Exploration of Hexagonal, Layered Carbides and Nitrides as Ultra-High Temperature Ceramics

Layered, hexagonal crystal structures, like zeta and eta phases, play an important role in ultra-high temperature ceramics, often significantly increasing toughness of carbide composites. Despite their importance open questions remain about their structure, stability, and compositional pervasiveness. We use high-throughput density functional theory to characterize the thermodynamic stability and elastic constants of layered carbides and nitrides M$_{n+1}$X$_{n}$ with $n$ = 1, 2, and 3, $M$ = Ta, Ti, Hf, Zr, Nb, Mo, V, W, Sc, Cr, Mn and $X$ = C, N. The stacking sequences explored are inspired by the possible use of MXenes as precursors to enable relatively low temperature processing of high-temperature ceramics. We identified 67 new hexagonal, layered materials with thermal stability comparable or better than previously observed zeta phases. To assess their potential for high temperature applications, we used machine learning and physics-based models with DFT inputs to predict their melting temperatures and discovered several candidates on par with the current state of the art zeta-like phases and five with predicted melting temperatures above 2500 K. The findings expand the range of chemistries and structures for high-temperature applications.诸如ζ相（zeta phase）、η相（eta phase）的层状六方晶体结构在超高温陶瓷中发挥着关键作用，通常可显著提升碳化物复合材料的韧性。尽管此类结构意义重大，但关于其晶体结构、热力学稳定性以及成分普适性仍存在诸多悬而未决的问题。本研究采用高通量密度泛函理论（high-throughput density functional theory），对n=1、2、3，M为Ta、Ti、Hf、Zr、Nb、Mo、V、W、Sc、Cr、Mn，X为C、N的M_{n+1}X_n型层状碳化物与氮化物的热力学稳定性与弹性常数开展了表征工作。本研究所探索的堆垛序列，灵感来源于以MXenes作为前驱体以实现高温陶瓷相对低温制备的潜在应用路径。本研究共发现67种新型层状六方晶体材料，其热稳定性可与此前报道的ζ相媲美，甚至更优。为评估此类材料在高温场景下的应用潜力，本研究结合机器学习与基于物理原理的模型，并以DFT计算结果作为输入，预测了它们的熔点；最终发现多款候选材料的性能可与当前主流类ζ相持平，另有5种材料的预测熔点超过2500 K。本研究成果拓展了高温应用领域可选用的化学体系与结构类型范围。