279 results found
Chen J, Ouyang M, Boldrin P, et al., 2020, Understanding the coarsening and degradation in a nanoscale nickel gadolinia-doped-ceria electrode for high-temperature applications., ACS Applied Materials and Interfaces, Vol: 12, Pages: 47564-47573, ISSN: 1944-8244
Nanostructure engineering is an effective approach to enhance the electrochemical performance of energy devices. While the high surface area of nanoparticles greatly enlarges the density of reaction sites, it often also leads to relatively rapid degradation as the particles tend to coarsen to reduce their high surface energy. Therefore, a nickel/gadolinia-doped-ceria (CGO) cermet electrode is studied, with a novel porous nanostructure consisting of nanoscale Ni (100 nm) and CGO (50 nm) crystallites, cosintered from nanocomposite precursor agglomerate particles. This electrode combines both high performance and excellent durability, with a total area-specific resistance (ASR) of 0.11 Ω cm2 at 800 °C and a stable ASR with up to 170 h ageing in humidified 5% H2-N2. Post-test analysis by 3D tomography shows that nickel coarsens and is responsible for the initial increase in ASR. However, the subsequent electrochemical performance is stable because reaction at the double phase boundaries (DPBs) on the surfaces of nanoscale CGO becomes dominant and is resistant to ageing. At this stage, the coarsened Ni network is also stabilized by the surrounding nanostructure. The dominant role of the DPB reaction is supported quantitatively using a continuum model with geometrical parameters obtained from 3D tomography.
Chen Z, Gong Z, Li Z, et al., 2020, Characterisation of indentation microstructures for porous SOFC cathodes, CERAMICS INTERNATIONAL, Vol: 46, Pages: 803-812, ISSN: 0272-8842
Chen J, Wang X, Boldrin P, et al., 2019, Hierarchical dual-porosity nanoscale nickel cermet electrode with high performance and stability, Nanoscale, Vol: 11, Pages: 17746-17758, ISSN: 2040-3364
Nano-structured metal-ceramic materials have attracted attention to improve performance in energy conversion applications. However, they have poor long-term stability at elevated temperatures due to coarsening of the metal nanoparticles. In this work we show that this can be mitigated by a novel design of the nano-structure of Ni cermet fuel electrodes for solid oxide cells. The strategy is to create a dual porosity microstructure, without the use of fugitive pore-formers, with micron-size pores to facilitate gas transport and nanoscale pores to control nano-particle coarsening. This has been achieved using a continuous hydrothermal synthesis and two-stage heat treatments to produce electrodes with micron size agglomerates of nano-structured porous Ni-Yttria-Stabilised-Zirconia (YSZ). This unique hierarchical microstructure combines enhanced electrochemical reaction in the high activity (triple phase boundary density 11 μm-2) nanoscale regions with faster gas diffusion in the micron-sized pores. The electrodes are aged at 800 °C in humidified 5% H2-N2 for up to 600 h. The electrochemical reaction resistance is initially 0.17 Ω cm2 but later reaches a steady long-term value of 0.15 Ω cm2. 3-D reconstruction of the electrodes after 10 h and 100 h of ageing reveals an increase in YSZ network connectivity and TPB percolation. This improvement is well-correlated to the 3-D tomography parameters using a physical model adapted from mixed conducting SOC air electrodes, which is also supported, for the first time, by numerical simulations of the microstructural evolution. These also reveal that in the long term, nickel coarsening is inhibited by the nanoscale entanglement of Ni and YSZ in the hierarchical microstructure.
Chen J, Ouyang M, Boldrin P, et al., 2019, Fabrication and Characterisation of Nanoscale Ni-CGO Electrode from Nano-Composite Powders, ECS Transactions, Vol: 91, Pages: 1799-1805, ISSN: 1938-6737
Chen J, Ouyang M, Boldrin P, et al., 2019, Fabrication and Characterisation of Nanoscale Ni-CGO Electrode from Nano-Composite Powders, 16th International Symposium on Solid Oxide Fuel Cells (SOFC-XVI)
Araki W, Wang X, Atkinson A, 2018, Can ferrroelasticity be evaluated by nanoindentation?, Journal of the European Ceramic Society, Vol: 38, Pages: 4495-4501, ISSN: 0955-2219
The present study investigated the possibility of evaluating ferroelastic mechanical characteristics by spherical indentation. Finite element simulation of spherical indentation, with a relatively large sphere, of a ferroelastic-plastic material was performed using characteristic bulk data of a typical ferroelastic oxide (LaSrCoFeO). The simulation results showed that the ferroelastic mechanical behaviour cannot be observed in the indentation load vs depth curve, but is clearly observable in the indentation stress vs indentation strain curve, which can be obtained reliably in experiments by estimating the contact radius using load-partial unloading sequences. The method can be reliable when the indentation stress is under the upper ferroelastic critical stress. Therefore, in principle ferroelastic mechanical characteristics could be evaluated by spherical indentation by obtaining the indentation stress vs indentation strain curve using partial unloading to estimate the contact radius, although the requirements are very difficult to satisfy in actual experiments.
Wang X, Chen Z, Atkinson A, et al., 2018, Numerical study of solid oxide fuel cell contacting mechanics, Fuel Cells, Vol: 18, Pages: 42-50, ISSN: 1615-6846
Assembly of a planar solid oxide fuel cell (SOFC) or solid electrolyzer (SOE) stack involves the lamination of cells and interconnect plates under an applied load. In most designs a pattern of ribs on the interconnector makes contact with a porous ceramic current collector layer on the air side. These localized contacts are regions of increased stress on the cells and can cause damage if the stresses become too large. In this paper the mechanical response of an anode-supported cell to localized loads from interconnector ribs is simulated. The simulations show that the critical stress for initiating and propagating a crack in the electrolyte (∼300MPa for a 10 μm thick electrolyte) is reached when the interconnector displacement reaches 20 μm (after touching the cathode) with reduced support, or 30 μm when in an oxidized state. The difference is due to the lower stiffness of the reduced support. The residual compressive stress in the electrolyte layer has a major protective effect for the electrolyte. It is concluded that fracture is very unlikely for a geometrically perfect contact, but if the contact is non-uniform due to manufacturing variability in the contact plate or cell, local displacements >∼20 μm can be dangerous. The simulations are used in an example of contacting geometry optimization.
Wang X, Atkinson A, 2017, Combining densification and coarsening in a Cellular Automata-Monte-Carlo simulation of sintering: methodology and calibration, Computational Materials Science, Vol: 143, Pages: 338-349, ISSN: 0927-0256
A hybrid Cellular Automata-Monte Carlo (CA-MC) approach is developed to simulate the sintering of particulate materials. The approach embodies a new, and physically realistic, way of simulating densification by grain boundary diffusion and collapse that takes into account the stresses arising from interactions with neighbouring particles (grains) by minimising the stored energy and energy dissipation rate using the variational principle. The parameters in the CA-MC simulations are calibrated in terms of measurable physical quantities by simulating the sintering of two identical contacting spheres, for which analytical solutions are well known and widely accepted. The use of the model is illustrated by simulating the densification of a randomly packed assembly of spherical particles. This demonstrates that the interactions between particles significantly inhibits shrinkage compared with that of two isolated spheres.
Hauth M, Lawlor V, Cartellieri P, et al., 2017, Production and reliability oriented SOFC cell and stack design, 15th International Symposium on Solid Oxide Fuel Cells (SOFC), Publisher: Electrochemical Society, Pages: 2231-2249, ISSN: 1938-5862
The paper presents an innovative development methodology for a production and reliability oriented SOFC cell and stack design aiming at improving the stacks robustness, manufacturability, efficiency and cost. Multi-physics models allowed a probabilistic approach to consider statistical variations in production, material and operating parameters for the optimization phase. A methodology for 3D description of spatial distribution of material properties based on a random field models was developed and validated by experiments. Homogenized material models on multiple levels of the SOFC stack were established. The probabilistic models were related to the experimentally obtained properties of base materials to establish a statistical relationship between the material properties and the most relevant load effects. Software algorithms for meta models that allow the detection of relationships between input and output parameters and to perform a sensitivity analysis were developed and implemented. The capabilities of the methodology is illustrated on two practical cases.
Chen J, Bertei A, Ruiz-Trejo E, et al., 2017, Characterization of Degradation in Nickel Impregnated Scandia-Stabilize Zirconia Electrodes during Isothermal Annealing, Journal of The Electrochemical Society, Vol: 164, Pages: F935-F943, ISSN: 1945-7111
This study investigates the stability of nickel-impregnated scandia-stabilize zirconia composite electrodes during isothermal annealing at temperatures from 600 to 950°C in a humidified hydrogen atmosphere (3 vol % water vapor). Typically an initial rapid degradation of the electrode during the first 17 h of annealing is revealed by both an increase in polarization resistance and a fall in electronic conductivity. Secondary electron images show a shift in nickel particle size toward larger values after 50 h of annealing. The declining electrochemical performance is hence attributed to nickel coarsening at elevated temperatures. Nickel coarsening has two microstructural effects: breaking up nickel percolation; and reducing the density of triple phase boundaries. Their impact on electrode area specific resistance is explored using a physical model of electrode performance which relates the macroscopic electrochemical performance to measurable microstructural parameters.
Chen Z, Wang X, Brandon N, et al., 2017, Spherical indentation of bilayer ceramic structures: dense layer on porous substrate, Journal of the European Ceramic Society, Vol: 37, Pages: 4763-4772, ISSN: 1873-619X
Spherical indentation of thin 8YSZ ceramic layers on porous substrates (NiO/Ni-8YSZ) was studied. Indentation-induced elastic and plastic deformation and damage of the bilayer was experimentally analysed. FE simulations of the indentation process were carried out using the Gurson model to account for densification of the porous substrates. The simulated load-depth responses were in excellent agreement with the measured ones. The resulting stress distributions showed that the damage to the YSZ initiates in a tensile region near the interface due to bending during loading at a failure stress of ∼2 GPa, which is consistent with pores of ∼1 μm size seen in the YSZ. Delamination occurs on unloading due to the elastic recovery of YSZ being greater than that of the substrates at a de-bonding stress of 120 MPa. Residual compressive stress in the YSZ inhibits crack opening displacements normal to the layer plane which is beneficial for application of these structures in SOFCs.
Bailey JJ, Heenan TMM, Finegan DP, et al., 2017, Laser-preparation of geometrically optimised samples for X-ray nano-CT, Journal of Microscopy, Vol: 267, Pages: 384-396, ISSN: 1365-2818
A robust and versatile sample preparation technique for the fabrication of cylindrical pillars for imaging by X-ray nano-computed tomography (nano-CT) is presented. The procedure employs simple, cost-effective laser micro-machining coupled with focused-ion beam (FIB) milling, when required, to yield mechanically robust samples at the micrometre length-scale to match the field-of-view (FOV) for nano-CT imaging. A variety of energy and geological materials are exhibited as case studies, demonstrating the procedure can be applied to a variety of materials to provide geometrically optimised samples whose size and shape are tailored to the attenuation coefficients of the constituent phases. The procedure can be implemented for the bespoke preparation of pillars for both lab- and synchrotron-based X-ray nano-CT investigations of a wide range of samples.
Chen Z, Atkinson A, Brandon N, 2017, Characterization of deformation and damage in porous sofc components via spherical indentation and simulation, 40th International Conference on Advanced Ceramics and Composites, Pages: 143-157
© 2018 by World Scientific Publishing Europe Ltd. The aim of this work is to present the methodology to characterize deformation and contact damage initiation and evolution in porous bulk and film components used in solid oxide fuel cells, based on indentation and simulation. Spherical indentation tests at a broad range of loads (50-10000 mN) were carried out on porous bulk and film electrodes with different levels of porosity, and on bilayer system. An axisymmetric model based on the Gurson model used for porous materials was developed to simulate the indentation processes. Elasticity and hardness of each component were reliably determined via both experiments and modelling. Inverse analysis via comparison of experimental indentation response curves and simulation-generated curves shows a very different relation between hardness and yield stress, compared with dense materials. Cracking behaviour was examined and appropriately explained by FEM results. Further insight of the deformation and damage behaviour was also obtained based on microstructural study using FIB-SEM. Overall, the study shows that the model developed in this work is highly applicable for the description the deformation and damage characteristics in porous bulk and film ceramics.
Atkinson A, 2017, Solid Oxide Fuel Cell Electrolytes-Factors Influencing Lifetime, Solid Oxide Fuel Cell Lifetime and Reliability: Critical Challenges in Fuel Cells, Pages: 19-35, ISBN: 9780081011027
This chapter addresses issues that influence the lifetime of electrolytes in solid oxide cells. These are the inherent stability of the material itself, degradation caused by interactions with other materials, and mechanical stability. The emphasis is on the common electrolyte materials, doped zirconia, and ceria. Inherent instability is reflected in a slow decrease in ionic conductivity with time at temperature and is particularly evident in 8. mol% yttria-stabilized zirconia electrolytes at high operating temperatures. Interactions causing degradation are commonly related to interdiffusion of elements from air electrodes leading to reaction products with low ionic conductivity or interdiffusion between layers in bilayer electrolytes. Most attention in this chapter is given to mechanical stability. The general methodology for assessing the mechanical failure of cells is outlined as they are subjected to thermal and chemical mismatch strains and stresses resulting from operation or cycling. The inclusion of time-dependent processes (such as creep and subcritical crack growth) is essential for assessing mechanical lifetime, but at present relevant data are not plentiful.
Chen J, Ruiz-Trejo E, Atkinson A, et al., 2017, Microstructural and Electrochemical Characterisation of Degradation in Nickel Impregnated Scandia-stabilised Zirconia Electrode during Isothermal Annealing, 15th International Symposium on Solid Oxide Fuel Cells (SOFC), Publisher: ELECTROCHEMICAL SOC INC, Pages: 1125-1137, ISSN: 1938-5862
Chen Z, Wang X, Brandon N, et al., 2016, Analysis of spherical indentation of porous ceramic films, Journal of the European Ceramic Society, Vol: 37, Pages: 1031-1038, ISSN: 1873-619X
Spherical indentation of a porous brittle La0.6Sr0.4Co0.2Fe0.8O3 ceramic film (porosity=39.7%) on a stiffer elastic Ce0.9Gd0.1O1.95 substrate is simulated by finite element modelling incorporating the Gurson model to account for densification. The simulated load-displacement curves, apparent elastic modulus E, indentation hardness H and densification profile are all in good agreement with experimental data for the film. The simulations show that E and H are not sensitive to film residual stress. However E is very sensitive to the indent depth-film thickness ratio f, although H is less so for f<0.3. The simulated dependence of E and H on f are highly consistent with experimental data, supporting the extrapolation of E and H measured for 0.1<f<0.3, to zero depth for good estimates of the film-alone properties. The inclusion of densification in the simulation makes only a small difference to E, but has a large influence on H as a function of indentation depth.
Bertei A, Ruiz-Trejo E, Tariq F, et al., 2016, Validation of a physically-based solid oxide fuel cell anode model combining 3D tomography and impedance spectroscopy, International Journal of Hydrogen Energy, Vol: 41, Pages: 22381-22393, ISSN: 1879-3487
This study presents a physically-based model for the simulation of impedance spectra in solid oxide fuel cell (SOFC) composite anodes. The model takes into account the charge transport and the charge-transfer reaction at the three-phase boundary distributed along the anode thickness, as well as the phenomena at the electrode/electrolyte interface and the multicomponent gas diffusion in the test rig. The model is calibrated with experimental impedance spectra of cermet anodes made of nickel and scandia-stabilized zirconia and satisfactorily validated in electrodes with different microstructural properties, quantified through focused ion beam SEM tomography. Besides providing the material-specific kinetic parameters of the electrochemical hydrogen oxidation, this study shows that the correlation between electrode microstructure and electrochemical performance can be successfully addressed by combining physically-based modelling, impedance spectroscopy and 3D tomography. This approach overcomes the limits of phenomenological equivalent circuits and is suitable for the interpretation of experimental data and for the optimisation of the electrode microstructure.
Chen Z, Wang X, Atkinson A, et al., 2016, Spherical indentation of porous ceramics: cracking and toughness, Journal of the European Ceramic Society, Vol: 36, Pages: 3473-3480, ISSN: 0955-2219
A combined experimental-numerical approach is used to characterise the fracture of a porous bulk ceramic material (La0.6Sr0.4Co0.2Fe0.8O3) with porosities of 5-45%, undergoing spherical indentation. The Gurson model was used in FEM to describe the densification of the porous material. Indentation-induced radial cracks were observed, when the applied nominal contact pressure exceeded threshold values, with no Hertzian ring-cone cracks found. FEM analysis indicated that the cracks propagated mainly during unloading, driven by the tensile hoop stress generated near the contact circle. The stress intensity at the crack tip was estimated using an approximate analysis of the FEM stress field to derive toughness values that were consistent with values determined by conventional methods, provided that the crack length is sufficiently large compared with the contact radius and can be measured accurately. The absence of ring-cone cracks in the elastic field during loading is due to the material’s high modulus-to-hardness ratio and the small indenter radius as predicted by established theory.
Vandeperre LJ, Wang X, Atkinson A, 2016, Measurement of mechanical properties using slender cantilever beams, Journal of the European Ceramic Society, Vol: 36, Pages: 2003-2007, ISSN: 1873-619X
The measurement of mechanical properties of materials only available in the form of thin sheets requires the use of load cells and displacement sensors of high sensitivity at low applied loads. These are available in testing platforms such as instrumented nano-indenters. In the current work, the elastic modulus and fracture toughness of thin cantilever beams of a representative brittle thin sheet material (300 μm thick NiO/YSZ support for a solid oxide fuel cell) were measured using a micro-/nano-indenter. The Young’s modulus and KIC were determined to be 139 ± 4 GPa and 2.13 ± 0.27 MPa m0.5 respectively using this method.
Chen Z, Wang X, Atkinson A, et al., 2016, Spherical Indentation of Porous Ceramics: Elasticity and Hardness, Journal of the European Ceramic Society, Vol: 36, Pages: 1435-1445, ISSN: 1873-619X
A combined experimental and numerical approach is used to characterise the elastic and plastic deformation of a porous bulk ceramic material (La0.6Sr0.4Co0.2Fe0.8O3, LSCF) with porosities in the range 5–45 vol%, undergoing spherical indentation. The Gurson model was used in FEM simulations to describe the densification of the porous material in the plastic zone under the indenter. The simulated indentation response curves, extracted elastic modulus, hardness and densification in the plastic zone all showed good agreement with corresponding experimental observations. The results show that the hardness increases with maximum indentation depth over a representative depth that depends on porosity. In this particular ceramic the hardness, at sufficiently large penetration depth, is approximately 1.7 times the uniaxial yield stress of the porous material.
Taub S, Neuhaus K, Wiemhoefer H-D, et al., 2015, The effects of Co and Cr on the electrical conductivity of cerium gadolinium oxide, Solid State Ionics, Vol: 282, Pages: 54-62, ISSN: 1872-7689
Selcuk A, Atkinson A, 2015, Measurement of mechanical strength of glass-to-metal joints, Fuel Cells, Vol: 15, Pages: 595-603, ISSN: 1615-6854
Wang X, Atkinson A, 2015, On the measurement of ceramic fracture toughness using single edge notched beams, Journal of the European Ceramic Society, Vol: 35, Pages: 3713-3720, ISSN: 1873-619X
Chen Z, Wang X, Giuliani F, et al., 2015, Microstructural characteristics and elastic modulus of porous solids, Acta Materialia, Vol: 89, Pages: 268-277, ISSN: 1359-6454
Porous La0.6Sr0.4Co0.2Fe0.8O3−δ ceramic films with different porosities were fabricated by constrained sintering on dense substrates of Gd-doped ceria at 900-1200 °C. The actual digital three dimensional microstructures of the as-sintered films were reconstructed using focused ion beam/scanning electron microscope tomography and their elastic moduli were calculated using finite element modelling based on the reconstructed microstructures. The calculated moduli were found to be in good agreement with experimental results. Porosity was found to be the primary factor influencing the elastic modulus. In order to explore the influence of microstructural features other than porosity the real microstructures, and artificial microstructures based on spherical mono-size particles, were coarsened numerically at constant porosity using a cellular automaton method. The simulation results showed that in the initial stages of sintering, when interparticle necks are small, the modulus increases with the neck size. However, as the coarsening increases further, the modulus becomes insensitive to the details of the microstructure and only depends on porosity. The results also show that simulation gives inaccurate results if the ratio of characteristic length of the simulated volume to the characteristic length of the microstructure is too small (less than approximately a factor of 8).
Atkinson A, 2015, Mechanical properties of fluorite-structured oxides, New Research Trends of Fluorite-Based Oxide Materials: From Basic Chemistry and Materials Science to Engineering Applications, Pages: 151-164, ISBN: 9781631173509
This chapter considers the mechanical properties of fluorite-structured oxides with particular emphasis given to cubic stabilized zirconia and doped ceria, both of which have potential application in electrochemical devices. They differ in that zirconia requires doping to stabilize the fluorite structure (ceria does not) and ceria has oxygen content depending on oxygen activity and temperature (zirconia does not). The mechanical properties considered are elasticity, plasticity, creep, and fracture. It is shown that all fluorite-structured oxides have similar mechanical properties at room temperature. However, there are interesting differences in behaviour between zirconia and doped ceria as the temperature is raised or doping levels and oxygen content (independently in the case of ceria) are changed. These differences are not understood, but are probably related to subtle differences in the way in which defect clusters respond to the applied stress in the two materials.
Chen Z, Wang X, Giuliani F, et al., 2015, Fracture toughness of porous material of LSCF in bulk and film forms, Journal of the American Ceramic Society, Vol: 98, Pages: 2183-2190, ISSN: 1551-2916
Ruiz-Trejo E, Boldrin P, Medley-Hallam JL, et al., 2015, Partial oxidation of methane using silver/gadolinia-doped ceria composite membranes, Chemical Engineering Science, Vol: 127, Pages: 269-275, ISSN: 1873-4405
Methane was partially oxidised to CO using oxygen permeated through a 1 mm thick silver/Ce0.9Gd0.1O2−x (Ag/CGO) composite membrane operating at 500–700 °C with air at 1 bar pressure. The membranes were fabricated by sintering ultrafine nanoparticles of gadolinia-doped ceria (<5 nm) coated with silver using Tollens׳ reaction. This unique combination led to dense composites with low content of silver (7 vol%), no reaction between the components and predominant metallic conductivity. When feeding 4% methane at 700 °C to a 1-mm thick Ag/CGO using Ni as reforming catalyst, the conversion reached 21% and the CO selectivity 92% with an estimated oxygen flux of 0.18 mL min−1 cm−2 (NTP). The samples were stable in carbon-containing atmospheres and under a large pO2 transmembrane pressure difference at temperatures below 700 °C for 48 h.
Ruiz-Trejo E, Atkinson A, Brandon NP, 2015, Metallizing porous scaffolds as an alternative fabrication method for solid oxide fuel cell anodes, Journal of Power Sources, Vol: 280, Pages: 81-89, ISSN: 1873-2755
A combination of electroless and electrolytic techniques is used to incorporate nickel into a porous Ce0.9Gd0.1O1.90 scaffold. First a porous backbone was screen printed into a YSZ electrolyte using an ink that contains sacrificial pore formers. Once sintered, the scaffold was coated with silver using Tollens' reaction followed by electrodeposition of nickel in a Watts bath. At high temperatures the silver forms droplets enabling direct contact between the gadolinia-doped ceria and nickel. Using impedance spectroscopy analysis in a symmetrical cell a total area specific resistance of 1 Ωcm2 at 700 °C in 97% H2 with 3% H2O was found, indicating the potential of this fabrication method for scaling up.
Wang X, Atkinson A, 2015, Simulation and prediction of 3-D microstructural evolution and long term performance of Ni-YSZ anode, Pages: 2867-2873, ISSN: 1938-6737
The electrochemical performance of SOFCs degrades over the operation time and a major contributor is microstructure coarsening in the anode. To enable long term durability prediction and improve the anode performance, it is necessary to understand its microstructural evolution under different operation conditions. In this contribution, we demonstrate that the 3D real microstructure evolution of a Ni-YSZ anode can be simulated and predicted using a cellular automaton approach. The same modelling approach is also used to determine the wettability of Ni on YSZ in the real anode. The 3D microstructures of as-reduced and aged anodes were reconstructed experimentally using the focused ion beam (FIB) slice and view method and it is shown that the simulation agrees well with the experimental results. Using these short time experimental observations as a calibration of the model, the long term microstructure evolution of the Ni-YSZ anode and its long term performance and lifetime are predicted.
Chen Z, Wang X, Giuliani F, et al., 2015, Analyses of microstructural and elastic properties of porous SOFC cathodes based on focused ion beam tomography, JOURNAL OF POWER SOURCES, Vol: 273, Pages: 486-494, ISSN: 0378-7753
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