National Institute Of Materials Physics - Romania

Publications

5781 articles found

3771. Quantum confinement model for phototransport processes in nanocrystalline porous silicon

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

Published: 2007, 2006 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOLS 1 AND 2, +, DOI:

The paper presents a quantum confinement model which describes the phototransport processes in nanocrystalline porous silicon. The model supposes that the nanocrystallites surface/interface acts as the walls of an infinite rectangular quantum well, introducing discrete confinement energy levels. This model is proved able to also describe the photoluminescence and the temperature dependence of the dark current in the same samples. Its results are in agreement with the Microstructure investigations.

3772. Effect of temperature on the main piezoelectric parameters of a soft PZT ceramic

Authors: Miclea, C; Tanasoiu, C; Amarande, L; Miclea, CF; Plavitu, C; Cioangher, M; Trupina, L; Miclea, CT; David, C

Published: 2007, ROMANIAN JOURNAL OF INFORMATION SCIENCE AND TECHNOLOGY, 10, 250, DOI:

The temperature behavior of the main piezoelectric parameters, such as the electromechanical coupling factor k(p), the mechanical quality factor Qm, the dielectric permittivity epsilon (real and imaginary part), the loss tangent tan delta, the piezoelectric charge constants d(33) and d(31) and the voltage constants 933 and 931 of a soft type piezoelectric material was investigated. The results showed that temperatures under 150 degrees C do not influence these parameters, which indicate that every transducer made from this type of material may be successfully used up to this temperature. Between 150 degrees C and 250 degrees C, the piezoelectric properties undergo more or less important changes, mainly due to the depoling effect and above that, at temperatures over 250 degrees C, they degrade very rapidly, tending to zero.

3773. Development of space instabilities of defect distribution at high fluences in neutron irradiated YBa2CU3O7-delta ceramics

Authors: Sandu, V; Popa, S; Sandu, E; Di Gioacchino, D; Tnipodi, P

Published: 2007, SIX INTERNATIONAL CONFERENCE OF THE BALKAN PHYSICAL UNION, 899, 647, DOI:

We investigated the behaviour of YBa(2)CU(3)o(7-delta) ceramics by multiharmonic ac-susceptibility. We have found an unexpected improvement of the intragranular response at the highest fluence 9.98 10(17) neutrons/cm(2). However, this fact is accompanied with a continuous suppression of the intergranular properties. We assume that this behavior is the result of the change of the nature of defect distribution at high fluence, when the uniform distribution of defects starts to develop space instabilities as a result of the subtle interplay of the in-cascade rate of interstitial loops production, cascade collapse of the vacancy loops, and the excess network bias.

3774. Mossbauer and optical investigation of Eu : YAG nanocrystals synthesized by a sol-gel method

Authors: Constantinescu, S; Georgescu, S; Bibicu, I; Chinie, AA; Stefan, AS; Toma, O

Published: 2007, ROMANIAN JOURNAL OF PHYSICS, 52, 307, DOI:

The morphological changes induced by the thermal treatments of YAG: Eu crystals synthesized by a sol-gel method have investigated using the Mossbauer and optical techniques. Both methods exhibit a drastic change of the spectral parameters at amorphous - garnet phase transition, followed by a slow evolution of these parameters with the increase of the annealing temperature. This slow evolution is due to the increase in size of the nanocrystals.

3775. Accelerate ageing of PZT-type ceramics

Authors: Toacsan, MI; Ioachim, A; Nedelcu, L; Alexandru, HV

Published: 2007, PROGRESS IN SOLID STATE CHEMISTRY, 35, 537, DOI: 10.1016/j.progsolidstchem.2007.01.018

Accelerate ageing of lead zirconate-titanate ferroceramics (PZT) by fast neutron irradiation is investigated. The ferroelectric behavior of Pb1-wMw(Zr,Ti)O-3, where w < 0.5 wt%, M = Li, Nb, Cr, Bi, Mn, Sb, La for Zr/Ti ratio = 51.5/48.5, prepared by solid state reaction, is strongly affected by fast neutron irradiation at Phi(i) = (10(13) divided by 10(18)) n/cm(2) integrated fluxes. Structural investigations (XRD and SEM) reveal modifications of the microstructure, porosity and elementary cell parameters. High irradiation levels induce tetragonal-cubic at the same time with a ferro-paraelectric transition. The micrographs show the evolution of 90 degrees and 180 degrees walls as well as herringbone structure typical for tetragonal distortions with irradiation level. A relation between macroscopic characteristics (piezoelectric and dielectric) and microscopic properties (crystallographic parameters, porous structure and grain morphology) as function of irradiation doses can be established using a model based on microstructural parameters. These results offer opportunities to understand the fundamentals of radiation effects upon piezoelectric ceramics. (c) 2007 Elsevier Ltd. All rights reserved.

3776. Mesoscopic Fano effect in an Aharonov-Bohm interferometer Coulomb-coupled to a nearby quantum dot

Authors: Tolea, M; Moldoveanu, V; Tanatar, B

Published: 2007, PHYSICA STATUS SOLIDI C - CURRENT TOPICS IN SOLID STATE PHYSICS, VOL 4, NO 2, 4, +, DOI: 10.1002/pssc.200673304

Motivated by the pionieering experiments of Buks et al. [Nature 391, 871 (1998)] we investigate the visibility of the Fano effect in a single-dot Aharonov-Bohm interferometer which is Coulomb-coupled to a nearby quantum dot. The latter acts as a 'Which Path Detector' and is coupled to two leads on which a finite bias is applied. Using the non-equilibrium Keldysh-Green function formalism we compute the currents through the detector and the interferometer. We take into account the first two contributions to the interaction self energy and emphasize the correction to the Landauer formula which appears beyond the single-particle approximation. Particular attention is given to the coherence properties of the interferometer in the presence of the electron-electron interaction between the embedded dot and the detector. We show that when the detector is subjected to a finite bias the amplitude of Aharonov-Bohm oscillations of the current through the interferometer decreases. The Fano line is in turn rather stable under interaction. Our results generalize an earlier work of Silva and Levit [Phys. Rev. B 63, 201309 (2001)] and complement the existing description of the controlled dephasing. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

3777. Broadband planar filters with enhanced couplings using defected ground structures

Authors: Militaru, N; Banciu, MG; Lojewski, G

Published: 2007, 2006 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOLS 1 AND 2, +, DOI:

In this paper a study of some microwave microstrip bandpass filters (BPF) using defected ground structures (DGS) is presented. This technique allows designs of tight couplings without the necessity of using very narrow coupling gaps. Based on the results of the study, four-pole cross-coupled planar microwave bandpass filters were designed, with a single of. with two ground slots. Compared to similar microstrip filters without defected ground, the simulated performances of these novel structures indicate some technological advantages.

3778. Aperture couplings in multilayer filtering structures

Authors: Militaru, N; Lojewski, G; Banciu, MG

Published: 2007, ISSCS 2007: INTERNATIONAL SYMPOSIUM ON SIGNALS, CIRCUITS AND SYSTEMS, VOLS 1 AND 2, +, DOI:

In multilayer structures apertures in metallic sheets can be used as coupling elements between resonators located on different layers, allowing very compact filter designs. In this paper, some novel types of planar microwave bandpass filters, using aperture couplings, in structures with three metallic layers, are considered. Couplings between two miniaturized resonators are investigated, by full electromagnetic field simulation. Based on the results of this study, several models of planar microwave bandpass filters, with a single or with two apertures, are designed, verified by simulation, fabricated and measured. Compared to the classical filters, these novel structures show a good compactness and can offer some technological advantages.

3779. Comparative study of the mass-loading effect on electrical parameters of gallium phosphate, quartz and langasite resonators

Authors: Mateescu, I; Krispel, F; Georgescu, S; Scott, K; Borca, E

Published: 2007, PROCEEDINGS OF THE 2007 IEEE INTERNATIONAL FREQUENCY CONTROL SYMPOSIUM-JOINTLY WITH THE 21ST EUROPEAN FREQUENCY AND TIME FORUM, VOLS 1-4, +, DOI: 10.1109/FREQ.2007.4319163

This paper presents some of the more relevant results concerning the mass-loading influence on the characteristics (effective mass-loading, motional inductance, quality factor) of AT-cut quartz, Y-cut langasite resonators and AT-cut gallium phosphate.

3780. Gas-sensing by Polymer-Iron nanocomposite materials

Authors: Tomescu, A; Simion, CE; Alexandrescu, R; Morjan, I; Scarisoreanu, M

Published: 2007, CAS 2007 INTERNATIONAL SEMICONDUCTOR CONFERENCE, VOLS 1 AND 2, PROCEEDINGS, +, DOI:

Electrical properties of new Polymer-Iron nanocomposite materials obtained by laser pyrolysis were investigated, in order to find applications for the detection of reducing gases. The aim of this paper was to prove the necessity of complex experimental investigation in order to point out the contribution of different types of adsorption. Usually resistance measurement cannot fully describe the surface interactions. The electron affinity changes are of real interest for in-field conditions which always involve humidity of surrounding gas-atmosphere. On the other hand, the nanocomposite materials can be a solution for specific gas-sensing properties.