- Email: [email protected]
aramid pulp composites with improved mechanical and thermal properties
Journal of Non-Crystalline Solids 454 (2016) 1–7
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Silica aerogel/aramid pulp composites with improved mechanical and thermal properties Zhi Li, Lunlun Gong, Congcong Li, Yuelei Pan, Yajun Huang, Xudong Cheng ⁎ State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230027, PR China
a r t i c l e
i n f o
Article history: Received 27 July 2016 Received in revised form 21 September 2016 Accepted 16 October 2016 Available online xxxx Keywords: Silica aerogel Composite Mechanical property Thermal property
a b s t r a c t For maintaining the integrality and improving the mechanical strength of fiber/silica aerogel composites without compromising thermal insulation properties, aramid pulp reinforced silica aerogel composites were prepared under ambient pressure drying by adding aramid pulps into silica sol directly. The microstructures indicated the ample fibrillated fibers of aramid pulps were inlaid in silica aerogel matrix which retained the integrality and nice interface adhesion, instead of separating the aerogel matrix into small fragments. As the aramid pulp content increased, the compressive strength was enhanced obviously up to about 1.2 MPa and the low thermal conductivity of 0.0232–0.0278 W·m−1·K−1 approximated linear growth. TG-DTA analysis indicated that the thermal stability of the as-prepared composites was about 260 °C which primarily depended on the thermal stability of pure silica aerogels. Thus it concluded that excellent silica aerogel composites with intact microstructures, improved mechanical strength and tailored thermal properties can be prepared by using aramid pulps as reinforcements. © 2016 Published by Elsevier B.V.
1. Introduction Because of inherent fragility and low strength [1], it is troublesome to use silica aerogels individually for thermal insulation [2–4], oilwater separation [5] and aerospace applications [6], etc. That is ultimately traced to their nanoporous structures [7]. To settle this problem, extensive researches suggest that using fibers to strengthen silica aerogels is one of the most frequent and effective methods to enhance mechanical properties [8–10]. For this purpose, Yang et al. [11–14] used ceramic fiber preforms to strengthen silica aerogels and the compressive strength about 1.8 MPa (at strain being ~0.5) was obtained for the supercritical dried specimen with a larger density of 0.29 g/cm3. Toledo Fernández prepared organic-inorganic hybrid aerogels/wollastonite composites under supercritical drying and measured the stress at failure as 0.56 MPa. Wu et al. [15,16] employed multilayer aligned glass fibers to fabricate aerogel composites which acquired a low thermal conductivity of 0.022–0.028 W·m−1·K−1 and an improved compressive strength (at strain being 0.5) ranging from 1.71–3.70 MPa. Besides, Wu et al. [17] also utilized electrospun nanofibers to synthesize flexible aerogel composites. Viewed from the preparation processes of these fiber/silica aerogel composites, most procedures are similar and differences mainly concentrate on fiber types and methods of adding fibers.
⁎ Corresponding author. E-mail address: [email protected] (X. Cheng).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.10.015 0022-3093/© 2016 Published by Elsevier B.V.
For the fiber types, inorganic fibers, e.g., ceramic fibers with an average diameter of 4–10 μm [11–14] and glass fibers with an average diameter of 12–20 μm [3,15,16], are the most commonly used. Due to the relatively thicker diameter and the brittleness of inlaid inorganic fibers, silica aerogel matrix usually cracks into small fragments with impairing the integrality. This results in forming material defects which decrease the mechanical property of aerogel composites. Besides, the generated aerogel chippings easily fall out from aerogel composites leading to aerogel dust in air during use. Hence, choosing appropriate fibers as reinforcements is vital for acquiring better mechanical performance for fiber/silica aerogel composites. For the methods of adding fibers, the primary approaches are: (i) directly adding original fibers into silica sol [12–14]; (ii) adding preprocessed fibers into silica sol, e.g., aligned glass fibers [15,16] and aramid fiber layers [18]; (iii) immersing fiber preforms into silica sol, e.g., ceramic fiber press forming products [11] and ceramic fiber felts [19]. For conveniently preparing fiber/silica aerogel composites, especially in industrial scale, a simpler and easier method of adding fibers into silica sol is required. However, it is unwonted to add fibers into silica sol directly for preparing aerogel composites. This is because the introduced fibers usually lead to precipitate themselves to the bottom of silica sol during the gelation, which counts against the uniform distribution of fibers. This disadvantage is far severer for fibers with a larger density and hydrophilia, such as inorganic fibers. These problems just attract our concern. According to the current investigations, the fibers with thinner diameters, nicer flexibility and less densities would become the potential
2
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
candidates for reinforcing silica aerogels. A significant phenomenon of reducing aerogel dust during use indicates that the compatibility with silica aerogels for flexible organic fibers (such as aramid fibers) is better than that of inorganic fibers according to the previous work [18]. It can be said that flexible organic fibers are more suitable for reinforcing silica aerogels on some extent [20]. As it is known to all, aramid fibers are one kind of organic fibers which present excellent flexibility and a low density. Aramid pulps (APs) derive from aramid fibers by fibrillation processes, i.e., stripping fibrillated fibers from original fibers by some physicochemical methods, which make APs possess a thinner diameter and a larger specific surface compared to those of original aramid fibers. The thinner diameter of APs will do a greater favor to retain the matrix (in composites) intact. Besides, the smaller density and shorter length of APs make them convenient to mixing with liquid and distribute uniformly, which simultaneously avoid their precipitation during the liquid phase. These advantages are very beneficial for the preparation of fiber reinforced silica aerogel composites. Thus using APs as reinforcements is a better choice for achieving our current research target. In this work, aramid pulp reinforced silica aerogel composites (AP/aerogels) were prepared simply by adding APs into silica sol directly and mixing evenly. The generated AP/aerogels presented significantly improved mechanical strength and tailored thermal properties. Importantly, the integrality of aerogel matrix was improved and the phenomenon of producing aerogel dust during use was well alleviated. 2. Experimental 2.1. Materials and preparation Aramid pulps (APs) with the length of 1.13 ± 0.03 mm were purchased from DuPont, USA. Tetraethoxysilane (TEOS), ethanol (EtOH) and trimethylchlorosilane (TMCS) used as precursors, solvent and surface modifier respectively, were chemical pure grade (Sinopharm Chemical Reagent Co., Ltd., SCRC, China). The main preparation process derived from a classic two-step, acidbase catalyzed sol-gel process. First, alcosol was prepared as per the specific formula and synthesis parameters reported in our previous work [18]. Before adding base catalyst, APs were added into the alcosol with rapidly stirring for 30 min until APs swelled completely and distributed evenly. Then 0.5 mol/L NH3·H2O (aq) as base catalyst was added into AP/alcosol mixture with stirring for 10 min. Subsequently, the AP/alcosol mixture was made air-tight in a 100 mL beaker for polycondensation and the gelation usually happened in 20 min to form AP/alcogels. After AP/alcogels aging with EtOH for 12 h, solvent exchange with n-hexane for 12 h and surface modification with 10% TMCS/n-hexane solution for 12 h, respectively, AP/aerogels were finally generated via ambient pressure at 80 °C and 100 °C for 8 h, respectively. The obtained samples have a circular pie shape with the average diameter of 45.3 mm and average height of 14.2 mm. The composites with various AP content (denoted as c) were prepared for performance characterization. Besides, aramid fibers (AFs) reinforced silica aerogel composites (AF/aerogels) as per Ref. [18] were also fabricated for the comparison between the two reinforcements. 2.2. Methods of characterization The microstructures of AP/aerogels were observed by field emission scanning electron microscope (SEM, SIRION200, FEI) to evaluate the morphology and integrality. Furthermore, the nanoporous structures of the specimens were studied by N2 adsorption-desorption measured at 77 K using a Tristar II 3020 M analyzer. The pore size distribution (PSD) and pore parameters of pure silica aerogels and AP/aerogels
were calculated by Brunauer-Emmitt-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method. The ideal density (ρid) is defined as the final sample mass divided by the volume of AP/alcogels while the bulk density (ρb) was measured from the final sample mass and volume. The porosity was calculated as per Eq. (1). Porosity ¼
1=ρb −1=ρs −1=ρap 100% 1=ρb
ð1Þ
Above, ρs and ρap are the density of silica skeleton and APs, usually 2200 kg·m−3 and 1450 kg·m−3, respectively. Electronic dynamic and static fatigue testing machine (E3000K8953, Instron) was used for uniaxial compression tests at room temperature. The thermal properties of AP/aerogels were measured with a thermal constants analyzer (Hot-Disk 2500, Sweden) at 25 °C and TG-DSC (SDT Q600, TA) with a heating rate of 10 °C/min from room temperature to 800 °C in air. 3. Results and discussion 3.1. Microstructure In Fig. 1a, the APs have an abundant of fibrillated fibers with the diameter of b1 μm while the AFs are just similar to cylindrical rods with a diameter of N10 μm in Fig. 1b. For the same length, the APs possess a larger aspect ratio than the AFs, which are more beneficial for strengthening aerogels [20]. Besides, the plentiful fibrillated fibers with the specific surface area up to 5–11 m2·g−1 increase the interface contacts between APs and aerogels which contributes to the combination between the two phases. The slender fibrillated fibers of APs are inlaid in the aerogels in Fig. 1c–d with distributing casually, either in the lamina plane or through the lamina plane, and without observably destroying the integrality of aerogel matrix. Inversely, most AFs lie flat in the aerogels with separating the aerogel matrix into small fragments and forming much cracks as shown in Fig. 1e. Viewed from the fiber distribution, the APs in aerogels are multidirectional and more uniform, without compromising the integrality of aerogel matrix, compared to the AFs. Just because of these, the APs and aerogels are combined tightly so that the phenomena of aerogel chippings falling out from aerogel composites decreases significantly. Furthermore, the finer microstructure is revealed in Fig. 1f with presenting the 3-D nanoporous network of pure silica aerogels. The N2 adsorption-desorption isotherms and pore size distribution (PSD) were used to investigate the microstructure further. The corresponding results for pure silica aerogels are presented in Fig. 2a–b. The N2 adsorption-desorption isotherms in Fig. 2a presents as the form of the typical type IV according with the mesoporous materials, and the hysteresis loops of the type H3 indicates the probable presence of slitlike interparticle pores [21]. The pore volume and pore area distribution with pore diameter are presented in Fig. 2b, with a sharp peak ranging from 5 to 10 nm, as an indication of the most probable pore size. The corresponding results for AP/aerogels are presented in Fig. 2c–d. It can be seen that the AP/aerogels nearly have the same N2 adsorption-desorption isotherm with the pure silica aerogels while the PSD for the AP/aerogels slightly shifts toward the smaller pore size with the most probable pore size still remaining between 5 and 10 nm. The pore parameters are calculated and listed in Table 1, in which the BET surface area, pore volume and average pore size of the AP/aerogels are slightly smaller than that of the pure silica aerogels. This just conforms to the trend as discussed in Fig. 2. Judging from these, we can drew that both the pure silica aerogels and the AP/aerogels are mesoporous materials which indicates the huge application prospects in numerous fields. From the nanoporous structure and pore size distribution, it is known that the introduced APs are compatible with silica aerogel without destroying the
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
3
Fig. 1. Microstructures of aramid pulps (a), aramid fibers (b), AP/aerogels (c) and (d), AF/aerogels (e) (reproduced with friendly permission Ref. [18]) and the nanoporous network of pure silica aerogels (f).
microstructure of aerogel matrix. On the contrary, the slender fibrillated fibers of APs retain the integrality of aerogel matrix, dramatically reducing the formation of aerogel dust during use.
depends on the relative change of volume and mass. Due to the higher increment on mass than that of volume, the bulk density of AP/aerogels still increases from 150.8 kg·m−3 to 161.5 kg·m−3 as well as the porosity decreases from 82.3% to 81.4% at last.
3.2. Density and porosity 3.3. Mechanical strength It is distinctly showed in Fig. 3 that the ideal density increases monotonously with the increasing AP content while the volume shrinkage decreases slightly first and then drastically decreases over 8% AP content. As APs are added into alcosol, a fast stirring and lasting soak make APs swell and distribute uniformly in the hybrid liquid system. The incremental volume due to the introduced APs is negligible compared to that of the original alcogel. Thus the rising of ideal density is primarily caused by the increasing mass of APs which just accounts for the approximative linear growth. As reported [9,15,16,18], APs in the aerogel matrix act as supporting skeleton which leads to a decreasing volume shrinkage. This effect is caused by the resistance to collapse of nanoporous skeleton during drying process. However, when the AP content is less than ~8%, the effect of supporting skeleton is not obvious that leads to a larger volume shrinkage. Further increasing the AP content to over ~8%, it is found that the volume shrinkage decreases rapidly which indicates the nice skeleton supporting effect. Fig. 4 presents the bulk density increases along with the rising AP content while the porosity presents an opposite trend. As the AP content increases, the effect of supporting skeleton in the aerogel matrix play a great role in reducing the volume shrinkage, which leads to increasing the volume of AP/aerogels. However, the bulk density of AP/aerogels
Fig. 5 is the stress-strain (σ −ε) graphs for each AP content composite. Seen from the four graphs, it is evident that all the σ − ε curves can be divided into two parts as marked, where ε = 0.15 and ε = 0.20 are the cut-off points for c ≤7.68% and c ≥10.03%, respectively. To be specific, the previous segments are nonlinear elastic stages while the subsequent segments are elastic-plastic stages. Furthermore, the average compressive strengths are ~ 0.83 MPa and ~ 1.10 MPa for the AP/aerogels with the AP content being 5.39% and 7.68%, respectively. Considering no obvious yielding when c ≥10.03%, the compressive strengths is substituted for σ0.5 which are ~1.16 MPa and ~1.22 MPa for AP/aerogels with the AP content being 10.03% and 11.99%, respectively. However, the maximum σ0.5 of the reported AF/aerogels was just ~0.18 MPa with the fiber content ranging from 1.58–11.40% [18]. Meanwhile, it is observed that the compressive strengths of AP/aerogels are enhanced with the increasing AP content. For APs, the thinner diameter not only alleviates the damages on aerogel matrix's microstructures but also is more beneficial for APs combining with aerogel matrix due to the improved interface contacts. Although there exists no chemical bonds, the increscent interfacial adhesion would facilitate the enhancement of the Van der Waals' force
4
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
Fig. 2. N2 adsorption-desorption isotherms and pore volume and pore area distribution, (a), (b) for pure silica aerogels and (c), (d) for AP/aerogels (c = 7.68%).
between the two phases [18], which renders APs and silica aerogels combining more tightly. In the previous section, the larger aspect ratios of APs have been referred which means larger fracture strains [20]. Since the APs in aerogel matrix are multidirectional and uniform, the compressive deformation is dominated by the APs bending, stretching and stripping out of aerogel matrix [20]. Through these mechanical behaviors, the external stress is transferred on the integral AF/aerogels, avoiding stress concentration. Under these mechanical effects of APs in composites, silica aerogels are reinforced and can stand larger external stress. As a consequence, the dispersive APs lead to a larger compressive strength for AP/aerogels which can be further enhanced by increasing AP content. For AFs, the cracks on aerogel matrix caused by the thicker diameter are observed evidently in Fig. 1e. When suffering from an external force, the initially formed cracks can be further induced to grow and spread, resulting in material failure at last. This is the basis of lower compressive strength for AF/aerogels.
α ¼ 0:00186c þ 0:1397; R2 ¼ 0:906
ð3Þ
s ¼ 0:00203c þ 0:1446; R2 ¼ 0:884
ð4Þ
From the fitting equations, the thermal conductivity completely complies with highly linear increase, ranging from 0.0232–0.0278 W·m− 1·K− 1 while the thermal diffusivity and volume specific heat range from 0.1521–0.1643 m2·s−1 and 0.1544–0.1702 MJ·m−3·K−1, respectively. Besides, the pure silica aerogel's thermal constants can be predicated by assuming c = 0, which correspond to the intercepts of the three fitting lines, namely, λ = 0.01959 W·m−1·K−1, α = 0.1397 m2·s−1 and s =0.1446s MJ·m−3·K−1, respectively. Actually, the APs in aerogels not only act as the supporting skeleton, but also produce new heat transfer passageways because the larger thermal conductivity of APs is more beneficial for transmitting heat.
3.4. Heat insulation properties The thermal conductivity (λ), diffusivity (α) and volume specific heat (s), all increase linearly with the AP content in Fig. 6 and the linear fitting equations have been listed as follow. λ ¼ 0:000634c þ 0:01959; R2 ¼ 0:997
ð2Þ
Table 1 Pore parameters of the pure silica aerogels and AP/aerogels (c = 7.68%).
Sample
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore size (nm)
Silica aerogels AP/aerogels
1067.2 955.4
4.3 4.1
13.4 12.8
Fig. 3. Ideal density and volume shrinkage of AP/aerogels with the AP content.
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
5
Fig. 4. Bulk density and porosity of AP/aerogels with the AP content.
As a consequence, more heat chooses to transfer through the new formed passageways instead of the aerogel's nanoporous network, which leads to an increasing thermal conductivity for AP/aerogels finally. Furthermore, the APs acting as the new channels render the heat transfer faster which is also beneficial for the thermal diffusion, resulting in an increasing thermal diffusivity. Due to the fact that the specific heat of APs is larger than that of pure silica aerogels, the increasing volume fraction of APs makes the average specific heat of unit volume AP/aerogels also increases at last. The linear increase of thermal conductivity, diffusivity and volume specific heat may ascribe to the distribution of APs which are now under further investigation. 3.5. Thermal stability Due to the excellent heat insulation properties, silica aerogels and their composites are usually used in thermal insulation system. For
Fig. 6. Variations of thermal conductivity, diffusivity and volume specific heat of AP/ aerogels with the AP content.
guaranteeing the use of aerogel products in safety, the thermal stability is an important parameter to study. The corresponding thermal stability analysis for the pure silica aerogels and AP/aerogels are presented in Fig. 7 and Fig. 8.
Fig. 5. Stress-strain curves of the various content AP/aerogels (two typical samples for each content).
6
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
Fig. 7. Thermal stability analysis (TG-DTA) for pure silica aerogels.
Fig. 7a shows that the primary weight loss concentrates between 260–460 °C during the heating process and the peak of weight loss rate is at ~ 280 °C. The DTA analysis is displayed in Fig. 7b that an obvious exothermic peak appears which just corresponds to the peak weight loss rate. The exothermic reaction is considered to be the oxidation of `Si-CH3 (TMS) groups on aerogels which results in the main weight loss [22]. The thermal stability analysis for AP/ aerogels is presented in Fig. 8a. In the infancy, a negligible weight loss from 50 °C to 200 °C indicates the evaporation of water and certain remained organic solvent [23]. The distinct differences for AP/aerogels are that the whole weight loss can be separated into two parts. At Stage I ranging from 250–418 °C, the weight loss is caused by the same reason, i.e., the oxidation of TMS groups on silica aerogels, which leads to the first exothermic peak in Fig. 8b. The weight loss between 418–563 °C at Stage II should be mainly ascribed to the pyrolysis of APs which causes the second exothermic peak in Fig. 8b simultaneously [24]. The comparisons on thermal stability between pure silica aerogels and AP/aerogels are made and the corresponding results are listed in Table 2. It can be seen that the onset temperature of the oxidation of TMS groups at Stage I are nearly the same at ~ 260 °C while the weight loss for pure silica aerogels and AP/aerogels are 9.1% and 7.8%, respectively. The peak temperature of TG and DTA for the pure silica aerogels is similar at ~ 280 °C while the peak temperature of TG for AP/aerogels clearly lags behind that of DTA. The same hysteresis at Stage II is observed with a temperature difference of about 43 °C. This phenomenon may be caused by the heat absorption of APs. From the above analysis, it can be drawn that the thermal stability of AP/aerogels primarily depends on the pure silica aerogel component and the introduced APs does not affect the thermal stability of AP/aerogels.
Fig. 8. Thermal stability analysis (TG-DTA) for AP/aerogels (c = 10.03%).
4. Conclusions In this study, we devoted to retaining the integrality and improving the mechanical strength of fiber/aerogels without compromising the thermal insulation properties. For this purpose, APs composed of extremely thin fibrillated fibers were used as reinforcements, through directly adding APs into alcosol, to prepare AP/aerogels composites. The microstructures demonstrated that APs were inlaid in composites without impairing the integrality of aerogel matrix. As the AP content increased, the bulk density of AP/aerogels slightly increased from 150.8 kg·m−3 to 161.5 kg·m−3 as well as the porosity decreased from 82.3% to 81.4%. The compressive strength increased distinctly reaching up to ~ 1.2 MPa. Meanwhile, the thermal conductivity, diffusivity and volume specific heat all approximated linear increase and the low thermal conductivity ranging from 0.0232–0.0278 W·m− 1·K−1 was still sustained. Furthermore, TG-DTA analysis indicated that the thermal stability of the AP/aerogels was about 260 °C which primarily depended on the pure silica aerogel component in composites.
Table 2 Thermal analysis data for pure silica aerogels and AP/aerogels (c = 10.03%).
Sample
Tonset (°C)
Tpeak of DTG (°C)
Tpeak of DTA (°C)
Weight loss (%)
Stage I Pure silica aerogels AP/aerogels
261.6 259.4
276.7 309.6
282.3 271.6
9.1 7.8
Stage II AP/aerogels
417.8
557.3
514.7
11.2
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
Acknowledgement This work was supported by the Fundamental Research Funds for the Central Universities (grant no. WK2320000032, WK6030000028). References [1] M.A.B. Meador, E.F. Fabrizio, F. Ilhan, A. Dass, G.H. Zhang, P. Vassilaras, et al., Crosslinking amine-modified silica aerogels with epoxies: mechanically strong lightweight porous materials, Chem. Mater. 17 (2005) 1085–1098. [2] D. Ge, L. Yang, Y. Li, J. Zhao, Hydrophobic and thermal insulation properties of silica aerogel/epoxy composite, J. Non-Cryst. Solids 355 (2009) 2610–2615. [3] B. Yuan, S. Ding, D. Wang, G. Wang, H. Li, Heat insulation properties of silica aerogel/ glass fiber composites fabricated by press forming, Mater. Lett. 75 (2012) 204–206. [4] J.G.R. Hansen, B.J. Frame, Aerogel-based thermal insulation, J. Non-Cryst. Solids 225 (1998) 254–259. [5] S. He, X. Cheng, Z. Li, X. Shi, H. Yang, H. Zhang, Green and facile synthesis of spongereinforced silica aerogel and its pumping application for oil absorption, J. Mater. Sci. 51 (2016) 1292–1301. [6] J.P. Randall, M.A. Meador, S.C. Jana, Tailoring mechanical properties of aerogels for aerospace applications, ACS Appl. Mater. Interfaces 3 (2011) 613–626. [7] A. Katti, N. Shimpi, S. Roy, H.B. Lu, E.F. Fabrizio, A. Dass, et al., Chemical, physical, and mechanical characterization of isocyanate cross-linked amine-modified silica aerogels, Chem. Mater. 18 (2006) 285–296. [8] H. Maleki, L. Durães, A. Portugal, An overview on silica aerogels synthesis and different mechanical reinforcing strategies, J. Non-Cryst. Solids 385 (2014) 55–74. [9] J. Feng, C. Zhang, J. Feng, Y. Jiang, N. Zhao, Carbon aerogel composites prepared by ambient drying and using oxidized polyacrylonitrile fibers as reinforcements, ACS Appl. Mater. Interfaces 3 (2011) 4796–4803. [10] K.E. Parmenter, F. Milstein, Mechanical properties of silica aerogels, J. Non-Cryst. Solids 223 (1998) 179–189. [11] S. Duoqi, S. Yantao, F. Jian, Y. Xiaoguang, H. Shiwei, M. Chunhu, et al., Experimental investigation on high temperature anisotropic compression properties of ceramicfiber-reinforced SiO2 aerogel, Mater. Sci. Eng. A-Struct. 585 (2013) 25–31. [12] X. Yang, Y. Sun, D. Shi, J. Liu, Experimental investigation on mechanical properties of a fiber-reinforced silica aerogel composite, Mater. Sci. Eng. A Struct. 528 (2011) 4830–4836.
7
[13] X. Yang, J. Wei, D. Shi, Y. Sun, S. Lv, J. Feng, et al., Comparative investigation of creep behavior of ceramic fiber-reinforced alumina and silica aerogel, Mater. Sci. Eng. AStruct. 609 (2014) 125–130. [14] X. Yang, Y. Sun, D. Shi, Experimental investigation and modeling of the creep behavior of ceramic fiber-reinforced SiO2 aerogel, J. Non-Cryst. Solids 358 (2012) 519–524. [15] H. Wu, Y. Liao, Y. Ding, H. Wang, C. Peng, S. Yin, Engineering thermal and mechanical properties of multilayer aligned fiber-reinforced aerogel composites, Heat Transf. Eng. 35 (2014) 1061–1070. [16] Y. Liao, H. Wu, Y. Ding, S. Yin, M. Wang, A. Cao, Engineering thermal and mechanical properties of flexible fiber-reinforced aerogel composites, J. Sol-Gel Sci. Techn. 63 (2012) 445–456. [17] H. Wu, Y. Chen, Q. Chen, Y. Ding, X. Zhou, H. Gao, Synthesis of flexible aerogel composites reinforced with electrospun nanofibers and microparticles for thermal insulation, J. Nanomater. 2013 (2013), http://dx.doi.org/10.1155/2013/375093. [18] Z. Li, X. Cheng, S. He, X. Shi, L. Gong, H. Zhang, Aramid fibers reinforced silica aerogel composites with low thermal conductivity and improved mechanical performance, Compos. Part A Appl. S. 84 (2016) 316–325. [19] A. Karout, P. Buisson, A. Perrard, A.C. Pierre, Shaping and mechanical reinforcement of silica aerogel biocatalysts with ceramic fiber felts, J. Sol-Gel Sci. Technol. 36 (2005) 163–171. [20] S.O. Kucheyev, M. Stadermann, S.J. Shin, J.H. Satcher, S.A. Gammon, S.A. Letts, et al., Super-compressibility of ultralow-density nanoporous silica, Adv. Mater. 24 (2012) 776–780. [21] F. Rojas, I. Kornhauser, C. Felipe, J.M. Esparza, S. Cordero, A. Domínguez, et al., Capillary condensation in heterogeneous mesoporous networks consisting of variable connectivity and pore-size correlation, Phys. Chem. Chem. Phys. 4 (2002) 2346–2355. [22] Z. Li, L. Gong, X. Cheng, S. He, C. Li, H. Zhang, Flexible silica aerogel composites strengthened with aramid fibers and their thermal behavior, Mater. Des. 99 (2016) 349–355. [23] A. Tadjarodi, M. Haghverdi, V. Mohammadi, Preparation and characterization of nano-porous silica aerogel from rice husk ash by drying at atmospheric pressure, Mater. Res. Bull. 47 (2012) 2584–2589. [24] F. Suarez-Garcia, S. Villar-Rodil, C.G. Blanco, A. Martinez-Alonso, J.M.D. Tascon, Effect of phosphoric acid on chemical transformations during Nomex pyrolysis, Chem. Mater. 16 (2004) 2639–2647.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Silica aerogel/aramid pulp composites with improved mechanical and thermal properties Zhi Li, Lunlun Gong, Congcong Li, Yuelei Pan, Yajun Huang, Xudong Cheng ⁎ State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230027, PR China
a r t i c l e
i n f o
Article history: Received 27 July 2016 Received in revised form 21 September 2016 Accepted 16 October 2016 Available online xxxx Keywords: Silica aerogel Composite Mechanical property Thermal property
a b s t r a c t For maintaining the integrality and improving the mechanical strength of fiber/silica aerogel composites without compromising thermal insulation properties, aramid pulp reinforced silica aerogel composites were prepared under ambient pressure drying by adding aramid pulps into silica sol directly. The microstructures indicated the ample fibrillated fibers of aramid pulps were inlaid in silica aerogel matrix which retained the integrality and nice interface adhesion, instead of separating the aerogel matrix into small fragments. As the aramid pulp content increased, the compressive strength was enhanced obviously up to about 1.2 MPa and the low thermal conductivity of 0.0232–0.0278 W·m−1·K−1 approximated linear growth. TG-DTA analysis indicated that the thermal stability of the as-prepared composites was about 260 °C which primarily depended on the thermal stability of pure silica aerogels. Thus it concluded that excellent silica aerogel composites with intact microstructures, improved mechanical strength and tailored thermal properties can be prepared by using aramid pulps as reinforcements. © 2016 Published by Elsevier B.V.
1. Introduction Because of inherent fragility and low strength [1], it is troublesome to use silica aerogels individually for thermal insulation [2–4], oilwater separation [5] and aerospace applications [6], etc. That is ultimately traced to their nanoporous structures [7]. To settle this problem, extensive researches suggest that using fibers to strengthen silica aerogels is one of the most frequent and effective methods to enhance mechanical properties [8–10]. For this purpose, Yang et al. [11–14] used ceramic fiber preforms to strengthen silica aerogels and the compressive strength about 1.8 MPa (at strain being ~0.5) was obtained for the supercritical dried specimen with a larger density of 0.29 g/cm3. Toledo Fernández prepared organic-inorganic hybrid aerogels/wollastonite composites under supercritical drying and measured the stress at failure as 0.56 MPa. Wu et al. [15,16] employed multilayer aligned glass fibers to fabricate aerogel composites which acquired a low thermal conductivity of 0.022–0.028 W·m−1·K−1 and an improved compressive strength (at strain being 0.5) ranging from 1.71–3.70 MPa. Besides, Wu et al. [17] also utilized electrospun nanofibers to synthesize flexible aerogel composites. Viewed from the preparation processes of these fiber/silica aerogel composites, most procedures are similar and differences mainly concentrate on fiber types and methods of adding fibers.
⁎ Corresponding author. E-mail address: [email protected] (X. Cheng).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.10.015 0022-3093/© 2016 Published by Elsevier B.V.
For the fiber types, inorganic fibers, e.g., ceramic fibers with an average diameter of 4–10 μm [11–14] and glass fibers with an average diameter of 12–20 μm [3,15,16], are the most commonly used. Due to the relatively thicker diameter and the brittleness of inlaid inorganic fibers, silica aerogel matrix usually cracks into small fragments with impairing the integrality. This results in forming material defects which decrease the mechanical property of aerogel composites. Besides, the generated aerogel chippings easily fall out from aerogel composites leading to aerogel dust in air during use. Hence, choosing appropriate fibers as reinforcements is vital for acquiring better mechanical performance for fiber/silica aerogel composites. For the methods of adding fibers, the primary approaches are: (i) directly adding original fibers into silica sol [12–14]; (ii) adding preprocessed fibers into silica sol, e.g., aligned glass fibers [15,16] and aramid fiber layers [18]; (iii) immersing fiber preforms into silica sol, e.g., ceramic fiber press forming products [11] and ceramic fiber felts [19]. For conveniently preparing fiber/silica aerogel composites, especially in industrial scale, a simpler and easier method of adding fibers into silica sol is required. However, it is unwonted to add fibers into silica sol directly for preparing aerogel composites. This is because the introduced fibers usually lead to precipitate themselves to the bottom of silica sol during the gelation, which counts against the uniform distribution of fibers. This disadvantage is far severer for fibers with a larger density and hydrophilia, such as inorganic fibers. These problems just attract our concern. According to the current investigations, the fibers with thinner diameters, nicer flexibility and less densities would become the potential
2
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
candidates for reinforcing silica aerogels. A significant phenomenon of reducing aerogel dust during use indicates that the compatibility with silica aerogels for flexible organic fibers (such as aramid fibers) is better than that of inorganic fibers according to the previous work [18]. It can be said that flexible organic fibers are more suitable for reinforcing silica aerogels on some extent [20]. As it is known to all, aramid fibers are one kind of organic fibers which present excellent flexibility and a low density. Aramid pulps (APs) derive from aramid fibers by fibrillation processes, i.e., stripping fibrillated fibers from original fibers by some physicochemical methods, which make APs possess a thinner diameter and a larger specific surface compared to those of original aramid fibers. The thinner diameter of APs will do a greater favor to retain the matrix (in composites) intact. Besides, the smaller density and shorter length of APs make them convenient to mixing with liquid and distribute uniformly, which simultaneously avoid their precipitation during the liquid phase. These advantages are very beneficial for the preparation of fiber reinforced silica aerogel composites. Thus using APs as reinforcements is a better choice for achieving our current research target. In this work, aramid pulp reinforced silica aerogel composites (AP/aerogels) were prepared simply by adding APs into silica sol directly and mixing evenly. The generated AP/aerogels presented significantly improved mechanical strength and tailored thermal properties. Importantly, the integrality of aerogel matrix was improved and the phenomenon of producing aerogel dust during use was well alleviated. 2. Experimental 2.1. Materials and preparation Aramid pulps (APs) with the length of 1.13 ± 0.03 mm were purchased from DuPont, USA. Tetraethoxysilane (TEOS), ethanol (EtOH) and trimethylchlorosilane (TMCS) used as precursors, solvent and surface modifier respectively, were chemical pure grade (Sinopharm Chemical Reagent Co., Ltd., SCRC, China). The main preparation process derived from a classic two-step, acidbase catalyzed sol-gel process. First, alcosol was prepared as per the specific formula and synthesis parameters reported in our previous work [18]. Before adding base catalyst, APs were added into the alcosol with rapidly stirring for 30 min until APs swelled completely and distributed evenly. Then 0.5 mol/L NH3·H2O (aq) as base catalyst was added into AP/alcosol mixture with stirring for 10 min. Subsequently, the AP/alcosol mixture was made air-tight in a 100 mL beaker for polycondensation and the gelation usually happened in 20 min to form AP/alcogels. After AP/alcogels aging with EtOH for 12 h, solvent exchange with n-hexane for 12 h and surface modification with 10% TMCS/n-hexane solution for 12 h, respectively, AP/aerogels were finally generated via ambient pressure at 80 °C and 100 °C for 8 h, respectively. The obtained samples have a circular pie shape with the average diameter of 45.3 mm and average height of 14.2 mm. The composites with various AP content (denoted as c) were prepared for performance characterization. Besides, aramid fibers (AFs) reinforced silica aerogel composites (AF/aerogels) as per Ref. [18] were also fabricated for the comparison between the two reinforcements. 2.2. Methods of characterization The microstructures of AP/aerogels were observed by field emission scanning electron microscope (SEM, SIRION200, FEI) to evaluate the morphology and integrality. Furthermore, the nanoporous structures of the specimens were studied by N2 adsorption-desorption measured at 77 K using a Tristar II 3020 M analyzer. The pore size distribution (PSD) and pore parameters of pure silica aerogels and AP/aerogels
were calculated by Brunauer-Emmitt-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method. The ideal density (ρid) is defined as the final sample mass divided by the volume of AP/alcogels while the bulk density (ρb) was measured from the final sample mass and volume. The porosity was calculated as per Eq. (1). Porosity ¼
1=ρb −1=ρs −1=ρap 100% 1=ρb
ð1Þ
Above, ρs and ρap are the density of silica skeleton and APs, usually 2200 kg·m−3 and 1450 kg·m−3, respectively. Electronic dynamic and static fatigue testing machine (E3000K8953, Instron) was used for uniaxial compression tests at room temperature. The thermal properties of AP/aerogels were measured with a thermal constants analyzer (Hot-Disk 2500, Sweden) at 25 °C and TG-DSC (SDT Q600, TA) with a heating rate of 10 °C/min from room temperature to 800 °C in air. 3. Results and discussion 3.1. Microstructure In Fig. 1a, the APs have an abundant of fibrillated fibers with the diameter of b1 μm while the AFs are just similar to cylindrical rods with a diameter of N10 μm in Fig. 1b. For the same length, the APs possess a larger aspect ratio than the AFs, which are more beneficial for strengthening aerogels [20]. Besides, the plentiful fibrillated fibers with the specific surface area up to 5–11 m2·g−1 increase the interface contacts between APs and aerogels which contributes to the combination between the two phases. The slender fibrillated fibers of APs are inlaid in the aerogels in Fig. 1c–d with distributing casually, either in the lamina plane or through the lamina plane, and without observably destroying the integrality of aerogel matrix. Inversely, most AFs lie flat in the aerogels with separating the aerogel matrix into small fragments and forming much cracks as shown in Fig. 1e. Viewed from the fiber distribution, the APs in aerogels are multidirectional and more uniform, without compromising the integrality of aerogel matrix, compared to the AFs. Just because of these, the APs and aerogels are combined tightly so that the phenomena of aerogel chippings falling out from aerogel composites decreases significantly. Furthermore, the finer microstructure is revealed in Fig. 1f with presenting the 3-D nanoporous network of pure silica aerogels. The N2 adsorption-desorption isotherms and pore size distribution (PSD) were used to investigate the microstructure further. The corresponding results for pure silica aerogels are presented in Fig. 2a–b. The N2 adsorption-desorption isotherms in Fig. 2a presents as the form of the typical type IV according with the mesoporous materials, and the hysteresis loops of the type H3 indicates the probable presence of slitlike interparticle pores [21]. The pore volume and pore area distribution with pore diameter are presented in Fig. 2b, with a sharp peak ranging from 5 to 10 nm, as an indication of the most probable pore size. The corresponding results for AP/aerogels are presented in Fig. 2c–d. It can be seen that the AP/aerogels nearly have the same N2 adsorption-desorption isotherm with the pure silica aerogels while the PSD for the AP/aerogels slightly shifts toward the smaller pore size with the most probable pore size still remaining between 5 and 10 nm. The pore parameters are calculated and listed in Table 1, in which the BET surface area, pore volume and average pore size of the AP/aerogels are slightly smaller than that of the pure silica aerogels. This just conforms to the trend as discussed in Fig. 2. Judging from these, we can drew that both the pure silica aerogels and the AP/aerogels are mesoporous materials which indicates the huge application prospects in numerous fields. From the nanoporous structure and pore size distribution, it is known that the introduced APs are compatible with silica aerogel without destroying the
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
3
Fig. 1. Microstructures of aramid pulps (a), aramid fibers (b), AP/aerogels (c) and (d), AF/aerogels (e) (reproduced with friendly permission Ref. [18]) and the nanoporous network of pure silica aerogels (f).
microstructure of aerogel matrix. On the contrary, the slender fibrillated fibers of APs retain the integrality of aerogel matrix, dramatically reducing the formation of aerogel dust during use.
depends on the relative change of volume and mass. Due to the higher increment on mass than that of volume, the bulk density of AP/aerogels still increases from 150.8 kg·m−3 to 161.5 kg·m−3 as well as the porosity decreases from 82.3% to 81.4% at last.
3.2. Density and porosity 3.3. Mechanical strength It is distinctly showed in Fig. 3 that the ideal density increases monotonously with the increasing AP content while the volume shrinkage decreases slightly first and then drastically decreases over 8% AP content. As APs are added into alcosol, a fast stirring and lasting soak make APs swell and distribute uniformly in the hybrid liquid system. The incremental volume due to the introduced APs is negligible compared to that of the original alcogel. Thus the rising of ideal density is primarily caused by the increasing mass of APs which just accounts for the approximative linear growth. As reported [9,15,16,18], APs in the aerogel matrix act as supporting skeleton which leads to a decreasing volume shrinkage. This effect is caused by the resistance to collapse of nanoporous skeleton during drying process. However, when the AP content is less than ~8%, the effect of supporting skeleton is not obvious that leads to a larger volume shrinkage. Further increasing the AP content to over ~8%, it is found that the volume shrinkage decreases rapidly which indicates the nice skeleton supporting effect. Fig. 4 presents the bulk density increases along with the rising AP content while the porosity presents an opposite trend. As the AP content increases, the effect of supporting skeleton in the aerogel matrix play a great role in reducing the volume shrinkage, which leads to increasing the volume of AP/aerogels. However, the bulk density of AP/aerogels
Fig. 5 is the stress-strain (σ −ε) graphs for each AP content composite. Seen from the four graphs, it is evident that all the σ − ε curves can be divided into two parts as marked, where ε = 0.15 and ε = 0.20 are the cut-off points for c ≤7.68% and c ≥10.03%, respectively. To be specific, the previous segments are nonlinear elastic stages while the subsequent segments are elastic-plastic stages. Furthermore, the average compressive strengths are ~ 0.83 MPa and ~ 1.10 MPa for the AP/aerogels with the AP content being 5.39% and 7.68%, respectively. Considering no obvious yielding when c ≥10.03%, the compressive strengths is substituted for σ0.5 which are ~1.16 MPa and ~1.22 MPa for AP/aerogels with the AP content being 10.03% and 11.99%, respectively. However, the maximum σ0.5 of the reported AF/aerogels was just ~0.18 MPa with the fiber content ranging from 1.58–11.40% [18]. Meanwhile, it is observed that the compressive strengths of AP/aerogels are enhanced with the increasing AP content. For APs, the thinner diameter not only alleviates the damages on aerogel matrix's microstructures but also is more beneficial for APs combining with aerogel matrix due to the improved interface contacts. Although there exists no chemical bonds, the increscent interfacial adhesion would facilitate the enhancement of the Van der Waals' force
4
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
Fig. 2. N2 adsorption-desorption isotherms and pore volume and pore area distribution, (a), (b) for pure silica aerogels and (c), (d) for AP/aerogels (c = 7.68%).
between the two phases [18], which renders APs and silica aerogels combining more tightly. In the previous section, the larger aspect ratios of APs have been referred which means larger fracture strains [20]. Since the APs in aerogel matrix are multidirectional and uniform, the compressive deformation is dominated by the APs bending, stretching and stripping out of aerogel matrix [20]. Through these mechanical behaviors, the external stress is transferred on the integral AF/aerogels, avoiding stress concentration. Under these mechanical effects of APs in composites, silica aerogels are reinforced and can stand larger external stress. As a consequence, the dispersive APs lead to a larger compressive strength for AP/aerogels which can be further enhanced by increasing AP content. For AFs, the cracks on aerogel matrix caused by the thicker diameter are observed evidently in Fig. 1e. When suffering from an external force, the initially formed cracks can be further induced to grow and spread, resulting in material failure at last. This is the basis of lower compressive strength for AF/aerogels.
α ¼ 0:00186c þ 0:1397; R2 ¼ 0:906
ð3Þ
s ¼ 0:00203c þ 0:1446; R2 ¼ 0:884
ð4Þ
From the fitting equations, the thermal conductivity completely complies with highly linear increase, ranging from 0.0232–0.0278 W·m− 1·K− 1 while the thermal diffusivity and volume specific heat range from 0.1521–0.1643 m2·s−1 and 0.1544–0.1702 MJ·m−3·K−1, respectively. Besides, the pure silica aerogel's thermal constants can be predicated by assuming c = 0, which correspond to the intercepts of the three fitting lines, namely, λ = 0.01959 W·m−1·K−1, α = 0.1397 m2·s−1 and s =0.1446s MJ·m−3·K−1, respectively. Actually, the APs in aerogels not only act as the supporting skeleton, but also produce new heat transfer passageways because the larger thermal conductivity of APs is more beneficial for transmitting heat.
3.4. Heat insulation properties The thermal conductivity (λ), diffusivity (α) and volume specific heat (s), all increase linearly with the AP content in Fig. 6 and the linear fitting equations have been listed as follow. λ ¼ 0:000634c þ 0:01959; R2 ¼ 0:997
ð2Þ
Table 1 Pore parameters of the pure silica aerogels and AP/aerogels (c = 7.68%).
Sample
BET surface area (m2/g)
Pore volume (cm3/g)
Average pore size (nm)
Silica aerogels AP/aerogels
1067.2 955.4
4.3 4.1
13.4 12.8
Fig. 3. Ideal density and volume shrinkage of AP/aerogels with the AP content.
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
5
Fig. 4. Bulk density and porosity of AP/aerogels with the AP content.
As a consequence, more heat chooses to transfer through the new formed passageways instead of the aerogel's nanoporous network, which leads to an increasing thermal conductivity for AP/aerogels finally. Furthermore, the APs acting as the new channels render the heat transfer faster which is also beneficial for the thermal diffusion, resulting in an increasing thermal diffusivity. Due to the fact that the specific heat of APs is larger than that of pure silica aerogels, the increasing volume fraction of APs makes the average specific heat of unit volume AP/aerogels also increases at last. The linear increase of thermal conductivity, diffusivity and volume specific heat may ascribe to the distribution of APs which are now under further investigation. 3.5. Thermal stability Due to the excellent heat insulation properties, silica aerogels and their composites are usually used in thermal insulation system. For
Fig. 6. Variations of thermal conductivity, diffusivity and volume specific heat of AP/ aerogels with the AP content.
guaranteeing the use of aerogel products in safety, the thermal stability is an important parameter to study. The corresponding thermal stability analysis for the pure silica aerogels and AP/aerogels are presented in Fig. 7 and Fig. 8.
Fig. 5. Stress-strain curves of the various content AP/aerogels (two typical samples for each content).
6
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
Fig. 7. Thermal stability analysis (TG-DTA) for pure silica aerogels.
Fig. 7a shows that the primary weight loss concentrates between 260–460 °C during the heating process and the peak of weight loss rate is at ~ 280 °C. The DTA analysis is displayed in Fig. 7b that an obvious exothermic peak appears which just corresponds to the peak weight loss rate. The exothermic reaction is considered to be the oxidation of `Si-CH3 (TMS) groups on aerogels which results in the main weight loss [22]. The thermal stability analysis for AP/ aerogels is presented in Fig. 8a. In the infancy, a negligible weight loss from 50 °C to 200 °C indicates the evaporation of water and certain remained organic solvent [23]. The distinct differences for AP/aerogels are that the whole weight loss can be separated into two parts. At Stage I ranging from 250–418 °C, the weight loss is caused by the same reason, i.e., the oxidation of TMS groups on silica aerogels, which leads to the first exothermic peak in Fig. 8b. The weight loss between 418–563 °C at Stage II should be mainly ascribed to the pyrolysis of APs which causes the second exothermic peak in Fig. 8b simultaneously [24]. The comparisons on thermal stability between pure silica aerogels and AP/aerogels are made and the corresponding results are listed in Table 2. It can be seen that the onset temperature of the oxidation of TMS groups at Stage I are nearly the same at ~ 260 °C while the weight loss for pure silica aerogels and AP/aerogels are 9.1% and 7.8%, respectively. The peak temperature of TG and DTA for the pure silica aerogels is similar at ~ 280 °C while the peak temperature of TG for AP/aerogels clearly lags behind that of DTA. The same hysteresis at Stage II is observed with a temperature difference of about 43 °C. This phenomenon may be caused by the heat absorption of APs. From the above analysis, it can be drawn that the thermal stability of AP/aerogels primarily depends on the pure silica aerogel component and the introduced APs does not affect the thermal stability of AP/aerogels.
Fig. 8. Thermal stability analysis (TG-DTA) for AP/aerogels (c = 10.03%).
4. Conclusions In this study, we devoted to retaining the integrality and improving the mechanical strength of fiber/aerogels without compromising the thermal insulation properties. For this purpose, APs composed of extremely thin fibrillated fibers were used as reinforcements, through directly adding APs into alcosol, to prepare AP/aerogels composites. The microstructures demonstrated that APs were inlaid in composites without impairing the integrality of aerogel matrix. As the AP content increased, the bulk density of AP/aerogels slightly increased from 150.8 kg·m−3 to 161.5 kg·m−3 as well as the porosity decreased from 82.3% to 81.4%. The compressive strength increased distinctly reaching up to ~ 1.2 MPa. Meanwhile, the thermal conductivity, diffusivity and volume specific heat all approximated linear increase and the low thermal conductivity ranging from 0.0232–0.0278 W·m− 1·K−1 was still sustained. Furthermore, TG-DTA analysis indicated that the thermal stability of the AP/aerogels was about 260 °C which primarily depended on the pure silica aerogel component in composites.
Table 2 Thermal analysis data for pure silica aerogels and AP/aerogels (c = 10.03%).
Sample
Tonset (°C)
Tpeak of DTG (°C)
Tpeak of DTA (°C)
Weight loss (%)
Stage I Pure silica aerogels AP/aerogels
261.6 259.4
276.7 309.6
282.3 271.6
9.1 7.8
Stage II AP/aerogels
417.8
557.3
514.7
11.2
Z. Li et al. / Journal of Non-Crystalline Solids 454 (2016) 1–7
Acknowledgement This work was supported by the Fundamental Research Funds for the Central Universities (grant no. WK2320000032, WK6030000028). References [1] M.A.B. Meador, E.F. Fabrizio, F. Ilhan, A. Dass, G.H. Zhang, P. Vassilaras, et al., Crosslinking amine-modified silica aerogels with epoxies: mechanically strong lightweight porous materials, Chem. Mater. 17 (2005) 1085–1098. [2] D. Ge, L. Yang, Y. Li, J. Zhao, Hydrophobic and thermal insulation properties of silica aerogel/epoxy composite, J. Non-Cryst. Solids 355 (2009) 2610–2615. [3] B. Yuan, S. Ding, D. Wang, G. Wang, H. Li, Heat insulation properties of silica aerogel/ glass fiber composites fabricated by press forming, Mater. Lett. 75 (2012) 204–206. [4] J.G.R. Hansen, B.J. Frame, Aerogel-based thermal insulation, J. Non-Cryst. Solids 225 (1998) 254–259. [5] S. He, X. Cheng, Z. Li, X. Shi, H. Yang, H. Zhang, Green and facile synthesis of spongereinforced silica aerogel and its pumping application for oil absorption, J. Mater. Sci. 51 (2016) 1292–1301. [6] J.P. Randall, M.A. Meador, S.C. Jana, Tailoring mechanical properties of aerogels for aerospace applications, ACS Appl. Mater. Interfaces 3 (2011) 613–626. [7] A. Katti, N. Shimpi, S. Roy, H.B. Lu, E.F. Fabrizio, A. Dass, et al., Chemical, physical, and mechanical characterization of isocyanate cross-linked amine-modified silica aerogels, Chem. Mater. 18 (2006) 285–296. [8] H. Maleki, L. Durães, A. Portugal, An overview on silica aerogels synthesis and different mechanical reinforcing strategies, J. Non-Cryst. Solids 385 (2014) 55–74. [9] J. Feng, C. Zhang, J. Feng, Y. Jiang, N. Zhao, Carbon aerogel composites prepared by ambient drying and using oxidized polyacrylonitrile fibers as reinforcements, ACS Appl. Mater. Interfaces 3 (2011) 4796–4803. [10] K.E. Parmenter, F. Milstein, Mechanical properties of silica aerogels, J. Non-Cryst. Solids 223 (1998) 179–189. [11] S. Duoqi, S. Yantao, F. Jian, Y. Xiaoguang, H. Shiwei, M. Chunhu, et al., Experimental investigation on high temperature anisotropic compression properties of ceramicfiber-reinforced SiO2 aerogel, Mater. Sci. Eng. A-Struct. 585 (2013) 25–31. [12] X. Yang, Y. Sun, D. Shi, J. Liu, Experimental investigation on mechanical properties of a fiber-reinforced silica aerogel composite, Mater. Sci. Eng. A Struct. 528 (2011) 4830–4836.
7
[13] X. Yang, J. Wei, D. Shi, Y. Sun, S. Lv, J. Feng, et al., Comparative investigation of creep behavior of ceramic fiber-reinforced alumina and silica aerogel, Mater. Sci. Eng. AStruct. 609 (2014) 125–130. [14] X. Yang, Y. Sun, D. Shi, Experimental investigation and modeling of the creep behavior of ceramic fiber-reinforced SiO2 aerogel, J. Non-Cryst. Solids 358 (2012) 519–524. [15] H. Wu, Y. Liao, Y. Ding, H. Wang, C. Peng, S. Yin, Engineering thermal and mechanical properties of multilayer aligned fiber-reinforced aerogel composites, Heat Transf. Eng. 35 (2014) 1061–1070. [16] Y. Liao, H. Wu, Y. Ding, S. Yin, M. Wang, A. Cao, Engineering thermal and mechanical properties of flexible fiber-reinforced aerogel composites, J. Sol-Gel Sci. Techn. 63 (2012) 445–456. [17] H. Wu, Y. Chen, Q. Chen, Y. Ding, X. Zhou, H. Gao, Synthesis of flexible aerogel composites reinforced with electrospun nanofibers and microparticles for thermal insulation, J. Nanomater. 2013 (2013), http://dx.doi.org/10.1155/2013/375093. [18] Z. Li, X. Cheng, S. He, X. Shi, L. Gong, H. Zhang, Aramid fibers reinforced silica aerogel composites with low thermal conductivity and improved mechanical performance, Compos. Part A Appl. S. 84 (2016) 316–325. [19] A. Karout, P. Buisson, A. Perrard, A.C. Pierre, Shaping and mechanical reinforcement of silica aerogel biocatalysts with ceramic fiber felts, J. Sol-Gel Sci. Technol. 36 (2005) 163–171. [20] S.O. Kucheyev, M. Stadermann, S.J. Shin, J.H. Satcher, S.A. Gammon, S.A. Letts, et al., Super-compressibility of ultralow-density nanoporous silica, Adv. Mater. 24 (2012) 776–780. [21] F. Rojas, I. Kornhauser, C. Felipe, J.M. Esparza, S. Cordero, A. Domínguez, et al., Capillary condensation in heterogeneous mesoporous networks consisting of variable connectivity and pore-size correlation, Phys. Chem. Chem. Phys. 4 (2002) 2346–2355. [22] Z. Li, L. Gong, X. Cheng, S. He, C. Li, H. Zhang, Flexible silica aerogel composites strengthened with aramid fibers and their thermal behavior, Mater. Des. 99 (2016) 349–355. [23] A. Tadjarodi, M. Haghverdi, V. Mohammadi, Preparation and characterization of nano-porous silica aerogel from rice husk ash by drying at atmospheric pressure, Mater. Res. Bull. 47 (2012) 2584–2589. [24] F. Suarez-Garcia, S. Villar-Rodil, C.G. Blanco, A. Martinez-Alonso, J.M.D. Tascon, Effect of phosphoric acid on chemical transformations during Nomex pyrolysis, Chem. Mater. 16 (2004) 2639–2647.