Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
1627104 | Journal of Alloys and Compounds | 2007 | 9 Pages |
Abstract
The effects of nanocompositing with sodium borohydride (NaBH4) additive on the synthesis and dehydriding properties of nanocrystalline magnesium hydride (β-MgH2) were studied. The nanocomposite hydrides with catalytic NaBH4 were processed by controlled reactive mechanical milling (CRMM) of Mg + X wt.% NaBH4 mixtures (X = 2, 10 and 20) in the magneto-mill Uni-Ball-Mill 5 under reactive hydrogen atmosphere. The X-ray diffraction (XRD) patterns of the as-milled nanocomposite powders reveal the presence of nanocrystalline β-MgH2 and NaBH4 having nanograins within the 12-14 and â¼55 nm range, respectively. Scanning electron microscopy shows pronounced particle size (expressed as equivalent circle diameter, ECD) reduction from ECD â¼9.5 μm for the as-received Mg powder to ECD â¼1 μm for the β-MgH2 + NaBH4 nanocomposite powders after 100 h of reactive milling. Both the β-MgH2 + 2 and 10 wt.% NaBH4 nanocomposites show a single differential scanning calorimetry (DSC) endothermic decomposition peak with the maximum at temperature â¼438 and â¼338 °C, respectively. A single peak at â¼338 °C clearly indicates that both hydrides are simultaneously decomposed acting catalytically on one another. The DSC curve for the MgH2 + 20 wt.% NaBH4 nanocomposite shows an endothermic peak triplet with the first small peak at â¼305 °C and the second large peak at â¼330 °C most likely from the decomposition of β-MgH2, and the third smaller peak at â¼405 °C possibly from the decomposition of NaBH4. Apparently, the presence of 10 and 20 wt.% NaBH4 in the nanocomposite mixture with β-MgH2 lowers the decomposition temperature by as large as 100 °C. XRD patterns show that the principal decomposition product after DSC test up to 500 °C is the elemental nanocrystalline Mg (nanograin size 30-69 nm) and additionally MgB2 compound in the MgH2 + 10 and 20 wt.% NaBH4 nanocomposites. After DSC decomposition of MgH2 + 20 wt.% NaBH4 nanocomposite the elemental Mg product exhibits unit cell volume expansion which suggests a formation of substitutional solid solution of Na in Mg.
Keywords
Related Topics
Physical Sciences and Engineering
Materials Science
Metals and Alloys
Authors
T. Czujko, R.A. Varin, Z. Wronski, Z. Zaranski, T. Durejko,