bluuebunny/crossref_metadata_2025 huggingface dataset:
The observation of critical behaviour in the thermal expansion of PbZr0.9Ti0.1O3
Authors: R Clarke  , A M Glazer
Published: July 2002

The lattice parameters have been determined as a function of temperature for single crystals of the ferroelectric PbZr0.9Ti0.1O3. By associating the difference in rhombohedral angle with the square of the order parameter, Ps, the critical index beta has been determined for the ferroelectric to paraelectric phase transition. This transition appears to be of second order with beta approximately=0.33 for (T0-T)/T0<or=0.06. This difference from the mean-field value ( beta =0.5) suggests the presence of critical fluctuations in the order parameter. Outside the critical region the order parameter deviates even further from mean-field behaviour. This is explained in terms of further fluctuations associated with a transition at a lower temperature.
A study of amplified acoustic flux in n-GaAs by Brillouin scattering
Authors: R S Sussmann  , B K Ridley
Published: July 2002

Using Brillouin-scattering techniques, the geometrical and spectral evolution of the acoustic flux is studied amplified from the background noise in n-type GaAs under the influence of an external electric field. Measurements are performed in oxygen-doped material over the temperature range 190-240K which corresponds to a resistivity range of about 30 to 2 Omega cm. The growth of the acoustic flux can be distinctly separated into a linear and a nonlinear regime. Some of the results are discussed in terms of some recent theories of parametric processes in the acoustoelectric effect. The validity of the phenomenological current-controlled model is also discussed. The evolution of the flux at high resistivities for long voltage pulses has been studied.
High-frequency magnon damping in Heisenberg antiferromagnets
Authors: K P Bohnen  , C R Natoli  , J Ranninger
Published: July 2002

The damping of zone-boundary magnons has been calculated for rutile and perovskite structure Heisenberg antiferromagnets for temperatures up to 0.8 TN. The formula obtained for the half width in terms of the temperature-dependent zone-boundary magnon frequency and the number of nearest-neighbour spins on opposite sublattice agrees well with the experimental data.
The choice of 'muffin-tine' sphere radii in APW band-structure calculations of VC
Authors: K Schwarz  , K Pechter  , A Neckel
Published: July 2002

A new self-consistent band-structure calculation for stoichiometric vanadium carbide is performed by means of the augmented plane wave method using a ratio of atomic sphere radii RC/RV=0.645. The results are compared with a similar calculation based on the ratio 0.899. It is demonstrated how the choice of sphere radii affects the potentials, the band structure and the charge distribution. A recommendation for determining the atomic sphere radii is given.
Electron-electron interaction, band-tailing and activity coefficients in doped compensated semiconductors
Authors: D C Herbert  , D T J Hurle  , R M Logan
Published: July 2002

Self-consistent calculations of band-tailing in doped compensated semiconductors have been performed. The inter-particle interactions are approximated by screened Coulomb interactions and the carrier density is allowed to respond to the fluctuations in impurity density. This leads to fluctuations in the screening length and in the screened Hartree-Fock contribution to the electronic potential. the fluctuations in carrier concentration considerably reduce the magnitude of the band-tailing. In fully compensated material, large potential fluctuations are obtained and the theory suggests that metallic droplets of the type proposed by Shklovskii and Efros may form at very low temperatures or very high doping levels. It is found that electronic activity coefficient effects are very weak, until the Fermi level is strongly affected by Fermi statistics.
Specific volume studies of the nematic liquid crystal: EBBA (and u.s. velocity)
Authors: B Bahadur  , S Chandra
Published: July 2002

The temperature variation of the specific volume of the nematic liquid crystal (p-ethoxybenzylidene)-p-n butylaniline (EBBA) has been recorded both in isotropic and nematic phases. The nematic-isotropic phase transition is concluded to be a first-order phase transition from the specific volume studies. Pre-transitional variations are found to occur only on the nematic side of the transition and can be explained in best manner on the basis of Maier-Saupe theory (1958). Values of A and Skare 17.9*10-9erg cm6and 0.450 respectively. Adiabatic compressibility ( betaad), Rao number (Rn) and van der Waals constant (b) have also been calculated from measurements of the ultrasonic velocity betaadand Rnalso show pre-transitional phenomena.
A LEED study of MgO(100). III. Theory at off-normal incidence
Authors: C G Kinniburgh
Published: July 2002

For pt.II see ibid., vol.8, p.2382 (1975). Dynamical LEED I(V) calculations have been performed for the MgO(100) surface for eight different off-normal incidence geometries. Using parameters identical to those which were found to give good agreement with experiment at normal incidence, similar good agreement is found over all the forty-eight spectra comprising the extended set of data. The result found previously, which was that the MgO(100) surface is little distorted from a simple termination of the bulk crystal, is confirmed, thereby emphasizing the value of the more economical normal incidence analysis.
Superfluid density and viscosity of 3He-A and B
Authors: P C Main  , C W Kiewiet  , W T Band  , J R Hook  , D J Sandiford  , H E Hall
Published: July 2002

The authors have used a torsion pendulum to measure the superfluid density and viscosity of liquid 3He at pressures of 18.2, 26.0 and 29.7 bar. It was found that rho s in the A-phase is not reproducible; the range of variation is about a factor of two, and large amplitudes tend to give low rho s, as would be expected for orientation of the l-vector by flow. In the B-phase, rho s is about 20% greater than any value found in the A-phase at the same reduced temperature. The viscosity below about 0.9 Tc is almost independent of pressure at a given value of (T/Tc); it is slightly lower in the B-phase than in the A-phase.
Layer-by-layer low-energy electron diffraction and two-dimensional space group symmetry
Authors: J Rundgren  , A Salwen
Published: July 2002

The wave field of LEED in a 2D structure is symmetrised with respect to the structure's 2D space group (2D structure meaning diperiodic structure composed of one or several 2D periodic overlayers lying in register on the 2D periodic face of a crystal). Attention is focused on the layer-KKR (Korringa-Kohn-Rostoker) method, the wave representations of which are beams outside layers and partial waves inside layers. It is shown how the field of beams and the field of partial waves are made globally symmetric representations of the 2D space group, how the 2D translations of an individual layer (giving rise to local 2D von Laue diffraction conditions) divide the beams into independent bundles and, further, how local point-group symmetry arranges the bundles in 'irreducible sequences' and combines the partial waves into 'irreducible showers'. The theory is applied to three examples of overlayer structures on surfaces of face-centered cubic crystals, and the coding of the scheme of symmetrisation is discussed.
The structure factor in liquid Sn
Authors: D Jovic  , I Padureanu
Published: July 2002

The structure factor in liquid Sn has been measured, using the neutron diffraction technique, in the temperature range from the melting point up to 870 degrees C. A wide region of momentum transfer 0.5 AA-1<Q<14 AA-1, has also been covered. Diffraction patterns show the presence of a double structure up to 500 degrees C. Comparison is made with the theoretical structure factor obtained from the hard-sphere interaction potential. From the experimental structure factor, the atomic radial distribution functions are derived.
Degeneration of ferromagnetic phase in films
Authors: E Praveczki
Published: July 2002

The anisotropic Heisenberg model of ferromagnetic films is investigated using the Green function method. A new type of approximation is applied for renormalizing the spin-wave energy. The renormalizing factor accounts for the magnetization and also for the transverse fluctuation of the magnetization, which makes it possible to describe the model correctly at all values of the anisotropy parameter, Delta , and the number of magnetic layers, n. In the special case of Delta =0 and n=1 (two-dimensional isotropic Heisenberg model), the approximation leads to the formulae obtained by Mubayi and Lange (1969) for describing the Stanley-Kaplan phase. It is concluded that the degeneration of the ferromagnetic phase (towards the Stanley-Kaplan phase) becomes observable when n<30 and Delta <0.001, simultaneously.
Capillary models for porous media I. Two-phase flow in a serial model
Authors: D Nicholson  , J H Petropoulos
Published: July 2002

The behaviour of a gas phase and combined gas and surface phase flow in a model microporous medium is examined. The model system consists of a bundle of cylindrical tubes each of which is constructed from a series of sections which have radii chosen from a distribution function. Structure factors are derived and calculated for this medium using mainly chi-square and Gaussian radius distribution functions, and their variation with the properties of the flow system is discussed. It is concluded that the particular aspect of pore structure examined in this paper is of considerable importance in the interpretation of surface flow results.
Thermoelectric and mean free path effects in thin metal films
Authors: G W Wilson
Published: July 2002

Measurements of the thermoelectric power and thickness of metal films can be used to calculate the mean free path of the conduction electrons without assumption of the bulk resistivity of the metal used in producing the films. The dependence of this mean free path on electron energy can also be found.
The epitaxial growth of zinc sulphide on silicon by vacuum evaporation
Authors: P L Jones  , C N W Litting  , D E Mason  , V A Williams
Published: July 2002

Epitaxial layers, up to 5 μm thick, of zinc sulphide have been grown on thermally cleaned (111) surfaces of single-crystal silicon disks. The layers have been shown by x-ray and electron diffraction to be single-crystal and a continuation of the silicon lattice. The importance of producing a clean silicon surface and of maintaining constant temperatures during the deposition has been demonstrated.
Formation and structure of the potassium
graphites
Authors: D E Nixon  , G S Parry
Published: July 2002

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=26·25 Å

stage 3
ABA| ACA| A
24·20 Å

stage 4
ABAB| BCBC| CACA| A
46·35 Å

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.
The electrical conductivity of magnesium oxide at low temperatures
Authors: T J Lewis  , A J Wright
Published: July 2002

The electrical conductivity of magnesium oxide single crystals has been measured for the first time in the temperature range 400-750°C. Results agree well with those obtained by other workers at higher temperatures and a single activation energy of about 2·8±0·1 ev seems to apply from 400 to 1800°C. Fields varying from 0·4 to 500 kv cm−1 were used and the maximum fields for ohmic conductivity were determined. Similar measurements were made on polycrystalline specimens with a large impurity content. The effects of impurities and the role of the electrodes are considered. The process of conduction at low temperatures is not yet fully understood, but it may be controlled by electrons hopping between impurity sites in the crystal rather than by oxygen ion transport as has previously been suggested.
A low-energy ion source for the deposition of chromium
Authors: B A Probyn
Published: July 2002

The feasibility of depositing metal films from a well-defined beam of positively charged chromium ions has been investigated. The design and development of an ion source suitable for this work are described and problems related to the extraction, focusing and deflection of such an ion beam are considered.

It is found that the desired focusing and deflection conditions cannot be achieved in practice because of certain fundamental limitations:

(i) A low-energy beam of less than 500 ev is necessary to obtain a deposit and prevent damage to the substrate by sputtering. Thus space-charge effects become predominant, causing spreading of the beam. (ii) A lens system having a very short focal length is necessary to give a high beam convergence angle. This limits the deflection obtainable without loss of definition. (iii) Energy spread within the beam may also contribute to poor focusing.

A minimum beam diameter of 0·5 mm has been obtained from an ion beam of energy 230 ev having a current density of 8 μA mm−2. Under these conditions the condensation coefficient was found to be approximately 25%, giving a deposition rate of 100 Å min−1.
Friction, wear and surface hardness in abrasion of binary solid-solution alloys in the copper-gold system
Authors: D S Lin  , H Wilman
Published: July 2002

The friction coefficient μ, the mass wear rateMand the surface micro-hardnessHDare measured in abrasion of Cu-Au alloys (in the fully abraded, and thus fully work-hardened, surface state), sliding on dry `smooth-cut' steel files at a load of about 1 kg and a speed of a few cm s−1.

μ andM/ρ (where ρ is the density) are found to be nearly the same as for a pure cubic metal of similarHD, and vary with alloy composition in a nearly parabolic manner analogous to that found by Sachs and Weerts for the shear stress of Ag-Au alloys. The deviations Δμ and Δ(M/ρ) from a Vegard's-law type of linear dependence on composition are approximately proportional to each other and to|Δa|, the lattice dimension deviation from Vegard's law, but they vary non-linearly with ΔHD. Similar results are indicated for the Cu-Zn and Cu-Ni alloy systems. On this basis the probable μ,M/ρ andHDcan be estimated for abrasion of face-centred-cubic alloys of other binary solid-solution systems. The stored energy in the abraded alloy surface regions is also estimated from μ andM.
Thermomagnetic effects in cadmium arsenide
Authors: D W G Ballentyne  , D R Lovett
Published: July 2002

Adiabatic and isothermal thermomagnetic coefficients have been obtained for cadmium arsenide in the temperature range 65-300°K. By assuming acoustic scattering and a single parabolic conduction band model, agreement between experiment and theory has been demonstrated over most of the temperature range, but results indicate the presence of mixed acoustic and impurity scattering at the lower temperatures. However, the measured values of thermal conductivity cannot be entirely explained as a function of temperature since there is an anomalous increase at higher temperatures.
Sub-critical measurements of buckling and migration area using spontaneous fission as the primary neutron source
Authors: M Ahmad  , M J Harris
Published: July 2002

Using spontaneous fission as the sole primary neutron source, measurements have been made of the lattice constants of a sub-critical assembly fuelled with natural uranium rods of 1·2 in. diameter and moderated by water (V
m/V
u=1·43). The experimental procedure is described and a simple two-group analysis is developed for interpreting the measurements. The thermal neutron fluxes were low, being of the order of 25 neutrons/cm2s; nevertheless the buckling was determined as -(6·80±0·42) 10−4 cm−2, a value in good agreement with that obtained from conventional exponential experiments.
Laser-induced electron emission from tungsten, tantalum and lanthanum hexaboride using ruby and argon lasers
Authors: L G Pittaway  , J Smith  , E D Fletcher  , B W Nicholls
Published: July 2002

A description is given of experiments carried out using pulsed ruby and argon lasers to investigate the possibility of producing pulses of high electron emission current densities from tantalum, tungsten and lanthanum hexaboride cathodes heated by a focused laser beam. Current densities as high as 104 A cm−2 were recorded using the ruby laser but subsequently it was shown, using the repetitively pulsed argon laser, that electron current densities of this order were obtained only when the cathode material was being melted. Lanthanum hexaboride was found to undergo changes on the surface when repetitively pulsed and was rejected in favour of the pure metals tungsten and tantalum. From these metals stable emission currents up to 1 mA peak were recorded from spots approximately 15 μm in diameter (i.e. 570 A cm−2) for several hours without incurring surface damage. Current density values of this order are near to those calculated from the Richardson equation for temperatures just below their melting points.

Experimental results have confirmed theoretical predictions regarding the delayed onset of emission, the delay in the peak emission relative to the peak light intensity and the extremely rapid termination of the emission pulse.
Far infra-red Faraday effect in rare-earth iron garnets
Authors: P G Frayne
Published: July 2002

The optical and magneto-optical properties of polycrystalline spinel ferrites and single-crystal rare-earth iron garnets have been determined at a wavelength of 337 μm using a HCN gas maser source. The relative performance of these materials as Faraday rotators in the far infra-red is discussed and the observed rotations are compared with theoretical predictions.
The effect of hydrogen on the electrical properties of
rutile
Authors: G J Hill
Published: July 2002

Hydrogen in rutile has two influences on the electrical properties:

Verneuil crystals contain hydrogen compensating the trivalent impurities.
Excess hydrogen acts as an electron donor, thereby increasing the
conductivity.

Removal of hydrogen above 650°C in vacuum causes structural changes
accompanied by enhanced conductivity and dielectric loss. There is also a
change in the activation energy for diffusion at this temperature. Values of
the coefficient of diffusion of protons in the temperature range 450-800°C
in crystals of differing impurity content are presented.

The position of the hydrogen, in the basal plane of the rutile lattice, is confirmed
by the dichroism of the absorption. Weight-loss experiments suggest that hydrogen
leaves the crystal in combination with oxygen in the proportions required to form
water.
The expansion of a laser-produced aluminum plasma and the heat penetration in multi-layered targets
Authors: A Zigler  , H Zmora  , Y Paiss  , J L Schwob
Published: July 2002

H-like and He-like Al lines and their satellites are obtained from Al and polyethylene layered targets irradiated by a Nd:glass laser at a power flux of 2*1013W cm-2on target. Two-dimensional spatial resolution is achieved using a flat crystal spectrograph incorporating a wire shadow technique. Expansion profiles in the lateral direction as a function of the Al layer location are measured and interpreted in terms of the temporal history of the interaction. By varying the location of the Al layer, the penetration depth of the heat wave front is also determined. It is found that in this power flux range, heat conduction can be explained by classical mechanisms.
Dissociative attachment coefficient in oxygen
Authors: K Masek  , L Laska  , T Ruzicka
Published: July 2002

The Boltzmann equation has been used to calculate the dissociative attachment coefficient in oxygen for E/N=10-200 Td and this has been compared with observations.
The development of electron avalanches in argon at high E/N values. I. Monte Carlo simulation
Authors: Y Sakai  , H Tagashira  , S Sakamoto
Published: July 2002

The behaviour of electron avalanches in argon when appreciable electron impact ionization occurs, is studied by a Monte Carlo simulation. The values of electron swarm parameters are obtained for steady-state Townsend, pulsed Townsend and time-of-flight experiments, by applying sampling techniques appropriate to the respective experiments. The results suggest that the value of an electron swarm parameter, such as the ionization frequency or the electron drift velocity, depends on the type of experiments for which there is appreciable electron impact ionization. The properties of electron avalanches in an electron energy non-equilibrium region, for example the variation of the electron energy distribution with distance from the cathode, and the effect of anisotropic scattering of electrons on the swarm parameters very close to the anode, are also studied.
An experimental study of the Timoshenko's shear coefficient for flexurally vibrating beams
Authors: T Kaneko
Published: July 2002

The validity of the shear coefficient K has been experimentally tested using a large number of rectangular glass beams and cylindrical phosphor-bronze beams. The theoretical expressions of K=(5+5m)/(6+5m) for rectangular beams and K=(6+12m+6m2)/(7+12m+4m2) for cylindrical beams, where m is Poisson's ratio, have been confirmed to yield the best agreement with experiment. For cylindrical beams, Timoshenko's theory has been shown to give good results over a fairly wide frequency range. For rectangular beams, it is suggested that a better expression should be proposed to fit the experimental results.
An optical method for measuring temperature and electric field in semiconductors
Authors: K G Germanova  , K P Marinova  , S D Stoianov
Published: July 2002

An optical method is proposed for measuring the spatial and time distributions of temperature and electric field in semiconductors. The method is based on the temperature and electric field shift of the fundamental absorption edge and has some advantages over known methods used in investigations of the above-mentioned characteristics. The optical method principle, the limits of its applicability and the sensitivity of the method are described. Experimental results for CdS crystals and examples for other applications are given.
Relaxation process of the electron velocity distribution in neon
Authors: K Kitamori  , H Tagashira  , Y Sakai
Published: July 2002

The relaxation process from an initial velocity distribution to the equilibrium distribution for electrons in neon is calculated by a finite difference method for the ratios of electric field to gas number density E/N between 56.6 and 566 Td (E/p0=20 and 200 V cm-1Torr-1at 0 degrees C) without using the usual two-term spherical harmonics expansion of the velocity distribution. The pulsed Townsend condition, in which the evolution of all the electrons involved in an avalanche is observed as a function of time only, is assumed. The results suggest that the electron velocity distribution reaches through randomisation the equilibrium distribution which has a structure with a minimum near the origin in the velocity space.
Re-thermalisation and flow of laser-produced plasmas in a uniform magnetic field
Authors: S Sudo  , T Sekiguchi  , K N Sato
Published: July 2002

Detailed time- and spatially-resolved measurements of the electron temperature and plasma density have been made, by means of laser Thomson scattering and other plasma diagnostics, for plasmas produced by laser from a thin wire target in an almost uniform magnetic field ( approximately 1 T). The physical processes involved in plasma/magnetic-field interactions are discussed. It has been found that a part of the translational kinetic energy of the ions in an earlier rapid plasma expansion phase is reconverted into the thermal energy of electrons as the results of the interactions, through the ohmic heating, by a circulating diamagnetic current flowing near the plasma outer boundary, as well as through subsequently shock heating (and additional compression). Also, the plasma expansion along magnetic field lines may be regarded as almost one-dimensional, and the time variation of electron temperature is adiabatic.
Carbon, oxygen and silicon impurities in gallium arsenide
Authors: M R Brozel  , R C Newman  , J B Clegg
Published: July 2002

The concentrations of carbon, oxygen and silicon, together with boron, aluminium, sulphur and potassium, in various samples of bulk gallium arsenide have been determined by spark source mass spectrometry (SSMS) calibrated by independent chemical methods. Hall effect measurements and localised vibrational mode (LVM) infrared absorption measurements have been made on the same samples. It is inferred that the carbon atoms which show LVM absorption are located on arsenic lattice sites and act as acceptors. Calibrations for the LVM lines for CAS, SiGa, AlGaand BGahave been obtained. All samples examined contained a very low concentration of oxygen in the range (1-4)*1015cm-3. Carbon concentrations of up to 1017cm-3were measured in deliberately doped material, while other crystals contained levels of 5*1015cm-3or lower.
The influence of laminar and turbulent flows upon the electrical characteristics of wall-stabilised arcs
Authors: R J Hill  , G R Jones
Published: July 2002

Measurements are reported of the electrical properties of wall-stabilised arcs subject to an imposed axial flow of nitrogen under steady current conditions. The current was in the range 10-60 A DC and the gas flow rate was 0-0.6 g s-1with stabilising bore radii of 2.5 mm and 5 mm. The results of the investigation confirm the existence of two axial zones corresponding to a flow entrainment region and a fully developed column region. The electric field strength along the fully developed region increases with mass flow rate as turbulent effects become more pronounced. An analytical arc model, based upon boundary layer approximations and taking account of molecular and turbulent radial diffusion and axial convection, is presented. Results of calculations using this model predict the trends observed experimentally.
The thermally modulated reflectance of gadolinium, terbium and dysprosium
Authors: R F Miller  , M Syms  , L S Julien
Published: July 2002

The thermoreflectance dispersion curves of opaque films of Gd, Tb, and Dy, have been measured at 315K and 97K over the range 1.45 to 3.2 eV. The response at 315K has been shown to be largely determined by the optical functions alphapand betapwhich relate the thermoreflectance response to Delta epsilon1and Delta epsilon2. When the temperature is lowered from 315K to 97K, pronounced structure develops in the spectra at 1.7 eV for Tb, 2.2 eV for Dy, and 1.6 and 2.8 eV for Gd; weaker structure is also evident at 2.0 eV for Gd. Measurements of the thermoreflectance as a function of temperature show the existence of anomalies at the ferromagnetic Curie point of Gd, and at the Neel points of Tb and Dy.
Measurement of the effective ionisation coefficient and the static breakdown voltage in SF6and nitrogen mixtures
Authors: H Itoh  , M Shimozuma  , H Tagashira  , S Sakamoto
Published: July 2002

The effective ionisation coefficient alpha /p20is measured for E/p20from 70 to 160 V cm-1Torr-1, and the static breakdown voltage Vsbetween parallel plates for p20d from 8 to 130 Torr cm, for SF6and nitrogen mixtures. It is found that the alpha /p20of the mixtures may be approximated by a linear function of the partial pressure of SF6at fixed E/p20and that Paschen's law holds for the p20d may be estimated with good accuracy from the Vsof the respective pure gases using an experimental formula of Takuma et al. (1972) which has been used for much larger p20d.
Molecular ion and metastable atom formations and their effects on the electron temperature in medium-pressure rare-gas positive-column plasmas
Authors: Y Ichikawa  , S Teii
Published: July 2002

A theoretical study has been made of the medium-pressure rare-gas positive-column plasma by taking into account the effects of molecular ion formation, volume recombination and ionisation due to metastable atoms, simultaneously. Calculated values of the electron temperature and the densities of metastable atoms, molecular and atomic ions, at various gas pressures for both He and Ne gases are presented. Numerical results show that in a rare-gas positive-column plasma, molecular ions become dominant in the discharge as the gas pressure increases above approximately 10 Torr. Theoretical values are compared with the experimental data and discussed. A new method to determine the three-body conversion rate coefficient is also proposed.
Measurement of the permittivity of nematic liquid crystals in magnetic and electric fields using extrapolation procedures
Authors: M G Clark  , E P Raynes  , R A Smith  , R J A Tough
Published: July 2002

An analysis of the continuum equations of nematic layers subject to large electric or magnetic fields leads to simple extrapolation procedures which allow the infinite field capacitance of the layers to be inferred. For a magnetic aligning field H the reciprocal of the capacitance should be plotted against 1/H, whereas for an aligning electric voltage V the capacitance should be plotted against 1/V except in certain cases where there is a large change in the dielectric permittivities between the frequencies of the aligning and measuring electric fields, when a plot of the reciprocal of the capacitance against 1/V should be used. The optimum experiments for accurate routine measurement of both principal electric permittivities of a small sample of nematic using a single cell are identified and two experimental arrangements described.
Electron swarm parameters in argon and krypton
Authors: H N Kucukarpaci  , J Lucas
Published: July 2002

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<or=E/N<or=566 Td for argon and 14<or=E/N<or=849 Td for krypton (1 Td=10-17V cm2). The results obtained from Monte Carlo simulation have been compared with the present and previously obtained data and a good fit has been achieved for the entire range of E/N. The simulation also produced results for the ionisation coefficient and the ratio of radial diffusion coefficient to mobility and these values have also been compared with other theoretical and experimental values, where available. The mean energy has also been computed and compared with the experimental breakdown voltage. This comparison has shown that equilibrium conditions exist in the region in which the present measurements have been made. As a result of this investigation, two sets of cross-sections are given which accurately simulate electron swarm parameters in argon and krypton respectively.
Non-linear mechanical response of various metals. VIII. Large elastic-plastic deformation leading to failure in simple torsion
Authors: E W Billington
Published: July 2002

For pt.VII see ibid., vol.14, p.1293 (1981). The Swift effect has been considered in relation to the simple torsion mode of deformation. The parameters appearing in a particular increment type constitutive equation for an isotropic incompressible strain hardening elastic-plastic solid with large deformation have been computed assuming a particular form of piece-wise continuous yield condition. These parameters are considered, where possible, in terms of modified Lobes variables. The Swift effect is also considered in relation to the fracture of various metals twisted in simple torsion. A tentative failure criterion for ductile materials is considered in relation to particular values of two of the parameters associated with the Swift effect.
Electron transport studies in SiO2films excited by a pulsed electron beam
Authors: D M Taylor  , A A Al-Jassar
Published: July 2002

Charge transport in steam-grown thin films of silicon dioxide (SiO2) is studied using a short-duration pulse of energetic electrons for generating hole-electron pairs in the oxide. Within the time resolution of the measuring circuitry ( approximately 1 ns) electron transport follows the excitation pulse exactly. It is shown that electron-beam-induced currents in SiO2are limited by geminate or columnar recombination processes and that the incorrect application of the normal photoconduction equation to experimental data leads to invalid thickness-dependent mobility-lifetime products ( mu tau ). The present measurements indicate that for electron mu tau >>1.2*10-13m2V-1and mu >1.2*10-4m2V-1s-1.
Magnetic domains in multilayered amorphous Co-P alloys
Authors: J M Riveiro  , M S Guijarro  , G Rivero  , M C Sanchez
Published: July 2002

A thorough study has been made of the magnetic structure of electrodeposited multilayered amorphous Co-P alloys. An unconventional electrolytic process has been used, employing a rectangular alternating current. The samples produced in this way are multilayered, and these layers are separated by very thin boundaries that have a greater P concentration. The results suggest that there is 'exchange interaction' between the layers. It has also been observed that the period and amplitude of the zig-zag of the 'charged walls' is strongly affected by a small energy variation. The origin of the in-plane magnetic anisotropy of the samples has also been investigated. Magnetic anisotropy is very susceptible to magnetic annealing. The results suggest that the authors are dealing with anisotropy induced by directional order.
Dielectric behaviour of isocyanate-terminated polymers
Authors: J G Krishna  , O S Josyulu  , J Sobhanadri  , R Subrahmaniam
Published: July 2002

The dielectric behaviour of isocyanate-terminated polymers is reported in this paper as a function of frequency (100 Hz-100 kHz), temperature (30-90 degrees C) and the concentration of the curing agent, 4,4'-methylene bis(ortho-chloroaniline) (MOCA) (1.2, 3.5 and 4.7%) which was used in preparing the polymer from the toluene di-isocyanate and castor oil. The dielectric constant and the loss were found to decrease as the MOCA concentration was increased. Distinct loss peaks were obtained. The influence of the curing agent on the dielectric properties of the polymer is discussed in detail. The most probable relaxation times were calculated from the complex arc plots of epsilon 'and epsilon ". From these arc plots it is established that the distribution of relaxation times becomes more and more symmetric with the increasing temperature. Activation energies were calculated. Maxwell-Wagner-Sillars interfacial polarisation is proposed as the probable mechanism.
Doping and electrical properties of Cd-doped crystals and LPE layers of Pb1-xSnxTe
Authors: E Silberg  , A Zemel
Published: July 2002

The doping characteristics and electrical properties of Cd-doped bulk crystals and LPE layers of Pb1-xSnxTe, 0<or=x<or=0.25, were studied using Hall-effect, resistivity and Cd-solubility measurements. The bulk crystals were doped by Cd diffusion in a two-temperature-zone furnace and the LPE layers by adding a small percentage of Cd to the growth solution. The behaviour of Cd in Pb0.8Sn0.2Te depends on the Cd concentration. At low Cd concentrations the Cd atoms compensate the acceptor metal vacancies of the undoped material, most probably by occupying the metal vacancy sites. At medium Cd concentrations most of the Cd atoms incorporate in the crystal as an electrically inactive impurity and only a small fraction act as active donors. At high Cd doping the electron concentration n saturates to a constant value independent of the thermal history of the sample prior to diffusion, independent of the carrier type and concentration of the doped material, independent of the doping procedure and independent of the doping temperature. Possible explanations for the saturation in n are discussed. At high Cd doping the low-temperature mobility is found to be limited by scattering of electrons by neutral Cd centres distributed uniformly throughout the crystal lattice. For low Cd doping, ionised native defect scattering with the inclusion of electrical compensation is the dominant scattering mechanism below 40K.
Mie calculations of the scattered light from a spherical particle traversing a fringe pattern produced by two intersecting laser beams
Authors: C R Negus  , L E Drain
Published: July 2002

Mie scattering calculations are described for a transparent spherical particle traversing a stationary fringe pattern produced by two intersecting laser beams as in a conventional, real fringe, LDA system. The scattered intensity and its modulation (visibility) are calculated for a range of different optical geometries and states of polarisation of the beams. In forward scatter the visibility is shown to be virtually independent of polarisation and refractive index, but strongly dependent on the geometry of the optical set-up. Optimum on-axis apertures incorporating stops are calculated for particle sizing. The authors demonstrate numerically that the dynamic range can be extended by the use of an off-axis aperture, or by the use of the intensity function which varies smoothly with particle size. In back scatter the authors show that the visibility technique can in fact be used for particle sizing provided the correct polarisations are used. The intensity function however shows strong irregularities in back scatter due to interference effects and cannot readily be used with monochromatic beams. A regularity in the back-scattered intensity distribution has been found for circularly polarised light and a simple explanation has been given. A particle sizing technique based on this principle is shown to be theoretically feasible.
Selfconsistent calculation of the hydrogen plasma parameters. II. Transient plasmas
Authors: N N Ljepojevic  , J M Labat
Published: July 2002

For pt.I see ibid., vol.15, p.621 (1982). For the particular case of a positive column glow discharge hydrogen plasma perturbed by a nanosecond pulse, the system of kinetic equations that includes also equations for electronic concentration and temperature has been numerically solved. One of the remarkable results is that the electron temperature, after initial rapid rise due to the perturbation, drops below the stationary value during the relaxation. The drop is more marked with stronger perturbation, and occurs earlier at higher initial gas pressures.
Calculation of SF6transport coefficients from revised data
Authors: J P Novak  , M Frechette
Published: July 2002

The transport coefficients of SF6have been recalculated in the range of reduced field between 35 and 225 V cm-1Torr-1using the latest cross-section data. While fitting of calculated and experimental ionisation coefficients has been used to determine missing data on excitation cross-sections, calculations of the drift velocity and the diffusion and attachment coefficients provided a means of checking the consistency of the model and the data. The calculated drift velocity proved to be about 20% lower than the experimental value. The diffusion coefficients agreed well but the attachment coefficients compared poorly with measured values. Good fitting was obtained by reducing the width of the SF6-attachment cross-section to 30% and increasing the high-energy attachment cross-sections by about 30%.
Ionisation growth in gases in strong uniform electric fields
Authors: L Friedland  , Yu M Kagan
Published: July 2002

The theory of electron multiplication in gases at large values of E/P is extended to cases where the characteristic energies of individual electrons are of the order of 102eV or more. The first Townsend ionisation coefficient is found analytically and is compared with existing experimental data.
A model for the cathode mechanism in low-current metal vapour arcs
Authors: B E Djakov
Published: July 2002

An analytical model is proposed for the cathode spot of a metal vapour arc. A number of physical parameters at the cathode surface and at the boundary of the cathodic plasma such as macroscopic velocity, temperatures, fluxes, particle densities and electric field can be determined from equations for conservation of electric charge, mass, momentum and energy, the equation for ion current to the cathode, bipolar space charge equation, Saha equation, electron emission equation, and evaporation equation. The cathode drop voltage and the current density serve as independent variables. The multiply charged ions are taken into account, and the jumps of some quantities over the first mean free path-due to non-equilibrium evaporation-are incorporated into the model. Numerical calculations are carried out for low-current copper vacuum arcs. The results are compared with experimental data.
The ionisation coefficient in Ar-Hg mixtures
Authors: A L J Burgmans  , A H M Smeets
Published: July 2002

The ionisation coefficient in Ar-Hg mixtures has been measured for E/n values between 6*10-21V m2and 2*10-18V m2and for a mercury atom fraction varying from 2*10-4to 2*10-2. The experiments were performed with a parallel-plate apparatus in which the current is measured as a function of the plate separation at constant electric field. The ionisation coefficient was found to be a function of E/n and the mercury atom fraction only. Small admixtures of mercury lead to a large increase in the ionisation coefficient due to Penning ionisation and associative ionisation. From the experimental results the effective rate coefficient for excitation to the appropriate argon states is obtained as a function of E/n, as well as the ratio of the rate coefficients for depopulation of these excited argon states by collisions with argon and mercury atoms respectively.
Departures from CLTE composition in a nitrogen arc at atmospheric pressure. I. Experimental results
Authors: J Bacri  , M Lagreca
Published: July 2002

The positive column of a wall stabilised arc burning in pure nitrogen at atmospheric pressure is spectroscopically studied. The behavior of local population number densities of atomic and molecular excited levels shows deviations from CLTE composition. These departures are mainly related to diffusion phenomena and radiation escape.
Electrical conductivity and charge carrier screening in weakly non-ideal argon plasmas
Authors: K Gunther  , S Lang  , R Radtke
Published: July 2002

A pulsed argon discharge as a stable source of weakly non-ideal plasmas is described in connection with the diagnostic necessities for conductivity measurements. The parameters overlap the range for stationary arcs and allow comparison with measurements in cascade arcs. The measured conductivities are explained using the binary collision model considering collisions with neutrals, excited atoms. A relation between the screening parameter and non-ideality is proposed which should be valid for all elements.
Measurements of the back-scattering and absorption of 15-60 keV electrons for transparent solid films at various angles of incidence
Authors: G Neubert  , S Rogaschewski
Published: July 2002

The fractions of 15 to 60 keV electrons back-scattered from and absorbed in transparent polycrystalline films of Al and Au have been measured at angles of incidence from 0 degrees , i.e. normal incidence to 80 degrees . Subtracting the sum of both these fractions from unity, the fraction of electrons transmitted through a film is determined. The experimental data for back-scattering and transmission are fitted by empirical expressions as functions of film mass-thickness, incident electron energy, and angle of incidence. Results for back-scattering are compared with the predictions of Niedrig's (1981) and Werner's (1982) analytic model.
The ionisation coefficient in Kr-Hg mixtures
Authors: A L J Burgmans  , H J H Merks-Eppingbroek
Published: July 2002

The ionisation coefficient in Kr-Hg mixtures has been measured for E/n values between 2*10-20V m2and 2*10-18V m2and for a mercury atom fraction varying from 8*10-4to 8*10-2. The experiments were performed with a parallel-plate apparatus in which the current is measured as a function of the plate separation at constant electric field. The ionisation coefficient was found to be a function of E/n and the mercury atom fraction only. The increase in the ionisation coefficient for small admixtures of mercury is mainly due to associative ionisation. From the experimental results the effective rate coefficient for excitation of the appropriate krypton states is obtained as a function of E/n, as well as the ratio of the rate coefficients for depopulation of these excited krypton states by nonionising collisions with krypton and ionising collisions with mercury atoms respectively.
Electron swarm parameters in oxygen and methane
Authors: S A J Al-Amin  , H N Kucukarpaci  , J Lucas
Published: July 2002

Experimental measurements of the drift velocity and the ratio of longitudinal diffusion coefficient to mobility for electrons have been obtained for the range of 25<or=E/N<or=848 Td for oxygen and 0.28<or=E/N<or=848 Td for methane (1 Td=10-17V cm2). The results obtained from a Monte Carlo simulation have been compared with the present and previously obtained data and a good fit has been achieved for the entire range of E/N. As a result of this investigation, two sets of cross sections are given which accurately simulate electron swarm parameters in oxygen and methane. The simulation also produced results for the ionisation coefficient and the ratio of radial diffusion coefficient to mobility and these values have also been compared with other theoretical and experimental values, where available. The mean energy has also been computed.
Influence of the sinusoidal field frequency on dielectrophoretic capture of a particle on a rod
Authors: L Benguigui  , A L Shalom  , I J Lin
Published: July 2002

Using a model describing dielectrophoretic capture of a single spherical particle on a single cylindrical rod, it is shown that the time taken by the particle to be collected is a function of the frequency of the applied sinusoidal field. For certain critical frequencies, a kind of anti-resonance is discerned with the time required for collection reaching a maximum. A parametric study of this phenomenon involving the competing forces (drag, gravity) was carried out using a numerical analysis. The phase of the sinusoidal field is also shown to be of great importance.
Boltzmann equation analysis of electron swarm behaviour in monosilane
Authors: Y Ohmori  , M Shimozuma  , H Tagashira
Published: July 2002

The electron swarm behaviour in monosilane is studied for E/p0from 0.2 to 300 V cm-1Torr-1by a Boltzmann equation method. A set of electron collision cross sections is determined by fitting the calculated values of the drift velocity, characteristic energy and effective ionisation coefficient to experimental values. The swarm parameters are calculated for the pulsed Townsend, steady-state Townsend and time-of-flight experiments. Moreover, the accuracy of the two-term approximation is checked by a Monte Carlo simulation. The results show that the calculated values of the effective ionisation coefficient, electron drift velocity and characteristic energy agree well with experimental ones and this suggests that the set of electron collision cross sections determined in the present analysis is an appropriate one, though no unique. It is also found that the electron energy distribution obtained in the present work is represented by a simple Druyvesteyn formula. The excitation frequencies for vibration and dissociation are also calculated and discussed.