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A novel formulation to explain heat propagation

Posted By Graphene Council, Thursday, February 13, 2020
Researchers at EPFL and MARVEL have developed a novel formulation that describes how heat spreads within crystalline materials. This can explain why and under which conditions heat propagation becomes fluid-like rather than diffusive. Their equations will make it easier to design next-generation electronic devices at the nanoscale, in which these phenomena can become prevalent.

Fourier's well-known heat equation describes how temperatures change over space and time when heat flows in a solid material. The formulation was developed in 1822 by Joseph Fourier, a French mathematician and physicist hired by Napoleon to increase a cannon's rate of fire, which was limited by overheating.

Fourier's equation works well to describe conduction in macroscopic objects (several millimeters in size or larger) and at high temperatures. However, it does not describe hydrodynamic heat propagation, which can appear in electronic devices containing materials such as graphite and graphene.

One of these heat-propagation phenomena is known as Poiseuille heat flow. This is where heat propagates within a material as a viscous-fluid flow. Another phenomenon, called "second sound," takes place when heat propagates in a crystal like a wave, similar to the way in which sound spreads through the air.

Since these phenomena are not described by Fourier's equation, until now researchers have analyzed them using explicit microscopic models, such as the Boltzmann transport equation. However, the complexity of these models means that they cannot be used to design complex electronic devices.

This problem has now been solved by Michele Simoncelli, a PhD student at EPFL, together with Andrea Cepellotti, a former EPFL PhD student now at Harvard, and Nicola Marzari, the chair of Theory and Simulation of Materials in the Institute of Materials at EPFL's School of Engineering and the director of NCCR MARVEL. They showed how heat originating from the atomic vibrations in a solid can be described rigorously by two novel "viscous heat equations", which extend Fourier's law to cover any heat propagation that is not diffusive.

"These viscous heat equations explain why and under which conditions heat propagation becomes fluid-like rather than diffusive. They show that heat conduction is governed not just by thermal conductivity, as described by Fourier's law, but also by a second parameter, thermal viscosity," says Simoncelli.

This breakthrough, published in Physical Review X, will help engineers design next-generation devices, particularly those that feature materials such as graphite or diamond in which hydrodynamic phenomena are prevalent. Overheating is the main limiting factor for the miniaturization and efficiency of electronic devices, and in order to maximize efficiency and predict whether a device will work - or simply melt - it is crucial to have the right model.

The results obtained by EPFL's team are timely. From the 1960s until recently, hydrodynamic heat phenomena had only been observed at cryogenic temperatures (around -260oC) and were therefore thought to be irrelevant for everyday applications. Already in 2015 Marzari and his colleagues predicted that this would be very different in two-dimensional and layered materials - a prediction that was confirmed with the publication in Science of pioneering experiments that found second-sound (or wavelike heat propagation) in graphite at temperatures around -170oC.

The formulation presented by the EPFL researchers yields results that line up closely with those experiments. Most important, they also predict that hydrodynamic heat propagation can also happen at room temperature, depending on the size and type of material.

Through their work, the EPFL researchers are providing new and original insight into heat transport, but also laying the groundwork for an understanding of shape and size effects - not only in next-generation electronic devices but also in "phononic" devices that control cooling and heating through engineered superstructures. Finally, the novel formulation can also be adapted to describe viscous phenomena involving electrons discovered in 2016 by Philip Moll, now a professor at EPFL's Institute of Materials.

Tags:  Andrea Cepellotti  Electronics  EPFL  Graphene  Michele Simoncelli  Nicola Marzari  Philip Moll  photonics 

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Directed evolution builds nanoparticles

Posted By Graphene Council, Thursday, March 7, 2019
Updated: Friday, March 1, 2019

The 2018 Nobel Prize in Chemistry went to three scientists who developed the method that forever changed protein engineering: directed evolution. Mimicking natural evolution, directed evolution guides the synthesis of proteins with improved or new functions.

First, the original protein is mutated to create a collection of mutant protein variants. The protein variants that show improved or more desirable functions are selected. These selected proteins are then once more mutated to create another collection of protein variants for another round of selection. This cycle is repeated until a final, mutated protein is evolved with optimized performance compared to the original protein.

Now, scientists from the lab of Ardemis Boghossian at EPFL, have been able to use directed evolution to build not proteins, but synthetic nanoparticles (Chemical Communications, "Directed evolution of the optoelectronic properties of synthetic nanomaterials").

These nanoparticles are used as optical biosensors – tiny devices that use light to detect biological molecules in air, water, or blood. Optical biosensors are widely used in biological research, drug development, and medical diagnostics, such as real-time monitoring of insulin and glucose in diabetics.

“The beauty of directed evolution is that we can engineer a protein without even knowing how its structure is related to its function,” says Boghossian. “And we don't even have this information for the vast, vast majority of proteins.”

Her group used directed evolution to modify the optoelectronic properties of DNA-wrapped single-walled carbon nanotubes (or, DNA-SWCNTs, as they are abbreviated), which are nano-sized tubes of carbon atoms that resemble rolled up sheets of graphene covered by DNA. When they detect their target, the DNA-SWCNTs emit an optical signal that can penetrate through complex biological fluids, like blood or urine.

Using a directed evolution approach, Boghossian’s team was able to engineer new DNA-SWCNTs with optical signals that are increased by up to 56% – and they did it over only two evolution cycles.

“The majority of researchers in this field just screen large libraries of different materials in hopes of finding one with the properties they are looking for,” says Boghossian. “In optical nanosensors, we try to improve properties like selectivity, brightness, and sensitivity. By applying directed evolution, we provide researchers with a guided approach to engineering these nanosensors.”

The study shows that what is essentially a bioengineering technique can be used to more rationally tune the optoelectronic properties of certain nanomaterials.

Boghossian explains: “Fields like materials science and physics are mostly preoccupied with defining material structure-function relationships, making materials that lack this information difficult to engineer. But this is a problem that nature solved billions of years ago – and, in recent decades, biologists have tackled it as well. I think our study shows that as materials scientists and physicists, we can still learn a few pragmatic lessons from biologists.”

Tags:  Ardemis Boghossian  biosensors  DNA  EPFL  Graphene  nanomaterials  optoelectronics 

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