Shedding light on solid-state microwave synthesis:In-situ Raman spectroscopy during formation of layered NaxCoO2

本研究的目的是了解微波辐射对层状无机材料高温合成过程中产物组成和形貌的影响。微波加热固体及其混合物是一种强大的合成技术，可以获得多种无机化合物。然而，所涉及的机制仍然没有得到很好的理解。为了填补这一空白，我们将开发一种独特而强大的拉曼光谱系统，与微波炉相结合，使我们能够在微波加热期间收集现场数据。层状氧化物家族NaxCoO2 （x = 1,0.74, 0.6, 0.5）将作为所有计划实验的模型系统，因为合成可以在空气中进行（不需要惰性气体），前体和不同的结构（取决于x）表现出不同的拉曼信号。将研究两种不同的合成方法，从固体粉末混合物开始的固态合成和从可溶性金属盐开始的溶胶-凝胶合成。这将有助于深入了解前体的性质和导致最终产物的不同类型的化学反应的作用。最后，将微波辅助和常规加热过程中获得的拉曼光谱数据进行比较，将揭示微波辐射对高温合成（固态和溶胶-凝胶）的影响。一旦原位测量系统被证明是成功的氧化物模型系统，原位分析将扩展到更具挑战性的层状三元金属碳化物（MAX相）的合成。关于形成机制和微波辐射对这些相的合成和形态的影响的知识将为层状无机材料（如氧化物和碳化物）的合成控制提供指导原则，并为这些材料的新成员/形态开辟道路。

The goal of the proposed work is to gain insights about the influence of microwave radiation on the formation mechanism as well as composition and morphology of the products during high-temperature synthesis of layered inorganic materials. Microwave heating of solids and mixtures thereof is a powerful synthesis technique to access a large variety of inorganic compounds. However, the involved mechanisms are still not well understood. In order to fill this void, we will develop a unique and powerful Raman spectroscopy system coupled to a microwave oven that will allow us to collect in-situ data during microwave heating. The layered oxide family NaxCoO2 (x = 1, 0.74, 0.6, 0.5) will serve as the model system for all planned experiments since the synthesis can be performed in air (no inert gas needed) and the precursors and different structures (depending on x) exhibit distinct Raman signals. Two different synthesis methods, solid-state synthesis starting from solid powder mixtures as well as sol-gel synthesis starting from soluble metal salts, will be investigated. This will enable gaining insights into the role of the nature of the precursors and the different types of chemical reactions that lead to the final products. Ultimately, the comparison of the obtained Raman spectroscopy data during microwave-assisted and conventional heating will shed light on the influence of the microwave radiation on high-temperature synthesis (solid-state and sol-gel). Once the in-situ measurement system has been proven successful for the oxide model system, the in-situ analysis will be extended to the more challenging synthesis of layered ternary metal carbides (MAX phases). The knowledge about the formation mechanisms and the influence of microwave radiation on the synthesis and morphology of these phases will provide guiding principles for the control of the synthesis of layered inorganic materials (e.g. oxides and carbides) and open a path to new members/morphologies of these classes of materials.