1,894 research outputs found
Quantum Photovoltaic Cells Driven by Photon Pulses
We investigate the quantum thermodynamics of two quantum systems, a two-level
system and a four-level quantum photocell, each driven by photon pulses as a
quantum heat engine. We set these systems to be in thermal contact only with a
cold reservoir while the heat (energy) source, conventionally given from a hot
thermal reservoir, is supplied by a sequence of photon pulses. The dynamics of
each system is governed by a coherent interaction due to photon pulses in terms
of the Jaynes-Cummings Hamiltonian together with the system-bath interaction
described by the Lindblad master equation. We calculate the thermodynamic
quantities for the two-level system and the quantum photocell including the
change in system energy, power delivered by photon pulses, power output to an
external load, heat dissipated to a cold bath, and entropy production. We
thereby demonstrate how a quantum photocell in the cold bath can operate as a
continuum quantum heat engine with the sequence of photon pulses continuously
applied. We specifically introduce the power efficiency of the quantum
photocell in terms of the ratio of output power delivered to an external load
with current and voltage to the input power delivered by the photon pulse. Our
study indicates a possibility that a quantum system driven by external fields
can act as an efficient quantum heat engine under non-equilibrium
thermodynamics.Comment: 10 pages, 8 figures, submitte
Preaching as interaction between church and culture: with specific reference to the Korean church
The Korean church, famous for her rapid growth, has begun to notice a downward trend in her growth rate since the mid-eighties. Although many reputable investigations have recently been carried out with regard to this downward slide, these investigations have overlooked the full meaning of preaching in the interaction between church and culture. In view of this, this study sets the following four aims: (1) to investigate the reasons behind church decline in terms of preaching in the interaction between church and culture in Korea; (2) to interpret preaching in the interaction between church and culture biblically, historically and theologically in order to understand the normative Christian perceptions and practices of preaching; (3) to attempt an integration between the descriptive and the normative; and (4) to propose developmental strategies for the Korean church. To achieve these purposes, two kinds of methods are employed in this study: (a) an analysis of preaching in the interaction between church and culture both in Korea and in the normative Christian sources, with the model advocated by D Browning (1991); and (b) qualitative interviewing as an empirical interpretation with a model based on the findings of Rubin&Rubin (1995). Five claims emerge from this study: (1) How do we reset the context of preaching? (2) How do we revise the present preaching theory of the Korean church? (3) How do we define and defend conversion preaching that is seemingly exclusive in contemporary pluralistic Korean society? (4) How do we rethink and re-establish the ecclesiology of the Korean church? (5) How do we formulate the Christian culture against or in the rage of worldly thoughts and cultures in Korea? This thesis concludes by proposing preaching as interaction and the preacher as an inter-actor between church and culture. Practical strategies are developed to answer the claims.Thesis (PhD (Practical Theology))--University of Pretoria, 2005.Practical Theologyunrestricte
Optimal Gaussian measurements for phase estimation in single-mode Gaussian metrology
The central issue in quantum parameter estimation is to find out the optimal
measurement setup that leads to the ultimate lower bound of an estimation
error. We address here a question of whether a Gaussian measurement scheme can
achieve the ultimate bound for phase estimation in single-mode Gaussian
metrology that exploits single-mode Gaussian probe states in a Gaussian
environment. We identify three types of optimal Gaussian measurement setups
yielding the maximal Fisher information depending on displacement, squeezing,
and thermalization of the probe state. We show that the homodyne measurement
attains the ultimate bound for both displaced thermal probe states and squeezed
vacuum probe states, whereas for the other single-mode Gaussian probe states,
the optimized Gaussian measurement cannot be the optimal setup, although they
are sometimes nearly optimal. We then demonstrate that the measurement on the
basis of the product quadrature operators XP+PX, i.e., a non-Gaussian
measurement, is required to be fully optimal.Comment: 13 pages, 6 figure
Automatic Review of Construction Specifications Using Natural Language Processing
Since construction specifications are normally over 1000 pages and are complicated and often inconsistent, reviewing them is a labor-intensive and time-consuming activity. Thus, the aim of this study was to automate the review process by comparing construction specifications with standard specifications using natural language processing. Standard specifications for road construction projects were collected from 43 different states in the U.S. and used as experimental data. Doc2Vec, cosine similarity, and named entity recognition (NER) were used to recognize construction objects, standard values, and execution conditions, which can be used to find specification errors. As an early stage of the research, most of related sentences were found from standard specifications with high relevancy, and the average F1 score of NER was 0.256. The research findings will contribute to enhancing the efficiency of checking for specification errors by automatically detecting abnormalities and the absence of specific standards.N
Effect of flow-field structure on discharging and charging behavior of hydrogen/bromine redox flow batteries
Designing and optimizing the flow-field structure for the liquid phase Br2/HBr electrolyte solution of H2/Br2 redox flow batteries (RFBs) is important for improving cell performance. In this study, two electrolyte flow modes, i.e. the flow-by and flow-through modes, are simulated by using a three-dimensional H2/Br2 RFB model. The model is first applied to real-scale H2/Br2 cell geometries and then validated against the experimental polarization curves acquired using the two different flow modes. The model predictions compare well with the experimental data and further highlight the advantages of using the flow-through mode relative to the flow-by mode. Detailed multi-dimensional contours of the electrolyte flow velocity and key species distributions reveal that more uniform diffusion and stronger convective transport are achieved by using the flow-through mode, which alleviates the ohmic loss associated with charge transport in the Br2 electrode
Microstructural evolution induced by micro-cracking during fast lithiation of single-crystalline silicon
h i g h l i g h t s Lithiation of Si results in various microstructures depending of crystal orientation. A complex vein-like microstructure of Li x Si was observed in {100} oriented Si. Micro-cracks provide a fast path for Li diffusion and cause a non-uniform lithiation. Crystalline Li x Si plays an important role in micro-crack generation. a r t i c l e i n f o t r a c t We report observations of microstructural changes in {100} and {110} oriented silicon wafers during initial lithiation under relatively high current densities. Evolution of the microstructure during lithiation was found to depend on the crystallographic orientation of the silicon wafers. In {110} silicon wafers, the phase boundary between silicon and Li x Si remained flat and parallel to the surface. In contrast, lithiation of the {100} oriented substrate resulted in a complex vein-like microstructure of Li x Si in a crystalline silicon matrix. A simple calculation demonstrates that the formation of such structures is energetically unfavorable in the absence of defects due to the large hydrostatic stresses that develop. However, TEM observations revealed micro-cracks in the {100} silicon wafer, which can create fast diffusion paths for lithium and contribute to the formation of a complex vein-like Li x Si network. This defect-induced microstructure can significantly affect the subsequent delithiation and following cycles, resulting in degradation of the electrode
Establishing ZIF-8 as a reference material for hydrogen cryoadsorption: An interlaboratory study
Hydrogen storage by cryoadsorption on porous materials has the advantages of low material cost, safety, fast kinetics, and high cyclic stability. The further development of this technology requires reliable data on the H2 uptake of the adsorbents, however, even for activated carbons the values between different laboratories show sometimes large discrepancies. So far no reference material for hydrogen cryoadsorption is available. The metal-organic framework ZIF-8 is an ideal material possessing high thermal, chemical, and mechanical stability that reduces degradation during handling and activation. Here, we distributed ZIF-8 pellets synthesized by extrusion to 9 laboratories equipped with 15 different experimental setups including gravimetric and volumetric analyzers. The gravimetric H2 uptake of the pellets was measured at 77â
K and up to 100â
bar showing a high reproducibility between the different laboratories, with a small relative standard deviation of 3â4 % between pressures of 10â100â
bar. The effect of operating variables like the amount of sample or analysis temperature was evaluated, remarking the calibration of devices and other correction procedures as the most significant deviation sources. Overall, the reproducible hydrogen cryoadsorption measurements indicate the robustness of the ZIF-8 pellets, which we want to propose as a reference material.M. Maiwald, J.â
A. Villajos, R. Balderas and M. Hirscher acknowledge the EMPIR programme from the European Union's Horizon 2020 research and innovation programme for funding. F. Cuevas and F. Couturas acknowledge support from France 2030 program under project ANR-22-PEHY-0007. D. Cazorla and A. Berenguer-Murcia thank the support by PID2021-123079OB-I00 project funded by MCIN/AEI/10.13039/501100011033, and âERDF A way of making Europeâ. K.â
N. Heinselman, S. Shulda and P.â
A. Parilla acknowledge the support from the National Renewable Energy Laboratory, operated by Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Funding provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Hydrogen and Fuel Cell Technology Office through the HyMARC Energy Materials Network
Interplay of Linker Functionalization and Hydrogen Adsorption in the MetalâOrganic Framework MIL-101
Functionalization of metalâorganic frameworks results in higher hydrogen uptakes owing to stronger hydrogenâhost interactions. However, it has not been studied whether a given functional group acts on existing adsorption sites (linker or metal) or introduces new ones. In this work, the effect of two types of functional groups on MIL-101 (Cr) is analyzed. Thermal-desorption spectroscopy reveals that the âBr ligand increases the secondary building unitâs hydrogen affinity, while the âNH2 functional group introduces new hydrogen adsorption sites. In addition, a subsequent introduction of âBr and âNH2 ligands on the linker results in the highest hydrogen-store interaction energy on the cationic nodes. The latter is attributed to a push-and-pull effect of the linkers
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