S. J. Campbell

Boron nitride nanotubes: Pronounced resistance to oxidation

This study was supported in part by a Discovery Research Grant from the Australian Research Council.

XANES calibrations for the oxidation state of iron in a silicate glass

Abstract Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% 57Fe2O3 quenched from melts equilibrated over a range of oxygen fugacities at 1409 °C. The Fe3+/ΣFe ratios were determined previously by 57Fe Mössbauer spectroscopy and vary between 0 (fully reduced) and 1 (fully oxidized). Using the Mössbauer results as a reference, various methods for extracting Fe3+/ΣFe ratios from XANES spectra were investigated. The energy of the 1s → 3d pre-edge transition centroid was found to correlate linearly with the oxidation state. Correlations also exist with the energy of the K absorption edge and the area of peaks in the derivative spectrum associated with the 1s → 4s and crest (1s → 4p) transitions. The Fe3+/ΣFe ratios determined from linear combinations of end-member spectra (Fe3+/ΣFe ~0 and ~1) were found to deviate significantly from the Mössbauer values. This may indicate the susceptibility of this method either to errors arising from the treatment of the background or to changes in Fe2+ or Fe3+ coordination with the Fe3+/ΣFe ratio. The general applicability of any XANES calibration for determining oxidation states is limited by variations in the Fe coordination environment, which affects both the intensity and energy of spectral features. Thus previous calibrations based on mineral spectra are not applicable to silicate glasses. Nevertheless, systematic trends in spectral features suggest that Fe3+/ΣFe values may be obtained from XANES spectra, with an accuracy comparable to Mössbauer spectroscopy, by reference to empirical calibration curves derived from compositionally similar standards.

A Mössbauer study of the oxidation state of Fe in silicate melts

Abstract Fe3+/ΣFe ratios were determined from Mössbauer spectra recorded for a series of 17 anorthitediopside eutectic glasses containing 1 wt% 57Fe2O3 quenched from melts equilibrated over a range of oxygen fugacities from fO₂ ~ 105 bars (Fe3+/ΣFe = 1) to 10-13 bars (Fe3+/ΣFe = 0) at 1682 K. Fe3+/Fe2+ was found to be proportional to fO₂ to the power of 0.245 ± 0.004, in excellent agreement with the theoretical value of 0.25 expected from the stoichiometry of the reaction Fe2+O + 0.25 O2 = Fe3+O1.5. The uncertainty in the Fe3+/ΣFe ratios determined by Mössbauer spectroscopy was estimated as ± 0.01 (1σ) from the fit of the data to the theoretical expression, which is significantly less than that quoted for previous measurements on silicate glasses; this results from fitting the spectra of a large number of systematically varying samples, which allows many of the ambiguities associated with the fitting procedure to be minimized. Fe3+/ΣFe ratios were then determined for samples of the anorthitediopside eutectic composition equilibrated at selected values of fO₂, to which up to 30 wt% Fe2O3 had been added. Fe3+/ΣFe was found to vary with ΣFe (or FeOT), but both the 1 wt% and high FeOT data could be satisfactorily fit assuming the ideal stoichiometry (i.e., Fe3+/Fe2+ ∝fO₂1/4) by the inclusion of a Margules term describing Fe2+-Fe3+ interactions. The large negative value of this term indicates a tendency toward the formation of Fe2+-Fe3+ complexes in the melt. The resulting expression, using the ideal exponent of 0.25, gave a fit to 289 Fe3+/ΣFe values, compiled from various literature sources, of similar quality as previous empirical models which found an exponent of ~0.20. Although the empirical models reproduce Fe3+/ΣFe values of glasses with high FeOT reasonably well, they describe the data for 1 wt% FeOT poorly. The non-ideal values of the exponent describing the dependence of Fe3+/ΣFe on fO₂ at high FeOT are an artifact of models that did not include a term explicitly to describe the Fe2+-Fe3+ interactions. An alternative model in which Fe in the silicate melt is described in terms of three species, Fe2+O, Fe3+O1.5, and the non-integral valence species Fe2.6+O1.3, was also tested with promising results. However, at present there is no model that fits the data within the assessed accuracy of the experimental measurements.

The magnetic behaviour of nanostructured zinc ferrite

We have investigated a series of nanostructured ZnFe2O4 samples produced by mechanical activation (mean particle sizes d ∼50-8 nm) by variable temperature neutron diffraction measurements (2-535 K) supported by DC magnetisation measurements (4.2-300 K). The systematic increase in the mean inversion parameter (c ∼0.04-0.43) with increasing milling time is accompanied by a gradual decrease in the occurrence of the long range antiferromagnetic ordering observed in the starting ZnFe2O4 material, as well as a gradual decrease in the related diffuse short range order peak. The neutron diffraction patterns of particles with d < ∼15 nm and c> ∼0.2 are consistent with the occurrence of ferrimagnetic order and exchange interactions of the type Fe3+A—O2−—Fe3+ [B]. Diagrams summarising the magnetic regions of nanostructured ZnFe2O4 are presented. The magnetic behaviour overall agrees well with the enhanced magnetisation and ferromagnetic behaviour reported for nanostructured, ultrafine and thin films of ZnFe2O4 by other groups using mainly magnetisation and Mössbauer spectroscopy measurements.

Structural evolution of ball-milled ZnFe2O4

Abstract Nanostructured zinc ferrite produced by milling in both low-energy and high-energy ball mills has been investigated by X-ray diffraction and Mossbauer effect spectroscopy. The lattice parameter of the milled products remains essentially unchanged from that of equilibrium ZnFe2O4 with the steady-state average particle size found to decrease to d=18(2) nm on low energy milling compared with d=8(1) nm on high energy milling. The room temperature Mossbauer spectra of the milled materials have been analysed using two doublets, one of which is considered to be associated primarily with the octahedral lattice sites. Spectral broadening is observed with decreasing particle size, particularly below d∼10 nm, for which the effects of magnetic hyperfine splitting become evident. The mean inversion parameter of nanostructured ZnFe2O4 is found to increase to c∼0.75 for particle sizes of d∼8 nm reflecting the systematic evolution of zinc ferrite from its normal spinel structure towards an inverse spinel structure on mechanical treatment as observed previously. The other factors which contribute to the Mossbauer spectra of nanostructured ZnFe2O4 (d∼8–70 nm) are discussed.

Magnetism of the nanostructured spinel zinc ferrite

Nanostructured zinc ferrite has been prepared by mechanical milling. The changes in the microstructure indicate a decrease in particle size and a simultaneous increase in the inversion parameter. Along with the structural changes, a magnetic transformation from the antiferromagnetic phase to a ferrimagnetic-like behaviour is observed by neutron diffraction.

Synthesis and structural evolution of tungsten carbide prepared by ball milling

Tungsten carbide has been synthesized directly by ball-milling tungsten powder and activated carbon in vacuum. The structural development of the WC phase with milling times up to 310 h has been followed using X-ray, neutron diffraction and scanning electron microscopy. Subsequent annealing (at 1000 °C for 1 and 20 h) of material milled for 90 h or longer, results in samples comprising almost entirely crystalline WC. The production of WC itself during milling results in enhanced iron contamination from the steel mill and balls on extended milling which were monitored by energy-dispersive X-ray and Mossbauer spectroscopies.

Magnetocaloric effect in layered NdMn2Ge0.4Si1.6

A giant magnetocaloric effect has been observed in NdMn2Ge0.4Si1.6 associated with the first-order magnetic phase transition from antiferromagnetism to ferromagnetism around TC=36K. The magnetic entropy change –ΔSM and adiabatic temperature change ΔTad have been determined from magnetization and specific heat measurements (B=0–5 T) with –ΔSM calculated by the Maxwell relation and Clausius–Clapeyron method. The values –ΔSMmax=12.3 J kg−1 K−1 and refrigerant capacity ∼95 J/kg for ΔB=0–2 T as derived from the Maxwell relation, together with the small hysteresis (thermal <0.5 K; magnetic field <0.1 T), indicate the potential of NdMn2Ge0.4Si1.6 for refrigeration applications.

Mechanochemical transformation of haematite to magnetite

Abstract The transformation of α-Fe 2 O 3 to Fe 3 O 4 on wet-milling α-Fe 2 O 3 under low energy conditions in vacuum has been investigated by Mossbauer effect spectroscopy. The structural transformation is discrete with no evidence of interdissolution of the 2 phases. The resultant ∼ 30 nm scale crystal blocks of magnetite exhibit hyperfine features characteristic of bulk Fe 3 O 4 although evidence for structural distortion of the octahedral (particularly) and tetrahedral iron sites is obtained. The results are consistent with a primarily physical origin for the transformation in which oxygen bonds on the cleaved α-Fe 2 O 3 surface are broken and oxygen released to the milling environment.

Magnetic properties in polycrystalline and single crystal Ca-doped LaCoO3

Polycrystalline (PC) and single crystalline (SC) Ca-doped LaCoO3 (LCCO) samples with the perovskite structure were synthesized by conventional solid-state reaction and the floating-zone growth method. We present the results of a comprehensive investigation of the magnetic properties of the LCCO system. Systematic measurements have been conducted on dc magnetization, ac susceptibility, exchange-bias, and the magnetocaloric effect. These findings suggest that complex structural phases, ferromagnetic (FM), and spin-glass/cluster-spin-glass (CSG), and their transitions exist in PC samples, while there is a much simpler magnetic phase in SC samples. It was also of interest to discover that the CSG induced a magnetic field memory effect and an exchange-bias-like effect, and that a large inverse irreversible magnetocaloric effect exists in this system.