH-0.47PT ceramics prepared by different methods were revealed.Large quantities of fine stripe submicron ferroelectric domains are observed in the PMN-PH-PT ceramics,which form large island domains.Their better ferroelectric and piezoelectric responses arise from the smaller domain size,which reduces the domain wall energy;and the higher coercive field E_c in the polarization-electric field?P-E?hysteresis loops results from the harder domain switching.In part four,the ferroelectric phase transition behaviors of the ferroelectrics were studied.Temperature-dependent Raman spectroscopy not only proves the occurrence of the ferroelectric to paraelectric phase transition around T_C of the PMN-PH-PT ceramics near MPB prepared by different methods,but also confirms the successive phase symmetry transitions,which correlate with the polar nanoregions?PNRs?or the multiple phase coexistence,revealing the origin of the enhanced electrical properties.Temperature-dependent Raman spectra and domain structures of the ferroelectric single crystals further confirm the ferroelectric phase transition characteristics of the ferroelectrics.Based on the analysis of the phase structure and electrical properties,the excellent dielectric,ferroelectric and piezoelectric properties of the PMN-PH-PT ceramics can be attributed to the fine stripe ferroelectric domains,and either the existence of the PNRs with low symmetry phase or the multiple phase coexistence around room temperature.The PMN-PH-PT ceramics exhibit relatively high T_C and outstanding piezoactivity,indicating their great potential in the transducer applications under elevated environmental temperatures.