National Institute Of Materials Physics - Romania

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5840 articles found

4981. Reactivity of the Bi, Sr, Ca, Cu oxalate powders used in BSCCO preparation

Authors: Popa, M; Totovana, A; Popescu, L; Dragan, N; Zaharescu, M

Published: 1998, JOURNAL OF THE EUROPEAN CERAMIC SOCIETY, 18, 1271, DOI: 10.1016/S0955-2219(98)00052-1

The solution chemistry of oxalate coprecipitation of Bi, Si, Ca, Cu has frequently! been used in the synthesis of Bi-based ceramic systems, although there are different opinions referring to some major aspects, such as coprecipitation pH and precipitating agent. In the present work we established the precipitation conditions of Bi, Sr, Ca, Cu individual oxalates and of the quaternary! mixture. The initial and thermally treated oxalate powders were characterized bq! XRD, IR, BET methods and TEM observation. The decomposition conditions were established based on a DTA/TG study. The powders resulted from the decomposition of Bi and Cu oxalates contain the corresponding oxides and the Ca and Sr oxalates decomposition lead to the carbonates obtaining. In each case a high reactivity and a very small powder particle size were observed. A very good powder homogeneity has also been noticed in the final mixture of oxalates. Compared to superconducting ceramic materials realized by a classic route, the reactive oxalate powders allow the decrease of both thermal treatment periods and temperature, emphasizing a high microstructure homogeneity of the ceramic body. (C) 1998 Elsevier Science Limited. All rights reserved.

4982. Amorphous Se/CdSe and SiOx/CdSe multilayers

Authors: Popescu, M; Sava, F; Lorinczi, A; Vateva, E; Nesheva, D; Tchaushev, G; Mihailescu, IN; Koch, PJ; Obst, S; Bradaczek, H

Published: 1998, FIFTH CONFERENCE ON OPTICS (ROMOPTO '97), PTS 1 AND 2, 3405, 964, DOI: 10.1117/12.312698

Amorphous multilayers based on Se and CdSe alternated sublayers were succesfully prepared by thermal vacuum evaporation and laser ablation with the periodicity of 22 nm. The multilayer structure is stable up to similar to 70 degrees C. In the samples prepared by laser ablation a large contraction of the multilayer stacking occurs by annealing. Amorphous SiOx/CdSe multilayers with the periodicity of similar to 15 nm were successfully prepared by thermal vacuum evaporation. The structure is stable up to 400 degrees C annealing temperature. Pulse excimer laser irradiation of the SiOx based multilayer produces an expansion of the interlayer distance with similar to 0.33%.

4983. Electrical and optical characterization of PbTiO3/Si heterostructures for applications in optoelectronics

Authors: Dragoi, V; Pintilie, L; Pintilie, I; Petre, D; Boerasu, I; Alexe, M

Published: 1998, FIFTH CONFERENCE ON OPTICS (ROMOPTO '97), PTS 1 AND 2, 3405, 851, DOI: 10.1117/12.312674

The ferroelectric/semiconductor heterostructures were fabricated by sol-gel deposition of lead titanate (PT) thin films on a single-crystalline p-type Si wafers. The ETT films were crystallyzed by a conventional thermal annealing for 30 min at temperatures ranging from 575 degrees C to 675 degrees C. Current-voltage and capacitance-voltage characteristics show a hysteresis which can be due to the spontaneous polarization of PbTiO3. The current-voltage characteristics exhibit a diode behaviour while the capacitance-voltage exhibits a large memory window, up to 3.5 V, for the films annealed at 600-650 degrees C. At the illumination with modulated light, a.c. photovoltage was detected on a broad range of wavelengths (0.35+4 mu m) for all samples. The spectral distribution of the photoelectric signal in the UV-Vis-IR domain shows two local maxima. A model is proposed to explain the observed experimental results.

4984. Photo-induced softening and hardening in Ge-As-S amorphous films

Authors: Popescu, M; Sava, F; Lorinczi, A; Skordeva, E; Koch, PJ; Bradaczek, H

Published: 1998, JOURNAL OF NON-CRYSTALLINE SOLIDS, 227, 722, DOI: 10.1016/S0022-3093(98)00236-1

Hardness, X-ray diffraction and transmission as well as infrared reflection measurements on virgin and ultraviolet (UV)-irradiated amorphous films of the GexAs40-xS60 system have been performed. The UV light induces film softening for x < 19 and film hardening for x > 19. The structure remains amorphous but the distance characteristic of medium range order decreases for x < 19 and increases for x > 19. Photo-contraction of the films (up to similar to 12%) has been revealed from IR spectra and X-ray transmission experiments. A main effect of UV light is the release of sulphur. (C) 1998 Elsevier Science B.V. All rights reserved.

4985. Theoretical model for carriers transport in nanocrystalline porous silicon films

Authors: Ciurea, ML; Iancu, V; Pavelescu, G; Baltog, I

Published: 1998, CAS'98 PROCEEDINGS - 1998 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 21ST EDITION, VOLS 1 AND 2, 112, DOI:

A quantum confinement model is proposed to explain the carriers transport in a nanowire network of porous silicon. The electron Hamiltonian is written as the sum of the longitudinal and the transversal contributions. The last one corresponds to a two-dimensional infinite cylindrical quantum well, whose energy levels determine the activation energies observed in the temperature dependence of the dark current. The model is in excellent agreement with the experimental data.

4986. Ageing and barriers in tin dioxide gas sensors

Authors: Ionescu, R

Published: 1998, EUROSENSORS XII, VOLS 1 AND 2, 700, DOI:

The time-evolutions of both the electrical resistance in air, and of the response to reducing gases lead to the conclusion that the resistance of tin dioxide gas sensors is not governed by intergranular barriers. A conductance model is discussed, with a surface resistance and a bulk resistance connected in parallel. The long-termed evolution of the sensor resistance (ageing) is attributed to oxygen diffusion into the bulk.

4987. Change of the optical properties of porous silicon by post anodization treatments

Authors: Pavelescu, G; Ciurea, ML; Mihut, L; Galeata, G; Lengyel, E; Baltog, I; Roger, JP

Published: 1998, CAS'98 PROCEEDINGS - 1998 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 21ST EDITION, VOLS 1 AND 2, 458, DOI:

Optical properties of as prepared porous silicon (PS) were compared with those of thermally treated and chemically cleaned samples. The change of the photoluminescence (PL) spectra correlated with ellipsometric. reflectance and Raman measurements leads us to conclude that the adsorbed species in the PS nanocrystalline structure play an important role in the origin of the PI emission, at least for as - anodized samples.

4988. Silicon linear image sensors for photometry

Authors: Cimpoca, V; Cengher, D; Crisan, C; Olariu, N; Oros, C; Cimpoca, M

Published: 1998, CAS'98 PROCEEDINGS - 1998 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 21ST EDITION, VOLS 1 AND 2, 70, DOI:

The paper describes a new linear image sensor for photometry by applying bipolar technology (Standard Buried Collector-SEC) to the manufacture of self-scanning linear photodiode arrays. On a monolithic chip we have designed 128 to 1024 linear photodiodes, demultiplexing system and signals processing system. A dynamic simulation and a real-time measurement have been implemented, that allow to take into account every operational phase of detection. The dialogue between the various functions is established by message's transmission, using common data areas stored on disks or in central memory of PC compatible computer.

4989. Cobalt-manganese oxide thin films thermistors obtained by MOD

Authors: Dragoi, V; Alexe, M

Published: 1998, CAS'98 PROCEEDINGS - 1998 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 21ST EDITION, VOLS 1 AND 2, 300, DOI:

Co1.4Mn1.6O4 (CMO) thin films were deposited onto silicon by Metalorganic Decomposition (MOD) method. Structure, composition and morphology were investigated by Electron Dispersive X-Ray Spectroscopy (EDX) and X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Electrical measurements showed possible applications as negative temperature coefficient (NTC) thermistors.

4990. A new method for discriminating different reducing gases with SnO2 sensors

Authors: Ionescu, R; Vancu, A; Moise, C; Tomescu, A

Published: 1998, CAS'98 PROCEEDINGS - 1998 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 21ST EDITION, VOLS 1 AND 2, 571, DOI:

By combining the changes induced by different reducing gases in electrical resistance with the corresponding changes of a second physical parameter it is possible to discriminate between groups off different reducing gases.