What’s the big deal with room temperature superconductivity?


by Sam Mugel

4th of October 2017


Image: cryogenically cooled superconductor levitating in a magnetic field.

When I was doing my undergrad, my professors used to joke that the surest way of obtaining a Nobel prize was to discover a room temperature superconductor.
Superconducting materials have exactly zero resistance to electrical currents, a property that is only possible thanks to a global behavior of its electrons. It is a delicate and inherently quantum mechanical phenomena which has only been achieved in materials cooled way below 0 °C (the current record for superconducting materials at normal pressure is bellow −135 °C).
These materials present exceptional features, among which:

  • Zero resistance to direct current;
  • Extremely high current densities;
  • High sensitivities to magnetic fields, making them precise magnetic sensors;
  • Exclusion of externally applied magnetic fields, useful for magnetic levitation;
  • Close to speed of light signal transmission.

Superconductors constitute a multi-billion dollar market, with applications in electrical power, transportation, medical imaging and wireless communications [1], all of which are possible thanks to state of the art cryogenic cooling techniques, which is nowadays relatively cheap and risk free.

So what is it that room temperature superconductors can achieve which cannot be done with present day superconductors?

This is the challenge that was issued to me and my team during the Quantum Ideas Factory, a three day workshop that took place at Heidelberg University, organized by Selim Jochim and Juris Ulmanis. As I was soon to find out, this seemingly benign open question was to spurt the excitement of students and professors alike and caused me a few sleepless nights.

Highly efficient motors

The electric motor was invented almost 200 years ago by Faraday, and we still use roughly the same architecture today. Typically, it is made of two parts: a conducting loop (the rotor), and a powerful magnet (the stator). An electric power source induces an alternating current in a conducting loop, causing it to rotate in the magnetic field.

Some present day motors include superconducting parts. These are typically lighter than standard motors, and present less losses. Superconducting motors, however, necessitate cryogenic cooling. This results in additional energy expenditure, and perhaps more importantly, extra weight and volume, making them unsuited for almost all small scale applications.
Imagine, however, if both the conducting loop and the magnet of our motor were built with room temperature superconducting materials. This would lead to a highly efficient and lightweight motor, in an extreme scenario, up to half the weight of standard electric motors, and with 60% less losses [2]. There are several applications in which this gain could make all the difference.

Solar powered airplanes

In July 2016, the solar powered airplane Solar Impulse 2 completed its first trip around the world, a feat only possible thanks to the light weight and extreme efficiency of its solar panels and motor. This single passenger airplane weighs a total of 1600 kg, has four engines, and a total of approximately 40-horsepower.

If we imagine replacing its motors by superconducting engines with the same weight, the airplane’s horsepower would go up to 83 hp! Assuming the structure weighs an extra 1000 kg due to the increased wing size and solar panels, we are left with 1400 kg free: enough to carry 9 extra people!

Space elevator

The space elevator is among the most gargantuan projects ever dreamed. It was first imagined in 1895 by Konstantin Tsiolkovsky, and has since become a possibility seriously studied by scientists, backed up by generous NASA funding. The motivation is simple: currently, it costs US$25,000 to bring a kilogram to geostationary orbit. It is estimated that a functioning space elevator could bring this price down to $220 per kilogram [3], opening up unparalleled possibilities for space exploration.
While different designs exist, the most commonly encountered is composed of a cable which is anchored to the equator and reaches out into space. A counterweight is attached at its end, which is subject to a sufficient centrifugal force for the cable to be taut. As a climber ascends it gains angular velocity, such that it can be released from the tower’s top into a in geostationary orbit (35,786 km altitude).

Besides the elevator’s construction, there are two main technical challenges which need to be overcome for this dream to become a reality. The first is of course the cable, which would have to be able to sustain almost 5000 km of its own weight (at sea level) to not collapse. Studies have suggested that carbon nanotubes [4] or diamonds nanothreads [5] might be a potential candidate for cable material.
The second great challenge is the climber which ascends the cable. Edwards estimated that a minimal functioning space elevator could be made of a thin ribbon able to support a ~620 kg climber [6]. The machine needs to be lightweight, while carrying enough fuel for its 35,000 km journey. One way around this problem is to power the climber using solar energy. The solar panels of Solar Impulse 2 have a peak production of 66 kW, enough to power a 35 kW motor, which is roughly the power of a motorcycle engine. Superconducting motors with this power output can be built to weigh around 100 kg [7]. At night, the engine could be powered through wireless power transfer, allowing the climber to complete its ascent in 10 days, with the capacity to carry around 220 kg of equipment and passengers.

Wearable devices

While standard superconductors are incredibly versatile materials, they require constant cryogenic cooling, making them unsuited for applications in the human body. By eliminating the need for cryocooling, we could extent the wide array of existing medical technologies to monitor patients in real time.
Currently, many medical devices use superconducting quantum interference devices (SQUIDs) to monitor the weak magnetic fields in the body for diagnostic. Magnetocardiography, for instance, applies this technology to estimate the risk of heart failure.
By designing SQUIDs which do not require cryocooling, we could build a device to be worn or implanted which would monitor the heart for the type of magnetic activity which is typically observed right before a heart attack. The device could then call for medical attention, drastically improving the patient's chances of survival. A device composed of a few hundred SQUIDs and an integrated circuit could in principle be run off of a watch battery and be no bigger than a pace-maker.
A similar device could be implanted in a patient’s head to detect tell-tale signs of an epilepsy seizure and administer a drug accordingly. This would significantly improve the quality of life of patients suffering from this condition. Alternatively, a circuit such as this one could simply be used to monitor a person’s brain activity in real time for research purposes, or it could be used to send impulses directly to artificial limbs.

Compact integrated circuits

In a much awaited scientific breakthrough, Andrea Caviglia and his colleagues recently designed the first superconducting transistor [8]. This works similarly to a normal transistor: a control, or gate voltage determines whether or not current can flow through an electrical junction. In a superconducting transistor, the junction can be made superconducting or electrically insulating by switching the gate voltage on or off (logical states 1 and 0).
The speed at which a standard transistor can switch is ultimately limited by the junction's resistance [8]. In a superconducting transistor, the channel does not heat up when a current flows through it, allowing it to be dramatically faster than standard transistors.
Due to the need for cryogenic cooling, however, superconducting transistors are not adapted for use in laptops and smartphones. There is therefore a high market barrier for this technology to be adopted. This is a situation where it is unlikely for this fantastic new technology to be available to the general public, unless superconducting transistors were designed which could function at room temperature.
Because superconducting integrated circuits do not dissipate electricity as heat, the need for cooling is eliminated altogether. Naturally, this dramatically reduces the device's energy consumption, as more than 40% of a servers' energy consumption can be dedicated to cooling [9]. This would also allow us to design compact 3D integrated circuits. These devices are formed of layered 2D circuits with a high degree of connectivity. This architecture makes them fast, efficient, and opens new design possibilities [10]. Being incredibly compact, however, means they are ultimately limited by the rate at which they can dissipate heat. Room temperature superconducting integrateds circuits circumvent this problem, and therefore could lead to entirely new designs for future electronic circuits.


References

[1]: Superconductivity, Present and Future Applications. Coalition for the Commercial Application of Superconductors, 2009.

[2]: Development of Ultra-Efficient Electric Motors. Final Technical Report covering work from April 2002 through September 2007.

[3]: The Spaceward Foundation. The Space Elevator FAQ. Mountain View, CA.

[4]: Bradley C. Edwards, Ph.D., The Space Elevator, 2000.

[5]: Julia Calderone, Liquid Benzene Squeezed to Form Diamond Nanothreads, 2014.

[6]: Edwards, Bradley Carl, The NIAC Space Elevator Program. NASA Institute for Advanced Concepts, 2003.

[7]: Oyama, H., Shinzato, T. Development of High-Temperature Superconducting Motor for Automobiles, 2016.

[8]: Paul Marks, First superconducting transistor promises PC revolution, 2008.

[9]: X. Zhang, T. Lindberg, N. Xiong, V. Vyatkin, A. Mousavi, Cooling Energy Consumption Investigation of Data Center IT Room with Vertical Placed Server, In Energy Procedia, Volume 105, 2017, Pages 2047-2052, ISSN 1876-6102.

[10]: J. Knechtel, O. Sinanoglu, I. M. Elfadel, J. Lienig, C. C. N. Sze, Large-Scale 3D Chips: Challenges and Solutions for Design Automation, Testing, and Trustworthy Integration, in IPSJ Transactions on System LSI Design Methodology, vol. 10, pp. 45-62, Aug. 2017

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