Phase transformations of the rhombohedral modification of boron nitride with shock compression

Features of crystal-orientated transformations of the rhombohedral modification of boron nitride (rBN) into dense modifications of the wurtzite (wBN) and sphalerite (spBN) types with shock compression in the range from 15 to 50 GPa are studied. Three compression schemes are used: in annular ampules (～15 GPa); in the ampules of K. M. Rabinin (25–35 GPa); in a plane shock wave (～50 GPa). With an increase in pressure from 15 to 50 GPa and a corresponding change in the loading scheme the degree of rBN transformation into dense modifications increases from 0 to 40%. There is a change in the rBN transformation mechanism: from longitudinal bending of layers (as a result of which wBN forms, and spBN is formed from it) to corrugation leading to spBN formation directly from rBN.     

The Influence of Activation Preprocessing of the B4C and SiC Powder Mixture with Al on the Phase Composition,Structure, and Properties of Hot-Pressed Composites

Powder Metallurgy and Metal Ceramics - The paper examines how the conditions of activation preprocessing (dry and wet grinding) of B4C–Al and SiC–Al powder mixtures in a planetary-ball...     

Electroerosion behavior of a ceramic based on boron carbide: Phase composition, structure, and properties of protective spark coatings

Measurements have been made on the erosion resistance of B₄C-15% TiB₂ materials made by reactive sintering on hot pressing of B₄C-TiO₂-C powder mixtures. The composite material is used as an electrode for treating steel and titanium surfaces in spark alloying. It is shown that the method of preparing the boron carbide influences the erosion resistance of the ceramic much less than it does for the mechanical properties. The surface layers of the electrode materials are affected by the electrical discharge and give rise to secondary structures because of chemical interaction between the electrode materials on the one hand and elements in the gas medium on the other. The properties of these structures are responsible for the erosion characteristics of the electrode material and for the compositions and qualitative properties of the coatings.     

Electron-microscopical investigations of the structure of the surface layers of titanium carbide after friction at 20–1400°c

Powder Metallurgy and Metal Ceramics -     

Valence-band electron structure in polycrystalline cubic boron nitride sintered in shock waves

Measurements have been made on the phase composition, valence-band electron-energy structure, the substructure, and the lattice parameters. Internal stresses have been determined in the BN_sp polycrystalline materials sintered in shock waves with various pressures at the fronts. The NK spectra show that the valence bands are shifted to energies that correspond to points of high symmetry U₁ and K₁ in the Brillouin zone. They represent sp hybridized states, which participate in direct bonds between the closest boron and nitrogen atoms. The reduction in the electron-state spaces in these energy regions is related to the internal stresses, which attain some gigapascals. These arise from the formation of point and line defects. The type of defect has a marked effect on the internal-stress sign and level.Translated from Poroshkovaya Metallurgiya, No. 7, pp. 94–101, July, 1993.     

Photonic bandgap structures with electronically controlled characteristics

Microwave filters based on periodic metallized ferrite-dielectric structures are described. The spectral characteristics of such a photonic bandgap filter can be electronically controlled on a real-time scale.