전구체 및 첨가제가 나노 α-Alumina 결정 형상에 미치는 영향

Abstract

Alumina ceramics are used in many areas of modern industry both as structure and functional materials because of their hardness, mechanical strength, good corrosion and low cost. It was demonstrated by recent works that nano-size plate-like particles can increase fracture toughness more significantly as compared with the ball-like particles. The object of this study is to prepare the plate-like nano α-Al₂O₃ by sol-gel method using different precursors(aluminum nitrate, aluminum sulfate) and fluoride additives (AlF₃, MnF₂, CaF₂). The crystalline structures of alumina particles were characterized by X-ray diffraction (XRD) and their particle size and morphology were observed by Field Emission Scanning Electron Microscope (FE-SEM). The temperature for complete transformation to α-Al₂O₃ changed with precursor and the transformation temperature is lower in the case of using aluminum nitrate precursor as compared to aluminum sulfate precursor. The particles obtained from aluminum nitrate precursor have elongated shape, while those obtained from aluminum sulfate precursor have spherical shape, and it could be attributed to the different characteristics of precipitate formed in the initial stage of reaction. The effect of AlF3 additive on modification of morphology of particles is different depending on the precursor. The change of morphology of α-alumina particles to the plate-like shape was observed in the case of using aluminum nitrate precursor, but not in the case of using aluminum sulfate precursor. The effect of additive on the boehmite-to-α-Al₂O₃ phase transition was investigated, and AlF₃, MnF₂ and CaF₂ were used as additives. These additives have different ionic radius: 0.05, 0.67 and 0.1 nm for AlF3, MnF2 and CaF₂, respectively. The crystalline phase formed by reaction with the additive at high temperature has a plate-like morphology. A clear relationship was found between the transition temperature and the difference in ionic radius between Al₂+ and the additive (Δr): The transition temperature increased as Δr increased. This result indicates that additives with larger ionic radii are more effective in suppressing the diffusion of Al₃+ and O₂- in γ-Al₂O3 to inhibit the grain growth to α-Al₂O₃, and consequently retarding the transformation into α-Al₂O₃.