Showing papers in "Journal of Physics D in 1981"

Theory of a plasma column sustained by a surface wave

Abstract: A self-consistent theory of a cylindrical plasma column sustained by a surface wave is derived which gives absolute values and radial profiles of electron density, electron temperature, HF electric field intensity and atomic excited state densities as a function of gas pressure, wave frequency and HF absorbed power. The basic equations are the continuity and the momentum transfer equations for both electrons and ions, the equations for the wave electric field and the power balance equation for electrons. The theory is applicable in the low attenuation regime. The plasma and field profiles are significantly different from the classical Bessel type and the field profiles of surface waves in homogeneous media, respectively. Depending on pressure the radial distribution of excited atoms is either flat or increases to reach a maximum near the wall.

High-energy electron distribution in an electron-beam-generated argon plasma

Abstract: The time evolution of high-energy electron distribution in an electron-beam-generated argon plasma is calculated. The distribution is derived for energy values above the threshold value of the first excited state (11.56 eV) from a reduced Boltzmann equation with no electron-neutral and electron-electron collisions. This equation can be numerically solved with a continuous source term taking account of all the new plasma electrons produced over the total energy range by primary electrons. As a result, the distribution reaches a steady state within a very short time tau s and its shape is pressure independent for a given current density, and a given energy of the beam, the evolution time tau s being inversely proportional to the pressure. Moreover, the energy distributions for given beam energy and pressure are in the same ratio as the primary-electron currents. An analytical approximation for the distribution tail is given as a function of the beam parameters (energy and current) and may be used for an electron-beam-generated Ar plasma as soon as these parameters are known. The branching ratios for energy deposition with electron-beam energy ranging between 103 and 106 eV is calculated. The relative influence of primary and secondary electrons is also discussed.

The influence of environment and pressure on the transition to dusting wear of graphite

Abstract: An examination has been made of the way in which the type of environment and its partial pressure influence the transition to high friction and wear-dusting-of graphite sliding against carbon. It is shown that the onset of dusting is invariably associated with the attainment of a critical temperature at the contacting asperities. This temperature increases with partial pressure of the vapour and, for a series of n-paraffins and alcohols, also increases with molecular size (chain length). From the variations in critical temperature with partial pressure, adsorption energies can be derived and these are broadly consistent with a mechanism of physical adsorption on graphite basal planes. It is suggested that the adsorbed vapour on basal planes functions as a 'reservoir' from which molecules can migrate to neutralise edge sites being continuously exposed during the normal, low-wear regime.

Power-law currents in some ZnO-Sn composite materials

Abstract: The dependence of current density on applied voltage has been studied in the composite system ZnO-Sn, where the proportion of Sn was 5% or 10% by weight. All samples showed a power-law dependence of current density on voltage, with the exponent being either equal to 2 or in the range 2.6-2.9. The conductivity was ascribed to traps of density 1*1016-2*1017 cm-3 which are situated below the conduction band edge of the ZnO. In some cases the traps were distributed exponentially according to the relation N(E)=N0exp(-E/kTt), where the temperature parameter Tt was typically about 500-600K; in other cases the traps were situated at a single level of 0.255-0.366 eV below the conduction band.

Depth profile composition studies of thin film CdS:Cu2S solar cells using XPS and AES

Abstract: Surface composition and depth profile studies of hemiplated thin film CdS:CuzS solar cells have been carried out using x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) techniques. These studies indicate that the junction is fairly diffused in the as-prepared cell. However, heat treatment of the cell at 210°C in air relatively sharpens the junction and improves the cell performance. Using the Cu(2p3p)/S(2p) ratio as well as the Cu(LVV)/(LMM) Auger intensity ratio, it can be inferred that the nominal valency of copper in the layers above the junction is Cut and it is essentially in the CUSS form. Copper signals are observed from layers deep down in the cell. These seem to appear mostly from the grain boundary region. From the observed concentration of Cd, Cu and S in these deeper layers and the Cu(LVV)/(LMM) ratio it appears that the signals from copper essentially originate partly from copper in CuS and partly from Cu2t trapped in the lattice. It is significant to note that the nominal valence state of copper changes rather abruptly from Cut to Cuz+ across the junction.

Thermal expansion reference data: silicon 300-850 K

Abstract: Measurements of the coefficient of linear thermal expansion of pure polycrystalline silicon were made with a polarization interferometer and a platinum resistance thermometer with an estimated error of less than 0.01 × 10-6 K-1.

Formation time and heating mechanism of arc cathode craters in vacuum

Abstract: With clean cathodes in UHV it is shown experimentally that (i) the arc craters are formed successively and displaced without spatial interruption, (ii) there are only a few active craters at one instant for currents below the spot-splitting limit (generally 1-2), and (iii) the crater formation is much faster than is compatible with the heat conduction time scale, the formation time amounting to only a few nanoseconds. From these observations and with the measured values of crater radii and formation times the heating mechanism of the craters is estimated. The calculations show that Joule heating is insufficient to explain the short time scale, therefore ion impact heating is concluded to be the dominant process. Also this energy source is effective only by heating thin layers of a well defined thickness (about 0.1 mu m) which are removed immediately after melting. Therefore, the melting front proceeds faster into the interior than by heat conduction. This model explains the high heat conduction losses at the cathode as measured by Daalder (1977), and also the reason for the arc spot movement.

Free convection flow through a porous medium bounded by a vertical surface

Abstract: Steady two-dimensional free convection flow through a porous medium bounded by a vertical infinite surface is considered. Expressions for the velocity, temperature and the rate of heat transfer are obtained. Effects of G (Grashof number) and K (permeability parameter) on the velocity and the rate of heat transfer are discussed.

Electron swarm parameters in argon and krypton

Abstract: Experimental measurements of the drift velocity and the ratio of longitudinal diffusion coefficient to mobility for electrons have been obtained for the range of 2.8

Rubber rolling on rough surfaces

Abstract: Earlier studies suggested that the adhesion of rubber to a substrate is increased by slight surface roughening. Rolling resistance experiments confirm this effect for very soft vulcanisates and unvulcanised rubber. Elastic analyses are shown not to predict the increased adhesion. The important factors suggested as giving rise to the increase for vulcanisates are stress relaxation during the contact period and loss at final detachment of the elastic energy built up in 'islands' of contact during separation. For the unvulcanised rubbers flow is shown to be the major factor. It leads to a high degree of intimate contact with rough surfaces, and also to energy dissipation around regions of locally higher adhesion where yielding occurs during peeling.

Structure and strength of carbon/carbon composites

Abstract: All types of polygranular carbon and graphite materials take advantage of the extremely high strength of sigma -bonds in elemental carbon. Manufacturing processes have been developed during the last century which allow the control and distribution of domains with anisotropic microstructure within the bulk material. Such polygranular graphite materials may be considered composites, consisting of the filler grain as skeleton and the binder carbon as matrix. However, the strength of sigma -bonds of elemental carbon is only partially utilised, because the fracture behaviour is controlled by crack propagation through the brittle and porous binder matrix. The basic problems encountered in manufacturing common polygranular carbon and graphite materials are also present in the fabrication of C/C composites. The carbon binder between the structural skeleton-now made of fibres instead of filler grains-remains the weakest part. Problems in manufacturing C/C composites and problems concerning fibre/matrix interactions are discussed. The possibilities and limitations in 'tailoring' carbon and graphite composites are considered.

On the role of electron attachment in the breakdown strength of gaseous dielectrics

Abstract: Experimental data on electron attachment and breakdown voltages Vs for two groups of strongly electron attaching gases-perfluorocarbons and freons-and some of their mixtures with electron slowing-down gases have been used in an effort to quantify the role of electron attachment on the breakdown strength of gases. The perfluorocarbons (PFC) considered have positive electron affinities (EAS) and capture electrons at thermal and near-thermal energies principally non-dissociatively. The freons capture slow electrons dissociatively producing mainly Cl-. Vs is strongly influenced by the magnitude and energy dependence of the attachment cross-section, sigma a( epsilon ). The increase in Vs with increasing attachment, however, seems to saturate, indicating an upper limit in Vs of approximately 2.5 times that of SF6. Electron detachment seems to be indicated for PFC and SF6.

On the determination of electrical conductivity in polyethylene

Abstract: There are significant differences in the field dependence of conductivity in low-density polyethylene as reported by various investigators. These differences are attributable partly to the presence of different kinds of impurities in variable concentrations in polyethylene samples of various origin, but are mainly due to the different measurement procedures adopted. The significance of the time which elapses after the application of voltage to the electrodes, as well as the effect of conditioning, e.g. thermal and voltage stressing of the specimen before and during the measurement, is discussed. Whether it is always necessary to obtain a steady-state current in order to determine the true conductivity is somewhat debatable.

A model based on nonlinear oscillations to explain jumps on creep curves

Abstract: A dislocation transformation model with three types of dislocations-namely the mobile, the immobile and those with clouds of solute atoms-is considered. Some physically reasonably reactions are postulated, leading to a coupled set of nonlinear differential equations for the rate of change of their densities. The basic idea of Cottrell's mechanism has been incorporated. It is shown that these equations admit a class of periodic solutions called limit cycles which are typical of nonlinear systems, suggesting that nonlinearity plays a fundamental role in the model. The rate equations are solved on a computer to obtain the oscillatory behaviour of the densities and hence leading to steps on the creep curve. The theory predicts that there is a range of temperature over which the phenomenon can occur, in agreement with the experiment of L.N. Zagorukuyko et al. (1977). The theory also reproduces other normal forms of creep curves.

Local field effects on voltage contrast in the scanning electron microscope

Abstract: The local field effects on voltage contrast in the scanning electron microscope (SEM) mean that local fields generated by a non-uniform potential distribution on specimen surface cause a variation in the secondary electron (SE) detector current. It causes some errors in the voltage measurement. The authors present a theory to calculate the SE detector current in the presence of the local fields. In the calculation, they assume that the field distribution above the specimen surface (metal electrodes with 8 mu m width and 12 mu m separation) is two-dimensional. Analysed models are a conventional SEM detector system (model A), a retarding-field energy analyser with an extraction field (model B) and a high-resolution energy analyser with an extraction field (model C). The results show that the local field effects could not be neglected even in models B and C with strong extraction fields. The calculated values of local field effects in models A and B agree well with the experimental ones. The dependence of local field effects on the electrode geometry is equivalent to that on the extraction field, though this dependency is not so strong.

The generation of electric currents by the laminar flow of dielectric liquids

Abstract: The electric current generated by the laminar flow of an insulating liquid through a cylindrical pipe has been calculated using a model that incorporates nonequilibrium boundary conditions at the wall. Analytical solutions to the model equations are derived in the limits of high and low electrical conductivity and numerical solutions are given at intermediate conductivities. The solutions depend on four parameters that are dimensionless representations of the electrical conductivity, the rate of adsorption of positive ions, the rate of adsorption of negative ions and the ratio of the diffusivities of positive and negative ions. The results are presented as graphs of current versus the electrical conductivity parameter for various values of the other three parameters. Physical reasons for the shapes of the current/conductivity curves are discussed. According to the model, the currents are generated by differences in the ionic diffusivities and/or by differences in the ionic adsorption rates at the wall.

Optical absorption studies on tin oxide films

Abstract: Transparent electrically conducting SnO2 films were prepared by the chemical vapour deposition technique. The films were deposited at substrate temperatures of 400, 450 and 500 degrees C by stannous chloride oxidation. The optical absorption studies, using unpolarised light, indicated that the deposition temperature is an important parameter influencing the optical properties of the films. The absorption edge for the films occurs at about 3.7 eV. The films also indicated direct transitions at about 3.95 eV and indirect transitions at about 3.27 eV.

Electron swarm parameters in helium and neon

Abstract: Experimental values have been given for both the drift velocity and the ratio of longitudinal diffusion coefficient to mobility for electrons in helium (1.4

Computation of electromagnetic flowmeter characteristics from magnetic field data

Abstract: A description is given of a method which may be used to predict the performance of a flowmeter subject to certain constraints. The flow channel is an insulating cylinder of circular cross-section, the contact electrodes are assumed to be mathematical points and the flow is restricted to the rectilinear axisymmetric case. The analysis includes the effect of a concentric annular stationary medium surrounding the liquid, but of different conductivity. The magnetic field may have any form, and this is found experimentally by measuring its normal component at a number of points on the cylindrical surface bounding the flow channel. The mathematical basis of the method is given, together with an outline of the computation and a description of the apparatus used to measure the magnetic field. The method can be used both as a means of 'dry calibrating' a given flowmeter and as an aid to flowmeter design.

A new method for the measurement of series resistance of solar cells

Abstract: A method is proposed which uses the observed nonlinearity in the short-circuit current versus light intensity curve at high intensities, to determine the series resistance of solar cells.

A percolation model of conduction in segregated systems of metallic and insulating materials - application to thick-film resistors

Abstract: An expression relating the resistivity, R, of a segregated system of conducting and non-conducting media to the volume fraction, V, of the conducting component has been derived by applying percolation theory to a simple but realistic model of the system microstructure. It is proposed that R varies as (p'-pc)- mu where pc is the percolation threshold, mu is the conductivity critical exponent for 3D systems and p' is the fraction of sites in the conducting region that are filled by conducting particles; p' is related to V. The expression can be used to fit the observed R-V curves (blending curves) of thick film resistor systems, which are known to have a segregated structure. The model is valid over a useful range of volume fractions and employs typical values for pc and mu .

Ultrasonic velocity dispersion in porous materials

Abstract: An analytic expression for the ultrasonic dispersion in porous materials is obtained. It is shown that size determination of pores of a few microns radius using ultrasonic dispersion should be possible with available techniques. For pores with radius 0.1-1 mu m higher frequencies, such as those characteristic of vapour-deposited transducers, will be necessary. The velocity dispersion and ultrasonic attenuation depend on averages of different powers of the pore radius. A measurement of both will give information on the mean square deviation in pore sizes as well as the average pore size.

Electron emission from molybdenum under ion bombardment

Abstract: Reports measurements of electron emission yields of clean molybdenum surfaces under bombardment with H+, H2+, D+, D2+, He+, N+, N2+, O+, O2+, Ne+, Ar+, Kr+ and Xe+ in the wide energy range 0.7-60.2 keV. The clean surfaces were produced by inert gas sputtering under ultrahigh vacuum. The results were compared with those predicted by a core-level excitation model. The disagreement found when using correct values for the energy levels of Mo is traced to wrong assumptions in the model. A substantially improved agreement with experiment is obtained using a model in which electron emission results from the excitation of valence electrons from the target by the projectiles and fast recoiling target atoms.

Thermoelectric properties of inhomogeneous materials

Abstract: The problem of determining the thermoelectric properties of a randomly inhomogeneous mixture of two thermoelectric materials is reduced to a simpler problem, the conductivity of a randomly inhomogeneous conductor.

Thermally stimulated currents in polyvinylidene fluoride. I. Unstretched alpha -form PVDF

Abstract: An unstretched alpha -form specimen of polarised PVDF shows four TSC peaks designated P1, P2, P3 and P4 in ascending order of temperature. The P1 and P2 peaks are associated with the dipolar depolarisation due to the alpha a and alpha e relaxations, respectively. The P3 and P4 peaks may both be attributed to interfacial polarisations formed by trapped carriers in the surface regions of PVDF crystals. The P3 peak shows a tendency to saturation in magnitude and a shift in peak temperature with poling field. The latter can be explained by the field-lowering of trap depth (the Poole-Frenkel effect). The P4 peak appears only in the specimen polarised at high fields which is expected to cause the field-induced structure change from the nonpolar alpha -form to the polar alpha -form. The polar beta -form specimen also shows a large TSC peak corresponding to the P4 peak in the alpha -form specimen. Therefore, the P3 and P4 peaks may be associated with nonpolar and polar alpha -form crystals, respectively.

Excess noise and refiring processes in thick-film resistors

Abstract: Excess noise in thick-film resistors (TFRs) fired from one to ten times under the same firing profile has been investigated The changes in structure and composition, which are responsible for variations in sheet resistivity and TCR, affect also the noise index of TFRS, sometimes in a substantial way, and the changes in noise index depend both on the resistor composition and the nature of the substrate Moreover, the excess noise appears to be correlated to physico-chemical phenomena induced by refiring in the bulk of the resistors, much more than to changes in the density of microdefects (or microvoids) in the resistors investigated

Momentum and energy exchange collision terms for interpenetrating bi-Maxwellian gases

Abstract: For application to aeronomy and space physics problems involving strongly magnetized plasma flows, we derived momentum and energy exchange collision terms for interpenetrating bi-Maxwellian gases. Collision terms were derived for Coulomb, Maxwell molecule, and constant collision cross-section interaction potentials. The collision terms are valid for arbitrary flow velocity differences and temperature differences between the interacting gases as well as for arbitrary temperature anisotropies. The collision terms had to be evaluated numerically and the appropriate coefficients are presented in tables. However, the collision terms were also fitted with simplified expressions, the accuracy of which depends on both the interaction potential and the temperature anisotropy. In addition, we derived the closed set of transport equations that are associated with the momentum and energy collision terms.

Time resolved DC electroluminescence studies in ZnS:Mn, Cu powder phosphors

Abstract: Time resolved studies of DC electroluminescence (EL) from ZnS:Mn, Cu powder phosphors are reported. An emission band arising from the Mn2+, 4T1 to 6AI transition and a very broad band attributed to radiative interband transitions of hot electrons are detected. The hot electron band is used as a probe of the onset of avalanching in the devices. EL emission is found to occur simultaneously for the two bands. Detailed studies of the EL onsets reveal that luminescence emission occurs, to within 100 ns, when the voltage across the high resistivity region is sufficient for avalanching, without any additional delay being present. This is consistent with the Mn2+ being excited by direct impact by hot electrons. The rate of decay of the Mn luminescence is found to increase with increasing Mn concentration. Various concentration quenching mechanisms are discussed to account for this behaviour. High current densities are deduced from the rate of increase of luminescence rise transients. Only 1 to 5% of the Mn ions in the phosphor can be excited.

Tunnelling currents in Langmuir-Blodgett monolayers of stearic acid

Abstract: Tunnelling currents in stearic acid monolayers stored at 15°C in dry He have been monitored over an 8 week period. Measurements were made as a function of both temperature and also applied voltage. It is deduced that the observed currents are associated with defects which are gradually annealed out.

An analysis of solid particle erosion mechanisms

Abstract: The erosion of a surface by a stream of solid particles is examined by considering the arrival times of the particles on the surface. The treatment first proposes a new concept, an 'impact zone', and then considers the arrival of the particles on one zone as a queuing process. The rate of erosion of ductile metals is predicted using simple arguments from probability theory. A comparison of the mechanisms of erosion has been made using this analysis as a basis for models. Results indicate that not all of the mechanisms are capable of predicting the incubation time effects which have been reported previously. Also discussed is the importance of the duration of the test and the concentration of eroding particles, or flux, striking the target. A recommendation is made for a new standard to be used when quoting flux levels in future publications.