贸易便利化对我国跨境电商发展的影响研究——基于RCEP成员国的分析

RCEP的生效标志着全球经贸规模最大、最具发展潜力的自由贸易区的正式落地,将为其成员国的外贸增长提供新动能。在疫情的冲击及贸易保护主义抬头的背景下,跨境电商仍然表现出强劲的增长韧性,但其作为新兴业态模式,仍面临着贸易非便利因素的阻碍。本文聚焦于贸易便利化以及各因素对跨境电商进出口发展的影响。运用主成分分析法建立贸易便利化综合评价体系,并对RCEP成员国2012—2019年的贸易便利化水平进行测度,比较成员国各项贸易便利化水平指标差异,随后基于修正后的引力模型对RCEP成员国2012—2019年的面板数据进行回归。研究表明,贸易便利化能够显著促进跨境电商进出口的发展,其效应要强于关税水平,且对出口端的影响更大;文化距离在跨境电商业态中的影响要强于地理距离。分指标回归的结果显示,在贸易便利化下的四个一级指标中,金融与电子商务对跨境电商发展的影响最大。基于以上实证结果,本文为中国推进RCEP贸易便利化建设提供新思路。

Partitioning Calculation Method of Short-Circuit Current for High Proportion DG Access to Distribution Network

Aiming at the problemthat the traditional short-circuit current calculationmethod is not applicable to Distributed Generation(DG)accessing the distribution network,the paper proposes a short-circuit current partitioning calculation method considering the degree of voltage drop at the grid-connected point of DG.Firstly,the output characteristics of DG in the process of low voltage ride through are analyzed,and the equivalent output model of DG in the fault state is obtained.Secondly,by studying the network voltage distribution law after fault in distribution networks under different DG penetration rates,the degree of voltage drop at the grid-connected point of DG is used as a partition index to partition the distribution network.Then,iterative computation is performed within each partition,and data are transferred between partitions through split nodes to realize the fast partition calculation of short-circuit current for high proportion DG access to distribution network,which solves the problems of long iteration time and large calculation error of traditional short-circuit current.Finally,a 62-node real distribution network model containing a high proportion of DG access is constructed onMATLAB/Simulink,and the simulation verifies the effectiveness of the short-circuit current partitioning calculation method proposed in the paper,and its calculation speed is improved by 48.35%compared with the global iteration method.

Structural parameter optimization for novel internal-loop iron–carbon micro-electrolysis reactors using computational fluid dynamics

It is generally recognized that internal-loop reactors are well-developed mass and heat-transfer multiphase flow reactors. However, the internal flow field in the internal-loop reactor is influenced by the structure parameter of the reactor, which has a great effect on the reaction efficiency. In this study, the computational fluid dynamics simulation method was used to determine the influence of reactor structure on flow field, and a volume-offluid model was employed to simulate the gas–liquid, two-phase flow of the internal-loop micro-electrolysis reactor. Hydrodynamic factors were optimized when the height-to-diameter ratio was 4:1, diameter ratio was9:1, draft-tube axial height was 90 mm. Three-dimensional simulations for the water distributor were carried out, and the results suggested that the optimal conditions are as follows: the number of water distribution pipes was four, and an inhomogeneous water distribution was used. According to the results of the simulation,the suitable structure can be used to achieve good fluid mechanical properties, such as the good liquid circulation velocity and gas holdup, which provides a good theoretical foundation for the application of the reactor.

功能化蒙脱土涂覆改性锂电池用聚丙烯微孔膜的性能

通过聚甲基丙烯酸甲酯(PMMA)插层改性蒙脱土(MMT)获得了功能化改性蒙脱土(MMT-PMMA),通过刮涂在聚丙烯(PP)微孔膜表面引入MMT-PMMA后,再通过浸涂在微孔膜表面引入单宁酸(TA),制备了具有高电解液润湿性和高耐热性的改性微孔膜(PP/MMT-PMMA/TA)。接触角测试结果显示,PP微孔膜改性后,电解液接触角由47°降至0°,且电解液吸收率由PP膜的97.5%提高至136.6%,证明PP/MMT-PMMA/TA微孔膜具有良好的电解液亲和性。将微孔膜在不同温度放置30 min测试其耐热性,结果表明,PP/MMT-PMMA/TA微孔膜的耐热性较改性前的PP微孔膜有显著提升。电池性能测试结果表明,经PP/MMT-PMMA/TA微孔膜组装的电池首次充放电容量(60.0 m Ah/g)相比改性前的PP微孔膜(41.3 mAh/g)提升了45%,且倍率性能更好。

Spatio-temporal Variation of PM_(2.5) in China from 1998 to 2016

With the rapid development of China's economy,people's demand for a healthy living environment is increasing,and air quality has gradually been widely concerned by all sectors of society.Using the big data of air quality monitoring from 1998 to 2016,based on the exploratory spatio-temporal analysis method,this paper explored the spatio-temporal evolution of PM_(2.5) at the national scale,and drew the following conclusions:①PM_(2.5) heavy pollution is mainly in central and eastern China,north and south China,and the pollution degree is relatively light in northwest and northeast China.Meanwhile,PM_(2.5) concentration in heavily polluted areas increased significantly over time,while PM_(2.5) concentration in low-polluted areas showed a long-term stable trend.②The number and area of cities with moderate and high PM_(2.5) pollution levels showed an inverted U-shaped curve from 1998 to 2016,and 2007 was the inflection point.③The spatial autocorrelation coefficient of PM_(2.5) is high over the years,and the spatial neighbor effect of PM_(2.5) is significant.The high-pollution clusters are mainly concentrated in the Beijing-Tianjin-Hebei region,the Yangtze River Delta and the Pearl River Delta,and the pollution concentration in these three regions has increased rapidly in recent years.It is necessary to focus on joint prevention and control.

纳米芳纶纤维/纳米氧化铝涂覆改性锂电池用聚丙烯微孔膜的性能

通过纳米芳纶纤维/纳米氧化铝对锂电池用聚丙烯(PP)微孔膜进行涂覆改性,以提高微孔膜亲电解液性能及耐热性能。将芳纶短纤在碱性二甲基亚砜溶液中分散制备纳米芳纶纤维分散液,加入纳米氧化铝涂覆在PP微孔膜表面获得改性微孔膜。通过扫描电镜对微孔膜表面微观形貌进行观察,结果表明,纳米氧化铝的加入改善了纳米芳纶纤维层的孔结构,提高了改性微孔膜的透气性能。接触角测试表明,改性微孔膜电解液接触角从未改性聚丙烯微孔膜的87°降低到0°,改性后微孔膜对电解液有更好的浸润效果。耐热测试结果证实,改性微孔膜的热收缩程度显著降低,在170℃放置30min,未改性微孔膜热收缩率为37.3%,改性微孔膜降低至13.4%,耐热性能明显提高。电池性能测试结果表明,改性涂层提升了锂电池的电池容量及容量稳定性。

The Influence of Ceramic Coating on Fatigue Crack Growth Behaviour of X17CrNi15-2 Stainless Steel

Fatigue crack growth tests were carried out on the SEN B3 precracked specimens, with dimensions in accordance with ISO 12108 requirements. The specimens were made of martensitic stainless steel, X17CrNi15-2, and some of them were modified by the ceramic coating deposition surface treatment. The effects of ceramic coating, on the fatigue crack growth behaviour of hollow shaft specimens, were experimentally investigated. Fatigue crack growth rates, da/dN, were characterised, using the power law relationship between da/dN (in mm/cycle) and the stress intensity factor range, ΔK (in MPa∙m0.5). The two constants of the correlation are 7.9768 × 10−9 and 2.8107 for the parent material, and those for the coated material are 2.4391 × 10−9 and 3.1990, respectively. Microstructural analyses were carried out on the tested specimens, which shows that the maximum hardness of the ceramic coating is higher than that of substrate by a factor of ~3.2. The dimple fracture dominates the final fracture mechanism for the parent material, and the combination of fatigue, ductile fracture and cleavage dominates the final fracture mechanism for the coated material, based on the SEM analyses. EDS tests’ results reveal that the parent material specimen shows higher levels of C at matrix regions along with Fe- and O-rich regions, compared with the coated material specimen.

Visible-light-induced decarboxylative alkynylation of carboxylic acids in batch and continuous flow

A green efficient photoredox-catalyzed decarboxylative alkynylation of carboxylic acids with alkynyl bromides has been developed.This broadly applicable protocol is presented whereinα-amino,aliphatic andα-oxy acids are converted into useful alkynylation products.The commercially-available organic photocatalyst 4CzIPN is used as the photocatalyst,organic base DBU is utilized as the base,and DMSO serves as solvent.This strategy features mild conditions,is metal-free,and is environmentally friendly.The batch and continuous-flow protocols described were applied to obtain a broader substrate scope of functionalization(more than 50 examples).Furthermore,we demonstrate that the use of microflow technology enhanced and intensified the reaction process,achieving significantly reduced reaction times(i.e.,10 min of residence time).