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Decompositions of solids are typically of the form: A(s) ----> B(s) + gases. Symmetry-controlled routes (based on known and hypothetical crystal structures) for transforming the solid reactant into the solid product were devised as possible decomposition pathways. Lattice energies of the reactants, of the postulated transient intermediate structures and of the final solid products were then estimated by crystal modelling procedures. Profiles of lattice energy changes during the proposed decomposition routes were constructed and any energy barriers were compared with experimental activation energies reported for the thermal decompositions. The crystal modelling was performed with the computer program WMIN. Calculation of the lattice energies involved the development of a model potential for the perfect lattice and the evaluation of the interatomic parameters. The potential was based on the Born model of ionic solids using the Buckingham potential (Ø(r)= Ae⁻r/p - C/r⁶) to describe the short-range energy contribution. Empirical fitting was used to establish reliable interatomic energy parameters. The reliability of the interatomic potentials was assessed by calculating crystal structures and lattice energies (which were not included in the fitting). The particular reactions selected for modelling were the decompositions of the alkaline-earth metal (Ca, Sr, Ba) peroxides and carbonates: M0₂(s) ---> MO(s) + ¹/₂0₂(g) MC0₃(s) ---> MO(s) + CO₂(g)The lattice energies calculated for the known structures were in good agreement with reported values, (except for Ba0₂ and BaC0₃) which provided support for the adequacy of the potential model used. Activation energies calculated for the decomposition of the carbonates were in the correct order but hlgher than experimental values, i. e., 422, 422, 465 and 499 kJ mol̄̄⁻¹ compared to the experimental values of 205, 87(?), 222 and 283 kJ mol̄̄⁻¹ for CaC0₃ (calcite), CaC0₃(aragonite), SrC0₃ and BaC0₃. The values calculated for the peroxides (91 and 100 kJ mol⁻¹ compared to the experimental values of 119 and 185 kJ mol⁻¹ for Sr0₂ and Ba0₂ respectively) were less satisfactory but could be a reflection of the poor structural data used for the peroxides. The significance of this approach to the modelling of solid decompositions is discussed.

The relationship between the fabric parameters, as independent variables, and the comfort related properties, as dependent variables, of commercial suiting fabrics, containing both natural and man-made fibres, have been studied. The fabric parameters measured in the study were mass, thickness, density, and air permeability. The comfort related properties, tested on a Permetest instrument, included water vapour permeability, water vapour resistance, and thermal resistance, with the moisture permeability index being derived. A total of 26 commercial suiting fabrics, covering a wide range of mass, as well as different fibre types and blends and fabric structures, was tested. The fibres covered, included wool, polyester, viscose, and cotton, while the fabric structures covered were 1x1 plain weave, 2x1 twill and 2x2 twill weave. The objectives of this study were to determine the empirical relationships between the measured fabric properties and the measured comfort related properties, and to determine the role, if any, of fibre type and fabric structure, since many claims are made in practice concerning the relative advantages, in terms of comfort, of a specific fibre type or blend, or fabric structure, over others, some of which being supported by research results. Since the fabrics covered, were commercial and were not engineered so that the different fabric parameters (independent variables) and fibre type and blend, as well as fabric structure could be varied independently of each other, the effects of the various fabric parameters on the fabric comfort related properties were determined and quantified by multiple regression analyses (multi-linear and multi-quadratic), and the best fit regression equations, and contribution of each parameter to the overall fit established. It was found that fabric thickness and fabric mass had the largest effect on the comfort related properties as measured here, fabric mass determining, to a large extent, water vapour permeability, and fabric thickness, thermal resistance. The rest of the fabric parameters included in the study had only a relatively small effect on the measured comfort related properties. The influence of fabric parameters, with particular reference to mass and thickness, on the measured comfort related properties, were much greater than that of fibre type or blend, or fabric structure. It was, therefore, concluded from this study, that the fabric parameters, as opposed to the intrinsic characteristics of a particular fibre, whether natural or man-made, largely determined those fabric comfort related properties measured here.

Continuous wave thermal propagation experiments were made with two single crystals of bismuth at frequencies up to 7 kHz. The experiments were performed at temperatures close to 4 K (i. e. close to the dielectric-like thermal conductivity peak). Accurate phase shift measurements were made in order to permit the detection of small departures from diffusive propagation. Attenuation measurements were also made. A summary of some microscopic theories of time-dependent thermal propagation in dielectric crystals is given. It is concluded that, for dielectric crystals in both the "hydrodynamic" and "ballistic" phonon gas regimes, the initial deviations from diffusive propagation will be described by a modified heat equation of the Vernotte type: [formula omitted] with appropriate identifications of the relaxation time. The possibility that the small numbers of charge carriers present in bismuth might lead to different forms of deviation is explored. Several types of thin-film insulating layers and superconducting alloy thermometers were investigated. Kodak Photo-Resist was found to be the most useful insulating material. This was used in conjunction with constantan heater films and Pb-In alloy thermometer films. The heat wave detection system employed a radio frequency thermometer bias current, a radio frequency tuned circuit, an envelope detector and phase-sensitive detection of the audio frequency heat wave signals. Heat wave phase lags were measured with a precision of 1°, using the phase-sensitive detector as a null detector. The measurements were analyzed in terms of a thermal transmission line model based on the modified heat equation given above. The electrical analogue of τ in such a model is L/R. A thermal leakage conductance term ⩋(electrical analogue G/C) was included in the model. The results at low frequencies were in excellent agreement with those expected on the basis of the transmission line model under conditions of diffusive propagation at high attenuations. Values of the apparent diffusivity obtained from these measurements were in reasonable agreement with the results of D. C. experiments made by other workers on comparable specimens. The quantity ⩋/ω was shown to be small at all frequencies used. Phase lag measurements at higher frequencies indicated significant departures from diffusive propagation in both crystals. (The crystals had different orientations.) The measurements in this range suggested a harmonic-wave-like mode of propagation. This mode appeared to break down at the highest frequencies examined. Evidence is presented to show that the observed deviations reflected thermal properties of the bismuth crystals rather than properties of the thin films, or spurious electrical effects. The apparent wave velocities were lower, and the corresponding relaxation times were longer than those predicted on the basis of the microscopic theories and from the diffusivity values obtained at low frequencies. In view of these numerical discrepancies, it is suggested that the wave-like mode could be a mode peculiar to the bismuth system, rather than the "second sound" mode predicted for ideal dielectrics. Some further experiments are suggested. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

In engineering practice it is important to know which factors affect the thermal and electrical resistance of metal contacts. This thesis is to investigate some of these factors such as surface roughness and contact pressure. Thermal electrical contact resistance ratios for metal contacts were calculated from the experimental data. The technical literature was searched, and several papers were found in which either thermal or electrical contact resistance was studied separately. However, none of the papers recorded data for both thermal and electrical resistances for the same samples. The information found in these papers has been used as a background for understanding the nature of thermal and electrical contact resistance. Both of these contact resistances are primarily a function of the load on the contact and the condition of the surfaces. At low pressures only a small fraction of the total gross area of the contacts is in metal-to-metal contact. Increasing the load, flattens the “hills” and reduces both the thermal and electrical contact resistance. This phenomenon is called “spreading resistance” since the flow of heat or electrical current must spread out after they pass through the restricted areas that are actually in contact. Another type of thermal and electrical resistance, which is called “interface resistance", is caused by a film of foreign material such as an oxide, etc. on the surfaces of the contacting “hills”. If the space between the “hills” of a contact is filled with air, there is a heat flow by convection currents. The literature indicates this quantity of heat flow is approximately one thousandth of the total heat flow through metal contacts. Since the only electrical current conduction mechanism acting between areas not in actual metallic contact is that due to thermionic emission, the electrical resistance for these areas will be extremely high at room temperature for which thermionic emission is negligible. The experimental apparatus to measure both the thermal and electrical contact resistances consists mainly of a bellows-actuated press which is operated remotely under a vacuum bell. The press pressure loads the sample metal wafers. A thin film heat meter is used to indicate the quantity of heat flowing through the metal contacts. The temperature drop caused by the contacts is measured with thermocouples. The temperature difference and the quantity of heat flowing is used to calculate the thermal contact resistance. A strain gage on the bellows-press stem measures the loading on the contact surfaces. Electrical probes are used to measure the electrica1 resistance across the contact surfaces. The thermocouples and electrical resistance probes are permanently installed in the outer two smooth copper wafers. This makes it possible to quickly change to other sets of sample wafers of other metals and finishes. In order to use this permanent arrangement, it is necessary to finish two mating surfaces of the particular set of metal wafers to be tested, similar to the permanent smooth copper wafers so that these two extra mating contact resistances can be found and thus be subtracted from the overall contact resistance. The data indicates that the thermal-electrical contact resistance ratio can be changed by changing the load on the contacts. The heat meter had performed very well, and this new method of measuring heat flow will undoubtedly become a standard method of measuring heat flux.

A thesis submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy / The cracking of mass and structural concrete due to thermal stress is a major problem in the concrete construction industry. Concrete will crack when the thermal stress exceeds tbe tensile strength of the concrete, Decisions on the type of concrete mix, cooling facilities and construction techniques to be used in the erection of a concrete structure can only be made if the thermal behaviour and strength of the concrete can be predicted during hydration. This thesis describes the development of a low cost, computer controlled, adiabatic calorimeter to determine tlte heat of hydration and a probe to determine the thermal conductivity or concrere samples. The main thrust of this thesis is the development of the thermal conductivity probe which, for the first time, can measure the thermal conductivity of concrete through all stages of hydration. A thermal model was also developed to verify the results, and the use of the calorimeter for temperature matched curing tests is also discussed. Results, obtained from the test procedures described, will provide far more accurate predictions of the temperatures in concrete structures than was possible in the past. / Andrew Chakane 2018

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