弛豫铁电铌镁酸铅的朗道系数及其临界行为的研究

发布时间：2019-06-10 01:02

【摘要】：弛豫铁电单晶通常具备比传统的压电材料更高的压电性能,这为它们在土木工程结构的健康检测等应用中提供了广阔的应用前景。虽然目前大部分学者都认为局部纳米极化区域是其具备高性能的主要原因,然而内在的机制并未被完全认知。另外,由于缺乏相应的朗道系数,相应的热力学理论计算也无法开展。结合实验观测的现象,获取朗道热力学系数,并结合实验结果进行理论预测推算,进而深入研究弛豫铁电单晶中巨压电的内在机理及在外场下的响应,这对开拓新一代压电单晶尤其是无铅压电单晶具有重要意义。基于此目的,本论文主要进行了以下的几项研究:(1)分析极性纳米微区在弛豫体里中的作用,确定其在不同温度下的状态和性质,并考虑其本质特性以及在弛豫铁电体中的作用,通过确定弛豫铁电里的相结构性质,推断纳米微区对巨压电效应的贡献。根据铌镁酸铅的组分-温度相图,可以将晶体的相结构分类为三方相,单斜相和四方相。结合不同温度以及电场下的相结构以及动力学行为,结合朗道唯象理论的特征性质分析获取朗道系数,并提出一系列具体的获取朗道系数的测试以及验证方法。(2)对于准同形相界偏左成份的PMN-0.30PT单晶,通过拟合双电滞回线,介电系数随温度变化的趋势以及三方和四方相发生转变的温度来拟合得到朗道系数。基于获取的朗道系数,利用热力学理论计算其铁电性能,并且与实验的结果进行对比。利用热力学的能量分布以及各种正则统计结果分析实验共存相,通过确定稳定相的含量,计算共存区的性能,进而与实验结果进行对比。(3)由于铁电朗道理论系数主要通过单畴结构的一些特征进行确定,因此需要确保单晶在极化后是单畴状态。采用偏光显微镜,可以进行微畴结构的观测。通过将晶体切成不同形状,对晶体施加不同方向的电场,结合晶体的光学特征,详细观测相变温度附近晶体畴结构的变化,分析归纳出晶体在形成单畴条件下的临界电场的大小。基于电场对畴结构的影响,本文在不同温度和电场下极化单晶样品,获取不同的畴结构。(4)对于常温下以为四方相为主的PMN-0.36PT单晶,通过拟合电滞回线,介电系数以及诱导三方相所需的临界电场获得朗道参数。利用所获取的朗道参数,理论计算随温度变化的铁电性能,并且与实验的结果进行对比。此外本文首次利用热力学理论推导了PMN-0.30PT和PMN-0.36PT薄膜的多畴结构相图,并分析其相图存在的差异和原因。
[Abstract]:Relaxor ferroelectric single crystals usually have higher piezoelectric properties than traditional piezoelectric materials, which provides a broad application prospect for their applications in health detection of civil engineering structures. Although most scholars believe that the local nano-polarization region is the main reason for its high performance, the internal mechanism has not been fully recognized. In addition, due to the lack of corresponding Landau coefficient, the corresponding thermodynamic theoretical calculation can not be carried out. Combined with the phenomenon observed by experiment, the thermodynamic coefficient of Landau is obtained, and the theoretical prediction and calculation are carried out with the experimental results, and then the internal mechanism of giant piezoelectric in relaxor ferroelectric single crystal and its response in external field are deeply studied. This is of great significance for the development of a new generation of piezoelectric single crystals, especially lead-free piezoelectric single crystals. For this purpose, the following studies have been carried out in this paper: (1) the role of polar nanoparticles in relaxor is analyzed, and the state and properties of polar nanoparticles at different temperatures are determined. Considering its essential characteristics and its role in relaxor ferroelectrics, the contribution of nano-microregions to the giant piezoelectric effect is inferred by determining the phase structure properties of relaxor ferroelectrics. According to the composition-temperature phase diagram of lead magnesium niobate, the phase structure of the crystal can be classified into trihedral phase, monoclinic phase and tetrahedral phase. Combined with the phase structure and dynamic behavior under different temperature and electric field, combined with the characteristic properties of Landau's phenomenological theory, the Landau coefficient is obtained. A series of concrete test and verification methods for obtaining Landau coefficient are put forward. (2) for PMN-0.30PT single crystal with left component of quasi-homomorphism phase boundary, the double electric hysteretic line is fitted. The Landau coefficient is obtained by fitting the trend of dielectric coefficient with temperature and the temperature of tripartite and tetrahedral phase transition. Based on the obtained Landau coefficient, the ferroelectric properties are calculated by thermodynamic theory and compared with the experimental results. The energy distribution of thermodynamics and various regular statistical results are used to analyze the coexistence phase of the experiment. By determining the content of the stable phase, the performance of the coexistence region is calculated. Furthermore, the results are compared with the experimental results. (3) because the theoretical coefficients of ferroelectric Landau are mainly determined by some characteristics of single domain structure, it is necessary to ensure that the single crystal is in a single domain state after polarization. The microdomain structure can be observed by polarizing microscope. By cutting the crystal into different shapes and applying electric fields in different directions to the crystal, combined with the optical characteristics of the crystal, the change of crystal domain structure near the phase transition temperature is observed in detail. The critical electric field of the crystal under the condition of forming a single domain is analyzed and summarized. Based on the effect of electric field on domain structure, different domain structures are obtained by polarizing single crystal samples at different temperatures and electric fields. (4) for PMN-0.36PT single crystals, which are mainly tetrahedral phase at room temperature, the hysteretic loop is fitted. The dielectric coefficient and the critical electric field needed to induce the tripartite phase are obtained to obtain Landau parameters. The ferroelectric properties varying with temperature are calculated theoretically by using the obtained Landau parameters, and the results are compared with the experimental results. In addition, the multi-domain structure phase diagram of PMN-0.30PT and PMN-0.36PT thin films is derived by thermodynamic theory for the first time, and the difference and reason of phase diagram are analyzed.
【学位授予单位】：哈尔滨工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：O738

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