R. I. Gulyaeva

Thermal expansion and phase transformations of copper sulfides

The microstructure and phase transformations of copper sulfides have been studied by thermal analysis, microstructural analysis, and x-ray diffraction during heating in an inert atmosphere. The unit-cell parameters of Cu1.8S, CuS, Cu1.96S, and a Cu1.8+xS (0 < x ≤ 0.2) solid solution have been determined as functions of temperature.

Composition, Structure, and Thermal Expansion of Ca3Fe4S3O6 and CaFeSO

The iron calcium oxysulfides Ca3Fe4S3O6 and CaFeSO are synthesized by reacting mixtures of CaO and FeS in an inert atmosphere. Their elemental compositions and lattice parameters are determined by x-ray microanalysis and x-ray diffraction. From the temperature-dependent lattice parameters of the oxysulfides (measured up to 1020 K), their thermal expansion coefficients are evaluated.

Thermal expansion of troilite and pyrrhotine in helium and air

High-temperature x-ray diffraction is used to determine the lattice parameters of synthetic troilite (sealed-ampule method) and pyrrhotine prepared via thermal dissociation of natural pyrite. The results are presented in the form of best fit polynomials in temperature. The gaseous environment (helium or air) is found to have a significant effect on the temperature variation of the lattice parameters.

Effect of boron and yttrium on the phase composition and the microstructure of natural Nb-Si composites

The phase formation in Nb-Si composites of a eutectic composition alloyed with 0.2–2.0 at % B and 0.9–6.0 at % Y is considered on model specimens prepared by vacuum arc melting. The phase composition of three-component alloys and the phase transformation temperatures are determined by physicochemical analysis, the specimen density and microhardness have been measured, and the content of alloying elements in the alloys has been determined. It is found that the solidus and liquidus temperatures of the alloyed alloys are almost unchanged within the yttrium and boron concentrations under study, and the difference between the densities of the model specimens and the base Nb-18.7 at % Si is ±1.6%. The introduction of yttrium and boron into the natural Nb-18.7 at % Si composite increases the microstructure dispersion and influences the composition of the strengthening phase: yttrium stabilizes high-temperature silicide Nb3Si at low temperatures, and boron, conversely, catalyzes its decomposition with formation of α-Nb5Si3.

Thermal expansion of CaZnSO

The temperature stability range of the zinc calcium oxysulfide CaZnSO has been determined in an inert atmosphere using high-temperature x-ray diffraction, thermogravimetry, and x-ray microanalysis. The lattice parameters and thermal expansion coefficients of CaZnSO have been measured in the range 298–1170 K and have been represented by best fit equations.

Thermal expansion and phase transformations of natural pyrrhotite

The phase transformations of natural hexagonal pyrrhotite have been studied by high-temperature x-ray diffraction and thermogravimetry during heating in an inert atmosphere, and its unit-cell parameters have been measured during heating to 1060 K.

Thermodynamics and kinetics of thermal dissociation of dolomite

The results of investigations into thermodynamic foundations and kinetics of thermal dissociation of dolomites of Ural deposits are presented. Dolomite samples are characteristic of the fraction and coarseness of calcite inclusions, which determines distinctions in their dissociation kinetics. It is established that dolomites dissociate according to the two-stage model limited by the reaction on a three-dimensional surface at the first stage and kinetic equation having the order close to 0.3 and 0.3 at the second stage.

Thermal magnetic hysteresis of nickel-doped pyrrhotites

The magnetic properties of polycrystalline natural pyrrhotite samples with low-nickel contents and synthesized Fe0.675Ni0.2S and Fe0.475Ni0.4S pyrrhotites are investigated. All of the investigated pyrrhotites are shown to have a thermal magnetic hysteresis in the region of 298–623 K. As a result, there is not only an increase in the difference between the magnetic moment values of the materials after a heating-cooling regime, but also a change in their magnetic state.

Chemism and kinetics of the oxidation of zinc-calcium oxysulfide

The sequence of phase transformations and the kinetics of the solid-phase (heating to 1273 K) oxidation of zinc-calcium oxysulfide CaZnSO with air are determined by thermodynamic, thermogravimetric, mass spectrometric, and X-ray diffraction analyses. The oxidation process is shown to be accompanied by the formation of the CaSO4 and ZnO phases depending on the heating conditions, as well as by the formation of CaO with SO2 evolution. The two-stage oxidation of CaZnSO is interpreted by the Avrami-Erofeev kinetic equations with activation energies of 190 and 422 kJ/mol.

Mechanism and Kinetics of the Thermal Oxidation of Natural Sphalerite

The oxidation of natural sphalerite on heating in an oxidative medium is studied by thermogravimetry coupled with scanning calorimetry, mass spectrometry of released gases, and X-ray powder diffraction analysis. The mechanism of sphalerite oxidation when the particle surface is equally accessible and sulfur dioxide is removed from the reaction zone is the formation of ZnO, ZnFe2O4, and SO2. The process is found to be one-stage, as determined by a nonisothermal kinetic method. The activation energies are from 293 to 317 kJ/mol depending on the model used. Natural sphalerite is oxidized in the kinetic regime, and the rate-determining steps are the formation and growth of new-phase nuclei.