I. D. Kovalev

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.

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.

Self-propagating crystallization waves in the TiCu amorphous alloy

Self-propagating crystallization waves are detected and experimentally demonstrated in the Ti50Cu50 amorphous alloy obtained by the melt spinning (ultrafast quenching) method. High-speed thermographic recording has shown that crystallization waves can appear spontaneously at the heating of an amorphous strip to 300–350°С or at the local initiation by a hot tungsten coil of a small segment of the strip preliminarily heated to 230–250°С. In the former case, the crystallization wave propagates at a velocity of ~7 cm/s; in the latter case, the crystallization wave propagates in a self-oscillation mode at an average velocity of ~1.2 cm/s. The temperature gradient across the wavefront is about 150°С. The samples crystallized in the self-oscillation mode have a characteristic banded structure with a smaller grain in depression regions. The crystallization product in all samples is the TiCu tetragonal intermetallic phase.

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.

SHS Casting of (Mo,W)Si2, (Mo,Nb)Si2, and (Mo,Ti)Si2 silicides: effect of activating 3CaO2 + 2Al additives

Abstract(Mo,W)Si2, (Mo,Nb)Si2, and (Mo, Ti)Si2 silicides and their solid solutions were prepared by the technique of SHS casting under 5 MPa of Ar pressure from thermit-like green mixtures containing the oxides of Mo, Nb, and Ti as well as activating 3CaO2 + 2Al additives. The synthesized composites were characterized by XRD and EDS.

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.

SHS of ultrafine and nanosized MoSi2 powders

Ultrafine MoSi2 powders were prepared by SHS reaction from the elements (under an Ar pressure of 5 atm) followed by optimized diminution process and chemical dispersion (hot alkali etching). Raw SHS products (cakes) were found to contain tetragonal MoSi2 and an admixture of Mo4.8Si3C0.6. Chemical dispersion of ground product yielded MoSi2 powders in the form of porous agglomerates. The precipitates containing 50% MoSi2 and 50% K2MoO4 were separated from etching solutions. The size of MoSi2 particles was below 100 nm.

SHS joining by thermal explosion in (Ni + Al)/Nb/(Ni + Al + Nb) sandwiches: Microstructure of transition zone

Thermal explosion in (Ni + Al)/Nb/(Ni + Al + Nb) sandwiches was found to results in formation of (a) NiAlNb composite containing NiAl, NiAlNb, and trace amounts of Nb5Ni and (b) good joining with a transition zone.

SHS of pyrochlore-type ceramic matrices for immobilization of actinide-containing nuclear wastes

Mineral-like Y2(Ti1–xZrx)2O7 ceramic matrices for immobilization of actinide–Zr–RE-containing high-level nuclear wastes (HLW) were prepared by SHS method. In experiments, HLW were modeled by a mixture of CeO2, La2O3, ZrO2, MnO2, and Fe2O3 powders. An increase in the HLW content of green mixture decreased the amount of Y2(Ti1–xZrx)2O7 in combustion product and increased that of ZrO2, LaTiO3, and CaTiO3; decreased the fractional substitutionality of Zr for Ti; and increased the product porosity. An increase in combustion temperature and suppression of heat sink during SHS reaction did not diminish markedly the porosity of synthesized ceramics.

Structure and properties of a composite material obtained by thermal explosion in a mixture of Ni + Al + Cr2O3

The synthesis of a composite material by thermal explosion in a reaction mixture of Ni + Al + Cr2O3 was studied. The thermodynamic parameters of combustion of the systems studied were estimated to predict the composition of the inorganic products (condensed or gaseous) of self-propagating high-temperature synthesis and calculate the adiabatic combustion temperature. It is shown that the synthesis process involves competing reactions in the sample volume which are responsible for the formation of a multiphase product. The influence of the content of Cr2O3 in the reaction system on the strength characteristics of the product synthesis was studied. The microstructure of the synthesized samples was examined, and their micro-hardness, toughness and residual porosity were determined. The possibility of obtaining a homogeneous material based on NiAl intermetallic compound containing dissolved chromium and chromium oxide nanoparticles is shown.