A novel bulk sol-gel process to prepare monolithic silica materials
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A novel bulk sol-gel process to prepare monolithic silica materials Ce Wang,a) Yahong Zhang, and Yunfeng Lu Department of Chemistry, Jilin University, Changchun, People’s Republic of China, 130023
Yen Wei Department of Chemistry, Drexel University, Philadelphia, Pennsylvania 19104 (Received 27 May 1999; accepted 12 July 1999)
A bulk sol-gel process has been developed to prepare monolithic silica materials at room temperature. The process involves an acid-catalyzed hydrolysis of tetraethyl orthosilicate in tetrahydrofuran containing aqueous HCl catalyst, followed by neutralization with carbonate salts, extraction with aliphatic nonpolar solvents, molding, gelation, and drying. This method shortens the processing time from weeks for the conventional sol-gel process to several days. The silica objects prepared from the process are transparent, crack-free, and of relatively low volume shrinkage (e.g., 52%) and high silica content (e.g., 82%).
I. INTRODUCTION
A typical sol-gel process proceeds through hydrolysis and polycondensation of metal oxide precursors such as tetraethyl orthosilicate (TEOS) in, usually, an alcoholic solution containing a base or acid catalyst, and molding and gelation of sol, followed by aging and drying of gel at ambient temperature, to afford inorganic materials such as silica.1,2 There are several severe drawbacks that have limited its commercial applications. For the preparation of bulk objects of metal oxides, the inherent problems include a very long processing time (e.g., ∼20 days or longer), an exceedingly large volume shrinkage (often 75% or higher), uncontrollable shape and dimension, and serious cracking of the materials.3 Scientists have been trying hard to solve these problems since the 1970s.4 A number of methods have been developed, including hypercritical drying,5,6 the use of drying control chemical additives (DCCA), 7–9 the “double processing” technique,10 formation of organic– inorganic hybrids or composites,11–13 etc. These methods have been successful to a certain extent in preventing the dry gel products from cracking as caused by volumeshrinkage-induced stress in the system. By removal of solvents and byproducts of the sol-gel reactions (i.e., water and alcohol) before gelation, monolithic and crackfree objects of polyacrylate–silica hybrid materials have been obtained with significantly lowered volume shrinkages.14 By increase of the pH value of reaction media,15 use of reactive solvents,16 and modification of the pre-
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II. EXPERIMENTAL A. Preparation of solution of colloidal silicates
For a typical procedure, 31.9 g of tetraethyl orthosilicate (TEOS), 6.75 ml of distilled water, 0.75 ml of 2 M HCl(aq), and 21 g of tetrahydrofuran (THF) were mixed at room temperature under nitrogen in a 250-ml
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cursor structures,17 the gelation time, i.e., the time needed for the system to lose its fluidity and become a gel, can be reduced and, therefore, t
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