Grain boundary impedance

Figure 15. Impedance spectrum of 10 mol% Y2C>3-doped zirconia according to Ref.96 The rhs semicircle represents the grain boundary impedance. Reprinted from S.P.S. Badwal, Solid State Ionics 76 (1995), 67-80. Copyright 1995 with permission from Elsevier.

Hence, in simple cases each bulk layer, each grain boundary plane, and both electrodes of the brick layer model sample, can be represented by separate RC elements (Fig. 7b). The RC elements of the n bulk layers can be combined to a single RC element with the -fold resistance and the 1 / -fold capacitance of a single layer. The n — 1 grain boundary impedances can also be summed, as can the two electrode impedances, and hence the model sample corresponds to a series connection of three RC elements (Fig. 7c) with... 

The quantitative analysis of conventionally obtained grain boundary impedance data is problematic if grain boundary properties strongly vary from boundary to bound-... 

A quantitative analysis of grain boundary impedances measured with macroscopic electrodes can be rather problematic if grain boundary properties vary from boundary to boundary (cf. Sec. 3.2). Hence, additional information on the distribution of grain boundary resistivities is often desired. Microelectrode measurements can yield such additional information (Sec. 4.2) and below a microcontact impedance spectroscopic study of grain boundaries in a polycrystal is exemplarily presented. The material of choice is again SrTiCE (0.2 mol % Fe-doped), which represents a model material for the technologically highly important class of perovskite-type titanates (see also above). 

For coarse-grained metals, dislocation movement and twinning are well known primary deformation mechanisms. Ultrafine, equiaxed grains with high-angle grain boundaries impede the motion of dislocations and... 

J. Fleig [2000] The Influence of Non-Ideal Microstructures on the Analysis of Grain Boundary Impedances, Solid State Ionics 131, 117-127. 

The Grain Boundary Impedance of Random Microstructures, Numerical Simulations and Implications for the Analysis of Experimental Data,... 

A Finite Element Study of the Grain Boundary Impedance of Different Microstructures, J. Electrochem. Soc. 

Aoki, M., Chiang, Y.-M., Kosacki, 1., Jong-Ren Lee, L., and Fuller, H. Liu, Y. (1996). Solute segregation and grain boundary impedance in high-purity stabilized zirconia. J Am Ceram Soc. 79 1169-1180. 

In 1995, Bonanos et al. [68] reviewed the literature and their own results and tabulated performances reported for a number of H2-O2 or H2-air fuel cells with 0.4- to 0.5-mm-thick, 10% 20% Gd- or Nd-doped BaCeOa electrolytes, Pt anodes, and Pt or Ag cathodes, operated at 800°C. At 700 mV cell voltage, they delivered from 70 to 285 mA/cm of current density, corresponding to 50-200 mW/cm of power density and effective electrolyte conductivities of around 10-40 mS/cm. These conductivities are, to a first approximation, in agreement with bulk (grain interior) conductivity data for doped BaCeOaS, indicating that electrode and grain boundary impedances were small. 

C. Kjolseth, H. Fjeld, P.I. Dahl, C. Estournes, R. Haugsrud, T. Norby, Space Charge theory applied to the grain boundary impedance of proton conducting BaZro.9Yo.i03. under publication. 

High bulk proton conductivity, high stability, and a wide ionic domain [47] therefore make Y-doped BaZrOs an interesting parent compound for the development of proton-conducting electrolytes for SOFC applications. Unfortunately, the unfavorable brittleness, the grain boundary impedance, and the increasing phase instability with increasing Y-dopant level remain problems to be solved. The addition of small amounts of BaCeOs or a compromise in the choice of the kind of dopant may help to reduce these problems. 

Fig. 3 Impedence spectra at 340°C of two 3 mol% Y203-Zr02 (3YSZ) compositions with different S1O2 impurities (O 20 ppm Si02, D 800 ppm Si02) showing the effect of impurity segregation at grain boundaries on the grain boundary impedance (arc on the right hand side) (Badwal et al. 1997).

Fig. 7.23. The contributions that this covers may include internal grain boundary impedances or electrode effects of nonblocking electrodes. In this way also transfer impedances of the nonblocked species at the blocking electrode can be approximately taken account of (Section 7.3.4e).

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