National Institute Of Materials Physics - Romania

Publications

847 articles found

451. Applicability of the Stoner-Wohlfarth Model for Ni-Fe Graded Thin Films

Authors: Kuncser, A; Schinteie, G; Ghica, C; Antohe, S; Kuncser, V

Published: MAR 2015, JOURNAL OF SUPERCONDUCTIVITY AND NOVEL MAGNETISM, 28, 969, DOI: 10.1007/s10948-014-2684-z

Fe-Ni graded films have been prepared by co-sputtering permalloy and Fe targets at different time-power sequences. Morpho-structural and magnetic investigations have been performed by transmission electron microscopy, Xray reflectometry, conversion electron Mossbauer spectroscopy and magneto-optic Kerr effect vector magnetometry, proving the thickness dependence of the properties. A magnetization reversal involving the displacement of Bloch-type walls is characteristic for a 115 nm thick Fe-Ni graded film whereas an in-plane coherent rotation of the spins, according to a Stoner-Wohlfarth mechanism was evidence for a 23 nm thick Fe-Ni graded film.

452. Ferromagnetic behaviour of bismuth germanate oxides glass-ceramic materials

Authors: Polosan, S; Negrea, R; Ciobotaru, IC; Schinteie, G; Kuncser, V

Published: FEB 25 2015, JOURNAL OF ALLOYS AND COMPOUNDS, 623, 196, DOI: 10.1016/j.jallcom.2014.10.104

The content of defects in Bi4Ge3O12 (BGO) glass-ceramic materials together with their ordering during crystallization induces ferromagnetic behaviours in these materials. The observed ferromagnetism has to be associated strictly with the GeO4 related defects in the atypical amorphous phase. The photoluminescence of BGO glass-ceramic materials can give useful information about the nature of defects, especially the oxygen vacancies from the GeO4 tetrahedra and Bi4Ge3O12 cubic structures. The concentration of these defects depends on annealing between 600 degrees C and 700 degrees C where orthorhombic and cubic phases coexist. The magnetization at saturation is higher in the samples dominated by the amorphous phase and the strength of the exchange interaction is higher in the samples dominated by the cubic phase, obtained over 700 degrees C, when the GeO4 tetrahedra are rearranged during crystallization together with their oxygen vacancies. (C) 2014 Elsevier B.V. All rights reserved.

453. New coordination polymers with chromato bridges: (1)(infinity)[Ni(phen)(H2O)(2)(mu-O2CrO2)] and (3)(infinity)[Mn(4,4 '-bipy)(H2O)(mu-O3CrO)]center dot H2O

Authors: Dumbrava, A; Olar, R; Badea, M; Maxim, C; Ghica, D; Andruh, M

Published: FEB 24 2015, INORGANICA CHIMICA ACTA, 426, 54, DOI: 10.1016/j.ica.2014.11.009

Two new coordination polymers have been assembled using the chromato ions as bridging ligands: (1)(infinity)[Ni(phen)(H2O)(2)(mu-O2CrO2)] 1, and (3)(infinity)[Mn(4,4'-bipy)(H2O)(mu-O3CrO)]center dot H2O 2. In crystal 1 the chromate ions act as bridges connecting two nickel ions through two oxygen atoms, resulting in infinite chains. 1,10-Phenanthroline acts as a blocking ligand. In compound 2, each chromato ion is connected to three manganese ions, resulting in layers which are parallel to the ab plane. The layers are constructed from {Mn3Cr3} meshes, each metal atom being shared between three other meshes. The structure is expanded into the third direction by connecting the inorganic layers through 4,4'-bipy spacers. The thermal decomposition of the two compounds has been investigated. (C) 2014 Elsevier B.V. All rights reserved.

454. Thermal stability, thermal expansion and grain-growth in exchange-coupled Fe-Pt-Ag-B bulk nanocomposite magnets

Authors: Nicula, R; Crisan, O; Crisan, AD; Mercioniu, I; Stir, M; Vasiliu, F

Published: FEB 15 2015, JOURNAL OF ALLOYS AND COMPOUNDS, 622, 870, DOI: 10.1016/j.jallcom.2014.10.181

Rare-earth free (RE-free) exchange coupling nanocomposite magnets are intensively studied nowadays due to their potential use in applications demanding stable high-temperature operation and corrosion resistance. In this respect, the FePt alloy system is one of the most actively addressed potential permanent magnet solutions. In FePt alloys, promising magnetic features arise from the co-existence of hard magnetic L1(0) FePt and soft magnetic L1(2) Fe3Pt phases emerged from the same metastable precursor. The present work deals with an in-situ temperature-resolved synchrotron radiation study of the thermal stability, thermal expansion and microstructure evolution in exchange-coupled FePtAgB alloys. The as-cast microstructural state as well as the optimized magnetic behavior are given as reference and correlated to the observed microstructural evolution with temperature. The melt-spun Fe48Pt28Ag6B18 alloy ribbons were examined in situ by synchrotron X-ray powder diffraction from ambient temperature up to 600 degrees C. The FePt-Fe3Pt exchange-coupled microstructure achieved by rapid solidification is not significantly altered during the high temperature exposure. The thermal expansion of the FePt L1(0) unit cell has been found to be strongly anisotropic, being essentially an in-plane expansion which may be seen as an anisotropic invar effect. For the FePt L1(0) phase, a significant deviation from linear thermal expansion is observed at the Curie temperature T-C = 477 degrees C. This non-linear behavior above T-C is tentatively linked to a diffusion/segregation mechanism of Ag. The promising hard magnetic properties as well as the direct formation of the L1(0) phase from the as-cast state are directly related to the presence of Ag in the intergranular regions. (C) 2014 Elsevier B.V. All rights reserved.

455. HfO2 with embedded Ge nanocrystals with memory effects

Authors: Palade, C; Slav, A; Lepadatu, AM; Maraloiu, AV; Teodorescu, VS; Ciurea, ML

Published: 2015, 2015 INTERNATIONAL SEMICONDUCTOR CONFERENCE (CAS), 48, DOI:

The charge storage properties of Ge nanocrystals-based MOS-like capacitors with tunnel and gate HfO2 are studied. HfO2/Ge/HfO2/Si trilayer structures were prepared by magnetron sputtering (in Ar) and subsequent rapid thermal annealing (650 degrees C). HfO2/Si structures were also prepared, some under similar conditions, while others were deposited in Ar:O-2. TEM investigations and C-V measurements were performed. TEM on annealed trilayers evidences the formation of ordered and precisely positioned array of Ge nanocrystals embedded in crystalline HfO2. The annealed Al/HfO2/Ge/HfO2/p-Si/Al capacitors present counterclockwise C-V hysteresis (0.8 V memory window) mainly given by Ge nanocrystals, with negligible contribution from crystallized-HfO2 traps.

456. Crystallographic holes: new insights for a beneficial structural feature for photocatalytic applications

Authors: Vajda, K; Kasa, Z; Dombi, A; Nemeth, Z; Kovacs, G; Danciu, V; Radu, T; Ghica, C; Baia, L; Hernadi, K; Pap, Z

Published: 2015, NANOSCALE, 7, 5786, DOI: 10.1039/c4nr07157c

One of the most fundamental aspects of the heterogeneous catalysis field is the manipulation of the catalysts' activity. In photocatalysis this is carried out by maximizing the right crystal plane of a semiconductor oxide. Until now, most of the papers have achieved this by a combination of different oxides, with noble metals and sometimes with carbon nanomaterials. In this work MWCNTs (multiwalled carbon nanotubes) were applied as "crystallization promoters" in a very simple, safe, one-step hydrothermal method. By this method TiO2 nano/micro crystals with exposed {001} facets were obtained in the first step. The next episode in the crystal manipulation "saga" was the modification of the (001) crystallographic plane's structure by creating ordered/own faceted "crystallographic holes". These elements are capable of further enhancing the obtained activity of titania microcrystals to a higher extent, as shown by the UV driven photocatalytic phenol degradation experiments. The appearance of the holes was "provoked" by simple calcination and their presence and influence were demonstrated by XPS and HRTEM.

457. Tuning the properties of Ge and Si nanocrystals based structures by tailoring the preparation conditions Review

Authors: Ciurea, ML; Lepadatu, AM

Published: JAN-MAR 2015, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 10, 87, DOI:

Ge and Si nanocrystals (NCs), quantum dots (QDs), and amorphous nanoparticles (NPs) have a significant role in the developement of micro- and nanoelectronic devices due to capability to tune their electrical and photoconductive properties by tailoring the morphology and structure parameters. Ge and Si NCs/QDs/NPs are zero-dimensional (0D) systems and the SiO2 films containing them are percolative systems. In this review, the role of deposition and annealing conditions in the morphology and structure of Ge and Si NCs/QDs/NPs embedded in amorphous SiO2 matrix is discussed for a wide variety of films and multilayered structures. The electrical and photoconductive properties of nanostructures deposited by different techniques as magnetron sputtering, ion implantation, chemical vapour deposition, sol-gel and molecular beam epitaxy, and subsequently annealed in conventional furnace or by rapid thermal annealing under different conditions are analised. We demonstrate how the electrical and photoconductive properties of nanostructures can be tuned by varying the deposition and annealing parameters. The role of Si-rich oxide and defects in the formation of Ge and Si NCs is shown and the role of defects in improving electrical properties and enhancing the photoconductivity of films and multilayered structures is highlighted. We evidence the contribution of quantum confinement effect and show that the most probable transport mechanisms in these percolative systems are tunnelling and hopping.

458. Analysis of bimodal thermally-induced denaturation of type I collagen extracted from calfskin

Authors: Staicu, T; Circu, V; Ionita, G; Ghica, C; Popa, VT; Micutz, M

Published: 2015, RSC ADVANCES, 5, 38406, DOI: 10.1039/c5ra02708j

The most widely occurring collagen in the extracellular matrix of the mammalian tissues - type I collagen has attracted huge interest from both theoretical and practical points of view. The results presented herein are mainly confined to a study of the thermal stability of type I collagen (CI) extracted from calfskin in acidic solutions (10(-2) M HCl). A two-step process associated with the heat-induced denaturation of CI was revealed by using ultra sensitive differential scanning calorimetry. The minor endothermic transition (peak temperatures of about 30.5-30.8 degrees C) has been assigned to the defibrillation of small supramolecular CI assemblies (occurring in solution) while the major endothermic transition (about 34.8-35.0 degrees C) has been ascribed to denaturation of CI, consisting of a complete unfolding of the native triple-helical conformation of the collagen molecule. The calorimetric data were supported by information obtained from capillary viscometry performed on the same solutions. The native state of CI (molecular integrity and triple-helical conformation) for all the systems studied in this paper was confirmed by transmission electron microscopy, ultraviolet circular dichroism and capillary viscometry. At the same time, based on the sodium dodecyl sulphate-polyacrylamide gel electrophoresis and an appropriate image processing of the corresponding electropherogram, a certain degree of intramolecular crosslinking of CI was detected, mainly between alpha 1 and alpha 2 chains of the collagen triple-helix. Such a feature seems to be caused by both the nature of the extraction procedure and the storage conditions.

459. STRAIN DRIVEN CHANGES OF DEFECT PARAMETERS IN HEAVY ION IMPLANTED Si

Authors: Palade, C; Lepadatu, AM; Slav, A; Ciurea, ML; Lazanu, S

Published: OCT-DEC 2015, DIGEST JOURNAL OF NANOMATERIALS AND BIOSTRUCTURES, 10, 1381, DOI:

We analyse the influence of the strain field on the parameters of trapping centres. The system under study is high resistivity Si implanted with Bi6+ and I6+ ions respectively, of 28 MeV kinetic energy, 3(O) off <100> axis orientation and 5x10(11) ions/cm(2) fluence. The strain field is the consequence of size and mass difference of the irradiation ions in respect to the atoms of the lattice, and the defects are produced during the slowing-down of ions, as a result of the energy transfer from the ion to Si atoms. These results are of interest for the design and manufacturing of microelectronic devices incorporating strain, particularly for quantum computers with qubits based on the interaction of electronic and nuclear spins of group-V donors in Si.

460. Nanostructured Titanium Doped Iron Oxide Photoelectrodes for Water Splitting

Authors: Miclea, C; Amarande, L; Cioangher, MC; Miclea, CT; Mihailescu, M; Radu, C; Ivanov, A

Published: 2015, ROMANIAN JOURNAL OF INFORMATION SCIENCE AND TECHNOLOGY, 18, 105, DOI:

Water splitting by means of semiconducting photoelectrodes and solar light represents a promising alternative to conventional fossil fuel economy. In this process the photoactive electrode absorb sunlight directly thus initiating the photochemical reaction which create excess electrons in the conduction band of the semiconducting electrode. Titanium doped iron oxide seems to be a promising semiconducting material for photoelectrodes. Consequently, we investigated the effect of Ti doping on the structure, electrical and photoelectrochemical properties alpha-Fe2O3. The Ti doped alpha-Fe2O3 were prepared by a slightly modified mixed oxide route, consisting in a prolonged mixing of the raw materials in a high energy planetary ball mill until the particles decreased to the nanometric sizes. Optimum results were obtained for samples doped with 5 at. % titanium and sintered at 1200 degrees C. Photocurrents as high as 8.4 mA/cm(2), for illumination from a 300 W xenon lamp, were recorded for such samples. It is hoped that such photoelectrodes show promise for water splitting.