Ice-templated porous alumina structures

Abstract

The formation of regular patterns is a common feature of many solidification processes involving cast materials. We describe here how regular patterns can be obtained in porous alumina by controlling the freezing of ceramic slurries followed by subsequent ice sublimation and sintering, leading to multilayered porous alumina structures with homogeneous and well-defined architecture. We discuss the relationships between the experimental results, the physics of ice, and the interaction between inert particles and the solidification front during directional freezing. The anisotropic interface kinetics of ice leads to numerous specific morphological features in the structure. The structures obtained here could have numerous applications, including ceramic filters and biomaterials, and could be the basis for dense multilayered composites after infiltration with a selected second phase.

Introduction

The formation of regular patterns is a common feature of many solidification processes, such as eutectic growth or unidirectional solidification of two-phase systems [1], [2]. Control of the regularity and size of the patterns is often a key issue with regards to the final properties of the materials. Hence, particular attention has been paid to the control of solidification microstructures in the presence of inert particles [3], both theoretically and experimentally, as this technique has wide application in cast materials. The final microstructure is directly related to the shape and behavior of the solidification front, which can either engulf or repel the inert particles, such as ceramic particles in a solidifying metal.

Porous materials have attracted considerable attention as a new class of materials with a wide range of applications, from bone substitutes to parts for the automotive industry. In these materials, control of the size and morphology of the porosity is often the critical factor. Cellular ceramics can be engineered to combine several advantages inherent from their architecture [4]: they are lightweight, can have open or closed porosity (making them useful as insulators or filters), can withstand high temperatures, and can exhibit high specific strength, in particular in compression [5]. Typical processing methods include foam or wood replication [6], [7], [8], [9], or direct foaming [10].

Of the many techniques used to prepare porous ceramics, freeze casting has not so far attracted much attention, although its simplicity certainly makes it appealing. The technique consists of freezing a liquid suspension (aqueous or otherwise), followed by sublimation of the frozen phase and subsequent sintering, leading to a porous structure with unidirectional channels in the case of unidirectional freezing, in which pores are a replica of the ice crystals [2] (in case of aqueous slurries). Although freeze casting has been applied to a wide variety of materials, such as alumina [11], [12], hydroxyapatite [13], silicon nitride [14], NiO-YSZ [15] or polymeric materials [16], a proper and rational control of the microstructure morphology has not yet been achieved.

In freeze casting, the particles in suspension in the slurry are ejected from the moving solidification front and pile up between the growing columnar or lamellar ice, in a similar way (Fig. 1) to salt and biological organisms entrapped in brine channels in sea ice [17]. The variety of materials processed by freeze casting suggests that the underlying principles of the technique are not strongly dependent on the materials but rely more on physical rather than chemical interactions. The phenomenon is very similar to that of unidirectional solidification of cast materials and binary alloys, with ice playing the role of a fugitive second phase.

The porosity of the sintered materials is a replica of the original ice structure. Since the solidification is often directional, the porous channels run from the bottom to the top of the samples. In addition, the pores exhibit a very anisotropic morphology in the solidification plane. The final porosity content can be tuned by varying the particle content within the slurry, and the size of porosity is affected by the freezing kinetics [11]. More elaborate experimental setups have been designed to obtain radially oriented porosity [15]. The surface of the channels is covered by dendritic-like features, probably related to the morphology of the ice front, though no direct interpretation has been probed so far.

The motivation for this work was therefore to investigate freeze casting of ceramic slurries, and in particular the relationships between the freezing conditions and the final microstructures, for moderate to highly concentrated suspensions, and to interpret the phenomenon in terms of the interaction between the solidification front and the inert ceramic particles. We have discovered how under proper control of the freezing conditions, porous multilayered ceramics can be obtained [2]. The experimental setup was inspired by those used for the two-dimensional freezing experiments of low-concentration solutions [18], [19], and modified for the processing of large three-dimensional samples, which may be characterized from a microstructural point of view. Alumina was used as a model material to investigate the relationships between the freezing conditions and the structure morphology. This inert oxide can be used to prepare stable and well-dispersed aqueous slurries with a wide range of solid contents.