16148069
Constrained Sintering in Fabrication of Solid Oxide Fuel Cells
Solid oxide fuel cells (SOFCs) are inevitably affected by the tensile stress field imposed by the rigid substrate during constrained sintering, which strongly affects microstructural evolution and flaw generation in the fabrication process and subsequent operation. In the case of sintering a composite cathode, one component acts as a continuous matrix phase while the other acts as a dispersed phase depending upon the initial composition and packing structure. The clustering of dispersed particles in the matrix has significant effects on the final microstructure, and strong rigidity of the clusters covering the entire cathode volume is desirable to obtain stable pore structure. The local constraints developed around the dispersed particles and their clusters effectively suppress generation of major process flaws, and microstructural features such as triple phase boundary and porosity could be readily controlled by adjusting the content and size of the dispersed particles. However, in the fabrication of the dense electrolyte layer via the chemical solution deposition route using slow-sintering nanoparticles dispersed in a sol matrix, the rigidity of the cluster should be minimized for the fine matrix to continuously densify, and special care should be taken in selecting the size of the dispersed particles to optimize the thermodynamic stability criteria of the grain size and film thickness. The principles of constrained sintering presented in this paper could be used as basic guidelines for realizing the ideal microstructure of SOFCs.
136858079
Materials and technologies for SOFC-components
Solid oxide fuel cells are a forward looking technology for a highly efficient, environmental friendly power generation. A SOFC is a multilayer structure consisting of ceramic and metallic materials with different electrical transport properties. All components have to show a well adjusted thermal expansion behavior, chemical compatibility of material interfaces and chemical stability in the prevailing atmospheres. The performance of SOFC single cells is not only determined by intrinsic material properties. There is a significant influence due to the fabrication technology respectively the microstructure at the electrode/electrolyte-interfaces. The performance of cells can only be improved by the application of elevated materials using appropriate technologies.
135789511
A numerical tool to estimate SOFC mechanical degradation: Case of the planar cell configuration
Abstract The stress state and cell degradation have been modelled considering the classical solid oxide fuel cells (SOFCs) materials (Ni–YSZ//YSZ//LSM). The anode-supported cell is found to be un-cracked after its elaboration. However, the high level of energy stored in the thin electrolyte layer lowers the cell robustness as it constitutes the driving force for interfacial delamination. The elaboration of the electrolyte supported cell leads to the anode layer cracking. The heating up to 800 °C and the cermet reduction allow relaxing the stress level in the cell. For the anode-supported cell, the cermet re-oxidation induces cathode damage as soon as the anodic strain exceeds 0.05–0.09% whereas the electrolyte fracture occurs for strain ranging between 0.12% and 0.15%. For the electrolyte supported cell, the cermet re-oxidation induces a triggered risk of anode/electrolyte delamination when the anodic strain is higher than 0.3–0.35%. The risk of rupture initiation in cell singularities has been also investigated. A pure material electrode/electrolyte singularity is harmless. A perpendicular corner at the electrode/electrolyte free edge constitutes a harmful singularity.
95288354
Materials and concepts for solid oxide fuel cells (SOFCs) in stationary and mobile applications
Abstract Solid oxide fuel cells (SOFCs) are a forward-looking technology for highly efficient, environmentally–friendly power generation. The electrochemically active single cell is a multilayer structure consisting of ceramic and metallic materials with different electrical transport properties. Materials for electrodes should exhibit electronic and ionic conductivity in combination with porosity and catalytic activity, whereas the electrolyte has to be a purely ionically conducting and gas-tight membrane. All components have to show well-adjusted thermal expansion behavior, chemical compatibility of material interfaces and chemical stability in the prevailing temperature and gas atmosphere. The performance and long-term stability of the single cell is significantly increased by the use of suitable materials, a proper design of the cell and an optimized microstructure at the electrode/electrolyte interfaces. The application of appropriate technologies for the production of single cells with optimized microstructure becomes even more important for highly efficient SOFCs operating in the medium and low temperature range.
136855189
Effect of internal and external constraints on sintering behavior of thin film electrolytes for solid oxide fuel cells (SOFCs)
Abstract The effect of internal and external constraints on the sintering behavior of yttria-stabilized zirconia (YSZ) thin film electrolytes is investigated for intermediate-temperature solid oxide fuel cells (SOFCs). The external constraints imposed by rigid substrates result in major process flaws, such as vertical and delamination cracks during film densification, in a chemical solution deposition (CSD) process. In this study, YSZ nanoparticles are incorporated into the precursor solution to impose internal constraints and control the shrinkage rate of the sol–gel matrix. Because the structural stability of the thin film is determined by the grain size distribution, it is critical to optimize the content and dispersion structure of the nanoparticles and the sintering temperature to obtain a gas-tight thin film electrolyte for SOFCs. A clustering model is proposed to obtain a comprehensive understanding of the sintering behavior of the YSZ thin-film electrolyte containing the nanoparticles in the CSD process.
135498853
Processing‐Related Fracture Origins: II, Agglomerate Motion and Cracklike Internal Surfaces Caused by Differential Sintering
Effects of differential sintering kinetics due to differential green density were studied by fabricating agglomerate-powder matrix specimens from two Al2O3/ZrO2 (30 vol%) powders and subjecting them to a cyclic sintering schedule to allow intermittent observations. Low-green-density compacts sintered faster than high-density compacts, although high-green-density compacts reached end-point densities first. Specimens containing low-green-density agglomerates produced circumferential cracklike voids at the agglomerate/matrix interface. Agglomerates with a higher green density than the matrix were subjected to compressive strains by the matrix to produce agglomerate motion when the resulting stress field was non-symmetric. Such agglomerates also produced a variety of cracklike separations. Implications regarding sintering kinetics and end-point densities are discussed. Implications regarding strength of sintered bodies are obvious, viz. strength can be proportional to the square root of the agglomerate size.
137226993
Thin film yttria-stabilized zirconia electrolyte for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition
Abstract A 500 nm thick thin film YSZ (yttria-stabilized zirconia) electrolyte was successfully fabricated on a conventionally processed anode substrate by spin coating of chemical solution containing slow-sintering YSZ nanoparticles with the particle size of 20 nm and subsequent sintering at 1100 °C. Incorporation of YSZ nanoparticles was effective for suppressing the differential densification of ultrafine precursor powder by mitigating the prevailing bi-axial constraining stress of the rigid substrate with numerous local multi-axial stress fields around them. In particular, adding 5 vol% YSZ nanoparticles resulted in a dense and uniform thin film electrolyte with narrow grain size distribution, and fine residual pores in isolated state. The thin film YSZ electrolyte placed on a rigid anode substrate with the GDC (gadolinia-doped ceria) and LSC (La 0.6 Sr 0.4 CoO 3− δ ) layers deposited by PLD (pulsed laser deposition) processes revealed that it had fairly good gas tightness relevant to a SOFC (solid oxide fuel cell) electrolyte and maintained its structural integrity during fabrication and operation processes. In fact, the open circuit voltage was 1.07 V and maximum power density was 425 mW/cm 2 at 600 °C, which demonstrates that the chemical solution route can be a viable means for reducing electrolyte thickness for low- to intermediate-temperature SOFCs.
70607604
A high performance ceria based interdiffusion barrier layer prepared by spin-coating
Abstract A multiple spin-coating deposition procedure of Ce0.9Gd0.1O1.95 (CGO) for application in solid oxide fuel cells (SOFCs) was developed. The thin and dense CGO layer can be employed as a barrier layer between yttria stabilised zirconia (YSZ) electrolyte and a (La, Sr)(Co, Fe)O3 based cathode. The decomposition of the polymer precursor used in the spin-coating process was studied. The depositions were performed on anode supported half cells. By controlling the sintering temperature between each spin-coating process, dense and crack-free CGO films with a thickness of approximately 1 μm were obtained. The successive steps of dense layer production was investigated by scanning electron microscopy. X-ray diffraction was employed to monitor the crystal structure of the CGO layer sintered at different temperatures. The described spin coated barrier layer was evaluated using an anode supported cell with a composite cathode made of La0.58Sr0.4Co0.2Fe0.8O3 (LSCF) and Ce0.9Gd0.1O1.95. The developed CGO layer, was sufficient to restrain reactions between the perovskite based electrode and the electrolyte.
96344710
Simulation of a composite cathode in solid oxide fuel cells
Incorporating the mechanistic model for oxygen reduction at YSZ/LSM interface, a complete micro-model for YSZ/LSM composite cathode considering all forms of polarization was developed which established the interrelationship among the transport phenomena, electrochemical processes and the microstructure of the composite cathode. The exchange current densities of the rate-limiting steps used in the simulation were obtained by fitting the proposed mechanistic model to the DC polarization curves. Simulation was conducted to predict the optimal design parameters, e.g. cathode thickness, particle size, particle size ratio and YSZ volume fraction, for a LSM/YSZ composite cathode. Except for the YSZ volume fraction beyond the percolation thresholds, the predicted results seem to be in good agreement with the experimental and literature data. Incorporating with reliable experimental data, the model can be used as a tool to guide the design of high performance cathodes.
97334202
Development of LSCF–GDC composite cathodes for low-temperature solid oxide fuel cells with thin film GDC electrolyte
Abstract La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) powder was prepared by glycine–nitrate combustion method. The electrochemical properties of porous LSCF cathodes and LSCF–Gd 0.1 Ce 0.9 O 1.95 (GDC) composite cathodes were evaluated at intermediate/low temperatures of 500–700 °C. The polarization resistance of pure LSCF cathode sintered at 975 °C for 2 h was 1.20 Ω cm 2 at 600 °C. The good performance of pure LSCF cathode is attributed to its unique microstructure—small grain size, high porosity and large surface area. The addition of GDC to LSCF cathode further reduced the polarization resistance. The lowest polarization resistance of 0.17 Ω cm 2 was achieved at 600 °C for LSCF–GDC (40:60 wt%) composite cathode. An anode-supported solid oxide fuel cell (SOFC) was prepared using LSCF–GDC (40:60 wt%) composite as cathode, GDC film (49-μm-thick) as electrolyte, and Ni–GDC (65:35 wt%) as anode. The total electrode polarization resistance was 0.27 Ω cm 2 at 600 °C, which implies that LSCF–GDC (40:60 wt%) composite cathode used in the anode-supported SOFC had a polarization resistance lower than 0.27 Ω cm 2 at 600 °C. The cell generated good performance with the maximum power density of 562, 422, 257 and 139 mW/cm 2 at 650, 600, 550 and 500 °C, respectively.
136997507
Fabrication of thin-film gadolinia-doped ceria (GDC) interdiffusion barrier layers for intermediate-temperature solid oxide fuel cells (IT-SOFCs) by chemical solution deposition (CSD)
Abstract A dense gadolinia-doped ceria (GDC) interdiffusion barrier layer as thin as 300 nm was successfully fabricated on a rigid anode/electrolyte bilayer substrate using the chemical solution deposition (CSD) process for intermediate temperature solid oxide fuel cells (SOFCs). Drying-related macro-defects were removed by employing drying control chemical additives (DCCA), which effectively relieved drying stresses. The major process flaws caused by the constraining effects of the rigid substrate were completely eliminated by the addition of GDC nanoparticles into the chemical solution, which suppressed the generation of microstructural anisotropy by mitigating the predominant bi-axial substrate constraints. As a consequence, a thin film GDC interlayer was successfully deposited with a high volumetric density, effectively preventing the chemical interaction between the electrolyte and cathode during the fabrication process and subsequent operation. The cell test and microstructural analysis confirmed excellent electrochemical performance and structural and chemical stability. The CSD process presented in this paper is considered to be a promising technology for the practical preparation of GDC thin film barrier layers for intermediate temperature SOFCs based on the film quality, processing costs and potential for large-scale production.
205244868
Extremely thin bilayer electrolyte for solid oxide fuel cells (SOFCs) fabricated by chemical solution deposition (CSD).
An extremely thin bilayer electrolyte consisting of yttria-stabilized zirconia (YSZ) and gadolinia-doped ceria (GDC) is successfully fabricated on a sintered NiO-YSZ substrate. Major processing flaws are effectively eliminated by applying local constraints to YSZ nanoparticles, and excellent open circuit voltage and cell performance are demonstrated in a solid oxide fuel cell (SOFC) at intermediate operating temperatures.
96223197
Characterization of electrical properties of GDC doped A-site deficient LSCF based composite cathode using impedance spectroscopy
Abstract The composite cathode consisting of A-site deficient perosvkite material La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3− δ (L58SCF) and Ce 0.8 Gd 0.2 O 2− δ (GDC) on YSZ electrolyte is studied for potential applications in intermediate/low-temperature solid oxide fuel cells (SOFCs). Impedance spectroscopy measurements are performed in air over the temperature range of 600–800 °C under open circuit potential. The results show that the addition of 40 wt.% GDC to L58SCF (L58SCF–GDC40) results in the lower polarization resistance (0.07 Ω cm 2 at 800 °C, 0.11 Ω cm 2 at 750 °C and 0.22 Ω cm 2 at 700 °C) than other composite cathodes and its activation energy values calculated for the low- and high-frequency arcs are 0.86 and 1.10 eV, respectively. The composite cathode exhibit high exchange current density and low charge-transfer resistance.
94986674
Quantitative three-dimensional microstructure of a solid oxide fuel cell cathode
Abstract Solid oxide fuel cells (SOFCs) are being actively developed world wide for clean and efficient electrical generation from fuels such as natural gas, hydrogen, coal, and gasoline. The cathode in state of the art SOFCs is typically a porous composite of electronically-conducting La 1− x Sr x MnO 3 (LSM) and ionically-conducting Y 2 O 3 -stabilized ZrO 2 (YSZ) that facilitates the critical oxygen reduction reaction. Here we describe the three-dimensional characterization and quantification of key structural parameters from an LSM-YSZ cathode, using imaging and volume reconstruction based on focused ion beam – scanning electron microscopy. LSM-YSZ-pore three-phase boundaries (TPBs) were identified. Approximately 1/3 of the TPBs were found to be electrochemically inactive, as they were on isolated LSM particles, yielding an active TPB density of 4.9 μm −2 . Cathode electrochemical modeling, which included a measured YSZ tortuosity of 3.4, yielded an effective TPB resistance of ≈2.5 × 10 5  Ω cm at 800 °C.
137089285
Inclusion size and sintering of composite powders
Ceramic powders containing dispersed inert particulates often exhibit dramatically lower densification rates than the pure powder. This decrease results from the shrinkage incompatibilities of the matrix and the inclusion during densification. The analyses of this phenomenon all predict that the shrinkage rate is only dependent on the overall volume fraction of the inert second phase, and not on the particle size. Experiments on ZnO containing dispersed SiC particles of different size showed that the retarding effect of the second phase increases significantly with decreasing particle size, at constant SiC volume fraction.
137246777
Analysis of Sintering of a Composite with a Glass or Ceramic Matrix
A recently developed theory for sintering of bimodal powders is applied to a composite where the matrix is either a glass or a ceramic. The filler phase is assumed to be rigid and nonsintering. It is shown that glass-matrix composites should have predictable sintering behavior which follows the rule of mixtures, but ceramic-matrix composites are expected to be more difficult to sinter, and their sintering behavior may deviate considerably from the rule of mixtures, even at low concentrations of the filler phase.
137531590
Viscoelastic stresses and sintering damage in heterogeneous powder compacts
Abstract A method for calculating viscoelastic stresses that develop around heterogeneities during sintering is developed. The method uses constitutive laws derived from experimental data obtained on porous, partially sintered bodies. Analysis of stresses using literature data for A1 2 O 3 reveal that the stresses around heterogeneities vary appreciably with density, particle size and temperature. The largest stresses occur in association with hard agglomerates.
138798406
Effect of Inclusions on Densification: II, Numerical Model
Experimental studies conducted in conjunction with a numerical analysis of strain-rate gradients have established the origin of the damage process that occurs upon the densification of powders containing nonsintering inclusions and reinforcements. The underlying phenomenon is the development of localized compressive strains in the porous matrix, both around high-aspect ratio reinforcements and between closely spaced reinforcements. These regions of compression densify first and then support grain growth. This process produces nondeformable networks that constrain the shrinkage of the adjacent, porous matrix. The constraint causes desintering and cracklike void formation in the lowdensity regions. The variables shown to be of importance are the volume fraction and aspect ratio of the reinforcements. The process is shown to be sensitive to the green density, such that a high initial density reduces initial damage and lowers the differential in densification.
137501762
Influence of thickness on the constrained sintering of alumina films
Alumina films prepared by tape casting were sintered freely and under geometrical constraint at 1350 °C. The effect of film thickness on sintering kinetics and microstructure development was investigated. A decrease in film thickness in the constrained case leads to enhanced retardation of densification and increased orientation of anisometric pores.
137148564
Anisotropic Microstructural Development During the Constrained Sintering of Dip‐Coated Alumina Thin Films
Alumina thin films were manufactured from aqueous slurries by dip coating. Film thickness as function of substrate withdrawal velocity could be correctly modeled by Landau and Levich's theory. Samples were sintered on a rigid substrate at 1350°C for different isothermal times to achieve relative densities from 84% to 97%. Microstructural analysis of polished cross sections revealed a continuous development of pore alignment, as expected by theoretical considerations. With increasing density pores become more anisometric and orientate along the thickness direction. A further preferential orientation of pores was found in the normal plane, apparently due to the coating process. Constraining conditions had less influence on the size and shape of the grains; they tend to become more equiaxed in the constrained plane, presumably due to the biaxial tensile stress state.
137605048
Constrained sintering: A delicate balance of scales
Sintering is a common technique for the production of ceramic, metallic and composite bodies and components. During this process it is known that internal stresses can arise from external constraint or from differential densification. These stresses can give rise to significant strain rate changes, distortions and damage in the fired parts. For quality control of sintered bodies, it is necessary to measure or predict the changes in deformation behaviour that occurs as the porous starting bodies undergo densification. Contributions of the continuum mechanical approach to this problem of constrained sintering are reviewed. Theoretical predictions as well as experimental techniques and results are detailed. Special emphasis is given to the development of anisotropy that can develop in the microstructure of a sintering body in response to the internal stresses.
136707395
TRANSIENT STRESSES IN BIMODAL COMPACTS DURING SINTERING
Abstract A method is described and used to evaluate the transient stresses in a sintering compact of ZnO containing a hard, dense dispersion of SiC. A hard second phase can severely limit densification rates by generating a mean hydrostatic stress, σh, which opposes the compressive sintering stress of the matrix. σh rapidly increases with increasing volume fraction, ƒ, of the second phase. The interface stress, σi., at the ZnO SiC boundary increases with decreasing ƒ, σi can attain large values, especially in the intermediate stage of sintering. The effect of these stresses on microstructural development is considered.
137566861
Sintering behavior of bi-modal powder compacts
Abstract Time dependent sintering of a bi-modal powder compact, consisting of two regions which sinter at different rates, is analyzed in detail. Complete solutions are obtained for the internal stress and for the densification rate. The important feature of the analysis is that it combines densification with deviatoric creep, since creep serves to relax the stress concentration produced by incompatible densification. The structure of the bi-modal compact is assumed to consist of a sphere of one type of material embedded in a matrix of the other material. Two cases are considered. In one case the matrix, and in the other the sphere, is assumed to sinter faster. The maximum tensile stress generated by incompatible sintering is found to be sensitive to a parameter, β, which is the ratio of the rate constant for creep, and the rate constant for densification. A large value of β reduces the magnitude of the stress. The generation of flaws as a result of this stress is considered. The influence of inhomogeneous sintering on the overall rate of shrinkage is calculated; we find that unless β is large the densification rate of the composite will deviate significantly from the simple rule-of-mixtures.
10394534
Calculation of interparticle spacing in colloidal systems.
Interparticle spacing (IPS) is a very important parameter for estimation of the viscosity of a suspension. A new equation for calculating IPS was derived on the assumption that each particle is surrounded by a virtual cell, which is the free volume that each particle can occupy. This idea was originated by Kuwabara and widely used for calculating electrophoretic mobility in concentrated suspensions. Our new IPS equation was evaluated and compared with a pre-existing IPS equation proposed by Dinger and Funk using experimental data from three commercial sources of titania. Our equation was found to give reasonable values, including cases where the equation of Dinger and Funk gave anomalous values.