Synthesis and characterization of MAX phases single crystals – MAXICRYST

Mn+1AXn phase is the generic formula for a family of layered ternary carbides and nitrides where M is an early transition element, A is an element of group III-A or IV-A and X is either C or N. The structure of these phases consists in layers of A element between Mn+1Xn layers. Mn+1AXn phases combine a unique set of properties: some of them are characteristic of the Mn+1Xn ceramic block (stiffness, resistance to oxidation…) while the others are typical of metallic materials and unique for ceramic ones (thermal and electrical conductivities, machinability, damage tolerance…). Such a combination of properties gives rise to many possible applications. The unusual macroscopic properties of MAX phases are closely related to the electronic and structural properties of the constituent atomic layers on the nanoscale. However, the investigation of their intrinsic properties and their anisotropy have so far been remained partially hindered by the lack of the availability of single crystals. The main goal of the present project is to remove the technological barrier limiting the development of large single crystals of MAX phases and to investigate their physical and mechanical properties.

The feasability of MAX phases synthesis using metallic melts as solvents has been clearly demonstrated by several authors. In the present project we aim to develop and adapt crystal growth techniques in order to promote and control the growth of large single crystals of MAX phases. To do so, the mechanisms responsible of growth have to be elucidated and controlled. The layered structure of MAX phases and particularly the weakly bonded A layers induces an original reactivity with a possibility of reversible exchange of A atoms. Therefore, the reverse case of the decomposition of MAX phases and particularly the exfoliation of the A atoms layer leading to two-dimensional graphene-like structures will also be carefully investigated.

The large size single crystals obtained during the first stage of the project will be characterized with the aim of acquiring intrinsic electronic and mechanical properties of MAX phases. With large size crystals, it will be possible to investigate the properties along different crystallographic orientation and to correlate the expected anisotropy of the properties to the anisotropy of the structure. It is worth noticing that, ab-initio calculations will be used in order to correlate the measured physical properties with the electronic structure of MAX phases. Of course, the physical properties of two-dimensional structures obtained by exfoliation of MAX phases will also be investigated by the same techniques as soon as they will be available.
We expect from this project a significant boost and progress in our academic knowledge of MAX phases and in their synthesis, and as a consequence to fill a substantial part of the gap still separating these materials from the application.