Toward Sustainable Revegetation in the Loess Plateau Using Coupled Water and Carbon Management

The"Grain-for-Green"project on the Loess Plateau is the largest revegetation program in the world.However,revegetation-induced land use changes can influence both water and carbon cycles,and the diverse consequences were not well understood.Therefore,the reasonability and sustainability of revegetation measures are in question.This study quantifies the impacts of revegetation-induced land use conversions on the water and carbon cycles in a typical watershed on the Loess Plateau and identifies suitable areas where revegetation of forest or grassland could benefit both soil and water conservation and carbon sequestration.We used a coupled hydro-biogeochemical model to simulate the changes of a few key components in terms of water and carbon by designing a variety of hypothetical land use conversion scenarios derived from revegetation policy.Compared to the baseline condition(land use in 2000),both sediment yield and water yield decreased substantially when replacing steep cropland with forest or grassland.Converting cropland with slopes larger than 25°,15°,and 6°to forest(CTF)would enhance the carbon sequestration with a negligible negative effect on soil water content,while replacing cropland with grassland(CTG)would result in a decline in net primary production but with a substantial increase in soil water content(3.8%-14.9%).Compared to the baseline,the soil organic carbon would increase by 0.9%-3.2% in CTF and keep relatively stable in CTG.Through testing a variety of hypothetical revegetation scenarios,we identified potential priority areas for CTF and CTG,where revegetation may be appropriate and potentially beneficial to conserving soil and water and enhancing carbon sequestration.Our study highlights the challenges in future water and carbon coupling management under revegetation policy,and our quantitative results and identification of potential areas for revegetation could provide information to policy makers for seeking optimal management on the Loess Plateau.

C/SiC Composites with a Self-healing SiBC Matrix Fabricated by Liquid Silicon Infiltration

Most researched continuous carbon fiber reinforced silicon carbide matrix composites(C/Si C)composites have porous matrix,which are fabricated by chemical vapor infiltration(CVI)or polymer infiltration and pyrolysis(PIP).The porous C/Si C composites exhibit higher fracture toughness,

Electromagnetic Wave Absorption Property of SiC NWs/GA-PDMS Composites

Ultralight graphene foam has triggered a new wave of discoveries.It is expected to be the most promising candidate for lightweight high performance microwave absorption(MA).Aligned-structured Si C nanowires(NWs)/graphene aerogel(GA)polydimethylsilox-ane(PDMS)composites are fabricated through a

In-Situ Synthesis of Polycrystalline Carbon Nanowires with Excellent Microwave Absorption Performance

To obtain a novel absorber with high absorption,small reflection and wide absorption band,carbon nanowire reinforced Si_3N_4composite ceramics(CNW/Si_3N_4)have been prepared by catalytic chemical vapor deposition.Results show that as-received carbon nanowires(CNW)have the pitted surface and unique hierarchically(micro-,

Adsorption behavior of sulfamethazine in an activated sludge process treating swine wastewater

Swine wastewater is an important pollution source of antibiotics entering the aquatic environment. In this work,the adsorption behavior of sulfamethazine（SMN）,a commonlyused sulfonamide antibiotic,on activated sludge from a sequencing batch reactor treating swine wastewater was investigated. The results show that the adsorption of SMN on activated sludge was an initially rapid process and reached equilibrium after 6 hr. The removal efficiency of SMN from the water phase increased with an increasing concentration of mixed liquor suspended solids,while the adsorbed concentration of SMN decreased. Solution pH influenced both the speciation of SMN and the surface properties of activated sludge,thus significantly impacting the adsorption process. A linear partition model could give a good fit for the equilibrium concentrations of SMN at the test temperatures（i.e.,10,20 and 30°C）. The partition coefficient（Kd） was determined to be 100.5 L/kg at 20°C,indicating a quite high adsorption capacity for SMN. Thermodynamic analysis revealed that SMN adsorption on activated sludge was an exothermic process. This study could help to clarify the fate and behavior of sulfonamide antibiotics in the activated sludge process and assess consequent environmental risks arising from sludge disposal as well.

Complex Permittivity and Microwave Absorbing Property of Si_3N_4-SiC Composite Ceramic

Si3N4-SiC composite ceramics were fabricated by chemical vapor infiltration using porous Si3Na ceramic as preform. The average grain size of SiC was 30 nm. Relationship between SiC content and relative complex permittivity of Si3Na-SiC within the frequency range of 8.2-12.4 GHz （X-band） was investigated. The average real part of relative complex permittivity ε′ of Si3N4-SiC increased from 3.7 to 14.9 and the relative imaginary part ε″ increased from 0.017 to 13.4 when the content of SiC increased from 0 to 10 vol.%. The Si3N4-SiC ceramic with 3 vol.% SiC achieved a reflection loss below -10 dB （90% absorption） at 8.0-11.4 GHz, and the minimum value was -27.1 dB at 9.8 GHz when the sample thickness was 2.5 mm. The excellent microwave absorbing abilities of Si3N4-SiC ceramic were attributed to the interfacial polarization at interface between Si3N4 and SiC and at grain boundary between SiC nanocrystals.

Dielectric and Electromagnetic Wave Absorbing Properties of Two Types of SiC Fibres with Different Compositions

The dielectric and electromagnetic properties of two types of SiC fibres with different compositions were investigated. The permittivity and electromagnetic shielding effectiveness （SE） of SiC fibre bundles were measured in 8.2-12.4 GHz by waveguide method. The reflection coefficient （RC） of unidirectional SiC fibre laminates was determined in 8-18 GHz using naval research laboratory （NRL）-arc method. Results showed that the electromagnetic wave （EMW） absorbing properties of SiC fibres were correlated with their composition, microstructure and instinct performance of electrical resistance. SiC fibres with higher content and greater size of nano-scale H-SiC showed higher permittivity, conductivity, SE and lower RC, which resulted in their better EMW absorbing ability, Le. the lower reflection to EMW.

Proportional Limit Stress and Residual Thermal Stress of 3D SiC/SiC Composite

Proportional limit stress （PLS） and residual thermal stresses （RTS） of 3D SiC/SiC composite were investigated. PLS was obtained by four different methods from the monotonic stress-strain response curve to get a convincing value. RTS in the SiC matrix was quantified by solving the geometric intersection point of the regression lines of hysteresis loops from the periodical loading-unloading-reloading cycle test curve. Classical ACK model and analytical formulas were used to analytically calculate the PLS and RTS of 3D SiC/ SiC composite. Good agreement between the experimental results and the analytical calculation was observed. And relationship between the PLS and the RTS of 3D SiC/SiC was discussed.

Microstructural evolution of carbon fiber reinforced SiC-based matrix composites during laser ablation process

In this work, four different carbon fiber reinforced SiC-based matrix composites(C/SiC) were prepared,and microstructure evolution during laser ablation process was characterized. Laser irradiation provided a special high-temperature environment up to 3500℃. For all four composites, different morphologies can be obtained in the transition region due to the oxidation of different matrices. While only needle-shaped carbon fiber and nanolayered carbon without any matrix remained in the central region, indicating that graphitization process occurred in the center, resulting from the high-temperature and low-oxygen environment in the laser process. Therefore, the laser ablation of C/SiC composites is controlled by chemical and physical erosion, and mainly by the physical erosion in the center.

Ultra-light, high flexible and efficient CNTs/Ti3C2-sodium alginate foam for electromagnetic absorption application

Ultra-light carboxylic functionalized multi-walled carbon nanotubes(CNTs-COOH) and Ti3C2 MXene hybrids modified sodium alginate(CNTs/Ti3C2-SA) based composite foams were prepared through ice-templated freeze-drying method. The microstructure of the synthesized CNTs/Ti3C2 hybrids and CNTs/Ti3C2-SA foams is characterized by the presence of CNTs inserted between MXene layers which prevents their restacking. The resultant CNTs/Ti3C2 hybrids exhibit a unique sandwich-like hierarchical structure. Scanning electron microscopy(SEM) images show that the CNTs/Ti3C2-SA foam exhibits a heterogeneous anisotropic microstructure and CNTs/Ti3C2 hybrids are homogeneously dispersed in the skeleton of the porous foam. In case that the content of the hybrids amounts 40 mg/cm^3, the CNTs/Ti3C2-SA foam possesses excellent electromagnetic(EM) absorption performance with a minimum reflection coefficient(RCmin) as low as-40.0 dB. In case of a sample thickness of 3.95 mm, the RCminreaches-24.4 dB and the effective absorption bandwidth covers the whole X band from 8.2 to 12.4 GHz. A control test shows that, with the same absorbent content, the CNTs/Ti3C2-SA foam exhibits a far better EM performance than that of CNT-free Ti3C2-SA foam.

Dielectric,Electromagnetic Interference Shielding and Absorption Properties of Si_3N_4-PyC Composite Ceramics

Pyrolytic carbon （PyC） was infiltrated into silicon nitride （Si3N4） ceramics by precursor infiltration and pyrolysis （PIP） of phenolic resin, and Ni nanoparticles were added into the phenolic resin to change the electric conductivity of Si3N4-PyC composite ceramics. Dielectric permittivity, electromagnetic interference （EMI） shielding and absorption properties of Si3N4-PyC composite ceramics were studied as a function of Ni content at 8.2-12.4 GHz （X-band）. When Ni nanoparticles were added into phenolic resin, the electric conductivity of the prepared composite ceramics decreased with increasing Ni content, which was attributed to the decrease of graphitization degree of PyC. The decrease in electric conductivity led to the decrease in both permittivity and EMI shielding effectiveness. Since too high permittivity is harmful to the impendence match and results in the strong reflection, the electromagnetic wave absorption property of Si3N4-PyC composite ceramics increases with increasing Ni content. When the content of Ni nanoparticles added into phenolic resin was 2 wt%, the composite ceramics possessed the lowest electric conductivity and displayed the most excellent absorption property with a minimum reflection loss as low as -28.9 dB.

Electromagnetic interference shielding properties of polymer derived SiC-Si3N4 composite ceramics

SiC-Si3N4 composite ceramics are successfully fabricated by pyrolysis of ferrocene-modified polycarbosilane(PCS) mixed with inert filler Si3N4 powders, followed by thermal treatment from 1100℃ to 1400℃ in Ar atmosphere. The porosity of SiC-Si3N4 ceramics decreases to 6.4% due to the addition of inert filler Si3N4. And the content and crystallization degree of free carbon and SiC derived from PCS are improved simultaneously with the increase of thermal treatment temperature. Finally, the free carbon and SiC interconnect, forming the conductive network. As a result, the electromagnetic interference(EMI) shielding performance of the as-prepared ceramic annealed at 1400℃ reaches up to 36 d B, meaning more than99.9% of EM energy is shielded. The low porosity and high EMI shielding performance enable SiC-Si3N4 composite ceramics to be a promising electromagnetic shielding and structural material.

Fabrication and Properties of Porous Si_3N_4 Ceramic with High Porosity

A novel process combining oxidation-bonding and sintering was developed to fabricate porous Si3N4 ceramic with high porosity. After sintering at 1800℃, the SiO2 in porous Si3N4 ceramic transforms into Si2N2O because of the reaction of SiO2 and Si3N4 . Due to the reaction of SiO2 and carbon, the porosity of porous Si3N4 ceramic increases obviously from 40.2% to 76.8% with the weight decreasing by 35.6%. As the porosity increases, the dielectric constant and loss of the porous Si3N4 ceramic decrease obviously from 3.08 to 1.61 and from 3.70×10（-3） to 0.74×10（-3） , but due to the production of Si2N2O whose mechanical properties are much higher than SiO2 and the increase of the bonding strength among Si3N4 particles, the flexural strength and the fracture toughness of the porous Si3N4 ceramic decrease from 55 to 39 MPa and from 0.7 to 0.5 MPa·m（1/2） , respectively, but its Vickers hardness increases from 1.2 to 1.3 GPa.