Wenle Li

Freeze casting of porous materials: review of critical factors in microstructure evolution

Abstract Freeze casting is a promising technique to fabricate porous materials with complex pore shapes and component geometries. This review is aimed to elaborate the fundamental principles of the porous microstructure evolution and critical factors that influence the fundamental physics involved in freeze casting of particulate suspensions. The discussion separately analyses homogeneous and directional freeze casting for both aqueous and non-aqueous systems. The effects of additives, freezing conditions, suspension solids loading and particle size on pore shape, size and morphology evolution are discussed. Special techniques based on modified freeze casting, such as freeze tape casting, double sided freeze casting and field directed freeze casting, are also included.

Porous Nanocomposites with Integrated Internal Domains: Application to Separation Membranes

Asymmetric membranes with layered structure have made significant achievements due to their balanced properties and multi-functionalities that come from a combination of multiple layers. However, issues such as delamination and substructure resistance are generated by the intrinsic layered structure. Here, we present a strategy to integrate the traditional layered structure into an asymmetric but continuous porous network. Through infiltrations of microparticles and nanoparticles to targeted regions, active domains are created inside the porous scaffold versus having them applied externally. The fabricated internal active domains are highly adjustable in terms of its dimensions, pore size, and materials. We demonstrate that it is a general method that can be applicable to a wide variety of particles regardless of their material, dimensions, or geometry. By eliminating the external layered structure, problems such as those mentioned above can be eliminated. This integration technique can be extended to other devices required a layered structure, such as solid oxide fuel cells and lithium ion battery.

Fabrication of Porous Nanocomposites with Controllable Specific Surface Area and Strength via Suspension Infiltration

Porous ceramics are promising candidates for a variety of applications, including separation membranes, catalyst supports, tissue engineering scaffolds, energy storage devices, and microelectronics. We describe a novel method for creating porous ceramics with controllable specific surface area and high strength. The fabrication procedure involves infiltrating aqueous suspensions of silica nanoparticles into a porous ceramic scaffold. The samples are then freeze-dried to maintain a homogeneous distribution of nanoparticles, followed by partial sintering to bond the infiltrated nanoparticles into place. By repeating this infiltration process multiple times, the specific surface area of the composite can be varied from less than one to well over 100 m(2)/g. It is also found that this infiltration increases the mechanical strength of the composite. Water flux experiments demonstrate the potential use of these materials as liquid membranes, with no detectable damage to the structure observed after these flux tests. While this initial work focused on silica nanoparticles and ceramic scaffolds, the basic approach would to applicable to a wide variety of other materials, meaning that the method described here would be generally applicable for creating porous materials with precisely controllable properties.

Positioning growth of scalable silica nanorods on the interior and exterior surfaces of porous composites

A novel yet straightforward one-pot synthesis technique was developed to grow silica nanorods on the interior and exterior surfaces of a porous, inorganic scaffold. Growth of the rods on the surface, versus in the bulk, was achieved by functionalizing the surface with chlorosilane molecules, which allowed the emulsion droplets in which the nanorods grow to anchor to the surface. Rods of 100–200 nm diameter and up to 2 μm in length could be grown uniformly over the surface with a typical surface density of 3 rods per μm2, resulting in an order-of-magnitude increase in the specific surface area (area per mass) of the porous material. It was also shown that the properties of the rods (e.g., size, surface density, shape) could be controlled by changing either the composition of the substrate material or the concentrations of key components in the reacting mixture. Furthermore, by selectively controlling the spatial location of the chlorosilane surface groups, the rods could be grown in specific locations inside the porous material.

Sintering of Porous Materials

This chapter focuses on the sintering of nanoparticle-based porous materials. First, nanoparticle-based porous materials are defined. Then, the fundamentals of sintering are briefly revisited but from a porous point of view, describing effects of parameters on porous structure and shrinkage, and the unique characteristics and challenges for the sintering of nanoparticle-based porous materials are addressed. The discussion on the sintering is divided into single composition systems and composites. For single composition systems, porous materials can be obtained by partial sintering, template-based sintering, or reaction sintering. For porous composite materials, the microstructures can be realized by direct sintering of composite systems, using nanoparticles as a second phase in the sintering of larger-sized particle matrix, or unique reaction processes. Finally, the current state of nanoparticle-based porous material sintering is summarized and future directions are analyzed.