D. P. F. de Souza

Rice Hull-Derived Silica: Applications in Portland Cement and Mullite Whiskers

Silica was extracted from rice hulls by three processing routes, starting with acid treatment followed by burning and milling. The amorphous white silica powder showed a surface area of 260 to 480 m2/g, purity above 99% and average particle size of 2.0 to 0.6 mm. This silica is suitable for the preparation of mullite whiskers employing the rare earth aluminosilicate glass technique. Due to its highly pozzolanic reaction, this prepared silica is used as an additive in high performance concrete.

Impure zirconia electrical conductivity enhancement by rare-earth minority ions in the Y2O3 RE2O3 ZrO2 system

Impure zirconia stabilized by 12 wt% yttria concentrate (85 wt% Y2O3 + 15 wt% rare-earth (RE)) was found to have high grain and grain-boundary electrical conductivities. The influence of the RE on the segregation of impurities was studied for four different compositions. Microstructure features are evidence for the enhanced segregation of impurities due to RE ions. The increased grain and grain-boundary conductivities are a consequence of the segregation of impurities.

Liquid phase sintering of RE2O3: YSZ ceramics Part I Grain growth and expelling of the grain boundary glass phase

Experiments on silicate liquid phase sintering of YSZ ceramics with addition of 0.5 mol % of rare earth ions have been done in order to study the effect of these ions on the kinetics of grain growth and the expulsion of glass through the grain boundaries. Kinetics follow a third power law in the following order YPr > YPrEr > YY > YEr. The expelled glass does not spread over the ceramic grains and its mass is inversely related to grain size. Glass phase separation inside the grain boundaries is found to be a necessary condition for glass expulsion.Experiments on silicate liquid phase sintering of YSZ ceramics with addition of 0.5 mol % of rare earth ions have been done in order to study the effect of these ions on the kinetics of grain growth and the expulsion of glass through the grain boundaries. Kinetics follow a third power law in the following order YPr > YPrEr > YY > YEr. The expelled glass does not spread over the ceramic grains and its mass is inversely related to grain size. Glass phase separation inside the grain boundaries is found to be a necessary condition for glass expulsion.

Caracterização microestrutural de materiais cerâmicos utilizando o programa de processamento digital de imagens Image J (Microstructural characterization of ceramic materials using the image digital processing software Image J)

Neste trabalho foram quantificados o tamanho de grao e a porosidade de uma amostra de zirconia estabilizada com itria (ZEI) sinterizada a 1500 oC/ 2 h e a fracao volumetrica das fases ZEI, NiO e poros de um composito de ZEI/NiO sinterizado a 1500 oC/ 2 h. Utilizando o programa de analise de imagens ImageJ foi possivel tratar as imagens removendo ruidos e evidenciando as regioes de interesse de analise. Apos as imagens serem analisadas, foi feito o tratamento estatistico dos dados como sugerido nas normas ASTM E 562-02 e ASTM E 1382-97. As fracoes volumetricas obtidas desta maneira foram 50 ± 7% de NiO, 32 ± 2% ZEI e 17 ± 7% poros. O tamanho medio de graos da ZEI foi 4,6 ± 0,2 µm e sua porosidade 1,5 ± 0,2%. Os resultados de porosidade foram comparados com os valores obtidos utilizando o metodo baseado no principio de Arquimedes. A diferenca entre os valores permitiu por este metodo quantificar somente a porosidade aberta, enquanto a analise de imagens quantifica tambem os poros fechados.

Liquid phase sintering of Re2O3 YSZ ceramics: Part II Grain boundary electrical properties

Grain boundary electrical properties of Y2O3 stabilised zirconia with small additions of Er2O3 and Pr2O3 sintered via silicate liquid phase were studied by the impedance spectroscopy technique. Grain boundary specific conductivity of the praseodymium doped samples was found to be independent of sintering time, while the erbium doped sample showed high anomalous conductivity for the 1.0 h sintered samples. The electrical behaviour is explained considering the grain boundary to be a series association of the glass film and the space charge region. Specific conductivity and Debye length of the space charge region of erbium doped samples were found to be 6.7 × 10−8 S/cm and 0.25 nm, respectively.Grain boundary electrical properties of Y2O3 stabilised zirconia with small additions of Er2O3 and Pr2O3 sintered via silicate liquid phase were studied by the impedance spectroscopy technique. Grain boundary specific conductivity of the praseodymium doped samples was found to be independent of sintering time, while the erbium doped sample showed high anomalous conductivity for the 1.0 h sintered samples. The electrical behaviour is explained considering the grain boundary to be a series association of the glass film and the space charge region. Specific conductivity and Debye length of the space charge region of erbium doped samples were found to be 6.7 × 10−8 S/cm and 0.25 nm, respectively.

Porcelanas elétricas aluminosas: parte II - desenvolvimento da microestrutura e sua influência no módulo de ruptura

The microstructure and bending strength of aluminous porcelains were analyzed at rising sintering temperatures. It was found that, in the presence of high alumina content, the particle rearrangement process during sintering is inhibited by the formation of a skeleton of alumina particles that are insoluble in the liquid phase. The bending strength increases along with the amount of crystalline phase, with the alumina particles contributing more effectively to this increase than the mullite crystals.

Influence of rare earths on the sintering of zirconia-yttria

Yttria stabilized zirconia sintered by silicate glass phase with addition of ytterbium or gadolinium was investigated. The grain growth rate was found to be higher in the gadolinium doped samples. The glass phase of the ytterbium doped sample showed partial crystallization on cooling. Grain conductivity was found to be highest in samples with a half hour of sintering, having decreased due to stabilizing ion migration from the grains to the grain boundary glassy phase. The differences observed in the sintering behavior and properties of the sintered bodies are attributed to the different ionic radii of the Yb+3 and Gd+3 ions.

Aspectos da Reologia e da Estabilidade de Suspensões Cerâmicas. Parte III: Mecanismo de Estabilização Eletroestérica de Suspensões com Alumina

The third and last part of this review about stabilization and rheological aspects of ceramic suspension gathers the knowledge in the two parts previously published. Here, the electrostatic and steric phenomena, related to the electrical double layer and polymeric molecules adsorption, respectively, are combined to explain the electrosteric stabilization mechanism of ceramic suspensions. The deflocculants used to afford such dispersion mechanism cover a specific kind of polymer, named polyelectrolytes, which are macromolecules that are ionizable in solution. The study of how the polyelectrolytes behave is justified due to the large use of this polymeric product in the ceramic industry. Polyacrylic acid (PAA) and polymethacrylic acid (PMAA) are examples of polyelectrolytes that are widely used in the processing of alumina based materials. The medium pH and ion concentration are strengthened, and the importance of the ionic strength is emphasized. As a practical application, the stability of alumina-PMAA system is presented. The viscosity and dispersion behaviour of the suspension are also reported. This study is concluded presenting results concerning the effect of molecular weigth on the suspension viscosity, and shows that it is not enough to select a polymer considering just its class. It is fundamental to specify the average molecular weight of the selected polymer.

The effect of Pr co-doping on the densification and electrical properties of Sr-LaAlO3

Perovskite-type materials have been investigated extensively in the attempt to discover new electrolyte materials for solid oxide fuel cells (SOFC) operating at low temperatures. These materials include LaAlO3-based perovskite, which, when adequately doped, presents high ionic conductivity. However, despite the high conductivity of doped LaAlO3, two characteristics limit its application as solid electrolyte: low sinterability and p-type electronic conductivity in oxidizing conditions. The purpose of this work was to investigate the effect of Pr co-doping on the densification and electrical conductivity of Sr-doped LaAlO3. The addition of Pr eliminated the deficiencies mentioned above. Pr in solid solution acts as a perfect sintering aid because promoted densification without forming secondary phase which should be detrimental for electrical conduction. The addition of Pr also increased the bulk electrical conductivity measured in air atmosphere but had no effect at low oxygen partial pressure. However, the addition of Pr had a strong effect on the grain boundary, improving the ionic grain boundary electrical conductivity at air atmosphere which was attributed to the Pr influence on the space charge layer.

Condutividade elétrica de CeO2-10 mol% Gd2O3 - x mol% Sm2O3 (0 < x < 2)

This work reports the electrical conductivity of CeO2-10 mol% Gd2O3 co-doped with x mol% Sm2O3 (0 < x < 2). The lattice parameter and the average grain size were insensitive to co-doping. However, the addition up to 1.0 mol% of co-dopant increased the grain electrical conductivity and above this level a decrease was detected in spite of the lattice parameters and average grain size remained constant. The grain boundary electrical conductivity increased systematically with the increase of co-doping. This behavior has been associated to the influence of co-doping in the space charge layer.