Long Shi

Flammability and oxidation kinetics of hydrophobic silica aerogels.

Silica aerogels (SAs) present great application prospects especially on thermal insulation, but their flammability is usually ignored. A combined study on the combustion behaviors and oxidation kinetics of hydrophobic silica aerogels prepared by ambient pressure drying (SA-apd) and supercritical drying (SA-sd) was performed by employing cone calorimeter and thermal analysis. The whole combustion process for SAs could be divided into three stages in which a fire propagation phenomenon was observed with the radial propagation velocity of 6.6-8.3cms-1. Current investigations forcefully demonstrated that hydrophobic SAs were combustible and easy to flashover when exposed to a heat flux higher than 25kWm-2. Compared between the two SAs, the SA-sd owned a less fire risk with presenting a less fire hazard and a lower smoke toxicity than those of SA-apd. The oxidation kinetics by Ozawa-Flynn-Wall method revealed that SA-sd had larger apparent activation energies than those of SA-apd which conformed to the thermal stability analysis by TG-DSC. Furthermore, a two-step combustion mechanism was proposed to explain the combustion behaviors of SAs.

Effects of shaft inclination angle on the capacity of smoke exhaust under tunnel fire

Vertical shaft is one of the most important approaches for smoke control under tunnel fires. However, the boundary layer separation is a common phenomenon of hampering the smoke exhaust for vertical shafts. A tilted shaft has been proposed to solve problems and improve the capacity of smoke exhaust. In this study, the effect of shaft inclination angle (θ decreases from 90° to 14°) and shaft height on the capacity of smoke exhaust was addressed numerically. A series of scenarios were simulated in a full-scale road tunnel. Numerical results showed that the tilted shaft could eliminate the boundary layer separation. However, small shaft inclination angle could lead to a relatively higher resistance to the smoke and a smaller cross-section area of shaft, which could have an adverse effect on the capacity. Under these two factors, an optimal inclination angle exists in the shaft of around 76° in this study. Based on the smoke flow characteristics and exhaust effect, the inclination angle was roughly divided into three regions. The main influence factor of the inclination angle on the mass flow rate of smoke in each region was examined. For a comprehensive consideration, the low and slightly tilted shaft was applied to tunnel fires, which can improve the capacity of smoke exhaust obviously.

Nanoflower-Like N-Doped C/CoS2 as High-Performance Anode Materials for Na-Ion Batteries

Novel nanoflower-like N-doped C/CoS2 spheres assembled from 2D wrinkled CoS2 nanosheets were synthesized through a facile one-pot solvothermal method followed by sulfurization. Ascribed to the optimized 3D nanostructure and rational surface engineering, the unique hierarchical structure of the nanoflower-like C/CoS2 composites showed an excellent sodium ion storage capacity accompanied by high specific capacity, superior rate performance and long-term cycling stability. Specifically, the conductive interconnected wrinkled nanosheets create a number of mesoporous structures and thus can greatly release the mechanical stress caused by Na+ insertion/extraction. Besides, it was observed from the experiments that many extra defect vacancies and Na+ storage sites are introduced by the nitrogen doping process. It was also observed that the crosslinked 2D nanosheets can effectively reduce the diffusion lengths of sodium ions and electrons, resulting in an outstanding rate performance (>700 mA h g-1 at 1 A g-1 and 458 mA h g-1 at even 10 A g-1) and extraordinary cycling stability (698 mA h g-1 at 1 A g-1 after 500 cycles). The results provide a facile approach to fabricate promising anode materials for high-performance sodium-ion batteries (SIBs).

Highly reversible Na ion storage in N-doped polyhedral carbon-coated transition-metal chalcogenides by optimizing the nanostructure and surface engineering

Transition-metal chalcogenides (TMCs) have been attracting widespread attention due to their high lithium/sodium storage capacity, wide availability, and enhanced safety. However, their practical applications are still suffering from high volume changes, poor electronic conductivity and low utilization of active materials, resulting in unsatisfactory electrochemical performance. In this study, a facile one-pot solvothermal method was developed to self-assemble and produce N-CoS2@C composites. It was found from the experiments that the developed 3D polyhedral carbon-coated structure of N-CoS2@C can effectively reduce the diffusion lengths of sodium ions and electrons. Carbon layer was also found firmly encapsulated the CoS2, where it can greatly release mechanical stresses under high volume change and also improve the electronic conductivity of active materials. The developed 3D polyhedral carbon-coated structure results in outstanding rate performance (738xa0mA h g−1 at 1 A g−1 reaching up to 86.2% theoretical capacity and 450 mA h g−1 even at 10 A g−1) and extraordinary cycle stability (559 mA h g−1 at 1 A g−1 after 1000 cycles) when used as anode materials for sodium-ion batteries (SIBs). The research outcomes provide a novel design strategy for high-performance TMC electrodes and also a facile approach to fabricate promising anode materials for high-performance SIBs.

Improving the performance of solar chimney by addressing the designing factors

Solar chimney is a reliable system totally based on solar energy to enhance natural ventilation in buildings, but challenge still exists to optimize its performance with the lowest cost. In this study, three designing factors, including configuration, installation conditions, and material usages, were reviewed to provide a technical guide for engineering applications. Regarding the configuration, the performance of solar chimney can be enhanced with a high cavity, an appropriate cavity gap (usually 0.2-0.3 m), equivalent inlet and outlet area, and height/gap ratio of 10-15. Regarding installation conditions, an optimum inclination angle of 45° was usually suggested, and large openings can enhance the performance while the increasing rate keeps decreasing. The principles of material usage are to maximum the heat absorption and reduce the heat losses, considering properties such as thermal conductivity, absorptivity, emissivity, transmissvity, and reflectivity.

Double-morphology CoS2 Anchored on N-doped Multichannel Carbon Nanofibers as High-Performance Anode Materials for Na-Ion Batteries

Na-ion batteries (NIBs) have attracted increasing attention given the fact that sodium is relatively more plentiful and affordable than lithium for sustainable and large-scale energy storage systems. However, the shortage of electrode materials with outstanding comprehensive properties has limited the practical implementations of NIBs. Among all the discovered anode materials, transition-metal sulfide has been proven as one of the most competitive and promising ones due to its excellent redox reversibility and relatively high theoretical capacity. In this study, double-morphology N-doped CoS2/multichannel carbon nanofibers composites (CoS2/MCNFs) are precisely designed, which overcome common issues such as the poor cycling life and inferior rate performance of CoS2 electrodes. The conductive 3D interconnected multichannel nanostructure of CoS2/MCNFs provides efficient buffer zones for the release of mechanical stresses from Na+ ions intercalation/deintercalation. The synergy of the diverse structural features enables a robust frame and a rapid electrochemical reaction in CoS2/MCNFs anode, resulting in an impressive long-term cycling life of 900 cycles with a capacity of 620 mAh g-1 at 1 A g-1 (86.4% theoretical capacity) and a surprisingly high-power output. The proposed design in this study provides a rational and novel thought for fabricating electrode materials.

Experimental investigation on the morphology of soot aggregates from the burning of typical solid and liquid fuels

Soot particles from the burning of typical fuels are one of the critical sources causing environmental problems and human disease. To understand the soot formation of these typical fuels, the size and morphology of soot aggregates produced from the burning of typical solid and liquid fuels, including diesel, kerosene, natural rubber (NR) latex foam, and wood crib, were studied by both extractive sampling and subsequent image analysis. The 2D and 3D fractal dimensions together with the diameter distribution of agglomerate and primary particles were analyzed for these four typical fuels. The average diameters of the primary particles were within 45–85xa0nm when sampling from different heights above the fire sources. Irregular sheet structures and flake-like masses were observed from the burning of NR latex foam and wood cribs. Superaggregates with a mean maximum length scale of over 100xa0μm were also found from the burning of all these four tested fuels. The fractal dimension of a single aggregate was 3 for all the tested fuels.