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  • 1
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    A coupling of martensitic and metamagnetic transitions with collective magneto-volume and table-like magnetocaloric effects (2014)
    American Institute of Physics (AIP)
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    Publication Date: 2014-08-13
    Description: A coupling of the first-order paramagnetic-to-induced-ferromagnetic martensitic and the second-order antiferromagnetic-to-ferromagnetic metamagnetic transitions was found in MnNi 0.8 Fe 0.2 Ge alloy. Based on the coupling, a magneto-volume effect driven by the martensitic transition and a table-like magnetocaloric effect generated by the successive magnetic phase transitions arise collectively. By using the magneto-volume effect, the internal stress in the volume-expansion martensitic transition was determined at 350 MPa. The magnetocaloric effect, with a wide working temperature range of 26 K around room temperature, shows a small hysteresis loss (5 J kg −1 ) and a large net refrigerant capacity (157 J kg −1 ).
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 2
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    Multiscale modeling and characterization for performance and safety of lithium-ion batteries (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-08-20
    Description: Lithium-ion batteries are highly complex electrochemical systems whose performance and safety are governed by coupled nonlinear electrochemical-electrical-thermal-mechanical processes over a range of spatiotemporal scales. Gaining an understanding of the role of these processes as well as development of predictive capabilities for design of better performing batteries requires synergy between theory, modeling, and simulation, and fundamental experimental work to support the models. This paper presents the overview of the work performed by the authors aligned with both experimental and computational efforts. In this paper, we describe a new, open source computational environment for battery simulations with an initial focus on lithium-ion systems but designed to support a variety of model types and formulations. This system has been used to create a three-dimensional cell and battery pack models that explicitly simulate all the battery components (current collectors, electrodes, and separator). The models are used to predict battery performance under normal operations and to study thermal and mechanical safety aspects under adverse conditions. This paper also provides an overview of the experimental techniques to obtain crucial validation data to benchmark the simulations at various scales for performance as well as abuse. We detail some initial validation using characterization experiments such as infrared and neutron imaging and micro-Raman mapping. In addition, we identify opportunities for future integration of theory, modeling, and experiments.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 3
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    Communication: Non-monotonic evolution of dynamical heterogeneity in unfreezing process of metallic glasses (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-07-30
    Description: The relaxation dynamics in unfreezing process of metallic glasses is investigated by the activation-relaxation technique. A non-monotonic dynamical microstructural heterogeneities evolution with temperature is discovered, which confirms and supplies more features to flow units concept of glasses. A flow unit perspective is proposed to microscopically describe this non-monotonic evolution of the dynamical heterogeneities as well as its relationship with the deformation mode development of metallic glasses.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics (AIP)
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  • 4
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    Multipactor threshold calculation of coaxial transmission lines in microwave applications with nonstationary statistical theory (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-08-15
    Description: This paper presents a statistical theory for the initial onset of multipactor breakdown in coaxial transmission lines, taking both the nonuniform electric field and random electron emission velocity into account. A general numerical method is first developed to construct the joint probability density function based on the approximate equation of the electron trajectory. The nonstationary dynamics of the multipactor process on both surfaces of coaxial lines are modelled based on the probability of various impacts and their corresponding secondary emission. The resonant assumption of the classical theory on the independent double-sided and single-sided impacts is replaced by the consideration of their interaction. As a result, the time evolutions of the electron population for exponential growth and absorption on both inner and outer conductor, in response to the applied voltage above and below the multipactor breakdown level, are obtained to investigate the exact mechanism of multipactor discharge in coaxial lines. Furthermore, the multipactor threshold predictions of the presented model are compared with experimental results using measured secondary emission yield of the tested samples which shows reasonable agreement. Finally, the detailed impact scenario reveals that single-surface multipactor is more likely to occur with a higher outer to inner conductor radius ratio.
    Print ISSN: 1070-664X
    Electronic ISSN: 1089-7674
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 5
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    Microscopic insight into the origin of enhanced glass-forming ability of metallic melts on micro-alloying (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-09-29
    Description: Extensive efforts have been made to develop metallic-glasses with large casting diameter. Such efforts were hindered by the poor understanding of glass formation mechanisms and the origin of the glass-forming ability (GFA) in metallic glass-forming systems. In this work, we have investigated relaxation dynamics of a model bulk glass-forming alloy system that shows the enhanced at first and then diminished GFA on increasing the percentage of micro-alloying. The micro-alloying did not have any significant impact on the thermodynamic properties. The GFA increasing on micro-alloying in this system cannot be explained by the present theoretical knowledge. Our results indicate that atomic caging is the primary factor that influences the GFA. The composition dependence of the atomic caging time or residence time is found to be well correlated with GFA of the system.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
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  • 6
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    Propagation characteristics of atmospheric-pressure He+O2 plasmas inside a simulated endoscope channel (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-11-26
    Description: Cold atmospheric-pressure plasmas have potential to be used for endoscope sterilization. In this study, a long quartz tube was used as the simulated endoscope channel, and an array of electrodes was warped one by one along the tube. Plasmas were generated in the inner channel of the tube, and their propagation characteristics in He+O 2 feedstock gases were studied as a function of the oxygen concentration. It is found that each of the plasmas originates at the edge of an instantaneous cathode, and then it propagates bidirectionally. Interestingly, a plasma head with bright spots is formed in the hollow instantaneous cathode and moves towards its center part, and a plasma tail expands through the electrode gap and then forms a swallow tail in the instantaneous anode. The plasmas are in good axisymmetry when [O 2 ] ≤ 0.3%, but not for [O 2 ] ≥ 1%, and even behave in a stochastic manner when [O 2 ] = 3%. The antibacterial agents are charged species and reactive oxygen species, so their wall fluxes represent the “plasma dosage” for the sterilization. Such fluxes mainly act on the inner wall in the hollow electrode rather than that in the electrode gap, and they get to the maximum efficiency when the oxygen concentration is around 0.3%. It is estimated that one can reduce the electrode gap and enlarge the electrode width to achieve more homogenous and efficient antibacterial effect, which have benefits for sterilization applications.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 7
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    Magnetostructural transition and magnetocaloric effect in MnNiSi-Fe2Ge system (2015)
    American Institute of Physics (AIP)
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    Publication Date: 2015-11-26
    Description: Magnetostructural transition from ferromagnetic orthorhombic phase to paramagnetic hexagonal phase can be obtained by chemically alloying appropriate amount of Fe 2 Ge into MnNiSi. The magnetostructural transition temperature is tunable in a wide temperature range of about 280 K. Saturation moment of the ferromagnetic orthorhombic phase increases from 2.62  μ B /f.u. to 3.17  μ B /f.u. with Fe 2 Ge-doping. The magnetostructural transition is accompanied by a large change of magnetization over 80 Am 2 /kg under magnetic field of 5 T. Relatively large magnetic entropy changes and working temperature ranges were observed in the vicinity of room temperature. Our findings suggest that MnNiSi-Fe 2 Ge material system is a promising platform for tunable magnetostructural transition and the associated magnetocaloric effect.
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
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  • 8
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    Windows open for highly tunable magnetostructural phase transitions (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-07-19
    Description: An attempt was made to tailor the magnetostructural transitions over a wide temperature range under the principle of isostructural alloying. A series of wide Curie-temperature windows (CTWs) with a maximal width of 377 K between 69 and 446 K were established in the Mn 1− y Co y NiGe 1− x Si x system. Throughout the CTWs, the magnetic-field-induced metamagnetic behavior and giant magnetocaloric effects are obtained. The (Mn,Co)Ni(Ge,Si) system shows great potential as multifunctional phase-transition materials that work in a wide range covering liquid-nitrogen and above water-boiling temperatures. Moreover, general understanding of isostructural alloying and CTWs constructed in (Mn,Co)Ni(Ge,Si) as well as (Mn,Fe)Ni(Ge,Si) is provided.
    Electronic ISSN: 2166-532X
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
    Published by American Institute of Physics (AIP)
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  • 9
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    The correlation between fragility, density, and atomic interaction in glass-forming liquids (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-07-21
    Description: The fragility that controls the temperature-dependent viscous properties of liquids as the glass transition is approached, in various glass-forming liquids with different softness of the repulsive part of atomic interactions at different densities, is investigated by molecular dynamic simulations. We show that the landscape of fragility in purely repulsive systems can be separated into three regions denoted as R I, R II , and R III , respectively, with qualitatively disparate dynamic behaviors: R I which can be described by “softness makes strong glasses,” R II where fragility is independent of softness and can only be tuned by density, and R III with constant fragility, suggesting that density plays an unexpected role for understanding the repulsive softness dependence of fragility. What is more important is that we unify the long-standing inconsistence with respect to the repulsive softness dependence of fragility by observing that a glass former can be tuned more fragile if nonperturbative attraction is added into it. Moreover, we find that the vastly dissimilar influences of attractive interaction on fragility could be estimated from the structural properties of related zero-temperature glasses.
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
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  • 10
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    Oxygen vacancy formation, crystal structures, and magnetic properties of three SrMnO3−δ films (2016)
    American Institute of Physics (AIP)
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    Publication Date: 2016-08-05
    Description: The crystal structures and magnetic properties of the 40 nm brownmillerite SrMnO 2.5 film, perovskite SrMnO 3-δ film, and mixed-phase film have been systematically investigated. The features of the oxygen vacancy ordering superstructure in the brownmillerite SrMnO 2.5 film are observed from HRSTEM as follows: the dark stripes with a periodicity of four (110) planes of the cubic perovskite appearing at an angle of 45° with the substrate-film interface and extra reflection spots in fast Fourier transformation patterns along the (001) plane. When annealing the brownmillerite SrMnO 2.5 film under higher oxygen pressure, the top portion undergoes structure transition into perovskite SrMnO 3-δ as seen in the mixed-phase film consisting of the perovskite SrMnO 3-δ phase dominating at the top part and the brownmillerite SrMnO 2.5 phase dominating at the bottom part. The magnetic properties and Mn valences of the brownmillerite SrMnO 2.5 film indicate that this film, similar to the bulk, is antiferromagnetic with T N at 375 K. However, the strained tetragonal perovskite SrMnO 3-δ film exhibits ferromagnetic behavior with Curie temperature at 75 K and a saturation magnetization of 2.5μ B /Mn at 2 K. Moreover, the top perovskite SrMnO 3-δ phase of the mixed-phase film also exhibits ferromagnetic behavior evidenced by the existence of the exchange bias. We propose that the ferromagnetic properties in both the perovskite SrMnO 3-δ film and the top perovskite SrMnO 3-δ phase in the mixed-phase film originate from Mn 3+ –Mn 4+ double exchange coupling. However, the formation of Mn 3+ differs for the two samples in that it is caused by oxygen vacancies in the former and the distribution of oxygen content across the film during annealing in the latter.
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    Electronic ISSN: 1077-3118
    Topics: Physics
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