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This dataset contains 3D data acquired with FIB-tomography from porous Zr-oxide. The pore structures of the ceramic materials were investigated in context with optimization of transport properties (aimed for low viscous flow/permeability and high electric conduction via liquid electrolyte in the pores), so that the Zr-oxide is suitable for application as liquid junction (diaphragm) in pH-sensors. Further details on the scientific background are given in Holzer et al., 2016, Materials & Design, 99, 314–327. (http://doi.org/10.1016/j.matdes.2016.03.034). The tomographs represent 9 ceramic materials that were produced with different sintering temperatures (1250, 1300, 1325 and 1350°C) and different sintering times (1, 2 and 3 hours). The study includes quantitative descriptions of relevant microstructure characteristics (porosity, tortuosity, constrictivity, hydraulic radius) and corresponding effective transport prorperties (permeability, conductivity). Each tomograph is presented as stack of 2D-tiff-images in two different versions: as gray-scale images (raw data) and as binarized images (segmented, with solid=black and pores=white). The voxel resolution is 10x10x10 nm for all 18 image stacks.

The performance of polymer electrolyte fuel cells (PEFC) strongly depends on a controlled water management within the porous layers. For this purpose we investigate liquid water transport in a commercial gas diffusion layer (SGL 25BA) on the pore scale. X-ray tomography experiments combined with pressure-induced water injection provide 3D images of the liquid water distribution inside the GDL at incremental pressure steps between 0 and 100 mbar. The breakthrough behavior of the liquid phase is highly anisotropic. In through-plane (tp) direction first bubble points appear at the outlet plane already at 5 mbar and the ‘breakthrough' then evolves continuously over an extended pressure range up to > 30 mbar. For in-plane (ip) direction the breakthrough is discontinuous and takes place at 27 mbar. Simulations of the intrusion process reveal that the different breakthrough behaviors are mainly triggered by different ip- and tp-transport distances. Short tp-transport distances through the thin gas diffusion layer (ca. 100 μm) are responsible for the characteristic continuous tp-breakthrough behavior, which is thus attributed to a so-called short-range effect. Dedicated methods for 3D-image analysis adapted to fibrous GDL microstructures were presented in part I. With these methods we quantify all microstructure characteristics that are relevant for liquid permeability. These characteristics of pore and liquid phases include size distributions of bulges and bottlenecks, connectivity, effective volume fractions, geodesic tortuosity, constrictivity and hydraulic radius. Quantitative relationships are established between these microstructure characteristics and the liquid permeability, which provide a better understanding of the underlying microstructure limitations for injection and liquid transport. For the in-plane direction the liquid permeability is limited to roughly a similar extent by tortuosity, constrictivity and effective volume fraction. In contrast, for through-pl

New quantitative relationships are established between effective properties (gas diffusivity, permeability and electrical conductivity) for a dry GDL (25 BA) from SGL Carbon with the corresponding microstructure characteristics from 3D analysis. These microstructure characteristics include phase volume fractions, geodesic tortuosity, constrictivity and hydraulic radius. The latter two parameters include information from two different size distribution curves for bulges (continuous PSD) and for bottlenecks (MIP-PSD). X-ray tomographic microscopy is performed for GDL at different compression levels and the micro-macro-relationships are then established for the in-plane and through-plane directions. The predicted properties based on these relationships are compared with numerical transport simulations, which give very similar results and which can be summarized as follows: Gas diffusivity is higher in the in-plane than in the through-plane direction. Its variation with compression is mainly related to changes of porosity and geodesic tortuosity. Permeability is dominated by variations in hydraulic radius. Through-plane permeability is slightly higher than in-plane. Anisotropy of electrical conductivity is controlled by tortuosity, which is higher for the through-plane direction. A table with new quantitative relationships is provided, which are considered to be more accurate and precise than older descriptions (e.g. Carman-Kozeny, Bruggeman), because they are based on detailed topological information from 3D analysis. Furthermore, when using these relationships as input for macro-homogenous modeling, this enables to simulate microstructure effects of real GDL (SGL 25 BA) more accurately. In future, the same methodology can be used to study micro-macro relationships in wet GDL and to predict relative liquid permeability and relative gas diffusivity.

Porous Diaphragms in pH-Sensors must meet apparently contradicting requirements such has high conductivity vs. low permeability and low outflow rate of the electrolyte vs. high flow velocity. In this study we intend to lay the foundations for knowledge-based materials design, so that the required materials properties can be achieved. This approach is based on a quantitative understanding of the relationships between 3D topological parameters with the corresponding effective transport properties (flow/permeability and conductivity). All transport relevant topological parameters (i.e. tortuosity, constrictivity, porosity and hydraulic radius) are determined by FIBtomography and 3D image analysis. Effective properties (conduction and flow) are determined a) by 3D numerical simulation and b) with experimental characterization. The experimental work includes fabrication and characterization of porous YSZ sintered at 1250, 1300 and 1350 ÅãC. Fundamental relationships are established by comparison of topological data with results from simulation and from experiment. The following design guidelines are then postulated: a) flow properties are adjusted independently from the conduction via manipulation of the hydraulic radius, b) high local flow velocity and at the same time relatively low volume outflow can be achieved by adjusting the constrictivity via manipulation of sintering conditions and with addition of pore former.

A parametric stochastic 3D model for the description of complex three-phase microstructures is developed. Such materials occur for example in anodes of solid oxide fuel cells (SOFC) which consist of pores, nickel (Ni) and yttria-stabilized zirconia (YSZ). The model is constructed using tools from stochastic geometry. More precisely, we model the backbones of the three phases by a certain class of random geometric graphs called beta-skeletons. This allows us to reproduce complete connectivity of all three phases as observed in experimental image data of a pristine Ni-YSZ anode as well as the prediction of volume fractions by model parameters. Finally a slightly generalized version of this model enables a good fit to experimental image data with respect to transport relevant microstructure characteristics and the length of triple phase boundary. Model validation is performed by comparing effective transport properties from finite element (FE) simulations based on 3D-data from the stochastic model and from tomography of real Ni-YSZ anodes. Moreover, the virtual, but realistic Ni-YSZ microstructures can be used for investigating the quantitative influence of microstructure characteristics on various physical properties and consequently on the performance of the anode material.

The analysis of big data is changing industries, businesses and research as large amounts of data are available nowadays. In the area of microstructures, acquisition of (3-D tomographic image) data is difficult and time-consuming. It is shown that large amounts of data representing the geometry of virtual, but realistic 3-D microstructures can be generated using stochastic microstructure modeling. Combining the model output with physical simulations and data mining techniques, microstructure-property relationships can be quantitatively characterized. Exemplarily, we aim to predict effective conductivities given the microstructure characteristics volume fraction, mean geodesic tortuosity, and constrictivity. Therefore, we analyze 8119 microstructures generated by two different stochastic 3-D microstructure models. This is – to the best of our knowledge – by far the largest set of microstructures that has ever been analyzed. Fitting artificial neural networks, random forests and classical equations, the prediction of effective conductivities based on geometric microstructure characteristics is possible.

Due to their high specific energy and high energy density, miniaturized low-temperature (350-550°C) solid oxide fuel cells, hereafter abbreviated “micro-SOFC”, are believed to constitute one of the technologies that could help satisfy the continuously increasing electric energy demand for mobile devices such as laptops and camcorders. Using thin film and MEMS technologies, cathode-electrolyte-anode layer assemblies as thin as 1 μm are deposited on silicon substrates that are micromachined to form arrays of free-standing membranes (surface area: 390x390 μm2 at ETH Zurich). Proof of concept was already established by several groups and high power densities of several hundreds of mW/cm2 have been reported at temperatures as low as 350 °C. In Switzerland, the OneBat® consortium consisting of eight research groups is working on the development of the micro-SOFC technology covering various aspects such as membrane fabrication and characterization, reformer catalysis, thermal management and system development. After a brief presentation of the consortium activities as well as the state-of-the-art of the micro-SOFC research worldwide, this contribution will lay emphasis on the core of the micro-SOFC technology, namely the electrochemical cells, and address key-aspects for their further development: fabrication and thermomechanical stability of free-standing membranes, development of cost-effective thin film deposition techniques, and development of thermally stable electrodes.

The microstructure influence on conductive transport processes is described in terms of volume fraction ε, tortuosity τ, and constrictivity β. Virtual microstructures with different parameter constellations are produced using methods from stochastic geometry. Effective conductivities σeff are obtained from solving the diffusion equation in a finite element model. In this way, a large database is generated which is used to test expressions describing different micro-macro relationships such as Archie's law, tortuosity, and constrictivity equations. It turns out that the constrictivity equation has the highest accuracy indicating that all three parameters (ε, τ, β) are necessary to capture the microstructure influence correctly. The predictive capability of the constrictivity equation is improved by introducing modifications of it and using error-minimization, which leads to the following expression: σeff = σ0^2.03ε^1.57β^0.72/τ^2 with intrinsic conductivity σ0. The equation is important for future studies in, for example, batteries, fuel cells, and for transport processes in porous materials.

This study investigates the influence of microstructure on the effective ionic and electrical conductivities of Ni-YSZ (yttria-stabilized zirconia) anodes. Fine, medium, and coarse microstructures are exposed to redox cycling at 950 ºC. FIB (focused ion beam)-tomography and image analysis are used to quantify the effective (connected) volume fraction (Φeff), constriction factor (β), and tortuosity (τ). The effective conductivity (σeff) is described as the product of intrinsic conductivity (σ0) and the so-called microstructure-factor (M): σeff = σ0 x M. Two different methods are used to evaluate the M-factor: (1) by prediction using a recently established relationship, Mpred = ε β^0.36/τ^5.17, and (2) by numerical simulation that provides conductivity, from which the simulated M-factor can be deduced (Msim). Both methods give complementary and consistent information about the effective transport properties and the redox degradation mechanism. The initial microstructure has a strong influence on effective conductivities and their degradation. Finer anodes have higher initial conductivities but undergo more intensive Ni coarsening. Coarser anodes have a more stable Ni phase but exhibit lower YSZ stability due to lower sintering activity. Consequently, in order to improve redox stability, it is proposed to use mixtures of fine and coarse powders in different proportions for functional anode and current collector layers.

Nanoporous 1-μm thin La0.6Sr0.4CoO3−δ (LSC) layers are deposited by spray pyrolysis and subsequently sintered at 600°C, 800°C, and 1000°C. A strontium- and oxygen-rich phase can be found within the pore network, which appears at low sintering temperatures. This so-called “secondary phase” occupies up to 20.7 vol.% of the LSC films for the 600°C annealing process. It does not hinder the electrochemical activity towards oxygen reduction of such layers that exhibit an area-specific resistance (ASR) as low as 0.13 Ω cm2 at 600°C in air. This result makes the spray pyrolysed LSC thin films promising candidates as intermediate-temperature solid oxide fuel cell cathodes. For higher sintering temperatures the secondary phase progressively disappears. A correlation between the inverse of the ASR and the whole LSC surface area (regardless of the presence of the secondary phase or not) is also evidenced. The increase of ASR with increasing sintering temperature is found to be primarily related to the exchange neutral flux density of the Sr-deficient LSC.

Oxide scale formation on metallic interconnects contributes to the overall degradation of solid oxide fuel cell (SOFC) stacks. On the anode side, thermally grown oxide scale might contain additional nickel, nickel oxide, or nickel chromium spinel phases – depending on the applied operation conditions. Ni originates from Ni-meshes, often applied as current collector, from Ni-containing anodes or from Ni-containing coatings. Ni particles released during thermo redox cycles from adjacent Ni-containing components might be interspersed into the oxide scale. This study aims at investigating the influence of Ni on the electrical conductivity of oxide scales. For this purpose pellets of Cr2O3 were mixed with different amounts of Ni and then investigated in-situ under both reducing and oxidizing gas atmospheres at 850°C. The formed crystals were analyzed using X-ray diffraction, whereas the resulting microstructures were quantified using scanning electron microscopy. During oxidation Ni is converted into NiO, and the latter interacts with Cr2O3 to form a NiCr2O4 spinel phase. Subsequent exposure to reducing conditions leads to an almost instantaneous decomposition of NiCr2O4 spinel, resulting in finely dispersed elementary Ni. This rearrangement of Ni by spinel decomposition leads to a significant improvement of the electrical conductivity of the Cr2O3 pellets compared to their initial state.

3D microstructure-performance relationships in Ni-YSZ anodes for electrolyte-supported cells are investigated in terms of the correlation between the triple phase boundary (TPB) length and polarization resistance (Rpol). Three different Ni-YSZ anodes of varying microstructure are subjected to eight reduction-oxidation (redox) cycles at 950 °C. In general the TPB lengths correlate with anode performance. However, the quantitative results also show that there is no simplistic relationship between TPB and Rpol. The degradation mechanism strongly depends on the initial microstructure. Finer microstructures exhibit lower degradation rates of TPB and Rpol. In fine microstructures, TPB loss is found to be due to Ni coarsening, while in coarse microstructures reduction of active TPB results mainly from loss of YSZ percolation. The latter is attributed to weak bottlenecks associated with lower sintering activity of the coarse YSZ. The coarse anode suffers from complete loss of YSZ connectivity and associated drop of TPBactive by 93%. Surprisingly, this severe microstructure degradation did not lead to electrochemical failure. Mechanistic scenarios are discussed for different anode microstructures. These scenarios are based on a model for coupled charge transfer and transport, which allows using TPB and effective properties as input. The mechanistic scenarios describe the microstructure influence on current distributions, which explains the observed complex relationship between TPB lengths and anode performances. The observed loss of YSZ percolation in the coarse anode is not detrimental because the electrochemical activity is concentrated in a narrow active layer. The anode performance can be predicted reliably if the volume-averaged properties (TPBactive, effective ionic conductivity) are corrected for the so-called short-range effect, which is particularly important in cases with a narrow active layer.

A methodology based on FIB-tomography and image analysis (IA) is proposed which allows quantification of all relevant morphological features that are necessary to predict effective transport properties in porous SOFC electrodes. These morphological Features are constrictivity, tortuosity, percolation factor and phase volume fraction. An M-factor can then be calculated which represents the ratio of effective over intrinsic conductivities. The methodology is used to describe effects of microstructure degradation in Ni-YSZ anodes which are caused by redox cycling at 950°C. The so calculated M-factors predict that because of redox cycling the effective electronic conductivity of nickel decreases from 3 to 1.2% which is mainly due to changes of percolation and constriction factors. Based on these results the effective electrical conductivity of nickel is predicted to be 685 S/cm before redox and 243 S/cm after 8 redox cycles. The predictions fit well with the experimental measurements that reveal 600 S/cm before and 200 S/cm after redox cycling at 950°C. For YSZ the M-factors obtained with 3D-analysis predict that the degradation causes a drop of the effective ionic conductivity from 7 to 0.6%, which is due to a change of the bottleneck dimensions. This finding contradicts the frequent interpretation of YSZ as a ‘rigid backbone’ that is not affected by microstructure degradation. Finally, the effective bulk gas diffusivity increases from 2 to 11% due to an increase of porosity associated with swelling of the anode.