M. Barsoum (PI), A. Ganguly (RA), E. Hoffman (RA); Drexel University, DMR Synthesis, Characterization & Modeling of Layered Ternary Carbides & Nitrides Graphitic Rose

Using a spherical nanoindenter we showed that graphite (top left) Using a spherical nanoindenter we showed that graphite (top left) behave like the ternary carbides and nitrides we discovered and have been investigating for about 8 years now. These carbides and nitrides, apart from being readily machinable, can also deform plastically and fully recover after the deformation dissipating substantial amounts of energy during each cycle. The graph (top middle) plots the energy dissipated during each cycle versus applied stress (both normalized) on a log-log plot. The excellent agreement between the values obtained for graphite, mica and Ti 3 SiC 2 is evidence that the same atomic mechanisms are occurring in all three materials. We call this new class of solids kinking nonlinear elastic, because they tend to deform by kinking as a deck of cards would if loaded parallel to the cards (top right). In another facet of our we are exploring why and how metal whiskers can grow with time (bottom left) - like human hair from the bottom up - out of the grain boundaries of Zr2InC (bottom right) - another MAX phase. We have shown that this process is essentially an oxidation reaction (paper is under review). It is worth noting this problem has been outstanding for 50 + years.

Impact of Research Graphite, ice,sapphire, layered silicates, such mica - and thus much of geology - and other layered solids such as the ternary carbides and nitrides, the so-called MAX phases that are the focus of our grant, were believed to behave differently when loaded. We have convincingly shown (Phys. Rev. Letters) that these extremely different solids all deform by kinking and thus constitute a new class of solids that we christened Kinking Nonlinear Elastic (KNE) solids. This insight allowed us to finally understand and explain the mechanical behavior of graphite and many geologically important materials. Given the diversity and ubiquity of KNE solids, it is fair to say that kinking plays an extremely but hitherto totally unrecognized role in our lives. For example, this insight will allow us to better understand earthquakes. In another facet of our work we have shown (Nature Materials) that it is possible to chlorinate Ti 3 SiC 2, and other MAX phases, to obtain nanoporous carbon wherein the pores size distributions are very narrow and tunable! The possibility of using this C for filtration, hydrogen storage and other applications is being explored. The room temperature spontaneous growth of low melting point metal whiskers, such as Sn, poses a serious reliability problem in the semiconducting industry; a problem that has become acute with the introduction of Pb-free technology. To date, this 50+ year old problem has resisted interpretation. We have shown that the driving force for this phenomenon is essentially a reaction of the sprouting metal with oxygen. These highlights represent a small fraction of our contributions. In the past 4 years we published more than 45 refereed publications in some of the most prestigious journals including Nature Materials and Phys. Rev. Letter.

Evolution of Indium Structures

In this movie we show what happens when a sample of Zr2InC is heated in a scanning electron microscope. The initial surface had a few whiskers. Upon heating, first the In whiskers start to grow rapidly up the melting point of In, at which time they become spheres. However, because the pressure inside small spheres is larger than inside large spheres, the small one disappear at the expense of the smaller ones. This phenomenon is called Ostwald ripening and as far as we are aware this is the first time this phenomenon has been observed in real time.

Reprinted with permission of The American Ceramic Society (