Nanostructured Materials最新文献

A co-precipitation method was investigated to manufacture 25 vol% YAG-Al₂O₃ nanocomposites with excellent strength characteristics at room temperature. The as-prepared powders were sintered by hot-pressing in N₂ to nearly full density with heating rate of 10°Cmin⁻¹ to 1400°C for 1h and at 30MPa pressure. The microstructure was characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and transmission electron microscope (TEM). Microstructural results of hot-pressed composites reveal that the intergranular YAG grains of micrometer size (≈800nm) were dispersed at the grain boundaries of a dense Al₂O₃ matrix, and most fine grain YAG grains of nanometric size (≈100nm) were entrapped within the Al₂O₃ grains in the composite. The room temperature fracture strength and fracture toughness of the hot-pressed composites were 611MPa and 4.53MPa·m^−1/2, respectively. These strength and fracture toughness values are higher than those reported in other studies for YAG-Al₂O₃ system. It is proposed that the improvement of mechanical property of hot-pressed nanocomposites is due to the reduction of the grain size of the matrix and the improved strength of the grain boundaries in 25 vol% YAG-Al₂O₃ nanocomposite.

Au, Pt, Pd, Ru and Ir nanoparticles with a narrow size distribution have been synthesized by chemical reduction of their corresponding metal species in ethylene glycol. In all cases, the average particle size was found to be smaller than 10 nm. Particle size was mainly controlled by varying the initial total metal concentration, the reaction temperature, and the concentration of PVP. With the exception of Ir, metal particle agglomeration and sintering was prevented by the addition of PVP, a well known protective agent that also aids particle dispersion.

This paper studies the stability of thin films of nanostructured titania at elevated temperatures. Thin films of titania are intended for use in many applications, including high-temperature gas sensing devices. The initial structure of the films consists of nanocrystalline anatase (∼8nm). Exposure of the films to temperatures > 400°C results in the nucleation and subsequent rapid growth of rutile grains (final grain size > 300nm). In-situ, hot-stage experiments in the TEM were carried out, which revealed details about the nucleation and abnormal grain growth processes in this system. It is argued that coarsening of the nano-structure following the transformation is a characteristic of polymorphic reactions from metastable to stable phases. The nucleation process is a critical rate-controlling process for maintaining nanosize grains in the transformed material.

Measurements of grain size in powders of the Cu-15at%Al, Cu-20at%Ni, Cu and Ni systems, milled for different times were conducted. X-ray diffraction (XRD) (Warren-Averbach method) and transmission electron microscopy were used for that purpose. From both techniques, distributions of grain diameter (length) were obtained, which permitted to compare both results. Such results were interpreted by considering previous studies of particle size evolution and microstructural evolution. A better comprehension of phenomena that occur in the mechanical alloying, as the grain refinement and the solid solution formation, is attained.

The grain size results obtained by the two techniques were coincident for the Cu-15at%Al system. This system did not present adherence of the powders to the milling media. In the case of the Cu-20at%Ni, Cu and Ni systems, that presented adherence, the results were coincident solely in the powder milled for 864 ks. This is explained due to the microstructural homogenization, reached until that time.

The grain size as a function of the milling time of the two classes of FCC systems, being referred to the adherence phenomenon, presented opposed trends. For example, for the Cu-at15at%Al system, the 70% of the population of grains had a grain size smaller than 7.5, 12 and 20 nm for the milling times of 180, 360 and 864 ks respectively. In the case of the Cu-20at%Ni, Cu and Ni systems, the 70% of the population of grains had a grain size smaller than 35, 22 and 16 nm for the milling times of 180, 360 and 864 ks respectively. These contrary trends reflect different mechanisms of grain size refinement.

A maximum value of grain size of 20 nm required to form the solid solution in the Cu-at15%Al and Cu-20%atNi systems was found.

Solid solutions of nanophase cerium oxides have been prepared and the relationship between their bulk crystal structure and surface characteristics has been studied at room temperature with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Dopants with a valence lower than +4, such as Ca²⁺, Nd³⁺, and Pb²⁺, introduce structural defects (oxygen vacancies) in the cerium oxide lattice, which has been found to affect the redox catalytic activity of the materials. The introduction of oxygen vacancies leads to the appearance of an O1s core level peak with a shift of 2.0–2.5 eV to higher binding energies as compared to the core level peak of the lattice oxygen in CeO₂. The intensity of this high binding energy O1s peak varies with the expected concentration of oxygen vacancies in the surface, the type of dopant cation and its concentration in the solid solution. It was found that carbonate and hydroxide species are responsible for the appearance of this O1s peak, presumably as a result of capping of oxygen vacancies at the surface. The implications of these surface groups on the catalytic activity of the materials are discussed.

Nanoscale α-Al₂O₃ powder has been prepared by the polyacrylamide gel method. Because polymer-network inhibit the aggregate of Al₂O₃, nanoscale α-Al₂O₃ powder with about 10 nm size can be obtained. Its calcination temperature is 1100°C, which is 100°C lower than general calcination temperature in other methods.

Based on microstructural observations by TEM and in particle size distribution done by sedimentation-photometry, a new grain size refinement mechanism for ductile powders in mechanical alloying is proposed. A 90–95% of the particle population was of submicrometric fragmented particles. These were detected from the beginning of the milling process up to 90 ks. It seems that the fragmentation of the original particles occurred under dynamic conditions to generate those submicrometric ones. Under these conditions the original grain size (100 nm to 350 nm) was preserved and a low level of dislocations was observed at these submicrometric particles. Once these submicrometric particles were deformed, grains smaller than 20 nm were observed. It seems from TEM results that the submicrometric fragmented particles were also deformed under dynamic conditions. This could be a new grain size refinement mechanism present in ductile metallic powder systems where the fragmentation is the dominant stage from the beginning of the milling up to some intermediate milling time.

In the Cu-20at%Ni, Cu and Ni systems where the particle coalescence process was the dominant stage during all the milling process, a derivation of the mechanism proposed by Hellstern [3] was identified. In our case, powders were mainly deformed by slip and not by shear.

It recognizes that the way to refine the grain size in milled powders is influenced at least by the metallic system used as well as by the equipment and the process conditions employed.

Gold nanoparticles have been synthesized by sol-gel technique. AFM analysis shows the presence of ≤ 12–14 nm and 40–60 nm sized particles for H₂S/not heated and H₂S/heated samples respectively. XPS analysis of H₂S/not heated sample reveals that the core-level Au 4f7/2 B.E. is shifted by +0.3 eV with increase in the FWHM by 0.2 eV relative to the respective bulk values of 84.0 eV and 1.1 eV. The results are interpreted in terms of the changes in the electronic structure due to finite cluster size and creation of +ve charge over the surface of cluster during the photoemission process itself. The electronic structure of Au nanoparticles (or clusters) produced by present sol-gel technique is compared with that of Au clusters deposited by evaporation method described in the literature.