Unidirectional Freezing of Ceramic Suspensions: In Situ X‑ray Investigation of the Effects of Additives Benjamin Delattre,* ,†,§ Hao Bai, † Robert O. Ritchie, †,‡ Joë l De Coninck, § and Antoni P. Tomsia † † Materials Sciences Division Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡ Department of Materials Science & Engineering University of California, Berkeley, California 94720, United States § Laboratoire de Physique des Surfaces et Interfaces, Universite ́ de Mons, Mons 7000, Belgium * S Supporting Information ABSTRACT: Using in situ X-ray radiography, we investigated unidirectional freezing of titanium dioxide suspensions. We showed how processing additives, which are generally used for ice-templating, strongly modified freezing dynamics during the solidification process. We observed and identified different freezing regimes by varying the amount of dispersant, binder, or poly(ethylene glycol) (PEG). We demonstrated that because each regime corresponds to a given final structure understanding the particle motion and redistribution at the ice-front level was essential. We also examined the transition from a random particles-entrapment regime to a well-defined lamellar regime and proposed and discussed two mechanisms by which additives might affect the solidification process. KEYWORDS: freeze-casting, ice-templating, ice lens, unidirectional freezing, ice growth, freezing dynamics 1. INTRODUCTION Freeze-casting, also known as ice-templating, is a shaping technique able to create highly complex porous structures with morphologies that differ depending on the solvent used. The process consists of casting the slurry into a mold and freezing it unidirectionally through application of a thermal gradient. Under specific conditions during solidification, particles are expelled by the growth of an ice front and are entrapped between ice crystals. After removing ice by sublimation, a green body with a highly anisotropic structure is obtained and eventually consolidated by sintering. This environmentally friendly process has received a great deal of attention during the past decade. 1−6 Freeze-casting offers the advantage of being applicable to a wide range of solid materials such as ceramics, metals, or polymers. Potential applications include filters, catalyst supports, storage systems, solid-oxide fuel cells, electrodes, infiltrated ceramic−metal composites, or bone- graft substitutes. The process and its underlying mechanisms have been the subject of several recent comprehensive reviews. 3−6 One of the most informative ways to examine the morphology of such freeze-cast structures, or scaffolds, is to use X-ray techniques. In recent years there has been significant progress in the fields of synchrotron X-ray radiography and tomography. This progress is linked with both advances in scientific computing that allow fast processing of complex algorithms to reconstruct objects in three dimensions and developments in CCD technology that enhance both resolution and time of acquisition. Deville et al. applied these techniques to investigate the solidification of alumina suspensions during the initial instant of freezing 7 followed subsequently by the so- called steady-state regime 8 and, finally, the instabilities at the solid−liquid interface. 9 More recently, the same group demonstrated the ability to use in situ 3D imaging to investigate the ice-crystal growth of colloidal silica suspen- sions. 10 The effect of supercooling and crystal growth during the initial freezing regime 11 was highlighted by Lasalle et al., who also described possible mechanisms to explain the redistribution of particles during the ice-templating process. 12 During the last few years, the use of secondary additives to modify the crystallization behavior of the water has raised significant interest, as have the highly complex porous structures 13−15 that can be created with this approach. However, until recently, cooling rates and solid loading-content volume were the most common experimental parameters manipulated to control freezing dynamics in studies dealing with unidirectional freezing. It is now well-established that in the case of low-solid-content suspensions subjected to slow ice- front velocities (i.e., <1 μm/s) ice grows with a planar interface and rejects all foreign bodies into the remaining liquid phase. 16 This leads to a system where the two components, ice and particles, are completely separated at the completion of the freezing process. This technique is sometimes used for water purification 17 or for concentrating solutions in the food industry. 18 Final scaffolds that have a cellular or lamellar macromorphology with porosity oriented along the temper- ature gradient typically require faster cooling rates (1−25 °C/ min) and higher solid contents (15−50% vol %), with those Received: September 4, 2013 Accepted: December 17, 2013 Published: December 17, 2013 Research Article www.acsami.org © 2013 American Chemical Society 159 dx.doi.org/10.1021/am403793x | ACS Appl. Mater. Interfaces 2014, 6, 159−166