National Institute Of Materials Physics - Romania

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5781 articles found

3611. Atomic structure of As25Si40Te35 glass

Authors: Kaban, I; Gruner, S; Jovari, P; Kehr, M; Hoyer, W; Delaplane, RG; Popescu, M

Published: AUG 22 2007, JOURNAL OF PHYSICS-CONDENSED MATTER, 19, DOI: 10.1088/0953-8984/19/33/335210

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.

3612. Crystallization processes and phase evolution in amorphous Fe-Pt-Nb-B alloys

Authors: Crisan, O; Crisan, AD; Randrianantoandro, N; Nicula, R; Burkel, E

Published: AUG 16 2007, JOURNAL OF ALLOYS AND COMPOUNDS, 440, L7, DOI: 10.1016/j.jallcom.2006.08.331

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.

3613. Functional carbonic structures with applications in cardio-vascular implantology

Authors: Marcov, D; Apetroaei, N; Verestiuc, L; Morosanu, C

Published: AUG 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2554, DOI:

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.

3614. Synthesis of iron-doped anatase-TiO2 powders by a particulate sol-gel route

Authors: Cernea, M; Valsangiacom, C; Trusca, R; Vasiliu, F

Published: AUG 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2652, DOI:

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.

3615. Thickness dependence of crystallization process for hydroxyapatite thin films

Authors: Mercioniu, I; Ciuca, S; Pasuk, I; Slav, A; Morosanu, C; Bercu, M

Published: AUG 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2538, DOI:

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.

3616. Maximization of the critical current of practical Nb3Sn wires through complex mechanical treatments at room temperature

Authors: Badica, P; Oguro, H; Awaji, S; Nishijima, G; Watanabe, K

Published: AUG 2007, SUPERCONDUCTOR SCIENCE & TECHNOLOGY, 20, 813, DOI: 10.1088/0953-2048/20/8/015

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).

3617. An algorithm for preparing bioactive fluorinated hydroxyapatite coatings by sol gel technique

Authors: Stan, GE; Ferreira, JMF

Published: AUG 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2542, DOI:

Fluorinated hydroxyapatite [Ca-10(PO4)(6)(OH)(x)F1-x] implantologic structures could constitute an alternative to classic hydroxyapatite [Ca-10(PO4)6(OH)21 ones in medical applications due to their bioactivity and low solubility. Bioactive apatites are usually used as coatings onto surface of biocompatible metals, leading to implants which take advantage of the coating's bioactivity and good mechanical properties of metals. In this work, powders and apatite type films were prepared on Ti6Al4V medical grade alloy using three sol-gel chemical routes by mixing different calcium and phosphorous precursors Ca(NO3)2, P2O5 and C6H15PO3, and two fluoride reagents: HPF6 and NH4F. The resulting calcium phosphates and fluorapatite/hydroxyapatite solid solution (FHA) compounds were investigated by SEM, XRD and evaluated in vitro in SBF Kokubo solution. An efficient preparation algorithm was designed for the Ca(NO3)(2+) C6H15PO3+ NH4F chemical route which produced monophasic powders with a high crystallinity degree. This chemical route led to FHA sol-gel coatings with bioactive potential.

3618. Phototransport and photoluminescence in nanocrystalline porous silicon

Authors: Iancu, V; Ciurea, ML; Stavarache, I; Teodorescu, VS

Published: AUG 2007, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 9, 2643, DOI:

The paper presents the phototransport and photoluminescence investigations on nanocrystalline porous silicon. Spectral dependence curves of the phototransport and photoluminescence were taken at room temperature. A typical phototransport curve presents several maxima and shoulders, while the photoluminescence curves present only one maximum. The temperature dependence of the phototransport was measured in the 85 - 250 K range for different wavelengths. All these curves present only one activation energy on the whole temperature interval. The experimental results are interpreted taking into account a quantum confinement model. Previous microstructure investigations proved that the studied nanocrystalline porous silicon is formed by a nanowire network. Then, the electron Hamiltonian can be split into the sum of a 1D longitudinal Bloch part and a 2D transversal part, the nanowire wall acting like a potential well. Thus, a number of discrete energy levels will appear into the band gap. Both phototransport and photoluminescence maxima are due to the transitions between these levels, corresponding to a mean nanowire diameter of 3.2 nm, in good agreement with the previous microstructure and electrical transport investigations. On the contrary, the temperature dependence of the phototransport is determined by the surface states located on the internal surface and/or the interface between nanocrystallites.

3619. Transient regime in nonlinear transport through many-level quantum dots

Authors: Moldoveanu, V; Gudmundsson, V; Manolescu, A

Published: AUG 2007, PHYSICAL REVIEW B, 76, DOI: 10.1103/PhysRevB.76.085330

We investigate nonstationary electronic transport in noninteracting nanostructures driven by a finite bias and time-dependent signals applied at their contacts to the leads. The systems are modeled by a tight-binding Hamiltonian, and the transient currents are computed from the nonequilibrium Green-Keldysh formalism. The numerical implementation is not restricted to weak coupling to the leads and does not imply the wideband limit assumption for the spectral width of the leads. As an application of the method we study in detail the transient behavior and the charge dynamics in single and double quantum dots connected to leads by a steplike potential, but the method allows as well consideration of nonperiodic potentials or short pulses. We show that when the higher-energy levels of the isolated system are located within the bias window of the leads, the transient current approaches the steady state in a nonoscillatory smooth fashion. At moderate coupling to the leads and fixed bias the transient acquires a steplike structure, the length of the steps increasing with system size. The number of levels inside a finite bias window can be tuned by a constant gate potential. We find also that the transient behavior depends on the specific way of coupling the leads to the mesoscopic system.

3620. Electrostatic electron piston pump with in-plane gate transistors

Authors: Draghici, M; Salloch, D; Melnikov, A; Wieck, AD

Published: AUG 2007, PHYSICA STATUS SOLIDI B-BASIC SOLID STATE PHYSICS, 244, 3014, DOI: 10.1002/pssb.200675613

We have demonstrated an electron pump with in-plane gate (IPG) transistors fabricated by focused ion beam (FIB) implantation on GaAs-A1(x)Ga(1-x)As modulation-doped heterostructure with a two-dimensional electron gas (2DEG). The maskless fabrication process does not require any alignment between the circuit elements including the sources, the drains and the gates of the IPG transistors. Although, the device is operating at low frequencies (tens of kHz), it is capable to produce a current more than one order of magnitude higher than other electron pumps based on Coulomb blockade and single electron tunneling effects. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.