Hydriding Chemical Vapor Deposition of Metal Hydride Nano-Fibers

Abstract

Here we show a novel chemical vapor synthesis technique, which uses high-pressure hydrogen and produces needle-shaped single crystalline made of metal hydride of MgH 2 . The principle of this method is based on the gas phase reaction of vaporized metal with high pressure hydrogen sublimating into solid metal hydride (Mg(g) þ H 2 (g) ! MgH 2 (s)). This can directly produce pure single-phased metal hydride of MgH 2 , while the conventional solid-gas reaction (Mg(s) þ H 2 (g) ! MgH 2 (s)) can hardly produce high purity hydride. The X-ray spectrum of as-synthesized product was MgH 2 with rutile structure. The scanning electron micrographs showed the interesting figure of the product; needle-shaped nano fibers with diameter less than 500 nm and length larger than 100 mm. Transmission electron micrograph and related electron diffraction pattern provided that the needle-shaped product was single crystalline of MgH 2 growing into [1 0 1] direction, into which Mg-layers and H-layers are alternately stacked up. The results appealed a revolutionary productive route for metal hydride, which offers many benefits for simplifying the productive procedure, minimizing processing time, saving energy, and upgrading the product.

Direct synthesis of MgH2 nanofibers at different hydrogen pressures

“…Transmission electron microscopy (TEM) measurements reveal that the abovementioned nanofibers are single crystals of MgH 2 in our previous study [19]. The results of this study show that the purity and morphology of nanofiber-like MgH 2 is quite dependent on hydrogen pressure.…”

“…The products deposited on the cooled substrate were collected using a sheet of paper in a manner described elsewhere [19]. The observed thickness, δ, of the sheet layers of the products, which were formed at different hydrogen pressures, deposited on the flange (see Fig.…”

“…MgH 2 can also be used in portable fuel cells, wherein it undergoes hydrolysis with pure water to produce 15.2 mass% of hydrogen [16,17]. Another example of the mobile application for MgH 2 is that it is used as a negative electrode in Li- [19,20].…”

Decomposition of Magnesium Hydride Fiber Observed Using TEM and <I>In-Situ</I> AFM

“…The MgH 2 synthesized by CVD consists of irregularly curving fiber and straight fiber of a single crystal with a diameter of several hundreds of nanometers and a length of several micrometers. [16][17][18] Such single crystal fiber of MgH 2 has also been obtained incidentally by a vaporliquid-solid (VLS) or vapor-solid (VS) process during hydrogen absorption and desorption cycles of Mg 19,20) or Mg-based alloys, 21) but the hydrogen storage properties of those fibers have not been measured because of the difficulty of separating the fiber from the base materials. Since the CVD method produces MgH 2 fiber separately from the Mg, it makes it possible to investigate the hydrogen storage properties of the MgH 2 fiber.…”

“…15) In order to eliminate these problems, we developed a direct reaction process based on chemical vapor deposition (CVD). 16) Using the CVD method, MgH 2 is successfully produced from Mg vapor and H 2 16) because H 2 can make direct contact with Mg without passing through any of the solid phases. The MgH 2 synthesized by CVD consists of irregularly curving fiber and straight fiber of a single crystal with a diameter of several hundreds of nanometers and a length of several micrometers.…”