Characterization of aluminum hydroxide (Al(OH)3) for use as a porogenic agent in castable ceramics

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

“…1) Al(OH) 3 particles are consolidated with the Al 2 O 3 (D-Al 2 O 3 , from calcined alumina, for instance) ones by means of pressing or use of a binding agent. 2) During dehydroxilation, Al(OH) 3 shrinks and decomposes into transition phases (F-Al 2 O 3 and N-Al 2 O 3 , for example), which generates intense cracking on the particles' surfaces [10][11][12][13][14]. The combination of these effects can generate porosity levels higher than 70 % [5,9], however, when the temperature reaches 1100-1200ºC, the transition Al 2 O 3 particles promote intense sintering with the D-Al 2 O 3 matrix, which reduces the total porosity levels at the same time the average pore size increases significantly [9,14].…”

“…2) During dehydroxilation, Al(OH) 3 shrinks and decomposes into transition phases (F-Al 2 O 3 and N-Al 2 O 3 , for example), which generates intense cracking on the particles' surfaces [10][11][12][13][14]. The combination of these effects can generate porosity levels higher than 70 % [5,9], however, when the temperature reaches 1100-1200ºC, the transition Al 2 O 3 particles promote intense sintering with the D-Al 2 O 3 matrix, which reduces the total porosity levels at the same time the average pore size increases significantly [9,14]. Consequences of this behavior involve reduction in permeability, mechanical strength, thermal shock resistance, and structure's ability for thermal insulation [2,5,9,14].…”

“…The combination of these effects can generate porosity levels higher than 70 % [5,9], however, when the temperature reaches 1100-1200ºC, the transition Al 2 O 3 particles promote intense sintering with the D-Al 2 O 3 matrix, which reduces the total porosity levels at the same time the average pore size increases significantly [9,14]. Consequences of this behavior involve reduction in permeability, mechanical strength, thermal shock resistance, and structure's ability for thermal insulation [2,5,9,14]. To overcome such drawbacks, several authors have proposed the use of in situ solid state reactions that generate large contents of pores and low density products concomitantly (to promote volumetric expansion and avoid the Al 2 O 3 matrix densification) [2].…”

“…At the first stages of heating and depending on the raw materials employed, intermediate reactions may take place and affect the mullite formation. For example, Al(OH) 3 , boehmite and kaolinite decomposition (200-450ºC) [11,14,29,33] and quartz allotropic change (520ºC) [16,17,31] generate pores and cracks in the structure, which increase the average interparticle space. The crystallization of amorphous silica into tridymit and cristobalite (800-1000ºC) reduces the interdifusion rate of Al 3+ , Si 4+ and O -2 ions and requires higher temperatures and longer periods for a complete reaction [31].…”

Structure and properties of porous ceramics obtained from aluminum hydroxide

“…The traditional method of creating highly porous materials is based on "burn-pore-forming" additives [3]. On the one hand this method is effective in terms of management of the pore volume, but on the other hand the possible formation of carbon on the inner surfaces of porous material is unacceptable for the manufacture of bio-implants and can lead to rejection of the implant.…”