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The electrochemical behavior of Ni-based SOFC anodes with different microstructures was studied. The anodes consisted either of a Ni gauze, a Ni pattern, a Ni paste, or a Ni -YSZ cermet. According to electrochemical impedance spectroscopy (EIS) measurements, the anodes are dominated by two main processes. The high frequency process decreases exponentially with an applied overpotential and is thermally activated with an activation energy of around 1 eV. The process could be attributed to the adsorption of hydrogen including charge transfer. The low frequency impedance arc arises only under a high overpotential applied between the working and the reference anode. The impedance of this process increases the higher the overpotential and is thermally activated with an activation energy of 0.5 eV. This process is assigned to the desorption of water., International Journal of Materials Research, 92 (7), ISSN:1862-5282, ISSN:2195-8556

Sintering of magnesium oxide ceramics to high density requires high temperatures (1500-1800 degrees C), resulting in coarse-grained microstructures, or additives that promote sintering. An alternative is to use a fine-grained MgO starting powder with an impurity level < 1 % that allows appropriate processing conditions. Here, we report about processing by slip casting, resulting in ceramic green bodies that can be sintered to closed porosity at 1600 degrees C. X-ray diffraction and scanning electron microscopy combined with energy-dispersive X-ray spectroscopy showed that the sintered bodies are single-phase. The process described here is suitable to sinter large substrates up to 200 mm x 200 mm and 96 % of the theoretical density with closed porosity. The MgO substrates are suitable for partial melt processing of high-quality Bi-2212 thick films., International Journal of Materials Research, 94 (3), ISSN:1862-5282, ISSN:2195-8556

The mixed conducting perovskite-type oxides BaxSr1-xCo1-yFeyO3-δ (BSCF) are intensively studied as potential high-performance solid oxide fuel cell cathode materials. The effect of different compositional variables and oxygen stoichiometry on the structure and thermodynamic stability of the BaxSr1-xCo1-yFeyO3-δ (x = 0.2, 0.4, 0.5, 0.6, 0.8; y = 0.2, 0.4, 0.6, 0.8, 1) perovskite-type compositions were investigated by solid electrolyte electrochemical cells method and scanning electron microscopy (SEM). The thermodynamic quantities represented by the partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen were obtained in the temperature range of 823–1273 K. The in situ change of oxygen stoichiometry and the determination of thermodynamic parameters of the new oxygen-deficient BSCF compositions were studied via coulometric titration technique coupled with electromotive force (EMF) measurements. The effect of A- and B-site dopants concentration correlated to the variation of oxygen stoichiometry on the thermodynamic stability and morphology of the BSCF samples was evidenced.

An error analysis of a splitting method applied to the Zakharov system is given. The numerical method is a Lie–Trotter splitting in time that is combined with a Fourier collocation in space to a fully discrete method. First-order convergence in time and high-order convergence in space depending on the regularity of the exact solution are shown for this method. The main challenge in the analysis is to exclude a loss of spatial regularity in the numerical solution. This is done by transforming the numerical method to new variables and by imposing a natural CFL-type restriction on the discretization parameters.

Journal of Electroceramics, 18 (1-2), ISSN:1385-3449, ISSN:1573-8663

Monatshefte für Chemie, 140 (9), ISSN:0026-9247, ISSN:1434-4475

For trigonometric and modified trigonometric integrators applied to oscillatory Hamiltonian differential equations with one or several constant high frequencies, near-conservation of the total and oscillatory energies are shown over time scales that cover arbitrary negative powers of the step size. This requires non-resonance conditions between the step size and the frequencies, but in contrast to previous results the results do not require any non-resonance conditions among the frequencies. The proof uses modulated Fourier expansions with appropriately modified frequencies., Comment: 26 pages

An error analysis of trigonometric integrators (or exponential integrators) applied to spatial semidiscretizations of semilinear wave equations with periodic boundary conditions in one space dimension is given. In particular, optimal second-order convergence is shown requiring only that the exact solution is of finite energy. The analysis is uniform in the spatial discretization parameter. It covers the impulse method which coincides with the method of Deuflhard and the mollified impulse method of Garcia-Archilla, Sanz-Serna, and Skeel as well as the trigonometric methods proposed by Hairer and Lubich and by Grimm and Hochbruck. The analysis can also be used to explain the convergence behavior of the Stormer--Verlet/leapfrog time discretization.

The crystallization kinetics of amorphous 3 and 8 mol% yttria stabilized zirconia (3YSZ and 8YSZ) thin films grown by pulsed laser deposition (PLD), spray pyrolysis and dc-magnetron sputtering are explored. The deposited films were heat treated up to 1000 °C ex situ and in situ in an X-ray diffractometer. A minimum temperature of 275 °C was determined at which as-deposited amorphous PLD grown 3YSZ films fully crystallize within five hours. Above 325 °C these films transform nearly instantaneously with a high degree of micro-strain when crystallized below 500 °C. In these films the t′′ phase crystallizes which transforms at T > 600 °C to the t′ phase upon relaxation of the micro-strain. Furthermore, the crystallization of 8YSZ thin films grown by PLD, spray pyrolysis and dc-sputtering are characterized by in situ XRD measurements. At a constant heating rate of 2.4 K min−1 crystallization is accomplished after reaching 800 °C, while PLD grown thin films were completely crystallized already at ca. 300 °C.

Geometric aspects play an important role in the construction and analysis of structure-preserving numerical methods for a wide variety of ordinary and partial differential equations. Here we review the development and theory of symplectic integrators for Hamiltonian ordinary and partial differential equations, of dynamical low-rank approximation of time-dependent large matrices and tensors, and its use in numerical integrators for Hamiltonian tensor network approximations in quantum dynamics., prepared for the Proceedings of ICM 2018 (Christian Lubich's plenary talk)

Journal of Materials Research, 9 (5), ISSN:0884-2914, ISSN:2044-5326

Journal of Materials Research, 8 (9), ISSN:0884-2914, ISSN:2044-5326

Trigonometric time integrators are introduced as a class of explicit numerical methods for quasilinear wave equations. Second-order convergence for the semi-discretization in time with these integrators is shown for a sufficiently regular exact solution. The time integrators are also combined with a Fourier spectral method into a fully discrete scheme, for which error bounds are provided without requiring any CFL-type coupling of the discretization parameters. The proofs of the error bounds are based on energy techniques and on the semiclassical G\aa rding inequality., Comment: 33 pages, 2 figures, comments welcome!! Version 3 has updates, typo corrections and simplifications due to referee reports Version 1 contains an extra example removed to shorten the paper overall

The chemical master equation is a fundamental equation in chemical kinetics. It is an ad- equate substitute for the classical reaction-rate equations whenever stochastic eects become relevant. In the present paper we give a simple argument showing that the solutions of a large class of chemical master equations, including all those in which elementary reactions between two and more molecules do not generate a larger number of molecules than existed before, are bounded in weighted '1-spaces. As an illustration for the implications of this kind of regularity we analyze the eect of truncating the state space. This leads to an error analysis of the nite state projection of the chemical master equation, an approximation that underlies many numerical methods.

The compound Ba0.6Sr0.4Co0.8Fe0.2O3−δ with perovskite structure has been studied focusing mainly on the thermodynamic stability in correlation to the change in the oxygen stoichiometry. The thermochemical behavior has been investigated from room temperature to 1300 K by thermal gravimetry and differential thermal analysis in air and by calorimetry in scanning mode in Argon. The temperature dependence of enthalpy (ΔHT298) in the temperature range of 300–900 K was measured by drop calorimetry. Thermodynamic properties represented by the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen have been obtained in the temperature range of 823–1273 K using solid electrolyte electrochemical cells (EMF). The influence of the oxygen stoichiometry on the thermodynamic properties was examined using a coulometric titration technique coupled with EMF measurements. The results are discussed based on the strong correlation between the energetic parameters and the charge compensation of the material system.

We propose a new and versatile synthesis route for the formation of particle-stabilized gel-core microcapsules. With this route, microcapsules can be produced in a large size range of a few to hundreds of micrometres. Due to the high mechanical stability developed upon gelation, the capsules can be harvested as dry powders. Implementing in-situ hydrophobization to modify the wetting properties of particles allows preparing capsule shells from almost any kind of particulate materials. In the present work, we demonstrate the formation of microcapsules with shells from 60 nm gamma alumina and 200 nm alpha alumina particles. These particle shells are combined with sodium acrylate and poly(ethylene glycol) based gel-cores. Shape, gel-core microstructure, and swelling of the capsules in deionized water are characterized. Modelling the release of a low molecular weight dye from the capsules reveals Fickian kinetics for 16 and polymer relaxation dominated kinetics for 280 μm sized capsules, respectively. Given the flexibility in materials selection, our processing route could lead to the design of new microcapsule systems for sensing and release applications.

In the first part of this study, the oxygen deficiency, ?, in Ba0.5Sr0.5Co0.8Fe0.2O3 - ? (BSCF) was measured by means of thermogravimetry as a function of oxygen partial pressure, p(O2), in the range of 1.1?10?6 < p(O2)/% < 41.67 at elevated temperatures ranging 873 ? T/K ? 1073. It was shown that ? becomes more pronounced with increasing T and with decreasing p(O2). The isotherms ? vs. p(O2) were determined. The second part of this study relates to the reaction of CO2 with Ba0.5Sr0.5Co0.8Fe0.8O3-? perovskite oxide in the absence and presence of O2 at temperatures ranging from 673 to 973 K also by thermogravimetry. The reactivity of CO2 with BSCF increased with increasing temperature and increasing exposure to CO2. Reaction of CO2 with BSCF was described by a equilibrium reaction isotherms. The results of X-ray diffractometry evidenced that the exposure to CO2 leads to the formation of carbonates.

The controlled assembly of nanostructures via shape instability mechanisms is a potential alternative to traditional top-down processes like e-beam lithography for nanostructuring surfaces. In this contribution, the dynamics of the nanostructuresʼ assembly via thin film agglomeration have been analyzed. Pt thin films with a thickness of 50 nm were deposited via magnetron sputtering on yttria-stabilized zirconia (YSZ) single crystals and subjected to heat treatments at 1023 K for times ranging from 10 to 130 min. The morphological evolution of Pt thin films has been investigated by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM), obtaining the hole growth dynamics and morphological parameters like the lateral correlation length and the nanostructuresʼ Minkowski functionals. The experimentally obtained morphology evolution is compared to the simulated evolution of thin film structures resulting from a cell dynamical system (CDS) model. Three main observations have been made. (i) The hole radius is found to scale as function of time t with a rate proportional to t − 3 4 [ log 3 t ] . This is in agreement with Srolovitzʼs instability theory describing hole growth predominated by surface diffusion. (ii) The morphological evolution of the Pt thin films has been analyzed as function of time t by means of Minkowski measures and the lateral correlation length. A discontinuity in the lateral correlation length and a significant deviation of the Minkowski functionals from the expected Gaussian behavior was found to be coupled with the coalescence of holes. (iii) By using the Ginzburg–Landau equation for the description of the fundamental diffusion process, the CDS model allows a computational reproduction of the experimentally obtained film morphologies in the early stages of agglomeration.

The mechanical properties of porous ceramics are greatly influenced by their microstructure. Therefore, mechanical behavior of highly porous ceramics is different from that of dense ceramics. In this work, we evaluate different mechanical testing methods such as static compression, Brazilian disc test and 3-point bending on their suitability for comparison of highly porous ceramic materials. It is shown that 3-point bending is more suitable than static compression or Brazilian disc testing, as the material exhibits no critical crack propagation under compressive loading. With 3-point bending tests, a quantitative comparison of the mechanical properties of foams with different microstructures and porosities is possible. Under cyclic compression the foams exhibit a very high degree of crack tolerance in combination with preservation of their structural integrity even at high strains of 10%.

The thermodynamic stability of the perovskite-type oxides in the Ba x Sr 1− x Co 0.8 Fe 0.2 O 3− δ (BSCF) system was investigated with varying Ba:Sr ratios ( x =0.2, 0.4, 0.5, 0.6, 0.8) and correlated with the charge compensation mechanism and the change in the oxygen stoichiometry of the materials. Thermodynamic properties represented by the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen have been obtained in the temperature range of 823–1273 K using solid electrolyte electrochemical cells (EMF) method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using a coulometric titration technique coupled with EMF measurements. The temperature dependence of enthalpy increment ( H T − H 298 ) in the temperature range of 700–900 K was measured by drop calorimetry. The energetic parameters allow for the correlation of the structural and electrical stability with the defect structures.

Thin cathodes for micro-solid oxide fuel cells (micro-SOFCs) are fabricated by spin-coating a suspension of La0.6Sr0.4CoO3–δ (LSC) nanoparticulates obtained by salt-assisted spray pyrolysis. The resulting 250 nm thin LSC layers exhibit a three-dimensional porous microstructure with a grain size of around 45 nm and can be integrated onto free-standing 3 mol.% yttria-stabilized-zirconia (3YSZ) electrolyte membranes with high survival rates. Weakly buckled micro-SOFC membranes enable a homogeneous distribution of the LSC dispersion on the electrolyte, whereas the steep slopes of strongly buckled membranes do not allow for a perfect LSC coverage. A micro-SOFC membrane consisting of an LSC cathode on a weakly buckled 3YSZ electrolyte and a sputtered Pt anode has an open-circuit voltage of 1.05 V and delivers a maximum power density of 12 mW cm–2 at 500 °C.

The crystallization upon thermal treatment of 8 mol% yttria-stabilized zirconia thin films deposited by RF-sputtering at room temperature is investigated. The as-deposited YSZ films are biphasic with crystallites of 5–10 nm in diameter building a columnar-like microstructure. Fully crystalline YSZ thin films are obtained after isothermal dwells in the temperature range 800–1100 °C. X-ray diffraction and Raman spectroscopy demonstrate that the crystalline films have a cubic structure retained even after high-temperature annealing while a strong texture is developed. Further, a stagnating grain-growth is observed, characterized by a final grain size of about 60 nm and a micro-strain of 0.15%. Given the observed films' micro-structural stability, their application in miniaturized electrochemical devices such as micro-solid oxide fuel cells or sensors can be foreseen.

ARTICLE DE JOURNAL Synthesis of bone like structured foams Krauss Juillerat Franziska; Engeli Roman; Jerjen Iwan; Sturzenegger Philip N.; Borcard Françoise; Juillerat Jeanneret Lucienne; Gerber Lemaire Sandrine; Gauckler Ludwig J.; Gonzenbach Urs T. Publié dans: Journal of the European Ceramic Society vol. 33 p. 1497 1505 Oxford: Elsevier Sci Ltd 2013 Here we present a processing route to produce multi structured ceramic foams based on the combination of particle stabilized foams with polymeric sponges to produce positive and negative templating structures. Polyester sponges are infiltrated with freshly produced calcium aluminate–alumina foams and upon sintering either positive templating structures are produced when wetting the sponges or negative templating foams with a percolating pore network are obtained when completely filling the sponges. Additionally by combining different layers of these particle stabilized foam infiltrated sponges various different structures can be produced including sandwich structures pore size gradients and ceramic bone like structures applying to different types of bone. The particle stabilized foams used were in situ self hardening calcium aluminate cement enriched alumina foams to obtain crack free samples with pore interconnections and tailorable pore sizes.

The electrical conductivity of dense and nanoporous zirconia-based thin films is compared to results obtained on bulk yttria stabilized zirconia (YSZ) ceramics. Different thin film preparation methods are used in order to vary grain size, grain shape, and porosity of the thin films. In porous films, a rather high conductivity is found at room temperature which decreases with increasing temperature to 120 °C. This conductivity is attributed to proton conduction along physisorbed water (Grotthuss mechanism) at the inner surfaces. It is highly dependent on the humidity of the surrounding atmosphere. At temperatures above 120 °C, the conductivity is thermally activated with activation energies between 0.4 and 1.1 eV. In this temperature regime the conduction is due to oxygen ions as well as protons. Proton conduction is caused by hydroxyl groups at the inner surface of the porous films. The effect vanishes above 400 °C, and pure oxygen ion conductivity with an activation energy of 0.9 to 1.3 eV prevails. The same behavior can also be observed in nanoporous bulk ceramic YSZ. In contrast to the nanoporous YSZ, fully dense nanocrystalline thin films only show oxygen ion conductivity, even down to 70 °C with an expected activation energy of 1.0 ± 0.1 eV. No proton conductivity through grain boundaries could be detected in these nanocrystalline, but dense thin films.

Bone substitute materials allowing trans-scaffold migration and in-scaffold survival of human bone-derived cells are mandatory for development of cell-engineered permanent implants to repair bone defects. In this study, we evaluated the influence on human bone-derived cells of the material composition and microstructure of foam scaffolds of calcium aluminate. The scaffolds were prepared using a direct foaming method allowing wide-range tailoring of the microstructure for pore size and pore openings. Human fetal osteoblasts (osteo-progenitors) attached to the scaffolds, migrated across the entire bioceramic depending on the scaffold pore size, colonized, and survived in the porous material for at least 6 weeks. The long-term biocompatibility of the scaffold material for human bone-derived cells was evidenced by in-scaffold determination of cell metabolic activity using a modified MTT assay, a repeated WST-1 assay, and scanning electron microscopy. Finally, we demonstrated that the osteo-progenitors can be covalently bound to the scaffolds using biocompatible click chemistry, thus enhancing the rapid adhesion of the cells to the scaffolds. Therefore, the different microstructures of the foams influenced the migratory potential of the cells, but not cell viability. Scaffolds allow covalent biocompatible chemical binding of the cells to the materials, either localized or widespread integration of the scaffolds for cell-engineered implants.

Yttria stabilized zirconia thin films are deposited on silicon single crystal substrates by aerosol assisted chemical vapor deposition from precursor solutions of zirconium and yttrium 2,4-pentanedionate in ethanol. Continuous films are obtained using pure oxygen, pure nitrogen, or mixtures of both as carrier gas. In the simultaneous presence of water and oxygen, crack formation is observed for films deposited at intermediate substrate temperatures (450 °C), while those deposited at low (300 °C) and high (600 °C) temperatures remain crack-free. Crack-free films can be deposited at 450 °C in a water-free setting, or in the presence of water using pure nitrogen as carrier gas. The addition of water to the precursor solutions also significantly reduces film growth rates.

New phase diagram data and a thermodynamic assessment of the CeO-Gd2O3-CoO system using the CALPHAD approach are presented. This information is needed to understand the surprisingly low sintering temperature (950°C–1050°C) of CeO2-based materials doped with small amounts of transition metal oxide (e.g., CoO). Experimental phase equilibria between 1100°C and 1300°C are reported based on the analysis of annealed and molten samples. No isolated compound exists in the ternary. At 1300°C the Co solubility in the ternary compounds Ce1−x−yGdxCoyO2−x/2−y (fluorite) is 2.7 mol% and is less than 1 mol% in the Gd2−xCexO3+x/2 (bixbyite). The Ce solubility in the perovskite GdCoO3−δ was found to be 1 mol%. The lowest temperature eutectic melt in the ternary has a composition of 57.2 mol% Co and 41.1 mol% Gd melting at an onset temperature of 1303 ± 5°C, which is close to the binary eutectic in the Gd2O3-CoO system at 60 ± 2 mol% Co and 1348 ± 1°C.

Microstructures of yttria stabilized zirconia thin films deposited by aerosol assisted chemical vapor deposition (AA-CVD) are correlated with the thermal decomposition behavior of the corresponding metal precursors, zirconium and yttrium 2,4-pentanedionate. Process conditions of AA-CVD are investigated with the aim of producing dense and compact YSZ thin films for applications as gas-tight electrolyte. Based on systematic cross sectional scanning transmission electron microscopy (STEM) investigations and conductivity measurements, the development of percolating nanoporosity is observed in samples prepared at temperatures between 350 degrees C and 600 degrees C at standard solution throughput. Compact columnar thin films with bulk conductivity are obtained at 600 degrees C by reducing the metal content of the precursor solution and at 450 degrees C by reducing the solution throughput.

The high absorption energies of partially wetted particles at fluid interfaces allow the production of macroporous composite materials from particle-stabilized foams. Competition between the different particle types determines how they are distributed in the foam lamella and allow the phase distribution to be controlled; a technique that is useful in the design and engineering of porous composites. Here, we report details on the effects of preferential and competitive adsorption of poly(vinylidene fluoride) (PVDF) and alumina (Al(2)O(3)) particles at the foam interfaces on the consolidated macroporous composite materials. By varying the relative composition and surface energies of the stabilizing particles, macroporous composite materials with a broad range of phase distributions are possible.

The microfabrication of Foturan ® glass ceramic as a potential substrate material for micro-solid oxide fuel cells (micro-SOFC) was investigated. Foturan ® was etched in 10% aqueous hydrofluoric (HF) acid solution at 25 °C with a linear rate of 22 ± 1.7 μm/min to create structures with an aspect ratio of 1:1 in 500 μm-thick Foturan ® substrates for micro-SOFCs. The concentration of the HF etchant was found to influence the etching rate, whereas the UV-exposure time creating nuclei in the glass for subsequent crystallization of the amorphous Foturan ® material had no significant influence on the etching rates. The surface roughness of the crystallized Foturan ® was determined by the crystallite size in the order of 10–15 μm. Free-standing micro-SOFC membranes consisting of a thin film Pt cathode, an yttria-stabilized-zirconia electrolyte and a Pt anode were released by HF etching of the Foturan ® substrate. An open-circuit voltage of 0.57 V and a maximum power density of 209 mW/cm 2 at 550 °C were achieved.

The electrical cross-plane conductivity of 8 mol% yttria stabilized zirconia (YSZ) thin films prepared by different deposition techniques, namely aerosol assisted chemical vapor deposition, wet spray pyrolysis (SP), and pulsed laser deposition (PLD), is correlated with their microstructure. Depending on deposition technique and process conditions, microstructures ranging from amorphous to randomly oriented nanocrystalline or columnar with preferred (111) orientation are obtained. Cross-plane AC impedance measurements of these thin films show that the oxygen ion conductivity of randomly oriented nanocrystalline samples is determined by the grain boundaries, which show significantly lower transport properties than the grain interior. In columnar microstructures, the conductivity is determined by ionic transport through the grains only. The same conduction behavior is found for amorphous and randomly oriented microstructures with grain sizes between 3 nm and 9 nm, indicating that no true size effects occur in 8 mol% YSZ. Grain and grain boundary conductivity determined for nanocrystalline 8 mol% yttria stabilized zirconia thin films of different microstructures.

Thin films from the Lan+1NinO3n+1 system exhibit favorable dielectric and electrochemical properties that may prove useful for a variety of devices ranging from ferroelectrics to low-temperature solid oxide fuel cells. The present work covers the compositional, microstructural and electrochemical characterization of thin lanthanum nickelate films deposited by spray pyrolysis. In accordance with the phase diagram, LaNiO3−δ or La4Ni3O10−δ films were obtained during annealing of spray deposited films at temperatures between 540 °C and 1100 °C. Whereas LaNiO3−δ films exhibited a high metallic conductivity, La4Ni3O10−δ films were semiconducting. Electrochemical impedance spectroscopy indicated an increase of the area specific oxygen reduction resistance with the annealing temperature. The study highlights how the phase and microstructure of thin films from the Lan+1NinO3n+1 system can be tailored by annealing of initially amorphous films. LaNiO3−δ films have a high potential for application in electrochemical devices operating at low temperatures where high electrical conductivity is required.

A simple method to predict the thermal characteristics of a micro-SOFC system is presented in this study. The basic design requirements for a thermally self-sustaining operation at a stack temperature of about 550 °C are assessed. Based on steady-state global energy and mass balances, the influence of the electrical efficiency, the overall air-to-fuel ratio and the heat losses on the operating temperature is discussed. It was found that at high electrical efficiencies and, hence, low heat release rates, a recuperator is needed to achieve the desired operating temperature. At lower electrical efficiencies, in contrast, disposing the released heat becomes an issue and an efficient cooling of the stack is required. Whether a recuperator or additional cooling components are necessary also depends on the electrical power output, the stack size and the thermal insulation specifications. The threshold between cooling and recuperator mode is of special interest, since this operating point allows a simple design of the thermal system without recuperator and only a minimum of air to be supplied to the system. This threshold efficiency, which is the maximal electrical efficiency that allows a thermally self-sustaining operation without recuperator, is above 50% under adiabatic operating conditions. In a non-adiabatic system, the threshold efficiency is reduced to about 45% in a 20 Wel and to about 20% in a 2.5 Wel system even with a state-of-the art thermal insulation. The considered thermal insulations dominate the system volume. Therefore, the ability of heat loss minimization is highly dependent on the targeted volumetric power density of the system.

Near-conservation over long times of the actions, of the energy, of the mass and of the momentum along the numerical solution of the cubic Schrodinger equation with small initial data is shown. Spectral discretization in space and one-stage exponential integrators in time are used. The proofs use modulated Fourier expansions.

Microstructures of yttria-stabilized zirconia (YSZ) thin fi lms deposited by spray pyrolysis at 370 ° C on sapphire are investigated. The as-deposited fi lms are predominantly amorphous and crystallize upon heating at temperatures above 370 ° C, developing grains in the range of 5 nm to several 100 nm. During post-deposition heat treatment up to 800 ° C, ∼ 50 vol% porosity develops in the center of the fi lms with gradients towards almost dense interfaces to the air and substrate. The reason for this porosity is the decomposition of residues from the precursor and the free volume liberated due to crystallization. Dense YSZ thin fi lms consisting of one monolayer of grains are obtained with annealing temperatures exceeding 1200 ° C. In gadoliniumdoped-ceria (CGO) thin fi lms similar microstructures and porosity are found after low-temperature heat treatments indicating that the precursor residues due to the deposition method are the main cause of the porosity. Grain growth stagnation in annealed thin fi lms is observed in both the YSZ and in CGO thin fi lms. Stagnating grain growth in the thin fi lms is rather caused by reduced grain boundary mobility, here predominately due to a “secondary phase”, i.e., pores, than to other effects. The stagnation ceases at higher annealing temperatures after densifi cation has taken place.

Microencapsulation is nowadays widely applied for the separation or controlled and targeted release of substances. In the present work we propose using particle-stabilized water-in-oil emulsions to produce microcapsules from calcium aluminate cement. First, we investigate the parameters determining the capsule size and size distribution. Then, the anhydrous particles at the water-oil contact line are hydrated resulting in the formation of a layer of precipitated calcium aluminate hydrate that strengthens the capsule shells. By manipulating the hydration reaction of the cement particles, the capsule shell thickness is varied between 200 and 1300 nm. For hydration temperatures of 3°C and 25°C, the capsule walls are mainly formed from Ca2Al2O5·8H2O, whereas Ca2Al2O5·8H2O and the stable γ-Al(OH)3 and Ca3Al2O6·6H2O phases are precipitated at 60°C. In addition, the impact of these manipulations on the yield of dry microcapsules is discussed. The highest yield of intact capsules observed in this study is 72 ± 3%. We believe that such microcapsules can lead to new applications, particularly in environments with harsh conditions or in inorganic matrixes.

Nanocrystalline yttria-stabilized-zirconia thin films with grain sizes smaller than 15 nm are fabricated by spray pyrolysis. Impedance spectroscopy is performed perpendicular to the thin film between room temperature and 600 °C. For a grain size of 13 nm, the grain and grain boundary contributions of the electrical conductivity can be discerned but only between 200 °C and 400 °C. For smaller grains, and higher or lower temperatures, the grain contribution cannot be resolved.

The cross-plane oxygen ion conductivity of yttria stabilized zirconia thin films prepared by aerosol assisted chemical vapor deposition is strongly influenced by the thin film microstructure. Thin films with highly textured columnar grains oriented parallel to the current direction exhibit significantly higher conductivity than thin films with randomly oriented nanocrystalline microstructure, where ionic transport proceeds through the numerous grain boundaries. The total conductivity of columnar AA-CVD thin films is consistent with literature values on oxygen ion conduction through chemically pure YSZ grains as determined for microcrystalline samples, while that of the nanocrystalline specimens is similar to corresponding specific grain boundary conductivities.

Thin film growth of yttria stabilized zirconia (YSZ) using atmospheric aerosol assisted chemical vapor deposition (AA-CVD) from β -diketonates is studied. The influence of nature and concentration of metal precursors and solvents on thin film growth, microstructure and composition is investigated as a function of deposition temperature. AA-CVD is able to produce smooth and homogeneous YSZ thin films of controlled thickness and stoichiometry. Amorphous, nanocrystalline or columnar microstructures can be obtained at deposition temperatures between 300 and 650 °C. In the same temperature regime, a transition from surface reaction to diffusion controlled film growth is observed. For applications as gas separating membranes, e.g. for micro-solid oxide fuel cell electrolytes, randomly oriented nanocrystalline microstructures with grain sizes in the range of 10 nm are promising.

The CALPHAD (Calculation of Phase Diagrams) framework to calculate the thermodynamic properties of multicomponent systems is extended to account for the change in thermodynamic properties by the size effect in small particle systems. We have applied this framework to the CeO 2 –CoO binary in order to elucidate the “liquid phase sintering like” fast densification at unusually low temperatures of cobalt oxide doped ceria. The phase diagram for 5 nm particles shows a significant drop of the eutectic from 1645 °C to 1327 °C. The size-dependent thermodynamic stability of intergranular films is examined using CALPHAD approach and melting temperature of 1180 °C is found for 1 mol% CoO doped CeO 2 corresponding to an intergranular film thickness between 1 and 4 nm. This result comes close to the unusual low sintering temperatures of 1100 °C, with a “liquid-sintering”-like characteristic that was previously experimentally observed for CeO 2 with small amounts of transition metal oxides such as CoO.

The two main mechanisms that cause the broadening of X-ray diffraction profiles in polycrystalline materials, i.e. those due to finite grain size and local strain inhomogeneities, are usually considered independently. In this paper, we discuss the potential interrelationship between them and propose a phenomenological equation which links the dispersion of strain distribution to grain size via the width of distorted regions near grain boundaries and the lattice disorder therein. The developed approach is applied to characterize crystallization processes in Gd-doped ceria films.

Electrochemical properties of La0.58Sr0.4Co0.2Fe0.8O3-delta (LSCF) thin films with well defined microstructures have been investigated. Symmetrical cells were characterized by impedance spectroscopy in the temperature range from 625 to 750 degrees C and the oxygen partial pressure, range from 10(-2) to 1 atm. The area specific resistance for dense films was invariant with thicknesses between 130 and 1200 nm indicating oxygen surface exchange as the process limiting oxygen transport in the films. The capacitances deduced from the impedance data show a linear dependence with the film thickness. Values were as high as 0.2 F cm(-2) suggesting a chemical capacitance. The oxygen non-stoichiometry was calculated using the measured chemical capacitance and a defect chemistry model. While dense films have an area specific resistance of 8 Omega cm(2) at 750 degrees C, porous films with an increase of the surface area by 26 times have only an area specific resistance of 0.38 Omega cm(2). It is shown that the polarization resistance of thin films is approximately proportional to the inverse of the surface area of the porous cathodes in the temperature regime 625 to 750 degrees C. The activation energy of the surface oxygen exchange process depends on the thin film microstructure as it decreased from 2.4 eV for dense films to 1.6 eV for porous films.

Photostructurable glass–ceramics are suitable for 3-dimensional microfabrication and, in some instances, can be used as an alternative material to silicon in the microfabrication of micro-electro-mechanical-system (MEMS) devices. Foturan ® is a photosensitive lithium–aluminium–silicate glass that can be structured by an exposure to UV light, followed by a thermal treatment and an etching step. In this work, the crystallisation kinetics and microstructure evolution of unexposed and UV-exposed Foturan ® are investigated by means of differential scanning calorimetry, X-ray diffraction and scanning electron microscopy. The glass transition temperature T g , the apparent activation energy of crystallisation E a and the colour of UV-exposed Foturan ® are discussed. It is shown that nucleation and crystallisation of UV-exposed Foturan ® can be steered either as a function of temperature (non-isothermal) or as a function of time (isothermal). This is essential for the photostructuring and etching of Foturan ® regarding the achievable feature sizes of 25 μm for MEMS applications.

We present a method to prepare metallic foams with a unique architecture of small pores and thin struts using wet foams and emulsions stabilized by metallic nanoparticles as templates.

Ultrathin 3 mol.% yttria-stabilised-tetragonal-zirconia-polycrystal (Y-TZP) foils with thicknesses of 1–10 μm are fabricated by a new wet-chemical processing route. The foils are free-standing, semi-transparent and flexible. The in-plane electrical conductivity of the Y-TZP foil is 0.03 S m −1 at 500 °C. Cross-plane impedance measurements with sputtered Pt electrodes yield two arcs, of which the high-frequency arc is attributed to the ohmic resistance of the electrolyte and the low-frequency arc to the electrode–electrolyte interface. A symmetrical micro-solid oxide fuel cell (SOFC) is designed using this ultrathin free-standing Y-TZP foil as the electrolyte and sputtered Pt electrodes. An open-circuit voltage of 0.98 V and a maximum power density of 12 mW cm −2 are measured at 500 °C. These results prove the feasibility of this approach to the fabrication of miniaturised planar SOFCs without the need for microfabrication.

Aqueous suspensions containing oppositely charged colloidal particles and amphiphilic molecules can form fluid dispersions, foams, and percolating gel networks, depending on the initial concentration of amphiphiles. While models have been proposed to explain the electrokinetic and flotation behavior of particles in the presence of long amphiphilic molecules, the effect of amphiphiles with less than six carbons in the hydrocarbon tail on the electrokinetic, rheological, and foaming behavior of aqueous suspensions remains unclear. Unlike conventional long amphiphiles (≥10 carbons), short amphiphiles do not exhibit increased adsorption on the particle surface when the number of carbons in the molecule tail is increased. On the basis of classical electrical double layer theory and the formerly proposed hemimicelle concept, we put forward a new predictive model that reconciles the adsorption and electrokinetic behavior of colloidal particles in the presence of long and short amphiphiles. By introducing in the classical Gouy-Chapman theory an energy term associated with hydrophobic interactions between the amphiphile hydrocarbon tails, we show that amphiphilic electrolytes lead to a stronger compression of the diffuse part of the electrical double layer in comparison to hydrophilic electrolytes. Scaling relationships derived from this model provide a quantitative description of the rich phase behavior of the investigated suspensions, correctly accounting for the effect of the alkyl chain length of short and long amphiphiles on the electrokinetics of such colloidal systems. The proposed model contributes to our understanding of the stabilization mechanisms of particle-stabilized foams and emulsions and might provide new insights into the physicochemical processes involved in mineral flotation.

Si-diffusion from Si-based substrates into yttria-stabilized-zirconia (YSZ) thin films and its impact on their microstructure and chemistry is investigated. YSZ thin films used in electrochemical applications based on micro-electrochemical systems (MEMS) are deposited via spray pyrolysis onto silicon-based and silicon-free substrates, i.e. Si x N y -coated Si wafer, SiO 2 single crystals and Al 2 O 3 , sapphire. The samples are annealed at 600 °C and 1000 °C for 20 h in air. Transmission electron microscopy (TEM) showed that the Si x N y -coated Si wafer is oxidized to SiO z at the interface to the YSZ thin film at temperatures as low as 600 °C. On all YSZ thin films, silica is detected by X-ray photoelectron spectroscopy (XPS). A particular large Si concentration of up to 11 at% is detected at the surface of the YSZ thin films when deposited on silicon-based substrates after annealing at 1000 °C. Their grain boundary mobility is reduced 2.5 times due to the incorporation of SiO 2 . YSZ films on Si-based substrates annealed at 600 °C show a grain size gradient from the interface to the surface of 3 nm to 10 nm. For these films, the silicon content is about 1.5 at% at the thin film's surface.

The oxidation of a cermet of screen-printed nickel (Ni) and gadolinia-doped ceria (CGO) with an approximate median porosity of 50 vol.% has been studied via in situ X-ray diffraction and focused ion beam nanotomography in the temperature range 773–848 K. The oxidation kinetics of Ni to NiO is found to be highly nonlinear with an apparent activation energy of 2.8(2) eV in this temperature range. The nonlinear oxidation kinetics found is in good agreement with theoretical works on oxide growth driven by nonlinear inbuilt fields. Stress-induced Kirkendall void formation has been identified as the physical process that enhances the oxidation of Ni/CGO cermets. Compressive stresses within the Ni matrix result from the thermal expansion mismatch of Ni and CGO and cause plastic deformation as they exceed the yield stress of the Ni matrix. The pore size distribution of Kirkendall voids formed has been measured by FIB nanotomography and shows a significant temperature dependence. It is shown that even one cycle of reoxidation changes irreversibly the microstructure of the cermet which can be interpreted as the onset and main contribution to the mechanical degradation of the cermet.