Free-standing microscale structures of nanocrystalline zirconia with biologically replicable three-dimensional shapes
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Christopher S. Gaddis, Ye Cai, and Kenneth H. Sandhagea) School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received 11 August 2004; accepted 1 November 2004)
Microscale zirconia structures with intricate three-dimensional (3D) shapes and nanoscale features were synthesized using diatom (single-celled algae) microshells as transient scaffolds. After exposure to a zirconium alkoxide-bearing solution and firing at 550–850 °C, silica-based diatom microshells were coated with a thin, continuous nanocrystalline zirconia layer. Predominantly tetragonal or monoclinic zirconia could be produced with appropriate heat treatments. Selective silica dissolution then yielded freestanding zirconia micro-assemblies that retained the microshell shape and fine features. Such hybrid (biological/synthetic chemical) processing may be used to mass-produce nanostructured micro-assemblies with a variety of 3D, biologically replicable shapes and tailored compositions for use in numerous applications.
Zirconia-based ceramics are technically important materials that are used, or considered for use, in a diverse range of applications such as oxygen sensors, oxygen pumps, electrochromic devices, fuel cells, thermalbarrier coatings, cutting tools, milling media, biomedical implants, ceramic membranes, and catalysts (e.g., isomerization of alkanes).1,2 Nanocrystalline zirconia-based ceramics can exhibit enhanced catalytic, mechanical, thermal, and electrical properties.3–6 The ability to massproduce three-dimensional (3D) micro-assemblies of nanocrystalline zirconia would be highly attractive for a number of applications (e.g., for low-temperature gas sensing, minimally-invasive biomedical devices, rapid catalysis, and controlled-shape reinforcements in composites).1–6 Appreciable global activity is underway to develop methods for assembling nanocrystal/nanoparticle-based structures with well-controlled chemistries, precise shapes, and fine features.7,8 Processing approaches that can be scaled up for high-throughput production of 3D microassemblies on a massive (up to tonnage) scale, while retaining structural precision on a fine (down to nanometer) scale, are needed. Numerous examples exist in nature of micro-organisms that, through sustained reproduction (repeated doubling), can assemble enormous numbers of microscale
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0046 282
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 2, Feb 2005 Downloaded: 17 Mar 2015
biomineralized structures with well-controlled 3D shapes and meso-to-nanoscale features.9 A particularly impressive range of such shaped bioclastic micro/nanostructures are generated by diatoms.10 Diatoms are unicellular algae that populate a diverse range of aquatic environments (marine and freshwater; arctic to equatorial conditions).10 Each diatom cell forms a rigid wall (frustule) consisting of a 3D microscopic assemblage of amorphous silica nanoparticles.10,11 Eac
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