Scope of the work
4
2.
Scope of the work
The work concentrates on thermo-mechanical aspects, in particular characterization of elastic,
fracture and creep behavior of sealants and anode substrates for application in planar SOFCs,
which should provide mechanical stability under typical operation conditions. Several
mechanical parameters have been determined and analyzed to get
insight into the mechanical
behavior and possible issues for the joining sealing, thermal cycling and long term reliability of
SOFC stacks and systems.
The work was carried out within the project “Production and Reliability Oriented SOFC Cell and
Stack Design (PROSOFC)”, supported financially by European Union in the 7
th
Framework
Programme. In order to fulfil the tasks within the project, that have been identified in the
consortium as critical for the reliability, different types of sealants
and anode substrates have
been investigated and compared within this work.
The mechanical stability of the anode substrate is obviously also crucial for reliable operation of
SOFCs. Contrary to the sealant materials, where the strength was investigated as parameter for
the comparison with SOFC stack stress simulations, permitting an assessment of failure
probabilities, the fracture toughness was selected as relevant parameter for the anode substrates,
since here the aim was to compare different materials, their advantages
and disadvantages and
associated mechanisms under different conditions and temperatures, without being affected by
the complexity of scatter of experimental data related to Weibull distribution of fracture stresses.
In particular, facture toughness and creep behavior as the major mechanical aspects were studied.
In this work, typical anode substrate materials (NiO-3YSZ supplied by the project partner TOFC
and NiO-8YSZ produced in Jülich) were analyzed in as-produced,
oxidized state, in reduced
state and some selected tests for the re-oxidized state, in terms of their fracture toughness at
room temperature and the currently typical stack operation temperature of 800°C.
Creep of
porous Ni-YSZ composite has been investigated under 4 % H
2
/Ar atmosphere at different
temperatures in the range of 800 to 900°C. Different loading configurations such as compression,
four-point bending and ring-on-ring bending have been used to assess the effect of compressive
and tensile stresses onto the materials creep. Ni-YSZ materials with different porosities and
Scope of the work
5
Ni/YSZ ratios were tested in order to investigate material composition and porosity effects. The
interpretation and analysis of the data was based on analytical and finite element analysis. The
results were systematically compared and discussed with the aid of microstructural investigations.
Since leakage of sealants might lead to a malfunction of the entire system, the structural integrity
of sealants is crucial for a reliable operation of SOFC stacks and systems. As application relevant
parameters, fracture properties and elevated temperature deformation
need to be assessed,
particularly for partially crystallized glass-ceramic sealants that might suffer from instability
issues at operation relevant temperatures due to viscoelastic deformation of the residual glass
phase. Specimens in stack typical as-joined and annealed state, representative for stack operation,
were studied. In this work, reinforced sealants, based on the glass matrix material “H” developed
in Jülich, which is composed of the BaO-CaO-SiO
2
ternary system,
with Ag particles
reinforcement and YSZ fiber reinforcement, were characterized. Bending tests were carried out
at room temperature and typical stack operation temperatures on specimen in a head-to-head
geometry in as-sintered and annealed state, yielding average fracture stresses. Torsion tests were
used to investigate shear strength. The results of mechanical analyses were supported by
advanced microstructural characterization to gain insight into annealing and filler reinforcement
effects. The results are further compared and discussed with respect to literature data and their
relevance for the application in SOFCs operated at different temperatures in the range of 600 to
800°C, considering typical current but also envisaged future lower stack operation temperatures.
Complementary fractographic analysis aided the interpretation of mechanical strength.
