Optical fibres for material processing lasers

Fibre transmission of high-power radiation has opened the laser a new dimension of material processing applications. The three-dimensional treatment revolutionised a far field of the production technology. One impressed example is the laser welding process in the automotive industry. The Nd–YAG solid state laser in combination with the optical fibre increase the automation and the flexibility of a whole industry. The next generation of Audis aluminium alloy car body is mainly welded by fibre-guided Nd–YAG laser beams. Since the past 10 years the maximum average laser power transmitted through optical fibres increase from some 100 W up to 10 kW. For industrial application only Nd–YAG lasers and recently the diode lasers are economically suited for fibre transmission.     

SiO2/PEG分散体系动态剪切流变行为

在动态应变条件下, SiO2/PEG200(聚乙二醇, 平均分子量为200)分散体系出现了剪切增稠现象. 剪切流变实验表明, 在两种情况下都出现了剪切增稠: 一种是在不同的恒定频率下应变扫描, 在临界应力γc出现的剪切增稠; 另一种是恒定的应变(γ0=500%)条件下频率扫描, 在临界频率棕c抑10 rad·s-1出现的剪切增稠. 在不同的恒定频率应变扫描条件下, 实验研究了储能模量(G’)和耗能模量(G’’)与应变的关系, 同时初步探讨了应变与不同恒定频率的函数关系. 在线性粘弹性区域内, G’和G’’满足G’∝ω0.57和G’’∝ω0.7指数关系. 在恒定的应变条件下, 发现模量和复数粘度与扫描频率具有强烈的依赖关系, 这些现象可以定性地通过“粒子簇”理论来解释.“粒子簇”理论认为这种剪切增稠的发生是由于形成了亚稳定、流动所导致的“粒子簇”, 使得粘度上升.     

The state space of short-range Ising spin glasses: the density of states

The correlations between the segments of a semidilute polymer solution are found to induce correlations in the positions of small particles added to the solution. Small means a diameter much less than the polymer's correlation length. In the presence of polymer the particles behave as if they attracted each other. It is shown how the polymer's correlation length may be determined from a scattering experiment performed on the spheres. Received: 7 July 1997 / Received in final form: 12 November 1997 / Accepted: 19 November 1997     

The adsorption of alkylpyridinium chlorides and their effect on the interfacial behavior of quartz

This research was directed at understanding cationic surfactant adsorption phenomena on wet-ground natural quartz, mainly with dodecylpyridinium chloride as the model surfactant. How these surfactant ions adsorb at the interface was delineated through measurements of adsorption isotherms, zeta potentials, suspension stability, contact angles, induction times, and flotation response. Hydrocarbon chain association of adsorbed surfactant ions (or self-association) leads to four distinct adsorption regions as the concentration of surfactant is increased in solution. The same four regions manifest themselves in the behavior of all of the interfacial processes studied. At low concentrations, adsorption is controlled primarily by electrostatic interactions, but when the adsorbed surfactant ions begin to associate into hemimicelles at the surface, hydrophobic chain interactions control the adsorption process. The results of experiments with alkylpyridinium chlorides of 12, 14 and 16 carbon atoms can be normalized in terms of their CMCs, which clearly show that surface aggregation phenomena are driven by the same hydrophobic interactions that lead to micelle formation in bulk solution.     

Differential Scanning Calorimetry and Hydrodynamic Study of Bacterial Viruses. About possible heat effects in hermetically closed calorimetric vessels with free volume above the liquid

The phase transitions occurring in aqueous solutions of macromolecules and their complex structures (like order-disorder transitions in proteins or nucleic acids solutions) are usually accompanied by small changes in their specific partial volumes. If the quantity of these substances in the closed calorimetric vessels is relatively large (few mg) and if the phase transition is accompanied by a high change of specific partial volume (like in solution of bacteriophages),it is possible to detect some imaginary heat effects in the DSC calorimeters which have closed (sealed) vessels with free volume above the liquid. This revised version was published online in July 2006 with corrections to the Cover Date.     

A new instrument adapted to in situ Raman analysis of objects of art

The analysis of precious artefacts and antiquities demands care in order to minimise the risk of accidental damage during measurement. Mobile fibre-optic-based Raman instruments offer a means to avoid destructive sampling and eliminate the need to transport artefacts for spectrochemical analysis. In this work we present a new mobile instrument developed and optimised for the in situ Raman investigation of objects of art and antiquities. The instrument is controlled by a portable computer. Selected mounts cover many types of artefacts. Newly written software routines organise spectra together with measurement parameters and facilitate calibration of the instrument. The present paper describes this new Raman instrument and discusses some challenges in the transition from a laboratory environment to in situ investigations in museums.     

Dynamical fluctuation effects in glassy colloidal suspensions

Fundamental understanding of heterogeneous dynamics in concentrated glassy hard sphere fluids and colloidal suspensions, even at the single particle level, requires major theoretical advances. Recent simulations and confocal microscopy experiments suggest strong nongaussian dynamical fluctuation effects and activated transport emerge well before an apparent kinetic glass transition is reached. New theoretical approaches that can predict the observable signatures of intermittent large amplitude motions and the associated fluctuation phenomena are discussed. Comparisons are made with experiments, computer simulations, and prior theory for average dynamical properties.     

A simple on-line fibre optics UV detector for microcolumn HPLC

Summary Modification of the UV absorbance detector for LC is described. It is based on a simple combination of a commercial photometer with optical fibres to conduct the light. The performance of the detector is illustrated by the analysis of polyaromatic hydrocarbons on a packed fused-silica column. The minimum detectable concentration for naphthalene was measured as 6·10^–6M.     

The effect of the concentration of dispersed particles on the mechanisms of low-frequency dielectric dispersion (LFDD) in colloidal suspensions

There are two mechanisms which are currently used to explain the low-frequency (kHz range) dispersion of the dielectric permittivity of suspensions in electrolyte solutions (LFDD). The first, the surface diffusion mechanism (SDM), associates the LFDD with the diffusion of bound ions along the particle surface caused by the applied electric field. The second, the volume diffusion mechanism (VDM), follows from the generalization for alternating fields of the classical theory of the relaxation effect in electrophoresis and associates the LFDD with the diffusion of free ions in the diffuse double layer. It has been found that VDM is much more strongly dependent on particle concentration than SDM, opening new possibilities for the investigation of each of these two mechanisms separately. The reason is that when the concentration of particles in suspension increases, the characteristic length for the propagation of volume diffusion processes decreases together with the decrease of the free electrolyte volume, whereas the characteristic length for the surface diffusion remains constant. Correspondingly, when particle concentration is raised, the relaxation time of the VDM effect must decrease, whereas it must remain constant for the SDM mechanism. Thus, by varying the concentration of particles in suspension, one can separate the dispersion curves of SDM and VDM. A simple model is elaborated which can be useful to predict the volume fraction dependence of the parameters of LFDD; in particular, its amplitude and critical frequency. The results are compared with experimental data obtained with polymer latex dispersions of volume fractions ranging from 3 to 16%. It is found that the dielectric behaviour (the volume fraction dependence of both the amplitude and critical frequency of LFDD) of the dispersions is reasonably well explained with our model, thus demonstrating that VDM prevails in the systems studied. Experimental data previously found by other authors are also discussed in the light of the model presented.     

Electrode Reactions of Catechol at Tyrosinase‐Immobilized Latex Suspensions

Tyrosinase was immobilized on polystyrene latex particles in order to control amounts of the enzyme. The tyrosinase‐coated latex particles were composed of the core polystyrene and four successive coating layers: polystyrene sulfonate, polyallylamine, tyrosinase and polyallylamine again, built up by the layer‐by‐layer technique. They showed catalytic currents for the enzymatic oxidation of catechol to o‐quinone. The enzyme activity per particle was evaluated as 2.3×10^?7 units from UV absorption of o‐quinone. The relation between the catalytic current and the concentration of catechol leads to a Michaelis‐Menten type kinetic equation. The layer‐by‐layer method was found to have a deactivating effect on enzyme catalysis. In spite of this, the catechol oxidation current was larger than the current from free tyrosinase at a common value of enzyme units per volume. This is ascribed to strong adsorption of the latex particles on the electrode, leading to the enhancement of the local concentration of tyrosinase.