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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Anharmonic properties of ionic crystals

Bruce, Alastair Douglas January 1973 (has links)
No description available.

Growth, electronics, processing andtoxicology of carbon nanotubes

Forrest, Gavin Alexander January 2006 (has links)
The growth of carbon nanotubes by chemical vapour deposition has been investigated using sodium chloride for facile post-CVD removal of the catalyst support. Saturated methanol vapour and acetylene gas have been used to grow tubes from bimetallic Co/Mo salts and cobalt-based nanoparticles at temperatures below 900°C. Multi-walled nanotubes and carbon fibres have been produced. Absence of single-walled tubes in the carbon product is shown to be characteristic of catalysts prepared using sodium chloride and other inorganic-salt supports, and the factors contributing to this outcome are explored. It is shown that sodium chloride actively inhibits carbon deposition and the formation of single-walled tubes in specific catalytic systems. L-α-phosphatidylcholine, a model for pulmonary surfactant, has been shown to disperse carbon nanotubes to a degree commensurate with commonly employed nanotube surfactants (e.g. Triton X-100 and SDS). The results obtained using this model surfactant have been used in understanding the potential interaction between nanotubes and the lung environment, and the relevance to issues surrounding the toxicology of carbon nanotubes is discussed. In collaboration with co-workers at the Scottish Microelectronics Centre, single-walled carbon nanotubes have been exposed to CF<sub>4</sub> and SF<sub>6</sub> plasmas to control their degree of functionalisation before substitution with 1,2-diaminoethane. The degree of amino functionalisation has been shown to depend on the degree of initial fluorination rather than oxygen or carbon defects and thereby presents a replicable route to n-type doping. The different types of nanotube-fluorine bonding produced by the plasma processes (e.g. covalent, semi-ionic) have been investigated as well as the effect of different plasmas on the doping process. Electrical characterisation has shown p-type semiconducting behaviour, for CF<sub>4</sub> functionalised tubes and n-type semiconducting behaviour for amino functionalised tubes. The degree of n-type behaviour increases with the amount of nitrogen attached.

Structural and dynamical studies of a ferroelectric crystal, KTaxNb1-xO3

Zaccai, Giuseppe January 1972 (has links)
No description available.

The electronic structure of disordered systems

Dooley, P. M. January 1976 (has links)
A theoretical study has been carried out on the electronic structure of concentrated disordered alloys and liquid semiconductors made up of components which are metallic in the pure liquid state (e.g. Mg-Bi, Li-Bi). For the latter there is strong evidence to suggest that chemical complexes form at the critical concentration (Mg3 Bi2, Li2Bi). The nature of the bonding is discussed in depth to reveal that it is unlikely that the bonding is ionic in the same sense as sodium chloride. Two separate calculations on the single hydrogen molecule in an electron gas show that the binding energy decreases with increasing electron number density. The assumption of the covalently bonded Mg3 Bi2 complex, and the associated change in binding energy with varying electron environment away from the critical composition, does not provide good agreement with experimental thermodynamic and electron transport data. Charge transfer within disordered systems is an effect associated with interatomic bonding. Its relationship with electronegativity difference and atomic cell size is considered in detail. No simple correlation is found to exist between charge transfer and electronegativity difference. A phenomenological study carried out on the liquid semiconducting Mg-Bi system has demonstrated that it is possible for an energy gap to occur at the critical composition with the filling of the s-p bands on the bismuth atoms. No presumptions of molecular formation are made. It does appear that this interpretation can provide a scheme, in terms of the positions of the elements within the periodic table, to explain the occurrence of many liquid semiconductors.

Optical and electrical instabilities in amorphous semiconductors

Hajto, Janos P. January 1993 (has links)
Studies have been carried out in the following areas: 1) Laser-induced crystallization The laser-induced crystallization of amorphous chalcogenide thin films has been studied, especially the kinetics and thermodynamics characterising the process. 2) Optical bistability and oscillatory phenomena The laser-amorphous semiconductor interaction has been studied especially c.w. laser induced optical bistabilities and oscillatory phenomena in transmittance and reflectance. Theoretical models of thermally induced optical bistability and oscillation have also been described. 3) Optical anisotropy The effect of linearly polarised c.w. laser light in inducing birefringence and dichroism in amorphous semiconductors has been studied. The kinetics of reorientation has also been investigated. A quantitative theory of optical anisotropy and reorientation has been described. 4) Current instabilities and electrical switching The effects of electrically induced current instabilities especially digital and analogue memory switching have been studied. A theoretical model has been described in terms of filament formation and tunnelling conduction in metal-amorphous silicon-metal structures. 5) Step-like current-voltage characteristics Unusual step-like current-voltage characteristics in electroformed metal-amorphous semiconductor-metal structures have been studied over a range of temperatures from 4.2 K to 400 K and theoretical models proposed.

Experimental studies of bubble domain materials

Thornley, Susan J. January 1976 (has links)
No description available.

Iso-electronic compounds and elements under high pressure

Maclean, John R. January 1998 (has links)
High pressure directly affects interatomic distances, offering a method of studying interatomic potentials, and the differing factors governing structural adoption. Using a combination of angle-dispersive x-ray powder diffraction and total energy quantum mechanical calculations, high pressure phase transitions and the structures of iso-electronic materials have been investigated. Synchrotron radiation and image-plate area detectors were used to collect data on high pressure phases of Si, Sb, Bi, PhS, PbSe and PbTe. A new method of extracting information, employing Monte-Carlo techniques from powder diffraction data was developed and employed in some of the structural solution. Both full-potential and pseudopotential <I>ab-initio</I> calculations have been used to compute electronic and structural properties of experimentally observed high pressure phases of Si, CuCl, CuI, CuBr and Sb. From the results of these calculations and experiments, a new high pressure phase of silicon (Si-XII or R8) has been observed, existing between the high pressure metallic β-tin structure and the metastable ambient pressure structure Si-III, also known as BC8. The pressure dependence, electronic structure and stability of this, the most complex structure of silicon, and all the relevant high pressure phases have been studied. Calculations were performed on structures adopted by the iso-electronic copper halide compounds, CuX (X=Cl,Br,I). The results of these calculations agree with results from previous experiments that the binary analogue of the BC8 structure observed in Si is a stable high pressure form, and also provide details on the nature of bonding and electronic structure exhibited by each material. Experiments and calculations performed on the group V elements Sb and Bi show that a monoclinic structure, obtained by distorting a previously proposed tetragonal structure, is adopted by both. Rietveld refinements of a further monoclinic high pressure form of Bi are given, supporting earlier experimental analysis.

A light scattering study of the borates Zn4B6O13 and Ni3B7O13I

Murray, Alan Fraser January 1978 (has links)
This work describes a light scattering study of two crystilline borates, a system for automated Brillouin spectroscopy, and a theoretical investigation of incommensurate-commensurate transitions. Firstly, the Raman spectrum of zinc metaborate (Zn4B6013) is reported and discussed, and a tentative assignment is made of vibrations of tha B-O framework. An interference feature in the spectrum is analysed in terms of models involving anharmonic phonon-phonon and phonon-continuum coupling. The inelastic and elastic light scattering spectrum of nickel-iodine buracite (N13B7013I) is presented for the first time, at temperatures between 6 K and 295 K. Anomalies in the phonon lifetimes and frequencies at 130 K are correlated with concomitant abnormalities in the structural, elastic and magnetoelectric properties, but a structural transition at this temperature is not indicated. The dynamics of the improper paraelectric-ferroelectric transition at 68 K are discussed, and possible symmetries considered. Furthermore, the data reveals a new transition at 7 K, which is thought to be both structural and magnetic. Critical modes are studied, and a strongly temperature dependent mode is identified, whose behaviour cannot be fully explained. A proven modular approach to computer control of experiments is presented, and a system for control of Brillouin scattering experiments is described. Examples of the resultant spectra are presented, along with some speculation as to future developments. Finally, the theory of incommensurate-commensurate lock-in transitions has been studied, and the method and conclusions are detailed. Two distinct types of lock-in transition are identified, and the corresponding distortion profiles are derived within a simple Landau theory.

Complex structures in tetrahedrally bonded semiconductors

Clark, Stewart January 1994 (has links)
Complex tetrahedral structures form good models for amorphous Group IV and III-V semiconductors. With a view of working towards examining non-crystalline materials, the structural, electronic and vibrational properties of complex tetrahedrally bonded semiconductors are investigated by various molecular dynamics techniques. First principles quantum mechanical molecular dynamics calculations are performed on two such structures and the effects of pressure on their behaviour is reported. A full free energy calculation using this method remains unfeasible and therefore an empirical bond charge model is used to calculate the full pressure-temperature phase diagram of the structures. Several surface reconstructions of a complex phase of silicon are then examined and the lowest energy surface of any silicon structure so far is found. Point defects in the diamond phase of silicon and carbon also give insight into various unusual bonding topologies that could be found in their amorphous phase. Structural and vibrational properties of several defects are considered. Finally, calculations on amorphous carbon and silicon at several densities are done and a comparison between the structural and electronic properties made. New bonding topologies are found in the structures including three centre bonding orbitals in the amorphous carbon models.

Raman and neutron scattering studies of phase transitions in ferroelectrics

Hosea, Thomas Jeffrey January 1978 (has links)
The complete Raman spectra, taken at high resolution, are described for lead germinate, Pb 5Ge3011, from low temperatures to above the ferroelectric phase transition. The line-shapes of the soft mode spectra are analysed with models of varying complexity. The underdamped line-shape has a three-peaked response and attempts at analysing this within simple anharmonic soft mode theory lead to divergences in the strength, damping and frequency. An appropriate line-shape is provided if low frequency coupling to phonon density fluctuations is considered and the resulting soft mode parameters recover their expected behaviours. The lifetime of the phonon density fluctuations, however, is much smaller than is expected and there is evidence that there are large contributions from previously unconsidered low frequency relaxation mechanisms at structural phase transitions. The complete Raman spectra are described for two crystals of barium doped lead germanate, (Pb1_XBax)5Ge3011, from low temperatures to above the respective transitions. The results are compared to those of the pure material, particularly with respect to the pronounced non-linear effects of barium doping on the soft mode and the transition temperature. A derivation is presented, for the form of the wave-vector dependence of the piezoelectrically coupled dielectric-elastic response, close to the ferroelectric phase transition in KD2PO4. Calculations of the resulting scattering cross-section give intensity maps in reciprocal space and these predictions are compared to the results of a neutron scattering experiment performed to measure the response in the [001] zone. It is shown how the presence of coupling provides an alternative method of obtaining the ferroelectric mode structure factor, and the experimental data, at (220), are analyzed to obtain this. The results are discussed, of a preliminary neutron scattering experiment to determine if the central peak in SrTiO3 can be attributed to precursory transformed surface regions.

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