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- Al2O3 (1)
- Electron microscopy (1)
- Grain size (1)
- Sol-gel processes (1)
- X-ray methods (1)
Electron paramagnetic resonance (EPR) of Fe3+ ions in Al2O3 is studied in powder samples prepared by different routes and/or modified by thermal or mechanical treatments, with different doping levels and grain sizes. The measurements are performed in various frequency bands (S, X, K, Q and W) and with bimodal detection in X-band. Simulations of the spectra are achieved with a code designed for computing EPR powder spectra described by any spin Hamiltonian including second-, fourth-and sixth-order ZFS terms (S ? 7/2). The linewidths, intensities and lineshapes are accounted for. The lineshape is Gaussian at low Fe3+ concentration whereas it is Lorentzian for higher concentration. The linewidths are interpreted as the superimposition of three main contributions: intrinsic linewidth, dipolar broadening and broadening due to lattice imperfections. The latter is tentatively interpreted in terms of quadrupolar spin Hamiltonian parameter distributions treated using first-order perturbation theory. Whatever the sample, only the b22 spin Hamiltonian parameter is found to be distributed around a mean zero value which corresponds to rhombic distortions. Angle and bond length distributions are tentatively extracted from the b22 distributions which gives some insight into the local order around the spin probe in relation to the preparation and treatment of the samples.
Direct coupling of an excimer laser to the cavity of an X-band ESR spectrometer enabled in situ observation of ultraviolet irradiation (248 nm) induced effects in aluminosilicate glasses. Under these conditions the radiation was absorbed by iron traces present in the glasses and finally electron/hole pairs were formed. The trapping and detrapping of electrons and holes, e.g. generation and decay of paramagnetic defects, could be followed in dependence on repetition rate of the laser (120 Hz), temperature during irradiations (4·2300 K) and composition of the glasses. The holes were trapped at SiO and AlOSi sites. Zn2+ and Cd2+ ions (likely as parts of ZnO or CdO aggregates) as well as Fe3+ ions served as effective electron traps. Additionally, changes of the dielectric properties of the samples during and after ultraviolet irradiations could be monitored directly. These are large if relatively mobile ions (e.g. Na+) are present in the sample. Comparison of ultraviolet to -irradiated samples shows that the type of traps present in the samples mainly depends on the glass composition. However, the relative amounts of stabilised holes and electrons essentially depend on the conditions and parameters of irradiation.
The transformation of doped or seeded pseudoboehmite to corundum was studied by combining thermal analysis, X-ray diffraction, transmission electron microscopy, and electron paramagnetic resonance spectroscopy. The temperature of phase transformation to corundum was lowered by about 130 °C when Fe3+ or corundum seeds were added to the sols. Action of Fe3+ ions depends on the actual degree of thermally induced transformation of pseudoboehmite via transition aluminas to corundum and the ability of these alumina phases to incorporate Fe3+ ions. These ions tend to aggregate with increasing iron concentration of the alumina phases and can work as nucleation centers. Small (not, vert, similar20 nm) corundum particles act as active nucleation sites whereas larger grains (200400 nm) also present in the samples are less effective. For the first time trapping and stabilization of NO2 molecules in transition aluminas formed by a solgel route was shown.