Vol 10, No 6 (2016)

Non-standard solutions for polarons in nonlinear lattices are investigated by analytical and numerical methods. These solutions represent polarons with several peaks in the envelope and are bound states of several solitons due to electron–phonon interactions.

The UV absorption spectrum of ethylene during the pulse heating of an ethylene–argon mixture to prepyrolysis temperatures in the range 950 < Т < 1285 K was studied by kinetic spectroscopy in a free-piston adiabatic compression unit. New intense light absorption bands were found at 210 < λ < 260 and 440 < λ < 490 nm. Ab initio quantum-chemical calculations of the mechanism of the thermal cis-trans isomerization of ethylene were performed. When this mixture is heated, the point symmetry group of the ethylene molecule in the ground state S₀ reduces to С₁, which is characteristic for ethylene at the minimum of the S₁ state, due to the second order Jahn–Teller effect.

Optical absorption spectra of the Bi⁺ impurity center isomorphically substituting Cs⁺ in CsCdBr3 are recorded over a wide temperature range (from 3.2 to 300 K). An analysis of the vibrational degrees of freedom of the impurity center within the framework of the model of a single effective phonon mode yields various spectroscopic parameters of the optical transition in the Bi⁺ ion, such as the zero-phonon transition energy, Huang–Rhys parameter, and effective phonon energy. The results are used to compare the properties of the Bi⁺ center in various crystalline matrices.

The thermal decomposition of 2,4,6-triazidopyridine in the melt is studied using thermogravimetry, manometry, mass spectrometry, and IR spectroscopy. In the temperature range of 120–160°C, the process obeys the first-order kinetic law, being described by the Arrhenius equation k [s^–1] = 10^{12.8 ± 0.4}exp[–(31200 ± 1500)]/RT with values of the parameters typical of the thermal decomposition of aromatic and heterocyclic azides. The reaction produces nitrogen, as the only gaseous product. Unlike the other heterocyclic azides, the decomposition of which is characterized by anomalously high values of the Arrhenius parameters, the thermal decomposition of 2,4,6-triazidopyridine yields a condensed product having a system of polyconjugated bonds with higher force characteristics. It is concluded that the decomposition of 2,4,6-triazidopyridine proceeds by a mechanism in which the rate-limiting step is the dissociation of the nitrogen molecule from the azide group to form a nitrene.

The results of detailed kinetic simulations of the formation of soot particles in the pyrolysis of n-hexane–argon mixtures and in the oxidation of fuel-rich (φ = 5) n-heptane–oxygen–argon mixtures behind reflected shock waves at pressures of 20–100 bar and a constant concentration of carbon atoms or a constant fraction of argon in the initial mixture within the framework of a modified reaction mechanism are reported. The choice of n-hexane and n-heptane for examining the effect of pressure on the process of soot formation was motivated by the availability for these hydrocarbons of experimental measurements in reflected shock waves at high pressures (up to ~100 bar). The temperature dependences of the yield of soot particles formed in the pyrolysis of n-hexane are found to be very weakly dependent on pressure and slightly shifting to lower temperatures with increasing pressure. In general, pressure produces a very weak effect on the soot formation in the pyrolysis of n-hexane. The effect of pressure and concentration of carbon atoms in the initial mixture on the process of soot formation during the oxidation of fuel-rich n-heptane mixtures behind reflected shock waves is studied. The results of our kinetic simulations show that, for both the pyrolysis of n-hexane and the oxidation of fuel-rich n-heptane–oxygen mixtures, the influence of pressure on the process of soot formation is negligible. By contrast, the concentration of carbon atoms in the initial reaction mixture produces a much more pronounced effect.

The interaction of iodine as electron acceptor with nortriptyline and imipramine drugs as electron donors has been investigated spectrophotometrically at various temperatures in chloroform and dichloromethane solutions. The observed time dependence of the charge–transfer band and subsequent formation of in solution were related to the slow transformation of the initially formed iodine: drug outer complex to an inner electron donor–acceptor (EDA) complex, followed by fast reaction of the inner complex with iodine to form a triiodide ion. The pseudo-first-order rate constants and activation parameters for the transformation process were evaluated from the absorbance-time data. Stoichiometrices of the complexes were defined by the Job’s method of the continuos variation and obtaind 2: 1 for iodine: drug complexes. The formation constants and molar absorptivities were evaluated from the absorbance-mole ratio data. Thermodynamic parameters of the complexes have been determined from the temperature dependence of the stability constant by Van’t Hoff equation.

The results of experimental studies of the effect of the shape of an organic water–coal fuel (OCWF) particle on its ignition delay time and the time of its complete burnout in a hot air flow are reported. Three most common shapes of real particles, such as spherical, ellipsoidal, and irregular-polyhedron-like, are considered. It is shown that the shortest ignition delay time and the time of complete burnout correspond to polyhedron- shaped OCWF particles. Conditions are identified under which this factor significantly influences the ignition characteristics. The experiments were carried out at initial particle sizes (averaged maximum values) of 0.5–5 mm and temperatures and velocities of the oxidant flow of 600–900 K and 0.5–5 m/s, respectively. The main components of the studied fuels were coal processing wastes and waste motor, turbine, and transformer oils.

A comparative experimental and theoretical study of the initiation of silver azide (SA) single crystals and pressed pentaerythritol tetranitrate (PETN)–metal nanoparticles by a neodymium laser pulse is performed. The main differences in the explosive decomposition of the samples are associated with the absence of the induction period and the presence of subthreshold effects in the initiation of PETN-based composites. By contrast, the initiation of SA single crystals always features an induction period, but no subthreshold effects. It is shown that the observed differences in the explosive decomposition are due to the fact that SA single crystals decompose by the chain explosion mechanism, whereas pressed PETN–metal nanoparticles samples, by the thermal explosion mechanism in the micro-hotspot mode. The kinetic parameters of the initiation of the decomposition reaction calculated within the framework of the existing model are consistent with the available experimental data. An experimental criterion for distinguishing between the chain and thermal (in micro-hotspot) mechanisms of the initiation of an explosion under the action of a laser radiation pulse is formulated, according to which the absence of the induction period and a pronounced manifestation of subthreshold effects are indicative of a thermal explosion, whereas the presence of the induction period and the absence of subthreshold effects are characteristic of a chain explosion.

The mechanical behavior and physicochemical transformations in a number of halide vinyl polymers and hydrogen-free inorganic oxidizers subjected to a mechanical impact are studied experimentally and theoretically to gain insights into explosive-like reactions in solids. Methodological issues of testing the explosibility of low-energy materials are considered.

An analysis of conditions for determining the thermophysical characteristics of energetic materials by the laser pulse method is performed. Based on the results of numerical solution of the heat conduction problems for a sample of a material irradiated with a short heating laser pulse corresponding to actual experimental conditions, the time dependences on the sample surface temperature are determined. The heat pulse duration required to determine the thermophysical properties of materials and the ignition delay time are compared. It is shown that the determination of the thermophysical characteristics of a typical energetic material by the laser pulse method is possible at pulse durations of no longer than 0.24 s.

The effect of the oxygen content in the oxidizing medium (O₂–N₂ mixture) on the characteristics of the combustion of gaseous fuel (hydrogen, methane)–oxidizing medium–hydrofluorocarbon (trifluoromethane CHF₃, pentafluoroethane C₂HF₅, or perfluorobutane C₄F₁₀) is experimentally studied. The oxygen concentrations in the oxidizing medium are 15, 20.6 (air), and 25 vol %. The dependences of the maximum explosion pressure ΔP_max, maximum explosion pressure rise rate (dP/dt)_max, and laminar burning velocity Sui on the hydrofluorocarbon content at various oxygen concentrations in the oxidizing medium are determined. It is shown that, at a relatively low deterrent content C_d (up to 30–50% of the minimum inhibitory concentration of the hydrofluorocarbon, i.e., up to 30–50% of the hydrofluorocarbon content at the “peak” point of the inhibition curve), the maximum explosion pressure is only slightly dependent on C_d. It is demonstrated that the larger the molecule of the hydrofluorocarbon, the higher is its inhibitory ability in terms of (dP/dt)_max and S_ui. A qualitative interpretation of the results is given, based on the concept of additional heat release in the flame front by reactions involving hydrofluorocarbons.

Film systems based on chitosan with different molecular weights (334 000 and 113000 Da) and amikacin, an aminoglycoside series antibiotic, are studied. The Ritger–Peppas, Hixson–Crowell, and Hopfenberg equations for describing the kinetics of drug release are tested. It is demonstrated that the release of amikacin from chitosan acetate films under conditions of concurrent diffusion and dissolution (hydrolysis) of the polymer matrix are more closely described by the Hopfenberg equation. It has been established that the isothermal annealing of the film causes a decrease in the kinetic constants characterizing the rate of drug release from the polymer matrix.

An approach based on quality reduction of the solvent, namely, the use of mixed water–ethanol solvents of variable volume ratio was used for control over the aggregation and formation of a physical net of bonds in chitosan succinamide solutions. A study of the molecular and supramolecular organization of chitosan succinamide in solution showed that this polymer, which does not aggregate in a dilute (to the crossover point) aqueous solution, aggregates in a mixed water–ethanol solvent even at high dilution. Polymer structurization in solution is accompanied by the formation of an additional net of physical bonds, whose nodes are aggregates of chitosan succinamide macromolecules formed in the mixed solvent. It leads to earlier formation of viscoelastic properties, which is ultimately reflected by the regularities of some physicochemical properties of materials formed from solution.

Molecular dynamics methods are used to study the kinetics of the migration of impurity atoms due to the diffusion of vacancies on a honeycomb-type lattice. A particular attention is paid to examining how the impurity diffusion coefficient depends on the coverage of vacancies ϑ_v. It is shown that, in the limit of vanishingly small concentration of vacancies, ϑ_v ≪ 1, this dependence is linear, with the simulation results being consistent with the predictions of our analytical theory. With increasing ϑ_v, the diffusion coefficient begins to grow nonlinearly, correlating with the increase in the size of the percolation clusters. Above the percolation threshold, the impurity diffusion coefficient tends rapidly to its value for a surface without obstacles.

The adsorption properties of two types of naphthenic acids (NAs), benzoic acid and cyclohexane carboxylic acid on four-nitrogen coordinated transition-metal (Mn, Fe, Co, Ni, Cu, and Zn) embedded graphene (TMN₄-G) were investigated in detail by means of density functional theory method. The calculation results indicate that NAs prefer the perpendicular adsorption configuration by bonding interactions between their carbonyl oxygen atom and TMN₄ active site, and could be chemisorbed on FeN₄-G, MnN₄-G, and ZnN₄-G. The FeN₄-G gives the strongest adsorption to the NAs, indicating it is the best adsorbent among them. Electron density maps further confirm that NAs are chemically adsorbed on the FeN₄-G surface, accompanied by electron transfer in the adsorption systems. The calculations indicate that benzoic acid has relatively stronger adsorption energy than that of cyclohexane carboxylic acid for the perpendicular adsorption on TMN₄-G surface.