Abstract
Unidirectional ice-templating produces materials with aligned, elongated pores via: (i) directional solidification of particle suspensions wherein suspended particles are rejected and incorporated between aligned dendrites, (ii) sublimation of the solidified fluid, and (iii) sintering of the particles into elongated walls which are templated by the ice dendrites. Most ice-templating studies utilize upward solidification techniques, where solid ice is located at the bottom of the solidification mold (closest to the cold source), the liquid suspension is above the ice, and the solidification front advances upward, against gravity. Liquid water reaches its maximum density at 4 °C; thus, liquid nearest the solid/liquid interface, at 0ºC, is less dense than warmer liquid above (up to 4 °C, above which, a density inversion occurs, and liquid density decreases with increasing temperature). The lower density liquid nearest the solidification front is thus expected to rise due to buoyancy, promoting convective fluid motion in the liquid during solidification. Here, we investigate the effect of solidification direction with respect to the direction of gravity on ice-templated microstructures to study the role of buoyancy-driven fluid motion during solidification. We hypothesize that, for upward solidification, the convective fluid motion that results from the liquid density gradient occurs near the solidification front. For downward solidification, we expect that this fluid motion occurs farther away from the solidification front. Aqueous suspensions of TiO 2 nanoparticles (10–30 nm in size, 10, 15, and 21 vol.%) are solidified upward (against gravity, with ice on bottom and water on top), downward (water on bottom, ice on top), and horizontally (perpendicular to gravity). Microstructural investigation of sintered samples shows evidence of buoyancy-driven, convective fluid flow during solidification for samples solidified upwards (against gravity), including (i) tilting of the wall (and pore) orientation with respect to the induced temperature gradient, (ii) ice lens defects (cracks oriented perpendicular to the freezing direction), and (iii) radial macrosegregation. These features are not observed for downward nor horizontal solidification configurations, consistent with the hypothesis that convective fluid motion does not interact directly with the solidification front for downward solidification.
Original language | English (US) |
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Pages (from-to) | 3180-3193 |
Number of pages | 14 |
Journal | Journal of the European Ceramic Society |
Volume | 39 |
Issue number | 10 |
DOIs | |
State | Published - Aug 2019 |
Funding
This work was supported by grants from NASA Office of Education and the Science Mission Directorate ( NNH15ZDA010C ), NASA\u2019s Physical Sciences Research Program ( 80NSSC18K0198 ), the Institute for Sustainability and Energy at Northwestern , and Northwestern University (NU) Office of the Provost . This work made use of the MatCI Facility and the EPIC facility (NUANCE Center) which are supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center at NU. The authors thank the following students for their assistance with experiments and ceramography: Mr. Youwu Fang (NU), Mr. Benjamin Richards (NU), and Ms. Catalina Young (NU, now at Purdue University). They also thank Dr. Christoph Kenel (NU) and Mr. Stephen Wilke (NU) for numerous useful discussions, and Prof. Peter Voorhees (NU) for helpful insights on radial macrosegregation in metal alloys. This work was supported by grants from NASA Office of Education and the Science Mission Directorate (NNH15ZDA010C), NASA's Physical Sciences Research Program (80NSSC18K0198), the Institute for Sustainability and Energy at Northwestern, and Northwestern University (NU) Office of the Provost. This work made use of the MatCI Facility and the EPIC facility (NUANCE Center) which are supported by the MRSEC program of the National Science Foundation (DMR-1121262) at the Materials Research Center at NU. The authors thank the following students for their assistance with experiments and ceramography: Mr. Youwu Fang (NU), Mr. Benjamin Richards (NU), and Ms. Catalina Young (NU, now at Purdue University). They also thank Dr. Christoph Kenel (NU) and Mr. Stephen Wilke (NU) for numerous useful discussions, and Prof. Peter Voorhees (NU) for helpful insights on radial macrosegregation in metal alloys.
Keywords
- Directional solidification
- Freeze-casting
- Ice banding
- Porous ceramics
- Rayleigh-Bénard convection
ASJC Scopus subject areas
- Ceramics and Composites
- Materials Chemistry