Table of contents

The anisotropy field distributions in powders of BaFe₁₂O₁₉ were measured by the remanent-torque-curve method. Milling of a powder increased the percentage of particles with low anisotropy fields and reduced the average anisotropy field H_A. Annealing the powder decreased the percentage of such particles and raised H_A. The intrinsic coercive force _MH_C was a single-valued function of H_A, for samples derived from a common starting powder. The points for samples derived from different, independently prepared powders did not all lie on the same curve, but it is suggested that each independently prepared powder has a curve of its own associated with it. These results are explained on the basis of lattice dislocations, introduced by milling and healed by annealing. These dislocations reduce the magnetocrystalline anisotropy, and a mechanism for this effect is briefly described.

A silicon crystal implanted with gold was heated to successively higher temperatures, reaching a maximum of 1250°C, and its surface examined by Auger emission spectroscopy (AES) and LEED at each stage. No recrystallization was observed until the crystal had been heated to 740°C, at which point the AES analysis could be interpreted in terms of the appearance of gold atoms at the surface. At 810°C the Auger peaks from gold were considerably larger than those from silicon, but decreased progressively thereafter as the temperature was raised until, at over 1000°C, the differential distribution was indistinguishable from that of clean silicon. The first LEED pattern observed, at 740°C, was not the Si(111)-7 pattern, but another familiar one, the Si(111)-(radical3 × radical3)-R30° pattern. With increasing temperature, the fraction one-third-order pattern spread over the whole surface and, above 900°C, was joined by another, a fraction one-fifth-order pattern, probably based on a domain structure. At the highest temperature no fractional orders were visible, only a Si (111)-(1 × 1) pattern, with good spot-to-background contrast. The normal 7-pattern could only be achieved by ion bombardment and annealing, not by heat treatment alone, suggesting that there was still a very high density of dislocations remaining from the implantation.

Properties of the chemically deposited layers of silver sulphide have been studied using gold contacts. The effect of field reversal, using one gold contact on the layers, response time, lux-ampere characteristics, photoconductivity in relation to biasing frequency, influence of the biasing voltage, effect of heat-treatment on the characteristics of the silver sulphide cells and the spectral characteristics are discussed.

The preparation of d.c. electroluminescent thin films of copper- and manganese-activated ZnS is described together with their electrical and luminescent characteristics. Films have been prepared from activated powders by sublimation in a horizontal two-zone vacuum furnace. Reproducible luminescent emission was obtained in the low-voltage operation range 10-20 V, and in a number of cases emission was observed at voltages as low as 7-8 V. Their electrical characteristics are governed by bulk conduction processes and show a strong dependence on deposition temperature. A theoretical model is proposed to explain the conduction mechanism in terms of bulk properties only. A relationship of the form V=A₀+A₁ ln I+A₂I^½ was found to give good agreement with the experimental results.

A luminescence line has been observed at 1·454 ev at 20°K in germanium-doped GaAs. Measurements have been made of the peak energy, intensity and half-width as a function of temperature. It is proposed that the centre responsible for the luminescence is an arsenic vacancy bound to a germanium atom on an arsenic site. Preliminary results indicate that a similar centre is present in silicon-doped GaAs grown from a gallium solution.

Studies of the pre-irradiation isothermal fading of glow peaks in thermoluminescent dosimetry grade lithium fluoride have shown that two peaks are associated with divalent impurity-positive ion vacancy dipoles, not one as previously thought. The decay of these peaks is shown to obey second-order kinetics with an activation energy of 0·85±0·05 ev.

The implications of these results for practical dosimetry are discussed.

Details are given of the pressure dependence of the resistance-temperature curve R(θ) of a sample of manganin in a strain-free mounting. The temperature of the resistance maximum increases linearly with pressure by about 5 degC for 1000 MN m⁻² (10 kbar). The curvature of R(θ) increases slightly with pressure.

The strain-free mounting described results in a guage of rapid response. There is a small change in resistance owing to an applied pressure cycle, the change decaying with a time constant of 340 hours.

Electrets were prepared by heating specimens of Perspex to 112°C and cooling them to room temperature while a forming field was continuously applied; these were compared with non-electrets, i.e. specimens heated in the same way but with no forming field applied. All were subsequently broken down by applying standard 1-50 μs impulse voltages.

The electric strength of the electrets was found to differ from that of the non-electrets, and it was also dependent on the polarity of the forming field. This is consistent with a stable volume distribution of homocharge contained within the electrets close to their electrodes.

The friction and dynamic properties of oriented crystalline polymers - linear and branched polyethylene - have been investigated at temperatures ranging from −180 to +149°C. Microhardness and dilatometric measurements were made and the structures were studied by x-ray and polarization methods. It is shown that orientation due to extension decreases friction and increases the shear storage modulus at temperatures below the melting range of the polymer. In the melting range orientation disappeared in the specimens and the dependences of friction and dynamic modulus on temperature practically coincide with the analogous dependences for unoriented specimens. The nature of the friction maxima of polyethylenes in the low and high temperature regions is discussed.

The internal vibration-damping characteristics of metallic materials at stress levels of engineering interest have been measured by exciting resonant longitudinal vibration in bar specimens and observing the consequent temperature rise. The method has proved suitable for all stress levels from a few hundred lbf in⁻² to stresses near the fatigue limit, and for the range of specific damping capacity encountered in most ferrous and non-ferrous materials. There are no extraneous effects from damping not inherent in the material itself.

The present paper describes the apparatus and techniques used, and summarizes the results of a few typical measurements. Detailed results obtained by this method are to be published separately.

The condition that nτ, the product of density and containment time, is sufficiently high to strip ions almost to their `steady-state' degree of ionization can be achieved in certain high-density pulsed devices of thermonuclear interest. One such plasma, that produced when a solid pellet is irradiated by a Q-spoiled laser beam, looks promising as a source of highly stripped ions. Present calculations show that the ions could be extracted for acceleration experiments with a mean charge exceeding ten.

Oxygen atoms have been produced by a 2·45 GHz discharge in an oxygen molecular gas flow, and e.p.r. techniques used to observe the decay of atoms in the flow tube. The decay is first order in atom concentration and proceeds in the gas phase by an ozone process with a rate constant of (4·5±0·2) × 10⁷ l² mol⁻² s⁻¹, with a small contribution from the quartz surface of the flow tube. From the extrapolation of an atom decay curve and by using the oxygen molecule as an e.p.r. calibration standard, absolute atom concentrations in the discharge have been obtained.

In the pressure range 1-10 torr (133-1330 N m⁻²) and with a discharge power of 15 W, the molecular dissociation varies from 0·02% to 0·9% atom-mole fraction with a maximum of 2% at 6 torr, and the percentage of the power dissipated in the gas used for dissociation varies from 0·05% to 5%. The yield obtained from a 15 W discharge with a flow velocity of 2·5 m s⁻¹ was approximately 4 g kwh⁻¹.

By following the same line of reasoning which the authors used in a previous paper, concerned with cylindrical probes, the ion current I_i to a spherical probe (radius r_p, bias voltage V) immersed in a collision-dominated flowing plasma of subsonic velocity v_f has been calculated to be

IMG1

where n_e is the electron density, μ_i the ion mobility, e the electronic charge and ₀ the permittivity of free space. This theory is only valid for conditions where the plasma sheath diameter r₀>>r_p.

A propane-air flame with a moving probe was used to investigate the above relationship. Reasonable agreement (considering the approximations made in deriving the theoretical relationship) was obtained between measured and theoretical ion currents.

The breakdown of a gas excited by a radiofrequency voltage of frequency 17·6 MHz has been studied in a cylindrical discharge tube of length 0·3 cm and radius 1·4 cm, in hydrogen, air, oxygen and carbon dioxide, within the pressure range 1-6 torr and in a transverse magnetic field varying from zero to 1800 G. The experimental conditions indicate that the discharge here is diffusion controlled. The values of α/p at different E/p values, calculated from the theory of Brown as well as from Kihara's theory, have been compared with the α/p values obtained in the literature. It is noted that α/p values calculated from Brown's theory are in better agreement with the published results than those calculated from Kihara's theory. The change of diffusion length in the presence of a transverse magnetic field has been obtained from measured E/p values, and comparison with theoretical values shows that there is quantitative agreement for small H/p, where H is the magnetic field. At higher magnetic fields the divergence has been ascribed to the inadequacy of the equivalent pressure concept and also to the uncertainty in the values of the molecular constants introduced by Kihara.

A new sampling technique has been developed which enables time-resolved electron energy distribution measurements to be made in a periodically varying plasma by means of the Langmuir-Druyvesteyn probe method. The method is illustrated with measurements made in moving striations in a neon discharge (p=0·32 torr, I=330 mA, 2R=7·0 cm); the validity of the technique for these discharge conditions is examined in detail.

An experiment has recently been described in which, by an `electrodynamic space-charge effect', the energy of a stream of electrons is transferred to electrons at the front of the stream. An attempt to confirm this acceleration method by a similar experiment has been negative.

A simple homogeneous-field magnetic prism spectrometer is described for use below the camera chamber of a conventional transmission electron microscope. The prism is designed for point-to-point focusing from the exit pupil of the electron microscope to the exit slit of the spectrometer. Energy spectra are scanned by varying the accelerating voltage to minimize chromatic effects in the microscope. Scanning coils are used to deflect an image or a diffraction pattern across an area-selecting aperture which also serves as the aperture stop for the spectrometer. A phase-sensitive periodic differential integrator is used to record the output from the scintillation counter detector.

The adaptation of an electron spectrometer to a transmission electron microscope affords another technique for the electron-optical study of materials, namely, selected-area electron spectrometry. A simple magnetic electron spectrometer, below the camera chamber of a transmission electron microscope, has been used in several preliminary applications, including evaluation of the H.T. supply of the electron microscope, measurement of specimen thickness, recording of electron diffraction patterns, studies of the angular distribution of energy loss electrons and microanalysis based on characteristic energy losses.

Losses at 50 Hz as a function of magnetic field and surface profile have been investigated in single crystal samples of niobium at 4·2°K using an electronic wattmeter. The losses in samples with smooth surfaces were less than 5 × 10⁻⁴ W Hz⁻¹ m⁻² at 1 × 10⁵ A m⁻¹ (1250 Oe). A sample with a cold-worked surface showed higher losses both before and after the surface was polished. It is concluded that the surface finish is of major importance in determining the magnitude of losses below the lower critical field.

Both LEED (low energy electron diffraction) and RHEED (reflection high energy electron diffraction) have been used to study the structure of nickel, vapour deposited in ultra-high vacuum onto clean (111) oriented silver surfaces. In contrast to a previous study by Gonzalez in 1967 performed at 10⁻⁵ torr, no evidence of an ordered alloy was found.

Attempts to produce large single crystals of antimony sulpho-iodide from the pure molten material are described along with attempts to alter the electrical characteristics. The results indicate that, except at very low growth rates, it is probably not possible to achieve the first objective. The second objective is also shown to be difficult to attain.

It is shown that the critical load to produce a ring crack varies with the coefficient of friction as well as with the normal load. The critical load is proportional to the radius of the ball indenter at small radii and is proportional to the square of the radius at large radii. The linear range decreases with increase in the coefficient of friction.

It appears that the negative result of Mathieson's attempt to confirm the acceleration effect reported by Heese and Raudorf is essentially due to mistakes in the design of his discharge device, inefficient electron optics and poor vacuum.

It is shown that the focal length of a phase zone plate is independent of the phase variation, but this does affect the intensity of the image.