Page 597 – Physics World

Home » Page 597

Ultraporous metal-organic frameworks could make clean energy carriers

A new metal-organic framework material based on aluminium can store large amounts of hydrogen and methane at relatively low pressures. The material might be used to carry clean energy in fuel cell-powered vehicles according to the researchers at Northwestern University in the US who developed it.

In 2017, transport vehicles (including cars, trucks, planes, trains and boats) overtook power plants as the largest source of greenhouse gas emissions in the US. The share of transport-based emissions increased still further in 2018, and the trend is expected to continue – making the search for transport-friendly alternative energy sources ever more important.

Methane and hydrogen are often touted as potential replacements for diesel and petrol fuels in vehicles. While methane is considered a “transition” fuel, since its combustion still emits carbon dioxide (albeit less than that of petrol), hydrogen has been hailed as “the fuel of the future” since burning it produces neither carbon dioxide nor particulate-based pollution.

The problem is that because both hydrogen and methane are gases at ambient temperatures, they need to be compressed and kept at high pressures (of 700 bar and 250 bar respectively) whenever they are transported and stored. Since such high pressures can create hazards for drivers and others involved in handling stored fuel, the maximum storage pressure limit for real-world vehicles has been set to 100 bar – significantly reducing the amount of gas that can be stored in a given space.

Storing increased amounts of gas without increasing the pressure

In recent years, researchers have investigated high-surface-area porous adsorbent materials as a means of increasing the amounts of gas that can be stored in a given volume without increasing the pressure. Metal-organic frameworks (MOFs), which have surface areas of 2000m²/g or more, are considered promising candidates. These highly crystalline materials are made up of organic molecules and metal ions or clusters that self-assemble into multidimensional structures, and are easy to design thanks to their tailorable pore chemistry and shape.

The Northwestern team, led by Omar Farha, used molecular simulations to inform the design of ultraporous MOFs based on trinuclear clusters, called NU-1501-M (where M is Al or Fe).

The researchers found that NU-1501-Al boasted high gravimetric (mass) and volumetric (size) storage performances for hydrogen and methane. Indeed, the material proved capable of storing 0.66 g of methane per gram of material at 100 bar and 270K – a value that exceeds the 0.5 g/g target set by the US Department of Energy (DOE) for developing the next generation of clean-energy automobiles. The material also has a high deliverable storage capacity of 14% by weight for hydrogen, which means it can store 14% of its own mass of hydrogen. While this figure seems low compared to its ability to store methane (66% by weight), it again surpasses the DOE target for 2020 of 4.5% by weight.

Storing gases with extreme efficiency in ‘crystalline sponges’

Nanosized pores mean a high surface area for gas adsorption

These high values are possible thanks to the material’s tiny pores, which measure less than 2.5nm across and thus offer a very high surface area for gas adsorption. As Farha notes, a one-gram sample of the material, with a volume equivalent to six M&M candies, has enough surface area to cover 1.3 American football fields.

This extensive surface area means that the team’s MOFs can store “tremendous” amounts of hydrogen and methane within their pores, Farha says, adding that the materials could deliver either gas to a car engine at lower pressures than are needed for current fuel-cell vehicles.

The new MOFs are detailed in Science.

Safe training: the linac simulator

The linear accelerator – or linac – that’s used to deliver radiation treatments to cancer patients is one of the most complicated devices that exists within healthcare. Training medical physicists on how to use such machines requires instruction in the fundamental physics of beam acceleration, and how these principles apply to the clinical device. Trainees also need access to the linac itself, but such systems are only available when not in use for patient treatments. The devices can be easily damaged if set up incorrectly, and risk unwanted radiation exposure if used in the wrong way.

Some years ago, these concerns inspired Marco Carlone, then a medical physicist at the Princess Margaret Cancer Centre in Canada, to develop a linac simulator, aptly named the SIMAC. The simulator is an online teaching tool that’s designed to train people to use medical linacs. The idea is to provide a real-time simulated radiation beam that’s accessible round the clock from any PC, and enables trainees to safely operate a virtual linac without any of the related risks. In October 2019, Carlone founded Linax Technologies to commercialize this simulator.

What was your motivation for developing SIMAC?

Today, linac training is mainly performed by hands-on learning. One of the reasons that I created an online simulator was to allow people to have access to a linac without having to actually be at one physically. Linacs are large, complicated machines and, in many ways, also highly dangerous. If used incorrectly they could expose the trainee to radiation. Another challenge when teaching people how to operate a linac is that if the device isn’t left in the correct state, it could harm a patient afterwards.

An online tool takes these problems away, allowing unsupervised access to linacs. The student doesn’t need to have someone with them to ensure that they are using the device correctly. They can also feel free to experiment with the device and learn how it works by changing parameters that you wouldn’t want to change on a machine that’s used in the clinic.

What made you decide to start a company?

I have a background in linear accelerator engineering, but about 20 years ago, I changed fields into medical physics. It was always apparent to me that there was a gap in approach between the people who design and build linacs, and the people who ultimately use them. The first time that I built a simulated linac environment was about six years ago, with grants from my institution and other sources. That was helpful in getting some people together to work on this, but as soon as the money ran out, the project slowed down quite a bit. At the same time, a lot of people had given me feedback that this approach was very useful. So, I thought it would be helpful to have a more continuous way of keeping the project going and keeping it sustainable through a commercial model.

There was a gap in approach between the people who design and build linacs, and the people who use them

Who are the target users for SIMAC?

At present, the linac simulator is designed for young medical physicists who would like to understand the functioning of the device, while learning how the physics of the accelerator relates to the clinical properties that they would likely see in the clinic. For example, users can see how adjusting the energy of the machine affects its clinical properties, which would be important for things like treatment planning or quality assurance.

One of my main drivers is making linac training and learning available to anyone who wants it. In developing countries, one of the challenges with these highly advanced machines is training people to use them when there isn’t anyone locally who can teach this and access to resources is sometimes difficult. The ability to teach people anywhere in the world is something I’ve been trying to do since this idea dawned on me.

I’m hoping that an online community, through a company like Linax Technologies, will allow people to have access to the materials they need in order to operate very complicated equipment, to create plans for their patients that are safe, and to communicate with anybody they’d like to in order to improve that.

Do you have any advice for someone wanting to commercialize their own research?

The most important thing that I have learned is to focus on the people who are going to use your product – it sounds like a simple thing to say but it’s sometimes difficult to understand. As an example, when I built the simulator, I thought that all medical physicists would be interested in this because they all recognize the importance of linacs and will want to understand how they work. And while that was true, I found that not many people were willing to spend money on it.

So one of the reasons why I’m focusing this on a specific segment – early-career medical physicists – is because those are the people who really need technologies like this, not just for their own knowledge, but also to help them with their career and to be more effective in what they do. It’s important to understand how people are using your product and what they are using it for. That will drive how you make decisions around how you develop it and what to focus on. The other problem with a start-up company is that you have very limited resources, and you have to make sure that you put your energy into things that will be valuable for the company moving forward.

How are you planning to develop SIMAC further?

Ultimately, the goal of Linax Technologies is to simplify radiotherapy. The name of my company is a hybrid of Linux and linac. Linux is an open-source operating system, and in my mind, radiotherapy is really an open-source community. We don’t do anything in radiotherapy unless the whole community endorses it, whether that be through the literature or via conferences. So what I’d like to do with Linax Technologies is provide a means to do that, starting with the linear accelerator, but hopefully expanding to other areas of radiotherapy and making the necessary information available to anyone who wants it.

It’s quite an ambitious goal. I’m starting very small with one core problem – how to make your linac work – but I’m hoping we’ll be able to expand that out into other areas that would be helpful for people, in developing countries or anywhere else.

Electric fields control spintronics devices

A new low-power technique for detecting the spin of electrons in a non-magnetic system could aid the development of spintronics devices that work using ferroelectricity rather than ferromagnetism. Such devices may eventually form the backbone of a new generation of efficient, low-energy computer processors, and thus help maintain progress in high-speed information processing.

For more than half a century, computing power has increased exponentially. Recently, however, this “Moore’s law” growth – named for Intel co-founder Gordon Moore, who predicted in 1965 that the number of transistors per square inch on integrated circuits would double every year for at least a decade – has slowed, as it is becoming increasingly difficult to make conventional transistors smaller than they already are. Researchers are therefore seeking ways of allowing computing power to continue to grow, even as traditional sized-based scaling runs up against fundamental limits.

Among the many solutions being investigated are those that seek to reduce power consumption in the field-effect transistors (FETs) that form the basis of modern silicon computer chips. One way to do this would be to replace conventional transistors with an alternative version that does not require a continuous power supply to maintain its ON or OFF state.

Spintronics circuits are smaller and more efficient

Spintronics technologies, which use electrons’ quantum spin (or intrinsic angular momentum) rather than their charge to store and process information, may offer a way to reach this goal. Since electronic spins can point “up” or “down”, this binary property can be used to perform logical operations in spintronic circuits in much the same way as electric charge is used in electronic circuits. The key advantage is that when an electron’s spin switches direction, its new state is stored permanently (that is, it is “non-volatile”). Hence, spintronics circuits do not require any additional input power for their states to remain stable.

Spintronic circuits do, however, have a major drawback for efficiency advocates. In spintronics, information is carried or manipulated via spin currents, which consists of electrons with opposite spins moving in opposite directions. These currents are usually generated using ferromagnetic materials. That’s a problem, because the magnetization of such materials cannot be switched except by applying very strong magnetic fields or currents. Hence, in any practical, switchable device, the energy advantage associated with non-volatile storage would quickly be wiped out.

A new way to control spins

Researchers at the Spintec Laboratory (CNRS/CEA/Université Grenoble Alpes) and the CNRS/Thales Laboratory led by Jean-Philippe Attané and Manuel Bibes have now developed a new, lower-power way to control spin currents. Their method uses the ultrathin layer of conducting electrons – known technically as a two-dimensional (2D) electron gas – that develops at the interface between strontium titanate (an electrical insulator in its pure state) and a covering layer of aluminium.

The researchers began by injecting a spin current from a ferromagnetic nickel-iron alloy into the strontium titanate (SrTiO₃). Once the electrons are confined in the 2D electron gas, their spin couples to their momentum thanks to a phenomenon known as spin-orbit interaction. “This effect converts the spin current into conventional charge current, which allows us to detect the injected spins,” Attané and Bibes explain.

SrTiO₃ behaves like a ferroelectric material

Next, the team applied a voltage across the insulating SrTiO₃ beneath the electron gas to tune the spin-orbit coupling, and thus the direction of the charge current. At this point, they discovered that the insulating SrTiO₃ was behaving like a ferroelectric material – that is, it had a permanent electric dipole moment, just as a ferromagnetic material has a permanent magnetic dipole moment. This is a significant benefit, since electric dipole moments can be oriented using electric fields, which are much easier to control than the magnetic fields required to switch the magnetization of ferromagnetic materials.

Attané and Bibes and their colleagues also confirmed that the direction of the SrTiO₃’s overall electrical polarization depends on the polarity of the applied voltage. This electrical polarization remains even when the electric field is switched off, meaning that they could permanently control the spin-orbit coupling and thus the direction of the charge current produced from the spin current.

The research is detailed in Nature.

Biochemical quantitative phase imaging goes photothermal

Optical imaging is widely used to image biological cells thanks to its non-destructive, label-free nature. A team of researchers in Japan has now combined two common optical techniques – quantitative phase microscopy and molecular vibrational imaging – to produce detailed images of live biological cells. The new approach could be used to observe how fundamental types of molecules are distributed in single cells, with potential applications in biology and medicine.

Quantitative phase imaging (QPI) is a technique that measures the 3D distribution of the refractive index in a transparent sample by calculating how light waves shift as they pass through the sample. It can thus be used to visualize the black-and-white outline of a biological cell as well as the outlines of the major structures inside. This information can then be used to reconstruct a 3D image of the cell.

Molecular vibrational imaging (MVI), for its part, provides information on biomolecular bonds based on Raman scattering or mid-infrared (MIR) light absorption. When molecules are excited with MIR light from a laser, they vibrate at a certain frequency and heat up their surroundings in a process known as the photothermal effect. By applying different wavelengths of MIR light to biological samples, researchers can selectively increase the temperature of specific types of chemical bonds, and so identify biostructures such as intracellular proteins, lipids or nucleic acids.

Both techniques combined

In their experiments, a team led by Takuro Ideguchi of The University of Tokyo combined QPI and MVI into a single, streamlined process. After taking a quantitative phase microscopy image of a live cell with their MIR light source turned off, they then repeat the measurement with the source turned on. The difference between the two images reveals both the outline of the major structures inside the cell and the exact locations of the type of molecule that was excited by the MIR light.

In their work, Ideguchi and colleagues say they were impressed when they first observed the molecular vibration signature characteristics of proteins, and then when this protein-specific signal appeared in the same location as the nucleolus — a structure within the cell’s nucleus in which high amounts of proteins would be expected.

Although it currently takes around 50 seconds or more to capture one complete image, the Tokyo team are confident that they can speed up the process by incorporating a higher-power MIR light source and a more sensitive camera into their experimental set-up.

The approach, which is detailed in Optica, could be used to study complex and fragile biological processes involved in cellular diseases and stem cell development, and to monitor drug delivery, the researchers say. In such applications, the cells need to be observed over long periods without disturbing them.

‘Photon crystals’ could be made using Rydberg atoms

Physicists in the US have come up with a way of making photons repel each other by sending them through an ultracold atomic gas. This astonishing feat could lead to the creation of “photon crystals” and exotic quantum states such as a Mott insulator.

Photons have zero rest mass and zero charge, and as they zip past each other at the speed of light, they barely feel each other’s effects. Physicists, however, have devised various means to amplify this tiny photon-photon interaction.

In 2013, a team led by Vladan Vuletić at the Massachusetts Institute of Technology and Mikhail Lukin of Harvard University created an attractive interaction by firing photons into an ultracold atomic gas, putting some atoms into an energetic “Rydberg state”. While travelling through the gas, the photons tended to stick together more often than they would have had the gas not been there.

Slow-moving quasiparticles

“Classically speaking, the photon comes in, gets absorbed by one atom and propels it to a Rydberg state,” explains Vuletić. “After a while, the photon gets released and absorbed by another atom, which stays in the Rydberg state a while and so on. Quantum mechanically, the gas is in a superposition state of all these possibilities.” These photon-atom couplings can be described as slow-moving quasiparticles called polaritons that interact with each other much more strongly than photons.

Using this technique to create repulsive interactions, however, is trickier. Polaritons interact by changing the local refractive index of the medium. To switch this interaction from attractive to repulsive, requires polaritons that shift the local refractive index in the opposite direction. While this can be achieved by changing how the incident light is detuned, this shift also gives the polaritons negative effective mass. “For one sign of detuning, the particles attract each other” explains Vuletić: “The other side of the detuning, you change both the interaction and the mass term – and the particles still attract each other.”

In 2014 Lukin’s group teamed-up with theoretical physicist Hans-Peter Buchler of the University of Stuttgart in Germany to develop a mathematical description of slow-light polaritons. Under certain specific conditions, the team proposed, repulsive interactions between the polaritons should dominate. Unfortunately, when Lukin and Vuletić’s groups tried to realize these conditions, they found it impossible: “We don’t think this proposal was wrong,” says Vuletić, “It just wasn’t practical in our system and required very, very high laser power that we were not able to reach.”

Two atomic states

After exploring several possibilities, the two groups have now devised and demonstrated a successful alternative scheme in which the photons are coupled simultaneously to two atomic states in the same atom. “Coupling the photons to matter not just once but twice gives us another free parameter where we were able to alter the mass of the particles independently from the index of refraction,” explains Vuletić. Using this, the researchers showed that, by tuning the wavelength of the incident light, they could alter whether or not photons exiting the gas were more or less likely to do so together than would have been expected by chance. This showed that they could control whether polaritons in the gas attracted or repelled each other. The researchers went on to demonstrate three-body repulsion between polaritons, which matched their theoretical predictions.

The researchers are now exploring several possible extensions of the work: “If you have repulsion at short distances and attraction at long distances you would create a molecular-like structure,” explains Lukin’s student Aditya Venkatramani, who along with Vuletić’s student Sergio Cantu were joint first authors of a paper in Nature Physics that describes the work.

This could create “crystalline order” in photon structures, potentially opening up experiments impossible in traditional matter. “If you have an atom interacting, it always has the same mass no matter how it travels around,” explains Cantu, “but here you could have a mass of one sign and magnitude in one direction and another sign and magnitude in another direction. It’s possible to change a lot of parameters that, in regular matter, are just given by your material.”

“It’s really cool,” says Mohammed Hafezi of the University of Maryland, College Park. He suspects the work will unlock many doors in photon many-body physics that were hitherto closed by the fact that all attractions between polaritons were attractive. He is excited, for example, by the potential of producing an exotic quantum state called a Mott insulator with photons: “You need repulsive interactions to have a traffic jam of photons,” he explains, “They’ve now achieved a repulsive bond, and if they manage to have a finite number of modes and to populate them, then they should get a Mott insulator.”

Micro-CT enables first non-destructive characterization of asthma medicines

3D XCT images of particles in inhalation-grade carriers that were sieved (left) and milled (right) during the manufacturing process. Each particle is shown with a different colour for visibility. (Courtesy: *Euro. J. Pharm. Biopharm.* 10.1016/j.ejpb.2020.02.013, CC BY 4.0)

Thirty years ago, over 180 million people worldwide were affected by asthma. Five years ago, the figure was approximately 350 million. It’s estimated that around 400 million people will be afflicted by 2025.

Asthma’s prevalence is increasing. Medicines delivered via inhalers effectively manage many peoples’ symptoms. But we still don’t know much about how an inhaler’s life-altering medicines look and behave at a fundamental level.

A group of UK researchers can now examine the materials in dry powder inhaler medicines more accurately and in more detail than ever before. Using a non-destructive imaging technique called X-ray micro-computed tomography (XCT), they have arrived at the first three-dimensional portrait of materials in dry powder inhaler medicines.

X-ray micro-CT of asthma medicines

Why dig into an asthma medicine’s physical structure? Because a medicine’s behaviour, which is dictated by its physical properties, impacts performance. For example, smaller particles, which can penetrate deep into the lungs, interact strongly with one another within inhalers, making it more difficult for them to aerosolize and reach the lungs to begin with. Other important properties include the particles’ shapes and roughness, and relationships between particles in space.

“Measurements [of these physical properties] are made for all inhaled formulations at some point during [the pharmaceutical] development, characterization and quality control process,” says Darragh Murnane, professor of pharmaceutics at the University of Hertfordshire. “The difference is that with XCT we don’t need to break up a tablet, we don’t need to open a capsule, we don’t need to spray an aerosol. We’re able to look inside a medicine as it’s shipped from the factory.”

Murnane and colleagues used a commercial XCT scanner that operates much like a medical CT system. A radiation source produces a cone of X-rays that hit a sample of the medicine. A shadow image is recorded on a detector, and by rotating the sample, images from different angles are collected and then reconstructed to form a volume image. From this, the researchers can make measurements of particle characteristics on sub-micrometre scales.

Validating the imaging technique

Dry powder inhalers contain a mixture of active ingredients and inert carriers that help disperse the active ingredients when a patient inhales. However, selecting which of these materials to use in the first XCT experiments wasn’t straightforward. Materials had to be large enough to allow the researchers to optimize the imaging and analysis procedures, and strong enough to resist damage in the presence of X-radiation.

Enter the so-called tablet-grade carriers, which contain particles slightly bigger than a dust mite, around 250 µm in diameter.

XCT images revealed the tablet-grade carrier particles’ distinct shapes, while differences in greyscale colours, or contrast, in the images reflected the different particles’ atomic weights (with brighter pixels signifying denser regions).

Even though XCT can study materials without altering or destroying them, the researchers found that their tablet-grade carrier XCT results matched those seen using other characterization techniques. The differences that they did see, such as undisturbed particle orientations and comprehensive views of individual particles, clarified XCT’s advantages.

XCT and SEM images — Images of two tablet-grade carriers produced by XCT (top row) and scanning electron microscopy (bottom row). Each particle is shown with a different colour for visibility. (Courtesy: *Euro. J. Pharm. Biopharm.* 10.1016/j.ejpb.2020.02.013, CC BY 4.0)

Satisfied, the researchers moved on to study a class of carriers that are more representative of those found in dry powder inhaler medicines.

Tinier particles, new challenges

Until recently, XCT systems could not distinguish between tiny, lightweight particles and air. Technological advances and X-ray optical lenses helped maximize image contrast by keeping the distances between scanner components and medicines short.

Still, the inhalation-grade carriers presented new challenges for XCT. The particles in these carriers are more complex than those in tablet-grade carriers. They are smaller in size, hovering around 100 µm, about the width of a human hair. They also have a much wider range of sizes, including a proportion of fine particles that are 10 µm or smaller.

“Consider a pixel size of 1 micrometre. A cubic particle with a 10-micrometre edge length would be spanned by 1000 voxels [equivalent to pixels in three-dimensional space], but a 100-micrometre cubic particle would be spanned by one million voxels,” explains Parmesh Gajjar, postdoctoral researcher at the University of Manchester.

That means that the fine particles in inhalation-grade carriers would have fewer voxels per particle in an XCT image than a larger carrier, making it difficult for researchers to identify individual particles. Despite these theoretical concerns, the team successfully separated and characterized individual inhalation-grade carrier particles.

Bolstered by this success, the researchers undertook their greatest challenge yet: imaging a blend of tablet-grade carrier and active ingredient, a mixture intended to mimic those in dry powder inhaler medicines.

Here, the researchers hit a wall. Initially, their XCT images didn’t provide enough contrast for them to separate and measure individual particles. However, by using different algorithms developed by their industry research partners, they were able to separate individual particles in the mixture. They presented these new results at the Digital Respiratory Drug Delivery meeting earlier this year.

“Taking the image, splitting the image, and identifying the different particles from within the image … was the main challenge from my point of view,” says Gajjar. “Because we’re looking at a carrier with a very low atomic weight, it’s hard to get [that] contrast.”

It’s a small world after all: XCT in the future

Asthma inhalers deliver medicine directly to the lungs; however, understanding this process and developing effective medicines is “extremely challenging”, Murnane says.

XCT is the first imaging technology to non-destructively produce 3D images, helping researchers understand how dry powder inhaler medicines behave during manufacture and how they aerosolize for inhalation into the lungs.

Even so, it takes hours for well-trained workers to take and analyse XCT images, and equipment is not yet commonplace. Because of these limitations, the researchers believe XCT might initially be used by a select number of laboratories to help improve the results of more traditional particle characterization techniques. XCT could also help characterize medicines that are under development, when active ingredients are often scarce, examine the relationships between different forms of asthma medicines, such as powders and aerosols, or compare medicines’ generic and name-brand forms.

“I don’t think this work is going to change the types of medicines that we formulate,” Murnane says. “But what it will do is allow us to understand the materials that we use much more robustly.”

The study is published in European Journal of Pharmaceutics and Biopharmaceutics.

Make spinning sprinklers and balloon rockets at home, lidar unveils huge Mayan structure, how to win Come Dine with Me

Are you looking for a fun physics activity to do with the kids this weekend? The Institute of Physics’ Melissa Brobby has just the thing – a self-spinning water sprinkler made from a milk carton. In the above video, she shows you how to make a sprinkler and tells you about the physics that makes it spin.

The video is part of the Institute’s Do Try This at Home series, which aims to make it easy for parents and carers to get their children excited about physics. If sprinklers are not your cup of tea, how about a balloon rocket – as described below by the Institute’s Mikey Jarrell.

If I had to name a physics-related technique that is most like magic, I would have to say lidar. This involves firing laser pulses at an object or landscape of interest and using the reflected light to create a 3D map of the object or landscape topography. The seemingly magic bit is that lidar can often see through vegetation – revealing what lies on a forest floor, to a lidar system flying overhead. This has proven very useful for archaeologists, who have used lidar to make some spectacular finds.

Now, lidar has revealed the largest and oldest Mayan structure known to archaeologists. The huge rectangular elevated platform was built between 1000–800 BC in Mexico’s Tabasco state and was found by Takeshi Inomata of the University of Arizona and colleagues. The discovery is described in Nature, where the team also explains how it used radiocarbon dating to work out the age of the structure.

Das Perfekte Dinner is a reality TV programme like the UK’s Come Dine with Me, in which people take turns hosting a dinner party for each other throughout a week. Each dinner is scored by the other contestants on the evening that is served.

Physicists Peter Blum and Marc Wenskat at the University of Hamburg have analysed results from the German show and have concluded that a contestant’s chances of winning are boosted if they host their dinner later in the week.

In a preprint uploaded to arXiv, they say that their finding is an example of the “secretary problem” that arises when things are rated consecutively using the same criteria. Apparently, the application of those criteria change as each scoring occurs, skewing the results.

Thermogalvanic hydrogel cools down electronic devices

A new thermogalvanic hydrogel can simultaneously cool down electronic devices and convert the waste heat that they produce into electricity. The material, developed by a team of researchers at Wuhan University in China and the University of California Los Angeles (UCLA) in the US, decreases the temperature of a mobile phone battery by 20 °C and retrieves 5 μW of electricity at fast discharging rates. This reduced working temperature ensures that the battery operates safely, while the amount of electricity harvested is enough to power the hydrogel’s cooling system.

Many electronic devices – including solar cells and light-emitting diodes, as well as phone batteries – generate significant amounts of heat during normal operation. Not only is most of this heat wasted, it can also lead to localized overheating, which decreases the devices’ efficiency and lifespan. In some cases, the excess heat can even cause devices to explode or catch fire.

Traditional ways of recovering waste heat, such as thermoelectric modules, involve adding extra thermal resistance. Unfortunately, this additional resistance prevents heat from dissipating, and thus increases the temperature of the electronic device’s core components. Removing heat tends to consume energy, especially if additional equipment like fans or pumps are required. This apparent conflict means that while researchers have previously succeeded in recovering waste heat from electronic devices, and in efficiently removing it, they have never accomplished both at the same time.

Separate thermodynamic cycles

Thermogalvanic cells, which consist of an electrolyte solution surrounded by two inert electrodes, show promise in reconciling the competing tasks of removing heat and converting it to electricity. In such a cell, electron-transferring (redox) reactions convert heat energy into electricity. Since the solvent in the cell’s electrolyte solution is only present to support ion transport and electron transfer, it can undergo a separate thermodynamic cycle without affecting the heat-to-electricity conversion process. Hence, the water molecules in aqueous electrolytes can be allowed to evaporate and condense, completing a cycle of heat absorption and release that cools down the cell even as the thermal-electric conversion process continues.

A team led by Kang Liu of Wuhan University and Jun Chen of UCLA’s Department of Bioengineering has now developed a hydrogel film to accomplish this task. The hydrogel is based on a polyacrylamide framework infused with potassium, lithium and bromine ions, as well as the ferricyanide ions Fe(CN)₆³⁻ and Fe(CN)₆⁴⁻.

When heated, the ferricyanide ions transfer electrons between the cell’s electrodes, generating electricity. At the same time, confined water in the hydrogel is allowed to freely evaporate, which removes a large amount of heat without affecting the thermal-electric conversion process. The positive lithium and negative bromine ions serve to control the system’s moisture balance, facilitating water absorption from the surrounding air and thus “regenerating” the hydrogel.

Battery cooling

To show that their new hydrogel film could cool a real-world device, the researchers attached it to a mobile phone battery during fast discharging of 2.2 C (where C is a measure of the rate at which a battery is discharged relative to its maximum capacity). They found that some of the waste heat was converted into 5 μW of electricity and that the temperature of the battery deceased by 20 °C.

The researchers found that their film, which measured around 12 x 30 x 3.6 mm, is robust, with a mechanical strength of 0.24 MPa. They also showed that it can be stretched up to 2–3 times its original length without suffering any damage. Full details of the new thermogalvanic hydrogel are reported in Nano Letters.

Exotic radioactive molecules could reveal physics beyond the Standard Model

The first spectroscopic study of radium monofluoride suggests that the radioactive molecule could be used to perform high-precision tests of the Standard Model of particle physics. The study was done by an international team of physicists working in the ISOLDE lab at CERN and could lead to a new upper limit being placed on the electric dipole moment of the electron – which could help explain why there is much more matter than antimatter in the universe.

Atomic and molecular spectroscopy allows physicists to make extremely precise measurements of some fundamental properties of both electrons and nuclei. As a result, spectroscopy offers a way of determining whether a particle like the electron conforms to the Standard Model of particle physics.

Radium monofluoride is a molecule of particular interest to physicists because in certain isotopic versions of the molecule, the radium nucleus is deeply asymmetrical – having a pear-shaped mass distribution. This, and the high mass of radium, means that it is ideal for studying the fundamental properties of the bound electrons – including whether the electron has an appreciable electric dipole moment.

Time reversal symmetry

It is well known that the electron has a magnetic dipole moment, which is a result of the particle’s “spin”, or intrinsic angular momentum. However, time reversal symmetry – a tenet of the simplest version of the Standard Model – forbids the electron from also having an electric dipole moment.

While more complicated versions of the Standard Model allow the electron to have an extremely small electric dipole moment, measuring a substantially higher value could point towards new physics beyond the Standard Model. This would be of great interest to cosmologists because it would reveal a fundamental symmetry breaking in the early universe that could explain why there is much more matter than antimatter in the cosmos.

The short-lived radium monofluoride molecules were created at ISOLDE, which produces beams of exotic radioactive particles that can be ionized and trapped with electromagnetic fields for further study. The team used the Collinear Resonance Ionization Spectroscopy (CRIS) instrument on ISOLDE, which enabled them to study even very low amounts of particles with high precision.

Short-lived molecules

The results provide the first spectroscopic information of radium monofluoride, including isotopologues – molecules that differ only in their isotopic composition – composed of radium isotopes with half lives as short as a few days.

Writing in Nature, nuclear physicist Ronald Garcia Ruiz and colleagues describe how they determined that the molecules have energy levels that should allow them to be laser cooled to temperatures just above absolute zero. This is a necessary condition to allow for the extremely high-precision measurements needed to find deviations from the Standard Model. Garcia Ruiz, who works at the Massachusetts Institute of Technology (MIT) and CERN, is currently setting up a new collaboration between MIT and ISOLDE, to reignite the quest to measure the electron’s electric dipole moment.

“We want to narrow the gap further between our most sensitive measurements, and the theoretically predicted value of the dipole moment,” says Gerda Neyens, a nuclear physicist at the KU Leuven in Belgium and ISOLDE’s head of research. “The value in the Standard Model is extremely small, and way out of the current experimental range. But by closing in we can already constrain on certain theories that predict much larger values.”

New technique uses food residues to date prehistoric pottery

Tiny traces of food left in ancient clay pots can now be used to date archaeological objects thanks to a novel combination of NMR spectroscopy and accelerator mass spectrometry (AMS). The new technique, developed by researchers at the University of Bristol, UK, overcomes previous challenges associated with dating pottery from the prehistoric era.

All living organisms absorb radioactive ¹⁴C from the atmosphere. Once an organism dies, radioactive decay causes the amount of this isotope present in its body to decrease at a known rate. Measuring the residual levels of ¹⁴C in an object made from formerly-living materials therefore enables scientists to determine how old it is.

AMS radiocarbon dating is one of the most widely-employed methods for estimating the age of archaeological objects up to 50 000 years old. It is mainly applied to samples of charred plant remains and bone collagen, and its adoption has transformed not only archaeology but also many other research disciplines, including climate science. However, objects primarily made from non-living materials, such as clay pots, are far harder to date using AMS – a distinct disadvantage, since pieces of pottery, or sherds, are among the artefacts most commonly recovered from archaeological sites.

Analysing individual fatty acids in food residues

A team led by Richard Evershed has now overcome this problem by analysing food residues that have been absorbed into (and are thus protected by) the clay matrix in pottery. These residues are typically left behind by cooking meat or milk, and they contain high amounts of palmitic (C_16:0) and stearitic (C_18:0) fatty acids. Indeed, these carbon-bearing compounds are often present in concentrations as high as milligrams per gram of clay.

The researchers isolated the fatty acids by cleaning sherds of cooking pots and then grinding them to a powder, which opens up the surface where the fatty materials (lipids) are preserved. Next, they extracted the lipids with organic solvents and separated the individual compounds in them using preparative capillary gas chromatography.

AMS-pottery-article — Team member Emmanuelle Casanova in the lab. Courtesy: University of Bristol

Evershed and his team checked the purity of their compounds using high-field NMR spectroscopy and mass spectrometry techniques before placing them in a tin capsule and burning them to make a graphite target. They then transferred this target to an accelerator mass spectrometer and used the instrument (which the lab acquired in 2016) to count the number of ¹⁴C atoms within the target. “Bringing together the state-of-the-art NMR to check the purity of the isolated compound and the latest AMS instrument was the major step forward that made these measurements possible,” Evershed says.

Neolithic vessels

The first samples Evershed’s team studied came from Neolithic vessels unearthed at archaeological sites in Somerset, UK; the World Heritage site of Çatalhöyük in Anatolia, Turkey; Lower Alsace in France; and Takarkori in the Sahara region of southwest Libya. Previous dating efforts based on pottery styles suggested that these vessels range in age from around 5500 to over 8000 years old, and the team found that their measurements agreed with these earlier estimates – at least to within a few decades.

Spurred on by this result, the team then analysed a collection of pottery recently excavated by workers from the Museum of London Archaeology in advance of building works there. These samples, which include 436 fragments from at least 24 different bowls, cups and pots, were thought to come from the Early Neolithic period, but their exact age was unknown.

Radiocarbon measurements on milk fats in these vessel fragments revealed that the pottery was 5500 years old. “The results indicate that the area around what is now Shoreditch High Street was already being used at this time by established farmers who ate cow, sheep or goat dairy products as a central part of their diet,” Evershed explains. “These people were likely to have been linked to the migrant groups who were the first to introduce farming to Britain from Continental Europe around 4000 BCE – just 400 years earlier.”

First technique to directly date pottery

Pottery was invented in the late Pleistocene epoch (2.6 million years BCE – 9700 BCE) and was an important factor in the development of food processing, Evershed says. Current techniques for dating it often depend on a careful examination of the object’s style and decorative features. However, this “typology” method provides only a crude estimate of a pot’s age – especially for earlier samples, which tend to be rather plain. Another method, known as association, relies on carbon-dating organic materials (such as charcoal, bone or seed) found lying next to the pottery and using the results to estimate the age of the pot itself. Again, this method is not very accurate, and is prone to errors.

The best method, Evershed says, is to date something from the pots themselves. Other researchers have previously done just that, by dating carbonized materials on the artefacts’ surfaces. However, Evershed notes that these residues are more easily contaminated by their environment than residues embedded within the clay.

“Our new technique is the first to date pottery directly,” he tells Physics World. “And since we are looking at fat residues, we can connect the date of these to food procurement practices. For example, dating milk residues form prehistoric sites gives us an insight into early animal management practices.”

Anchoring “floating chronologies”

Members of the Bristol team, who report their work in Nature, now plan to investigate whether they can apply the technique to other fatty residues such as plant oils or beeswax. Another possibility would be to use it to date mummies, which contain lipids in their soft tissues and bandaging. A third application might involve dating archaeological bones, as these also contain lipids.

“The importance of this advance to the archaeological community cannot be overstated,” Evershed says. While pottery typology makes it possible to arrange objects in chronological order according to their decorative characteristics, he points out that these chronologies are hard to pin down to specific calendar dates. “Our technique allows us to anchor these ‘floating chronologies’ in real calendric time and assign a proper date to them,” he says.

Browse articles by content type

Ultraporous metal-organic frameworks could make clean energy carriers

Storing increased amounts of gas without increasing the pressure

Storing gases with extreme efficiency in ‘crystalline sponges’

Nanosized pores mean a high surface area for gas adsorption

Safe training: the linac simulator