Nanostructured Materials最新文献

We have produced Ag-Nb clusters by a facing-target type plasma-gas-condensation cluster source as our first step toward alloy cluster formation. The Ag-Nb clusters have been deposited on substrates and examined by a transmission electron microscope with a nano-beam energy dispersive X-ray analysis. We have obtained Ag-Nb alloy clusters with the sizes range between 5 and 10 nm in diameter. Their chemical compositions are broadly dispersed and partitioned into Ag-rich and Nb-rich ones, being consistent with the immiscible type equilibrium phase diagram. This result suggests that alloy cluster formation is driven by the alloy phase stability.

The addition of 17 at.% of elemental Ni powders to the cryogenic attritor milling of Metglas Fe₇₈B₁₃Si₉ slowed the mechanical crystallization of the α-Fe and Fe₂B phases. Transmission electron microscopy (TEM) observation and energy dispersive spectroscopy (EDS) analysis indicated that no more than 3.60 at.% of Ni dissolved into the Metglas, which was well within the equilibrium solubility limit of Ni in α-Fe. It is proposed that the addition of Ni impede mechanical crystallization during attritor milling by inhibiting bending and wear-like processes which could otherwise cause crystallization.

Amorphous cobalt nanoparticles were synthesized by laser-inductive vaporization condensation technique. The nanoparticles are in fine size dispersion and the average particle size is 20 nm. When heating the amorphous cobalt nanoparticles in O₂ ambient condition there is a sharp exothermic reaction in temperature range from 2072°C to 2972°C with a peak temperature of 260°C and exothermic enthalpy of 100.08 kJ/mol. When heating in Ar ambient condition, the amorphous cobalt nanoparticles may begin to transform from the amorphous solid to a supercooled liquid state at about 167°C and keep the supercooled liquid state in a wide temperature span from 167°C to 277°C, followed by crystallization at 277°C. The crystallization takes place in the temperature span from 277°C to 477°C. The corresponding exothermic heat is equal to 23.2 kJ/mol, which is larger than the heat of fusion of bulk cobalt (14.4 kJ/mol).

Hemispherical-grained Si (HSG) coating was deposited on an amorphous Si layer by using a rapid thermal chemical vapor deposition (RTCVD) process. The formation of the HSG coating consists of “seeding” and subsequent isothermal annealing stages. The microstructure and surface morphology of the HSG coating was studied after various stages of its formation by TEM and HRSEM techniques. The “seeding” process results in formation of nanometer size Si crystals on the substrate. Each Si crystal is covered with a thin amorphous Si film which forms a hemisphere shape. The annealing process results in growth and crystallization of polycrystaline Si hemispheres. The RTCVD process enables high control of the size and crystallinity of the Si hemispheres. The mechanism of formation and growth of the HSG coating is discussed and compared with other fabrication methods.

Two kinds of Cu/Nb nanocomposite wires were investigated using field ion microscopy (FIM) and 3D atom probe. These two techniques revealed for the first time the nanoscale microstructure of nanocomposite wire cross sections. FIM investigations confirmed the Cu and Nb texture and the disorientation between (111) Cu and (110) Nb planes. Low angle Nb/Nb grain boudaries were also observed. Thanks to 3D atom probe, parts of niobium fibres and copper channels a few nanometer width were mapped out in 3D. Smooth Cu/Nb interfaces were attributed to stress-induced diffusion. Shear bands, observed perpendicular to the wire axis, were attributed to tracks of moving dislocations in a copper channel.

DC magnetron sputtering of silicon was carried out at 175 watts in argon gas at pressures from 100 mtorr to 900 mtorr. Sputter deposits were collected on an array of transmission electron microscopy (TEM) grids placed between the sputter source and a cold-finger located 10.5 cm above the sputter source. The resulting deposits were analyzed by TEM, and the morphologies were found to include granular films, well-defined particles 5–13 nm in diameter, and transition morphologies between that of particles and granular films. The resulting morphology map, as a function of sputtering pressure and TEM-grid location, indicated that particles were more likely to form at higher pressures and locations farther from the source. In addition, results obtained by varying the cold-finger temperature and the sputter-source/cold-finger distance indicated that the temperature of the cold-finger can play a significant role in particle formation at small sputter-source/cold-finger separations. Deposits with granular-film morphologies were amorphous under most conditions. However, deposits containing particles exhibited the diamond-cubic crystalline phase under some conditions as well as the amorphous phase.

The polyoxocation Al₁₃O₄(OH)₂₄(H₂O)₁₂⁷⁺, known as an Al₁₃ cluster, has been synthesized by controlled hydrolysis of aluminum(III) aqueous solution at 70 ^oC, and pH between 5.0 and 6.0. The formation of the polyoxocation Al₁₃O₄(OH)₂₄(H₂O)₁₂⁷⁺ in solution was experimentally confirmed by ²⁷Al NMR. The Al₁₃ cluster-containing powder was obtained by precipitating the polyoxoaluminate ion from the solution by the addition of a suitable salt, e.g. sodium sulphate or ammonia oxalate. The obtained powder was characterized by TGA, XRD, and SEM. It was found that the sulfate-precipitated powder had three different crystal structures, i.e. monoclinic fiber, monoclinic rectangle shaped crystal and tetrahedral shaped crystal, while the oxalate powder obtained from the oxalate precipitation was a spherical-particles powder with particle size around 200 nm. XRD of the Al₁₃-sulfate showed that the powder consisted of a cubic phase with a unit cell size of 17.9Å. After calcination of the Al₁₃-sulfate and Al₁₃-oxalate salts at 1000 °C, the powder was converted to α-alumina. The α-alumina powder obtained from the oxalate route was about 100 to 200 nm in diameter.