Development of a newly designed ultraviolet Raman scattering system for in-site observation of structure at very high temperature

在高温下原位观察陶瓷的结构变化对控制材料的性能和成分具有重要意义。在陶瓷领域，由于x射线衍射法是最常用的相识别方法，该技术已扩展到高温下陶瓷结构的原位观察。然而，x射线衍射技术并不总是适用于像陶瓷这样的系统，其中氧原子的位置在决定性能和相变方面起着至关重要的作用，或者由超细粉末组成的样品。因此，需要开发一种替代x射线衍射技术的方法，使我们能够在高温下进行研究。为了实现这一要求，我们开发了一种新设计的拉曼散射系统，并结合了紫外线激发光源。在普通的拉曼散射实验中，传统的可见光氩激光器被用作拉曼光谱的激发源。最常用于拉曼散射的波长包括488纳米的蓝线和515纳米的绿线。然而，当这些可见光用于高温下的拉曼散射实验时，来自热辐射的非常强的背景会严重干扰相对较弱的拉曼散射的观测。为了克服这一严重的问题，我们设计了一种新的拉曼散射系统，该系统结合了波长为363 nm的激发源，使得在高于1500℃的温度下测量拉曼光谱成为可能。我们成功地测量了各种高温材料（铪、氧化锆和钙钛矿）的拉曼光谱，并清楚地观察到它们在高温下的原位相变。例如，在加热条件下，hafnia的单斜向四方转变在2080 K左右完成，而在冷却条件下，单斜相在2018 K出现。

It is of prime importance to observe change in structure of ceramics in-situ at high temperature for controlling materials properties and compositions. In the field of ceramics, since the X-ray diffraction method is most frequently used for phase identification, the technique has been extended to in-situ observation of ceramic structure at high temperature. However, X-ray diffraction techniques are not always suitable for systems like ceramics, where positions of oxygen atoms play a crucial role in determining properties and phase transformations, or for samples comprised of ultra fine powders. For this reason, it is required to develop an alternative to the X-ray diffraction technique, which enables us to carry out studies at high temperatures. To achieve this requirement, we have developed a newly designed Raman scattering system combined with an ultra-violet excitation light source. In ordinary Raman scattering experiments, conventional visible light Ar lasers have been used as excitation sources for Raman spectra. Wavelengths most frequently used for Raman scattering include the blue line at 488 nm and the green line at 515 nm. However, in case where these visible lights are used for Raman scattering experiments at high temperature, a very strong background coming from thermal emissions interfere quite severely with the observation of relatively weak Raman scattering. To overcome this serious problem, we have designed a new Raman scattering system combined with an excitation source having a wave length 363 nm, which made possible to measure Raman spectra at temperatures even higher than 1500℃. We have successfully measured Raman spectra of various high temperature materials (hafnia, zirconia, and perovskite) and clearly observed their phase transformations in situ at high temperatures. For instance, the monoclinic-to-tetragonal transformation of hafnia finished around 2080 K on heating, while the monoclinic phase appeared at 2018 K on cooling.