Effect of Thermal Treatment on the Structure and Properties of Hydroxyapatite

X-ray diffraction analysis and transmission electron microscopy were used to demonstrate that hydroxyapatite produced by the precipitation method is a powder composed of anisometric particles with longitudinal size of 70–100 nm and transverse size of 7–9 nm. The particles are constituted by crystallites with longitudinal sizes of 22–24 nm and transverse sizes of 8–10 nm. At a temperature of 600°C, the crystallite sizes grow, and the volume porosity and specific surface area decrease. Synthetic hydroxyapatite has a low thermal stability as compared with hydroxyapatite of biological origin.     

Acoustic field of a vertical cylindrical antenna in an inhomogeneous waveguide with an impedance lower boundary

An expansion of the field of a vertical antenna located in an inhomogeneous waveguide in terms of the normal waves of a homogeneous reference waveguide is obtained. The frequency dependence of the radiation resistance is analyzed numerically for various antenna depths and sound velocity profiles. Variations in the radiation resistance are correlated with the variations in the sound velocity.     

Highly selective substitutions in 2,3-dichloropyrazine. A novel general approach to aloisines

A highly efficient synthesis of the potent CDKs (cyclin-dependent kinases) inhibitors, aloisines (substituted 5H-pyrrolo[2,3-b]pyrazines) is presented. The method is based on highly selective monosubstitution of a single chlorine atom in 2,3-dichloropyrazine with lithiated ketones, esters, and nitriles followed by co-cyclization of the resulting intermediates with primary amines or hydrazines.     

Growth Rate anisotropy of the K2(S,Cr)O4 Mixed Crystals Grown from Aqueous Solution

The ratio of growth rates of the K₂(S,Cr)O₄ crystals in the [100] and [001] directions (the anisometry coefficient) is proportional to the initial effective supersaturation of the solutions from which the crystals were grown. The values of the factors of asymmetry and excess of distribution of the crystal anisometry coefficient are affected by the composition of solution. The change in the values of these factors is caused by the change in the value of the initial supersaturation of solution according to the specific features of the solubility diagram.     

Anodic oxide-film of the Co—Sn—O composition

Properties and electrocatalytic activity of thin-film (∼2 μm) oxide coatings on aTi support are considered by the example of the anodic chlorine evolution reaction. The coatings are obtained by thermally decomposing Co and Sn chloride solutions of different compositions on a Ti plate. The Co and Sn oxides are present in all studied compositions as individual phases and form no compounds or solid solutions. The oxide coating containing 70 at. % Co and 30 at. % Sn reveals a maximum electrocatalytic activity. The existence of the optimum composition is attributed to a favorable effect tin dioxide has on the formation of a transition layer between the Ti support and oxide coating. Tin dioxide hampers the Ti diffusion in the main coating and its subsequent oxidation, thus diminishing the negative impact titanium oxides have on the coating     

Effect of coalescence and of the character of the initial oxide on the photoluminescence of ion-synthesized Si nanocrystals in SiO<Subscript>2</Subscript>

The photoluminescence intensity (PLI) related to Si nanocrystals in a SiO₂: nc-Si system synthesized by ion implantation is studied experimentally and theoretically as a function of the Si+ ion dose at various annealing temperatures T_ann (1000–1200°C). The dose corresponding to the maximum PLI is found to decrease with increasing T_ann. These data are explained in terms of a model taking into account the coalescence of neighboring nanocrystals and the dependence of the probability of radiative recombination of quantum dots on their size. It is found that, when silicon oxide is grown in a wet atmosphere, the photoluminescence spectrum contains an additional band (near 850 nm), which is related to shells around the nanocrystals. This band weakens abrupily after high-temperature annealing in an oxidizing atmosphere (air).