Prof. Claudio Di Leo 's Team Working on Two Grants to Improve Numerical Analysis for Microstructures

Prof. Claudio Di Leo 's Team Working on Two Grants to Improve Numerical Analysis for Microstructures
Claudio DiLeo
Prof. Claudio V. Di Leo

Prof. Claudio Di Leo's Multiphysics Mechanics of Materials (M3) Lab has turned the intersection of mechanics and chemistry into a very busy interchange. And he doesn't see it dying down anytime soon.

"We received an NSF grant to develop next generation lithium-metal battery electrodes, which is big, because we need batteries that are safe but also have high energy density. They are going to play a huge role in things like urban air mobility - the use of battery-powered vertical lift vehicles to transport people around and between cities. We can't have them blow up or fail," he said.

"But at the heart of our work is the coupling between chemistry and mechanics -- something that occurs in many engineering systems, not just batteries. The problems we tackle on the microstructure level make it a lot more complex. And a lot more interesting."

It Starts with the Batteries.

The performance and safety of next gen lithium batteries poses some unique challenges.

When Di Leo looks at these batteries he sees a mix of physical and electro-chemical systems that are in constant flux. The relatively recent replacement of a liquid with a solid-state electrolyte  has improved the safety of these batteries - liquid electrolytes are notoriously volatile -- but the resulting mechanical stresses impact performance. Inside this new solid-state environment, the routine transport of ions between electrodes now causes loads to build up and physically deform the electrodes -- a process that eventually leads to their fracture and failure.


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"The liquid electrolyte used to redistribute itself to accommodate these increased loads. With solid-state electrolytes, it's a stiffer environment. You give up the volatility of the liquid, but you run into problems with electrochemical performance, which is essentially: how much capacity does your battery have before it cracks? This is a strength of the M3 lab. We are not just mechanicians. We are interested in how mechanics impact the electro-chemical performance of a system. And vice versa."

Engineering Volume: Going Small to Get Big

DiLeo’s team built electrode models that replaced the traditional graphite with amorphous silicate -- a material whose active particles can store 10 times more lithium ions. That, by itself, greatly boosted the electro-chemical performance of the batteries; the silicate-based batteries have a much higher energy density.

 “The problem then, was that the silicate particles had this humongous deformation,” DiLeo explained.

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To resolve that problem, DiLeo’s team turned to the micro-structure: the tiny particles that make up the electrode. Could they re-arrange those particles such that their eventual deformation would not cause an outward expansion?

"Instead of random particles distributed and maybe bumping into each other, we created electrodes whose active particles are architected to form a nice neat lattice -- tiny crossbeams, arches, domes, and spirals,” Di Leo explains.

“When stimulated, these materials can exploit elastic instabilities of their geometry. They can buckle, bend, and collapse such that the electrode will neither destruct nor expand. When it wants to expand, it buckles inward into free space. From the outside, it retains a solid shape."

Building on this model, DiLeo and his colleagues from CalTech broadened their scope a bit. Could other materials be reconfigured to enable (and reverse) novel functionalities? Their paper, "Electrochemically Reconfigurable Architected Materials" published in the journal Nature, pointed to that possibility. In it, they presented the ways in which a silicon-coated tetragonal microlattice buckled in a predictable fashion when stimulated by lithiation, an electrochemical process.

Their conclusion – “that a variety of reconfigurational degrees of freedom can be achieved through micro-architecture design” – opened up a very rich line of research for the M3 lab.  

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Beyond Batteries: Tuning a Material's Properties Through Electrochemistry

Rubber – a normally malleable material – will become stiff when charged by a lithium battery. Remove the electric charge, and it will become soft again. DiLeo explains this as the result of electrochemistry: lithiation temporarily alters the underlying lattice structure of the rubber. And this, in turn, alters the rubber’s mechanical properties. Remove the charge, and the rubber will revert to a more malleable state.

Di Leo's research is constructing models that will allow the M3 team to look at the impact that other electrochemical stimuli might have on the mechanical properties of other materials.

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“It could be sodiation lithiation, or any number of processes,” DiLeo points out. "Once again, it's that meeting of chemistry and mechanics, and we're learning some very interesting things."

One of the properties that DiLeo’s group is focusing on is wave propagation. In a perfectly formed lattice, an external stimulus such as a lithium charge, might travel throughout the material. Or not. Differently architected lattices can prevent waves of a certain frequency from passing through various zones. DiLeo points out that these 'wave band gaps' create unique opportunities for his team. The M3 lab is particularly interested in the wave gaps that form in the amorphous silicate electrodes when a battery is lithiated.

"We don't have this yet, but we are envisioning a nanoarchitected material that has tunable wave gaps. This means you could tune it so that the electro-chemical waves no longer pass through certain sections. You might want to do this to stop the degradation of a material or to stop the degradation of a material that is behind it."

Finding the Chemical-Mechanic Connection in Hydrogen Embrittlement

Another grant from the US Air Force Academy's Center For Aircraft Structural Life Extension (CAStLE) is allowing DiLeo  and Prof. Josh Kacher from Georgia Tech's School of Materials Science Engineering to probe the chemical and mechanical properties at play in the hydrogen embrittlement of corroded metals. Embrittlement occurs when diffused hydrogen atoms collect along the boundaries of a metal’s component crystals. Eventually, this weakens those boundaries, causing it to crack or fracture under minimal stress.

Their proposal, "Experimentally Validated Numerical Frameworks for Understanding and Predicting Microstructural Effects on Environmental Induced Cracking of Aluminum Magnesium Alloys" outlines a four-year study that should produce some very practical analysis tools for countless microstructures that could be produced in the future.

“We don’t know exactly what’s going on with environmental induced cracking -- why it's degrading -- but  we do know it  degrades the mechanical properties, causing the material to fail. And this is important, because when we send a ship out into the ocean, and these little corrosion pits form in the metal that's exposed to the ocean, those corrosion pits are susceptible to hydrogen embrittlement. We know it's going to fail."

Forestalling that failure through the manufacture of more resistant microstructures is the goal of the collaboration: Catcher is performing the experiments and Di Leo is simulating those same experiments to develop an evidenced-based theory that can vary the inputs and predict the results.

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Di Leo's team is building models that simulate different microstructure properties - like grain distribution, grain volume, and grain size - and predicts their susceptibility to hydrogen embrittlement.

"If we can tell a metallurgist what size particles are optimal, then they can change their casting process to create that-sized crystal," Di Leo explains.

Thus Catcher and Di Leo will begin to identify what properties are working in different alloys. When they run a computer-generated mesh microstructure through Di Leo's numerical framework, they can compare the results to Catcher's experiment's to validate the framework.

"The output of this process will be a calibrated theory that, for any given microstructure, will tell you what you want to know, which is 'how it will become susceptible to environmental cracking- hydrogen embrittlement?'" he said. "And then what we're going to take that theory and  we're going to feed it computer-generated microstructures to get an assessment of each microstructure's resistance to environmental cracking.

"Ultimately, they all fail, but when we have more data, we'll be able to leverage it to create less susceptible alloys and forestall that failure."

 

 

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