Chirality-Induced Spin Selectivity of Chiral 2D Perovskite Enabling Efficient Spin-Dependent Oxygen Evolution Reaction.

The sluggish and complex multi-step oxygen evolution reaction remains an obstacle to bias-free photoelectrochemical water-splitting systems. Several theoretical studies have suggested that spin-aligned intermediate radicals can significantly enhance the kinetic rates for oxygen generation. Herein, it is reported that the chirality-induced spin selectivity phenomena can become an impressive approach by adopting chiral 2D organic-inorganic hybrid perovskites as a spin-filtering layer on the photoanode. This chiral 2D perovskite-based water-splitting device achieves enhanced oxygen evolution performance with a reduced overpotential of 0.14 V, high fill factor, and 230% increased photocurrent compared to a device without a spin-filtering layer. Moreover, combined with a superhydrophobic patterning strategy, this device realizes excellent operational stability by sustaining ≈90% of the initial photocurrent, even after 10 h.

Scalable, solvent-free transparent film-based air filter with high particulate matter 2.5 filtration efficiency.

Over the course of the COVID-19 pandemic, people have realized the importance of wearing a mask. However, conventional nanofiber-based face masks impede communication between people because of their opacity. Moreover, it remains challenging to achieve both high filtration performance and transparency through fibrous mask filters without using harmful solvents. Herein, scalable transparent film-based filters with high transparency and collection efficiency are fabricated in a facile manner by means of corona discharging and punch stamping. Both methods improve the surface potential of the film while the punch stamping procedure generates micropores in the film, which enhances the electrostatic force between the film and particulate matter (PM), thereby improving the collection efficiency of the film. Moreover, the suggested fabrication method involves no nanofibers and harmful solvents, which mitigates the generation of microplastics and potential risks for the human body. The film-based filter provides a high PM2.5 collection efficiency of 99.9 % while maintaining a transparency of 52 % at the wavelength of 550 nm. This enables people to distinguish the facial expressions of a person wearing a mask composed of the proposed film-based filter. Moreover, the results of durability experiments indicate that the developed film-based filter is anti-fouling, liquid-resistant, microplastic-free and foldability.

A Scalable Haze-Free Antireflective Hierarchical Surface with Self-Cleaning Capability.

The lotus effect indicates that a superhydrophobic, self-cleaning surface can be obtained by roughening the topography of a hydrophobic surface. However, attaining high transmittance and clarity through a roughened surface remains challenging because of its strong scattering characteristics. Here, a haze-free, antireflective superhydrophobic surface that consists of hierarchically designed nanoparticles is demonstrated. Close-packed, deep-subwavelength-scale colloidal silica nanoparticles and their upper, chain-like fumed silica nanoparticles individually fulfill haze-free broadband antireflection and self-cleaning functions. These double-layered hierarchical surfaces are obtained via a scalable spraying process that permits precise control over the coating morphology to attain the desired optical and wetting properties. They provide a "specular" visible transmittance of >97% when double-side coated and a record-high self-cleaning capability with a near-zero sliding angle. Self-cleaning experiments on photovoltaic devices verify that the developed surfaces can significantly enhance power conversion efficiencies and aid in retaining pristine device performance in a dusty environment.

High-Temperature Carbonized Ceria Thermophotovoltaic Emitter beyond Tungsten.

Thermophotovoltaics (TPVs) require emitters with a regulated radiation spectrum tailored to the spectral response of integrated photovoltaic cells. Such spectrally engineered emitters developed thus far are structurally too complicated to be scalable, are thermally unstable, or lack reliability in terms of temperature cycling. Herein, we report wafer-scale, thermal-stress-free, and wavelength-selective emitters that operate for high-temperature TPVs equipped with GaSb photovoltaic cells. One inch crystalline ceria wafers were prepared by sequentially pressing and annealing the pellets of ceria nanoparticles. The direct pyrolysis of citric acid mixed with ceria nanoparticles created agglomerated, pyrolytic carbon and ceria microscale dots, thus forming a carbonized film strongly adhering to a wafer surface. Depending on the thickness of the carbonized film that was readily tuned based on the amount of citric acid used in the reaction, the carbonized ceria emitter behaved as a tungsten-like emitter, a graphite-like emitter, or their hybrid in terms of the absorptivity spectrum. A properly synthesized carbonized ceria emitter produced a power density of 0.63 W/cm2 from the TPV system working at 900 °C, providing 13 and 9% enhancements compared to tungsten and graphite emitters, respectively. Furthermore, only the carbonized ceria emitter preserved its pristine absorptivity spectrum after a 48 h heating test at 1000 °C. The scalable and facile fabrication of thermostable emitters with a structured spectrum will prompt the emergence of thermal emission-harnessed energy devices.

Water penetration dynamics through a Janus mesh during drop impact.

Here, we study the water penetration dynamics through a Janus membrane with opposite wettability, i.e., (super-) hydrophobic on one side and (super-) hydrophilic on the other side, during drop impact. It is demonstrated that the penetration dynamics through the membrane consists of two temporally distinct events: dynamic pressure driven penetration dynamics on a shorter timescale and capillary pressure driven penetration dynamics on a longer timescale. For penetration under dynamic pressure, the threshold velocity for the penetration is dependent on the wettability of the impact side, such that a smaller impact velocity is required for water penetration when a water drop is impinged onto the superhydrophobic side over the superhydrophilic side. We demonstrate that this difference in the penetration dynamics upon drop impact can still be accounted for by the balance between the dynamic pressure and the capillarity pressure after adjusting the relative magnitude of the two contrasting pressures required for the penetration. Meanwhile, it is demonstrated that the penetration dynamics under capillary pressure is governed by the balance between the capillary pressure and the viscous pressure while the penetration mainly proceeds through the penetration area, which is formed during short-time penetration, showing the dynamic coupling between the two penetration dynamics. By elucidating the penetration dynamics on a Janus membrane, we believe that our results can help in designing Janus membranes for various fluidic applications such as oil-water separation, aeration, and water harvesting.

Performance Analysis of Gravity-Driven Oil-Water Separation Using Membranes with Special Wettability.

A membrane with selective wettability to either oil or water has been utilized for highly efficient, environmentally friendly membrane-based oil-water separation. However, a predictive model, which can be used to evaluate the overall separation performance of the membrane, still needs further development. Herein, we investigate three separation performance parameters, that is, separation efficiency, liquid intrusion pressure, and mass flux in particular, as a function of pore geometry and liquid properties using metallic meshes whose surface wettability is modified by scalable spray coating. We show that the prepared membrane exhibits a separation efficiency over 98% below the intrusion pressure, while the intrusion pressure increases with the decrease of pore size of the membrane. Particularly, we develop a semi-empirical model for the mass flux through the membrane. As application examples of our performance analysis, we successfully predict the separation time for one-way and two-way gravity-driven separation of the oil-water mixture, the decrease of the mass flux due to membrane fouling, and the maximum allowable separation capacity of the given membrane. This work can help to design optimal membrane-based oil-water separation systems for actual industrial applications by providing a selection guideline for separation membranes.

Signal to noise ratio of upgraded imaging bolometer for KSTAR.

An InfraRed imaging Video Bolometer (IRVB) was installed on KSTAR in 2012 having a ∼2 µm × 7 cm × 9 cm Pt foil blackened with graphite and a 5 mm × 5 mm aperture located 7.65 cm from the foil with 16 × 12 channels and a time resolution of 10 ms. The IR camera was an Indigo Phoenix (InSb, 320 × 256 pixels, 435 fps, <25 mK). In 2017, the IRVB was upgraded by replacing the IR camera with a FLIR SC7600 (InSb, 640 × 512 pixels, 105 fps, <25 mK). The aperture area was reduced by approximately half to 3.5 mm × 3.5 mm, and the number of channels was quadrupled to 32 × 24. A synthetic image derived using the projection matrix for the upgraded IRVB from a Scrape Off Layer Plasma Simulator (SOLPS) model with 146 kW of total radiated power had a maximum signal of 7.6 W/m2 and a signal to noise ratio (SNR) of 11. Experimental data for a plasma with parameters similar to the SOLPS model (total radiated power of 158 kW) had a maximum signal of 12.6 W/m2 and noise equivalent power density (SNR) of 0.9 W/m2 (14).

Forward projection matrix derivation through Monte-Carlo ray-tracing of KSTAR infra-red imaging video bolometer (IRVB).

The infrared imaging video bolometer (IRVB) as a foil bolometry technique can be an alternative solution to the conventional resistive bolometer due to its electro-magnetic immunity and 2D plasma profiles. The plasma profile of the IRVB cannot be directly derived from the foil images due to the difference between the foil pixel number and the plasma pixel number and the line integrated nature of the incident rays on the foil. So, it needs tomography such as the Phillips-Thikhonov algorithm. The projection matrix constructing the foil image from the plasma very directly influences the tomography. So, the projection matrix needs to be constructed precisely. For the technique calculating the precise projection matrix, a forward Monte-Carlo ray-tracing method is proposed here, and this technique can provide the detailed descriptions of the foil image. And it can give enhanced performance in the reconstructions of the plasmas with spatially localized power.

Reconstruction of radiation profiles near the plasma boundary using an infrared imaging video bolometer in KSTAR.

An infrared imaging video bolometer using tomographic inversion can provide the total radiated power and 2-D radiation profiles, which are crucial information for impurity seeding experiments. Because large amounts of impurities exist at the plasma edge, accurate reconstruction of the radiation profiles near the material boundary is an important issue. In this study, two methods of boundary condition treatment are compared. One involves the exclusion of plasma pixels outside the boundary before reconstruction, whereas the other excludes these pixels after reconstruction. Phantom reconstruction tests are performed with D-shaped and divertor radiation profiles, and the second method shows an improvement in the boundary-reconstruction results compared with the first method. Using the second method, the radiation profiles of krypton (Kr) seeded H-mode plasmas in KSTAR are reconstructed. A significant amount of input power is dissipated through the Kr radiation. The 2-D reconstructed radiation profiles show that Kr mostly accumulates at the plasma core rather than at the edge.

Condensation Heat-Transfer Performance of Thermally Stable Superhydrophobic Cerium-Oxide Surfaces.

We introduce a thin (<200 nm) superhydrophobic cerium-oxide surface formed by a one-step wet chemical process to enhance the condensation heat-transfer performance with improved thermal stability compared to silane-treated surfaces. The developed cerium-oxide surface showed a superhydrophobic characteristic with a low (<5°) contact angle hysteresis because of the unique surface morphology and hydrophobicity of cerium oxide. The surface was successfully incorporated to popular engineering materials including copper, aluminum, and steel. Thermal stability of the surfaces was investigated by exposing them to hot (∼100 °C) steam conditions for 12 h. The introduced ceria surfaces could maintain active dropwise condensation after the thermal stability test, whereas silane-treated surfaces completely lost their hydrophobicity. The heat-transfer coefficient was calculated using the thermal network model incorporating the droplet size distribution and morphology obtained from the microscopic measurement. The analysis shows that the suggested cerium-oxide surfaces can provide approximately 2 times and 5 times higher heat-transfer coefficient before and after the thermal stability test, respectively, mainly because of the decrease in the thermal conduction resistance across droplets. The results indicate that the introduced nanostructured cerium-oxide surface is a promising condenser coating to enhance the droplet mobility and the resulting condensation heat-transfer performance for various thermal and environmental applications, especially those being exposed to hot steam conditions.

Examinations of electron temperature calculation methods in Thomson scattering diagnostics.

Electron temperature from Thomson scattering diagnostic is derived through indirect calculation based on theoretical model. χ-square test is commonly used in the calculation, and the reliability of the calculation method highly depends on the noise level of input signals. In the simulations, noise effects of the χ-square test are examined and scale factor test is proposed as an alternative method.

Conceptual design of new polychromator on Thomson scattering system to measure Zeff.

To measure the Z(eff) with electron temperature (T(e)) and electron density (n(e)) profiles at the same time and the same position in the KSTAR tokamak, we design a new polychromator for Thomson scattering system that has additional function. The additional function is measuring bremsstrahlung intensity to calculate Z(eff) independent of Thomson signals. For this new polychromator, we design and fabricate a collimation lens set, and interference filter that has center wavelength of 523 nm and 2 nm FWHM. Finally, we change the lenses, detector diodes, and add the bremsstrahlung filter on the KSTAR edge Thomson scattering polychromator. Then this new polychromator was tested by Tungsten light and monochromator.

Fabrication of helical long-period fiber gratings by use of a CO2 laser.

We present a method of helical long-period fiber grating (H-LPFG) fabrication by use of a CO2 laser for use as an optical torque sensor. A conventional optical fiber grating has periodic vertical index changes along its fiber axis, but a H-LPFG has a screw-type index modulation. The helical index modulation is obtained with the asymmetric index change caused by a single-side laser beam exposure. The H-LPFG shows peak shifts with codirectional or contradirectional torsion to the helix. Also, the polarization-dependent loss is measured to be relatively small compared with that of a conventional long-period fiber grating.

Discrimination of temperature and strain with a single FBG based on the birefringence effect.

We will demonstrate a new technique to discriminate the temperature and strain effects using a single fiber Bragg grating (FBG). The birefringence is typically induced during FBG inscription, and it is manifested as polarization-dependent loss (PDL), and it is defined as the maximum change in the transmitted power for polarizations. Two independent measurements of the resonance wavelength shift and the changes of PDL can discriminate those effects.