National Institute Of Materials Physics - Romania

We report results of a powder neutron diffraction study on the crystal structure and local magnetic moments in Nd3Co13-x NixB2 compounds with Nd3Ni13B2- type structure. The largest 3d magnetic moment resides at the 4 h sites. Nd magnetic moment is reduced with respect to the free ion value. Preferential Ni substitution for Co is also found at the 4h site. The refined magnetization direction is consistent with the T-x phase diagram derived from magnetization data. (c) 2007 Elsevier B. V. All rights reserved.

Intermetallic Fe-Pt-Nb-B alloys with 3 different compositions have been synthesized by rapid solidification technique and their phase structure was characterized by means of X-ray diffraction and Mossbauer spectrometry. It is shown that Fe68Pt21Nb2B9 and Fe65Pt25Nb2B8 as-east samples consist mainly of A1 soft magnetic f.c.c. Fe-Pt phase, while the Fe68Pt13Nb2B17 as-cast sample exhibits topological short-range order, typical for amorphous ribbons. Crystallization processes in the amorphous sample and phase evolution with the temperature have been studied using differential scanning calorimetry (DSC). The occurrence of exothermic peaks is related to structural transformations in the alloys and the crystallization process is shown to be highly dependent upon the heating rate in the DSC process. (C) 2006 Elsevier B.V. All rights reserved.

The structure of amorphous silicon and amorphous hydrogenated silicon has been investigated starting from a large model of amorphous silicon (2052 atoms). The modifications induced in a-Si model by when hydrogen is introduced have been revealed. The diffusion of hydrogen leads to a nanostructuring effect in the continuous random network of amorphous silicon. The effect of hydrogen on the distributed distortion defects in silicon network has been modelled. The most strained bonds have been broken and hydrogen wasd attached to the dangling bonbds on silicon. A free energy relaxation process was carried out in the frame of a Monte - Carlo - Metropolis procedure. There was demonstrated that, by nanostructuring, the total free energy of the model decreases. The variation of the bond stretching and bond bending energy as a function of the number of broken bonds indicated a dominant contribution of the bond bending term. The modelling shows, undoubtedly, the disappearance of many structural defects that probably are related to the deep electronic states in the gap of amorphous silicon.

The bulk polymerization reaction of N-vinylcarbazole (VK) at 70 degrees C in the presence of single-walled carbon nanotubes (SWNTs) leads to a new composite, whose optical properties were studied by photoluminescence (PL), surface enhanced Raman scattering (SERS) and Fourier transform infrared (FTIR) spectroscopies. A dramatic reduction of the poly(N-vinylcarbazole) (PVK) PL efficiency and a change in the vibrational structure of the PL spectrum of this polymer were observed by adding SWNTs to the synthesis mixture. Steric hindrance effects were evidenced both in SERS spectra of the VK when it interacts mechanico-chemically with SWNTs and in FTIR spectra of the un-doped PVK/SWNTs' composites. Cyclic voltammetry was used to demonstrate the doping process of PVK in PVK/SWNTs' composite. (c) 2007 Elsevier Ltd. All rights reserved.

Glassy As25Si40Te35 has been studied by x-ray and neutron diffraction as well as x-ray absorption spectroscopy ( EXAFS) at As and Te K-edges. Simultaneous modelling of the four independent measurements by means of the reverse Monte Carlo ( RMC) simulation technique allowed the separation of partial pair distribution functions and estimation of the corresponding coordination numbers. It is shown that the atomic structure of As25Si40Te35 glass can be presented as a three-dimensional network of twofold coordinated Te, threefold coordinated As and fourfold coordinated Si atoms.

Fe-Pt system is nowadays widely studied due to its potential applications as magnetic recording media. The hard magnetic FePt L1(0) phase has extremely promising potential as permanent magnet with high magnetocrystalline anisotropy. Of recent interest is also the developing of the hard magnetic phase from an amorphous precursor by appropriate crystallization processes. The melt-spun amorphous Fe68Pt13Nb2B17 alloy has been submitted to dynamical annealing and its phase transformation during the process has been monitored by differential scanning calorimetry and in situ energy-dispersive X-ray diffraction of the synchrotron radiation. In the first stage of crystallization, alpha-Fe and cubic FePt phases are formed from the amorphous precursor. At around 600 degrees C superlattice Bragg reflections corresponding to tetragonal FePt are indexed in the XRD spectra and a-Fe phase diminishes drastically. Finally, between 900 degrees C and 975 degrees C the tetragonal superlattice peaks disappear and cubic FePt phase is formed again. This reversible order-disorder transformation is accompanied by a strong uniaxial lattice expansion of the cubic FePt unit cell. The system show promising features for the co-existence of hard and soft exchange coupled magnetic phases crystallized from FePt-based amorphous precursors. (c) 2006 Elsevier B.V. All rights reserved.

The hemocompatibility of an implant is determined by its surface properties. Diamond like carbon surfaces (DLC) are slowing thrombin development due to their low values of surface energy. Magnetron sputtering (MS) and plasma enhanced chemical vapor deposition (PECVD) are the techniques successfully used in this type of applications. In this paper we have studied carbonic hemocompatible adherent structures onto metallic substrates of stainless steel and titanium. The carbonic structures have been characterized by FTIR, AFM, scratch test and pull-test adherence test. Surface energy was estimated by the contact angle method and the clotting time by the droplet method.

TiO2 anatase doped with 0.5 at%Fe powders were prepared by sol-gel technique. Titanium (IV) butoxide and iron (III) acetylacetonate were used as starting precursors, n-butanol as solvent, acetylacetone is added as a chelating agent and acetic acid to decrease the kinetics of the hydrolysis and condensation of Ti(O-Bu-n)(4). The evolution of the network bonds and the structural characterization of the gel were studied by thermogravimetric analysis (TG), differential thermal analysis (DTA), differential thermogravimetric analysis (DTG), Fourier transform infrared spectroscopy(FTIR) scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) techniques, respectively. A gelprecusor nanopowder with the mean grain size of about 15-20 nm was obtained. In order to prepare a Fe-cloped TiO2 material with the structure of the anatase, a thermal treatment at 450 degrees C, 3 h in air, was fund enough.

Hydroxyapatite (HAP) thin films (0.2 pm, 0.3 pm and 1.2 mu m thickness) were grown by RF magnetron sputtering in argon atmosphere onto silicon substrates at high deposition rates (0.6 mu m/h). Crystalline HAP films were obtained using a low temperature (100 degrees C) followed by post-deposition annealing at 300 degrees C, 450 degrees C, 500 degrees C and 550 degrees C in environmental air for 1 hour. An important influence of the films thickness upon the crystallization degree was noticed at intermediate annealing temperatures, as obtained from XRD measurements. For low and high temperatures similar values were obtained with a better crystallization degree for the thinner films. FTIR absorption led to the same conclusion considering the shape of stretching and bending PO4 lines. This suggests that the crystallization process has a diffusion component besides usual thermal activation process.

Relaxation of the thermal residual strain in Nb3Sn wires is realized through cycles of multiple torsion loadings ('pre-torsion') at room temperature. As a consequence, the critical current, I-c, is enhanced. This effect is stronger than previously reported mechanical treatments of multiple bending ('pre-bending'). The maximum I-c is attained for complex mechanical treatments of pre-torsion and pre-bending. Complex treatments allow efficient and relatively independent control of residual strain relaxation over all three directions of the wire, resulting in enhancement of I-c towards the theoretical limit of the material. Our findings have an immediate positive impact on the performance of Nb3Sn wires and, hence, on their applications (e.g. react-and-wind coils).