In-situ synthesis and textural evolution of the novel carbonaceous SiC/mullite aerogel via polymer-derived ceramics route
Introduction
Within all the different kinds of aerogels, carbon aerogels can maintain their mesoporous structures at elevated temperatures, therefore possess the most excellent thermal stability under inert atmosphere at over 2000 °C [1], [2], [3]. The excellent thermal stability and their low bulk density as well as the good thermal insulation property makes aerogels to be the most promising material in advanced industrial devices, space vehicles and hypersonic vehicles [4], [5], [6]. However, oxidation easily happens for carbon aerogels under oxidizing atmosphere at temperatures over 400 °C, thus their applications can be limited for sometimes. The most conventional solution to this problem is to add a coating with high emissivity on the surface of carbon based materials [7], however, it may cause damage to the mesoporous structures of carbon aerogels. Thus, some researchers have focus on fabricating carbon based composite aerogel to improve the oxidation resistance of carbon aerogel. One of the conventional used material is carbon based silicon carbide (SiC), because SiC is a kind of strong covalent composite, which possesses excellent properties such as low bulk density, good thermal shock resistance and retention of good mechanical strength under high temperatures [8]. In addition, some amount of amorphous silica inside the matrix can further increase the oxidation resistance of carbon aerogel, in considering that SiO2 is thermal stable under oxidation atmosphere. Therefore, Mohamad [9], [10] et al. developed a novel C/SiO2/SiC ternary aerogel based on novolac/silica hybrid hyperporous material. The results showed that with the addition of SiC, the starting temperature of oxidation process is improved for 26 °C, compared with the carbon aerogel. However, the oxidation improvement is not evident and the SiC formation temperature is as high as 1500 °C, which is some kind of high energy consumption. In addition, the SiO2/SiC aerogel derived after carbon combustion of the carbonaceous SiO2/SiC aerogel under air atmosphere is not easy to keep its original monolithic property, which also limited the applications of SiC/SiO2 aerogel.
Mullite is another kind of material with high melting temperature, good stability, lower oxygen permeability, good erosion resistance at high temperature atmospheres [11]. In addition, the thermal expansion of mullite (4.4–5.6×10−6/°C) is close to SiC (4.3–5.4×10−6/°C), and they have good chemical compatibility [12]. Therefore, the introduce of mullite may be a good way to decrease the formation temperature of SiC phase, as well as increasing the oxidation resistance of carbon based SiC aerogel. In addition, mullite formation may be also favorable to its monolithic property after carbon combustion due to the structural effect of mullite phase inside the material. Therefore, Kai [8] et al. fabricated a mullite/SiC oxidation protective coating for carbon/carbon composite using a hydrothermal electrophoretic method. Results showed that the mullite/SiC coating displayed an excellent oxidation resistance, which can protect carbon/carbon composite from oxidation with a much lower mass loss rate of only 4.89×10−4g/cm2 h after 1773 K for 322 h. Yang [13] et al. prepared a mullite/SiC coating on graphite via chemical vapor reaction. Results showed that the mass gain of the coated sample is 0.085% after oxidation at 1150 °C for 109 h and thermal shock cycling between 1150 °C and room temperature for 12 times. In addition, Omid [14] et al. also presented a mullite bonded porous SiC ceramics via a sol-gel assisted in situ reaction bonding and found that mullite bonding can be useful for lower sintering temperature of SiC ceramics. However, the studies by now are mainly focused on mullite/SiC composite coating with different techniques for further improvement of the oxidation resistance or mullite bonding SiC composite for decrease the sintering temperatures of SiC phase. Mullite formation is caused by alumina introduced and the silica layer resulting from SiC at elevated temperatures under oxidation atmosphere. The in-situ synthesis of mullite/SiC composite via CF/SiO2/AlOOH as the precursor have been seldomly reported so far as we know.
Herein, we presented a sol-gel method for in-situ synthesis of carbonaceous SiC/mullite aerogel via polymer-derived ceramics route (PDCR). Sol-gel method assures a better fine scale mixing and thus the homogenous structures. The SiC and mullite are both formed via the PDCR from catechol-formaldehyde/silica/alumina hybrid composite aerogel (CF/SiO2/AlOOH). Mullite is derived from the reaction of alumina and silica inside the composite material, while SiC is formed by carbothermal reduction process. The effects of the reactants concentrations on the physicochemical properties of the carbonaceous SiO2/Al2O3 aerogel and SiC/mullite aerogel are investigated. The mechanism of the textural and structural evolution for the novel carbonaceous SiC/mullite is also proposed based on the experimental results.
Section snippets
Synthesis
Catechol (C), formaldehyde (F), aluminum chloride hexahydrate (AlCl3·6H2O), anhydrous alcohol (EtOH), and 3-aminopropyltriethoxysilane (APTES) were used as raw materials. All of the reagents and solvents were analytical grade and used as received without further purification. The C and F were used as the carbon source while APTES was used as silicon source as well as the gelation agent. AlCl3·6H2O was involved as the alumina source for mullitization. EtOH was used as the solvent, which was used
Results and discussion
Fig. 1 shows the bulk densities and mass losses during carbonization under flowing argon atomosphere with differnet reactants concentrations. The reactants concentration is defined as the following equation:
It can be found that the bulk densities of the resulting CF/SiO2/AlOOH composite aerogel increase in direct proportion with the increase of reactants concentrations. The fitting curve can be calculated as B=0.678C-0.04. After
Conclusions
A novel carbonaceous SiC/mullite composite aerogel is derived from catechol-formaldehyde/silica/alumina hybrid aerogel (CF/SiO2/AlOOH) via polymer-derived ceramics route (PDCR). Reactants concentration of 25% is determined as the optimal condition in considering of the mullite formation and bulk densities of the preceramic aerogels. Spherical silica particles is also produced during mullitization process, and amorphous silica is remained after this reaction. With further heat treatment at 1400
Acknowledgments
This work was financially supported by the Industry Program of Science and Technology Support Project of Jiangsu Province (BE2014128), the Prospective Joint Research Program of Jiangsu Province (BY2015005-01), the Major Program of Natural Science Fund in Colleges and Universities of Jiangsu Province (15KJA430005), the Aeronautical Science Foundation of China (201452T4001), the Program for InnovativeResearch Team in University of Ministry of Education of China (No. IRT_15R35), Jiangsu
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These two authors contributed equally to this work and should be considered co-first authors.