National Institute Of Materials Physics - Romania

The molecular mobility of 4-butyl- and 4-pentyl-4'-cyanophenyl benzoate (CP4B, CP5B) and their composites prepared from aerosil A380 was investigated by broadband dielectric spectroscopy in a large temperature range. Thermogravimetric and infrared investigations were additionally performed. High silica density (larger than 7 g aerosil/1 g of liquid crystal) was selected to observe a thin layer adsorbed on the surface of the silica particles. The data were compared with those of the member of the series with six carbon atoms in the alkyl tail. Bulk CP4B and CP5B show the dielectric behaviour expected for liquid crystals. For the composites one relaxation process is observed at frequencies much lower than those for the corresponding bulk, which was assigned to the dynamics of the molecules in a surface layer. The temperature dependence of the relaxation rates (and of the dielectric strength) shows a crossover behaviour with two distinguished regimes. At higher temperatures the data obey the Vogel-Fulcher-Tammann law, whereas an Arrhenius law is observed at lower temperature, in a close similarity to the behaviour of a constrained dynamic glass transition. The estimated Vogel and crossover temperature is independent on the tail length, while the activation energy for the low temperature branch increases weakly with increasing the alkyl tail. (C) 2010 Elsevier B. V. All rights reserved.

Structural and optical properties of (Co0.01Zn0.99)Al2O4 and Zn(Al(1.99)Ln(0.01))O-4 (where Ln(III)=Eu(III), Tb(III)) spinels obtained by a variant of the complexation method-tartarate route were studied. The tartarate precursors have been characterized by IR, UV-vis and luminescence spectroscopy. The doped aluminium based spinets have been investigated by X-ray diffraction (XRD), transsmision electron microscopy (TEM), infrared spectroscopy (IR), ultraviolet visible spectroscopy (UV-vis) and luminescence spectroscopy. Pure spinel nanometer powders were obtained. Under excitation at 380 nm the nanoparticles exhibits strong luminescence corresponding to D-5(0) -> F-7(2) transition of Eu3+ ions and respectively, to D-5(4) -> 7 F-j (j = 3, 4, 5, 6) transition of Tb3+ ions. No blue emission from the D-5(3) state were observed. (C) 2010 Elsevier B.V. All rights reserved.

Negative ion production is investigated in a chamber with transversal magnetic filter operated in dc or inductively coupled plasma (ICP) modes in Ar/SF6 gas mixtures. Plasma parameters are evaluated by mass spectrometry and Langmuir probe for different discharge conditions. The density ratio of negative ion to electron exceeded 300 in dc mode while it was below 100 in the ICP mode. The possibility to apply a large positive bias to an electrode without affecting the plasma potential and the transition from a negative sheath to anodic glow are also investigated. The etching rates by positive and negative ions are evaluated on silicon substrate for different Ar/SF6 gas ratios. The etching rate by negative ions was with less than 5% smaller than that by positive ions. (C) 2010 American Institute of Physics. [doi:10.1063/1.3452357]

The nematic liquid crystalline mixture E7 was confined with similar filling degrees to molecular sieves with constant composition but different pore diameters (from 2.8 to 6.8 nm). Fourier transform infrared analysis proved that the E7 molecules interact via the cyanogroup with the pore walls of the molecular sieves. The molecular dynamics of the system was investigated by broadband dielectric spectroscopy (10(-2)-10(9) Hz) covering a wide temperature range of similar to 200 K from temperatures well above the isotropic-nematic transition down to the glass transition of bulk E7. A variety of relaxation processes is observed including two modes that are located close to the bulk behavior in its temperature dependence. For all confined samples, two relaxation processes, at frequencies lower than the processes observed for the bulk, were detected. At lower temperatures, their relaxation rates have different temperature dependencies whereas at higher temperatures, they seem to collapse into one chart. The temperature dependence of the slowest process (S-process) obeys the Vogel-Fulcher-Tammann law indicating a glassy dynamics of the E7 molecules anchored to the pore surface. The pore size dependence of both the Vogel temperature and fragility revealed a steplike transition around 4 nm pore size, which indicates a transition from a strong to a fragile behavior. The process with a relaxation rate in between the bulklike and the S-process (I-process) shows no dependence on the pore size. The agreement of the I-process with the behavior of a 5CB surface layer adsorbed on nonporous silica leads to the assignment of E7 molecules anchored at the outer surface of the microcrystals of the molecular sieves. (C) 2010 American Institute of Physics. [doi:10.1063/1.3432775]

CuO/Al2O3 and NiO/Al2O3 catalysts prepared by incipient wetness impregnation were used for the oxidation of Reactive Black 5 (RB5) in aqueous solution. Removal of the dye was assessed by High Performance Liquid Chromatography (HPLC) and Total Organic Carbon (TOC) measurements and the generation of the hydroxyl radicals in the process was evaluated by chemiluminescence measurements. To put in evidence the interaction RB5 - catalyst and the surface species formed onto catalysts during the oxidation, Diffuse Reflectance Infrared Fourier Transform (DRIFT) analysis was performed. A different behavior of the two catalytic systems was revealed by the comparative analysis of the data obtained from the adsorption and oxidation tests. Only CuO/Al2O3 was effective in the RB5 degradation, NiO/Al2O3 acted as a simple adsorbent. In the presence of CuO/Al2O3, at H2O2 concentration of 40 mM the azo dye was totally eliminated from both solution and catalyst surface after 4 h, with a mineralization degree higher than 90%. However, a strong inhibition of the catalytic oxidation of RB5 was observed in the presence of phosphate ions. In the conditions of hydrogen peroxide excess, the rate equation in the case of copper catalyst was simply expressed by a pseudo-first order equation and the model was found to fit well the data. The amount of copper leached from catalyst during the oxidation process was only 1.0-1.6% per cycle leading to the conclusion that the decrease of the dye mineralization with the number of cycles has to be explained mostly by the surface covering with the reaction products, at least to a certain extent. (C) 2010 Elsevier B.V. All rights reserved.

Simple Fe-Co ferromagnetic films and Fe-Mn antiferromagnetic films of different compositions were grown on epitaxial Si substrates with Ta buffer layers via thermo-ionic vacuum arc method. Fe-Co/Fe-Mn bilayer structures were obtained by following similar growing conditions as for the simple films. The magnetic behavior concerning easy axis distribution, anisotropy energy and coercive field of the Fe-Co films exchange coupled to the antiferromagnetic layers were discussed with respect to the magnetic behavior of the simple, uncoupled, ferromagnetic films. The composition of the antiferromagnetic films has a sensible influence on the magnetic parameters of the exchange coupled ferromagnetic films. The largest coupling and the narrowest easy axis distribution are induced in the bilayer system with equatomic composition of the Fe-Mn antiferromagnetic layer. (C) 2010 Elsevier B.V. All rights reserved.

We report on pulsed laser deposition of TiO2 films on glass substrates in oxygen, methane, nitrogen and mixture of oxygen and nitrogen atmosphere. The nitrogen incorporation into TiO2 lattice was successfully achieved, as demonstrated by optical absorption and XPS measurements. The absorption edge of the N-doped TiO2 films was red-shifted up to similar to 480 nm from 360 nm in case of undoped ones. The photocatalytic activity of TiO2 films was investigated during toxic Cr(VI) ions photoreduction to Cr(III) state in aqueous media under irradiation with visible and UV light. Under visible light irradiation, TiO2 films deposited in nitrogen atmosphere showed the highest photocatalytic activity, whereas by UV light exposure the best results were obtained for the TiO2 structures deposited in pure methane and oxygen atmosphere. (C) 2009 Elsevier B.V. All rights reserved.

Aluminum doped Zinc Oxide (AZO) and Lithium doped Zinc Oxide (LZO) thin films are obtained by Pulsed Laser Deposition (PLD) method. These films are characterized by using Spectroscopic Ellipsometry (SE), X-ray Diffraction (XRD) and Photoluminescence (PL). By modeling the ellipsometry spectra we get the dielectric functions, the optical band gap E(g), and the electrical properties. Our results show the influence of the processing parameters on the optical and structural properties of doped ZnO thin films. The post-annealing treatment applied to AZO thin films, changes strongly the optical properties, by lowering the resistivity and red-shifting the band gap. (C) 2009 Elsevier B.V. All rights reserved.

Two series of cobalt substituted Ni-Fe-Ga ferromagnetic shape memory alloys with lower gallium content (< 27 at. %) were studied by differential scanning calorimetry, x-ray diffraction, and thermomagnetic measurements. Co substitution for Fe or Ni promotes an increase in the Curie temperatures and a variation in the martensitic transformation (MT) temperatures, in accord with the alloy valence electron concentration change. For alloys with MTs below room temperature, a field dependent thermomagnetic hysteresis was evidenced and discussed in connection with the effect of cobalt substitution, on the magnetic hardness of the martensite phase. A direct interpretation of the evolution of the thermomagnetic hysteresis versus the applied field was provided. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3429231]

The short-circuit photocurrent was measured in ferroelectric capacitors of polycrystalline and epitaxial quality. The interest was to study the possible relation between photocurrent and back-switching phenomena due to ferroelectric polarization imprint, as suggested by Pintilie [J. Appl. Phys. 101, 064109 (2007)]. An interesting relation between the shape of the ferroelectric hysteresis loop and the shape of the photocurrent spectral distribution was found. In polycrystalline samples, the shape of spectral distribution and the sign of photocurrent are changing in time, although the hysteresis is almost symmetrical. However, the hysteresis is not rectangular as in the case of epitaxial films. This behavior suggests a subtle relation between polarization back-switching and photocurrent. In epitaxial samples a peculiar dependence between photocurrent and polarization imprint was found. All these are explained assuming the presence of an internal field, possibly generated by charged defects, which can change its direction and magnitude under illumination, with consequence on the orientation and magnitude of the ferroelectric polarization, and on the sign/shape of the short-circuit photocurrent spectral distribution. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3445877]