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Home » Page 1404

Passing of a legend

goldhaber.jpg

By Matin Durrani

I recently received a copy of the 15 April issue of the Brookhaven Bulletin – the newsletter of the Brookhaven National Laboratory in the US – which described the forthcoming 100th birthday celebration of the physicist Maurice Goldhaber (right, image courtesy of Brookhaven National Laboratory).

The birthday bash took place on 18 April, as planned, so I was so sad to learn, as I did yesterday via the New York Times, that Goldhaber sadly died on 11 May.

Born in Austria on 18 April 1911, Goldhaber was one of the last survivors of the glittering pre-war era that saw so many revolutions in physics.

According to Brookhaven’s online tribute, Goldhaber had worked at the University of Cambridge in the UK with the Nobel-prize-winning physicist James Chadwick, where in 1934 Goldhaber became the first person to measure accurately the mass of the neutron.

After obtaining his PhD from Cambridge in 1936, Goldhaber moved to the US, joining the University of Illinois. He arrived at Brookhaven in 1950, going on to serve as lab director from 1961 to 1973.

In 1957 Goldhaber famously discovered that neutrinos have a left-handed helicity, which means that their intrinsic angular momentum, or “spin”, is in the opposite direction to their momentum. That experiment was cited by Brookhaven historian and Physics World columnist Robert P Crease in his collection of most beautiful experiments of all time

By all accounts, Goldhaber was one of those physicists who saw physics as not just a job but his life. Although he retired in 1985, Goldhaber continued to go in to Brookhaven most days until he was well into his 90s. He won numerous awards and prizes, sharing the Wolf Prize in 1991 with Valentine Telegdi for their “separate seminal contributions to nuclear and particle physics, particularly those concerning the weak interactions involving leptons”. He was also awarded a US National Medal of Science.

Goldhaber was not alone in his love for physics: he was part of a family of four generations of physicists, including his son Fred Goldhaber and brother Gerson.

The climate science rap

By Michael Banks

Well it had to come didn’t it? There have been quite a few science raps over the last few years touching on nuclear physics, the American astronomer Edwin Hubble and even the Large Hadron Collider at the CERN particle-physics lab, so it seems about right there is now one about climate change.

The rap video for I’m a climate scientist was produced by the Australian current affairs television programme Hungry Beast.

Featuring lines such as “climate change is caused by people, Earth unlike Alien has no sequel”, the video features a raft of climate scientists doing their best Beastie Boys impression.

I will let you decide whether using rap as a means of communicating climate science is a worthwhile endeavour.

Positive feedback boosts eye’s ability to see

Researchers in the US have discovered a new feedback mechanism that allows the human eye to be sensitive enough to see small details in a scene while also being able to detect the large contrast between bright and dark objects. The process involves boosting the output of certain light receptors in the retina while damping down others – and could help with the design of digital vision systems.

The retina of the human eye senses light using about 100 million photoreceptors and a host of neuronal tricks to convert light signals into a useful picture of the world. One of the most basic processes to occur in the eye is contrast enhancement, which allows the eye to resolve bright and dark objects. This is done directly in the retina by a negative feedback mechanism between neighbouring light-sensitive cells.

The first line of signal processing occurs in the star-shaped horizontal cells just below the surface of the retina. Each of these cells receives input from about 100 photoreceptors (rods and cones) on the retina. A photoreceptor responds to darkness with a burst of a neurotransmitter called glutamate. This chemical causes the horizontal cell to depolarize, and the resulting voltage change shuts off the signalling channels of other nearby photoreceptors, preventing them from emitting more glutamate. This has the result of isolating and sharpening the original signal within a circle of quiet – allowing the eye to see a dark object on a bright background and vice versa.

This process is called “lateral inhibition” and helps the eye to detect the edges of objects. The downside of this mechanism, however, is that it also reduces the maximum strength of the optical signal. This should theoretically cause a loss of dynamic range and limit the eye’s ability to pick out faint details – something that doesn’t happen in a real eye.

Positive feedback at work

Now Richard Kramer, Skyler Jackman at the University California at Berkeley and colleagues at the University of Nebraska and University of Massachusetts have discovered that the eye is able to pick out fine details using an unexpected positive feedback process. They found that cones exposed directly to glutamate, far from reducing their neurotransmitter production, increased their own production of glutamate four-fold.

Upon further investigation, the team discovered that while the glutamate-mediated change in horizontal cell voltage did have a negative feedback effect, glutamate exposure also caused an additional, more subtle change in the horizontal cells, increasing the number of calcium ions in nearby regions. It’s thought that this triggers an increase of calcium ions within the cones themselves, boosting the production of glutamate very locally: in just the initially firing photoreceptor and perhaps a few of its immediate neighbours. “This recoups the signal strength lost to negative feedback, while preserving edge detection and contrast enhancement,” explained Jackman.

The discovery of a secondary signalling system is particularly surprising because the retina has received a lot of attention from researchers. “The positive feedback circuit is very susceptible to damage, and disappears when the retina is studied using more traditional preparations, involving slices of the retina,” explains Jackman. “This may explain why positive feedback has not previously been observed in such a well studied circuit.”

Feedback circuits and other processing at a cellular level have several advantages for the central nervous system. First, it saves time because picking out important features helps the brain to interpret what it is seeing quickly, and make appropriate decisions. Second, there is an issue of information flow: 100 million photoreceptors are connected to a mere million nerve channels. This requires a slimming-down of data without losing anything important.

Emulating the eye

It is these virtues of the eye that researchers in digital vision want to copy. Neuroscience and visual prosthetics experts Stephen Hicks at the University of Oxford said, “This finding could well have a positive and relatively immediate effect in computer vision for intelligent systems.” He added, “In computer vision we perform a process similar to the eye’s lateral inhibition to identify and enhance the edges in a scene before putting them in context.” He believes that this work will give researchers “new ideas for implementing a fast approximation of the boundaries between objects in a video feed, which would improve everything from robot–human interactions to video surveillance”.

The work is described in PLoS Biology.

Exoplanet seems right for life…or does it?

A planet orbiting a star 20 light-years from Earth could have the right conditions for sustaining life. Simulations carried out by a team of scientists in France suggest that the planet, called Gliese 581d, could harbour liquid water, clouds and rainfall, as well as winds that distribute the heat it absorbs from its star. However, the researchers also admit that the simulations might be wrong and the planet could have little or no atmosphere – or even be cloaked in a thick layer of hydrogen and helium.

First observed in 2007, Gliese 581d is thought by some astronomers to be a rocky planet with a mass at least seven times that of Earth, making it a “super Earth”. It is one of more than 500 extrasolar planets (exoplanets) that astronomers have spotted orbiting stars other than the Sun. However, none of these exoplanets has been shown to be both Earth-like and to orbit within its star’s “habitable zone”, where conditions on the planet would be just right for life to emerge.

Now simulations of the climate on Gliese 581d, which have been carried out by Robin Wordsworth, François Forget and colleagues at the Laboratoire Météorologique Dynamique and the University of Bordeaux, suggest that the exoplanet might be able to harbour life. Indeed, the team describe Gliese 581d as “the first discovered terrestrial-mass exoplanet in the habitable zone”.

Gliese 581d is one of six exoplanets thought to orbit the red-dwarf star Gliese 581. It receives about a third of the energy that the Earth receives from the Sun and is also thought to have a hot side that always faces its star and a cold, dark side. The large temperature difference between the two hemispheres was expected to make it difficult for the planet to sustain the thick atmosphere needed for life.

Atmospheric models

Wordsworth and colleagues simulated conditions on Gliese 581d using a 3D model of the atmosphere that is similar to those used to study the Earth’s climate. These work on the basis that the planet has a climate dominated by the greenhouse effects of carbon dioxide and water, which the researchers think is a reasonable assumption given that the climates of Venus, Earth and Mars are defined by these gases. The resulting simulations suggest that Gliese 581d could have a thick atmosphere – and that it could be warm enough to have oceans, clouds (both water and carbon dioxide) and rainfall.

One key driver towards habitability, according to the researchers, is the red colour of the exoplanet’s parent star. Rayleigh scattering in a planet’s atmosphere usually tends to reflect incoming blue light back into space. However, Gliese 581 emits little blue light and therefore the exoplanet absorbs a greater percentage of its star’s light compared with the Earth and the Sun. Simulations of circulation within the atmosphere suggest that much of this heat could be transported to the dark side of the exoplanet, perhaps preventing the atmosphere there from condensing completely.

If the simulations are correct, conditions on Gliese 581d would be very different to those here on Earth. The dense atmosphere would let little light get to the surface, which would be in a perpetual murky red twilight – according to the researchers.

Or maybe it is not habitable

The team admits, however, that conditions on Gliese 581d could be very different to that described in the simulations. The exoplanet may have little or no atmosphere, thanks to a fierce stellar wind from Gliese 581 during its early years. Alternatively, Gliese 581d could have a thick layer of hydrogen and helium in its atmosphere, which would lead to a much less-hospitable climate.

To gain a better understanding of the exoplanet’s atmosphere, the team has come up with a wish list of spectroscopy measurements of the exoplanet’s atmosphere that it hopes will be performed by astronomers in the future. Although the researchers believe that the measurements are beyond the capability of current ground- and space-based telescopes, the exoplanet’s close proximity to Earth means that the next generation of instruments could shed more light on Gliese 581d.

The research is described in Astrophys. J. Lett. 733 L48.

Cosmic-ray detector blasts off on Space Shuttle

An instrument for detecting cosmic rays – and possibly even dark matter – has finally been lifted into orbit on board the Space Shuttle Endeavour. The Alpha Magnetic Spectrometer (AMS), which is the brainchild of the Nobel-prize-winning physicist Samuel Ting, will soon be installed on the International Space Station (ISS). Ting first came up with the idea for the AMS in the 1990s but a series of setbacks, including the Columbia shuttle disaster in 2003, has led to the mission being continually delayed.

The launch of the AMS also marks the end of an era in space exploration, as this is the final mission of NASA’s Space Shuttle programme – which began with the launch of Columbia in April 1981. The lift-off from Kennedy Space Center in Florida involved celebrations commemorating the 30-year Space Shuttle programme.

Costing $2bn and weighing seven tonnes, the AMS detector uses a 0.15 T cylindrical magnet 1 m in diameter and 1 m in height to sort incoming particles according to their momentum and charge. The direction of bend of the particle tracks through the magnet’s bore depends on whether the particles are matter or antimatter, while the gradient of the bend is determined by their speed. This will allow the detector to distinguish between vast numbers of different types of cosmic-ray particle.

Searching for dark matter

Physicists are particularly interested in high-energy positrons (anti-electrons), which could be produced by collisions of dark-matter particles in the Milky Way. However, the ability of the experiment to detect dark matter is controversial. The magnet inside the detector was supposed to be an 0.87 T superconducting device, which the project’s scientists had spent nearly a decade designing and building. But in 2010 the researchers suddenly decided to revert to the weaker permanent magnet that had been flown on a test flight aboard the Space Shuttle in 1998.

The change was made in response to the decision to extend the lifetime of the ISS to 2020 and perhaps beyond. The superconducting magnet would only have had a three-year supply of liquid-helium coolant, leaving the AMS inoperative for most of the ISS’s lifetime. In addition, tests of the AMS at CERN in early 2010 revealed that the detector heated up more than expected – which would have reduced the time that the helium held out.

Some critics claim that the new configuration will make the experiment less likely to make discoveries such as the detection of dark matter, while others insist that the changes made at such a late stage could make failure more likely.

Seeking strangelets

The AMS could also detect strangelets, which are ultra-dense clumps comprising large numbers of up, down and strange quarks. This new form of matter was first proposed in 1984 by Edward Witten, but has yet to be seen by a succession of experiments. Strangelets could be produced when high-energy cosmic rays strike Earth’s atmosphere. The particles are expected to have a very high mass-to-charge ratio, which means that they should take a nearly straight path through the AMS.

AMS uses a series of silicon sheets positioned one on top of the other across the magnet’s bore to sense the position of particles as they travel through the magnet. To optimize for the replacement magnet as much as possible, the AMS team has shifted two of these planes so that they now lie well outside the magnet’s bore. The AMS researchers claim that the momentum resolution of the new configuration will be within 10% of that possible with the superconducting device.

The team also says that the extended running time of the experiment will allow it to gather about six times more data and boosts its chances of seeing rare cosmic-ray events. In addition, the mission could extend over an entire solar cycle, allowing it to study the effect of the Sun on cosmic-ray fluxes.

Evidence that cosmic rays seed clouds

By firing a particle beam into a cloud chamber, physicists in Denmark and the UK have shown how cosmic rays could stimulate the formation of water droplets in the Earth’s atmosphere. The researchers say this is the best experimental evidence yet that the Sun influences the climate by altering the intensity of the cosmic-ray flux reaching the Earth’s surface.

The now conventional view on global warming, as stated by the Intergovernmental Panel on Climate Change, is that most of the warming recorded in the past 50 years has been caused by emissions of manmade greenhouse gases. But some scientists argue that the Sun might have a significant influence on changes to the Earth’s climate, pointing out that in centuries past there has been a close correlation between global temperatures and solar activity.

However, changes to the Sun’s brightness are believed to have altered temperatures on Earth by no more than a few hundredths of a degree in the last 150 years. Researchers have therefore been investigating ways that the Sun could indirectly modify the Earth’s climate, and one hypothesis, put forward by Henrik Svensmark of the National Space Institute in Copenhagen, posits a link between solar activity and cosmic-ray flux.

According to Svensmark, cosmic rays seed low-lying clouds that reflect some of the Sun’s radiation back into space, and the number of cosmic rays reaching the Earth is dependent on the strength of the solar magnetic field. When this magnetic field is stronger (as evidenced by larger numbers of sunspots), more of the rays are deflected, fewer clouds are formed and so the Earth heats up; whereas when the field is weaker, the Earth cools down.

Building clouds

The latest experiment provides evidence for a major component of this theory – how ionization enhances cloud formation. To be converted into droplets and form clouds, water vapour in the Earth’s atmosphere needs some kind of surface on which to condense, and this is usually provided by tiny solid or liquid particles already present in the atmosphere, including aircraft emissions. Svensmark’s theory suggests that cosmic rays can enhance this process by ionizing molecules in the atmosphere that then draw molecules of water vapour to them until the aggregate is large enough to act as a condensing surface.

To reproduce this process in the lab, Svensmark and his colleagues filled a 0.05 m3 stainless-steel vessel with a mixture of gases representing an idealized atmosphere – oxygen and nitrogen plus trace amounts of water vapour, sulphur dioxide and ozone. They then shone ultraviolet light into the vessel in order to generate the sulphuric-acid molecules around which water molecules could aggregate, and irradiated the mixture with a beam of 580 MeV electrons supplied by the University of Aarhus’s ASTRID storage ring.

By removing samples from the vessel and counting the number of gas clusters that measured at least 3 nm across, the researchers found that the beam led to a significant increase in the rate at which clusters were produced. They say that the electrons, like cosmic rays in the real atmosphere, are ionizing molecules in the air and so cause water molecules to stick together. Furthermore, the researchers found that this effect also took place when they used a radioactive sodium source, which produces gamma rays, and as such claim that similar measurements in the future will not require expensive accelerators.

Team member Jens Olaf Pepke Pedersen of the National Space Institute at the Danish Technical University explains that to prove the link between cosmic rays and cloud formation, the experiment will need to be carried out for longer in a bigger vessel. This would determine whether the clusters grow to about 100 nm, at which point they would be large enough to act as cloud-condensing nuclei. He says that the chamber being used in the CLOUD experiment at CERN, which has a volume of some 26 m3, might be large enough.

Clouded science

According to Pedersen, if it can be shown that the clusters reach the scale of micrometres, Svensmark’s hypothesis will have been proven. Then, he explains, it would be a question of finding out the significance of the effect. “There is so much that is not known about cloud formation, so it is possible that it could be an important component of global warming,” he says.

However, there are problems with the cosmic-ray hypothesis. One is that although there was a clear correlation between global temperatures and the intensity of cosmic rays reaching the Earth’s surface (as measured by neutron counters) prior to 1970, that correlation has broken down over the last 40 years. Another problem is that a claimed correlation between cosmic rays and global low cloud cover – as revealed in satellite observations – that was put forward by Svensmark to support his theory has been questioned by a number of researchers, who have found that the correlation only holds over specific regions of time and space.

Indeed, Chris Folland, a climate researcher at the UK’s Met Office, says it is not clear to what extent cosmic rays could really enhance cloud formation, given the vast numbers of naturally occurring particulates within the atmosphere that could act as cloud-condensing nuclei. He also says that even if there is a noticeable effect on cloudiness, this effect could be either positive or negative, arguing that cosmic rays might be expected to have a larger affect on higher-altitude clouds, which tend to warm the planet by preventing radiation from escaping into space. “Low-level clouds generally cool the surface climate, but it’s not clear why they should be preferentially affected by cosmic rays,” he adds, “given that there is some effect on overall cloudiness.”

The research has been published in Geophysical Research Letters.

The Big Bang on the big screen

By Michael Banks

If you are in the US and stuck for things to do this weekend, then you might well think about catching the noir film The Big Bang, which is released today.

Starring Antonio Banderas, who plays private detective Ned Cruz, and directed by Tony Krantz, the film features Cruz searching for a missing stripper named Lexie Permisson (played by Sienna Guillory) while contending with unsavoury Russian boxers and brash police detectives.

And the physics connection? Well apart from a café in the film called Planck’s Constant Café, the movie’s resident madman is Sam Elliott, played by Simon Kestral, who, with the help of a particle physicist, has built a proton collider under the New Mexico desert to search for the Higgs boson. The film then sees Cruz heading to the underground “military base” to find Permisson.

From the trailer the physics in the movie seems to be fairly accurate. “In 27 hours I am going to find something that theoretically should exist but no-one has ever seen,” says Kestral. “Funny,” replies Cruz. “That is exactly what I am looking for.”

Before heading off to the nearest cinema, however, you might want to read this less than favourable review of the film in the New York Times, which calls the movie a “jumble of notions tossed into a hat”, with the picture being a “low point for Mr Banderas”.

Well, at least it contains some accurate physics, which probably makes for a change.

N.B. The film is rated R (“under 17, requires accompanying parent or adult guardian”) so take note when watching the above trailer.

Unzipped graphene reveals its secrets

Researchers in the US have made the first precise measurements on the “edge states” of graphene nanoribbons. These states have been predicted to have extraordinary properties and the work could help build improved nanoscale devices in the future.

Graphene is a sheet of carbon just one atom thick and nanoribbons of this material are strips of graphene just nanometres across. Physicists believe that, depending on the angle at which they are cut, such ribbons should have a range of different – and technologically useful – electronic, magnetic and optical properties. These properties include band gaps, such as those found in semiconductors, that do not exist in larger sheets of graphene.

However, until now, scientists have been unable to test these predictions because they could not study the atomic-scale structure at the edges of cut nanoribbons – and therefore ensure their samples have the appropriate edges. This is because as-produced nanoribbons are typically disordered structures with only short stretches of straight edges.

Unzipping carbon

Michael Crommie’s team at the Lawrence Berkeley National Laboratory (LBNL) and the University of California, Berkeley (UCB) has overcome this problem by looking at specially made nanoribbons with smooth edges using a scanning tunnelling microscope (STM). These ribbons were obtained from Hongjie Dai’s group at Stanford University, where they were produced by chemically unzipping carbon nanotubes (rolled up sheets of graphene) – a technique that produces well-ordered, straight edges along the entire length of a nanoribbon.

The researchers discovered that these ribbons support 1D electronic edge states and that electrons in these states are confined to the nanoribbon edge and have an energy gap. “This kind of behaviour has been predicted for many years but never experimentally verified,” Crommie told physicsworld.com.

The LBNL–UCB team began by spin coating the nanoribbons onto clean gold crystals. Next, the scientists cooled the nanoribbon-decorated gold crystals down to 6 K and imaged them with an STM. “We were able to see the atomic-scale structure of the nanoribbons and use the STM to measure the local density of states of the edge states – that is, we measured ‘where’ the electrons are,” explains Crommie. “In other words, by measuring the current at the STM tip at different locations near the nanoribbon edge, we were able to determine the spatial distribution of electrons confined near the edge.”

“Nanoribbon edge states are real”

Research teams around the world have predicted that the novel electronic, optical and magnetic properties of such nanoribbons edges could be exploited, in principle, to make new types of devices – such as spin-valves, nanoribbon switches, detectors and photovoltaics from graphene. “Our new experimental results bolster the pursuit of these applications because we now know that the nanoribbon edge states are real,” says Crommie.

“The work could also help us better understand the basic physics of what happens at the edges of graphene samples”, he adds. Edges are as important and as useful as any other part of graphene, especially as the size of nanostructure-based devices is reduced to atomic length scales. “Understanding graphene edge behaviour, however, has lagged behind other graphene research because of the difficulties of preparing and probing smooth graphene edges,” says Crommie. “Our new results advance our ability to control and characterize graphene-edge nanostructures and so help to push the field forward and spur new ideas and applications.”

Xiaoting Jia of the Massachusetts Institute of Technology, who was not involved in the work, can see its merits. “This work is a big step towards understanding and controlling the unique electronic properties in graphene nanoribbon edges, and opens up many opportunities in the electronics, spintronics and optical applications of graphene nanoribbons,” he says.

Crommie’s team is now interested in modifying graphene edges in different ways – for example through electronic doping. “We want to explore nanoribbon edge behaviour under different conditions, both to test theories regarding behaviour in the materials and to perhaps discover new, unexpected phenomena,” reveals Crommie. “One of our goals is to fabricate nanoribbon devices that allow us to simultaneously probe atomic-scale nanoribbon structure and device performance, and to correlate these properties.”

The results were detailed in Nature Physics 10.1038/nphys1991.

Mysterious ‘superflares’ confound astronomers

By Tushna Commissariat

Most of us with an interest in astronomy would recognize the Crab Nebula in images and videos quite readily. The supernova remnant, first seen on Earth in the year 1054, consists of a super-dense neutron start that spins about 30 times an second, making it a pulsar that swings a beam of radiation towards Earth, like a lighthouse.

NASA’s Fermi Gamma-ray Space Telescope is one of many that look for high-energy radiation sources, and recently the Crab Nebula has caught its eye. The past seven months have seen some rather dramatic variations within the nebula, with Fermi and other telescopes noticing X-ray flares a hundred times brighter than seen ever before.

Since 2009 Fermi has detected several short-lived gamma-ray flares at energies greater than 100 million electron volts (eV), which is much higher than the flares seen before. On 12 April Fermi detected a flare that grew about 30 times more energetic than the nebula’s normal gamma-ray output and about five times more powerful than previous outbursts. On 16 April an even brighter flare erupted, which lasted for a few days before the activity died out.

“These superflares are the most intense outbursts we’ve seen to date and they are all extremely puzzling events,” says Alice Harding of NASA’s Goddard Space Flight Center. “We think they are caused by sudden rearrangements of the magnetic field not far from the neutron star, but exactly where that’s happening remains a mystery.”

When Fermi noted the variances in 2010 it alerted NASA’s Chandra X-ray Observatory, which began routinely monitoring the nebula to identify X-ray emissions associated with the outbursts. When Fermi scientists alerted the astronomers at Chandra about the spike in April, a pre-planned set of observations using the observatory was initiated.

Unfortunately, no clear evidence was seen for correlated flares in the Chandra images, so the reason for the sudden extreme variations is still a mystery. Theorists have deduced that the flares must arise within about one-third of a light-year from the neutron star, but efforts to locate them more precisely have been unsuccessful.

Scientists believe the flares occur as the intense magnetic field near the pulsar undergoes sudden structural changes. Such changes can accelerate electrons to velocities near the speed of light. As these relativistic electrons interact with the magnetic field, they emit gamma rays. To account for the observed emission, scientists say the electrons must have energies 100 times greater than can be achieved in any particle accelerator on Earth. This makes them the highest energy electrons associated with any source within our galaxy.

Take a look at the wonderful video by NASA that shows the changes as seen by Chandra, as well as some spectacular shots of the nebula.

‘Activated’ graphite oxide boosts supercapacitors

Researchers in the US have discovered a new form of carbon produced by “activating” expanded graphite oxide. The material is full of tiny nanometre-sized pores and contains highly curved atom-thick walls throughout its 3D structure. The team has also found that the material performs exceptionally well as an electrode material for supercapacitors, allowing such energy-storage devices to be used in a wider range of applications.

Capacitors are devices that store electric charge on two conducting surfaces separated by an insulating gap – the larger the surface area of the capacitor, the greater its capacity to hold charge. Charging a capacitor requires electrical energy, which is recovered when the device is discharged. Supercapacitors, also known as electric double-layer capacitors or electrochemical capacitors, store more charge thanks to the double layer formed at an electrolyte–electrode interface when a voltage is applied. Although already used in applications such as mobile phones, these devices are currently limited by their relatively low energy storage density compared with batteries.

Now, Rodney Ruoff and colleagues at the University of Texas at Austin and scientists at the Brookhaven National Laboratory, the University of Texas at Dallas and QuantaChrome Instruments have synthesized a new form of porous carbon with a very high surface area. The carbon consists of a continuous 3D porous network with single-atom-thick walls, with a significant fraction being “negative curvature carbon” similar to inside-out buckyballs. The researchers used the material to make a two-electrode supercapacitor with high gravimetric densities of capacitance, energy capacity and power per unit mass. What is more, the team claims that the process used to make this form of carbon can be scaled up to produce industrial quantities of the material.

Expanded with microwaves

Ruoff and co-workers begin by converting samples of graphite into graphite oxide, which they expand using microwaves to generate what they have dubbed “microwave-expanded graphite oxide” (MEGO). The MEGO is then treated with potassium hydroxide so that its surface is covered (or decorated) with the chemical. After heating at 800 °C for about an hour in an inert gas, “activated MEGO” or aMEGO is obtained.

“What is quite surprising is that the [potassium hydroxide] remarkably restructures the carbon so that a 3D porous structure is generated with essentially no edge atoms,” Ruoff told physicsworld.com. “Every wall in the structure is one atom thick and all the carbon atoms there are sp2-bonded.”

The researchers used aMEGO as the carbon for electrodes in a supercapacitor – mixing it with different electrolytes. They obtained “exceptional” gravimetric energy densities that are about four times higher than that of state-of-the-art conventional supercapacitors, for example those based on porous activated carbon, on the market today.

Best BET

The porous carbon produced also has a “BET” (Brunauer–Emmett&nadash;Teller) surface area of up to 3100 m2/g. For comparison, typical activated-carbon materials have BET surface areas in the range of 1000 to 2000 m2

And that is not all: the material is also very stable and continues to work at 97% capacitance even after 10,000 constant current charge/discharge cycles.

“The Texas work shows an important increase in energy capacity on a gravimetric basis, but unfortunately the graphene material has relatively low density. It will be interesting to see if further work yields higher-density materials with corresponding improvements in volumetric energy density,” says John Miller of the capacitor maker JME and Case Western Reserve University, who was not involved in the research.

Ruoff and colleagues are optimistic and now plan to further improve the new carbon and hope to obtain further funding so that they can carry on conducting more fundamental research on generating still better materials based on similar types of structures. “We also hope to optimize performance in other electrical energy-storage systems in parallel,” reveals Ruoff.

The work is reported in Science.

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