Abstract: [Background] Magnesium hydroxide and magnesium oxide are both important metal compounds with many excellent properties which make them have great application prospects in various fields of industry and environmental protection, such as flame-retardant, antibacterial agents, water/gas treatment, catalysts.[Purpose] This paper focus on analysis of the microstructure of magnesium hydroxide at different calcining temperatures, especially the changes of vacancy clusters and pores before and after phase transition temperature.[Methods] Nano-magnesium hydroxide was prepared by direct precipitation method with the addition of polyethylene glycol (PEG) surfactant and nano-magnesium oxide was obtained by calcining nano-magnesium hydroxide. The microstructure and defects of both magnesium hydroxide and magnesium oxide were investigated by positron annihilation lifetime spectroscopy (PALS), X-ray diffraction (XRD), thermal gravimetric analysis (TGA), field emission scanning electron microscopy (FESEM) and nitrogen adsorption-desorption experiment.[Results] The original magnesium hydroxide particle prepared by direct precipitation method has flake -like morphology with about 15 nm thickness. Corresponding magnesium oxide is a sphere-like particle with a size of about 30 nm and good dispersivity. The positron lifetime measurement found that two long lifetime components τ₃ and τ₄ reflect the information of micropores and mesopores in the samples, respectively. When the nano-magnesium hydroxide transformed to nano-magnesium oxide in the temperature range of 250~300℃, relative intensity I₄ increases significantly and I₃ decreases rapidly, indicating that the number of mesopores increases suddenly, while the number of micropores decreases rapidly. The pore size calculated by the improved Tao-Eldrup model is consistent with the pore size (2~4 nm) measured by nitrogen adsorption-desorption experiment.[Conclusion] Flake-like nano-magnesium hydroxide was synthesized by precipitation method with the addition of PEG surfactant. Sphere-like nano-magnesium oxide was obtained by calcining nano-magnesium hydroxide in the temperature range of 300~700℃. During the phase transition from nano-magnesium hydroxide to nano-magnesium oxide, a large number of micropores are migrated and integrated into larger mesoporous pores because of growth and recombination of the grain. On the other hand, the removement of water molecules promotes formation, migration and aggregation of the vacancy clusters and microvoids.

Key words: Positron annihilation, Magnesium hydroxide, Magnesium oxide, Phase transition, Pore