Synthesis, Characterization, and Modeling of the Mn+1AXn Layered Ternary Carbides and Nitrides

近年来，PI发现了一类新的陶瓷材料，最好的描述是多晶纳米层酸盐。这些固体是三元层状六方碳化物和氮化物，通式为：Mn+1axn，其中n=1-3，M是早期过渡金属，A是A族元素，X是碳和/或氮。到目前为止，已知的大约50个相中有15个已经合成和表征。这些相具有它们各自的化学计量二元金属碳化物或氮化物的许多有利属性：它们具有弹性刚性、导电性和导热性。然而，在机械上，它们完全不同：它们易于加工，相对柔软，抗热冲击，并具有异常的损伤容忍度。它们将易加工性与良好的机械性能结合在一起，特别是在T 1000℃下。此外，它们将热各向同性与机械各向异性结合在一起。这种不同寻常的性质组合可追溯到它们的层状结构，特别强的MX键的金属共价性质，以及相对较弱的M-A键，特别是剪切。到目前为止，表征最好的三元系是Ti3SiC2。尽管在理解这些固体方面取得了明显进展，但仍有几个根本问题尚未解决。其中最主要的是：i)在许多这样的系统中大约在1100℃发生脆塑性转变的物理起源；ii)在较高温度下观察到的Ti2AlN0.5C0.5中固溶体软化效应的性质；iii)键与单晶弹性常数以及电和热性能之间的关系。这项研究项目将合成和表征以前从未合成过的大约35个三元化合物中的大多数，以试图了解它们的结构-性质-成键关系。制作和表征将在PI位于德雷克塞尔的实验室进行。为了进行结构的理论建模，PI与Nadia Medvedeva博士领导的俄罗斯团队合作，后者最近发表了一篇关于Ti3SiC2及其固体溶液的全势线性松饼锡轨道计算的优秀论文。最终目标是改进理论模型，以便能够预测弹性、热学和电学性质。PI已经探索出了一种可归类为纳米叠层的新型陶瓷材料。这些材料结合了陶瓷的一些更好的热、化学、电和弹性特性，但几乎没有它们的缺点，如脆性。由于这些不同寻常的特性，这些陶瓷材料可能会产生广泛的技术影响。

In recent years, the PI has identified a new class of ceramic material best described as polycrystalline nanolaminates. These solids are ternary layered hexagonal carbides and nitrides with the general formula, M n+1 AX n , where n = 1 to 3, M is an early transition metal, A is an A-group element and X is either carbon and/or nitrogen. To date 15 of the roughly 50 phases known to exist have been synthesized and characterized. These phases share many of the advantageous attributes of their respective stoichiometric binary metal carbides or nitrides: they are elastically stiff, electrically and thermally conductive. Mechanically, however, they cannot be more different: they are readily machinable, relatively soft, resistant to thermal shock and unusually damage tolerant. They combine ease of machinability with good mechanical properties, especially at T 1000 degrees C. Furthermore, they couple thermal isotropy with mechanical anisotropy. This unusual combination of properties is traceable to their layered structure, the metallic-covalent nature of the MX bonds that are exceptionally strong, together with M-A bonds that are relatively weak, especially in shear. The best-characterized ternary to date is Ti3 SiC2. Despite clear progress in understanding these solids several fundamental issues remain unresolved. Primary amongst them are: i) the physical origin of the brittle-to-plastic transition that occurs at approximately 1100 degrees C in many of these systems; ii) the nature of a solid-solution softening effect observed at higher temperatures in Ti 2 AlN0.5C0.5 ; iii) the relationship between the bonding and the single crystal elastic constants and the electrical and thermal properties. This research project will synthesize and characterize most of the 35 or so ternaries that have never been synthesized before in an attempt to understand their structure-properties-bonding relationships. The fabrication and characterization will take place in the PI's laboratory at Drexel. To carry out the theoretical modeling of the structure, the PI has teamed with the Russian team headed by Dr. Nadia Medvedeva who recently published an excellent paper on the full potential linear-muffin-tin orbital calculations of Ti3 SiC2 and its solid solutions. The ultimate goal is to refine the theoretical model to allow for the prediction of the elastic, thermal and electrical properties. A new class of ceramic material that can be classified as nano-laminates has been explored by the PI. These materials combine some of the better thermal, chemical, electrical and elastic characteristics of ceramics, with few of their drawbacks, such as brittleness. Because of these unusual properties, these ceramic materials are likely to have a broad technological impact.