Extension of conventional solid state methods: Microwave synthesis of transition metal carbides

在这个项目中，我们将探索微波加热作为一种合成工具来获取各种二元和更复杂的过渡金属碳化物。固体微波反应已被广泛用于制备多种无机化合物，这些化合物甚至经常表现出改进的性质。基于我们使用家用微波炉进行微波反应的经验（例如半赫斯勒金属间化合物和氧化物荧光粉），我们将把我们的努力扩展到另一类化合物，即过渡金属碳化物。在这里，一个实验室级的微波反应堆是这个项目过程中使用的核心设备。虽然家用微波炉已经取得了可喜的成果，但只有研究级微波炉才能使我们充分利用微波加热技术。与家用微波炉相比，它有许多优点，最重要的是可以监控反应过程中的温度，而且安全标准要高得多。这两个方面对于这里提出的项目都非常重要，因为它们将使高度可重复的合成以及实验室中的安全反应程序成为可能。由于不同形式的碳与微波辐射的良好耦合行为导致快速的加热速率，碳化物是利用微波加热合成碳化物的理想候选者之一。研究了二元过渡金属碳化物，如Fe3C， Co2C，以评估各种可能的合成方法，从元素直接反应和碳热还原各自的氧化物。然后我们将转向六方MAX相，这是更复杂的碳化物，通式为Mn+1AXn，其中M是早期过渡金属，A是A族元素（主要在13和14族中），X是碳或氮。这些化合物中有许多是已知的，并得到了充分的研究。然而，并没有太多的工作致力于寻找与后期过渡金属（如Mn， Fe和Co）的额外MAX相。因此，我们将重点关注已知MAX化合物与后期过渡金属的可能固溶体，并对其磁性特别感兴趣。此外，所谓的MXene化合物也将被研究。它们是MAX相的剥离版本，其中A层已被蚀刻掉。由于这些化合物中的一些表现出半导体而不是金属的行为，它们有望表现出有前途的热电性质。

In the proposed project, we will explore microwave heating as a synthesis tool to access a variety of binary and more complex transition metal carbides. Solid state microwave reactions have been widely used to prepare a multitude of inorganic compounds that often even exhibit improved properties. Based on our experience with microwave reactions using a domestic microwave oven (e.g. half-Heusler intermetallic compounds and oxide phosphors), we will extend our efforts to an additional class of compounds, namely transition metal carbides. Here, a laboratory-grade microwave reactor is the central piece of equipment that will be used during the course of this project. Although promising results can already be achieved with a domestic microwave oven, only the research-grade microwave will allow us to take full advantage of the microwave heating technique. It offers a number of advantages in contrast to the domestic microwave oven, most importantly the possibility to monitor the temperature during the reaction as well as much higher safety standards. Both aspects are of major importance for the here proposed project because they will enable highly reproducible syntheses as well as safe reaction procedures in the laboratory.Due to the excellent coupling behavior of different forms of carbon with the microwave radiation leading to rapid heating rates, carbides are among the ideal candidates to exploit microwave heating for their synthesis. Binary transition metal carbides, such as Fe3C, Co2C, will be studied to evaluate the various possible synthesis methods, direct reaction from the elements and carbothermal reduction of the respective oxides. We will then move towards the hexagonal MAX phases that are more complex carbides with the general formula Mn+1AXn, where M is an early transition metal, A is an A-group element (mostly in groups 13 and 14) and X is either carbon or nitrogen. Many of these compounds are known and well studied. However, not that much work has been devoted to finding additional MAX phases with later transition metals, such as Mn, Fe and Co. We will therefore focus on possible solid solutions of known MAX compounds with later transition metals and are particularly interested in their magnetic properties. Besides, so called MXene compounds will be also be investigated. They are an exfoliated version of the MAX phases where the A layer has been etched away. Since some of these compounds exhibit semiconducting instead of metallic behavior, they are expected to show promising thermoelectric properties.