National Institute Of Materials Physics - Romania

Publications

5840 articles found

5311. PROMOTION OF HIGH-TC PHASE IN B-DOPED (BI,PB)2SR2CA2CU3OY SUPERCONDUCTORS PREPARED BY THE MELT-QUENCHING METHOD

Authors: FRUTH, V; ZAHARESCU, M; ALDICA, G

Published: MAR-JUN 1993, APPLIED SUPERCONDUCTIVITY, 1, 707, DOI: 10.1016/0964-1807(93)90280-F

We prepared superconducting Bi1.6Pb0.4-xBxSr2Ca2CU3Oy (x=0, 0.1, 0.2, 0.3 and 0.4) ceramics by the melt-quenching method and examined the effect of B addition on the formation of the high-Tc phase.The temperatures and sequence of formation of the superconducting phases within this composition were determined using simultaneous differential thermal analysis (DTA) and thermogravimetric analysis (TGA), X- ray powder diffraction (XRD). It was found that the bulk ceramics with x=0.1 and 0.2 exhibited superconductivity with a critical temperature range of 95-100 K. The lowering of the partial melting temperature caused by the coexistence of Pb and B elements is effective for the formation mechanism of the high-Tc phase.

5312. EFFECT OF THERMAL-TREATMENT AND SWEEPING ON THE ELECTRICAL-CONDUCTIVITY OF QUARTZ CRYSTALS

Authors: ILIESCU, B; TOPA, V; TANASE, AT

Published: 1993, CRYSTAL RESEARCH AND TECHNOLOGY, 28, 1168, DOI: 10.1002/crat.2170280822

The effect of thermal treatment and sweeping at different temperatures on the current peaks occurring during the heating-up phase of the electrodiffusion runs are studied. Arrhenius plots of swept synthetic quartz are discussed.

5313. MEYER-NELDEL CORRELATION IN SEMICONDUCTORS AND MOTT MINIMUM METALLIC CONDUCTIVITY

Authors: POPESCU, C; STOICA, T

Published: DEC 15 1992, PHYSICAL REVIEW B, 46, 15071, DOI: 10.1103/PhysRevB.46.15063

The paper outlines conditions under which the statistical shift of the Fermi level within the density-of-states (DOS) distribution may result in the Meyer-Neldel correlation. Sufficient conditions have been found: (i) The DOS should have, around the Fermi level, two competing exponential slopes; (ii) The temperature range for the statistical shift should correspond to kT values higher than at least one of these two characteristic energies associated with the slopes. Under such conditions, the Fermi level is no longer the dominant energy for carrier-concentration changes. Relating the Mayer-Neldel correlation to DOS parameters is an essential step in a correct deduction of Mott's minimum-metallic-conductivity value from experiment.

5314. MAGNETIC-PROPERTIES OF XCEO2-(1-X)(3B2O3.PBO) GLASSES

Authors: BURZO, E; ARDELEAN, I; MATULESCU, D

Published: NOV 15 1992, JOURNAL OF MATERIALS SCIENCE LETTERS, 11, 1497, DOI: 10.1007/BF00729270

5315. RADIOACTIVE-WASTE TREATMENT PRODUCTS STUDIED BY MOSSBAUER-SPECTROSCOPY

Authors: SPANU, V; TURCANU, CN; FILOTI, G

Published: NOV 2 1992, JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY-LETTERS, 166, 320, DOI: 10.1007/BF02163837

Mossbauer effect was used for the characterization of the radioactive waste treatment products, e.g. precipitate formed during the treatment of LLAW (Low Level Active Waste) using iron compounds and their conditioned matrix obtained by cementation.

5316. PERIODIC STRUCTURES IN A TWISTED NEMATIC LIQUID-CRYSTAL CELL UNDER DC EXCITATION

Authors: FRUNZA, S; MOLDOVAN, R; BEICA, T; GIURGEA, M; STOENESCU, DN

Published: NOV 1 1992, EUROPHYSICS LETTERS, 20, 411, DOI: 10.1209/0295-5075/20/5/005

Some new results concerning the appearance of a periodic structure in a pi/2-twist cell filled with MBBA under d.c. excitation are reported. This structure consists of rectilinear domains whose orientation varies with the applied voltage around the normal to the molecular orientation in the middle of the cell and this variation covers a large range of angles even for fixed polarity of the excitation. When the polarity is changed, the sense of the domain orientation is also changed.

5317. ANCHORING ENERGY AND EASY DIRECTION OF NONUNIFORM SURFACES

Authors: BARBERO, G; BEICA, T; ALEXEIONESCU, AL; MOLDOVAN, R

Published: NOV 1992, JOURNAL DE PHYSIQUE II, 2, 2024, DOI:

The effect of a periodic distribution of easy directions on the surface macroscopic properties of nematic samples are considered. Our analysis shows that the effective surface energy of the sample contains an interference term coming from the two different easy axis. This allows to introduce an effective uniform easy direction, depending on the two easy directions and containing an interference term too, as suggested a few years ago by some researchers.

5318. TEXTURE IN RIETVELD REFINEMENT

Authors: POPA, NC

Published: OCT 1 1992, JOURNAL OF APPLIED CRYSTALLOGRAPHY, 25, 616, DOI: 10.1107/S0021889892004795

A texture model suitable for use in Rietveld-refinement programs is proposed. It is based on the series expansion of the pole distribution function in symmetrized harmonics. The model was tested on a textured plate sample of Al2O3.

5319. HOPPING CONDUCTIVITY OF THE FIBONACCI-CHAIN QUASI-CRYSTAL - COMMENT

Authors: DULEA, M; ALDEA, A

Published: OCT 1 1992, PHYSICAL REVIEW B, 46, 8643, DOI: 10.1103/PhysRevB.46.8642

This Comment refers to a recent publication on the ac hopping conduction of the Fibonacci chain [Phys. Rev. B 43, 1183 (1991)]. The renormalization procedure proposed by its authors gives a correct formula for the conductivity [Eq. (15)]. Nevertheless, their conclusion regarding the vanishing of the dc conductivity disagrees with the low-frequency asymptotics previously derived by us for the same model, and also disagrees with the general result given by the resistance-network analogy of the Miller-Abrahams rate equations. Their result assumes the factorization of the frequency in Eq. (15), which we prove is not true. We show also that their numerical curves, which apparently support their conclusion, are strongly influenced by errors in the low-frequency range.

5320. A METHOD FOR MEASURING THE NEUTRON LIFETIME WITH STORED ULTRACOLD NEUTRONS

Authors: STOICA, AD

Published: SEP 1 1992, NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT, 320, 607, DOI: 10.1016/0168-9002(92)90959-8

A method for measuring the neutron lifetime is proposed, using simultaneous detection of electrons and upscattered neutrons generated during the ultracold neutron storage.