Sheng Pang,Zhenqian Zhang,Liangwei Cong,Youpeng Xu,Guoyu Qian,Dong Wang,Laishi Li,Yusheng Wu,Zhi Wang
The conventional method for synthesizing Ti 2 SC depends on the reaction between TiS and TiC, which requires the utilization of costly Ti, TiC, or TiS 2 powders. Utilizing TiO 2 as the raw material for the synthesis of Ti 2 SC enables the cost-effective preparation of this compound. However, the optimization of the TiO 2 /C/X n S m (sulfides) system is of utmost importance when aiming to synthesize Ti 2 SC through carbothermal-sulfurization of TiO 2 , as the formation temperature of TiS and TiC greatly depends on the X n S m . In this study, the identification of the thermodynamically dominant region for the formation of TiS and TiC in the TiO 2 /C/X n S m systems was conducted by utilizing various sulfides (63 types). Considering factors such as the challenges associated with removing by-products, accessibility, and toxicity, FeS 2 was ultimately chosen as a highly promising candidate. Thanks to the low melting point of FeS 2 , Ti 3 O 5 , the preliminary carbothermal reduction product of TiO 2 , has achieved rapid-deep deoxidation and phase reconstruction processes in the molten phase. Meanwhile, unlike the conventional solid-solid sintering reaction , which often results in residual reactants that are challenging to eliminate. This novel one-step phase transformation process that occurs in the molten phase exhibits minimal impurity phases, apart from the by-product Fe. Ti 2 SC with a purity greater than 99 wt% was obtained when the TiO 2 /C/FeS 2 was pickled to remove Fe after sintering at 1600 °C for 5 min. This work will provide the theoretical basis and a new path for the low-cost preparation of sulfur-containing MAX phases.
Sheng Pang,Liangwei Cong,Youpeng Xu,Yiwei Sun,Dong Wang,Guoyu Qian,Zhi Wang
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 Ti 2 SC. Instead, the synthesized Ti 2 SC consistently exhibits a polyhedral structure (PS). Furthermore, even with hot-pressing sintering, the purity of Ti 2 SC 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 Ti 2 SC within the Ti/TiC/FeS 2 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 Ti 2 SC and the growth of Ti 2 SC crystals. As a result, PS-Ti 2 SC and HOS–Ti 2 SC with purity greater than 99 wt% were successfully prepared. The formation of PS-Ti 2 SC can be attributed to the inadequate growth of each crystal plane at relatively lower temperatures (1500 °C). While the synthesis of HOS–Ti 2 SC 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 Ti 2 SC 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.
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