V. B. Zelyavskii

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 (pshc = 20–36 GPa, Tshc = 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.

Crystallization of boron nitride from solution in a lithium borate melt

We have investigated the reaction between disordered (turbostratic) boron nitride and lithium carbonate under a nitrogen atmosphere using x rays and electron microscopy. Lithium borate and cyanamide were found to be the reaction products at a temperature of 650°C. Spontaneous crystallization of highly, ordered graphitic boron nitride occurs from solution in a melt of lithium borates at 1000°C. Three-dimensional ordering and crystal size increase with elevation of the temperature to 1450°C.

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.

Phase composition and structure of composite powders based on solid solutions of SiC and AlN

The structure of SiC–AlN powders is investigated by x-ray diffraction and transmission electron microscopy methods. The powders were produced by joint carbon reduction and nitriding of silicon and aluminum oxide mixtures. The results show that a mixture of solid solutions forms during joint SiC and AlN synthesis at 1700°C, with SiC forming β (3C) and α (2H) modifications with different grain morphology. The fiber form is characteristic of β-SiC, whereas the grains of the solid solution based on SiC have a predominantly equiaxed form. α-SiC grain dimensions are considerablys smaller than those of AlN.

Structure of polycrystals produced by sintering nanocrystalline powders of cubic and wurtzitic boron nitrides

The structure and some properties of polycrystals produced by sintering nanocrystalline powders of the dense modifications of shock—wave-synthesized BN have been studied. The sintering was conducted at a static pressure of 7.7 GPa and temperatures from 1100 to 1800° C. The highest density (3 g/cm3) and microhardness (up to 20 GPa) have been exhibited by polycrystals produced by sintering the powder containing wurtzitic and cubic modifications in amounts that are approximately equal. In the temperature range from 1100 to 1300° C the wurtzitic phase transformed into the cubic one. In this temperature range the average size of cBN grains changed from 20 to 50 nm. The structure of compacts is characterized by the presence of grain (grain-boundary) interlayers 2–5 nm in thickness.

Nature of the real structure of graphite-like BN and its transformation into a wurtsite modification under impact compression

The dependence of the interlayer distance d on the degree of unidimensional disorder γ in graphite-like BN was determined. It was established that d=0.33273±0.00005 nm at 20°C in the perfectly ordered structure (γ=0); the value of d increases uniformly with increase of γ, and at γ=1 it equals 0.343±0.001 nm. It was also established that the yield of wurtsite phase produced by impact compression of the graphite-like modification sharply decreases with increase of γ, and for γ>0.2 it becomes equal to zero. This result is explained in terms of the nature of martensitic transformations in layered structures.

Shock-wave synthesis of diamond nanofibers and their structure

Diamond nanofibers produced by high-temperature shock compression of graphite nanofibers in the presence of KCl at pressures of 25–35 GPa and temperatures of 3000–3500 K have been considered. The synthesized fibers have been shown to consist of randomly oriented nanograins of diamond with the amorphous phase impurity, whose content decreases as the impact compression pressure increases.

Phase transformations of disordered structures of graphite-like Boron Nitride under high-temperature shock compression

The pattern of phase transformations in boron nitride under high-temperature shock compression has been studied using a previously proposed method for high-temperature shock-wave synthesis of high-pressure phases followed by rapid quenching. Fine powders of turbostratic and partially ordered graphite-like BN were used as initial structures. Shock compression was carried out in ring devices at a pressure of 30 GPa and a temperature above 2500 K. A mixture of dense phases (wurtzitic and sphaleritic) was found to form from the graphite-like structures under those conditions; the total yield of those phases and the relative amount of the sphaleritic modification are considerably higher when turbostratic BN is the starting material. Both of the dense phases formed have a nanocrystalline grainstructure. The wurtzitic phase does not transform into the sphaleritic phase under those conditions, which points to cubic BN forming directly from the graphite-like structures.

FEATURES OF QUANTITATIVE X-RAY DIFFRACTION ANALYSIS OF WEAKLY ABSORBING POWDER PRODUCTS WITH A DEFECTIVE STRUCTURE

The methods of quantitative phase analysis of low-absorbing powders by X-ray diffraction have been developed. The approach used accounts for the dependence of line intensity absorption factor on the Bragg angle and sample porosity and thickness. It has been shown that the use of traditional methods without reference to this dependence results in considerable errors in quantitative determination of phase content and in phase structure investigation. The application of proposed methods is illustrated by phase analysis of products of boron nitride shock compression.