R. W. McCallum

Hole filling, charge transfer and superconductivity in Nd1+xBa2-xCu3O7+δ

Abstract Unlike YBa2Cu3O7−δ (Y123), the Nd-Ba-Cu-O system exhibits a solid solution, Nd1+xBa2−xCu3O7+δ (Nd123ss), for 0.04⩽x⩽0.6. However, the superconducting properties decrease nonlinearly for increasing x, and Tc can also be varied by lowering the annealing temperature without changing the low-temperature oxygen soak. The changes in microstructure and Tc with increasing x are analogous with Y123 with increasing δ, yet for fully oxygenated Nd123ss samples, electron energy loss spectroscopy (EELS) demonstrates that the total unoccupied density of states below the unfilled band is constant for all x. The neutron diffraction and the inert gas fusion analysis support an overall charge balance for all x. Tc is modeled in terms of a forced oxygen disorder resulting from Nd3+ atoms on the Ba sites relocating chain oxygens to anti-chain sites. The variability in Tc as a function of x and heat treatments can be explained by the number of four-fold coordinated Cus on the chains which can be varied by differing the amounts of paired and unpaired Nd3+ substituting for Ba2+, resulting in more or less charge transfer from the planes to the chains.

Metal-bonded Co-ferrite composites for magnetostrictive torque sensor applications

A new class of magnetomechanical sensor materials, Co-ferrite (CoO/spl middot/Fe/sub 2/O/sub 3/) and metal-bonded (Ag, Ni, Co) Co-ferrite composites, has been investigated. These materials exhibited magnetostriction in excess of 200 ppm and high d/sub 33/ (/spl part//spl lambda///spl part/H)/sub /spl sigma// coefficient, 1.3/spl times/10/sup -9/ A/sup -1/ m, at low applied field (<100 kA/m). Selected compositions were formed into test samples in the form of rings brazed to stainless steel through-shafts. Changes of surface axial magnetic field in response to applied torque as high as 64 AN/sup -1/ m/sup -2/ were observed for a demonstration sample of dimensions 25 mm OD, 12.5 mm ID, and 8 mm high. A hysteresis of /spl plusmn/0.5 N/spl middot/m was observed. These materials appear to be promising candidates for torque sensors, and other magnetostrictive sensor and actuator applications.

Cerium: An Unlikely Replacement of Dysprosium in High Performance Nd–Fe–B Permanent Magnets

Replacement of Dy and substitution of Nd in NdFeB-based permanent magnets by Ce, the most abundant and lowest cost rare earth element, is important because Dy and Nd are costly and critical rare earth elements. The Ce, Co co-doped alloys have excellent high-temperature magnetic properties with an intrinsic coercivity being the highest known for T ≥ 453 K.

Substitutions for Cu in YBa2(Cu1−xMx)3O7−δ (M = Fe, Co, Al, Cr, Ni, and Zn)

Abstract Substitutions for copper in YBa 2 (Cu 1− x M x ) 3 O 7− δ (1:2:3) were investigated. It was found that indirect methods to determine solubility into 1:2:3 typically used in the study of intermetallic compounds (observed variation with composition of superconducting transition temperature and of lattice parameters) are not entirely reliable. Optical micrography was used to asses the morphology of the sintered polycrystalline specimens. Differential thermal analysis provides melting point data signaling the presence of impurities which melt at lower temperatures. Electron microprobe studies were undertaken to determine actual M content of the superconducting phase and to check for the presence of impurity phases. Estimates of solubility limits are made. Transmission electron microscope studies were used to determine the twin structure and investigate the occurrence of a tweed pattern. Superconducting transition temperature as a function of x was determined below the solubility limits. In several cases T c s observed in this work differ from some previous reports. Possible sources of these discrepancies are discussed. Variation of lattice parameters as a function of x is reported. Also described are electrical resistivity and DC susceptibility measurements for selected samples.

Effect of atmosphere and rare earth on liquidus relations in RE-Ba-Cu oxides

In the processing of the high temperature superconductors RE{sub 1}Ba{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}}, a knowledge of the liquidus temperatures is required in order to avoid liquid formation during the initial reactions of the starting materials. We have investigated the invariant points on the liquidus surface of the RE-Ba-Cu-O systems for RE=Y, Er, Gd, and Nd in oxygen, air, and argon. While the temperatures of the low melting reactions are almost independent of the rare earth species, they are heavily dependent on oxygen partial pressure. In addition, the peritectic decomposition temperature of the REBa{sub 2}Cu{sub 3}O{sub 7} phase was found to be a function of rare earth with a significantly higher value for the Nd compound than for the other rare earths.

Site-preference and valency for rare-earth sites in (R-Ce)2Fe14B magnets

Rare-earth (R) permanent magnets of R2Fe14B have technological importance due to their high energy products, and they have two R-sites (Wyckoff 4f and 4g, with four-fold multiplicity) that affect chemistry and valence. Designing magnetic behavior and stability via alloying is technologically relevant to reduce critical (expensive) R-content while retaining key properties; cerium, an abundant (cheap) R-element, offers this potential. We calculate magnetic properties and Ce site preference in (R1−xCex) 2Fe14B [R = La,Nd] using density functional theory (DFT) methods—including a DFT + U scheme to treat localized 4f-electrons. Fe moments compare well with neutron data—almost unaffected by Hubbard U, and weakly affected by spin-orbit coupling. In La2Fe14B, Ce alloys for 0≤x≤1 and prefers smaller R(4f) sites, as observed, a trend we find unaffected by valence. Whereas, in Nd2Fe14B, Ce is predicted to have limited alloying (x≤0.3) with a preference for larger R(4g) sites, resulting in weak partial ordering and s...

Origins of coercivity in the amorphous alloy Nd-Fe-Al

Results obtained from detailed studies of the microstructure and devitrification behavior, as revealed by advanced electron microscopy and synchrotron x-ray diffraction techniques, were compared with magnetic and thermal analysis characterization to obtain insight into the appreciable coercivity of nominally-amorphous melt-spun ribbon of Nd/sub 60-2/3x/Fe/sub 30-1/3x/Al/sub 10+x/(-2</spl times/<6.5). The as-spun microstructure consists of an amorphous Nd-rich, magnetically-soft matrix with clusters of approximate 1.2 nm diameter that apparently confer the hard magnetic properties to the alloy. The clusters are tentatively identified as sub-unit-cells of Nd/sub 6/Fe/sub 13-x/Al/sub 1+x/, by virtue of their unique and analogous temperature-dependent magnetic character as well as considerations of the ternary phase relationships during solidification.

Transition metal carbide formation in the Nd2Fe14B system and potential as alloying additions

Abstract Precipitates controlling grain growth in rapidly solidified Nd2Fe14B-type magnets are commercially important for inhibiting grain growth during heat treatments or thermomechanical processing. Compound additions of Group IVA, VA, and VIA transition metals along with C were added to the Nd2Fe14B (2-14-1) system. Transition metal carbide (TMC) formation was found for the Group IVA and VA transition metals. All the TMC precipitates observed formed with a 1:1 stoichiometry. To be successful as alloying additions in the 2-14-1 system, additional criteria besides carbide formation are necessary. The alloying ability of each transition metal carbide system was graded using criteria that dealt with phase stability, liquid and equilibrium solid solubility, and high temperature carbide stability. The Group IVA TMC alloys satisfied the proposed alloying criteria and were found to be excellent additions. The Group VA alloys, while forming carbides, only partially satisfied the proposed alloying criteria. Finally, the Group VIA alloys did not form TMCs.

Mechanical and high‐temperature (920 °C) magnetic field grain alignment of polycrystalline (Ho,Y)Ba2Cu3O7−δ

The effects of uniaxial compression, high‐temperature (920 °C) heat treatment, and heat treatment plus magnetic field on grain alignment in bulk ceramic pellets of HoBa2Cu3O7−δ and YBa2Cu3O7−δ were studied and separated. Uniaxially cold pressing prereacted powder into pellets at room temperature is found to yield significant grain alignment on the flat end surfaces of the pellets perpendicular to the pressing axis, where the c axes of the grains are parallel to this axis, in agreement with previous reports. A simple method to quantitatively determine the degree of surface grain alignment from the powder x‐ray diffraction data from these surfaces is used. These data were augmented by x‐ray rocking curve measurements. The degree of alignment of the cold pressed samples was found to increase with increasing pressure up to our pressure limit of 310 MPa. However, bulk magnetization anisotropy measurements on these pellets indicate that the degree of bulk alignment is only about 1/3 to 1/2 that inferred from the above surface measurements, which in turn suggests that the degree of alignment decreases with distance from the flat end surfaces. We find that a pronounced additional surface grain alignment is produced by heat treating the cold‐pressed samples at high temperature (920 °C). We also observed a significant influence during the high‐temperature annealing of a small (0.7 T) applied magnetic field on the alignment of the surface grains. A model to explain these observations is presented and discussed.The effects of uniaxial compression, high‐temperature (920 °C) heat treatment, and heat treatment plus magnetic field on grain alignment in bulk ceramic pellets of HoBa2Cu3O7−δ and YBa2Cu3O7−δ were studied and separated. Uniaxially cold pressing prereacted powder into pellets at room temperature is found to yield significant grain alignment on the flat end surfaces of the pellets perpendicular to the pressing axis, where the c axes of the grains are parallel to this axis, in agreement with previous reports. A simple method to quantitatively determine the degree of surface grain alignment from the powder x‐ray diffraction data from these surfaces is used. These data were augmented by x‐ray rocking curve measurements. The degree of alignment of the cold pressed samples was found to increase with increasing pressure up to our pressure limit of 310 MPa. However, bulk magnetization anisotropy measurements on these pellets indicate that the degree of bulk alignment is only about 1/3 to 1/2 that inferred from th...

Flux growth at ambient pressure of millimeter-sized single crystals of LaFeAsO, LaFeAsO1−xFx, and LaFe1−xCoxAsO

Millimeter-sized single crystals of LaFeAsO, LaFeAsO1−xFx, and LaFe1−xCoxAsO were grown in NaAs flux at ambient pressure. The detailed growth procedure and crystal characterizations are reported. The as-grown crystals have typical dimensions of 3×4×0.05–0.3 mm3 with the crystallographic c-axis perpendicular to the plane of the platelike single crystals. Various characterizations confirmed the high quality of our LaFeAsO crystals. Co and F were introduced into the lattice leading to superconducting LaFe1−xCoxAsO and LaFeAsO1−xFx single crystals, respectively. This growth protocol is expected to be broadly applicable to grow other RMAsO (R=rare earth, M=transition metal) compounds.