水文序列相依变异识别的RIC定阶准则——以自回归模型为例

水文过程相依性是水文变异的主要表现形式之一,应用自回归模型对其进行拟合时合理确定模型阶数是一个难点问题。本文在分析AIC和BIC准则的基础上,提出了一种以原序列与其相依成分的相关系数作为拟合度指标,同时借用信息熵形式的函数式,作为模型不确定性度量指标的自回归模型定阶准则(简称RIC准则)。以AR(1)、AR(2)、AR(3)和AR(4)模型为例进行统计试验,将不同序列长度下该准则的定阶准确率与其他定阶准则进行比较,试验结果表明,RIC准则对于上述模型均具有较好的适应性,且定阶准确率远高于AIC准则,其中对于前三阶模型RIC准则优于BIC准则,但四阶模型略低于BIC准则。RIC准则的优势是可以同时满足模型定阶、相依程度分级与模型检验的需求,将其应用于实测水文序列分析,结果显示,该准则能较准确地识别自回归模型的阶数,且符合提出的"相依有变异而残差无变异的最小阶数"的检验标准。     

Monolith free-standing plasmonic PAN/Ag/AgX(X = Br,I)nanofiber mat as easily recoverable visible-light-driven photocatalyst

Most efficient visible-light-responsive photocatalysts are in the form of powder, leading to the tedious separation from the reaction media. Herein, we developed a versatile method for the general synthesis of free-standing polyacrylonitrile(PAN)/Ag/AgX(X = Br, I) nanofiber mats, where fibrous PAN/Ag precursor was firstly prepared via an electroless plating strategy, followed by a direct elemental halogenation with Br_2/H_2 O or I_2/ethanol solution. The as-obtained PAN/Ag/AgX nanofiber mats exhibit exceptional photocatalytic activity toward degradation of organic pollutants. Furthermore, the flexibility enables it to be easily recovered after the reaction was completed. This work provides new insights into the fabrication of membrane-based photocatalysts on a large scale.     

The enhanced photocatalytic activity of Na0.9Mg0.45Ti3.55O8 co‐loaded with silver and platinum

Noble metals Ag and Pt were loaded on Na_0.9Mg_0.45Ti_3.55O₈ (NMTO) by chemical bath deposition method, which was synthesized firstly in our recent work. The scanning electron microscopy and transmission electron microscopy results show that Ag and Pt nanoparticles were distributed on the different surfaces of NMTO. NMTO loaded with Ag and Pt can efficiently enhance the separation of photogenerated electron‐hole pairs, exhibiting much higher photodegradative ability for methylene blue and rhodamine B than the pure NMTO. The best weight concentrations of Ag and Pt are 9.00% and 3.70%, respectively. The electrostatic field is built in the Schottky barrier between NMTO and noble metals, leading to the energy band bending. Then, photogenerated electrons and holes are efficiently separated to enhance the photocatalytic activity.     

Fabrication of high-purity HfSi2 powder via molten salt-assisted magnesium thermal reduction

In this work, high-purity HfSi₂ powders were successfully fabricated via a molten salt-assisted magnesium thermal reduction method using HfO₂ and Si as raw materials. The effects of reaction temperature and time on the formation of HfSi₂ were systemically investigated. The morphological and phase composition of as-prepared HfSi₂ powders were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and nitrogen/oxygen determinator. The results indicated that as-obtained HfSi₂ powders possess the orthorhombic structure with approximately 6.4 μm in size and the oxygen content as low as 0.20%. This work can provide a novel route to fabricate the high-purity transition metal silicides powders.     

Logarithmic decrease of adhesion force with lateral dynamic revealed by an AFM cantilever at different humidities

Adhesion forces between a tipless cantilever and an Au film were determined to investigate the influence of lateral velocity by recording force curves with an atomic force microscope at 20%–90% relative humidities. The sample was moved laterally, forth and back, with a frequency of 0.001–100 Hz and scan distances of 0.8, 8, and 80 μm to achieve a velocity ranging over 7 orders of magnitude. Experimental results show that at low lateral velocities (between 1.6 nm/s and 1–10 μm/s), the adhesion force either increases or decreases or remains stable with the lateral velocity without a certain characteristic trend. However, after a critical velocity, the adhesion force decreases logarithmically with the lateral velocity (between 1–10 and 16,000 μm/s). The decreasing magnitude can be as large as 97.3% of the maximum adhesion force. This decrease is well-explained by the contact time dependence of water bridges formed by capillary condensation.     

ADS熔盐散裂靶中子学性能研究

与传统加速器驱动次临界系统（ADS）采用金属靶作为散裂中子靶的设计不同，加速器驱动次临界熔盐堆（AD-MSRs）采用靶堆一体的设计，直接使用燃料熔盐作为散裂中子靶。由于熔盐靶的中子学性能直接影响AD MSRs的能量放大系数、核废物的嬗变和核燃料增殖的效率，所以本研究基于MCNPX程序，详细计算了高能质子轰击氟盐和氯盐两种熔盐靶产生的散裂中子产额、散裂中子能谱、能量沉积分布以及散裂产物等中子学性能，并与液态Pb和铅铋共熔体（LBE）两种液态金属靶进行了对比。计算结果表明，熔盐靶在散裂中子产额上与液态金属靶有一定的差距，但熔盐靶内能量沉积分布的梯度较小，更有利于靶区的热量导出。与液态Pb和LBE靶相比，熔盐靶的散裂产物中包含更多的气体以及高质量数的α发射体核素。     

A library of carbon-supported ultrasmall bimetallic nanoparticles

Small-sized bimetallic nanoparticles that possess numerous accessible metal sites and optimal geometric/electronic structures show great promise for advanced synergetic catalysis but remain synthetic challenge so far. Here, an universial synthetic method is developed for building a library of bimetallic nanoparticles on mesoporous sulfur-doped carbon supports, consisting of 24 combinations of 3 noble metals (that is, Pt, Rh, Ir) and 7 other metals, with average particle sizes ranging from 0.7 to 1.4 nm. The synthetic strategy is based on the strong metal-support interaction arising from the metal-sulfur bonding, which suppresses the metal aggregation during the H₂-reduction at 700 °C and ensure the formation of small-sized and alloyed bimetallic nanoparticles. The enhanced catalytic properties of the ultrasmall bimetallic nanoparticles are demonstrated in the dehydrogenation of propane at high temperature and oxidative dehydrogenations of N-heterocycles.

     

基于连续介质损伤力学的7075铝合金高温成形极限理论预测与数值仿真

开展7075铝合金高温单向拉伸试验和成形极限试验，获得了不同温度和应变率的应力-应变曲线和成形极限曲线。结果表明，在较高的温度和应变率时7075铝合金的强度减小、成形性提高。为描述7075铝合金高温损伤演化过程，提出一种改进的连续介质损伤模型，并建立了耦合损伤的多轴统一黏塑性本构模型。基于试验结果，运用NSGAII遗传算法标定了模型中的参数，标定后的本构模型可以很好地预测7075铝合金的高温热力行为和极限应变。通过有限元软件Abaqus的用户材料子程序VUMAT，该本构模型被编入到Abaqus软件中进行数值仿真计算。结果表明，仿真获得的成形极限曲线和应变场分布与试验和理论结果吻合度好，进一步证明了所建立的耦合损伤的多轴本构模型的正确性及其在高温成形极限有限元仿真中的适用性。     

Metal–Organic Frameworks Based on Group 3 and 4 Metals

Metal–organic frameworks (MOFs) based on group 3 and 4 metals are considered as the most promising MOFs for varying practical applications including water adsorption, carbon conversion, and biomedical applications. The relatively strong coordination bonds and versatile coordination modes within these MOFs endow the framework with high chemical stability, diverse structures and topologies, and interesting properties and functions. Herein, the significant progress made on this series of MOFs since 2018 is summarized and an update on the current status and future trends on the structural design of robust MOFs with high connectivity is provided. Cluster chemistry involving Y, lanthanides (Ln, from La to Lu), actinides (An, from Ac to Lr), Ti, and Zr is initially introduced. This is followed by a review of recently developed MOFs based on group 3 and 4 metals with their structures discussed based on the types of inorganic or organic building blocks. The novel properties and arising applications of these MOFs in catalysis, adsorption and separation, delivery, and sensing are highlighted. Overall, this review is expected to provide a timely summary on MOFs based on group 3 and 4 metals, which shall guide the future discovery and development of stable and functional MOFs for practical applications.     

低阶煤热解条件对高炉喷吹半焦燃烧性能及动力学特性的影响

采用热重分析法研究了不同热解条件下半焦的燃烧性能和动力学特征，利用Ozawa法求取动力学参数。结果表明，热解温度越低、保温时间越短时，半焦的燃烧性能越好；热解升温速率对半焦燃烧过程的反应程度影响不大；粒度越大，燃烧性能差异性越明显。热解温度对半焦燃烧性能影响较大，550℃是本研究中制备高燃烧反应性半焦的适宜热解温度。两种不同粒度原煤制得的半焦均随转化率增大，活化能减小。1~3 mm原煤在热解温度为550℃时所得半焦在燃烧过程中符合反应级数模型，化学反应为限制性环节，反应最概然机理函数为f(α)=(1–α)2。