Levan Chkhartishvili

Boron nitride nanosystems of regular geometry

The explicit expressions in term of B-N bond length are obtained for atomic site coordinates and intersite distances in regular boron nitride nanotubes and fullerenes. The radii of single-walled BN nanotubes and single-shelled BN fullerenes are estimated, and their most stable associations in form of double-walled nanotubes and double-shelled fullerenes are predicted. The differences between radii of regular boron nitride fullerenes with indexes of (n + 3) and (n) are almost equal to the interlayer distance in layered boron nitride structures. Description made for the boron nitride nanosystems of regular geometries may serve as basis for further ground state and electron structure calculations.

On quasi-classical estimations of boron nanotubes ground-state parameters

The explicit expressions in term of B-B bond length are obtained for atomic sites coordinates and intersite distances in boron nanotubes of regular geometry. Using the charge and potential distributions in boron atom parameterized within initial quasi-classical approximation, the boron-boron pair potential curve is constructed. It leads to the boron-boron interatomic distance, binding energy, and relative vibration frequency equilibrium values which only slightly are deviated from averaged bond length in structural units of boron and boron-rich solids, dissociation energy, and oscillatory quantum of the diboron molecule, respectively. This curve together with obtained formulas should be useful for calculating of boron nanotubes geometrical and other ground-state parameters.

Isotopic composition dependences of lattice constant and thermal expansion of β-rhombohedral boron

Boron isotope substitution in boron-containing crystalline structures is important because many properties of these materials are structure sensitive. In the present work, we studied the unit cell expansion at the fixed temperature and temperature dependence of thermal expansion for the β-rhombohedral modification of boron, when 11B is replaced by 10B isotope. Theoretical analysis of experimental results associates the observed changes in structures of the β-rhombohedral boron samples of various isotope compositions with isotope effects rather than with technologically concomitant impurities.

Estimation of the isotopic effect on the melting parameters of boron

It is shown that the isotopic composition has a noticeable effect on the melting parameters of boron. The differences in melting temperature and enthalpy of the monoisotopes 10B and 11B are estimated at ∼16 K and ∼0.0021 eV/at, respectively.

Concentrations of radiation defects with almost isoenergetical levels in silicon

Isochronous annealing of radiation defects with almost isoenergetical levels (indistinguishable from each other by means of temperature dependence of the majority charge carriers concentration) in silicon is studied. The concentrations of G- and A-centres with a level E c−0.17 eV in n-Si and some (unidentified) vacancy-type and V2+B complexes with a level E v+0.22 eV in p-Si are determined.

Nanoparticle Near-Surface Electric Field

Theoretical studies show that surface reconstruction in some crystals involves splitting the surface atomic layer into two—upper and lower—sublayers consisting of atoms with only positive or only negative effective electric charges, respectively. In a macroscopic crystal with an almost infinite surface, the electric field induced by such a surface-dipole is practically totally concentrated between the sublayers. However, when the material is powdered and its particles are of sufficiently small sizes, an electric field of a significant magnitude can be induced outside the sublayers as well. We have calculated the distribution of the electric field and its potential induced at the surface of a disc-shaped particle. The suggested novel nanoscale effect explains the increase in physical reactivity of nanopowders with decreasing particle sizes.

Neutron Detectors Based on 10B-Containing Nanomaterials

Due to of deep penetration ability and strong secondary ionization, neutron-radiation is the mostly dangerous ionizing radiation for humans and environment. Developments in materials science provide neutron-sensor devices with enhanced selectivity and sensitivity. Among them, detectors based on boron-containing thin films are the naturally best because of special neutron-capture properties of 10B isotope. In this work, there are evaluated the key physical-technical characteristics of neutron detectors made from 10B-enriched semi-conducting materials: thickness of the effective working layer ~10 μm, releasing rate of the 10B – n interaction products ~1015/cm3 s, electron–hole pairs generating rate in process of neutron absorption ~1022/cm3 s, rate of rise in the temperature ~10 K/s, and device mean operating time ~10−4 s.

Estimation of atomic charges in boron nitrides

Для бинарных соединений выведено выражение эффективных зарядов, зависящее от числа молекул в элементарном параллелограме, площади его сечения поперек внешнему полю, модули Юнга и диэлектрической проницаемости. Для нитридов бора были получены следующие полуэмпирические оценки: h-BN – 0.35 и 0.09, c-BN – 0.49, и w-BN – 0.76 и 0.50.

Anomalously high capacitance of β-rhombohedral boron induced by structural defects

Capacitance and dielectric loss tangent measurements have been carried out for β-rhombohedral boron within the frequency range 102 – 104 Hz and temperature interval 77 – 373 K. Different specimens were studied: high-purity macrocrystals, vapor-liquid-grown faceted crystals, zone-melted single crystals, and crucible-melted polycrystals. Within the whole ranges of measurements, the capacitance of all samples was 10–60 times larger than their geometric capacitance. Temperature dependences of the capacitance reveal two-level step-like behavior. Location of levels is independent from the applied electric field frequency, but at higher frequencies, the capacitance steps shift to lower temperatures. For all frequencies, an abrupt rise in capacitance takes place at one and same temperature ~ 290 K. At low temperature, dielectric losses are negligible, but they increase towards the room temperature. At ~ 290 K (i.e., simultaneously with the abrupt rise in capacitance) their value abruptly falls down. Dielectric properties of β-rhombohedral boron are discussed on the basis of generalized barrier model of the heterogeneous semiconductor within its three-layer version. Planar defects like twins and stacking faults characteristic for real crystals are assumed to produce local elastic stresses sufficient for the local lowering in conductivity due to the piezoresistive effect. Consequently, layers adjacent to the planar defects should represent low-conducting inclusions in the relatively high-conducting matrix. In addition, the lesser-conducting barrier layers should appear at their boundaries. Effect at ~ 290 K seems to be related with a symmetry-restoring phase transition driven by the occupation patterns of certain atomic sites in β-rhombohedral crystalline boron lattice.

Effect of concentrated light on morphology and vibrational properties of boron and tantalum mixtures

Heating a mixture of boron (impurities: carbon ∼ B50C2, boric acid – H3BO3) and tantalum (Ta) powders in nitrogen flow in a xenon high-flux optical furnace was performed. As-received powder composed of h-BN, H3BO3, TaB2, B9H11 and a number of other phases including β-rhombohedral boron, apparently, heavily doped with Ta. FT–IR examination of any sample of the material reveals the complicated vibration spectrum containing, in particular, an absorption band near 2260 cm−1. The shapes of these bands are different for samples because powders were synthesized at different temperatures. Known, that in β-rhombohedral boron lattice, there are nano-sized voids of different types, which allow an accommodation of single atoms or small groups of atoms. Theoretical calculations performed by the method of quasi-classical type yields the same value, 2260 cm−1, for the vibrations frequency of Ta atoms in D-type crystallographic voids in β-rhombohedral boron lattice. Since, Ta atoms are known to prefer accommodation just in D-voids the experimentally detected bands can be identified with localized vibrations of Ta atoms.