Heat-conducting crystals could help computer chips stay cool

US researchers have created a new way to whisk heat away from the circuitry in a computer’s innards to the outside environment, thus preventing computer chips from overheating and causing permanent damage.

Described in the journal Science, their breakthrough lies in crystals made of boron arsenide — a material that has excellent thermal properties that can effectively dissipate the heat generated in electronic devices.

Most of today’s computer chips are made of the element silicon, a crystalline semiconducting material that does an adequate job of dissipating heat. But silicon, in combination with other cooling technology incorporated into devices, can handle only so much — and as consumers demand smaller, faster and more powerful electronic devices that draw more current and thus generate more heat, the issue of heat management is reaching a bottleneck.

Diamond has the highest known thermal conductivity, at around 2200 watts per meter-Kelvin (W/mK), compared to about 150 W/mK for silicon. But although diamond has been incorporated occasionally in demanding heat-dissipation applications, the cost of natural diamonds and structural defects in manmade diamond films make the material impractical for widespread use in electronics. It is also an electrical insulator and, when paired with a semiconductor device, expands at a different rate than the device does when it is heated.

Now, researchers at The University of Texas at Dallas have collaborated with the University of Illinois at Urbana-Champaign and the University of Houston on a potential solution, growing a semiconducting crystal from two relatively common mineral elements — boron and arsenic. Their boron arsenide crystals were found to have far higher thermal conductivity than any other semiconductors and metals currently in use, including silicon, silicon carbide, copper and silver.

In 2013, researchers at Boston College and the US Naval Research Laboratory predicted that boron arsenide could potentially perform as well as diamond as a heat spreader. One such researcher was David Broido, a theoretical physicist at Boston College and one of the authors of the current paper, who proposed that the combination could yield a high thermal conductivity crystal, defying the conventional theory that ultrahigh lattice thermal conductivity could only occur in crystals composed of strongly bonded light elements, limited by anharmonic three-phonon processes.

But according to Paul Ching-Wu Chu, founding director of the Texas Center for Superconductivity at UH, combining boron with arsenic is a complex challenge. Boron arsenide needs to have a very specific structure and low defect density for it to have peak thermal conductivity, he said, so that its growth happens in a very controlled way.

“The mismatch between the physical properties of boron and arsenic makes the synthesis of boron arsenide extremely difficult and boron arsenide single crystals almost impossible,” said Chu.

In 2015, Dr Bing Lv — then a researcher at UH — successfully produced such boron arsenide crystals, but the material had a fairly low thermal conductivity of around 200 W/mK. Now a faculty member at UT Dallas, Dr Lv has been working for the past three years to optimise the crystal-growing process to boost the material’s performance.

Dr Lv worked with Dr Sheng Li and Xiaoyuan Liu, both from UT Dallas, to create the high thermal conductivity crystals using a technique called chemical vapour transport, in which raw boron and arsenic are placed in a chamber that is hot on one end and cold on the other. Inside the chamber, another chemical transports the elements from the hot end to the cooler end, where they combine and condense into small crystals.

Previous reported efforts to synthesise boron arsenide have yielded crystals measuring less than 500 µm — too small for useful application — but the researchers have since reported growing crystals larger than 4 x 2 x 1 mm. A larger crystal could be produced by extending the growing time beyond the 14 days used for the experiment, they said.

The researchers also had to combine extensive materials characterisation and trial-and-error synthesis to find the conditions that produced crystals of high enough quality. Eventually they succeeded, achieving thermal conductivity up to 1000 W/mK — second only to diamond in bulk materials, according to Dr Lv.

“To jump from our previous results of 200 W/mK up to 1000 W/mK, we needed to adjust many parameters, including the raw materials we started with, the temperature and pressure of the chamber, even the type of tubing we used and how we cleaned the equipment,” he continued.

Dr David Cahill and Dr Pinshane Huang’s research groups at the University of Illinois at Urbana-Champaign played a key role in the work, using electron microscopy and a technique called time-domain thermoreflectance to determine if the lab-grown crystals were free of the types of defects that cause a reduction in thermal conductivity.

“We measure the thermal conductivity using a method developed at Illinois over the past dozen years called ‘time-domain thermoreflectance’ or TDTR,” said Dr Cahill. “TDTR enables us to measure the thermal conductivity of almost any material over a wide range of conditions and was essential for the success of this work.”

“We measured dozens of the boron arsenide crystals produced in this study and found that the thermal conductivity of the material can be three times higher than that of the best materials being used as heat spreaders today,” added Illinois postdoctoral researcher Qiye Zheng.

The tiny crystals of boron arsenide, like the one shown here imaged with an electron microscope, have high thermal conductivity. Image credit: University of Texas at Dallas.

The way heat is dissipated in boron arsenide and other crystals is linked to the vibrations of the material. As the crystal vibrates, the motion creates packets of energy called phonons, which can be thought of as quasiparticles carrying heat. Dr Lv said the unique features of boron arsenide crystals — including the mass difference between the boron and arsenic atoms — contribute to the ability of the phonons to travel more efficiently away from the crystals.

The researchers believe their discovery has the potential to address a range of technological challenges, including cooling electronic devices and nanodevices. The next step in the work will include trying other processes to improve the growth and properties of this material for large-scale applications, Dr Lv said.

“I think boron arsenide has great potential for the future of electronics,” he said. “Its semiconducting properties are very comparable to silicon, which is why it would be ideal to incorporate boron arsenide into semiconducting devices.”

Top image credit: ©stock.adobe.com/au/Ben R