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Abstract: The major challenge in the development of plasma sprayed components for SOFC was the fabrication of dense electrolytes by atmospheric plasma_spraying (APS) avoiding cracks typically generated during the spraying process. Compared to conventional plasma_sprayed ceramics both the number of micro_cracks and segmentation cracks have to be reduced considerably to achieve sufficiently low leakage rates. Based on a detailed understanding of the formation of plasma_sprayed coatings appropriate process conditions have been established to reach the target values. The major influencing factors are the particle properties in the plasma jet, the substrate temperature, the kind of movement of the gun during deposition, the particle size distribution of the used YSZ powder, and the powder feeding rate. In addition, also the used APS gun plays a major role. With the Triplex II gun it was possible to produce rather thin (≤ 40 μm) YSZ layers with sufficiently low leakage rates without any thermal post_treatment. As substrates tape_casted ferritic steel sheets have been used. The anode was also sprayed by APS using a separate injection of NiO and YSZ. Electrochemical performance was measured after wet_chemically applying a LSCF cathode. The values of the measured open circuit voltage (OCV) were comparable to the ones of sintered cells, indicating the high density of the produced electrolytes. The optimized cells showed at 0.7 V cell voltage a power density of 500 mW/cm2 @ 1073 K. [Copyright &y& Elsevier]

Abstract: Nonstoichiometric yittrium-substituted SrTiO3 samples with a nominal composition of Sr x Y0.07TiO3± δ (x =0.87–1.01) and Sr0.895Y0.07Ti x O3 − δ (x =1.00–1.20) were prepared and investigated. Phase impurities were found in samples with too high A-site or B-site deficiency. The electrical properties of the samples sintered in reducting atmosphere at different temperatures were analyzed with a view to their potential use as anode materials for solid oxide fuel cells (SOFCs). The redox properties of selected samples were investigated in detail. Samples with small A-site deficiency were considered to be the best candidates for SOFC anode materials. [Copyright &y& Elsevier]

Growing demands on the quality of thermally sprayed coatings require reliable methods to monitor and optimize the spraying processes. Thus, the importance of diagnostic methods is increasing. A critical requirement of diagnostic methods in thermal spray is the accurate measurement of temperatures. This refers to the hot working gases as well as to the in-flight temperature of the particles. This article gives a review of plasma and particle temperature measurements in thermal spray. The enthalpy probe, optical emission spectroscopy, and computer tomography are introduced for plasma measurements. To determine the in-flight particle temperatures mainly multicolor pyrometry is applied and is hence described in detail. The theoretical background, operation principles and setups are given for each technique. Special interest is attached to calibration methods, application limits, and sources of errors. Furthermore, examples of fields of application are given in the form of results of current research work. [ABSTRACT FROM AUTHOR]

Powder injection parameters such as gas flow, injection angle, and injector position strongly influence the particle beam and thus coating properties. The interaction of the injection conditions on particle properties based on DPV-2000 measurements using the single-cathode F4 torch is presented. Furthermore, the investigation of the plasma plume by emission computer tomography is described when operating the three-cathode TriplexPro™ torch. By this imaging technology, the three-dimensional shape of the radiating plasma jet is reproduced based on images achieved from three CCD cameras rotating around the plume axis. It is shown how the formation of the plasma jet changes with plasma parameters and how this knowledge can be used to optimize particle injection. [ABSTRACT FROM AUTHOR]

The very low pressure plasma spray (VLPPS) process operates at a pressure range of approximately 100 Pa. At this pressure, the plasma jet interaction with the surrounding atmosphere is very weak. Thus, the plasma velocity is almost constant over a large distance from the nozzle exit. Furthermore, at these low pressures the collision frequency is distinctly reduced and the mean free path is strongly increased. As a consequence, at low pressure the specific enthalpy of the plasma is substantially higher, but at lower density. These particular plasma characteristics offer enhanced possibilities to spray thin and dense ceramics compared to conventional processes which operate in the pressure range between 5 and 20 kPa. This paper presents some examples of gas-tight and electrically insulating coatings with low thicknesses <50 μm for solid oxide fuel cell applications. Furthermore, plasma spraying of oxygen conducting membrane materials such as perovskites is discussed. [ABSTRACT FROM AUTHOR]

Atmospherically sprayed thermal barrier coatings (TBCs) are nowadays an essential part in modern gas turbines. However, a design integrated use of these coatings is only possible with reliable lifetime models. In this paper, a model is outlined which describes the major failure in TBCs associated with the growth of a thermally grown oxide (TGO) on the bond coat (BC). An essential part of the model is a simplified description of the crack growth as a result of thermal cycling and TGO growth. In addition, the energy release rate for the system is calculated and compared to an estimated critical energy release rate reduced by the crack growth. If both are equal, failure is assumed. The results of the modeling are compared to thermal cycling experiments partly applying a thermal gradient. BC temperatures and also microstructures of the ceramic topcoat have been varied and the influence on the cyclic life studied. [ABSTRACT FROM AUTHOR]

Abstract: The interdependence between plasma spray process parameters and porosity of YSZ coating microstructures was investigated with simultaneous consideration of the deposition efficiency. Based on a factorial experimental plan, the argon plasma gas flow, the current, the interaction of argon flow and current, and the spray distance for the Triplex II plasma gun were found to yield the main contributions to porosity as well as to deposition efficiency. Each of these three process parameters has a significant individual effect on the in-flight particle velocities and temperatures. The contribution to the effects on porosity arises almost exclusively from the particle temperature. Regarding the deposition efficiency, the larger contribution originates from the particle velocity. To achieve a targeted high porosity at reasonable deposition efficiency a simple linear regression model was applied yielding an argon flow of 50slpm and a current of 470A at a spray distance of 200mm as the optimum parameter set. The average particle temperature estimated for this optimum is just above the melting temperature. At this setting, a porosity of 17.7% and a deposition efficiency of 32.5% may be expected. At a greater spray distance and lower power density (lower current and/or higher argon plasma gas flow) the deposition efficiency was observed to drop considerably. The cooling of the particles here becomes critical, i.e. the particles are only partly molten. This was verified by an analysis of the density distributions of measured in-flight particle temperatures. [Copyright &y& Elsevier]

Conventional thermal barrier coating (TBC) systems consist of a duplex structure with a metallic bondcoat and a ceramic, heat-isolative topcoat. Several recent research activities are concentrating on developing improved bondcoat or topcoat materials; for the topcoat especially, those with reduced thermal conductivity are investigated. Using advanced topcoat materials, the ceramic coating can be further divided into layers with different functions. One example is the double-layer system in which conventional yttria-stabilized zirconia (YSZ) is used as bottom and new materials such as pyrochlores or perovskites are used as topcoat layers. These systems demonstrated an improved temperature capability compared to standard YSZ. In addition, new functions are introduced within the TBCs. These can be sensorial properties that can be used for an improved temperature control or even for monitoring remaining lifetime. Further increased application temperatures will also lead to efforts for a further improvement of the reflectivity of the coatings to reduce the radiative heat transfer through the TBC. [ABSTRACT FROM AUTHOR]

Abstract: This paper provides a short overview on the different aspects of SOFC materials, especially of new cathode materials, ranging from considerations of material properties to long-term phenomena after cell operation. Special emphasis is laid on the reproducible manufacturing of cathode microstructures, which has to be optimised for each individual composition or powder quality. If this is not performed a comparison of various cathode materials can be misleading. The microstructure can be tailored by varying the sintering conditions, which results in different particle sizes. Besides the improvement of power density, the long-term stability of cathodes is discussed in more detail. For (La,Sr)(Co,Fe)O3 (LSCF) cathodes, the predominant factor determining voltage losses during cell operation is the temperature. The mechanism responsible for voltage deterioration is a slow decomposition of the LSCF perovskite due to partial de-mixing of strontium. [Copyright &y& Elsevier]

Abstract: The recently introduced three-cathode TriplexPro™-200 atmospheric plasma spray gun was tested with yttria-stabilized zirconia. The effects on the particle characteristics (velocity and surface temperature) were measured by time-of-flight measurement and two-color pyrometry, respectively (DPV-2000 diagnostic system) while the arc current intensity, the plasma gas flow, and its composition were systematically varied. With typical spray parameters of the Triplex® II, using the TriplexPro™-200 with the 9.0 mm nozzle the particle characteristics were found to be almost the same while the nozzle still provides power reserves. In this case, helium may be dispensed with entirely. Using the 6.5 mm or the 5.0 mm nozzle operation is limited to a maximum 500 A current, which significantly increased particle velocities, but it was not possible to keep the particles fully molten. The investigated range of powder grain size (d 50 =58 μm) is to be considered with these parameters. Further possibilities are to be expected with the 6.5 mm and 5.0 mm nozzle using smaller-sized or lower-melting powders. Operating the TriplexPro™-200 gun with the 11.0 mm nozzle and/or nitrogen as secondary plasma gas instead of helium, increased particle temperatures and velocities are achievable. This offers extended possibilities to spray high-melting oxide ceramics like yttria-partially stabilized zirconia. [Copyright &y& Elsevier]

Abstract: An approach to measure and assess the oxygen contents of vacuum plasma spraying process atmospheres is introduced. Hereby it is envisaged to monitor vacuum plasma spraying processes and to control the uptake of oxygen in metal alloy coatings. Coating experiments were performed with Ni–Co–Cr–Al alloy bondcoat powders to proof the effectiveness of this approach. Additional experimental coatings made of NiTi powders show the benefit of controlling the oxygen contents especially for metallic alloys being highly affine to oxygen. [Copyright &y& Elsevier]

Abstract: Single phase (La,Sr)(Al,Mn)O3 perovskites were synthesized and their electrical conductivity was studied as a function of temperature and oxygen partial pressure. The electrical conductivity was found to increase with Mn content, temperature and oxygen partial pressure, thus (La, Sr)(Al, Mn)O3 may be regarded as a diluted system of (La,Sr)MnO3. The highest electrical conductivity at 810 °C in air and humidified Ar/4% H2 was observed for (La0.8Sr0.2)0.94Al0.5Mn0.5O3 and reached 12 and 2.7 S/cm, respectively. [Copyright &y& Elsevier]

Abstract: The oxygen uptake and release of La1−xSrxCo1−yFeyO3−δ (x=0.2, 0.4; y=0, 0.8, 1) perovskite powders was examined with temperature-programmed techniques. In addition to the commonly used temperature-programmed reduction (TPR), temperature-programmed oxidation (TPO) was used to examine oxygen adsorption on the perovskites that had previously been partially reduced. While perovskite reduction started slowly at an offset temperature of about 300 °C, the re-oxidation already started below 100 °C resulting in a distinct re-oxidation peak. The measured oxygen stoichiometry changes were in the range of δ=0.03–0.19. For the re-oxidation process, surface-controlled kinetics was assumed, allowing the application of conventional temperature-programmed desorption (TPD) procedures to determine the activation energy of the adsorption process. Simulation of the peak profiles confirmed the predominant surface kinetics. The determined activation energies were in the range of 0.46–0.63 eV. For the measurements presented here, the demands made on preparation and experimental equipment are much lower than for other surface exchange characterisation techniques and allow a pre-selection of interesting perovskite materials for use in solid oxide fuel cells or as exhaust catalysts. [Copyright &y& Elsevier]

Inkjet printing was applied to manufacture silica-based gas separation membranes, which were coated on a pore-graduated alumina substrate with a mesoporous γ-alumina interlayer. A silica sol diluted by 1-propanol was used to print the membrane layer followed by thermal treatment in a rapid thermal processing furnace. The membrane thickness was varied between 30 and 110 nm by conducting one, two, and three coating steps. In the latter case, H2 permeance in the range of 2.0 × 10−8-3.3 × 10−8 mol/s·m2·Pa combined with H2/CO2 selectivities in the range of 15-25 were achieved, proving the concept that inorganic gas separation membranes can be successfully processed by inkjet printing. [ABSTRACT FROM AUTHOR]

Cavitation is a severe wear mechanism in technical applications where parts are in contact with rapidly flowing liquids. Examples are turbine blades in hydropower plants or pump components. Coating exposed surfaces with wear-resistant materials is an effective measure for extending lifetime in the case of cavitation attack. NiTi is an attractive material for such coatings considering its clearly pronounced damping behavior based on its pseudoelastic properties. A promising processing route for coating net-shaped components with NiTi is low-pressure plasma spraying (LPPS). In the present work, NiTi layers were produced by LPPS, starting from pre-alloyed NiTi powder. Cavitation resistance was investigated in relation to LPPS parameters, layer thickness and specific surface treatment. Increased cavitation resistance was demonstrated compared to UTP 730, an established cavitation protection material. The study was accompanied by comprehensive characterization of microstructure and phase transformation behavior of the NiTi coatings. [ABSTRACT FROM AUTHOR]

Abstract: A key to the development of metal-supported solid oxide fuel cells (MSCs) is the manufacturing of gas-tight thin-film electrolytes, which separate the cathode from the anode. This paper focuses the electrolyte manufacturing on the basis of 8YSZ (8 mol.-% Y2O3 stabilized ZrO2). The electrolyte layers are applied by a physical vapor deposition (PVD) gas flow sputtering (GFS) process. The gas-tightness of the electrolyte is significantly improved when sequential oxidic and metallic thin-film multi-layers are deposited, which interrupt the columnar grain structure of single-layer electrolytes. Such electrolytes with two or eight oxide/metal layers and a total thickness of about 4 μm obtain leakage rates of less than 3 × 10−4 hPa dm3 s−1 cm−2 (Δp: 100 hPa) at room temperature and therefore fulfill the gas tightness requirements. They are also highly tolerant with respect to surface flaws and particulate impurities which can be present on the graded anode underground. MSC cell tests with double-layer and multilayer electrolytes feature high power densities more than 1.4 W cm−2 at 850 °C and underline the high potential of MSC cells. [Copyright &y& Elsevier]

Abradable coatings have been used in low- and high-pressure sections of jet engine compressors for more than 40 years. Today, they are also used in the high-pressure turbine of jet engines and are gaining more interest for applications in industrial gas turbines. They minimise the clearance between the rotating blade tips and the stationary liners. Aside from being abradable, the coatings have to be mechanically stable and withstand high thermo-mechanical loadings. A typical material used in engines today is yttria-stabilised zirconia (YSZ). This material advantageously combines a suitable thermal conductivity with a high thermal expansion coefficient, but shows a temperature capability limited to 1200°C in long-term applications. Typical abradable coating thicknesses are above 1mm. With increasing coating thickness and limited cooling efficiency leading to high surface temperatures, there is a risk of premature failure. As a result, new ceramic materials have been developed with better high-temperature capability. The present work investigates an atmospheric plasma sprayed ceramic double-layer coating system composed of 7YSZ as an intermediate layer and magnesia alumina spinel as a top layer. This double-layer system was sprayed onto disc-shaped Inconel 738 superalloy substrates, which were coated with a vacuum plasma sprayed MCrAlY bondcoat. The lifetime of the coating system was assessed via thermal gradient cycling testing with surface temperatures above 1400°C. During cycling, the samples showed a typical failure mechanism with exfoliation of thin coating lamellae starting from the coating surface. This failure mechanism was not observed in thermal barrier or abradable coatings in the past. The failure mechanism was analysed and mismatch stress calculations were carried out. [ABSTRACT FROM AUTHOR]

Mixed proton-electron conductors (MPEC) can be used as gas separation membranes to extract hydrogen from a gas stream, for example, in a power plant. From the different MPEC, the ceramic material lanthanum tungstate presents an important mixed protonic-electronic conductivity. Lanthanum tungstate La6-xWO12-y (with y = 1.5x + δ and x = 0.5-0.8) compounds were prepared with La/W ratios between 4.8 and 6.0 and sintered at temperatures between 1300 and 1500 °C in order to study the dependence of the single-phase formation region on the La/W ratio and temperature. Furthermore, compounds substituted in the La or W position were prepared. Ce, Nd, Tb, and Y were used for partial substitution at the La site, while Ir, Re, and Mo were applied for W substitution. All substituents were applied in different concentrations. The electrical conductivity of nonsubstituted La6-xWO12-y and for all substituted La6-xWO12-y compounds was measured in the temperature range of 400-900 °C in wet (2.5% H2O) and dry mixtures of 4% H2 in Ar. The greatest improvement in the electrical characteristics was found in the case of 20 mol % substitution with both Re and Mo. After treatment in 100% H2 at 800 °C, the compounds remained unchanged as confirmed with XRD, Raman, and SEM. [ABSTRACT FROM AUTHOR]

Abstract: The overall performance of ionic conducting electrolyte layers is a key factor for determining the power density of solid oxide fuel cells (SOFCs). The aim of this work is to investigate high performance SOFC electrolyte layers developed in our lab via a low cost wet-chemical processing method. In this paper, SOFCs with bulk ionic conductivity dominated thin-film electrolyte demonstrate superior electrochemical performances. Conventional materials for SOFCs are applied in this work: Ni–YSZ cermet as the anode, yttria-stabilized zirconia (YSZ) as the electrolyte, gadolinia-doped ceria (CGO) as the Sr-diffusion barrier layer, and LSCF or LSC as the cathode. At 0.7 V and 600 °C, single cells with an active LSCF and LSC cathode area of 4 × 4 cm2 obtain a power density of 0.7 and 1.4 W cm−2, respectively. According to electrochemical impedance spectroscopy (EIS), the ohmic resistance of the single cells is almost one order of magnitude lower than the conventionally fabricated SOFCs. Due to the improved performance of the electrolyte, SOFCs are able to deliver high power output at reduced operating temperature and increased cell voltage. [Copyright &y& Elsevier]

The article discusses various reports published within the issue, including the use of sensors in thermal spraying, the effectiveness of high velocity oxygen fuel (HVOF) to produce high performance bond coats and the developments of thermal barrier coatings.

Lanthanum zirconate (LaZrO) has been proposed as a promising material for thermal barrier coatings. During atmospheric plasma spraying (APS) of LaZrO a considerable amount of LaO can evaporate in the plasma flame, resulting in a non-stoichiometric coating. As indicated in the phase diagram of the LaO-ZrO system, in the composition range of pyrochlore structure, the stoichiometric LaZrO has the highest melting point and other compositions are eutectic. APS experiments were performed with a TriplexPro™-200 plasma torch at different power levels to achieve different degrees of evaporation and thus stoichiometry. For comparison, some investigations on gadolinium zirconate (GdZrO) were included, which is less prone to evaporation and formation of non-stoichiometry. Particle temperature distributions were measured by the DPV-2000 diagnostic system. In these distributions, characteristic peaks were detected at specific torch input powers indicating evaporation and solidification processes. Based on this, process parameters can be defined to provide stoichiometric coatings that show good thermal cycling performance. [ABSTRACT FROM AUTHOR]

The thin film low pressure plasma spray process (LPPS-TF) has been developed with the aim of efficient depositing uniform and thin coatings with large area coverage by plasma spraying. At high power input (~150 kW) and very low pressure (~100 Pa) the plasma jet properties change considerably and it is even possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This relatively new technique bridges the gap between conventional plasma spraying and physical vapor deposition. In addition, the resulting microstructures are unique and can hardly be obtained by other processes. In this paper, microstructures made by LPPS-TF are shown and the columnar layer growth by vapor deposition is demonstrated. In addition to the ceramic materials TiO, AlO or MgAlO, the focus of the research was placed on partially yttria-stabilized zirconia. Variations of the microstructures are shown and discussed concerning potential coating applications. [ABSTRACT FROM AUTHOR]

In the field of thermal barrier coatings (TBCs) for gas turbines, suspension plasma sprayed (SPS) submicrometer-structured coatings often show unique mechanical, thermal, and optical properties compared to conventional atmospheric plasma sprayed ones. They have thus the potential of providing increased TBC performances under severe thermo-mechanical loading. Experimental results showed the capability of SPS to obtain yttria stabilized zirconia coatings with very fine porosity and high density of vertical segmentation cracks, yielding high strain tolerance, and low Young's modulus. The evolution of the coating microstructure and properties during thermal cycling test at very high surface temperature (1400 °C) in our burner rigs and under isothermal annealing was investigated. Results showed that, while segmentation cracks survive, sintering occurs quickly during the first hours of exposure, leading to pore coarsening and stiffening of the coating. In-situ measurements at 1400 °C of the elastic modulus were performed to investigate in more detail the sintering-related stiffening. [ABSTRACT FROM AUTHOR]

Abstract: Perovskite Ba(Mg1/3Ta2/3)O3, BMT has promising bulk properties (thermal conductivity ~2W/m-K and coefficient of thermal expansion ~11×10−6/K at 1473K) for thermal barrier coating (TBC) applications at high temperature. However, during atmospheric plasma spraying (APS), such material was found to lose constituents due to the differences of vapor pressures resulting to non-stoichiometric composition of deposited coatings. To investigate the extent of phase decomposition at spray distance and varying electric arc current, different feedstock powders were plasma sprayed into water and collected for chemical, microstructural and phase analyses. When the electric arc current was decreased from 500A to 300A, the decomposition of the powders was reduced and the microstructure of the deposited coatings was improved. The thermal cycling lifetime of the deposited coatings at ~1250°C surface temperature is also higher. [Copyright &y& Elsevier]

Abstract: The application of a thin film electrolyte layer with a thickness in the micrometer range could greatly improve current solid oxide fuel cells (SOFCs) in terms of operating temperature and power output. Since the achievable minimal layer thickness with conventional powder coating methods is limited to ∼5μm, a variety of thin film methods have been studied, but results on regular large-scale anode substrates are still lacking in the literature. In this paper, a wet coating process is presented for fabricating gas-tight 1–2μm thick 8YSZ electrolyte layers on a regular NiO/8YSZ substrate, with a rough surface, a high porosity and a large pore size. These layers were deposited in a similar way as conventional suspension based layers, but the essential difference includes the use of coating liquids (nano-dispersion, sol) with a considerably smaller particle size (85nm, 60nm, 35nm, 6nm). Successful deposition of such layers was accomplished by means of an innovative coating process, which involves the preparation of a hybrid polyvinyl alcohol/8YSZ membrane by dip-coating or spin-coating and subsequently burning out the polymer part at 500°C. Results from He leak tests confirmed that the sintered layers posses a very low number of defects and with values in the range 10−4–10−6 (hPadm3)/(scm2) the gas-tightness of the thin film layers is satisfactory for fuel cell operation. Moreover, preliminary results have also indicated a potential reduction of the sintering temperature from 1400°C to the range 1200–1300°C, using the presented coating process. [Copyright &y& Elsevier]

Thermal spray coatings from liquid feedstock such as suspensions and solution precursors have received increasing interest due to the unique coating properties obtainable by these processes. Several research groups are working on the basis of plasma as well as on high-velocity oxy-fuel approaches to manufacture advanced nanostructured and nanophased materials. These activities are reflected in various recent publications and conference presentations about feedstock preparation, equipment and process design, modeling techniques, in-process diagnostics, coating characterization, and emerging applications. This article will review these recent developments to give an up-to-date overview and to trace the current trends. [ABSTRACT FROM AUTHOR]

Thin-film La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF58428) exhibits high oxygen permeability due to its high ionic and electronic conductivity. In order to increase the oxygen flux, a thin-film membrane on a structural substrate is beneficial. Different Ni-based alloys were studied as potential substrate materials. The chemical compositions and thermal expansion coefficients of Nibased alloys were measured in this study. LSCF58428 layers were screen printed on Ni-based alloys and cofired at a high temperature in air. Microstructural and elemental analyses of samples were conducted using a scanning electron microscope and energy-dispersive X-ray spectroscope. X-ray diffraction was used to investigate the phase compositions. The Ni-based alloy, MCrAlY (with M = Ni, Co), with a high Al content showed better chemical compatibility with perovskite material at high temperatures than other Ni-based alloys. A reaction occurred between Sr in the perovskite and the alumina-based surface layer on MCrAlY. However, the reaction zone was found to be stable in mid-term annealing at 800°C in air. Hence, it is expected that this reaction will not prevent application as an oxygen transport membrane. Three different cofiring atmospheres were investigated. Air was found to be the only possible cofiring atmosphere. [ABSTRACT FROM AUTHOR]

solid oxide fuel cell (SOFC) with a thin-film yttria-stabilized zirconia (YSZ) electrolyte was developed and tested. This novel SOFC shows a similar multilayer set-up as other current anode-supported SOFCs and is composed of a Ni/8YSZ anode, a gas-tight 8YSZ electrolyte layer, a dense Sr-diffusion barrier layer and a LSCF cathode. To increase the power density and lower the SOFC operating temperature, the thickness of the electrolyte layer was reduced from around 10 μm in current cells to 1 μm, using a nanoparticle deposition method. By using the novel 1 μm electrolyte layer, the current density of our SOFC progressed to 2.7, 2.1 and 1.6 A/cm at operation temperatures of 800, 700 and 650°C, respectively, and out-performs all similar cells reported to date in the literature. An important consideration is also that cost-effective dip-coating and spin-coating methods are applied for the fabrication of the thin-film electrolyte. Processing of 1 μm layers on the very porous anode substrate material was initially experienced as very difficult and therefore 8YSZ nanoparticle coatings were developed and optimized on porous 8YSZ model substrates and transferred afterwards to regular anode substrates. In this paper, the preparation of the novel SOFC is shown and its morphology is illustrated with high resolution SEM pictures. Further, the performance in a standard SOFC test is demonstrated. [ABSTRACT FROM AUTHOR]

The very low-pressure plasma Spray (VLPPS) process has been developed with the aim of depositing uniform and thin coatings with coverage of a large area by plasma spraying. At typical pressures of 100-200 Pa, the characteristics of the plasma jet change compared to conventional low-pressure plasma-spraying processes (LPPS) operating at 5-20 kPa. The combination of plasma spraying at low pressures with enhanced electrical input power has led to the development of the LPPS-TF process (TF = thin film). At appropriate parameters, it is possible to evaporate the powder feedstock material providing advanced microstructures of the deposits. This technique offers new possibilities for the manufacturing of thermal barrier coatings (TBCs). Besides the material composition, the microstructure is an important key to reduce thermal conductivity and to increase strain tolerance. In this regard, columnar microstructures deposited from the vapor phase show considerable advantages. Therefore, physical vapor deposition by electron beam evaporation (EB-PVD) is applied to achieve such columnar-structured TBCs. However, the deposition rate is low, and the line-of-sight nature of the process involves specific restrictions. In this article, the deposition of TBCs by the LPPS-TF process is shown. How the evaporation of the feedstock powder could be improved and to what extent the deposition rates could be increased were investigated. [ABSTRACT FROM AUTHOR]

Abstract: Plasma-sprayed ceramic coatings, used as thermal barrier or abradable coatings in high-pressure stages of gas turbines, are exposed to high thermo-mechanical loading due to harsh operating conditions. Under certain conditions they also have to withstand attack by calcium-magnesium-alumino-silicate (CMAS) deposits resulting from the ingestion of siliceous minerals with the intake air. Resistance to this kind of attack becomes more important at higher temperatures, when the melting temperature of the CMAS deposits is exceeded and a penetration into the coating microstructural features (cracks and pores) can take place. During cooling the CMAS solidifies and the coating loses its strain tolerance, which can lead to coating failure. Although the basic principles of failure seem to be understood, a detailed analysis of the mechanisms and the possibilities of avoiding delamination are still lacking, especially because there are as yet no adequate test beds. This paper investigates the possibility of testing such coatings in a burner rig test facility under thermal gradient cycling conditions and at the same time CMAS deposition. This novel and unique test approach promises a coating screening and characterization test under service conditions. The CMAS rig was established because the test conditions simulated here are closer to actual engine conditions, as compared to previous tests with primarily CMAS deposition and subsequent thermal furnace testing. The experimental setup of this new test approach is described and the applicability is confirmed. Furthermore, a first evaluation for plasma-sprayed coatings is presented. A significantly reduced lifetime was found for samples tested with CMAS attack in comparison to tests with water deposition only. The lifetime was also reduced compared to older results without any corrosive media. A decrease of nearly two orders of magnitude was found. A microstructural evaluation of the coatings is presented and the mechanisms and reasons for the very early failure are discussed. [Copyright &y& Elsevier]

Abstract: Benefits and limitations of process diagnostics are investigated for the suspension plasma spraying of yttria-stabilized zirconia thermal barrier coatings. The methods applied were enthalpy probe measurements, optical emission spectroscopy, and in-flight particle diagnostic. It was proved that the plasma characteristics are not affected negatively by the injection of the ethanol based suspension since the combustion of species resulting from ethanol decomposition achieves a gain of plasma enthalpy. Furthermore, the conditions of the suspension injection into the plasma were found to be optimum as a significant content of evaporated powder material could be detected. Regarding the void content and segmentation crack density of the coatings, the in-flight particle diagnostic showed that the spray distance should be dimensioned in a way that the molten particles reach the substrates just before solidification starts. [ABSTRACT FROM AUTHOR]

Abstract: During the last decade a number of ceramic materials, mostly oxides have been suggested as new thermal barrier coating (TBC) materials. These new compositions have to compete with the state-of-the-art TBC material yttria stabilized zirconia (YSZ) which turns out to be difficult due to its unique properties. On the other hand YSZ has certain shortcomings especially its limited temperature capability above 1200°C which necessitates its substitution in advanced gas turbines. In the paper an overview is tried on different new materials covering especially doped zirconia, pyrochlores, perovskites, and aluminates. Literature results and also results from our own investigations will be presented and compared to the requirements. Finally, the double-layer concept, a method to overcome the limited toughness of new TBC materials, will be discussed. [ABSTRACT FROM AUTHOR]

This article summarises recent developments in solid oxide fuel cell research regarding materials, processing and microstructure–property relationships. In the materials section, the various cell and stack materials are briefly described, i.e. electrolytes, electrodes, contact and protective layers, interconnects and sealing materials. The section on processing gives an overview of manufacturing technologies for cells including a view of different substrate materials and designs. Besides the widely used planar cell designs, the technologies for tubular designs are also described. In addition, the technologies are grouped with respect to the support, e.g. metal- or ceramic–metal (cermet anode substrate)-supported SOFCs. Finally, special emphasis is laid on the microstructure of functional layers which primarily govern the power output of the SOFC. [ABSTRACT FROM AUTHOR]

Abradable coatings in compressor and high-pressure stages of gas turbines must provide specific hardness and porosity values to achieve an optimal cut-in of the blade tips. A fractional factorial experimental plan was designed to investigate the influence of the plasma spraying parameters argon flow rate, current, spraying distance and powder feed rate on these properties of magnesia spinel. Based on the results, magnesia spinel coatings with low (~400 HV0.5), medium (~600 HV0.5) and high hardness (~800 HV0.5) could be reliably manufactured. Further incursion rig tests confirmed the dependence of the rub-in behavior and abradability on the coating characteristics and process parameters, respectively. Process monitoring was also applied during plasma spraying of magnesia spinel abradables on batches of turbine components. The recorded particle characteristics and coating properties showed a good reproducibility of the spraying process. [ABSTRACT FROM AUTHOR]

Abstract: Donor-substituted SrTiO3 ceramic materials were investigated as the anodes of solid oxide fuel cells (SOFCs). Sr0.89Y0.07TiO3−δ (SYT) samples with good electrical conductivity and redox stability were prepared. The thermal and chemical expansions of SYT are both compatible with YSZ electrolyte. Half cells consisting of a flat anode substrate and an electrolyte layer with outer dimensions of 5cm×5cm were fabricated, Ni particles were infiltrated on the pore walls within the ceramic anode framework, and the redox stability of the half cell was tested by He leakage tests after redox cycling. Ti diffusion but no Sr migration was found between the anode and electrolyte layers. Sr0.895Y0.07Ti x O3±δ (x =1.00–1.20)–YSZ composites with a volume ratio 2:1 were prepared to investigate the influences of this interdiffusion between YSZ and SYT materials. The results indicate that the conductivity of SYT decreases because of the Ti diffusion, and a small amount of Ti excess can solve this problem. [Copyright &y& Elsevier]

Suspension plasma spraying (SPS) offers the manufacture of unique microstructures which are not possible with conventional powdery feedstock. Due to the considerably smaller size of the droplets and also the further fragmentation of these in the plasma jet, the attainable microstructural features like splat and pore sizes can be downsized to the nanometer range. Our present understanding of the deposition process including injection, suspension plasma plume interaction, and deposition will be outlined. The drawn conclusions are based on analysis of the coating microstructures in combination with particle temperature and velocity measurements as well as enthalpy probe investigations. The last measurements with the water cooled stagnation probe gives valuable information on the interaction of the carrier fluid with the plasma plume. Meanwhile, different areas of application of SPS coatings are known. In this paper, the focus will be on coatings for energy systems. Thermal barrier coatings (TBCs) for modern gas turbines are one important application field. SPS coatings offer the manufacture of strain-tolerant, segmented TBCs with low thermal conductivity. In addition, highly reflective coatings, which reduce the thermal load of the parts from radiation, can be produced. Further applications of SPS coatings as cathode layers in solid oxide fuel cells (SOFC) and for photovoltaic (PV) applications will be presented. [ABSTRACT FROM AUTHOR]

Perovskite-type SrZrO3 has been investigated as a candidate material for thermal barrier coating application. During plasma spraying of SrZrO3, SrO volatilized more than ZrO2 and the coating composition deviates from initial stoichiometry. In this investigation, partial evaporation was investigated by spraying SrZrO3 powders into water. The influences of spraying current, distance and particle size of the powder on the partial evaporation were also investigated in a quantitative way. With optimized spraying parameters, stoichiometric SrZrO3 coating was produced by adding an excess amount of Sr in the precursors before plasma spraying to compensate for the volatilized component. [ABSTRACT FROM AUTHOR]

The occurrence of monoclinic zirconia phase has an important impact on the performance of thermal barrier coatings (TBC) of yttria-stabilized zirconia (YSZ). Therefore, a reliable method is needed to detect its contents and to investigate also its spatial distribution within the parent microstructure. This was the motivation to apply cathodoluminescence (CL) spectroscopy. YSZ coatings with different porosities were manufactured by atmospheric plasma spraying. CL analysis yielded monoclinic phase contents of 5.2 ± 1.6% for the high-porous sample and 3.4 ± 0.5% for the low-porous sample. The results were qualitatively confirmed by x-ray diffraction (XRD). However, due to its lower detection sensitivity the XRD results are quantitatively on lower level. Owing to its synthesis method, the applied powder feedstock showed a considerable content of monoclinic phase. The lower the particle temperatures were the larger fraction of monoclinic phase remained untransformed. This has to be considered when spraying high-porous TBCs. [ABSTRACT FROM AUTHOR]

In an effort to improve the performance of heat engines at high temperatures, advanced surface coatings have been developed from complex perovskites. Materials of Ba(Mg1/3Ta2/3)O3 and La(Al1/4Mg1/2Ta1/4)O3 composition were synthesized and applied as ceramic topcoats of thermal barrier coating (TBC) systems by atmospheric plasma spraying (APS) in single layer and in double-layer combination with conventional yttria stabilized zirconia (YSZ). Microstructural and phase analyses reveal that plasma spraying of complex perovskites is accompanied with the formation of vertical crack networks and secondary oxide phases which influence the failure mechanism of the TBCs. The low value of fracture toughness for the complex perovskites and the thermally grown oxide at the topcoat-bondcoat interface of the TBCs are, however, the major factors which lead to the coating failure on thermal cycling at about 1250 °C. [ABSTRACT FROM AUTHOR]

Abstract: Highly porous TiO2 coatings have been produced by suspension plasma spraying on ITO coated glass substrates. The deposition process could be optimized so that fine/nano grained highly porous coatings were obtained. Mean crystallite sizes well below 50 nm could be achieved in the coatings for the anatase phase. Special emphasis was on the establishment of a high volume fraction of the desired anatase phase. The best coatings showed values of about 90% of anatase phase. With these coatings photovoltaic cells could be produced. Different levels of layer thickness and porosity as well as contents of anatase phase have been investigated. In the paper details on the microstructure of the cells as well as the photovoltaic performance will be given. [Copyright &y& Elsevier]

High-performance solid oxide fuel cells require a thin and gas-tight electrolyte membrane that must be coated on a porous and relatively rough support. A pretreatment of the delivered submicronmeter electrolyte powder of 8 mol%-yttria-stabilized zirconia (8YSZ) yielded a reduced sintering mismatch between the anode substrate made from NiO/8YSZ and the electrolyte coating. Furthermore, it also enhanced the powder packing inside the green film. Constrained sintering usually leads to inadequate film density and an unfavorable pore deformation and orientation. It was demonstrated that these limitations can be resolved by using a coshrinking substrate in a planar cell design. Relative densities of >97% were achieved, which are higher than those for free-standing layers. Additionally, the camber behavior was investigated in dependence of the temperature program with and without gravity effects, giving an overall suggestion for the cofiring parameters of the electrolyte. [ABSTRACT FROM AUTHOR]

Abstract: The production of high quality powder metallurgical NiTi alloys with elevated phase transformation temperatures is challenging. During processing, an unavoidable pickup of impurity elements (especially oxygen and carbon) results in a decrease of phase transition temperatures and in the formation of brittle secondary phases. We introduce a processing route including melting, gas atomization and hot isostatic pressing for binary NiTi shape memory alloys which minimizes these problems. We demonstrate that the microstructure of the Ti-rich NiTi alloy contains precipitates of Ti2Ni type which can be exploited to dissolve oxygen picked up during later process stages. In this study, three powder fractions with different grain sizes and impurity contents were subjected to hot isostatic pressing. The evolution of microstructures and material properties was studied by chemical analysis, microscopy, differential scanning calorimetry, and mechanical testing. Exploiting the solubility of oxygen in Ti2Ni, the processing route presented in the present paper succeeds in producing powder metallurgical NiTi shape memory alloys with good structural and functional properties. [Copyright &y& Elsevier]

Perovskite-type SrZrO3 was investigated as an alternative to yttria-stabilized zirconia (YSZ) material for thermal barrier coating (TBC) applications. Three phase transformations (orthorhombic↔pseudo-tetragonal↔tetragonal↔cubic) were found only by heat capacity measurement, whereas the phase transformation from orthorhombic to pseudo-tetragonal was found in thermal expansion measurements. The thermal expansion coefficients (TECs) of SrZrO3 coatings were at least 4.5% larger than YSZ coatings up to 1200°C. Mechanical properties (Young's modulus, hardness, and fracture toughness) of dense SrZrO3 showed lower Young's modulus, hardness, and comparable fracture toughness with respect to YSZ. The “steady-state” sintering rate of a SrZrO3 coating at 1200°C was 1.04 × 10−9 s−1, which was less than half that of YSZ coating at 1200°C. Plasma-sprayed coatings were produced and characterized. Thermal cycling with a gas burner showed that at operating temperatures ∼1250°C the cycling lifetime of SrZrO3/YSZ double-layer coating (DLC) was more than twice as long as SrZrO3 coating and comparable to YSZ coating. However, at operating temperatures >1300°C, the cycling lifetime of SrZrO3/YSZ DLC was comparable to the optimized YSZ coating, indicating SrZrO3 might be a promising material for TBC applications at higher temperatures compared with YSZ. [ABSTRACT FROM AUTHOR]

Abstract: NiTi alloys produced by different powder metallurgical (PM) methods are investigated concerning transformation behavior and shape memory characteristics. Effects of purity, heat treatments and processing conditions are taken into account. It is shown that the deformability of PM-NiTi is increased after a long-term treatment at 1250°C when the two different types of secondary phases resulting from impurities of oxygen and carbon become larger in size, and their distribution is therefore more isolated. In the case of Ni-rich NiTi material, the mechanical behavior is further influenced by the precipitation of metastable phases (Ni4Ti3) at intermediate temperatures (350 and 450°C), which reduce the deformability, again resulting in reduced elongations at fracture. The stress level and the elongation at fracture are reduced by a residual porosity. [Copyright &y& Elsevier]

Abstract: Perovskites resulting from discrete changes in composition within the quasi-ternary system La0.8Sr0.2MnO3−δ –La0.8Sr0.2CuO2.4+δ –La0.8Sr0.2FeO3−δ were investigated under constant experimental conditions with the objective of obtaining an overview of the variation of the properties relevant for possible future applications. Nineteen nominal perovskite compositions within this system were systematically selected and synthesized under identical conditions by the Pechini method. The experimental data obtained on quantitative chemical analysis, powder X-ray diffraction, electrical conductivity and thermal expansion are presented collectively for the first time to facilitate comparisons. The formation and distribution of the different crystallographic phases at 950°C within this quasi-ternary system are shown. The DC electrical conductivity is strongly influenced by the Cu content and increases up to 276Scm−1 for La0.8Sr0.2CuO2.4+δ . The thermal expansion is dominated by the Cu/Mn ratio and is almost independent of the Fe content. [Copyright &y& Elsevier]

Abstract: It was shown in part I (Variation of the cathode composition) [A. Mai, V.A.C. Haanappel, S. Uhlenbruck, F. Tietz, D. Stöver, Solid State Ionics 176 (2005), 1341], that an interlayer is needed between yttria-stabilised zirconia (YSZ) electrolytes and (La,Sr)(Co,Fe)O3−δ (LSCF) cathodes in order to prevent undesired chemical reactions between these materials. This interlayer makes it possible to benefit from the superior electrochemical properties of the LSCF perovskites in solid oxide fuel cells (SOFCs). In this study the influence of a Ce0.8Gd0.2O2−δ (CGO) interlayer on the electrochemical performance of LSCF-type SOFCs is investigated in more detail. For screen-printed and sintered interlayers the grain size as well as the sintering temperature affected the electrochemical performance. The use of a fine powder with a mean particle size of d 50 =0.2 μm and sintered at 1250 °C resulted in the best performance. Furthermore, reactive sputtering resulted in dense CGO interlayers at low deposition temperatures, which led to improved properties regarding diffusion inhibition and electrochemical performance. SOFCs with sputtered interlayers gave power densities of up to 0.9 W/cm2 at 700 °C and 0.7 V, with H2 +3% H2O as fuel gas (approx. 10% fuel utilisation). [Copyright &y& Elsevier]

Abstract: A novel processing method to produce a carbon-fibre-reinforced silicon carbide (C/SiC) is presented and the mechanical properties are evaluated. The low-cost process yields a reaction bonded silicon carbide matrix but avoids efficiently reactive damage of the carbon fibres. Thus, the C/SiC-material shows a fibre-dominated behaviour due to high fibre strength and matrix porosity. The characteristics and mechanical properties of this C/SiC are highlighted. The discussion deals with the particular mechanical characteristics of porous-matrix materials relating to similar materials investigated hitherto. [Copyright &y& Elsevier]

Abstract: A carbon-fibre-reinforced carbon (C/C) composite filled with silicon carbide (SiC) particles was produced, using the winding technique. A porous matrix consisting of glassy carbon, SiC particles and small pores in homogeneous distribution was produced, ensuring a damage-tolerant fracture behaviour. Neither fibre coatings nor further infiltration steps were necessary for the preparation of the composite, which makes this a low-cost process making no limitations on wall thickness. The microstructure and homogeneity of the material is shown using micrographs, mercury porosimetry and BET measurements. Bending tests, as well as single-edge notch beam (SENB) tests point to high values for the bending strength and formal fracture toughness. A damage-tolerant fracture behaviour is demonstrated by the load–displacement curves and fracture surfaces. [Copyright &y& Elsevier]

Abstract: The properties and the applicability of iron- and cobalt-containing perovskites were evaluated as cathodes for solid oxide fuel cells (SOFCs) in comparison to state-of-the-art manganite-based perovskites. The materials examined were La1−x−y Sr x Co0.2Fe0.8O3− δ (x =0.2 and 0.4; y =0−0.05), La0.8Sr0.2FeO3− δ , La0.7Ba0.3Co0.2Fe0.8O3− δ and Ce0.05Sr0.95Co0.2Fe0.8O3− δ . The main emphasis was placed on the electrochemical properties of the materials, which were investigated on planar anode-supported SOFCs with 8 mol% yttria-stabilised zirconia (8YSZ) electrolytes. An interlayer of the composition Ce0.8Gd0.2O2− δ was placed between the electrolyte and the cathode to prevent undesired chemical reactions between the materials. The sintering temperatures of the cathodes were adapted for each of the materials to obtain similar microstructures. In comparison to the SOFCs with state-of-the-art manganite-based cathodes, SOFCs with La1− x − y Sr x Co0.2Fe0.8O3− δ cathodes achieved much higher current densities. Small A-site deficiency and high strontium content had a particularly positive effect on cell performance. The measured current densities of cells with these cathodes were as high as 1.76 A/cm2 at 800 °C and 0.7 V, which is about twice the current density of cells with LSM/YSZ cathodes. SOFCs with La0.58Sr0.4Co0.2Fe0.8O3− δ cathodes have been operated for more than 5000 h in endurance tests with a degradation of 1.0–1.5% per 1000 h. [Copyright &y& Elsevier]

The concept of anode-supported planar solid oxide fuel cells (SOFCs) involves different fabrication procedures and several sintering steps. This contribution is an attempt towards minimizing the number of sintering steps. By adopting an improved technique a pre-firing step was eliminated. Cells obtained following this co-firing technique showed relatively higher power density (∼1 A cm-2 at 0.7 V) at an operating temperature of 800 °C compared to the cells prepared by the conventional technique. In the present investigation, cells as large as10 cm×10 cm were prepared and characterized following the new co-firing approach. Detailed microstructural, leak-rate testing (gastightness of the sintered electrolyte film) and electrochemical characterizations were carried out. [Copyright &y& Elsevier]