1,334 research outputs found
Quantitatively Analyzing Phonon Spectral Contribution of Thermal Conductivity Based on Non-Equilibrium Molecular Dynamics Simulation I: From Space Fourier Transform
Probing detailed spectral dependence of phonon transport properties in bulk
materials is critical to improve the function and performance of structures and
devices in a diverse spectrum of technologies. Currently, such information can
only be provided by the phonon spectral energy density (SED) or equivalently
time domain normal mode analysis (TDNMA) methods in the framework of
equilibrium molecular dynamics simulation (EMD), but has not been realized in
non-equilibrium molecular dynamics simulations (NEMD) so far. In this paper we
generate a new scheme directly based on NEMD and lattice dynamics theory,
called time domain direct decomposition method (TDDDM), to predict the phonon
mode specific thermal conductivity. Two benchmark cases of Lennard-Jones (LJ)
Argon and Stillinger-Weber (SW) Si are studied by TDDDM to characterize
contributions of individual phonon modes to overall thermal conductivity and
the results are compared with that predicted using SED and TDNMA. Excellent
agreements are found for both cases, which confirm the validity of our TDDDM
approach. The biggest advantage of TDDDM is that it can be used to investigate
the size effect of individual phonon modes in NEMD simulations, which cannot be
tackled by SED and TDNMA in EMD simulations currently. We found that the phonon
modes with mean free path larger than the system size are truncated in NEMD and
contribute little to the overall thermal conductivity. The TDDDM provides
direct physical origin for the well-known strong size effects in thermal
conductivity prediction by NEMD
An integrated inventory model for supply chain management
A thesis submitted for the degree ofMaster of Science by research ofthe University of LutonImproved integration of logistics processes across multiple companies of a supply chain is of increasing interest and importance. With modern information technologies, more and more companies intend to implement a logistics alliance strategy for co-operations in the supply chain. However, the implementation of the strategy highly depends on the integrated logistics models available. To this end, extensions of existing models may be required to facilitate the entire supply chain rather than individuals.
Inventory management is one of the most important parts of logistics management. In this project, an integrated inventory model is built for a supply chain with a manufacturer, multiple upstream factories and mul1iple downstream vendors. Based on some assumptions, all the individual inventory behaviours are considered together to suggest an overall optimised plan to minimise the total inventory cost of the supply chain. Then, extensions are made to the integrated inventory model for practical considerations. A numerical analysis is conducted to compare the optimised results of the integrated model with the results of some existing models. Finally, conclusions and future perspectives are drawn
Thermal conductivity reduction in core-shell nanowires
Nanostructuring of thermoelectric materials bears promise for manipulating physical parameters to improve the energy conversion efficiency of thermoelectrics. Using nonequilibrium molecular dynamics, we investigate how the thermal conductivity can be altered in core-shell nanocomposites of Si and Ge. By calculating the phonon vibrational density of states and performing normal mode analysis, we show that the thermal conductivity decreases when phonon-transport becomes diffusion-dominated and unveil a competition between modes from the various regions of the nanocomposite (core, interface, and shell). The effects of nanowire length, cross section, and temperature on thermal conductivity are explicitly considered. Surprisingly, the thermal conductivity variation with nanowire length is much weaker than in pure nanowires. Also, the thermal conductivity is almost independent of temperature in the wide region between 50 and 600 K, a direct result of confinement of the core by the shell. These results suggest that core-shell nanowires are promising structures for thermoelectrics
Visible two-dimensional photonic crystal slab laser
The authors describe the fabrication and performance of photonic crystal lasers fabricated within thin membranes of InGaP/InGaAlP quantum well material and emitting in the visible wavelength range. These lasers have ultrasmall mode volumes, emit red light, and exhibit low threshold powers. They can be lithographically tuned from 650 to 690 nm. Their cavity volumes of approximately 0.01 µm3 are ideally suited for use as spectroscopic sources
Investigation of Stick Propellant Internal Perforation Erosive Burning on Interior Ballistics Performances
In this study, a thorough investigation of a stick propellant internal perforation erosive burning on interior ballistic performances is presented via extending the previous work of author. The stick propellant combustion process and the internal perforation erosive burning are revealed by numerical simulations. Different factors with respect to the stick propellant, including propellant length, internal perforation diameter and loading density are analysed in detail. Stick propellant length and the internal perforation diameter have a significant influence on the ballistic performance, the longer and smaller internal propellant have a greater erosive burning effect to the ballistic performance. Loading density has very weak influence on the ballistic
The UTE and ZTE Sequences at Ultra-High Magnetic Field Strengths: A Survey
UTE (Ultrashort Echo Time) and ZTE (Zero Echo Time) sequences have been
developed to detect short T2 relaxation signals coming from regions that are
unable to be detected by conventional MRI methods. Due to the high
dipole-dipole interactions in solid and semi-solid tissues, the echo time
generated is simply not enough to produce a signal using conventional imaging
method, often leading to void signal coming from the discussed areas. By the
application of these techniques, solid and semi-solid areas can be imaged which
can have a profound impact in clinical imaging. High and Ultra-high field
strength (UHF) provides a vital advantage in providing better sensitivity and
specificity of MR imaging. When coupled with the UTE and ZTE sequences, the
image can recover void signals as well as a much-improved signal quality. To
further this strategy, secondary data from various research tools was obtained
to further validate the research while addressing the drawbacks to this
approach. It was found that UTE and ZTE sequences coupled with some techniques
such as qualitative imaging and new trajectories are very crucial for accurate
image depiction of the areas of the musculoskeletal system, neural system, lung
imaging and dental imaging
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