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Freeze-casting is a technique that exploits the highly anisotropic solidification behavior of a solvent (generally water) in a well-dispersed slurry to controllably template a directionally porous ceramic.〔(), Microstructural Control of Self-Setting Particle-Stabilized Ceramic Foams〕〔(), Mass transfer in graded microstructure solid oxide fuel cell electrodes〕〔(), Ice-templated porous alumina structures〕〔 By subjecting an aqueous slurry to a directional temperature gradient, ice crystals will nucleate on one side of the slurry and grow along the temperature gradient. The ice crystals will redistribute the suspended ceramic particles as they grow within the slurry, effectively templating the ceramic. Once solidification has ended, the frozen, templated ceramic is placed into a freeze-dryer to remove the ice crystals. The resulting green body contains anisotropic macropores in an exact replica of the sublimated ice crystals and micropores found between the ceramic particles in the walls. This structure is often sintered to consolidate the particulate walls and provide strength to the porous material. The porosity left by the sublimation of solvent crystals is typically between 2 - 200 μm. == Overview == The first observation of cellular structures resulting from the freezing of water goes back over a century 〔(), Uber das Ausfrieren von Hydrosolen〕 but the first reported instance of freeze-casting in the modern sense was in 1954 when Maxwell et al.〔(), Preliminary Investigation of the "Freeze-Casting" Method for Forming Refractory Powders〕 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 wasn’t until 2001, when Fukasawa et al.〔(), Synthesis of Porous Ceramics with Complex Pore Structure by Freeze-Dry Processing〕 created directionally porous alumina castings, that the idea for using freeze-casting as a means of creating novel porous structures really took hold. Since that time, research has grown considerably with 100s of papers coming out within the last decade.〔(), Ice templating, freeze casting: Beyond materials processing〕 Because freeze-casting is a physical process, the techniques developed for one material system can be applied to a wide range of materials.〔〔 Additionally, due to the inordinate amount of control and broad range of possible porous microstructures that freeze-casting can produce, the technique has found its niche in a number of disparate fields such as tissue scaffolds,〔(), Bio-materials by freeze casting〕〔 photonics,〔(), Honeycomb Monolith-Structured Silica with Highly Ordered, Three-Dimensionally Interconnected Macroporous Walls〕 metal-matrix composites,〔(), Experimental study of particle incorporation during dendritic solidification〕 dentistry,〔(), Freeze Casting of High Strength Composites for Dental Applications〕 materials science,〔(), Dispersion, connectivity and tortuosity of hierarchical porosity composite SOFC cathodes prepared by freeze-casting〕〔(), Processing of Hierarchical and Anisotropic LSM-YSZ Ceramics〕〔(), Lightweight and stiff cellular ceramic structures by ice templating〕 and even food science 〔(), Fast Dispersible Cocoa Tablets: A Case Study of Freeze-Casting Applied to Foods〕 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 are able to move by Brownian motion away from the front. The resultant structure contains no macroporosity. If you were to moderately increase the solidification speed, size of the particles or solid loading, 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 specific conditions of the system. It is typically 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.〔(), Freeze-Casting of Porous Ceramics: A Review of Current Achievements and Issues〕 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)''.〔 where ''Δσ'' is the change in free energy of the particle, is the surface potential between the particle and interface, ''σpl'' is the potential between the particle and the liquid phase and ''σsl'' 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 ''(vc)'' 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 vc as a function of particle size where .〔 The transition from a porous R (lamellar) morphology to one where the majority of particles are entrapped occurs at ''vc'', which was defined by Deville et al.〔〔 to be: where ''a0'' is the average intermolecular distance of the molecule that is freezing within the liquid, ''d'' is the overall thickness of the liquid film, ''η'' is the solution viscosity, ''R'' is the particle radius and ''z'' is an exponent that can vary from 1 to 5.〔〔(), On the development of ice-templated silicon carbide scaffolds for nature-inspired structural materials〕 As expected, we see that ''vc'' decreases as particle radius ''R'' goes up. Waschkies et al.〔(), Investigation of structure formation during freeze-casting from very slow to very fast solidification velocities〕 studied the structure of dilute to concentrated freeze-casts from low (< 1 μm s−1) to extremely high (> 700 μm s−1) solidification velocities. From this study they were able to generate morphological maps for freeze-cast structures made under various conditions. Maps such as these are excellent for showing general trends but they are quite specific to the materials system from which they were derived.For most applications where freeze-casts will be used after freezing, binders are needed to supply strength in the green-state. The addition of binder can significantly alter the chemistry within the freezing environment, depressing the freezing point and hampering particle motion leading to particle entrapment at speeds far below the predicted ''vc''.〔 Assuming however that we are operating at speeds below vc and above those which produce a planar front, we will achieve some sort of cellular structure with both ice-crystals and walls composed of packed ceramic particles. The morphology of this structure is tied to a number of variables but the most influential is the temperature gradient as a function of time and distance along the freezing direction. Freeze-casts have at least three apparent morphological regions.〔(), Morphological instability in freezing colloidal suspensions〕 At the side where freezing initiates is a nearly isotropic region with no visible macropores dubbed the Initial Zone (IZ). Directly after the IZ is the Transition Zone (TZ), where macropores begin to form and align with one another. The pores in this region may appear randomly oriented. The third zone is called the Steady-State Zone (SSZ), macropores in this region are aligned with one another and grow in a regular fashion. Within the SSZ, the structure is defined by a value λ that is the average thickness of a ceramic wall and its adjacent macropore.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Freeze-casting」の詳細全文を読む スポンサード リンク
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