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

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

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.

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.

Proposed design of Freeze-casting device

Gantt chart

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

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

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.

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 (30-100 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

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)

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

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

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

Any questions? Final statements