Plasma sprayed coatings of Raney nickel alloys developed as electrodes for hydrogen evolution electrodes in alkaline media, exhibit poor resistance to electrochemical erosion. The aim of this work is to develop an understanding of the correlation between plasma spray process parameters and coating quality and with that improve the electrochemical performance of the coatings. Air plasma spraying with TriplexPro gun was performed using NiAlMo powders. Plasma parameters were varied and particle inflight velocity and temperature was measured by Accuraspray. Coatings were developed for conditions in which particles in-flight temperatures were comparable but in-flight velocities differed. Electrochemical tests were performed for evaluating the effect of different velocities on electrode performance. Coating attained with particles having higher velocity exhibited better electrochemical performance and durability. The microstructure and elements map before and after the electrochemical test performed by SEM and EDX show that the coatings with higher velocity particles led to microstructure that enabled better activation of the electrodes and higher surface area for reactions.

The current paper reports self-healing plasma sprayed Mgspinel (MgAl 2 O 4 ) coatings. The coatings were used for electrical insulation in high temperature fuel cells. A range of potential self-healing additives consisting of SiC+X (where X was BaO, CaO, ZnO, Y 2 O 3 , GeO 2 , Ta 2 O 5 , V 2 O 5 ) were characterized and SiC+Y 2 O 3 was initially selected for coating development. Coatings of spinel with 20wt% additive were developed using vacuum plasma spraying (VPS) or atmospheric plasma spraying (APS). In the developed coatings, self-healing was demonstrated after heat treatment at 1050°C in air for 10 hour. Thermophysical and thermomechanical properties of self-healing coatings were determined and compared to spinel coatings. Lastly, a modelling technique is presented to simulate the effective elastic moduli of the coatings. Numerical results based on microstructural simulations showed good agreement with experimental data.

Perovskite-type LSM and LSCF deposits were developed for oxygen electrode for solid oxide fuel cell and high temperature water electrolyzer by atmospheric plasma spraying (APS) using different feedstock powders. The deposits were tailored to exhibit high oxygen catalytic activity, oxygen surface exchange and diffusion rates, gas permeability and electronic-ionic conductivity. Deposits did not exhibit undesired secondary phases that may form in plasma. Promoting partial melting of the surface of the particles ensured interlayer cohesion and very porous deposit. In SOFC mode cells with LSCF cathodes operating at 800 °C had more than 700 mW/cm² power densities at 0.7 V, which was 35% better than that of cells with LSM cathode. When operating in electrolyzer mode at 800 °C the cells with LSCF oxygen electrode also proved significantly enhanced electrochemical performance compared to cells with LSM oxygen electrode. At a current density of 1 A/cm 2 the voltage for water splitting was reduced to around 1.4 V at an operating temperature of 800 °C and to 1.28 V at 850 °C.

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.

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.

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.

By means of Schlieren photography, enthalpy probe, mass spectrometry and the particle measuring system DPV 2000 the influence of the internal and external anode nozzle and torch geometry, on plasma jet quality for atmospheric plasma spraying was investigated. It turned out that there is a strong geometrical effect of the inner contour and that with a proper expansion of the hot core of the plasma jet a considerable improvement of the melting and deposition quality can be obtained. Also the outer torch contour is of influence on the spray process because it controls the formation and the intensity of turbulence and the interaction of the plasma jet with its surrounding and hence the cold gas entrainment.

In this paper a process based on both Thermal Plasma Chemical Vapor Deposition (TPCVD) and Suspension Plasma Spraying (SPS) is applied on r.f. induction thermal plasma for α/β-SiC ceramic synthesis and deposition. The starting materials are low-cost liquid disilanes. The resulting coatings are investigated by means of SEM and XRD. Results on the influence of the processing parameters (i.e. pressure, spray distance, substrate temperature, plasma gas nature and composition, precursor composition, atomization parameters) on the coating phase and microstructure are shown. Control of the microstructure (or nanostructure) as well as of the phase content, namely the ratio α/β can be achieved. A processing route presenting the elementary steps of SiC TPCVD is also proposed.

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 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.

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.

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.

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.

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.

Fine (median size 6 μm and 0.3 μm) cobalt spinel (Co 3 O 4 ) powders were processed suspended in a suitable liquid phase. Suspensions exceeding 50 wt.% solid phase content were successfully injected into an inductively coupled plasma. Spheroidized powders with large particle size (up to 80 μm) were prepared, and cobalt oxide coatings were produced by this novel RF-SPS method. The microstructural features of the coatings can be controlled by parameter optimization similarly to plasma spraying of dry powders. Numerous variations of the physical and chemical conditions of the process were performed in an attempt to overcome the main disadvantage of the process, i.e. the decomposition of the spinel phase to CoO. So far, the spinel phase could be reestablished only by a post-treatment of the deposited coatings with atomic oxygen in the RF plasma.

LaMnO 3 powders and coatings have been prepared by reactive suspension plasma spraying (SPS) of MnO 2 powders and LaCl 3 solutions. A 40 kW inductively coupled plasma with an oxygen plasma sheath gas has been used. Water and ethanol have been tested as the liquid phase in the SPS process. High perovskite content (70-90%) has been achieved for both powders and coatings when spraying a suspension of fine MnO 2 powder in a saturated ethanol solution of LaCl3 with a 1:1 molar ratio of La and Mn. Materials obtained by a 1100 °C oven treatment have been used as reference during the study. The reactor pressure was varied from 30 to 80 kPa. Low pressure was found to be necessary to suppress the formation of undesired phases in the powders and coatings obtained. A plasma post treatment of the coatings results in an increase of the perovskite content.