Advanced aluminum graphite composites have unique thermal properties due to opposing coefficients of thermal expansion of aluminum and graphite. The thermal and mechanical properties of such composites are anisotropic due to directional properties of graphite fibers and their designed orientation. A joint with different fiber orientations would theoretically produce an isotropic material for thermal management. This paper presents results for welding and brazing of the composite using different joining techniques. Laser welding of Al-Gr composite showed that a power density above 30kW/mm2 gives a weld with microstructure defects. Also the laser beam melts the matrix and delaminates the graphite fibers. The molten aluminum reacts with graphite to form aluminum carbide (Al4C3). The joint strength is compromised when laser welding at optimal conditions to minimize the carbide formation. Also porosity and redistribution of graphite fibers is seen during laser welding. These defects prompt us to consider a low temperature joining. Brazing is considered since the low melting temperature of a filler material suppresses the formation of Al4C3 while minimizing pores and microstructural defects in the joint. Microstructural study and shear test are performed to analyze the joints. Shear strengths of brazed joints are determined to be 20-21MPa which is comparable to the composite shear strength (46.5MPa in x-y plane and 19MPa in z plane). The fracture surface is found to be mostly on the composite rather than in brazed material or along the interface. Also, the microstructural study showed no Al4C3 formation and minimal porosity in the brazed region. These results show a successful joining of the composite using laser brazing and resistance brazing methods.

Via the viewpoint of fuel and oxidant supply in this study, we compare heterogeneous carbon fiber bipolar plates with graphite bipolar plates that apply to fuel cell. In operating condition with different gas inlet pressure and compressing pressure, we study the penetrability of reactant gases that come into the carbon cloth under the rib of a bipolar plate. Eventually the output voltage and power density are measured to prove the advantages of the new bipolar plate. The experimental results show that carbon fiber bipolar bunch in low compressing pressure 2bar already display high gas penetrability. Its dimensionless flow rate is about quadruple of graphite bipolar plates. The reactant gas can enter the carbon cloth either from the side or from the top of the penetrating carbon fiber bipolar bunch. In addition, carbon fiber bipolar plates are affected slightly by compressing pressure. Further, the total electrical resistant of carbon fiber bipolar plates with carbon cloth already decreases to 18.5mΩ*cm² in low compressing pressure 2bar. Therefore, by appling the new bipolar plate, the fuel cell in compressing pressure 2bar and inlet fuel pressure 1.15bar(absolute pressure) can developed a power rate 180mW/cm². Concerning graphite bipolar plates, we can find that compressing pressure increase from 1bar to 4bar due to the reduction in total resistance so the output voltage and power density can increase to maximum value 113mW/cm2. However, while we augment more compressing pressure, the influence in reducing total resistance is much smaller than that in reducing the porosity of carbon cloth. Therefore, the output power density decreases. Also, output voltage of carbon fiber bipolar plates at 0.5mA/cm2 is 0.38 V and is higher than that of graphite bipolar plate 0.2 V.

Thèse doctorat : Génie des Matériaux : Villeurbanne, INSA : 2004. / Titre provenant de l'écran-titre. Bibliogr. p. 187-195.

Thèse de doctorat : Physique et Chimie de la Matière et des Matériaux : Nancy 1 : 2007. / Titre provenant de l'écran-titre. Bibliogr.

Reproduction de : Thèse de doctorat : Mécanique et matériaux : Reims : 2004. / Titre provenant de l'écran titre. Bibliogr. f. 153-158.

Thesis (M. S.)--Mechanical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Dr. Steve Johnson; Committee Member: Dr. Andrew Makeev; Committee Member: Kyriaki Kalaitzidou. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Graphite oxide (GO), a carbon material prepared in one step from low cost commercial materials, and graphene oxide have been found to catalyze a wide range of reactions including oxidations, hydrations, and dehydrations, as well as cationic or oxidative polymerizations. Applicable in both small molecule and polymer chemistry, this single, metal-free catalyst shows remarkable breadth, including the combination of the aforementioned reactions in an auto-tandem fashion to form advanced substrates, such as chalcones, from simple starting materials. Some of these reactions, such as the selective oxidation of alcohols to aldehydes, have been shown to be dependent on the presence of molecular oxygen, suggesting that this may be the terminal oxidant. Aside from its eminently valuable reactivity, the use of GO as a catalyst also presents practical advantages, such as its heterogeneous nature, which facilitates separation of the catalyst from the desired product. The use of this simple material in synthetic chemistry, as well as others like it, is distinct from other forms of catalysis in that the active species is carbon-based, heterogeneous and metal-free (as confirmed by ICP-MS and other spectroscopic techniques). This has led us to propose the term “carbocatalyst” to describe such materials. With dwindling supplies of precious metals used in many common organic reactions, the use of inexpensive and widely available carbocatalysts in their place will ensure that commercial processes of fundamental importance can continue unabated. Moreover, as we have shown with just one material, carbons are capable of facilitating a broad range of reactions. / text

In this thesis, electrode binders as vital components in the fabrication of composite electrodes for lithium-ion (LIB) and lithium-sulfur batteries (LiSB) have been investigated. Poly(vinylidene difluoride) (PVdF) was studied as binder for sulfur-carbon positive electrodes by a combination of galvanostatic cycling and nitrogen absorption. Poor binder swelling in the electrolyte and pore blocking in the porous carbon were identified as origins of low discharge capacity, rendering PVdF-based binders an unsuitable choice for LiSBs. More promising candidates are blends of poly(ethylene oxide) (PEO) and poly(N-vinylpyrrolidone) (PVP). It was found that these polymers interact with soluble lithium polysulfide intermediates generated during the cell reaction. They can increase the discharge capacity, while simultaneously improving the capacity retention and reducing the self-discharge of the LiSB. In conclusion, these binders improve the local electrolyte environment at the electrode interface. Graphite electrodes for LIBs are rendered considerably more stable in ‘aggressive’ electrolytes (a propylene carbonate rich formulation and an ether-based electrolyte) with the poorly swellable binders poly(sodium acrylate) (PAA-Na) and carboxymethyl cellulose sodium salt (CMC-Na). The higher interfacial impedance seen for the conventional PVdF binder suggests a protective polymer layer on the particles. By reducing the binder content, it was found that PAA-Na has a stronger affinity towards electrode components with high surface areas, which is attributed to a flexible polymer backbone and a higher density of functional groups. Lastly, a graphite electrode was combined with a sulfur electrode to yield a balanced graphite-sulfur cell. Due to a more stable electrode-electrolyte interface the self-discharge of this cell could be reduced and the cycle life was extended significantly. This example demonstrates the possible benefits of replacing the lithium metal negative electrode with an alternative electrode material.

No description available.

The focus of this research is characterizing a new material system composed of carbon and graphite foams, which has potential in a wide variety of applications encompassing aerospace, military, offshore, power production and other commercial industries. The benefits of this new material include low cost, light weight, fire-resistance, good energy absorption, and thermal insulation or conduction as desired. The objective of this research is to explore the bulk material properties and failure modes of the carbon foam through experimental and computational analysis in order to provide a better understanding and assessment of the material for successful design in future applications. Experiments are conducted according to ASTM standards to determine the mechanical properties and failure modes of the carbon foam. Sandwich beams composed of open cell carbon foam cores and carbon-epoxy laminate face sheets are tested in the flexure condition using a four point setup. The primary failure mode is shear cracks developing in the carbon foam core at a critical axial strain value of 2,262 με. In addition to flexure, the carbon foam is loaded under tensile and shear loads to determine the respective material moduli. Computational analysis is undertaken to further investigate the carbon foam's failure modes and material characteristics in the sandwich beam configuration. Initial estimates are found using classical laminated plate theory and a linear finite element model. Poor results were obtained due to violation of assumptions used in both cases. Thus, an additional computational analysis incorporating three dimensional strain-displacement relationships into the finite element analysis is used. Also, a failure behavior pattern for the carbon foam core is included to simulate the unique failure progression of the carbon foam on a microstructure level. Results indicate that displacements, strains and stresses from the flexure experiments are closely predicted by this two parameter progressive damage model. The final computational model consisted of a bond line (interface) study to determine the source of the damage initiation, and it is concluded that damage initiates in the carbon foam, not at the bond line.