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.

Nanostructured YSZ+NiO functional layers for SOFC anodes were developed by air plasma spraying. Spray processing was controlled to conserve finely porous and nanostructure of the feedstock in the deposit. The optimized deposits exhibiting high gas permeability, suitable high temperature electronic conductivity, enhanced triple phase boundaries and catalytic activity. The results were compared with conventional NiO+YSZ and Ni-C+YSZ anodes. YSZ electrolyte layer was deposited onto the anodes for electrochemical testing at 800°C in static conditions. Impedance spectroscopy measurements were performed to collect data on the polarization resistance and catalytic behavior of these anode layers. It was established that enlarged reaction zone, provided by high specific surface area of the nanostructured anodes, and finely porous microstructure, led to lower polarizations and enhanced cell performance by more than 60% compared to conventional cells.

Elastic properties of 20 and 40 µm thick deposits of yttria fully stabilized zirconia (YSZ), fabricated by vacuum plasma spraying (VPS) and air plasma spraying (APS) with modified injection system were investigated at room temperature by nanoindentation, and 4 point flexion test and at 800°C by 4 point bend test. The data was correlated with structural analysis of different YSZ deposits. At room temperature, E values of VPS YSZ deposit decreased from 237 ± 6 to 105 ± 5 GPa on increasing nanoindentation load from 1 mN to 450 mN. The results indicated change from intrinsic to defect-dependent E values with increasing load. Despite lower porosity of VPS deposit (6 ± 1%) compared to that of APS (24 ± 1%), E values, measured by flexion test at room temperature and at 800°C, of former were 35 ± 1 and 16 ± 1 and of latter were 55 ± 1 and 18 ± 1 GPa respectively. The interlamellar sliding, parallel to applied load, was considered as prime reason of lower rigidity of deposits.

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.

The cermet coatings constituted by a metallic phase mixed with a ceramic one allows obtaining coating properties with a wide range of variations. This is particularly interesting for graded coatings produced by APS technique with two different injection ports devoted to each kind of powder. By monitoring the powder feed rates independently it is relatively easy to obtain the requested composition and also the gradient as a function of the coating thickness. In the present work 600 ìm graded coatings were produced with 5 or 6 layers, each one with a specific composition. The hardness properties of the whole coating were evaluated by the Knoop technique. Also each layer was produced separately and characterized as well. These coatings were devoted to thermo-mechanical applications. In this work the metallic phase is always NiCr 80/20 wt% and the ceramic phase is zirconia doped with yttria or ceria as well as Al 2 TiO 5 . The powders used as a standard have a size range included between 40 and 10 µm. Smaller size ranges were evaluated to produce NiCr / ZrO 2 , Y 2 O 3 graded coatings and were compared to those obtained with the standard powder. In this last part a post heat treatment of the coating was evaluated against the mechanical properties.

In a previous work, the authors determined that different mechanisms control in-flight particle oxidation in the core of a plasma jet and in its plume. In the core, convective movements within the liquid particle govern the oxidation kinetics if the plasma-to-liquid particle kinematic viscosities ratio is greater than 55 and the particle Reynolds number (Re) is more than 20. Convective movements entrain the oxide and dissolved oxygen from the particle surface towards its interior forming oxide nodules, and the fresh liquid metal is continuously renewed at the surface. Higher particle reactivity can be achieved in the plasma core. Convective movements cease in the plasma plume and the mass transfer from particle surface to its interior ends. Only classical surface oxidation continues, and the formed oxide covers the surface of the particles. The reaction kinetics are diffusion controlled and the oxidation rate decreases. In this present study, two austenitic stainless steel powders were air plasma sprayed using a dc plasma gun (PTF4 type) and were collected in an argon atmosphere. This paper reviews the influence of plasma spraying parameters on the oxide content in the collected particles. The studied parameters were plasma current intensity, hydrogen gas content in the plasma gas, and sprayed particle size. From the results, it can be concluded that plasma spray conditions favoring higher plasma to particle kinematic viscosities ratio and particle Re results in higher convective oxidation of particles in the plasma core.

Air engulfment by the plasma jet in Air Plasma Spraying (APS) causes in-flight oxidation of metallic particles. This oxidation, often complex and difficult to explain by classical diffusion-controlled oxidation, is governed by several mechanisms. This paper highlights the possible in-flight oxidation mechanisms in metallic particles with a focus on convective oxidation. Two different types of austenitic stainless steel particles, Metco 41C (-106+45 µm) and Techphy (-63+50 µm) were air plasma sprayed using a dc plasma gun (PTF4 type) and were collected in an argon atmosphere. Preliminary experiments indicated that different mechanisms are likely to occur during the in-flight oxidation of particles. Mass transfer from surface to interior of particle occurred forming oxide islands in particles. The mass transfer is governed by convective movements inside liquid particles within plasma jet core due to higher plasma-particle kinematic viscosities ratio and particles Reynolds number higher than 20. The islands were composed of metastable phases consisting of mixed oxide of Fe and Cr, likely in a nonstoichiometric form of FeCr 2 O 4 . Convective movements within particles cease roughly outside of the plasma jet core and classical surface oxidation was found to be the dominating phenomenon forming the surface oxide layer. Moreover, the molten surface oxide outside the jet core is entrained to the tail of the particle if plasma conditions promote higher particle temperature, velocity and Re number.