Lamellar structures in directionally solidified naphthalene suspensions

To investigate naphthalene as a suspending fluid for freeze-casting applications, sterically stabilized suspensions of copper microparticles suspended in liquid naphthalene are directionally solidified in a Bridgman furnace. Colonies of nearly particle-free naphthalene lamellae, interspersed with particle-enriched interlamellar regions, are predominantly aligned in the direction of the imposed thermal gradient. As furnace translation velocities decrease from 80 to 6.5 μm s⁻¹, the thickness of naphthalene lamellae increases. For the lowest velocity, a transition to a lensing microstructure (with naphthalene bands aligned perpendicular to the solidification direction) is observed in central regions of samples. For all velocities, the naphthalene lamellae show (i) secondary dendritic arms on one of their sides and (ii) are thinnest within core regions relative to peripheral regions (closest to the crucible walls). Together, these observations suggest the presence of buoyancy-driven convection during solidification.

Graphical Abstract