Precursor scavenging of the resistive grain-boundary phase in 8 mol% yttria-stabilized zirconia: Effect of trace concentrations of SiO2

The influence that trace concentrations Of SiO2 have on improving grain-boundary conduction via precursor scavenging using additional heat treatment at 1200 degreesC for 40 h before sintering was investigated. At a SiO2-impurity level (SIL) less than or equal to 160 ppm by weight, the grain-boundary resistivity (p(gb)) decreased to 20% of its value, while no improvement in grain-boundary conduction was found at a SIL greater than or equal to 310 ppm. The correlation between the resistance per unit grain-boundary area, p(gb), and average grain size indicated that the inhomogeneous distribution of the siliceous phase in the sample with a SIL greater than or equal to 310 ppm. hampered the scavenging reaction.

Scavenging of siliceous grain-boundary phase of 8-mol%-ytterbia-stabilized zirconia without additive

The grain-boundary conductivity (sigma (gb),) of 8-mol%-ytterbiastabilized zirconia increased markedly with heat treatment between 1000 degrees and 1300 degreesC with a slow heating rate (0.1 degreesC/min) before sintering. The extent of the sigma (gb) improvement was the same or larger than that via Al2O3 addition. The heat treatment did not affect the grain-interior conduction when sintered at 1600 degreesC, while Al2O3-derived scavenging significantly did, given the larger increment of total conductivity in the heat-treated sample. The formation of a silicon-containing phase in a discrete form was suggested as a possible route of scavenging the resistive phase from the correlation between average grain size and sigma (gb).

Precursor scavenging of resistive grain-boundary phase in 8 mol % ytterbia-stabilized zirconia

The grain-boundary conduction of 8 mol % ytterbia-stabilized zirconia (8YbSZ) was improved markedly by precursor scavenging via the two-stage sintering process. The most significant increase in the grain-boundary conductivity was found when the sample, whose conductivity was higher than that via Al2O3-derived scavenging, was heat-treated at 1250degreesC for greater than or equal to 20 h. The formation of a stable Si-containing phase such as ZrSiO4 during the first-stage heat-treatment was suggested as one probable scavenging route from the optimal heat-treatment temperature (HTT), long duration time (>20 h) at HTT, and the stability of the formed phase up to sintering temperatures (1500degrees C). (C) 2002 The Electrochemical Society.

HRTEM study of orthorhombic zirconia in MgO-PSZ

High-resolution transmission electron microscopy (HRTEM) was used to study the phase of orthorhombic ZrO2 formed in magnesia partially stabilized zirconia (MgO-PSZ) during HRTEM specimen preparation. Based on the three reported crystal structures of orthorhombic ZrO2, with the space groups Pbcm, Pbc2(1) and Pbca, here it is shown that orthorhombic ZrO2 formed in MgO-PSZ has the Pbcm structure.

Synthesis and stabilization of nanocrystalline zirconia with MSU mesostructure

In the presence of nonionic block-copolymer surfactant, nanocrystalline zirconia particles with MSU mesostrucmre were synthesized by a novel solid-state reaction route. The zirconia particles possess a nanocrystalline pore wall, which renders higher thermal stability compared to an amorphous framework. To further enhance its stability, laponite, a synthetic clay, was introduced. Laponite acts as an inhibitor to crystal a growth and also as a hard template for the mesostructure. High surface area and ordered pore structure were observed in the stabilized zirconia. The results show that the formation of the MSU structure is attributed to reverse hexagonal micelles, which are the products of the cooperative self-assembly of organic and inorganic species in the solid-state synthesis system with crystalline water and hygroscopic water present.

Influence of nano-structure on electrolytic properties in CeO2 based system

Doped ceria (CeO2) compounds are fluorite type oxides that show oxygen ionic conductivity higher than yttria stabilized zirconia, in oxidizing atmosphere. In order to improve the conductivity, the effective index was suggested to maximize the oxygen ionic conductivity in doped CeO2 based oxides. In addition, the true microstructure of doped CeO2 was observed at atomic scale for conclusion of conduction mechanism. Doped CeO2 had small domains (10-50 nm) with ordered structure in a grain. It is found that the electrolytic properties strongly depended on the nano-structural feature at atomic scale in doped CeO2 electrolyte.

The martensitic transformation in ceramics - its role in transformation toughening

This paper reviews the current knowledge and understanding of martensitic transformations in ceramics - the tetragonal to monoclinic transformation in zirconia in particular. This martensitic transformation is the key to transformation toughening in zirconia ceramics. A very considerable body of experimental data on the characteristics of this transformation is now available. In addition, theoretical predictions can be made using the phenomenological theory of martensitic transformations. As the paper will illustrate, the phenomenological theory is capable of explaining all the reported microstructural and crystallographic features of the transformation in zirconia and in some other ceramic systems. Hence the theory, supported by experiment, can be used with considerable confidence to provide the quantitative data that is essential for developing a credible, comprehensive understanding of the transformation toughening process. A critical feature in transformation toughening is the shape strain that accompanies the transformation. This shape strain, or nucleation strain, determines whether or not the stress-induced martensitic transformation can occur at the tip of a potentially dangerous crack. If transformation does take place, then it is the net transformation strain left behind in the transformed region that provides toughening by hindering crack growth. The fracture mechanics based models for transformation toughening, therefore, depend on having a full understanding of the characteristics of the martensitic transformation and, in particular, on being able to specify both these strains. A review of the development of the models for transformation toughening shows that their refinement and improvement over the last couple of decades has been largely a result of the inclusion of more of the characteristics of the stress-induced martensitic transformation. The paper advances an improved model for the stress-induced martensitic transformation and the strains resulting from the transformation. This model, which separates the nucleation strain from the subsequent net transformation strain, is shown to be superior to any of the constitutive models currently available. (C) 2002 Elsevier Science Ltd. All rights reserved.

Electrolytic properties and nanostructural features in the La2O3-CeO2 system

Doped ceria (CeO2) compounds are fluorite-type oxides which show oxide ionic conductivity higher than yttria-stabilized zirconia in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in applications of these materials for low or intermediate temperature operation of solid-oxide fuel cells (SOFCs). In this study, the effective index was suggested to maximize the ionic conductivity in La2O3-CeO2 based oxides. The index considers the fluorite structure, and combines the expected oxygen vacancy level with the ionic radius mismatch between host and dopant cations. Using this approach, the ionic conductivity of this system has been optimized and tested under operating conditions of SOFCs. LaxCe1-xO2-delta (x = 0.125, 0.15, 0.175, and 0.20), (LaxSr1-x)(0.175)Ce0.825O2-delta (x = 0.1, 0.2, and 0.4), and (La1-xSr0.2Bax)(0.175)Ce0.825O2-delta (x 5 0.03, 0.05, and 0.07) were prepared and characterized as the specimens with low, intermediate, and high index, respectively. The ionic conductivity was increased with increasing suggested index. The transmission electron microscopy analysis suggested that partial substitution of alkaline earth elements in place of La into Ce site contributes to a decrease of microdomain size and an improvement of conductivity. (La0.75Sr0.2Ba0.05)(0.175)Ce0.825O1.891 with high index and small microdomains exhibited the highest conductivity, wide ionic domain, and good performance in SOFCs. (C) 2003 The Electrochemical Society.

Influence of particle morphology on nanostructural feature and conducting property in Sm-doped CeO2 sintered body

Doped ceria (CeO2,) compounds are fluorite type oxides, which show oxide ionic conductivity higher than yttria stabilized zirconia (YSZ), in oxidizing atmospheres. As a consequence of this, considerable interest has been shown in application of these materials for 'low (500-650 degreesC)' or 'intermediate (650-800 degreesC)' temperature operation, solid oxide fuel cells (SOFCs). In this study, the authors prepared two kinds of nanosize Sm-doped CeO2 particles with different morphologies: one type was round and the other was elongated. Processing these powders with different morphology produced dense materials with very different ionic conducting properties and different nanoscale microstructures. Since both particles are very fine and well dispersed, sintered bodies with high density (relative density >95% of theoretical) could be prepared using both types of powder particles. The electrical conductivity of sintered bodies prepared from these powders with different starting morphologies was very different. Materials prepared from particles having a round shape were much higher than those produced using powders with an elongated morphology. Measured activation energies of the corresponding sintered samples showed a similar trend; round particles (60 kJ/mol), elongated particles (74 kJ/mol). While X-ray diffraction (XRD) profiles of these sintered materials were identical, diffuse scatter was observed in the back.-round of selected area electron diffraction pattern recorded from both sintered bodies. This indicated an underlying structure that appeared to have been influenced by the processing technology. Detailed observation using high-resolution transmission electron microscopy (HR-TEM) revealed that the size of microdomain with ordering of cations in the sintered body made from round shape particles was much smaller than that of the sintered body made from elongated particles. Accordingly, it is concluded that the morphology of doped CeO2 powders strongly influenced the microdomain size and electrolytic properties in the doped CeO2 sintered body. (C) 2004 Elsevier B.V. All rights reserved.

Synthesis and characterization of nano-hetero-structured dy doped CeO2 solid electrolytes using a combination of spark plasma sintering and conventional sintering

Doped ceria (CeO2) compounds are fluorite-type oxides that show oxide ionic conductivity higher than yttria-stabilized zirconia in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in application of these materials for low (500 degrees-650 degrees C) temperature operation of solid oxide fuel cells (SOFCs). To improve the conductivity in dysprosium (Dy) doped CeO2, nano-size round shape particles were prepared using a coprecipitation method. The dense sintered bodies with small grain sizes (< 300 nm) were fabricated using a combined process of spark plasma sintering (SPS) and conventional sintering (CS). Dy-doped CeO2 sintered body with large grains (1.1 mu m) had large micro-domains. The conductivity in the sintered body was low (-3.2 S/cm at 500 degrees C). On the other hand, the conductivity in the specimens obtained by the combined process was considerably improved. The micro-domain size in the grain was minimized using the present process. It is concluded that the enhancement of conductivity in dense specimens produced by the combined process (SPS+CS) is attributable to the microstructural changes within the grains.

Design of high quality doped CeO2 solid electrolytes with nanohetero structure

Doped ceria (CeO2) compounds are fluorite related oxides which show oxide ionic conductivity higher than yttria-stabilized zirconia in oxidizing atmosphere. As a consequence of this, a considerable interest has been shown in application of these materials for low (400-650 degrees C) temperature operation of solid oxide fuel cells (SOFCs). In this paper, our experimental data about the influence of microstructure at the atomic level on electrochemical properties were reviewed in order to develop high quality doped CeO2 electrolytes in fuel cell applications. Using this data in the present paper, our original idea for a design of nanodomain structure in doped CeO2 electrolytes was suggested. The nanosized powders and dense sintered bodies of M doped CeO2 (M:Sm,Gd,La,Y,Yb, and Dy) compounds were fabricated. Also nanostiructural features in these specimens were introduced for conclusion of relationship between electrolytic properties and domain structure in doped CeO2. It is essential that the electrolytic properties in doped CeO2 solid electrolytes reflect in changes of microstructure even down to the atomic scale. Accordingly, a combined approach of nanostructure fabrication, electrical measurement and structure characterization was required to develop superior quality doped CeO2 electrolytes in the fuel cells.

Ba0.5Sr0.5Co0.8Fe0.2O3-delta ceramic hollow-fiber membranes for oxygen permeation

Setf-supported asymmetric hollow-fiber membranes of mixed oxygen-ionic and electronic conducting perovskite Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) were prepared by a combined phase-inversion and sintering technique. The starting inorganic powder was synthesized by combined EDTA-citrate complexing process followed by thermal treatment at 600 degrees C. The powder was dispersed in a polymer solution and then extruded into hollow-fiber precursors through a spinneret. ne fiber precursors were sintered at elevated temperatures to form gastight membranes, which were characterized by SEM and gas permeation tests. Performance of the hollow fibers in air separation was both experimentally and theoretically studied at various conditions. The results reveal that the oxygen permeation process was controlled by the slow oxygen surface exchange kinetics under the investigated conditions. The porous inner surface of the prepared perovskite hollow-fiber membranes considerably favored the oxygen permeation. The maximum oxygen flux measured was 0.031 mol-m(-2).s(-1) at 950 degrees C with the sweep gas flow rate of 0.522 mol(.)m(-2).s(-1). To improve the oxygen flux of BSCF perovskite membranes, future work should be focused on surface modification rather than reduction of the membrane thickness. (c) 2006 American Institute of Chemical Engineers.

Microstructures and electrolytic properties of yttrium-doped ceria electrolytes: Dopant concentration and grain size dependences

The microstructures and electrolytic properties of YxCe1-xO2-x/2 (x = 0.10-0.25) electrolytes with average grain size in the range 90 nm-1.7 mu m were systematically investigated. Through detailed transmission electron microscopy characterization, nanosized domains were observed. The relationship of the domains, the doping level and grain sizes were determined, and their impacts on the electrolytic properties were systematically studied. It was found that the formation of domains has a negative impact on the electrolytic properties, so that electrolytic properties can be adjusted through careful control of domain formation, doping level and grain size. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

A method to lower the detection limit for optical beat signals from a frequency-dithered stabilized laser

A narrow absorption feature in an atomic or molecular gas (such as iodine or methane) is used as the frequency reference in many stabilized lasers. As part of the stabilization scheme an optical frequency dither is applied to the laser. In optical heterodyne experiments, this dither is transferred to the RF beat signal, reducing the spectral power density and hence the signal to noise ratio over that in the absence of dither. We removed the dither by mixing the raw beat signal with a dithered local oscillator signal. When the dither waveform is matched to that of the reference laser the output signal from the mixer is rendered dither free. Application of this method to a Winters iodine-stabilized helium-neon laser reduced the bandwidth of the beat signal from 6 MHz to 390 kHz, thereby lowering the detection threshold from 5 pW of laser power to 3 pW. In addition, a simple signal detection model is developed which predicts similar threshold reductions.

Super-CHO - A cell line capable of autocrine growth under fully defined protein-free conditions

Chinese Hamster Ovary (CHO) cells are widely used for the large scale production of recombinant biopharmaceuticals. Growth of the CHO-K1 cell line has been demonstrated in serum-free medium containing insulin, transferrin and selenium. In an attempt to get autocrine growth in protein-free medium, DNA coding for insulin and transferrin production was transfected into CHO-K1 cells. Transferrin was expressed well, with clones secreting approximately 1000 ng/10(6)cells/24h. Insulin was poorly expressed, with rates peaking at 5 ng/10(6)cells/24h. Characterisation of the secreted insulin indicated that the CHO cells were incompletely processing the insulin molecule. Site-directed mutagenesis was used to introduce a furin (prohormone converting enzyme) recognition sequence into the insulin molecule, allowing the production of active insulin. However, the levels were still too low to support autocrine growth. Further investigations revealed insulin degrading activity (presumably due to the presence of insulin degrading enzymes) in the cytoplasm of CHO cells. To overcome these problems insulin-like growth factor I (instead of insulin) was transfected into the cells. IGF-1 was completely processed and expressed at rates greater than 500 ng/10(6)cells/24h. In this paper we report autonomous growth of the transfected CHO-K1 cell line expressing transferrin and IGF-1 in protein-free medium without the addition of exogenous growth factors. Growth rates and final cell densities of these cells were identical to that of the parent cell line CHO-K1 growing in insulin, transferrin, and selenium supplemented serum-free media.