Stochastic phase lockings in a relaxation oscillator forced by a periodic input with additive noise: A first-passage-time approach

Noise effects on phase lockings in a system consisting of a piecewise-linear van der Pol relaxation oscillator driven by a periodic input are studied. The problem of finding the period of the oscillator is reduced to the first-passage-time problem of the Ornstein-Uhlenbeck process with time-varying boundary. The probability density functions of the first-passage time are used to define the operator which governs a transition of an input phase density after one cycle of the oscillator. Phase lockings in a stochastic sense are investigated on the basis of the density evolution by the operator.     

Limit cycle analysis of a class of strongly nonlinear oscillation equations

The limit cycle of a class of strongly nonlinear oscillation equations of the form % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiGc9yrFr0xXdbba91rFfpec8Eeeu0x% Xdbba9frFj0-OqFfea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs% 0dXdbPYxe9vr0-vr0-vqpWqaaeaabiGaciaacaqabeaadaqaaqGaaO% qaaiqadwhagaWaaiabgUcaRmXvP5wqonvsaeHbbjxAHXgiofMCY92D% aGqbciab-DgaNjab-HcaOiaadwhacqWFPaqkcqWF9aqpcqaH1oqzca% WGMbGaaiikaiaadwhacaGGSaGabmyDayaacaGaaiykaaaa!50B8!\[\ddot u + g(u) = \varepsilon f(u,\dot u)\] is investigated by means of a modified version of the KBM method, where is a positive small parameter. The advantage of our method is its straightforwardness and effectiveness, which is suitable for the above equation, where g(u) need not be restricted to an odd function of u, provided that the reduced equation, corresponding to =0, has a periodic solution. A specific example is presented to demonstrate the validity and accuracy of our 09 method by comparing our results with numerical ones, which are in good agreement with each other even for relatively large .     

Glass capillary gas chromatography of amino acid enantiomers

Glass capillaries coated with Chirasil-Val, a chirally functionalised polysiloxane, are capable in principle of resolving all protein amino-acid enantiomers in a single run and within a short analysis time, thus allowing for example the quantitative amino acid determination by enantiomer labelling. The elution characteristics of the individual amino acids however are also dependent upon the chemical nature of the capillary wall surface, and a surface pretreatment is found to be necessary if all protein amino acids are to be analysed. Of the various methods of pretreatment tested, etching of borosilicate glass with gaseous HCl followed by deposition of colloidal silicic acid is considered to be the most suitable.     

Influence of mechanical properties of pectin films on charge density and charge density distribution in pectin macromolecule

The hydration and mechanical properties of citrus pectin films were examined in conditions relevant to those in the plant cell wall. The pectins used for this study varied in the degree of esterification (DE) (high or low) and charge distribution on the backbone (random or block). The hydration of the films was controlled in an osmotic pressure experiment using polyethylene glycol solutions (PEG 20000). Hysteresis tests at constant deformation rate (stress vs deformation) were used for investigating the mechanical behaviour of films. Mechanical and hydration properties of pectin films were examined as a function of charge density, charge density distribution and counterion environment—K⁺, Ca²⁺, Mg²⁺. Swelling decreased with increasing counterion concentration. The effect is stronger in the case of Ca²⁺ and Mg²⁺ for low esterified pectins and therefore crosslinks from divalent ions could be assumed. The crosslink effect is confirmed in mechanical experiments where an increase in the film tensile modulus is observed with increasing counterion concentration. It is shown for the first time that in case of highly concentrated pectin solutions Mg²⁺ cations also act as a crosslinker for pectin macromolecules.     

盐水振荡研究的新发现

在对化学振荡的研究中,无论是溶液均相氧化还原形成的振荡反应,还是界面扩散受阻形成的界面振荡,都普遍采取各种电极法来监测。前文[1,2]研究了各种不同电极监测液膜振荡的变化,本文将讨论不同的电极方法在监测盐水振荡现象过程中的新发现。1970年,地球物理学家Martin[3]发现把     

电沉积AuInSe2半导体薄膜上的电化学振荡现象

研究了电沉积制得的ＡｕＩｎＳｅ２半导体薄膜上过氧化氢阴极还原过程中产生的电化学振荡行为，对影响该振荡行为的一些因素如半导体薄膜的后处理、溶液组成、传质、光照、化学浸渍作用等进行了分析，同时采用外界周期性光照和外接小幅度正弦波电位来调节振荡频率，为金铟硒半导体薄膜发展成为光电传感器件提供了一定的理论与实践基础．     

葡萄糖、半乳糖和乙醇恒电流氧化过程电位振荡的EQCM研究

采用电化学石英晶体微天平(EQCM)研究了0.5 mol•L^-1 NaOH水溶液中铂电极上葡萄糖、半乳糖和乙醇恒电流氧化过程中伴随的电位振荡行为. 两个糖体系的电位振荡过程伴随EQCM频率的同步振荡响应, 而乙醇体系中相应的频率响应却非常小；三个体系振荡过程的同步动态电阻响应均很小, 表明振荡过程频率响应主要为质量效应. 虽然葡萄糖和半乳糖结构相似, 电位和频率振荡的幅度相当, 但频率波数和周期明显不同, 表明电位振荡行为对两者呈现良好的分子识别能力. 本文也讨论了相关振荡机理和NaOH浓度效应及碱性介质中铂电极电化学过程, 提出了所形成的铂氧化物主要是PtO₂-3H₂O_ad以及两糖体系振荡过程中糖酸根阴离子伴随着高/低电位在铂电极上吸/脱附的新观点.     

MoS2纳米管包覆单壁碳纳米管束的制备

MoS₂ nanotube coated SWNT (Single wall carbon nanotube) bundles have been successfully prepared by adsorbing (NH₄)₂MoS₄ onto SWNT bundles and subsequent heat treatment under H₂ at 900 ℃ in a tube furnace. The morphologies, structure and composition of the as-prepared sample were investigated by XRD, SEM, HRTEM coupled with EDS. The formation mechanism has also been preliminarily discussed.     

Sensitive Voltammetric Response of p‐Nitroaniline on Single‐Wall Carbon Nanotube‐Ionic Liquid Gel Modified Glassy Carbon Electrodes

An ionic liquid (i.e., 1‐butyl‐3‐methylimidazolium hexafluorophosphate, BMIMPF₆)‐single‐walled carbon nanotube (SWNT) gel modified glassy carbon electrode (BMIMPF₆‐SWNT/GCE) is fabricated. At it the voltammetric behavior and determination of p‐nitroaniline (PNA) is explored. PNA can exhibit a sensitive cathodic peak at ?0.70 V (vs. SCE) in pH 7.0 phosphate buffer solution on the electrode, resulting from the irreversible reduction of PNA. Under the optimized conditions, the peak current is linear to PNA concentration over the range of 1.0×10^?8–7.0×10^?6 M, and the detection limit is 8.0×10^?9 M. The electrode can be regenerated by successive potential scan in a blank solution for about 5 times and exhibits good reproducibility. Meanwhile, the feasibility to determine other nitroaromatic compounds (NACs) with the modified electrode is also tested. It is found that the NACs studied (i.e., p‐nitroaniline, p‐nitrophenol, o‐nitrophenol, m‐nitrophenol, p‐nitrobenzoic acid, and nitrobenzene) can all cause sensitive cathodic peaks under the conditions, but their peak potentials and peak currents are different to some extent. Their peak currents and concentrations show linear relationships in concentration ranges with about 3 orders of magnitude. The detection limits are 8.0×10^?9 M for p‐nitroaniline, 2.0×10^?9 M for p‐nitrophenol, 5.0×10^?9 M for o‐nitrophenol, 5.0×10^?9 M for m‐nitrophenol, 2.0×10^?8 M for p‐nitrobenzoic acid and 8.0×10^?9 M for nitrobenzene respectively. The BMIMPF₆‐SWNT/GCE is applied to the determination of NACs in lake water.     

Reverse permeation of sodium ions driven by pH difference across a liquid membrane: time dependencies of membrane resistance and membrane potential in a reverse permeation system

The diffusional flux of sodium ions across a liquid membrane was observed as a reverse permeation phenomenon: sodium ions were transported across the membrane against their own concentration difference. A supported liquid membrane having stearic acid as an ionic carrier was used. The internal aqueous phase contained NaCl and HCl and the external aqueous phase contained NaOH of the same initial concentration as NaCl in the internal aqueous phase. The reverse permeation occurred with a long time delay. During the delay, sodium ions flowed from the acidic to alkaline solution. The diffusion coefficient of sodium ion estimated from the flux equation taking into account the Donnan equilibrium at the interface was found to be much greater than that in the membrane solvent, 1-octanol. In the same membrane system as for the flux measurement, the membrane conductance and the membrane potential were measured as a function of time. The time dependence of the membrane potential in the presteady state showed a biphasic behavior. The initial rapid phase could be attributed to the change in the phase boundary potential and the subsequent slow step to the change in the diffusion potential within the membrane. Before the steady membrane potential had been reached, the reverse permeation of sodium ions against their own concentration difference was not observed. During the slow relaxation process of the membrane potential, the membrane resistance decreased to approach the steady state. Moreover, the oscillation of membrane potential abruptly started at a time in the slow step of the potential change and continued during the steady state. It was suggested that, at the presteady state, the increase in the amount of water in the membrane would drive a drastic change in the state of the liquid membrane in the filter pore, e.g. an inverted micellar structure making.