Controlled freezing and freeze drying: a versatile route for porous and micro-/nano-structured materials

Abstract

Freeze drying is a process whereby solutions are frozen in a cold bath and then the frozen solvents are removed via sublimation under vacuum, leading to formation of porous structures. Pore size, pore volume and pore morphology are dependent on variables such as freeze temperature, solution concentration, nature of solvent and solute, and the control of the freeze direction. Aqueous solutions, organic solutions, colloidal suspensions, and supercritical CO 2 solutions have been investigated to produce a wide range of porous and particulate structures. Emulsions have recently been employed in the freeze drying process, which can exert a systematic control on pore morphology and pore volume and can also lead to the preparation of organic micro-and nano-particles. Spray freezing and directional freezing have been developed to form porous particles and aligned porous materials. This review describes the principles, latest progress and applications of materials prepared by controlled freezing and freeze drying. First of all the basics of freeze drying and the theory of freezing are discussed. Then the materials fabricated by controlled freezing and freeze drying are reviewed based on their morphologies: porous structures, microwires and nanowires, and microparticles and nanoparticles. The review concludes with new developments in this area and a brief look into the future.

Cryogels: Morphological, structural and adsorption characterisation

“…Qian and Zhang [20] reviewed the basis of freeze-drying and the theory of freezing which can be applied to describe morphological and structural features of cryogels. As it was mentioned above, impurities (organics, salts and other solutes) have low solubility in ice crystals in contrast to the bulk aqueous solutions.…”

“…The cryoconcentration and the confined space effects lead to the freezing/melting point depression of the solution near the ice surface (i.e. in pores, voids between ice crystallites), and the supercooling zones can form there [1,20]. This can result in breakage of the planar interface [22,23], which is of importance in destruction of biological objects and other systems during their freezing/thawing.…”

Biomimetic Materials by Freeze Casting

“…Freeze casting is a relatively simple, inexpensive, and adaptable technique to fabricate bulk porous scaffolds and hybrid composites. Deville,14 Gutierrez et al, 15 Qian and Zhang, 16 and Li et al 17 provide excellent reviews of general freeze casting topics, including the processing principles, materials, structures, properties, and applications. This article focuses on freeze casting as it relates to different biomimetic materials and processing modifications inspired by nature.…”

“…[14][15][16][17] Figure 1 shows the basic processing steps involved in freeze casting porous ceramic scaffolds and hybrid ceramic-polymer composites-the main focus of this article. First, a liquid slurry containing ceramic powders and a freezing vehicle (e.g., water) is mixed with a dispersant (e.g., surfactant) and binder (e.g., long-chain polymer), which aid in colloid dispersion and green body integrity, respectively (Fig.…”

“…Other potential applications for freeze-cast materials with complex shapes and designer microstructures include separation filters, insulators, sensors, electrodes, catalyst supports, fuel cells, and piezoelectric devices. [14][15][16][17] …”