Calculation of conductivities from the corresponding diffusion
coefficients by means of the Nernst–Einstein equation confirms the experimentally
obtained values as well (with an error of approximately
one order of magnitude).
4. Summary
Phase-pure nanocrystalline powders of doped apatite type lanthanumsilicate
(ATLS) with and without dopants (Al, Fe,Mg) on the silicon
site La10 − ySi6 − x(Fe, Al,Mg)xO26 + z were successfully prepared and
sintered into dense pellets withmicro-meter sized grains. The obtained
apatite type lanthanum silicate LSO (La9.33Si6O26) electrolyte and the
Al-, Fe- and Mg-doped compositions LASO (La9.83Al1.5Si4.5O26), LFSO
(La9.83Fe1.5Si4.5O26), and LMSO (La9.93Mg0.9Si5.1O26) have been investigated
in terms of their oxygen self-diffusion and total conductivity.
Oxygen self-diffusivities and surface exchange coefficients and the
respective activation enthalpies have been estimated, and all samples
exhibited a typical Arrhenius behaviour of the surface exchange coefficients
meaning that the surface incorporation reaction is thermally activated.
The doped samples exhibit higher surface concentrations as well
as higher surface exchange coefficients as compared to the undoped
sample. This indicates that doping has a direct influence to the oxygen
incorporation reaction.
Oxygen self-diffusivities and incorporation reaction rates as well
as corresponding activation enthalpies have been estimated and
discussed. It has been found that doping enhances the self-diffusion in
all cases as compared to the undoped material. The trend found in this
study is LSO «LFSO b LASO ≤ LMSO. The diffusivities of the ATLS electrolyte
materials are higher compared to yttria stabilized zirconia which
is the state-of-the-art electrolyte material. The activation enthalpy for
oxygen diffusion in YSZ is higher as compared to the ATLS used in this
study.
Conductivity studieswere alsomade on samples of the samecomposition
and density. We observed that aluminium- and magnesiumdoped
samples have better conductivity than the undoped counterpart.
The conductivity values are in the expected order compared with literature
data and confirmed by using the Nernst–Einstein equation. The
role of iron on the conduction mechanism is unclear, but it shows a
positive influence on the conductivity. This behaviour is in agreement
with conductivity measurement results in the literature, where it has
been found that doping with aluminium and magnesium leads to an
increased ionic conductivity.
Acknowledgments
We gratefully acknowledge the funding by the Landesinitiative
Brennstoffzellen Niedersachsen, Germany in the frame of the
MONOLITH-project and by the European Commission in the frame of
the FP6 project MatSILC.
Calculation of conductivities from the corresponding diffusioncoefficients by means of the Nernst–Einstein equation confirms the experimentallyobtained values as well (with an error of approximatelyone order of magnitude).4. SummaryPhase-pure nanocrystalline powders of doped apatite type lanthanumsilicate(ATLS) with and without dopants (Al, Fe,Mg) on the siliconsite La10 − ySi6 − x(Fe, Al,Mg)xO26 + z were successfully prepared andsintered into dense pellets withmicro-meter sized grains. The obtainedapatite type lanthanum silicate LSO (La9.33Si6O26) electrolyte and theAl-, Fe- and Mg-doped compositions LASO (La9.83Al1.5Si4.5O26), LFSO(La9.83Fe1.5Si4.5O26), and LMSO (La9.93Mg0.9Si5.1O26) have been investigatedin terms of their oxygen self-diffusion and total conductivity.Oxygen self-diffusivities and surface exchange coefficients and therespective activation enthalpies have been estimated, and all samplesexhibited a typical Arrhenius behaviour of the surface exchange coefficientsmeaning that the surface incorporation reaction is thermally activated.The doped samples exhibit higher surface concentrations as wellas higher surface exchange coefficients as compared to the undopedsample. This indicates that doping has a direct influence to the oxygenincorporation reaction.Oxygen self-diffusivities and incorporation reaction rates as wellas corresponding activation enthalpies have been estimated anddiscussed. It has been found that doping enhances the self-diffusion inall cases as compared to the undoped material. The trend found in thisstudy is LSO «LFSO b LASO ≤ LMSO. The diffusivities of the ATLS electrolytematerials are higher compared to yttria stabilized zirconia whichis the state-of-the-art electrolyte material. The activation enthalpy foroxygen diffusion in YSZ is higher as compared to the ATLS used in thisstudy.Conductivity studieswere alsomade on samples of the samecompositionand density. We observed that aluminium- and magnesiumdopedsamples have better conductivity than the undoped counterpart.The conductivity values are in the expected order compared with literaturedata and confirmed by using the Nernst–Einstein equation. Therole of iron on the conduction mechanism is unclear, but it shows apositive influence on the conductivity. This behaviour is in agreementwith conductivity measurement results in the literature, where it hasbeen found that doping with aluminium and magnesium leads to anincreased ionic conductivity.AcknowledgmentsWe gratefully acknowledge the funding by the LandesinitiativeBrennstoffzellen Niedersachsen, Germany in the frame of theMONOLITH-project and by the European Commission in the frame ofthe FP6 project MatSILC.
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