Étude électrique de couches minces de polymère obtenues dans une décharge luminescente II: Caractérisation électrique sous champ alternatif

The aim of this study was to investigate the electrical characteristics under alternating stress of a metal-polymer-metal structure, the dielectric of which was a thin layer created by a glow discharge in styrene.

The study, carried out in the 250 Hz–100 kHz range between −90° and 150°C, indicates two different domains for the variations of ε_r and ε_r'. Using bi-logarithmic coordinates the variation of the a.c. conductivity σ_ac(Ω) obeys the law σ_ac(Ω)∝Ω for temperatures lower than 50°C.

In order to explain this result, two hypotheses are proposed: that dipolar fluctuations occur and that a hopping process takes place.     

Dimensionally scaled penetration experiments to extract projectile sizes from space exposed surfaces

Impact experiments were conducted which employed soda-lime glass projectiles (50, 150, 1000 and 3175 μm in diameter; D_p) and aluminum (1100 series) and Teflon (FEP) targets of variable thickness (T; ranging from thick infinite halfspace targets [D_p/T < 0.1] to foil thicknesseof a few microns [D_p/T > 100]). The objectives of these impact experiments were determine, at constant impact velocity, the relationships between the diameter of the resulting penetration hole (D_h), the foil thickness (T) and the projectile size (D_p). We found that D_h, and other morphologic features such as rim structures in aluminum or spall phenomena in Teflon exhibit a systematic relationship to the target thickness. This relationship is described by polynomial fits which permit unique solutions for unknown projectile sizes (D_p) from the measurement of T and D_h on space-exposed surfaces.     

Reaction between the tetragonal tungsten bronze niobate K2Sr4Nb10O30 and the perovskite Pb(Mg1/3Nb2/3)O3

The search for dielectric materials with a high dielectric constant and ′_r = ƒ(T) curves with a flat profile fitting the X7R specification is still ongoing. Promising results were obtained by mixing compounds with closely related structures, such as the tetragonal tungsten bronze (TTB) niobate K₂Sr₄Nb₁₀O₃₀ and the perovskite Pb(Mg_1/3Nb_2/3)O₃ (PMN). The present study, based on three methods of synthesis, explores the origin of the spreading out of the dielectric curves ′_r = ƒ(T). For the composition 10x K_0.2Sr_0.4NbO₃ (KSN) + (1 − x)Pb(Mg_1/3Nb_2/3)O₃ (PMN) with x = 0.3–0.6, the three synthesis methods provided similar characteristics and for the highest perovskite ratio (x = 0.3), the ′_r = ƒ(T) curve exhibits a flat profile. When lithium is used as a sintering agent, ′_r = ƒ(T) curves present a linear dependency with the temperature. These materials are also characterized by a structural and a microstructural inhomogeneity. Two phases TTB and perovskite type, different from KSN and PMN, are present after calcination and sintering, but not evenly distributed. The PbO loss during sintering also contributes to the evolution of the properties of the material.     

Stress relaxation in the interface between creep particle and elastic matrix-strengthening mechanism

The effect of a power law creep particle on interface behavior between the particle and elastic matrix is investigated by stress analysis. Using the results obtained through the stress analysis, the forces due to interaction of an applied stress and stress concentration with an edge dislocation are determined. The direct interaction between the edge dislocation and the creeping particle is studied under fully relaxed stress conditions. Through the investigation the following results are derived. Stress relaxation in the interface can be caused by power law creep along or by diffusion, or a combination of both mechanisms. The degree of stress relaxation caused by diffusion can be defined in terms of the relaxation time for both boundary diffusion and volume diffusion. The amount of stress relaxation caused by the power law creep particle is characterized by the quantity ₂ which is a function of Γ₀ = 2(1/√3)^{1 + m} × (σ_∞/2μ)^m(σ₀/σ_∞t^m), where m is strain rate hardening exponent, σ_∞ is applied stress, μ is the shear modulus, σ₀ is the material constant of the power law creep particle, and t is elapsed time. The value ₂ = 1.0 corresponds to the fully relaxed condition and ₂ = −0.6 corresponds to the initial state. The time to reach a fully relaxed condition is very sensitive to the strain rate exponent, with the smaller m values leading to longer times. The stress state of complete relaxation in the elastic matrix is equivalent to the solution of a void in an elastic matrix superposed on the solution of positive surface traction on the void. This result is identical to that obtained by Srolovitz et al. [Acta. Metall.32, 1979 (1984)]. When the stress is completely relaxed in the particle, all stress components (σ_r, σ_θand σ_rθ) are relaxed, while in the matrix relaxations are observed only for σ_rand σ_θ. The critical resolved shear stress and critical stress to climb the dislocation in the neighborhood of the particle exceed the Orowan stress. Also, the particle attracts the dislocation. Therefore the strengthening of a power law creep particle in an elastic matrix is caused by the Orowan mechanism and by attraction of the dislocation.     

Addition of “Living” Polymers onto C60.

Various “living” polymers were grafted onto C₆₀ The number of arms of the so obtained “star” molecules can be controlled by stoechiometry and/or by varying the reactivity of the carbanion on the “living” chain against a double bond on the C₆₀. Even the oxanion of “living” polyethylenoxide is able to add onto the reactive double bonds on C₆₀. In some conditions, the carbanions present on these alkaline salts of grafted fullerenes becomes able to initiate anionic polymerization of vinyl monomers. Using “living” poly(phenylvinylsulfoxide) as a precursor polymer for PA, polyacetylene chains could be attached to the fullerene.     

AlxGa−xAs/GaAs heterostructure characterization by wet chemical etching

Selective wet chemical etching of the Al_xGa_1−xAs/GaAs system has been applied to heterostructure characterization. Samples of LPE grown AlGaAs/GaAs laser double-heterostructures and separate confinement heterostructures as well as antiresonant reflecting optical waveguides heterostructures were treated with “I2 solution” (I₂:KI:H₂O) and hydrochloric acid. These compounds selectively etch the ternary Al_xGa_1−xAs layers, but with different “threshold composition” x_th values (the x value is that above which the etching rate of a given compound increases sharply). Selectively etched samples have been examined by SEM. The experimental dependence of etching rate on the x value for I2 solution has been derived. From this dependence, the x composition of any ternary layer can be estimated simply. Observations were made of the “microscopic” properties of the heterostructure, such as the smoothness of the interfaces and the uniformity of layers. All imperfections resulting from the growth process, such as interface perturbations or compositional nonuniformity of layers, are clearly seen. An additional advantage of this etching technique is its simplicity. It allows quick examination of grown heterostructure for the selection of wafers for further processing.     

The dynamic elastic buckling of a circular arch with finite displacements and initial imperfections

The equilibrium equations for elastic circular arches are established using the principle of virtual work. The nonlinear partial differential equations of motion are solved using a finite difference method (Park's method for time difference). The dynamic stability of a hinged and a clamped elastic circular arch with a uniform step load is analysed with finite deformations and initial geometric imperfections. Results show that the buckling mode varies with the value of the arch half angle, θ₀. The boundary condition and initial imperfection amplitude also effect the buckling mode. A nearly perfect arch usually buckling with a “direct” buckling form, while an imperfect arch with an “indirect” buckling form. The effect of θ₀ on the ratio p_d/p_s (p_d is the dynamic critical load and p_s the static critical load) is shown for different initial imperfections and different boundary conditions.     

C.V.D. Coating of Alumina Reinforcing Particles to Improve the Wear Resistance of H.S.S. Based Composites

C.V.D. coating of the reinforcing ceramic particles used in particulate metal matrix composites allows the control of reactivity at the particle/matrix interface. Wear resistant high speed steel-based composites containing uncoated A1₂0₃, uncoated TiC and C.V.D. coated A1₂0₃ were liquid phase sintered, then characterized using “pin-on-disc” wear testing. TiC or TiN C.V.D. coatings of A1₂0₃ were tested to determine die increase in reactivity of the particles with the liquid phases formed during sintering. This resulted in a porosity decrease at the particle/matrix interface in addition to a better ceramic/metal cohesion due to improved wettability. Reactivity and wettability were studied using differential thermal analysis, electron microprobe analysis, transmission electron microscopy, and image analysis. Results from pin-on-disc wear testing illustrated the role of the C.V.D. coating on the wear behavior of the studied materials. Lower wear rates were obtained with the composites containing TiC or TiN-coated Al₂0₃. These results showed that there is a relation between wettability of ceramic particles by the metallic phases and wear resistance of the composites.     

Varistors based on n-BaTiO3 ceramics

Stable and highly reproducible voltage-limiting characteristics have been observed at room temperature for polycrystalline ceramics prepared from donor-doped BaTiO₃ solid solutions containing isovalent lattice substitute ions that lower the Curie point T_c. When the ambient temperature T_a is decreased such that T_a < T_c, the same ceramics show current-limiting behaviour. The leakage current, the breakdown voltage and the non-linear coefficient ( = 30−50) could be varied with grain-boundary layer (GBL) modifiers and postsintering annealing. The magnitude of the abnormally high dielectric constant (ε_r 10⁵) indicates the prevalence of GBL capacitance in these ceramics. Analyses of the current-voltage relations show that GBL conduction at T_a < T_c corresponds to tunnelling across asymmetric barriers formed under steady state Joule heating. At T_a > T_c, trap-related conduction gives way to tunnelling across symmetric barriers as the field strength increases.     

Biomimetic ion-channel sensors based on host-guest molecular recognition in Langmuir-Blodgett membrane assemblies

Biomimetic “ion-channel sensors” based on host-guest molecular recognition were constructed by incorporating several kinds of receptor molecules in Langmuir-Blodgett (LB) molecular assemblies deposited directly on glassy carbon electrodes. The receptors used were valinomycin, bis(crown ether)s, macrocyclic polyamine and cyclodextrin polyamine. The binding of charged stimulants (metal ions, inorganic and organic anions) to these receptors induced a marked increase (or decrease) in the ion permeability of the membranes, as detected by cyclic voltammetry using [Fe(CN)₆]^4- or [Ru(bpy)₃]²⁺ as marker ion. Such a mode of response, corresponding to model “channel opening (or closing)”, can be explained on the basis of the charge-charge interaction involving the stimulant, marker ion and lipid.     

Structure and ferroelectric properties of Bi(Zn1/2Ti1/2)O3-(Bi1/2K1/2)TiO3 perovskite solid solutions

Lead-free piezoelectric ceramics based on χBi(Zn_1/2Ti_1/2)O₃-(1-χ)(Bi_1/2K_1/2)TiO₃ were obtained via solid state processing techniques. A single perovskite phase with tetragonal symmetry was obtained for Bi(Zn_1/2Ti_1/2)O₃ (BZT) substitutions up to 20 mol%. The maximum density was 97.1% at the composition of χ = 0.1. The dielectric measurement indicated that the transition temperature decreased linearly with increasing BZT content. The P-E loops revealed an increase in remanent polarization (P_r) with the addition of BZT. The maximum planar coupling coefficient, κ_r, for the χ = 0.1 composition was 21.6 and the piezoelectric coefficient, d₃₃, for χ = 0, χ = 0.05, and χ = 0.1 was 108, 185, and 235 pm/V, respectively. Overall, the dielectric and piezoelectric properties showed significant improvement when BZT was added.     

The influence of the stress state on the plastic zone size

“Linear elastic” fracture mechanics is based on the “plane strain” condition which means dε_zz = 0. Really stress states can occur in structures or components where dε_zz > 0. Two examples are discussed. Using the v. Mises criterion and the Sneddon equations the influence of an external stress σ^e_zz acting parallel to the crack front on the plastic zone size is calculated and demonstrated by examples.     

Reduction of field emission in superconducting cavities with high power pulsed RF

A systematic study is presented of the effects of pulsed high power RF processing (HPP) as a method of reducing field emission (FE) in superconducting radio frequency (SRF) cavities to reach higher accelerating gradients for future particle accelerators. The processing apparatus was built to provide up to 150 kW peak RF power to 3 GHz cavities, for pulse lengths from 200 μs to 1 ms. Single-cell and nine-cell cavities were tested extensively. The thermal conductivity of the niobium for these cavities was made as high as possible to ensure stability against thermal breakdown of superconductivity.

HPP proves to be a highly successful method of reducing FE loading in nine-cell SRF cavities. Attainable continuous wave (CW) fields increase by as much as 80% from their pre-HPP limits. The CW accelerating field achieved with nine-cell cavities improved from 8–15 MV/m with HPP to 14–20 MV/m.

The benefits are stable with subsequent exposure to dust-free air. More importantly, HPP also proves effective against new field emission subsequently introduced by cold and warm vacuum “accidents” which admitted “dirty” air into the cavities.

Clear correlations are obtained linking FE reduction with the maximum surface electric field attained during processing. In single cells the maximums reached were E_peak = 72 MV/m and H_peak = 1660 Oe. Thermal breakdown, initiated by accompanying high surface magnetic fields is the dominant limitation on the attainable fields for pulsed processing, as well as for final CW and long pulse operation.

To prove that the surface magnetic field rather than the surface electric fields is the limitation to HPP effectiveness, a special two-cell cavity with a reduced magnetic to electric field ratio is successfully tested. During HPP, pulsed fields reach E_peak = 113 MV/m (H_peak = 1600 Oe) and subsequent CW low power measurement reached E_peak = 100 MV/m, the highest CW field ever measured in a superconducting accelerator cavity.     

Formation of MgAl2O4 in Al2O3-(Al-4wt.%Mg) composites

The Al₂O₃ particles are introduced into the Al-4wt.%Mg melt by the “vortex” method. After being cast, Al₂O₃-(Al-4wt.%Mg) composites are remelted at 700, 750, 800 and 850°C for different residence times to investigate the formation of MgAl₂O₄ (spinel).

The results show that MgAl₂O₄ is the unique interface of the Al₂O₃-(Al---Mg) composites held at 700–850°C. Fine MgAl₂O₄ crystals grow on the surface of the Al₂O₃ particle but, as the holding temperature and the residence time increase, some spinels will form themselves into pyramidal shape. The MgAl₂O₄ grows not only at the matrix-particle interface but also on the surface of the composite specimens. The formation reactions of interfacial MgAl₂O₄ are as follows: Mg(1) + 2Al(1) + 2O₂(g) = MgAl₂O₄(s)3Mg(1) + 4Al₂O₃(s) = 3MgAl₂O₄(s) + 2Al(1) Both of them are equally important.     

Information geometry in functional spaces of classical and quantum finite statistical systems

Statistical states and bounded random variables (observables) of finite physical systems can be represented in real Banach spaces L_s¹ and L_s^∞, respectively. Since both norms are Krein-weak, the solution of the estimation problem in these spaces is not necessarily unique. The latter property occurs on the Hilbert-Schmidt space L_s² which is connected with the Onicescu information energy and the method of least squares. The square information is only an approximation of the “true” logarithmic Shannon information which induces a “logarithmic” asymmetric geometry by means of the concept of relative information (gain of information). This geometry was known in the classical case as the asymmetric Pythagorean geometry (Chentsov [10]) and is approximated by the Riemannian geometry of Fisher's information (Kullback and Leibler [45]). This paper shows that a similar geometric construction is also possible in the quantum case. The fundamental formulae of the quantum case are given (they differ in some details from the classical ones), and possible physical applications are shortly sketched.     

Fractal properties of percolating film structures

We simulate nucleation and growth processes of thin films on the basis of the so-called rate equation approach allowing “atoms” to diffuse and rearrange whereby enhancing their co-ordination number. The resulting percolating structures are different from those obtained by the “pure” percolation model where “atomic diffusion” is not taken into account. However, the fractal properties for p = p_c are the same as for the percolation model with the fractal dimension of d_f = 1.896 and for random walks of d_w = 2.87. Moreover, d_f and d_w are independent on the diffusion time we choose for our simulations.     

Composite of Carboxyl-Modified Multi-walled Carbon Nanotubes and TiO2 Nanoparticles: Preparation and Photocatalytic Activity

The composite of carboxyl-modified multi-walled carbon nanotubes (MWNT-COOHs) and TiO₂ nanoparticles was prepared by improved solvothermal process. X-ray diffraction (XRD), transmission electron microscope (TEM) and UV-Vis spectra were used to characterize the products. The results show that MWNT-COOHs were “welded” by the TiO₂ nanoparticles attached to the open ends of MWNT-COOHs. Compared with pure TiO₂ nanoparticles, the composite displays higher photocatalytic activity with 99.9% of degradation ratio of copper sulfophthalocyanine after 3 h irradiation.     

Evolution of Superconducting and Normal-State Properties in C60 “Polymer” Doped with K

Abstract:

Bulk superconductivity was observed in a new iodine-treated aggregated form of solid C₆₀, believed to be a “polymer”, by doping with alkali metal. Evolution of superconducting (T_c = 17.2 K) and normal-state properties, has been studied as a function of doping with K. The normal-state dc susceptibility exhibited a complex behavior with the increasing K uptake, evolving from an enhanced paramagnetism at low doping level, to a weakly temperature-dependent diamagnetism at the optimum doping level for superconductivity, and ending with a strong temperature-independent diamagnetism in over-doped samples. The measured superconductivity parameters, such as T_c, London penetration depth and the Ginzburg-Landau coherence length, are compared with those of the monomer K₃C₆₀. and the main factors determining the differences between the two systems are discussed.     

THE REACTION OF C60F20 WITH ANTHRACENE: FORMATION OF AN OXIDISED-ANTHRACENE 1 : 1 COMPLEX

The reaction of C₆₀F₂₀ (“Saturnene”) with anthracene yields a white 1 : 1 cycloadduct (1) in which two oxygen atoms have inserted into the anthracene framework due to fullerene-catalysed oxidation; the ¹⁹F NMR spectrum of the product shows evidence of through-cage homoconjugation.     

Hypervelocity impact of rod projectiles with L/D from 1 to 32

Beside a short remark on the “hydrodynamic theory of rod projectiles”, the paper deals with the terminal ballistic behaviour of cylindrical projectiles against semi-infinite targets. Experimental data of EMI, completed by results of some other authors, are presented. Crater parameters like depth, diameter and volume and their dependence on projectile velocity (up to 5000 m/s), projectile and target material properties, as well as L/D-ratios (1–32), will be discussed. Mainly the projectile materials steel and tungsten sinter-alloys are considered. Target materials are mild steel and high strength steel, an Al-alloy and a tungsten sinter-alloy. The results show that the influence of material density on the crater dimensions is considerably greater than the influence of strength. The L/D ratio determines the velocity dependence of crater depth, diameter and volume. At high velocities in the hydrodynamic regime, the crater depth of short cylinders (L/D 1) is approximately proportional to v_p^2/3 (V_p=projectile velocity). With increasing L/D-ratio, the slope of the penetration curves decreases and converges for rods (L/D 1) versus a saturation, i. e. becomes nearly independent on v_p. A consequence of this saturation is the existence of a so-called “tangent velocity”, above which an optimal increase of efficiency is only realized by increasing the projectile mass and not the velocity. Furthermore, ballistic limits of real targets like single plates and symmetric double plates meteorite bumper shield) are taken into account. The expected better performance of “segmented rods” is also discussed.