Preparation of silica aerogels with high temperature resistance and low thermal conductivity by monodispersed silica sol

Journal Pre-proof Preparation of silica aerogels with high temperature resistance and low thermal conductivity by monodispersed silica sol

Huafei Cai, Yonggang Jiang, Jian Feng, Sizhao Zhang, Fei Peng, Yunyun Xiao, Liangjun Li, Junzong Feng PII:

S0264-1275(20)30174-X

DOI:

https://doi.org/10.1016/j.matdes.2020.108640

Reference:

JMADE 108640

To appear in:

Materials & Design

Received date:

11 November 2019

Revised date:

11 March 2020

Accepted date:

12 March 2020

Please cite this article as: H. Cai, Y. Jiang, J. Feng, et al., Preparation of silica aerogels with high temperature resistance and low thermal conductivity by monodispersed silica sol, Materials & Design (2020), https://doi.org/10.1016/j.matdes.2020.108640

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© 2020 Published by Elsevier.

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Preparation of silica aerogels with high temperature resistance and low thermal conductivity by monodispersed silica sol Huafei Cai1, Yonggang Jiang*,1, Jian Feng*,1, Sizhao Zhang2, Fei Peng1, Yunyun Xiao1, Liangjun Li1, Junzong Feng1 Science and Technology on Advanced Ceramic Fibers and Composites Laboratory, College of Aerospace Science and Engineering, National University of Defense Technology, Changsha, 410073, Hunan, P. R. China, 2 China-Australia International Institute for Minerals, Metallurgy and Materials, Jiangxi University of Science and Technology, Nanchang, 330013, Jiangxi, P. R. China

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Abstract

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Silica aerogels with different particle size were prepared by monodispersed silica sol for the first time. The properties of the silica aerogels under normal and high temperature were systematically studied. Among the four different particle size aerogels, the SA-2 sample consisted of 20.31±1.21 nm particles with narrow size distribution had high temperature resistance and low thermal conductivity. Compared with the traditional acid-base two-step prepared silica aerogels, the monodisperse silica aerogels can significantly increase the temperature resistance while maintaining a low thermal conductivity (0.02723 W‧ m-1‧ K-1). The sturdy skeleton structure formed by the interconnection of large particles can effectively inhibit the viscous flow between aerogel particles and avoid pore collapse caused by skeleton failure which can maintain a stable structure until 900 °C and retain a relatively complete structure at 1000 °C with only about 35% volume shrinkage after 2 h heat treatment. At 1100 °C, the viscous flow between aerogel particles and pore collapse cannot be effectively suppressed, and the pure silica aerogels can only be used during a short period at 1100 °C. The results will be meaningful for the design of super thermal insulation materials with high temperature resistance and low thermal conductivity. Keywords: Silica aerogel; monodispersed; temperature resistance; thermal conductivity; volume shrinkage; viscous flow

1. Introduction Silica aerogel is a unique ultra-light weight nanoporous material with ultra-low density (~3 kg/m-3), high specific surface area (~1200 m2/g), high porosity (~99%), and very low thermal conductivity (~0.013 W/m‧ K) [1-6]. Because of these unique characteristics, they can be applied in many various fields, such as environmental protection [7-9], adsorption catalysis [10-12], energy saving in construction [13-15] and super thermal insulation [16, 17]. As thermal insulation materials, SiO2 aerogels can effectively inhibit solid and gaseous heat conduction because of the amorphous slender skeleton and unique nanometer-sized pore structure, which make them widely * Corresponding author. Fax +86 073184576578

E-mail addresses: Yonggang Jiang ([email protected]), Jian Feng ([email protected]) 1

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used as high efficiency thermal insulation in various fields up to 800 °C [18-21]. However, the shrinkage and deformation of silica aerogels above 800 °C which caused by the particle agglomeration and pore collapse during the sintering process will seriously limits their application at higher temperature. The conventional pure silica aerogels have about 20% volume shrinkage at 800 °C within 0.5 h and has about 50% volume shrinkage at 900 °C within 0.5 h [22-25]. Therefore, it attracts many scholars to study the methods to improve the heat resistance of silica aerogels at high temperature. Silica aerogels are typical amorphous porous materials with three-dimensional reticular structure formed by the interconnection of aerogel particles, and the sintering behavior of silica aerogels is primarily caused by viscous flow [26-28]. At high temperature, the silica aerogel particles will flow viscously under the action of high surface tension, which will lead to the neck formation and fusion of aerogel particles; and the aerogel particles will grow and the pores will collapse with prolonged heating, leading to the decrease of the specific surface area and the macroscopic shrinkage and deformation. In order to improve the temperature resistance of silica aerogels, it is necessary to inhibit the viscous flow of aerogels at high temperature. Although the viscous flow can be effectively suppressed by adding reinforcement such as fibers [21, 29-31], it is also significant to improve the temperature resistance of the silica aerogel itself. At present, the main method to improve the temperature resistance of silica aerogels is to add high temperature resistance components like metallic oxides [32]. The high temperature resistance components such as Al2O3 [33-37], ZrO2 [38-40], Y2O3 [41] and TiO2 [42-44] can inhibit the viscous flow and particle growth by forming the aerogel skeleton together with silica, thus maintaining the nanoporous structure and improving the temperature resistance of aerogel. Among the high temperature components, Al2O3 is the most widely used applied. Feng et al. [35] prepared Al2O3-SiO2 aerogel with different Si/Al ratios, and found that the Al2O3-SiO2 aerogel will form an intermediate state different from the pure silica aerogel and pure alumina structure at the optimal ratio, which can effectively inhibit the viscous flow between the aerogel particles and pore collapse, therefore, it can maintain intact skeleton and high surface area after heat treatment. Zu et al. [40] prepared ZrO2-SiO2 aerogel by sol-gel method, and the addition of ZrO2 inhibited the viscous flow and particle growth of aerogel particles, therefore, it showed high thermal stability and can maintain a 172 m2/g specific area after heat treatment at 1000 °C; but the aerogel still exhibited obvious volume shrinkage because the partial fusion of particles and collapse of pores formed by slender skeleton. Zhang et al. [41] prepared Y2O3-SiO2 aerogel with high structural stability and high surface area, and it can maintain a 643.79 m2/g specific area after heat treatment at 900°C. The higher thermal stability of Y2O3-SiO2 aerogel than pure SiO2 aerogel (138.7 m2/g) is attributable to the high-temperature resistance of Y2O3 which effectively inhibit the viscous flow between the aerogel particles. But the aerogel skeleton is too slender which may cause severe pore collapse over 900°C. In addition, there is also many researches devoted to improving the temperature resistance of silica aerogels by adding heterogeneous components [45-50]. Although the temperature resistance of silica aerogels can be improved effectively by adding high temperature resistant components, the composition and structure of silica aerogels will be significantly changed by adding high temperature resistant components. The thermal conductivity of aerogels will obviously increase after doping caused by the addition of high thermal conductivity doped solid phase and the change of pore structure [31-33, 51]. Under the same conditions, the thermal conductivity of the composites with pure silica aerogels as matrix is 2

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2. Experimental details

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obvious lower than that of those doped with heterogeneous elements silica aerogels. [30, 52] Therefore, it is necessary to find a more effective way to maintain the excellent performance of the silica aerogel while increasing the temperature resistance of the silica aerogel. Silica aerogels exhibit similar sintering behavior for heat treatment before 900 °C [53, 54]. The silica aerogel will shrink obviously and the aerogel particles will fuse together obviously during the initial stage of the heating process. After a period of heat treatment, the silica aerogel particles will no longer grow up obviously and the aerogel will not shrink obviously again with the prolongation of heat treatment time [21]. According to the sintering behavior of silica aerogels, it can be predicted that increasing the secondary silica aerogel particle size may contribute to an improvement in the temperature tolerance and reduce shrinkage of the silica aerogel in that temperature range while maintaining the pure silica aerogel characteristics. At present, there are no reports on the preparation and properties of different particle size silica aerogels with high temperature resistance and low thermal conductivity. In this work, silica aerogels with different particle size were designed and prepared by monodispersed silica sol for the first time. Monodispersed silica sol has the characteristics of single particle shape and narrow particle size distribution, and it can effectively reduce the number of small particles in silica aerogel, reduce the sintering driving force and inhibit the shrinkage of silica aerogel at high temperature. By studying the properties of different particle size silica aerogels, the formulation of silica aerogel with high temperature resistance and low thermal conductivity was determined. In addition, the sintering behavior and temperature resistance mechanism of the silica aerogels were also systematically studied.

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2.1 Materials Tetraethoxysilane (TEOS; analytical reagent (AR) grade), ethanol (EtOH; AR grade), and ammonia (25 wt%; AR grade) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (China). Note that the water used in all the experiments is deionized water, and all of the reagents in this study were used as received without further purification. 2.2 Methods (1) Preparation of monodispersed silica sol In this paper, the mono-dispersed silica sol was synthetized via a modified Stöber method reported by Cai et al [55], in which TEOS was used as precursor, ethanol as solvent, ammonia as catalyst and deionized water as hydrolyzing agent. First, water, ethanol and ammonia were mixed and heated to a specified temperature. Second the TEOS was heated to a specified temperature. Then the TEOS and the component solvent are poured simultaneously and rapidly into a beaker heated in a constant temperature water bath in order to ensure uniformity of mixing, and the original monodispersed silica sol was obtained by stirring for 24h. Finally, after vacuum concentration and solvent replacement by ethanol, the monodispersed silica sol with high concentration (18~20 wt%) and a single ethanol solvent system was obtained. The specific parameters were shown in Table 1.

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Journal Pre-proof Table 1. Preparation parameters and properties of the mono-dispersed silica sol.

Sample

TEOS concentration (mol/L)

R (H2O/TEOS)

NH3 concentration (mol/L)

Temperature (°C)

Sol particle size (nm)

1 2 3 4

0.4 0.4 0.4 0.4

60 60 60 60

0.10 0.15 0.20 0.25

75 75 75 75

6.98±0.79 13.29±1.08 18.36±1.71 23.73±2.05

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(2) Preparation of silica aerogel samples The silica aerogel was prepared by the following steps: (1) place the monodispersed silica sol in a 50 °C water bath for gelling; (2) age the samples for two days at 50 °C after gelling, (3) dry the samples with ethanol supercritical drying (265 °C, 7 MPa), and (4) heat the samples at 500 °C for 2 h in an air atmosphere to remove the organic residue. The preparation procedure of the monodispersed silica aerogels was shown in Figure 1.

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Figure 1. Preparation procedure of the monodispersed silica aerogels. The monodisperse silica samples used for property characterization and test were randomly sampled at three different positions of monodisperse aerogels to reduce the experimental error caused by the chance of sampling. The silica aerogel samples used for high temperature characterization was heat treated in a muffle furnace. The aerogel samples were divided into very small pieces and placed flat on the bottom of the crucible. When the furnace temperature began rising to the required temperature, the crucible containing silica aerogel samples was quickly placed in the furnace and heated for the desired time. Subsequently, the samples were taken out directly and air cooled, and the heat treatment of silica aerogels was complete. c The bulk density of the samples before and after heating was measured by using Archimedes method. During the bulk density test, the test conditions must keep consistent, and it needed to repeat the experiment five times and take the average value to determine the final results. First, the mass of the silica aerogel samples was determined to be m1. In a water-filled beaker placed on an electronic microbalance electronic balance, a cylinder with holes and was placed vertically into the beaker, with everything being submerged in the water. The weight was recorded as m2 (buoyancy of the empty container). After removing the container, it was dried, and the silica aerogel was loaded into it. Then, the filled container was immersed in the water and the weight was recorded as m3 (the total buoyancy of the container and silica aerogels). It should be noted that the samples were hydrophilic due to the oxidation of the organic groups. Therefore, samples 4

Journal Pre-proof should be modified by hydrophobic treatment before being placed in the water container. The hydrophobic treatment process of the samples was according to the procedure reported by Feng et al [56]. The bulk density of the silica aerogel sample can be obtained by:

a 

m1 H O m3  m2 2

(1)

Wherein, ρa is the density of silica aerogel (kg/m3); and ρH2O is the density of distilled water, (1.003 g/cm3, 25 °C, 1 atm). The volume shrinkage (η) of silica aerogels was characterized by the change of volume density at different temperatures:

V2   1 1 V1 2

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Wherein, V1 is the original volume of silica aerogel; V2 is the volume of silica aerogel after heat treatment; ρ1 is the original bulk density of silica aerogel; and ρ2 is the bulk density of silica aerogel after heat treatment. The microstructure of the samples was observed by using a high-resolution field emission (SEM Zeiss Sigma 500, Carl Zeiss Co. Ltd., Germany), and three different regions were selected for observation to ensure that the typical morphology of monodisperse silica was presented. The sample was fixed on the sample table with conductive adhesive; the metal (Pt) spraying time was 180s, and the voltage was 20kV. The size distribution of silica aerogel particles before and after heat treatment was measured by Image J software, and the particle size was determined by the diameter of the aerogel particle circumferential circle, and no less than 100 particles were selected to the particle size measurement. The thermal conductivity of silica aerogels was carried out on a Hot Disk 2500 thermal constant analyzer, the samples were measured for three times and then the standard deviation was calculated. The thermal properties of the samples were analyzed by thermogravimetric analyzer-differential scanning calorimeter (TG-DSC, SDTQ600, TA Co. Ltd., USA) from 25 °C to 1200 °C with a 10 °C/min heating rate in air. The chemical surface groups on the silica aerogel samples were measured by Fourier transform infrared spectrometer (FT-IR, Nicolet avatar 360, Nicolet Co. Ltd., USA) using pressed KBr pellets, and the wavenumber measurement rage was 4000 cm−1-400 cm−1. The specific surface area of the silica aerogel samples was determined from the adsorption isotherm using Brunauer-Emmett-Teller (BET) theory in a surface and porosity analyzer (ASAP 2460, Micromeritics Co. Ltd., USA) at N2 environment. The pore size distribution of the silica aerogel samples was derived from desorption branches of the isotherms by applying Barret-Joyner-Halenda (BJH) model. The crystal structure of silica aerogels was analyzed by Bruker D8 Advance X-ray diffraction instrument (Cu Kα ray, 40 kV); the scanning speed was 0.2 °s-1, and the scanning range is 10°~80°.

3. Results and discussion 3.1 Microstructure and properties of the silica aerogels The microstructure of silica aerogels with different particles was shown in Figure 2. The silica aerogels prepared by mono-dispersed silica sol had a typical three-dimensional nanoporous structure. Compared with the traditional acid-base two-step prepared aerogels (The diameter of aerogel particles is generally less than 5 nm) [57-61], the aerogel prepared by monodispersed 5

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silica sol had larger particles with narrow size distribution. The skeleton of the SA-1 aerogel sample prepared by small particle size mono-dispersed silica sol consisted of many stacked and aggregated nanoparticles having a 13.59±0.87 nm average particle size, which seem to be more likely to form large pores. The skeleton of the SA-2, SA-3 and SA-4 aerogel samples was formed by the interconnection of individual particles with 20.31±1.21, 28.64±2.06 and 34.61±2.43 nm average particle size respectively (Table 2). It can be observed that when the particle size of the monodispersed silica sol was increased, the skeleton structure of the aerogel was changed from the multi-particle stack into single particle connected to each other. In addition, with the increase of aerogel particles, macropores were more likely to appear in the pore structure of aerogels.

Figure 2. SEM images of silica aerogels and TEM images of mono-dispersed silica sol (inset) with different particle size. Figure 3 (a) showed the nitrogen sorption isotherms the silica aerogels with different particle size. It can be seen that the curve shape of the silica aerogel samples exhibited a mesoporous structure according to the classification of the International Union of Pure and Applied Chemistry [62]. With the increase of silica aerogel particles, the specific surface area of aerogel decreased as shown in Table 2. The specific surface area of SA-1aerogels with small particles was not very high for the multi-particle stack. Figure 3 (b) showed the distribution of the silica aerogel diameter pore size with different particle size. It can be seen that the silica aerogels with different particle size had a wide pore size distribution and almost the same most probable diameter around 34.33 nm. Besides, there were some small size pores formed by the irregular stack of aerogel particles. With the increase of silica aerogel particles, the average pore size of aerogel decreases gradually as shown in Table 2, and the average pore size were all in the mesoporous range. Table 2 showed the physical properties of silica aerogels with different particle size. It can be 6

Journal Pre-proof seen that the bulk density of the different particle size silica aerogels prepared by the monodispersed silica sol with the same solid content was almost the same. Silica aerogels prepared by monodispersed silica sol at the same density had low thermal conductivity (0.02681~0.03176 W‧ m-1‧ K-1, 25 °C). With the increase of silica aerogel particles, the thermal conductivity increases gradually. This is mainly due to the decrease of the specific surface area and average pore size.

400 200 0 0.0

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dv/d(logW)(cm3‧g-1‧nm-1)

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13.59±0.87 20.31±1.21 28.64±2.06 34.61±2.43

Thermal conductivity (W‧ m-1‧ K-1)

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Bulk density (g‧ cm-3) 0.217 0.206 0.213 0.221

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Figure 3. Nitrogen sorption isotherms (a) and size distribution (b) of silica aerogels with different particle sizes. Table 2. The physical properties of silica aerogels with different particle size.

0.02681 0.02723 0.02962 0.03176

Surface area (m2‧ g-1)

most probable aperture (nm)

Average pore size (nm)

295.67 286.63 219.71 209.15

37.06 34.33 34.33 34.33

28.34 25.41 33.91 39.00

Density was a key parameter that reflected the structure of the silica aerogel during heat treatment. When heated after a short period of time (0.5 h), as shown in Figure 4 (a), there was no significant increase in the density of the aerogels until 1000 °C; when the temperature was raised to 1100 °C, the density of the aerogels increased especially the SA-1 sample composed of small particles; and when the temperature rises further to 1200 °C, the density of aerogel increased significantly, and there is no obvious difference in the density of each sample. When heated after a long period of time (2 h), as shown in Figure 4 (b), it can be seen that the density of silica aerogel samples did not increased significantly until 1000 °C, but increased obviously at 1100 °C; and the density of the silica aerogel was close to compact density when the temperature increased to 1200 °C. The volume shrinkage of the aerogels followed a trend similar to that of the density. The volume shrinkage of silica aerogels increased obviously with the increase of temperature, especially the SA-1 sample composed of small particles. The volume shrinkage of silica aerogels was relatively small (within 15%) until 900 °C after 2 h heat treatment as shown in Figure 4 (c) and (d). The silica aerogels except the SA-1 sample had less than 20% volume shrinkage at 1000 °C after 0.5 h heat treatment and had less than 40% volume shrinkage at 1000 °C after 2 h 7

Journal Pre-proof heat treatment. When the temperature rose to 1100 °C, the silica aerogels except the SA-1 sample had less than 40% volume shrinkage after 0.5 h heat treatment but exceeded 80% volume shrinkage after 2 h. The volume shrinkage of silica aerogels would exceed 80% in 0.5 h heat treatment at 1200 °C. From the above results it can be concluded that the shrinkage characteristics of the silica aerogels except the SA-1 sample composed of small particles had no particularly obvious difference at high temperature. In addition, SA-2 sample had larger specific surface area and lower thermal conductivity among the three samples consisted of big particles, therefore SA-2 sample consisted of 20.31±1.21 nm particles have excellent comprehensive properties with high temperature resistance and low thermal conductivity.

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Figure 4. Bulk density of silica aerogels after 0.5 h (a) and 2.0h (b) heat treatment at different temperature; volume shrinkage of silica aerogels after 0.5 h (c) and 2.0h (d) heat treatment at different temperature. 3.2 Structural evolution at high temperature Figure 5 showed the morphology and structure of SA-2 sample before and after heat treatment observed by high resolution SEM. Compared with the untreated sample as shown in Figure 5 (a), the morphology and structure after holding the temperature at 800 and 900 °C for 2 h (Figure 5 b and c) were almost no difference, and the aerogel maintained the structure of single particle connected to each other, which indicated that the silica aerogel prepared by monodispersed silica sol had good temperature resistance during this temperature range. At 1000 °C, the morphology and structure of the aerogel began to change. After holding the temperature at 1000 °C for 0.5 h (Figure 5 d), the aerogel structure changed a little with some particles beginning to fuse together, but there was no significant change in the pore structure; after holding for 2 h (Figure 5 e), the 8

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silica aerogel particles fused together partially and the number of pores decreased obviously. When the temperature was raised to 1100 °C, the morphology and structure of the aerogel changed more obviously and rapidly. After holding the temperature at 1100 °C for 0.5 h (Figure 5 f), there are obvious clusters and fusion between particles, and the number and the size of pores decreased simultaneously. With prolonged heating (1h at 1100 °C), as shown in Figure 5 g, the silica aerogel particles were fused further, the pores especially the macropore were seriously collapse and the number and the size of pores decreased obviously. After maintaining heating for 2 h, the aerogel particles almost completely fused together and nanoporous structure was almost disappeared. When the temperature rose further to 1200 °C, the aerogel particles completely fused together and the pore structure almost completely disappeared in a short time (within 0.5 h).

Figure 5. SEM images of SA-2 sample before (a) and after (b ~ i) heat treatment. Figure 6 (a) showed the nitrogen sorption isotherms at different heat treatment temperatures. It can be seen that silica aerogel samples after heat treatment at different temperature still exhibited a mesoporous structure according to the classification of the International Union of Pure and Applied Chemistry [62]. Figure 6 (b) showed the distribution of the silica aerogel diameter pore size at different heat treatment temperatures. After holding the temperature at 800 and 900 °C for 2 h, the aerogel samples similar pore size distributions to the untreated samples and the specific surface area had no obvious decrease as shown in Table 3. After holding the temperature at 1000 °C for 2 h, the aerogels maintained a wide pore size distribution but the pore volume decreased, and the most probable diameter changed from 34.33 nm to 25.25 nm, in addition, the specific surface area reduced from 286.63 to 222.08 m2/g. After holding the temperature at 1100 °C for 2 h, the pore volume decreased obviously, the pore size distribution became more concentrated with the macropore almost disappeared, and the specific surface area reduced from 9

Journal Pre-proof 286.63 to 114.89 m2/g. When the sample was heated to 1200 °C, the pore structure can hardly be measured, which indicated that the pore structure completely disappeared, and the silica aerogels were in a dense state. It can be seen that the aerogel samples had high structural stability after 2 h heat treatment when the temperature at 800 and 900 °C; the aerogel structure changed obviously but can maintain part of the pore structure after heating for a long time at 1000 and 1100 °C; and the porous structure of aerogel disappeared in a very short period of time and silica aerogels cannot be used at this temperature at 1200 °C. Therefore, in order to further determine the high temperature stability of aerogels after different time heat treatment, the properties of aerogels at 1000 and 1100 °C were systematically studied.

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Figure 6. Nitrogen sorption isotherms (a) and size distribution (b) of SA-2 sample after different temperature heat treatments. Table 3. The physical properties of SA-2 sample before and after heat treatment.

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Sample SA-2 SA-2(800°C, 2 h) SA-2(900°C, 2 h) SA-2(1000°C, 2 h) SA-2(1100°C, 2 h) SA-2(1200°C, 0.5 h)

286.63 280.56 271.35 222.08 114.89 6.39

Average pore diameter (nm)

most probable aperture (nm)

Volume shrinkage (%)

25.41 24.06 24.53 19.97 13.06 10.23

34.33 34.33 34.33 25.25 17.20 17.20

0 3.41 ± 1.21 12.41 ± 2.01 35.14 ± 4.93 83.82 ± 5.08 89.86 ± 5.67

It can be seen from Figure 7 (a) and Figure 7 (b) that the density and volume shrinkage of SA-2 sample increased slowly with the prolongation of heating time at 1000 °C. After 0.5 h heating at 1000 °C, the volume shrinkage of aerogel was 18.11±4.98%, and the volume shrinkage of aerogel was only about 35% after 2 h heat treatment. The aerogel maintained mesoporous structure (Figure 7 (c)) and had a wide pore size distribution (Figure 7 (d)) all the heating time, and the specific surface area decreased slowly with the pore volume and the most probable pore diameter gradually decrease in the heating process at 1000 °C. The above results indicated that the aerogel sample can maintain a relatively stable structure and show a good temperature resistance at 1000 °C.

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Volume absorbed (cm3/g)

(e)

SA-2(25℃ ) SA-2(1000℃ -0.5h) SA-2(1000℃ -1.0h) SA-2(1000℃ -2.0h)

700

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Specific surface area (m2/g)

Volume absorbed (cm3/g)

(c)

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Heating time (h)

Heating time (h)

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Pore width (nm)

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SA-2(25℃ ) SA-2(1100℃ -0.5h) SA-2(1100℃ -1.0h) SA-2(1100℃ -2.0h)

2.5 2.0 1.5 1.0 0.5 0.0 0

20 40 60 80 100 120 140 160 180 200

Pore width (nm)

Relative pressure (P/P0)

Figure 7. Bulk density (a) and volume shrinkage (b) of SA-2 sample after different durations of heat treatment; (c) Nitrogen sorption isotherms, BET specific surface area (inset) and (d) pore size distribution of SA-2 sample after different durations of heat treatment at 1000 °C; (e) Nitrogen sorption isotherms, BET specific surface area (inset) and (f) pore size distribution of SA-2 sample after different durations of heat treatment at 1100 °C. When the temperature rose to 1100 °C, as shown in Figure 7 (a) and Figure 7 (b), the density and volume shrinkage of SA-2 sample increased rapidly with the prolongation of heating time. After 0.5 h heating at 1100 °C, the volume shrinkage of aerogel exceeded 40%, and the volume shrinkage increased rapidly and finally exceeded 80% with the aerogel density closed to compact density. Figure 7 (e) presents the nitrogen sorption isotherms and BET specific surface area of silica aerogel at different heat treatment times at 1100 °C. It can be seen that for longer heating time, the silica aerogel maintained a mesoporous structure, and the specific surface area rapidly decreased. In Figure 6 (f), the distribution of the silica aerogel pore diameter for different heat 11

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treatment times at 1100 °C is shown. After 0.5 h heating, the pore volume decreased rapidly with the most probable pore diameter being 29.45 nm. When the heat treatment was extended to 1h, the pore volume especially the macropore volume decreased obviously, and the pore size distribution was narrower with 20.06 nm being the most probable pore diameter. After 2 h heating, the pore volume further decreased with the volume of the macropores completely disappeared. The above results indicated that the aerogel sample cannot maintain a relatively stable structure and aerogels can only be used during a short period at 1100 °C. 3.3 Thermal and chemical characteristics Figure 8 showed the TG-DSC of original SA-2 sample (untreated after ethanol supercritical drying) in air. There was an obvious weightlessness processes occurred before 500 °C. Also, exothermic phenomena appeared in the DSC curve. The weight loss was caused by the removal of unreacted organic groups, thus the exothermic peak is caused by the oxidative decomposition of organic remnants in the sample. The residual organic groups on the surface of silica aerogels will oxidize and eventually form Si–OH. For temperatures beyond 500 °C, the sample undergoes continuous loss of weight, but there was no obvious endothermic peak or exothermic peak on the DSC curve beyond 500 °C. This phenomenon may be caused by the polycondensation of Si–OH on the surface and inside of the silica particles. With the increase in temperature, the Si–OH bonds continuously condensed and eventually formed Si–O–Si bonds. Therefore, the weight of the sample was continuously reduced at temperatures between 600 °C and 1200 °C.

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Temperature (℃ )

Figure 8. TG-DSC curves of original SA-2 sample from 25 to 1200 °C. The XRD pattern of silica aerogel before and after rapid heating was presented in Figure 9. A single broad peak appeared at approximately 2θ = 23° and was present before and after heating, which was a typical characteristic for amorphous silica. It can be concluded that the silica aerogels remained amorphous after heat treatment from 800 °C to 1200 °C, and no phase transition process occurred in the silica aerogel after heat treatment. The nanostructure change of silica aerogel after heat treatment was not related to crystal transformation.

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Figure 9. XRD patterns of SA-2 sample at different heat temperature. Figure 10 showed the FT-IR spectra of the silica aerogel treated at different rapid heating temperatures. At room temperature, peaks from the stretching and bending vibrations of the –OH group on the surface of the silica aerogel were detected at 3450 and 1635 cm−1. Also, peaks caused by asymmetrical stretching, symmetrical stretching, and bending of Si–O–Si were found at 1085 cm−1, 800 cm−1 and 465 cm−1. In addition, there were a few Si–OH stretching peaks at 960 cm−1 [63-65]. With the increase of temperature, the Si–OH peak gradually diminished, and eventually disappeared at 1100 °C, and the new Si–O–Si groups formed by the polycondensation reaction of connected Si–OH.

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H-OH Si-O-Si

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Si-O-Si Si-OH

1100℃ -2h 1000℃ -2h 900℃ -2h 800℃ -2h

465

960 802

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3450

25℃ 1635

Transmittance (a.u.)

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wave numbers (cm-1) Figure 10. FT-IR spectra of SA-2 sample at different heat temperature. 3.4 Temperature resistance mechanism The skeleton of silica aerogels is formed by the interconnection of secondary particles; therefore, the sintering behavior of aerogel is mainly caused by sintering between secondary particles [66, 67]. Pure silica aerogels are amorphous materials [26], so viscous flow is the main 13

Journal Pre-proof reason for the sintering of silica aerogels at high temperature. At high temperature, the atoms on the aerogel particle surface will migrate to the junction of the aerogel particles and form the sintering neck. The sintering driving force between the interconnected particles can be expressed by the chemical potential [21]. The chemical potential ( 1 ) of atoms under a circular curved surface with a positive radius of curvature can be expressed by the following formula: (3-1) 1  0  2 / r1 The chemical potential ( 2 ) of atoms at the sintering neck with a negative radius of curvature can be expressed by the following formula: (3-2) 2  0  2 / r2

0 is the chemical potential of plane,  is the atomic weight,  is the surface free energy, r1 is the particle surface curvature, r2 is the sintering neck curvature. There will be a potential energy difference (  ) between the particle surface and the sintering   2 1/ r1 +1/ r2 

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Wherein,

(3-3)

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The small-sized particles possess a higher sintering driving force, as indicated in equation (3-3). The silica aerogels prepared by acid-base two-step method has pearl chain structure formed by the connection of small particles with each other, and it is prone to fuse between the particles at high temperature[68-71]; in addition, for the slender skeleton, the pores in the silica aerogels is more prone to collapse caused by skeleton failure under sintering stress. The above is the reason for the shrinkage and deformation of silica aerogels at high temperature.

Figure 11. Structure change of SA-2 silica aerogels prepared by mono-dispersed silica sol at high temperature. As indicated in equation (3-2), when the particle size increases, the sintering driving force between particles will decrease obviously. Figure 11 shows the structure change of SA-2 silica aerogels prepared by mono-dispersed silica sol at high temperature. The silica aerogels prepared by mono-dispersed silica sol have a large particle size with narrow size distribution. This unique structure can effectively decrease sintering driving force and inhibit the viscous flow between 14

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aerogel particles. Therefore the silica aerogels prepared by mono-dispersed silica sol can maintain a stable structure until 900 °C. In addition, compared with the aerogel prepared by acid-base two-step method, the aerogel prepared by monodisperse silica sol has sturdy skeleton structure with stronger supporting effect, so it can effectively avoid pore collapse caused by skeleton failure at high temperature. Therefore, although the fusion phenomenon caused by viscous flow occurs at 1000 °C, the skeleton structure remains relatively complete and the shrinkage of aerogel increases gradually with the increase of heating time, eventually the volume shrinkage of aerogel is only about 35% after 2 h heat treatment. When the temperature further rises to 1100 °C, the higher temperature further increases the sintering driving force and the viscous flow between the aerogel particles cannot be effectively suppressed, in addition, the softening of the skeleton caused by the viscous flow will lead to a significant pore collapse in the aerogels, therefore the aerogels shrink significantly with the prolongation of heating time and eventually close to the dense state. When the temperature reaches 1200 °C, the aerogel will shrink rapidly and reach a dense state in a very short time (within 0.5 h) due to the severe viscous flow and rapid pore collapse, and pure silica aerogels cannot be used at this temperature.

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Silica aerogels with different particle size were designed and prepared by monodispersed silica sol for the first time, and the properties of the silica aerogels under normal and high temperature are systematically studied. The silica aerogels prepared by mono-dispersed silica sol have a large particle size with narrow size distribution. Among the four different particle aerogels, the SA-2 sample consisted of 20.31±1.21 nm particles had high temperature resistance and low thermal conductivity simultaneously. Compared with the traditional acid-base two-step prepared aerogels, the specific surface area of the silica aerogels prepared by monodispersed silica sol has an obvious decrease (286.63 m2/g), but the aerogels significantly increase the temperature resistance while maintaining a low thermal conductivity (0.02723 W‧ m-1‧ K-1). The sturdy skeleton structure formed by the interconnection of large particles can effectively inhibit the viscous flow between aerogel particles and avoid pore collapse caused by skeleton failure. The silica aerogels prepared by mono-dispersed silica sol can maintain a stable structure until 900 °C and retain a relatively complete structure at 1000 °C with only about 35% volume shrinkage after 2 h heat treatment. At 1100 °C, the viscous flow between aerogel particles and pore collapse cannot be effectively suppressed, and the pure silica aerogels can only be used during a short period at 1100 °C. The further work will focus on the preparation of silica aerogel composites which are expected to has higher temperature resistance and lower thermal conductivity among high temperature thermal insulation materials.

Conflict of interest No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication.

Acknowledgment This work was supported by Hunan Provincial Natural Science Foundation of China (No.2018JJ2469).

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Credit Author Statement

Huafei Cai did most of the experiments and wrote the manuscript; Yonggang Jiang and Jian Feng gave the guidance on experimental design; Sizhao Zhang, Fei Peng, Yunyun Xiao, Liangjun Li and Junzong Feng assisted in performance testing and

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SiO2 aerogels with high temperature resistance are prepared by controllable particle size monodispersed silica sol. Thermal conductivity of SiO2 aerogels is as low as 0.02723 W·m-1·K-1 despite forming large particle structure. The aerogel large particle skeleton structure can inhibit the viscous flow between aerogel particles. Pore collapse caused by skeleton failure can be obviously inhibited depending on formed sturdy skeletons.

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