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  • (2007) Tsai, Ping-Han
    Since the beginning of the industrial revolution in the late 18th century, there's an abrupt increase in need for energy and resources. Despite of advantages it offers, waste heat or energy generated by machines or vehicles exhaust is evolved into the surrounding atmosphere, resulting in the evolution of the Greenhouse effect. With rampant global warming, thermoelectricity - an environmentally benign technology that recovers waste heat and converts it directly into useful electricity or vice versa - has gradually become an interesting topic for researchers. Regardless, it has yet to be widely commercialised mainly due to its inferior energy conversion efficiency to the conventional power generators and cooling devices. The aim of this project was to explore ways of enhancing the efficiency of thermoelectric materials. NaC0204 is one of the state-of-the-art p-type thermoelectric materials to date due to its superior thermoelectric properties and stability at elevated temperatures. As a result of its layered structure, NaC0204 is highly anisotropic in thermoelectric properties. Literatures revealed that the thermoelectric properties along the in-plane direction are superior to that along the out-of-plane direction, which emphasise the significance of texture. In this work, the texturing mechanisms and the texturing process as a function of sintering duration were studied in detail and the synthesis procedure of highly textured NaC0204-; materials was optimized. The results indicate that the extent of texture increased with increasing process duration and then reached the maximum when the materials were sintered at 900°C for 12 hrs. The texturing kinetics obeys an exponential relationship, suggesting that the texturing processing is dominated by self-diffusion mechanism. The experimental evidence also shows that the extent of texture decreased when the annealing time exceeded 12 hrs. The mechanisms of such deterioration were highly due to the nature of grain growth in ceramics. Given that the Co ions in NaxC020 4-; have different valence between 3+ and 3.48+ depending on the Na content and have spins, it is expected that oxygen vacancies formed in the lattice would cause charge-spin interaction, which would have a profound effect on the thermoelectric properties. As part of current study, the oxygenation kinetics and possible mechanisms were investigated. Room temperature X-ray diffraction study shows a decrease in FWHM and a certain left shift of spectra for the oxygenated samples, suggesting an increase in d-spacing and lattice distortion as a result of oxygen deficiency. The extent of oxygen saturation increased with decreasing annealing temperature. The high oxygenation flux in the first three hours of annealing was possibly due to lower energy requirement for oxygen to occupy its vacant sites than that to interstitially occupy a site. These experimental results could be used to further study the oxygen dependence of thermoelectric performance. Moreover, the investigation of temperature effects on lattice structure of NaC02O4-; with/without oxygenation was carried out to understand oxygen contribution to the lattice structure and also thermal expansion behaviours. Refined high temperature XRD results revealed that lattice parameter a decreased and c increased over the temperature range investigated after oxygenation due to Coulomb interaction between the constituent ions. Linear thermal expansion coefficient along the a- b-axis direction is circa 1.105 x 10-5K-1 for the sample before oxygenation and 1.06 x 10-5K-1 after oxygenation. An almost three-fold increase was observed for the thermal expansion coefficient along the c-axis direction for the samples before and after oxygenation, suggesting that higher oxygen concentration suppresses the thermal expansion coefficient in all directions.

  • (2007) Liu, Chang
    This thesis reports a study on the effects of size distribution, moisture content and oil addition on bulk density and angle of repose of coal. The experimental work includes four stages. The first stage is to develop reliable experimental techniques. The results confirm that ASTM cubic foot test is reliable for measurement of bulk density and angle of repose if properly operated, although the latter is better measured in a piling process. Stages 2 and 3 are to investigate the effects of size distribution by using -3.55mm% for stage 2 and mean size do.s for stage 3, water content and oil addition on bulk density and angle of repose of coal. For each of them, empirical equations are formulated to predict bulk density and angle of repose. The results indicate that the fraction -3.55mm cutting size in stage 2 does not affect bulk density significantly, while the increase of do.s decreases bulk density to a minimum and then increases. Particle size distribution does not affect angle of repose much. The increase of moisture content decreases bulk density and increases angle of repose significantly. The increase of oil addition increases bulk density while decreases angle of repose significantly. The correlation between bulk density and angle of repose can also be observed: the higher bulk density, the lower angle of repose. There are other variables affecting bulk density and angle of repose. They include oil type, absorption time discharging height and external loading. Their effects on bulk density and angle of repose are quantified in stage 4. The results suggest that, a higher discharging position or larger external loading increase bulk density significantly. Angle of repose decreases when increase the height of discharging position. Diesel oil performed better than waste oil addition in terms of bulk density enhancement. For most of the cases examined, bulk density and angle of repose become stable after ~24 hours oil absorption time.

  • (2007) Tsai, Ping-Ju (Ben)
    AB5 hydrogen storage alloys have been intensively studied due to its superior ability to store hydrogen and release at ambient conditions. It is also a major component in the negative electrode of Ni-MH batteries. However, it has poor high rate capability and cycle life stability. Carbon nanotubes (CNTs) were found to store a tremendous amount of hydrogen, owing to the fact that they possess very large surface areas. It is because the hydrogen storage capacity is in general highly dependent on the surface area of the storing materials. The aim of this project has been to investigate the effect on electrochemical behaviours of Ab5 negative electrode in Ni-MH batteries by adding carbon nanotubes. The research also studied the influence of the ball milling treatments applied to both the Ab5 and CNTs. La0.59Ce0.27Nd0.08Pr0.06 (Ni0.76Mn0.08Al0.01Co0.15)5 AB5 alloy powder was used as active material in the negative electrode in the Ni-MH batteries, CNTs were used as additive, nickel powers as conductor in a three-electrode cell. Electrodes with compositions of AB5 + x wt.% CNTs (x=0, 5, 10) were studied. Activation, high rate capability and cycle life stability were investigated. The three-electrode cell in an open container with 6 M of KOH as electrolyte was connected to charge/discharge machine where galvanostatically charging and discharging took place. Hydrogenation of ball milled and as-received AB5 alloy powders were examined by conventional volumetric method. Morphology of AB5 and CNTs was examined by scanning electron microscopy (SEM) and transition electron microscopy (TEM), respectively. The phase identification and crystal lattice parameters were analysed by multi-purpose X-ray diffraction before and after ball milling treatments for both materials. The chemical composition of Ab5 alloy powders was tested using ICP chemical method. The results show the addition of CNTs in negative electrode in a Ni-MH battery enhanced the specific discharge capacity remarkably. A maximum discharge capacity of 252 mAh/g was observed for electrode with low energy ball-milled (LEBM) Ab5 with 5 wt.% of CNTs. This was due to the superior properties and great surface area of CNTs which allow more hydrogen to be stored and diffused onto the surface. Not only CNTs could act as a hydrogen reservoir in the negative electrode, it also acted as a conductor by building a conductive network between active material and nickel powders, and hence an increase in discharge capacity. However, the milling on CNTs alone will not improve the electrochemical properties of the electrode. In contrary, the activation profiles, high rate capability and cycle stability have been enhanced significantly when Ab5 alloy powders were ball-milled. The possible explanation is the smaller particle size and rough surface (and hence large surface area) obtained after ball milling induces a better hydrogen diffusion between the particles, as a result of shorter distance between particles after ball milling. Ball milling treatments on AB5 alloy powders did not improve the hydrogen absorption capacity. A highest value of 1.27 wt.% was observed for LEBM alloy powders. Ball milled samples have a slightly lower plateau pressure as compared with that of as-received alloy powders. In addition, only 4% of the maximum absorption capacity was lost after 10 repeated absorption and desorption cycles due to pulverisation of the particle over cycling. It can be concluded that LEBM Ab5 with addition of 5 wt.% CNTs, can significantly improve the electrochemical properties of negative electrode in Ni-MH batteries.

  • (2007) Yuan, Fei (Fred)
    Metal matrix composites can produce mechanical and physical properties better than those of the monolithic metal. Titanium alloys are widely used matrix materials as they can offer outstanding specific strength, corrosion resistance and other advantages over its competitors, such as aluminium, magnesium and stainless steel. In past decades, titanium matrix composites served in broad areas, including aerospace, military, automobile and biomedical industries. In this project, a revised powder metallurgy method, which contains cold isostatic pressing and hot isostatic pressing, was adopted to refine the microstructure of monolithic titanium. It was also used to manufacture titanium matrix composites. TiH2 powder was selected as the starting material to form Ti matrix and the reinforcements were sub-micron and nano-metric TiB particles. Mechanical properties and microstructure of commercial titanium composites exhaust valves from Toyota Motor Corporation have been studied as the reference of properties of titanium composites manufactured in this project. It has been shown that tensile strength and hardness of exhaust valves increase about 30% than those of similar matrix titanium alloys. Examination on powder starting materials of this project was also carried out, especially the dehydrogenation process shown in the DSC result. Mechanical properties and microstructures of titanium matrix composites samples in this project, as related to the process parameter, have also been investigated. The density of these samples reached 96% of theoretical one but cracks were found through out the samples after sintering. Fast heating rates during the processing was suspected to have caused the crack formation, since the hydrogen release was too fast during dehydrogenation. Hardness testing of sintered samples was carried out and the value was comparable and even better than that of commercial exhaust valves and titanium composites in literature. Microstructure study shows that the size of reinforcements increased and the size of grains decreased as the increasing amount of TiB reinforcements. And this condition also resulted in the increasing amount of the acicular alpha structure.