کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
5369388 | 1388428 | 2008 | 9 صفحه PDF | دانلود رایگان |
![عکس صفحه اول مقاله: Effect of mixed Catalysts system on TEOS-based silica aerogels dried at ambient pressure Effect of mixed Catalysts system on TEOS-based silica aerogels dried at ambient pressure](/preview/png/5369388.png)
In the present paper, the experimental results on the effect of mixed Catalysts system on the physical properties of the TEOS-based silica aerogels, are reported and discussed. The aerogels were produced by the single-step as well as two-step sol-gel process followed by atmospheric pressure drying. In the single-step process, only the NH4F was used as a catalyst, whereas in the two-step process, NH4F as well as a mixed catalysts, i.e., NH4F and NH4OH, were used after 12 h of acid (oxalic acid) addition. Effect of various exchanging solvents, viz., xylene, toluene, heptane or hexane and silylating agents, viz., MTMS, TMES, TMCS, HMDSO or HMDZ on the physical properties of the as prepared aerogels were studied. The volume of the NH4OH, the molar ratios of MeOH/TEOS and HMDZ/TEOS were varied from 0.2 to 1 ml, and 5.5 to 27.5, 0.34 to 0.9, respectively, by keeping the volume of NH4F and the concentrations of NH4F and NH4OH constant at 0.5 ml, 0.1 and 1 M, respectively. Remarkable results were obtained by using mixed catalyst system, hexane as exchanging solvent and surface chemical modification with 5% HMDZ in hexane. The aerogels were characterized by bulk density, optical transmission, thermal stability and contact angle measurements. The surface chemical modification was confirmed by Fourier Transform Infrared (FTIR) spectroscopy. The microstructural studies of the aerogels were done by Transmission Electron Microscopy (TEM), which revealed highly ramified self-similar polymeric structure in large length scale. The thermal stability of the aerogels were tested using TG-DT analyses. It was found that low bulk density (0.065 g/cm3), superhydrophobic (153°), high thermal stability (380 °C) and high optical transmission (95%) of the as produced aerogels obtained at the molar ratio of TEOS:MeOH:oxalic acid:NH4F:NH4OH:HMDZ for 1:16.5:0.81:0.62:0.63:0.41, respectively.
Journal: Applied Surface Science - Volume 255, Issue 5, Part 2, 30 December 2008, Pages 3019-3027