Thermal energy harvesting and solar energy conversion utilizing carbon-based nanomaterials
This dissertation provides details of carbon-based nanomaterial fabrication for applications in energy harvesting and generation. As energy demands increase, and concerns about mankind's environmental impact increase, alternative methods of generating energy will be widely researched. Carbon-based nanomaterials may be effective in such applications as their fabrication is often inexpensive and they have highly desirable electrical, mechanical, and thermal properties. Synthesis and characterization of carbon nanotube thermal interfaces on gadolinium foils is described herein. Total thermal interface resistances of carbon nanotube coated gadolinium were measured using a one-dimensional reference calorimeter technique, and the effect of hydrogen embrittlement on the magnetic properties of gadolinium foils is discussed. The samples generated in this study were consistently measured with reduced total thermal interface resistances of 55-70% compared to bare gadolinium. Characterization of gadolinium foils in a cooling device called a magneto thermoelectric generator was also performed. A gadolinium shuttle drives the device as it transitions between ferromagnetic and paramagnetic states. Reduced interface resistances from the carbon nanotube arrays led to increased shuttle frequency and effective heat transfer coefficients. Detailed theoretical derivations for electron emission during thermal and photo-excitation are provided for both three-dimensional and two-dimensional materials. The derived theories were fitted to experimental data from variable temperature photoemission studies of potassium-intercalated graphitic nanopetals. A work function reduction from approximately 4.5 eV to 2 – 3 eV resulted from potassium intercalation and adsorption. While changes in the electron energy distribution shape and intensity were significant within 310 – 680 K, potassium-intercalated graphitic petals demonstrate very high thermal stability after heating to nearly 1000 K. Boron nitride modification of the nanopetals was performed in an effort to minimize deintercalation of potassium from the nanopetal lattice and while multiple work functions were present within the electron energy distribution, massive reductions in emission intensity took place above 580 K. Finally, a device for measuring the current density during photoemission was also developed and photoemission induced by a solar simulator at room temperature produced currents on the order of 1 nA/cm 2 resulting in a quantum efficiency of approximately 8.0x10 –8 electrons emitted per photon of illumination.
Fisher, Purdue University.
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