Batch versus Flow Photochemistry: A Revealing Comparison of Yield and Productivity

The use of flow photochemistry and its apparent superiority over batch has been reported by a number of groups in recent years. To rigorously determine whether flow does indeed have an advantage over batch, a broad range of synthetic photochemical transformations were optimized in both reactor modes and their yields and productivities compared. Surprisingly, yields were essentially identical in all comparative cases. Even more revealing was the observation that the productivity of flow reactors varied very little to that of their batch counterparts when the key reaction parameters were matched. Those with a single layer of fluorinated ethylene propylene (FEP) had an average productivity 20 % lower than that of batch, whereas three‐layer reactors were 20 % more productive. Finally, the utility of flow chemistry was demonstrated in the scale‐up of the ring‐opening reaction of a potentially explosive [1.1.1] propellane with butane‐2,3‐dione.     

Different defect morphologies in polyethylene crystal induced by surface physicochemical properties

The physicochemical properties of surfaces have a great effect on the micro-morphologies of the crystal structures which are in contact with them.Understanding the interaction mechanism between the internal driving forces of the crystal and external inducing forces of the surfaces is the prerequisite of controlling and obtaining the desirable morphologies.In this work,the dynamic density functional theory was applied to construct the free energy functional expression of polyethylene(PE) lattice,and the micro-dynamic evolution processes of PE lattice morphology near the surfaces with different properties were observed to reveal the interaction mechanism at atomic scale.The results showed that the physical and chemical properties of the external surfaces synergistically affect the morphologies in both the defect shapes and the distribution of the defect regions.In the absence of the contact surfaces,driven by the oriented interactions among different CH2 groups,PE lattices gradually grow and form a defect-free structure.Conversely,the presence of contact surfaces leads to lattice defects in the interfacial regions,and PE lattice shows different self-healing abilities around different surfaces.     

Demystifying terms for understanding bioelectrochemical systems towards sustainable wastewater treatment

Bioelectrochemical systems (BESs) have been intensively studied in the past decade, but precise understanding of BESs performance is hindered by unclear definition of several key parameters. Herein, we analyze and discuss three sets of terms about conversion efficiency, energy performance, and pilot scale. It is suggested that ‘Coulombic recovery’ can avoid the misleading results because of different organic removals, compared with ‘Coulombic efficiency.’ Power density is not a suitable term to describe energy performance of BESs, and energy production/consumption should be reported in the energy unit such as kWh. Pilot-scale BESs should meet several criteria, including hydraulic capacity, use of actual wastewater, non-laboratory condition, and long-term operation. Proper use of those terms is strongly encouraged and will be critically important to BESs research and development.     

Real‐Time Atomic‐Scale Visualization of Reversible Copper Surface Activation during the CO Oxidation Reaction

By using in situ aberration‐corrected environmental transmission electron microscopy, for the first time at atomic level, the dynamic evolution of the Cu surface is captured during CO oxidation. Under reaction conditions, the Cu surface is activated, typically involving 2–3 atomic layers with the formation of a reversible metastable phase that only exists during catalytic reactions. The distinctive role of CO and O₂ in the surface activation is revealed, which features CO exposure to lead to surface roughening and consequently formation of low‐coordinated Cu atoms, while O₂ exposure induces a quasi‐crystalline CuOx phase. Supported by DFT calculations, it is shown that crystalline CuOx reversibly transforms into the amorphous phase, acting as an active species to facilitate the interaction of gas reactants and catalyzing CO oxidation.     

Study on wave dispersion characteristics of piezoelectric sandwich nanoplates considering surface effects

In this study, the wave propagation properties of piezoelectric sandwich nanoplates deposited on an orthotropic viscoelastic foundation are analyzed by considering the surface effects (SEs). The nanoplates are composed of a composite layer reinforced by graphene and two piezoelectric surface layers. Utilizing the modified Halpin-Tsai model, the material parameters of composite layers are obtained. The displacement field is determined by the sinusoidal shear deformation theory (SSDT). The Euler-Lagrange equation is derived by employing Hamilton’s principle and the constitutive equations of piezoelectric layers considering the SEs. Subsequently, the nonlocal strain gradient theory (NSGT) is used to obtain the equations of motion. Next, the effects of scale parameters, graphene distribution, orthotropic viscoelastic foundation, and SEs on the propagation behavior are numerically examined. The results reveal that the wave frequency is a periodic function of the orthotropic angle. Furthermore, the wave frequency increases with the increase in the SEs.

     

硫化丁苯橡胶纳观界面耐磨机理及参数影响分析

硫化橡胶因其良好的力学和物理化学性能而被广泛作为摩擦副的基础材料. 本文提出了一种硫化交联算法, 实现了C—C键的硫化互交联和自交联, 构建了硫化丁苯橡胶的分子动力学磨损模型, 从微观摩擦学的角度阐明了硫化交联结构对改善丁苯橡胶磨损性能的机理, 研究了不同界面参数对硫化橡胶微观磨损性能的影响. 结果发现 硫化使丁苯橡胶分子链的界面黏附能力和活动能力更弱, 拉伸和解缠能力更低, 磨损过程中界面累积能量更低, 更不容易脱离橡胶基体, 因此可以表现出更好的摩擦学性能, 更强的抗磨损性能; 随着速度的增大, 硫化橡胶的磨损率降低, 与宏观实验结果一致, 原因是硫化橡胶的原子分布函数和相互作用能随着速度增大而降低, 说明橡胶分子链的黏附能力和活动能力随着速度增加趋弱, 温升更低, 导致较低的磨损率; 压入深度对磨损率的影响规律则呈现相反的结果和趋势.     

高居里温度铁电单晶PIN-PT的机电性能

弛豫铁电单晶Pb(In_1/2Nb_1/2)O₃-PbTiO₃(PIN-PT)相较于常用的Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃(PMN-PT)具有更高的居里温度,在高稳定性、高性能的传感器、换能器方面具有应用前景。本工作采用谐振法研究了[001]方向极化的0.66PIN-0.34PT铁电单晶的全矩阵机电性能参数。0.66PIN-0.34PT 单晶的三方-四方相变温度(T_RT)约为160 ℃,居里温度(T_C)约为260 ℃,室温压电系数d₃₃、d₃₁、d₁₅分别为1 340 pC/N、-780 pC/N、321 pC/N,介电常数ε^T₃₃、ε^S₃₃、ε^T₁₁、ε^S₁₁分别为2 700、905、2 210、1 927,机电耦合系数 k₃₃、k₃₁、k₁₅、k_t分别为 87%、58%、38%、61%。其纵向压电常数(d₃₃)和纵向机电耦合系数(k₃₃)小于 PMN-PT 单晶,但是横向压电性能(d₃₁)和剪切压电性能(d₁₅)都略高于PMN-PT单晶。另外,研究了机电耦合性能随温度的变化趋势,发现0.66PIN-0.34PT单晶在150 ℃以下有较好的温度稳定性。     

DMD红外场景产生器非均匀性校正方法研究

非均匀性是影响数字微镜器件(Digital Micro-mirror Device,DMD)红外场景产生器投射红外场景质量的主要因素之一,DMD红外场景产生器必须经过非均匀性校正才能满足复杂环境下红外成像设备内场仿真试验的应用要求.给出了DMD红外场景产生器的非均匀性校正流程;提出了一种更适合于现有测试条件的变尺度稀疏...     

对数周期偶极子天线全空间可变相位中心

导出了直线阵列在全空间中存在可变相位中心的条件,得出了由非相似单元组成的直线阵列(如对数周期偶极子天线)不存在全空间可变相位中心,但在E面和H面独立存在各自的可变相位中心的结论.证明了对数周期偶极子天线(LPDA)在E面和H面有不同的可变相位中心.采用矩量法(MoM)分析了LPDA的E面和H面可变相位中心,计算结果表明:虽然在LPDA的主波束范围内其相位中心变化很小,可以作为"视在相位中心",但是这种"视在相位中心"仍然在E面和H面是不重合的.此外,"视在相位中心"随着频率沿着集合线移动.     

Prediction of resolvent mode shapes in supersonic turbulent boundary layers

This work applies resolvent analysis to compressible zero-pressure-gradient turbulent boundary layers with freestream Mach numbers between 2 and 4, focusing exclusively on large scale motions in the outer region of the boundary layer. We investigate the effects of Mach number on predicted flow structures, and in particular, look at how such effects may be attributed to changes in mean properties. By leveraging the similarity between the compressible and incompressible resolvent operators, we show that the shape of the streamwise velocity and temperature components of resolvent response modes in the compressible regime can be approximated by applying ideas from wavepacket pseudospectral theory to a simple scalar operator. This gives a means of predicting the shape of resolvent mode components for compressible flows without requiring the singular value decompositions of discretized operators. At a Mach number of 2, we find that accurate results are obtained from this approximation when using the compressible mean velocity profile. At Mach numbers of 3 and 4, the quantitative accuracy of these predictions is improved by also considering a local effective Reynolds number based on the local mean density and viscosity.