Phase transformations of carbon-black in high-temperature shock compression

The carbon-black transformations into diamond and amorphous carbon phase having an intermediate density of 2.9 g/cm³ in high-temperature shock compression at 20–32 GPa and 2500–3500 K have been studied. The conditions of compression that ensure the maximum yield of these phases have been defined. The transformation regularities have been analyzed under the assumption that the amorphous phase is an intermediate structure on the way to the transformation of turbostratic carbon into diamond.     

Deformation processes during high-pressure sintering of the diamond powders produced by catalytic synthesis

Deformation structures formed in diamond grains during polycrystalline sample sintering at 7.7 GPa were studied using TEM. A number of deformation features were observed in diamond during sintering in the temperature range 700–2500 °C. Based on these data a sequence for the structure formation processes in diamond grains under P-T treatment was ascertained.     

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 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.     

Effects of grain size and disorder in graphite-like structures on high-pressure martensitic transformations

Nucleation is considered in martensitic transformations of graphite-like structures into diamond ones. The pressure dependence of the critical nuclear size has been determined. Studies have also been made on the effects of initial defectiveness on the martensitic transformations at various pressures. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(406), pp. 96–102, March–April, 1999.     

Formation of long-period and unidimensionally disordered structures in the AlN-SiC-O system

An investigation was made of the formation of long-period structures (LPS) in solid AlN-SiC solutions with added oxygen. It was found that the crystallographic parameters of these structures are similar to those of sialon. A structural model was developed which connects LPS with the formation of structural and chemical inhomogeneities in the basic 2H-structure. It was established that a considerable fraction of LPS exhibit packing defects which are unidimensionally disordered.Institute of Problems of Materials Science, National Academy of Sciences of the Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, No. 5–6, pp. 97–101, May–June, 1994.