Electrical characterization of in situ polymerized polyaniline thin films

The alternating conductivity, sigma*(f) = sigma'(f) + i sigma ''(f), of in situ polymerized polyaniline thin films doped with hydrochloric acid, deposited on top of an interdigitated gold line array previously deposited on glass substrates, were measured in the frequency (f) range between 0.1 Hz to 10 MHz and in the temperature range from 100 to 430 K. The results for sigma'(f) are typical of a disordered solid material: for frequencies lower than a certain hopping frequency gamma(hop), log[sigma'(f)] is frequency-independent rising almost linearly for in logf > gamma(hop). A master curve was thus obtained by plotting the real component of the conductivity using normalized scales sigma'(f)/sigma(dc) and f/gamma(hop) which is indicative of a single process operating in the whole frequency range. An expression encompassing the conduction through a disordered structure taken from previous random free energy barrier model for hopping carriers, as well a dielectric function to represent the capacitive behavior of the PAni was employed to fit the experimental results. The dielectric constant and activation energy for hopping carriers were obtained as function of the polymer doping level. (c) 2007 Elsevier B.V. All rights reserved.

Photoreflectance studies of optical transitions in cubic GaN grown on GaAs(001) substrates

The optical properties of cubic GaN epitaxial layers were investigated by modulated photoreflectance (PR) and photoluminescence in the temperature interval from 5 to 300 K. The epilayers were grown on GaAs(001) substrates by molecular beam epitaxy using a nitrogen RIF-activated plasma source. The PR spectra show a transition which is well fitted using the third-derivative functional form of the unperturbed dielectric function, which we interpret as band-to-band transition. Our results allow determination of the temperature dependence of the main gap of c-GaN and give insights into the residual strain in the film, as well as allow us to estimate the binding energy of the complex formed by an exciton bound to a neutral acceptor. (C) 2003 Elsevier B.V. B.V. All rights reserved.

Determinação de parâmetros ópticos de materiais por análise de Kramers-Kronig de espectros de infravermelho

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Caracterização de propriedades de vidros polarizados do sistema P2O5 – B2O3 – CaO – TiO2

Pós-graduação em Ciência dos Materiais - FEIS

Modelos não-lineares de teoria de campos e mundos-brana

Pós-graduação em Física - FEG

Investigation in SrTiO3-CaTiO3-PbTiO3 ternary thin films by dielectric proprieties and Raman spectroscopy

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Electric and dielectric behavior of CaCu3Ti4O12-based thin films obtained by soft chemical method

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Effects of crystallization conditions on dielectric and ferroelectric properties of PZT thin films

This paper reports studies on dielectric and ferroelectric properties of lead zirconate titanate (PZT) thin films crystallized by conventional thermal annealing (CTA) and rapid thermal annealing (RTA) in air, oxygen and nitrogen atmospheres to better understand, control and optimize these properties. The dielectric constant (epsilon) and dissipation factor (tan delta) values, at a frequency of 100 kHz; for film crystallized in air by CTA process, were 358 and 0.039, respectively. Considering the same frequency for film crystallized in air by RTA, these values were 611 and 0.026, respectively. The different dielectric values were justified by a space-charge or interfacial polarization in films, often characterized as Maxwell-Wagner type. This effect was also responsible to dispersion at frequencies above 1 MHz in film crystallized in air by CTA process and film crystallized by RTA in oxygen atmosphere. The film crystallized by RTA under nitrogen atmosphere presented an evident dispersion at frequencies around 100 Hz, characterized by an increase in both epsilon and tan delta. This dispersion was attributed to conductivity effects. The remanent polarization (P-r) and coercive field (E-c) were also obtained for all films. Films obtained from RTA in air presented higher P-r (17.8 muC cm(-2)) than film crystallized from CTA (7.8 muC cm(-2)). As a function of the crystallization atmospheres, films crystallized by RTA in air and nitrogen presented essentially the same P-r values (around 18 muC cm(-2)) but the P-r (3.9 muC cm(-2)) obtained from film crystallized under oxygen atmosphere was profoundly influenced.

Modeling the dielectric response of lanthanum modified lead zirconate titanate ferroelectric ceramics-an approach to the phase transitions in relaxor ferroelectrics

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Features of dielectric response in PMN-PT ferroelectric ceramics

In this paper, electrical and structural properties were reported for pyrochlore free (1-x)[Pb(Mg1/3Nb2/3)O-3] - xPbTiO(3) (PMN-PT) (with 35 mol% PbTiO3) ceramics obtained from fine powders. Dielectric studies were focused on the investigation of the complex dielectric permittivity (epsilon '-i epsilon '') as a function of frequency and temperature. The effects of the dc applied electric field on dielectric response were also investigated. Results revealed a field dependence dielectric anomaly in the dielectric permittivity curves (epsilon(T)) in the low dc electric field region, which in turn prevails in the whole analysed frequency interval. To the best of our knowledge, these properties for the PMN-PT ceramic system have not been reported before as in this work. The results were analysed within the framework of the current models found in the literature.

Investigation on dielectric response of PMN ceramics around paraelectric-ferroelectric diffuse phase transition

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Investigation of nonlinear dielectric properties in Sr0.75Ba0.25Nb2O6 relaxor ferroelectric thin films

The ac and dc driving fields dependence of the dielectric permittivity for the strontium barium niobate relaxor ferroelectric thin films has been investigated. The nonlinear dielectric properties were obtained by using the measurements of the dielectric permittivity of the material as a function of the ac and dc "bias" electric field amplitude in wide frequency (100 Hz-10 MHz) and temperature (50-450 K) intervals. The results hint the existence of a true mesoscopic dielectric relaxor response in the ferroelectric thin film, which is very similar to those observed in bulk relaxor ferroelectrics. An anomalous behavior of the NL dielectric response was observed when submitted to moderate dc electric fields levels,,indicating a crossover from paraelectric to a glasslike behavior on cooling the sample toward the freezing transition. The obtained results were analyzed within the framework of the models proposed in the current literature.

Dielectric and ferroelectric characteristics of barium zirconate titanate ceramics prepared from mixed oxide method

Barium zirconium titanate (BZT) ceramics were prepared by mixed oxide method. X-ray diffraction showed the presence of a single phase while Raman scattering confirmed structural transitions as a function of different Zr/Ti ratio. The addition of Zr strongly influenced the crystal structure and electrical properties of the ceramics. A typical hysteresis loops were observed for all investigated compositions. BZT ceramics with 15 mol% Zr have shown a ferroelectric to paraelectric transition at around 77 degrees C. (C) 2007 Published by Elsevier B.V.

Temperature dependence of dielectric properties for Ba(Zr0.25Ti0.75)O-3 thin films obtained from the soft chemical method

Ba(Zr0.25Ti0.75)O-3(BZT) thin films prepared by the polymeric precursor method (PPM) were annealed at 500, 600, and 700 degrees C for 4h. All films crystallized in the perovskite structure present a crack-free microstructure. Dielectric properties of the BZT thin films were investigated as a function of frequency and applied voltage. The dielectric constant of the films were 36, 152 and 145 at 1 kHz, while the dielectric loss were 0.08, 0.08, and 0.12 at 1 MHz. (c) 2007 Elsevier B.V. All rights reserved.

Dielectric loss and phase transition of sodium potassium niobate ceramic investigated by impedance spectroscopy

The dielectric permittivity of Na0.80K0.20NbO3 ceramic was investigated by impedance spectroscopy. The dielectric characterization was performed from room temperature to 800 degreesC, in the frequency range 5 Hz-13 MHz. The bulk permittivity was derived by the variation of the imaginary part of the impedance as a function of reciprocal angular frequency. The permittivity values as a function of temperature showed two maxima. The first maximum is very similar at 200degreesC and the second one positioned at around 400degreesC, which was associated to Curie's temperature. The evolution of the complex permittivity as a function of frequency and temperature was investigated. At low frequency dispersion was investigated in terms of dielectric loss. The Na0.80K0.20NbO3 showed a dissipation factor between 5 and 40 over a frequency range from 1 to 10(2) kHz. (C) 2002 Elsevier B.V. B.V. All rights reserved.