(Source: Supercond. Sci. Tech/Univ. of Cambridge)
New superconducting material could open the door to commercial viability in some applications

A new superconductor made by an international team of researchers from the University of CambridgeFlorida State University (FSU), and the Boeing Comp. (BA) have bumped the record for strongest superconductor to 17.6 Tesla (T), breaking the previous held record by 0.36 T, or roughly 2.1 percent.  While that may not sound like much, it's the first increase in the mark in more than a decade, with the last record (17.24 T) set in 2003.

The new superconductor is about 550,000 times stronger than the Earth's relatively low-density field, about 3,500 times the density of a fridge magnet, and over the twice as strong as the CMS Magnet of the Large Hadron Collider.  Previously, even small bumps have led to new commercialization prospects for superconductors.  And given the well-documented and carefully thought out manufacturing process, this study has given those looking to commercialize a pretty clear roadmap.

I. Superconductors -- The Road to Discovery

Superconductivity is a phase transition that causes electrical resistance to reduce to zero, as the title suggests.  The phase transition occurs due to quantum effects, but there's a catch -- it can only operate in very pure, homogenous materials and in most known superconductors is only observed at very low temperatures.

Most metals react to form oxides and other compounds.  A notable exception is mercury, which behaves somewhat like a noble gas due to its filled electron shell.  As a result, mercury forms weak bonds and is relatively easy to extract in pure elemental form.  When chilled to 4K it starts to superconduct.

To get there you need something to cool to such a low temperature, which required the invention of the refrigeration cycle.  Gases when they are throttled -- allowed to expand in volume in order to reduce pressure -- see an increase in entropy (disorder) and a decrease in temperature, if the throttling is done in a very well-insulated system.  This is the Joule–Thomson effect.  The cooled gas can then be used to cool another substance -- like your food.

The catch is that you have to expand the gas at below a certain temperature -- its so called "inversion temperature" (Ti), where the gas begins to behave ideally.  For any given gas, this temperature is equal to about 27/4th of the critical temperature (TC), the temperature above which liquid and gas coexist at the same density to form a homogeneous single-phase fluid. 

For diatomic nitrogen at standard pressure (N@ 1 atm), Ti = 621 K, or a little more than twice as hot as room temperature (273.14 K).  For diatomic oxygen (O2 @ 1 atm), Ti = 764 K [source].  So at room temperature air -- composed primarily of oxygen and nitrogen -- can be directly compressed and expanded to make it a liquid and provide cooling.

II. Cooler Than Ice

But where things get tricky is helium.  Its inversion temperature is 51 K.  But scientists devised a way around that -- the Hampson–Linde cycle.  The Curious Physicist has a good overview of how this technology works -- but in short the key trick is to just do basically everything you'd do with the standard Siemens cycle except it's all done on a vapor, which is allowed to slowly liquify in one part of the cycle.

The method was first devised independently by not one, but two researchers in 1895 -- German Professor Carl von Linde of Technische Hochschule München (Univ. of Munich) and William Hampson, a UK lawyer turned self-educated scientist and engineer.
JT Cooler
Another diagram of a cooler using the Linde-Hampson cycle [Image Source: Wikimedia Commons]

A little over a decade after Hampson–Linde cycle came on the scene in the lab, they became available to other researchers.  In 1908 a Dutch physics professor by the name of Professor Heike Kamerlingh Onnes began to experiment with the idea of liquifying helium -- which has a super-cold temperature of around 4 K (with the "around" bit being due to isotopal impurity).  Working in his lab at the University of Leiden he quickly succeeded in becoming the first to liquify helium, recording a temperature of around 4.2 K sometime in 1908.

Professor Onnes
Professor Heike Kamerlingh Onnes [Image Source: Wikimedia Commons]

What he would witness two years later, in 1911, was an amazing, if accidental discovery.  Mercury is in liquid form at room temperature, but freezes at ~234 K.  Like all metals the resistance decreases as temperature decreases.  But in 1911 there was still a lively debate among physicists over whether the conductivity reached some non-zero minima at absolute zero temperature -- as the slope of the resistance vs. temperature plot would suggest.  Others believed that the resistance would drop sharply to zero as you approached absolute zero.

The helium cooling provided a perfect means to solidify a sample of supercooled metal mercury in order to find out which hypothesis was correct.  But at 4.2 K something even more remarkable was witnessed -- the sample showed no electrical resistance at all, something no one had predicted.

In his lab notebook he wrote:

Mercury has passed into a new state, which on account of its extraordinary electrical properties may be called the superconductive state.

Recognizing the importance, in 1911 he published his discovery [PDF] (it's a good read -- especially the part where he recognizes a young Albert Einstein's Planck-vibrator theories were key to the phenomena he was witnessing).  In 1913, for the work he was awarded the Noble Prize in physics. His student Professor Willem Hendrik Keesom -- who he groomed as his successor -- was the first to solidify helium in 1926, the same year his mentor passed away.

The liquification of helium and Professor Onnes' pioneering findings were intriguing, but the only immediate discovery was the superconductance of lead in 1913.  These discoveries shed light on one early promise -- magnetic field generation.  Zero resistance is one properties of superconductors that has led to commercialization.  Current passing through a wire loop creates a magnetic field, a kind of a nonpermanent magnet called a solenoid.  

With superconductors you can channel vast amounts of current through the material.  You're only limited by the creation of thermal instabilities that can heat and melt a material.  Hence superconductors are today used in most of the world's most powerful magnets, including the CMS magnet used in the LHC experiment, the world's strongest magnet.

III. Type II Superconductors, High Temperature Superconducting, and Magnetic Field Storage

A discovery in 1933 by Professor Fritz Walther Meissner and another German scientist -- Robert Ochsenfeld -- discovered the so-called "Meissner effect" -- which holds part of the key to a second kind of superconductivity's commercialization and the one that's directly relevant to the University of Cambridge team's new work.

The potential of the Meissner effect was fully understood nearly a decade layer.  In 1941, niobium nitride was found to superconduct at 16 K.  Researchers would later discover that this material differed from the lead and mercury pure metal superconductors in another way; it could store magnetic fields.  The previously discovered superconductors resisted weak magnetic fields.  When they were exposed to a very strong magnetic field, they underwent a phase transition and broke down.

NbN was a little different.  It would resist weak fields, but as the field strength intensified, it formed so-called "magnetic vortices" trapping the field.  This was incredibly important as it meant that superconductors could store magnetic energy on a thin layer non-superconducting phase on their surface.  The only catch was that if you exposed this kind of superconductor to progressively stronger magnetic fields, it would reach a point where eventually superconductivity broke down as it did in lead and mercury.

Soviet physics Professor Alexei Alexeyevich Abrikosov theorized the existence of Type I and Type II superconductors.  Type I were the non-magnetic ones like solid lead or mercury.  Type II were the ones capable of storing moderately strong magnetic fields on their surface, like NbN.  The theory he applied actually predated the discovery of NbN, so the neat part here was that physicists actually predicted the possibility of these substances before they discovered them.

Superconductors began to see commercial applications in the 1960s and 1970s and a number of compounds and elemental metals were found to superconduct.  In 1986 it was discovered that cuprate oxides doped with certain other metals, including rare earth metals, were capable of acting as Type II superconductors at much higher temperatures.  The first such superconductor to be found was cuprate lanthanum barium copper oxide (BCO) La2-xBaxCuO4 which at a Tc of 35 K shattered the previous record of 23 K.

The race was on, and since even higher temperature cuprate oxides in this class have been found.  In a sort of beautiful symmetry the current record holder is actually a mercury derived BCO -- HgBa2Ca2Cu3O8+x.  It superconducts at below 135 K at ambient (standard) pressure, or at 153 K if you crank the pressure up to 150 kilobar (roughly 148,000 times standard pressure).

In 2008 a team claimed to see superconduction at Tc=185.6 K (-125.6 ºF) in (Sn1.0Pb0.5In0.5)Ba4Tm5Cu7O20 under a much lower pressure -- 1.000 to 1.250 kilobars.  However, claims regarding that indium superconductor remain controversial.

While much glory has been awarded in the temperature race as scientists continue to try to find a room temperature superconductor (or at least one capable of superconducting at the temperature of frozen water), others have focused on commercializing the strengths of Type II superconductors -- lossless current transmission, magnetic field generation, and magnetic field storage.

IV. A Superconductive Silver Lining

The Cambridge team focused their efforts on the latter application.  Their formulation uses a slightly different kind of BCO superconductor is made of gadolinium barium copper oxide (GdBCO) doped with bits of AgO2 -- silver oxide.

Like all BCO superconductors, the metal oxides act as a superconducting quantum lattice at low temperatures, with the other compounds (typically barium and rare earth metals) acting as spacers.

The progress of the superconductor was supervised by Cambridge Professor Yun-Hua Shi -- an expert in the field.  Professor David Cardwell led the overall design and testing effort and was the senior author of a paper on the work.  Professor John Durrell -- a Cambridge research associate -- was the first author of the paper.

A whopping total of 8 other researchers also contributed to the study and the paper on it.  Among them were a researcher from Boeing (the company who also cofunded the study) and FSU Professor Eric Hellstrom.  The growth, assembly of the finished superconductor structures, and testing was performed at FSU's National High Field Magnet Laboratory.

Superconductor disc
One of the prepared superconductor alloy discs. [Image Source: Univ. of Cambridge]

To coax their formulation into reaching the record field strength, the team first used an unusually high silver percentage -- 15 percent by weight.  This allows for an increase in so-called "flux-pinning centers" in the material.  Prof. Shi describes:

The development of effective pinning sites in GdBCO has been key to this success.

Samples of the carefully controlled metal mix were grown using top-seeded melt growth, which preserves single grains of metal, capable of storing much stronger fields.  Metal grain crystals were machined into discs 3 mm thick (about one-third as thick as that smartphone you might have in your pocket) and 24 mm in diameter.  Two discs were stacked with measurement equipment in between.

Superconductor stack
The finished steel-reinforced record setting stack [Image Source: Supercond. Sci. Tech./Univ. of Cambridge]

Aside from the extra silver and growth technique the other trick to breaking the record involved heating a stainless steel 304 ring and affixing it around the stack.  That techniqued allowed a reinforcing prestress of 250 megapascals (MPa) (2500 bar, or roughly 2500 times standard pressure).

V. Storing Over 500,000 Times the Earth's Magnetic Field in Two Tiny Metal Discs

The researchers then set out to find how much field the construct could store.  At 28 K the team exposed the samples to a field strength of 16 Tesla.  When the field was shut off one sample managed to retain a field strength of 15.4 T, while the other cracked, leading to it storing a weaker 10 T.

Looking to get a higher storage, they bumped the temperature next down to 17.6 K.  That proved the trick.  At 17.8 Teslas -- almost 560,000 times the Earth's magnetic field strength/flux density -- one of the samples managed to store 17.6 Teslas, after the source field was deactivated.

Superconducting Graph
[Image Source: Supercond. Sci. Tech./Univ. of Cambridge]

The team continued to study that star stack and found it stored 10 T at 55 K and extrapolating from their data that it could potentially continue to store 17.6 T all the way up to 32 K, with careful controls.

The storage was quite long-term.  Even after about an hour it had only dropped to around 17.5 Tesla -- still more than the previous record.  The field strength was steady for that sample, also, at 16 mm using Hall sensors, where as it dropped off sharply in the broken samples.  That indicated that careful growth is necessary not only to ensure an attractive field storage, but also to ensure that the range of the field is sufficient to make it commercially useful.

The sample was remagnetized and cracked at 18 Tesla at 26 K, leading the researchers to conclude that the achieved storage -- 17.6 Tesla was around the maximum possible before the brittle sample was overcome by stresses.

The team suggested that while the stainless steel ring method was likely unable to be improved upon do to the unparalleled yield strength and stiffness of the 304 alloy, that future samples could maybe store more field if they were internally reinforced (the previous record from Professor Masato Murakami of the Shibaura Institute of Technology in Japan had used a somewhat similar 26 mm two-disc stack of YBa2 Cu3O7-X (YBCO) reinforced with carbon fiber.

So there's at least one clear route with the record setting material to reaching even more impressive storage.

VI. The Final Frontier -- Commercial Energy Storage

The study on the work was published [PDF] in the field's most prestigious journal, Superconductor Science and Technology.

Professor Cardwell says of the work:

The fact that this record has stood for so long shows just how demanding this field really is.  There are real potential gains to be had with even small increases in field.

This work could herald the arrival of superconductors in real-world applications.  In order to see bulk superconductors applied for everyday use, we need large grains of superconducting material with the required properties that can be manufactured by relatively standard processes.

This record could not have been achieved without the support of our academic and industrial colleagues and partners.  It was a real team effort, and one which we hope will bring these materials a significant step closer to practical applications.

And what does all this mean?  Well you could induce current from various was -- solar photovoltaics, steam from nuclear energy driving induced current, etc. -- and then use a superconducting magnet to fill the superconducting stack with intense magnetism, which you could then harvest by using the field to induce electrical current later in the day, once you've stopped producing.

In terms of time to market, clearly we're a ways off, given that the team struggled to have the sample survive a handful of magnetization cycles.  But if the reinforcement and growth techniques could be simplified, this could bump superconductors a lot closer to becoming a cheap, reliable form of power storage, hence eliminating the problems from sporadic power generation methods like solar and wind.  For now we're stuck with batteries, but superconductors like this one loom as a bright and promising solution on the horizon.

Edit: A previous version of the piece referred to the superconductor as doped with gold oxide (AuO2).  It's actually doped with silver oxide (AgO2).  The article has been corrected.

Sources: Superconductor Science and Technology [PDF], University of Cambridge [press release]

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