Development of densification-resistant castable porous structures from in situ mullite

Preparation and Characterization of Mullite-Alumina Structures Formed "In Situ" from Calcined Alumina and Different Grades of Synthetic Amorphous Silica

“…Comparing to the other SAS grades, MS particles have a much lower internal mesoporosity (IP = 10.7 %) and specific surface area. Due to this, its reduces the amount of voids amongst particles at the compacting step 1,6,8,48 . During the initial heating, no significant densification was observed up to 1000ºC (Fig.…”

“…1d) particles are spherical, dense and individualized and their broad particle size distribution ranges from 50 to 500 nm. They are obtained as a precipitated by-product during the production of elemental silicon and are widely used in civil construction for enhancing the packing of high performance concrete, and in refractories for the production of mullitebased bricks and castables 1,7,8,15,38,39,48,49 . After physical characterization, the SAS particles were combined with calcined alumina to produce in situ mullite dense parts.…”

“…Its technological importance suggests that it is important to understand how these different physical properties could affect mullite formation. Therefore, in this study, four grades of SAS 8,15,38,39 (Table 1, Fig. 1) were tested regarding their ability in generating dense structures of in situ mullite after a reaction with fine calcined alumina.…”

“…Its formation is a complex set of high-temperature (800-1400°C) solid state reactions [22][23][24] between silica (SiO 2 ) and alumina (Al 2 O 3 ) [25][26][27][28][29] based on the mutual diffusion of Al 3+ , Si 4+ and O -2 ions amongst particles [30][31][32][33] . Various studies have pointed out the characteristics of the silica source (crystallinity 35 , particle size distribution 25 , presence of impurities 26,27 ) and processing conditions (heating rate 28 , temperature of the thermal treatment 29 ) affect directly the yield, kinetics and morphology of the products formed 2,[7][8][9][10] . For instance, reactions between coarse alumina and micronized quartz particles (45 µm average size) generate porous mullite at temperatures above 1300ºC 9,28,31 , whereas the combination of finer nanoparticles of alumina and amorphous silica produced by alcoxi-based precursors reduced the range to 800-1100ºC and promote greater densification [35][36][37] .…”

“…Mullite (3Al 2 O 3 .2SiO 2 or Al 6 Si 2 O 13 ), one of the main raw materials for the ceramic industry [1][2][3][4] , is an input for the production of refractory materials 1,[5][6][7][8][9] , catalyst support 10,11 , electronic and optic devices [12][13][14] , thermal insulators 8,9,15 , hot gas filters (above 600°C) [16][17][18] and biological and biomedical scaffolds [19][20][21] . Its formation is a complex set of high-temperature (800-1400°C) solid state reactions [22][23][24] between silica (SiO 2 ) and alumina (Al 2 O 3 ) [25][26][27][28][29] based on the mutual diffusion of Al 3+ , Si 4+ and O -2 ions amongst particles [30][31][32][33] .…”