Publications

Phase transition and field driven hysteresis evolution of a two-dimensional Ising grid consisting of ferroelectric-antiferroelectric multilayers that take into account the long range dipolar interactions were simulated by a Monte-Carlo method. Simulations were carried out for a 1 + 1 bilayer and a 5 + 5 superlattice. Phase stabilities of components comprising the structures with an electrostatic-like coupling term were also studied. An electrostatic-like coupling, in the absence of an applied field, can drive the ferroelectric layers toward 180A degrees domains with very flat domain interfaces mainly due to the competition between this term and the dipole-dipole interaction. The antiferroelectric layers do not undergo an antiferroelectric-to-ferroelectric transition under the influence of an electrostatic-like coupling between layers as the ferroelectric layer splits into periodic domains at the expense of the domain wall energy. The long-range interactions become significant near the interfaces. For high periodicity structures with several interfaces, the interlayer long-range interactions substantially impact the configuration of the ferroelectric layers while the antiferroelectric layers remain quite stable unless these layers are near the Neel temperature. In systems investigated with several interfaces, the hysteresis loops do not exhibit a clear presence of antiferroelectricity that could be expected in the presence of anti-parallel dipoles, i.e., the switching takes place abruptly. Some recent experimental observations in ferroelectric-antiferroelectric multilayers are discussed where we conclude that the different electrical properties of bilayers and superlattices are not only due to strain effects alone but also due to long-range interactions. The latter manifests itself particularly in superlattices where layers are periodically exposed to each other at the interfaces.

In this paper, surface enhanced Raman scattering (SERS) studies on the chemical polymerization of 3,4-ethylene dioxythiophene (EDOT) in the presence of single-walled carbon nanotubes (SWNTs) are reported. Both a non-covalent and covalent functionalization of SWNTs with poly(3,4-ethylene dioxythiophene) (PEDOT) is invoked as a result of the following variations induced in the SERS spectra of PEDOT and SWNTs: (i) the appearance of a Raman line at 140 cm(-1) indicating the formation of a new covalent bond between PEDOT and SWNTs; (ii) an increase in the intensity of the Raman line at 705 cm(-1), associated with the deformation vibration mode of C-S-C bond, the result of a steric hindrance effect induced by the bonding of PEDOT on SWNTs; and (iii) the enhancement of the Raman band with maximum at 1540 cm(-1) (G(-) component) in SWNTs when the PEDOT weight in the PEDOT/SWNTs composite increases. Using the PEDOT/SWNTs composite as a positive electrode and an electrolytic solution containing LiPF6, the charge-discharge characteristics of the rechargeable lithium cells are determined. High specific discharge capacity are reported for the PEDOT/SWNTs composite (ca. 218 mAh g(-1)) in comparison with PEDOT doped with FeCl4- ions (ca. 25 mAh g(-1)).

We have investigated by DC magnetization measurements and frequency-dependent AC susceptibility the critical current density (J (c)), pinning force (F (p)) and pinning potential in thick (1.3-1.6 mu m) YBa2Cu3O7-delta (YBCO) films grown by Pulsed Laser Deposition on SrTiO3 substrates decorated with LaNiO3 nanodots deposited by a few (5-15) laser pulses, in comparison with those of a 1 mu m thick YBCO reference sample. Experiments show that the highest improvement of superconducting properties was achieved for films grown on substrates decorated with 10 laser pulses on the LaNiO3 target, which have, at 77.3 K, a J (c) of 40-125% higher than in pure YBCO in fields between 1 and 2 T, and F (p) increased by 40%. These results could be important for further improvement of current-carrying capability of coated conductors for in-field power applications.

Experimental evidence that nuclear magnetic resonance (NMR) can detect structural changes of piezoelectric La3Ga5SiO14 induced by dilute paramagnetic ions is presented. Gd3+ and Eu3+ cations have been incorporated into La3Ga5SiO14 monocrystals. As expected, the line-width of the tetrahedral Si-29 magic angle spinning (MAS) NMR spectra as well as the inverse of the T-2 relaxation time of Ga-71 increases with the concentration of the paramagnetic ions. A surprising result is shown by Ga-71 multiple quantum (MQ) MAS NMR spectrum, which changes with the concentration of paramagnetic ions. The changes in the Ga-71 MQMAS spectra can be explained by a more ordonated distribution of Ga ions inside the oxygen tetrahedra. The Ga-71 MQMAS NMR spectra allow identification of the one octahedral and two tetrahedral Ga sites. (C) 2009 Elsevier Inc. All rights reserved.

The electrochemical polymerization of N-ethyl carbazole (EK) on carbon nanotubes (CNs) films immersed in a LiClO4/acetonitrile solution was studied by cyclic voltammetry. Cyclic voltammo-grams recorded on a blank Pt electrode were compared with those obtained when CNs films were deposited on a Pt plate. In the latter case, the oxidation maximum is not clearly observed. Using the Raman scattering studies, oligomers of poly(N-ethyl carbazole) (PEK) on CNs are shown to be formed. A covalent functionalization of CNs with the PEK oligomers is demonstrated by FTIR spectroscopy studies. The formation of new C-C covalent bonds between CNs and the benzene ring of PEK induces steric hindrance effects that are shown in FTIR spectra by the increase in the intensity of the absorption band at 1072 cm(-1) attributed to the vibration of C-H in plane deformation of aromatic ring. The influence of the monomer and the supporting electrolyte concentration as well as the sweep rate on the polymerization conditions in the case of the composites based on the PEK oligomers and CNs were also investigated.

We investigated the vortex dynamics in the noncentrosymmetric superconductor Li2Pt3B in the temperature range 0.1-2.8 K. Two different logarithmic creep regimes in the decay of the remanent magnetization from the Bean critical state have been observed. In the first regime, the creep rate is extraordinarily small, indicating the existence of an unconventional very effective pinning mechanism. At a certain time, a vortex avalanche occurs that increases the logarithmic creep rate by a factor of about 5 to 10 depending on the temperature. This may indicate that certain barriers against flux motion are present and they can be opened under increased pressure exerted by the vortices.

The new tetranuclear complexes [Fe(3)Ln(mu(3)-O)(2)(CCl3COO)(8)(H2O)(THF)(3)]center dot THF (Ln = Ce-III (1), Pr-III (2), Nd-III (3)) and [Fe(3)Ln(mu(3)-O)(2)(CCl3COO)(8)(H2O)(THF)(3)]center dot THF center dot C7H16 (Ln = Sm-III (4), Eu-III (5), Gd-III (6), Tb-III (7), Dy-III (8), Ho-III (9), Lu-III (10) and Y-III (11)) have been prepared. All compounds were prepared by the reaction between [Fe2BaO(CCl3COO)(6)(THF)(6)] and the corresponding Ln(III) nitrate salt. The crystal structures of 1-4, 8 and 9 have been determined; these isostructural molecules have a non-planar {Fe(3)Ln(mu(3)-O)(2)} "butterfly" core. Magnetic susceptibility measurements show dominant intramolecular antiferromagnetic exchange interactions for all the complexes. Fe-57 Mossbauer spectroscopy shows three different environments for the Fe-III metal ions, all in their high-spin state S = 5/2 (confirming that no electron transfer from Ce-III to Fe-III occurs in 1). At the time scale of the Mossbauer spectroscopy (about 10(-7) s), evidence of magnetization blocking, i.e. slow relaxation of the magnetization, is observed below 3 K for 7, which was confirmed by ac susceptibility measurements. (C) 2009 Elsevier Ltd. All rights reserved.

Lead free ceramics in the system (1-x)KNN x(Bi2O3,CaO,CuO) with 0.0 <= x <= 0.1 were synthesized by the usual solid state reaction and their structural and piezoelectric properties were investigated Dense samples with densities of about 4 68 g/cm(3) were prepared by sintering at an optimum temperature of 1150 degrees C for 6 h Only perovskite phase was detected by X-ray investigation in all samples A typical MPB where orthorombic-tetragonal phases coexist was detected for a doping level of 0.06 where the piezoelectric parameters showed maximum values Planar coupling factor, as much as 0.46, piezoelectric charge constant of 270 pC/N and mechanical quality factor Q(m) of 75 were recorded for the samples situated within the MPB.

Among the great number of sol-gel prepared nanomaterials, TiO2 has attracted significant interest due to its high photocatalytic activity, excellent functionality, thermal stability and non-toxicity. The photocatalytic degradation of pollutants using un-doped and doped TiO2 nanopowders or thin films is very attractive for applications in environmental protection, as a possible solution for water purification. The present work describes a comparative structural and chemical study of un-doped TiO2 and the corresponding S- and Ag-doped materials. The photocatalytic activity was established by testing the degradation of organic chloride compounds from aqueous solutions. Sol-gel Ag-doped TiO2 coatings, prepared by co-gelation and sol-gel Ag-doped TiO2 coatings obtained from nanopowders were also compared. Their structural evolution and crystallization behaviour (lattice parameters, crystallite sizes, internal strains) with thermal treatment were followed by thermal analysis, X-ray diffraction, transmission electron microscopy, atomic force microscopy and specific surface areas measurements. X-ray photoelectron spectroscopy analyses were performed to characterize the surface composition and S or Ag speciation, which was used to interpret the catalytic data.

Lead titanate (PT) ceramics co-doped with 8 to 13 mol% Ni-Nb, by substituting titanium ions, were prepared by conventional ceramic method and the influence of Ni-Nb doping level on their mopho-structural, electromechanical and dielectric properties was investigated. Dense tetragonal PT ceramics were obtained for 8 to 12 mol% Ni-Nb co-doping. Lattice anisotropy decreased from 1.053 to 1.046 with increasing doping level. Large thickness coupling factors of 0.4 were obtained for 8, 11, and 12 mol% Ni-Nb. Large electromechanical anisotropy of 10 was obtained for 8 mol% Ni-Nb. It was found that Ni-Nb co-doping has both softening and hardening effects as a function of donor or acceptor behavior of different amounts of Ni-Nb. For 13 mol% Ni-Nb, segregation of a small amount of a secondary phase occurs, lattice constants and tetragonality significantly decrease and the resulting ceramic becomes very hard.