Synthesis of a novel Al2O3–SiO2 composite aerogel with high specific surface area at elevated temperatures using inexpensive inorganic salt of aluminum
Introduction
Alumina aerogels are nano-porous solid materials with low bulk densities, low thermal conductivities and amorphous network structures. Due to their high porosity and large specific surface area at elevated temperatures, they have broad application prospects in advanced aircraft, spacecraft and catalyst carriers at high temperatures [1]. However, the main problem for alumina aerogel is their high thermal conductivity compared with that of silica aerogel [2], [3], their limited heat resistance and a drastic decrease in the specific surface area after heat treatment at temperatures over 1000 °C due to phase transitions of alumina part. Therefore, maintaining the nanostructures and improving the specific surface areas at elevated temperatures are the main focus of alumina research. Two possible methods were reported for the suppression of the alumina phase transition: the first method involves the addition of other elements, such as Si, La, Ba, or Zr [4], [5], [6], whereas the second method involves a decrease in the bulk density of the alumina powders [7]. Thus, many researchers have focused on Al2O3–SiO2 aerogel with a significant low bulk density and a silica-doped effect to obtain a composite with high heat resistance and a large specific surface area at elevated temperatures. Osaki et al. [8], [9], [10] proposed a method using aluminum isopropoxide as alumina source and tetraethyl orthosilicate as the silicon source to prepare Al2O3–SiO2 aerogel. The addition of silica particles surrounding the γ-Al2O3 inhibited the contact of Al2O3 particles, decreased the probability of crystal nucleation, and significantly inhibited the α-Al2O3 phase transition. It possessed a large specific surface area of 47 m2/g at 1200 °C; however, the alumina alkoxides were very expensive and were readily hydrolyzed by water, which produced aluminum mono- or tri-hydroxides. Because the formed mono-hydroxide is the only material that can be peptized to a clear sol, a highly complex procedure is needed to obtain a clear solution [8], [9], [10]. Hurwitz et al. [11] directly employed boehmite powder as an alumina source to prepare Al2O3–SiO2 aerogel. The specific surface area was as high as 33 m2/g at 1300 °C, whereas the phase transition of γ-Al2O3/η-Al2O3 occurred as early as 600 °C. The boehmite powder had to be dispersed in 0.09 M nitric acid solution or water and sonicated using an ultrasonic processor, which was also a complex procedure.
The objective of this study is to develop a new and convenient sol–gel method that facilitates the fabrication of Al2O3–SiO2 composite aerogel, which possesses a high specific surface area at elevated temperatures. The monolithic Al2O3–SiO2 composite aerogel was prepared by simply mixing the silica sol and alumina sol in one pot, followed by gel, aging, solvent exchange and C2H5OH supercritical drying. Al2O3–SiO2 sols were synthesized using cheap inexpensive inorganic salt (AlCl3·6H2O) and tetraethoxysilane (TEOS) as precursors, ethanol (EtOH) as a solvent, and propylene oxide (PO) as a network forming agent; no catalyst was employed in the total procedure. Details of the synthesis and discussions of Al2O3–SiO2 composite aerogel, which was heat-treated at different temperatures, are provided in the next section.
Section snippets
Synthesis
Al2O3–SiO2 sols were prepared according to the following three steps. Firstly, appropriate amount of C2H5OH (17 ml, 0.3 mol) and H2O (15.2 ml, 0.86 mol) were mixed, and then AlCl3·6H2O (4.83 g, 0.02 mol) was dissolved in the mixture, stirring for about 30 min for complete hydrolysis. Secondly, TEOS (0.56 ml, 0.0025 mol) was added to a mixture composing of H2O (0.18 ml, 0.01 mol) and C2H5OH (2.32 ml, 0.04 mol). The mixed solution was continuously stirred for about 90 min at 50 °C and then cooled to room
Results and discussion
Fig. 1 exhibits the Macrograph and schematic representation of the growth mechanism of Al2O3–SiO2 composite aerogel via C2H5OH supercritical drying. The as-dried aerogel is crack-free and translucent, and the minimum density is 0.053 g/cm3. The theoretical density can be calculated as the following equation:where , are the masses of the boehmite and the SiO2 aerogel part and , , , are the volumes of EtOH, TEOS, H2
Conclusions
A new, safe and straightforward sol–gel method for the production of Al2O3–SiO2 composite aerogel is investigated. As discussed, SiO2 is amorphous and Al2O3 primarily exists as polycrystalline boehmite, which consists of fibrous particles and weblike microstructures. As the heat treatment temperature increases to 600 °C, structural transition from boehmite to γ-Al2O3 occurs within the composite. When the composite is heat-treated at 1100 °C, SiO2 crystallization occurs and the mullite phase
Acknowledgments
This work was supported by the Priority Academic Program Development of Jiangsu Higher Education Institution (PAPD), the Program for Changjiang Scholars and Innovative Research Team in University (IRT1146), Jiangsu Planned Projects for Postdoctoral Research Funds (1402016A) and the State key Laboratory of Materials-Oriented Chemical Engineering (No. KL11-09). Any opinions, findings, and conclusions or recommendations expressed in this paper are those of the authors and do not necessarily
References (27)
- et al.
Preparation and characterization of monolithic alumina aerogels
J. Non-Cryst. Solids
(2011) - et al.
Synthesis of high porosity, monolithic alumina aerogels
J. Non-Cryst. Solids
(2001) - et al.
Maintenance of large surface area of alumina heated at elevated temperatures above 1300 °C by preparing silica-containing pseudoboehmite aerogel
J. Non-Cryst. Solids
(2001) - et al.
Silica-doped alumina cryogels with high thermal stability
J. Non-Cryst. Solids.
(2007) - et al.
Infrared-opacified Al2O3–SiO2 aerogel composites reinforced by SiC-coated mullite fibers for thermal insulations
Ceram. Int.
(2015) - et al.
Synthesis of a novel porous material comprising carbon/alumina composite aerogels monoliths with high compressive strength
Micropor. Mesopor. Mater.
(2013) - et al.
Fabrication of mullite-bonded porous SiC ceramics via a sol–gel assisted in situ reaction bonding
J. Eur. Ceram. Soc.
(2014) - et al.
Synthesis and characterization of sol–gel derived ZrO2 doped Al2O3 nanopowder
Ceram. Int.
(2007) - et al.
Influences of heat-treatment on the microstructure and properties of silica-titania composite aerogels
J. Porous Mater.
(2014) - et al.
Improvement of thermal stability of alumina by addition of zirconia
Catal. Lett.
(1999)
Synthesis and characterization of pure and yttrium-containing alumina aerogels
J. Mater. Chem.
High surface area alumina aerogel at elevated temperatures
J. Chem. Soc. Faraday Trans.
Alumina gels that form porous transparent Al2O3
J. Mater. Sci.
Cited by (129)
Microstructural evolution of a novel TiN ceramic aerogel derived from the organic/inorganic hybrid with excellent anti-oxidation and thermal insulation property
2024, Journal of the European Ceramic SocietyNew strategy to further enhance the thermal insulation performance of silica aerogel: Decline the macropore content between skeletons
2024, Journal of Non-Crystalline SolidsPreparation, mechanical, acoustic and thermal properties of silica composite aerogel using wet-laid glass fiber felt as scaffold
2024, Composites Part A: Applied Science and ManufacturingAnalysis of heat transfer characteristics of sandwich cylindrical shell with an aerogel-metal-rubber composite core
2024, International Journal of Thermal Sciences