National Institute Of Materials Physics - Romania

Temperature induced surface alignment transitions in nematic liquid crystals are analysed by means of an elastic model in which the effective splay-bend surface-like elastic constant is taken into account. Experimental data in hybrid cells are analysed and the value of the effective splay-bend elastic constant is found to be in good agreement with that obtained in homogeneous samples for the same liquid crystal. The validity of the model is successfully checked on data from the literature, for such different temperature dependences as those of MBBA at the free surface and on two types of SiO(x) deposited surfaces, and for E7 on a fluoropolymeric surface, exhibiting a temperature induced surface alignment transition from planar to homeotropic.

ZSM-5 samples with a silicon-to-aluminium atomic ratio of 25 were impregnated with gallium, indium or thallium salts. The acidic and textural properties of the samples were determined either before or after reduction. The acidity was investigated by infrared spectroscopy using pyridine (Py) as test molecule, titration with KOH or Py as well as ammonia adsorption. Differences were revealed concerning the dispersion of impregnated species and the distribution of the acidic sites. The sites containing ionic thallium were spectroscopically identified; the other sites were indirectly evidenced. Under our impregnation conditions, gallium species seem to enter inside the zeolite to a higher extent than was reported for similar samples. The contribution of indium to the sample acidity was reduced because it is deposited as a distinct phase onto the external surface. An adsorption mechanism operating during impregnation was proposed on the basis of electrochemical measurements; it was used to account for the acidity data. The textural properties were interpreted in terms of the deposition of the impregnation species near the pore mouth, hindering the entrance of the test molecules.

We compare a set of results concerning our measurement of the optical absorption (OA), photoconductivity (PC), thermally stimulated currents (TSC) and thermally stimulated luminescence (TSL) on the same crystal. We also investigate the effects of various preillumination treatments at room or liquid nitrogen temperature. We conclude that illumination with infrared light (rather than solely red) plays an important role which has been underestimated in the past.

The effects of pulsed excimer laser irradiation (lambda = 308 nm, tau = 10 ns) on the magnetic and structural properties of amorphous and nanocrystalline Fe73.5Cu1Nb3Si22.5-xBx (x = 6 and 9) were investigated by transmission Mossbauer spectroscopy, scanning electron microscopy and energy-dispersive X-ray analysis. In the amorphous specimens, two types of compositionally-dependent kinetics have been deduced for the laser-induced changes to the easy magnetization axis: controlled magnetic anisotropy (x = 9) and random distribution of magnetic moment directions (x = 6). Micrometer-size crystalline precipitates of alpha-Fe have been observed in the Fe73.5Cu1Nb3Si16.5B6 sample irradiated at the highest repetition rate. Pulsed excimer laser treatment of nanocrystalline Fe73.5Cu1Nb3Si13.5B9 has been found to enhance the fraction of crystalline Fe-Si alloy (18 at. %) with DO3 structure, and to determine a random distribution of magnetic moment directions in the remaining amorphous grain boundary phase. Excimer laser processing is suggested as a powerful tool for controlling the magnetic anisotropy and phase equilibrium in amorphous and nanocrystalline materials

Alignment layers containing copper(II) complexes with acrylic acid or poly polyvinylpyrrolidone show homeotropic orientational properties. This behaviour is supposed to be due to the particular structure of the complex copper(II) ions. We give spectroscopic arguments to support the anchoring of liquid crystal (LC) molecules by coordination to the copper ion within the alignment layer.

An impedance spectroscopy experimental characterization of a new synthetized Palladium complex organo-metallic liquid crystal is reported. It has been found that although organo-metallic complexes modify some of the physical properties of the liquid crystals, namely transition temperatures, values of the relaxation times and conductivity, they still have, in the nematic phase, the general behaviour of the usual liquid crystals. In particular the low frequency conductivity is very small, so one can conclude that the considered organo-metallic liquid crystal behaves like a good insulator. The temperature dependence of the static parallel and perpendicular components of the dielectric tensor are reported.

This paper presents a new method for obtaining electric twins in quartz crystal: thermal treatment in a temperature gradient at about 500 degrees C, that means under phase transition point. + 5X quartz crystal plates for resonators were used; a temperature gradient of 15-25 degrees C was applied for 4-5 hours along the Y axis (the place length). For + 5X resonators, the diminution of the resonant frequency and the increase of the inductance were plotted as a function of the level of twinning.

The electron paramagnetic resonance spectra of Cr3+ in LiNbO3 co-doped with ZnO or CaO (approximate to 6%) have been studied. Both impurities give rise to the same effect as that observed in MgO-doped samples: a strong diminution of the anisotropic (axial) spectrum reported for LiNbO3:Cr and the appearance of an intense and isotropic line. The origin of this line in the co-doped samples is discussed.