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Science Notebook

One step closer to room-temperature superconductors?

Print edition : Jun 12, 2022 T+T-

One step closer to room-temperature superconductors?

A high-temperature (liquid nitrogen cooled) superconductor levitating above a permanent magnet (TU Dresden).

A high-temperature (liquid nitrogen cooled) superconductor levitating above a permanent magnet (TU Dresden). | Photo Credit: Henry Muhlfordt

In the search for room-temperature superconductors, scientists are constantly on the lookout for new materials that show superconductivity at temperatures and pressures that are easily achieved and maintained for real-life applications. Two independent groups of researchers, one Chinese and the other Chinese-plus-American, have discovered a superconducting property in a new class of materials called clathrate metal hydrides. Interestingly, both teams experimentally found a superconducting phase in clathrate calcium hydride (CaH 6) at temperatures of over 200 degrees kelvin (zero degrees C corresponds to 273 degrees K) and under very high pressures.

The first group, from Jilin University, China, led by Liang Ma, published its results in the April 20 issue of Physical Review Letters. The other group, led by Zhiwen Li of the Beijing National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences, reported its findings in the May 23 issue of Nature Communications.

The existence of high-temperature superconducting phases in metal hydrides under very high pressures has been demonstrated earlier. These are “superhydrides” of either rare-earth elements or actinides. A superhydride is a compound that contains an abnormally large amount of hydrogen compared with normal chemical compounds. In 2019, one such compound, lanthanum superhydride (LaH 10), was shown to superconduct at temperatures of up to 260 K but only when subjected to pressures greater than 170 gigapascal (GPa), about 1.7 million atmospheres. ( See also:“Impossible” superconductor synthesised)

The elements in superhydrides act as anchor sites that hold the compounds’ many hydrogen atoms in an arrangement known as clathrate, a cage-like structure. This led researchers to wonder whether hydrides containing main-group elements or other transition metals could also form clathrate structures with superconducting phases at high temperatures.

CaH 6 was of particular interest because in 2012 it was predicted that this material could have superconducting phases at temperatures as high as 220 K or more at a pressure of about 150 GPa.

Earlier attempts to synthesise the compound failed because of obstacles such as high reactivity between calcium and hydrogen, which resulted in hydrides with a low hydrogen content. Ma and colleagues solved this issue by using ammonia borane (BH 3NH 3) as the hydrogen source, which allowed them to synthesise the compound by direct reaction between calcium and hydrogen at a high temperature and pressure. Both the groups adopted almost similar techniques, including the hydrogen source.

Solid-state batteries

Solid-state batteries are soon expected to replace lithium-ion batteries, the staple of smartphones and laptops. But on repeated or excessive use, these next-gen batteries develop thin filaments called dendrites, which can short-circuit the batteries and render them useless. Researchers at the Indian Institute of Science (IISc), Bengaluru, in collaboration with Carnegie Mellon University, Pittsburg, Pennsylvania, have devised a novel strategy to make them last longer and charge faster. The study was published in the June 2 issue of the journal  Nature Materials

Conventional lithium-ion batteries contain a liquid electrolyte (typically made of a lithium salt dissolved in an organic solvent) sandwiched between a positively charged electrode (cathode) made of a lithium compound of a transition metal (such as iron and cobalt) oxide and a negatively charged electrode (anode) made of graphite. When the battery is being charged or is discharging, lithium ions shuttle between the two electrodes. One major safety issue is that the liquid electrolyte can catch fire at high temperatures.

Are lithium ion batteries on the way out?
Are lithium ion batteries on the way out? | Photo Credit: EMMANUAL YOGINI

Solid-state batteries use a solid ceramic electrolyte instead of liquid and a metallic anode made of lithium instead of graphite. In fact, ceramic electrolytes perform better at high temperatures. Lithium is also lighter and stores more charge than graphite, which can significantly cut down the battery cost.   

“Unfortunately, when you add lithium, it forms these filaments that grow into the solid electrolyte, and short out the anode and cathode,” the IISc press release quotes Naga Phani Aetukuri of the Solid State and Structural Chemistry Unit of the IISc, and corresponding author of the study.   

To investigate this phenomenon, dendrite formation was artificially induced by repeatedly charging hundreds of battery cells. Examining thin sections sliced out from the lithium-electrolyte interface under a scanning electron microscope, the team found that long before the dendrites formed, microscopic voids developed in the lithium anode during discharge. The team computed that the currents concentrated at the edges of these microscopic voids were about 10,000 times larger than the average currents across the battery cell, which was likely creating stress on the solid electrolyte and accelerating the dendrite formation. To ensure that voids did not form, the researchers introduced an ultra thin layer of a refractory metal—metals that are resistant to heat and wear such as tungsten and molybdenum—between the lithium anode and solid electrolyte. These metals do not alloy with lithium. “The refractory metal layer shields the solid electrolyte from the stress and redistributes the current to an extent,” said Aetukuri.

The computational analysis carried out by the Carnegie Mellon collaborators showed that the refractory metal layer indeed delayed the growth of the microscopic lithium voids. The researchers said that the findings were a critical step forward in realising practical and commercial solid-state batteries.

Liver transplants

Normothermic (having normal body temperature) machine perfusion (NMP) technologies are emerging as an important technique in organ preservation and transplantation. In a global first, a multidisciplinary research group in Zurich—involving a collaboration between University Hospital Zurich (USZ), ETH-Zurich, and the University of Zurich (UZH)—treated an originally damaged human liver in an NMP machine for three days outside the body and then implanted the recovered organ into a cancer patient in May 2021. One year later, the patient is doing well. The study was reported in the May 31 issue of Nature Biotechnology.

According to the USZ press release, the machine, which was developed in-house, mimics the human body in order to provide ideal conditions for human livers. A pump serves as the heart, an oxygenator replaces the lungs, and a dialysis unit takes over the functions of the kidneys. In addition, numerous hormone and nutrient infusions perform the functions of the intestines and the pancreas. Like the diaphragm in the human body, the machine also moves the liver to the rhythm of human breathing. 

In January 2020, the group demonstrated for the first time that perfusion technology makes it possible to store a liver outside the body for several days. It enables, for example, antibiotic or hormonal therapies or optimisation of liver metabolism. Usually, once livers are removed from their donors, they can be stored on ice for up to 12 hours. This narrow window makes it difficult to match the organs to people needing a transplant, and many donor livers become unusable before a patient can be found. The new technique could allow donor livers to be stored for up to 12 days, the scientists believe.

In the case reported in Nature Biotechnology, the donor liver from a 29-year-old woman had been rejected by all other transplant centres because it had a lesion. The new technique allowed time for a biopsy and a successful treatment of the lesion. The recipient was a 62-year-old male with serious liver conditions, including advanced cirrhosis and severe portal hypertension. Once transplanted into his body, the liver began functioning normally within three days. The patient was discharged after 12 days. An assessment a year later found no sign of liver damage, injury, or rejection.

The technology could significantly increase the number of liver transplants.

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