Papadakis Integrated Sciences Building, Room 108

“Synthesis and behavior under ion irradiation of MAX phase thin films”

Matthieu Bugnet Ph.D.

Post Doc, Canadian Centre for Electron Microscopy

Abstract

M_n+1AX_n phases (M is an early transition metal, A an A-group element, X either carbon or nitrogen, and n = 1-3) are potential candidates for applications as structural materials in future nuclear reactors thanks to their exceptional mechanical and thermal properties, their strong damage tolerance at high temperature and refractoriness. Recently, promising results such as a strong tolerance with respect to ion irradiation induced damage have been obtained on Ti₃SiC₂, Ti₃(Si,Al)C₂ and Ti₃AlC₂. However, even though a structural damage is evidenced, its nature is not clearly determined and the influence of the chemical composition has not been widely investigated.

The work presented here aims to bring a better understanding of the behavior of MAX phases under extreme environment such as ion irradiation. First we focus on the growth of model materials, selected titanium-based and chromium-based MAX phases epitaxial thin films, using magnetron sputtering. In a second step, the behavior of these films is studied under low energy (150-340 keV) ion irradiation, using ex situ (Ar²⁺) and in situ (Xe²⁺, in a TEM) ion beams. The microstructural modifications are investigated by electron energy loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) in addition to X-ray diffraction (XRD) and transmission electron microscopy (TEM). The C-K and Al-K near-edge fine structures in Ti₃AlC₂ evidence that Ti₆C octahedra layers are very resistant to irradiation damage and on the contrary, aluminium layers are strongly disordered. A similar behavior is suggested for Ti₂AlC, Ti₂AlN, and also partly for Cr₂AlC. This study indicates strong complementarities of local (EELS, XAS) and global (XRD) probes. Although titanium based materials are still crystalline after irradiation at high fluence, chromium based compounds rapidly amorphize. Of particular interest for nuclear applications, it is evidenced that the initial crystalline structure of most compounds is recovered after post-irradiation annealing at 700°C. We suggest that the behavior of these materials in radiation environment can be tuned by the chemical composition and the stacking sequence.

Biography

 Matthieu Bugnet is a postdoctoral fellow in the Department of Materials Science and Engineering and in the Canadian Center for Electron Microscopy at McMaster University. He received his B.Sc., M.Sc. and Ph.D. in physics and materials science at the University of Poitiers (France). He worked on the synthesis of MAX phase thin films, their behavior under ion irradiation, and the investigation of their electronic structure by electron energy loss spectroscopy (EELS) and ab initio simulations. He came to McMaster University in November 2011 and his research is currently focused on aberration corrected transmission electron microscopy, high resolution EELS and ab initio simulations of EEL spectra of oxide materials.

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