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Synthesis of monolithic aerogel-like alumina via the accumulation of mesoporous hollow microspheres
Microporous and Mesoporous Materials 202 (2015) 234–240
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis of monolithic aerogel-like alumina via the accumulation of mesoporous hollow microspheres Li-ang Wu, Xvsheng Qiao, Shuo Cui, Zhanglian Hong, Xianping Fan ⇑ State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 7 July 2014 Received in revised form 25 September 2014 Accepted 7 October 2014 Available online 16 October 2014 Keywords: Aerogel-like alumina Monolith Hollow sphere High temperature stability Ambient drying
a b s t r a c t Monolithic aerogel-like alumina was synthesized via the accumulation of mesoporous hollow microspheres through an epoxide-driven sol–gel method. The addition of propylene oxide to the solution controls the gelation process, while the aging time and H2O/ethanol molar ratio in the initial component determine the morphology of the gel. The bulk density of the as-prepared sample was 0.133 g/cm3, and the BET surface area was 505.6 m2/g, which were very close to the values for the common alumina aerogel synthesized through the supercritical drying method. The microstructural evolution of the aerogel-like material as a function of aging time was carefully studied by SEM and TEM. The transition from solid particles to urchin-like mesoporous hollow microspheres can be explained as a self-templating process according to the Ostwald ripening mechanism. In addition, the samples were heat treated to 800 °C, 1000 °C and 1200 °C for 2 h. With an increase of the heat treatment temperature, the crystalline phases of the aerogel-like alumina varied from c-phase alumina to h-phase alumina. The microstructures as well as the physical properties, such as the thermal conductivity and elastic modulus, of samples heat-treated at different temperatures were also measured and revealed that the samples exhibited excellent stability against high temperature. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Thermal insulation materials have attracted an enormous amount of attention for their potential applications in aerospace and energy conservation during the last decade [1–3]. However, some limitations still exist in traditional thermal insulation materials because these materials cannot meet the increasing requirements for their density, porosity and mechanical properties in some hi-tech areas. A monolithic aerogel is a synthetic porous ultralight material and is famous for its high specific surface area, high porosity, low density and extremely low thermal conductivity. Monolithic aerogels appear to be appropriate candidates for future thermal insulation materials. Thus far, monolithic-aerogelbased materials have been widely studied and are expected to be utilized in thermal insulation [4], catalysis [5], magnetics [6], electronics [7] and optics [8]. For a long time, monolithic aerogels were only fabricated using the supercritical drying (SCD) method [9] because this process was able to diminish the capillary force built up in the pore walls of the gels and avoid the collapse of most of the pore volume during the drying process under ambient conditions. However, complicated
⇑ Corresponding author. http://dx.doi.org/10.1016/j.micromeso.2014.10.015 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.
facilities and extreme conditions caused the SCD technique to become a dangerous, expensive, multi-step and time- and energy-consuming procedure, which greatly limited its applications in industrial production. The weak high-temperature stability of the aerogels also limited their applications for thermal insulation. Alumina aerogels [10] undergo significant shrinkage and lose their extraordinary properties when heat-treated at high temperatures (>1000 °C) for a long time because of the severe sinter process caused by their high surface free energy. Consequently, it is very difficult for aerogels to be used at high temperature. It is necessary to find a new material with extraordinary physical properties and excellent high-temperature stability. Recently, Gash and co-workers [11,12] demonstrated a new route to obtain crack-free monolithic porous aerogels of various metal oxides including Al2O3 using an inorganic metal salt as the precursor through the SCD method. Typically, epoxides were added to aqueous solutions of inorganic metal salts to initiate gelation through a ring-opening reaction and worked as an acid scavenger to gradually increase the solution pH. The uniform increase in the solution pH results in homogeneous gelation to produce a monolithic gel. This method would be very useful for designing various monolithic aerogels with different microstructures and functions. The high-surface-area alumina aerogels [3], monolithic
L.-a. Wu et al. / Microporous and Mesoporous Materials 202 (2015) 234–240
Al2O3 with well-defined macropores and mesostructured skeletons [13] and monolithic alumina aerogel synthesized through ambient drying [14] were already synthesized through this method. However, some of them required SCD method and the weak high temperature stability still greatly limited their extensive applications. In this paper, we demonstrate a new facile route to synthesize monolithic aerogel-like alumina with high-temperature stability via the accumulation of mesoporous hollow microspheres through ambient drying. Materials with hollow structures have been widely studied recently because of their unique structure. These materials can be useful in drug storage and release [15], catalysis [16], separation [17], and lithium-ion batteries [18]. Many papers have described the formation of hollow spherical materials: traditional template-based methods, including those using hard [15] and soft [19] templates, and self-templating methods [18], which have been widely used in obtaining a hollow structure without an extra template. Nevertheless, nearly all of the materials with hollow structures were presented as powders or films; bulk materials with a hollow structure have been rarely reported. At the same time, alumina aerogel has been synthesized through ambient drying. In this study, the monolithic porous alumina gel was accumulated with urchin-like hollow spheres; the gel was crack-free even when dried at a rapid rate under ambient pressure drying conditions. The as-dried monolithic porous alumina gel exhibited aerogel-like properties such as a low bulk density, high surface area, high porosity and low thermal conductivity, even though the microstructure was absolutely different from a traditional aerogel. In addition, the aerogel-like alumina also exhibited excellent physical property stabilities even after being heat-treated at high temperature (up to 1200 °C). 2. Experimental 2.1. Synthesis of aerogel-like alumina Aluminum chloride hexahydrate (Aldrich) was used as the aluminum source, and a mixture of distilled water and ethanol (Sinopharm Chemical Reagent Co., Ltd.) was used as the solvent. Propylene oxide (Sinopharm Chemical Reagent Co., Ltd.) was added to initiate the condensation reaction. Isopropanol (Sinopharm Chemical Reagent Co., Ltd.) was used for the aging process. All of the reagents were used as received. Aluminum chloride hexahydrate (4.35 g, 18 mmol) was first dissolved in the mixture of distilled water and ethanol (using various H2O/EtOH molar ratios: 1:0.48, 1:1.18, 1:0.2, the corresponding real volume rations of H2O, EtOH is 4 ml, 5.5 ml; 2 ml, 7.5 ml; 6 ml, 2.5 ml). Propylene oxide (7 ml, 0.1 mol) was then rapidly added into the mixture solution, and stirring was continued for 20 s. The resultant homogeneous solution was sealed and maintained at 60 °C for gelation. As this reaction is an exothermic reaction and the solution would reach to about 60 °C just several second after the epoxide was added to the procurer. Then, the transparent solution turned into a translucent gel quite rapidly (in approximately 20 s). After gelation, the wet gel was aged with fivefold isopropanol for 2 days at 60 °C. Subsequently, the gel was dried by evaporation under ambient pressure condition. After the gel was completely dried, it was heat treated at various temperatures in air with a heating rate of 1 °C/min.
JEM-1200EX at 120 keV (JEOL Japan). The TEM samples were deposited onto carbon-coated, 3-mm diameter, copper electron microscope grids and dried in air. Nitrogen adsorption measurements (ASIC-2 Quantachrome Instruments USA) were performed to obtain the pore properties such as the BET-specific surface area and pore size distribution. The adsorption branch was used for the Barrett–Joyner–Halenda (BJH) calculation. Before measurement, the sample was outgassed under vacuum at 200 °C for 4 h. The bulk density was determined by measuring the rate of volume and weight of a rectangle sample. XRD analysis with Cu Ka (0.154 nm, X-98, Rigaku Japan) radiation was performed to identify the crystalline phases. All of the measurements were performed for the powder specimens prepared by grinding monolithic samples. Stress–strain curves were generated using a universal material tester (Zwieklroell Germany) with a load of 20 kN; the tests were performed in triplicate, and the speed was 1 mm/min. The elastic modulus was calculated based on the strain values between 100 and 500 N of stress. The thermal conductivities were measured using the hot-wire method (TC 3000 XIATECH China) at room temperature. 3. Results and discussion 3.1. Formation and characterization of aerogel-like alumina The monolithic aerogel-like alumina was synthesized using a very convenient procedure based on the epoxide-driven method established by Gash [12,13]. In the previous work, epoxide was used as the gelation initiator to induce the gelation of a metal salt solution, acting as an irreversible proton scavenger through a ring-opening reaction. In this work, as we utilized a relatively high metal-ion concentration and propylene oxide/metal-ion molar rate, the transition from a clear solution to opaque gel was completed in a very short time (approximately 20 s). This approach is quite different from the traditional aerogel preparation with epoxide-driven sol–gel process [3], which requires a long gelation time for the complete reaction. An opaque monolithic aerogel-like material could be obtained after the wet gel was aged for 2 days and dried under ambient pressure conditions. Fig. 1 presents a photograph of the as-dried monolithic aerogellike alumina. Fig. 2 presents SEM images of alumina gels prepared using various H2O/ethanol molar ratio. With an appropriate ratio of H2O/ethanol (1:0.48), urchin-like hollow microspheres can be obtained, the diameter of these hollow microspheres was about 1 2 lm (Fig. 2a); these hollow microspheres attached to each other and accumulated to form an opaque monolithic gel. When the H2O/ethanol ratio was low (1:1.18), the gelation time became
2.2. Characterization FE-SEM (S-4800, Hitachi, Japan) was used to observe the morphology of the gel structure using a slight Pt coating on the sample. TEM analysis was performed in bright-field mode using a
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Fig. 1. Photographs of monolithic aerogel-like alumina.
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Fig. 2. SEM images of alumina gels with various H2O/EtOH molar ratios: (a) 1:0.48, (b) 1:1.18, and (c) 1:0.2.
extremely short (with gelation occurring almost directly after the addition of the epoxide), and the gel preferred to form a solid structure with nanopores (Fig. 2b), resulting in a transparent gel. If the ratio was high (1:0.2), the gelation time was too long, and only some ill-defined precipitates could be acquired (Fig. 2c). As observed in Fig. 3, with the use of the epoxide that acted as a proton scavenger, solutions of common hydrated metal salts undergo hydrolysis and condensation reactions to form metal oxide gel networks [20]. During the process, ethanol dramatically increases the rate of reaction, which may be due to the improved nucleophilicity of the counterion (Cl ) in ethanol versus water [3], resulting in a more rapid opening of the protonated ring. Moreover, the decreased solubility of the sol particles in ethanol would also lead to a rapid condensation of the formed oligomers to yield solid gel networks [21]. Thus, in this study, with a high concentration of ethanol in the starting solution, the hydrolysis and condensation of the hydrated Al3+ was very fast after the epoxide was added to the solution, resulting in a solid and transparent gel. In contrast, when the gelation was performed in a water-dominated system, the transformation from solution to gel occurred much more slowly, and only some badly connected gel networks could be obtained. Therefore, the accurate selection of the H2O/ethanol ratio would lead to an appropriate gelation rate
and was demonstrated to be very important in the formation of hollow microspheres. The aging time of the gels also had a significant effect on the formation of hollow microspheres. Fig. 4 presents the SEM and TEM images of the structural evolution of the microspheres with increasing aging time. The monolithic gel is observed to consist of many badly attached solid spheres directly after the gelation (Fig. 4a and b). If the wet gel was dried in this state, significant shrinkage would occur, and the gel would crack into fragile particles. For the gel aged in isopropanol for 24 h, a core–shell structure can be observed (Fig. 4c and d). It was remarkable that an urchinlike structure on the surface of the microspheres started to emerge at this stage. Finally, when the gel was immersed in isopropanol for 48 h, the solid spheres completely transformed into urchin-like hollow spheres (Fig. 4e and f). Fig. 4g presents the XRD patterns of the alumina gels after various aging times. The structure of the gels changed from an amorphous phase to a crystalline phase (boehmite) with increasing aging time. Based on this phenomenon, the structural evolution can be explained as a self-templating process according to the Ostwald ripening mechanism. Fig. 5 illustrates the formation process of the urchin-like hollow microspheres. First, the solid spheres (formed directly after the gelation) were comprised of numerous smaller amorphous
Fig. 3. General reaction scheme for the epoxide-driven sol–gel process.
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Fig. 4. SEM (left) and TEM (right) images of alumina gels after various aging times: (a, b) 0 h, (c, d) 24 h, (e, f) 48 h. (g) XRD patterns of alumina gels with various aging times. (h) Nitrogen adsorption–desorption isotherms of the aerogel-like alumina which were prepared at 60 °C for different aging time. The inset shows the corresponding pore size distribution curve of the sample that aging time was 48 h calculated using the BJH method.
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Fig. 5. Scheme of structural evolution of the hollow spheres.
nanoparticles. According to Ref. [22], the solid particle could self-transform to a hollow structure by a redistribution of matter essentially within the same morphological boundary. In particular, hollow interior structures can be obtained by localized Ostwald ripening or differential diffusion (Kirkendall effect) within metastable solid microspheres [23,24]. The nanoparticles in the solid sphere thus tended to be dissolved and recrystallized onto the surface of the sphere through a diffusion pathway. A crystalline shell could be formed on the solid sphere, and the thickness of the crystalline shell increased gradually as the amorphous core became progressively depleted. Finally, the core of the sphere was completely dissolved, and a crystalline shell with a hollow inner was formed [22]. It was noticeable that during the aging process, the badly joined solid spheres started to connect with each other to form a stronger skeleton through recrystallization. This strengthened interacting urchin-like hollow structure could endure large capillary forces during the drying procedure under ambient pressure conditions, leaving a robust crack-free monolithic aerogel-like alumina structure. Fig. 4h shows the nitrogen adsorption–desorption isotherms of the aerogel-like alumina which were prepared at 60 °C for different aging time. The corresponding BJH pore size distribution of the sample that aging time was 48 h is depicted in the inset. The corresponding specific surface area calculated using the BET method was 4.0 m2/g (0 h), 370.7 m2/g (24 h), 505.6 m2/g (48 h). The BET surface area of the sample that aging time was 48h was similar to that of a common alumina aerogel [3] and mainly
contributed by the mesopores in the skeleton. The increasing BET surface area with aging time indicated that the mesopores were formed in the skeleton gradually, just as the SEM and TEM images revealed. The isotherm was type IV according to the IUPAC classification, indicating the existence of mesopores. A H2-type hysteresis loop was observed, which suggests the presence of ink-bottle mesopores. As revealed by the BJH pore size distribution, the median pore size was 10.5 nm, and pores were distributed in the mesopore region. The relatively sharp pore size distributions originated from the urchin-like structure. 3.2. High-temperature stability of aerogel-like alumina The high-temperature stability of the aerogel-like alumina was evaluated through heat treatment at 800 °C, 1000 °C and 1200 °C for 2 h. Fig. 6 shows photographs of the as-dried and 1200 °C heat-treated monolithic aerogel-like alumina. It is clear that the samples changed less in appearance and does not show much shrinkage after heat-treatment. Fig. 7a–c presents SEM images of the aerogel-like alumina heat-treated at various temperatures, and Fig. 7d presents the corresponding XRD patterns. Although the aerogel-like alumina transformed into c-phase alumina or hphase alumina with an increase of the heat-treatment temperature, the morphology of the urchin-like hollow spheres exhibited excellent stability against high temperature (up to 1200 °C). This finding indicated that the aerogel-like alumina was quite stable even when heat-treated at high temperature for a long time.
Fig. 6. Photographs of (a) as-dried and (b) 1200 °C heat-treated monolithic aerogel-like alumina.
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Fig. 7. SEM images of aerogel-like alumina that was heat-treated for 2 h at (a) 800 °C, (b) 1000 °C, and (c) 1200 °C. (d) XRD patterns and (e) stress–strain curves of the aerogellike alumina heat-treated for 2 h at various temperatures. (f) Nitrogen adsorption–desorption isotherms of aerogel-like alumina heat-treated at 800 °C, 1000 °C, 1200 °C, respectively.
Table 1 Bulk densities, room-temperature thermal conductivities and elastic moduli of the aerogel-like alumina materials heat-treated for 2 h at various temperatures and of traditional alumina and silica aerogels. Aerogel-like alumina
a b
Heat treatment temperature (°C)
25
800
1000
1200
Bulk density (g cm 3) Thermal conductivity (Wm Elastic modulus (MPa)
0.133 0.036 –
0.114 0.038 1.51
0.098 0.040 1.66
0.150 0.043 1.89
1
K
1 a
)
Alumina aerogelsb
Silica aerogelsb
0.037 0.029 0.55
0.040 0.031 0.43
The thermal conductivity was measured at P = 1 atm, 25 °C. Cited from Ref. [19].
Fig. 7e shows the stress–strain curves of the aerogel-like alumina heat-treated at various temperatures. The strength of the samples increased significantly with increasing heat-treatment temperature, which could be attributed to the reinforcement of the bonding force among the accumulated hollow spheres from the sintering process. Fig. 7f shows the nitrogen adsorption– desorption isotherms of the aerogel-like alumina which were heat-treated at 800 °C, 1000 °C, 1200 °C, respectively. It is clear that the adsorption–desorption isotherms of the samples kept unchanged when they were heated to 800 °C and 1000 °C, indicating that the microstructure of the alumina aerogel-like have not
affected by the heat-treatment at these temperatures. But when the sample was heated to 1200 °C, the area between loop-circles and the adsorbed volume of nitrogen at P/P0 = 1 in adsorption– desorption isotherms was reduced significantly. This means that some mesopores on the surface of hollow microspheres have disappeared, it was mainly caused by the sintering process and the crystalline phases change at 1200 °C. Table 1 lists the bulk densities, room-temperature thermal conductivities and elastic moduli of the aerogel-like alumina samples that were heat-treated at various temperatures. The properties of select traditional alumina or silica aerogels [25] are also presented
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in Table 1. The bulk density almost remained unchanged with increasing heat-treatment temperature, which also indicated the excellent high-temperature stability of the aerogel-like alumina. Table 1 indicates that the room-temperature thermal conductivities of the aerogel-like alumina samples were extremely low and similar to the values for the traditional alumina or silica aerogels. Moreover, the elastic moduli of the aerogel-like alumina samples were 200% higher than those of the traditional alumina or silica aerogels because of the strong skeleton formed through the recrystallization process and the reinforcement of the bonding force among the accumulated hollow spheres during the sintering process. 4. Conclusions In summary, we have successfully synthesized monolithic aerogel-like alumina with high-temperature stability via the accumulation of mesoporous hollow microspheres through ambient drying. The prepared monolithic aerogel-like alumina was crack-free and exhibited extraordinary physical properties similar to traditional alumina aerogels. The structural evolution of the hollow structure could be described through a self-templating method according to the Ostwald ripening mechanism. The room-temperature thermal conductivities and elastic moduli of the aerogel-like alumina samples that were heat-treated at various temperatures were observed to be near or superior to those of traditional alumina aerogels, which indicated that these aerogellike alumina materials could be used at higher temperature (up to 1200 °C). Thus, these aerogel-like alumina materials could replace aerogels in thermal insulation, catalysis and liquid separation applications, especially at high temperature (>1000 °C), where the traditional aerogels lose their extraordinary properties during the severe sintering process due to their high-surface free energy. In addition, because many metal oxides have been synthesized through the epoxide addition method, it can be expected that the synthesis method in this study can be used to fabricate a series
of monolithic aerogel-like metal oxides via the accumulation of mesoporous hollow sphere. References [1] A.E. Aliev, J. Oh, M.E. Kozlov, A.A. Kuznetsov, S. Fang, A.F. Fonseca, R. Ovalle, M.D. Lima, M.H. Haque, Y.N. Gartstein, M. Zhang, A.A. Zakhidov, R.H. Baughman, Science 323 (2009) 1575–1578. [2] J.L. Mohanan, I.U. Arachchige, S.L. Brock, Science 307 (2005) 397–400. [3] T. Baumann, A. Gash, S. Chinn, A. Sawvel, R. Maxwell, J. Satcher Jr., Chem. Mater. 17 (2005) 395–401. [4] J.K. Lee, G.L. Gould, W. Rhine, J. Sol–Gel Sci. Technol. 49 (2009) 209–220. [5] J.G. Seo, M.H. Youn, J.C. Jung, I.K. Song, Int. J. Hydrogen Energy 35 (2010) 6738– 6746. [6] R.T. Olsson, M.A.S.A. Samir, G. Salazar-Alvarez, L. Belova, V. Ström, L.A. Berglund, O. Ikkala, J. Nogues, U.W. Gedde, Nat. Nanotechnol. 5 (2010) 584–588. [7] K.T. Nam, D.W. Kim, P.J. Yoo, C.Y. Chiang, N. Meethong, P.T. Hammond, Y.M. Chiang, A.M. Belcher, Science 312 (2006) 885–888. [8] H.M. Xiong, Y. Xu, Q.G. Ren, Y.Y. Xia, J. Am. Chem. Soc. 130 (2008) 7522–7523. [9] R. Pekala, J. Mater. Sci. 24 (1989) 3221–3227. [10] N. Al-Yassir, R. Le Van Mao, Appl. Catal. A 7 (2007) 275–283. [11] A. Gash, T. Tillotson, J. Satcher Jr., J. Poco, L. Hrubesh, R. Simpson, Chem. Mater. 13 (2001) 999–1007. [12] A. Gash, T. Tillotson, J. Satcher Jr., L. Hrubesh, R. Simpson, J. Non-Cryst. Solids 285 (2001) 22–28. [13] Y. Tokudome, K. Fujita, K. Nakanishi, K. Miura, K. Hirao, Chem. Mater. 19 (2007) 3393–3398. [14] L. Gan, Z. Xu, Y. Feng, L. Chen, J. Porous Mater. 12 (2005) 317–321. [15] Y. Zhu, T. Ikoma, N. Hanagata, S. Kaskel, Small 6 (2009) 471–478. [16] C.Y. Cao, Z.M. Cui, C.Q. Chen, W.G. Song, W. Cai, J. Phys. Chem. C 114 (2010) 9865–9870. [17] A. Leo, S. Smart, S. Liu, J.C. Diniz da Costa, J. Membr. Sci. 368 (2011) 64–68. [18] X.W. Lou, Y. Wang, C. Yuan, J.Y. Lee, L.A. Archer, Adv. Mater. 18 (2006) 2325– 2329. [19] X.J. Wu, D. Xu, Adv. Mater. 22 (2010) 1516–1520. [20] C.N. Chervin, B.J. Clapsaddle, H.W. Chiu, A.E. Gash, J.H. Satcher Jr., S.M. Kauzlarich, Chem. Mater. 17 (2005) 3345–3351. [21] C.N. Chervin, B.J. Clapsaddle, H.W. Chiu, A.E. Gash, J.H. Satcher Jr., S.M. Kauzlarich, Chem. Mater. 18 (2006) 4865–4874. [22] J. Yu, H. Guo, S.A. Davis, S. Mann, Adv. Funct. Mater. 16 (2006) 2035–2041. [23] H. Cölfen, Rapid Commun. 22 (2002) 219–252. [24] Y. Sun, Y. Xia, Science 298 (2002) 2176–2179. [25] J. Poco, J. Satcher, J. Non-Cryst. Solids 285 (2001) 57–63.
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis of monolithic aerogel-like alumina via the accumulation of mesoporous hollow microspheres Li-ang Wu, Xvsheng Qiao, Shuo Cui, Zhanglian Hong, Xianping Fan ⇑ State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 7 July 2014 Received in revised form 25 September 2014 Accepted 7 October 2014 Available online 16 October 2014 Keywords: Aerogel-like alumina Monolith Hollow sphere High temperature stability Ambient drying
a b s t r a c t Monolithic aerogel-like alumina was synthesized via the accumulation of mesoporous hollow microspheres through an epoxide-driven sol–gel method. The addition of propylene oxide to the solution controls the gelation process, while the aging time and H2O/ethanol molar ratio in the initial component determine the morphology of the gel. The bulk density of the as-prepared sample was 0.133 g/cm3, and the BET surface area was 505.6 m2/g, which were very close to the values for the common alumina aerogel synthesized through the supercritical drying method. The microstructural evolution of the aerogel-like material as a function of aging time was carefully studied by SEM and TEM. The transition from solid particles to urchin-like mesoporous hollow microspheres can be explained as a self-templating process according to the Ostwald ripening mechanism. In addition, the samples were heat treated to 800 °C, 1000 °C and 1200 °C for 2 h. With an increase of the heat treatment temperature, the crystalline phases of the aerogel-like alumina varied from c-phase alumina to h-phase alumina. The microstructures as well as the physical properties, such as the thermal conductivity and elastic modulus, of samples heat-treated at different temperatures were also measured and revealed that the samples exhibited excellent stability against high temperature. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Thermal insulation materials have attracted an enormous amount of attention for their potential applications in aerospace and energy conservation during the last decade [1–3]. However, some limitations still exist in traditional thermal insulation materials because these materials cannot meet the increasing requirements for their density, porosity and mechanical properties in some hi-tech areas. A monolithic aerogel is a synthetic porous ultralight material and is famous for its high specific surface area, high porosity, low density and extremely low thermal conductivity. Monolithic aerogels appear to be appropriate candidates for future thermal insulation materials. Thus far, monolithic-aerogelbased materials have been widely studied and are expected to be utilized in thermal insulation [4], catalysis [5], magnetics [6], electronics [7] and optics [8]. For a long time, monolithic aerogels were only fabricated using the supercritical drying (SCD) method [9] because this process was able to diminish the capillary force built up in the pore walls of the gels and avoid the collapse of most of the pore volume during the drying process under ambient conditions. However, complicated
⇑ Corresponding author. http://dx.doi.org/10.1016/j.micromeso.2014.10.015 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.
facilities and extreme conditions caused the SCD technique to become a dangerous, expensive, multi-step and time- and energy-consuming procedure, which greatly limited its applications in industrial production. The weak high-temperature stability of the aerogels also limited their applications for thermal insulation. Alumina aerogels [10] undergo significant shrinkage and lose their extraordinary properties when heat-treated at high temperatures (>1000 °C) for a long time because of the severe sinter process caused by their high surface free energy. Consequently, it is very difficult for aerogels to be used at high temperature. It is necessary to find a new material with extraordinary physical properties and excellent high-temperature stability. Recently, Gash and co-workers [11,12] demonstrated a new route to obtain crack-free monolithic porous aerogels of various metal oxides including Al2O3 using an inorganic metal salt as the precursor through the SCD method. Typically, epoxides were added to aqueous solutions of inorganic metal salts to initiate gelation through a ring-opening reaction and worked as an acid scavenger to gradually increase the solution pH. The uniform increase in the solution pH results in homogeneous gelation to produce a monolithic gel. This method would be very useful for designing various monolithic aerogels with different microstructures and functions. The high-surface-area alumina aerogels [3], monolithic
L.-a. Wu et al. / Microporous and Mesoporous Materials 202 (2015) 234–240
Al2O3 with well-defined macropores and mesostructured skeletons [13] and monolithic alumina aerogel synthesized through ambient drying [14] were already synthesized through this method. However, some of them required SCD method and the weak high temperature stability still greatly limited their extensive applications. In this paper, we demonstrate a new facile route to synthesize monolithic aerogel-like alumina with high-temperature stability via the accumulation of mesoporous hollow microspheres through ambient drying. Materials with hollow structures have been widely studied recently because of their unique structure. These materials can be useful in drug storage and release [15], catalysis [16], separation [17], and lithium-ion batteries [18]. Many papers have described the formation of hollow spherical materials: traditional template-based methods, including those using hard [15] and soft [19] templates, and self-templating methods [18], which have been widely used in obtaining a hollow structure without an extra template. Nevertheless, nearly all of the materials with hollow structures were presented as powders or films; bulk materials with a hollow structure have been rarely reported. At the same time, alumina aerogel has been synthesized through ambient drying. In this study, the monolithic porous alumina gel was accumulated with urchin-like hollow spheres; the gel was crack-free even when dried at a rapid rate under ambient pressure drying conditions. The as-dried monolithic porous alumina gel exhibited aerogel-like properties such as a low bulk density, high surface area, high porosity and low thermal conductivity, even though the microstructure was absolutely different from a traditional aerogel. In addition, the aerogel-like alumina also exhibited excellent physical property stabilities even after being heat-treated at high temperature (up to 1200 °C). 2. Experimental 2.1. Synthesis of aerogel-like alumina Aluminum chloride hexahydrate (Aldrich) was used as the aluminum source, and a mixture of distilled water and ethanol (Sinopharm Chemical Reagent Co., Ltd.) was used as the solvent. Propylene oxide (Sinopharm Chemical Reagent Co., Ltd.) was added to initiate the condensation reaction. Isopropanol (Sinopharm Chemical Reagent Co., Ltd.) was used for the aging process. All of the reagents were used as received. Aluminum chloride hexahydrate (4.35 g, 18 mmol) was first dissolved in the mixture of distilled water and ethanol (using various H2O/EtOH molar ratios: 1:0.48, 1:1.18, 1:0.2, the corresponding real volume rations of H2O, EtOH is 4 ml, 5.5 ml; 2 ml, 7.5 ml; 6 ml, 2.5 ml). Propylene oxide (7 ml, 0.1 mol) was then rapidly added into the mixture solution, and stirring was continued for 20 s. The resultant homogeneous solution was sealed and maintained at 60 °C for gelation. As this reaction is an exothermic reaction and the solution would reach to about 60 °C just several second after the epoxide was added to the procurer. Then, the transparent solution turned into a translucent gel quite rapidly (in approximately 20 s). After gelation, the wet gel was aged with fivefold isopropanol for 2 days at 60 °C. Subsequently, the gel was dried by evaporation under ambient pressure condition. After the gel was completely dried, it was heat treated at various temperatures in air with a heating rate of 1 °C/min.
JEM-1200EX at 120 keV (JEOL Japan). The TEM samples were deposited onto carbon-coated, 3-mm diameter, copper electron microscope grids and dried in air. Nitrogen adsorption measurements (ASIC-2 Quantachrome Instruments USA) were performed to obtain the pore properties such as the BET-specific surface area and pore size distribution. The adsorption branch was used for the Barrett–Joyner–Halenda (BJH) calculation. Before measurement, the sample was outgassed under vacuum at 200 °C for 4 h. The bulk density was determined by measuring the rate of volume and weight of a rectangle sample. XRD analysis with Cu Ka (0.154 nm, X-98, Rigaku Japan) radiation was performed to identify the crystalline phases. All of the measurements were performed for the powder specimens prepared by grinding monolithic samples. Stress–strain curves were generated using a universal material tester (Zwieklroell Germany) with a load of 20 kN; the tests were performed in triplicate, and the speed was 1 mm/min. The elastic modulus was calculated based on the strain values between 100 and 500 N of stress. The thermal conductivities were measured using the hot-wire method (TC 3000 XIATECH China) at room temperature. 3. Results and discussion 3.1. Formation and characterization of aerogel-like alumina The monolithic aerogel-like alumina was synthesized using a very convenient procedure based on the epoxide-driven method established by Gash [12,13]. In the previous work, epoxide was used as the gelation initiator to induce the gelation of a metal salt solution, acting as an irreversible proton scavenger through a ring-opening reaction. In this work, as we utilized a relatively high metal-ion concentration and propylene oxide/metal-ion molar rate, the transition from a clear solution to opaque gel was completed in a very short time (approximately 20 s). This approach is quite different from the traditional aerogel preparation with epoxide-driven sol–gel process [3], which requires a long gelation time for the complete reaction. An opaque monolithic aerogel-like material could be obtained after the wet gel was aged for 2 days and dried under ambient pressure conditions. Fig. 1 presents a photograph of the as-dried monolithic aerogellike alumina. Fig. 2 presents SEM images of alumina gels prepared using various H2O/ethanol molar ratio. With an appropriate ratio of H2O/ethanol (1:0.48), urchin-like hollow microspheres can be obtained, the diameter of these hollow microspheres was about 1 2 lm (Fig. 2a); these hollow microspheres attached to each other and accumulated to form an opaque monolithic gel. When the H2O/ethanol ratio was low (1:1.18), the gelation time became
2.2. Characterization FE-SEM (S-4800, Hitachi, Japan) was used to observe the morphology of the gel structure using a slight Pt coating on the sample. TEM analysis was performed in bright-field mode using a
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Fig. 1. Photographs of monolithic aerogel-like alumina.
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Fig. 2. SEM images of alumina gels with various H2O/EtOH molar ratios: (a) 1:0.48, (b) 1:1.18, and (c) 1:0.2.
extremely short (with gelation occurring almost directly after the addition of the epoxide), and the gel preferred to form a solid structure with nanopores (Fig. 2b), resulting in a transparent gel. If the ratio was high (1:0.2), the gelation time was too long, and only some ill-defined precipitates could be acquired (Fig. 2c). As observed in Fig. 3, with the use of the epoxide that acted as a proton scavenger, solutions of common hydrated metal salts undergo hydrolysis and condensation reactions to form metal oxide gel networks [20]. During the process, ethanol dramatically increases the rate of reaction, which may be due to the improved nucleophilicity of the counterion (Cl ) in ethanol versus water [3], resulting in a more rapid opening of the protonated ring. Moreover, the decreased solubility of the sol particles in ethanol would also lead to a rapid condensation of the formed oligomers to yield solid gel networks [21]. Thus, in this study, with a high concentration of ethanol in the starting solution, the hydrolysis and condensation of the hydrated Al3+ was very fast after the epoxide was added to the solution, resulting in a solid and transparent gel. In contrast, when the gelation was performed in a water-dominated system, the transformation from solution to gel occurred much more slowly, and only some badly connected gel networks could be obtained. Therefore, the accurate selection of the H2O/ethanol ratio would lead to an appropriate gelation rate
and was demonstrated to be very important in the formation of hollow microspheres. The aging time of the gels also had a significant effect on the formation of hollow microspheres. Fig. 4 presents the SEM and TEM images of the structural evolution of the microspheres with increasing aging time. The monolithic gel is observed to consist of many badly attached solid spheres directly after the gelation (Fig. 4a and b). If the wet gel was dried in this state, significant shrinkage would occur, and the gel would crack into fragile particles. For the gel aged in isopropanol for 24 h, a core–shell structure can be observed (Fig. 4c and d). It was remarkable that an urchinlike structure on the surface of the microspheres started to emerge at this stage. Finally, when the gel was immersed in isopropanol for 48 h, the solid spheres completely transformed into urchin-like hollow spheres (Fig. 4e and f). Fig. 4g presents the XRD patterns of the alumina gels after various aging times. The structure of the gels changed from an amorphous phase to a crystalline phase (boehmite) with increasing aging time. Based on this phenomenon, the structural evolution can be explained as a self-templating process according to the Ostwald ripening mechanism. Fig. 5 illustrates the formation process of the urchin-like hollow microspheres. First, the solid spheres (formed directly after the gelation) were comprised of numerous smaller amorphous
Fig. 3. General reaction scheme for the epoxide-driven sol–gel process.
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Fig. 4. SEM (left) and TEM (right) images of alumina gels after various aging times: (a, b) 0 h, (c, d) 24 h, (e, f) 48 h. (g) XRD patterns of alumina gels with various aging times. (h) Nitrogen adsorption–desorption isotherms of the aerogel-like alumina which were prepared at 60 °C for different aging time. The inset shows the corresponding pore size distribution curve of the sample that aging time was 48 h calculated using the BJH method.
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Fig. 5. Scheme of structural evolution of the hollow spheres.
nanoparticles. According to Ref. [22], the solid particle could self-transform to a hollow structure by a redistribution of matter essentially within the same morphological boundary. In particular, hollow interior structures can be obtained by localized Ostwald ripening or differential diffusion (Kirkendall effect) within metastable solid microspheres [23,24]. The nanoparticles in the solid sphere thus tended to be dissolved and recrystallized onto the surface of the sphere through a diffusion pathway. A crystalline shell could be formed on the solid sphere, and the thickness of the crystalline shell increased gradually as the amorphous core became progressively depleted. Finally, the core of the sphere was completely dissolved, and a crystalline shell with a hollow inner was formed [22]. It was noticeable that during the aging process, the badly joined solid spheres started to connect with each other to form a stronger skeleton through recrystallization. This strengthened interacting urchin-like hollow structure could endure large capillary forces during the drying procedure under ambient pressure conditions, leaving a robust crack-free monolithic aerogel-like alumina structure. Fig. 4h shows the nitrogen adsorption–desorption isotherms of the aerogel-like alumina which were prepared at 60 °C for different aging time. The corresponding BJH pore size distribution of the sample that aging time was 48 h is depicted in the inset. The corresponding specific surface area calculated using the BET method was 4.0 m2/g (0 h), 370.7 m2/g (24 h), 505.6 m2/g (48 h). The BET surface area of the sample that aging time was 48h was similar to that of a common alumina aerogel [3] and mainly
contributed by the mesopores in the skeleton. The increasing BET surface area with aging time indicated that the mesopores were formed in the skeleton gradually, just as the SEM and TEM images revealed. The isotherm was type IV according to the IUPAC classification, indicating the existence of mesopores. A H2-type hysteresis loop was observed, which suggests the presence of ink-bottle mesopores. As revealed by the BJH pore size distribution, the median pore size was 10.5 nm, and pores were distributed in the mesopore region. The relatively sharp pore size distributions originated from the urchin-like structure. 3.2. High-temperature stability of aerogel-like alumina The high-temperature stability of the aerogel-like alumina was evaluated through heat treatment at 800 °C, 1000 °C and 1200 °C for 2 h. Fig. 6 shows photographs of the as-dried and 1200 °C heat-treated monolithic aerogel-like alumina. It is clear that the samples changed less in appearance and does not show much shrinkage after heat-treatment. Fig. 7a–c presents SEM images of the aerogel-like alumina heat-treated at various temperatures, and Fig. 7d presents the corresponding XRD patterns. Although the aerogel-like alumina transformed into c-phase alumina or hphase alumina with an increase of the heat-treatment temperature, the morphology of the urchin-like hollow spheres exhibited excellent stability against high temperature (up to 1200 °C). This finding indicated that the aerogel-like alumina was quite stable even when heat-treated at high temperature for a long time.
Fig. 6. Photographs of (a) as-dried and (b) 1200 °C heat-treated monolithic aerogel-like alumina.
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Fig. 7. SEM images of aerogel-like alumina that was heat-treated for 2 h at (a) 800 °C, (b) 1000 °C, and (c) 1200 °C. (d) XRD patterns and (e) stress–strain curves of the aerogellike alumina heat-treated for 2 h at various temperatures. (f) Nitrogen adsorption–desorption isotherms of aerogel-like alumina heat-treated at 800 °C, 1000 °C, 1200 °C, respectively.
Table 1 Bulk densities, room-temperature thermal conductivities and elastic moduli of the aerogel-like alumina materials heat-treated for 2 h at various temperatures and of traditional alumina and silica aerogels. Aerogel-like alumina
a b
Heat treatment temperature (°C)
25
800
1000
1200
Bulk density (g cm 3) Thermal conductivity (Wm Elastic modulus (MPa)
0.133 0.036 –
0.114 0.038 1.51
0.098 0.040 1.66
0.150 0.043 1.89
1
K
1 a
)
Alumina aerogelsb
Silica aerogelsb
0.037 0.029 0.55
0.040 0.031 0.43
The thermal conductivity was measured at P = 1 atm, 25 °C. Cited from Ref. [19].
Fig. 7e shows the stress–strain curves of the aerogel-like alumina heat-treated at various temperatures. The strength of the samples increased significantly with increasing heat-treatment temperature, which could be attributed to the reinforcement of the bonding force among the accumulated hollow spheres from the sintering process. Fig. 7f shows the nitrogen adsorption– desorption isotherms of the aerogel-like alumina which were heat-treated at 800 °C, 1000 °C, 1200 °C, respectively. It is clear that the adsorption–desorption isotherms of the samples kept unchanged when they were heated to 800 °C and 1000 °C, indicating that the microstructure of the alumina aerogel-like have not
affected by the heat-treatment at these temperatures. But when the sample was heated to 1200 °C, the area between loop-circles and the adsorbed volume of nitrogen at P/P0 = 1 in adsorption– desorption isotherms was reduced significantly. This means that some mesopores on the surface of hollow microspheres have disappeared, it was mainly caused by the sintering process and the crystalline phases change at 1200 °C. Table 1 lists the bulk densities, room-temperature thermal conductivities and elastic moduli of the aerogel-like alumina samples that were heat-treated at various temperatures. The properties of select traditional alumina or silica aerogels [25] are also presented
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in Table 1. The bulk density almost remained unchanged with increasing heat-treatment temperature, which also indicated the excellent high-temperature stability of the aerogel-like alumina. Table 1 indicates that the room-temperature thermal conductivities of the aerogel-like alumina samples were extremely low and similar to the values for the traditional alumina or silica aerogels. Moreover, the elastic moduli of the aerogel-like alumina samples were 200% higher than those of the traditional alumina or silica aerogels because of the strong skeleton formed through the recrystallization process and the reinforcement of the bonding force among the accumulated hollow spheres during the sintering process. 4. Conclusions In summary, we have successfully synthesized monolithic aerogel-like alumina with high-temperature stability via the accumulation of mesoporous hollow microspheres through ambient drying. The prepared monolithic aerogel-like alumina was crack-free and exhibited extraordinary physical properties similar to traditional alumina aerogels. The structural evolution of the hollow structure could be described through a self-templating method according to the Ostwald ripening mechanism. The room-temperature thermal conductivities and elastic moduli of the aerogel-like alumina samples that were heat-treated at various temperatures were observed to be near or superior to those of traditional alumina aerogels, which indicated that these aerogellike alumina materials could be used at higher temperature (up to 1200 °C). Thus, these aerogel-like alumina materials could replace aerogels in thermal insulation, catalysis and liquid separation applications, especially at high temperature (>1000 °C), where the traditional aerogels lose their extraordinary properties during the severe sintering process due to their high-surface free energy. In addition, because many metal oxides have been synthesized through the epoxide addition method, it can be expected that the synthesis method in this study can be used to fabricate a series
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