Solid Vacuum Research Articles

Theoretical and experimental study of surface waves on solid–fluid interfaces when the fluid sound velocity is larger than the shear wave velocity in the solid

Two surface waves can propagate on a solid–fluid interface: the Rayleigh and the Stoneley waves. The problem is well known when the fluid sound velocity cF is lower than the velocities in the solid (cS and cL). The Scholte–Stoneley dispersion equation has been studied when cF is greater than the shear wave velocity cS. The numerical results obtained for PVC–water and PlexiglasR–water show that there might exist two surface waves whose velocities are lower than cS. The two corresponding roots of the dispersion equation tend to the Rayleigh root of the solid–vacuum interface as the density of the fluid tends to zero. The structure of the two waves has been studied using the formalism of the evanescent plane waves. They are theoretically propagating without attenuation along the interface and exponentially decaying along the direction normal to the interface inside both media. These two waves have been experimentally observed. They exponentially decay along the direction normal to the interface inside both media. The experimental arrival times are in very good agreement with the theoretical ones.

Summary Abstract: High-resolution electron microscopy of the solid–vacuum interface

Summary Abstract: High-resolution electron microscopy of the solid–vacuum interface

Summary Abstract: Bridging the gap between solid–solid and solid–vacuum interfaces: A study of buried Si/αSi interfaces

Summary Abstract: Bridging the gap between solid–solid and solid–vacuum interfaces: A study of buried Si/αSi interfaces

Modification of electric field at the solid insulator–vacuum interface arising from surface charges on the solid insulator

The surface of a solid insulator in vacuum (and in a high gas pressure) becomes electrically charged when subjected to a high voltage stress. The surface charge density is proportional to the applied voltage. The magnitude of the surface charge density depends on the secondary electron emission characteristic, the geometrical shape, and the material of the insulator. The field enchancement, contributing to a lower withstand voltage of solid insulators in vacuum (and in high pressure gases), due to the presence of surface charge is computed along the surface of the solid insulator and at the triple electrode-solid insulator-vacuum junction near the anode and cathode. Different patterns of surface charge density distribution has been considered in order to evaluate their effects on the field enhancement. The polarity, the magnitude and the shape of the distribution of the surface charge density has been found to have a considerable effect on the field enhancement. The observed influence of the charge distribution patterns on the electric field suggests that in order to obtain the experimentally observed field enhancements, the insulator surface must possess a positive surface charge except in the region close to the anode, where it is negatively charged. The electric field and the potential distributions of the interfacial boundary between the solid insulator and vacuum is computed for different realistic surface charge distributions. The computed electric field at the triple junction for varying values of the dc applied field is then compared with the measured values reported in the literature. Good agreement is obtained. The effects of the magnitude of surface charge density, the applied field and the length of the solid insulator on the field enhancement at the triple junctions are also investigated. Nine different solid insulator materials having a relative permittivity in the range 2.1–13 and electron impact energies in the range 20–60 eV are examined. The effect of the surface charge on the withstand voltage of the insulating vacuum gap bridged by a solid insulator is discussed.

Influence of Radius of Curvature, Contact Angle and Material of Solid Insulator on the Electric Field in Vacuum (and Gaseous) Gaps

The insulation strength of a vacuum gap bridged by a solid insulator is lower than that in the absence of the solid insulator. The triple junction formed by the solid insulator, electrode, and vacuum in vacuum devices (or a gas in high-pressure apparatus) has a critical influence on the withstand voltage level of the insulating gap. The electric field is calculated using the charge simulation technique at the triple junction as a function of the junction geometry. The dependence of the field is studied for a varying contact angle a between the solid insulator and the electrode in the range 0 to 90°, for a varying ratio r/D of the radius of curvature r of the rounded edge of the solid insulator at the junction to the length of the insulator D, at a fixed r/D and varying r and D and for different practical insulator materials having relative dielectric permittivities ε£ in the range 2.1 to 12000. It has been found that the electrical field at the electrode-solid insulatorvacuum junction has a strong dependence on both the contact angle and the solid insulator material. The tangential and normal components of the electric fields are determined at the solid insulator-vacuum boundary and away from the boundary both inside the solid insulator and in the vacuum. Large enhancements in the electric field at the triple junction are found. The effects of the parameters α, r, and εr on the with-stand voltage of the insulating gap are discussed.

X-ray diffraction from adsorbed iodine on graphite

Our knowledge of surface structure at the solid–vacuum interface has been acquired largely through high-vacuum analytical techniques. Unfortunately these may not be applied in situ to solid–liquid or solid–high-pressure gas interfaces which are of greater practical importance. Ex situ investigations have been made but the transfers involved probably result in surface restructuring. It is therefore apparent that in situ techniques are required. Raman spectroscopy1 is applicable but lacks versatility, whilst neutron diffraction2 and SEXAFS3 are promising but depend on a reactor and synchrotron respectively. X-ray diffraction, on the other hand, is routinely performed and the theory is well developed though it has been assumed to be insensitive to surfaces. We report here the first observation of X-ray diffraction from a two-dimensional adsorbate formed at the solid–liquid interface, I2 on carbon, using position-sensitive detection to enhance sensitivity.

The Solid–Vacuum Interface (Proceedings of the 4th International Symposium on Surface Physics), edited by A. Van Oostrom and M. J. Sparnaay. Reviewer

<i>The</i> <i>Solid</i>–<i>Vacuum</i> <i>Interface</i> (<i>Proceedings</i> <i>of</i> <i>the</i> 4<i>th</i> <i>International</i> <i>Symposium</i> <i>on</i> <i>Surface</i> <i>Physics</i>), edited by A. Van Oostrom and M. J. Sparnaay. Reviewer

Effect of reconstruction during epitaxial growth

Surface reconstructions are first-order phase transitions observed at the solid–vacuum interface for low index surfaces of many covalently bonded materials. It is generally held that they result in the removal of bonds dangling into vacuum from the ideal surface and in a strengthening of back bonds in the crystal. Consequently, they are also expected to occur when the crystal is growing from its own melt or some other phase into which it cannot satisfy its dangling bonds. This consideration leads to prediction of rate-of-growth and orientation-of-growth substrate effects upon the perfection of the resultant crystal. Such effects are observed in good agreement with theory in several instances.

Dewatering Base Metal Mine Drainage Sludge

Abstract Sludge dewatering studies were carried out at the Brunswick Mining and Smelting Corporation’s No. 12 mill as part of a pilot plant program on the removal of heavy metals from base metal mining effluents. A mobile sludge dewatering trailer located on site was used to assess the feasibility of basket centrifugation, solid bowl centrifugation, vacuum filtration and pressure filtration as possible methods of dewatering. The effect of control variables on performance, and the applicability of existing bench-scale methods to design were studied for each process. Cake solids of 25% and suspended solids recoveries of 95% were attainable on all four pilot-scale dewatering units. The use of a basket centrifuge may not be practical due to very short cycle times which result from the high sludge feed solids concentration (7%). The addition of cationic polyelectrolyte was necessary to achieve recoveries in excess of 75% with the centrifuges, but was not required with either the vacuum filter or the filter press. The filter press, which produced a filter cake of 38 to 45% and a filtrate quality of 0 to 10 mg/1 of suspended solids, produced the best cake solids and effluent quality. However, the vacuum filter would require the lowest operating cost of units tested. Bench-scale work included penetration tests, Buchner funnel tests, capillary suction time, leaf filter tests and bench filter tests. Scale-up from bench to pilot plant was not possible for centrifugation with existing methodology. Filter yields and pressing time were predicted from bench tests for the vacuum filter and pressure filter respectively, and compared with the results obtained. Specific sludge characteristics such as zeta potential, particle size distribution and rheology were also determined and related to solids concentration and level of polymer addition.

Use of Auger Electron Spectroscopy and Inert Gas Sputtering for Obtaining Chemical Profiles

An apparatus is described in which surface chemical analysis by Auger electron spectroscopy (AES) is carried out simultaneously with inert gas sputtering to obtain composition profiles normal to the solid–vacuum interface. Chemical profiles are presented for as-evaporated and air-heated nichrome thin film resistors.

Rayleigh Waves on Curved Surfaces

Rayleigh waves, which are elastic surface waves occurring at the interface between a solid elastic body and vacuum, are investigated. The exact solution of some canonical elastodynamic problems in a circular cylindrical geometry is given, from which the Rayleigh wave contribution is isolated. The dispersion—i.e., the dependence of the velocity of propagation of these waves on the local radius of curvature (of the interface)—is investigated. The propagation velocity diminishes as the ratio between wavelength and radius of curvature diminishes, reaching an asymptotic value at “zero wavelength.” A comparison is made with the small-wavelength asymptotic approach of Keller and Karal, which does not take dispersion into account. It is found that the effects of dispersion should not be ignored down to rather small wavelengths. Moreover, it is found that there occurs a “critical” or “cutoff” wavelength, above which no proper Rayleigh wave can exist. Thus, a Rayleigh wave that propagates on an interface of variable curvature towards a point of minimum radius may be scattered and partially transformed into body waves.

Optical lattice vibrations in finite ionic crystals: II

A theory for the polariton modes (long-wave optical phonons coupled to photons) in finite ionic crystals of arbitrary shape is given. The theory is exemplified by detailed calculations for the case of a spherical specimen. Stationary polariton states of three types are obtained, which, for small enough crystals, are characterized by (i) large amplitudes inside the solid (bulk modes), (ii) large amplitudes near the surface of the solid (surface modes) and (iii) small amplitudes inside the solid (vacuum modes). The absorption spectrum of spherical diatomic samples is calculated and it is found that the polariton modes cause absorption in regions which are forbidden for infinite crystals. The generalization of the theory to the case of complex cubic crystals is given and applied to interpret the absorption spectrum of small SrTiO3 spheres.

Gas Content of Solid Aluminum by Solid Extraction and Vacuum Fusion

The determination of hydrogen in aluminum by two methods, solid extraction and vacuum fusion, and the preparation of suitable samples are described. Analyses of similar aluminum specimens by the two methods agree. In addition, results by solid extraction agree with those obtained on duplicate samples exchanged with the Research Laboratories of the British Aluminium Co. Little additional hydrogen is evolved in the vacuum fusion of a sample already degassed by a solid extraction.