Showing papers in "Journal of Physics D in 1968"
TL;DR: In this paper, the problem of statistically choosing free times between the collisions, when the velocity cannot be assumed constant during a free time, was studied and the free times can be chosen by the use of a piecewise velocity-independent mean collision frequency chosen to be greater than the actual collision frequency, combined with the assumption that an appropriate part of the collisions results in zero momentum transfer.
Abstract: The stochastic computer simulation of ion motion in a gas subjected to an electric field involves the problem of statistically choosing free times between the collisions, when the velocity cannot be assumed constant during a free time. It is shown how free times can be chosen, without any approximations or numerical integrations, by the use of a piecewise velocity-independent mean collision frequency chosen to be greater than the actual collision frequency, combined with the assumption that an appropriate part of the collisions results in zero momentum transfer.
319 citations
TL;DR: In this paper, it was shown that the carbon fibres reduce the friction and wear of polymers by preferentially supporting part of the applied load, and in addition by smoothing the surface of the steel counterface.
Abstract: Thermosetting and thermoplastic polymers have been reinforced with carbon fibres of high strength and elastic modulus, and their friction and wear properties examined. During dry sliding against steel, randomly oriented chopped fibres reduce both the coefficients of friction and the wear rates of polymers to levels which are approximately independent of the matrix material. Fibre orientation is also important, and minimum wear is obtained when the fibres are normal to the sliding surface. Carbon fibre reinforced polymers are less abrasive than those containing glass, exhibit lower friction and have greater moduli of elasticity; the flexural strengths are also often greater, and particularly so with polytetrafluoroethylene (PTFE). It is concluded that the carbon fibres reduce the friction and wear of polymers by preferentially supporting part of the applied load, and in addition by smoothing the surface of the steel counterface.
205 citations
TL;DR: In this article, a two-valence-band model for room temperature electrical and thermal properties of p-type SnTe for the carrier concentration range 4 × 1019-2 × 1021 cm−3 have been quantitatively explained using the Cohen's dispersion law for energy surfaces with rotation plus reflection symmetry.
Abstract: Room-temperature electrical and thermal properties of p-type SnTe for the carrier concentration range 4 × 1019-2 × 1021 cm−3 have been quantitatively explained using a two-valence-band model in which the principal light mass band is always highly non-parabolic and the heavy mass band is parabolic. Band parameters of the model were determined. The theory was based on Cohen's dispersion law for energy surfaces with rotation plus reflection symmetry. The close similarity in band structure between SnTe and PbTe was established. It appeared that the 300°K band parameters of SnTe were very similar to the 0°K band parameters of PbTe, given by Rogers, but the energy separation between valence band edges was much greater in SnTe. Intravalley acoustic-mode lattice scattering was always dominant at room temperature.
159 citations
TL;DR: In this paper, the complex refractive index of a thin film was put into a simpler form, which greatly simplifies the programming for a computer solution of the equations, and also leads to some useful approximate formulae.
Abstract: The very complicated formulae which relate the components of the complex refractive index of a thin film to the measurable optical reflection and transmission coefficients have been put into a much simpler form. This greatly simplifies the programming for a computer solution of the equations, and also leads to some useful approximate formulae.
132 citations
TL;DR: In this article, the authors studied the progressive intercalation of graphite by potassium by x-ray diffraction methods and showed that the graphite layer stacking sequences and repeat distances for stages 2, 3 and 4 are stage 2 AB| BC| CA| A c=2625 A stage 3 ABA| ACA| A 2420 A stage 4 ABAB| BCBC| CACA| A 4635 A within each intercalate layer the potassium atoms are distributed irregularly over the centres of carbon hexagons in adjacent graphite layers.
Abstract: The progressive intercalation of graphite by potassium has been studied by x-ray diffraction methods. It has been established that potassium graphites of stages 4, 3, 2 and 1 (with intercalated potassium layers separated by 4, 3, 2 and 1 graphite layers respectively) can be formed successively within a single graphite sample encapsulated with excess potassium. Each stage was prepared in turn by adjusting the temperature difference between the graphite and the potassium in a specially designed double furnace. The graphite layer stacking sequences and repeat distances for stages 2, 3 and 4 are stage 2 AB| BC| CA| A c=2625 A stage 3 ABA| ACA| A 2420 A stage 4 ABAB| BCBC| CACA| A 4635 A Within each intercalate layer the potassium atoms are distributed irregularly over the centres of carbon hexagons in adjacent graphite layers. The fully ordered structure of the stage 1 compound, A α A β A γ A δ A, has been confirmed.
128 citations
TL;DR: In this article, the efficiency of production of characteristic x radiation is reported for the Kα lines of carbon, aluminium, titanium, iron, nickel, copper, germanium, molybdenum, silver and tin, for the Lα1 lines of neodymium, tantalum and gold, and for the Mα line of gold.
Abstract: Measurements of the efficiency of production of characteristic x radiation are reported for the Kα lines of carbon, aluminium, titanium, iron, nickel, copper, germanium, molybdenum, silver and tin, for the Lα1 lines of neodymium, tantalum and gold, and for the Mα line of gold. Satisfactory agreement with the experimental values of others and with theoretical predictions is obtained for the K shell. However, the experimental production efficiencies for the L shell are a factor of 2 greater than the theoretical values. Measurements of the LII and LIII ionization cross sections of gold, also reported here, show this parameter to be the source of the discrepancy between theory and experiment. The experimental technique used during the course of this work to determine characteristic x-ray intensities has, in addition, given new values of the absolute intensity of the continuous x-ray spectrum for twelve elements.
118 citations
TL;DR: In this paper, the problem of aerosol sampling with a thin-walled tube and a bulky head is broken down into sampling with small tubes and sampling with large tubes, where there is a change of sampling efficiency when the direction of the tube is altered.
Abstract: No general solution exists of the very complicated problem of aerosol sampling. In this paper the problem is broken down into sampling with a thin-walled tube and sampling with a bulky head. Tube sampling can be classified into `small tubes', which give the same results independent of their orientation, and `large tubes' where there is a change of sampling efficiency when the direction of the tube is altered. The limits of radius of a `small tube' sampling with an efficiency near to 100% are laid down in terms of particle size and suction speed; tables are given for evaluating the sampling efficiency in a wind. Calm air is defined by the wind speed being less than a value which is dependent on the sampling speed and particle size. The sampling efficiency of a `large tube' may depend on its orientation owing to the fall of particles due to gravity or to a wind blowing past the tube. Isokinetic sampling is necessary to obtain correct samples in strong winds; the effect on efficiency of sampling due to departure from the isokinetic conditions is discussed. The conditions are deduced for accurate sampling with a bulky suction head containing a small orifice: if the orifice faces downwards a critical suction speed exists below which no sample is obtained; at higher speeds sampling is selective and depends on the falling speed of the particles. When a bulky head is used in a cross wind there is no effect due to wind when the orifice is in the top. For a downwards facing orifice the effect of wind is to cause sampling to start at a lower rate of suction and to be more efficient at lower rates of suction. At high rates of suction the wind has no effect.
101 citations
TL;DR: In this paper, the lattice parameter a0 of thin single crystals of molybdenum disulphide was shown to be independent of crystal thickness and to have the value 31475 A at 293°K.
Abstract: The lattice parameter a0 of thin single crystals of molybdenum disulphide was shown to be independent of crystal thickness and to have the value 31475 A at 293°K. The variation of a0 with temperature was measured from 77 to 373°K.
88 citations
TL;DR: In this article, it was shown that electrons, photoexcited in the GaAs, can diffuse through the coating of Cs-O without a significant loss in number, and that the work function of such coatings is at least as low as that of GaAs-Cs.
Abstract: High-yield photoemission, already reported for GaAs-Cs, is also observed for GaAs-Cs-O. Spectral distributions of yields from both GaAs and tungsten with surface coatings of Cs-O indicate that the work function of such coatings is at least as low as that of GaAs-Cs. Photoemissive yields from GaAs-Cs-O have approximated to the highest observed from GaAs-Cs and have exhibited greater stability. It is concluded that electrons, photoexcited in the GaAs, can diffuse through the coating of Cs-O without a significant loss in number.
80 citations
TL;DR: In this article, the analysis of Sakurai is extended to include variable-energy flows with particular reference to cylindrical spark channel formation, and a new analysis in Lagrange coordinates enables the blast-wave decay to be studied and the number of differential equations to be reduced.
Abstract: The analysis of Sakurai is extended to include variable-energy flows with particular reference to cylindrical spark channel formation. A new analysis in Lagrange coordinates enables the blast-wave decay to be studied and the number of differential equations to be reduced. A new approximation for finding the velocity of the shock front is presented.
76 citations
TL;DR: In this paper, the number of ionizations on a given j shell in any electron-bombarded target has been calculated analytically by means of the Bethe formulae, and a general formula for the atomic number correction has been derived in the form of an equation using parameters that are functions of target composition and accelerating voltage.
Abstract: The number of ionizations on a given j shell in any electron-bombarded target has been calculated analytically by means of the Bethe formulae. A general formula for the atomic number correction has been derived in the form of an equation using parameters that are functions of target composition and accelerating voltage. This equation allows the calculation of the penetration effect to be readily carried out. The electron range, in terms of beam energy, has also been calculated. Finally, it is shown that the correction is different in the case of thin-target analysis.
TL;DR: In this paper, the theoretical treatment of a positive column using fluid equations to describe the ion motion and including ion inertia is extended to the case where quasi-neutrality is not assumed.
Abstract: The theoretical treatment of a positive column using fluid equations to describe the ion motion and including ion inertia is extended to the case where quasi-neutrality is not assumed. Calculations have been performed in the approximation of electron temperature large compared with ion temperature. The range of discharges from collisionless to collision-dominated and from quasi-neutral to freely-diffusing can then be treated. Charged-particle, ion-velocity and potential distributions have been obtained. The relevance of the calculations is examined in the low-current (low number density) case when electron temperature becomes dependent on current in an active discharge. Firm conclusions cannot be drawn because of the lack of experimental data at low enough pressures.
TL;DR: In this article, the theoretical analysis of the temperature coefficient of capacitance of most insulators is outlined and it is shown that, for given ranges of permittivity, the analysis can be greatly simplified.
Abstract: The temperature coefficient of capacitance (TCC) of most insulators is dominated by five components. The theoretical analysis of these components is outlined and it is shown that, for given ranges of permittivity, the analysis can be greatly simplified. This simplified treatment is shown to apply to widely different types of compound. Since TCC depends on permittivity, the permittivity of a variety of materials has been rationalized by reference to their `I' factor, which is defined as mean atomic number per cubic angstrom.
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TL;DR: In this article, an x-ray interferometer working in the low absorption Laue-case (transmission geometry) and the extensive application of spring-strip elastic design, combine to give high intensity with almost perfect stability and backlash-free operation.
Abstract: As a first step in the establishment of an absolute scale of length in the region of 10−10 m an x-ray interferometer has been used to measure distances. In preliminary experiments several hundred high-contrast x-ray moire fringes have been recorded, enabling distances from 0 to 01 μm to be measured with a precision of ±002 A. Two novel features, x-ray interferometers working in the low absorption Laue-case (transmission geometry) and the extensive application of spring-strip elastic design, combine to give high intensity with almost perfect stability and backlash-free operation.
TL;DR: In this paper, the dependence of the yield stress of binary solid solution crystals on temperature and alloy concentration is explained in terms of a simple model in which the breakaway of dislocations from short rows of pinning points is rate determining.
Abstract: The dependence of the yield stress of binary solid solution crystals on temperature and alloy concentration is explained in terms of a simple model in which the breakaway of dislocations from short rows of pinning points is rate determining. With an assumed binding energy of about 001 ev between dislocations and zinc atoms the calculated yield stress of α-brass single crystals agrees quantitatively with measured values in the range 0-400°K. The yield stress of dilute solid solutions, particularly iron alloys, and a possible bearing of the present theory on the problem of the temperature dependence of the Peierls stress, are discussed briefly.
TL;DR: In this paper, a weighted geometric mean of the maximum and minimum values of the individual thermal conductivities of the sample is proposed for obtaining its effective thermal conductivity, which can demonstrate the structure and distribution of phases in the sample.
Abstract: Experimental values of the effective thermal conductivity obtained by Sugawara on calcareous sandstone partially filled with water are compared with values calculated by using the geometric mean equation of Lichteneker extended to a system having three phases and also with values obtained from a relation given by Brailsford and Major. The calculated values disagree with the experimental values. A weighted geometric mean of the maximum (weighted arithmetic mean of the individual thermal conductivities) and minimum (weighted harmonic mean of the individual thermal conductivities) values of the thermal conductivity of the sample is proposed here for obtaining its effective thermal conductivity. It is held that such an equation can demonstrate the structure and distribution of phases in the sample.
TL;DR: In this article, the authors adapted the equation of vertical flow to describe the evaporation of water from a semi-infinite porous material during the second stage of drying when water loss is limited by the distribution of water in the material.
Abstract: The equation of vertical flow is adapted to describe the evaporation of water from a semi-infinite porous material during the second stage of drying when water loss is limited by the distribution of water in the material. Flow measurements on long columns of aggregates of six soil materials confirm the theory, and enable the transport parameters controlling flow to be found over the range of volumetric water content θ (cm3 water/cm3) from aggregate saturation θC to dryness (i.e. from near pF 2 to near pF 7). Evaporation from porous materials initially at a uniform water content θC produces profiles of θ invariant with zt−1/2 (where z is the depth in cm and t the time in seconds) which differ for each material. As θ decreases from θC diffusivities (D cm2 s−1) calculated from these profiles fall to a minimum, then rise to a subsidiary maximum close to dryness, before falling again as θ approaches zero. Such behaviour accords with prediction. Hydraulic conductivities (k cm s−1) fall continuously as θ decreases from θC to zero.
TL;DR: For helium gaps between 1.5 and 250 mm, the discharge process exhibited features common to both spark and glow discharges as discussed by the authors, and the term ''pseudo-glow'' was applied to this type of discharge.
Abstract: For helium gaps between 015 and 250 mm the discharge process exhibited features common to both spark and glow discharges; the term `pseudo-glow' was applied to this type of discharge. Above and below these gap settings spark discharges having variable breakdown voltages were found to occur.
TL;DR: In this article, measurements of the nuclear magnetic resonance second moment for the proton resonance of a number of drawn polyethylene films are used to obtain orientation functions for the films, on the basis of a model in which a drawn film is regarded as an aggregate of units which are aligned by the drawing process.
Abstract: Measurements have been made of the anisotropy in the nuclear magnetic resonance second moment for the proton resonance of a number of drawn polyethylene films. The results are used to obtain orientation functions for the films, on the basis of a model in which a drawn film is regarded as an aggregate of units which are aligned by the drawing process. The experimental results for low-density polyethylene are compared with the prediction of the affine deformation theory, in which the unique axes of the units undergo the same changes of direction as lines connecting pairs of material points in a body undergoing uniaxial deformation, without change of volume. Reasonable qualitative agreement is obtained. Orientation functions are then obtained directly from the experimental results and used to predict the mechanical anisotropy of the films. Good agreement is obtained with previously reported mechanical data. Finally an attempt is made to obtain the shape of the distribution function at each draw ratio. With the exception of the lowest draw ratio film this is less successful, due to the small number of spherical harmonic functions which can be obtained from the anisotropy of the second moment.
TL;DR: In this article, a modified critical state model of type II superconductors is described, which takes account of Hc1 and surface sheath currents. But the model implies zero power dissipation for peak applied fields less than Hc 1+ΔH and rapidly increasing dissipation just above this region, asymptotically approaching that derived from the Bean-London model.
Abstract: A modified form of the critical-state model of type II superconductors is described which takes account of Hc1 and surface sheath currents. The penetrating flux, hysteresis loop characteristics, a.c. losses and rate of penetration of flux into the superconductor when exposed to a sinusoidal magnetic field are evaluated. The model implies zero power dissipation for peak applied fields less than Hc1+ΔH and rapidly increasing dissipation just above this region, asymptotically approaching that derived from the Bean-London model.
TL;DR: In this article, the phase transformation at 68°C found in materials based on vanadium dioxide has been used to produce devices exhibiting an S-type current-voltage characteristic.
Abstract: Using the phase transformation at 68°C found in materials based on vanadium dioxide, devices exhibiting an S-type current-voltage characteristic have been produced. The characteristic has an unstable differential negative-resistance region which leads to fast switching (20 ns). Devices have been made by the fusion of mixtures of vanadium pentoxide with other oxides on to thin wires in a reducing atmosphere, by depositing vanadium dioxide particles between conductors using a dilute varnish suspension, and by reactive sputtering of vanadium in low oxygen pressures to give thin films. Theoretical expectations based on a thermal explanation are in good agreement with the observed static and dynamic characteristics. A total cycle time of 700 ns has been obtained, most of which is derived from the turn-off time. On/off resistance ratios over 1000 have been observed.
TL;DR: In this paper, a technique for calculating the elastic constants of a uniaxial fiber-reinforced composite in which long fibres are arranged on a square lattice is developed.
Abstract: A technique is developed for calculating the elastic constants of a uniaxial fibre-reinforced composite in which long fibres are arranged on a square lattice. Infinite series Airy stress functions are utilized. The boundary value problem for a unit cell is solved numerically by truncating the infinite series and satisfying the boundary conditions at discrete points. For certain systems of practical interest graphical results are presented for the variation of elastic constants with fibre volume fraction.
TL;DR: In this paper, high-modulus RAE carbon fibres were placed in a resin matrix on an aluminium backing and strained, and the behaviour near breaks in the fibres was examined.
Abstract: High-modulus RAE carbon fibres were placed in a resin matrix on an aluminium backing and strained. The behaviour near breaks in the fibres was examined. If the interface was weak the broken fibre ends slid back in their holes in the resin and the load in the fibre was transferred by friction at the interface over a distance of about 12 mm. The mean frictional shear force was about 6 MN m−2 (800 lbf in−2). There was little interaction between adjacent fibres in small bundles, and three-dimensional composites broke in a fibrous manner. If the interface was strong the load was transferred from broken fibres mainly elastically over a distance of about 130 μm. In bundles of a few fibres a crack in one spread to the others, and three-dimensional composites broke in a brittle manner.
TL;DR: A high-temperature x-ray study of pure iron has shown that a small but easily detectable, lattice parameter anomaly, representing a decrease in the mean linear thermal expansion coefficient, occurs over a narrow range of temperature (similar 35 degc) about the Curie point.
Abstract: A high-temperature x-ray study of pure iron has shown that a small, but easily detectable, lattice parameter anomaly, representing a decrease in the mean linear thermal expansion coefficient, occurs over a narrow range of temperature (similar 35 degc) about the Curie point. The total volume change associated with the ferromagnetic-paramagnetic transition in pure iron obtained from the experimental measurements gave values in good agreement with those calculated directly from volume magnetostriction data, although for an alternative method of calculation the agreement was poor.
TL;DR: In this article, the ionization distribution in depth has been determined experimentally for two elements, lead and titanium, at various incident electron energies, by the sandwich tracer technique previously used by Castaing and Descamps.
Abstract: The ionization distribution in depth has been determined experimentally for two elements, lead and titanium, at various incident electron energies, by the sandwich tracer technique previously used by Castaing and Descamps. The experimental curves of the ionization distribution in depth at 29 kv for lead and titanium are compared with curves calculated from the distribution in depth and energy of the incident electrons computed by Bishop. Agreement between the experimental and theoretical results is good. The surface ionization straightphi0, which is mainly due to backscattering of the electrons, has been determined at various accelerating voltages for titanium, nickel and lead. straightphi0 is a function of the atomic number of the anticathode and of the accelerating voltage. Good agreement is obtained between the experimental values and values calculated by Duncumb and Melford. The x-ray absorption correction curves f(×) for titanium and lead have been computed from the experimental curves of the distribution in depth of the characteristic emission. These are compared with the curves predicted by the `Philibert correction'.
TL;DR: In this article, single-crystal x-ray results are given for specimens of Ti-207 at.%Nb. These enable their known current-carrying capacity in the superconducting state to be interpreted in terms of the response of phase structure to metallurgical treatment.
Abstract: Single-crystal x-ray results are given for specimens of Ti-207 at.%Nb. These enable their known current-carrying capacity in the superconducting state to be interpreted in terms of the response of phase structure to metallurgical treatment, and especially in terms of the transformation products obtained by quenching from the β phase (body-centred cubic) and subsequent ageing. Strain from heavy cold-work is shown to favour the precipitation of particles of the equilibrium hexagonal phase, α-Ti, on subsequent ageing. The α-Ti precipitate is believed to contribute to flux pinning. Quenching from the β phase causes transformation to αdouble prime, an orthorhombic metastable phase, which can maintain high currents. Ageing at low temperatures for short times causes reversion of αdouble prime to β with a fall in current-carrying capacity. Further ageing brings about the β->ω transformation, where ω is a hexagonal metastable phase, and the appearance of ω can be correlated with a recovery of current-carrying capacity. A new metastable phase, with a tetragonal structure, is reported for Ti-25 at.% Nb in the as-worked condition.
TL;DR: The Monte Carlo model for electron scattering described in an earlier paper has been used to calculate the absorption and back-scattering corrections met in electron-probe x-ray microanalysis as mentioned in this paper.
Abstract: The Monte Carlo model for electron scattering described in an earlier paper has been used to calculate the absorption and back-scattering corrections met in electron-probe x-ray microanalysis. Although agreement with experimental data is on the whole good, the calculated values for the correction factors are not sufficiently accurate for general use. However, in the case of light element analysis where very high absorption corrections are needed, the corrections calculated from Monte Carlo data are the best available at present. Values for the back-scattering correction factor R calculated for incident beam angles 225° and 45° to the normal surface are given.
TL;DR: In this paper, the polarization features of light reflected by soil, desert sand, white sand and water under different conditions of illumination with natural (unpolarized) and linearly polarized light have been investigated in three narrow spectral intervals (bandwidth similar 150 A) centred on wavelengths 3975, 5000 and 6050 A.
Abstract: The polarization features of light reflected by soil, desert sand, white sand and water under different conditions of illumination with natural (unpolarized) and linearly polarized light have been investigated in three narrow spectral intervals (bandwidth similar 150 A) centred on wavelengths 3975, 5000 and 6050 A. A simple `rotating-analyser' type photoelectric reflectometer was used in the measurements. The data were acquired in computer compatible format to facilitate Fourier analysis of the photosignal. The degree of polarization and relative intensity variations have been determined from a knowledge of the Fourier coefficients. The polarization of light reflected by soil, desert sand and white sand exhibits pronounced wavelength dependence. There is over-all similarity in the behaviour of soil and desert sand. White sand shows a strong tendency to behave like an ideal diffuser (Lambert surface). The hypothesis of scattering of light by an `optically rough, locally smooth' surface has to be invoked to explain the reflection characteristics of an apparently smooth surface of water.
TL;DR: In this paper, the refractive index and absorption coefficient of single crystals of SnSe2 grown by the vapour transport technique were measured and the energy gap was found to be 100±003 ev, due to forbidden indirect transitions.
Abstract: Measurements are reported of the refractive index and absorption coefficient of single crystals of SnSe2 grown by the vapour transport technique. The energy gap is found to be 100±003 ev, due to forbidden indirect transitions. The crystals are n-type with an electron mobility of 33 cm2 V−1 S−1 at room temperature. A value for the electron effective mass is estimated as (04±02) m0.
TL;DR: In this paper, the dark-current statistics and other properties are measured for an International Telephone and Telegraph FW 130 photomultiplier both cooled and at room temperature, and the tube was found to have a constant non-thermal residual count rate.
Abstract: In order to measure low light flux with a photomultiplier tube it is necessary to know the properties of the dark current of the tube. In this paper the dark-current statistics and other properties are measured for an International Telephone and Telegraph FW 130 photomultiplier both cooled and at room temperature. The tube was found to have a constant non-thermal residual count rate. The pulse height distribution of these non-thermal dark-count pulses was shown to be the same as for the room-temperature dark current and to correspond to electrons emitted from the cathode. The dark-count pulses were shown to have a Poisson counting distribution both at room temperature and when the tube was cooled. Finally, having demonstrated that the cooled dark current was random, a low light flux giving 0?043 photoelectrons/s was detected in the presence of a cooled dark current (at maximum efficiency) of 0?459 counts/s.