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Energetics, Kinetics, and Optical Absorption of Point Defects in Sapphire

Hornak, Mark, Hornak
http://orcid.org/0000-0002-4601-4892
http://rave.ohiolink.edu/etdc/view?acc_num=osu1471861567

Abstract Details

2016, Master of Science, Ohio State University, Materials Science and Engineering.
In an effort to increase the safety and reliability of current and future nuclear reactors and fuel reprocessing facilities, there is a demand for novel, in-situ instrumentation. Such instrumentation would be responsible for providing real time temperature and pressure data inside of the confined spaces of a reactor core, but would also be exposed to the extreme high temperatures and radiation fields associated with those environments. Fiber optic materials have been suggested for use in these applications due to their high degree of immunity to electromagnetic radiation and small spatial profile. To withstand such extreme conditions, conventional fiber optic materials are not able to be used. Single crystalline a-Al2O3, or sapphire, is a prime candidate to replace these existing materials due to its relatively high melting temperature, strength, corrosion resistance, and optical transparency. As a crystalline material, however, sapphire is still susceptible to some degree of radiation damage, which could have an adverse effect on the optical properties. Upon bombardment from radiation from a nuclear reactor, crystalline materials tend to form point defects, which can act as absorption centers for transmitted light in fiber optic applications. Therefore, in order to validate sapphire as a potential material for use in such nuclear applications, it is important to first examine the formation, evolution, and effects point defects have on the optical properties of the material. To understand the formation of point defects in sapphire, it is especially important to accurately calculate the elemental chemical potentials of both oxygen and aluminum. Previous methods used to calculate these quantities, such as the heat of formation method, have produced negative values of formation energy, which would indicate a defect being more stable than its host material. In this work, a method based on atomic concentration balancing is proposed and used in conjunction with density functional theory calculations to determine the formation energies of various point defects in stoichiometric sapphire. Additionally, the formation energies are calculated for the O-rich and Al-rich nonstoichiometric limiting cases using the constitutional defect and heat of formation methods, for comparison purposes. In order to incorporate the effect of temperature, phonon calculations and the quasi-harmonic approximation are used to calculate formation free energies as a function of temperature for point defects in stoichiometric sapphire. These results are used to explain which defect are expected to form under the operating conditions for the proposed sapphire fiber optic materials and suggest that the poor optical properties of sapphire above ~1400 °C The time evolution of defect content in sapphire is then explored in this work, by the use of a kinetic rate theory model. The model considers a system of partial differential equations (PDEs) that explain the relationship between defect concentration and the variables of time and temperature, and solves these equations using the PDE solver Alamode. Formation energies and free energies calculated using the constitutional defect and concentration-compensated methods are used as inputs to determine the concentrations of the various point defects in both nonstoichiometric and stoichiometric sapphire, respectively. The results from this model give an understanding of which defect species are most prevalent in the material under certain thermal annealing conditions, and also provide information about the special distribution of defects within a sapphire fiber cross-section. Finally, the effects of individual point defect species on the optical properties of sapphire are determined using density functional theory calculations. The optical absorption for each point defect is determined through the calculation of the frequency-dependent dielectric function. From this data, added attenuation spectra are calculated for all the considered defect species, which correlate to signal loss in fiber optic materials. The individual attenuation spectra are discussed, and then used with the defect concentration data for stoichiometric sapphire calculated using the rate theory model, to construct a total defect attenuation spectrum. This data is then used to determine which regions of the optical spectrum point defects interact with, providing a direct link between point defect formation and added attenuation.
Wolfgang Windl, Ph.D. (Advisor)
Thomas Blue, Ph.D. (Committee Member)
76 p.
Materials Science
Energetics, Kinetics, Optical Absorption of Point Defects, Sapphire

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Citations

  • Hornak, Hornak, M. (2016). Energetics, Kinetics, and Optical Absorption of Point Defects in Sapphire [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1471861567

    APA Style (7th edition)

  • Hornak, Hornak, Mark. Energetics, Kinetics, and Optical Absorption of Point Defects in Sapphire. 2016. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1471861567.

    MLA Style (8th edition)

  • Hornak, Hornak, Mark. "Energetics, Kinetics, and Optical Absorption of Point Defects in Sapphire." Master's thesis, Ohio State University, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=osu1471861567

    Chicago Manual of Style (17th edition)

osu1471861567
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