J.C. Ho

Large zinc cation occupancy of octahedral sites in mechanically activated zinc ferrite powders

The cation site occupancy of a mechanically activated nanocrystalline zinc ferrite powder was determined as (Zn0.552+Fe0.183+)tet[Zr0.452+Fe1.823+]octO4 through analysis of extended x-ray absorption fine structure measurements, showing a large redistribution of cations between sites compared to normal zinc ferrite samples. The overpopulation of cations in the octahedral sites was attributed to the ascendance in importance of the ionic radii over the crystal energy and bonding coordination in determining which interstitial sites are occupied in this structurally disordered powder. Slight changes are observed in the local atomic environment about the zinc cations, but not the iron cations, with respect to the spinel structure. The presence of Fe3+ on both sites is consistent with the measured room temperature magnetic properties.

Ferrimagnetic zinc ferrite fine powders

Partially inverted fine powders of zinc ferrite (ZnFe/sub 2/O/sub 4/) were produced by chemical aerogel synthesis. These powders showed magnetic ordering at temperatures to 250 K. Upon ball milling, a much higher inversion parameter was attained, yielding ferrimagnetic powders having small remanence and coercive fields at room temperature. Due to the small mean particle size, superparamagnetic behavior was seen at temperatures above 25 K. Powder properties were characterized by X-ray diffraction, electron microscopy, calorimetry, magnetometry, and Mossbauer effect spectroscopy techniques.

A Mössbauer evaluation of cation distribution in titanomagnetites

Abstract Peak area analyses of Mossbauer spectra were used to directly appraise models of cation distribution in synthetic titanomagnetites, Ti x Fe 3− x O 4 (0 x x 3+ ions on the tetrahedral sites of the cubic spinels was effectively delineated, and on the basis of its area fraction a new cation distribution is proposed.

The origin and future of composite aluminium conductors

A composite conductor with high-purity aluminium filaments in an aluminium alloy matrix has been developed. It retains a very high electrical resistance ratio and yield strength when cooled by liquid hydrogen. In typical cryogenic magnetic applications, the aluminium composite is lighter in weight and more versatile than superconductors at any temperature, as long as large flows of cryogenic hydrogen are permissible. >

Principles of application of high temperature superconductors to electromagnetic launch technology

A review is presented of advances in the performance of bulk high-temperature superconductors (HTSC) which permit conductor and magnet development at practical magnetic fields to be pursued for high-current applications such as electromagnetic launchers (EMLs). While early hopes for a superconductor critical temperature (T/sub c/) approaching room temperature have not been fulfilled, numerous HTSC with T/sub c/ between 60 K and 125 K exist which can be successfully processed. Some of these HTSC are well enough understood that small conductors and coils may be fabricated for operation near 20 K. Numerous physics, magnetic flux mechanics, materials processing, and structural support issues remain for resolution before large-scale coils made of HTSC can be operated at high energy storage density at temperature well above 20 K. Properties and materials processing of HTSC and their relation to EML applications technology are described. >

Structural and magnetic characterization of aerogel-produced Ge0.5Fe2.5Oy nanoparticles

Abstract Fine powders of Ge 0.5 Fe 2.5 O y with nearly uniform grain size were synthesized through the sol–gel supercritical drying technique, and were characterized by XRD, TEM, SQUID, and Mossbauer spectroscopy. Each grain is comprised of a spinel-crystallite core of mean size 11.2xa0nm and a thin amorphous shell. Such a complex structure yields collinear ferrimagnetism in the core and canted ferrimagnetism with multiple configurations in the shell. At low temperatures, the low-field dependence of magnetization as it approaches saturation suggests the presence of a weak random anisotropy, and the high-field dependence suggests an increase in the magnetic moments of molecules. Both tetrahedral and octahedral sites are occupied by Ge 4+ and Fe, mainly trivalent, cations. The charge balance is presumably provided by cation and/or anion vacancies.

Crystallographic, magnetic and calorimetric studies of Ho5Si2Ge2

Abstract Following the discovery of a giant magnetocaloric effect in Gd 5 (Si,Ge) 4 , attention has been extended to R 5 (Si,Ge) 4 with R being other rare-earth elements. In this work, X-ray structural analyses, low- and high-field magnetization measurements and zero-field calorimetric measurements were carried out on Ho 5 Si 2 Ge 2 . Specific heat data were also obtained for nonmagnetic Lu 5 Si 2 Ge 2 as a reference material. In contrast to the general trend of having a ferromagnetic order in the R 5 (Si,Ge) 4 series, Ho 5 Si 2 Ge 2 actually becomes antiferromagnetically ordered with a Neel temperature T N near 25xa0K. Moreover, an anomalous behavior below T N also prevails in the temperature dependence of both magnetization and specific heat, suggesting further transitions from the antiferromagnetic to other complex magnetic structures.

Role of calcium plumbate during the formation of 2223 phase in the Bi(Pb)srcacuo system

Abstract There are conflicting reports regarding the melting point of Ca 2 PbO 4 in the literature. By performing differential thermal analysis under various atmospheres, we found that the melting point of Ca 2 PbO 4 is severely depressed due to the reduced partial pressure of oxygen. At an oxygen partial pressure of one atm., Ca 2 PbO 4 melts at 1055°C and this temperature is reduced to about 833°C in zero oxygen partial pressure (e.g. in nitrogen). Ca 2 PbO 4 is formed as an impurity in the initial stage during the preparation of pure 2223 phase from the Bi(Pb)srcacuo system in air. However, the usual sintering temperatures (840°-850°C) at which the 2223 phase is synthesized in air are not high enough to melt Ca 2 PbO 4 . Thus, the melting of Ca 2 PbO 4 is not responsible for the growth of 2223 phase from a liquid medium. Ca 2 PbO 4 and the lead-enriched 2223 phase were found to be compatible with each other at 845°C in air. Utilization of reduced partial pressure of oxygen will inhibit the growth of Ca 2 PbO 4 and promote the formation of the lead-containing, 2223 phase by modifying the reaction paths that would not otherwise be possible if higher oxygen partial pressure were used.

Aspects of forming metal-clad melt-processed Y-Ba-Cu-O tapes

The authors describe the manufacture of a tape of Y-Ba-Cu-O up to 25-cm long in nickel cladding using partial-melt processing. Magnetic and transport current measurements of the tapes were performed. Transport critical currents at 4.2 K of greater than 600 A have routinely been carried by these tapes. Magnetization measurements indicate that these high currents can be maintained up to 40 K. Aspects of selecting a criterion for establishing the transport critical current are discussed as they relate to applications.

Bi-based high temperature superconducting tapes by cold rolling method

Bi-based high-temperature superconductor doped by lead with and without silver addition was used to produce tape approximately 1.2-cm wide and up to 20-cm long. Powder was calcined at 800 degrees C, 810 degrees C, and 835 degrees C for 50 h each time and loaded into silver tubes with o.d.=6.35 mm and i.d.=4.35 mm. Tapes with nominal thicknesses of 0.5 mm, 0.35 mm, and 0.2 mm were produced by the cold rolling technique. Direct transport current, DC magnetization, and AC susceptibility measurements were performed to characterize the superconducting properties of the tapes. The authors observed a significant increase in the magnetization critical current density with increasing number of calcinations and decreasing thickness of the tape, which is equivalent to the introduction of deformations into the system, thus increasing the number of pinning centers. A silver addition (3 wt.%) increases connectivity between grains and creates much cleaner grain boundaries.