L.S. Chumbley

Liquid metal extraction of Nd from NdFeB magnet scrap

This research involves using molten magnesium (Mg) to remove neodymium (Nd) from NdFeB magnet scrap by diffusion. Mg was melted over pieces of NdFeB scrap and held at temperatures in the range 675–705 °C for 2–8 h. The Mg was allowed to solidify, and the castings were then sectioned and characterized using scanning electron microscopy, x-ray diffraction, and chemical analysis. Nd was found to have diffused out of the solid scrap into the molten Mg. The thickness of the diffusion layer was measured, the diffusion of Nd through the NdFeB scrap into liquid Mg was described, and the diffusion coefficient of Nd in liquid Mg was estimated.

Characterization of the phase relations and solid solution range of the Bi2Sr2Ca1Cu2Oy superconductor

Abstract The phase relationships and solid-solution region of the Bi 2 Sr 2 Ca 1 Cu 2 O y (2212) superconductor were investigated for compositions around the ideal 2212 stoichiometry at 865°C in one bar of oxygen. Changes in the starting composition were accommodated by changes in the types and composition of the secondary phases present, the number of intergrowths within the 2212 phase, and the solid solution composition of the 2212 phase. Compositional analysis of the superconducting phase was carried out in the transmission electron microscope (TEM) in order to separate the effects of solid-solution changes and the presence of intergrowths upon measured compositions. The actual solid-solution region determined from TEM measurements showed most of the solid solution substitution to occur between Sr and Ca. The highest superconducting transition temperatures ( T c ) were found in annealed samples for which the measured Sr+Ca content of the 2212 phase was closest to the ideal stoichiometric ratio of 3 (42.86 at.%). No correlation was found between the transition temperature and lattice parameters of the superconducting phase.

Solid solution region of the Bi2Sr2CaCu2Oy superconductor

Abstract Measurements were made of the compositional range of the Bi 2 Sr 2 CaCu 2 O x superconductor in which the effects of intergrowths and grain-boundary phases were eliminated. At 800°C, the solid solution extends from Bi 2.06 Sr 2.09 Ca 0.83 Cu 2.01 O y to Bi 2.09 Sr 1.13 Ca 1.79 Cu 1.99 O y . Most of the cation interchange occurs between Sr and Ca while only a small variation in the Cu content was observed. In addition, substitution of Bi for Sr and/or Ca extends the range of compositions, although the reverse substitution of Sr or Ca for Bi does not seem to occur. The degree of interchange between Sr and Ca was found to decrease slowly with increasing temperatures in pure oxygen. In low oxygen partial pressures (0.1% 0 2 ), a significant portion of the Ca-rich solid solution region was found to be unstable.

Crystallization of Bi–Sr–Ca–Cu–O glasses in oxygen

A detailed study of the crystallization process for compositions near Bi 2 Sr 2 Ca 1 Cu 2 O y was undertaken using differential thermal analysis (DTA), transmission and scanning electron microscopy (TEM and SEM), and x-ray diffraction (XRD). Glasses prepared by a splat-quench technique were free of secondary phases in most cases. A two-step crystallization process in oxygen was observed in which partial crystallization of the glass occurs initially with the nucleation of “2201” and Cu 2 O, and is completed with the formation of SrO, CaO, and Bi 2 Sr 3− x Ca x O y . No specific thermal event could be associated with the formation of the “2212” phase. Rather, formation occurs via conversion of 2201 into 2212. This was a kinetically limited process at temperatures below 800 °C as other phases were found to evolve in addition to the 2212 phase during extended anneals. In contrast, a nearly full conversion to the 2212 phase occurred after only 1 min of annealing at 800 °C and above. However, changes in resistivity data, secondary phases, and the measured 2212 composition upon extended anneals at 865 °C showed that considerably longer heat treatments were necessary for the sample to reach its equilibrium state.

Directional isothermal growth of highly textured Bi2Sr2CaCu2Oy

For Bi2Sr2CaCu2Oy (2212), it is shown that an oxygen gradient, as opposed to a temperature gradient, can be used to produce large bulk forms of the 2212 superconductor with highly textured microstructures from an oxygen‐deficient melt held at a constant temperature. Material produced in this manner was found to have transition temperatures between 85 and 92 K, high critical current densities below 20 K, and modest critical current densities at 77 K.

Deformation induced defects in ReBa2Cu3O7−δ

Abstract In this paper, we report the microstructural characterization of bulk rare-earth ( Re = Nd or Y ) Re123 ceramic superconductors subjected to different levels of stress at various confining pressures, strain rates, and temperatures. This includes several classes of samples: (i) deformed under constant stress, (ii) deformed at a constant strain rate of 10 −4 s −1 , (iii) hand-ground in a mortar and pestle, and (iv) shock-compacted at 167 kbar. Deformation conditions for these samples spanned a wide range of processing conditions: 10 -7 to 10 −5 s −1 in strain rate, temperatures of 25°C to 1020°C, and pressures from 1 atm to 167 kbar. Dislocations were always observed to lie on the (001) planes having Burgers vectors of 〈100〉 and possibly 〈110〉. Small amounts of strain can be accommodated by deformation twinning on the {110} planes. High temperature anneal of the Y123 results in the formation of extrinsic stacking faults with R = 1 6 [031] , with an intercalation of an extra CuO layer offsetting the Ba atoms by half a unit cell in the b -axis direction. These stacking faults are not observed in the Nd123. Further work is in progress to better understand the interactions of the line and planner defects on the superconducting properties of these materials.

Analysis of interdiffusion of Dy, Nd, and Pr in Mg

The diffusion characteristics of Mg–rare-earth diffusion couples were studied. Cylinders of pure Mg and rare earth (Dy, Nd, and Pr) were abutted and annealed at 500 °C for 100 h or 300 h. Point-by-point composition profiles were collected starting in pure Mg, across the diffusion zone, and ending in the pure rare earth, using energy dispersive x-ray spectroscopy with a scanning electron microscope. The intermetallic phases that resulted due to diffusion were identified and compared to existing phase diagrams, for which the data is limited. For each diffusion couple, a plot of concentration versus distance perpendicular to the original plane of contact was obtained and analyzed using the Boltzman–Matano method. The interdiffusion coefficients for each set of phases were then calculated. The results show that diffusion through the intermetallic phases is much slower than is expected in a solid solution.

Crystallization and stability of the Bi 2 Sr 2 CaCu 2 O x superconductor

A basic study of the stability of the Bi2Sr2CaCu2Ox phase has been carried out in order to identify the composition and processing conditions for optimum superconducting properties. Analytical electron microscopy has been used to follow the crystallization of this phase during annealing in the solid state as well as from the melt. The crystal chemistry and phase purity is found to depend strongly on the processing conditions. Significant differences in crystallization behavior may be related to kinetic limitations of oxygen transport. The Bi2Sr2CaCu2Ox phase has also been subjected to thermomechanical processing in order to improve our understanding of the deformation processes involved during fabrication of wires and tapes. It has been found that mechanical deformation can have significant effects on the microstructure of the material which in turn result in changes in superconducting properties.

The upper critical field of HoBa2Cu3O7−δ for H ∥ c

Abstract The upper critical field H c2 of a grain-aligned HoBa 2 Cu 3 O 7−δ specimen has been measured as a function of temperature near the critical temperature, T c , for applied field H [Verbar] c. H c2 exhibited a(1−T/T c ) 1/2 temperature dependence, which is expected for a thin-film superconductor. This lends support to the idea that the superconducting pair potential is changed near twinning planes in HoBa 2 Cu 3 O 7−δ .