Influence of Atmospheric Plasma-Sprayed Composite Cathode on the Performance of Solid Oxide Fuel Cells

Ni-Al2O3 cermet supported tubular solid oxide fuel cells(SOFCs)with different cathodes were fabricated by thermal spraying.The anode,electrolyte and cathode were deposited by atmospheric plasma spraying(APS)aimed at reducing the manufacturing cost of SOFCs.Three porous composite cathodes of lanthanum strontium manganite(LSM)and yttria-stabilized zirconia(YSZ),lanthanum strontium cobalt oxide(LSC)and YSZ,LSC and scandia-stabilized zirconia(ScSZ)were prepared to investigate influence of cathode constitutions on the cell’s performance.The electrode polarization can be improved through using a composite cathode.The maximum power density of the cell with APS YSZ electrolyte and LSC/ScSZ composite cathode is increased about 12%than pure LSM cathode.The maximum output power density of the cell with APS ScSZ electrolyte and LSC/ScSZ composite cathode reaches 1.0 W/cm 2 at 1000 o C.The further optimization of the performance of plasma-sprayed composite cathode can be made through improving the interface contact between YSZ electrolyte and composite cathode.     

Manufacturing of high performance solid oxide fuel cells (SOFCs) with atmospheric plasma spraying (APS)

The potential of atmospheric plasma spraying (APS) technology has been investigated for the manufacture of anode, electrolyte and cathode of a solid oxide fuel cell. As the substrate a tape-casted FeCr alloy was used. It turned out that all layers can be applied by this technique, however, the APS cathode layer, although applied by suspension plasma spraying led to cells with rather low performance. Much better cell characteristics could be obtained by using screen-printed LSCF cathodes, which do not need any additional thermal treatment.Anode layers with high electrochemical activity were produced by separate injection of NiO and YSZ powders. The manufacturing of gastight electrolyte layers was a key-issue of the present development. As APS ceramic coatings typically contain microcracks and pores their leakage rate is not sufficiently low for SOFC applications.Based on the understanding of the formation of defects during spraying an optimized spraying process was developed which led to highly dense coatings with the appearance of a bulk, sintered ceramic. Open cell voltages above 1 V proofed the low leakage rates of the rather thin (< 50 μm) coatings. With these cells having a screen-printed cathode an output power of 500 mW/cm² could be achieved at 800 °C.It turned out that the long-term stability of the metal substrate based APS SOFCs was rather poor. The aging of the cells was probably due to interdiffusion of anode and substrate material. Hence, diffusion barrier was applied by APS between substrate and anode. These layers were very effective in reducing the degradation rate. For these cells the output power reached 800 mW/cm².     

Permeability and Microstructure of Suspension Plasma-Sprayed YSZ Electrolytes for SOFCs on Various Substrates

Yttria-stabilized zirconia electrolyte coatings for solid oxide fuel cells were deposited by suspension plasma spraying using a range of spray conditions and a variety of substrates, including finely structured porous stainless steel disks and cathode layers on stainless steel supports. Electrolyte permeability values and trends were found to be highly dependent on which substrate was used. The most gas-tight electrolyte coatings were those deposited directly on the porous metal disks. With this substrate, permeability was reduced by increasing the torch power and reducing the stand-off distance to produce dense coating microstructures. On the substrates with cathodes, electrolyte permeability was reduced by increasing the stand-off distance, which reduced the formation of segmentation cracks and regions of aligned and concentrated porosity. The formation mechanisms of the various permeability-related coating features are discussed and strategies for reducing permeability are presented. The dependences of electrolyte deposition efficiency and surface roughness on process conditions and substrate properties are also presented.     

Splat Morphology of Yttria-Stabilized Zirconia Droplet Deposited Via Hybrid Plasma Spraying

The splat morphology of yttria-stabilized zirconia (YSZ) droplets deposited by dc-rf hybrid plasma spraying (HYPS) was studied. Two types of YSZ powder were used, namely fused and crushed powder (FC) and hollow spherical powder (HOSP). The three-dimensional shape of more than 600 disk-shaped splats on preheated substrates was evaluated using a laser-scanning microscope to determine the splat diameter, thickness, and their dimensionless forms. The HOSP showed a higher degree of flattening than the FC. Both the FC with a powder size distribution of 45-75 μm and the HOSP of 30-120 μm can be used as spray materials in the HYPS to achieve a coating design based on fully molten particles. The effect of the substrate temperature on the splat morphology was similar to that of atmospheric dc plasma spraying; however, the transition from a splashed shape to a disk shape gradually occurred at higher substrate temperatures.     

Microstructure and Electrochemical Behavior of La0.8Sr0.2MnO3 Deposited by Solution Precursor Plasma Spraying

采用液料等离子喷涂方法(SPPS)制备固体氧化物燃料电池多孔La0.8Sr0.2MnO3(LSM)阴极。用SEM观察LSM的微结构,用XRD研究其相结构。考察了喷涂距离和热处理温度对LSM微结构的影响规律。结果表明,SPPSLSM在1050℃热处理2h后形成连续的具有微纳介孔结构的涂层,且LSM具有单一的钙钛矿结构。利用电化学交流阻抗谱方法研究了LSM极化行为。微结构对极化性能有显著影响,1000℃时,LSM在喷涂距离为60mm时具有最佳的电化学性能,阴极极化电阻约为0.3Ω·cm2。通过工艺的控制,SPPS可以实现SOFC阴极相和微结构的优化。     

TiB2-SiC粉末喷雾造粒及其等离子喷涂沉积机理研究

利用喷雾干燥对TiB_2-SiC复合粉末进行造粒,研究了浆料固含量、粘结剂含量及SiC含量对喷雾干燥粉体颗粒形貌等的影响。采用大气等离子喷涂技术,以抛光的石墨为基体,在不同预热温度和不同喷距下对TiB_2-SiC粉末进行粒子收集,研究不同工艺参数对TiB_2-SiC粒子铺展形貌的影响,并制备了TiB_2-SiC涂层。结果表明:当浆料固含量为50%,粘结剂含量为5%,SiC含量为10%时,喷雾造粒获得球形度高、流动性好的TiB_2-SiC粉末;随着基体预热温度的升高,喷距的增大,扁平粒子的溅射逐渐减弱,形成规则的圆盘状粒子;在等离子焰流作用下,TiB_2-SiC粒子熔化加速并与基体发生碰撞,熔融粒子扁平化,急速冷却凝固,不断堆叠、搭接为宏观涂层。     

Atmospheric Plasma Spraying Low-Temperature Cathode Materials for Solid Oxide Fuel Cells

Atmospheric plasma spraying (APS) is attractive for manufacturing solid oxide fuel cells (SOFCs) because it allows functional layers to be built rapidly with controlled microstructures. The technique allows SOFCs that operate at low temperatures (500-700 °C) to be fabricated by spraying directly onto robust and inexpensive metallic supports. However, standard cathode materials used in commercial SOFCs exhibit high polarization resistances at low operating temperatures. Therefore, alternative cathode materials with high performance at low temperatures are essential to facilitate the use of metallic supports. Coatings of lanthanum strontium cobalt ferrite (LSCF) were fabricated on steel substrates using axial-injection APS. The thickness and microstructure of the coating layers were evaluated, and x-ray diffraction analysis was performed on the coatings to detect material decomposition and the formation of undesired phases in the plasma. These results determined the envelope of plasma spray parameters in which coatings of LSCF can be manufactured, and the range of conditions in which composite cathode coatings could potentially be manufactured.     

Cold Spray Coating of Submicronic Ceramic Particles on Poly(vinyl alcohol) in Dry and Hydrogel States

We report an approach using cold spray technology to coat poly(vinyl alcohol) (PVA) in polymer and hydrogel states with hydroxyapatite (HA). Using porous aggregated HA powder, we hypothesized that fragmentation of the powder upon cold spray could lead to formation of a ceramic coating on the surface of the PVA substrate. However, direct spraying of this powder led to complete destruction of the swollen PVA hydrogel substrate. As an alternative, HA coatings were successfully produced by spraying onto dry PVA substrates prior to swelling in water. Dense homogeneous HA coatings composed of submicron particles were obtained using rather low-energy spraying parameters (temperature 200-250 °C, pressure 1-3 MPa). Coated PVA substrates could swell in water without removal of the ceramic layer to form HA-coated hydrogels. Microscopic observations and in situ measurements were used to explain how local heating and impact of sprayed aggregates induced surface roughening and strong binding of HA particles to the molten PVA substrate. Such an approach could lead to design of ceramic coatings whose roughness and crystallinity can be finely adjusted to improve interfacing with biological tissues.     

Deposition Behavior of Semi-Molten Spray Particles During Flame Spraying of Porous Metal Alloy

Porous 316L stainless steel deposits were fabricated by flame spraying semi-molten particles with different melting degrees and spray angles to understand the deposition behavior of semi-molten spray particles. The effects of spray angle relative on the deposition efficiency and deposit porosity were investigated. The morphology of individual splats deposited on flat surface at different angles was examined. The results show that the spray angle had a significant influence on the deposit porosity, pore structure, and deposition efficiency. The slipping of solid core in semi-molten spray particle was clearly observed when semi-molten particles impacted on the polished substrate with an inclined angle. A random model was proposed to simulate the process of particle deposition. It was found that after considering the effects of both solid particle slipping upon impact and particle melting degree, the porosity calculated by simulation with the model agreed well with the experimental observation.     

Effect of Substrate Concave Pattern on Splat Formation of Yttria-Stabilized Zirconia in Atmospheric Plasma Spraying

Splat morphology of yttria-stabilized zirconia (YSZ) on a microconcave-patterned substrate was investigated by both numerical and experimental approaches under a dc-rf hybrid-plasma spray condition. The spreading behavior of molten droplets on a microdimple pattern was numerically simulated in a three-dimensional form. For comparison, impact of a YSZ droplet onto a microdimple pattern of a quartz glass substrate was studied in situ utilizing thermal emissions from the droplet. Concave aspects of a substrate surface play an important role in fingering/splashing of a spreading droplet as well as convex patterns. The main mechanism that causes splashing is likely due to the slipping of a spreading droplet at the edge of concave patterns. The viscosity decrease of the spreading droplet enhances the droplet splash.     

Development of a Ni/Al2O3 Cermet-Supported Tubular Solid Oxide Fuel Cell Assembled with Different Functional Layers by Atmospheric Plasma-Spraying

A cermet-supported tubular configuration amenable to preparation by a relatively low-cost thermal spraying process is proposed. An Al₂O₃-Ni cermet thick deposit prepared by flame spraying is employed as both support tube and anode current collector. Atmospheric plasma spraying (APS) has been employed to prepare the anode, cathode, and stabilized ZrO₂-based electrolyte with the aim of reducing manufacturing costs. Gas-tightness of the APS electrolyte has been achieved by a postdensification process. The effects of the densification process on the gas-tightness of the plasma-sprayed YSZ electrolyte and the open-circuit voltage of the SOFC have been investigated. The effects of the microstructures of the plasma-sprayed anode, electrolyte, and cathode on the performance of the SOFC test cell have been investigated.     

Splat Morphology and Influence of Feeding Rate During Reactive Plasma Spray of Aluminum Powder

Fabrication of aluminum nitride (AlN) coatings using conventional plasma spraying processes directly has been deemed impossible. It is attributed to the thermal decomposition of the AlN feedstock particles during spraying without a stable melting phase. Using the reactivity of the plasma (reactive plasma spraying: RPS) showed a promising consideration for in situ formation of AlN thermally sprayed coatings. Several AlN-based coatings were fabricated through the RPS of aluminum powders in the N₂/H₂ plasma. The focus of this study is in discussing the morphology of splat deposition during the nitriding of Al particles. Furthermore, the influence of the feeding rate during the RPS and nitriding of Al powders will be investigated. The nitride content, as well as the unreacted molten Al phase, strongly influences splat deposition and morphology during the RPS of Al. The collected splats can be divided into reacted, partially reacted, and unreacted splats. The reacted splats tend to show a disk or egg-shell shape. The partially reacted mainly had outside nitride shells and an unreacted molten Al part in the center. The unreacted splats tended to show a splash shape. The main controlling factor is the time of the droplet impact on the substrate during the reaction sequence. The particle size and spray distance showed significant effects on the splat formation due to their effect on the nitriding conversion and the melting behavior of the particles during RPS nitriding. The powder feeding rate was investigated through increasing the injection rate and by using a low carrier gas flow rate. Increasing the powder feeding rate significantly improved the coating thickness. However, it suppressed the nitriding conversion of the large Al particles. Thus, with increasing the amount of the powder in the plasma, the Al molten particles are easily aggregated and agglomerate together upon colliding on the substrate with an AlN shell on the surface. This prevents the N₂ from having access to all of the aggregated particles. Therefore, the fabricated coatings using large Al particles consist of surface AlN layers and the central parts of AlN and Al composite layers. On the other hand, it was possible to fabricate about 500-μm-thick AlN coatings using fine Al particles of 15 μm and increasing the feeding rate. Using the fine particles improved the nitriding reaction due to the improvement of the surface area (the reaction area). Moreover, the nitriding process of the Al particles with increasing the feeding rate was also investigated.     

Influence of dopant on the behavior under thermal cycling of two plasma- sprayed zirconia coatings Part 2: residual stresses

The evolution of coating morphology and surface residual stresses was followed for three different pow-ders: zirconia stabilized with 8 wt% yttria (YSZ), 9.9 wt% dysprosia (DSZ), and 9.8 wt% ytterbia (YbSZ). The YSZ reference powder was fused and crushed (-45 +22 μm), and the other two were agglom-erated and sintered (-90 +10 μm). According to the size distributions and manufacturing process, the plasma-sprayed YSZ particles were fully molten, resulting in dense coatings with good contact between the splats; the DSZ and, especially, the YbSZ particles were partially molten. In general, the surface residual stresses were slightly compressive before thermal cycling. The YSZ and DSZ coatings were insensitive to aging (600 h in air at room temperature), as shown by the surface stress evolution, which was not the case for YbSZ coatings. Six hundred furnace thermal cycles from 1100 °C to room temperature indicated excellent behavior of YSZ and DSZ coatings, with almost no variation of sur-face residual stresses, compared to a high dispersion for YbSZ coatings with the development of macrocracks parallel and perpendicular to the substrate within the coating. Part 1 of this article was printed in the Journal of Thermal Spray Tech-nology, Vol 5 (No. 4), 1996, p 431-438.     

Development and characterization of vacuum plasma sprayed thin film solid oxide fuel cells

The vacuum plasma spraying (VPS) process allows the production of thin solid oxide fuel cells (SOFCs) with low internal resistances. This enables the reduction of the cell operating temperature without a significant decrease in power density. Consequently, the long-term stability of the cells can be improved and low-cost materials can be used. Different material combinations and spray parameter variations were applied to develop thin-film SOFCs, which were plasma sprayed in a consecutive deposition process onto different porous metallic substrates. The use of Laval nozzles, which were developed at the German Aerospace Center (DLR), and the use of conical F4V standard nozzles enable the fabrication of thin gas tight yttria- and scandia-stabilized ZrO₂ (YSZ and ScSZ) electrolyte layers and of porous electrode layers with high material deposition rates. The optimization of the VPS parameters has been supported by laser doppler anemometry (LDA) investigations. The development of the plasma-sprayed cells with a total thickness of approximately 100 μm requires an overall electrical and electrochemical characterization process of the single layers and of the completely plasma-sprayed cell assembly. The plasma-sprayed cell layers reveal high electrical conductivities. The plasma-sprayed cells show very good electrochemical performance and low internal resistances. Power densities of 300 to 400 mW/cm² at low operating temperatures of 750 to 800 °C were achieved. These cells can be assembled to high performance SOFC stacks with active cell areas up to 400 cm², which can be operated at reduced temperatures and good long-term stability.     

Characterization of YSZ solid oxide fuel cells electrolyte deposited by atmospheric plasma spraying and low pressure plasma spraying

Yttria doped zirconia has been widely used as electrolyte materials for solid oxide fuel cells (SOFC). Plasma spraying is a cost-effective process to deposit YSZ electrolyte. In this study, the 8 mol% Y₂O₃ stabilized ZrO₂ (YSZ) layer was deposited by low pressure plasma spraying (LPPS) and atmospheric plasma spraying (APS) with fused-crushed and agglomerated powders to examine the effect of spray method and particle size on the electrical conductivity and gas permeability of YSZ coating. The microstructure of YSZ coating was characterized by scanning electron microscopy and x-ray diffraction analysis. The results showed that the gas permeability was significantly influenced by powder structure. The gas permeability of YSZ coating deposited by fused-crushed powder is one order lower in magnitude than that by agglomerated powder. Moreover, the gas permeability of YSZ deposited by LPPS is lower than that of APS YSZ. The electrical conductivity of the deposits through thickness direction was measured by potentiostat/galvanostat based on three-electrode assembly approach. The electrical conductivity of YSZ coating deposited by low pressure plasma spraying with fused-crushed powder of small particle size was 0.043 S cm⁻¹ at 100 °C, which is about 20% higher than that of atmospheric plasma spraying YSZ with the same powder. This article was originally published inBuilding on 100 Years of Success, Proceedings of the 2006 International Thermal Spray Conference (Seattle, WA), May 15–18, 2006, B.R. Marple, M.M. Hyland, Y.-Ch. Lau, R.S. Lima, and J. Voyer, Ed., ASM International, Materials Park, OH, 2006.     

Flattening of droplets and formation of splats in thermal spraying: A review of recent work—Part 1

Properties of the coatings developed during thermal spraying are essentially determined by rapid solidification of splats formed as a result of impingement of the melted powder particles onto a substrate surface. The processes of flattening droplets and formation of splats in thermal spraying have been studied intensively during the last two decades. The last review on this topic was published at the end of 1994. Since then many papers have been dedicated to investigating splat formation, taking into account such important issues as roughness of the substrate surface, wetting phenomena, and splashing. This review, consisting of two parts, includes the main results obtained since 1994 and examines the influence of solidification of the lower part of the splat, substrate roughness, wetting at the substrate-coating interface, substrate deformation, oxidation, and splashing on the dynamics of flattening of droplets and the formation of splats. Flattening of composite powder particles, splat-substrate interaction, and development of splat-substrate adhesion and splat porosity are discussed. Part 1 of the review covers the following issues, which significantly influence the droplet flattening and splat formation: droplet solidification during flattening and roughness of the substrate surface, composite morphology of the powder particles, and oxidation processes. The results provide a better understanding of the thermal spray processes to increase their efficiency.     

Fabrication of finely structured silicon-supported SOFC with anode deposited by multi-phase plasma spraying

The mechanically mixed NiO/YSZ powder was usually used as the anode material of atmospheric plasma sprayed (APS) solid oxide fuel cells (SOFC). Big particles and the non-uniform distribution of the pores were observed in the resultant anode layer. To overcome the limitations, a method of fabricating anode layer by multi-phase plasma spraying (MPS) was proposed in this paper. The NiO and YSZ powders were delivered into plasma jet by a separate injection, where nitrogen carrier was employed to feed micrometer-sized NiO powder and liquid carrier was to feed submicrometer-sized YSZ powder. Suspension plasma spraying (SPS) was applied to fabricate dense electrolyte layer. The microstructure and composition of coatings were characterized by SEM and EDS. The results showed that finely structured anode layer with small particle size (d ∼ 2 μm) was achieved by the MPS method. The MPS anode layer was porous with the porosity of 32.1% while the APS anode layer was 22.6%. Three kinds of elements (Ni, Y, Zr) were observed in the MPS anode layer and the NiO content was calculated to be 49.6 wt%. In the SPS process, the suspension flow rate was matched to the plasma gas flow rate to obtain proper injection condition.     

Hot Corrosion Behavior of a High Velocity Arc-Sprayed Fe-Cr-B-C Coating

A Fe-Cr-B-C coating was prepared by electric arc spraying process to prevent the boiler tubes from hot corrosion at elevated temperatures.A hot corrosion resistance test was conducted in a mixed molten salt of Na2SO4 and K2SO4(7:3)at 700 ℃for a total period of 156 h.The microstructure and phases of the coatings before and after exposed to the hot corrosion were investigated by scanning election microscopy(SEM),optical microscopy(OM)and X-ray diffraction(XRD).The hardness and porosity were analyzed.The hot corrosion behavior of the coatings was examined by the measurement of corrosion mass gain and the observation of corrosion morphology.The results show that some splats of particles are formed on flat substrate surfaces and the coatings have a dense typical layer structure of electric arc thermally spraying deposits.Some amorphous phase exist in the coating.The coatings have an excellent resistance to hot corrosion.The formation of oxides of chromium on the exposed surface may be contributing better resistance to hot corrosion.The corrosion of the coatings follows the oxidation and sulfidation mechanism.     

Thermal Spray Using a High-Frequency Pulse Detonation Combustor Operated in the Liquid-Purge Mode

Experiments on thermal spray by pulsed detonations at 150 Hz were conducted. Two types of pulse detonation combustors were used, one operated in the inert gas purge (GAP) mode and the other in the liquid-purge (LIP) mode. In both modes, all gases were supplied in the valveless mode. The GAP mode is free of moving components, although the explosive mixture is unavoidably diluted with the inert gas used for the purge of the hot burned gas. In the LIP mode, pure fuel-oxygen combustion can be realized, although a liquid-droplet injector must be actuated cyclically. The objective of this work was to demonstrate a higher spraying temperature in the LIP mode. First, the temperature of CoNiCrAlY particles heated by pulsed detonations was measured. As a result, the spraying temperature in the LIP mode was higher than that in the GAP mode by about 1000 K. Second, the temperature of yttria-stabilized zirconia (YSZ) particles, whose melting point was almost 2800 °C, heated by pulsed detonations in the LIP mode was measured. As a result, the YSZ particles were heated up to about 2500 °C. Finally, a thermal spray experiment using YSZ particles was conducted, and a coating with low porosity was successfully deposited.     

Microstructure and Electrical Conductivity of Atmospheric Plasma-Sprayed LSM/YSZ Composite Cathode Materials

Yttria stabilized zirconia and lanthanum strontium manganate (YSZ/LSM) have been employed to fabricate the composite cathode layer for solid oxide fuel cells (SOFCs). In the present study, the YSZ/LSM composite coating was deposited by atmospheric plasma spray (APS) using the mechanical blending LSM and YSZ with ratios of 50:50, 40:60, and 20:80 wt.%. The electrical conductivity of the composite coating was measured by the means of direct current (DC) measurement in the temperature range of 500-900 °C. The electrical conductivity of the YSZ-50%LSM coating ranged from 2.17 to 3.60 S/cm along the direction parallel to the coating surface at the temperature range. For the same specimen, the electrical conductivity perpendicular to the plane is less than one-tenth of that in the plane. The anisotropy of the electrical conductivity is attributed to the phases of different properties in the composite coating and the APS coating structure characteristics. The results also showed that the electrical conduction of the composite was strongly influenced by the YSZ content. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.