SOFCs for mobile applications require short starting times and capability of withstanding several and severe cycles. For such applications metallic cassette type cells with low weight and thermal capacity are beneficial where the active cell part is set in interconnects consisting of two sheets of ferritic steel. These cells are stacked serially to get higher voltage and power. This approach needs interconnect sheets that are electrically insulated from each other to prevent electrical short circuit. The technology discussed here is to use brazed metals, as sealants, and ceramic layers, as electrical insulators, which are vacuum plasma sprayed on the cassette rims. For reliable insulating layers, a variety of deposits were developed, starting from cermet-spinel multilayers with various compositions and constituents, where reactive metals (such as Ti, Zr) were part of the coatings, to pure ceramic layers. The qualities and characteristics of these coatings were investigated which included electric insulation at room temperature and at 800 °C (SOFC operating temperature), wettability of different brazes towards these deposits, phase stability and peeling strength. The single steps of development, characteristics of the insulating layers for SOFCs as well as some challenges that have to be taken into account in the process are described.

Solid oxide fuel cells (SOFCs) are one of the options as auxiliary power units (APU) in transportation, e.g. in vehicles or in aircraft. In particular, metal supported SOFCs consisting of metallic frames and substrates coated with plasma sprayed functional layers have shown an excellent stability concerning redox cycling. In order to provide sufficient power, these single cells have to be assembled to stacks. To prevent short-circuiting the frame of each cell has to be electrically insulated from the neighbouring one. For that purpose a ceramic coating is applied on each metal frame by thermal spraying before it is brazed to other stack components. Such layers should at one hand show good wetting and adhesion to the silver based brazing materials. On the other hand it should maintain sufficient electrical resistance even at the fuel cell operating temperature. As the applied solder, which connects the cells and seals the gas manifold simultaneously, is an excellent electrical conductor, it is mandatory to prevent the brazing material from penetrating into the deposit. In this paper a description of the design and experiences with these plasma sprayed insulating layers is given.

Thermal plasma jets interact intensively with the surrounding atmosphere. This interaction leads to strong changes of the plasma jet properties affecting the resulting products. Modification of the nozzle parameters and conditions at the exit of the torch helps to vary and better control the process of plasma jet and ambient air interaction. In the present study, the DC arc plasma torch was equipped with a modified anode nozzle (M2.5) and a surrounding shroud at the jet exit. The process of air entrainment was investigated when shroud gas was supplied producing a protecting envelope to reduce the air entrainment. Schlieren photography and the enthalpy probe with the mass spectrometer were applied to study the influence on plasma jet behavior. The effect of shroud nozzle geometry as well as the effect of the shroud gas flow rate was investigated. Likewise, influence of gas shrouding on the resulting coatings was studied.

This paper presents selected research results of the DFG founded project group, consisting of four institutes focusing on diagnostic methods in thermal coating processes. The aim of this group is to characterize the Atmospheric Plasma Spraying (APS) process by means of diagnostic methods so that – based on the requirement profile of the coating – appropriate adjusting of the process parameters can be realized. For this purpose, different diagnostic tools like Particle Shape Imaging, Laser Doppler Anemometry, Schlieren Technique, Particle Image Velocimetry, Enthalpy Probe, DPV 2000 and Thermography were qualified and adjusted to each other. Most of the results presented in this article are limited to the area close to the substrate which is difficult to handle with diagnostic methods. Additionally, new achievements concerning nozzle design and system enhancements are introduced.

Structured surfaces are needed for improving the heat transfer in a lot of industrial processes. The paper deals with the production and the characterization of coatings with rough and porous surface. Structured coatings of inconel or copper are deposited on copper tubes by means of radio-frequency vacuum plasma spraying. The microstructure as well as the surface roughness of the coatings are investigated. Boiling experiments are carried out on the coated tubes to measure their heat transfer coefficient. The results show that the coated tubes exhibit improved heat transfer values in comparison to smooth tubes. The enhancement ratio can reach more than 20 for inconel coatings.

The aim of the project group consisting of four research centers and founded by the DFG (German Research Society) is to characterize the plasma spraying process by means of diagnostic methods so that, based on the requirement profile of the coating, appropriate adjusting of the process parameters can be realized. For this purpose, different, partly newly-developed diagnostic tools, like Particle Shape Imaging, Laser Doppler Anemometry, Streak Technique, Particle Image Velocimetry, Enthalpy Probe, DPV 2000 and Thermography were qualified and adjusted to each other. The new results presented in this article are limited to the areas of particle injection and substrate which are difficult to handle with diagnostic methods.

This paper presents research highlights obtained over the past three years in the course of a DFG-funded project on new and emerging diagnostic methods for thermal coating. It describes the tools and techniques used, the particle and substrate variables monitored, the accuracy of each measurement, and various associations with coating properties. Paper text in German.

This paper investigates the potential of radio frequency thermal plasma chemical vapor deposition for producing Sr-doped La-Mn-perovskite and yttria-doped zirconia layers for solid-oxide fuel cells. Aqueous solutions were used as starting materials and were injected into the hot plasma core by means of an air-assist atomizer. Test results show how the microstructure, dopant distribution, and phase purity of the resulting layers depends both on process conditions and the material system. Paper includes a German-language abstract.

This paper investigates the influence of plasma spraying parameters on the gas tightness and ionic conductivity of YSZ electrolyte layers in thin-film solid oxide fuel cells. Measurement data obtained from the layers are correlated with electrochemical test results, providing helpful information on how the performance of SOFC stacks can be improved. Paper includes a German-language abstract.

DC plasma spraying with its products has gained a high technical importance. With the availability of technically reliable high-frequency plasma torches whose basic development can be traced back to about 40 years ago, some of the disadvantages of the DC spray method are no longer existing or can be avoided to a great extent. This paper describes the principle, construction, and function of high-frequency plasma torches in which the plasma is generated by induction and metallic electrodes are not required (as is the case with conventional DC plasma torches). Typical examples of HF plasma spray application are discussed. Paper includes a German-language abstract.

This paper compares the features of direct current and radio frequency (RF) plasma spraying, but also the properties of aluminum coatings that are reinforced with short carbon fibers and manufactured using these two methods. It shows that the embedding of the fibres during the RF plasma spray process is very poor due to the low velocity of the plasma jet. Paper includes a German-language abstract.

A project group consisting of four institutes and funded by the DFG has started a new approach to investigate the complicated processes (phenomena) involved in thermal coating in order to control these methods and here especially the plasma spraying in a better way and to make use of them more efficiently. This approach starts from the desired quality of the layer to be produced and derives the required conditions and features of plasma and spray material prior to deposition to get this quality. To obtain well-defined interrelationships several modern and specially modified diagnostic methods have to be used and theoretical considerations performed as well. These and the state of the work are described. Paper text in German.

This paper reports on the synthesis of SiC material through the decomposition of silanes in a thermal high frequency (HF) plasma. The process is based on thermal plasma technology for chemical deposition from the gas phase and on suspension plasma spray technology, in which a liquid or suspension is injected axially and atomized in the plasma flame. The liquid silane then decomposes, and forms SiC with some gaseous by-products such as HCl. Various plasma parameters were varied, for example the plasma power level, the plasma gas composition, the chamber pressure, and the silane composition. The paper also presents first investigations into the elementary and phase composition as well as the morphology of the powders and coatings. Paper includes a German-language abstract.

This paper deals with the production of porous functional coatings by means of high-frequency plasma spraying. This technology is presented for the first time in connection with controlled, open, porous, and graded structures. Materials such as pure metals (molybdenum and titanium), high-temperature alloys (Inconel), and ceramics (yttrium oxide-stabilized zirconium oxide) have been processed with several powder fractions with extremely different process parameters (container pressure, power, plasma gas composition). The samples were tested for their characteristic properties (porosity, effective pore sizes and permeability). The status of development is reported and potential attractive industrial applications are mentioned. Paper includes a German-language abstract.

More than 20 years the Institute of Technical Thermodynamics of the German Aerospace Center (DLR) in Stuttgart has been active in the field of plasma spray technology with improvements of spray equipment and their application. At the beginning the DC vacuum plasma spray method was in the center of interest and knowledge gained from rocket technology helped to develop supersonic nozzles for the plasma spray torches producing plasma jets with increased velocity and improved laminarity in order to get denser coatings with higher quality. In the meantime also nozzles for subsonic conditions with controlled expansion of the plasma jet leading to considerably increased deposition efficiency were developed as compatible parts for already existing equipment and made available on the market. In the next step also the DC plasma torches themselves have been improved. Recently a modern equipment for RF plasma technology could be developed and installed, where some new ideas could be realised. Paper text in German.

Suspensions of cobalt spinel (Co3O4) powders were rf plasma sprayed to form electrocatalytically active anode layers. Stable cobalt oxide suspensions of low viscosity exceeding 50 wt% solid phase have been processed. A spheroidization study revealed the formation of large spherical powder particles (- 30 + 80 µm). Cobalt oxide coatings were produced by rf suspension plasma spraying. The porosity was controlled by optimizing spray distance and reactor pressure. The main disadvantage of the thermal plasma processing of cobalt spinel is that the decomposition of the spinel phase into CoO could not be prevented, not even with the application of an 80% oxygen plasma. However, with a relatively low power oxygen plasma post-treatment, the deposited CoO layers can be oxidized to Co3O4, greatly improving the electrochemical performance of the anode layers.

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.

The central components for solid oxide fuel cells (SOFC) are the electrodes-electrolyte multilayer arrangement (PEN) and the separating bipolar plates. The PEN (Positive electrode- Electrolyte-Negative electrode) assembly consists of a dense gastight yttria-stabilized zirconia (YSZ) electrolyte and porous electrodes for which usually Ni-YSZ cermet anode and Sr-doped LaMnO 3 cathode layers are used. The various PEN units are connected in a cell stack by bipolar plates which are either metallic or ceramic ones. Furthermore, a protective layer on the metallic bipolar plates consisting of a chromium alloy is required to prevent chromium evaporation leading to a rapid and strong degradation of the SOFC performance. At the DLR Stuttgart both the DC and the RF vacuum plasma spraying technique have been further developed and adapted to meet the requirements for the manufacture of the different SOFC components. The DCVPS process using specially developed Laval-like nozzles is especially appropriate to the production of thin and dense coatings as required for the electrolyte and the protective layers. However, applying special spray parameters and nozzles it is also possible to deposit porous electrode layers. The production of the entire PEN arrangement in one consecutive DC-VPS process is the objective of the actual development. On the other hand, the RF plasma spray technique is suitable for the near net-shape production of bulk components such as the metallic bipolar plate. The development of the deposition processes for the production of SOFC components using DC and RF plasma spray methods and the results obtained concerning PEN fabrication, deposition of protective layers and the near net-shape production of metallic bipolar plates are presented in the paper.

A chromium alloy as used for the metallic bipolar plate of a solid oxide fuel cell was processed by RF-plasma spraying to dense free-standing parts. The plasma spray parameters were successfully adapted for two different types of powder. The layer properties, particularly the porosity and the splat shape were investigated in dependence on the spray angles. All the coatings produced with off-normal spray angles show higher porosity increasing from spray angles of 60° to 30°. The splat orientation changes from parallel to the inclined surfaces to almost perpendicular to the plasma jet axis with shallower angles.