Effects of Gravity-Driven Convection on Microstructural Development during Directional Solidification of Particle Suspensions

Scotti, Kristen

doi:https://doi.org/10.21985/n2-mskm-zq38

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Effects of Gravity-Driven Convection on Microstructural Development during Directional Solidification of Particle Suspensions

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Directional freeze-casting is a porous materials fabrication technique that is used to create materials with complex, three-dimensional pore structures. Particle suspensions are solidified under a thermal gradient, promoting anisotropic growth of dendrites and incorporation of particles within interdendritic space. A fully-solidified directional freeze-cast structure is composed of dendrites that are separated by particle-packed walls and arranged in colonies. After solidification, the solidified fluid is removed via sublimation and the resulting particle scaffold is sintered to densify particle-packed walls. As microstructures are templated during the freezing process, microstructural parameters can be tuned via modifications to solidification conditions and suspension characteristics. However, the interdependency of these relationships is not well-understood; thus, predictive control over microstructural development during solidification is currently limited. Here, the effects of gravity-driven convective fluid motion, that arises during solidification, are studied to better understand processing-structure relationships in these materials. First, a systematic investigation on the effect of solidification direction (with respect to gravity) on microstructures templated during the freeze-casting solidification process is conducted using aqueous particle suspensions. Solidifying in a buoyancy-unstable configuration is found to promote defects in resulting materials which can be avoided by solidifying in the buoyancy-stable configuration; defects observed include microstructural tilting, asymmetric dendritic features on particle walls, and lensing (cracking, in sintered materials). Next, the same aqueous suspensions are used to test the effect of initial suspension temperature on microstructural development. Radial segregation (variation in pore and wall width as a function of radial distance) is observed for all sample types, but the magnitude is greatest for samples that are solidified using an initial suspension temperature that is expected to produce a double fluid density gradient in the liquid. That is, the double-fluid density gradient scenario is correlated with greatest degree of inhomogeneity in pore width distribution across the diameter of these samples, while samples solidified under conditions that promote the smallest gradient in fluid density exhibit lesser variations in pore width. Subsequent work tests the suitability of using naphthalene as a suspending fluid for freeze-casting suspensions and the viability of resulting suspensions for microgravity flight testing on the International Space Station. It is shown that naphthalene can be used as a suspending fluid for freeze-casting suspensions and the resulting structures are primarily lamellar. While solidified in a buoyancy-stable (with respect to fluid density), radial microstructural segregation is still observed and attributed to radial temperature gradients during solidification, a driver for buoyancy driven convective fluid motion. Asymmetric dendritic features on particle walls are also observed and are attributed to interdendritic convection. Next, a naphthalene particle suspension that is electrosterically stabilized is tested and compared to results obtained with a sterically-stabilized suspension. Electrosterically-stabilized suspensions show increased stability relative to sterically stabilized naphthalene/particle suspensions. Finally, freeze-thaw stability tests are conducted for naphthalene/particle suspensions stabilized via steric and electrosteric mechanisms. Freeze-thaw suspensions (subjected to an initial freeze in an ultrasonic bath, with temperatures held at either 10 or 50°C) are directionally solidified, particle sedimentation during solidification is quantified, and microstructures of solidified structures are investigated. It is found that electrosterically stabilized suspensions perform better under freeze-thaw testing relative to sterically stabilized suspensions. Sterically-stabilized suspensions subjected to freeze-thaw exhibit increased particle sedimentation relative to no-freeze-thaw suspensions and microstructural images show that earlier-to-solidify regions are particle enriched relative to later regions for freeze-thaw suspensions. For electrosterically stabilized suspensions, sedimentation is found to be negligible and microstructural parameters (including lamellae thickness and particle fraction) do not change significantly for structures solidified using freeze-thaw suspensions relative to the no-freeze thaw condition.

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