Browsing by Author "Zhao, Yao"

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    Key steps towards carbon nanotube-based conductors
    (2012) Zhao, Yao; Barrera, Enrique V.
    Making a robust carbon nanotube-based conductor as a replacement of copper in electricity grids can initiate a paradigm shift in energy transmission. This dissertation identifies four fundamental factors for making carbon nanotube-based conductors as functionalization, dispersion, concentration and processing. These four factors are discussed in detail by studying four separate systems: nanotube/epoxy composites, nanotube/porous medium density polyethylene (MDPE) composites, nanotube/high density polyethylene (HDPE) composites and pure nanotube cables. In nanotube/epoxy composites, homogeneous dispersion of nanotubes and a strong interface between nanotubes and epoxy matrix were simultaneously achieved through the development of a novel nanotube functionalization. While the degree of functionalization was high, the process was non-destructive to the mechanical properties of the nanotubes. In addition, the functional groups constructed covalent bonds with the epoxy matrix and also made dispersing the nanotubes much easier. As a result, the composites reinforced by the functionalized nanotubes had better mechanical properties than the samples reinforced by the raw nanotubes. In nanotube/porous MDPE composites, the degree of nanotube dispersion reached a level of 1 micron for nanotube agglomerate size within the matrix. This successful dispersion was primarily attributed to creating the porous MDPE. The pore size was tuned to be as small as 1 micron so that the sub-micron long HiPco nanotubes could easily penetrate into the matrix. The nanotube/porous MDPE composites obtained enhancement both in mechanical strength and electrical conductivity compared to the control samples. In nanotube/HDPE composites, the nanotube conducting networks were studied. Conductivity of the composites with the loading ratio at the percolation threshold was not sufficiently high for conductor applications. Nanotube/HDPE composite wires with higher loading ratios up to 40 wt% were prepared. Key factors for improving the formation of the conducting networks were identified. Through optimization in processing, maximum conductivity of ∼10 3 S/m was achieved. Pure nanotube cables were prepared by a solid spinning procedure, which showed the potential to make macroscopic cables of various length and thickness. The pure nanotube cables circumvented the bottleneck in improving conductivity for composite systems, in which polymer in-between the nanotubes caused high contact resistance. The pure nanotube cables reached conductivity as high as ∼10 6 S/m. Through iodine doping, conductivity further was enhanced so that the specific conductivity of the doped cables exceeded that of metals such as copper. As a result of applying the knowledge learned from study of the four fundamental factors, a macroscopic carbon-nanotube cable was created. It reached an unprecedented conductivity as high as ∼10 7 S/m. Mechanically it was more robust than steel, but with 1/6 the weight. This advanced nanotube-based conductor can have a wide spectrum of applications such as transmission lines and low dimensional connecting wires.
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    Optimization of FLASH proton beams using a track-repeating algorithm
    (Wiley, 2022) Wang, Qianxia; Titt, Uwe; Mohan, Radhe; Guan, Fada; Zhao, Yao; Yang, Ming; Yepes, Pablo
    Background: Radiation with high dose rate (FLASH) has shown to reduce toxicities to normal tissues around the target and maintain tumor control with the same amount of dose compared to conventional radiation. This phenomenon has been widely studied in electron therapy, which is often used for shallow tumor treatment. Proton therapy is considered a more suitable treatment modality for deep-seated tumors. The feasibility of FLASH proton therapy has recently been demonstrated by a series of pre- and clinical trials. One of the challenges is to efficiently generate wide enough dose distributions in both lateral and longitudinal directions to cover the entire tumor volume. The goal of this paper is to introduce a set of automatic FLASH proton beam optimization algorithms developed recently. Purpose: To develop a fast and efficient optimizer for the design of a passive scattering proton FLASH radiotherapy delivery at The University of Texas MD Anderson Proton Therapy Center, based on the fast dose calculator (FDC). Methods: A track-repeating algorithm, FDC, was validated versus Geant4 simulations and applied to calculate dose distributions in various beamline setups. The design of the components was optimized to deliver homogeneous fields with well-defined diameters between 11.0 and 20.5 mm, as well as a spread-out Bragg peak (SOBP) with modulations between 8.5 and 39.0 mm. A ridge filter, a high-Z material scatterer, and a collimator with range compensator were inserted in the beam path, and their shapes and sizes were optimized to spread out the Bragg peak, widen the beam, and reduce the penumbra. The optimizer was developed and tested using two proton energies (87.0 and 159.5 MeV) in a variety of beamline arrangements. Dose rates of the optimized beams were estimated by scaling their doses to those of unmodified beams. Results: The optimized 87.0-MeV beams, with a distance from the beam pipe window to the phantom surface (window-to-surface distance [WSD]) of 550 mm, produced an 8.5-mm-wide SOBP (proximal 90% to distal 90% of the maximum dose); 14.5, 12.0, and 11.0-mm lateral widths at the 50%, 80%, and 90% dose location, respectively; and a 2.5-mm penumbra from 80% to 20% in the lateral profile. The 159.5-MeV beam had an SOBP of 39.0 mm and lateral widths of 20.5, 15.0, and 12.5 mm at 50%, 80%, and 90% dose location, respectively, when the WSD was 550 mm. Wider lateral widths were obtained with increased WSD. The SOBP modulations changed when the ridge filters with different characteristics were inserted. Dose rates on the beam central axis for all optimized beams (other than the 87.0-MeV beam with 2000-mm WSD) were above that needed for the FLASH effect threshold (40 Gy/s) except at the very end of the depth dose profile scaling with a dose rate of 1400 Gy/s at the Bragg peak in the unmodified beams. The optimizer was able to instantly design the individual beamline components for each of the beamline setups, without the need of time intensive iterative simulations. Conclusion: An efficient system, consisting of an optimizer and an FDC have been developed and validated in a variety of beamline setups, comprising two proton energies, several WSDs, and SOBPs. The set of automatic optimization algorithms produces beam shaping element designs efficiently and with excellent quality.