Literature survey and applications of aerogels. Chapter - 2

Transcription

1 Literature survey and applications of aerogels

2 Literature survey and 25 Literature survey and applications of aerogels 2.1 Introduction Aerogels, the lightest transparent solids known, are a class of the low density ~0.03 g/cc solid-state materials obtained from gel by the replacement of liquid component with air maintaining the network structure intact. They are also known as frozen smoke or air-glass. Thomas Graham [1] in 1864 showed that water in the silica gel could readily be replaced by organic liquids. This fact led Kistler at Stanford University in California, USA [2] to replace the liquid in a wet gel with gas by extracting the liquid from the gel. But, the main difficulties arising during the evaporative drying of silica gels are the shrinkage of the gel, capillary flow and vapor diffusion. In this case, capillary tensions become of great importance. For a cylindrical pore, the tension P exerted by a meniscus of radius r m is shown in fig γ LV Fig. 2.1 Effect of pore meniscus The capillary tension on a meniscus is given by equation (2.1) where γ LV is the interficial tension of the liquid-vapor interface and θ is contact angle: γ LV cos θ P = -2 r m --- (2.1)

3 Literature survey and 26 Therefore, the capillary pressure can be avoided by extracting the liquid component of a gel by supercritical drying. This allows the liquid to be slowly drawn off without causing the solid matrix in the gel to collapse from capillary action. In 1931, to develop the first aerogels, Kistler used sodium silicate precursor and this process of supercritical drying. By increasing the temperature and pressure the liquid is forced into a supercritical fluid state where by dropping the pressure instant gasification and removal of the liquid inside the aerogel occurs, avoiding damage to the delicate threedimensional network because in the supercritical state there is no difference between phases of the substance and hence the capillary pressure is zero. While this can be done with ethanol, the high temperatures (260 o C) and pressures (100 bars) lead to hazardous processing conditions. However, because of very tedious and time consuming procedures of supercritical drying, there was no follow up interest in the field of aerogels. Later on, in 1968, a team of researchers headed by Prof. S. J. Teichner at the University of Lyon, France, succeeded in producing silica aerogels using silicon alkoxides (Tetraethoxysilane; TEOS or Tetramethoxysilane; TMOS) within a day [3]. But, these chemicals are very costly when taking into account of commercialization as given in the table 2.1. Later on, in 1985, an alternative method for aerogel drying was suggested by Tewari et al. [4] in which the solvent present in the gel before drying (generally alcohol) is replaced by a liquid having critical point close to ambient temperature. Liquid CO 2 was found to be the most practical choice since it has a low critical temperature (31 o C) and moderate pressure (73 bars). One of the most limiting parameters associated to this technique is the fact that washing of nanoporous gels with liquid CO 2 is time consuming because of the very low gel permeability. Also, the solvent contained in the nanoporous structure of the gel must be miscible with liquid CO 2. If it is not the case, a solvent exchange has to be made prior to the washing with liquid CO 2 and extraction in its supercritical conditions. Hence, supercritical CO 2 drying method is still expensive if we consider the above facts, which restricts the growing technology of the aerogels. To overcome these difficulties, silica aerogels have to be prepared at ambient pressure. A combination of sodium silicate and ambient pressure

4 Literature survey and 27 drying would result in the development of low cost silica aerogels. The ambient pressure drying method of the aerogel preparation using sodium silicate involves the main three steps as (i) gel formation, (ii) washing / solvent exchange / surface modification, and (iii) ambient pressure drying of surface modified gels. Table 2.1 Cost of precursors used for aerogel preparation Precursors Cost (Rs.) Tetraethoxysilane (TEOS) 2000/L Tetramethoxysilane (TMOS) 4000/25g Sodium silicate (Na 2 SiO 3 ) 70/L 2.2 Need of ambient pressure drying The ambient pressure on an object is the pressure of the surrounding medium, such as a gas or liquid, which comes into contact with the object. While the supercritical state exists only above the critical pressure and temperature of the fluid where there is no difference in the state of matter. In short, the supercritical drying is related to very high pressures and temperatures (above the ambient conditions). Traditionally, silica aerogels are synthesized using supercritical drying method. But supercritical drying method has certain limitations in terms of its cost efficiency, process continuity and safety because a high temperature and pressure are needed to approach the critical point. If liquid CO 2 is used then it is limited due to the fact that washing of nanoporous gels with liquid CO 2 is time consuming because of the very low gel permeability. To overcome these difficulties, silica aerogels have to be prepared by a commercially attractive ambient pressure drying method. 2.3 Why to use sodium silicate? Sodium silicate (or sodium metasilicate), Na 2 SiO 3, also known as water glass or liquid glass is an inorganic compound and readily soluble in water, producing an alkaline solution having ph in the range of The structure of sodium silicate is given in fig. 2.2.

5 Literature survey and 28 NaO O Na Si O O Fig. 2.2 Structure of sodium silicate (Na 2 SiO 3 ) As shown in the structure of sodium silicate, the polar NaO groups are attached on the surface while the single O (oxygen) atom forms the chain with other sodium silicate molecules. The polar NaO groups of sodium silicate make it to dissolve in the water easily. Uniqueness of sodium silicate is that it can undergo three very distinct chemical reactions [5]. These reactions have been named as: Hydration/dehydration Gelation Surface charge modification These reactions allow silicate to acts as matrix binder. Gelation occurs rapidly when the ph of liquid silicate falls below Silicate species begin crosslinking to form polymers. Although the bond formed by the polymerized silicate is not as strong as the bond formed by the dehydration, it has a higher degree of water resistance. This reaction can play a role in agglomeration where the surface of material being agglomerated is acidic environment. Most importantly, the sodium silicate dissolves in water, is very low cost and easy to handle as compared to silicon alkoxides (TEOS or TMOS) (Table 2.1). Detailed preparation conditions of the aerogels using comparatively low cost sodium silicate precursor are very essential for the commercial exploitation of the aerogels. The cost of the final product is due to the raw material used for the preparation of product itself. In fact, around 60% of the cost of aerogels is due to the precursor. The rest of the chemicals such as methanol, hexane, etc., are less expensive. Therefore, one can think of using low cost sodium silicate in place of silicon alkoxides for economic purpose like Cabot aerogels.

6 Literature survey and Necessity of surface chemical modification (Silylation) Aerogels by themselves are hydrophilic, but chemical treatment can make them hydrophobic. They absorb moisture from humid surroundings and they usually undergo a structural change, such as contraction, and deteriorate which limits the long time applications of the aerogels. But the degradation can be prevented by making them hydrophobic. Therefore, the surface of aerogels can be rendered hydrophobic with appropriate surface chemical modification. For this purpose the surface -OH groups should be converted into -O-Si-R groups with the help of silylating agents as shown in fig Due to surface modification, repulsion takes place between end R groups in the silica gel and the capillary pressure becomes nearly zero during drying and hence the original structure of the final aerogel product is maintained. Generally, the organosilane compounds of the type R n SiX 4-n (R: alkyl group, X: halide group and n=1 to 3) like trimethylchlorosilane (TMCS) or hexamethyldisilazane (HMDZ) are used for surface modification of the gels. These compounds are immiscible and or non-reactive with water as they contain non-polar alkyl groups. Therefore, the surface modification cannot be achieved in the hydrogels without solvent (alcohol) exchange. Fig. 2.3 Surface chemical modification of silica gel 2.5 Literature survey of the aerogels prepared by ambient pressure drying With the advancement in the aerogel research, in 1995, Prakash et al reported the preparation of ambient pressure dried TEOS based films by surface chemical modification of wet silica films prior to drying [6]. In continuation to this, Deshpande et al in 1996 have prepared the aerogels using TEOS precursor by many solvent exchanges and by surface chemical

7 Literature survey and 30 modification at ambient pressure [7]. Later on Schwertfeger et al. [8] prepared the aerogels at ambient pressure using sodium silicate by ionexchange method. The ion exchange is necessary because Na + ions are poorly soluble in alcohol and trapped in the gel network tend to decrease the transparency of the dried gels. Further, many research groups have prepared the sodium silicate based aerogels by ambient pressure drying employing the same method [8-11]. A brief literature survey of the aerogels prepared by ambient pressure is given as follows. Schwertfeger et al. [8] had prepared the aerogels by passing two liters of a water-glass solution SiO 2 (content 8 wt%, Na 2 O:SiO 2 ratio 1:3.3.) through an ion exchange as shown in fig The ion exchange column was filled with Duolitee, a special styrene divinyl-benzene-copolymer. The collected silicid acid had a ph in the range of 2.3. The hydrogel was prepared under nearly neutral conditions by adding 1.0 N aqueous NaOH. The solution was stirred for 2 min and transferred in small polyethylene vessels. After gelation about 10 min. the hydrogel was aged for 1.5 h in closed polyethylene vessels. For the silylation the aged hydrogel sticks were placed together with hexamethyldisiloxane (HMDSO) and trimethylchlorosilane in a beaker. The resulting silylated lyogel sticks were dried in a nitrogen steam of 1500 l/h at 200 o C for 1 h. Fig. 2.4 Schematic preparation of silica aerogels by Schwertfeger

8 Literature survey and 31 Ackerman et al. [9] followed Schwertfeger and prepared the aerogels using sodium silicate (SiO 2 contents 6 wt%) in a two or three step procedure. In the first step dilute sodium silicate solution was passed trough an ion exchange column and the collected silicic acid was neutralized with sodium hydroxide at ph values ranging from 5-7. Residual salt byproduct was negligible and the resulting hydrogels were processed into granular form, acidified, silylated and dried at ambient pressure. Lee et al. [10] had synthesized the silica aerogels as shown in a flow chart in fig Water Glass D. I. Water Water Glass Solution (8wt% SiO 2 ) Ion-exchange NH 4 OH (1M) Silica sol Gelation Aging t 60 o C in D.I. Water Solvent exchange / Surface modification SiO 2 Aerogel Ambient Drying Fig. 2.5 Schematic preparation of silica aerogels by Lee In this process, about 0.7 liter of water-glass solution (SiO 2 contents 8 wt%, Na 2 O:SiO 2 molar ratio 1:3.3) went through an ion exchange column filled with 1 liter of Amberlite. The collected silica sols had a ph in the range of 2.7. For gelation, 1.0 M NH 4 OH solution was added to the silica sol. The

9 Literature survey and 32 sols, which had ph values ranging from 3.5 to 5.0 with increments of 0.5, were stirred for 30 seconds respectively, and then transferred into polypropylene. After gelation in closed vessels, the wet gels were aged for 1 day in D.I. water in order to strengthen the gel structure and were immersed in isopropyl alcohol (IPA) / trimethylchlorosilane (TMCS) / n-hexane solution. The modified gels were dried at room temperature for h and then heat treated at 230 o C for 1 h in air. Rao et al. [11] prepared the silica aerogels using sodium silicate (SiO 2 contents 10 wt%, SiO 2 :Na 2 O molar ratio 3.4) and hexamethyldisilazane (HMDZ) in heptane for silylation. The sodium silicate solution was passed through an ion-exchanger, to replace Na + ions by H + ions. Before the ion exchange, the ph of the solution was ~13 and ~2 after passing the solution through ion exchanger. The aquogels were prepared by adding 1M NH 4 OH to the ion exchanged silicic acid while stirring. The amount of NH 4 OH was adjusted until the ph of the sol equaled 4.6. After gelation the aquogels were aged in airtight Teflon containers. The aged aquogels were washed with pure ethanol 4X24 h in dry n-heptane at 50 o C to facilitate the surface derivatization because HMDZ reacts with H 2 O and C 2 H 5 OH, but does not react with n-heptane. The surface modification was done by immersing the gels in 20% HMDZ in heptane for 12 h at 50 o C. Finally the gels were washed with n-heptane 4X 24 h at 50 o C in order to remove any unreacted HMDZ before drying at ambient pressure at 200 o C for 24 h. In this way the preparation of silica aerogels by ambient pressure drying using sodium silicate is possible. The silica aerogels show same properties as the aerogels prepared by that of supercritical drying route. Hence the ambient pressure drying route helps in large scale production of silica aerogels. 2.6 Applications of silica aerogels Silica aerogels are a class of extremely low density ( g/cm 3 ) materials characterized by an open cross-linked silica network with particles usually < 10 nm and pore size usually < 50 nm in diameter. They have low thermal conductivity ( W/mK), with high porosity (80 -

10 Literature survey and %) and large inner surface area ( m 2 /g), low refractive index (1-1.08), low sound velocity (~100 m/s) etc. Owing to these extra ordinary properties, they have wide potential applications in many fields. The following applications for aerogels are associated with certain properties or features of aerogel materials [12] Optical property applications Aerogels have been used to prepare ultrapure, full density silica glass by sintering at temperatures below the melting temperature of silica. Translucent aerogels have been proposed for solar covers and collectors and transparent aerogels have been considered by many for use in solar windows. Ultralow density aerogels have been proposed as lightweight mirror backings Thermal insulator applications Aerogel materials exhibit the lowest thermal conductivities of any of the solid or porous materials. This key property of the material leads to many applications including insulation for architectural purposes, piping, heat and cold storage appliances and devices, automotive exhaust pipes, transport vehicles and vessels. Commercial sources of aerogels have characterized their materials for thermal insulation applications Acoustical and mechanical applications All of the aerogel varieties have unusual acoustic and mechanical behavior, which could be exploited for various applications. The two applications that have been used or proposed are: acoustic impedance matching for more efficient ultrasonic devices, and sound absorption (anechoic chambers). Aerogels have also been proposed as a shock absorbing material Porosity and surface area applications The high porosity (>85%) and very large surface areas (>400 m 2 /g) available in aerogel materials, lead to applications as filters, absorbing media waste containment, encapsulation media, and hydrogen fuel storage.

11 Literature survey and 34 Aerogels have been recognized for many years as excellent catalysts and catalyst supports Space applications Aerogels have already captured cosmic dust while on the European Retrieval Carrier (EURECA) Satellite and in Space Shuttle experiments and they are now proposed for a mission to capture cometary dust in NASA s STARDUST project. Lightweight silica aerogels have also been proposed as a contaminant collector to protect space mirrors from volatile organics.

12 Literature survey and 35 References [1] T. Graham, J. Chem. Soc., 17 (1864) 318 [2] S. S. Kistler, Nature, 127 (1931) 741 [3] G. A. Nicolaon and S. J. Teichner, Bull. Soc. Chem., France, 5 (1968) 1906 [4] P.H. Tewari, A.J. Hunt, K.D. Lofftus, Mater. Lett. 3 (1985) 363. [5] R. K. Iller, The Chemistry of Silica, John Wiley & Son Inc., (1979) [6] S. S. Prakash, C. J. Brinker, A. J. Hurd, S. M. Rao, Nature, 374 (1995) 439 [7] R. Deshpande, D. Smith, C. J. Brinker, US Patent No. 5, 565 (1996) 142 [8] F. Schwertfeger, D. Frank, M. Schmidt, J. of Non-Cryst. Solids 225 (1998) 24 [9] W. C. Ackerman, M. Vlachos, S. Rouanet, J. Fruendt, J. of Non-Cryst. Solids, 285 (1998) 264 [10] C. J. Lee, G. S. Kim, S. H. Hyun, J. Mater. Sci., 37 (2002) 2237 [11] A. P. Rao, A. V. Rao, M. M. Kulkarni, J. of Non-Cryst. Solids, 350 (2004) 224 [12] L. W. Hrubesh, J. of Non-Cryst. Solids, 225 (1998) 335

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