S-shaped Characteristics Research Articles

Coulomb Drag by Injected Ballistic Carriers in Graphene n+−i−n−n+ Structures: Doping and Temperature Effects

Carrier Drags Dirac dispersion law in graphene enables the Coulomb drag of quasi equilibrium electrons by injected ballistic electrons. The drag effect determines the shape of the current–voltage characteristics (monotonous or S-shaped). The S-shaped characteristics enable switching, current filamentation and negative differential resistance at terahertz frequencies. The applications of these new effects include voltage limiting, switching, terahertz detection, amplification and generation, and novel circuit solutions integrating the S-type graphene switches with conventional graphene transistors and diodes. More details can be found in article number 2100535 by Victor Ryzhii and co-workers.

Energy efficient green synthesized MOF-801 for adsorption cooling applications

Utilization of water as a refrigerant in adsorption cooling systems is a challenging task due to the unavailability of suitable adsorbents. Recently, a new class of porous material, metal–organic frameworks (MOFs), is heightening interest as the potential adsorbent for water adsorption due to its crucial S-shaped adsorption isotherm characteristics. Additionally, the high surface area, large pore volume, and higher affinity towards water vapor outshined MOFs over other porous materials to be used as adsorbent in water-based adsorption cooling system (ACS). However, their industrial production could be obstructed due to economic and ecological limitations. Therefore, in this study, an energy-efficient, environment friendly, rapid and ultrasonication assisted room temperature green synthesis procedure for MOF-801 is reported with additional finding of optimum synthesis condition. Several characterization techniques are employed to ensure the structural stability of the synthesized MOFs and their applicability in ACS. The reaction time of the green synthesized MOF-801 was reduced by eight times than the conventional one while ensuring excellent system performance, which makes this process promising for industrial production.

S-Shaped Current-Voltage Characteristics of n+ - i - n - n+ Graphene Field-Effect Transistors due to the Coulomb Drag of Quasiequilibrium Electrons by Ballistic Electrons

We demonstrate that the injection of the ballistic electrons into the two-dimensional electron plasma in lateral n$^+$-i-n-n$^+$ graphene field-effect transistors (G-FET) might lead to a substantial Coulomb drag of the quasi-equilibrium electrons due the violation of the Galilean and Lorentz invariance in the systems with a linear electron dispersion. This effect can result in the S-shaped current-voltage characteristics (IVs). The resulting negative differential conductivity enables the hysteresis effects and current filamentation that can be used for the implementation of voltage switching devices. Due to a strong nonlinearity of the IVs, the G-FETs can be used for an effective frequency multiplication and detection of terahertz radiation.

CFD analysis of flow pattern in S-Shape region for low specific speed Francis turbine

In this research, a Francis turbine with low specific speed was analyzed by unsteady numerical simulation in case of S-Shape region appearing model test. It was found that S-Shape characteristics curve was caused by an increase of head. Firstly, the head was decreased in accordance with a decrease of flow rate. However, once a centrifugal force became larger, then a reverse flow appeared between guide vane and runner and the head was increased. And then, head was decreased with flow rate decreasing. In this research, operating conditions of flow rate were defined as three regions, first half of S-Shape region, just S-Shape region and last half of S-Shape region. The operating condition, where the head was decreased in accordance with the decrease of flow rate, was defined as the first half of S-Shape region. The operating condition, where the head was just being increased in accordance with the decrease of flow rate, was defined as the just S-Shape region. The operating condition after passing of the just S-Shape region was defined as the last half of S-Shape region. It was revealed that those three operating conditions showed a spectrum distribution of pressure fluctuation differently each other. A flow visualization analysis with numerical results was conducted to investigate the difference of pressure spectrum in three operating conditions. At the first half of S-Shape region, two groups of reverse flow were caused at chamber between guide vane and runner and revolving in same direction as runner rotation. At the just S-Shape, the reverse flows were circumferentially connected to build a long curved string. At the last half of S-Shape, the reverse flows were fully connected to build a round ring shape.

Hydraulic interaction of two parallel pump-turbines in constant-speed oscillation: Measurement, simulation, and sensitivity analysis

The S-shaped characteristic of a pump-turbine poses a latent risk to problems such as failure to start the unit, excessive water hammer pressure after load rejection, severe pressure pulsation, and intense vibration. However, S-shaped characteristics of a pump-turbine are usually investigated by static model tests, while the scope of this method is relatively limited. In a previous study exploring the S-shaped characteristic curve of pump-turbines, the constant-speed oscillation (CSO) was experimentally discovered. This transient process can be utilized to acquire the dynamic trajectory of the unit at a fixed rotational speed in the turbine mode, turbine brake mode, and reverse pump mode. On this basis, this study investigated the hydraulic oscillations between two pump-turbines after the net flow of the system was reduced to zero when the two units were operated in parallel. It was found that the two pump-turbines had an equal oscillation cycle, but their transient trajectories were significantly different. One of the runners oscillated only in the reverse pump mode, whereas the other runner oscillated in the turbine, turbine brake, and reverse pump modes. This process was termed the CSO interaction (CSOI). Subsequently, a one-dimensional rigid column simulation and a sensitivity analysis were performed to analyze the CSOI further, thereby verifying that the S-shaped characteristic curve was the root cause of the oscillation period and amplitude variation of the unit in the CSOI. The runner with the more prominent S-shaped characteristic curve tends to operate in the reverse pump mode during oscillation. The novelty of the work is that a new hydraulic interaction between pump-turbines, CSOI, was discovered. CSOI provides a metric for comparing the intensities of the S-shaped characteristic curves of two pump-turbines experimentally or theoretically.

Mechanisms of Cultural Evolution in the Songs of Wild Bird Populations.

Young songbirds draw the source material for their learned songs from parents, peers, and unrelated adults, as well as from innovation. These learned songs are used for intraspecific communication, and have well-documented roles for such functions as territory maintenance and mate attraction. The songs of wild populations differ, forming local “dialects” that may shift over time, suggesting that cultural evolution is at work. Recent work has focused on the mechanisms responsible for the cultural evolution of bird songs within a population, including drift, learning biases (such as conformity and rare-form copying), and selection (including sexual selection). In many songs or song repertoires, variability is partitioned, with some songs or song segments being stable and consistent, while others vary within the population and across time, and still others undergo population-wide transitions over time. This review explores the different mechanisms that shape the cultural evolution of songs in wild populations, with specific reference to a long-term investigation of a single population of philopatric Savannah sparrows. Males learn a single four-segment song during their 1st year and sing the same song thereafter. Within this song, the buzz segment is a population marker, and may be stable for decades – variant forms occur but eventually disappear. In contrast, the middle segment is highly variable both within the population and over time; changes in relative prevalence of different forms may be due to cultural drift or a rare-form learning bias. Within the introductory segment, a high note cluster was replaced by a click train between 1982 and 2010, following an S-shaped trajectory characteristic of both selective sweeps in population genetics and the replacement of one form by another in human language. In the case of the Savannah sparrows, this replacement may have been due to sexual selection. In subsequent generations, the number of clicks within trains increased, a form of cultural directional selection. In contrast to the narrowing of a trait's range during directional selection in genetic systems, variation in the number of clicks in a train increased as the mean value shifted because improvisation during song learning allowed the range of the trait to expand. Thus, in the single short song of the Savannah sparrow, at least four different mechanisms appear to contribute to three different types of cultural evolutionary outcomes. In the future, it will be import to explore the conditions that favor the application of specific (and perhaps conditional) learning rules, and studies such as the ongoing song seeding experiment in the Kent Island Savannah sparrow population will help in understanding the mechanisms that promote or repress changes in a population's song.

Grid synchronization of variable speed pump-turbine units in turbine mode

The S-shaped characteristics of the pump-turbine may cause instability and thus leads to difficulties in grid synchronization. This paper develops a complete model for a pumped storage power plant and studies the start-up and grid synchronization procedure of two 300 MW variable speed units at no load in turbine mode. Based on the grid-voltage-oriented vector control method, the stator voltage of the doubly-fed induction machine is controlled to meet the grid connection requirements. Compared with the fixed speed units, the simulation results show that for a pump-turbine with typical S-shaped characteristics, the fixed speed unit cannot meet the grid connection requirements due to the unstable unit speed. Whereas, the variable speed unit can quickly reach synchronization with the grid voltage. For a pump-turbine without typical S-shaped characteristics, the synchronous unit can meet the grid connection requirements, but it takes a longer time and the stator voltage is not stable enough, while the variable speed unit is 11 times faster and more stable. The effects of PI parameters on the synchronization process of a variable speed unit are also studied. The results show that with suitable PI control parameters, the synchronization process can be accelerated without large overshoot of the rotor power.

Unstable S-shaped characteristics of a pump-turbine unit in a lab-scale model

Unusual flow characteristics such as reverse flow, rotating stalls, flow recirculation, and stationary vortexes can induce high dynamic forces and torque variations on the entire system, creating a positive slope in the discharge-head curve. To avoid these problems, the present study investigates the hydrodynamic characteristics of a lab-scale model of a Francis type pump-turbine unit in the transition region. To verify the simulation, its results were compared with those of a laboratory-scale experiment performed over various operating ranges. The differences between the experimental and numerical speed, discharge, and torque factors were compared. The numerical analysis was well-matched the experimental tendencies in the overall operating and transition regions. The reliability of the simulations was within 4%. The unsteady RANS equations in the SAS–SST model were discretized for a detailed analysis of the pressure and internal flow characteristics. Under the runaway condition and low-discharge conditions, the frequency spectra of the pressure fluctuations were remarkable at low-frequency related to the rotating stall and blade passing frequency. These results represent a rotating stall with a frequency propagation of approximately 60% of the rotational speed of the runner. In case of the internal flow field, some blade loading distributions developed a positive shape while others developed a negative shape under the runaway condition. Although a rotating stall formed under the low-discharge condition, the form under this condition differed from that developed under runaway conditions, owing to backflow and the single stall cell.

A Comparative Study of the Scale Effect on the S-Shaped Characteristics of a Pump-Turbine Unit

S-shaped characteristics in turbine mode are prone to instabilities in associated transient processes. A single value of the speed factor corresponds to multiple values of the discharge factor, having the possibility of changing the operating point among the turbine, turbine brake, and reverse pump modes. Because of this characteristic, the S-shaped curves induce instability in transient processes. Understanding the hydraulic behavior of a turbine on the four-quadrant characteristic is important since it provides detailed performance information through the whole discharge range of the turbine. This study was numerically and experimentally investigated the scale effect on the S-shaped characteristics in the turbine transition region. The four-quadrant characteristic curves (full- and laboratory-scale) in the turbine mode were predicted by numerical simulations. To verify the predicted results, a laboratory-scale experiment was performed in the turbine, turbine brake, and reverse pump modes. Although the full-scale experiment was performed in the normal operating head range, the scale effect can be validated by comparing steady operating points between the two models. Based on the verified results, the internal flow and pressure pulsation characteristics were determined at the operating point in a specific transition region.

Theoretical study of exciton-exciton annihilation dynamics in the approximation of weak coupling

Dynamics of exciton-exciton annihilation (EEA) in molecular aggregates is closely related to its luminescence characteristics and energy transfer. It is meaningful to uncover energy and charge transfer process in molecular systems. Therefore, studying the dynamics of exciton is important for simulating photosynthesis in nature and analyzing the transport process of photocarriers. In this paper the weak coupling approximation is adopted to obtain the rate equation in the framework of density matrix theory. The relation among the intermolecular distance, exciton state density, excited state dipole moment and exciton-exciton annihilation dynamics is studied by the rate equations. It is found that the decrease of intermolecular distance leads the generation rate of higher-order excited states to increase, resulting in the obvious S-shaped decay characteristics. Moreover, the dipole moment of the higher-order excited state is the key factor of the exciton fusion process, and the greater the exciton density, the more easily the exciton fusion process occurs. Therefore, the reduction of intermolecular distance and the increase of the dipole moment of the higher-order excited state make the nearest neighbor molecules have a strong coupling, resulting in a high generation rate of the higher-order excited state. It is found that the evolution processes of the first excited state in different exciton densities are consistent with the experimental results of the excitation of OPPV7 monomer (PPV oligomers of 7) at a low excitation energy, and the excitation of OPPV7 aggregates at different excitation energy levels. It can be observed that the exciton decay rate is faster under the excitation of the strong external field. Using the quantum wave packet under optical excitation as the initial state, the excited state dynamics is simulated at different exciton energy levels. It is found that the exciton state can maintain good locality within a few hundreds of femtoseconds, which shows that the exciton state is a coherent superposition state, and its local characteristics are related to the excitation energy level. These conclusions are applicable to the aggregations whose single molecule has an energy level of <inline-formula><tex-math id="M1">\begin{document}${E_{mf}} \approx 2{E_{me}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20211242_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="22-20211242_M1.png"/></alternatives></inline-formula>, and also provide a reasonable reference for the exciton-exciton annihilation process under optical excitation.

Influence of rotational inertia on the runner radial forces of a model pump‐turbine running away through the S‐shaped characteristic region

To enhance the power grid's stability and safety, pumped-storage power stations (PSPSs) have to keep themselves stable and safe strictly. However, instabilities happen frequently in some PSPSs due to the S-shaped characteristics of pump-turbines. When the operating point goes through the S-shaped region, intensive radial forces on the runner could cause shaft swing of the turbine-generator unit. In this study, the runaway transient scenarios of a pump-turbine in a model PSPS were simulated by using a one-dimensional and three-dimensional coupled computational fluid dynamics model, and the influence of the unit rotational inertia on the runner radial forces was investigated. The results show that when the pump-turbine runs in the S-shaped region, large rotational inertia is easier than a small one to induce abrupt increases in runner radial forces. The reason is that large rotational inertia gives slower changes in rotational speed and discharge, providing enough time to generate unstable and uneven flow patterns in the pump-turbine. This indicates that apart from the running away region in the characteristics plane, the running away duration is also essential for the transient instability of a pump-turbine. The finding is different from the traditional understandings and should be considered when selects the rotational inertia of a pump-turbine unit.

Prediction of the Number of Patients Infected with COVID-19 Based on Rolling Grey Verhulst Models.

The outbreak of a novel coronavirus (SARS-CoV-2) has caused a large number of residents in China to be infected with a highly contagious pneumonia recently. Despite active control measures taken by the Chinese government, the number of infected patients is still increasing day by day. At present, the changing trend of the epidemic is attracting the attention of everyone. Based on data from 21 January to 20 February 2020, six rolling grey Verhulst models were built using 7-, 8- and 9-day data sequences to predict the daily growth trend of the number of patients confirmed with COVID-19 infection in China. The results show that these six models consistently predict the S-shaped change characteristics of the cumulative number of confirmed patients, and the daily growth decreased day by day after 4 February. The predicted results obtained by different models are very approximate, with very high prediction accuracy. In the training stage, the maximum and minimum mean absolute percentage errors (MAPEs) are 4.74% and 1.80%, respectively; in the testing stage, the maximum and minimum MAPEs are 4.72% and 1.65%, respectively. This indicates that the predicted results show high robustness. If the number of clinically diagnosed cases in Wuhan City, Hubei Province, China, where COVID-19 was first detected, is not counted from 12 February, the cumulative number of confirmed COVID-19 cases in China will reach a maximum of 60,364–61,327 during 17–22 March; otherwise, the cumulative number of confirmed cases in China will be 78,817–79,780.

Bipolar effects in snapback mechanism in advanced n-FET transistors under high current stress conditions

This work models high current snapback behavior in n-FET transistors with bottom body contact under high current stress at the drain for ZRAM (Zero capacitor RAM). We analyze 2D current flow in n-FET near the pinch-off region and relate the results to the S-shaped snapback characteristics under high injection avalanche generated carriers conditions. The role of surface bipolar effects on the first snapback phenomenon in the GG-NMOS (Gate-Grounded NMOS) is investigated. A novel physical insight of the bipolar activity is modeled through current flow and barrier lowering at the source-substrate junctions. Moreover, the coupling of electron and hole injection is described. Finally, a model for surface potential inside the substrate before and after snapback is derived and compared with TCAD simulation results. A very good agreement is observed between the model and the TCAD results.

Advantage analysis of variable-speed pumped storage units in renewable energy power grid: Mechanism of avoiding S-shaped region

Pumped storage units (PSU) as the most efficient energy storage device (ESD) play an important role in absorbing renewable energy (RE) sources. Nevertheless, pump-turbines have inherent S-shaped characteristics caused by their reversible design which seriously threaten the operation stability of PSU. In recent years, the variable-speed pumped storage units (VSU) have been proposed as an alternative to the fixed-speed pumped storage units (FSU). Therefore, it is a meaningful subject to evaluate the operation advantage of VSU, which has not been adequately quantified. This paper aims to analyse the advantages of VSU in operation stability by revealing the mechanism of avoiding trapping in the S-shaped region. Firstly, a precise coupling model for VSU with nonlinear pump-turbine characteristics is established and the operation areas in turbine mode are analysed. Then, the mechanism of how the trap of S-shaped region could be avoided is revealed and the regulation performance of the control system is quantified by comparing with FSU. Based on a real Chinese VSU, three scenarios are studied, including turbine start-up process, power regulation and turbine closing process. The results indicate that the S-shaped region could be escaped effectively by operating the VSU. The periodic oscillations times and setting time index of rotational speed in turbine start-up process are 0.5 and 20.74 s respectively, which are much smaller than the values of FSU. VSU outperforms the FSU in the aspect of power regulation performance: much smaller average and standard deviation of power difference for wind and load disturbance. Moreover, VSU can also obtain a significantly higher assessment in closure process: the maximum water pressure deviation is only 40% of FSU.

Effect of Wall Thickness on Stress-Strain Response and Buckling Behavior of Hollow-Cylinder Rubber Fenders.

In this study, the effect of wall thickness (15–25 mm) on the stress–strain response of hollow-cylinder rubber fenders were investigated by conducting monotonic compression tests. It was found that a progressive increase in lateral bending deformation was observed during monotonic compression. Simultaneously, the extent of the lateral deflection decreased notably with an increasing wall thickness. From the experimental results, the fact is accepted that buckling occurred in the tested fender due to the fact that the ratio of the height to the wall thickness was higher than four in all of the considered cases. Moreover, an s-shape profile appeared in the stress–strain curves, which became clearer as the wall thickness was reduced from 25 to 15 mm. To assess the performance of fenders objectively, an energy-effectiveness index, , was introduced to quantify the energy absorption capacity of the fender. From the experimental observations, it was inferred that the contact area of the folded inner surface of the fender produced under compression generated an additional reaction force and affected the shape of the stress–strain curve since the measured load consisted of two reaction forces: one caused by the self-contact area, and the other resulted from the compression-bending deformation that occurred in the side wall of the fender. To examine this assertion, a finite element analysis (FEA) was conducted and confirmed the effect of the reaction force on the sensitivity of the s-shape characteristic of the stress–strain curve. Finally, a polynomial regression was conducted and the calculated results based on the fourth-degree stress polynomial function correlated very well with the measured stress–strain curves.

Two different types of S-shaped J-V characteristics in organic solar cells

In this paper, S-shaped characteristics of organic solar cells (OSCs) are analyzed by a drift–diffusion model which includes charge carrier surface recombination and thermal injection on the anode and cathode through boundary conditions. By varying surface recombination velocities (SRVs) for electrons and holes on both contacts and the injection barrier heights for majority carriers, two different S-shaped deviations in OSCs J-V characteristics were observed. The first type of S-shaped J-V characteristic manifests the S-shape bending in the vicinity of the voltage axis, after which it rises almost exponentially. This kind of S-shape deformation was found to arise from the finite SRVs. The second type of the S-shaped J-V characteristic also makes a kink near the voltage axis but proceeds to grow monotonically, having only one saddle point. The S-shaped J-V curve of this kind turned out to be the consequence of the electron injection barrier height larger than 0.2 eV. The validity of our model is confirmed by comparing the simulated J-V curves with the experimentally obtained data. The model has been applied to the ITO/PEDOT:PSS/P3HT:PCBM/Al and ITO/PEDOT:PSS/P3HT:ICBA/Al solar cells. For the P3HT:PCBM based solar cells, the regular J-shaped J-V curves were measured, while for the P3HT:ICBA solar cells, the J-V curves with anomalous S-shape behavior were recorded. All experimentally obtained J-V curves were reproduced very well with our model. It seems that the ITO/PEDOT:PSS/P3HT:ICBA/Al solar cell S-shaped J-V curve originates from the electron barrier height on the cathode contact, rather than from the low surface recombination on the electrodes.

Graphical-analytical method for constructing load characteristics

The article discusses a graphical-analytical method for constructing the load characteristics of a three-element resonant circuit in the current stabilization mode. The current stabilization mode is observed when compensating the negative section of the S-shaped characteristics of the parallel resonant circuit of the connected sequence with a linear inductance and with a linear capacitor characteristic. The equation of the load mode in a dimensionless form represents the equation of an ellipse, which makes it possible to construct the necessary characteristics of a three-element resonant circuit for various types of load.

Graphical-analytical method for constructing load characteristics

The article discusses a graphical-analytical method for constructing the load characteristics of a three-element resonant circuit in the current stabilization mode. The current stabilization mode is observed when compensating the negative section of the S-shaped characteristics of the parallel resonant circuit of the connected sequence with a linear inductance and with a linear capacitor characteristic. The equation of the load mode in a dimensionless form represents the equation of an ellipse, which makes it possible to construct the necessary characteristics of a three-element resonant circuit for various types of load.

Sputter-Deposited Titanium Oxide Layers as Efficient Electron Selective Contacts in Organic Photovoltaic Devices

Organic photovoltaics (OPVs) has recently reached power conversion efficiencies of 17.3%, making it a green technology that not only offers short energy payback times and diverse photovoltaic integration schemes, but also can deliver competitive power outputs. OPVs typically employs electron selective contact layers made from low work function n-type metal oxide semiconductors, such as titanium oxide (TiO2) or zinc oxide (ZnO), developed from a variety of deposition techniques. However, in the case of TiO2 interlayers, the appearance of unwanted s-shape characteristics has been reported extensively in the literature in the past, for a variety of different deposition methods used. It has been shown that the s-shape arises from negatively charged chemisorbed oxygen, and that it can be deactivated by UV light illumination, which, however, is hardly compatible with real-life OPV application. In this work, we introduce sputtered crystalline titanium oxide layers as efficient s-shape-free electron selective extraction layers in organic solar-cell devices. We demonstrate that the onset of crystallization takes place at substrate growth temperatures of approximately 100 °C for the TiOx thin films, and that the crystallization onset temperature correlates well with a strong increase in device performance and the removal of any s-shape characteristics. Optical, structural, compositional, and electronic energy-level characterizations of the TiOx layers are shown in the present work, and point to the formation of an oxide with a low surface-defect density, developed from the sputter–crystallization process. Importantly, well-functioning s-shape-free PTB7:PC70BM devices are demonstrated for TiOx growth temperatures of 155 °C.

Investigation on reversible pump turbine flow structures and associated pressure field characteristics under different guide vane openings

The use of reversible pump turbines (RPT) within pumped storage power plants goes with prolonged periods of off-design operating conditions, which leads to the onset of operating mode-dependent instabilities. In order to decrease the gravity of RPT flow instabilities and associated damages or even completely eliminate them, a deep understanding of its onset and development mechanism is needed. In line with this, the present study seeks to numerically investigate the onset and development mechanism of RPT unsteady flow structures as well as the evolutional characteristics of associated pressure pulsations throughout the RPT complete flow passage, under off-design conditions for three GVOs namely 17, 21, and 25 mm. The study results showed that low torque operating conditions and associated vaneless space back flow structures were the trigger of flow unsteadiness onset within the RPT vaneless space, the instabilities which grew to cause the S-shape characteristics appearance. Moreover, the runner flow unsteadiness was found to decrease with the GVO increase. On the other hand, the GVO increase worsened the pressure pulsation levels within RPT flow zones, where pressure pulsations within the vaneless space and flow zones in its vicinities were found to be the most sensitive to GVO changes.