Growth and characterization of single crystalline lithium niobate-tantalate (Li(Nb,Ta)O3) solid solutions for high-temperature sensor and actuator applications

由于压电传感器在多参数现场监测和过程控制方面具有诸多优势，因此越来越受到人们的关注。特别是，对于适用于温度高达1000°C的气体成分，温度和压力的敏感，坚固且具有成本效益的传感器有很高的需求。压电致动器的位移可以通过施加的电压来调节，例如能量转换，航空航天和其他经济上重要的工业应用，如高温喷射喷嘴或用于涡轮机或太阳能聚光器过程控制的致动器组件。在任何情况下，同时需要优异的热稳定性和大的压电系数。在实际应用中，普通压电材料的应用温度是有限的。相关的多晶陶瓷在约200°C以上表现出热不稳定性。尽管来自langasite （La3Ga5SiO14）家族的石英型晶体在高温下表现出优异的稳定性，但由于其压电系数低，因此不适合用作执行器。铌酸锂（LiNbO3）和钽酸锂（LiTaO3）已被认为是高温传感和致动器件的潜在材料。这两种晶体都具有很高的压电系数，但它们的使用受到LiNbO3的热不稳定性和LiTaO3的低居里温度的限制。在这方面，Li(Nb,Ta)O3固溶体可能克服上述单个化合物的限制。我们的初步工作支持了这一假设。双方研究伙伴的共同目标是开发高温稳定的Li(Nb,Ta)O3固溶体，可应用于传感和驱动应用。这种单晶的生长以及它们的化学和结构表征，包括Nb/Ta比的检查，结构的完善，线和体缺陷将由俄罗斯合作伙伴实现。此外，还计划对声波场进行分析，以确定Nb/Ta比对波传播的影响。德国合作伙伴专注于高温特性，从而确定电气和机电特性，研究原子传输并开发缺陷化学模型。输运动力学分析将由稳定的示踪同位素18O和6Li进行，这允许与机电损失数据相一致来判断高温稳定性。此外，双方将用不同的方法确定Li(Nb,Ta)O3的选定材料常数作为温度的函数，以验证结果。双方的工作紧密相连，应该在第一个项目中期就提供Nb/Ta比最稳定的Li(Nb,Ta)O3的信息，为改进晶体的生长提供反馈。

Piezoelectric sensors are attracting increasing interest since they offer numerous advantages for multiparameter in-situ monitoring and process control. In particular, there is a high demand in sensitive, robust and cost-effective sensors for gas composition, temperature and pressure that are suitable for applications at temperatures up to 1000°C. Piezoelectric actuators whose displacement can be adjusted by an applied voltage are required for e.g. energy conversion, aerospace and other economically significant industrial applications such as high-temperature injection nozzles or actuator-based components for process control of turbines or solar concentrators. In any case, excellent thermal stability and large piezoelectric coefficients are required simultaneously.In practice the application temperature of common piezoelectric materials is limited. Related polycrystalline ceramics show thermal instability above about 200°C. Even though quartz type crystals from langasite (La3Ga5SiO14) family show excellent stability at high temperatures, they are not suitable for actuators due to their low piezoelectric coefficients.Lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) have been recognized as potential materials for high-temperature sensing and actuating devices. Both crystals possess high piezoelectric coefficients, however their usage is limited by thermal instability of LiNbO3 and low Curie temperature of LiTaO3. In this respect Li(Nb,Ta)O3 solid solutions possibly overcome the above-mentioned restrictions of the individual compounds. This hypothesis is supported by our preliminary work.The joint objective of both research partners is to develop high-temperature stable Li(Nb,Ta)O3 solid solutions that can be applied in sensing and actuating applications. The growth of such single crystals as well as their chemical and structural characterization, including examination of the Nb/Ta ratio, structure perfection, and line and bulk defects will be realised by the Russian partner. Further, analysis of the acoustic wave fields to determine the influence of Nb/Ta ratio on wave propagation is planned from this party. The German partner focuses on high-temperature properties, thereby determining electrical and electromechanical properties, investigating the atomic transport and developing a defect chemical model. The analysis of transport kinetics will be performed by the stable tracer isotopes 18O and 6Li, which allows in concert with data about electromechanical losses to judge the high-temperature stability. Further, selected material constants of Li(Nb,Ta)O3 will be determined as a function of temperature by both partners with different methods to validate the results. The work of both partners is closely linked and should provide information on Li(Nb,Ta)O3 with the most stable Nb/Ta ratio to provide feedback for growth of improved crystals already in the middle of the first project period.