Dual regulation of crystal structure and purity of Ti2SC in the Ti/TiC/FeS2 system via enhancing reactant contact and controlling crystal plane growth rates

Abstract

Typically, the X-ray diffraction spectrum of a highly oriented structure (HOS) MAX phase displays a prominent (002) characteristic peak. Surprisingly, this principle does not hold true for Ti2SC. Instead, the synthesized Ti2SC consistently exhibits a polyhedral structure (PS). Furthermore, even with hot-pressing sintering, the purity of Ti2SC remains relatively low. The primary factors contributing to anomalous crystal structures and low purity are insufficient crystal growth and limited reactant contact. In this study, we successfully achieved dual regulation of the crystal structure and purity of Ti2SC within the Ti/TiC/FeS2 system, which was accomplished by enhancing reactant contact and controlling the growth rates of crystal planes. Notably, the intermediate products FeS and Fe, existing in a liquid phase at varying temperatures, played a crucial role in both the formation of Ti2SC and the growth of Ti2SC crystals. As a result, PS-Ti2SC and HOS–Ti2SC with purity greater than 99 wt% were successfully prepared. The formation of PS-Ti2SC can be attributed to the inadequate growth of each crystal plane at relatively lower temperatures (1500 °C). While the synthesis of HOS–Ti2SC necessitates the utilization of both a liquid phase and high temperature, as the epitaxial growth mechanism followed by vertical growth has been demonstrated to facilitate the generation of highly oriented crystals. The liquid phase facilitates the swift growth of Ti2SC crystals along the (001) crystal plane, while the (002) crystal plane necessitates a higher temperature (1760 °C) for relatively rapid growth. This work will open a new pathway for the development of the MAX phase from the perspective of crystal structure regulation and high purification preparation.