Nanoengineering Super Heat-Resistant, Strong Alumina Aerogels

Nov 20, 2013 - limits its high-temperature applications. Herein, super heat-resistant, strong alumina aerogels are prepared via a novel acetone-anilin...
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Nanoengineering Super Heat-Resistant, Strong Alumina Aerogels Guoqing Zu, Jun Shen,* Liping Zou, Wenqin Wang, Ya Lian, Zhihua Zhang, and Ai Du Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology, Pohl Institute of Solid State Physics, Tongji University, Shanghai 200092, P.R. China S Supporting Information *

ABSTRACT: Because of ultralow thermal conductivity, excellent catalytic activity, and better heat resistance than silica aerogel, alumina-based aerogel has drawn great interest as thermal insulators and catalysts. However, it is too fragile and sinters above 1000 °C (it shrinks drastically, >50%, and leaves the surface area as low as 10−70 m2/g at 1300 °C), which badly limits its high-temperature applications. Herein, super heat-resistant, strong alumina aerogels are prepared via a novel acetone-aniline in situ water formation (ISWF) method combined with novel modification techniques: supercritical fluid modification (SCFM) and hexamethyldisilazane gas phase modification. The heat resistance of alumina aerogel is enhanced up to 1300 °C via this method. The shrinkage of the optimized alumina aerogel is reduced to as low as 1 and 5% and the corresponding surface area reaches up to 152−261 and 125−136 m2/g after being heated to 1200 and 1300 °C for 2 h, respectively. The strength is significantly increased by more than 120% through SCFM. It also exhibits excellent thermal insulation properties at temperatures up to 1300 °C. This may significantly contribute to their practical ultrahigh-temperature applications in thermal insulations, catalysts, catalyst supports, etc. KEYWORDS: alumina aerogel, sol−gel, specific surface area, heat resistance, supercritical fluid modification

1. INTRODUCTION As a kind of nanoporous material with a high porosity of more than 90%, a high specific surface area, and extremely low density, aerogels show ultralow thermal conductivity and excellent catalytic activity and have drawn great interest in a wide range of applications such as thermal insulation, catalysts, catalyst supports, etc.1−3 However, practical applications of aerogels have always been restricted due to their fragility and sintering behavior during heat treatment. With the development of catalytic reactions and thermal insulations, hightemperature heat-resistant aerogels are in urgent need. For example, some catalytic reactions have been applied at temperatures of over 1000 °C for the treatment of gas emissions or catalytic combustion.4,5 Another example is that the required working temperature of insulators applied to the supersonic vehicles is as high as 1200 °C or even higher owing to the friction of high-speed vehicles and atmosphere. Because silica aerogels sinter at temperatures above 600 °C,6 significant efforts have been dedicated to the preparation of more heatresistant aerogels. It was found that alumina based aerogels exhibit enhanced thermal and chemical stability at high temperatures due to their unique nanostructure.7−10 With the exception of silicon carbide aerogels currently,11 the heat resistance of alumina aerogels is better than that of any other oxide aerogels, including those of titania, zirconia, niobia, etc.12−14 Alumina aerogels are usually prepared by a sol−gel synthesis route from hydrated alumina salts or aluminum alkoxides.15−27 The aluminum alkoxides derived aerogels show better thermal stability at high temperatures than those derived from hydrated alumina salts.15,18,26 However, preparation of alumina aerogel © 2013 American Chemical Society

monoliths from aluminum alkoxides is typically difficult because of the complex chemical pathways leading to gelation, high reactivity of the aluminum alkoxide to water and the susceptibility to cracking during drying.15 Up to now, only a few papers have described the preparation of monolithic alumina based aerogels using an aluminum alkoxide route that have high heat resistance.8,15,16 The specific surface area of pure alumina aerogel shows drastic decrease after heat treatment above 1000 °C, namely ∼100 m2/g at 1200 °C and 10−40 m2/ g at 1300 °C.8 It has been reported that heat resistance of alumina aerogels can be enhanced by adding additives such as silica, phosphoric oxide, barium oxide, lanthanum oxide or SiC whiskers.25,28 But the specific surface area is still decreased to 114.3 and 48.6−71.8 m2/g at 1200 and 1300 °C, respectively, and the shrinkage is not mentioned. According to reports, alumina-modified silica aerogels have heat resistance only to 1000 °C.29 J.F. Poco et al. have described monolithic alumina aerogels with shrinkage of 2% after heat treatment at 1050 °C.15 However, all the attempts have not described the nonshrinking alumina aerogels above the temperature of approximate1000 °C. Some important properties (e.g., strength, hydrophobicity, etc.) of aerogels can be improved via postgelation modification methodology, also known as cross-linking method. Deposition of polymeric cross-linkers on the skeletal structure of aerogels is an efficient method for enhance strength with minimum compromise in the internal porosity, specific surface area and Received: August 16, 2013 Revised: November 19, 2013 Published: November 20, 2013 4757

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Table 1. Sol−Gel Parameters of Typical Alumina Aerogels Obtained in the Present Study step one

step two

step three

sample

ASB (g)

EtOH (mL)

H2O (mL)

EtOH (mL)

HNO3 (mL)

H2O (mL)

acetone (mL)

aniline (mL)

A1 A2

4.64 4.64

12 3.5

0.15 0.1

0.5 0.3

0.05 0.03

0.1

2.0 2.0

1.0 2.0

thermal conductivity.30 There are several chemical routes to synthesis polymeric cross-linked aerogels including crosslinking of hydroxyl-rich surfaces with isocyanate-derived polymers and cross-linking of amine-rich surfaces with isocyanate-, epoxide-, and styrene-derived polymers.31 The strength of inorganic aerogels can also be enhanced by postgelation treatment with a hydrolyzable alkoxide such as tetraethylorthosilicate (TEOS).32 Going a step further, we introduce this postgelation modification methodology into the preparation of heat-resistant, strong alumina aerogels. This postgelation modification methodology realizes the manipulation of primary and secondary particles in nanoscale by introducing functional modifiers. It is well-known that the sinter behavior of metal oxide aerogels are mainly attributed to the nanoparticle evolutions during heat treatment. For this consideration, except for strength improvement, the transformation route of metal oxide aerogels may also be changed to prevent sintering by choosing appropriate modifiers and developing certain modification route to nanocast coatings on alumina surface. We recently have developed a novel sol−gel route for the synthesis of heat-resistant, strong metal oxide aerogels. This novel sol−gel route involves one new synthesis method of aerogels, namely acetone-aniline in situ water formation (ISWF) method, as well as two postgelation modification methodologies: supercritical fluid modification (SCFM) technique and hexamethyldisilazane (HMDS) gas phase modification technique. The acetone-aniline ISWF technique can realize slow in situ release of water, which leads to lower the hydrolysis and condensation of highly reactive metal alkoxide and thus transparent metal oxide gels with uniform microstructure could be obtained without using any chelating agents. The SCFM technique refers to chemical modification by partially hydrolyzed metal alkoxide or silica alkoxide such as tetraethoxysilane (TEOS) during supercritical fluid drying (SCFD). Supercritical condition of SCFM not only renders modification much faster and more complete but also enhances the degree of crystallinity of metal oxide aerogel, which can effectively improve its heat resistance. Gas phase modification is an efficient method to modify 3D nanoparticles.33,34 HMDS gas phase modification can further improve its heat resistance by restricting crystal growth upon heat treatment. In the work presented here, alumina aerogel are prepared through this sol− gel route. Robust and transparent alumina wet gels are easily prepared via the acetone-aniline ISWF method. Monolithic alumina aerogels are then obtained via high temperature SCFD with SCFM technique, of which the modifiers used are partially hydrolyzed aluminum trisec butoxide (ASB) and TEOS. After heat treatment at 1000 °C for 2 h, they are further modified by HMDS at 70 °C. The alumina aerogels obtained via this approach show high specific surface area, excellent mechanical strength, and super heat resistance up to 1300 °C.

Corporation (China). ASB was obtained from Zhejiang Ultrafine Powders &Chemicals Corporation (China). The polycrystalline mullite fiber was obtained from Tianjin Morgan Kundom High-tech Development Co., Ltd. All of the chemical reagents were used as received. Gel Preparation via Acetone-Aniline in Situ Water Formation (ISWF) Method. The alumina wet gels were prepared according to the following three steps. First, Alumina sec-butoxide (ASB) was dissolved in ethanol and distilled water at 60 °C with stirring. After turning clear, the mixture was then cooled to room temperature. Second, ethanol-diluted nitric acid was added into the mixture with stirring. Third, an appropriate amount of solution of methanol, distilled water, acetone and aniline with a certain molar ratio was added into above mixture. The mixture was stirred for 10 min and then poured into a glass beaker where the gel typically formed within 3 h. Table 1 details the sol−gel parameters for the synthesis of high heatresistant alumina aerogels. Supercritical Fluid Drying (SCFD) with SCFM. After aging and solvent exchanged with ethanol for a few days, the wet gel was put into an autoclave containing certain amount of ethanol with partially hydrolyzed Alumina sec-butoxide (ASB) and tetraethoxysilane (TEOS). The molar ratio of the total ethanol in autoclave to modifier ASB was approximate 460:1. The partially hydrolyzed ASB was prepared by dissolving ASB in the mixture of ethanol and distilled water at 60 °C with stirring for 5 min. The molar ratio of ASB: EtOH: H2O was kept at 1:14.5:0.6. The partially hydrolyzed TEOS was prepared by mixing TEOS, ethanol, distilled water and HNO3 in a molar ratio of TEOS:EtOH:H2O:HNO3 = 1:3.8:1:0.035, and stirring for 10 min. The molar ratio of the added ASB to Al3+ in alumina aerogel was in range 0−0.7:1. The molar ratio of added ASB to TEOS was kept at 4.39:1. After it was sealed, ultra pure, dry nitrogen gas was flushed in the autoclave to produce an oxygen free atmosphere and as a safety precaution. The autoclave temperature was raised to 300 °C at a rate of 1 °C/min while the pressure rose and was controlled at ∼15 MPa. It was maintained at 300 °C and 15 MPa for 2 h, and the autoclave was then decompressed slowly at a rate of 30 kPa/min. Finally, the system was cooled to room temperature naturally and the alumina aerogel was removed. HMDS Gas Phase Modification. The as-prepared alumina aerogel was heated to 1000 °C for 2 h. After cooling down to room temperature, it was placed in a closed container, into which 5 mL of HMDS per gram of aerogel was injected. The HMDS was put beneath aerogel monolith, and the aerogel was separated from HMDS by putting the aerogel on a wire mesh in the container. The closed container was then heated to 70 °C. After treated for 4 days at 70 °C, it was taken out and heated at 100 °C for about 2 h to remove the residual HMDS and chemicals. Finally, the modified alumina aerogel was heated to 1200 °C and then to 1300 °C for 2 h respectively. It is reported that density of alumina-based aerogel largely affects the thermal stability to sintering,28 to characterize this effect alumina aerogels with different densities were prepared. Samples were given the prefix A and the numbers that follow indicate different densities. The character after the hyphen denotes that the alumina aerogel is modified by partially hydrolyzed ASB and TEOS during drying, and the numbers that follow clarify the molar ratio of the added ASB to Al3+ in alumina aerogel. The last character H denotes the alumina aerogel with modification by HMDS. Characterization. The bulk density of alumina aerogels was determined by ρ = M/V where ρ, M, and V are bulk density, mass and volume (obtained by V = πD2h/4, where D and h are diameter and height of the aerogel disk) of the aerogels respectively. The morphology of the sample was characterized by a transmission

2. EXPERIMENTAL SECTION Materials. Ethanol, distilled water, TEOS, acetone, aniline, HMDS, and HNO3 (68%) were purchased from Sinopharm Chemical Reagent 4758

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electron microscope (TEM, JEOL-1230) and a scanning electron microscope (SEM, XL30FEG, Netherland). The nature of phases in the aerogels was analyzed by powder X-ray diffraction (XRD) in a Rigata/max-C diffractometer using the Cu−Kα radiation. Organic groups were investigated by a Fourier transform infrared spectroscope (FTIR, TENSOR27, Germany). The pore size distribution and specific surface area were measured by a N2 adsorption analyzer (TriStar 3000, USA) using the BET nigrogen adsorption/desorption technique. The elastic modulus was measured by a dynamic mechanical analyzer (DMA8000, USA). The thermal conductivities were measured by a hotdisk thermal analyzer (TPS2500, Sweden).

As can be seen from Figure S2 in the Supporting Information, there is no N―C or NC bands found in the FTIR spectra. The water is generated uniformly in the sol and would react with surrounding ASB and thus hydrolysis of ASB takes place. The hydrolysis reaction is shown in eq 2. Al(OC4 H 9)3 + nH 2O → Al(OH)n (OC4 H 9)3 − n + nC4 H 9OH

(2)

The product of the hydrolysis reaction is expected to undergo condensation reactions in the presence of acid catalysts (eqs 3 and 4)

3. RESULTS AND DISCUSSION Formation of Gels. Scheme 1 shows the sol−gel process via the acetone-aniline in situ water formation (ISWF) method

−Al − OC4 H 9 + HO − Al − → Al − O − Al − +C4 H 9OH

Scheme 1. (a) Sol−Gel Process via Acetone-Aniline in Situ Water Formation (ISWF) Method; (b) Chemical Strategies for the Synthesis of Super Heat-Resistant Alumina Aerogel

(3)

−Al − OH + HO − Al − → −Al − O − Al − +H 2O (4)

The slow water formation leads to lower the hydrolysis and condensation of aluminum alkoxide and thus transparent alumina gels with uniform microstructure could be obtained without using any chelating agents. The molar ratio of the total amount of added water and in situ formed water to ASB, namely n in eq 2, is kept in range of (1.3−2.5):1 so that ASB can be adequately hydrolyzed. The reason we choose this values is that condensation reaction during sol−gel process would release water as is shown in eq 4, so the molar ratio of the practically needed water to ASB is below 3:1. Because each mole of acetone or aniline will produce one mole of water, the formed water is effectively controlled by the amount of added acetone and aniline. In addition, the residual aniline can be readily oxidized in this system during age of wet gel, which is confirmed by the color changes of the gel during age when the aniline is excess. This would introduce new impurity and should be avoided. To render aniline to react completely, excess acetone is added. The molar ratio of acetone to aniline is in range (1.25−2.5):1. From FTIR spectra of the as-prepared alumina aerogel (see Figure S2 in the Supporting Information), we can see that the band of −CH3 is weak and these bands of Al−O−H and A−O are intense. These OH ligands can only have been formed from water, which confirms the dehydration reaction of acetone and aniline. This acetone-aniline ISWF method could also be applied to prepare other metal oxide aerogels from highly reactive metal alkoxide such as zirconium n-butoxide, tetra-nbutyl titanate, etc. In the first step during sol−gel process, to dissolve ASB in ethanol, a small amount of distilled water needs to be added. The molar ratio of added H2O to ASB is in the range (0.2− 0.9):1. When the molar ratio of H2O to ASB is blow 0.2:1, ASB could not be completely dissolved in ethanol. When the molar ratio of H2O to ASB is above 0.9:1, only precipitation Al(OH)3 is obtained. This is because aluminum alkoxides are readily hydrolyzed by water, producing aluminum mono- or trihydroxides. Only the monohydroxide thus formed can be peptized to a clear sol. In this step, temperature is another important factor. When the temperature is below 50 °C, the solubility of ASB in ethanol is poor. It is found that the temperature of >50 °C is better for solving ASB in ethanol. Therefore, we maintain the temperature at 60 °C. In this system, nitric acid not only catalyzes the hydrolysis and condensation of ASB but also catalyzes the dehydration

and chemical strategies for the synthesis of super heat-resistant alumina aerogel. In order to obtain transparent and rigid alumina wet gels with uniform microstructure from aluminum alkoxide, a chelating agent such as acetylacetone, ethyl acetoacetate, or acetic acid is usually used.9,16,35 Although it prolongs the gelation time and produces the transparent wet gels, it introduces organic ligands bonded to Al, which lowers the cross-linking density and skeletal strength. Meanwhile, the organic ligands would decompose when heat treated, which results in lower thermal stability. For this reason, we introduce a novel acetone-aniline in situ water formation (ISWF) technique into the sol−gel process of synthesizing alumina aerogels. During gel preparation, appropriate amounts of acetone and aniline are added. Little or even no water is added to the sol solution, because acetone and aniline would react under acidic conditions to slowly release water in situ (eq 1).This reaction not only releases water that we want but also

generates byproduct imine (C6H5NC(CH3)2). The byproduct imine does not react with ASB, which can be confirmed by the FTIR spectra (see Figure S2 in the Supporting Information) of the as-prepared alumina aerogel. 4759

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Additional modifier continues to accumulate on and around secondary particles. HMDS Gas Phase Modification. During heat treatment at elevated temperature, the constituent particles of the alumina aerogel are pulled together through condensation of neighboring hydroxyl groups, which allows these particles to grow to a size much larger than that of the original particles. This usually results in a series of phase transitions, decrease of surface area and even sintering of the alumina aerogel. Crystal growth upon heat treatment can be effectively inhibited by introduction of another functional group that can produce small particles that restrict growth of grain boundaries at elevated temperatures.31 For this reason, in addition to partially hydrolyzed ASB, we introduce an appropriate amount of partially hydrolyzed TEOS during SCFM. After heated at 1000 °C for 2 h, the alumina surface would contain more −OH due to the decomposition of the residual organic groups. The alumina aerogel is then treated by another modifier, namely HMDS, at 70 °C so that the heat resistance is further enhanced. During HMDS gas phase modification, liquid phase HMDS is expected to vaporize and react with the hydroxyl groups on the alumina surface (including not only Al−OH but also Si−OH introduced through SCFM as described above) to form methyl siloxyl groups on the alumina surface (eq 9)38,39Decomposition of

reaction of acetone and aniline. It is found that the more nitric acid is added the faster the acetone reacts with aniline and the faster the gelation occurs. The molar ratio of HNO3 to ASB is kept in range (0.02−0.05):1. In this case, the gelation time is approximate 2−3 h for both samples A1 and A2. Supercritical Fluid Modification (SCFM). During SCFD, maintaining the alumina aerogels at high temperature and high pressure causes the boehmite crystals to develop more and leads to larger specific surface area of the alumina aerogels as well as structural strengthening.28 This makes the alumina aerogel more heat-resistant because the formed boebmite crystals change the transformation route of the alumina aerogel to prevent sintering. For this reason, we keep the supercritical drying at 300 °C and 15 MPa, which is far above the supercritical point of ethanol (243 °C, 6.38 MPa). Alumina aerogel with boehmite (AlO(OH)) structure contains a great amount of Al−OH in the alumina surface.18,36 Therefore, we developed a supercritical fluid modification (SCFM) technique in which partially hydrolyzed ASB and TEOS are introduced into the autoclave and the Al−OH in the alumina surface may react with partially hydrolyzed ASB and TEOS during SCFD. The molecular formulas of partially hydrolyzed ASB and TEOS could be expressed as Al(OH)x(OC4H9)3−x and Si(OH)y(OC2H5)4−y, respectively. According to the molar ratio of ASB or TEOS to water shown in the Experimental Section, the initial value of x or y is approximately 1.0. It should be noted that a large amount of ethanol in the autoclave contains approximate 0.3% water. Considering the total ethanol in the autoclave, the molar ratio of the water contained in 99.7% ethanol to modifier ASB is about 3.6:1. Therefore, Al(OH)x(OC4H9)3−x and Si(OH)y(OC2H5)4−y would probably undergo further hydrolysis when the temperature and pressure are raised during SCFD, resulting in larger values of x and y. During SCFM, these reactions are assumed to take place:

these siloxyl groups in air above 350 °C would be expected to produce SiO2 particles in the alumina surface, which could restrict the alumina particle growth during heat treatment and which could form a mullite phase at sufficiently high temperatures. The modification mechanism is schematically represented in Scheme 2.

−Al − OH + (OH)x Al(OC4 H 9)3 − x → −Al − O − Al(OH)x − 1(OC4H 9)3 − x + H 2O

Scheme 2. Schematic Representation of Modification Mechanism of Primary Alumina Particles of Heat-Resistant Alumina Aerogel

(5)

−Al − OH + (OC4 H 9)3 − x Al(OH)x → −Al − O − Al(OH)x (OC4H 9)2 − x + C4H 9OH

(6)

−Al − OH + (OH)y Si(OC2H5)4 − y → −Al − O − Si(OH)y − 1(OC2H5)4 − y + H 2O

(7)

−Al − OH + (OC2H5)4 − y Si(OH)y → −Al − O − Si(OH)y (OC2H5)3 − y + C2H5OH

(8)

The formed products in eqs 5−8 such as −Al−O−Al(OH)x−1(OC4H9)3−x are presumed to react further with partially hydrolyzed ASB and TEOS. Hence, deposition and growth of the interparticle cross-linker on the alumina surface take place under supercritical conditions, which leads to more boehmite particles, a higher degree of crystallinity and high strength of alumina aerogel. Monolithic alumina aerogels treated via this method would be presumed to possess better heat resistance. According to Dhairyashil P. Mohite’s report,37 the accumulation of the cross-linking modifier follows the hierarchical structure of alumina. The modifier forms conformal coating around primary particles and then fills secondary particles.

Morphology and Appearance of Alumina Aerogel. The morphology of the typical alumina aerogels is shown in the TEM images (Figure 1). From TEM images we can see that the as-prepared unmodified alumina aerogel (A1) exhibits randomly interconnected networks made up of nanometersized leaf-like particles with thickness of 1−3 nm and length of 20−80 nm, whereas the as-prepared modified alumina aerogel (A1-A0.7-H) shows larger leaflike particles with thickness of 2− 6 nm and length of 50−250 nm. After heat treatment at 1200 4760

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Figure 1. TEM images of the alumina aerogels before and after heat treatment at high temperatures: (a) unmodified sample A1, (b) A1A0.7-H, (c) A2-A0.7-H.

Figure 2. Photograph of typical alumina aerogels before and after heat treatment at high temperatures: (a) unmodified sample A1, (b) unmodified sample A2, (c) A1-A0.7-H, (d) A2-A0.7-H.

°C for 2 h, the unmodified alumina aerogel (A1) is composed of much thicker rodlike particles with thickness of 6−16 nm and length of 40−120 nm, whereas the modified one (A1-A0.7H) retains the leaflike particles with thickness of 4−8 nm and length of 80−350 nm. This confirms that alumina particle growth upon heat treatment is restricted after modification. Electron diffractions of the alumina aerogels (the inset in Figure 1a,b) show that the as-prepared modified alumina aerogel shows several distinct reflections that are not observed in unmodified one, which indicates the higher degree of crystallinity of the modified alumina aerogel compared to that of the unmodified one. Nevertheless, it should be noted that the degree of crystallinity of the modified alumina aerogel becomes lower than that of the unmodified one after heat treatment at 1200 °C for 2 h, which is confirmed by the weaker electron diffraction spot. In addition, the alumina aerogel with larger density (A2-A0.7-H) shows a higher degree of crystallinity compared to that of the sample with lower density (A1-A0.7-H), and it still retains the leaflike particles even after heat treatment at 1300 °C for 2h as can be seen in Figure 1c. The appearances and textural properties of the alumina aerogels after drying and heat treatment are shown in Figure 2 and Table 2, respectively. The as-prepared monolithic alumina aerogels are translucent or opaque depending on the extent of modification by ASB and TEOS. They become less transparent with more added modifier ASB and TEOS. This is because that more secondary particles will cluster together, forming larger domains (see Figure S1 in the Supporting Information) that promote light scattering and haziness. From Figure 2, we can also clearly see the shrinkage of alumina aerogels after heat treatment. The unmodified alumina aerogel shows drastic shrinkage (∼43% linear) after heat treatment at 1200 °C for 2 h, whereas the modified one shrinks little after heat treatment at 1200 °C and even at 1300 °C for 2 h. Heat Resistance at High Temperatures. It is found that supercritical fluid modification (SCFM) significantly improves

the heat resistance of alumina aerogels. Keeping the molar ratio (4.39:1) of ASB to TEOS and the amount (5 mL per gram of aerogel) of HMDS constant, the more modifier (ASB and TEOS) added, the less the alumina aerogel shrinks after heat treatment above 1000 °C. As is shown in Figure 3a, b and Table 2, the alumina aerogel without SCFM (A1-H) shrinks as much as 32% and 51% after heat treatment at 1200 and 1300 °C for 2 h, respectively, whereas the corresponding modified aerogel (A1-A0.7-H) shrinks only 2 and 9% when the molar ratio of added modifier ASB to Al in aerogel is 0.7:1. This is because more boehmite crystals are produced with more added modifier ASB during SCFM and crystal growth upon heat treatment is effectively restricted by more TEOS enhancing the heat resistance and thus lowering the shrinkage. The presence of boehmite crystals in modified alumina aerogel is confirmed by XRD analysis below. The molar ratio of the modifier ASB to TEOS is another important factor that significantly affects the heat resistance of the resulting alumina aerogel. When no ASB is added, the goal of more boehmite particles and higher degree of crystallinity could not be realized during SCFM and thus the heat resistance could not be effectively enhanced. With no ASB modification, the aerogel shrinks about 40% after heat treatment at 1200 °C for 2 h. When no TEOS is added, the crystal growth upon heat treatment could not effectively be restricted and thus the heat resistance could not be effectively enhanced. It is found that when the molar ratio of modifier ASB to TEOS is 4.39:1, the alumina aerogel shows excellent heat resistance. Therefore, we have maintained the molar ratio of ASB to TEOS at this value for all the samples modified by this method. Correspondingly, the finally obtained alumina aerogel after SCFM and HMDS gas phase modification owns a small number of silica and the molar ratio of Al to Si is approximate 12.4:1 for sample A1A0.7-H (see Figure S3 and Table S1 in the Supporting Information). 4761

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Table 2. Textural Properties of the Alumina Aerogels after Drying and after Heat Treatment at 1200 and 1300 °C (in parentheses) SBETc (m2/g) sample

ρinit (mg/cm )

A1 A1-H A1-A0.2-H A1-A0.5-H A1-A0.7-H A2-A0.5-H A2-A0.7-H aluminaf

61 61 65 71 78 120 135 37

a

3

Einitb

(MPa)

0.93 0.93 1.2 1.6 2.3 4.7 5.6 0.55

ΔL/L0e (%)

dd (nm)

300 °C

1200 °C (1300 °C)

300 °C

1200 °C (1300 °C)

1000 °C

1200 °C (1300 °C)

525 525 468 349 317 235 214 376

87(62) 95 261 185 166(136) 159 152(125)

27.2 27.2 16.2 16.6 15.8 13.7 13.1

7.1(6.5) 7.5 7.4 9.2 9.9(9.1) 12.2 12.6(11.8)

10 10 3 0 0 0 0 2

43(59) 32(51) 15 5 2(9) 2 1(5) 33(54)g

a Initial density before heat treatment. bInitial elastic modulus. cBET specific surface area obtained from nitrogen adsorption measurements. dMean pore diameter obtained from nitrogen adsorption branch and Barrett−Joyner−Halenda (BJH). eShrinkage after heat treatment. L0 is the original diameter of the sample and ΔL is diameter decrement. fAlumina aerogel prepared by J. F. Poco et al; gThe shrinkage of alumina aerogel at 1200 and 1300 °C is not mentioned in the report, and this values are obtained from heat treatment of the alumina aerogel that is prepared ourselves according to J. F. Poco’ method.

as follows. First, higher density increases the contact points and thus develops more boehmite particles, which strengthens the skeletons and hinders the sintering at high temperature. Second, the higher density provides less free pore volume to collapse. The FTIR spectra for sample A2-A0.5-H after HMDS gas phase modification (see Figure S2 in the Supporting Information) confirms that HMDS has reacted with the hydroxyl surface groups. From Table 2 we can see that sample A1-H shows lower shrinkage than that of A1 after heat treatment at 1200 °C, confirming that heat resistance of alumina aerogel is further improved by HMDS gas phase modification. Figure 4 provides the XRD patterns for typical alumina aerogels. The as-prepared unmodified sample A2 shows broad

Figure 3. (a) Influence of molar ratio of modifier ASB to Al in aerogel during SCFM on shrinkage and specific surface area of alumina aerogels after heat treatment at 1200 °C for 2 h. All the samples are derived from the wet gel of A1 and treated by the same amount of HMDS. The molar ratio of added ASB to TEOS is kept at 4.39:1. (b) Shrinkage of alumina aerogels with heat treatment temperature.

It should be noted that the specific surface area after heat treatment at 1200 °C does not decrease with increase of modifier ASB and TEOS during SCFM, which can be seen in Figure 3a and Table 2. The specific surface area shows a maximum value (261 m2/g) when the molar ratio of modifier ASB to Al in aerogel is 0.2:1. This could be explained as follows. Because the shrinkage is reduced with the increase in the amount of modifier, the surface area after heat treatment is enhanced at the start. However, with further increasing the amount of modifier, more nanoparticles would cluster together, which reduces mesopores as well as specific surface area. In addition, although the as-prepared modified alumina aerogel shows lower surface area compared to the unmodified one, the specific surface area is much higher than that of the unmodified aerogel after heat treatment at temperatures above 1000 °C. For example, the specific surface area of the sample with molar ratio of modifier ASB to Al of 0.7:1 (A1-A0.7-H) is 317 m2/g, which is lower than that of the unmodified sample A1 (525 m2/ g). However, surface areas of 166 m2/g and 136 m2/g are retained after heat treatment at 1200 and 1300 °C for 2 h, respectively, which is much higher than that of the unmodified aerogel (87 m2/g and 62m2/g respectively). Density also plays an important role on heat resistance of alumina aerogel. As we can see from Table 2, sample A1-A0.5H (71 mg/cm3) shrinks 5%, whereas sample A2-A0.5-H (120 mg/cm3) shrinks only 2% after heat treatment at 1200 °C for 2 h. This suggests that higher density leads to less shrinkage. By comparing sample A1-A0.7-H with A2-A0.7-H, we may draw the same conclusion. This is probably attributed to two reasons

Figure 4. XRD patterns for (a) unmodified sample A2 and (b) A2A0.7-H after drying and heat treatment at high temperatures. (⧫) boehmite, (■) δ-Al2O3, (▲) θ-Al2O3, (•) α-Al2O3.

diffraction peaks at 15, 28, 38, 50, 56, 65, and 72 that correspond to boehmite AlO(OH).18,41−43 The as-prepared modified sample A2-A0.7-H, by contrast, gives more narrow and intense peaks, indicating higher degree of crystallinity than that of unmodified sample A2. The boehmite structure of the modified sample is also confirmed by FTIR spectra (see Figure S2 in the Supporting Information). After heat treatment at 1000 °C for 2 h, the phase of A2-A0.7-H is δ-Al2O3, whereas modified sample A2 is changed to θ-Al2O3.44 After heat treatment at 1200 °C for 2 h, θ-Al2O3 emerges for both modified and unmodified samples.45 It is worth noting that the modified alumina aerogel exhibits weaker peaks of θ-Al2O3 than those of unmodified one, which indicates the lower degree of crystallinity of modified alumina aerogel compared to the unmodified one after heat treatment at 1200 °C. After further 4762

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heat treatment at 1300 °C for 2 h, A2-A0.7-H still remains θAl2O3, whereas unmodified sample A2 not only shows θ-Al2O3 but also includes an α-Al2O3 phase. This indicates that transformation to α-Al2O3 is inhibited during heat treatment at 1300 °C after modification. Both XRD and TEM analysis confirm that the as-prepared alumina aerogel shows high degree of crystallinity and crystal growth upon heat treatment is effectively restricted after SCFM and HMDS gas phase modification. Alumina particles are connected by “necks” (the secondary particle necks can be seen from Figure S1a in the Supporting Information), which are reduced because some particles would cluster together (Figure S1b in the Supporting Information shows some clustered secondary particles) after modification by partially hydrolyzed ASB and TEOS.40 On the convex surfaces of nanoporous materials, the vacancy concentrations (n) are given by eq 10 n = noexp(Ωγ /ρkT )

same as that after drying (13.1 nm). This also confirms the highest heat resistance for sample A2-A0.7-H. Sample A2-A0.7H also exhibits excellent mechanical strength. The elastic modulus is as high as 5.6 MPa, which is more than 120% higher than that of the unmodified one (2.5 MPa). High-Temperature Thermal Insulations. The thermal conductivity of sample A2-A0.7-H at room temperature is as low as 0.031 W/mK, which is similar to those of reported silica (∼0.031 W/mK) and alumina (∼0.029 W/mK) aerogels.15 This indicates that it is an excellent thermal insulator. Because of its super heat resistance and nanoporous microstructure at ultrahigh temperature, it also shows excellent high-temperature thermal insulation property. This is confirmed by the hightemperature thermal insulation test as shown in Figure 6. A

(10)

where n0 is the vacancy concentration on the flat surface, Ω is the atom volume, γ is the surface free energy, ρ is the surface curvature, k is the Boltzmann constant, and T is the absolute temperature.46 Because the surface curvature is minus at the neck, the vacancy concentration is large, which is favorable for rearrangement of atoms. This means that α-phase nucleation is easy to occur at the neck. Because the necks are reduced after modification by hydrolyzed ASB and TEOS during SCFM, the α-phase nucleation is inhibited during heat treatment at elevated temperatures up to 1300 °C, and the heat resistance is enhanced accordingly. Among the samples listed in Table 2, A2-A0.7-H shows the highest heat resistance. It shrinks only 1 and 5% in linear after heat treatment at 1200 and 1300 °C for 2 h, respectively. This shrinkage is much lower than that of the reported one (the shrinkage of the alumina aerogel prepared according to J. F. Poco’ method is 33% at 1200 °C and 54% at 1300 °C) (Table 2). As can be seen from Table 2 and Figure 5, compared to unmodified sample A1, A1-A0.2-H with less added modifier ASB and TEOS and A1-A0.7-H with lower density, A2-A0.7-H exhibits the smallest variation of average pore diameter and pore size distribution before and after heat treatment at 1200 and 1300 °C. Its average pore diameter after heat treatment (12.6 nm at 1200 °C and 11.8 nm at 1300 °C) is nearly the

Figure 6. (a) Photograph of the high-temperature thermal insulation test of the super heat-resistant alumina aerogel (A2-A0.7-H). The alumina aerogel effectively insulates the flower against the fire with temperature up to 1300 °C. (b) Temperature changes of upper surface of sample A2-A0.7-H, polycrystalline mullite fiber felt (122 mg/cm3), and board (245 mg/cm3) with firing time at 1300 °C. All the samples have the same shape and size (φ44 mm × 8 mm).

fresh flower is put on the heat-resistant alumina aerogel (with thickness of only 8 mm), under which the fire is burning with temperature up to 1300 °C. The flower does not show any damage after continuous firing for 5 min. We further test its thermal insulation property by measuring the temperature changes of upper surface of the sample with firing time at 1300 °C. It is compared with traditional high-temperature thermal insulators. The traditional insulators with heat resistance above 1200 °C are mainly refractory fibers (including aluminosilicate fiber, mullite fiber, alumina fiber, zirconia fiber, etc.) and porous ceramics (including ZrO2, SiC, Al2O3 ceramics, etc.). In the test, we choose one typical traditional fiber: polycrystalline mullite fiber in the form of either felt (122 mg/cm3) or board (245 mg/cm3). All the samples are disk-like monoliths and have the same size (φ44 mm ×8 mm). As can be seen from Figure 6b that the temperature of upper surface of sample A2-A0.7-H

Figure 5. Pore size distributions for the samples (a) unmodified sample A1, (b) A1-A0.2-H, (c) A1-A0.7-H and (d) A2-A0.7-H. 4763

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is only 75 °C after firing for 5 min at 1300 °C, which is much lower than those of polycrystalline mullite fibers (290 °C for the felt and 233 °C for the board after firing for 5 min). This indicates that the high-temperature thermal insulation property of the super heat-resistant alumina aerogel is much better than those of traditional high-temperature thermal insulators such as polycrystalline mullite fiber felt and board.

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4. CONCLUSIONS In summary, monolithic alumina aerogels are prepared via a novel acetone-aniline ISWF method combined with SCFM and HMDS gas phase modification techniques. The obtained alumina aerogels show super heat resistance and excellent thermal insulation property up to 1300 °C. Three main reasons contribute to the improved heat resistance and excellent mechanical strength. First, acetone-aniline ISWF leads to transparent and robust alumina gels with uniform microstructure. Second, alumina aerogel with a high degree of crystallinity identified by XRD as boehmite, and high strength is obtained after SCFM. Third, crystal growth upon heat treatment is effectively restricted through modification by TEOS during SCFD and HMDS after drying. This synthesis method could also be applied to prepare other heat-resistant, strong metal oxide aerogels. Further works are underway to (a) measure the hightemperature thermal conductivity; (b) study the long-term behavior (e.g., after 200, 500, or more hours at higher temperatures); (c) investigate possible temperature shocks during use of the alumina aerogels in severe industrial environments; and (d) apply this method to preparation of other heat-resistant, strong metal oxide aerogels such as titania aerogel, zirconia aerogel, etc.

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ASSOCIATED CONTENT

S Supporting Information *

SEM images, FTIR spectra, and EDX elements analysis data for typical alumina aerogels. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is financially supported by National Natural Science Foundation of China (11074189, U1230113), National key Technology R&D Program of China (2013BAJ01B01) and Shanghai Committee of Science and Technology (11 nm0501600, 11 nm0501300).

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REFERENCES

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