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Biological and synthetic self-assembly processes can yield macro-to-microscale structures with a variety of morphologies and fine patterned features. For example, intricate three-dimensional (3-D) microscale silica-based, chitin-based, and sporopollenin-based structures with finely-patterned (micro-to-nanoscale) features are formed by diatoms (a type of algae), butterflies, and pollen-generating plants, respectively. Synthetic self-assembly approaches have yielded micro-scale structures with periodically-spaced meso-to-nanoscale pores (e.g., inverse opal structures). While such self-assembled structures can be attractive for certain applications, materials that are readily formed by these processes may not possess preferred chemistries for a broader range of uses.

The scalable fabrication of structures with complex 3-D morphologies and with a range of tailorable chemistries may be accomplished by separating the processes for structure formation and for chemical tailoring; that is, solid structures with a desired 3-D morphology may first be assembled in a readily-formed chemistry and then converted into a new functional chemistry via a morphology-preserving process. Two general approaches for such shape-preserving conversion of synthetic and biogenic structures will be discussed: fluid/solid reactions and conformal coating methods. Macroscopic 3-D preforms of porous WC (formed by green machining, 3-D printing, or gel-casting) have been converted into dense, near net-size ZrC/W composites via pressureless infiltration by, and reaction with, Zr2Cu liquid: 0.5Zr2Cu(l) + WC(s) => ZrC(s) + W(s) + 0.5Cu(l)

Microscale SiO2 structures (diatom frustules; photolithographically-patterned SiO2) have been converted into porous-wall Si replicas via a magnesiothermic displacement reaction (followed by selective MgO dissolution): 2Mg(g) + SiO2(s) => 2MgO(s) + Si(s) A layer-by-layer surface sol-gel process has been used to convert bio-organic templates (e.g., pollen particles, butterfly scales) into functional oxide replicas. Such chemically-tailored 3-D microscale and macroscale materials can be attractive for a variety of applications in optics, sensors, aerospace, and renewable energy (e.g., heat exchangers for concentrated solar power; Caccia, et al., Nature, 562, 406, 2018).

Kenneth H. Sandhage received a B.S. in Metallurgical Engineering with Highest Distinction from Purdue University and a Ph.D. in Ceramics from the Massachusetts Institute of Technology. After working as a Senior Scientist on the processing of optical fibers at Corning, and oxide superconductors at American Superconductor Corp., he joined the Dept. of Materials Science and Engineering at Ohio State University (1991). From 2003-2015, he was a faculty member in the School of MSE at the Georgia Institute of Technology, where he was the B. Mifflin Hood Professor. In 2015, Sandhage joined Purdue University as the Reilly Professor of Materials Engineering. Sandhage’s research is focused on the gas/solid and liquid/solid reaction processing, and conformal coating (via wet (bio)chemical strategies), of biogenic and synthetic structures to yield functional 3-D materials for energy, optical, medical, and aerospace applications. This research has yielded several patented methods for fabricating complex-shaped, chemically-tailored materials, including the Displacive Compensation of Porosity (DCP) and Biological Assembly and Shape-preserving Inorganic Conversion (BASIC) processes. Sandhage is a Fellow of the American Ceramic Society.

Birck Nanotechnology Center

Researchers should cite this work as follows:

• M. Caccia, M. Tabandeh-Khorshid, G. Itskos, A. R. Strayer, A. S. Caldwell, S. Pidaparti, S. Singnisai, A. D. Rohskopf, A. M. Schroeder, D. Jarrahbashi, T. Kang, S. Sahoo, N. R. Kadasala, A. Marquez-Rossy, M. H. Anderson, E. Lara-Curzio, D. Ranjan, A. Henry & K. H. Sandhage, Ceramic–metal composites for heat exchangers in concentrated solar power plants, Nature 562, 406–409 (2018).

• Kenneth H. Sandhage (2018), "Shape-Preserving Transformation of Synthetic and Biogenic Structures into Chemically-Tailored 3-D Macroscopic and Microscopic Materials," https://nanohub.org/resources/29064.

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