Aubert Thierry, Kokanyan Ninel, Elmazria Omar
Université de Lorraine-CentraleSupélec, LMOPS, F57070 Metz, France.
Université de Lorraine-CNRS, Institut Jean Lamour (UMR 7198), F54000 Nancy, France.
Sensors (Basel). 2021 Sep 6;21(17):5978. doi: 10.3390/s21175978.
Langasite crystals (LGS) are known for their exceptional piezoelectric properties at high temperatures up to 1000 °C and more. In this respect, many studies have been conducted in order to achieve surface acoustic wave (SAW) sensors based on LGS crystals dedicated to high-temperature operations. Operating temperatures of more than 1000 °C and 600 °C for wired and wireless sensors, respectively, have been reached. These outstanding performances have been obtained under an air atmosphere since LGS crystals are not stable in high-temperature conditions under a low-oxygen atmosphere due to their oxide nature. However, if the stability of bulk LGS crystals under a high-temperature air atmosphere is well established, the surface deterioration under such conditions has been hardly investigated, as most of the papers dedicated to LGS-based SAW sensors are essentially focused on the development of thin film electrodes that are able to withstand very elevated temperatures to be combined with LGS crystals. Yet, any surface modification of the substrate can dramatically change the performance of SAW sensors. Consequently, the aim of this paper is to study the stability of the LGS surface under a high-temperature air environment. To do so, LGS substrates have been annealed in an air atmosphere at temperatures between 800 and 1200 °C and for durations between one week and one month. The morphology, microstructure, and chemical composition of the LGS surface was examined before and after annealing treatments by numerous and complementary methods, while the surface acoustic properties have been probed by SAW measurements. These investigations reveal that depending on both the temperature and the annealing duration, many defects with a corolla-like shape appear at the surface of LGS crystals in high-temperature prolonged exposure in an air atmosphere. These defects are related to the formation of a new phase, likely an oxiapatite ternary compound, the chemical formula of which is LaGaSiO. These defects are located on the surface and penetrate into the depth of the sample by no more than 1-2 microns. However, SAW measurements show that the surface acoustic properties are modified by the high-temperature exposure at a larger deepness of at least several tens of microns. These perturbations of the LGS surface acoustic properties could induce, in the case of LGS-based SAW sensors operating in the 434 MHz ISM band, temperature measurement errors around 10 °C.
硅酸镧镓晶体（LGS）以其在高达1000℃及更高温度下出色的压电性能而闻名。在这方面，已经开展了许多研究，以实现基于LGS晶体的用于高温操作的表面声波（SAW）传感器。有线和无线传感器分别实现了超过1000℃和600℃的工作温度。由于LGS晶体因其氧化物性质在低氧气氛的高温条件下不稳定，这些出色的性能是在空气气氛中获得的。然而，尽管块状LGS晶体在高温空气气氛下的稳定性已得到充分证实，但在这种条件下的表面劣化情况却鲜有研究，因为大多数致力于基于LGS的SAW传感器的论文主要集中在能够承受非常高温度的薄膜电极的开发上，以便与LGS晶体结合。然而，基板的任何表面改性都可能极大地改变SAW传感器的性能。因此，本文的目的是研究LGS表面在高温空气环境下的稳定性。为此，将LGS基板在空气气氛中于800至1200℃的温度下退火，持续时间为一周至一个月。通过多种互补方法对退火处理前后LGS表面的形貌、微观结构和化学成分进行了检查，同时通过SAW测量探测了表面声学特性。这些研究表明，取决于温度和退火持续时间，在空气气氛中高温长时间暴露时，LGS晶体表面会出现许多花冠状缺陷。这些缺陷与一种新相的形成有关，可能是一种氧磷灰石三元化合物，其化学式为LaGaSiO。这些缺陷位于表面，向样品深度渗透不超过1 - 2微米。然而，SAW测量表明，在至少几十微米的更大深度处，高温暴露会改变表面声学特性。对于工作在434 MHz ISM频段的基于LGS的SAW传感器而言，LGS表面声学特性的这些扰动可能会导致约10℃的温度测量误差。
