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The doping of hydroxyapatite with various substituent ions can give this material new and useful properties. Nonetheless, local distortions of structure after doping can change the properties of the material. In this work, the thermal stability of copper-substituted hydroxyapatite synthesized by the mechanochemical method was investigated. In situ diffraction analyses showed that copper ion diffusion during the heating of Cu-substituted hydroxyapatite promotes phase transformations in the substituted hydroxyapatite. The behavior of copper ions was studied in samples with ratios (Ca + Cu)/P = 1.75 and 1.67. It was found that in both cases, single-phase Cu-substituted hydroxyapatite with the general formula Ca10−xCux(PO4)6−y(CO3)y(OH)2−yOy is formed by the mechanochemical synthesis. When heated at approximately 600–700 °C, the lattice loses copper cations, but at higher temperatures, CuO diffusion into the hydroxyl channel takes place. Cuprate-substituted hydroxyapatite with the general formula Ca10(PO4)6(OH)2−2x(CuO2)x forms in this context. At 1200 °C, the sample is single-phase at (Ca + Cu)/P = 1.75. Nonetheless, slow cooling of the material leads to the emergence of a CuO phase, as in the case of (Ca + Cu)/P = 1.67, where the material contains not only CuO but also Cu-substituted tricalcium phosphate. In the manufacture of ceramic products from Cu-substituted hydroxyapatite, these structural transformations must be taken into account, as they alter not only thermal but also biological properties of such materials.

Zinc-substituted hydroxyapatite samples with a preset degree of substitution (Са10 –хZnx(PO4)6(OH)2, x = 0.1–2.0) have been synthesized by a mechanochemical method. The composition of starting components ((Zn(H2PO4)2 ⋅ 2Н2О and ZnO) and water of crystallization have been shown to influence the apatite structure formation process and the degree of zinc substitution for calcium in the mechanochemical synthesis and annealing processes. When ZnO was used as a dopant, beta-tricalcium phosphate was observed to form as a reaction intermediate directly in the mill after 5–10 min of the reaction mixture milling and disappear after 20 min of milling. The final synthesis product—zinc-substituted hydroxyapatite—remains single-phase for x ≤ 0.8 in the case of Zn(H2PO4)2 ⋅ 2Н2О and x ≤ 0.25 in the case of ZnO. After annealing, the zinc-substituted hydroxyapatite is single-phase only in the case of zinc phosphate using at x ≤ 0.1.

In the present work, the formation kinetics of hydroxyapatite Ca10(PO4)6(OH)2 during ball milling of mixtures of calcium phosphates with calcium oxide or calcium hydroxide was studied. The conversion degrees of reaction mixtures containing calcium dihydrogenphosphate/dicalcium phosphate/tricalcium phosphate/calcium pyrophosphate and calcium oxide/calcium hydroxide were determined for different milling times. The formation kinetic data were analyzed together with the Gibbs free energies of the corresponding reactions. It was found that the kinetic factors dominate at the initial formation stage of hydroxyapatite due to the acid-base interactions during the “soft” mechanochemical synthesis. A technologically attractive synthesis route of Ca10(PO4)6(OH)2 was suggested based on the “soft” mechanochemical synthesis via reactions between СаНРО4 and СаО or Са(ОН)2.

This paper presents the results of the mechanochemical synthesis of hydroxyapatite (HAP) with Zn2+, Cu2+, K1+ and Cs1+ as substituents for the calcium ion and (ZrO4)4-, (SiO4)4- and (AlO3)3- anion substituents for the phosphate group with the substituent concentrations in the range of 0.04–2.0 mol per 1 mol of HAP. The use of Si-substituted HAP as a coating material promotes accelerated osteosynthesis and osteointegration of implants into the bone tissue. Zirconium and aluminum ions can promote the adhesion of coatings to ceramic implants. The replacement of calcium ions by Zn2+, Cu2+, K1+ and Cs1+ in the HAP structure plays an antimicrobial role preventing inflammatory processes.

The work shows that single-phase hydroxyapatite doped with zinc or copper ions can be obtained using a solid-phase mechanochemical synthesis after 30 ​min of processing the reaction mixture in a planetary ball mill. To understand the mechanism of incorporating doping elements in the apatite structure, two substitution reactions have been investigated, namely replacement of the calcium and hydroxide ions with copper or zinc ions. The research has shown that, in the synthesized material, the dopant ions mainly take the sites of calcium ions. The degree of substitution can be up to 2 ​mol of substituent per mol of hydroxyapatite. The obtained samples can be used to produce medical bioresorbable items and coatings with antibacterial properties.

This paper presents the results of research on the regularities of the formation of carbon nanofibrous materials, as well as the influence of the composition of the model mixture of hydrocarbons С 1 –С 4 on the morphological features and textural characteristics of the resulting carbon nanofibres (CNF). One of the most urgent issues faced by the oil and gas industry is how to increase the processing depth of associated petroleum gas (APG). In the Russian Federation alone, the annual economic losses from unutilised APG during processing are estimated at 2 billion dollars. It is known that the composition of APG can vary greatly, depending on the oil and gas field. In particular, the methane concentration, the main component of natural gas and associated gas, varies from 25 to 95 vol.%. In this study, we sought to identify the main factors that determine the morphology and structure of CNF, and to develop approaches in processing actual hydrocarbon mixtures to produce a carbon product with the desired or predictable characteristics. Such an approach can serve as a basis for resource-saving catalytic technology utilizing C 1 –C 4 hydrocarbons that are currently disposed of as post-combustion flare gas. The research results clearly demonstrate that the morphology and texture of the carbon materials obtained by the decomposition of hydrocarbon mixtures depend on the composition of the reaction gas. The key components in this case are methane and the resultant hydrogen: their relationship at a specific temperature determines the direction of the reversible reaction of carbon hydrogenation: С + 2H 2 = СH 4 . The decisive factor in the decomposition of hydrocarbons yielding carbon materials is the degree of deviation of the methanation reaction from equilibrium which can be influenced in two ways. The addition of hydrogen to the reaction gas promotes the driving force of the carbon hydrogenation reaction, resulting in defective high-surface carbon fibres. Increasing the concentration of methane in the reaction gas composition, in contrast, reduces the driving force of methanation until the process is suppressed, which promotes a more structured form of the CNF, without defects.

This work shows that mechanochemical synthesis allows the obtaining of hydroxyapatite in which fluorine ions replace hydroxyl ions either partially or completely after 30 ​min of synthesis. The crystal structure of the materials obtained and their thermal stability when heated in the air were studied. It has been shown that the fluorine ions in the hexagonal channel of hydroxyapatite hinders diffusion of hydroxide ions at high temperatures, which leads to increased thermal stability of the apatite.