ABSTRACT:This paper describes the direct synthesis of magnesium hydride (MgH 2 ) nanofibers by hydriding chemical vapor deposition (HCVD), in which the effect of hydrogen pressure on the production rate, the composition and the shape of products obtained were examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET). The XRD patterns showed that the main product in each case was MgH 2 ; in particular, the products formed at 2, 3 and 4 MPa were highly pure. In contrast, at a hydrogen pressure of 1 MPa, unhydrided Mg was deposited along with MgH 2 . The SEM images also revealed orientation of the as-deposited products; higher pressures of 3 and 4 MPa caused the formation of straight and curved nanofibers, and lower ones of 1 and 2 MPa, highly curved nanofibers and nanorods with a few straight nanofibers. With pressurizing hydrogen, not only the BET specific surface areas of the products but also the production rate increased. The results also appealed that HCVD could control the shape/size of MgH 2 nanofibers by changing the pressure via only a single operation.

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.

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