Mechanical properties of silica aerogels measured by microindentation: in¯uence of sol±gel processing parameters and carbon addition M. Moner-Girona a , E. Mart  õnez b , A. Roig a , J. Esteve b , E. Molins a, * a Institut de Ci  encia de Materials de Barcelona ICMAB-CSIC), Campus Universitat Aut  onoma de Barcelona, E-08193 Bellaterra, Catalunya, Spain b Departament de F  õsica Aplicada i  Optica, Universitat de Barcelona, Avgda. Diagonal 647, E-08028 Barcelona, Catalunya, Spain Abstract The response to deformation of several types of silica aerogels is addressed. Samples were produced by sol±gel and hypercritical extraction of the solvent. The in¯uence of the silicon alkoxide concentration, the solvent, the drying procedure and the addition of carbon on the sample's mechanical behavior are studied. Continuous loads and dis- placements were recorded using a nanoindenter. The experimental equipment has extreme sensitivity and small loads and penetration depths are possible. Young's moduli, hardness values and elastic parameters for all samples are re- ported. Initial conditions for the TMOS sol±gel polymerization process, such as the solvent and metal alkoxide molar ratio, have a pronounced in¯uence on the density of the aerogels, which is re¯ected in their mechanical behavior. Solvent selection and details of the drying procedure cause variations in the aerogel hardness and Young's modulus. Power function relationships have been found between these mechanical parameters and the aerogel densities. It is found that the addition of a small amount of activated carbon to the sol 2%) has a pronounced in¯uence on the Young's modulus. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 62.20.x; 81.05.Rm 1. Introduction A major drawback of aerogels is that they are fragile and brittle. Eorts are being made to im- provetheirmechanicalpropertiesandtoovercome the diculties associated with measuring such properties. In our previous work [1] we demon- strated that dynamic depth-sensing microindenta- tioncanbeasuitabletechniqueforthemechanical characterization of the aerogels, since it is a non- destructive technique and does not require any sample machining or processing prior to mea- surement.Thistechnique,mostlyusedforthin®lm characterization, has several advantages. Auto- matic depth sensing of the indenter penetration makes it unnecessary to register real images of the indentation mark on the sample. This is a crucial pointconsideringthatthehightransparencyofthe aerogels makes it extremely dicult to get well- contrastedimages.Italsoallowstheapplicationof extremelysmallloadstothesample,preventingthe formation of cracks in the aerogels. Journal of Non-Crystalline Solids 285 2001) 244±250 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +34-93 580 1853; fax: +34-93 580 5729. E-mail address: elies.molins@icmab.es E. Molins). 0022-3093/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0022-309301)00462-8