Papers by Keyword: Nanocrystalline

Abstract: In this study, titanium dioxide nanoparticles were synthesized for possible application in enhanced oil recovery. Sol-gel method was employed with titanium (IV) isopropoxide as the precursor. The prepared materials were characterized using Powder X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), High-Resolution Transmission electron microscopy (HRTEM) and Brunauer–Emmet–Teller (BET) techniques. Reaction parameter such as calcination temperature was varied during the preparation to obtain the uniform TiO₂ nanoparticles with the smallest particle size and high surface area. The results of study revealed that 400 °C is the optimum calcination temperature in preparing TiO₂ nanoparticles producing the smallest crystallite and particle sizes. XRD results indicated that the nanoparticles have formed anatase phase at 400 °C and achieved low crystallite size of 7.27 nm with the smallest average particle size of 19.53 nm through FESEM and HRTEM observations. BET analysis had achieved the highest surface area 103.64 m²/g.

Abstract: In this paper, a kind of FeCrBSiMnMoW amorphous nanocrystalline composite coating was prepared on the steel matrix by arc spraying, and the microstructure and properties of the coating were studied. The results show that the coating owns compact structure and low porosity. The coating is composed of amorphous phase and α (Fe, Cr) nanocrystalline phase, and the nanocrystalline phase is uniformly distributed in the amorphous matrix. The initial crystallization temperature is above 587.5 °C, which indicates the coating has good thermal stability. The Vickers hardness of the coating is around HV_0.3= 1150. The wear mechanism of the coating is abrasive wear with a friction coefficient of 0.434 and the wear loss rate of only 3.3×10^-5 mm³/Nm, which shows excellent wear resistance of this kind of coating.

Abstract: Mechanochemical synthesis is a simple and effective method to prepare ceramic compounds with nanosize. The present work was aimed at investigating the application of the mechanochemical method to synthesize nanocrystalline hydroxyapatite (HA). The shortest milling time required for synthesizing HA, using Ca (OH)₂ and (NH₄)₂HPO₄ as precursor materials was also established. The synthesized samples were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) to determine the phases evolved, functional groups present and to assess the size and morphology of the particles, respectively. Further, the thermal stability of the synthesized powders was investigated by heating to a temperature of 900 °C with a dwell time of 2 h. The broadening of the XRD peaks was used to find out the crystallite size and Williamson-Hall plots were used to estimate the lattice strain. The XRD and FTIR results demonstrated that the complete formation of the HA phase by mechanochemical method has started within a milling time of 30 min using Ca (OH)₂ and (NH₄)₂HPO₄ as precursors and the Ca/P ratio of the HA increased with increasing milling time. The TEM micrographs demonstrated that the HA particles are nanosized, non-spherical and highly agglomerated.

Abstract: Spark Plasma Sintering (SPS) was studied as a means to consolidate Nd-Fe-B powders, previously subjected to grain refinement by HDDR (Hidrogenation–Disproportionation–Dessorption–Recombination). The sintering process was carried out under 60 MPa constant pressure, varying the maximum processing temperature from 500 °C to 900 °C with a holding time of 5 min. Densification was observed above 600 °C related to the melting of Nd-rich phase. The magnetic properties are clearly related to microstructure coarsening associated with the SPS temperature regime. A monotonic decrease for coercivity (H_cj) was observed as a function of maximum SPS operating temperature with values varying from maximum of 750 kA/m at 500 °C to less than 200 kA/m for SPS at 900 °C. Remanence (B_r) and maximum energy product (BH)_max showed optimum values for intermediate temperatures, since these properties benefit from the densification developed by SPS.

Abstract: This study highlights on a convenient and optimised method for the preparation of nanocrystalline magnesium oxide (MgO) catalyst via sol-gel combustion method. Nanocrystalline MgO was prepared by using polyvinyl alcohol (PVA) as a complexing agent and metal nitrate (Mg (NO ₃ )₂.6H₂O) as a precursor. The obtained MgO powder was calcined at 200 °C, 400 °C, 600 °C and 800 °C. All the MgO calcined samples including commercial MgO were characterised using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-Ray diffraction (XRD) and N₂ adsorption-desorption Brunauer–Emmett–Teller (BET). From FTIR analysis, the appearance of a peak at 3700 cm^-1 represent the O-H stretching bonded with Mg and the broad absorption peak at 3421 cm^-1 indicates O-H stretching band which is due to the absorption of moisture from the surrounding. (BET) results indicate the MgO sample that has been calcined at 400 °C shows the largest surface area. SEM images show there is porosity in all MgO powder. While XRD patterns revealed that higher temperature of calcination gives higher crystallinity of the MgO samples.

Abstract: The bamboo-like FeVO₄ nanocrystallines were synthesized by a two-step method of the microwave hydrothermal-calcination, using Fe (NO₃)₃·9H₂O and NH₄VO₃ as raw materials. The physical and photophysical properties of the as-prepared photocatalysts were fully characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), UV-vis diffuse reflectance spectra and photoluminescence (PL) analysis. The photocatalytic activities were evaluated by the decolorization of RhB solution under UV and visible light irradiation. The results reveal that the precursor solution concentration is 0.15 mol/L, the molar ratio n (Fe)/n (V) is 1, pH=3.0. The microwave hydrothermal reaction is at 180 °C for 120 min and then calcinated under 550 °C for 3 h so as to obtain the triclinic FeVO₄ nanocrystalline. Along [120] and [110], the fore and aft phases of the crystal orientation are bonded self-assembly to grow into the bamboo-like nanocrystalline with the energy gap of 2.42 eV. Under the UV-light irradiation for 240 min, the degradation rate of RhB is up to 91.2%. Adding 0.1 mL H₂O₂ to the solution, the out-phase photo-fenton reaction occurs and the degradation rate to RhB can reach to 98.8% after 8 h visible-light irradiation.

Abstract: Nanocrystalline surface layer about 10~15μm thick was fabricated on the surface of Ti6Al4V sheet by means of the surface mechanical attrition treatment (SMAT). The average grain size was about 10nm and the grain characteristic presented equiaxed morphology. The nanocrystalline surface layer could be perfectly maintained below 550°C in the following thermal stability analysis. Neutral salt mixture was about 21% NaCl, 31% BaCl₂ and 48% CaCl₂ and additionally 5% Na₂CO₃ of total was utilized. After carburizing process, a continuous charcoal grey carburized layer was composed of TiC and carbon supersaturated solid solution, the hardening layer was about 10~15μm thick. The hardness of the outermost surface reached 1000HV, which was much higher than its coarse-grained counterpart in the same carburizing condition. The experimental result indicated that the carburizing kinetics was obviously enhanced by nanocrystalline surface layer assistance. Furthermore, the neutral molten salt-bath carburizing was verified that it could be performed in a relatively low temperature of 650°C.

Abstract: Nanostructured Cu-Ag alloys with bimodal grain size distribution were prepared and their tensile deformation behaviors were studied. The alloys were processed by hot isostatic pressing of blends of nanoand micrometer-sized powder particles. The microstructure of the alloys consisted of nanograins with an average grain size of 40 nm and coarse-grains with an average grain size of 30 um. The bimodal structured alloy exhibited high tensile strengths 522 MPa and a large plastic strain to failure approximately 30%. Simultaneously, Their tensile stress-strain curves displayed a long work-hardening region, and their tensile ductility increased with increasing coarse-grained volume fraction. The high strength primarily results from the contribution of nanograins, while the enhanced ductility may be attributed to the improved strain hardening capability by the presence of coarse grains.

Abstract: The existence of exchange coupling or magnetostatic coupling between two phases, one magnetically hard and other magnetically soft is a way for obtaining higher maximum energy product (BH_max). In this study, it is discussed the microstructures for obtaining either exchange coupling or magnetostatic coupling in real materials. One relevant condition is that the assumptions of the Stoner-Wohlfarth model should be obeyed, with both phases, hard and soft, with dimensions less than single domain particle size. A real possibility is enveloping a spherical grain of hard phase with a first shell of soft phase, and another second shell or layer of a paramagnetic phase. The paramagnetic phase may play relevant role promoting magnetically decoupling between the grains.

Abstract: (NH₄)₂Mo₄O₁₃ and h–MoO₃ nanocrystalline powders were synthesized by precipitation method at a varied pH range from 5.0 to 1.0. The crystal structure, morphology and optical property of samples were determined by X–ray diffractometer (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and UV–vis diffuse reflectance spectrophotometer (UV-vis DRS). At pH 2.0, 3.0 and 5.0, homogenous plate–like (NH₄)₂Mo₄O₁₃ was seen, whereas the heterogeneous hexagonal rod–shaped MoO₃ was found at very low pH of 1.0 and 1.5. Band gap energy of the synthesized (NH₄)₂Mo₄O₁₃ and h–MoO₃ were 3.38 and 3.18 eV, respectively. Photochromic properties of the products were illustrated by color difference before and after UV irradiation using CIE Lab color system. The synthesized h–MoO₃ provided a strong photochromic performance, while the (NH₄)₂Mo₄O₁₃ showed non–photochromic properties. Intercalation of H⁺ in h-MoO₃ were studied using electrochemical characterization by cyclic voltammetry (CV). The diffusion coefficient of the samples increases with decreasing pH of the solution.