Intrinsic and extrinsic control of freeze casting

Freeze casting is a versatile material fabrication process that is known for its ability to create porous scaffold structures from effectively any constituent material. One of its advantages is that a wide variety of alterations to the processing conditions can result in drastic changes to the final micro- and macro-structure of the freeze-cast scaffolds and, therefore, its properties. Here, the authors present a novel view on these numerous control methods, through the concept that each of these methods for controlling freeze casting can either be considered to be an intrinsic or extrinsic control method. Intrinsic control methods act within the freezing process by altering the characteristic repulsive and attractive forces that govern the interactions between the solid loading particles and approaching freezing front during freezing of the slurry. Extrinsic control methods act upon the freeze process through the application of additional forces or energies, often external to the freezing process. This new system of understanding control over freeze casting is presented so as to inspire new work on the advanced control of freeze-cast scaffolds, specific work into extrinsic control methods and the interactions between intrinsic and extrinsic control methods. (C) 2018 Brazilian Metallurgical, Materials and Mining Association. Published by Elsevier Editora Ltda.