Home – Home – Jornals – Chemistry for Sustainable Development 2024 number 5

Many branches of science and technology need new materials. Different methods are used to obtain them. In particular, one of the promising methods used both at the stage of preparation for the thermal synthesis of materials and as an independent method is the mechanical activation of solids. Research carried out by domestic and foreign scientific organisations is devoted to the development of fundamental principles of mechanochemical technologies. This review summarises and formulates some patterns of mechanochemical reactions, such as the reactions of hydrated compounds and mechanically stimulated solid-phase thermal reactions, and indicates the most effective ways of their use. Examples of the synthesis of materials with valuable properties for use in modern technology are presented.

The present review is concerned with advances in the area of hydroxyapatite mechanochemistry. The features of the mechanochemical method for obtaining this mineral, the reactions proceeding in the reaction chamber, the parameters affecting the process efficiency are considered, and the advantages of the method are listed. It has been shown that rapid mechanochemical synthesis of not only stoichiometric hydroxyapatite, but also various options of substitution in its structure within a wide range of concentrations can be carried out in a high-energy planetary mill at a high frequency of vial rotation.

We consider the influence of mechanical activation and radiation-thermal sintering on the synthesis of indium lead tantalate, used in optical electronics and capacitor-based technologies, and the system (1 - х )Pb₂YbNbO₆ + х Pb₂FeNbO₆ ( х = 0.1), the solid solutions of which exhibit high di-, piezo- and magnetoelectric characteristics. The conditions of perovskite and pyrochlore phase formation during both mechanochemical synthesis and subsequent sintering within a wide temperature range with different electron beam exposure times are investigated. To reduce the electrical conductivity of the ceramic, lithium carbonate, which is also a stabiliser of perovskite structure, was introduced into the batch. It has been shown that the characteristics of the final product are determined by the chosen conditions of batch preparation for sintering. The methods of such preparation differing in the order of mechanical activation and the conditions for introducing the components are compared. The optimal conditions have been established that make it possible to obtain piezoceramics with the largest amount of the perovskite phase and maximum density. The prospects of using radiation-thermal sintering on several piezoceramic systems were assessed. It has been established that a number of intermediate phases are formed during radiation-thermal synthesis, though these phases never appeared during conventional sintering. The results obtained in the work suggest that sintering of some lead-containing samples at an accelerator under the action of the electron beam is a promising method to synthesise a monophase complex oxide with perovskite structure. The developed method is of practical interest for obtaining piezoceramics.

The stages of development and application of mechanical activation, mechanochemical and autoclave-based technologies for the purpose of enhancing the efficiency of industrial chemical and technological processes are considered. The development of technologies for processing rare metal raw materials and products containing precious metals is shown, accompanied in parallel by the development of technological equipment based on mechanical activation and autoclave treatment. The developed processes and equipment were assimilated into actual technological charts and transferred for operation at industrial enterprises.

In recent years, new classes of lithium-excess cathode and anode materials with disordered rock-salt structure (DRX), in which any channels for Li⁺ ion diffusion are absent, have attracted increased interest. In DRX cathode materials of Li _y (Me₁Me₂)_{2- y} O₂ composition, Li⁺ and transition metal ions (Me) are equally likely to occupy the same octahedral positions in the lattice, and the diffusion of Li⁺ ions occurs by hopping from one octahedron to another through an intermediate tetrahedron (o-t-o diffusion). The presence of fluorine in oxyfluorides DRX-F (Li_{1+ x} (MeMn³⁺)_{1- x} O_{2- y} F _y , where Me = Ti⁴⁺, Nb⁵⁺) affects the local ordering of Mn³⁺ ions, the stability of the redox couple O^2-/O^- and its contribution into specific capacity. On the other hand, high-entropy oxides (HEO), which are single-phase oxide systems containing five and more cations, were synthesised by mixing five oxides CoO, CuO, MgO, NiO and ZnO in equimolar ratios, followed by thermal treatment at 1000 °C. Stabilisation of single-phase solid solutions Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2O and Li _x (Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)_{2- x} O_{2- x} F _x (0 ≤ x ≤ 1) with a rock-salt crystal structure turned out to be possible due to the large contribution of the entropy of mixing to the Gibbs free energy. Another group of anode materials are high-entropy oxides with a spinel structure (HES), for example (Cr_0.2Fe_0.2Mn_0.2Co_0.2Ni_0.2)₃O₄ obtained by solid-state synthesis at T = 500-1000 °C in air. After ball grinding, the particle size is reduced to 20 nm. The initial specific capacity during cycling in the 0.01-3.0 V range is 1333.6 mA×h/g and decreases to 329.9 mA×h/g after 20 cycles. A comparative investigation of synthesis conditions, crystal structure, morphology and electrochemical characteristics has been carried out for the four classes of high-entropy oxides: 1) Li _y (MeMn³⁺)_{2- y} O₂, где Me = Ti⁴⁺, Nb⁵⁺; 2) Li_{1+ x} (MeMn³⁺)_{1- z} O_{2- y} F _y , где Me = Ti⁴⁺, Nb⁵⁺; 3) Li _x (Co_0.2Cu_0.2Mg_0.2Ni_0.2Zn_0.2)_{2- x} O_{1- x} F _x ; 4) (Cr,Fe,Mn,Co,Ni)₃O₄.

Electrochemical methods in combination with the technique of electrode surface renewal by cutting in dilute aqueous solutions of 0.1 М NaClO₄ with the addition of dodecanethiol (DDT) at controlled potential were used in the work. This made it possible not only to overcome the problem of obtaining an oxide-free metal surface but also to carry out in situ investigation of the formation dynamics and properties of the resulting DDT layers by voltammetry and chronoamperometry, and to determine the conditions for the rapid formation of strong insulating nanofilms on nickel and cobalt. The trends in the influence of various factors (the nature of electrode metal, DDT concentration, the presence of oxygen in solution, the addition of ethanol) on the kinetics of formation and blocking properties of DDT nanofilms are established. The processes of adsorption on Co and Ni were determined to be similar, but adsorption on nickel is slower. For instance, at the dodecanethiol concentration of 10^-3 mol/L, nanofilm formation on Ni lasts for ~30 min, and on Co for ~10 min . It is shown that the adsorption behaviour of DDT on these metals strongly depends on the concentration of DDT and ethanol in solution and less significantly on the presence of oxygen in the solution. The presence of oxygen in the solution slows down this process and slightly reduces the inhibitory properties of nanofilms. The introduction of ethanol into a DDT-containing solution of sodium perchlorate leads to significant acceleration of the process of dodecanethiol self-assembly on nickel and cobalt in a wide ethanol concentration range. The best insulating properties of monolayers are obtained in water-ethanol solutions with ethanol concentration 40-60 % on DDT/Ni electrodes and 20-50 % on DDT/Co electrodes.

The processes of potato starch swelling, dispersion, dissolution and hydrolysis during thermal heating of water - starch suspension in a thermal shaker and during mechanical treatment in a hydro-pulse reactor are considered. The conditions of mechanical treatment and heating were chosen to provide the same heating rates of the suspension in both installations. This allowed us to separate the influence of thermal and mechanical effects on starch grains and to find that during mechanical processing at temperatures below gelatinisation point, dispersion of starch grains occurs, followed by gelatinisation, while when heated, at first gelatinisation of starch grains occurs, and then their destruction is observed. It is shown that under mechanical action about 70 % of amylose from starch granules passes into solution after treatment for 40 min, while no noticeable release of amylose into the solution is observed as a result of thermal treatment for the same time. It is found that during mechanical processing of the water-starch suspension, both amylose and amylopectin are immediately released into the solution, while during heating, amylose is primarily dissolved, and only after heating the suspension for 340 min at 95 °С the dissolution (leaching) of amylopectin begins. The rate of the initial stage of acid hydrolysis of starch at 90 ºC and pH 2.4 under mechanical processing is approximately 3 times higher than in the case of suspension heating in the thermal shaker. With longer treatment, the rate of mechanochemical acid hydrolysis decreases and becomes only 1.3 times higher than the rate of thermal hydrolysis.

Using the mechanochemical method, supramolecular compositions of a potential anthelmintic, artesunate (ARS) with sodium glycyrrhizinate (Na₂GA), were obtained in different mass ratios of the components included in ARS-Na₂GA complex (1 : 5, 1 : 10, and 1 : 20). The formation of an inclusion complex of ARS with Na₂GA micelles was confirmed by NMR relaxation and the nuclear Overhauser effect (NOESY). It has been established that the ARS-Na₂GA complex (1 : 10) has optimal physicochemical properties for further biological studies on the helminths Opisthorchis felineus , which cause opisthorchiasis in humans and animals. Experiments in vitro showed that ARS-Na₂GA complex (1 : 10) had a more pronounced inhibition of motility both in invasive individuals of O. felineus (metacercariae) and in adult trematodes (maritae), compared with pure ARS, and even at low concentrations (0.1-10 µg/mL) for maritae. Despite the fact that the dose of ARS in the composition of ARS-Na₂GA (1:10) was 11 times lower, the efficiency coefficient (IC₅₀) declined 2.4-fold (for metacercariae) and 1.45-fold (for maritae) with respect to pure ARS. The results obtained indicate the availability of using the new complex for further studies on O. felineus -infected animals.

The article is concerned with the preparation of medium-temperature highly conductive composite proton electrolytes of the new type by introducing a nanodiamond (ND) additive to cesium dihydrogen phosphate, and investigation of their properties. The data on changes in the structural properties of the salt in the composite, morphology, mechanical strength and proton conductivity depending on the composition (1- x )CsH₂PO₄- x ND (where x = 0-0.98, the molar fraction of ND) are considered. Valuable information explaining the mechanism of nanocomposite formation was obtained: the salt protons are partially bound to OH nanodiamond groups, which results in the arrangement of a weaker hydrogen bond system of the salt. It has been shown that there is no chemical interaction between the components in the composites, and the structure of CsH₂PO₄ ( P 2₁/ m ) is preserved during dispersion and partial amorphisation of the salt with an increase in the mole fraction of ND. The composites are characterised by the uniform particle distribution. The introduction of small ND concentrations leads to the stabilization of the size of salt particles (250±20 nm) in nanocomposites, as a result of the interfacial surface interaction of the components. Based on the change in the enthalpy of the superionic phase transition and X-ray diffraction data, the proportion of the amorphous phase in the composites was estimated to increase substantially with an increase in ND molar fraction, reaching 50 % at x = 0.8. A significant increase in the proton conductivity of the low-temperature phase of CsH₂PO₄ is observed, up to 3.5 orders of magnitude with a maximum at x = 0.9, and a decrease at x > 0.95 due to the conductor-insulator percolation effect. The superionic conductivity of the composites does not change up to x = 0.7 (11.7 vol% ND) and remains close to the value characteristic of initial salt CsH₂PO₄ (~10^-2 S/cm). An assessment of the strength characteristics of nanocomposites using the Vickers method shows that, due to the high hardness of nanodiamonds, the microhardness of the composites is significantly higher than that of initial CsH₂PO₄ even with a small content of ND additives ( x = 0.3, which corresponds to 2.64 vol%). The studied composite electrolytes have high proton conductivity, chemical stability and mechanical strength required for medium-temperature proton membranes of the new type for fuel cells.

The results of studying high-temperature oxygen desorption from an YBaCo₂O_6-δ oxide in the quasi-equilibrium regime are presented. The solid-phase synthesis of the oxide, ways and methods of its characterisation are described. The data obtained by quasi-equilibrium oxygen release technique allowed us to provide a reliable description of oxygen desorption from the oxide and to calculate the change in oxygen content in the oxide. Oxygen content in YBaCo₂O_6-δ is determined as a continuous function of temperature and the partial pressure of oxygen within the ranges of 600-900 °C and 0.2-10^-5 atm, respectively. The data obtained make it possible to identify the conditions of phase transitions in the oxide and determine the stability regions of the conductive phase, which is necessary for the development of cathode materials for solid oxide fuel cells.

The aim of this work was to develop a simple method to prepare silver micro- and nanoparticles that can be used to fabricate electroconductive compositions, including pastes, inks, and adhesives. To achieve this goal, the reduction of silver alkyl carboxylates (SAC) with different lengths and structures of the methylene chain by ethylene and propylene glycol was studied. The possibility of preparing monodisperse silver nanoparticles without using polymers as dispersants and stabilisers was shown. The particles obtained were characterized by X-ray diffraction analysis and electron microscopy. It was found that complete reduction of the straight short-chain alkyl SAC occurred at lower temperature (100-130 °C) than long-chain ones (130-170 °C). The particles synthesised under these conditions were spherical, and their average size decreased from 70 to 16 nm with an increase in the length of methylene chain. In the case of branched SAC structure, the reduction with polyols started at lower temperatures and was accompanied by the formation of larger silver particles 150 to 450 nm in size. It was shown that the characteristics of particles obtained by the reduction of linear SAC were almost independent of the synthesis conditions, whereas in the case of branched analogues the temperature, synthesis time and polyol type affected the size of the particles formed. Based on the data obtained, a mechanism for the formation of silver nanoparticles was proposed. Silver particles synthesised according to the developed procedure can find application as metal fillers in 2D and 3D printing inks and pastes for the manufacture of functional components and devices, in conductive adhesive compounds, in polymer composites, in medicine and biology.

High-temperature ceramic materials C_f/ZrB₂-SiC consisting of a refractory ZrB₂-SiC matrix reinforced with continuous carbon fibres (C_f) are of intense interest as candidates for the development of next-generation thermal protection for propulsion systems designed to operate under extreme conditions of temperature, mechanical load, and aggressive gas flows. The focus of these studies is on the oxidative and ablative stability, which is crucial for the practical application of high-temperature materials. In the present work, high-temperature C_f/ZrB₂-SiC composites were prepared by the ceramic prepreg method based on carbon fibre tow impregnation with ceramic slurry, formation of unidirectional ceramic ribbons, followed by their lay-up, pyrolysis, and silicon melt infiltration. The morphology and phase distribution over the volume of ceramic C_f/ZrB₂-SiC composites have been studied by electron microscopy with energy-dispersive spectroscopy performed at different accelerating voltages. For the first time, the behaviour of such materials under the conditions of exposure to high-speed plasma flow at temperatures as high as 2100 °С has been investigated. Air pressure was 0.35 MPa, air flow rate was 6 m³/h. The composite demonstrates rather stable behaviour up to 2000 °С for 300 s. A comparative analysis of the composite microstructure before and after gas dynamic tests has been carried out. The phase and elemental composition, as well as the cross-sectional morphology of the composite, are determined. It is shown that the ablative stability of the composite is due to the formation of complex microstructure, in which several sublayers can be distinguished, and each of them prevents oxygen diffusion inside the composite. Temperature increase up to 2100 °С leads to a significant degradation of the composite. The obtained data can be used for further improvement of the composition of high-temperature composites suitable for the stable functioning of power system parts and units under extreme conditions.

An efficient procedure for sample preparation and determination of rare earth elements (REE) in the brown coals of the Azeysk by a highly sensitive method, mass spectrometry with inductively coupled argon plasma, has been developed. The relative standard deviations in REE determination by the semi-quantitative and quantitative methods do not exceed 15 and 8 wt%, respectively. The total content of rare earth elements is 0.92 ± 0.07 kg/t of coal, which was verified by an independent spectrophotometric method with аrsenazo III.

Cocrystals of betulin with dicarboxylic acids were obtained using mechanochemical treatment with the addition of small amounts of organic solvents. The formation of cocrystals was confirmed by powder X-ray diffraction, thermal analysis, and IR spectroscopy methods. The presence of a solvent, the optimal components ratio, and the duration of mechanical processing are important for the formation of cocrystals. In order to choose a solvent for preparing betulin cocrystals, the solvents of different polarity were compared. It has been shown that cocrystals are formed if solvents that can effectively dissolve dicarboxylic acid are used during mechanical processing. For comparison with the mechanochemical method, betulin cocrystals were obtained by heating a mixture of starting reagents. Morphological changes observed by scanning electron microscopy upon heating the reaction mixtures also indicate the formation of cocrystals. It has been shown that when cocrystals are dissolved in water, solutions with an increased concentration of betulin are formed, and an increase in the length of the aliphatic chain of the acid leads to a decrease in the rate of betulin release into the solution.

The process of metal bismuth dissolution in nitric acid was studied, and a comparison was made over environmentally friendly methods of obtaining bismuth nitrate solutions by the preliminary oxidation of bismuth, as well as by its dissolution in the presence of urea or ammonium nitrate. It has been shown that the dissolution of bismuth in the presence of ammonium nitrate at a molar ratio of ammonium nitrate to bismuth equal to 1.5-3.5 and a temperature of 70±5 °C allows eliminating the release of nitrogen oxides into the gas phase and obtaining a solution with a high concentration of bismuth. The feasibility of two-stage hydrolytic processing of a bismuth-containing nitrate solution in order to purify bismuth from impurity metals is shown. The first stage is the aqueous hydrolysis of a bismuth-containing solution by adding a bismuth nitrate solution to water heated to 60 °C at a volume ratio of water to bismuth-containing solution 9 : 1, and the second stage is additional precipitation of bismuth from the mother solution after aqueous hydrolysis by adding ammonium carbonate solution to reach pH 1 at a temperature of 55±5 °C. Based on the research carried out, ecologically safe technologies have been developed for the production of bismuth oxohydroxonitrate [Bi₆O₅(OH)₃](NO₃)₅∙3H₂O and bismuth nitrate pentahydrate Bi(NO₃)₃∙5H₂O, as well as specially pure bismuth(III) oxide. As a result of the reduction of bismuth(III) oxide in molten sodium hydroxide containing sulphur at a temperature of 500 °C and at a ratio of bismuth(III) oxide/sodium hydroxide/sulphur equal to 1: 1.31: 0.20, high-purity metallic bismuth was obtained.

Physicochemical and pharmacological properties of mechanochemically synthesised supramolecular systems/complexes of the guest-host type have been studied at the institutes of the Siberian Branch of the Russian Academy of Sciences in cooperation with Chinese Zhejiang University of Technology. The guest is a molecule of a medicinal substance, and the host is a carrier particle - a macromolecule of polysaccharide, saponine micelle, silicon dioxide particle, etc. The strengthening of the pharmacological effect of such structures is achieved by increasing the water-solubility and trans-membrane permeability of drug molecules. The most effective hosts among the studied carriers are plant metabolites - glycyrrhizic acid and its salts, as well as polysaccharide arabinogalactan from Larix Siberica wood. An original solid-phase mechanochemical technology has been developed to obtain water-soluble supramolecular systems from the solid dispersions of components. In this case, supramolecular systems are formed in the process of solid-phase synthesis, or by dissolving the obtained dispersions in aqueous media. As a result of studies of a large number of widely used drugs of various pharmacological classes, it has been shown that the inclusion of drug molecules in these supramolecular systems can significantly increase the bioavailability, effectiveness and safety of their action and reduce the effective therapeutic dose of drugs significantly (by a factor of 2-150), and decrease (down to complete disappearance in some cases) harmful side effects. In this paper we give a brief overview of the studies carried out mainly over the last 10 years.