Nanostructured Materials最新文献

Various multicomponent alloys containing Ni-Fe-Si-B and refractory metals have been rapidly solidifed in order to check their tendency to produce glassy phases and their devitrification behaviour has been studied. Among these a Ni₃₆Fe₃₂Ta₇Si₈B₁₇ alloy presents primary crystallization with production of fcc nanocrystals embedded in the glassy matrix. The kinetics of the transformation is determined at 530 °C showing that it consists in two steps: fast nucleation and growth of nanocrystals plus homogenization of the remaining matrix with crystal coarsening. Evidence based on TEM observations, XRD patterns and DSC experiments, are reported in favour of a homogeneous mechanism of nucleation. Faceting of nanocrystals is observed for the first time in such transformations and is attributed to a stress effect on the particles.

The magnetic hysteresis properties of granular Cu_100-xCo_x alloys (x = 5 to 20) containing nanometric Co particles, prepared under different quenching conditions and submitted to various anneals, have been measured at room temperature. Major loops (vertex field 1 × 10⁴ Oe) and minor loops (vertex field between 1 × 10² and 9 × 10³ Oe) were measured by vibrating-sample magnetometry. The features of major loops were analysed in terms of sample composition and structural state; the dependence of remanence and coercivity of minor loops on vertex field has been analysed in detail. A novel graphical representation method, particularly suitable for describing the details of the narrow hysteresis loops observed in granular systems has been proposed and checked.

Gas absorbing materials (getters) find several applications in modern vacuum technology; in particular to maintain required vacuum levels in evacuated and sealed enclosures. The gas absorbing properties of these getters depend on the physico-chemical nature of their surfaces. The aim of this work is to study the absorption properties of commercial Zr-based alloys (non-evaporable getters) after mechanical alloying by means of a high vacuum planetary ball milling equipped with an in-situ compaction facility. The main aim was to refine the grain size and to develop particular defect structures to enhance the getter properties. The results have shown an improvement of the specific pumping speed of the ball milled commercial Zr-Zr(V,Fe)₂ alloy with respect to the starting microcrystalline material. In particular, under our experimental conditions, the specific pumping speed vs absorbed gas curve presents a maximum after 2 hours of milling; prolonged milling reduced the pumping speed of the alloyed material. This behaviour is explained in terms of two opposing simultaneous chemical and structural effects.

Two amorphous compositions of the Co-B system, which present quite different magnetic behaviour, have been studied in the as-cast state and after low temperature annealings. The Co₈₀B₂₀ amorphous alloy submitted to thermal treatments undergoes a primary crystallisation which transforms the material into a two phase system composed of nanocrystals of cobalt embedded in a Co-B amorphous matrix. The role of the cobalt grains on the magnetisation processes and the coercive field is analysed for temperature ranging from 77 to 300 K. It is found that the hardening effect of the grains diminishes on increasing the temperature. The large crystalline anisotropy of the Co grains as well as the magnetic state of the grain boundary which transmits the exchange interaction, are mainly responsible for the observed behaviour.

The microstructural, mechanical and magnetic properties of the ball milled and annealed Fe-40Al at% B2 alloys have been studied. The disordering process is characterized by an alloy hardening and a para-ferromagnetic transition, which depend mainly on the grain refinement and the creation of antisite defects. The reordering process shows a continuous softening of the nanocrystalline alloy and the recovery of the paramagnetic state, which is essentially governed by the annihilation of antisite defects. The correlation between the microstructural, mechanical and magnetic properties make magnetic measurements a suitable, non-destructive, tool to study the microstructural and mechanical state of FeAl nanocrystalline alloys.

Thin film multilayers, constituted by ten Fe/Al bilayers with Fe thickness in the range 15 ÷ 30 Å and constant Al thickness of 20 Å, have been obtained by electron beam evaporation in ultra high vacuum. Sample characterization was performed using transmission electron microscopy, surface Mössbauer spectroscopy and alternating gradient force magnetometry. For iron thickness ≥ 20 Å the periodicity of multilayers is maintained, while for 15 Å the periodicity is almost completely lost and fine particles showing a superparamagnetic behavior begin to form. In all samples the magnetization is laying in the film plane, and, moreover, it shows an easy and hard direction at 90°. The magnetic behavior is typical of poly-domain systems.

Tin oxide thin films, deposited on alumina or silicon, are commonly employed as sensors of pollutant gases in the atmosphere. The response of these materials in gas sensing applications depends on the microstructural and morphological characteristics of the film, which are dependent on the specific production process and deposition conditions. Sensor sensitivity and selectivity can be improved by doping the oxide layer with small quantities of Pt. Tin oxide films were deposited on polycrystalline alumina substrates by reactive r.f. magnetron sputtering at different values of the substrate temperature during the process. Transmission electron microscopy was employed to investigate the microstructure of the samples, before and after Pt doping. The TEM results are discussed in relation to the behavior of the films in sensing applications.

Phase nucleation in irradiated metallic alloys is studied by the atomistic Segregation-Charge Transfer (SCT) model, taking into account that when bombardment conditions favour the development of dense collision cascades the system explores kinetic and thermodynamic regions conventionally not accessible. Locally non-equilibrium compositional and charge density profiles evolve at the interface between a cascade and the crystalline matrix, driven by the interface enrichment of one of the film constituents. Charge relaxation to (meta)stable equilibrium is simulated by Charge Transfer Reactions (CTR), each involving a couple of dissimilar atoms of the initial alloy. Both the amount of energy needed in a single CTR and the local strain undergone by the target as a consequence of ion formation are calculated. Considering a set of nineteen alloys which are reported to be amorphized and seventeen ones which remain crystalline under ion bombardment, for both structure stability parameters threshold values are found, which allow to separate crystalline from vitrified alloys.

The structural evolution of three silicon oxycarbide glasses was studied by X-ray diffraction (XRD) as a function of the pyrolysis temperature. Three compositions were prepared by the sol-gel method and pyrolysed at 1000°C under atmosphere of Ar. The black glasses obtained correspond respectively to i) silicon oxycarbide network with excess of C, ii) stoichiometric SiC_xO_2(1-x) where x = 0.3, and iii) silicon oxycarbide network with deficiency of C, i.e. with excess of Si. At this stage of the treatment, the samples are made up of a single and amorphous phase. A phase separation occurs after further pyrolysis in the high temperature range 1200–1500°C, leading to the formation of nanocrystalline β-SiC and amorphous SiO₂. We used a cubic silica structure factor to model the component due to amorphous silica. This enabled us to apply the Rietveld method to all patterns and to obtain a satisfactory fit of the experimental data. From these refinements, the amount of each phase (crystalline or amorphous) can be determined, based on the assumption that the electron density of the model agrees with the actual amorphous phase. A comparison is also made with results from chemical analysis and ²⁹Si Magic Angle Spinning NMR found in the litterature. Concerning the crystalline component β-SiC, its average crystallite size and microstrain were also evaluated. The evolution of the phase separation was then reported versus the pyrolysis temperature and seems to suggest a nucleation-and-growth mechanism.

Co-Cu multilayer granular films deposited onto SiO₂/Si substrates have been submitted to thermal treatments in scanning mode. Their magnetisation, resistance and magnetoresistance have been studied after applying two different heating rates. The analysis of experimental data points out that only with the lower heating rate the properties of the films are strongly modified. In this case, indeed, XPS measurements show the presence of interdiffusion between atomic species of the substrate and metallic species inside the film. XPS data are presented to show the modifications in composition and chemical environment produced by thermal treatments.