Effect of Doping Concentration on Grain Boundary Conductivity of Samaria Doped Ceria Composites

This work aims to study the effect of Sm3+ doping concentration on the grain boundary ionic conductivity of ceria. The materials were prepared by a modified co-precipitation method, where molecular water associated with the precursor has been utilized to facilitate the hydroxylation process. The synthesized hydroxide / hydrated oxide materials were calcined and the green body (pellet) has been sintered at high temperature in order to achieve highly dense (~ 96 %) pellet. The structural analyses were done using XRD and Raman spectroscopy, which confirm the single phase cubic structure of samaria doped ceria (SDC) nanoparticles and the surface morphology of sintered samples was studied using FESEM. The ionic conductivity was measured by AC impedance spectroscopy of the sintered pellets in the temperature range of 400-700 °C, which shows superior grain boundary conductivity. The grain boundary ionic conductivity of around 0.111 S/cm has been obtained for 15SDC composition at 600 °C.

Effect of Reduced Atmosphere Sintering on Blocking Grain Boundaries in Rare-Earth Doped Ceria

Rare-earth doped ceria materials are amongst the top choices for use in electrolytes and composite electrodes in intermediate temperature solid oxide fuel cells. Trivalent acceptor dopants such as gadolinium, which mediate the ionic conductivity in ceria by creating oxygen vacancies, have a tendency to segregate at grain boundaries and triple points. This leads to formation of ionically resistive blocking grain boundaries and necessitates high operating temperatures to overcome this barrier. In an effort to improve the grain boundary conductivity, we studied the effect of a modified sintering cycle, where 10 mol% gadolinia doped ceria was sintered under a reducing atmosphere and subsequently reoxidized. A detailed analysis of the complex impedance, conductivity, and activation energy values was performed. The analysis shows that for samples processed thus, the ionic conductivity improves when compared with conventionally processed samples sintered in air. Equivalent circuit fitting shows that this improvement in conductivity is mainly due to a drop in the grain boundary resistance. Based on comparison of activation energy values for the conventionally processed vs. reduced-reoxidized samples, this drop can be attributed to a diminished blocking effect of defect-associates at the grain boundaries

The Effect of Tartaric Acid and Citric Acid as a Complexing Agent on Defect Structure and Conductivity of Copper Samarium Co-Doped Ceria Prepared by a Sol-Gel Auto-Combustion Method

Copper samarium co-doped ceria (CSDC) (Cu0.01Sm0.19Ce0.80O2−δ) nanoparticles were synthesized via a sol-gel auto-combustion of metal nitrates without a complexing agent (DI) and with tartaric acid (TA) or citric acid (CA). The solid oxide formation of CSDC/DI corresponds to the endothermic stage, whereas that of CSDC/TA and CSDC/CA matches the exothermic stage caused by the decomposition of the metal cross-linking and carbon combustion. The cross-linking occurs more extensively in the CA case as more heat is released in CA than in the TA route. The as-synthesized morphology of CSDC/DI reveals both layered structures and small agglomerated particles, whereas CSDC/TA and CSDC/CA show dense xerogel and porous xerogel, respectively. The cubic fluorite structure for calcined CSDCs was confirmed by XRD. From Raman analysis, calcined CSDC/CA has the lowest amount of copper segregation and the highest relative total oxygen vacancy concentration [ V O • • ]total, whereas calcined CSDC/DI has the highest amount of copper segregation and the lowest [ V O • • ]total. For all samples, copper segregation promotes densification, albeit to varying degrees. The relative densities of CSDC/DI, CSDC/TA, and CSDC/CA pellets are 82.8 ± 2.4%, 95.5 ± 1.8%, and 97.8 ± 0.9%, respectively. The sintered CSDC/DI has the lowest density because some copper segregates and liquid copper in interparticle spaces could evaporate earlier than samples containing a complexing agent, whereas sintered CSDC/CA has the highest density because Cu could slowly diffuse from the Cu-Sm-Ce solid solution to grain boundary regions and then precipitate as CuO. The specific grain boundary conductivity is predominantly influenced by CuO along grain boundaries, which reduces specific grain boundary conductivity and increases the enthalpy of association (ΔHa) at 250–350°C; however, it rarely impacts total grain boundary conductivity at temperatures higher than 400°C. CSDC/CA has slightly higher total conductivity than CSDC/TA despite having more CuO segregation because it has higher density and V O • • .

Transport and Dielectric Properties of Mechanosynthesized La2/3Cu3Ti4O12 Ceramics

La2/3Cu3Ti4O12 (LCTO) powder has been synthesized by the mechanochemical milling technique. The pelletized powder was conventionally sintered for 10 h at a temperature range of 975–1025 °C, which is a lower temperature process compared to the standard solid-state reaction. X-ray diffraction analysis revealed a cubic phase for the current LCTO ceramics. The grain size of the sintered ceramics was found to increase from 1.5 ± 0.5 to 2.3 ± 0.5 μm with an increase in sintering temperature from 975 to 1025 °C. The impedance results show that the grain conductivity is more than three orders of magnitude larger than the grain boundary conductivity for LCTO ceramics. All the samples showed a giant dielectric constant (1.7 × 103–3.4 × 103) and dielectric loss (0.09–0.17) at 300 K and 10 kHz. The giant dielectric constant of the current samples was attributed to the effect of internal barrier layer capacitances due to their electrically inhomogeneous structure.

Samples of Ba1−xSrxCe0.9Y0.1O3−δ, 0 < x < 0.1, with Improved Chemical Stability in CO2-H2 Gas-Involving Atmospheres as Potential Electrolytes for a Proton Ceramic Fuel Cell

Comparative studies were performed on variations in the ABO3 perovskite structure, chemical stability in a CO2-H2 gas atmosphere, and electrical conductivity measurements in air, hydrogen, and humidity-involving gas atmospheres of monophase orthorhombic Ba1−xSrxCe0.9Y0.1O3−δ samples, where 0 < x < 0.1. The substitution of strontium with barium resulting in Ba1−xSrxCe0.9Y0.1O3−δ led to an increase in the specific free volume and global instability index when compared to BaCe0.9Y0.1O3−δ. Reductions in the tolerance factor and cell volume were found with increases in the value of x in Ba1−xSrxCe0.9Y0.1O3−δ. Based on the thermogravimetric studies performed for Ba1−xSrxCe0.9Y0.1O3−δ, where 0 < x < 0.1, it was found that modified samples of this type exhibited superior chemical resistance in a CO2 gas atmosphere when compared to BaCe0.9Y0.1O3−δ. The application of broadband impedance spectroscopy enabled the determination of the bulk and grain boundary conductivity of Ba1−xSrxCe0.9Y0.1O3−δ samples within the temperature range 25–730 °C. It was found that Ba0.98Sr0.02Ce0.9Y0.1O3−δ exhibited a slightly higher grain interior and grain boundary conductivity when compared to BaCe0.9Y0.1O3−δ. The Ba0.95Sr0.05Ce0.9Y0.1O3−δ sample also exhibited improved electrical conductivity in hydrogen gas atmospheres or atmospheres involving humidity. The greater chemical resistance of Ba1−xSrxCe0.9Y0.1O3−δ, where x = 0.02 or 0.05, in a CO2 gas atmosphere is desirable for application in proton ceramic fuel cells supplied by rich hydrogen processing gases.