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@PHDTHESIS{Stanislowski:51107,
author = {Stanislowski, Michael},
title = {{V}erdampfung von {W}erkstoffen beim {B}etrieb von
{H}ochtemperatur-{B}rennstoffzellen ({SOFC})},
volume = {54},
school = {RWTH Aachen},
type = {Dr. (FH)},
address = {Jülich},
publisher = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
reportid = {PreJuSER-51107},
isbn = {3-89336-438-2},
series = {Schriften des Forschungszentrums Jülich. Reihe
Energietechnik / Energy Technology},
pages = {IV, 154 S.},
year = {2006},
note = {Record converted from VDB: 12.11.2012; RWTH Aachen, Diss.,
2005},
abstract = {Due to their high efficiencies and versatility planar solid
oxide fuel cells (SOFCs) offer a lot of advantages compared
to other fuel cell systems and combustion engines. One of
the main problems concerning the development of state of the
art planar SOFCs are the occurrence of ageing effects in
long term application. To a great deal these effects are
caused by the release of volatile Cr-species from metallic
interconnects which leads to an inhibition of the
electrochemical processes at the cathode resulting in a
rapid degradation of the cell performance. A goal in further
development of SOFC-systems is the reduction of the
operation temperature of the cell from currently 800°C to
700°C and below. For this purpose alternative electrolyte
materials with higher oxygen ion conductivities have to be
developed. Doped lanthanum gallates have been identified as
promising materials. However for these materials a depletion
of Ga by vaporization has been observed under anodic
conditions which may lead to a destruction of their
electrolyte properties. The aim of this work is the study of
the vaporization processes leading to the mentioned
degradation effects. For this purpose an experimental setup
according to the transpiration method has been developed.
Concerning the vaporization of chromium the Cr release rates
of the main ferritic interconnect alloys, namely Crofer 22
APU, ZMG 232, E-Brite, IT-10, IT-11, IT-14 and Ducrolloy as
well as a variety of Ni- and Co-base superalloys and
stainless steels with different contents of Al, Si, Ti, Mn,
W, Ni and Co were measured at 800°C in air and compared to
each other. The alloys that form an upper layer of
Cr-Mn-spinel on top of the grown chromia scale showed a
reduction of the Cr release by 61-75 $\%$ compared to pure
chromia scales whereas alloys with an outer
Co$-{3}$O$_{4}$(s) scale had a by more than 90 \% reduced Cr
release. For the former alloys a significant vaporization of
Mn under anodic conditions could be detected. On the base of
empirical data it could be extrapolated that in order to
achieve the long term requirements for SOFC-systems
additional coatings are necessary. Tests with sputtered
perovskite and metal coatings showed that the Cr release
could be reduced by up to three orders of magnitude by
coatings of Co, Ni or Cu. Additionally the Cr vaporization
rates of alumina forming and alitized alloys was
investigated. Due to the high electrical resistance of
alumina these materials are not suitable for interconnect
but for other components like heat exchanges, pipes, pumps
etc. After short oxidation times these materials showed Cr
release rates that are by more than 3 orders of magnitude
lower than those of chromia forming materials. Concerning
the vaporization of doped lanthanum gallates the
vaporization rates of the elements Ga, Mg, Sr and La were
measured as function time, temperature, gas flow rate and
stoichiometry for compositions of the type
La$_{(1-x)}$Sr$_{x}$Mg$_{y}$(Fe$_{z}$)Ga$_{(1-y-z)}$O$_{3-\delta}$(s)
with x=0,1-0,3, y=0,12-0,32 and z=0-0,08. As a consequence
of the depletion of Ga the formation of the impurity phases
LaSrGaO$_{4}$(s) and LaSrGa$_{3}$O$_{7}$(s) was observed.
Kinetic parameters were obtained by fitting the results to a
vaporization model with imposed solid state diffusion. In
order to elucidate the quality of thermodynamic calculations
equilibrium measurements were performed with pure
Cr$_{2}$O$_{3}$(s) and Ga$_{2}$O$_{3}$(s) in air or
Ar/H$_{2}$ respectively as function of temperature and
humidity. For the vaporization of Ga$_{2}$O$_{3}$(s) it
could be shown that GaOH(g) is the most abundant Ga species
in the gas phase. The determined partial pressures for
CrO$_{2}$(OH)$_{2}$(g) and CrO$_{3}$(g) or GaOH(g)
respectively and the derived enthalpies and entropies of
reaction for CrO$_{2}$(OH)$_{2}$(g) and GaOH(g) showed
significant differences compared to the thermodynamical data
in the literature.},
cin = {IWV-2},
ddc = {620},
cid = {I:(DE-Juel1)VDB2},
pnm = {Rationelle Energieumwandlung},
pid = {G:(DE-Juel1)FUEK402},
typ = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
url = {https://juser.fz-juelich.de/record/51107},
}