分类: 物理学 >> 核物理学 提交时间: 2016-09-14
摘要: We infer the emission positions of twin kilohertz quasi-periodic oscillations (kHz QPOs) in neutron star low mass X-ray binaries (NS-LMXBs) based on the Alfven wave oscillation model (AWOM). For most sources, the emission radii of kHz QPOs cluster around a region of 16-19 km with the assumed NS radii of 15 km. Cir X-1 has the larger emission radii of 23-38 km than those of the other sources, which may be ascribed to its large magnetosphere-disk radius or strong NS surface magnetic field. SAX J1808.4-3658 is also a particular source with the relative large emission radii of kHz QPOs of 20 - 23 km, which may be due to its large inferred NS radius of 18 - 19 km. The emission radii of kHz QPOs for all the sources are larger than the NS radii, and the possible explanations of which are presented. The similarity of the emission radii of kHz QPOs (16-19 km) for both the low/high luminosity Atoll/Z sources is found, which indicates that both sources share the similar magnetosphere- disk radii.
提交时间: 2017-05-02
分类: 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间: 2017-05-02
摘要: Zinc hexacyanoferrate (ZnHCF) which is a dimorphic (cubic or rhombohedral) Prussian blue analogue and can be intercalated by both monovalent and divalent ions,is a promising cathode material for rechargeable aqueous metal-ion batteries.In this paper, a simple co-precipitation method is developed to tune the particle morphology of ZnHCF by adjusting the dropping speed at room temperature. Three polyhedral ZnHCF particles, with cubooctahedral, truncated octahedral or octahedral shapes, are obtained at room temperature. Structural transformation from cubic phase of as-prepared ZnHCF to rhombohedral phase is observed by further dehydration of the sample at 70 °C, whereas the dehydrated ZnHCF crystals still hold the identical polyhedral shape as that of the cubic phase particles. Then the in uence of shape and facets on electrochemical performance is studied for polyhedral ZnHCF with rhombohedral structure (RZnHCF). RZnHCF sample with cubooctahedral shape possesses the best rate capability and cyclic stability comparing with RZnHCF particles having truncated octahedral or octahedral shapes. Furthermore, the structure of cuboctahedron RZnHCF particles during electrochemical cycling has been monitored with ex situ X-ray di raction to demonstrate the reversible zinc-ion intercalation mechanism.
分类: 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间: 2017-05-02
摘要: A well-dispersed noble metal on perovskite support is expected to show good catalytic ability for NO oxi- dation under oxygen-rich conditions, which is an important step in reducing NO in lean burn exhaust gas. In this study, we investigated the catalytic oxidation of NO to NO2 on a Pt4/CaTiO3 (001) model to eval- uate interfacial effect. Results showed that O2 preferred Pt–Ti interface sites, and that the decomposition of O2 required a dissociation activation barrier of 0.97 eV. The decomposed O–O on the interface required nearly no activation barrier in the subsequent NO oxidation. However, the most rate-limiting step involved the desorption of the second formed NO2 from interfacial Pt4/CaTiO3 (0 0 1), which yielded a sig- nificant desorption barrier of 2.63 eV. These findings can help understand the oxidation process of NO to NO2 on noble-metal–perovskite interface.
分类: 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间: 2017-05-02
摘要: In the present work, we investigated the structural and catalytic properties of a prototype system Pt-doped CaTiO3 by means of first principles calculations. We paid particular attention to the aggregation and penetration of Pt on different surfaces of CaTiO3, and subsequent CO oxidation by surface oxygen atoms on Pt-doped CaTiO3. Our calculations indicate that CO oxidation can potentially take place when Pt is doped on the first layer of CaTiO3(001). The activation barriers are calculated to be 0.20–0.45 eV. The possibly induced O vacancy on the surface will produce a magnetic behavior by breaking the spin density symmetry due to one Pt–O bond cleavage. Our study is expected to provide an insight into the catalytic behavior of Pt ions in Pt-doped perovskite toward the oxidation of exhaust gas.
分类: 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间: 2017-05-02
摘要: Three-dimensional (3D) graphene networks are attracting ever-increasing attention in the field of energy storage because their unique architecture at macroscopic scales is beneficial for effective electron and ion transport. Herein, a novel interconnected 3D graphene mesh network (3D GMN) was successfully designed and fabricated by folded Ni meshes assisted chemical vapor deposition method. The structure parameters of 3D GMN can be controlled well by tuning the period of Ni mesh and the electroplating time. With the increase of the density of 3D GMN, the electrical conductivity of 3D GMN and the thermal conductivity of 3D GMN/epoxy composite are greatly improved compared to that of the 3D graphene foam. This 3D GMN enables the high capacity of 57 mA h g 1 in an aluminum ion battery at the ultra- high rate of 40C with capacity retention of 96.5% after 200 cycles.
分类: 天文学 >> 天体物理学 提交时间: 2024-05-20
摘要: Context : Understanding the mechanisms that launch and shape powerful relativistic jets from supermassive black holes (SMBHs) in high-redshift active galactic nuclei (AGN) is crucial for probing the co-evolution of SMBHs and galaxies over cosmic time. Aims :We study the high-redshift ($z=3.396$) blazar OH~471 to explore the jet launching mechanism in the early Universe. Methods : Using multi-frequency radio monitoring observations and high-resolution Very Long Baseline Interferometry imaging over three decades, we study the milliarcsecond structure and long-term variability of OH~471. Results : Spectral modelling of the radio flux densities reveals a synchrotron self-absorbed spectrum indicating strong magnetic fields within the compact core. By applying the flux freezing approximation, we estimate the magnetic flux carried by the jet and find that it reaches or exceeds theoretical predictions for jets powered by black hole spin energy via the Blandford-Znajek mechanism. This implies that OH~471 was in a magnetically arrested disk (MAD) state where the magnetic flux accumulated near the horizon regulates the accretion flow, allowing efficient extraction of black hole rotational energy. Conclusions : Our study demonstrates the dominance of MAD accretion in powering the prominent radio flares and relativistic jets observed in the radio-loud AGN OH~471 and statistical studies of large samples of high-redshift AGN will shed light on the role of MAD accretion in launching and accelerating the earliest relativistic jets.
分类: 物理学 >> 普通物理:统计和量子力学,量子信息等 提交时间: 2017-05-02
摘要: This study investigates the structural stability of small Pd@Pt core@shell octahedral nanoparticles (NPs) and their shell thickness dependent catalytic performance for p-chloronitrobenzene hydrogenation with H2. The 6−8 nm Pd@Pt octahedral NPs are prepared by a sequential reduction method, and the characterization results confirm that Pd@Pt octahedral NPs with one to four atomic Pt layers can be controllably synthesized. The Pd@Pt octahedral NPs with one atomic Pt layer demonstrate excellent structural stability with the maintenance of core−shell structures as well as high catalytic stability during cycle to cycle catalytic p-chloronitrobenzene hydrogenation reactions. The alumina-supported Pd@Pt octahedral NPs illustrate a superior catalytic performance relative to individual Pt and Pd and their physical mixtures. Theoretical calculations by density functional theory suggest that the unexpected structural stability for Pd@Pt octahedral NPs with thin Pt shells and their corresponding catalytic stability during hydrogenation reactions can be ascribed to the strong binding between Pt surfaces and reactants/products in catalytic reactions. The enhanced catalytic performance of Pd@Pt octahedral NPs possibly originates from the core−shell interaction, which adjusts the electronic state of surface Pt atoms to be suitable for selective p-chloronitrobenzene hydrogenation.