Publications

Orientational transitions induced by temperature variation, in cells with nematic liquid crystal having limiting substrates with controlled topography, are presented. The variable topography was obtained by SiOx deposition in vacuum at an incidence angle around 85 degrees, or by depositions of polyvinyl alcohol (PVA) with different thicknesses, on glasses with the same SiOx deposition. The experimental results are explained by a phenomenological model based on the Landau-de Gennes free energy expansion, taking into account the anisotropy of the substrate.

The effect of niobium and lithium doping on the structure and piezoelectric properties of some samples with the following compositions PbZr0.051 Nb3Li)(x)Ti0.49-xO3, with 0.015 less than or equal to x less than or equal to 0.050, was investigated. The materials were prepared by the usual ceramic technique using high purity raw materials. Disc shaped samples of each composition were sintered at temperatures between 1200 and 1300 degrees C, for 4 h. The samples structure was determined by X-ray diffractometry and the piezoelectric properties were determined by means of resonance-antiresonance method, using a HP 4194A impedance gain phase analyser. A morphotropic phase boundary (MPB) with the optimum piezoelectric properties was found around the compositions with x=0020. The optimum values for relative dielectric constant epsilon(r), planar electromechanical coupling factor k(p), and mechanical quality, factor Q(m) of about 1700, 0.59 and 75, respectively, were found for these compositions. The lattice constants a and c of rhombohedral and tetragonal phases were determined by means of an X-ray CuK alpha diffractometer. The results are discussed in terms of the domain easy switching phenomena and high poling degree, brought about by the A-vacancies produced by the NbLi pairs doping. (C) 1999 Elsevier Science Limited. All rights reserved.

Three ceramic magnetic compositions within the CuZnTi ferrite system, having the Curie temperatures centered around 60, 80 and 100 degrees C, respectively, were investigated in order to determine the time-temperature stability of their main magnetic properties. Permeability and the slope of permeability around their Curie points were determined on ring shaped samples, before and after being subjected to long term (over 6000 h) ageing at their Curie temperature and for a shorter time of about 50h at higher temperatures up to 700 degrees C. Rates of changes of 33, 66 and 100 ppm h-l of the maximum slopes of permeability were found for the samples aged at 60, 80 and 100 degrees C, respectively. The samples aged at temperatures higher than 200 degrees C show a rather sudden decrease of maximum slope of permeability, but no shift of the Curie temperature and the working point, corresponding to the temperature where the slope has the maximum value. This is the most interesting result as concerns the use of such magnetic temperature sensors for the construction of highly sensitive temperature controllers, for example ultrathermostats. The results are discussed in terms of the migration processes of the cations, especially Cu2+,from metastable positions on which they were frozen during the rapid cooling of the sample from the sintering temperature, to the more stable ones, namely the octahedral sites into the spinel lattice. (C) 1999 Elsevier Science Limited. All rights reserved.

Tl2+(6s(1))-type of paramagnetic centres, produced by low temperature X-ray irradiation, were observed in the low temperature ferroelectric phases of Rb2ZnCl4 : TICl crystals. The difference between the spin-Hamiltonian parameters of the main centre, determined in the two phases, is attributed to the symmetry lowering at phase transition.

The structure of the heteronuclear iron acetate [Fe2CoO(CH3COO)(6)(3-Cl-Py)(3)]. 1/4 . 3-Cl-PyX1/4(CH3)(2)CO . 1/2H(2)O was determined by X-ray structure analysis. The crystal has a molecular structure and is monoclinic with lattice parameters a = 21.034(4), b = 8.398(2), c = 23.360 Angstrom, beta = 98.28(3)degrees, R = 0.0656, space group P2(1)/c. The trinuclear complex [Fe2CoO(CH3COO)(6)(H2O)(3)] has a structure typical for iron(III) mu(3)-O compounds with iron atoms lying at the vertices of the equilateral triangle centered by an oxygen atom. The metal atoms are each coordinated to four oxygen atoms of the four bridging carboxy groups, the bridging oxygen atom (mu(3)-O), and the coordinated 3-chloropyridine molecule which is trans relative to the latter atom. According to Mossbauer spectroscopy data, the iron(III) ions are in the high-spin state. The value of (mu(ef))(mole)/mu(3) at room temperature and its temperature dependence suggest that the resulting magnetic exchange interaction between the paramagnetic centers of the cluster is antiferromagnetic.

Aliquots of waste water from power stations were analyzed by a FT-IR spectroscopic procedure and showed both qualitatively and quantitatively the presence of cyclohexylamine and morpholine. The detection limit and the sensitivity of the method were investigated and the validity of the Lambert-Beer law was checked. To maintain the selectivity of the method the pH must be maintained around 6.

Ferroelectric Bi4Ti3O12 (BiT) thin films of different thicknesses were deposited on p-type Si substrates using the Chemical Solution Deposition (CSD) method. The films were crystallized by the conventional thermal annealing for 30 min at temperatures in the 500-700 degrees C range. It was found that the shape of the photoconductive signal spectral distribution is dependent on the film thickness. For thin films (150 nm) four peaks were observed (400, 500, 860 and 1075 nm) and the photoconductive signal occurs only if the Si substrate is negatively biased. For thicker films (500 nm) only two peaks were observed (370 and 1075 nm) and the photoconductive signal occurs no matter the polarity of the applied voltage. (C) 1999 Elsevier Science Limited. All rights reserved.

Transmission electron microscopy of Er implanted LiNbO3 crystals reveals a structural modification due to the electron bi am irradiation, which consists of the decomposition of LiNbO3 in niobium oxide nanometric size domains. The corresponding electron diffraction patterns do fit at best with the NbO2 simple tetragonal phase. The orientation of the oxide nuclei is along the pseudo-cubic directions of the lithium niobate.

We study a first order transition of the anchoring direction imposed by PVA coating of SiO layer. Contrary to the model of anchoring competition, no vanishing of the anchoring strength was observed close to the transition. We explain this behaviour by memory effects.

The normal-incommensurate phase transition in Rb2ZnCl4 was studied by Rayleigh scattering of Mossbauer radiation. The discontinuity in resonance effect (epsilon) proves a stepwise variation of the inelastic component of the scattered radiation. The result is discussed in connection with the photon-phonon interaction. (C) 1999 Elsevier Science Ltd. All rights reserved.