General conclusions and the scope of future work Chapter -10
General conclusion and 157 General Conclusions and the scope of future work 10.1 Introduction The first step in the preparation of silica aerogels is the sol-gel process to prepare the gel which is followed by drying of the gel by supercritical (SC) or ambient pressure drying (APD). Conventionally, silica aerogels are prepared by the energy intensive and sophisticated expensive supercritical hydrocarbon or liquid CO 2 drying method using alkoxide precursors namely Tetraethoxysilane (TEOS) or Tetramethoxysilane (TMOS) which restricts the commercialization of aerogels [1-3]. Recent developments have shown the great potential in the ambient pressure drying as an alternative method employing sodium silicate (Na 2 SiO 3 ) as a purely inorganic precursor [4-6]. The properties of aerogels are quite similar in both the drying methods. Therefore, now a days aerogels are prepared by surface chemical modification of the gels derived from sodium silicate precursor and ambient pressure drying. To obtain hydrophobic low density and low thermal conductivity aerogels for thermal insulation and liquid marble formation purpose, various sol-gel parameters, processing parameters namely washing, shaking, various solvents, silylating agents and drying method were varied along with doping the gel with TiO 2 powder. 10.2 General conclusion The general process of ion-exchange method to remove sodium salt in the preparation of silica aerogels is replaced by the simple washing and shaking methods. Hence the large scale production of silica aerogels using sodium silicate is possible by employing a simple and inexpensive ambient pressure drying technique. As stated earlier, the native aerogels are hydrophilic in nature and their structure gets deteriorated due to the moisture in the atmosphere. For long term applications, it is necessary to make the surface of aerogels hydrophobic. Therefore, detailed study of solvent exchange/surface modification of sodium silicate based aerogels using
General conclusion and 158 surface-modification and ambient pressure drying was done. Superhydrophobic, low density and semitransparent silica aerogels were obtained using a sodium silicate precursor by an ambient pressure drying method. Both, the gel washings with water and sol-gel parameters have striking effects on the physical properties of the silica aerogels. It was observed that for more gel washing times, the optical transmission (%) of the aerogel improved. Increasing the silylation period and TMCS percentage reduces the density of the aerogels. Also, the 50 % hexane (or methanol) in the silylating mixture produced the low density aerogels. From FTIR spectra of the aerogels, it was observed that the intensity of OH bond at 1600 and 3400 cm -1 decreased and C-H bond at 2960, 1450 cm -1, Si-C bond at 840 and 1260 cm -1 increased with increase in the silylation period. The TGA-DTA showed that the silica aerogels were thermally stable up to around 435 o C. The aerogels with density ~ 0.084 g/cc, porosity ~ 95 %, thermal conductivity ~ 0.090 W/m.K, hydrophobicity ~146 o, and optical transmission ~ 50% were obtained for the molar ratio of Na 2 SiO 3 :H 2 O:Tartaric acid:tmcs at 1:146.67:0.86:9.46 respectively, with 4 times gel washing with water in 24 h, 3 h aging, 24 h silylation period and 50 % hexane (or methanol) in silylating mixture by ambient pressure drying method. Further, silica aerogels were obtained by catalyzing the hydrolysis and condensation of sodium silicate with different acid catalysts varying their concentration followed by simultaneous solvent exchange, surface modification and ambient pressure drying. Strong acids (HCl, HNO 3, H 2 SO 4 ) requires a longer gelation time than the weak acids (HF, CH 3 COOH, HCOOH, C 3 H 6 O 2, H 3 PO 4, C 4 H 6 O 6, C 6 H 8 O 7 ). In particular, among all the acids the citric acid produced low density (0.086 g/cc) silica aerogels. These aerogels are obtained for the molar ratio of Na 2 SiO 3 :H 2 O:Citric acid:tmcs at 1:146.67:0.72:9.46 respectively. They have thermal conductivity ~ 0.09 W/m.K and hydrophobicity ~148 o. These aerogels exhibited large specific surface area ~ 719 m 2 /g with mesopores in their network. And these aerogels are thermally stable up to a temperature of around 420 o C. The influence of washing temperature and protic solvents on the porosity was confirmed from TEM images. FT-Raman spectra of the silica aerogels dried at RT, 50, 200 o C and 50, 200 o C clarified the difference in the
General conclusion and 159 hydrophobicity. The opaque silica aerogels with low granular density (0.053 g/cm 3 ), low thermal conductivity (0.068 W/mK) and better hydrophobicity (150 o ) were obtained for the molar ratio of Na 2 SiO 3 :H 2 O:Citric acid:tmcs at 1:146.67:0.72:9.46 by washing the gels with water at 50 o C for 24 h, using methanol as a solvent and drying the silylated alcogels washed with hexane at 50, 200 o C. These aerogels were thermally stable up to 430 o C and showed slight decrease in hydrophobicity after one month storage in 85% humidity at 30 o C. The hydrophobic powder of these aerogels forms the liquid marbles. Moreover, the reduction in processing time of the doped sodium silicate based ambient pressure dried aerogels employing a shaker has been studied. It is observed that there is a tremendous influence of shaking on the removal of sodium from the pores of gel and hence on the physical properties of the aerogels. EDAX and FTIR studies revealed the presence of titanium in the silica matrix depending on the doping concentration. The TEM images of the silica aerogels clearly indicate the influence of TiO 2 doping on the porosity and density of aerogels. N 2 adsorption-desorption isotherms and pore size distribution profiles of the doped aerogels implies that the increase in TiO 2 loading reduces the surface area and pore volume of aerogel. The superhydrophobic (152 o ) aerogels with low granular bulk density (0.047 g/cm 3 ), low thermal conductivity (0.057 W/mK) and high surface area (638 m 2 /g) were obtained for the molar ratio of Na 2 SiO 3 :H 2 O:TiO 2 :Citric acid:tmcs at 1:146.67:6X10-5 :0.54:9.46 respectively by shaking the gels in presence of water at the speed of 150 RPM for 4 h. The thermal stability of these aerogels was up to around 435 o C. The processing time of these aerogels was reduced from 4 days to 2 days. These aerogels can be applied as thermal insulating materials in ovens and refrigerators in place of glass wool and PUF. It is concluded that the transparency of the aerogels can be controlled by washing the gels with NH 4 F and modifying them with various silylating agents along with aprotic solvents. The TEM images of the silica aerogels clearly show the influence of NH 4 F and silylating agents on the density and porosity of the aerogels. FTIR spectra of the aerogels silylated with HMDSO and TMCS clarify their hydrophilic and hydrophobic nature respectively. The
General conclusion and 160 translucent (55%) aerogels with comparatively high density (0.089 g/cc) and better hydrophobicity (144 o ) were obtained for the molar ratio of Na 2 SiO 3 :H 2 O:Citric acid:tmcs at 1:146.67:0.72:9.46 by shaking the gels in presence of NH 4 F at 50 o C for 4 h and silylating using TMCS and hexane. These aerogels are thermally stable up to around 400 o C. 10.3 Scope for future work in the field of aerogels The production of insulating materials through aerogel preparation using sodium silicate precursor and ambient pressure can develop into a substantial market for residential and commercial applications. The excellent thermal properties of silica aerogel make it an obvious choice for super insulation windows, heat exchangers, refrigerators, furnaces and ovens, water jackets. They are nonflammable, nontoxic, light weight and thermally stable up to around 435 o C. A big push forward can be expected in the next few years. The possibilities originating from ambient-pressure drying will provide strong impetus to preparative chemical research, because aerogels can now be prepared with standard laboratory equipment. The washing and shaking methods of sodium removal after gel formation, surface modification and ambient pressure drying have made the preparation process much simpler and a big step forward from an economic point of view. The most important area for the application of aerogels are all kinds of thermal and acoustic insulation [7, 8]. Further, one more use of aerogels is in the formation of liquid marbles for the transport of microfluids [9]. The new drying techniques will probably make the technical preparation much cheaper and will thus make aerogels more competitive.
General conclusion and 161 References [1] S. S. Kistler, Nature, 127 (1931) 741 [2] G. A. Nicolaon and S. J. Teichner, Bull. Soc. Chem., France, 5 (1968) 1906 [3] P.H. Tewari, A.J. Hunt, K.D. Lofftus, Mater. Lett. 3 (1985) 363 [4] F. Schwertfeger, D. Frank, M. Schmidt, J. of Non-Cryst. Solids, 225 (1998) 24 [5] C. J. Lee, G. S. Kim, S. H. Hyun, J. Material Science, 37 (2002) 2237 [6] A. P. Rao, A. V. Rao, M. M. Kulkarni, J. of Non-Cryst. Solids 350 (2004) 224 [7] J. Gross, J. Fricke, J. Non-Cryst. solids, 145 (1992) 217. [8] K.Y. Jang, K. Kim, R. S. Upadhye, J. Vac. Sci. Technol. A8 (3) (1990) 1732 [9] P. Aussillous, D. Quere, Nature, 411 (2001) 924
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