Elektrische und optoelektrische monolithisch integrierte Schaltungen auf InP-basierenden Heterostrukturen
Hodel, Uwe (Corresponding author)
Publikationen vor 2000; PRE-2000; Retrocat
Jülich Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag 2001
114 p.
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Berichte des Forschungszentrums Jülich 3854
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Please use the identifier: http://hdl.handle.net/2128/24125 in citations.


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(III) compounds at the triple phaseboundary cathode/electrolyte/oxidant after penetrating the cathode material, leading to adegradation of the electrochemical properties of the cell.In this work the vaporization of volatile chromium species was studied by the transpirationmethod. Transpiration experiments were carried out under equilibrium as well as nonequilibriumconditions. Equilibrium studies were carried out in air, using Cr$_{2}$O$_{3}$(s) (powder) atdifferent water partial pressures (between 0.7 mbar and 0.3 bar) and temperatures (between400 °C and 950 °C). Thermodynamic calculations based on the measured temperaturedependence yielded an enthalpy of reaction of $\Delta_{r}$H$_{948}$ = 122.85 kJ/mol for the formation of thevolatile chromium species CrO$_{2}$(OH)$_{2}$(g) (dominating in the vapour over Cr$_{2}$O$_{3}$(S) under theseexperimental conditions).Transpiration experiments under non-equilibrium conditions were carried out at a watervapour pressure of p(H$_{2}$O) = 0.02 bar at temperatures of 850 °C and 950 °C. The chromiumvaporization rate of metallic interconnect materials forming different oxide scales as well asthe chromium vaporization of different coated alloy samples (perovskite coatings onCr5Fe1Y$_{2}$O$_{3}$ and X10CrA118 substrates) were determined. Perovskite coatings are used as adiffusion barrier against volatile chromium(VI) species vaporizing from the interconnectsurface. In this work the chromium retention potential of vacuum plasma sprayed coatings(scale thickness 25-30 $\mu$m) was investigated by comparison of chromium transport rates ofcoated and uncoated interconnect samples. As a result of this experiments a chromiumretention of more than 99 % was found for protective perovskite coatings (e.g. La$_{0.9}$Sr$_{0.1}$,Cr=$_{3}$)under cathodic operating conditions of the SOFC.Investigations of the scale morphology of perovskite coatings revealed a time dependentdensification 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 protectivescale. This was confirmed by $^{53}$Cr tacer experiments with protective perovskite scales basedon lanthanum chromfite which were carried out to obtain chromium self diffusion coefficients.For the scale composition of La$_{0.9}$Sr$_{0.1}$CrO$_{3}$ at a temperature of 1000 °C a chromium diffusioncoefficient of 2.59 $\cdot$ 10$^{-16}$ cm$^{2}$/s was determined. Considering a scale thickness of 30 $\mu$m,from this diffusion coefficient, a break through time of 550 years can be calculated. Accordingto these results, grain boundary diffusion as well as bulk diffusion are too slow to competewith gas phase transport of chromium through the protective scale.Considering the results of this work it is possible to reduce the chromium deposition rate inthe cathode by a factor of more than two orders of magnitude if the SOFC operatingtemperature is decreased by 100 °C from 950 °C to 850 °C and if proper substrate/coatingcombinations are used.