Reaction Mechanism And Phase Formation Of FeSe Superconductor During Sintering Process

The FeSe superconductor, one of the iron-based superconductors, is the best candidate to study the mechanism of iron-based superconductor due to its simple structure, easy preparation and non-toxic, providing a new way of studying the mechanism of superconductivity as well. At present, the researches on FeSe mainly focus on improving superconductivity. However, studies on phase formation and reaction mechanism are rarely reported. In this study, phase formation process, as well as growth mode of the initial stages for undoped and Te doped FeSe are investigated by means of thermal analysis, scanning electron microscopy and other analytical tools, and corresponding reaction mechanism is analyzed by modified FWO method, which is one of the multiple rate scanning methods. The main conclusions are as follows:The majority of Se becomes amorphous during the ball milling process of Fe-Se mixed powders. In the sintering process,β-FeSe superconducting phase lastly forms after the successively formation of FeSe₂, Fe₃Se₄, andα-FeSe. The growth mode ofβ-FeSe phase follows two-dimensional nucleation growth mechanism, thus exhibits layered grains.The most fitting function for initial reaction of Fe-Se mixed powders is G ( ) 2[ ln(1 )]1/2, indicating a random nucleation followed by a subsequent growth, and each particle having only one core. The activation energy and pre-exponential factor both decrease with the increasing fractionα.Substitution of Se for Te into lattice could be observed after the ball milling process of Fe-Se-Te mixed powders. In the sintering process of Fe-Se-Te mixed powders, besides FeSe₂ and Fe₃Se₄, FeTe1.45 forms as a new intermediate phase, while the superconductive FeSe0.5Te0.5 phase, which is the final product, shows layered growth mode of two-dimensional nucleation and growth. The proportion of tetragonal phase is increased resulted from Te doping.The reaction mechanism for initial reaction of Te doped FeSe converts to two-dimensional phase boundary reaction, and the function is G ( ) 2[1 (1 )1 /2]. The activation energy and pre-exponential factor both increase with the increasing fractionαafter Te doping, and the values are all higher than that of FeSe system.