Crystal Growth And Photoelectric Properties Of Relaxor-based Ferroelectric Material PRN-PMN-PT

In the past decade,the relaxor-based ferroelectric materials such as PMN-PT single crystal have been developed systematically.Compare to the traditional ferroelectric ceramics PZT,the relaxor-based ferroelectric materials exhibit a very high piezoelectric constant(d₃₃～1500 p C/N)and Electromechanical coupling coefficient（k～0.9）,So the novel piezoelecric materials are valuable to be applied in the piezoelecric devices with high performance.The rare earth doped Pb Ti O₃-based single crystals have been showing a great interest for exploring the novel relaxor-based ferroelectric materials.The relaxor-based ferroelectric single crystals,doped rare earth ions with different ways,could be expectable to possess both desirable electrical and lumiescence properties as well as even higher piezoelectric constants.This thesis reports the research work on the crystal growth and photoelectric properties of relaxor-based ferroelectric material PRN-PMN-PT.Based on the binary solid solution PMN-PT adding a PRN component,the ternary solid solution polycrystalline materials are synthesized via solid state reaction at elevated temperature.A series of novel relaxor-based ferroelectric single crystals with the nominal composition of PRN-PMN-PT have been grown by the modified vertical Bridgman process.In as-grown single crystals with a perovskite structure,the rare earth ions such as Sm³⁺、Eu³⁺and Nd³⁺are doped effectively into the B-sites in the crystal lattice of the solid solution A（B₁B₂）O₃.The nominal composition of as-grown ternary solid solution crystal should be identified as PSm N-PMN-PT,PEN-PMN-PT and PNd N-PMN-PT.The electrical and lumiescence properties of the relaxor-based ferroelectric single crystal have been characterized systematically.The research results acquired in this work are summarized as follows:（1）According to the composition ratios of the lead-based solid solution near morphotropic phase boundary,the nominal composition of Sm³⁺ions doped ternary solid solution is designed as x Pb(Sm_1/2Nb_1/2)O₃-（0.70?x）Pb(Mg_1/3Nb_1/3)O₃-0.30Pb Ti O₃（x=0.01,0.02,0.03）.Using the high purity reagents of Sm₂O₃,Pb O,4Mg CO₃·Mg（OH）₂·4H₂O,Nb₂O₅,and Ti O₂ as the initial materials.The ternary solid solution polycrystalline materials are synthesized via solid state reaction at elevated temperature.A series of novel relaxor-based ferroelectric single crystals PSmN-PMN-PT have been grown by the modified vertical Bridgman process.By means of X-ray powder diffraction analysis,as-grown single crystals are identified as the solid solution with perovskite structure,in which Sm³⁺ions are doped effectively into the B-sites in the crystal lattice of the ternary solid solution.It proves that the perovskite structure of the solid solution has not been changed by the doped component Pb(Sm_1/2Nb_1/2)O₃.（2）A series of（001）-oriented crystal wafers were fabricated from as-grown crystal boules.After the silver-coated crystal wafers had been polarized electrically,the electrical and lumiescence properties of the wafer samples were measured systematically.It is verified that the diagram of dielectric permittivity versus temperature show two board abnormal dielectric peaks,which could be attributed to the ferroelectric phase transitions.The dielectric peak located at the temperature of 60～70^oC is correspond to the phase transition from rhombohedral to tetragonal,while the dielectric peak located at the Curie temperature of 118～140^oC is correspond to the phase transition from tetragonal to cubic.The dielectric peaks exhibit an evident frequency dispersion phenomenon in the diagram,which just proves the relaxation ferroelectric nature of these piezoelectric materials.The polarization-electric field（P-E）hysteresis loops show that the crystal wafers possess a coercive field of 4～7 kV/cm and a residual polarization of 10～14mC/cm².After the crystal wafer has been polarized under direct current electrical field,the wafer sample exhibits a piezoelectric constant d₃₃above 2000 pC/N.However,the polarized wafer can possess a piezoelectric constant d₃₃ as high as 3920 pC/N as the sample has been polarized with alternating current voltage.This single material with giant piezoelecric effect is promising for the practical application in the piezoelectric devices with high sensitivity.（3）The absorption spectra of the crystal wafers show a series ofabsorption peaks associated with the energy level transition of Sm³⁺ions,which also proves that Sm³⁺ions have been doped effectively into the crystal lattice.Under the photonic excition at the central wavelength of 408nm,the fluorescence spectra measured for the crystal wafers exhibit four emission peaks located at the central wavelength of 562 nm,598 nm,644 nm and 705 nm,respectively.The fluorescenceemission peaks of Sm³⁺ions can be attributed to the energy level transition of⁴G_5/2→⁶H_5/2,⁴G_5/2→⁶H_7/2,⁴G_5/2→⁶H_9/2 and ⁴G_5/2→⁶H_11/2.As for the wafer samples corresponding to PSm N component ratio of 0.01,0.02 and 0.03,the decay time of 598 nm fluorescence emission are measured to be 686ms、553ms、563ms,respectively.The absorption intensities and fluorescence emission intensities show a rising tendency with the increasing concentration of Sm³⁺ions as PSm N component ratio is increasing.（4）According to the composition ratio of 0.1PEuN-0.69PMN-0.30PT,the polycrystalline materials are synthesized via solid state reaction at elevated temperature.A relaxor-based ferroelectric single crystal PSm N-PMN-PT have been grown by the modified vertical Bridgman process.（001）-oriented crystal wafer was fabricated from the middle region of as-grown crystal boule.The electrical properties and fluorescence spectrum of the crystal wafers with（001）orientation were characterized.Based on the fact that Eu³⁺ions are doped into the crystal lattice via the component of Pb(Eu_1/2Nb_1/2)O₃,the fluorescence spectrum exhibits the characteristic emission peaks of Eu³⁺ions.After the crystal wafer has been polarized under direct current electrical field,the wafer sample exhibits a piezoelectric constant d₃₃of 1500～1700 p C/N,while the polarized wafer polarized with alternating current voltage can possess a piezoelectric constant d₃₃ of 1900～2100 pC/N.The polarization-electric field hysteresis loops measured under the voltage of 10 kVshow that the crystal wafers possess a coercive field of 7.10 k V/cm and a residual polarization of 12.81μC/cm².（5）The polycrystalline material with a nominal composition of 0.1PNdN-0.69PMN-0.30PT are synthesized via solid state reaction at elevated temperature.A relaxor-based ferroelectric single crystal PNd N-PMN-PT have been grown by the modified vertical Bridgman process.In order to acquired the electrical and fluorescence properties of as-grown crystal,the measuring experiments were performed on a（001）-oriented crystal wafer fabricated from the middle region of as-grown crystal boule.The fluorescence spectrum exhibits the characteristic emission peaks of Nd³⁺ions,which supports that Nd³⁺ions have been doped into the crystal lattice via the component of Pb(Nd_1/2Nb_1/2)O₃.After the crystal wafer has been polarized under direct current electrical field,the wafer sample exhibits a piezoelectric constant d₃₃of1500～1900 p C/N,while the polarized wafer polarized with alternating current voltage can possess a piezoelectric constant d₃₃ of 2000～2500 pC/N.