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Ultralight, thermal insulating, and high-temperature-resistant mullite-based nanofibrous aerogels
Chemical Engineering Journal 360 (2019) 464–472
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Ultralight, thermal insulating, and high-temperature-resistant mullite-based nanofibrous aerogels
T
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Ruili Liua, Xue Dongb, , Shuangtian Xiea, Tao Jiaa, Yunjia Xuea, Jiachen Liua, Wei Jingc, ⁎ Anran Guoa, a
School of Materials Science and Engineering, Key Lab of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, China b College of Aeronautical Engineering, Civil Aviation University of China, Tianjin 300300, China c Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China
H I GH L IG H T S
nanofibrous aerogels were firstly fabricated. • Mullite-based aerogels show a unique multilevel pore structure. • The aerogels exhibit an ultralow density and ultralow thermal conductivity. • The • The aerogels show an excellent high-temperature thermal stability.
A R T I C LE I N FO
A B S T R A C T
Keywords: Aerogel Mullite fiber Thermal insulation High temperature resistance
The fabrication of insulation materials with ultralow thermal conductivity and excellent thermal stability at high temperatures (higher than 1200 °C) has remained an extremely challenging. In this study, we reported the manufacturing of mullite-based nanofibrous aerogels via the gel-casting and freeze drying methods using the electrospun nanofibers with different alumina/silica molar ratios (3:2, 3:1 and 3:0) as the matrix and silica sols as the high temperature binders. The formation process of the mullite-based fibrous aerogel and effect of aerogel composition on the sample physical and mechanical properties were investigated. All mullite-based nanofibrous aerogel show a similar multilevel pore structure. The minor pores were formed by the overlaps of nanofibers and were the fundamental porous structure of the aerogel, while the major pores was caused by the sublimation of the ice crystal. This unique multilevel pore structure make the mullite-based nanofibrous aerogels exhibit an ultralow density (34.64–48.89 mg/cm3) and low thermal conductivity (0.03274–0.04317 Wm−1 K−1) although the sintering temperature was as high as 1400 °C much higher than the service temperature of the traditional nanoparticle aerogel. In addition, besides controlling the fabrication parameters, the physical and mechanical properties could be also tuned by adjusting the composition of the nanofibers. The research of this work provides a new insight into the development of high efficient thermal insulation materials used at high temperatures.
1. Introduction Traditional aerogels usually refer to solid materials with nanoporous network structure which was composed of nanoparticles conglomerating together and are well known for their ultralow density, ultralow thermal conductivity and ultrahigh porosity, playing a key role in thermal insulation and preservation systems of various aircrafts, missiles and furnaces [1,2]. However, the 3D nanoporous microstructure of the traditional aerogel would collapse at high temperatures
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due to the ultrahigh activity of nanoparticals, and therefore the service temperature of the traditional aerogels are often below 1200 °C in air atmosphere [3,4]. Although researchers have tried to use ceramic nanoparticals with excellent thermal stability as the raw materials to prepare other ceramic aerogels, such as ZrO2 aerogels and Al2O3 aerogels, the thermal stability of these aerogels didn’t improve dramatically due to the limitation of their intrinsic nanoparticle-based nanoporous structure [5,6]. Recently, a new kind of aerogel called nanofibrous aerogel was
Corresponding authors. E-mail addresses: [email protected] (X. Dong), [email protected] (A. Guo).
https://doi.org/10.1016/j.cej.2018.12.018 Received 28 August 2018; Received in revised form 30 November 2018; Accepted 4 December 2018 Available online 05 December 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.
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because this kind of precursor exhibited an excellent spinnability. However, the crystallization temperature of metal alkoxide is usually quite low because the elements existing in the starting material are mixed at the atomic level. For example, many groups [28–31] have successfully fabricated nanoscale mullite fibers by electrospinning a monophasic mullite sol. However, the results suggested that when the sintering temperature increased to higher than 1200 °C, the mullite grain size in the fibers would increase obviously, resulting in a degradation of the fiber mechanical property. Our group recently reported the fabrication of mullite nanofibers by electrospinning a diphasic mullite precursor [32] and the corresponding mullite nanofibers exhibited an excellent high temperature resistance and was a promising starting material for the fabrication of mullite nanofibrous aerogel. In this work, the thermal stability mullite-based fibers were firstly fabricated from the diphasic mullite precursor and then were used as the starting materials to construct the nanofibrous aerogels via the gelcasting and freeze drying methods. The formation process of the mullite-based fibrous aerogel was investigated and the effect of aerogel composition on the sample physical and mechanical properties was discussed.
successfully fabricated with nanofibers as the main starting materials [7–9]. Different from the nanoparticle-based porous structure of the traditional aerogel, the three-dimensional network structure of the nanofibrous aerogel is constructed by the overlapping and entangling of many nanofibers. In the beginning, the fabrication of nanofibrous aerogel mainly focused on the functional materials, such as carbon nanotube aerogels [10,11], carbon nanofiber aerogel [12,13] and carbon microbelt aerogel [14] and few studies focused on its thermal insulation properties. In 2014, Ding’s group [15] firstly suggested the possibility of using electrospun fibers as the starting material to fabricate the nanofibrous aerogel, which largely broaden the category of the nanofibrous aerogels. The obtained PAN-based nanofibrous aerogel exhibited an extremely low density of 0.12 mg/cm3 and low thermal conductivity of 0.026 Wm−1 K−1. However, since the aerogel contained a large number of polymer fibers, the aerogel can be only used below 600 °C. Then his group [16] successfully fabricated the SiO2 nanofibrous aerogel with ultralow density (10 mg/cm3) and low thermal conductivity (0.025 Wm−1 K−1) and demonstrated that the sample shows an excellent flexibility at 1100 °C. However, a clear fusion was observed after the sample was sintered at 1200 °C due to dramatically growth of the silica crystal in nanofibers. Therefore, this kind of aerogels can’t also be used at 1200 °C. At the same time, Wu’s group [17] successfully prepared a ZrO2 nanofiber sponge with ultralow density (15 mg/cm3) and low thermal conductivity (0.027 Wm−1 K−1) and suggested that the sample could be used as high as 1300 °C. However, as described in its experimental section, the sintering temperature of the ZrO2 sponge was only 800 °C and the thermal stability of the sample was tested by the methane flame. This characterization is quite unreasonable. At high temperature, the nanocrystal in ceramic fibers would grow rapidly, resulting in the serious degradation of the fiber mechanical strength, finally leading to collapse of the nanofibrous aerogel. This is the main reason why the nanofibrous aerogel couldn’t withstand the high temperature. Therefore, in order to investigate the thermal stability of the aerogel, the sample should be sintered at a certain temperature for several hours in the furnace rather than just using a methane flame to test the sample for only several minutes because the testing time of the methane flame was too short for the growth of ceramic crystal. Therefore, it is difficult to demonstrate the ZrO2 sponge could be used at 1300 °C for a long time. The above-mentioned researches fully demonstrate that the nanofibrous aerogels exhibit similar ultralow density and ultralow thermal conductivity with the traditional solid nanopartical aerogels; however the service temperature of the ceramic nanofibrous aerogel is still too low due to the crystal growth of the ceramic fiber at high temperature. Therefore, it can be speculated that if the nanofibers used was changed to a ceramic nanofibers with excellent thermal stability, the resultant aerogel would exhibit both low thermal conductivity and excellent high-temperature stability. Porous mullite fibrous ceramics, which is constructed by the commercially mullite fibers and exhibits low density, low thermal conductivity and excellent thermal stability [18–22], have been widely used as the high temperature thermal insulation materials in the insulation systems of space shuttles and high-temperature furnaces and the service temperature of this kind of thermal insulation materials could reach as high as 1500 °C in air. However, the density and thermal conductivity of the porous mullite fibrous ceramics is still much higher than that of the traditional solid nanopartical aerogel because the diameter of mullite fiber used is quite large about 10–20 μm consequently resulting in a thick and heavy “scaffold” of the fibrous ceramics. Therefore, it is conceivable that if the diameter of the mullite fiber used could decrease to nanoscale size, this kind of porous mullite nanofibrous ceramic would become the above mentioned mullite nanofibrous aerogels. In recent years, electrospinning is a kind of emerging technology for preparing nanoscale fibers [23–27]. Almost all the ceramic nanofibers were fabricated using the metal alkoxide as the staring materials
2. Materials and methods 2.1. Fabrication process of mullite-based nanofibers Mullite-based (or alumina) nanofibers were prepared by electrospinning a diphasic mullite precursor with different molar ratios of alumina to silica. In a typical fabrication process of the mullite nanofibers with the alumina/silica molar ratio of 3:2, 2.1 g of aluminum trisec-butoxide, 0.1692 g of polyhydromethylsiloxane were dissolved in the mixture of 10 g of isopropanol, 1 g of N,N-dimethylformamide and 1.5 g of ethylacetoacetate by vigorous stirring for 30 min. Then 1.53 g of polyvinylpyrrolidone powders (PVP-K90, Mw = 1300000) was added into the solution under continuous stirring until the PVP completely dissolved into the solution. The electrospinning apparatus was set up horizontally and the fibers were electrospun under an applied electric field, generated using a high voltage power supply. The feeding rate was set at 0.5 ml/h, the working distance was 12 cm, and the working voltage was 12 kV. After the electrospinning process, the as-spun mullite precursor fibers were cross-linked at 200 °C for 1 h in air to make them infusible and then sintered at different temperatures from 800 °C to 1500 °C with a heating rate of 2 °C/min. The mullite-based nanofiber with the alumina/silica molar ratio of 3:1 and the pure alumina nanofiber were also fabricated with the same method. 2.2. Fabrication process of mullite nanofiber-based aerogel Fig. 1 shows the typical fabrication process of the mullite-based nanofibrous aerogel. Firstly, the as-spun precursor fibers were calcined at 800 °C to remove the organic component and were dispersed into individual fibers using a high speed homogenizer with the speed of 3000 r/min for 5 min. Then the dispersion was dried at 120 °C and short aluminum silicate fibers were obtained. During the gel-casting process, 0.05 g of short aluminum silicate fibers and 0.02 g of agar were dissolved in 2 ml of silica sols (30 wt%) in the mould at 90 °C; then the mould was frozen in a refrigerator for 12 h followed by a freeze drying for 10 h, consequently obtaining a high strength green body. Finally, the green samples were sintered at 1400 °C for 2 h with a heating rate of 2 °C/min in air. 2.3. Characterization The morphology of the sample was observed using a Hitachi S-4800 field-emission electron microscope. Fourier transform infrared spectroscopy (FT-IR) spectra of the fibers sintered at different temperatures ranging from room temperature to 1500 °C were taken using a Fourier 465
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Fig. 1. Fabrication process of the mullite-based nanofibrous aerogel.
3.1. Formation process of the mullite-based fibrous aerogel
transform infrared spectroscope (FTIR, TRNSOR27, Germany). Phase transformation processes of fibers and aerogels after heat treatment at different temperatures was investigated via a X-ray Diffraction (XRD, Rigaku D/Max-2500, Japan). The compression tests were examined using a universal testing machine. The bulk density of the aerogel was calculated by measuring the geometry volume and mass of the sample. The true density of sample was measured by the Micromeritics AccuPyc 1330 pycnometer, and the porosity of aerogel was calculated according to the bulk density and true density of aerogel. Thermogravimetry (TG) of a green body of the aerogel was carried out from room temperature to 1500 °C in air with a thermal analyzer (Netzsch Sta 449C, Germany). The pore size distribution of the aerogel was investigated by mercury intrusion porosimetry (AutoPore IV 9510, America). The sample thermal conductivity at 18 °C, 400 °C, 600 °C, 800 °C and 1000 °C were tested by a Hot Disk TPS2500s instrument. The samples used for the test were 13.7 mm in diameter and 6.5 mm in thickness. It should be mentioned that in general the thermal conductivity measurement for the insulation materials (the thermal conductivity lower than 0.1 Wm−1 K−1) can be guaranteed only by means of steady state methods, such as the heat flow method and guarded hot plate method; however these methods require a large size sample, which is quite difficult for us to prepare. Therefore, the thermal conductivity in this work was measured by the hot disk method, a kind of non-steady methods, which has been extensively adopted by other similar works [16,33]. The sample size and thickness have a great influence on the results and therefore the values should be used for relative comparison under the same measurement condition.
As shown in Fig. 1, the whole fabrication process was divided into two parts, the fabrication of electrospun nanofibers and the construction of the aerogel. In this section, the mullite-based nanofibrous aerogel fabricated using the ceramic fibers with the alumina/silica mole ratio of 3:1 as the starting materials was selected as the typical example to illustrate the formation process of the fibrous aerogel. 3.1.1. Characterization of mullite-based nanofibers As illustrated in the introduction section, the selection of suitable ceramic fibers is a key point of fabricating the high temperature resistant aerogel. At high temperature, the nanocrystal in ceramic fibers would grow rapidly, resulting in the serious degradation of the fiber mechanical strength, finally leading to collapse of the nanofibrous aerogel. This is the main reason why the nanofibrous aerogel couldn’t withstand the high temperature. Therefore, obtaining nanofibers with excellent high temperature resistance is the basis for preparing mullitebased nanofibrous aerogel. According to our previous research, in order to maintain the high thermal stability of the mullite fibers, the electrospinning mullite precursor should be the diphasic sol. Compared with our previous work [32], the silica source in this work was changed from a commercial solid silica resin to the liquid polymethylhydrogensiloxane (PHMS) because the PHMS was more compatible with the alumina source. Therefore, it is necessary to investigate the type of mullite precursor and the crystallization temperature of the mullite crystal. The main difference between the diphasic sol and monophasic sol is that the diphasic sol is composed of Al-O-Al and Si-O-Si linkages, while the monophasic sol is composed of Al-O-Si linkages. Fig. 2 shows the FTIR spectra of the electrospun mullite-based nanofibers with the alumina/silica molar ratio of 3:1. As shown in the FTIR spectra of the asspun fibers, peaks at 813 cm−1 and 1090 cm−1, which were assigned to the Al-O-Al and Si-O-Si linkages respectively, could be clearly observed and no peaks corresponding to the Al-O-Si linkage appeared, thus demonstrating that the precursor used is the diphasic sol. In addition, the peaks of C]O and CeN existing in PVP at 1670 cm−1 and 1290 cm−1 and the peaks at 3500 cm−1 and 2900 cm−1 assigned to the OH and CHn groups were observed in the green body, and then disappeared when the sintering temperature increased to 800 °C, indicating that the free water, adsorbed water and organics in the green body would burn off before 800 °C. Moreover, when the temperature increased to 1300 °C, peaks at 1180 cm−1 assigned to the Si-O-Al linkage was observed, suggesting that the mullite crystallization temperature was about 1300 °C.
3. Results and discussion Mullite is a series of minerals composed of aluminum silicates with the alumina and silica within a certain range and can also form composites with silica and alumina. Normally, the thermal insulation property of the composite improves gradually with the increase of the silica content and the thermal stability of the composite improves gradually with the increase of alumina content, which means the composite with high alumina content exhibits a better thermal stability, while the one with high silica content exhibits a better thermal insulation property. Therefore, in this work, three kinds of mullite-based nanofibrous aerogels with different composition were fabricated using different starting ceramic fibers (mullite nanofibers with the alumina/ silica mole ratio of 3:2, mullite-alumina composite nanofibers with the alumina/silica mole ratio of 3:1 and alumina nanofibers) to investigate the effect of aerogel composition on the sample properties. 466
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Fig. 2. FTIR spectra of as-spun mullite-based nanofibers with the alumina/silica molar ratio of 3:1 before and after sintering at different temperatures.
Fig. 4. SEM images of as-spun mullite-based nanofibers with the alumina/silica molar ratio of 3:1 before and after sintering at different temperatures. (a) Before sintering, (b) 800 °C, (c) 1400 °C, (d) 1500 °C.
a promising starting materials used to fabricate the mullite-based nanofibrous aerogels. 3.1.2. Characterization of the mullite-based nanofibrous aerogel After electrospinning, the resultant sample was a non-woven fiber mat rather than the individual fibers; therefore, in order to fabricate the nanofibrous aerogel, the obtained non-woven fiber mat should be dispersed into individual fibers. In this work, the fiber mat was dispersed by the high speed dispersion and it is found that the fiber composition would influence the length of the obtained individual fibers. Fibers sintered at high temperatures would become fragile and after the high speed dispersion the length of the fiber would become too short to construct the 3D skeleton structure; however, fibers sintered at low temperatures would dissolve into the water due to the existence of the organic component, consequently influencing the fiber morphology. Fig. S3 shows SEM images of the ceramic fibers after calcinated at different temperatures followed by the high speed dispersion treatment. When the calcination temperature was 600 °C, a clear fusion of the fiber was observed due to the existence of organic component in fibers. However, if the calcination temperature was 1000 °C, the fiber morphology could preserve but the fiber length was too short because the fiber became too fragile. Therefore, 800 °C was set as the best calcination temperature and the resultant fibers exhibited both an integrate fiber morphology and an acceptable length to diameter ratio. After calcination, the aluminum silicate fiber mat was dispersed into individual fibers using a high speed homogenizer and the length of the fiber was about 15 μm as shown in Fig. 5a. During the gel-casting process, the agar, short fibers and silica sols formed a uniform suspension at 90 °C and when the temperature of the suspension cooled to the room temperature, the agar would precipitate from the water, forming a polymeric network and the suspension became an elastic solid gel. Then the solid gel was frozen in a refrigerator followed by a freeze drying. The water existing in the gel would transform into the ice at low temperature and then converted into gases through the sublimation during the freeze drying process, leaving many large pores [15,16]. Fig. 5b and c show the SEM images of the green sample and it can be clearly seen that the sample exhibited a macroporous structure and each cell wall was constructed by the interlocked fibers connected by the agar. With the increase of sintering temperature, the agar decomposed completely and the silica sols transformed into the silica acting as the high temperature binders bonding the lapped short fibers. As shown in Fig. 5d and e, the mullite-based nanofibrous aerogel exhibited an obvious multilevel pore structure with a major pore size of approximately 10–100 μm and a minor pore size of about 0.5–10 μm. The major pores were generated by the sublimation of the ice crystal
Fig. 3. XRD patterns of mullite-based nanofibers with the alumina/silica molar ratio of 3:1 after sintering at different temperatures.
In order to further determine the mullite crystallization temperature, the phase transformation process of the electrospun fiber was studied by XRD analysis. As shown in Fig. 3, when the sintering temperature was lower than 1000 °C, no crystal phase was formed in the fiber. However, when the calcination temperature was 1100 °C, two weak characteristic peaks of the γ-alumina (PDF#75-0921) at about 46° and 73° were observed. After sintering at 1300 °C, the characteristic peak corresponding to mullite phase (PDF#79-1275) appeared which further verified the conclusion that the mullite formation temperature was about 1300 °C. As the temperature continues to rise to 1500 °C, the γ-alumina transformed into corundum (PDF#74-1081) and the mullitebased fiber was finally composed of mullite and alumina because the mullite precursor contains an excessive amount of aluminum source. Fig. 4 shows the SEM images of as-spun mullite-based nanofibers before and after sintering at different temperatures. It can be seen from Fig. 4 that the fiber diameter gradually decreased with the increase of sintering temperature from room temperature to 1400 °C, which was mainly caused by the pyrolysis of organics in the green body and the ceramic densification. The average diameters of the green body and fibers after sintering at 800 °C and 1400 °C were about 750 nm, 460 nm and 410 nm, respectively. However, the average fiber diameter (450 nm) after sintering at 1500 °C is a little higher than that of fibers sintered at 1400 °C due to significantly growth of the mullite crystal on the fiber surface [34]. In addition, the surface of the fiber became rougher with the continuous rise of the sintering temperature which was resulted from the crystallization process of the mullite. It should be mentioned that the mullite-alumina composite fibers sintered 1400 °C still shows a smooth surface suggesting that this kind of ceramic fiber is 467
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Fig. 5. SEM images of (a) the individual nanofibers after mechanical dispersion, (b, c) green body, and (d, e) mullite-based nanofibrous aerogel sintered at 1400 °C. (f) Photograph of the mullite-based nanofibrous aerogel.
and the minor pores mainly originated from the overlapping of the ceramic fibers in aerogels. Fig. 5f shows the photograph of the fibrous aerogel after sintering at 1400 °C and the sample could stand on the plastic fluff due to the ultralow density. Fig. 6 shows the TG and DSC curves of the green body from room temperature to 1500 °C. The first weight loss of the sample occurring between 25 °C and 210 °C was about 7.2% resulted from the removal of the free water. The second weight loss (210–680 °C) about 22.62% was caused by the decomposition of the agar and silica sol. The obvious exothermic peak at about 1000 °C on the DSC curve was resulted from the phase transformation, that is, the formation of mullite phase. The XRD patterns of the green body and samples sintered at different temperatures were shown in Fig. 7. A broad background which indicates the existence of the amorphous SiO2 was observed in the green body. When the sintering temperature increased to 1000 °C, the pattern of the sintered body was composed of mullite characteristic peaks and alumina characteristic peaks. When the temperature was above 1000 °C, the alumina characteristic peaks gradually disappeared from the patterns, which demonstrated that the excessive aluminum source existing in the fibers could react with the silica derived from the silica sol, forming the mullite phase. Therefore, due to the introduction of additional silica source (silica sol), the final composition of the aerogel was the mullite and silica phase rather than the mullite and alumina phase as analyzed from Fig. 3. As shown in Figs. 8 and 9a, the shrinkage of the green sample after freeze drying was only 1.61% and this ultralow shrinkage during the
Fig. 7. XRD patterns of the green body before and after sintering at different temperatures.
Fig. 8. Variation of the linear shrinkage and density of the nanofibrous aerogel with the increasing of sintering temperature.
freezing drying process ensured the high porosity of the green sample. In addition, the strength of the green sample could reach to 15 KPa due to the formation of agar polymeric network although the sample density was only 34.64 mg/cm3, and this high porous green sample can be machined easily due to its high strength. The linear shrinkage and density of sample increased with the increasing of sintering
Fig. 6. TG and DSC curves of the green body of the mullite-based nanofibrous aerogel. 468
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Fig. 9. (a) Compressive strength and thermal conductivity of the nanofibrous aerogel sintered at different temperatures; (b) Thermal conductivity of the mullitebased nanofibrous aerogel at different test temperatures.
diphasic sol and the organic component disappeared when the sintering temperature increased to 800 °C. The peak at 1180 cm−1 belonging to the Si-O-Al linkage was observed for the first time in fibers sintered at 1000 °C, suggesting that the mullite phase was formed at about 1000 °C. The XRD analysis (Fig. 10b) shows that the characteristic peaks of the mullite phase (JCPDS#15-0776) firstly appeared when the calcination temperature reached 1000 °C, further indicating that the mullite crystallization temperature was about 1000 °C. Besides, as the temperature continued to increase, the mullite characteristic peak became sharper and only mullite phase existed in fibers at last. The FTIR spectra and XRD pattern of as-spun alumina nanofibers before and after sintering at different temperatures are shown in Fig. 10c and d, respectively. Results indicate that the pure alumina phase formed at 1100 °C. SEM images of the mullite fibers and alumina fibers after sintering at different temperatures were shown in Figs. S1 and S2 in the supplementary materials and shows that the resultant fibers were quite uniform. XRD patterns of another two kinds of mullite-based nanofibrous aerogels after sintering at 1400 °C are shown in Fig. 11. For the aerogel fabricated with the alumina fibers (the molar ratio of alumina to silica was 3:0), the final fibrous aerogel was composed of mullite and alumina phase and no characteristic peaks of silica phase was observed, indicating that the silica sol used as the high temperature binder completely reacted with the aluminum source in the fibers forming the mullite phase. For the aerogel fabricated with the mullite fibers (the molar ratio of alumina to silica was 3:2), as expected the final phase of the aerogel were mullite and silica. In addition, the silica phase content in this sample was obvious much higher than he silica content in the aerogels fabricated using the fibers with the alumina/silica molar ratio of 3:1 (see Fig. 7). Fig. 12 shows SEM images of another two mullite-based nanofibrous aerogels after sintering at 1400 °C. It clearly shows that all mullitebased nanofibrous aerogels exhibited an obvious multilevel porous structure, just like the structure shown in Fig. 5d and e. Moreover, the crystallization of the fibrous aerogel fabricated using the nanofiber with the alumina/silica molar ratio of 3:2 was quite evident, and as the molar ratio of alumina to silica increased, the crystallization of the fibers become was not as evident as before. The mechanical and thermal conductivity of porous ceramics were determined by two things, the sample composition and the sample porous structure. Therefore, in order to analyze the effect of aerogel composition on the aerogel properties, three different aerogels with similar density were fabricated through adjusting the fabrication parameters, such as the solid content during the gel casting process and the addition amount of the agar. As shown in Table 1, three different mullite-based nanofibrous aerogels exhibited similar densities (about 48 mg/cm3) and porosities (about 98.9%). The Mercury intrusion porosimetry testing results (Fig. 13) indicates that the pore size distributions of three different mullite-based nanofibrous aerogels were quite similar, showing a multilevel porous structure with a minor pore size of
temperature due to the decomposition of the agar and the sintering densification of the sample. The compressive strength of the sample firstly decreased to 13.54 KPa when the sintering temperature was 1000 °C due to the decomposition of agar polymeric network and then increased gradually with the increase of sintering temperature resulting from the formation of silica phase acting as the high temperature binder between the interlocked fibers. The basic mechanisms of heat transfer are mainly divided into three types: conduction, convection and radiation. At room temperature, the radiative heat transfer and convective heat transfer could be negligible, and the conduction is the most important means of heat transfer. Therefore, as to the mullite-based nanofibrous aerogels, the thermal conductivity at room temperature was mainly determined by the conduction of solid content (ceramic fibers) and the gas content (pores). As the thermal conductivity of gas phase was much lower than that of solid phase, the gas content (porosity) of the sample was the main factor that influences the sample thermal conductivity. For the green sample, the pores formed by the overlaps of nanofibers were filled by the agar (Fig. 5b) so that the thermal conductivity of the green body was slightly larger than that of sample sintered at 1000 °C. When the sintering temperature increased from 1000 °C to 1400 °C, the thermal conductivity increased steadily because the shrinkage of the sample led to the decrease of the sample porosity. Fig. 9b shows the thermal conductivity of the mullite-based nanofibrous aerogel at different test temperatures. It can be seen that the thermal conductivity of the mullite-based nanofibrous aerogel increased with the increase of the test temperature, mainly due to the increasing thermal radiation of the gas at high temperature [35,36]. Moreover, when the test temperature increased to 1000 °C, the thermal conductivity of the aerogel was still quite low, about 0.07341 Wm−1 K−1. All these results indicates that the mullite-based fibrous aerogel fabricated using the as-spun fibers with the alumina/silica molar ratio of 3:1 after sintering at 1400 °C exhibited a relative high compressive strength (18.08 KPa), ultralow density (47.41 mg/cm3), ultralow roomtemperature thermal conductivity (0.0426 Wm−1 K−1) and ultralow thermal conductivity (0.07341 Wm−1 K−1) at high temperature (1000 °C), and was a promising high-temperature insulation materials used at high temperatures in an air atmosphere.
3.2. Effect of fiber composition on the properties of nanofibrous aerogel The above section provides a detailed introduction about the formation process and properties of the mullite-based nanofibers aerogel fabricated with the as-spun fibers with the alumina/silica molar ratio of 3:1 as the starting materials. In this section, aerogels were fabricated using another two kinds of nanofibers and the effect of aerogel composition on the sample properties were investigated. The FTIR spectrum of as-spun mullite nanofibers with the alumina/ silica molar ratio of 3:2 (Fig. 10a) indicates that the precursor is also a 469
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Fig. 10. FTIR spectra and XRD patterns of (a, b) as-spun mullite nanofibers with the alumina/silica molar ratio of 3:2, and (c, d) as-spun alumina nanofibers before and after sintering at different temperatures.
Fig. 12. SEM images of mullite-based nanofibrous aerogels fabricated using nanofibers with different alumina/silica molar ratios after sintering at 1400 °C. (a, b) 3:2 and (c, d) 3:0.
Fig. 11. XRD patterns of mullite-based nanofibrous aerogels fabricated using nanofibers with different alumina/silica molar ratios after sintering at 1400 °C.
0.5–10 μm and a major pore size of 5–100 μm. The minor pores were mainly formed by the overlaps of nanofibers, which was the fundamental porous structure of the aerogel, and the major pores were caused by the sublimation of the ice crystal. Apparently, this pore size distribution was consistent with the observation results from the corresponding SEM images (Figs. 5 and 12). Therefore, in case of the similar density, porosity and pore size distribution of the above three fibrous aerogels, the difference in the sample properties can be attributed to their different components. Normally, the mullite-based composite with high silica content shows a low thermal conductivity and the one with high alumina content exhibited an excellent high temperature stability and high strength. In this study, the composition of the aerogel is directly
determined by the composition of the starting materials and the XRD analyses also prove this conclusion that the alumina content in aerogel increased with the increase of the alumina/silica mole ratio of the electrospun nanofibers used. The thermal conductivity and compressive strength of the aerogels were illustrated in Table 1. All samples exhibited a low density (47.16–48.89 mg/cm3), low thermal conductivity (0.03812–0.04317 Wm−1 K−1) and relative high strength (14.70–18.77 kPa) even when the sintering temperature was as high as 1400 °C, which was much higher than the service temperature (lower than 1200 °C) of traditional nanoparticle aerogel. In addition, as expected the thermal conductivity decreased and compressive strength increased with the decrease of the alumina/silica molar ratio due to the increase of alumina content in the aerogel. Therefore, we can easily 470
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Table 1 Properties of mullite-based nanofibrous aerogels fabricated using nanofibers with different alumina/silica molar ratios after sintered at 1400 °C. Molar ratio of alumina to silica
Density (mg/cm3)
Porosity (%)
Thermal conductivity (W/mK)
Compressive strength (kPa)
3:2 3:1 3:0
47.67 ± 0.07 48.89 ± 0.08 47.16 ± 0.06
98.94 ± 0.01 98.96 ± 0.02 98.95 ± 0.01
0.03812 ± 0.00010 0.04036 ± 0.00020 0.04317 ± 0.00010
14.70 ± 0.04 17.79 ± 0.05 18.77 ± 0.06
Acknowledgement The authors would like to acknowledge the financial support of National Natural Science Foundation of China (Project No. 51502196 and Project No. 51872194). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2018.12.018. References [1] M. Koebel, A. Rigacci, P. Achard, Aerogel-based thermal superinsulation: an overview, J. Sol-Gel Sci. Technol. 63 (2012) 315–339. [2] R. Baetens, B.P. Jelle, A. Gustavsen, Aerogel insulation for building applications: a state-of-the-art review, Energy Build. 43 (2011) 761–769. [3] D. Huang, C. Guo, M. Zhang, L. Shi, Characteristics of nanoporous silica aerogel under high temperature from 950°C to 1200°C, Mater. Des. 129 (2017) 82–90. [4] J. Ma, F. Ye, C. Yang, J. Ding, S. Lin, B. Zhang, Q. Liu, Heat-resistant, strong alumina-modified silica aerogel fabricated by impregnating silicon oxycarbide aerogel with boehmite sol, Mater. Des. 131 (2017) 226–231. [5] Z. Zhao, A. Dairong Chen, X. Jiao, Zirconia aerogels with high surface area derived from sols prepared by electrolyzing zirconium oxychloride solution: comparison of aerogels prepared by freeze-drying and supercritical CO2(l) extraction, J. Phys. Chem. C 111 (2007) 18738–18743. [6] G.Q. Zu, J. Shen, X.Q. Wei, X.Y. Ni, Z.H. Zhang, J.C. Wang, G.W. Liu, Preparation and characterization of monolithic alumina aerogels, J. Non-Cryst. Solids 357 (2011) 2903–2906. [7] B. Sun, Y.Z. Long, F. Yu, M.M. Li, H.D. Zhang, W.J. Li, T.X. Xu, Self-assembly of a three-dimensional fibrous polymer sponge by electrospinning, Nanoscale 4 (2012) 2134–2137. [8] H.Y. Mi, X. Jing, B.N. Napiwocki, Z.T. Li, L.S. Turng, H.X. Huang, Fabrication of fibrous silica sponges by self-assembly electrospinning and their application in tissue engineering for three-dimensional tissue regeneration, Chem. Eng. J. 331 (2018) 652–662. [9] J. Xiao, W.Y. Lv, Y.H. Song, Q. Zheng, Graphene/nanofiber aerogels: performance regulation towards multiple applications in dye adsorption and oil/water separation, Chem. Eng. J. 338 (2018) 202–210. [10] M.B. Bryning, D.E. Milkie, M.F. Islam, L.A. Hough, J.M. Kikkawa, A.G. Yodh, Carbon nanotube aerogels, Adv. Mater. 19 (2007) 661–664. [11] C. Hoecker, F. Smail, M. Pick, A. Boies, The influence of carbon source and catalyst nanoparticles on CVD synthesis of CNT aerogel, Chem. Eng. J. 314 (2017) 388–395. [12] Z.Y. Wu, C. Li, H.W. Liang, J.F. Chen, S.H. Yu, Ultralight, flexible, and fire-resistant carbon nanofiber aerogels from bacterial cellulose, Angew. Chem. Int. Ed. Engl. 52 (2013) 2925–2929. [13] Z.Y. Wu, C. Li, H.W. Liang, Y.N. Zhang, X. Wang, J.F. Chen, S.H. Yu, Carbon nanofiber aerogels for emergent cleanup of oil spillage and chemical leakage under harsh conditions, Sci. Rep. 4 (2014) 4079. [14] H. Bi, X. Huang, X. Wu, X. Cao, C. Tan, Z. Yin, X. Lu, L. Sun, H. Zhang, Carbon microbelt aerogel prepared by waste paper: an efficient and recyclable sorbent for oils and organic solvents, Small 10 (2014) 3544–3550. [15] Y. Si, J. Yu, X. Tang, J. Ge, B. Ding, Ultralight nanofibre-assembled cellular aerogels with superelasticity and multifunctionality, Nat. Commun. 5 (2014) 5802. [16] Y. Si, X. Wang, L. Dou, J. Yu, B. Ding, Ultralight and fire-resistant ceramic nanofibrous aerogels with temperature-invariant superelasticity, Sci. Adv. 4 (2018) eaas8925. [17] H. Wang, X. Zhang, N. Wang, Y. Li, X. Feng, Y. Huang, C. Zhao, Z. Liu, M. Fang, G. Ou, H.J. Gao, X.Y. Li, H. Wu, Ultralight, scalable, and high-temperature-resilient ceramic nanofiber sponges, Sci. Adv. 3 (2017) e1603170. [18] H. Schneider, J. Schreuer, B. Hildmann, Structure and properties of mullite - a review, J. Eur. Ceram. Soc. 28 (2008) 329–344. [19] J. Zhang, X. Dong, F. Hou, H.Y. Du, J.C. Liu, A.R. Guo, Effects of fiber length and solid loading on the properties of lightweight elastic mullite fibrous ceramics, Ceram. Int. 42 (2016) 5018–5023. [20] X. Dong, G.F. Sui, J.C. Liu, S.E. Ren, M.C. Wang, H.Y. Du, Mechanical behavior of fibrous ceramics with a bird’s nest structure, Compos. Sci. Technol. 100 (2014) 92–98. [21] X. Dong, J.C. Liu, R.H. Hao, A.R. Guo, Z.G. Hou, M.X. Liu, High-temperature elasticity of fibrous ceramics with a bird's nest structure, J. Eur. Ceram. Soc. 33 (2013) 3477–3481.
Fig. 13. Pore size distributions of mullite-based nanofibrous aerogels fabricated using nanofibers with different alumina/silica molar ratios.
tune the properties of mullite-based aerogel by adjusting the composition of electrospun fibers. It should be mentioned that the whole synthesis process of this kind of aerogel was quite complicated, including electrospinning, fiber calcination, dispersion, gel casting, freeze-drying, and sintering. Considering the industrial feasibility, further work should be done to simplify this process. The electrospinning, aerogel forming process and sintering were the fundamental fabrication process and couldn’t be omitted; therefore efforts could be made to simplify the fiber calcination, dispersion, gel casting and freeze-drying into one process. The role of fiber calcination is to prevent the fiber from dissolving into the water (gel casting solution) and the role of freeze-drying is to prevent the large shrinkage during the drying process. Therefore, a suitable forming method should prevent both the melting of electrospun fiber and the large shrinkage of the sample during the drying process. 4. Conclusions In conclusion, mullite-based nanofibrous aerogels were firstly fabricated by gel-casting and freeze drying method using the electrospun nanofibers with different alumina/silica molar ratios. All mullite-based nanofibrous aerogels exhibited an obvious a multilevel pore structure with a major pore with size of 10–100 μm generated by the sublimation of the ice crystal and a minor pore with size of 0.5–10 μm originated from the overlapping of the ceramic fibers in aerogels. Due to this special microstructure, the mullite-based nanofibrous aerogel still shows an ultralow density (34.64–48.89 mg/cm3) and ultralow roomtemperature thermal conductivity (0.03274–0.04317 Wm−1 K−1) even when the sintering temperature was 1400 °C, which was much higher than the service temperature (lower than 1200 °C) of traditional nanoparticle aerogel. In addition, the properties of mullite-based aerogel can be adjusted by controlling the composition of electrospun fibers. The sample can be used as the high temperature insulation materials in different thermal protection systems to replace the traditional ceramic fiber brick, which could dramatically decrease the weight and volume of the thermal protection system of various industrial furnace and hypersonic vehicle. 471
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Ceram. Int. 39 (2013) 9079–9084. [30] M.M.A. Zadeh, M. Keyanpour-Rad, T. Ebadzadeh, Effect of viscosity of polyvinyl alcohol solution on morphology of the electrospun mullite nanofibers, Ceram. Int. 40 (2014) 5461–5466. [31] Z. Chen, Y. Gu, P. Aprelev, K. Kornev, I. Luzinov, J. Chen, F. Peng, P. Joy, Mullitenickel magnetic nanocomposite fibers obtained from electrospinning followed by thermal reduction, J. Am. Ceram. Soc. 99 (2016) 1504–1511. [32] X. Dong, J.Q. Liu, X.T. Li, X.J. Zhang, Y.J. Xue, J.C. Liu, A.R. Guo, Electrospun mullite nanofibers derived from diphasic mullite sol, J. Am. Ceram. Soc. 100 (2017) 3425–3433. [33] L. Su, H.J. Wang, M. Niu, X.Y. Fan, M.B. Ma, Z.Q. Shi, S.W. Guo, Ultralight, recoverable, and high-temperature-resistant SiC nanowire aerogel, ACS Nano 68 (2018) 165–169. [34] H.Y. Wei, H. Li, Y. Cui, R.L. Sang, H.Y. Wang, P. Wang, J.L. Bu, G.X. Dong, Synthesis of flexible mullite nanofibres by electrospinning based on nonhydrolytic sol-gel method, J. Sol-Gel Sci. Technol. 82 (2017) 718–727. [35] S. Chabi, V.G. Rocha, E. García-Tuñón, C. Ferraro, E. Saiz, Y. Xia, Y. Zhu, Ultralight, strong, three-dimensional SiC structures, ACS Nano 10 (2016) 1871–1876. [36] Y. Liu, Y. Liu, W.C. Choi, S. Chae, J. Lee, B. Kim, M. Park, H.Y. Kim, Highly flexible, erosion resistant and nitrogen doped hollow SiC fibrous mats for high temperature thermal insulators, J. Mater. Chem. A 5 (2017) 2664–2672.
[22] W.J. Zang, T. Jia, X. Dong, J.C. Liu, H.Y. Du, F. Hou, A.R. Guo, Preparation of homogeneous mullite-based fibrous ceramics by starch consolidation, J. Am. Ceram. Soc. 101 (2018) 3138–3147. [23] F. Li, Z. Kang, X. Huang, G.J. Zhang, Synthesis of ZrB2, nanofibers by carbothermal reduction via electrospinning, Chem. Eng. J. 234 (2013) 184–188. [24] A.M. Bazargan, S.M.A. Fateminia, M.E. Ganji, M.A. Bahrevar, Electrospinning preparation and characterization of cadmium oxide nanofibers, Chem. Eng. J. 155 (2009) 523–527. [25] J. Shen, Z. Li, Y.N. Wu, B.R. Zhang, F.T. Li, Dendrimer-based preparation of mesoporous alumina nanofibers by electrospinning and their application in dye adsorption, Chem. Eng. J. 264 (2015) 48–55. [26] W. Ma, W. Yu, X. Dong, J.X. Wang, G.X. Liu, Electrospinning preparation and upconversion luminescence properties of LaOBr:Er3+ nanofibers and nanoribbons, Chem. Eng. J. 244 (2014) 531–539. [27] N. Sarlak, M.A.F. Nejad, S. Shakhesi, K. Shabani, Effects of electrospinning parameters on titanium dioxide nanofibers diameter and morphology: an investigation by Box-Wilson central composite design (CCD), Chem. Eng. J. 210 (2012) 410–416. [28] J. Wu, H. Lin, J.B. Li, X.B. Zhan, J.F. Li, Fabrication and characterization of electrospun mullite nanofibers, Mater. Lett. 63 (2009) 2309–2312. [29] M.M.A. Zadeh, M. Keyanpour-Rad, T. Ebadzadeh, Synthesis of mullite nanofibres by electrospinning of solutions containing different proportions of polyvinyl butyral,
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Ultralight, thermal insulating, and high-temperature-resistant mullite-based nanofibrous aerogels
T
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Ruili Liua, Xue Dongb, , Shuangtian Xiea, Tao Jiaa, Yunjia Xuea, Jiachen Liua, Wei Jingc, ⁎ Anran Guoa, a
School of Materials Science and Engineering, Key Lab of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, China b College of Aeronautical Engineering, Civil Aviation University of China, Tianjin 300300, China c Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China
H I GH L IG H T S
nanofibrous aerogels were firstly fabricated. • Mullite-based aerogels show a unique multilevel pore structure. • The aerogels exhibit an ultralow density and ultralow thermal conductivity. • The • The aerogels show an excellent high-temperature thermal stability.
A R T I C LE I N FO
A B S T R A C T
Keywords: Aerogel Mullite fiber Thermal insulation High temperature resistance
The fabrication of insulation materials with ultralow thermal conductivity and excellent thermal stability at high temperatures (higher than 1200 °C) has remained an extremely challenging. In this study, we reported the manufacturing of mullite-based nanofibrous aerogels via the gel-casting and freeze drying methods using the electrospun nanofibers with different alumina/silica molar ratios (3:2, 3:1 and 3:0) as the matrix and silica sols as the high temperature binders. The formation process of the mullite-based fibrous aerogel and effect of aerogel composition on the sample physical and mechanical properties were investigated. All mullite-based nanofibrous aerogel show a similar multilevel pore structure. The minor pores were formed by the overlaps of nanofibers and were the fundamental porous structure of the aerogel, while the major pores was caused by the sublimation of the ice crystal. This unique multilevel pore structure make the mullite-based nanofibrous aerogels exhibit an ultralow density (34.64–48.89 mg/cm3) and low thermal conductivity (0.03274–0.04317 Wm−1 K−1) although the sintering temperature was as high as 1400 °C much higher than the service temperature of the traditional nanoparticle aerogel. In addition, besides controlling the fabrication parameters, the physical and mechanical properties could be also tuned by adjusting the composition of the nanofibers. The research of this work provides a new insight into the development of high efficient thermal insulation materials used at high temperatures.
1. Introduction Traditional aerogels usually refer to solid materials with nanoporous network structure which was composed of nanoparticles conglomerating together and are well known for their ultralow density, ultralow thermal conductivity and ultrahigh porosity, playing a key role in thermal insulation and preservation systems of various aircrafts, missiles and furnaces [1,2]. However, the 3D nanoporous microstructure of the traditional aerogel would collapse at high temperatures
⁎
due to the ultrahigh activity of nanoparticals, and therefore the service temperature of the traditional aerogels are often below 1200 °C in air atmosphere [3,4]. Although researchers have tried to use ceramic nanoparticals with excellent thermal stability as the raw materials to prepare other ceramic aerogels, such as ZrO2 aerogels and Al2O3 aerogels, the thermal stability of these aerogels didn’t improve dramatically due to the limitation of their intrinsic nanoparticle-based nanoporous structure [5,6]. Recently, a new kind of aerogel called nanofibrous aerogel was
Corresponding authors. E-mail addresses: [email protected] (X. Dong), [email protected] (A. Guo).
https://doi.org/10.1016/j.cej.2018.12.018 Received 28 August 2018; Received in revised form 30 November 2018; Accepted 4 December 2018 Available online 05 December 2018 1385-8947/ © 2018 Elsevier B.V. All rights reserved.
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because this kind of precursor exhibited an excellent spinnability. However, the crystallization temperature of metal alkoxide is usually quite low because the elements existing in the starting material are mixed at the atomic level. For example, many groups [28–31] have successfully fabricated nanoscale mullite fibers by electrospinning a monophasic mullite sol. However, the results suggested that when the sintering temperature increased to higher than 1200 °C, the mullite grain size in the fibers would increase obviously, resulting in a degradation of the fiber mechanical property. Our group recently reported the fabrication of mullite nanofibers by electrospinning a diphasic mullite precursor [32] and the corresponding mullite nanofibers exhibited an excellent high temperature resistance and was a promising starting material for the fabrication of mullite nanofibrous aerogel. In this work, the thermal stability mullite-based fibers were firstly fabricated from the diphasic mullite precursor and then were used as the starting materials to construct the nanofibrous aerogels via the gelcasting and freeze drying methods. The formation process of the mullite-based fibrous aerogel was investigated and the effect of aerogel composition on the sample physical and mechanical properties was discussed.
successfully fabricated with nanofibers as the main starting materials [7–9]. Different from the nanoparticle-based porous structure of the traditional aerogel, the three-dimensional network structure of the nanofibrous aerogel is constructed by the overlapping and entangling of many nanofibers. In the beginning, the fabrication of nanofibrous aerogel mainly focused on the functional materials, such as carbon nanotube aerogels [10,11], carbon nanofiber aerogel [12,13] and carbon microbelt aerogel [14] and few studies focused on its thermal insulation properties. In 2014, Ding’s group [15] firstly suggested the possibility of using electrospun fibers as the starting material to fabricate the nanofibrous aerogel, which largely broaden the category of the nanofibrous aerogels. The obtained PAN-based nanofibrous aerogel exhibited an extremely low density of 0.12 mg/cm3 and low thermal conductivity of 0.026 Wm−1 K−1. However, since the aerogel contained a large number of polymer fibers, the aerogel can be only used below 600 °C. Then his group [16] successfully fabricated the SiO2 nanofibrous aerogel with ultralow density (10 mg/cm3) and low thermal conductivity (0.025 Wm−1 K−1) and demonstrated that the sample shows an excellent flexibility at 1100 °C. However, a clear fusion was observed after the sample was sintered at 1200 °C due to dramatically growth of the silica crystal in nanofibers. Therefore, this kind of aerogels can’t also be used at 1200 °C. At the same time, Wu’s group [17] successfully prepared a ZrO2 nanofiber sponge with ultralow density (15 mg/cm3) and low thermal conductivity (0.027 Wm−1 K−1) and suggested that the sample could be used as high as 1300 °C. However, as described in its experimental section, the sintering temperature of the ZrO2 sponge was only 800 °C and the thermal stability of the sample was tested by the methane flame. This characterization is quite unreasonable. At high temperature, the nanocrystal in ceramic fibers would grow rapidly, resulting in the serious degradation of the fiber mechanical strength, finally leading to collapse of the nanofibrous aerogel. This is the main reason why the nanofibrous aerogel couldn’t withstand the high temperature. Therefore, in order to investigate the thermal stability of the aerogel, the sample should be sintered at a certain temperature for several hours in the furnace rather than just using a methane flame to test the sample for only several minutes because the testing time of the methane flame was too short for the growth of ceramic crystal. Therefore, it is difficult to demonstrate the ZrO2 sponge could be used at 1300 °C for a long time. The above-mentioned researches fully demonstrate that the nanofibrous aerogels exhibit similar ultralow density and ultralow thermal conductivity with the traditional solid nanopartical aerogels; however the service temperature of the ceramic nanofibrous aerogel is still too low due to the crystal growth of the ceramic fiber at high temperature. Therefore, it can be speculated that if the nanofibers used was changed to a ceramic nanofibers with excellent thermal stability, the resultant aerogel would exhibit both low thermal conductivity and excellent high-temperature stability. Porous mullite fibrous ceramics, which is constructed by the commercially mullite fibers and exhibits low density, low thermal conductivity and excellent thermal stability [18–22], have been widely used as the high temperature thermal insulation materials in the insulation systems of space shuttles and high-temperature furnaces and the service temperature of this kind of thermal insulation materials could reach as high as 1500 °C in air. However, the density and thermal conductivity of the porous mullite fibrous ceramics is still much higher than that of the traditional solid nanopartical aerogel because the diameter of mullite fiber used is quite large about 10–20 μm consequently resulting in a thick and heavy “scaffold” of the fibrous ceramics. Therefore, it is conceivable that if the diameter of the mullite fiber used could decrease to nanoscale size, this kind of porous mullite nanofibrous ceramic would become the above mentioned mullite nanofibrous aerogels. In recent years, electrospinning is a kind of emerging technology for preparing nanoscale fibers [23–27]. Almost all the ceramic nanofibers were fabricated using the metal alkoxide as the staring materials
2. Materials and methods 2.1. Fabrication process of mullite-based nanofibers Mullite-based (or alumina) nanofibers were prepared by electrospinning a diphasic mullite precursor with different molar ratios of alumina to silica. In a typical fabrication process of the mullite nanofibers with the alumina/silica molar ratio of 3:2, 2.1 g of aluminum trisec-butoxide, 0.1692 g of polyhydromethylsiloxane were dissolved in the mixture of 10 g of isopropanol, 1 g of N,N-dimethylformamide and 1.5 g of ethylacetoacetate by vigorous stirring for 30 min. Then 1.53 g of polyvinylpyrrolidone powders (PVP-K90, Mw = 1300000) was added into the solution under continuous stirring until the PVP completely dissolved into the solution. The electrospinning apparatus was set up horizontally and the fibers were electrospun under an applied electric field, generated using a high voltage power supply. The feeding rate was set at 0.5 ml/h, the working distance was 12 cm, and the working voltage was 12 kV. After the electrospinning process, the as-spun mullite precursor fibers were cross-linked at 200 °C for 1 h in air to make them infusible and then sintered at different temperatures from 800 °C to 1500 °C with a heating rate of 2 °C/min. The mullite-based nanofiber with the alumina/silica molar ratio of 3:1 and the pure alumina nanofiber were also fabricated with the same method. 2.2. Fabrication process of mullite nanofiber-based aerogel Fig. 1 shows the typical fabrication process of the mullite-based nanofibrous aerogel. Firstly, the as-spun precursor fibers were calcined at 800 °C to remove the organic component and were dispersed into individual fibers using a high speed homogenizer with the speed of 3000 r/min for 5 min. Then the dispersion was dried at 120 °C and short aluminum silicate fibers were obtained. During the gel-casting process, 0.05 g of short aluminum silicate fibers and 0.02 g of agar were dissolved in 2 ml of silica sols (30 wt%) in the mould at 90 °C; then the mould was frozen in a refrigerator for 12 h followed by a freeze drying for 10 h, consequently obtaining a high strength green body. Finally, the green samples were sintered at 1400 °C for 2 h with a heating rate of 2 °C/min in air. 2.3. Characterization The morphology of the sample was observed using a Hitachi S-4800 field-emission electron microscope. Fourier transform infrared spectroscopy (FT-IR) spectra of the fibers sintered at different temperatures ranging from room temperature to 1500 °C were taken using a Fourier 465
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Fig. 1. Fabrication process of the mullite-based nanofibrous aerogel.
3.1. Formation process of the mullite-based fibrous aerogel
transform infrared spectroscope (FTIR, TRNSOR27, Germany). Phase transformation processes of fibers and aerogels after heat treatment at different temperatures was investigated via a X-ray Diffraction (XRD, Rigaku D/Max-2500, Japan). The compression tests were examined using a universal testing machine. The bulk density of the aerogel was calculated by measuring the geometry volume and mass of the sample. The true density of sample was measured by the Micromeritics AccuPyc 1330 pycnometer, and the porosity of aerogel was calculated according to the bulk density and true density of aerogel. Thermogravimetry (TG) of a green body of the aerogel was carried out from room temperature to 1500 °C in air with a thermal analyzer (Netzsch Sta 449C, Germany). The pore size distribution of the aerogel was investigated by mercury intrusion porosimetry (AutoPore IV 9510, America). The sample thermal conductivity at 18 °C, 400 °C, 600 °C, 800 °C and 1000 °C were tested by a Hot Disk TPS2500s instrument. The samples used for the test were 13.7 mm in diameter and 6.5 mm in thickness. It should be mentioned that in general the thermal conductivity measurement for the insulation materials (the thermal conductivity lower than 0.1 Wm−1 K−1) can be guaranteed only by means of steady state methods, such as the heat flow method and guarded hot plate method; however these methods require a large size sample, which is quite difficult for us to prepare. Therefore, the thermal conductivity in this work was measured by the hot disk method, a kind of non-steady methods, which has been extensively adopted by other similar works [16,33]. The sample size and thickness have a great influence on the results and therefore the values should be used for relative comparison under the same measurement condition.
As shown in Fig. 1, the whole fabrication process was divided into two parts, the fabrication of electrospun nanofibers and the construction of the aerogel. In this section, the mullite-based nanofibrous aerogel fabricated using the ceramic fibers with the alumina/silica mole ratio of 3:1 as the starting materials was selected as the typical example to illustrate the formation process of the fibrous aerogel. 3.1.1. Characterization of mullite-based nanofibers As illustrated in the introduction section, the selection of suitable ceramic fibers is a key point of fabricating the high temperature resistant aerogel. At high temperature, the nanocrystal in ceramic fibers would grow rapidly, resulting in the serious degradation of the fiber mechanical strength, finally leading to collapse of the nanofibrous aerogel. This is the main reason why the nanofibrous aerogel couldn’t withstand the high temperature. Therefore, obtaining nanofibers with excellent high temperature resistance is the basis for preparing mullitebased nanofibrous aerogel. According to our previous research, in order to maintain the high thermal stability of the mullite fibers, the electrospinning mullite precursor should be the diphasic sol. Compared with our previous work [32], the silica source in this work was changed from a commercial solid silica resin to the liquid polymethylhydrogensiloxane (PHMS) because the PHMS was more compatible with the alumina source. Therefore, it is necessary to investigate the type of mullite precursor and the crystallization temperature of the mullite crystal. The main difference between the diphasic sol and monophasic sol is that the diphasic sol is composed of Al-O-Al and Si-O-Si linkages, while the monophasic sol is composed of Al-O-Si linkages. Fig. 2 shows the FTIR spectra of the electrospun mullite-based nanofibers with the alumina/silica molar ratio of 3:1. As shown in the FTIR spectra of the asspun fibers, peaks at 813 cm−1 and 1090 cm−1, which were assigned to the Al-O-Al and Si-O-Si linkages respectively, could be clearly observed and no peaks corresponding to the Al-O-Si linkage appeared, thus demonstrating that the precursor used is the diphasic sol. In addition, the peaks of C]O and CeN existing in PVP at 1670 cm−1 and 1290 cm−1 and the peaks at 3500 cm−1 and 2900 cm−1 assigned to the OH and CHn groups were observed in the green body, and then disappeared when the sintering temperature increased to 800 °C, indicating that the free water, adsorbed water and organics in the green body would burn off before 800 °C. Moreover, when the temperature increased to 1300 °C, peaks at 1180 cm−1 assigned to the Si-O-Al linkage was observed, suggesting that the mullite crystallization temperature was about 1300 °C.
3. Results and discussion Mullite is a series of minerals composed of aluminum silicates with the alumina and silica within a certain range and can also form composites with silica and alumina. Normally, the thermal insulation property of the composite improves gradually with the increase of the silica content and the thermal stability of the composite improves gradually with the increase of alumina content, which means the composite with high alumina content exhibits a better thermal stability, while the one with high silica content exhibits a better thermal insulation property. Therefore, in this work, three kinds of mullite-based nanofibrous aerogels with different composition were fabricated using different starting ceramic fibers (mullite nanofibers with the alumina/ silica mole ratio of 3:2, mullite-alumina composite nanofibers with the alumina/silica mole ratio of 3:1 and alumina nanofibers) to investigate the effect of aerogel composition on the sample properties. 466
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Fig. 2. FTIR spectra of as-spun mullite-based nanofibers with the alumina/silica molar ratio of 3:1 before and after sintering at different temperatures.
Fig. 4. SEM images of as-spun mullite-based nanofibers with the alumina/silica molar ratio of 3:1 before and after sintering at different temperatures. (a) Before sintering, (b) 800 °C, (c) 1400 °C, (d) 1500 °C.
a promising starting materials used to fabricate the mullite-based nanofibrous aerogels. 3.1.2. Characterization of the mullite-based nanofibrous aerogel After electrospinning, the resultant sample was a non-woven fiber mat rather than the individual fibers; therefore, in order to fabricate the nanofibrous aerogel, the obtained non-woven fiber mat should be dispersed into individual fibers. In this work, the fiber mat was dispersed by the high speed dispersion and it is found that the fiber composition would influence the length of the obtained individual fibers. Fibers sintered at high temperatures would become fragile and after the high speed dispersion the length of the fiber would become too short to construct the 3D skeleton structure; however, fibers sintered at low temperatures would dissolve into the water due to the existence of the organic component, consequently influencing the fiber morphology. Fig. S3 shows SEM images of the ceramic fibers after calcinated at different temperatures followed by the high speed dispersion treatment. When the calcination temperature was 600 °C, a clear fusion of the fiber was observed due to the existence of organic component in fibers. However, if the calcination temperature was 1000 °C, the fiber morphology could preserve but the fiber length was too short because the fiber became too fragile. Therefore, 800 °C was set as the best calcination temperature and the resultant fibers exhibited both an integrate fiber morphology and an acceptable length to diameter ratio. After calcination, the aluminum silicate fiber mat was dispersed into individual fibers using a high speed homogenizer and the length of the fiber was about 15 μm as shown in Fig. 5a. During the gel-casting process, the agar, short fibers and silica sols formed a uniform suspension at 90 °C and when the temperature of the suspension cooled to the room temperature, the agar would precipitate from the water, forming a polymeric network and the suspension became an elastic solid gel. Then the solid gel was frozen in a refrigerator followed by a freeze drying. The water existing in the gel would transform into the ice at low temperature and then converted into gases through the sublimation during the freeze drying process, leaving many large pores [15,16]. Fig. 5b and c show the SEM images of the green sample and it can be clearly seen that the sample exhibited a macroporous structure and each cell wall was constructed by the interlocked fibers connected by the agar. With the increase of sintering temperature, the agar decomposed completely and the silica sols transformed into the silica acting as the high temperature binders bonding the lapped short fibers. As shown in Fig. 5d and e, the mullite-based nanofibrous aerogel exhibited an obvious multilevel pore structure with a major pore size of approximately 10–100 μm and a minor pore size of about 0.5–10 μm. The major pores were generated by the sublimation of the ice crystal
Fig. 3. XRD patterns of mullite-based nanofibers with the alumina/silica molar ratio of 3:1 after sintering at different temperatures.
In order to further determine the mullite crystallization temperature, the phase transformation process of the electrospun fiber was studied by XRD analysis. As shown in Fig. 3, when the sintering temperature was lower than 1000 °C, no crystal phase was formed in the fiber. However, when the calcination temperature was 1100 °C, two weak characteristic peaks of the γ-alumina (PDF#75-0921) at about 46° and 73° were observed. After sintering at 1300 °C, the characteristic peak corresponding to mullite phase (PDF#79-1275) appeared which further verified the conclusion that the mullite formation temperature was about 1300 °C. As the temperature continues to rise to 1500 °C, the γ-alumina transformed into corundum (PDF#74-1081) and the mullitebased fiber was finally composed of mullite and alumina because the mullite precursor contains an excessive amount of aluminum source. Fig. 4 shows the SEM images of as-spun mullite-based nanofibers before and after sintering at different temperatures. It can be seen from Fig. 4 that the fiber diameter gradually decreased with the increase of sintering temperature from room temperature to 1400 °C, which was mainly caused by the pyrolysis of organics in the green body and the ceramic densification. The average diameters of the green body and fibers after sintering at 800 °C and 1400 °C were about 750 nm, 460 nm and 410 nm, respectively. However, the average fiber diameter (450 nm) after sintering at 1500 °C is a little higher than that of fibers sintered at 1400 °C due to significantly growth of the mullite crystal on the fiber surface [34]. In addition, the surface of the fiber became rougher with the continuous rise of the sintering temperature which was resulted from the crystallization process of the mullite. It should be mentioned that the mullite-alumina composite fibers sintered 1400 °C still shows a smooth surface suggesting that this kind of ceramic fiber is 467
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Fig. 5. SEM images of (a) the individual nanofibers after mechanical dispersion, (b, c) green body, and (d, e) mullite-based nanofibrous aerogel sintered at 1400 °C. (f) Photograph of the mullite-based nanofibrous aerogel.
and the minor pores mainly originated from the overlapping of the ceramic fibers in aerogels. Fig. 5f shows the photograph of the fibrous aerogel after sintering at 1400 °C and the sample could stand on the plastic fluff due to the ultralow density. Fig. 6 shows the TG and DSC curves of the green body from room temperature to 1500 °C. The first weight loss of the sample occurring between 25 °C and 210 °C was about 7.2% resulted from the removal of the free water. The second weight loss (210–680 °C) about 22.62% was caused by the decomposition of the agar and silica sol. The obvious exothermic peak at about 1000 °C on the DSC curve was resulted from the phase transformation, that is, the formation of mullite phase. The XRD patterns of the green body and samples sintered at different temperatures were shown in Fig. 7. A broad background which indicates the existence of the amorphous SiO2 was observed in the green body. When the sintering temperature increased to 1000 °C, the pattern of the sintered body was composed of mullite characteristic peaks and alumina characteristic peaks. When the temperature was above 1000 °C, the alumina characteristic peaks gradually disappeared from the patterns, which demonstrated that the excessive aluminum source existing in the fibers could react with the silica derived from the silica sol, forming the mullite phase. Therefore, due to the introduction of additional silica source (silica sol), the final composition of the aerogel was the mullite and silica phase rather than the mullite and alumina phase as analyzed from Fig. 3. As shown in Figs. 8 and 9a, the shrinkage of the green sample after freeze drying was only 1.61% and this ultralow shrinkage during the
Fig. 7. XRD patterns of the green body before and after sintering at different temperatures.
Fig. 8. Variation of the linear shrinkage and density of the nanofibrous aerogel with the increasing of sintering temperature.
freezing drying process ensured the high porosity of the green sample. In addition, the strength of the green sample could reach to 15 KPa due to the formation of agar polymeric network although the sample density was only 34.64 mg/cm3, and this high porous green sample can be machined easily due to its high strength. The linear shrinkage and density of sample increased with the increasing of sintering
Fig. 6. TG and DSC curves of the green body of the mullite-based nanofibrous aerogel. 468
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Fig. 9. (a) Compressive strength and thermal conductivity of the nanofibrous aerogel sintered at different temperatures; (b) Thermal conductivity of the mullitebased nanofibrous aerogel at different test temperatures.
diphasic sol and the organic component disappeared when the sintering temperature increased to 800 °C. The peak at 1180 cm−1 belonging to the Si-O-Al linkage was observed for the first time in fibers sintered at 1000 °C, suggesting that the mullite phase was formed at about 1000 °C. The XRD analysis (Fig. 10b) shows that the characteristic peaks of the mullite phase (JCPDS#15-0776) firstly appeared when the calcination temperature reached 1000 °C, further indicating that the mullite crystallization temperature was about 1000 °C. Besides, as the temperature continued to increase, the mullite characteristic peak became sharper and only mullite phase existed in fibers at last. The FTIR spectra and XRD pattern of as-spun alumina nanofibers before and after sintering at different temperatures are shown in Fig. 10c and d, respectively. Results indicate that the pure alumina phase formed at 1100 °C. SEM images of the mullite fibers and alumina fibers after sintering at different temperatures were shown in Figs. S1 and S2 in the supplementary materials and shows that the resultant fibers were quite uniform. XRD patterns of another two kinds of mullite-based nanofibrous aerogels after sintering at 1400 °C are shown in Fig. 11. For the aerogel fabricated with the alumina fibers (the molar ratio of alumina to silica was 3:0), the final fibrous aerogel was composed of mullite and alumina phase and no characteristic peaks of silica phase was observed, indicating that the silica sol used as the high temperature binder completely reacted with the aluminum source in the fibers forming the mullite phase. For the aerogel fabricated with the mullite fibers (the molar ratio of alumina to silica was 3:2), as expected the final phase of the aerogel were mullite and silica. In addition, the silica phase content in this sample was obvious much higher than he silica content in the aerogels fabricated using the fibers with the alumina/silica molar ratio of 3:1 (see Fig. 7). Fig. 12 shows SEM images of another two mullite-based nanofibrous aerogels after sintering at 1400 °C. It clearly shows that all mullitebased nanofibrous aerogels exhibited an obvious multilevel porous structure, just like the structure shown in Fig. 5d and e. Moreover, the crystallization of the fibrous aerogel fabricated using the nanofiber with the alumina/silica molar ratio of 3:2 was quite evident, and as the molar ratio of alumina to silica increased, the crystallization of the fibers become was not as evident as before. The mechanical and thermal conductivity of porous ceramics were determined by two things, the sample composition and the sample porous structure. Therefore, in order to analyze the effect of aerogel composition on the aerogel properties, three different aerogels with similar density were fabricated through adjusting the fabrication parameters, such as the solid content during the gel casting process and the addition amount of the agar. As shown in Table 1, three different mullite-based nanofibrous aerogels exhibited similar densities (about 48 mg/cm3) and porosities (about 98.9%). The Mercury intrusion porosimetry testing results (Fig. 13) indicates that the pore size distributions of three different mullite-based nanofibrous aerogels were quite similar, showing a multilevel porous structure with a minor pore size of
temperature due to the decomposition of the agar and the sintering densification of the sample. The compressive strength of the sample firstly decreased to 13.54 KPa when the sintering temperature was 1000 °C due to the decomposition of agar polymeric network and then increased gradually with the increase of sintering temperature resulting from the formation of silica phase acting as the high temperature binder between the interlocked fibers. The basic mechanisms of heat transfer are mainly divided into three types: conduction, convection and radiation. At room temperature, the radiative heat transfer and convective heat transfer could be negligible, and the conduction is the most important means of heat transfer. Therefore, as to the mullite-based nanofibrous aerogels, the thermal conductivity at room temperature was mainly determined by the conduction of solid content (ceramic fibers) and the gas content (pores). As the thermal conductivity of gas phase was much lower than that of solid phase, the gas content (porosity) of the sample was the main factor that influences the sample thermal conductivity. For the green sample, the pores formed by the overlaps of nanofibers were filled by the agar (Fig. 5b) so that the thermal conductivity of the green body was slightly larger than that of sample sintered at 1000 °C. When the sintering temperature increased from 1000 °C to 1400 °C, the thermal conductivity increased steadily because the shrinkage of the sample led to the decrease of the sample porosity. Fig. 9b shows the thermal conductivity of the mullite-based nanofibrous aerogel at different test temperatures. It can be seen that the thermal conductivity of the mullite-based nanofibrous aerogel increased with the increase of the test temperature, mainly due to the increasing thermal radiation of the gas at high temperature [35,36]. Moreover, when the test temperature increased to 1000 °C, the thermal conductivity of the aerogel was still quite low, about 0.07341 Wm−1 K−1. All these results indicates that the mullite-based fibrous aerogel fabricated using the as-spun fibers with the alumina/silica molar ratio of 3:1 after sintering at 1400 °C exhibited a relative high compressive strength (18.08 KPa), ultralow density (47.41 mg/cm3), ultralow roomtemperature thermal conductivity (0.0426 Wm−1 K−1) and ultralow thermal conductivity (0.07341 Wm−1 K−1) at high temperature (1000 °C), and was a promising high-temperature insulation materials used at high temperatures in an air atmosphere.
3.2. Effect of fiber composition on the properties of nanofibrous aerogel The above section provides a detailed introduction about the formation process and properties of the mullite-based nanofibers aerogel fabricated with the as-spun fibers with the alumina/silica molar ratio of 3:1 as the starting materials. In this section, aerogels were fabricated using another two kinds of nanofibers and the effect of aerogel composition on the sample properties were investigated. The FTIR spectrum of as-spun mullite nanofibers with the alumina/ silica molar ratio of 3:2 (Fig. 10a) indicates that the precursor is also a 469
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Fig. 10. FTIR spectra and XRD patterns of (a, b) as-spun mullite nanofibers with the alumina/silica molar ratio of 3:2, and (c, d) as-spun alumina nanofibers before and after sintering at different temperatures.
Fig. 12. SEM images of mullite-based nanofibrous aerogels fabricated using nanofibers with different alumina/silica molar ratios after sintering at 1400 °C. (a, b) 3:2 and (c, d) 3:0.
Fig. 11. XRD patterns of mullite-based nanofibrous aerogels fabricated using nanofibers with different alumina/silica molar ratios after sintering at 1400 °C.
0.5–10 μm and a major pore size of 5–100 μm. The minor pores were mainly formed by the overlaps of nanofibers, which was the fundamental porous structure of the aerogel, and the major pores were caused by the sublimation of the ice crystal. Apparently, this pore size distribution was consistent with the observation results from the corresponding SEM images (Figs. 5 and 12). Therefore, in case of the similar density, porosity and pore size distribution of the above three fibrous aerogels, the difference in the sample properties can be attributed to their different components. Normally, the mullite-based composite with high silica content shows a low thermal conductivity and the one with high alumina content exhibited an excellent high temperature stability and high strength. In this study, the composition of the aerogel is directly
determined by the composition of the starting materials and the XRD analyses also prove this conclusion that the alumina content in aerogel increased with the increase of the alumina/silica mole ratio of the electrospun nanofibers used. The thermal conductivity and compressive strength of the aerogels were illustrated in Table 1. All samples exhibited a low density (47.16–48.89 mg/cm3), low thermal conductivity (0.03812–0.04317 Wm−1 K−1) and relative high strength (14.70–18.77 kPa) even when the sintering temperature was as high as 1400 °C, which was much higher than the service temperature (lower than 1200 °C) of traditional nanoparticle aerogel. In addition, as expected the thermal conductivity decreased and compressive strength increased with the decrease of the alumina/silica molar ratio due to the increase of alumina content in the aerogel. Therefore, we can easily 470
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Table 1 Properties of mullite-based nanofibrous aerogels fabricated using nanofibers with different alumina/silica molar ratios after sintered at 1400 °C. Molar ratio of alumina to silica
Density (mg/cm3)
Porosity (%)
Thermal conductivity (W/mK)
Compressive strength (kPa)
3:2 3:1 3:0
47.67 ± 0.07 48.89 ± 0.08 47.16 ± 0.06
98.94 ± 0.01 98.96 ± 0.02 98.95 ± 0.01
0.03812 ± 0.00010 0.04036 ± 0.00020 0.04317 ± 0.00010
14.70 ± 0.04 17.79 ± 0.05 18.77 ± 0.06
Acknowledgement The authors would like to acknowledge the financial support of National Natural Science Foundation of China (Project No. 51502196 and Project No. 51872194). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2018.12.018. References [1] M. Koebel, A. Rigacci, P. Achard, Aerogel-based thermal superinsulation: an overview, J. Sol-Gel Sci. Technol. 63 (2012) 315–339. [2] R. Baetens, B.P. Jelle, A. Gustavsen, Aerogel insulation for building applications: a state-of-the-art review, Energy Build. 43 (2011) 761–769. [3] D. Huang, C. Guo, M. Zhang, L. Shi, Characteristics of nanoporous silica aerogel under high temperature from 950°C to 1200°C, Mater. Des. 129 (2017) 82–90. [4] J. Ma, F. Ye, C. Yang, J. Ding, S. Lin, B. Zhang, Q. Liu, Heat-resistant, strong alumina-modified silica aerogel fabricated by impregnating silicon oxycarbide aerogel with boehmite sol, Mater. Des. 131 (2017) 226–231. [5] Z. Zhao, A. Dairong Chen, X. Jiao, Zirconia aerogels with high surface area derived from sols prepared by electrolyzing zirconium oxychloride solution: comparison of aerogels prepared by freeze-drying and supercritical CO2(l) extraction, J. Phys. Chem. C 111 (2007) 18738–18743. [6] G.Q. Zu, J. Shen, X.Q. Wei, X.Y. Ni, Z.H. Zhang, J.C. Wang, G.W. Liu, Preparation and characterization of monolithic alumina aerogels, J. Non-Cryst. Solids 357 (2011) 2903–2906. [7] B. Sun, Y.Z. Long, F. Yu, M.M. Li, H.D. Zhang, W.J. Li, T.X. Xu, Self-assembly of a three-dimensional fibrous polymer sponge by electrospinning, Nanoscale 4 (2012) 2134–2137. [8] H.Y. Mi, X. Jing, B.N. Napiwocki, Z.T. Li, L.S. Turng, H.X. Huang, Fabrication of fibrous silica sponges by self-assembly electrospinning and their application in tissue engineering for three-dimensional tissue regeneration, Chem. Eng. J. 331 (2018) 652–662. [9] J. Xiao, W.Y. Lv, Y.H. Song, Q. Zheng, Graphene/nanofiber aerogels: performance regulation towards multiple applications in dye adsorption and oil/water separation, Chem. Eng. J. 338 (2018) 202–210. [10] M.B. Bryning, D.E. Milkie, M.F. Islam, L.A. Hough, J.M. Kikkawa, A.G. Yodh, Carbon nanotube aerogels, Adv. Mater. 19 (2007) 661–664. [11] C. Hoecker, F. Smail, M. Pick, A. Boies, The influence of carbon source and catalyst nanoparticles on CVD synthesis of CNT aerogel, Chem. Eng. J. 314 (2017) 388–395. [12] Z.Y. Wu, C. Li, H.W. Liang, J.F. Chen, S.H. Yu, Ultralight, flexible, and fire-resistant carbon nanofiber aerogels from bacterial cellulose, Angew. Chem. Int. Ed. Engl. 52 (2013) 2925–2929. [13] Z.Y. Wu, C. Li, H.W. Liang, Y.N. Zhang, X. Wang, J.F. Chen, S.H. Yu, Carbon nanofiber aerogels for emergent cleanup of oil spillage and chemical leakage under harsh conditions, Sci. Rep. 4 (2014) 4079. [14] H. Bi, X. Huang, X. Wu, X. Cao, C. Tan, Z. Yin, X. Lu, L. Sun, H. Zhang, Carbon microbelt aerogel prepared by waste paper: an efficient and recyclable sorbent for oils and organic solvents, Small 10 (2014) 3544–3550. [15] Y. Si, J. Yu, X. Tang, J. Ge, B. Ding, Ultralight nanofibre-assembled cellular aerogels with superelasticity and multifunctionality, Nat. Commun. 5 (2014) 5802. [16] Y. Si, X. Wang, L. Dou, J. Yu, B. Ding, Ultralight and fire-resistant ceramic nanofibrous aerogels with temperature-invariant superelasticity, Sci. Adv. 4 (2018) eaas8925. [17] H. Wang, X. Zhang, N. Wang, Y. Li, X. Feng, Y. Huang, C. Zhao, Z. Liu, M. Fang, G. Ou, H.J. Gao, X.Y. Li, H. Wu, Ultralight, scalable, and high-temperature-resilient ceramic nanofiber sponges, Sci. Adv. 3 (2017) e1603170. [18] H. Schneider, J. Schreuer, B. Hildmann, Structure and properties of mullite - a review, J. Eur. Ceram. Soc. 28 (2008) 329–344. [19] J. Zhang, X. Dong, F. Hou, H.Y. Du, J.C. Liu, A.R. Guo, Effects of fiber length and solid loading on the properties of lightweight elastic mullite fibrous ceramics, Ceram. Int. 42 (2016) 5018–5023. [20] X. Dong, G.F. Sui, J.C. Liu, S.E. Ren, M.C. Wang, H.Y. Du, Mechanical behavior of fibrous ceramics with a bird’s nest structure, Compos. Sci. Technol. 100 (2014) 92–98. [21] X. Dong, J.C. Liu, R.H. Hao, A.R. Guo, Z.G. Hou, M.X. Liu, High-temperature elasticity of fibrous ceramics with a bird's nest structure, J. Eur. Ceram. Soc. 33 (2013) 3477–3481.
Fig. 13. Pore size distributions of mullite-based nanofibrous aerogels fabricated using nanofibers with different alumina/silica molar ratios.
tune the properties of mullite-based aerogel by adjusting the composition of electrospun fibers. It should be mentioned that the whole synthesis process of this kind of aerogel was quite complicated, including electrospinning, fiber calcination, dispersion, gel casting, freeze-drying, and sintering. Considering the industrial feasibility, further work should be done to simplify this process. The electrospinning, aerogel forming process and sintering were the fundamental fabrication process and couldn’t be omitted; therefore efforts could be made to simplify the fiber calcination, dispersion, gel casting and freeze-drying into one process. The role of fiber calcination is to prevent the fiber from dissolving into the water (gel casting solution) and the role of freeze-drying is to prevent the large shrinkage during the drying process. Therefore, a suitable forming method should prevent both the melting of electrospun fiber and the large shrinkage of the sample during the drying process. 4. Conclusions In conclusion, mullite-based nanofibrous aerogels were firstly fabricated by gel-casting and freeze drying method using the electrospun nanofibers with different alumina/silica molar ratios. All mullite-based nanofibrous aerogels exhibited an obvious a multilevel pore structure with a major pore with size of 10–100 μm generated by the sublimation of the ice crystal and a minor pore with size of 0.5–10 μm originated from the overlapping of the ceramic fibers in aerogels. Due to this special microstructure, the mullite-based nanofibrous aerogel still shows an ultralow density (34.64–48.89 mg/cm3) and ultralow roomtemperature thermal conductivity (0.03274–0.04317 Wm−1 K−1) even when the sintering temperature was 1400 °C, which was much higher than the service temperature (lower than 1200 °C) of traditional nanoparticle aerogel. In addition, the properties of mullite-based aerogel can be adjusted by controlling the composition of electrospun fibers. The sample can be used as the high temperature insulation materials in different thermal protection systems to replace the traditional ceramic fiber brick, which could dramatically decrease the weight and volume of the thermal protection system of various industrial furnace and hypersonic vehicle. 471
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