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在线监测和过程控制方面具有许多优点，因此引起了人们越来越大的兴趣。特别是，对适用于温度高达1000°C的气体成分、温度和压力的灵敏、坚固且经济实惠的传感器的需求很高。例如，在能源转换、航空航天和其他重大经济工业应用中，需要具有可通过施加电压调节位移的压电致动器，例如用于涡轮机或太阳能聚光器过程控制的高温喷嘴或基于致动器的部件。在任何情况下，都要求具有良好的热稳定性和较大的压电系数，而在实际应用中，常用的压电材料的应用温度是有限的。尽管La3Ga5SiO14家族的石英型晶体在高温下表现出很好的稳定性，但由于它们的压电系数较低，因此不适合作为执行器。NbO_3和LiTaO_3被认为是潜在的高温传感和执行器件材料。这两种晶体都具有较高的压电系数，但LiNbO_3的热不稳定性和LiTaO_3的低居里温度限制了它们的应用。在这方面，Li(Nb，Ta)O_3固溶体可能克服了单个化合物的上述限制。这一假设得到了我们前期工作的支持。双方研究伙伴的共同目标是开发可应用于传感和驱动应用的高温稳定的Li(Nb，Ta)O3固溶体。这种单晶的生长以及它们的化学和结构表征，包括Nb/Ta比、结构完整性以及线和体缺陷的检查，将由俄罗斯合作伙伴实现。此外，还计划对声波波场进行分析，以确定Nb/Ta比对波传播的影响。德国合作伙伴专注于高温性能，从而确定电气和机电性能，研究原子传输，并开发缺陷化学模型。传输动力学的分析将由稳定示踪同位素18O和6Li进行，这使得可以结合有关机电损失的数据来判断高温稳定性。此外，Li(Nb，Ta)O_3的选定材料常数将由双方以不同的方法确定为温度的函数，以验证结果。这两个合作伙伴的工作密切相关，应提供关于具有最稳定的Nb/Ta比率的Li(Nb，Ta)O_3的信息，以便为已经在第一个项目期中期的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.