Academic literature on the topic 'High thermal conductivity'

This thesis describes research undertaken to improve a technique for the measurement of the thermal conductivity of molten materials. The research follows on from the work of previous researchers who designed and tested an instrument for the measurements of the thermal conductivity of molten metals up to 750 K. The previously used transient hot-wire technique, which consisted of the experimental measurement of the voltage response of a sensor and a subsequent inverse Unite element analysis, has been significantly upgraded. The experimental part of the technique has been improved by the introduction of a new design of the sensor for the measurement of the thermal conductivity. Both the new and the original designs have been used to investigate the same material samples in order to demonstrate the robustness and repeatability of the experimental technique. Additionally, the finite element analysis employed has also undergone various major improvements and resulted in a new finite element model which not only represents the true geometry of the experimental device but also employs a more accurate solution of the transient, conductive heat transfer. The significant upgrade of the technique and the availability of two different sensor designs have helped to uncover systematic errors which could not have been previously identified and may have resulted in deviations of the measured thermal conductivity. Five original sensors and five sensors with the new design have been used to investigate the thermal conductivity of molten indium, tin and lead at various temperatures up to 750 K. The results have been compared to previously published data and the discrepancies have been discussed and explained. Each metal has been measured using at least two sensors and the consistency of the measured data has also been verified by using two different samples of pure tin. Besides the pure metals, the thermal conductivity of several metal alloys currently used in industry has been investigated within the same temperature range. The overall uncertainty of the measurements of the thermal conductivity is estimated to be ±3 %.

Apparatus to measure the thermal conductivity of YBa(_2)Cu(_3)O(_7-δ) at temperatures between 20K and 120K has been designed and constructed. The thermal conductivity is measured using a longitudinal steady state heat flow technique. Thermal conductivity measurements have been performed upon a sample of YBa(_2)Cu(_3)O(_7-δ) which has been subjected to a series of heat treatments in order to remove oxygen from the material. The measurements show conclusively that the thermal conductivity of YBa(_2)Cu(_3)O(_7-δ) is very strongly influenced by the oxygen content of the material. A reduction of the oxygen content of the material results in a substantial lowering of the thermal conductivity. To explain this result, a quantitative model has been constructed; the model demonstrates that consideration of the changes in phonon interactions alone cannot account for the differences in the behaviour of the thermal conductivity of YBa(_2)Cu(_3)O(_6) and YBa(_2)Cu(_3)O(_7). In addition; the model, shows that there must be a significant carrier contribution to the thermal conductivity in both the normal and superconducting states. A physical process has been proposed which provides the required large carrier contribution below T(_c). Further studies have been performed on a series of samples of YBa(_2)Cu(_3)O(_7-δ) which were sintered at slightly different temperatures. Qualitative analysis of the physical properties, of these samples has been performed.

This research describes the preparation of electrically and thermally conductive polymer composites. The filler used is short carbon fibers. These were dispersed in methyl methacrylate (MMA) and settled under different vibrational and gravitational forces, resulting in well packed sediments. To improve further the dispersability of the fiber/MMA system, steric stabilization was attempted by using organic dispersants of increasing chain length. Subsequent polymerization of the dense sediments produced composites with high fiber volume fractions. The electrical and thermal conductivities of these composites were studied. Fiber size, distribution, orientation and volume fraction are shown to have a profound influence on these properties. A general effective media equation, which relates percolation and effective media theories, is shown to describe the electrical conductivity of the composites. The specific thermal conductivity of the high fiber fraction composites is greater than that of stainless steel. Applications include electronic packaging and electromagnetic interference shielding.

Thermal Barrier Coatings (TBC) are used to shield hot sections of gas turbine engines, helping to prevent the melting of metallic surfaces. TBC is a sophisticated layered system that can be divided into top coat, bond coat, and the super-alloy substrate. The highly heterogeneous microstructure of the TBC consists of defects such as pores, voids, and cracks of different sizes, which determine the coating’s final thermal and mechanical properties. The service lives of the coatings are dependent on these parameters. These coatings act as a defensive shield to protect the substrate from oxidation and corrosion caused by elevated temperatures. Yttria Stabilized Zirconia (YSZ) is the preferred thermal barrier coating for gas turbine engine applications. There are a certain number of deposition techniques that are used to deposit the thermal coating layer on the substrate; commonly used techniques are Air Plasma Sprayed (APS) or Electron Beam Physical Vapour Deposition (EB-PVD). The objective of this thesis is to model an optimized TBC that can be used on next-generation turbine engines. Modelling is performed to calculate the effective thermal conductivity of the YSZ coating deposited by EB-PVD by considering the effect of defects, porosities, and cracks. Bruggeman’s asymmetrical model was chosen as it can be extended for various types of porosities present in the material. The model is used as an iterative approach of a two-phase model and is extended up to a five-phase model. The results offer important information about the influence of randomly oriented defects on the overall thermal conductivity. The modelled microstructure can be fabricated with similar composition to have an enhanced thermal insulation. The modelling results are subsequently compared with existing theories published in previous works and experiments. The modelling approach developed in this work could be used as a tool to design the porous microstructure of a coating.

The presented thesis focuses on study of transport and thermoelastic properties of materials under conditions of planetary interiors by means of high-pressure experimental tools and finite element modeling, and their role in the dynamics and states of cores of terrestrial planets. Experiments in laser-heated diamond anvil cell (LHDAC) in combination with numerical simulations of heat transfer in DAC are shown to yield information on thermal conductivity of a pressurized sample. The novel technique consists of one-sided laser heating and double-sided temperature measurements and utilizes a precise determination of several parameters in course of the experiment, including the sample geometry, laser beam power distribution, and optical properties of employed materials. The pressure-temperature conditions at the probed portion of the sample are, however, not uniform. To address this problem, thermal pressure in the laser-heated diamond anvil cell and anisotropic thermal conductivity originating from the texture development upon uniaxial compression have been studied by means of numerical simulations. The method for determination of thermal conductivity is applied to iron at pressures up to 70 GPa and temperatures of 2000 K, meeting the Earth’s lower mantle conditions and covering Mercury’s entire core. The obtained results are extrapolated to the conditions of the Earth’s core-mantle boundary using a theoretical model of the density dependence of thermal conductivity of metals and published values on Grüneisen parameter and bulk modulus. After considering the effect of minor core elements, the obtained value at these conditions supports case for the downward revision of the thermal conductivity in the core. From the point of view of core dynamics and energy budget, the lower thermal conductivity implies more favorable conditions to drive the dynamo. Similar scenario applies for Mercury where, for high values of thermal conductivity, heat flux conducted along the iron-core adiabat exceeds the actual heat flux through the core-mantle boundary. This leads to a negative rate of entropy production in the core that makes it impossible to sustain the dynamo process presumably responsible for the observed magnetic field of Mercury.

Previous measurements show that the thermal conductivity of $Y Ba sb2 Cu sb3 O sb{7- delta}$ in the basal plane is anisotropic with a large peak in the superconducting state. The magnitude of this anisotropy in the superconducting and normal states, and the dominant mechanism for heat conduction in the superconducting state are currently the subject of debate. We have measured the thermal conductivity of high quality $Y Ba sb2 Cu sb3 O sb{7- delta}$ for deoxygenated, twinned and detwinned samples along the a and b axes to shade light on this issue. We were able to measure the electrical and thermal conductivity using the same contacts and hence determine the Lorenz number L = $ kappa$p/T accurately.<br>Attributing the normal state anisotropy in the heat transport to electrons in the Cu-O chains, the Lorenz number takes on its full Sommerfeld value i.e. $L = L sb0.$ Under this assumption, the phonon conduction is about the same in the superconducting and deoxygenated samples.<br>Our results are discussed in connection with the two possible mechanisms for heat conduction in the superconducting state. We find that although a strong case can be made for the "electronic scenario" whereby the peak is due to rapidly increasing electron mean free path below $T sb{c},$ it is still not compelling at this stage.<br>In addition, it is found that the thermal conductivity along the a and b axes is isotropic at low temperatures, with a nonzero linear term in $ kappa,$ indicative of some uncondensed electrons as $T rightarrow$ 0. This low temperature isotropy contradicts previous explanations in terms of non-superconducting chains.