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Customized Porosity in Ceramic Composites via Freeze Casting


Kuo, Claire Taijung (2021) Customized Porosity in Ceramic Composites via Freeze Casting. Dissertation (Ph.D.), California Institute of Technology.. doi:10.7907/88p1-5v79.


Freeze casting is a facile pore-forming technique for ceramics as it affords great tunability in pore structure including size, morphology, wall thickness, tortuosity, and alignment. Nevertheless, similar to any other pore-forming techniques, it has limitations in terms of the range of accessible properties. For example, a porous lamellar structure is highly permeable but easily fractures, while the dendritic structure is the opposite. This research seeks to provide strategies used with freeze casting to achieve a combination of properties that go beyond the current limitations and create optimized pore structures with a specific focus on three properties: strength, permeability, and surface area.

Such strategies utilize two composite material principles. First, particle reinforcement was implemented to optimize the mechanical and transport properties. Second, surface area was increased with hierarchical design for enhanced capture or catalysis applications. To optimize the mechanical and transport properties, we reinforced high-permeability lamellar structures with reinforcement fillers of silicon carbide (SiC) whiskers and carbon nanotubes (CNTs). The two fillers afford two different mechanisms of reinforcement: structural and material reinforcement.

Additions of 30 vol.% SiC whiskers increased the compressive strength by 325% at a small expense in permeability. Shear failure, common in lamellar structures, was prevented by the interwall bridges produced via particle engulfment during freezing. These bridges were demonstrated by the change in microstructure, stress-strain behavior, and fracture surfaces. A 2D in-situ solidification experiment was conducted to observe solidification and particle engulfment directly. We proposed a modified engulfment model to account for the complexity stemming from high-aspect ratio particles and non-planar freezing fronts. Reasonable agreement was found between the model, the simulation based on the model, and the experimental values from the freeze-casting and 2D-solidification experiments.

Freeze-casting with CNTs was explored as an alternative reinforcement strategy, but one which maintains the original pore structure. CNTs were pushed aside by the freezing front to pore walls due to their small diameters for low CNT concentration composites (<4.5 wt.%) such that the original pore structures remained. The compressive strength increased, albeit by smaller percentages (118% for 4.3 wt.%) than those with SiC whiskers. The increase was attributed to the toughening of pore walls with no diminishing effect on permeability. In addition, CNTs changed the electrical conductivity by ten orders of magnitude with the addition of 8.2 wt.% of the reinforcement.

Finally, conformal coatings via self-assembly of block copolymers (BCP) were produced by infiltration into a freeze-cast lamellar structure and significantly increased the surface area of the underlying scaffold. A bimodal pore size distribution with nanometer-size pores from the BCP self-assembly and micron-size pores from freeze casting was observed. An increase in compressive strengths was achieved with the introduction of pore hierarchy while retaining permeability of the macroporous structure due to enlarged lamellar spacings from the infiltration process.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Freeze casting; Ceramic composites; Preceramic polymer; Carbon nanotubes; SiC whisker; Porous materials; Block copolymer
Degree Grantor:California Institute of Technology
Division:Engineering and Applied Science
Major Option:Materials Science
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Faber, Katherine T.
Thesis Committee:
  • Fultz, Brent T. (chair)
  • Kornfield, Julia A.
  • Johnson, William L.
  • Faber, Katherine T.
Defense Date:15 December 2020
Funding AgencyGrant Number
National Science FoundationDMR-1411218
J Yang & Family FoundationUNSPECIFIED
Record Number:CaltechTHESIS:12242020-000539491
Persistent URL:
Related URLs:
URLURL TypeDescription adapted for Chapter 4. adapted for Chapter 5.
Kuo, Claire Taijung0000-0002-3720-968X
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:14042
Deposited By: Tai Jung Kuo
Deposited On:05 Jan 2021 19:34
Last Modified:02 Nov 2021 16:55

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