Hollow Engineering to Co@N-Doped Carbon Nanocages via Synergistic Protecting-Etching Strategy for Ultrahigh Microwave Absorption

Rational manipulation of hollow structure with uniform heterojunctions is evolving as an effective approach to meet the lightweight and high-performance microwave absorption for metal-organic frameworks (MOFs) derived absorbers. Herein, a new and controlled synergistic protecting-etching strategy is proposed to construct shelled ZIF-67 rhombic dodecahedral cages using tannic acid under theoretical guidance, then hollow Co@N-doped carbon nanocages with uniform heterojunctions and hierarchical micro-meso-macropores are obtained via a pyrolysis process, which addresses the shortcomings of using sacrificing templates or corrosive agents. The outer Co@N-doped carbon shell, composed of highly dispersive core-shell heterojunctions, possesses micro-mesopores while the inner hollow macroporous cavity endows the absorbers with lightweight characteristics. Accordingly, the maximum reflection loss is −60.6 dB at 2.4 mm and the absorption bandwidth reaches 5.1 GHz at 1.9 mm with 10 wt% filler loading, exhibiting superior specific reflection loss compared with the vast majority of previous MOFs derived absorbers. Furthermore, this synergistic protecting-etching strategy provides inspiration for precisely creating a hollow void inside other MOFs crystals and broadens the desirable candidates for lightweight and high-efficient microwave absorbers.     

均质多孔介质中纳米颗粒吸附层的水流滑移模型

考虑纳米颗粒吸附边界层对管径大小的影响和不同流态时的岩心渗透率的差异,建立了基于纳米颗粒吸附的均质多孔介质水流滑移数学模型(K-L MⅡ)。测试了油基和水基两种纳米流体的减阻效果,两组岩心水相渗透率的增幅平均分别为46.5%和76%。利用K-L MⅡ和K-L MⅠ计算了两组岩心中的水流滑移长度,大约在20–80 nm左右。结果显示,两者的差异较大,最大相差100%,表明不宜用K-L MⅠ计算具有吸附层厚度影响的滑移长度。利用K-L MⅡ解释了纳米颗粒吸附造成孔道减小而注水量增加的矛盾,为阐述纳米减阻机理提供了理论依据。K-L MⅡ也适用于吸附层厚度为0的水流滑移效应,具有普适性。     

级配不连续碎砾石粗细颗粒区分粒径比较分析

粗细颗粒区分粒径是级配不连续碎砾石细粒含量判别法判定渗透变形类型重要指标 ,目前通常使用的有定值法的2mm和颗粒级配曲线中平缓段最小粒径(平均粒径),为进一步验证这2个典型方法和量值的符合性、适宜性 ,统计了26个工程121个试样的渗透变形试验结果进行比较.结果表明 :2种方法和量值的渗透变形类型判断结果与试验结果符合率较高 ,分别为85 .5% 和84 .6% ,均可作为粗细颗粒区分粒径 ,但若和级配连续碎砾石渗透变形类型判断综合考虑 ,推荐2mm作为级配不连续碎砾石粗细颗粒区分粒径.     

Possible Ignition of Particles Ejected into the Gap in Explosive Welding of Titanium

Processes that occur in the welding gap during explosive welding are analyzed. It is shown that metal particles flying out into the gap due to the jetformation effect can ignite in shockcompressed air. For most metals, the energy released thereby is small and has no significant influence on weld formation. In titanium welding on large areas, surface sections located far from the place of detonation initiation, which experience a longterm action of a hot air flow, can dissolve a large amount of oxygen and nitrogen. If particles from these sections enter the gap, it can lead to chemical reactions with formation of TiO₂ and TiN by the mechanism of internal combustion. The energy released in the gap per unit area is commensurable with and even greater than the kinetic energy of the accelerated (flyer) plate. Local bulging and rupture of metal observed in practice can be explained by ignition and combustion of gassaturated titanium particles in the welding gap.     

沉淀法制备纳米级粒子的研究——化学原理及影响因素

本文详细介绍了沉淀法制备纳米级粒子的化学原理，重点讨论了反应物浓度，反应温度，表面活性剂，ｐＨ值及反应时间等诸因素对粒子大小，形状的影响。     

Bubble sizes in agitated air-alcohol systems with and without particles: Turbulent and transitional flow

In heterogeneous catalytic processes, the relationship between bubble size and various process parameters is important in determining interfacial area, hence mass transfer rate. This paper presents the results of an experimental study of bubble sizes in air-alcohol dispersions with and without the presence of catalyst particles in a batch stirred reactor at different agitation speeds and mean specific energy dissipation rates. Factors investigated include catalyst type, particle size and concentration. It has been found that:

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Since there are large differences in the viscosity of the various alcohols used, by dividing them into two groups based on the flow type (turbulent or transitional), a consistent correlation has been found for mean bubble size as a function of Weber number, We, for each region, though neither correlation agrees with the precise relationship suggested by Kolmogoroff's theory of locally, homogeneous, isotropic turbulence.

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At the same We number, the size is bigger in transitional flow than in turbulent. The functionality of the correlation of d₃₂/D versus We is similar whether particles are present or not.

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In fully developed turbulent flow, the bubble size is slightly reduced when catalyst particles are present, independent of catalyst size or type.

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In transitional flow, the effect of catalyst particles on bubble size is much stronger, the size being reduced by approximately 30%.

     

环氧树脂增韧改性的研究现状

综述了改性环氧树脂的最近研究状况，概述了橡胶粒子、热塑性树脂及热致液晶聚合物等对环氧树脂改性方法。对环氧树脂改性的方向进行了展望。     

影响粉末流化的本征作用力及其调节

本文归纳了人们对粒间力的分析和计算，指出了影响颗粒间作用进而影响其流化质量的各种因素，并从改善微细物料流化质量的角度，综述了近年来国内外流化微细物料的诸多措施，将其归结为两大类型：外力场方法与本征途径，以作为进一步探求微细物料流态化的研究基础．     

掺混纳米微粉的聚碳硅烷熔融纺丝工艺研究

运用功率超声将纳米Ｎｉ粉均匀分散到聚碳硅烷（ＰＣＳ）中，通过熔融纺丝制备出了直径约１５～３５μｍ的有机纤维。研究了超声过程对纳米微粉分散状态的影响及掺混型ＰＣＳ熔融纺丝的工艺条件。     

常压烧结莫来石／氧化锆／碳化硅复相陶瓷的研究

本文对莫来石／氧化锆／碳化硅复相陶瓷进行了Ｎ２气氛中常压烧结的研究。实验结果表明：ＳｉＣ粒子添加量≤２０ｖｏｌ％，材料均可致密烧结并可获得均匀的微观结构。ＳｉＣ粒子的加入使材料人力学性能较莫来石／氧化锆陶瓷有明显的提高，并在ＳｉＣ含量为１０ｖｏｌ％时达到峰值，室温强度和断裂韧性分别为６０１ＭＰａ和５．８ＭＰａ＾Ｃ２，接近热压材料。