Phase stability and ionic conductivity of cubic xNb2O5-(11-x)Sc2O3-ZrO2 (0 ≤ x ≤4) (original) (raw)
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Temporal stability of oxygen-ion conductivity in 1Nb2O5-10Sc2O3-89ZrO2
Journal of the European Ceramic Society, 2018
A slight Nb2O5 co-doping in 11Sc2O3-89ZrO2 was earlier reported to stabilize the highsymmetry cubic phase completely and enhances the conductivity significantly. The present work looked at the temporal stability of conductivity in 1Nb2O5-10Sc2O3-89ZrO2 (1Nb10ScSZ) for the electrolyte application in solid oxide fuel cells. In-situ conductivity measurement was done using impedance spectroscopy at 650°C in the air for 2000 h. A substantial conductivity loss (29%) was observed in the first 1000 h. Following which, conductivity remained relatively stable for the last 1000 h. Impedance analysis showed that the main contribution to the conductivity degradation was from grain conductivity. Phase analysis performed using XRD, TEM and Raman spectroscopy revealed that both the unaged and aged 1Nb10ScSZ samples consisted of metastable t″-phase. However, the extent of tetragonality was found to increase after ageing. The formation of low-symmetry phase was suggested to be the reason for the grain conductivity loss in 1Nb10ScSZ.
Microscopy and Microanalysis, 2013
As the presence of Sc 2 O 3 and CeO 2 is known to largely enhance the ionic conductivity in the temperature range of 600-8008C, compared with the conventional yttria-stabilized ZrO 2 , Sc 2 O 3 &CeO 2stabilized ZrO 2 provide its applicability as electrolytes in solid oxide fuel cells. The current study introduces the methodology to synthesize Sc 2 O 3 &CeO 2-stabilized ZrO 2 powders by using co-precipitation technique or high-temperature hydrothermal reaction, and further describes the structural characterization of the zirconia powders synthesized by the above-mentioned two methods. The co-precipitation technique was found to allow obtaining powders of cubic phase, whereas high-temperature hydrothermal synthesis results in the presence of a monoclinic phase as well. The scanning transmission electron microscope observations also confirm that the size of the synthesized ZrO 2 powders in this study is found to be much smaller than that of commercially available powders.
Journal of the American Ceramic Society, 2008
X-ray diffraction (XRD) and electron microscopy investigations have been performed on Sc 2 O 3-stabilized ZrO 2 as-sintered and after aging in air or in wet-forming gas at 8501C for 1000 h. Some tetragonal to monoclinic transformation had occurred in the near-surface regions of 4 mol% Sc 2 O 3 samples after aging; the phase transition was more severe for samples aged in the forming gas ambient. A decrease of B20% in electrical conductivity accompanied the aging. In 6 mol% Sc 2 O 3 samples, although no cubic to tetragonal transformation was detected, both the electrical conductivity and the activation energy for ionic conductivity decreased significantly during aging. Ten mole percent Sc 2 O 3 samples did not show appreciable change in electrical conductivity due to aging, although some near-surface cubic to rhombohedral transformation did occur. Sharpening of the (400) t XRD peak of Sc 2 O 3-stabilzed tetragonal ZrO 2 accompanies the change(s) in the electrical conductivity.
X-ray diffraction (XRD) and electron microscopy investigations have been performed on Sc 2 O 3 -stabilized ZrO 2 as-sintered and after aging in air or in wet-forming gas at 8501C for 1000 h. Some tetragonal to monoclinic transformation had occurred in the near-surface regions of 4 mol% Sc 2 O 3 samples after aging; the phase transition was more severe for samples aged in the forming gas ambient. A decrease of B20% in electrical conductivity accompanied the aging. In 6 mol% Sc 2 O 3 samples, although no cubic to tetragonal transformation was detected, both the electrical conductivity and the activation energy for ionic conductivity decreased significantly during aging. Ten mole percent Sc 2 O 3 samples did not show appreciable change in electrical conductivity due to aging, although some near-surface cubic to rhombohedral transformation did occur. Sharpening of the (400) t XRD peak of Sc 2 O 3 -stabilzed tetragonal ZrO 2 accompanies the change(s) in the electrical conductivity.
Scientific Reports
In this study, the Ho-substituted BaZrO 3 electrolyte ceramics (BaZr 1-x Ho x o 3-δ , 0.05 ≤ x ≤ 0.20) were synthesized through a low-cost flash pyrolysis process followed by conventional sintering. The effects of Ho-substitution in BaZrO 3 studied in terms of the structural phase relationship, microstructure and electrical conductivity to substantiate augmented total electrical conductivity for intermediate temperature solid oxide fuel cells (IT-SOFCs). The Rietveld refined X-ray diffraction (XRD) patterns revealed that pure phase with Pm m 3 space group symmetry of cubic crystal system as originated in all samples sintered at 1600 °C for 8 h. The Raman spectroscopic investigations also approved that Ho incorporation in BaZro 3 ceramics. Field Emission Scanning Microscopic (FESEM) study informed a mixture of fine and coarse grains in the fracture surface of Ho-substituted BaZrO 3 sintered samples. The relative density and average grain size of samples were observed to decrease as per the addition of Hosubstitution in BaZro 3 ceramics. The electrical conductivity study was accomplished by Electrical Impedance Spectroscopy (EIS) under 3% humidified O 2 atmosphere from 300 to 800 °C. Furthermore, the total electrical conductivity of BaZr 0.8 Ho 0.2 o 3-δ ceramic was found to be 5.8 × 10 −3 S-cm −1 at 600 °C under 3% humidified atmosphere, which may be a promising electrolyte for IT-SOFCs. Recently, the proton conductive oxide ceramics have fascinated worldwide attention due to widespread applications in intermediate temperature solid oxide fuel cells (IT-SOFCs), hydrogen separation and electrolysis of steam, etc. In this context, the rare-earth cerates and zirconates with the perovskite-type A(II)B(IV)O 3 crystallographic structure are the two foremost families of proton-conducting oxides for electrochemical applications 1-4. Generally, in these categories of oxide materials, oxygen vacancies are increased by replacement of tetravalent cation B(IV) by trivalent cation M(III) as given in the Eq. (1) using Kröger-Vink notation.
Chemical Vapor Deposition, 2012
Dense, crack-free thin films (<5 mm) of the nanostructured scandia-zirconia system (Sc 2 O 3 :ZrO 2 ) stabilized in the cubic-fluorite phase (c-ZrO 2 ) are deposited through conventional low-pressure metal-organic(LP-MO) CVD by using b-diketonate metal complexes as precursors [(Zr(tmhd) and Sc(tmhd) 3 , with -tmhd ¼ 2,2,6,6-tetramethyl-3,5-heptanedionate]. The compositional (energy dispersive X-ray spectroscopy -EDX), structural (X-ray diffraction -XRD) and morphological (field emission gunenvironmental scanning electron microscopy -FEG-ESEM) analyses, confirmed the growth of dense partially and fully stabilized ZrO 2 , a suitable electrolyte for solid oxide fuel cells (SOFC). Results of impedance spectroscopy, which investigates the electrical conductivity of coating, deposited as thin as possible to guarantee the uniform covering of a porous substrate, are reported. Results of thin films of yttria-zirconia system (Y 2 O 3 :ZrO 2 ), deposited with the same method, are also reported for comparison.
Solid State Ionics, 1998
In search for better ionically conducting ceramics for oxygen separators, fuel cells and sensors, the electrical conductivity and microstructure of the 9 mol% (Sc O-Y O)-ZrO system with varying Sc O / Y O ratios has been investigated in 2 3 2 3 2 2 3 2 3 detail with XRD, SEM, TEM and conductivity measurements as a function of temperature. The stability of electrolyte compositions was studied by continuously monitoring conductivity as a function of time at 850 and 10008C. Impedance spectroscopy was employed for determining the contribution of the grain boundary resistivity. The role of alumina additions to selected Sc O-Y O-ZrO compositions was studied as alumina is known to reduce the grain boundary resistivity by 2 3 2 3 2 scavenging silica impurities and enhance mechanical properties in zirconia-based systems. Al O-containing compositions 2 3 show much higher conductivity degradation compared with alumina-free materials. This behaviour has been investigated in more detail with XRD and TEM analysis.
Microscopy and Microanalysis, 2013
As the presence of Sc2O3 and CeO2 is known to largely enhance the ionic conductivity in the temperature range of 600–800°C, compared with the conventional yttria-stabilized ZrO2, Sc2O3&CeO2-stabilized ZrO2 provide its applicability as electrolytes in solid oxide fuel cells. The current study introduces the methodology to synthesize Sc2O3&CeO2-stabilized ZrO2 powders by using co-precipitation technique or high-temperature hydrothermal reaction, and further describes the structural characterization of the zirconia powders synthesized by the above-mentioned two methods. The co-precipitation technique was found to allow obtaining powders of cubic phase, whereas high-temperature hydrothermal synthesis results in the presence of a monoclinic phase as well. The scanning transmission electron microscope observations also confirm that the size of the synthesized ZrO2 powders in this study is found to be much smaller than that of commercially available powders.