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Freeze casting, also known as thermally induced phase separation, is a generic method of materials processing, covering several different approaches. Freeze-casting methods can be classified in four categories:


 * Freeze-casting of dense ceramics
 * Freeze-casting of porous ceramics
 * Freeze-casting of porous polymers
 * Freeze-gelation

Freeze-casting of ceramics

 * 1) Slurry preparation. The ceramic powder must be dispersed in the solvent (usually water). The resulting suspension must be stable enough to avoid any segregation effect occuring during the solidification stage. Additives, such as deflocculant, plasticizers or binders, are commonly used to optimise the stability of the slurry.
 * 2) Solidification. During this stage, the starting slurry is solidified. This stage is critical with regards of the final structure and porosity of the pieces. During solidification, continuous crystals of solvent are formed, and the ceramic particles in suspension are rejected from the growing crystals and entrapped between them.
 * 3) Sublimation of the solvent. Once the solification is completed, the obtained piece is stored under conditions of low temperature and reduced pressure, to avoid melting of the solvent. Sublimation, i.e., conversion of the solid solvent into its gas form, can then occured. Porosity is created where the solvent crystals were. The obtained porosity is hence a directly replica of the solvent crystals. Special equipment such as freeze-dryer are usually used.
 * 4) Consolidation at high temperature. Usually referred to as sintering. This stage is necessary to bind the ceramic particles alltogether and provide strength to the processed pieces. The microporosity, present between the compacted particles, is removed, while the macroporosity, created by the sublimated solvent, is retained.

The process of freeze-casting of dense ceramics is very similar. The only difference is the higher solid content of the slurry (more powder added to the solvent), so that the ice crystals cannot reject the ceramic particles. Instead, the ceramic particles are trapped, the distribution of ceramic particles in the solidified body is homogeneous and no macroporosity is created. The remaining microporosity can be removed by the sintering stage, so that dense pieces are obtained, replicating fine details of the mold.

Freeze-casting of porous polymers
The process is similar to that of porous ceramics, with phase separation between the polymer and the solvent occuring during the solidification stage. Two noticeable differences are found. First, the polymer must be dissolved in a first stage (instead of putting particles in suspension). Second, after sublimation, no consolidation stage is necessary, the pieces are ready to be used.

Structure and Properties
porous structure

high strength

The Science
For the ice to act as a template, two conditions must be satisfied :
 * 1) The solidification front must have a non-planar morphology, so that particles can be collected in between the crystals.
 * 2) The particles must be rejected from the solidification front.

Applications
Freeze-casting of dense ceramics is being used as a near net-shape processing technique. Dense ceramic pieces with small details can be obtained, without further machining stage.

The usually anisotropic porous structure has been identified of interest in several fields of applications.
 * Biomaterials  . Several advantages of the process are of interest for the biomaterials field (bone substitute, tissue engineering): the open nature of the structure, the potentialities to tailor micro- and macro-porosity, along with the use of hydroxyapatite as a starting material. The directionnality of the structure could potentially provide an easier access of the cells to the core of the pieces. The biological response of these materials is still being assessed.
 * Materials for chemical engineering and energy sources, such as solid oxide fuel cells, electrodes, catalysts, sensors, filtration/separation devices or photocatalysis   . The open nature of the structure, with good connectivity and low tortuosity, along with the good mechanical properties, make freeze-casted porous ceramics appealing candidates for these applications.

The main application of freeze-casted porous polymers so far is tissue engineering. Biodegradable scaffolds made of collagen, gelatin or polylactide polymers (e.g., PLA, PLGA) can be processed by freeze-casting. Of interest here is the absence of organic solvents and the in situ incorporation of biological agents (e.g., antibiotic) to functionnalise the final components.