UNIVERSITY OF CINCINNATI

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1 UNIVERSITY OF CINCINNATI Date: I,, hereby submit this work as part of the requirements for the degree of: in: It is entitled: This work and its defense approved by: Chair:

2 HIGH TEMPERATURE SEALS FOR SOLID OXIDE FUEL CELLS A dissertation submitted to the Division of Graduate Studies and Research of University of Cincinnati in partial fulfillment of the requirements for the degree of DOCTORATE OF PHILOSOPHY (Ph.D.) In the Department of Material Science and Engineering of the College of Engineering 2006 By Shailendra S. Parihar B.E. (Metallurgical Engineering), Ravishankar University, India Committee Chair: Dr. Raj N. Singh

3 ABSTRACT Solid Oxide Fuel cells (SOFCs) represent a clean and efficient alternative to existing methods of energy production. But, they need hermetic seals to prevent fueloxidant mixing within the stack. Glasses are attractive options for fabrication of these high temperature seals but suffer from their inherent brittleness and tend to crack during thermal cycling. In this study, an innovative concept of self-healing glass seals is developed to solve the problem of cracking of glasses in a SOFC seal. Rationale behind this concept is that a glass of suitable viscosity characteristics can flow and heal cracks at SOFC operating temperatures and thus can provide seals which can self-repair. A novel method, based on in-situ video imaging of cracks on the glass surface during high temperature treatment is developed and used to select and evaluate the suitability of different glasses for making self-healing seals. Promising glasses are studied experimentally to determine kinetics of healing of Vickers indented cracks at various temperatures. In addition, the effect of crystallization of glass on its healing kinetics is studied. A model is developed for crack healing behavior and is used to validate the experimental data. Studies on Cracks healing and crystallization of glasses showed that glasses with no crystallization tendency show fast crack healing response, whereas glasses which crystallize display sluggish healing. A glass displaying fast healing kinetics and good stability against crystallization is used to fabricate self healing glass seals for SOFCs. Seals fabricated using this glass not only remained hermetic but also maintained their self i

4 healing ability for as long as 3000 hours at C and 300 thermal cycles between room temperature and C. These results clearly indicated that self-healing glasses are promising candidates for SOFC seals. Key Words: Solid Oxide Fuel Cells, Glass Seals, Self-Healing Glasses, Seal Leak Testing. ii

5 ACKNOWLEDGMENT My deepest and most sincere thanks goes to my advisor, Prof. Raj N. Singh, one of the best educator I have seen during student life, and a man I shall respect rest of my life. Without his guidance and support, this dissertation would have not been possible. I would also like to thank him for his patience towards me during my difficult times, and his valuable advices made me not only more scientifically aware but also helped me in becoming a better person in life. I would also like to give special thanks to my committee members, Dr. Wim J Van Ooij, Dr. Ray Y. Lin, and Dr. Rodney D. Roseman for their valuable insights and guidance throughout the compilation of this dissertation. I am highly thankful to University of Cincinnati, and Department of Energy SECA program for financial support of this research work. I would also like to thank members of our research group, Dr. Dibakar Das, Dr. Niraml Govindraju, Dr. Sandeep Chavan, Indrajit Dutta, Ratandeep Kukreja, Vidhyasagar Jayaseelan, Li Guo, and Sandeep Singh for all their help and support over the past few years. I would like to extend my deep gratitude to my parents for their love and support and all the sacrifices they have made to give me a better life. I do not have words to thank my brother and sister who always believed in my abilities and extended their unconditional affection to me. Finally, this dissertation is devoted to my wife, Abhilasha. Her devotion, affection, and understanding have made my life easier. iii

6 TABLE OF CONTENTS ABSTRACT i ACKNOWLEDGEMENT iii TABLE OF CONTENTS iv LIST OF TABLES vi LIST OF FIGURES vii 1. INTRODUCTION LITERATURE REVIEW Fuel Cell Solid Oxide Fuel Cells (SOFCs) Introduction Components of SOFCs Designs of SOFCs Planar Solid Oxide Fuel Cell Different Types of Seals in Planar SOFC Functional Requirements of Metal-Ceramic Seals Sealing Options Glasses as SOFC Sealants Desired Properties of a SOFC Sealant Glass Designing of SOFC Glass Sealant Composition Glass Compositions used as SOFC Sealants Problem of Cracking of Glass Sealants OBJECTIVES AND APPROACHES EXPERIMENTAL PROCEDURES Selection of Materials Thermal Expansion Measurements Stability of Glasses against Crystallization Self-Healing Study on Glasses Kinetics and Mechanism of Crack Healing in Glasses iv

7 4.6 Stability of Seal Interfaces Fabrication of Seals Leak Testing of Seals RESULTS AND DISCUSSIONS Thermal Expansion Behavior Crystallization Behavior of Glasses Self-Healing Behavior of Glasses Kinetics and Mechanism of Crack Healing Mechanism of Crack Healing Model of Crack Healing Analysis of Crack Healing Data Verification of Crack Healing Model Effect of Crystallization on Crack Healing Seal Interfaces Seal Fabrications Performance of Seals CONCLUSIONS FUTURE WORK REFERENCES APPENDIX v

8 LIST OF TABLES Table No. Title Page No. Table 2.1 Generic set of requirements for SOFC seals 20 Table2.2 Desired properties of a SOFC sealant glass 29 Table 2.3 Influence of various oxides on the properties of glass 33 Table 4.1 Composition and basis of selection of glasses 46 Table 4.2 Metals and their criteria of selection 46 Table 5.1 Self- healing response of glasses 63 vi

9 LIST OF FIGURES Figure No. Title Page No. Figure 1.1 Different types of seals in a SOFC 1 Figure 2.1 Schematic diagram of fuel cell operation 4 Figure 2.2 Schematic diagram of the operating principle of a 7 Solid Oxide Fuel Cell Figure 2.3 Seal-less tubular design of solid oxide fuel cell 12 Figure 2.4 Segmented-cell-in-series design of solid oxide fuel cell 13 Figure 2.5 Monolithic cell design of solid oxide fuel cell 14 Figure 2.6 Planar stack design of solid oxide fuel cell 15 Figure 2.7 Cell-to-edge design of planar SOFC 16 Figure 2.8 Cell-to-frame design of planar SOFC 17 Figure 2.9 Different types of seals in a planar SOFC stack 18 Figure 4.1 Design of seal 50 Figure 4.2 Flow chart of seal fabrication procedure 52 Figure 4.3 Schematic diagram of seal testing set up. 53 Figure 5.1 Thermal expansion characteristics of glasses and YSZ. 55 Figure 5.2 Thermal expansion characteristics of metals and YSZ. 56 Figure 5.3 Crystallization behavior of glass 1 57 Figure 5.4 Crystallization behavior of glass 2 58 Figure 5.5 Crystallization behavior of glass 3 59 Figure 5.6 Crystallization behavior of glass 4 60 vii

10 Figure 5.7 Crystallization behavior of glass 5 61 Figure 5.8 Crystallization behavior of glass 6 62 Figure 5.9 Still photographs of crack on glass sample before and after healing 63 Figure 5.10 Softening point and healing temperature of glasses. 64 Figure 5.11 Different stages of crack healing in glass 65 Figure 5.12 Analogy between a crack tip and neck region between two spheres 66 Figure 5.13 Contact groove between two solid spheres. 66 Figure 5.14 Relation between a and W 69 Figure 5.15 Flow between parallel plates 69 Figure 5.16 Vickers indented crack on glass surface 77 Figure 5.17 Crack length as a function of time at C 78 Figure 5.18 Predicted and experimental values of time coefficient 79 Figure 5.19 Crack healing rate of glass 1 and glass 8 80 Figure 5.20 Crack healing response of glass-ceramic 81 Figure 5.21 Glass-YSZ interface 82 Figure 5.22 Glass-SS 430 interface 83 Figure 5.23 Glass-Nickel interface 83 Figure 5.24 Glass-Crofer interface 84 Figure 5.25 Pressure-Time plot for the glass seal at room temperature 86 Figure 5.26 Pressure-Time plot for the glass seal during first thermal cycle 87 Figure 5.27 Pressure-Time plot for the seal during third thermal cycle 88 viii

11 1. INTRODUCTION A fuel cell is an energy conversion device that produces electricity and heat by electrochemical combination of a fuel with an oxidant. 1 Among various types of fuel cells, solid oxide fuel cells (SOFCs) offer advantage of highest efficiency, potentially long life, construction from readily available materials, production of high grade waste heat, possibility of internal reforming, and no problem with electrolyte management. 2 There are different designs of SOFCs among which planar and tubular designs are more common. Planar configuration of SOFC is superior to other configurations in terms of efficiency and power density, 3 but requires different types of hermetic seals to prevent fuel-oxidant mixing and to provide electrical insulation to stacks. 4 Figure 1.1 shows different types of seals which are needed to make a planar SOFC functional. Metal-Metal Seal Electrolyte Ceramic-Metal Seal Metal Interconnect Metal Frame Metal Interconnect Metal Endplate Fuel Air Air Fuel Figure 1.1 Different types of seals in a SOFC. Seals required for a planar SOFC can be classified into metal-metal, ceramicceramic, and metal-ceramic seals. Among these seals metal-ceramic seals are particularly challenging because of their severe functional requirements as well as difficulty in engineering suitable material and associated processing optimization. 1

12 Functional requirements for metal-ceramic seals are very stringent, because these seals should be leak tight at fuel cell operating temperature (800 0 C). In addition, seal should have mechanical stability (robustness under external forces), chemical stability in fuel cell environments, thermal cycling/thermal shock resistance, chemical inertness with fuel cell components, high electrical insulation properties, acceptable sealing temperatures, and low cost. 5,6 There are two main approaches for making metal-ceramic seals such as compressive seals and rigid seals. In compressive seals, seal is not rigidly bonded to the fuel cell components, so an exact match of thermal expansion is not required. However, these seals require constant application of external pressure in order to maintain gas tightness. Because these seals are not bonded to the surfaces to be sealed their leak tightness is not good, and their performance further deteriorates with thermal cycling. In the case of rigid seals, no external pressure is required as seal is bonded with the components to be sealed. But, these seals have more stringent requirements for thermal expansion matching. Glasses are the main materials used for rigid seals, because properties of the glasses can be easily tailored by controlling their composition and they have the ability to flow and bond with other materials. But, glasses suffer from their inherent brittleness, which can result in cracking of glass seals during thermal cycling or shock. If cracks are generated in the glass it can cause leakage from the seal leading to degradation in cell performance. Current sealing technologies of both compressive and rigid type are not able to produce seals which can remain hermetic for long duration of time at SOFC operating conditions. Therefore, development of gas tight seals which can operate for longer times is crucial to realization and application of SOFC technology. 2

13 In this study, a new concept of self-healing glass seals is developed to minimize the problem of glass cracking, by utilizing the ability of a glass to flow and self-heal cracks at higher temperatures Utilization of self healing behavior of a glass in SOFC sealing application requires that the glass should have suitable viscosity at fuel cell operating temperatures and it should not crystallize even after long time exposure at higher temperatures. Seals demonstrating self-healing behavior are fabricated and tested using this approach. The research work reported in this dissertation is primarily focused on: 1) Fabrication and testing of self-healing glasses and seals for SOFCs, 2) Determination of the mechanism and kinetics of crack healing in glasses, and 3) Evaluation of self-healing glass seals concept for application in SOFCs. These studies have provided a significant understanding of crack healing behavior of glasses and their application towards making reliable seals for SOFCs. 3

14 2. LITERATURE REVIEW 2.1 Fuel Cells A fuel cell is an energy conversion device that converts chemical energy of a fuel and an oxidant into electrical energy. Fuel cell operates much like a battery, but, unlike a battery, it doesn't consume electrode material or require electrical recharging. In fact, a fuel cell can continuously generate power as long as fuel is supplied to it. 12 In principle, fuel cell is a fairly simple device consisting of two electrodes (anode and cathode) separated by an ion conducting electrolyte. Fuel is fed to the anode side where it gets oxidized and electrons are released to the external circuit. Oxidant is fed to the cathode side where it gets reduced and electrons are accepted from the external circuit. This flow of electrons from the anode side to the cathode side through the external circuit produces electricity. A schematic diagram of operating principle of a fuel cell is shown in Fig.2.1. Fuel Oxidant Anode Electrolyte Cathode e - e - External Load Direct current, Exhaust gases, Heat Figure 2.1. Schematic diagram of fuel cell operation 1 Fuel cells were first discovered in mid 19 th century. 13 However, their development lacked the necessary impetus until recently when they were realized as an effective means of energy production. Some of the reasons, which have resulted in greater attention towards fuel cell technology now-a-days, are discussed below. 4

15 1. Technical reasons: Because a fuel cell converts the chemical energy of the fuel directly to the electrical energy without the intermediate step of conversion into thermal energy, its conversion efficiency is not subjected to the Carnot limitation. Due to this reason fuel cells offer much greater energy efficiency as compared to the traditional means of electricity production. 2. Environmental reasons: Most of the current energy generation techniques are based on the principle of combustion of fossil fuels. Combustion of these fuels generate huge amount of pollutants, which is not conducive to the environment. But, fuel cells can operate without causing any environmental pollution. Fuel cells are also noiseless, which means they can reduce noise pollution too. 3. Futuristic reasons: It is well known that the availability of fossil fuels is limited and eventually these fuels will get exhausted. In order to fulfill our energy requirements in future we need to explore new ways of energy production, which does not solely rely on fossil fuel. Fuel cells can use renewable energy sources and can well be a sustainable energy generation technique for the future. 4. Economic Reasons: Energy requirements of modern human society are expected to grow at a very fast rate. In order to fulfill the requirements, installation of new power production plants is necessary. But, traditional power plants need big capital investment. On the other hand, fuel cells can be installed economically for decentralized power production, which means a number of small plants, each plant providing power to a localized area. This approach gives two other advantages, a) Heat produced during power production can be used for heating of buildings and water, unlike bigger power plants where produced heat energy is 5

16 mostly wasted (heat energy can not be transported to large distances). This heat can be constructively utilized in the case of fuel cell based power plants. Furthermore, this cogeneration of heat makes fuel cells even more energy efficient. b) Large scale power outage can be avoided, if power is generated in a decentralized manner only small local areas will be affected if there is a problem with either power generation or distribution. 5. Geopolitical Reasons: There is a high degree of economic dependence of industrialized nations on oil, and the oil reserves are highly centralized to certain parts of the world. This means any political instability in small region can greatly affect the world economy. This may not be an ideal situation and appropriate measures are needed in order to make the world less dependent on oil. Fuel cell technology can be an attractive way of doing so. Currently, there are different types of fuel cells in different stages of development. These fuel cells can broadly be divided into two categories: fuel cells which operate at lower temperatures, and fuel cells which operate at higher temperatures. Low temperature fuel cells (Alkaline Fuel Cells 14, Proton Exchange Membrane Fuel Cells 15, and Phosphoric Acid Fuel Cells 16 ) show lower electrical efficiency and power density, compared to high temperature fuel cells (Molten Carbonate Fuel Cells 17, and Solid Oxide Fuel Cells 18 ), although they are suited to automotive and portable electrical systems because they can be started quickly. For continuous big scale energy production, high temperature fuel cells are more suitable. Among high temperature fuel cells, solid oxide fuel cells seem to be better candidate for continuous energy productions as compared to the molten carbonate fuel cells because the later uses liquid electrolyte which makes 6

17 electrolyte management difficult and offers lesser power density and low electrical efficiency as compared to the solid oxide fuel cell Solid Oxide Fuel Cells Introduction Figure 2.2 shows schematic diagram of the operating principle of a solid oxide fuel cell. 20 The cell is composed of a ceramic electrolyte sandwiched between two porous electrodes. Electrolyte material is a conductor of oxygen ions at higher temperature. In order to run the cell, air is supplied to the cathode side while fuel is supplied to the anode side. When an oxygen molecule reaches the cathode/electrolyte interface, it catalytically acquires four electrons from the cathode and splits into two oxygen ions. These oxygen ions diffuse into the electrolyte material and migrate to the other side of the cell where they encounter and react with the hydrogen and carbon mono-oxide molecules at the anode/electrolyte interface. The reaction between oxygen ions and fuel molecules, gives off water, carbon dioxide, heat, and most importantly electrons. The electrons transport through the anode to the external circuit and back to the cathode, providing electrical energy in the external circuit. Figure 2.2 Schematic diagram of the operating principle of a solid oxide fuel cell 20 7

18 In order to produce appreciable amount of electrical energy, it is required that the oxygen ion conductivity of the electrolyte is sufficiently high. The conductivity of the electrolyte increases with the temperature and in case of YSZ (Yttrria Stabilized Zirconia), which is the most common electrolyte for SOFCs, conductivity is sufficient at or above the temperature of C. 21 The main characteristic of SOFCs is therefore their high operating temperature, which is needed to attain an adequate level of ionic conductivity in the ceramic electrolyte. There are dual advantages to this high operating temperature. 1 First, it allows internal reforming of various hydrocarbon gases without using catalysts based on noble metals. 22 This gives a distinct advantage, the ability to use multiple types of fuels for SOFC operation. Absence of noble metals in the SOFC also reduces the overall cost of the system. Second, it provides a source of intense heat that can be used without difficulty in a cogeneration system, either with or without a gas turbine. 23 But, there are several drawbacks also: the heating up process is long, and therefore these cells may not be particularly suitable for short repetitive operations. On the other hand, due to high operating temperatures, the development of this type of cell necessarily involves problem solving related to the thermal, chemical, and mechanical instability of constituent materials at higher temperatures. Problems related to stackassembly and gas-tightness at higher temperatures, are also needed to be solved. Although the operating concept of SOFCs is rather simple, the development of materials for the individual components presents enormous challenge. Each material must have the desired electrical properties required to perform its function in the cell. Different components must have enough chemical and structural stability in order to endure severe conditions during fabrication and subsequent operation at high temperatures. Reactivity 8

19 and inter-diffusion between the components must be as low as possible. The thermal expansion coefficients of the components must be close to one another to minimize thermal stresses, which could lead to cracking and mechanical failures. And finally, the materials must be cost-effective Components of SOFCs Cathode The cathode material must be porous in order to allow oxygen molecules to reach the electrode/electrolyte interface. Most commonly used cathode material is lanthanum manganite (LaMnO 3 ), a p-type pervoskite. 24 Typically, it is doped with either alkali earth or rare earth elements (eg. Sr, Ce, Pr) to enhance its conductivity. 25 Most often it is doped with strontium and referred as LSM (La 1-x Sr x MnO 3 ). 26 The conductivity of these perovskites is predominantly electronic (none or very little ionic conductivity), a desirable feature since the electrons from the open circuit flow back through the cell via the cathode to reduce the oxygen molecules, forcing the oxygen ions through the electrolyte. Electrolyte Once the molecular oxygen has been converted to oxygen ions it must migrate through the electrolyte to the fuel side of the cell. In order for such migration to occur, the electrolyte material should have high ionic conductivity. Electrolyte must also be fully dense to prevent short circuiting of reacting gases through it. Electrolyte should also be as thin as possible to minimize resistive losses in the cell. Similar to the other cell materials, it must be chemically, thermally, and structurally stable across a wide temperature range. 9

20 There are several candidate materials: Yttrria Stabolized Zirconia (YSZ), 27 doped cerium oxide, 28 and doped bismuth oxide. 29 Of these, the first two are more promising. Bismuth oxide-based materials have high oxygen ion conductivity and hence can offer lower operating temperatures (less than C), but these materials easily get reduced under low oxygen partial pressure conditions. 30 YSZ has emerged as the most suitable electrolyte material. Yttria serves the dual purpose of stabilizing zirconia into the cubic structure at higher temperatures and also provides oxygen vacancies at the rate of one vacancy per mole of dopant. A typical dopant level is 8-10 mol% yttria. Cerium oxide has the advantage of high ionic conductivity at much lower temperatures (under C). This temperature range significantly broadens the choice of materials for the other components, which can be made of much less expensive and more readily available materials. The problem with this electrolyte is its higher susceptibility for reduction on the anode (fuel) side. 31 Anode Anode materials should meet most of the same set of requirements as cathode materials in terms of electrical conductivity, thermal expansion compatibility, and porosity. But, additionally these materials should be stable under reducing atmosphere. The reducing conditions combined with electrical conductivity requirements make metals attractive candidates for anode material. Among different metals, most of the development has been focused on nickel owing to its abundance and affordability. However, its thermal expansion (13.3 x 10-6 /C) compared with YSZ (10.6 x 10-6 /C) is too high to pair it in pure form with YSZ; moreover, it tends to sinter and close off its porosity at operation temperatures. 32 These problems have been solved by making the 10

21 anode out of a Ni-YSZ composite. 33 The YSZ provides structural support for separated Ni particles, preventing them from sintering together while matching the thermal expansions. Adhesion of the anode to the electrolyte is also improved. Although Ni-YSZ is currently the anode material of choice, nickel still has a disadvantage: it catalyzes the formation of graphite from hydrocarbons. 34 The deposition of graphite residues on the interior surfaces of the anode reduces its usefulness by destroying one of the main advantages of SOFCs, namely their ability to use unreformed fuel sources. Interconnect In order to generate enough voltage and current, several fuel cells need to be connected together and a mechanism for collection of electrical current needs to be provided, hence the need for interconnects arises. The high operating temperature of the cell combined with the severe environment means that interconnects must meet the most stringent requirements of all the cell components: High electrical conductivity, no porosity (to avoid mixing of fuel and oxygen), thermal expansion compatibility, and chemical inertness with the fuel cell components. It will be exposed simultaneously to the reducing environment of the anode side and the oxidizing atmosphere of the cathode side. For a SOFC operating at about C, the material of choice is LaCrO 3 doped with a rare earth element (Ca, Mg, Sr, etc.) to improve the conductivity. 35 The strong economic incentives of using traditional metals as interconnect is driving the development of intermediate and low temperature SOFCs. At the operating temperatures over C range, interconnects made of nickel based alloys such as Inconel 600 are possible. 36 At ~800 0 C, ferritic steels can be used. 37 At even lower temperatures (~700 0 C), 11

22 it becomes possible to use stainless steels, which are comparatively inexpensive and are readily available Designs of SOFCs There are four principle designs for SOFC stack: tubular design, segmented cell design, monolithic design, and planar design. These designs differ in the amount of dissipative losses within the individual cells, in the manner of sealing between fuel and oxidant channels and in the configuration of cell-to-cell electrical connections. Steps of fabrication and assembly also differ from design to design. Tubular design: In this design, the cell components are configured as thin layers on a closed one end thin tubular support (Fig.2.3). 39 In order to operate the cell, air is introduced inside the tube while fuel flows outside the support tube. Un-reacted fuel is then combusted by oxidant stream coming out of the cell. Figure 2.3 Seal-less tubular design of solid oxide fuel cell 39 This type of design does not require any high temperature seal and the problem of gas tightness at higher temperatures is eliminated. But problem with this type of design 12

23 lies on the fact that the current path through the cell is relatively long which results in greater resistive losses. On the other hand, thickness of the support tube restricts the amount of oxygen which can be transported to the cathode/electrolyte interface and thus limits the maximum current generated by the cell. 40 Segmented cell design: This type of design is somewhat similar to tubular design but differs with the later in respect of supporting structure. Unlike tubular design in which cells are supported on a porous ceramic tube, in this design cells are self-supported because they are arranged in such a way that one cell fits into another and forms a tubular self-supporting structure (Fig.2.4). 41 The interconnect provides both sealing and electrical connection between the anode of one cell and the cathode of the next cell. Figure 2.4 Segmented-cell-in-series design of solid oxide fuel cell 41 13

24 This design offers improved efficiency as compared to tubular design because better fuel utilization is possible due to thinner support structure. For the same length of tubular cell and segmented cells in series, output power in case of later is more. But, this design suffers from some drawbacks: 1. although self-supporting electrolyte reduces the losses arising from obstacles in gas transport but can significantly increase the resistive losses. 2. The benefit of increased power due to cells in series rapidly diminishes with the number of the cells in the series. 3. Gastight seals at both ends of the stack are needed to separate fuel and oxidant. In addition, gastight seals must be maintained between each cell at the interconnecting areas. Monolithic Design: This design consists of thin cell components formed into a compact corrugated structure (Fig.2.5). 42 In this design, flow of gases can either be arranged in co-flow or cross-flow configuration. The cross-flow version results in a reduction in power density when compared with that of the co-flow, but offers a simpler way of introducing gases into and out of the cell structure. Figure 2.5 Monolithic cell design of solid oxide fuel cell 42 14

25 The key features of this design are high power density and absence of any seal. The main drawback of this design is the difficulty in fabricating the corrugated structure. Structural integrity of such a multilayer corrugated structure depends on the expansion behavior of all four cell components. Any significant mismatch in expansion and shrinkage of these different materials can cause stresses in fired body and can result in cracking during processing or operation. Planar Design: In this design, flat plates of cell assembly (anode/electrolyte/cathode) and interconnect are alternatively arranged to form a stack. Interconnect has ribs on both sides, which forms gas channels and also acts as bipolar gas separator contacting the anode of one cell in one side and cathode of adjoining cell on the other side (Fig.2.6). 43 Figure 2.6 Planar stack design of solid oxide fuel cell 43 15

26 The planar design offers improved performance and power density relative to tubular and segmented cell designs. This design also offers simpler fabrication and more structural reliability as compared to the monolithic design. Because of in-plane conduction, internal resistance losses of planar SOFCs are independent of cell area, and thus they can be termed as modular units. Furthermore, this design allows inspection of individual cell components during manufacturing of the cells, and thus ensures better quality control. 2.3 Planar Solid Oxide Fuel Cell Planar design obviously offers some great advantages but it requires hermetic sealing at the edge of different plates. Number and types of these seals depend on specific stack design. There are different stack designs for planar SOFCs but two designs are more common. In the cell-to-edge design (Fig.2.7), 44 the footprint of the cell matches that of the separator plate, each with the same pattern of openings for the transport of fuel and air through the stack. Sealing is required along the interfaces between each cell and adjacent separator plate. Openings in the seal for gas transport changes alternatively from one side of the cell to other side. Stack can be either electrolyte or anode supported depending upon the fact that gas manifold holes are chosen to be made on extended electrolyte or anode. Figure 2.7 Cell-to-edge design of planar SOFC 44 16

27 In more common cell-to-frame design the cell is smaller than the separator plate, contains no holes, and is joined to an intermediary component, a metallic window frame, which incorporates the necessary gas porting. In this case two seals are employed, one between the cell and metallic window frame to form a cassette repeat unit and a second between two adjacent cassette units (Fig.2.8). Figure 2.8 Cell-to-frame design of planar SOFC Seals in Planar SOFC In addition to these obvious seals, planar SOFCs require some other seals also. Figure 2.9 shows cross-sectional view of a planar SOFC, stacked in cell-to-window frame design. 45 These seals can be categorized as metal-metal, and metal-ceramic seals. All of these seals should be hermetic at higher temperatures (650 0 C C) for very long times (5,000 hours-40,000 hours). Metal-metal sealing can be readily achieved using metal joining, brazing and soldering techniques, although effect of high temperature oxidation of metallic components on survivability of these seals is a big issue. But, in case of metalceramic seals, things are more complicated due to the brittle nature of ceramic component. Hermetic joining of thin, electrochemically active ceramic cell to a metallic body without destroying chemical or mechanical integrity of the cell is very challenging. 17

28 Primary function of metal-ceramic seals in a SOFC stack is to avoid mixing of fuel and oxidant gases within the cell. In addition, they have several other functions and requirements, which are discussed in next section. Metal Interconnect Metal Frame S1 S2 S3 Ceramic Spacer S4 Metal Endplate S5 Fuel Air Air Fuel S1: Cell assembly to metal frame seal. S2: Metal frame to metal interconnect seal. S3: Metal frame to ceramic spacer seal. S4: Ceramic spacer to metal interconnect seal. S5: Metal frame to metal endplate seal. Figure 2.9 Different types of seals in a planar SOFC stack Functional requirements of Metal-Ceramic seals Functional requirements for metal-ceramic seals are very stringent, as these seals should be leak tight at fuel cell operating temperature (800 0 C) for a very long duration of time. In addition, seal should have mechanical stability (robustness under external forces), chemical stability in fuel cell environments, good resistance against thermal 18

29 cycling/thermal shock, chemical inertness with adjacent fuel cell components, electrical insulation properties, acceptable sealing temperatures, and low cost. Typical conditions under which SOFC devices are expected to operate and to which these metal-ceramic seals will be exposed include: an average operating temperature of C, continuous exposure to an oxidizing atmosphere on the cathode side and a wet reducing atmosphere on the anode side, an anticipated device lifetime of nearly hours, and a number of thermal cycles between room temperature and C. Hermeticity of these seals at SOFC operating temperature is paramount. Any leakage from the seal leads to reduced system performance, lower power generation efficiency, and poor fuel utilization. Leaks can also cause local hot spot or worse, widespread internal combustion within the stack, both of which induce accelerated degradation of the device. These seals are also supposed to provide electrical insulation to the stack, hence electrical resistance of these seals at higher temperature is of big importance. Any severe chemical reaction between the sealing material and other cell components can drastically affect stability and performance of the cell, hence chemical reactivity of these seals should be as low as possible. Stack assembly procedure have tremendous impact on seal fabrication step. In order to mitigate inter-diffusion and interfacial reactions between different layers of a cell (anode, cathode, and electrolyte), sealing temperature can not be very high. Additional considerations during sealing step include processing and material cost, which should be as low as possible. 19

30 Table 2.1 summarizes generic set of requirements for SOFC seals broken down by functional category. This table also summarizes the properties desired from the seal material for practical realization of these requirements. Table 2.1 generic set of requirements for SOFC seals Different sealing options for SOFC stack The options for sealing and joining the ceramic and metal components in a SOFC stack can be broadly classified into rigid bonded seals, compressive seals, and compliant bonded seals. Each offers advantages and has some limitations Rigid Bonded Seals In rigid bonded sealing, the sealant forms a joint that is non-deformable at room temperature. Because the final joint is brittle, it is susceptible to fracture when exposed to tensile stresses generated during non-equilibrium thermal events or due to thermal expansion mismatch between the sealant and adjacent substrates. Hence, the sealant must be tailored to match its coefficients of thermal expansion (CTE) to that of the adjacent substrates. Even a modest degree of thermal expansion mismatch can cause substantial stress and cracking of the sealant, which can lead to fuel and air mal-distribution in the 20

31 stack and poor system performance. For these reasons, the metal stack components (i.e., frames, separators, and spacers) are typically fabricated from ferritic stainless steel (CTE of K 1 ) to approximately match the composite CTE of the cell ( K 1 ), depending on whether the cell is electrolyte- or anode-supported). Glass and Glass-Ceramic Sealants Most common sealants employed in joining SOFC stacks are high-temperature glasses and glass-ceramics These materials tend to display acceptable stability in the reducing and oxidizing atmospheres of the stack, are generally inexpensive, can be readily applied to the sealing surfaces as a powder dispersed in a paste or a tape cast sheet, typically exhibit good wetting behavior on both yttria-stabilized zirconia (YSZ) and stainless-steel surfaces, are electrically insulating, and can be engineered to exhibit a CTE matching those of the adjacent SOFC components in the final joint. They are characterized by a glass transition temperature (Tg) above which the mechanical properties of these material changes from brittle to plastic. However, the brittle nature of glasses below glass transition temperature makes these seals vulnerable to cracking. Other problem in these materials is related to their phase stability after prolonged exposure to higher temperatures. Typically, glasses tend to crystallize at SOFC operating temperature, which can result in dramatic change in their thermal expansion behavior. In order to minimize this problem attempts have been made to select the glass composition in such a way that the thermal expansion behavior of newly formed crystalline phases is similar to the parent glass. Glasses of these compositions form glass-ceramics which are stable at SOFC operating temperatures. On the other hand Glass crystallization can be advantageous for several reasons: the resulting material is typically stronger than the 21

32 starting glass and by controlling the kinetics of crystallization and the product phases that form, it is possible to tailor the properties of the resulting glass-ceramic sealant. Various glass-forming systems have been considered as SOFC sealants, including those based on phosphates, 51 borates, 52 and silicates However, prior work has shown that phosphate and borate glasses are not sufficiently stable in the humidified fuel gas environment, tending to undergo significant corrosion through the formation of volatile species as well as by reacting with and degrading the various cell materials. To date, the best results have been obtained using compositions based on silica with various modifiers added to increase CTE, to modify softening behavior, and to improve adhesion and joint strength. Various silicate glasses and their performance as a SOFC sealant is discussed in detail in next section. There are several challenges in developing acceptable glasses and glass-ceramics for SOFC sealing. The first challenge in developing a useful glass sealant is in designing the proper glass composition which will result into the material with desired properties. Coefficient of thermal expansion, glass softening temperature, wetting behavior and viscosity of the glass at sealing temperature, all must fall under desired range. For example, the glass must be fluid enough at the temperature of sealing to wet the sealing surfaces, yet not so fluid that it flows out from between the substrates and results in open gaps and subsequent leaks. Additionally for compositions intended for formation of glass-ceramics, optimization of initial glass composition and the heating schedule is required in order to control the rate of crystallization and the nature of the crystalline phases. 22

33 The second key challenge in developing a useful glass, glass-ceramic sealant is to understand, how to stabilize the material s CTE as a function of time at SOFC operating temperature. There can be two approaches to achieve it. First, by selecting a glass which remains vitreous for very long time at SOFC operating temperatures and second, by designing the glass-ceramic composition whose devitrification response is less timedependant, which means the devitrification process completes during sealing process and it does not continue beyond that. The third and perhaps most critical challenge with designing of glass, glassceramic sealant compositions is to control their reactivity with metal components. Glasses and glass-ceramics generally adhere well to YSZ with little chemical interaction, but tend to form interfacial reaction products such as barium chromate (BaCrO 3 ) with the oxide scales of the metal. With long-term exposure at the stack operating temperature, these phases thicken and become porous, yielding interfaces that are often weak and susceptible to thermo-mechanically induced cracking. Most failures observed in full-scale stacks initiate along the metal/sealant interface. This problem can be mitigated by designing new metals which form more adherent and less reactive oxide layers. Glass composition can also be tailored to reduce the amount of reactive components in it. Ceramic Sealants High-temperature cements and sealants formed by reaction bonding can also be used as SOFC sealants. High temperature ceramic cements such as Duco and Sauereisen cements have been used in small-scale cell testings, 55 but they do not display the degree of CTE matching required for stack fabrication and often crack when cooled to room temperature. Ceramic sealants formed by in-situ reaction have also been tried as an 23

34 alternative method of rigid bonded sealing. 56 But, this approach requires heat treatment at very high temperatures in order to attain satisfactory joint. However, the use of preceramic polymer precursors significantly lowers the temperatures required for joining. 57 These precursors are typically organo-silane polymers that convert to SiC or SiO x C y when heated to higher temperatures. But, the pyrolysis of these polymers is accompanied by the formation of gaseous reaction products and high volume shrinkage, which often causes pores and cracks in the seal Compressive Seals Compressive seals employ deformable materials that do not bond to the SOFC components but instead serve as gaskets. Thus, sealing results when the entire stack is compressively loaded. Because the sealing material conforms to the adjacent surfaces and is under constant compression during use, it forms a dynamic seal. That is, the sealing surfaces can slide past one another without a disruption in hermeticity and the individual stack components are free to expand and contract during thermal cycling with no need to consider CTE matching. Additionally, they offer the potential for mid-term stack repair by releasing the compressive load, disassembling the stack, and replacing the damaged cell or separator components. However, to employ compressive seals in a SOFC stack, a load frame is required to maintain the desired level of compression on the stack over the entire period of operation and hence the stack components must be capable of withstanding the sealing load. The load frame introduces several complexities in stack design, including oxidation of the frame material, load relaxation due to creep, and increased weight and thermal mass resulting into reduced specific power of the system. 24

35 Metal Gaskets Noble metals such as gold and silver may be viable in forming hermetic seals under external pressure at stack operating temperatures. But definitely they are not cost effective way of doing so. Additionally, severe degradation in seal performance has been observed with silver seals, because silver has very poor stability under dual (oxidizing and reducing atmosphere) conditions. 58 Oxidation-resistant alloys such as stainless steel and nickel-based super-alloys are also been tried in gasket form but very little information is available about the effectiveness of these seals. However, an obvious disadvantage with these materials is their high electrical conductivity which can potentially lead to internal shortening. Mica-Based Seals An alternative to metal-based gaskets is the use of mica-based materials. 59 Micas belong to a class of layered minerals known as phyllo-silicates and are composed of cleavable silicate sheets. These materials are well known for their high resistivity. The commercial mica papers exhibited very poor sealing characteristics even under high compressive loads. Subsequent studies demonstrated that the primary leak paths in the compressed mica seal are along the interfaces with the ceramic and metal sealing surfaces 60 and that sealing can be greatly improved by incorporating a compliant interlayer such as a deformable metal or glass at these interfaces. 61 Under equivalent loading conditions, the leak rates in these hybrid seals are several orders of magnitude lower than in the commercial mica papers. However, reactivity of these inter-layers is a matter of concern. Additionally, there are potential concerns about the effectiveness of 25

36 compressive sealing concept because sealing stress may not distribute uniformly over the larger sealing area or from one end of the stack to the other Other Sealing Options Brazing One of the most reliable methods of joining dissimilar materials is brazing. In this technique, a filler metal with a liquidus temperature well below that of the materials to be joined is heated to a point at which it becomes molten and under capillary action fills the gap between the sealing surfaces. When cooled, a solid joint forms 62. Active metal brazing is a specialized version of this technique that employs a reactive element such as titanium to facilitate wetting between the filler metal and a ceramic sealing surface. 63 Unfortunately, active metal brazing is typically conducted in an inert or vacuum environment and therefore complicates the sealing process. In addition, the ceramic-metal joint produced by this technique is not sufficiently resistant to oxidation and degrades when exposed to high-temperature. Compliant Bonded Seal This method employs a thin stamped metal foil that is bonded to both sealing surfaces. Unlike a mica gasket, this seal is non-sliding. When properly designed, the foil yields or deforms under modest thermo-mechanical loading and limits the transfer of these stresses to the adjacent ceramic and metal components. 64 But, information available about this sealing concept is very limited. Overall, in comparison to other sealing techniques, glass and glass-ceramics based sealing technology appear to be more promising and mature because seals of these materials can provide very good leak tightness along with excellent environmental 26

37 stability. 65 Additionally, they meet most of the other requirements desired by a suitable seal material (e.g. high electrical resistance, low cost). But definitely, there are the problems related to the cracking of these materials. In the next section, first the properties desired from a suitable SOFC sealant glass are identified. Then an approach for designing proper glass, glass-ceramic compositions in order to achieve desired properties is discussed. And finally, a potential solution of the problem of cracking of these materials is discussed. 2.4 Glasses as Sealants for SOFCs Desired properties of a SOFC sealant glass Two of the most important properties, required to be in desired range for a suitable SOFC glass sealant are the glass softening temperature (Ts) and coefficient of thermal expansion (CTE). Softening temperature of the glass should be low enough to facilitate proper spreading of the glass on the sealing surfaces at sealing temperature, but it should be high enough to avoid squeezing out of the glass from the cell stack at SOFC operating temperature. Ideally, Glass should have the softening temperature in the range of (550 0 C C). Coefficient of thermal expansion (CTE) of glass should closely match with other fuel cell components namely yttrria stabilized zirconia (YSZ) and the metals. YSZ has a CTE of 10.6x 10-6 / 0 C, so glass should have CTE in the range of (9-12 ppm/ 0 C). Another desired property expected from a Suitable SOFC sealant glass is high electrical resistance at SOFC operating temperatures. Generally, glasses have very high electrical resistance at ambient temperature, but at higher temperatures their electrical resistance goes down due to the increased mobility of different ionic species. Electrical 27

38 resistivity of a sealant glass should at least be more than 2000 Ω-cm at C 66. Sealant glass should also have suitable viscosity characteristics. Viscosity of the sealant glass at sealing temperature should be in the range of Pa s, and it should not be less than 10 4 Pa s at SOFC operating temperature. Glass should also show good wetting behavior with sealing surfaces of both the metal and YSZ. In order to facilitate proper spreading of glass on the sealing surfaces (required for formation of true intimate interface between glass and sealing surfaces), glass should have a wetting angle of less than 90 0 at the sealing temperature. Sealant glass should also have a high degree of chemical inertness towards any reaction with metal and YSZ both under oxidizing and reducing condition. Additionally, sealant glasses should have long term chemical and phase stability under SOFC operating conditions, which means it should have minimal evaporation losses at higher temperatures along with stable glassy phase which does not crystallize with time at SOFC temperatures. Finally, seals made using glasses should be highly hermetic and leakage rate through these seals should be as low as possible. Leakage rate through SOFC seals can be directly measured either by pressure drop test or by measuring the flow rate of leakage. A leakage rate determined using pressure drop test can be expressed as (pressure drop/ differential pressure. time. seal length). A leakage rate of 10-7 (Pa / differential Pa. second. cm) should be an acceptable leak rate. 66 If seal leakage rate is determined by flow rate measurement then it can be expressed in the units of (sccm / differential pressure. cm). A leakage rate determined using this method should be acceptable if its value is less than 10-7 sccm / differential Pa. cm. Table 2.2 Lists all of the desired properties, a SOFC sealant glass should have. 28

39 Table 2.2. Desired properties of a SOFC sealant glass. Property Expansion Coefficient Softening Temperature Electrical resistance Viscosity Wetting angle Chemical reactivity with YSZ and metal Crystallization tendency Evaporation losses Leak rate Desired value range 9-12 ppm/ 0 C C C > 2000 Ω-cm at C Pa s at sealing temperature > 10 4 Pa s at C < 90 0 at sealing temperature Negligible Negligible for glasses Negligible after initial crystallization for glass-ceramics Negligible 10-7 Pa / s.cm ( per differential pressure) 10-7 sccm / cm (per differential pressure) Designing of SOFC glass sealant composition Designing of a suitable SOFC glass sealant composition requires thorough understanding of effect of each individual glass component on the glass properties. So in order to finalize a suitable glass composition, we should at-least qualitatively know the effect of different oxides on glass properties. Following discussion is an attempt to first summarize the qualitative influence of different components on the glass properties and then further discussion deals with the quantification of these influences. Any multi-component oxide glass comprises of glass network forming, glass network modifying, and intermediate oxides. Glass network forming oxides are responsible for building up continuous three dimensional random networks in the glass structure. Network modifying oxides are incapable of building up a continuous network and effect of these oxides is usually to weaken the glass network. On the other hand, 29

40 intermediate oxides are although not capable of forming glass network but they can take part in glass network. Glass network formers SiO 2, B 2 O 3, P 2 O 5, V 2 O 5, As 2 O 3, GeO 2, SeO 2, Sb 2 O 3, TeO 2, are the main oxide glass formers. Except SiO 2, B 2 O 3, and P 2 O 5 all other oxides are unstable at C as they get reduced by Hydrogen. 67 Use of P 2 O 5 as a glass former for SOFC sealant glasses is limited, as phosphate based glasses exhibit low CTE values 51 and low strength along with chemical instability in humid conditions. 68 P 2 O 5 when used in conjunction with SiO 2, increases the crystallizing tendency of the silicate glass. 69 Glasses based on B 2 O 3 glass former, exhibit low softening temperature and they are soluble in water. 68 B 2 O 3 when used in conjunction with SiO 2, decreases the viscosity and crystallization tendency of the silicate glass. But, it forms volatile compounds in the presence of water vapors. 70, 71 SiO 2 is preferred glass former as it is stable at higher temperatures in both reducing and oxidizing atmospheres. Glass network modifiers Alkali-metal oxides (Li 2 O, Na 2 O, K 2 O) act as fluxing agents and increase the coefficient of thermal expansion while reducing softening temperature of the glass. But these oxides can cause severe chemical reaction between glass and adjacent fuel cell components (metal), which can result into the enhanced volatility of chromium, 72,73 and poisoning of cathode. Additionally, these metal ions are small in size and their diffusion coefficient at SOFC operating temperature is very high which can cause drastic drop in electrical resistance of glass at higher temperatures. Li 2 O is the most active flux and 30

41 increases the tendency of devitrification by reducing the glass viscosity. Similarly, Na 2 O, and K 2 O if present in higher amounts, increase the crystallization tendency of the glass. Among all alkali-metal ions, diffusion coefficient of Cs + ion is least and reported to be similar to alkali earth ions. 74,75 In addition, Cs + ion charge is only half of alkali earth ions, therefore the driving force for Cs + in electric field is half compared to alkali earth ions. Cs 2 O is the modifying oxide that leads to the best glass formation 76 among all alkali and alkali earth oxides. Cs 2 O does not lead to phase separation in ternary alkali borosilicate glasses as other alkali and alkali earth oxides do. 77 The problems with Cs 2 O are chemical stability problem in water, evaporation, and its strong basicity. 67 Among alkali earth metal Oxides MgO, CaO, SrO, and BaO, all can be useful addition in SOFC sealant glasses. MgO decreases the viscosity of the glass around C 69 and reduces the devitrification tendency of the glass. CaO acts as a flux and reduces glass viscosity while increasing thermal expansion coefficient. Among the alkali earth metal oxides, SrO and BaO cause the greatest reduction in viscosity in the glass softening range, they increase the CTE 81 and show better glass formation 75 with lesser phase separation tendency 75 than do MgO and CaO. But, the problem with the BaO is that it forms mechanically weak celsian phase (BaAl 2 Si 2 O 8 ) which has very low CTE (2.29ppm/k). Among transition metal oxides, Sc 2 O 3 varies its oxidation state and can cause electronic conduction. 69 Y 2 O 3 increases expansion and glass transition temperature. Rare earth oxides have similar properties as Y 2 O These oxides increase the glass transition temperature and expansion coefficient. Among the rare earth oxides La 2 O 3, is reported to increase the thermal expansion considerably (similar to SrO and BaO). 83 La 2 O 3 is also reported to increase CTE and Tg more than Y 2 O 3 and Nd 2 O Other rare earth oxides 31

42 are similar as BaO and SrO. For rare earth oxides, the glass formation and thermal expansion increases with increasing ionic radius, while the viscosity decreases. Hence, only oxide with bigger ionic size can cause greater increase in thermal expansion as compared to BaO or SrO. Ce +3, Nd +3 and Pr +3 are the three biggest ions in rare earth group. 82 CeO 2 may vary its oxidation state in SOFC condition and can cause electronic conduction. 69 Other two oxides Nd 2 O 3, and Pr 2 O 3 increase the thermal expansion coefficient more than BaO. Use of Bi 2 O 3 and PbO is limited due to the concerns regarding toxicity of these materials. 85 Intermediate oxides Although transition metal oxides may help in improving the adhesion of glass with metals. 86 but most of these metal oxides give rise to electronic conduction in glasses at higher temperatures by electron hopping process. 69 Fe 2 O 3 and Cu 2 O are known to cause electronic conduction in glasses. 87 TiO 2 gets reduced by H 2 at C 67 and also stimulates crystallization 86 by acting as a nucleating agent 88 it can also induce phase separation in glasses and can influence the phase formations at the interface. 88 TiO 2 addition reduces the CTE of glass. Cr 2 O 3 reduces the surface tension of glass and thus facilitates better flow behavior of glass 89 but it can get oxidized and react with BaO to 90 form BaCrO 4 which is known to cause cathode poisoning. ZnO is reduced by dry H 2 at temperatures as low as C. However, wet H 2 has no influence even at temperatures significantly higher than C, 67 but carbon containing gases, can reduce ZnO under both wet and dry conditions. 67 ZnO is known to decrease the viscosity as well as CTE. ZnO lowers viscosity with less effect on CTE than B 2 O ZrO 2 reduces CTE and 32

43 increases the viscosity. 77 ZrO 2 is also known to increase the chemical durability of silicate glasses even when present in very small amounts. But, ZrO 2 is responsible for poor joining properties 91 and it promotes crystallization. 86 HfO 2 is very similar to ZrO 2 in many of its properties. 81 Ta 2 O 5 acts as a good flux. Al 2 O 3 acts as flux and retards crystallization if it is present in small amounts. 92 It also increases chemical durability of the glass. 93 Table 2.3 summarizes the Influence of various oxides on the property of silicate glass. Table 2.3: Influence of various oxides on the properties of silicate glass Constituent SiO 2 B 2 O 3 Al 2 O 3 La 2 O 3 CaO ZnO TiO 2, ZrO 2,Ni CuO,MnO,NiO Cr 2 O 3, V 2 O 5 BaO,SrO MgO Influence on property Main constituent of glass acts as glass former Reduces viscosity and chemical durability of glass, increases expansion Retards crystallization, improves flux. Used to control the viscosity characteristics, Increases CTE, shortens working range Reducing agent, improves flux Crystallization nucleation agents Improves adhesion Reduces surface tension Flux, increases CTE Increases CTE, Makes glass longer Thermal expansion coefficient of a glass is a continuous function of glass composition and each oxide has a predictable affect on the thermal expansion coefficient of the glass. Oxide with smaller valence cation increases the thermal expansion of the glass more than the oxide with higher valence cation (e.g. Na 2 O increases the expansion value more than MgO). For similar valence of cations, bigger size cation has more incremental effect than the smaller size cation (e.g. Cs 2 O increases the expansion more than Na 2 O). On the other hand, calculation of other properties like glass softening 33

44 temperature or glass transition temperature is not that simple. These properties are dictated by rather complicated structural factors, and approach for their calculation is empirical rather than based on scientific parameters. Different glass databases and models are available, which can be used to calculate these properties with reasonable degree of accuracy Overall, effect of a particular oxide on glass softening temperature mainly depends on the valence of its cation and is virtually independent of size of cation. Oxides with higher valence cations, have less effect in lowering of glass softening temperature (e.g. glass containing Na 2 O has lower softening temperature as compared to glass containing MgO in same molar amount) Glass compositions used for SOFC sealant Glasses based on all three of the primary glass forming oxides (B 2 O 3, P 2 O 5, and SiO 2 ) have been investigated as potential SOFC sealants. B 2 O 3 based glasses tend to exhibit excessive volatization in SOFC environment. Glasses with high B 2 O 3 content in the SrO La 2 O 3 Al 2 O 3 B 2 O 3 SiO 2 system have been tried for SOFC sealant application but they show very low softening point. P 2 O 5 -based glasses can be adjusted to minimize volatilization, but their CTEs are too low, and they have low mechanical strength. Among SiO 2 based glasses, alkali silicates tend to be very reactive towards SOFC components and alkali boro-silicates have very low CTE. 111 But, alkaline earth silicate and alkaline earth containing alumino-silicate glasses are promising, 112 but these glasses have strong tendency for crystallization and formation of glass-ceramics. Most of the glasses studied in these two systems contain BaO. 34

45 In the case of barium-containing glasses, the crystallization increases thermal expansion. This increase in the coefficient of thermal expansion (CTE) is due to formation of barium silicate (BaSiO 3 ), which has a large coefficient of thermal expansion (9-13 ppm/ 0 C). 113 In BaO-Al 2 O 3 -SiO 2 glass system, glass crystallizes to form celsian (BaAl 2 Si 2 O 8 ) in addition to barium silicate. 114 The two common forms of celsian are monoclinic and hexagonal. Hexacelsian has high coefficient of thermal expansion (9-11 ppm/ 0 C), while monocelsian has a very low coefficient of thermal expansion (3 ppm/ 0 C). When other alkali earth oxides are added into barium containing aluminosilicate glasses, they behave differently than BaO. Strontium forms solid solutions with barium in the celsian crystal structures and stabilizes the monocelsian phase. 115 But, other alkalineearth oxides do not dissolve in the celsian phase, they rather form new phases. Calcium oxide addition results in the formation of an additional phase, barium calcium orthosilicate (Ba 3 CaSi 2 O 8 ), 116 which has a large coefficient of thermal expansion (12-14 ppm/ 0 C). If calcium oxide is used without barium oxide, wollastonite 117 (CaSiO 3 ) (4-9 ppm/ 0 C) and anorthite (CaAl 2 Si 2 O 8 ) 118 are formed. The addition of magnesium oxide results in the formation of enstatite (MgSiO 3 ) and silica (quartz or cristobablite) along with celsian. 119 Cristobalite formation is problematic as it can cause cracking. Overall, there is a very strong tendency of barium containing glasses to form celsian phase which is not desirable. Magnesium aluminosilicates without barium oxide form cordierite (Mg 2 Al 4 Si 5 O 18 ) which has a very low coefficient of thermal expansion (1 ppm/ 0 C). 120 Glasses in the CaO-SrO-SiO 2 and CaO-SrO-ZnO-SiO 2 with small amounts of Al 2 O 3 and some other oxides, are also developed for SOFC sealant application. 121 It is found that the glasses containing ZnO crystallize faster than glasses without ZnO. For 35

46 glasses containing ZnO, there is no obvious correlation between ZnO content and stability of the glass in reducing atmosphere. But, these glasses are not tested in reducing atmosphere with humidity. In humid conditions, it is known that ZnO is not stable. On the other hand, addition of B 2 O 3 makes these glasses unstable in reducing atmosphere. Glasses in this system crystallize to form calcium zinc silicate if ZnO is present and calcium silicate if ZnO is not present. Proper control of crystallization kinetics of the glasses is very important to achieve the desirable properties. The crystallization kinetics can be controlled with the addition of nucleating agents. Control of the crystallization also includes control of the specific phases formed. In MgO-Al 2 O 3 -SiO 2 glass system, crystalline phase of cordierite (Mg 2 Al 4 Si 5 O 18 ) is formed which has very low thermal expansion coefficient (2 ppm/ 0 C). In order to stop formation of the undesirable phases, nucleating agents like TiO 2, ZrO 2, Cr 2 O 3, Ni can be used to control and change the crystallization kinetics. Nucleating agents have high field strength and hence they can assist in phase separation causing regions which are rich in their concentration, eventually changing the crystallization kinetics. Addition of TiO 2 in MgO-Al 2 O 3 -SiO 2 glasses suppresses the formation of cordierite. 122 Proneness of glass towards crystallization can be expressed in terms of the activation energy (E a ) required for crystal growth. For boroaluminosilicate glass system addition of TiO 2, Cr 2 O 3 and Ni increases the E a value in the order of Ni > Cr 2 O 3 > TiO 2. While addition of ZrO 2 decreases the E a value. As the alkaline earth metal changes from Ba to Ca to Mg, the activation energy of crystal growth increases significantly, this can be explained by increasing field strength. This is the reason why BaO and CaO containing glasses have higher tendency towards crystallization as compared to MgO 36

47 containing glasses. 122 Al 2 O 3 can both increase and decrease the E a value of the glass depending on its amount in the glass. When Al 2 O 3 is present in smaller amount (up to 5 mol %) it acts as network former and increases the E a value of the glass. But, when amount of Al 2 O 3 in the glass is higher it acts as network modifier and thus decreases the E a value. This dual role of aluminum has led to reports of aluminum both inhibiting and enhancing crystallization. 114 Aluminum additions have also been reported to inhibit cristobalite formation, 88 which can cause cracking. In MgO containing glasses, Al 2 O 3 content of the glass is a determining factor in the competition between cristobalite and cordierite phase formation. 122 Chromium is shown to suppress the formation of cordierite (Mg 2 Al 4 Si 5 O 18 ) phase. 122 Barium containing glasses in the BaO-CaO-Al 2 O 3 -SiO 2 glass system are extensively studied for their compatibility with YSZ and promising interconnect alloys. 90 Barium containing glasses form a homogenous continuous interface with YSZ. No significant reaction occurs between glass and YSZ and strong bonding is achieved due to mechanical interlocking of two surfaces. With chromia forming alloys, like stainless steel 446 and AL29-4C, barium containing glasses do not form homogeneous continuous interface. The extent and nature of interaction between the glass and metal depend on the alloy matrix composition, exposure conditions, and proximity of the interface to the ambient air. At the edges of metal-glass interface, where oxygen is available, glass reacts with metal and forms BaCrO 4 which leads to separation of the glass from stainless steel due to the thermal expansion mismatch as BaCrO 4 has very high thermal expansion coefficient (α a =16.5, α b =33.8, α c =20.4 x 10-6 / 0 C). In the areas beyond edges, glass bonds well with the metal and a clearly discernible interface with reaction zone of Cr-rich 37

48 solid solution results because of the dissolution of Cr from the alloy scale into the glass. Glass-metal interface is also characterized by porosity due to the formation of vapor species via the interaction of Cr with dissolved water and alkaline oxide residues in the glass. Among chromia forming alloys, compatibility of Crofer22 APU with barium containing glass is better as compared to stainless steel 446 and AL29-4C. Glass does not interact excessively with the metal in the edge areas to form BaCrO 4 and in the interior areas formation of the Cr- rich solid solution and porosity is less. Better compatibility of Crofer22 APU can be attributed to the unique scale formed on the surface comprising of (Mn,Cr) 3 O 4 spinel top layer and chromia-rich sub-layer. 90 But, Crofer22 APU is also not immune to BaCrO 4 formation and the formation of porosity. These problems start to occur after prolonged exposure to higher temperature. Crofer22 APU-glass interface does not show extensive porosity because Crofer has a protective oxide layer at the top, which suppresses the Cr diffusion. Similarly, if the metal surface is oxidized before sealing the problem of porosity is minimized as protective oxide layer suppresses the Cr diffusion. 90 With austenitic chromia forming alloys (Nicrofer 6025), barium containing glass reacts but the extent of reaction is less compared to the ferritic chromia forming alloys. But, porosity formation is extensive and pores get agglomerated leading to the formation of crack. The interfacial zone containing Cr-rich solid solution is thinner as compared to ferritic chromia forming alloys. In case of alumina forming alloys no BaCrO 4 formation takes place and there is absence of any extensive interfacial reaction, although the interface is marred with extensive presence of porosity and Al gets segregated at the interface. Alumina forming 38

49 alloys show low reactivity, because their oxide scale acts as a protective layer and Cr diffusion is suppressed. But, they show large porosity as Al or the other elements in the alloy react with absorbed water to form hydrogen. Alumina forming alloys show more porosity than the chromia forming alloys because the reaction between Al and water is even more thermodynamically favorable than the reaction between Cr and water. Although all alkali earth metal oxides MgO, CaO, BaO act as modifiers of the glass structure but they have very different field strength and electro-negativity, which results in differences in their crystallization tendency and reactivity with other materials. Glasses containing BaO and CaO show vigorous crystallization and reaction with the metals. CaO containing glasses are found to react with YSZ to form m-zro 2, 88 which is an undesirable phase because it has very high specific volume as compared to YSZ. This interaction between CaO containing glasses and YSZ is due to the diffusion of Ca into the YSZ. MgO containing glasses on the other hand show lesser tendency towards crystallization as well as less severe interaction with metals and YSZ. MgO containing glasses do not form any obvious reaction product with YSZ. Similarly, they also do not seem to form any interfacial product with chromia forming steels. But, interaction with steel leads to the change in crystallization kinetics of these glasses. Glass, when in contact with chromia forming steel, seem to crystallize to form cristobalite at the expense of cordierite. It is proposed that chromium from the steel acts as inhibitor for the growth of cordierite phase. MgO containing glasses react with chromium alloys (Cr 5Fe 1Y 2 O 3 ) to form spinel of approximate composition (Mg, Fe) (Cr, Al) 2 O MgO containing glasses also appear to perform better with chromia forming steels in aspect of cathode poisoning caused due to evaporation of chromium containing 39

50 species. Mg +2 has smaller size and higher mobility than other alkali earth ions and it can migrate easily to the metal-glass interface due to the driving force of heat of oxide formation at the metal surface. Oxide layer of these alloys contain Cr +2 species, which has a strong tendency to go to higher valence state, which can cause Mg +2 to convert to magnesium metal, resulting in the formation of magnesium rich layer at the interface. This layer of magnesium can act as a barrier layer for diffusion of chromium resulting in lesser formation of volatile reaction products. Migration of magnesium to the interface will also cause the reduction in openness of glass structure resulting in increased viscosity of the glass to further inhibit chromium diffusion Cracking of glasses Glasses are intrinsically brittle, and they crack very easily under tensile stresses. When glasses are used as a sealant for SOFCs, they are bound to experience stresses. These stresses can be generated either due to non-equilibrium thermal events in the SOFC stack or due to thermal expansion mismatch between glass and adjoining sealing surfaces. Once stresses in the glass body exceed the ultimate strength of the glass, cracking of glass will take place. Generation of these cracks in the glass body is particularly problematic in case of SOFC seals because it results into the loss of hermeticity of the seals. Hermeticity of the SOFC seals is of paramount importance and hence approaches to minimize the cracking of glass body are highly desirable. There can be two approaches to solve the problem of glass cracking. First, crack minimization approach: which can be achieved by either reinforcing the glass with strong fibers or by optimizing the processing conditions to minimize the amount of flaws in the glass body. This approach can be useful to reduce the generation of the crack up to a degree but will 40

51 not be helpful in eliminating the cracks in case they develop. Other approach can be termed as crack healing approach. This approach is based on the healing of already existing cracks in the glass body. Viscosity of the glass decreases with increasing temperature. At the temperature when viscosity of the glass is sufficiently low, it starts to flow. This flow of glass can be utilized to heal the cracks in its body. The glass with appropriate viscosity at SOFC operating temperature can be used to make seals which will have the ability to self-heal. Other advantage of this sealing concept is the possibility of effectively sealing materials of very different thermal expansion characteristics. But, glasses for this type of seals should have some unusual characteristics. 1. Viscosity of the glass at SOFC operating temperature should fall in the desired range. If viscosity is too low then glass can not be used as a sealant because it will squeeze out of the SOFC stack during its prolonged exposure at higher temperature. And if viscosity of the glass is very high then it will not be able to display its self-healing ability. 2. Viscosity characteristics of the glass should not change with time at SOFC operating temperature. When glasses are exposed to higher temperature they start to crystallize. Crystallization of glasses severely reduces their ability to flow because of increase in its viscosity. Hence, glasses for self-healing seals should have a good stability against crystallization. Glasses which have these characteristics can be very good materials for SOFC sealant application, because they can offer some of the functionalities which are 41

52 unavailable by other materials. Some of the advantages offered by these types of glass materials are discussed below- 1. Seals fabricated from self-healing glasses can be repaired in case of development of any leakage through the seal. Glass will flow and close the cracks causing leakage. 2. If sealing material gets debonded from the surfaces to be sealed, then at higher temperatures glass will flow and will re-establish the bond. This functionality is otherwise unachievable by other glass or glass-ceramic materials. 3. Flow of glass will remove any stresses from the seal and hence even the materials which have very different expansion characteristics can be hermetically sealed against each other. In this research work, self-healing glass seal concept is developed and used to fabricate seals for solid oxide fuel cells. 42

53 3. OBJECTIVES AND APPROACHES There are three principle objectives of this research work. 1. Fabrication of self-healing glass seals for Solid Oxide Fuel cells: Primary objective of this research work is to successfully fabricate a hermetic seal between a chosen metal sheet and a sintered Yttrria Stabilized Zirconia (YSZ) membrane. This seal should be leak tight at higher temperatures and under thermal cycles for long duration of time under SOFC operating conditions. Additionally, this seal should have the ability to self-repair itself in case any leak develops through it during its extended operation. 2. Determination of mechanism and kinetics of crack healing in glasses: In order to demonstrate self-healing seal, leaky seal will have to be heat treated at higher temperatures. To decide the suitable temperature-time profile of this heat treatment, information about the dependence of crack healing rate on the temperature is essential. Variation of crack healing rate with temperature for a series of glasses is experimentally measured, and sequential changes in crack morphology during healing is analyzed to develop a model of kinetics of crack healing in glasses at higher temperature. Experimentally measured crack healing rates are then used to validate the developed model. This model can be used to predict the crack healing rate of glass at a given temperature. 3. Evaluation of self-healing glass seals concept under SOFC operating conditions: If self-healing seals were to be a feasible option for SOFC sealing, these seals should not only show long term hermeticity in oxidizing, reducing and dual atmospheres but should also retain their self-healing ability for long duration of time. 43

54 Hence, long term stability of these seals depends upon thermo- physical stability of the sealant glass (which is stability of the glass against crystallization), mechanochemical stability of different interfaces in the seal assembly, and the ability of these glass seals to retain their self-healing response. All of these factors are experimentally determined to evaluate the potential of this sealing concept for SOFCs. Approaches used in accomplishing these goals are summarized below: 1. Fabrication of self-healing glass seals: Promising metals and glass compositions which were expected to have thermal expansion in the desired range of (9-12 ppm/ 0 C), were selected and their thermal expansions were experimentally measured to ensure close expansion compatibility of these materials with YSZ. Stability of the selected glasses against crystallization was determined by annealing the glasses at C for various times and then X-ray diffraction studies were done to identify any crystalline phase if present. Self-healing response of all the glasses was determined by in- situ video imaging of crack on glass surface during heat treatment. Using these results a glass which was stable against crystallization, showed fast self healing response and had close match in thermal expansion with YSZ, was selected for seal fabrication. Seal fabrication process was then optimized by determining optimum temperature, atmosphere, and pressure conditions for seal making. Fabricated seals were then tested by pressure drop test under various conditions of temperature, pressure and atmospheres to evaluate seal performance. 2. Mechanism and kinetics of crack healing in glasses: Glasses (glass 1, glass4) and glass-ceramic (glass 5, and glass 8) samples were indented using Vickers indent. Due 44

55 to the indent, cracks were generated on the surface of glass samples. These samples were then annealed at different temperatures for varying time. The variation in crack length at different temperature with increasing time was recorded for all of the samples. By analyzing the morphological evolution and length of the cracks, mechanism and kinetic model of crack healing in glasses was developed. 3. Evaluation of self-healing glass seal concept: In order to determine the performance of the seal in SOFC conditions, seals were tested under different temperature, pressure, and atmospheric conditions during prolonged annealing at C and continuous thermal cycling between room temperature and C. 45

56 4. EXPERIMENTAL PROCEDURES 4.1 Selection of Materials Main criteria for selection of specific glass compositions were coefficient of thermal expansion and softening point of the glass. Glass compositions which were expected to have thermal expansion coefficient in the range of ppm/ 0 C, and softening point in the range of C C were selected. Overall nine different glasses were chosen to determine their suitability for SOFC sealing application. Table 4.1 lists the nominal compositions and the basis of selection for these glasses. Table 4.1: composition and basis of selection of glasses For metals, thermal expansion coefficient, oxidation resistance and cost were main criteria for selection. Four metals were selected in this study, basis of their selection along with their compositions are listed in Table 4.2. Table 4.2: Metals and their criteria of selection 8 mol% Yttria Stabilized Zirconia (YSZ) was chosen as the ceramic membrane of the seal, because it is the most common electrolyte for SOFCs. 46

57 4.2 Thermal Expansion Measurement In order to experimentally determine the expansion behavior of the selected materials, dilatometric experiments were done using a horizontal differential dilatometer. For measurements on glasses, dense glass samples of 1 inch length made by tape casting / sintering method were used. For thermal expansion measurement on metals, metal sheet was rolled and pressed and 1 inch long sample was cut from it. YSZ membranes were made using tapes fabricated from 8 mol% yttria stabilized zirconia (YSZ) powder. After binder removal YSZ tapes were sintered at C for 3 hours to make dense YSZ membranes. Dense bar shaped samples of 1 inch length were used for expansion measurements on all the samples. Thermal expansion measurements on all of the samples were done under an inert atmosphere of argon gas at a heating rate of 3 0 C/min. 4.3 Stability of Glasses against Crystallization If glass seals are to maintain their self-healing response for long duration of time under SOFC conditions, then the glass used to fabricate these seals must be stable against crystallization under these conditions. If glass is not stable against crystallization, then newly formed crystalline phases will severely impair its ability to self-heal. In order to determine the stability of these glasses against crystallization, sintered samples of the glasses were annealed in air at C for 500 hours. Presence of any crystalline phase was then identified by X-ray diffraction study on annealed glass samples. 4.4 Self Healing Study on Glasses If final seals are expected to show self healing response under SOFC conditions, then glasses which are being used to fabricate these seals must have self-healing capability. In order to determine the self-healing ability of selected glasses, an innovative 47

58 approach based on in-situ video imaging of self-healing response of glasses was used. For this study, a scratch was intentionally made on the surface of the glass samples using a diamond scribe, and then these samples were placed inside a furnace. CCD camera was attached to the furnace, which provides the unique capability of in-situ examination and video recording of the changes in the size and shape of the crack during continuous heating or during a hold at any temperature. Samples were heated at a rate of 5 0 C/min from the room temperature to the desired healing temperature and during this heating, video showing the state of the crack was continuously recorded. These experiments gave very useful information about the self-healing response of the glasses. Temperature at which appreciable healing started, temperature at which healing completed, time needed for complete healing, and the degree of healing, were extracted from these experiments. 4.5 Kinetics and Mechanism of Crack Healing in Glasses Two compositions of pure glass (glass 4 and glass 1), one glass composition with 30% YSZ (glass 7), and one glass-ceramic composition (glass 5) were selected for this study. Selection of these specific compositions gave the opportunity for systematic study of crack healing behavior of materials with varying glass content. Pure glass compositions had 100% glass content, while glass+ YSZ mixture had 70 wt% glass content. Glass-ceramic composition had very minimal glass content. Sintered samples of these glasses and glass-ceramics were polished with 0.5 micron alumina paste. Polished samples were then indented using a Vickers indent under a load of 10N for 15 seconds. Each sample was indented for 5 indents nearly equally spaced 3mm from each other. Optical micrographs of the indents were taken after these indents were made. Then these samples were heat treated at different temperatures for varying times. After each heat 48

59 treatment cracks were observed under optical microscope and crack length was recorded as a function of time at a given temperature. Mechanism of crack healing was determined based on the sequential changes in crack length and morphology upon heat treatment. Mathematical modeling of these sequential changes gave a model which was used to predict the crack healing rate at any given temperature. This model was then verified using experimental data obtained by crack healing experiments. 4.6 Stability of the Seal Interfaces To determine stability of glass-metal and glass-ysz interface under SOFC operating conditions, coupons of glass with metal and glass with YSZ were prepared. These coupons were annealed for 500 hours at C, and the cross section was studied under scanning electron microscope to determine the stability of sealing materials in contact with each other. 4.7 Fabrication of Seals In order to generate favorable stress pattern at the different interfaces of the seal, thickness of different components of seal should be optimum. Seal assembly was made up of three distinct layers of different materials of metal, glass, and YSZ. Metal layer was in the form of a thin sheet of 200 micron thickness. Glass layer was thin laminates of glass tapes of 250 micron thickness, while YSZ was in the form of a dense membrane of 250 micron thickness. Dimensions of the layers and their arrangement in seal assembly are shown in the Fig It was very important to optimize the conditions during seal fabrication. The optimum sealing conditions were dependent upon the composition of glass and metal 49

60 selected. During seal making process the variables such as sealing temperature, heating and cooling rate, atmospheric conditions and applied load were controlled. 7.5 cm 2.54 cm 7.5 cm 1.9 cm 3.2 cm 1.3 cm 2.54 cm YSZ Glass Metal 3.2 cm cm cm 0.02 cm Figure 4.1: Design of seals. Sealing temperature for seal making should not be very high so as to avoid unnecessary crystallization of the glass and to avoid any type of phase transformation in the metal, which can generate stresses in the seal during cooling. At sealing temperature, the glass should show no or very little crystallization because if glass will crystallize during seal making then it will not be able to flow and form a good bond. Tendency of any particular glass to remain in the glassy state during heating can be expressed quantitatively by the expression for glass forming tendency. Glass forming tendency = (Tc Tg) / Tg. Where, Tc is the glass crystallization temperature and Tg is the glass transition temperature. For sealing glasses this difference in crystallization temperature and glass transition temperature should be high. 50

61 Heating and cooling rates should also be decided carefully. As faster cooling rates can result in uneven heating of the seal assembly causing generation of stresses and poor bonding, and slow heating rate can result in increased crystallization of glass if Tc and Tg for the glass are not very different. During cooling it is preferable to have a slow cooling rate because at this stage stresses start to develop. Atmospheric conditions are also very important because for bonding to occur there should be the formation of oxide scale on the metal surface, which can be achieved either by prior controlled oxidation of the metal surface or by performing sealing process in air. Sealing in air may not be preferable because it can cause excessive oxidation of the metal, resulting in oxide layer of poor adhesion on the metal. It is preferable to perform seal making in an inert atmosphere, but care must be taken to avoid chemical reduction of anode material. Application of external load may also be necessary in order to achieve good sealing, because first, due to gravity glass tends to bond better with the surface below than the surface placed on top of it. And second, application of load can help the spreading of the glass on bonding surface. Amount of load should not be too high otherwise glass may flow out of the sealing area. For seal fabrication, all three constituents of the seal (glass, metal, and YSZ) were first prepared according to specifications of seal design. Glass laminates were first heated to C at a heating rate of 2 0 C/min and kept there for 2 hours to remove organic binders and to give some strength to glass laminate in order to facilitate easier handling. After binder removal, the glass laminates were placed in-between the metal and YSZ layers and this assembly was heated at a rate of 5 0 C/min to a temperature of C, 51

62 where it was held for 2 hours before it was cooled down to room temperature at a rate of 5 0 C/min. Seal fabrication was done under inert atmosphere of argon gas. Flow chart of seal fabrication process is shown in the Fig 4.2. YSZ Powder Glass Powder Metal Sheet Powder Slurry Powder Slurry Window Frame Tape Casting Tape Casting Tape lamination Tape lamination Binder Burnout Window Frame Sintering Binder Burnout Dense Sintered YSZ Piece Dense YSZ + Glass Window Frame + Metal Window Frame Heat Treatment to C in Inert Atmosphere Hermetic Seal Figure 4.2: Flow chart of seal fabrication procedure. 4.8 Leak Testing of Seals The seals were tested at room temperature and higher temperatures by pressurizing the seal from one side and monitoring the change in pressure of that side as a 52

63 function of time using a pressure transducer. A hermetic seal shows a very little or no change in the pressure as a function of time. The seals were also tested after thermal cycling at different temperatures to assess the effect of thermal cycling on hermeticity. Seal assembly was attached to steel housing with which a steel tube was connected. This assembly of seal and steel housing was placed inside a closed end tube furnace. Other end of the tube furnace was sealed against a metal flange with openings for vacuum pump, thermocouple, gas entry into the furnace and gas entry into the steel housing assembly. This set up gives the ability to control the temperature and pressure of the furnace as well as the pressure inside and outside of the steel housing. By controlling the pressure inside the furnace tube and steel housing, one can control the pressure difference ( P) experienced by the seal. Initial seal testing was done at room temperature for 20 hours at a P ~ 150 Torr. Higher temperature leak tests were done for 1 hour at P of Torr. A simplified diagram of seal testing set up is shown in the Fig.4.3. Figure 4.3: Schematic diagram of seal testing set up. In order to demonstrate self-healing ability of seals, leaky seals were heat treated to C for 30 minutes under no differential pressure across the seal. After the completion of heat treatment seals were again tested to check if they can withstand 53

64 differential pressure without any leakage. If seals show hermetic response, it means they are healed by heat treatment. In order to determine stability of self-healing seals, one of the seal which showed self-healing response during short term testing was selected for testing under different temperatures and atmospheric conditions. During this long-term testing, seal was continuously thermally cycled between room temperature and C. Effect of thermal cycling on hermeticity of the seal and effect of exposure at higher temperature on ability of the seal to self-heal was investigated using previously discussed pressure drop test. Main aim of this long term testing was to determine whether seal retains its ability to selfheal after long time exposure at C and how many thermal cycles seal can withstand without getting irreparably damaged. 54

65 5. RESULTS AND DISCUSSIONS 5.1 Thermal Expansion Behavior Thermal expansion characteristics of glasses and YSZ are shown in Fig 5.1. All of the glasses have close match in thermal expansion with YSZ, but their softening points are different. Thermal expansion characteristics of metals and YSZ are shown in Fig 5.2. Difference in thermal expansion of metals with respect to YSZ is higher than the glasses. Nickel and SS 304 have a very big difference in thermal expansion with respect to YSZ. But, Crofer and SS 430 have a close match with YSZ. Expansion Expansion (ppm) Glass 1 Glass 4 glass1 glass2 glass3 glass4 glass5 glass7 glass6 glass8 glass9 YSZ Glass 5 YSZ Glass 7 Glass 9 Glass 8 Glass 6 Glass 2 Glass Temperature ( 0 C) Figure 5.1: Thermal expansion characteristics of glasses and YSZ. 55

66 Expansion Expansion (ppm) SS304 Ni Crofer YSZ SS Temperature ( o C) Figure 5.2: Thermal expansion characteristics of metals and YSZ. 5.2 Crystallization Behavior of Glasses Glasses for self-healing glass seals should exhibit minimal or no crystallization under SOFC operating temperatures. Glasses selected in this study were studied for their crystallization behavior in order to determine their stability at higher temperatures. Selected glasses were annealed in air at C for different times and X-ray diffraction studies were conducted on annealed glass samples to detect the presence of any crystalline phase. Glass 1 Glass 1 did not show much stability against crystallization at C. Just after 10 hours at C, it starts to crystallize and forms 2BaO.3SiO 2 (Fig.5.3). Barium silicate phases exhibit large expansion coefficients (9-13 ppm/ 0 C) but progressive increase in the 56

67 amount of these phases at C, is not desirable because expansion behavior of glass will change with time at SOFC operating temperature. Furthermore, due to the crystallization of this glass its self-healing ability will be negatively affected. Figure 5.3: Crystallization behavior of glass 1. Glass 2 Glass 2 crystallizes to form Zinc aluminum oxide, for this glass also crystallization starts very early at C, and just after 10 hours at the temperature (Fig. 5.4), crystalline phase of ZnO.Al 2 O 3 appears. ZnO.Al 2 O 3 phase has undesirably low thermal expansion coefficient which will result in decrease in thermal expansion coefficient of the glass with increasing time at C. 57

68 Figure 5.4: Crystallization behavior of glass 2. Glass 3 Glass 3 is also not stable against crystallization at C, and starts to form Zinc aluminum oxide after just 10 hours at C (Fig 5.5). Small cations of high field strength tend to be surrounded by more ordered arrangement of oxygen ions than the large cations of low field strength. Due to this more ordered structure of the glasses containing small cations, devitrification tendency of these glasses is high. Glass 2 and Glass 3 contain sufficient amount of zinc oxide. Zinc ion has fairly high field strength, and this ion exert a marked ordering effect on surrounding oxygen ions. Due to this effect exerted by zinc ions, glass 2 and glass 3 have higher tendency for devitrification. 58

69 Figure 5.5: Crystallization behavior of glass 3. Glass 4 Glass 4 exhibits very good stability against crystallization at C, and it does not form any crystalline phase even after annealing for 500 hours (Fig 5.6). This is a very desirable feature of this glass, because its thermo-physical properties will not change with time at SOFC operating temperatures. Absence of any devitrification in this glass also ensures the retention of fast self-healing response exhibited by this glass. Because, this glass does not crystallize at C, it exhibits better flow behavior on metals and YSZ. Glasses which crystallize tend to show poor flow characteristics due to the formation of crystal phases which increase the viscosity of the glass. 59

70 Glass 4 annealed in air at C 1100 Annealed at C for 500 hours 900 Annealed at C for 400 hours Intensity (a.u.) Annealed at C for 300 hours Annealed at C for 200 hours Annealed at C for 100 hours 100 As sintered Theta Figure 5.6: Crystallization behavior of glass 4. Glass 5 Glass 5 is a glass ceramic and it contains barium alumino-silicate phases in both hexacelsian and monocelsian form (Fig 5.7). Hexacelsian has a higher coefficient of thermal expansion in the range of (7-8 ppm/ 0 C), while monocelsian exhibit very low expansion in the range of (2-3 ppm/ 0 C). This glass-ceramic also forms an additional phase of barium lanthanum silicate, which also exhibits low expansion. Due to high ceramic content this glass-ceramic does not exhibit fast self-healing response, but glasses in this system show good chemical stability in the SOFC operating conditions. Celsian and Hexacelsian phases have similar chemical composition but they differ in their crystal structure. Due to different structures these two phase show dissimilar expansion behavior. 60

71 Figure 5.7: Crystallization behavior of glass 5. Glass 6 Glass 6 crystallizes after 50 hours at C to form calcium zinc silicate and strontium containing calcium alumino-silicate (Fig 5.8). These phases have higher expansion coefficient and expansion characteristics of the glass are not altered drastically due to the crystallization. This glass is an invert glass which means, instead of glass former, glass modifiers are responsible for formation of glass network. Low connectivity of this type of glass readily causes formation of crystalline phases in the parent glass. Crystallization in the glass increases the viscosity and retards self-healing. Due to start of crystallization only after sufficient long time at C (50 hours), this glass does not suffer from the problem of glass crystallization during seal making. If glass crystallizes during seal fabrication it does not flow and bond to the surfaces properly. 61

72 Figure 5.8: Crystallization behavior of glass Self Healing Behavior Self healing studies on the glasses show that glass heals over a range of temperatures. Generally, for the glasses without any crystalline species, healing temperature is close to its softening point. For pure glass sample healing is rapid and it takes nearly minutes for cracks to completely disappear. On the other hand glasses which have crystalline phases, show a large difference between healing temperature and softening point. These glasses heal very slowly and sometimes healing is incomplete. In-situ video recording of crack during healing process also provided the ability to check if glass can be healed completely or not. Still photographs from the recorded video of the crack on the glass sample before and after healing were used to determine the degree of healing. Most of the glasses heal completely provided that the temperature is high enough and sufficient time was allowed during crack healing experiment. Figure 62

73 5.9 shows the condition of a crack on the glass surface before and after healing treatment. This crack has healed completely as shown in the figure on the right. Glass Unhealed Glass Healed Figure 5.9: Still photographs of crack on glass sample before and after healing. Unique test set up for healing study allowed in situ crack monitoring, which gave useful information on the start and the end healing temperatures. Table 5.1 summarizes data on the healing study on glasses. Table 5.1: Self healing response of glasses Glass Glass Type Softening (Ts) Healing start (T1) Healing end (T2) Comment Glass1 Glass C C C Complete Healing Glass2 Glass C C C Complete Healing Glass3 Glass C C C Complete Healing Glass4 Glass C C C Complete Healing Glass5 Glass-ceramic C C C Very Slow Healing Glass6 Glass C C C Complete Healing Glass8 Glass-ceramic C C C Incomplete Healing The mean healing temperature, which is the average of start and end of healing temperatures, is plotted against softening point of glasses in Fig. 5.10, which shows that the glasses without any crystallization have values of healing temperatures very close to 63

74 the softening point. On the other hand, glasses with crystalline phases do not show this trend Softening point Vs healing temparture Softening Point ( 0 C) Glass-Ceramics Healing Temperature ( 0 C) Figure 5.10: Softening point and healing temperature of glasses. 5.4 Kinetics and Mechanism of Crack Healing Mechanism of Crack Healing Detailed crack healing experiments on glass samples showed that the crack healing of the glass can be divided into three stages. In the first stage, crack tip blunting is observed along with some other changes in crack morphology, which results in cylinderization of the crack. In the second stage, this cylindrical crack gets filled by viscous flow of glass and results in spherodization of the crack. In the third stage, spherical crack cavity also gets filled and results in crack free surface. These stages of healing are shown in Fig

75 Fresh Crack Stage 1 Stage 2 Stage 3 Complete healing Figure 5.11: Different stages of crack healing in glass Model of Crack Healing Healing of cracks generated by Vickers indent on a glass surface, can be divided into three stages as mentioned earlier. Total time required for healing of indented crack can then be determined by addition of time required for completion of each of these three individual stages. Stage 1 It is reasonable to assume that when blunting of crack tip occurs during healing, other parts of the crack also go through some morphological changes. It is not possible to differentiate between the end of the process of crack tip blunting and start of crack regression, because both of these processes can be occurring at the same time. And even if crack regression (up to the time crack becomes cylindrical in shape) follows crack tip blunting, it is very hard to draw a timeline between them. So, for simplicity in 65