National Institute Of Materials Physics - Romania

Publications

5975 articles found

5281. Polymer degradation within the glass transition range

Authors: Chipara, MI; Chipara, MD; Romero, JR

Published: 1996, IEEE 1996 ANNUAL REPORT - CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA, VOLS I & II, 820, DOI:

Polymer degradation around the glass transition temperature exhibits frequently an anomalous behaviour. Starting from the hypothesis that such anomalies are related to the glass transition phenomenon, the most relevant data reported in the literature are reviewed and a theoretical description, based on the free volume approximation is suggested. The degradation process is split in three steps. It is proved that the dominant contributions come from the defect's diffusion step and from the proper chemical stage, which may be supposed to be statistically independent. It is explain, using an extended WLF equation why these anomalies are not located exactly at the glass transition temperature. The agreement between theory and experimental data reported elsewhere is good.

5282. Coulomb interaction effects on the magnetoconductivity of laterally modulated two-dimensional electron systems

Authors: Manolescu, A; Gerhardts, RR; Tornow, M; Weiss, D; vonKlitzing, K; Weimann, G

Published: 1996, SURFACE SCIENCE, 361, 516, DOI: 10.1016/0039-6028(96)00458-X

For a 2D electron system in a 1D periodic potential modulation, a complicated structure of Shubnikov-de Haas peaks has been measured and qualitatively explained by a transport calculation including interaction effects in the Hartree-Fock approximation.

5283. Photoelectric properties of PbTiO3/Si heterostructures

Authors: Alexe, M; Pintilie, L; Pintilie, I; Pignolet, A; Senz, S; Hesse, D

Published: 1996, FERROELECTRIC THIN FILMS V, 433, 430, DOI: 10.1557/PROC-433-425

Photoelectric properties of PbTiO3/Si heterostructures were investigated over a broad range of wavelengths. The spectral distribution of the a.c. open-circuit voltage was measured for wavelengths ranging from 0.35 mu m to 4 mu m. Two local peaks are observed at 0.38 mu m and 0.9 mu m which can be attributed to band-to-band excitations in PbTiO3 and Si respectively. The shape of the a.c. signal depends both on the light intensity and on the wavelength. When the heterojunction is reverse-biased, a d.c. photocurrent was observed which has a similar spectral distribution as the a.c. open-circuit voltage.

5284. A study of hydrogenated amorphous silicon (a-Si:H)/electrolyte interface by means of impedance spectroscopy

Authors: Dragoe, D; Maracine, L; Popescu, V; Popescu, C

Published: SEP-OCT 1996, REVUE ROUMAINE DE CHIMIE, 41, 748, DOI:

The a-Si:H/electrolyte interface was studied using impedance spectroscopy (IS) method. The measurements were made in the frequency range of 100 Hz - 300 kHz and the potential range of -0.6 V - +0.8 V. The equivalent circuit suggested by the experimental curves allowed us to calculate the main parameters which characterize the interface and the mechanism through which the space charge region supplies carriers to the interface.

5285. Dielectric and DSC investigations of 4-n-substituted benzoic and cyclohexane carboxylic acids .1. Textural changes in homologous 4-n-alkoxybenzoic acids

Authors: Frunza, L; Frunza, S; Petrov, M; Sparavigna, A; Torgova, SI

Published: 1996, MOLECULAR CRYSTALS AND LIQUID CRYSTALS SCIENCE AND TECHNOLOGY SECTION C-MOLECULAR MATERIALS, 6, 223, DOI:

We present the results of dielectric- and differential scanning calorimetry investigations on several homologous of 4-n- alkoxybenzoic acids: in particular the 4-n-heptyl-, -octyl-, -nonyloxybenzoic acids are studied. Our measurements confirm the existence in the nematic phase of a textural transition, which on cooling changes a well ordered state, f.i. a twisted state, to a highly distorted one, characterized by the appearance of a multidomain structure. Such a transition turns out to be reversible, with a small hysteresis. The transition exhibits a clear evidence for the 4-n-octyl- and the 4-n-nonyloxybenzoic acids. On the contrary, the presence of this transition in the 4-n-heptyloxybenzoic acid is suggested by a small peak only in the curve of the dielectric permittivity. The effect of the simultaneous presence in the nematic phase of open and closed dimers and of cybotactic clusters can explain the observed dielectric behavior.

5286. On the purification of cesium iodide

Authors: Dan, C; Mitroaica, G; Apostol, E

Published: JAN-FEB 1996, REVUE ROUMAINE DE CHIMIE, 41, 83, DOI:

Purifying methods of cesium iodide are presented along with the reasons of their choice. Fractional recrystallization, drying in vacuum, chemical treatment with iodine and refining zone were used in this sequence. Impurity analysis regarded chlorine, bromine, iron and heavy metals. Finally the results show very low halogen level and reasonable level for metal.

5287. Factors influencing the electric conductance of SnO2 gas sensors

Authors: Ionescu, R; Vancu, A

Published: 1996, CAS '96 PROCEEDINGS - 1996 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 19TH EDITION, VOLS 1 AND 2, 495, DOI:

5288. Determination of atomic local order in thyroid hormones by extended x-ray absorption fine structure {EXAFS} for radiation dose estimates

Authors: Orton, BR; Vorsatz, D; Macovei, D

Published: 1996, ACTA ONCOLOGICA, 35, 899, DOI: 10.3109/02841869609104043

Dosimetry from I-125 emitting Auger electrons in the follicles of the thyroid will improve when their complete interaction with the molecular structure about the Auger emitting atoms is known to Angstrom distances. Extended x-ray absorption fine structure (EXAFS) can provide this information. EXAFS experiments gave intermolecular and intramolecular distance for pure solid L-thyroxine (T-4) (C15H11I4NO4) using a model based on crystalline T-4 hydrochloride monohydrate (C15H11I4NO4HCl.H2O). For a solution of L-thyroxine the structure consists of the intramolecular distances found for T-4 and an additional shell of four carbon atoms distances of 2.01 Angstrom. Atomic density functions from Fourier transformed EXAFS measurements are suitable for estimating very short range interactions of Anger electrons in thyroid thyroglobulin over a radial distance of 7.71 Angstrom. Examination of density functions provided a plausible argument for the difference in biological effect between I-131 and I-125 in the thyroid.

5289. Symmetric SQW structure with low confinement factor for high power laser diodes

Authors: Buda, M; Iordache, G; Vlaicu, M; Cengher, D; Diaconescu, D

Published: 1996, CAS '96 PROCEEDINGS - 1996 INTERNATIONAL SEMICONDUCTOR CONFERENCE, 19TH EDITION, VOLS 1 AND 2, 210, DOI:

5290. Alkali defect centers and current density peaks in quartz sweeping

Authors: Iliescu, B; Vedda, A; Spinolo, G

Published: 1996, CRYSTAL RESEARCH AND TECHNOLOGY, 31, 219, DOI: 10.1002/crat.2170310217

The paper presents the current density peaks during the heating-up phase for different kind of quartzes: clear natural from Brazil, smoky natural, and synthetic quartz from Sawyer Research Product (Electronic and Premium Q), Toyo, romanian and polish ones, and compares the phenomenon in vacuum and in air for the same applied electric field and gives a model, with which the process could be described.