Further crosslinking of the fluorinated polyimide was examined to separate the cure reactions from degradation and to determine the optimum post curing conditions. Glass transition/melting temperatures were ascertained using DSC, while weight loss during curing and Td were determined using TGA. Furthermore, the mechanical properties were measured using an Instron to relate to the thermal properties to find the optimum curing conditions. The polyimide resin exhibited the best post-curing conditions for further crosslinking for 8 hours at 410 degress C based on Tg, thermal stability, and mechanical properties. Blister temperatures, resulting from rapid heating, were obtained by monitoring changes in transverse thickness expansion using two different techniques. Both techniques employed showed similar blister temperatures in relation to the amount of absorbed moisture, regardless of sample size. The polyimide resin exhibited blister temperatures ranging from 225 - 362 degrees C, with 1.7 - 3.0 wt% absorbed moisture, and the polyimide composite had blister temperatures from 246 - 294 degrees C with 0.5 - 1.5 wt% moisture. Weight loss of the fluorinated polyimide and its corresponding polyimide carbon fiber composite under elevated temperature was examined. Weight loss as a function of exposure temperature and time was measured using TGA and by pre- and post-weighing of specimens treated in an oven. Both techniques showed similar weight loss trends as a function of time and temperature, but TGA showed much greater weight loss due to greater surface area to volume (i.e., small sample size). The neat polyimide resin and carbon fiber composite exhibited negligible weight loss at temperatures below 430 degrees C for exposure times up to 20 minutes. Transition-metal carbides were initially synthesized by carbothermal reduction of transition-metal halides and polymer precursor mixtures, at temperatures that range from 900 to 1500 degrees C in an argon atmosphere. TaC was synthesized from TaBr5, as a model carbide for this process. Significant (> 40 vol%) amounts of TaC were formed at reaction temperatures as low as 900 degrees C for one hour, with greater times and temperatures leading to > 90 vol% yield. Universality of method was also proven by using other various transition-metal halide salts (NbBr5, WCl4, and WCl6) with the polyimide.

The characteristics of the proton exchange membrane fuel cell (called PEMFC) stacks made with the graphite unipolar/bipolar plates are studied in this thesis. Using pure hydrogen as fuel, certain experimental work is conducted to help us to understand the factors which influence on the performance of a HFC stack. The experimental work under various operating conditions starts from single cell stacks to multi-cell stacks. The maximum power is about 200 W, which is made with two 10-cell stacks in series. For simplification, all of the flow channels in the cathode are open type in which air is directly supplied from ambient by fan. The comparison of the performance of two single cells, which are made with both a graphite unipolar plate and a new-type carbon fiber unipolar plate, is conducted. The total resistances of the two types of bipolar plates with gas diffusion layers are tested to help us to understand their strong or weak points. The experimental results display that the double inlets has better performance than the single inlet due to larger entrance space. Increasing the applied torque will reduce the contact resistance between bipolar plate and diffusion layer and also the gaps between the fibers of carbon cloth. Reducing the contact resistance is helpful in increasing the performance of the cell, but reducing the gaps between fibers will inhibit the entering of reactive gas and is unfavorable for performance; therefore, the proper torque is necessary to obtain the best voltage output. When air is used as an oxidizer and the flow channel is an open type channel, the fan in high rotating speed is helpful at high current density. The high air volume flow rate can supply sufficient oxidizer and avoid the decay of the voltage output at high current density. At the current density 1 A/cm2, the power density of the single-cell stack is about 400 mW/cm2 and the power density of the 10-cell stack is down to about 310 mW/cm2 in our experiment. The rib of the carbon fiber unipolar/bipolar plate is soft, so there is no deformation in the gas diffusion layer in stack assembly. Only slight compression is needed to assemble a stack; therefore, the reactive gas can easily flow into the most of active area. This type unipolar/bipolar plate is made with low density plastic except that the rib is made with carbon fiber bunches. Thus the new plate is weight light, cost low and volume small. So it is quite possible that the new-type of carbon fiber plate is used as substitution for the graphite bipolar plate in the future. In that case the light, low cost and high performance choice can be achieved.

Magnesium alloys have attracted considerable attention owing to its low density of ~1.7 g/cm3. On the other hand, the carbon fiber (CF) reinforced polyether ether ketone (PEEK) polymer composites possess extraordinary specific strength and stiffness along the longitudinal (or fiber) direction. It follows that the combination of Mg/CF/PEEK would offer an alternative in forming a high specific strength and stiffness composite. In the first part of this study, the low density and high performance Mg-based laminated composites were fabricated by means of sandwiching the AZ31 Mg foils with the carbon-fiber/PEEK prepreg through hot pressing. Proper surface treatments of AZ31 sheet using CrO3 base etchants are necessary in order to achieve good interface bonding characteristics. The resulting Mg base laminated composite, with a low density of 1.7 g/cm3, exhibits high modulus of 75 GPa and tensile strength of 932 MPa along the longitudinal direction. The experimentally measured tensile modulus and strength data along both the longitudinal and transverse direction are within 90-100% of the theoretical predictions by rule of mixtures, suggesting that the bonding between layers and the load transfer efficiency are satisfactory. The flexural stress and modulus along the longitudinal direction are 960 MPa and 54.6 GPa, respectively, suggesting a sufficiently high resistance against bending deflection. The peel strengths are about 2.75 and 4.85 N/mm along the longitudinal and transverse directions, respectively, superior to that of the epoxy-resin-adhered and carbon-fiber-reinforced aluminum laminated composites. Polymer nanocomposites have attracted considerable attention during the past decade due to their versatile and extra-ordinary performances. The polymer nanocomposites can be prepared by the well-known sol-gel method. It is well known that PEEK is a good solvent resistant polymer. Hence, it is impossible to fabricate the PEEK nanocomposite by means of sol-gel method. In the second part of this study, the PEEK nanocomposites filled with nano-sized silica or alumina measuring 15-30 nm to 2.5-10 weight percent were fabricated by vacuum hot press molding at 400oC. The resulting nanocomposites with 5-7.5 wt% SiO2 or Al2O3 nanoparticles exhibit the optimum improvement of hardness, elastic modulus, and tensile strength by 20-50%, with the sacrifice of tensile ductility. With no surface modification for the inorganic nanoparticles, the spatial distribution of the nanopartilces appears to be reasonably uniform. There seems no apparent chemical reaction or new phase formation between the nanoparticle and matrix interface. The crystallinity degree and thermal stability of the PEEK resin with the addition of nanopartilces were examined by X-ray diffraction, differential scanning calorimetry, and thermogravity analyzer, and it is found that a slight decrease in crystallinity fraction and a higher degradation temperature would result in as compared with the prestine PEEK.

A new type of heterogeneous carbon fiber bunch bipolar plate developed in our lab is applied to portable pure hydrogen proton exchange membrane fuel cell stacks. Several different types of bipolar plate structures have been designed, and the voltages and currents of these fuel cell stacks are measured to compare their performance. The new type of heterogeneous carbon fiber bunch bipolar plate is well in low contact resistance, weight low, small volume and the flexible geometry shape. Due to its flexible structure of carbon fiber bunch, the compressing pressure is small while assembling stack so that the electrode can not be over compressed and out of shape. Therefore the high porosity of diffusion layer can be keep and reaction gas can enter and distribute to all reaction areas easily. For using to portable equipments, a small 6-cell flat type of fuel cell stack are developed firstly. The total weight is about 75g and the total volume is about 68cm . The second stack is cylinder-type(I) fuel cell stack. The total weight is about 60g and the total volume is about 71cm . The third stack is cylinder-type (II). The total weight has been reduced to about 20g and the total volume has been reduced to about 30cm . Above three kinds of the 6-cell stacks the total electrode area is 13.5cm . Using Nafion, the catalyst content anode Pt 0.4mg/cm , cathode Pt 1.0mg/cm , On room temperature and inlet hydrogen gauge pressure 0.15atm air-breathing, total output power of the cylinder (II) can reach 1.85W, and the power density of unit area can reach about 137mW/cm^2.

In this thesis the numerical method is adopted to study the flow characteristics of reactants, when the newly developed heterogeneous composite carbon bipolar plate and the traditional hard surface bipolar plate are applied to fuel cells. The simulation in this study includes the distributions of the velocity and pressure of oxidizers flowing in a parallel or serpentine flow channel under several inlet gas flow rates and pressures. The difficulty to supply reactant to the active area under the ribs is also studied in this thesis. From these studies we can understand the strong and weak points of the newly developed bipolar plates and the traditional bipolar plates better. The simulation results display that the gaseous reactants or products can permeate through the gaps between carbon fibers into or out of active area under the rib, which is formed with carbon fiber bunches. Therefore more reactant gases can be supplied with the heterogeneous carbon fiber bipolar plates than the graphite bipolar plates. In addition, the higher efficiency of fuel cells can be obtained with the new plate, especially in high power density. The pressure distribution in making use the heterogeneous carbon fiber bipolar plates are more uniform, and the pressure drop is also less than the traditional bipolar plates. For large fuel cells the current distribution will be more uniform and the pumping power will be less. The reactant gases can flow through the gaps of carbon fibers and the porous carbon cloth into the catalyst layers by convection and diffusion. So no matter what type of the flow channels used the needed oxidizer is much less with the new bipolar plate than with traditional one. The flow fields of the two types of bipolar plates are quite similar, but the gas needed to supply is also much less with the new plate. Because of the advantages mentioned above, we believe that the heterogeneous carbon fiber bipolar plate is better than the traditional graphite bipolar plate. Keywords: fuel cell, heterogeneous carbon fiber bipolar plate,hard surface bipolar plate

We study a novel structure employing the woven continuous carbon fiber (CCF) epoxy composite with high electromagnetic (EM) shielding. The influences of wove type, number and angle of overlapped plates upon the shielding effectiveness (SE) of wove CCF epoxy composite are investigated. The minimum SE of the single, double, and triple plain or balanced twill woven CCF composite plates were measured to be as high as 50 dB, 60 dB, and 70 dB, respectively. More than 100 dB of SE was obtained for the triple overlapped plain wove CCF composite at frequency of 0.9 GHz. The weight percentage of single CCF composite plate required for electronic application was 4.8% only, which was less than one quarter of the carbon fiber (CF) content and the performance of SE was 10 dB higher in comparison with long CF filled liquid crystal polymer composites. The SE calculated theoretically is consistent with that measured by the experiment. We have demonstrated a new woven CCF epoxy composite with high EM shielding. This work may lead to the development of effective shielding for plastic optical transceiver modules to prevent electromagnetic interference (EMI) for use in low cost and lightwave communication systems.

In this study a three-dimensional mathematical model is developed to simulate the flow field and mass transfer in a PEM fuel cell. In the model, the effects of the different flow channel structures in heterogeneous composite carbon fiber bipolar plates and traditional hard surface bipolar plates on the performance are studied. The results show that, the cell performance with the heterogeneous composite carbon fiber bipolar plates have better performance than that with the traditional hard surface bipolar plates, whether in the parallel flow channel structures or the serpentine flow channel structures. The reason is that, the heterogeneous composite carbon fiber ribs are porous material, so it allows the reactants and products transport uniformly even in the rib zone. This greatly improved the mass transfer and the gases distribution in the fuel cell. With the traditional bipolar plates, the reactants can only enter the reaction zone from the side of carbon cloth under ribs, so that the performance in this area under rib is relatively poor. In the simulation of the flow channel structures, we detect that, due to the single inlet serpentine flow channel have stronger convective effects that forced reactants to flow through the whole reaction zones, so it has better performance at high current density than in the singles inlet parallel flow channel. In addition, the results also show that, higher fuel stoichiometric number and operated pressure and properly humidified at anode will all improve the performance of the fuel cell.

This study addresses three major parts: (1) to establish the technology for the preparation of tubular ceramic membrane substrates; (2) to establish the technology for the preparation of tubular carbon/ceramic membranes; and (3) to reclaim water from chemical mechanical polishing (CMP) wastewaters by a combined treatment system of a novel simultaneous electrocoagulation/electrofiltration (EC/EF) process coupled with laboratory-prepared tubular composite membranes (TCMs) and evaluate its feasibility of water recycling and operating cost. First, in this work the green substrates of tubular porous ceramic membranes consisting of corn starch were prepared using the extrusion method, followed by curing, drying, and sintering processes. Experimental results have demonstrated that an addition of starch granules to the raw materials would increase the porosity, pore size, and permeability of the sintered matrices but accompanied by a decrease of the compressive strength. It revealed that the membrane substrates with desired pore sizes and permeability could be obtained by adding a proper amount of corn starch. The nominal pore sizes of the prepared membrane substrates were ranging from 1 to 2 £gm. The membrane substrates thus obtained are suitable for crossflow microfiltration applications. Second, the carbon/alumina TCMs and carbon fibers/carbon/alumina TCMs were obtained by the chemical vapor deposition (CVD) method resulting in a pore size distribution of 2 to 20 nm and a nominal pore size ranging from 3 to 4 nm. Besides, during the CVD process the reaction temperature was found to be the main factor for influencing the pore size of carbon fibers/carbon/alumina TCMs and the type of carbon fibers. When the reaction temperature was above or equal to 1000 ¢J, the pore size of TCMs increased due to the pyrolysis of thin carbon layers. The ¡§Tip-Growth¡¨ mechanism was found for tubular carbon fibers formation under such conditions. On the other hand, ¡§Base-Growth¡¨ (also known as ¡§Root-Growth¡¨) mechanism was found for curved and irregular carbon fibers formation when reaction temperature was under or equal to 950 ¢J. Third, for reclaiming water from CMP wastewaters, experimental results of laboratory-prepared carbon/alumina TCMs incorporated into the custom-made EC/EF treatment module used was found to be capable of treating oxide-CMP wastewater in a proper manner. Permeate thus obtained had a turbidity of below 0.5 NTU and the removal efficiencies of TS (total solids content) and Si were 80% and 93 %, respectively. Further, for understanding the applicability of fractional factorial design and Taguchi experimental design, two laboratory-prepared carbon fibers/carbon/alumina TCMs (i.e., Tube B and Tube E obtained from two different preparation conditions) incorporated into the EC/EF treatment module were chosen for evaluating the performance of CMP wastewaters treatment. Permeate obtained based on the fractional factorial design of experiments had a turbidity of below 1.0 NTU and the removal efficiencies of TOC (total organic carbon), Cu and Si were all above 80 % except for the TS (i.e., ranging from 72 to 74%). Permeate obtained based on the Taguchi experimental design had a turbidity of below 0.3 NTU and the removal efficiencies of TS, TOC, Cu and Si were ranging from 82 to 91%. Apparently, similar optimum operating conditions were obtained from the fractional factorial design and Taguchi experimental design. Permeate thus obtained could be reused as the make-up water of cooling towers. The operating cost of Cu-CMP wastewater treatment based on a total water reclaim of 600 m3 per day was determined to be NT$ 98 (i.e., US$ 3.22) and NT$ 35 (i.e., US$ 1.05) per m3 of permeate for Case 1 (i.e., the filtration area of 0.0189 m2 in one EC/EF module) and Case 2 (i.e., the filtration area of 0.0801 m2 in one EC/EF module), respectively.

This study elucidates the effect of various processing parameters on polyacrylonitrile (PAN) /carbon nanotube (CNT) composite fiber structure and properties. Interaction between PAN and MWNT enabled the gel-spun PAN/MWNT composite fiber to be drawn to a higher draw ratio, than the control PAN fiber, resulting in the composite fiber tensile strength value as high as 1.3 GPa. PAN/MWNT composite fibers were stabilized and carbonized, and the resulting fibers have been characterized for their structure and properties. The effect of precursor fiber shelf-time on the mechanical properties of the gel-spun PAN/MWNT composite fibers is also reported. A rheological study of PAN-co-MAA/few wall nanotube (FWNT) composite solution has been conducted. At low shear rates, the network of FWNTs contributes to elastic response, resulting in higher viscosity and storage modulus for the composite solution as compared to the control solution. On the other hand, at high shear rates, the network of FWNTs can be broken, resulting in lower viscosity for the composite solution than that for the control solution. Larger PAN crystal size (~16.2 nm) and enhanced mechanical properties are observed when the fiber was drawn at room temperature (cold-drawing) prior to being drawn at elevated temperature (~ 165 °C; hot-drawing). Azimuthal scan of wide angle X-ray diffraction (WAXD) and Raman G-band intensities were used for the evaluation of Herman's orientation factor for PAN crystal (fPAN) and FWNT (fFWNT), respectively. Significantly higher nanotube orientation was observed than PAN orientation at an early stage of fiber processing (i.e during spinning, cold-drawing). Differential scanning calorimetry (DSC) revealed that PAN-co-MAA fiber can be converted into cyclic structure at milder conditions than those for PAN. Continuous in-line stabilization, carbonization, and characterization of the resulting carbon fibers were carried out. Rheological and fiber spinning studies have also been carried out on PAN-co-MAA/VGCNF (vapor grown carbon nano fiber). The diameter of PAN-co-MAA/VGCNF composite fiber is smaller than that of the PAN-co-MAA control fiber with same draw ratio due to the suppressed die-swell in the presence of VGCNF. The mechanical properties of PAN-co-MAA control and PAN-co-MAA/VGCNF composite fibers were characterized. Crystalline structure and morphology of the solution-spun PAN-co-MAA/VGCNF fibers are characterized using WAXD and scanning electron microscopy (SEM), respectively. The volume fraction of PAN-CNT interphase in PAN matrix has been calculated to illustrate the impact of CNTs on structural change in PAN matrix, when ordered PAN molecules are developed in the vicinity of CNTs during fiber processing. The effect of PAN-CNT interphase thickness, CNT diameter, and mass density of CNT on volume fraction of PAN-CNT interphase has been explored.

Carbon fibers contain more than 90 wt. % carbon. They have low density, high specific strength and modulus, and good temperature and chemical resistance. Therefore, they are important candidate as reinforcement materials. Carbon fiber is made by pyrolysing precursor polymers. Polyacrylonitrile (PAN) which has been used as precursor to produce high strength carbon fiber is used as precursor in this study. The theoretical tensile strength of carbon fibers can reach over 100 GPa. Currently, the best commercial carbon fibers reach only 7.5 GPa. To make good quality carbon fiber and to narrow the gap between theoretical values and currently achieved experimental properties, the entire manufacturing process including fiber spinning, stabilization and carbonization, needs to be improved optimized. In this dissertation, the stabilization processes of gel-spun PAN/carbon nanotubes (CNTs) composite fibers are studied. PAN/CNT (1 wt. % CNT) composite fibers are spun by dry-jet gel-spinning. Three types of CNTs with different number of walls and varying catalyst content are used as additives. The effect of different types of CNTs on the properties of the stabilized fibers was compared. It is found that the CNTs with the highest surface area shows the best reinforcement efficiency on the tensile modulus, and reduces the formation of β-amino nitrile. The residual catalyst in the range of 1 to 4 wt. % shows little effect on the mechanical properties of the stabilized fibers. Stabilization involves complex chemical reactions, including cyclization, oxidation, dehydration, and cross-linking. These complex reactions are separated by using different gas environments during stabilization. The cross-linking reaction has the highest activation energy among all stabilization reactions, and requires a temperature higher than 300 DegC to be completed. The effect of applied tension on the stabilized fiber properties are investigated, and it is found that higher tension leads to better properties for the stabilized fiber, including higher Young's modulus, higher orientation, less formation of β-amino nitrile, and less shrinkage. The relationship between stabilization conditions and the mechanical properties of the carbonized fiber is investigated, and the methods to identify optimum stabilization conditions are proposed. It is observed that the highest tension should be applied during both stabilization and carbonization, and the mechanical properties of the resulting carbon fibers are increased if fibers are further stabilized at a temperature of ~ 320 DegC to improve the cross-linking degree as compared with the fibers only stabilized at 255 DegC. The optimum stabilization time depends on both the stabilization temperature and on the applied tension. A new characterization method by monitoring the dynamic mechanical properties, while stabilization is in progress is used to narrow down the range of the optimum stabilization time. Also, the effect of carbonization temperature on the ultimate carbon fiber properties is studied in the batch process carbonization. Preliminary studies are carried out to find the relationship between the structure and properties of precursor fibers and the tensile strength of carbon fibers, including mechanical properties and co-monomers of precursor fibers.