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

近年来，PI已经确定了一类新的陶瓷材料，最好描述为多晶纳米层压材料。 这些固体是具有通式Mn +1AX n的三元层状六方碳化物和氮化物，其中n = 1至3，M是前过渡金属，A是A族元素，X是碳和/或氮。 迄今为止，已知存在的大约50个阶段中有15个已经合成和表征。 这些相共享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：它们是弹性刚性的、导电的和导热的。 然而，在机械上，它们却截然不同：它们易于加工，相对柔软，耐热冲击，并且具有异常的耐损伤性。 它们结合了联合收割机的易加工性和良好的机械性能，特别是在T 1000 ℃下。 此外，他们耦合热各向同性与机械各向异性。 这种不寻常的性质组合可追溯到它们的层状结构，MX键的金属共价性质非常强，M-A键相对较弱，特别是在剪切中。 迄今为止，最好的三元化合物是Ti 3 SiC 2。 尽管在理解这些固体方面取得了明显进展，但一些基本问题仍然没有解决。 其中主要有：i）在这些系统中的许多系统中在大约1100摄氏度下发生的脆塑性转变的物理起源; ii）在较高温度下在Ti 2 AlN0.5C0.5中观察到的固溶体软化效应的性质; iii）键合和单晶弹性常数与电和热性能之间的关系。 该研究项目将合成和表征以前从未合成过的35种左右的三元化合物中的大多数，以试图了解它们的结构-性质-键合关系。 制造和表征将在德雷克塞尔的PI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，该团队最近发表了一篇关于Ti 3 SiC 2及其固溶体的全势能线性松饼轨道计算的优秀论文。 最终目标是完善理论模型，以预测弹性，热和电性能。PI已经探索了一类新的陶瓷材料，可以归类为纳米层压材料。这些材料联合收割机结合了陶瓷的一些更好的热、化学、电和弹性特性，而几乎没有它们的缺点，例如脆性。 由于这些不寻常的特性，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.