Freiburg i. Br., Univ., Diss., 200.

Thesis (M.S.)--University of Wisconsin--Madison, 1965. / eContent provider-neutral record in process. Description based on print version record. Bibliography: l. 48.

Made available in DSpace on 2014-06-11T19:25:34Z (GMT). No. of bitstreams: 0 Previous issue date: 2007-04-27Bitstream added on 2014-06-13T20:33:16Z : No. of bitstreams: 1 nahime_bo_me_ilha.pdf: 1294512 bytes, checksum: ffc3048caf6b741e23555fbf3b2fbd5e (MD5) / Na última década, cresceu consideravelmente o interesse pela produção de filmes finos ferroelétricos, em virtude do grande potencial que estes materiais apresentam para a produção de dispositivos de memória ferroelétrica não-voláteis. O titanato de bismuto, Bi4Ti3O12 (BIT) é um ferroelétrico com estruturas de camadas de bismuto, conhecido por apresentar elevada temperatura de Curie (Tc), próxima de 650ºC, e excelentes propriedades de fadiga ferroelétrica. Este trabalho teve como objetivo a síntese de filmes finos de BIT e o estudo das suas propriedades estruturais. Os filmes estudados foram preparados a partir de um método químico, similar ao método Pechini, e depositados sobre substratos de Si(100). Os filmes foram cristalizados em forno convencional entre 500°C e 700°C, por períodos entre 1 e 12 horas. As técnicas de difração de raios-X (DRX) e Espectroscopia no Infravermelho por Transformada Fourier (FT-IR), foram usadas como ferramentas de investigação. Para os filmes tratados termicamente entre 400°C e 700°C, observouse a presença das fases cristalinas Bi4Ti3O12 e Bi2Ti2O7 (paraelétrica). Para os filmes tratados termicamente a temperaturas mais elevadas (700°C) e tempos mais prolongados (10 horas), observou-se uma tendência de desaparecer a fase Bi2Ti2O17. Aliados a estes resultados, o aumento da intensidade do pico (117) do BIT e o decréscimo da sua respectiva largura a meia altura, para os filmes cristalizados a 700°C, sugerem que nestas condições são obtidos filmes mais bem cristalizados. Um estudo usando a técnica FT-IR demonstrou a presença de uma banda de absorção claramente evidenciada em torno de 700 cm-1. Esta banda está associada ao modo stretching da ligação Ti-O e torna-se consideravelmente mais estreita quando a resina polimérica precursora foi tratada termicamente a temperaturas mais elevadas. Este fato também comprovou que temperaturas em torno de 700°C são favoráveis para a... / In the last decade the interest for the production of ferroelectric thin films increased considerably because of the great potential that these materials present for the production of devices of no-volatile ferroelectric memory. The bismuth titanate, Bi4Ti3O12 (BIT), is a ferroelectric with structures of bismuth layers, known by presenting high temperature of Curie (Tc), close of 650ºC, and excellent properties of ferroelectric fatigue. This work had as objectives the synthesis of thin films of BIT and the study of their structural properties. The studied films were prepared starting from a chemical method, similar to the Pechini method, and deposited on substrata of Si(100). The films were crystallized in conventional oven between 500°C and 700°C for periods between 1 and 12 hours. The techniques of X-ray diffraction (DRX) and Fourier Transform Infrared (FT-IR) Spectroscopy were used as investigation tools. It was observed the presence of the crystalline phases Bi4Ti3O12 and Bi2Ti2O7 (paraelectric) for the films termically treated between 400°C and 700°C. To the films termically treated to higher temperatures (700°C) and more lingering times (10 hours) a tendency of the phase Bi2Ti2O7 to disappear was observed. Allied to these results, the increase of the intensity of the pick (117) of the BIT and the decrease of its respective width to half height, for the crystallized films to 700°C suggests that, in these conditions, better crystallized films are obtained. A study using the FT-IR technique demonstrated the presence of an absorption band clearly evidenced around 700 cm-1. This band is associated to the stretching way of the Ti-O connection and becomes considerably narrower when the precursory polimeric resin was termically treated to higher temperatures. This fact also proved that temperatures around 700°C are favorable for obtaining a mono-crystalline phase of Bi4Ti3O12.

An investigation into the processes taking place at the surface interface of ferroelectric Pb(Zr1-x,Tix)O3 immersed in metal salt solution under ultraviolet illumination is presented. The semiconducting and switchable dipolar nature of this material allows the spatial separation and control of photo-induced reduction and oxidation across its surface interface. These properties can be of application in novel techniques such as the controlled growth of metallic nanoparticles across specific polar domains. 70nm thick Pb(Zr0.3,Ti0.7)O3 samples, PZT(30/70), are manufactured using the sol gel methodology, two crystallographic orientations being produced. The orientation being controlled by the substrate used; Si was used for [111] orientation and MgO for [100]. The initial work with wideband ultraviolet light shows that the reduction and growth of silver on the PZT surface is greatly influenced by the structure of the film. The crystallographic orientation of the film affects metal deposition such that on [111] films the metal deposits only on positive domains, where as the [100] films experience deposition on both positive and negative domains. This is shown to be due to the difference in width of the space charge region, Δw = 4.4nm, between the [111] and [100] samples so that the negative domain on [100] samples have 10 19 times higher chance of electron tunnelling compared to the [111]. It is also shown that grain boundaries have the greatest effect on the growth of metal, with a metal cluster growth rate 51 times faster than elsewhere on the surface. This increased rate of growth is due to the effect a grain boundary has on the surrounding area, the energy band bending at the boundary attracting charge carriers from the grains around it. The interface types ranked from greatest to lowest influence are grain boundaries, positive domains, domain boundaries and finally interphase boundaries.ii It is shown that the stern layer, strongly adsorbed charged ions of opposite sign to the surface charge, at the PZT/solution interface act as an insulating layer to metal reduction. The accumulation of photoexcited charge carriers at points along grain boundaries causes the surface potential gradient to alter and allows metal reduction and thus clusters to nucleate. The energy required to cause this variation is investigated by use of narrow band, 5nm bandwidth, ultraviolet. For energy from 4.4eV to 5 eV, it is found there is an increase in the average silver cluster cross sectional area by a ratio of ca 1.6 to 1 for both the [111] and [100] orientations of PZT. Finally it is shown that the type of metal salt used in the photochemical process affects the type of reaction that takes place at the sample surface. For a cation to reduce on positive domains its reduction potential needs to be below the bottom edge of the conduction band of PZT. Chloride salts, that sit above the conduction band, cause decomposition of the negative domains. Use is made of these effects to find the position of the bottom of the conduction band for PZT. It is found that across similar [111] PZT samples FeCl2 can both reduce on positive domains and decompose negative domains, this puts the bottom of the conduction band for PZT(30/70) between 4.06 and 4.36 eV from vacuum. It is also discovered that the type of anion affects the decomposition of the negative domains. Nitrate salts with cations above the conduction band cause no decomposition whereas chlorides do. The decomposition is shown to be the loss of Pb from the negative surface.

Compósitos cerâmicos planares de titanato zirconato de bário, BaTi1-xZrxO3, foram produzidos e os efeitos da quantidade de Zr4+ em suas propriedades funcionais foram estudados. As amostras foram fabricadas pelo método convencional de processamento cerâmico e pela técnica de deposição de fitas cerâmicas a partir de BaTi1-xZrxO3 com x=0, 0,05, 0,1, 0,15 e 0,2 sintetizados pelo método hidrotermal e pelo método dos precursores poliméricos. Foram realizadas caracterizações estrutural (difração de Raios X pelo método do pó e espectroscopia Raman), composicional (espectroscopia por dispersão de energia), microestrutural (microscopia eletrônica de varredura, ensaios de dilatometria) e funcional (permissividade elétrica, coeficiente piezoelétrico d33, coeficiente piroelétrico e histerese ferroelétrica). Além disso, a regra da mistura de fase foi utilizada para prever a permissividade elétrica dos compósitos e as tensões mecânicas internas e seus efeitos na permissividade elétrica e na temperatura de Curie de policristais de BaTiO3 foram simulados. As análises dos dados e as discussões foram realizadas considerando o modelo fenomenológico de Devonshire, a modificação de Forsbergh deste modelo para incluir efeitos de tensões mecânicas bidimensionais e o modelo de policristal tetragonal de BaTiO3 sugerido por Buessem. Os resultados mostram a existência de tensões residuais bidimensionais que surgem após o resfriamento dos compósitos devido às diferenças nos coeficientes de expansão térmica das fases constituintes. Os métodos de obtenção das amostras afetaram o tamanho final dos grãos e as espessuras das interfaces dos compósitos, sendo que, em geral, eles são menores e mais finos nas fitas cerâmicas homogêneas do que nas correspondentes cerâmicas homogêneas. Os compósitos apresentaram deslocamento da Tc para maiores temperaturas e aumento do grau de difusividade da transição. A presença das tensões mecânicas residuais e as características microestruturais, juntamente com os modelos utilizados, explicam qualitativamente a permissividade elétrica obtida. Concluí-se que a quantidade de Zr4+ modifica o comportamento das camadas durante a sinterização e altera o coeficiente de dilatação. Estas mudanças geram tensões mecânicas residuais que afetam a microestrutura e as propriedades funcionais dos compósitos. Portanto, a produção de compósitos cerâmicos ferroelétricos deve considerar a correlação existente entre microestrutura e tensões residuais para que suas propriedades sejam otimizadas. / Planar BaTi1-xZrxO3 ceramic composites had their functional properties investigated. These composites were obtained by the conventional ceramic processing technique and by tape casting technique. Furthermore, BaTi1-xZrxO3 x=0, 0.05, 0.1, 0.15 and 0.2 were synthesized by the polymeric precursors method and by the hydrothermal synthesis to study the synthesis influences and to study the effect of Zr4+ on the functional properties of the composites. Structural characterization (X-Ray powder diffraction and Raman spectroscopy), compositional analysis (energy dispersive X-Ray spectroscopy), microestrutural evaluation (scanning electron microscopy, dilatometry measurements) and functional properties characterization (electrical permittivity, piezoelectric coefficient d33, pyroelectric coefficient and ferroelectric hysteresis) were performed. Furthermore, the composites electrical permittivity was predicted by the simple mixture and the internal mechanical stress distribution and it´s effects on both, electrical permittivity and Curie´s temperature (Tc) of BaTiO3 polycrystals, were simulated. The analyses and discussions were supported by 1- Devonshire´s phenomenological theory, 2- Forsbergh´s modification to Devonshire´s theory to include the two-dimensional stress effects on Tc and 3- Buessem´s BaTiO3 tetragonal polycrystal model. The results show that the sintered composites present two-dimensional residual stresses after cooling due to the constrained sintering of the layers and their thermal expansion coefficient mismatch. The methods of sample preparation led to differences in grain size and interface thickness, with the homogeneous ceramic tapes presenting smaller grain sizes and thinner thickness than the conventional homogeneous ceramics. On the other hand, the composites showed a Tc shift to higher temperatures, a more diffuse phase transition and residual mechanical stresses. Concluding, the Zr4+ substitution of Ti4+ modifies the layer´s sintering behavior and their thermal expansion coefficient. These changes lead to microstructure modifications that affect the functional properties of planar BaTi1-xZrxO3 composites. Thus, the design of ferroelectric composites should take into consideration the correlations between microstructure and residual stresses in order to optimize their functional properties.

Orientador: Claudia Longo / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Quimica / Made available in DSpace on 2018-08-14T02:32:28Z (GMT). No. of bitstreams: 1 Miranda_BarbaraSantosde_M.pdf: 1864005 bytes, checksum: 098b7327374f16552656f05bf9aef20f (MD5) Previous issue date: 2009 / Resumo: As propriedades de eletrodos de filmes finos e porosos dos titanatos, H2Ti2O5; H2Ti4O9 e H2Ti8O17 foram investigadas, visando sua aplicação em dispositivos para conversão de energia solar. Os titanatos foram sintetizados por reações de estado sólido, pelo aquecimento de misturas de TiO2 e K2CO3 seguida por tratamento com HCl para a troca iônica e lavagem com água. Análises por Difração de Raios-X indicaram a presença dos precursores, sugerindo que a reação de estado sólido não foi completa. Dependendo das condições de síntese, obtiveram-se produtos com estrutura e morfologia muito distintos. O aquecimento de TiO2:K2CO3 2:1 a 640 °C por 8 horas resultou na mistura H2Ti2O5/H2Ti4O9, constituido por bastoes (100 nm de diâmetro, 500 nm de comprimento) e partículas esféricas. Bastões com mesmo diâmetro e comprimento de 5 mm foram observados para H2Ti4O9 (razão molar de reagentes 4:1, 800 °C por 20 horas). Para o H2Ti8O17, sintetizado a partir da razão 8:1 sob 1100 °C por 4 horas, observaram-se predominantemente folhas. A Energia de band gap foi estimada em 3,5 eV para o H2Ti2O5 e 3,2 eV para os outros titanatos. Filmes porosos dos titanatos foram depositados em eletrodos transparentes. Na ausência de irradiação, em solução aquosa de Na2SO4 0,1 M, os eletrodos apresentaram corrente capacitiva baixa e reações de desprendimento de hidrogênio e de oxigênio em -0,1 V e 1,3V; em K4Fe(CN)6, observou-se a reversibilidade da reação redox apenas em velocidades de varredura < 10mV.s. Sob irradiação policromática, os eletrodos apresentaram fotocorrente positiva e potencial de circuito aberto negativo, comportamento típico de semicondutor de tipo-n. No estudo de possíveis aplicações para o eletrodo de H2Ti4O9, observou-se baixa atividade fotocatalítica para a degradação de fenol. Este eletrodo apresentou, porém, alta adsorção para um corante de rutênio, resultado promissor para aplicação em células solares de semicondutor sensibilizado por corante / Abstract: Thin porous films electrodes of titanates H2Ti2O5, H2Ti4O9 and H2Ti8O17 were prepared and investigated. The titanates were synthesized through solid state reactions by heating mixtures of TiO2 and K2CO3; after HCl treatment for ion exchange, the samples were rinsed with water. X-ray diffraction and scanning electron microscopy indicated that, depending on the synthesis conditions, different titanates were obtained. Also, precursors were identified, meaning the solid state reaction was not complete. Heating of TiO2:K2CO3 2:1 at 640 °C for 8 hours resulted on the mixture H2Ti2O5/H2Ti4O9, composed by rods (100 nm diameter, 500 nm length) and spherical particles. Rods with the same diameter and 5 mm length were observed for H2Ti4O9 (molar ratio of reagents 4:1, 800 °C for 20 hours). For H2Ti8O17, synthesized from ratio 8:1 at 1100 °C for 4 hours, mainly sheets were observed. Band gap energy was estimated at 3,5 eV for H2Ti2O5/H2Ti4O9 and 3,2 eV for the others titanates. Thin porous films of titanate were deposited on transparent electrodes. In the dark, the electrodes presented low capacitive current and hydrogen and oxygen evolution reactions at -0,2V and 1,4V (vs. Ag/AgCl) in Na2SO4 0,1 M aqueous solution; also, the reversibility of redox reaction in K4Fe(CN)6, was observed only at slow scanning rates. Under polychromatic irradiation, the electrodes presented positive photocurrents and negative open circuit potential, a typical behavior of n-type semiconductor. Studies of possible applications for the H2Ti4O9 electrode revealed low activity for phenol photocatalytic degradation. Nevertheless, this electrode showed high adsorption of a ruthenium-based dye, a promising result for application on dye-sensitized solar cells / Mestrado / Físico-Química / Mestre em Química

Three systems of electrospun composite membranes with piezoelectric polymer polyvinylidene fluoride (PVDF) as matrix incorporating: 1) Carbon based fillers: carbon nanotube (CNT) and graphene oxide (GO); 2) Ceramic based fillers-barium titanate (BT), zinc oxide (ZnO) and nanoclays (halloysite and bentonite); 3) Cellulosic fillers: microcrystalline cellulose (MCC) and nanocrystalline cellulose (NCC) at different loadings were prepared by electrospinning process. Influence of filler type and loading on total PVDF crystallinity (Xc), relative fraction of β phase (piezoelectric phase) in total crystalline PVDF (Fβ), volume fraction of β phase in the samples (vβ) and piezoelectric coefficient d33 were characterised and analysed. Correlation between vβ and piezoelectric performance (d33) will be focused by this work. A common situation was observed for the composites-d33 increased while vβ is reduced by the fillers, so it can be concluded that d33 of the composites is not totally up to their vβ, there are other factors that need to be taken into account. For example, for carbon based filler like CNT, it increased electric conductivity of sample during and after electrospinning process, making it easier for charges produced by β crystals from inside of sample to be transferred to surfaces of the sample, and possibly promoting orientation of β crystals in d33 direction, therefore enhanced d33 of the composites though β phase formation was significantly hindered by inclusion of CNT; For piezoelectric ceramic fillers like BT and ZnO, a possible combined piezoelectricity from filler and β phase PVDF might enhanced d33 though less β phase was formed; And for non-piezoelectric and non-conductive fillers, enhancement in orientation of β crystals might play a major role in promotion of d33. Keywords: electrospinning; polyvinylidene fluoride (PVDF); nanocomposites; piezoelectric coefficient d33; energy harvesting.

The electric vehicle itself today outlives its battery, necessitating battery replacement. Lithium titanium oxide (LTO) has, in this context, been suggested as a new anode material in heavy electric vehicle applications due to intrinsic properties regarding safety, lifetime and availability. The work presented here is focused on the LTO electrode/electrolyte interface. Photoelectron spectroscopy (PES) has been applied to determine how and if the usage of LTO could prevent extensive anode-side electrolyte decomposition and build-up of a surface layer. The presence of a solid electrolyte interphase (SEI) comprising LiF, carbonates and ether compounds was found in half-cells utilizing a standard ethylene:diethylcarbonate electrolyte with 1 M LiPF6. Via testing of symmetrical LTO-LTO cells, the stability of the formed SEI was put in to question. Moreover, the traditional polyvinylidene difluoride (PVdF) binder was replaced by more environmentally benign carboxylmethyl cellulose (CMC) and polyacrilic acid (PAA) binders in LTO electrodes, and it was found that CMC helped to form a more stable surface-layer that proved beneficial for long term cycling. Following the half-cell studies, full-cells were investigated to observe how different cathodes influence the SEI of LTO. The SEI in full-cells displayed characteristics similar to the half-cells, however, when utilizing a high voltage LiNi0.5Mn1.5O4 cathode, more electrolyte decomposition could be observed. Increasing the operational temperature of this battery cell generated even more degradation products on the LTO electrodes. Mn was also found on the anode when using Mn-based cathodes, however, it was found in its ionic state and did not significantly affect the composition or behavior of the observed SEI layer. Furthermore, by exchanging the electrolyte solvent for propylene carbonate, the thickness of the SEI increased, and by replacing the LiPF6 salt for LiBF4 the stability of the SEI improved. Thus is it demonstrated that such a passivation can be beneficial for the long-term surface stability of the electrode. These findings can therefore help prolong the lifetime of LTO-based battery chemistries.

Low temperature sintering has become a very important research area in ceramics processing and sintering as a promising process to obtain grain size below 100nm. For electronic ceramics, low temperature sintering is particularly difficult, because not only the required microstructure but also the desired electronic properties should be obtained. In this dissertation, the effect of liquid sintering aids and particle size (micrometer and nanometer) on sintering temperature and Positive Temperature Coefficient Resistivity (PTCR) property are investigated for Ba1-xSrxTiO3 (BST) doped with 0.2-0.3mol% Sb3+ (x = 0.1,0.2,0.3,0.4 and 0.5). Different sintering aids with low melting point are used as sintering aids to decrease the sintering temperature for micrometer size BST particles. Micrometer size and nanometer size Ba1-xSrxTiO3 (BST) particles are used to demonstrate the particle size effect on the sintering temperature for semiconducting BST. To reduce the sintering temperature, three processes are developed, i.e. 1 using sol-gel nanometer size Sb3+ doped powders with a sintering aid; 2 using micrometer size powders plus a sintering aid; and 3 using nanometer size Sb3+ doped powders with sintering aids. Grain size effect on PTCR characteristics is investigated through comparison between micrometer size powder sintered pellets and nanometer size powder sintered pellets. The former has lower resistivity at temperatures below the Curie temperature (Tc) and high resistivity at temperatures above the Curie temperature (Tc) along with higher ñmax/ñmin ratio (ñmax is the highest resistivity at temperatures above Tc, ñmin is the lowest resistivity at temperatures below Tc), whereas the latter has both higher ñmax and ñmin. Also, ñmax/ñmin is smaller than that of pellets with larger grain size. The reason is that the solid with small grain size has more grain boundaries than the solid with large grain size. The contribution z at room temperature and high temperature and a lower ñmax/ñmin ratio value.

Dans le contexte énergétique actuel, les piles à combustible à oxyde solide sont très prometteuses comme source d’énergie alternative pour la production d’électricité. Quelques verrous restent cependant à lever pour améliorer leur durabilité, notamment en termes de matériaux d’électrode. Ce travail de thèse s’est intéressé à la caractérisation de nouveaux matériaux d’anode et de cathode. La substitution partielle du titane par du vanadium dans le titanate de lanthane La4Ti2O10 de structure cuspidine a permis de conduire à des matériaux d’anode prometteurs. Des résistances spécifiques surfaciques (ASR, Area Specific Resistance) de l’ordre de 0,2 W.cm2 ont été obtenues à 750°C sous hydrogène. Au niveau de la cathode, nos recherches ont porté sur deux types de cobaltites : une pérovskite de formulation Ba1-xCo0,9Fe0,2Nb0,1O3-d avec x = 0 et 0,1 et un matériau innovant Ba2Co9O14. Dans les deux cas, après optimisation de la microstructure des électrodes, les ASR sont inférieure à 0,1 W.cm2 à 700°C. / In the current energy context, solid oxide fuel cells hold great promise as an alternative energy source for electricity generation. However, bottlenecks remain to improve their sustainability, particularly in terms of electrode materials. This work focused on the characterization of new anode and cathode materials. The partial substitution of titanium by vanadium in the lanthanum titanate La4Ti2O10 of cuspidine structure has led to promising anode materials with Aera Specific Resistance (ASR) of the order of 0.2 W.cm2 at 750 ° C under hydrogen. At the cathode, our research has focused on two types of cobaltites: a perovskite Ba1-xCo0,9Fe0,2Nb0,1O3-d with x = 0 and 0.1 and an innovative material Ba2Co9O14. In either case, after optimization of the microstructure of the electrodes, ASR less than 0.1 W.cm2 at 700 ° C were obtained.