J.S. Abell

Nd-Fe-B Permanent Magnets: Hydrogen Absorption/Desorption Studies (HADS) on Nd16Fe76B8 and Nd2Fe14B

The hydrogen absorption/desorption (HAD) behaviours of alloys based on Nd15Fe77B8 (Neomax composition) and the stoichiometric phase Nd2Fe14B have been examined in an attempt to relate these effects to the microstructures of these alloys. The HAD processes were studied by microbalance, Differential Thermal Analysis (DTA) and mass spectroscopy and these showed that the absorption process in the Nd16Fe76B8 alloy is multi-stage and occurs readily at room temperature. The Nd2Fe14B alloy on the other hand could not be activated at room temperature and hydrogen absorption could only be achieved at elevated temperatures. This confirmed the crucial role of the grain boundary phase(s) in the activation process of the Nd16Fe76B8 alloy. The mass spectroscopy/vacuum degassing studies showed a two stage desorption process whereby hydrogen was first desorbed from the matrix (Nd2Fe14B) phase and then from the grain boundaries. The mass spectroscopy peaks indicated that almost equal amounts of hydrogen were desorbed from the matrix and the grain boundary phases.

The growth of YBa2Cu3O7−δ single crystals with the aid of NaCl-KCl flux

Abstract Different quantities of mixed alkali halide flux have been used to grow single crystals of YBa 2 Cu 3 O 7−δ . The superconducting properties of the crystals have been determined in the as-grown state and after post growth oxygen annealing, using AC susceptibility. The crystals were found to be relatively large in size, 8 × 7 × 2 mm, and were confirmed as being single crystals by the use of Laue X-ray diffraction. The size, the quality and the superconducting transition temperature of the crystals were found to depend not only on the proportion of flux but also on the precise heat treatment cycle, involving the maximum temperature and cooling rate. X-ray micro-analyses have confirmed the presence and identity of a number of inclusions.

Preparation of rare earth materials, crystals and specimens

Abstract The reactive nature of rare earth metals is frequently noted but is usually only a problem during the preparative stages. In terms of processing of rare earth metals the dominant feature is their reactivity: vapour pressure and phase transformations are two other parameters that have a major influence. We present our state-of-art position in the preparation of specimens of rare earth materials. Production to a given materials requirement can involve various processes including refining, alloying and crystal growth, where the basic requirement is the use of a non-contaminating environment, e.g. ultra high vacuum. We have extended our refining studies to include vacuum re-melting, sublimation, zone-refining and solid state electrotransport. Considerable technical developments have also been achieved and our latest equipment includes, for instance, the capability of RF vertical float zoning in UHV and pressurised melt-growth. Following the processing stages we are concerned with actual specimen fabrication and report recent developments to improve surface preparation, particularly to deal with brittle materials.

A study of the LiYF4-LiErF4 psuedo-binary system

The constitution and structure of compounds represented by the general formula LiY1−x Erx F4 (0 < x< 1) have been studied by X-ray diffraction, magnetic susceptibility and differential thermal analysis (DTA). X-ray diffraction shows that the bct Scheelite structure is retained across the system with only a very slight change in the lattice parameters. Large changes in room temperature magnetic susceptibilities as a function of x have been observed and these have been used to provide a very sensitive composition check on the compounds. DTA profile analysis shows that, for samples prepared in a carefully purified environment, the solidus-liquidus separation is so small that an approximation to congruent melting can be made for all the compounds in the system. A comparison with material prepared under unpurified conditions provides information on the effects of contamination upon the melting behaviour.

The preparation, microstructures and magnetostrictive properties of Samfenol-D

Abstract A high-pressure arc furnace has been used to prepare button- and rod-shaped samples of Sm 0.86 Dy 0.14 Fe y alloys. Sm losses as a result of volatilisation during melting were found to be lower than those encountered when using conventional arc melting techniques, while higher arc currents could be employed leading to more effective melting. The effects of annealing on the microstructures and the magnetomechanical coupling factor, k 33 , have also been investigated. X-Ray diffraction and scanning electron microscopy analysis indicated that annealing samples at 850xa0°C under vacuum for periods up to 10 h gives a pseudo-single phase microstructure. A double partial zoning technique, followed by annealing, was devised to prepare rod samples with high magnetomechanical coupling factors.

Assessment of the crystal lattice perfection of rare earth and intermetallic single crystals by x-ray topography

Abstract Single crystals of a number of rare earths and intermetallics used for bulk magnetic measurements have been examined by X-ray diffraction topography using the Bragg geometry. Experiments were performed using both a conventional X-ray source and synchrotron radiation. Tb grown by solid state electrotransport (SSE) from high purity start material had the highest perfection as well as the highest residual resistance ratio. SSE refined Dy was of somewhat inferior perfection while Ho crystals cut from an ingot not SSE processed were of very much poorer quality. The reduction in lattice strain as cutting damage was removed was monitored. A significant decrease in perfection of the Tb was noted over several months following thermal cycling through the Curie temperature. Increase in surface strains due to oxidation was most marked in Gd and the Gd-Tb alloys. In contrast no polishing problems occurred with RAl 2 crystals grown by the Czochralski method. Clean images, showing little long range strain, were obtained from TbAl 2 , NdAl 2 and GdAl 2 .

A characterisation of the solidification behaviour of a LiY0.416Er0.5Tm0.067Ho0.017F4 Stockbarger single crystal

Abstract It is shown that the broadening of differential thermal analysis peaks and the nature of certain void defects, observed in Stockbarger-grown crystals of the complex lithium-fluoride/rare-earth-fluoride mixtures, are consistent with compositional changes due to segregation of lithium fluoride. The observed melting reactions suggest that, whilst this material must be a solid solution, if there is a liquidus-solidus separation then it is sufficiently small for an approximation to congruently melting behaviour to be made. No evidence for peritectic behavior was found.

The primary crystallization field and growth of Bi-2212 crystals in platinum and gold crucibles

Abstract We report that the crucible material used is crucial for the growth of Bi 2.2 Sr 2 CaCu 2 O 8.3+ x (2212) single crystals. With a Pt-crucible, the platinate phase of composition (Sr,Ca) 2 Pt 0.6 Cu 0.4 O 3.6 forms due to corrosion. The Au-crucible corrosion rate is an order of magnitude lower than that of a Pt-crucible. The use of an Au-crucible made it possible to reproducibly grow melt-free 2212 crystals by the TSSG method. On investigation, we found that 2212 primary crystallization field (PCF) intersects the “2212-0 14 x 24 - 119 x 5-CuO” tetrahedron. The temperature range of the “2212-melt” equilibrium is determined to be 859–883°C. According to the obtained X-ray topography data, the domain misorientation in the crystals occurs around the a -axis (0.5°) and is, practically, absent around the b - and c -axes (less than 0.1°). It is suggested that such misorientation anisotropy is a consequence of the incommensurate modulation in this compound. We report the general dependence of T c on annealing conditions. The highest T c =91xa0K and lowest Δ T c =1xa0K were observed on crystals annealed at 400°C in 1×10 −4 xa0atm oxygen.

Crystal growth of iron-based compounds by the cold-crucible Czochralski technique

Abstract Bulk single crystals of NbFe2, HoFe2 and ( Tb Dy ) Fe 2 have been grown from a melt with a cold-crucible Czochralski technique. The as-grown crystals were characterised by X-ray diffraction, optical and scanning electron microscopy. The microstructural defects of the pseudo-binary ( Tb Dy ) Fe 2 crystals have been studied by transmission electron microscopy. The as-grown NbFe2 crystals with C14 hexagonal structure contain many thermal cracks, but no secondary phases. Both X-ray lattice parameter measurement and EDX/SEM analysis show consistent stoichiometric composition. However, both HoFe2 and ( Tb Dy ) Fe 2 have C15 cubic Laves phase structure and form by peritectic reactions with a narrow separation between the liquidus and solidus lines. By adopting a slow growth rate, a planar growth front has been maintained and bulk single crystals have been achieved. The as-grown crystals of ( Tb Dy ) Fe 2 contain many needle-shaped Widmanstatten precipitates formed by a solid state reaction. Microstructural study by transmission electron microscopy revealed the presence of many {111} plane stacking faults and a few twin boundaries. A large number of interfacial dislocation networks have been observed on the Widmanstatten precipitates. The presence of thermal cracks, microstructural defects and Widmanstatten precipitates in the Laves phase compounds will strongly affect the measurement of the magnetic properties of the crystals.

Removal of surface strain from rare earth intermetallic compounds by ion-beam planing

The use of argon ions to remove the surface damage introduced into brittle rare earth intermetallic compounds by lapping and polishing is described. The ion-beam planing technique allows strain-free surfaces to be prepared over large areas (∼ 4cm2) without degradation of surface quality. The effectiveness of the technique applied to both single crystal and polycrystalline material has been demonstrated by monitoring the quality of electron channelling patterns generated from the surfaces. Microstructural detail not evident from other characterization methods has been revealed.