Freeze-casting is cool. How cool you ask? Well it’s so cool it’s cold, freezing in fact. Using freeze-casting researchers worldwide have made beautiful porous structures like the ones shown below. It’s quick, easy, relatively “green” and very versatile.
How does freeze-casting work?
First, start with a suspension of particles called a slurry. It doesn’t matter what these particles are made of as long as they remain suspended in the slurry. Imagine flour and water mixed together. Now if you reduce the temperature on one side of this slurry this produces a temperature gradient in the suspension. Basically it means that one side is cold and one side is less cold. At some point, once it’s cold enough on the “cold” side, ice crystals will form and grow towards the “less cold” side. The magic of freeze-casting lies with the fact that ice has a significantly smaller affinity for foreign material (i.e. particles) than water does. In layman’s terms “the ice doesn’t like having particles in it as much as liquid water does”. The growing ice crystals will push the particles out of the way in a process known as “templating”.
Faster freezing = Finer structure
Slower freezing = Coarser structure
As the ice crystals grow through the suspension, the particles get trapped in the spaces between the growing crystals until you end up with a structure alternating between ice crystals and ceramics walls. This will continue until the suspension is completely frozen.
After a freeze-cast sample is completely frozen, it is placed in a freeze-dryer which removes the large ice crystals through a process known as sublimation while leaving the ceramic walls intact. The entire casting is now only held together by interparticle attractive forces (Van Der Waals) and any added organic binders (I often use polyethylene glycol). These added long chain polymers wrap around and weave in between the templated particles, holding them together. The structure can now be densified using high temperatures (sintering) or used as is.
For further reading check out the excellent work done by Sylvain Deville: