Publications

The steady-state conductance G of thick-film SnO2 chemoresistive gas sensors is measured for different CO concentrations c in air, in the 0-300 ppm range, at 0 and 100% relative humidity. The conductances G0 at 0 and G at 120 ppm CO in normal air are measured as a function of time in non-steady-state conditions, during the first two hours of stabilization, after keeping the sensors unheated (out of operation) for seven days. The time dependence of the conductance at sudden changes of the CO concentration between c = 0 and c not-equal 0 in normal air is also measured. Experimental results indicate that the ratio (G2-G0(2))/G0 depends almost linearly on c down to very low values (about 1 ppm), it is fairly independent of humidity in the steady state, and of time during the stabilization stage. This can offer a possibility for using SnO2 sensors not only for the detection, but also for the quantitative measurement, of small concentrations of a combustible gas in air.

A theory of correlated hopping diffusion on lattices with site-exclusion interaction is presented. We develop a variational procedure to estimate the diffusion constant D. We derive a method of obtaining several sequences of decreasing upper bounds for D. The method is extensively discussed for the particular sequence that starts with the mean-field approximant D(MF). The first term of a second sequence D(MF) approximately is obtained by a suitable renormalization of the jump rates and is closer to D than D(MF). This modified mean-field approximation enables us to formulate an improved version of the Miller-Abrahams equivalence to a resistor network that includes correlation effects.

The dimensionality of the dissipation process in the Bi2Sr2Ca2Cu3O10 high-T(c) superconducting compound was analyzed. This material shows two-dimensional superconductivity at high temperatures, almost up to T(c). Critical-current-anisotropy studies performed on ceramic samples with preferential crystallite orientation revealed the absence of strong intrinsic pinning. The dissipation is caused only by the magnetic field component perpendicular to the CuO2 planes.

The formation mechanism of definite compounds, under non-isothermal conditions, in binary and pseudobinary systems of the type Bi2O3-MO and CuO-MO, where M = Sr, Ca and Sr + Ca and in the pseudoternary system Bi2O3-(Sr,Ca)O-CuO has been studied. It has been established that the components of systems containing Bi2O3 and CaO exhibit much higher reactivity than those of the systems containing SrO and CuO. In the pseudoternary system superconducting phases are formed at low temperature with preferential participation of calcium. with increasing temperature incorporation of Sr and Cu in a preformed network takes place.

In the paper the properties of some Pd-Sm/gamma Al2O3 catalysts with different contents of palladium obtained by the successive method of impregnation have been studied. The metallographic microscopy determinations showed that by impregnating samarium upon a Pd/gamma Al2O3 supported catalyst a penetrating front, mostly different from the one of samarium, has resulted depending on the existent concentration of palladium upon the support and also a different migration front of the platinic metal. From the electronic microscopy determinations, electronic diffraction and EXAF3, the state in which the samarium is found after impregnation and after the drying and reduction, respectively, was established. Based on these determinations a mechanism of samarium impregnation was suggested.

Structural, electrical and magnetic data are reported on superconducting polycrystalline GdBa2Cu3Oapproximately7 materials obtained from Gd(NO3)3 .6 H2O, CuO and BaCO3 powders by a solid state reaction. The samples are single phase with orthorhombic distorted perovskite structure and with a critical temperature of approximately 90 K.

Using a new method, based on nanosecond-pulse reflection, we measured the temperature dependence of the permeability of the superconducting samples. The transition temperature and the superconducting phase content were determined.

An anion exchange of NO3- with CO3(2-) anions was evidenced by X-ray diffraction, chemical analysis and IR spectroscopy, during washing of Ni-Al hydroxycompounds (HCPs) precipitated at pH values less than 7. The anionic composition of the resulting HCP is determined by the pH value of the washing water. In the 'increasing pH' method of HCP preparation, the precipitation process is shown to be accompanied by a process of anion exchange. The degree of disorder of the interlayer and in the brucite-like layer stacking is discussed in terms of the IR spectroscopy data and XRD line broadening.

Electron localization and optical transmission in one-dimensional systems with two components distributed according to the Rudin-Shapiro sequence are investigated. The nature of the eigenstates in the diagonal tight-binding model is studied by making use of nonlinear recurrence relations satisfied by the associated transfer matrices and their traces. It is shown that the wave functions display a wide range of features going from weak to exponential localization. Numerical computations lead to the conjecture that the localization property is generic. Nevertheless, a countable dense set of critical values of the on-site potential amplitude is found for which a class of extended states exists. Accordingly, the numerical investigation of the time evolution of the electronic wave packets displays a subdiffusive behavior when the potential amplitude becomes critical. Finally, a study of the optical properties of the Rudin-Shapiro dielectric multilayer shows strong similarities with the behavior of the disordered multilayers regarding the development of gaps in the transmission spectra.

The minimum observed in the magnetic field dependence of the transport critical current density of high-T(c) superconducting ceramics at relatively low applied field values was analysed. This anomalous behaviour results from the influence of pinning assisted demagnetisation effects at low fields and an increase of the intergranular pinning force at higher fields, through vortex-vortex interactions. The low voltage level I-V characteristics of the investigated samples can be described in terms of thermally activated flux creep at grain boundaries.