Rapid synthesis of nanocrystalline magnesium chromite and ferrite ceramics with concentrated sunlight

High-temperature refractory ceramics and catalysts such as MgM2O4 (M = Cr, Fe) are produced conventionally via energy-intensive solid-state syntheses (using 0.44-10 GJ electricity for sintering per ton oxide, equivalent to combustion of 48-1088 kg coal per ton oxide). This article reports rapid production of 17 ± 2 mol% MgFe2O4 and 8.6 ± 0.9 mol% MgCr2O4 after 30 min at 1200 °C employing 0.82 kW m−2 sunlight concentrated at a geometric ratio of about 900 m2 m−2 using a Fresnel lens. Solar radiation promotes the diffusion-limited ferrite formation (42 ± 5 μmol MgFe2O4 per mol Fe2O3 s−1 vs. 26 ± 3 μmol mol−1 s−1 in absence of sunlight) while the transition metals promote the reduction of Mg2+. The nanocrystalline and macroporous spinel has a specific surface area of 9.7-11.9 m2 g−1 (in the order of sol-gel synthesis methods) and is stable under extreme conditions, i.e., high temperature, solar radiation, and reducing agents.