National Institute Of Materials Physics - Romania

Publications

5840 articles found

4691. Optical and morphological propel-ties of laser photo-deposited hydrogenated CNx thin films

Authors: Crunteanu, A; Charbonnier, M; Romand, M; Mugnier, J; Sandu, C

Published: DEC 15 2000, APPLIED SURFACE SCIENCE, 168, 51, DOI: 10.1016/S0169-4332(00)00581-X

The optical and morphological properties of hydrogenated carbon nitride thin films obtained by ArF excimer laser photodeposition (lambda = 193 nm) from different ammonia/acetylene mixtures were investigated. The optical properties of the films depend on the nitrogen content. The band gap deduced from measurements of the optical absorption coefficient in the UV-VIS spectrum is found to be superior to 2.3 eV. Film thicknesses and refraction indexes were investigated by m-lines spectroscopy The refraction index varies from 1.66 to 1.72. The microstructure of the deposited films as revealed by scanning and transmission electron microscopy is discussed as a function of the deposition parameters. 2000 Elsevier Science B.V. All rights reserved.

4692. Mossbauer spectroscopy study of Ti-Zr-Ni-Fe icosahedral and approximant phases

Authors: Nicula, R; Kuncser, V; Jianu, A; Filoti, G; Burkel, E

Published: DEC 15 2000, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 294, 541, DOI: 10.1016/S0921-5093(00)01160-6

Quaternary Ti53Zr21Ni13Fe13 alloys were prepared by melt-spinning using peripheral wheel speeds omega =19.5, 24.2 and 30m/s. The icosahedral (i-) phase formation was studied using high resolution synchrotron radiation powder diffraction. An amorphous phase component is present irrespective of the rapid solidification conditions, its volume fraction decreasing towards lower quenching speed values. The amorphous phase coexists with a b.c.c. rational approximant phase (W-phase). For omega below 20 m/s the formation of an i-phase was also noticed. Transmission Fe-57 Mossbauer spectroscopy measurements indicate that the number of asymmetrical Fe positions decreases with decreasing w in agreement with the improved structural perfection of the W-phase and with the formation of an additional i-phase. (C) 2000 Elsevier Science B.V. All rights reserved.

4693. Pulsed laser deposition of hydroxyapatite thin films on Ti-5A1-2.5Fe substrates with and without buffer layers

Authors: Nelea, V; Ristoscu, C; Chiritescu, C; Ghica, C; Mihailescu, IN; Pelletier, H; Mille, P; Cornet, A

Published: DEC 15 2000, APPLIED SURFACE SCIENCE, 168, 131, DOI: 10.1016/S0169-4332(00)00616-4

We present a method for processing hydroxyapatite (HA) thin films on Ti-5Al-2.5Fe substrates. The films were grown by pulsed laser deposition (PLD) in vacuum at room temperature, using a KrFdouble dagger excimer laser. The amorphous as-deposited HA films were recrystallized in ambient air by a thermal treatment at 550 degreesC. The best results have been obtained when inserting a buffer layer of ceramic materials (TiN, ZrO2 or Al2O3) The films were characterized by complementary techniques: grazing incidence X-ray diffraction (GIXRD), scanning electron microscopy (SEM), cross-section transmission electron microscopy (XTEM), SAED, energy dispersive X-ray spectroscopy (EDS) and nanoindentation. The samples with buffer interlayer preserve the stoichiometry are completely recrystallized and present better mechanical characteristics as compared with that without buffer interlayer. (C) 2000 Published by Elsevier Science B.V.

4694. Laser ablation synthesis of Al-based icosahedral powders

Authors: Nicula, R; Jianu, A; Grigoriu, C; Barfels, T; Burkel, E

Published: DEC 15 2000, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 294, 89, DOI: 10.1016/S0921-5093(00)01204-1

The laser ablation (LA) synthesis of ultra-fine-grained powders with dominant icosahedral structure has been attempted using Al-Cu-Fe and Al-Pd-Mn targets. The structure of the Al-Cu-Fe LA powders was examined using synchrotron radiation diffraction experiments which revealed the presence of the icosahedral Al-Cu-Fe phase. The Al-Pd-Mn powders were studied using scanning electron microscopy (SEM) and energy dispersive synchrotron radiation diffraction. For Al-Pd-Mn, the icosahedral phase is also a constituent phase of the LA alloy powders. Our results indicate that LA may be successfully used for obtaining quasicrystalline-based ultra-fine grained powders and/or surface coatings, while also providing a basis for the powder metallurgy processing of these materials. (C) 2000 Elsevier Science B.V, All rights reserved.

4695. Equivalent pyroelectric coefficient of a pyroelectric bimorph structure

Authors: Pintilie, L; Pintilie, I; Matei, I

Published: DEC 15 2000, JOURNAL OF APPLIED PHYSICS, 88, 7271, DOI: 10.1063/1.1327284

A detailed formula for the equivalent pyroelectric coefficient of a bimorph structure acting as a pyroelectric detector was deduced. It is shown that electric fields occur in the structure when it is subjected to a temperature variation, due to the unequal charges generated by the two components. The field dependence of the ferroelectric polarization has to be considered in this case and the (partial derivativeP/partial derivativeE) derivative appears explicitly in the deduced formula. The effect is that the equivalent pyroelectric coefficient has different values for heating and cooling. This could lead to a redressing effect on the pyroelectric signal if the temperature difference varies periodically. The equivalent pyroelectric coefficient is, also, thickness dependent. This fact offers the possibility of designing structures with high values of pyroelectric coefficient compared with the values of the component phases. (C) 2000 American Institute of Physics. [S0021- 8979(00)04701-6].

4696. Icosahedral phase formation in Ag-substituted Ti-Zr-Ni rapidly-quenched alloys

Authors: Nicula, R; Ponkratz, U; Jianu, A; Schick, C; Burkel, E

Published: DEC 15 2000, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 294, 92, DOI: 10.1016/S0921-5093(00)01205-3

The formation of icosahedral phases in Ag-substituted Ti-Zr-Ni alloys was investigated by in situ annealing diffraction experiments using synchrotron radiation. The as-quenched (Ti1-xAgx)-Zr-Ni alloys (x=2, 5, 7 and 10 at.%) are amorphous and/or in a mixed nanostructured state. Upon heating, the Ti-Zr-Ni-Ag alloys transform to either a crystalline or a quasicrystalline state, depending on the Ag content. Differential scanning calorimetry (DSC) studies were performed to identify the nature and sequence of the phase transitions. (C) 2000 Elsevier Science B.V. All rights reserved.

4697. Magnetization and level statistics at the quantum Hall liquid-insulator transition in the lattice model

Authors: Nita, M; Aldea, A; Zittartz, J

Published: DEC 15 2000, PHYSICAL REVIEW B, 62, 15370, DOI: 10.1103/PhysRevB.62.15367

Statistics of level spacing and magnetization are studied for the phase diagram of the integer quantum Hall effect in a two-dimensional finite lattice model with Anderson disorder.

4698. Crossover from elastic to plastic vortex creep across the second magnetization peak of high-temperature superconductors

Authors: Miu, L; Noji, T; Koike, Y; Cimpoiasu, E; Stein, T; Almasan, CC

Published: DEC 1 2000, PHYSICAL REVIEW B, 62, 15176, DOI: 10.1103/PhysRevB.62.15172

We investigated the relaxation of the irreversible magnetization of YBa2Cu3O7-delta, Pb2Sr2Y0.53Ca0.47Cu3O8, and Bi2Sr2CaCu2O8+delta single crystals with significant quenched disorder in the region of the second magnetization peak. It was found that for an applied magnetic field between the onset field and the peak field the relevant current-density dependence of the activation energy exhibits a sudden change, which can be interpreted as a crossover from elastic to plastic vortex creep. The evolution of this change with magnetic field illustrates the increase of the collective pinning barrier between the onset field and the peak field. The observed increase of the collective pinning barrier is Limited by the plastic barrier at the peak field. This appears to be a general behavior, and may have important consequences on the interpretation of the thermally induced vortex phase transition at high magnetic fields.

4699. Many-body fermion systems in the Floquet formalism

Authors: Lungu, RP; Manolescu, A

Published: DEC 2000, PHYSICA SCRIPTA, 62, 445, DOI: 10.1238/Physica.Regular.062a00433

A general fermion system interacting with a time-periodic external held is discussed in the second quantization formalism transposed in the Fock-Floquet space. The method simulates the evolution of the system with the help of an auxiliary parameter which is analog to the time and defines the vectors associated to the states of the system, such that the matrix elements in the Hilbert space have a physical correspondence. Assuming that only the principal quasi-energies have physical relevance, the ground state of the free system is constructed in the occupation-numbers representation and an analog of Wick's theorem is derived. The perturbation series of the Green's functions is subsequently obtained, and the linear response of a given observable is expressed in terms of the correlation functions. The results have formal similarities to those of the standard theory of the many-particle conservative systems.

4700. Mn-site doping effects in the giant magnetoresistive perovskite La2/3Ca1/3MnO3: An EPR investigation

Authors: Giurgiu, LV; Grecu, MN; Filip, X; Darabont, A; Indrea, E; Deac, IG; Filip, C; Ordean, R; Popescu, GD; Surducan, E; Blasco, J; Papavassiliou, G

Published: DEC 2000, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 2, 378, DOI:

The effects of aluminium doping on Mn site in the magnetoresistive La-2/3 Ca-1/3 Mn1-x, AlxO3 (x less than or equal to 0.05) powders have been investigated by EPR. The temperature and x dependence of the resonance linewidth has been investigated and discussed in terms of exchange narrowing and spin-lattice relaxation. In the paramagnetic regime, the exchange coupling integral, J, between Mn3+ and Mn4+ ions shows a decrease with increasing x. it could arises from weakening of the double-exchange interaction by aluminium doping which reduces the exchange field at the Mn sites.