National Institute Of Materials Physics - Romania

We demonstrate that several kinds of colored T centers can be prepared by an electrical coloring of Pb-doped KCl crystals. Ta is diamagnetic and it contains predominantly the [Pb-]2 species. Te is paramagnetic and we tentatively assign it to [Pb2]- units perturbed by Ca2+. Oxygen centers are thought to occur in Tb.

Even in the absence of interfering gases, the lack of stability in the response of SnO2-based gas sensors does not allow their use as measuring devices. The cause is the dependence of the conductance of the sensor, in the absence of the reducing ps, on ambient atmosphere conditions and on ageing. As a result, at the same value of the concentration of the reducing ps, the conductance of the sensor measured at different moments has different values. In this paper, based on previous calculations, a formula for conductance is derived: the parameters which depend on the ambient atmosphere conditions and on ageing, on one hand, and the sensor constants, on the other hand, are separated. Based on this formula a calibration curve, valuable at gas concentrations higher than 20 ppm, is proposed: the ratio between the conductance of the sensor in the presence of the reducing gas and the conductance in the presence of a known gas concentration versus the concentration of the reducing gas. The results obtained for C2H5OH and CO showed that during a week, using the proposed calibration curve, the errors in the measurements of the gas concentration were less than 7%.

The optical properties of Er3+ doped LiNbO3, LiNbO3(MgO) and LiNbO3(ZnO) have been studied. Polarized absorption and laser excited luminescence spectra at liquid-He temperature show different structure dependent on the co-dopants (MgO, ZnO). The relationship between the optical characteristics and the impurity sites location is discussed.

Being unselective, SnO2-based gas sensors can be used by means of appropriate methods. Various techniques, e.g., pattern recognition or time modulation of the sensor temperature are used. The lack of selectivity is compensated also when one uses these sensors in chromatographic systems. The chromatographic column itself separates the gas mixture into a time sequence, so the sensor receives the different gases at different moments; the first report was made by Figaro Engineering. We used SnO2-based gas sensors as chromatographic detectors; in this paper a complete description of our chromatographic system and of the chromatograms is given. The gas samples we analysed were CO/air mixtures in the 1 to 100 ppm range. The reproducibility was very good; the CO concentration was measured with a maximum error of 10% during a working day. The detection limit was below 1 ppm which is a very good result.

Two Pd-Dy/gamma-Al2O3 catalysts, with 1 wt.% Dy and various Pd loadings (0.1 and 0.5 wt.%) have been investigated by EXAFS at the Dy L(III)-edge, and compared with 1 % Dy/gamma-Al2O3. The Dy-carrier interaction was found to change with the Pd dispersity.

Phonon- and random-phason-type structural disorder was measured in Al-transition metal (Tm) ternary alloys, prepared with Al excess by rapid quenching. The disorder parameters correlate with the calculations on the energy stability of Al12TM icosahedra. Thus, substitution of 'unstable' elements (Fe and Co) for 'stable' elements (Cr and Mn) causes a strong increase in both types of disorder, leading finally to crystallization. This chemical trend is modulated around special compositions of the TM sublattice. The degree of structural order correlates with the thermal stability of icosahedral phases.

We introduce a theoretical analysis of the temporal and spatial evolution of the surface topography of solids following interference between incident and scattered pulsed laser beams. The essential role played by the nonlinear delayed feedback in the laser-radiation-surface system is considered. We show that it finally determines the surface topography evolution from pulse to pulse. In order to complete the analysis, numerical calculations have been conducted under the hypothesis of strong attenuation of laser radiation into the sample and of a limited heat diffusion during the action of a laser pulse. We predict an evolution from very simple to complex (chaotic) structures under multiple-pulse-laser irradiation of solid surfaces. This evolution is determined by some key irradiation parameters; initial surface microrelief, incident laser intensity, and the number of applied laser pulses. Experiments were performed in order to check the main predictions of the theoretical analysis. The system of transversal excited atmospheric pressure-CO2 laser radiation (lambda = 10.6 mum)-interacting with fused silica was chosen as appropriate for performing test experiments. Optical microscopy studies of laser-treated zones evidenced special modifications of the surface topography in good accordance with the conclusions following from the theoretical analysis. The theoretical analysis is also in good agreement with some available data from the literature, at the same time providing a coherent interpretation of previously unexplained behaviors.

The peculiarities of the photocapacitance effect in dry processed Cu2S-CdS heterojunction, have been investigated. The light flux dependence of capacitance is analysed. The main effect of low-level illumination is the narrowing of the 'intrinsic' layer that is formed as a consequence of Cu diffusion from the Cu2S layer. It was found that, in photocapacitance effects, a major role is played by the intermediate recombination centres present in the near-interface region.

The time-dependence of the conductance G(t) of SnO2:Pt:Sb gas sensors after introducing CO in atmospheres containing oxygen and water vapour was investigated. Depending on experimental conditions, different G(t) behaviours were observed over different temperature ranges: either monotonic increases towards steady-state values, or self-sustained oscillations. Part I describes and analyzes the non-oscillatory behaviour, Part II the oscillatory behaviour. A unitary model is proposed, which offers a qualitative explanation for both, by taking into account an assembly of interdependent surface phenomena, including adsorption, desorption, spill-over and chemical reactions of oxygen, carbon monoxide and water. The role of water vapour is emphasized. Though speculative, the model is consistent with a broad range of experimental results and it also allows for some general considerations about utilizing the SnO2:Pt:Sb material as CO sensor and as oxidation catalyst.