The effect of solidification direction with respect to gravity on ice-templated TiO2 microstructures

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 on top of the ice, and the solidification front advances upward, against gravity. Liquid water reaches its maximum density at 4°C; thus, liquid nearest the cold source 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 cold source is expected to rise due to buoyancy, promoting convective fluid motion 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 a 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 TiO2 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 parallel 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.

Low-tortuosity and graded lithium ion battery cathodes by ice templating

“…Methods of maintaining a constant pore diameter and pore fraction have been investigated for templating-based processing methods, but achieving this while preserving scalability is difficult. 54 Instead, we now consider how this gradient microstructure can be exploited. This would allow a judgement on the need for further efforts to either promote or suppress porosity and help to assess the scalability of the DIT processing.…”

Freeze‐cast honeycomb structures via gravity‐enhanced convection

“…This article is protected by copyright. All rights reserved and radial micro-segregation, caused by the convective flow 17 . Another study demonstrated that different gravitational forces (micro-, lunar and Martian gravity) affect lamellar spacings 18 .…”

“…Scotti et al investigated the effect of the freezing direction with respect to gravity. Microstructures were found to contain tilted lamellar walls, ice lens formation, and radial micro‐segregation, caused by the convective flow 17 . Another study demonstrated that different gravitational forces (micro‐, lunar, and Martian gravity) affect lamellar spacings 18 .…”

Micromechanical modeling of compressive behavior in freeze‐casted ceramic

“…Another important morphological attribute of the anisotropic ice-templated foams is the misalignment of the lamellar walls, that develops during the ice crystal growth. 5,[22][23][24] Furthermore, the loading conditions applied to ice-templated foams or composites are not expected to coincide with the unidirectional lamellar structure, therefore, it is important to characterize the off-axis compressive behavior of these materials. Different research groups investigated the effect of off-axis loading in various composite materials 25 , however, a few studies exist for the ice-templated ceramics.…”

“…The 3‐point bending load‐displacement response measured by Fielden was used to calculate stiffness of a single lamellar wall and was used for developing the present micromechanical model. Another important morphological attribute of the anisotropic ice‐templated foams is the misalignment of the lamellar walls, that develops during the ice crystal growth 5,22‐24 . Furthermore, the loading conditions applied to ice‐templated foams or composites are not expected to coincide with the unidirectional lamellar structure, therefore, it is important to characterize the off‐axis compressive behavior of these materials.…”