A. V. Kurdyumov

Structural mechanisms of rhombohedral BN transformations into diamond-like phases

Abstract Structural features and mechanisms of transformation of graphite-like rhombohedral boron nitride modification (rBN) into the diamond-like wurtzite (wBN) and zinc blende (zBN) types have been studied. The puckering mechanism of the rBN→zBN transformation in shock waves was first proved structurally. It has been found that in static compression of CVD rBN (8 GPa, 20–2600 °C), transformations occur in the following sequence: rBN→(S,T)→Ig→wBN→Id→cBN, where (S,T) are structures deformed via basal shears or twinning, and Ig and Id are intermediate graphite- and diamond-like structures, respectively. The structure of wBN formed from rBN has been shown to be much more disordered than that of wBN obtained from the BN hexagonal modification (hBN).

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.

CATALYTIC SYNTHESIS OF GRAPHITE-LIKE BORON NITRIDE

It is well-known [1, 2] that lithium compounds have a strong effect on the process of formation of boron nitride by the carbothermal method from highly dispersed charges produced by the dissolution method. It is therefore interesting to examine the effect of these compounds on the formation and ordering of the structure of boron nitride in other methods of preparation.Investigations were carried out into the process of nitriding boron and a mixture of boric acid with soot in the presence of lithium compounds. Initial substances were amorphous boron with a purity of 98.6, boric acid of ChDA grade, grade 514 soot, grade ChDA lithium carbonate and hydroxyl, and dried nitrogen and ammonia. Charges were prepared by mixing the components by the dried method. The ratio of the boric acid and soot in the charges was constant (7:2), and the concentration of the addition was varied in the range from 1 to 58% (by mass). Nitriding was carried out in the temperature range 800–1500°C in a gas flow with an optimum speed of 2 litres/h. Temperature was measured with a platinum-platinum/rhodium thermocouple and regulated using an Sh-4501 device. Nitriding products were examined by chemical and x-ray analysis and also electron microscopy.

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.

Shock Synthesis of Ternary Diamond-Like Phases in the B ― C ― N System

Nanodispersed powders of the ternary graphite-like phases BC1.28N and BC2.14N have been used along with mixtures of powders of turbostratic BN and C in high-temperature shock compression followed by sharp quenching. The yield of diamond-like phases has attained 50% by volume. The excess of the graphite-like phase has been removed by treatment with molten alkalis containing nitrates and then with concentrated HClO4. The shock and chemical treatments alter the compositions of the diamond-like phases towards BCN in both of the BCxN specimens on account of the segregation of the excess carbon followed by the dissolution of it. Precision lattice-parameter measurements and line shape analysis indicate that the diamond-like phase is a BN ― C substitutional solid solution. Treatment of a mechanical mixture of the graphite-like phases of carbon and BN under the same p and T conditions led to the formation of a mixture of cubic boron nitride and diamond.

Wurtzitic boron nitride thermal stability and transformation into the rhombohedral boron nitride modification when heated

Abstract Samples of wurtzitic boron nitride (wBN) were obtained from the rhombohedral BN (rBN) modification as a result of phase transformation under high pressures. Structure features and kinetics of wBN→graphite-like BN transformation in heating have been investigated. The nature of the low thermal stability of wBN is discussed.

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.

Physical principles of shock-wave synthesis of superhard phases and their structure

This paper reviews work carried out at the Institute for Problems of Materials Science, Ukraine National Academy of Sciences, in developing physical bases for shock-wave synthesis of superhard phases of carbon (diamond, lonsdaleite) and boron nitride (wurtzite and sphalerite modifications). The effect of phase transformation mechanisms on structure features of the phases that are obtained under shock compression conditions is considered.