Papers by Keyword: Phase Transformation

Abstract: This study aims to optimize the production conditions for forming graphene directly on a quartz substrate, using a carbon 60 (C₆₀) thin film as a solid carbon source. In this experiment, we focused on the relationships between the thickness of the C₆₀ film and the nickel (Ni) catalyst film and the heat treatment conditions. As the thicknesses of the C₆₀ and Ni catalyst films increased, high-crystallinity multi-layered graphene was formed, however the optical transparency of the graphene film decreased. Scanning Electron Microscopy (SEM) observations and Raman scattering spectroscopy showed that after changing the atmosphere of the heat-treatment from an argon (Ar) gas to an Ar+ hydrogen (H₂) gas, the optical transparency of the graphene film was remarkably improved, due to the migration and vaporization of the Ni film, and due to etching of the multi-layered graphene.

Abstract: Microstructural design is generally applied to improve the mechanical property of titanium alloy by introducing different phase transformations and thermomechanical treatments. Aside from the martensitic and diffusion transformation, the occurrence of massive transformation occurs in Ti alloy. Massive transformation is categorized as civilian phase transformation, which resulted in the change of crystal structure of an alloy with a given composition without changing the chemical composition of its initial phase. It happened when the body centered-cubic β phase changed into hexagonal closed-pack α phase without decomposing into α+β. Massive transformation involves a diffusion and growth mechanism in a short-range and generally occurs during the introduction of high cooling rates to restrict the full diffusion mechanism. Owing to the nature of a rapid cooling rate as a requirement for massive transformation, the massive phase is normally found together with the product of martensitic transformation. On the other hand, the product of massive transformation is observed as a blocky grain with a featureless characteristic using optical microscopy and. Phase identification using electron backscattered diffraction shows that the region of α_m shows only the presence of the α phase. It was reported for containing a high dislocation density similar to martensitic transformation. Specifically, in Ti alloy, the higher magnification using scanning electron microscopy shows fine sub-lamellar morphology, which observed as a combination product morphology between martensitic and diffusion transformation. It resulted in the mechanical property of the massive phase is between those two morphologies. Hence, it brings a new perspective on designing the microstructure of Ti alloy, which can be used to improve the mechanical property of Ti alloy.

Abstract: The effect of cooling rates during a double stage solution treatment (DSST) on the volume fraction of the massive phase (α_m) in Ti-6Al-4V alloy was successfully confirmed in the present study. The morphology of Ti-6Al-4V alloy depends on the cooling rates during the cooling from the β region. The α_m, which has a transformation characteristic between martensite (α′) and α diffusion, is reported to be a potential method for obtaining a fine lamellar α/β by thermal decomposition. The different fraction of α_m was found after DSST with the first stage was conducted above the β-transus temperature at 1050 °C, followed by second annealing at different temperatures in the α+β region. It was found that the formation of α_m exists in a specific temperature region. A longer period in this region, which was calculated based on different cooling rates during DSST, will increase the fraction of α_m in the specimen. All specimens after DSST contain α_m with the α width of approximately 1μm and white-dot particles, which is predicted to be V-enriched precipitates. The DSST can be a potential method for producing a high fraction of α_m, which can be thermally decomposed into a fine lamellar α/β, introducing a Ti-6Al-4V alloy with superior mechanical properties.

Abstract: Titanium dioxide (TiO₂) is known as one of the widely used catalysts in photocatalysis process. Recently, the photocatalysis of TiO₂ has been implied in water purification and treatment, particularly dyes and organic compounds degradations. Naturally, the TiO₂ can be found in three phases including anatase, rutile and brookite; each phase has its own specific properties such as grain size, stability and band gap energy. In this work, the effect of calcination temperature on the structure, morphology and photocatalytic activity were investigated. The data suggested that the anatase/rutile ratio of TiO₂ can be controlled through the calcination process. The phase transformation data strongly indicated the liner function between percentage of rutile phase and calcination temperature. The BET analysis provided the consistent data with XRD patterns by showing that the specific surface area was decreased by increasing calcination temperature. The photodegradation of methylene blue under UV irradiation proved that the mixed phase of anatase/rutile ratio at 78.5/21.5 provided the highest photocatalytic activity. The phase composition ratio can influence the nanoparticles properties including band gap, specific surface area and energy band structure. Therefore, the control of anatase/rutile ratio was an alternative to enhance the photocatalytic activity of TiO₂ nanoparticles for dyes and organic compounds degradations.

Abstract: Lightweight ultra-fine grained (<1 μm size) SiC-ZrO₂(3Y₂O₃) composites, with a combination of high hardness, high bending strength and high fracture toughness, were successfully prepared by high energy mechanical milling followed by heat treatment. The SiC-ZrO₂(3Y₂O₃) composites exhibitied high hardness (1707 MPa), high bending strengh (as high as 1689 MPa) and high fracture toughness (up to approximately 12.6 MPa.m^1/2). Such a combination of mechanical properties was attributed to the fine microstructure with a distinct feature consisting of almost continuous network of ZrO₂(3Y₂O₃) phase around SiC grains, or we call harmonic microstructure. It has been demonstrated that a combination of these unique microstructural characteristics was very effective in supressing the initiation of cracks and governing the path of their subsequent growth during fracture, leading to excellent combination of mechanical properties.

Abstract: Effect of Ta-alloying on microstructure, martensitic transformation, mechanical property and shape memory effect of Ni₅₄Mn₂₅Ga_17.5Ta_0.5 alloy has been systematically investigated. The results show that the substructure of Ni-Mn-Ga alloy significantly changed, which was converted from the plate martensite to the lath martensite. Compression tests show that a compressive strength of 1380 MPa with a fracture strain up to 21.92% can be achieved in the Ni₅₄Mn₂₅Ga_17.5Ta_0.5 alloy at room temperature. This is no changed martensite structure with non-modulated T martensite. In addition, the martensitic transition temperature obviously decreases from 350 °C to 208 °Cand hysteresis loop increases about 20 °Cwhen Ta substituted of Ni. The shape memory effect increased with the increase of pre-deformation, nevertheless, the shape recovery ratio appeared firstly increases and then decreases. When the pre-deformation is 10%, 15%, 20%, the shape memory effect of the alloy is 5.1%, 6.8% and 10%, respectively.

Abstract: The purpose of this study was to evaluate the effects of different polishing techniques on the topographical features and phase transformation of monolithic zirconia. Four brands of zirconia were ground and polished using one of four systems. All zirconia specimens were ground with a fine-grit diamond bur (GB) prior to polishing procedures. The surface roughness and phase transition (XRD) were evaluated, and surface characterizations (SEM and XPS) were performed. The highest roughness was obtained with the Tob system. The strongest diffraction peak in the obtained XRD patterns was at 2θ=30.246°. No monoclinic phase change was found in any group. The XPS analysis showed that the atomic percentages of yttrium in the specimens of Cercon before and after polishing were the highest of any sample. All the polishing systems tested may not adversely affect the phase transformation of monolithic zirconia. The Tob system resulted in the highest roughness. The XPS analysis showed that grinding and polishing had some effects on the properties of zirconia from a microscopic point of view.

Abstract: Multiferroics, with two or more coexisting ferroic orders (ferroelectric, ferro (antiferro)-magnetic) in a single phase, display promising photovoltaic characteristics which can be utilised in solar energy harvesting. However, the efficacy is seriously challenged due to their wide band gap, far from the ideal value of ~1.52 eV for photovoltaic applications, resulting in overall unimpressive performance. In the present work, an approach towards imparting multiferroism in an otherwise non-ferroic system was adopted through strain engineering. Bulk SrMnO₃ (SMO) is antiferromagnetic-paraelectric. However, our previous first-principles studies predicted high-pressure phase transformation from bulk non-polar phase to a tetragonal polar phase. In light of the above, SMO was synthesised hydrothermally at 200°C for 96 h using water-soluble nitrate salts of strontium and manganese. FESEM study reveals the formation of hexagonal bipyramid shaped SMO crystals with elongated 1-D features. Powder x-ray diffraction studies and subsequent Rietveld refinement confirm the presence of hexagonal (P6₃/mmc) as well as tetragonal (P4mm) phases. Energy dispersive x-ray analysis (EDAX) confirms Sr/Mn ≈ 1, the stoichiometric ratio. UV-VIS spectroscopy was utilised to estimate the optical bandgap of the as-grown sample which was found to be in the range of 1.4-1.5 eV. Temperature-dependent magnetisation plot indicates the magnetic transition temperature, ~275K.

Abstract: In this work, we simulated and analysed phase transformations in the structure of nanosized bimetallic titanium-containing clusters during the cooling process. The results demonstrate the predominantly α+β crystalline structure of the TiAl nanoalloy after cooling, and the TiV nanoalloy has an amorphous structure. The glass transition temperatures for bimetallic systems TiAl and TiV for various compositions were determined.

Abstract: A high-temperature sealing adhesive was prepared using kyanite, andalusite, and magnesia fully stabilized zirconia (Mg-FSZ) as raw materials and nanozirconia (ZrO₂) sol as binder. The adhesive is chemically near-neutral and suitable for bonding acid, neutral, or alkaline materials at 1560 °C. The mineral composition, volume expansion, and air permeability of multiple thermal cycles were analyzed. Results showed that the volume expansion caused by the mullitization of kyanite and andalusite can compensate for the sintering shrinkage high temperature. MgO could gradually desolvatize from the Mg-FSZ particles and react with SiO₂ and Al₂O₃ to produce a stable forsterite and magnesium aluminate spinel. Hence, Mg-FSZ became unstable, and cubic ZrO₂ transformed into monoclinic ZrO₂ when cooled, leading to a volume expansion of the adhesive, which ensured the sealing effect. A larger critical particle size of Mg-FSZ can provide the adhesive with a more persistent volume expansion during thermal cycles due to the more durable desolvatization of MgO. The nanoZrO₂ sol binder can improve the sintering of the adhesive and bonding of the joint, resulting in a low gas permeability.