Papers by Keyword: Phase Transition

Abstract: The ZrW_2-xMo_xO₈ (0≤x≤2) system has a unique property - a negative coefficient of thermal expansion (CTE), that is it shrinks when heated. Such materials solve the problem of undesirable thermal expansion of composites in many areas. In contradistinction to ZrW₂O₈ and ZrMo₂O₈, this system has anomalously low CTE values [1,2] and adverse phase transition is shifted to a lower temperature, unlikely area for operation [3,4]. At the same time, the fundamental question remains about the mechanism of formation of ZrW_2-xMo_xO₈(0≤x≤2). According to [5,6], pure cubic ZrW₂O₈ and ZrMo₂O₈ are obtained by thermal decomposition of the precursor ZrM₂O₇(OH)₂· 2H₂O, where (M = Mo, W). However, the structure of the precursor is extremely sensitive to various changes in the synthesis process. To date, there are a number of works devoted to the hydrothermal synthesis of precursors ZrW_2-xMo_xO₇(OH)₂·2H₂O (0≤x≤2) solid solutions, however, not all formulations were obtained during the hydrothermal reaction. The study of their thermal properties, as well as structural and phase transformations are not fully studied. Within the paper it was first time proposed to obtain full number of nanosized materials by the hydrothermal method and also to study the nature of the thermal expansion of each member of the solid solution. The obtained data will solve many technical problems associated with dimensional nonvariance in the electronic, oil and gas, optical industry, medicine and rocket engineering.

Abstract: We have studied temperature-induced phase transitions in PbZrO₃ thin films by X-ray diffraction. By tracing the temperature dependence of superstructure reflections we show that the onset of antiferroelectric ordering takes place highly continuously on cooling and another, presumably ferroelectric phase is present at high temperatures, between the antiferroelectric and cubic phases. To clarify the possible reason for this behavior, we have investigated the X-ray diffraction profile with momentum transfer along the normal to the film surface and carried out the relevant simulation using the formalism of scattering by inhomogeneously deformed sample. From the analysis it follows that the near-interface layer is effectively compressed along the normal to the film. We associate this observation with the presence of dislocations through which the film is relaxed. The results suggest that the reasons for the phase transition sequence modification in thin films can be associated with inhomogeneous distribution of stress and defects in the near-interface area.

Abstract: In this paper, Ga₂O₃ thin films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD). There was phase transition for samples grown with different flow rates of triethyl-gallium (TEGa) and deionized water (H₂O). It is found that ε-Ga₂O₃ is difficult to coalesce and the phase mixture by β­Ga₂O₃ takes place if the flow rates of TEGa and H₂O are too high. However, by using multiple-step growth method, the film became fully coalesced. High-quality ε-Ga₂O₃ thin film with atomically flat surface and multilayer morphology was obtained.

Abstract: The work reports the results of neutron diffraction measurements of NiMnGe:T systems where T is Cr or Ti. All investigated compounds have the helicoidal magnetic structure with the propagation vector k = (k_a,0,0). The values of the k_a component decrease with increasing Cr content and increase with increasing Ti content. For all compounds, except the sample with x = 0.18 in Cr-system, the helicoidal order is stable up to the Néel temperature. The obtained data are analysed based on simple model in which the magnetic interactions are described by two exchange integrals J₁ > 0 for first and J₂ < 0 for second neighbouring moments. This model clears up different dependence of k_a component in different systems.

Abstract: Complete the composition design and laboratory smelting of high strength weathering steel for railway freight car body of grade 500MPa. The actual continuous cooling transformation curve (CCT curve and cold speed of 0.5-50 °C/s) was measured through the Gleeble3500 thermal simulation test machine with expansion method and alloy phase method, observing the microstructures by optical microscope and SEM. The hardness of the samples under different cooling rates were measured by Vivtorinox hardness tester. Study on the effect of cooling rates on the microstructures and hardness of the steels.

Abstract: A new simple universal form of the Kelvin equation, which can be used near the gas-liquid critical point and the correction of the pressure for vapor outside the porous medium are used for the N₂ meniscus effective curvature radius calculation at the phase equilibrium in mesoporous silica SBA-15, on the basis of the capillary condensation experimental data.

Abstract: A model based on the generation and recombination of defect was developed to describe the stability of stacking faults and basal plane dislocation loops in crystals with layered polytype structures. The stability of the defects configuration was analysed for stacking faults surrounded by Shockley and Frank partial dislocation as well as Shockley dislocation dipoles with long range elastic fields. This approach allows the qualitative prediction of defect subsystems in polytype structure in external fields.

Abstract: High strength, low density, and excellent corrosion resistance are the main properties that make titanium attractive for a variety of applications. The phase structures and phase transitions of titanium, which are of tremendous scientific and technological interest, have attracted a great deal of attention for many years. In addition to hexagonal close packed α-Ti, high temperature phase β-Ti with body-centered cubic structure and ω-Ti with the hexagonal structure of high-pressure phase, the face-centered cubic structure, which is not in the P-T diagram of titanium, is observed in ultrathin films. In the present paper, the Ti films prepared by magnetron sputtering on MgO(111) single crystal substrate were investigated by means of X-Ray Diffraction (XRD) and High-Resolution Transmission Electron Microscope (HRTEM). The results showed that the Ti films grow epitaxial with a face centered cubic (fcc) structure even the thickness is up to about 50nm. With the thickness increases, the Ti films transformed to hexagonal close packed (hcp) structure and showed an epitaxial growth along (002)_hcp-Ti direction. The results show that the onset thickness of fcc-hcp structure transformation is 50-100nm. The temperature and power of sputter affect the formation of fcc-Ti.

Abstract: A new simple universal form of the Kelvin equation that can be used even near the gas-liquid phase transition critical point is shown. The correction of the chemical potential, pressure, and density outside the porous medium is presented and taken into account for the CO₂ meniscus curvature radius calculation at capillary condensation in mesoporous silica MCM-41, known [1] for its wide range of applications.

Abstract: The phase-field method is a very powerful tool to model the phase transformation and microstructural evolution of solids at mesoscopic scale. However, several important phenomena, like defect formation, grain boundary motion, or reconstructive phase transitions require an atomic scale study. Recently an approach called the quasi-particle approach, based on the Atomic Density Function theory was developed to incorporate the atomic-level crystalline structures into standard continuum theory for pure and multicomponent systems. This review focuses on the description of different computational methods used to model microstructural evolution and self-assembly phenomena at mesoscopic and atomistic scales. Various application examples of these methods are also presented.