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Solid oxide fuel cells (SOFCs) enable environmentally friendly energy to be produced with high efficiency. The market entry of SOFC systems depends on the functionality of the components and on the costs. The SOFC has not yet reached market maturity. This presentation focuses on the possibilities for manufacturing SOFCs with high power outputs and long-term durability by using manufacturing technologies feasible in industry. For the past 15 years, FZ Jülich has been developing large-size so-called anode-supported SOFCs (up to 200 x 200 mm²) with reproducibly high power output (> 2 A/cm² at 800°C). Novel technologies for high-capacity manufacturing such as tape casting and roller coating have been introduced. Additionally, newly developed thin-film techniques have led to power outputs of more than 3 A/cm² at 800°C and more than 1.5 A/cm² below 700°C. These high power densities open up new possibilities for the operation of SOFCs at low temperatures to ensure low degradation and therefore long lifetimes.

The green density and roughness of green ceramic layers determine their mechanical and micro structural properties after final sintering. These properties can be measured precisely by laser profilometry. The green density of thin layers (20-50 µm) could quickly be determined as accurately as 0.5% theoretical density. The influence of paste parameters (powder conditioning, solid content, binder, and dispersing agent) on the green density was studied systematically for electrolyte pastes (8 mol.-% yttria stabilised zirconia = 8YSZ) typically used in solid oxide fuel cell applications. It could be shown that a minimal binder content is required to achieve acceptable green densities. Pre-calcination of the powder yielded also significantly higher film densities. Dispersant agents contributed to a smoother surface in any case, however a significant effect on the packing density was only observed for the fine, non-calcined powder.

A new method was developed for producing highly porous NiTi for use as an implant material. The combination of the space‐holder technique with the metal injection molding process allows a net‐shape fabrication of geometrically complex samples and the possibility of mass production for porous NiTi. Further, the porosity can be easily adjusted with respect to pore size, pore shape, and total porosity. The influence of the surface properties of powder metallurgical NiTi on the biocompatibility was first examined using human mesenchymal stem cells (hMSCs). It was found that pre‐alloyed NiTi powders with an average particle size smaller than 45 μm led to the surface properties most suitable for the adhesion and proliferation of hMSCs. For the production of highly porous NiTi, different space‐holder materials were investigated regarding low C‐ and O‐impurity contents and the reproducibility of the process. NaCl was the most promising space‐holder material compared to PMMA and saccharose and was used in subsequent studies. In these studies, the influence of the total porosity on the mechanical properties of NiTi is investigated in detail. As a result, bone‐like mechanical properties were achieved by the choice of Ni‐rich NiTi powder and a space‐holder content of 50 vol% with a particle size fraction of 355–500 μm. Pseudoelasticity of up to 6% was achieved in compression tests at 37 °C as well as a bone‐like loading stiffness of 6.5 GPa, a sufficient plateau stress σ25of 261 MPa and a value for σ50of 415 MPa. The first biological tests of the porous NiTi samples produced by this method showed promising results regarding proliferation and ingrowth of mesenchymal stem cells, also in the pores of the implant material.

Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bondcoat and a ceramic, heat isolative topcoat. In modern TBCs the ceramic topcoat is further divided into layers with different functions. One example is the double layer system in which conventional yttria stabilized zirconia (YSZ) is used as bottom and new materials as pyrochlores or perovskites are used as topcoat layers. These systems demonstrated an improved temperature capability compared to standard YSZ. Examples of such systems will be shown. In modern gas turbines the increased temperatures and gas pressures lead to an increased fraction of radiative heat flow. Coatings with increased reflectivity can be used to avoid the direct heating of the metallic substrates by this radiation. An effective method to produce such coatings is suspension plasma spraying. These reflective coatings are deposited on top of the TBC system and will lead to a further grading and improved performance of the coating.

Abstract: Wear at the electrode surfaces of a one-cathode plasma torch changes the characteristic fluctuation pattern of the plasma jet. This affects the trajectory of the particles injected into the plasma jet in a non-controllable way, which degrades the reproducibility of the process. Time-based voltage measurements and Fourier analysis were carried out on a one-cathode F4 torch at different wear conditions to determine the evolution of wear dependant characteristics. A significant correlation is observed between increasing torch wear and decreasing voltage roughness and high frequency noise. Furthermore, by means of particle diagnostic systems, the change in the particle velocity and temperature has been measured. The variations of the particle characteristics are significant and thus an influence on the sprayed coating microstructure is to be expected.

Abstract: Comparative in-flight measurements of particles plasma sprayed by F4 and Triplex II guns were carried out using the diagnostic systems DPV-2000 and Accuraspray-g3. The comparison of mean particle velocities and temperatures as well as intensity profiles of the plume show a good agreement confirming their accuracy. However, the varying operating principles especially the deceased dimensions of the measurement volumes have to be considered carefully when evaluating the results. Furthermore, some applications of the diagnostic systems DPV-2000 and Accuraspray-g3 are shown. Finally some application limits, which were identified for certain powder compositions at higher plasma power levels will be discussed.

Up to now, Solid Oxide Fuel Cell (SOFC) materials and processing does not meet the cost goals for commercialization. This resulted in a worldwide increase in R&D activities dealing with advanced materials and effective manufacturing methods. The present paper describes efforts to process novel SOFC materials as well as optimization of well known ones. The R&D trends are explained for key components such as anode, electrolyte, cathode, contact- and protective layers. Typical SOFC manufacturing methods include tape casting, extrusion, calendaring and axial pressing. Each of these techniques has advantages and limitations. Examples for the highly efficient use of these methods are given for electrolyte supported cells as well as anode and cathode supported designs. An evaluation in reference to automation, process complexity and costs is given under the present limiting factors. Exemplary the processing by tape casting and the micro structural fine tuning of an advanced anode-supported system is discussed in detail. To produce the layered components of an SOFC, techniques like screen printing, wet powder spraying, PVD and CVD are under development. While the layer properties are excellent, PVD and CVD are nowadays too expensive in some cases, due to the low deposition rates. If thin layers are required, these techniques become interesting under cost considerations. The effectiveness of a PVD interlayer between electrolyte and high power density cathodes is shown in comparison to a sintered layer. In thin electrolyte concepts, the cathode becomes the power limiting component at operating temperatures below around 750°C. Thus new cathode materials and adjusted processing parameters are under development. The possibilities to manufacture advanced cathode layers by screen printing, wet powder spraying and other wet chemical methods are discussed. As an example screen printing of LSCF is described which results in a high power density cathode layer for low temperature SOFC operation. Finally, future needs to achieve the technical and economic goals are summarized.

Due to their direct conversion of electrochemical into electrical energy solid oxide fuel cells (SOFCs) have great potential for a future additional energy supply. Even in the last two years numerous developers of SOFCs, both industry and research institutions, have demonstrated long-term stable operation of stacks of various dimensions (ranging from 1 to 125 kWel, with durations of up to 25000 hours of operation). Besides technical proof, single component availability (cells, bipolar plates, sealing…), stable and low-aging operation, as well as cost efficient manufacturing of the components is becoming more and more evident in preparation for a market launch. Close cooperation between SOFC stack developers, SOFC users and manufacturers of powders, semifinished parts or stack components is a prerequisite for success. Within a collaboration project funded by the German Federal Ministry of Economics and Labor (BMWA) the development of an SOFC as an auxiliary power unit (APU) is being promoted. The industrial users are BMW for automotive applications and Liebherr for use in construction vehicles or aircraft. The content of this presentation will be the transfer of the manufacturing knowledge developed at Research Center Jülich to CeramTec; including on the one hand the problems and limitations and, on the other hand the successes and positive perceptions. In detail, the transfer of, for example tape casting and screen printing will be addressed, powder characteristics concerning paste or slip formulation and special tests with reference to SOFCs are presented, and single cell tests of various cells manufactured with different powders or fabrication processes are described. Additionally, some remarks will concern different priorities in either R&D or industry (e.g. R&D: high power density; industry: reproducibility), process windows for manufacturing and the search for alternative fabrication methods.

The lifetime under thermal cycling of a system consisting of an air plasma sprayed thermal barrier coating (TBC) deposited on a metallic bondcoat (BC) is determined by the subcritical growth of micro-cracks near the interface between both coatings. This growth mainly occurs during the cooling down phase, as shown by the acoustic emission monitoring during the thermal cycling. The factors controlling the stress level leading to the crack growth are the local curvature of the metallic-ceramic interface, the growth of an oxide scale (TGO) at such interface and the sintering of the TBC, the two last processes occurring during the high temperature cycle phase. Implementing all these factors, a model based on Finite Element Method (FEM) calculations is presented where growing cracks are incorporated by assigning soft properties to the FEM cells occupied by the cracks. Determining the growth direction for the maximum energy release rate at every cooling down step, the current crack extension during the cycling is tracked until it reaches a characteristic length corresponding to the TBC failure. The influence by the metallic-ceramic interface roughness and by the temperature gradient across the TBC is discussed.

Abstract: In 1995 a fuel cell program was established at Forschungszentrum Jlich (FZJ) to combine various and independently existing activities on generic fuel cell research. As of this year, the materials development as well as the component manufacturing was focused on anode-supported cells. This contribution reviews the investigations on many materials and the fabrication processes applied for anode substrates, anodes, electrolytes and cathodes. Continuous effort on each of the components resulted in a steady progress in quality, size and performance of the solid oxide fuel cells (SOFCs). The electrochemical performance of anode-supported cells (ASC) with “conventional” electroceramic materials like lanthanum manganite as cathode, yttria-stabilized zirconia (YSZ) as electrolyte and a NiO/YSZ composite as anode and anode substrate was improved from about 0.15 W/cm2 at 800∘C to 1.2 W/cm2 by optimization of processing and microstructure. Considering new perovskite materials like lanthanum cobaltite/ferrite as cathode, a power density of 1.4–1.7 W/cm2 has been obtained. In addition to these strongly focused activities on ASC, FZJ has carried out R + D projects together with industrial companies for the planar and tubular electrolyte-supported cell systems (Siemens, Dornier). This review, however, also discusses efforts that were less successful for anode-supported SOFCs. As examples are mentioned here the lanthanum gallates as electrolyte materials or low-cost NiO/Al2O3 substrates, which are not dimensionally stable in fuel gas atmosphere.

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Silver (Ag) at 0.1–2.0 wt% was incorporated into cathodes for solid oxide fuel cells as a catalyst for oxygen reduction. A novel processing route for Ag incorporation ensuring a very homogeneous Ag ion distribution is presented. From the results of X-ray powder diffraction it can be concluded that the La 0.65Sr 0.3MnO 3−δ perovskite phase is already formed at 900 °C. The solubility of Ag in the crystal lattice in this type of perovskite was below 1 wt%. The electrochemical tests of these materials show that there is only a slight catalytic effect of Ag. Scanning electron microscopy reveals a low mechanical contact of the cathode grains to the electrolyte due to the low cathode sintering temperature that was chosen.

Abstract: It is generally known that the porosity of thermal barrier coatings is essential to guarantee a sufficiently high strain tolerance of the coating during thermal cycling. However, much less is known about the influence of the specific morphology of porosity, such as microcracks and typically larger pores, on the performance of the coatings. Both features are usually formed during plasma spraying of yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). In this investigation, the influence of microcracks on the thermal cycling behavior was studied. The amount of microcracks within YSZ thermal barrier coatings was changed by changing the powder-feeding rate. Only small changes of the total porosity were observed. Mercury porosimetry served as a tool to investigate both the amount of microcracks and pores in the coating. Additionally, microcrack densities were determined from metallographical investigations. A linear dependence between the amount of fine pores determined by Hg porosimetry and the crack density was obtained for one set of coatings. Thermal cycling TBC specimens with different microcrack densities were produced and tested in a gas burner test facility. At high surface temperatures (above 1300 °C), failure occurred in the ceramic close to the surface. Under these conditions, the samples with increased horizontal microcrack densities showed a significant increase of thermal cycling life.

In solid oxide fuel cells (SOFCs) the interconnects electrically link air and fuel electrodes on either side to produce a practical electrical power output. The long-term stability of intermediate temperature (650–800 °C) SOFC operation strongly depends on the composition of the ferritic steel interconnection material and the steel/ceramic interface. During high-temperature operation the Cr-containing ferritic steel forms an oxide scale at its surface, thereby causing high ohmic electrical contact resistance when connected to the surface of an electronically conducting ceramic cathode material. In the long run, the vaporization of Cr species from these oxide scales also affects the cathode activity, eventually leading to cell deterioration. One way of overcoming the problem is to incorporate another electronically conducting ceramic compliant layer, commonly known as the contact layer, between the cathode and metallic interconnect. In this contribution, LaNi0.6Fe0.4O3was tested as a cathode contact material. Its performance at 800 °C in the form of a ~50 µm thick film applied on two ferritic steel compositions was examined. After 600 h of testing, contact resistances of 60 and 160 mΩ cm2were obtained. The different values are explained by the variation in steel composition.

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The manufacture of fine-grained SiC ceramics by using nanophase SiC powders with particle sizes of about 20 nm is described. Conventional sintering of these powders led to extreme grain growth and hence the nanophase microstructure was destroyed. Pressure-assisted sintering was applied to reduce the sintering temperature and also grain growth. Hot isostatic pressing (HIPing) of samples with about 1 wt carbon and 1 wt boron addition at temperatures below 1700°C and with pressures up to 350 MPa resulted in densities of more than 95 of the theoretical density (TD) and grain sizes of 150 nm. A further reduction of grain size became possible by an optimized high-temperature heat treatment of the samples prior to HIP. This procedure removed at least partially the oxygen layer on the surface of the nanophase SiC particles. Samples with densities of more than 97 TD and grain sizes below 80 nm were produced. Grain sizes were measured with scanning and transmission electron microscopes; both methods gave similar values. Grain sizes determined from the peak broadening of X-ray diffraction peaks showed in some cases lower values. This was attributed to the relatively high amount of twinning boundaries and stacking faults produced during crystal growth. The influence of the grain size on different mechanical and thermal properties was investigated. Vickers hardness and indentation fracture toughness were measured for samples with different densities and grain sizes. The results revealed that besides grain size features like density andor oxygen content can strongly influence hardness and fracture toughness. Similar results have been found for the wear resistance of fine-grained materials. Results of pin-on-disk type experiments showed the importance of a high density of the samples for high wear resistance. Thermal diffusivity measurements were performed for samples with different grain sizes up to 1400°C. A large decrease with decreasing grain size was found at room temperature. At higher temperatures the difference in the thermal diffusivity of fine- and large-grained materials was reduced.

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Physical vapor deposition (PVD) technologies allow for a variety of microstructural morphologies. PVD is especially qualified for the production of dense ceramic layers at comparatively moderate substrate temperatures. This paper discusses the physical vapor deposition of SOFC functional layers, comprising perovskites, NiO/8YSZ and composite electrolyte layers, with a thickness between 300 nanometres and several micrometres and lateral dimensions of 100 mm⋅100 mm. Magnetron sputtering and electron beam evaporation were used as PVD techniques, with and without additional ion bombardment of the layers during deposition. Scanning electron microscopy analysis revealed that an additional ion bombardment during layer growth increasingly promotes the creation of a continuous and dense layer morphology instead of a columnar microstructure.

This paper reports a study on the preparation of thin nano-structured 8YSZ membrane films, which can be sintered to full density at a lower temperature (~ 1200{degree sign}C) than currently applied in SOFC manufacturing (> 1400{degree sign}C). For the deposition process, four different coating liquids with varying particle size, covering the range from 85 nm to 6 nm, were prepared. In the first part, it is demonstrated that nano-dispersions with a particle size of e.g. 85 or 60 nm can be used for the formation of dense YSZ membranes with a thickness of 1 - 2 µm on a regular macroporous SOFC anode, but the layers can only be sintered to full density at 1400{degree sign}C. In the second part, ultra-thin YSZ membranes are prepared by sol-gel coating (particle size 35 and 6 nm). These layers show a thickness < 500 nm and a very tight mesoporous or microporous structure in the calcined state (pore size < 5 nm), which allows a sintering treatment in the desired temperature range (~ 1200{degree sign}C).