Synergistic Relay Reactions To Achieve Redox-Neutral α-Alkylations of Olefinic Alcohols with Ruthenium(II) Catalysis

Herein, we report a ruthenium-catalyzed redox-neutral α-alkylation of unsaturated alcohols based on a synergistic relay process involving olefin isomerization (chain walking) and umpolung hydrazone addition, which takes advantage of the interaction between the two rather inefficient individual reaction steps to enable an efficient overall process. This transformation shows the compatibility of hydrazone-type “carbanions” and active protons in a one-pot reaction, and at the same time achieves the first Grignard-type nucleophilic addition using olefinic alcohols as latent carbonyl groups, providing a higher yield of the corresponding secondary alcohol than the classical hydrazone addition to aldehydes does. A broad scope of unsaturated alcohols and hydrazones, including some complex structures, can be successfully employed in this reaction, which shows the versatility of this approach and its suitability as an alternative, efficient means for the generation of secondary and tertiary alcohols.     

A simple image-reject mixer based on two parallel phase modulators

A simple photonic microwave image-reject mixer (IRM) using two parallel phase modulators is proposed. First, a photonic microwave mixer with phase shift ability is achieved using two parallel phase modulators (PMs), an optical bandpass filter, three polarization controllers, three polarization beam splitters and two balanced photodetectors. At the output of the mixer, two frequency downconverted signals with tunable frequency difference can be obtained. By adjusting the phase difference as 90° and utilizing an electrical 90° hybrid, the useless components can be eliminated, and the image reject operation is realized. The key advantage of the proposed scheme is the usage of PM, which avoid the DC bias shifting problem and make the system simple and stable. A simulation is performed to verify the proposed scheme, a relative ??90° or 90° phase shift can be obtained between the two output ports of the photonic microwave mixer, at the output of the IRM, 60 dB image-reject ratio is obtained.     

Tunable microwave photonics filter with dual pass-band based on PM-FBG and PS-FBG

In this paper, a dual pass-band microwave photonics filter with simple, commercial structure is proposed and demonstrated. The key devices are the specially designed polarization maintaining fiber Bragg grating and the phase shift fiber Bragg grating. They are employed to extract out two orthogonally polarized sidebands and remove the undesired sideband, respectively. The simulation results show that without any extra operations or electrical processing, the dual pass-band can be achieved with the two central frequencies of 3.5 GHz and 8 GHz when the frequency spacing between the two orthogonally polarized sidebands is 12 GHz, their 3-dB bandwidth are about 500 MHz. The central frequencies of the two pass-bands can be simply tuned by adjusting the frequency spacing in a range of 4 GHz. In addition, the spurious free dynamic ranges for the two pass-bands are 75.71 dB Hz^2/3 and 70.17 dB Hz^2/3 respectively. Finally, a brief experiment is also carried out to demonstrate the feasibility.     

三聚氯氰催化及溶剂效应实现温和高效的酮肟贝克曼重排反应

与其他溶剂的效果显著不同,以六氟异丙醇为溶剂时,低用量三聚氯氰可在室温下高效催化酮肟的贝克曼重排反应得到相应的酰胺产物.本方法操作简便,催化剂用量少,溶剂可回收再利用,条件温和,官能团兼容性好,适用底物范围广,产物收率高,是一种制备酰胺化合物的相对绿色实用的方法.     

Error rate performance analysis of optical links between high altitude platforms with misalignment fading by Hoyt distribution

The more reality misalignments model with Hoyt distribution is used to analyze performance for optical links between high altitude platforms. Compared with the recent work in terrestrial FSO, it is proved that the variance ratio have more significant effect on error performance for optical links over long distance. Then, the expression for the pairwise error probability is analyzed and applied to obtain upper bounds on the BER performance for coded optical communications. Simulation results show that coding technique can improve error performance of optical links, but it also can enhance the influences of the variance ratio.     

双音调制下星上微波光子系统的交调失真分析

马赫-曾德尔调制器的非线性会严重恶化星上微波光子系统的性能。建立了包括光源、马赫-曾德尔调制器和光电探测器的双音调制理论模型,利用傅里叶级数展开、傅里叶变换和Gegenbauer加法定理,推导出了调制器非线性失真的严格通用解析解。根据该解析解,可精确地预计不同调制方式下星上微波光子系统的非线性失真,优化系统性能。分析结果表明,双边带和单边带调制条件下,将调制器输出信号保留至二次谐波能较好地近似计算三阶交调失真比和三阶交调截点,三阶交调失真比随调制系数的增大先增大后减小。当调制系数小于1.4674时,单边带调制三阶交调失真比至少比双边带调制小6dB,适合于星上微波光子系统的应用。     

Advances in thermoelectrics

Thermoelectric generators, capable of directly converting heat into electricity, hold great promise for tackling the ever-increasing energy sustainability issue. The thermoelectric energy conversion efficiency is heavily dependent upon the materials’ performance that is quantified by the dimensionless figure-of-merit (ZT). Therefore, the central issue in the research of thermoelectric materials lies in continuously boosting the ZT value. Although thermoelectric effects were discovered in the nineteenth century, it was only until the 1950s when classic materials like Bi₂Te₃ and PbTe were developed and basic science of thermoelectrics was established. However, the research of thermoelectrics did not take a smooth path but a rather tortuous one with ups and downs. After hiatus in the 1970s and 1980s, relentless efforts starting from the 1990s were devoted to understanding the transport and coupling of electrons and phonons, identifying strategies for improving the thermoelectric performance of existing materials, and discovering new promising compounds. Rewardingly, substantial improvements in materials’ performance have been achieved that broke the ZT limit of unity. Meanwhile, advancements in fundamental understanding related to thermoelectrics have also been made. In this Review, recent advances in the research of thermoelectric materials are overviewed. Herein, strategies for improving and decoupling the individual thermoelectric parameters are first reviewed, together with a discussion on open questions and distinctly different opinions. Recent advancements on a number of good thermoelectric materials are highlighted and several newly discovered promising compounds are discussed. Existing challenges in the research of thermoelectric materials are outlined and an outlook for the future thermoelectrics research is presented. The paper concludes with a discussion of topics in other fields but related to thermoelectricity.     

高碳含量新型亚微米无孔二氧化硅材料的修饰方法及其在反相加压毛细管电色谱平台上的应用

亚微米无孔二氧化硅(NPS)材料具有小粒径及表面光滑形状规整等特点,是一种性能优异的色谱材料,但其存在比表面积小、修饰效率低的问题.针对此设计了一种具有高碳含量的修饰方法:以3-缩水甘油基氧基丙基三甲氧基硅烷(GPTS)作为硅烷偶联剂,聚乙烯亚胺(PEI)作为聚合物包覆层,并以硬脂酰氯修饰得到一种氨基包覆的具有C18碳...