Subnanosecond magnetization reversal of magnetic nanoparticle driven by chirp microwave field pulse

We investigate the magnetization reversal of single-domain magnetic nanoparticle driven by linear down-chirp microwave magnetic field pulse. Numerical simulations based on the Landau-Lifshitz-Gilbert equation reveal that solely down-chirp pulse is capable of inducing subnanosecond magnetization reversal. With a certain range of initial frequency and chirp rate, the required field amplitude is much smaller than that of constant-frequency microwave field. The fast reversal is because the down-chirp microwave field acts as an energy source and sink for the magnetic particle before and after crossing over the energy barrier, respectively. Applying a spin-polarized current additively to the system further reduces the microwave field amplitude. Our findings provide a new way to realize low-cost and fast magnetization reversal

Macroscopic quantum tunneling and phase diffusion in a La2−x_{2-x}Srx_xCuO4_4 intrinsic Josephson junction stack

We performed measurements of switching current distribution in a submicron La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) intrinsic Josephson junction (IJJ) stack in a wide temperature range. The escape rate saturates below approximately 2\,K, indicating that the escape event is dominated by a macroscopic quantum tunneling (MQT) process with a crossover temperature $T^{*}\approx2\,$K. We applied the theory of MQT for IJJ stacks, taking into account dissipation and the phase re-trapping effect in the LSCO IJJ stack. The theory is in good agreement with the experiment both in the MQT and in the thermal activation regimes.Comment: 9 pages, 7 figure

Transformer-less local grid based 11 kV SCHB multilevel converter for renewable energy systems

Due to the variable nature of renewable energy resources and power demand by the consumers, it is difficult to operate a power system installed with only one type of renewable energy resource. The local grid based renewable generation may be the only solution to overcome this problem. In this regard, an 11 kV Series Connected H-Bridge (SCHB) multilevel Voltage Source Converter (VSC) is proposed, which is the transformer less, cost effective solution to interface the renewable generation system to the local grid directly. This paper presents the design, simulation and analysis of a Five Level (5L)-SCHB and an Eleven Level (11L)-SCHB VSC for an 11 kV local grid based renewable energy systems. The performance, cost, modulation scheme and harmonic spectra of the converter are the bases for analysis. © 2011 IEEE

Effects of different types of feeds on growth and production of tiger shrimp, Penaeus monodon at Bagerhat region, Bangladesh

An experiment was carried out in farmers' gher (shrimp farm) at Bagerhat sadar upazilla, Bagerhat to ascertain the effects of three different types of feeds on the production and economics of brackishwater shrimp, Penaeus monodon for a period of 120 days. There were three treatments such as T1 (BFRI dough feed containing of 30% fish meal, 10% protein conc., 10% soya meal, 15% mustard oil cake, 18% rice bran, 5% maize, 10% wheat flour, 1% oyster shell powder and 1% vitamin premix), T2 (Commercial diet Saudi-Bangla grower) and T3 (Saudi-Bangla special feed). Each treatment had two replicates and the stocking of shrimp in each gher was 3 nos/m². Water quality parameters did not differ significantly among the treatments except water depth. Average production and net return of shrimp in different treatments varied from 404.0 to 509.0 kg/ha and Tk. 56,493.99-Tk. 84,209.60, respectively. T2 showed significantly (p<0.05) the highest production and economic return. The result of the study implied that T2 is more suitable and economically viable than that of other treatments for shrimp farming

Performance and cost comparison of NPC, FC and SCHB multilevel converter topologies for high-voltage applications

The rapid increase in global energy consumption and the impact of greenhouse gas emissions have accelerated the renewable energy technology into a more competitive area. Due to the variable nature of renewable energy resources and power demand by the consumers, grid based renewable generation has gained significant popularity in the world. High-voltage converter can interconnect the renewable systems to the grid directly without introducing a lossy, costly and bulky transformer. Three popular multilevel converter topologies: Neutral Point Clamped (NPC), Flying Capacitor (FC) and Series Connected H-Bridge (SCHB) have successfully made their way into the industry and therefore can be considered a mature and proven technology for low and medium voltage applications. But most of them are not suitable for high-voltage applications. This paper presents the comparison of a Five-Level (5L)-NPC, a 5L-FC, a 5L-SCHB, an Eleven-Level (11L)-NPC, an 11L-FC and an 11L-SCHB topologies for an 11 kV Voltage Source Converter (VSC). The comparison is made in terms of number of semiconductors, semiconductor cost, Total Harmonic Distortion (THD), filter size and control complexity. © 2011 IEEE

A medium-frequency transformer with multiple secondary windings for grid connection through H-bridge voltage source converters

Although the power output of today's wind turbine has exceeded 7 MW, the voltage rating of the most common generator is below 700 V. A low-frequency transformer is commonly used to step-up the voltage to the grid voltage level, e.g. 11 kV or 33 kV. These heavy and bulky low-frequency transformers significantly increase the volume and weight of nacelle. To achieve a compact and light nacelle, a medium-voltage converter with series-connected H-bridge (SCHB) topology would be an attractive technology for future wind turbines. However, the SCHB converter requires multiple isolated and balanced DC sources, which makes the application not straightforward. As an alternative approach to generate multiple isolated and balanced sources a prototype transformer with six secondary windings, 1.26 kVA and 10 kHz, is designed and developed for 1 kV five levels SCHB multilevel converters. The experimental results show that the proposed system may be attractive in grid based renewable energy systems. © 2012 IEEJ Industry Appl Soc

Computer Simulation of Current Forces on Motion of Floating Production Storage and Offloading in Irregular Waves

This paper presents the effect of current forces on the motion of forces on Floating Production Storage and Offloading (FPSO) in irregular waves. The objective of this research is to compute the motion of FPSO in irregular waves by time domain simulation including the effect of current forces. A study is made on the slowly varying oscillations of a moored single body system in a current and waves. Linear potential theory is used to describe the fluid motion, and three-dimensional source distribution techniques are applied to obtain the hydrodynamic forces and transfer function of the wave exciting forces. OCIMF (1994) data are used for estimation of the current forces. The non-linear time domain simulations have been carried out in irregular waves. Based on it, slowly varying motion responses are examined including the effect of the current forces. Several environmental conditions, such as the current angle of attack, current velocity, significant wave height and mean wave period are considered, which may significantly affect FPSO motion in surge, sway and yaw moments. It is found that the effect of current forces is quite significant when the current velocity is increased. In this simulation, while the current velocity is increased to 3.0 meter/seconds, the impact on FPSO motion is quite significant, which should be taken into consideration from the point of view of safety, failure of mooring systems, operating responses and the dynamic positioning of the FPSO

Hysteresis-Free Nanosecond Pulsed Electrical Characterization of Top-Gated Graphene Transistors

We measure top-gated graphene field effect transistors (GFETs) with nanosecond-range pulsed gate and drain voltages. Due to high-k dielectric or graphene imperfections, the drain current decreases ~10% over time scales of ~10 us, consistent with charge trapping mechanisms. Pulsed operation leads to hysteresis-free I-V characteristics, which are studied with pulses as short as 75 ns and 150 ns at the drain and gate, respectively. The pulsed operation enables reliable extraction of GFET intrinsic transconductance and mobility values independent of sweep direction, which are up to a factor of two higher than those obtained from simple DC characterization. We also observe drain-bias-induced charge trapping effects at lateral fields greater than 0.1 V/um. In addition, using modeling and capacitance-voltage measurements we extract charge trap densities up to 10^12 1/cm^2 in the top gate dielectric (here Al2O3). Our study illustrates important time- and field-dependent imperfections of top-gated GFETs with high-k dielectrics, which must be carefully considered for future developments of this technologyComment: to appear in IEEE Transactions on Electron Devices (2014

Design and comparison of 11 kV multilevel voltage source converters for local grid based renewable energy systems

Because the availability of renewable energy is highly variable and the power demand by the consumers could have a very different characteristic, it is very desirable to connect a renewable generation system to the grid. In this respect, the 11 kV multilevel Voltage Source Converter (VSC) has no heavy transformer and is the cost effective solution to interfacing a renewable generation system to the local grid directly. This paper presents the design and comparison of a Five-Level Neutral Point Clamped (5L-NPC), a Five-Level Flying Capacitor (5L-FC), a Five-Level Series Connected H-Bridge (5L-SCHB), an Eleven-Level Neutral Point Clamped (11L-NPC), an Eleven-Level Flying Capacitor (11L-FC), and an Eleven-Level Series Connected H-Bridge (5L-SCHB) VSC for an 11 kV local grid based converter. The cost of power semiconductors and capacitors, modulation schemes and harmonic spectra of the converters are the bases for comparison. © 2011 IEEE

Simulation of PV array characteristics and fabrication of microcontroller based MPPT

In photovoltaic (PV) systems, the PV array represents about 57% of the total cost of the system, so it is very desirable to operate the PV panel at the maximum power point (MPP). The output characteristics of the solar array are nonlinear and vitally affected by solar radiation, temperature, and load conditions. In this respect simulation analysis of PV array is essential to understand the output characteristics of PV array such that it can operate at the maximum power producing point for a given atmospheric condition. This paper explores the output characteristics of a PV array in MATLAB environment and the fabrication of a microcontroller based simple maximum power point tracker (MPPT). The fabricated MMPT has the capability to track the MPP and the use of the MPPT enhances the output power by 20%. ©2010 IEEE