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Super-Compressibility of Ultralow-Density Nanoporous Silica S. O. Kucheyev,* M. Stadermann, S. J. Shin, J. H. Satcher Jr., S. A. Gammon, S. A. Letts, T. van Buuren, and A. V. Hamza Porosity generally embrittles ceramics, and low-density nanoporous oxides typically exhibit very brittle behavior. In contrast to such expectations, we find that an effective fracture strain of nanoporous silica increases with increasing porosity. At ultralow relative densities of 50%. We attribute such a super-compressible behavior to consequences of an increase in the average aspect ratio of ligaments with decreasing monolith density. These results have important implications for designing novel supercompressive materials and for understanding observations of super-compressibility for other low-density nanoporous systems such as carbon-nanotube-based nanofoams. Understanding effects of porosity on mechanical properties of solids has been a subject of numerous previous investigations, driven by their important technological implications. Indeed, most brittle structural materials, such as masonry materials, ceramics, and bones, are to some extent porous, with the size of pores and/or ligaments often being at the nano scale. Porosity of different materials covers a very wide range, from zero (i.e., full density solids) to >99% for aerogels (AGs). The AGs are representative materials for the limiting case of low-density/high-porosity systems with submicron uniformity. They are sol-gel-derived solids made from nanoscale ligaments randomly interconnected into a macroscopic three-dimensional structure with open-cell porosity tunable up to ∼99.95%.[2] Numerous previous studies[2] have focused on conventional silica AGs with densities above ∼50 mg cm−3, first made by Kistler a number of decades ago.[3] Ligaments in these AGs are made of amorphous SiO2 with variable surface hydroxylation. Successful synthesis of ultralow-density[4] silica AGs has also been reported.[5–7] Ultralow-density nanofoams are currently of interest for thermonuclear fusion energy applications as scaffolds for condensed hydrogen fuel layers in fusion targets.[8] They are also attractive materials for solid-state targets for ultrabright x-ray lasers,[9] energy absorbing structures,[10] compliant electrical contacts,[11] and electromechanical devices.[12] Poor mechanical properties of nanofoams limit their use in these applications.
Dr. S. O. Kucheyev, Dr. M. Stadermann, Dr. S. J. Shin, Dr. J. H. Satcher Jr., S. A. Gammon, Dr. S. A. Letts, Dr. T. van Buuren, Dr. A. V. Hamza Lawrence Livermore National Laboratory Livermore, California 94551, U.S.A E-mail:
[email protected]
DOI: 10.1002/adma.201103561 776
wileyonlinelibrary.com
We are aware of only two previous experimental reports on mechanical properties of ultralow-density silica AGs studied by acoustic velocity measurements[13] and uniaxial compression.[5] Both these previous reports[5,13] have focused exclusively on elastic properties, and, the elastic data reported has relatively large scatter. This is related to challenges of mechanical characterization of ultralow-density silica. Indeed, the elastic modulus (E) of AGs decreases superlinearly with decreasing monolith density. As a result, silica AGs with densities of 50%, has been reported.[19–21] These observations of supercompressibility have been attributed to unique mechanical properties of CNTs.[19–21] In this work, we study the mechanical deformation behavior of silica AGs with a wide range of relative densities of 0.05 − 15%. We show, that, in contrast to a generally accepted belief that porosity embrittles a material, an effective fracture strain of silica AGs increases with a reduction in the monolith density. Aerogels with densities of 0.1) for nanowires,[30] they have large ε f r of >50% for ξ > 20 and, hence, can be bent to small radii. The simplest cubic cell model[27] also predicts an increase in ε f r with decreasing den√ sity according to the following scaling law: ε f r ∝ 1/ ρ . This is close to the indentation strain scaling law observed experimentally (Figure 3): ε∗ ∝ ρ −0.34±0.01. The fact that compressibility of nanofoams is determined by the effective aspect ratio of ligaments can be applied to designing novel super-compressive nanostructures for specific applications. It can also explain observations of super-compressibility in other low-density nanoporous material systems, such as CNTbased nanofoams[19–21] as well as vanadia[31] and even organic[32] AGs that have large effective aspect ratio of ligaments. However, additional systematic studies of the three-dimensional geometry and structure of nanofoams are currently required for accurate quantitative modeling of their mechanical deformation, particularly in the still poorly understood regime of inelastic deformation.
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2012, 24, 776–780
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Experimental Section Single-step AGs were prepared by a conventional base-catalyzed sol-gel chemistry.[2] This method is not practical for the synthesis of monolithic silica AGs with densities
