Compact fiber optic accelerometer

A compact fiber optic accelerometer based on a Michelson interferometer is proposed and demonstrated. In the proposed system, the sensing element consists of two single-mode fibers glued together by epoxy, which then act as a simple supported beam. By demodulating the optical phase shift, the acceleration is determined as proportional to the force applied on the central position of the two single-mode fibers. This simple model is able to calculate the sensitivity and the resonant frequency of the compact accelerometer. The experimental results show that the sensitivity and the resonant frequency of the accelerometer are 0.42 rad/g and 600 Hz, respectively.     

基于等离子体气动激励的斜劈诱导激波控制

基于弧光等离子体气动激励,采用不同的放电通道间距、放电通道数、放电直流输入电压、斜劈劈角、有无磁场作用等激励条件,实验研究了在超音速来流条件下(马赫数为2.2)斜激波位置、角度、强度的变化规律。结果表明:施加等离子体气动激励后,激波的起始位置平均前移1～8 mm,激波角平均减小4%～8%,激波强度平均减弱8%～26%。这主要是由于等离子体气动激励产生高温高压的表面等离子体层,使边界层分离点逆气流前移,改变了原有激波系结构,使原有的激波位置前移,激波角减小;同时由于局部的高温导致当地音速增大,使得当地马赫数减小,上述两个原因均可导致激波强度减弱。     

Multiscale dewetting of triblock copolymer thin film induced by solvent vapor

Multiscale dewetting of poly(styrene‐b‐ethylene/butylenes‐b‐styrene) (SEBS) triblock copolymer thin films induced by volatile solvent vapor treatment were observed in this study. Film rupture occurred at first and produced macroscopic holes. Near‐regular droplets (which represented a compromise between complete disorder and perfect order) could be formed at the last stage. The mechanism of solvent‐driven dewetting was discussed by comparing with that of thermal‐induced dewetting. Similar to thermal‐induced dewetting, the block copolymer thin films initially break up through the nucleation of holes that perforated the films. The rapid growing holes became unstable and formed nonequilibrium fingering patterns. The films exhibit autophobic or autodewetting phenomena. The velocity of the holes growth was nearly a constant (3.3 μm/min). The stages of the dewetting were quite similar to that found for homopolymer and block copolymer thin films dewetting on solid or liquid substrates under thermal treatment. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2874–2884, 2005     

HTO/Cellulose Aerogel for Rapid and Highly Selective Li+ Recovery from Seawater

To achieve rapid and highly efficient recovery of Li⁺ from seawater, a series of H₂TiO₃/cellulose aerogels (HTO/CA) with a porous network were prepared by a simple and effective method. The as-prepared HTO/CA were characterized and their Li⁺ adsorption performance was evaluated. The obtained results revealed that the maximum capacity of HTO/CA to adsorb Li⁺ was 28.58 ± 0.71 mg g⁻¹. The dynamic k₂ value indicated that the Li⁺ adsorption rate of HTO/CA was nearly five times that of HTO powder. Furthermore, the aerogel retained extremely high Li⁺ selectivity compared with Mg²⁺, Ca²⁺, K⁺, and Na⁺. After regeneration for five cycles, the HTO/CA retained a Li⁺ adsorption capacity of 22.95 mg g⁻¹. Moreover, the HTO/CA showed an excellent adsorption efficiency of 69.93% ± 0.04% and high selectivity to Li⁺ in actual seawater. These findings confirm its potential as an adsorbent for recovering Li⁺ from seawater.     

Formal Diels-Alder reactions of chalcones and formylcyclopropanes catalyzed by chiral N-heterocyclic carbenes

Highly enantioselective (formal) hetero-Diels-Alder reactions between chalcones and formylcyclopropanes are disclosed. The challenging N-heterocyclic carbene (NHC)-bounded enolate intermediates from formylcyclopropanes were captured for new C-C bond forming reactions. The reaction products were obtained with high diastereo- and enantioselectivities and could be easily transformed to optically pure multisubstituted cyclohexane derivatives.     

新型无线局域网双频段微带贴片天线设计

 新型无线局域网双频段微带贴片天线采用50 Ω微带线进行馈电,采用在贴片切槽设计方式,实现了微带天线在双频段工作的要求。其中心工作频率分别为2.44 GHz和5.20 GHz,相应的工作带宽分别达到了200 MHz和1.20 GHz。该天线具有小尺寸、结构简单、制作简单,低成本和在各工作频点处具有良好的方向性等优点,而且只需简单修改辐射贴片上的缝隙尺寸参数,就可以实现对天线高频段和低频段的调节。     

Chemo-selective cross reaction of two enals via carbene-catalyzed dual activation

A dual catalytic chemo-selective cross-coupling reaction of two enals is developed. One enal (without α-substitution) is activated by an NHC catalyst to form an acylazolium enolate intermediate that undergoes Michael-type addition to another enal molecule bearing an alkynyl substituent. Mechanistic studies indicate that non-covalent interactions between the alkynyl enal and the NHC·HX catalyst play important roles in substrate activation and enantioselectivity control. Many of the possible side reactions are not observed. Our reaction provides highly chemo- and diastereo-selective access to chiral lactones containing functionalizable 1,3-enyn units with excellent enantioselectivities (95 to >99% ee).

An NHC-catalyzed dual activation of two different enals is disclosed with both covalent and non-covalent activation pathways involved.

The development of chemo-selective reactions of two or more substrates bearing similar functional groups remains a classic challenge in organic synthesis.¹ Enals (α,β-unsaturated aldehydes) are common building blocks that offer multiple useful modes of reactions. For instance, enals are readily used as Michael acceptors in many reactions including organic catalytic reactions mediated by amines.² In the area of N-heterocyclic carbene (NHC) organocatalysis,³ enals are used as precursors of several NHC-bound intermediates, including Breslow acyl anion intermediates,⁴ homoenolate intermediates,⁵ enolate intermediates,⁶ and acylazolium intermediates.⁷ Somewhat surprisingly, on the other hand, there is little success in using enals as Michael acceptors to react with any of these NHC-bound intermediates.⁸ Elegant studies in this direction are from Scheidt, in which they showed that in the presence of an NHC catalyst, a homo coupling reaction of enals (with one enal molecule as the Michael acceptor) occurred effectively (Fig. 1a, top side).^8a,c Berkessel reported an intramolecular reaction of two enal moieties (in one molecule) to form a bicyclic lactone adduct in the presence of an achiral NHC catalyst (Fig. 1a, bottom side).^8b To the best of our knowledge, the intermolecular Michael addition reaction of two different enal substrates mediated by NHC catalysts has not been reported.⁹ Possible reasons for the difficulties of enals to behave as effective Michael acceptors likely include: (a) the relatively low electrophilicity of the α,β-unsaturated bonds of enals under the typical NHC catalytic conditions and (b) the presence of competing reactions involving both the alkene and aldehyde moieties of enals.Open in a separate windowFig. 1NHC-catalyzed reactions (a) with enals as Michael acceptors, (b) via cross intermolecular reactions of two enals, and (c) bio-active molecules bearing alkyne units.Here we disclose the first cross intermolecular reaction of two enals catalyzed by NHC catalysts (Fig. 1b). We envisioned that installation of an alkynyl substituent at the α-position of an enal can likely promote its reactivity as a Michael acceptor.¹⁰ The presence of an α-substituent can interrupt π-conjugations and thus minimize its reactivity via the corresponding enal-derived enolate/homoenolate intermediate formed with NHC, as shown by Bode, Glorius and others.^6b,11 In addition, the alkynyl substituent can promote hydrogen-bonding interactions to increase the electrophilicity of the enal to react as a Michael acceptor, as observed in Jørgensen''s amine-catalyzed reactions.¹² In our present study, a non-linear effect was observed regarding enantiomeric excesses of the NHC catalyst and the catalytic reaction product. The reaction enantioselectivity was also found to be sensitive to solvents and bases. These results suggested that the NHC and its azolium salt pre-catalyst (NHC·HX) played dual roles in our reaction: one is to activate the α-unsubstituted enal via the formation of the NHC-bound enolate intermediate,⁶ the other is to activate the α-alkynyl substituted enal via the acidic proton of the chiral NHC·HX (Fig. 1b, intermediate I & transition state TS-I).¹³ With respect to applications, carbon–carbon triple bonds are found in a good number of bioactive molecules such as cleviolide, (+)-prelaureatin, and oxamflatin (Fig. 1c).¹⁴ We demonstrated that our products containing these alkynyl units could be readily transformed into a diverse set of molecules.Cinnamaldehyde 1a and α-alkynyl enal 2a were chosen as the model substrates to search for suitable cross coupling reaction conditions (Table 1). The reactions were first carried out with Et₃N as the base and THF as the solvent. When aminoindanol derived azoium salt A¹⁵ was used as the NHC pre-catalyst, the desired formal [4 + 2] product (3a) was obtained in a very encouraging yield (52%) with excellent ee and dr values (entry 1). The reactions appeared to be very sensitive to the structure of the NHC pre-catalysts, as similar azolium salts with N-phenyl or N–C₆F₅ substituents (B¹⁶ and C¹⁷) were completely ineffective, leading to no product formation (entries 2 & 3). Additional studies on the NHC pre-catalysts finally revealed that introduction of a Br substituent in the indane phenyl ring of the catalyst (D)¹⁸ led to 3a in 85% yield with 99% ee as nearly a single diastereomer (entry 4). Replacing Et₃N with DIEA led to similar results (entry 5). Very interestingly, when the bases were replaced with DABCO or K₃PO₄, a significant drop in the enantioselectivity was observed (entries 6 & 7; see the ESI^† for more details). Changing the solvent from THF to CHCl₃ or EtOAc has moderate effects on reaction yields (entries 8 & 9).Optimization of reaction conditions^a