P. de Rango

Catalytic effect of additives on the hydrogen absorption properties of nano-crystalline MgH2(X) composites

Hydrogen sorption properties of MgH2–X nanocomposite powders (X=V, Nb, Ti, TiCN) are conditioned by the milling step. The respective roles of the additives and the milling time have been dissociated. The study reveals details not only on the hydrogen sorption process but also on the particle size reduction, distribution and agglomeration, that depend on the nature of the additives. In addition to the effect related to the extra-elements, the milling time has to be optimised to lead to the best particles size reduction without any agglomeration of the grains.

Phase stability and neutron diffraction studies of Laves phases Zr(Cr1−xMx)2 with M = (Cu0.5Ni0.5) and 0 < x < 0.2 and their hydrides

Abstract Laves phases of composition Zr(Cr 1− x M x ) 2 with M = (Cu 0.5 Ni 0.5 ) and 0 ⩽ x ⩽ 0.2 have been synthesised with the hexagonal C14- and/or cubic C15-type structures. By X-ray and neutron diffraction we have determined the structures of the alloys, and in particular the sites for the 3d metal substitution. The position of the hydrogen atoms in the structures and the occupation of the different sites of the hydrides have been determined. Purity and quality of polycrystalline alloys have been checked by using scanning electron microscopy and energy dispersive X-ray analysis techniques. The effects of the M/Cr substitution have been studied for phase stability, maximum hydrogen content and hydrogen site stability and analyzed in view of applications as electrodes in H M batteries.

Structural studies of Laves phases Zr(Cr1−xNix)2 with 0≤x≤0.4 and their hydrides

Abstract Hexagonal C14-type Laves phases of the series Zr(Cr 1− x Ni x ) 2 with 0≤ x ≤0.4 have been synthesised. Purity and quality of polycrystalline alloys have been checked using SEM and EDX techniques. By X-ray and neutron diffraction we have analyzed the structure of the alloys, and determined the sites for the Ni substitution. The positions of hydrogen atoms in the structure and the occupation of the different interstitial sites of the hydrides have been determined. The effects of the Ni/Cr substitution have been studied with regard to the phase stability, the maximum hydrogen content and the hydrogen site stability.

Magnetically melt textured YBa2Cu3O7−δ: Disagreement between Jc determined by magnetic and direct transport measurements

Abstract Textured YBa 2 Cu 3 O 7−δ (Y-123) ceramic material has been prepared from a mixture of Y 2 BaCuO 3 , BaCuO 2 and CuO, by solidification in a magnetic field of 7 T. The application of the magnetic field during solidification allows one to control the orientation of the crystals within the material. An annealing temperature above 1040°C is necessary to obtain a well textured ceramic. The composition of the starting material has an influence on the form of the sample: an excess of 25% by weight of Y 2 BaCuO 3 allows the initial form of the sample to be preserved. Electrical transport and magnetization measurements are reported. Transport critical current densities of 7200 A/cm 2 in a magnetic field of 2 T and 4300 A/cm 2 in 6 T in the case H ∥ ab planes have been observed at 77 K. A transport critical current density of 1600 A/cm 2 in 6 T in the case H ∥ c -axis has been measured, in spite of the fact that critical currents deduced from magnetic measurements using the Bean model are negligible for fields in excess of 5 T.

Transport properties of magnetically textured YBa2Cu3O7−δ

Abstract A new process has been developed in which bulk textured YBa 2 Cu 3 O 7 − δ ceramics have been prepared by solifification in a magnetic field. The electrical transport properties of this field textured material are discussed. The temperature dependence of the electrical resistivity in fields of up to 4.5 T are presented. Transport critical current densities at 77 K in magnetic fields of up to 7 T are reported. For currents flowing along the a – b planes and a magnetic field of 6.9 T applied parallel to both the a – b planes and to the direction of current flow, transport critical current densities of 15 200 A/cm 2 were observed. Variations in the critical current density with the magnitude and direction of the applied field are also discussed.

Hydrogen in hard magnetic materials

Abstract The rare-earth/transition metal alloys exhibiting high permanent magnet properties are able to reversibly absorb significant amounts of hydrogen. This affects markedly the fundamental characteristics of such intermetallics, and can be used as a probe to understand better the basis of the magnetic interactions. In addition, structural, mechanical and chemical aspects of the hydrogenation process are used to improve the synthesis of appropriate powders for the magnet technologies.

A study of the use of a magnetic field to control the microstructure of the high-temperature superconducting oxide YBa2Cu3O7-δ

Abstract This paper demonstrates that, by taking advantage of the anisotropy in the paramagnetic susceptibility of the high-temperature superconducting oxide YBa2Cu3O7 -δ it is possible to prepare bulk textured samples of this material by solidification in a magnetic field. The effects of increasing the annealing temperature employed, varying the strength of the annealing field and altering the temperature range in which an annealing field is applied on the properties of the material obtained are discussed.

Bulk textured rare earth-Ba2Cu3O7- delta prepared by solidification in a magnetic field

Experiments are reported in which single-crystal grains of rare earth-Ba2Cu3O7- delta (R-123) have been aligned in liquid silver by a magnetic field. They demonstrate that there exists a significant anisotropy in the paramagnetic susceptibility of many R-123 superconducting oxides close to their peritectic decomposition temperature. A magnetic field has also been used to induce and control the direction of alignment within bulk Y-123 and Er-123 material. The authors report magnetization versus field measurements, and observations made using X-ray spectroscopy and scanning electron microscopy which have been used to characterize the nature and degree of alignment obtained. Magnetic field texturing offers an alternative technique for producing bulk textured high-Tc superconductors, in which the direction of preferred orientation can be easily controlled.

Pressure and magnetic field effects on the crystallographic texture and electrical conductivity of the compound

The compound Bi4.V0:85Co0:15/2O11 figures among the solid electrolyte materials exhibiting a high oxygen ion conduction. It belongs to the well known BIMEVOX family whose structure originates from the aurivillius phase Bi2MoO6. The layered structure of these compounds results in a larger bidimensional conductivity in the (a; b) planes than along the c axis. This physical characteristic is clearly observed in a single crystal, whereas it is completely inhibited in misoriented powders. This paper deals with the means to retrieve this anisotropic conduction from free powders. Powder orientations were obtained using a high magnetic field and a uniaxial pressure. The correlation between the crystallographic texture and the electrical conductivity of this compound is reported here for the first time.

Anisotropic and coercive HDDR NdFeB powders prepared under magnetic field

Abstract High coercivity NdFeB powder with H cj up to 14 kOe was achieved after optimisation of the two latest phases of the Hydrogen—Disproportionation—Desorption—Recrystallisation (HDDR) process. The progression of the solid gas reaction (NdFeB/H 2 ) as analysed by a thermomagnetic balance, has given the possibility to monitor the disproportionation reaction. The first step was to define favourable conditions to develop a controlled but uncompleted disproportionation reaction. The second step was the application of an high magnetic field of μ 0 H = 7 T during the DR process. Anisotropic NdFeB powder was crystallised.