April 13, 2024

A First Complete Roadmap for Elastic Deformation Engineering

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The “map”, or phonon stability limit, is a graphical representation that plots the regions of stability of a crystal as a function of strain. This map helps scientists and engineers determine the conditions under which a material can exist in a given phase and when it can fail or transition to another phase. By analyzing the phonon stability limit, researchers can understand the properties of materials under extreme conditions and design new materials with desired characteristics. Credit: Zhe Shi et al

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The “map”, or phonon stability limit, is a graphical representation that plots the regions of stability of a crystal as a function of strain. This map helps scientists and engineers determine the conditions under which a material can exist in a given phase and when it can fail or transition to another phase. By analyzing the phonon stability limit, researchers can understand the properties of materials under extreme conditions and design new materials with desired characteristics. Credit: Zhe Shi et al

Without a map, it can be virtually impossible to know not just where you are, but where you’re going, and this is especially true when it comes to material properties.

For decades, scientists have understood that although bulk materials behave in certain ways, these rules can break down for materials at the micro and nano scales, and often in surprising ways. One such surprise was the discovery that, for some materials, applying even modest strains—a concept known as elastic strain engineering—to the materials can dramatically improve certain properties, as long as these strains remain elastic and do not relax through plasticity, fracture. or phase transformations. Micro- and nano-scale materials are especially good at retaining strains applied in elastic form.

However, the precise way to apply these elastic deformations (or equivalently, residual stress) to achieve certain material properties was less clear – until recently.

Using a combination of first-principles calculations and machine learning, a team of MIT researchers has developed the first map of how to tune crystalline materials to produce specific thermal and electronic properties.

Led by Ju Li, Battelle Energy Alliance Professor of Nuclear Engineering and professor of materials science and engineering, the team described a framework for understanding precisely how changing a material’s elastic strains can adjust properties such as thermal and electrical conductivity. The work is described in an open access article published in PNAS.

“For the first time, using machine learning, we were able to delineate the full six-dimensional limit of ideal strength, which is the upper limit for elastic deformation engineering, and create a map for these electronic and phononic properties,” Li says. “Now we can use this approach to explore many other materials. Traditionally, people create new materials by altering the chemistry.”

“For example, with a ternary alloy, you can change the percentage of two elements, so you have two degrees of freedom,” he continues. “What we’ve shown is that diamond, with just one element, is equivalent to a six-component alloy, because you have six degrees of elastic deformation freedom that you can adjust independently.”

Small tensions, big material benefits

The paper builds on a foundation established as early as the 1980s, when researchers first discovered that the performance of semiconductor materials doubled when a small elastic strain – just 1% – was applied to the material.

While this discovery was quickly commercialized by the semiconductor industry and is now used to boost the performance of microchips in everything from laptops to cellphones, that voltage level is very small compared to what we can achieve now, says Vannevar Professor Subra Suresh. Bush. of Engineering Emeritus.

In a 2018 Science article, Suresh, Dao and colleagues demonstrated that the 1% voltage was just the tip of the iceberg.

As part of a 2018 study, Suresh and colleagues demonstrated for the first time that diamond nanoneedles could withstand elastic strains of up to 9% and still return to their original state. Later, several groups independently confirmed that microscale diamond can indeed deform elastically by approximately 7% in strain reversibly.

“Once we showed that we could bend diamonds at the nanoscale and create deformations on the order of 9 or 10 percent, the question was: what to do with it,” says Suresh. “It turns out that diamond is a very good semiconductor material… and one of our questions was: If we can mechanically deform diamond, can we reduce the band gap from 5.6 electron volts to two or three? Or can we achieve this all the way to zero, where does it begin to conduct like a metal?”

To answer these questions, the team first turned to machine learning in an effort to get a more accurate picture of exactly how deformation changed the material’s properties.

“Tension is a big space,” explains Li. “You can have tensile strain, you can have shear strain in multiple directions, so it’s a six-dimensional space, and the phonon band is three-dimensional, so in total there are nine tunable parameters. So we’re using machine learning, for the first time, to create a complete map to navigate electronic and phononic properties and identify boundaries.”

Armed with this map, the team later demonstrated how the deformation could be used to drastically alter the diamond’s semiconducting properties.

“Diamond is like the Mount Everest of electronic materials,” says Li, “because it has very high thermal conductivity, very high dielectric breakdown strengths, and very high support mobility. What we have shown is that we can crush Mount Everest of controlled manner. …then we show that through strain engineering you can improve the thermal conductivity of diamond by a factor of two or worsen it by a factor of 20.”

New map, new apps

In the future, the findings could be used to explore a range of properties of exotic materials, Li says, from drastically reducing thermal conductivity to superconductivity.

“Experimentally, these properties are already accessible with nanoneedles and even microbridges”, he states. “And we’ve seen exotic properties like reducing the (thermal conductivity) of diamond to just a few hundred watts per meter-Kelvin. Recently, people have shown that it’s possible to make superconductors at room temperature with hydrides if you compress them to a few hundred gigapascals. , so we find all kinds of exotic behavior once we have the map.”

The results could also influence the design of next-generation computer chips, capable of running much faster and cooler than current processors, as well as quantum sensors and communications devices. As the semiconductor manufacturing industry moves toward increasingly dense architectures, Suresh says the ability to tune a material’s thermal conductivity will be particularly important for heat dissipation.

While the paper could inform the design of future generations of microchips, Zhe Shi, a postdoctoral fellow in Li’s lab and the paper’s first author, says more work will be needed before these chips make it into the average laptop or cell phone.

“We know that 1% voltage can provide an order of magnitude increase in your CPU clock speed,” says Shi. “There are a lot of manufacturing and device issues that need to be resolved for this to become realistic, but I think it’s definitely a great start. It’s an exciting start to what could lead to significant advances in technology.”

More information:
Zhe Shi et al, Phonon stability limit and network thermal conductivity deep elastic deformation engineering, Proceedings of the National Academy of Sciences (2024). DOI: 10.1073/pnas.2313840121

Diary information:
Proceedings of the National Academy of Sciences

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