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Thermo‐mechanical Behavior of Glass Based Seals for Solid Oxide Fuel Cells
A dissertation submitted to the
Graduate School
Of the University of Cincinnati
In partial fulfillment on the
Requirements for the degree of
Doctor of Philosophy
In the Materials Science program
Of the College of Engineering and Applied Science
By
Sandeep K. Singh
M.S., Indian Institute of Technology, 2005
Committee Chair: Dr. Raj Singh
ABSTRACT
Availability of durable seals for planar solid oxide fuel cells (pSOFC) is one of the main limitations to the fabrication of reliable fuel cell stacks. Glass and glass ceramics are often used as sealing materials to seal cell components, because of their flexibility in matching coefficient of thermal expansion with sealing components. In this work glass and glass composites containing ceramic fillers with suitable thermal, chemical and sealing behaviors are explored for use in a novel concept on self-repairable seals. This concept requires a glass with low viscosity at the SOFC operating temperature, which contradicts with the requirement of creep resistance.
A silicate glass with self-healing capability is selected based on their thermo- mechanical properties as the starting material for seals, but this glass may suffer from excessive flow at elevated temperatures, which leads to squeezing of the glass.
Therefore, to alleviate this glass composite with fillers and suitable thermo-mechanical properties are promising because the glass matrix can flow to mitigate stresses and heal cracks, whereas the filler controls the flow behavior of the glass and improves the creep resistance and seal strength. In order to make glass composites, a range of ceramic fillers such as Al2O3, MgO, and YSZ are considered. The effect of filler materials and filler volume fraction on the thermo-mechanical properties such as coefficients of thermal expansion, glass softening temperature and viscosity are investigated.
Sessile drop method is used to investigate the glass and glass composite properties useful for sealing. The variation of contact angles with temperature and time are measured from which activation energy for wetting, work of spreading, viscosity and surface tension are obtained. The surface tension values are determined based on
i sessile drop model by following Dorsey and Porter approaches. A new approach based on sessile drop model and using wetting dynamics is investigated to determine viscosity at elevated temperatures. The viscosity values are also obtained using a number of different approaches such as sessile drop method, creep measurement, Vogel-Fulcher-
Tamman (VFT) and Moynihan model equations. The experimentally measured viscosity values are fitted with modified-VFT model equation to construct viscosity-temperature curve over a wide range.
Long term thermal, chemical, electrical resistivity and volatilization stabilities of sealing glass are evaluated in dual oxidizing and wet reducing environments at 8000 C.
The chemical compatibility of a self healing glass with cell components shows no microstructural evidence of any new reaction phases present at the interface. The measured electrical resistivity of glass at 8000 C is 105 cm and is stable over the test period of 1000 h. The weight loss stability test showed negligible weight loss (1 wt. % for the time period of 5 years). Based on these results, the selected glass and glass composites with YSZ filler seem to be a promising candidate for SOFC seals.
KEYWORDS: Glass Seals, Glass Composites, Sessile Drop, Wetting, Surface Tension,
Viscosity, Work of Spreading, Creep.
ii iii ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to my advisor Prof. Raj N. Singh for the continuous support of my Ph.D. study and research, for his patience, motivation, enthusiasm, and immense knowledge. I appreciate all his contributions of time, ideas, and funding to make my Ph.D. experience productive and stimulating. The joy and enthusiasm he has for his research was contagious and motivational for me even during tough times in the Ph.D. pursuit. I am also thankful for the excellent example he has provided as a successful Materials Scientist and Professor.
Besides my advisor, I would like to thank the rest of my Ph.D. dissertation committee members, Dr. R. C. Buchanan, Dr. R. D. Roseman, and Dr. V. N. Shanov for their encouragement and insightful comments. Mentoring experience during PFF program under Dr. R. D. Roseman is always appreciable.
I thank my colleagues Dr. Nirmal Govindraju, Chaitanya Kane, Akshay, Parveen,
Pravahan, for the stimulating discussions and for all the fun we have had in the last four years.
I would like to thank my parents who raised me with a love of science and supported me in all my pursuits. I wish to thank my entire extended family, my father-in- law, mother-in-law, sister-in-law for providing a loving environment for me.
Last but not least, I am indebted to my loving, supporting and encouraging wife
Roli Singh whose continuous support during each and every stages of life is so appreciated. Without her unflagging love, encouragements, understanding and support it would have been impossible for me to finish this work.
iv TABLE OF CONTENTS
TITLE PAGE NO.
ABSTRACT i
ACKNOWLEDGEMENTS iv
TABLE OF CONTENTS v
LIST OF TABLES ix
LIST OF FIGURES x
LIST OF ABBREVIATIONS xiv
LIST OF SYMBOLS xv
1. INTRODUCTION 1
1.1. Fuel Cells 1
1.2. Different Types of Fuel Cells 1
1.3. Solid Oxide Fuel Cell 4
1.3.1. Cell and Stack Designs 6
1.3.2. Interconnect Materials 9
1.4. Sealing Concepts and Materials 12
1.5. Sealing Technology 15
1.5.1. Compressive Seals 15
1.5.2. Compliant Bonded Seals 17
1.5.3. Rigid Seals 18
2. LITERATURE REVIEW 22
2.1. Silicate Based Glass Seals 22
2.2. Composite Sealants 28
2.3. Challenges for Glass-Based Sealants 29
v 2.4. Measurement Techniques of Surface Tension 30
2.4.1. The Capillary Rise Method 31
2.4.2. The Drop Weight Method 31
2.4.3. The Fiber Elongation Method 32
2.4.4. The Dipping Cylinder Method 32
2.4.5. Maximum Bubble Pressure Technique 34
2.4.6. Drop Shape Methods 35
2.4.6.1. The Sessile Drop Technique 35
2.4.6.2. The Pendant Drop Technique 36
2.5. Viscosity Measurement Techniques 38
2.5.1. Rotation Viscometers 38
2.5.2. Falling Sphere Viscometers 39
2.5.3. Fiber Elongation Viscometers 39
2.5.4. Beam Bending Viscometers 40
2.5.5. Parallel Plates 41
2.5.6. Viscosity from Creep Data 41
2.5.7. Other Viscometers 42
3. OBJECTIVES AND APPROACHES 43
4. EXPERIMETAL PROCEDURES 45
4.1. Materials Selections 45
4.2. Materials Processing 46
4.3. Characterization Techniques 47
4.3.1. Dilatometric Measurements 47
4.3.2. X-Ray Diffraction 47
vi 4.3.3. Differential Scanning Calorimetry 47
4.3.4. SEM-EDAX Analysis for Interfacial Study 48
4.3.5. Three Point Beam Bend Test 48
4.3.6. Weight Loss Stability Test 48
4.3.7. Wettability Test using Sessile Drop Method 49
4.3.8. Electrical Resistivity Test 50
5. RESULTS AND DISCUSSIONS 52
5.1. Thermal Expansion Behavior of SOFC Components 52
5.1.1. Thermal Expansion of SOFC Materials 52
5.1.2. Thermal Expansion of Ceramic Fillers 53
5.1.3. Thermo-mechanical Behavior of Glass Composites 53
5.1.4. Long Term Annealing Behavior of Glass and 55 Glass Composite Seals
5.2. Wetting Behavior of Glass/Glass Composites with Cell Components 57
5.2.1. Activation Energy for Wetting/Spreading 63
5.2.2. Work of Spreading of Glass/Glass Composite 65
5.3. Sessile Drop Model for Surface Tension and Viscosity 67
5.3.1. Determination of Surface Tension 62
5.3.2. Determination of Viscosity 73
5.4. Viscosity of Glass and Glass Composite 81
5.4.1. Viscosity from VFT Model Based on Dilatometric Data 81
5.4.2. Viscosity from Moynihan Model Based on DSC Data 83
5.4.3. Viscosity from Creep Data 86
5.4.4. Viscosity Comparisons Based on Experimental and 88 Model Equations
vii 5.4.5. Activation Energy for Viscous Flow 90
5.5. Microstructural and Interfacial Analysis 93
5.5.1. Glass-YSZ Electrolyte Interface 93
5.5.2. Glass-Crofer22 Interconnect Interface 94
5.5.3. Glass-441 SS Interface 95
5.5.4. Glass- Spinel ( Mn1.5Co1.5O4 ) Coated 441 SS Interface 96
5.5.5. Glass- Aluminized 441 SS Interface 98
6. CONCLUSIONS 100
7. FUTURE DIRECTIONS 104
8. REFERENCES 105
9. LIST OF PUBLICATIONS 118
10. APPENDIX 119
10.1. Stability against Weight Loss of Glass in Dual Oxidizing 119 and Wet Reducing Environments
10.2. Stability of Electrical Resistivity of Glass/Glass-Metallic 120 Interconnects
viii LIST OF TABLES
Table No. Title Page No.
Table 1.1 Coefficient of Thermal Expansion (CTE) of Some 12 Materials for SOFC.
Table 1.2 Functional Requirements of Sealing Systems for 15
SOFCs.
Table 1.3 Overview of Potential Sealing Technology for SOFCs. 21
Table 4.1 Chemical Composition of Metallic Interconnects used. 45
Table 4.2 Physical parameters of Particulate Fillers. 46
Table 5.1 Sealing Temperature, Bonding Nature and 61 Crystallization of Seals.
Table 5.2 Surface Tension of Silicate Glasses at Respective 73 Temperatures.
Table 5.3 Sample Dimensions and Creep Test Conditions for 87 Glass/G+10YSZ.
ix LIST OF FIGURES
Figure No. Title Page No.
Figure 1.1 Types of Fuel Cells. 2
Figure 1.2 Schematic representation of a planar SOFC and the 5 electrochemical reactions.
Figure 1.3 Schematic of a SOFC designs (a) Tubular and (b) 7 Planar.
Figure 1.4 Planar cell concept (a) electrolyte and (b) anode 8 supported.
Figure 1.5 (a) Different types of seals (b) cross flow structure 13 (c) single cell.
Figure 2.1 Glass transition temperature (Tg ) and coefficient of 24 thermal expansion (CTE) of sealant glasses for SOFCs. The frame represents the target range as defined by Geasee.
Figure 2.2 Measurement techniques to determine surface 33 tension.
Figure 2.3 Measurement techniques to determine viscosity. 40
Figure 4.1 Schematic diagram of the apparatus used for glass 49 seal annealing and weight loss experiment in wet reducing environment.
Figure 4.2 Schematic of the set-up for sessile drop method 50 used for wetting/spreading experiment.
Figure 4.3 Schematic of the set-up for electrical resistance 51 measurements.
Figure 5.1 Thermal expansions data for SOFC materials (a) 52 expansion and (b) CTE.
Figure 5.2 Thermal expansion curves of filler materials (a) 54 thermal expansion and (b) CTE.
Figure 5.3 Effect of different types of fillers on (a) CTE and (b) 54 softening temperature.
x Figure 5.4 Glass + Al2O3 composite : effect of annealing on (a) 56 expansion and (b) phase formation.
Figure 5.5 Glass + MgO composite : effect of annealing on (a) 56 expansion and (b) phase formation.
Figure 5.6 Glass + YSZ composite : effect of annealing on (a) 57 expansion and (b) phase formation.
Figure 5.7 In situ photographic images of glass and glass 58 composites tested on YSZ and metallic substrate at different temperatures.
Figure 5.8 In situ images after relaxation for seal samples at 59 different temperatures (a) glass on YSZ substrate (b) glass on SS 441 metal substrate (c) glass composite on YSZ substrate (d) glass composite on SS 441 metal substrate.
Figure 5.9 Wetting behavior of glass and glass composite on 60 cell components (a) contact angle vs temperature (b) contact angle vs time.
Figure 5.10 Time evolution of contact angle for glass spreading 62 on cell components (a) glass on YSZ and (b) glass on SS 441 metal.
Figure 5.11 Time evolution of contact angle for glass with 10 62 wt.% YSZ filler spreading on cell components (a) glass on YSZ (b) glass on SS 441 metal.
Figure 5.12 Logarithmic plot of contact angle versus time : (a, b) 64 glass tested on YSZ and on metal and (c, d) glass/10 wt.% YSZ composite on YSZ and on metal substrate, respectively.
Figure 5.13 Glass and glass-composite (a) Arrhenius plots for 65 contact angle and (b) activation energy for wetting in SOFC operating range.
Figure 5.14 Thermodynamic work of spreading of seals on cell 67 components.
Figure 5.15 (a) Sessile drop and (b) definition of co-ordinates 68 systems.
Figure 5.16 Schematics of geometrical parameters based on 69 Dorsey and Porter approach.
xi Figure 5.17 Image profiles of glass at different temperatures for 70 surface tension calculation using Dorsey and Porter equation based on sessile drop model.
Figure 5.18 Image profiles of G+10 wt.% YSZ composite at 71 different temperatures for surface tension calculation using Dorsey and Porter equation based on sessile drop model.
Figure 5.19 Variation of surface tension of glass with (a) 71 temperature and (b) time.
Figure 5.20 Variation of surface tension of G-10YSZ with (a) 72 temperature and (b) time.
Figure 5.21 Schematics to describe wetting dynamics using the 74 hydrodynamic model.
Figure 5.22 (a) Viscous spreading, (b) fluid velocity profile in the 75 liquid wedge close to the triple line, and (c) motion of the liquid in the vicinity of the triple line ( : liquid molecule sliding) over a wedge.
Figure 5.23 Time evolution of drop radius for glass spreading on 77 cell components (a) glass on YSZ and (b) glass on SS 441 metal.
Figure 5.24 Time evolution of drop radius for glass with 10 wt. 77 % YSZ filler spreading on cell components (a) glass on YSZ and (b) glass on SS 441 metal.
Figure 5.25 Capillary number vs. contact angle of (a) glass and 79 (b) glass composite.
Figure 5.26 Spreading velocity vs. contact angle of (a) glass 79 and (b) glass composite.
Figure 5.27 Viscosity temperature data based on the sessile 80 drop model.
Figure 5.28 Determination of Tg and Ts from dilatometer curve. 83
Figure 5.29 ' 84 DSC profile of glass and determination of Tg ,Tg .
Figure 5.30 Viscosity-temperature curves for glass based on 85 VFT and Moynihan models.
xii Figure 5.31 Glass and glass composites viscosity (a) based on 86 Moynihan model and (b) comparison of viscosity obtained from different approaches.
Figure 5.32 Strain versus time plot based on creep data for (a) 87 glass and (b) glass composite.
Figure 5.33 Viscosity versus temperature plot based on the 88 creep rate data.
Figure 5.34 (a) Viscosity comparison based on experimental 89 and model equations (b) VFT Fit of experimental data.
Figure 5.35 Activation energy for viscous flow for glass and 91 glass composite based on the VFT and Moynihan approaches (a) temperature dependence and (b) in SOFC operating range.
Figure 5.36 Activation energy in the softening range (a) 92 Arrhenius plots of glass and glass composite and (b) effect of YSZ filler and model comparison.
Figure 5.37 Glass-YSZ interface annealed at 8000 C in air for 93 3500 hrs.
Figure 5.38 Glass-Crofer22 interface annealed at 8000 C in air 94 for 1000 hrs.
Figure 5.39 Glass-441 stainless steel interface annealed at 8000 95 C in air for 1000 hrs.
Figure 5.40 Glass-spinel coated 441 SS interface annealed at 96 8000 C in air for 1000 h.
Figure 5.41 Spinel coated 441 SS cross-section (a) as received 97 (b) after 1000 hrs annealing.
Figure 5.42 Glass-aluminized 441 SS interface annealed at 98 8000 C in air for 1000 hrs.
Figure 10.1 Weight loss of glass as a function of annealing time 120 tested at 8000 C showing (a) wt. loss per unit area and (b) weight of glass with time.
Figure 10.2 (a) Resistivity versus temperature plot of sealing 121 glass (b) Long term resistivity stability of sealing glass at 8000 C in air.
xiii LIST OF ABBREVIATIONS
AFC Alkaline Fuel Cell
APU Auxiliary Power Units
CHP Combined Heat and Power
CTE Coefficient of Thermal Expansion
DSC Differential Scanning Calorimetry
EVD Electrochemical Vapor Deposition
EDAX Energy Dispersive X-ray Analysis
IT Intermediate Temperature
LSM Strontium doped Lanthanum Magnetite
MCFC Molten Carbonate Fuel Cell
MTS Mechanical Testing System
ODS Oxide Dispersion Strengthening
PAFC Phosphoric Acid Fuel Cell
PEMFC Polymeric Electrolyte Membrane Fuel Cell
PNNL Pacific Northwest National Laboratory ppm Part Per Million pSOFC Planar Solid Oxide Fuel Cell
SEM Scanning Electron Microscopy
SOFC Solid Oxide Fuel Cell
SS Stainless Steel
VFT Vogel- Fulcher-Tamman
XRD X-ray Diffraction
8YSZ 8 mol% Yttria Stabilized Zirconia
xiv LIST OF SYMBOLS
E0 Nernst Potential
F Faraday constant p Oxygen Partial Pressure O2
R Molar gas constant
T Absolute temperature
Z Number of charge transfer, oxidation state
Tg Glass transition temperature
Ts Dilatometric softening temperature
Tm Melting temperature
f Field strength a Metal (cation)-oxygen (anion) distance
Coefficient of thermal expansion
0 Equilibrium contact angle
D Dynamic Contact angle h Height of a droplet image
rmax Half of the droplet width
LV Surface tension of liquid-vapor interface
SV Surface tension of solid-vapor interface
SL Surface tension of solid-liquid interface p Pressure
R1 , R2 Principal radii of curvatures
xv g Acceleration due to gravity z Ordinate giving depth below the apex of the drop
glass , Density of glass or liquid x Distance from axis of symmetry to the interface
Angle of rotation
Dorsey Surface tension based on Dorsey approach
Porter Surface tension based on Porter approach
X i Oxides in mol %
gi ( ) Additive factors of surface tension for oxides
Ca Capillary number
U Spreading velocity, triple line velocity
Fdrive Driving force for spreading
Fvis Viscous force u Fluid velocity
u (z) Velocity gradient x z
, glass Viscosity of glass
A small cut-off near the contact line
R Radius of drop
AVFT , BVFT , TVFT VFT parameters
(Tg ) Viscosity value at glass transition temperature
(Ts ) Viscosity value at dilatometer softening temperature
xvi
(Tm ) Viscosity value at glass melting temperature
0 Viscosity at infinite temperature (T )
' End point of glass transition temperature Tg
TSOFC SOFC operating temperature
I c Moment of inertial of rectangular beam
V Linear deflection rate in creep test
L Span length in 3 point flexure test
P Applied load in creep test
A Cross sectional area of bar
act Activation energy for viscous flow Eviscous
VFT Activation energy for viscous flow based on VFT model Eact
Moyniha Activation energy for viscous flow based on Moynihan Eact model Heating rate
Soft Activation energy near softening range Eact
Frequency factor
Ew Activation energy for wetting/spreading
Wsp Thermodynamic work of adhesion
G Gibbs free energy of the system m , n Wetting/spreading exponents
Stress (MPa)
Creep strain rate (s-1)
xvii 1. Introduction
1.1 Fuel Cells
A fuel cell is an electrochemical energy conversion device. It produces electricity from external supplies of fuel (on the anode side) and oxidant (on the cathode side) with water and heat as its by-products. As long as fuel is supplied, the fuel cell will continue to generate power. Compared to battery, it does not need to be recharged, and continuously produces power, when supplied with fuel and oxidant [1]. Fuel cells, because of their high efficiencies, low noise and non-pollutant output, modular construction to suit load and excellent load following capability, promise to revolutionize the power generation industry with a shift from central power stations and long transmission lines to dispersed power generation at user sites [2]. A fuel cell device comprises an anode (exposed to fuel), electrolyte, and a cathode (exposed to oxidant).
The electrolyte separates anode and cathode and facilitates the ionic transport required for the oxidation of fuel. Electron flow in the external circuit produces usable electrical power.
1.2. Different Types of Fuel Cells
Usually, fuel cells are classified according to their working temperature or to the electrolyte employed as shown in Fig. 1.1 [3]. A brief descriptions of different types of fuel cells are discussed below and more detailed informations are available in [4, 5].
Alkaline Fuel Cells (AFC)
The alkaline fuel cells use concentrated (85 wt. %) KOH as the electrolyte for high temperature operation (2500C) and less concentrated (35-50 wt. %) for lower temperature operation (< 1200 C). The electrolyte is a porous matrix saturated in an
1 aqueous alkaline solution of KOH that conducts OH- ions. Cathode and anode are made of Carbon, NiO/Ni or polymer with a wide range of electrocatalysts such as Ni, Ag, metal oxides, and noble metals. AFC show a high electrical efficiency of 60 to 70% and generate 10 to 100 kW power. Due to required use of high purity gases and noble metal catalysts, the technology is expensive and the application is restricted to the aerospace application. This type of fuel cell has been used in space program (Apollo and Space
Shuttle) since the 1960s.
SOFC: Solid Oxide Fuel Cell MCFC: Molten Carbonate Fuel Cell PAFC: Phosphoric Acid Fuel Cell PEMFC: Polymeric Electrolyte Membrane Fuel Cell AFC: Alkaline Fuel Cell
Figure 1.1. Types of Fuel Cells [3].
Polymeric Electrolyte Membrane Fuel Cells (PEMFC)
A polymeric electrolyte membrane fuel cell uses a thin ( 50m ) proton conducting polymer membrane such as perfluorosulfonated acid polymer or polybenzimidazol fiber (typically Nafion®) as the electrolyte. The membrane is covered with a catalyst layer of platinum or Pt/Ru, cathode and anode diffusion layers are made of graphite, and bipolar plates of graphite, polymer or metal are used as interconnects.
Oxygen or air can be used as oxidants, and hydrogen or methanol as fuels. Operating
2 temperature is typically between 60 and 800 C. Main issues are the water management for the membrane performance, and poisoning of the catalyst by CO (<1-10 ppm acceptable). The efficiency of PEMFCs lies between 40 to 55% for systems in power range of a few watts to 250 kW. PEM fuel cells are a potential candidate for automotive applications, for small-scale distributed stationary power generation, and for portable power applications.
Phosphoric Acid Fuel Cells (PAFC)
The use of an acid electrolyte, concentrated phosphoric acid, enables use of
CO2- containing gases.The electrodes are made of carbon paper bonded with a polymer (PTFE), coated with a finely dispersed platinum catalyst. They are operated with air as an oxidant, and hydrogen or reformed methane as fuel gas. PAFCs have an electrical efficiency of 40 to 60%, for power generation of up to 1-10 MW. With a combined heat and power efficiency of up to 70%, the PAFC has been well established in the stationary fuel cell market and systems of 200 kW are commercialized in the USA and in Japan.
Molten Carbonate Fuel Cells (MCFC)
Molten Carbonate Fuel Cells are high-temperature fuel cells that use an electrolyte composed of a molten carbonate salt mixture (e.g., Li2CO3, K2CO3 or
Na2CO3) suspended in a porous, chemically inert ceramic matrix of beta alumina.
Cathodes are composed of Ag2O or lithium-coated NiO, anodes of Ni or Ni with 10 wt.
% Cr. Due to high operating temperatures (600-7000C), methane can be internally converted to hydrogen, and cells are relatively insensitive to fuel impurities. The disadvantage is highly corrosive environment which reduces the cell life. The electrical efficiency lies between 55 to 65% and power generation of up to 100 MW can be built.
3 Solid Oxide Fuel Cell (SOFC)
Solid Oxide Fuel Cells rely on a dense, gas-tight oxide ceramic electrolyte, typically yttria-stabilized zirconia (YSZ) that shows high ionic conductivity at temperature higher than 7000 C. The electrolyte is sandwiched between two porous, electronically conducting electrodes. The cathode is typically made of a perovskite ceramic and the anode is a cermet of Ni-YSZ. This type of high-temperature fuel cells are insensitive to
CO, which make them highly fuel-flexible and a wide variety of fuel gases such as hydrogen, methane and other hydrocarbons can be utilized. High electrical efficiencies of 55 to 65% are possible for power generation of up to 100 MW. Combined heat and power systems (CHP) and hybrid systems with gas turbines can reach efficiencies of
70%. Due to high operating temperature in the rage of (500-1000)0C, solid oxide fuel cells (SOFCs) are the most efficient devices for the electrochemical conversion of chemical energy of hydrocarbon fuel into electricity, and have been gaining increasing attention in recent years for clean and efficient distributed power generation [6]. Due to fuel flexibility and high efficiency of these cells, SOFCs have wider applications in residential, transportation and military sectors. Their applications in auxiliary power units
(APU) utilizing gasoline or diesel as fuel also promises to bring SOFCs into the consumer product automotive and recreational vehicle market.
1.3. Solid Oxide Fuel Cell
High temperature Solid Oxide Fuel Cell is composed of a dense, gas-tight oxide ceramic electrolyte, which is ionic conducting and remains electrically insulating at high temperatures. The electrolyte is layered in between two porous electronically conducting electrodes, a ceramic cathode and a cermet anode. A schematics of solid
4 oxide fuel cell is presented in Fig. 1.2. Each electrode should have a high catalytic activity for adsorption, dissociation and electrochemical reactions of the oxidant or fuel species. The cathode side is exposed to air, while a fuel gas is injected on the anode side. The oxygen partial pressure gradient between both hermetically separated chambers is the driving force for the electrochemical process.
CO + H ObH + CO CH4, H2, 2 2 2 CO, CO2 Fuel
H2 H2 H2 H2 H2 Permeable anode e- e- 2-b - 2H2 + 2O 4e +2H2O
O2- O2- O2- O2- O2- Dense Electrolyte O2- O2- O2- O2- O2- e- e- -b 2- O2 + 4e 2O
O2 Permeable cathode O2 O2 O2 O2
depleted O2 Air or O2 Oxidant Heat
Figure 1.2. Schematic representation of a planar SOFC and the electrochemical reactions.
Oxygen (O2 ) molecules are adsorbed on the catalytic surface of the cathode
2 2 materials, dissociated and are ionized toO , following the reaction: O2 4e 2O .
Oxygen ions then diffuse through the electrolyte to the anode side. On the anode side, the fuel molecules reach at the three-phase boundaries (metal/oxide/atmospheres).
2 They are adsorbed and react with the diffused ions: H 2 O H 2O 2e and
2 CO O CO2 2e . Electrons charge the anode negatively compared to the cathode.
The resulting voltage can be used to feed an external circuit, returning the electrons to the cathode side. The global reaction in the fuel cell is hence given by:
2H 2 O2 2H 2O and 2CO O2 2CO2 .
5 The open cell voltage of the fuel cell is given by the Nernst potential for oxygen
F V F RT V pO (cathode) 2 partial pressures from both sides [4]: E0 G W lnG W ; where R is the gas H zF X G p (anode) W H O2 X
constant, z the number of charge transferred (forO2 , z 4 ), F the Faraday constant
and p(O2 ) the oxygen partial pressure at the respective electrodes. Under standard
0 21 operational use (T 800 C , p (O2 )cathode 0.2bar , p (O2 ) anode 10 bar ), the open cell voltage is about 1.1 V. The open cell voltage is an indicator of the cell performance, and will also give information on the eventual gas leakage or electrical short. It is affected by the oxygen partial pressure on both sides of the cell, which will change in case of a failure of the hermetic sealing.
1.3.1. Cell and Stack Designs
In order to obtain high voltage and high power from SOFCs, single cells are stacked together in different ways. SOFC can be classified into three major designs based on the unit cell stacking type, i.e. tubular, planar and monolithic [1, 6]. At present, only the tubular and planar designs are being investigated. A schematic of tubular and planar SOFCs are shows in Fig.1.3.
Tubular SOFC
In Tubular designs, the cell components are configured as thin layers on a closed-one-end tubular support (Fig. 1.3a) [7]. For cell operation, oxidant is introduced to the cell near the closed end, and the fuel flows on the outside of the support tube.
The tubular support is generally made of a porous (35% porosity) 15-mol% CaO- stabilized ZrO2 (dimension: inner diameter ~ 12 to 13 mm, wall thickness ~1- to 1.5 mm and length ~ 36-cm to 1-m). The tube is overlaid with a 1.4 mm porous (~35 % porosity)
6 strontium-doped (10 mol%) LaMnO3 and a 40 m gas-tight Y2O3-stabilized (10 mol%
Y2O3) ZrO2 electrolyte layer cover the cathode. The cathode is covered with a 40 m gastight, magnesium-doped LaCrO3 interconnect layers and 100 m nickel/stabilized
ZrO2 cermet anode covers the entire electrolyte surface.
(a) (b)
Tubular SOFC
Planar SOFC Figure 1.3. Schematic of SOFC designs (a) Tubular and (b) Planar [1].
Individual cells are bundled in series and parallel electrical connection to form a basic power-generating building block. One distinct feature of this design is that, each single cell is built as a unit structure and does not require gastight seals for ceramics at high temperatures [8]. This allows some freedom of thermal expansion and minimizes the problem of cracking caused by thermally induced stresses in connected cells.
However, the sealless tubular design has a relatively long current path through the cells, which enhances the cell resistance and limit the power density of the system. Further the processing technology such as Electrochemical Vapor Deposition (EVD) is expensive and difficult to upscale. The tubular design has advanced sufficiently for testing power plants up to 200 kW electrical power with an electrical efficiency of about
53 % [9].
7 Planar SOFC
The planar concept is favored by the majority of developers because of their high power densities due to the short transport paths across the cell. An additional advantage is relatively simple fabrication technology, with the possibility of using low cost processes such as tape-casting or screen-printing facilitating a large-scale production potential [10]. The major problems with planar design include sealing the stacks to prevent fuel and oxidant gases mixing and the thermal mismatch between ceramic components, which leads to cracking during thermal cycling. Planar SOFC designs can be divided into two categories: electrolyte and electrode supported configurations (Fig. 1.4). (a) (b) Electrolyte ~ 10 m Cathode ~ 50 m Cathode ~ 50 m Electrolyte ~ 200 m Anode Substrate ~ 1500 m Anode ~ 50 m
Anode functional layer ~ 5 10 m Figure 1.4. Planar cell concepts (a) electrolyte and (b) anode supported [11].
Electrolyte-supported cells offer a strong support from a dense but thick electrolyte (thickness >100 m ), but require a high operating temperature of about 900-
10000 C to minimize the ohmic losses of the electrolyte. Anode- or cathode- supported cells with thin electrolytes (5 20 m ) allow operating temperatures lower than 8000 C.
High-conducting anode cermets have been favored as substrate but have a lower stability due to potential re-oxidation [12]. Externally supported configurations include
8 interconnect-supported cells that provide a lower electrical transport limitation than anode-supported cells and stronger metallic support. However a problematic issue is the interconnect oxidation and an alternative is to improve the support properties of materials which have oxidization-resistance properties [13]. The advantages of reduced operation temperature include a wider choice of materials. Especially low-cost metallic materials for interconnects, reduced thermal stresses, hence improved reliability and longer cell life, together with reduced cell costs. In addition, metallic interconnect plates are easy to machine and internal manifold for gas distribution and gas channels can be carved into the plates to provide uniform distribution of gas and air on the superposed layers of the stack. Descriptions of the materials used in the planar SOFC are given in the literatures [3, 4]. Interconnect materials are of particular importance as the sealing is applied on interconnect parts, and their interaction is one of the most critical compared to other components of the fuel cell.
1.3.2. Interconnect Materials
Interconnect plates provide electrical connection between two successive cells, separation between both atmospheres and distribution of the fuel and oxidant gases.
The materials for interconnects are selected based on their high electronic conductivity, low thermal expansion mismatch to cell components (coefficient of thermal expansion,
CTE of about 10.5 12.5 ppm / 0 C between room temperature and an operating temperature of 8000 C [1, 14] and their chemical stability in both high and low oxygen partial pressures corresponding to cathode and anode atmospheres, respectively. The materials should show a high thermal conductivity to allow a uniform heat distribution across the stack, good chemical compatibility with adjacent stack components, high
9 creep resistance and stability in both oxidizing and wet reducing environments. Different types of interconnect materials were successfully developed for different ranges of operating temperatures. For high operating temperatures of 900 to 10000 C, perovskite based ceramic materials are favored, which includes chromites materials (e.g., LaCrO3 and YCrO3), since they can easily be modified to fulfill the main requirements mentioned above. The electrical conductivity of these perovskites is improved by substituting
2+ 2+ 2+ divalent ions, such as Sr , Ca or Mg , on either the A or the B sites of the ABO3 lattice and CTE values of the perovskites can also be modified by doping with aliovalent ions such as Co2+ that increases markedly the CTE. The replacement of a trivalent by a divalent ion is compensated by the formation of tetravalent chromium ions at high oxygen partial pressure. However, this change of valence state, leads to a volume expansion generating mechanical stresses with the interconnects [15, 16]. Further drawbacks of chromite materials are related to their fabrication process, due to their brittleness and lack of proper workability, they are difficult to sinter up to high density under high oxygen partial pressures [17].
With the decrease of operating temperatures to 8000 C and below, metallic interconnects show several advantages over the ceramic materials due to their high electrical and thermal conductivity, their easier fabrication with low machining costs.
However, commercially available high temperature oxidation resistant alloys could not directly be used due to their high CTE compared to the ceramic cell components. In addition, their electrical conductivity is controlled by the oxide scale formed at high temperature. Hence among alloys forming alumina, chromia or silica protective layers, both Al and Si containing alloys were excluded due to their insulating oxide layers and
10 chromia forming alloys were selected for their relatively high electrical conductivity of the oxide layer (102 to 101 S.cm1 at 9000 C ) compared to that of an alumina scale
(108 to 106 S.cm1 at 9000 C ) [17].
Several types of alloys such as Ni-and Cr-based alloys, ferritic stainless steels have been developed by oxide dispersion strengthening (ODS) and tested as interconnect plates. The first commercial composition Cr5Fe1Y2O3 (named Ducrolloy) was developed for high temperatures ( 900 10000 C ) [18], and then successive FeCr- based compositions (e.g., IT-10, IT-11 and IT-14) for intermediate temperatures
( 700 9000 C ) [19, 20]. The alloys are fabricated by powder metallurgy, mechanical alloying by high-energy ball milling process [21]. The thermal expansion coefficient of
Ducrolloy is similar to the 8YSZ electrolyte and for one of IT-alloys CTE matches that of the anode cermet (Ni/8YSZ). Those alloys showed a good oxidation resistance at operating temperatures, low electrical resistance and good mechanical properties.
However, high chromium vaporization in air at high temperatures is a critical issue, which leads to degradation of the cathode-to-electrolyte interface [22, 23].
The potential candidates for metallic interconnects are high chromium (> 17 wt.%) containing ferritic stainless steels that present a high oxidation resistance together with CTEs close to the cell components. The chromium content has to be higher to ensure the formation of a protective chromium oxide layer and to achieve low
CTE values. Quadakkers et al. [21, 24] investigated ferritic steels with Cr contents of
(16-25) wt. % and various reactive element (RE) additions to achieve a low oxide scale growth rate. Small amounts of Mn and Ti were added to promote the (Cr,Mn)3O4 spinel formation that decreases the formation of volatile Cr-species. Formation of thin oxide
11 scales with a very low electrical resistance was obtained for alloys of the type
FeCrMn(Ti/La) and in the commercial steel Crofer22APU [24,25]. Further information on the development and oxidation behavior of metallic interconnect materials are found in references [21, 26-28].
The current materials for the SOFC may consist of yttria-stabilized zirconia
(YSZ) electrolyte, a strontium-doped lanthanum magnanite (LSM) cathode, a Ni-YSZ cermet anode, a strontium-doped lanthanum chromite interconnect or bipolar plate and insulating seals and manifolds made of heat-resistant metallic alloys such as crofer-
22APU and ferritic steel [29]. Some materials properties and their uses in SOFC are listed in Table 1.1.
Table 1.1.Coefficient of Thermal Expansion (CTE) of Some Materials for SOFC [29]. Materials use in SOFC Temperature range (0C) CTE (ppm/0C) Inconel 600 Metallic Hardware 25-1000 16.7 SS 430 Metallic Hardware 25-1000 11.4 SS 446 Metallic Hardware 25-1000 12.6 Crofer22APU Metallic Hardware 25-1000 11.5 Ceramic perovskite Interconnect 25-1000 10.6-11.1 8%YSZ Electrolyte 25-1000 10-11 Soda lime glass Sealant 25-800 9.0 Glass/Glass ceramics Sealant 25-900 10-12 depending on the composition
1.4. Sealing Concepts and Materials
The main functions of sealing materials for solid oxide fuel cells are to prevent the gas mixing outside the cell, and to guaranty electrical insulation between two successive interconnect plates. Solid oxide fuel cells typically work under a oxygen partial pressure gradient across a dense, gas-tight electrolyte, leaks that appear because of fabrication defects or due to the degradation of components or interfaces during operation reduces the system performance drastically, and lead to complete
12 failure of the stack [30]. Local short-circuiting between two current collectors plates could also lead to local hot spots, and degrade the cell performance. A key challenge in assembling planar SOFC systems is hereby to create and maintain a hermetic seal between a stack of ceramic layers and the metallic interconnect plate. Seal designs and materials depend on the cell and stack configurations. Figure 1.5 shows different type of seals which are needed to make planar SOFC functional. Common seals include: (a) cell to metal frame (S1), which could include sealing of the edges of the cells and/or sealing to a particular cell layer; (b) metal frame to metal interconnect (S2); (c) frame/ interconnect pair to electrically insulating spacer (S3); (d) stack to base manifold plate
(S4), and (e) cell electrode edge or electrolyte to interconnect edge. Metal-metal seals can be fabricated using metal joining, soldering and brazing techniques, but one should take care of oxidation issue as these seals have to function at high temperature for longer time. For the fabrication of metal-ceramic and ceramic-ceramic seals, these techniques can not be used because of brittle nature of ceramics.
(a)(b) Fuel Gas (c) Interconnect S2 Interconnect Glass S3 S1 Ceramic Spacer Cell Air Metal Frame Anode Electrolyte Electrolyte Cathode Air S4 Metal Endplate Cell Fuel Air Air Air Fuel Interconnect Manifold Figure 1.5.(a) Different types of seals (b) cross flow structure (c) single cell [29].
Among the above discussed seals, metal-ceramic seals are challenging due to their wide functional requirements as well as problems in selecting materials for seal.
13 For metal-ceramic seals, glasses are widely used because the thermal expansion coefficient of glasses can be tailored according to the joining components. An essential problem is the difference between the coefficients of thermal expansion (CTE) of the different layers of the cell (10.5 12.5 ppm / 0 C ) and the interconnect plates
(11.5 13 ppm / 0 C ), which generates stresses upon thermal cycling from an operating temperature of about 8000 C to room temperature. In the case of a rigid bonded seal, the material should have a thermal expansion matching those of the sealing components such as electrolyte and interconnect in order to minimize interfacial stresses that would cause cracking in the seal [31, 32]. Alternatives are to use either a non-bonded compressive seal system that would dissipate stresses by elastic deformation, or a ductile or viscous seal that allows plastic relaxation of stresses.
Within the fuel cell stack, an effective sealant must be compatible with the thermal expansion behavior of fuel cell components, i.e. it should have a suitable coefficient of thermal expansion (CTE) that matches those of the other cell components, and it must have no or little chemical reaction with the joining components (such as YSZ electrolyte and metallic interconnects), high chemical stability, and should be stable against evaporation in both wet reducing (reducing fuel gas + H2O) and oxidizing atmosphere. Also, the sealant must be electrically insulating to prevent any short circuit.
Moreover, these properties must be kept unchanged over the required life-time of
40,000 h (~ 5 years) at elevated temperatures, together with hundreds of thermal cycles. The functional requirements of sealing systems for SOFCs can be summarized as follows and presented in Table 1.2.
14 Table 1.2. Functional requirements of sealing systems for SOFCs. Chemical Properties : Mechanical Properties: Long term stability under dual oxidizing Hermetic sealing or low leakage and wet reducing atmospheres at 8000 C CTE matching with cell Limited or no reactivity with adjacent components ( 1.5 ppm / 0 C ) or components, no volatilization dissipation of thermal stresses High bond strength to joined Electrical Properties : components Electrical insulation between Tolerance to thermo-mechanical interconnect plates, high resistivity and externally applied stresses
Design and fabrication: Simple processing and design flexible High reliability, reproducibility, Low cost
1.5. Sealing Technology
Different sealing approaches have been considered to fulfill those demanding requirements and can be classified into three categories: compressive seals, compliant bonded seals and rigid bonded seals. The materials and sealing concepts tested for application in SOFCs were successively reviewed by several authors [29, 33, 34]. Brief overviews of different sealing technologies are described in the following sections.
1.5.1. Compressive Seals
A major advantage of compressive seals is that the seals are not rigidly fixed to the other cell components, so an exact match of thermal expansion is not required.
However, to employ compressive seals in a planar SOFC stack, a load frame is required to maintain the desired level of compression on the stack over the entire period of operation [33]. The load frame introduces several complexities in stack design,
15 including oxidation of the frame material, load relaxation due to creep, and increased weight, which reduces specific power and thermal response of the overall system.
These factors increase system cost and seriously limit the use of compressive seal. In this approach, metallic compressive seals and mica based compressive seals were evaluated as sealing SOFCs.
Metal gaskets
One approach is to use flat metal gaskets made of a ductile, non-oxidizing noble metals such as gold or silver, which provide a hermetic seal deformation at operating temperature [35, 36]. However, the durability of such seal is doubtful as the leak rate of silver seal was found to degrade under thermal cycling [37]. Another approach is to use deformable shapes that allow for the use of less ductile metals, fabricated from superalloys, such as corrugated, C-shape or hollow tube gaskets [35]. However an obvious disadvantage of metal gaskets is the high electrical conductivity of materials and cost of noble metals.
Mica-based seals
Micas belong to a family of layered minerals composed of cleavable silicate sheets and show remarkable high resistivity and uniform dielectric constant. Different types of mica have been investigated as sealants, such as muscovite
(KAl2(AlSi3O10)(F,OH)2), phlogopite (KMg3(AlSi3O10)(F,OH)2), Vermiculite (Mg, Fe,
Al)3(Al,Si)4O10(OH)2.4H2O and the difference between these phases is the higher CTE of phlogopite [38]. Among different forms of micas available, cleaved crystals showed lower leak rates than mica papers [38]. Commercial papers exhibited poor sealing characteristics even under high compressive loads and subsequent studies showed that
16 the primary leak path lie along the interface with adjacent components, so that sealing could be improved by coating the sealing surface with a compliant interlayer, such as a ductile metal or glass [39, 40]. It was shown that the characteristics of phlogopite paper could be improved by infiltrating the mica particles with a wetting agent such as
Bi(NO3)3 or H3BO3 [41]. However the reactivity of such agents with interconnect materials was detrimental and recent results of hybrid seals prepared using phlogopite paper together with barium aluminosilicate interlayer showed minimal leakage under a low applied pressure of 0.34 MPa and undergo over 1000 thermal cycles [42].
1.5.2. Compliant Bonded Seals
Compliant sealants form a bond that can undergo plastic deformation at operating temperatures and, in this way stresses arise from thermal expansion mismatch between the cell components can be relaxed to a certain extent.
Brazing
Brazing is a wide-spread and reliable method to join dissimilar materials. The principle of brazing is based on a filler material with a liquidus temperature well below that of materials to be joined. By heating to its melting point, the sealant fills the gap between the sealing surfaces by capillary action and a solid joint is obtained upon cooling to room temperature. In case of reactive metal brazing, a reactant metallic filler materials (e.g. titanium) is used to enhance the wettability of ceramic sealing surfaces.
However, the method cannot be directly applied to SOFCs as reactive metal brazing is usually conducted either in inert or in high vacuum atmospheres, which can degrade the performance of cathode materials significantly. In addition ceramic-metal joints produced by such a technique showed excessive oxidation in air atmospheres [43].
17 Therefore alternative brazing techniques conducted in air were developed recently for use in solid state electrochemical devices. The method utilizes the combination of a noble metal with an oxide that is partially soluble in metal and favors the wetting of the ceramic sealing surface by a eutectic low melting point. The Ag-CuO system shows such a eutectic [44] together with a high-temperature oxidation resistance and metal ductility. A balance between good wetting of the ceramic surface and high strength of the ceramic-metal joints was found for the composition containing 4 to 10 mol% of CuO
[45]. Such seals were optimized to withstand heating and cooling with fast rates through repeated cycles with no detected degradation of both the hermeticity and the joint strength [46]. There are however concerns about the stability of silver-based alloys in dual atmosphere environment and silver volatility. Both oxygen and hydrogen are soluble in silver and could form pores along grain boundaries due to the formation of water vapor [47].
Wet seals
The reduction of the operating temperature to 600-8000 C broadens the scope of potential sealant systems and similar technologies like in MCFCs. One possibility is to use so-called “wet seal” that are fluid state into a porous matrix at operating temperature, the more detail about this type of sealing technology can be found elsewhere [48].
1.5.3. Rigid Seals
Rigid seals, on the other hand, do not require applied pressure and forms a joint that is non-deformable at room temperature. Due to the inherent brittle nature of the materials, which are glass/glass-ceramics, ceramics or composites, seals are sensitive
18 to tensile stresses, caused by thermal expansion mismatches between the sealant and adjacent components. In this case, the sealant must be tailored to match the CTE of the interconnect plates and ceramic cell (9.5 12.5 ppm / 0 C between room temperature and
8000 C) to avoid any thermal stresses. These materials tend to display acceptable stability in the oxidizing and reducing 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 exhibited 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.
Glasses and glass-ceramics sealants
Glass or glass-ceramic materials are the most commonly used sealants for
SOFCs [49, 50,51-55] because the properties can be easily tailored by manipulating glass compositions [56, 57], and also because glasses are cost effective and simple to be processed. Glasses are typically distinguished by the glass formers, like phosphate glasses, borate glasses, and silicate glasses. All the three types of glasses have been explored for SOFC sealing [50, 58, 59]. It has been shown that the phosphate glasses have very low thermal expansion coefficient (6-7 ppm/0C) and at SOFC temperatures, glasses purely based on phosphate are not stable. Glasses based purely on phosphate as glass former tend to volatilized and reacted with Ni/YSZ-based anode to form nickel phosphate and zirconium oxyphosphate [58]. Additionally, these phosphate glasses crystallizes to form meta- or pyrophosphates, both of which exhibit low stability in humidified fuel gas at temperature greater than 7000 C. Borate based glasses and glass-ceramics have also been considered as potential sealing candidate for pSOFC
19 and they have good coefficient of thermal expansion (10-11 ppm/0C) in SOFC sealing requirement range. However, investigations conducted by Ley et al. [60] indicate that the boron undergoes significant reaction with humidified fuel gas to form gaseous species such as B2(OH)2 and B2(OH)3 at operating temperature. These species are unstable and volatilizes, and borate based glasses shows weight loss as much as a
20% weight loss and exhibit extensive interactions with stack components under both air and wet reducing environment [61]. Most of research work focused on silicate based glass or glass ceramics seal due to their stability in cell atmosphere. Detailed description of silicate based glass seals are discussed in literature review section.
Ceramic sealants
High-temperature ceramic sealants include cements that are formed by reaction bonding. Ceramic adhesive cements have been used in small-scale cell testing [34], but often cracked upon cooling to room temperature because they do not match the CTE of cells. Ceramic sealants formed by in-situ reaction have been synthesized from polymer precursors to reduce the joining temperatures and these precursors are typically organosilane polymers that convert to SiC or SiOxCy when thermally treated in the temperature range of 800-14000 C [62]. The application of such sealant in fuel cell was tested [63] and showed that the materials are stable at operating temperature. However, the Pyrolysis of such polymer precursors produced extensive gaseous products and high volume shrinkage, which generated pores and cracks within the seal [64].
An overview of the relative advantages and drawbacks of the different sealing technologies and materials are given in Table 1.3.
20 Table 1.3. Overview of potential sealing technology for SOFCs [33, 34]. Sealing Technology Pros Cons Plastic stress relaxation Non-adapted CTE Metal gaskets Enable disassembling Permanent gas leakage
Creep under load
Elastic High leakage rates Mica-based seals High permanent load
Elastic Electrical conductivity Metal-mica seals Compressive Seals Plastic stress relaxation High permanent load
Enable disassembling Complex processing
Gas tightness Electrical conductivity Metal Brazing Plastic stress relaxation Chemical reactivity
Paste, sheet processing Processing (T, atmosphere)
Gas tightness Electrical insulation, Wet seals
Compliant Seals Plastic self healing Chemical reactivity
Electrical insulation Control of phase content Glass-ceramic sealants Gas tightness Chemical reactivity Paste processing Brittle fracture behavior
Pyrolysis processing Ceramic sealants Chemically inert Non-adapted CTE
Brittle fracture behavior
Filler phase stability Rigid bonded Seals Composite Sealants Tailored CTE Control of viscous flow
Brittle fracture behavior
21 2. Literature Review
2.1. Silicate Based Glass Seals
Among different types of glasses, silicate-based glasses [33] received much attention due to their thermal expansion matching with SOFC components, stability in dual SOFC environment and better chemical compatibility with SOFC components. Two types of approaches are currently being pursued to seal SOFC using silicate based glasses. In one approach glass ceramics is used, where seals are designed to soften and flow at a temperature above that required for stack operation in order to form a hermetic seal via chemical bonding with the cell components. During the sealing operation, the glass partially or fully crystallizes to form a rigid, bonded seal.
Crystallization is advantageous because the final resulting materials are stronger than the starting glass. However, because the final joint is brittle, it is susceptible to fracture when exposed to operating temperature due to the thermal expansion mismatches between sealant and adjacent components and also due to thermal cycling events [65,
66]. Due to rigidity of glass ceramic seal it is not possible to repair the seal and if cracking occurs the whole components need to be replaced, which increases the system cost and limits the use of glass ceramics as a viable seal for stable long term operation.
Another type of a novel approach [29] is to start with a glass, which does not crystallize and remains soft (viscous) at operating temperatures. In this case viscous flow and proper wetting helps to seal the adjacent components. The advantages of using this soft glass sealing technology is that at the operating temperature, the glass remains viscous and can help relieving the stresses that may arise due to thermal
22 gradient and mismatch in CTE of sealing components. Also, this type of glass can heal the cracks that may generate due to thermal cycling or thermal gradient without replacing the sealing material. The detailed study and rationale behind crack healing of this type of glass seals and their hermeticity can be found elsewhere [29,67].
Among various properties, the main criteria for selection of suitable glass seal
compositions are the glass transition temperature (Tg ) and the coefficient of thermal expansion (CTE). Glass must show viscous behavior to enable sealing by softening, while maintaining a mechanical rigidity not to flow out. The flow characteristics of glass
is characterized by either glass transition (Tg ) or softening temperature (Ts ). In the scientific literature, glass sealants with low softening temperature show high thermal expansion [68] and for specific applications such as SOFC, the development of new sealing materials is required. Geasee et al. [68] suggested the target range for both
parameters Tg and CTE, which is shown in Fig 2.1 and most of these glass compositions are barium, strontium or lanthanum-containing boro or aluminosilicates based having large CTE [60, 69-72].
The thermal expansion behavior depends on the chemical composition of the parent glass. Glasses in the systems BaO-MO-SiO2 (M=Mg, Zn) show increase in CTE with increasing BaO content [72] and this increase is due to the lower field strength, as defined by Dietzel [73] ( f z / a 2 , where z = oxidation state and a Me O distance
in ), of Ba 2 (0.27) compared to Mg 2 (0.47) , Zn 2 (0.45) . The lower field strength of modifier ions weakens the SiO2 network structure and increases the CTE value.
However the thermo-mechanical properties of glasses change at high temperature due
23 to crystallization and this requires proper control of thermal expansion behavior resulting from different phases. In binary or ternary glass systems, the effective CTE can be predicted by thermodynamic calculations of existing phases and residual glass [74].
However, for a multi-component glass, this becomes more challenging due to complexity from several kinetic factors, which intervene in the CTE equation and influence the physical properties of the material.
CTE ( 10-6 / 0C)
Figure 2.1. Glass transition temperature (Tg ) and coefficient of thermal expansion (CTE) of sealant glasses for SOFCs [55, 63-66]. The frame represents the target range as defined by Geasee [68].
Glasses based on barium/strontium aluminosilicates easily crystallize and form celsian (RAl2Si2O8, R = Ba, Sr) and barium silicates [69, 71, 72, 75]. In this system, both
0 the high-temperature metastable hexagonal ( 0 7 8 ppm / C ) and stable 201000 C
0 monoclinic ( 0 2 3 ppm / C [76]) phases form, which have low coefficients of 20300 C thermal expansion. In some composition, quartz or cristobalite can also appear; having
24 0 0 very large CTE ( 0 23.3ppm / C and 27.1 ppm / C , respectively [50]). Cristobalite 20600 C is particularly problematic due to a displacive transformation that occurs upon cooling with a large volume decrease causing cracking [61]. The formations of these detrimental phases affect the material properties during long-term annealing by reducing the coefficient of thermal expansion [71] and ultimately affect the thermo-mechanical integrity [61].
Besides controlling the crystalline phase content of the glass-ceramic, proper control of the crystallization kinetics are also important. Glass seals are usually processed by powder technologies, which require sintering to make it dense and wetting of the substrate prior to crystallization is desirable [77-79]. If crystallization occurs before complete sintering and proper wetting, a high porosity or a poor adherence of the seal will result [72, 79]. Among earth-alkaline glasses, barium-based glasses showed faster crystallization compared to calcium- and magnesium-based glasses [69]. For barium as a network modifier, the lower activation energy for crystallization was observed and this was attributed to lower field strength of Ba 2 (0.27) compared to
2 2 Ca (0.36) and Mg (0.47) , which weakens the SiO2 network structure. As a result, barium aluminosilicate glasses crystallize at lower temperature compared to other earth- alkaline based glasses and their crystallization kinetics can be modified by addition of nucleating agents such as TiO2, ZrO2, Cr2O3 and Ni [69, 80]. Small additions of TiO2,
Cr2O3 and Ni as a nucleating agents enhanced the activation energy for crystallization, whereas ZrO2 decreased it. Additional nucleating agents also control the phase content of glass-ceramics. In case of magnesium aluminosilicate glasses, chromium and titanium oxide as nucleating agents inhibit the formation of cordierite (Mg2Al4Si5O18),
25 which has a very low CTE (1 ppm / 0 C [50]). Titania and zirconia both decreased the
CTE of barium and magnesium-aluminosilicate based glasses [69, 71], but showed
contrasted effects on the glass transition: TiO2 addition decreased the Tg while ZrO2 additions increased it.
In order to tailor the viscous behavior of glass compositions, an additional glass- forming and flux oxides are needed. Glass compositions that match the simultaneous
criteria for Tg and CTE are essentially alumino- and borosilicate glasses (based on
Fig.2.1). In earth-alkali modified silica glasses, aluminum oxide (up to 10 wt.%) was introduced to extend the glass-forming region [56] and higher contents of aluminum oxide led to fast crystallization upon cooling. Depending on the glass composition, aluminum can adopt either a tetrahedral coordination (at low amounts) and play the role of a glass former, or six-fold coordinated (at higher amounts) and act as a glass modifier
[56]. Due to its dual role, aluminum has been reported both to inhibit [69] and to enhance crystallization [69, 70] forming hexacelsian, having low CTE and critical to thermo-mechanical properties.
The chemical compatibility of glass seal materials with yttria-stabilized zirconia electrolyte is generally good. The reactivity of glasses, which contains alkaline species, with YSZ [80] can be avoided due to crystallization of the glass sealant [78]. Earth-alkali boro- and aluminosilicate glasses show a minimum interaction with 8YSZ, limited diffusion of yttrium and zirconium into the glass [71, 78, 82, 83] and no diffusion of the glass species into the electrolyte. In barium, calcium and magnesium aluminosilicate glasses with nickel and YSZ as filler material showed phase formation after annealing at
0 1000 C for 1000 h [84]. It was observed that, in both air and humidified Ar /4% H2 only
26 zirconia reacted with Ba, Ca and Si to form mixed zirconate phases [84].The compatibility of glasses with interconnect materials is more problematic due to the high reactivity of alkaline-earth oxides with chromium and among earth-alkali aluminosilicate glasses, barium oxide based glasses show the highest reactivity whereas glasses based on magnesium oxide show the least interaction with a chromium based alloy [68,
84]. In case of magnesium aluminosilicates, a mixed spinel (Mg,Fe)(Cr,Al)2O4 phase is observed at the interface, where chromate phases like Ca3Cr2(SiO4)3 and BaCrO4 are formed in calcium and barium aluminosilicates glasses [84]. Due to high chemical activity of barium oxide at high oxygen partial pressure, formation of barium chromate are the most extensive in air, whereas in a reducing atmosphere chromium dissolves into the glass [85, 86]. BaCrO4 has an orthorhombic structure with coefficients of
0 0 0 thermal expansion a 16.5 ppm / C , b 33.8 ppm / C and c 20.4 ppm / C [86].
Due to the large mismatch with the CTE of interconnect materials, this lead to cracking at the interface between glasses and interconnect.
In addition to the compatibility with other components, the sealant should withstand the stringent atmospheric conditions such as air on the cathode side, and wet fuel on the anode side. Several studies reported on an anomalous oxidation behavior of metallic interconnect materials under dual atmosphere exposure, where a fast growth of ferrous oxide was observed on the air side of the seal [81, 87-89] and this started to react with the alkaline elements from the sealant. This leads to new phase formation at the glass-ceramic-to-steel interface resulting in a local volume change which caused a crack formation. Nevertheless barium calcium aluminosilicate glasses have been successfully used as sealants in fuel cell stacks operated at 8000 C beyond 6000 h of continuous operation [87-89].
27 2.2. Composite Sealants
Composite sealant materials are an alternative way to solve the problems raised by the glass-ceramic approach. By proper selection of filler material either an inert or a reactive, an improved thermo-mechanical properties of resulting glass composite can be obtained [90]. The CTE and flow/deformation behavior of the sealant can be optimized by adjusting the chemical nature and volume fraction of the filler. The filler materials can also act as nucleation sites and can control the kinetics of the glass crystallization. In a first attempt, mica-based glass-ceramic composites were used and good thermal expansion match was obtained to seal the 8YSZ electrolyte to La0.8Ca0.22CrO3 interconnect [90]. However the long-term stability of the composite was not investigated and extensive reaction was observed at the interface. Furthermore the same material tested with ferritic stainless steel in hydrogen at 9000 C showed chemical reactions due to the evaporation of alkali species contained in the composite [91]. The possibility to adapt the CTE by the composite approach was demonstrated with glass-metal composites [92] and in this case the CTE of the composites was tailored by a variation of the amount of metal fillers to match that of a metallic substrate. Another concept is based on the thermodynamic calculation of domains of coexistent constitutional compounds in a glass system developed by Conradt [74]. The properties of glass system such as 2BaO.3SiO2-BaO.2SiO2-CaO.SiO2 can be tailored by adding proper amount of crystalline BaSi2O5, without modification of the parent glass phase. The stability of the phase is, however, not guaranteed and in a multicomponent glass system, additional species may also react with the compound and form detrimental phases. For instance, the compound BaSi2O5 was not stable in MgO-CaO-Fe2O3-Al2O3-
28 SiO2 glass, but transformed into celsian BaAl2Si2O8 upon thermal treatment, which degraded the thermal expansion behavior of the glass-ceramic [92]. Other attempts to tailor the CTE of composite glass sealants were not successful due to similar problems of phase stability [93-94].
2.3. Challenges for Glass-Based Sealants
Due to the stringent requirements for the sealing material, e.g., seal materials should be stable and capable of a service life of more than 40,000 h and hundreds of thermal cycles for stationary systems, or at least 5,000 h and 3,000 thermal cycles for transportation systems, the fundamental difficulty in fabricating planar SOFCs is how to effectively seal the anode/electrolyte/cathode assembly together with the interconnect to create a hermetic and stable stack. Glass or glass–ceramic materials are the most commonly used sealants for SOFCs. Although many advances have been made for glass-based seals, several challenges remain unsolved. Firstly, glass-based seals have very poor thermal stability, where the thermal properties of the glass based seals change continuously with the time of exposure at high temperatures. Silicate based glasses are easily crystallized at SOFC operating temperatures, which might cause a significant change in the thermal expansion coefficient. Even if, a glass ceramics with well developed crystalline phase is considered as seal material, the crystalline phase may not have the same CTE as parent glass, also the developed crystalline phase may react with cell components and undesirable phase (s) may form at the interface. This change of the CTE value and other thermal properties can induce significant thermal stress at the sealing interface, and will ultimately cause the failure of the seal as well as the stack. So, the thermal stability of current state of the art sealing glass/glass–
29 ceramics needs to be further improved. Secondly, glass/glass ceramics based seals often have poor chemical compatibility with other SOFC components (especially with interconnects) at SOFC operating temperatures. As demonstrated by Yang et al. [85], extensive reactions were observed between a glass-based seal and metal alloys, where the reactions generated pores up to 200 m aligned along the metal–glass interface. In some cases, the reaction also caused separation of the glass from the alloy matrix; possibly due to the thermal expansion mismatch between new developed phase
BaCrO4 at the interface and metal interconnect. The extensive formation of interfacial pores and interface separation will greatly reduce interfacial bonding strength, which would be detrimental to structural stability of the SOFC stacks. Therefore, reliable seals with much better thermal stability, good chemical compatibility, reasonable mechanical strength, which can function for longer time and shows self repairing behavior are still needed.
In sealing technology, the sealing parameters, such as wetting angle, surface tension, work of adhesion and viscosity are important, since these parameters determine the sealing ability and strength of the final seal-cell joint. In particular, surface tension and viscosity are most important parameters and measurement of these properties require special techniques, which is time consuming or instrumentation limited. A brief review of the measurement techniques of surface tension and viscosity are presented in the following sections.
2.4. Measurement Techniques of Surface Tension
The surface tension of a liquid can be measured by a variety of methods [95-97].
These are based on the change in profile of a drop as it deforms under gravity (sessile
30 drop, pendant drop), capillary action (capillary height method), the maximum force to pull an object from the liquid (Wilhelmy plate method, Du Nouy ring), or the pressure required to form a bubble in the liquid (maximum bubble pressure method).This section summarizes the general description of the most widely used techniques for measuring surface tension for high-temperature materials. The general nature of these techniques is illustrated in Fig. 2.2. The principles governing each of these approaches are discussed in the following section.
2.4.1. The Capillary Rise Method:
The capillary rise technique is the oldest method used to measure the surface tension of liquids. When a capillary tube is immersed in a liquid, the liquid rises (or falls) until the pressure difference across the curved meniscus is balanced against the hydrostatic pressure. Surface tension, , is then described by the following equation:
2cos / r g h ; rg h / 2cos (1.1) where is the contact angle formed by the liquid on the capillary tube material, g is the gravitational constant, h is the capillary rise, is the density difference between the liquid and the gas phase, and r is the radius of the capillary. At high temperatures, the difficulty in determining r , h , and limits the application of the technique [97].
2.4.2. The Drop Weight Method:
The general relationship between the weight of a drop that falls from a tube/rod and the surface tension of the liquid was described by Tate [98] and given by:
W ad bd 2 (1.2) where W is the weight of the drop, d is the diameter of the tube/rod, a is a number related to surface tension and b is a constant that depends on the cohesion of the
31 liquid. Lecrenier [99] modified the technique to determine the surface tension of glass drops that formed from a circular orifice at the bottom of a crucible (crucible drop weight method): (1 r / R)(W / 2 r) (1.3) where r is the orifice radius and R is the radius of curvature at the widest point of the drop. Based on this method the surface tension results obtained are too high, raising serious doubts about the accuracy [100, 101].
2.4.3. The Fiber Elongation Method:
This method developed by Berggren [102] and relied on suspension of a glass fiber in a furnace and observing the progressive changes due to thermal expansion, contraction due to surface tension, and final lengthening when gravitational forces overcame surface tension [103]. Just prior to the final lengthening stage the fiber diameter is a maximum and the forces of surface tension and gravity are equal so that:
W (1.4) d c
where W is the weight of the fiber below the heated zone, and d c is the critical, maximum fiber diameter. Like the drop weight method, this technique is simple but not very accurate [104].
2.4.4. The Dipping Cylinder Method:
Du Nouy described a technique where a ring is dipped into the liquid [105].The ring method is more applicable to low-viscosity liquids and for molten glasses that uses a platinum cylinder. The forces acting on a thin-walled cylinder are:
2 2 2 (r1 r2 ) F h g2 (r1 r2 ) (1.5)
where r1 and r2 are the outer and inner radii of the tube, h is the cylinder immersion depth, is the liquid density, and g is the gravitational constant.
32
dc
(a) Capillary Rise (b) Drop Weight (c) Fiber Elongation
(d) Dipping Cylinder (e) Maximum Bubble Pressure
Pendant drop Sessile drop (f) Drop Method Figure 2.2. Measurement techniques to determine surface tension [98-122].
33 Babcock [100] derived the surface tension of glasses from the maximum force required
to extract the cylinder from the liquid surface, Fm :
f Fm / 4 r (1.6) where r is the average radius of the tube, and f is a correction factor that depends on the size and shape of the ring. This is one of the more widely used techniques for the accurate determination of the surface tension of high-temperature liquids or melts.
Babcock found excellent agreement with the maximum bubble pressure technique when the maximum force method was used [106]. The maximum force method as described by Shartsis and Smock [107] works best for high-viscosity glass melts.
2.4.5. Maximum Bubble Pressure Technique:
This technique is applied for glass melts [108,109] and the basic principle is that
the gas pressure, Pmax , required for a small hemispherical bubble to break away from the tip of a platinum capillary tube immersed in the melt to a known depth is the sum of the hydrostatic pressure and the capillary pressure:
Pmax Ph P gh 2 / r (1.7) where g is the gravitational constant, h is the depth of immersion, is the density difference between the liquid and the gas phase, and r is the radius of the capillary.
The main experimental difficulty associated with this technique is the accurate positioning of the capillary to a known depth [97]. In addition, if the bubbles are not formed slowly enough, an overestimate of the surface tension is obtained [104] and
Kingery states that the contact angle formed between the liquid and the tube material should be less than 30 degrees. Ellefson and Taylor limited the maximum bubble pressure technique to liquids with viscosities below 50 to 70 poises [110]. Closely
34 related to the maximum bubble pressure technique is the bulb method devised by
Pietenpol [111]. In this method, the gas pressure required to retain the dimensions of a hollow glass bulb is equated to the capillary pressure. It is important to ensure that an equilibrium size is reached since the readings are viscosity-dependent when the bulb is increasing or decreasing in size. Consequently, this method has not been widely used.
2.4.6. Drop Shape Methods:
There are two variants of drop shape techniques in use depending on whether the liquid rests on a horizontal solid surface (sessile drop) or is hanging from a tube or rod (pendant drop). Surface tension tends to force a liquid into a spherical shape, while gravity tends to flatten the sessile drop or elongates the pendant drop. The shape of these drops is completely described given the liquid mass, density, and surface tension, but the second-order differential equation that describes this balance cannot be integrated [100, 112]. Two widely used approaches, the Bashforth and Adams method and the Dorsey method, are based on the numerical solutions of the differential equation that describes the balance of capillary and hydrostatic forces. The numerical solutions to the problem are presented in table form compiled by Padday [113]. Prokop et al. [114] provided a recent reassessment of the drop shape analysis problem.
2.4.6.1. The Sessile Drop Technique
The schematic for sessile drop is shown in Fig. 2.2(f). Measurement of the maximum diameter of the drop, the distance from the maximum diameter to the drop apex, the diameter of the drop at its base, and the height of the drop allow determination of the contact angle, surface tension, and drop volume. Interpolation between values listed in the Bashforth and Adams tables [115] allows determination of
35 the shape factor, , and a scale factor, b . The density of the liquid, , is a critical part of the surface tension calculation and is usually determined by an independent method.
Insertion of the appropriate values into the following equation yields the surface tension:
(g b 2 ) / (1.8) where g is the acceleration due to gravity, is shape factor and b is a scale factor.
The size of the sessile drop does affect the error associated with the determination of surface tension when the Bashforth and Adams approach is used, largely due to its effect on the drop shape factor, . Kingery et. al. [116] stated that the Bashforth and
Adams and Dorsey methods should yield deviations of ± 2–3% and ± 5%, respectively, for metals and ionic materials having drop diameters of about 0.8 cm. The sessile drop technique has not been as widely used for the measurement of the surface tension of high-temperature liquids as some of those discussed earlier. With attention to experimental details, however, it is a relatively simple technique that yields accurate results [117].
2.4.6.2 The Pendant Drop Technique:
When a nonwetted substrate cannot be identified for the sessile drop technique, the pendant drop technique can be used and the surface tension is determined using the following equation:
2 g Jd m (1.9)
where J is the drop shape factor, g is the gravitational constant, is the liquid
density, and d m is the maximum diameter. Values of J can be found in tables as a
function of the ratio d s : d m (see Fig. 2.2(f)) [118].
36 The most important techniques used to determine the surface tension of molten metals and alloys are the sessile drop method, the pendant drop method and the maximum bubble pressure method [119].The preference among these techniques are the sessile drop, pendant drop, and drop weight methods, since they cover the broadest range of temperature for liquid metals and alloys. Errors as low as 1.5–2 % can be expected with the proper execution of the drop-shape techniques [120]. It is likely that the recent improvements to the sessile drop technique that were discussed in the previous section would improve its standing, although no recent comparative study has been made. The fiber elongation method [121] originally yielded surface tension values for soda-lime-silicate glass that were approximately half of those determined by the maximum bubble pressure method. Parikh demonstrated that the fiber elongation can be used for glasses if proper precautions are taken [122]. The capillary rise technique should probably be restricted to special cases where other techniques cannot be used due to the difficulty of determining the capillary radius, capillary rise and contact angle at elevated temperatures. Whatever technique is used, it should be made clear that proper attention must be paid to sources of chemical contamination (atmosphere, materials of construction, sample preparation, liquid contact material) as well as the physical requirements discussed above.
In this research work sessile drop technique is used to determine the surface tension based on drop profile image analysis. The values for all relevant geometrical parameters used to calculate were experimentally determined and used for surface tension determination. This technique is commonly used for the evaluation of the surface tension of liquid metals as it allows an accurate determination at high temperatures, is relatively easy to perform, and the mathematical treatment of the
37 results is simple. The sessile-drop method is based on the measurement of the profile of a drop or sample wetting on a flat non-deformable substrate. By monitoring the shape and wetting behavior with temperature, the surface tension can be calculated.
2.5. Viscosity Measurement Techniques
The viscosity of glass is highly temperature dependent, so the measurement of viscosity over a wide temperature range requires the use of several different techniques, due to the restriction to a limited range of viscosity values. Common viscometers are based on direct measurement of the viscosity using a rotation viscometer, the rate of decent of a falling sphere, or the rate of deformation of a plate, fiber, or beam. The following sections discuss the basic principles of different types of techniques for viscosity measurement.
2.5.1. Rotation Viscometers:
Rotation viscometers [123] are commonly used at room temperature to measure the viscosity of a wide variety of liquids in the range of 1 to 104 Pa s. Use of these viscometers at higher temperatures (up to 16000 C) require the use of noble metal such as platinum or platinum alloys to construct the parts exposed to the melt. These viscometers consist of a small cylinder, or spindle, which is rotated inside a large cylindrical crucible or the crucible is rotated and the torque exerted on the spindle by the melt is measured. The viscosity range covered by this method can be extended by measuring the time required for the spindle to rotate through a defined angle of deflection (103.5 to 106.5 Pa s), or by measuring the torque required to twist the spindle through a small angle (104.5 to 109 Pa s).The viscosity is determined from measurements of the torque,T , on the spindle and using the following equation:
1 C 1 1 SC T S D TD T (1.10) 4 L E r 2 R 2 UE w U
38 where L and r are the length and radius of the spindle, respectively, R is the inner radius of the cylinder holding the melt, and w is the angular velocity of the spindle rotation.
2.5.2. Falling Sphere Viscometers:
Viscosity can be measured directly through the determination of the resistance of a liquid to the motion of a sphere falling through the liquid under the influence of gravity.
The viscosity is given by the Stokes equation [124]:
2 r 2 g ( ) (1.11) 9 v s m where r is the radius of the sphere, g is the gravitational constant, v is the velocity of
the sphere, and s and m represents the densities of the sphere and melt, respectively. This method yields viscosity data in the range of 1 to 106 Pa s.
2.5.3. Fiber Elongation Viscometers:
The most widely used viscometers are based on measurements of the rate of elongation of a fiber of known dimensions under a known load. This technique was first proposed by Lillie and is now an ASTM standard [125]. A schematic of the experimental setup is shown in Fig. 2.3 (b). This method can be used for viscosities ranging from 105 to 1012 Pa s. Since the method requires formation of a long fiber of a specimen, it is well suited for many easily-worked commercial glasses, but difficulties in the formation of good fibers from many experimental compositions often limit the use of this method for basic research studies.
Fiber elongation measurements are based on the rate of elongation, dL / dt , where L is the fiber length, of a fiber of cross-sectional area, A , which is suspended vertically in a furnace. The elongation rate is determined by the viscosity of the melt and
39 the applied stress, F / A , where F is the force applied to the fiber. The viscosity is then given by the expression:
LF (1.12) 3A(dL / dt) The large surface to volume ratio of the fiber frequently results in compositional changes at the fiber surface, by either reaction with atmospheric gases or by evaporation of melt components.
(a) Rotational Viscometer (b) Fiber Elongation
(c) Beam Bending (d) Parallel plates Figure 2.3. Measurement techniques to determine viscosity [122-129].
2.5.4. Beam-Bending Viscometers:
The beam-bending method was described by Jones [126] and Hagy [127] and is now an ASTM standard [128]. A schematic of the beam bending technique is shown in
Fig. 2.3. (c). In this technique a small beam of known cross-sectional area A , is placed
40 in 3-point bending configuration with a load M , applied at the center of the beam. The viscosity is given by the expression:
gL3 C AL S D M T (1.13) 1440I cV E 1.6 U
where L is the length of the specimen between the support span, I c is the moment of inertia of the beam, V is the deflection rate of the mid-point of the beam, and is the density of the material. The second term in parentheses, AL /1.6 accounts for the contribution of the mass of the beam to the bending load. This term is frequently negligible when compared to the added mass, M , and is often neglected in the calculation, especially for viscometers which use very small samples. The easy of sample preparation for the beam-bending method makes this technique particularly suitable for research studies.
2.5.5. Parallel Plates:
The parallel plate technique [129] is an important method because it covers the intermediate viscosity range (104-108) Pa s. A schematic of the experimental technique is shown in Fig. 2.3 (d). The equation which relates the viscosity to the deformation is:
2 mh5 (1.14) dh 3V &'2 h3 V dt where m is the applied load, h is the specimen thickness and V is the specimen volume.
2.5.6. Viscosity from Creep Data:
Viscosity in the glass softening range can also be determined based on the creep data. Under a constant compressive load, either in three or four point beam bending
41 configuration, the deformation with time is monitored. Based on strain rate data in steady state regime and applied compressive stress, the viscosity can be calculated using the following equation [130,131]:
(1.15) 3
3PS 6h( and (1.16) 2bh 2 S 2 where is the viscosity, is the stress, strain, S is the length of support span, b is the width of test sample, h is the thickness of test sample, ( is the maximum deflection of the center of the sample, P is applied load at a given point on load deflection curve
and is strain rate.
2.5.7. Other viscometers:
A number of other methods are occasionally used for viscosity measurements.
The most common are the parallel plate viscometer, used in 105-108 Pa s range, the penetration viscometer, used in the 108-1012 Pa s range, and the torsion viscometer, used in the 1011-1014 Pa s range. Although each of these methods has advantages under specific conditions, none have gained wide acceptance in the glass community.
In this research work an innovative approach based on the sessile drop measurement was used to determine the viscosity of glass at solid oxide fuel cell operating temperature. Also, viscosity was determined based on creep data and these experimental data were well fitted with modified-VFT model equation. The obtained values were compared from different empirical model equations such as VFT and
Moynihan model.
42 3. Objectives and Approaches
The primary objectives of this research work is to perform a fundamental study on glass and glass composites useful as seals for solid oxide fuel cell for joining the ceramic electrolyte and metallic interconnects. The glass and glass composites seals are designed to be chemically compatible with the cell components, to have suitable thermo-mechanical properties (thermal expansion coefficient, softening temperature), high electrical resistivity, no volatilization tendency (minimum or no weight loss) and good sealing (wettability) ability with sealing components. Another goal is to investigate the effects of different types of fillers added to glass and filler volume fractions on coefficient of thermal expansion, softening temperature and their sealing ability.
Thermo-physical properties of glasses are also important in growing number of industrial applications for high temperature sealing and joining. Viscosity and surface tension are two important properties affecting sealing behavior. At elevated temperatures, glass becomes viscous and the surface tension is the parameter which controls the wetting and sealing mechanism, whereas viscosity provides information about creep resistance.
Study of the wetting behavior of seals on cell components and determination of the surface tension, viscosity, activation energy for wetting, activation energy for viscous flow and work of adhesion are important for understanding the sealing behavior. Equally important is the validation of the experimentally measured viscosity of the glass and glass composites using several existing models such as VFT and Moynihan. The approaches that are used to accomplish these objectives are as follows:
43 1. Select silicate based glass, whose thermo-mechanical properties (e.g. thermal
expansion coefficient and glass softening point) are in the range suitable for
0 0 SOFC sealing (CTE ~ 9 12 ppm/ C ,Ts ~550 750 C ), fabricate glass composites
containing 10-50 wt.% using different ceramic particulate fillers such as Al2O3,
MgO, YSZ. Investigate the effect of different types of fillers and filler volume
fractions on the thermo-mechanical properties such as CTE, softening
temperature, and viscosity and long term thermal and chemical stability of the
fabricated glass composites at 8000C.
2. Study wetting behavior of glass and glass composites on sealing components
e.g. YSZ and ferritic stainless steel to ensure the adherence and bonding
behavior and determine surface tension of the glass at SOFC operating
temperature using sessile drop technique (based on Dorsey and Porter
equations).
3. Determine viscosity of the glass and glass-composites based on VFT model
(using dilatometric data), Moynihan model (using DSC data), sessile drop
technique (using hydrodynamic model), measure viscosity by beam bending test
(creep data), and compare viscosity obtained from different approaches.
4. Characterize weight loss, electrical resistivity and microstructure of glass/cell
component interface.
44 4. Experimental Procedures
4.1. Materials Selections
Glass: Selection of a silicate glass was based on their close thermal expansion matching with cell components such as ceramic electrolyte 8YSZ.
Electrolyte: 8 mol% Yttria stabilized Zirconia, due to high ionic conductivity and well developed material as an electrolyte for solid oxide fuel cell.
Interconnect Materials: The stainless steel (SS) based alloys are potential candidate for SOFC interconnects because of their good mechanical properties, ease of fabrication, much lower cost, good thermal and electronic conductivities, and better thermal expansion compatibility. Materials such as crofer22, coated and uncoated 441 stainless steel materials were used as interconnect. Sample 441 coated and uncoated were received from PNNL (Pacific Northwest National Laboratory). The chemical composition of the interconnect materials are presented in Table 4.1 [27, 132]. Prior to experiment, all metallic interconnect samples were preoxidized at 8500C for 2 h to form an oxide layer.
Table 4.1.Chemical composition of metallic interconnects used [27,132]. Wt.% Fe Cr Mn Si C Ti P S Ni Nb La N Crofer22 Bal 22.8 0.45 - 0.005 0.08 0.016 0.002 - - 0.06 - 441 SS Bal 18.0 0.35 0.34 0.009 0.27 0.023 0.002 0.3 0.5 - 0.02
Ceramic Particulate Fillers:
The selection of ceramic filler materials were based on their high temperature stability and close match to thermal expansion of glass matrix. Three different types of ceramic particulate fillers -Al2O3 (Corundum), MgO (Periclase) and YSZ were selected.
The physical properties of as received materials are given in Table 4.2.
45 Table 4.2. Physical parameters of particulate fillers. 2 0 0 Particulates Supplier Purity Density BET area (m /g)/ CTE (RT-800 C) (ppm/ C) Fillers (g/cm3) Particle size(μm) Vendor/measured 8YSZ Tosho Inc. 99.9% 5.90 5.48/0.2 9.8/9.4 MgO Cerac Inc. 99.5% 3.58 83.29/5 13.8/13.13 -Al2O3 Sigma 99.0% 3.95 1.02/10 8.0/7.76
4.2 Material Processing
The raw materials for preparing the glass batches were made from reagent grade
SiO2, Al2O3 and carbonates obtained from Sigma Aldrich Inc, USA (99.9% purity). The thoroughly mixed batches were taken in a Pt-Rh (10%) crucible and melted in an electric furnace at 14000 C for 2 h and quenched in cold water. The glass frits were ball milled using zirconia balls in a dry state and then sieved through mesh to collect the fine powders of size (20-40) μm. Glass slurry was prepared by mixing glass powder in organic solvents, binder and plasticizer. Slurry was cast on a polymeric film to make glass tapes. The dried tape was cut into 50mm50mm dimensions and several layers of tapes were laminated using laminating press to form a laminated piece of glass sample.
The laminated sample were then used for making samples for different measurements.
Glass composites containing 0, 10 20, 30 and 50 wt. % of ceramics particulate fillers were prepared. A dry mixing process was employed in order to properly disperse the particulates fillers in the glass matrix. Dry mixing was conducted for 24 h in a ball milling process containing zirconia balls. The well mixed composite powders were further used for making tapes using tape casting process and samples were prepared for further processing and characterizations. In order to measure the thermal expansion of glass/glass composites a total three samples of rectangular shape having dimension of 25mm6mm 4mm were prepared. For weight loss test and chemical stability test, a piece of glass sample (dimension ~15mm15mm ) was placed on sintered 8 % YSZ
46 substrate (dimension ~ 20mm20mm ) and on metallic interconnect (dimension
~ 20mm20mm ). The glass samples for each measurement were sintered in air at
8000C at a rate of 30C/min, holding for 2 h, and then cooling to room temperature at the same rate.
4.3. Characterization Techniques
4.3.1. Dilatometric Measurements
The thermal properties of the glass/glass composites, such as glass transition
temperature Tg , the coefficient of thermal expansion (CTE) and dilatometric softening
temperature Ts of the sintered and annealed samples were determined from dilatometric measurements using a double push rod dilatometer (Theta Industries, Inc,
NY). The experiments were performed at a heating rate of 30C/min in Argon environment. An equal-length of sapphire sample was used as a standard to calibrate the dilatometer in order to obtain accurate thermal expansion data of the samples.
4.3.2. X-ray Diffraction
Fabricated glass and glass composites samples annealed at 8000 C for different periods of time were tested using X-ray diffractometer (X'Pert Pro,) with Cu-K radiation to analyze the phase.
4.3.3. Differential Scanning Calorimetry
Differential Scanning Calorimeter (DSC) traces were acquired in a DSC/TGA instrument (STA409PC NETZSCH) using alumina crucibles. The samples were heated at a rate of (10-30)0C/min from room temperature to 10000C in a flowing Argon gas (30
47 ' ml/min). Glass transition temperatures Tg , end of transition temperature Tg for glass and glass composites at different heating rates were acquired to calculate the activation energy for viscous flow over the softening temperature range.
4.3.4. SEM-EDAX Analysis for Interfacial Study
The long term annealed samples were cross-sectioned, mounted, and polished using conventional metallographic techniques and final finishing was done using 1 μm diamond paste. The polished cross sectioned samples were analyzed by scanning electron microscopy (SEM). Elemental line scans were performed using energy dispersive x-ray analysis (EDX) at a voltage of 15 KV.
4.3.5. Three Point Beam-Bend Test
For viscosity measurement, the deflection of sintered samples as a function of time (creep) at elevated temperatures was determined using three-point bending-beam measurements. Samples were loaded into a commercial MTS system and heated at
100C/min from room temperature to the test temperature. A load of 10 N was applied and the deflection was then measured isothermally in inert atmosphere as a function of time. After completion of the creep experiment, the sample was cooled to room temperature.
4.3.6. Weight Loss Stability Test
The weight loss was measured by annealing samples in a wet reducing environment using a set up (shown in Fig. 4.1). The samples were placed into an alumina boat and a mixture of gas (4% H2 + 96 % Ar) was sent into the furnace after
48 bubbling through deionized water (DI) held at a constant temperature of 320C so that the atmosphere contains ~ 6% H2O. After flowing the gas mixture over the sample, the gas mixture was exhausted by bubbling through the DI water. Weight-loss measurements of sample as a function of time were monitored, from which the normalized weight loss per unit exposed surface area was determined.
Furnace Alumina Tube Outlet
Water Bubbler
H2O
A r g Glass sample o n Inlet Ar Water + Bubbler H O 2 Vacuum H2 pump Silicone oil Constant temperature Bath
Figure 4.1. Schematic diagram of the apparatus used for annealing and weight loss experiment in a wet reducing environment.
4.3.7. Wettability Test using Sessile Drop Method
For sessile drop experiment, the glass/glass composite powder was pressed into a pellet with dimensions of 5mm in diameter and 5mm in height .The pressed pellet
49 was placed on a sintered YSZ plate and on preoxidized 441-stainless steel. The samples were heated in a tubular air furnace at the rate of 200C/min and at each temperature, starting from the softening point to 10500C, photographs of sample showing wetting profiles were acquired. The set up used is shown in Fig. 4.2.
Furnace Quartz Tube
Camera
Glass Sample
YSZ /Metal substrate
Thermocouple Ceramic Substrate Temperature Read out Box
Figure 4.2. Schematic of the setup for sessile drop method used for wetting/spreading experiment.
4.3.8. Electrical Resistivity Test
Experiments were performed with glass sample alone and glass sample sandwiched between different types of the metallic interconnects plates. The electrical measurements were conducted using a simple two-probe technique. Pt electrodes measuring 20 mm 20 mm in dimension were used as electrodes for glass sample.
Sample from glass laminate was cut and sandwiched between two Pt foils and heat
50 treated in air at 8000 C (dwell time: 2 h heating /cooling rate: 20 C/min). The specimen was placed in an alumina ring to prevent flow of the glass at the elevated temperatures.
A schematics of the electrical resistivity test setup is shown in Fig. 4.3.
1 K Ohm (+) V Thermocouple ( )
Quartz Tube
Metallic Interconnect Alumina Ring
Glass Furnace
Figure 4.3. Schematic of the set-up for electrical resistance measurements.
51 5. Results and Discussions
5.1. Thermal Expansion Behavior of SOFC Components
5.1.1. Thermal Expansion of SOFC Materials
A knowledge of thermal expansion is important for making seals, which operates
at a high temperature of 8000 C. The SOFC components that need to be considered in
order to make seals are ceramic electrolyte, metallic interconnects and glass. Figure 5.1
shows a series of thermal expansion curves obtained using a dilatometer for materials
used in SOFC. From the thermal expansion curves, the measured CTE are 9.40
ppm/0C, 11.50 ppm/0C, 12.50 ppm/0C and 10.64 ppm/0C for 8YSZ, Crofer22, 441
stainless steel and sealing glass, respectively.
12000 20 (a) Thermal Expansion of SOFC components (b) Coefficient of Thermal Expansion 18 of SOFC components 10000 441 SS 16 2 r2 441 SS fe 14 8000 ro C C)
Z 0 12 S Crofer22 8Y 6000 10
8YSZ 8 Glass-G
4000 T CTE (ppm/ Expansion (ppm) s 6 Glass T 4 2000 g 2 0 0 200 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 Temperature (0C) Temperature (0C) Figure 5.1. Thermal expansion data for SOFC materials (a) expansion and (b) CTE.
The thermal expansion curve for a glass yields three important pieces of
information: the thermal expansion coefficient, the glass transition temperature, and the
dilatometric softening temperature. The thermal expansion coefficient indicates the
52 relation between the volume of a glass and its temperature. The glass transition
temperature (Tg ) indicates the onset of visco-elastic behavior, while the dilatometric
softening temperature (Ts ) indicates the onset of flow. When fuel cell stacks are cooled
to room temperature, stresses begin to develop as the temperature drops below Tg and with further decrease in temperature, the stress enhances significantly due to the thermal expansion mismatch between the cell components. Therefore, to minimize the
total stress produced, Tg or Ts should be as low as practicable with adequate rigidity at
0 the cell operating temperature of around 800 C . The glass transition temperature (Tg )
0 0 and dilatometric softening temperature (Ts ) are 522 C and560 C , respectively. The
thermo-mechanical properties (CTE, Tg , Ts ) of the sealing glass are in the desired range of sealing materials.
5.1.2. Thermal Expansion of Ceramic Fillers
The selection of filler materials to design glass composite seals is based on their thermal expansion matching with glass matrix and their high temperature stability. The thermal expansion curves for ceramic fillers Al2O3, MgO, and 8-YSZ were experimentally measured as shown in Fig. 5.2. The CTE values in the range of (RT-
0 0 0 0 800) C are 13.13 ppm/ C, 9.4 ppm/ C and 7.76 ppm/ C for MgO, 8YSZ and Al2O3, respectively.
5.1.3. Thermo-mechanical Behavior of Glass Composites
Glass composites were fabricated with different types of fillers and coefficients of thermal expansion and softening temperature were measured using a dilatometer.
Figure 5.3 shows the variation in coefficient of thermal expansion and softening
53 temperature with the amount of fillers for different types of fillers. For comparison the
CTE values calculated using the rule-of-mixtures are also plotted.
12000 20 (a) Thermal expansion of filler materials (b) Coefficient of thermal expansion 18 of filler materials 10000 gO 16 M Z ) S 14 MgO 8000 8Y C) 0
ppm 12 ( O 3 Al 2 8YSZ on on 6000 10
i Al O 8 2 3 CTE (ppm/
xpans 4000
E 6 4 2000 2 0 0 200 300 400 500 600 700 800 900 1000 300 400 500 600 700 800 900 1000 0 0 Temperature ( C) Temperature ( C) Figure 5.2.Thermal expansion curves of filler materials (a) thermal expansion and (b) CTE.
900 (a) Variation in CTE with different fillers (b) Softening temperature with fillers 14.4 850 C)
13.6 0 ( G+YSZ-Exp 800 12.8 G+MgO-Exp YSZ C) 0 G+Al O -Exp 750 2 3 MgO 12.0 Al O 2 3 700 11.2
CTE (ppm/CTE 650 10.4
9.6 600 Softening Temperature Softening G+YSZ-mixture rule 8.8 G+MgO-mixture rule 550 G+Al O -mixture rule 2 3 8.0 500 0 102030405060 0 102030405060 Filler (wt.%) Filler (wt.%) Figure 5.3.Effect of different types of fillers on (a) CTE and (b) dilatometric softening temperature.
54 Figure 5.3 (b) shows the increase of the softening temperature with filler content, because the fillers inhibit flow of the glass. Also, by the use of different types of fillers and filler volume fractions a wide variety of CTE can be obtained to match the sealing components. These results show that the CTE of composites can be controlled by the use of fillers.
In case of MgO filler (30 wt.%), tremendous increase in softening temperature is due to the crystalline phase formation (such as quartz, MgSiO3, Mg2SiO4) during sintering, which prevents glass from flowing. Also, deviation in CTE from mixture rule can be seen for higher amount of MgO filler (> 20 wt. %) due to the high expansion of phases such as quartz and forsterite upon reaction with the glass.
In case of alumina filler, there is a decrease in CTE and deviation from the rule- of- mixtures. For higher amount of alumina contents, the value of CTE is lower than the predicted from the rule-of-mixtures, and this is due to the formation of a low expansion orthoclase phase, which was observed in XRD pattern.
In case of YSZ filler, the experimental values are close to the mixture rule, but for higher amounts of filler, CTE value is lower than the prediction from the rule-of-mixtures.
The density of glass/YSZ composites for higher amount of fillers (30 wt. % and 50 wt. % filler) are (90-93) % that of theoretical values and this leads to initiation of pores, which may be the reason for lower CTE. Also, there is an increase in softening temperature from 5600 C (for pure glass) to 7400 C for higher amount of YSZ filler (50 wt.%).
5.1.4. Long Term Annealing Behavior of Glass and Glass Composite Seals
In order to assess the thermal and phase stability of the designed glass and glass composite seals, the seals were annealed at 8000 C in air. At each time interval
55 the thermal expansion and phases were monitored using dilatometer and XRD to determine the thermal and phase stability. Figures (5.4-5.6) show the effect of annealing time on thermal and phase stability of glass and glass composites.
12.0 (a) Effect of annealing time on CTE at 8000 C (b) XRD pattern of G + 30 wt.% Al O particulate 2 3 Glass + Al O Composites 0 11.2 2 3 annealed in air at 800 C O 10.4 G+10 Alumina O A : -Al O G+20 Alumina 2 3 O : Orthoclase C) G+30 Alumina 0 9.6 1000 hrs
8.8 O A O,A
CTE (ppm/ CTE O A
8.0 (a.u.) Intensity O O O A A 7.2 A A O O OA AO OO OO O O OO 6.4
0 200 400 600 800 1000 1200 10 20 30 40 50 60 70 Annealing time (hrs) 2 (Degrees)
Figure 5.4. Glass + Al2O3 composites: effect of annealing on (a) expansion and (b) phase formation.
13.8 (a) Effect of annealing time on CTE at 8000 C (b) XRD pattern of G + 30 wt.% MgO particulate Glass + MgO Composites annealed in air at 8000 C 13.2 F G+10 MgO F F : Mg SiO G+20 MgO F 2 4 E : MgSiO G+30 MgO F 3 12.6 Q : Quartz F F F F F F C) F 0 F F F F F F 12.0 F F F FF F 1000 hrs Intensity (a.u.) Intensity CTE (ppm/ CTE 11.4 500 hrs A
10.8 F FEQ FQ As sintered
10.2 0 200 400 600 800 1000 1200 10 20 30 40 50 60 70 80 90 Annealing time (hrs) 2 (Degrees) Figure 5.5 Glass+ MgO composites: effect of annealing on (a) expansion and (b) phase formation.
56 12.4 (a) Effect of annealing time on CTE at 8000 C (b) XRD pattern of G + 30 wt.% YSZ particulate Glass + YSZ Composites 0 12.0 annealed in air at 800 C
11.6 Y Y : YSZ G+10YSZ G+20YSZ JCPDS file no. 48-0224 Y 11.2 G+30YSZ G+50YSZ C) 0 Y 10.8 Y Y
Y Y Y 10.4 1000 hrs CTE (ppm/
10.0 (a.u.) Intensity
9.6 500 hrs 9.2 As sintered 8.8 0 200 400 600 800 1000 120010 20 30 40 50 60 70 80 90 Annealing time (hrs) 2 (Degrees) Figure 5.6. Glass + YSZ composite: effect of annealing on (a) expansion and (b) phase formation.
Long term annealing of glass composites shows that the alumina and MgO fillers are not so effective in terms of their thermal expansion and phase stability. Alumina reacts with glass and forms low expansion monoclinic orthoclase phase having CTE ~
6.1 ppm/0C. MgO reacts with the glass and forms high expansion phases such as quartz and forsterite. Glass composites with YSZ filler show thermal and chemical stability so that they can be used as a seal without degrading the glass properties.
5.2. Wetting Behavior of Glass/ Glass composites with Cell Components:
In sealing technology, proper wetting of sealing material with sealing surface is an important criterion that needs to be evaluated. Good wetting/spreading is required to ensure interfacial contact between two sealing surfaces. The wetting/spreading behavior of sealing glass with cell components (YSZ and 441 SS metallic interconnect) was examined by monitoring the changes in shape of a glass pellet on cell components.
The influence of temperature and time on flow behavior of glass was evaluated by
57 measuring the contact angle. Also, a glass composite sample (10 wt. % YSZ filler) was evaluated to observe the effect of filler material on contact angle and flow behavior.
Contact angle was determined by analyzing the image and measuring the height and
radius of the droplet shape using formula [133]: 0 2*arctan(h / rmax ) ; where 0 is the
contact angle, h is the height of a droplet’s image and rmax is the half of its width (more details are discussed in section 5.5). Figure 5.7 shows the photographic images of glass and glass composites with 10 wt.% YSZ filler tested on YSZ electrolyte substrate and on metallic interconnect at different temperatures.
Glass on YSZ electrolyte Substrate
5000C 5500C 6000C 6500C 7000C 7500C 8000C Glass on 441SS Metal Substrate
5000C 5500C 6000C 6500C 7000C 7500C 8000C G+10YSZ on YSZ electrolyte Substrate
5000C 5500C 6000C 6500C 7000C 7500C 8000C G+10YSZ on 441SS Metal Substrate
5000C 5500C 6000C 6500C 7000C 7500C 8000C Figure 5.7. In situ photographic images of glass and glass composites tested on YSZ and metallic substrates at different temperatures (Magnification: 3X).
Figure 5.8 shows some in-situ images of glass and glass composite at different temperatures after equilibrium is reached.
58 a
b
c
d
T = 7500 CT = 8000 CT = 8500 CT = 9000 C Figure 5.8. In situ images after relaxation of seal samples at different temperatures (a) glass on YSZ substrate (b) glass on SS 441 metal substrate (c) glass composite on YSZ substrate, and (d) glass composite on SS 441 metal substrate (Magnification: 5X).
Based on these images, the contact angle was measured using the formula
0 2*arctan(h / rmax ) . The measured contact angles as a function of temperature and time are plotted in Figure 5.9. A reduction in contact angle with increasing temperature is observed for the pure glass and glass composite containing YSZ filler on both substrates. The addition of 10 wt. % YSZ powder increased the contact angle when compared with the pure glass at 8000 C. The contact angles of glass on YSZ and on metal substrates are 630 and 570, respectively, whereas for glass composite contact angles are 740 and 670 on YSZ and metal substrates, respectively. The measured contact angles are less than 900, which is important for good wetting of glass/glass composite seals on sealing components.
59 140 140 (a) Variation in contact angle with temperature (b) Time evolution of the contact angle 0 120 at 800 C 120 Glass on YSZ 100 Glass on metal Glass Composite on YSZ 100 80 Glass Composite on Metal
60 80
Glass on YSZ Contact angle (deg) 40
Glass on metal Contact angle (deg) 60 20 Glass composite on YSZ Glass composite on Metal 0 40 0 100 200 300 400 500 600 600 650 700 750 8000 850 900 950 Temperature ( C) Time (min) Figure 5.9. Wetting behavior of glass and glass composite on cell components (a) contact angle versus temperature, and (b) contact angle versus time.
From the plot, it can be seen that the difference in contact angle of samples tested on YSZ and metal varies. These differences are because of different surface energy of metal and ceramic YSZ substrates. Wetting occurs when the surface energy of the solid surface is reduced by contact with the glass. The relative order of surface energy values is metals > ceramics > silicate glasses. The equilibrium relationship between contact angle and surface energy in the wetting environment is given by
cos 0 ( SV SL ) / LV ; the driving force for the wetting is ( SV SL ), which is acting on the periphery of the glass. Since the lowest surface energy configuration for the glass is
sphere, if the enhanced driving force for wetting exceeds LV , then spreading occurs,
which can happen when SV is greater than LV .
In order to understand the joining/bonding behavior of developed glass composite containing different fillers with cell components, a square piece of sample from laminates was cut and placed on a sintered piece of YSZ plate and on preoxidized
60 metallic interconnect (Crofer22). All specimens were heated to different temperatures to
observe the sealing and wetting behaviors. The sealing ability, sealing parameters and
phase stability of tested glass composites are presented in Table 5.1.
Table 5.1.Sealing temperature, bonding nature and crystallization of seals. Seal Materials Joining/Bonding Bonding Bonding Phase stability Temperature (0C) with YSZ with Interconnect (Crofer22) Glass 8000 C/ 2 hrs No crystallization 0 Glass-10 wt.% Al2O3 800 C/ 2 hrs Crystallization 0 Glass-20 wt.% Al2O3 800 C/ 4 hrs Crystallization 0 Glass-30 wt.% Al2O3 800 C/ 10 hrs Crystallization 0 Glass-50 wt.% Al2O3 >1200 C × × N/A Glass-10 wt.% MgO 8000 C/ 2 hrs No crystallization Glass-20 wt.% MgO 8000 C/ 4 hrs Crystallization Glass-30 wt.% MgO 10500 C/ 2 hrs Crystallization Glass-50 wt.% MgO >11500 C × × Crystallization Glass-10 wt.% YSZ 8000 C/2 hrs No crystallization Glass-20 wt.% YSZ 8000 C/4 hrs No crystallization Glass-30 wt.% YSZ 8000 C/10 hrs No crystallization Glass-50 wt.% YSZ 9500 C/ 2 hrs No crystallization
Based on this analysis, glass composites with YSZ filler show promise for making viable
seals, which can act as self healing seals without degrading the glass properties.
Further investigations and discussions are focused on glass and glass composites with
YSZ fillers.
Figures 5.10 and 5.11 show the contact angle relaxation curves for glass and
glass composite (10 wt.% YSZ filler) tested at different temperatures. Higher contact
angles were observed during the initial stages of the relaxation for glass and glass
composite. The relaxation of the contact angle was sharp during initial stages, and it
became gradual as the system approached the equilibrium. The contact angle
decreases with increase in temperature for both glass and glass composite. At lower
temperatures, viscous forces dominated the spreading due to high viscosity of glass
61 and glass composite and at lower temperature, longer periods of time are generally required to achieve equilibrium.
120 120 (a) Time evolution of the contact angle (b) Time evolution of the contact angle 110 110 at different temperatures at different temperatures 100 100 Glass on YSZ Glass on Metal 90 90 80 80 , deg) , deg) D D 70 70 60 60
50 50 40 40 Contact angleContact ( Contact angle ( 30 (t) ~ t-m 30 (t) ~ t-m D D 20 20 T = 9000 C T = 8500 C T = 9000 C T = 8500 C 10 10 T = 8000 C T = 7500 C T = 8000 C T = 7500 C 0 0 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Time (min) Time (min) Figure 5.10. Time evolution of contact angle for glass spreading on cell components (a) on YSZ, and (b) on SS 441 metal.
120 120 (a) Time evolution of the contact angle (b) Time evolution of the contact angle 110 at different temperatures 110 at different temperatures 100 G + 10 wt.% YSZ filler on YSZ 100 G + 10 wt.% YSZ filler on Metal 90 90 , deg) , deg) D D 80 80
70 70
60 60
50 -m 50 Contact angle ( (t) ~ t Contact angle ( D 40 40 (t) ~ t-m D T = 9000 C T = 8500 C 30 30 T = 9000 C T = 8500 C T = 8000 C T = 7500 C T = 8000 C T = 7500 C 20 20 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Time (min) Time (min)
Figure 5.11. Time evolution of contact angle for glass with 10 wt.% YSZ filler spreading on cell components (a) on YSZ, and (b) on SS 441 metal.
62 In higher temperature regime, spreading was faster due to lower viscosity. When system reached at equilibrium, the negligible change in the base radius was observed due to viscous forces. Due to lower viscosity of the glass it spreads faster than the glass composite. The spreading rate increased due to the increase in temperature of the substrate. This is due to the fact that an increase in temperature reduces both viscosity of the glass and surface energy, resulting in enhanced spreading. Glass shows the higher base radius as compared to higher viscosity glass composite. In case of glass composite, having higher viscosity offer greater resistance to flow during spreading as compared to lower viscosity glass, resulting in a higher equilibrium contact angle.
5.2.1. Activation Energy for Wetting/Spreading
The contact angle dependence on time and temperature can be empirically
m expressed as: D ~ t (time dependent part of contact angle: Figs. 5.10 and 5.11) and
0 ~ exp( Ea / KT ) (Arrhenius type temperature dependent part of contact angle : Fig.
5.9), 0 is the equilibrium contact angle at a particular temperature, m is the exponent
describing the relaxation process, Ea is the activation energy of the process, R is the
m gas constant (8.314 J/mol K), and T is the absolute temperature; i.e. D 0t and
0 exp( Ea / KT ) . A logarithmic (ln-ln) plot of contact angle ( D ) vs time will be a
straight line and the slope of ln( 0 ) vs 1/T can be used to determine the activation energy for spreading (wetting). Combining the time and temperature dependence of
m contact angle together, we have D {t exp( (Ea / m) / KT )} , where Ea / m Ew is the activation energy for spreading (wetting). Figure 5.12 shows the ln-ln plot of contact angle versus time and Fig. 5.13 shows Arrhenius plots for contact angles; from the slope the value of activation energy for wetting can be obtained.
63 8.0 8.0 (a) ln-ln plot of contact angle vs time (b) ln-ln plot of contact angle vs time 7.2 at different temperatures 7.2 at different temperatures Glass on YSZ Glass on Metal 6.4 6.4
5.6 5.6 , deg) , deg) 4.8 4.8 D D
4.0 4.0 ln ( ln ln ( ln
0 3.2 900 C 3.2 9000 C 0 850 C 8500 C 2.4 0 2.4 800 C 8000 C 0 1.6 750 C 1.6 7500 C
56789101112 56789101112 ln (t, sec) ln (t, sec) 7.0 7.0 (c) ln-ln plot of contact angle vs time (d) ln-ln plot of contact angle vs time 6.5 at different temperatures 6.5 at different temperatures 6.0 Glass composite on YSZ 6.0 Glass composite on Metal 5.5 5.5 5.0 5.0 , deg) , deg) D D 4.5 4.5 ln ( ln 4.0 ( ln 4.0 0 3.5 T = 900 C 3.5 T = 9000 C 0 T = 850 C T = 8500 C 3.0 0 3.0 T = 800 C T = 8000 C 0 2.5 T = 750 C 2.5 T = 7500 C 2.0 2.0 5 6 7 8 9 10 11 12 5 6 7 8 9 10 11 12 ln (t, sec) ln (t, sec) Figure 5.12. Logarithmic plot of contact angle versus time: (a, b) glass tested on YSZ and on metal and (c, d) glass/10 wt. % YSZ composite on YSZ and on metal substrates, respectively.
The wetting activation energies of glass on YSZ and on metal substrate are 248
KJ/mol and 318 KJ/mol, respectively, whereas for glass composite (10 wt.% YSZ filler) wetting activation energies are 370 KJ/mol and 397 KJ/mol on YSZ and metal substrate, respectively. The wetting activation energy for glass composite is higher than glass,
64 which is due to the presence of rigid filler materials inhibiting the spreading of glass.
Also, the activation energy for wetting is temperature dependent and decreases with temperature because of low contact angle at high temperature.
5.4 900 (a) Arrhenius plots of the contact angle for 0 5.2 (b) Activation energy for wetting at 800 C glass and glass composite 800 5.0 700 Glass on YSZ 4.8 Glass on YSZ Glass on Metal Glass on Metal Glass composite on YSZ 4.6 600 Glass composite on YSZ Glass composite on Metal 4.4 Glass composite on Metal 500 , Deg) 0 4.2
(KJ/mol) 400 W
ln ( ln 4.0 E 3.8 300 E Glass (on YSZ) = 318 KJ/mol 3.6 w Glass 200 E (on metal) = 248 KJ/mol 3.4 w 0 G+10YSZ T = 800 C E (on YSZ) = 397 KJ/mol -m w 100 = {t exp(-E /RT)} 3.2 G+10YSZ D 0 w E (on metal) = 370 KJ/mol = * exp(E /RT) 3.0 w 0 D 0 a 0.84 0.88 0.92 0.96 1.00 700 750 800 850 900 950 0 1000/T(K-1) Temperature ( C) Figure 5.13. Glass and glass-composite (a) Arrhenius plots for contact angle and (b) activation energy for wetting in SOFC operating range.
5.2.2. Work of Spreading of Glass/Glass Composite
The thermodynamic criterion of liquid spreading over the surface of a solid is the diminishing free energy of the system,G , due to increasing surface of contact between the liquid and the solid. At a constant temperature and volume, the change in free energy of the system because of spreading of the liquid is [139]:
dG LV dALV SV dASV LS dALS (5.1)
where LV etc. are surface free energies (liquid-vapor, etc.), and A is the surface area
+ of the border (liquid-vapor, etc). The condition for spreading is LV LS SV 0 .
65 At equilibrium the free energy of the system should have minimum value, dG 0, and the equilibrium contact angle of wetting is:
cos SV LS (5.2) 0 LV
The work of spreading or adhesion characterizes the bonding forces between the liquid and solid and equals the work of separation of liquid and solid along the interphase border. It also characterizes the interaction between two phases at the contact and is given by
WSp SV LV LS (5.3)
Combining equation (5.2-5.3) gives
,- WSp LV 1 cos 0 (5.4)
An interface is stable when WSp is positive i.e. when forming the surface results in a decrease in the total free energy of the system. It is therefore, experimentally feasible to obtain quantitative values of the bonding strength between different glass-to-cell components. The surface tension values determined in proceeding section 5.5 and were used to calculate the work of spreading of glass and glass composite on cell components. The thermodynamic works of spreading for seals were calculated and are presented in Fig. 5.14. Higher value of work of spreading represents stronger bond between seal and cell components. The calculated values of thermodynamic work of spreading for glass and glass composite at SOFC operating temperature (8000 C ) are
0.39 J / m 2 , 0.34 J / m 2 on YSZ substrate and 0.42 J / m 2 , 0.38 J / m 2 on metallic interconnect, respectively. In case of glass, low contact angles and good spreading on cell components were observed and this leads to high work of spreading compare to glass composite.
66 0.65 Thermodynamic work of spreading on 0.60 cell components at different temperatures
0.55 Glass on YSZ Glass on Metal Glass composite on YSZ ) 2 0.50 Glass composite on Metal
(J/m 0.45 Sp W 0.40
0.35 W = [1 + cos( )] Sp LV 0 0.30 T = 8000 C 0.25 700 750 800 850 900 950 Temperature (0C) Figure 5.14. Thermodynamic work of spreading of seals on cell components.
5.3. Sessile Drop Model
5.3.1. Determination of Surface Tension
Based on the well known sessile drop experiments, the shape of the sessile drop is used as a fundamental basis to determine the surface tension. The determination of surface tension by means of the sessile-drop method is accomplished by equating the hydrostatic pressure of the liquid at any point to the excess pressure due to the curvature and tension of surface at the point of interest. The method employed consists of calculating surface tension from the dimensions of a sessile drop. The governing equations for surface tension are summarized in the following section following the method outlined by Kingery [116]. As illustrated in Fig. 5.15, the shape of a sessile drop depends on equilibrium between forces of surface tension and of gravity. The fundamental equation of surface tension at any point requires:
C 1 1 S D T gz c (5.5) E R1 R2 U
67 where is the surface tension, R1 , R2 are principal radii of curvatures, g is the acceleration due to gravity, is the density of glass or liquid, z is the ordinate giving depth below the apex of the drop (shown in Fig. 5.15) and c is a constant. If R is the radius of curvature of a meridional section and is the angle between R and the axis
x C 1 sin S of revolution, z , then R R ; R ; D T gz c (5.6) 1 2 sin E R x U
(a) O X (b) O X z x z R
R2
R1 Z Z Figure 5.15. (a) Sessile drop and (b) definition of co-ordinates systems [116,135].
If R0 is defined as the radius of curvature at the origin, at the origin R R0 ,
2 x / sin R0 and z 0 . From equation (5.6) c (5.7) R0
From equations (5.6) and (5.7), the equation of the equilibrium surface will lead to the following fundamental equation also known as Laplace equation [116]:
C 1 sin S 2 D T gz (5.8) E R x U R0
1 F 1 sin 2 V gz 4 G W (5.9) H R x R0 X
68 The differential equations describing the drop profile can be obtained by a variational approach-such as a minimization of the sum of the gravitational and surface potential energies [135]. Then, the equation for surface tension reduces to:
1 F1 d 2 V gz 4 G (xsin) W (5.10) H x dx R0 X
This is the general equation for determination of surface tension from the sessile drop method. By measuring the respective dimension from the obtained photographic image of spherical contour (Fig. 5.16), Dorsey [136] simplified the above equation by using the
maximum radius of the contour, rmax and the measured distance from the top to the intersection of the axis with a 450 tangent to the contour ( z ), h is the distance between a specimen peak and a level (plane) of specimen longest chord.
450 z ( x , z ) h r r max c Figure 5.16. Schematics of geometrical parameters based on Dorsey and Porter approach.
Based on the different geometrical factors and iteration of image profile, Dorsey deduced an equation which leads to a fundamental equation also known as Dorsey equation:
C S Dorsey 2 D 0.0520 T (r z) g. glass .rmax D 0.1227 0.0481f T; f 0.4142 (5.11) E f U rmax
69 The parameter rmax is also defined as a half of specimen longest chord parallel to the base. Porter developed an equation based on the same approach with different factors. The Dorsey’s equation is not suitable for small drops, since the value of
(r z) f 0.4142 , is close to zero and negative values of surface tension often rmax appear. The Better results are achieved by using the Porter’s equation [137]:
CC S2 C S3 F C S2 VS Porter 2 DD h T D h T G D h T WT g. glass .rmax D T 0.660D T 1 4.05D T (5.12) D G WT EE rmax U E rmax U H E rmax U XU
Figures 5.17 and 5.18 show the image profiles for glass and glass composite at different test temperatures. Based on the image profiles and using equations (5.11) and
(5.12), the values of measured surface tensions are plotted in figures (5.19-5.20).
Figure 5.17. Image profiles of glass at different temperatures for surface tension calculation using Dorsey and Porter equation based on sessile drop model.
70 Figure 5.18. Image profiles of G + 10 wt.% YSZ composite at different temperatures for surface tension calculation using Dorsey and Porter equation based on sessile drop model.
0.330 0.330 (a) Surface Tension with Temperature (b) Surface Tension with Time 0.320 Glass 0.320 at 9000 C Glass 0.310 0.310 Porter Porter Dorsey 0.300 Dorsey 0.300 0.290 0.290
0.280 0.280
0.270 at 8000 C 0.270 Surface Tension (N/m) Dorsey Surface Tension (N/m) 0.260 = 0.288 N/m 0.260 Porter = 0.281 N/m 0.250 0.250 850 900 950 1000 1050 1100 0 100 200 300 400 500 600 700 Temperature (0C) Time (Minutes) Figure 5.19. Variation of surface tension of glass with (a) temperature and (b) time.
71
0.330 0.330 (a) Surface Tension with Temperature (b) Surface Tension with Time 0.320 0.320 at 9000 C G + 10 wt.% YSZ Porter ) G + 10 wt.% YSZ 0.310 0.310 Porter m Dorsey Dorsey (N/ 0.300 0.300 on i 0.290 0.290
ens T 0.280 0.280
ace 0 f 0.270 at 800 C 0.270
ur Dorsey Surface Tension (N/m) S = 0.296 N/m 0.260 0.260 Porter = 0.295 N/m 0.250 0.250
850 900 950 10000 1050 1100 0 100 200 300 400 500 600 700 Temperature ( C) Time (Minutes) Figure 5.20. Variation of surface tension of G-10YSZ with (a) temperature and (b) time.
The experimentally measured values of surface tension of glass and glass composite were extrapolated to determine the value at 8000 C. The surface tension
0 Porter values for glass obtained by using sessile drop model at 800 C are LV 0.281N / m
Dorsey and LV 0.288 N / m , respectively based on Porter and Dorsey’s approaches. The values of surface tension for glass composite with 10 wt.% YSZ filler are
Porter Dorsey LV 0.295 N / m and LV 0.296 N / m , respectively.
Comparison from theory and literature
The experimentally obtained values were compared with the value obtained by
additive principle calculation using the formula [138]: LV B X i gi ( ) ; where X i is the
content of the oxides in mol% and gi ( ) is the additive factors of surface tension for respective oxides in the considered multi components glass system. The values of the additive factors were obtained from the literature as provided by Dietzel [138]. The
72 values of surface tension are LV 0.273 N / m and LV 0.269 N / m based on the additive calculation and using SciGlass6.0 software [139]. Some of data for surface tension for silicate glasses are collected from literature (Table 5.2) and compared with the experimentally obtained values; the reported values agree well with our results.
Table 5.2.Surface Tension of silicate glasses at respective temperatures [140]. Silicate based glass system (mol %) Surface tension Temperature (0C) LV (N / m) 25.0Na2O 75.0SiO2 0.298 1020 1412 36.0Na2O 64.0SiO2 0.292 900 19.5Na2O 80.5SiO2 0.276 1100 16.7K 2O 83.3SiO2 0.224 1100 16.0Na2O 20.0K 2O 64.0SiO2 0.236 1300 16.7Na2O 4.1K 2O 9.6CaO 69.6SiO2 0.292 1200 16.6Na2O 4.4Al2O3 9.6CaO 69.4SiO2 0.315 1200 16.9Na2O 2.7B2O3 9.8CaO 70.6SiO2 0.298 1200 16.8Na2O 3.5MgO 9.7CaO 70.0SiO2 0.316 1200 19.8Na2O 5.9BaO 74.3SiO2 0.296 1300
5.3.2. Determination of Viscosity
The wetting of solid by liquid/glass is actually considered as the displacement of a solid surface by liquid involving the contact line (where, liquid, solid and vapor seem to meet) movement. The dynamics of a spreading liquid are controlled by the details of the fluid motion very near the moving contact line (liquid/solid/vapor interface). A model, valid in the small capillary number limit, describes the fluid motion and viscous interfacial deformation near the moving contact line. Capillary number represents the relative effect of viscous forces versus surface tension acting across an interface between a liquid and a vapor phase. In all these phenomena, surface tension forces are important and the details of the fluid motion very near the contact line control the spreading. Figure 5.21 depicts the schematics used to describe dynamics of wetting using hydrodynamic model.
73 (a) Non equilibrium (b) Equilibrium z Young’s equation LV LV LV LV
U h U liquid SV SL D x SV SL liquid D SV SL 0 0 SV SL LV cos D Solid R (t) LV cos 0 Solid
Figure 5.21. Schematics to describe wetting dynamics using hydrodynamic model.
In Fig. 5.21, D , is the dynamic contact angle, which depends on time and velocity of triple line at test temperature and after some time reaches at equilibrium
contact angle 0 (which is the contact angle at equilibrium as defined by Young’s equation). In case of spreading, the driving force for spreading per unit length of triple line is given by the change in the surface and interfacial energy of the system resulting from a lateral displacement of the triple line. This driving force for spreading is the out- of-balance surface tensions, which occur when the equilibrium is disturbed. From Fig.
5.21(a), a simple force balance results in the following equation:
Fdrive SV SL LV cos D (t) (5.13)
From Fig. 5.21(b): cos SV SL ; cos (5.14) 0 SV SL LV 0 LV
Combining equation (5.13) and (5.14), the driving force for spreading is given by
Fdrive LV [cos 0 cos D ] (5.15)
This is balanced by the braking force due to viscous effects within the liquid and is characterized by the viscosity. The viscous force is the resisting force, which is opposing the spreading driving force. Viscous spreading is well defined in the
74 framework of De Gennes model [141], where the flow very near the contact line is assumed as a wedge advancing over a solid surface. The fluid velocity in the wedge region is calculated by a “lubrication” approximation [142] where the fluid velocity u
parallel to the interface varies parabolically between u 0 at z 0 and u umax at z h , at the surface (h: the drop height) as shown in Fig.5.22.
(a) liquid h D (t) Solid R (t) z (b)
umax Z=h (c) gradient Fvis
du x (z) h liquid Vapor dz D (t) Z=0 x Solid velocity R (t) Figure 5.22. (a) Viscous spreading, (b) fluid velocity profile in the liquid wedge close to the triple line, and (c) motion of the liquid in the vicinity of the triple line ( : liquid molecule sliding) over a wedge.
Based on De Gennes [141] assumption and considering the liquid wedge advancing over a solid surface is a nearly thin flat film, the velocity profile can be approximated as Poiseuille type flow (Fig. 5.22(b)). Assuming no-slip at the solid surface and no stress at the free surface (i.e. the solid-liquid boundary), the boundary u x (z) conditions are: No shear at the free liquid surface i.e. | 0 and no slip at the z z h solid surface i.e. u x (z) |z0 0 . Then the viscous force (resistance) per unit length of the triple line is given by [143]:
75 3 U C R S F lnD T (5.16) vis D E U where is small cut-off distance near the contact line (Fig. 5.22(c)), without which, the viscous dissipation in the liquid wedge would diverge [144], corresponds to a thickness of the meniscus immediately close to the liquid front on the solid surface (this length is of the order of molecular size of liquid element, the values used here are
3 10 R 10 m and 10 m ), D is the dynamic contact angle and 0 is the equilibrium contact angle. Typical values of the term ln (R / ) reported in the literature lie between
10 and 20 [144], U is the triple line velocity ( dR / dt ), and R is the radius of drop.
For steady state spreading, the work done by surface forces = energy dissipated
due to viscous impedance, i.e. Fdrive Fvis
3 U C R S ; [cos cos ] lnD T (5.17) LV 0 D D E U
Based on this analysis, and following De Gennes assumption, the hydrodynamic
0 models for dynamics of wetting for small contact angle ( D 135 ) can be described as
[145]:
9 U C R S 3 3 lnD T (5.18) D 0 LV E U
Where the capillary number depends on dynamic contact angle by the following
1 equation [146]: C ( 3 3 ) ( 1350 ) (5.19) a C R S D 0 D 9lnD T E U
Based on sessile drop experiments, the values of dynamic contact angles and equilibrium contact angles are measured and used to calculate the capillary number. The drop radius and contact angle evolutions with time were recorded between
76 750-9000C and used to determine the velocity of triple line movement. Figures 5.23 and
5.24 show the time evolution of drop radius at different temperatures.
12.0 12.0 (a) Time evolution of the drop radius (b) Time evolution of the drop radius at different temperatures at different temperatures 10.5 Glass on YSZ 10.5 Glass on Metal
9.0 Viscous 9.0
Capillary
7.5 Capillary 7.5
Viscous Drop radius(R,Drop mm) Drop radius(R,Drop mm) 6.0 6.0 R(t) ~ tn R(t) ~ tn T = 9000 C T = 8500 C T = 9000 C T = 8500 C 4.5 T = 8000 C T = 7500 C 4.5 T = 8000 C T = 7500 C
0 100 200 300 400 500 600 0 100 200 300 400 500 600 Time (min) Time (min)
Figure 5.23. Time evolution of drop radius for glass spreading on cell components (a) glass on YSZ, and (b) glass on SS 441 metal.
7.5 12 (a)Time evolution of the drop radius (b)Time evolution of the drop radius at different temperatures 11 at different temperatures 7.0 G + 10 wt.% YSZ filler on YSZ substrate G + 10 wt.% YSZ filler on Metal 10
6.5 Viscous 9
8 6.0 Capillary
7 Capillary
5.5 6 Viscous
Drop radius (R, mm) radius Drop n
Drop radius (R, radius (R, mm) Drop R(t) ~ t R(t) ~ tn 5 5.0 0 0 0 0 T = 900 C T = 850 C 4 T = 900 C T = 850 C T = 8000 C T = 7500 C T = 8000 C T = 7500 C 4.5 3 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Time (min) Time (min) Figure 5.24. Time evolution of drop radius for glass with 10 wt.% YSZ filler spreading on cell components (a) on YSZ, and (b) on SS 441 metal.
77 The spreading behavior of glass and glass composite on different substrates and temperatures consisted of an initial capillary regime followed by the viscous regimes. Significant spreading was observed in glass and glass composite at higher temperatures. The higher spreading area is indicative of the spreading of seal material during relaxation resulting in a lower contact angle.
Glass/glass composite started spreading rapidly with a relatively high velocity resulting in a sharp increase of base area during early stages of spreading. However, within a period of 1 h, the spreading rate was significantly reduced, indicating the stabilization condition. This is due to the attainment of equilibrium between the various surface forces.
Based on measured drop radius and contact angles with respect to time, the spreading velocity and capillary numbers were calculated using equation (5.19). The plot of capillary number and spreading velocity versus dynamic contact angle of the glass and glass composite (10 wt. % YSZ) are shown in Fig. 5.25 and Fig. 5.26, respectively. In case of glass, the spreading velocity is initially high due to less viscous drag but as the system approaches towards equilibrium contact angle, the process slow down and spreading velocity reduces sharply. However, in case of glass with YSZ filler, the spreading velocity does not drop as sharply due to inhibition of flow because of the rigid filler. Capillary number depends on the ratio of viscous and interfacial forces.
However, the surface tension does not change much with filler addition, so the capillary number will be mostly affected by viscous effect.
78 130 130 (a) Capillary number vs contact angle (b) Capillary number vs contact angle 120 at different temperatures 120 at different temperatures 110 Glass 110 G + 10 wt.% YSZ 100 100 , deg) , deg) D D 90 90 80 80
70 70
60 60 0 0 T = 900 C Contact angle ( Contact angle ( T = 900 C 0 50 T = 8500 C 50 T = 850 C 0 T = 8000 C T = 800 C 40 40 0 T = 7500 C T = 750 C 30 30 10-5 10-4 10-3 10-2 10-1 100 10-5 10-4 10-3 10-2 10-1 100 Ca Ca Figure 5.25. Capillary number vs. contact angle of (a) glass, and (b) glass composite.
120 120 (a) Spreading Velocity vs contact angle (b) Spreading Velocity vs contact angle 110 at different temperatures 110 at different temperatures Glass G + 10 wt.% YSZ 100 100
, deg) 90 90 D , deg) D
80 80
70 70
0 60 0 60 T = 900 C T = 900 C 0 0 T = 850 C
Contact angleContact ( T = 850 C 50 50 0 0 T = 800 C Contact angleContact ( T = 800 C 0 0 T = 750 C 40 T = 750 C 40 30 30 10-12 10-10 10-8 10-6 10-4 10-2 100 10-12 10-10 10-8 10-6 10-4 10-2 100 U(m/s) U(m/s) Figure 5.26. Spreading velocity vs. contact angle of (a) glass, and (b) glass composite.
79 By substituting value of capillary number from equation (5.19) into equation (5.18) and rearranging equation, an equation for viscosity can be obtained as:
C S log (T) logCa ( , ,T ) logD LV T (5.20) D 0 EU ( ) U
Using equation (5.20), viscosity at different temperatures can be calculated. Figure 5.27 shows the viscosity temperature plot for glass and glass composite measured based on sessile drop model.
10 Viscosity of glass and glass composite 9 based on sessile drop model 8 Glass G+10 wt.% YSZ Linear fit-Glass 7 Linear Fit-G-10YSZ 6 , Pa S)
5 log ( log 4
3 log Glass (Pa.s) = - 6.579 + 11.867 (103/T) 2 log G+10YSZ (Pa.s) = - 10.234 + 16.612 (103/T) 1 0.80 0.84 0.88 0.92 0.96 1.00 1.04 1.08 103/T(K-1) Figure 5.27. Viscosity-temperature data based on the sessile drop model.
The measured viscosity data can be best fitted with the following equations:
C 103 S Glass: log (Pa.s) 6.579 11.867D T (5.21) E T (K) U
C 103 S Glass + 10 wt. % YSZ: log (Pa.s) 10.234 16.612D T (5.22) E T (K) U
80 The value of viscosity determined over a range of temperatures also gave an activation energy value of 227 KJ/mol and 318 KJ/mol for glass and glass composite with 10 wt. %
YSZ filler, respectively.
5.4. Viscosity of Glass and Glass composite
5.4.1. Viscosity from VFT Model Based on Dilatometric Data
The Vogel-Fulcher-Tamman (VFT) model is based on the free volume of the melt structure. According to this model, below the glass transition temperature, the molecules are packed together at their maximum density since no molecular motion is possible. As the temperature increases, it increases the free volume which provides space for molecular motion and hence viscous flow. VFT equation of viscosity is an empirical expression, which describes viscosity data at intermediate temperatures over many orders of magnitude with a high degree of accuracy. According to VFT equation
[147], the viscosity dependence on temperature is given by:
B log ( ) A VFT (5.23) glass VFT T TVFT
where AVFT and BVFT are constants and TVFT is the temperature where the viscosity
should diverge in case the melt is cooled below Tg . Equation (5.23) requires three parameters and these parameters can be calculated if viscosity at the three characteristics temperatures are known. The temperature and viscosity dependence of
VFT parameters can be written as follows:
T (T T ) KT (T T ) T m s g g s m (5.24) VFT K(Ts Tm ) Tg Ts
81 C S (Tg ) logD T E (T ) U K s (5.25) C (T ) S logD s T D T E (Tm ) U
C (T ) S (T T )(T T ) B logD g T g VFT m VFT (5.26) VFT D T E (Tm ) U (Tm Tg )
B A log&' (T ) VFT (5.27) VFT g (Tg TVFT )
where (Tg ) , (Ts ) and (Tm ) are viscosity at glass transition temperature (Tg ) ,
dilatometric softening temperature (Ts ) and melting temperature (Tm ) , respectively.
An approach was adopted to determine the three unknown parameters based on the dilatometric measurement. For silicate based glass, it is well known that there is a relationship between the glass transition temperature and glass melting temperature
(i.e.Tm 1.5Tg ) and several experimental results were validated using this rule [148].
This relationship can be used to determine the melting temperature of glass. Tg and Ts can be obtained from dilatometer curve as per ASTM standard [149] as shown in Fig.
5.28, and Tm can be estimated using Tm 1.5Tg . The viscosity values for any glass material at these three characteristic temperatures are fixed and independent of
12.08 10.39 3 materials: the reported values are10 Pa.s , 10 Pa.s and 10 Pa.s at Tg , Ts and Tm ,
0 0 0 respectively [67]. The values are Tg 522 C ,Ts 560 C and Tm 920 C .
Then, the final VFT equation based on the dilatometric analysis becomes:
5679.49 log ( ) 4.72 (5.28) glass T 456.97
82 6000 Dilatometric Expansion curve of Glass-G @ heating rate of 3 K/min 5000
T =5220C 4000 g T =5600C s 3000
2000 Thermal Expansion (ppm) Expansion Thermal 1000
T T 0 g s 225 300 375 450 525 600 675 Temperature (0C)
Figure 5.28. Determination of Tg and Ts from dilatometer curve.
5.4.2. Viscosity from Moynihan Model Based on DSC Data
Moynihan [150] proposed a method for estimating the entire viscosity versus temperature curve from a single differential scanning calorimeter (DSC) measurement.
Based on the assumption that the atomic and molecular arrangements taking place during structural relaxation are similar to those occurring during viscous flow in response of a shear stress, they derived a relation between the width of the transition region and the temperature dependent shear viscosity. They have shown that the onset and completion temperatures for the glass transition in the DSC curve occur at approximately the same viscosity for every glass for a given heating rate (i.e. the
12 viscosity at the glass transition is fixed: (Tg ) 10 Pa.s ). Following assumption suggested by Angell [151], that the viscosity at very high temperatures approaches the same value for all liquids, i.e. log 0 (Pa.s) 5 , where 0 is the viscosity at high temperature ( T ), they derived the following expression for a heating rate of 10
K/min [152]:
83 14.2 log glass (T) 5 (5.29) F I Y V G L L W G L L W ' 1 G(0.147)(T Tg )J Z 1W G L C 1 1 SL W (T ' )2 D T G L g D ' TL W H K ETg Tg U[ X
' where Tg is the glass transition temperature and Tg is the end point of the glass transition determined from DSC curve. The output of DSC signal is proportional to the heat capacity of the material and the maximum or overshoots in the DSC curve is a kinetic phenomena. To determine the lower end of the glass transition region for DSC heating curve, the extrapolated temperature of onset of rapid rise of heat capacity
versus temperature curve gives the value of Tg , and the upper end of the transition
' region, the extrapolated temperature of completion of the overshoots gives Tg . Figure 5.
29 shows a DSC profile of glass obtained at a heating rate of 10 K/min, from which the
' 0 0 values of Tg and Tg were determined as 530 C and 590 C, respectively.
0.54 DSC profile of Glass-G at heating rate of 10 K/min 0.51 1st order derivative of DSC curve Heat capacity (arb unit.) 0.48
V/mg) 0 Exo T =530 C g 0.45 T '=5900C g
0.42 Heat Flow ( Heat
0.39 T T ' g g
400 450 500 5500 600 650 700 Temperature ( C) ' Figure 5.29. DSC profile of glass and determination of Tg and Tg .
84 Based on these analyses, the temperature dependent viscosities were obtained using model equations 5.28 and 5.29 as shown in Fig. 5.30. At SOFC operating
0 4.5 temperature, the viscosity values for glass are (TSOFC 800 C) 10 Pa.s and
0 4.6 (TSOFC 800 C) 10 Pa.s , based on the VFT and Moynihan models, respectively. The
0 4.53 values measured using sessile drop experiment is (TSOFC 800 C) 10 Pa.s , which is also in close agreement with the values obtained by above approaches.
16 Viscosity Temperature curve of sealing glass 14 VFT model 12 Moynihan Model
10 at 8000C = 104.5 Pa.S VFT at 8000C = 104.6 Pa.S 8 Moynihan
(Pa.s)
6 log 4
2
0
400 600 800 1000 1200 1400 0 Temperature ( C) Figure 5.30. Viscosity-temperature curves for glass based on VFT and Moynihan models.
DSC run was performed on glass composite with 10 wt% YSZ filler and temperature dependent viscosity was obtained using Moynihan model as shown in Fig.
5.31. The viscosity value for glass composite (with 10 wt.% YSZ) at 8000C based on
Moynihan model equation is 105.52 Pa.s , in good agreement with the value
105.18 Pa.s obtained from the sessile drop model. For glass composite, it can be
85 seen from Fig. 5.31 that the addition of 10 wt.% YSZ filler into the glass increases
0 4.6 0 5.52 viscosity from (TSOFC 800 C) 10 Pa.s to . (TSOFC 800 C) 10 Pa.s . This result shows the addition of filler can improve the viscosity at SOFC operating temperature, which is desirable for offering creep resistance. For comparison, the viscosity values for glass and glass composite with 10 wt.% YSZ filler in the temperature range of (750-
900)0C obtained using sessile drop, VFT and Moynihan models are presented in Fig.
5.31(b). A good agreement is observed for the viscosity values derived from different approaches.
16 8 (a) Viscosity vs Temperature plot (b) Viscosity of glass and glass composite 14 based on Moynihan model in SOFC operating range Glass and Glass composite 7 Calculated from different models 12 VFT-Glass 10 6 Moynihan-Glass Sessile-Glass , Pa.s) 8 , Pa.s)
5
6 log ( log log ( log 4 4 G + 10 wt.% YSZ at 8000C
2 3 Glass Moynihan-G+10 wt.% YSZ-composite 0 Sessile-G+10 wt.% YSZ-composite 2 400 600 800 1000 1200 1400 700 750 800 850 900 950 0 Temperature (0C) Temperature ( C) Figure 5.31. Glass and glass composites viscosity (a) based on Moynihan model and (b) comparison of viscosity obtained from different approaches.
5.4.3. Viscosity from Creep Data
Creep behavior of glass is important for assessing flow behavior of the sealing glass subjected to stack loads. The creep behavior of the glass and glass composite sample were measured near the glass softening temperatures in a 3-point flexure mode at an applied load of 10 N. The sample dimensions of creep test samples and other parameters are presented in Table 5.3.
86 Table 5.3. Sample dimensions and creep test conditions for glass/G+10YSZ. Sample Length Width Thickness Stress Test temperature Strain rate (S-1) ID. (mm) (mm) (mm) (MPa) (0C) #1 40.183 5.664 2.489 12.822 580 1.079×10-6 #2 40.183 6.604 2.261 13.334 600 3.007×10-6 #3 40.183 6.833 2.286 12.603 615 5.352×10-6 #4 35.636 4.624 2.674 13.607 610 7.170×10-7 #5 35.636 4.624 2.674 13.607 640 3.600×10-6 #6 35.688 4.572 2.674 13.763 650 5.331×10-6
Using the deflection rate data, we can calculate the viscosity for the samples at test temperatures. Figure 5.32 shows the creep data over a range of temperatures and these data were used to calculate the viscosity of samples. By using the steady state
condition ( (T) / ), the viscosity at test temperatures were calculated for glass and glass composite as shown in Fig. 5.33. In case of glass composite, the viscosity is higher than for a pure glass and this increase is as a result of the YSZ crystalline filler phase that controls the flow behavior of glass composite.
0.09 0.09 (a) Strain versus Time plot of Glass (b) Strain versus Time plot of G+10YSZ 0.08 Load = 10 N in 3 point flexure mode 0.08 Load = 10 N in 3 point flexure mode
0.07 0 0.07 T = 580 C T = 6100 C 0 0.06 T = 600 C 0.06 T = 6400 C 0 T = 615 C 0 0.05 0.05 T = 650 C
#6
Strain #3 #5 0.04 Strain 0.04
0.03 #2 0.03
0.02 #1 0.02 0.01 0.01 #4 0.00 0.00 0 1000 2000 3000 4000 5000 6000 7000 0 1000 2000 3000 4000 5000 6000 7000 Time (sec) Time (sec) Figure 5.32.Strain versus time plot based on creep data for (a) glass and (b) glass composite.
87 17 Viscosity Temperature plot from creep data 16 Glass G+10 wt.% YSZ 15 Linear fit-Glass Linear Fit-G-10YSZ 14
13 (Pa.s) 12 log log 11
10 Glass 3 log (Pa.s) = - 5.459 + 15.353 (10 /T) 9 log G+10YSZ (Pa.s) = - 4.859 + 16.171 (103/T) 8 1.06 1.08 1.10 1.12 1.14 1.16 1.18 1.20 103/T (K-1) Figure 5.33. Viscosity versus temperature plot based on the creep data.
The viscosity data can be fitted to the following equations:
C 103 S Glass: log (Pa.s) 5.459 15.353D T (5.30) E T (K) U
C 103 S Glass + 10 wt. % YSZ: log (Pa.s) 4.859 16.171D T (5.31) E T (K) U
The values of viscosity determined from creep data over a range of temperatures gave an activation energy value of 294 KJ/mol and 310 KJ/mol for glass and glass composite with 10 wt. % YSZ filler, respectively.
5.4.4. Viscosity Comparisons Based on Experimental and Model Equations
Figure 5.34 shows the viscosity-temperature plots of glass and glass composite with 10 wt. % YSZ filler based on creep and sessile drop experiment. All the viscosity values measured and calculated from different model equations are also presented.
88 27 27 (a) Viscosity comparision (b) Viscosity experimental data 24 Experimental and Model equations 24 Modified-VFT model fit Experimental data-Glass 21 21 4330 Experimental data-G+10YSZ log Glass (Pa.s) 4.72 a T 615 18 t 18 a Modified VFT-Eq (5.32) 4860 d G 10YSZ log (Pa.s) 4.72 15 p Modified VFT-Eq (5.33) 15 T 615 e e r (Pa.s) (Pa.s)
C
12 12 Glass-Exp log log 9 log 9 Creep data G+10YSZ-Exp VFT-Dil 6 Moynihan model 6 Eq (5.28) Eq (5.29): G-10YSZ 3 3 Moynihan-DSC 0 Eq (5.29) Sessile drop data 0 Sessile drop data
400 600 800 1000 1200 1400 400 600 800 1000 1200 1400 Temperature (0C) Temperature (0C)
Figure 5.34. (a) Viscosity comparison based on experimental and model equations and (b) VFT fit of experimental data.
Comparison of viscosity values determined from sessile drop model agrees well with VFT, Moynihan model equations and provide a way to predict viscosity at
SOFC operating temperature range. However, the viscosity value calculated based on creep data does not agree with VFT model equation (5.28) and Moynihan model equation (5.29).These results suggests that the VFT model equation constructed based on dilatometric analysis and Moynihan model equation using DSC data provide better fit to higher temperature regime and fails to predict the viscosity values in lower temperature regime. The complete experimental data set obtained from the creep measurement and sessile drop can be best fitted to a modified VFT model equation and are shown in Fig. 5.34 (b).
4330 Glass: log Glass (Pa.s) 4.72 (5.32) T 615
4860 Glass + 10 wt.% YSZ: log G 10YSZ (Pa.s) 4.72 (5.33) T 615
89
where AVFT ( 4.72) and TVFT ( 615 K) are fixed and parameter BVFT represents the
activation energy term and increases with the YSZ filler. The values of BVFT parameters are 4330 K and 4860 K for glass and glass composite with 10 wt. % YSZ filler, respectively. This modified VFT equation provides a way to determine viscosity in wide temperature range.
5.4.5. Activation Energy for Viscous Flow
act The activation energy for viscous flow ( Eviscous ) is defined at constant pressure by [153]:
C ln S E act RD T (5.34) viscous E 1/T U P
Thus, the temperature dependent of activation energy for viscous flow based on VFT and Moynihan equations (5.28) and (5.29) are given by:
T 2 E VFT 2.303RB (5.35) act VFT 2 (T TVFT )
I T Y 2 g L TTg T (1 ) L Moynihan ' ' T Eact 4.807RTgTg (Tg Tg )J Z (5.36) L Tg L [(0.147T T )(1 ) T ' (T ' T )]2 KL g T g g g [L
Based on equations (5.35) and (5.36), temperature dependence of activation energy for viscous flow were obtained as shown in Fig. 5.35. In case of glass composite with 10 wt.% YSZ filler, an increase in activation energy for viscous flow is due to the presence of filler, which increases viscosity. At SOFC operating temperature the activation energy for viscous flow of glass is 273 KJ/mol and of glass composite (10 wt.
% YSZ filler) is 318 KJ/mol based on the Moynihan model.
90
800 800 (a) Temperature dependent activation energy (b) Activation energy for viscous flow for viscous flow 700 in SOFC operating range 700 VFT-Glass Moynihan-Glass Eact at 8000C (Glass) = 329 KJ/mol 600 VFT Moynihan-G+10YSZ act 0 E at 800 C (Glass) = 273 KJ/mol 600 Moynihan Eact at 8000C (G+10YSZ) = 318 KJ/mol 500 Moynihan (KJ/mol) 500 (KJ/mol) 400
viscous viscous act 400 act 300 E E
300 200 VFT-Glass Moynihan-Glass 100 200 Moynihan-G+10YSZ 0 400 600 800 1000 1200 1400 650 700 750 800 850 900 950 1000 0 0 Temperature ( C) Temperature ( C) Figure 5.35. Activation energy for viscous flow for glass and glass composite based on the VFT and Moynihan approaches (a) temperature dependence and (b) SOFC operating range.
Following the Kissinger method [152], the activation energy for viscous flow near
glass softening temperature (Ts ) can be determined by running DSC at different heating
rates and using the following relation between heating rate and the glass softening
temperature [152] :
C T 2 S C RA7 S E soft 1 lnD s T lnD T act (5.37) D T D soft T E U E Eact U R Ts
where is the heating rate, A is a constant, v is frequency factor, R is the gas
soft constant, Eact is activation energy for viscous flow in the softening range and Ts is the
& 2 ' glass softening temperature. A plot of ln Ts / versus 1/Ts yields a straight line with a
soft 7 soft slope of ( Eact / R ) and an intercept of ln (RA / Eact ) . This method can be used to
determine the activation energy for viscous flow near the glass softening temperature.
Figure 5.36 shows the activation energy based on Kissinger method and for comparison
91 the value of activation energy obtained based on Moynihan model near softening range
and at SOFC operating temperature are also plotted.
12.4 600 (a) Determination of activation energy in the (b) Activation energy for viscous flow of Glass and Glass composite 12.0 softening range based on Kissinger approach 550 Kissinger Moynihan 11.6 Glass G+10YSZ 500 11.2 )
/ 2 (KJ/mol) s 10.8 450 soft act ln(T E 10.4 400
10.0 E soft (Glass) = 426 KJ/mol 350 act 9.6 E soft (G+10YSZ) = 482 KJ/mol act 1.16 1.18 1.20 1.22 1.24 1.26 1.28 1.30 300 Glass G+10YSZ 103/T (K-1) s Figure 5.36. Activation energy in the softening range (a) Arrhenius plots of glass and glass composite and (b) effect of filler and model comparison.
The values of activation energy calculated in the softening range based on
Kissinger approaches are 426 KJ/mol and 482 KJ/mol for glass and glass composite
with 10 wt.% YSZ filler. These values are approximately close to the values of 400
KJ/mol and 437 KJ/mol for glass and glass composite (10 wt.% YSZ filler) calculated
from Moynihan model. This result provides evidence that the activation energy
calculated from approach using equation (5.37) is valid for glass and glass composite.
Small deviation is understood as the Moynihan equation depends both on start and end
of glass transition region and the presence of particle fillers broadens the transition
region. From the plot, it is evident that the activation energy at Ts for glass composite
increased compare to pure glass and this is expected as the particulate filler acts as a
barrier to the molecular transitions of glassy unit.
92 5.5. Microstructural and Interfacial Analysis
The coefficient of thermal expansion of selected glass G (~ 10.64 ppm/0C) is close to that of YSZ electrolyte (~ 9.8 ppm/0C) and at the same time not far off from that of the interconnect materials (Crofer22APU: ~ 11.8 ppm/0C, stainless steel 441: 12.5 ppm/0C). Therefore, it is expected that this glass would result in good adhesion between glass-electrolyte as well as the glass-interconnects. It was indeed found that the glass adheres well with YSZ electrolyte and the metallic interconnects under the optimized sealing conditions of the present work. The interfacial region between sealing glass and the cell components were evaluated by annealing samples on cell components such as
YSZ electrolytes and different types of metallic interconnects. The annealing was done in air at 8000 C for 1000 h for interconnects and 3500 h for YSZ electrolyte and the cross sections of annealed sample were analyzed by SEM-EDAX analysis to study the interfacial behavior. A defect free structure without any major microcracks at the interface was observed for all seal.
5.5.1. Glass-YSZ Electrolyte Interface
A characteristic microstructure of the interface formed between 8YSZ sintered plate and glass after heat treatment at 8000 C for 3500 h in air is shown in Fig. 5.37.
Figure 5.37. Glass-YSZ interface annealed at 8000 C in air for 3500 hrs.
93 The good matching of the CTEs of YSZ and glass resulted in a continuous interface with no cracks or gaps. There is no evidence of any formation of a reaction zone at the interface. The line scan profiles also suggest the long term stability of the glass-YSZ interface and no diffusion of any element from glass into YSZ or vice versa occurred. This means that the sealing glass is stable with YSZ. The small black dots in
YSZ region is the intrinsic porosity of YSZ membrane itself.
5.5.2. Glass- Crofer22 Interconnect Interface
Crofer metallic interconnect plate was first preoxidized at 8500 C in air for 2 hrs to form a chromia scale layer and then preoxidized sample was used for compatibility test with glass seal. Figure 5.38 shows the SEM images of glass and crofer interface annealed in air for 1000 hrs at 8000 C. A preoxidized layer consisting of chromia sub- layer can be seen from the micrograph.
Figure 5.38. Glass-Crofer interface annealed at 8000 C in air for 1000 hrs.
The SEM elemental line profiles indicate that during exposure to air after 1000 hrs at
8000 C, there was no diffusion of iron into chromia layer or no diffusion of any species into the glass. A good bonding between glass and Crofer interface was also observed without any crack at the interface. From the literature, other sealing glasses have shown
94 detrimental effect with chromia forming metallic interconnects and new phase such as
BaCrO4, was observed, which eventually lead to failure of seal due to very high thermal expansion of BaCrO4 phase [85, 86]. In our case, for the considered sealing glass no such phase formation or any adverse reaction between interconnect and sealing glass was observed.
5.5.3. Glass-441 SS Interface
Figure 5.39 shows the cross sectioned of glass-441 stainless steel interface annealed in air at 8000 C for 1000 hrs. The metal sample was preoxidized before testing glass seal on interconnect material.
Figure 5.39. Glass-441 stainless steel interface annealed at 8000 C in air for 1000 hrs. The glass-441 SS interface has a region enriched in Cr and O indicating formation of the chromia scale on 441 SS as expected from chromia forming metallic alloy. If there is any phase formation due to the reaction between the sealing glass and the metallic interconnect, it would be visible at the interface and it might suggest the existence of adverse reactions [154]. However, no phase formation was observed as evidenced by line scan and interface micrograph. There is no evidence of Fe or other elemental diffusion from the 441 SS into the glass. The glass shows chemical and
95 mechanical stability against chromium dissolution from the oxidized 441 SS and good adhesion/bonding was observed between glass and metal.
5.5.4. Glass- Spinel ( Mn1.5Co1.5O4 ) Coated 441 SS Interface
SEM images of the interface and elemental line scan profiles of glass in contact
0 with Mn1.5Co1.5O4 - coated 441 SS annealed in air at 800 C for 1000 h is presented in
Fig. 5.40. After such a prolong heat treatment in contact with glass the outer coating
layer of Mn1.5Co1.5O4 is detached from the bulk coating. However, the inner part of coating is still visible in image and provides protective coating for any Cr diffusion towards the glass.
Figure 5.40. Glass-spinel coated 441 SS interface annealed at 8000 C in air for 1000 hrs.
The glass-metal interface has three regions. A chromia layer next to the 441 SS
followed by a Mn1.5Co1.5O4 layer of inner coating. After this there is a thicker region in
contact with the glass. This region is formed because of the dissolution of Mn1.5Co1.5O4 spinel into the glass and has a small amount of Mn and Co along with Si from the glass.
Higher magnification of this region shows particles of (Mn, Co) spinel dispersed into the
96 glass with larger particles near the Mn1.5Co1.5O4 spinel coating (left) and smaller particles
near the glass (right) indicating the different degree of dissolution of Mn1.5Co1.5O4 spinel
into the glass. The Mn1.5Co1.5O4 -spinel coating is still there on the 441 SS but may get dissolved when samples are annealed for longer times. The crack between chromia layer and inner part of spinel coating suggests the flaws initiated during the manufacturing of coating process or due to the stress induced detachment of coating,
which lead to the crack formation. These results indicate that Mn1.5Co1.5O4 spinel coating may not be stable in contact with this glass but appears to be adherent. It can be
possible that as-deposited Mn1.5Co1.5O4 coating is not fully dense; oxygen can diffuse readily through the coating during initial stages of annealing to form a chromia scale
beneath of Mn1.5Co1.5O4 oxide coating. In order to confirm this, spinel coated 441 SS sample without glass was heat treated for 1000 hrs in air at 8000 C and cross sectioned sample was analyzed using SEM-line scan. Figure 5.41 shows the cross sectioned
SEM images of spinel coated 441 SS annealed for 1000 hrs in air at 8000 C.
Figure 5.41. Spinel coated 441 SS cross-section (a) as received (b) after 1000 hrs annealing.
97 It is evident from the micrograph that the coating is adherent to the 441SS and average coating thickness is 22 μm. The coating has three regions, one region close to metal part of thickness ~ 8 μm, then a porous region (thickness ~ 6 μm) and again coating region of thickness 8 μm. The SEM line scan performed at the interface of
441SS and the coating shows that there is no diffusion of Cr species from the metal into the coating and Fe is also found at a uniform concentration throughout the coating. For as received sample, the thickness of Cr2O3 oxide scale near 441 SS zone is ~ 1μm, however, after 1000 hrs heat treatment in air at 8000 C, the coating is still existing but the chromia scale thickness is now ~ 2.2μm. The crack between chromia layer and inner part of spinel coated layer suggests that the coating is the weakest link mechanically in metal and coating. This can be improved by the optimum coating thickness (4-6) μm and by the elimination of the porous region in the bulk of the coating.
5.5.5. Glass-Aluminized 441 SS Interface
Figure 5.42 shows the SEM-line scan profile of glass-aluminized 441 SS interface after 1000 hrs heat treatment at 8000 C in air.
Figure 5.42. Glass-aluminized 441 SS interface annealed at 8000 C in air for 1000 hrs.
98 It can be seen that chromia scale formation is promoted and aluminized coating is not effective but glass adheres well with metal. Higher magnification image shows the concentration of Al is very low and no aluminized coating is visible on 441 SS. Some enrichment of Si is observed near glass metal interface but no diffusion of any element into the glass was observed. These results show good stability of the chromia scale against glass in terms of the chemical and mechanical properties.
99 6. Conclusions
In this research work, fundamental studies of thermo-mechanical behaviors of glass and glass composite for seals were explored. An innovative approach for determining material properties useful for sealing such as surface tension and viscosity
(important in high temperature sealing technology) were investigated and experimental results were validated with several existing models. The following conclusions can be derived from this research work.
1. Glass and glass composites containing Al2O3, MgO, and YSZ fillers were
synthesized for applications as seals in a SOFC. The thermal expansion and
phase stability of glass and glass composites were determined. The developed
glass and glass composites containing YSZ fillers showed thermal and phase
stability in dual oxidizing and reducing environments at 8000C. Glass composites
with YSZ filler can be used to control the CTE and viscosity of the glass
composite. Due to inert nature of YSZ with glass matrix, composite can be used
as self healing seal without degrading the other properties.
2. The wetting behavior of glass/glass-YSZ composites were evaluated using
sessile drop method, and surface tension, viscosity, wetting activation energy
and work of spreading were calculated. The measured surface tension at 8000 C
was 0.281 N/m and 0.296 N/m for the glass and glass-10 wt.% YSZ composite,
respectively. The wetting activation energies for glass was (248-318) KJ/mol,
which were close to the activation energy for viscous flow in the glass (~ 319
KJ/mol). Similarly the wetting activation energies for glass-10 wt.% YSZ
composite of (370-397) KJ/mol were close to the activation energy for viscous
100 flow in the glass composite of ~ 371 KJ/mol. The contact angles for glass on YSZ
and on 441 SS metal substrates were 630 and 570, respectively. In contrast, for
glass composite (10 wt.% YSZ) contact angles were 740 and 670 on YSZ and
441 SS metal substrate, respectively. Addition of YSZ filler (10 wt. %) increased
the contact angle and showed that the spreading kinetics can be controlled by
using appropriate unreactive fillers, without degrading the glass matrix property.
Higher values of work of spreading represented stronger bond between glass
and cell components. The calculated values of thermodynamic work of spreading
for glass and glass-10 wt.% YSZ composite seals at SOFC operating
temperature (8000 C ) were 0.39 J / m 2 , 0.34 J / m 2 on YSZ substrate and
0.42 J / m 2 , 0.38 J / m 2 on 441 SS metallic interconnect, respectively. These
results demonstrated that by carefully selecting the additive and the glass matrix,
the sealing behavior and strength of the seal can be controlled.
3. Glass and glass-10 wt.% YSZ composite viscosity was determined using
different approaches. The viscosity values obtained from sessile drop model
0 4.53 0 5.18 were (TSOFC 800 C) 10 Pa.s and (TSOFC 800 C) 10 Pa.s for glass and
glass composite (10 wt. % YSZ filler), respectively. The value for glass composite
0 5.52 obtained based on the Moynihan equation was (TSOFC 800 C) 10 Pa.s ,
which was in good agreement with the data obtained from sessile drop model.
The values of viscosity of glass obtained from VFT and Moynihan models at
0 0 4.5 0 4.6 800 C, were (TSOFC 800 C) 10 Pa.s and (TSOFC 800 C) 10 Pa.s ,
respectively. These values are also in close agreement with the values obtained
from the sessile drop model. The incorporation of filler increased the viscosity,
101 which showed that the addition of filler can be used to control the viscosity at the
SOFC operating temperature, and mitigate the creep phenomena, which is
undesirable for long term application. The viscosity-temperature dependence
over the entire range of temperatures can be best described by a modified-VFT
4330 4860 log Glass (Pa.s) 4.72 and log G 10YSZ (Pa.s) 4.72 for the T 615 T 615
glass and glass composite (10 wt.% YSZ filler), respectively.
4. The chemical compatibility of a self healing glass with cell components after long
term annealing (1000 h) at 8000 C showed good bonding and crack free interface
between the glass and cell components (YSZ electrolyte and chromia based
metallic interconnects). In case of metallic material (Crofer22, 441 SS) without
any oxide coating good bonding and no new phase formation was observed at
the interface. But for oxide coated (spinel coated) 441 SS interconnects,
dissolution of Mn and Co species from coating into the glass was observed,
which can be improved by optimization of the oxide coating thickness and
manufacturing process.
5. Electrical resistivity of the sealing glass at 8000 C was 105 cm , which remained
fairly constant over time. However, in case of sandwich structure with different
types of 441 SS metallic interconnects (aluminized and spinel coated) the
measured resistivity was in the range (104 105 ) cm . This reduction was due to
dissolution of Co, Mn species from coating into the glass. Overall, the considered
sealing glass provides good chemical and electrical compatibility with cell
components under SOFC operating condition.
102 6. The weight loss test for sealing glass in air and in wet reducing environment at
8000 C was estimated to be ~ 1% over the period of 40,000 hrs (~ 5 yrs), so
vaporization is not an issue for the considered glass system.
All these results confirmed the thermal, chemical and volatility stability of selected self healing glass for SOFC application. It is evident from the results so far obtained that the sealing glass and glass composites with YSZ filler were stable in SOFC environment at 8000 C and can be used as a promising candidate for SOFC sealing.
103 7. Future Directions
The sessile drop approach used to determine the sealing parameters and to study the wetting kinetics of glass/glass composite can be further pursued to study the different types of glass systems. Glass and glass composites developed in this research work can be used to determine the viscosity using creep data and a master equation relating viscosity and filler contents can be derived. Based on the fundamental studies done in this research work, some of the suggestions for further study in this field are:
• Glass composites with YSZ as filler can be further used to make seal and their
hermeticity can be tested using leak test.
• Compatibility of glass composite with interconnect materials can be tested by
annealing the composite seal in dual oxidizing and reducing environment.
• Self healing behavior of developed glass and glass composites can be performed
by in situ SEM analysis and effect of filler volume fraction on healing time and
healing condition can be studied.
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117 9. List of Publications
1. S. K. Singh and R. N. Singh, "Viscous Glass Composite Seal for Solid Oxide Fuel Cell”, Materials and Energy Conference: The Ohio innovation summit, UCEAO, Columbus, OH, E 32, April 20 (2010).
2. S. K. Singh and R. N. Singh, "Viscous Glass Composite Seal for Solid Oxide Fuel Cell”, University of Cincinnati, Poster Forum, p 34, March 5 (2010).
3. S. K. Singh and R. N. Singh, " Compatibility of a Self Healing Glass with Metallic Interconnects”, University Clean Energy Alliance of Ohio (UCEAO), Putting The Pieces Together: The New Energy Paradigm in Research, Education, Business and Public Policy, Coulombs, OH, April 8-9 (2009).
4. S. K. Singh and R. N. Singh, "Solid Oxide Fuel Cell and Sealing Issues", 3rd Annual Graduate Student Interdisciplinary Conference, University of Kentucky, Lexington, KY, p 205-7, April 3 (2009).
5. S. K. Singh and R. N. Singh, "Compatibility of a Self Healing Glass Seal with SOFC Components”, University of Cincinnati, Poster Forum, p 112, March 7, (2009).
6. S. K. Singh and R. N. Singh, "Hermetic Glass Seals for Solid Oxide Fuel Cell", 2nd Annual Meeting and Workshop, University Clean Energy Alliance of Ohio (UCEAO), Columbus, OH, April 24 (2008).
7. S. K. Singh and R. N. Singh, "Hermetic Glass Seals for Solid Oxide Fuel Cell", University of Cincinnati, Poster Forum, p 121, March 6 (2008).
8. S. K. Singh and J. Kumar, "On the Structural, Magnetic and Oxygen Desorption Response of BaxSr1 xFe0.8Co0.2O3 (x = 0, 0.5 and 1) Oxide" Journal of Alloys and Compounds, 481 (1-2), p 455-461 (2009).
9. S. K. Singh and J. Kumar, "Ac Conductivity, Dielectric Losses, Permittivity Behavior of BaxSr1 xFe0.8Co0.2O3 (x = 0, 0.5 and 1) Ceramics" Journal of Materials Science, 42(6), p 2105-2111 (2007).
10. S. K. Singh and J. Kumar, "Magnetic and Dielectric Behaviors of Ba0.2Sr0.8Co0.8Fe0.2O3- Oxide” Journal of Physics and Chemistry of Solids, 67 (8), p 1687-1691 (2006).
118 10. Appendix
10.1. Stability against Weight Loss of Glass in Dual Oxidizing and Wet Reducing Environments
The weight loss test is very important due to the concern of vaporization of glass constituents in long-term application. The samples were tested for extended times at
SOFC operating temperature (~8000C) in dual oxidizing and reducing environments. For the wet reducing tests, Argon + 4 wt. % H2 was bubbled through a 32°C water bath to create a 6% simulated steam atmosphere. The weighed samples were heated in a temperature controlled tabular electric furnace and tested up to 5000 hours in oxidizing and reducing environments. During the experiments, the test specimens were cooled to room temperature at intervals of 500 hrs and then weighed using a precision weighing balance (balance sensitivity = 0.001 mg). The experimental data were used to calculate
the weight losses of the samples ( w w0 wt ) per unit area of the exposed surface
A ( w / A , mg / cm2 ) and plotted as a function of time. Figure 10.1 shows the weight loss per unit area with annealing time for glass tested in oxidizing and reducing environments.
Brow et al. [155] reported that their sealing glass have shown degradation in terms of weight loss due to volatile species and different fits to data such as parabolic rate and linear rate were used to determine the weight loss up to 40,000 hrs, which is typically the life of a SOFC. In the present case of selected glass, the weight loss is not much over the period of tested time. The maximum weight loss after 5000 hrs testing is
3.0 mg cm2 and 3.5mg cm 2 in air and in wet reducing environment, respectively. The experimental data are extrapolated for the time period of 40,000 hrs (~5 years). The
119 extrapolated weight loss is ~ 1 % in both oxidizing and reducing environments and this weight loss is negligible compared to the other glasses, which has been tested and showed a maximum weight loss of 20-25 % [155], so the further investigation for the weight loss for our glass is not required.
10 0.6168 0 (a) Wt. loss per unit exposed area at 8000 C (b) Wt. loss of Glass at 800 C
wt. loss in air 0.6164 wt. loss in air 8 wt. loss in wet reducing environment wt. loss in wet reducing environment ) 2 0.6160
6 0.6156
0.6152 4 Wt. loss(mg/cm Wt. Wt. of glass (gm) 0.6148 2 0.6144
0 0.6140 0 1000 2000 3000 4000 5000 6000 0 1000 2000 3000 4000 5000 6000 Time (hrs) Time (hrs) Figure 10.1. Weight loss of glass as a function of annealing time tested at 8000C showing (a) wt. loss per unit area and (b) weight of glass with time.
10.2. Stability of Electrical Resistivity of Glass/Glass-Metallic Interconnects
In order to prevent any short circuiting during functioning SOFC, the sealing materials should be highly resistive. The electrical resistivity of the glass under an applied dc electric field was measured between 25-8000C. These measurements were done at an applied voltage of 1 volt, which is typically the cell voltage in a SOFC.
Figure 10.2(a) shows measured resistivity between (300-8000) C. The decrease in resistivity value with increasing temperature could be due to ionic conductivity associated with the migration of alkali (R+) and alkaline (R2+) ions. However, the measured resistivity value at 8000 C is high enough ( 6105 cm at 8000 C) for the application of SOFC and within the range of SOFC sealing criteria [72].
120 9 35 10 Glass 800 700 600 500 400 300 200 8 Temperature (0C) 10 Glass between Aluminized 441 SS 30 Glass between Spinel coated 441 SS 107 25 106
8 5 20 0 = 4.3X10 Ohm.cm 500 C 10 7 0 = 7.3X10 Ohm.cm
600 C 4 6
(Ohm.cm) 0 = 7.6X10 Ohm.cm 10 700 C 15 5 0 = 6.1X10 Ohm.cm 3 ln ln 800 C 10 10 Resistivity (Ohm.cm) Resistivity 102 5 101 (a) Electrical resistivity of glass (b) Stability of electrical resistivity at 8000 C 100 0.8 1.0 1.2 1.4 1.6 1.8 2.0 0 150 300 450 600 750 900 1050 1200 103/T (K-1) Time (hours)
Figure 10.2. (a) Resistivity versus temperature plot of sealing glass (b) Long term resistivity stability of sealing glass at 8000 C in air.
Even though the sealing glass have relatively high resistivity as compared to all the cell components, but it has been observed that excessive interaction of sealing glass with metallic interconnects such as Crofer22APU, may lead to a rapid degradation of stack components due to the short circuiting phenomena [87,156]. This is because of the formation and growth of voluminous Fe-oxide (e.g Fe3O4, FeO ) near the boundary between the interconnect, glass and air. This results in the formation of a bridge with extremely small electrical resistance between the adjacent metallic plates and thus results in short circuiting. The electrical resistance of glass tested upto 1000 hrs at 8000
C and in contact with metallic interconnects are shown in Fig. 10.2(b). It can be seen from the plot that the resistivity values increase initially due to polarization effect and reach constant value after 50 hrs.
Now a days, in order to prevent the chromium vaporization, an oxide coating is applied on metallic interconnects [157], so keeping in mind this issue, two samples of
121 441 SS coated with two different oxide materials were used to evaluate the compatibility of this sealing glass with interconnects. The electrical resistance of glass was measured in sandwiched condition between two different types of metallic interconnects and the resistivity data measured upto 1000 hrs at 8000 C are also shown in Fig. 10.2(b). It can be seen that the resistivity values of glass sandwiched between (CoMn) spinel coated
441 SS lies in the range of (104 105 ).cm , where as the glass tested alone shows higher resistivity. As already discussed in section 5.7, in case of glass-spinel coated 441
SS interconnects sample, the long term exposure at 8000 C, leads to the dissolution of spinel (Co. Mn) oxide species into the glass. These transition metals are highly conductive at SOFC operating temperature and dissolution in glass may degrade the resistive nature of sealing glass. The decrease in resistivity is because of the dissolution of these transition metals (Co, Mn) into the glass. However, the electrical resistivity of the glass is still fairly high after 1000 hrs of test duration. Also, glass sandwiched between aluminized coated 441 SS has little higher resistivity compared to spinel- coated sample and this means that the aluminized coating is more effective and this glass has good compatibility with aluminized 441 SS interconnect. However, the resistivity is less than the pure glass. As observed from the interface analysis the aluminized coating was also not so effective and it may be possible that some chromium is getting dissolved into the glass and contributing to decreased resistivity of glass in contact with metallic interconnect.
122