Publications

The present work reports the magnetic properties and Mossbauer spectra of soft ferrites MnaZnbFe1-a-b2+Fe23+O4 with Ti4+ substitutions. The addition of Ti4+ to manganese-zinc ferrites induces considerable improvements of properties such as the linear dependence of permeability versus temperature, and small disaccommodation of the permeability. Samples with Mn0.57Zn0.34Fe0.09+x2+TixFe2-2x3+O4 composition (where x = 0.00, 0.02, 0.04, 0.06, 0.08, 1.00) were prepared by the usual ceramic technology. The presence of Ti ions strongly influences the magnetic anisotropy, as evidenced by the temperature dependence of the permeability. A sublattice assigned to Ti4+Fe2+ pairs appeared in the Mossbauer spectra, which also gave information on the local surroundings of iron ions. The appearance of a secondary maximum of permeability (SMP) at negative temperatures or a constant slope over a wide temperature range is typical for mu(i)(t) curves. Increasing Ti4+ contents shift the SMP to higher temperatures because of a positive contribution to K1 of the Ti4+Fe2+ pairs at octahedral sites, in agreement with the hyperfine magnetic fields observed for ferrites with large titanium substitutions.

A strong non-stoichiometry of pure f c c CeO2 was induced by laser irradiation. The increase of laser power and/or energy density had a saturable effect on particle size growth. The possibility of CeO2 reduction to A-Ce2O3 by laser irradiation was demonstrated. Particles of stable Ce7O12 phase were observed in all specimens irradiated at low laser-power densities. An epitaxial relationship between triclinic Ce11O20 and cubic Ce12O22 phases was found. The controversial C-Ce2O3 Phase was detected at the limits of a b c c particle. An unknown b c c phase of acicular morphology, strongly related to C-Ce2O3, was also registered. The dose dependence of CeO2 structural modifications obtained by laser irradiation as a function of laser energy density variation could be explained by a simple defect aggregation model implying lattice defects (oxygen vacancies and Ce3+ ions).

Adherent and transparent unhydrogenated DLC layers were deposited by means of r.f. and d.c. magnetron sputtering at low deposition temperatures. The optical gap value increases from 0.7 to 2.2 eV when deposition pressure is increased from 10(-2) to 10(-1) Torr. In high gap materials, large amounts of sp3 hybridized carbon atoms are present. When Al contacts are evaporated in coplanar configuration on these high gap materials, rectifying and photoelectrical effects are observed.

The homogeneous-inhomogeneous transitions of a two-dimensional electron gas in a strong perpendicular magnetic field are studied in the particular case when half of the states in a spin splitted Landau level are occupied. To this end the dielectric susceptibility in the homogeneous states as well as the free energy in the modulated states are calculated. Unless the g factor dos not exceed a certain value the charge-density waves are stable in an interval of finite temperatures.

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.