Gadolinia (Gd2O3) doped ceria (GDC) and GDC co-doped with Eu2O3 (Eu-GDC) and Nd2O3 (Nd-GDC) were processed in a single step by citrate-nitrate solution combustion synthesis and a detailed impedance spectroscopy analysis was carried out to understand the conduction behavior. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed the nanocrystallinity in the as-prepared powders for all compositions. Dense sintered pellets were obtained by conventional sintering at lower temperatures compared to microcrystalline ceria. The Arrhenius plots of bulk conductivity showed two different slopes indicating presence of different conduction mechanisms at lower and higher temperatures. At lower temperatures, the conduction mode was found to be ionic and it changed to mixed ionic and electronic conduction at higher temperatures. The mixed conductivity was confirmed by the half-tear drop shape of the impedance plots. The grain boundary conductivity, on the other hand, exhibited ionic character and was influenced by the space-charge layer and defect chemistry. The conductivity was improved in Nd-GDC while Eu-GDC showed similar conductivity compared to GDC. © 2016 Elsevier Ltd

Ranjit Bauri

Department of Metallurgical and Materials Engineering

Arrhenius plots

Combustion synthesis

Electrolytes

Europium

Grain boundaries

High resolution transmission electron microscopy

Ionic conductivity

Nanocrystals

Sintering

Spectroscopy

Transmission electron microscopy

X ray diffraction

X ray powder diffraction

CERIA ELECTROLYTE

Co-doping

Conduction mechanism

CONVENTIONAL SINTERING

GRAIN BOUNDARY CONDUCTIVITY

Impedance spectroscopy

IONIC AND ELECTRONIC CONDUCTION

Redox

Gadolinium

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IIT Madras

Electrochimica Acta

New multicomponent equiatomic rare earth oxides (ME-REOs) containing 3-7 rare earth elements (Ce, Gd, La, Nd, Pr, Sm and Y) in equiatomic proportions are synthesized using nebulized spray pyrolysis. All the systems crystallized as a phase pure fluorite type (Fm3m) structure in spite of the high chemical complexity. A nominal increase in the lattice parameter compared to CeO2 is observed in all ME-REOs. X-ray photoelectron spectroscopy performed on the ME-REOs confirmed that all the constituent rare earth elements are present in the 3+ oxidation state, except for Ce and Pr which are present in 4+ and in a mixed (3+/4+) oxidation state, respectively. The presence of Ce4+ contributes substantially to the observed stability of the single phase structure. These new oxide systems have narrow direct band gaps in the range of 1.95-2.14 eV and indirect band gaps in the range of 1.40-1.64 eV, enabling light absorption over the entire visible spectral range. Furthermore, the oxygen vacancy concentration rapidly increases and then saturates with the number of rare earth elements that are incorporated into the ME-REOs. The lowering of the band gap is found to be closely related to the presence of multivalent Pr. Interestingly, the band gap values are relatively invariant with respect to the composition or thermal treatments. Considering the high level of oxygen vacancies present and the observed low band gap values, these new material systems can be of importance where the presence of oxygen vacancies is essential or in applications where a narrow band gap is desirable. © 2017 The Royal Society of Chemistry.

Dalton Transactions

Multicomponent equiatomic rare earth oxides with a narrow band gap and associated praseodymium multivalency

Nanocrystalline 10 mol% gadolinia doped ceria (GDC) was processed and sintered in air and oxygen to study the role of sintering atmosphere on densification and the redox reaction (Ce4+â†" Ce3+) and its effect on the conduction behavior. The density of the air-sintered sample was higher than the oxygen-sintered sample. X-ray photoelectron spectroscopy (XPS) confirmed the presence of higher fraction of Ce3+ and Raman spectroscopy revealed higher concentration of oxygen vacancies in GDC sintered in air indicating higher degree of reduction in air compared to oxygen atmosphere. Impedance spectroscopy studies revealed breaks in the slope of the Arrhenius plot of bulk conductivity indicating presence of different conduction mechanisms which can be related to the redox and defect chemistry. GDC sintered in oxygen atmosphere showed higher ionic conductivity and lower activation energy than the air sintered sample in lower temperatures whereas it showed higher activation energy in the higher temperature range owing to a change in the redox and defect chemistry. © 2013 Elsevier Ltd and Techna Group S.r.l.

Ceramics International

Effect of sintering atmosphere on densification, redox chemistry and conduction behavior of nanocrystalline Gd-doped CeO2 electrolytes

Samaria doped ceria (SDC) co-doped with Nd2O3 (Nd-SDC) and doubly co-doped with Nd2O3 and Pr 2O3 (Pr-Nd-SDC) was prepared by solution combustion synthesis. X-ray diffraction studies confirmed the nanocrystalline nature of the powder with cubic fluorite phase. Raman spectroscopy showed the characteristic cubic fluorite peak (464 cm-1) of ceria and the vacancy concentration calculated from the peak area ratio (A550/A464) was found to be higher in Pr-Nd-SDC (0.556) compared to Nd-SDC (0.168). The powder was compacted into discs and sintered to obtain dense pellets. The sintering temperature was found to be considerably lower than that required for microcrystalline SDC. The electrical conductivity was obtained by impedance spectroscopy. The doubly co-doped sample (Pr-Nd-SDC) exhibited higher conductivity and lower activation energy compared to the co-doped sample (Nd-SDC). © The Electrochemical Society.

ECS Transactions

Rare earth co-doped nanocrystalline ceria electrolytes for intermediate temperature solid oxide fuel cells (IT-SOFC)

Sc2O3 stabilized zirconia (ScSZ) was processed by combustion synthesis. X-ray diffraction was carried for analyzing the phase and crystallinity of the processed powder. The results indicate the effectiveness of the combustion synthesis process to produce nanocrystalline ScSZ from the precursor salts without any intermediate calcination step. Comparison with the X-ray diffraction pattern of a commercial 8YSZ sample showed that the process is also effective in producing the desired cubic fluorite phase. The powder was compacted and sintered to produce a dense electrolyte pellet. The sintering temperature was found to be considerably lower compared to conventional microcrystalline zirconia ceramics. The electrical conductivity of the ScSZ electrolyte was found to be higher compared to 8YSZ processed under same conditions. The Arrhenius plot of conductivity yielded two activation energies corresponding to low and high temperature regions. The activation energy of the 10ScSz electrolyte was found to be considerably lower than the 8YSZ sample. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

International Journal of Hydrogen Energy

Synthesis and characterization of nanocrystalline ScSZ electrolyte for SOFCs

Journal of Alloys and Compounds

Record-low sintering-temperature (600 °C) of solid-oxide fuel cell electrolyte

Acta Materialia

Enhanced mass diffusion phenomena in highly defective doped ceria

Processing and conduction behavior of nanocrystalline Gd-doped and rare earth co-doped ceria electrolytes

Journal	Data powered by TypesetElectrochimica Acta
Publisher	Data powered by TypesetElsevier Ltd
ISSN	00134686
Open Access	No