National Institute Of Materials Physics - Romania

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5840 articles found

5231. DESIGN OF A 1 W, SINGLE FILAMENT LASER-DIODE

Authors: PETRESCUPRAHOVA, IB; BUDA, M; VANDEROER, TG

Published: SEP 1994, IEICE TRANSACTIONS ON ELECTRONICS, E77C, 1478, DOI:

A design of a high power laser structure is presented which is based on an increase of the cavity length as well as a maximization of the stripe width. This requires a low value for the modal attenuation coefficient and a low optical confinement factor. A model is presented from which the modal gain, the confinement factor, the active region thickness, the stripe width, the length and the reflection coefficients can be calculated. A variant for all design parameters needed to reach 1 W emission in the fundamental lateral mode is given. These values are used to design the epitaxial structure.

5232. EXPERIMENTAL-DETERMINATION OF THE EFFECTIVE SPLAY-BEND ELASTIC-CONSTANT

Authors: BARBERI, R; BARBERO, G; GIOCONDO, M; MOLDOVAN, R

Published: SEP 1994, PHYSICAL REVIEW E, 50, 2099, DOI: 10.1103/PhysRevE.50.2093

The temperature-induced surface alignment transitions in nematic liquid crystals are analyzed. It is shown that it is possible to interpret the experimental data by means of an elastic model in which the effective splay-bend surfacelike elastic constant is taken into account. This effective elastic constant is defined as the sum of the usual splay-bend and spontaneous splay elastic constant. In this framework the experimental data seem to indicate that the easy axis characterizing the nematic-substrate interaction is independent of the liquid crystal. It depends only on the evaporation geometry (we use flat glass plates vacuum coated by SiO) and on the surfactant dissolved in the liquid crystal. The value of the effective splay-bend elastic constant is obtained for three different liquid crystals in the nematic phase.

5233. TRAPPING LEVELS IN BI12SIO20 CRYSTALS

Authors: PETRE, D; PINTILIE, I; BOTILA, T; CIUREA, ML

Published: AUG 15 1994, JOURNAL OF APPLIED PHYSICS, 76, 2219, DOI: 10.1063/1.357637

Localized levels play a major role in the electro-optic properties of Bi12SiO2O (BSO) crystals. The activation energy of trapping levels was studied by different laboratories using various methods (e.g., thermally stimulated currents and photoinduced current transient spectroscopy). A more sensitive investigation of traps in undoped BSO single crystals has been performed by optical charging spectroscopy. The presence of traps in the energy range 0.2-1.1 eV was found, and the results are in good agreement with previous studies. On the other hand, this method led us to suggest that the trapping levels observed can be both electron traps and hole traps. For deep trapping levels at higher temperatures, a strong temperature dependence of the cross section was observed.

5234. LAALO3 THIN-FILMS DEPOSITED ON SILICON AND SAPPHIRE AS BUFFER LAYERS FOR YBA2CU3O7-X

Authors: SANDU, V; JAKLOVSZKY, J; MIU, D; DRAGULINESCU, D; GRIGORIU, C; BUNESCU, MC

Published: AUG 15 1994, JOURNAL OF MATERIALS SCIENCE LETTERS, 13, 1225, DOI: 10.1007/BF00241019

5235. ON A MODIFIED KIM ANDERSON MODEL OF THE CURRENT-VOLTAGE CHARACTERISTICS OF HIGH-TEMPERATURE SUPERCONDUCTING CERAMICS

Authors: CRISAN, A

Published: AUG 1994, JOURNAL OF SUPERCONDUCTIVITY, 7, 691, DOI: 10.1007/BF00721648

In comparison with the ''classical'' Kim-Anderson model for the current-voltage characteristics of type-II superconductors, another model with a less sharp pinning potential is presented. In addition to the approximate solution, for currents close to the critical one, a complete solution of this model is derived, which better fits the experimentally obtained current-voltage characteristics.

5236. INVESTIGATION OF THE PHASE-COMPOSITION IN SINTERED LANTHANA-DOPED (ZR,SN) TIO4 CERAMICS

Authors: VASILIU, F; MOISA, S; GROZEA, D; BUNESCU, C

Published: JUN 15 1994, JOURNAL OF MATERIALS SCIENCE, 29, 3341, DOI: 10.1007/BF00356682

Ceramic materials of the ZrO2-SnO2-TiO2 system, modified by La2O3 and ZnO additions, were investigated by X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectrometry. All the samples sintered at 1330, 1360 and 1400-degrees-C contain a (Zr(x)Sn(z))TiO4 (x+z = 1, 0 < z less-than-or-equal-to 0.4) solid solution coexisting with ZrTiO4, the last gradually disappearing with rising temperature. At 1400-degrees-C, the (Zr0.8Sn0.2)TiO4 solid solution matrix was not quite homogeneous, containing small amounts of a grain-boundary phase assumed to be La2(Zr,Sn,Ti)2O7 and the expected TiO2-basis solid solution rich in Zn and La. The evolution of the chemical composition of (Zr(x)Sn(z))TiO4 solid solution to (Zr0.8Sn0.2)TiO4 and the simultaneous disappearance of ZrTiO4 are thought to explain the variation trends of lattice parameters with increasing sintering temperature.

5237. AN X-RAY STUDY OF GENERALIZED CANTOR SUPERLATTICES

Authors: JARRENDAHL, K; BIRCH, J; DULEA, M; ARWIN, H; SUNDGREN, JE

Published: JUN 15 1994, THIN SOLID FILMS, 246, 125, DOI: 10.1016/0040-6090(94)90740-4

Amorphous Ge/Si and crystalline Mo/V superlattices have been grown by dual-target d.c. magnetron sputtering with modulations according to finite approximants of various generalized Cantor sequences. The samples were investigated with both conventional powder X-ray diffractometry and high-resolution X-ray diffractometry as well as transmission electron microscopy. The experimental X-ray results were compared with derived analytical expressions for the locations and scalings of the diffraction peaks. Results from the X-ray measurements and the analytical expressions were also compared with numerical reflectivity simulations. The analytical and numerical calculations show good agreement with the experimental results.

5238. ESR INVESTIGATIONS ON ION-BEAM IRRADIATED POLYCARBONATE

Authors: CHIPARA, MI; GRECU, VV; NOTINGHER, PV; ROMERO, JR; CHIPARA, MD

Published: JUN 1994, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS, 88, 422, DOI: 10.1016/0168-583X(94)95393-7

Electron spin resonance (ESR) investigations with a polycarbonate solid state nuclear detector, irradiated with oxygen ions, are reported. The nature of the paramagnetic defects induced by irradiation is discussed. The temperature dependence of resonance line parameters is studied. From the experimental data, obtained by ESR, spectroscopy, the activation energy for defect recombination, the average isotropic exchange integral between paramagnetic defects as well as the average distance between defects, are estimated. Correlations with latent tracks structure are discussed.

5239. EXISTENCE OF ENANTIOMERIC PHASE-SEPARATION IN A 3-DIMENSIONAL LATTICE-GAS MODEL

Authors: HUCKABY, DA; PITIS, R; SHINMI, M

Published: JUN 1994, JOURNAL OF STATISTICAL PHYSICS, 75, 995, DOI: 10.1007/BF02186754

A three-dimensional lattice gas model for enantiomeric phase separation is introduced. The enantiomeric molecules (d and 1) are the two nonsuperimposable mirror images having the molecular structure C(AB)2, where C is a tetrahedrally bonded carbon atom with one bond to each end of two AB groups. The lattice gas model consists of a body-centered cubic lattice, each site of which can be either vacant or occupied by a molecule oriented so that the A and B groups point toward neighboring lattice sites. Pairs of molecules interact with short-range, orientationally-dependent interactions. For a domain of interaction parameters, the Pirogov-Sinai extension of the Peierls argument is used to prove that d-rich and l-rich phases exist in the model at sufficiently low temperature. For another domain of interaction parameters, at sufficiently high chemical potential there is an infinite number of ground states, each containing a racemic mixture of d and l molecules.

5240. RADIOACTIVE-WASTE TREATMENT PRODUCTS STUDIED BY MOSSBAUER-SPECTROSCOPY .2. IRON HYDROXIDE PRECIPITATION SYSTEMS

Authors: SPANU, V; TURCANU, CN

Published: JUN 1994, JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY-ARTICLES, 181, 200, DOI: 10.1007/BF02037559

Different iron hydroxide precipitation processes simulating radioactive waste treatment have been investigated by Mossbauer spectroscopy at room temperature and at 80 K. Magnetic oxides (hematite or magnetite) partially affected by superparamagnetic relaxation have been observed. The crystallization degree and the particle size depend on the concentration and the addition order of chemicals. Much smaller particles were precipitated with Ca(OH)2 than with NaOH as neutralization reagent.