National Institute Of Materials Physics - Romania

The particle density fluctuations generated by a weak unidirectional periodic potential in a two-dimensional interacting electron system, in a strong perpendicular magnetic field, are calculated using a local-density description of the exchange-correlation energy for the lowest Landau level. The correlation effects are discussed with respect to previous results obtained in the Hartree and Hartree-Fock approximations. In the spatial regions where the local filling factor is in the range 0.3-0.7, the correlation corrections cancel the exchange contributions, such that the Hartree picture is shown to be qualitatively correct. In these regions, the external potential is strongly screened by the electronic system. In the other spatial regions the density has higher order oscillations with a specific wavelength, which disappear with increasing temperature or the external potential amplitude. Possible experimental evidence of these oscillations in modulated structures is discussed.

The paper presents the experimental results got by adding some inorganic and organic additives to the negative active mass of the lead-sulphuric acid battery plates. The effect of some surface active substances, lignin derivatives,was rendered evident by determining the specific surface of the active masses in their non-formed or formed state, or following some loading-unloading experimental cycles.

The effects of neutron irradiation on the critical temperature and critical current density at 77 K and zero magnetic field of high-temperature superconductors was studied in Li-doped YBa2Cu3O7-x. Damage and thermal neutron reactions with Li produce an insignificant increase of the critical temperature for Li inserted as flame-melted Li2CuO2 while for the others it remains nearly unaffected. The critical transport current density of the sintered YBa2Cu3O7-x remains unaffected for samples with Li2CuO2 and shows a decrease for LiF-doped samples.

Spectral properties of - Delta + V(x), where V(x) lies in a 'neighbourhood' of the periodic case and 'describes various 'models of disorder', are studied. We prove the exponential decay of generalized eigenfunctions corresponding to energies in the resolvent set of the 'unperturbed' periodic Hamiltonian, as well as the stability of the essential spectrum for the 'dislocation disorder' in two dimensions.

The effect of annealing duration in oxygen flow on the superconducting characteristics of YBa2Cu3Osimilar to 7 bulk ceramics was investigated by structural, electrical, magnetic, and electronic microscopy investigations. The long-time annealing is deleterious for the superconductivity in YBa2Cu3Osimilar to 7 ceramics. The optimum time interval for annealing is from 20 to 70 h at 950 degrees C, and within this range the maximum value of T-c is obtained.

Optical charging spectroscopy and thermally stimulated depolarization measurements were performed on metal-insulator-semiconductor-insulator-metal structures with a ZnS:Mn active layer. Two groups of trapping centres situated at every insulator-semiconductor interface were in evidence. Their depths were found to be centred at 0.55 eV and 0.875 eV respectively in the forbidden gap of a ZnS:Mn film. A partial thermal healing of these interface states was observed.

Electrochemical laboratory investigations were used as a screening tool for selection of metallic materials applicable to concrete reinforcement. Simultaneous examination of FeCrXP9C11 (X = 4; 7; 9), FeB13Si9, FeB13.5Si3.5C2 as compared to classical carbon steels aas conducted in simulated concrete corrosive electrolyte: Ca(OH)(2) sat. +/- NaCl +/- y Na2SO4 +/- CO2 sat. (x = 5; 50; 100 g/L, y = 1; 10 g/L). Indications are that FeCrXP9C11 amorphous alloys are highly resistant materials. Their resistance to the most aggressive media is 100-1000 times greater than of FeBSi(C) alloys and carbon steels.

The results presented in this paper are obtained using SnO2-based gas sensors as chromatographic detectors. The tests were performed during a temperature cycle consisting in steps of one week at 350, 400, 450, 400 and 350 degrees C. The analysed gas samples were CO-air mixtures with CO concentration between 0 and 50 ppm. The advantages of the used experimental set-up are the stable ambient atmospheric conditions (the constant flow of the stable carrier gas) and the separation properties of the chromatographic column which compensate the intrinsic lack of selectivity of SnO2-based gas sensing layers; the chromatographic column separates the gas mixture into a time sequence, so the sensor receives the different gases at different moments. The compositions of the used eluent gas - O-2 technical grade which also contains percents of N-2 and about 40 ppm of CH4 - and of the analysed sample can be examined in the same experimental run of the response to O-2, CH4 and CO because, when gases are present in the sample which are also present in the carrier gas, differences appear between the concentrations in the gas sample and in the carrier gas in the ambient atmosphere of the sensor at their corresponding moments in the time sequence. It was found that the relative sensitivities to O-2 and CH4 increase when the temperature increases; the relative sensitivity to CO decreases when the temperature increases and is strongly influenced by the temperature cycle. Its value at 350 degrees C and 40 ppm is around 40% before the cycle and around 16% after. It was also found that after the period of heating at 450 degrees C, the conductance at 400 and 350 degrees C became an increasing function of time; for a week this process did not stop. Using the results listed above, an explanation is given: at 450 degrees C the characteristics of the surface of the sensing layer are modified by a new equilibrium of surface oxygen vacancies.