1,274 research outputs found
Selective engram co-reactivation in idling brain inspires implicit learning
No full text富山大学博士（医学）Article富山大学・富生命博甲第130号・MOHAMED HUSSEIN YOUSSEF ALY MOSTAFA・2021/03/23・★論文非公開
Intrinsic point-defect equilibria in tetragonal ZrO[subscript 2]: Density functional theory analysis with finite-temperature effects
No full textWe present a density functional theory (DFT) framework taking into account the finite temperature effects to quantitatively understand and predict charged defect equilibria in a metal oxide. Demonstration of this approach was performed on the technologically important tetragonal zirconium oxide, T-ZrO[subscript 2]. We showed that phonon free energy and electronic entropy at finite temperatures add a nonnegligible contribution to the free energy of formation of the defects. Defect equilibria were conveniently cast in Kröger–Vink diagrams to facilitate realistic comparison with experiments. Consistent with experiments, our DFT-based results indicate the predominance of free electrons at low oxygen partial pressure (P[subscript O2]≤10[superscript −6] atm) and low temperature (T≤1500 K). In the same regime of P[subscript O2] but at higher temperatures, we discovered that the neutral oxygen vacancies (F-centers) predominate. The nature of the predominant defect at high oxygen partial pressure has been a long-standing controversy in the experimental literature. Our results revealed this range to be dominated by the doubly charged oxygen vacancies at low temperatures (T≤1500 K) and free electrons at high temperatures. T-ZrO[subscript 2] was found to be hypostoichiometric over all ranges of T and PO2, mainly because of the doubly charged oxygen vacancies, which are responsible for inducing n-type conductivity via a self-doping effect. A range of 1.3 eV in the band gap of T-ZrO[subscript 2] starting from the middle of the gap toward the conduction band is accessible to the chemical potential of electrons (Fermi level) by varying T and PO[subscript 2] without extrinsic doping. The approach presented here can be used to determine the thermodynamic conditions that extremize certain desirable or undesirable defects to attain the optimal catalytic and electronic performance of oxides.United States. Dept. of Energy (Contract DE-AC05-00OR22725
Predicting self-diffusion in metal oxides from first principles: The case of oxygen in tetragonal ZrO₂
No full textTheoretical prediction of self-diffusion in a metal oxide in a wide range of thermodynamic conditions has been a long-standing challenge. Here, we establish that combining the formation free energies and migration barriers of all charged oxygen defects as calculated by density functional theory, within the random-walk diffusion theory framework, is a viable approach to predicting oxygen self-diffusion in metal oxides. We demonstrate this approach on tetragonal ZrO2 by calculating oxygen self-diffusivity as a function of temperature and oxygen partial pressure or, alternatively, temperature and off-stoichiometry. Arrhenius analysis on the isobaric (or constant off-stoichiometry) self-diffusivities yields a spectrum of effective activation barriers and prefactors. This provides reconciliation for the wide scatter in the experimentally determined activation barriers and prefactors for many oxides.United States. Dept. of Energy (Contract No. DE-AC05-00OR22725)National Science Foundation (U.S.) (XSEDE Science Gateways program, research allocation (TG-DMR120025)
Oxygen self-diffusion mechanisms in monoclinic
No full textIn this work, we quantify oxygen self-diffusion in monoclinic-phase zirconium oxide as a function of temperature and oxygen partial pressure. A migration barrier of each type of oxygen defect was obtained by first-principles calculations. Random walk theory was used to quantify the diffusivities of oxygen interstitials by using the calculated migration barriers. Kinetic Monte Carlo simulations were used to calculate diffusivities of oxygen vacancies by distinguishing the threefold- and fourfold-coordinated lattice oxygen. By combining the equilibrium defect concentrations obtained in our previous work together with the herein calculated diffusivity of each defect species, we present the resulting oxygen self-diffusion coefficients and the corresponding atomistically resolved transport mechanisms. The predicted effective migration barriers and diffusion prefactors are in reasonable agreement with the experimentally reported values. This work provides insights into oxygen diffusion engineering in ZrO₂-related devices and parametrization for continuum transport modeling
Predicting point defect equilibria across oxide hetero-interfaces: model system of ZrO2/Cr2O3
No full textWe present a multi-scale approach to predict equilibrium defect concentrations across oxide/oxide hetero-interfaces. There are three factors that need to be taken into account simultaneously for computing defect redistribution around the hetero-interfaces: the variation of local bonding environment at the interface as epitomized in defect segregation energies, the band offset at the interface, and the equilibration of the chemical potentials of species and electrons via ionic and electronic drift-diffusion fluxes. By including these three factors from the level of first principles calculation, we build a continuum model for defect redistribution by concurrent solution of Poisson's equation for the electrostatic potential and the steady-state equilibrium drift-diffusion equation for each defect. This model solves for and preserves the continuity of the electric displacement field throughout the interfacial core zone and the extended space charge zones. We implement this computational framework to a model hetero-interface between the monoclinic zirconium oxide, m-ZrO[subscript 2], and the chromium oxide Cr[subscript 2]O[subscript 3]. This interface forms upon the oxidation of zirconium alloys containing chromium secondary phase particles. The model explains the beneficial effect of the oxidized Cr particles on the corrosion and hydrogen resistance of Zr alloys. Under oxygen rich conditions, the ZrO[subscript 2]/Cr[subscript 2]O[subscript 3] heterojunction depletes the oxygen vacancies and the sum of electrons and holes in the extended space charge zone in ZrO[subscript 2]. This reduces the transport of oxygen and electrons thorough ZrO[subscript 2] and slows down the metal oxidation rate. The enrichment of free electrons in the space charge zone is expected to decrease the hydrogen uptake through ZrO[subscript 2]. Moreover, our analysis provides a clear anatomy of the components of interfacial electric properties; a zero-Kelvin defect-free contribution and a finite temperature defect contribution. The thorough analytical and numerical treatment presented here quantifies the rich coupling between defect chemistry, thermodynamics and electrostatics which can be used to design and control oxide hetero-interfaces
Route to understand the corrosion and hydrogen pickup of zirconium alloys
No full textThesis: Ph. D., Massachusetts Institute of Technology, Department of Nuclear Science and Engineering, 2014.Cataloged from PDF version of thesis.Includes bibliographical references (pages 172-178).The performance of zirconium alloys in nuclear reactors is compromised by corrosion and hydrogen pickup. The thermodynamics and kinetics of these two processes are governed by the behavior of point defects in the ZrO₂ layer that grows natively on these alloys. In this thesis, we developed a general, broadly applicable framework to predict the equilibria of point defects in a metal oxide. The framework is informed by density functional theory and relies on notions of statistical mechanics. Validation was performed on the tetragonal and monoclinic phases of ZrO₂ by comparison with prior conductivity experiments. The framework was applied to four fundamental problems for understanding the corrosion and hydrogen pickup of zirconium alloys. First, by coupling the predicted concentrations of oxygen defects in tetragonal ZrO₂ with their calculated migration barriers, we determined oxygen self-diffusivity in a wide range of thermodynamic conditions spanning from the metal-oxide interface to the oxide-water interface. This facilitates future macro-scale modeling of the oxide layer growth kinetics on zirconium alloys. Second, using the computed defect equilibria of the tetragonal and monoclinic phases, we constructed a temperature-oxygen partial pressure phase diagram for ZrO₂. The diagram showed that the tetragonal phase can be stabilized below its atmospheric transition-temperature by lowering the oxygen chemical potential. This work adds a new explanation to the stabilization of the tetragonal phase at the metal-oxide interface where the oxygen partial pressure is low. Third, using the developed framework, we modeled co-doping of monoclinic ZrO₂ with hydrogen and a transition metal. Our modeling predicted a volcano-like dependence of hydrogen (proton) solubility on the first-row transition metals, which is consistent with a set of systematic experiments from the nuclear industry. We discovered that the reason behind this behavior is the ability of the transition metal to p-type-dope ZrO₂ and hence lower the chemical potential of electron. Therefore, the peak of the hydrogen solubility in monoclinic ZrO₂ also corresponds to an increased barrier for hydrogen gas evolution on the surface. This explanation opens the door to physics-based design of resistant zirconium alloys, and qualitatively consistent with the monoclinic ZrO₂. Finally, we uncovered the interplay between certain hydrogen defects and planar compressive stress which tetragonal ZrO₂ experiences on zirconium alloys. The stress enhances the abundance of these defects, while these same defects tend to relax the stress. This interplay was used to propose an oxide fracture mechanism by which hydrogen is picked up.by Mostafa Youssef Mahmoud Youssef.Ph. D
Interview with Abdel Tawab Youssef
No full textلقاء مع عبد التواب يوسف الأديب و المؤلف المصري و الذى إشتهر بالكتابة في أدب الأطفال بمناسبة إنتخابه أمين عام لإتحاد كتاب مصر. أجرى هذا اللقاء حسن شمس الدين.An interview with Abdel Tawab Youssef, Egyptian children's book author, on the occasion of his election as Secretary General of Egypt Writers' Union. Interview conducted by Hassan Shams El Din
Doping in the Valley of Hydrogen Solubility: A Route to Designing Hydrogen-Resistant Zirconium Alloys
No full textHydrogen pickup and embrittlement pose a challenging safety limit for structural alloys used in a wide range of infrastructure applications, including zirconium alloys in nuclear reactors. Previous experimental observations guide the empirical design of hydrogen-resistant zirconium alloys, but the underlying mechanisms remain undecipherable. Here, we assess two critical prongs of hydrogen pickup through the ZrO[subscript 2] passive film that serves as a surface barrier of zirconium alloys; the solubility of hydrogen in it—a detrimental process—and the ease of H[subscript 2] gas evolution from its surface—a desirable process. By combining statistical thermodynamics and density-functional-theory calculations, we show that hydrogen solubility in ZrO[subscript 2] exhibits a valley shape as a function of the chemical potential of electrons, μ[subscript e]. Here, μ[subscript e], which is tunable by doping, serves as a physical descriptor of hydrogen resistance based on the electronic structure of ZrO[subscript 2]. For designing zirconium alloys resistant against hydrogen pickup, we target either a dopant that thermodynamically minimizes the solubility of hydrogen in ZrO[subscript 2] at the bottom of this valley (such as Cr) or a dopant that maximizes μ[subscript e] and kinetically accelerates proton reduction and H[subscript 2] evolution at the surface of ZrO[subscript 2] (such as Nb, Ta, Mo, W, or P). Maximizing μ[subscript e] also promotes the predomination of a less-mobile form of hydrogen defect, which can reduce the flux of hydrogen uptake. The analysis presented here for the case of ZrO[subscript 2] passive film on Zr alloys serves as a broadly applicable and physically informed framework to uncover doping strategies to mitigate hydrogen embrittlement also in other alloys, such as austenitic steels or nickel alloys, which absorb hydrogen through their surface oxide films.United States. Dept. of Energy. Energy Innovation Hub for Modeling and Simulation of Nuclear Reactors. Consortium for Advanced Simulation of Light Water Reactors (Contract DE-AC05-00OR22725)MIT-China Scholarship Council (Fellowship
Electro-chemo-mechanical effects of lithium incorporation in zirconium oxide
No full textUnderstanding the response of functional oxides to extrinsic ion insertion is important for technological applications including electrochemical energy storage and conversion, corrosion, and electronic materials in neuromorphic computing devices. Decoupling the complicated chemical and mechanical effects of ion insertion is difficult experimentally. In this work, we assessed the effect of lithium incorporation in zirconium oxide as a model system, by performing first-principles based calculations. The chemical effect of lithium is to change the equilibria of charged defects. Lithium exists in ZrO_{2} as a positively charged interstitial defect, and raises the concentration of free electrons, negatively charged oxygen interstitials, and zirconium vacancies. As a result, oxygen diffusion becomes faster by five orders of magnitude, and the total electronic conduction increases by up to five orders of magnitude in the low oxygen partial pressure regime. In the context of Zr metal oxidation, this effect accelerates oxide growth kinetics. In the context of electronic materials, it has implications for resistance modulations via ion incorporation. The mechanical effect of lithium is in changing the volume and equilibrium phase of the oxide. Lithium interstitials together with zirconium vacancies shrink the volume of the oxide matrix, release the compressive stress that is needed for stabilizing the tetragonal phase ZrO_{2} at low temperature, and promote tetragonal-to-monoclinic phase transformation. By identifying these factors, we are able to mechanistically interpret experimental results in the literature for zirconium alloy corrosion in different alkali-metal hydroxide solutions. These results provide a mechanistic and quantitative understanding of lithium-accelerated corrosion of zirconium alloy, as well as, and more broadly, show the importance of considering coupled electro-chemo-mechanical effects of cation insertion in functional oxides
Polarizing Oxygen Vacancies in Insulating Metal Oxides under a High Electric Field
No full textWe demonstrate a thermodynamic formulation to quantify defect formation energetics in an insulator under a high electric field. As a model system, we analyzed neutral oxygen vacancies (color centers) in alkaline-earth-metal binary oxides using density functional theory, Berry phase calculations, and maximally localized Wannier functions. The work of polarization lowers the field-dependent electric Gibbs energy of formation of this defect. This is attributed mainly to the ease of polarizing the two electrons trapped in the vacant site, and secondarily to the defect induced reduction in bond stiffness and softening of phonon modes. The formulation and analysis have implications for understanding the behavior of insulating oxides in electronic, magnetic, catalytic, and electrocaloric devices under a high electric field.National Science Foundation (U.S.) (Grant DMR–1419807
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