Electrochemical Performance of Carbon-Rich Silicon Carbonitride Ceramic as Support for Sulfur Cathode in Lithium Sulfur Battery

As a promising matrix material for anchoring sulfur in the cathode for lithium-sulfur (Li-S) batteries, porous conducting supports have gained much attention. In this work, sulfur-containing C-rich SiCN composites are processed from silicon carbonitride (SiCN) ceramics, synthesized at temperatures from 800 to 1100 °C. To embed sulfur in the porous SiCN matrix, an easy and scalable procedure, denoted as melting-diffusion method, is applied. Accordingly, sulfur is infiltrated under solvothermal conditions at 155 °C into pores of carbon-rich silicon carbonitride (C-rich SiCN). The impact of the initial porosity and microstructure of the SiCN ceramics on the electrochemical performance of the synthesized SiCN-sulfur (SiCN-S) composites is analysed and discussed. A combination of the mesoporous character of SiCN and presence of a disordered free carbon phase makes the electrochemical performance of the SiCN matrix obtained at 900 °C superior to that of SiCN synthesized at lower and higher temperatures. A capacity value of more than 195 mAh/g over 50 cycles at a high sulfur content of 66 wt.% is achieved

Single-source-precursor synthesis of high temperature stable SiC/C/Fe nanocomposites from a processable hyperbranched polyferrocenylcarbosilane with high ceramic yield

NSFC [50802079, 51072169]; Natural Science Foundation of Fujian Province of China [2011J01330]Hydrosilylation of vinyl ferrocene with allylhydridopolycarbosilane was used to synthesize a processable hyperbranched polyferrocenylcarbosilane (HBPFCS), which was characterized by combination of gel permeation chromatography, Fourier transform infrared (FT-IR) spectroscopy, and nuclear magnetic resonance (NMR) spectroscopy. The polymer-to-ceramic transformation of the HBPFCSs was then investigated by FT-IR and C-13 MAS NMR spectroscopy as well as by thermal gravimetric analysis (TGA). A self-catalytic effect of ferrocenyl units in the HBPFCS skeleton on dehydrocoupling was found during a curing process at 170 degrees C resulting in a high ceramic yield of ca. 80% at 1200 degrees C in Ar. Finally, microstructures and magnetic properties of the final ceramics were studied by techniques such as X-ray diffraction, energy dispersive spectroscopy, Raman spectroscopy, transmission electron microscopy and vibrating sample magnetometry. The final ceramic (pyrolysis temperature >= 900 degrees C) is characterized by a microstructure comprised of a SiC/C/Fe nanocomposite. Turbostratic carbon layers located at the segregated alpha-Fe crystal boundary avoid interdiffusion and explain the exclusive existence of alpha-Fe in a SiC/C matrix even at 1300 degrees C. Variations of the iron content in the HBPFCSs and of the pyrolysis conditions facilitate the control of the composition and ceramic micro/nanostructure, influencing in particular magnetic properties of the final SiC/C/Fe nanocomposite ceramic

Water level affects availability of optimal feeding habitats for threatened migratory waterbirds

Extensive ephemeral wetlands at Poyang Lake, created by dramatic seasonal changes in water level, constitute the main wintering site for migratory Anatidae in China. Reductions in wetland area during the last 15 years have led to proposals to build a Poyang Dam to retain high winter water levels within the lake. Changing the natural hydrological system will affect waterbirds dependent on water level changes for food availability and accessibility. We tracked two goose species with different feeding behaviors (greater white‐fronted geese Anser albifrons [grazing species] and swan geese Anser cygnoides [tuber‐feeding species]) during two winters with contrasting water levels (continuous recession in 2015; sustained high water in 2016, similar to those predicted post‐Poyang Dam), investigating the effects of water level change on their habitat selection based on vegetation and elevation. In 2015, white‐fronted geese extensively exploited sequentially created mudflats, feeding on short nutritious graminoid swards, while swan geese excavated substrates along the water edge for tubers. This critical dynamic ecotone successively exposes subaquatic food and supports early‐stage graminoid growth during water level recession. During sustained high water levels in 2016, both species selected mudflats, but also to a greater degree of habitats with longer established seasonal graminoid swards because access to tubers and new graminoid growth was restricted under high‐water conditions. Longer established graminoid swards offer less energetically profitable forage for both species. Substantial reduction in suitable habitat and confinement to less profitable forage by higher water levels is likely to reduce the ability of geese to accumulate sufficient fat stores for migration, with potential carryover effects on subsequent survival and reproduction. Our results suggest that high water levels in Poyang Lake should be retained during summer, but permitted to gradually recede, exposing new areas throughout winter to provide access for waterbirds from all feeding guilds

Microsphere Pattern Prepared by a "Reverse" Breath Figure Method

通讯作者地址: Xiong, XP (通讯作者), Xiamen Univ, Coll Mat, Dept Mat Sci & Engn, Xiamen 361005, Peoples R China 地址: 1. Xiamen Univ, Coll Mat, Dept Mat Sci & Engn, Xiamen 361005, Peoples R China 电子邮件地址: [email protected] have reported all interesting method, named reverse breath figure, for the preparation of polymeric microsphere patterns. By the same procedure as breath figure, instead of under a humid atmosphere, linear and star-shaped poly(styrene-block-butadiene) copolymers dissolved in solvents such as toluene, trichloroform, and dichloromethane were cast onto the surface of a glass substrate in methanol or ethanol vapor. After the complete evaporation of the solvent, microspheres with the diameters ranging from hundreds of nanometers to several micrometers were prepared. The microsphere patterns are the reverse of the honeycomb porous structure of breath Figure. The mechanism of the microsphere formation has been studied to show that when the surface tension of the polymer solution is 1.5 mN/m higher than that of the condensed liquid, microsphere patterns call be prepared, whereas a honeycomb porous film of breath Figure call be obtained when the Surface tension of the polymer solution is lower than that of the condensed liquid. The viscosity of the polymer solution is also an important factor to influence the fabrication of the microsphere patterns.National Natural Science Foundation of China 2084400

Topologically-protected refraction of robust kink states in valley photonic crystals

Recently discovered valley photonic crystals (VPCs) mimic many of the unusual properties of two-dimensional gapped valleytronic materials such as bilayer graphene or MoS2. Of the utmost interest to optical communications is their ability to support topologically protected chiral edge (kink) states at the internal domain wall between two VPCs with spectrally overlapping bandgap zones and opposite half-integer valley-Chern indices. We experimentally demonstrate the robustness of the kink states in VPCs that support degenerate transverse-electric-like (TE) and transverse-magnetic-like (TM) topological phases, thus enabling polarization multiplexing in a single topological waveguide. The propagation direction of the kink states is locked to the valleys of the reverse Brave lattice and, therefore, cannot be reversed in the absence of inter-valley scattering. At the intersection between the internal domain wall and the external edge separating the VPCs from free space, the kink states are shown to exhibit >97% out-coupling efficiency into directional free-space beams. This constitutes the first experimental demonstration of meron-like valley-projected topological phases with half-integer valley-Chern indices.Comment: 19 pages, 4 figure

Charting stability space

A comprehensive chemical space of potential inorganic ternary metal nitrides has been explored by computational methods as a guideline for their experimental synthesis and discovery

Enhanced hydrogen evolution reaction catalyzed by carbon‐rich Mo4.8Si3C0.6/C/SiC nanocomposites via a PDC approach

In this study, mesoporous carbon‐rich Mo4.8Si3C0.6/C/SiC ceramic nanocomposites were successfully prepared via a single‐source precursor route, starting from allylhydridopolycarbosilane (AHPCS, SMP‐10), bis(acetylacetonato) dioxomolybdenum (VI) [MoO2(acac)2], and divinylbenzene (DVB). Besides, polystyrene (PS) was used as a pore former. The obtained carbon‐rich single‐source precursor/PS mixtures were pyrolyzed at 1100°C, and then annealed at 1350°C‐1600°C to fabricate a series of carbon‐rich Mo4.8Si3C0.6/C/SiC ceramics comprised of high carbon content above 50 wt%. In comparison to the carbon‐poor materials, the carbon‐rich samples retain the higher specific surface area up to 214.6‐304 m2/g at higher annealing temperatures (1350°C‐1600°C) due to the enhancement of carbothermal reaction. The carbon‐rich samples synthesized at 1500°C, denoted as SM/Mo/PS/DVB 2‐1‐4‐2 1500 exhibit enhanced electrocatalytic performance with ultra‐low overpotentials of 119 mV vs reversible hydrogen electrode at a current density of 10 mA cm−2 in acidic media, which is superior to that of the Mo4.8Si3C0.6/C/SiC ceramic (138 mV) with lower carbon content reported in our previous study. Therefore, our porous materials comprised of high carbon content and Nowotny phase (Mo4.8Si3C0.6, NP) are considered as promising catalysts for the hydrogen evolution reaction (HER)

The fate and role of in situ formed carbon in polymer-derived ceramics

Polymer-derived ceramics (PDCs) have been intensively studied for nearly 50 years due to their unique advantages to producing ceramic fibers, coatings, foams, nanocomposites and for additive manufacturing. A phenomenon associated with the polymer-to-ceramic transformation process using organo-substituted silicon polymers as the starting material has been widely reported, namely, in situ formation of carbon within the generated silicon-based ceramic matrix. Interestingly, the precipitation of carbon depends to a great extent on the molecular structure of the preceramic polymer and significantly affects the composition, crystallization and decomposition behavior, microstructural evolution as well as the related structural and functional properties of PDCs. Thus, this review article highlights the recent progress in the PDC field with the focus on the fate and role of the in situ formed carbon. Firstly, a brief summary of the synthesis and processing of PDCs is provided, followed by the microstructural characterization of the formed ceramics. The in situ formation of carbon, precursor-carbon-morphology relation and high-temperature evolution of the carbon will be summarized. Secondly, the influence of the segregated carbon on the microstructure and its associated properties of the PDCs will be comprehensively highlighted. Finally, potential advanced structural and functional applications of the PDCs related to the carbon are evaluated

Single-source-precursor synthesis and high-temperature evolution of a boron-containing SiC/HfC ceramic nano/micro composite

A boron-containing SiHfC(N,O) amorphous ceramic was synthesized upon pyrolysis of a single-source-precursor at 1000 degrees C in Ar atmosphere. The high-temperature microstructural evolution of the ceramic at high temperatures was studied using X-ray powder diffraction, Raman spectroscopy, solid-state nuclear magnetic resonance spectroscopy and transmission electron microscopy. The results show that the ceramic consists of an SiHfC(N,O)-based amorphous matrix and finely dispersed sp(2)-hybridized boron-containing carbon (i.e. ByC). High temperature annealing of ByC/SiHfC(N,O) leads to the precipitation of HfCxN1-x nanoparticles as well as to beta-SiC crystallization. After annealing at temperatures beyond 1900 degrees C, HfB2 formation was observed. The incorporation of boron into SiHfC(N,O) leads to an increase of its sintering activity, consequently providing dense materials possessing improved mechanical properties as compared to those of boron-free SiC/HfC. Thus, hardness and elastic modulus values up to 25.7 +/- 5.3 and 344.7 +/- 43.0 GPa, respectively, were measured for the dense monolithic SiC/HfCxN1-x/HfB2/C ceramic nano/micro composite