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Self-regulating high temperature superconducting (HTS) flux pump is one of the most effective methods to compensate the current degradation of a closed HTS magnet for its stable operation. In this study, to understand the working principle as well as the influential factors regarding to its operation performance, a basic circuit model has been established. Based on that, we found the saturation load current is strongly related to the added secondary resistance, which could offset the DC bias current induced in the charging loop. Then, we built a prototype experimentally, in which, part of the load coil was used as a HTS bridge and bifilar bridge design has been adopted to minimize the possible flux fluctuation in the load loop. In the experiment, the dependence of the charging performance on the bridge length as well as the added secondary resistance at different frequencies has been clarified. The results show that, the load current is remarkably impacted by the aforementioned parameters. Specifically speaking, with an increasing added secondary resistance, the operating frequency needs to be higher to ensure the secondary current can follow the primary current, which is crucial for maintaining a larger load current.

Carbonyl reductase (CR)-catalyzed asymmetric reduction offers an approach for producing t-butyl 6-cyano-(3R,5R)-dihydroxyhexanoate ((3R,5R)-1b), which serves as a key building block in atorvastatin...

(S)-2-chlorophenylglycine methyl ester ((S)-1) is a key chiral building block of clopidogrel, which is a widely administered antiaggregatory and antithrombotic drug. Herein, Protease 6SD was covalently immobilized on multi-walled carbon nanotubes (MWCNT), and the as-prepared immobilizate P-6SD@NH2-MWCNT was applied in the enantioselective resolution of (R,S)-1 to yield (S)-1. In order to overcome the poor solubility of (R,S)-1 in aqueous solution, a novel triphasic reaction system constituting P-6SD@NH2-MWCNT, aqueous phase and methyl tert-butyl ether (MTBE) as the organic phase was constructed, which simultaneously improved the substrate solubility and the immobilizate recyclability. Under the optimized reaction conditions, P-6SD@NH2-MWCNT catalyzed 10 mM (R,S)-1 for 2 h, yielding optically pure (S)-1 (>99.0 % ees) with 70.74 % conversion of the (R,S)-1. Moreover, P-6SD@NH2-MWCNT can be reused for 15 batches, displaying an exquisite recycling performance. It is for the first time that enantiomerically pure (S)-1 was successfully synthesized by protease-catalyzed one-step resolution.

Graphene with low oxygen content via an electrochemically anode exfoliation is desired. Here, an electrochemical exfoliation method to realize the simultaneous electrochemical preparation and reduction of graphene with low oxygen content is proposed using a flexible graphite sheet (FGS). Under water bath, the FGS is soaked into a NaBH4 solution under an electric field of 3 V/cm for 3, 9, 15 or 21 h (Gr-3/Gr-9/Gr-15/Gr-21). The C/O ratios of the FGS, Gr-3, Gr-9, Gr-15 and Gr-21 increase in turn by the X-ray photoelectron spectroscopy. The comparison between the electrochemical properties of the FGS, Gr-3, Gr-9, Gr-15 and Gr-21 indicates that the Gr-15 possesses a specific capacitance of 181 F/g and capacitance retention of 93.8% over 3000 cycles, which shows better electrochemical performances than the FGS, Gr-3, Gr-9 and Gr-21. The symmetric supercapacitor based on the Gr-15 achieves an energy density as high as 22.56 Wh/kg at a power density of 0.85 kW/kg.

In order to charge the superconducting magnets, the means of current leads has been utilized widely. Nevertheless, due to the losses in high-temperature superconducting (HTS) magnets, the need for compensating the current decay periodically promotes the development of HTS flux pumps. Among them, transformer-rectifier type flux pump grabs attention owing to the simple rectifying component. However, it is the principle of bridge rectifying that may lead to an unavoidable induced direct current which self-evidently would make a negative impact on the charging speed. To this regard, this paper proposed the means of combining the current-controlled transformer-rectifier type flux pump with a secondary resistor to achieve an improved charging performance. With a verification experiment carried out, it was confirmed that the proposed method is viable. And there exists an optimal resistance -0.9 mΩ, which is essential to speed the charging progress up.

The rational design of highly active and stable atomically dispersed M-X4 (M = Fe, Co, Ni, etc., X = C, N) -based catalysts holds promises for wide application in almost all realms of catalysis. Despite great effort in the construction of specific M-X4 centers, the possible effect of non-coordinated heteroatoms on the catalytic activity of metal centers has been rarely explored. Herein, we develop a new type of M-X4 catalyst composed of Fe-N4 centers and non-coordinated B heteroatoms (FeNC+B) and find the key role of non-coordinated B adjacent to Fe-N4 centers in tailoring their electron density and final catalytic selectivity. The experimental and theoretical results demonstrated that non-coordinated boron atoms could decrease the electron density of Fe-N4 centers to a suitable level and thus boost the selective production of nitriles from amine oxidation by depressing the formation of imines due to the flattened energy barrier of the reversible conversion of imines back to amines. As a reusable heterocatalyst, the state-of-the-art FeNC+B catalyst provides a turn-over frequency (TOF) value of 21.6 molbenzonitrile·molFe−1·h−1 (100 °C), outpacing that of bench-marked nonnoble-metal-based homogeneous catalyst by a factor of 3.4.

The activation of C–H bonds is a central challenge in organic chemistry and usually a key step for the retro-synthesis of functional natural products due to the high chemical stability of C–H bonds. Electrochemical methods are a powerful alternative for C–H activation, but this approach usually requires high overpotential and homogeneous mediators. Here, we design electron-deficient W2C nanocrystal-based electrodes to boost the heterogeneous activation of C–H bonds under mild conditions via an additive-free, purely heterogeneous electrocatalytic strategy. The electron density of W2C nanocrystals is tuned by constructing Schottky heterojunctions with nitrogen-doped carbon support to facilitate the preadsorption and activation of benzylic C–H bonds of ethylbenzene on the W2C surface, enabling a high turnover frequency (18.8 h−1) at a comparably low work potential (2 V versus SCE). The pronounced electron deficiency of the W2C nanocatalysts substantially facilitates the direct deprotonation process to ensure electrode durability without self-oxidation. The efficient oxidation process also boosts the balancing hydrogen production from as-formed protons on the cathode by a factor of 10 compared to an inert reference electrode. The whole process meets the requirements of atomic economy and electric energy utilization in terms of sustainable chemical synthesis. Designing electrode materials for mild and additive-free activation of C–H bonds is of great challenge. The authors report the application of electron-deficient W2C nanocrystal electrodes to boost the dissociation of C-H bonds toward the efficient alkoxylation and hydrogen evolution reactions.

The traditional NH3 production method (Haber-Bosch process) is currently complemented by electrochemical synthesis at ambient conditions, but the rather low selectivity (as indicated by the Faradaic efficiency) for the electrochemical reduction of molecular N2 into NH3 impedes the progress. Here, we present a powerful method to significantly boost the Faradaic efficiency of Au electrocatalysts to 67.8% for the nitrogen reduction reaction (NRR) by increasing their electron density through the construction of inorganic donor-acceptor couples of Ni and Au nanoparticles. The unique role of the electron-rich Au centers in facilitating the fixation and activation of N2 was also investigated via theoretical simulation methods and then confirmed by experimental results. The highly coupled Au and Ni nanoparticles supported on nitrogen-doped carbon are stable for reuse and long-term performance of the NRR, making the electrochemical process more sustainable for practical application.

Due to the local emissions and transportations of air pollution from the most polluted regions such as the North China Plain and Yangtze Delta metropolitan, Xuzhou is becoming one of the most polluted cities in East China. The sources and formation processes of nitrogen-containing aerosols are therefore very complex. Two continuous aerosol measurement campaigns were conducted in this industrial city during the wintertime and summertime of 2016, to investigate the chemical compositions and potential sources of total nitrogen (TN, including 89% inorganic nitrogen and 11% organic nitrogen) in PM2.5. Abrupt enhancements of nitrogen-containing aerosols (e.g., NO3− and NH4+) were found in the winter, and nitrate became as a dominant contributor in high pollution days (e.g., PM2.5 > 150 μg m−3). Nitrogen oxidation ratios (NOR) correlated significantly with aerosol liquid water content (ALWC), which was estimated by ISORPROPIA-II model. This suggested heterogeneous process might be an important pathway in nitrate formation during the high PM2.5 days. The nitrogen isotope composition (δ15N) in TN varied from −1.3 to +13.2‰ with a mean value of 6.9 ± 3.6‰ during the wintertime. An isotope-based source apportionment approach was then developed using a Bayesian isotope mixing model (SIAR) with chemical compositions as an important constrain, which improved accuracy and reduced the overall uncertainties in estimations of TN sources. From this optimized model, we identified six major sources including NH3 from combustion-related emissions (49%), NH3 derived from animal wastes (6%), NH3 from urban volatilization (3%), NOx derived from coal combustion (33%), NOx from biomass burning (5%) and NOx from vehicles (3%). Our results demonstrated that ambient NOx was dominated by coal combustion. Since NOx and NH3 are important precursors for ammonium nitrate aerosols, controlling of combustion related NOx and NH3 emissions might be an important way to reduce PM2.5 levels in this region.

Several conditions of two-step chemical reduction method for simultaneously determination of stable nitrogen and oxygen isotope compositions in atmospheric nitrate (NO3−) particles, such as the pH, reaction time, type of cadmium powder and concentration of NaCl, were optimized. The nitrite (NO3−) in the samples was reduced to nitrous oxide (N2O) through the improved method and the produced N2O was measured for their isotope compositions (δ15N and δ18O) by coupling an autosampler and a pre-concentration device with the MAT253 isotope ratio mass spectrometer. The standard curves of the nitrogen (R2 = 0.9999) and oxygen (R2 = 0.9996) isotope ratios of N2O and NO3− were established by measuring the international standards with known isotope ratios, and the nitrogen and oxygen isotope ratio of nitrates of the sample could be therefore converted. The principle of this method was clear, and only required very small amounts of samples (i.e., 0.8 μg of N). The standard deviations of δ15N and δ18O were 0.08‰ and 0.24‰, respectively. This method was not only suitable for atmospheric aerosol samples, but also for the other environmental samples such as rainwater and snow samples with low concentrations. This study provided a novel method for better understanding emission sources and transformation pathways of atmospheric nitrate.

A stronger, cooler wood One good way to reduce the amount of cooling a building needs is to make sure it reflects away infrared radiation. Passive radiative cooling materials are engineered to do this extremely well. Li et al. engineered a wood through delignification and re-pressing to create a mechanically strong material that also cools passively. They modeled the cooling savings of their wood for 16 different U.S. cities, which suggested savings between 20 and 50%. Cooling wood would be of particular value in hot and dry climates. Science , this issue p. 760

Singlet oxygen as an activated oxygen species played an important role in organic synthesis. Suitable catalyst for converting ubiquitous oxygen molecule to singlet oxygen under mild conditions has attracted a wide range of attention. Herein, carbon dots have been confined into mesopores of silicalite-1 nanocrystals framework and acted as active sites for generation of singlet oxygen. The high oxygen-adsorption capacity of zeolite nanocrystals facilitated the photocatalytic generation rate of singlet oxygen, outpacing the free-standing carbon dots for 14-fold. The integrated carbon dot-zeolite nanocrystal hybrid also exhibited a special size-dependent selectivity for organic synthesis by using the in situ formed and confined singlet oxygen as active oxygen species.

We described an effective way to generate a Co3O4 mesocrystal array with well-developed porosity, simply by uniting a coupled interface with hydrazine treatment. Due to the fast electron transfer and sufficient active sites, the Ti mesh-supported Co3O4 nanoneedles electrode could provide a current density of 49.9 mA cm−2 at 570 mV OER overpotential and has exceptionally high stability.

Summary Real-world applications of radiative sky cooling require thoughtful design of the system, along with clear understanding of weather effects on system performance. This work explores application of radiative sky cooling based upon a low-cost radiative cooling metamaterial that can be scalably manufactured. A radiative cooled-cold collection (RadiCold) module is developed to cool water to 10.6°C below ambient at noon under stationary conditions. The effects of different weather conditions (wind speed, precipitable water, and cloud cover) on the performance of radiative cooling have been investigated. A kilowatt (kW)-scale RadiCold system with 13.5 m2 radiative cooling surface area is then built and demonstrated to provide a maximum cooling power of 1,296 W at night, and an average cooling power of 607 W at noon (12–2 p.m.) under 952 W/m2 average solar irradiance at 26.5 L/(h⋅m2) volumetric flow rate. A building-integrated RadiCold system is proposed to provide continuous day-and-night cooling.

As a new type of heterogeneous catalyst with "homogeneous-like" activity, single-site transition-metal materials are usually treated as integrated but separate active centers. A novel grouping effect is reported for single Ni-N4 sites in nitrogen-doped carbon (Ni/NC), where an effective ligand-stabilized polycondensation method endows Ni/NC nanocatalysts with a high content of single-site Ni up to 9.5 wt %. The enhanced electron density at each single Ni-N4 site promotes a highly efficient hydrogen transfer, which is exemplified by the coupling of benzyl alcohol and aniline into N-benzylaniline with a turnover frequency (TOF) value of 7.0 molN-benzylaniline molmetal-1 h-1 ; this TOF outpaces that of reported stable non-noble-metal-based catalysts by a factor of 2.

Ba(Co1/3(Nb1-xW5x/6)2/3)O3 (06+. The Ba(Co1/3(Nb1-xW5x/6)2/3)O3 solid solution is formed with the small substitution of W6+ (x≤0.01). With the increase of x, the impurity phase BaWO4 occurrs and the grains become less homogeneous. Combined with the analysis of X-Ray Diffraction, the cation ordering degree on B-site increases in the sample of x=0.01, which leads to the decrease of the dielectric loss. However, the second phase BaWO4 inhibits the formation of the ordered structure with the reduction of sinterability. Due to the formation of BaWO4, the values of εr and τf of all samples decrease subsequently. The optimized microwave dielectric properties of Ba(Co1/3(Nb1-xW5x/6)2/3)O3 ceramics were obtained in the samples of x=0.01: εr=33.8, Q×f =110761 GHz, τf = -17 ppm/°C.

Designing functional and reusable electrode materials to replace homogeneous versions of scarifying additives or mediators in electrochemical organic synthesis is of great importance for both the fundamental academics and practical production of various useful compounds. Herein, we rediscover the function of nitrogen-doped carbon monoliths (NxC) as de novo anode materials to trigger a new reaction path for the synthesis of acetals from the direct coupling of alcohols without the assistance of any mediator, oxidant, acid, or organic ligand. The lowered valence bands of carbon monoliths induced by doping nitrogen are of key importance for polarizing and activating the pre-adsorbed O–H groups of benzyl alcohol molecules to boost the high Faradaic efficiency (71% at 1.8 V versus Ag/AgCl) and reusability of NxC anodes, meeting the requirements of green chemistry.

The lazy way to keep cool in the sun Passive radiative cooling requires a material that radiates heat away while allowing solar radiation to pass through. Zhai et al. solve this riddle by constructing a metamaterial composed of a polymer layer embedded with microspheres, backed with a thin layer of silver (see the Perspective by Zhang). The result is an easy-to-manufacture material near the theoretical limit for daytime radiative cooling. The translucent and flexible film can be made in large quantities for a variety of energy technology applications. Science , this issue p. 1062 ; see also p. 1023

Predicting tropospheric cloud formation and subsequent nutrient deposition relies on understanding the sources and processes affecting aerosol constituents of the atmosphere that are preserved in cloud water. However, this challenge is difficult to address quantitatively based on the sole use of bulk chemical properties. Nitrogenous aerosols, mainly ammonium (NH4+) and nitrate (NO3-), play a particularly important role in tropospheric cloud formation. While dry and wet (mainly rainfall) deposition of NH4+ and NO3- are regularly assessed, cloud water deposition is often underappreciated. Here we collected cloud water samples at the summit of Mt. Tai (1545 m above sea level) in eastern China during a long-lasting biomass burning (BB) event and simultaneously measured for the first time the isotopic compositions (mean ±1σ) of cloud water nitrogen species (δ15N-NH4+ = −6.53 ‰ ± 4.96 ‰, δ15N-NO3- = −2.35 ‰ ± 2.00 ‰, δ18O-NO3- = 57.80 ‰ ± 4.23 ‰), allowing insights into their sources and potential transformation mechanism within the clouds. Large contributions of BB to the cloud water NH4+ (32.9 % ± 4.6 %) and NO3- (28.2 % ± 2.7 %) inventories were confirmed through a Bayesian isotopic mixing model, coupled with our newly developed computational quantum chemistry module. Despite an overall reduction in total anthropogenic NOx emission due to effective emission control actions and stricter emission standards for vehicles, the observed cloud δ15N-NO3- values suggest that NOx emissions from transportation may have exceeded emissions from coal combustion. δ18O-NO3- values imply that the reaction of OH with NO2 is the dominant pathway of NO3- formation (57 % ± 11 %), yet the contribution of heterogeneous hydrolysis of dinitrogen pentoxide was almost as important (43 % ± 11 %). Although the limited sample set used here results in a relatively large uncertainty with regards to the origin of cloud-associated nitrogen deposition, the high concentrations of inorganic nitrogen imply that clouds represent an important source of nitrogen, especially for nitrogen-limited ecosystems in remote areas. Further simultaneous and long-term sampling of aerosol, rainfall, and cloud water is vital for understanding the anthropogenic influence on nitrogen deposition in the study region.

Production of ammonia is currently realized by the Haber–Bosch process, while electrochemical N2 fixation under ambient conditions is recognized as a promising green substitution in the near future. A lack of efficient electrocatalysts remains the primary hurdle for the initiation of potential electrocatalytic synthesis of ammonia. For cheaper metals, such as copper, limited progress has been made to date. In this work, we boost the N2 reduction reaction catalytic activity of Cu nanoparticles, which originally exhibited negligible N2 reduction reaction activity, via a local electron depletion effect. The electron-deficient Cu nanoparticles are brought in a Schottky rectifying contact with a polyimide support which retards the hydrogen evolution reaction process in basic electrolytes and facilitates the electrochemical N2 reduction reaction process under ambient aqueous conditions. This strategy of inducing electron deficiency provides new insight into the rational design of inexpensive N2 reduction reaction catalysts with high selectivity and activity., Electrocatalytic nitrogen reduction is promising for ammonia production, but electrocatalysts are limited by low efficiency and high cost. Here, the authors report electron-deficient copper nanoparticles, induced by rectifying contact with polyimide, for selective reduction of nitrogen to ammonia.

A nitrogen-thermal approach via the reaction between transition metal species and N dopants affords us the ability to optimize the tradeoff between the number of exposed transition metal/carbon (exemplified by cobalt in this work) boundaries and the most pronounced interfacial rectifying contact to achieve the highly efficient and selective hydrogenation and dehydrogenation of N-heterocycle compounds in a reversible manner.

In this paper, we report a mid-wave infrared (MWIR) and long-wave infrared (LWIR) dual-band photodetector capable of voltage-controllable detection band selection. The voltage-tunable dual-band photodetector is based on the multiple stacks of sub-monolayer (SML) quantum dots (QDs) and self-assembled QDs. By changing the photodetector bias voltages, one can set the detection band to be MWIR, or LWIR or both with high photodetectivity and low crosstalk between the bands.

In the paper, we discuss the theories of the information acquisition and analysis and the information quality of the case-based reasoning (CBR) personalized recommendation system. We also take a deep study of the key techniques of acquiring and analyzing information quality. With research results of this paper, combined with the content-based recommendation technology and recommendation results of collaborative filtering, a CBR-based personalized combinatorial recommendation algorithm is designed.

Radiative sky cooling cools an object on the earth by emitting thermal infrared radiation to the cold universe through the atmospheric window (8–13 μm). It consumes no electricity and has great potential to be explored for cooling of buildings, vehicles, solar cells, and even thermal power plants. Radiative sky cooling has been explored in the past few decades but limited to nighttime use only. Very recently, owing to the progress in nanophotonics and metamaterials, daytime radiative sky cooling to achieve subambient temperatures under direct sunlight has been experimentally demonstrated. More excitingly, the manufacturing of the daytime radiative sky cooling material by the roll-to-roll process makes large-scale deployment of the technology possible. This work reviews the fundamental principles of radiative sky cooling as well as the recent advances, from both materials and systems point of view. Potential applications in different scenarios are reviewed with special attention to technology viability and benefits. As the energy situation and environmental issues become more and more severe in the 21st century, radiative sky cooling can be explored for energy saving in buildings and vehicles, mitigating the urban heat island effect, resolving water and environmental issues, achieving more efficient power generation, and even fighting against the global warming problem.

Surfaces with tunable topological features enable important applications, such as optical devices, precision metrology, adhesion, and wetting. In this study, we demonstrate a facile method to fabricate and control the surface morphologies by combining thin film wrinkling and thermal expansion. This approach utilizes self-assembled surface wrinkling induced by shape recovery of shape memory polymers (SMPs) and localized thermal expansion caused by Joule heating. Recovering the prestrain in the SMP substrate induces global wrinkling of the thin film on the substrate. Joule heating in the SMP by a heating wire embedded in the substrate induces thermal expansion of the substrate in a localized area, which leads to the disappearance of the wrinkling pattern. This effect is reversed when heating is stopped, leading to reversible and repeatable tuning of the surface morphology in a controllable localized surface region. With metal coating, the SMP surface can be switched from specular to diffuse reflectance in response to external Joule heating. Finally, we demonstrate a smart micromirror device with its diffuse reflectance tunable between 13.5% and 81.9% in the visible light region. This approach provides a method to modulate surface diffusivity by controlling its surface morphologies, with potential applications in optical display and optical microelectromechanical systems devices.

Shape memory polymers (SMPs) can remember different shapes and can be recovered to their permanent shapes from temporary shapes with appropriate stimuli, such as heat, humidity, and electrical field. Using programmed thermal responsive SMPs as substrates, we demonstrate a self-assembly fabrication method for programmable surface wrinkling within a highly confined area that is accurately controllable. Different from global wrinkling reported in most of the literature, Joule heating through a heating wire embedded in the SMP substrate leads to temperature increase and thus recovery in a highly confined area of the SMP substrate, inducing localized wrinkling of the stiff thin film on SMPs. The patterns show good sinusoidal profiles, with the wrinkling wavelength and amplitude decreasing gradually with the distance from the heat source. The surface wrinkling area can be accurately tuned by controlling the heat input, such as power and duration. Based on this unique surface wrinkling phenomenon, we demonstrate a nonuniform reflective optical grating device, whose peak wrinkling wavelength and amplitude decrease gradually away from the heat source. This study offers a simple method to fabricate programmable localized wrinkling patterns, with potential applications in surface engineering, advanced manufacturing, optical gratings, and other demanding areas.

Optical devices with tunable specular optical transmittance have recently attracted great interest due to their wide range of applications. However, the reported methods of realizing tunable optical transmittance suffer from complex fabrication processes, high cost, unstable materials, or low tuning range. In this study, a simple, cheap, and highly effective approach to achieve large tuning range of optical transmittance through harnessing surface wrinkling–cracking patterns on polydimethylsiloxane (PDMS) films is reported. The surface wrinkling–cracking patterns are induced by stretching ultraviolet/ozone-treated PDMS films and can effectively scatter the light transmitted through the films. With moderate 50% uniaxial tensile strain, the optical transmittance can be tuned between 92 and 9.2%. The films can be reversibly tuned between transparent and opaque for 1000 cycles without losing structural integrity and optical performance, with promising application in smart windows. By patterning the surface wrinkling–cracking patterns, an elastomeric switchable display is also demonstrated, which can be turned “ON” and “OFF” by mechanical strain. The material design demonstrated in this work offers a promising means to dynamically tune the optical properties of functional materials via strain-controlled surface topography. This study can find applications in various optical devices and systems that require tunable optical properties.