Physical properties of piezoelectric oxides

在功能材料领域，压电、铁电和多铁系统由于其潜在的应用，如传感器、电子存储器或超声波换能器，过去已经多次或多或少地进行了详细的研究。这类晶体包括氧化物化合物，它们结晶为压电体或铁电体。到目前为止，压电或铁电氧化物的弹性特性在某些情况下已经作为温度的函数进行了研究，但在大多数情况下没有进行进一步的研究。这尤其适用于温度相关的声阻尼现象。通过弹性张量的温度依赖性，即热弹性系数，可以特别敏感地测量机械性能的变化，因此可以借助超声波技术。压电特性也可以通过超声技术，特别是谐振超声光谱（RUS）来推导，因为机电耦合与它们的弹性特性密切相关。该项目的目的是使用RUS技术作为温度的函数来研究选定的氧化晶体（硅长石，辉长石，fresnoite, γ - lialo2, LiGaO2）的弹性和压电特性，以便分析这些材料在宽温度范围内的弹性和热弹性行为的基本特征。此外，对温度相关的声阻尼现象以及晶体结构与热弹性性质的相关性的理解将会加深。由于弹性性质反映了晶体结构的三维键合系统，因此诸如RUS之类的超声技术为检测结构不稳定性和相变提供了高灵敏度的探针。结果将提供以下信息：(1)在这里研究的氧化物（如绿长石、硅灰石）中可能出现的温度依赖的声阻尼，正如已经在硅灰岩中观察到的那样；(2)在与超声阻尼相关的高温范围内，硅灰岩中是否会发生显著的结构变化；(3)在超声阻尼温度极限以上直至熔点时，硅质岩的弹性张量和压电张量随温度的变化情况。另一方面涉及伽马- lialo2和LiGaO2的锂扩散增加的温度依赖关系（环境温度到熔点）和弹性和压电性能，以及可能发生的超声波阻尼。为了阐明压电效应频繁下降的可能原因，将对这些化合物的一部分进行高温电导率和HT-DK测量，以及高温结构测定和温度相关的粉末shg测量。

In the field of functional materials, piezoelectric, ferroelectric and multiferroic systems have repeatedly been investigated in more or less detail in the past because of their potential applications, e.g., as sensors, electronic memories or ultrasonic transducers. This group of crystals includes oxide compounds that crystallize as piezoelectrics or ferroelectrics. Up to now, elastic properties of piezoelectric or ferroelectric oxides have been investigated in some cases as a function of temperature, but in most cases no further investigations have been carried out. This applies in particular to the temperature-dependent acoustic damping phenomena. The changes in mechanical properties can be measured particularly sensitively via the temperature dependence of the elasticity tensor, i.e., the thermoelastic coefficients, and thus with the aid of ultrasound techniques. The piezoelectric properties can also be derived using ultrasound techniques, in particular resonant ultrasound spectroscopy (RUS), as the electromechanical coupling is closely linked to their elastic properties.The aim of the project is to investigate the elastic and piezoelectric properties of selected oxidic crystals (sillenites, gehlenite, fresnoite, gamma-LiAlO2, LiGaO2) using the RUS technique as a function of temperature in order to analyze the basic features of the elastic and thermoelastic behaviour of these materials in a wide temperature range. Furthermore, the understanding of temperature-dependent acoustic damping phenomena and the correlation of crystal structure and thermoelastic properties will be deepened. Since the elastic properties reflect the three-dimensional bonding system of the crystal structure, ultrasound techniques such as RUS provide a highly sensitive probe for the detection of structural instabilities and phase transformations. The results will provide information on (1) the possible occurrence of temperature-dependent acoustic damping in the oxides (e.g. gehlenite, fresnoite) investigated here as was already observed for the sillenites, (2) whether a significant structural change occurs in the sillenites in the high-temperature range associated with the ultrasound damping, and (3) how the temperature-dependent elastic and piezoelectric tensor components of the sillenites change above the temperature limit of the ultrasound damping up to the melting point. Another aspect deals with the temperature-dependent correlation of increasing lithium diffusion of gamma-LiAlO2 and LiGaO2 (ambient temperature to melting point) and the elastic and piezoelectric properties, and a possible occurrence of ultrasound damping. In order to elucidate the possible reasons for a frequently occurring decrease of the piezoelectric effect, high-temperature conductivity and HT-DK measurements together with high-temperature structure determinations and temperature dependent powder-SHG measurements will be performed on a part of these compounds.