Synthesis of hollow AlN microsphere by hydrothermal and carbothermal reduction–nitridation hybrid technique

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The synthesis of hollow AlN microsphere was studied because of its potential applications in various areas associated with high surface area and lightweight. A combined technique of hydrothermal and carbothermal reduction–nitridation (CRN) was adopted to synthesize spherical precursor and to transform to AlN, respectively, using Al-nitrate, sucrose, and urea. The precursor having different morphology were obtained after hydrothermal treatments at 180 °C for 12 h, depending on starting material combinations, where urea/Al molar ratio = 1 resulted in amorphous hollow microsphere. By CRN at 1500 °C for 4 h in a flowing N2 atmosphere, approximately 5 μm-sized hollow microsphere composed of AlN, α-Al2O3, or γ-AlON could be synthesized. CRN at higher temperature (1600 °C for 4 h) or for longer holding time (1500 °C for 10 h) is required to obtain pure AlN microsphere. Further tests at nine different hydrothermal conditions were performed to find an optimal synthesis condition, while the possible formation mechanism for hollow AlN microspheres was also proposed based on observations.