The effect of mechanical milling on the solid state reactions in the barium oxalate-iron(III) oxide system

The formation of barium hexaferrite, BaFe₁₂O₁₉, from a 1:6 molar ratio mixture of barium oxalate and iron oxide has been investigated. Thermogravimetry (TGA), high temperature X-ray powder diffraction (HT-XRPD), differential scanning calorimetry (DSC) and micro-Raman spectroscopy have been used to determine the effect of mechanical activation on the solid state reactions occurring during heating. The resulting magnetic properties were investigated measuring hysteresis loops. For the activated mixtures, the mass loss is over at ≈600 °C i.e. well below the temperature where BaCO₃ spontaneous decomposition is complete (T>850 °C). Such a noticeable temperature lowering is a consequence of the high energy milling enhancing the formation of BaFe₂O₄. After heating the milled mixture to 850 °C, BaFe₁₂O₁₉ was rapidly formed from the BaFe₂O₄ and residual Fe₂O₃. Starting from an unmilled mixture, only minor amounts of BaFe₁₂O₁₉ were formed by heating to 850 °C. The favourable formation of barium hexaferrite, when starting from milled powders, has been confirmed by micro-Raman spectroscopy. The powder from the activated sample was shown to have far better magnetic properties than the unactivated sample.