Growth Mechanism for the Controlled Synthesis of MgH2/Mg Crystals via a Vapor–Solid Process

Abstract

In this paper, we continue the work on the controlled growth of MgH2/Mg nano/microstructures, in which we highlight the growth mechanism from thermodynamics and kinetics, by a catalyst-free vapor–solid method of hydriding chemical vapor deposition (HCVD), which is a process of the evaporation–condensation of Mg under high-pressure H2. We easily achieved control of both the morphology and composition, including nano/microsized powders of MgH2 or a mixture of MgH2 and Mg, nanosized straight and curved MgH2 fibers, irregular bulk microstructures of Mg, hexagonal microplates of Mg, and microdendritic Mg, of the products by adjusting three experimental parameters: evaporation temperature, deposition temperature, and H2 pressure. The products were compositionally separated into three categories, Mg, MgH2, and a mixture of both, based on the growth temperature within the thermodynamic P–T diagram of the Mg–H system. A growth mechanism for the HCVD process was proposed from the perspectives of thermodynamics and kinetics, comprising a two-step reaction on the deposition substrate of the first Mg condensation, Mg(g) → Mg(s), with subsequent hydrogenation, Mg(s) + H2(g) ↔ MgH2(s). The composition of the products was controlled by controlling the reaction rates of these two steps, in which these reaction rates were determined by the atmospheric conditions of deposition temperature, H2 pressure, and evaporation temperature. These Mg/MgH2 nano/microstructures may find applications in hydrogen storage and batteries. By choosing suitable experimental parameters, such as evaporation temperature, deposition temperature, and H2 pressure, this simple vapor-solid method may be extended to the synthesis of nano/microstructures of other metal hydrides.