Ab Initio Calculations of New α-L5-7a and β-L5-7a Graphyne Polymorphic Varieties

Calculations of the structural and energy parameters, band structure and density of electronic states of new structural varieties of graphyne have been performed by the density functional theory method. The initial structure of the nine polymorphs was theoretically constructed on the basis of the 5-7a graphene layer. As a result of the calculations, the structure of only five graphyne layers was found to be stable: α-L5-7a, β1-L5-7a, β2-L5-7a, β3-L5-7a and β4-L5-7a. The structure of layers γ1-L5-7a, γ2-L5-7a, and γ3-L5-7a is transformed into the structure of graphene layers by geometric optimization, and the graphyne layer γ4-L5-7a is transformed sp+sp2 layer L3-6-13. The sublimation energy of the stable graphyne polymorphs varies from 6.66 to 6.78 eV/atom. The density of electronic states at the Fermi energy level for all α-L5-7a and β-L5-7a layers of graphyne is different from zero, so the new graphyne polymorphs should have metallic properties.

SIMULATION OF POLYMORPHIC VARIETIES OF HEXAGONAL GRAPHENE FUNCTIONALIZED BY HYDROXYL GROUPS

Моделирование кристаллической и электронной структуры слоев гексагонального графена, на поверхность которых были химически адсорбированы гидроксильные группы, было выполнено методом теории функционала плотности. В результате расчетов была установлена возможность устойчивого существования пяти структурных разновидностей COH - L слоев. Слоевая плотность изменяется от 1,62 до 1,72 мг/м. Длина водород-кислородной связи варьируется в диапазоне от 1,046 до 1,079 Å, а углерод-кислородной связи - от 1,455 до 1,465 Å. Ориентация O - H связей относительно плоскости слоев может варьироваться в зависимости от выбора элементарной ячейки слоя. Минимальной энергией сублимации и равной 18,69 эВ/(COH) обладает слой COH-L - T4, а максимальную энергию сублимации 18,93 эВ/(COH) имеет слой COH - L - T1. Электронная структура всех COH слоев характеризуется наличием прямой запрещенной зоны на уровне энергии Ферми, изменяющейся в диапазоне от 3,02 до 4,56 эВ. Computer simulation of the crystal and electronic structure of hexagonal graphene layers, on the surface of which hydroxyl groups, chemically adsorbed, was performed by the density functional theory method. As a result of calculations, the possibility of the stable existence of five structural varieties of COH - L layers was established. The layer density varies from 1,62 to 1,72 mg/m. The length of the hydrogen-oxygen bond varies in the range from 1,046 to 1,079 Å, and the carbon-oxygen bond-from 1,455 to 1,465 Å. The orientation of the -OH bonds relative to the surface of the layers can vary depending on the choice of the unit cell of the layer. Layer COH - L - T4 has the minimum sublimation energy equal to 18,69 eV/(COH), and layer COH - L - T1 has the maximum sublimation energy 18,93 eV/(COH). The electronic structure of all COH layers is characterized by the presence of a direct band gap at the Fermi energy level, varying in the range from 3,02 to 4,56 eV.

Закономерности образования карбидов титана и вольфрама из продуктов электровзрывного разрушения проводников

A series of electrical explosions in propane-butane of single and strand-connected Ti and W conductors with various diameters was carried out. Electrophysical characteristics of the explosion revealed that resistive heating of conductors is characterized by two monotonically ascending sections on the voltage – current curves separated by a flat segment (plateau), which corresponds to relatively stable electrical resistivity of refractory metals in the liquid state. The energy deposited by changing the power input into the conductor during its resistive heating, which can be higher or lower than its sublimation energy and can be regulated by changing the external adjustable parameters of the discharge circuit, is a key indicator determining the structural-phase state of destructed and chemically synthesized products after the explosion. The conditions are achieved under which micro- and nano-sized powder products do not contain residual metals and consist of carbide phases completely (TiC with an average microhardness of 29580 MPa as a result of the titanium explosion and a mixture of W2C+WC1-х, in which stabilized high-temperature non-stoichiometric cubic carbide WC1-x dominates, with an average microhardness of 16770 MPa as a result of tungsten explosion).

INVESTIGATION OF NEW POLYMORPHIC VARIETIES OF BORON NITRIDE WITH DIAMOND-LIKE STRUCTURES

В работе проведено теоретическое исследование новых полиморфных разновидностей нитрида бора, имеющих алмазоподобные структуры. В результате расчетов методом теории функционала плотности в градиентном приближении была установлена возможность устойчивого существования четырех новых структурных разновидностей нитрида бора: BN - LA4, BN - LA5, BN - LA6 и BN - LA7 . Энергия сублимации новых BN фаз варьируется в диапазоне от 16,85 до 17,84 эВ/(BN), ширина запрещенной зоны - от 4,34 до 6,07 эВ. Объемная плотность BN полиморфов изменяется от 3,020 до 3,452 г/см. The article presents a theoretical study of new polymorphic varieties of boron nitride with diamond-like structures. As a result of calculations by the density functional theory method in the gradient approximation, the possibility of stable existence of four new structural varieties of boron nitride: BN - LA4, BN - LA5 , BN - LA6, and BN - LA7 was established. The sublimation energy of new BN phases varies in the range from 16,85 to 17,84 eV/(BN), the band gap is from 4,34 to 6,07 eV. The bulk density of BN polymorphs varies from 3,020 to 3,452 g/cm.

Метод определения параметров парного межатомного потенциала

Disadvantages of methods known from the literature for determining 4 parameters of the paired interatomic potential of Mie-Lennard-Jones in relation to crystals are indicated. A new method is proposed for determining the parameters of this potential from the thermoelastic properties of the crystal. In this method the parameters are determined by the best coincidence of calculated values with experimental data: 1) of the sublimation energy of the crystal at zero temperature (T = 0 K) and pressure (P = 0); 2) of coefficient of thermal expansion and isothermal elastic modulus, which were measured at P = 0 and T = 300 K; 3) of the dependence of the isotherm T = 300 K state equation from volume of P(300 K, V). The method was tested on iron and gold and showed good results. By this method also were determined the interatomic potential parameters for refractory metals: Nb, Ta, Mo, and W. The results obtained also made it possible to determine more precisely such properties of these metals as the sublimation energy, the Debye temperature, and the surface energy.

The Formation and Migration Energy of Bivacancy in fcc Metals

In the work we propose a method for determining of the formation energy of bivacancy using molecular dynamics method. The key moment of the method for determining of the formation energy of bivacancy is the use of the value ζ, the minimum work that must be spent to remove one atom to infinity from the kink in the monatomic step on the surface of the crystal, calculated indirectly through the experimental data on the formation energy of the vacancy and the sublimation energy. The energy of migration of bivacancy in the work was determined from the temperature dependence of the diffusion coefficient when one bivacancy was introduced into the calculation block.

Influence of Ultra-Violet Radiation on Sublimation Energy of Silver Chloride (AgCl)

Influence of silver chloride synthesis conditions on its heat of sublimation after ultra-violet irradiation was explored. It was determined that silver chloride synthesized in excess of silver ions after irradiation was characterized by heat of sublimation about 6813 J/g, which meant that the original value increased in ~ 1.8 times. At the same time silver chloride synthesized in excess of chloride ions was characterized by decrease in value of heat of sublimation by ~ 2 times. Growth of heat of sublimation of silver chloride with excess of silver ions is connected with energy expenditure on sublimation due to decrease in proportion of surface which is not covered with silver. Reduction of heat of sublimation of silver chloride with excess of chloride ions is explained by absence of silver solid surface film and catalytic effect of silver klasters formed during the process of irradiation.