Table of contents

The structural stability of amorphous metallic alloys exposed to particle irradiation is of some current interest. In particular the ability of glassy alloys to resist irradiation damage and to avoid induced crystallisation will determine their usefulness in such environments. In the work reported the authors find that a NiTi glass subjected to neutron and proton fluences of 10¹⁷ cm^-2 shows changes in its mode of crystallisation but no change in crystallisation temperature.

The authors prove a theorem that relates the energies of unit transmission through a finite, repeated one-dimensional structure to the band structure of the associated infinite lattice formed by periodic repetition of the one-dimensional structure.

The temperature dependence of the J-V characteristics of an In/poly(N-methylpyrrole) junction device has been measured in order to investigate the junction properties. The barrier height and the effective Richardson constant of the junction have been determined to be 0.38 eV and 1.3*10^-4 A cm^-2 K^-1 by the Richardson plots. The extremely small effective Richardson constant has been due to the existence of the composite layer, acting as a tunnelling one, formed by the reaction of the indium with the polymer surface.

This letter reports the attempts made to conceive and develop solid state varistors with positive voltage coefficient of resistance through particulate composite materials.

The main photoluminescence (PL) peak energies and half-widths of bound-exciton and free-electron-to-acceptor transitions in Al_0.28Ga_0.78As have been measured precisely at various temperatures between 10 and 200K. In bound-exciton transitions, the emission peaks shift toward lower energies with the increase in temperature accompanied by the small increase of half-widths. On the other hand, in free-electron-acceptor transitions, the emission peaks are independent of temperature changes accompanied by large increases of half-widths.

For pt.II see ibid., vol.18, p.1971 (1985). The investigation of the magnetic field at the magnetic centre of the cylindrical saddle-shaped coil system described in preceding papers is completed by considering field points far away from the magnetic centre. The calculation of numerical values using closed expressions is discussed. The magnetic field mod B mod is calculated for the optimal coil which is defined as the coil that gives the most uniform field in a small volume at the coil centre. In order to improve the uniformity of the magnetic field in a large volume, the criterion of the maximum usable volume is applied. The coil shape is varied until the useable volume is maximal under the condition that the magnetic field deviation does not exceed a given value. The coil parameters defining the optimal coil with respect to a sphere with a certain radius are evaluated. The procedure may be modified for applications in NMR imaging.

Electron collisional quenching rate coefficients and radiative lifetimes of the 7¹S, 6¹D, 5¹D, 5³D and 6³S states of Cd I are investigated by means of time-resolved, laser-induced fluorescence spectroscopy in the positive column of a helium-cadmium discharge (i=0-200 mA, P_He=1-6 Torr, n_Cd=10¹²-3*10¹⁴ cm^-3). The collisional rate coefficients measured for the first time are 5.2+or-0.6, 2.3+or-0.8, 07+or-0.2 in units of 10^-5 cm³ s^-1 for the 6¹D, 7¹S and 5¹D states respectively. Assuming that these quenching collisions are due to collisions towards the neighbouring levels connected by a dipolar interaction, hydrogenic formulas based on the Born-Bethe cross section correctly fit experimental values. The radiative lifetimes deduced from this experiment are found to be in good agreement with those of previous works. In particular, the radiation processes are, for the 5D and 6S triplet states, the dominant part of the quenching. Finally, from the peak amplitude of the LIF signals it is shown that the low-lying 5P states can be described in the discharge by a coronal equilibrium whereas the upper levels are in a collisional one.

High-pressure argon plasmas have been generated by mechanical gas compression. Experimentally determined values for kinematic data: minimum volume, time of piston motion, piston speed as well as for the pressure of the test gas are compared with the results of theoretical approaches. Temperatures up to 11000K are derived from spectroscopic measurements of the continuous emission by applying a multichannel spectrometer, and at lower conditions optical depth determinations were also carried out. The comparison with theoretical results shows that radiation losses and boundary layers may not be neglected.

The drift of electrons in argon, krypton and xenon is studied for the reduced electric field range of interest for electroluminescence based radiation detectors (E/p=0.5 to 5 V cm^-1 Torr^-1), using a unidimensional Monte Carlo model which is described in detail. Appropriate factors are introduced in this model to take into account the three-dimensional nature of the electron scattering process. Electron drift velocities, reduced light output and excitation and scintillation efficiencies are obtained. Some detailed information on drift times and drift distances between elastic and inelastic collisions is also presented.

Friction measurements are reported for the sliding of rubber over individual glass lenses, wavy-surface glass and abrasive paper. Two different apparatuses were employed to obtain friction data. All the experiments were made with the same vulcanised rubber compound moulded with smooth test surfaces to facilitate contact area observations. The measurements covered large ranges of sliding speed (0.001 to 10 mm s^-1), ambient temperature (-60 to +90 degrees C), normal load (0.01-57 N) and contact geometry (R=1-18 mm). Some results are given as a function of the sliding velocity by use of the WLF transform. It was found that, particularly for smooth surfaces, the friction varied only slightly with rate (WLF parameter). For the test arrangements employed the viscoelastic response of the rubber compound appeared to be of less importance than the geometry of contact, except when operating conditions were near to the glassy region of the rubber. A simple plot of friction coefficient as a function of the Hertz contact area brought together all the diverse measurements. In the course of experimentation the influence of the method of surface cleaning upon friction was investigated. New insight was also obtained concerning the nature and magnitude of the static friction of rubber.

A two-dimensional numerical model of the medium pressure (0.1<or=p<or=10 Torr) flowing glow discharge positive column plasma has been constructed. The axial and radial plasma density distributions and the degree of ionisation required to sustain the positive column have been calculated for a steady state ambipolar diffusion-controlled discharge acting over a finite portion of an infinite cylinder in which there is laminar gas flow. The influence of volume recombination, discharge region length and the gas flow velocity has been investigated.

A detailed analysis of current and voltage 'noise' in a low-current vacuum arc is presented. The occurrence of typical instabilities in arc current is shown to be correlated to the DC arc lifetime. Influence of the parameters of the feeding circuit on the instability behaviour is investigated and explained qualitatively. A model of the interaction between circuit and arc is proposed on the basis of which arc extinction seems to be a result of an accumulation of instabilities at low currents (I<20 A).

The damage caused by low-energy argon bombardment of silicon has been investigated by Rutherford back-scattering and transmission electron microscopy. At the doses required for sputter cleaning, gas bubbles are formed even at argon energies as low as 2 keV. The bubbles, and the argon they contain, cannot be annealed out below 1100 degrees C, although a significant amount of gas is trapped more weakly in other sites and is released at lower temperatures. The stability of the bubbles is explained in terms of the knock-in of oxygen from the native oxide during bombardment. The formation of bubbles cannot be avoided unless the angle of incidence of the argon is almost glancing.

A specimen-transducer-waveguide set was constructed in which the ringing-down, reflections, and resonance are minimised. The transducer and the AE detection system were calibrated and used to measure the forces induced during martensitic transformation in a beta -CuZnAl alloy. Forces between 0.1 and 2 mN were measured while the amplitude analysis of demodulated emission bursts yields the relation (P/P₀)¹2/=1-0.86 log (A/A₀).

The effects of post-deposition heat treatment on some polycrystalline samples of SnO₂ thin films prepared by the pyrosol process have been reported. It is observed that heat treatment in different ambients have remarkable effects on the film resistivity. No appreciable change of carrier concentration and optical transmission have been noticed, therefore the change of resistivity is considered to be due to the change of mobility of the carriers. The Hall mobility in SnO₂ thin films, having carrier concentration >or approximately=10¹⁹ cm^-3, is thermally activated in the investigated temperature range of 27 to 250 degrees C. The oxygen diffusion mechanism at the grain boundary of the films has been discussed critically to explain the observed mobility data. It is concluded that the role of the oxygen diffusion process in the film is only to modulate the inter-grain boundary potential energy barrier height.

The DC conductivity of disordered dielectric films is theoretically investigated assuming that there are two subsystems of charged species involved in the conduction process: one ensuring an electronic hopping transport and another of much less mobile carriers forming an inhomogeneous and non-stationary space charge in the dielectric. A system of differential equations specifying current density, electric field strength and boundary conditions for the dielectric is evolved and digitally simulated assuming its specific features, such as stiffness, turning point presence, etc. The results are compared with experimental data on DC conductivity of polar and non-polar thin film dielectrics, with special emphasis on anodic oxide films.

The impedance associated with recombination of free electrons and photoholes in surface states has been calculated. Two types of surface states have been considered: the so-called 'ordinary' surface states that are already present in the dark, and the surface states associated with intermediates of the photo-anodic decomposition. In both cases the result for the parallel equivalent surface state capacitance was characterised by a peak as a function of the electrode potential.

A 2D mathematical model for the magnetic leakage field of surface-breaking cracks in ferromagnetic materials is developed. The expressions are shown to be valid for 3D defects provided they are at least twenty times as long as they are deep. The model is tested by comparison with measured leakage fields for the range of artificial cracks. The magnetic forces experienced by a small magnetic particle in a leakage field are also calculated and the level of magnetic field strength required for magnetic particle crack inspection is discussed.

The current study has examined the thermoluminescent properties of polycrystalline beta "-Al₂O₃. The material is utilised as a solid electrolyte in the Na-S cell. The work has examined the thermoluminescence technique with regard to its ability to assess changes in the point defect distribution within the solid electrolyte. beta "-Al₂O₃ fabricated with the incorporation of various divalent cation additives has been characterised with thermoluminescence. The additives employed have been those used to stabilise the beta "-phase of the alumina structure. The effect that cell operation has upon the thermoluminescent properties has also been investigated. General conclusions are drawn with regard to how both additives and cell operation effect the point defect distribution within the solid.

Optical transmittance and Auger Electron Spectroscopy (AES) analyses are used to derive some thermodynamic data from alkali antimonide compounds deposited on glass substrates as thin layers. These measurements are performed in the course of the deposition of the layers as well as during their thermal dissociation (linear temperature-time dependence) under UHV conditions. The optical and AES measurements recorded during the dissociation process are analysed in terms of dissociation energies and equilibrium alkali vapour pressures (P_Cs, P_K, P_Na) over the compounds. The dissociation phenomenon is analysed and considered to be neither alkali-diffusion limited nor layer morphology limited. Dissociation behaviours of alkali antimonides with respect to temperature and time are deduced and discussed from the above results. It is shown that P_K=5*10^-10 Torr and P_Na=1*10^-11 Torr at 200 degrees C over the Na₂KSb. Without alkali pressures, at 200 degrees C, 6% of a Na₂KSb layer is dissociated within 1 h whereas 50% of a NaK₂Sb one is dissociated. The dissociation energy, Phi , of a Cs monolayer deposited on the top of the Na₂KSb compound is also considered and is about Phi =35+or-1 kcal mol^-1. At room temperature, the corresponding value of P_Cs is 2*10^-17 Torr. Finally optical transmittance measurements associated with the AES analysis are shown to be powerful tools for the measurement of dissociation energies and the assessment of related phenomena.

Field electron emission measurements have been made on composite emitters consisting of electrolytically etched tungsten micropoint cathodes overlayed by a 40-200 nm thick layer of epoxy resin. Their emission properties include (a) an initial switch-on effect at threshold fields of approximately 10⁹ V m^-1, (b) a subsequent reversible I-V characteristic that gives a linear FN plot at low fields (<or approximately=10⁸ V m^-1) and saturated emission at high fields >or approximately=4*10⁸ V m^-1, (c) electron spectra whose FWHM and energy shift is strongly field dependent, (d) single-spot emission images. This unusual pattern of behaviour has been interpreted in terms of a hot electron emission mechanism resulting from field penetration in the dielectric overlayer. Consideration is also given to the technological significance of such composite microemitters.