Journal of The Less Common Metals最新文献

The isothermal section (500 °C) of the phase diagram of the ternary system Cu-Sn-Y (40 at.% Y or less) was investigated by X-ray diffraction, differential thermal microanalysis, optical microanalysis and electron probe microanalysis techniques. The isothermal section consists of 12 single-phase regions, 25 two-phase regions and 14 three-phase regions. Three new ternary compounds were found in this isothermal section: the θ phase (Y₈Cu₇Sn₅), the β phase (Y₃Cu₄Sn₃) and the γ phase (Y₁₉Cu₆₆Sn₁₅). They are all non-variable compounds. The θ phase is an orthorhombic system with $\text{a = 9.702}\text{A}\text{̊}$, $\text{b = 7.285}\text{A}\text{̊}$ and $\text{c = 6.545}\text{A}\text{̊}$.

The maximum solid solubility of tin in α-Cu at 500 °C is about 9.0 at.% and that of yttrium in α-Cu at 500 °C (with 4.3 at.% Sn) is about 1.5 at.%.

YBa₂Cu₃O_∼7 (YBCO) powder of particle size distribution smaller than 80 μm has been coated with silver by a secondary ion beam deposition technique. Sintered Ag-YBCO composites prepared from this powder consist of homogeneously distributed orthorhombic phase, Ag₂O, silver and a small amount of other phases. The composites are superconducting at about 92 K, with a sharp superconducting transition.

The structure of LiCu₂O₂ was solved using two sets of X-ray diffraction twin-crystal data. Extended twinning creates virtual tetragonal symmetry. The compound crystallizes in Pnma (62) with unit-cell parameters $\text{a = 5.72}\text{A}\text{̊}\text{, b = 2.86}\text{A}\text{̊}$ and $\text{c = 12.4}\text{A}\text{̊}$ with a certain homogeneity range. The structure consists of LiCu^IIO₂ layers interleaved by layers of Cu^I connected to oxygen in an almost linear coordination. Lithium and copper(II) have five oxygen neighbours in pyramidal arrangements that run as parallel bands through edge connections. Electron diffraction was used for characterizing the twinning.

The dependence of the Curie temperature T_c and saturation magnetization σ_s on composition was studied in the alloys Nd₁₄Fe_{78 − x}T_xC₈ (T ≡ Co, Ni, Mn, Cr). The effects on T_c and σ_s of the substitution of these 3d elements for iron were found to be similar to the effects on T_c and σ_s of the corresponding borides. Mössbauer spectra of the tetragonal Nd₂Fe₁₄C compound were taken at various temperatures and fitted with a set of six subspectra owing to six non-equivalent iron sites. It was found that the average hyperfine fields at 295 K and 4.2 K are 28.04 T and 32.36 T, the corresponding average iron moments being about 1.89 μ_B and 2.19 μ_B, respectively.

Silicon, titanium and boron carbide layers were formed on carbon fibres by reactive chemical vapour deposition (RCVD) using mixtures of SiCl₄-H₂-Ar, TiCl₄-H₂ and BCl₃-H₂ gases. Two types of fibres were considered: fibres made using a polyacrylonitrile precursor (ex-pan) and fibres made using a tar precursor (ex-brai). The mechanical properties of the coated fibres were determined on monofilaments. The high temperatures involved in RCVD resulted in degradation of the fibres. In order to use these fibres as reinforced aluminium-matrix composites, the influence of thermal treatment in air or in the presence of metal was investigated. In both cases, the coating decreased the carbon reactivity. In air, at 450°C, the properties of the fibres increased after a few minutes of treatment, whereas at 600°C, the tensile strength decreased in Une with the microcrystallinity of the oxides obtained. In the presence of liquid aluminium, the silicon carbide protects the fibres as long as there is no chemical reaction between the two components. In particular, silicon oxide, present as an impurity in the coating, easily reacts with the metal to produce defects on the fibre surface.

Different samples of nominal composition Er_{2 − x}Y_xBaNiO₅ (0 ≤ x ≤ 2) were synthesized and characterized by X-ray powder diffraction. These oxides, which are isostructural, crystallize with orthorhombic symmetry, space group Immm. The linear increase observed in the lattice parameters with increasing values of x is due to the larger ionic radius of the Y³⁺ ion compared with the Er³⁺ ion.

The magnetic susceptibility in all cases obeys Curie-Weiss behavior between 300 K and 40 K. The magnetic moments obtained agree fairly well with those expected for the only contribution of Er³⁺ which confirms the strong antiferromagnetic interactions Ni-O(2)-Ni along the a-axis which we reported earlier for the one-dimensional Y₂BaNiO₅. The three-dimensional antiferromagnetic interactions operative in the undoped Er₂BaNiO₅ are slightly diminished in the substituted Er_{2 − x}Y_xBaNiO₅, as can be deduced from evolution of T_χmax for these samples.