Yunfei En

Biomimetic Ant-Nest Electrode Structures for High Sulfur Ratio Lithium–Sulfur Batteries

The lithium-sulfur (Li-S) rechargeable battery has the benefit of high gravimetric energy density and low cost. Significant research currently focuses on increasing the sulfur loading and sulfur/inactive-materials ratio, to improve life and capacity. Inspired by natures ant-nest structure, this research results in a novel Li-S electrode that is designed to meet both goals. With only three simple manufacturing-friendly steps, which include slurry ball-milling, doctor-blade-based laminate casting, and the use of the sacrificial method with water to dissolve away table salt, the ant-nest design has been successfully recreated in an Li-S electrode. The efficient capabilities of the ant-nest structure are adopted to facilitate fast ion transportation, sustain polysulfide dissolution, and assist efficient precipitation. High cycling stability in the Li-S batteries, for practical applications, has been achieved with up to 3 mg·cm(-2) sulfur loading. Li-S electrodes with up to a 85% sulfur ratio have also been achieved for the efficient design of this novel ant-nest structure.

Effects of surface tension and axis stress on piezoelectric behaviors of ferroelectric nanowires

The effects of surface tension and axis stress on piezoelectric behaviors of ferroelectric nanowires with radius polarization were investigated by the time-dependent Ginzburg-Landau theory. When surface tension increases, both of coercive field and remnant strain decrease. The larger the surface tension is, the more they decrease. The axis compressive stress enhances the coercive field and remnant strain, while the axis tensile stress has contrary effect. The reason for the stress-modulated piezoelectricity is that radius polarization is forced by axis compressive stress but restrained by surface tension and axis tensile stress. The research is useful for ferroelectric nanostructures in strain engineering.

Reliability Investigations of AlGaN/GaN HEMTs Based on On-State Electroluminescence Characterization

In this paper, we investigated ON-state electroluminescence (EL) characteristics in fresh GaN-based HEMTs under different ON-state bias conditions. It demonstrated that: 1) the intensity of the ON-state EL emitted by the GaN-based HEMTs with relatively high VDS (VDS > 10 V) monotonically increased as the gate voltage increased; and 2) the distribution of the intensity of the ON-state EL was uniform along the gate and across the device before stress tests. The degradation process and mechanisms of the devices in the high temperature operation (HTO) stress were also analyzed based on the ON-state EL characterization, the electrical measurement, and failure analysis. It demonstrated the following. First, the ON-state EL distribution of the device after step HTO (S-HTO) stress tests was no longer uniform along one of the fingers, which indicated that the degradation was related to this finger. This hypothesis was confirmed by the SEM observation after the deprocess of the gate metal and the SiN passivation layer. Second, a new degradation mechanism of AlGaN/GaN HEMTs that caused the abrupt degradation in the S-HTO stress was identified and related to the emergent, localized, and jagged crack under the gate metal. Third, there was no abrupt degradation in the long-term HTO (LT-HTO) stress test. However, the ON-state EL intensity of the inner fingers in the device after the LT-HTO stress test decreased more than that of the outer fingers, which meant that the degradation of the inner fingers was faster than that of the outer fingers as a result of the higher temperature caused by self-heating in the inner region of the device. Finally, the gradual degradation in the LT-HTO stress test was ascribed to the gradual formation of structural damage along the drain side of the whole gate edge, which was confirmed by the SEM observation.

Effect of solder joint parameter on vibration fatigue reliability of high density PCB assembly

In order to explore the influence of solder joint parameter on vibration fatigue reliability of high density PCB assembly, a three-dimensional simulation model which was almost completely in keeping with test samples was created, and the modal parameters were obtained through modal test and finite element method respectively. Based on the verification of FEM model correctness, through designed different solder joint parameters, such as height, diameter, and material, and based on GJB-150.16-86 vibration test standard, random vibration finite element simulation was being done, and vibration parameters were picked up and analyzed. The result shows that more PCB fixations could increase the first frequency, decrease the deformation of PCB. Moreover, solder joint stress and strain decreases with solder joint height decreasing and diameter increasing, but when height and diameter reaches a certain value, stress and strain will almost keep constant. Under the same conditions, the order of the strains of the joints is: lead solder joints (Sn63Pb37) > mixed solder joints (79% SAC and 21% SnPb) > lead-free solder joints (SAC305). Consequently, more PCB fixations and solder joints which have smaller height, larger diameter or have lead-free will decrease stress and strain of key positions of PCB assembly, and BGA (ball grid array) devices should be placed near the region which has more fixation positions. Finally, based on stress and strain failure criteria, it can make some references for predicting and optimizing random vibration fatigue life of solder joints, guiding BGA layout and selecting solder joints.

A novel hardware Trojan detection method based on side-channel analysis and PCA algorithm

Malicious modification of integrated circuits, namely Hardware Trojan, has emerged as a major security threat. Since it is extremely difficult to detect the presence of such small Trojan circuits using only logic test or side-channel analysis, this paper presents a novel test method combining logic test and side-channel test together. An 18-bit CORDIC IP core is adopted as a golden circuit, while a 2-bit counter is applied as a Trojan circuit. The automatic test platform is set up with Xilinx FPGA, LabVIEW software, and high precision oscilloscope. Meanwhile, the power traces of power supplies Vccint or Vccaux in FPGA are both monitored to enhance the detection sensitivity. The relevant test flow chart is also depicted in detail. Experimental results demonstrate that the novel test method can easily achieve about 0.1% Trojan detection sensitivity when principal component analysis approach is adopted as the data processing algorithm.

Using Termination Effect to Characterize Electric and Magnetic Field Coupling Between TEM Cell and Microstrip Line

In this paper, we change terminal loads of a microstrip line to separate electric field coupling and magnetic field coupling between a microstrip line and a transverse electromagnetic (TEM) cell during radiated emission measurement. Such separation can also be validated by another terminal load. After validation, the effect of termination on both couplings is investigated, and the requirements of termination impedance to suppress the two couplings are revealed. In addition, we find that the linear physical models deriving both lumped coupling parameters only hold below a limited frequency.

Investigation of Threshold Ion Range for Accurate Single Event Upset Measurements in Both SOI and Bulk Technologies

Experimental evidences are presented showing obvious differences in threshold ion range for silicon-on-insulator (SOI) and bulk static random access memories (SRAMs). Single event upset (SEU) cross sections of SOI SRAMs start to decline off the Weibull curve at ion ranges of 20.7 μm to 40.6 μm, depending on the ion species and also the thickness of metallization layers. Whereas for the bulk SRAMs, threshold range of Bismuth beam is unexpectedly larger than 60.4 μm. Underlying mechanisms are further revealed by Monte Carlo simulations and in-depth analysis. The relative location of ions Bragg peak to the sensitive region and also the position of ion LET in the σ-LET curve of test device turn out to be two key parameters in determining the threshold ion range which can explain the experimental results. Significant discrepancies are observed in the deposited energy spectrums in sensitive regions of bulk SRAM by ions at different sides of the Bragg peak, but with almost the same LET at die surface (all with ion range larger than 30 μm). Energy straggling of incident ions at the die surface is considered by Monte Carlo calculations. Implications for hardness assurance testing are also discussed. A formula is proposed for calculating the “worst case” threshold ion range.

Supply voltage dependence of single event upset sensitivity in diverse SRAM devices

Experimental evidences are presented showing the variety of supply voltage dependence of single event upset (SEU) sensitivity in diverse SRAM devices. Devices under test (DUTs) from Alliance Memory, ISSI and IDT companies with different technologies were irradiated by several kinds of heavy ions at Heavy ion Research Facility in Lanzhou (HIRFL) cyclotrons. For the Alliance 256 kb SRAM device, SEU cross section increases by more than one order of magnitude as supply voltage decreases from 5.0 V to 3.0 V. SEU data of Alliance 64 kb SRAM also exhibits significant supply voltage dependence. The reduction of critical charge is the predominant factor worsening the device performance. While for the Alliance 8 Mb, ISSI 2 Mb and IDT 256 kb SRAM devices, no obvious trend was observed, which is attributed to the negligible net contribution of competing mechanisms. Those results suggest that the worst-case supply voltage for evaluation of SEU sensitivity depends on the test devices.

The Function of IR thermal imaging technology for device and circuit reliability research

Along with the infrared technical rapid development, infrared radiation (IR) thermal imaging technology is widely applied in device and circuit reliability research. For device and circuit reliability study, IR thermal imaging technology is best for getting the peak temperature and temperature distribution. With the peak temperature and temperature distribution, the thermal information of the devices were evaluated and optimized to meet the design requirement. Secondly, IR thermal imaging technology is used to position hotspot for failures analysis, example for especially increases in elevated leakage currents. However, IR thermal imaging technology can provide evidence of excessive temperature rise due to power dissipation. It can identify root-cause of failures at temperatures below expectations.

End of discharge time prediction for Li-ion battery

How to predict the end of discharge time for Li-ion battery is one of the most important problems in the field of battery health management. This paper proposes a linear regression model to describe the relationship between the end of discharge time and discharge cycle. Moreover, the model verification shows that the discharge cycle has significant effect on the end of discharge time and that the proposed model has good fitness in fitting Li-ion discharge data. This paper also presents the end of discharge time predictions for Li-ion battery based on the linear regression model.