Carbon clusters

Some of the most significant discoveries and achievements concerning the mass spectra and gas phase ion chemistry of carbon clusters are reviewed. These include (1) nanosecond and femtosecond laser ionizations; (2) ion structures through ion/molecule reactions, ion chromatography, and computational methods; (3) carbon cluster cooling through radiative decay, dissociative decay, and thermionic emission; (4) mechanisms and energetics of fragmentation reactions; (5) endohedral fullerenes including recent data on ion beam implantation, and (6) ion chemistry as a function of the fullerene charge state.     

Non-linear vibrations of cantilever beams with feedback delays

A comprehensive investigation of the effect of feedback delays on the non-linear vibrations of a piezoelectrically actuated cantilever beam is presented. In the first part of this work, we examine the linear and non-linear free responses of a beam subjected to a delayed-acceleration feedback. We show that the trivial solution loses stability via a Hopf bifurcation leading to limit-cycle oscillations. We analyze the stability of the dynamic response in the postbifurcation, close to the stability boundaries by examining the nature of the Hopf bifurcation and away from the stability boundaries by using the method of harmonic balance and Floquet theory. We find that, increasing the gain for certain feedback delays may culminate in quasiperiodic and chaotic oscillations of the beam.In the second part, we analyze the effect of feedback delays on a beam subjected to a harmonic base excitations. We find that the nature of the forced response is largely defined by the stability of the trivial solutions of the unforced response. For stable trivial solutions (i.e., inside the stability boundaries of the trivial solutions), the homogeneous response emanating from the feedback diminishes, leaving only the particular solution resulting from the external excitation. In this case, delayed feedback acts as a vibration absorber. On the other hand, for unstable trivial solutions, the response contains two co-existing frequencies. Depending on the excitation amplitude and the commensurability of the delayed-response frequency to the excitation frequency, the response is either periodic or quasiperiodic.     

Collision between high and low viscosity droplets: Direct Numerical Simulations and experiments

Binary droplet collisions are of importance in a variety of practical applications comprising dispersed two-phase flows. In the present work we focus on the collision of miscible droplets, where one droplet is composed of a high viscous liquid and the other one is of lower viscosity. This kind of collisions take place in, for instance, spray drying processes when droplets with different solid content collide in recirculation zones. The aim of this paper is to investigate the details of the flow inside the colliding droplets. For this purpose, two prototype cases are considered, namely the collision of equal sized droplets and the collision between a small and highly viscous droplet and a bigger low viscous droplet. A new experimental method has been developed in order to visualize the penetration and mixing process of two colliding droplets, where a fluorescence marker is added to one liquid and the droplets are excited by a laser. The results show a delay in the coalescence which takes place during the initial stage of a collision of droplets with different viscosities. Direct Numerical Simulations based on the Volume-of-Fluid method are used to study these collisions and to allow for a more detailed inspection of the mixing process. The method is extended to consider a second liquid with a different viscosity. In order to reproduce the delay of coalescence, an algorithm for the temporal suppression of the coalescence is applied. A predictive simulation of the delay is not possible, because the extremely thin air gap separating the droplets cannot be resolved by the numerics. This approach is validated by comparison with experimental data. The results provide local field data of the flow inside the collision complex, showing in particular a pressure jump at the liquid–liquid interface although no surface tension is present. The detailed analysis of the terms in the momentum balance show that the pressure jump results from the viscosity jump at the liquid–liquid interface.     

聚己内酯/聚酯丙烯酸酯共混物的辐射交联及其形状记忆行为

研究了聚酯丙烯酸酯 (PEA)类多官能团物质与聚己内酯 (PCL)共混物的辐射交联效应 .分别考察了多官能团PEA的用量、官能团数目、辐射交联剂量等因素对聚己内酯的辐射交联规律、动态力学性能、形状记忆行为等性能的影响 .结果表明 ,PCL PEA共混物的辐射交联规律不再遵从Charlesby Pinner关系式 ,而是符合陈欣方 刘克静 唐敖庆关系式 .多官能团PEA的加入可以显著提高PCL辐射交联的效率 ;相同剂量时 ,官能团数目越多、用量越大的样品 ,生成的凝胶含量越多 ,强化辐射交联效应越大 .DMA分析表明 ,PCL强化辐照交联后的弹性模量和耐热性能显著提高 .强化交联PCL在其熔点以上都呈现出高弹态平台 ,可以实现形状记忆 .且交联度较高 ,形状记忆恢复速率较快 .     

Column packings with immobilized methyl polysiloxane phases for gas chromatography

Summary The free radical cross-linking procedure of stationary phase immobilization introduced by Grob et al. for capillary columns has been adapted for the preparation of column packings on diatomaceous supports. Immobilization of SE-30 and OV-1 on typical gas chromatography supports: Polsorb B-AW, Chromosorb W-AW, Chromosorb G-AW and Gas Chrom Q was investigated. Before immobilization, the supports (excluding Gas Chrom Q) were purified by extraction with 6 M HCl in a Soxhlet extractor and silanized with hexamethyldisilazane vapour at 300°C.The influence of the type and concentration of peroxide on the efficiency of immobilization of the silicone was studied. In order to obtain over 95% immobilization a minimum of 2% w/w of the peroxide in the stationary liquid is required.Properties of packings with immobilized phase were compared with those of related conventional packings. Adsorption activity, column efficiency and thermal stability were used for this comparison.     

Cross-Linked Thermoplastic Blends of Polyethylene with an Elastomer. 1. Deformation and Strength Characteristics

Results of an experimental investigation into changes of some mechanical characteristics of binary blends of high- and low-density polyethylenes with an elastomer (ethylene-propylene-dicyclopentadiene terpolymer) in the course of -radiation and chemical cross-linking are reported for a wide range of component ratios. Data on changes in the tensile strength and total, residual, and reversible strains at break, as well the microhardness and other characteristics are obtained. The effect of component ratio and cross-linking on these characteristics is discussed.__________Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 2, pp. -, March–April, 2005.     

Time scales in linear delayed differential equations

The aim of this paper is to apply and justify the so-called aggregation of variables method for reduction of a complex system of linear delayed differential equations with two time scales: slow and fast. The difference between these time scales makes a parameter ε>0 to appear in the formulation, being a mathematical problem of singular perturbations. The main result of this work consists of demonstrating that, under some hypotheses, the solution to the perturbed problem converges when ε→0 to the solution of an aggregated system whose construction is proposed.     

NMR characterization of styrene-divinylbenzene gel beads using freezing point depression of benzene

Freezing processes of benzene in the presence of styrene-divinylbenzene gel beads were investigated by proton nuclear magnetic resonance spectroscopy. Some portion of benzene in the pore did not freeze below its freezing point, which was detected until ?80°C. Temperature dependence of the amount of unfrozen benzene was discussed in relation to the pore size and the cross-linking density of gel beads. The small pore size and high crosslinking density increased the amount of unfrozen benzene, whereas the polymer matrix concentration showed little effect on the freezing processes. © 1995 John Wiley & Sons, Inc.     

Solid polymeric electrolyte of PVC–ENR–LiClO<Subscript>4</Subscript>

The ionic conductivity of PVC–ENR–LiClO₄ (PVC, polyvinyl chloride; ENR, epoxidized natural rubber) as a function of LiClO₄ concentration, ENR concentration, temperature, and radiation dose of electron beam cross-linking has been studied. The electrolyte samples were prepared by solution casting technique. Their ionic conductivities were measured using the impedance spectroscopy technique. It was observed that the relationship between the concentration of salt, as well as temperature, and conductivity were linear. The electrolyte conductivity increases with ENR concentration. This relationship was discussed using the number of charge carrier theory. The conductivity–temperature behaviour of the electrolyte is Arrhenian. The conductivity also varies with the radiation dose of the electron beam cross-linking. The highest room temperature conductivity of the electrolyte of 8.5 × 10⁻⁷ S/cm was obtained at 30% by weight of LiClO₄. The activation energy, E _a and pre-exponential factor, σ _o, are 1.4 × 10⁻² eV and 1.5 × 10⁻¹¹ S/cm, respectively.