The major objectives of this study were to examine electrical conduction properties of BaTiO3-based multilayer ceramic ( MLC ) capacitors in order to gain a better understanding of the conduction transport mechanisms inside the devices. The experiments involved mainly leakage current versus time measurements under both low temperature-low voltage stress and high temperature-high voltage stress. It was established that leakage current conduction in a MLC capacitor under temperature-voltage stress can be divided into three different conduction regions due to different mechanisms. Those regions are polarization current, DC conduction current and degradation current. The polarization current decreases with time as a power law relation, i.e. Ic(t) t-m where the exponent value m is strongly dependent on the type of capacitor and temperature, but is only weakly dependent on the applied voltage. It has been proposed that two degradation models ( a charge carrier concentration model and a reduction of grain boundary barrier height model ) can explain the degradation behavior for the Z5U devices tested. Degradation measurements indicate that the lifetime for Z5U capacitors can be described by Minford's expression. However, these models account only partly for X7R degradation. X7R behavior, is characterized by an early power law time dependence, followed by exponential voltage dependence. The most probable conduction transport mechanism in X7R capacitors is small polaron hopping, while grain boundary transmission may be the predominant conduction transport mechanism in Z5U capacitors. / Master of Science

The doping of BiFeO3 with lanthanide elements like Ho, with a radius smaller than Bi, is ideal to improve the ferroelectric and magnetic properties of BiFeO3, which in principle can cause structural distortions of the lattice that improve the electrical and magnetic properties. In this work, we report on the temperature dependence of the structural and magnetic properties of Ho substituted BiFeO3 (BHFO) samples, which have been investigated by X-ray diffraction (XRD) and Mössbauer spectroscopic techniques. The XRD and Mössbauer measurements were done at room temperature on the as-synthesized BHFO samples and after annealing the samples in Argon up to 1073 K. The resultant XRD patterns have shown that BHFO is of rhombohedral R3m space group, with a majority Bi25FeO90 phase and a minority Bi2Fe4O9 phase. These two phases are attributed to the local stoichometry fluctuations in BiFeO3 (BFO). A new phase was evident in the XRD spectra after annealing the sample between 673 – 873 K; this has been assigned to the octahedral B-site of Fe3O4. The Mössbauer spectra were characterized by broadened features and the magnetic hyperfine splitting patterns were indicative of magnetic ordering mostly probably screwed or slightly antiferromagnetic ordering. The spectra were fitted with two symmetric sextets (S1 & S2) which were present in all annealed samples, a symmetric sextet (S3) which was observable at annealing temperatures greater than 673 K, a Lorentzian doublet (D) and a single line (SL) which were present in all spectra. The extracted hyperfine parameters of sextet S1 are consistent with those of rhombohedral BiFeO3 and are characteristic of magnetically ordered Fe3+. At TA > 673 K, a third sextet S3 was assigned to the high symmetry cubic spinel phase. The paramagnetic doublet D was attributed to the Bi25FeO40 phase and the singlet line SL to the Bi2Fe4O9 phase which has been observed previously in the studies of BiFeO3 and other BiFeO3 doped systems. The isomer shift and quadrupole splitting values of the paramagnetic doublet D corresponds to an oxidation state of Fe3+, while the isomer shift of S1 remained fairly constant up to TA = 623 K then decreased gradually after the appearance of S3 indicating an increase of the s-electron density at the Fe nucleus. The quadrupole splitting of S2 showed no systematic change with annealing temperature, however at TA > 623 K this parameter changed dramatically to a negative value with a slightly larger magnetic field. The distribution of the isomer shift and the difference in the quadrupole splitting values and signs are due the variation in the angles between the principal axis of the electric field gradient (EFG) and the spin direction. The hyperfine fields of S1 and S2 remained fairly constant for all measured samples, however at TA > 623 K the hyperfine field of S3 showed a slight increase which could be due to Ho being substituted at the Fe site in BiFeO3. In addition, in-situ Mössbauer measurements at temperatures in the range 300 – 748 K were made on the BHFO samples. The room temperature spectrum showed similar features as observed on the annealing series of measurements. The hyperfine magnetic fields of the two sextet components (S1 and S2) decreased with increasing temperature and finally collapsed at T > 588 K. The hyperfine fields of both the S1 and S2 components decreased systematically with temperature to a field distribution just below the Néel temperature. From our measurements, we estimated the Néel temperature for BHFO to be in the range 598 – 617 K. The isomer shift for all spectral components showed a linear decrease with increasing temperature which closely followed the usual second order Doppler shift variation with temperature. The S1 and S2 spectral components present at room temperature disappeared just before the Néel temperature resulting in the area fraction of the paramagnetic doublet D dominating the spectrum. From the site populations, an average Debye temperature (θD) was estimated to be 240 ± 81 K for BHFO which is lower than the value of 340 ± 50 K cited for BiFeO3.

O objetivo deste trabalho foi implementar um sistema para a determinação do coeficiente piroelétrico em materiais poliméricos. O método consiste em variar a temperatura da amostra de forma senoidal e medir a corrente em função do tempo. Através do sinal da corrente elétrica e da sua diferença de fase com a temperatura, separa-se a corrente elétrica de origem piroelétrica, a partir da qual se determina o coeficiente piroelétrico. Apresenta-se a caracterização do sistema abrangendo aspectos como o intervalo de temperatura das medidas e amplitude e freqüência da oscilação da temperatura. O método foi empregado para realizar medidas exploratórias com o polímero ferroelétrico poli(fluoreto de vinilideno), PVDF, polarizado pela aplicação de uma rampa de tensão, e com o polímero poli(metacrilato de metila), PMMA, com corante polar, polarizado com o triodo de corona. Nas medidas foram variados os parâmetros do processo de polarização elétrica tais como o campo elétrico máximo e a temperatura / An experimental setup to measure the pyroelectric coefficient of polymers was implemented. The method is based on the measurement of the electric current generated by the sample when its temperature was set to oscillate sinusoidally. Using the measured current and its phase difference related with the temperature thepyroelectric current was obtained, from that the pyroelectric coefficient was determined. It is shown the characterization of the experimental system such as the temperature amplitude and frequency interval of operation. The method was applied in exploratory measurements employing the ferroelectric polymer poly(vynilidene fIuoride), PVDF, poled by a ramp voltage, and the poly(methyl metacrylate), PMMA, having a polar dye, poled with the carona triode. Measurements were performed far different poling conditions such as the maximum applied field and temperature

Esta dissertação descreve os métodos empregados na preparação por solução de filmes de copolímeros P(VDF-TrFE) nas composições molares 60/40, 70/30 e 80/20. Foram utilizadas as técnicas de deposição por centrifugação (spin coating deposition) e de espalhamento por extensor, usando acetona e metil-etil-cetona (MEK) como solventes. Filmes de boa qualidade só foram obtidos quando se acelerou o processo de evaporação do solvente, através da passagem de um fluxo de ar seco na câmara onde os filmes eram produzidos. Medidas de espectroscopia de infra-vermelho confirmaram a ausência de solvente nos filmes, já que observou-se que o pico 1715 cm-1 (carbonilha) característico do MEK não aparece nos resultados com filmes. A caracterização dos filmes fabricados foi feita através de medidas de calorimetria diferencial exploratória de varredura (DSC), difração de raios-X, espectroscopia de infra-vermelho e cromatografia líquida por exclusão de tamanho (GPC). Nos termogramas de DSC puderam ser identificadas as transições de fase ferro-paraelétrica (na temperatura de Curie, Tc) e de fusão. Os valores de Tc e da temperatura de fusão, Tf, encontrados são consistentes com os valores da literatura, com Tc aumentando com o conteúdo de VDF, ao passo que o oposto ocorre com Tf, devido ao fato que o aumento do percentual de TrFE na região cristalina aumenta a estabilidade da fase paraelétrica. Os difratogramas de Raios-X apresentaram, basicamente, um ombro largo correspondente à porção amorfa do filme, e um pico alto e estreito devido a parte cristalina do filme. No ajuste dos difratogramas através de uma função de Lorentz modificada, os resultados para as amostras de 60/40 e 70/30 foram ajustadas com três picos, o que parece indicar que pode haver duas fases cristalinas para estas amostras. Já os resultados para a amostra de 80/20 foram ajustados com apenas dois picos. O tratamento térmico das amostras, a diversas temperaturas, mostrou influenciar fortemente as propriedades dos filmes. Os seus efeitos mais importantes são o aumento da temperatura de Curie e da percentagem de cristalinidade do filme com a temperatura de tratamento / This dissertation describes the experimental methods employed in the fabrication of thin films (up to 40 micra) of the copolymer P(VDF-TrFE) for three different VDF contents, Viz. 60/40, 70/30 and 80/20. The copolymers were dissolved in either acetone or methyl-ethyl-ketone(MEK) and deposited on a solid substrate using spin coating or a sliding-knife. Good quality films could only BR obtained after dry air was flushed into the chamber where the films were fabricated. Infra-red spectrocopy measurements confirmed the abscene of solvent in the samples, as the 1715 cm-1 peak from MEK is not present in the results for the films. The film characterization was carried out using differential scanning calorimetry (DSC), X-ray diffraction, infra-red spectrocopy and gel permeation chromatography (GPC). Phase transitions could be identified in the DSC thermography which can be related to the ferro-paraelectric transition (at the Curie temperature, Tc) and to the melting of the sample. The values for Tc and the melting temperature, Tf), were consistent with those published in the literature, with Tc increasing with the contents of VDF, while the opposite is true for Tf because the increase in the TrFE contents causes the stability of the paraelectric phase to increase. The X-ray diffractograms displayed, basically, one broad halo due to the amorphous part of the film, and a eak due to its crystalline region. The diffractograms were fitted using a modified Lorentz function, where three peaks had to be assumed for the 60/40 and 70/30 samples. This appears to indicate that there may exist two crystalline phases in these samples. The results for 80/20 samples, on the other hand, were fitted with two peaks. The thermal treatment of the samples, under various temperatures, proved to influence strongly the properties of the films. Its main effects are the increase in the Curie temperature and the increase in the degree of crystallinity of the films with increasing temperature of thermal treatment

. / .

First principles calculations provide a powerful tool for sorting out the interplay of chemical composition and structure with the physical properties of materials. In this dissertation, I discuss the physical properties and their microscopic basis within this framework for following illustrative examples. (i) The Zintl phase hydrides, where I find H is anionic and the formation of covalent sp2 bonds in the Al/Ga/Al-Si planes, which is a highly unusual bonding configuration for these elements. (ii) PbTe, which shows strong coupling between the longitudinal acoustic and transverse optic modes that may explain its low thermal conductivity. (iii) The double perovskites BiPbZnNbO6 and BiSrZnNbO6, where introducing size disorder at A-site prevents the BO6 octahedra from tiling and enhances the polar behavior. (iv) FeSe, which shares the salient electronic and magnetic features of other Fe superconductors and cannot be described as a conventional electron phonon superconductor. (v) NbFe2, which is near a magnetic quantum critical point and shows strong competition between various magnetic orderings that may explain its unusual non-Fermi liquid behavior at very low temperatures. (vi) The nickel analogues of Fe superconductors LaNiPO and BaNi2As2, where I show that superconductivity is of conventional electron-phonon type in contrast to the Fe-based superconductors. (vii) Noncentrosymmetric LaNiC2, which I find is a conventional electron-phonon superconductor with intermediate coupling.

Thesis (Ph.D.)--City University of Hong Kong, 2009. / "Submitted to Department of Physics and Material Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy." Includes bibliographical references (leaves 140-167)

No description available.

No description available.

Thesis (Ph.D.)--McMaster University (Canada), 1994. / Source: Dissertation Abstracts International, Volume: 56-08, Section: B, page: 4527. Adviser: G. C. Weatherly.