based/Silver Glass Composites Seals for Solid Oxide Fuel Cells Yaneth Aguilar-Diaz

183 Pages·2011·47.01 MB·English

Evaluation of Silica-based/Nickel and Borate-
based/Silver Glass Composites Seals for Solid
Oxide Fuel Cells

Yaneth Aguilar-Diaz

Department of Mining and Materials Engineering,
McGill University, Montreal

August, 2011

A thesis submitted to McGill University in partial fulfillment of the
requirements of the degree of Doctor of Philosophy.

© Yaneth Aguilar-Diaz 2011
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A
BSTRACT

The increasing demand for energy and the necessity to overcome the depletion
of fossil fuel supplies requires that alternative energy sources be developed.
Solid Oxide Fuel Cells (SOFCs) are one of the alternative technologies to
minimise our dependence on fossil fuel due to their numerous advantages
including high efficiency, long-term stability, fuel flexibility and low emissions.
However, the development of reliable sealing techniques remains a crucial
challenge to overcome to allow usable efficiency and facilitate
commercialization.
Sealing technology has been object of research for several years. Nevertheless,
the optimal solution is yet to be found. The use of a glass composite approach is
attractive as it allows the possibility of engineering the properties of the seal, by
independently adjusting the particle size distribution and volume fraction of the
additives. In the present work, the interaction between various SiO based
2
glasses with nickel and B O based glasses with silver were studied. Results as
2 3
a function of additive particle size distribution (7-100 microns) and volume
fraction (0-18%) will be presented.
Micrographs, X-ray patterns and CTE measurements showed that the proposed
systems have adequate characteristics for usage as seal for fuel cells due to the
inertness of the additive particles with the respective glass matrix and
predictable long-term chemical and thermal stability.
The use of DTMA as a technique to calculate the onset of residual stresses,
explores the influence of the additive and its interfacial interactions on the
dissipation of energy during deformation. The multi-frequency test lead to an
i
activation energy for stress relaxation between 400 and 600 kJ/mol depending
on the different additive content. Furthermore, the temperature difference
between de T and the onset of residual stresses was calculated showing that
g
increments on the additive content results on a larger temperature range that
allows stress relaxation.
The mechanical response under compression test was also investigated to
identify the potential deformation of a stack during service. The results showed
that the glass composites can experience large deformations during the entire
service cycle and not only during the isothermal service hold. Moreover, the
microstructure in terms of crystalline phase evolves with the test temperature
and the applied force, showing an increase of the crystals volume fraction when
either the temperature of the applied load increase. The microstructures showed
that the additive is getting aligned during deformation, providing an increased
resistance to compression against flow of the viscous glass composite.
Finally the measurement of the residual stresses as function of cooling rate and
additive content revealed that the residual stresses development is minimised
for a combination of service conditions including cooling rate under 20 °C/min
and glass composite containing a minimum of 12 %vol. Such operating
conditions should contribute to maximise the service life of a SOFC stack.
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R
ÉSUMÉ

La demande croissante en énergie et la nécessité de surmonter les défis
d'épuisement des réserves de combustibles fossiles exigent que des sources
d'énergies alternatives soient développées. Les piles à combustible à électrolyte
solide sont l'une des technologies alternatives pour réduire notre dépendance
aux combustibles fossiles en raison de leurs nombreux avantages, y compris
leur haute efficacité, stabilité à long terme, flexibilité dans le choix du carburant
et leurs faibles émissions. Cependant, le développement de techniques fiables
pour joindre les composantes demeure un défi important à relever pour obtenir
une efficacité utilisable et pour faciliter la commercialisation.
Les technologies de jointage ont été l'objet de recherches depuis plusieurs
années. Néanmoins, la solution optimale demeure encore à être trouvée.
L’approche du composite de verre est intéressante car elle permet la possibilité
d’optimiser les propriétés du joint en ajustant de façon indépendante la
distribution de la taille des particules et la fraction volumique des additifs. Dans
le présent travail, l'interaction entre des différents verres composites SiO avec
2
nickel, et des verres composite B O avec de l’argent ont été étudiés. Les
2 3
résultats, en fonction de la distribution de la taille des particules (7-100
microns) et la fraction volumique des additifs (0-18%) seront présentés.
Les micrographies, la diffraction des rayons X et les calculs du coefficient
d’expansion thermique ont démontré que les systèmes proposés ont les
caractéristiques adéquates pour leur utilisation en tant que joint pour les piles
à combustible, en raison de l'inertie des particules d’additif avec la matrice de
verre, et en raison de la prévisibilité à long terme de la stabilité chimique et
thermique.
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L'utilisation de l’analyse thermomécanique dynamique comme technique pour
calculer l'apparition de contraintes résiduelles, explore l'influence de l'additif et
ses interactions relatives à l’égard de la dissipation de l'énergie pendant la
déformation. Les tests à multifréquences a mené à une énergie d'activation
variant entre 400 et 600 kJ/mole pour la relaxation des contraintes, er ce en
fonction des différentes quantités d’additifs. De plus, la différence de
température entre T et l'apparition de contraintes résiduelles a été calculée, et
g
démontre que des incréments de quantités d’additif résultent en un
augmentation de l’interval de températures pouvant permettre la relaxation des
contraintes.
Les résultats du point de vue mécanique, pour des tests de compression ont
également été étudiés afin d'identifier les déformations potentielles des
assemblages durant l’opération. Les résultats ont montré que les composites de
verre peuvent subir de grandes déformations au cours du cycle entier
d’opération et non pas seulement pendant la période isotherme. De plus, la
microstructure, en termes de phases cristallines, évolue avec la température
d'essai et la force appliquée, montrant une augmentation de la fraction
volumique des cristaux avec l'augmentation de la température et de la charge
appliquée. Les microstructures ont démontré que les particules s’alignent
pendant la déformation, offrant une résistance accrue contre la compression,
résultant de l'écoulement du composite de verre visqueux.
Enfin, le calcul des contraintes résiduelles en fonction de la vitesse de
refroidissement et de la fraction volumique d’additifs a révélé que le
développement des contraintes résiduel est minimisé dans quelques
combinaisons de conditions d’opération, incluant un taux de refroidissement en
dessous de 20 °C/min et une fraction volumique minimum de 12% en additif.
Ces conditions d’opérations devraient contribuer à maximiser la durée de vie
des assemblages de piles à combustible à électrolyte solide.

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A
CKNOWLEDGMENTS

I want to acknowledge CONACyT (National Council of Science and Technology in
México); for granting me a scholarship and and therefore make my studies at McGill
University possible.
I would like to express my most sincere gratitude to Prof. Mathieu Brochu. Many Thanks
for the guidance and confidence deposited on me; I am thankful for the chance to
participate in several conferences.
Sincere thanks to Dean Robin A. L. Drew for all the helpful comments and suggestions.
Also I would like to express my gratitude to Prof. Richard K. Brow from the Department
of Materials Science & Engineering in the University of Missouri-Rolla for providing the
glass for this research.
I want to express my gratitude to the technicians of the different Laboratories at McGill:
Helen Campbell, Monique Riendeau, and Ray Langlois, they provided valuable help on
the use of several research equipment. Thank you all.
Finally I want to thank my fellow graduate students: Dr. Farzad Jalilian, who gave me a
warm welcome and show me how to move around McGill, PhD Paula Proa for her
valuable help and friendship, my girl friends PhD C. Ramona Vintila and PhD C. Lydia
Aguirre for helpful and passionate discussions besides all the moments that made this
experience even more treasured. Also my officemates Le Hong, Faramarz, Mehdi, Amir,
and all the people that I got to meet during this time; you guys enriched my student
experience at Mcgill and made these years very enjoyable.
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T C
ABLE OF ONTENTS

ABSTRACT i
RÉSUMÉ iii
ACKNOWLEDGEMENTS v
TABLE OF CONTENTS vi
LIST OF FIGURES ix
LIST OF TABLES xiii
1. INTRODUCTION 1
2. LITERATURE REVIEW 4
2 .1 ENERGY AND ENVIROMENT 4
2 .2 FUEL CELLS 6
2.2.1 Fuel Cell History 6
2.2.2 Fuel Cell Efficiency 9
2.2.3 Fuel Cell Classification 10
2.3 SOLID OXIDE FUEL CELLS 11
2.3.1 SOFC Components 12
2.3.2 Stack Designs 16
2.3.2.1 Seal-less Tubular Design 17
2.3.2.2 Segmented Cell in Series Design 18
2.3.2.2 Monolithic Design 21
2.3.2.1 Flat Plate Design 22
2.4 REQUIREMENTS AND APPROACHES FOR SEALING SOFCs 24
2.4.1 Seal Requirements 24
2.4.2 Sealing Techniques 27
2.4.2.1 Compressive Seals 27
2.4.2.2 Bonded Compliant seals 30
2.4.2.3 Glass Seals 31
2.4.2.3.1 Invert glasses 34
2.4.2.4 Glass Ceramic Seals 35
2.4.2.5 Glass Composite Seals 37
2.4.2.6 Brazing 40
2.5 THERMAL STRESSES ON pSOFCs 42
2.5.1 Residual Stresses 42
2.5.2 Thermal Stresses on SOFC’s 43
3. OBJECTIVES 50
3.1 Long term stability of the glass composite seals 51
3.2 Potential absorption of the residual stresses by a glass composite 51

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3.3 Assessment of the mechanical behaviour of composites above Tg 52
3.4 Estimation of the residual stresses development 52
4. EXPERIMENTAL PROCEDURE 53
4.1 STARTING MATERIALS 53
4.1.1 Preparation of the Glass 55
4.2 GLASS – COMPOSITE MIXTURES 57
4.3 CERMET PREPARATION 58
4.4 WETTING, FLOW AND LONG TERM STABILITY 58
4.5 CHARACTERIZATION TECHNIQUES 59
4.5.1. Microstructural Characterization 60
4.5.2. Thermo Mechanical Analysis 60
4.5.3. X-Ray Diffraction 62
4.5.4. Differential Thermal Analysis 63
5. LONG TERM STABILITY OF GLASS COMPOSITE SEALS 64
5.1. INITIAL POWDER CHARACTERIZATION. 64
5.1.1 Thermal Properties Of Glasses 66
5.1.1.1. Crystallization Kinetics 68
5.2. WETTING AND CHEMICAL REACTIVITY 71
5.2.1 Wetting 71
5.2.2. Chemical Reactivity and Microstructure 73
5.2.2.1 Reactivity Sealing in Air 73
5.2.2.2 Reactivity Sealing in Ar/H 75
2
5.2.2.3 Reactivity Induced by Thermal Cycling 79
5.2.2.4 Reactivity of SiO2 based glasses with Ni Additives 80
5.2.2.5 Reactivity of B O based glasses with Ag Additives 83
2 3
5.3. LONG TERM STABILITY 85
5.3.1 SiO Based Glasses 85
2
5.3.2 B O Based Glasses 86
2 3
5.4. THERMAL EXPANSION COEFFICIENT MISMATCH 89
6. DETERMINATION OF THE ONSET OF RESIDUAL STRESSES TEMPERATURE IN
94
GLASS COMPOSITES
6.1 VISCOELASTICITY ON GLASSES 94
6.2 DEVELOPMENT OF THE ANALYTICAL PROCEDURE 98
6.3 INVESTIGATION OF THE GLASS COMPOSITE SEALS UNDER DMTA
101
STRESS
6.3.2 Effect of Frequency on DTMA Response 103
6.3.3 Effect of Particle Size 104
6.4 COMPARISON WITH OTHER TECHNIQUES USED TO MEASURE
109
RESIDUAL STRESS FORMATION
7. MECHANICAL BEHAVIOUR OF GLASS COMPOSITE SEALS ABOVE T 111
g
7.1 MECHANICAL COMPRESSION TESTS 115
7.1.1 Flow Behaviour 115
7.1.2 Mechanical Compression Tests with Ramp Load 116
7.1.3 Mechanical Compression Tests at Constant Load 121
8. RESIDUAL STRESS DEVELOPMENT 128
8.1 EFFECTS OF RESIDUAL STRESSES ON A MATERIAL’S MECHANICAL
128
PROPERTIES
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8.1.1 Residual Stress Measuring Methods 129
8.1.2 X-Ray Diffraction Method 129
8.2 METHODOLOGY FOR LATTICE STRAIN CALCULATIONS 130
8.3 LATTICE STRAIN AS A FUNCTION OF COOLING RATE AND
134
PARTICLE SIZE
8.3.1 G67L – Ni Composite Lattice Strain 135
8.3.2 G38 – Ni Composite Lattice Strain 137
8.3.3 G27 – Ni Composite Lattice Strain 139
8.3.4 Composite Lattice Strain by the Scherrer Method 141
8.4. STRESS AS A FUNCTION OF COOLING RATE AND PARTICLE SIZE 144
8.4.1 Composite Residual Stresses 144
8.4.2 Composite Stress by the Scherrer Method 148
9. CONCLUSIONS 154
9.1 LONG TERM STABILITY OF GLASS COMPOSITE SEALS 154
9.2 ABSORPTION OF RESIDUAL STRESSES BY THE USE OF GLASS
155
COMPOSITES
9.3 MECHANICAL BEHAVIOR OF GLASS COMPOSITE ABOVE TG 155
9.4 RESIDUAL STRESS DEVELOPMENT 156
10. CONTRIBUTION TO ORIGINAL KNOWLEDGE 157
11. REFERENCE LIST 158

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