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K2-18b: A Habitable Zone Exoplanet 124 Lightyears Away

Krishna Amin (St Catharine’s). February 27, 2020. 

In a paper published today (27 Feb), researchers from the Institute of Astronomy revealed findings on the interior and atmospheric composition of exoplanet K2-18b, orbiting an M-dwarf (‘low-mass’) star in the habitable zone, only 124 lightyears away from Earth.

K2-18b’s density, between those of Earth and Neptune, suggested a hydrogen-rich outer envelope surrounding a rocky interior. Previous studies of similar planets proposed temperatures of around 250-300 Kelvin (-23 to 27 °C) – similar to those found on Earth. Given these properties, the authors detected the presence of water and the absence of methane and ammonia and did not find ‘strong evidence’ for clouds in the atmosphere.

The interior of the planet was modelled with an inner iron layer, an outer silicate layer, a water layer and a hydrogen/helium layer. Notice the similarities to Earth’s own structure: iron core, silicate mantle and crust, oceans, some sort of atmosphere. Consideration of variations on the model (i.e. different compositions and masses of different layers) resulted in three ‘representative classes’ defining K2-18b that include a ‘range of possible compositions’: rocky world, mini-Neptune and water world.

Life as we know it can survive in a huge range of harsh conditions, from pressures of ~1000 atmospheres and temperatures of ~400 K (127 °C). Whether or not K2-18b is habitable depends on the extent of the hydrogen/helium atmosphere. Many solutions to the data give water at the atmosphere-ocean boundary – the surface of the water layer – to be in the ‘supercritical’ phase, but some give water in the liquid or gas phases. The ‘water world class’ has liquid water approaching normal conditions (27 °C, 1-10 atmospheres) under a thin hydrogen/helium atmosphere, a description seemingly like that of Earth. Furthermore, chemical disequilibrium – the absence of methane and ammonia – indicates the possibility of biochemical processes, although other explanations exist. The authors argue that the search for biosignatures – signs of life – should not be limited to smaller rocky worlds as larger planets such as K2-18b have the potential to host life.

Their paper:
Nikku Madhusudhan et al 2020 ApJL 891 L7.

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Novel Solid-State Alternative to Commerical Cooling

Krishna Amin (St Catharine’s). October 16, 2019.

Material science researchers at the University of Cambridge have recently reported in Nature the development of a new design of electrocaloric (EC) cooling devices (which can change in temperature when an electric field is applied) which works over a wider and more usable temperature range and a larger effect than previous proposals.

The new designs make use of multi-layer capacitors (MLCs) made of an assembly of films of the well-known EC material PST. Their highly-ordered PST was able to ‘access large EC effects’ while representing a potential macroscopic option. Recorded temperature changes peaked at 5.5°C near room temperature, and showed changes of 3°C over temperatures ranging from 21°C – 197°C. Compared to current macroscopic cooling devices – ones that make use of magnetocaloric (MC, which change in temperature when a magnetic field is applied) cooling using bulky, permanent magnets and gadolinium (Gd – a lanthanide metal) – these data fare well, suggesting that EC cooling using MLCs of highly ordered PST could provide a cost-effective alternative to current MC designs.

This has exciting implications for cooling consumer electronics and solar cells and can be used to reduce below room temperature using doped PST. Furthermore, the new design could lead to alternatives to current methods of air conditioning, which are relatively energy-intensive.

Their paper:
Nair, B., Usui, T., Crossley, S. et al. Large electrocaloric effects in oxide multilayer capacitors over a wide temperature range. Nature 575, 468–472 (2019).

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Greater Efficiency from Less Ordered Solar Cells

Krishna Amin (St Catharine’s). December 2, 2019. 

Cantabrigian researchers have suggested that solar cells can have increased efficiency if their chemical compositions are less ordered.

The work, published in Nature Photonics, comes from an international team of scientists, led by Dr Samuel Stranks and Dr Felix Deschler, with members from the Cavendish Laboratory, Department of Material Sciences and Metallurgy, Department of Earth Sciences and the Department of Chemical Engineering and Biotechnology.

Solar cells are traditionally made of crystalline silicon, but perovskite solar cells have recently emerged as promising alternatives, with efficiencies of above 25%. The lead or tin-based materials are easier to source and manufacture than their silicon predecessors, thus reducing costs and increasing the feasibility of solar cells as an energy source.

Less structurally refined products were found to be further increasing the efficiency of perovskite solar cells by creating pockets which can localise charge, created by either sunlight in a solar cell or electrical currents in an LED. It is then easier to extract that energy from the material.

The localisation of the charge is stabilised by chosen cations in the surrounding material, so next steps for the team involve improving performance through identifying ideal cations, as well as finding the right conditions for taming the ‘chaos’ that lends augmented efficiency. Furthermore, if perovskite solar cells are to become widespread in the future, they will need to reduce their sensitivity to water, a trait for which silicon still holds the advantage.

Their paper:
Feldmann, S., Macpherson, S., Senanayak, S.P. et al. Photodoping through local charge carrier accumulation in alloyed hybrid perovskites for highly efficient luminescence. Nat. Photonics 14, 123–128 (2020).

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Nausea-free AR

Krishna Amin (St Catharine’s). September 29, 2019.

Cambridge researchers at the Centre for Photonic Devices and Sensors and their colleagues from Huawei Technologies Duesseldorf GmbH have developed a new AR headset that eliminates nausea during use and improves image quality and the field of view, with implications for numerous industries.

Accommodation-free displays, also known as Maxwellian displays, keep the displayed image sharp regardless of the viewer’s focal distance. However, they typically suffer from a small eye-box and limited effective field of view (FOV) which requires careful alignment before a viewer can see the image. Their paper presents a high-quality accommodation-free head mounted display (aHMD) based on pixel beam scanning for direct image forming on the retina, using narrow, collimated pixel beams. It has an enlarged eye-box and FOV for easy viewing by replicating the viewing points with an array of beam splitters.

A prototype aHMD built using this concept shows high definition, low colour aberration 3D augmented reality (AR) images with a FOV of 36∘, a marked improvement on holographic displays, with poor image quality and a limited FOV of 4.9 degrees. The advantage of the proposed design over other head mounted display (HMD) architectures is that the high image quality is unaffected by changes in eye accommodation, and the approach to enlarge the eye-box is scalable. Most importantly, such an aHMD can deliver realistic three-dimensional (3D) viewing perception without vergence-accommodation conflict (VAC), which causes nausea, dizziness, eyestrain and inaccurate depth perception. Viewing the accommodation-free 3D images with the aHMD presented in this work was comfortable for viewers and did not cause the VAC symptoms commonly associated with conventional stereoscopic 3D or HMD displays, even for all-day use. Furthermore, the aHMD put forward in their paper delivers all the above improvements in a relatively compact system – previous solutions had resulted in impractically bulky optics. This new development in the world of AR has promising implications in almost every conceivable field, including education, sports and the military.

Their paper:
Pawan K. Shrestha, Matt J. Pryn, Jia Jia, et al., “Accommodation-Free Head Mounted Display with Comfortable 3D Perception and an Enlarged Eye-box,” Research, vol. 2019, Article ID 9273723, 9 pages, 2019.

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An Artifical Leaf

Krishna Amin (St Catharine’s). November 16, 2019. 

Researchers from the Department of Chemistry announced a few weeks ago in Nature Materials the development of a new method of syngas production, paving the way for greener production practices.

Syngas is a crucial intermediate in the production of complex hydrocarbons, with applications extending to pharmaceuticals and fertilisers. The conventional reforming of methane to syngas is highly energy intensive and not always very efficient. Photoelectrochemical (PEC) production of syngas, a mixture of CO and H2, is an attractive green method of enabling a cyclical carbon economy. Current attempts, however, have been hindered by the high overpotential, low selectivity and cost of their catalysts.

The new method, proposed by Virgil Andrei, Bertrand Reuillard and Erwin Reisner, makes use of cobalt (II) meso-tetrakis(4-methoxyphenyl)porphyrin (CoMTPP), a molecular catalyst avoiding the sustainability question by using cobalt, a commonly available earth metal. CoMTPP is immobilised onto carbon nanotube (CNT) sheets (buckypaper), and the combination is employed in electrodes, perovskite-based photocathodes and perovskite-BiVO4 PEC tandem devices.

The result is tuneable syngas production through a tandem PEC device that reduces CO2 to CO through coupling to the oxidation of water to O2. Light intensity as low as 0.1sun still permits reduction, so that syngas could be produced during all day regardless of weather conditions, potentially answering the questions of economics and reliability that might hinder the development of this technology into a dominant source of hydrocarbon product.

Their paper:
Andrei, V., Reuillard, B. & Reisner, E. Bias-free solar syngas production by integrating a molecular cobalt catalyst with perovskite–BiVO4 tandems. Nat. Mater. 19, 189–194 (2020).

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