Ceria Ceramics Research Articles

Monitoring of water dissolution into high temperature protonic conductors

Water dissolution into a doped strontium cerate ceramics was monitored with Raman spectroscopy. Since a Raman band at 630 cm − 1 is observed when the ceramics possess oxygen vacancies, and a relative intensity of the band to that of the main band at 345 cm − 1 ( I 630/ I 345) is proportional to its oxygen vacancy concentration, the parameter I 630/ I 345 was examined for the understanding of water dissolution behavior into the lattice. The parameter for quenched samples after annealing decreased gradually with an increase in water vapor partial pressure, P(H 2O) during annealing due to water dissolution, resulting in disappearance of some oxygen vacancies. An in situ monitoring of water dissolution into this ceramics was also performed employing an atmosphere-controlled system.

Sinter-active nanocrystalline CeO2 powder prepared by a mixed fuel process: Effect of fuel on particle agglomeration

A modified combustion process, namely a mixed fuel process making use of a mixture of two fuels, such as citric acid and glycine has been developed to prepare nanocrystalline ceria powders. The effect of the mixed fuel and the different fuel to oxidant ratios on the decomposition characteristics of the gels were investigated by simultaneous thermal analysis experiments. It was established from various characterization techniques that the ceria powder prepared through the mixed fuel process has got the optimum powder characteristics, namely, a surface area of 33.33 m2/g and a crystallite size of 14 nm compared to the powders produced through the combustion process using a single fuel like glycine or citric acid. Such powders when sintered at 1250°C resulted in pellets with densities in the range of 94–96% of theoretical density. In this paper, we have carried out systematic studies on the sintering of ceria powders prepared by different approaches. The sintered ceramic from mixed fuel batch, exhibited and retained relative density more than 95% up to 1250°C and this data clearly underscores the ability of this process in developing ceria ceramics with increased stability against reduction.

Sintering of gadolinia-doped ceria prepared by mechanochemical synthesis

Gadolinia-doped ceria (GDC) ceramics were prepared by mechanochemical synthesis of ceria and gadolinia (10 mol.%) powders with subsequent uniaxial pressing and pressureless sintering. The mechanochemical synthesis was conducted in an attritor with different dispersion media: neutral (distilled water), basic (aqueous solution of ammonia), acidic (aqueous solution of propionic acid), and non-polar medium (xylene with stearic acid). The effect of suspension composition and attritor milling time on the sintering behaviour and ionic conductivity of GDC was studied. An attritor milling time of 3–6 h was evaluated as the optimum milling time from the viewpoint of further sintering behaviour. Longer milling times resulted in nanometre-sized powders, whose sintering behaviour deteriorated due to powder agglomeration. The appearance of agglomerates in the non-polar medium was reduced. The ionic conductivity of the samples was evaluated by impedance spectroscopy and correlated with their microstructure.

Synthesis, densification and electrical properties of strontium cerate ceramics

Powders of pure and 5% ytterbium substituted strontium cerate (SrCeO 3/SrCe 0.95Yb 0.05O 3− δ ) were prepared by spray pyrolysis of nitrate salt solutions. The powders were single phase after calcination in nitrogen atmosphere at 1100 °C (SrCeO 3) and 1200 °C (SrCe 0.95Yb 0.05O 3− δ ). Dense SrCeO 3 and SrCe 0.95Yb 0.05O 3− δ materials were obtained by sintering at 1350–1400 °C in air. Heat treatment at 850 and 1000 °C, respectively, was necessary prior to sintering to obtain high density. The dense materials had homogenous microstructures with grain size in the range 6–10 μm for SrCeO 3 and 1–2 μm for SrCe 0.95Yb 0.05O 3− δ . The electrical conductivity of SrCe 0.95Yb 0.05O 3− δ was in good agreement with reported data, showing mixed ionic–electronic conduction. The ionic contribution was dominated by protons below 1000 °C and the proton conductivity reached a maximum of ∼0.005 S/cm above 900 °C. In oxidizing atmosphere the p-type electronic conduction was dominating above ∼700 °C, while the contribution from n-type electronic conduction only was significant above ∼1000 °C in reducing atmosphere.

Effect of Y and Dy co-doping on electrical conductivity of ceria ceramics

The effect of co-joint additions of Y and Dy on the electrical conductivity of ceria ceramics was studied by impedance spectroscopy. Powder materials were synthesized by aqueous coprecipitation to obtain Ce 1− x (Y 0.5Dy 0.5) x O 2− δ with 0 ≤ x ≤ 0.15. Sintering of powder compacts was carried out at 1450 °C for 4 h. The main results show that high densification (∼95% of theoretical value) was attained for all compositions. Raman spectroscopy results evidenced the solid solution formation among cerium, yttrium and dysprosium. The electrical conductivity of sintered specimens varies with the total dopant content. A maximum conductivity value was obtained for a lower dopant content considering the behaviour of singly doped ceramics. Moreover, sintered specimens with the same total dopant content exhibit similar electrical conductivity values suggesting that the concentration of oxygen vacancies play a major role in the conduction process of co-doped ceramics.

Electrical properties of nanocrystalline Sm-doped ceria ceramics

Electrical properties of nanocrystalline materials are usually different than that of microcrystalline. This phenomenon is not fully understood and therefore it is of continuous interest in the scientific community. In this paper, the results of investigation of nanocrystalline Sm-doped ceria prepared by a net shape technology are presented. The net shape technology is a relatively new ceramic preparation method, which combine powder and polymer precursor methods. The ceramics prepared in this way can be dense and nanocrystalline. Net-shaped prepared Sm-doped ceria was fabricated in the form of bulk ceramics and film on platinum foil. Their electrical properties were investigated and compared with the results obtained from microcrystalline sample.

Effect of Bi 2O 3 addition on the sintering and microstructural development of gadolinia-doped ceria ceramics

The effect of small amounts (0.2–2.0 wt.%) of bismuth oxide on the sintering behavior and microstructural development of Ce 0.9Gd 0.1O 1.95 (GDC) submicronized powders has been studied using XRD for the lattice parameter measurements, the constant heating rate (CHR) method in air to monitor the shrinkage kinetics of powder compacts, and scanning electron microscopy (SEM) to study the microstructure of the sintered samples. Sintering of GDC compacts was significantly improved by adding small amounts of Bi 2O 3 (≤2.0 wt.%), and samples of doped-GDC sintered at 1200–1400 °C for 2–4 h were dense bodies (98–99.5% of theoretical density). Measurements showed that the addition of Bi 2O 3 could reduce the sintering temperature by about 250–300 °C lower than that for undoped-GDC samples. A liquid phase-assisting mechanism was assumed as the main cause for the enhancement of the densification process. The average grain size of doped-GDC sintered samples grew with the increasing of Bi 2O 3 addition up to 1.0 wt.%, and then decreased indicating a poor wetting properties of the formed liquid phase.

Local structures around Y and Ce cations in 10 mol% Y 2O 3 doped ceria ceramics by EXAFS spectroscopy

Local structures around host Ce and dopant Y cations in 10 mol% Y 2O 3 doped ceria solid solutions have been investigated by room and high temperature EXAFS spectroscopy. The results show that the local structures around the Ce cation in doped ceria samples are similar to that in the fluorite CeO 2 structure though the coordination numbers of Ce–O tend to be smaller than 8. The local structures around Y cation, however, are significantly different from those around Ce cation, and show more resemblance to that around Y cation in the C-type Y 2O 3 structure. A more accurate description of the local structures around Y cation in doped ceria was given by analyzing Y–K edge EXAFS spectra based on the C-type Y 2O 3 structure.

Microstrain and self-limited grain growth in nanocrystalline ceria ceramics

The grain coarsening in dense nanocrystalline gadolinia-doped ceria (CGO) and CeO2 was studied for the grain size regime from 10 to 250nm. Dense ceria-based thin films were prepared by spray pyrolysis and pulsed laser deposition on sapphire substrates. The small grain-sized CGO and CeO2 microstructures were surprisingly stable and showed self-limited grain growth and relaxation of the microstrain with time. In this regime, material transport occurs via grain boundary diffusion leading to densification and grain coarsening in the thin films. A continuous transition to common grain growth kinetics was observed for larger grains at higher temperatures.

Rapid densification by using Bi 2O 3 as an aid for sintering of gadolinia-doped ceria ceramics

Densification process and microstructure development in Bi 2O 3-doped gadolinia ceria (GDC) submicrometer powders have been studied by the constant heating rate (CHR) method in air and by SEM observations. Densification of GDC was strongly enhanced by adding small amounts of Bi 2O 3, and samples of GDC containing ≤1 wt% Bi 2O 3 sintered at 1200–1400 °C for 2 to 4 h were near theoretically dense bodies (98–99.8% of theoretical), i.e., about 250–300 °C lower than that for undoped-GDC ceramics. A transient liquid phase-assisting mechanism was assumed to be the main cause for such an improvement. The grain size of GDC ceramics was hardly affected by the Bi 2O 3 additions during sintering. A total electrical conductivity as high as 4 S m −1 at 700 °C for the doped-GDC samples sintered at 1400 °C, was measured.

Characterization of Porous Titania and Ceria Ceramics by Small-Angle Neutron Scattering

Titania powder was prepared by a solution based hydroxide precipitation technique using the tetrachloride precursor and ammonia as the precipitating agent. Ceria powder was prepared by homogeneous precipitation method using cerium nitrate and hexamethylenetetramine. The particle size distribution of titania and ceria powders was measured by laser light scattering. The powder particles were found to follow a log-normal size distribution and the average size of the particles for both the powders was ∼0.7 microns. Small-angle neutron scattering (SANS) was used to study the sintering behaviour and evolution of pore morphology in sintered compacts of titania and ceria by varying the sintering temperature/time. The SANS profiles were analyzed in the light of poly-disperse spherical pore model using log-normal size distribution. The pore sizes in both the ceramic compacts varied from 10 to 1000 nm and the mean value of the distribution shifted from a lower, 50 nm to a higher, 350 nm size. This trend could be attributed to both densification and grain growth occurring in the compacts with the progress of sintering.

Enhanced Ionic Conductivity in Nanostructured, Heavily Doped Ceria Ceramics

AbstractThe electrical properties of nanostructured, heavily yttria‐ or samaria‐doped ceria ceramics are studied as a function of grain size using electrochemical impedance spectroscopy (EIS). A remarkable enhancement in the total ionic conductivity of about one order of magnitude is found in nanostructured samples, compared with the intrinsic bulk conductivity of conventional microcrystalline ceramics. This effect is attributed to the predominance of grain‐boundary conduction in the nanostructured materials, coupled with an increase in the grain‐boundary ionic diffusivity with decreasing grain size.

Fast low-temperature consolidation of bulk nanometric ceramic materials

Abstract Dense zirconia and ceria ceramics with grain sizes approaching 10 nm have been obtained using a high-pressure modification of the field activated sintering (spark plasma sintering). Pressures up to 1 GPa and sintering times of 5 min have been used. The densification has been performed at temperatures as low as 930 °C for fully-stabilized zirconia, 815 °C for samaria-doped ceria, and 675 °C for pure ceria. Relative densities greater than 98% were achieved for all materials. The method overcomes the deleterious effect of powder agglomeration, producing fine grain size for these ceramics in bulk form.

Fracture properties of (CeO2)1−x(RO1.5)x (R = Y, Gd, and Sm; x = 0.02–0.20) ceramics

The fracture properties of rare earth (yttrium, gadolinium, and samarium) doped ceria ceramics were systematically investigated using the modified small punch method at room temperature in air and at high temperatures in argon gas. The samples prepared by oxalate co-precipitation method showed higher fracture strength than those by solid-state reaction method. In addition, the sintering temperature dependence of gadolinium doped ceria indicated that the highest fracture strength was attained by sintering at 1600 °C. Furthermore, it turned out that the fracture strength was influenced by the dopant concentration rather than the kind of dopant. The relationship between mechanical properties and defect structures are discussed from the dopant concentration, sintering temperature and testing temperature dependence of the fracture strength.

Gadolinium-Doped Ceria Ceramics with a Submicron Structure for Electrochemical Applications

The effect of the composition and density of ceramic materials on the electrical conductivity is investigated for solid electrolytes that are sintered from Ce1 − x GdxO2 − δ (x = 0.09–0.31) nanopowders and have a submicron structure (grain size, 0.1–0.3 µm). The powders are prepared through evaporation of a target with a pulsed CO2 laser, followed by magnetic pulsed compaction. The ceramic materials are synthesized at temperatures in the range 1100–1300°C in air. The effect of the composition on the electrical conductivity is studied for the ion-conducting ceramic samples in the density range up to a limiting value (99.8% of the X-ray density).

Electronic Conductivity of La‐Doped Ceria Ceramics

Lanthanum‐doped ceria powder with a composition Ce0.8La0.2O1.9 was prepared by heating the oxalate solid solution (Ce0.8La0.2)2(C2O4)3 at 873 K in air. As‐prepared powder was densified to 96%–97% relative density by sintering in air at 1773 K for 4 h. The electronic current of the disk sample was measured in a temperature range from 773 to 1113 K by the direct current polarization method using a Hebb–Wagner ion‐blocking cell. A linear relationship, which was theoretically predicted, was measured between log σe (electronic conductivity) and E (applied voltage) in the applied voltage range of 0.2–1.0 V. The σe was proportional to PO2−1/4.3≈PO2−1/4.6 in the oxygen partial pressure range of 10−2–10−8 Pa, and to PO2−1/6.7≈PO2−1/7.1 in the oxygen partial pressure range of 10−7.5–10−22 Pa. The apparent activation energy of the electronic conduction was 1.87–1.94 eV. The hole conduction was also measured in the Po2 range of 102–105 Pa. The transport number of oxide ion was 0.96–1.00 at 773–1113 K under an oxygen partial pressure of 10−5 Pa.

Influence of the electrode nature on conductivity measurements of gadolinia-doped ceria

Ten percent and 20% Gd-doped ceria ceramics were prepared by the oxalate coprecipitation method. The grain interior, grain boundary and total conductivities were determined by impedance spectroscopy using different contact electrodes. The purpose of this paper is to verify which contact material is the best in these studies and to determine the temperature range in which they can be used. Four different electrode materials were studied: platinum, gold and silver in the form of the commercial inks or paints and sputtered platinum. While the intragranular response is similar for all the electrode materials, the grain boundary semicircle on the complex plane impedance spectra is dependent upon the electrode nature, however, the activation energies are independent of the electrode nature. Our studies indicate that gold is suitable at temperatures below 800 °C, silver up to 700 °C, and sputtered platinum up to ∼400 °C and that platinum ink can be used at higher temperatures when only the total electrolyte resistance is needed but gives poorly resolved grain boundary semicircles on the complex plane plots.

High density BaCeO3 ceramics sintered using microwave irradiation

AbstractThe goal of this work was to investigate the capabilities of high-power microwaves in manufacture of small-grained barium cerate ceramics. Having compared the samples sintered in the microwave and the thermal fields, we inferred that the microwave sintering occurs at far higher rates. It is also noteworthy that microwave sintering provides a record-breaking low temperature of barium cerate sintering (900°C). Having studied the effect of a microwave field on the microstructure of sintering products, we found that high-power microwave processing significantly reduces the average grain size of barium cerate ceramics.

Densification and electrical conductivity of fast fired manganese-doped ceria ceramics

Reactive pure and manganese-doped (5% and 10 at.%) ceria nanosized powders were prepared by the polymeric precursor technique. Physical properties of powder materials were studied by X-ray diffraction, nitrogen adsorption, and diffuse reflectance infrared Fourier transform spectroscopy. Characterization of powder compacts after fast firing at 1200 °C for 5 min was carried out by scanning electron microscopy and impedance spectroscopy measurements. The bulk apparent density of sintered pellets was determined for pellets of different compositions sintered at 1200 °C. A gradual decrease of the particle size occurs with increasing doping content. Relatively high values of apparent density were obtained after fast firing doped specimens at 1200 °C. DRIFT spectra evidence that a fraction of Mn ions was segregated onto particles surface. The electrical resistivity of sintered pellets reveals different mechanisms of conduction depending on the Mn content.

Oxygen partial pressure dependence of creep on yttria-doped ceria ceramics

A creep test was performed on yttria-doped ceria (YDC), which was investigated as an electrolyte for solid oxide fuel cells (SOFC). The test was performed using a four-point bending method and with testing conditions including a constant load under a temperature range of 1050–1125 °C, an applied stress of 20–50 MPa, and an oxygen partial pressure of P(O 2)=0.21–10 −15.2 atm. The results showed that the creep rate increased with decreasing P(O 2), and showed a peak at P(O 2)=10 −13.6 atm. The activation energy for creep was increased from 264 to 333 kJ/mol with decreasing P(O 2) from 0.21 to 10 −14.5 atm. These values were much higher than the reported activation energy of oxygen anion diffusion in ceria. Thus, it could be suggested that the creep mechanism is controlled by cation diffusion. The creep mechanism changed from Nabarro-Herring creep to grain boundary sliding in a reducing atmosphere when the P(O 2) was lower than 10 −14.5 atm.