J.C.C. Abrantes

The effect of cobalt oxide sintering aid on electronic transport in Ce0.80Gd0.20O2−δ electrolyte

Abstract Additions of 2 mol% CoO 1.333 into gadolinia-doped ceria (CGO) solid electrolyte considerably improve sinterability and make it possible to obtain Ce 0.8 Gd 0.2 O 2− δ ceramics with 95–99% density at 1173–1373 K. The effect of cobalt oxide on the total electrical conductivity in air is negligible if the sintering is performed at 1173 K, although p-type electronic conduction measured at 900–1200 K increases with doping by 10–30 times. When increasing the sintering temperature up to 1773 K, grain growth in Co-containing CGO ceramics is accompanied with a decrease in both ionic and electron-hole transport. The oxygen ion transference numbers under oxygen/air gradient vary in the range 0.89–0.99. The n-type conductivity measured by the ion-blocking technique is lower for Co-containing materials than for undoped CGO, suggesting that the electrolytic domain can, to some extent, be enlarged by cobalt oxide additions. The relative role of both p- and n-type electronic contributions to the total conductivity of CGO increases with increasing temperature. The results show that Co-doped materials can still be used as solid electrolyte for intermediate-temperature electrochemical applications, when the operation temperature is 770–970 K.

Applicability of the brick layer model to describe the grain boundary properties of strontium titanate ceramics

AbstractImpedance spectroscopy was used to evaluate the bulk and grain boundary resistances (R B and R gb ) of undoped strontium titanateceramicsinairandinN 2 .Thehighfrequencycontributionofthespectradoesnotdependsignificantlyontheaveragegrainsizeorgrainsize distributions, and was thus ascribed to the bulk behaviour. The grain boundary results obtained for several samples vary with theaverage grain size, and these e•ects nearly agree with predictions by a simple brick layer model. The deviations from this model mayincrease with decreasing temperature, and are somewhat greater in N 2 than in air. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Electrical resistance; Grain boundaries; Impedance spectroscopy; SrTiO 3 1. IntroductionThe barrier character of grain boundaries of strontiumtitanate ceramics play an important role on low tempera-ture applications such as multilayer capacitors. 1 Thisinterest thus prompted investigations to separate the bulkand grain boundary contributions by comparing impe-dance spectra obtained for single crystals, bicrystals, andceramic samples,

An alternative representation of impedance spectra of ceramics

Abstract Suitable approximations are derived to extract the relevant parameters of the bulk, grain boundary, and electrode contributions of impedance spectra from Z′ vs. Z″/f plots. Corrections are proposed for cases where the difference between bulk and grain boundary relaxation frequencies and/or the difference between grain boundary and electrode relaxation frequencies is relatively small. In these cases, the Nyquist plot (Z″ vs. Z′) shows overlapped arcs, but inspection of alternative Z′ vs. Z″/f representations may reveal contributions with relatively small differences in relaxation frequencies. These plots and the corresponding derivative dZ′/d(Z″/f) may be useful to interpret cases showing a significant distribution of grain boundary relaxation frequencies.

Conductivity of CGO and CSO ceramics obtained from freeze-dried precursors

Ce0.8Gd0.2O1.9 (CGO) and Ce0.8Sm0.2O1.9 (CSO) have been prepared as polycrystalline materials using a freeze-dried precursor. This method yields amorphous nanometric powders. Crystallization of the fluorite phase occurred on heating at 600 8C or higher temperatures. The grain size of freeze-dried powders increases to about 100 nm after calcination at 800 8C, or about 200 nm after firing at 1000 8C. Freeze-dried powders were used to prepare dense ceramic disks by sintering at 1400 8C. Some disks were sintered at 1000 8C by adding small amounts of cobalt nitrate solution to assist the densification. The electrical conductivity results obtained for these gadolinia-doped ceria and samaria-doped ceria ceramics are similar to those obtained for CGO pellets obtained from commercial nanopowders (Rhodia). Though the bulk conductivity of CSO is probably higher than that of CGO, its grain boundary conductivity is inferior, and tends to control the overall behaviour, at least at relatively low temperatures. # 2003 Elsevier Science Ltd. All rights reserved.

Representations of impedance spectra of ceramics Part I. Simulated study cases

Abstract The present work demonstrates that the representation of log(tg(δ)) vs. log(f) may be best suited to reveal the bulk, grain boundary, and electrode contributions of impedance spectra, and to extract the relevant parameters. Alternative representations of Z″ vs. Z′, log(Z″) vs. log(f), log(M″) vs. log(f), and log(A″) vs. log(f) often have a limited applicability. The log(tg(δ)) vs. log(f), and log(A″) vs. log(f) plots show peaks or minima at frequencies that are somewhat displaced relative from the true relaxation frequencies, or transitions from the electrode to grain boundary, or grain boundary to bulk contributions. Suitable criteria are proposed to account for these differences.

Evaluation of SrTi1−yNbyO3+δ materials for gas sensors

Abstract Porous samples were used to evaluate the dependence of electrical conductivity on the oxygen partial pressure. Results obtained with these porous samples revealed that Nb for Ti substitution suppresses the p-type contribution even in oxidizing conditions, and enhances the n-type conductivity. The conductivity of porous SrTi1−yNbyO3+δ samples can be described by the power law dependence on p−1/4O2, except possibly in very reducing conditions. The response time of porous samples is short, as required for resistive oxygen sensors.

Representations of impedance spectra of ceramics. Part II. Spectra of polycrystalline SrTiO3

Simultaneous representations of M0 ,Z 0 ,A 0, and tg(d)) vs. frequency were used to analyze impedance spectroscopy data of SrTiO3 and to demonstrate the applicability of these representations. The log(tg(d)) vs. log(f)) representation usually shows several local extremes (maxima or minima), which can be interpreted to obtain a wide range of relevant parameters, even for cases in which the Nyquist plot (Z0 vs. Z9) shows overlapped bulk and grain boundary arcs. Peaks of log(tg(d)) indicate electrode to grain boundary transition, and/or the grain boundary to bulk transition. Local minima can be used to extract the grain boundary or the electrode relaxation frequencies. The admittance representation is slightly less useful. The modulus representation shows the bulk contribution term, but does not reveal the grain boundary or contributions of electrode terms.

Effects of Mn-doping on the structure and biological properties of β-tricalcium phosphate

Doping calcium phosphates with trace elements that exist in bone tissues is beneficial in terms of cell-material interactions and in vivo performance of the bone grafts made thereof. Manganese (Mn) is an essential element for normal growth and metabolism of bone tissues, but studies reporting the effects of Mn-doping calcium phosphates are scarce. The present study investigated the influence of Mn-doping on the structure, morphology and biological properties of β-tricalcium phosphate [β-Ca3(PO4)2] (β-TCP). Mn-doped (MnTCP) powders, with Mn contents varying from 0 to 10 mol%, were obtained through an aqueous precipitation method followed by heat treatment at 800 °C. The successful incorporation of Mn into β-TCP structure was proved through quantitative X-ray diffraction (XRD) phase analysis coupled with structural Rietveld refinement. Increasing Mn concentrations led to decreasing trends of a- and c-axis lattice parameters, and Mn-doping also significantly affected the morphology of β-TCP powders. In vitro proliferation and differentiation assays of MC3T3-E1 osteoblastic-like cells, grown in the presence of the powders, revealed that the biological benefits of Mn-doped β-TCP are limited to lower Mn incorporation levels and potentially related to their surface microstructure. The Mn1-βTCP composition revealed the best set of bioactivity properties, potentially a good candidate for future applications of β-TCP materials in osteoregeneration.

Behavior of strontium titanate ceramics in reducing conditions suggesting enhanced conductivity along grain contacts

In reducing conditions the resistance of dense strontium titanate ceramic samples decreases markedly with decreasing average grain size. This trend is interpreted on assuming enhanced conductivity along grain contacts in parallel with a RC bulk term. The temperature dependence is consistent with this model and indicates that the bulk may still control the overall behavior at sufficiently high temperatures (typically for T> 700 � C). The resistivity of fine grained porous samples is intermediate between the results obtained for fine grained dense samples and dense samples with large average grain size. An extended model is thus proposed to describe the behavior of porous samples by assuming a series association of isolated channels connected by the interior of adjacent grains. # 2002 Elsevier Science Ltd. All rights reserved.

New Oxide-Ion Conductors Ln2+xTi2-xO7-x/2 (Ln = Dy – Lu; x=0.096)

New oxide-ion conductors Ln2+xTi2-xO7-x/2 (Ln = Dy-Lu, x = 0.096; 35.5 mol. % Ln2O3· 64.5 mol.% TiO2)-LANTIOX with a disordered pyrochlore structure are obtained by coprecipitation with next thermal annealing at 1600°C. Their ionic conductivity in air at 740°C attains 5·10-3 S/cm for Ln2.096Ti1.904O6.952 (Ln= Lu, Yb), 10-3 S/cm for Ln2,096Ti1,904O6,952 (Ln=Tm, Er, Ho) and 10- 4S/cm for Dy2,096Ti1,904O6,952 due to the presence of defects in both the cation and anion sublattices. The materials contain ~4.8-10.6% LnTi anti-site defects. Ln2.096Ti1.904O6.952 ceramics are shown to be stable in the temperature range of 1400 to 1700°C. 1400°C-samples Ln2+xTi2-xO7-x/2 have lower ionic conductivity then 1600°C and 1690°C samples.