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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 MoS
2 the through-plane thermal conductivity was reduced to 57 ± 3 mW/m/K, and for WS
2, 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-MoS
2-MoSe
2-MoS
2 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.)
References:
- Shi En Kim, Fauzia Mujid, Akash Rai, Fredrik Eriksson, Joonki Suh, Preeti Poddar, Ariana Ray, Chibeom Park, Erik Fransson, Yu Zhong, David A. Muller, Paul Erhart, David G. Cahill, and Jiwoong Park, "Extremely anisotropic van der Waals thermal conductors," Nature, vol. 597 (September 30, 2021), pp. 660-665, https://doi.org/10.1038/s41586-021-03867-8. This is an open access article with a PDF file here.
- Louise Lerner, "UChicago scientists create material that can both move and block heat," University of Chicago Press Release, September 29, 2021.
- Wenyu Yuan, Kan Ueji, Takashi Yagi, Takahiko Endo, Hong En Lim, Yasumitsu Miyata, Yohei Yomogida, and Kazuhiro Yanagi, "Control of Thermal Conductance across Vertically Stacked Two-Dimensional van der Waals Materials via Interfacial Engineering, ACS Nano (Advanced Online Publication, September 29, 2021), https://doi.org/10.1021/acsnano.1c03822. Supporting Information as a 2.3 MB PDF file can be found here.
- Atomic Scale 'lasagna' keeps heat at bay, Tokyo Metropolitan University Press Release, October 23, 2021.
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