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Modulus–density scaling behaviour and framework architecture of nanoporous self-assembled silicas

Nature Materials volume 6, pages 418–423 (2007)Cite this article

Abstract

Natural porous materials such as bone, wood and pith evolved to maximize modulus for a given density1. For these three-dimensional cellular solids, modulus scales quadratically with relative density2,3. But can nanostructuring improve on Nature’s designs? Here, we report modulus–density scaling relationships for cubic (C), hexagonal (H) and worm-like disordered (D) nanoporous silicas prepared by surfactant-directed self-assembly. Over the relative density range, 0.5 to 0.65, Young’s modulus scales as (density)n where n(C)<n(H)<n(D)<2, indicating that nanostructured porous silicas exhibit a structure-specific hierarchy of modulus values D<H<C. Scaling exponents less than 2 emphasize that the moduli are less sensitive to porosity than those of natural cellular solids, which possess extremal moduli based on linear elasticity theory4. Using molecular modelling and Raman and NMR spectroscopy, we show that uniform nanoscale confinement causes the silica framework of self-assembled silica to contain a higher portion of small, stiff rings than found in other forms of amorphous silica. The nanostructure-specific hierarchy and systematic increase in framework modulus we observe, when decreasing the silica framework thickness below 2 nm, provides a new ability to maximize mechanical properties at a given density needed for nanoporous materials integration5.

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Figure 1: Modulus–density scaling relationships.
Figure 2: Cross-sectional and plan-view TEM images.
Figure 3: Simulated trends in mechanical and structural properties of amorphous silica confined to nanometre-scale slabs with thicknesses ranging from 2 to 1 nm.
Figure 4: Raman spectra of self-assembled nanostructured films, a high-surface-area silica xerogel and conventional amorphous silica.
Figure 5: Solid-state magic-angle-spinning 29Si–NMR spectra of D, H and C nanostructured silica films prepared by evaporation-induced self-assembly using 5wt% Brij-56 surfactant (as in Fig. 2).

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Acknowledgements

We thank D. Doshi (UNM), A. Gibaud (Université du Maine), B. Smarsly (Max Planck Institute of Colloids and Interfaces) and R. Köhn (Munich) for GISAXS and SRSAXS, N. Liu (UNM) for molecular simulations of the cyclo-tetrasiloxane and G. Scherer (Princeton University) for many useful discussions. This work was supported by the US Department of Energy Office of Science, Air Products and Chemicals, Incorporated, Sematech, the US Air Force (FA9550-04-1-0087-CJB; F49620-03-1-0406-ST), the Army Research Office (DAAD19-03-1-0227-CJB), NSF (0402867-DJL) and the University of New Mexico/Rutgers/NSF Ceramics and Composites Research Center. TEM investigations were carried out in the Department of Earth and Planetary Sciences at the University of New Mexico. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the US DOE.

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Authors and Affiliations

  1. Sandia National Laboratories, Advanced Materials Laboratory, 1001 University Blvd SE, Albuquerque, New Mexico 87106, USA

    Hongyou Fan, Thomas Buchheit, David Tallant, Roger Assink, Regina Simpson & C. Jeffrey Brinker

  2. The University of New Mexico/NSF Center for Micro-Engineered Materials and Department of Chemical and Nuclear Engineering, Albuquerque, New Mexico 87131, USA

    Hongyou Fan, Christopher Hartshorn, Dave J. Kissel & C. Jeffrey Brinker

  3. Department of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106, USA

    Daniel J. Lacks

  4. Department of Chemistry and Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey 08544, USA

    Salvatore Torquato

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Contributions

H.F. prepared the nanostructured silicas, devising the means to achieve three different architectures at each density, carried out TEM and other structural characterization and oversaw the experimental work. C.H. under the guidance of T.B. carried out the nano-indentation experiments. R.S. and D.T. carried out Raman spectroscopy on thin-film nanostructures and (earlier) bulk silica gels. R.A. carried out 29Si magic-angle-spinning NMR spectroscopy. D.J.K. developed acoustic means to measure Poisson’s ratio of thin-film samples. D.J.L. carried out molecular simulations. S.L. carried out theoretical analyses of modulus–density scaling relationships. C.J.B. directed the research and contributed to the interpretation of the combined experimental, modelling and theoretical studies.

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Correspondence to C. Jeffrey Brinker.

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Fan, H., Hartshorn, C., Buchheit, T. et al. Modulus–density scaling behaviour and framework architecture of nanoporous self-assembled silicas. Nature Mater 6, 418–423 (2007). https://doi.org/10.1038/nmat1913

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