Capturing particulates from diesel engine exhausts, providing channels for gaseous transport in fuel cells, and retaining heat via insulation all rely on porous materials. Each of these applications requires porous networks with distinctive size, shape, roughness, and connectivity to control and ease the flow or filtering or insulating capabilities. Many energy-related applications also require materials (ceramics) that maintain strength and robustness to temperatures in excess of 1500 C (2732 F). Advanced ceramics produced from preceramic polymers surpass conventional materials for high-temperature applications. This project focuses on the use of preceramic polymers to create new high-temperature porous ceramics by freeze casting. The polymers are dissolved in organic solvents, that when frozen in a controlled manner and converted to a ceramic, produce directionally porous networks that are optimized for flow characteristics and mechanical integrity. By developing an understanding of the chemistry of these freeze-casting systems, creating versatile pore architectures is possible, thereby expanding the use of porous ceramics for environmental and energy needs. Students involved in this research are trained in state-of-the-art ceramic processing and characterization methods, as preparation for the technical workforce. As an added benefit, porous materials produced by these methods resemble those found in nature and offer striking images that afford opportunities for science education through art.
TECHNICAL DETAILS: The use of preceramic polymers to create silicon- and boron-based ceramics provides a route to high-temperature creep- and chemically-resistant materials. Traditionally, large shrinkages accompany the conversion to ceramic by polymer pyrolysis often resulting in cracking. To deal with this issue, components are limited to thin fibers, plates, and foamed porous solids. This research builds on the foundation of ceramic processing from preceramic polymers, and extends their formability using directional freeze casting. Compared to other porous material-forming methods, directional freeze casting affords anisotropic continuous pores, whose size, shape and tortuosity can be tuned, and hence, opens new avenues for energy-related applications. To make freeze casting viable for preceramic polymers, it is necessary to develop a fundamental understanding of polymer-solvent pairs and crosslinking catalysts using spectroscopic measurements and real-time microscopic observation of phase separation and solidification. An added benefit of preceramic polymers is their ability to be functionalized using reactive additives and fugitive phases, resulting in more complex chemistries and hierarchical microstructures for greater functionality. These materials provide a new class of freeze-cast solids where their pore network characteristics are of particular interest; they are explored using synchrotron techniques and permeability experiments. Moreover, their mechanical properties, consisting of anisotropic pores and complex pore wall structures, are assessed with respect to conventional mechanics models for porous materials. The research seeds an international collaboration with Prof. Paolo Colombo (Universita di Padova, Italy), an internationally recognized expert in preceramic polymers and porous ceramics, and provides significant international training experiences for students.
