Influence of high pressures on the structure of polycrystalline aluminum nitride

Translated from Poroshkovaya Metallurgiya, No. 3(339), pp. 38–44, March, 1991.     

Silicon Carbide Formed in Mixtures of Silicon with Carbon in Various Structural Modifications Under Shock Compression

Powder Metallurgy and Metal Ceramics - The interaction of silicon with carbon in various structural modifications under shock compression in cylindrical recovery fixtures was studied. X-ray...     

The influence of the shock compression conditions on the graphite transformations into lonsdaleite and diamond

Shock compression-induced phase transformations of graphite into carbon dense modifications in cylindrical recovery containers (p _shc = 20–36 GPa, T _shc = 1800–3500 K) have been studied. The dependences of the lonsdaleite and diamond yields on the compression conditions have been established. The results obtained have been analyzed taking into account the formation mechanisms of the dense phases and special features of their shock-wave synthesis.     

Structure of the dense amorphous carbon phase synthesized in a mixture with diamond as a result of shock compression of carbon black

Soft X-ray emission and absorption spectroscopy methods were used in the present work to study experimentally the electronic structure of the dense amorphous carbon phase synthesized in a mixture with diamond as a result of shock compression of carbon black. The X-ray emission C Kα bands, representing the energy distribution of the C 2p-like states, have been recorded for the amorphous carbon phase and, for comparison, for diamond, lonsdaleite, graphite, and carbon black. For the amorphous carbon phase, diamond and lonsdaleite the spectra of quantum yields of the X-ray photoeffect in the area of the C K absorption edge have been recorded as well. Total and partial densities of states of the carbon H-6 and bct-4 phases, diamond and lonsdaleite have been calculated using the first-principles self-consistent full potential linearized augmented plane wave (FP-LAPW) method. Comparison on a common energy scale of the X-ray emission C Kα bands and C K quantum yields of the dense amorphous carbon phase as well as the theoretical curves of partial C 2p-like states of the carbon H-6 and bct-4 phases reveals that, the electronic structure of the amorphous carbon phase under consideration is well described by that of the carbon bct-4 phase. The experimental X-ray emission and absorption spectroscopy data indicate that the amorphous carbon phase studied is metallic, being in agreement with the results of the theoretical FP-LAPW calculations.     

Structure formation patterns in carbamide synthesis of nanocrystalline graphite-like boron nitride

We have used chemical analysis, x-ray diffractometry, transmission and scanning electron microscopy, and also measurement of the specific surface area to study the characteristic patterns of nitriding and structure formation in graphite-like BN powders synthesized by the carbamide method under various conditions. We have studied the effect on the synthesis process from the ratio of boric acid to carbamide in the starting mixture, preliminary grinding of the charge, the nitriding temperature and time. We show that the major factor determining the composition and structure of the product synthesized at 900–1200 °C is the ratio of the starting components. Carbon-free BN of stoichiometric composition is formed for a 1: 3 ratio of boric acid to carbamide as a result of two-hour nitriding at 1200 °C. The graphite-like BN obtained is characterized by a turbostratic nanocrystalline structure with a specific surface area of 100 m²/g and high nanoporosity of the powder particles.     

Development of a Method for High-Temperature Shock Compression of Powder Mixtures to Obtain Cubic Boron Nitride

We used shock compression of a mixture of boron nitride powder with an alkali halide salt as an additive to realize the phase transformation of turbostratic BN to the cubic modification. We have experimentally studied the effect of the type and amount of the additive and also the initial density of the mixture on the yield of the cubic phase of BN. We have roughly calculated the pressure and temperature arising upon shock compression of powder mixtures in an annular cylindrical storage ampul. We have shown that melting of the additive has a substantial effect on development of the phase transformation in boron nitride under shock loading conditions. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 107–118, July–August, 2005.