Experimental study on dynamic load magnification factor for ballastless track-subgrade of high-speed railway

The magnitude of dynamic load produced by high-speed trains depends on many factors,of which train speed is the most critical one.However,it is quite difficult to determine the effect of train speed on dynamic load using the theoretical methods due to the complexity of the interaction between vehicle and track-subgrade.Thus large-scale model test has gradually become an important approach for studying dynamic responses of ballastless track-subgrade of high-speed railway.In this study,a full-scale model of ballastless track-subgrade was constructed in accordance with the design and construction standards for Shanghai-Nanjing intercity high-speed railway line firstly.Then,the dynamic strain of slab and the dynamic earth pressure of subgrade were measured by conducting single wheel axle excitation test.In addition,the relationship between the dynamic load magnification factor（DLF） and the train speed was obtained.Finally,the DLF of track-subgrade under different train speeds was proposed,similar to that given by German Railway Standard.

Investigation of mechanical failure performance of a large-diameter shield tunnel segmental ring

The control criteria for structural deformation and the evaluation of operational safety performance for large-diameter shield tunnel segments are not yet clearly defined.To address this issue,a refined 3D finite element model was established to analyze the transverse deformation response of a large-diameter segmental ring.By analyzing the stress,deformation,and crack distribution of large-diameter segments under overload conditions,the transverse deformation of the segmental ring could be divided into four stages.The main reasons for the decrease in segmental ring stiffness were found to be the extensive development of cracks and the complete formation of four plastic hinges.The deformation control value for the large-diameter shield tunnel segment is chosen as 8%o of the segment's outer diameter,representing the transverse deformation during the formation of the first semi-plastic hinge(i.e.,the first yield point)in the structure.This control value can serve as a reinforcement standard for preventing the failure of large-diameter shield tunnel segments.The flexural bearing capacity characteristic curve of segments was used to evaluate the structural strength of a large-diameter segmental ring.It was discovered that the maximum internal force combination of the segment did not exceed the segment ultimate bearing capacity curve(SUBC).However,the combination of internal force at 9°,85°,and 161°of the joints,and their symmetrical locations about the 0°-180°axis exceeded the joint ultimate bearing capacity curve(JUBC).The results indicate that the failure of the large-diameter segment lining was mainly due to insufficient joint strength,leading to an instability failure.The findings from this study can be used to develop more effective maintenance strategies for large-diameter shield tunnel segments to ensure their long-term performance.

Review on photocatalytic and electrocatalytic artificial nitrogen fixation for ammonia synthesis at mild conditions: Advances,challenges and perspectives

The ammonia synthesis from nitrogen and water under ambient conditions is one of the most inviting but challenging reaction routes.Although nitrogen is abundant in the atmosphere and the ammonia synthesis reaction is exothermic on the thermodynamics,the conversion of N2 to ammonia is actually hard to proceed owing to the chemical inertness and stability of N2 molecules.In industry,ammonia synthesis is carried out by the Haber-Bosch process under harsh conditions (300-500 ℃,20-30 MPa) associated with the requirement of substantial energy input and the enormous emission of greenhouse gases (e.g.,CO2).Recently,a growing number of studies on photo(electro)catalytic and electrocatalytic nitrogen reduction reaction (NRR) in aqueous solution have attracted extensive attention,which holds great promise for nitrogen fixation under room temperature and atmospheric pressure.However,the very low efficiency and ambiguous mechanism still remain as the major hurdles for the development of photochemical and electrochemical NRR systems.Here we provide an overview of the latest progresses,remaining challenges and future prospects in photocatalytic and electrocatalytic nitrogen fixation.Moreover,this review offers a helpful guidance for the reasonable design of photocatalysts and electrocatalysts towards NRR by combining theory predictions and experiment results.We hope this review can stimulate more research interests in the relatively understudied but highly promising research field of NRR.

Prediction of shield tunneling-induced ground settlement using machine learning techniques

Predicting the tunneling-induced maximum ground surface settlement is a complex problem since the settlement depends on plenty of intrinsic and extrinsic factors.This study investigates the efficiency and feasibility of six machine learning(ML)algorithms,namely,back-propagation neural network,wavelet neural network,general regression neural network(GRNN),extreme learning machine,support vector machine and random forest(RF),to predict tunneling?induced settlement.Field data sets including geological conditions,shield operational parameters,and tunnel geometry collected from four sections of tunnel with a total of 3.93 km are used to build models.Three indicators,mean absolute error,root mean absolute error,and coefficient of determination the(7?2)are used to demonstrate the performance of each computational model.The results indicated that ML algorithms have great potential to predict tunneling-induced settlement,compared with the traditional multivariate linear regression method.GRNN and RF algorithms show the best performance among six ML algorithms,which accurately recognize the evolution of tunneling-induced settlement.The correlation between the input variables and settlement is also investigated by Pearson correlation coefficient.

High-performance Li-ion capacitor based on black-TiO2-x/graphene aerogel anode and biomass-derived microporous carbon cathode

Lithium-ion capacitor (LIC) has been regarded as a promising energy storage system with high powder density and high energy density.However,the kinetic mismatch between the anode and the cathode is a major issue to be solved.Here we report a high-performance asymmetric LIC based on oxygen-deficient black-TiO2-x/graphene (B-TiO2-x/G) aerogel anode and biomass derived microporous carbon cathode.Through a facile one-pot hydrothermal process,graphene nanosheets and oxygen-vacancy-rich porous B-TiO2-x/G nanosheets were self-assembled into three-dimensional (3D) interconnected B-TiO2-x/G aerogel.Owing to the rich active sites,high conductivity and fast kinetics,the B-TiO2-x/G aerogel exhibits remarkable reversible capacity,high rate capability and long cycle life when used as anode material for lithium ion storage.Moreover,density functional theory (DFT) calculation reveals that the incorporation of graphene nanosheets can reduce the energy barrier of Li^+ diffusion in B-TiO2-x.The asymmetric LIC based on B-TiO2-x/G aerogel anode and naturally-abundant pine-needles derived microporous carbon (MPC) cathode work well within a large voltage window (1.0-4.0 V),and can deliver high energy density (166.4 Wh·kg^-1 at 200 mA·g^-1),and high power density (7.9 kW·kg^-1 at 17.1 Wh·kg^-1).Moreover,the LIC shows a high capacitance retention of 87% after 3,000cycles at 2,000 mA·g^-1.The outstanding electrochemical performances indicate that the rationally-designed LICs have promising prospect to serve as advanced fast-charging energy storage devices.

Controlled growth and photoconductive properties of hexagonal SnS2 nanoflakes with mesa-shaped atomic steps

We demonstrated the controlled growth of two-dimensional （2D） hexagonal tin disulfide （SnS2） nanoflakes with stacked monolayer atomic steps. The morphology was similar to flat-topped and step-sided mesa plateaus or step pyramids. The SnS2 nanoflakes were grown on mica substrates via an atmospheric-pressure chemical vapor deposition process using tin monosulfide and sulfur powder as precursors. Atomic force microscopy （AFM）, electron microscopy, and Raman characterizations were performed to investigate the structural features, and a sequential layer-wise epitaxial growth mechanism was revealed. In addition, systematic Raman characterizations were performed on individual SnS2 nanoflakes with a wide range of thicknesses （1-100 nm）, indicating that the A1g peak intensity and Raman shifts were closely related to the thickness of the SnS2 nanoflakes. Moreover, photoconductive AFM was performed on the monolayer-stepped SnS2 nanoflakes, revealing that the flat surface and the edges of the SnS2 atomic steps had different electrical conductive properties and photoconductive behaviors. This is ascribed to the dangling bonds and defects at the atomic step edges, which caused a height difference of the Schottky barriers formed at the interfaces between the PtIr-coated AFM tip and the step edges or the flat surface of the SnS2 nanoflakes. The 2D SnS2 crystals with regular monolayer atomic steps and fast photoresponsivity are promising for novel applications in photodetectors and integrated optoelectronic circuits.

Controllable growth and flexible optoelectronic devices of regularly-assembled Bi2S3 semiconductor nanowire bifurcated junctions and crosslinked networks

Regularly assembled structures of nanowires, such as aligned arrays, junctions and interconnected networks, have great potential for the applications in logical circuits, address decoders, photoelectronic devices and transparent electrodes. However, for now it is still lack of effective approaches for constructing nanowire bifurcated junctions and crosslinked networks with ordered orientations and high quality. Herein, we report the controlled growth of Bi2S3 semiconductor nanowire bifurcated junctions and crosslinked networks with well-aligned directions and high crystalline degree by utilizing the proportional lattice match between nanowires and substrates. Taking advantages of the “tip-to-stem splice” assembly of individual nanowires, the precise orientation alignments of Bi2S3 semiconductor nanowire bifurcated junctions and crosslinked networks were successfully realized. The controlled growth mechanism and structural evolution process have been elucidated by detailed atomic structure characterizations and modeling. The highly crystal quality and direct energy bandgap of as-assembled photodetectors based on individual bismuth sulfide nanowires enabled high photoresponsivity and fast switch time under light illumination. The three-terminal devices based on nanowire bifurcated junctions present rapid carrier transport across the junction. The flexible photodetectors based on nanowire crosslinked networks show very minimal decay of photocurrent after long-term bending test. This work may provide new insights for the guided construction and regular assembly of low-dimensional ordered functional nanostructures towards advanced nanotechnologies.