CHANGE IN THE STRUCTURE OF TITANIUM NANOCLUSTERS UNDER THERMAL EXPOSURE: MOLECULAR DYNAMIC SIMULATION

Исследование структуры нанокластеров при различных температурах является актуальной задачей современного материаловедения. Данный факт обусловлен перспективой их применения при создании материалов с уникальными физическими, механическими, химическими и эксплуатационными свойствами. Компьютерное моделирование проводилось методом классической молекулярной динамики в программном комплексе LAMMPS. Для описания межатомного взаимодействия в кластере использовалась модификация многочастичного потенциала Финниса-Синклера. Проведено изучение структуры нанокластеров титана различного размера. Они получены при различных скоростях охлаждения из жидкого состояния. Увеличение скорости охлаждения приводит к формированию субблочной структуры и росту числа атомов с неупорядоченным окружением. Они обусловлены тем, что большие скорости охлаждения препятствуют равновесному протеканию процессов перестройки атомной структуры с формированием дальнего порядка. Областей с икосаэдрической структурой не обнаружено. Показано, что температура кристаллизации и энергия связи уменьшаются при убывании размера нанокластера. Рост скорости охлаждения увеличивает разницу температур точек начала и конца кристаллизации, соответственно. Результаты моделирования свидетельствуют о менее выраженной размерной зависимости температуры кристаллизации - её оценочное значение для макроскопической системы (810 К) гораздо ниже значения для массивного титана (1940 К). Investigation of the structure of nanoclusters at different temperatures is an urgent task of modern materials science. This fact is due to the prospect of their application in the creation of materials with unique physical, mechanical, chemical and operational properties. Computer simulation was carried out by the method of classical molecular dynamics in the LAMMPS software package. To describe the interatomic interaction in the cluster, a modification of the Finnis-Sinclair many-body potential was used. The structure of titanium nanoclusters of various sizes has been studied. They are obtained at various cooling rates from the liquid state. An increase in the cooling rate leads to the formation of a subblock structure and an increase in the number of atoms with a disordered environment. They are due to the fact that high cooling rates impede the equilibrium process of rearrangement of the atomic structure with the formation of long-range order. No regions with an icosahedral structure were found. It is shown that the crystallization temperature and binding energy decrease with decreasing nanocluster size. An increase in the cooling rate increases the temperature difference between the start and end points of crystallization, respectively. The simulation results indicate a less pronounced dimensional dependence of the crystallization temperature - its estimated value for a macroscopic system (810 K) is much lower than the value for bulk titanium (1940 K). Keywords: nanocluster, binding energy, crystallization temperature, cooling rate, structure, molecular dynamics method.

REGULARITIES OF STRUCTURE FORMATION IN BIMETALLIC NANOPARTICLES WITH DIFFERENT CRYSTALLIZATION TEMPERATURES

В данной работе исследуются закономерности структурообразования на примере биметаллических наночастиц Au - Ag, Ti - Al, Ti - V. Данные биметаллические наночастицы обладают различным размерным несоответствием и различной температурой кристаллизации. Проведены серии молекулярно-динамических экспериментов, по результатам которых проанализированы конечные конфигурации с наименьшей энергией и получены концентрационные зависимости энергии смешения. Анализ концентрационных зависимостей энергии смешения позволяет прогнозировать составы и размеры биметаллических наночастиц, которые могут проявлять нестабильность, как например для биметаллических наночастиц Ti - V. Асимметричность отдельных концентрационных зависимостей энергии смешения свидетельствуют о специфических структурных превращениях, характерных именно для данного состава и размера. Установлено, что для биметаллических наночастиц Au - Ag, Ti - Al характерна структурная сегрегация, и она активно проявляется при малых концентрациях более легкоплавкого компонента. Конкурирующими фазами в данном случае выступают ГЦК и ГПУ фазы. Кроме того, для средних из рассматриваемых в статье размеров исследована зависимость температуры кристаллизации от состава биметаллических наночастиц. In this work, of the structure formation was investigated using Au - Ag, Ti - Al, Ti - V bimetallic nanoparticles as the patterns. These bimetallic nanoparticles have different atomic size mismatches and different crystallization temperatures. A series of molecular dynamics experiments was carried out. Based on their results, the final configurations with the lowest energy were analyzed and the concentration dependences of the mixing energy were obtained. An analysis of the concentration dependences of the mixing energy makes it possible to predict the compositions and sizes of bimetallic nanoparticles, which can exhibit instability, such as for Ti - V bimetallic nanoparticles. The asymmetry of individual concentration dependences of the mixing energy is evidence of specific structural transformations characteristic for the given composition and size. It has been established that structural segregation is characteristic for Au - Ag,Ti - Al bimetallic nanoparticles and it is actively manifested at low concentrations of a more low-melting component. The competing phases in this case are fcc and hcp phases. In addition, for the average sizes considered in the article, the dependence of the crystallization temperature on the composition of bimetallic nanoparticles was investigated.

Low-Temperature Rheology and Thermoanalytical Investigation of Lubricating Greases: Influence of Thickener Type and Concentration on Melting, Crystallization and Glass Transition

This study investigates crystallization, melting and glass transition of Li- and Ca-12-hydroxystearate greases in relation to the pour point of the corresponding oils. The base oils for the greases are mineral oil, polyalphaolefin, alkylated naphthalene, propylene glycol, and trimellitate. For the mineral oil-based greases the crystallization temperature Tc increases and the melting temperature Tm decreases upon addition of thickener. The pour point of the mineral oil then is 3 K below Tc and does not properly define the lowest application temperature for mineral oil (MO) based greases. Both thickeners induce a small increase of the glass transition temperature (1–3 K) of the synthetic oils polyalphaolefin, alkylated naphthalene, propylene glycol. The pour point of the base oils correlates well with the onset of the glass transition in the corresponding grease indicated by a sharp increase in grease viscosity. Pure trimellitate with unbranched alkyl chains does not crystallize upon cooling but shows noticeable supercooling and cold crystallization. As the percentage of thickener in corresponding greases increases, more oil crystallizes upon cooling 20 K above the crystallization temperature of the trimellitate without thickener (−44 °C). Here, the thickener changes the crystallization behavior from homogeneous to heterogeneous and thus acts as a crystallization nucleus. The pour point of the base oil does not provide information on the temperature below which the greases stiffen significantly due to crystallization.

PREPARATION OF STABLE COMPOSITES WITH NANOPARTICLES OF METALS IN BIOMEDICAL MATRICES

The unique properties of metal nanoparticles are increasingly used in the pharmaceutical, medical, food, textile and paint industries. It is known that the properties of nanoparticles depend on the shape, size, concentration. Agglomeration processes that occur during the production and storage of nanoparticles lead to a reduction in the size of the specific area to the volume of nanoparticles, which in turn leads to the loss of the unique properties of nanoparticles. In the process of analysis of domestic and foreign literature it was found that the problem of agglomeration processes of nanoparticles is relevant. One of the possible approaches to solving the agglomeration of synthesized nanoparticles, which is proposed in this work is the use of matrices of carriers. In this case, the matrix can be liquids: monomers, oils of vegetable origin or medical substances that are constituent tablets, capsules, ointments, soaps, and paints.The morphology, size, structure and time stability of silver and copper nanoparticles in the obtained dispersed systems were studied. Examples of stability of composites based on glycerol, polytetrahydrofuran (PTGF), oils of synthetic and vegetable origin with nanoparticles of metals with a size of 15… 52 nm are presented. The stability of dispersed systems has been improved through the use of two approaches: the use of surfactants and dispersed systems based on matrices with low crystallization temperature

Effects of La on Thermal Stability, Phase Formation and Magnetic Properties of Fe–Co–Ni–Si–B–La High Entropy Alloys

The microstructure, phase formation, thermal stability and soft magnetic properties of melt-spun high entropy alloys (HEAs) Fe27Co27Ni27Si10−xB9Lax with various La substitutions for Si (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) were investigated in this work. The Fe27Co27Ni27Si10−xB9La0.6 alloy shows superior soft magnetic properties with low coercivity Hc of ~7.1 A/m and high saturation magnetization Bs of 1.07 T. The content of La has an important effect on the primary crystallization temperature (Tx1) and the secondary crystallization temperature (Tx2) of the alloys. After annealing at relatively low temperature, the saturation magnetization of the alloy increases and the microstructure with a small amount of body-centered cubic (BCC) phase embedded in amorphous matrix is observed. Increasing the annealing temperature reduces the magnetization due to the transformation of BCC phase into face-centered cubic (FCC) phase.

Potential of Transforming Sodalite from Synthesis Kaolin with a Mild Condition of the Hydrothermal Method

The hydrothermal synthesis method is one of the successful methods for transforming kaolin into sodalite with various parameters considered. The variation of alkalinity source of 2-3 Molarity of sodium hydroxide and crystallization time (12-16 hours) was considered an important parameter that influences the formation of sodalite. It is reported in this research that the process of synthesizing sodalite to transform into an amorphous stage (Metakaolinization phase) began with beneficiation of kaolin to remove the impurities and calcination by 6500C for four hours. The synthesis was done through an aging process (400C, 24 hours), and the crystallization temperature was 1000C. The kaolin was characterized by XRD, FESEM, FTIR, PSA, TGA, followed with metakaolin by XRD, FESEM, FTIR, and the end-product by XRD, FESEM, FTIR, and PSA. The crystallinity percentage of sodalite slightly increased by increasing the molarity and crystallization time, but quartz remains in the end-product.

The effect of isothermal crystallization on mechanical properties of poly(ethylene 2,5-furandicarboxylate)

The effects of isothermal crystallization temperature/time on mechanical properties of bio-based polyester poly(ethylene 2,5-furandicarboxylate) (PEF) were investigated. The intrinsic viscosity, crystallization properties, thermal properties, and microstructure of PEF were characterized using ubbelohde viscometer, X-ray diffraction, polarizing optical microscope, differential scanning calorimetry, and scanning electron microscopy. The PEF sample isothermal crystallized at various temperatures for various times was denoted as PEF-T-t. The results showed that the isothermal crystallization temperature affected the mechanical properties of PEF-T-30 by simultaneously affecting its crystallization properties and intrinsic viscosity. The isothermal crystallization time only affected the crystallization properties of PEF-110-t. The crystallinity of PEF-110-40 was 17.1%. With small crystal size, poor regularity, and α′-crystal, PEF-110-40 can absorb the energy generated in the tensile process to the maximum extent. Therefore, the best mechanical properties can be obtained for PEF-110-40 with the tensile strength of 43.55 MPa, the tensile modulus of 1,296 MPa, and the elongation at a break of 13.36%.

Study on crystallization kinetics of dry fractionation products of beef tallow

The dry fractionation beef tallow and their products were analyzed in the dynamic thermodynamic analysis, isothermal analysis and crystallization kinetics analysis in this experiment. Through the dynamic thermodynamic analysis by DSC, the possibility of fine fractionation of beef tallow at 25 °C and 42.9 °C crystallization temperature was obtained. The dynamic thermodynamic analysis of dry fractionation products was carried out, and the linear functions of peak temperature and melting/crystallization rate of beef tallow and its stearic acid mixture were constructed. The crystallization temperature and melting point were obtained by linear function. The isothermal crystallization kinetic model was used to calculate and fit the experimental data by the Avrami model. Beef tallow and its stearic mixture were fitted with the Avrami equation to obtain R 2 ≥ 0.98. This analysis provides an innovative idea and method for thermodynamics and crystallization kinetics of beef tallow.