본 연구는 뮬라이트화 공정에서 나타나는 발열 반응의 특성, spinel 상의 조성과 구조, 뮬라이트 전미분체의 뮬라이트화 공정에 대하여 고찰하였다. Mullite 전미분체는 4배위체의 SiO₄와 6배위체의 AlO₆가 결합하고 있음을 알 수 있었다. 스피넬상의 조성은 2Al₂O₃·SiO₂에 근사함을 알 수 있었고, 4개의 Si⁴⁺ 이온과 4개의 Al³⁺ 이온이 8개의 유효한 4배위체 자리를 차지하고 있으며, 16개의 유효한 6배위체 자리 중 12자리를 Al³⁺ 이온이 차지하고 있고 나머지 4개의 6배위체 자리는 양이온 결손 결자점으로 존재한다. 따라서 스피넬상의 이온 분포는 □₄Al^vi₁₂[Al₄Si₄]^ivO₃₂이며, □는 6배위체 자리중 빈 격자점을 나타낸다. 스피넬상을 거친 시료는 1,100 ℃ 부근에서 T-Mullite로 상전이가 일어났으며, 계속하여 하소온도를 증가시키면 1,250 ℃ 부근에서 O-Mullite가 생성되었고, 위와 같은 mullite화 공정에는 Al³⁺ 이온의 배위체 변화와 비정질 SiO₂의 확산에 의한 결정의 조성 변화가 3Al₂O₃·2SiO₂ mullite가 생성될 때까지 계속 일어남을 알 수 있었다.

This study is to clarify the existing outstanding problems concerning (i) the nature of the exothermic reaction, (ii) the structure and composition of the spinel phase, and (iii) the mechanism of mullite formation. The structure of pre-mullite particles could be interpreted to be one where SiO₄ tetrahedron and AlO₆ octahedron are bonded at random by sharing oxygen atom. The composition of the spinel phase was most likely 2Al₂O₃·SiO₂ and it is regarded that 4 Si⁴⁺ and 4 Al³⁺ occupy the available 8 tetrahedral sites and 12 Al³⁺ occupy the available 16 octahedral sites leaving 4 octahedral sites being left vacancy. On this basis, the composition of the unit cell of spinel is □₄Al^vi₁₂[Al₄Si₄]^ivO₃₂, where □ represents a vacancy in the octahedral coordination state. The sample passed through the spinel phase was transformed into the T-Mullite at around 1100 ℃ and transformation of T-Mullite to O-Mullite could take place about 1250 ℃. In the mullitization process as mentioned above, change of coordination number of Al³⁺ ion and diffusion of SiO₂ into the crystal occured continuously with increasing calcination temperature until the 3:2 mullite is formed.

Mullite Mullitization Sol-Gel Process Spherical Powder Mixed Solvent Partial Hydrolysis

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