ECI Digital Archives

Title

Effect of (external) electric fields on heterogeneous solid state reactions – Special role of grain boundary diffusion

Authors

Carsten Korte, Institut für Energie- und KlimaforschungFollow

Conference Dates

March 6-11, 2016

Abstract

During ope­ra­tion ceramic materials are of­ten ex­posed to high elec­tri­cal fields, which create a se­cond dri­ving force for mobile com­po­nents in ad­di­tion to the chemical po­ten­tial gra­dient. Due to the ongoing miniaturisation in modern appli­ca­tions, interfaces gain in importance for the materials pro­per­ties. So­lid state reac­tions, neg­li­gi­ble on ma­cro­scopic length sca­les, be­come more im­portant on the nanoscale and thus be­come a frequent source of materials degra­da­tion

In this contribution the influence of an electric field on the kinetics and the morphological evo­lution of (hetero­ge­ne­ous) solid state model reactions will be highlighted [1-4]. Experiments on the reac­tion couples MgO + In₂O₃ (for­ming one product: MgIn₂O₄) and Al₂O₃ + Y₂O₃ (forming three pro­ducts:YAG, YAM and YAP) were per­formed in thin film technique. Using linear tran­sport theory, a time in­de­pen­dent growth rate for the product layer(s) is ex­pec­ted, de­pen­ding on the magnitude and the direction of the ionic current through the electrochemical cell and the dif­fe­rence of the ionic transference numbers in the product phase. This is generally different com­pa­red to solely diffu­sion control­led reactions without electric fields, where the reaction rate decreases with increasing pro­duct layer thickness. In a spinel forming reaction an en­han­ced growth rate for the product layer is predicted, when the di­va­lent cations are more mobile and the tri­va­lent oxide is attached to the catho­de side (Fig. 1).

The role of grain boun­da­ries as fast dif­fu­sion paths is highly em­pha­si­sed. The mor­pho­logy of the pro­duct layer is sig­ni­fi­cant­ly dif­fe­rent compared to a non-field-driven reaction. An analysis shows that the ionic transference num­bers in (large angle) grain boun­daries differs significantly from the bulk phase, cau­sing locally different growth rates of the product layer. In case of a spinel forming reaction the divalent oxide tends to grow along grain boundaries through the complete product layer, reaching the trivalent oxide.

In solid states reactions forming more than one product layer, applying an additional electric field opens a pos­si­bi­lity to control the product formation. Driven by an electric field, the growth kinetics of the product layer depends on the difference of the ionic transference numbers, without field on the Nernst-Planck coupled conductivities. In case of the reaction between Al₂O₃ + Y₂O₃ (three pro­ducts:YAG, YAM and YAP)the formation of the perovskite phase (YAP) can be selectively en­han­ced when connecting the Y₂O₃ layer to the cathode side (Fig. 2).

Recommended Citation

[1] M. T. Johnson, C B. Carter and H. Schmalzried, J. Am. Ceram. Soc 83, 1768-1772 (2000) [2] C. Korte, N. Ravishankar, C. B. Carter and H. Schmalz¬ried, Solid State Ionics 148(1-2), 111-121 (2002) [3] C. Korte, N.D. Zakharov and D. Hesse, Phys. Chem. Chem. Phys 5(25), 5530-5535 (2003) [4] C. Korte, and B. Franz and D. Hesse, Phys. Chem. Chem. Phys. 7(2), 413-420 (2005)