V. I. Ponomarev

Effect of mechanical activation on thermal explosion in Ni-Al mixtures

The effect of mechanical activation (MA) on thermal explosion in equimolar Ni-Al mixtures was studied by time-resolved XRD. MA was also found to increase the burning velocity and decrease the ignition temperature. Thermal explosion in non-activated Ni-Al mixtures was found to proceed via formation of liquid (melted) intermediate products, while that in the activated mixtures gave no liquid intermediates.

Ordering of carbon atoms in boron carbide structure

Boron carbide crystals have been obtained in the entire compositional range according to the phase diagram by self-propagating high-temperature synthesis (SHS). Based on the results of X-ray diffraction investigations, the samples were characterized by the unit-cell metric and reflection half-width in the entire range of carbon concentrations. A significant spread in the boron carbide unit-cell parameters for the same carbon content is found in the data in the literature; this spread contradicts the structural concepts for covalent compounds. The SHS samples have not revealed any significant spread in the unit-cell parameters. Structural analysis suggests that the spread of parameters in the literary data is related to the unique process of ordering of carbon atoms in the boron carbide structure.

Thermal decomposition of TiH2: A TRXRD study

Thermal decomposition of SHS-produced TiH2 powder in vacuum at temperatures below 570°C was explored by time-resolved XRD. The process got started with transformation of starting TiH2 into solid solution of hydrogen in β-Ti (β-Ti[H]) and then followed by the polymorphic β-Ti[H] → α-Ti[H] transition and subsequent hydrogen elimination to yield α-Ti. Marked changes in the phase composition of starting TiH2 were found to get started around 450°C.

Dynamics of Phase Formation during Combustion of Zr and Hf in Air

The processes of phase formation taking place during combustion of Zr and Hf in air have been explored by time-resolved XRD (TRXRD) at a time resolution of 0.1 s. In case of Zr combustion, the final product was found to form in two stages: first the tetragonal high-temperature modification of ZrO2 is formed (in the combustion wave) and then the latter undergoes polymorphic transition (behind the combustion wave) into its monoclinic modification. In combustion of Hf, product formation proceeds in three stages: the low-temperature monoclinic modification of α-HfO2 is formed in the combustion wave, which is followed by its transformation into the tetragonal β-HfO2 and backward conversion of the latter into α-HfO2 behind the combustion wave. The obtained data were comparatively analyzed with those reported for combustion of Ti in air.

SHS-produced boron carbide: Some special features of crystal structure

Boron carbides with carbon contents (σ) of 7–24 at. % were prepared by SHS method and their lattice parameters were determined by high-precision XRD analysis. An unusually wide spread in the literature data on lattice parameters of boron carbide as a function of σ was associated with the process of crystal ordering caused by gradual replacement of boron by carbon atoms. For SHS-produced boron carbide, the above spread turned out minimal. Lattice parameter c was found to attain the unusually high values of 12.20–12.31 s was observed at σ = 13.2%. The widths of diffraction lines from boron carbide were found to depend on σ and attain their maximum values at σ = 13.2% when the lattice is most disordered. Our results can make a basis for elaborating the means for regulating the structure/properties of boron carbide.

The mechanism of formation of copper aluminide in the thermal explosion mode

The mechanism of formation of copper aluminides in the thermal explosion mode was studied. A molded mixture of copper and aluminum powders was heated to the self-ignition temperature by the radiation of a tungsten heater. The phase transformations were detected by time-resolved X-ray diffraction (TRXRD). The kinetics of heat release due to the chemical reaction was studied by measuring the temperature of the sample. The macrokinetic stages of the process were revealed, and the apparent activation energy of each stage was estimated.

SHS of boron carbide: Influence of combustion temperature

Boron carbides of varied composition were synthesized through magnesiothermic SHS reaction at temperatures between 1500 and 2500°C and characterized by XRD. Variation in synthesis temperature was found to change the lattice parameters of boron carbide, which was associated with the disordering of the crystal structure.

About Nonuniqueness of Combustion Modes in the Heterogeneous Systems

The multiple chemical reaction paths taking place in the combustion wave of the ternary heterogeneous systems (Ti-N2-02 and Nb-B-02), were experimentally studied. The Time-Resolved X-Ray Diffraction method was used for in-situ investigation of the mechanisms of phase formation during combustion in these systems. It was shown, that variation of the ignition temperature may lead to the significant changes of combustion temperature and velocity. It was experimentally confirmed that, this effect of nonuniqueness of the combustion parameters is a result of change in the chemical reaction path occurring in the combustion front. Analysis of the obtained data showed that this changing of reaction path may be a result of either thermodynamic or kinetic considerations depending on the investigated system.

SHS of single crystals in the B-C-Mg system: Crystal structure of new modification of B25C4Mg1.42 = [B12]2[CBC][C2]Mg1.42

A new modification of B25C4Mg1.42, [B12]2[CBC][C2]Mg1.42, was prepared by magnesiothermic SHS and characterized by XRD. This compound was found to have the following crystallographic parameters: a = 9.626(1), b = 11.329(1), c = 8.966(1) Å, β = 105.80(3)°, V = 940.8(2) Å3, space group P21/b, Z = 4, R = 0.032. SHS-produced crystals exhibited high acid resistance and hardness and can be recommended as a starting compound for synthesis of other modifications of carboboride.

Crystal structure features in a new compound C4B25Mg1.42

The composition of C4B25Mg1.42 crystal obtained by self-propagating high-temperature synthesis was determined using X-ray diffraction. This is the first crystalline structure where all boron atoms in the В12 icosahedron occupy crystallographically independent positions; this circumstance allowed us to analyze the effect of substituents on bond lengths in the icosahedron. The crystal structure features, including the channels filled with disordered Mg atoms and the spread of В—В endo- and exo-bond lengths in the icosahedra, are described. A crystallochemical analysis of pair bonds has been performed for the first time.