Percolation behaviour of ultrahigh molecular weight polyethylene/multi-walled carbon nanotubes composites

The electrical conductivity of polymer/multi-walled carbon nanotubes (MWCNTs) composites in a powder and in a hot-pressed compacted state, prepared by mechanical mixing, was studied. The semicrystalline ultrahigh molecular weight polyethylene (UHMWPE) was used as a polymer matrix. The data clearly evidence the presence of a percolation threshold φ_c at a very small volume fraction of the MWCNTs φ in a polymer matrix, φ_c ≈ 0.0004-0.0007. The ultralow percolation threshold in UHMWPE/MWCNTs thermoplastic composites was explained by high aspect ratio of the nanotubes and their segregated distribution inside the polymer matrix. The method of composite preparation effects the values of percolation threshold concentration φ_c and critical exponent t. A noticeable positive temperature coefficient of resistivity (PTC effect) was observed in the region of temperatures higher than melting point. It was explained by influence of thermal expansion of the polymer matrix and independence from the melting process that is a result of specific structure of conductive phase.     

A monopole mass filter with a hyperbolic V-shaped electrode

In this article we compare the classical monopole mass filter of von Zahn and the monopole mass filter with a hyperbolic V-shaped electrode. The experimental results and those of computer simulation for both mass spectrometers are presented. We show that the replacement of a conventional 90 degrees V-shaped electrode by an electrode with a hyperbolic profile substantially improves the peak shape of any given mass, and increases the mass resolution by a factor of 3-4 and the abundance sensitivity by a factor of 100. The potential of high analytical performance combined with electroforming techniques for electrode manufacture indicate future practical uses of such instruments. Copyright 1999 John Wiley & Sons, Ltd.     

Stability of the aqueous suspensions of nanotubes in the presence of nonionic surfactant

Stability of aqueous suspensions of multiwalled carbon nanotubes (MWNTs) and their percolation behavior are investigated. Nanotubes of aqueous suspensions show a strong tendency to aggregation and networking into electroconductive clusters. The percolation threshold of the electrical conductivity is rather low and of order phi approximately 0.01 (where phi is the volume fraction), which can be explained by the high aspect ratio of MWNTs. Strong influence of the nonionic surfactant Triton X-305 on the colloidal stability of aqueous suspensions of MWNTs is observed. Addition of surfactant exerts a stabilizing effect at surfactant concentration C(s) proportional to the weight concentration C of MWNTs, C(s) approximately C mol/dm3. The transient behavior of electrical conductivity in the aqueous suspensions is explained by fractal aggregation processes. The fractal dimension is shown to be sensitive to the surfactant concentration C(s).     

Justification of the Kirchhoff hypotheses and error estimation for two-dimensional models of anisotropic and inhomogeneous plates, including laminated plates

Asymptotic analysis of the problem describing deformation ofa thin cylindrical plate with clamped lateral side is performed.The problem is considered under the most general statement withthe plate being laminated and consisting of an arbitrary numberof nonhomogeneous and anisotropic (21 elastic moduli) layers.Explicit integral representations of the differential operatorswhich form the two-dimensional model of the plate are derived.In the case when the elastic moduli of each of the layers areconstant, these integral representations turn into algebraicones. The asymptotic procedure is justified with the help ofa weighted inequality of Korn's type. The error estimates obtainedgive a rigorous mathematical proof of both of Kirchhoff's hypotheses(kinematic and static) and shed light on the well-known intrinsicinconsistency of two of the hypotheses.