查看更多>>摘要：Alinhac solved a long-standing open problem in 2001 and established that quasilinear wave equations in two space dimensions with quadratic null nonlinearities admit global-in-time solutions,provided that the initial data are compactly supported and sufficiently small in Sobolev norm.In this work,Alinhac obtained an upper bound with polynomial growth in time for the top-order energy of the solutions.A natural question then arises whether the time-growth is a true phenomenon,despite the possible conservation of basic energy.In the present paper,we establish that the top-order energy of the solutions in Alinhac theorem remains globally bounded in time.
查看更多>>摘要：In the two-dimensional(2D)melting transition of colloidal systems,the hexatic-isotropic(H-I)transition can be either first-order or continuous.However,how particle dynamics differs at the single-particle level during these two different melting transitions remains to be disclosed.In this work,by Brownian dynamics(BD)simulations,we have systematically studied the dynamic behavior of corner-rounded hexagons during the H-I transition,for a range of corner-roundness ζ=0.40 to 0.99 that covers the crossover from the continuous to first-order nature of H-I transition.The results show that hexagons with ζ ≤ 0.5 display a continuous H-I transition,whereas those with ζ ≥ 0.6 demonstrate a first-order H-I transition.Dynamic analysis shows different evolution pathways of the dominant cluster formed by migrating particles,which results in a droplet-like cluster structure for ζ=0.40 hexagons and a tree-like cluster structure for ζ=0.99 hexagons.Further investigations on the hopping activities of particles suggest a cooperative origin of migrating clusters.Our work provides a new aspect to understand the dependence of the nature of H-I transition on the roundness of hexagons through particle dynamic behavior.
查看更多>>摘要：The photogenerated charge carrier separation and transportation of inside photocathodes can greatly influence the performance of photoelectrochemical(PEC)H2 production devices.Coupling TiO2 with p-type semiconduc-tors to construct heterojunction structures is one of the most widely used strategies to facilitate charge separation and transportation.However,the band position of TiO2 could not perfectly match with all p-type semiconductors.Here,taking antimony selenide(Sb2Se3)as an example,a rational strategy was developed by introducing a vio-logen electron transfer mediator(ETM)containing polymeric film(poly-1,1'-dially-[4,4'-bipyridine]-1,1'-diium,denoted as PV2+)at the interface between Sb2Se3 and TiO2 to regulate the energy band alignment,which could inhibit the recombination of photogenerated charge carriers of interfaces.With Pt as a catalyst,the constructed Sb2Se3/PV2+/TiO2/Pt photocathode showed a superior PEC hydrogen generation activity with a photocurrent density of-18.6 mA cm-2 vs.a reversible hydrogen electrode(RHE)and a half-cell solar-to-hydrogen efficiency(HC-STH)of 1.54％at 0.17 V vs.RHE,which was much better than that of the related Sb2Se3/TiO2/Pt photo-cathode without PV2+(-9.8 mA cm-2,0.51％at 0.10 V vs.RHE).
查看更多>>摘要：Mechanically interlocked polymers(MIPs)are promising candidates for the construction of elastomeric mate-rials with desirable mechanical performance on account of their abilities to undergo inherent rotational and translational mechanical movements at the molecular level.However,the investigations on their mechanical properties are lagging far behind their structural fabrication,especially for linear polyrotaxanes in bulk.Herein,we report stretchable poly[2]rotaxane elastomers(PREs)which integrate numerous mechanical bonds in the polymeric backbone to boost macroscopic mechanical properties.Specifically,we have synthesized a hydroxy-functionalized[2]rotaxane that subsequently participates in the condensation polymerization with diisocyanate to form PREs.Benefitting from the peculiar structural and dynamic characteristics of the poly[2]rotaxane,the representative PRE exhibits favorable mechanical performance in terms of stretchability(～1200％),Young's mod-ulus(24.6 MPa),and toughness(49.5 MJ/m3).Moreover,we present our poly[2]rotaxanes as model systems to understand the relationship between mechanical bonds and macroscopic mechanical properties.It is concluded that the mechanical properties of our PREs are mainly determined by the unique topological architectures which possess a consecutive energy dissipation pathway including the dissociation of host-guest interaction and con-sequential sliding motion of the wheel along the axle in the[2]rotaxane motif.
查看更多>>摘要：Two-dimensional MXenes are key high-capacitance electrode materials for micro-supercapacitors(MSCs)cater-ing to integrated microsystems.However,the narrow electrochemical voltage windows of conventional aqueous electrolytes(≤ 1.23 V)and symmetric MXene MSCs(typically ≤ 0.6 V)substantially limit their output voltage and energy density.Highly concentrated aqueous electrolytes exhibit lower water molecule activity,which in-hibits water splitting and consequently widens the operating voltage window.Herein,we report ultrahigh-voltage aqueous planar asymmetric MSCs(AMSCs)based on a highly concentrated LiCl-gel quasi-solid-state electrolyte with MXene(Ti3C2Tx)as the negative electrode and MnO2 nanosheets as the positive electrode(MXene//MnO2-AMSCs).The MXene//MnO2-AMSCs exhibit a high voltage of up to 2.4 V,attaining an ultrahigh volumetric energy density of 53 mWh cm-3.Furthermore,the in-plane geometry and the quasi-solid-state electrolyte en-abled excellent mechanical flexibility and performance uniformity in the serially/parallel connected packs of our AMSCs.Notably,the MXene//MnO2-AMSC-based integrated microsystem,in conjunction with solar cells and consumer electronics,could efficiently realize simultaneous energy harvesting,storage,and conversion.The findings of this study provide insights for constructing high-voltage aqueous MXene-based AMSCs as safe and self-sufficient micropower sources in smart integrated microsystems.
查看更多>>摘要：Exploitable or potentially exploitable deposits of critical metals,such as rare-earth(REE)and high-field-strength elements(HFSE),are commonly associated with alkaline or peralkaline igneous rocks.However,the origin,trans-port and concentration of these metals in peralkaline systems remains poorly understood.This study presents the results of a mineralogical and geochemical investigation of the Na-metasomatism of alkali amphiboles and clinopyroxenes from a barren peralkaline granite pluton in NE China,to assess the remobilization and redis-tribution of REE and HFSE during magmatic-hydrothermal evolution.Alkali amphiboles and aegirine-augites from the peralkaline granites show evolutionary trends from sodic-calcic to sodic compositions,with increasing REE and HFSE concentrations as a function of increasing Na-index[Na#,defined as molar Na/(Na+Ca)ratios].The Na-amphiboles(i.e.,arfvedsonite)and aegirine-augites can be subsequently altered,or breakdown,to form hydrothermal aegirine during late-or post-magmatic alteration.Representative compositions analyzed by in-situ LA-ICPMS show that the primary aegirine-augites have high and variable REE(2194-3627 ppm)and HFSE(4194-16,862 ppm)contents,suggesting that these critical metals can be scavenged by alkali amphiboles and aegirine-augites.Compared to the primary aegirine-augites,the presentative early replacement aegirine(Aeg-Ⅰ,Na#=0.91-0.94)has notably lower REE(1484-1972)and HFSE(4351-5621)contents.In contrast,the late hy-drothermal aegirine(Aeg-Ⅱ,Na#=0.92-0.96)has significantly lower REE(317-456 ppm)and HFSE(6.44-72.2 ppm)contents.Given that the increasing Na# from aegirine-augites to hydrothermal aegirines likely resulted from Na-metasomatism,a scavenging-release model can explain the remobilization of REE and HFSE in peralkaline granitic systems.The scavenging and release of REE and HFSE by Na-metasomatism provides key insights into the genesis of globally significant REE and HFSE deposits.The high Na-index of the hydrothermal aegirine might be useful as a geochemical indicator in the exploration for these critical-metals.
查看更多>>摘要：Financial incentives play a key role in promoting renewable energy investments that can help China achieve the'dual carbon'goal.The national emissions trading scheme(ETS)and the renewable energy portfolio standard(RPS)are two existing market-based policy instruments that can generate stable expected returns for low-carbon projects.This paper studies the interactive distribution effects of these two market-based instruments.We use the micro-level thermal power plant data to investigate the abatement effects of the national ETS,in which the details show that the existing rate-based ETS will result in higher negative impacts on power units,whose installed capacities are smaller than 400 MW.The interactive distribution effects between the two markets will occur when the permit allocation standards of the national ETS become stricter than the existing ones.Provinces in Eastern China and Northern China will face high pressure on costs in both ETS and RPS markets.When the levels of the permit allocation standards are set as 70％of the existing ones and the carbon price is assumed to be 200 yuan/ton in 2030,the annual market size of the national ETS will be nearly 100 billion yuan,and the annual market size is predicted to be 250 billion yuan.In the existing rate-based national ETS,the China Certified Emission Reduction(CCER)mechanism will have an offsetting effect,which should be taken into serious consideration during the policy-making processes in the future.
查看更多>>摘要：By conducting a two-dimensional experimental study,this paper aims to enhance the understanding of the mech-anism of sand convective motions in the vicinity of a wall subjected to long-term cyclic lateral loadings.The experimental tests were conducted in a rectangular sandbox with a transparent front-wall,through which the process of sand particle motions could be recorded by using a high-resolution digital camera.The images were processed with a high time-resolved PIV(Particle Image Velocimetry)system.Based on the experimental data,this work(1)presents the sand flow field in the convective zones;(2)provides means to describe the convection mechanism;(3)proposes the relationships between the loading conditions and dimensions of the region with intense sand movement;and(4)elaborates the similarity of the sand flow velocity structure within the sand convective zones.
查看更多>>摘要：Raptors can change the shape and area of their wings to an exceptional degree in a fast and efficient manner,surpassing other birds,insects,or bats.Some researchers have focused on the functional properties of muscle skeletons,mechanics,and flapping robot design.However,the wing motion of the birds of prey has not been measured quantitatively,and synthetic bionic wings with morphing abilities similar to raptors are far from reality.Therefore,in the current study,a 3D suspension system for holding bird carcasses was designed and fabricated to fasten the wings of Falco Peregrinus with a series of morphing postures.Subsequently,the wing skeleton of the falcon was scanned during extending motions using the computed tomography(CT)approach to obtain three consecutive poses.Subsequently,the skeleton was reconstructed to identify the contribution of the forelimb bones to the extending/folding motions.Inspired by these findings,we propose a simple mechanical model with four bones to form a wing-morphing mechanism using the proposed pose optimisation method.Finally,a bionic wing mechanism was implemented to imitate the motion of the falcon wing-divided into inner and outer wings with folding and twisting motions.The results show that the proposed four-bar mechanism can track bone motion paths with high fidelity.
查看更多>>摘要：The ionic environment of body fluids influences nervous functions for maintaining homeostasis in organisms and ensures normal perceptual abilities and reflex activities.Neural reflex activities,such as limb movements,are closely associated with potassium ions(K+).In this study,we developed artificial synaptic devices based on ion concentration-adjustable gels for emulating various synaptic plasticities under different K+ concentrations in body fluids.In addition to performing essential synaptic functions,potential applications in information pro-cessing and associative learning using short-and long-term plasticity realized using ion concentration-adjustable gels are presented.Artificial synaptic devices can be used for constructing an artificial neural pathway that con-trols artificial muscle reflex activities and can be used for image pattern recognition.All tests show a strong relationship with ion homeostasis.These devices could be applied to neuromorphic robots and human-machine interfaces.
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