Publications

Lignin isolated from Miscanthus x giganteus using acidic (FAL) and basic (AL) conditions was thereafter subjected to catalytic depolymerization under thermal or ultrasonic activation. The characterization of lignin samples was achieved by thermogravimetric analysis and FTIR. Three different classes of catalysts containing nickel as the active species have been prepared in this respect: (i) nano-Ni(0) by reduction of NiCl2 with NaBH4 under ultrasonication, (ii) Fe3O4-(NiMgAlO)(x) and (NiAlO)(x) by calcination of Mg(Ni)-Al hydrotalcite incorporating Fe3O4, followed by reduction with hydrogen, and (iii) NiO(1 1 1) nanosheets by reduction of Ni(NO3)(2) with urea using benzyl alcohol as a structure directing agent. The catalysts were characterized by XRD and XPS techniques. Reduced mixed oxides displayed a moderate activity while a significant increase in conversion (up to 90%) was observed with nano-Ni(0) and NiO(1 1 1) nanosheets catalysts. The conversion and the mass distribution of the reaction products were strongly related to the procedure used for the extraction of lignin. In all the reactions AL samples had a greater extent of depolymerization. The performance from the tested catalysts under ultrasonic conditions was inferior to those tested under conventional heating conditions. The nature of the solvent was also found to be very important to this process, with the ionic liquid, 1-butyl-3-methylimidazolium acetate ([BMIM]OAc) demonstrating the best results in autoclave conditions and methanol under ultrasonic conditions. (C) 2012 Elsevier B. V. All rights reserved.

N-doped ZnO (ZnO: N) thin films, intended to be used as one of the layers in solar cell applications were deposited by r.f. sputtering, using ZnN target (99.9% purity), on silicon and fused silica substrates. In the gas flow composition, Ar was kept constant (50%) and the O-2/N-2 ratio was varied as: 40%/10%, 25%/25% and 10%/40%. After deposition, rapid thermal annealing (RTA) at 400 and 550 degrees C for 1 min in N-2 ambient has been performed. The RTA impact on the optical and microstructural properties of ZnO: N thin films have been investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM) coupled with selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDX), UV-vis-NIR spectroscopy, UV-vis-NIR spectroscopic ellipsometry (SE) and infrared ellipsometry (IR-SE). The as-deposited (ad) ZnO: N films are polycrystalline with preferentially oriented columnar crystals. After RTA we found ZnO: N films with improved crystallinity and fewer boundary defects. We report optical constants of ZnO: N from UV to IR spectral range as well as the infrared active phononic modes. (C) 2012 Elsevier B. V. All rights reserved.

The hysteretic properties of metal-ferroelectric-semiconductor (MFS) structures based on Pb(Zr0.2Ti0.8)O-3 (PZT) and ZnO films were studied with respect of the quality of the PZT-ZnO interface. The films were grown by pulsed laser deposition (PLD) on platinized silicon (Pt/Si) substrate and on single crystal, (001) oriented SrTiO3 (STO) substrates. The structural analysis has revealed that the PZT-ZnO stack grown on single crystal STO is epitaxial, while the structure grown on Pt/Si has columnar texture. The temperature change of the capacitance-voltage (C-V) hysteresis direction, from clockwise at low temperatures to counter clockwise at high temperatures, was observed at around 300K in the case of the MFS structure grown by PLD on Pt/Si substrate. This temperature is lower than the one reported for the case of the PZT-ZnO structure grown by sol-gel on Pt/Si substrate (Pintilie et al., Appl. Phys. Lett. 96, 012903 (2010)). In the fully epitaxial structures the C-V hysteresis is counter clockwise even at 100K. These findings strongly points out that the quality of the PZT-ZnO interface is essential for having a C-V hysteresis of ferroelectric nature, with negligible influence from the part of the interface states and with a memory window of about 5V at room temperature. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4765723]

Gas sensing peculiarities of chromiumoxide (Cr2O3) nanoparticles, obtained by hydrothermal reaction at moderate temperature and subsequent thermal annealing, were studied. The structure and morphology of the obtained Cr2O3 samples were investigated by X-ray diffraction and transmission electron microscopy techniques. Gas sensing measurements revealed their selective sensitivity to nitrogen dioxide (NO2) at 200 degrees C relative to carbon monoxide (CO), both in dry and humid air. Higher selectivity towards NO2 is attributed to the material surface reactivity even at low operating temperatures. This property was experimentally highlighted by work function changes, as a complementary investigation method to basic electrical resistance. Thus, it was observed that the surface reactivity of Cr2O3 (calcinated at 1000 degrees C) towards NO2 is higher compared to CO and less influenced by the relative humidity of the surrounding atmosphere. The present study reveals possible applications in the selective detection of NO2 based on Cr2O3. (C) 2012 Elsevier B.V. All rights reserved.

Ba(Zn1/3Ta2/3)O-3 (BZT) thin films were grown on Pt-coated Si substrates at 500 degrees C substrate temperatures by pulsed electron beam deposition method and post-annealed at 600 and 650 degrees C for 1 h. The X-ray diffraction patterns indicate that the as-grown films are partially crystallized but single-phase cubic perovskite structure was formed in annealed films. The temperature dependence of the dielectric constant of the BZT films was recorded in the -100 to + 100 degrees C range. The annealing treatment induces a decrease of the temperature coefficient of the dielectric permittivity with an order of magnitude, from 2000 to 100 ppm/degrees C. The influence of the annealing treatments on the temperature behavior of the BZT films was evidenced; a dielectric constant of about 21 at room temperature was obtained for the films annealed at 650 degrees C. (C) 2012 Elsevier B. V. All rights reserved.

The influence of target-substrate distance during pulsed laser deposition of indium zinc oxide (IZO), indium tin oxide (ITO) and aluminium-doped zinc oxide (AZO) thin films grown on polyethylene terephthalate (PET) substrates was investigated. It was found that the properties of such flexible transparent conductive oxide (TCO)/PET electrodes critically depend on this parameter. The TCO films that were deposited at distances of 6 and 8 cm exhibited an optical transmittance higher than 90% in the visible range and electrical resistivities around 5 x 10(-4) Omega cm. In addition to these excellent electrical and optical characteristics the films grown at 8 cm distance were homogenous, smooth, adherent, and without cracks or any other extended defects, being suitable for opto-electronic device applications. (C) 2012 Elsevier B. V. All rights reserved.

We review, based on our results, the problems and solutions for the growth of thin films and composite heterostructures emphasizing the general growth aspects and principles vs specifics for each material or heterostructure. The materials used in our examples are Bi2Sr2Ca2Cu3O10, Bi2Sr2CaCu2O8, YBa2Cu3O7, (Sr, Ca)CuO2, (Ba, Ca) CuO2, and Bi4Ti3O12. The growth method was metal organic chemical vapor deposition (MOCVD). The presented thin films or heterostructures have c- and non-c-axis orientations. We discuss the implications of the film-substrate lattice relationships, paying attention to film-substrate lattice mismatch anisotropy and to film-film lattice mismatch, which has a significant influence on the quality of the non-c-axis heterostructures. We also present growth control through the use of vicinal substrates and two-temperature (template) and interrupted growth routes allowing significant quality improvements or optimization. Other key aspects of the growth mechanism, that is, roughness, morphology, and interdiffusion, are addressed. It is concluded that the requirements for the growth of non-c-axis heterostructures are more severe than those for the c-axis ones. (C) 2012 The Japan Society of Applied Physics

The occurrence of the second peak (SP) on the dc magnetization curves of iron pnictide and chalcogenide single crystals (the 'fishtail' effect) has been interpreted in surprisingly different ways, involving, for example, a thermally induced square-to-rhombic vortex lattice transition, a crossover between single vortex pinning in the low magnetic field range and pinning of interacting vortices at higher fields, or has been treated similarly to the peak effect close to the upper critical magnetic field. Here we show that the SP in FeSe1-xTex single crystals is related to the well documented order-disorder transition in the vortex system, and its evolution in the low-temperature T domain is described by a dynamic energy balance relation. The main argument supporting this approach is the observed strong increase of the field for the onset of the SP with decreasing T down to 2 K, with an inflection-like point located between 3 and 4 K, in agreement with the two-band superconductivity reported for FeSe1-xTex.

Mixtures of MgB2 and metal or oxide additions with starting compositions of (MgB2)(M2O3)(x), x = 0.0025, 0.005, 0.015, and (MgB2)(M)(y), y = 0.01, M = Bi, Sb, were processed by spark plasma sintering (SPS). As-obtained samples are composites with high density exceeding 94% of the theoretical values. Secondary phases indicate similar reactions for samples with Bi- or Sb-based additions. However, samples show very different superconducting characteristics depending on the addition type and amount. A direct correlation with the melting temperature of the addition could not be revealed, although some aspects will be discussed. From the critical current density (J(c)) and irreversibility field (H-irr) enhancement viewpoints, optimum additions are oxides for x = 0.0025; 0.005. Both oxides improve Jc at high fields, but Sb2O3 is effective up to 10 K, whereas Bi2O3 is effective up to 30 K. Metal additions decrease J(c) and H-irr when compared with a pristine MgB2 sample. (C) 2012 The Japan Society of Applied Physics

High permittivity materials allow good miniaturization of microwave components, but the dielectric resonator antenna using these materials has the drawback of a reduced bandwidth. To overcome this disadvantage we wanted to obtain a relatively wideband high permittivity DRA using the superior modes. Using HFSS simulations, we identified some of the superior modes that appear in cylindrical DRAs situated on a conductor ground plane and excited by a symmetric rectangular slot made in the ground plane. We identified seven superior modes and, for all of them, the measured and theoretically calculated resonant frequencies did not differ by more than 6%. Knowing the possible excited modes in the antenna and the relations for their resonant frequencies, one can predict the operating frequencies of a cylindrical DRA excited by a central rectangular slot.