Journal of The Less Common Metals最新文献

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.

The conditions of milling are of crucial importance in determining the kind of solid state reaction that can take place, among various possible paths, in a given system submitted to the mechanical alloying (MA) process. A model correlating the energy transfer during milling with the experimental parameters (rotation speed, mass and diameter of the balls) has been verified with the Pd-Si system. The composition Pd₈₀Si₂₀ has been milled, starting from pure elemental powders, in different milling conditions and the MA process followed by X-ray and differential scanning calorimetry measurements. It has been found that for high levels of energy transfer the system evolves towards the formation of the intermetallic Pd₃Si. At lower levels of energy transfer, formation of a stable intermetallic phase is hindered and the reaction product is a fully amorphous phase. The amorphous structure of the MA sample has been compared with an amorphous melt-spun ribbon of the same composition.

K₂[GeAs₂], Rb₂[SnAs₂], Cs₂[SiP₂], Cs₂[SiAs₂] and Cs₂[SnAs₂] were prepared from molten mixtures of the elements under an argon atmosphere in Al₂O₃ crucibles. The compounds form needle-shaped crystals, which are sensitive to moisture. The structures are of the K₂[SiP₂] type and contain the polyanionic chains ${}_{\mathrm{\infty }}{}^1\text{[SiP}{}_{\mathrm{<mtext>4</mtext><mtext>2</mtext>}}{}^{\mathrm{2-}}\text{],}{}_{\mathrm{\infty }}{}^1\text{[SiAs}{}_{\mathrm{<mtext>4</mtext><mtext>2</mtext>}}{}^{\mathrm{2-}}\text{],}{}_{\mathrm{\infty }}{}^1\text{[GeAs}{}_{\mathrm{<mtext>4</mtext><mtext>2</mtext>}}{}^{\mathrm{2-}}\text{] and}{}_{\mathrm{\infty }}{}^1\text{[SnAs}{}_{\mathrm{<mtext>4</mtext><mtext>2</mtext>}}{}^{\mathrm{2-}}\text{]}$ composed of edge-sharing tetrahedra, isoelectronic and isostructural with SiS₂. The structures were refined from X-ray single-crystal data: space group Ibam (No. 72), Z = 4. K₂[GeAs₂]: a = 1329.2(4) pm, b = 702.8(3) pm, c = 654.8(3) pm Rb₂[SnAs₂]: a = 1397.4(4) pm, b = 745.4(3) pm, c = 694.2(3) pm Cs₂[SiP₂]: a = 1412.7(4) pm, b = 746.2(3) pm, c = 619.6(3) pm Cs₂[SiAs₂]: a = 1442.5(4) pm, b = 757.1(3) pm, c = 642.0(3) pm Cs₂[SnAs₂]: a = 1465.7(4) pm, b = 773.3(3) pm, c = 695.3(3) pm

Phase relationships in the system Mo-Pd in the temperature range 880–1100°C were re-investigated using ultra-rapidly solidified samples. The finely divided microstructures produced using this technique were found to achieve equilibrium on annealing much more rapidly than conventionally prepared materials. Optical microscopy, X-ray diffraction methods and scanning and analytical electron microscopy were used for phase characterization. The solubility limits of the molybdenum-rich and palladium-rich terminal solid solutions were established. No intermediate phases were found at the temperatures investigated.