Yttrium silicates are among the candidates for protection of silicon-based ceramics in high temperature and moist environments due to chemical and mechanical compatibility with substrate, low volatility and moisture resistance. Here we reported on the development of yttrium silicate coatings by sol precursor plasma spraying. The use of a sol feedstock allowed easy composition flexibility. The microstructure and the structure of as-sprayed and heat-treated coatings were investigated. Finer microstructure was obtained compared to micrometric powder plasma spraying traditionally used to produce environmental barrier coatings (EBC). XRD analyses on as-sprayed coatings revealed amorphous or crystalline layers depending on plasma parameters. In EBC application, a volume change from crystallization or phase transformation was envisaged to be damaging due to induced stresses and fully crystalline phases are a key durability requirement for EBC from conventional plasma spraying. Yttrium silicates are characterized by an important polymorphism and the ability to form amorphous coatings. Therefore, special attention was so paid to the amorphous degree of the coatings.

Homogenous mixtures of Ce 0.8 Gd 0.2 O 1.9 (GDC) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF6428) nanopowders were successfully synthesized using radio frequency (RF) induction plasma by axial injection of a solution. Two kinds of powders with different mass ratio of GDC/LSCF, such as 3/7 and 6/4, were obtained. The crystallinity and morphological features of the powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The particles are almost globular in shape with a diameter lower than 100nm and the BET specific areas around 20m 2 /g. In addition, suspensions, made with the composite nanopowders and ethanol, were used to deposit some cathode coatings using suspension plasma spray method. Several initial results of the coatings are also presented. The coatings are homogeneous and porous with cauliflower structures.

Ceramic functional coatings are frequently applied to structure materials in a wide range of mechanical and chemical applications for their specific properties of hardness, passive resistance to chemical aggression and low thermal conductivity. The counter part of those specific properties is a low mechanical and thermal shock resistance. Thus it is of prime importance to investigate the fundamental mechanisms, which governs the failure under thermal shock and thermal cycle conditions to increase the range of reliable industrial applications. The work presented in this paper deals with the Finite Element Modelling [FEM] of the transient thermal fields and associated stresses developed in ceramic coatings during firstly the post deposition cooling step and secondly during thermal cycling conditions. Alumina [Al 2 O 3 ] and Partially Stabilized Zirconia [PSZ] coatings, which are both widely used in mechanical and chemical industries, are under investigation.

The system studied in this work is based on a Gadolinia-doped Ceria (GDC) matrix and a Nickel oxide cermet as the anode material where both layers are RF plasma deposited. The challenge lies in the optimization of the anode deposition parameters to obtain a controlled porosity thick layer and to process the electrolyte material in such a way as to completely eliminate both open and closed porosity in the relatively thin deposit. The starting materials were nitrate based solutions, prepared so as to obtain final anode spray deposits of NiO-GDC (35%Wt) with the aimed for electrolyte composition of Gd0.2Ce0.8O1.90. In order to optimize these anode deposits, the plasma power, the “stand off” distance, the concentration of the salts solution, as well as substrate cooling, were studied. Similar parameters were also studied for the electrolyte deposition process but, in order to assure attaining high deposit density, the supersonic RF torch nozzle was used for the spray processing. The interactions between all of these parameters and their influence on the deposition process, for both the anode and the electrolyte layers, is examined and discussed.

This work is aimed at acquiring knowledge on successive deposition of alumina and nickel coatings on a metallic substrate using an RF plasma torch. The composition and morphology of various Ni/Al 2 O 3 steam reforming catalysts, prepared by plasma spraying of precursor nickel suspensions, are studied in this work. As Ni/γ-Al 2 O 3 is a well known reforming catalyst it was decided to study the deposition of a particular catalytic formulation on a metallic substrate. The purpose of this study was to find the means of building 2D monolithic catalysts by plasma deposition. Since 2D monolithic catalysts are non-porous, their specific areas available for hosting the catalytic reactions are low. Thus, It is essential to have very good dispersion of the active catalytic sites. Such dispersion is only possible only if the deposit can take place at the nanometer scale. The suspension plasma spraying is assessed for its ability to produce such structures. This method permits the formation of nanocrystalline structures within a few seconds. The main difficulty is the control of the various spraying parameters in order to obtain satisfactory deposition. The Ni/γ-Al 2 O 3 catalysts so obtained are characterized by scanning electron microscopy, field electron microscopy and X-Ray diffraction.

In this work, metal-based thermal barrier coatings (MBTBCs) for use in low heat rejection diesel engines have been produced, using high frequency induction plasma spraying (IPS) of iron-based nanostructured alloy powders. Important advances have been made over recent years to the development of ceramic-based thermal barrier coatings (TBCs) for diesel engines, but they are not yet applied in mass production situations. Besides the important economic considerations, the reliability of ceramic TBCs is also an issue, being associated with the difficulty of predicting their “in-service” lifetime. Through engineering of the nano/amorphous structure of MBTBCs, their thermal conductivity can be made as low as those of ceramic-based TBCs, with reduced mean free paths of the electrons/phonons scattering. In this work, nano/amorphous structured coatings were deposited by IPS using the following spray parameters: spraying distance (200mm), plasma gas composition (Ar/N 2 -85/15, by volume %), IPS torch power (25kW), and powder feed-rate (16g/min.). The structure and properties of the deposited layers were characterized through SEM (Scanning Electron Microscopy) observations. The thermal diffusivity (α) properties of the MBTBCs were measured using a laser flash method. Density (ρ) and specific heat (Cρ) of the MBTBCs were also measured, and their thermal conductivity (k) calculated (k =αρCp). The thermal conductivity of MBTBCs, with 7.5% total porosity, was found to be 1.22 W/m/K. The heat treatment study showed that phase transformation started at 650oC, and grain size growth from nano- to micron- scales occurred at around 1000°C under static exposure conditions. Thermal expansion coefficient (TEC) of MBTBCs was 15E-6 /K, which is close to the TEC of cast iron and thus, closer to the TEC values of aluminium alloys than are conventional TBCs. Fracture toughness of MBTBCs has also been assessed by use of Vickers hardness tests, with a 100 g load for 15 s, and the results show that there are no measurable crack developments around “indented” areas on all samples of MBTBCs tested.

A particle-sampling probe has been designed and constructed for the continuous collection of nano-powders produced by the plasma spray synthesis (PSS) process. The probe comprises a powder sampling line (inner tube), a quench gas line (outer tube) and a water-cooling jacket surrounding the outer tube. A sample holder is disposed at the exit of the inner tube to hold a standard 3.08 mm diameter TEM copper grid which is used to collect the powders by means of the pumping pressure differential. Quenching gas is introduced to the probe, via the outer tube to quench and entrain the as-synthesized clusters. After each sample collection event, the inner tube can be cleaned in-situ by means of a water injection, and then dried using a compressed gas flow. The results obtained to date indicate that the sampling probe location in the plasma reactor and the quenching gas flow rate employed are the most important parameters involved in the satisfactory operation of the sampling probe.

The synthesis of nanoscale particles has received considerable attention because of the potential for new materials and unique properties. The novel properties and the numerous applications of nanophase materials, especially ceramic nanopowders, have attracted many scientists and engineers to invent and explore the preparation methods of ceramic nanoparticles. Induction plasma is used to synthesize cathode materials for fuel cells. Solid oxide fuel cells (SOFCs) are very promising energy conversion systems. SOFCs are based on an oxide-ion conducting electrolyte and they offer a clean, low-pollution technology to electrochemically generate electricity at high efficiencies. These fuel cells provide many advantages over traditional energy conversion systems including high efficiency, reliability, modularity, fuel adaptability, and very low levels of SOx and NOx emissions. It has been found that La1-xSrxMO3-d, (M= Fe, Co etc) are perovskite materials widely considered as the Intermediate Temperature SOFC cathode materials of choice. In particular, La0.6Sr0.4Co0.2Fe0.8O3-δ is extensively used for IT-SOFCs because its thermal expansion coefficient is relatively close to that of the common electrolytes. In this paper, the nanopowders of SOFC cathode materials were synthesized by thermal plasma spray technique. The results of their structure, morphology and particle size distributions will be presented. Abstract only; no full-text paper available.

Influence of induction plasma gas composition on Ti coatings microstructure and composition has been studied. Plasma sprayed Ti powder and coatings were prepared by using induction plasma system. Spheroidization of irregular shaped particles was observed in the powder collected after spraying with Ar-air plasma. Microstructures of the coatings were analyzed by a high-resolution scanning electron microscope. Image analysis of the backscattered images of the cross-sectional view of the coatings was used to calculate coating porosities. X-ray diffraction analysis was also performed to identify secondary phases, which have been formed in the coatings during plasma spraying. Partially induced nanostructures were observed in fractured areas in the coatings. The nanostructures were preferentially formed on the surface of splats. X-ray diffraction pattern reveals that a secondary phase has been formed in the coatings during spraying with air vol. at 16 % and 20 % in plasma gas. Changes of the coating density and thermal diffusivity were studied with respect to the formation of secondary phases and induced nanostructures.

The aim of this paper is to compare the impact of plasma deposition processes onto Yttria Stabilized Zirconia (YSZ) electrolytes’ properties. Indeed, physical properties and microstructure of plasma sprayed coatings depend on the in-flight treatment of the particles in the plasma jet. Investigations of the relationships between spraying conditions and the in-flight properties of the particles upon impact for Air Plasma Spraying (APS) and Vacuum Plasma Spraying (VPS) were carried out using a MacLean & Anderson design of experiment. The In-flight particle properties were determined using the DPV 2000. For each type of plasma spraying process, coatings were elaborated with optimized plasma spraying conditions that allowed reaching the highest particle velocity upon impact keeping the highest temperature as possible. Coatings properties were then, evaluated by SEM and impedance spectroscopy.

The aim of this paper is the optimization of Yttria Stabilized Zirconia (YSZ) particle injection in a supersonic induction plasma torch to improve the reproducibility of this plasma deposition process. Indeed, the optimization is necessary to eradicate clogging in the supersonic nozzle due to the constriction of the nozzle. Investigations of the relationships between the parameters of particle’injection and the in-flight properties of the particles are carried out using an ANOVA experiment design. The in-flight particle properties were determined using two commercial systems, the DPV 2000 associated with the CPS 2000 (temperature, velocity and diameter of in-flight cold particles) and the Control Vision system (divergence of plasma jet imagery). First results have shown the necessity of a new design of the injection probe; development that allowed to reach a good reproducibility in terms of particle velocity and molten state.

Nanopowders of amorphous SiO 2 , with typical particle sizes of 30-80 nm, were treated under non-equilibrium plasma conditions created by a capacitively coupled (CC) RF discharge in pure methane or ethane. The gas flow rate was varied between 0.02-0.06 slpm, with reactor pressures maintained between 1000 and 5000 Pa, and applied RF power inputs between 700 and 1500 W. The plasma properties were monitored through measurements of the C 2 rotational and the atomic hydrogen excitation temperatures. The compositions of the gases that passed through the plasma were analyzed by mass-spectrometry. In spite of the evidence indicating the presence of C n H 2n+2 and C n H 2n (n=1-3) species, as well as acetylene, in the discharge, the homogeneous formation of soot was not observed. At the same time, introduced nanoparticles acted as centers for the inception and growth of C:H thin coatings in the form of polymer-like hydrocarbon layers, whose thickness lay between < 5 - 30 nm. The results of TEM, IR spectroscopy, thermo-gravimetric and precision calorimetric analyses performed on the plasma treated powders provided evidence for the formation of an amorphous, high density C:H matrix on particles' surfaces.

Relationships between the electrical properties of thermally sprayed titania coatings and their microstructure have been investigated. As far as possible, a broad range of microstructures was produced by using various processes of plasma spraying with different powder size ranges and variations of the plasma operating parameters. The two spraying processes consisted of DC plasma spraying and RF plasma spraying. Physical properties of plasma-sprayed coatings are generally influenced by their microstructure. But the electrical properties of plasma-sprayed titania coatings are known to be strongly influenced by their stoichiometry. It is the reason why coatings with identical stoichiometry were compared. It was found that electrical resistivity was directly linked to the quality of the contact between the splats and their density through the titania plasma-sprayed coatings.

In the continuing progress of fuel cell technology, CeO 2 double doped electrolytes appears to be promising for lowering the SOFC's working temperatures. Ceria electrolytes have better ionic conductivities than YSZ but, at low oxygen partial pressures, the chemical reduction of ceria leads to increasing electronic conduction. Double doping of the ceria increases the electrolytic conduction range without changing its conductivity. To avoid stress development within the ceria crystallographic structure, the dopants mix must have a mean ionic radius as close as possible to the critical ionic radius. Ceria electrolytes with various compositions and dopant concentrations are synthesized with a combinatorial chemistry approach. To synthesize new electrolytes, solution plasma spraying with nitrate salt precursor is used. The reaction is completed and nanocrystalline thin layers of ceramic are formed in the plasma. Comparative studies of plasma spraying techniques, with YSZ powder plasma spraying as electrolyte reference, were performed. Also, comparative impedance spectroscopy measurements are to be performed to validate the double doping hypothesis and thence to identify the best electrolytes in the suite of over 300 new materials.

Ultrafine MoSi 2 powders have been synthesized from commercial MoSi 2 powders by using an Ar-H 2 induction plasma. Reactor pressure and plate power were taken as the experimental parameters to optimize the phase as well as the size distribution of ultrafine MoSi 2 powders. The powders were collected from porous metal fibers. They were composed of both metastable hexagonal structure (β-MoSi 2 ) and stable tetragonal structure (α-MoSi 2 ) with small levels of Mo 5 Si 3 and free silicon.

A thermal cycling test rig and procedure was designed in order to predict the life expectancy of Thermal Barrier Coatings (TBC) under thermal cycling conditions similar to those meet in combustion chambers. Two 2kW-halogen lamps highly focused on the TBC were used to expose the surface of the coating to an intense heat flux. A 25x100 mm TBC is Air Plasma Sprayed (APS) centered onto a substrate 25x370 mm. The thermal cycling can be done either under inert or oxidizing atmosphere in order to separate oxidation-induced acoustic emissions from that resulting from the mismatch of the Coefficient of Thermal Expansion (CTE) of the coating compared to that of the substrate. Two transducers located at each end of the substrate monitor the Acoustic Emission (AE) signals emitted by crack initiation and/or propagation, were recorded and analyzed in order to deduce available information about TBC behavior under thermal load. The use of two transducers with a time of flight approach provides a valuable means of identifying both the crack formation and its location. This thermal cycling test is adequate for the study of various samples, like welded substrates coated with TBC or TBC coated around holes. The presence of cracks is observed using metallography preparation and microscopic observation.

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.

A thermal cycling test was designed in order to predict thermal barrier coatings (TBC) life of combustion chambers. The thermal cycle is produced by two 2kW halogen lamps highly focused on the TBC. A 25X100 mm TBC is plasma sprayed centered onto a substrate 25X300 mm. The thermal cycling can be done either under argon atmosphere or air in order to be able to discriminate the oxidation induced acoustic emissions from the expansion mismatch. Two transducers located at each end of the substrate monitor the acoustic signals emitted by crack initiation and/or propagation. The advantage of using two transducers is that with a time of flight approach cracking phenomena can be located along the TBC. This process allows the study of welded substrates coated with TBC, TBC coated around holes and to check for the presence of cracks by using metallography preparation. The challenge of the test is to use the early cycles emission signatures in order to predict the long term durability of the TBC.

A study on induction plasma shape forming with yttria stabilized zirconia (YSZ) was conducted as part of an effort to develop a new method for producing nuclear fuel. YSZ was selected because its melting point is similar to that of UO2. Nuclear fuel pellets were made using a large (70 mm) induction plasma flame that sprays more than 100 pellets simultaneously and a small (10 mm) supersonic plasma flame that produces one pellet at a time. Process optimization for the large induction plasma flame was done based on chamber pressure, plasma plate power, powder spraying distance, sheath gas composition, probe position, and particle size. The best results were 97.11% theoretical density (TD) for 5-mm thick pellets. For the single pellet approach, densities as high as 99% TD have been obtained in 12-mm thick free-standing pellets.

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.