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Fabrication and characterization of aluminum silicate fiber–reinforced hollow mesoporous silica microspheres composites
Microporous and Mesoporous Materials 152 (2012) 104–109
Contents lists available at SciVerse ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Fabrication and characterization of aluminum silicate fiber–reinforced hollow mesoporous silica microspheres composites Qiang Chen a, Shubin Wang a,b,⇑, Zhen Li a a b
School of Materials Science and Engineering, Beihang University, Beijing 100191, China Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 6 September 2011 Received in revised form 27 November 2011 Accepted 29 November 2011 Available online 4 December 2011 Keywords: Hollow mesoporous silica microspheres Composite Aluminum silicate fiber Mechanical properties Thermal conductivity
a b s t r a c t Aluminum silicate fiber (ASF)/hollow mesoporous silica microspheres (HMSM) composites were fabricated by a vacuum filtration technique, followed by pressureless sintering at 1100 °C. Properties namely: bulk density, microstructure, mechanical strength and thermal conductivity were investigated with the ASF content. Results showed that bulk density of the resultant composites increased from 0.17 to 0.26 g/cm3 with the increase of ASF content, and the ASF was intimately bonded to the HMSM matrix. When the ASF content was 15 wt.%, the composites exhibited the highest compressive strength (5.07 MPa) and flexural strength (3.32 MPa), then the mechanical strength decreased on further addition (20 wt.%). The thermal conductivity of composites ranged from 0.048 to 0.081 W/(m K) at room temperature, which makes aluminum silicate fiber/hollow mesoporous silica microspheres composites a promising choice for thermal insulation. Crown Copyright Ó 2011 Published by Elsevier Inc. All rights reserved.
1. Introduction In recent years the research on improving thermal insulation property of materials has been paid more and more attention to increase the energy efficiency, which is of great importance and necessity in modern industry and society. Besides, specialized applications such as the heat protection of space shuttles and nuclear reactor also require good thermal performance [1]. The focus was set on microporous materials like fumed silica [2–4] and silica aerogel [5–9]. High porosity and nanoscale porous nature in these materials can decrease the solid and the gaseous thermal conductivity, which lead to a particularly low thermal conductivity (<0.02 W/(m K)). Further reduction of thermal conductivity can be realized if the materials are evacuated. These microporous materials utilized for thermal insulation have been proven successfully and have already played an important role in the field of high performance thermal insulation materials. However, poor mechanical properties such as brittleness and low strength limit their applications in the load bearing area. Hollow microspheres with dimensions from nanometers to micrometers are becoming a focus in nanoscience and nanotechnology. Due to their special properties, such as specific surface area, prominent optical activities and the ability of encapsulating, ⇑ Corresponding author at: School of Materials Science and Engineering, Beihang University, Beijing 100191, China. Tel./fax: +86 0182316500. E-mail address: [email protected] (S. Wang).
hollow microspheres have been applied in catalysis, chromatography, protection of biologically active agents, fillers (or pigments/ coatings), waste removal, and large bimolecular-release systems [10–16]. In addition, the special hollow structure endows hollow microspheres high porosity and excellent thermal insulating property (poor solid–solid contact), which makes hollow microspheres a promising candidate in insulation fields [17–19]. Hollow mesoporous silica microspheres (HMSM), in particular, have attracted significant attention because of easy fabrication, controlled particle size and low thermal conductivity [20–24]. Compared to the conventional mesoporous silica materials, HMSM exhibit a much higher load bearing property [25]. Moreover, it is possible to design the pore structure and control the porosity for porous materials prepared by hollow microspheres [26,27]. But up to date, most of the researchers have concentrated on the fabrication of HMSM with different methods. Little work has been done to prepare porous materials using HMSM as raw material. In the present study, a novel thermal insulation material was fabricated by sintering micro-sized HMSM. Aluminum silicate fiber (ASF) was introduced into HMSM matrix to act as a stiff skeleton and to improve the mechanical properties of HMSM by reinforcement [28]. The low intrinsic thermal conductivity of ASF [29] can also minimize the negative effect of the filler on the thermal insulating property. The preparation process of ASF/HMSM composites was reported in detail. The effects of fiber content on the mechanical and thermal insulating properties were also investigated and discussed.
1387-1811/$ - see front matter Crown Copyright Ó 2011 Published by Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2011.11.053
Q. Chen et al. / Microporous and Mesoporous Materials 152 (2012) 104–109
2. Experimental 2.1. Materials Styrene (St) was purchased from Beijing Chemical Reagent Co. (China) and purified by treating with 5 wt.% aqueous NaOH to remove the inhibitor. A 2-(methacryloyloxy)ethyltrimethylammonium chloride (MTC, 72%aq) aqueous solution was supplied from Alfa Aesar. 2,2-Azoisobutyronitrile (AIBN, 98%, Aldrich USA) was recrystallized from tetrahydrofuran (THF) before use. Poly(vinyl pyrrolidone) (PVP, K30) with a molecular weight of 40,000, TEOS, absolute ethanol and an aqueous ammonia solution (25 wt.%) were purchased from Beijing Chemical Reagent Co. (China). Aluminum silicate fiber was purchased from Shandong Huolong Ceramic Fiber Co. (China) and chopped to produce suitable length fiber as described by Fu et al. [30]. Diameters of the fiber varied from 1 lm to 5 lm, as did the fiber lengths, which averaged about 200 lm. Ultrapure water from a Milli-Q water system was used throughout the experiment. 2.2. Preparation of HMSM The polymer bead template method was used to produce HMSM with an average diameter of around 1 lm, as described by Li and Wang [31]. In a typical process, 1.5 g of stabilizer (PVP), 0.2 g of initiator (AIBN), 5.0 g of H2O, 5.0 g of St, and 22.5 g of ethanol (EtOH) were charged into a 500 ml three-necked flask. The reaction solution was deoxygenated by bubbling nitrogen gas at room temperature for approx 30 min with a stirring rate of 100 r/min, and was then heated to 70 °C, followed by addition of 5.0 g of St and 22.5 g of EtOH and 0.39 g of MTC. The reaction was continued for 24 h and the mixture was then cooled to 50 °C, after which 1.0 ml of ammonia and 12 g of TEOS were added quickly and the mixture was reacted for around 1 h. The obtained particles were separated from the reaction medium by centrifuging at 6000 r/min, and were washed several times by absolute ethanol and deionized water to get rid of the unreacted St and PVP dissolved in water or other impurities. Finally, PS cores were removed by calcinations at 500 °C in air by an increment rate of 5 °C/min. Thus, HMSM were obtained and then preserved for following experiments. 2.3. Preparation of the ASF/HMSM composites Aluminum silicate fiber–reinforced hollow mesoporous silica microspheres composites were molded by vacuum filtration method as illustrated in Fig. 1. In this process, HMSM and ASF were added into an aqueous PVA solution (2.5 wt.%) orderly. To study
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effects of the content of fiber on the mechanical and thermal properties of composites, 0, 5, 10, 15 and 20 wt.% ASF were introduced. In order to make the fiber disperse uniformly, the weight fraction of fiber in solution was below 1 wt.%. The mixture was subsequently sonicated using a high intensity sonicator (Sonicator 3000 from Misonix Inc.) for 1 h to disperse ASF. The uniformly dispersed suspension was then poured through a filter paper in a Buchner funnel. The solid was trapped by the filter and the liquid was drawn through the funnel into the flask below, by a vacuum. After molding, the wet green bodies were dried at 100 °C for 5 h in an oven. Finally, the samples were sintered in an air atmosphere at 1100 °C for 1 h, using a heating rate of 5 °C/min, and then slowly furnace-cooled back to room temperature. In the end, aluminum silicate fiber–reinforced hollow mesoporous silica microspheres composites were fabricated. 2.4. Characterization Transmission electron microscopy (TEM) images were taken on a Hitachi H-600 electron microscope operated at 200 kV. For the TEM measurements, the samples were dispersed in ethanol with the aid of supersonic and then dried on a holey carbon film Cu grid. Scanning electron microscopy (SEM) images were taken on a Apollo 300 scanning electron microscope at 15.0 kV. Prior to testing, the samples were sputtered with a conducing layer of gold. N2 adsorption–desorption isotherm measurements were carried out on a TriStar 3000 apparatus (Micromeritics, USA) at 77 K under a continuous adsorption condition. Samples were degassed at 200 °C for 6 h before taking the measurements. Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) analyses were used to determine the surface areas, pore volumes and pore sizes of the products. The bulk density was measured using the Archimedes method in water. The apparent porosity in the samples was measured according to ASTM Stand Test Method C830-00 [32] by soaking in water. The X-ray diffraction (XRD) patterns were obtained with a D/ maxIIIA X-ray diffractometer (Rigaku, Japan) using Cu Ka radiation. Data were collected in continuous scan mode from 0.6° to 8° with a 0.02° sampling interval. Mechanical tests were performed on a universal testing machine. The compressive test specimens (10 mm 10 mm 10 mm) were compressed between two stainless steel platens with a crosshead speed of 1 mm/min. The specimens for flexural test were machined into test bars with dimensions of 3 mm 4 mm 35 mm before a three-point-bending test was conducted at a loading rate of 0.5 mm/min. Prior to testing, all surfaces of the test bars were polished and the edges were beveled. Each strength value was averaged over three measurements. The thermal properties of aluminum silicate fiber–reinforced hollow mesoporous silica microspheres composites were measured by laser flash method. A LFA 427 (NETZSCH Corp., German) was applied to measure the thermal conductivity of samples. The temperature was maintained at 23 °C during the whole experiment. Thermal conductivity, was determined according to the relation,
k ¼ qC a
ð1Þ
where q is the sample density, C is heat capacity and a is thermal diffusivity. 3. Results and discussion 3.1. Characteristics of ASF and prepared HMSM
Fig. 1. Schematic illustration of the vacuum filtration method.
ASF used in this study is a non-crystalline dense material. Fig. 2(a) shows SEM micrograph of ASF. It can be seen that the
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Q. Chen et al. / Microporous and Mesoporous Materials 152 (2012) 104–109
Fig. 2. (a) SEM micrograph of ASF and (b) TEM micrograph of prepared HMSM.
Fig. 3. (a) Nitrogen adsorption–desorption isotherm and pore size distribution (inset) of HMSM and (b) X-ray diffraction patterns of HMSM.
ASF has smooth surface and their diameters range from 1 to 5 lm. The prepared HMSM is finely dispersed and free-flowing. The bulk density of HMSM is about 0.13 g/cm3, which was measured according to Archimedes displacement method using distilled water. Typical TEM image of prepared HMSM is shown in Fig. 2(b). It can be seen that the prepared HMSM have good spherical structure and narrow size distribution. The average size of HMSM is about 1 lm. In Fig. 2(b), we can also see that the silica spheres are hollow and the shell thickness of the spheres is about 30 nm. The nitrogen adsorption–desorption isotherm (Fig. 3(a)) for HMSM is essentially of type IV with a hysteresis loop. The presence of hysteresis loop in high relative pressures (P/P0 = 0.70–0.98) is characteristic of a mesoporous structure. Moreover, the peaks of BJH pore size distribution for HMSM appear at about 5.36 nm (Fig. 3(a) inset), indicating that HMSM are consisted of mesoporous structure. In addition, HMSM with a specific surface area of 223 m2/g and pore volume of 0.29 cm3/g was obtained using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. Small-angle XRD pattern of HMSM is shown in Fig. 3(b). Clearly, no characteristic Bragg diffraction peaks at 2h below 8° is observed, possibly indicating the disordered mesostructure of the present HMSM [33]. 3.2. Bulk density and apparent porosity As listed in Table 1, the bulk density of the resultant composites ranges from 0.17 to 0.26 g/cm3. Since the density of ASF is 1.8 g/ cm3 which is much higher than that of HMSM (0.13 g/cm3), the bulk density of the composites increases as the content of fiber in-
creases. In contrast to bulk density, apparent porosity of the composites decreases with the increase of the fiber content, which is because that the dense fiber filled the pores and decreased the porosity. However, the decrease of apparent porosity is slight when 20 wt.% fiber is added. During the vacuum filtration molding process, the appearance of voids between the HMSM matrix and fiber, which is caused by bridging effect of the fiber, is inevitable. As a result, the addition of fiber may lead to big pores. When the fiber content is up to 20 wt.%, it is difficult to make the fiber disperse uniformly in the HMSM matrix and the aggregation of the fiber leads to a not apparent decrease of porosity. 3.3. Mechanical properties The mechanical properties of porous composites generally depend on the density, which in turn depends on matrix to filler ratio. In the case of aluminum silicate fiber–reinforced hollow mesoporous silica microspheres composites, the content of fiber also plays an important role in determining the mechanical properties. Fig. 4 plots the variation of mechanical strength of the composites as a function of fiber content. From Fig. 4, it is evident that fiber-free HMSM materials exhibit a very low strength. The compressive strength and flexural strength are 0.93 and 0.75 MPa, respectively. This can be ascribed to the following reasons. Firstly, despite the HMSM are bonded during the sintering process (Fig. 5(a)) and the bonded structure can make contributions to the strength of the materials, the intrinsic low bearing capacity and brittleness of HMSM lead to a low strength. Secondly, it is well known that the strength of the porous
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Q. Chen et al. / Microporous and Mesoporous Materials 152 (2012) 104–109 Table 1 Bulk density and apparent porosity of ASF/HMSM composites. Content of fiber (wt.%)
Bulk density (g/cm3)
Apparent porosity (%)
0 5 10 15 20
0.17 0.20 0.22 0.25 0.26
78.6 73.3 69.1 66.8 65.7
sponding to 15 wt.% of fiber. The highest compressive and flexural strength values are 5.07 and 3.32 MPa, which are about five times and four times both the strength of fiber-free HMSM materials, respectively. Moreover, the compressive strength is much higher than that of fiber–reinforced silica aerogel (1.0 MPa at 25% strain) as reported in Ref. [7]. Further addition of ASF develops a much better bearing capacity of the whole composites and brings an increase in strength. At the same time, more big pores are formed by further addition of fiber (Fig. 6(a)–(d)), and the appearance of big pores will result in a decrease in strength. When the fiber content reaches up to 20 wt.%, the adverse effect caused by the big pores (Fig. 6(d) and (e)) counterbalances part of the reinforcing effect of fiber and leads to a reduction of strength compared to materials containing 15 wt.% fiber. Yet, the value is still higher than that of the fiber-free materials. 3.4. Thermal insulating properties
Fig. 4. Variation of mechanical strength of ASF/HMSM composites with different contents of fiber.
materials is dependent on phase composition and microstructural features, especially porosity [34]. As shown in Fig. 5(a), many pores are observed in samples, which lead to a high porosity and are also responsible for the low strength. Nevertheless, due to the better load-bearing capacity of the fibrous reinforcements, both compressive and flexural strength increase sharply when 5 wt.% fiber is added. Fig. 5(b) shows the microstructure of fiber–reinforced HMSM composites. It can be seen that ASF are embedded in the HMSM matrix and act as a stiff skeleton. This structure can effectively sustain the load that is transferred onto them from the HMSM matrix. Besides, the firm bond between ASF and HMSM (Fig. 5(b)) can make the crack propagation path longer compared with fiber-free composites and lead to higher strength [35]. It can also be seen that the added fiber resulted in big pores around the fiber (Fig. 5(b)), which have a negative influence on material strength, but this effect can be neglected compared with the fiber reinforcement in a low fiber content. As Fig. 4 shows, both compressive and flexural strength increase with further addition of fiber and show a maximum value corre-
The thermal conductivity is a key factor for evaluating the thermal insulating materials. Fig. 7 plots thermal conductivity as a function of the content of fiber. It can be observed that thermal conductivity of the fiber-free HMSM materials shows a lowest value of 0.048 W/(m K), and then increases to 0.064 W/(m K) at 5 wt.% fiber content. At the 15 wt.% fiber content, the thermal conductivity of the composites increases to a highest value of 0.081 W/ (m K). When the content of fiber is further increased to 20 wt.%, the thermal conductivity decreases a little. The behavior of the thermal conductivity of the composites can be explained by the mechanism of heat transfer. The basic mechanisms of heat transfer are generally considered to be conduction, convection and radiation. Conduction is the main mechanism of heat transfer for porous material at room temperature when the bubble diameter is less than 4 mm and the other two can be neglected [36]. This can also apply to HMSM materials since the diameter of HMSM is only 1 lm and even the biggest pore size is less than 40 lm (Fig. 6(e)). It is known that gas is less conductive compared to conduction within a solid or between solid objects in contact. Thus, the thermal conductivity of materials can be expressed by the equation:
k ks us þ kg ug
and kg ks ;
ks us
so ks us þ kg ug ð2Þ
where k is the thermal conductivity of the system, ks is the thermal conductivity of the solid, kg is the thermal conductivity of the gas, us is volume percentage of the solid, ug is volume percentage of the gas. In the case of fiber-free HMSM materials, silica is the only solid component. The heat transfer of fiber-free HMSM material is conducted mainly by the solid thermal convection between silica spheres and within the shells, as shown schematically in Fig. 8(a).
Fig. 5. SEM micrographs of HMSM materials: (a) without fiber and (b) containing 5 wt.% fiber.
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Q. Chen et al. / Microporous and Mesoporous Materials 152 (2012) 104–109
Fig. 6. SEM micrographs of ASF/HMSM composites with different fiber contents: (a) 5 wt.%; (b) 10 wt.%; (c) 15 wt.%; (d) 20 wt.% and (e) a low magnification image of (d).
rials composed of filled component and matrix component, the thermal conductivity can be calculated from the equation:
k ¼ kc
Fig. 7. Thermal conductivity of ASF/HMSM composites with different contents of fiber.
The high volume percentage of gas leads to the low volume percentage of silica, which in turn leads to the low thermal conductivity of HMSM materials. What is more, the thermal conductivity of composites generally increase as the content of fiber increases. For heterogeneous mate-
1 þ 2ud ð1 ðkc =kd ÞÞ=ðð2kc =kd Þ þ 1Þ 1 ud ð1 ðkc =kd ÞÞ=ðð2kc =kd Þ þ 1Þ
ð3Þ
where kc is the thermal conductivity of the matrix component, kd is the thermal conductivity of the filled component, and ud is the volume fraction of the filled component [37,38]. In the case of ASF/ HMSM composites, the solid thermal conduction can be considered to spheres–spheres conduction, spheres–fiber conduction and fiber–fiber conduction, as shown schematically in Fig. 8(b) (here, assuming HMSM is a unit). Liao et al. [24] reported that the thermal conductivity of powder hollow silica spheres depended on the internal diameter and density. HMSM applied in the research has an identical internal diameter and a constant density (0.13 g/cm3) which lead to the constancy of thermal conductivity of HMSM. The thermal conductivity of ASF is about 0.15 W/(m K)), which is much higher than that of HMSM. So, kc =kd < 1, 1 kc =kd > 0 and ð1 ðkc =kd ÞÞ=ðð2kc =kd Þ þ 1Þ is a positive and constant value. Thus, Eq. (3) can be simplified as,
k ¼ kc
1 þ 2aud 1 aud
ð4Þ
where k is the thermal conductivity of the composites, kc is the thermal conductivity of the matrix component, a is a positive and con-
Q. Chen et al. / Microporous and Mesoporous Materials 152 (2012) 104–109
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Fig. 8. Schematic illustration of heat transfer process of materials: (a) without fiber and (b) containing fiber.
stant value and ud is the volume fraction of the filled component. According to Eq. (4) it can be concluded that the thermal conductivity of the composites raises as the volume fraction of fiber increases. Hence the thermal conductivity generally increases as the content of fiber increases. However, the thermal conductivity decreases a little for composites containing 20 wt.% fiber. This is probably because that the big pores (Fig. 6(e)) caused by the fiber entanglement leaded to a slight decrease of ud when 20 wt.% fiber was introduced. 4. Conclusion The ASF/HMSM composites had been prepared through the process of first molded by vacuum filtration method, then sintered at 1100 °C for 1 h. The bulk density of the composites ranged from 0.17 to 0.26 g/cm3 and the ASF was intimately bonded to the HMSM matrix. With increasing the ASF content, the mechanical strength increased first and then decreased, and showed a maximum corresponding to 15 wt.% of ASF. The highest compressive and flexural strength were 5.07 and 3.32 MPa, respectively. The thermal conductivity of composites ranged from 0.048 to 0.081 W/(m K) at room temperature. The high mechanical strength and low thermal conductivity make ASF/HMSM composites a good candidate for thermal insulation. References [1] K.W. Suh, C.P. Park, M.J. Maurer, M.H. Tusim, R. De Genova, R. Broos, D.P. Sophiea, Adv. Mater. 12 (2000) 1779. [2] H. Abe, I. Abe, K. Sato, M. Naito, J. Am. Ceram. Soc. 88 (2005) 1359. [3] V. Meynen, P. Cool, E.F. Vansant, Micropor. Mesopor. Mater. 125 (2009) 170. [4] E.F. Voronin, V.M. Gunko, N.V. Guzenko, E.M. Pakhlov, L.V. Nosach, R. Leboda, J. Skubiszewska-Zieba, J. Colloid Interf. Sci. 279 (2004) 326. [5] X. Lu, M.C. Ardunini, J. Kuhn, Science 255 (1992) 971. [6] M. Schmidt, F. Schwertfeger, J. Non-Cryst. Solids 225 (1998) 364. [7] X.G. Yang, Y.T. Sun, D.Q. Shi, Mater. Sci. Eng. A 528 (2011) 4830.
[8] D. Haranath, G.M. Pajonk, P.B. Wagh, Mater. Chem. Phys. 49 (1997) 129. [9] P.B. Sarawade, J.K. Kim, A. Hilonga, D.V. Quang, H.T. Kim, Micropor. Mesopor. Mater. 139 (2011) 138. [10] K.J.C. Bommel, J.K. Jung, S. Shinkai, Adv. Mater. 13 (2001) 1472. [11] M. Hartmann, Chem. Mater. 17 (2005) 4577. [12] J. Huang, Y. Xie, B. Li, Y. Liu, Y. Qian, A. Zhang, Adv. Mater. 12 (2000) 808. [13] G. Sun, K. Li, J. Wang, H. He, J. Wang, Y. Li, Y. Liu, J. Gu, Micropor. Mesopor. Mater. 139 (2011) 207. [14] T.H. Zheng, J.B. Pang, G. Tan, J.B. He, G.L. McPherson, Y.F. Lu, V.T. John, J.J. Zhan, Langmuir 23 (2007) 5143. [15] X. Sun, J. Liu, Y. Li, Chem. Eur. J. 12 (2006) 2039. [16] Y. Wang, X.G. Li, Z.Y. Xue, L.S. Dai, S.H. Xie, Q.Z. Li, J. Phys. Chem. B 114 (2010) 5747. [17] M.S. Allen, R.G. Baumgartner, J. Fesmire, Cryogenic Engineering Conference, Anchorage, Alaska, September 2003. [18] Y. Wang, Y. Xia, Nano Lett. 4 (2004) 2047. [19] S. Kidambi, J.H. Dai, M.L. Bruening, J. Am. Chem. Soc. 126 (2004) 2658. [20] Y. Zhu, J. Shi, H. Chen, W. Shen, X. Dong, Micropor. Mesopor. Mater. 84 (2005) 218. [21] M. Chen, L. Wu, S. Zhou, B. You, Adv. Mater. 18 (2005) 801. [22] Y. Le, J. Chen, J. Wang, L. Shao, W. Wang, Mater. Lett. 58 (2004) 2105. [23] Z.G. Teng, Y.D. Han, J. Li, F. Yan, W.S. Yang, Micropor. Mesopor. Mater. 127 (2010) 67. [24] Y.C. Liao, X.F. Wu, H.D. Liu, Y.F. Chen, Thermochim. Acta 526 (2011) 178. [25] E. Ozcivici, R.P. Singh, J. Am. Ceram. Soc. 88 (2005) 3338. [26] D.T. Queheillalt, D.J. Sypeck, H.N.G. Wadley, Mater. Sci. Eng. A 323 (2002) 138. [27] I. Thijs, J. Luyten, S. Mullens, J. Am. Ceram. Soc. 87 (2003) 170. [28] T. Ogasawara, T. Ishikawa, T. Matsuzaki, J. Am. Ceram. Soc. 86 (2003) 830. [29] M. Schmucker, B. Kanka, H. Schneider, J. Eur. Ceram. Soc. 20 (2000) 2491. [30] R.L. Fu, H.P. Zhou, L. Chen, Y. Wu, J. Mater. Sci. 34 (1999) 3605. [31] Z. Li, S.B. Wang, J. Inorg. Mater. 26 (2011) 885. [32] Standard Test Methods for Apparent Porosity, Liquid Absorption, Apparent Specific Gravity and Bulk Density of Refractory Shapes by Vacuum Pressure, ASTM C830-00. [33] J.X. Yang, D.D. Hu, Y. Fang, C.L. Bai, H.Y. Wang, Chem. Mater. 18 (2006) 4902. [34] M. Boaro, J.M. Vohs, R.J. Gorte, J. Am. Ceram. Soc. 86 (2003) 395. [35] B. John, D. Mathew, B. Deependran, G. Joseph, C.P.R. Nair, K.N. Ninan, J. Mater. Sci. 46 (2011) 5017. [36] B. Li, J. Yuan, Z.G. An, Mater. Lett. 65 (2011) 1992. [37] S. Tian, Physical Properties of Materials, Beijing University of Aeronautics and Astronautics Press, Beijing, 2004 (in Chinese). [38] S.B. Wang, R.Y. Luo, Y.F. Ni, Mater. Sci. Eng. A 527 (2010) 3392.
Contents lists available at SciVerse ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Fabrication and characterization of aluminum silicate fiber–reinforced hollow mesoporous silica microspheres composites Qiang Chen a, Shubin Wang a,b,⇑, Zhen Li a a b
School of Materials Science and Engineering, Beihang University, Beijing 100191, China Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 6 September 2011 Received in revised form 27 November 2011 Accepted 29 November 2011 Available online 4 December 2011 Keywords: Hollow mesoporous silica microspheres Composite Aluminum silicate fiber Mechanical properties Thermal conductivity
a b s t r a c t Aluminum silicate fiber (ASF)/hollow mesoporous silica microspheres (HMSM) composites were fabricated by a vacuum filtration technique, followed by pressureless sintering at 1100 °C. Properties namely: bulk density, microstructure, mechanical strength and thermal conductivity were investigated with the ASF content. Results showed that bulk density of the resultant composites increased from 0.17 to 0.26 g/cm3 with the increase of ASF content, and the ASF was intimately bonded to the HMSM matrix. When the ASF content was 15 wt.%, the composites exhibited the highest compressive strength (5.07 MPa) and flexural strength (3.32 MPa), then the mechanical strength decreased on further addition (20 wt.%). The thermal conductivity of composites ranged from 0.048 to 0.081 W/(m K) at room temperature, which makes aluminum silicate fiber/hollow mesoporous silica microspheres composites a promising choice for thermal insulation. Crown Copyright Ó 2011 Published by Elsevier Inc. All rights reserved.
1. Introduction In recent years the research on improving thermal insulation property of materials has been paid more and more attention to increase the energy efficiency, which is of great importance and necessity in modern industry and society. Besides, specialized applications such as the heat protection of space shuttles and nuclear reactor also require good thermal performance [1]. The focus was set on microporous materials like fumed silica [2–4] and silica aerogel [5–9]. High porosity and nanoscale porous nature in these materials can decrease the solid and the gaseous thermal conductivity, which lead to a particularly low thermal conductivity (<0.02 W/(m K)). Further reduction of thermal conductivity can be realized if the materials are evacuated. These microporous materials utilized for thermal insulation have been proven successfully and have already played an important role in the field of high performance thermal insulation materials. However, poor mechanical properties such as brittleness and low strength limit their applications in the load bearing area. Hollow microspheres with dimensions from nanometers to micrometers are becoming a focus in nanoscience and nanotechnology. Due to their special properties, such as specific surface area, prominent optical activities and the ability of encapsulating, ⇑ Corresponding author at: School of Materials Science and Engineering, Beihang University, Beijing 100191, China. Tel./fax: +86 0182316500. E-mail address: [email protected] (S. Wang).
hollow microspheres have been applied in catalysis, chromatography, protection of biologically active agents, fillers (or pigments/ coatings), waste removal, and large bimolecular-release systems [10–16]. In addition, the special hollow structure endows hollow microspheres high porosity and excellent thermal insulating property (poor solid–solid contact), which makes hollow microspheres a promising candidate in insulation fields [17–19]. Hollow mesoporous silica microspheres (HMSM), in particular, have attracted significant attention because of easy fabrication, controlled particle size and low thermal conductivity [20–24]. Compared to the conventional mesoporous silica materials, HMSM exhibit a much higher load bearing property [25]. Moreover, it is possible to design the pore structure and control the porosity for porous materials prepared by hollow microspheres [26,27]. But up to date, most of the researchers have concentrated on the fabrication of HMSM with different methods. Little work has been done to prepare porous materials using HMSM as raw material. In the present study, a novel thermal insulation material was fabricated by sintering micro-sized HMSM. Aluminum silicate fiber (ASF) was introduced into HMSM matrix to act as a stiff skeleton and to improve the mechanical properties of HMSM by reinforcement [28]. The low intrinsic thermal conductivity of ASF [29] can also minimize the negative effect of the filler on the thermal insulating property. The preparation process of ASF/HMSM composites was reported in detail. The effects of fiber content on the mechanical and thermal insulating properties were also investigated and discussed.
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Q. Chen et al. / Microporous and Mesoporous Materials 152 (2012) 104–109
2. Experimental 2.1. Materials Styrene (St) was purchased from Beijing Chemical Reagent Co. (China) and purified by treating with 5 wt.% aqueous NaOH to remove the inhibitor. A 2-(methacryloyloxy)ethyltrimethylammonium chloride (MTC, 72%aq) aqueous solution was supplied from Alfa Aesar. 2,2-Azoisobutyronitrile (AIBN, 98%, Aldrich USA) was recrystallized from tetrahydrofuran (THF) before use. Poly(vinyl pyrrolidone) (PVP, K30) with a molecular weight of 40,000, TEOS, absolute ethanol and an aqueous ammonia solution (25 wt.%) were purchased from Beijing Chemical Reagent Co. (China). Aluminum silicate fiber was purchased from Shandong Huolong Ceramic Fiber Co. (China) and chopped to produce suitable length fiber as described by Fu et al. [30]. Diameters of the fiber varied from 1 lm to 5 lm, as did the fiber lengths, which averaged about 200 lm. Ultrapure water from a Milli-Q water system was used throughout the experiment. 2.2. Preparation of HMSM The polymer bead template method was used to produce HMSM with an average diameter of around 1 lm, as described by Li and Wang [31]. In a typical process, 1.5 g of stabilizer (PVP), 0.2 g of initiator (AIBN), 5.0 g of H2O, 5.0 g of St, and 22.5 g of ethanol (EtOH) were charged into a 500 ml three-necked flask. The reaction solution was deoxygenated by bubbling nitrogen gas at room temperature for approx 30 min with a stirring rate of 100 r/min, and was then heated to 70 °C, followed by addition of 5.0 g of St and 22.5 g of EtOH and 0.39 g of MTC. The reaction was continued for 24 h and the mixture was then cooled to 50 °C, after which 1.0 ml of ammonia and 12 g of TEOS were added quickly and the mixture was reacted for around 1 h. The obtained particles were separated from the reaction medium by centrifuging at 6000 r/min, and were washed several times by absolute ethanol and deionized water to get rid of the unreacted St and PVP dissolved in water or other impurities. Finally, PS cores were removed by calcinations at 500 °C in air by an increment rate of 5 °C/min. Thus, HMSM were obtained and then preserved for following experiments. 2.3. Preparation of the ASF/HMSM composites Aluminum silicate fiber–reinforced hollow mesoporous silica microspheres composites were molded by vacuum filtration method as illustrated in Fig. 1. In this process, HMSM and ASF were added into an aqueous PVA solution (2.5 wt.%) orderly. To study
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effects of the content of fiber on the mechanical and thermal properties of composites, 0, 5, 10, 15 and 20 wt.% ASF were introduced. In order to make the fiber disperse uniformly, the weight fraction of fiber in solution was below 1 wt.%. The mixture was subsequently sonicated using a high intensity sonicator (Sonicator 3000 from Misonix Inc.) for 1 h to disperse ASF. The uniformly dispersed suspension was then poured through a filter paper in a Buchner funnel. The solid was trapped by the filter and the liquid was drawn through the funnel into the flask below, by a vacuum. After molding, the wet green bodies were dried at 100 °C for 5 h in an oven. Finally, the samples were sintered in an air atmosphere at 1100 °C for 1 h, using a heating rate of 5 °C/min, and then slowly furnace-cooled back to room temperature. In the end, aluminum silicate fiber–reinforced hollow mesoporous silica microspheres composites were fabricated. 2.4. Characterization Transmission electron microscopy (TEM) images were taken on a Hitachi H-600 electron microscope operated at 200 kV. For the TEM measurements, the samples were dispersed in ethanol with the aid of supersonic and then dried on a holey carbon film Cu grid. Scanning electron microscopy (SEM) images were taken on a Apollo 300 scanning electron microscope at 15.0 kV. Prior to testing, the samples were sputtered with a conducing layer of gold. N2 adsorption–desorption isotherm measurements were carried out on a TriStar 3000 apparatus (Micromeritics, USA) at 77 K under a continuous adsorption condition. Samples were degassed at 200 °C for 6 h before taking the measurements. Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) analyses were used to determine the surface areas, pore volumes and pore sizes of the products. The bulk density was measured using the Archimedes method in water. The apparent porosity in the samples was measured according to ASTM Stand Test Method C830-00 [32] by soaking in water. The X-ray diffraction (XRD) patterns were obtained with a D/ maxIIIA X-ray diffractometer (Rigaku, Japan) using Cu Ka radiation. Data were collected in continuous scan mode from 0.6° to 8° with a 0.02° sampling interval. Mechanical tests were performed on a universal testing machine. The compressive test specimens (10 mm 10 mm 10 mm) were compressed between two stainless steel platens with a crosshead speed of 1 mm/min. The specimens for flexural test were machined into test bars with dimensions of 3 mm 4 mm 35 mm before a three-point-bending test was conducted at a loading rate of 0.5 mm/min. Prior to testing, all surfaces of the test bars were polished and the edges were beveled. Each strength value was averaged over three measurements. The thermal properties of aluminum silicate fiber–reinforced hollow mesoporous silica microspheres composites were measured by laser flash method. A LFA 427 (NETZSCH Corp., German) was applied to measure the thermal conductivity of samples. The temperature was maintained at 23 °C during the whole experiment. Thermal conductivity, was determined according to the relation,
k ¼ qC a
ð1Þ
where q is the sample density, C is heat capacity and a is thermal diffusivity. 3. Results and discussion 3.1. Characteristics of ASF and prepared HMSM
Fig. 1. Schematic illustration of the vacuum filtration method.
ASF used in this study is a non-crystalline dense material. Fig. 2(a) shows SEM micrograph of ASF. It can be seen that the
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Fig. 2. (a) SEM micrograph of ASF and (b) TEM micrograph of prepared HMSM.
Fig. 3. (a) Nitrogen adsorption–desorption isotherm and pore size distribution (inset) of HMSM and (b) X-ray diffraction patterns of HMSM.
ASF has smooth surface and their diameters range from 1 to 5 lm. The prepared HMSM is finely dispersed and free-flowing. The bulk density of HMSM is about 0.13 g/cm3, which was measured according to Archimedes displacement method using distilled water. Typical TEM image of prepared HMSM is shown in Fig. 2(b). It can be seen that the prepared HMSM have good spherical structure and narrow size distribution. The average size of HMSM is about 1 lm. In Fig. 2(b), we can also see that the silica spheres are hollow and the shell thickness of the spheres is about 30 nm. The nitrogen adsorption–desorption isotherm (Fig. 3(a)) for HMSM is essentially of type IV with a hysteresis loop. The presence of hysteresis loop in high relative pressures (P/P0 = 0.70–0.98) is characteristic of a mesoporous structure. Moreover, the peaks of BJH pore size distribution for HMSM appear at about 5.36 nm (Fig. 3(a) inset), indicating that HMSM are consisted of mesoporous structure. In addition, HMSM with a specific surface area of 223 m2/g and pore volume of 0.29 cm3/g was obtained using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively. Small-angle XRD pattern of HMSM is shown in Fig. 3(b). Clearly, no characteristic Bragg diffraction peaks at 2h below 8° is observed, possibly indicating the disordered mesostructure of the present HMSM [33]. 3.2. Bulk density and apparent porosity As listed in Table 1, the bulk density of the resultant composites ranges from 0.17 to 0.26 g/cm3. Since the density of ASF is 1.8 g/ cm3 which is much higher than that of HMSM (0.13 g/cm3), the bulk density of the composites increases as the content of fiber in-
creases. In contrast to bulk density, apparent porosity of the composites decreases with the increase of the fiber content, which is because that the dense fiber filled the pores and decreased the porosity. However, the decrease of apparent porosity is slight when 20 wt.% fiber is added. During the vacuum filtration molding process, the appearance of voids between the HMSM matrix and fiber, which is caused by bridging effect of the fiber, is inevitable. As a result, the addition of fiber may lead to big pores. When the fiber content is up to 20 wt.%, it is difficult to make the fiber disperse uniformly in the HMSM matrix and the aggregation of the fiber leads to a not apparent decrease of porosity. 3.3. Mechanical properties The mechanical properties of porous composites generally depend on the density, which in turn depends on matrix to filler ratio. In the case of aluminum silicate fiber–reinforced hollow mesoporous silica microspheres composites, the content of fiber also plays an important role in determining the mechanical properties. Fig. 4 plots the variation of mechanical strength of the composites as a function of fiber content. From Fig. 4, it is evident that fiber-free HMSM materials exhibit a very low strength. The compressive strength and flexural strength are 0.93 and 0.75 MPa, respectively. This can be ascribed to the following reasons. Firstly, despite the HMSM are bonded during the sintering process (Fig. 5(a)) and the bonded structure can make contributions to the strength of the materials, the intrinsic low bearing capacity and brittleness of HMSM lead to a low strength. Secondly, it is well known that the strength of the porous
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Q. Chen et al. / Microporous and Mesoporous Materials 152 (2012) 104–109 Table 1 Bulk density and apparent porosity of ASF/HMSM composites. Content of fiber (wt.%)
Bulk density (g/cm3)
Apparent porosity (%)
0 5 10 15 20
0.17 0.20 0.22 0.25 0.26
78.6 73.3 69.1 66.8 65.7
sponding to 15 wt.% of fiber. The highest compressive and flexural strength values are 5.07 and 3.32 MPa, which are about five times and four times both the strength of fiber-free HMSM materials, respectively. Moreover, the compressive strength is much higher than that of fiber–reinforced silica aerogel (1.0 MPa at 25% strain) as reported in Ref. [7]. Further addition of ASF develops a much better bearing capacity of the whole composites and brings an increase in strength. At the same time, more big pores are formed by further addition of fiber (Fig. 6(a)–(d)), and the appearance of big pores will result in a decrease in strength. When the fiber content reaches up to 20 wt.%, the adverse effect caused by the big pores (Fig. 6(d) and (e)) counterbalances part of the reinforcing effect of fiber and leads to a reduction of strength compared to materials containing 15 wt.% fiber. Yet, the value is still higher than that of the fiber-free materials. 3.4. Thermal insulating properties
Fig. 4. Variation of mechanical strength of ASF/HMSM composites with different contents of fiber.
materials is dependent on phase composition and microstructural features, especially porosity [34]. As shown in Fig. 5(a), many pores are observed in samples, which lead to a high porosity and are also responsible for the low strength. Nevertheless, due to the better load-bearing capacity of the fibrous reinforcements, both compressive and flexural strength increase sharply when 5 wt.% fiber is added. Fig. 5(b) shows the microstructure of fiber–reinforced HMSM composites. It can be seen that ASF are embedded in the HMSM matrix and act as a stiff skeleton. This structure can effectively sustain the load that is transferred onto them from the HMSM matrix. Besides, the firm bond between ASF and HMSM (Fig. 5(b)) can make the crack propagation path longer compared with fiber-free composites and lead to higher strength [35]. It can also be seen that the added fiber resulted in big pores around the fiber (Fig. 5(b)), which have a negative influence on material strength, but this effect can be neglected compared with the fiber reinforcement in a low fiber content. As Fig. 4 shows, both compressive and flexural strength increase with further addition of fiber and show a maximum value corre-
The thermal conductivity is a key factor for evaluating the thermal insulating materials. Fig. 7 plots thermal conductivity as a function of the content of fiber. It can be observed that thermal conductivity of the fiber-free HMSM materials shows a lowest value of 0.048 W/(m K), and then increases to 0.064 W/(m K) at 5 wt.% fiber content. At the 15 wt.% fiber content, the thermal conductivity of the composites increases to a highest value of 0.081 W/ (m K). When the content of fiber is further increased to 20 wt.%, the thermal conductivity decreases a little. The behavior of the thermal conductivity of the composites can be explained by the mechanism of heat transfer. The basic mechanisms of heat transfer are generally considered to be conduction, convection and radiation. Conduction is the main mechanism of heat transfer for porous material at room temperature when the bubble diameter is less than 4 mm and the other two can be neglected [36]. This can also apply to HMSM materials since the diameter of HMSM is only 1 lm and even the biggest pore size is less than 40 lm (Fig. 6(e)). It is known that gas is less conductive compared to conduction within a solid or between solid objects in contact. Thus, the thermal conductivity of materials can be expressed by the equation:
k ks us þ kg ug
and kg ks ;
ks us
so ks us þ kg ug ð2Þ
where k is the thermal conductivity of the system, ks is the thermal conductivity of the solid, kg is the thermal conductivity of the gas, us is volume percentage of the solid, ug is volume percentage of the gas. In the case of fiber-free HMSM materials, silica is the only solid component. The heat transfer of fiber-free HMSM material is conducted mainly by the solid thermal convection between silica spheres and within the shells, as shown schematically in Fig. 8(a).
Fig. 5. SEM micrographs of HMSM materials: (a) without fiber and (b) containing 5 wt.% fiber.
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Fig. 6. SEM micrographs of ASF/HMSM composites with different fiber contents: (a) 5 wt.%; (b) 10 wt.%; (c) 15 wt.%; (d) 20 wt.% and (e) a low magnification image of (d).
rials composed of filled component and matrix component, the thermal conductivity can be calculated from the equation:
k ¼ kc
Fig. 7. Thermal conductivity of ASF/HMSM composites with different contents of fiber.
The high volume percentage of gas leads to the low volume percentage of silica, which in turn leads to the low thermal conductivity of HMSM materials. What is more, the thermal conductivity of composites generally increase as the content of fiber increases. For heterogeneous mate-
1 þ 2ud ð1 ðkc =kd ÞÞ=ðð2kc =kd Þ þ 1Þ 1 ud ð1 ðkc =kd ÞÞ=ðð2kc =kd Þ þ 1Þ
ð3Þ
where kc is the thermal conductivity of the matrix component, kd is the thermal conductivity of the filled component, and ud is the volume fraction of the filled component [37,38]. In the case of ASF/ HMSM composites, the solid thermal conduction can be considered to spheres–spheres conduction, spheres–fiber conduction and fiber–fiber conduction, as shown schematically in Fig. 8(b) (here, assuming HMSM is a unit). Liao et al. [24] reported that the thermal conductivity of powder hollow silica spheres depended on the internal diameter and density. HMSM applied in the research has an identical internal diameter and a constant density (0.13 g/cm3) which lead to the constancy of thermal conductivity of HMSM. The thermal conductivity of ASF is about 0.15 W/(m K)), which is much higher than that of HMSM. So, kc =kd < 1, 1 kc =kd > 0 and ð1 ðkc =kd ÞÞ=ðð2kc =kd Þ þ 1Þ is a positive and constant value. Thus, Eq. (3) can be simplified as,
k ¼ kc
1 þ 2aud 1 aud
ð4Þ
where k is the thermal conductivity of the composites, kc is the thermal conductivity of the matrix component, a is a positive and con-
Q. Chen et al. / Microporous and Mesoporous Materials 152 (2012) 104–109
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Fig. 8. Schematic illustration of heat transfer process of materials: (a) without fiber and (b) containing fiber.
stant value and ud is the volume fraction of the filled component. According to Eq. (4) it can be concluded that the thermal conductivity of the composites raises as the volume fraction of fiber increases. Hence the thermal conductivity generally increases as the content of fiber increases. However, the thermal conductivity decreases a little for composites containing 20 wt.% fiber. This is probably because that the big pores (Fig. 6(e)) caused by the fiber entanglement leaded to a slight decrease of ud when 20 wt.% fiber was introduced. 4. Conclusion The ASF/HMSM composites had been prepared through the process of first molded by vacuum filtration method, then sintered at 1100 °C for 1 h. The bulk density of the composites ranged from 0.17 to 0.26 g/cm3 and the ASF was intimately bonded to the HMSM matrix. With increasing the ASF content, the mechanical strength increased first and then decreased, and showed a maximum corresponding to 15 wt.% of ASF. The highest compressive and flexural strength were 5.07 and 3.32 MPa, respectively. The thermal conductivity of composites ranged from 0.048 to 0.081 W/(m K) at room temperature. The high mechanical strength and low thermal conductivity make ASF/HMSM composites a good candidate for thermal insulation. References [1] K.W. Suh, C.P. Park, M.J. Maurer, M.H. Tusim, R. De Genova, R. Broos, D.P. Sophiea, Adv. Mater. 12 (2000) 1779. [2] H. Abe, I. Abe, K. Sato, M. Naito, J. Am. Ceram. Soc. 88 (2005) 1359. [3] V. Meynen, P. Cool, E.F. Vansant, Micropor. Mesopor. Mater. 125 (2009) 170. [4] E.F. Voronin, V.M. Gunko, N.V. Guzenko, E.M. Pakhlov, L.V. Nosach, R. Leboda, J. Skubiszewska-Zieba, J. Colloid Interf. Sci. 279 (2004) 326. [5] X. Lu, M.C. Ardunini, J. Kuhn, Science 255 (1992) 971. [6] M. Schmidt, F. Schwertfeger, J. Non-Cryst. Solids 225 (1998) 364. [7] X.G. Yang, Y.T. Sun, D.Q. Shi, Mater. Sci. Eng. A 528 (2011) 4830.
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