Synthesis and Characterization of CeO2- and LaNbO4-based Ionic Conductors

Abstract

Ceramic electrolytes that conduct either oxygen ions or protons at intermediate temperatures are important materials for use in e.g. solid oxide fuel cells, the prime candidate to produce electricity by electrochemical reactions. There are still challenges with respect to the cell performance which have to be solved before solid oxide fuel cells become a commercial success. One of the main topics in this field concerns the enhancement of ionic conductivity at suitable operation temperatures. During the last decade nanocrystalline materials have received considerable attention. Nanocrystalline ionic conductors may have higher ionic conductivity compared to traditional ceramics with grain sizes in the micrometer range.

The aim of this work has been to develop electrolyte ceramic materials with a designed microstructure. CeO2- and LaNbO4-based materials have been prepared through a complete route, from preparation of powders to densification of ceramics including characterization of selected properties. CeO2-based materials are oxygen ion conductors and show higher ionic conductivity compared to the more common yttria stabilized zirconia (YSZ), thus a lower operation temperature is possible. LaNbO4-based materials have recently been suggested as a promising proton conductor stable in CO2/H2O atmosphere.

In Paper I, powder synthesis of nanocrystalline CeO2-based powders (CeO2, Ce0.8Gd0.2O1.9 and Ce0.8Sm0.2O1.9) using combustion synthesis with glycine as fuel and nitrate as oxidizer is reported. The influence of glycine to nitrate (G/N) ratio on the pure CeO2-based powders was investigated. The influence of calcination temperature on crystallite size, surface area and carbonate species remaining from combustion reaction was studied, with special attention to powders prepared using a near-stoichiometric G/N-ratio. A G/N-ratio of 0.55 and calcination at 550°C in oxygen flow resulted in high quality powders with a crystallite size of ~10 nm with low degree of agglomeration due to the vigorous combustion. The G/N-ratio influenced the densification behavior of the powders. A G/N-ratio of 0.55 resulted in excellent sintering properties with an onset of sintering at ~600°C and fully dense materials were obtained at ~1300°C.

In Paper II, three different sintering techniques have been used to prepare dense (>95%) CeO2-based materials from the high-quality powders described in Paper I: Spark plasma sintering, hot pressing and conventional sintering. The three different sintering techniques resulted in different grain sizes, ranging from 160 nm by spark plasma sintering, to 50 μm by conventional sintering mainly due to difference in sintering temperature and the applied pressure. The materials were reduced after hot pressing and a minor reduction was observed after spark plasma sintering. The materials were easily reoxidized at temperatures above 200°C. The electrical conductivity, measured by van der Pauw method, revealed no clear dependence on grain size, but instead a dependence on the sintering method used. The substituted materials prepared by hot pressing had a lower electrical conductivity and higher activation energy compared to the materials prepared by both conventional and spark plasma sintering. Thus, it is proposed that the reduction of Ce observed during hot pressing might be detrimental for the ionic conductivity even after reoxidation. Hardness and fracture toughness, measured by Vickers indentation, were more influenced by chemical composition than the grain size of the materials. Higher fracture toughness and lower hardness were observed for pure CeO2 compared to the substituted materials.

A novel route to prepare large quantity of sub-micron LaNbO4-based powders by spray pyrolysis is presented in Paper III. An aqueous solution containing stable La-EDTA complex and Nb-malic acid complex was spray pyrolysed using an in-house spray pyrolysis unit. The pure, nonagglomerated powders had a particle size of ~0.1 μm, narrow particle size distribution and high purity after calcination at 800°C.

The sintering behavior, microstructure, phase content and electrical conductivity of La1-xAxNbO4 (x = 0, 0.005 and 0.02 and A = Ca, Sr and Ba) prepared by spray pyrolysis is presented in Paper IV. The powders had excellent sintering properties and achieved high density after conventional sintering at 1200°C or as low as 1050°C by hot pressing at 25 MPa. A grain size down to 0.4 μm was achieved by hot pressing. The acceptor doped materials had a more homogenous microstructure due to secondary phases inhibiting grain growth compared to pure LaNbO4. Liquid secondary phase was formed at elevated temperatures in acceptor doped LaNbO4, resulting in tremendous grain growth (~70 μm) and microcracking in La0.98Ba0.02NbO4. The solubility of Sr on La-site in LaNbO4 was determined to 1% at 1500°C, and similar low solubility of CaO and BaO in LaNbO4 was inferred. Protons were found to be the main charge carrier up to 1000°C in wet hydrogen. Higher grain boundary resistivity was observed compared to previous work, possibly due to lower sintering temperature resulting in secondary phases due to lower solubility of AO.

The thermal and mechanical properties of LaNbO4-based materials are presented in Paper V. The materials possessed a ferroelastic to paraelastic phase transition at ~500°C and the linear thermal expansion was significant lower for the paraelastic compared to the ferroelastic phase. The pure LaNbO4 had a significantly lower hardness compared to acceptor doped (Ca, Sr and Ba) LaNbO4 due to large grain size and microcracking. The fracture toughness of La0.98Sr0.02NbO4, measured by SEVNB method, was 1.7±0.2 MPa·m1/2. The ferroelastic properties were confirmed by non-linear stress-strain relationship and remnant strain. The remnant strain decreases with increasing temperature and increasing acceptor doping. The latter was possibly due to secondary phases pinning the ferroelastic domain boundaries.