Molten salt synthesis and formation mechanisms of ternary V-based MAX phases by V-Al alloy strategy

V2AlC and V4AlC3 are obtained in much milder conditions by a new facile approach using low-cost V-Al alloys and carbon black with the assistance of molten salts. Density functional theory (DFT) calculations show that our strategy is feasible and thermodynamically superior to the traditional method, with the following X-ray diffraction and Scanning Electron Microscopy fully revealing the phase and morphology evolutions of both reaction processes. Unreported formation mechanisms are revealed for the first time. Al8V5 transforms directly into V2AlC with both lattices perfectly overlapped due to the existence of multiple parallel planes between V2AlC and Al8V5 with consistent interplanar spacings, which leads to the overlapped phenomenon of the diffraction spots as well. For V4AlC3, an embedded structure of V2AlC-VC is first formed through an aluminum removing behavior in V2AlC layers, causing the increase of the VC content, and V4AlC3 is transformed by the embedded V2AlC-VC structure through diffusion and interface migration with a possible climb behavior of Frank partial dislocation.

Automated summary

A new facile approach using low-cost V-Al alloys and carbon black with molten salts assistance was able to obtain V2AlC and V4AlC3 in milder conditions. The method is shown to be thermodynamically superior to the traditional method, with X-ray diffraction and Scanning Electron Microscopy revealing phase and morphology evolutions. Unreported formation mechanisms were also revealed for the first time.