Materials about Science, Science about Materials

One of the best parts of the March for Science movement for me has been learning about all the different types of scientists and their research. Both the #MyScienceWill and #ActualLivingScientist hashtags have impressed on me just how small my little bubble of science is and how important it is to expand my own scientific awareness. There have been calls for years now for scientists to do more to disseminate their work to the public, but what about scientists sharing more with each other? Why don’t more biologists share their work with astronomers or quantum physicists geek out with ornithologists? Maybe they do and I’m just unaware of it. In either case, this blog article is my small attempt to open up my own bubble and give a little bit of information about my field of study, materials science and engineering. 

 

A Materials Scientist by Any Other Name….

 

Without fail, the first question a materials scientist or engineer will hear is “What’s that?” and each of us will probably say something a little bit different, but here’s my take. Materials science is the study of a material’s structure and properties and how processing can alter those properties. It’s mostly a combination of physics, chemistry, and sometimes biology, in the case of biomaterials like hip implant coatings or 3D-printed organs. You may or may not have heard too much about us, but materials scientists and the fruits of our labors are everywhere. Boeing has long been a mecca for materials engineers with their work in aircraft alloys and now with the mostly composite 787 Dreamliner. The silicon industry employs thousands of materials people to design and build smaller transistors, higher efficiency solar-cell materials, and brighter LEDs. Sporting companies employ us to help make lighter bikes, harder golf clubs, and high-performance fabrics. Even the paint on your walls has been engineered by a materials scientist at some point.

 

The next question after “What’s a materials scientist” is almost inevitably “what’s your favorite material?” The answer I’ve given has evolved over time from steel to Nickel-Titanium (shape-memory alloy) to Cadmium Selenide nanoparticles (rainbow quantum dots) but for the past five years or so, the answer I’ve always given has been ice—plain, run-of-the-mill, solid water.

Rainbow CdSe quantum dots fluorescing under UV light.

Rainbow CdSe quantum dots fluorescing under UV light.

Ice, Baby, Ice

During my PhD, my research focused on using ice to make unique, porous microstructures for a variety of applications using a process called freeze casting. As anyone who’s looked closely at a snowflake knows, they tend to grow much wider than they are thick. That’s why we have snowflakes not snowpucks. This growth behavior is due to ice’s hexagonal crystal structure, which favors growth along the lowest energy crystallographic direction. We can exploit the directional solidification behavior of ice to create a wide range of amazing crystal morphologies, each with its own unique properties.

Phase diagram of ice morphologies as a function of temperature and supersaturation of solvents. Example freeze-cast microstructures on the right. Credit to Deville et al. 2013

Phase diagram of ice morphologies as a function of temperature and supersaturation of solvents. Example freeze-cast microstructures on the right. Credit to Deville et al. 2013

 

To create a freeze-cast, you suspend particles in a water-based suspension and then apply a directional temperature gradient to the suspension. As the ice crystals nucleate and grow, they push the particles in the suspension aside. Think of these growing crystals as thousands of tiny snowplows pushing the particles in the suspension aside. The result is a material that alternates between particle walls and ice crystals. Freeze drying the freeze-cast suspension removes the ice, leaving you with a directionally porous particulate material. The following phase diagram illustrates the steps typically required to freeze cast a material.

 

Process to make freeze-casts. Materials other than ceramics can be used as long as they can be suspended in water.

Process to make freeze-casts. Materials other than ceramics can be used as long as they can be suspended in water.

There are many applications for such a material. Personally, I was experimenting to see if I could develop more efficient and robust solid oxide fuel cell electrodes. The idea being that the large channels could guide gases efficiently into the electrode where they could combust. Freeze-cast materials are also being studied for oil/water separation filters, next-generation bone scaffolds (since their porous channels are perfect for osteoblasts to grab onto), and high-surface-area batteries.

 

Much of materials science boils down to kinetics versus thermodynamics. It’s a question of what does the system want to happen versus what can it actually do. Freeze casting requires balancing those competing forces to get a desirable result. There is only a small window of thermodynamic and kinetic conditions that will allow freeze casting. Lower the temperature too quickly and you’ll end up engulfing all your particles. Freeze too slowly and you’ll push all your particles to the top of the suspension leaving you an unstructured pile of particles. You have to find that sweet spot in the middle and once you do you can make a wide range of different structures, all with different properties simply by changing the freezing rate or adding some additives to the suspension. Isn’t ice amazing!?

(1) Freezing too slowly and pushing particles. (2) Freeze-casting regime. (3) Freezing too quickly and trapping particles.

(1) Freezing too slowly and pushing particles. (2) Freeze-casting regime. (3) Freezing too quickly and trapping particles.

Now, I know there are those of you out there that probably have fantastic ideas for how these types of structures could be used and I’d love to hear them. Again, that’s why we need to talk to each other and learn from one another. A biologist or geologist will look at a material scientist’s work with completely different eyes. Maybe you’ve solved problems that I’ve been struggling with for months. Scientists DO need to do a better job speaking with the public about their work, but science also can benefit from greater levels of interdisciplinary communication. The March for Science has reminded me of just how many different fields there are and how little I know about any of them other than my own. For my part, I’m currently figuring out what scientific periodicals I want to subscribe to, and I’m looking forward to sitting down for a beer with my fellow March for Science organizers and asking how the heck this CRISPR thing, a new way to edit DNA they won’t stop talking about, works.

For more information:

Ice templating, freeze casting: Beyond materials processing: Deville 2013

Freeze-Casting Wiki - Note: Normally I wouldn’t cite Wikipedia, but I actually wrote the article here, which is essentially my dissertation introduction.

Generic Materials Science Information - Again, this is Wikipedia, but this is a nice comprehensive collection of information about materials science.

 

About the Author: This week’s blog post was written by Aaron Lichtner, a member of the March for Science leadership team. Aaron got his PhD in Materials Science and Engineering from the University of Washington and currently works for Nordstrom as a data scientist doing Image Analysis.