GoKawiilOnce solidification has ended, the frozen, templated composite is placed into a freeze-dryer to remove the ice. The resulting green body contains anisotropic macropores in a replica of the sublimated ice crystals and structures from micropores to nacre-like packing [10] between the ceramic or metal particles in the walls. The walls templated by the morphology of the ice crystals often show unilateral features. [11] These together build a hierarchically structured cellular structure. [12] This structure is often sintered for metals and ceramics, and crosslinked for polymers, to consolidate the particulate walls and provide strength to the porous material. The porosity left by the sublimation of solidified fluid is typically between 2–200 μm. [13]
Freeze-casting , also frequently referred to as ice-templating , is a technique that exploits the highly anisotropic solidification behavior of a solvent (often, but not exclusively, water) in a well-dispersed solution or slurry to controllably template directionally porous ceramics , [1] [2] [3] [4] polymers, [5] [6] metals [7] and their hybrids. [8] By subjecting an aqueous suspension or slurry to a directional temperature gradient, ice crystals will nucleate on one side and grow along the temperature gradient. The ice crystals will redistribute the dissolved substance and the suspended particles as they grow within the solution or slurry, effectively templating the ingredients that are distributed in the suspension or slurry. [9]
Overview [ edit ]
The first observation of cellular structures resulting from the freezing of water goes back over a century,[14] but the first reported instance of freeze-casting, in the modern sense, was in 1954 when Maxwell et al.[15] attempted to fabricate turbosupercharger blades out of refractory powders. They froze extremely thick slips of titanium carbide, producing near-net-shape castings that were easy to sinter and machine. The goal of this work, however, was to make dense ceramics. It was not until 2001, when Fukasawa et al.[16] created directionally porous alumina castings, that the idea of using freeze-casting as a means of creating novel porous structures really took hold. Since that time, research has grown considerably with hundreds of papers coming out within the last decade.[17]
The principles of freeze casting are applicable to a broad range of combinations of particles and suspension media. Water is by far the most commonly used suspension media, and by freeze drying is readily conducive to the step of sublimation that is necessary for the success of freeze-casting processes. Due to the high level of control and broad range of possible porous microstructures that freeze-casting can produce, the technique has been adopted in disparate fields such as tissue scaffolds,[18][19] photonics,[20] metal-matrix composites,[21] dentistry,[22] materials science,[23][24][25] and even food science.[26]
There are three possible end results to uni-directionally freezing a suspension of particles. First, the ice-growth proceeds as a planar front, pushing particles in front like a bulldozer pushes a pile of rocks. This scenario usually occurs at very low solidification velocities (< 1 μm s−1) or with extremely fine particles because they can move by Brownian motion away from the front. The resultant structure contains no macroporosity. If one were to increase the solidification speed, the size of the particles or solid loading moderately, the particles begin to interact in a meaningful way with the approaching ice front. The result is typically a lamellar or cellular templated structure whose exact morphology depends on the particular conditions of the system. It is this type of solidification that is targeted for porous materials made by freeze-casting. The third possibility for a freeze-cast structure occurs when particles are given insufficient time to segregate from the suspension, resulting in complete encapsulation of the particles within the ice front. This occurs when the freezing rates are rapid, particle size becomes sufficiently large, or when the solids loading is high enough to hinder particle motion.[4] To ensure templating, the particles must be ejected from the oncoming front. Energetically speaking, this will occur if there is an overall increase in free energy if the particle were to be engulfed (Δσ > 0).
For freeze casting to yield aligned pores, solid particles must be rejected by the solidification front. Otherwise ice‐templating would not occur, as particles will be distributed homogeneously throughout the frozen system. Depending on the speed of the freezing front, particle size and solids loading there are three possible morphological outcomes: (a) planar front where all particles are pushed ahead of the ice, (b) lamellar/cellular front where ice crystals template particles or (c) particles are engulfed producing no ordering. [ 27 ]
Δ σ = σ p s − ( σ p l + σ s l ) {\displaystyle \Delta \sigma =\sigma _{ps}-(\sigma _{pl}+\sigma _{sl})}
where Δσ is the change in free energy of the particle, σ ps is the surface potential between the particle and interface, σ pl is the potential between the particle and the liquid phase and σ sl is the surface potential between the solid and liquid phases. This expression is valid at low solidification velocities, when the system is shifted only slightly from equilibrium. At high solidification velocities, kinetics must also be taken into consideration. There will be a liquid film between the front and particle to maintain constant transport of the molecules which are incorporated into the growing crystal. When the front velocity increases, this film thickness (d) will decrease due to increasing drag forces. A critical velocity (v c ) occurs when the film is no longer thick enough to supply the needed molecular supply. At this speed the particle will be engulfed. Most authors express v c as a function of particle size where v c ∝ 1 R {\displaystyle v_{c}\propto {\tfrac {1}{R}}} . The transition from a porous R (lamellar) morphology to one where the majority of particles are entrapped occurs at v c , which is generally determined as:[3]
v c = Δ σ d 3 η R ( a 0 d ) z {\displaystyle v_{c}={\frac {\Delta \sigma d}{3\eta R}}\left({\frac {a_{0}}{d}}\right)^{z}}
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