Synthesis of a powder precursor in the form of hydroxyhydrogel for reaction sintering of BN–mullite composite

Highly refractory mullite obtained through the route of hydroxyhydrogels

“…The dried colloidal silica, the aluminium hydroxide gel, the water, the antifoaming agent and the dispersant were mixed at a high shear stress in order to achieve an "intimate contact mix" between the alumina and the silica particles, to induce the aluminium hydroxide and colloidal silica molecules hydrolysis. It is believed that a polymerization process takes place at this stage (Li et al, 1997) and silica and alumina macro-molecules are formed (Basu et al, 2006), as it is shown in Fig. 1.…”

“…Some factors that affect the process are the type of precursor, the pH, the temperature and the thermal treatment length. To generate very fine sizes particles in an aqueous medium, the principles of chemistry of colloids were used in this route (Basu et al, 2006;Meher et al, 2005;BurgosMontes et al, 2007;Wasan et al, 2005;da Costa et al, 2003). The hydroxyhydrogels were prepared from colloidal silica (Glassven, SiO 2 , 99.8%; superficial area of 200 m 2 /g), aluminum hydroxide gel (DisproAlquímicos; 9.5% of Al 2 O 3 ) and some additives like antifoams, dispersants and flocculants.…”

“…Under high shear, the precursors form colloidal or molecular aqueous solutions that polymerize into an inorganic network. Then, with the addition of a gelling agent at room temperature the network is stabilized and immobilized (Samanta et al, 2001;Basu et al, 2006;Meher et al, 2005). The material was dried and thermal treated according to the DTA data of each case.…”

“…New technologies such as sol-gel, hydrothermal synthesis, pyrolysis, CVD, ceramic powders combustion and synthesis of hydroxyhydrogels have been developed (Somiya and Hirata, 1991;Samanta et al, 2001;Ismail et al, 1987;Pask et al, 1987;Segal, 1989;Tucker et al, 1990;Basu et al, 2006;Bhattacharyya and Ghatak, 2007;Kurajica et al, 2007).…”

Mechanical Properties and Thermal Shock Resistance of Alumina/Hexagonal Boron Nitride Composite Refractories

“…[6,7] h-BN containing composites are potential materials due to their excellent corrosion and thermal shock resistance (TSR), mechanical tolerance, and machinability. [8][9][10][11] For example, Si 3 N 4 -BN [12][13][14] and Sialon-BN [15] composites have already been used as special refractories nozzles, tubes, and break rings for the continuous casting of steel. However, the decomposition of Si 3 N 4 and Sialon by dissolution of silicon into molten steel restricted their wide application.…”