High‐Surface‐Area Alumina Ceramics Fabricated by the Decomposition of Al(OH)₃

Abstract

Al 2 O 3 -based porous ceramics with high surface areas were fabricated by adding Al(OH) 3 to the starting powder, followed by pressureless sintering at temperatures >1100°C. Three types of starting powders were used in the present study: pure (OH) 3 . The addition of Al(OH) 3 considerably increased the surface area of the porous Al 2 O 3 , and the addition of ZrO 2 further increased the surface area; a surface area as high as 36.39 m 2 /g was obtained, and the high surface area was retained at higher temperature. The pore size distribution of the specimens with high surface area was bimodal, with one peak at ϳ100 nm and the other, which contributed most of the surface area, at ϳ10 nm. X-ray analysis showed that in the sample with the fine pores and high surface area, there was -Al 2 O 3 phase produced by the decomposition of Al(OH) 3 , presumably because the phase transformation of -Al 2 O 3 to ␣-Al 2 O 3 was incomplete after low-temperature sintering. Moreover, the porous Al 2 O 3 with high surface area retained superior mechanical properties, attributed to the good sinterability of the fine ␣-Al 2 O 3 powder used in the present study. The sintered specimens could be large and designed to any shape, because pressureless sintering was used for fabrication. The present approach provides a new way of fabricating porous Al 2 O 3 ceramics that could be widely used as catalyst supports in industry, especially for high-temperature catalysis.

Fabricating Porous Ceramics

“…The porous alumina-base catalyst carriers were generally prepared with the raw materials of g-Al 2 O 3 , d-Al 2 O 3 , and y-Al 2 O 3 transitional phases due to the larger surface areas compared to those of the stable a-Al 2 O 3 phase [61]. The porous alumina-base catalyst carrier is normally prepared by sintering the transitional Al 2 O 3 phases with the addition of a phase transformation inhibitor, or by the sol-gel method.…”

“…However, the obtained porous Al 2 O 3 takes the form of thin membranes, sheets, and small balls. The preparation of open-cell Al 2 O 3 -based porous ceramic products with higher specific surface areas have been described in a couple of papers [61,62]. Al(OH) 3 is added to the raw materials and then fired above 1,100 C without applying pressure.…”

Fabrication of Microporous Corundum-Spinel Refractory Castables by Foam-Gelcasting Methods

“…So far, Yan et al (Ref 7) have worked out a high-strength corundum-mullite ceramics which MgCO 3 is used as pore-forming agent. Deng et al (Ref 8,9) optimized pore structure to improve the mechanical properties of the porous alumina ceramic. However, it is too difficult to obtain great pore parameters in light-weight refractories by traditional methods such as direct sintering with addition of pore-forming agents (Ref 5, 10) and polymeric sponge impregnation process (Ref 11).…”

Desenvolvimento e caracterização de cerâmicas porosas moldáveis à base de alumina-mulita para uso como isolamento térmico em altas temperaturas

“…O compósito bifásico pode ser preparado por prensagem da mistura dos dois componentes ou através da formação de uma suspensão coloidal a qual é posteriormente processada por colagem ou moldagem direta. (DENG et al, 2001a;HONGO, 1988;MIŚTA;WRZYSZCZ, 1999;ZHOU;SNYDER, 1991 Figura 2.6 -Representação do método da decomposição de compostos inorgânicos hidroxilados para produção de cerâmicas porosas Fonte - Salomão (2012) No entanto, quando a temperatura de sinterização e/ou uso ultrapassa 1000ºC, há uma significativa redução de porosidade que ocorre mais intensamente nas amostras com maior teor de hidróxido de alumínio. Essa redução está associada à sequência de transformações de fases que acompanha a decomposição da bayerita (Al(OH) 3 ) (COBLE; KINGERY, 1956;MIŚTA;WRZYSZCZ, 1999;ZHOU;SNYDER, 1991): O aumento da porosidade (que pode ser alcançada através da redução da densidade verde, redução do tempo ou temperatura de sinterização) reduz significativamente a resistência da amostra e de outras propriedades mecânicas.…”

“…Isto se deve ao fato de que ao sofrer o aquecimento inicial o hidróxido de alumínio se decompõe formando aluminas de fases intermediárias e sofre uma contração volumétrica. Essa mudança volumétrica pode causar um colapso no empacotamento local e diminuir a conectividade das partículas, diminuindo assim sua densificação (AKLOUCHE; ACHOUR; TABET, 2008;DENG et al, 2001a;DONG et al, 2008). Para o material contendo uma composição intermediária de alumina e mulita eletrofundida (0,25 SMT) o que se verifica é um comportamento mais próximo ao da composição 0,4-SMT (composição estequiométrica para formação da mulita).…”