Nanostructured Materials最新文献

The process utilizing metal-organic chemical vapor deposition (MOCVD) was conducted in a fluidized Al₂O₃ powder bed for the preparation of nano-Mo ceramic composites. During the process, Mo species were deposited in fine Al₂O₃ ceramic powder using a pyrolysis of Mo carbonyl. The composition and crystallinity of the intermediate phases of Mo₂C_xO_y, and the microstructure of the coated particles and coated layer were analyzed using XRD/SEM/TEM techniques. The granulated powder was then treated by H₂ reduction, pressureless sintering or hot-pressing in a vacuum, which could achieve densities better than 99% T.D. The densification, wear, and microstructural properties of the dense nano Mo-composites were then investigated and discussed. It is seen that the nano-inclusion of Mo grains inhibited the grain growth of the alumina matrix, which had a mean grain size of either 4.9 μm or 1.2 μm, as the volume fraction of Mo increased from 0 vol% to 5 vol%. The wear resistance of the nano-Mo/Al₂O₃ was approximately 2 times better than that of pure Al₂O₃. Through an understanding of the pyrolysis of Mo(CO)₆ and grain growth kinetics of Mo-species growth kinetics, the morphology and size of the Mo grains in ceramic composites can be modified.

Isothermal annealing of a Ni₈₁P₁₉ amorphous alloy performed at 600 K for 1200 s resulted in the formation of a nanocrystalline state with 10 nm average grain size. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements were performed to study the influence of further heat treatments. Linear-heating DSC scan starting from the nanocrystalline state revealed a broad exothermic contribution between 600 K and 770 K, corresponding to a grain-growth process. Isothermal annealing of the nanocrystalline alloy yields the complete formation of fcc-Ni and Ni₃P phases. This process took place by normal grain-growth at low annealing temperatures, while abnormal grain-growth was observed with increasing annealing temperatures.

Magnetization curves were measured on Ni₃Fe and Fe₇₅Al_12.5Ge_12.5 nanocrystals of different grain sizes. These materials were prepared by high-energy ball milling, followed by annealing at various temperatures. The alloy compositions were chosen because they have low magnetostriction in bulk form, implying that strain in the samples should have little effect on their magnetic properties. The M-H magnetization curves were used to obtain the coercivity, the maximum permeability, and the saturation magnetization. Differences in these magnetic properties were related to changes in grain size and internal RMS strain. In spite of the low bulk magnetostriction of these materials, the internal stress controlled the coercivity. The changes in permeability, however, were not as expected from the trend in grain size. We suggest that the powder morphology, plays an important role in determining the soft magnetic properties of these nanocrystalline alloys.

Pure amorphous SiO₂ powder with nanometer size particles was exposed to various heat treatments up to 1200°C. The microstructure, particle size, dielectric constant and electrical resistivity of the powder were characterized after each heat treatment. It was found that the dielectric constant of the powder is higher compared to that of amorphous SiO₂ thin films. This enhancement is correlated with higher density of Si dangling bonds, which contribute to the polarization of the material. A major decrease in the dielectric constant takes place during heating up to 600°C where neither growth nor crystallization of the particles occur but only pronounced reduction in the density of the Si dangling bonds is observed. Pronounced growth and initial crystallization to a cristobalite phase of the powder particles occur at about 1100°C and have a minor effect on the dielectric constant. The Si dangling bonds also serve as electrical conducting centers in the powder and their annihilation due to the heat treatments is well observed as an increase in the electrical resistivity of the powder.