Non-metallic cold-cathode electron emission from composite metal-insulator microstructures

A detailed investigation has been undertaken into a field-induced electron emission (FIEE) mechanism that occurs at microscopically localised `sites' on uncoated, dielectric-coated and composite-coated metallic cathodes. An optical imaging technique has been used to observe and characterize the spatial and temporal behaviour of the populations of emission sites on these cathodes under various experimental conditions, e.g. pulsed-fields, gas environment etc. This study has shown that, for applied fields of 20MVm^-1, thin dielectric (750AA) and composite metal-insulator (MI) overlayers result in a dramatic increase in the total number of emission sites (typically 30cm^-2), and hence emission current. The emission process has been further investigated by a complementary electron spectroscopy technique which has revealed that the localised emission sites on these cathodes display field-dependent spectral shifts and half-widths, i.e. indicative of a `non-metallic' emission mechanism. Details are also given of a comprehensive investigation into the effects of the residual gas environment on the FIEE process from uncoated Cu-cathodes. This latter study has revealed that the well-known Gas Conditioning process can be performed with a wide range of gas species (e.g. O_2, N_2 etc), and furthermore, the degree of conditioning is influenced by both a `Voltage' and `Temperature' effect. These experimental findings have been shown to be particularly important to the technology of high-voltage vacuum-insulation and cold-cathode electron sources. The FIEE mechanism has been interpreted in terms of a hot-electron process that is associated with `electroformed' conducting channels in MI, MIM and MIMI surface microstructures.

Spectroscopic studies of field-induced electron emission from isolated microstructures

A detailed investigation has been undertaken into the field induced electron emission (FIEE) mechanism that occurs at microscopically localised `sites' on uncoated and dielectric coated metallic electrodes. These processes have been investigated using two dedicated experimental systems that were developed for this study. The first is a novel combined photo/field emission microscope, which employs a UV source to stimulate photo-electrons from the sample surface in order to generate a topographical image. This system utilises an electrostatic lens column to provide identical optical properties under the different operating conditions required for purely topographical and combined photo/field imaging. The system has been demonstrated to have a resolution approaching 1m. Emission images have been obtained from carbon emission sites using this system to reveal that emission may occur from the edge triple junction or from the bulk of the carbon particle. An existing UHV electron spectrometer has been extensively rebuilt to incorporate a computer control and data acquisition system, improved sample handling and manipulation and a specimen heating stage. Details are given of a comprehensive study into the effects of sample heating on the emission process under conditions of both bulk and transient heating. Similar studies were also performed under conditions of both zero and high applied field. These show that the properties of emission sites are strongly temperature and field dependent thus indicating that the emission process is `non-metallic' in nature. The results have been shown to be consistent with an existing hot electron emission model.

Acoustic emission and yield in sand

An experimental investigation into the Acoustic Emission (AE) response of sand has been undertaken, and the use of AE as a method of yield point identification has been assessed. Dense, saturated samples of sand were tested in conventional triaxial apparatus. The measurements of stresses and strains were carried out according to current research practice. The AE monitoring system was integrated with the soil mechanics equipment in such a way that sample disturbance was minimised. During monotonically loaded, constant cell pressure tests the total number of events recorded was found to increase at an increasing rate in a manner which may be approximated by a power law. The AE response of the sand was found to be both stress level and stress path dependent. Undrained constant cell pressure tests showed that, unlike drained tests, the AE event rate increased at an increasing rate; this was shown to correlate with the mean effective stress variation. The stress path dependence was most noticeable in extension tests, where the number of events recorded was an order of magnitude less than that recorded in comparable compression tests. This stress path dependence was shown to be due to the differences in the work done by the external stresses. In constant cell pressure tests containing unload/reload cycles it was found that yield could be identified from a discontinuity in the event rate/time curve which occurred during reloading. Further tests involving complex stress paths showed that AE was a useful method of yield point identification. Some tests involving large stress reversals were carried out, and AE identified the inverse yield points more distinctly than conventional methods of yield point identification.

A study of field emission from cadmium sulphide

It was decided to investigate field emission from cadmium sulphide because many workers have found that the agreement between theory and experiment for this material, and other semiconductors, is poor. An electron energy analyser, similar to those used in most of the previously reported experiments, was, therefore, built. The performance of the analyser was thoroughly investigated both theoretically and practically and the results of these investigations were used in conjunction with a tungsten emitter. Excellent agreement was obtained between the usually accepted total energy distribution for tungsten and the corresponding .distribution measured with the present analyser. A method of obtaining reliable cadmium sulphide emitter was developed. These emitters were then used in the analyser and it was found that the agreement between theory and experiment was poor. Previous explanations of the lack of agreement are considered and are found to be doubtful. The theory of field emission from semiconductors is reviewed and possible reasons for the discrepancy between theory and experiment are proposed. Finally, further experiments are described which should prove or disprove the conclusions arrived at in this work.