Table of contents

The results of impact energy tests described by Bader et al (1973) fit in well with existing theory, and indicate that direct shear tests on composites correlate well with shear stresses occurring at the fibre-matrix interface.

A scanning light spot technique was used to measure the transfer efficiency of 8- and 64-bit charge-coupled device shift registers. Efficiencies of 99·87±0·03 for 64-bit devices are reported.

An arithmetical error in the derivation of a theoretical expression by Celasco and Mazzetti (1972) has led to a prediction inconsistent with experimental results.

An approximate expression (Kaczer and Tomas 1972) for the energy of cylindrical 180° domain walls in uniaxial ferromagnetic material predicts that long cylindrical reverse domains aligned in the easy direction should be metastable against collapse to zero radius. It is shown here by a scaling argument, without approximation, that these domains are unstable in a uniaxial continuum.

The preparation of the thermoluminescent material PbSO₄ doped with 0·06 mol% Sm is described. Diffuse reflection spectra show that exposure to γ or UV radiation induces colour centres with absorption maxima at 390 and 540 nm. They are bleached when the material is heated. The glow emission spectrum is typical of other samarium-doped sulphates. The glow curve has a large composite peak at 160°C which could be utilized for image storage or for radiation dosimetry.

Single crystals of tin oxide (SnO₂) have been grown by a vapour-phase transport method and subsequently doped with europium. The cathodoluminescence of these crystals has been measured at room temperature. The spectrum comprises the intrinsic emission band of the host lattice and bands due to the presence of the europium ion; the emission bands due to the europium ion are reported for the first time. The europium is incorporated in the host lattice as Eu³⁺.

Single crystals of tin oxide (SnO₂) have been grown by a vapour-phase transport method and subsequently doped with terbium ions. The cathodoluminescence of these crystals has been measured at room temperature (300 K). The spectra comprise the intrinsic emission band of the host lattice and bands due to the presence of the terbium ion; the emission bands due to the terbium ion are reported for the first time. The terbium is incorporated in the host lattice as Tb³⁺.

Samples of the metallic spinels CuCr₂S₄ and CuCo₂S₄ have been prepared and examined by x-ray diffraction. In each case the spinel pattern was accompanied by a number of weak foreign reflections consistent with the presence of a single impurity phase, identified as γ-CuCrS_2-x and CoS₂ respectively. Careful structural investigation of CuCo₂S₄ suggests that the space group Fd3m may be satisfactory for this compound. The structure parameters are then a=9·478±0·005 Å, u=0·388±0·001 and B=0·4±0·1 Ų.

The influence of fibre modulus and surface treatment on the modulus, damping and strength of unidirectional, cross-ply (0°/90°) and ±15° angle-ply laminates has been investigated.

Fibre modulus was the dominating parameter when the fibres lay in or near the direction of the direct stresses. In general, the higher the fibre modulus, the higher was the composite modulus and the lower was the damping.

Surface treatment affects the properties when the tensile or shear bond between the fibre and the matrix becomes important, eg in the flexural strength of 90° specimens. However, the damping is not significantly affected by surface treatment since the test stresses did not put the bond into jeopardy.

Different lamination geometries and fibre moduli may be used to give the same stiffness in one direction, but quite different damping values. In flexure, for example, a 0° specimen with a Young's modulus of 100 GN m⁻² has a specific damping capacity of about 0·35%, whereas a ±15° angle-ply composite with the same stiffness has about four times more damping.

A study is described of the fracture of vitreous carbon. The fracture surface energy was obtained by an approximate method, the velocity of crack propagation was measured by the ultrasonic fracture velocity measurement technique of Kerkhof (1952), and the microhardness was measured using a diamond indenter. The fracture surface energy was estimated to be 12 J m⁻². This is greater than for soda-lime glass by a factor of approximately three. Cracks propagating in a uniaxial tensile field accelerated to a maximum crack velocity of 1400 m s⁻¹. This value is slightly less than for soda-lime glass, but is approximately the same fraction of the stress wave velocity as is the value for glass. For both materials these fractions are considerably smaller than predicted for a linearly elastic solid, and evidence is presented which supports the alternative view that fracture can be regarded as a flow process (Marsh 1963). At high velocities, crack branching took place. Vitreous carbon fracture surfaces were examined with a scanning electron microscope, and their appearance was found to be similar to that of glass.

Investigations of ultrasonic velocity and absorption are carried out on bismuth-antimony alloys with a concentration of 0-60 at%antimony using the composite oscillator method and the pulse method respectively. The elastic constants are estimated from velocity and density measurements. Anomalous variations of elastic constants and ultrasonic attenuation are observed at a certain critical concentration region, and these are interpreted in terms of changes in lattice parameter and carrier concentration. Studies of the temperature variation of Young's modulus and ultrasonic attenuation are also carried out at some of the concentrations. The observed anomalies in ultrasonic properties confirm the peculiar electronic properties of these alloys in that concentration range.

The paper is concerned with the measurement by an ultrasonic pulse technique (Markham 1970) of the elastic stiffness constants of oriented polyvinyl chloride sheet over a range of degrees of orientation produced by drawing. The technique involved immersing the sample in a tank of water containing an ultrasonic transmitter and receiver, and measuring the difference in transit time between the two, with and without the sample. In general, two types of wave were propagated through the sample, and their velocities were determined, from which the stiffness constants were deduced. The frequency was 5 MHz. Measurements of the optical birefringence were also made.

The effect of drawing the initially elastically isotropic sheet was most marked in the case of c₃₃ (axis 3 being the draw direction) which increased from 6·3 to 11·8 GN m⁻², and least marked in the case of c₄₄ which decreased from 1·8 to 1·6 GN m⁻². The material was found to have transverse elastic isotropy about the draw direction, and the other three stiffness constants changed by intermediate percentages.

The variation of stiffness constants with draw ratio was fitted quite well by a theory of Ward (1962), except for a discrepancy in the case of the unoriented material.

A study has been made of the velocity of propagation of ultrasonic waves in uniaxially aligned carbon-fibre-reinforced resin using a variety of experimental techniques. It has been confirmed that the behaviour of transversely polarized waves is analogous to that reported in hexagonal crystals, and also that longitudinal waves can easily be propagated parallel or perpendicular to the fibre direction. In other directions, however, the quasilongitudinal or L wave is strongly attenuated and a slower wave which could be called the pseudo-L wave has been identified. This depends on the strong shear coupling between the fibres and the matrix material and has no analogy in the crystal case. As a demonstration of this effect it has been shown that a suitably shaped piece of carbon-fibre-reinforced plastic can act as an efficient converter of compressional waves to shear, or vice versa, at a given frequency.

Currents decaying with time according to a 1/t law were observed in polystyrene films when a direct voltage was applied. Upon voltage removal, discharge currents were recorded, equal in magnitude to the charge currents. The currents were strongly dependent on the texture of the edges of the evaporated gold electrodes. Study of the currents as a function of several parameters (voltage, electrode size, electrode shape and film thickness) leads to the conclusion that previous proposals about the origin of transient currents in similar insulating films are not applicable to the present case. It is shown that the carrier currents are confined to flow on the surfaces of the film. Moreover, if the surface resistivity is assumed to grow exponentially with the distance from the electrode edge, the predicted current agrees well with the experimental results.

A scanning electron microscope was operated in the stroboscopic mode at about 9·0 GHz, with estimated beam diameter of 0·3 μm, and beam pulse lengths of 10 ps. Gunn domains in transverse devices were examined for velocity and spatial width.

The anomalous behaviour of powdered barium titanate is confirmed by the use of the formula for mixtures. Systematic measurements of the dielectric properties of BaTiO₃ have been done for two cases: embedded grains and fine-grained pressed powder. The influence of the size and shape of the grains, and of the method of preparation of the samples, has been studied.

A study of switching effects in vapour-grown zinc sulphide platelets (1-100 μm thickness) contacted with silver or indium is described. Depending on temperature, both `monostable' and `bistable' switching was observed. In the `bistable' region crystals exhibited a variety of metastable conduction states; these are discussed in terms of filamentary behaviour and a `variable donor density' model in the accompanying paper (II). This paper attempts to develop an overall model of the switching by concentrating on transient behaviour. Some attempt is made to describe the type of centre necessary to explain the observed properties.

The results of an extensive investigation of the low-field DC current-voltage characteristics of vapour-grown zinc sulphide platelet (1-100 μm thickness) switching devices is described in an attempt to characterize and understand the end states of the switching. A wide range of metastable states was observed, typical characteristics exhibiting a range of current-voltage laws, including the Poole-Frenkel effect. A detailed analysis of the results in terms of conduction band theory, whilst not categorically ruling out impurity conduction, does strongly support an explanation of the conduction in terms of filamentary behaviour and a `variable donor density' model. A fuller account of switching in zinc sulphide is given in the accompanying paper (I).

Current-voltage measurements have been carried out at room temperature on anodized tantalum thin films ranging in thickness from 120 to 400 nm. The tantalum was prepared by getter sputtering in such a way as to reduce the possibility of impurities and defects being present as was found in tantalum foil. The films were found to be rectifying, the forward direction of current flow being with the aluminium counter-electrode positive with respect to the base tantalum. The conduction mechanism is believed to be space-charge-limited in the forward direction and Schottky-controlled emission in the reverse direction. The current in the forward direction was on average two orders of magnitude higher than that in the reverse direction, and the mobility of the electrons in the space-charge-limited mode, as obtained from steady-state and transient measurements, is very low, being of the order of 10⁻¹⁷ m² V⁻¹ s⁻¹. The results in general support those of Aris and Lewis (1973) and show that localized conduction does not play an important part in the conduction process.

Approximate formulae are derived for the dielectric constants of composite materials consisting of fibre-like inclusions embedded in an isotropic continuous matrix. Both the axially symmetric and isotropic cases are treated. It is suggested that the results may be useful for fibre, whisker or polymer composites. Likely geometries for which the formulae may be valid are discussed.

Thermo-galvanomagnetic measurements have been performed on single crystals of cadmium arsenide prepared by vapour growth techniques. It has been found that the electrical properties of this semiconductor are not noticeably anisotropic. Furthermore, thermal diffusivity measurements at liquid nitrogen temperature and room temperature show that the thermal conductivity is also isotropic within the limits of experimental error. Thus there is no preferred orientation for cadmium arsenide in thermal detectors based on the Nernst effect. It is possible that the relevant figure of merit has its highest value at an electron concentration of about 3×10²³ m⁻³, the lowest concentration that has been achieved up to the present time.

The critical breakdown field, ie the product of field enhancement and (mean) applied electric field at breakdown, between copper electrodes in vacuum is shown to increase with the number of low-current (<1A) conditioning arcs. A similar effect is not observed on materials with smaller ranges of hardness (stainless steel, molybdenum, etc). A hypothesis is put forward that a local hardening of emitting regions occurs in copper due to the mechanical stresses induced into the surface at arc points. Experimental evidence supporting this is presented along with complementary experimental results for deliberately hardened copper surfaces.

A simple model which enables the backscattering factor r and the energy-dependent part of the ionization cross section, ϕ(E), of a solid to be measured by means of Auger spectroscopy has been developed by Gallon (1972). This model has been used to estimate these two parameters for carbon, silicon, selenium, silver, gadolinium and gold, ie a set of elements spanning a large range of Z values.

The backscattering factor r was found to increase with atomic number Z, as expected. The variation of r with reduced energy U is compared with the calculations of Bishop (1967).

The refractive indices of ferric borate have been measured by a nondestructive method at room and liquid-nitrogen temperatures. Details of the method used and a discussion of the results are given.

Multiphoton absorption processes have been studied theoretically. It has been shown that the processes above five-photon absorption do not occur in the hydrocarbons using the present available laser power. It has been further shown that different input laser energy is needed to induce different process orders. The limits for various process orders have been estimated, along with the process emission wavelengths. Theoretical expressions have been derived which completely explain the experimental results using Feynman diagrams. The discrepancy noted by Pantell et al (1967) between their theoretical and experimental results has been discussed. The difference between the emission and the absorption frequencies in various process orders has been explained using the above diagrams. Spin analysis has been used to calculate the nature of the final state, the polarization of fluorescence, and the wavefunction. The results have been verified using spin analysis, dipole moment concepts, and Huckel's theory.

A theory has been developed for the temperature dependence of intensity of cathodoluminescence in `n'-type GaAs in the range 150-500 K. The dependence is seen to arise almost entirely from the temperature dependence of τ_r the radiative lifetime. The theoretical dependence has been computed and compared with experimental results and shows good agreement in this range.

A solid-state electrochemical cell with pellets of Ag₄KI₅ as the electrolyte, silver foil as the anode and a mixture of I₂ and graphite as the cathode is described. This cell, Ag|Ag₄KI₅| I₂+C, has an open circuit voltage OCV of 680 mV. Cell characteristics under different load conditions are reported. Polarization studies, a possible cell mechanism and the practical utility of the cell are also reported and discussed.

A number of alloys of neighbouring elements in the periodic table have been analysed by electron probe microanalysis to study the validity of the correction procedure in such alloys. The enhancement of Ni radiation in the analysis of Ni-Cu alloys observed by other workers has been confirmed. However, similar effects are not found in either Ti-V or Fe-Co alloys, where similar enhancements of Ti and Fe radiations are expected by analogy with the Ni-Cu system because of a large atomic number effect. Conversely, in Mn-Fe alloys there is an unusually large enhancement of Mn radiation, although the atomic number effect is expected to be either negligible or to result in an attenuation. The enhancement of both Ni and Mn is shown to be consistent with a combination of fluorescence and atomic number effects. It is concluded that there is no evidence for any abnormally large x-ray enhancement which cannot be corrected for in any of the systems studied.