碱金属掺杂的石墨相氮化碳在可见光光催化制氢中的比较研究(英文)

利用半导体光催化技术将太阳能转化为清洁化学能源是解决能源危机和环境问题的最有潜力的途径之一.过去几十年,许多半导体包括氧化物、硫化物和氮化物均表现出光催化活性.然而,半导体光催化的实际应用仍然受制于其较低的太阳能转化效率.解决上述问题的方法之一是发展高效的可见光光催化制氢材料.近年来,石墨相氮化碳(g-C_3N_4)作为一种聚合物半导体材料,受到了光催化研究人员的广泛关注.g-C_3N_4具有可见光吸收能力、合适的导带价带位置、良好的热稳定性和化学稳定性,且制备方法简单和结构易调控,是一种极具潜力的光催化制氢材料.然而g-C_3N_4仍然仅能吸收波长450 nm以下的光,且其光生电子和空穴极易复合,因而光催化制氢效率较低.目前,研究人员采用了多种改性方法来增强g-C_3N_4的光催化性能,其中通过元素掺杂进行能带结构调控是一种非常有效的策略.而碱金属原子(Li,Na和K)被认为可有效进入g-C_3N_4的内部结构,通过引入缺陷来拓宽g-C_3N_4的光吸收范围和提高光生电荷的分离效率.不过到目前为止,尚未见系统的比较研究来深入理解不同碱金属元素掺杂的g-C_3N_4在可见光光催化制氢中的性能差异.本文采用X射线衍射(XRD)、氮气吸附-脱附测试、紫外可见漫反射光谱(UV-visDRS)、时间分辨荧光光谱(TRPL)、X射线光电子能谱(XPS)、光电化学测试和光催化制氢测试等表征和测试手段比较研究了不同碱金属元素掺杂的g-C_3N_4在结构、光学性质、能带结构、电荷转移能力和光催化性能等方面的差异.XRD结果表明,碱金属掺杂可导致g-C_3N_4的层间距离增大,且碱金属原子半径越大,g-C_3N_4的层间距离越大.氮气吸附-脱附测试结果表明,碱金属掺杂可提高g-C_3N_4的比表面积,其中Na掺杂的最高.UV-vis DRS和XPS谱结果表明,依Li,Na,K的顺序,碱金属掺杂导致g-C_3N_4带隙逐渐变窄,使得可见光吸收能力逐渐增强,且其导带和价带位置逐渐下移.TRPL和光电化学测试结果显示,碱金属掺杂有效抑制了g-C_3N_4的光生载流子复合和促进了光生载流子的转移,其中Na掺杂的g-C_3N_4的光生载流子利用效率最高.可见光光催化制氢实验表明,碱金属掺杂显著提升了g-C_3N_4的光催化性能,其中以Na掺杂的g-C_3N_4性能最佳,其产氢速率(18.7mmol h–1)较纯的g-C_3N_4(5.0mmol h–1)可提高至3.7倍.由此可见,g-C_3N_4的掺杂改性是一个对其微结构和能带结构的优化调控过程,最终获得最优的光催化性能.

Ultra-lightweight ceramic scaffolds with simultaneous improvement of pore interconnectivity and mechanical strength

The high porosity and interconnectivity of scaffolds are critical for nutrient transmission in bone tis-sue engineering but usually lead to poor mechanical properties.Herein,a novel method that combines acid etching(AE)with selective laser sintering(SLS)and reaction bonding(RB)of Al particles is pro-posed to realize highly improved porosity,interconnectivity,mechanical strength,and in vitro bioactivity in 3D Al_(2)O_(3) scaffolds.By controlling the oxidation and etching behaviors of Al particles,a tunable hol-low spherical feature can be obtained,which brings about the distinction in compressive response and fracture path.The prevention of microcrack propagation on the in situ formed hollow spheres results in unique near elastic buckling rather than traditional brittle fracture,allowing an unparalleled compressive strength of 3.72±0.17 MPa at a high porosity of 87.7%±0.4%and pore interconnectivity of 94.7%±0.4%.Furthermore,scaffolds with an optimized pore structure and superhydrophilic surface show excellent cell proliferation and adhesion properties.Our findings offer a promising strategy for the coexistence of out-standing mechanical and biological properties,with great potential for tissue engineering applications.

Mechanical Properties and Oxidation Behaviors of Self-Healing SiCf/SiC-SiBCN Composites Exposed to H_(2)O/O_(2)/Na_(2)SO_(4)Environments

The oxidation behaviors and their influence on the mechanical properties of self-healing SiCf/SiC-SiBCN composites were investigated in H_(2)O/O_(2)and H_(2)O/O_(2)/Na_(2)SO_(4)environments at 1200‒1350℃for 100 h.As the temperatures increase from 1200 to 1350℃,the oxidation rate constants increase from 0.45×10^(–7)to 1.58×10^(–7)mg^(2)/(mm^(4) h)in H_(2)O/O_(2),and from 1.02×10^(–7)to 3.42×10^(–7)mg^(2)/(mm^(4) h)in H_(2)O/O_(2)/Na_(2)SO_(4).The involvement of Na_(2)SO_(4)leads to the formation of a loose lamellar oxide layer,the breakage of the SiBCN/CVI-SiC interface and the decrease in the oxide viscosity,thus accelerating the oxidation of the composites.The composites show the maximum retention rate of strength(102%,535.71 MPa)after oxidation in H_(2)O/O_(2)at 1200℃due to the good self-healing capacity of the produced glass,while the minimum(82%,430.56 MPa)in H_(2)O/O_(2)/Na_(2)SO_(4)at 1350℃caused by the severe microstructural corrosion derived from Na_(2)SO_(4).

Conductivity and tribological properties of IL-PANI/WS_(2) composite material in lithium complex grease

An ionic liquid-polyaniline/tungsten disulfide(IL-PANI/WS_(2))composite was synthesized in 1-butyl-3-methylimidazole tetrafluoroborate(LB104)aqueous solution by in-situ polymerization and characterized by Fourier transform infrared spectroscopy.A current-carrying friction and wear tester was used to study the tribological properties of steel-steel and copper-copper friction pairs lubricated by an IL-PANI/WS_(2) lithium complex grease(LCG).After the experiment,scanning electron microscope was used to observe the surface morphology of the wear scar on the steel and copper plates,and X-ray photoelectron spectrometer was used to analyze the elemental composition of the wear scar surface.The results show that compared with greases containing IL-PANI and WS_(2),greases containing IL-PANI/WS_(2) exhibit better antiwear performance when lubricating steel-steel friction pairs and better tribological performance and electrical conductivity when lubricating copper-copper friction pairs.Therefore,it can be concluded that WS_(2) and IL-PANI have a synergistic effect.

Design, Preparation and Properties of Carbon Fiber Reinforced Ultra-High Temperature Ceramic Composites for Aerospace Applications： A Review

Carbon fiber reinforced ultra-high temperature ceramic （UHTC） composites, consisting of carbon fibers embedded in a UHTC-matrix or a C-SiC-UHTC-matrix, are deemed as the most viable class of materials that can overcome the poor fracture toughness and thermal shock resistance of monolithic UHTC ma- terials, and also improve the oxidation resistance and ablation resistance of C/C and C/SiC composites at ultra-high temperatures. In this review, we summarize the different processing routes of the compos- ites based on the UHTC introducing methods, including chemical vapor infiltration/deposition （CVI/D）, precursor infiltration and pyrolysis （PIP）, reactive melt infiltration （RMI）, slurry infiltration （SI）. in-sito reaction, hot pressing （HP）, etc; and the advantages and drawbacks of each method are briefly dis- cussed. The carbon fiber reinforced UHTC composites can be highly tailorable materials in terms of fiber. interface, and matrix. From the perspective of service environmental applications for engine propul- sions anti hypersonic vehicles, the material designs （mainly focusing on the composition, quantity, structure of matrix, as well as the architecture of carbon fibers, UHTCs and pores）, their relevant processing routes and properties （emphasizing on the mechanical and ablation properties） are discussed in this paper. In addition, we propose a material architecture to realize the multi-function through changing the distri- bution of carbon fibers, UHTCs and pores, which will be an important issue for future development of carbon fiber reinforced UHTC composites.

Synthesis of monolithic carbon aerogels with high mechanical strength via ambient pressure drying without solvent exchange

A simple,fast and cost-effective method for monolithic carbon aerogels(CAs) preparation was proposed through sol-gel polycondensation of resorcinol with fo rmaldehyde in a basic aqueous solution followed by ambient pressure drying without solvent exchange,and carbonization.The microstructure and network strength of CAs were tailored by adju sting the catalyst concentration([resorcinol]/[sodium carbonate] in the range of 300-2000),water content([deionized water]/[resorcinol] equals to 17 and 24,respectively),and gelation temperature(Tgel in the range of 30-90℃).Resultantly,the CAs with a wide range of density(0.30-1.13 g/cm3),high specific surface area(465-616 m2/g),high compressive strength(6.5-147.4 MPa)and low thermal conductivity(0.065-0.120 W·m-1 K-1) were obtained in this work.Moreover,the largesized CAs(100×100×20 mm3) can also be prepared by this method since the formed robust skeleton network can resist shrinkage/collapse of pore structure and prevent cracking during drying.The improved mechanical strength and monolithic forming abilities could be mainly attributed to the uniform arrangement of carbon particles and pores,fine particle size,abundant network structure and enhanced particle neck.

Deposition kinetics and mechanism of pyrocarbon for electromagnetic-coupling chemical vapor infiltration process

Although the electromagnetic-coupling chemical vapor infiltration(E-CVI)has been proven of a highefficiency technique for producing carbon fiber reinforced pyrocarbon(Py C)matrix(C/C)composites,a deep understanding of the deposition kinetics and mechanism of Py C matrix is still lack.In this work,a deposition model with uniform electric field but gradient magnetic field was set up by using unidirectional carbon fiber bundles as the substrates to investigate the deposition kinetics and mechanism.Meanwhile,the polarizability,and the chemical adsorption and dehydrogenation barriers of hydrocarbon were simulated based on the density functional theory(DFT)and the Climb-image nudged elastic band method,respectively.The E-CVI process exhibited extremely high Py C deposition rates of 8.7,11.5,16.5 and 22.7 nm/s at 700,750,800 and 850℃,respectively,together with a significantly low apparent activation energy of 57.9 k J/mol within the first 5 min.The Py C deposited at different temperatures with different time shows a smooth laminar structure with low coherent length and graphitization degree.The theoretical calculation and simulation results indicated that the electrons existing on the carbon fibers and the accelerated motion of radicals with preferred orientation forced by the derived magnetic field have reduced the energy barrier for the deposition process,thereby resulting in low apparent activation energy and high Py C deposition rate.The results of this work may shed a light on how to better direct the preparation of C/C composites by E-CVI process with high quality and efficiency.

Effect of C/SiC Volume Ratios on Mechanical and Oxidation Behaviors of C_(f)/C-SiC Composites Fabricated by Chemical Vapor Infiltration Technique

C/SiC volume ratios in carbon fiber-reinforced carbon-silicon carbide(C_(f)/C-SiC)composites may influence greatly mechanical and oxidation properties of the composites,but have not been well investigated yet.Herein,C_(f)/C-SiC composites with different C/SiC volume ratios were fabricated by chemical vapor infiltration(CVI)technique through alternating the thickness of a pyrocarbon(PyC)interlayer.The composites with C/SiC volume ratios of 0.37 and 0.84 exhibited the better comprehensive mechanical properties.The CS0.37 showed the highest flexural strength of 340.6 MPa,and CS0.84 had the maximum tensile strength of 139.1 MPa.The excellent mechanical properties were closely related to the relatively low C/SiC volume ratios and porosities,optimum interfacial bonding and reduced matrix micro-cracks.The composite with a low C/SiC volume ratio of 0.10 showed the best anti-oxidation performance due to its high SiC content.The oxidation mechanisms at 1100℃and 1400℃were discussed by considering the effect of the C/SiC volume ratios,pores and matrix micro-cracks,oxidation of carbon phase and SiC.

Persistent excitation of spin waves for kπ-state skyrmions

In this study,we investigated the micromagnetic dynamics of kπ-state skyrmions in a magnetic nanodot under a circular spinpolarized current and found an excited spin wave that can propagate persistently along the direction of the radius toward the center.This dynamic process is associated with two energetically favorable states in an oscillating period of spin waves.In this case,the spin-polarized current plays a role similar to effective perpendicular magnetic anisotropy and decreases the minimum energy in the magnetic system.Our findings provide insight into understanding the dynamic behaviors of topological magnetic textures.

Optimizing magnetic/dielectric matching in permalloy/carbonized cotton fiber composites by strain-tunable ferromagnetic resonance and defect-induced dielectric polarization

Electromagnetic losses in composites could be synergistically controlled by permeability and permittivity,associated with multiple ferromagnetic resonances and dielectric polarization.However,it is still challenging for simultaneous tunability for both the terms in a magnetic/dielectric composite system.Here,we demonstrate the tunable ferromagnetic resonances and the enhanced dielectric losses at gigahertz frequencies in permalloy/carbonized cotton fiber composites with different annealing temperatures.It is theoretically confirmed that the stress field acting on the magnetic permalloy layer increases with increasing temperature because of the shrinkage of the dielectric carbonized cotton fibers,resulting in multiple ferromagnetic resonances,in which there is a linear relationship(f=1.52×σ+9.38)between the resonance frequency(f)and the stress(σ).The present work provides a fundamental insight into understanding the micromagnetic dynamics of the magnetic/dielectric composite system.

Facile preparation of a SiC@SiO_(2)nanowire-toughened ZrB_(2)-SiC/SiC bilayer coating with good interfacial bonding,high toughness,and excellent cyclic ablation resistance on C/CA composites

Preparing antioxidant coatings to address the inherent oxidation sensitivity of carbon fiber-reinforced carbon aerogel(C/CA)composites is a feasible way to promote their application in oxidizing environments as thermal insulation materials.However,preparing the coatings with excellent oxidation and ablation resistance while avoiding evident damage to the C/CA substrate still remains a challenge.Herein,a SiC@SiO_(2)nanowire-toughened ZrB2–SiC/SiC bilayer coating with a large thickness of 500μm was prepared on C/CA using a one-step low-temperature reaction sintering method,which simultaneously formed a sintered outer layer with even-distributed nanowires and a siliconized gradient inner layer.By courtesy of the synergic thermal response of the layers and the crack deflection induced by the nanowires,the resulting coating has moderate residual compressive stress of 0.08–1.22 GPa in the interface,high interfacial bonding strength of 6.02 MPa,and good fracture toughness of 4.36 MPa·m1/2.Benefited from the optimum components and improved structure,the coating shows excellent cyclic ablation resistance with linear ablation rates of 0.1μm/s at 1650℃for 1500 s(300 s×5 cycles)and 0.4μm/s at 1850℃for 900 s(300 s×3 cycles).The one-step preparation strategy contributes to little damage to the substrate,thus showing the well-preserved mechanical and thermal insulation properties.