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As computer processors have continued to shrink down to sizes where billions of transistors are on single chip, heat has increasingly become a bigger factor in their performance. If those CPUs did not get as hot in the first place, then much less energy would be needed to keep them cool. Managing that heat is one of the biggest roadblocks for new devices like computer processors or LEDs.
Computers heat up because the electrons that travel through the processors and circuits generate heat as they move around through, for example, their interaction with lattices. Heat degrades computing performance, so keeping computer processors from getting too hot is why smartphones have a heat sink, or why desktops have fans to blow hot air out. Large data centers with thousands of computers require a lot of additional energy for their high-tech cooling systems.
A Heat Shield Just 10 Atoms Thick To Protect Electronic Devices
Excess heat given off by smartphones, laptops and other electronic devices can be annoying, but beyond that it contributes to malfunctions and, in extreme cases, can even cause lithium batteries to explode. To guard against such ills, engineers often insert glass, plastic or even layers of air as insulation to prevent heat-generating components like microprocessors from causing damage or discomforting users.
Now, Stanford researchers have shown that a few layers of atomically thin materials, stacked like sheets of paper atop hot spots, can provide the same insulation as a sheet of glass 100 times thicker. In the near term, thinner heat shields will enable engineers to make electronic devices even more compact than those we have today, said Eric Pop, professor of electrical engineering and senior author of a paper published Aug. 16 in Science Advances.
The heat we feel from smartphones or laptops is actually an inaudible form of high-frequency sound. If that seems crazy, consider the underlying physics. Electricity flows through wires as a stream of electrons. As these electrons move, they collide with the atoms of the materials through which they pass. With each such collision an electron causes an atom to vibrate, and the more current flows, the more collisions occur, until electrons are beating on atoms like so many hammers on so many bells – except that this cacophony of vibrations moves through the solid material at frequencies far above the threshold of hearing, generating energy that we feel as heat.
UCLA team develops boron arsenide based thermal management solution
With that goal in mind, the UCLA team set out to develop a semiconductor material that is far better at managing heat that current best performing ones. This UCLA team reported for the first time, the experimental realization of boron arsenide free of defects with the highest thermal conductivity (1300 W/mK) among all common semiconductor materials and metals. Heat that concentrates in hot spots in computer chips is quickly dissipated and flushed away because of its unique structural and thermal properties. The new material is three-times more conducting than silicon carbide and copper, the current best materials in use at heat management industry.
In addition to the big technology impact, this study also reveals important physics of thermal transport mechanisms. Thermal properties in solids can be described by the interactions of phonons, i.e. the quantum mechanical modes of lattice vibrations. For many decades, theorists consider that three-phonon process governs thermal transport, and the effects of four-phonon and higher-order processes were believed to be negligible, which actually is the true case for most common materials.
This study makes significant impact to the theory field by showing that high-order anharmonicity through four-phonon process makes important contribution in defect-free BAs single crystals. The conclusion has been supported by their experimental measurement, compared with ab initio calculations from independent research groups and Hu’s group. Furthermore, the study probed the ballistic thermal transport physics and explained the origin of ultrahigh thermal conductivity of BAs due to its long phonon mean free paths.
“This achievement and celebration should go to the whole field,” Hu added, “There are many other leading research groups making progress towards this target. In particular, this success exemplifies the power of combining experiments and ab initio theory in new materials discovery, and I believe this approach will continue to push the scientific frontiers in new materials discovery for many areas including energy, electronics, and photonics applications.”
Thinking about heat as a form of sound inspired the Stanford researchers to borrow some principles from the physical world. From his days as a radio DJ at Stanford’s KZSU 90.1 FM, Pop knew that music recording studios are quiet thanks to thick glass windows that block the exterior sound. A similar principle applies to the heat shields in today’s electronics. If better insulation were their only concern, the researchers could simply borrow the music studio principle and thicken their heat barriers. But that would frustrate efforts to make electronics thinner. Their solution was to borrow a trick from homeowners, who install multi-paned windows – usually, layers of air between sheets of glass with varying thickness – to make interiors warmer and quieter.
“We adapted that idea by creating an insulator that used several layers of atomically thin materials instead of a thick mass of glass,” said postdoctoral scholar Sam Vaziri, the lead author on the paper.
Atomically thin materials are a relatively recent discovery. It was only 15 years ago that scientists were able to isolate some materials into such thin layers. The first example discovered was graphene, which is a single layer of carbon atoms and, ever since it was found, scientists have been looking for, and experimenting with, other sheet-like materials. The Stanford team used a layer of graphene and three other sheet-like materials – each three atoms thick – to create a four-layered insulator just 10 atoms deep. Despite its thinness, the insulator is effective because the atomic heat vibrations are dampened and lose much of their energy as they pass through each layer.
To make nanoscale heat shields practical, the researchers will have to find some mass production technique to spray or otherwise deposit atom-thin layers of materials onto electronic components during manufacturing. But behind the immediate goal of developing thinner insulators looms a larger ambition: Scientists hope to one day control the vibrational energy inside materials the way they now control electricity and light. As they come to understand the heat in solid objects as a form of sound, a new field of phononics is emerging, a name taken from the Greek root word behind telephone, phonograph and phonetics.
“As engineers, we know quite a lot about how to control electricity, and we’re getting better with light, but we’re just starting to understand how to manipulate the high-frequency sound that manifests itself as heat at the atomic scale,” Pop said.
Revolutionizing electronics thermal management technologies with boron arsenide
Professor Yongjie Hu and his group at UCLA MAE, for the first time, experimentally realized a new compound single crystal, boron arsenide (BAs) free of defects and observed the highest isotropic thermal conductivity beyond all metals and semiconductors. This study (Science Magazine, “Experimental Observation of High Thermal Conductivity in Boron Arsenide”) established a new benchmark thermal materials that could potentially revolutionize thermal management technologies in electronics and photonics.
The team of UCLA engineers have developed a thermally ultra-conducting semiconductor material that could dramatically reduce the heating temperature and efficiently remove the waste heat generated by computers and other electronic or photonic devices. It is more effective at drawing heat away from hotspots than any other semiconductor or metal and could potentially revolutionize the current technology paradigms for electronics thermal management. The study was led by Professor Yongjie Hu of mechanical and aerospace engineering, and all other authors are UCLA graduate students of Hu’s research group (H-Lab): Joonsang Kang, Man Li, Huan Wu, and Huuduy Nguyen.
New metamaterial enhances natural cooling without power input
A team at the University of Colorado Boulder (CU-Boulder) in the US developed a new metamaterial film out of glass microspheres, polymer and silver, that provides cooling without needing a power input. Radiative cooling is the natural process through which objects shed heat in the form of infrared radiation. All materials at room temperature emit infrared at wavelengths of 5–15 μm. However, the process is not typically very efficient because it is counteracted by external influences that heat the object, such as sunlight and air currents. Air, meanwhile, absorbs and emits very little radiation with wavelengths 8–13 μm. The Earth cools itself at night by emitting infrared through this “atmospheric window” and into space.
While night-time radiative cooling materials, including a pigment paint, have been successfully developed, a daytime version has proved challenging. The problem is that the materials absorb sunlight, which quickly exceeds the cooling power and instead heats the surface. So the challenge for the CU-Boulder researchers was to create a material that both reflects sunlight and also allows infrared emission.
They created a thin, flexible material with two layers; a sheet of polymer polymethyl pentene containing randomly dispersed silicon-dioxide (SiO2) glass microspheres 8 μm in diameter and a 200 nm-thick silver coating. The combination of the two layers is only 50 μm thick. The polymer-microsphere film is transparent to the whole solar spectrum but radiates infrared. The broad collective resonance among the microspheres ensures the film is highly emissive of infrared within the atmospheric range of 8–13 μm. This property therefore enhances the naturally occurring radiative cooling. Meanwhile, sunlight travels through the metamaterial and is reflected back by the silver coating, which prevents any solar heating.
Researchers also developed low cost production method for the material. “The key innovation of this work is to produce the designed material at scale using the roll-to-roll process,” explains Yang. The researchers used a roll-to-roll extruder to distribute the microspheres in the polymer and a roll-to-roll sputtering machine to apply the silver coating. This means they are able to produce large amounts of the material in mere minutes. “When produced at scale, we estimate that the material cost is only $0.50 per m2 (yes, 50 cents per square metre), since it can be produced at 100 square metres per minute,” adds Yang.
Field tests in Boulder, Colorado and Cave Creek, Arizona, revealed that the film’s average cooling power was more than 110 W/m2 over 72 hours. Even in the midday Sun, its average was 93 W/m2. This is roughly equivalent to the electricity generated by a typical solar panel of the same area. The glass-polymer sheet has many potential cooling applications. By applying it to a solar-panel’s surface, the film could not only cool the panel but also recover an additional one or two per cent of solar efficiency, because overheating hampers the ability to convert solar energy. “That makes a big difference at scale,” says Xiaobo Yin, another researcher on the project.
