Mechanochemical preparation, sintering aids and hybrid microwave sintering in the proton conductor Sr0.02La0.98Nb1-xVxO4-δ, x = 0, 0.15

Mechanochemical preparation of Sr0.02La0.98Nb1-xVxO4-δ was demonstrated for values of x = 0 and 0.15. Crystallinity could be improved by calcining at 1073 K. On cooling, the high temperature scheelite phase was retained to room temperature. Several novel sintering additives for LaNbO4 materials have been tested. The most successful were Cu3Nb2O8 and CuV2O6, which reduced the temperature of maximum shrinkage rate to temperatures ∼1173 K. The additive CuV2O6 was shown to increase densification and promote grain growth. A mechanosynthesised sample of Sr0.02La0.98Nb0.85V0.15O4-δ + 2 mol% CuV2O6 could be densified to ∼ 90% that of the theoretical by hybrid microwave sintering at 1168 K for 5 min. The scheelite phase in this material was retained to room temperature, against the normal thermodynamic tendency. Impedance spectroscopy in wet and dry, nitrogen and oxygen atmospheres suggested that the bulk conductivity of this material is unaffected by the sintering aid, whereas the grain boundary conductivity was impaired and exhibited n-type conductivity behaviour, characteristic of the additive. The concentration of this promising additive should be reduced in further work.

Graphical abstractComparison of XRD patterns of dense Sr0.02La0.98Nb0.85V0.15O4-δ with 2 mol% of CuV2O6 additive formed by hybrid microwave firing (MW) and an additive free sample prepared by solid-state reaction (SSR). The markers identify (♦) LaNbO4 scheelite phase and (×) LaNbO4 fergusonite phase.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Mechanosynthesis of Sr0.02La0.98Nb1-xVxO4-δ materials. ► Novel sintering additives for LaNbO4-based materials. ► Retention of scheelite phase to room temperature by mechanochemical processing. ► Electrical properties of scheelite Sr0.02La0.98Nb0.85V0.15O4-δ materials.