Inorganic Materials

α-Si3N4-based powder composites containing 3 wt % Y2O3–Al2O3 sintering aids with 2 : 1, 1 : 1, and 3 : 5 stoichiometries have been prepared by the Pechini process and spray drying. We have studied four ceramic samples produced by spark plasma sintering of the powder composites. The sintering process was run under a load of 70 MPa at a heating rate of 50°C/min as long as shrinkage of the powder composites continued. We have analyzed the shrinkage behavior of the powder composites during the sintering process; evaluated the activation energy for sintering; and measured the density, microhardness, and fracture toughness (cracking resistance) of the ceramic samples. The highest hardness, HV = 16.5 GPa, in combination with a fracture toughness KIc = 3.8 MPa m1/2, was offered by the material with an Y2O3 : Al2O3 molar ratio of 3 : 5 sintered at 1860°C. The corresponding densification factor was 99.0%. The starting mixture for this material was synthesized using spray drying.

The electrical properties, magnetic susceptibilities, and low-temperature photoluminescence spectra of semi-insulating CdTe{Cl} single crystals grown from the melt in graphitized and nongraphitized quartz ampules were compared. The crystals contaminated with carbon were found to be inhomogeneous and contain carbon-related deep centers affecting their detector performance. The possible mechanisms of compensation and defect complexing were discussed.

The effect of process-induced oxygen impurity on the electrical and optical properties of tetragonal CdP2 was studied using crystals grown from the stoichiometric melt and by sublimation. Irradiation of CdP2 with 14-MeV electrons was shown to cause a type conversion and to give rise to a broad, featureless emission band at 1.72 eV. The influence of isochronal anneals on the defect structure of CdP2 was examined. Annealing at 100°C was found to give rise to an emission at 1.82 eV.

We have studied the radiation resistance of BaF2:Ce3+ scintillator single crystals and ceramics at gamma doses from 106 to 108 rad. It has been shown that the ceramics produced using the self-fluorinating precursor BaF2 ∙ HF is more radiation-resistant.

SiO2-based aerogels have been produced be removing a solvent (ethanol or hexafluoroisopropanol) from lyogels both above and below the critical temperature of the alcohols (in the range 210–260 and 160–220°C, respectively). The resultant materials have been characterized by low-temperature nitrogen adsorption measurements, X-ray diffraction, thermal analysis, scanning electron microscopy, X-ray microanalysis, and small-angle and ultrasmall-angle neutron scattering. The results demonstrate that removing the solvent 20–30°C below the critical temperature of the solvent yields silica that is characterized by higher specific porosity and has the same or a larger specific surface area in comparison with the aerogels produced by drying under supercritical conditions. The nature of the solvent used and the solvent removal temperature influence the size and aggregation behavior of primary clusters and the cluster aggregate size in the aerogels.

Optically homogeneous single crystals of tetragonal cadmium diphosphide (β-CdP2) are prepared by vapor-phase growth, and their electrical conductivity, dielectric permittivity, and loss tangent are measured during slow heating from 78 to 400 K in the [001] and [100] directions at frequencies of 102, 103, 104, and 106 Hz. The permittivity of the crystals increases with temperature. The electrical properties of β-CdP2 are shown to be anisotropic.

Chemical vapor deposition of niobium from NbCl5–H2–O2 mixtures onto heated substrates was studied. Data are presented on the microstructure, phase composition, and microhardness of metastable oxygen-containing Nb.