Showing papers by "Wei Ji published in 2013"

Real-Space Identification of Intermolecular Bonding with Atomic Force Microscopy

Abstract: We report a real-space visualization of the formation of hydrogen bonding in 8-hydroxyquinoline (8-hq) molecular assemblies on a Cu(111) substrate, using noncontact atomic force microscopy (NC-AFM). The atomically resolved molecular structures enable a precise determination of the characteristics of hydrogen bonding networks, including the bonding sites, orientations, and lengths. The observation of bond contrast was interpreted by ab initio density functional calculations, which indicated the electron density contribution from the hybridized electronic state of the hydrogen bond. Intermolecular coordination between the dehydrogenated 8-hq and Cu adatoms was also revealed by the submolecular resolution AFM characterization. The direct identification of local bonding configurations by NC-AFM would facilitate detailed investigations of intermolecular interactions in complex molecules with multiple active sites.

A theoretical analysis of the effect of the hydrogenation of graphene to graphane on its mechanical properties

Abstract: We investigated the effect of the hydrogenation of graphene to graphane on its mechanical properties using first-principles calculations based on density-functional theory. The hydrogenation reduces the ultimate strengths in all three tested deformation modes – armchair, zigzag, and biaxial – and the in-plane stiffness by 1/3. The Poisson ratio was reduced from 0.178 to 0.078, a 56% decrease. However, the ultimate strain in zigzag deformation was increased by 8.7%. The shear mode elastic constants are more sensitive than longitudinal ones to hydrogenation. The fourth and fifth order longitudinal mode elastic constants are inert to the hydrogenation, in contrast to a large decrease of those in second and third order. The hydrogenation does not change the monotonic decrement of the Poisson ratio with increasing pressure, but the rate is tripled. Our results indicate that graphene–graphane systems could be used for hydrogen storage with high speed of charge–discharge of hydrogen.

A Theoretical Analysis of the Effect of the Hydrogenation of Graphene to Graphane on Its Mechanical Properties

Abstract: We investigated the effect of the hydrogenation of graphene to graphane on its mechanical properties using first-principles calculations based on density-functional theory. The hydrogenation reduces the ultimate strengths in all three tested deformation modes – armchair, zigzag, and biaxial – and the in-plane stiffness by 1/3. The Poisson ratio was reduced from 0.178 to 0.078, a 56% decrease. However, the ultimate strain in zigzag deformation was increased by 8.7%. The shear mode elastic constants are more sensitive than longitudinal ones to hydrogenation. The fourth and fifth order longitudinal mode elastic constants are inert to the hydrogenation, in contrast to a large decrease of those in second and third order. The hydrogenation does not change the monotonic decrement of the Poisson ratio with increasing pressure, but the rate is tripled. Our results indicate that graphene–graphane systems could be used for hydrogen storage with high speed of charge–discharge of hydrogen.

Mechanical properties of g -GaN: a first principles study

Abstract: We investigate the mechanical properties of pro- posed graphene-like hexagonal gallium nitride monolayer (g-GaN) using first-principles calculations based on density- functional theory. Compared to the graphene-like hexagonal boron nitride monolayer (g-BN), g-GaN is softer, with 40 % in-plane stiffness, 50 %, 46 %, and 42 % ultimate strengths in armchair, zigzag, and biaxial strains, respectively. How- ever, g-GaN has a larger Poisson's ratio, 0.43, about 1.9 times that of g-BN. It was found that the g-GaN also sus- tains much smaller strains before rupture. We obtained the second-, third-, fourth-, and fifth-order elastic constants for a rigorous continuum description of the elastic response of g-GaN. The second-order elastic constants, including in- plane stiffness, are predicted to monotonically increase with pressure while the Poisson's ratio monotonically decreases with increasing pressure. The sound velocity of a compres- sional wave has a minima of 10 km/s at an in-plane pres- sure of 1 N/m, while as a shear wave's velocity monotoni- cally increases with pressure. The tunable sound velocities have promising applications in nano waveguides and surface acoustic wave sensors.

First-principles study of the effects of mechanical strains on the radiation hardness of hexagonal boron nitride monolayers

Abstract: We investigate the strain effect on the radiation hardness of hexagonal boron nitride (h-BN) monolayers using density functional theory calculations. Both compressive and tensile strains are studied in elastic domains along the zigzag, armchair, and biaxial directions. We observe a reduction in radiation hardness to form boron and nitrogen monovacancies under all strains. The origin of this effect is the strain-induced reduction of the energy barrier to displace an atom. An implication of our results is the vulnerability of strained nanomaterials to radiation damage.

Chemically Tuning Mechanics of Graphene by BN

Abstract: With finite bandgaps, g-BNC, a boron nitride monolayer (g-BN) phase within a graphene layer, is a promising semiconductor for next generation electronics. We report its mechanics dependence of the g-BN concentration, including the high order elastic constants and mechanical failure, through a first-principles study based on density functional theory. The in-plane stiffness as well as third order elastic constants of graphene can be linearly tuned with g-BN concentration. The longitudinal mode elastic constants are sensitive to the BN modification, in contrast to the shear mode elastic constants. This study may provide guidance in optimizing the mechanics of graphene-based nanodevices.

The Effect of Cryogenic Treatment on the Microstructure and Properties of Ti-6Al-4V Titanium Alloy

Abstract: The effect of cryogenic treatment on the microstructure and properties of Ti-6Al-4V has been studied in this paper. The program controlled SLX cryogenic box was used to conduct the cryogenic treatment and the subsequent low temperature temper. The scanning electron microscope was used to study the morphology of microstructure and fracture surface. As the results show that the cryogenic treatment increases the elongation of Ti-6Al-4V from 16.5 percent to 24.5 percent, at the same time, the strength increases slightly, this indicates that cryogenic treatment can improve the comprehensive mechanical properties. The microstructure measurement revealed that there is a tendency of reduction in the precipitated particles after cryogenic treatment. The cross section is flat and the size of dimples is more uniform. It is concluded that the change in the precipitation particle had a great influence in the mechanical properties.

Apoptotic responses of zebrafish (Danio rerio) after exposure with microcystin-LR under different ambient temperatures.

Abstract: Microcystins (MCs) can cause evident hepatic apoptosis In vitro studies indicated that uptake of MC by isolated hepatocytes was dramatically reduced as ambient temperature dropped, and some studies presented a hypothesis that differences in core body temperatures in animals result in diverse uptake of MC, as well as different toxic effects Thus far, however, few in vivo studies have been conducted to investigate the effects of temperature on MC-induced hepatocyte apoptosis in fish, a typical poikilotherm In the present study, zebrafish were treated with MC-LR, an MC metabolite, at three water temperatures (12, 22 and 32 degrees C), and evident differences in apoptotic profiles were observed Damage to liver ultrastructures revealed temperature-dependent early-stage patterns of apoptosis Flow-cytometric analysis indicated that hepatocyte apoptotic rates varied with a temperature-dependent effect The transcription levels of some apoptosis-related genes were determined using quantitative real-time polymerase chain reaction, and significantly elevated gene expressions of P53, Bcl-2, Bax and caspase-3 were found in the 12 and 32 degrees C groups Results of the present study indicate that different ambient temperatures can lead to various toxic effects of MCs on hepatic apoptosis in fish Copyright (c) 2012 John Wiley & Sons, Ltd

Pebble Flow and Coolant Flow Analysis Based on a Fully Coupled Multiphysics Model

Abstract: In pebble bed reactors (PBRs), pebble flow and coolant flow are highly correlated, and the behavior of each flow is strongly influenced by pebble-coolant interactions. Simulation of both flows in P...

Simultaneous enhancement of phonons modes with molecular vibrations due to Mg doping of a TiO2 substrate

Abstract: The Raman behavior of Mg-doped TiO2 (0%, 1%, 3% and 5%) nanoparticles (NPs) prepared by a sol–gel method was investigated. It was found that the Mg2+ ions were in situ doped into the TiO2 NPs. Normal Raman and surface enhanced Raman scattering (SERS) spectra have been measured. We observe the enhanced Raman intensity of both the adsorbed molecules and the phonon modes of the Mg–TiO2 NPs when 4-mercaptobenzoic acid (4-MBA) is the probing molecule adsorbed on the Mg–TiO2 NPs. The results showed that the Mg doping concentration and the contents of the surface defects play an important role in the synergistic enhanced phenomenon. The performances of these substrates strongly depend on the charge-transfer resonance mechanism, which originates from the electron and energy transfer between the semiconductor's and the molecule's interfaces.

Evaluation of vectorized monte carlo algorithms on gpus for a neutron eigenvalue problem

Abstract: Conventional Monte Carlo (MC) methods for radiation transport computations are 'history-based', which means that one particle history at a time is tracked. Simulations based on such methods suffer from thread divergence on the graphics processing unit (GPU), which severely affects the performance of GPUs. To circumvent this limitation, event-based vectorized MC algorithms can be utilized. A versatile software test-bed, called ARCHER - Accelerated Radiation-transport Computations in Heterogeneous Environments - was used for this study. ARCHER facilitates the development and testing of a MC code based on the vectorized MC algorithm implemented on GPUs by using NVIDIA's Compute Unified Device Architecture (CUDA). The ARCHER{sub GPU} code was designed to solve a neutron eigenvalue problem and was tested on a NVIDIA Tesla M2090 Fermi card. We found that although the vectorized MC method significantly reduces the occurrence of divergent branching and enhances the warp execution efficiency, the overall simulation speed is ten times slower than the conventional history-based MC method on GPUs. By analyzing detailed GPU profiling information from ARCHER, we discovered that the main reason was the large amount of global memory transactions, causing severe memory access latency. Several possible solutions to alleviate the memory latency issue are discussed. (authors)

An Update of ARCHER, a Monte Carlo Radiation Transport Software Testbed for Emerging Hardware Such as GPUs

Abstract: Heterogeneous computing systems involving the graphics processing unit (GPU) and other accelerators such as the coprocessor are playing an increasingly important role in scientific computing. However, none of the existing production Monte Carlo (MC) radiation transport codes were designed to take advantage of such heterogeneous computer architectures. In this paper, we describe the development of a software package, called ARCHER Accelerated Radiation-transport Computations in Heterogeneous EnviRonments, which is designed as a software testbed for research on how to accelerate Monte Carlo calculations using the Nvidia GPU and the Intel Xeon Phi coprocessor.

A neutronic feasibility study of the AP1000 design loaded with fully ceramic micro-encapsulated fuel

Abstract: A neutronic feasibility study is performed to evaluate the utilization of fully ceramic microencapsulated (FCM) fuel in the AP1000 reactor design. The widely used Monte Carlo code MCNP is employed to perform the full core analysis at the beginning of cycle (BOC). Both the original AP1000 design and the modified design with the replacement of uranium dioxide fuel pellets with FCM fuel compacts are modeled and simulated for comparison. To retain the original excess reactivity, ranges of fuel particle packing fraction and fuel enrichment in the FCM fuel design are first determined. Within the determined ranges, the reactor control mechanism employed by the original design is directly used in the modified design and the utilization feasibility is evaluated. The worth of control of each type of fuel burnable absorber (discrete/integral fuel burnable absorbers and soluble boron in primary coolant) is calculated for each design and significant differences between the two designs are observed. Those differences are interpreted by the fundamental difference of the fuel form used in each design. Due to the usage of silicon carbide as the matrix material and the fuel particles fuel form in FCM fuel design, neutron slowing down capability is increased in the new design,more » leading to a much higher thermal spectrum than the original design. This results in different reactivity and fission power density distributions in each design. We conclude that a direct replacement of fuel pellets by the FCM fuel in the AP1000 cannot retain the original optimum reactor core performance. Necessary modifications of the core design should be done and the original control mechanism needs to be re-designed. (authors)« less

Adsorption of PTCDA and C60 on KBr(001): electrostatic interaction versus electronic hybridization

Abstract: The adsorption of functional molecules on insulator surfaces is of great importance to molecular electronics. We present a systematical investigation of geometric and electronic properties of PTCDA and C60 on KBr(001) using DFT and non-contact atomic force microscopy. It was found that electrostatics is the primary interaction mechanism for PTCDA and C60 adsorbed on KBr. It was thus concluded that alkali-halides is a competitive candidate to be adopted to support low polarizability molecules, such as PTCDA, in future electronics.

Continuously tunable electronic structure of transition metal dichalcogenides superlattices

Abstract: Two dimensional transition metal dichalcogenides (TMDC) have very interesting properties for optoelectronic devices. In this work we theoretically investigate and predict that superlattices comprised of MoS$_{2}$ and WSe$_{2}$ multilayers possess continuously tunable electronic structure having direct band gap. The tunability is controlled by the thickness ratio of MoS$_{2}$ versus WSe$_{2}$ of the superlattice. When this ratio goes from 1:2 to 5:1, the dominant K-K direct band gap is continuously tuned from 0.14 eV to 0.5 eV. The gap stays direct against -0.6% to 2% in-layer strain and up to -4.3% normal-layer compressive strain. The valance and conduction bands are spatially separated. These robust properties suggest that MoS$_{2}$ and WSe$_{2}$ multilayer superlattice should be an exciting emerging material for infrared optoelectronics.

Development of gpu-based monte carlo code for fast ct imaging dose calculation on cuda fermi architecture

Abstract: This paper describes the development of a Graphics Processing Unit (GPU) accelerated Monte Carlo photon transport code, ARCHERGPU, to perform CT imaging dose calculations with good accuracy and performance. The code simulates interactions of photons with heterogeneous materials. It contains a detailed CT scanner model and a family of patient phantoms. Several techniques are used to optimize the code for the GPU architecture. In the accuracy and performance test, a 142 kg adult male phantom was selected, and the CT scan protocol involved a whole-body axial scan, 20-mm x-ray beam collimation, 120 kVp and a pitch of 1. A total of 9 10 8 photons were simulated and the absorbed doses to 28 radiosensitive organs/tissues were calculated. The average percentage difference of the results obtained by the general-purpose production code MCNPX and ARCHERGPU was found to be less than 0.38%, indicating an excellent agreement. The total computation time was found to be 8,689, 139 and 56 minutes for MCNPX, ARCHERCPU (6-core) and ARCHERGPU, respectively, indicating a decent speedup. Under a recent grant funding from the NIH, the project aims at developing a Monte Carlo code with the capability of sub-minute CT organ dose calculations.

Contact detection acceleration in pebble flow simulation for pebble bed reactor systems

Abstract: Pebble flow simulation plays an important role in the steady state and transient analysis of thermal-hydraulics and neutronics for Pebble Bed Reactors (PBR). The Discrete Element Method (DEM) and the modified Molecular Dynamics (MD) method are widely used to simulate the pebble motion to obtain the distribution of pebble concentration, velocity, and maximum contact stress. Although DEM and MD present high accuracy in the pebble flow simulation, they are quite computationally expensive due to the large quantity of pebbles to be simulated in a typical PBR and the ubiquitous contacts and collisions between neighboring pebbles that need to be detected frequently in the simulation, which greatly restricted their applicability for large scale PBR designs such as PBMR400. Since the contact detection accounts for more than 60% of the overall CPU time in the pebble flow simulation, the acceleration of the contact detection can greatly enhance the overall efficiency. In the present work, based on the design features of PBRs, two contact detection algorithms, the basic cell search algorithm and the bounding box search algorithm are investigated and applied to pebble contact detection. The influence from the PBR system size, core geometry and the searching cell size on the contact detection efficiency is presented. Our results suggest that for present PBR applications, the bounding box algorithm is less sensitive to the aforementioned effects and has superior performance in pebble contact detection compared with basic cell search algorithm.

The impact of fuel particle size distribution on neutron transport in stochastic media

Abstract: This paper presents a study of the particle size distribution impact on neutron transport in threedimensional stochastic media. An eigenvalue problem is simulated in a cylindrical container consisting of fissile fuel particles with five different size distributions: constant, uniform, power, exponential and Gaussian. We construct 15 cases by altering the fissile particle volume packing fraction and its optical thickness, but keeping the mean chord length of the spherical fuel particle the same at different size distributions. The tallied effective multiplication factor (keff) and flux distribution along axial and radial directions are compared between different size distributions. At low packing fraction and low optical thickness, the size distribution has a significant impact on radiation transport in stochastic media, which can cause as high as ~270pcm difference in keff value and ~2.6% relative error difference in peak flux. As the packing fraction and optical thickness increase, the impact gradually dissipates.