Controlled shape of magnesium hydride synthesized by chemical vapor deposition

Three types of products were synthesized from Mg vapor and H2 using chemical vapor deposition (CVD) at temperatures of 570–740 K and hydrogen pressures of 1–6 MPa. A CVD reactor with a moderate temperature gradient was developed, and the fluid behavior in the reactor was simulated to discuss the effects of temperature effects on product shape and phase accurately. The products were divided in terms of the temperature and hydrogen pressure at which they were synthesized. (1) At temperatures higher than or close to the formation equilibrium temperature of conventional MgH2 powder at the hydrogen pressure applied, hexagonal particles of Mg were obtained. (2) When the temperature and hydrogen pressure were near equilibrium, curved fibers of Mg, MgH2 or both were produced. (3) Straight fibers of MgH2 were obtained at temperatures lower than the equilibrium temperature at the hydrogen pressure applied. Straight fibers of MgH2 were directly synthesized from Mg vapor and H2, while curved fibers were produced through hydrogen absorption by Mg fibers. These results indicate that MgH2 can be synthesized in a uniform straight fibrous shape by controlling temperature and H2 pressure during CVD.

Research highlights▶ CVD products of MgH2 were divided in terms of temperature and hydrogen pressure during CVD. ▶ When the temperature and hydrogen pressure were near equilibrium, curved fibers of Mg and/or MgH2 were produced. ▶ Straight fibers of MgH2 were obtained at temperatures lower than the equilibrium at the hydrogen pressure applied. ▶ Straight fibers of MgH2 were directly synthesized from Mg vapor and H2. ▶ Curved fibers were produced through hydrogen absorption by Mg fibers.