Electron hole conductivity of gadolinia doped ceria

The electron hole conductivity of gadolinia doped ceria (GCO) was estimated from electrochemical permeability measurements, at temperatures in the range of 800 to 1000°C. The transient behaviour of the cell (while recovering from reducing to oxidising conditions) was used to assess the accuracy of the experimental procedure, and some of these results still suggested the presence of leaks in the sealings. However, the temperature dependence of steady state results indicates that electrochemical permeability prevails, and these results were thus used to estimate the hole conductivity. These results showed that the hole conductivity of GCO at 1000°C is about two orders of magnitude higher than reported for yttria stabilized zirconia, and the activation energy (129 kJmol⁻¹) is slightly lower.     

Magnesia stabilized zirconia doped with boron,ceria and gadolinia

In this study, magnesia stabilized zirconia based nanocrystalline ceramics were produced through a polymer precursor route using gadolinium and boron. The powders were characterized during the various steps by structural and morphological techniques (FT-IR, XRD, and SEM). XRD results proved that a tetragonal phase is predominant for all samples with varying magnesium contents and no monoclinic zirconia solid solution appears. The crystallite sizes of the samples were calculated using Scherrer equation. The smallest crystallite size was obtained for the sample containing Zr/Mg/Ce/Gd/B ratio of 82/0/10/8/0. The lattice parameters were calculated for cubic, tetragonal, hexagonal, and orthorhombic structures. SEM results show all the samples have spherical grains. The average grain diameters were calculated for all the samples. The smallest average grain diameter was obtained for the sample containing Zr/Mg/Ce/Gd/B ratio of 82/0/10/8/0.     

Synthesis of nano-sized gadolinia doped ceria powder by aerosol flame deposition

In this work, aerosol flame deposition method was applied to deposit spherical and dense gadolinia-doped ceria (GDC) particles in the sub-micron range for the electrolyte application in solid oxide fuel cell. The particle size distribution was dependent on processing parameters such as the concentration of the precursor solution, the hydrogen gas flow rate, the oxygen gas flow rate, and flame conditions. GDC electrolyte thin layer was also fabricated from the liquid source materials by AFD method. Microstructure of synthesized powder was characterized using XRD and SEM.     

Preparation of nanocrystalline gadolinia doped ceria powders by combustion synthesis process

Abstract

Nanocrystalline gadolinia doped ceria powder was prepared by a combustion synthesis process using polyvinyl alcohol as fuel, with the aim of obtaining nanosized crystallites of this solid solution with high specific surface area. The structures of the polymer gel and the calcined materials were investigated by Fourier transform infrared spectroscopy. Residual carbon content and specific surface area determinations were carried out on calcined powders. Microstructural characterisation was done by X-ray diffractometry and scanning electron microscopy. The results demonstrate that nanocrystalline particles of the solid solution can be prepared with low carbon content and high specific surface area by this one step synthesis technique.     

Toward nanograined gadolinia doped ceria via two-step sintering at ultralow temperature and its electrical conductivity

Highly dense (>98%) and nanograined (∼60 nm) gadolinia doped ceria are obtained from ultrafine powders by adopting two-step sintering (TSS) procedure at an ultralow temperature of 750 °C with a dwell time of 20 h, which is the lowest sintering temperature for ceria family without sintering aids up to now. Electrochemical impedance spectroscopy investigations suggest that the electrical conductivities of densified electrolytes are closely related to sintering temperature and grain size, and GDC900-750 exhibits the highest total electrical conductivity of 3.640 S m⁻¹ at 700 °C in air. Fitting calculation indicates partial grain-size dependence of oxygen vacancy association enthalpy and grain-size independence of oxygen ion migration enthalpy. Grain boundary maturity influences on grain boundary conductivity to some extent, and younger grain boundary endues the densified electrolytes with higher grain boundary conductivity.     

Tape casting of nanocrystalline ceria gadolinia powder

A ceramic ceria gadolinia solid solution membrane for solid oxide fuel cells was fabricated by tape casting using a nanopowder of 37 nm average particle size. A novel combination of solvent and dispersant was used to disperse the nanoparticles. The polymer was added in a dilute stage to guarantee a homogeneous distribution. After casting a remarkable densification of the cast tape suspension from a solid loading of 20 up to 42 vol.% was observed during drying. The green tape was sintered to >92% theoretical density and was dense towards perfusion. The resulting grain size in the sintered specimen still was <200 nm.     

Lower down both ohmic and cathode polarization resistances of solid oxide fuel cell via hydrothermal modified gadolinia doped ceria barrier layer

Hydrothermal reaction in Cerium and Gadolinium solution as an optimization method is developed and first reported for the densification of gadolinia doped ceria, the barrier layer between Zirconia electrolyte and (La,Sr)(Co,Fe)O_3-δ cathode. This method is based on the hydrothermal reaction for nano particles in-situly grown on porous surface, to improve barrier layer density, alongside the sintering of cathode at 1075 °C. As a result, the ohmic resistance is prominently decreased by ～16.4 % at 750 °C for electrolyte supported symmetrical cell. Whereas, the cathode polarization resistance is decreased by as much as a factor of ～3 from 0.3702 Ω·cm² to 0.1325 Ω·cm² at 750 °C and $p_{O_2}=0.21atm$. Furthermore, the anode supported cell exhibits higher open circuit voltage, smaller area specific resistance, elevated performance output and less degradation. And this modified barrier layer shows reduced Sr migration in 300 h operation at 750 °C. The hydrothermal reaction is demonstrated to prepare denser and sintering-active barrier layer with faster oxygen ion transfer and better interface connection, with large-scale application prospects and cost-competitiveness.     

Electrical conductivity of praseodymia doped ceria

The electrical conductivity () and oxide ion transference number (t ₀) of praseodymia doped ceria systems were measured. The former increased rapidly with the praseodymia content, while the latter decreased. At 600° C, for instance, CeO₂ and Ce_0.6Pr_0.4O₂ under 0.21 atm of oxygen were 2.0×10^–5 and 3.6×10^–2 S cm^–1; andt ₀ in them were 0.59 and 0.11, respectively. This mixed conductor having high electrical conductivity might be useful as a fuel cell electrode if it could be combined with a suitable solid electrolyte.     

Catalytic activity of pure and doped ceria

Structural and phase changes accompanying the thermal dehydration of pure and doped hydrated eerie oxide have been followed by thermogravimetric analysis, differential thermal analysis, and X-ray diffraction studies. Doping has been affected by either Zn²⁺, Al²⁺ or U⁶⁺ ions. Texture investigation has been studied by analysis of nitrogen adsorption isotherms. Variation in porosity due to annealing and/or doping, as well as mechanisms of foreign ion incorporation, have been correlated with catalyst activity towards decomposition of isopropanol.     

Studies on electrodeposited nickel–yttria doped ceria composite coatings

Incorporation of ceria particles into the Ni matrix was found to improve the corrosion resistance of pure Ni coatings. With the aim of further improving the corrosion resistance of Ni-ceria, yttria was doped with ceria and used as distributed phase. About 8-mol% yttria doped ceria (8YDC) particles synthesized by a solution combustion process were dispersed in a nickel sulfamate bath and electrodeposition was carried out to prepare Ni–8YDC composite coatings at various current densities. The microhardness of the composite coatings was determined. Optical microscopy confirmed the incorporation of 8YDC particles into the Ni matrix. Potentiodynamic polarization and electrochemical impedance spectroscopy were used to characterize the corrosion behavior of the Ni–8YDC coatings. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Analysis (EDAX) were used to characterize the corroded samples. The results were compared with those for Ni and Ni–CeO₂ coatings. The wear behavior of Ni–8YDC was studied. Wear tracks were characterized by MicroRaman Spectroscopy.     

Solvent-deficient method lowers grain-boundary resistivity of doped ceria

Nanoparticles of gadolinium-doped cerium oxide (GDC) were synthesized using solvent-deficient method and their sinterability and electrical properties were investigated using the powder and cold sintering process. The GDC powder was uniaxially pressed into cylindrically-shaped pellets with a mixture of nitric acid and hydrogen peroxide at 200°C to encourage particle arrangement during forming process. These bulk samples were annealed using two different temperature profiles: at 800°C for 5 hours and at 1300°C for 1 minute—800°C for 5 hours. The samples produced using HNO₃/H₂O₂ mixture showed higher relative density than ones without it. Ionic conductivity of the sample sintered through the two-step profile was obtained from electrochemical impedance spectroscopy. Although the grain conductivity for the samples (8.0 × 10⁻³ S cm⁻¹ at 500°C, and 3.3 × 10⁻² S cm⁻¹ at 700°C) is on par with a conventionally sintered sample, the measured total conductivity (3.9 × 10⁻³ S cm⁻¹ at 500°C, and 2.5 × 10⁻² S cm⁻¹ at 700°C) is about 10 times higher than the conventionally sintered one and is comparable to the values seen in the previous studies for GDC which employed higher sintering temperature, pointing to the effectively lower grain-boundary impedance. This result could be attributed to no significant phase segregation along grain boundaries due to the low-temperature processing.     

Electrospinning and characterisation of ceria doped yttria fibres

Abstract

Homogeneous inorganic-organic composite fibres were produced using electrospinning technique from alcoholic solutions containing polyvinyl butyral and precursors of yttrium and cerium ions. Upon heat treatment, ceria and yttria doped ceria fibres retaining the original morphological features observed in the as spun composition were obtained. X-ray diffraction was used to identify the crystalline phases of the final products. Scanning electron microscopy, thermogravimetric analysis, differential thermal analysis and Brunauer-Emmett-Teller analysis were used to study the ceramic phase formation and the evolution of morphological features of the fibres. Thus, several micrometres long, uniform ceria and yttria doped ceria fibres of high phase purity were produced. The CeO₂ and the CeO₂ with Y₂O₃ fibres presented average diameter that ranged from 19 to 25?μm, and the distribution of specific surface ranged from 33 to 43?m²?g^?1.     

Electrophoretic deposition of doped ceria: Effect of solvents on deposition microstructure

Electrophoretic deposition of doped ceria has been carried out in non-aqueous solvent to prepare coatings on different substrates and free standing films. It has been found that uneven deposition occurred in ethanol, while in butanol deposition yield is low having very little variation with deposition time. On the other hand, good deposit obtained in acetyl acetone medium, but had a porous structure. The best result however was obtained in mixed solvent. Effect of adding charge modifying additives in the ceria suspension on the deposit microstructure has been studied. Mechanism of charging in the non-aqueous medium to modify the surface properties of the suspended particles has been discussed.     

Interdiffusion at electrochemical interfaces between yttria-stabilized zirconia and doped ceria

Integration of doped ceria into fuel electrode-supported solid oxide cells is challenging due to high sintering temperatures leading to undesirable interdiffusion between the layers.We investigate the influence of the dopant in ceria X_0.1Ce_0.9O_1.95 (10XDC, X = Y, Gd or Sm) on the interdiffusion with yttria-stabilized zirconia (8YSZ). Powder mixtures of 8YSZ and 10XDC were sintered at temperatures between 1000 and 1400 °C to quantify the phase formation. Interdiffusion in layered systems sintered at 1400 °C was investigated by SEM. Symmetrical Ni-10XDC cells with an 8YSZ electrolyte were analyzed using impedance spectroscopy. Despite small differences in the interdiffusion behavior, different dopants do not lead to significant changes in the cell impedance.Notably, the presence of NiO in the fuel electrode leads to enhanced interdiffusion kinetics of 10XDC with 8YSZ and the formation of porosity at the electrolyte interface. The detrimental influence of these microstructural changes on the electrode performance was investigated.     

Synthesis and characterization of neodymium doped ceria nanocrystalline ceramic structures

In this study, a new method to synthesize neodymium doped ceria ceramic nanopowders by the electrospinning of the hybrid polymers solution of their composite precursor was put forward. Calcined and sintered nanopowders were characterized by FT-IR, XRD, BET, SEM, and AFM techniques. According to the XRD analysis, the obtained powders are single phase and independent of the dopant concentration in the range investigated. The crystallite sizes were calculated using Scherrer equation. Moreover, lattice parameters, dislocation densities and microstrain values were calculated. BET results show that the increase of the neodymium doped content decrease the surface area of the composite powders, confirming the highly ordered micro and mesostructure. SEM and AFM results show that the samples have spherical grains. According to the surface roughness measurements, the increase in the amount of neodymium and the decrease in the amount of cerium decreased the surface roughness.     

Multiscale modeling of the ionic conductivity of acceptor doped ceria

The ionic conductivity of acceptor doped ceria is strongly influenced by grain boundaries and interfaces, with most experiments showing a conductivity decrease in these regions. Classical models explain this observation by the formation of space charge layers, that are depleted of mobile ionic charge carriers. However, some experiments demonstrate an increase in ionic conductivity and recent models show that the space charge layers can also be enriched in mobile ionic species. Because of these discrepancies, it is still not certain whether nanocrystalline or thin film ceria can offer superior ionic conductivity or not. Recently, we have demonstrated by means of Monte Carlo simulations that the ionic conductivity in space charge layers can indeed exceed the bulk value. In this work, we combine these Monte Carlo simulations with a continuum model to predict charge carrier concentration profiles. This multiscale approach allows for a realistic prediction of the grain boundary ionic conductivity.     

Ionic conductivity of space charge layers in acceptor doped ceria

The ionic conductivity of acceptor doped ceria is strongly influenced by grain boundaries and interfaces. Most experiments show a decrease in ionic conductivity and an increase in electronic conductivity in these regions. Classical models explain this observation by the formation of space charge layers that are depleted of mobile ionic charge carriers and enriched in small polarons. However, some experiments demonstrate an increase in ionic conductivity and recent models show that the space charge layers can also be enriched in mobile ionic species. Because of these contradictions, it is still not clear whether nanocrystalline or thin film ceria can offer superior ionic conductivity or not. To aid this debate, we calculate the ionic conductivity of yttrium doped ceria in regions of net charge density using kinetic Monte Carlo simulations. Through an appropriate choice of the charge densities, these calculations allow to demarcate the possible conductivity gains from space charge layers.     

Synthesis,densification and characterization of Ag doped ceria nanopowders

Nanosized Ag-doped ceria (Ce_1-xAl_xO_2-δ)powders (0.1 ≤ x ≤ 0.04) were obtained by self-propagating room temperature reaction. The solid solubility of Ag into ceria lattice was the highest reported so far. X-ray diffraction analysis and field emission scanning microscopy results showed that the doped samples are single phase solid solutions with fluorite-type structure and all prepared powders were nanometric in size. The average size of Ce1-xAgxO2-▯ particles lies at about 4 nm. Raman spectra revealed an increase in the amount of oxygen vacancies with the increase of Ag concentration, such as is foreseen. The thermal stability of solid solution was followed by XRD. Microstructure development was studied by scanning electron microscopy. By controlling the processing variables, it was possible to obtain high density samples with homogeneous microstructure at low sintering temperature.     

Two-step sintering of Sm2O3 doped ceria stabilized zirconia

Effect of Sm₂O₃ addition and two-step sintering of Ceria Stabilized Zirconia (CSZ) on microstructure and mechanical properties were investigated in the present work. Samaria doped CSZ (SmCSZ) nanopowders were prepared by co-precipitation synthesis from their respective nitrate salts. Synthesized powders were calcined at 1000?°C for 2?h and then compacted to ?10?mm pellets using a uniaxial hydraulic press. Single step & two-step sintering methods were used to sinter the compacted pellets. Powders and sintered pellets were characterized for phase and microstructure using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) technique. Rietveld method was used for quantification of obtained phases. The hardness of the sintered samples was evaluated by Vicker's hardness tester, and toughness was estimated by indentation fracture toughness method. Samples sintered using two-step sintering method shown optimum hardness and toughness (up to 1288 HV10 and 5.37?MPa?m^1/2) values compared to conventionally sintered samples because of reduced grain size.     

Synthesis of ceria doped nanozirconia powder by a polymerized complex method

Nanocrystalline ceria-doped zirconia powders with mesoporous structure and tetragonal phase have been synthesized by a polymerized complex method based on the Pechini process. This method is based on the gelling of nitrate solutions by the addition of citric acid and ethylene glycol, followed by a calcination process. The X-ray diffraction (XRD), nitrogen adsorption (BET), Transmission Electron Microscopy (TEM), Thermogravimetric (TG) and differential thermal analysis (DTA) were adopted for the characterization of the synthesized materials. The effects of citric acid to ethylene glycol, calcination temperature and ceria content on the properties of the ceria-doped zirconia have been investigated. X-ray diffraction analysis of the samples showed the stabilization of the tetragonal phase at room temperature because of their small crystallite sizes. These samples showed a mesoporous structure with low porosity.