National Institute Of Materials Physics - Romania

Publications

5714 articles found

2921. Magnetoelectric properties of multiferroic cobalt ferrite and soft piezoelectric PZT particulate composites

Authors: Miclea, C; Tanasoiu, C; Amarande, L; Miclea, CF; Plavitu, C; Cioangher, M; Trupina, L; Miclea, CT; Tanasoiu, T; Susu, M; Voicu, I; Malczanek, V; Ivanov, A; David, C

Published: FEB 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 276, DOI:

Multiferroic composite materials, consisting of piezoelectric and piezomagnetic phases, show electric polarization in the presence of a magnetic field and magnetization on applying an electric field, i.e. piezomagnetic phase causes polarization while piezoelectric phase causes changes in magnetization. The figure of merit of such multiferroic particulate composites is the magnetoelectric voltage coefficient representing the amount of polarization induced by a given magnetic field. In the present investigation we prepared and studied the properties of bulk particulate composites made from cobalt ferrite (COF) and a soft piezoelectric material (PZT). Composite samples, according to the formula xCOF center dot(1-x)PZT with 0 <= x <= 1 were prepared by mixing the COF and PZT powders, pressing them as discs and then sintered at 1200 degrees C for 3 hours. Structural, magnetic, piezoelectric and magnetoelectric characteristics of the sintered composites were determined as a function of the mass fraction of COF into PZT and magnetic field. The ME coefficient shows a maximum value of about 70 mV/cm.Oe for composites containing 40 % COF and 60 % PZT, in a DC magnetic field of 3 kOe at a frequency of 1 KHz of the ac magnetic field. Some of the dielectric and piezoelectric properties of the composite samples, as a function of the mass fraction x, were also determined.

2922. Organic Photovoltaic Cells Based on ZnO Thin Film Electrodes

Authors: Ghica, C; Ion, L; Epurescu, G; Nistor, L; Antohe, S; Dinescu, M

Published: FEB 2010, JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, 10, 1326, DOI: 10.1166/jnn.2010.1836

Due to its wide band-gap (ca. 3.4 eV), ZnO is a possible candidate material to be used as transparent electrode for a new class of photovoltaic (PV) cells. Also, an increased interest for the photovoltaic properties of several organic monomers and polymers (merocyanines, phthalocyanines and porphyrins) was noticed, because of their high optical absorption in the visible region of the spectrum allowing them to be used as potential inexpensive materials for solar cells. Preparation and properties of CuPc (copper phthalocyanine) based photovoltaic cells using ZnO thin films as transparent conductor electrodes are presented in this paper. ZnO layers are grown by pulsed laser deposition, while the organic layers are obtained by thermal evaporation. Structural characterization is performed by electron microscopy. Optical and transport properties of the mutilayered structures are obtained by electrical and spectro-photometric measurements. The influence of the ZnO-polymer interface on the external quantum efficiency (EQE) of the photovoltaic cell is clearly evidenced by our measurements.

2923. Four-band printed antenna for personal communication systems

Authors: Cazanaru, D; Szilagyi, A; Banciu, MG

Published: FEB 2010, OPTOELECTRONICS AND ADVANCED MATERIALS-RAPID COMMUNICATIONS, 4, 204, DOI:

The paper describes a four-band, small-dimensions printed antenna intended to be integrated into modern wireless systems. The printed antenna has resonant slots, at slightly different frequencies. in order to provide a simultaneous response at four frequency bands in the operating spectral region. This type of antennas is proposed for the emerging multi-mode multi-band transceivers, which operate over a wide range of bands required by modern telecommunication systems. Measured data including return loss, antenna gain and radiation patterns are in good agreement with the simulated data.

2924. Iron Oxide-Based Nanoparticles with Different Mean Sizes Obtained by the Laser Pyrolysis: Structural and Magnetic Properties

Authors: Morjan, I; Alexandrescu, R; Dumitrache, F; Birjega, R; Fleaca, C; Soare, I; Luculescu, CR; Filoti, G; Kuncer, V; Vekas, L; Popa, NC; Prodan, G; Ciupina, V

Published: FEB 2010, JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY, 10, 1234, DOI: 10.1166/jnn.2010.1863

Nano-sized iron oxide-based particles have been directly synthesized by the laser induced pyrolysis of a mixture containing iron pentacarbonyl/air (as oxidizer)/ethylene (as sensitizer). In this paper we further demonstrate the possibility to vary the chemical composition and the nanoparticle dimensions of the iron oxide-based materials by handling the oxidation procedure in the frame of the laser pyrolysis process. Thus, nanoparticles with major maghemite/magnetite content may change composition into mixtures with variable amounts of three components: major gamma-Fe2O3/Fe3O4 iron oxide, metallic Fe and cementite Fe3C. By X-ray diffraction (XRD) it is found that the relative proportion of these phases differs in function of the reaction temperature (laser power). As revealed by transmission electron microscopy (TEM), mean particle sizes between about 4 nm and 6 nm and between about 9 and 11 nm may be prepared by varying the oxidation procedure and the laser power, respectively. By the controlled heating of samples (maximum temperature 185 degrees C), increased crystallinity for the gamma-Fe2O3/Fe3O4 oxide phase was found as well as an increase of the mean particle diameters. The examination of the magnetization curves for samples obtained for different laser powers indicates notable differences in the magnetic behavior and parameters. The temperature dependent Mossbauer measurements confirm the formation of larger particles at higher laser power densities as well as the presence of inter-particle magnetic interactions. On this basis, the estimation of phase composition for the different representative samples is given.

2925. Soft type PZT material used to fabricate a high intensity air siren

Authors: Miclea, C; Iuga, A; Tanasoiu, C; Trupina, L; Cioangher, M; Miclea, CT; Malczanek, V; Susu, M; Voicu, I

Published: FEB 2010, JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS, 12, 281, DOI:

A high quality soft type PZT material was designed and produced in order to be used as sensor for a high intensity air siren, Two thin disks from this material were assembled into a parallel bimorph type sensor with an intermediate metallic plate for the siren. This assembly behaves like an acoustic resonator in the high frequency audio range. This structure transforms the radial mode of vibration into a flexural one, with a much lower frequency. A mechanical impedance adapter was used in order to assure a good match between the oscillating ceramic and the surrounding air. A theoretical approach was outlined, which allows the evaluation of the resonance frequency for the working transducer. The theoretical and experimental data agree to a satisfactory degree up to the input errors assumed by the model. Acoustical and impedance measurements were made in order to appreciate the influence of the metallic plate on the frequency. The bimorph necessary for making the siren was designed as a parallel structure and the corresponding transducer was measured and evaluated. In order to increase the acoustic emission a frontal cardboard horn was added to the resonator.

2926. Epoxidation of cyclohexene with O-2 and isobutyraldehyde catalysed by cobalt modified hydrotalcites

Authors: Angelescu, E; Ionescu, R; Pavel, OD; Zavoianu, R; Birjega, R; Luculescu, CR; Florea, M; Olar, R

Published: JAN 15 2010, , DOI: 10.1016/j.molcata.2009.09.009

2927. Incorporation and localization of substitutional Mn2+ ions in cubic ZnS quantum dots

Authors: Nistor, SV; Stefan, M; Nistor, LC; Goovaerts, E; Van Tendeloo, G

Published: JAN 15 2010, PHYSICAL REVIEW B, 81, DOI: 10.1103/PhysRevB.81.035336

Multifrequency electron paramagnetic resonance (EPR) and high resolution transmission electron microscopy (HRTEM) investigations were performed on small (2 nm) cubic ZnS nanocrystals (quantum dots-QDs) doped with 0.2% mol Mn2+, self-assembled into a mesoporous structure. The EPR data analysis shows that the substitutional Mn2+ ions are localized at Zn2+ sites subjected to a local axial lattice distortion, resulting in the observed zero-field-splitting parameter vertical bar D vertical bar = 41 x 10(-4) cm(-1). The local distortion is attributed to the presence in the second shell of ligands of a stacking fault or twin, which alters the normal stacking sequence of the cubic structure. The HRTEM results confirm the presence of such extended planar defects in a large percentage of the investigated QDs, which makes possible the proposed substitutional Mn2+ impurity ions localization model. Based on these results it is suggested that the high doping levels of Mn2+ ions observed in cubic ZnS and possible in other II-VI semiconductor QDs prepared at low temperatures can be explained by the assistance of the extended lattice defects in the impurities incorporation.

2928. One-pot synthesis of an unusual manganese-lanthanide-ferrocene cluster: A combination of d-, f-metals and an organometallic fragment

Authors: Mereacre, V; Ako, AM; Filoti, G; Bartolome, J; Anson, CE; Powell, AK

Published: JAN 13 2010, POLYHEDRON, 29, 247, DOI: 10.1016/j.poly.2009.07.046

Reaction of the preformed cluster [Mn6O2(Piv)(10)(4-Me-py)(2.5)(PivH)(1.5)] with Nd(NO3)(3)center dot 6H(2)O, N-butyldiethanolamine (bdeaH(2)) and ferrocene-1,1'-dicarboxylic acid (fcdcH(2)) resulted in the formation of the first 3d-4f complex incorporating organometallic ferrocene [Mn4Nd4(OH)(4)(fcdc)(2)(Piv)(8)(bdea)(4)]center dot H2O; we report the X-ray structure and preliminary magnetic studies of this high-nuclearity cluster. (C) 2009 Elsevier Ltd. All rights reserved.

2929. Annealing study of a bistable cluster defect

Authors: Junkes, A; Eckstein, D; Pintilie, I; Makarenko, LF; Fretwurst, E

Published: JAN 11 2010, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 612, 529, DOI: 10.1016/j.nima.2009.08.021

This work deals with the influence of neutron and proton induced cluster related defects on the properties of n-type silicon detectors. Defect concentrations were obtained by means of Deep Level Transient Spectroscopy (DLTS) and Thermally Stimulated Current (TSC) technique, while the full depletion voltage and the reverse current were extracted from capacitance-voltage (C-V) and current-voltage (I-V) characteristics. The annealing behaviour of the reverse current can be correlated with the annealing of the cluster related defect levels labeled E4a and E4b by making use of their bistability. This bistability was characterised by isochronal and isothermal annealing studies and it was found that the development with increasing annealing temperature is similar to that of divacancies. This supports the assumption that E4a and E4b are vacancy related defects. In addition we observe an influence of the disordered regions on the shape and height of the DLTS or TSC signals corresponding to point defects like the vacancy-oxygen complex. (C) 2009 Elsevier B.V. All rights reserved.

2930. INFLUENCE OF POLARIZING ELECTRIC FIELDS ON THE ELECTRICAL AND OPTICAL PROPERTIES OF POLYMER-CLAY COMPOSITE SYSTEM

Authors: Rosu, C; Manaila-Maximean, D; Donescu, D; Frunza, S; Sterian, AR

Published: JAN 10 2010, MODERN PHYSICS LETTERS B, 24, 73, DOI: 10.1142/S0217984910022238

We examined the electrical and optical properties of a new material containing polymer-clay nano-composite dispersed in a nematic liquid. The clay (Cloisite-type) was modified by copolymerization of maleic anhydride and divinyl benzene, using azobisisobutytironitrile as the initiator. The final polymer-clay nano-composite has an intercalated structure according to XRD patterns. We measured the thermally stimulated depolarization currents and determined the activation energies. Simultaneously we measured the optical transmission and we studied the influence of the previously applied polarizing electric fields.