Hirofumi Kakuta

Crystal structure of novel hydrides in a Mg-Ni-H system prepared under an ultra high pressure

The crystal structure of novel hydrides in the Mg–Ni–H system has been studied using a powder X-ray diffraction and transmission electron microscopy. A cubic-anvil-type apparatus was utilized to prepare samples. The new hydride with a chemical composition of around MgH2–60 at% Ni was synthesized at 1073 K for 2 h under a pressure as high as 5 GPa. From TGA analysis, the new hydride was found to be Mg2Ni3H3.4. Orthorhombic and monoclinic crystal systems with a primitive cell were proposed as possible symmetries of the new hydride. X-ray and electron diffraction patterns of the new hydride were indexed in an orthorhombic structure with a=0.8859(4), b=1.3740(5), c=0.4694(2) nm. Moreover, decomposition of the hydride into Mg2Ni was observed by the transmission electron microscopy.

Effect of synthesis pressure on hydride phases in Mg–M systems (M=Mn, Y)

Abstract High-pressure synthesis of the hydrides in Mg–M (M = Mn, Y) systems and the influence of applied pressures during synthesis on present phases and their crystal structures have been studied. In Mg–Mn system, it was found that the crystal structure of Mg3MnHy changed from hexagonal structure (a = 0:47107(4) nm and c = 1:0297(1) nm) to monoclinic structure (a = 0:8819(8) nm, b = 0:4658(4) nm, c = 0:4678(5) nm and b= 105:6(1)8) in a pressure range of 3–3.5 GPa. This crystal structural change was reversible with respect to pressure. The Mg3MnHy synthesized under 5 GPa was stable up to around 620 K. From thermogravimetric and fusion extraction analyses, the hydrogen content was determined as Mg3MnH5.0–5.6. In Mg–Y system, the high-pressure hydride (MgY2Hy) with yellowish color was synthesized at 1073 K for 2 h under 3 GPa or higher. This phase exhibited an FCC-type structure with a cell parameter of a = 0:516 nm. Its hydrogen content was determined to be about 3.7 mass%, corresponding to a chemical formula of MgY2H7.8. The hydride was partially dehydrogenated at around 600 K, and the amount of hydrogen partially desorbed was 1.4 mass%. The FCC-type structure was stable even after the partial dehydrogenation.

Oxygen Permeable Ce0.8Gd0.2 O 1.9 ­ CoFe2 O 4 Thin Films Prepared on Porous Ce0.8Gd0.2 O 1.9 Substrates

We propose the use of porous substrates and thin films of oxygen permeable ceramics as a method for separating oxygen from air. The porosity and pore size of various Ce 0 . 8 Gd 0 . 2 O 1 . 9 (CGO) porous substrates were modified in an attempt to optimize the oxygen permeable flux of CGO-CoFe 2 O 4 thin films. The oxygen permeable flux density of the CGO-CoFe 2 O 4 thin film coated on the porous CGO substrate was found to be 4.7 μmol cm - 2 s - 1 under a O 2 partial pressure difference (-log(P O I I /P O I )) of 4.5 at 1000°C. This value is about 20 times higher than that of the 1 mm thick bulk material.

Synthesis and Crystal Structure of New Hydrides in Mg-RE Systems under High-Pressure (RE = La, Ce, Pr)

The high-pressure synthesis of new hydrides of Mg-RE-H systems, where RE = La, Ce and Pr, were conducted by using a cubic-anvil-type apparatus, and their crystal structure, thermal stabilities and hydrogen contents were investigated. In MgH2-xmol%REH (REH = LaH3, CeH2.5 and PrH3), new hydrides with primitive tetragonal structure were synthesized around x = 25 - 33 under GPa-order high pressures. The lattice constants were a = 0.8193 nm, c = 0.5028 nm, a = 0.8118 nm, c = 0.4979 nm and a = 0.8058 nm, c= 0.4970 nm at x = 25 in Mg-La, Ce and Pr systems, respectively. The hydrogen contents of the novel compounds were 4.1 mass%, 3.7 mass% and 3.9 mass% in Mg-La, Ce and Pr systems, respectively, and the chemical formulas were found to correspond to Mg3LaH9, Mg3CeH8.1 and Mg3PrH9. The new hydrides decomposed into Mg and rare-earth hydride at about 600 K (Mg3LaH9: 614 K, Mg3CeH8.1: 609 K, Mg3PrH9: 630 K) with an endothermic reaction.