I. V. Kolbanev

Structure of mechanically activated high-energy Al + polytetrafluoroethylene nanocomposites

The structure of high-energy Al/polytetrafluoroethylene nanocomposites prepared by mechanochemical synthesis is studied by X-ray diffraction analysis, scanning electron microscopy, atomic force microscopy, and chemical analysis. It is revealed that the composite consists of aluminum particles with sizes of 100–150 nm separated by the polymer layers. The formation of nanocomposite is accompanied by the accumulation of dislocations with the density ρ = (4 ± 1.5) × 1010 cm−2. Upon the shock-wave initiation of activated samples, Al + (-C2 F4-) → AlF3 + C reaction propagates in detonation-similar regime at supersonic speed. The velocity of detonation is the highest at the stoichiometric component ratio.

Defective structure and reactivity of mechanoactivated magnesium/fluoroplastic energy-generating composites

Mechanical activation has been employed to produce highly reactive energy-saturated Mg/(-C2F4-)n composites, chemical transformations in which are initiated by either heating or shock-wave loading. The structure and reactivity of these composites have been analyzed with the use of X-ray diffraction, microscopy, thermogravimetry, calorimetry, and the measurement of combustion and detonation velocities. Mechanical activation is accompanied by the formation of a magnesium/fluoroplastic composite structure with the intercomponent contact area as large as 6 m2/g and accumulation of chaotically arranged dislocations to concentrations as high as 6 × 1010 cm−2, basal and prismatic deformation stacking faults (the maximum probabilities of their formation are 2.1 and 1.4%, respectively), and boundaries of coherent-scattering regions in magnesium. In fluoroplastic, disordering and partial amorphization of the structure take place. Mechanical activation leads to a dramatic increase in the propagation velocity of Mg + (-C2F4-)n → MgF2 + C chemical reaction in the explosive combustion regime (to 400 m/s) and the development of knocking combustion, in which the reaction propagates at a velocity as high as 1100 m/s. The optimal dose of mechanical activation (7–8 kJ/g), at which the maximum velocity of reaction propagation is reached, has been determined. The use of a “slow” heating in the cell of a calorimeter in combination with the mass-spectral analysis of evolved gases has made it possible to distinguish processes of three types in the thermally activated interaction between magnesium and fluoroplastic. The formation of MgF2 at temperatures below 300°C seems to be due to the interaction between defects in magnesium (dislocations and stacking faults) and macromolecules. The reaction occurring at 300–420°C with a slight thermal effect is caused by the solid-phase interaction between magnesium and fluoroplastic brought in contact with one another. The main contribution to the conversion is made by the processes that take place at temperatures above 420°C and are relevant to the thermal depolymerization of fluoroplastic. The layered structure of the composite and the large area of the intercomponent contact ensure the penetration of gaseous products of depolymerization into the bulk of magnesium particles and the completeness of the interaction.

Defect structure of nanosized mechanically activated MoO3

Defect structure of mechanically activated MoO3 has been studied with the use of X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance, laser granulometry, and adsorption methods. Two stages of mechanical activation have been distinguished. At mechanical activation doses below 1 kJ/g, the fracture of oxide particles is the main process. At this stage, MoO3 particle sizes decrease from 30 μm to 60 nm and specific surface area linearly increases to 30 m2/g, the sizes of coherent-scattering regions decrease to 18 nm, paramagnetic centers are accumulated, and the Raman spectral bands corresponding to three different types of Mo-O bonds widen and shift. At doses above 1 kJ/g, the main process consists in the friction and aggregation of particles, which is accompanied by some reduction in the specific surface area and an increase in the particle sizes. At the stage of friction, the phase transition from an orthorhombic modification to a monoclinic modification of MoO3 occurs seemingly due to a shift of one layer of the material in plane (100). The shift is accompanied by the accumulation of lattice microstrains in the same plane, formation of “stressed” Mo-O-Mo bridge bonds, and a substantial rise in the concentration of Mo5+ radicals. The maximum total concentration of paramagnetic centers is 1 × 1018 g−1. It may be assumed that the radicals are formed due to the rupture of the most stressed molybdenum-oxygen bridge bonds.

Defective structure, plastic properties, and reactivity of mechanically activated magnesium

The genesis of a defective structure (particle size, size of coherent scattering regions (CSRs), dislocation concentrations, and two types of deformation and twin stacking defects (SDs)) of magnesium during its mechanical activation in a vibrating mill in the presence of liquid additions was studied by X-ray diffraction (XRD) analysis, microscopy, and adsorption (BET) method. The dynamic mechanical properties were checked for the activated samples using a K-44-2 vertical impact machine. The ability of magnesium to be oxidized in air was checked by heating it in the cell of a differential scanning calorimeter. At mechanical activation doses of less than 5 kJ/g, the accumulation of chaotically arranged dislocations and deformation SDs was accompanied by an increase in the plasticity of the material. At higher doses, polygonization of dislocations led to a drastic decrease in the CSR size and dislocation run, leading to embrittlement of the material. The changes in the mechanical properties were confirmed by symbatic changes in the outer particle size and showed themselves on the pressure oscillograms during the impulse loading of pressed Mg layers. Mechanical activation led to an increase in the level of oxidation of magnesium with oxygen, but did not affect the temperature of the start of oxidation. A method for activating magnesium with additions was suggested and led to the formation of highly disperse magnesium samples with the oxidation temperature lowered by 150°C.

Thermal relaxation of defects in nanosized mechanically activated МоО3

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.

Structure and reactivity of mechanoactivated Mg (Al)/MoO3 nanocomposites

X-ray diffraction and thermal analyses, microscopy, and specific surface area measurements are used to study the formation, structure, and reactivity of mechanoactivated Mg/MoO3 and Al/MoO3 nanocomposites during slow heating (10°C/min). The optimal mechanoactivation dose is determined. The mechanoactivated Mg/MoO3 composite is a dense mixture of two nanosized components with a contact surface of ~8 m2/g (upper estimate). The area of the contact surface between the components of the Al/MoO3 composite is less than 2 m2/g, with the sample consisting of micron-sized aluminum flakes coated with nanoparticles oxide nanoparticles. When heated, the Mg/MoO3 system explodes, with the temperature of explosion being determined by the heating conditions. The minimum temperature of conversion is ~250°C, close to the temperature of autoignition of fuel–air mixtures promoted by these additives. The Al/MoO3 system is characterized by a phased progress of the reaction in the temperature range of 200 to 1000°C. The reasons for the differences in the reactivity of the mixtures are discussed.

Promotion of the self-ignition of fuel–air mixtures with mechanoactivated Al (Mg)–MoO3 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.

Kinetics of mechanical activation of Al/CuO thermite

AbstractnThe general aspects of the mechanical activation (MA) of Al/CuO thermite compositions based on micron-sized particles and nanopowders of the starting components have been analyzed using X-ray diffraction and hydrogen titration. The latter method has been employed to evaluate the amount of residual oxygen in CuO and Cu2O from the weight loss during heating in H2. The reactivity of the activated mixtures was assessed using DSC and TG in combination with mass spectrometric analysis. In addition, we have measured the ignition temperature, burning velocity, and brightness temperature of the reaction products. The results demonstrate that mechanical activation leads to the fragmentation of the components, mixture homogenization, and the formation of a composite, producing “weakly bound” oxygen in CuOn and causing partial reaction between the components. The total exothermic heat effect in DSC scans, burning velocity, and brightness temperature as functions of specific milling dose (D) have an extremum. The highest reactivity is observed near Du2009=u20092xa0kJ/g, where a sufficient defect density in the components and good mixture homogenization are ensured, but the degree of MA-induced conversion does not exceed 10%. The burning velocity then reaches 400–700xa0m/s, and the brightness temperature is 3400–3800xa0°C. The milling dose dependence of the self-ignition temperature has no extremum. The self-ignition temperature steadily decreases with increasing milling dose, even though the ignition knock “power” falls off. The use of nanoparticulate starting components does not appear reasonable.