A new method, called Cloud of Points (COP) Reconstruction, is proposed in the present work to extend the meshfree method to simulate viscous flows. With the characters of viscous flows, the anisotropic COP structure is distributed in boundary layer. The proposed method can improve the anisotropic COP structure to isotropic COP structure and reduce the condition number of the least square coefficient matrix for conventional meshfree method. The values of the new reconstructed points are calculated by the Lagrange interpolation. The accuracy and the robustness of the presented meshfree solver are demonstrated on a number of standard test cases, including the functions with analytical gradients and the viscous flows past NACA0012 airfoil. The comparison of the simulation results with the experimental data and other numerical simulation data are also investigated.

The transient impact hypothesis was extended, and the oblique collision model was established by considering the tangential slip. In order to solve this problem, the oblique-impact equations for cam-follower were transformed into a linear complementarity problem. Impulsive control method was employed to control or anti-control the nonlinear responses. The simulation results show that the cam-follower system performs very complex nonlinear characteristics, such as period, quasi-period and chaos responses. Using the impulsive control method, the nonlinear responses of the cam-follower system can be controlled to P(n, n) and P(∞, n) or anti-controlled to chaos.

Social networks facilitate the transmission of hepatitis C virus (HCV) in people who inject drugs (PWID). The aim of this study was to assess how certain network structural characteristics are related to HCV infections in PWID and to determine the most susceptible individuals for HCV transmission in a network of PWID. PWID (N = 80) from central China were recruited from a previous follow-up case-control study. Demographic and behavioural information was obtained from a computerized database for each group. HCV RNA was extracted from blood specimens. Sequences were used to construct a phylogenetic tree and to determine genetic distances. Socio-metric social links were established between participants. Network measures were calculated using UCINET. Three HCV genotypes were identified, covering five subtypes. The density of the social networks for the whole sample (N = 80), case group (n = 31) and control group (n = 49) was 0.038, 0.054 and 0.008, respectively. PWID infected with HCV were in frequent contact with others within their group. There were four pairs of nodes with genotypic distances of 0.000 that were identified and clustered in subtypes 6a and 1b; each subject pair was linked and found in one clique. Three of the five most active nodes were infected with HCV. These three nodes served as a bridge, contributing to the connection of other nodes. These findings identify susceptible individuals for HCV transmission in PWID based on their frequent contact with others in the network. These results provide data that could be used for modelling HCV transmission patterns and in public health policies.

Human infection with the emerging avian influenza A(H7N9) virus in China in 2013 has raised global concerns. We conducted a retrospective descriptive study of 27 confirmed human influenza A(H7N9) cases in Jiangsu Province, to elaborate poultry-related exposures and to provide a more precise estimate of the incubation periods of the illness. The median incubation period was 6 days (range 2–10 days) in cases with single known exposure and was 7·5 days (range 6·5–12·5 days) in cases with exposures on multiple days, difference between the two groups was not significant (Z = −1·895, P = 0·058). The overall median incubation period for all patients was estimated to be 7·5 days (range 2–12·5 days). Our findings further highlight the necessity for public health authorities to extend the period of medical surveillance from 7 days to 10 days.

The EU-Rotate_N model was developed as a tool to estimate the growth and nitrogen (N) uptake of vegetable crop rotations across a wide range of European climatic conditions and to assess the economic and environmental consequences of alternative management strategies. The model has been evaluated under field conditions in Germany and Norway and under greenhouse conditions in China. The present work evaluated the model using Italian data to evaluate its performance in a warm and dry environment. Data were collected from four 2-year field rotations, which included lettuce (Lactuca sativa L.), fennel (Foeniculum vulgare Mill.), spinach (Spinacia oleracea L.), broccoli (Brassica oleracea L. var. italica Plenck) and white cabbage (B. oleracea convar. capitata var. alba L.); each rotation used three different rates of N fertilizer (average recommended N1, assumed farmer's practice N2=N1+0·3×N1 and a zero control N0). Although the model was not calibrated prior to running the simulations, results for above-ground dry matter biomass, crop residue biomass, crop N concentration and crop N uptake were promising. However, soil mineral N predictions to 0·6 m depth were poor. The main problem with the prediction of the test variables was the poor ability to capture N mineralization in some autumn periods and an inappropriate parameterization of fennel. In conclusion, the model performed well, giving results comparable with other bio-physical process simulation models, but for more complex crop rotations. The model has the potential for application in Mediterranean environments for field vegetable production.

The effects of source field plates on AlGaN/GaN High Electron Mobility Transistor reliability under off-state stress conditions were investigated using step-stress cycling. The source field plate enhanced the drain breakdown voltage from 55V to 155V and the critical voltage for off-state gate stress from 40V to 65V, relative to devices without the field plate. Transmission electron microscopy was used to examine the degradation of the gate contacts. The presence of cracking that appeared on both source and drain side of the gate edges was attributed to the inverse piezoelectric effect. In addition, a thin oxide layer was observed between the Ni gate contact and the AlGaN layer, and both Ni and oxygen had diffused into the AlGaN layer. The critical degradation voltage of AlGaN/GaN High Electron Mobility Transistors during off-state electrical stress was determined as a function of Ni/Au gate dimensions (0.1-0.17μm). Devices with different gate length and gate-drain distances were found to exhibit the onset of degradation at different source-drain biases but similar electric field strengths, showing that the degradation mechanism is primarily field-driven. The temperature dependence of sub-threshold drain current versus gate voltage at a constant drain bias voltage were used to determine the trap densities in AlGaN/GaN high electron mobility transistors (HEMTs) before and after the off-state stress. Two different trap densities were obtained for the measurements conducted at 300-493K and 493-573K, respectively.

By pumping AlGaN/GaN HEMTs with below band-gap light we observe changes in drain current that correspond to the trapping and detrapping of carriers within the band-gap. These changes in drain current are indicators of trap density, since the energy from a specific wavelength of light pumps traps whose activation energies are less than or equal to that of the light source.

AlGaN/GaN HEMTs on SiC with dual submicron gates with widths of 125nm, 140nm, or 170nm, are DC-stressed under three different conditions along a load line: VGS=0, VDS=5 (on-state), VGS=-2, VDS=9.2 and, VGS=-6, VDS=25 (off-state). The stress tests are interrupted at 20% degradation and the optically pumped comparisons to the baseline are measured.

This paper describes the optical pumping technique and results from experiments of AlGaN/GaN HEMTs under the three DC stress biases along a load line.

Within the framework of the dielectric-continuum model, the equations of motion for p-polarization field in wurtzite quantum well are solved for the propagating optical phonon modes. The polarization eigenvector and the dispersion relation are derived by using the determinate method. The dispersion relation and phonon modes properties of the propagating optical phonons are investigated for GaN/AlN single QW. The numerical results show that the propagating phonons have a quantization character by a quantum number n that define its order. The propagating optical phonons are more dispersive for decreasing n, and the bands formed by the dispersion curves are narrower for higher order modes. The wave vector and width of quantum well have more important effect on phonon energy, with the increase of wave vector or width of quantum well, the energy of propagating optical phonon modes decrease in TO domain (between A1 (TO) and E1 (TO) frequency of GaN), and increase in LO domain (between GaN A1 (LO) and E1 (LO) frequency of GaN). The strain effects of QW structures have clear influence on the dispersion behavior of propagating optical phonons. The frequency of the propagating optical phonon modes is increased due to the strains of the QW structure.

The development of reliable and thermally stable Ohmic and Schottky contacts to ZnO is one of the critical issues related to the fabrication of ZnO-based UV light emitters/detectors and field effect transistors. To date, a number of different metallization schemes and surface cleaning procedures prior to metal deposition have been examined for rectifying contacts on n-ZnO. While these reports have shown that low reactive metals such as Au, Ag and Pd form rectifying contacts with Schottky barrier heights in the 0.6-0.8 eV range, the thermal stability of these contacts is usually extremely poor, with degradation occurring even at 60°C for Au/n-ZnO. One approach to achieving increased barrier heights that has proven successful for GaAs, InP, InGaAs and other compound semiconductors is the use of cryogenic deposition temperatures. In this context, we report in this work on the effect of cryogenic temperature metal deposition on the contact properties of Pd, Pt, Ti, and Ni on single-crystal n-type ZnO. Deposition at both room and low temperature produced contacts with Ohmic characteristics for Ti and Ni metallizations. In comparison, both Pd and Pt contacts showed rectifying characteristics after deposition. All rectifying contacts exhibited barrier heights around 1-2 eV and idealities between 1 and 2. Low temperature deposition gave higher resistances in comparison to room temperature deposition for all cases. Larger contacts also corresponded to an increase in resistance. Changes in contact behavior were measured on Pd to anneal temperatures of ∼300°C, showing an increase in barrier height along with a decrease in ideality with increasing temperature. This difference with annealing temperature is in sharp contrast to previous results for Au contacts to ZnO. There were no differences in near-surface stoichiometry for the different deposition temperatures; however low temperature contacts demonstrated some cracking in Pt and Pd, probably due to surface stress.

The development of new etching and contact metallurgies for the ZnO/ZnMgO/ZnCdO materials system and various approaches for realizing ZnO LEDs are reviewed. ZnO nanorod MOSFETs and pH sensors have been demonstrated. In addition, selective detection of hydrogen with Pt-coated single ZnO nanorods is discussed discussed. The Pt-coated single nanorods show a current response approximately a factor of three larger at room temperature upon exposure to 500ppm H2 in N2 than thin films of ZnO. The power consumption of these sensors can be very small (in the nW range) when using discontinuous coatings of Pt. Once the Pt coating becomes continuous, the current required to operate the sensors increases to the μW range. The ZnO nanorods are insensitive to oxygen in the measurement ambient.

Many of the recent advances in enhancing the thermoelectric figure of merit are linked to nanoscale phenomena with both bulk samples containing nanoscale constituents and nanoscale materials exhibiting enhanced thermoelectric performance in their own right. Prior theoretical and experimental proof of principle studies on isolated quantum well and quantum wire samples have now evolved into studies on bulk samples containing nanostructured constituents. In this review, nanostructural composites are shown to exhibit nanostructures and properties that show promise for thermoelectric applications. A review of some of the results obtained to date are presented.

An updated compilation of published and new data of major-ion (Ca, Cl, K, Mg, Na, NO3, SO4) and methylsulfonate (MS) concentrations in snow from 520 Antarctic sites is provided by the national ITASE (International Trans-Antarctic Scientific Expedition) programmes of Australia, Brazil, China, Germany, Italy, Japan, Korea, New Zealand, Norway, the United Kingdom, the United States and the national Antarctic programme of Finland. The comparison shows that snow chemistry concentrations vary by up to four orders of magnitude across Antarctica and exhibit distinct geographical patterns. The Antarctic-wide comparison of glaciochemical records provides a unique opportunity to improve our understanding of the fundamental factors that ultimately control the chemistry of snow or ice samples. This paper aims to initiate data compilation and administration in order to provide a framework for facilitation of Antarctic-wide snow chemistry discussions across all ITASE nations and other contributing groups. The data are made available through the ITASE web page (http://www2.umaine.edu/itase/content/syngroups/snowchem.html) and will be updated with new data as they are provided. In addition, recommendations for future research efforts are summarized.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.

Single ZnO nanowire metal-oxide semiconductor field effect transistors (MOSFETs) were fabricated using nanowires grown by site selective Molecular Beam Epitaxy. When measured in the dark at 25°C, the depletion-mode transistors exhibit good saturation behavior, a threshold voltage of ∼-3V and a maximum transconductance of order 0.3 mS/mm. Under ultra-violet (366nm) illumination, the drain-source current increase by approximately a factor of 5 and the maximum transconductance is ∼ 5 mS/mm. The channel mobility is estimated to be ∼3 cm2 /V.s, which is comparable to that reported for thin film ZnO enhancement mode MOSFETs and the on/off ratio was ∼25 in the dark and ∼125 under UV illumination.

Gram quantities of titania (TiO2) nanotubes, with typical outside diameter about 9 nm, wall thickness about 2.5 nm, and length about 600 nm, were synthesized from anatase nano- and micro-powder using the hydrothermal method. The crystallization, structure, and phase stability of the nanotubes at high temperatures were systematically studied. A morphology change from nanotube to nanowire was observed at 650°C. The as-prepared nanotubes were usually contaminated with sodium impurities, other TiO2-derived phases and were poorly crystallized, but under optimized synthesis conditions the impurity phases was completely removed, resulting in highly crystallized pure nanotubes. The volume filling fraction of the autoclave as well as the concentration of the acid treatment were found to be particularly important for controlling the purity and crystallinity of the resulting nanotubes.

Both MgO and Sc2O3 are shown to provide low interface state densities (in the 1011 eV-1 cm-2 range) on n- and p-GaN, making them useful for surface passivation layers to mitigate current collapse in GaN/AlGaN high electron mobility transistors(HEMTs) and also gate dielectrics for metal-oxide semiconductor(MOS) devices. Clear evidence of inversion has been demonstrated in gate-controlled MOS p-GaN diodes using both types of oxide. Charge pumping measurements on diodes undergoing a high temperature implant activation anneal show a total surface state density of ∼3 × 1012 cm-2. On HEMT structures, both oxides provide effective passivation of surface states and these devices show improved output power. The MgO/GaN diodes and Sc2O3 passivated HEMT are also found to be quite radiation-resistant, making them attractive for satellite and terrestrial communication systems requiring a high tolerance to high energy(40MeV) protons.

High figure of merit has been reported in superlattice structures in recent years mainly due to a large reduction in thermal conductivity, while maintaining or even enhancing the power factor. These findings suggest the possibility of using nanocomposites to reduce a material's thermal conductivity and thereby increasing the thermoelectric figure of merit. The current work reports on the thermal conductivity of various Si-Ge nanocomposites synthesized in a simple and straightforward process that allows one to exploit nanoscale physics while rapidly producing macroscale devices. Composite synthesis consisted of combining Si nanoparticles approximately 70 nm in diameter with micron-sized Ge particles in various atomic ratios via hotpressing to form macroscale samples. Room-temperature thermal conductivity was measured for each of the nanocomposite samples and compared with the values of SiGe bulk alloys. In this preliminary work, we observed a significant reduction in bulk thermal conductivity in such materials.

High quality undoped AlGaN/GaN high electron mobility transistors(HEMTs) structures have been gorwn by Hydride Vapor Phase Epitaxy (HVPE). The morphology of the films grown on Al2O3 substrates is excellent with root-mean-square roughness of ∼0.2nm over 10×10μm2 measurement area. Capacitance-voltage measurements show formation of dense sheet of charge at the AlGaN/GaN interface. HEMTs with 1μm gate length fabricated on these structures show transconductances in excess of 110 mS/mm and drain-source current above 0.6A/mm. Gate lag measurements show similar current collapse characteristics to HEMTs fabricated in MBE- or MOCVD grown material.

The deterministic growth of ZnO nanorods using molecular beam epitaxy is reported. The process is catalyst-driven, as single crystal ZnO nanorod growth is realized via nucleation on Ag islands that are distributed on a SiO2-terminated Si substrate surface. Growth occurs at substrate temperatures on the order of 300-500°C. The nanorods exhibit diameters of 15-40 nm and lengths in excess of 1 μm. Nanorod placement can be predefined via location of metal catalyst islands or particles. This, coupled with the relatively low growth temperatures needed, suggests that ZnO nanorods could be integrated on device platforms for numerous applications, including chemical sensors and nanoelectronics.

The effect of oxygen pressure on MgO grown by RF plasma assisted gas-source molecular beam epitaxy was investigated. Increasing oxygen pressure was found to decrease the growth rate, improve the morphology and reduce the Mg/O ratio to near that obtained from bulk single crystal MgO. By contrast, the electrical characteristics of MgO/GaN diodes showed continual improvement in breakdown field and interface state density as the pressure was decreased. The lowest pressure tested, 1×10−5Torr, produced the lowest Dit, 3×1011 eV−1cm−2, and the highest VBD, 4.4 MV/cm. Cross sectional transmission electron microscopy of the MgO grown at the lowest pressure showed the initial 40 monolayers to be epitaxial, with the remainder of the layer appearing to be fine grained poly-crystal. Comparisons with films grown using an electron cyclotron resonance (ECR) plasma suggest that higher ion energies are desirable for obtaining the best electrical characteristics.