Publications

Coloured TiO2(Ni2+) thin coatings doped with transitional metals (Ni) were deposited on different substrates (silicon wafers, glass, aluminum) by the sol-gel dip coating process. Complementary techniques (XPS, SE, TEM, XRD, Absorption Spectroscopy) were used to characterize the surface stoichiometry and optical properties, as well as the structure and texture of the thermally treated gels. The presence of fully oxidized film in the outer layers, crystallization in heat treatment to anatase and rutile, formation of porous and coloured films due to the influence of the dopant and the reaction with the substrate, as well as a decrease by three orders of magnitude of the resistivity of Ni-doped films, are reported.

A new method of processing the experimental data in the interference method for the determination of birefringence of liquid crystals which allows obtaining results in good agreement with those from prism or wedge methods, is presented.

Polymer degradation around the glass transition temperature exhibits frequently an anomalous behaviour. Starting from the hypothesis that such anomalies are related to the glass transition phenomenon, the most relevant data reported in the literature are reviewed and a theoretical description, based on the free volume approximation is suggested. The degradation process is split in three steps. It is proved that the dominant contributions come from the defect's diffusion step and from the proper chemical stage, which may be supposed to be statistically independent. It is explain, using an extended WLF equation why these anomalies are not located exactly at the glass transition temperature. The agreement between theory and experimental data reported elsewhere is good.

For a 2D electron system in a 1D periodic potential modulation, a complicated structure of Shubnikov-de Haas peaks has been measured and qualitatively explained by a transport calculation including interaction effects in the Hartree-Fock approximation.

Photoelectric properties of PbTiO3/Si heterostructures were investigated over a broad range of wavelengths. The spectral distribution of the a.c. open-circuit voltage was measured for wavelengths ranging from 0.35 mu m to 4 mu m. Two local peaks are observed at 0.38 mu m and 0.9 mu m which can be attributed to band-to-band excitations in PbTiO3 and Si respectively. The shape of the a.c. signal depends both on the light intensity and on the wavelength. When the heterojunction is reverse-biased, a d.c. photocurrent was observed which has a similar spectral distribution as the a.c. open-circuit voltage.

The a-Si:H/electrolyte interface was studied using impedance spectroscopy (IS) method. The measurements were made in the frequency range of 100 Hz - 300 kHz and the potential range of -0.6 V - +0.8 V. The equivalent circuit suggested by the experimental curves allowed us to calculate the main parameters which characterize the interface and the mechanism through which the space charge region supplies carriers to the interface.

We present the results of dielectric- and differential scanning calorimetry investigations on several homologous of 4-n- alkoxybenzoic acids: in particular the 4-n-heptyl-, -octyl-, -nonyloxybenzoic acids are studied. Our measurements confirm the existence in the nematic phase of a textural transition, which on cooling changes a well ordered state, f.i. a twisted state, to a highly distorted one, characterized by the appearance of a multidomain structure. Such a transition turns out to be reversible, with a small hysteresis. The transition exhibits a clear evidence for the 4-n-octyl- and the 4-n-nonyloxybenzoic acids. On the contrary, the presence of this transition in the 4-n-heptyloxybenzoic acid is suggested by a small peak only in the curve of the dielectric permittivity. The effect of the simultaneous presence in the nematic phase of open and closed dimers and of cybotactic clusters can explain the observed dielectric behavior.

Purifying methods of cesium iodide are presented along with the reasons of their choice. Fractional recrystallization, drying in vacuum, chemical treatment with iodine and refining zone were used in this sequence. Impurity analysis regarded chlorine, bromine, iron and heavy metals. Finally the results show very low halogen level and reasonable level for metal.