Table Of ContentThermo-Mechanical Properties of Glass-Ceramic Solid Oxide Fuel
Cell Sealant Materials
Yilin Zhao
Thermo-Mechanical Properties of Glass-Ceramic Solid Oxide Fuel
Cell Sealant Materials
Von der Fakultät für Maschinenwesen der Rheinisch-Westfälischen Technischen Hochschule Aachen zur
Erlangung des akademischen Grades einer Doktorin der Ingenieurwissenschaften genehmigte Dissertation
vorgelegt von
Yilin Zhao
Berichter: Univ. Prof. Dr.-Ing. Tilmann Beck
Univ. Prof. Dr.-Ing. Lorenz Singheiser
Tag der mündlichen Prüfung: 02. September 2013
„Diese Dissertation ist auf den Internetseiten der Hochschulbibliothek online verfügbar.“
Kurzfassung
Die zuverlässige Funktion von Hochtemperaturbrennstoffzellen-Stapeln ist sehr stark vom
strukturellen Zusammenhalt des Dichtungsmaterials abhängig. Dabei sind Versagen und
Verformung Aspekte, die insbesondere für Glaskeramik-Dichtungsmaterialien bestimmt werden
müssen. Die vorliegende Studie konzentriert sich dabei auf die mit H-P (YSZ teilchenverstärkt)
und B abgekürzten Glaskeramiken. Härteeindruckstests wurden zur Bestimmung von
elastischem Modul, Härte und Risszähigkeit verwendet. Biege- und Drucktests wurden basierend
auf balkenförmigen und stirnseitig gefügten Proben bei Raumtemperatur und typischen
Betriebstemperaturen zur Herleitung von Bruchfestigkeits- und Viskositätswerten durchgeführt,
ähnlich wie in der Brennstoffzellenstapel-Anwendung für unterschiedliche Kristallisationsgrade
und Auslagerungstemperaturen. Die Resultate zeigten, dass ein Abfall der Festigkeit des
teilkristallisierten H-P Dichtungsmaterials bei betriebsrelevanten Temperaturen mit der viskosen
Verformung des Materials in Zusammenhang steht, wohingegen stabile Festigkeiten für
ausgelagertes H-P und nicht ausgelagertes B Material bei Betriebstemperaturen ein Ergebnis des
stärkeren Kristallisationszustands der Materialien sind. Die experimentell als Funktion der
Belastungsrate bestimmten Festigkeitsdaten der Dichtungsmaterialien H-P und B wurden zur
Bewertung der Empfindlichkeit hinsichtlich unterkritischen Risswachstums verwendet und
ergaben Versagenszeiten als Funktion der statischen Last anhand eines Festigkeits-
Wahrscheinlichkeits-Zeit Diagramms. Fraktographische Analysen basierend auf einer
Kombination on optischer-, konfokaler- und Rasterelektronenmikroskopie erlaubten ein
Verständnis der Versagensursprünge.
Abstract
The reliable operation of solid oxide fuel cell stacks depends strongly on the structural integrity
of the sealing materials. In this respect, failure and deformation are aspects which need to be
assessed in particular for glass ceramic sealant materials. The current study concentrates on the
glass ceramics abbreviated H-P (YSZ particle reinforcement) and B. Indentation tests have been
carried out to determine elastic modulus, hardness and fracture toughness. Bending tests and
compressive tests were carried out at room temperature and typical stack operation temperature
for bar and head-to-head jointed glass ceramic sealant specimens, similar as in a stack
application with different degree of crystallization and annealed temperature, from which
fracture stresses and viscosity values are derived. The results reveal a decrease of the strength for
the partially crystallized sealant H-P at operation relevant temperatures that can be associated
with the viscous deformation of the material, whereas a stable strength was obtained for the
annealed sealant H-P and non-annealed sealant B at operation relevant temperatures that can be
associated with the enhanced crystallization of the materials. The experimentally obtained
strength data of sealant H-P and B as a function of loading rate are used to assess the effect of
subcritical crack growth yielding a failure time under static loading via a strength-probability-
time plot that can be used for life time prediction. Fractographic analyses based on a combination
of optical, confocal and scanning electron microscopy gives insight into the failure origins.
Contents
1 INTRODUCTION ................................................................................................................... 1
2 LITERATURE REVIEW ........................................................................................................ 3
2.1 Solid Oxide Fuel Cells ..................................................................................................... 3
2.1.1 Anode ........................................................................................................................ 7
2.1.2 Cathode ..................................................................................................................... 8
2.1.3 Electrolyte ................................................................................................................. 8
2.1.4 Interconnector ........................................................................................................... 8
2.2 Sealant techniques ............................................................................................................ 9
2.3 Glass ceramic sealants .................................................................................................... 10
2.3.1 Glass material.......................................................................................................... 11
2.3.2 BaO-CaO-SiO ternary system................................................................................ 16
2
2.4 Mechanical properties .................................................................................................... 18
2.4.1 Young’s modulus .................................................................................................... 20
2.4.1.1 Indentation ....................................................................................................... 20
2.4.1.2 Bending test ..................................................................................................... 22
2.4.1.3 Impulse excitation technique ........................................................................... 22
2.4.2 Hardness .................................................................................................................. 24
2.4.3 Fracture toughness .................................................................................................. 25
2.4.3.1 Indentation ....................................................................................................... 25
2.4.3.2 Griffith criterion .............................................................................................. 28
2.4.4 Strength ................................................................................................................... 29
2.4.5 Creep ....................................................................................................................... 32
2.4.5.1 Diffusion creep ................................................................................................ 33
2.4.5.2 Viscous creep ................................................................................................... 33
2.4.6 Statistical lifetime evaluation .................................................................................. 36
2.4.6.1 Subcritical crack growth .................................................................................. 36
2.4.6.2 Strength-probability-time diagram .................................................................. 37
3 EXPERIMENTAL................................................................................................................. 39
3.1 Material .......................................................................................................................... 39
3.1.1 Processing of glass bars .......................................................................................... 39
3.1.2 Preparation of joined specimens ............................................................................. 41
3.1.3 Preparation of head-to-head joined specimens ....................................................... 41
3.1.4 Thermal treatment ................................................................................................... 43
3.2 Microstructural characterization .................................................................................... 44
3.2.1 Microscopy ............................................................................................................. 44
3.2.2 X-ray diffraction ..................................................................................................... 44
3.2.3 DTA ........................................................................................................................ 44
3.3 Indentation test ............................................................................................................... 45
3.3.1 Slow crack growth test ............................................................................................ 46
3.4 Mechanical characterization ........................................................................................... 47
3.4.1 Impulse excitation technique .................................................................................. 47
3.4.2 Bending test ............................................................................................................ 48
3.4.3 Compressive test ..................................................................................................... 49
4 RESULTS AND DISCUSSION ............................................................................................ 52
4.1 Microstructure ................................................................................................................ 52
4.2 X-ray analysis ................................................................................................................. 55
4.3 Thermal analysis ............................................................................................................ 57
4.4 Young´s modulus ........................................................................................................... 59
4.4.1 As joined material ................................................................................................... 59
4.4.2 Stacks with sealant H-P .......................................................................................... 61
4.4.3 Sintered bars............................................................................................................ 66
4.5 Hardness ......................................................................................................................... 69
4.6 Fracture toughness.......................................................................................................... 72
4.6.1 As-joined sealant ..................................................................................................... 72
4.6.2 Stacks ...................................................................................................................... 76
4.7 Geometry effect .............................................................................................................. 77
4.8 Crack deflection ............................................................................................................. 80
4.9 Fracture strength ............................................................................................................. 84
4.9.1 Sintered bars............................................................................................................ 84
4.9.2 Head-to-head joined sealant .................................................................................... 86
4.9.2.1 Room temperature ........................................................................................... 86
Description:annealed sealant H-P and non-annealed sealant B at operation relevant temperatures that can be associated with the .. interconnect is currently achieved by glass-ceramic sealants [2], which appear to fulfill the For penny shape flaws in a uniform stress field, Y is independent of c and given by:.
