Showing papers in "Journal of Physics D in 1988"

Stable glow plasma at atmospheric pressure

Abstract: A stable glow plasma at atmospheric pressure has been achieved for plasma treatment under selected conditions, for example the structure of electrodes, the kind of dilute gas, and the frequency of power. The surface fluorination of PET film and carbon thin-film deposition by such a plasma is described here. The surface energy can be controlled by plasma treatment of various concentrations (O2/CF4/He) in the same way as the lower pressure method that has been reported by Kogoma and co-workers (1987). Active species in plasma are identified by emission spectroscopy.

On the resuspension of small particles by a turbulent flow

Abstract: Presents a new approach to the way small particles are resuspended from a surface exposed to a turbulent flow. In contrast to current force balance models, this approach recognises the influence of turbulent energy transferred to a particle from the resuspending flow. This energy maintains the particle in motion on the surface within a surface adhesive potential well. The particle is detached from the surface when it has accumulated enough vibrational energy to escape from the well. Such considerations lead to a formula for the rate constant for long-term resuspension.

Velocity and attenuation of ultrasound in suspensions of particles in fluids

Abstract: The theory of the scattering of compression waves in viscous fluids is examined. The effects of fluid viscosity, of differences in density and in elastic modulus between the particles and the fluid, of heat transfer and of concentration are considered. Ultrasonic phase velocity and attenuation are derived. Results for the phase velocity are compared with several other formulations. The feasibility of using ultrasound to characterise suspensions is discussed.

Boundaries and probes in electronegative plasmas

Abstract: A criterion for the positive-ion velocity at the boundary of an electronegative plasma is discussed. The case considered is that in which the negative-ion density obeys the Boltzmann relation. The appropriate physical solution is determined in circumstances where a triple-valued mathematical solution is found. The work has much in common with models of two-electron-temperature plasmas. Under conditions where the theory is valid, the ratio of negative-ion density to positive-ion density can be determined from Langmuir probe characteristics.

Gas sensitivity of some metal phthalocyanines

Abstract: Previous studies of the gas sensitivity of CuPC (copper phthalocyanine) and ZnPC have been extended to include CoPC and NiPC, with particular reference to ammonia detection. Ammonia sensitivity is found to depend on the crystal structure of the films, ammonia concentration and temperature. The alpha - beta phase transition is found to give different film morphologies for ZnPC and NiPC. alpha -NiPC is the only PC studied that shows an increase in resistance on exposure to air. The other PCs show enhanced conductivity resulting from hole injection associated with oxygen adsorption. Surface conduction dominates bulk properties for all the PCs. Ammonia adsorption decreases conductivity due to the catalytic removal of surface oxygen but the response of the various PCs varies by two orders of magnitude. The low response of ZnPC is thought to be associated with the formation of a ZnPC-NH3 surface complex. CoPC and NiPC offer the highest response for low ammonia concentrations in air and the improved linearity of the response-concentration characteristics with SiO2 sublayers suggests that these may be viable materials for the fabrication of an integrated NH3 detector.

The analysis of the electrical conductivity of graphite conductivity of graphite powders during compaction

Abstract: The conductivity of four graphite powders was measured as a function of the volume fraction of graphite (or air) along (axial) and across (transverse) the direction of compression. The conductivity varies by between two and four orders of magnitude during these measurements. The results are shown to fit an equation which expresses the conductivity of the mixture as a function of the conductivity of the graphite and two morphology parameters. One parameters is the critical volume fraction, at which the graphite-air mixture would become insulating (percolation threshold) and the other an exponent is a combination of the effective demagnetisation coefficients of the grains and the critical volume fraction. Electron micrographs of the powder grains are shown and volume orientation percentage measurements of the graphite using X-rays were made in order to try to understand the results better.

Microgeometry of random composites and porous media

Abstract: For practical applications of variational bounds to the effective properties of composite materials, the information available is often not that required by the formulae for the optimal bounds. It is therefore important to determine what can be said rigorously about various unknown material properties when some other properties are known. The key quantities to be analysed are the parameters zeta and eta , depending on the microgeometry through integrals of the three-point correlation functions. The physical significance of these parameters for two-phase composites and porous media is elucidated by examining the various relationships between them and material properties. The bounds on conductivity due to Beran (1965) and the bounds on elastic constants due to Beran and Molyneux (1966) and to McCoy (1970), as well as those of Milton and Phan-Thien (1982), are considered. For the special case of porous media, the formulae simplify greatly and the resulting analytical relationships between transport properties and geometrical parameters are easily interpreted. In particular, it is shown that the microgeometry parameter zeta places limits on the pore space connectivity. Examples of bounds on one effective material property from measurements of another are also derived. These include bounds correlating the effective electrical or thermal conductivities and the effective shear modulus with the effective bulk modulus. These bounds are somewhat more restrictive than the well known bounds of Hashin and Shtrikman (1962). For porous materials, measurements of bulk modulus provide bounds on electrical formation factor and vice versa.

Ion and electron energy analysis at a surface in an RF discharge

Abstract: A retarding field analyser has been used to examine ion and electron energies at the grounded electrode of a capacitively coupled RF discharge in argon. The onset of collisional scattering of the ion distribution was observed at 0.05 mbar. A Tonks-Langmuir-type model has been used to generate a theoretical collision-free energy distribution which compares well with results at low pressure. The energies of electrons incident on the electrode were not consistent with the high-energy tail of a simple Maxwellian distribution in the plasma, and are better modelled by a two-temperature distribution.

Mechanical properties of SiO2 aerogels

Abstract: Ultrasonic and static compression experiments were performed in the frequency range 50 kHz to 1 MHz in order to investigate the mechanical behaviour of silica aerogels as a function of internal gas pressure pG and external stress Pext The measurement of longitudinal and transverse sound velocities allows the Young's modulus and the Poisson's ratio for aerogels of different densities to be determined upon variation of pG and Pext For low-density aerogels ( rho approximately=100 kgm-3) the authors found a decrease in sound velocity upon evacuation Surprisingly, the sound velocity decreases upon uniaxial compression with small loads The most important finding from the static compression experiments is that low-density aerogels display creeping with a time constant of about 45 min

The high-efficiency (17.1%) WSe2 photo-electrochemical solar cell

Abstract: WSe2 crystals (photo-electrodes) have been grown via a vapour transport technique, employing SeCl4 as transporter photo-electrochemical (PEC) solar cells with n-WSe2/I-, I2/Pt have been fabricated. In order to improve upon the efficiency, the effective WSe2 crystal surfaces have been etched in aqua-regia. The authors have obtained a high PEC conversion efficiency, up to 17.1%. This corresponds to the most efficient WSe2-based PEC solar cell reported so far. Evidence and arguments have been presented to show that the high efficiency ( approximately 17.1%) is achieved due to the combined effect of improved crystal quality, obtained through a SeCl4 transporter, and a decrease in the density of surface steps consequent to photo-etching.

Preparation of YBa2Cu3O7-? by citrate synthesis and pyrolysis

Abstract: A recent preparation method by citrate synthesis in combination with pyrolysis has been applied to the new high-Tc superconductors. A step-by-step procedure is given to obtain the ultra-fine and homogeneous powder. X-ray diffraction analyses were performed to characterise the powder and to study the influence of the calcination temperature. Resistance measurements exhibit complete superconductivity at 91.2 K.

Electron transport parameters in argon and its momentum transfer cross section

Abstract: The drift velocity and longitudinal diffusion coefficient of electrons in argon were measured over the range of E/N from 0.25 to 50 Td at room temperature by a double-shutter drift tube with variable drift distance. A momentum transfer cross section for the argon atom which was consistent with both of the present electron swarm parameters was derived over the range of electron energy from 2.5 to 15 eV.

Quasi-neutral theory of positive columns in electronegative gases

Abstract: A theory of the positive column in electronegative gases based on fluid-type momentum equations to describe charged particle motion is presented. It is assumed that quasi-neutrality conditions prevail and the ion inertial terms are neglected. The positive ions are assumed to be created by electron collisions with neutral molecules and the negative ions to be formed by dissociative electron attachment and destroyed by detachment in reactions with neutral species. The mathematical formulation consists of a two-point boundary value problem involving two independent parameters, functions of collisional and transport data, and two eigenvalues. One of these is the central ratio of the negative ion density to the electron density, while the other is related to the ionisation-loss balance and embodies a discharge characteristic for the maintenance field. These eigenvalues and the radial density distributions of the charged species were calculated for a wide range of variation of the independent parameters. An application of the theory to a positive column in oxygen is given as an illustrative example.

A point and line source model of laser keyhole welding

Abstract: The keyhole formed during penetration welding with a laser is modelled so far as its thermal absorption characteristics are concerned by the combination of a point and a line source. In this way a representation of the broader surface section of the weld is obtained in addition to the lower section. A simple analytical form for the temperature distribution is obtained and possible weld profiles are found numerically from this. These are compared with examples of actual welds; it is possible in this way to estimate the proportion of the laser power absorbed in each section of the weld. Graphs and some simple approximations are given which allow it to be done easily for a given weld.

The spreading dynamics of a liquid drop on a viscoelastic solid

Abstract: The liquid-fluid tension at the edge of a liquid drop may provoke a wetting ridge on a sufficiently soft solid substrate. Many soft solids such as polymers, and in particular elastomers, are viscoelastic and display hysteretic characteristics in a strain cycle. Such a strain cycle can be present near the advancing wetting ridge of a spreading drop. By considering that the excess capillary free energy is consumed during spreading by both viscous dissipation in the liquid and by mechanical, viscoelastic hysteresis in the solid, it is shown how spreading properties may be modified. An important degree of wetting hysteresis may result. In addition absolute spreading rates are reduced.

The strongly superlinear dose dependence of thermoluminescence in synthetic quartz

Abstract: The 110 degrees C thermoluminescence (TL) peak of unfired, synthetic quartz (Sawyer Research Products) has been found to be strongly superlinear with the dose of excitation. Quadratic and nearly cubic dependences have been observed with both beta - and gamma -irradiation. Firing the samples at 300 degrees C or higher increased the sensitivity substantially and removed the superlinearity. The fired samples have been found to have a pre-dose (sensitisation) effect similar to natural quartz, however, the pre-dose effect is weaker than in many of the other types of quartz. A model of competition during the heating phase, previously suggested to explain a quadratic dose dependence, is now shown to be capable of explaining the very strong superlinearity observed with the unfired sample. Furthermore, the model can also be used to account for the observed shifts in the temperature of the TL peak, the linearity and sublinearity of the fired samples, and their higher sensitivity.

Magnetic properties and microstructure of AISI 1000 series carbon steels

Abstract: The magnetic properties of a number of specimens of AISI 1000 series plain carbon steels have been measured as functions of chemical composition, microstructure and heat treatment. The results showed that for a given morphology of the carbides the magnetic properties such as coercivity and permeability varied smoothly as a function of carbon content. Coercivity increased with carbon content by 2.0 Oe for every 0.1 wt.% carbon added at low carbon contents while initial permeability decreased by 57% on the addition of 0.2 wt.% carbon in the form of lamellar carbides (pearlite), but by only 21% when those carbides were in the form of spheroidised particles. In all cases the spheroidised specimens were found to be magnetically softer than the lamellar specimens and the difference increased with the amount of carbon present. Grain size did not seem to affect the magnetic properties significantly above a carbon content of 0.2 wt.%, although it was known that grain size is a significant factor in determining magnetic properties in iron alloys with lower carbon content.

Electron swarm development in SF6. I. Boltzmann equation analysis

Abstract: The electron swarm behaviour in SF6 gas is studied for E/N values from 141 to 707 Td by a three-term Boltzmann equation method, in which the effect of generation and loss of electrons due to ionisation and attachment is considered properly. A consistent set of electron collision cross sections, which gives the swarm parameter values in agreement with previous measurements, is determined considering the latest cross section data. The calculation is performed mainly for the steady-state Townsend condition. The validity of the results obtained has been confirmed by a Monte Carlo simulation carried out parallel to the analysis. The present results are also compared with those of the usual two-term Boltzmann analysis. It is found that the two-term approximation is fully valid for deduction of the swarm parameters for E/N values as considered despite the fact that SF6 is a strongly electronegative gas.

The effect of compressive plastic deformation on the magnetic properties of AISI 4130 steels with various microstructures

Abstract: The magnetic properties of AISI 4130 steel after eight heat treatments have been investigated. Properties such as hysteresis loss, coercivity, and initial permeability were found to be closely interrelated. Furthermore, they were each dependent on the hardness of the material. A relationship between the hardness and permeability was found which was microstructure independent. These parameters were also found to change in a systematic way with plastic deformation and this result can be used for non-destructive evaluation. Correlations between the magnetic parameters were also found to change in a systematic way with plastic deformation and this result can be used for non-destructive evaluation. Correlations between the magnetic parameters revealed relationships which depended on the microstructure of the material. The residual stress in steels can be determined from the changes in maximum differential permeability.

Studies on the fractal dimension and fracture toughness of steel

Abstract: The slit island method is employed to measure fractal dimensions DF of fractured surfaces under plane strain conditions with the help of an image analysis technique for two high-strength steels under different heat treatment conditions and at different test temperatures. It is shown that the fractured surfaces are of fractal character. The values of DF decrease smoothly with an increase of the logarithm of fracture toughness K1C. This relationship may reflect the changes in the microstructure that occurred during different heat treatments and the changes in the fracture mechanism that occurred at different test temperatures. The fractal dimension DF can be regarded as a measure of fracture toughness in steel. Finally, the experimental results are explained in terms of the relation of critical crack extension force to the true total area of the irregular fracture surfaces.

Computer simulation of acoustic wave scattering from rough surfaces

Abstract: Describes the computer simulation of acoustic wave scattering from rough surfaces. A moving average is performed on a set of uncorrelated random numbers giving a set of random correlated numbers representing the height of a rough surface at intervals along the surface. The distribution of the initial data, together with the correlation of the moving average weights, determines the statistics of the generated surfaces. Surfaces are rough in one direction only, i.e. 'furrowed', with random heights and spacings of the furrows. The scattering of acoustic waves from these surfaces is studied using Kirchhoff theory. This gives an expression for the total wave field at any surface point in terms of the local surface height and gradient. The Helmholtz scattering integral is then evaluated over the surface by summing the contributions from each surface point. This gives the amplitude or intensity of the far-field scattered wave.

Departures from equilibrium in a free-burning argon arc

Abstract: A magnetogasdynamic arc model due to Kovitya and Lowke has been combined with a collisional-radiative model of the argon atom to show how intense radiation from an arc can produce excited-state densities in the boundary of the arc which are far in excess of the densities corresponding to thermodynamic equilibrium Quantitative predictions based on this model have been compared with measurements recently reported by Farmer and Haddad There are encouraging similarities between theory and experiment, although it is suggested that the theory could be further improved by including the effect of resonance radiation which is scattered in the arc boundary

The load-bearing area of a hardness indentation

Abstract: An experimental investigation is described in which it is shown that the piled-up region of a hardness indentation in metals also supports the indenter load and that the normal pressure is distributed uniformly over the projected area of the indentation. It is cautioned that if the piled-up region is not taken into account, as has been done by some investigators working with ultra-low hardness testing machines, then the estimated hardness values may have significantly high errors.

An investigation of the structures and molecular dynamics of natural waxes. II. Carnauba wax

Abstract: For pt.I see ibid., vol.21, p.1421 (1988). Structural aspects and the molecular dynamics of carnauba wax are reported. X-ray diffraction results show that it crystallises in an orthorhombic unit cell. DSC and proton spin-lattice measurements as a function of temperature revealed a premelting solid-solid transition at 335 K. The percentage of carbon atoms in methyl and methylene groups is approximately 12% less than in beeswax, whereas it has a much higher ester and carbonyl content. The average chain length in carnauba wax is 50 carbon atoms. Spin-spin relaxation measurements revealed that the reorientational-translational motion of the chains is much more hindered than in beeswax. The motional parameters associated with the reorientations of the methyl groups have been isolated and compared with these parameters for similar motions in Fischer-Tropsch waxes and beeswax. Spin-lattice relaxation measurements in the rotation frame showed that the activation energy for the dangling motion of the chain ends is approximately 20% higher than in beeswax. The liquid content of the wax has also been determined as a function of temperature by employing a simple pulse technique.

Enhanced cold-cathode emission using composite resin-carbon coatings

Abstract: A new type of composite resin-carbon field-emitting cathode (FEC) is described which is found to switch on at applied fields as low as approximately 1.5 MV m-1, and subsequently has a reversible I-V characteristic with stable emission currents of >or=1 mA at moderate applied fields of typically

The influence of plastic deformation on Barkhausen effects and magnetic properties in mild steel

Abstract: The rectified Barkhausen noise envelope, induction rate dB/dt signal, and hysteresis curve have been measured simultaneously for an annealed and plastically deformed mild steel specimen. The two peaks in the Barkhausen noise envelope of the annealed specimen occurring in the knee regions of the hysteresis curve are due to domain nucleation and annihilation, while the single-peak in the Barkhausen noise envelope of the deformed specimen at the coercive field is due to the dislocation pinning occurring during irreversible wall displacement. The increase of the coercive field with the plastic strain is attributed to the increase in the dislocation density. The strain dependence of the Barkhausen noise energy is explained in terms of the energy released during domain nucleation and annihilation, and the hysteresis loss in terms of pinning effects of a moving wall. The variations of the frequency spectrum with the plastic deformation and magnetising frequency are characterised by the Barkhausen noise exhibiting a clustering and overlapping nature.

Contact electrification-why is it variable?

Abstract: Research into the causes of triboelectrification is hampered by the well known problem of irreproducibility. Charge transfer by contact of a metal to a polymer, for example, may vary widely from one contact to another, even on the surface of a single polymer sample. Such variation may, in principle, be a result of place-to-place variations in the properties of the polymer; or it might be a result of changes in the properties of the metal (e.g. work function); or to variations in the process of charge transfer (e.g. variable amounts of back-tunnelling, or material transfer). The authors have made a detailed study of the variation in charge transfer for a variety of metal and polymer samples in order to decide among these possibilities. They find clear evidence that the charge transfer is associated with variations in the properties of the polymer surface. There are strong correlations between the fluctuations in the charge transfer from different metals, and between neighbouring places on the polymer surface. However, the variability of charge transfer is not a consequence of adventitious contamination, as is often suggested. Their observations show that the mean charge and the variation about the mean are little influenced by the surface treatment; moreover, if the charge transfer for a particular metal/polymer combination is scaled to the mean charge the spread about the mean is closely similar, both in magnitude and form, for a wide range of polymers. They conclude that the fluctuations in charge transfer must be much more closely connected with the basic mechanism of charge transfer than has hitherto been supposed.

Alpha and gamma RF capacitance discharges in N2 at intermediate pressures

Abstract: Measurements have been made on the current and voltage characteristics and the time-resolved and time-averaged visible emission of alpha-( alpha -) and gamma- ( gamma -) type RF capacitative discharges in N2 at frequencies between 10 and 60 MHz over the pressure range 10 to 100 Torr. A time-dependent model is proposed for alpha -type discharges that yields electron energy and density, internal potentials, discharge current and power dissipation given the measured parameters of inter-electrode voltage and separation, gas pressure and RF frequency. The model predicts considerable energy dissipation in the near electrode sheaths and elucidates the increasing stability of alpha discharges for higher input powers as the RF frequency is raised.

Optimisation and fabrication of grating beamsplitters

Abstract: Optimisation of certain holographic 1 to N and 1 to N*N beamsplitters (Dammann gratings) with the aid of simulated annealing and damped least-squares algorithms is considered. Structures with N=5 to 53 uniform beams have been calculated. A photolithographic technique for fabrication of these elements is also presented, and its accuracy analysed with the aid of numerical simulations and actual test results.

An investigation of the structures and molecular dynamics of natural waxes. I. Beeswax

Abstract: A detailed NMR investigation, supported by DSC and X-ray diffraction measurements, of the combwax of the African bee apis mellifera adansonii is reported. Proton spin-lattice relaxation times in the laboratory and rotating frames, as well as the proton spin-spin relaxation time, have been measured as a function of temperature (333 K>T>110 K). The liquid content of the wax has also been determined as a function of temperature by employing a simple pulse technique. The motional parameters associated with the reorientations of the methyl groups and the chains have been isolated and compared with these parameters for similar motions in Fischer-Tropsch waxes. The T1 rho results revealed a minimum which is associated with the dangling motion of chain ends in the amorphous zone of the wax. The relaxation results strongly suggest that beeswax is branched to a much higher degree than Fischer-Tropsch waxes, including oxidised waxes. The high-resolution 13C spectrum of beeswax in the solid state shows that it resembles oxidised Fischer-Tropsch hard wax closely. The major difference is that a higher percentage of carbon atoms are involved in ester groups in beeswax. Oxidised hard wax contains a higher fraction of carbon atoms with double carbon-carbon bonds. The average chain length in beeswax, determined by ebullioscopic methods, is 40 carbon atoms.