Browsing by Subject "Radiation effects"
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Item Characterization of neutron flux spectra for radiation effects studies(2011-05) Graham, Joseph Turner; Landsberger, SheldonThe effects of neutron displacement damage on materials are sensitive to neutron energy spectra. In controlled neutron damage experiments, a well characterized neutron flux spectrum is critical in determining the equivalent dose for displacement damage. Two techniques were used to characterize the neutron flux spectra in the University of Texas at Austin TRIGA research nuclear reactor. The first technique uses a standard method of measuring the reaction rates of two identical metal foils, one of which was irradiated in a Cd cover, the other of which was irradiated bare. Assuming an analytic form of the neutron spectrum the reaction rates were used to determine an approximate spectrum. The second technique uses the reaction rates measured from a set of activated metal foils along with two spectral unfolding techniques to approximate and then refine the neutron spectrum. A Matlab code was developed which fits radiative capture reaction rates to an approximate spectrum using a least squares approach. The result was used as an initial guess in a second Matlab code which refines the epithermal and fast energy ranges of the spectrum using reaction rates from threshold reactions. Errors in the reaction rates calculated from the resulting spectrum to the measured reaction rates were used to assess the accuracy of the final neutron spectrum.Item Radiation Effects on Low-dimensional Carbon System(2013-08-05) Wang, JingIon irradiation has been known to be an effective tool for structure modification with micro/nano-scale precision. Recently, demonstrations have been made for nano-machining, such as the cutting and welding of carbon nanotubes. Understanding the fundamental effects of ion irradiation on carbon nanotubes is critical for advancing this technique as well as for scientific curiosity. Molecular dynamics modeling was performed to study irradiation stability, structural changes, and corresponding thermal properties. In our study, Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) was used to perform atomic scale simulation. In order to understand size and geometry effects on carbon damage creation, the threshold energy of displacement was calculated as a function of recoiling angles for both single-walled and multi-walled nanotubes. A strong directional dependence was found to exist in different shells of multi-walled carbon nanotubes. We found that carbon atoms on the innermost tube were more susceptible to be displaced toward the center of axis. The calculation matrix was further extended to nanotubes having different diameters for a full-scale understanding of the creation of defects. Besides studies on defects creation, thermal properties of carbon nanotubes were studied via a simplified model of the carbon nanotube network. Thermal conductivity, were found to be increased nearly one order of magnitude in carbon nanotube networks after irradiation and subsequent annealing. All the modeling results were compared with experimental observations either obtained from this project as a parallel study or from previous works, for the purpose of verification and validation. For experimental works, atomic scale characterization was performed by using transmission electron microscopy and the thermal conductivity measurement was characterized by using laser flash technique. Through a combination of modeling and experimentation, we proved that ion beam techniques can be used to enhance thermal conductivity in carbon nanotube bundles by inter-tube defects mediated phonon transport.Item A study of the ferroelectric properties of neutron irradiated lead zirconate titanate(2013-08) Graham, Joseph Turner; Landsberger, Sheldon; Ferreira, Paulo J. (Paulo Jorge)Lead zirconate titantate (PZT) is an electroceramic material with many important technological applications in sensing and computer memory. Some of these applications require the PZT based devices to operate in radiation fields where they will be exposed to a high flux of energetic, heavy and light, charged and uncharged particles. The risk to any device exposed to ionizing radiation is the accumulation of displacement and ionization damage. Significant damage accumulation over time can lead to property drifts and, in some cases, failure of the device to perform properly. The goal of the undertaking recounted in this dissertation was to study changes in the ferroelectric properties of PZT exposed to the neutron field of a research nuclear reactor and to help develop an understanding for the type of radiation induced defects that play a dominant role in the degradation process. Thin film PZT capacitors were prepared using a wet chemical technique. The capacitors were then irradiated in a 1 MW TRIGA research nuclear reactor at the University of Texas at Austin up to a maximum 1 MeV equivalent neutron flux of 5.2 x 10¹⁵ cm⁻². Following irradiation, electronic characterization of polarization-electric field hysteresis loops, first order reversal curves, and small-signal permittivity were performed to ascertain tendencies between irradiation dose and ferroelectric properties. The measurements indicate a drop in remanent polarization, a loss of domain wall mobility, shifts in local switching fields and the formation of dipolar defects. These effects are all attributed to the introduction of defects into the material through displacement damage cascades. Numerical models of the damage cascades were performed to determine the displacement concentration. Comparison of those values and the primary recoil spectrum with typical survival rates found in the literature suggest that both free point defects as well as defect clusters are produced in comparable if not larger concentrations. It is proposed that defect clusters play a more significant role in ferroelectric property change than previously believed.