Directional solidification of aqueous TiO2 suspensions under reduced gravity

Abstract

Porous materials exhibiting aligned, elongated pore structures can be created by directional solidification of aqueous suspensions-where particles are rejected from a propagating ice front and form interdendritic, particle-packed walls-followed by sublimation of the ice and sintering of the particle walls. Theoretical models that predict dendritic lamellae spacing-and thus wall and pore width in the final materials-are currently limited due to an inability to account for gravitydriven convective effects during solidification. Here, aqueous suspensions of 10-30 nm TiO 2 nanoparticles are solidified on parabolic flights under micro-, lunar, and Martian gravity and compared to terrestrially-solidified samples. After ice sublimation and sintering, all resulting TiO 2 materials exhibit elongated lamellar pores replicating the ice dendrites. Increasing the TiO 2 fraction in the suspensions leads to decreased lamellar spacing in all samples, regardless of gravitational acceleration. Consistent with previous studies of microgravity solidification of binary metallic alloys, lamellar spacing decreases with increasing gravitational acceleration. Mean lamellar spacing for 20 wt.% TiO 2 nanoparticles suspensions under micro-, lunar, Martian, and terrestrial gravity are, respectively: 50±8, 34±11, 30±6, and 23±9 μm, indicating that gravity-driven convection strongly affects lamellae spacing under terrestrial gravity conditions. Gravitational effects on lamellar spacing are highest at low TiO 2 fractions in the suspension; for 5 wt.% TiO 2

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

“…Given that thermal buoyancy flow is stabilized for downward solidification and partially stabilized for horizontal solidification, these observations provide strong evidence that buoyancy-driven fluid flow increases the propensity of ice lens formation during upward solidification (as discussed in the introduction, Stoke's sedimentation velocity of nanometric spheres in water is negligible). These observations are also consistent with results which we reported previously [43], wherein ice lens defects were observed for 20 wt.% TiO2 suspensions solidified upward under normal terrestrial gravity (1g) but were not observed for suspensions solidified upward under reduced gravity (where buoyancy-driven fluid motion is reduced). is observed to increase with increasing: (i) initial particle fraction in the suspension, (ii) vertical distance from the first-to-solidify region (in contact with the aluminum mold base) to the last-tosolidify region, and (iii) radial distance from the outer wall to the center of the specimen.…”

“…For example, if mold walls are more thermally conductive than solid ice, the interface is expected to be concave because latent heat of solidification is preferentially evacuated through the mold wall [62]. Here (and previously [43]), the thermal conductivity of the PVC mold (~0.2 W/m K) is much lower than that of the ice (~2.2 W/m K); thus, if a thermal conductivity mismatch was responsible for interface curvature observed previously (and inferred here), we would expect the curvature to be convex, rather than concave. We posit that the concave curvature observed previously (and inferred here) results from the macroscopic convection pattern itself.…”

“…Fig. 12 shows a modified version of the phase diagram provided by these authors which includes data points calculated for our current experiments (star markers) and our previous work [43] (triangle markers). Predictions are based on the relationship between three dimensionless parameters identified by the authors; the Darcy coefficient (D), the film coefficient (F), and a dimensionless parameter Φ, which is given by:…”

“…In a previous study using parabolic flights [43], we investigated the effect of gravity during upward solidification of aqueous suspensions (5-20 wt.%) of nanometric TiO2 by comparing sintered microstructures of materials solidified under normal terrestrial gravity (1 g) to those solidified under reduced gravity (i.e., Martian, lunar and micro-gravity, corresponding to 0.38, 0.16 and ~0g). Ice lens defects were observed in samples solidified from the most concentrated, 20 wt.% TiO2 suspensions that were solidified upward under 1g, but not under any of the reduced gravity conditions.…”

Controls on microstructural features during solidification of colloidal suspensions

“…Colloidal particles are forced into a variety of morphologies when a suspension is frozen [1,2]: soil is compacted between ice lenses during frost heave [3]; metal-matrix composites with tailored microstructure can be fabricated by freeze casting [4][5][6][7]; ice templating (freeze-casting using water as the suspending fluid) is a growing technology to produce bio-inspired, micro-porous materials [8][9][10]; cells and tissue can be damaged during cryosurgery [11]; and the properties of phase change materials can be improved by nanostructures in thermal energy storage applications [12]. Commonly observed structures are ice lenses transverse to the direction of solidification (figures 1 (a),(b)) and dendrites aligned with the direction of solidification (figure 1 (c)).…”