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@PHDTHESIS{Froitzheim:2197,
      author       = {Froitzheim, Jan},
      title        = {{F}erritic {S}teel {I}nterconnectors and {T}heir
                      {I}nteractions with {N}i {B}ase {A}nodes in {S}olid {O}xide
                      {F}uel {C}ells ({SOFC})},
      volume       = {16},
      issn         = {1866-1793},
      school       = {RWTH Aachen},
      type         = {Dr. (Univ.)},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {PreJuSER-2197},
      isbn         = {978-3-89336-540-1},
      series       = {Schriften des Forschungszentrums Jülich : Energie $\&$
                      Umwelt / Energy $\&$ Environment},
      pages        = {169 S.},
      year         = {2008},
      note         = {Record converted from VDB: 12.11.2012; Aachen, RWTH, Diss.,
                      2008},
      abstract     = {In recent years high Cr ferritic steels such as Crofer 22
                      APU became the most widespread construction materials for
                      solid oxide fuel cell (SOFC) interconnects mainly due to low
                      cost and the ease of fabrication compared to ceramic
                      materials. It was shown that optimum properties with respect
                      to oxide scale growth and adherence could only be obtained
                      by very low, carefully controlled concentrations of minor
                      alloying additions such as Al and Si. This required
                      sophisticated alloy manufacturing methods, including vacuum
                      induction melting, resulting in higher manufacturing costs.
                      The high alloy purity also has as result a low creep
                      strength of the material which indirectly resulted in
                      increased oxidation rates of thin components. The aim of the
                      present project was the development of a ferritic steel with
                      improved creep strength and less restrictions with respect
                      to alloy purity (especially Al or Si levels) than in the
                      case of Crofer 22 APU For this purpose modifications of
                      Crofer 22 APU with various amounts of W, Nb, Al and Si were
                      investigated with respect to SOFC relevant properties. The
                      alloying additions W and Nb were chosen because they are
                      potentially suitable to increase creep strength by solid
                      solution strengthening (W) and precipitation strengthening
                      (Nb). The investigations included oxidation tests in air and
                      anode atmosphere at 800 and 900°C for up to 10000h, creep
                      tests, investigation of the electrical resistance of the
                      surface oxide scale as well as measurements of the
                      coefficient of thermal expansion (CTE). While the addition
                      of W did not have a major effect on the oxidation behaviour,
                      the addition of Nb resulted in significantly increased
                      oxidation rates. Further it could be shown that the Nb
                      containing precipitates of the Laves phase type had a high
                      affinity for Si which results in a reduced amount of Si in
                      the alloy matrix. Thus, in Nb containing alloys Si
                      impurities can be tolerated which causes lower alloy
                      manufacturing costs. At the same time Si additions suppress
                      the adverse effect of Nb on the oxidation behaviour by
                      stabilising the Laves phase which results in an oxidation
                      rate which, for 1-2mm thick specimens, is almost identical
                      to that of Crofer 22 APU. Due to the higher creep strength
                      the new alloys exhibited only a slight thickness dependence
                      of the oxidation rates with the consequence, that for thin
                      components of a few tenths of a mm the oxidation rates for
                      the new alloy were smaller than for Crofer 22 APU. Thus, if
                      the amounts of W, Nb and Si were carefully adjusted the new
                      material had, compared to Crofer 22 APU, similar CTE,
                      similar ductility and lower oxidation rates for thin
                      components. Apart from the interconnect material itself,
                      issues related to compatibility of the interconnect with
                      other cell components are a major problem in the SOFC. While
                      compatibility related issues on the cathode side are widely
                      known and have been extensively studied, diffusion related
                      problems on the anode side have been hardly addressed until
                      now. In the second part of the project a number of metallic
                      coatings were investigated to check their potential
                      suitability for inhibiting interdiffusion processes between
                      ferritic steel and Ni base anode or wire mesh. However, none
                      of the tested coatings could fulfil the requirements. In
                      contrast, CeO$_{2}$ turned out to be an efficient diffusion
                      barrier, and its electronic conductivity appeared to be
                      sufficient for SOFC purposes. Measurements of ceria coatings
                      on Crofer 22 APU substrates showed that a considerable part
                      of the area specific resistance is related to the thermally
                      grown oxide scale on the steel surface and to interfacial
                      resistances. The latter can be reduced substantially if an
                      intermediate Cu coating is applied between the steel
                      substrate and CeO$_{2}$ barrier layer.},
      cin          = {IEF-2},
      ddc          = {333.7},
      cid          = {I:(DE-Juel1)VDB810},
      pnm          = {Rationelle Energieumwandlung},
      pid          = {G:(DE-Juel1)FUEK402},
      typ          = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
      url          = {https://juser.fz-juelich.de/record/2197},
}