New Quaternary MAX Phase Thin Films: Understanding the Thermally Induced Microstructural Evolutions and Reaction Mechanisms in Nanostructured Multilayers via Experimental Combinatorial Study

MAX相材料（Mn+1AXn，M =过渡金属，A = A族元素，X =碳或氮，n =1，2，3）是具有结合金属和陶瓷属性的独特性质的原子层状化合物。单个原子层的多元素合金化是合成新型四元MAX相的有力工具，并提供了多种机会，通过定制其局部化学和结构复杂性来广泛调节其性能。最近，除了随机固溶体结构之外，还发现了M层上的新的化学有序四元（M′M″）n+1AXn结构，包括面外有序（称为o-MAX）和面内有序（称为i-MAX）。然而，在单相结构中合成这种块状四元MAX相仍然是一个巨大的挑战。四元MAX相材料的薄膜合成是材料科学中的一个新兴研究领域，而化学有序的四元MAX相薄膜还未见报道。在本论文中，我们的目标是在三个四元模型体系（Cr-M″-Al-C，M″：V，Ti，Zr）中合成单相和潜在的基面织构的四元（M′M″）n+1AXn薄膜材料。 这将通过利用用于薄膜前体设计和合成的实验组合方法，通过适当的处理工艺将纳米级元素多层与预定义的纳米结构化架构（“薄膜前体”）转化来实现。本论文的研究工作将首先以四元无规固溶体MAX相的合成为目标，然后将致力于两种复杂有序结构的合成。该计划的科学目标是：1）了解纳米结构多层膜在热处理过程中形成不同四元MAX相薄膜的微结构演变及其潜在反应机制，以及2）探索四元MAX相薄膜的组成-微结构-性能关系。薄膜前体的热处理将涵盖各种加热速率和动力学（即非常低以及快速加热速率）下的退火实验，包括使用新型薄膜量热计的选定实验。科学工作将以原子级分辨率的密集薄膜表征方法为基础。因此，拟议的研究将有助于开发合适的薄膜前驱体架构和热处理工艺的合成相纯四元MAX相薄膜。科学成果将有助于理解局部化学（成分和化学有序）和微观结构对这种新型MAX相薄膜材料性能的作用。

MAX phase materials (Mn+1AXn, M = transition metal, A = A group element, X = carbon or nitrogen, n =1, 2, 3) are atomically layered compounds that possess unique properties combining attributes of both metals and ceramics. Multi-elemental alloying of individual atomic layers is a powerful tool to synthesize novel quaternary MAX phases and offers manifold opportunities to widely tune their properties through tailoring their local chemistry and structural complexity. Recently, apart from random solid solution structures, new chemically ordered quaternary (M′M″)n+1AXn structures on M layers have been discovered, including out-of-plane ordering (referred to as o-MAX) and in-plane ordering (referred to as i-MAX). However, the synthesis of such bulk quaternary MAX phases in single-phase structure remains a great challenge. Thin-film synthesis of quaternary MAX phase materials is an emerging research field in materials science, and, chemically ordered quaternary MAX phase thin films have not been reported yet. In this proposal, we aim to synthesize single-phase and, potentially, basal-plane textured quaternary (M′M″)n+1AXn thin film materials in three quaternary model systems (Cr-M″-Al-C with M″: V, Ti, Zr). This will be achieved by transformation of nanoscale elemental multilayers with pre-defined nanostructured architectures (“thin film precursors”) through appropriate treatment processes, utilizing an experimental combinatorial approach for the thin film precursor design and synthesis. The research work will target at first the synthesis of quaternary random solid solution MAX phases, and will then address the synthesis of the two complex ordered structures. The scientific objectives of this proposal are: 1) to understand the microstructural evolutions and their underlying reaction mechanisms during thermal processing of the nanostructured multilayers towards the formation of different quaternary MAX phase thin films, and, 2) to explore the composition–microstructure–properties relationship of the quaternary MAX phase thin films. The thermal processing of the thin film precursors will cover annealing experiments at various heating rates and kinetics (i.e. very low as well as fast heating rates), including selected experiments with a novel thin film calorimeter. The scientific work will be based on intensive thin film characterization methods with atomic-scale resolution. Thus, the proposed research will contribute to developing suitable thin film precursor architectures and thermal treatment processes for the synthesis of phase-pure quaternary MAX phase thin films. The scientific outcome will lead to an understanding of the role of local chemistry (composition and chemical ordering) and microstructure on the properties of such new MAX phase thin film materials.