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Meeting Paris Agreement requires asset stranding

Discover why meeting the Paris Agreement on climate change requires asset stranding in this video abstract in Environmental Research Letters (ERL) from Alexander Pfeiffer, Cameron Hepburn, Adrien Vogt-Schilb and Ben Caldecott. “Our results can help companies and investors re-assess their investments in fossil-fuel power plants, and policymakers strengthen their policies to avoid further carbon lock-in,” write the researchers in ERL. ERL comes to you from Physics World parent IOP Publishing.

Video courtesy CC-BY 3.0, Alexander Pfeiffer, Cameron Hepburn, Adrien Vogt-Schilb and Ben Caldecott 2018 Committed emissions from existing and planned power plants and asset stranding required to meet the Paris Agreement Environ. Res. Lett. 13 054019 10.1088/1748-9326/aabc5f

 

Quantifying changes in carbon stocks and forest structure from Amazon degradation

Discover more about quantifying long-term changes in carbon stocks and forest structure from Amazon forest degradation in this video abstract published in Environmental Research Letters (ERL) by Danielle I Rappaport, Douglas C Morton, Marcos Longo, Michael Keller, Ralph Dubayah and Maiza Nara dos-Santos. The team combined annual time series of Landsat imagery and high-density airborne lidar data to characterize the variability, magnitude, and persistence of Amazon forest degradation impacts on aboveground carbon density and canopy structure. ERL comes to you from Physics World parent IOP Publishing.

Video courtesy CC-BY 3.0, Danielle I Rappaport, Douglas C Morton, Marcos Longo, Michael Keller, Ralph Dubayah and Maiza Nara dos-Santos 2018 Quantifying long-term changes in carbon stocks and forest structure from Amazon forest degradation Environ. Res. Lett. 13 065013 10.1088/1748-9326/aac331

Niobium tungsten oxides help make faster-charging batteries

Is there an alternative to nanostructuring electrode materials to speed up ionic diffusion in lithium-ion batteries and so increase their maximum power output and charging rates? Yes, according to researchers at the University of Cambridge in the UK, who have shown that two complex niobium tungsten oxides can intercalate large amounts of lithium even when the oxides are microns in size. The new work will be important for developing systems that require high power delivery and/or fast charging (think of a mobile phone that could be fully charged in just minutes). It could also have implications for solid-state energy storage for example, in electric cars, and grid-scale storage for solar power.

Ubiquitous in portable electronics, rechargeable lithium-ion batteries consist of two electrodes – anode and cathode – separated by an electrolyte. When the battery is being charged with electrical energy, lithium ions move from the cathode through the electrolyte to the anode, where they are absorbed and stored into the bulk of the anode material.

The maximum power output and the rate at which these batteries can be charged and discharged is typically limited by solid-state ionic transport in the electrodes. To speed up ionic diffusion and so reach high power and rapid charging, the size of the active particles in the electrodes is often reduced to the nanoscale – so that the lithium ions have a shorter distance to travel. Nanoparticles are difficult to pack together, however, which affects the battery’s volumetric energy density. They are also difficult to process and can undergo unwanted chemical reactions with the electrolyte, so ultimately reducing battery life and increasing cost. 

“Nanoscaling and nanostructuring are simply not needed”

A team of researchers led by Clare Grey has now found that two complex “block” or “bronze-like” niobium tungsten oxides (Nb16W5O55 and Nb18W16O93) can intercalate large quantities of lithium at high rates even when the particles are as big as 30 microns in size. Measurements of how lithium ions move through the materials (using a technique called pulse field gradient NMR) point to room-temperature diffusion rates that are several orders of magnitude higher than those in typical electrode materials such as Li4Ti5O12 and LiMn2O4. This rapid solid-state lithium transport in the bulk materials leads to extremely high volumetric capacities and rate performance so nanoscaling and nanostructuring are simply not needed, says study lead author Kent Griffith.

Most conventional negative electrodes in lithium-ion batteries are made of graphite, which has a high energy density, explains Grey. However, when charged at high rates, spindly lithium fibres known as dendrites form. These can create a short circuit and cause the batteries to catch fire, or even explode.

“In high-rate applications, safety is a bigger concern than under any other operating circumstances,” she says. “These materials and potentially others like them, would definitely be worth looking at for fast-charging applications where you need a safer alternative to graphite.”

“One of the most important aspects of our work is the discovery that energy storage material candidates with unconventional structures can boast exceptional performance,” Griffith tells Physics World. “This means that complex, relatively unexplored regions of composition/structure phase space may hold promise for next-generation devices. In particular, breaking away from the often-held belief that nanoscaling is mandatory to achieving high-rate storage may help the transition from new materials to applications.”

Higher densities at high discharge/charge rates

One of the reported niobium tungsten oxides is built from 4 x 5 blocks of (Nb,W)O6 octahedra that are connected along shear planes, he adds. The other, the “bronze-like” phase, is held open by pillars of oxygen that prop open the atomic layers.  Both compounds are easily synthesized and allow lithium ions to quickly move through them in 3D. Their architectures also make them more rigid than other battery materials.

“They may be able to safely offer higher densities at high discharge/charge rates and could thus be used in systems requiring high power delivery and/or fast charging, possibly in combination with lower rate, high-energy-density batteries,” he says. The fact that they can readily be produced without any additional chemicals or solvents is another point in their favour.

The team, which includes researchers from the Advanced Photon Source at the Argonne National laboratory in the US and the Diamond Light Source in Didcot in the UK, says that it is now busy conducting further electrochemical tests on niobium tungsten oxides and optimizing them for specific applications. “We will also be applying the design principles and advanced characterization techniques employed in this work to unearth new material compositions and chemistries,” says Griffith.

The research is detailed in Nature 10.1038/s41586-018-0347-0.

Black hole hologram appears in a graphene flake

Much research on black holes is theoretical since it is difficult to make actual measurements on real black holes. Such experiments also need to be undertaken over decades or longer. Physicists are therefore keen to create laboratory systems that are analogous to these cosmic entities. New theoretical calculations by a team in Canada, the US, UK and Israel have now revealed that a material as simple as a graphene flake with an irregular boundary subjected to an intense external magnetic field can be used to create a quantum hologram that faithfully reproduces some of the signature characteristics of a black hole. This is because the electrons in the carbon material behave according to the Sachdev-Ye-Kitaev model.

Some of the most important unresolved mysteries in modern physics come from the “incompatibility” between Einstein’s theory of general relativity and the theory of quantum mechanics. General relativity describes the physics of the very big (the force of gravity and all that it affects: spacetime, planets, galaxies and the expansion of the Universe). The theory of quantum mechanics is the physics of the very small – and the other three forces, electromagnetism and the two nuclear forces.

“In recent years, physicists have gleaned important new insights into these questions through the study of the SYK model,” explains Marcel Franz of the University of British Columbia in Canada, who led this research effort. “This model is an illustration of a type of ‘holographic duality’ in which a lower-dimensional system can be represented by a higher dimensional one. In our calculations, the former is N graphene electrons in (0+1) dimensions and the latter the dilation gravity of a black hole in (1+1) dimensional anti-de Sitter (AdS2) space.”

Remarkably, this model accurately describes the physical characteristics of black holes for large values of N (larger than 100 ideally). These characteristics include non-zero residual entropy, and fast scrambling of quantum information at the black hole singularity (the region beyond which not even light can escape the tug of its gravity).

Irregular boundary

Franz and colleagues’ simple experimental realization of the SYK model involves electrons in a graphene flake (a sheet of carbon just one atom thick) that has an irregular boundary. It must be irregular in order to imprint randomness onto the electrons, Franz tells Physics World. “We need this because random structure of electron-electron interactions is the essential requirement of the SYK model.

“Unlike earlier solid-state system proposals to demonstrate this model, our device does not require advanced fabrication techniques and should therefore be relatively straightforward to assemble using existing technologies.”

When a magnetic field B is applied to graphene a number of interesting quantum phases are produced. At a simple (“non-interacting”) level, the field simply reorganizes the single-particle electron states in the material into Dirac Landau levels with certain energies. When the graphene flake is sufficiently small (ideally around 100-200 nm in size), the electrons in the n=0 Landau level are described by the SYK model thanks to the so-called Aharonov-Casher construction.

The team, which includes researchers from Tel-Aviv University in Israel, the Rudolf Peierls Centre for Theoretical Physics in Oxford in the UK, and Microsoft Research in Santa Barbara in the US, says that it is now busy trying to better understand the behaviour of graphene electrons in the SYK regime. “We hope that our theory results will motivate experimentalists to study graphene flakes of the type required to produce SYK physics,” says Franz.

The black hole hologram is described in Physical Review Letters 10.1103/PhysRevLett.121.036403.

Meeting Paris climate goals could see more people hungry

The UN’s Paris Agreement is currently the most comprehensive global effort to limit climate change. But some of the land-use measures that would help meet the agreement’s objectives could slow the decline of global hunger. That’s according to researchers in Japan, who have proposed a set of food security policies.

“People who are involved in climate change research, its policy making, and international negotiation are mainly considering climate change, but more attention should be paid to the unintended consequences of climate change mitigation,” says Shinichiro Fujimori of Kyoto University, Japan.

Rates of hunger in developing nations have declined in recent decades. Long periods of relative political stability along with economic growth have seen the number of people experiencing hunger fall by 184 million since 1990, to some 795 million in 2015, even as population has increased. However, Fujimori and colleagues discovered that certain measures consistent with the objectives of the Paris Agreement could put this trend at risk.

Signed in 2015, the Paris Agreement aims to limit the increase in global average temperature to well below 2 °C above pre-industrial levels. Some of the most important measures that could help meet this goal relate to land use, including re-planting trees in areas recently cleared, and increasing biofuels production. Ultimately, note Fujimori and colleagues, many of these actions would take place on former agricultural land.

With less space available for food production as a result, the researchers predict that an increase in food prices could follow. Without preventative measures, in a 2 °C scenario an additional 84 million people could be at risk of hunger by 2050.

What’s more, naïve mitigation policies such as simply pricing greenhouse gas emissions could increase the cost of agricultural commodities, Fujimori and colleagues write in a paper in Environmental Research Letters, because greenhouse gas emissions generated by their production are penalized. “Carbon pricing without consideration of the effects on specific sectors can be a threat for low-income people,” says Fujimori.

Fujimori and colleagues advocate that the Paris Agreement must incorporate global food security policies to avoid adverse side-effects. Among the measures they suggest are increased international aid from more developed nations, taxes on biofuels, and a reallocation of income to less developed nations to account for a lower income from agriculture.

The team believes these changes would not significantly impact the costs of climate change mitigation. The suggested food security policies would likely require less than 0.1% extra expenditure globally, socioeconomic analysis revealed, compared with a currently-estimated welfare loss of 3.7%. Additionally, international aid would account for just 0.5% or so of high-income nations’ total GDP.

Fujimori hopes that his team’s research will contribute to deliberation about the effects climate change mitigation will have on global society.

“This study is one of the examinations trying to figure out multi-sectoral policy effects,” he says. “This type of research is related to a broader sense of sustainable development and is now highly demanded. We hope that this study’s approach can contribute to stimulating thinking about other sectors’ similar research.”

The team reported the results in Environmental Research Letters.

DoseTracker evaluates motion-induced dose errors during radiotherapy

Danish researchers have developed and validated a four-dimensional (4D) dose reconstruction and evaluation system that tracks radiation dose delivered to abdominal or thoracic tumours affected by motion. The system represents the first real-time, delivery-specific quality assurance (QA) performed at a linear accelerator during radiation delivery to moving targets (Med. Phys. 10.1002/mp.13037).

Performing QA of treatment plans adapted for anticipated tumour motion is a time-consuming process, and does not guarantee accuracy since the tumour motion may differ during treatment. A better approach would be to perform the QA during treatment using the actual tumour motion. As such, the team, from Aarhus University Hospital, developed the system specifically to monitor radiation dose delivery errors caused by tumour motion during radiation delivery.

The system, named DoseTracker, is designed to work with both tracking and non-tracking systems on conventional linear accelerators. DoseTracker is based on a fast motion-including (FMI) dose algorithm, which is a hybrid between a series of point calculations and a pencil beam convolution algorithm.

For dose evaluations, principal investigator Thomas Ravkilde and colleagues used a spatially focused analysis, the γ-test of the cumulative dose, due to its usefulness at all times during beam delivery. They note that while dose-volume histograms (DVHs) could be a more clinically relevant measure, they might not be meaningful for a partially delivered fraction, and could obscure spatial distribution of dose peaks and valleys within a structure.

Summarizing how the system works, the authors explain that DoseTracker receives linac parameters and target positions as unified data protocol (UDP) messages, and reconstructs the transient dose and cumulative dose to a phantom, with the measured motion (the actual dose) and without motion (the planned dose). An independent loop calculates the motion-induced γ error of the latest available cumulative dose distributions by comparing reconstructed doses with and without motion.

Positions of calculation points can be shifted independently between each transient dose reconstruction, to mimic translational, rotational or deformational motion. This capability is needed to ensure the accuracy of motion-including dose reconstruction.

System validation

The authors validated DoseTracker using the original five-field coplanar volumetric-modulated arc therapy (VMAT) plans of five patients with liver cancer, delivered with and without multi-leaf collimator (MLC) tracking to a motion stage carrying a Delta4 dosimeter. The liver motion during the treatments had previously been recorded by kilovoltage intrafraction monitoring (KIM). The motion stage reproduced the KIM-measured tumour motions in 3D, while optical monitoring guided the MLC tracking and provided the position signal to DoseTracker.

The researchers performed 90,560 actual and 90,560 planned dose reconstructions online and in real time, and 5237 γ evaluations, with median computation times of 30 ms and 1.2 s, respectively. The mean difference between reconstructed and measured doses was –1.2% for transient doses and –1.5% for cumulative doses. DoseTracker’s ability to predict γ–fail rates above a given threshold had a mean sensitivity of 96.8% and a specificity of 99.2%.

“DoseTracker accurately predicted the instantaneous validity of the ongoing radiation treatment throughout beam delivery with high frequency and high specificity for all clinically relevant γ–fail rate action thresholds,” the authors write. “Reconstructed doses and dose evaluations were all benchmarked and validated against simultaneous phantom measurements.”

 Ravkilde and colleagues believe that their system will be appropriate for treatments with real-time motion adaptation by gating or tumour tracking and for treatments without motion adaptation. They suggest that DoseTracker will resolve an inherent problem associated with pre-treatment plan-specific QA for MLC tracking, which cannot determine MLC leaf positions of future fractions. DoseTracker would provide a seamless form of in-treatment QA at every fraction.

When used in treatments without motion adaptation, DoseTracker would alert for treatment interruption in cases of large motion-induced errors, or could provide dose-based gating instead of conventional geometrically-based gating.

“We have been preparing for clinical use of the system,” Ravkilde tells Physics World. “During validation and testing, we can use an infrared block meant for use on the patient surface, but this does not provide the internal motion of the tumour in a patient. So we have worked on integration of combined optical and sparse monoscopic imaging with kilovoltage X-rays with DoseTracker.”

The team’s aim is to provide real-time online motion-including dose evaluation during radiotherapy, on a conventional linear accelerator and using only widely available standard equipment. “We also aim to use DoseTracker for real-time QA in clinical trials of tumour tracking where all of the necessary information, including tumour motion, is readily available by definition,” says Ravkilde. The team hopes to reach these goals within a year.

The researchers have also been working to refine the algorithm for real-time calculations on patient anatomy. “Our preliminary work shows good results on patient anatomy, even with very few additions to the original algorithm. The most challenging problem to solve for real-time use on patient anatomy is adequate inclusion of tissue heterogeneity,” Ravkilde says.

‘Oleo Sponge’ mops up oil spills

An easy-to-manufacture sponge-like material, made from a household material, has been shown to effectively clean up oil spills on ocean surfaces. Researchers at the Argonne National Laboratory in the US have used the material, dubbed the “Oleo Sponge”, to completely remove a naturally-occurring sheen of oil, and they now hope to commercialize the technology on a worldwide scale.

Oil spills are a notorious environmental and economic issue, typically leaving a micron-thick sheen of oil on the surface of the water. Previous methods to remove this sheen have included skimming it away using floats, burning it, and collecting oil particles with chemical-based thickening agents. But these methods are largely impractical, since the layer of oil is too thin to be completely removed, and some of the approaches introduce environmental hazards of their own. “Despite the industry’s best intentions, oil spills continue to happen, and existing clean-up methods are surprisingly inadequate,” says Argonne researcher Seth Darling.

The Argonne team set out to find a clean-up technique that is both effective and environmentally friendly, and they explored how polyurethane foam – a material commonly used in padded furniture – could provide a possible starting point. As well as its large interior surface area, polyurethane has the right combination of mechanical properties to enable it to absorb large amounts of oil.

To improve its absorbent properties further, the team used a newly-developed technique called sequential infiltration synthesis – which infuses polymers with inorganic materials through repeated exposure to gaseous precursor molecules. The researchers used the technique to grow a thin layer of metal-oxide primer on the foam’s interior surface. This sponge-like material readily binds to the organic molecules in oil, which should make it a practical and non-invasive solution for capturing oil sheens.

The Argonne researchers tested the material on a naturally-occurring sheen of organic molecules off the coast of southern California. After deploying a two-foot-square Oleo Sponge onto the sheen, emulating an emergency clean-up, the team could see no visible trace of oil on the surface of the water. After the experiment, the sponge was simply wrung out ready for repeated use, while the oil it contained could be disposed of safely.

The Argonne team has already produced Oleo Sponge in small quantities, but they now hope to scale the technology and manufacture much larger quantities for a worldwide market. “This technology has so many applications,” says Darling. “We are excited about the opportunities for other environmental remediation applications and beyond, which makes us that much more motivated to keep working on it.”

Lunar eclipse, most popular time to order take-away and Emmy Noether’s theorem

Tonight marks the longest lunar eclipse this century with the spectacular event being visible for most people except those in North America. A total lunar eclipse is when the Sun, Earth and Moon perfectly line up. The Moon then turns a reddish brown because some of the sunlight going through Earth’s atmosphere is bent around the edge of our planet and falls onto the Moon’s surface. For those in the UK, the eclipse will begin around 9pm BST. For more information, see this BBC story.

You might be thinking to order take-away to enjoy tonight’s lunar spectacle. You might remember last month when we wrote about the physics of pizza, well now a team of scientists at Aberdeen University have analysed a database of Google searches to determine when people are mostly likely to think about ordering food. They looked at food-related queries such as “pizza delivery” or “Chinese delivery” that were made in five countries: Australia, Canada, India, the UK and US. They discovered that there is one particular time when people wanted take-away — 7pm, which was consistent across the five countries. This was then followed by another spike at 2am, which we guess is likely to be university students.

And finally, 23 July marked 100 years since Emmy Noether published her theorem relating conservation laws to symmetries in nature. The Perimeter Institute for Theoretical Physics in Waterloo, Canada, have put together this handy guide that explains the idea for those from kindergarten to PhD.

Composite chitin film could replace plastic packaging

Researchers at the Georgia Institute of Technology have developed a material from two naturally-occurring polymers that could offer an environmentally-friendly alternative to traditional plastic packaging for food and consumer goods. The material is made by layering cellulose – the most abundant biopolymer on Earth – with chitin, which is widely found in shellfish, insects and fungi.

Cellulose is a linear homopolymer of glucose, while chitin is a linear homopolymer of N-acetyl glucosamine. Both biopolymers are found in the hierarchical structures of plant cells and crustacean exoskeletons, respectively, from which nanocrystals and nanofibres can be extracted. Cellulose nanocrystals and nanofibres, and chitin nanowhiskers and nanofibres, have a high Young’s modulus and tensile strength as well as being good barriers to many gases, including oxygen.

The research team, led by Carson Meredith, created films from the two biopolymers by suspending cellulose nanocrystals (CNCs) and chitin nanofibres (ChNFs) – which are negatively and positively charged, respectively – in water, and then spraying them onto a compostable substrate in alternating layers. The processing technique is compatible with roll-to-roll coating.

67% reduction in oxygen permeability

Meredith and colleagues found that the material shows up to a 67% reduction in oxygen permeability compared to some forms of polyethylene (PET), which is one of the most common plastics employed in transparent food packaging today. This improvement comes from the crystalline structure of the nanofibres and means that it could, in theory, keep foods fresher for longer, say the researchers. The most impermeable composites were the ones that were made of three alternating layers of ChNF-CNC-ChNF).

“It’s difficult for a gas molecule to penetrate a solid crystal, because it has to disrupt the crystal structure,” says Meredith. “Something like PET on the other hand has a significant amount of amorphous or non-crystalline content, so there are more paths and it is easier for a small gas molecule to find its way through.”

Scaling up

It is not all plain sailing though, and the researchers say that they now need to develop a scaled-up manufacturing process so that their film is cost competitive with traditional packaging materials. “What is more, while industrial processes to mass produce cellulose are mature, methods to produce chitin are still in their infancy,” explains Meredith. “And, more research is also needed to improve the material’s ability to block water vapour.

Another important point: “Ground, coarse powders, of cellulose and chitin have a generally-recognized as safe (GRAS) status with the US Food and Drug Administration (FDA) and can be used as food additives,” he tells Physics World. “However, some have asked whether a material derived from shellfish would cause allergies in shellfish-sensitive people. In the packaging we propose, a typical shellfish allergy response is unlikely because the chitin is so highly purified. However, the question of whether purified chitin or cellulose fibres could be released from the packaging and induce their own allergy response should be addressed.”

The spray-coated multilayer CNC-ChNF nanofibre film is detailed in ACS Sustainable Chemistry and Engineering 10.1021/acssuschemeng.8b01536.

What is time?

Regular readers of the magazine will be familiar with “Lateral Thoughts” – Physics World’s long-running column of humorous or otherwise offbeat essays, puzzles, crosswords, quizzes and comics, all written by our readers – that appears on the back page each month. For this month’s special issue on the theme of “time – the abiding mystery”, siblings Eugenia Viti and Ivan Viti have crafted a comic that takes a wry look at the physics and metaphysics of time. Eugenia is a cartoonist, illustrator and writer living in Chicago. Follow her on instagram (ayokdit) to see how much pizza she eats. It’s a lot. Ivan has a PhD in physics and is currently working as a postdoctoral researcher at the University of Notre Dame, Indiana. He lives with his wife, two cats, two dogs and one goldfish.

Comic about time
(Comic by Eugenia Viti and Ivan Viti)
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