Tikalon Blog by Dev Gualtieri

Thermal Management

December 6, 2021

The only sounds that young computer users associate with their devices are music, audio from podcasts and steaming videos, and the beeps and boops of video games. Early users of desktop computers will remember the sounds of their modems as they connected to the Internet through the telephone system. Early computer professionals will remember the dull drone of the specialized ventilation systems designed to extract the heat from mainframe computers. electronic devices are inefficient, and they generate heat as well as data; fortunately, much less heat today than in the past.

The low voltage field effect transistors that populate the computer chips of today are far more efficient than the junction transistors of the TTL and ECL logic chips of the past. In the 1970s, the next level supercomputer was envisioned as a "hairy, smoking golf ball." It was golf ball sized, since the limitation imposed by the speed of light requires a small dimension for fast transit of data between components. The hair represented the necessary interconnections to the outside world; and, it would be smoking, because of the waste heat generated.

Play it as it lies. Rule No. 13 of the Rules of Golf, jointly governed by The Royal and Ancient Golf Club of St Andrews and the United States Golf Association state, with some exceptions, that a golf ball should be played where it lands without any change. The Rules of Golf also state that a golf ball, which must be spherically symmetric, shall have a diameter of not less than 1.680 inches (42.67 mm). It takes light of speed 299,792,458 meters/second 7.11659 x 10^-11 seconds (71 picoseconds) to travel one golf ball radius (2.1335 x 10^-2 meters).

(Left image, a Wikimedia Commons image obtained from Flickr Golf Pictures. Click for larger image.)

One memorable thermal management experience that I had was with a Linux server that went into thermal shutdown during a lengthy compile. The root cause of this was misalignment of the duct that directed airflow from one of two cooling fans to the CPU heatsink. Handheld computing devices don't need the active cooling of forced air flow, but they do need a way to extract heat from the billions of transistors buried inside computer chips to the relatively cool outside world. Thermal conduction by integrated metals such as copper is only marginally effective. That's why scientists are researching better ways to direct waste heat through circuit chips.

A team of scientists from the University of Chicago (Chicago, Illinois), the University of Illinois at Urbana-Champaign (Urbana, Illinois), the Chalmers University of Technology (Gothenburg, Sweden), and Cornell University (Ithaca, New York) has researched a method to create an anisotropic thermal conductor.[1-2] Anisotropic thermal conductors are characterized by the ratio between the thermal conductivities along a fast direction and a slow direction, and this research demonstrated a thermal anisotropy ratio of 900.[1-2] Such an anisotropic thermal conductor would be useful in channeling heat from specific areas of an integrated circuit to its periphery without overly heating other portions of the chip. This would allow the more rapid transistor switching that increases chip speed but generates more heat.

Artist's representation of an atomic scale anisotropic thermal conductor.

Art, which is far easier to create today because of digital technology, enhances our daily lives, and I've always enjoyed art with a scientific theme.

This image represents the randomly twisted crystalline layer created to produce a thermal conductor with high anisotropy.

(Image by Daniel Spacek / Pavel Jirak) / Chalmers University. Access the neuroncollective.com website for many interesting images of this type.)

Some natural crystalline materials, such as graphite and hexagonal boron nitride, have a thermal conductivity anisotropy with ratios of 340 and 90, respectively, but most materials have anisotropy ratios that are much smaller.[1] One technique for obtaining anisotropy is by creation of inorganic superlattices, but these engineered materials have a room temperature anisotropy of less than 20.[1] Graphite and transition metal dichalcogenides are layered van der Waals materials that have excellent in-plane thermal conductivities. The present study examined methods of significantly decreasing their out-of-plane thermal conductivity while keeping the in-plane thermal conductivity high.[1]

The research team stacked ultra-thin layers of crystalline sheets with successive layers rotated slightly to create a material whose atoms aligned in one direction but not another other.[2] The interlayer rotation was random, and this gave a room-temperature thermal anisotropy for MoS₂ close to 900.[1] The interlayer rotations impede plane-to-plane thermal transport, while the in-plane thermal conductivity is maintained.[1] Says study first author, Shi En Kim, a graduate student at the University of Chicago,

"Think of a partly-finished Rubik’s cube... What that means is that within each layer of the crystal, we still have an ordered lattice of atoms, but if you move to the neighboring layer, you have no idea where the next atoms will be relative to the previous layer - the atoms are completely messy along this direction."[2]

For MoS₂ the through-plane thermal conductivity was reduced to 57 ± 3 mW/m/K, and for WS₂, 41 ± 3 mW/m/K.[1] The measured in-plane thermal conductivity for MoS2 films is close to the crystalline value.[1] When a nanofabricated gold electrode was covered by such an anisotropic films, overheating of the electrode was prevented, and heat was blocked from reaching the device surface.[1] This research was funded by the U.S. Air Force Office of Scientific Research, the National Science Foundation, the Samsung Advanced Institute of Technology, and the Camille and Henry Dreyfus Foundation.[2]

Another study of transition metal dichalcogenide layered van der Waals materials was undertaken by physicists at the Tokyo Metropolitan University (Tokyo, Japan) and the National Institute of Advanced Industrial Science and Technology (Tsukuba, Japan).[3-4] They stacked atomically thin layers into van der Waals heterostructures.[4] They likewise found that a mismatch between layers significantly reduce heat transport in stacks of four-layers.[4]

Heterostructures of alternating layers of molybdenum disulfide and molybdenum diselenide have an atomic mismatch between adjacent layers, and this gave a heat transfer between layers an order of magnitude less than for strongly bound layers made by chemical vapor deposition.[4] vertically stacked MoSe₂-MoS₂-MoSe₂-MoS₂ heterostructures showed the lowest thermal conductivity of 1.5 mW/m/K.[3]

Heat transfer through different four layer heterostructures. The heterostructures were formed by chemical vapor deposition (CVD), annealing of weakly bonded layers, weakly bonded layers, and alternating layers of MoSe₂ and MoS₂

(Created using Inkscape from data in ref. 3.[3] Click for larger image.)

Thermal Management

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