Journal of The Less Common Metals最新文献

Cold deformation of YBa₂Cu₃O_{7 − x} (phase 123, T_c = 88.5–92 K) powders and strips causes partial decomposition of the 123 phase, a reduction in the degree of orthorhombicity of the structure up to almost complete degradation, and a decrease in T_c. When they are deformed, yttrium high temperature superconductors acquire basal (001) [110] texture with high pole density (13–15 arbitrary units), low scattering angle (±6°–7° from the normal direction), and a weak preference for a, b or a + b in the rolling direction; traces of (139) orientations may also be found. This texture is known to be favourable for increasing j_k. The combined effect of cold deformation and a carbon-containing binder leads, however, to a complete loss of superconductivity at 77 K or above. Depending on the regime of subsequent annealing, the following effects may be observed: degradation of the orthorhombic structure with a decrease in T_c; restoration of the orthorhombic structure and T_c of the 123 phase with complete or substantial loss of basal-type texture; change in texture type or retention of the basal texture (p_{α = 0}⁽⁰⁰⁵⁾ = 6–7 arbitrary units) with restoration of the orthorhombic lattice and T_c of the 123 phase. The appearance of a set of orientations from (139) to (001) in the deformation texture is evidence for the process of recrystallization grain growth. This suggests the appearance of plastic deformation of the ceramics (most probably of the shear type). With long annealing and thermocycling, the basal texture changes in accordance with the theory of compromise recrystallization grain growth up to the restoration of basal texture.

Solid state phase equilibria at 800 °C in the Co-Ga-As system have been investigated by X-ray powder diffraction methods. GaAs forms equilibria with CoGa₃, CoGa, CoAs₃, CoAs₂ and CoAs. The presence of a ternary phase with a homogeneity range around the composition Co₃Ga_0.5As_1.5 has been established.

Use of the dissolution of magnesium metal in liquid ammonia enabled novel magnesium-containing hydrogen storage materials to be prepared. The dispersion of magnesium atoms in ammonia matrices at 77 K using a metal vapour technique gave rise to a homogeneous solution of dissolved magnesium metal in liquid ammonia. Hydrogen storage materials prepared using the solvent power of ammonia are of two types. (i) Magnesium was crystallized out of the solution of dissolved magnesium in liquid ammonia; the crystallization was carried out in the presence or absence of catalytically active nickel powders (respectively Mg-Ni or Mg); (ii) Magnesium was highly dispersed on an active carbon (AC) support by impregnating the AC with the solution (referred to as $\text{Mg}\text{AC}$). Using AC on which catalytically active nickel had been previously supported, the preparation of samples was further extended to include binary systems ($\text{Mg-}\text{Ni}\text{AC}$). These samples were investigated in connection with the preparation methods. The magnesium samples were extremely active toward hydrogen absorption. For the Mg-Ni and $\text{Mg-}\text{Ni}\text{AC}$ samples an increase in nickel markedly accelerated the initial hydriding rates of the parent magnesium metal.

Neutron diffraction measurements have been performed on the ternary stannides TbMn₆Sn₆ and HoMn₆Sn₆ of HfFe₆Ge₆ type structure (space group, P6/mmm). This structure can be described as a filled derivative of the CoSn B35-type structure. Each of the rare earth and manganese atoms is successively distributed in alternate layers stacked along the c axis with the sequence Mn-R-Mn-Mn-R-Mn. Owing to the manganese atom coordination of the rare earth, this structure appears closely related to the CaCu⁶- and ThMn₁₂-type structures. This study confirms that both the rare earth and the manganese sublattices order simultaneously above room temperature. In the whole temperature range studied (2–300 K), TbMn₆Sn₆ and HoMn₆Sn₆ exhibit a collinear ferrimagnetic arrangement. At 300 K, the magnetic structure consists of a stacking of ferromagnetic (001) layers of rare earth and manganese with the coupling sequence Mn(+)R(−)Mn(+)Mn(+)-R (−)Mn(+) (μ_Mn ≈ 2.0(1)μ_B, μ_Tb = 4.9(1)μ_B, μ_Ho = 4.7(1)μ_B). For HoMn₆Sn₆, the magnetic moments lie in the (001) plane while in TbMn₆Sn₆, they deviate by φ = 15° from the c axis. Below 300 K, the two compounds exhibit a spin rotation process and at 2 K the magnetic moments are along [001] and at 50° from this axis for TbMn₆Sn₆ and HoMn₆Sn₆ respectively μ_Mn = 2.4(1)μ_B, μ_Tb = 8.6(1)μ_B, μ_Ho = 8.4(1)μ_B).

The results are discussed in terms of Ruderman-Kittel-Kasuy-Yoshida exchange interactions and compared with those obtained previously for the parent RFe₆Al₆ and RCo₆ compounds.

Redistribution of the components in U-Zr and U-Pu-Zr alloys occurs under a steep temperature gradient. The phenomenon was analysed as a thermal diffusion process accompanying phase separation. A unique combination of thermodynamic and transport properties was found to be responsible for the pronounced demixing of these alloys under thermal gradient.

X-ray powder diffraction studies of the Zr(Cr_{1 − x}Cu_x)₂ system revealed the existence of a single-phase region up to the composition Zr(Cr_0.5Cu_0.5)2. All the ternary alloys crystallize with a Friauf-Laves structure of the MgCu₂ type, differing from the binary compound ZrCr₂ which exhibits polymorphism and crystallizes with the hexagonal structure of the MgZn₂ type and the cubic structure of the MgCu₂ type. It was also found that single-phase alloys react readily with hydrogen to form hydrides having 2–3 hydrogen atoms per formula unit under a hydrogen pressure of 1.0 MPa at room temperature. Pressure—composition hydrogen desorption isotherms were determined over the temperature range 296–473 K. In general, the dissociation equilibrium pressure increases and hydrogen capacity decreases with increasing copper content. The hydrogen sorption properties of the systems described in this paper and of similar systems based on ZrCr₂ are briefly discussed.

For boron carbide the limit of the homogeneity range at the carbon-rich side has not yet been firmly established. During the last 20 years carbon-rich limiting compositions have been reported corresponding to B₄₃C (i.e. “B₁₃C₃”), B_4.0C (i.e. “B₁₂C₃”) and B_3.6C (i.e. “B₁₁C₃”). By means of quantitative electron probe microanalysis (EPMA) the composition of the boron carbide phase in the following samples has been determined:

• 1.

  (i) boron-carbon diffusion couples, prepared by hot isostatic pressing (HIP) of the pure elements at above 2000 °C;

• 2.

  (ii) fused, regular boron carbide, obtained by carbothermic reduction of boric oxide at above 2500 °C (ESK electrofurnace process);

• 3.

  (iii) dense boron carbide compacts made by pressureless sintering of carbothermic boron carbide powder or by HIP of magnesiothermic boron carbide powder at above 2000 °C.

In each sample the carbon-rich limiting composition has been determined as B_4.3C (18.8 at.% C) which corresponds to B₁₃C₂·C and is probably a solid solution of carbon in B₁₃C₂. The B₁₂C₃ and B₁₁C₃ compositions could not be found. For quantitative analysis a standard sample has been used, which was obtained by arc melting the pure elements with a B: C atomic ratio of 4.39. Also indirect B: C atomic ratio determinations via chemical analyses of crushed samples have produced evidence to support the limit 4.3 found by EPMA.

The low temperature (α) form of Co₂As has been investigated by single-crystal X-ray methods. It crystallizes in a hexagonal unit cell. The space group is $\text{P}\text{6}\text{2m}$ (number 189), with lattice parameters $\text{a = 11.9821(7)}\text{A}\text{̊}$ and $\text{c = 3.5822(5)}\text{A}\text{̊}$ and Z = 12. The crystal structure is closely related to, but distinctly different from, the high temperature Fe₂P-type structure.