In 1911, a Dutch scientist discovered a class of materials which, at temperatures near absolute zero, could conduct electricity with no resistance and therefore zero loss of power. Superconductors can transmit electrical current with absolutely no loss of energy. If scientists can develop them to function at room temperature, they may one day replace anything requiring electricity—including entire power grids. One HTS application is already a reality: stronger research magnets, which scientists use to study materials, disease, complex molecules like oil and proteins, among many other areas.
Conventional superconductors consist of simple metals, such as niobium, lead, or mercury, which become superconducting when cooled to below a characteristic “critical temperature” close to absolute zero—4.2 K in the case of mercury. These became known as Low Temperature Superconductor (LTS) materials. Since the 1960s a Niobium-Titanium (Ni-Ti) alloy has been the material of choice for commercial superconducting magnets. More recently, a brittle Niobium-Tin intermetallic material has emerged as an excellent alternative to achieve even higher magnetic field strength. These Conventional superconductors like niobium-tin and niobium-titanium operate only at temperatures so low that you need expensive (and non-renewable) liquid helium. to create them. That puts a limit on how much you can do with these “low-temperature” superconductors.
A room-temperature superconductor is a material that is capable of exhibiting superconductivity at temperatures around 77 degrees Fahrenheit. Current superconductors work when cooled near absolute zero, and the warmest superconductor, hydrogen sulfide, works at -95 degrees Fahrenheit. Others have claimed to have invented a room-temperature superconductor in the past.
In 1986, J. G. Bednorz and K. A. Müller discovered oxide based ceramic materials that demonstrated superconducting properties as high as 35K. This was quickly followed in early 1997 by the announcement by C. W. Chu of a cuprate superconductor functioning above 77K, the boiling point of liquid nitrogen.
Since then, extensive research worldwide has uncovered many more oxide based superconductors with potential manufacturability benefits and critical temperatures as high as 135K. A superconducting material with a critical temperature above 23.2K is known as a High Temperature Superconductor (HTS), despite the continuing need for cryogenic refrigeration for any application. Scientists continue to hunt for new materials that superconduct under various conditions (in addition to lowering the thermostat, increasing pressure can change a material into a superconducting state).
Last year, two Indian scientists claimed to have made a room-temperature superconductor using particles of gold and silver. Other physicists are using pressurized lanthanum and hydrogen.
One question driving their work is the same question asked by scientists studying any other high-temperature superconductor: How do they work? While scientists have known for decades how low-temperature superconductors work , they don’t yet understand what’s behind high-temperature superconductors.
High Temperature Superconductor (HTS)
These materials called Superconductors have unique properties including, Zero resistance to direct current; Extremely high current carrying density; Extremely low resistance at high frequencies; Extremely low signal dispersion; High sensitivity to magnetic field; Exclusion of externally applied magnetic field; Rapid single flux quantum transfer; and Close to speed of light signal transmission.
Magnesium diboride (MgB2) was discovered to be a high Tc superconductor in 2001. It actually falls somewhere between a low-temperature and high-temperature superconductor. It appears to work, at least in part, like a low-temperature superconductor: via a phenomenon known as Cooper pairs. Scientists at CERN have demonstrated that, when cold, it’s an outstanding conductor, findings that suggest it could be used in long-distance power transportation. “MgB2 is cheap, abundant and lightweight,” said engineering professor Eric Hellstrom, who studies high Tc superconductors at the ASC. “For that combination of properties, there may be a special niche of potential applications.”
New superconducting compounds, however, can operate at much higher temperatures than conventional superconductors. First came BSCCO, shorthand for bismuth strontium calcium copper oxide — or “bisco” (rhymes with Crisco). Within a few years of the Bednorz/Muller discovery, engineers were making wire of it, which is now used in some very high-tech transmission cable, transformers, motors and generators and magnets.
There are different types of BSCCO, depending on its exact chemical composition, including Bi-2223 and Bi-2212. The MagLab is using the latter to build a novel research magnet dubbed the Platypus, a high-field (24-tesla) instrument for nuclear magnetic resonance research. Elsewhere, scientists are working with Bi-2223 to build current leads for research magnets. Both of these forms of BSCCO superconduct well above liquid nitrogen temperatures. Of all classes of high-temperature superconductors, cuprates continue to boast the highest transition temperature — the point below which they are superconducting.
A cousin of BSCCO is ReBCO, another cuprate. In this high-Tc superconductor, the barium, copper and oxygen found in BSCCO is joined by one of the rare earth elements (hence the “Re”).
One variation is YBCO, the Y standing for the rare earth element yttrium. In collaboration with industry partners, MagLab scientists and engineers have done extensive research and development on ReBCO and YBCO, which has a transition temperature of about -181 degrees Celsius. MagLab engineers developed a windable tape out of YBCO and used it in a magnet that broke the world record in 2017 for an HTS coil operating inside a high-field resistive magnet.
A newly available superconducting material – a steel tape coated with a compound called yttrium-barium-copper oxide, or YBCO – has allowed scientists to produce smaller, more powerful magnets. YBCO is used in another novel MagLab magnet. Projected to crush the existing world record for the strongest superconducting magnet when completed in 2017, the 32-tesla magnet will feature about 6 miles of YBCO tape. Like the Platypus, it will combine high-temperature and low-temperature superconducting magnet coils. YBCO is also currently in use in numerous applications, including power transmission cables, motors, generators, bearings and microwave filters for cell phone towers.
Also called ferropnictides, iron-based superconductors (IBSCs) are iron-containing compounds that are relative newcomers to the high Tc family. They feature layers of iron and a pnictide, such as arsenic or phosphorus. Turns out it isn’t, and that IBSCs have properties that are helpful for turning them into current-carrying wires. They also have a respectable transition temperature (although not as high as the cuprates).
Navy files for patent on room-temperature superconductor
Salvatore Cezar Pais , a scientist working for the U.S. Navy has filed for a patent on a room-temperature superconductor, representing a potential paradigm shift in energy transmission and computer systems. The application claims that a room-temperature superconductor can be built using a wire with an insulator core and an aluminum PZT (lead zirconate titanate) coating deposited by vacuum evaporation with a thickness of the London penetration depth and polarized after deposition.
An electromagnetic coil is circumferentially positioned around the coating such that when the coil is activated with a pulsed current, a non-linear vibration is induced, enabling room temperature superconductivity.
“This concept enables the transmission of electrical power without any losses and exhibits optimal thermal management (no heat dissipation),” according to the patent document, “which leads to the design and development of novel energy generation and harvesting devices with enormous benefits to civilization.”
Graphene on the way to superconductivity
Grapheme, the exotic, strictly two-dimensional material conducts electricity well, but is not a superconductor. But scientists at HZB have found evidence that double layers of graphene have a property that may let them conduct current completely without resistance. They probed the band structure at BESSY II with extremely high resolution ARPES and could identify a flat area at a surprising location. Their research is published in Science Advances.
In April 2018, a group at MIT in the U.S. showed that it is possible to generate a form of superconductivity in a system of two layers of graphene under very specific conditions. To do this, the two hexagonal nets must be twisted against each other at a 1.1 degree angle. Under this condition, a flat band forms in the electronic structure. The preparation of samples from two layers of graphene with such an exactly adjusted twist is complex, and not suitable for mass production. Nevertheless, the study has attracted a lot of attention among experts.
But there is one more, much simpler way of flat band formation. This was shown by a group at the HZB around Prof. Oliver Rader and Dr. Andrei Varykhalov with investigations at BESSY II. The samples were provided by Prof. Thomas Seyller, TU Chemnitz. There they are produced using a process that is also suitable for the production of larger areas and in large quantities: A silicon carbide crystal is heated until silicon atoms evaporate from the surface, leaving first a single-layer of graphene on the surface, and then a second layer of graphene. The two graphene layers are not twisted against each other, but lie exactly on top of each other.
At BESSY II, the physicists are able to scan the so-called band structure of the sample. This band structure provides information on how the charge carriers are distributed among the quantum-mechanically permitted states and which charge carriers are available for transport at all. The angle-resolved photoemission spectroscopy (ARPES) at BESSY II enables such measurements with extremely high resolution.
Via an exact analysis of the band structure, they identified an area that had previously been overlooked. “The double layer of graphene has been studied before because it is a semiconductor with a band gap,” explains Varykhalov. “But on the ARPES instrument at BESSY II, the resolution is high enough to recognize the flat area next to this band gap.”
“It is an overseen property of a well-studied system,” says first author Dr. Dmitry Marchenko. “It was previously unknown that there is a flat area in the band structure in such a simple well-known system.”
This flat area is a prerequisite for superconductivity, but only if it is situated exactly at the so-called Fermi energy. In the case of the two-layer graphene, its energy level is only 200 milli-electron volts below the Fermi energy, but it is possible to raise the energy level of the flat area to the Fermi energy either by doping with foreign atoms or by applying an external voltage, the so-called gate voltage.
The physicists have found that the interactions between the two graphene layers and between graphene and the silicon carbide lattice are jointly responsible for the formation of the flat band area. “We can predict this behavior with very few parameters and could use this mechanism to control the band structure,” adds Oliver Rader.
For the first time, researchers were able to model the behavior of electrons, which are responsible for superconductors’ ability to conduct electricity. Understanding this puzzling phenomenon, Bansil said, could be the critical step necessary toward designing superconductors that work at room temperature.
The base material for the first superconductor ever discovered is lanthanum copper oxide, which is an insulator, meaning it does not conduct electricity. But current mathematical models for superconductors predict lanthanum copper oxide to be a metal. The problem lies in the inability of these models to explain how the parent compound transitions from an insulator to a metal. The transition occurs when researchers add strontium atoms to the lanthanum copper oxide. “Suddenly it become the world’s best conductor—a superconductor,” Bansil said. For the first time, his team was able to model both the insulating state and the transition to the metallic state when strontium is added to the parent compound, he explained.
John Perdew, a professor of physics and chemistry at Temple University who was not involved in the research, called the paper a “major breakthrough” that could lead to the discovery of new superconducting materials that can operate at higher temperatures. Those materials could eventually enable the creation of energy-saving power lines, Perdew said.