Current Transport Models Research Articles

DRIFT-INDUCED DECELERATION OF SOLAR ENERGETIC PARTICLES

We investigate the deceleration of Solar Energetic Particles (SEPs) during their propagation from the Sun through interplanetary space, in the presence of weak to strong scattering in a Parker spiral configuration, using relativistic full orbit test particle simulations. The calculations retain all three spatial variables describing particles' trajectories, allowing to model any transport across the magnetic field. Large energy change is shown to occur for protons, due to the combined effect of standard adiabatic deceleration and a significant contribution from particle drift in the direction opposite to that of the solar wind electric field. The latter drift-induced deceleration is found to have a stronger effect for SEP energies than for galactic cosmic rays. The kinetic energy of protons injected at 1 MeV is found to be reduced by between 35 and 90% after four days, and for protons injected at 100 MeV by between 20 and 55%. The overall degree of deceleration is a weak function of the scattering mean free path, showing that, although adiabatic deceleration plays a role, a large contribution is due to particle drift. Current SEP transport models are found to account for drift-induced deceleration in an approximate way and their accuracy will need to be assessed in future work.

Current Transport Through Lead–Borosilicate Interfacial Glass Layers at the Screen–Printed Silver-Silicon Front Contact

During the firing of screen-printed silver contacts in industrial crystalline silicon solar cells, an interfacial glass layer is formed between the silver bulk contact and the silicon substrate. By means of numerical modeling, we investigate conduction mechanisms across the interfacial glass layer, which consists of lead–borosilicate glass enclosing silver colloids. As there is a considerable lack of parameters for the interfacial glass layer, we revisit properties of similar materials such as as-grown crystalline and polycrystalline lead monoxide. We revise characteristics of applications that use similar glasses, such as photodetectors, thick-film transistors, and surface passivation. We propose that current transport via the interfacial glass is similar to the one in thick-film resistors based on ruthenium-doped lead–borosilicate glass. Device simulations are performed using direct tunneling, trap-assisted tunneling, and a conducting insulator model for current transport across the interfacial glass layer. The tunneling mechanisms are only effective for thin interfacial glass layers up to 2 nm. A combination of direct tunneling and the conducting insulator modeling is well suited to model interfacial glass layers between 0.5 and 1000 nm. Applications to specific contact resistance measurements show that the resistivity of the interfacial glass is in the range of thick-film resistors of similar lead monoxide content.

Quantum theory of space charge limited current in solids

We present a quantum model of space charge limited current transport inside trap-free solids with planar geometry in the mean field approximation. We use a simple transformation which allows us to find the exact analytical solution for the steady state current case. We use our approach to find a Mott-Gurney like behavior and the mobility for single charge carriers in the quantum regime in solids.

Exploring Possible Missing Sinks of Nitrate and Its Precursors in Current Air Quality Models —A Case Simulation in the Pearl River Delta, China, Using an Observation-Based Box Model

Nitrate and its precursor (gaseous HNO3) in China are generally overestimated by current chemical transport models in comparison to in-situ observations. In this study, we used an observation-based box model and in-situ measurements at a rural site in southern China to investigate possible missing sinks of nitrate. We found that a heterogeneous reaction of HNO3 to NOx on soot can better balance the NOx/HNO3 chemistry in models and improve model performance with regard to nitrate, particularly where an additional HONO source (the heterogeneous reaction of NO2 on soot) is incorporated into current models. Through a series of sensitivity simulations, the uptake coefficients of heterogeneous reactions were suggested to be 3.0 × 10−3 and 1 × 10−4 for HNO3 and HONO over southern China in fall, respectively. A 3-D simulation with the suggested uptake coefficients further confirmed that heterogeneous reactions significantly decreased the nitrate concentrations (PNO3) in southern China, by up to 10 μg m−3 (50%-80% of simulated PNO3) in polluted regions (e.g., the Yangtze River). In contrast to nitrate and HNO3, NOx concentrations in China were enhanced, which partly explained the underestimation of NO2 in current models compared to satellite observations.

The effects of temperature on growth, development and settlement of northern rock sole larvae (Lepidopsetta polyxystra)

AbstractNorthern rock sole (Lepidopsetta polyxystra) is a commercially important fish in the North Pacific and a focal species in understanding larval transport to nursery grounds in the Bering Sea. However, the temperature‐dependent vital rates and settlement dynamics for this species have not been described in detail. We reared northern rock sole larvae in the laboratory to measure growth, condition, development and settlement parameters across four temperatures (2, 4, 7 and 10°C). Both length and mass‐measured growth rates increased with temperature and were best described by non‐linear regression. Residuals of the length–mass relationships were positively related to temperature, indicating larval condition also increased with temperature. Larval development and settlement were largely size dependent, resulting in reduced larval stage duration and earlier settlement at higher temperatures owing to more rapid growth at elevated temperatures. However, larvae at colder temperatures were less developed at a given size, but more likely to settle at smaller sizes than larvae reared in warmer conditions. These temperature–response parameters can be used to refine current and future transport models for northern rock sole larvae under changing environmental conditions in the North Pacific.

Forward Current Transport Mechanisms of Ni/Au—InAlN/AlN/GaN Schottky Diodes

We fabricate two Ni/Au-In0.17Al0.83N/AlN/GaN Schottky diodes on substrates of sapphire and Si, respectively, and investigate their forward-bias current transport mechanisms by temperature-dependent current-voltage measurements. In the temperature range of 300–485 K, the Schottky barrier heights (SBHs) calculated by using the conventional thermionic-emission (TE) model are strongly positively dependent on temperature, which is in contrast to the negative-temperature-dependent characteristic of traditional semiconductor Schottky diodes. By fitting the forward-bias I–V characteristics using different current transport models, we find that the tunneling current model can describe generally the I–V behaviors in the entire measured range of temperature. Under the high forward bias, the traditional TE mechanism also gives a good fit to the measured I–V data, and the actual barrier heights calculated according to the fitting TE curve are 1.434 and 1.413 eV at 300K for InAlN/AlN/GaN Schottky diodes on Si and the sapphire substrate, respectively, and the barrier height shows a slightly negative temperature coefficient. In addition, a formula is given to estimate SBHs of Ni/Au—InAlN/AlN/GaN Schottky diodes taking the Fermi-level pinning effect into account.

(Invited) Dielectrics for Graphene Transistors for Emerging Integrated Circuits

In this paper, we present the use of high-k dielectric material in design and physical modeling of following two types of transistors for emerging integrated circuits: graphene nano ribbon (GNR) tunnel field-effect transistors (TFETs) and graphene short channel FETs.Graphene nanoribbon field effect transistor (GNR FET) is one of the promising devices for emerging integrated circuits because of its exceptional electronic properties such as the large carrier mobility and planar structure. However, the problem is a high drain-source leakage current in standby state due to small band gap of graphene and short channel effect (SCE) enforced by the transistor feature size. We have proposed a new GNR FET structure which has two side gates with a lower work function than the main gate on top of the SiO2 and HfO2 insulating layers. The main gate and two side gates have work functions of Φ1=4.8eV and Φ2=4.5eV, respectively. The GNR FET width and length has been chosen 1.5nm and 15nm, where the source and drain regions are doped with a molar fraction of ionized donors equal to 5×10-3/cm3. The lengths of the main and side gates are 2.5nm and 1.25nm while those of insulator layers have been used as a variable. We have simulated the proposed GNR FET using the self-consistent solution of three-dimensional Poisson–Schrödinger equation, within the Non-equilibrium Green’s function (NEGF) formalism. In our new GNR FET structure, the two side gates induce the inversion layers next to drain and source regions, which increase the controllability of the main gate on the channel potential barrier by providing an effective screen for the main gate to prevent the change in drain current due to the change in drain voltage. We have found that the application of the HfO2 in combination of SiO2 in the proposed GNR FET not only shifts the drain source leakage current to the lower values by several orders of magnitude but also make the leakage current sensitive to the proportionality of thicknesses of SiO2 and HfO2. The proposed GNR FET introduce a minimum peak in the drain source leakage current much below that of a common GNR FET with the same insulator structure. The minimum leakage currents of the proposed structure for total insulator thickness of 10Å, 15 Å and 20Å are approximately 6×10-13A, 4×10-12A and 5×10-11A, respectively while those of a common GNR FET are 1.5×10-10A, 9×10-10A and 5×10-9A, respectively. The minimum peak in leakage current has been obtained for HfO2 thickness = 8Å and 14 Å corresponding to the total insulator thickness of 10 Å and 15 Å, respectively, while it shifts to purely HfO2 insulator layer for that of 20 Å. The proposed GNR FET can diminish the short channel effect and prevent an undesirable increase in the leakage current by decreasing the channel length. Effect of dielectric on performance of GNR TFETs is also studied through an analytical current transport model which we have developed for designing of integrated circuits. It is observed that choice of dielectric material and corresponding thickness along the lateral gate position severely modifies performance of GNR TFET. We have found that for constant oxide thickness of 0.76nm, threshold voltage reduces from 0.176V to 0.054V for dielectric permittivity of 2.5 to 47.5. We have also noticed that the performance of GNR TFET studied through threshold voltage and sub-threshold slope depends on selection of both dielectric material and their thickness. Specific results on influence of choice of dielectric in designing of GNR TFET will be presented included in the paper and presentation.

(Invited) Dielectrics for Graphene Transistors for Emerging Integrated Circuits

In this work, we present the use of high-k dielectric material in designing a novel structure of graphene nanoribbon field-effect transistor (GNR FET) to reduce short channel effects (SCE) for emerging integrated circuits. We have also studied influence of high-k dielectric on ON-state current density, threshold voltage and subthreshold slope of GNR tunneling field effect transistors (TFET). In the GNR FET structure, two side metal gates with lower work function than the main gate on top of the silicon dioxide (SiO2) and hafnium oxide (HfO2) insulating layers are used in a conventional double-gate (DG) GNR FET topology to provide virtual extensions to source and drain regions while these are fixed biased and independent of the main gate bias. We have found that the application of HfO2 in combination with SiO2 in the proposed GNR FET not only shifts the drain source leakage current to the lower values by several orders of magnitude but also makes the leakage current sensitive to the proportionality of thicknesses of SiO2 and HfO2 layers. This improvement makes the structure a more suitable configuration than the typical GNR FET for the integrated circuit design. In GNR TFET structure, SiO2, Al2O3 and HfO2 are considered as dielectric material and their corresponding effects on transistor are studied. By changing dielectric parameters the oxide capacitance changes, followed by the change in threshold voltage. With semi-classical current transport model such variations are closely observed for the proposed GNR TFET model.

Memory and Electrical Properties of (100)‐Oriented AlN Thin Films Prepared by Radio Frequency Magnetron Sputtering

The (100)‐oriented aluminum nitride (AlN) thin films were well deposited onto p‐type Si substrate by radio frequency (RF) magnetron sputtering method. The optimal deposition parameters were the RF power of 350 W, chamber pressure of 9 mTorr, and nitrogen concentration of 50%. Regarding the physical properties, the microstructure of as‐deposited (002)‐ and (100)‐oriented AlN thin films were obtained and compared by XRD patterns and TEM images. For electrical properties analysis, we found that the memory windows of (100)‐oriented AlN thin films are better than those of (002)‐oriented thin films. Besides, the interface and interaction between the silicon and (100)‐oriented AlN thin films was serious important problem. Finally, the current transport models of the as‐deposited and annealed (100)‐oriented AlN thin films were also discussed. From the results, we suggested and investigated that large memory window of the annealed (100)‐oriented AlN thin films was induced by many dipoles and large electric field applied.

Analyzing experimental data and model parameters: implications for predictions of SOA using chemical transport models

Abstract. Despite critical importance for air quality and climate predictions, accurate representation of secondary organic aerosol (SOA) formation remains elusive. An essential addition to the ongoing discussion of improving model predictions is an acknowledgement of the linkages between experimental conditions, parameter optimization and model output, as well as the linkage between empirically-derived partitioning parameters and the physicochemical properties of SOA they represent in models. In this work, a "best available" set of SOA modeling parameters is selected by comparing predicted SOA yields and mass concentrations with observed yields and mass concentrations from a comprehensive list of published smog chamber studies. Evaluated SOA model parameters include existing parameters for two product (2p) and volatility basis set (VBS) modeling frameworks, and new 2p-VBS parameters; 2p-VBS parameters are developed to exploit advantages of the VBS approach within the computationally-economical and widely-used 2p framework. Fine particulate matter (PM2.5) and SOA mass concentrations are simulated for the continental United States using CMAQv.4.7.1; results are compared for a base case (with default CMAQ parameters) and two best available parameter cases to illustrate the high- and low-NOx limits of biogenic SOA formation from monoterpenes. Results are discussed in terms of implications for current chemical transport model simulations and recommendations are provided for future modeling and measurement efforts. The comparisons of SOA yield predictions with data from 22 published chamber studies illustrate that: (1) SOA yields for naphthalene, and cyclic and > C5 straight-chain/branched alkanes are not well represented using either the newly developed or existing parameters for low-yield aromatics and lumped alkanes, respectively; and (2) for four of seven volatile organic compound+oxidant systems, the 2p-VBS parameters better represent chamber data than do the default CMAQ v.4.7.1 parameters. Using the "best available" parameters (combination of published 2p and newly derived 2p-VBS), predicted SOA mass and PM2.5 concentrations increase by up to 15% and 7%, respectively, for the high-NOx case and up to 215% (~3 μg m−3) and 55%, respectively, for the low-NOx case. Percent bias between model-based and observationally-based secondary organic carbon (SOC) improved from −63% for the base case to −15% for the low-NOx case. The ability to robustly assign "best available" parameters in all volatile organic compound+oxidant systems, however, is critically limited due to insufficient data; particularly for photo-oxidation of diverse monoterpenes, sesquiterpenes, and alkanes under a range of atmospherically relevant conditions.

Evaluation of travel behaviors in order to prepare Travel Plans for residential areas and schools of Gyor

The municipality of Gyor participated in the last five years in two Intelligent Energy Europe projects (Pro Motion project, Travel Plan plus project), which aim was to deliver transport-sector energy savings by creating a new approach to site-based mobility management across Europe. During the development of the two projects there has been worked out two Local Travel Plans, one for the residential area Menfocsanak, in the city, Gyor and one for the educational institutions of Gyor. The first one is a so called ‘area travel plan’ and the second one is a so-called ‘work travel plan’. The aims of these studies were to reduce CO2 emission caused by private car users and to promote sustainable mobility choices for the residents of the area and for students. In order to solve the future mobility challenges in a sustainable way it is necessary to analyze the existing travel demand and modal-split caused by target groups (residents and students).It is important to develop Local Travel Plans for residential areas and schools, based on quantitative travel behavior data, which can be used for modeling of current and future transport to provide optimized solutions for daily mobility. It is very important to collect data from various sources according to the needed detail of a Travel Plan.

Sediment transport and development of banner banks and sandwaves in an extreme tidal system: Upper Bay of Fundy, Canada

Multibeam sonar mapping and geophysical and geological groundtruth surveys were coupled with tidal current and sediment transport model calculations to investigate the sediment transport and formation processes of the complex seabed features off the Cape Split headland in the upper Bay of Fundy. The Cape Split banner bank, composed of coarse to very coarse sand, is a southwest–northeast oriented, large tear-drop shaped sand body with superimposed sand waves that show wavelengths from 15 to 525m and heights from 0.5 to 19m. Isolated and chains of barchan dunes occur on top of a shadow bank to the southeast of the banner bank. The barchan dunes are composed of well-sorted medium sand and are oriented northwest–southeast. Their mean height and width are 1.5 and 55m, respectively. A gravel bank, with an elongated elliptical shape and west–east orientation, lies in the Minas Passage erosional trough east of the headland to form the counterpart to the sandy Cape Split banner bank. The southern face is featureless but the northern face is covered by gravel megaripples.Tidal model predictions and sediment transport calculations show that the formation of the banner bank and the gravel bank are due to the development of the transient counter-clockwise and clockwise tidal eddies respectively to the west and east of the headland. The formation of barchan dunes is controlled by the nearly unidirectional flow regime in outer Scots Bay. Sand waves on the flanks of the Cape Split banner bank show opposite asymmetry and the barchan dunes are asymmetric to the northeast. The tidal current and sediment transport predictions corroborate bedform asymmetry to show that sand wave migration and net sediment transport is to southwest on the northern flank of the banner bank but to northeast on the southern bank. Long-term migration of the Scots Bay barchan dunes is to the northeast. Spring-condition tidal currents can cause frequent mobilization and high-stage transport over the banner bank and barchan dunes. Strong currents in Minas Passage can cause infrequent low-stage transport over the megarippled northern face but are not high enough to mobilize the coarser gravels on the southern face of the gravel bank.

Modeling mass transfer using surface morphology in full-scale reverse osmosis membrane processes

The influence of surface characteristics on membrane process performance is considered significant and is not well understood. Current mass transport models generally assume constant mass transfer coefficients (MTCs) based on a homogeneous flat surface. This study evaluated membrane mass transfer by incorporating surface morphology into a diffusion-based model assuming that the MTCs are dependent on the thickness variation of the membrane’s active layer. Concentration polarization is also affected by this nonuniform surface property and was incorporated into the model. A simulation was performed using parameters from a full-scale 4.5 million gallon per day brackish water reverse osmosis membrane process. The process was simulated by modeling one thousand uniform slices of the membrane channel and the permeate water quality was determined locally through a finite difference approach. It was determined that solute mass transport is controlled by diffusion in the nonhomogeneous thinner regions (membrane valleys) of the active layer. This nonuniform surface affected the concentration polarization layer, where more solutes tended to accumulate within the valleys than on the ridges. Prediction of the permeate total dissolved solids concentration was accurate, ranging between 5 and 15%, as measured as an average percent difference between predicted and actual values.

Effects of sorption kinetics on the fate and transport of pharmaceuticals in estuaries

Many current fate and transport models based on the assumption of instantaneous sorption equilibrium of contaminants in the water column may not be valid for certain pharmaceuticals with long times to reach sorption equilibrium. In this study, a sorption kinetics model was developed and incorporated into a water quality model for the Patuxent River Estuary to evaluate the effect of sorption kinetics. Model results indicate that the assumption of instantaneous sorption equilibrium results in significant under-prediction of water column concentrations for some pharmaceuticals. The relative difference between predicted concentrations for the instantaneous versus kinetic approach is as large as 150% at upstream locations in the Patuxent Estuary. At downstream locations, where sorption processes have had sufficient time to reach equilibrium, the relative difference decreases to roughly 25%. This indicates that sorption kinetics affect a model’s ability to capture accumulation of pharmaceuticals into riverbeds and the transport of pharmaceuticals in estuaries. These results offer strong evidence that chemicals are not removed from the water column as rapidly as has been assumed on the basis of equilibrium-based analyses. The findings are applicable not only for pharmaceutical compounds, but also for diverse contaminants that reach sorption equilibrium slowly.

A Thermal Model for Carbon Nanotube Interconnects.

In this work, we have studied Joule heating in carbon nanotube based very large scale integration (VLSI) interconnects and incorporated Joule heating influenced scattering in our previously developed current transport model. The theoretical model explains breakdown in carbon nanotube resistance which limits the current density. We have also studied scattering parameters of carbon nanotube (CNT) interconnects and compared with the earlier work. For 1 µm length single-wall carbon nanotube, 3 dB frequency in S12 parameter reduces to ~120 GHz from 1 THz considering Joule heating. It has been found that bias voltage has little effect on scattering parameters, while length has very strong effect on scattering parameters.

Influence of in-Grown Stacking Faults on Electrical Characteristics of 4H-SiC Pin Diode with Long Carrier Lifetime

The electrical characteristics of 4H-SiC pin diodes with 8H-type in-grown stacking faults are investigated. The pin diodes have epilayers with low Z1/2 center concentration formed by using the carbon implantation process. The forward voltage drops of the diode with 8H-type in-grown stacking faults are larger than those of the diode without a 8H-type in-grown stacking fault. At room temperature, the differential on-resistance of the pin diode with 8H-type in-grown stacking faults is larger than the value calculated from donor concentration in the drift layer by using the current transportation model of the unipolar device. Meanwhile, the differential on-resistances of the pin diode with 8H-type in-grown stacking faults decrease with an increase in temperature and become smaller than the calculated value at temperature of more than 200 °C.

Suspended sediment fluxes in the radial sand ridge field of South Yellow Sea

Zhang C. K., Yang Y. Z., Tao, J. F., Chen, Y. P., Yao, P., Su. M., 2013. Suspended sediment fluxes in the radial sand ridge field of South Yellow Sea.The radial sand ridge at the South Yellow Sea is one of unique geomorphic units in the world. It is famous of its radial current pattern and high suspended sediment concentration, forming abundant tidal flat resources which could be reclaimed for the land use. In order to make a scientific plan for the land reclamation, it is necessary to quantitatively understand the distribution of suspended sediment fluxes in this field. In this study, a 2D tidal current and suspended sediment transport model is developed and validated by the hydrologic survey data measured in 2006. The suspended sediment fluxes are calculated by the period-averaged multification of the simulated tidal current and sediment concentration in two consecutive tidal periods. The numerical results show that, in either winter or summer season, there has a positive sediment flux comes from the north and the south boundaries, indicating a net gain of sediment for the sand ridge field from both boundaries. The numerical results also show that a clockwise sediment flux system exists in the middle of the sand ridge field, but a small portion of sediment leaves the sand ridge area at the north wing. The overall gain of the sediment in this field is positive, implying that the radial sand ridge is still in the growing process.

Modeling the distribution of ammonia across Europe including bi-directional surface–atmosphere exchange

Abstract. A large shortcoming of current chemistry transport models (CTM) for simulating the fate of ammonia in the atmosphere is the lack of a description of the bi-directional surface–atmosphere exchange. In this paper, results of an update of the surface–atmosphere exchange module DEPAC, i.e. DEPosition of Acidifying Compounds, in the chemistry transport model LOTOS-EUROS are discussed. It is shown that with the new description, which includes bi-directional surface–atmosphere exchange, the modeled ammonia concentrations increase almost everywhere, in particular in agricultural source areas. The reason is that by using a compensation point the ammonia lifetime and transport distance is increased. As a consequence, deposition of ammonia and ammonium decreases in agricultural source areas, while it increases in large nature areas and remote regions especially in southern Scandinavia. The inclusion of a compensation point for water reduces the dry deposition over sea and allows reproducing the observed marine background concentrations at coastal locations to a better extent. A comparison with measurements shows that the model results better represent the measured ammonia concentrations. The concentrations in nature areas are slightly overestimated, while the concentrations in agricultural source areas are still underestimated. Although the introduction of the compensation point improves the model performance, the modeling of ammonia remains challenging. Important aspects are emission patterns in space and time as well as a proper approach to deal with the high concentration gradients in relation to model resolution. In short, the inclusion of a bi-directional surface–atmosphere exchange is a significant step forward for modeling ammonia.

Numerical Simulation of Maximum Scour Depth of the Riverbed above a Subway Tunnel Crossing the Tidal River

To ensure the secure buried depth of the subway tunnel which was routed underneath the Fenghuahe River, a two-dimensional tidal current and sediment transportation model was established to simulate the maximum scour depth of the riverbed. The conclusions were shown as follows: the most disadvantageous hydrologic condition was a combination of the 200-year return period flood in the Fenghuahe River and the 5-year highest tide in the Yonghe River. The maximum scour depth of the riverbed above the subway tunnel was 1.19 m and the lowest elevation of the riverbed cross-section was -8.56m. The results could provide technical support for the safety of subway design.

Two-Dimensional Numerical Model for Scalar Transport by Finite Volume Method on Unstructured Grid

Based on arbitrarily unstructured shallow water current model and finite volume method, a high-order non-oscillatory transport model is constructed. The stability and non-oscillatory condition of the transport model are also analyzed in theory. Usually, the maximum allowable time steps of current model and transport model are different, so an unsynchronized technique is used for the flow and the transport model computations to improve computational efficiency. In addition, the conservation of transport model during the unsynchronized computation is ensured by the time integration of currents continuity equation. Apply this model to the salinity transport simulation of Oujiang estuary, numerical results show that the water level, velocity and salinity agree well with the measured data.