Lipka, Stephen M.

The intercalation of anions into carbon fiber from organic electrolytes containing lithium salts was studied. The reversible intercalation of anions into carbon could lead to the possible substitution of conventional metal oxide cathode materials in lithium-ion cells. EWC300 was selected as the most suitable carbon fiber material based on data from preliminary tests. Experiments were performed with LiClO4 in EC/DMC and LiPF6 in EC/DMC electrolytes. Slow scan cyclic voltammetry (0.1 mV/s) and galvanostatic charge/discharge experiments at various C rates were used. Intercalation of PF6- occurred by staging and was highly dependent on the current density. High current density (20 mA/g) was necessary to reach potentials over 5 V vs Li to achieve intercalation capacities over 80 mAh/g. Powder x-ray diffraction revealed that carbon fibers became less crystalline after anions were intercalated into their structure. Scanning electron microscopy showed longitudinal cracking on the carbon fibers after 120 cycles indicating dimensional instability.

A novel carbon nano-fiber was prepared using catalytic vapor phase growth. Electrochemical capacitors were assembled using these fibers. Physical analysis was conducted on the carbon nano-fibers and electrochemical analysis was performed on capacitors made from these carbon nano-fibers. Scanning electron microscopy revealed that the nano-fibers had diameters ranging from 20nm to 400nm. X-ray diffraction showed the nano-fibers were more ordered than some commercial carbon fibers. BET adsorption yielded specific surface areas of the nano-fibers at around 400 m$\sp2$/g. Electrochemical studies including cyclic voltammetry and electrochemical impedance spectroscopy indicated that capacitors made from carbon nano-fibers were promising for practical use. Further modification/activation of the carbon nano-fibers was conducted and capacitors made from these materials were also evaluated.

A novel electrode structure for Li-ion batteries was created by thermally treating nonwoven polyacrylonitrile (PAN) fabric in an inert atmosphere. Rapid carbonization of PAN fiber, stabilized in an inert atmosphere, produced a fused carbon fiber structure having a reversible specific capacity for lithium above 300 mAh/g; with charge reversibility between 95 and 100%. Chemical, structural and electrochemical analyses, including electrochemical impedance spectroscopy were performed on carbon fiber electrodes. Scanning electron microscopy revealed grooves and striations on the surfaces of carbon fibers prepared from PAN precursors stabilized in oxygen; but could not resolve any surface features on the fused carbon fibers, prepared from PAN precursors stabilized in an inert atmosphere. The fused, porous carbon fibers are believed to contain a large fraction of carbon chains with relatively few carbon planes.

A number of fibrous carbon materials have been investigated as intercalation host materials. Commercially available rayon fiber (synthetic cellulose) based carbon fibers were synthesized for use as anode material in lithium-ion batteries. The effects of oxidation and carbonization temperature, heating ramp rate, soak time and gaseous atmosphere during thermal treatment on the electrochemical performance of the carbon fibers were studied. Intercalation/deintercalation experiments were performed to evaluate the electrochemical performance of the carbon fibers. The highest reversible capacity and lowest irreversible capacity loss were obtained for carbon fibers carbonized at 1100C at a ramp rate of 10C/min held at soak times of 1 and 5 hours. Electrolyte containing 1M LiPF6 in 70/30 v/o EC:DMC proved to be most suitable for these carbon fiber materials. The influence of electrolyte composition (solvent and salt) on the reversible and irreversible capacities as well as on the intercalation/deintercalation potential profile were also studied.

Electrochemical double-layer capacitors were constructed using low surface area carbon fibers that are commercially available. The fibers were made from polyacrylonitrile (PAN) and pitch and vary from low (LC = 14A, d002 = 3.54A) to high (LC = 169A, d002 = 3.40A) crystallinity. High energy densities (up to 7.83 Wh/kg) were obtained by electrochemically intercalating HSO4- ions between the graphene planes of the carbon fibers. The intercalation process was strongly influenced by the crystallinity of the carbon fiber and by the precursors from which the fiber was manufactured. All the pitch fibers had a higher structural order and a higher carbon content than the PAN fibers. A total of 10 capacitors were constructed. Nine of these were constructed from fibers that were electrochemically activated and one was constructed from fiber as received. 38 w/o sulfuric acid was used as the electrolyte for each of these capacitors. Performance of the capacitors decreased as the structural order and carbon content of the fibers decreased.

The codeposition of a smooth and uniform coating of copper and molybdenum was successfully achieved on T-650 carbon fiber. The effect of various plating parameters on the electrodeposition of copper and molybdenum such as plating bath chemistry, current density, and pulse frequency were studied. By adjusting the aforementioned variables, qualitative and quantitative analysis was conducted to evaluate the deposit smoothness, uniformity, and wetting characteristics. Qualitative analysis of the deposits were made using scanning electron microscopy and energy dispersive spectroscopy. Quantitative analysis of the deposit coating was conducted using inductively coupled plasma chemical analysis, dewetting tests, X-ray diffraction, transmission electron microscopy, and auger electron spectroscopy. Based on the results, a plating line was designed and constructed for the continuous deposition of copper and molybdenum onto carbon fiber tows.

The stress corrosion cracking susceptibility of austenitic stainless steels SS304L, SS316L and SS904L was studied in an acidified seawater environment by slow strain rate testing at 24, 38 and 66$\sp\circ$C. Fractographic evidence of SCC susceptibility was obtained using scanning electron microscopy. The degree of susceptibility to SCC for each alloy in these environments is discussed based on the mechanical parameters, fractography and anodic polarization behavior. The results showed that SS904L performed better than SS304L and SS316L in the aforementioned environments.

This research evaluated the applicability of electrochemical impedance spectroscopy (EIS) as a non-destructive technique to predict and characterize the degradation of carbon fiber reinforced polymer (CFRP) composites exposed to aqueous environments at ambient and 6.2 $\pm$ 0.3 MPa. Changes in EIS data were related to water uptake into the composite material as a function of exposure time. Electrochemically induced damage (both anodic and cathodic) were also evaluated using impedance measurements. Three point flexure tests with concurrent EIS measurements were employed to study the effect of stresses on water uptake and mechanical degradation. Visual observation of the extent of damage (i.e., fiber-matrix debonding) was made using scanning electron microscopy (SEM) and correlated with EIS observation.

Full nickel-hydrogen (Ni-H2) boilerplate batteries were cycled and impedance measurements were made at different states-of-charge (SOC), electrolyte concentrations and charge/discharge rates. Experiments were conducted on cells containing new and cycled (11,000 cycles) electrodes. Additionally, an EIS study of Ni-H2 flightweight IPV satellite cells was performed. A number of experiments were conducted on silver oxide-metal hydride batteries. The interest was focused on both negative and positive electrodes and upon the system itself. This work was preliminary and aided in describing the general performance of the battery. For analysis, the data was fitted to an equivalent electrical circuit using the Nonlinear Least Squares Method (NLSM). The correlation between theoretical and empirical data was sufficiently good.

The stress corrosion cracking (SCC) tendencies of several engineering alloys were studied in an acidified seawater environment as a function of applied strain rate and electrolyte temperature. The selected alloys included austenitic stainless steels 304L, 316L, 904L and A-286 (an iron-based superalloy at two heat treatments yielding ultimate tensile strengths of 130 and 200 ksi), Inconel 718 (220 ksi ultimate tensile strength) and Hastelloys C-22 and C-276. The slow strain rate test technique was used to evaluate the SCC strain rate dependency of each alloy at extension rates of 4.7 x 10^-6, 4.7 x 10^-4 and 4.7 x 10^-3 mm/sec. The effect of electrolyte temperature was evaluated at 38C and 60C at a single extension rate of 4.7 x 10^-5 mm/sec. Control specimens were tested in a laboratory air environment at an extension rate of 4.7 x 10^-5 mm/sec. Various mechanical parameters of the specimens tested in the corrosive medium were compared with those of control specimens to quantify the degree of cracking. Fractographic evidence of SCC was obtained using scanning electron microscopy (SEM). An attempt was made to correlate SCC tendency with the alloy's passivation kinetics and microstructure. Atmospheric exposure testing was performed in a simulated space shuttle launch pad environment for selected alloys.