Influence of ordered carbon-vacancy networks on the electronic structures and elastic properties of Nb₄AlC_3−x

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• 作者：Hui Zhang ; Tao Hu ; Zhaojin Li ; Yanhui Zhang ; Minmin Hu ; Xiaohui Wang and Yanchun Zhou
• 刊名：Journal of the American Ceramic Society
• 出版年：2017
• 出版时间：February 2017
• 年：2017
• 卷：100
• 期：2
• 页码：724-731
• 全文大小：2411K
• ISSN：1551-2916

文摘

Carbon-vacancy-bearing Nb₄AlC_3−x has the best high-temperature mechanical robustness among MAX phases. The existing form of the vacancies has been long overlooked. Recently, the vacancies in Nb₄AlC_3−x have been identified to be ordered, establishing an ordered compound Nb₁₂Al₃C₈. Here, the spatial distribution of the ordered vacancies and their influences on bonding characteristics and elastic properties are unraveled by thoroughly comparing Nb₁₂Al₃C₈ and vacancy-free Nb₄AlC₃. In Nb₁₂Al₃C₈, the carbon vacancies break only relatively weak Nb–C bonds and form ordered equilateral triangular carbon-vacancy networks (OETCVNs) to maximize the bond strengthening effect. The networks slightly shift partial and total density of states toward the Fermi energy level, and bring about a feature of “de-metallization”. Meanwhile, the presence of OETCVNs results in the softening of elastic modulus, decreasing of the anisotropy of Young's modulus, yet increasing that of shear modulus. These results shed lights on the carbon-vacancy ordering behavior of MAX phases, and provide an opportunity to tailor their electronic structures and elastic properties through defect engineering.