弛豫铁电单晶宽频(10mHz-20GHz)介电性能研究

Compared with traditional ferroelectric lead zirconate titanate ceramics, relaxor ferroelectric single crystals with the composition near the morphotropic phase boundary (MPB) region have attracted much attention due to their superior piezoelectric and dielectric properties. Up to date, the routine piezoelectric, dielectric and elastic properties of relaxor ferroelectric single crystals have been widely studied. However, the origin of high properties of them near the MPB region is still under debate. Many representative opinions based on intrinsic crystal lattice and extrinsic domain wall motion have been proposed to understand this phenomenon. It is very difficult to separate one mechanism from the other by means of experiments. In this proposal, we will study the relationship between the broadband (10mHz-20GHz) dielectric properties and the corresponding phase/domain structures of the relaxor ferroelectric single crystals. Making use of the microwave dielectric relaxation, we could separate the domain wall motion from the crystal lattice contribution to the dielectric properties. Based on these experiments results, we could identify the origin of the high properties of the relaxor ferroelectric single crystals with the composition near MPB. We will also investigate different response characteristics govern by different domain wall motion mechanisms in such a wide frequency range, and verify presence of different regions of domain wall dynamics in relaxor ferroelectric single crystals, predicted by recent universal theoretical models. To the application, a deeper knowledge of the relationship between the broadband dielectric properties and external field could be greatly beneficial to improvement of the dielectric performance of the relaxor ferroelectric single crystals, which is the basis of the development of the novel microwave dielectric devices.

同传统的铁电锆钛酸铅陶瓷材料相比，弛豫铁电单晶材料因其准同型相界成分附近的高压电与介电性能而被广泛的关注。目前，弛豫铁电单晶材料的常规压电、介电、弹性性能已被广泛深入的研究。而对于其准同型相界成分附近高性能的起源现在依然存在争议。代表性的观点多从本征晶格与非本征畴壁移动的角度来解释这一现象。实验上将二者定量的加以区分依然存在很大的难度。本项目将从弛豫铁电单晶的宽频(10mHz-20GHz)介电性能与相结构/畴结构的关系入手，利用微波介电弛豫现象将本征晶格与非本征畴壁移动对于介电性能的贡献加以区分。并以此为突破口，探讨弛豫铁电单晶高介电性能的起源问题。同时研究不同畴壁移动机理在此频率范围内的响应特性，并与最近提出的普适畴壁动力学理论模型相验证。在应用方面，本项目将归纳弛豫铁电单晶宽频介电性能与外场的关系，探索优化介电性能的技术路线，为开发新的微波介电器件提供材料研究的基础。

同传统的铁电锆钛酸铅陶瓷材料相比，弛豫铁电单晶材料因其准同型相界成分附近的高压电与介电性能而被广泛的关注。目前，弛豫铁电单晶材料的常规压电、介电、弹性性能已被广泛深入的研究。而对于其准同型相界成分附近高性能的起源现在依然存在争议。代表性的观点多从本征晶格与非本征畴壁移动的角度来解释这一现象。实验上将二者定量的加以区分依然存在很大的难度。本项目将从弛豫铁电单晶的宽频(10mHz-20GHz)介电性能与相结构/畴结构的关系入手，利用微波介电弛豫现象将本征晶格与非本征畴壁移动对于介电性能的贡献加以区分。并以此为突破口，探讨弛豫铁电单晶高介电性能的起源问题。研究表明除低频(100 kHz到1 MHz区间)压电效应导致的压电谐振在介电谱上的影响外，该单晶在高频频率测试窗口(>1MHz)表现出一种与频率几乎无关的介电响应特性，介电常数从10 MHz到1.8 GHz稳定在160，介电常数虚部为1.3~3.2之间。该数值为铁电单晶的本征晶格在GHz频段的响应，对比单畴弛豫铁电单晶低频(1 kHz)1000左右介电常数，我们可以认为弛豫铁电单晶材料中本征晶格与介电性能的贡献不足20%，即非本征畴壁移动对于介电性能起主导作用。该数据极大地支持了我们关于微波介电弛豫起源的分析，既铁电体中的微波介电弛豫来源于畴壁的谐振与晶粒的共振。对于畴壁的谐振我们可以通过极化的方法在一定程度上减少晶体内部的畴壁密度，从而消弱或者去除其畴壁谐振对于介电弥散的影响。在陶瓷材料中晶粒共振天然存在无法消除，而对于铁电单晶而言，晶界的缺失为使我们完全不用担心晶粒共振对于介电弥散的影响。因此本实验数据证实了我们在本项目立项之初的推论，即在极化的铁电单晶材料中我们有可能在微波段获得一种无频率弥散的介电响应，且其损耗也表现出很好的频率稳定性。该发现为铁电单晶材料作为微波介电材料的应用打下了良好基础。