Boron nitride (BN) nanotubes have the same nanostructure as carbon nanotubes but are found to exhibit significant resistance to oxidation at high temperatures. Our systematic study has revealed that BN nanotubes are stable at 700 °C in air and that some thin nanotubes (diameter less than 20 nm) with perfect multiwalled cylindrical structure can survive up to 900 °C. Thermogravimetric analysis reveals an onset temperature for oxidation of BN nanotubes of 800 °C compared with only 400 °C for carbon nanotubes under the same conditions. This more pronounced resistance of BN nanotubes to oxidation is inherited from the hexagonal BN and also depends on the nanocrystalline structure. This high level of resistance to oxidation allows promising BN nanotube applications at high temperatures.

Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% 57 Fe 2 O 3 quenched from melts equilibrated over a range of oxygen fugacities at 1409 ∞C. The Fe 3+ /SFe ratios were determined previously by 57 Fe Mössbauer spectroscopy and vary between 0 (fully reduced) and 1 (fully oxidized). Using the Mössbauer results as a reference, various methods for extracting Fe 3+ /SFe ratios from XANES spectra were investigated. The energy of the 1s AE 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 AE 4s and crest (1s AE 4p) transitions. The Fe 3+ /SFe ratios determined from linear combinations of end-member spectra (Fe 3+ /SFe ~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 Fe 2+ or Fe 3+ coordination with the Fe 3+ /SFe 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 Fe 3+ /SFe 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.

Nickel ferrite (NiFe2O4) nanoparticles with an average crystallite size of about 8.6 nm were prepared by mechanochemical synthesis (mechanosynthesis). In-field Mössbauer spectroscopy and high-resolution TEM studies revealed a nonuniform structure of mechanosynthesized NiFe2O4 nanoparticles consisting of an ordered core surrounded by a disordered grain boundary (surface) region. The inner core of a NiFe2O4 nanoparticle is considered to possess a fully inverse spinel structure with a Néel-type collinear spin alignment, whereas the surface shell is found to be structurally and magnetically disordered due to the nearly random distribution of cations and the canted spin arrangement. As a consequence of frustrated superexchange interactions in the surface shell, the mechanosynthesized NiFe2O4 exhibits a reduced nonsaturating magnetization, an enhanced coercivity, and a shifted hysteresis loop. The study also demonstrates that one can tailor the magnetic properties of mechanosynthesized NiFe2O4 particles by suitably controlling their size. The thickness of the surface shell of about 1 nm estimated from size-dependent magnetization measurements is found to be in good agreement with that obtained from high-resolution TEM and Mössbauer experiments. On heating above 673 K, the mechanosynthesized NiFe2O4 relaxes to a structural and magnetic state that is similar to the bulk one.