Study On The Growth And Phase Transition Of Relaxor Ferroelectric Single Crystals

Relaxor ferroelectric single crystals , such as Pb(Mg1/3Nb2/3)-PbTiO3 (abbreviated as PMNT) or Pb(Zn1/3Nb2/3)-PbTiO3 (abbreviated as PZNT), have been reported to exhibit an extremely large piezoelectric constant and excellent electrostrictive properties. Such excellent performance makes it fully substitute the traditional piezoelectric ceramics and points to a revolution in ultrasonic transducers, actuators and micro-positioners, making relaxor-based piezocrystals the most promising materials for a broad range of advanced applications. However, it is difficult to grow the high quality single crystals because of the lack of valid thermodynamic data.In this paper, PMNT, PZNT single crystals in the vicinity of the morphotropic phase boundary were obtained by high-temperature solution technique. The growth, structure and phase stability of single crystals were studied. The results are shown as follows:1. By high-temperature solution technique, the PMNT, PZNT single crystals with perovskite structure were successfully prepared and the crystals showed size from 0.5mm to 5mm, and color from light yellow to brown.2. As-grown PMNT single crystal were pure perovskite structure and regular pseudo-cubic while PZNT showed an arrow-head shape with the formation of pyrochlore phases.3. Processing conditions were optimized in order to increase the size and yield of perovskite crystals by a high temperature flux solution method. The optimum mole ratio of flux to composition was found to be 6:4, which can restrain the formation of pyrochlore phase. It was revealed that B2O3 additive played an important role in increasing the size of single crystals.4. Phase structure stability of PMNT, PZNT single crystals were systematically studied by X-ray diffraction (XRD) method and differential scanning calorimetry (DSC). Under no excess PbO flux environment, the perovskite phase of PMNT single crystals were more stable than that of PZNT, but it was found to be the decomposition of perovskite crystals into pyrochlore crystals at 1250℃. The pure perovskite PZNT crystals prepared by the PbO flux method were thermodynamically metastable. yielding pyrochlore and PbO over high temperature.5. The formation mechanism of the ferroelectric perovskite in PZNT was examined by X-ray diffraction method. Both of pyrochlore phases of Pb3Nb4O13 and Pb1.83Zn0.29Nb1.71O6.39 were obtained during the growth of single crystals, butPb3Nb4O13 existed during middle temperature period while Pb existed during high temperature peroid.6. Domain configuration in PMNT single crystals were observed using scanning electronic microscope (SEM).7. The defects existing in piezoelectric single crystals were analyzed. It is shown that the morphology of PMNT, PZNT crystals was related to a layer growth mechanism controlled by two-dimensional nucleation which can be depicted by the theoretical model of periodic bond chains (PBC).8. The temperature autocontrol system of middle-temperature resistance furnace was designed and assembled, which meet experimental requirements.