1. A Custom-made Device for Unidirectional Freezing of Ceramic Suspensions
Senior Design Team
John Simmonds
Joe Hastings
Michael Harty
Michael Beachy
Michael Cook (MET Department)
Department of Mechanical and Aerospace Engineering
Old Dominion University, Norfolk, VA 23529

2. Background and motivation
Lightweight mechanically tolerant materials are required for but not limited to aerospace and automobile industries, biomedical implants, impact protection systems for military personnel and vehicles, and energy storage.
Natural materials such as bone, wood and shells are made out of weak constituents, yet exhibit an excellent synergy of high stiffness, strength and damage-tolerance.
Such mechanical property synergy is not available in the current engineered materials.
Investigations on natural materials indicate that unprecedented mechanical properties and property synergy are linked to their unique hierarchical microstructures.
Bamboo microstructure
Compact bone
Spongy bone
Abalone shell
Multilayered microstructure

3. Bio-inspired materials design
Background and motivation
Among the various natural materials, nacre (mother of pearl) has received significant attention for bio-inspired materials design.
Nacre has a dense multilayered structure containing 95 vol.% CaCO3 and 5 vol.% organic phase.
Although, nacre’s main material constituent is a weak ceramic phase (CaCO3), its hierarchical multilayered dense structure gives rise to unusual combination of high stiffness, strength and toughness (damage-tolerance).
Abalone shell
Multilayered dense microstructure
However, biostructure-enhanced materials design to develop robust engineering materials is an extremely challenging endeavor.
Additive manufacturing and freeze-casting are two promising processing techniques.

4. Freeze-casting to design nacre-inspired materials
Background and motivation
Freeze-casting to design nacre-inspired materials
Ceramic slurry
Sintering
Sublimation
Cold surface
Lamellar crystals
Growth direction
Temperature 
Gas
Liquid
Solid
Pressure 
iii ii i iv
Porous multilayered ceramic
Freeze-casting involves unidirectional freezing of particulate suspensions to multilayered porous structures.
Length-scale features of freeze-cast structures can be controlled by exploiting the freezing kinetics and suspension composition.
Goal of this project is to design a custom-made freeze-casting device enabling fabrication of multilayered porous ceramics to foster bio-inspired materials research at Old Dominion University.
Processed porous materials can be directly employed for impact energy absorption, energy storage, and biomedical implants. Additional post- processing can lead to strong-tough materials for various applications.

5. Proposed design of Freeze-casting device

6. Gantt chart

7. Cold finger design stages
Initial design
(rectangular plate)
Circular cold finger
For more uniform temperature distribution
Final cold finger
Teflon mounting block added to reduce heat transfer

8. Thermal insulation for inner dewar wall and mold
Finite Element Analysis
Multiple thermal gradients with initial design
Mold
Thermal gradients after introduction of inner dewar wall insulation
Mold insulation
Unidirectional thermal gradient after introduction of insulation for inner dewar wall and mold
Inner Dewar wall insulation

9. Mold types for freeze-casting
Initially, molds for freeze-casting were made from polydimethylsiloxane (PDMS) polymer.
PDMS mold dimensions: ID 12 mm and height 14 mm.
Although, PDMS molds worked fine, PDMS is expensive and mold making process is time consuming (> 8 hrs).
Later, it was also decided to increase the dimensions of the molds significantly, which will further increase the cost and processing time.
PDMS molds
Teflon mold
Currently, Teflon is used as the mold material.
Teflon is much cheaper than PDMS polymer.
Hollow Teflon tubes were purchased from McMaster and cut to the required length.
Teflon mold dimensions: ID 19 mm and height 48 mm.

10. Cold finger temperature measurements
T type thermocouple (from Omega) is currently used to measure temperature changes of the cold finger.
These temperature measurements are used for calculations of average cooling rates of the cold finger and freezing front velocities during unidirectional freezing of ceramic suspensions.
Cold finger initially (first few minutes) cools rapidly and then reaches an almost steady-state that is maintained for rest of the durations ( minutes depending on the experimental set up).
Cooling rates of the cold finger are calculated from the steady-state part of the curves.
T type Thermocouple
Data acquisition system
Cooling curve of cold finger
Initial rapid cooling
Steady-state cooling

11. Freezing front velocity (FFV)
Ceramic suspension composition and freezing front velocity (FFV) are two critical parameters employed to control the microstructures of the freeze- cast porous multilayered materials.
Critical microstructural parameters: layer thickness and interlayer gap.
FFV: velocity at which solvent crystals travel through the suspension.
FFV = (specimen height / time taken for freezing of suspension).
Cold finger cooling rate is controlled by adjusting the gap between the liquid N2 surface and cold finger (shown by red arrow).
The higher the cooling rate the greater is the FFV.
Freeze-casting set up
Control of freezing kinetics
Ceramic layers
Distance from liquid N2 surface (mm)

12. Intermediate freeze-casting set up
Equipment design stages
Current freeze-casting
set up
Model freeze-casting set up
Intermediate freeze-casting
set up

13. Freeze-cast processing steps
Ball milling
De-airing of ceramic slurry
Unidirectional freezing of ceramic slurry
Sintered samples
Sintering
Freeze drying - sublimation

14. Microstructures of freeze-cast porous Al2O3 ceramic
Perpendicular to ice growth direction
Parallel to ice growth direction

15. Any questions?
Final statements