Ambient pressure drying

In addition to the standard sol-gel synthesis procedure, the preparation of aero-gel-hke APD materials requires a modified gel chemistry that essentially consists of either chemical surface modification and/or pore strengthening. The main issue addressed by such modification schemes is to prevent structural collapse due to the stresses formed as a consequence of high capillary pressures during drying, which normally leads to irreversible shrinkage and densification. 

Densification occurs as a result of condensation of reactive surface chemical groups on network particles that appear as a result of the shrinkage. In order to avoid this reaction, a treatment of the internal surface is then required to protect reactive surface groups such as Si-OH and thus prevent freezing-in of collapsed structures by condensation. In silica gels, the surface modification is commonly done through a chemical reaction with reactive alkylsilane compounds to form hydrolytically stable Si-R groups on the surface. 

Evaporative drying of sol-gel has been reported to be very inefficient in maintaining the original pore structure if no pretreatment modifications are carried out. Together with the pretreatment of the samples already described, the sub-critical pressure drying can be performed by two different methods direct evaporation or controlled atmosphere drying. 

---

Ultra thin microporous carbon films are derived via the pyrolysis of phenolic precursors. The latter can be prepared from resorcinol-formaldehyde resins using a base catalyst. After several hours at 50°C of curing, the solution forms a stable polymeric film. Followed by a solvent exchange and ambient pressure drying, the film is pyrolysed in argon atmosphere at temperatures above 800°C. The result is an electrically conducting polymeric carbon film, the structure of which resembles the organic precursor, but shows microporosity in addition. Hereby, films with thicknesses of > 5 microns and sufficient mechanical stability can be made. 

Keywords fast gelation, hydrogensilsesquioxanes, aerogel, ambient pressure drying, highly porous hybrid... 

Land VD, Harris TM, Teeters DC (2001) Processing of low-density silica gel by critical point drying or ambient pressure drying. J Non-Cryst Solids 283 11-17... 

Parvathy Rao A, Pajonk GM, Venkastewara Rao A (2005) Effect of preparation conditions on the physical and hydrophobic properties of two step processed ambient pressure dried sUica aerogels. J Mater Sci 40 3481-3489... 

Parvathy Rao A, Venkateswara Rao A, Pajonk GM (2005) Hydrophobic and Physical Properties of the Two Step Processed Ambient Pressure Dried Silica Aerogels with Various Exchanging Solvents. J Sol Gel Sci Technol 36 285-292... 

Parvathy Rao A, Venkastewara Rao A, Pajonk GM (2007) Hydrophobic and physical properties of the ambient pressure dried silica aerogels with sodium silicate precursor using various surface modification agents. Appl Surf Sci 253 6032-6040... 

Shi F, Wang L, Liu J (2006) Synthesis and characterization of silica aerogels by a novel fast ambient pressure drying process. Mater Lett 60 3718-3722... 

Wei, T, Chang, T, Lu, S, Chang, Y (2007) Preparation of monolithic silica aerogel of low thermal conductivity by ambient pressure drying. J Am Ceram Soc 90(7) 2003-2007. 

Rao, A P, Rao, A V, Pajonk, G (2005) Hydrophobic and physical properties of the two step processed ambient pressure dried silica aerogels with various exchanging solvents. J Sol-Gel Sci Technol 36(3) 285-292. 

Rao, A P, Rao, A V, Gurav, J L (2008) Effect of protic solvents on the physical properties of the ambient pressure dried hydrophobic silica aerogels using sodium silicate precursor. J Porous Mater 15(5) 507-512. 

Gurav J. L., A. Venkateswara Rao, Nadargi D. Y., Park H. H. Ambient pressure dried TEOS-bsLsed silica aerogels good absorbents of organic liquids, J. Mater Sci 45, 503-510 (2010)... 

Sodium Silicate Based Aerogels via Ambient Pressure Drying... 

---