Porous mullite/alumina-layered composites with a graded porosity fabricated by camphene-based freeze casting

Abstract

A freeze casting technique was processed to fabricate porous mullite/alumina-layered composite with a gradient in porosity and pore size. In this work, a camphene/coal fly ash slurry system with an appropriate addition of Al2O3 was used. The pore channels with circular-shaped cross-sections were aligned along the solidification direction of molten camphene and connected with each other. The pore morphology was influenced by the starting solids loading and sintering temperature. The compressive strength of monolithic specimen greatly decreased when the porosity increased. However, the layered composites with a graded porosity exhibited improved compressive strength with minor decrease in porosity, probably due to the formation of residual compressive stress and relatively dense glass phase in the outer layer.

Freeze casting – A review of processing, microstructure and properties via the open data repository, FreezeCasting.net

“…Confidence intervals are shown in brackets and p values for regression models and coefficient values are expressed as one, two, three and four stars for p< 0.05, 0.01, 0.001, and 0.0001; ns indicates the value was not significant. [6, 49, 107, 109, 111, 112, 117, 120, 124, 148, 149, 184, 185, 187, 195-197, 199, 217-220, 224, 228, 239, 240, 245, 261, 282-286, 295, 296, 298, 299, 302, 304, 309-312, 316, 320, 324, 325, 328, 329, 336, 339, 341, 343-345, 353, 379, 384, 386, 387, 390, 392, 393, 396, 403, 405, 408, 414, 415, 425, 429, 432, 433, 446, 448 Green 221 73±90 42 [54, 155, 191, 223, 281, 287-289, 294, 297, 307, 314, 321, 390, 401, 417, 420, 423, 439-441, 445, 459 Sintered 30 31±67 8 [296,357,396,418,568,653,691,692] Honeycomb Green 13 3±3 3 [350,352,424,459,887,888] Sintered 39 18±24 10 [351, 429, 430, 433, 569, 593, 647] Cellular Green 15 16±29 8 [314,553,802,804,863] Sintered 19 50±63 25 [226,492,696,710,720,795] Equiaxed Green ---Sintered 8 95±11 98 [698,699] Table 7. Micromechanical model and regression parameters describing relationships between relative density and compressive strength of freeze-cast metals.…”

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

“…This liquid phase is conducive to the formation of secondary mullite and also has a positive effect on the sintering of the mullite. Moreover, the presence of Fe 2 O 3 , MgO, and CaO promotes the growth of the mullite crystals by reducing the liquid viscosity [100]. It should be noted that Ca 2+ and Mg 2+ ions can be replaced by the 4 NH  ions in the process of the ion exchange as well [45,101].…”

Synthesis of hierarchically porous mullite ceramics with improved thermal insulation via foam-gelcasting combined with pore former addition

“…separation/adsorption, catalysis, thermal insulation, energy storage and so on [1][2][3][4][5][6][7][8][9]. So far, various techniques have been developed to fabricate porous mullite ceramics, including sacrificial template [10], gelcasting [5,11], freeze casting [12,13], particle stacking [14], pore-former addition [15,16] and foam-gelcasting [17][18][19][20][21][22][23]. Ahmad et al [10] prepared bimodal porous mullite ceramics with 65.3-70% porosity and pore size ranges of 0.1-0.5 μm and 2-10 μm at 1600°C using the sacrificial template method.…”

“…On the other hand, Yuan et al [5] manufactured porous fibrous mullite ceramics with an apparent porosity of 61.1-71.7% and thermal conductivity of 0.378-0.467 W m −1 K −1 via a gelcasting process. By using fly ash and Al 2 O 3 as the main raw materials, Kim et al [13] synthesised porous mullite/aluminalayered ceramics with gradient porosity/pore size. In addition, by using 25 wt-% of carbon black as a pore former, Liu et al [16] fabricated a porous alumina ceramic with 63% porosity, an average pore size of 14 μm and thermal conductivity of 0.192 W m −1 K −1 .…”

“…separation/adsorption, catalysis, thermal insulation, energy storage and so on [1-9]. So far, various techniques have been developed to fabricate porous mullite ceramics, including sacrificial template [10], gelcasting [5,11], freeze casting [12,13], particle stacking [14], pore-former addition [15,16] and foam-gelcasting [1723]. Ahmad et al.…”