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Freeze-casting, also frequently referred to as ice-templating, freeze casting, or freeze alignment, is a technique that exploits the highly anisotropy solidification behavior of a solvent (generally water) in a well-dispersed solution or slurry to controllably template directionally porous ceramics, polymers, metals and their hybrids. By subjecting an aqueous solution 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 solution or slurry.
Once solidification has ended, the frozen, templated composite is placed into a freeze-dryer to remove the ice crystals. The resulting green body contains anisotropic macropores in a replica of the sublimated ice crystals and structures from micropores to nacre-like packing between the ceramic or metal particles in the walls. The walls templated by the morphology of the ice crystals often show unilateral features. These together build a hierarchically structured cellular structure. This structure is often sintering 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 solvent crystals is typically between 2–200 μm.
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, photonics, metal-matrix composites, dentistry,[2] , Freeze Casting of High Strength Composites for Dental Applications materials science,[3] , Dispersion, connectivity and tortuosity of hierarchical porosity composite SOFC cathodes prepared by freeze-casting[4] , Processing of Hierarchical and Anisotropic LSM-YSZ Ceramics[5], Lightweight and stiff cellular ceramic structures by ice templating and even food science.
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. 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, σps 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 is generally determined as:
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. As expected, vc decreases as particle radius R goes up.
Waschkies et al. 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 frozen 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 cellular structure with both ice-crystals and walls composed of packed ceramic particles. The morphology of this structure is tied to some variables, but the most influential is the temperature gradient as a function of time and distance along the freezing direction.
Freeze-cast structures have at least three apparent morphological regions.[6], 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.
There are colonies of similarly aligned ice crystals growing throughout the suspension. There are fine lamellae of aligned z-crystals growing with their basal planes aligned with the thermal gradient. The r-crystals appear in this cross-section as platelets but in actuality, they are most similar to columnar dendritic crystals cut along a bias. Within the transition zone, the r-crystals either stop growing or turn into z-crystals that eventually become the predominant orientation, and lead to steady-state growth. There are some reasons why this occurs. For one, during freezing, the growing crystals tend to align with the temperature gradient, as this is the lowest energy configuration and thermodynamically preferential. Aligned growth, however, can mean two different things. Assuming the temperature gradient is vertical, the growing crystal will either be parallel (z-crystal) or perpendicular (r-crystal) to this gradient. A crystal that lays horizontally can still grow in line with the temperature gradient, but it will mean growing on its face rather than its edge. Since the thermal conductivity of ice is so small (1.6 - 2.4 W mK−1) compared with most every other ceramic (ex. Al2O3= 40 W mK−1), the growing ice will have a significant insulative effect on the localized thermal conditions within the slurry. This can be illustrated using simple resistor elements.
When ice crystals are aligned with their basal planes parallel to the temperature gradient (z-crystals), they can be represented as two resistors in parallel. The thermal resistance of the ceramic is significantly smaller than that of the ice however, so the apparent resistance can be expressed as the lower Rceramic. If the ice crystals are aligned perpendicular to the temperature gradient (r-crystals), they can be approximated as two resistor elements in series. For this case, the Rice is limiting and will dictate the localized thermal conditions. The lower thermal resistance for the z-crystal case leads to lower temperatures and greater heat flux at the growing crystals tips, driving further growth in this direction while, at the same time, the large Rice value hinders the growth of the r-crystals. Each ice crystal growing within the slurry will be some combination of these two scenarios. Thermodynamics dictate that all crystals will tend to align with the preferential temperature gradient causing r-crystals to eventually give way to z-crystals, which can be seen from the following radiography taken within the TZ.
When z-crystals become the only significant crystal orientation present, the ice-front grows in a steady-state manner except there are no significant changes to the system conditions. It was observed in 2012 that, in the initial moments of freezing, there are dendritic r-crystals that grow 5 - 15 times faster than the solidifying front. These shoot up into the suspension ahead of the main ice front and partially melt back. These crystals stop growing at the point where the TZ will eventually fully transition to the SSZ. Researchers determined that this particular point marks the position where the suspension is in an equilibrium state (i.e. freezing temperature and suspension temperature are equal). We can say then that the size of the initial and transition zones are controlled by the extent of supercooling beyond the already low freezing temperature. If the freeze-casting setup is controlled so that nucleation is favored at only small supercooling, then the TZ will give way to the SSZ sooner.
Both A and υ are used as fitting parameters as currently there is no way of calculating them from first principles, although it is generally believed that A is related to slurry parameters like viscosity and solid loading while n is influenced by particle characteristics.
Initially, the materials system is chosen based on what sort of final structure is needed. This review has focused on water as the vehicle for freezing, but there are some other solvents that may be used. Notably, camphene, which is an organic solvent that is waxy at room temperature. Freezing of this solution produces highly branched dendritic crystals. Once the materials system is settled on however, the majority of microstructural control comes from external operational conditions such as mold material and temperature gradient.
To ensure highly anisotropic, yet predictable solidification behavior within the SSZ, dynamic freezing patterns are preferred. Using dynamic freezing, the velocity of the solidification front, and, therefore, the ice crystal size, can be controlled with a changing temperature gradient. The increasing thermal gradient counters the effect of the growing thermal buffer imposed by the growing ice front. It has been shown that a linearly decreasing temperature on one side of a freeze-cast will result in near-constant solidification velocity, yielding ice crystals with an almost constant thickness along the SSZ of an entire sample. However, as pointed out by Waschkies et al. even with constant solidification velocity, the thickness of the ice crystals does increase slightly over the course of freezing. In contrast to that, Flauder et al. demonstrated that an exponential change of the temperature at the cooling plate leads to a constant ice crystal thickness within the complete SSZ, which was attributed to a measurably constant ice-front velocity in a distinct study. This approach enables a prediction of the ice-front velocity from the thermal parameters of the suspension. Consequently, if the exact relationship between the pore diameter and ice-front velocity is known, an exact control over the pore diameter can be achieved.
The non-overlapping growth directions also help to explain why dendritic textures are often seen in freeze-casts. This texturing is usually found only on the side of each lamella; the direction of the imposed temperature gradient. The ceramic structure left behind shows the negative image of these dendrites. In 2013, Deville et al. made the observation that the periodicity of these dendrites (tip-to-tip distance) actually seems to be related to the primary crystal thickness.
These morphological instabilities can trap particles, preventing full redistribution and resulting in inhomogeneous distribution of solids along the freezing direction as well as discontinuities in the ceramic walls, creating voids larger than intrinsic pores within the walls of the porous ceramic.
The mechanical behavior of the freeze casted structure can be classified into distinct regions. At low strains, the lamallae follow a linear elastic behavior. Here, the lamellae bend under a compressive stress, and thus deflect. According to Ashby, this deflection can be calculated from single beam theory, in which each of the cellular sections are idealized to be cubic shaped where each of the cell walls are assumed to be beam-like members with a square base. Based on this idealization, the amount of bending in the cell walls under a compressive force is given by where is the length of each cell, is the second moment of area, is the Young's modulus of the cell wall material and is a geometry dependent constant. Furthermore, we find that the Young's modulus of the entire structure is proportional to the square of the relative density: . This shows that the density of the material is an important factor when designing structures that can withstand loads, and that the Young's modulus of the structure is heavily determined by the porosity of the structure. Past the linear region, the lamellae start to buckle elastically and deform non-linearly. In a stress-strain curve, this is shown as a flat plateau. The critical load at which buckling begins is given by: where where is the fracture stress for the bulk material. These models demonstrate that the bulk material selection can drastically impact the mechanical response of freeze casted structures under stress. Other microstructural features such as the lamellar thickness, pore morphology and degree of macroporosity can also heavily influence the compressive strength and Young's modulus of these highly anisotropic structures.
Another emerging and promising application of freeze casting is the production of porous foams for green hydrogen generation through advanced thermochemical processes like Chemical Loop Combustion (CLC) and the Steam Iron Process (SIP). These processes leverage the unique properties of porous metal structures, such as optimized reaction kinetics, enhanced thermal efficiency, and sustainability. In Chemical Loop Combustion (CLC), foams made from materials like iron oxides act as oxygen carriers, enabling fuel combustion without direct air contact, separating CO₂ for capture while producing high-purity hydrogen. Similarly, in Steam Iron Process (SIP), dendritic pore structures ensure efficient water vapor distribution and maximize hydrogen yield. The precise control over porosity and thermal properties afforded by freeze casting, along with the use of eco-friendly solvents like camphene, positions these foams as a vital innovation for scalable and sustainable hydrogen production, contributing to the fight against climate change.
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