Papers by Keyword: Phase Transition

Abstract: The paper studies applicability of individual particles of Ag-Cu nanoalloys as data bits in the next generation memory devices constructed on the phase change memory principle. To fulfill this task, the structure formation was simulated with the molecular dynamics method on cooling from the melt of Ag-Cu nanoparticles of the diameter of 2.0 – 8.0 nm of different chemical compositions (with copper content in the alloy from 10 to 50 percent), based on the modified tight-binding potential (TB-SMA). The authors investigated the influence of the size effects and the heat removal rate on the formation of the clusters structure. The investigation showed that different internal structures can be developed upon cooling from the liquid phase, so there were determined some criteria of their stability. Clusters with copper content of not more than 10% and diameters of more than 6.0 nm were isolated from the entire set of the considered particles.

Abstract: Ferroelectric ceramic solid solutions Li_xNa_1-xTa_yNb_1-yO₃ (х = 0.17; у = 0 – 0.5) with the perovskite structure have been obtained by the thermobaric synthesis method. Particularities of their microstructure, elastic properties, electrical conductivity and permittivity have been researched. It has been established that an increase in the thermobaric synthesis temperature leads to a decrease in the Young’s modulus value. Specific static conductivity values have been determined; charge carrier activation enthalpies Н_а have been calculated. The Curie temperature of the samples has been determined to decrease with an increase in tantalum content. A Ferroelectric ceramic solid solution Li_0.17Na_0.83Ta_0.1Nb_0.9O₃ was shown to undergo four structure phase transitions in the temperature range 300-820 К. A Li_0.17Na_0.83Ta_0.1Nb_0.9O₃ has been shown to be a high temperature superionic. Possible mechanisms of the detected phenomena are discussed.

Abstract: In this work, the microstructure evolution of as-cast NAB under different electropulsing parameters were studied. The microstructure of the electropulsing treatment (EPT) sample was characterized by mircohardness test and optical microscopy. The results show that compared with heat treatment, when the peak current density reaches 5.84×10⁸A/m² (no significant change in the structure when the peak current density is lower), the β' phase region undergo phase transition in a shorter time. When the peak current density reaches 7.25×10⁸A/m², the sample is significantly affected by the Joule heating effect, and the κ_Ⅲ and κ_Ⅳ phases are successively dissolved to form Widmanstätten α structure. As the β' phase increases and the Widmanstätten α structure forms, the hardness value of the microstructure increases by 80%.

Abstract: Partially stabilized zirconia (PSZ) materials were fabricated using 4 wt% CaO, 3 wt% MgO, and 5.4 wt% Y₂O₃ as stabilizing agents together with monoclinic zirconia powder. The physical properties, phase compositions, and microstructures of the Ca-PSZ, Mg-PSZ, and Y-PSZ samples were investigated by X-ray diffraction, scanning electron microscopy, and energy spectrum analysis. A crucible method was used to explore the relationship between the stabilizing agent and erosion resistance to alkaline steel slag. The results revealed that the zirconia materials stabilized by different stabilizing agents showed obvious differences in their bulk densities, apparent porosities, microstructures, and erosion resistances to alkaline steel slag. The structure of Y-PSZ showed highest density, containing a small number of uniformly distributed pores. In terms of Mg-PSZ, the intergranular bonding in its structure was observed to not be close, and the sample contained some cracks, but no pores. A large number of intragranular pores and a small number of overall pores was observed in Ca-PSZ, resulting in this material having the lowest bulk density. The pores and cracks provide the path to penetrate and diffuse for alkaline steel slag, which weakens the corrosion resistance of PSZ materials. The phase composition of the affected layers in all of the samples after corrosion was almost completely transformed from monoclinic phase to cubic phase, and the phase transition of both the original and transition layers was not obvious due to the formation of a slag film. Y-PSZ did not react with components of the steel slag such as SiO₂ and Al₂O₃, showing the best corrosion resistance to alkaline steel slag.

Abstract: Differential scanning calorimetry (DSC) was used to study non-isothermal kinetics of α→β transformation of Zr-0.5wt%Sn-0.15wt%Nb-0.5wt%Fe-0.25wt%V alloy. The DSC curves were measured from room temperature to 1030 °C at the heating rate of 15, 20, 30, 50°C /min respectively. The Flynn-Wall-Ozawa (FWO) method was used to get the activation energy (E) of α→β transformation at different conversion ratios. Then the values of activation energy obtained were modified by Ozawa iterative equation. The kinetic mechanism functions of α→β transformation were investigated by Criado-Ortega methods. The results show that the activation energy is related to conversion ratios. It means α→β transformation is not a simple one-step reaction but a complex multi-step reaction. The most probable kinetic mechanism functions are different in different temperature ranges, which are -ln(1-x) for ≤830 °C, [-ln(1-x)]^1/2 for 834~848 °C, [-ln(1-x)]^2/5for 850~856 °C and [-ln(1-x)]^1/3 for 858~868 °C respectively.

Abstract: La_0.8Sr_0.2MnO₃ was prepared to investigate the infrared emissivity of the sample under 0.76-2.5 μm and 2.5-500 μm infrared fields. The sample exhibited characteristic peaks of rhombohedral perovskite structure at 298-318 K, and split of the main peak weakened with increasing temperature. Intensity of the ferromagnetic resonance peak enhanced with increasing temperature, and the peak was shifted toward high magnetic field. A weak paramagnetic resonance peak appeared at 318 K, indicating that ferromagnetic-paramagnetic transition was occurring. The temperature of the sample under 2.5-500 μm field was higher than that under 0.76-2.5 μm. The emissivity of the sample increased with radiation time under 2.5-500 μm, but it had no obvious changes at about 0.665 under 0.76-2.5 μm. The emissivities at the same temperature fields were higher than those under 0.76-2.5 μm and 2.5-500 μm fields, respectively. It suggested that 0.76-2.5 μm and 2.5-500 μm radiations had inhibition effect on emissivity of La_0.8Sr_0.2MnO₃.

Abstract: Study on microstructure appearance of 3 mm thick TC4 titanium alloy welded join by common electron beam and pulsed electron beam were carried out. Experimental results show that pulsed electron beam improved grain size and decreased the cooling velocity of weld metal by oscillation and fast cooling effect, the acicular martensite decomposes β phase and transforms to finer and more α′ phase, which shows an interwoven pattern.

Abstract: A new simple universal form of the Kelvin equation, which can be used near the gas-liquid phase transition critical point, and the correction of the pressure and density for gas phase fluid outside the porous medium are taken into account for the oxygen meniscus effective curvature radius calculation at the phase equilibrium in mesoporous silica MCM-41, on the basis of the capillary evaporation and condensation experimental data.

Abstract: In order to improve the corrosion resistance, bioabsorbable magnesium (Mg) alloy was immersed in an aqueous solution with three times of Ca²⁺ and HPO₄^2- in comparison with those of conventional SBF at the moderate temperature. After immersion in the solution, the whole surface of Mg alloy was coated with plate-like crystals consisting of octacalcium phosphate (OCP). Then the OCP-coated Mg alloy was immersed in the alkali solution. Although significant change of morphology was not observed, the OCP formed on the Mg alloy was transformed to hydroxyapatite (HAp) under alkali condition.

Abstract: As a promising third generation semiconductor material, gallium nitride (GaN) has become a research hotspot in optoelectronic field nowadays. In this paper, GaN thin films were grown by radio frequency (RF) planar magnetron sputtering of a powder GaN target in a pure nitrogen atmosphere at (0.2 – 2.0) Pa, (10 - 100) W onto various substrates such as GaAs (100), Si (100), Si (111), Al₂O₃(0001) and glass without any buffer layer. A clear phase transition from the metastable cubic zinc-blende (c - ZB) to the stable hexagonal wurtzite (h - WZ) dependence on substrates has been found in the GaN thin films. And the phase transition of GaN films were studied by X-ray diffraction (XRD), photoluminescence (PL) and Raman spectroscopy.