Verdampfung von Werkstoffen beim Betrieb von Hochtemperaturbrennstoffzellen (SOFC)
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 Co3O4(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)SrxMgy(Fez)Ga(1-y-z)O3-d(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 im-purity phases LaSrGaO4(s) and LaSrGa3O7(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 Cr2O3(s) and Ga2O3(s) in air or Ar/H2 respectively as function of temperature and humidity. For the vaporization of Ga2O3(s) it could be shown that GaOH(g) is the most abundant Ga species in the gas phase. The determined partial pressures for CrO2(OH)2(g) and CrO3(g) or GaOH(g) respectively and the derived enthalpies and entropies of reaction for CrO2(OH)2(g) and GaOH(g) showed significant differences compared to the thermodynamical data in the literature.
info:eu-repo/classification/ddc/620, Ingenieurwissenschaften, Festoxidbrennstoffzelle, Verdampfung, Degradation, Chromoxid, Hitzebeständige Legierung, Gallate, Galliumoxide, Alu, Transpirationsmethode, Transpiration method
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