Special features of the electron structure of ultradisperse powders of cubic boron nitride

ing factors. The quantum dimensional effect determined by the absence of the long-range order in the volume of the particle leads to extensive changes in the EES in comparison with the thick material, especially to a reduction of the width of the valency band and appearance of new energy states forbidden in the thick materials [2]. The electrons with the sp-symmetry are more sensitive to the dimensional effect than the electrons of the d-symmetry, and the structure of the EES of the small particles simulates to a certain degree the EES of the surface of the thick crystal. However, these effects are detected at particle sizes smaller than i0 nm. The second factor is a reduction of the interatom spacing and deformation of the crystal lattice in small particles under the effect of surface tension which may cause corresponding deformation of the EES [4]. The third factor is a high Laplace pressure inside the small particles. For high-modulus materials with a higher surface energy (such as, for example, diamond, dense modifications of BN, refractory carbides and nitrides) with particles of 0.i ~m this pressure exeeds 1GPa and can cause barich changes in the EES [4, 5]. The final factor is an increase of the relative contribution on the free surface to the overall pattern of the EES of the material with a reduction of the particle size. It is well-known that the EES on the surface reflects the states of structural vacancies and is characterized by the presence of high-density nonbonding states in the area of the ceiling of the valency zone [6, 7]. As a result of theseEES changes, the material in the ultradispersed state assumes special physicochemical properties [2], especially lower sintering temperatures and higher sintering rates [i, 8], and a high catalytic activity. The most informative methods of experimental examination of the EES of the surface in the thick crystals are electron spectroscopy with angular resolution and electron Auger spectroscopy. However, for materials in the ultradisperse state it is more efficient to use ultrafine x-ray spectroscopy (with a slightly greater depth of excitation of the spectrum) because the efficient cleaning of the surface of a set of ultradisperse particles to remove adsorbed impurities associated with considerable difficulties and the surface state provides a relatively large contribution to the total spectrum of the valency bands of the materials. In [3] this method was used successfully for examining the EES of the UDP of a monoatomic refractory material (diamond). The EES in the ultradispersed polyatomic chemical compounds has not as yet been studied by the spectral methods. In this work, the method of ultrafine x ray emission spectroscopy based on K-bands of boron and nitrogen was used to examine the EES in the populated part of the valency band of the UDP of cubic boron nitride of the El'bor grade. The powders were synthesized from hexagonal BN with a graphite-like structure in industrial high-pressure systems; magnesium diboride was used as a catalyst/solvent [9]. The BNcu b crystals were separated from the rem