The first complete map for elastic strain engineering. MIT News

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Without a map, it can be nearly impossible to know not only where you are, but also where you are going, and this is especially true when it comes to material properties.

For decades, scientists have understood that while bulk materials behave in certain ways, those rules can be broken for micro- and nano-scale materials, and often in surprising ways. One of those surprises was the discovery that, for some materials, applying even modest strains—a concept known as elastic strain engineering—changed certain properties dramatically on the materials. Recovery may occur, provided that those strains remain elastic and do not overcome plasticity, fracture, or phase change. Micro- and nano-scale materials are particularly good at elastically sustaining applied strains.

However, how to apply those elastic strains (or equivalently, residual stresses) 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 liThe team, led by Battelle Energy Alliance Professor of Nuclear Engineering and Professor of Materials Science and Engineering, described a framework for understanding precisely how changing the elastic stress on a material can fine-tune properties such as thermal and electrical conductivity. Could. This work is described in an open-access paper PNAS,

“For the first time, using machine learning, we have been able to characterize the full six-dimensional range of ideal strength, which is the upper limit of elastic strain engineering, and generate a map for these electronic and phononic properties, ” Lee says. “We can now use this approach to detect 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 adds. “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 strain freedom that you can freely tune.”

Small Stress, Big Material Benefits

The paper builds on the foundation laid in the 1980s, when researchers first discovered that the performance of semiconductor materials doubled when a small – only 1 percent – ​​elastic strain was applied to the material.

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

in 2018 Science In the paper, Suresh, Dao and colleagues demonstrated that the 1 percent strain was just the tip of the iceberg.

As part of a 2018 study, Suresh and his colleagues demonstrated for the first time that diamond nanoneedles could withstand up to 9 percent elastic strain and still return to their original state. Later, several groups independently confirmed that microdiamonds can indeed be reversibly deformed under tension by about 7 percent.

“Once we showed that we could bend nanoscale diamonds and create strains on the order of 9 or 10 percent, the question was what do you do with it,” Suresh says. “It turns out that diamond is a very good semiconductor material… And one of our questions was, if we could mechanically strain diamond, could we get the band gap down from 5.6 electron-volts to two or three can do? Or can we bring it all the way down to zero, where it starts behaving like a metal?

To answer those questions, the team first turned to machine learning to get a more accurate picture of how stress changed material properties.

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

With that map, the team later demonstrated how strain could be used to dramatically change the semiconductor properties of diamond.

“Diamond is like the Mount Everest of electronic materials,” says Lee, “because it has very high thermal conductivity, very high dielectric breakdown strength, a very large carrier mobility. What we’ve shown is that we can climb the Mount Everest.” ‘Can be crushed in a controlled manner… So we show that by strain engineering you can either improve the thermal conductivity of diamond by a factor of two, or make it worse by a factor of 20.’

New map, new applications

Going forward, the findings could be used to explore many exotic material properties, Li says, ranging from dramatically reduced thermal conductivity to superconductivity.

“Experimentally, these properties are already accessible with nanoneedles and even microbridges,” he says. “And we have seen exotic properties, such as reducing the (thermal conductivity) of diamond to only a few hundred watts per meter-kelvin. Recently, people have shown that you can produce room-temperature superconductors with hydrides if you squeeze them to a few hundred gigapascals, so all kinds of exotic behavior have been found 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 today’s processors, as well as quantum sensors and communications devices. As the semiconductor manufacturing industry moves toward denser and denser architectures, Suresh says the ability to adjust a material’s thermal conductivity will be especially important for heat dissipation.

While the paper could inform the design of future generations of microchips, Zhe Shi, a postdoc in Li’s lab and first author of the paper, says more work will be needed before those chips make their way into the average laptop or cellphone. .

“We know that 1 percent stress can cause an order of magnitude increase in the clock speed of your CPU,” Shi says. “There are a lot of manufacturing and equipment issues that need to be solved to make this realistic, but I think it’s definitely a great start. “This is an exciting start that could lead to significant advances in technology.”

This work was supported by funding from the Defense Threat Reduction Agency, NSF Graduate Research Fellowship, Nanyang Technological University School of Biological Sciences, National Science Foundation (NSF), MIT Vannevar Bush Professorship, and Nanyang Technological University Distinguished University Professorship. ,

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