Theses and Dissertations at Montana State University (MSU)
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Item Aluminate spinels for use as catalyst enhancement of solid oxide fuel cells(Montana State University - Bozeman, College of Engineering, 2019) Zachariasen, Marley Sarria; Chairperson, Graduate Committee: Stephen W. SofieThe growing necessity to find clean, efficient power sources has led to the advancement of technology in various fields of renewable energy. The field of electrochemical energy conversion, better known has Hydrogen Fuel Cell energy, has shown promise in replacing fossil fuels. This technology is fuel flexible, emits no harmful products, and generates power at efficiencies double or triple that of the Carnot combustion cycle widely used in automotive propulsion and large scale combustion power generation. However, the power production is limited by the short life expectancy of the components used to convert the chemical energy of the fuel into an electrical current. Two mechanisms work simultaneously during fuel cell operation to degrade the anodic electrode of the cell. The coarsening of the catalyst metal particles reduces the total active area of the anode while contaminants from the fuel deposit on the anodes remaining active areas, blocking fuel from the locations where the reaction takes place. Recent studies have shown that doping the industry standard fuel cell anode, Ni/YSZ, with a compound known as Aluminum Titanate (ALT) increases the overall resiliency of the cell. When heat-treated, ALT disassociates in to aluminum and titanium oxides which are then able to go into solution with the material components of the anode. These new secondary phases were shown to increase the strength and overall power output of the cell while decreasing the rate at which the catalyst coarsens. The electrochemical enhancements were attributed to the aluminum based secondary phase, known as nickel aluminate, a spinel structured compound which undergoes unusual reduction and catalytic transport kinetics. This work assesses the viability of transferring these enhancement effects to various other cermet anode systems by individually exchanging the ceramic ion conductor and metal electrocatalyst. The electrochemical performance and degradation, as well as mechanical properties, were evaluated for Ni/GDC anodes doped with ALT and alumina. In addition, synthesis and reduction behavior of cobalt and copper aluminate spinels were analyzed for similarities with nickel aluminate.Item Investigation of the mechanical properties of aluminum titanate (Al 2TiO 5) doped NI-YSZ solid oxide fuel cell anodes(Montana State University - Bozeman, College of Engineering, 2019) McCleary, Madisen Wynn; Chairperson, Graduate Committee: Roberta Amendola; Roberta Amendola was a co-author of the article, 'Effect of aluminum titanate (Al 2TiO 5) doping on the mechanical performance of solid oxide fuel cell Ni-YSZ anode' in the journal 'Fuel Cells' which is contained within this thesis.; Roberta Amendola was a co-author of the article, 'Investigation of the kinetics of the solid-state reduction process of undoped and aluminum titanate (Al 2TiO 5) doped NiO-YSZ anodes for solid oxide fuel cells' in the journal 'Ceramics international' which is contained within this thesis.; Roberta Amendola, Stephen Walsh and Benjamin McHugh were co-authors of the article, 'Analysis of the mechanical strength and failure mode of undoped and aluminum titanate (Al 2TiO 5, ALT) doped-Ni-YSZ solid oxide fuel cell anodes under uniaxial and biaxial strength testing conditions' submitted to the journal 'Materialia' which is contained within this thesis.; Roberta Amendola and Benjamin McHugh were co-authors of the article, 'Effect of redox cycling on the mechanical performance of undoped and aluminum titanate (Al 2TiO 5, ALT) doped NiO-YSZ solid oxide fuel cell anodes' which is contained within this thesis.; Roberta Amendola was a co-author of the article, 'Mechanical performance of aluminum titanate (Al 2TiO 5, ALT) doped NiO-YSZ SOFC half cells' which is contained within this thesis.Recently, there has been growing interest in anode supported Solid Oxide Fuel Cells (SOFCs) because of improved single cell performance. In these systems, the anode layer is the thickest and provides the mechanical strength of the stack. Nickel-yttria stabilized zirconia (Ni-YSZ) composites are widely used as anode material but, in contrast to the vast amount of data available on their electrochemical properties, little data on the mechanical performance exists. This dissertation work focuses on the use of secondary materials added to traditional Ni-YSZ anodes to enhance SOFC anode mechanical performance. Small amounts of, aluminum titanate (Al2TiO5, ALT), added to the NiO-YSZ system during the manufacturing process, results in a material that is over 50% stronger than the native Ni-YSZ. Samples with different geometries have been fabricated and tested in uniaxial and biaxial strength testing apparatuses. Advanced microscopy techniques and Weibull statistical analyses have been used to properly characterize the mechanical performance, the failure mechanism and to elucidate chemical compositions. This work has found that the enhanced strength resulting from ALT is related to the development of secondary phases: Al2O3 reacts with NiO to form NiAl2O4 while TiO2 preferentially reacts with YSZ to form a solid YSZ framework defined as the 'rough phase' that add stiffness to the system and persists upon reduction. The mechanical behavior of reduced samples has been related to the partial reduction of NiAl2O4 which results in the formation of Ni nanoparticles within an Al2O3 matrix ('small particle phase'). This phase is characterized by a high strength interface while adding ductility and crack deflection ability to the system. ALT was also found responsible for changing the Ni-YSZ system failure mechanism from an intergranular to a transgranular fashion indicating the material toughness increased. During cyclic operational testing, ALT has potential for mechanical stabilization through porosity development with secondary phase formation. Testing of ALT anodes with YSZ electrolyte material showed increased strength over similar native assemblies. This dissertation work lays the foundation for future research into the effects of ALT doping on the SOFC system and how this material could be tailored for even further increases in strength.Item Fabrication and assessment of anode supported SOFCS doped with aluminum titanate via electrochemical and non-destructive micro-indentation testing(Montana State University - Bozeman, College of Engineering, 2019) Kent, John Parker; Chairperson, Graduate Committee: Stephen W. SofieCeramic-metal (cermet) composites are the most promising electrochemical anodes for commercial implementation in solid oxide fuel cells (SOFC). Recent advances at MSU in cermet formulations utilizing aluminum titanate (ALT) dopants in nickel oxide (NiO)-yttria stabilized zirconia (YSZ) anodes has shown substantial performance gains in degradation rates as well as mechanical behavior when evaluated in low power density electrolyte supported cell (ESC) geometries and bulk anode forms through modulus of rupture and equibiaxial flexure. The benefits associated with ALT are due to the formation of secondary phases of nickel aluminate and zirconium titanate in NiO-YSZ cermets that form during processing. Cermet modulus of rupture studies are rigorous, can span multiple months, and requiring hundreds of samples when studying the effects of both thermal and redox cycling on SOFC anodes to achieve statistically significant results. The use of non-destructive methods such as micro-indentation to examine the strength and toughness of doped and differently processed cermet anodes can rapidly speed up the analysis of mechanical properties including the mechanical support characteristics of higher power density anode supported cell (ASC) geometries targeted by industrial SOFC developers. The aim of this study was to examine non-destructive micro-indentation testing in evaluating cermet anode materials in both oxidized and reduced state in direct contrast with traditional destructive methods. Extending the current state of ALT anode doping by utilizing these rapid assessment methods, this work examines mechanical properties degradation and fracture toughness under multiple thermal and redox cycles. Additionally, this work details the framework for cell fabrication methods that were developed to process ASCs with state of the art 5 micrometer electrolytes for the first evaluation of ALT doping of SOFCs in this high power cell configuration.Item Sintering in ceramics and solid oxide fuel cells(Montana State University - Bozeman, College of Engineering, 2017) Hunt, Clay Dale; Chairperson, Graduate Committee: Stephen W. Sofie; David Driscoll, Adam Weisenstein and Stephen W. Sofie were co-authors of the article, 'Nickel nitrate and molybdenum oxide as a yttria-stabilized zirconia sintering aid' in the journal 'Processing, properties, and design of advanced ceramics and composites' which is contained within this thesis.; Marley Zachariasen, David Driscoll and Stephen W. Sofie were co-authors of the article, 'Current degradation rate quantification of solid oxide fuel cells with and without aluminum titanate' which is contained within this thesis.; David Driscoll and Stephen W. Sofie were co-authors of the article, 'Constant rate of heating definition of the undefined function of density of the Wang and Raj equation for 8YSZ' which is contained within this thesis.; David Driscoll and Stephen W. Sofie were co-authors of the article, 'Constant rate of heating definition of undefined density function for 8YSZ with a sintering aid' which is contained within this thesis.; David Driscoll and Stephen W. Sofie were co-authors of the article, 'Constant temperature definition of the undefined density function for 8YSZ' which is contained within this thesis.; David Driscoll and Stephen W. Sofie were co-authors of the article, 'Constant temperature definition of the undefined density function of 8YSZ with a sintering aid' which is contained within this thesis.Nature's propensity to minimize energy, and the change in energy with respect to position, drives diffusion. Diffusion is a means by which mass transport resulting in the bonding of the particles of a powder compact can be achieved without melting. This phenomenon occurs in powdered materials near their melting temperature, and is referred to as 'sintering'. Because of the extreme melting temperature of some materials, sintering might be the only practical means of processing. The complexity and subtlety of sintering ceramics motivated the evaluation of empirical data and existing sintering models. This project examined polycrystalline cubic-zirconia sintering with and without transition-metal oxide additions that change sintering behavior. This study was undertaken to determine how sintering aids affect the driving force, and activation energy, the energy barrier that must be overcome in order for an atom or ion to diffuse, of the densification occurring during sintering. Examination of commercially-available cubic-zirconia powder sintering behavior was undertaken with dilatometry, which allows monitoring of the length change a material undergoes as it sinters, and with scanning electron microscopy, which facilitates the study of sintered-sample microstructure. MATLAB algorithms quantifying sintering results were developed. Results from this work include proposed definitions of a 26-year-old undefined function of density factor in a well-accepted mathematical model of sintering. These findings suggest activation energy is not changing with density, as is suggested by recent published results. The first numerical integration of the studied sintering model has been performed. With these tools, a measure of the activation energy of densification of cubic-zirconia with and without the addition of cobalt-oxide as a sintering aid has been performed. The resulting MATLAB algorithms can be used in future sintering studies. It is concluded that sintering enhancement achieved with cobalt-oxide addition comes from reduction in activation energy of densification of cubic-zirconia. Further, it is suggested that the activation energy of densification does not change with material density. This conclusion is supported by the sensitivity of the numerical integration of the aforementioned sintering model to changes in activation energy.Item Stabilization of metallic catalyst microstructures against high-temperature thermal coarsening(Montana State University - Bozeman, College of Engineering, 2016) Driscoll, David Robert; Chairperson, Graduate Committee: Stephen W. Sofie; Clay D. Hunt, Julie E. Muretta and Stephen W. Sofie were co-authors of the article, 'Thermally stabilized nickel electro-catalyst introduced by infiltration for high temprature electrochemical energy conversion' in the journal 'Transactions of the Electrochemical Society' which is contained within this thesis.; Cameron H. Law and Stephen W. Sofie were co-authors of the article, 'Design and synthesis of metallic nanoparticle-ceramic support interfaces for enhancing thermal stability' in the journal 'Ceramic transactions' which is contained within this thesis.; Stephen W. Sofie was a co-author of the article, 'Stabilization of nano-scale metallic microstructure against thermal coarsening' in the journal 'Ceramic transactions' which is contained within this thesis.; Melissa D. McIntyre, Martha M. Welander, Stephen W. Sofie and Robert A. Walker were co-authors of the article, 'Enhancement of high temperature metallic catalysts : aluminum titanate in the nickel-zirconia system' in the journal 'Applied catalysis A: general' which is contained within this thesis.; Thesis contains two articles of which David Robert Driscoll is not the main author.; Melissa D. McIntyre, Martha M. Welander, Daniel E. Perea, Robert A. Walker and Stephen W. Sofie were co-authors of the article, 'Aluminum oxide processed as a beneficial additive in SOFC anodes' submitted to the journal 'Journal of the electrochemical society' which is contained within this thesis.; Clay D. Hunt, Daniel E. Perea, and Stephen W. Sofie were co-authors of the article, 'Diffusion caging : thermodynamic arrest of Ostwald ripening' submitted to the journal 'Advanced Materials' which is contained within this thesis.The size and shape of metal particulate at high temperature is dictated by surface energy. In systems containing very small metal particles, smaller particles shrink and disappear as they grow into larger particles in a process referred to as coarsening. Coarsening causes irreversible degradation in a number of important systems including automotive catalytic converters and solid oxide fuel cells (SOFC) through a loss of catalyst (metal) surface area. This phenomenon is exemplified by nickel metal catalyst that is supported on ytrria-stabilized zirconia (YSZ) which represents a materials system critical to the function of SOFCs. It has been demonstrated that additions of aluminum titanate (ALT) to the Ni-YSZ system with subsequent thermal treatment can act to stabilize the geometry of Ni on YSZ. In demonstration SOFCs, ALT has increased the time required for the first 10% of degradation by a factor of 115. This work has sought to elucidate the mechanisms by which ALT imparts increased stability. The work contained here demonstrates that ALT easily decomposes to Al 2O 3 and TiO 2. During thermal treatment, the alumina reacts with NiO to form nickel aluminate and the titania interacts with the YSZ where it can form Zr 5Ti 7O 24 -- a mixed ion electron conducting phase. In this way, the Al and Ti components of ALT have been determined to act independently where alumina appears to be dominant in microstructural stabilization. During cell operation, the nickel aluminate decomposes to nickel metal decorated with alumina nano-particulate. This geometry forms the basis of 'diffusion caging' as a stabilization mechanism which is the subject of Chapter 8. The role of titania appears to be less important except when processing occurs in a way that facilitates formation of the MIEC phase. However, Ni-YSZ cermets have also shown a strength enhancement when doped with ALT. This strength enhancement is likely due to the influence of titania (Chapter 7). Future work has the potential to extend concepts discussed here to a number of high temperature catalytic systems.Item The evolution of laves phase precipitation in AISI 441 under SOFC operating conditions and the effects on oxide growth(Montana State University - Bozeman, College of Engineering, 2016) Zimny, Christopher Isadore; Chairperson, Graduate Committee: Roberta AmendolaRecent research on solid oxide fuel cell technologies -- a type of high temperature fuel cell -- has resulted in the reduction of operating temperatures from around 1000°C to an intermediate range of 600-800°C. This reduction in temperature allows previously unviable materials to be investigated for use with this system. Interconnects, the component that separates the anode and cathode of adjacent fuel cells, benefit from this advancement as metals may now be utilized in place of the ceramic interconnects traditionally used in solid oxide fuel cell systems. In separating the anode and cathode, interconnects are necessarily subjected to what is known as dual atmosphere exposure: the exposure to the fuel, H 2, on one side of the material and the oxidant, air, on the other side. These extreme operating conditions have unusual effects on materials. AISI 441 is one promising ferritic stainless steel alloy for use as an interconnect; however, under dual atmosphere exposure, AISI 441 sees accelerated and anomalous oxide growth, exceeding that of similar ferritic stainless steels. This study investigated the oxide and Laves phase precipitate evolution of AISI 441 subjected to various environments including single, dual, and vacuum environments at a temperature of 800°C. Additionally, the relationship between the precipitates and the oxide formation was observed. Analysis was performed with a variety of equipment including a field emission scanning electron microscope. In environments containing oxygen, the oxide thickness of AISI 441 was found to increase over time and saw accelerated oxide formation in the dual atmosphere test, corroborating previous research. This study went on to investigate the formation of Laves phase precipitate Fe 2Nbin the microstructure of AISI 441 as a result of atmosphere. Laves phase materials are under research as hydrogen storage materials, suggesting the existence of these phases may facilitate hydrogen transport and storage in materials used as interconnects. Increased hydrogen transport could potentially explain the accelerated and anomalous oxidation of AISI 441 compared to other ferritic stainless steels. Over the time frames tested, the environments had no apparent effect on the Laves phase precipitation; further testing should investigate Laves phase precipitation between 0 and 1000 minutes.Item Investigation of chemical anchoring of nickel catalyst networks by aluminum titanate additives(Montana State University - Bozeman, College of Engineering, 2011) Law, Cameron Hunter; Chairperson, Graduate Committee: Stephen W. Sofie; Stephen W. Sofie was a co-author of the article, 'Anchoring of infiltrated nickel electro-catalyst by addition of aluminum titanate' in the journal 'Journal of the Electrochemical Society' which is contained within this thesis.; Stephen W. Sofie was a co-author of the article, 'Chemical anchoring of infiltrated nickel metal catalysts for improved stability at high temperature' in the journal 'Journal of the Electrochemical Society' which is contained within this thesis.; Stephen W. Sofie was a co-author of the article, 'Investigation of aluminum titanate for chemical anchoring of infiltrated nickel catalyst in solid oxide fuel cell anode systems' in the journal 'Journal of Power Sources' which is contained within this thesis.; Stephen W. Sofie, Zane Townsend and Max Lifson were co-authors of the article, 'Sintering performance of YSZ ceramics with transitionmetal oxide sintering aid' in the journal 'TMS Supplemental Proceedings' which is contained within this thesis.Electrocatalysts are incorporated into a plethora of technologies and material systems such as catalytic converters, reforming systems, multilayer ceramic capacitors, and solid oxide fuel cells (SOFCs). In SOFCs, nickel is commonly the catalyst of choice due to its chemical stability, high catalytic activity, and lower cost. While traditional SOFCs have a bulk mixture of nickel and yttria stabilized zirconia (YSZ) with at least 33 vol% nickel, solution infiltrated anode SOFCs have several benefits including lower nickel vol% to satisfy percolation, better mechanical strength and CTE matching that can improve redox cycling. Coarsening of the fine nickel metal catalyst with microstructures below 1 micrometer have shown a strong propensity to coarsen from thermal migration at temperatures above 700°C. This migration induced degradation by decreasing particle surface area and nickel network connectivity for electrical conduction. Utilizing metastable oxide additives as a minor dopant in the anode cermet system, novel methods of anchoring the metal phase to porous YSZ ceramic scaffolds have been identified as a means to engineer infiltrated anodes for improved performance. Less than 10 wt% aluminum titanate (ALT-Al 2TiO 5), added to YSZ by mechanical mixing, has shown a stepwise process in the formation of NiAl 2O 4 at 1100°C and ZrTiO 4 at 1205°C for chemically binding the nickel phase to YSZ. XRD, SEM, TEM, and EDS characterization coupled with FIB sample preparation has been utilized to identify the size, morphology, composition and temperature at which the anchoring phases form. Area specific resistance tests of component anodes indicate a decrease in degradation of at least 521%/1000 hr compared to infiltrated specimens without the ALT additive. Electrochemical tests of electrolyte supported cells (ESC) show higher initial performance of cells doped with ALT and at least a 1400%/1000 hr reduction in performance degradation at the same nickel loading content.Item Investigation of aluminosilicate refractory for solid oxide fuel cell applications(Montana State University - Bozeman, College of Engineering, 2010) Gentile, Paul Steven; Chairperson, Graduate Committee: Stephen W. Sofie; Paolo R. Zafred and Stephen W. Sofie were co-authors of the article, 'Progress in understanding silica transport process and effects in solid oxide fuel cell performance' in the journal 'Proceedings of the ASME eight international fuel cell science, engineering & technology conference in Brooklyn, New York, USA'. Its abstract is contained within this thesis.; Stephen W. Sofie, Camas F. Key, and Richard J. Smith were co-authors of the article, 'Silicon volatility from alumina and aluminosilicates under solid oxide fuel cell operating conditions' in the journal 'International Journal of Applied Ceramic Technology' which is contained within this thesis.; Stephen W. Sofie was a co-author of the article, 'Investigation of aluminosilicate as a solid oxide fuel cell refractory' in the journal 'Journal of Power Sources' which is contained within this thesis.Stationary solid oxide fuel cells (SOFCs) have been demonstrated to provide clean and reliable electricity through electro-chemical conversion of various fuel sources (CH 4 and other light hydrocarbons). To become a competitive conversion technology the costs of SOFCs must be reduced to less than $400/kW. Aluminosilicate represents a potential low cost alternative to high purity alumina for SOFC refractory applications. The objectives of this investigation are to: (1) study changes of aluminosilicate chemistry and morphology under SOFC conditions, (2) identify volatile silicon species released by aluminosilicates, (3) identify the mechanisms of aluminosilicate vapor deposition on SOFC materials, and (4) determine the effects of aluminosilicate vapors on SOFC electrochemical performance. It is shown thermodynamically and empirically that low cost aluminosilicate refractory remains chemically and thermally unstable under SOFC operating conditions between 800°C and 1000°C. Energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) of the aluminosilicate bulk and surface identified increased concentrations of silicon at the surface after exposure to SOFC gases at 1000°C for 100 hours. The presence of water vapor accelerated surface diffusion of silicon, creating a more uniform distribution. Thermodynamic equilibrium modeling showed aluminosilicate remains stable in dry air, but the introduction of water vapor indicative of actual SOFC gas streams creates low temperature (<1000°C) silicon instability due to the release of Si(OH) 4 and SiO(OH) 2. Thermal gravimetric analysis and transpiration studies identified a discrete drop in the rate of silicon volatility before reaching steady state conditions after 100-200 hours. Electron microscopy observed the preferential deposition of vapors released from aluminosilicate on yttria stabilized zirconia (YSZ) over nickel. The adsorbent consisted of alumina rich clusters enclosed in an amorphous siliceous layer. Silicon penetrated the YSZ along grain boundaries, isolating grains in an insulating glassy phase. XPS did not detect spectra shifts or peak broadening associated with formation of new Si-Zr-Y-O phases. SOFC electrochemical performance testing at 800-1000°C attributed rapid degradation (0.1% per hour) of cells exposed to aluminosilicate vapors in the fuel stream predominately to ohmic polarization. EDS identified silicon concentrations above impurity levels at the electrolyte/active anode interface.Item Development of a novel high performance electrolyte supported solid oxide fuel cell(Montana State University - Bozeman, College of Engineering, 2007) Gentile, Paul Steven; Chairperson, Graduate Committee: Stephen W. SofieHigh power solid oxide fuel cell (SOFC) stacks are based on the planar design concept to yield high specific power densities. The key engineering challenges to planar stack reliability and robust operation is attaining low resistance interconnection of individual cells in series and hermetic sealing of interconnects. While stack design and contact paste development is paramount to address this issue, the basic design of the fuel cell introduces limitations. State-of-the-art anode supported cells (ASC) yield high power densities due to low ASR thin electrolytes, however, the asymmetrical design, anode/electrolyte CTE mismatch, and thick support anode yields undesirable cell camber and fuel transport issues. These deficiencies lead to poor interconnect contact, non-optimal sealing surfaces, and poor fuel utilization, which can mitigate the key benefit of the ASC. Conversely, the electrolyte supported cell (ESC) presents a host of advantages from ease of processing, large diameter scale-up potential, mechanical robustness, optimal seal contact surface, thin electrodes, and minimal cell curvature with the key obstacle arising from high cell ASR due to the thick structural electrolyte. MSU has developed a novel cell concept that merges the benefits of the ASC and ESC designs.Item Robust copper braze for hermetic sealing of solid oxide fuel cells(Montana State University - Bozeman, College of Engineering, 2008) Ator, Danielle Elizabeth; Chairperson, Graduate Committee: Stephen W. SofieSolid oxide fuel cells (SOFCs) are becoming of increasing interest as a primary power source in today's industrial market. The voltage of a single cell under load is approximately 0.7 volts necessitating the use of many cells in series to generate useful electrical potentials, which gives rise to the SOFC stack. One of the key technical challenges in improving the long term performance and reliability of stacks is in the effective sealing of stack interfaces, particularly in planar stacks for which a hot seal (700-900°C) is required. SOFC stack seals must be: resistant to oxidation/volatilization in oxidizing and reducing atmospheres, must wet and bond to the joining members (both ceramic and metal), form a hermetic seal to prevent hydrogen leakage, and have a coefficient of thermal expansion (CTE) close to that of the adjoining components to limit thermally induced stresses. Active metal copper-based brazes present a novel approach to sealing SOFCs by means of robust mechanical/thermal properties providing strong, hermetic braze-interconnect and braze-YSZ interfaces. A commercially available active braze alloy utilizing no precious metal additives was tested and compared to custom synthesized braze compositions fabricated and tested at MSU. Two testing configurations were evaluated for this sealing study, utilizing dense YSZ substrates joined to 25.24mm, 430SS coupons as well as 25mm 440SS pressure test fixtures. Active braze alloys require a protective atmosphere to facilitate chemical bonding with YSZ and results show excellent performance in moderate vacuum (10-4 to 10-5 mbar) and argon atmosphere. Sample characterization was performed by electron microscopy, energy dispersive x-ray spectroscopy, pressurized rupture and leak tests, differential thermal analysis, thermal gravimetric analysis and thermodynamic evaluation. Robust copper-based brazes show potential for the use of sealing in SOFC applications. The brazes display desirable characteristics for sealing applications including the formation of chemically bonded braze joints, formation of a protective oxide barrier and high strength properties. Evidence of silicon diffusion into the YSZ may be problematic for long-term SOFC operation, however, development of a siliconfree braze has yielded excellent performance near that of the commercially available brazing powder.