Microstructure and property of porous mullites with a whiskers framework obtained by a sol–gel process
Introduction
Porous mullites (3Al2O3·2SiO2) possess high temperature stability, high corrosion resistance, chemical stability and a low thermal expansion coefficient at elevated temperatures [1], [2]. Consequently, the applications of these materials include high-temperature sealing, hot-gas separation and diesel particulate filters [3], [4], [5]. High porosities are often required for thermal insulation and sufficient permeability in these applications [4]. However, low strengths at the high porosities limit practical applications [6].
Whiskers as a reinforcement or framework in porous mullite ceramics have attracted extensive attentions to improve the strength of the porous mullites because of their nearly perfect structure and high strength. Zhu [7] and Li [8] prepared porous mullites with whiskers as reinforcement; mechanical strengths of 81.2±3.2 and 100 MPa were obtained at porosities of only 48.6±0.5% and 50% due to the sintering densification of equiaxed particles, respectively. As the whiskers as reinforcement can’t retain high porosities during the sintering of the equiaxed particles, whiskers frameworks gradually attracted more attentions due to their lap-joint structure with a high porosity. Liu [6] produced a mullite fiber/whiskers framework with a porosity of approximately 82% by slurry-filtration and heat-treating; the product had a compressive strength of approximately 1 MPa due to the presence of polycrystalline fibers. Li [9] synthesized a framework of single crystalline aluminum borate whiskers, which resulted in a porosity of 85% and a flexural strength of 2.2 MPa. Xu [10], [11] prepared a mullite whiskers framework in porous ceramics with a compressive strength of 4.98 MPa at a porosity of 83%. Although the high strength was obtained at the high porosity, the weak bonding by solid sintering limited the advantage of the mullite whisker framework to improve its strength. Hence, the fabrication of a whiskers framework with a strong bond is meaningful to enhance the strength of porous mullites.
Vapor–solid methods can enhance whisker growth [12], [13], and several methods have been reported for the preparation of mullite whiskers. Choi [14] synthesized mullite whiskers by heating a mixture of SiO2 and silicon in an alumina tube reactor under a flow of H2 and CF4. Okada [15], [16] synthesized mullite whiskers by firing AlF3 as a catalyst and using tetraethoxysilane (TEOS) and Al(NO3)3·9H2O as the starting materials in an airtight container. In these methods, Al2O3 and SiO2 are converted to AlOF and SiF4 vapors catalyzed by F ions, followed by the formation of intermediates Al2SiO4Fx(OH)2−x(topaz, x=0–1) with an acicular morphology. Preferential growth of mullite whiskers along the (001) direction finally occurs with release of F. Zhu [7] and Li [8] fabricated porous mullites with whiskers as reinforcement with the addition of AlF3 as a whisker-forming agent. However, no one fabricated porous mullites with a whiskers framework by Okada's method.
In order to form high porosities of mullites, sol–gel methods have been widely used due to the gel network structure with a high porosity. Guo [17] and Ding [18] prepared porous mullites with a 42% porosity by sol–gel and a 92.9% porosity by gel freeze drying, respectively. Compared with adding pore-forming agent [11], foaming method [19] and so on, the sol–gel method in this paper removes the process to burn out sacrificing templates because TEOS as SiO2 source could allow green bodies form a high porosity through its intrinsic sol–gel transition. As cracks often occur during gel drying, freeze [18] and supercritical drying [20] were used to solve it. But, the dependence on equipments and freeze intermediates limits their operations to a certain extent. A method by gel slowly drying could simplify the operation and also avoid obvious cracks via reducing the evaporation rate of solvents, where the shrinkage of gel networks could compensate the cracks [21]. To our best knowledge, there are few reports that gel slowly drying was employed to fabricate porous mullites.
In this paper, we report a novel approach to fabricate porous mullites with a whiskers framework by the vapor–solid reaction sintering of a slowly dried aerogel block shaped by the SiO2 sol with nanosized α-Al2O3 and AlF3 powders. The effect of heating temperatures on porosity, whisker formation, microstructure, compressive strength and fracture morphology was characterized and are discussed.
Section snippets
Raw materials and process
Porous mullite ceramics were prepared by a sol–gel process, as shown in Fig. 1. An ethanol slurry of 2 mol/L distilled H2O and 0.5 mol/L TEOS (AR, Sinopharm Chemical Reagent, China) was stirred for 3 days to form a SiO2 sol. α-Al2O3 (20 nm, 99% purity, Chaowei Technology Co. Ltd., China) and AlF3 powders (AR, Sinopharm Chemical Reagent, China) were used as sources of Al and a whisker-forming agent, respectively. Another ethanol slurry containing 0.6 mol/L Al2O3, 0.3 mol/L AlF3 and 1 mol/L NH3·H2O was
Results and discussion
The variations of the weight loss, linear shrinkage and porosity of the samples before and after heat treatment are presented in Table 1. Due to the low solid content (6.6%) of the wet gel precursor, the green body occupied a high porosity of 76.2% and a low density of 0.64 g/cm3. The weight loss after heat treatment was very high due to the presence of AlF3, and the formation of mullite was accompanied by the vapors leaking of AlF3, AlOF and SiF4. Weight losses of 28.4% and 28.9% were measured
Conclusions
The effect of heating temperatures on the fabrication of porous mullite ceramics with AlF3 addition was studied. The in situ formation of the mullite whiskers from fluoride was the result of the vapor–solid reaction, which facilitated the formation of whiskers at the relatively low temperatures of 1100 °C. Porosities of 80.2–84.1% for the porous mullites with a whiskers framework were obtained at 1100–1600 °C. The whiskers growth and their bonding were enhanced by increasing heating temperatures,
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant nos. 51272205, 51302208), by the Doctoral Fund of the Ministry of Education of China (Grant no. 20130201120003), and by the Central University Fund Project (Grant no. 2014G1251026).
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