Publications

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.

Pd-Ln/gamma-Al2O3 (Ln = Sm, Gd, Dy, Ho, Tm) catalysts containing 0.1%Pd -1%Ln, 0.3%Pd -1% Ln and respectively, 0.5% Pd -1% Ln (percentage in wt.%) were investigated. Introduction of Ln leads to an increase of the vinylbenzenes conversion comparatively with monometallic Pd/gamma-Al2O3 catalysts. The turnover frequency (TOF) calculus of the latter catalysts suggests that there are no differences among these. In the case of bicomponent Pd-Ln/gamma-Al2O3 catalysts, if one considers that the reaction occurs only on the palladium particles, then introduction of Ln seems to increase the TOF and vinylbenzenes hydrogenation seems to be controlled by the mass transfer processes as indicated by the Koros-Nowak criterion. However, by taking Ln into consideration when calculate the TOF, it results that bicomponent catalysts behave like those monocomponent and the slope of the Koros-Novak dependence is included in a range typical for kinetically controlled reactions. This behavior could be associated to direct participation of Ln to reaction using hydrogen spilled over from the palladium particles.

Aliquots of waste water from power stations were analyzed by a special procedure in order to put in evidence the presence of renex, a nonionic detergent. The investigations were performed spectroscopically by FTIR technique. Qualitative and quantitative features of the procedure were established as specific peaks, detection limit and method sensibility. The validity of Lambert-Beer law was checked. Taking into account the selectivity of the method it results the necessity of separating renex from nitrogenous bases in the analyzed water.

Hydrogenation of a prostaglandin intermediate has been carried out on some Ru-molecular sieves prepared via deposition of RuCl3. Characterization of these catalysts using different tools like WAXS, XPS or EXAFS indicated differences due to both the different metal loading and the different structure and topology of the investigated molecular sieves (L, APO-34 and ZSM-5). The prostaglandin intermediate has two kinds of unsaturation, a double C=C bond and a prochiral C=O bond. Therefore hydrogenation could imply two different selectivities. On simple Ru-molecular sieves, hydrogenation occurs with different reaction rates and selectivities as a function of the catalysts characteristics. Thus, d.e. of about 100% for a selectivity to the alcohol of 99% were obtained for Ru(12wt.%)-L. However, on these catalysts only the epi configuration was obtained. Modification of the catalysts using L(+) tartaric acid leads to the increase of the reaction rate and the decrease of the d.e. Adding of the pivalic acid to the reaction mixture determined a supplementary increase of the reaction rate. More important in this case is the change of the stereoselectivity, the main product becoming the natural configuration.

C-axis oriented and disordered aluminum nitride layers have been deposited by RF, respectively DC magnetron reactive sputtering mode. The same deposition conditions were used for gas flow and pressure. The differences between the two growing modes are explained by the influence of ion bombardment upon the growing layers, which is stronger in the RF mode in comparison with the DC one, at the same deposition rate.

We obtained a multilayer heteroepitaxial structure of In-0.53 Ga-0.47 As-type deposed on (100) oriented We selected deliberately different thickness and doping concentrations df each layer for their potential applications in very high frequency bipolar transistors. A sequence of six epilayers were realized by vapour phase epitaxy. The characterization of crystallographic and semiconducting properties in correlation with technological processes is performed by TSC-method and by double crystal InP substrate. We propose a kinematics model of X-ray diffraction in heteroepitaxial multilayer structures of InP/InGaAs. It is based on a discontinuous lattice misfit and elastic strain distribution. The model fits well the rocking curves obtained for both (004) and (224) reflecting planes. The values of the lattice misfit and the elastic strains in the plane of lamellae were calculated.