Room-temperature creep of nanoporous silica
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We show that low-density nanoporous silica monoliths (aerogels), in contrast to the case of fulldensity silica, exhibit pronounced time-dependent deformation during indentation at room temperature. Logarithmic indentation creep and stress relaxation are revealed, with an exponential dependency of the creep constant on the applied stress. Such time-dependent deformation is attributed to stress corrosion fracture of nanoligaments that have a large surface-to-bulk atomic fraction.
I. INTRODUCTION
Mechanical properties of low-density nanoporous solids, such as aerogels (AGs), have been a subject of numerous previous studies (see, for example, Refs. 1–10 and references therein). Depth-sensing indentation (DSI) has emerged as a very convenient tool for the evaluation of mechanical properties of AGs.4–7,9,10 A low-density nanoporous system that has received the most attention is the silica (SiO2) AG. All previous DSI studies of silica AGs have been performed at room temperature.4,5,7,10 In this case, negligible time-dependent deformation for fulldensity silica is expected because room temperature is much lower than silica’s glass transition temperature (~1170 °C).11 Hence, with an exception of our recent report,10 all such previous studies have not been concerned with controlling strain rates and with consequences of possible time-dependent deformation. In this work, we show that, in contrast to our expectations based on the behavior of full-density silica, nanoporous silica AGs exhibit pronounced time-dependent deformation effects during room temperature DSI experiments. We attribute such time-dependent deformation to stress corrosion fracture of nanoligaments. It is consistent with a large surface-to-bulk atomic fraction of nanoligaments amenable to stress corrosion fracture and with our recent findings10 suggesting that inelastic deformation of silica AGs during DSI is controlled by nanoligament fracture rather than by plastic flow. II. EXPERIMENTAL
The monolithic silica AGs studied in this work were prepared, as reported in detail previously,10 by base catalyzed hydrolysis of tetramethylorthosilicate [Si(OCH3)4] in an aqueous methanol solution. An average bulk density a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2010.68 J. Mater. Res., Vol. 26, No. 6, Mar 28, 2011
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of the AG of ~340 mg cm 3 (corresponding to ~15% of the full-density silica) was determined by measuring the dimensions and mass of the monolith. The original monolith, a right cylinder with a diameter and height of ~20 and 2 cm, respectively, was shaped to ~2 2 2 cm3 cubes by fracturing along scratches made on monolith surfaces. The fracture surfaces prepared in this way were free from “skin layers” (i.e., surface layers of a densified and fractured material) and contamination debris present on surfaces of as-cast and machined silica AG monoliths, respectively.10,12 Such fracture surfaces were subjected to indentation in an MTS XP Nanoindenter (MTS Syst
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