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Thermophotovoltaic cells top 40 per cent efficiency

The first thermophotovoltaic cells with an efficiency of more than 40% – higher than any existing solid-state heat engine, and exceeding even the average efficiency of turbine-based power generation – have been fabricated by researchers at the Massachusetts Institute of Technology (MIT) and the US National Renewable Energy Laboratory (NREL). The cells, which are two-junction devices made from III-V semiconducting materials with electronic bandgaps between 1.0 and 1.4 eV, use back surface reflectors to divert unusable sub-bandgap radiation back to the source, and are optimized for heat sources at temperatures of 1900–2400 °C. According to their developers, the cells could be integrated into renewable energy systems for low-cost thermal grid storage.

Thermophotovoltaic (TPV) devices use photovoltaic cells to convert the predominantly infrared light emitted by hot objects (at 600 °C or more) into electrical energy. They can operate with higher-temperature heat sources than those used by turbines, and their range of possible sources is very broad, including combustion, nuclear reactions, waste heat, heat stored in a thermal energy storage system and solar radiation via an intermediate radiation absorber. All these sources are, in principle, more reliable than wind or solar energy generated directly from sunlight, both of which are intermittent.

New efficiency of 41.1%

The first TPVs were made from an integrated back surface reflector and a tungsten source emitting at 2000 °C. These devices had an efficiency of just 29%, and despite subsequent advances, TPVs have struggled to exceed the 32% mark and operate at temperatures below 1300 °C. The theory of TPVs, however, predicts that their efficiencies can exceed 50%, so researchers suspected there was room for improvement.

The new TPV cells, which were developed by a team led by Asegun Henry and Alina LaPotin of MIT’s Department of Mechanical Engineering, have a maximum efficiency of 41.1% and operate at a power density of 2.39 W/cm2 using a heat source emitting at 2400 °C. The devices were fabricated by researchers at the National Renewable Energy Lab (NREL) using a technique called organometallic vapour phase epitaxy.

The first of the team’s two cell designs uses top and bottom junctions made from AlGaInAs and GaInAs grown on a GaAs substrate. In this design, the AlGaInAs has a bandgap of 1.2 eV and the GaInAs a bandgap of 1.0 eV, and their lattices are mismatched with respect to the crystallographic lattice constant of the substrate. The second design combines a lattice-matched 1.4 eV GaAs top cell with a lattice-mismatched 1.2 eV GaInAs bottom cell.

The cells’ high efficiency comes from a combination of factors, LaPotin tells Physics World. “The first is the use of multi-junction cells that allow us to convert different energy bands of the incident spectrum more efficiently by reducing so-called thermalization losses,” she explains. “The second is the use of materials with a bandgap that is higher than those typically employed for TPVs, along with higher heat-source temperatures.”

“Usually, TPV targets bandgaps of around 0.7 eV with source temperatures lower than 1300 °C,” LaPontin continues. “Since there is an almost constant ‘penalty’ you pay on the voltage produced, moving to higher bandgaps (of 1.0 to 1.4 eV) confers an advantage. Indeed, as you move to higher bandgaps, you get higher voltage and the penalty becomes a smaller fraction of the total voltage, thereby leading to a higher overall efficiency.”

Other factors contributing to the device’s high efficiency include the use of a high-reflectance back surface reflector to send sub-bandgap radiation back to the heat source, as well as the high-quality fabrication techniques developed at NREL, LaPontin says.

More funding required

Henry says that the team’s device represents the first time that the efficiency of TPVs has reached 40% and the first time that any solid-state heat engine has ever demonstrated an efficiency higher than the average efficiency from turbine-based power generation in the US. The comparison to the average turbine is key, he says, because turbines currently have a near-monopoly on large-scale power production thanks to their low cost and efficiency. “This is the first time in history that another technology has shown a similar efficiency, lower cost and scalability, such that it can compete with turbine-based heat engines,” he says.

In Henry’s view, TPV technology merits more attention from the science and engineering community and “really deserves to get funding” for research and development. He notes that research into generating energy via an alternative method involving streams of supercritical CO2 has received around $100m in US government funding.

For their part, the MIT researchers have turned to developing thermal batteries that are compatible with their TPV technology, which Henry says could be “a huge part of the solution to mitigating climate change”. “Thermal batteries are an extremely low-cost, grid-scale energy storage technology that can enable full penetration of renewables onto the grid,” he explains. “The main applications for the cells we have demonstrated in our work is for these types of batteries.”

Another option for TPVs would be to combine them with hydrogen-fuel technology. “Some of the advantages of TPVs over turbines in this context include lower cost, faster response times, low maintenance, fuel flexibility and the ability to be cost effective at smaller power-generation scales – on the order of 10 MW,” Henry says.

The researchers now plan to test their cells in a thermal battery prototype system and pilot demonstration. They also hope to further improve the cells’ efficiency to 50% by increasing the fraction of unusable radiation they reflect to 97–98%.

The present work is detailed in Nature.

Nanoscale Möbius strips made from carbon, quantum tipple supports STEM outreach

In 2017, Kenichiro Itami and colleagues made the first carbon nanobelts, which can be as little as three atoms thick. Based at Nagoya University in Japan, Itami’s team has managed to tweak the topology of their nanobelts to create Möbius carbon nanobelts (see figure). Making the nanobelts was not an easy task and took 14 chemical reaction steps. They confirmed the Möbius structure using a chiral separation technique and circular dichroism spectroscopy.

It looks like Itami and colleagues created their Möbius nanobelts for the sheer joy of being the first to do so, which is wonderful.  Although they do point out that newly discovered forms of carbon have in the past opened doors to new science and technology. They also believe that the techniques they developed to create Möbius carbon nanobelts could be used to make other interesting and useful carbon nanostructures. They describe their work in an open access paper in Nature Synthesis.

Described as out of this world, Dark Matter Quantum Lager is the latest tipple from Canada’s Wellington Brewery. “Of course, we used Comet and Galaxy hops in this refreshingly crisp lager, to give the beer a zesty grapefruit and tangerine boost,” says the website of the brewery, which is in Guelph, Ontario. Keeping with the extra-terrestrial theme, meteorites from Namibia are added during the boil portion of the beermaking process.

Food-grade glitter

Some cans of the lager contain “food-grade glitter” and anyone who cracks one of these open can claim a prize from the brewery (although there is no mention of what the prize is). But the real winner from the launch of this beer is Royal City Science, which will receive 50 cents for every can of Dark Matter Quantum Lager sold. The Guelph-based organization describes itself as “a hub for informal science, technology, engineering, and mathematics (STEM) education to ignite curiosity, build confidence, and inspire the inner scientist in us all”.

Earlier this week, the brewery and Royal City Science hosted the “Physics of Fizz” public event with University of Guelph physicist Joanne O’Meara and brewer Mitch Marquis. If you happen to be in Southern Ontario, there are more events planned at Guelph breweries and pubs in May and June.

Nobel-prize-winning nuclear physicist Ben Roy Mottelson dies aged 95

The American-Danish physicist and Nobel laureate Ben Roy Mottelson died on 13 May at the age of 95. Mottelson made critical contributions to determining and understanding that certain nuclei could have asymmetrical shapes. The work earned him a share of the Nobel Prize for Physics in 1975.

Born in Chicago on 9 July 1926, Mottelson received a BSc in physics from Purdue University in 1947. He then carried out a PhD in nuclear physics at Harvard University under the supervision of Julian Schwinger. After graduating in 1950, Mottelson  moved to the Institute for Theoretical Physics in Copenhagen, joining CERN’s theoretical study group in Copenhagen in 1953.

Four years later, Mottelson took up a post at the newly formed Nordic Institute for Theoretical Physics (Nordita), where he remained for the rest of his career.

A ‘collective model’

It was in the early 1950s that Mottelson carried out his Nobel-prize-winning research, the roots of which can be traced back to work done by the Danish scientist Niels Bjerrum, who in 1912 was the first to recognize that the rotation of molecules is quantized. Later, in 1938, Edward Teller and John Wheeler observed similar features in the spectra of excited nuclei, which they suggested was caused by the nucleus rotating.

Mottelson
(Courtesy: Niels Bohr Institute)

But a more complete explanation for this effect had to wait until 1950 and 1951 when Åage Bohr from the University of Copenhagen and James Rainwater from Columbia University pointed out that the rotation is a consequence of the nucleus deforming from its spherical shape. Their work challenged the widely accepted theory that all nuclei are perfectly spherical.

Bohr and Rainwater independently began to develop a theoretical model of the nucleus that combines the individual and collective motions of the neutrons and protons inside it. They did this by bringing together the liquid-drop model of the nucleus – which pictures the nucleus as an incompressible fluid – with the shell model.

Using this “collective model” of the nucleus, the duo showed how individual nucleon orbits can exist in a nucleus with non-spherical, liquid-drop properties. Mottelson then worked with Bohr to confirm that the theoretical models agreed with experimental data of energy levels in certain nuclei.

For their work, Bohr, Mottelson and Rainwater shared the 1975 Nobel Prize for Physics “for the discovery of the connection between collective motion and particle motion in atomic nuclei and the development of the theory of the structure of the atomic nucleus based on this connection”.

In addition to the Nobel prize, Mottelson also received the Atoms for Peace Award in 1969 and the John Price Wetherill Medal from the Franklin Institute in 1974. Mottelson worked with Bohr on a two-volume monograph –Nuclear Structure. The first volume – Single-Particle Motion – appeared in 1969 while the second volume, Nuclear Deformations, was published in 1975.

Control of mechanical quantum resonators reaches new levels of precision

New levels of precision control over the quantized energy levels of mechanical resonators have been achieved by teams in the US and Switzerland, who independently measured the number of phonons in a cavity without disturbing it. In addition, the US group produced an entangling gate comprising two nanomechanical oscillators. The work could potentially have implications for quantum networking and quantum error correction.

Just as electromagnetic energy is quantized into propagating photons, acoustic energy propagates in quanta called phonons. The science of photon behaviour – called quantum electrodynamics – is an important branch of modern physics because it provides a relativistic description of the interaction of light with matter. Scientists have used the theory in a variety of applications such as atomic clocks and quantum computation.  In recent years, scientists have begun applying some of the same concepts to phonons in a field called quantum acoustodynamics. Last year, for example, two groups independently used laser-based measurements to entangle the oscillations of membranes in cavities.

Quantum acoustodynamics is attractive for quantum networking and quantum information processing for several reasons. First, whereas it is extremely difficult to isolate photons from unwanted thermal and electrical noise in a superconducting qubit, sound only propagates within a medium. Therefore, isolated mechanical resonators can have a much longer lifetime, which could make them useful in quantum memory applications and quantum error correction. Despite this isolation, such resonators can also be interfaced with a wide variety of different quantum technologies – and this could prove invaluable for connecting superconducting, trapped ion or atomic quantum computers. “Everything that we have in our physical world talks to mechanical vibrations,” explains Uwe von Lüpke of ETH Zurich.

Changing energy

Reading out the energy level of a mechanical resonator, however, poses a challenge. The simplest way is to tune another system such as an electronic circuit or laser onto a resonance – much as one could ascertain the frequency of a laser by pumping it at multiple frequencies and finding out which one worked. This creates a problem, however: “If you have this resonant energy exchange, you’re actually changing the energy of the resonator,” explains von Lüpke.

In the new research, two groups – one at ETH Zurich led by Yiwen Chu and one at Stanford University in California headed by Amir Safavi-Naeini – have independently made “quantum non-demolition” measurements of the states of mechanical resonators. They interfaced mechanical resonators with superconducting qubits through piezo-electric materials, which expand when subjected to electric currents.

They did not, however, tune the frequency of the current oscillations into resonance with the mechanical oscillation. Instead, they utilized the fact that the number of phonons in the cavity alters its resonant frequency. Therefore, by measuring the relative phases of the oscillations between the cavity and the qubit (effectively measuring how far apart their frequencies were), they could determine the number of phonons in the cavity.

Long stability

“This effect is typically used to read out superconducting qubits,” explains von Lüpke, who is first author on the paper describing the ETH Zurich work. This type of measurement is only possible in the so-called strong dispersive regime, in which the coupled phononic-electronic states are stable long enough to allow sufficiently precise measurements of the frequency of the mechanical resonator. The two groups are the first to reach this regime.

The groups’ approaches, however, were different. The ETH Zurich team used bulk density waves in a sapphire wafer. The Stanford group fabricated two nanoscale, periodic “phononic crystals” on a lithium niobate chip. These are analogous to photonic crystals in that they preferentially support specific phonon frequencies. Like the ETH Zurich group, the Stanford team interfaced their phononic crystal resonators with a qubit and performed quantum non-demolition measurements of the number of phonons in each one.

The Stanford group then used the fact that both acoustic resonators were connected to the same electronic qubit to perform an entangling gate operation, which is a measurement of the qubit that left the two resonators in an entangled state. This could potentially be useful in quantum error correction, allowing the states of superconducting qubits to be stored in longer-lived mechanical qubits and removing the need for large amounts of redundancy to protect the integrity of quantum algorithms.

Towards hybrid technologies

“I think that’s what’s really motivated both of our groups to develop these sorts of heterogeneously integrated devices,” says Amir Safavi-Naeini. “That’s the direction where a lot of the field is going – towards these hybrid technologies.”

Warwick Bowen of the University of Queensland in Australia believes that both teams have achieved significant progress in creating their systems. “I think they’re very different, and they’ll have different applications,” he says. He points out that the bulk acoustic waves utilized by the ETH Zurich group can generate very long lifetimes, which could be useful in quantum memories – especially as a superconducting circuit could be mounted directly on the mechanical resonator. However, the Stanford group has already demonstrated entanglement generation (albeit with less than 60% fidelity, compared with over 99% fidelity required in viable quantum gates).

Bowen also says that the miniaturization of the phononic crystal approach has inherent benefits. “Nanoscale is good in terms of being able to pack more devices into the same area…and because they’re so much smaller it turns out they have a much stronger interaction with light. So, if you wanted to build a quantum interface between microwaves and light, this system is much better suited to that.”

The Stanford research is described in Nature. The ETH Zurich work is unveiled in Nature Physics.

Geometric distortion characterization using QUASAR™ MRID3D in radiation oncology: 1.5T MRI and 0.35T MR-LINAC

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MR-guided radiation therapy (MRgRT) has been running routine clinical practice. However, MRgRT requires robust imaging spatial integrity to achieve accurate dose calculations and high-quality plan delivery.

Throughout this webinar, we will review how to characterize the spatial integrity of 1.5T MRI and 0.35T MR-LINAC using QUASAR™ MRID3D Geometric Distortion Analysis System in radiation oncology.

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Taeho Kim

Taeho Kim, PhD, DABR, is an associate professor of radiation oncology and joined the medical physics faculty in 2018. He currently has two primary interests in medical physics research: MRI-guided radiotherapy and respiratory motion management in clinical practice. Kim received his PhD in physics in 2007 from Washington University in St Louis. He completed two post-doctoral fellowships, one in MRI (radiology, University of Utah) and the other in radiation therapy (radiation oncology, Stanford University). He completed his medical physics residency in the CAMPEP-accredited medical physics programme at the University of Virginia. He worked on several medical physics research projects including respiratory motion management using MRI-guided audiovisual biofeedback, an integrated MRI-linear accelerator project, and quasi-breath-hold biofeedback in radiation oncology. He has extensive clinical research experience in medical physics and MRI from several universities.

Bioactive materials and regenerative repair

Biomedical Materials (BMM) will host a webinar focusing on bioactive materials and regenerative repair.

Prof. Rongrong Zhu, board member of BMM will be the chair for the webinar. Along with two others speakers, Prof. Huiling Cao from Southern University of Science and Technology and Prof. Yuxiao Lai from Shenzhen Institute of Advanced Technology of Chinese Academy of Sciences, they will share their research on bioactive materials and regenerative repair and perspectives.

Please note, this webinar will be in Chinese.

Left to right: Rogrong Zhu, Huiling Cao and Yuxiao Lai

Rongrong Zhu is professor at Tongji University. She focuses her research on bioactive materials and regenerative repairs.

Huiling Cao is professor at Southern University of Science and Technology. She focuses on the study of skeleton development and homeostasis regulation and related bone diseases (i.e. osteoporosis, osteoarthritis), mechanisms of sugar and fat metabolism, and more.

Yuxiao Lai is professor at Shenzhen Institute of Advanced Technology of Chinese Academy of Sciences. She is mainly focusing her research on the R&D of orthopaedic biomaterials. She leads TMC integrates medicine, life science, material science, imaging, and mechanics. She focuses on the translational research of technology and products for the clinical application in orthopaedics.

About this journal

Biomedical Materials publishes original research findings and critical reviews that contribute to our knowledge about the composition, properties, and performance of materials for all applications relevant to human healthcare.

Editor-in-chief: Jianwu Dai, Center for Regenerative Medicine and Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, People’s Republic of China.

China Initiative continues to prosecute scientists with links to China

A federal jury in Illinois has found Mingqing Xiao, a mathematician at Southern Illinois University Carbondale, guilty of failing to report a Chinese bank account to tax authorities. Xiao’s sentencing on the tax charges is set for 11 August but his lawyers say they will appeal. Xiao’s case may be one of the last that is brought by the US government under its controversial China Initiative. 

The China Initiative was introduced in 2018 to combat efforts by China’s government to acquire US technology illegally. The US Department of Justice (DOJ) applied it in a few successful prosecutions of individuals charged with theft of technology and intellectual property. But several failed cases against academic scientists of Chinese descent, who had received funds for research projects from Chinese universities, created concern that the department had targeted the scientists for their ethnicity rather than their actions. Protests by Asian-American groups persuaded the DOJ to change the programme’s name to “a strategy for countering nation-state threats”. 

Yet the DOJ has continued to take researchers such as Xiao, who is a naturalized US citizen, to court. “The evidence established that Dr Xiao concealed foreign work and hid more than $100,000 of foreign assets in an account in China, and he was properly prosecuted and held accountable,” notes US attorney Steven Weinhoeft.

The US government, however, failed to prove three related grant fraud cases that charged Xiao with lying to his university and the National Science Foundation – from which he obtained $151,099 in grants – about his ties to China’s Shenzhen University. US district judge Staci Yandle threw out two of those charges and the jury quickly acquitted Xiao of the third. 

According to Xaio’s legal team, the failure of those three grant fraud cases represents “a complete rebuke” of the China Initiative. “We are thankful that those counts were rejected by the court and the jury as we believe that they were unjust, improperly motivated, and unsupported by the facts and the law,” the team declared in a statement.

Xiao’s legal counsel Michelle Nasser told Physics World that the team was “very disappointed” in the verdicts on the tax charges and intends to appeal. 

Xiao is currently on administrative leave at his university but has received significant support from the community. Students and faculty members have held protests on his behalf, with the rallying cry “I stand with Ming”.

A GoFundMe page set up to help fund his legal expenses notes that he “has contributed enormously to our local community for the past eight years”. As of today, the page has received over $42,000 towards its target of $350,000. 

Meanwhile, another China Initiative case hangs in the balance. In early April a federal jury found University of Kansas chemical engineer Franklin Tao – the first academic charged under the China Initiative – guilty on three charges of wire fraud and one of making a false statement regarding his work for Fuzhou University.

However, comments by US district judge Julie Robinson before and after the jury’s announcement have persuaded Tao’s lawyer, Peter Zeidenberg, that Robinson might overturn the verdict. 

Black-hole physics and that iconic ‘shadow’ image, balloons and rockets probe the atmosphere’s acoustic duct

In this episode of the Physics World Weekly podcast, we meet Shep Doeleman, who is the founding director of the Event Horizon Telescope. He explains how he and his colleagues obtained that iconic image of the “shadow” of the supermassive at the centre of the Milky Way. Based in the US at the Harvard & Smithsonian Center for Astrophysics, Doeleman explains what the image tells us about the physics of black holes, and he looks forward to the day when we can watch “movies” of dancing black-hole shadows.

Also in this week’s podcast is the geophysicist Sarah Albert, who studies sound propagation in the atmosphere by listening in on rocket launches using high-altitude balloons. Albert works at Sandia National Laboratories in the US and talks about what her research reveals about an atmospheric phenomenon called the “acoustic duct”.

Netherlands invests €1.1bn in the photonic-chip industry

The Dutch photonic chip industry has been boosted with €1.1bn of public and private investment. The cash, which includes €470m from the Dutch government’s National Growth Fund, will be used by PhotonDelta over six years to create hundreds of photonic start-up companies and scale up photonic-chip production to encourage development of new photonic applications. 

Photonic chips – also known as photonic integrated circuits (PICs) – are the photonic equivalent of electronic integrated circuits and use small structures called waveguides to transport photons over the chip. These chips can have similar or better functionality than their optical equivalents, and can therefore be used to create smaller, faster and more energy-efficient devices.

PICs can also process and transmit data much faster and more effectively than their electronic counterparts. The production process is also carried out using automatic wafer-scale technology, which cuts costs by allowing them to be mass produced. Photonics chips could have a range of applications from high-speed communications to quantum computing. 

PhotonDelta, which consists of 26 companies, 11 technology partners and 12 R&D partners, specializes in two major PIC production platforms – indium phosphide and silicon nitride – and is already collaborating with the Belgium institute Imec to build a pilot line for silicon photonics.

These three platforms create PICs with different functionalities. Indium phosphide has an active layer and can be used to create lasers, amplifiers and detectors for example for high-speed communication. Silicon nitride can build low-loss waveguides for applications such as biosensing or quantum computing. Silicon photonics are mostly passive devices and benefit from being CMOS compatible and using the same production facilities as standard integrated circuits. 

With the new funding, PhotonDelta hopes by 2030 to include over 200 companies and give the Netherlands a wafer-production capacity of more than 100,000 per year. PhotonDelta director Ewit Roos says that the Netherlands has acquired a globally distinctive position in integrated photonics and that it is now time for a series of follow-up steps. “In this way, we can stay ahead of the competition and strengthen Europe’s intended strategic autonomy in photonics,” he says.

‘Micronovae’ explosions on white dwarfs caused by localized accretion

The mystery of why small explosive bursts occur on some accreting white dwarf stars appears to have been solved by a team of astronomers led by Simone Scaringi at the UK’s Durham University. The team examined bursts of light from three white dwarf systems and noticed that their evolution was similar to X-ray bursts that occur on the surfaces of some neutron stars. The team suggests that these “micronovae” on white dwarfs are caused by matter falling onto the poles of the stars – and that this could be a common phenomenon.

Several times over the past 40 years, astronomers have spotted bursts of optical and ultraviolet light from the binary-star system TV Columbae (TV Col). In less than an hour, this light trebles in brightness, before fading within roughly 10 h.

TV Col is known to contain a white dwarf with a moderately strong magnetic field. The white dwarf is accreting material from its companion star, which is known to produce nova explosions. These are not powerful enough to blow the white dwarf apart, but they can trigger runaway thermonuclear explosions across the entire surface of a star as energy is released through hydrogen fusion. When this happens, the star can shine brightly for several weeks.

Fraction of the energy

The bursts on TV Col release about one millionth of the energy of novae. This could mean that the accreted material is falling onto just a small fraction of the white dwarf’s surface, causing localized fusion. Now, Scaringi and colleagues have found evidence for this explanation.

Looking at data gathered by NASA’s Transiting Exoplanet Survey Satellite (TESS), the team discovered that two other systems produce bursts similar to TV Col. One system is called EI Ursae Majoris and is already known to contain an accreting white dwarf with a moderately strong magnetic field.

The other system is ASASSN-19bh, which little had been known about. Now Scaringi and colleagues have used the European Southern Observatory’s Very Large Telescope in Chile to confirm that ASASSN-19bh does indeed contain a white dwarf with similar characteristics to that of TV Col and EI Ursae Majoris.

Further clue

A further clue to the origin of the bursts came from the observation that time variations in the brightness of bursts from all three systems bear a strong resemblance to X-ray bursts that sometimes occur on the surfaces of accreting neutron stars.

In these neutron stars, accreting material is confined into columns by the extreme magnetic fields of the stars. This means that the material only falls onto the polar regions of those stars. Scaringi and colleagues believe that a similar process is taking place on the three white dwarfs. The result is that thermonuclear burning only occurs in the stars’ polar regions, and this is likely to be the cause of the observed micronovae.

The team believes that these bursts could be common on white dwarfs. To study them in more detail, the astronomers now aim to capture more micronovae in large-scale surveys, and make quick follow-up measurements with instruments including the VLT.

The research is described in Nature.

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