National Institute Of Materials Physics - Romania

Parallel and series pyroelectric bimorph structures, with 2-2 connectivity, are theoretically analysed to obtain the electric fields inside the two component phases. The proposed model takes into consideration the possibility of an electrical interaction between the two phases. It is demonstrated that the value of the internal electric field, due to the pyroelectric charge generated during heating of the bimorph structure, is considerably different than the case where each of the two phases is heated separately. It is shown that the value and the orientation of the electric fields inside the two phases of the bimorph depend on the value of the pyroelectric coefficients and dielectric constants of the two phases. The most interesting result is obtained in the case of the series bimorph, where the orientation of the electric field inside the phase with a lower pyroelectric coefficent is opposite to the orientation of spontaneous polarisation of this phase.

The crystal structure of Bi-2212 phase was studied by XRD, TEM, HREM and SAED. Bi-2212 specimens with different Sr/Ca ratios have the same displacive incommensurate Bi modulation vector q(1) approximate to 0.21b*+c* but exhibit different superconducting critical temperatures T-c. The structural modulation does not influence the T-c value which is controlled by the cation concentration and the oxygen content. A Pb addition leads to a superposition of the incommensurate Pb-modulation characterized by a modulation vector q(2) approximate to +/- 0.16 b*, on the already observed Bi modulation. Typical structural disordering (stacking faults, Bi-layer delineation effects, a-b plane misorientation) was found.

The paper describes some results concerning technology and behaviour of X-and gamma-ray N+PP+ silicon detectors used in physics research, industrial and medical radiography and non-destructive testing. These detectors work at the room-temperature and can be used individually to detect X-and soft gamma-rays, or coupled with scintillators for higher incoming energies. Electrical characteristics of these photodiodes, their modification after exposure to radiation and results of spectroscopic X-and gamma-ray measurements are discussed Devices manufactured under this technology proved to be stable after an exposure in high intensity gamma field with the dose range of 10 krad-5 Mrad: Nuclear radiation resistance was studied by irradiation with Co-60 gamma source (1.17, 1.33 MeV) at dose rates of 59 krad/hour and 570 krad/hour. Results indicate that proposed structures enable the development of reliable silicon detectors to be used in a high gamma-radiation environments encountered in a lot of applications.

The aim of the experiments presented in this paper was to choose from the project-recommended types of steel, the most suitable one for manufacturing the inner surfaces of the fire tube of an inert gas generator (IGG). Typically, such generators are used on oil tankers for maintaining an inert gas cushion above the oil's surface in order to prevent accidental combustion. For this approach, we employed the Thm Layer Activation (TLA) technique. Several steel samples have been activated either by 14 MeV proton or 8.5-12.5 MeV deuteron beams. The principle of measuring method, including the calibration procedure for transforming the resulted decreases in radioactivity into material loss data, have been presented in detail in the paper. The real working conditions-exposure to H(2)O, CO(2), and SO(x) compounds (ca. 1-2.5% in regular Diesel oil) having temperatures up to 700 degrees C were assured by mounting the steel samples in a similar tube to the one used offshore. For high accuracy results, the experiment required a dedicated calibration set-up, designed particularly for this application. The main outcome of the experiment was a classification of the different kinds of steels used (brands and/or manufacturers will not be specified in the paper) in accordance with the resistance to erosive corrosion induced by fuel-oil burning products. (C) 1997 Elsevier Science Ltd. All rights reserved.

This work presents a unitary mode to describe orientational structures of extreme free energy for unidimensional distortions of a chiral nematic layer with weak tilted anchoring at limiting surfaces, using structure parameters. The case of an applied electric field was also considered. A generalised expression for surface energy involving both polar and azimuthal deformations was introduced. A classification of possible structures having monotonous or non-monotonous variations of the tilt angle is presented The solving of the equation system that links the structure parameters in each case is given.

This paper presents the optical field behaviour of relatively large stripes, long laser diodes. The devices were designed to operate in the fundamental lateral mode. The stripe width, of order of 16 mu m, involves a very low refractive index step at the ridge edges and the operation of the diode in weak index guided regime. This index profile and subsequently, the optical field distribution is very easily disturbed by carrier injection or by temperature The paper aims to characterise the influence of direct current intensity on lateral far field.

The electrical transport of stabilised porous silicon layers was investigated. The samples were stored in ambient for 1,5-2 years, A MIS-like C-V characteristic, a strong rectifying I-V curve and I-T dependence with two activation energies were obtained, Thermally stimulated depolarization currents have an activation energy of 0.81-0.87 eV. The electrical properties are discussed in the frame of a quantum confinement model, keeping into account the surface component.

Using the Thermally Stimulated Current (TSC) method, the activation energy of trapping levels in sol-gel deposited lead titanate films was determined. Four levels were found in films annealed at 550 degrees C and 600 degrees C. Their activation energies are: E-1 = 0.23 eV, E-2 = 0.30 eV, E-3 = 0.34 eV and E-4 = 0.40 eV. For the films annealed at 650 degrees C only one level (E-2) was observed. It was found that the TSC magnitude decreases if the same sample is subjected to several cycles of cooling and heating between 100 K and 373 K in vacuum. This suggests a ''healing'' of the traps, probably due to the desorption of some gaseous species from the surface of the ferroelectric thin film.

Electron spin resonance investigations on the thermal degradation in air of stabilized and crude polyepichlorohydrins are reported. The temperature dependence of resonance line parameters, in the temperature range 250-500 degrees C, has been investigated. The activation energy has been estimated for the total free-radical generation processes. lit situ experiments on crude and stabilized polyepichlorohydrins are reported. For the sample stabilized with amines the appearance of nitroxide-like free radicals has been noticed. (C) 1997 Elsevier Science Limited.