Vollet D R, Donatti D A, Domingos R N, de Oliveira I
Department of Physics, IGCE-UNESP, 13500-230 Rio Claro, SP, Brazil.
Ultrason Sonochem. 1998 Jun;5(2):79-81. doi: 10.1016/s1350-4177(98)00011-x.
Diphasic gel in the mullite composition was prepared from a colloidal sol of boehmite mixed with a hydrolyzed tetraethoxisilane (TEOS) solution. The boehmite sol was obtained by peptization of a poorly crystallized or very small mean crystallite size (approximately 34 A) precipitate, resulting from the reaction between solutions of aluminum sulfate and sodium hydroxide. Ultrasound was utilized in the processes of the TEOS hydrolysis and the boehmite peptization, and also for complete homogenization of the mixture to gel. The wet gel is almost clear and monolithic. The gel transparency is lost on drying, when syneresis has ended, so that the interlinked pore structure starts to empty and is recovered upon water re-absorption. Cracking closely accompanies this critical drying process. Differential thermal analysis (DTA) and X-ray diffraction (XRD) show that the solid structure of the gel is composed of an amorphous silica phase, as a matrix, and a colloidal sized crystalline phase of boehmite. Upon heat treatment, the boehmite phase within the gel closely follows the same transition sequence as in pure alumina shifted towards higher temperatures. Orthorhombic mullite formation was detected at 1300 degrees C.
莫来石成分的双相凝胶是由勃姆石胶体溶胶与水解的四乙氧基硅烷（TEOS）溶液混合制备而成。勃姆石溶胶是通过对硫酸铝溶液和氢氧化钠溶液反应生成的结晶性差或平均微晶尺寸非常小（约34埃）的沉淀物进行胶溶而获得的。在TEOS水解和勃姆石胶溶过程中使用了超声，并且超声也用于使混合物完全均匀化为凝胶。湿凝胶几乎是清澈且整块的。当脱水收缩结束时，干燥过程中凝胶透明度会丧失，使得相互连接的孔隙结构开始排空，并且在重新吸水后恢复。在这个关键的干燥过程中会伴随出现开裂。差示热分析（DTA）和X射线衍射（XRD）表明，凝胶的固体结构由作为基质的无定形二氧化硅相和胶体尺寸的勃姆石结晶相组成。经过热处理后，凝胶中的勃姆石相遵循与纯氧化铝相同的转变顺序，但向更高温度偏移。在1300℃时检测到正交莫来石的形成。
