Home – Home – Jornals – Chemistry for Sustainable Development 2016 number 2

Oxides of the composition Ba_0.5Sr_0.5Co_{0.8 - x} Mo _x Fe_0.2O_{3 - d} (BSCFM) (0 < x < 0.15), obtained by partial substitution of cobalt in BSCF structure by multicharged molybdenum cations, were studied. It was shown that doping by Mo⁺⁶ cations ( x > 0.06) leads to the formation of the heterophase system cubic perovskite-double perovskite. With the help of diffraction in situ and ex situ methods, the processes involved in the interaction of cathode BSCFM5 material with the electrolyte CGO material were studied.

Relying on the analysis of experimental data on the structural changes in hexagonal cobaltate YBaCo₄O₇ under saturation with oxygen up to the composition of YBaCo₄O₈ and YBaCo₄O_8.4, significant atomic groups in Co sublattice were marked out for generalized description of the existing distortions. The sequence of structural transformations accompanying oxygen insertion is clearly presented; the maximal saturation degree that is possible within the framework of this model is estimated.

Perovskite-like compounds with the general formula La_{1 - x} Bi _x Mn_{1 - y} M _y O_{3 + } (M = Fe, Ni, Cu; x = 0.1-0.3; у = 0.0-0.2,  х =  у = 0.1) were obtained by solid-phase synthesis and characterized by means of XPA, TG and dilatometry. For solid solutions with rhombohedral structure, the existence of two regions in the linear dependence of unit cell volume on temperature was explained by thermal expansion of samples.

Results of the synthesis of multilevel nanometre-sized cermets with interpenetrative structure based on -Bi₂O₃/Ag, for selective medium-temperature oxygen membranes needed for novel clean coal-based power engineering are presented. In particular, the size of crystallites of the components was decreased to 10-20 nm; the composition of the ceramics was improved by adding Pr; silver is doped heterogeneously by fluorite, with the conservation of compatibility and with increased fraction of silver in cermet with interpenetrative structure; nanocermets are modified with Pd. In spite of the significant improvement of the composite characteristics in comparison with known cermet 60Bi_0.75Er_0.25O_1.5/40Ag, the problem of making a real material for membranes has not been solved. Membrane degradation in all positions at T ~ 600 °C comes to the forefront. Along with optimization of the composition and method to obtain membrane material, it is proposed to decrease the working temperature to T ~ 500 °C.

Aluminium oxide systems were studied using a set of X-ray diffraction methods: Rietveld method, modelling of diffraction patterns of one-dimensionally disordered materials, modelling of fine dispersed substances, method of the radial distribution of electron density. Modelling of diffraction patterns of aluminium hydroxide and the curves of the radial distribution of electron density of oxides and hydroxides was carried out. Using the data of previous modelling, the real structure of aluminium hydroxides with different physicochemical properties and oxides obtained from them was studied. The results may be used in the analysis of any real object containing aluminium oxide systems, first of all nanometre-sized ones.

Nanopowders of α-Al₂O₃ were synthesized through crystallization of aluminium hydroxide gel with nanoparticles introduced into it as seedings. Depending on the gel crystallization conditions, the powder consists either of weakly aggregated spherical particles or of polygons with irregular shapes. It was established that particle shape is determined by the composition of the gas phase during crystallization, in particular by water vapour pressure. The temperature at which the gel is completely crystallized into the α-phase depends on the content of micro-impurities in the solution from which the gel is formed. Under optimal synthesis conditions, α-Al₂O₃ nanopowders with particle size about 100 nm are reproducibly obtained. The powders exhibit good moldability and sintering ability to form almost pore-free ceramics at 1300 ^oC.

Comparative analysis of two methods EXAFS and PDF to study the phase composition and local structure (interatomic distances and coordination numbers) of materials including nanomaterials was carried out. The shortcomings and advantages of the methods are considered.

Effect of mechanical activation (MA) of hexagonal boron nitride ( h BN) powder with different intensities in attritor and in a planetary ball mill (PBM) on the structure and phase composition of materials based on this compound was studied. Mechanical activation of h BN powder was carried out in order to decrease the parameters of synthesis of cubic boron nitride ( c BN) due to the formation of nanostructure in h BN. The stoichiometry of h BN is observed to decrease during MA due to partial decomposition. Mechanical treatment of h BN powders in PBM proceeds more intensively and leads to the formation of particles with crystallite size 5-10 times smaller than the size of crystallites formed during treatment in attritor. The specific surface increases by a factor of 3-6. After agglomeration of powders after MA at a pressure above 7 GPa, the phase composition of the resulting material is independent of the intensity of MA and is characterized by the presence of the cubic phase of BN alone.

Results of chemisorption measurements and experiments on hydrogen thermodesorption carried out with Pt/γ-Al₂O₃ samples with Pt nanoclusters (size <1 nm) are presented. The reasons of the reversible changes of adsorption properties were studied. The results are compared with the data for platinum nanoparticles (with the size mainly 2-3 nm) deposited on the “inert” oxide (SiO₂). The high sensitivity of chemisorption methods and temperature-programmable desorption to the changes in the state of supported metal clusters during thermal treatment of the samples in reducing and neutral media was demonstrated. It was shown that hydrogen adsorption on ultrafine systems is activated, and the adsorption capacity of samples determined from oxygen-hydrogen titration may decrease during multiple titration procedures. A strong effect of Н₂О on hydrogen thermodesorption from Pt/γ-Al₂O₃ samples reduced at high temperature was observed. The obtained results are interpreted taking into account the structural changes of nanoclusters in the low-temperature region and the formation of oxygen vacancies at the metal - support interface under the action of H₂ at high temperature.

Intra-mold modification of 110G13L steel was carried out using various dispersed modifying agents containing boron, tungsten, titanium carbides, ferrotitanium, and carbon as the active phase. To improve wettability, modifying agents were treated preliminarily with copper powder in AGO-3 planetary centrifugal mill. It was discovered that the microstructure of the formed steel samples is composed of austenite and carbides released inside the grains and along their boundaries. A significant increase in the point of maximal load by 14.5, 18.0 and 9.0 % was demonstrated by the steel samples treated with the modifying agents containing boron carbide (concentration of the active phase: 0.056 %), a mixture of tungsten and titanium carbides (0.033 %) and titanium carbide (0.083 %), respectively. In addition, the use of a mixture of tungsten and titanium carbides allowed us to increase relative elongation by 40 %. The use of the modifying agents containing boron carbide and a mixture of tungsten and titanium carbides promotes uniform distribution of carbides and a substantial decrease in grain size; the grain size (according to the State Standard GOST 5639) corresponds to two points (the actual grain diameter is 0.387-0.694 and 0.338-0.581 mm, respectively). The grain size in the reference sample is more than three points (the actual grain diameter is 2.325-3.168 mm). The grain size in other modified steel samples corresponds to three points and more.

Structural-chemical changes in high-silica zeolite-containing rocks from the Kholinskoye and Shivyrtuy deposits (Russia) under the action of mechanical activation in a laboratory vibrogrinder were studied by means of differential scanning calorimetry, thermogravimetry, densimetry, air permeability measurement, atomic emission spectroscopy with inductively coupled plasma, infrared spectroscopy, X-ray phase analysis. The kinetic parameters of dehydration of initial and mechanically activated natural zeolites were calculated (apparent activation energy, pre-exponential factor), as well as their thermodynamic potentials (enthalpy of formation, entropy and Gibbs energy), and disordering degree. The degree of carbonization of the samples under study was estimated with the help of the analysis of infrared spectra and thermogravigrams. It was demonstrated that the change of the reactivity of natural zeolites as a result of mechanochemical treatment is determined by the phase composition off initial samples.

Nickel boride Ni₃B is a promising material for making self-regulated heating elements. In the present work, the synthesis of nickel boride Ni₃B and its compaction were carried out for the first time by means of electric spark agglomeration. Two approaches were considered for obtaining sintered materials based on Ni₃B: 1) reactive electric spark agglomeration of the mixture of 3Ni + B subjected to preliminary mechanical treatment; 2) obtaining Ni₃B by means of thermal explosion in mechanically treated powder mixture, followed by compaction of the resulting product with the help of electric spark agglomeration. It was established that the material based on Ni₃B, obtained by reactive agglomeration, is characterized by lower content of admixture phase Ni₂B and higher relative density and microhardness in comparison with the material obtained by sintering the product of thermal explosion at the same temperature. Specific electric conductance of agglomerated materials based on Ni₃B is about 10^-3 Ohm x cm.

The samples of MoS₂ with added small amounts of polar liquids were synthesized for the first time by means of soli-phase mechanical activation of commercial molybdenite. Physicochemical characteristics of the catalysts are discussed that have been measured by means of sedimentation analysis, XSA, XPES, TEM, thermal analysis. The high activity of the catalyst with small added amount of methanol was discovered in the model reaction of dibenzothiophene hydrogenolysis. It is demonstrated that the level of detect content does not determine the hydrodesulphurising ability of monocatalysts in the model reaction. Interconnection between the physicochemical characteristics of the catalysts and their activity in the model reaction was studied. Conclusions concerning the reaction routes were made on the basis of analysis of the composition of hydrodesulphurising agents by means of gas chromatography-mass spectrometry.

The interaction of solid bismuth oxohydroxonitrates having the composition [Bi₆O₄(OH)₄](NO₃)₆ x 4H₂O (I), [Bi₆O₅(OH₃)](NO₃)₅ x 3H₂O (II) and [Bi₆O₄(OH)₄](NO₃)₆ x H₂O (III) with the aqueous solutions of citric acid was studied by means of X-ray phase analysis, electron microscopy and atomic emission spectroscopy. The conditions for the formation of bismuth citrate having the composition BiC₆H₅O₇ were determined.

Mechanochemical synthesis of anhydrous sodium thiosulphate Na₂S₂O₃ was carried out for the first time by means of mechanical activation of the stoichiometric powder mixture of sodium sulphate (Na₂SO₃) and sulphur (S) in the ball planetary mill “Aktivator-2SL”. By means of iodometric titration, X-ray phase analysis and thermal analysis, it was established that under relatively mild conditions of mechanical activation for about 1 h the degree of transformation in the solid-phase reaction Na₂SO₃ + S = Na₂S₂O₃ reaches 95 %.

For steel and copper as example, the processes of structure formation are considered, taking place under the conditions of intense plastic deformation which is initiated by different kinds of dynamic loading in the zone of contact between the metals of different kinds. It is shown that intense plastic deformation promotes an increase in mutual solubility metals in each other and the formation of atomically ordered phases. In the case of ultrahigh pressure, the formation of atomically ordered phases was not detected. This is connected with the fact that the free atomic volume in the liquid phase, formed in the cumulative jet, exceeds the free volume during solid-phase interaction. A set of the models of structure formation under the gradient conditions can be united within the framework of the scheme “shear-turn”.

Effect of the nature of decomposition of the particles of cobalt azide deposited in the pores of carbon matrix on the phase composition and particle size characteristics of the products is considered. Porous carbon material of Sibunit type, of TU-145 grade (pore diameter 10-15 nm) was used as the matrix. The studies were carried out by means of X-ray diffraction, X-ray fluorescence analysis, complexometry, and thermogravimetry. It was established that mainly the Co₃O₄ phase is formed as a result of the slow decomposition of cobalt azide nanoparticles, while the product of explosive decomposition contains the metal phase and CoO.

Dependencies of the critical energy density of the initiation of explosive decomposition of PETN-Co nanocomposite on nanoparticle radius and the duration of the pulse of the first harmonics of neodymium laser were calculated using the microfocal model of thermal explosion. For each pulse duration, the minimal values of the critical energy density for the initiation and the corresponding radii of nanoparticles were determined. The calculation was carried out in two versions: 1) with the coefficient of absorption efficiency equal to 1, which is independent of nanoparticle radius; 2) taking into account the dependence of absorption efficiency coefficient on the radius of cobalt nanoparticle in a transparent matrix with the refractive index equal to 1.54 (PETN), calculated within the framework of Mie theory. For pulse duration shorter than 100 ns, calculation results differ substantially: neglect of the regularities of absorption by cobalt nanoparticles leads to a decrease in the minimal energy density by a factor of more than 17 for pulse duration 0.1 ns, while taking into account the dependence of the coefficient of absorption efficiency on particle radius, the energy threshold in this range decreases only by a factor of 3. The account of the dependence of laser pulse absorption efficiency coefficient on nanoparticle radius allows solving the paradox of small particles.

Products of the interaction of diphenolic compounds (diph) hydroquinone, resorcinol, pyrocatechin with silicon oxide with the formation of chains of the general formula diph-(Si(OH)₂-diph) _n were studied by means of computer modeling. For each compound, inertia radius and other spatial characteristics were calculated; the fractal dimensionality was determined within the row of compounds. It was concluded on the basis of the results that for the compounds of hydroquinone and resorcinol series the shape of the polymer chain is a rod, while for the pyrocatechin series its shape is a stochastic clew.

Supramolecular structure of arabinogalactan and its complexes with betulin diacetate was studied. Using the methods of small-angle X-ray scattering and laser ablation, the presence of arabinogalactan associates in solution was demonstrated. The formation of these associates may proceed due to the interaction either with water molecules or with betulin diacetate molecules.

Effect of mechanical activation of brown and oxidized coal in the presence of alkaline reagent and abrasive material in different concentrations was demonstrated. Solid-phase mechanochemical reactions of humic acids with the salts of heavy metals were carried out. The features of structural transformations of humic acids during mechanical activation were revealed.

Effect of the conditions of mechanochemical activation on the efficiency of enzymatic hydrolysis of carbohydrates incorporated into highly lignified plant raw material - reed stem biomass was studied. Complicated supramolecular structure and high lignin content in the structure have a negative effect on enzymatic hydrolysis. The model conditions of fragile grinding and plastic deformation of the material under different temperature of mechanical activation are considered. The samples obtained by mechanical activation of lignocellulose material are characterized by increased reactivity in enzymatic hydrolysis due to an increase in the surface area and disordering of the crystalline regions in cellulose. Optimal conditions for mechanical activation of plant raw material for obtaining highly reactive products were established.