Section 1. Inorganic/composite aerogels Strengtheningofsilicagelsandaerogelsbywashingandaging processes M.-A. Einarsrud a, * , E. Nilsen a , A. Rigacci b , G.M. Pajonk c , S. Buathier c , D. Valette d , M. Durant d , B. Chevalier e ,P.Nitz f , F. Ehrburger-Dolle g a DepartmentofChemistry,NorwegianUniversityofScienceandTechnology,7491Trondheim,Norway b EcoledesMinesdeParisCENERG,SophiaAntipolis,B.P.207,06904Sophia-Antipoliscedex,France c UMR5634CNRSUniversit e ClaudeBernardLyon1,43boulevarddu11Novembre1918,69622Villeurbanne,France d PCAS,B.P.111,91161Longjumeau,France e CSTB24rueJosephFourier,38400SaintMartind'H eres,France f FraunhoferInsituteforSolarEnergySystems,Oltmannsstr.5,D-79100Freiburg,Germany g ICSI,CNRS,15rueJeanStacky,F-68057Mulhousecedex,France Abstract Gels were prepared from a polyethoxydisiloxane precursor by using HF as a catalyst. During washing in water solution a signi®cant increase in the permeability of the gels was observed, showing that dissolution-reprecipitation occurs. After washing, the gels were further soaked in a solution of polyethoxydisiloxane precursor to strengthen and stien the gel. As expected, a signi®cant enhancement of the mechanical properties of the wet gels was observed. It is also interesting to note, however, that the permeability does not decrease below the value for the as-prepared gels. Hence,apromisingprocesshasbeendevelopedwhereboththestinessandthestrengthhavebeenincreasedaswellas thepermeability.Theincreaseinpermeabilityisofimportancetofacilitatethesupercriticaldryingprocess.Reasonably successfulscalingupofthesupercriticaldryingofthesegelstolaboratoryscalehasbeenachieved,andmonolithicand transparentgelsareobtained.Opticalpropertieshavebeenmeasuredonlaboratoryscaleaerogels.Thecorresponding results have been correlated with structural characteristics measured by small-angle X-ray scattering SAXS). Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 82.33.Ln 1. Introduction This work is a part of the EU HILIT Highly Insulating and Light Transmitting Aerogel Glaz- ingforWindow)project,themainaimofwhichis to develop and investigate the pre-industrial pro- duction and application of monolithic silica aero- gel in highly insulating and light transmitting glazings for windows. The production process is based on a direct supercritical drying route using CO 2 and the use of commercially available poly- ethoxydisiloxane silica precursors. For this appli- cation,aerogeltilesofminimumsize60 60 cm 2 arenecessaryandhencethemechanicalproperties of both the wet and dried gel are of importance. Journal of Non-Crystalline Solids 285 2001) 1±7 www.elsevier.com/locate/jnoncrysol * Correspondingauthor.Tel.:+47-73594002;fax:+47-73590 860. E-mail address: mari-ann.einarsrud@chembio.ntnu.no M.-A. Einarsrud). 0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0022-309301)00423-9