http://hdl.handle.net/1721.1/7582 http://hdl.handle.net/1721.1/49807 Flame synthesis of carbon nanotubes and metallic nanomaterials Height, Murray John, 1975- Carbon nanotubes are a remarkable material with many appealing properties. Despite the appeal of this material, there are few synthesis techniques capable of producing nanotubes in large quantities at low-cost. The broad objective of this study was to examine the potential of a premixed flame for the synthesis of carbon nanotubes with the view that flame synthesis may prove a means of continuous production at low-cost. The specific approach focused on the formation of metallic nanoparticles in flames; identification of nanotube formation zones, time scales, and transition conditions; characterization of material properties; and the development of a formation mechanism and associated flame-model. Carbon nanotube formation requires a source of carbon, a source of heat and the presence of metal particles. A fuel-rich flame is a high-temperature, carbon-rich environment and addition of metal is likely to give conditions suitable for nanotube growth. This study considered a premixed acetylene/oxygen/15 mol% argon flame doped with iron pentacarbonyl (Fe(CO)₅) vapor (typically 6100 ppm), operated at 50 Torr pressure and 30 cm/s cold gas feed velocity. The flame was investigated with regard to the growth of metal particles and subsequent formation and growth of carbon nanotubes. Thermophoretic samples were extracted from the flame at various heights above burner (HAB) and analyzed using transmission electron microscopy (TEM). HAB is representative of residence time in the flame. Size distribution and number density data were extracted from TEM images using a quantitative image analysis technique. The mean particle size for a precursor concentration of 6100 ppm was observed to increase from around 2 to 4 nm between 20 and 75 mm HAB. (cont.) The particle number density results showed a decreasing number density with increasing HAB, giving a complementary picture of the particle dynamics in the flame. Single-walled carbon nanotubes (SWNT) were also observed to form in the premixed flame. Thermophoretic sampling and TEM analysis gave insight into nanotube formation dynamics. Nanotube structures were observed to form as early as 30 mm HAB (20 ms) with growth proceeding rapidly within the next 10 to 20 mm HAB. The growth-rate for the nanotubes in this interval is estimated to be between 10 and 100 ptm per second. The upper region of the flame (50 to 70 mm HAB; 35 to 53 ms) is dominated by tangled web structures formed via the coalescence of individual nanotubes formed earlier in the flame. The nanotube structures are exclusively single-walled with no multi-walled nanotubes observed in any of the flame samples. The effect of carbon availability on nanotube formation was tested by collecting samples over a range of fuel equivalence ratios at fixed HAB. The morphology of the collected material revealed a nanotube formation 'window' of 1.5 < < 1.9, with lower dominated by discrete particles and higher favoring soot-like structures. These results were also verified using Raman spectroscopy. A clear trend of improved nanotube quality (number and length of nanotubes) is observed at lower . More filaments were observed with increasing concentration, however the length (and quality) of the nanotubes appeared higher at lower concentrations ... Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2003. Includes bibliographical references. http://hdl.handle.net/1721.1/49806 Strategies of public diplomacy : an assessment of the current U.S. public diplomacy strategy in light of a directional, elite-oriented model and two historical cases Crow, Ryan Michael, 1980- Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Political Science, 2003. Includes bibliographical references. http://hdl.handle.net/1721.1/49805 Observational constraints on the number, albedos, size, and impact hazards of the near-Earth asteroids Stuart, Joseph Scott, 1971- This work provides a statistical description of the near-Earth asteroids (NEAs) in terms of number, orbital parameters, reflectance spectra, albedos, diameters, and terrestrial and lunar collision rates. I estimate the size and shape of the NEA population using survey data from the Lincoln Near-Earth Asteroid Research project including more than 1300 NEA detections. The NEA population is more highly inclined than previously estimated and the total number of NEAs with absolute magnitudes (H) brighter than 18 is 1227 +170/-90. The absolute magnitude and orbital parameter distributions for the NEAs are combined with reflectance spectra and albedo measurements. I obtain a debiased estimate of the fraction of NEAs in each of 10 taxonomic complexes, and a debiased average albedo for each. The number of NEAs larger than 1 km is 1090 +/- 180. Next, I determine the impact frequency, collision velocity distribution and collision energy distribution for impacts of NEAs into the Earth and Moon. Globally destructive collisions ([approx.] 1021 J) of asteroids 1 km or larger strike the Earth once every 0.60 +/- 0.1 Myr on average. Regionally destructive collisions with impact energy greater than 4x1018 J ([approx.] 200 m diameter) strike the Earth every 47,000 +/- 6,000 years. The rate of formation of craters expected from the NEAs is found to be in close agreement with the observed number of craters on the Earth and Moon. (cont.) These results combine the largest set of NEA discovery statistics from a single survey, the largest set of physical data on NEAs, and corrections for observational bias. The result is a comprehensive estimate of the total NEA population in terms of orbital parameters, absolute magnitudes, albedos, and sizes. This improved description of the NEAs will help us to plan surveys to find and study the remaining undiscovered NEAs, to connect the NEAs to their origins in the main-belt, to connect the NEAs to meteorite samples, to compare the lunar and terrestrial cratering record to the current population of potential impactors, and to understand the magnitude of the NEA impact hazard to the Earth's biosphere. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2003. Includes bibliographical references (p. 132-144). http://hdl.handle.net/1721.1/49804 Preservation without boundaries : transitioning Bhutan's traditional settlements into an urban environment Loeb, Rachel L. (Rachel Lauren), 1973- Thesis (M.C.P.)--Massachusetts Institute of Technology, Dept. of Urban Studies and Planning, 2002. Includes bibliographical references (p. 111-114).