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A solid oxide fuel cell (SOFC) research capability has been developed that facilitates measuring the electrochemical performance of an operating SOFC while simultaneously performing x-ray diffraction on its cathode. The evolution of this research tool’s development is discussed together with a description of the instrumentation used for in-situ x-ray diffraction (XRD) measurements of operating SOFC cathodes. The challenges that were overcome in the process of developing this capability, which included seals and cathode current collectors, are described together with the solutions that are presently being applied to mitigate them.

A reactive air brazing (RAB) technique utilizing silver-copper oxide (Ag-CuO) alloys has previously been developed for joining ceramics components used in high temperature devices ranging from oxygen separation membranes, gas turbines and combustion engines. The application of the Ag-CuO system as a brazing material is limited by its solidus and liquidus temperatures, which are known to be in the range of 935 C and 967 C. Some joined ceramic components may be used in devices, which require further processing steps, or may be used in applications, that exceed these temperatures. It has been found that the addition of palladium to the silver copper oxide system will increase solidus and liquidus temperatures of the resulting alloy. In our work, we are studying the effects of palladium addition on the wetting properties of Ag-CuO braze system on alumina. Quality of brazing is evaluated through microstructural analysis and bending strength of brazed joints created with alumina. The presentation will include processing, and characterization of Ag-CuO brazed system with and without palladium addition on alumina.

Recently a new method of ceramic brazing has been developed. Based on a two-phase liquid composed of silver and copper oxide, brazing is conducted directly in air without the need of an inert cover gas or the use of surface reactive fluxes. Because the braze displays excellent wetting characteristics on a number ceramic surfaces, including alumina, various perovskites, zirconia, and ceria, we were interested in investigating whether a metal-reinforced ceramic matrix composite (CMC) could be developed with this material. In the present study, two sets of homogeneously mixed silver/copper oxide/ceria powders were synthesized using a combustion synthesis technique. The powders were compacted and heat treated in air above the liquidus temperature for the chosen Ag-CuO composition. Metallographic analysis indicates that the resulting composite microstructures are extremely uniform with respect to both the size of the metallic reinforcement as well as its spatial distribution within the ceramic matrix. The size, morphology, and spacing of the metal particulate in the densified composite appears to be dependent on the original size and the structure of the starting combustion synthesized powders.

A new brazing technique is currently being developed specifically for sealing solid state electrochemical devices such as planar solid oxide fuel cells (pSOFCs). Referred to as air brazing, this metal-based joining technique is unique in that it can be conducted directly in air without the use of inert gases or fluxing agents to prevent/remove surface scale during the joining process. To date, several developmental silver-based filler metal compositions have shown good joint strength, excellent stability under high-temperature oxidizing conditions, and superior thermal cycling performance in comparison with glass-based seals. However, there are concerns about exposing silver alloys to a high-temperature, hydrogen-rich environment, particularly with respect to potential embrittlement under these conditions. This paper discusses initial results obtained from a series of experiments designed to characterize the effect of simultaneous hydrogen/air exposure on silver-based air brazes at high temperature.

Complex high temperature devices such as planar solid oxide fuel cell (pSOFC) stacks often require a two-step sealing process. For example, in pSOFC stacks the oxide ceramic fuel cell plates might be sealed into metallic support frames in one step. Then the frames with the fuel plates sealed to them would be joined together in a separate sealing step to form the fuel cell stack. In this case, the initial seal should have a sufficiently high solidus temperature that it will not begin to remelt at the sealing temperature of the material used for the subsequent sealing step. Previous experience has indicated that, when heated at a rate of 10°C/min, Ag-CuO reactive air braze (RAB) compositions have solidus and liquidus temperatures in the approximate range of 925 to 955°C. Therefore, compositionally modifying the original Ag-CuO braze with Pd-additions such that the solidus temperature of the new braze is between 1025 and 1050°C would provide two RAB compositions with a difference in melting points large enough to allow reactive air brazing of both sets of seals in the fuel cell stack. This study determines the appropriate ratio of Pd to Ag in RAB required to achieve a solidus in the desiredmore » range and discusses the wettability of the resulting Pd-Ag-CuO brazes on YSZ substrates. The interfacial microstructures and flexural strengths of Pd-Ag-CuO joints in YSZ will also be presented.« less

One of the key limiting issues in designing and fabricating a high performance planar solid oxide fuel cell (pSOFC) stack is the development of the appropriate materials and techniques for hermetically sealing the metal and ceramic components. There are essentially two standard methods of sealing: (1) by forming a rigid joint or (2) by constructing a compressive “sliding” seal. While short-term success has been achieved with both techniques, it is apparent that to meet the long-term operational needs of stack designers, alternative sealing concepts will need to be conceived. Described below are two alternative pSOFC sealing methods that have been developed at Pacific Northwest National Laboratory.

Mixed ionic/electronic conducting perovskite oxides such as lanthanum strontium cobalt ferrite (LSCF) are strong candidates for potential use in a number of electrochemical devices, including gas separation membranes and solid oxide fuel cells (SOFC). Underlying the excitement over the these novel ceramics is the engineering challenge of effectively incorporating them into practical devices. Taking full advantage of the unique properties of advanced ceramics such as mixed conducting oxides depends in large part on being able to develop reliable joining techniques. Earlier studies have indicated that Ag-CuO reactive air braze (RAB) compositions are effective in joining to LSCF. Meanwhile, it has been found that small additions of as little as 0.5 mol% titanium oxide to Ag-CuO RAB compositions cause a dramatic increase in the wettability of RAB on many oxide ceramic surfaces. Therefore the wettabilty of Ag-CuO-TiO2 brazes on LSCF substrates will be examined and the flexural strength, microstructure, and conductivity of joints in LSCF made using Ag-CuO-TiO2 brazes will be discussed. Long-term aging effects on conductivity and microstructure will also be presented.

Developing reliable, hermetic ceramic-to-metal seals has become a key issue to the future success of planar solid oxide fuel cell (pSOFC) technology. In the course of designing and fabricating operating pSOFC stacks at Pacific Northwest National Laboratory over the past several years, we have relied on a simple, but extremely useful seal testing technique that allows us to screen through the numerous variables involved in developing glass seals. Here we discuss the design of the test apparatus and the procedure we employed when using the test to analyze the effects of various processing parameters on the hermeticity and durability of our glass seals.

Reactive air brazing (RAB) technique was developed as an effective alternative for the joining of complicated ceramic parts. The most important advantage of RAB over conventional active metal brazing is that joining operation of RAB technique can be conducted without using controlled atmosphere. It has been reported by us that the reactive component (copper) in the Ag-CuO braze is reactively to modify faying surfaces of alumina, improving the wettability with the oxide and potentially increasing bond strenth between braze and ceramics. In this work, the effects of CuO content on wetting behavior of Ag-CuO braze with yttria-stabilized zirconia (YSZ) substrates and the mechanical properties of brazed YSZ have been investigated. The results of this study to date will be discussed.

Mixed ionically/electrically conducting oxides such as lanthanum calcium manganate are currently being investigated for use as electrodes in a number of high temperature devices, including solid oxide fuel cells. Traditionally, glass seals have been employed as a means of hermetically joining the ceramic components that lie at the heart of these devices to the heat resistant metal frame that gives the device its structural integrity. The present paper outlines an alternative brazing technique for joining these dissimilar materials and discusses recent findings regarding the wettability of the braze on the two joining surfaces, as well as the strength of the resulting joint.

Anode-supported solid oxide fuel cells (SOFCs) were prepared and tested identically except for variations in the A-site stoichiometry of the LSCF cathode. A commercial supplier provided lanthanum strontium cobalt ferrite (LSCF) powders that were stoichiometric [La0.6Sr0.4Co0.2Fe0.8O3-δ], A-site deficient [(La0.6Sr0.4)1-xCo0.2Fe0.8O3-δ], and Sr-deficient [La0.6Sr0.4-xCo0.2Fe0.8O3-δ], in which the imposed deficiency (x) was 0.02, 0.05, and 0.1 in the non-stoichiometric cases. Multiple 1,000 hour tests were run using each LSCF cathode composition. The performance and stability of the cells will be discussed.

A significant challenge in developing and manufacturing planar solid oxide fuel cells is in hermetically sealing the electrochemically active ceramic PEN to the metallic body of the device. One means of doing this is by chemically bonding the electrolyte surface of the PEN to the metal frame, thus forming a fixed or static seal. Typically a glass or glass-ceramic material is used in making this type of seal. However because of their brittle nature, there are number of manufacturing and operational problems that are encountered when using these materials. In an effort to efficiently screen potential solutions to these problems, a seal rupture test was developed. Details of the test procedure, the test device, and its use in thermal cycle testing various planar solid oxide fuel cell sealing concepts will be discussed.

We have developed a deformable seal for planar solid oxide fuel cells (pSOFCs) that can accommodate significant thermal mismatch between the adjoining components and still remain hermetic. Essentially composed of a thin stamped metal foil bonded to both sealing surfaces, the seal offers a quasi-dynamic mechanical response to thermally generated stresses. It remains well adhered to both faying surfaces, i.e. non-sliding, but readily yields or deforms under modest thermo-mechanical loading, thereby mitigating the transfer of these stresses to the adjacent ceramic and metal components. Initial thermal testing demonstrates that the seal retains its original hermeticity and strength after a number of rapid cycles between {approx}75 C and 750 C.

The feasibility of brazing LSCoF ceramic substrates with silver-vanadium pentoxide (Ag-V{sub 2}O{sub 5}) alloys for use in solid oxide fuel cell (SOFC) applications was studied. Preliminary testing was also performed on the Ag-V{sub 2}O{sub 5}/alumina (Al{sub 2}O{sub 3}) system in order to determine the feasibility of brazing LSCoF to Fecralloy{trademark} with an Ag-V{sub 2}O{sub 5} alloy since an Al{sub 2}O{sub 3} layer is formed on the Fecralloy surface prior to brazing. Sessile drop tests were performed using liquid Ag with 1 to 20 weight percent (wt%) V{sub 2}O{sub 5} additions at either 1000 or 1100 C. Similar to previous results for Ag-CuO alloys, additions of V{sub 2}O{sub 5} resulted in a decrease in the apparent contact angle and a transition from non-wetting to wetting behavior for both substrate materials. Mechanical test samples were fabricated by brazing using Ag with 1, 2 and 5 wt% V{sub 2}O{sub 5} additions. For the Ag-V{sub 2}O{sub 5}/LSCoF samples, the joint fracture strengths ranged from 10 to 30% of the monolithic LSCoF fracture strength ({sigma}{sub joint} = 26 {+-} 9 MPa versus {sigma}{sub LSCoF} = 151 {+-} 24 MPa). An SEM/EDS microstructural analysis of the brazed cross-sections indicated the formation of a vanadium rich reactionmore » product at the braze/LSCoF interface. It is hypothesized that the reaction product provided improved chemical bonding at the interface that resulted in the modest joint strengths. In the the Ag-V{sub 2}O{sub 5}/Al{sub 2}O{sub 3} system, although the contact angles were less than 90{sup o}, all of the brazed Al{sub 2}O{sub 3} samples failed to bond during brazing or fractured during sample preparation. These poor adhesion results indicate potential problems with the Fecralloy{trademark}/Ag-V{sub 2}O{sub 5}/LSCoF system.« less

The feasibility of joining porous alumina (Al{sub 2}O{sub 3}) bodies to monolithic Al{sub 2}O{sub 3} using Ag-CuO and Ag-V{sub 2}O{sub 5} alloys via reactive air brazing (RAB) was examined for a nanoporous filter application. Brazing for these systems is complicated by the conflicting requirements of satisfactory wetting to fill the braze gap, while minimizing the infiltration of the porous body. By varying the firing time, temperature, and initial powder size, porous bodies with a range of pore microstructures were fabricated. The wettability was evaluated via sessile drop testing on monolithic substrates and porous body infiltration. Porous Al{sub 2}O{sub 3}/monolithic Al{sub 2}O{sub 3} brazed samples were fabricated, and the microstructures were evaluated. Both systems exhibited satisfactory wetting for brazing, but two unique types of brazing behavior were observed. In the Ag-CuO system, the braze alloy infiltrated a short distance into the porous body. For these systems, the microstructures indicated satisfactory filling of the brazed gap and a sound joint regardless of the processing conditions. The Ag-V{sub 2}O{sub 5} alloys brazed joints exhibited a strong dependence on the amount of V{sub 2}O{sub 5} available. For Ag-V{sub 2}O{sub 5} alloys with large V{sub 2}O{sub 5} additions, the braze alloy aggressively infiltrated the porousmore » body and significantly depleted the Ag from the braze region resulting in poor bonding and large gaps within the joint. With small additions of V{sub 2}O{sub 5}, the Ag infiltrated the porous body until the V{sub 2}O{sub 5} was exhausted and the Ag remaining at the braze interlayer bonded with the Al{sub 2}O{sub 3}. Based on these results, the Ag-CuO alloys have the best potential for brazing porous Al{sub 2}O{sub 3} to monolithic Al{sub 2}O{sub 3}.« less

A standard combustion-based approach typically used to synthesize nanosize oxide powders has been modified to prepare composite oxide-metal powders for subsequent densification via sintering or hot-pressing into ceramic- or metal-matrix composites. Copper and cerium nitrate salts were dissolved in the appropriate ratio in water and combined with glycine, then heated to cause autoignition. The ratio of glycine-to-total nitrate concentration was found to have the largest effect on the composition, agglomerate size, crystallite size, and dispersivity of phases in the powder product. After consolidation and sintering under reducing conditions, the resulting composite compact consists of a well-dispersed mixture of sub-micron size reinforcement particles in a fine-grained matrix.

Aluminum foils were used to braze ceramic plates directly in air. Since aluminum forms an inert oxide layer, aluminum air brazing can provide significant benefits in ceramic joining such as chemical tolerance in dual atmospheres. In using aluminum to air braze, the key factor is optimizing the brazing conditions in order to obtain good interface between braze and substrates by controling the kinetics of melting and oxidation of aluminum and sintering of oxide scale. In this study, joint strength was found to be largely dependent on brazing conditions such as temperature and hold time. Effects of these conditions on the mechanical properties and the microstructure of the Al-brazed ceramic joints will be discussed in this paper.