1 Introduction

In 1931, silica aerogel was firstly prepared by Kistler [1] via a Supercritical Drying (SPD) technology, in an autoclave with an excess of liquid and above the critical temperature. Silica aerogel is a nanostructured material with low thermal conductivity (~0.015 W/m K, 25 °C), low bulk density (~0.03–0.1 g/cm3) and high specific surface area (~1000 m2/g). In order to improve the mechanical performance, silica aerogels were reinforced by particles [2–5], whiskers, [6, 7] and fibers [8–15], the SPD-silica nanocomposites with low thermal conductivity and high mechanical performance expands its uses especially when exposed to vibration and compression as an excellent thermal insulation materials [16–20].

However, the SPD processing is expensive, hazardous, restricting the commercial exploitations of silica aerogel nanocomposites [21]. Today, one of the most focusing areas is silica aerogel nanocomposites synthesized via Ambient Pressure Drying (APD) technology to further decrease the manufacturing cost. Recently, the APD-silica aerogel nanocomposites were reported and reinforced by particles [22–27], nanotubes [28], foams [29, 30], and fibers (including cotton) [31, 32], aramid fibers [33, 34], polyester fibers [35], nonwoven fibers [36], glass wool [37], aluminosilicate glass fiber [38], silica fibers [39], mullite fibers [40], glass fibers [41–44], carbon fibers [45, 46], and boehmite nanofiber [47]. For example, Liu GW [40] demonstrated the super insulation mullite fiber/silica aerogel nanocomposites with low thermal conductivity (500 °C, 0.0393 W/m K) and good mechanical property (storage modulus of 12.5 MPa) in 2016.

For the surface modification and drying procedure as reported in all above APD-silica aerogel nanocomposites papers, the wet silica gels were commonly immersed into a modification agent (such as trimethylchlorosilane (TMCS)/n-hexane) by constant volume ratio to make the gels hydrophobic, and were dried slowly at ambient pressure. It is reported that the solvent-modification agents molar ratio [48–50] and drying temperature ramp [31, 51, 52] had a significant effect on the APD-silica aerogel, and fiber diameter had a significant effect on the radiative conductivity of fibrous materials [53]. But the effects of gel treatment and drying temperature, modification agent molar ratio on the APD-silica aerogel nanocomposites reinforced by smaller diameter fibers had received little attention. In this paper, we report a new ambient-dried silica aerogel nanocomposites reinforced by smaller diameter microglass fiber mat, as-prepared nanocomposites have a lower thermal conductivity and high mechanical performance, their effects of gel treatment and drying temperature, modification agent molar ratio and microglass fibers volume content on the composites structure and properties were investigated.

2 Experimental

2.1 Materials

Tetraethoxysilane (TEOS), ethanol (EtOH), n-hexane, hexamethyldisilazane (HMDZ), hydrochloric acid (37%), and ammonia (27%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Sinopharm, China). Deionized water was used to prepare HCl (aq) and NH3·H2O (aq) which were used as the acid and base catalysts, respectively. The glass fiber used in this research was microglass fiber mat (Sinoma, Nanjing, China), with the fiber diameter of 2–4 μm and thermal conductivity of 0.036 W/m K at room temperature.

2.2 Synthesis of ambient-dried microglass fibers/silica aerogel nanocomposites

Silica alcogel was synthesized by two step (acid–base) Sol–Gel Process [10]. Figure 1 illustrated the schematic overview of ambient-dried microglass fibers/silica aerogel nanocomposites. Firstly, the microglass fiber mat was immersed into the silica sol in vacuum. Microglass fibers volume content (f c ) was controlled with 4.5, 6.8, and 9.1%, respectively. After gelation, the fiber/gels were washed in EtOH twice in 24 h in order to exchange pore water and also strengthen the network of the gels. The slylation of wet fiber/gels was carried out by immersing into a HMDZ/n-hexane silylating agent solution (25% HMDZ in n-hexane) for 4 times in 96 h. Finally, surface modified wet fiber/gels were exchanged in n-hexane twice in 24 h in order to remove the unreacted silylating agent, and dried at ambient pressure at high temperature (see Table 1, Drying programmer) controlled by gradient multi-segment programmers’ oven, evaporating the trapped solvent from the gel network and to get hydrophobic silica aerogel and nanocomposites, the temperature ramp rate of oven is 1 °C/min.

Fig. 1
figure 1

Schematic overview of ambient-dried microglass fibers/silica aerogel nanocomposites

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Table 1 Effect of drying temperature on the silica aerogel
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To study the physical properties of the APD-silica aerogel nanocomposites, silylating agent/TEOS molar ratio named as HT, was varied from 2, 4, and 6 by keeping total volume of alcogel constant.

2.3 Characterization

The aerogel bulk density (ρ a ) was obtained by the Archimedes method [54]. The aerogel porosity (P a ) was obtained according to the following formula:

$${P_a}{\rm{ = }}\left( {1 - \frac{{{{\it{\rho }}_a}}}{{{{\it{\rho }}_s}}}} \right) \times 100\%$$

Wherein, ρ s is the compact density of the silica (2.19 g/cm3, 25 °C, 1 atm).

The shrinkage of the aerogels (ΔV a , %) and nanocomposites (Δd c , %) was obtained according to the following formulas:

$$\Delta {V_a} = \left( {1 - \frac{{{V_{aerogel}}}}{{{V_{alcogel}}}}} \right) \times 100\%$$
$${V_{aerogel}} = \frac{{{m_{\rm{l}}}}}{{{{\it{\rho }}_a}}}$$
$$\Delta {d_c} = \left( {1 - \frac{{{d_c}}}{{{d_f}}}} \right) \times 100\%$$

Wherein, V alcogel is the wet gel volume (cm3), V aerogel is the aerogel volume (cm3). d c is the thickness of the nanocomposites (mm), d f is the thickness of the microglass fiber mat (mm).

The bending strength of APD-silica aerogel nanocomposites was carried out with WDW-100 Electronic Universal Testing Machine (Bairoe), with the samples dimensions of 120 × 20 × 10 mm. The crosshead rate was 0.5 mm/min, five specimens for each kind of sample were used. The microstructure of the aerogel and nanocomposites were investigated by a Hitachi S4800 Scanning Electron Microscope (SEM) after coating the samples with a thin platinum layer. The thermal conductivities of nanocomposites were determined by a hotplate apparatus (YB/T 4130–2005) with sample dimensions of Φ180 × 20 mm. Nitrogen sorption measurements were performed to obtain pore properties with a QuadraSorb SI (Quantachrome, USA) analyzer.

3 Results and discussion

3.1 Effect of gel treatment temperature on the silica aerogel

Table 2 illustrated the wet gel treatment temperature effect on the silica aerogel. The results showed that gel treatment temperature has a significant effect on the APD-silica aerogel. The aerogel density and volume shrinkage treated at 25 °C was higher than that of 50 °C. When increasing the treatment temperature, the aerogel density decreased from 0.329 to 0.17 g/cm3, and volume shrinkage decreased from 66.9 to 38.3%. Consequently, homogeneous and translucent bulk aerogel could be obtained at 50 °C (Fig. 2). The reasons are that the diffusivity increased at high temperature for homogeneous hydrogels according to the theory of diffusion in gels [55, 56] and the higher coarsening of the gel network occurs at 50 °C [57]. Increasing the gel treatment temperature will make a higher solvent diffusivity, a faster exchanging process, and a more absolutely replacement of end -H from Si-OH group by Si-(CH3)3 group from HMDZ within the gel [49].

Table 2 Effects of gel treatment temperature on the silica aerogel
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Fig. 2
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Silica aerogels with different gel treatment temperature

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3.2 Effect of drying temperature on the silica aerogel

Table 1 demonstrated the effect of drying temperature on the silica aerogel. Low density and high porosity of silica aerogel could be obtained via APD technology with a gradient multi-segment drying process (50 °C × 8 h, 80 °C × 8 h, 100 °C × 24 h, 180 °C × 8 h), the temperature ramp rate is 1 °C/min. Lower density and volume shrinkage, higher porosity APD-silica aerogels were prepared with higher second-segment-temperature drying or gradient multi-segment drying process. The density decreased from 0.248 to 0.129 g/cm3, and porosity increased from 88.7 to 94.1% with the gradient multi-segment drying process. The results consistent with the drying behavior of silica gels [58]. In the first drying stage named as “constant rate period”, lowering drying temperature which decreasing the drying rate, solvent volatilized slowly and gel network remained well structure. If drying is too rapid, the gel body will warp or crack. In the second drying stage named as “falling rate period”, the solvent surface tension and vapor pressure were reduced via gradient increasing and higher drying temperature. There is enough time for gel network to adjust its structure according to the changing of gas-liquid interface, which decreases the shrinkage and cracks of the gel.

The SEM image and pore size distribution, nitrogen adsorption isotherms of the retrieved APD-silica aerogel are shown in Fig. 3, the silica aerogel is dried with the gradient multi-segment drying process (50 °C × 8 h, 80 °C × 8 h, 100 °C × 24 h, 180 °C × 8 h). It is clear that there is uniformity and homogeneity of silica particle and porous network (Fig. 3a). According to International Union of Pure and Applied Chemistry recommendations [59], the adsorption-isotherm classification of the APD-silica aerogel (Fig. 3b) is Type IV isotherm, and the adsorption hysteresis is Type H2 loop, which is associated with capillary condensation taking place in mesopores. The specific surface area of the aerogel was 731.76 m2/g, and the average pore size of the aerogel was 20.0 nm, smaller than that of air mean free path (66 nm, 1 atm, 23 °C) [60], it is advantageous to reduce the thermal conductivity of the aerogel. Such nano-structured particle and pores results in high homogeneous and translucent silica aerogel (Fig. 2) with lower density (Table 1).

Fig. 3
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SEM image a and pore size distribution, nitrogen adsorption isotherms b of retrieved APD-silica aerogel

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3.3 Effect of HT on the silica nanocomposites

Mahadik DB [61] demonstrated that by making use of surface modifying silylating reagents via TMCS and HMDZ, the apparent surface free energy of APD-silica aerogels can be tuned in a wide range from 5.5892 to 0.3073 mJ/m2, higher concentration of silylating reagents induced lower surface free energy. Consequently, the surface polar –OH groups from wet fibers/silica gel must be replaced absolutely by non-polar –CH3 groups, in order to get non-multi cracks nanocomposites with high mechanical performance.

As can be seen from Fig. 4a, the bending strength was improved with increasing HT, microglass fiber volume content of f c is 9.1%. The bending strength of the nanocomposites is 1.4 MPa with the HT of six. Figure 4b shows excellent toughness of the nanocomposites with ductile fractures. The SEM of nanocomposites illustrated that fewer cracks and more silica matrix can be found with higher HT (Fig. 5). The microglass fibers adhere to the silica aerogel matrix with a stronger interface, which significantly improves the mechanical properties of the nanocomposites [62].

Fig. 4
figure 4

Effect of HT on the bending strength a and the typical bending load-displacement curve b of the nanocomposites (HT = 6)

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Fig. 5
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SEM of nanocomposites prepared with different surface modification molar ratio (HT)

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3.4 Effect of fibers volume content (f c) on the silica nanocomposites

Figure 6 illustrates the effect of fibers volume content (f c ) on the thermal conductivity of nanocomposites. The thermal conductivity of the nanocomposites increased from 0.013 W/m K (200 °C) to 0.022 W/m K (650 °C) with the rise of temperature, decreased from 0.0265 W/m K (650 °C, f c  = 4.5%) to 0.022 W/m K (650 °C, f c  = 9.1%) with increasing fiber volume content, which is lower than that of APD-silica aerogel composites strengthened with mullite fibers [40]. One of the reasons is that the retrieved nanocomposites were reinforced by microglass fibers with smaller diameters [53], results in lower radiative conductivity. Higher the fiber’s volume content, higher will be the ability of depressing radiative heat conduction. Another reason is that the fibers were surrounded by APD aerogel (Fig. 4), gas convection was also depressed heavily by silica nano pore network.

Fig. 6
figure 6

Effect of fibers volume content (f c ) on the thermal conductivity of nanocomposites

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On the other hand, the bending strength of the nanocomposites increased with the rising of fiber content (Fig. 7). When microglass fibers are used as the reinforcement in the nanocomposites, they received outer strength and withstand the load, and hence the higher fiber content and higher mechanical performance. Consequently, the retrieved nanocomposite is an excellent thermal insulation material with lower thermal conductivity and high mechanical performance.

Fig. 7
figure 7

Effect of fiber content on the bending strength of the nanocomposites

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4 Conclusions

A new ambient-dried silica aerogel nanocomposites reinforced by smaller diameter microglass fiber mat, with a lower thermal conductivity and high mechanical performance, was synthesized. Effects of gel treatment and drying temperature, modification agent molar ratio and microglass fibers volume content on the nanocomposites structure and properties were investigated.

Increasing the gel treatment temperature with a gradient multi-segment drying process, the aerogel density and volume shrinkage decreased rapidly. Homogeneous and translucent bulk aerogel could be obtained with the density of 0.129 g/cm3, specific surface area of 731.76 m2/g and average pore size of 20 nm. Fewer cracks, more silica matrix and stronger fiber/silica interface, which significantly improves the mechanical performance of the nanocomposites with the bending strength of 1.4 MPa (HT = 6). The thermal conductivity of the APD-silica aerogel nanocomposites decreased and the bending strength increased with increasing fibers volume content. The retrieved nanocomposites is an excellent thermal insulation material with lower thermal conductivity (0.022 W/m K, 650 °C, f c  = 9.1%) and high mechanical performance.