Compositional and Hollow Engineering of Silicon Carbide/Carbon Microspheres as High-Performance Microwave Absorbing Materials with Good Environmental Tolerance

Microwave absorbing materials(MAMs)characterized by high absorption efficiency and good environmental tolerance are highly desirable in practical applications.Both silicon carbide and carbon are considered as stable MAMs under some rigorous conditions,while their composites still fail to produce satisfactory microwave absorption performance regardless of the improvements as compared with the individuals.Herein,we have successfully implemented compositional and structural engineering to fabricate hollow Si C/C microspheres with controllable composition.The simultaneous modulation on dielectric properties and impedance matching can be easily achieved as the change in the composition of these composites.The formation of hollow structure not only favors lightweight feature,but also generates considerable contribution to microwave attenuation capacity.With the synergistic effect of composition and structure,the optimized SiC/C composite exhibits excellent performance,whose the strongest reflection loss intensity and broadest effective absorption reach-60.8 dB and 5.1 GHz,respectively,and its microwave absorption properties are actually superior to those of most SiC/C composites in previous studies.In addition,the stability tests of microwave absorption capacity after exposure to harsh conditions and Radar Cross Section simulation data demonstrate that hollow SiC/C microspheres from compositional and structural optimization have a bright prospect in practical applications.

Study of poly-disperse aerosols deposition in turbulent flow with different Reynolds numbers

The turbulent deposition mechanism is one of the main mechanisms of aerosol deposition in nuclear power plant tubes.An experimental study of poly-disperse aerosol deposition in a horizontal tube is conducted,where the nominal Reynolds number(Re)is in a range of 3600–200,000.The aerosol deposition velocity first increases and then decreases with the increase of Res,and at high Re,particle rebound occurs during aerosol deposition in the tube.When the Re is low,the aerosol deposition velocity increases with the increase of aerosol diameter.When the Re is greater than 60,000,the deposition velocity first increases and then decreases with the increase of aerosol diameter due to particle surface rebound.A new aerosol deposition model has been developed by establishing the energy conservation equation of the rebounded particles in the viscous sublayer.The calculated results of the new model are in good agreement with these experimental results,and the error between the aerosol deposition velocity calculated by the model and experimental results is between−60%and 150%.

Biocatalytic stereoselective synthesis of methyl mandelates by engineering a cytochrome P450 hydroxylase

Chiral methyl mandelates are useful synthons in organic transformation and pharmaceutical synthesis.Green synthesis of these valuable compounds by direct C–H activating oxidative hydroxylation has attracted keen interest.Described herein is achieving the stereoselective and efficient bio-hydroxylation of methyl 2-phenylacetates to the chiral methyl mandelates by directed evolution of the cytochrome P450DA hydroxylase.In the present study,a new colorimetric high-throughput screening assay was successfully developed based on a dualenzyme cascade for the engineering of the P450DA's hydroxylation activity.Several beneficial variants with enhanced bio-hydroxylation activity were created by combining random mutagenesis and site-saturated/directed mutagenesis strategies.Whole-cell bio-hydroxylation of various methyl 2-phenylacetates using the best septupletmutant P450DA-11 yielded the corresponding chiral methyl mandelates in up to 92%isolated yields and>99%ee.