磁场作用下弛豫铁电晶体生长过程中的偏析研究

Relaxor ferroelectric crystal with large size has been grown successfully for decade, but it is not been widely used as expected. The major reason is that the composition of the crystal grown by Bridgman method is not homogeneous, which leads to the non-uniformity of electric properties of the crystal. Based on the situation, in this project static magnetic field is introduced to the growth process of PMN-PT and PMN-PSN-PT crystals. The convection in the melt is restrained by Lorentz force, which keeps gathering of lower-melting-point composition in the melt in front of S/L interface. The gathering reduces the composition segregation along axial direction during the crystal growth, which increases the homogeneity of the composition and the properties of the crystal. Other crystal defect is decreased with the help of the local effects of static magnetic field on the interface and surface. During crystal growth under static magnetic field the convection pattern in the melt and its effect on the composition segregation, defect distribution and electric properties of crystal are studied. The crystal growth technique with Bridgman method is optimized. The PMN-PT and PMN-PSN-PT crystals with large size and homogeneous composition are grown. The characteristics of dielectric, piezoelectric and ferroelectric of the grown crystal are studied. Meanwhile, the relationship among the phase structure, defect, domain configuration and the characteristics of dielectric, piezoelectric, ferroelectric are studied.

大尺寸弛豫铁电晶体的成功生长已有十多年的历史，但至今应用并不象预想的那样广泛，主要原因是Bridgman法生长的晶体中组分不均匀，从而导致其电学性能不一致。针对这一背景，本项目拟将直流静磁场引入PMN-PT和PMN-PSN-PT晶体生长过程，借助于直流静磁场的洛仑兹力抑制固-液界面前沿熔体的对流，保持固-液界面前沿低熔点组分的富集，以减小晶体生长过程中的轴向组分偏析，提高晶体组分和性能的均匀性；通过直流静磁场对界面和表面的微区作用，减少其它晶体缺陷。拟主要研究直流静磁场作用下弛豫铁电晶体生长过程中的熔体对流形态及其对晶体组分偏析、缺陷分布和电学性能的影响规律；优化弛豫铁电晶体的Bridgman法生长技术，生长组分均匀、大尺寸的PMN-PT和PMN-PSN-PT晶体；研究所生长晶体的介电、压电、铁电特性及其与相结构、缺陷、电畴组态之间的内在联系。

本项目主要针对大尺寸弛豫铁电单晶生长过程中的组分偏析问题，拟通过将直流静磁场引入PMN-PT单晶生长过程中，利用洛伦兹力对高温熔体对流的抑制作用，达到提高晶体组分均匀性的目的。基于此，首先研究了PMN-32PT晶体沿轴向和径向的Nb、Mg、Ti等元素的分布规律和组分变化对晶体电学性能的影响。研究表明，沿晶体轴向，Ti元素的偏析比较严重，晶体的电学性能的变化主要来源于Ti元素的变化；而在晶体径向，Nb/Mg元素的偏析相对较大，晶体电学性能的变化主要由Nb/Mg元素的变化引起的。其次，系统地研究了PMN-32PT弛豫铁电晶体生长过程中高温熔体的电阻温度特性，评估利用磁场抑制高温熔体对流的可行性。结果表明，随着温度的升高，PMN-32PT历经了从绝缘体到导体的转变，在高温下（1315℃左右）PMN-32PT熔体的电阻率约为9.4×10-3Ω·m，为磁场抑制熔体对流提供了有利条件。最后，设计并搭建了缩腰式长方体晶体生长炉和横向磁场装置，通过ANSYS有限元软件模拟了磁场分布晕图，并结合晶体生长过程中抑制熔体对流所需的洛伦兹力，优化线圈电流、线圈匝数等参数。结果显示，用高质量的含锆1460和1260纤维板和氧化铝空心球耐火砖配合多晶莫来石纤维替代传统的耐火材料，缩腰式晶体生长炉内温度能满足晶体生长的需求，同时最小化了磁极间距；当线圈电流为40A、匝数为1194匝时，相对磁场强度较大且耗电功率较小，磁场强度为0.17T，能够满足晶体生长的需求。此外，作为辅助研究内容，还开发了稀土掺杂的PT基的多元弛豫铁电晶体和陶瓷以及铋层状高温铁电陶瓷。本项目的研究结果对于提高弛豫铁电晶体成分均匀性、稳定该类晶体性能、进一步将该类晶体推向实用化具有重要意义，同时为晶体和其它材料多功能性的实现提供了可参考路径。