Khay Wai W See

Tuning the Band Gap in Silicene by Oxidation

Silicene monolayers grown on Ag(111) surfaces demonstrate a band gap that is tunable by oxygen adatoms from semimetallic to semiconducting type. With the use of low-temperature scanning tunneling microscopy, we find that the adsorption configurations and amounts of oxygen adatoms on the silicene surface are critical for band gap engineering, which is dominated by different buckled structures in √13 × √13, 4 × 4, and 2√3 × 2√3 silicene layers. The Si-O-Si bonds are the most energy-favored species formed on √13 × √13, 4 × 4, and 2√3 × 2√3 structures under oxidation, which is verified by in situ Raman spectroscopy as well as first-principles calculations. The silicene monolayers retain their structures when fully covered by oxygen adatoms. Our work demonstrates the feasibility of tuning the band gap of silicene with oxygen adatoms, which, in turn, expands the base of available two-dimensional electronic materials for devices with properties that is hardly achieved with graphene oxide.

Evaluation of persistent-mode operation in a superconducting MgB2 coil in solid nitrogen

We report the fabrication of a magnesium diboride (MgB2) coil and evaluate its persistent-mode operation in a system cooled by a cryocooler with solid nitrogen (SN2) as a cooling medium. The main purpose of SN2 was to increase enthalpy of the cold mass. For this work, an in situ processed carbon-doped MgB2 wire was used. The coil was wound on a stainless steel former in a single layer (22 turns), with an inner diameter of 109 mm and height of 20 mm without any insulation. The two ends of the coil were then joined to make a persistent-current switch to obtain the persistent-current mode. After a heat treatment, the whole coil was installed in the SN2 chamber. During operation, the resultant total circuit resistance was estimated to be <7.4 × 10−14 Ω at 19.5 K ± 1.5 K, which meets the technical requirement for magnetic resonance imaging application.

A comprehensive study of the pinning mechanisms of MgB2 wires treated with malic acid and their relationships with lattice defects

The effects of sintering temperature on the lattice parameters, structural strain, critical temperature (Tc), critical current density (Jc), irreversibility field (Birr), upper critical field (Bc2), and resistivity (ρ) of MgB2 wires treated with 10 wt.% malic acid (C4H6O5) are investigated in this paper. The a-lattice parameter of the sample treated with malic acid was drastically reduced, to 3.0745 A, as compared to those for the undoped samples. Reduction in the a-lattice parameter is related to crystalline imperfections arising from carbon substitution—as confirmed by x-ray diffraction and Raman spectra—which play a vital role in enhancing Jc, Bc2 and Birr. We have also analyzed the pinning mechanisms, and concluded that flux pinning is dominated by point and correlated pinning at lower and higher magnetic fields, respectively, for the carbon-doped samples sintered at both 700 and 900 ° C. The degree of flux pinning enhancement and the ratio RHH (Bc2/Birr) have been estimated to guide us towards further enhancement of Jc. We argue that δl and δTc pinning mechanisms, based on variation of the mean free path (l) and the critical temperature, respectively, coexist in the MgB2 wires treated with malic acid, regardless of the sintering temperature. The δl pinning is predominant at lower operating temperatures, and δTc pinning starts close to Tc, which means that spatial variation in the charge carrier mean free path is the main mechanism responsible for the flux pinning in the MgB2 wires treated with malic acid that were sintered at 700 and 900 ° C.

MgB2 superconducting joints for persistent current operation

High-performance superconducting joints are essential for realizing persistent-mode magnets. Herein, we propose a concept and fabrication of such superconducting joints, which yielded reliable performance in the operating temperature range of 4.2–25 K. MgB2–MgB2 joints in magnets are known to result in deterioration of localized electrical, thermal, and mechanical properties. To overcome these problems, the ends of the two wires are inserted into a pellet press, which is then filled with a mixture of unreacted magnesium and boron powders, followed by heat treatment. The critical current capacity and joint resistance were precisely evaluated by the standard four-probe method in open-circuit and by field-decay measurements in a closed-loop, respectively. These joints demonstrated up to 66% of the current-carrying capacity of unjoined wire at 20 K, 2 T and joint resistance of 1.4 × 10−12 Ω at 4.2 K in self-field.

Transport critical current of MgB2 wires: pulsed current of varying rate compared to direct current method

The measurement of transport critical current (Ic) for MgB2 wires and tapes has been investigated with two different techniques, the conventional four-probe arrangement with direct current (DC) power source, and a tailored triangle pulse at different rates of current change. The DC method has been widely used and practiced by various groups, but suffers from inevitable heating effects when high currents are used at low magnetic fields. The pulsed current method has no heating effects, but the critical current can depend on the rate of the current change (dI/dt) in the pulse. Our pulsed current measurements with varying dI/dt show that the same values of Ic are obtained as with the DC method, but without the artifacts of heating. Our method is particularly useful at low field regions which are often inaccessible by DC methods. We also performed a finite element method (FEM) analysis to obtain the time dependent heat distribution in MgB2 due to the electric potential produced at the current contacts to the superconducting sample and its gradient around the contacts. This gradient is defined as the current transfer length (CTL) of the samples and leads to Joule heating of the wire near the contacts. The FEM results provide further evidence of the limitation of the DC method in obtaining high transport critical current.

Online SoC estimation of lithium ion battery for EV/BEV using Kalman filter with fading memory

A novel algorithm based on fading Kalman filter to estimate the state of charge (SoC) of Li-ion battery used in electric vehicles is proposed and validated in this paper. Online identification of batterys electric model parameters followed by open circuit voltage estimation by fading Kalman filter resulted in accurate SoC estimation. The experimental results obtained from actual driving cycle in real-time reveal the robust performance of the proposed algorithm.

A Single-Switch Quadratic Buck–Boost Converter With Continuous Input Port Current and Continuous Output Port Current

A single-switch quadratic buck–boost converter with continuous input port current and continuous output port current is proposed in this paper. Compared with the traditional buck-boost converter, the proposed converter can obtain a wider range of the voltage conversion ratio with the same duty cycle. Moreover, the proposed converter can operate with continuous input port current and continuous output port current compared to the existing counterparts with inherently discontinuous input port current and discontinuous output port current. The operating principle and steady-state performance of the proposed converter under continuous inductor current mode is analyzed in detail. Then, the comparison between the proposed converter and the existing quadratic buck–boost converters has been conducted to demonstrate the unique features of the proposed one. Finally, experimental results from a prototype built in the lab are recorded to verify the effectiveness and validity of the proposed quadratic buck–boost converter.

SOC estimation for LiFePO 4 battery in EVs using recursive least-squares with multiple adaptive forgetting factors

This work presents a novel technique which is simple yet effective in estimating electric model parameters and state-of-charge (SOC) of the LiFePO4 battery. Unlike the well-known recursive least-squares-based algorithms with single constant forgetting factor, this technique employs multiple adaptive forgetting factors to provide the capability to capture the different dynamics of model parameters. The validity of the proposed method is verified through experiments using actual driving cycles.

A Δ-Y Hybrid Impedance Network Boost Converter With Reduced Input Current Ripple

This letter is to present a Δ-Y hybrid impedance network based boost converter with reduced input current ripple. Comparing to the existing impedance network based boost converters to enhance the output port characteristic, i.e., voltage conversion ratio, of the traditional boost converter (TBC), the proposed converter applies the impedance network to improve its input port performance, i.e., the input current ripple issue. The proposed Δ-Y hybrid impedance network consists of the TBCs main inductor, an additional coupled inductor, and an additional resonant inductor and capacitor pair. Although the peak currents flowing through the diode and the switch are higher in comparison to the TBC, the proposed converter can remain TBCs voltage conversion feature and effectively reduce the input current ripple and the average current of the main inductor with the application of the Δ-Y hybrid impedance network. The operating principle of the proposed converter is explained and the principle of current ripple reduction is analyzed in detail. Experimental results from a prototype are carried out to validate the effectiveness of the proposed converter.

Systematic Derivation of Dead-Zone Elimination Strategies for the Noninverting Synchronous Buck–Boost Converter

The noninverting synchronous buck-boost converter is preferable to operate in buck and boost operating modes to obtain a high operating efficiency. However, the dead zone, which degrades the performance of the converter, will occur when the converter shifts from buck operating mode to boost operating mode or vice versa. Therefore, the origin of the dead zone is derived in this paper by analyzing the relationship between the voltage conversion ratio and the duty cycles of the switches. Based on this, a series of three-mode and four-mode modulation schemes are systematically derived to completely eliminate the dead zone. The ripple and average value of the inductor current under different modulation schemes are investigated to evaluate the performance of these modulation schemes. To demonstrate the effectiveness of the proposed modulation schemes, two implementations of a four-mode modulation scheme are presented and experimentally tested as the examples for all modulation schemes. Experimental results correspond well with the theoretical analysis in both implementations over the entire input voltage range.