The concept of field-dependent tensors (tensors the components of which depend on the direction of an applied field) is generalized to define quantities which have different tenserial character in different subspaces (field subspace and geometrical or physical subspace) of the whole space. The transformation laws for these field-dependent tensors are worked out and the weighted and relative field dependent tensors are defined. The calculus of field dependent tensors is established through which the operations of addition, subtraction, inner and outer multiplication, contraction and differentiation of field dependent tensors are defined and the conditions under which each operation can be performed are discussed, and the quotient law for field dependent tensors is also worked out. The effect of magnetic crystal symmetry on the forms of field-dependent tensors is considered and a generalized Neumann's principle is defined. Furthermore, it is proved that in working out the magnetic point group of symmetry operations, identification of the magnetic moment inversion operator with the time-inversion operator is not correct. The effect of the symmetry operations of the magnetic point groups on the field dependent tensors representing the transport properties of a magnetic crystal is considered. Different prescriptions (A, B and C) given by different workers for finding the magnetic symmetry restricted forms of the magnetoconductivity tensor σ(_ij)(B) are discussed, the objections to each prescription are pointed out and then a new prescription (D) is given. Following prescription D, the restriction on the forms of the magnetoconductivity tensor σ(_ij)(B) the magnetoresistlvlty tensor σ(_ij)(B), the magnetothermoelectric power σ(_ij)(B) the magnetothermal conductivity K(_ij)(B) and the magneto-Peltier effect π(_ij)(B) imposed by the symmetry of crystals belonging to each magnetic point group are found. Finally the way in which the permittivity of a crystal belonging to a magnetic point group can be represented by a field dependent tensor is discussed.

No description available.

Materials based on ceria (Ce02) are used in many catalytic applications [2-5]. These applications make use of the unusual properties of ceria, namely, the ability to shift between its two oxidation states, Ce(III) and Ce(lV), and the high mobility of bulk oxygen species - properties that allow ceria to behave as an oxygen buffer. It is important therefore that we understand more fully how vacancies (surface vacancies are the sites of catalytic activity) segregate to ceria surfaces and also how oxygen atoms move in the ceria surface. In this study, we investigate vacancy segregation and focus on studying transport at or near the {Ill} surface of ceria, chosen because it is the most stable surface [8] and hence most prevalent. Many applications take advantage of the high oxygen storage capacity (OSe) of ceria. We propose a new polycrystalline multilayered nanotube structure that could go some way to further unlocking the oxygen storage capabilities of the material. We illustrate how our simulation models are constructed and further investigate the potential reactivity of the new structure, by comparing predictions of vacancy cluster segregation behavior to that predicted for the most stable flat {Ill} surface.

No description available.

This study of the thermal desorption of zinc from GaAs was carried out in ultra-high-vacuum using molecular beam and mass spectrometric techniques. Preliminary experiments showed that the GaAs substrate may be heated to 930K without altering greatly the stoichiometry of the surface, while above this temperature it will evaporate with preferential loss of arsenic. The sticking coefficient of zinc on GaAs was found to increase with coverage, reaching a value between 0.8 and 1 at high coverages, and the pumping speed of the vacuum system for zinc was found to be very high. Auger electron spectroscopy of the GaAs substrates was carried out to determine the contaminants present on the surface. The desorption spectra of zinc from GaAs were studied as a function of initial zinc coverage. Care was taken to match the experimental conditions with those assumed in the theory of thermal desorption. On all GaAs substrates used, a low temperature zinc desorption peak was seen which grew with zinc coverage without saturating. For the cleanest specimen used, this peak was analysed, and, from Arrhenius plots, the desorption energy of the zinc was found to be about 16kcal/mole over a range of coverage from about 5 x 10[15] to 1.4 x 10[17] atoms/sq. cm. This main peak was probably due to evaporation from the nuclei of zinc on the surface. During the last stages of desorption the desorption energy of the zinc fell from about 16kcal/mole at a coverage of 5 x 10[15] atoms/sq. cm to 6-8 kcal/mole at about 5 x 10[14] atoms/sq. cm. As the number of desorption runs increased the nature of the GaAs became permanently changed: this may have been due to diffusion of zinc into the top few atomic layers of the substrate. The nature of the adsorbate-adsorbent and adsorbate-adsorbate interactions complicated the analysis and this allowed comparisons to be made between the various methods of analysing thermal desorption curves.

Long range interactions between the atoms in a rare gas crystal are developed in a many-body multipole series formalism. The various interaction terms are expressed as the product of a geometric part, which depends only upon the lattice structure, and a multipole interaction constant appropriate to the atomic species. The interaction constants for Neon, Argon, Krypton and Xenon are calculated by means of time-dependent Brueckner-Goldstone perturbation theory and provide results of a higher expected accuracy than previous calculations and in many cases provide results previously unavailable. Lattice summations are performed for the geometric part of the interactions. To overcome the slow convergence of the tripledipole lattice summation, the finite summation range is extended, and and extrapolation to infinite range is accomplished analytically by means of a continuum model for the crystal. The contributions to the various properties of the rare gas crystals of three-body multipole long range interactions are calculated. In particular the lattice energy, the harmonic zero point vibrational energy, the elastic constants C[11], C[12] and C[44] and the bulk modulus are evaluated for each of the rare gas crystals at the absolute zero. The effects of the high order multipole terms are shown to be not negligible. The quantity delta = (C[44] - C[12])/C[12], which is always positive when calculated with only two-body forces, is reduced when three-body interactions are included. In the cases of A, Kr and Xe, delta in fact becomes negative. For the only experimental data available delta is found to be negative for Argon providing strong evidence for the existence of three-body forces. In the final chapter a number of points arising from the thesis are discussed and some indication is given of the possible direction of future developments in this area of research.

In this investigation a wide range of disorder has been studied in various solid materials. It has been shown that normally brittle materials, including diamond, could be deformed mechanically, and the retained strain relieved by annealing. However, a far greater degree of disorder was exhibited by materials subjected to conditions severely restricting crystal growth. Conventional X-ray line broadening methods were used for the analysis of the disorder induced in the brittle materials. As these analytical methods were inappropriate in the other cases, the potentialities of diffraction computation, based on the Debye interference function were studied, with particular reference to Raney nickel. Alignment order also formed an important microstruetural feature of several of the materials investigated. In cortical bone the texture was found hardly to vary in the bones examined. The textural changes produced by the severe cold-rolling of polyethylene were modified by the addition of several types of branches, whilst the internal texture of shock-quenched diamond particles indicated the structural relationship between the synthetic diamond and the parent graphite.

The interactions between point and extended defects in b.c.c. metals have been studied using real space computer simulation techniques. With such techniques a model crystal is set up into which the defects are introduced and the equilibrium atomic configurations and energies are found by minimizing the potential energy of the system. For the present purposes, the interatomic interactions were defined using five different empirical potentials matched to physical properties of alpha-iron, molybdenum and tungsten. Initially, various properties of vacancies and divacancies were studied under controlled conditions of uniaxial and hydrostatic stress, The properties were the energy associated with the formation of a single vacancy, the binding energies of the 1/2 , and 1/2 divacancies and the migration energies of these defects, The uniaxial stresses were directed along several distinct axes for each defect and in general the changes induced in the properties were anisotropic and larger than those produced by equivalent hydrostatic stresses, The interactions of vacancies and divacancies with the two possible types of {112} twin boundary were then considered. Interaction and migration energies were obtained as a function of defect separation and uniaxial stress applied perpendicular to the interfaces. For both single and divacancies, the maximum binding energies for the two boundaries occur when the vacancies lie adjacent to the interface. In addition, marked changes in vacancy migration energy result for specific jumps near the boundaries. Finally, the interactions of single vacancies with the 1/2 {110} edge dislocation were studied. The dominant interaction mechanism was shown to be of the inhomogeneity type, and for certain vacancy locations within the dislocation slip plane, the position of the centre of the core was seen to move. The vacancy sites for which this behaviour occurs are correlated with the locations of the 'fractional' components of the Burgers vector, into which the dislocation core is split.

No description available.

It has been the purpose of the present work to measure diffusion rates in metallic grain boundaries; to find whether these rates are influenced by relative motion in the boundary plane of the contiguous crystals, and to interpret the results of the investigation in terms of recent theories of metal structure. The back- ground to this work is of a dual nature and is concerned with the form of intercrystalline boundaries and with diffusion in strained systems. Accordingly a review will be made of past and present grain boundary theories and of the present position with regard to the effect of straining on measurements of bulk diffusion coefficients.