Showing papers in "Journal of Physics D in 1989"

Microporous materials with negative Poisson's ratios. I. Microstructure and mechanical properties

Abstract: A microporous, anisotropic form of expanded polytetrafluoroethylene has been found to have a large negative major Poisson's ratio. The value of Poisson's ratio varies with tensile strain and can attain values as large as -12. The microporous structure of the material is described and the mechanisms that lead to this large negative Poisson's ratio are identified. Micro-rotational degrees of freedom are observed, suggesting that a micropolar elasticity theory may be required to describe the mechanical properties.

Reduction of oxides of iron, cobalt, titanium and niobium by low-energy ion bombardment

Abstract: Ion-beam bombardment of solid surfaces with rare gas ions is used extensively in conjunction with XPS and AES for surface cleaning and depth profiling. It is sometimes not appreciated that the process can give rise to extensive chemical changes in the surface to be examined. The purpose of this investigation is to study this ion-induced chemical damage. As a vehicle for this, changes in the elemental chemical states associated with some transition metal oxides have been examined using XPS, when the oxides were subjected to progressive periods of argon ion bombardment. Chemical reduction of all oxides investigated has been observed and the data have been compared with theoretical predictions from a current model for preferential oxygen removal. Measurements of the core electron energy levels in oxides and some hydroxides of iron, cobalt, titanium and niobium have been made and the results are tabulated.

A collisional-radiative model applicable to argon discharges over a wide range of conditions. I. Formulation and basic data

Abstract: A collisional-radiative model with an extended region of applicability is developed for an argon atom plasma. Atom-atom inelastic collisions and diffusion losses of the metastable states along with the electron-atom inelastic collisions and radiative processes are considered in this model, taking into account 65 effective levels. Among the analytical expressions used for the corresponding cross sections, special attention is paid to those determining the set of cross sections for excitation by electrons from the ground state, owing to the possibility of utilising the formulae recommended in kinetic modelling studies of discharges in argon or in mixtures including argon atoms. The numerical method developed makes it possible to investigate the mechanisms by which the excited levels are populated in a non-equilibrium argon plasma characterised (even in the case of a non-Maxwellian electron distribution) by a set of parameters, such as the electron kinetic temperature Te, the atom temperature Ta, the ion temperature Ti, the electron number density ne, the ground state atom population n1, the discharge tube (or the plasma column) radius R and the optical escape factors Lambda mn and Lambda m, which are dependent only on the quantities Ta, n1 and R in many cases of practical interest.

Microporous materials with negative Poisson's ratios. II. Mechanisms and interpretation

Abstract: For pt.I see ibid., vol.22, p.1877-82 (1989). In a previous paper the morphology of a microporous material made from expanded poly(tetrafluoroethylene) was described and results presented for its mechanical behaviour. The material was shown to be highly anisotropic and exhibited a large negative Poisson's ratio. In this paper a simple model for the microstructure is described to account for this effect. The model is based on an interconnected array of anisotropic particles that deforms so as to produce a large transverse displacement under longitudinal tensile loading. Very good agreement is found between the model and experimental results, providing an explanation for the variation of Poisson's ratio with tensile strain, in terms of changes in material morphology.

Scattering of ultrasound by emulsions

Abstract: Ultrasonic scattering theory is used to relate the ultrasonic velocity and attenuation of emulsions to their physical properties (e.g. droplet size and volume fraction). This theory includes visco-inertial and thermal scattering mechanisms and corrections due to multiple scattering. The physical significance of the scattering mechanisms is discussed so as to highlight the factors which influence ultrasonic measurements in these systems. Theoretical predictions were compared with experimental measurements of velocity and attenuation in sunflower oil in water emulsions with varying droplet radii (0.14-0.74 mu m), mass fraction (0-0.5) and frequency (1.25-10.0 MHz) at 293.1 K. Appreciable excess attenuation and velocity dispersion were observed which could be attributed to thermal scattering. The relationship between the measurable ultrasonic parameters and the physical properties of emulsions means ultrasonic scattering should prove a useful means of investigating these systems.

Extraction of extinction coefficient of weak absorbing thin films from special absorption

Abstract: The extinction coefficient of a weak absorbing film can be extracted from special absorption using a simple formula, provided the substrate is a kind of glass of refractive index within the range of 1.5-1.7.

Prospects of chemically deposited metal chalcogenide thin films for solar control applications

Abstract: Solar control coatings, required for architectural glazing applications in warm climates, must provide controlled optical transmission ( approximately 10-50%) of the solar radiation in the visible region and should reflect efficiently in the infrared (>0.7 mu m) region to create a cool interior in the buildings. Thin films of PbS and CuxS on glass substrates, deposited from chemical baths, are shown to possess excellent solar control characteristics-superior or comparable to the metallic solar control coatings. For example, for an acceptable range of integrated optical transmittance ( approximately 10-20%) in the visible region, the integrated infrared reflectance for AM2 solar spectrum for the different glazings are: PbS coated glass, 50%; CuxS coated glass, 14%; stainless steel/Cu coated glass, 25% and tinted glass, 4%. The CuxS and PbS coatings also have the advantage of giving pleasant reflected colours (golden, purple, blue, etc), which improves the cosmetic appearance. This paper presents the basic requirements of solar control coatings and provides a comparison of the characteristics of PbS and CuxS coatings against commercially available coatings.

Rough surfaces: gaussian or exponential statistics?

Abstract: The authors make use of a numerical method for generating random rough surfaces to study the effects of sampling interval on measured surface correlation functions. This numerical investigation avoids some of the complications present in experimental studies of the effects of this parameter, such as instrument resolution and measurement reproducibility. The numerical technique is used to generate surfaces with exponential correlation functions and surfaces with gaussian correlations. The short-wavelength fluctuations on the exponential surfaces are clearly seen, these arising from the high-frequency tail in the surface power spectrum. The authors are able to quantify the sampling interval necessary to record this short-range surface behaviour. They conclude that the sampling interval must be at least as small as one tenth of the surface correlation length, for these high-frequency variations to be recorded and hence for the inherent exponential nature of the surface to be measured.

Specimen inertia in high strain-rate compression

Abstract: Previous theories of the inertial contribution to measured stress in dynamic compression of cylindrical specimens have been extended to allow for anvil motion as well as specimen strain. A numerical model has been developed to estimate the inertial stress from this theory, and it was found that the new term can have the largest contribution. Because of the effects of wave propagation and dispersion found in experimental records, an estimate of the inertial errors from this numerical simulation is probably more reliable than an estimate derived directly from the experimental measurements.

Kinetic simulation of gaseous species created by an electrical discharge in dry or humid oxygen

Abstract: A kinetic simulation of various neutral species created by the corona effect in dry or humid oxygen is presented. The physical conditions of the pulsed electrical discharge used are: atmospheric pressure, a gas discharge channel temperature in the 200 to 800 K range and an electron density Ne=5*1015 cm-3 or 1016 cm-3 with the electron energy assumed to be larger than 5 eV. The presence of water vapour leads to a non-negligible H2O2 production. Temperature and humidity have a cumulative effect on the O3 production. With multiple pulses, the maximal O3 concentration is obtained for a finite number of pulses.

Calculation of the optical constants of a thin layer upon a transparent substrate from the reflection spectrum

Abstract: A method has been proposed for calculation of the optical constants of a thin layer upon a transparent substrate only from the reflection spectrum into the region of transmission of the layer. The envelopes of the maxima of the spectrum RM and of the minima Rm are used, taking into account the finite dimensions of the substrate. The algorithm of the calculations is similar to the one proposed by Swanepoel (1983) using the transmission spectrum. The method makes it possible to calculate the thickness of the layer and the refractive index n with an error of 0.2%. The correct description of the dispersive relationship of n is obtained for an amorphous layer Ge19As21S60.

Tensile network microstructures exhibiting negative Poisson's ratios

Abstract: A group of tensile microstructures are modelled that exhibit negative Poisson's ratios. These microstructures are a combination of anisotropic or isotropic particles, tensile springs and topologically constraining rods or strings. A range of structures are modelled, one of which contains an internal rotational degree of freedom. In all cases topological interactions occur between first- and second-nearest neighbour particles to produce cooperative transverse expansions under longitudinal loading.

The study of a ferrofluid exhibiting both Brownian and Neel relaxation

Abstract: The low-field complex susceptibility curves of ferrofluids have in general a Debye-type profile with a loss component which peaks at a frequency from which it is possible to estimate the average relaxation time of the particles in the ferrofluid. The authors report on magnetic measurements on a water-based ferrofluid whose loss curve displayed two distinct peaks. It is concluded that one peak is due to Brownian relaxation and the other is due to a combination of Brownian and Neel relaxation.

Electron and vibrational kinetics in the hydrogen positive column

Abstract: The electron energy distribution function in H2 and the vibrational distribution function of H2(X, v) molecules have been self-consistently calculated for the typical operating conditions of a low-pressure, moderate current, hydrogen positive column by solving the Boltzmann equation together with a system of rate balance equations for the vibrational levels. This system takes into account e-V, V-V and V-T processes, the latter including both intermolecular and molecule-atom processes. The formulation provides a relationship between the characteristic vibrational temperature, Tv, the degree of ionisation, delta e, the fractional concentration of dissociated atoms, delta a, and the ratio of the electric field to the gas density, E/N. At low E/N the superelastic e-V collisions strongly enhance the high-energy tail of the electron energy distribution which results in a significant increase in the electron excitation rates, especially in the rate of dissociation by electron impact. The authors present calculations of electron transport parameters and excitation rates, of the power transfer by the electrons via the various collisional mechanisms, of the total power transferred to the translational and rotational modes by electron collisions and V-T and non-resonant V-V processes, and of the total dissociation rate.

Dependence of electrical impedance of cement-based materials on their moisture condition

Abstract: This paper outlines an investigation into monitoring the moisture condition of cement-based materials using impedance spectroscopy techniques. Impedance measurements are made over the frequency range 20 Hz-110 MHz, and several parameters identified which could characterise the moisture condition of such materials.

Glass-metal nanocomposite synthesis by metal organic route

Abstract: Glass-metal nanocomposites incorporating ultrafine particles of iron, nickel, cobalt and manganese, respectively, in a silica glass matrix have been prepared by heat treatment of a gel derived from a sol containing silicon tetraethoxide and a suitable metal organic compound. Metal particles in all the nanocomposites are isolated and spherical-shaped with diameters ranging from 3 to 10 nm. Films of these nanocomposites with thickness of the order of a few micrometres have been prepared on glass slides by a simple dip-and-pull technique. Optical absorption spectra of the nanocomposite films have been measured over the wavelength range 200 to 2000 nm. Effective medium theories of Maxwell-Garnett and Bruggeman, respectively, have been used to calculate theoretically the optical absorption of these materials. The Maxwell-Garnett theory gives results which are in better agreement with experimental data than those obtained from Bruggeman formalism. The filling factor f as estimated from the least-squares fit of the experimental results with the Maxwell-Garnett theory has a value in the range 1 to 4%.

Electrochromic properties of conducting polyanilines

Abstract: A study has been made of the electrochromic behaviour of polyaniline films of potential use in passive display devices, and a preliminary examination of the properties of substituted polyanilines has also been carried out. Films of 'emeraldine-type' conducting polyanilines have been grown electrochemically from aqueous solutions onto glass substrates coated with gold or indium tin oxide. The authors have studied the electrochemistry of their redox reactions in acidic media to examine the kinetics and reversibility. Cyclic voltammetry was used to determine the electrode potentials for growth and redox switching and to obtain information on the electrode kinetics. The effect of electrode potential on the optical absorption spectra was observed, and was found to be consistent with polaron and bipolaron formation. The electrochromic colour contrast and switching times were measured as a function of pH; the requirements were somewhat conflicting, but a satisfactory compromise could be obtained around pH=0. By switching the electrodes over a constant potential step but from a variety of starting potentials, the influence on switching times of the initial oxidation state, and hence of the film conductivity, was investigated. Under suitable circumstances electronic conductivity, ionic transport or interfacial charge-transfer can limit the switching speed, but response times (for 50% transmission change) as short as 15 ms have been obtained without full optimisation. Of the numerous substituted polyanilines tried, only simple alkyl and alkoxy derivatives formed good polymeric films, and only the 2-ethoxy and 2-methoxy polymers gave good electrochromic behaviour. Switching times for these two were very fast, being less than 2.5 ms for both oxidising and reducing potential changes.

Electrical and optical properties of reactively evaporated indium tin oxide (ITO) films-dependence on substrate temperature and tin concentration

Abstract: The dependence of the electrical and optical properties of reactively evaporated indium tin oxide films on substrate temperature and tin concentration has been studied. Films of resistivity as low as 3*10-4 Omega cm and high visible transparency of about 91% have been obtained at higher deposition temperatures ( approximately 300 degrees C) for tin concentrations of 10-15 wt.% in the source alloy. Hall mobility in the films has been found to decrease with the addition of tin and increase with deposition temperature. An observed increase in carrier density with substrate temperature has been viewed as being due to the possibility that higher deposition temperature favours the incorporation of tin as Sn4+ in the films. In the optical band-gap calculation the effect of collision broadening, which is typical for a degenerate semiconductor, has been included and found to be significant.

Preparation of ohmic contacts to semiconducting diamond

Abstract: Low resistance ohmic contacts were manufactured on a natural semiconducting diamond (type IIb), by implanting boron ions at a temperature of 200 degrees C. This was followed by annealing at 1200 degrees C and removal of the top ion-damaged layer by etching in a hot acids solution. The thin, highly doped surface which remained could then be contacted with metals which need not interact chemically with the diamond surface.

Electrically conducting organic films and beads based on conducting latex particles

Abstract: Electrically conducting films have been prepared by casting aqueous dispersions of mixed polymer lattices on supporting glass microscope slides; one of the lattices is conducting (polypyrrole or polyaniline) and the other film-forming (a 1:1 co-polymer of polymethylmethacrylate and polybutylacrylate). The polypyrrole particles, being roughly spherical in shape, give a percolation threshold where the conductivity rises sharply at a weight fraction of about 20%, whereas the polyaniline particles, being needle-shaped, have a much lower percolation threshold at about 5%. In addition, the preparation of composite beads, having a non-conducting polymer core (e.g. polymethylmethacrylate) and a conducting shell (polypyrrole), has been carried out.

Some optical properties of evaporated zinc telluride films

Abstract: The absorption and transmission of ZnTe thin films are measured at wavelengths of 0.32-2.3 mu m. The optical constants (n, K) are measured in this range, although the short wavelength refractive index n measurement is limited by the band gap. A theoretical calculation of n near the band gap is made from experimental parameters. An explanation is given for the abnormal decrease of refractive index near the band gap. The effects of film thickness and doping with impurities like In, PbCl2, BaF2 have also been investigated. Effective crystallite size and strain have been determined by the method of variance analysis of the X-ray diffraction line profile on the same films. Lattice constants have also been calculated using the Nelson-Riley plots. It has been observed that there is an increase in optical band gap with decrease in crystallite size, increase in strain and decrease in lattice constant value.

Numerical studies on the optimisation of volume-produced H- ions in the single-chamber system

Abstract: Optimisation of volume-produced H- ions is studied by using a set of particle balance equations in a steady-state hydrogen plasma with a single-chamber system. The dependence of production of both H- ions and vibrationally excited hydrogen molecules H2* (vibrational level V) on plasma parameters (i.e. electron temperature Te, electron density ne, hydrogen gas pressure p, density ratio nfe/ne of fast primary electrons ef to slow plasma electrons e, energy of fast electron Efe, etc.) is explored because it is expected that H- ions are produced by the following two-step process, i.e. H2+ef to H2*(V)+Ef; H2*(V)+e to H-+H. Particular attention is also paid to wall effects, i.e. neutral particles-wall interaction, on H- production. So, a wall recombination coefficient gamma 1 for H and a wall de-excitation collision parameter gamma 2 for H2*(V) are treated as numerical parameters. It is confirmed that most H- ions are produced by the above-mentioned two-step process, and that the presence of ef with energies in excess of 40 eV is reasonable for H2*(V) production. With increasing Te (above 1 eV), H- yield decreases monotonically. Besides, ne, nfe/ne and p have some optimum values for H- production. However, the optimum condition for H- formation is not compatible with that for H2*(V) production. Another significant point is that the ion species ratios depend strongly on the wall parameters, i.e. gamma 1 and gamma 2. For H- production, the optimum condition is that gamma 1 approximately=1 and gamma 2<<1.

Optical centres related to nitrogen, vacancies and interstitials in polycrystalline diamond films grown by plasma-assisted chemical vapour deposition

Abstract: The authors have used cathodoluminescence spectroscopy to examine point defects present in polycrystalline diamond films grown by the decomposition of a methane-hydrogen mixture in a microwave plasma. For films grown with low methane concentrations (0.3%) the bright-blue luminescence is predominantly due to donor-accepted pair recombination, together with some weak zero-phonon lines. As the CH4 concentration in the gas mixture is increased the cathodoluminescence spectra from the resulting films contain a number of additional zero-phonon lines. Some of these are unique to this type of diamond; other lines at 2.156 eV, 2.807 eV, 3.188 eV and 4.582 eV are associated with optical centres that have been studied in diamonds produced by high-pressure synthesis, and indicate that the diamond films contain carbon interstitials, nitrogen-vacancy and nitrogen-interstitial centres. The widths of the zero-phonon lines in the diamond films suggest that the material is heavily strained.

Electron excitation rates and transport parameters in direct-current N2 discharges

Abstract: The electron transport parameters and excitation rate coefficients for individual N2(X, v-Y, v') transitions (Y denoting an excited electronic state) were calculated for typical operating conditions of a stationary N2 discharge at low pressures (0.1

Method for determining the optical constants of a thin film on a transparent substrate

Abstract: A method has been proposed for exact calculation of the optical parameters n and k of a thin film on a transparent substrate by simultaneously solving the equations for two of the envelopes of the transmission and reflection spectra. The singularity of the solution and the convergency area are proved, and the influence of the inaccuracies in drawing the envelopes upon the errors made during the calculation of n and k is also investigated. The method is most accurate for the envelopes of the transmission minima and the reflection maxima in the region of strong and medium absorption. The accuracy of other methods which use data from the transmission or reflection envelopes has also been investigated. The present method describes correctly the change of n in the region of medium and strong absorption for an amorphous film with composition Ge28As12S60.

Characterisation of a low-pressure oxygen discharge created by surface waves

Abstract: Low-pressure oxygen discharges created by surface waves are characterised. Three discharge characteristics, the electron-neutral collision frequency nu , the maintaining electric field Eeff and the power required to maintain one electron theta , are deduced from the measurement of the electron density. They are independent of the microwave power and are studied as functions of the tube diameter (8-52 mm), the pressure (0.05-2 Torr) and the excitation frequency (390 or 210 MHz). The concentrations of ground state and singlet molecular oxygen are measured by VUV absorption and the concentration ratio of singlet to ground state molecular oxygen is about 10%, whatever the microwave power and the pressure. The ground state atomic oxygen concentration is measured downstream from the discharge by VUV absorption. The dissociation rate increases with the microwave power but remains less than 5%. A power balance model in the discharge, i.e. calculation of theta , is performed and yields a relation between theta /N and Eeff/N (n being the total neutral density), which is in good agreement with the experimental results. Finally, similarity laws are shown in the form theta /N and Eeff/N (Nr), where r denotes the tube radius.

Electron excitation rates and transport parameters in high-frequency N2 discharges

Abstract: The electron energy distribution function, transport parameters and rate coefficients for excitation and ionisation in N2 under the action of a uniform HF field were calculated by solving the Boltzmann equation together with a set of rate balance equations for the populations in the Nx(X,v) vibrational levels. The treatment is restricted to situations in which the electrons do not lose appreciable energy during a cycle of field oscillation but applies to a broad range of applied frequencies, omega , bridging the gap between the limiting cases of few and many collisions per oscillation. The case of DC applied field previously investigated is obtained from the present formulation for omega =0. The effects of changes in omega and in vibrational excitation on the electron kinetics are investigated in detail, thus opening the way to firmly based theoretical interpretations of the properties of N2 discharges sustained by HF fields.

Child-Langmuir sheath structure around wedge-shaped cathodes

Abstract: The steady state Child-Langmuir sheath around a wedge-shaped cathode immersed in a plasma is calculated numerically. The ions drawn from the plasma may be employed to sputter material from the cathode, or may implant into the cathode, depending upon the magnitude of the applied negative voltage. The ion impact rate onto the cathode reaches 2.2 times higher near a square edge and 3.6 times higher near a knife edge than the rate for planar surfaces of the cathode. However, for a knife edge the rate falls to zero at the edge. All the ions strike the cathode with the same kinetic energy, but the angle of impact is nonperpendicular near the edge, which could reduce the implantation depth and increase sputtering.

An analysis of the laser-plasma interaction in laser keyhole welding

Abstract: In penetration welding with a laser, a plasma is formed in the keyhole. In the energy exchange process it is to be expected that a substantial part of the power is absorbed directly at the walls of the keyhole, but another exchange mechanism is the interaction of the laser beam with the plasma. A simple model of this part of the process is described and its properties are investigated. It is found to be consistent, and estimates are obtained for the parameters. The existence of a linking intensity is deduced. This relates the power absorbed by the workpiece per unit thickness of the workpiece to the laser power available at each cross section of the keyhole.

Synthetic-aperture radar imaging

Abstract: Synthetic-aperture radar (SAR) has become an important tool for remote sensing of the environment. However, until recently SAR systems, particularly airborne ones, have been incapable of yielding focused undistorted imagery consistently. Before any meaningful image understanding can be attempted it is essential to define the imaging function under all operating conditions. Some imaging problems are totally predictable and can be corrected in principle. The most serious difficulties are associated with unknown variations in the aircraft dynamics. In this review the author outlines the physical principles underlying SAR imaging, including a self-consistent scheme for the production of near-perfect images based on motion compensation from inertial navigation systems and a data-dependent autofocus and phase-correction technique.