The effect of solids and dispersant loadings on the suspension viscosities and deposition rates of suspension plasma sprayed YSZ coatings

Suspension plasma spraying (SPS) is a promising modification of traditional plasma spraying techniques that uses small (≤ 2 μm) particles suspended in a liquid to fabricate coatings with fine microstructures and controlled porosity rapidly and without the need for post-deposition heat treatments. These qualities make SPS an interesting new technique to manufacture solid oxide fuel cell (SOFC) active layers. However, in order to be able to manufacture layers with good microstructures, the properties of the feedstock suspension must be optimized to enhance particle dispersion and improve feedability. This study uses a pressurized gas delivery system to feed aqueous YSZ suspensions containing an organic dispersant to a Northwest Mettech Axial III axial injection suspension plasma spray system. Three different dispersant types (polyacrylic acid (PAA), polyethylene imine (PEI) and 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTCA)) were characterized and the effects of solid loadings, dispersant type, and dispersant concentration on suspension properties such as viscosity and feedability, and layer characteristics such as microstructure and deposited thickness were examined.     

Suspension Plasma Sprayed Sr<Subscript>2</Subscript>Fe<Subscript>1.4</Subscript>Mo<Subscript>0.6</Subscript>O<Subscript>6−<Emphasis Type="Italic">δ</Emphasis></Subscript> Electrodes for Solid Oxide Fuel Cells

In this study, suspension plasma spraying (SPS) was applied to deposit double perovskite Sr₂Fe_1.4Mo_0.6O_6?δ (SFM) which can be used as both cathode and anode for solid oxide fuel cells. The effects of SFM concentration on the electrode phase composition, microstructure, and catalytic performance were investigated. The electrodes showed a dense structure when it was deposited at a concentration of 0.05 mol/L. The cathode performance was limited by the limited three-phase boundaries and poor gas diffusion. At 750 °C, cathode polarization (R _pc) was 0.19 Ω cm². When the SFM concentration increased to 0.075 mol/L, the deposits revealed a porous microstructure with well-bonded fine particles. As a result, the R_pc decreased significantly to 0.078 Ω cm² at 750 °C. However, when the SFM concentration was further increased to 0.1 mol/L, the R _pc increased owing to the limited interface bonding between the non-molten particles. As a result, it was found that the SFM suspension concentration should be optimized to achieve a highly active SFM by SPS process. Moreover, when the optimized deposit was employed as an anode and tested in a hydrogen atmosphere, it showed anode polarization resistance (R_pa) of 1.5 Ω cm² at 750 °C.     

先进陶瓷涂层结构调控及其在固体氧化物燃料电池中的应用

等离子喷涂技术可以对陶瓷涂层的微观结构进行调控设计,因此在制备固体氧化物燃料电池方面具有独特的优势。基于等离子喷涂方法,可以直接制备或经过后处理获得致密的电解质涂层。采用等离子喷涂技术也可以制备高性能的多孔阳极和阴极,并可对钙钛矿结构阴极材料的成分和晶体结构进行调控。文中介绍了目前国内外采用涂层制备电池的方法,主要探讨了热喷涂方法制备电解质涂层的特点,对存在的问题和可行思路进行了讨论,并探讨了基于提高三相反应界面长度来制备高性能电极的方法。由于固体氧化物燃料全电池各功能层都有可能通过热喷涂方法制备,因此该方法在固体氧化物燃料电池结构设计具有巨大的潜力。     

Suspension Plasma Spray and Performance Characterization of Half Cells with NiO/YSZ Anode and YSZ Electrolyte

The use of a liquid feedstock carrier in suspension plasma spray (SPS) permits injection of fine powders, providing the possibility of producing sprayed coatings that are both thin and dense and have fine microstructures. These characteristics make SPS an attractive process for depositing highly efficient electrodes and electrolytes for solid oxide fuel cell (SOFC) applications. In this study, NiO-yttria stabilized zirconia (YSZ) anode and YSZ electrolyte half cells were successfully deposited on porous Hastelloy X substrates by SPS. The NiO-YSZ anode deposition process was optimized by design of experiment. The YSZ electrolyte spray process was examined by changing one parameter at a time. The results from the design-of-experiment trials indicated that the porosity of the as-deposited coatings increased with an increase of suspension feed rate while it decreased with an increase of total plasma gas flow rate and standoff distance. The deposition rate increased with an increase of total plasma gas flow rate, suspension feed rate, and standoff distance. The microstructure examination by SEM showed that the NiO and YSZ phases were homogeneously distributed and that the YSZ phase had a lamellar structure. It was observed that the density of the YSZ electrolyte layer increased as input power of the plasma torch increased. Electrochemical characterization of the fabricated cells indicated that an open cell voltage of 0.989 V at 500 °C and a peak power of 0.610 W/cm² at 750 °C were reached.     

Microstructure and Electrochemical Behavior of a Structured Electrolyte/LSM-Cathode Interface Modified by Flame Spraying for Solid Oxide Fuel Cell Application

Plasma-sprayed yttria-stabilized zirconia (YSZ) can be potentially employed as electrolyte layers in solid oxide fuel cells (SOFCs). The formation of a structured electrolyte surface characterized by convex micro deposits generated by only partially molten particles at spraying will increase the specific surface area and subsequently improve the output performance of SOFCs. However, using completely molten YSZ particles during plasma spraying leads to the formation of locally flat surface. In this study, flame spraying was employed to deposit YSZ particles on YSZ substrate using surface-melted particles. The deposition was carried out at different spray distances on YSZ substrate preheated to 650 °C. The surface and cross-section morphology of YSZ particles were characterized by SEM. The electrochemical behavior of single cell with the structured cathode was characterized by the electrochemical impedance spectroscopy. The results show that spray distance exhibits significant influence on the morphology of deposited YSZ particles. The cathode polarization of a structured cathode was decreased by about 30-43% compared to a flat cathode at different temperatures.     

Controlling Spray Torch Fluid Dynamics—Effect on Spray Particle and Coating Characteristics

In order to identify means to improve plasma spray consistency, various modifications to the design of a commercial plasma torch nozzle have been investigated. The modifications consist of preparing anode inserts with grooves in the axial direction (spline insert), and introducing a fraction of the plasma gas through a ring of micro-nozzles surrounding the anode nozzle (micro-jet ring). Different designs for each modification have been investigated, and these modifications have also been paired with a modified upstream gas injector. For each of the modified designs, a wide range of characteristics have been measured for the arc, the plasma jet, the in-flight particles, and the coating. The results show that most nozzle modifications lead to higher particle temperatures and velocities. The plasma jet is significantly elongated by using some of the modified nozzles, and the cold gas entrainment somewhat reduced. Each of the nozzle modifications can be easily implemented offering an economical way to enhance process reliability.     

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.     

双丝电弧喷涂中粒子交叉飞行行为的在线测量及分析

采用两根具有不同熔点的材料分别作为双丝电弧喷涂的阳极和阴极丝材,采用SprayWatch-2i热喷涂监测系统,通过对融熔粒子飞行中温度的在线测量分析,直接验证了粒子的＂交叉飞行＂现象.利用能谱仪（EDS）定量研究了两极的材料和氧化物在喷涂沉积丘中的分布,并分析了这些分布特征在不同喷涂距离上的变化规律,结果发现,沿喷涂丘的横截面,来自阴极和阳极的扁平化粒子以接近反对称的方式分布.在所测量的两个喷涂距离上,各种成分（包括氧化物）的含量随喷涂距离变化不大.而且电弧喷涂涂层中具有约10 %左右的氧化物含量.     

喷嘴形状对Al2O3-3TiO2粒子扁平化及其涂层性能的影响*

采用SprayWatch在线监测系统测量了F6大气等离子喷枪在不同喷嘴条件下产生的等离子射流中Al2O3-3TiO2粒子的温度和速度。利用201不锈钢和Q235钢作为基体,分别用来收集粒子和制备涂层。分析了不同喷嘴对飞行粒子温度和速度的影响,并通过扫描电镜(SEM)对扁平粒子的铺展程度和涂层显微组织进行了分析,并对比了涂层的结合强度、显微硬度和磨损失重量的差异。结果表明:在相同的测量位置,圆柱形喷嘴喷出粒子的速度比Laval喷嘴条件下的高出一倍,但是温度比Laval喷嘴条件下略低。圆柱形喷嘴获得的扁平粒子比Laval喷嘴获得的扁平粒子铺展程度要大;圆柱形喷嘴获得的涂层的孔隙率及磨损失重量比Laval喷嘴制备的小,其涂层的结合强度、显微硬度均高于Laval喷嘴制备的涂层。     

Characteristics of a Plasma Torch Designed for Very Low Pressure Plasma Spraying

Unlike atmosphere plasma spraying (APS), very low pressure plasma spraying (VLPPS) can only weakly heat the feed materials at the plasma-free region exit of the nozzle. Most current VLPPS methods have adopted a high power plasma gun, which operates at high arc currents up to 2500?A to remedy the lower heating ability, causing a series of problems for both the plasma torch and the associated facility. According to the Kundsen number and pressures distribution inside of the nozzle in a low-pressure environment, a plasma torch was designed with a separated anode and nozzle, and with the powder feed to the plasma jets inside the nozzle intake. In this study, the pressures in the plasma gas intake, in the nozzle intake and outside the plasma torch were measured using an enthalpy probe. For practice, SUS 316 stainless steel coatings were prepared at the plasma currents of 500-600?A, an arc voltage of 50?V and a chamber pressure of 1000?Pa; the results indicated that coatings with an equiaxed microstructure could be deposited?in proper conditions.     

Effect of Powder Injection Location on Ceramic Coatings Properties When Using Plasma Spray

The effect of powder injecting location of the plasma spraying on spraying properties was studied. Three different powder-injecting methods were applied in the experiment. In the first method, the particles were axially injected into the plasma flow from the cathode tip. In the second method, the particles were radially injected into the plasma flow just downstream of the anode arc root inside the anode nozzle. In the third method, the particles were radially injected into the plasma jet at the nozzle exit. The alumina particles with a mean diameter of 20 μm were used to deposit coatings. Spraying properties, such as the deposition efficiency, the melting rate of the powder particles, and the coating quality were investigated. The results show that the spraying with axial particle injecting can heat and melt the powder particles more effectively, produce coatings with better quality, and have higher deposition efficiency. 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.     

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.     

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。     

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.     

Induction Plasma Synthesis of Nano-Structured SOFCs Electrolyte Using Solution and Suspension Plasma Spraying: A Comparative Study

In this paper, two plasma spraying technologies: solution plasma spraying (SolPS) and suspension plasma spraying (SPS) were used to produce nano-structured solid oxide fuel cells (SOFCs) electrolytes. Both plasma spraying processes were optimized in order to achieve the thin gas-tight electrolytes. The comparison of the two plasma spraying processes is based on electrolyte phase, microstructure, morphology, as well as on plasma deposition rate. The results show that nano-structured thin electrolytes (~5 μm thick) have been successfully SPS deposited on porous anodes with a high deposition rate. Compared to the electrolytes produced by SolPS, the SPS-deposited electrolyte layer is much denser. During the SPS process, fine droplets of 0.5-1 μm in diameter impact on the surface of the coating and penetrate into the pores of the anode. As the stresses are reduced on the resulting 0.5-2 μm splats, there is no apparent microcracks network on the splats, this resulting in highly gas-tight coatings. It is demonstrated that the SPS process is beneficial for the improvement of the performance of the films to be used as SOFC electrolytes.     

Asymmetric Melting Behavior in Twin Wire Arc Spraying with Cored Wires

Asymmetric melting behavior of the electrodes is a process-related feature of the Twin Wire Arc Spraying (TWAS) technique since the heating of the negative connected wire is different from that of the positive connected wire. Due to these differences in melting behavior, a tracking of particle velocity and temperature for both electrodes individually is very important. Particle velocity and temperature have been recorded from anode side and cathode side by positioning the tracking device respectively. To draw the whole picture of the spraying, jet, particles have been tracked also from the top side of the spray gun. The goal of this study is to have an experimental data setup for model building and simulation of depositing process in TWAS. Corresponding measuring devices have been employed to investigate the TWAS process by spraying of massive and cored wires.     

Cold Spray Deposition of Copper Electrodes on Silicon and Glass Substrates

Copper lines with widths varying from 150 to 1500 μm were deposited onto crystalline silicon wafers and soda-lime glass plates by cold spraying copper particles with 1 μm average diameter through a mask. This direct deposition method yielded high-aspect-ratio electrodes with minimum shadowing effects and maximum electrode-to-silicon contact area. The copper lines had triangular cross sections with aspect ratios (height/width) ranging from 0.1 to 1.1, depending on the number of spray gun passes. Copper particles were densely packed with increasing the width of the masking slit. This study presents the potential use of the cold spray technology in printing lines as front electrodes in solar cell applications.     

Highly Segmented Thermal Barrier Coatings Deposited by Suspension Plasma Spray: Effects of Spray Process on Microstructure

Effects of the ceramic powder size used for suspension as well as several processing parameters in suspension plasma spraying of YSZ were investigated experimentally, aiming to fabricate highly segmented microstructures for thermal barrier coating (TBC) applications. Particle image velocimetry (PIV) was used to observe the atomization process and the velocity distribution of atomized droplets and ceramic particles travelling toward the substrates. The tested parameters included the secondary plasma gas (He versus H₂), suspension injection flow rate, and substrate surface roughness. Results indicated that a plasma jet with a relatively higher content of He or H₂ as the secondary plasma gas was critical to produce highly segmented YSZ TBCs with a crack density up to ~12 cracks/mm. The optimized suspension flow rate played an important role to realize coatings with a reduced porosity level and improved adhesion. An increased powder size and higher operation power level were beneficial for the formation of highly segmented coatings onto substrates with a wider range of surface roughness.