National Institute Of Materials Physics - Romania

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.

Magnetic materials in the CuZnTi ferrite system with the chemical composition Cu1-xZnxTiyFe2-yO4, with 0.5 less than or equal to x less than or equal to 0.62 and 0 less than or equal to y less than or equal to 0.05 were investigated as a function of sintering temperature and time and cooling speed in order to see the influence of these factors upon the Curie temperature and the permeability behaviour around the Curie point. The quality of the magnetic temperature transducers made from such materials is directly connected with the value of the slope of the mu(T) curve. The higher the slope the more sensitive will be the transducer. Slopes as high as 50%degrees C-1 were obtained by a proper cooling of the samples. this value bring the highest reported so far for such materials. The results are discussed in terms of the ionic distribution of the Cu, Zn and Ti ions over the tetrahedral and octahedral sites of the spinel lattice. The materials proved to be stable in time if the temperature does not exceed 200 degrees C, but higher temperatures may induce irreversible change in the material structure due to cation migration and electron transfer from copper ions to iron ions.

An analysis is given of the way in which thickness fluctuations in a multilayer can affect the grazing incidence X-ray reflectivity pattern. It is shown that peak shifts, broadening and height variations can result. They alter in a predictable way the parameter values derived from a fit with an ideal lattice. A principle for designed Bragg reflectors is also proposed.

The effect of intermediate grounding and sintering environment (air or sintered powder) on the superconducting characteristics of Bi(Pb)-Sr-Ca-Cu-O hulk ceramics obtained by spray-frozen freeze drying technique was investigated by X-ray diffraction, electric and magnetic measurements. Intermediate grounding leads to higher scattering coefficients of the superconducting characteristics. This result does not depend on sintering environment. Improved results are obtained when sintering is performed by immersing pellets "in powder" rather than placing them "on powder". The powder used as sintering environment was pyrolized precursor powder calcinated in air for 100 h. (C) 1998 Elsevier Science Ltd. All rights reserved.

Modeling of the I(V, T) dependence in thin p-n semiconductor structures shows that various behaviors in its shape factor can be correlated with and explained through the voltage dependence of the carrier concentration product np and its components, at the zero field injection-point, for various conditions controlled by thickness and doping. A ''second exponential'' high injection range exp (qV/nkT), with a shape factor n as high as four appears to be a consequence of the departure from quasineutrality due to significant minority carrier space charge, favored by low doping and thickness. In thin structures, the presence of a second exponential range is associated with the presence of a quadratic range (I similar to V-2) at very high current densities.