PMN-PT-based Ferroelectric Crystal Component Segregation And Its Structure And Performance Research

Relaxor ferroelectric single crystals(100-x)Pb(Mg1/3Nb2/3)O3-xPbTiO3(PMN-xPT)near morphotropic phase boundary(MPB)have been widely investigated because of the excellent dielectric,piezoelectric and electromechanical performances.Most researches in basic theoretical were focused on the origin of high electrical properties,the structural phase transition near MPB,and the modification.However,the systematic researches on the component segregation and the relationships among elements distribution,structure and performance were scarce.On the other hand,the debate about the structural and the origin of high electrical performance still was remained in PMN-xPT ferroelectric crystal because of their complex structures and mulltiple B-site cations.In our work,the influences of component segregation on structure and performance were systematically analyzed for PMN-xPT based crystals.The main results and conclusions were listed as following:Based on the electron probe X-ray microanalyzer(EPMA),the relationships between element distribution and macroscopic electrical properties were studied along[100],[010],[011]and[011]orientation on(001)and(111)crystal face for nominal chemical component PMN-32PT crystal grown along[111]orientation.On the(001)crystal face,the PT content was almost a constant in the range of 6-8 mm along[100]orientation,and 10-12 mm along[010].On the(111)crystal face,the PT content was almost unchanged along[011]in the tange of 14 mm.Although the PT content was unchanged,the electric properties of crystals still was fluctuated,indiating that the properties of crystals can depend on the distribution of Mg,Nb and Pb.Moreover,the electric properties were positively correlated with Mg,and negatively correlated with Nb.Based on the pseudo-binary phase diagram of PMN-xPT,the influences of Ti content on the electrical properties and structure of crystals near morphotropic phase boundary(MPB,30 ? x ? 35)were studied.The dielectric constant first was increased and then decreased at room temperature with increasing the Ti content from x = 30 to 35 at room temperature.The poled PMN-31PT and unpoled PMN-33PT displayed the high dielectric constant(?r?6000),indicating two dielectric abnormal regions in MPB.The monoclinic MA might be the dominant phase structure for unpoled PMN-31PT crystal at room temperature,instead of Mc,and the sequence R?M?T?C phase transformations were suggested with temperature increased.For the poled PMN-31PT,temperature induced MA?Mc?T?C.Applying a critical compressive stress along the direction of[001],the MA transforms into R.When the compressive stress reached 20 MPa,MA would change to O phase.Under the 14 MPa,the sequence phase transitions R?MA?Tr?MC?T ? C were proposed with the increase of temperature.According to the disorder of B-site elements in PMN-xPT,the evolutions of microstructure of PMN-33PT crystal were investigated in the low temperature range from 5 K to 300 K by the dielectric measurement,micro-Raman,and the fractal cluster theory,which was advantage to understanding the outstanding dielectric properties in PMN-33PT.The results showed that the ultrahigh dielectric constant of PMN-33PT crystal at room-temperature was attributed to the microstructure instability and polarization rotation at low temperature range due to the inhomogeneity and disorder of the B-site cations.The coupling of polar nanoregions and phonon vibration makes the structure unstability at low temperature microstructure.The variable microstructures are helpful to the polarity rotation,which improved the dielectric performance for PMN-33PT,rather than the contribution of monocline M phase.The relationships among the phase structure,domain morphology and electrical properties were studied systematically in PMN-PT crystals modified by Pb(Sc1/2 Nb1/2)O3(PSN).Compared with the PMN-PT,higher phase transition temperature(TR-T=121 ?)and coercive field(Ec = 400 V/mm),and larger diffuse phase transition were observed in PSN-PMN-PT,indicating its potential application in large power and high temperature range,which can be attributed the enhanced order-degree in B-site cations and large free-energy by introduction the Sc element.The studies of domain size effect showed that the smalldomain size reflected the good piezoelectric at low field,but it was deviation for high electric-field.Compared with the normal poling process,the high temperature poling process was beneficial to increase the piezoelectric stability,but decrease the temperature stability.The defects hinder the migration of domain wall,and influence the evolution of polarization,further affecting the macroscopic electrical properties of the crystal.The electrical properties and charge transport were studied in PSN-PMN-PT crystal and NBT ceramics,and the differences between them were revealed.At low temperature,the crystal had lower leakage current than that of ceramics.In crystal,the conduction carrier was electron,while the carrier was oxygen vacancy in ceramics.On the contrary,in 'the high temperature range,the crystal showed the ionic conduction and great conductivity.The domain was pinned by charge defect,resulting in the increase of coercive field and the decrease of dielectric loss.Moreover,the coupling between charge defect and domain increased the stability of multi-domain structure.