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silica aerogel composites fabricated by the sol–gel method via ambient pressure drying
Materials and Design 85 (2015) 438–443
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Microstructure and properties of the Si3N4/silica aerogel composites fabricated by the sol–gel method via ambient pressure drying Haixia Yang, Feng Ye ⁎, Qiang Liu, Ye Gao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
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Article history: Received 22 February 2015 Received in revised form 21 June 2015 Accepted 8 July 2015 Available online 13 July 2015 Keywords: Microstructure Composites Mechanical properties Dielectric properties Thermal insulation
a b s t r a c t Si3N4 particle reinforced silica aerogel composites have been fabricated by the sol–gel method via ambient pressure drying. The microstructure and mechanical, thermal insulation and dielectric properties of the composites were investigated. The effect of the Si3N4 content on the microstructure and properties were also clarified. The results indicate that the obtained mesoporous composites exhibit low thermal conductivity (0.024– 0.072 Wm−1 K−1), low dielectric constant (1.55–1.85) and low loss tangent (0.005–0.007). As the Si3N4 content increased from 5 to 20 vol.%, the compressive strength and the flexural strength of the composites increased from 3.21 to 12.05 MPa and from 0.36 to 2.45 MPa, respectively. The obtained composites exhibit considerable promise in wave transparency and thermal insulation functional integration applications. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction With the development of high-speed and long-duration spaceflight aircrafts in aerospace technology, the radomes or wave-transparent windows in these aircrafts are affected by aerodynamic heating and are subjected to high temperatures for extended periods of time. To protect the interior electronic equipment from the high temperature, the materials for these radomes or wave-transparent windows should be fabricated by wave-transparent and thermal insulation functional integration materials. Silica aerogels have attracted much attention for their excellent properties, such as high specific surface area (500– 1200 m2/g), low density (approximately 0.003–0.5 g/cm3), low thermal conductivity (0.005–0.1 Wm− 1 K− 1), ultra-low dielectric constant (1.0–2.0) and low loss tangent (10−2–10−4) [1–3]. Due to such characteristics, silica aerogels can be used as wave-transparent and thermal insulation functional integration materials; however, it is difficult to obtain mass production due to the low strength. Moreover, the mesoporous structure of silica aerogels would be destroyed above 1073 K and the heat insulation performance would be decreased. Silica aerogels with density of 0.12 g/cm3 can easily be destroyed by a stress of 31 kPa [4]. To overcome these problems, improvement of the strength and thermal stability is necessary. Many efforts have focused on improvement of the mechanical properties of the silica aerogels by using structural reinforcement [5–7], fiber reinforcement [8–10], polymer cross-linking [11–14] or other chemical or physical means [15–19]. However, it is difficult to form the fiber-reinforced silica aerogel ⁎ Corresponding author. Tel.: +86 451 86402040; fax: +86 451 86414291. E-mail address: [email protected] (F. Ye).
http://dx.doi.org/10.1016/j.matdes.2015.07.041 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
composites into complex shapes. On the other hand, the polymer cross-linked aerogels may be used for wave-transparent materials, but their application is limited due to thermal pyrolysis of the polymer occurring above 1073 K. Moreover, researchers have only concentrated on the thermal insulation properties of the silica aerogels, but little work has focused on the dielectric properties, which are crucial for wave-transparent materials. Silicon nitride is a promising structural–functional material for radomes because of its excellent mechanical and dielectric properties [20–22]. It is possible to use Si3N4 particles as reinforcement to improve the mechanical properties of silica aerogels while maintaining the low dielectric constant and low loss tangent. In this work, new mesoporous Si3N4/silica aerogel composites were obtained by a sol–gel process under ambient pressure drying. By adding dispersants and using planetary ball milling technology, untreated Si3N4 particles were dispersed in the silica aerogels. The microstructure, and mechanical, thermal insulation and dielectric properties of the Si3N4/ silica aerogel composites with different Si3N4 contents were investigated.
2. Experimental procedure 2.1. Raw materials Si3N4 powders (98%, d = 0.5 μm, UBE Co., Ltd., Japan) was used as the starting material. Tetraethoxysilane (TEOS, 98%, AR), ethanol (EtOH, 99%, AR), polyacrylic acid (PAA, LR), oxalic acid, isopropanol, N,N-dimethylformamide (DMF), n-hexane, ammonia (30%, AR) and trimethylchlorosilane (TECS) were purchased from Sinopharm Chemical
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Fig. 1. Scheme of the processing routes used for the fabrication of the Si3N4/silica aerogel composites in this study.
Reagent Co., Ltd., China. All chemicals were used as received without further purification.
2.2. Preparation process Fig. 1 shows the scheme of the processing routes for the fabrication of the Si3N4/silica aerogel composites in this study. Tetraethoxysilane, ethanol, water, and oxalic acid were mixed together with a molar ratio of 1:8:1:10−3 respectively. The mixture was stirred at room temperature for 24 h, and then additional water, N,N-dimethylformamide and ammonia solution were added. The TEOS/H2O/DMF/NH3·H2O molar ratio was 1:3:0.5:10− 2 respectively. After stirring for 30 min, Si3N4 powders with different volume fractions (i.e., 2, 5, 10, 15 and 20 vol.%) and PAA (Si3N4/PAA mass ratio was 100:1) were added into the silica sol, and the mixture was wet-milled using high-purity Si3N4 balls. Next, the mixture was poured into a mold just before the silica sol started to gel. Wet gel was formed by condensation within 20 h. The cured Si3N4/silica aquagel was aged in a mixture of tetraethoxysilane and ethanol (volume ratio of 7:3) at 323 K for 24 h and was subsequently treated by placing it in a mixture of isopropanol and n-hexane (volume ratio of 1:1) at 333 K for 24 h. After that, it was transferred to a mixture of n-hexane and trimethylchlorosilane with a ratio of 10:1 for 24 h at 333 K. Finally, the Si3N4/silica aquagel was placed in nhexane at 333 K for 72 h by slowly volatilizing and sintering in an air furnace at 973 K for 2 h to obtain Si3N4/silica aerogel composites.
2.3. Characterization Crystalline phases were identified by X-ray diffraction analysis (X' PERT PRO MPD, PANalytical Analytical Instruments, Netherlands). The microstructures of the specimens were characterized by scanning electron microscopy (Helios Nanolab 600i, FEI Co., Oregon, USA) and transmission electron microscopy (Tecnai G2F30, FEI Co., Oregon, USA). Nitrogen adsorption isotherms were measured by a gas adsorption analyzer (Gemini VII 2390, Micrometics Co., USA) at 77 K in the relative pressure range (p/p0) 0.05–0.99. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method [23]. The pore size distributions of the samples were calculated using the Barrett, Joyner and Halenda (BJH) method [24]. The mean pore diameter DBJH was obtained from the nitrogen adsorption isotherm and the BJH method. The bulk density (ρ) of the Si3N4/silica aerogel composite was calculated from the dimensions and weight of the samples. The porosity of the composite was calculated by (1 − ρ/ρs) × 100%. ρs is the skeletal density, which is obtained by the theoretical density of 3.18 g/cm3 and 2.2 g/cm3 for Si3N4 and silica 2.2 g/cm3, respectively. Three samples were examined to determine the average porosity. The linear shrinkage
Table 1 The physical properties of the composites with different Si3N4 contents. Specimens
Volume fractions of the Si3N4 (%)
Density (g/cm3)
Porosity (%)
Linear shrinkage (%)
1 2 3 4 5 6
0 2 5 10 15 20
0.14 0.21 0.32 0.49 0.64 0.82
93.6 90.5 85.8 78.7 72.7 65.3
7.8 6.9 6.2 5.5 4.9 4.4
Fig. 2. XRD patterns of the Si3N4/silica aerogel composites with different volume fractions of Si3N4 particles.
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Fig. 3. SEM images of the Si3N4/silica aerogel composites with different volume fractions of Si3N4 particles: (a) 0%; (b) 2%; (c) 5%; (d) 10%; (e) 15%; (f) 20%.
of the composites was determined by measuring the diameter of the dried monolithic sample and comparing it to its initial diameter when the wet gel had aged but not dried. The compressive strength of the specimens was tested with a load parallel to the freezing direction at a crosshead speed of 0.5 mm/min on specimens (6 mm × 6 mm × 8 mm). The flexural strengths of the specimens were tested by a three-point bending strength test with a load parallel to the freezing direction and a span of 30 mm at a crosshead speed of 0.5 mm/min on specimens (4 mm × 3 mm × 35 mm). The thermal conductivity of the specimens, with diameters of 30 mm and thicknesses of 5 mm, was determined by a thermal physical property parameter testing instrument (Model DRXII-PS, Xiangtan Huafeng instrument Co. LTD, China) at room temperature in air. The dielectric constant and loss tangent were measured by test equipment (Model N5230A, Agilent Technologies USA).
3. Results and discussion The density, porosity and linear shrinkage of the specimens are listed in Table 1. The results show that the density of the specimens increased from 0.14 g/cm3 to 0.82 g/cm3, whereas the corresponding porosity decreased from 93.6% to 65.3% as the volume fractions of Si3N4 particles increased from 0% to 20%. The linear shrinkage was 7.8% when drying the pure silica hydrogel to silica aerogel. The dry shrinkage of the composites decreased with increasing Si3N4 content, indicating that the Si3N4 particles provided a strong support and restrained shrinkage during drying. Fig. 2 shows the XRD patterns of the pure silica aerogels and the composites. The XRD pattern of the pure silica aerogels shows a broad peak between 20° and 30°, which indicates that the aerogels exist in
Fig. 4. TEM images and HRTEM images of the Si3N4/silica aerogel composites with different volume fractions of Si3N4 particles: (a) 0%; (b) 10%; (c) 20%; (d) the Si3N4 particles in the composites (20 vol.%); (e) electron diffraction pattern of (d); (f) HRTEM images of (d).
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Fig. 5. Nitrogen adsorption isotherms and pore size distributions of the composites with different Si3N4 contents.
the form of amorphous phase. After adding Si3N4, the XRD patterns show crystal diffraction peaks which correspond to α-Si3N4 and βSi3N4 phases. With the increasing Si3N4 content, the relative diffraction intensity of the crystal diffraction peaks increase, and the intensity of amorphous peaks decrease. 3.1. Microstructure of the Si3N4/silica aerogel composites The SEM images of the samples are shown in Fig. 3, indicating that the composites possess nanoporous structure. Some particles were dispersed within the silica aerogel matrix and adhered to the silica network. According to the XRD analysis, these particles are silicon nitride. The Si3N4 particles are enveloped by silica aerogel nanoparticles. There is a significant difference in the morphology of the pure silica aerogel and the composites. TEM micrographs of the composites are presented in Fig. 4. The pure silica aerogels exhibited a sponge-like [25] microstructure with nanoparticles (7–12 nm) stacked and interconnected into a porous three dimensional network. When the volume fraction of Si3N4 increased, the size of the aerogel nanoparticles increased and the pores decreased. The diffraction patterns (Fig. 4(e)) proved that the larger particles were crystalline, the crystal phase and structure were determined by XRD analysis. The HRTEM image (Fig. 4(f)) shows that the Si3N4 particle is embedded into amorphous silica and their interface is clean. The nitrogen adsorption isotherms and pore size distributions of the Si3N4/silica aerogel composites are displayed in Fig. 5. As shown in Fig. 5(a), all composites show adsorption isotherms of type IV, indicating a well-developed mesoporous structure [26]. With the increasing volume fraction of the Si3N4, the adsorption quantity gradually decreases due to the decreased porosity of the composites. The inset image in Fig. 5(a) shows the specific surface areas (SBET) of the pure silica aerogels and the composites. Pure silica aerogels show a higher SBET of 790 m2/g. As the volume fraction of Si3N4 increased from 2% to 20%, the specific surface area decreased from 322 to 64 m2/g. The results are in accordance with the fact that SBET can be affected by the particle
size [27]. As shown in Figs. 3 and 4, Si3N4 particles added in the silica aerogel increase the particle size, leading to a decrease of the SBET of the composites. On the other hand, the density of the Si3N4 powder (3.18 g/cm3) is approximately 22.7 times larger than pure silica aerogels (0.14 g/cm3). Adding Si3N4 powder sharply increases the mass of the samples in a certain volume of space but slightly increases surface area; thus, the specific surface area (the surface area per unit mass) shows a downward trend. Fig. 5(b) shows the pore size distribution of the composites. Pure silica aerogel has a narrow pore size distribution with the peak pore size (Dp) approximately 5 nm. As the Si3N4 particles are added, the composites show multi-peak distributions and an increase of Dp. The peak height decreases with the increase of the Si3N4 content, indicating the decrease of the pore volume and the decrease of the number of pores with a certain size. According to the results of the SEM and TEM, the silica aerogel particle size increased with the increasing of Si3N4 content, which caused the increasing of pore size of the composites. The adding of Si3N4 particles and poly acrylic acid interferes with the original sol– gel process which may cause different degrees of monomer polycondensation, resulting in pore diameter show multi-peak distribution. The inset image in Fig. 5(b) shows that the mean pore diameters of the composites are varied by the content of the Si3N4 but are still smaller than 20 nm. 3.2. Mechanical properties of the Si3N4/silica aerogel composites The mechanical properties of the composites are shown in Fig. 6. The reported compressive strength of unmodified silica aerogels was 0.04 MPa in Ref. [28], but the flexural strength was not presented. In this study, the compressive strength of the composites was 0.24 MPa when the volume fraction of the Si3N4 was 2%; the flexural strength could not be obtained. The mechanical properties of the composites increase with the increasing Si3N4 content. As the volume fractions of Si3N4 particles increased to 20%, the compressive strength and the flexural strength of the specimens increased to 12.05 MPa and 2.45 MPa,
Fig. 6. Compressive strengths and flexural strengths of the Si3N4/silica aerogel composites.
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Fig. 7. Thermal conductivities of the Si3N4/silica aerogel composites.
respectively. Si3N4 particles occupied some places of the pores in the aerogels, resulting in an increase of the density, leading to the increased strength of the composite. The increase of the strength is attributed to the support of Si3N4 particles before fracture and the increase of the elastic modulus associated with the density of the bulk material [29]. When cracks propagate to Si3N4 particles, stress concentration occurs and causes the weak interfaces between the Si3N4 particles and the matrix to debond and crack to deflect, which would absorb more fracture work and improve the mechanical properties. These results indicate that the mechanical properties of aerogels can be optimized by exploiting the support of some solid particles to create relatively robust aerogel composites. The pure unmodified silica aerogels have extremely low strength when the porosity is up to 90%, which tends to be extremely fragile. It is reported that the pure silica aerogels with a density of 0.112 g/cm3 exhibited a low compressive strength of 0.018 MPa [30]. In conventional methods, the glass fibers or other ceramic fibers were used as reinforcement to improve the mechanical properties of silica aerogels. Yuan et al. [8] reported the silica aerogel/glass fiber composites (0.53 g/cm3) with a compressive strength of 1.2 MPa. This preparation process includes preparing silica aerogel powders and press forming. In our study, the gel molding method was used to obtain the Si3N4/silica aerogel composites which exhibits a relatively higher compressive strength of 6.37 MPa (the density of 0.49 g/cm3). In addition, unlike fibers the Si3N4 particles were used as reinforcements without pretreatment. The method in our work can easily obtain Si3N4/silica aerogel composites with higher mechanical properties. 3.3. Thermal insulation properties of the Si3N4/silica aerogel composites The thermal conductivities of the Si3N4/silica aerogel composites are shown in Fig. 7. In this study, the prepared pure silica aerogels could not be made into standard units to test the thermal conductivity; thus, the reported data in the literature are referenced. The thermal conductivity of the silica aerogels is approximately 0.014 Wm− 1 K−1 in Ref. [31].
After adding Si3N4 particles, the thermal conductivities of the composites tended to increase. The thermal conductivity of the composites increases from 0.024 to 0.072 Wm−1 K−1 as the volume fraction of Si3N4 increases from 2% to 20%. The increase of the thermal conductivity is attributed to the increase of the solid phase conduction caused by the solid mass and pore wall thickness increase [32]. On the other hand, the particle size increased and the SBET decreased with the increasing volume fraction of Si3N4, indicating that the number of interfaces per unit length of heat path decreased. Thus, the thermal conductivity of the solid phase increases [33]. Fortunately, the composites inherited the nanometer pore structure of the silica aerogels, and their average pore size (less than 15 nm in Fig. 5(b)) is less than the mean free path of gas molecules (69 nm); therefore, the composites still possess good thermal insulation properties. For the aerogel composites, the optimal range of thermal conductivity is generally 0.012–0.1 Wm−1 K−1 at room temperature. The range of thermal conductivity of the Si3N4/silica aerogel composites is 0.024– 0.072 Wm−1 K−1, when the volume fraction of Si3N4 particles is 2%– 20%. The mechanical properties of the composites increase with Si3N4, but the thermal insulation properties decrease. Based on above analysis and discussion, the composites with definite strength and thermal conductivity can be obtained by adjusting content of Si3N4 according to the practical application environment. 3.4. Dielectric properties of the Si3N4/silica aerogel composites To obtain excellent wave-transparent ability, the dielectric constant and loss tangent of the materials must be no more than 4.0 and 0.01, respectively [34]. Fig. 8 shows the dielectric constant and loss tangent of the silica aerogel composites as a function of the frequency. With increasing Si3N4 content, the dielectric constant of the composites increases from 1.55 to 1.85. For well-distributed composites, the dielectric constant can be calculated according to the mixing law [35]. The dielectric constant of air is approximately 1; thus, the higher the porosity is in the materials, the lower the dielectric constant that will be obtained. As the Si3N4 content increased, the porosity of the composites decreased, resulting in an increase of the dielectric constant. The loss tangent of the composite varied slightly with the addition of Si3N4. 4. Conclusions We presented a detailed investigation of the microstructural, and mechanical, thermal insulation and dielectric properties of Si3N4/silica aerogels prepared by the sol–gel method. The mechanical properties are highly dependent on the densities of the composites with the Si3N4 content varying from 2 to 20 vol.%. Compressive strength as high as 12.05 MPa and flexural strength as high as 2.45 MPa were obtained. Due to the nanometer pore structure that was inherited from the silica aerogels, the composites exhibited excellent properties of thermal insulation; the thermal conductivity as low as 0.024 Wm−1 K−1. Meanwhile, the composites also showed a low dielectric constant (b1.9) and a low
Fig. 8. Dielectric constant and loss tangent of the Si3N4/silica aerogel composites as functions of frequency.
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loss tangent (b7 × 10−3). The Si3N4/silica aerogel composites have potential application in the aerospace field for thermal insulation and wavetransparent properties. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51021002). References [1] L.W. Hrubesh, L.E. Keene, V.R. Latorre, Dielectric-properties of aerogels, J. Mater. Res. 8 (1993) 1736–1741. [2] Hajar Maleki, Luias Durães, António Portugal, An overview on silica aerogels synthesis and different mechanical reinforcing strategies, J. Non-Cryst. Solids 385 (2014) 55–74. [3] A. Jain, S. Rogojevic, S. Ponoth, N. Agarwal, L. Matthew, W.N. Gill, et al., Porous silica materials as low-k dielectrics for electronic and optical interconnects, Thin Solid Films 398 (2001) 513–522. [4] M.F. Bertino, J.F. Hund, J. Sosa, G. Zhang, C. Sotiriou-Leventis, N. Leventis, A.T. Tokuhiro, J. Terry, High resolution patterning of silica aerogels, J. Non-Cryst. Solids 333 (2004) 108–110. [5] A.V. Rao, S.D. Bhagat, H. Hirashima, G.M. Pajonk, Synthesis of flexible silica aerogels using methyltrimethoxysilane (MTMS) precursor, J. Colloid Interface Sci. 300 (2006) 279–285. [6] D.Y. Nadargi, S.S. Latthe, H. Hirashima, A.V. Rao, Studies on rheological properties of methyltriethoxysilane (MTES) based flexible superhydrophobic silica aerogels, Microporous Mesoporous Mater. 117 (2009) 617–626. [7] P.R. Aravind, G.D. Soraru, High surface area methyltriethoxysilane-derived aerogels by ambient pressure drying, J. Porous. Mater. 18 (2011) 159–165. [8] B. Yuan, S.Q. Ding, D.D. Wang, G. Wang, H.X. Li, Heat insulation properties of silica aerogel/glass fiber composites fabricated by press forming, Mater. Lett. 75 (2012) 204–206. [9] D.Q. Shi, Y.T. Sun, J. Feng, X.G. Yang, S.W. Han, C.H. Mi, Y.G. Jiang, H.Y. Qi, Experimental investigation on high temperature anisotropic compression properties of ceramic-fiber-reinforced SiO2 aerogel, Mater. Sci. Eng. A 585 (2013) 25–31. [10] X.G. Yang, Y.T. Sun, D.Q. Shi, Experimental investigation and modeling of the creep behavior of ceramic fiber-reinforced SiO2 aerogel, J. Non-Cryst. Solids 358 (2012) 519–524. [11] H.L. Yang, X.M. Kong, Y.R. Zhang, C.C. Wu, E.X. Gao, Mechanical properties of polymer-modified silica aerogels dried under ambient pressure, J. Non-Cryst. Solids 357 (2011) 3447–3453. [12] H. Guo, M.A.B. Meador, L. Mccorkle, D.J. Quade, J. Guo, B. Hamilton, M. Cakmak, G. Sprowl, Polyimide aerogels cross-linked through amine functionalized polyoligomeric silsesquioxane, Appl. Mater. Interfaces 3 (2011) 546–552. [13] B. Markus, N. Theresa, R. Gudrun, Cross-linked monolithic xerogels based on silica nanoparticles, Chem. Mater. 25 (2013) 3648–3653. [14] F. Sabri, J. Marchetta, K.M. Smith, Thermal conductivity studies of a polyurea crosslinked silica aerogel-RTV655 compound for cryogenic propellant tank applications in space, Acta Astronaut. 91 (2013) 173–179. [15] J. Cai, S.L. Liu, J. Feng, S. Kimura, M. Wada, S. Kuga, L.N. Zhang, Cellulose–silica nanocomposite aerogels by in situ formation of silica in cellulose gel, Angew. Chem. Int. Ed. 51 (2012) 2076–2079.
443
[16] D.J. Boday, R.J. Stover, B. Muriithi, M.W. Keller, J.T. Wertz, K.A.D. Obrey, D.A. Loy, Formation of polycyanoacrylate–silica nanocomposites by chemical vapor deposition of cyanoacrylates on aerogels, Chem. Mater. 20 (2008) 2845–2847. [17] C.Q. Hong, J.C. Han, X.H. Zhang, J.C. Du, Novel nanoporous silica aerogel impregnated highly porous ceramics with low thermal conductivity and enhanced mechanical properties, Scr. Mater. 68 (2013) 599–602. [18] D.J. Boday, R.J. Stover, B. Muriithi, M.W. Keller, J.T. Wertz, K.A.D. Obrey, D.A. Loy, Strong, low-density nanocomposites by chemical vapor deposition and polymerization of cyanoacrylates on aminated silica aerogels, Appl. Mater. Interfaces 7 (2009) 1364–1369. [19] L.F. Hu, C.A. Wang, Y. Huang, Porous YSZ ceramics with unidirectionally aligned pore channel structure: lowering thermal conductivity by silica aerogels impregnation, J. Eur. Ceram. Soc. 31 (2011) 2915–2922. [20] C. Kawai, A. Yamakawa, Effect of porosity and microstructure on the strength of Si3N4: designed microstructure for high strength, high thermal shock resistance, and facile machining, J. Am. Ceram. Soc. 80 (1997) 2705–2708. [21] T. Ohji, Microstructural design and mechanical properties of porous silicon nitride ceramics, Mater. Sci. Eng. A 498 (2008) 5–11. [22] G. Ziegler, J. Heinrich, G. Wotting, Relationships between processing, microstructure and properties of dense and reaction-bonded silicon nitride, J. Mater. Sci. 22 (1987) 3041–3086. [23] S. Brunauer, P.H. Emmett, E. Teller, Adsorption of gases in multimolecular layers, J. Am. Chem. Soc. 60 (1938) 309–319. [24] E.P. Barrett, L.G. Joyner, P.P. Halenda, The determination of pore volume and area distributions in porous substances: 1. computations from nitrogen isotherms, J. Am. Chem. Soc. 73 (1951) 373–381. [25] G.Y. Wu, Y.X. Yu, X. Cheng, Y. Zhang, Preparation and surface modification mechanism of silica aerogels via ambient pressure drying, Mater. Chem. Phys. 129 (2011) 308–314. [26] K. Kaneko, Determination of pore size and pore size distribution: 1. Adsorbents and catalysts, J. Membr. Sci. 96 (1994) 59–89. [27] S.J. Gregg, K.S.W. Sing, Adsorption, surface area and porosity, 2nd ed. Academic Press, London, 1982. [28] Y.N. Duan, C.J. Sadhan, L. Bimala, P.E. Matthew, Reinforcement of silica aerogels using silane-end-capped polyurethanes, Langmuir 29 (2013) 6156–6165. [29] S.O. Kucheyev, M. Stadermann, S.J. Shin, J.H. Satcher Jr., S.A. Gammon, S.A. Letts, T. van Buuren, A.V. Hamza, Super-compressibility of ultralow-density nanoporous silica, Adv. Mater. 24 (2012) 776–780. [30] Z.S. Deng, J. Wang, A.M. Wu, J. Shen, B. Zhou, High strength SiO2 aerogel insulation, J. Non-Cryst. Solids 225 (1998) 101–104. [31] J.C.H. Wong, H. Kaymak, S. Brunner, M.M. Koebel, Mechanical properties of monolithic silica aerogels made from polyethoxydisiloxanes, Microporous Mesoporous Mater. 183 (2014) 23–29. [32] C. Rudaz, R. Courson, L. Bonnet, S. Calas-Etienne, H. Sallée, Tatiana Budtova, Aeropectin: fully biomass-based mechanically strong and thermal superinsulating aerogel, Biomacromolecules 15 (2014) 2188–2195. [33] J. Bourret, N. Tessier-Doyen, B. Naït-Ali, F. Pennec, A. Alzina, C.S. Peyratout, D.S. Smith, Effect of the pore volume fraction on the thermal conductivity and mechanical properties of kaolin-based foams, J. Eur. Ceram. Soc. 33 (2013) 1487–1495. [34] C.R. Zou, C.R. Zhang, B. Li, S.Q. Wang, F. Cao, Microstructure and properties of porous silicon nitride ceramics prepared by gel-casting and gas pressure sintering, Mater. Des. 44 (2013) 114–118. [35] M.B.H. Othman, M.R. Ramli, L.Y. Tyng, Z. Ahmad, H.M. Akil, Dielectric constant and refractive index of poly (siloxane-imide) block copolymer, Mater. Des. 32 (2011) 3173–3182.
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Materials and Design journal homepage: www.elsevier.com/locate/jmad
Microstructure and properties of the Si3N4/silica aerogel composites fabricated by the sol–gel method via ambient pressure drying Haixia Yang, Feng Ye ⁎, Qiang Liu, Ye Gao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 22 February 2015 Received in revised form 21 June 2015 Accepted 8 July 2015 Available online 13 July 2015 Keywords: Microstructure Composites Mechanical properties Dielectric properties Thermal insulation
a b s t r a c t Si3N4 particle reinforced silica aerogel composites have been fabricated by the sol–gel method via ambient pressure drying. The microstructure and mechanical, thermal insulation and dielectric properties of the composites were investigated. The effect of the Si3N4 content on the microstructure and properties were also clarified. The results indicate that the obtained mesoporous composites exhibit low thermal conductivity (0.024– 0.072 Wm−1 K−1), low dielectric constant (1.55–1.85) and low loss tangent (0.005–0.007). As the Si3N4 content increased from 5 to 20 vol.%, the compressive strength and the flexural strength of the composites increased from 3.21 to 12.05 MPa and from 0.36 to 2.45 MPa, respectively. The obtained composites exhibit considerable promise in wave transparency and thermal insulation functional integration applications. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction With the development of high-speed and long-duration spaceflight aircrafts in aerospace technology, the radomes or wave-transparent windows in these aircrafts are affected by aerodynamic heating and are subjected to high temperatures for extended periods of time. To protect the interior electronic equipment from the high temperature, the materials for these radomes or wave-transparent windows should be fabricated by wave-transparent and thermal insulation functional integration materials. Silica aerogels have attracted much attention for their excellent properties, such as high specific surface area (500– 1200 m2/g), low density (approximately 0.003–0.5 g/cm3), low thermal conductivity (0.005–0.1 Wm− 1 K− 1), ultra-low dielectric constant (1.0–2.0) and low loss tangent (10−2–10−4) [1–3]. Due to such characteristics, silica aerogels can be used as wave-transparent and thermal insulation functional integration materials; however, it is difficult to obtain mass production due to the low strength. Moreover, the mesoporous structure of silica aerogels would be destroyed above 1073 K and the heat insulation performance would be decreased. Silica aerogels with density of 0.12 g/cm3 can easily be destroyed by a stress of 31 kPa [4]. To overcome these problems, improvement of the strength and thermal stability is necessary. Many efforts have focused on improvement of the mechanical properties of the silica aerogels by using structural reinforcement [5–7], fiber reinforcement [8–10], polymer cross-linking [11–14] or other chemical or physical means [15–19]. However, it is difficult to form the fiber-reinforced silica aerogel ⁎ Corresponding author. Tel.: +86 451 86402040; fax: +86 451 86414291. E-mail address: [email protected] (F. Ye).
http://dx.doi.org/10.1016/j.matdes.2015.07.041 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
composites into complex shapes. On the other hand, the polymer cross-linked aerogels may be used for wave-transparent materials, but their application is limited due to thermal pyrolysis of the polymer occurring above 1073 K. Moreover, researchers have only concentrated on the thermal insulation properties of the silica aerogels, but little work has focused on the dielectric properties, which are crucial for wave-transparent materials. Silicon nitride is a promising structural–functional material for radomes because of its excellent mechanical and dielectric properties [20–22]. It is possible to use Si3N4 particles as reinforcement to improve the mechanical properties of silica aerogels while maintaining the low dielectric constant and low loss tangent. In this work, new mesoporous Si3N4/silica aerogel composites were obtained by a sol–gel process under ambient pressure drying. By adding dispersants and using planetary ball milling technology, untreated Si3N4 particles were dispersed in the silica aerogels. The microstructure, and mechanical, thermal insulation and dielectric properties of the Si3N4/ silica aerogel composites with different Si3N4 contents were investigated.
2. Experimental procedure 2.1. Raw materials Si3N4 powders (98%, d = 0.5 μm, UBE Co., Ltd., Japan) was used as the starting material. Tetraethoxysilane (TEOS, 98%, AR), ethanol (EtOH, 99%, AR), polyacrylic acid (PAA, LR), oxalic acid, isopropanol, N,N-dimethylformamide (DMF), n-hexane, ammonia (30%, AR) and trimethylchlorosilane (TECS) were purchased from Sinopharm Chemical
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Fig. 1. Scheme of the processing routes used for the fabrication of the Si3N4/silica aerogel composites in this study.
Reagent Co., Ltd., China. All chemicals were used as received without further purification.
2.2. Preparation process Fig. 1 shows the scheme of the processing routes for the fabrication of the Si3N4/silica aerogel composites in this study. Tetraethoxysilane, ethanol, water, and oxalic acid were mixed together with a molar ratio of 1:8:1:10−3 respectively. The mixture was stirred at room temperature for 24 h, and then additional water, N,N-dimethylformamide and ammonia solution were added. The TEOS/H2O/DMF/NH3·H2O molar ratio was 1:3:0.5:10− 2 respectively. After stirring for 30 min, Si3N4 powders with different volume fractions (i.e., 2, 5, 10, 15 and 20 vol.%) and PAA (Si3N4/PAA mass ratio was 100:1) were added into the silica sol, and the mixture was wet-milled using high-purity Si3N4 balls. Next, the mixture was poured into a mold just before the silica sol started to gel. Wet gel was formed by condensation within 20 h. The cured Si3N4/silica aquagel was aged in a mixture of tetraethoxysilane and ethanol (volume ratio of 7:3) at 323 K for 24 h and was subsequently treated by placing it in a mixture of isopropanol and n-hexane (volume ratio of 1:1) at 333 K for 24 h. After that, it was transferred to a mixture of n-hexane and trimethylchlorosilane with a ratio of 10:1 for 24 h at 333 K. Finally, the Si3N4/silica aquagel was placed in nhexane at 333 K for 72 h by slowly volatilizing and sintering in an air furnace at 973 K for 2 h to obtain Si3N4/silica aerogel composites.
2.3. Characterization Crystalline phases were identified by X-ray diffraction analysis (X' PERT PRO MPD, PANalytical Analytical Instruments, Netherlands). The microstructures of the specimens were characterized by scanning electron microscopy (Helios Nanolab 600i, FEI Co., Oregon, USA) and transmission electron microscopy (Tecnai G2F30, FEI Co., Oregon, USA). Nitrogen adsorption isotherms were measured by a gas adsorption analyzer (Gemini VII 2390, Micrometics Co., USA) at 77 K in the relative pressure range (p/p0) 0.05–0.99. The specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method [23]. The pore size distributions of the samples were calculated using the Barrett, Joyner and Halenda (BJH) method [24]. The mean pore diameter DBJH was obtained from the nitrogen adsorption isotherm and the BJH method. The bulk density (ρ) of the Si3N4/silica aerogel composite was calculated from the dimensions and weight of the samples. The porosity of the composite was calculated by (1 − ρ/ρs) × 100%. ρs is the skeletal density, which is obtained by the theoretical density of 3.18 g/cm3 and 2.2 g/cm3 for Si3N4 and silica 2.2 g/cm3, respectively. Three samples were examined to determine the average porosity. The linear shrinkage
Table 1 The physical properties of the composites with different Si3N4 contents. Specimens
Volume fractions of the Si3N4 (%)
Density (g/cm3)
Porosity (%)
Linear shrinkage (%)
1 2 3 4 5 6
0 2 5 10 15 20
0.14 0.21 0.32 0.49 0.64 0.82
93.6 90.5 85.8 78.7 72.7 65.3
7.8 6.9 6.2 5.5 4.9 4.4
Fig. 2. XRD patterns of the Si3N4/silica aerogel composites with different volume fractions of Si3N4 particles.
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Fig. 3. SEM images of the Si3N4/silica aerogel composites with different volume fractions of Si3N4 particles: (a) 0%; (b) 2%; (c) 5%; (d) 10%; (e) 15%; (f) 20%.
of the composites was determined by measuring the diameter of the dried monolithic sample and comparing it to its initial diameter when the wet gel had aged but not dried. The compressive strength of the specimens was tested with a load parallel to the freezing direction at a crosshead speed of 0.5 mm/min on specimens (6 mm × 6 mm × 8 mm). The flexural strengths of the specimens were tested by a three-point bending strength test with a load parallel to the freezing direction and a span of 30 mm at a crosshead speed of 0.5 mm/min on specimens (4 mm × 3 mm × 35 mm). The thermal conductivity of the specimens, with diameters of 30 mm and thicknesses of 5 mm, was determined by a thermal physical property parameter testing instrument (Model DRXII-PS, Xiangtan Huafeng instrument Co. LTD, China) at room temperature in air. The dielectric constant and loss tangent were measured by test equipment (Model N5230A, Agilent Technologies USA).
3. Results and discussion The density, porosity and linear shrinkage of the specimens are listed in Table 1. The results show that the density of the specimens increased from 0.14 g/cm3 to 0.82 g/cm3, whereas the corresponding porosity decreased from 93.6% to 65.3% as the volume fractions of Si3N4 particles increased from 0% to 20%. The linear shrinkage was 7.8% when drying the pure silica hydrogel to silica aerogel. The dry shrinkage of the composites decreased with increasing Si3N4 content, indicating that the Si3N4 particles provided a strong support and restrained shrinkage during drying. Fig. 2 shows the XRD patterns of the pure silica aerogels and the composites. The XRD pattern of the pure silica aerogels shows a broad peak between 20° and 30°, which indicates that the aerogels exist in
Fig. 4. TEM images and HRTEM images of the Si3N4/silica aerogel composites with different volume fractions of Si3N4 particles: (a) 0%; (b) 10%; (c) 20%; (d) the Si3N4 particles in the composites (20 vol.%); (e) electron diffraction pattern of (d); (f) HRTEM images of (d).
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Fig. 5. Nitrogen adsorption isotherms and pore size distributions of the composites with different Si3N4 contents.
the form of amorphous phase. After adding Si3N4, the XRD patterns show crystal diffraction peaks which correspond to α-Si3N4 and βSi3N4 phases. With the increasing Si3N4 content, the relative diffraction intensity of the crystal diffraction peaks increase, and the intensity of amorphous peaks decrease. 3.1. Microstructure of the Si3N4/silica aerogel composites The SEM images of the samples are shown in Fig. 3, indicating that the composites possess nanoporous structure. Some particles were dispersed within the silica aerogel matrix and adhered to the silica network. According to the XRD analysis, these particles are silicon nitride. The Si3N4 particles are enveloped by silica aerogel nanoparticles. There is a significant difference in the morphology of the pure silica aerogel and the composites. TEM micrographs of the composites are presented in Fig. 4. The pure silica aerogels exhibited a sponge-like [25] microstructure with nanoparticles (7–12 nm) stacked and interconnected into a porous three dimensional network. When the volume fraction of Si3N4 increased, the size of the aerogel nanoparticles increased and the pores decreased. The diffraction patterns (Fig. 4(e)) proved that the larger particles were crystalline, the crystal phase and structure were determined by XRD analysis. The HRTEM image (Fig. 4(f)) shows that the Si3N4 particle is embedded into amorphous silica and their interface is clean. The nitrogen adsorption isotherms and pore size distributions of the Si3N4/silica aerogel composites are displayed in Fig. 5. As shown in Fig. 5(a), all composites show adsorption isotherms of type IV, indicating a well-developed mesoporous structure [26]. With the increasing volume fraction of the Si3N4, the adsorption quantity gradually decreases due to the decreased porosity of the composites. The inset image in Fig. 5(a) shows the specific surface areas (SBET) of the pure silica aerogels and the composites. Pure silica aerogels show a higher SBET of 790 m2/g. As the volume fraction of Si3N4 increased from 2% to 20%, the specific surface area decreased from 322 to 64 m2/g. The results are in accordance with the fact that SBET can be affected by the particle
size [27]. As shown in Figs. 3 and 4, Si3N4 particles added in the silica aerogel increase the particle size, leading to a decrease of the SBET of the composites. On the other hand, the density of the Si3N4 powder (3.18 g/cm3) is approximately 22.7 times larger than pure silica aerogels (0.14 g/cm3). Adding Si3N4 powder sharply increases the mass of the samples in a certain volume of space but slightly increases surface area; thus, the specific surface area (the surface area per unit mass) shows a downward trend. Fig. 5(b) shows the pore size distribution of the composites. Pure silica aerogel has a narrow pore size distribution with the peak pore size (Dp) approximately 5 nm. As the Si3N4 particles are added, the composites show multi-peak distributions and an increase of Dp. The peak height decreases with the increase of the Si3N4 content, indicating the decrease of the pore volume and the decrease of the number of pores with a certain size. According to the results of the SEM and TEM, the silica aerogel particle size increased with the increasing of Si3N4 content, which caused the increasing of pore size of the composites. The adding of Si3N4 particles and poly acrylic acid interferes with the original sol– gel process which may cause different degrees of monomer polycondensation, resulting in pore diameter show multi-peak distribution. The inset image in Fig. 5(b) shows that the mean pore diameters of the composites are varied by the content of the Si3N4 but are still smaller than 20 nm. 3.2. Mechanical properties of the Si3N4/silica aerogel composites The mechanical properties of the composites are shown in Fig. 6. The reported compressive strength of unmodified silica aerogels was 0.04 MPa in Ref. [28], but the flexural strength was not presented. In this study, the compressive strength of the composites was 0.24 MPa when the volume fraction of the Si3N4 was 2%; the flexural strength could not be obtained. The mechanical properties of the composites increase with the increasing Si3N4 content. As the volume fractions of Si3N4 particles increased to 20%, the compressive strength and the flexural strength of the specimens increased to 12.05 MPa and 2.45 MPa,
Fig. 6. Compressive strengths and flexural strengths of the Si3N4/silica aerogel composites.
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Fig. 7. Thermal conductivities of the Si3N4/silica aerogel composites.
respectively. Si3N4 particles occupied some places of the pores in the aerogels, resulting in an increase of the density, leading to the increased strength of the composite. The increase of the strength is attributed to the support of Si3N4 particles before fracture and the increase of the elastic modulus associated with the density of the bulk material [29]. When cracks propagate to Si3N4 particles, stress concentration occurs and causes the weak interfaces between the Si3N4 particles and the matrix to debond and crack to deflect, which would absorb more fracture work and improve the mechanical properties. These results indicate that the mechanical properties of aerogels can be optimized by exploiting the support of some solid particles to create relatively robust aerogel composites. The pure unmodified silica aerogels have extremely low strength when the porosity is up to 90%, which tends to be extremely fragile. It is reported that the pure silica aerogels with a density of 0.112 g/cm3 exhibited a low compressive strength of 0.018 MPa [30]. In conventional methods, the glass fibers or other ceramic fibers were used as reinforcement to improve the mechanical properties of silica aerogels. Yuan et al. [8] reported the silica aerogel/glass fiber composites (0.53 g/cm3) with a compressive strength of 1.2 MPa. This preparation process includes preparing silica aerogel powders and press forming. In our study, the gel molding method was used to obtain the Si3N4/silica aerogel composites which exhibits a relatively higher compressive strength of 6.37 MPa (the density of 0.49 g/cm3). In addition, unlike fibers the Si3N4 particles were used as reinforcements without pretreatment. The method in our work can easily obtain Si3N4/silica aerogel composites with higher mechanical properties. 3.3. Thermal insulation properties of the Si3N4/silica aerogel composites The thermal conductivities of the Si3N4/silica aerogel composites are shown in Fig. 7. In this study, the prepared pure silica aerogels could not be made into standard units to test the thermal conductivity; thus, the reported data in the literature are referenced. The thermal conductivity of the silica aerogels is approximately 0.014 Wm− 1 K−1 in Ref. [31].
After adding Si3N4 particles, the thermal conductivities of the composites tended to increase. The thermal conductivity of the composites increases from 0.024 to 0.072 Wm−1 K−1 as the volume fraction of Si3N4 increases from 2% to 20%. The increase of the thermal conductivity is attributed to the increase of the solid phase conduction caused by the solid mass and pore wall thickness increase [32]. On the other hand, the particle size increased and the SBET decreased with the increasing volume fraction of Si3N4, indicating that the number of interfaces per unit length of heat path decreased. Thus, the thermal conductivity of the solid phase increases [33]. Fortunately, the composites inherited the nanometer pore structure of the silica aerogels, and their average pore size (less than 15 nm in Fig. 5(b)) is less than the mean free path of gas molecules (69 nm); therefore, the composites still possess good thermal insulation properties. For the aerogel composites, the optimal range of thermal conductivity is generally 0.012–0.1 Wm−1 K−1 at room temperature. The range of thermal conductivity of the Si3N4/silica aerogel composites is 0.024– 0.072 Wm−1 K−1, when the volume fraction of Si3N4 particles is 2%– 20%. The mechanical properties of the composites increase with Si3N4, but the thermal insulation properties decrease. Based on above analysis and discussion, the composites with definite strength and thermal conductivity can be obtained by adjusting content of Si3N4 according to the practical application environment. 3.4. Dielectric properties of the Si3N4/silica aerogel composites To obtain excellent wave-transparent ability, the dielectric constant and loss tangent of the materials must be no more than 4.0 and 0.01, respectively [34]. Fig. 8 shows the dielectric constant and loss tangent of the silica aerogel composites as a function of the frequency. With increasing Si3N4 content, the dielectric constant of the composites increases from 1.55 to 1.85. For well-distributed composites, the dielectric constant can be calculated according to the mixing law [35]. The dielectric constant of air is approximately 1; thus, the higher the porosity is in the materials, the lower the dielectric constant that will be obtained. As the Si3N4 content increased, the porosity of the composites decreased, resulting in an increase of the dielectric constant. The loss tangent of the composite varied slightly with the addition of Si3N4. 4. Conclusions We presented a detailed investigation of the microstructural, and mechanical, thermal insulation and dielectric properties of Si3N4/silica aerogels prepared by the sol–gel method. The mechanical properties are highly dependent on the densities of the composites with the Si3N4 content varying from 2 to 20 vol.%. Compressive strength as high as 12.05 MPa and flexural strength as high as 2.45 MPa were obtained. Due to the nanometer pore structure that was inherited from the silica aerogels, the composites exhibited excellent properties of thermal insulation; the thermal conductivity as low as 0.024 Wm−1 K−1. Meanwhile, the composites also showed a low dielectric constant (b1.9) and a low
Fig. 8. Dielectric constant and loss tangent of the Si3N4/silica aerogel composites as functions of frequency.
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loss tangent (b7 × 10−3). The Si3N4/silica aerogel composites have potential application in the aerospace field for thermal insulation and wavetransparent properties. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 51021002). References [1] L.W. Hrubesh, L.E. Keene, V.R. Latorre, Dielectric-properties of aerogels, J. Mater. Res. 8 (1993) 1736–1741. [2] Hajar Maleki, Luias Durães, António Portugal, An overview on silica aerogels synthesis and different mechanical reinforcing strategies, J. Non-Cryst. Solids 385 (2014) 55–74. [3] A. Jain, S. Rogojevic, S. Ponoth, N. Agarwal, L. Matthew, W.N. Gill, et al., Porous silica materials as low-k dielectrics for electronic and optical interconnects, Thin Solid Films 398 (2001) 513–522. [4] M.F. Bertino, J.F. Hund, J. Sosa, G. Zhang, C. Sotiriou-Leventis, N. Leventis, A.T. Tokuhiro, J. Terry, High resolution patterning of silica aerogels, J. Non-Cryst. Solids 333 (2004) 108–110. [5] A.V. Rao, S.D. Bhagat, H. Hirashima, G.M. Pajonk, Synthesis of flexible silica aerogels using methyltrimethoxysilane (MTMS) precursor, J. Colloid Interface Sci. 300 (2006) 279–285. [6] D.Y. Nadargi, S.S. Latthe, H. Hirashima, A.V. Rao, Studies on rheological properties of methyltriethoxysilane (MTES) based flexible superhydrophobic silica aerogels, Microporous Mesoporous Mater. 117 (2009) 617–626. [7] P.R. Aravind, G.D. Soraru, High surface area methyltriethoxysilane-derived aerogels by ambient pressure drying, J. Porous. Mater. 18 (2011) 159–165. [8] B. Yuan, S.Q. Ding, D.D. Wang, G. Wang, H.X. Li, Heat insulation properties of silica aerogel/glass fiber composites fabricated by press forming, Mater. Lett. 75 (2012) 204–206. [9] D.Q. Shi, Y.T. Sun, J. Feng, X.G. Yang, S.W. Han, C.H. Mi, Y.G. Jiang, H.Y. Qi, Experimental investigation on high temperature anisotropic compression properties of ceramic-fiber-reinforced SiO2 aerogel, Mater. Sci. Eng. A 585 (2013) 25–31. [10] X.G. Yang, Y.T. Sun, D.Q. Shi, Experimental investigation and modeling of the creep behavior of ceramic fiber-reinforced SiO2 aerogel, J. Non-Cryst. Solids 358 (2012) 519–524. [11] H.L. Yang, X.M. Kong, Y.R. Zhang, C.C. Wu, E.X. Gao, Mechanical properties of polymer-modified silica aerogels dried under ambient pressure, J. Non-Cryst. Solids 357 (2011) 3447–3453. [12] H. Guo, M.A.B. Meador, L. Mccorkle, D.J. Quade, J. Guo, B. Hamilton, M. Cakmak, G. Sprowl, Polyimide aerogels cross-linked through amine functionalized polyoligomeric silsesquioxane, Appl. Mater. Interfaces 3 (2011) 546–552. [13] B. Markus, N. Theresa, R. Gudrun, Cross-linked monolithic xerogels based on silica nanoparticles, Chem. Mater. 25 (2013) 3648–3653. [14] F. Sabri, J. Marchetta, K.M. Smith, Thermal conductivity studies of a polyurea crosslinked silica aerogel-RTV655 compound for cryogenic propellant tank applications in space, Acta Astronaut. 91 (2013) 173–179. [15] J. Cai, S.L. Liu, J. Feng, S. Kimura, M. Wada, S. Kuga, L.N. Zhang, Cellulose–silica nanocomposite aerogels by in situ formation of silica in cellulose gel, Angew. Chem. Int. Ed. 51 (2012) 2076–2079.
443
[16] D.J. Boday, R.J. Stover, B. Muriithi, M.W. Keller, J.T. Wertz, K.A.D. Obrey, D.A. Loy, Formation of polycyanoacrylate–silica nanocomposites by chemical vapor deposition of cyanoacrylates on aerogels, Chem. Mater. 20 (2008) 2845–2847. [17] C.Q. Hong, J.C. Han, X.H. Zhang, J.C. Du, Novel nanoporous silica aerogel impregnated highly porous ceramics with low thermal conductivity and enhanced mechanical properties, Scr. Mater. 68 (2013) 599–602. [18] D.J. Boday, R.J. Stover, B. Muriithi, M.W. Keller, J.T. Wertz, K.A.D. Obrey, D.A. Loy, Strong, low-density nanocomposites by chemical vapor deposition and polymerization of cyanoacrylates on aminated silica aerogels, Appl. Mater. Interfaces 7 (2009) 1364–1369. [19] L.F. Hu, C.A. Wang, Y. Huang, Porous YSZ ceramics with unidirectionally aligned pore channel structure: lowering thermal conductivity by silica aerogels impregnation, J. Eur. Ceram. Soc. 31 (2011) 2915–2922. [20] C. Kawai, A. Yamakawa, Effect of porosity and microstructure on the strength of Si3N4: designed microstructure for high strength, high thermal shock resistance, and facile machining, J. Am. Ceram. Soc. 80 (1997) 2705–2708. [21] T. Ohji, Microstructural design and mechanical properties of porous silicon nitride ceramics, Mater. Sci. Eng. A 498 (2008) 5–11. [22] G. Ziegler, J. Heinrich, G. Wotting, Relationships between processing, microstructure and properties of dense and reaction-bonded silicon nitride, J. Mater. Sci. 22 (1987) 3041–3086. [23] S. Brunauer, P.H. Emmett, E. Teller, Adsorption of gases in multimolecular layers, J. Am. Chem. Soc. 60 (1938) 309–319. [24] E.P. Barrett, L.G. Joyner, P.P. Halenda, The determination of pore volume and area distributions in porous substances: 1. computations from nitrogen isotherms, J. Am. Chem. Soc. 73 (1951) 373–381. [25] G.Y. Wu, Y.X. Yu, X. Cheng, Y. Zhang, Preparation and surface modification mechanism of silica aerogels via ambient pressure drying, Mater. Chem. Phys. 129 (2011) 308–314. [26] K. Kaneko, Determination of pore size and pore size distribution: 1. Adsorbents and catalysts, J. Membr. Sci. 96 (1994) 59–89. [27] S.J. Gregg, K.S.W. Sing, Adsorption, surface area and porosity, 2nd ed. Academic Press, London, 1982. [28] Y.N. Duan, C.J. Sadhan, L. Bimala, P.E. Matthew, Reinforcement of silica aerogels using silane-end-capped polyurethanes, Langmuir 29 (2013) 6156–6165. [29] S.O. Kucheyev, M. Stadermann, S.J. Shin, J.H. Satcher Jr., S.A. Gammon, S.A. Letts, T. van Buuren, A.V. Hamza, Super-compressibility of ultralow-density nanoporous silica, Adv. Mater. 24 (2012) 776–780. [30] Z.S. Deng, J. Wang, A.M. Wu, J. Shen, B. Zhou, High strength SiO2 aerogel insulation, J. Non-Cryst. Solids 225 (1998) 101–104. [31] J.C.H. Wong, H. Kaymak, S. Brunner, M.M. Koebel, Mechanical properties of monolithic silica aerogels made from polyethoxydisiloxanes, Microporous Mesoporous Mater. 183 (2014) 23–29. [32] C. Rudaz, R. Courson, L. Bonnet, S. Calas-Etienne, H. Sallée, Tatiana Budtova, Aeropectin: fully biomass-based mechanically strong and thermal superinsulating aerogel, Biomacromolecules 15 (2014) 2188–2195. [33] J. Bourret, N. Tessier-Doyen, B. Naït-Ali, F. Pennec, A. Alzina, C.S. Peyratout, D.S. Smith, Effect of the pore volume fraction on the thermal conductivity and mechanical properties of kaolin-based foams, J. Eur. Ceram. Soc. 33 (2013) 1487–1495. [34] C.R. Zou, C.R. Zhang, B. Li, S.Q. Wang, F. Cao, Microstructure and properties of porous silicon nitride ceramics prepared by gel-casting and gas pressure sintering, Mater. Des. 44 (2013) 114–118. [35] M.B.H. Othman, M.R. Ramli, L.Y. Tyng, Z. Ahmad, H.M. Akil, Dielectric constant and refractive index of poly (siloxane-imide) block copolymer, Mater. Des. 32 (2011) 3173–3182.