Thermal relaxation of defects in nanosized mechanically activated МоО<Subscript>3</Subscript>

The regularities of the thermal relaxation of structural defects (paramagnetic centers and microdistortions), as well as the sizes of coherent-scattering regions and the external surface, of mechanically activated МоО₃ have been studied with the use of X-ray diffraction, electron paramagnetic resonance, and adsorption/desorption methods. It has been revealed that heating of activated samples at temperatures below 450°C is accompanied by the death of paramagnetic centers, annealing of microdistortions, and liberation of molecular oxygen. It has been assumed that oxygen results from the rupture of deformed Mo–O–Mo bridge bonds formed by its atoms. Above 450°C, recrystallization processes occur, which are accompanied by an increase in the sizes of the coherent-scattering regions and the MoO₃ (monoclinic) → MoO₃ (orthorhombic) phase transition. The thermal stability of the external particle surface depends on mechanical activation conditions. For samples activated at early stages of activation (fracture regime), the specific surface area decreases by more than an order of magnitude, when a temperature of 450°C is reached. At higher activation doses (friction regime), the sample is not sintered in the same temperature range.     

Mechanical activation of aluminum: 2. Size,shape, and structure of particles

The structure of active Al/C composite prepared by the mechanochemical method from aluminum and graphite powders (10–30 wt % C) is studied by scanning and transmission electron microscopies, atomic force microscopy, local elemental analysis, X-ray diffraction, as well as by adsorption and sedimentation measurements. It is established that the particles of chemically active Al/C composite represent porous plate-like aggregates with a mean size smaller than 2.5–5 m and composed of nanocrystalline aluminum blocks with a size of 15–60 nm distributed in a loose amorphous carbon. Single aluminum particles with a size of up to 10 nm are also observed; however, their weight fraction is small.Translated from Kolloidnyi Zhurnal, Vol. 66, No. 6, 2004, pp. 819–828.Original Russian Text Copyright © 2004 by Streletskii, Pivkina, Kolbanev, Borunova, Leipunskii, Pshechenkov, Lomaeva, Polunina, Frolov, Butyagin.     

Mechanisms of reactions between > Si=O groups and CO2, N2O, and HC≡CH molecules: An experimental and quantum chemical study

IR spectroscopy and quantum chemical calculations are used to study the directions and kinetics of reactions between silanone groups (≡Si-O)₂Si=O and CO₂, N₂O, and acetylene molecules. IR bands are assigned on the basis of the calculation of vibrational spectra of model low-molecular systems. Quantum chemical methods are used to obtain the data on the shapes of potential energy surfaces of these systems (intermolecular complexes and transition states). These data are used to interpret kinetic data. The silanone group is inclined to the formation of relatively stable (~10 kcal/mol) intramolecular complexes with CO₂, N₂O, and acetylene molecules. Their geometries and electronic structures are determined.     

Thermal Transformations in Mechanically Activated MeOx/C Systems (Me = Mo,Mn, Bi,and V)

Colloid Journal - Thermogravimetry and calorimetry in combination with mass spectroscopy, as well as X-ray diffraction, have been employed to study thermal transformations in mechanically activated...     

Promotion of the self-ignition of fuel–air mixtures with mechanoactivated Al (Mg)–MoO<Subscript>3</Subscript> particles

The ignition delay times of heptane–air and diesel oil–air mixtures with and without additives of mechanoactivated Mg–MoO³, Al–MoO³, and Mg–fluoroplastic nanopowders are measured using a rapid-mixture-injection setup. At temperatures below a certain threshold value, the metal–MoO³ additives produce practically no effect on the ignition delay time, whereas at higher temperatures, these additives sharply reduce the ignition delay time, down to the resolution time of the experimental method. The promoting efficiency of the small heterogeneous additives tested is many times superior to that of the known homogeneous promoters. Magnesium–fluoroplastic additives are demonstrated to produce no promoting effect on the ignition of the fuel–air mixtures studied. The mechanism of the action of the heterogeneous additives on the gasphase self-ignition of fuels is discussed.     

Interactions in the NdF<Subscript>3</Subscript>-Nd<Subscript>2</Subscript>O<Subscript>3</Subscript>-MF<Subscript>2</Subscript> (M = Ba,Sr) systems

Isothermal anneals (at 873 K) and powder X-ray diffraction were used to study isothermal sections of phase diagrams of the NdF₃-Nd₂O₃-MF₂ (M = Ba, Sr) systems. In studying the NdF₃-Nd₂O₃-BaF₂ system, classical solid-phase synthesis was supplemented with mechanochemical activation of feedstock mixtures or BaF₂ activated with gaseous hydrogen fluoride was used. In both systems, a solid solution with the fluorite structure based on MF₂ and NdOF phases, a solid solution with the tysonite structure based on NdF₃, and an ordered fluorite-related phase Ba₄Nd₃F₁₇ were found. The NdOF-based solid solutions were shown to have polymorphism: β_trig ai α_cub at ≈800 K; a new trigonal phase of these solid solutions has been discovered. The effect of a dimensional factor $\left( {R_{Ba^{2 + } } > R_{Sr^{2 + } } } \right)$\left( {R_{Ba^{2 + } } > R_{Sr^{2 + } } } \right) on phase formation and unit cell parameters of the solid solutions was traced.     

Detonation in an aluminum-Teflon mixture

Detonation in an aluminum-fluoroplastic-4 (Teflon) mixture is studied experimentally. To increase reactivity, the initial mixture is pretreated in a mechanochemical activator. As a result, a mechanically activated composite is obtained in the form of thin aluminum layers in a Teflon matrix. The action of a shock wave on a composite sample initiates the steady detonation regime, in which the initial and final substances are in the condensed state. Depending on the percentage composition and density of the mixture, the detonation velocity varies from 700 to 1300 m/s for the speed of sound below 100 m/s in the initial composition. The steady detonation velocity changes insignificantly when sample pores are filled with helium instead of air. The results prove that it is possible in principle to reach the steady detonation regime in reactive condensed mixtures forming final reaction products in the solid state.     

Mechanochemical Preparation of Highly Dispersed MeOx/C Composites as Materials for Supercapacitors and Ion Batteries

Colloid Journal - Potential of mechanochemical methods has been studied as applied to the development of promising materials of electrodes for supercapacitors with MeOx/C composition (Me = Mo, Mn,...     

Explosive compositions based on the mechanoactivated metal-oxidizer mixtures

Detonation-like regimes in mechanoactivated energetic composites (MAECs) were experimentally studied. The test MAECs consisted of layers of a metal (Al, Mg) and Teflon mixed at the submicron and nano levels. The systems reacted to form solid final products. MAECs are appreciably superior to ordinary mixtures in chemical transformation rate. The burning of MAECs occurs in an explosive regime at a velocity of 300–400 m/s, with the temperature of the products being as high as 4000 K. When initiated with a HE charge, porous MAECs detonate in the steady regime. Depending on the composition and density of the samples, the detonation velocity varies from 700 to 1300 m/s, values markedly higher than the speed of sound in the initial mixture. Detonation is controlled by a hot spot mechanism, more specifically, by relay reaction propagation by jets of products.