In this work, a dense YSZ coating with homogenous structure was produced by atmospheric plasma spraying (APS) on a NiO-YSZ anode supported by a porous metal substrate. A powder separator was used to limit fluctuations in the particle stream, thus stabilizing the process. During feeding, the YSZ powder was separated into fine particles that were fed directly to the plasma and coarse particles that were collected and reused. The coatings obtained exhibited high homogeneity both across the surface and through the thickness. SEM images showed that the YSZ layer was crack-free and that its porosity ratio rivaled that of vacuum plasma sprayed zirconia when two separators were used. Based on these observations and the results of gas leak rate measurements, the potential of online powder separation to improve SOFC electrolyte performance has been established.

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.

Ceramic layers, such as yttria-stabilized zirconia or scandia-stabilized zirconia, used for functional layers of solid oxide fuel cells, i.e. the gas tight oxygen ion conductive electrolyte or as ceramic component in the porous cermet anode, were obtained by the Solution Precursor Plasma Spray (SPPS) process. The influence of different solvent types on microstructure was analyzed by comparison of coatings sprayed with water-based solution to ethanol-based one. Use of solvent with low surface tension and low boiling point enhances splat formation, coating microstructure and crystalline structure. Parameter adjustment to receive coatings from nitrate solutions with ethanol as solvent was carried out. Results of Raman spectroscopy indicate that an intermediate of both nitrates (zirconyl and scandium nitrate hydrate) was deposited.

This paper describes recent improvements in metal supported solid oxide fuel cells produced by plasma spraying. By optimizing the process, incorporating nano-micro structured electrodes, and introducing additional layers to prevent interlayer diffusion, the power density of these cells has doubled, voltage has increased to 1.1 V, degradation rates have been cut in half, and redox stability has improved. Although upscaling to larger substrates initially caused distortion in the metal supports, the problems were successfully overcome.

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.

High heat load on the surface of electrodes in DC arc plasma torches results in strong wear of electrodes. This takes place during the start up and shutting down procedure as well as during long-term running of the torch and results in arc voltage reduction. Such changes in voltage lead to a modification of the plasma jet properties and effect adversely reproducibility of deposits, which are undesirable for industrial applications. The present work offers a new approach to compensate cathode wear in DC arc gas stabilized plasma torches by using moveable anode. With this design of torch, the distance between the cathode tip and anode can be varied. Voltage fluctuation analysis show the possibility of a reasonable arc modification by approaching the nozzle to the cathode caused by a change of the flow field inside the arc chamber and the arc dynamics. Smaller cathode tip-nozzle distances result in higher arc voltage with reduced fluctuations accompanied by a positive effect on the plasma jet properties. Thus, the negative influence of cathode wear on generated plasma properties can be suppressed and the lifetime of the cathode can be extended by simple approaching the nozzle.

This paper focuses on development of the anode layer for SOFCs by plasma spraying. The composite (cermet) anode, developed by thermal spraying, consisted of nickel and YSZ. The effect of different plasma spraying technologies on the micro structural characteristics and the electrochemical behavior of the anode layer were investigated. Coatings were fabricated by spraying nickel-coated graphite or nickel oxide with YSZ using a Triplex II plasma torch under atmospheric conditions as well as a standard F4-torch under atmospheric or soft vacuum conditions. The investigations were directed in order to have an open micro porous structure, higher electrical conductivity and catalytic activity of anode deposits. Porosity was investigated by measuring the gas permeability. SEM and XRD technologies were applied to examine the morphology, microstructure and composition of the layers. Electrical conductivity measurements were carried out to determine ohmic losses within the anode layer. Most promising layers were analyzed by measuring the electrochemical behavior to obtain information about catalytic activity and performance.

Using a D-optimal design of experiments (DOE), influence of feedstock powder and plasma gases were examined on deposition efficiency, gas tightness and electrochemical behavior of vacuum plasma sprayed YSZ for SOFC electrolytes. In-flight particle temperature and velocity, measured by on-line particle diagnostics, were correlated with plasma and deposit properties. Electrochemical testing of cells was performed to determine the influence of gas tightness and microstructure of electrolyte deposit on cell behavior.

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.

Reproducibility of plasma spraying process and resulting deposits is one of the most essential requirements for its application in industrial environment. In most cases, only controlling process input parameters does not suffice for achieving stability and reproducibility of the process. Besides short time plasma instabilities such as arc root fluctuation, process deviations in longer time operations may arise due to electrodes wear and need to be identified by online diagnostics of the plasma spraying process. In this study two online diagnostic systems were employed to investigate the long-term behaviour of a F4-type plasma gun with a convergent and divergent Laval-nozzle. A Plasma Instability Analysis (PISA) system was applied to identify electrode wear and plasma fluctuation phenomena within the gun whereas Particle Flux Imaging (PFI) was used to correlate the electrode wear with the plasma jet characteristics at the nozzle exit. The long-term performance of electrodes was investigated under continuous operation and under periodic reignition of the torch. A good correlation between aberrations in the plasma process observed with PFI and changes in the frequency spectra detected by PISA was found.

Plasma jets for thermal spraying are strongly affected in a negative sense by the interaction with the relatively static surrounding atmosphere, particularly at atmospheric spray conditions. Turbulences at the jet fringes arise resulting in entrained cold gas, in slowing and cooling down of the jet and in causing eventually its disintegration. All means suppressing or delaying this phenomenon, called cold gas entrainment, help to improve the interaction of plasma and spray material and hence lead to better product quality and higher deposition efficiency of the process. To observe the cold gas entrainment, to investigate the thermal and kinetic properties of DC plasma jets at different operating conditions and to study the effect of plasma source and powder injection modifications a diagnostic equipment with Schlieren optics, enthalpy probe and mass spectrometry was installed. By modification of the internal and external anode nozzle contours and also by application of a shroud nozzle around the plasma jet exit encouraging results with reduced penetration of cold ambient air into the jet could be obtained.

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.