L. del Campo

Phase transformations of crystalline SiO2 versus dynamic disorder between room temperature and liquid state

The sequence of phase transitions of crystalline silica has been probed by infrared emission spectroscopy. The lattice dynamics, deeply impacted by the low frequency dynamic disorder, exhibit with increasing temperature signs of inhomogeneous broadening of the symmetry allowed normal modes. High frequency supplementary components are also activated. The analysis with a causal Voigt dielectric function model within the framework of hard mode spectroscopy allowed a fine characterization of solid–solid phase transitions. We also report experimental evidence showing that the occurrence of the intermediate ill-defined region above 1300 K is concomitant with the reactivation of the low frequency dynamic disorder; a behavior change that on the way explains the appearance of the negative thermal expansion regime.

Infrared optical properties of α-alumina with the approach to melting: γ-like tetrahedral structure and small polaron conduction

The normal spectral emittance of α-Al2O3 single crystal has been measured from room temperature up to the liquid state and from 20 cm−1 up to 10 000 cm−1, in two polarization configurations. The spectra were fitted with a semi-quantum dielectric function model. AlO4 structure units are revealed within the phonon spectral range more than a hundred degrees below the melting point when heating from the solid state. In parallel, the anomalous increase of emittance observed within the transparency spectral range with the approach to melting appears strongly correlated. Implications on the electronic structure are discussed: the existence of small polaron conduction is suggested which has never been mentioned before.

Al2O3/Al Cermets by Plasma Spraying: Optical Response of Experimental and Numerically Represented Materials

Optical properties of plasma-sprayed coatings and numerically represented samples were studied at wavelengths ranging from visible to mid-infrared. The paper focuses on Al2O3 and Al2O3/Al cermet coatings with different metal concentrations. Microstructure and composition of the samples were characterized in order to explain their optical response that is highly dependent on volume and/or surface scattering as a function of the wavelength range. 2D scanning electron microscopy and 3D x-ray microtomography images were exploited to get statistical data in order to numerically represent simplified samples from the complex microstructure of plasma-sprayed coatings. A Monte Carlo ray-tracing model, based on geometrical optical laws, was then applied to reproduce experimental trends of the acquired optical spectra. Good agreement with the experimental data was obtained.