Lancaster Team Tests LEGO® as Superior Sub-Kelvin Insulator

The Ultra Low Temperature Physics Group at Lancaster University in the UK has achieved the record for coldest LEGOs in the universe by cooling the plastic toys to 4 mK in early December. Dr. Dmitry Zmeev and PhD student Joshua Chawner tested the properties of four LEGO blocks and a LEGO character using Lancaster’s dilution refrigerator, the world’s best performing dilution refrigerator. The results have promising implications for thermal insulation, especially in the construction of quantum computers.

The pair spoke with Cold Facts on January 17 to discuss the unconventional experiment. “Very often our non-physicist friends are surprised when they learn that outer space is actually not the coldest place in the universe. It’s here on Earth, in a physics lab,” said Zmeev, lecturer and Engineering and Physical Sciences Research Council research fellow at Lancaster University. “We were thinking of a way to tell the world, and LEGO is one of the most beloved and famous objects one can think of, so we knew the public would love to see it subjected to these low temperatures.”

“We actually saw a box of LEGOs at the pub,” adds Chawner. “We thought, ‘Hey, maybe we can set a world record while we’re at it. Maybe we can produce the coldest LEGOs in the universe. We can make this figure the world’s first cryonaut.’”

The team turned to the university’s dilution refrigerator built by Professor George Pickett 20 years ago. Professor Pickett is a Fellow of the Royal Society and former head of the physics department at Lancaster University. He has been recognized by Nobel winners for his work in low temperature physics, particularly for his work with superfluid helium-3. His refrigerator holds the record for coldest dilution refrigerator at 1.6 mK. The device utilizes baths of liquid nitrogen and helium to cool the center of the fridge down to 1 K. Liquid helium-3 is mixed with helium-4 and then separated to reach temperatures in the millikelvin range before superfluid helium-3 is added. Adiabatic demagnetization of copper nuclei further cools the fridge down to roughly 100 microkelvins, or 0.0001 K.

The group discovered that the LEGO blocks were remarkable thermal insulators. “I think the result that we got was beyond our expectations,” says Zmeev. “LEGO turned out to be a much better insulator than Vespel or Macor, which are the current best performers—and most expensive.”

“We knew that all plastics are fairly good thermal insulators at low temperatures, but the LEGO bricks were better. The contact area between the individual bricks is actually very small. The incredible precision and intricacy of the LEGO block design improves thermal insulation, yet remains very robust,” says Chawner. “You would know how robust if you’ve accidentally stepped on one with a bare foot.”

Zmeev and Chawner measured the thermal conductivity by simply injecting heat into one side of the four-block LEGO to observe how easily thermal energy flowed in the coldest part of the fridge. The results indicated that the process is not easy at all. Tests demonstrated an effective thermal conductivity of K = (8.7 ± 0.3) × 10 -5 T 1.75±0.02 WK −1 m −1 , meaning 400 nanowatts of power (0.0000004 W) can heat an experimental area of 5 cm2 to over 1 kelvin, without any significant change to the base temperature of the dilution refrigerator. The team published their results in Scientific Reports.

The team says their results have promising implications for low temperature research. “Suppose you need to study a system in a wide range of temperatures from 100 millikelvins to a few kelvins,” Zmeev postulates. “You would normally use a dilution refrigerator for this measurement. It is often the case that there are a few different systems attached to the same dilution refrigerator that you want to measure as well, and you do not want to heat those up, so you cannot change the universal fridge temperature. A thermal insulator allows the changing of the temperature of just one experiment without affecting the others. You can use a range of materials, but nobody tried to use ABS [acrylonitrile butadiene styrene, the common thermoplastic polymer used to make LEGOs] structures before. Now we have shown that it works very well.”

Another potential application is the development of quantum computers, which rely on dilution refrigerators to maintain extremely low temperatures. “Thermal insulators are used in the design of dilution refrigerators to thermally separate the parts of the refrigerator that sit at different temperatures,” Chawner explains. “The better these parts are thermally isolated, the better the performance of the refrigerator as unwanted heat leaks can prevent the fridge cooling down further. A problem we are trying to solve with quantum computing is how to make it more reliable and stable over time. Having a cooler fridge and reducing thermal noise helps increase the stability and reliability of the delicate quantum circuitry used in quantum computers.”

The team is applying their results and continuing to develop materials for similar uses. Namely, they are producing ABS insulators in unique shapes via 3D printers. “We have tried a few different configurations and they all perform fairly well, almost as well as LEGO,” concludes Zmeev. “LEGO is still the winner so far per unit of mass, but 3D printed shapes are more customizable—they can also be retrofitted to current fridge experiments to provide an instant upgrade. We are optimistic about the performance of these samples and are currently preparing a publication about the properties of 3D printed shapes at millikelvin temperatures.”

The Lancaster team has produced an explanatory video to share and showcase their results. Watch it at http://2csa.us/j4