Nanostructured Materials最新文献

Structures of alloys prepared by an inert-gas condensation and compaction method and subsequently rolled were examined with interest in the deformation mechanism. The initial grain size is found to be smaller in the alloy than in the single component material, but XRD measurements indicate that a normal preferred orientation of rolling is developed in the Ag/Fe alloy slightly but not in the Ag at all. The grain growth occurs more or less but hardness is not increased by deformation in both materials. Strain contrasts along grain boundaries are observed by TEM, but tangled dislocations are found in grain interior of neither of the deformed materials. The observation has indicated that the dominant mechanism of deformation in the examined nanocrystalline materials, especially in the single component material, is considered to be grain boundary sliding accomplished by a diffusional process.

Visible photoluminescence from silicon nanocrystals embedded in SiO₂ matrix by ion beam mixing was investigated. Photoluminescence spectra of ion beam mixed SiO₂/Si/SiO₂ films excited by an Ar-laser (457.9 nm) showed more intense luminescence with a peak centered at 720 nm than that prepared by the conventional ion implantation method. The formation of nanocrystals in SiO₂ matrix was confirmed by cross-sectional high resolution transmission electron microscopy. The red luminescence is attributed to the silicon nanocrystals produced by ion beam mixing.

The effects of the geometry, thermal treatment and the amplitude of applied magnetic field on the coercivity behaviour of glass covered Fe_73.4-xCu₁Nb_3.1Si_13.4+xB_9.1 (x = 0; 1.1 and 1.6) have been investigated. Variations of the coercivity, H_c, with the geometry (d/D ratio, d –diameter of the metallic nucleus, D-total diameter, d/D ranging from 0.1 to 0.9) are attributed mainly to the internal stress acting on the metallic nucleus owing to the glass coating. Annealing temperature (300–700 °C) dependencies of H_c exhibit some peculiar differences with respect to that of the classical FINEMET ribbon, especially in the temperature range of 400–500 °C (very beginning of the nucleation of the nanocrystallization process) and at 650–700 °C (magnetic hardening owing to the iron borides segregation) depending in both cases on ratio d/D. With decreasing of d/D ratio the transition soft-hard magnetic character takes place in a narrower annealing temperature range with a rectangular hysteresis loop of H_c≈2400 A/m. The evolution of H_c with the applied magnetic field amplitude, H_o (<2500 A/m) for the as-prepared and treated at low temperature samples results to be quite linear. This linear behaviour of H_c (H_o) allows to assign a nucleation of domain walls mechanism mainly for the magnetization process connecting with a magnetic bistable behaviour, while the non-linear H_c (H_o) curves of samples with x = 1.6 treated above 650 °C and 700 °C suggest that different mechanism takes place.

Grain growth kinetics for nanostructured nickel was studied by means of isothermal and isochronal annealing and TEM. The range of temperature for the isothermal and isochronal annealing has been selected between 603K and 683K. The isothermal curves for grain size show that an explosive grain growth occurs in the early stage of annealing, but the growth returns normal beyond grain size of 600 nm. The change in microstructure in the annealed specimens has been investigated by TEM. The activation energy for the grain growth was found to be 102 kJ/mol from the plot of grain growth rate vs. 1/T, which is comparable to the activation energy for diffusion at the grain boundaries. The time exponent was obtained from the plot of grain size difference vs. annealing time, and found to be in the range of 1/6–1/7.

A novel method to produce powders and films from ceramic targets is presented. It is shown that the laser spark atomizer (LINA-SPARK™), LSA, combines the advantages of operating at atmospheric pressure and ambient temperature to produce highly uniform films with a web-like structure which can be deposited at relatively high deposition rates (ca. 1 μm·min⁻¹). The preparation of powder particles and coating starting from commercial ceramic specimen (Al₂O₃) is investigated. Highly agglomerated nanopowders and homogeneous mesoporous coatings forming a web-like structure are prepared using this new technique. The formation of the nanoparticles is explained following a liquefaction process which generates an aerosol, followed by the cooling/solidification of the droplets which results in the formation of a smoke. The general dynamic of both the aerosol and the smoke favors the aggregation process and micrometer-sized fractal-like particles are formed. The laser spark atomizer can be used to produce highly mesoporous thick films. The porosity can be modified by the carrier gas flow rate thus enabling for a control of the microstructure of the coatings which make these nanoparticulate thick films suitable candidates for application in membrane technology, catalysis and lithium ion batteries. ZrO₂ and SnO₂ nanoparticulate thick films are also synthesized successfully using this new process with quite identical microstructure.

Wear resistant multilayers consisting of the components TiN and Al₂O₃ are deposited on WC-Co-hardmetal substrates by plasma assisted chemical vapour deposition (PACVD) using different variants. Cross sections of the layered system are investigated by means of analytical transmission electron microscopy (TEM). Correlations between the layer formation and the nanostructure of the layers are revealed. Electron energy loss spectroscopy (EELS) in the scanning mode is used for the investigation of nanoscale interface regions, whereby both element- and bonding specific signals are used for characterization.

Amorphous Fe_73.5Cu₁Nb₃Si_13.5B₉ ribbons prepared by melt-spinning were annealed at 500°C for different periods. The dependence on annealing time (t) of the average grain size (d) and volume fraction (V_c) of α-Fe(Si) phase, as well as the Young’s modulus (E) of the ribbons were investigated by X-ray diffraction (XRD) and tensile test, respectively. The grain size of the α-Fe(Si) phase grew in stages with annealing time, exhibiting plateaus between rapid growth periods. The Young’s modulus oscillated with annealing time and the peaks in the E-t curve appeared at the times when grain growth slow down, i.e. the plateau region. The chemical compositions of the amorphous layer surrounding the growing α-Fe(Si) phase and that of the amorphous matrix were estimated. The time taken for the composition equilibration between the amorphous shell ahead of the crystallization front and the amorphous matrix was calculated with Fick’s diffusion equation. The calculated time had the same order of magnitude as that of the period of the staged-growth of the α-Fe(Si) phase. This staged-growth process, controlled by the diffusion of niobium, and the periodical structural relaxation of the amorphous matrix give rise to the oscillating behavior of the E vs. t relationship.