Large scale alignment of alumina platelets en route to porous nacre-like alumina by ice-templating

Abstract

Ice templating, being a highly versatile technique contributes largely to the development of biomimetic structures. Biomimetic materials are the synthetic materials that possess one or more design and functional aspects of natural materials like abalone nacre, bone, wood. The present research aimed to replicate the inorganic part of nacre, by taking as-synthesized alpha-alumina platelets as primary building blocks. Ice templating was carried out under large vertical temperature gradient in a custom-built setup, that resulted in large-scale ordered lamellar structure; mainly due to the fast and straightforward self-assembly of platelets during directional solidification. The structural wavelength and lamella thickness, the critical parameters to define the ordered lamellar microstructure, were measured to be 16.23 ± 2.0 μm and 2.79 ± 1.0 μm respectively. The resultant green scaffold was subsequently pressed (i.e. cold-compaction) in a direction perpendicular to the ice-front growth and sintered at 1550 °C/4h. Although the nature of the compression response (before and after intermediate cold compaction step) did not change considerably, but large differences in the strength, work of fracture and modulus were obtained. This is mainly due to the large reduction of porosity as well as large-scale alignment of alumina platelets in bulk, resulted from the additional intermediate cold-compaction step. The microstructure of the sintered specimen displayed large-scale parallel alignment of anisotropic alumina platelets, closely resembles to the inorganic counterpart of natural nacre.