Abstract The process of the Membrane Electrode Assembly in the Proton Exchange Membrane Fuel Cell and the controllable variables: the pressure, the temperature, and the time of the hot pressure in the producing period would be discussed here. The experimental result of the MEA revealed that for Nafion112 and Nafion117 membranes, the conditions under the hot pressure are the same in the temperature. However, the pressure used in Nafion112 should be lower than Nafion117. In this case, the better function of the MEA can be achieved. Because Nafion112 is thinner, its water in the process of the hot pressure would be lost with extreme ease. This has a very serious impact on the function of the MEA. Therefore, to improve the MEA¡¦s function, the MEA should be boiled by water after being fabricated. The outcome of the research showed that for the purpose of improving the function of the MEA, some humidifier structure adding to the design of STACK is necessary. In order to be familiar to the related practical skills of STACK, PEMFC is brought into use for an electric bicycle in this research. Because the maximum power of STACK is only 150W, which is almost equal to the one-third power of the electric bicycles available such as the 400W of GIANT-Lafree electric bicycle. Besides, the speed of the electric bicycle is too slow when it is operated by itself, but it will be run more smoothly by the means of the assistant power. If the PEMFC electric bicycle can make more useful STACK and be redesigned, not be composed as it used to be, the use of the PEMFC electric bicycle will be steadier, more efficient, and more beneficial to the environment.

The goal of this PhD thesis was to develop dense aluminium compound-cubic boron nitride composites with a high cBN content. To achieve this goal, two different strategies were used: infiltration of cBN preforms and hot pressing of cBN-Al mixtures. The particle size of the cBN and the amount of aluminium were systematically varied and the influence of these parameters on densification and selected properties was evaluated. A basic understanding of the product that was formed over certain temperatures and times provides information that can be used in optimizing the infiltration and hot pressing of cBN with Al. For this reason, the reaction kinetics between Al and cBN was initially investigated. The reaction kinetics of the chemical interactions between Al and cBN was investigated in detail using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The reaction was studied using samples containing 50 volume percent of Al and 50 volume percent of cBN (12 μm) hot pressed at 800 oC. The prepared samples were allowed to react isothermally at temperatures between 1 000 oC and 1 400 oC under nitrogen (N2) and argon (Ar) atmospheres. It was found that the degree and rate of the reactions increased with increasing temperature in both N2 and Ar atmospheres. The degree of aluminium nitride (AlN) formation was considerably higher under N2 than under Ar. The difference in the formation of AlN between the two atmospheres was attributed to the reaction of N2 gas with the sample due to the open porosity. The infiltration of partially hexagonalized cBN matrix with molten Al was studied. The samples were found to have a density higher than 97% of the theoretical density. It was found that the amount of soft hBN phase present in the sample (due to hexagonalization) increases at temperatures higher than 1 300 oC, resulting in a hardness of the final material of Hv10 = 6.5 ± 4.8 GPa. Because of thie poor hardness this route was abandoned. Cubic boron nitride powder (12 μm) without hexagonal boron nitride (hBN) was also infiltrated with aluminium. The infiltrated samples were found to exhibit a density higher than 96% of the theoretical density. The Al reacted with cBN and no hBN was observed at temperatures below 1 400 oC. The resulting product showed a Vickers hardness of Hv10 = 14.4 ± 1.6 GPa compared with Hv10 = 6.5 ± 4.8 GPa obtained with the partially hexagonalized cBN matrix. Infiltration of 3 μm cubic boron nitride increased the Vickers hardness to 22.0 ± 0.6 GPa. However, this infiltration was not very reproducible. Al-cBN cermets were hot pressed at temperatures between 800 oC and 1 750 oC and at a pressure of 50 MPa in vacuum. The effect of the particle size of the starting powders, as well as the effect of the starting compositions and temperature, was investigated. The materials could only densify up to 80 – 92% of the theoretical density. After hot pressing at 800 oC, only Al and cBN could be observed by XRD, whereas higher hot-pressing temperatures resulted in the formation of AlN and AlB2 which retard the densification. The microstructure of the hot-pressed materials was studied using SEM. It was observed that there are oxide layers at the interface between the Al and cBN phases. The presence of these oxide layers prevented the Al from spreading, thereby preventing full densification.

Thesis advisor: Zhifeng Ren / Solid state cooling and power generation based on thermoelectric principles are regarded as one of the technologies with the potential of solving the current energy crisis. Thermoelectric devices could be widely used in waste heat recovery, small scale power generation and refrigeration. It has no moving parts and is environmental friendly. The limitation to its application is due to its low efficiency. Most of the current commercialized thermoelectric materials have figure of merit (ZT) around 1. To be comparable with kitchen refrigerator, ZT is required at room temperature. Skutterudites have emerged as member of the novel materials, which potentially have a higher ZT. In the dissertation, my investigation will be focused on the optimization of CoSb₃ &ndash based skutterudites. Starting with Co and Sb elements, CoSb₃ will form through a high energy ball mill. Unfortunately, even after 20 hours, only a small percentage of the powders have transformed in into CoSb₃. Then the powders will be compacted into bulk samples by DC-controlled hot press. CoSb₃ single phase will form after press. Characterization of the structure and thermoelectric properties will be presented with details. The effects of synthesis conditions on thermoelectric properties of skutterudites were studied and discussed. Several possible methods of improving the ZT of N type skutterudites were applied. The highest obtained ZT thus far is about 1.2 from Yb doped CoSb₃. For a group of samples with nominal composition Yb_xCo₄Sb₁₂, the increased Yb concentration in our samples not only enhanced the power factor due to electron doping effect but also decreased the thermal conductivity due to a stronger rattling effect. In addition, the increased grain boundary density per unit volume due to the small grains in our bulk skutterudite materials may have also helped to enhance the phonon scattering and thus to reduce the thermal conductivity. Single and double doping methods with different combinations were also tried. So far, none of them have surpassed ZT of 1.2. Mixing different materials with Yb_0.35Co₄Sb₁₂ so far to increase the phonon scattering was also performed. No dramatic thermal conductivity reduction was observed. Small amounts of Fe/Mn substitution on Co sites will decrease the power factor to undesired degrees. Some results with Nd filled P type sample will be briefly introduced. P type samples are also obtained through substitution on Sb site. Preliminary work on preparing the electrode for CoSb₃ will be presented in the dissertation. CoSi₂ has low resistivity, and a similar coefficient of thermal expansion (CTE) as of doped CoSb₃. It is good electrode candidate. DC controlled hot press is used to make the contact. Thermal stability of the contact was tested. Small cracks will form in the contact area, further improvement is necessary. Finally, my previous work on ZnO nanowire growth is briefly introduced. Large throughput of ZnO nanowire could be obtained with NaCl as the support to promote the conversion of Zn powder to ZnO. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Thesis advisor: Zhifeng Ren / Thermoelectric (TE) technology is an environment-friendly one due to reduction of carbon emission, which can be widely used either for power generation or for refrigeration. Basically applications of TEs are based on TE effects, which involve the transition between heat and electricity. Despite the superior advantages of being solid state and providing a clean form of energy, TE technology so far only finds its niche area of application due to the relatively less efficiency compared to traditional methods. The efficiency of a thermoelectric device is solely determined by the dimensionless figure-of-merit (ZT) of thermoelectric materials. According to the definition, ZT is equal to square of Seebeck coefficient times electrical conductivity times absolute temperature divided by thermal conductivity. Therefore, a good thermoelectric material should possess high Seebeck coefficient and electrical conductivity while low thermal conductivity, so called phonon glass electron crystal (PGEC). In bulk materials, it is challenging to further improve ZT or independently vary individual parameters without affecting others, mainly due to the interrelated relationships among these three parameters. Fortunately, nano approach gives us some independent control in parameters adjustment. One important aspect of nano idea lies in the fact that enhanced boundary scattering due to the increased intensities of interfaces arising from nano-sized grains could reduce the thermal conductivity more than the electrical conductivity, which is practically realized in our material system. Since the introduction of nano idea, large ZT as high as above two has been achieved in the superlattice system. Due to the high fabrication cost of superlattices, they are not scalable for mass production. Theoretical calculations indicate that thermal boundary resistance is the main mechanism for the low thermal conductivity in superlattices, rather than the periodicity. Basically, we hope to achieve the supplattice-like ZT in the less costly bulk nanograined materials, based on the idea that reduction of thermal conductivity which is responsible for ZT enhancement in superlattices can be realized in bulk materials with embedded nanostructures as well. Inspired by the nanocomposite idea, in my thesis work I applied the technique of ball milling and then hot press to various thermoelectric materials, from low temperature to high temperature, demonstrating the feasibility of the approach. By ball milling alloyed ingot into nanopowders and DC hot pressing them, we have achieved a 62-89% ZT improvement for p-type half-Heusler samples, mainly due to the significantly enhanced Seebeck coefficient and partially due to the moderately reduced thermal conductivity. Microstructure studies indicated that increased boundaries due to smaller nano-sized grains is the cause for change of parameters. For our ball milled samples, the trend of decreasing thermal conductivities with increasing ball milling time is observed, further substantiating our nano-approach idea because longer ball milling time gives rise to smaller grain sizes and thus stronger boundary scattering. By applying the same technique to n-type half-Heuslers, we also successfully obtained pronounced enhancement in ZT especially at medium and low temperature ranges, which might be useful in medium temperature power generation. By ball milling a mixture of individual constituent elements into alloyed nanopowders and then DC hot pressing them, we did not gain improvement in ZT initially for n-type BiTeSe system mainly due to the simultaneous reduced power factor with the thermal conductivity. Considering anisotropic properties of the n-type BiTeSe single crystal and randomization effect of ball milling process, we repressed the as-pressed bulk samples in a bigger diameter die, during which lateral flow took place, resulting in preferred grain orientation. As a result, a 22% improvement in the peak ZT from 0.85 to 1.04 at 125 oC in n-type Bi₂Te_2.7Se_0.3 has been successfully achieved, arising from the more enhanced power factor than the thermal conductivity. Compared with single crystal, we benefit from the small nano-sized grains in bulk materials. Taking into account the in-plane power factor of single crystal, we still have much room for further ZT improvement if more ab orientation is promoted into the disk plane and/or the crystal plate size and thickness are reduced. By applying our technique of ball milling and then hot press to p-type skutterudites system, we have achieved a peak ZT of 0.95 at 450 <super>o</super>C in NdFe_3.5Co_0.5Sb₁₂, which is comparable to that of the state-of-the-art ingot. Our approach has the advantage of being less costly and more time-efficient compared to traditional fabrication methods. Besides, even lower thermal conductivity and hence higher ZT can be expected, provided that the nanosize of the precursor powder is preserved during hot press. The nanocomposite idea has been substantiated and the feasibility and generality of our ball milling and then hot press approach has been demonstrated, based on the thermoelectric properties data we obtained and the microstructure studies we carried out from various thermoelectric material systems, from low temperature to high temperature. We believe that continued effort in the area of thermoelectrics by our approach should be paid with superlattice-like ZT if ingenious methods are devised to control the grain growth during consolidation. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

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This research is to develop procedures on the fabrication of membrane electrode assembly (MEA), which is the heart of the Proton Exchange Membrane Fuel Cell. Sensitivity studies of the manipulated variable, such as pressure, temperature, and time, in the hot press process, which is adopted in the assembling on the performance of the MEA are also performed. The developed products on the cleaning of membrane as well as the hot press of MEA have been verified through many experiments. The tests of the MEA¡¦s thus produced reveal that temperature and pressure in hot press process have significant influence on MEA performance. Both have to be kept in a suitable range. Optimal operating conditions in the hot press process may be achieved by conducting more experiments and a detail understanding on the internal structure variation of membrane under high pressure and temperature condition.