Fabrication and characterization of mullite-whisker-reinforced lightweight porous materials with AlF3·3H2O

Open-cell mullite ceramic foams derived from porous geopolymer precursors with tailored porosity

“…℃ The porosity and mechanical characteristics of the mullite prepared using various pore-forming methods and sintered at about 1400 are compared in ℃ Table 3. We can observe that within the sintering temperature range of 1350-1450 , the compressive strength for ℃ the mullite ceramics (S4) obtained in this work was similar or even higher than those of other mullite foams at similar porosity levels [111][112][113][114][115][116][117][118][119], although higher strength than the one for the sample S4, even associated with higher porosity, can be also achieved [104,[120][121][122][123][124]. However, we can state that these mullite foams obtained from the porous geopolymers possessing high mechanical strength, high open porosity, and homogeneous microstructures are competitive at similar porosity levels and sintering temperatures because of using lower-cost raw materials [112,121], being manufactured using easier foaming steps [104,111,121] and not requiring additional sintering aids (SiC and B 4 C) [111,115,121] nor various types of mullite seeds (fiber [111,112,115,121], whisker [104], and powders [114,117,118,120,122,124]).…”

Low sintering shrinkage porous mullite ceramics with high strength and low thermal conductivity via foam‐gelcasting

“…The reason for this is that the difference in porosity between the two is not significant, but the addition of AlF 3 reacts with the abundant but lower strength SiO 2 to generate a strong mullite phase, which increases the compressive strength. At 30 wt%, the compressive strength increases slightly due to the generation of many whiskers, which form more stress points by lapping each other and dispersing the stress [28,30]. The thermal conductivity gradually decreases with increasing AlF 3 content, Comparison of the properties of porous mullite ceramics is listed in Table 2.…”

“…The thermal conductivity gradually decreases with increasing AlF 3 content, Comparison of the properties of porous mullite ceramics is listed in Table 2. 14,15,28,29,[38][39][40][41][42][43] It can be found that porous mullite ceramics prepared from sapphire with aluminum fluoride additives have relatively high porosity and have a low linear shrinkage rate. The compressive strength and thermal conductivity are not much different from those of other porous ceramics.…”

“…The researchers found that AlF 3 can be used as an additive that not only reacts with free silica to form alumina, which in turn continues to react to form mullite but also produces gaseous substances that catalyze the formation of mullite whiskers, achieving whisker enhancement and improving the performance of porous ceramics. [28][29][30][31][32][33] In this study, low sintering shrinkage porous mullite ceramics were prepared by the foam-gelcasting method using kyanite as raw material and AlF 3 as additive, through volume expansion from phase transition and gas generated from the reaction. The effects of AlF 3 addition on the shrinkage, phase composition, porosity, compressive strength, and thermal conductivity of porous mullite ceramics were investigated (if not specified, shrinkage in the text indicates sintering shrinkage).…”

“…The free silica generated by decomposition will have an impact on the performance of porous ceramics, so an aluminum source can be introduced to further react with silica to generate mullite, reduce the impurity content, and enhance the ceramic performance. The researchers found that AlF 3 can be used as an additive that not only reacts with free silica to form alumina, which in turn continues to react to form mullite but also produces gaseous substances that catalyze the formation of mullite whiskers, achieving whisker enhancement and improving the performance of porous ceramics 28–33 …”