Lunlun Gong

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.

A fast synthesis of silica aerogel powders-based on water glass via ambient drying

In this paper, we report the experimental results of the silica aerogel powders prepared by water glass and ambient pressure drying within 4 h. Water glass was hydrolyzed in water using HCl as the acid catalyst. In order to reduce the consumption of modifier and preparation period, the new technological process of mechanical crushing and filtration was introduced and solvent exchange/surface modification process can be rapidly carried out in this way. Furthermore, we utilized water glass instead of NH3·H2O as base catalyst. To some extent, it avoided impurities into the aerogels. Base concentration proved to have a very important influence on the physical and chemical properties of aerogels. As a result, when the dilution ratio of water glass as base catalyst is 3, aerogels with well-developed mesoporous structure (mean pore size of ~ 20 nm) and super hydrophobicity (contact angle of ~ 162°) can be obtained.Graphical Abstract

CoS2 Nanoparticles Wrapping on Flexible Freestanding Multichannel Carbon Nanofibers with High Performance for Na-Ion Batteries

Exploration for stable and high-powered electrode materials is significant due to the growing demand for energy storage and also challengeable to the development and application of Na-ion batteries (NIBs). Among all promising electrode materials for NIBs, transition-mental sulfides have been identified as potential candidates owing to their distinct physics-chemistry characteristics. In this work, CoS2 nanomaterials anchored into multichannel carbon nanofibers (MCNFs), synthesized via a facile solvothermal method with a sulfidation process, are studied as flexible free-standing electrode materials for NIBs. CoS2 nanoparticles uniformly distributed in the vertical and horizontal multichannel networks. Such nanoarchitecture can not only support space for volume expansion of CoS2 during discharge/charge process, but also facilitate ion/electron transport along the interfaces. In particular, the CoS2@MCNF electrode delivers an impressively high specific capacity (537.5 mAh g-1 at 0.1 A g-1), extraordinarily long-term cycling stability (315.7 mAh g-1 at at 1 A g-1 after 1000 cycles), and excellent rate capacity (537.5 mAh g-1 at 0.1 A g-1 and 201.9 mAh g-1 at 10 A g-1) for sodium storage. Free-standing CoS2@MCNF composites with mechanical flexibility provide a promising electrode material for high-powered NIBs and flexible cells.

Facile construction of the aerogel/geopolymer composite with ultra-low thermal conductivity and high mechanical performance

In this work, we have successfully prepared a lightweight, highly hydrophobic and superb thermal insulating aerogel/geopolymer composite by a sol–gel immersion method. After silica aerogel was impregnated, the composite exhibited nano-porous structures. Moreover, scanning electron microscopy observations revealed that the aerogel particles were tightly anchored on the geopolymer surface. With several excellent properties (bulk density: 306.5 g cm−3, thermal conductivity: 0.0480 W m−1 K−1 and maximum compressive strength: 0.79 MPa) the as-prepared composite shows great potential to be applied in the thermal insulation field.

Fabrication of cordierite foam ceramics using direct foaming and slip casting method with plaster moulds

The cordierite foam ceramics were successfully fabricated using direct foaming and slip casting method with plaster moulds. Kaolin, attapulgite and magnesium oxide were used as starting materials with Arabic gum added as the dispersant. The samples were sintered at 1200°C, and then the microstructure, porosity, bulk density and thermal conductivity were characterised. The results show that the cordierite foam ceramics had a porous structure of open cells and the struts had abundant small pores. The maximum open porosity achieved 87·65% with a bulk density of 329 kg m− 3, and the thermal conductivity was as low as 0·095 W (m K)− 1. Therefore, these cordierite foam ceramics show promise for use as the thermal insulator.

Effect of Opening Geometry on the Heat Transfer Characteristics for External Flames Impinging on an Exterior Wall

The heat transfer characteristics of external flames impinging on an exterior vertical wall have been investigated experimentally. The external flame was produced by the combustion of liquid fuel n-heptane in a compartment and then ejected from the compartment opening. The effect of opening geometry on the heat flux distribution is studied. Results indicate that the opening aspect ratio, which is defined as the ratio of opening height to width, has great influence on the heat flux distribution on the exterior wall. With increasing the opening width, the external flame inclines to the wall side and becomes shallower, which improves the heat transfer to the wall. A correlation is developed for correlating the heat flux to the flame height.

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).

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.

Effect of Starch on Sintering Behavior for Fabricating Porous Cordierite Ceramic

Abstract Porous cordierite ceramics were prepared with starch as pore-forming agent by solid-state method. The green bodies were sintered at 1,100–1,400 °C for 2 h. The characterization was focused on thermal analysis, phase evolution, sintering behavior, porosity and micro-structural changes. The results show that cordierite becomes the main crystallization phase at 1,200 °C. The shrinkage behavior shows the most obvious dependence on the sintering temperature and starch content, and it can be divided into three stages. Moreover, the open porosity increases with the increase of starch content, but the pore-forming effectivity decreases. Nevertheless, compared with the open porosity curves, the bulk density curves are more in line with the linear rule. The microphotographs show the densification process with the sintering temperature and the variation of pore connectivity with the starch content.