Search

Over the past decades, many efficient methodologies have been developed that allow for the enantioselective synthesis of chiral sulfinyl compounds. However, the enantioselective deoxygenation of hexavalent sulfones for the formation of chiral sulfinyl compounds still remains one of the major challenges in the fields of asymmetric synthesis and organosulfur chemistry. Here we have demonstrated that a synergistic combination of organocatalysis and the incorporation of a cyano group into the sulfone generates a chiral sulfinic species as an active intermediate. A wide range of chiral sulfinates with high enantioselectivities could then be acquired using alcohols as nucleophiles, and the subsequent transformations allowed the collective preparation of a variety of chiral sulfinyl compounds. Density functional theory calculations revealed that the catalytic cycle involves a quinuclidine-assisted stepwise 1,2-cyano group transfer, base-assisted intermolecular substitution with alcohol and regeneration of the active catalyst. The enantioselectivity was determined by the cyano migration step.

Fault diagnosis is becoming increasingly important in improving the reliability of power electronic devices. The research in this paper focuses on the issue of the faulty operation that can occur after partial IGBT open-circuit faults in three-phase PWM rectifier circuits. To promptly and effectively diagnose faults and to determine their locations, a fault diagnosis method based on transient characteristics and random forest classification is proposed. First, the characteristics of single and double IGBT open-circuit faults in three-phase PWM rectifier circuits are analyzed. It is discovered that these faults do not immediately manifest. Instead, they exhibit fault characteristics in the corresponding time sequence. Then the random forest classifier is trained using transient fault samples from the three-phase PWM rectifier circuit. Finally, generalized testing is performed on data that was not involved in the training process, with an accuracy rate of over 98%. The use of frequency distribution graphs for visual analysis of the diagnostic results solves the problem of diagnosing multiple IGBT open-circuit faults.

In order to meet the requirements of weight, vibration, and size in the field of infrared imaging, Lihan Cryogenics has developed a lightweight and compact free-piston Stirling cooler. The cooler weighs 265g and includes a built-in electronic controller and active balancer. The system comprises a gas-bearing moving magnet compressor and a low-temperature rod-less displacer, are arranged in-line to have axial symmetry (SymCoolTM). This is also designed to be IDCA (integrated dewar cooler assembly) optional to meet industry-standard. Furthermore, an active vibration controller is developed to reduce vibration, utilizing an active motor on the same axis as the compressor piston to offset vibrations. The refrigeration performance is experimentally tested under different heat dissipation and cold head working temperatures. The test results show that the cooler acquires 0.5W cooling capacity at 77K with 12.6W power consumption, and the power consumption of the active vibration controller is less than 3W.

The applications of a newly designed chiral naphthyl-C2-indole bifunctional phosphine organocatalyst in stereoselective formal [4 + 2] cycloaddition reactions were reported. The chiral naphthyl-C2-indole skeleton was introduced to bifunctional phosphine organocatalysis for the first time, and excellent stereocontrol was achieved in two types of formal [4 + 2] cycloaddition reactions. With the optimal catalyst, a series of chiral spirooxindole and hydrodibenzofuran architectures were produced in moderate to good yields with excellent stereoselectivities (up to >99% ee, >20:1 dr).

An unprecedented highly diastereoselective dimerization of 2-alkynylnaphthols is presented to furnish bridged polycyclic compounds containing a bicyclo[3.2.1]octane moiety with good to excellent yields. The reaction proceeded under mild conditions using N-iodosuccinimide as a promoter, simultaneously constructing one new C–O bond and two new C–C bonds. A tetra-substituted vinylidene ortho-quinone methide intermediate was likely involved, and the steric hindrance of substituents played a critical role in this transformation.

An organocatalyzed enantioselective γ-elimination process has been achieved and applied in the kinetic resolution of peroxides to access chiral peroxides and epoxides. The reaction provided a pathway for the preparation of two useful synthetic and biologically important structural motifs through a single-step reaction. A range of substrates has been resolved with a selectivity factor up to 63. The obtained enantioenriched peroxides and epoxides allowed a series of transformations with retained optical purities.

The pure AgNbO3photocatalyst with intrinsic electric field has aroused much interest due to its potential in promoting photocatalytic activity, but the performance was generally restricted by its relative fast recombination of photo-induced charges. Herein, Ag(1–3x)NdxNbO3(x = 0, 0.01, 0.02, 0.03, 0.04, 0.05) powders with good crystalline structures were prepared by conventional solid-state reaction. The photocatalytic capacity of AgNbO3powders was observably improved by Nd3+doping due to the enhanced separation of photogenerated charges. In contrast to the pristine one, the photocatalytic efficiency of Ag(1–3x)NdxNbO3with x = 0.02 (ANN002) raised to 2.78 times (98.6% within 30 min) and the corresponding reaction rate constant increased to 9.87 times (0.1905 min−1within 15 min) under visible light. The built-in electric field arising from the ferroelectricity of ANN002 may effectively restrain the recombination of photogenerated electron–hole pairs. The superoxide radicals (·O2-), holes (h+) and hydroxyl radicals (·OH) are involved in the rhodamine B (RhB) degradation activity, and ·O2-and h+play the major part. Besides, ANN002 achieved the complete RhB degradation under sunlight within 15 min and was able to degrade two other pollutants (methylene blue and tetracycline) under visible light within 30 min, indicating its potential application in photocatalytic field.

Organocatalytic selenosulfonylation of the C–C double bond of α,β-unsaturated ketones to construct two contiguous stereogenic centers in an aqueous medium was described. A series of α-selenyl and β-sulfonyl ketones with various functional groups were synthesized in good yields and enantioselectivities with saturated NaCl solution as the solvent. In addition, this protocol had been successfully scaled up to a decagram scale via a simple workup procedure.

Observing dynamic change patterns and higher-order complexities from remotely sensed images is warranted, but the main challenges include image inconsistency, plant phenological differences, weather variations, and difficulties of incorporating natural conditions into automatic image processing. In this study, we proposed a new algorithm and demonstrated it by producing 2002–2008 and 2010 land-cover maps in heterogeneous Southern California based on an existing 2009 land-cover map. The new algorithm improves the baseline land-cover map quality by discarding potential bad land-cover pixels and dividing each land-cover type into several subclasses. Time series Landsat images were used to detect changed and unchanged areas between baseline year and target year t. Subsequently, for each individual year t, each pixel that was identified as unchanged inherited the baseline classification. Otherwise, each pixel in the changed areas was classified by a similar surrogate majority classifier. The demonstration results in Southern California showed that the land-cover temporal pattern captured the observed successional stages of the ecosystem very well. The accuracy assessment had an overall classification accuracies ranging from 81% to 86% and overall kappa coefficients ranging from 0.79 to 0.83.

With the continued development and proliferation of renewable energy systems worldwide, particularly wind and photovoltaic (PV) generation, computer simulation models for these technologies to be used in large interconnected power-system stability analyses have been a key focus over the past several years. Such computer simulation models are used by power-system planners and operators to simulate various scenarios of large interconnected systems?e.g., continental Europe and the western or eastern North American grid?to assess the stability and reliability of the bulk electric power system. In Europe, much of this work has been led by efforts within the International Electrotechnical Commission, while in North America, most of the major efforts in modeling has gone through the Renewable Energy Modeling Task Force (REMTF) of the Western Electricity Coordinating Council (WECC). The total nameplate capacity of wind generation installed worldwide has surpassed 360,000 MW, and PV generation has surpassed 130,000 MW, with the top three countries in terms of cumulative total capacity being China, the United States, and Germany.

Nonactivated arenes, such as benzene derivatives, are chemically inert due to their intrinsic aromaticity and low polarity. The catalytic asymmetric dearomatization (CADA, coined by You and co-workers) of the nonactivated arenes represents a formidable challenge. We herein demonstrated an organocatalytic asymmetric dearomatizing hetero-Diels–Alder reaction of benzene derivatives. The tunable regioselectivity of this strategy allowed delivery of a diversity of stereochemically complex polycyclic compounds and oxahelicenes with excellent stereoselectivity. The high complexity and three-dimensionality of the products are crucial for their potential applications in materials science and drug discovery. Mechanistic studies suggested that this reaction proceeds through a chiral tetra-substituted vinylidene ortho-quinone methide (VQM) intermediate, which is extremely active to overcome the loss of aromaticity of benzene derivatives with concomitant chirality transfer.

Short-term monthly mean temperature (Tm) and short-term daily mean temperature (Td) rather than long-term monthly and daily mean temperature (Tmand Td) are preferred for some ecosystem studies such as carbon source and sink, pine beetle mortality, and snow melting. The recent progress of modeling Tmand Td(based on the previous work on Tm) supported by climatologically aided interpolation (CAI) is reported over the mountainous Yellowstone National Park. With the spatial scale of a 30 m digital elevation model (DEM), the slope, aspect, and shadows cast by surrounding topography, which could not be well captured by very coarse DEM, could be taken into account. Data from 12 months (Jan-Dec 2008) and 12 dates (25 Jan-Dec 2008) were used to demonstrate the approach. Inverse distance weighting (IDW) interpolations of limited temperature anomalies were adopted to represent the deviations from normality. Tm, as a preexisting climatology surface, was added to deviations in order to model Tm. Linear temporal interpolation of adjacent Tmwas used to create a climatology surface, which was then added to deviations in order to model Td. Results show the mean absolute errors (MAEs) for Tmranged from 0.75° C to 1.78° C, while the MAEs for Tdranged from 1.14° C to 2.02° C. The four factors of elevation, seasonal change of lapse rate, temperature difference caused by variation in solar radiation, and preexisting climatology surface for the CAI approach were comprehensively considered in this approach.

Spatially distributed surface temperature over complex topography is important to many ecological processes, but it varies spatially and temporally in complex ways and is difficult to measure at landscape scales at tens of meters resolution. Our goal is to develop a methodology that accurately predicts surface temperature in mountain ecosystems. First, we modeled monthly incoming solar radiation (insolation) based on topography and observed variation in atmospheric conditions, and accounting for site latitude, elevation, and surface orientation (slope and aspect), daily and seasonal shifts in sun angle, and effects of shadows cast by surrounding topography. Then we investigated the ability to predict monthly average temperature from lapse rates together with energy input from insolation. Monthly lapse rates are not constant and have a seasonal variation.Monthly temperature is not highly correlated with the corresponding monthly insolation; however, it is highly correlated with a one-month-lag insolation, enabling calculation of a lag-corrected linear regression. We used this regression to calculate 30 m resolution surface temperature maps for mountain areas at Yellowstone National Park. The comparison between our modeled temperature and observed temperature shows the mean average errors range from 0.90°C to 1.49°C. Our approach, with consideration of influences of topography and variable atmospheric conditions on energy from insolation, and with appropriate time lags, greatly improves the ability to calculate spatially distributed temperature regimes in complex terrain.

Spin injection performance in GaN film was systematically investigated through the three-terminal Hanle spin precession measurements with the comparison of varied tunnel barrier thickness, spin injector materials, and channel widths. Spin relaxation time and diffusion length were optimized to 37.3 ps and 139.0 nm at room temperature, respectively. With the optimal spin injector, a 12% spin polarization was obtained in a four-terminal non-local spin valve device. By applying the optimized spin injector structure to the spin-light emitting diodes, room temperature circular polarization ratios of 6.2% and 9.2% for Co and Fe spin polarizers were respectively achieved at surface-emission.

Circular polarization in nitride-based light-emitting diodes is investigated and regulated through the spin injection. Ferromagnet/dielectric are grown on n-type GaN as electron injector. Ultrathin quantum wells with quantum confinement effect are designed for enhancement of the circular polarizations in electroluminescence. By modulating the tunneling layer thickness and injection polarizer material, the preferred spin injection structure is determined. Room temperature circular polarization for surface-emission is optimized to 9.0% at most under a vertical magnetic field for the spin injecting contact. Spin relaxation time and diffusion length of about 34.9 ps and 134.5 nm, respectively, are revealed through the non-local three-terminal spin valve measurements.

Multi-walled carbon nanotubes (MWCNTs) respond as brittle materials at room temperature (RT) and fail upon any imposed strain for elongation. By following up our previous study on electron-beam (e-beam)-induced non-uniform elongation of MWCNTs (Khan et al. in J Phys Condens Matter 30:385302, 2018), here, it is illustrated that through moderate irradiation to a high enough intensity e-beam, dramatic uniform elongations can be attained for thin enough MWCNTs at RT, where homogeneous plastic flow of carbon atoms along the tube shells in the tube axis direction takes place with uniform inward diffusion of tube shells. The study also reports well-fitting results for quantitative kinetic relationship of the shrinkage time consequences under nanoscaled curvature, revealing predominant effect of much greater surface energy. These findings are purposeful for processing CNTs and other conventional nanotubes for their utilization in nanodevices and nanotechnology.