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@PHDTHESIS{Gindorf:29351,
      author       = {Gindorf, Christian},
      title        = {{U}ntersuchungen zur {C}hromfreisetzung aus metallischen
                      {I}nterkonnektorwerkstoffen für die
                      {H}ochtemperaturbrennstoffzelle},
      volume       = {3853},
      issn         = {0944-2952},
      school       = {Techn. Hochsch. Aachen},
      type         = {Dr. (FH)},
      address      = {Jülich},
      publisher    = {Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag},
      reportid     = {PreJuSER-29351, Juel-3853},
      series       = {Berichte des Forschungszentrums Jülich},
      pages        = {IV, 100 p.},
      year         = {2001},
      note         = {Record converted from VDB: 12.11.2012; Aachen, Techn.
                      Hochsch., Diss., 2001},
      abstract     = {Metallic interconnect materials (chromium-based alloys and
                      chromium-containing steels) form volatile chromium species
                      under operating conditions of the Solid Oxide Fuel Cell.
                      These volatile Cr(VI) species are reduced to solid Cr(lII)
                      compounds at the triple phase boundary
                      cathode/electrolyte/oxidant after penetrating the cathode
                      material, leading to a degradation of the electrochemical
                      properties of the cell. In this work the vaporization of
                      volatile chromium species was studied by the transpiration
                      method. Transpiration experiments were carried out under
                      equilibrium as well as nonequilibrium conditions.
                      Equilibrium studies were carried out in air, using Cr203(s)
                      (powder) at different water partial pressures (between 0.7
                      mbar and 0.3 bar) and temperatures (between 400 °C and 950
                      °C). Thermodynamic calculations based on the measured
                      temperature dependence yielded an enthalpy of reaction of
                      OrH94s = 122.85 kJ/mol for the formation of the volatile
                      chromium species Cr02(OH)2(g) (dominating in the vapour over
                      Cr203(S) under these experimental conditions) .
                      Transpiration experiments under non-equilibrium conditions
                      were carried out at a water vapour pressure of p(H20) = 0.02
                      bar at temperatures of 850 °C and 950 °C. The chromium
                      vaporization rate of metallic interconnect materials forming
                      different oxide scales as well as the chromium vaporization
                      of different coated alloy samples (perovskite coatings on
                      Cr5FelY203 and X10CrAl18 substrates) were determined.
                      Perovskite coatings are used as a diffusion barrier against
                      volatile chromium(VI) species vaporizing from the
                      interconnect surface . In this work the chromium retention
                      potential of vacuum plasma sprayed coatings (scale thickness
                      25-30 [,m) was investigated by comparison of chromium
                      transport rates of coated and uncoated interconnect samples.
                      As a result of this experiments a chromium retention of more
                      than 99 $\%$ was found for protective perovskite coatings
                      (e.g. Lao.9Sro.,Cr03) under cathodic operating conditions of
                      the SOFC. Investigations of the scale morphology of
                      perovskite coatings revealed a time dependent densification
                      process. Due to that densification process possible values
                      of more than 99 $\%$ were observed after different annealing
                      times. The vaporization of volatile chromium(VI) species
                      from coated interconnect specimens is caused by cracks and
                      pores in the protective scale. This was confirmed by 53Cr
                      tacer experiments with protective perovskite scales based on
                      lanthanum chromfite which were carried out to obtain
                      chromium self diffusion coefficients . For the scale
                      composition of Lao.9Sro.1CrO3 at a temperature of 1000 °C a
                      chromium diffusion coefficient of 2.59 - 10-16 cm2/s was
                      determined. Considering a scale thickness of 30 [tm, from
                      this diffusion coefficient, a break through time of 550
                      years can be calculated. According to these results, grain
                      boundary diffusion as well as bulk diffusion are too slow to
                      compete with gas phase transport of chromium through the
                      protective scale. Considering the results of this work it is
                      possible to reduce the chromium deposition rate in the
                      cathode by a factor of more than two orders of magnitude if
                      the SOFC operating temperature is decreased by 100 °C from
                      950 °C to 850 °C and if proper substrate/coating
                      combinations are used},
      cin          = {IWV-2},
      cid          = {I:(DE-Juel1)VDB2},
      pnm          = {Werkstoff- und Bauteilentwicklung für die
                      Hochtemperatur-Brennstoffzelle},
      pid          = {G:(DE-Juel1)FUEK22},
      typ          = {PUB:(DE-HGF)11 / PUB:(DE-HGF)3},
      url          = {https://juser.fz-juelich.de/record/29351},
}