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The high ionic conductivity in lithium lanthanum titanate perovskite ceramics, Li3xLa(2/3)-xTiO3 (LLTO), is well-known for the x ≈ 0.11 lithium concentration. The grain conductivity is approximatel...

Made available in DSpace on 2021-06-25T10:11:57Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-01-15 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) The effects of Yttrium-doped Barium Zirconate (BZY) synthesis steps by solid-state reaction and the number of starting reactants (2 or 3) on the characteristics of the synthesized ceramic powders were investigated. The slow diffusion rate associated with this synthesis route and the calcination steps used in this work lead to an incomplete synthesis reaction. This condition proved to be useful to improve densification during sintering for undoped samples by emulating the reactive sintering technique with transient reactions originating from the residual BaCO3 reactant. The effects of a low concentration (1 mol%) of ZnO as sintering aid on microstructure and electrical conductivity were also investigated and used for comparison to undoped samples. Electrical properties were analyzed and correlated with microstructural and compositional characteristics. Dense undoped BZY samples (99% of relative density) with high proton conductivity (2.4×10−2S/cm at 600 °C) were obtained by sintering at 1500 °C, while ZnO-doped samples achieved high relative density (97%) at only 1300 °C, but with lower electrical conductivity (7.8×10−3 S/cm at 600 °C). Graduate Program in Materials Science and Engineering Department of Materials Engineering Federal University of São Carlos (UFSCar) São Paulo State University (UNESP) School of Sciences and Engineering São Paulo State University (UNESP) School of Sciences and Engineering CAPES: 001 CAPES: 88882.332711/2019–01

Made available in DSpace on 2020-12-12T01:14:02Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-06-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) The effects of powder characteristics and conformation methods on green microstructure have been investigated, and microstructural evolution was analyzed based on the green density and the different sintering techniques. Colloidal processing of nanopowders with narrow particle size distribution (15 ± 5 nm) can be more problematic than dry processing and can lead to more heterogeneous microstructures. Two-step sintering can be used to obtain dense nanostructured samples with more flexibility than microwave sintering and still at lower temperatures than conventional sintering. The sinterability of Ce0.8Gd0.2O1.9 is significantly improved by Zn addition, which can reduce sintering temperature to values as low as 800, 1150 and 1200 °C for two-step, microwave and conventional sintering, respectively, in order to achieve relative densities above 90%. Microscopic grain boundary conductivity is shown to be improved in nanostructured samples, despite the higher grain boundary density. São Paulo State University (UNESP) School of Sciences and Engineering Graduate Program in Materials Science and Engineering Department of Materials Engineering Federal University of São Carlos São Paulo State University (UNESP) School of Sciences and Engineering

Materiais com estrutura do tipo perovskita tem sido extensamente investigados na tentativa de encontrar novos eletrolitos para Celulas a Combustivel de Oxido Solido que operem em baixas temperaturas. Dentre esses materiais destaca-se o LaAlO3 que, quando apropriadamente dopado, apresenta condutividade ionica consideravel. Neste trabalho LaAlO3 puro e dopado com Sr, Ba e Ca foram estudados. O metodo utilizado para preparacao dos pos foi o de reacao no estado solido. Analises microestrutural, estrutural e eletrica das amostras sinterizadas foram realizadas por microscopia eletronica de varredura, difracao de raios X e espectroscopia de impedância, respectivamente. Entre os diferentes tipos de dopantes, a amostra dopada com Sr foi a que apresentou maior condutividade, tanto do grao ( a 800°C) como total ( a 800°C). Em todas as amostras dopadas, a condutividade total e controlada pelo contorno de grao e as microestruturas sao heterogeneas e o grau de heterogeneidade depende do dopante. Palavras-chave: eletrolito, SOFC, perovskita, aluminato de lantânio.

Yttria Stabilized Zirconia (YSZ) high chemical and thermomechanical stability under Solid Oxide Fuel Cells (SOFCs) standard operational conditions and its high electrolytic domain indicate that an interesting alternative for lowering SOFCs operating temperature would be the development of high conductive YSZ-based electrolytes compared to conventional electrolytes through microstructure engineering. In this work, YSZ/Al2O3 micrometric multilayer films obtained by spin coating presented electrical conductivity 3.5 times higher than traditional YSZ ceramics. The multilayer film was formed by three layers of YSZ with 3.7 µm of thickness each were intercalated by two 7.4 µm thick layers of Al2O3. Therefore, the high electrical conductivity observed in nanometric multilayer films obtained by advanced techniques was achieved, even with lower magnitude, with ceramic suspensions and in the micrometric scale. Computer simulation shows higher stresses on YSZ than on Al2O3 layers, correlating the heterofilms higher conductivity with the stress imposed on the YSZ layer during sintering.

Tape casting is an established forming technique for several industries, however, researches focus more on slurry composition. In this work, the combined use of design of experiment and materials characterization techniques showed tape casting process parameters have great influence on the microstructure and mechanical properties of green tapes. Formulation and processing optimization allowed obtaining YSZ green tapes with good mechanical characteristics and homogeneous microstructure without laminating step. The optimized sintering schedule and sintering load allowed obtaining planar electrolytes with high density, tensile strength, and electrical conductivity. This work provides an environmental friendly procedure for large-scale production of SOFCs planar electrolytes.

Made available in DSpace on 2018-12-11T17:06:13Z (GMT). No. of bitstreams: 0 Previous issue date: 2016-09-01. Added 1 bitstream(s) on 2021-07-15T15:02:57Z : No. of bitstreams: 1 S1516-14392016000501057.pdf: 1441816 bytes, checksum: b43599be8b24b67a6482589c1cbf547b (MD5) In this work, cobalt and zinc-doped Ce0.8Gd0.2O1.9 samples were prepared starting from a commercial nanopowder and compared to the undoped material. The powder samples were pressed and afterwards sintered by a two-step procedure, before characterization by X-Ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Impedance Spectroscopy (IS) in air. Cobalt or zinc additions are effective as sintering aid, allowing peak sintering temperatures around 1000°C to reach densifications above of 93% of theoretical density, showing no evidence for the presence of secondary phases. The total conductivity at 800 °C of pressed Zn-doped samples (6.7x10-2 S/cm) and Co-doped samples (7.5x10-2 S/cm) is similar for undoped samples (7.2x10-2 S/cm) showing that Zn and Co has a positive effect on densification without compromising the electrical conductivity. Department of Materials Engineering Federal University of São Carlos - UFSCAR Department of Applied Physics Campinas Institute of Physics State University of Campinas - UNICAMP Department of Biosystems Engineering São Paulo State University - UNESP Tupã Campus Department of Biosystems Engineering São Paulo State University - UNESP Tupã Campus

MgAl2O4 is a humidity sensing material, which high melting point requires high sintering temperatures. In this work, manganese oxide was used as a sintering aid to obtain MgAl2O4 samples sintered at 1350°C with a microstructure suitable for humidity sensing. Impedance spectroscopy analyses were performed at several levels of relative humidity, indicating that this correlation is possible within the range of 10-80% of relative humidity.

GDC-YSZ bi-layer SOFC electrolytes are an interesting manner to protect GDC electrolytes of the low oxygen partial pressure present at anode-electrolyte interface. This oxygen partial pressure condition and high temperatures during the operation of SOFC permit GDC electrolyte reduction injuring the performance of SOFC. In this work, GDC-YSZ bi-layer electrolytes were prepared using double tape casting technique. Single layers of YSZ and GDC electrolytes were also prepared by the same technique for comparison. All samples were sintered at 1600 °C/2 h in air and analyzed by SEM and impedance spectroscopy technique. A good adhesion was established between the layer GDC and YSZ and no cracks were detected despite the differential firing shrinkage. The electrical conductivity of GDC-YSZ bi-layer sample at 350 °C was 7.98x10-5 S.cm-1 and for GDC and YSZ single layer samples electrical conductivities were 1.89x10-4 S.cm-1 and 4.71x10-5 S.cm-1, respectively. These results show that GDC-YSZ bi-layer electrolyte is superior to YSZ single layer electrolyte.

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.

A condutividade elétrica de CeO2-10 mol% Gd2O3 co-dopado com x mol% de Sm2O3 (0 < x < 2) foi investigada. O aumento da concentração total de dopantes com a co-dopagem não promoveu variação significativa do parâmetro de rede e do tamanho médio de grão. A adição de até 1,0% em mol de co-dopante favoreceu a condutividade elétrica do grão e acima deste teor ocorreu decréscimo, apesar de o parâmetro de rede e o tamanho médio de grão se manterem constantes. A condutividade elétrica do contorno de grão teve um aumento sistemático com a adição do co-dopante, indicando a influência da co-dopagem na camada de carga espacial. 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.

Adding ZnO reduces sintering temperature of yttria stabilized zirconia. Adding up to 0.5 wt% of ZnO is possible to densify to 8 mol% yttria stabilized zirconia (TZ8Y) to 95% of relative density at 1300 °C, besides, the electrical conductivity increases about 30% at 800 °C when compared to pure TZ8Y with the same relative density and average grain size. These results show that TZ8Y co-doped with ZnO can be a potential electrolyte to solid oxide fuel cells and electrolyzer cells.

Sixteen different thermal profiles were used to exploit the role of thermal treatment on the structure, microstructure and electrical conductivity of yttrium- and gadolinium-doped ceria electrolytes. Samples were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and impedance spectroscopy in air. The bulk and grain boundary conductivity dependence on grain size and thermal treatment deviated from usual trends. Consistent changes in the fluorite lattice parameter with thermal history, reported here for the first time, reveal a tendency for solid solution demixing. Formation of localized concentrations of dopant cations (both in bulk and grain boundaries), explains the apparently abnormal electrical performance of these materials after prolonged thermal treatments.

Impedance spectroscopy analysis was performed under different oxygen pressures, on dense YSZ/ZnO composites with a ZnO content varying from 3 to 25 vol% and two concentrations, 8 and 10 mol% of Y2O3 in YSZ. Raman spectroscopy showed that the cubic zirconia phase containing 8 mol% Y2O3 is not stable in contact with ZnO, under the selected sintering conditions (1300°C, 2 h), in contrast to that containing 10%. The impedance diagrams reveal a peculiar blocking effect associated with the ZnO grains. Its magnitude is quasi independent of the large variations of the ZnO conductivity, associated with changes in the equilibrium oxygen pressure. On the other hand, its relaxation frequency markedly depends on the non-stoichiometry of this compound. These peculiar properties are interpreted in terms of a pseudo-capacitive behavior of the electronically conducting ZnO in contact with the YSZ ionic conductor. This contact forms an electrode, the potential of which being determined by the non-stoichiometry of ZnO.

Neste trabalho foram quantificados o tamanho de grão e a porosidade de uma amostra de zircônia estabilizada com ítria (ZEI) sinterizada a 1500 ºC/ 2 h e a fração volumétrica das fases ZEI, NiO e poros de um compósito de ZEI/NiO sinterizado a 1500 ºC/ 2 h. Utilizando o programa de análise de imagens ImageJ foi possível tratar as imagens removendo ruídos e evidenciando as regiões de interesse de análise. Após as imagens serem analisadas, foi feito o tratamento estatístico dos dados como sugerido nas normas ASTM E 562-02 e ASTM E 1382-97. As frações volumétricas obtidas desta maneira foram 50 ± 7% de NiO, 32 ± 2% ZEI e 17 ± 7% poros. O tamanho médio de grãos 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 método baseado no princípio de Arquimedes. A diferença entre os valores permitiu por este método quantificar somente a porosidade aberta, enquanto a análise de imagens quantifica também os poros fechados.

Yttrium and gadolinium-doped ceria-based electrolytes (20 at% dopant cation) with and without small Ga2O3-additions (0.5 mol%) were fired at peak temperatures of 1250 and 1300 °C, or following a two-step sintering profile including one peak temperature and subsequent dwell at 1150 °C, 10 h. All materials were characterized by scanning electron microscopy, X-ray diffraction and impedance spectroscopy in air, in the temperature range 200–800 °C. Average grain sizes in the range 150–250 nm and densifications up to about 94% were found dependent on the sintering profile and presence of Ga. The grain boundary arcs in the impedance spectra increased significantly with Ga-doping, cancelling the apparently positive role of Ga on bulk transport, evidenced mostly in the case of yttrium-doped materials.

Samaria-doped ceria-based composites with a 2:1 addition of Li and Na carbonates (or simple Na carbonate as admixture) were prepared mixing nanosized powders of the ceramic phase with the remaining constituents. Samples fired at relatively low temperatures (below 700 °C) were characterized by X-ray diffraction, scanning electron microscopy combined with energy dispersive spectroscopy and impedance spectroscopy in air. These composites showed a complex but homogeneous distribution of both phases, with one ceramic skeleton of bonded nanosized grains surrounded by the carbonate-based phase. Impedance spectroscopy data was used to confirm the impressive electrical conductivity of these materials, but also to put into evidence the complex nature of the charge transport process, clearly deviating from classical electrolytes.

Yttria–zirconia doped ceria, 10% ZrO 2 –10% Y 2 O 3 –CeO 2 (mol%) (CZY) and 0.5 mol% alumina-doped CZY (CZYA), prepared through oxide mixture process, were sintered by isothermal sintering (IS) and two-step sintering (TSS) having as variable the temperature and soaking time. The electrical conductivity of sintered samples was investigated in the 250 to 600 °C temperature range by impedance spectroscopy in air atmosphere. The microstructure was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Alumina, as additive, improves the grain boundary conductivity of samples sintered at temperatures lower than 1500 °C. Concerning the sintering mode, two-step sintering (TSS) proved to be a good procedure to obtain CZYA samples with high electrical conductivity and density (> 95%) at relatively low sintering temperature and long soaking time.

Placas cerâmicas densas, com espessuras variando entre 10 µm e ~ 1 mm, podem ser obtidas pela técnica de colagem fita que é uma técnica de conformação cerâmica, de custo relativamente baixo. A etapa mais crítica deste processo de conformação é a preparação de suspensões com elevado teor de sólidos, ~ 30% em massa, e, também, elevado teor de aditivos orgânicos, tais como ligante e plastificante. Dentre os vários produtos cerâmicos que podem ser produzidos pela técnica de colagem de fita, substratos eletrônicos e eletrólitos planares para pilhas a combustível de óxido sólido (PaCOS) destacam-se pelo elevado consumo e pelo valor agregado, respectivamente. Neste trabalho, a quantidade apropriada de aditivos em suspensões de alumina e suspensões de zircônia estabilizada com ítria (8 e 10% em mol) foi determinada através da avaliação do comportamento reológico das suspensões. Embora exista semelhança no tamanho médio de partículas das matérias primas, foi determinado que cada matéria prima necessita de quantidade específica de aditivos orgânicos para conferir às suspensões o comportamento pseudoplástico. Placas obtidas pela colagem das suspensões foram analisadas antes e após a sinterização. Os resultados obtidos mostram que a densidade, o tamanho de grão e a distribuição de poros são sensíveis ao teor de ligante. O tamanho de grão das placas é menor que os grãos desenvolvidos em peças obtidas por prensagem e, por conseqüência, a incidência de poros intragranulares também é muito menor.

A alta temperatura de operação da PaCOS pode causar a formação de fases resistivas isolantes na interface eletrólito-catodo o que contribui para a perda de eficiência da pilha, podendo-se citar como exemplo a formação de zirconatos na interface manganita de lantânio dopada com estrôncio (LSM) - zircônia estabilizada com ítria (ZEI). Outro aspecto importante em PaCOS é o uso de material de eletrólito com uma condutividade iônica maior do que a da zircônia em temperaturas mais baixas. Essa característica é importante para a redução da temperatura de operação da PaCOS minimizando os problemas relacionados com a alta temperatura de operação, sendo Céria dopada com ítria (CY) um exemplo desse tipo de material. Por isso, para o uso de CY como eletrólito em PaCOS é importante avaliar a possibilidade de formação de fase isolante envolvendo esse material e o material do catodo, como perovskitas de lantânio. Para avaliar possíveis reações entre céria dopada com ítria e dois tipos de perovskitas usadas como catodo, LSM and LSCF, foi realizado, neste trabalho, a dopagem de CY com dois conjuntos de íons: La, Sr, Mn e La, Sr, Fe, Co. A dopagem provocou aumento da condutividade elétrica do grão enquanto que a condutividade efetiva do contorno de grão decresceu. Contudo a condutividade total não foi comprometida com a dopagem. A dopagem com La, Mn e Sr forneceu amostra de Céria dopada com Itria mais estáveis em alta temperatura e atmosfera redutora.

Yttria-doped ceria ceramics were prepared and reduced in an oxygen-deficient (argon) ambient. Electrical characterization through impedance spectroscopy revealed ionic-type conduction processes in as-sintered samples, with grain–bulk and grain–boundary activation energies (H) of about 1.00 eV and 1.05 eV, respectively. Electrical results from the reduced materials showed a predominant electronic-type, relatively high conductivity for the grains (H=0.52 eV), in contrast to a still ionic-like, relatively poor conductivity for the grain boundaries (H=0.95 eV). With the support of the results processed after re-oxidizing the materials in air combined with information from literature, this apparently contradictory behavior is discussed in terms of electron trapping at (Ce3+: $V_{\text{O}^{..}}$ )-type defect complexes. The overall work strongly supports the idea that surfaces (e.g., grain boundaries) in polycrystalline ceria are indeed much more sensitive to redox interactions than lattice.

Strontium-doped lanthanum manganite (LSM) has good properties that qualify it for cathode application in solid oxide fuel cells (SOFCs). Although LSM and YSZ are thermally compatible, composite cathodes made from these materials have been used instead of pure LSM. In this work, the electrical conductivity of La 0.7 Sr 0.3 MnO 3 (LSM), Ce 0.8 Y 0.2 O 2 (CY) and LSM– x vol.% CY ( x = 30 and 70) composites was investigated. The electrical conductivity of the samples was measured by the dc four-probe technique, using Pt electrodes and YSZ electron-blocking electrodes. Measurements were taken in a temperature range of 250–800 °C in air. The samples were also characterized by XRD and SEM. The use of two types of electrodes allowed for an evaluation of the ionic and electronic conductivity of LSM, found to be 6.3 × 10 −4 and 1.1 × 10 2 S cm −1 , respectively, at 800 °C. CY showed only ionic conduction of 7.2 × 10 −2 S cm −1 at 800 °C.

This work consisted of the preparation of pure and ZnO-doped tin titanate zirconate (ZTS) by oxide mixture. Samples were sintered at temperatures ranging from 1200 to 1450 °C. An analysis was made of the influence of sintering on the microstructural development and dielectric properties of these materials at high frequencies. The liquid phase composition present during sintering was shown to be dependent on the ZnO percentage. This percentage determines grain composition and, hence, dielectric performance at high frequencies. A maximum loaded quality factor, Q l , of 1645 was found for 1.0 wt% ZnO-doped ZTS sintered at 1400 °C.

Nucleation and growth of mullite whiskers in the La 2 O 3- Al 2 O 3- SiO 2 system were investigated in the 1500°-1700°C temperature range. A differential thermal analysis (DTA) showed that the mullitization temperature decreases from 1350°C to 1240°C as a result of lanthania doping. In the temperature range of 1250°-1500°C, most of the mullite grains have an Al 2 O 3 /SiO 2 = 1.5 composition throughout the ceramic body; however, from 1400°C upward, the number of anisotropic grains with the Al 2 O 3 /SiO 2 = 1.3 composition begins to increase. The concentration of alumina in the composition of the grain-boundary phase decreases as firing temperatures increase. At temperatures > 1500°C, alumina grains and whiskers grow on the internal and external surfaces of the ceramic body with the characteristic Al 2 O 3 /SiO 2 = 1.3 composition. Removal of the mullite whisker layer by acid attack revealed an alumina-rich, rosace-like patterned microstructure correlated with the process of whisker nucleation and growth. In the early stages, whisker growth rates were found to be near 60 μm/h. Experimental evidence pointed to nucleation inside the thin glass layer on the external surface.

A microestrutura e o módulo de ruptura de porcelanas aluminosas foram analisados com o aumento da temperatura de sinterização. Foi determinado que quando o teor de alumina é elevado o processo de rearranjo das partículas durante a sinterização é inibido devido à formação do esqueleto de partículas de alumina as quais são insolúveis na fase líquida. O módulo de ruptura aumenta com a quantidade de fase cristalina sendo que as partículas de alumina contribuem de forma mais efetiva para este aumento do que os cristais de mulita.

Porcelanas são cerâmicas multifásicas produzidas a partir de matérias-primas naturais. O desenvolvimento da microestrutura dessas cerâmicas é dependente da composição, das características das matérias-primas e do processamento cerâmico. Devido à complexidade da porcelana, várias aspectos relacionados com a evolução da microestrutura e os seus efeitos nas propriedades não são ainda bem entendidos, apesar do grande número de publicações sobre este assunto. A parte I deste artigo é uma revisão da literatura sobre porcelanas aluminosas e enfoca o efeito das composições nas propriedades elétricas e mecânicas, como também a influência de alguns aditivos no desenvolvimento da microestrutura.

The local structure around Zr, Ce and dopant atoms (Fe and Ni) in the ZrO2–CeO2 system investigated by X-ray absorption spectroscopy (XAS) is reported to better understand the tetragonal phase stabilization process of zirconia. The first coordination shell around Zr atoms is not sensitive to the introduction of dopants or to an increase in the ceria content (from 12 to 20 mol%). Ce ions maintain the eight-fold coordination as in CeO2, but with an altered bond distance. The formation of vacancies resulting from reduction of Ce atoms can be discarded, because XANES spectra clearly show that Ce ions are preferentially in a tetravalent state. XANES and EXAFS experiments at the Fe K-edge evidence that the local order around Fe is quite different from that of the Fe2O3 oxide. On the one hand, ab initio EXAFS calculations show that iron atoms form a solid solution with tetragonal ZrO2. The EXAFS simulation of the first coordination shell around iron evidences that the substitution of zirconium atoms by iron ones generates oxygen vacancies into the tetragonal network. This is a driven force for the tetragonal phase stabilization process. For Ni doped samples, EXAFS results show that Ni–O mean bond length is similar to the distance found in the oxide material, i.e., NiO compound. Besides this result, no evidence of similar solid solution formation for Ni-doped systems has emerged from the EXAFS analysis.

Sintered zirconia–alumina composites, prepared in a wide range of compositions, are studied in terms of their electrical response. Both grain conductivity and dielectric constant show the typical characteristics expected from the percolation theory, with v c =0.14±0.2 as the critical zirconia volume fraction for the onset of conduction. When the conducting zirconia phase is calcined prior to forming the composite, the whole system still shows a strongly reduced conduction response even for zirconia volume fractions ( v ) in the range of 0.4–0.5, after which it is considerably enhanced for v =0.7. These results are discussed in terms of (i) the influence of the material's microstructure and (ii) the effect of stress resulting from the alumina sintering on the calcined zirconia grains on the overall electrical response of the composite.

Mn K-edge X-ray absorption spectroscopy was used to determine the local structure at the Mn site in LaMnO 3 , La 0.7 Sr 0.3 MnO 3 and La 0.5 Sr 0.5 MnO 3 samples. The X-ray absorption near-edge spectra (XANES) show an intermediate oxidation state of the Mn atoms, the edge position indicating an average valence of ∼3.4 for the LaMnO 3 sample and an average valence of ∼3.7 for the La 0.7 Sr 0.3 MnO 3 and La 0.5 Sr 0.5 MnO 3 samples. Extended X-ray absorption fine structure (EXAFS) analyses show that the local order around Mn atoms in the La 0.7 Sr 0.3 MnO 3 sample can be described as a regular MnO 6 octahedron or as a distorted octahedron with two different Mn-O bond lengths. With regard to the La 0.5 Sr 0.5 MnO 3 sample, the local structure around Mn atoms is appropriately described by a regular MnO 6 octahedron. For this sample, the mean Mn-O bond length is shorter and is similar to that of Mn-O in the SrMnO 3 .

Células a combustível de óxido sólido é uma tecnologia promissora de geração de energia limpa e utilizam zircônia estabilizada com ítria como eletrólito. Devido à alta temperatura de operação convencional, a busca por novos materiais de eletrólito tem sido feita e apontam a céria como um substituto. Entretanto, pós micrométricos de céria devem ser sinterizados a alta temperatura gerando fases secundárias que inibem a condutividade elétrica. Neste trabalho, amostras de nanopós de Ce0,8Gd0,2O1,9 puro e co-dopado com óxido de cobalto foram sinterizadas pelo processo em duas etapas. O uso de Co3O4 como aditivo de sinterização se mostrou efetivo na densificação (96,8% da densidade teórica). No entanto, a co-dopagem foi deletéria para a condutividade elétrica dos contornos de grão, embora tenha favorecido a condutividade do grão em comparação com amostras não co-dopadas. Solid oxide fuel cells are one of the most promising technologies for energy generation, are mainly based on designs that employ yttria-stabilized zirconia (YSZ) as electrolyte. Efforts have been done to find new electrolyte materials with higher electrical conductivity, lowering the SOFC operating temperature. Ceria based electrolytes have received great attention as a substitute for YSZ. However, ceria-based ceramics prepared with micrometric powders must be sintered at high temperatures yielding secondary phases, which are detrimental to electrical conduction. In this work, pure and cobalt oxide doped samples were prepared from Ce0.8Gd0.2O1.9 nanopowder and sintered through two-step method. Cobalt oxide addition was effective in densification (96.8% of theoretical density), however, it was deleterious to grain boundary conductivity although it has improved grain conductivity comparing to Co-free samples.

Gd-doped ceria (CGO) nanopowders were mixed with up to 1 mol% of Zn (as nitrate) to exploit the role of this dopant on sintering and electrical performance. All samples were prepared by two-step sintering schedules to try to combine the advantages of both effects on low-temperature densification. Structural and microstructural analyses were complemented with impedance spectroscopy (wide range of temperatures and oxygen partial pressures) and ion-blocking measurements. The combination of all data suggests that Zn is preferentially located in the grain boundary region, playing a significant role on sintering. Zn additions (I mol%) allowed for a 100 degrees C lower sintering temperature with respect to pure CGO. A marginal effect on the oxide-ion conductivity and a small enhancement (less than 20%) in n-type electronic conductivity was found between Zn-free and 1 mol% Zn-doped CGO. The removal of Zn from CGO after sintering is discussed. (C) 2013 Elsevier B.V. All rights reserved.

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.

Mullite whiskers and anisotropic grains that were derived from erbia-doped aluminum hydroxide–silica gel were studied. Firing 3.0-mol%-erbia-doped isostatically pressed pellets at 1600°C for 1.0–8.0 h resulted in a high surface concentration of mullite whiskers. Their c-axes were aligned preferentially along the pellet surface; the maximum length was 50 μm, and the maximum aspect ratio was 23. The pellet surface was fully covered by mullite whiskers, and small anisotropic grains with a low aspect ratio were observed in the bulk. The voids that were observed in the fracture surfaces were covered fully by mullite whiskers. The large number of voids resulted in an apparent density of 1.60 g/cm3 in the sintered pellets. The molar ratio of alumina to silica in the whiskers was in the range of 1.30–1.45 (an average value of 1.31), regardless of whether the alumina/silica powder compositions were mixed in a 3:2 or 2:1 ratio.

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.

The Al2O3-ZrO2 composite was studied by impedance spectroscopy, a non destructive technique that was found to be sensitive to the composite's microstructure. The observed decrease in the zirconia grain and grain boundary conductivities points to compression on zirconia grain by alumina matrix. This effect increased with decreased concentration of zirconia in the composite. Measurements were taken of composites above the percolation threshold for vacancy conduction along the zirconia grains. The effect of densification and grain growth on the composite's conduction was measured. The changes in the zirconia grain and grain boundary specific conductivities were found to be correlated.

The addition of iron oxide on ceria-stabilised zirconia was studied by X-ray photoelectron spectroscopy (XPS). Zirconia presents a monoclinic to tetragonal phase transition at 1443 K, which is accompanied by a volume variation of approximately 3–5%, preventing the formation of a dense polycrystalline monoclinic ceramic. Ceria-stabilised tetragonal zirconia presents high toughness and can be applied as a structural material. However, CeO 2 –ZrO 2 has low sinterability, so it is important to investigate the effect of sintering dopants, such as iron, copper and manganese ions, which could improve the sinterability and the mechanical properties of the ceramic. In previous studies, it was shown that the addition of 0.3 mol% of Fe 2 O 3 helped in sintering the ceramic, and enhanced the electrical conductivity, although it was not determined if this enhancement was due to ionic or electronic contribution. In this work, we employed XPS to characterise ZrO 2 + x mol% CeO 2 +0.3 mol% Fe 2 O 3 , x =12 and 20, ceramics sintered at 1450 and 1600°C in order to better understand the influence of iron in the stabilisation of the tetragonal phase and electrical conductivity.

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.

Expelling of the liquid phase during sintering of ZrO2-6.5 mol% Y2O3- 0.5 mol% Pr2O3 ceramic was observed as a result of grain coarsening. ZrO2- 7.0 mol% Y2O3 samples, without Pr2O3 addition, do not show this effect under the same sintering conditions. The expelling process is caused by surface tension forces and attracting van der Waals forces between the grains, coupled with the existence of two glass phases on the grain boundaries. The amount of expelled glass phase increases with grain growth, but saturates above 16 mm average grain size.

Esta terceira e última parte da revisão sobre os aspectos reológicos e de estabilização de suspensões com pós cerâmicos vem reunir a aplicação dos conhecimentos adquiridos nas primeiras duas partes publicadas anteriormente. Aqui, os fenômenos eletrostático devido à dupla camada elétrica, e estérico, relacionado à adsorção de moléculas poliméricas, são combinados para explicar o mecanismo eletroestérico de estabilização de suspensões cerâmicas. Os defloculantes que atuam através desse mecanismo abrangem uma classe específica de polímero denominada polieletrólitos, a qual é constituída por macromoléculas ionizáveis quando em solução. O estudo da forma com que os polieletrólitos atuam justifica-se devido à larga utilização desta classe de polímeros na indústria cerâmica. Os ácidos poliacrílico (PAA) e polimetacrílico (PMAA) são exemplos de polieletrólitos amplamente utilizados no processo de materiais à base de alumina. Dá-se destaque à influência do pH do meio e da presença de íons, sendo novamente aqui importante o conceito de força iônica da suspensão. Como aplicação prática, apresenta-se a estabilidade do sistema alumina-PMAA, reportando-se sobre o comportamento da viscosidade e da efetiva defloculação da suspensão. Este estudo é concluído apresentando resultados do efeito do peso molecular sobre a viscosidade, chamando atenção para o fato de que não basta definir apenas a classe de polímero a ser usada, sendo também fundamental especificar o peso molecular médio do polímero selecionado.

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.

Neste trabalho é abordada a escolha da pasta de eletrodo para realização da caracterização elétrica de amostras de BaCeO3 dopado com ítrio e contendo ZnO como aditivo de sinterização. A sinterização ocorreu via fase líquida. Foi utilizada uma pasta de eletrodo de platina e outra de prata cujas temperaturas de cura são 1100 °C e 750 °C, respectivamente. A análise das interfaces amostra-eletrodo por microscopia eletrônica de varredura revelou que durante a cura da pasta de platina, devida a elevada temperatura, ocorreu exsudação de fase que permaneceu entre a superfície original da amostra e o eletrodo. Por conseqüência, durante a caracterização elétrica por espectroscopia de impedância foi detectada polarização adicional na interface amostra-eletrodo comprometendo a veracidade dos valores de resistividade elétrica do material em análise o que não ocorreu com o uso do eletrodo de prata devida sua menor temperatura de cura. This work deals with the choice of electrodes for realizing the electrical characterization of Yttrium doped-BaCeO3 sample which contains ZnO as sintering aid. The sintering occurred through liquid phase. It were used platinum and silver pastes which curing temperatures are 1100 °C and 750 °C, respectively. Scanning electron microscopy characterization of sample-electrode interfaces showed that during the curing of the platinum paste, due to the high temperature, occurred phase exudation which remained between the primary sample surface and electrode. Consequently, electrical characterization by impedance spectroscopy detected additional polarization at interface sample-electrode becoming non-veracious the electrical resistivity values of the material under analysis. No additional polarization was detected for silver electrodes due to the lowest curing temperature.