Nanostructured Materials最新文献

The nanoparticles γ-Fe₂O₃ coated with surfactants, DBS, ST and CTAB, repsectively, were synthesized by microemulsion method. The coated ferric oxides, which show enhanced nonlinear optical properties compared with their bulk counterpart, were probed by EXAFS in solution forms. The x-ray absorption spectra for Fe K edge were measured at room temperature by help of synchrotron radiation light source. Extended x-ray absorption fine structure analysis shows that the coating layers of organic molecules interact with surface atoms of nanoparticles, the Fe-O bond length was extended.

The isothermal grain growth of nanocrystalline SnO₂, prepared by the sol-gel route was investigated at various temperatures between 500°C and 800°C. Grain growth data were analyzed using two different models. A conventional grain growth model for polycrystalline materials yields an extremely low activation energy of 47 kJ/mol, but large grain growth exponent n from 5 to 11. These values exceed the rational region deduced from conventional theory. An alternative model is based on the assumption that the ordering of the interface regions in nanocrystalline SnO₂ occurs simultaneously with grain growth by structural relaxation. This structural relaxation model describes the grain growth kinetics satisfactorily and also yields a low activation energy of 31 kJ/mol appropriate for the rearrangement of atoms.

Processing of self-assembled magnetic nanocrystals has vitally scientific and technological interests. In this paper, three different kinds of self-assemblies of cobalt nanocrystals were processed using particles processed by thermal decomposition of cobalt carbonyl in toluene. In case 1, two-dimensional self-assembly of cobalt nanocrystals with an average particle size of 9.2 nm and polydispersity of 9% has been formed. In case 2, macroscopic whisker structures composed of self assembled cobalt nanocrystals (with an average size of 8 nm) were formed during the thermal decomposition under an externally applied magnetic field. The whiskers can be aligned in the longitudinal axis direction during drying using an external magnetic field. In case 3, one-dimensionally ordered, densely packed self-assemblies have also been prepared using cobalt nanocrystals with irregular shape and larger particle sizes (10–20 nm). The formation of these different structures is attributed to the magnetic properties of cobalt particles with different sizes.

Nanophase and coarse-grain titanium films were prepared on Mo substrates by Ion Beam Assisted Deposition. Ion implantation of helium was used to simulate the behavior of helium in titanium films. Helium ions (100 keV) were implanted into the titanium films with a dose of 5 × 10¹⁷/cm². The retained helium in the titanium films after implantation was detected by Enhanced Proton Back Scattering (EPBS). The microstructure of titanium films and the distribution of helium bubbles were studied by Transmission Electron Microscopy (TEM). It was found that helium bubbles were formed in the coarse-grain titanium films with a size distribution of 1 to 10 nm and a numerical density of 5 to 25 × 10²²/m³. No helium bubble was observed in the nanophase titanium films. A discussion is presented for helium bubble formation.

Refining grain size to the nanocrystalline level has been suggested as a way of improving strength while enhancing ductility and toughness. In the present study, nanocrystalline bulk FeAl has been prepared by mechanical alloying and hot forging. Powders quickly reach a state of partial order during milling, and low temperature annealing is sufficient to chemically homogenise and give full order. Contamination during milling leads to the formation of carbide and oxide particles, which stabilise fine grains during heating. Bulk materials show grain sizes of 20nm to 100nm depending on the consolidation temperature. Hardness and compression strength show little change over this grain size range. Fracture toughness stays high down to moderately small grain sizes, falling only for consolidation at the lowest temperatures. There appears to be a reasonable range of fine grain sizes (40–100nm) where good interparticle bonding and high densities can be achieved leading to good strength and toughness.

The Raman and Mössbauer spectra of nano-SnO₂ specimens with crystallite size 3–16 nm were measured at room temperature. Specimens were annealed at different temperatures in oxygen atmosphere and vacuum respectively. Both the Raman and Mössbauer spectra of the specimens consist of crystalline and interface (or surface) components. The effects of oxygen deficiency on the Raman spectra are presented and correlated with the changes of the Sn-O bond properties, which were obtained from the corresponding Mössbauer spectra.

Barium titanate (BaTiO₃) ceramic/ceramic nanocomposite thick films (∼16μm) for ultrasonic transducer applications were prepared by a modified sol-gel process. Nano-sized BaTiO₃ powder was dispersed in a sol-gel matrix of BaTiO₃ to form a composite solution. Films were prepared by spin coating and then annealed at various temperatures. The crystallization of the composite film was studied by X-ray diffraction. The dielectric permittivity and the ferroelectric properties of the film were also measured.

Equal channel angular pressing (ECAP) and high pressure torsion (HPT) are two severe plastic deformation (SPD) processes that have been used to process ultrafine-grained (UFG) materials. In this investigation, we have attempted to combine these two processes to refine the grain size of coarse-grained pure titanium. ECAP processing was first carried out at 500–450 °C to refine the grain size to about 300 nm. Further processing by HPT resulted in finer grain size and higher dislocation density. The second step, HPT processing, also increased the microhardness, ultimate strength, yielding strength and ductility of the UFG pure titanium.

The electrical properties and the microstructure of nanocomposite materials composed of nano-sized amorphous silica powder and micron-sized tungsten powder were studied as a function of composition and heat treatment temperature. The dielectric constant and the electrical conductivity of the nanocomposites are higher than those of pure silica powder at all compositions. The percolation threshold of this system is about 20% vol. W. Above this composition a sharp increase in the dielectric constant and in the electrical conductivity occurs. This increase becomes even sharper after the nanocomposites are heat treated at 950°C for 1 hour. No changes in the microstructure are observed up to this temperature. It is suggested that the electrical properties of the nanocomposites are dominated mainly by dangling bonds at the W/SiO₂ interfaces. Further research should be undertaken to investigate the factors controlling the dielectric constant and the electrical conductivity in order to obtain high capacitance materials with high dielectric constant and low electrical conductivity.

A new approach for gas-phase combustion synthesis of nanosized particles using a novel hybrid burner facility is demonstrated. The basis of the synthesis technique is to use both a premixed flame and a diffusion flame to control the synthesis environment. Specific experimental results for silica (SiO₂) production from silane/hydrogen/oxygen/argon (SiH₄/H₂/O₂/Ar) flames are presented. A parametric study of several burner conditions was conducted, and the subsequent effects on the particles produced were determined. Particle morphology was examined using transmission electron microscopy (TEM). The results indicated a broad variation in particle size and structure as a function of the burner operating conditions (in particular, reactant stoichiometry and flame geometry). Particle structures were aggregated with primary particles varying from 6–8 nm in size (high oxygen concentration conditions) to larger more continuous structures with primary particles 18–20 nm in size (low oxygen concentration conditions). Bulk material properties were examined using Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA), nitrogen adsorption (BET) and x-ray diffraction (XRD).