Synthesis of nanocrystalline Mn–Zn ferrite powders through thermolysis of mixed oxalates

Nanocrystalline Mn–Zn ferrite powders were synthesized by thermal decomposition of an oxalate precursor. Two polymorphs of a mixed Mn–Zn–Fe oxalate dihydrate were obtained by precipitation of metal ions with oxalic acid: monoclinic α-(Mn, Zn, Fe)3(C2O4)3·6H2O is obtained after precipitation and ageing at 90 °C, whereas the orthorhombic β-type is formed after precipitation at room temperature. The morphology of the oxalate crystals can be controlled by the precipitation conditions. The α-polymorph of the mixed oxalate consists of prismatic and agglomerated particles. The β-oxalate forms non-agglomerated crystallites of submicron size. Thermal decomposition of the oxalate at 350 °C in air results in an amorphous product. Nanosize Mn–Zn ferrite powders are formed at 500 °C and a mixture of haematite and spinel is observed at 750 °C. The thermal decomposition of the mixed oxalate is monitored by thermal analysis, XRD and IR-spectroscopy. The morphology of the oxalate particles is preserved during thermal decomposition; the oxide particle aggregates display similar size and shape as the oxalates. The primary particles are much smaller; their size increases from 3 nm to 50 nm after decomposition of the oxalates at 350 and 500 °C, respectively. The powder synthesized by decomposition at 500 °C was sintered at 1150 °C to dense and fine-grained Mn–Zn ferrites.