No description available.

No description available.

No description available.

In the present investigation, measurements were made at liquid-helium temperatures on single-crystal bismuth samples which had a stair-step geometry in order to study several thicknesses during one helium run. These samples were also thinned to extend the thickness range of the steps to a thinner region. In addition J.E. Parrott's theory is extended to include a diagonal anisotropic relaxation-time tensor and the effect of holes on the size effect. A discussion of the theory of Parrott, and the extension of Parrott's theory in connection with the experimental results is presented.

No description available.

The anistropy of the orbital and spin properties of conduction electrons in InSb has been measured simultaneously using a cyclotron-resonance type experiment. This represents the first time that the anistropy of effective mass in InSb has been directly measured using an optical method.

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. May 27, 2016. / Heat transfer in a wall with temperature dependent thermal conductivity and internal heat generation is considered. We rst focus on the steady state models followed by the transient heat transfer models. It turns out that the models considered are non-linear. We deliberately omit the group-classi cation of the arbitrary functions appearing in the models, but rather select forms of physical importance. In one case, thermal conductivity and internal heat generation are both given by the exponential function and in the other case they are given by the power law. We employ the classical Lie point symmetry analysis to determine the exact solutions, while also determining the optimal system for each case. The exact solutions for the transient models are di cult to construct. However, we rst use the obtained exact solution for the steady state case as a benchmark for the 1D Di erential Transform Method (DTM). Since con dence in DTM is established, we construct steady state approximate series solutions. We apply the 2D DTM to the transient problem. Lastly we determine the conservation laws using the direct method and the associated Lie point symmetries for the transient problem / MT2016

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of requirements for the degree of Master of Science. August 8, 2014. / We study the nonlinear models arising in heat transfer in extended surfaces (fins) and in solid slab (hot body). Here thermal conductivity, internal generation and heat transfer coefficient are temperature dependent. As such the models are rendered nonlinear. We employ Lie point symmetry techniques to analyse these models. Firstly we employ Lie point symmetry methods and determine the exact solutions for heat transfer in fins of spherical geometry. These solutions are compared with the solutions of heat transfer in fins of rectangular and radial geometries. Secondly, we consider models describing heat transfer in a hot body, for example, a plane wall. We then employ the preliminary group classification methods to determine the cases of the arbitrary function for which the principal Lie algebra is extended by one. Furthermore we the exact solutions.

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering, 2017. / In traditional two-dimensional shock wave theory the reflection of a shock wave off a surface is treated as an adiabatic process and that the reflection surface is perfectly rigid and smooth with an inviscid flow of the fluid. In reality it has been found that these assumptions are not entirely accurate, and that although they are a good indication in the regular and irregular reflection domains of shock waves over the surface, viscous and thermal effects are present within the flow field. It has been experimentally shown that the transition of regular reflection to irregular reflection exceeds the theoretical limit, which is known as the von Neumann paradox. This paradox has largely been accounted for in the formation of a viscous boundary layer behind the reflected shock wave, based on numerous experimental and computational studies. However, the thermal effects in the reflection process have largely been neglected as the assumption of heat transfer between the post-shock wave gas and the reflection surfaces is assumed to be invalid. These thermal effects were investigated by testing materials with a varying range of thermal conductivities (1.13 to 401 W/mK) and similar surface roughness’s below the suggested limit for hydraulic smoothness. Each experiment placed two test pieces at the same incidence angle, symmetrically in the shock tube. This allowed flow properties to be exactly the same for the two materials being tested with a single plane shock wave. Test Mach numbers ranged from 1.2790 to 1.3986, with tests conducted at shock wave incidence angles of 36◦, 40◦, 60◦ and 70◦. This allowed both the regular and irregular reflection domains to be tested. Shadowgraph images were created using a z-configuration optical set up. These shadowgraph images were analysed quantitatively based on the angles measured as well as qualitatively based on flow features and symmetry. Both the quantitative and qualitative tests indicated that there was a difference in the angles between the reflected shock waves and surfaces based on the material thermal conductivity. In the quantitative tests the value of this angle was larger for materials with a lower thermal conductivity, and smaller for ones with a higher thermal conductivity for the regular reflection cases. In the irregular reflection cases the angle between the reflected and incident shock waves was larger for materials with a higher thermal conductivity. The materials with midrange thermal conductivities had reflection angles that lay within the bounds of the glass and copper angle values. The qualitative images supported these findings showing asymmetry in materials with different thermal conductivities with the intersection of reflected shock waves lying closer to the material with a higher thermal conductivity. Control experiments using test pieces made from an identical material showed no bias due to the location of the test piece in the shock tube / XL2018

Thesis advisor: Zhifeng Ren / Silicon germanium alloys (SiGe) have long been used in thermoelectric modules for deep-space missions to convert radio-isotope heat into electricity. They also hold promise in terrestrial applications such as waste heat recovery. The performance of these materials depends on the dimensionless figure-of-merit ZT (= S2σ T/ κ), where S is the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity, and T is the absolute temperature. Since 1960 efforts have been made to improve the ZT of SiGe alloys, with the peak ZT of n-type SiGe reaching 1 at 900 - 950 C. However, the ZT of p-type SiGe has remained low. Current space-flights run on p-type materials with a peak ZT ~ 0.5 and the best reported p-type material has a peak ZT of about 0.65. In recent years, many studies have shown a significant enhancement of ZT in other material systems by utilizing a nanostructuring approach to reduce the thermal conductivity by scattering phonons more effectively than electrons. Here we show, using a low-cost and mass-production ball milling and direct-current induced hot press compaction nanocomposite process, that a 50% improvement in the peak ZT, from 0.65 to 0.95 at 800 - 900C is achieved in p-type nanostructured SiGe bulk alloys. The ZT enhancement mainly comes from a large reduction in the thermal conductivity due to the increased phonon scattering at the grain boundaries and crystal defects formed by lattice distortion, with some contribution from the increased electron power factor at high temperatures. Moreover, nanocomposite approaches have been used to study the thermoelectric properties of other material systems such as bismuth telluride (Bi-Te), bismuth antimony (Bi-Sb), and half-Heusler phases. We observed a significant improvement in peak ZT of nanostructured p- and n-type half-Heusler compounds from 0.5 to 0.8 and 0.8 to 1.0 respectively. The ZT improvement is mainly due to the reduction of thermal conductivity. This nanostructure approach is applicable to many other thermoelectric materials that are useful for automotive, industrial waste heat recovery, space power generation, or solar power conversion applications. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.