Depth-sensing indentation response of ordered silica foam
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Andreas Stein Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455
Robert F. Cook Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received 16 June 2003; accepted 8 October 2003)
Depth-sensing indentation was applied to three-dimensionally ordered silica foams of two different pore diameters—500 nm and 850 nm—formed by colloidal crystal templating. The contact responses of indentations with Berkovich and hemispherical indentation tips are presented over a load range of 1 mN to 100 mN. Scanning electron microscopy images of residual indentation impressions showed homogeneous deformation for small loads in which the peak displacement was shallow relative to the film–substrate interface. The characteristics of the load–displacement responses changed from periodic discontinuities, associated with cell wall fracture and pore collapse, to smooth and increased stiffness, as a result of densification due to the accumulation of material under the indentation tip and proximity (and contact) of the substrate. Load–displacement responses were translated into pressure–volume space, in which the average pressure during indentation is a measure of the crushing pressure of the cell walls.
I. INTRODUCTION
Macroporous structures in the form of threedimensionally ordered foams (“inverse opals”) can be synthesized in many material compositions including insulating oxides, semiconductors, and metals.1–5 Such foams can be synthesized by colloidal crystal templating—a simple, efficient, and relatively inexpensive method:2–3 (i) a colloidal crystal template is prepared by ordering monodisperse spheres [e.g., polystyrene, poly(methylmethacrylate), silica] into a close-packed array; (ii) interstices in the array are then infiltrated with a fluid that is solidified, resulting in an intermediate composite structure; and (iii) an ordered foam is produced after removing the template by calcination or extraction. The schematic diagrams in Figs. 1(a)–1(c) illustrate the general synthesis procedure, with details specific to the materials of this study. The ordered foam structures synthesized using this method consists of a skeleton surrounding uniform close-packed macropores that are interconnected through voids that form as a result of the contact between the template spheres prior to infiltration of the skeleton phase. Average pore dimensions and cell wall character (thickness, amorphous or nanocrystalline) are dependent on the template sphere diameter, cell wall composition, and processing conditions.1,3,4 260
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 1, Jan 2004 Downloaded: 18 Mar 2015
Potential applications that utilize the large surface area and periodic structure of these ordered foams include photonic crystals, sensors, catalysts, catalyst supports, sorbents, bioactive materials, host materials for drug release, and battery materials.2,6–10 The periodic structures with repeat distances of a few hundred nanometers allow for inte
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