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

Drexel University, Philadelphia PA

Investigators

Abstract

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.