Showing papers in "Contemporary Physics in 1969"

Under what conditions can a glass be formed

Abstract: Summary Generally substances are more stable in a crystalline than in a glassy state. Therefore, to form a glass, crystallization must be bypassed. Under certain conditions, the melts of many substances can be cooled to the glass state. Whether or not the melt of a given material forms a glass is determined principally by a set of factors which can be specified to some extent in the laboratory, namely, the cooling rate, - T, the liquid volume, v], and the seed density, ps and upon a set of materials constants: the reduced crystal–liquid interfacial tension, α the fraction, f, of acceptor sites in the crystal surface, and the reduced glass temperature, Trg . The glass-forming tendency will be greater the larger are - T and Trg and the smaller are v]. ps, and f. The number and variety of substances which have been prepared in a glassy or ‘amorphous solid’ form have been greatly increased with techniques in which the material is condensed from solution on to a surface held well below its glass temperature. T...

Conduction and switching in non-crystalline materials

Abstract: The mechanism by which electrons move in crystalline semiconductors is now understood in some detail. The theoretical concepts necessary for the understanding of conductions in non-crystalline materials involve new ideas, which are discussed in this article. These include ‘localized states’ and ‘thermally activated hopping’. Some experiments designed to test the theory are described. Certain switching devices which make use of the electrical properties of non-crystalline materials are described, and theoretical explanations are suggested.

Mechanical Properties of Ceramic Materials

Abstract: The strength and fracture behaviour of ceramics are considered with particular reference to oxides. Ceramics fracture in a brittle manner and the strength of most materials is about two orders of magnitude less than the theoretical strength. Strength is discussed conveniently in terms of a modified Griffith equation and the important variables are an effective surface energy and an initial crack size. Both these variables can be related to microstructural features. The effective surface energy is relatively insensitive to structure. The Griffith crack size is more dependent on structure but the controlling factors are only partly understood. Further advances require more detailed information about the role of grain and phase boundaries and the relationship to Griffith cracks.

Electrical conduction in discontinuous metal films

Abstract: Electrical conduction in ultra-thin metal films is examined and the part played by an anomalous quantum mechanical tunnelling process is investigated. The process varies greatly with temperature, because the discrete metal particles forming the film are very small. The work required to remove a charge from such a particle being an inverse function of the particle size. The nature of the conduction path between the metal particles is considered, and it is shown that conduction occurs through the substrate supporting the film. Experimental results for gold, platinum end chromium films on soda and alumino-borosilicate glasses are analysed in terms of a proposed model. The consistency of the values of the barrier heights between the metal particles and the glass substrate, supports the quantum mechanical tunnelling model.

Scanning electron microscopy

Abstract: Scanning electron microscopy is a method of microscopy that permits resolution better than that of the optical microscope (about 100 A) while examining one surface of a bulk specimen. The technique depends on electronic application to microscopy and has been developed over many years by electronic engineers rather than physicists or microscopists. The main field of application so far has been non-biological, but two biological examples are given here. The reference list includes the main papers on scanning electron microscopy written in this laboratory.

Thermal conductivity: A review of some important developments

Abstract: Some recent developments are presented which relate to the thermal conductivity of metals and its measurement. These include the influence of sample size on the electronic thermal conductivity of very pure metals, the thermal conductivity minimum of aluminium and a few other metals at sub-normal temperatures, the high-temperature thermal conductivity and increasing Lorenz function of platinum, with particular reference to the experiments of Flynn and O'Hagan (1967), the thermal conductivity of molten metals and the recently reported Lorenz functions decreasing with increase in temperature to well below the theoretical value, and the encouraging results of an investigation in progress at the Thermophysical Properties Research Center (TPRC) on direct electrical heating methods for the measurement of metallic thermal conductivities to high temperatures. Modern computer techniques avoid the restrictions and approximations introduced in the many existing methods and allow thermal conductivity to be ac...

The Josephson effects

Abstract: This article reviews some of the experimental work on the Josephson effects. These effectsoccur in both superconductivity and superfluidity. In superconductivity they are found when two superconductors are separated by a thin insulating barrier (∼ 10−9 m thick), or connected by a weak link of metal. A supercurrent can flow between the superconductors, and electromagnetic and magnetic fields can interact with this supercurrent. The interactions demonstrate quantum effects in a particularly striking way, have led to a better understanding of the superconducting state, and have also found some interesting applications.

Critical point phenomena

Abstract: During the last decade considerable effort has been expended in the study of critical phenomena. These investigations are especially rewarding because they provide important links between several diverse branches of physics. For example, antiferromagnets, pure fluids, binary solutions and alloys all exhibit analogous critical behaviour. It is now firmly established that the classical phenomenological theories that have been proposed to describe these systems fail in the critical region: mainly because they do not account for the fluctuation phenomena which occur close to critical points, and are a poor approximation for systems with shortrange forces. Considerable progress has been made in formulating more realistic models, for example the king and Heisenberg ferromegnets, and some success has been achieved in obtaining solutions for these models. Similar theories may be postulated to describe the critical behaviour of other systems. Recent progress in theoretical understanding and refinement of ...

Ion implantation :A new method of doping semiconductors–I

Abstract: Summary A novel method for doping semiconductors is ion implantation where impurities are injected in the form of high-energy ions. The range of these ions is well defined and is a function of ion energy and mass, the nature of the crystal and its orientation to the beam. In addition t o its application to semiconductor device manufacture, ion implantation involves a good deal of interesting physics over a wide field, and in order to include the important aspects the article has been divided into two parts. In Part I the current theory for the range of heavy ions in amorphous and crystalline materials was summarized with the aid of a simple model that demonstrated some of the important features. Some experimental results for the ranges of particles in semi- conductors were given and the various methods used to obtain them were reviewed. In addition, typical apparatus used for ion implantation was described with particular reference to ion sources. In Part II some of the interesting techniques used to stud...

Modern electrostatic generators

Abstract: The fundamental principles involved in electrostatic power generations are described and the main features of modern machines discussed. Recent developments in the fields of liquid dielectrics and aerosol physics are now being applied to electrostatic generations. Some of the more promising devices may lead to eleotro static machines which have power densities an order of masnitude greater than that of existing machines and to generators of great mechanical simplicity.

The zone plate and its role in holography

Abstract: The classical Fresnel zone plate, with its alternate opaque and transparent zones, behaves like a lens which is simultaneously both converging and diverging. Improvements in the performance of such zone plates has recently led to some attention being given to their potential use as lenses in certain applications. The image formation involved in holography is strikingly similar to that characteristic of the zone plats and a hologram may be considered as a generalized zone plate. In this respect the sine-wave zone plate, in which the amplitude transmission is continuously distributed according to a sine-wave, is of primary importance. By examining the properties of such a plate it is possible to understand holographic image reconstruction.

Clusters in Liquid Metals

Abstract: The two divergent approaches to the structure of liquid metals, which regard them as being either condensed-gas or quasi-crystalline, are inadequate to explain the properties and behaviour of liquid metals. In this article the concept of entropy is used to suggest that some form of near-solid association of large numbers of atoms, not amenable to any rigorous mathematical treatment, is retained in the liquid state. The implications of ‘Bernel's concept of the statistical geometrical structure of liquid metals are discussed. This concept regards a liquid as a network of holes or Voronoi polyhedra and suggests that metallic liquids when heated should exhibit a second-order transformation because their thermal expansion is accompanied by a progressive change in atomic coordination. A second-order transformation is indeed suggested by the experimentally determined values of the specific heat of certain liquid metals. Other evidence in support of this contention is also presented. The work of Kumar an...

A kinetic liquid-simulator

Abstract: Devised primarily as a teaching aid, a two-dimensional kinetic simulator of monatomic liquids is described in which atomic binding is simulated by using oil-covered ball-bearings and thermal energy by vibrating them on rough-moulded glass. The measured velocity distribution and radial distribution curves are reported, and the simulator's use in studying viscosity and diffusion quantitatively and melting qualitatively is outlined.

Ultrasonic waves in liquids

Abstract: This paper begins with a general discussion of wave propagation in a fluid followed by an account of the accompanying attenuation and relaxation effects. The attenuation and relaxation effects due to viscosity, heat conduction and molecular processes are then discussed in some detail. Next there is a description of the various experimental techniques followed by an account of ultrasonic cavitation and a brief summary of other work.

Archimedes' principle in gases

Abstract: A simple microscopic deduction of Archimedes' principle as applied to gases is given in terms of the kinetic theory, and some conventional explanations of barometric effects are queried.

Bulk and surface transport in liquids—An introductory treatment

Abstract: Summary Eyring's thermodynamic treatment of diffusive and viscous flow in liquids is reviewed here in simple statistical terms. The requirement for dynamic equilibrium between the bulk and surface layers of a liquid is shown to lead to a surface tension, while capillary rise follows on demanding equality of diffusion rates along the wall, The simple models employed here predict quantities which are in order-of-magnitude agreement with experiment.

Polarized electron beams

Abstract: A beam of electrons may have the spins of the individual electrons preferentially polarized in a specified direction. The resulting state of polarization of the electron beam is described by a polarization vector p. This polarization vector is affected in direction and magnitude by macroscopic electric and magnetic fields. The relevance of this to precise measurement of the electronic g-factor, end to the problems of handling polarized beams (as in accelerating systems), is discussed. Some spinsensitive interactions, and various methods of producing intense highly polarized electron beams, are described. In a final paragreph an outline is given of the mathematical formalism used in describing polarization phenomene.

Progress towards a nuclear fusion reactor

Abstract: The article starts by outlining the physical conditions under which nuclear fusion reactions are most likely to be sustained, pointing out the sharp difference between the plasma state of matter necessary for nuclear fusion reactions and the thermodynamically more relaxed conditions sufficient for nuclear fission reactions. The confinement of hot dense plasmas by magnetic fields is the principal scientific problem to be solved, and progress in this work under the main headings of open-ended magnetic confinement systems and closed magnetic confinement systems is described. The article ends by reviewing the technological problems of nuclear fusion reactors, which are the next major obstacle to be overcome once the confinement problem is solved, and the economic possibilities of these reactors as electricity power producers.

Magnetic surface levels

Abstract: Magnetic surface levels represent a new fundamental quantum phenomenon. Its essence is that in the surface layer of a metal placed in a weak magnetic field a system of energy levels arises for electrons moving along shallow arcs whose ends rest upon the surface of the metal. This phenomenon is due to the specular reflection of electrons at the metal surface, which gives rise to a quantized periodic motion of electrons along these ‘skipping’ trajectories. The spectrum of the system of magnetic surface levels manifests itself as an oscillatory dependence of the surface-impedance on the weak magnetic field. The impedance oscillations are due to resonant absorption of microwave radiation in electron transitions between the levels.

The physics and mechanics of soil

Abstract: Soil has its origin in physical processes of comminution and abrasion of rock masses and in chemical processes of solution and recrystallization of rock minerals. After formation it is transported and distributed by a variety of agencies before subaerial or subaqueous deposition. Generally, soils experience a number of cycles of sedimentation, uplift, erosion and redeposition. Their mechanical behaviour under the stresses imposed by engineering structures varies with the type of soil. Both the deformation and failure of soils are of interest in the design of structures and these properties must be studied by special laboratory tests. The solution of a soil engineering problem therefore involves a field investigation and sampling programme to define the boundaries of the soil involved, laboratory testing of the soil, and analytical or computer calculations to determine the displacements and stresses which will occur in the soil and structure. Problems of particular interest include slope failures, the design of earth dams, retaining walls and piled structures, the displacement and failure of soil under buildings and the determination of soil properties on the surface of the moon and other planets.

Flight Through the Air at Very High Speeds

Abstract: The changes which its own speed causes in the properties of air in the neighbourhood of a very high-speed aircraft can alter the aerodynamics of tho flow around the vehicle. This paper outlines the nature of these changes in air properties, mentions the techniques by which they can be studied in the laboratory, and also indicates their significance for flight at very high speeds.