Entropy in the Kerr-Newman Black Hole

Entropy of the Kerr-Newman black hole is calculated via the brick wall method with maintaining careful attention to the contribution of superradiant scalar modes. It turns out that the nonsuperradinat and superradiant modes simultaneously contribute to the entropy with the same order in terms of the brick wall cutoff $\epsilon$. In particular, the contribution of the superradiant modes to the entropy is negative. To avoid divergency in this method when the angular velocity tends to zero, we propose to intr oduce a lower bound of angular velocity and to treat the case of the angular momentum per unit mass $a=0$ separately. Moreover, from the lower bound of the angular velocity, we obtain the $\theta$-dependence structure of the brick wall cutoff, which natu rally requires an angular cutoff $\delta$. Finally, if the cutoff values, $\epsilon$ and $\delta$, satisfy a proper relation between them, the resulting entropy satisfies the area law.Comment: 16 pages, Latex, no figures, References are included, Subsection A and B are reduced to subsection A, Abstract is rewritten, Minor corrections are include

Study on corrosion properties of WC-Ni cold spray coatings to mitigate flow-accelerated corrosion of carbon steels in nuclear power plants

Department of Nuclear EngineeringIn secondary system of pressurized water reactors (PWRs), carbon steel undergoes severe corrosion owing to high temperature and pressure, and fast flow velocity. These circumstance, named as flow-accelerated corrosion (FAC), induces corrosion and eventually failure of power piping. Flow velocity brings about destruction of oxide layer by electrochemical corrosion and mechanical shear stress at the oxide/water interfaces. The piping system in the secondary circuit of PWRs mainly consists of carbon steels being vulnerable to FAC because Fe, main element of carbon steel, forms Fe3O4 dissolved to secondary water system by changing to Fe ion. To mitigate corrosion of carbon steel, chemical and mechanical treatments to carbon steel or changing secondary water chemistry has been widely studied. Addition of Cr and/or Mo to carbon steel components is one of the adequate countermeasure to mitigate FAC by anodic passivation. Mechanical treatments are controlling surface roughness because reducing surface roughness can decrease flow turbulence at the oxide/water interface. Increasing pH, dissolved oxygen contents, and addition of advanced corrosion inhibitor to secondary system can decrease corrosion rate of carbon steels. However, maintaining homogeneous chemistry condition of water is hardly possible and changing chemical composition of carbon steel needs not only research of corrosion properties but also other mechanical properties. In contrast, application of coatings to the steel surface can be easily applied to the secondary system. Therefore, thermal spray to steel surface using less corrosive metal is widely used. Among them, cold spray has advantage with lower process temperature than that of other spray coatings which leads to low residual stress and oxidation or decarburization of coating. In this study, tungsten carbide (WC) and nickel (Ni) particles are used as feedstock materials for the coating. WC exhibits high corrosion resistance in acidic aqueous media due to the phase stability. Moreover, the oxidation resistance of WC in O2 containing atmosphere at high temperature is applied to industry which is exposed to severe corrosive environments However, in alkaline solution, the dissolution of WC increases by forming WO4-. Moreover, high melting temperature and poor plastic deformation make cold spray coating difficult. To suppress the corrosion of and increase the deposition efficiency of the coating, Ni, which has better plastic deformation and lower corrosion potential and melting temperature than WC, is used as a binder metal. Ni acts like a sacrificial metal to WC so corrosion and oxidation of Ni occur before those of WC. To evaluate the potential of the coating as an effective corrosion barrier, series of corrosion tests at simulated secondary water chemistry is carried out and quantitative analysis is conducted. Surface and cross-section morphologies of the coatings before and after FAC simulation tests is investigated by scanning electron microscope and X-ray photoelectron microscopy. Electrochemical experiments are carried out for quantitative analysis. Cold spray coatings using WC-10Ni with different Ni contents have rough and porous surface but significant changes of surface is not observed by SEM image. However, lower current density is measured at 25Ni coating than carbon steel, P22, 20Ni and 30Ni coating from potentiodynamic polarization experiments in pH 9.3 ETA solution at room temperature. After 2 weeks FAC experiments, weight loss of coating is increased with increases Ni contents in coating but weight of 20Ni coating is increased. On the other hands, weight loss occurs at 20Ni and 30Ni coating and weight of 25Ni coating is increased after 4 weeks FAC experiments. Although weight change of coatings is smaller than carbon steel after 2 weeks and 4 weeks FAC experiments, weight change of 25Ni coating shows better corrosion performance than P22 comparing with 20Ni and 30Ni coating. As a result, Ni has lower corrosion potential than WC and it induces galvanic corrosion of Ni and prevents WC corrosion or oxidation at early stage of corrosion process. Moreover, oxidation product of Ni, Ni(OH)2 is dissolved into water by Ni ion because of poor thermal stability of Ni(OH)??2. Therefore, high Ni contents in coating increases weight loss during the early stages by Ni(OH)2 dissolution. However, after 4 weeks of immersion, WC oxidizes and transforms into WO3 increasingly which is influenced by Ni contents. 30Ni coating induces continuous weight loss during FAC simulation because of low WC contents which means weight gain by WO3 formation is smaller than weight loss by Ni(OH)2 dissolution. Therefore, weight loss occurs at 30Ni coating. Contrarily, 20Ni coating do not protect WC dissolution after 2 weeks because the absence of Ni and Ni(OH)2.accelerates WC oxidation and WO3 dissolution to water. To be concluded, WC-10Ni + 25Ni coating specimens exhibit the best corrosion resistive performance, the lower corrosion current density and the small weight change than other coatings. Therefore, WC-10Ni + 25Ni coating is possibly suggested to one of the option to mitigate corrosion of carbon steels.ope

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School of Energy and Chemical Engineering (Energy Engineering)Since the Earth was formed, CO2 level has steadily increased as mankind began using fossil fuels, now exceeding 408 ppm. The increased CO2 has a significant impact on climate change by increasing the global temperature due to the greenhouse effect by approximately 1 oC, and it increased sea level by more than 30 cm for the last century. In response to this crisis, there have been great efforts to reduce and utilize CO2 as a resource. The considerable research has been focused on the chemical conversion of CO2 into high value-added carbon compounds and the electrochemical utilization on metal-CO2 batteries adopting carbon capture, utilization and sequestration (CCUS) technology. However, it has been pointed out that it cannot be an effective greenhouse gas abatement technology due to low conversion rate with high energy requirement. In order to meet an energy society liberated from fossil fuels, we should be able to utilize and store energy using unlimited fuel which is environment friendly. This research focuses on developing advanced electrocatalysts for efficient utilization of oxygen via polymer electrolyte membrane fuel cell (PEMFC) and Meta (Li, Na, Zn)-air batteries. In addition to energy production, the rationally engineered electrochemical devices were further invented to efficiently reduce and utilize carbon dioxide in atmosphere.ope

Einstein Structure of Squashed Four-Spheres

It is known that the moduli space of Einstein structures is an isolated point so that an Einstein structure has no infinitesimal Einstein deformations. We examine the rigidity of the Einstein structure by considering deformations of the round four-sphere $\mathbb{S}^4$. We show that any deformation of the round four-sphere $\mathbb{S}^4$ causes it to deviate from the Einstein structure, except trivial deformations at most only changing the size of the sphere.Comment: v2: 22 pages, 1 figure, references adde

Efficient CO2 Utilization via a Hybrid Na-CO2 System Based on CO2 Dissolution

Carbon capture, utilization, and sequestration technologies have been extensively studied to utilize carbon dioxide (CO2), a greenhouse gas, as a resource. So far, however, effective technologies have not been proposed owing to the low efficiency conversion rate and high energy requirements. Here, we present a hybrid Na-CO2 cell that can continuously produce electrical energy and hydrogen through efficient CO2 conversion with stable operation for over 1,000 hr from spontaneous CO2 dissolution in aqueous solution. In addition, this system has the advantage of not regenerating CO2 during charging process, unlike aprotic metal-CO2 cells. This system could serve as a novel CO2 utilization technology and high-value-added electrical energy and hydrogen production device

Generalization of Instanton-Induced Inflation and Dynamical Compactification

It was shown that Yang-Mills instantons on an internal space can trigger the expansion of our four-dimensional universe as well as the dynamical compactification of the internal space. We generalize the instanton-induced inflation and dynamical compactification to general Einstein manifolds with positive curvature and also to the FLRW metric with spatial curvature. We explicitly construct Yang-Mills instantons on all Einstein manifolds under consideration and find that the homogeneous and isotropic universe is allowed only if the internal space is homogeneous. We then consider the FLRW metric with spatial curvature as a solution of the eight-dimensional Einstein-Yang-Mills theory. We find that open universe $(k=-1)$ admits bouncing solutions unlike the other cases $(k=0, +1)$.Comment: 30 pages, 5 figure