Abstract Stabilized bismuth oxide with fluorite structure is considered a promising electrolyte material for intermediate temperature solid-oxide fuel cells (SOFCs) due to its high oxygen ion conductivity. The ternary system, Bi2O3-Er2O3-WO3, is of particular interest because it is ionically conductive as well as thermally stable. This study investigates the quality of Bi2O3-Er2O3-WO3 (EWSB) electrolyte produced by plasma spraying. The phase structure and cross-sectional microstructure of plasma-sprayed EWSB were characterized by XRD and SEM. The as-sprayed EWSB was found to have a dense microstructure with well bonded lamellae. XRD analysis showed the formation of EWSB with δ-phase and a trace of β-phase, while the β-phase disappeared after annealing at 750°C for 10h. Electrical property tests revealed that the plasma-sprayed electrolyte also had excellent ionic conductivity (0.26 S cm-1 at 750 °C), making it a strong candidate for use in SOFCs at intermediate temperatures.

Abstract One of the most important innovations in solid oxide fuel cells is to have a membrane-like cell unit and to use a thin and dense electrolyte with reduced internal resistance. This paper deals with thin yttrium-stabilized zirconium oxide layers. An experimental study is carried out on vacuum-sprayed zirconium dioxide-8wt% yttrium oxide layers. The investigation of the process showed that layers of low porosity were obtained with a fine powder and an optimized plasma gas composition. Increased He flow, spray performance, and short spacing resulted in lower porosity and surface roughness. With optimized spraying conditions, thin YSZ layers with a thickness of 10-20 gm, lesser than 1% porosity, and an average roughness of 1 gm are achieved. Paper includes a German-language abstract.

Abstract Materials researchers discovered a series of promising oxide ceramic materials to serve for example in the field of solid oxide fuel cells (SOFC). This paper investigates the potential of the chemical vapor deposition (Thermal Plasma Chemical Vapor Deposition) supported by thermal high-frequency plasma for the deposition of Sr-doped La-Mn-Perovskite cathodes and Yttria-doped zirconia (YSZ) to demonstrate electrolytes of the SOFC. Aqueous solutions that evaporate completely in the plasma are used as starting materials. The microstructure of the layers produced depends on the position in the plasma jet at which the critical supersaturation of the precursor vapor is exceeded. Globular or stalky layers with growth rates of up to 30 micrometer/min are deposited. The homogeneity of the dopand distribution and the phase purity of the layers are excellent in the case of the YSZ. However, they have to be further improved for the perovskite layers through process modifications. Paper includes a German-language abstract.

Abstract The combination of the internal combustion engine for drive purposes with an auxiliary power unit (APU) for the electrical vehicle on-board power supply is seen as a particularly promising future automotive concept. Although a first demonstration vehicle has already been realized, the market launch of APUs based on solid electrolyte fuel cells (SOFC) depends very much on the use of an economical manufacturing process suitable for series production. To obtain 2-dimensional images of the heat distribution on SOFC surfaces during plasma spraying, a vacuum-sealed IR-camera was implemented in the low pressure vacuum plasma spraying chamber. The thermographic data obtained should be used as boundary conditions for a finite element model in order to calculate the stresses in the ceramic fuel cells resulting from the plasma spraying process. Paper includes a German-language abstract.

Abstract Plasma spray is a fast and inexpensive process for fabricating high performance yttria stabilized zirconia (YSZ) electrolyte for solid oxide fuel cells. In this paper, free-standing electrolyte disks consisting of YSZ with a diameter of 13 mm are produced by means of plasma spraying. The YSZ electrolyte disks are subjected to post-heat treatment at 1200, 1400, and 1500 deg C with holding times of 3 and 9 minutes with the help of a single and multi-cycle sintering process (Spark Plasma Sintering, SPS). The microstructure of the original and the post-treated electrolyte disks is examined by means of SEM. The mechanical properties and thermal conductivity of the electrolytes are measured to determine the effect of sintering. The results show that an SPS aftertreatment converts the lamellar structure of the plasma-sprayed panes into a coaxial granulated structure. The thermal conductivity of the electrolytes is significantly improved after SPS treatment. Paper includes a German-language abstract.

Abstract A novel concept for a thin-film cell was developed at DLR Stuttgart, which is applied to a porous metallic carrier substrate in a multistage, integrated vacuum plasma spraying process. This allows large-area cell units with 20-30 micrometer thick electrolyte layers to be deposited in a simple manner. The properties of the thin plasma-sprayed electrolyte layers with their lamellar structure differ greatly from the properties of the layers produced by sintering. This paper focuses on the investigation of the influence of the process parameters in plasma spraying on the gas tightness and on the ionic conductivity of the electrolyte layers. The measurement results of the sprayed coatings examined are correlated with the results of the electrochemical measurements of the cells. The investigations contribute to improving the electrochemical performance of Solid Oxide Fuel Cells stacks, which can be operated at reduced temperatures from 750 to 800 deg C. Paper includes a German-language abstract.

Abstract Ceria (CeO2) based electrolytes have been considered for use in solid oxide fuel cells (SOFC) for more than 20 years. There are however some limitations to this usage that this study has tried to address, indeed the study objective has been that of synthesizing and thermal spraying thin layers (50 - 100 µm) of doped CeO2 by the technique of suspension plasma spraying, using radio frequency (RF) plasma technology. Various dopant combinations and concentrations have been selected for this work in order to increase the useful partial oxygen pressure range for satisfactory ionic conductivity development, thereby increasing the anionic conductivity and preventing CeO2 reduction in fuel cell service. Ceria possesses the fluorite crystal structure at low temperatures but does not have enough oxygen vacancies to be a good ionic conductor. In ceria the cerium have 4+ oxidation state within the fluorite structure, and by substituting a certain amount of Ce4+ ions by trivalent dopant ions, oxygen vacancies are induced into the structure. Recent studies have demonstrated that at low temperatures doped ceria seems to be a better electrolyte than doped zirconia. Also, it seems that dopants with ionic radii close to Ce4+ ions give rise to better ionic conductivities. The doped ceria conductivity increases with the dopant concentration because more oxygen vacancies are created, but at higher concentrations vacancy ordering occurs which results in decreased ionic conductivity.

Abstract Solid oxide fuel cells (SOFC) are expected to gain a high importance as direct converters for transforming chemical into electrical energy. They have the potential of working with considerably higher efficiency and much less environmental problems compared to systems used so far. SOFCs of present technology operate at temperatures in the range of 950 °C. Besides an increase in performance and stability, a main precondition for a technical breakthrough of SOFCs is a drastic reduction of their production costs. Approaches are the use of less-expensive materials, new SOFC designs with thinner components and the improvement of presently applied production routes, or their replacement by other techniques such as thermal spray methods. DC- and RF-VPS show very attractive properties particularly if the cell will be manufactured in one consecutive combined process. The state of SOFC spray design will be described together with results of the process adaptation and the SOFC components development.

Abstract The atmospheric pressure plasma spray processes for functional layers of the tubular solid oxide fuel cell are developed to build a fuel cell structure consisting of air electrode, ceramic electrolyte, and fuel electrode. Further more the characteristics of each film are also investigated. The layers of LSM (La 0.65 Sr 0.35 MnO3) air electrode and Ni/8YSZ fuel electrode have porosities of 23 ~32 % sufficient for supplying fuel and oxidant gases efficiently to electrochemical reaction interfaces. The measured electrical conductivities of the electrodes are higher than 90 S/cm at 1000 °C, which satisfy the requirement as the current collecting electrodes. The YSZ electrolyte film has a high ionic conductivity of 0.07 S/cm at 1000 °C, but shows a bit too porous to block the oxygen molecule penetration through it. A unit tubular SOFC is fabricated by the optimized plasma spray processes for depositing each functional film and forming a porous cylindrical supporting tube of the cell, and turns out to have a promising capability of electricity generation.

Abstract A new plasma spray process was developed for the rapid deposition of very dense electrolyte layers for solid oxide fuel cells (SOFCs). The dense yttria-stabilized zirconia (YSZ) film was prepared by a center-injection low pressure plasma spraying (CI-VPS) process on various substrates in a triple-torch reactor. For deposition on porous substrates, an intermediate layer was applied using conventional atmospheric plasma spraying (APS) to close the large pores in the substrate. The films were characterized by XRD, SEM, and EMPA. The porosity of the film was analyzed by computerized image analysis of the micrographs. The film was also fractured by four-point bending to characterize the nature of bonding of layer-to-layer and within the deposit. The film analysis showed that YSZ layers with porosities of 0.3 % could be obtained at very high deposition rates with the CI-VPS process, with a very good functional performance of the layer as an electrolyte. Building of a complete SOFC by successive deposition of an atmospheric pressure sprayed porous cermet film, the dense YSZ electrolyte layer, and a porous perovskite film is discussed.

Abstract Perovskite-type LaMnO 3 powders and coatings have been prepared by a novel technique, the reactive suspension plasma spraying (SPS) using an inductively coupled plasma of about 40 kW plate power and an oxygen plasma sheath gas. Suitable precursor mixtures were found on the basis of solid state reactions, solubility and the phases obtained during the spray process. Best results were achieved by spraying a suspension of fine MnO 2 powder in a saturated ethanol solution of LaCl 3 with a 1:1 molar ratio of La and Mn. Low reactor pressure was helpful in order to diminish the amount of corrosive chlorine compounds in the reactor. As-sprayed coatings and collected powders showed perovskite contents of 70-90%. After a post-treatment with an 80% oxygen plasma an almost pure LaMnO 3 deposit was achieved in the center of the incident plasma jet.

Abstract In this paper, supersonic RF induction plasma deposition of Yttria Stabilized Zirconia (YSZ) has been developed in order to produce dense solid electrolyte membranes for Solid Oxide Fuel Cells (SOFC). Different RF induction plasma torch configurations were tested. The results show that high density layers could be obtained using a supersonic Laval nozzle integrated on a standard torch. 50 to 100 um YSZ coatings with porosity of near 1% could be obtained using this technique at relatively high deposition rates (10g/min.). Attention has been given to the thin coating porosity measurement by using a proper calibration and back scattered electron micrographs of the deposit cross-section coupled with image analysis. Absolute porosity has been measured by using this technique described in another paper of the same conference.