National Institute Of Materials Physics - Romania

Synthesis and characterization of iron oxide nanoparticles coated with a large molar weight dextran for environmental applications are reported. The first experiments involved the synthesis of iron oxide nanoparticles which were coated with dextran at different concentrations. The synthesis was performed by a co-precipitation technique, while the coating of iron oxide nanoparticles was carried out in solution. The obtained nanoparticles were characterized by using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction spectrometry, Fourier transform infrared spectroscopy and superconducting quantum interference device magnetometry. The results demonstrated a successful coating of iron oxide nanoparticles with large molar weight dextran, of which agglomeration tendency depended on the amount of dextran in the coating solution. SEM and TEM observations have shown that the iron oxide nanoparticles are of about 7 nm in size.

The divertor of a fusion reactor like DEMO requires materials able to withstand high heat fluxes and neutron irradiation for several years. For the water cooling concept of this essential part of the reactor, the most likely plasma facing material will be W, while the heatsink material considered is CuCrZr or an improved version of such a Cu-based alloy. To realize W-Cu alloy joints able to withstand thousands of thermal cycles can be difficult due to the difference between the thermal expansion coefficients of these materials. In this work we investigate the possibility to realize such joints by using W-Cu functional gradient materials (FGMs) produced from nanometric and micrometric metallic powders mixtures and consolidated by spark plasma sintering at about 900 degrees C. Morphological and thermal properties investigations, performed for typical compositions, shows that the best results are obtained using powders with micrometric dimensions. A resulting 1 mm thick, 3 layers W-Cu FGM produced by this simple method shows a remarkable almost constant thermal conductivity value of 200 W m(-1) K-1, from room temperature up to 1000 degrees C.

DEMO fusion reactor divertor is expected to extract a heat flux of about 10 MW/m(2). One of the most promising concept design for it is the W-monoblock, which should be connected to a CuCrZr or an advanced Cu ODS alloy pipe passing through the W component. Since the optimum operating temperature windows for W and existing Cu alloys are far away from overlapping, a suited interface is needed to keep the adjacent materials as close as Possible to their respective temperature operating windows. The interface material should therefore have a low enough thermal conductivity to act as a thermal barrier and a thermal expansion coefficient suited to protect the W-pipe joint from stresses induced by the different thermo-mechanical properties of W and Cu-alloys. As interface materials we have considered Cu-ZrO2 composites produced by powder metallurgy route. Such materials can be realized in an unexpected large compositional range (up to at least 90% ZrO2 volume concentration) and be easily further joined to both W and Cu-alloys by an electrical field assisted technology. We analyse their microstructure and thermo-physical properties both as single materials and included in W-thermal barrier-CuCrZr 3-layers systems in comparison to those of previously produced Cu-ased composites and commercially available Cu foams.

Samples of MgB2 with relative density above 95% were obtained by spark plasma sintering (SPS) at 1150 A degrees C for a heating time of 100 A degrees C/min and a maximum pressure of 95 MPa. Dwell time was of 1, 4, 7, 10, and 20 min. Samples show that dwell time has a low influence on superconducting characteristics. However, small differences were observed and they are discussed from the viewpoint of density, structural, microstructural, critical current density, irreversibility field, and pinning type. The dominant contribution for all investigated samples is given by point and delta T (c) pinning.

We explored in-gap states (IGSs) in perovskite oxide heterojunction films. We report that IGSs in these films play a crucial role in determining the formation and properties of interfacial two-dimensional electron gas (2DEG). We report that electron trapping by IGSs opposes charge transfer from the film to the interface. The IGS in films yielded insulating interfaces with polar discontinuity and explained low interface carrier density of conducting interfaces. An ion trapping model was proposed to explain the physics of the IGSs and some experimental findings, such as the unexpected formation of 2DEG at the initially insulating LaCrO3/SrTiO3 interface and the influence of substitution layers on 2DEG.

MoO2-Fe2O3 nanoparticle system was successfully synthesized by mechanochemical activation of MoO2 and alpha-Fe2O3 equimolar mixtures for 0-12 hours of ball milling time. The study aims at exploring the formation of magnetic oxide semiconductors at the nanoscale. X-ray powder diffraction (XRD), Mossbauer spectroscopy and magnetic measurements were used to study the phase evolution of MoO2-Fe2O3 nanoparticle system under the mechanochemical activation process. The Mossbauer studies showed that the spectrum of the mechanochemically activated composites evolved from a sextet for hematite to sextet and a doublet upon duration of the milling process with molybdenum dioxide. Recoilless fraction was determined using our dual absorber method and was found to decrease with increasing ball milling time. Magnetic measurements recorded at 5 and 300 K in an applied magnetic field of 50,000 Oe showed the magnetic properties in the antiferromagnetic and canted ferromagnetic states. The Morin transformation was evidenced by zero-field cooling-field cooling (ZFC-FC) measurements in 200 Oe and the transformation characteristic temperatures were shifted to lower values.

We report on the formation of the Ti3SiC2 nanolaminate phase in the Ti-Si-C thin film system, using a UHV magnetron sputtering technique with top mounted sample holder, from elemental and compound targets. The formation of the Ti3SiC2 (or 312 phase) has been evidenced by detailed X-ray diffraction analysis followed by full-profile quantitative analysis of the obtained diffractograms. It has been proven that for deposition temperatures as low as 500 degrees C, there is a significant amount of 312 phase obtained in the deposited films, in co-existence with the majority TiC phase. This amount increased to about 21% when the deposition temperature was raised to 650 degrees C. The ternary 312 phase becomes predominant at around 60% relative abundance for slight off-stoichiometric, Si increased, content of the alloy, for temperatures as low as 650 degrees C. The conditions for improving the relative abundance of the 312 phase within our experimental setup are pointed out and explained in terms of a nucleation and growth model of nanostructure formation from the amorphous precursor (?).

Local atomic configuration, phase composition and atomic intermixing in Fe-rich Fe1-xCrx and Fe1-xMox ribbons (x = 0.05, 0.10, 0.15), of potential interest for high-temperature applications and nuclear devices, are investigated in this study in relation to specific processing and annealing routes. The Fe-based thin ribbons have been prepared by induction melting, followed by melt spinning and further annealed in He at temperatures up to 1250 degrees C. The complex structural, compositional and atomic configuration characterisation has been performed by means of X-ray diffraction (XRD), transmission Mossbauer spectroscopy and differential scanning calorimetry (TG-DSC). The XRD analysis indicates the formation of the desired solid solutions with body-centred cubic (bcc) structure in the as-quenched state. The Mossbauer spectroscopy results have been analysed in terms of the two-shell model. The distribution of Cr/Mo atoms in the first two coordination spheres is not homogeneous, especially after annealing, as supported by the short-range order parameters. In addition, high-temperature annealing treatments give rise to oxidation of Fe (to haematite, maghemite and magnetite) at the surface of the ribbons. Fe1-xCrx alloys are structurally more stable than the Mo counterpart under annealing at 700 degrees C. Annealing at 1250 degrees C in He enhances drastically the Cr clustering around Fe nuclei.

Ferromagnetic shape memory alloys (FSMA) have attained strong interest over the last years and Fe-Pd alloys seems to be more suited for engineering and medical applications, due to their improved ductility and biocompatibility compared to the well-known Ni2MnGa alloy. The shape memory effect in disordered Fe-Pd (30 at.% Pd) is associated with FCC-FCT thermoelastic martensitic transformation. The melt-spinning technique enables to get ribbons with the FCC meta-stable structure preventing the precipitation of undesirable BCT irreversible phase and subsequent proper thermal treatments could improve the characteristic parameters of the martensitic transformation. The present work reports the effect of the rapid solidification (via melt-spinning technique) and the different thermal treatments on the microstructure, martensitic transformation and magnetic properties of the Fe69.4Pd30.6 ribbons. The samples were investigated by calorimetry, X-ray diffractometry, scanning electron microscopy and magnetometry. Two different structures induced by the distinct thermal treatments and responsible for the characteristic behavior of the martensitic transformation, were noticed and discussed in details. The high temperature treatment for short time stabilized the FCC phase and slightly decrease the martensitic transformation temperature, while the annealing at low temperature for longer time promotes the reduction of the amount of transforming FCC phase by its partial decomposition in the stable phases, causing the fall of the heat of transformation.

We report unusual effect of channeled magnetic flux motion in YBa2Cu3O7-delta/PrBa2Cu3O7-delta superlattices grown by pulsed laser deposition. Magneto-optical imaging reveals that flux moves along a set of parallel and perpendicular lines, while optical microscopy does not show any features on the surface that may be linked to this effect. In contrast, scanning electron microscopy registers sub-micron fractures in the superlattices corresponding to these flux lines, but the magnetic flux channels are much wider than the width of these fractures. To further clarify the origin of flux channels, electrical transport measurements on the superlattices have been performed. Their current-voltage characteristics reveal the presence of distinctive branches related to the flux motion along the selective channels, following which magnetic flux crosses sample in a shortest and least resistive way. The application of very large current overheated the superlattice along these channels, evaporating superconducting material and exposing, wider than in the superconductor, fractures in the substrate. It is concluded that motion of flux in the channels is controlled not only by the presence of nano-fractures in YBa2Cu3O7-delta/PrBa2Cu3O7_delta, but also by stress developed in the superconducting material due to the fracturing of the substrate.