National Institute Of Materials Physics - Romania

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5840 articles found

5021. Indirect enhancement of PbS photoconductivity by ferroelectric field effect in a PbS/PbTiO3/Si heterostructure

Authors: Pintilie, I; Pintilie, L; Dragoi, V; Petre, D; Botila, T

Published: AUG 25 1997, APPLIED PHYSICS LETTERS, 71, 1106, DOI: 10.1063/1.119740

A lead sulphide/lead titanate/silicon (PbS/PbTiO3/Si) heterostructure was manufactured by successive deposition of PbTiO3 and PbS thin film on a single crystalline, p-type Si substrate. Chemical methods were used for deposition. A three electrode configuration was used to control the photoconductivity of PbS thin film by field effect, through the ferroelectric PbTiO3 thin film. The spectral distribution of the photoconductive signal shows two maxima, situated at 1.1 and 2.45 mu m. It was found that the photoconductive signal at 1 mu m varies of about eight times when the voltage applied on the Si substrate is varied between -1 and +1 V. At 2 mu m the photoconductive signal is almost independent of the applied voltage on the Si substrate. The observed results are explained considering a ferroelectric field effect by which the photogenerated carriers in Si influence the photoconductive signal in the PbS thin film. (C) 1997 American Institute of Physics.

5022. Physics of optimal resonant tunneling

Authors: Racec, PN; Stoica, T; Popescu, C; Lepsa, M; vandeRoer, TG

Published: AUG 15 1997, PHYSICAL REVIEW B, 56, 3597, DOI: 10.1103/PhysRevB.56.3595

The optimal resonant tunneling, or the complete tunneling transparence of a biased double-barrier resonant-tunneling (DBRT) structure, is discussed. It is shown that its physics does not rest on the departure from the constant potential within the barriers and well, due to the applied electric field, but on the effective symmetry of the rectangular-barrier profile, which approximates the real potential profile for the corresponding applied bias.

5023. SEM studies on BSCCO superconducting ceramic produced by spray frozen, freeze drying technique

Authors: Bunescu, MC; Aldica, G; Badica, P; Vasiliu, F; Nita, P; Mandache, S

Published: AUG 1 1997, PHYSICA C, 281, 197, DOI: 10.1016/S0921-4534(97)00633-3

Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) have been used to evidence the occurrence, morphology and microcomposition of the superconducting phases (Bi,Pb)(2)Sr2Ca2Cu3O10+delta (2223) and (Bi,Pb)(2)Sr2CaCu2O8+delta (2212), and of other non-superconducting phases, in the sintered pellets obtained from nitrate solution by spray frozen, freeze drying technique. For decomposition of the nitrate powder four different heat treatments were used. Superconducting and structure properties of the pellets have been tested by AC susceptibility measurements (610 Hz, 0.5 Oe) and X-ray diffraction analysis, respectively. A correlation between the SEM and EDS observations and the superconducting properties has been established. (C) 1997 Elsevier Science B.V.

5024. ESR investigations on ion beam irradiated polymers

Authors: Chipara, MI

Published: AUG 1997, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 131, 90, DOI: 10.1016/S0168-583X(97)00382-0

Electron spin resonance investigations on ion beam irradiated polymers are reviewed from a critical point of view. The nature of free radicals produced by heavy ion bombardment is discussed in detail, in connection with the main hypotheses (thermal spikes and secondary electrons). ESR investigations on cellulose nitrate irradiated with accelerated oxygen ions are reported. From the temperature dependence of resonance line parameters, the presence of exchange interaction is noticed and the generation of free radicals at 200 degrees C is studied. (C) 1997 Elsevier Science B.V.

5025. Radiation effects on polyoctenamer

Authors: Chipara, MI; Georgescu, L; Oproiu, C; Chipara, MD; Niculescu, A; Galatanu, N; Reyes, JR; Secu, CE

Published: AUG 1997, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 131, 191, DOI: 10.1016/S0168-583X(97)00339-X

The effect of electron beams on polyoctenamer is investigated by viscosimetry, swelling and electron spin resonance spectroscopy. From these experimental investigations it has been deduced that the main degradation process in polyoctenamer is its crosslinking during electron beam irradiation in air, at room temperature. (C) 1997 Elsevier Science B.V.

5026. Thermal stability of amorphous Se/CdSe multilayers

Authors: Popescu, M; Sava, F; Lorinczi, A; Vateva, E; Nesheva, D; Koch, PJ; Gutberlet, T; Uebach, W; Bradaczek, H

Published: AUG 1997, SOLID STATE COMMUNICATIONS, 103, 434, DOI: 10.1016/S0038-1098(97)00207-X

The thermal stability of amorphous Se/CdSe multilayers has been investigated by X-ray diffraction. The multilayer structure is stable up to similar to 60 degrees C. After annealing at higher temperatures hexagonal selenium and hexagonal CdSe phases with nanometric crystallite size were revealed. (C) 1997 Elsevier Science Ltd.

5027. Localization and charge conversion of copper in Rb2ZnCl4:Cu crystals: An ESR and optical absorption study

Authors: Stefan, M; Nistor, SV; Grecu, NM; Schoemaker, D

Published: AUG 1997, PHYSICA STATUS SOLIDI B-BASIC RESEARCH, 202, 1011, DOI: 10.1002/1521-3951(199708)202:2<999::AID-PSSB999>3.0.CO;2-5

Electron spin resonance and optical studies reveal the presence of both Cu2+ and Cu+ centers in Rb2ZnCl4:Cu single crystals grown from melt and their conversion by X- or gamma-irradiation. Two types of paramagnetic Cu2+ centers with different concentrations and production properties have been identified. The more abundant Cu2+(I) center consists of a Cu2+ ion substituting for Zn2+ at the center of a ZnC42- tetrahedron. The less abundant Cu2+(II) center seems to be situated at a Rb site. Production experiments strongly suggest that during the crystal growth copper enters the Rb2ZnCl4 lattice as Cu2+, mainly at Zn2+ sites, part of it being afterwards converted to Cu+ precursor centers. The presence of a neighboring-charge compensating anion vacancy and its departure during the radiolytic Cu+(I) --> Cu2+(I) conversion seems to play an essential role in the stabilization of the Cu+(I) and Cu2+(I) centers, respectively.

5028. On an anomalous kinetic in irradiated polymers around the glass transition temperature

Authors: Chipara, MI

Published: AUG 1997, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 131, 187, DOI: 10.1016/S0168-583X(97)00340-6

Anomalies occurring in irradiated polymers within the glass transition range, as reported by various authors are critically reviewed. A theoretical description for such processes, is developed within the free volume approximation. The discrepancies between the temperature at which such anomalies are noticed and the glass transition temperature have been ascribed to the fact that the volume of radiation-induced radicals is not equal with the segmental one. The agreement between experiment and theory is good. (C) 1997 Elsevier Science B.V.

5029. Preparation and structure of lanthanum stearate films

Authors: Popescu, M; Gutberlet, T; Kastowsky, M; Koch, PJ; Bradaczek, H

Published: JUL 30 1997, THIN SOLID FILMS, 304, 326, DOI: 10.1016/S0040-6090(97)00073-4

Lanthanum stearate films were prepared by the conventional Langmuir-Blodgett method and by a dragging procedure. Multilayers have been obtained and studied by X-ray diffraction. The electron density distribution normal to the bilayer plane has been calculated. (C) 1997 Elsevier Science S.A.

5030. Investigation of the light induced refractive index changes in Fe:PVA

Authors: Kuncser, V; Filoti, G; Avramescu, A; Podgorsek, R; Biebricher, M; Franke, H

Published: JUL 25 1997, JOURNAL OF ALLOYS AND COMPOUNDS, 257, 292, DOI: 10.1016/S0925-8388(97)00021-2

The light induced modifications in thin films of Fe:PVA were studied by optical and Mossbauer spectroscopy. Under the UV exposure both the refractive index and the absorption coefficient are changing. The real part of the refractive index is measured using m-line Spectroscopy. As a function of UV exposure, the optical absorption as well as the Mossbauer spectra were measured. A correlation between the optical and the measured Fe2+/Fe3+ ratio is discussed. (C) 1997 Elsevier Science S.A.