Table of contents

Pentagonal growth has been observed for beta boron. From geometrical relations between the icosahedron and the dodecahedron, the pentagonal faces are thought to consist of {11bar above 3}, {201} and {2bar above 01bar above} planes. The present results imply that any crystals having an icosahedral or a dodecahedral arrangement of atoms can show pentagonal growth.

The time dependence of the photochromic effect (at 479 nm) in erbium (silicate-based) glasses was found to be strongly influenced by the Er³⁺ concentration. A marked decrease in the photochromic relaxation time from 12·2 ms at 1 wt% Er₂O₃ concentration to 3·6 ms at 10 wt% was observed.

The stresses in toroidal magnet coils of anisotropic structure are analysed. Consideration is given to shear forces and to the correct choice of the elastic constants of a composite coil. It is concluded that for many practical purposes some of the earlier approximate analyses may be adequate.

Thin films of silver of thickness 58-212 nm (580-2125 Å) were prepared by evaporation on glass substrates at a residual air pressure of similar 10⁻⁵ Torr, and x-ray diffraction line profiles were chart-recorded on a Geiger counter x-ray diffractometer at room temperature using Cu K_α radiation. The peak positions of 111, 200, 220 and 311 reflections were measured, and from the peak shifts of adjacent pairs of reflections the lattice parameter changes, residual stresses and stacking fault concentrations in the films were determined as a function of film thickness. In relatively thin films (<100 nm), an appreciable decrease in the lattice parameter from the bulk value and a predominance of extrinsic stacking faults were observed. The residual stresses in the plane of the films were found to be compressive in nature, varying from similar 0·17 to similar 0·01×10¹⁰ N m⁻² (similar 24 to similar 1×10⁴ psi) with the increase in thickness. The results have been analysed and discussed in the light of existing work on thin metallic films.

The crystalline structure of thin films of ZnS, prepared by reactive cathodic sputtering, has been studied in relation to the parameters of the synthesis: anode-cathode voltage, H₂S ratio in the gaseous mixture, anode and cathode temperatures.

The principal structure types of ZnS are cubic zinc-blende, which is normally obtained at temperatures lower than 1000°C, and hexagonal wurtzite, obtained at higher temperatures. With reactive cathodic sputtering one can obtain either blende, or wurtzite, at low temperatures by modification of one parameter.

It is shown that sputtered ZnS films have the wurtzite structure when the values of the parameters are such that an excess of zinc is condensed for instance in the case of a high voltage, or at a low H₂S ratio. They have the blende structure in the opposite case. The blende structure shows preferential orientation, with the [111] axis perpendicular to the substrate, when the H₂S ratio is very high.

Descriptions are given of a jet-thinning apparatus which rotates the specimen in the jet stream, and a method of producing continuously chlorinated methanol via a double concentric emulsifier jet. The system has been successfully used to produce uniform electron microscope specimens from a variety of semiconductor materials, and by double-sided thinning, buried heterojunction dislocation networks have been examined.

Examples are also given of the use of the apparatus for preparing shallow angle taper sections for examination by chemical staining methods. With the use of the internal interference fringes, the depth of of staining at p-n junctions etc has been measured to better than 500 Å. The advantages of chemically prepared taper sections over conventional mechanically lapped specimens are discussed.

Stress relaxation measurements on single fibres of regenerated cellulose (viscose), which had a low degree of anisotropy, are described. These were made in the temperature range +20 to −185°C for the time interval 0·03 to 1000 s using tensile strains of the order of 0·3%.

The results are discussed in relation to existing reaction rate theories, and it is concluded that they cannot be explained in terms of such theories if only a single relaxation process is considered. A distribution of activation energies can be derived from the data if the system is regarded as approximately linear. A modified Ferry transform is used for this purpose, the theory of which is discussed. The distribution so derived is bimodal and corresponds approximately to fully bonded glucose rings moving singly and in pairs. Measurements on other hydrogen-bonded systems may help to establish the validity of this result.

The effective elastic moduli of a particle-filled material are analytically derived from a consideration of the deviations of the particle and matrix materials from either parallel or series connection. In the analysis an equivalent cell is defined based upon the operation of a force or deformation in an elementary cell representative of the composite; in addition, so-called particle and matrix stresses and strains are introduced. These parameters enable an expression for the respective energies of the constituents to be derived from both the parallel and series connection approach. The equation of both expressions gives the elastic modulus of the composite. This holds for six of the nine moduli. The other three are derived from separate considerations. The expressions obtained involve the way in which the particles are arranged, their shape and volume fraction, and the relevant elastic moduli of both materials. The numerical results are in good agreement with experimental values.

The temperature dependences from 4·2 to 300 K of the seven adiabatic elastic constants of CaWO₄ have been determined by measurements of ultrasound wave velocities using the `sing-around' and pulse superposition methods. The adiabatic elastic constants have been calculated from the measured ultrasound wave velocities by a perturbation method using just seven of the eleven measured velocities and by an iterative method using all the measured velocities; agreement has been found between the two sets of values. The complementary data on the thermal expansion and heat capacity at constant pressure of CaWO₄ have been used with the present results to find the isothermal elastic constants, the heat capacity at constant volume, the Grüneisen parameters, and the Debye characteristic temperature at 0 K. The Debye characteristic temperature has been found from the heat capacity at constant volume from 10 to 300 K after subtraction of the Einstein heat capacity terms from the total heat capacity.

The extraction of a stainless steel wire embedded in an epoxy resin has been studied as a function of the embedded length. The models developed to elucidate the dependence of the debonding stress and the pull-out stress on the embedded length are applied successfully to the experimental results. The nonlinear variation of the debonding stress with the embedded length is caused by the distribution of shear stress along the wire. The variation of pull-out stress with embedded length is caused by the decrease of the frictional stress due to the Poisson contraction of the wire under tensile stress. The effect of wire contraction on the friction process was confirmed by using different initial wire tensions. The residual compressive stress at the interface due to resin shrinkage and the coefficient of friction were determined from the analysis of the pull-out process.

The interpretation of creep as an evolutionary Markov process, involving a distribution of energy-barrier heights governing dislocation movement (Feltham 1968), is extended to encompass work-hardening under increasing stresses, by making explicit allowance for the operation of dislocation sources. The widely observed, linear, work-hardening law, derivable from the model, is shown to imply the strain invariance of the normalized distribution of the energy-barrier levels. This invariance appears to be a consequence of a mode of `microdeformation' in which the rate of storage of `elastic' energy with respect to strain tends towards a minimum.

Fatigue curves for onset of damage and final separation were obtained under zero-tension loading for plain specimens of GRP laminates, and compared with those obtained for specimens containing a circular hole. The stress concentrator was found to be fully effective in initiating damage, but the effect on ultimate failure was smaller. The results for damage of the specimens with a hole were compared with those predicted from the tangential edge stress distribution and the fatigue properties of the plane material. Agreement was found to be reasonable.

Between 30 and 290 K the thermal expansion of vitreous silica is shown to be particularly sensitive to thermal history. Values given for the linear coefficient α were determined relative to copper, and existing reference data for copper are discussed. α for silicon has also been measured relative to copper from 55 K to room temperature. Values of α at 283 K are reported for a number of samples of copper, and for Ag, Au, Al, Pt, Pd, MgO, NaCl and CaF₂.

Electric currents induced in wurtzite crystals by steady streams of cathode rays are studied experimentally, with the view to elucidating the electronic properties of these crystals. It is found that currents are carried in surface channels as well as through the bulk of the crystals, the range of the bulk currents being limited to about 200 μm. Crystals grown from the melt, from the vapour phase and naturally grown platelets are used. The platelets have built-in fields which influence the currents.

A new method of laser modulation is discussed, which depends on certain plasma effects in semiconductors. In the presence of low-frequency background noise, an incident electromagnetic wave of frequency near the plasma frequency can be anomalously absorbed because it puts energy into damped electrostatic plasma waves. This anomalous absorption process is highly resonant and requires near-matching of the incident wave frequency and the frequency of the excited plasma waves. If the frequency of the electron plasma waves is shifted in and out of resonance by a modulating agency, the absorption coefficient can get strongly modulated, leading to amplitude modulation of the electromagnetic wave. The frequency shift can be achieved by the application of suitable electric fields to the semiconductor.

Mixtures consisting of conducting particles dispersed in an insulating medium are considered, and the electrical conduction which can occur by particles touching is related to the proportion of conducting material present in the mix, and its conductivity.

It is found that limiting values of conduction can be obtained for a random dispersion and also for the situation where the conducting material lies in interconnecting channels, both systems being observed in practical mixtures.

The way in which the basic relationship can be modified by particle size, shape and orientation, contact resistance and particle packing is also considered.

Samples of heavily worked, cold-drawn niobium wire were heat-treated at temperatures between 200 and 700°C to produce changes in the magnetization behaviour of the material. An investigation was then carried out to determine the influence of these changes on the AC power losses produced in the wires at 4·2 K due to 50Hz transport currents of up to 120 A. The results show that the power losses at a particular current increase as the field of initial penetration into the niobium, H_p, decreases.

Gallium telluride single crystals have been grown and cleaved to form thin platelet specimens. Small-area contacts on opposite faces produce threshold switching characteristics when high fields are applied across them, while large-area contact produce memory switching. Detailed measurements of conductivity under DC and pulsed conditions are presented, and the threshold switching is interpreted in terms of a thermal mechanism.

The current injected into SiO₂ as a result of avalanche breakdown of an underlying p-n junction, with doping typically used in MOSTS, has been investigated. By use of a simple model, an expression has been derived for the dependence of this current on the gate and reverse voltages; reasonable agreement with experiment is obtained.

An apparatus is described for measuring drift velocities v- in ionic crystals at high electric fields by the transient method. In zone-refined crystals of AgCl, v- rises linearly with the field strength F as long as F<1·2×10⁵ V cm⁻¹; then it converges to a constant value. The shape of the transit current pulses makes sure that impact ionization does not occur at any field below the dielectric strength. Therefore most probably the mechanism of dielectric breakdown in ionic crystals is not collision ionization.

Experiments were carried out to measure the secondary electron emission yield of thin films of CsI. The vacuum-evaporated layers gave reproducible gain factors of about 20. The energy E_pm of the primary electrons at which the gain is maximum is found to be 3·3 keV. The large value of E_pm suggests a large escape depth L for secondary electrons. On the basis of experimental results, L is estimated to be 56 nm.

The thermoluminescent properties of LiF (TLD-100) have been investigated as a function of plastic deformation. Without exposure to ionizing radiation, plastic deformation up to fracture does not induce a thermoluminescent response. After subsequent exposure to ionizing radiation, the intensity of the thermoluminescent glow peaks decreases with the amount of plastic deformation. This is explained in terms of a relative decrease in the concentration of free dipoles and low-order dipole complexes which act as trapping centres for peaks 2 and 5. This decrease is a result of an interaction of these defect structures with the dislocations introduced during deformation.

Quantitative determinations of A1, Ti, Ag and Au deposits down to 10⁻³ monolayers thickness on thin carbon substrates have been performed by electron micro-probe analysis with spatial resolution better than 2 μm. A linear relationship between characteristic x-ray intensity and nominal deposit thickness is shown between 10⁻³ and 10⁺² monolayers. The lower detection limit was investigated as a function of the electron energy. Experimental results are compared with theory. Relations and references are given which enable one to estimate the sensitivity of the method for other substrate-deposit combinations.