Browsing by Author "Shirpour, Mona"

Now showing 1 - 1 of 1

Open Access
Grain boundary characterization of electroceramics : acceptor-doped BaZrO3, an intermediate temperature proton conductor
(2011) Shirpour, Mona; Maier, Joachim (Prof. Dr.)
Acceptor doped BaZrO3 exhibit a high bulk proton conductivity of 2.10-2 S/cm to 5.10-5 S/cm in the temperature range of 500°C to 100°C. Nevertheless, highly resistive grain boundaries and notoriously small grain sizes seriously hamper the application of this material as an electrolyte in intermediate-temperature solid oxide fuel cells. Up to now, none of the usual methods (such as powder synthesis via chemical methods, reactive sintering, and sintering aids) could considerably improve the specific GB conductivity of this material. Investigation on sintering properties showed that acceptor-doped BaZrO3 exhibits a limited grain growth resulting in grain sizes 10 times smaller than undoped BaZrO3. Larger grains, by decreasing the GB number density, can improve the total conductivity of BaZrO3. Nevertheless, due to the limited range for the expected grain size increase, a modification of specific GB conductivity seems much more effective. This work is focused on characterization of grain boundaries to get a better understanding of the blocking effect in this perovskite structure proton conductor. It was attempted to check the most frequently reported origins for the blocking character of GBs in ion conductors: (i) Presence of continuous secondary/amorphous phase at GB (ii) Distorted crystallographic structure of grain boundary region which can result in a low water solubility and/or lower proton mobility in GB region (iii) Inhomogeneous distribution of dopant cations in GB region (iv) Depletion of protonic charge carriers in space charge layers which form to compensate an excess charge in the GB core Based on TEM images showing clean GB, secondary phases at the GB could be ruled out as origin for the blocking character. The similar change of bulk and GB conductivity upon switching from dry to wet atmosphere indicated that hydration behaviors and proton mobility in the GB region do not significantly differ from bulk. Due to high vapor pressure of barium oxide, the conventionally sintered samples typically suffer a certain barium loss due to the long-term high-temperature sintering step. The effect of barium loss is more pronounced for the GB conductivity than for the bulk. Although some extra barium can be added in the starting composition to compensate the loss, composition control is not easy using the conventional sintering method. This situation could be improved by “Spark Plasma Sintering” which allows one to minimize Ba deficiency due to much shorter sintering times. The sintered ceramic achieves a higher density and offers the possibility of applying an additional post-heat treatment. EDXS-TEM studies on GBs with different heat treatments (resulting in strongly different conductivities) showed that Y as well as Sc dopants segregate to the GB region, and that GBs with a higher amount of segregation exhibit a lower resistivity. Y+3 and Sc+3 cations were chosen due to their different ionic radius; the large Y+3 cation exhibits a large size mismatch in Zr+4 site in contrast to Sc+3 with a negligible size mismatch. The comparison revealed that for those two dopants segregation is mainly driven by electrostatic forces due to a positively charged core, which is responsible for depletion of protons, holes and oxygen vacancies in the adjacent space charge zones. The segregated dopants increase the GB conductivity partly by directly compensating the positive charge in the core, and partly by accumulation in the space charge zone decreasing the thickness of the depletion layer. Further evidence for a positive core charge comes from measurements on strongly reduced n-conducting Y-doped BaZrO3, where the blocking GB character disappears. This finding is a clear evidence for a positively charged core and consequently related depletion/accumulation layers in adjacent space charge zones. The expected non-linear current-voltage character of the proposed space charge zones was confirmed studying the electrical behavior of GBs under DC-bias. For this purpose, a large grain sample, allowing to apply a reasonable DC bias over each GB, was prepared in an Infrared Image Furnace. The observed voltage-dependent GB resistances and capacitances are consistent with a Schottky-type barrier at the interface. Two models, with and without interface states, are discussed to explain voltage dependence of the GB capacitance. Based on these findings, as an alternative core charge compensation, the Ba-site was doped with Cs+ cations. has a high tendency to segregate to the GB region due to its size mismatch (rCs+ = 1.88 Å, rBa+2 = 1.61 Å) and electrostatic interaction with positive core. The conductivity measurements showed that only 1 at.% of Cs can increase GB conductivity of Y-doped BaZrO3 by more than 2-3 orders of magnitude. In summary, from the systematic structural, chemical and electrical characterization a positive GB core charge resulting in space charge depletion of protons could be identified as the major causes for the blocking effect of GBs in acceptor-doped BaZrO3. This investigation has been successfully used to increase conductivity of GB by adjusting heat treatment or by applying adequate GB decoration. These results may be extended to other electroceramic materials for which GB properties are important.