Molten salt shielded synthesis and formation mechanism of Ti2AlC in NaCl–KCl medium

Abstract

Herein, a highly crystalline Ti2AlC was synthesized via the improved molten salt synthesis method called molten salt shielded synthesis. To achieve this goal, the mixture of Ti, Al, and graphite and KCl–NaCl eutectic composition salt was heated at 1000, 1050, and 1100 °C for 0.5, 1, and 1.5 h. The X-ray diffraction (XRD) patterns showed that the optimum condition for obtaining the more crystalline Ti2AlC was achieved at 1100 °C for 1.5 h. Such phase identification, and transmission electron microscopy (TEM) images, proved that applying a protective carbon layer on the surface of salt led to inhibiting the diffusion of oxygen into the surface of the green pellet. As a result, the crystallinity of Ti2AlC improved, while the content of undesirable compounds such as Al2O3 and TixOy decreased drastically. In order to shed light on the Ti2AlC synthesis mechanism, differential thermal analysis (DTA) was employed. The DTA curve revealed that the Ti2AlC formation completed in three levels. First, the partial dissolution of Ti in KCl–NaCl salt followed by a reaction with liquid Al resulted in the TiAl formation. Next, Ti(II) reacted in-situ on the surface of graphite that resulted in the non-stoichiometric TiC (TiC1-x) formation, and, at last in a reaction between TiAl and TiC1-x, Ti2AlC phase formation took place at 940 °C.