On the influence of silica type on the structural integrity of dense La9.33Si2Ge4O26 electrolytes for SOFCs

Apatite-type rare earth based oxides, such as R-doped lanthanum oxides of general formula La_9.33(RO₄)₆O₂ with R = Ge, Si, exhibit high ionic conductivity and low activation energy at moderate temperatures, when compared to the yttria-stabilized zirconia electrolyte making them potential materials to be used in the range 500–700 °C, for intermediate temperature solid oxide fuel cells (IT-SOFCs). In this study, dense oxyapatite-based La_9.33Si₂Ge₄O₂₆ electrolytes have been successfully prepared either by electrical sintering at 1400 °C or microwave hybrid sintering at 1350 °C for 1 h from La₂O₃, SiO₂ and GeO₂ powders dry milled at 350 rpm for 15 h in a planetary ball mill. The densification behaviour of the apatite-type phase synthesized by mechanical alloying was found to be dependent on the grade of SiO₂ used: either pre-milled quartz powder or amorphous nanosized fumed silica. The influence of the silica type on the La_9.33Si₂Ge₄O₂₆ integrity was assessed by dynamic Young's modulus, microhardness and indentation fracture toughness measurements. A good correlation between the degree of densification (as observed by SEM/EDS) and the resulting mechanical properties could be established. Pre-milling of quartz powder has favoured higher densification rates to be attained suggesting that both Fe content, resulting from the dry milling (as determined by PIXE analyses) and crystallinity of SiO₂ do promote densification of these electrolytes thereby improving their structural integrity.