A. N. Streletskii

Hallmarks of mechanochemistry: from nanoparticles to technology

The aim of this review article on recent developments of mechanochemistry (nowadays established as a part of chemistry) is to provide a comprehensive overview of advances achieved in the field of atomistic processes, phase transformations, simple and multicomponent nanosystems and peculiarities of mechanochemical reactions. Industrial aspects with successful penetration into fields like materials engineering, heterogeneous catalysis and extractive metallurgy are also reviewed. The hallmarks of mechanochemistry include influencing reactivity of solids by the presence of solid-state defects, interphases and relaxation phenomena, enabling processes to take place under non-equilibrium conditions, creating a well-crystallized core of nanoparticles with disordered near-surface shell regions and performing simple dry time-convenient one-step syntheses. Underlying these hallmarks are technological consequences like preparing new nanomaterials with the desired properties or producing these materials in a reproducible way with high yield and under simple and easy operating conditions. The last but not least hallmark is enabling work under environmentally friendly and essentially waste-free conditions (822 references).

Promising energetic materials composed of nanosilicon and solid oxidizers

A method for production of mechanically activated energetic compositions consisting of nanosilicon and solid inorganic oxidizers is developed. For the compositions prepared, both high-speed burning and detonation are observed. The propagation of the reaction is accompanied by a high energy release, comparable to the heat of explosion of aluminized high explosives. The compositions are highly sensitive to thermal stimuli and capable of rapid deflagration-to-detonation transition. The results obtained in the work suggest that nanosilicon-based formulations as promising energetic materials for a wide range of applications, from initiating compositions in blasting caps to compositions for small charges in microsystem devices.

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.

Combustion and detonation of mechanoactivated aluminum–potassium perchlorate mixtures

The properties of mechanoactivated energetic composites based on aluminum and potassium perchlorate with high rates of self-sustaining chemical reactions under conditions of combustion and detonation are examined. The results of experiments on studying the combustion, deflagration-to-detonation transition, and sensitivity to friction of these composites are reported. The activation duration and aluminum content in the mixture are varied. The experiments on the deflagration-to-detonation transition of mechanoactivated composites are supplemented by the results of numerical simulations. The calculations and experiments on the dynamics of development of a blast wave and on the steady detonation velocity are found to be in qualitative agreement. It is shown that the velocity of the observed process is significantly (by about 40%) lower than the normal detonation velocity obtained from thermodynamic calculations.

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.

The effect of the mechanical activation dose on the defective structure of artificial graphite

Energy parameters (dose D and the work of surface formation) have been determined for the formation of a defective structure as a result of mechanical activation of graphite. Graphite activation has been shown to be a two-stage process: at low doses (D ≤ 20 kJ/g), the disruption and shift of graphite particles are the main processes, which are accompanied by a reduction in particle size, formation of meso- and micropores, and a rise in the BET specific surface area to 450–550 m2/g predominantly due to the development of a slitlike mesoporosity. At the same time, the crystalline structure of graphite is transformed into a turbostrate one with a concomitant increase in the lattice parameter and a decrease in the sizes of coherentscattering regions. The shape of diffraction lines can be described under the assumption that several fractions with greatly different degrees of defectiveness coexist in graphite. At higher doses, turbostrate graphite is transformed into X-ray amorphous carbon with a concomitant decrease in the specific surface area and meso- and microporosity. The defects resulting from the mechanical activation cannot be completely annealed at 2800°C. The main parameter of mechanical activation is the dose of supplied energy D = Jgt (Jg is the specific power consumption, and t is the duration of the activation). The curves describing accumulation of different defects can be represented in the form of a unified dependence on the dose for the Jg and, accordingly, t values varied by more than an order of magnitude (Jg = 1.7–22 W/g).