Synthesis of Magnesium Borates by Mechanically Activated Annealing

Different magnesium borate compounds such as MgB₄O₇, Mg₂B₂O₅, and Mg₃B₂O₆ were synthesized from B₂O₃/MgO powder blends by mechanically activated annealing (M2A). Milling experiments were carried out in a SPEX 8000 D Mixer/Mill, and mechanically activated powders were subsequently annealed under air. The effects of B₂O₃/MgO mole ratios (1/1, 1/2, 1/3, and 2/1), milling durations (30 minutes and 1 and 2 hours), annealing durations (2 and 5 hours), and annealing temperatures (973 K, 1173 K, and 1273 K (700 °C, 900 °C, and 1000 °C)) on the formation and microstructure of magnesium borates were investigated. Compositional, microstructural, and thermal characterizations were conducted by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Temperature-product mole amount relations of the B₂O₃/MgO powder blends were interpreted by thermochemical software. MgB₄O₇ and Mg₂B₂O₅ crystalline phases were identified in the M2A’d powders in the B₂O₃/MgO mole ratios of 1/1, 1/2, and 2/1. Mg₂B₂O₅ and Mg₃B₂O₆ crystalline phases were detected in the M2A’d B₂O₃/MgO: 1/3 powders in addition to unreacted MgO.