Analytical evaluation of DC capacitor RMS current and voltage ripple in neutral-point clamped inverters

The sizing of the DC-link capacitor in a three-level inverter is based on the RMS current flowing through it. This paper analyses the DC-link capacitor RMS current in a neutral-point clamped (NPC) inverter and expresses the same as a function of modulation index, line-side current amplitude and power factor. Analytical closed-form expressions are derived for the capacitor RMS current for single-phase half-bridge, single-phase full-bridge and three-phase three-leg topologies of a three-level inverter. The worst-case capacitor current stress is determined for each topology based on the analytical expressions. Further, analytical expressions are derived for the RMS values of low-frequency and high-frequency capacitor currents. These expressions are then used to estimate voltage ripple across the DC capacitor for sinusoidally modulated three-phase NPC inverter. The analytical expressions for the RMS current and voltage ripple are validated experimentally over a wide range of operating points.     

一种用于声呐发射机的超级电容被动均压简易实用电路

超级电容器的性能不一致性会造成处于串联模式的电容组中个别电容器首先发生过压失效,进而加速整个储能组的失效。现有的均压电路一般工作在线性状态,电路也较复杂,不便于调试和生产。介绍了一种仅由5个元器件组成的简易、实用、高效的开关型被动均压电路,分析了其工作原理及均压特性。该电路在声呐发射机的储能电源中可长期使用,很好地保护了超级电容组。     

Modeling for multilevel switching in oxide-based bipolar resistive memory

We report a physical model for multilevel switching in oxide-based bipolar resistive memory (ReRAM). To confirm the validity of the model, we conduct experiments with tantalum-oxide-based ReRAM of which multi-resistance levels are obtained by reset voltage modifications. It is also noticeable that, in addition to multilevel switching capability, the ReRAM exhibits extremely different switching timescales, i.e. of the order of 10(-7)?s to 10(0)?s, with regard to reset voltages of only a few volts difference which can be well explained by our model. It is demonstrated that with this simple model, multilevel switching behavior in oxide bipolar ReRAM can be described not only qualitatively but also quantitatively.     

Minimum voltage for threshold switching in nanoscale phase-change memory

The size scaling of the threshold voltage required for the amorphous-to-crystalline transition in phase-change memory (PCM) is investigated using planar devices incorporating individual GeTe and Sb2Te3 nanowires. We show that the scaling law governing threshold switching changes from constant field to constant voltage scaling as the amorphous domain length falls below 10 nm. This crossover is a consequence of the energetic requirement for carrier multiplication through inelastic scattering processes and indicates that the size of PCM bits can be miniaturized to the true nanometer scale.     

Chiral pattern selection induced by a fokker-planck diffusion law

It is pointed out that in a reaction-diffusion system near a chiral symmetry-breaking instability, a weak drift term in the diffusion law can induce a systematic selection between macroscopic three-dimensional patterns of opposite chirality. When the cause of the drift term is an external field, the orientation of the field with respect to the system determines the final chiral choice. The possible appearance of a drift term in the diffusion law, arising from the intrinsic asymmetry of the underlying material is also considered. In a one-dimensional system with periodic boundary conditions it is shown that the drift term may account for systematic selection between waves rotating in opposite directions     

Design and evaluation of a compensating algorithm for the secondary voltage of a coupling capacitor voltage transformer in the time domain

This study describes the design and evaluation of a compensating algorithm for the secondary voltage of a coupling capacitor voltage transformer (CCVT) in the time domain by considering the hysteresis characteristics of the core. The major error of the CCVT is the voltage across the capacitor and the tuning reactor caused by a non-sinusoidal exciting current. The proposed algorithm estimates the voltage across the capacitor and the reactor by considering the effect of the hysteresis characteristics of the core and adds it to the measured secondary voltage to obtain the correct voltage. The algorithm reduces the errors of the CCVT significantly both in the steady state and during a fault. The performance of the algorithm is verified under the various fault conditions by varying the fault distance, the fault inception angle and the fault impedance with the EMTP generated data. Test results clearly indicate that the algorithm can increase the accuracy of a CCVT significantly under the fault conditions as well as in the steady state. The algorithm helps to improve the performance of a protection relay or a metering device.     

西林电桥(QS1)测试中常忽视的问题

通过西林电桥（QS1型）对电力设备测试结果可靠性及其试验方法的评价，介绍了在其试验环节存在容易忽视的常见问题，分析了这些问题产生的根本原因，对其试验结果产生的影响进行了具体的说明。根据对电力设备介质损失角正切值tgδ试验的实际，介绍了使用西林电桥（QS1）测量各种电力设备介质损失角正切值tgδ的方法、注意事项及规范要求，并提出如何将各项试验结果全面综合分析判断，对被试设备的绝缘状况和缺陷性质作出科学的结论。     

微量稀土对高压阳极电容铝箔织构演变的影响

采用晶体取向分布函数方法（ODF）和X射线衍射测定板面晶面衍射强度研究和分析了在高纯铝加入微量稀土（Y）对箔材织构演变的影响。研究结果表明：添加微量稀土，能改变高纯铝箔中变形织构组分的相对强度，随稀土含量增加，黄铜取向密度增加，S织构组分减少，成品退火箔材中立方织构含量随稀土含量增加就相应减少。当稀土含量为0.0030%时，成品退火箔材中立方织构取向密度最大。其作用机理是稀土元素Y化学活性强，能与铁等微量杂质元素形成一系列化合物，析出后可净化基体，降低基体中铁等杂质的浓度，消除杂质元素对高纯铝箔中立方织构形成的不利影响。     

Controlled multilevel switching and artificial synapse characteristics in transparent HfAlO-alloy based memristor with embedded TaN nanoparticles

Atomic layer deposition technique has been used to prepare tantalum nitride nanoparticles (TaN-NPs)and sandwiched between Al-doped HfO2 layers to achieve ITO/HfAlO/TaN-NP/HfAlO/ITO RRAM device.Transmission electron microscopy along with energy dispersive spectroscopy confirms the presence of TaN-NPs.X-ray photoelectron spectroscopy suggests that part of TaN converted to tantalum oxynitride(TaOxNy) which plays an important role in stable cycle-to-cycle resistive switching.Charge trapping and oxygen vacancy creation were found to be modified after the inclusion of TaN-NPs inside RRAM struc-ture.Also,HfAlO/TaOxNy interface due to the presence TaN-NPs improves the device-to-device switching reliability by reducing the probability of random rupture/formation of conductive filaments (CFs).DC en-durance of more than 103 cycles and memory data retention up to 104 s was achieved with an insignif-icant variation of different resistance states.Multilevel conductance was attained by controlling RESET voltage with stable data retention in multiple states.The volatile threshold switching was monitored af-ter controlling the CF forming at 200 nA current compliance with high selectivity of～103.Synaptic learn-ing behavior has been demonstrated by spike-rate-dependent plasticity (SRDP).Reliable potentiation and depression processes were observed after the application of suitable negative and positive pulses which shows the capability of the TaN-NPs based RRAM device for transparent synaptic devices.     

Induction of coherent magnetization switching in a few atomic layers of FeCo using voltage pulses

The magnetization direction of a metallic magnet has generally been controlled by a magnetic field or by spin-current injection into nanosized magnetic cells. Both these methods use an electric current to control the magnetization direction; therefore, they are energy consuming. Magnetization control using an electric field is considered desirable because of its expected ultra-low power consumption and coherent behaviour. Previous experimental approaches towards achieving voltage control of magnetization switching have used single ferromagnetic layers with and without piezoelectric materials, ferromagnetic semiconductors, multiferroic materials, and their hybrid systems. However, the coherent control of magnetization using voltage signals has not thus far been realized. Also, bistable magnetization switching (which is essential in information storage) possesses intrinsic difficulties because an electric field does not break time-reversal symmetry. Here, we demonstrate a coherent precessional magnetization switching using electric field pulses in nanoscale magnetic cells with a few atomic FeCo (001) epitaxial layers adjacent to a MgO barrier. Furthermore, we demonstrate the realization of bistable toggle switching using the coherent precessions. The estimated power consumption for single switching in the ideal equivalent switching circuit can be of the order of 10(4)k(B)T, suggesting a reduction factor of 1/500 when compared with that of the spin-current-injection switching process.     

Temperature influence and reset voltage study of bipolar resistive switching behaviour in ZrO2 thin films

We have fabricated ZrO₂ thin films by sol–gel deposition and annealed them at 300, 500 and 700 °C. Reproducible I–V curves can be obtained for the device Cu/ZrO₂/ATO which is measured at room temperature (300 K). During the RESET operation, R _L and R _H values can be controlled by the RESET voltage. Moreover, the Cu/ZrO₂/ATO device which the ZrO₂ thin film annealed at 300 °C can be measured as resistive switching sweeps at 200, 100 and 50 K. It was found that the ratio of R _off/R _on reduced when the measured temperature decreased. When the I–V measurement temperature decreases, R _on decreases obviously which is typical for electronic transportation in a Cu metal. It is indicated that the Cu metallic conduction filament has been formed in the ZrO₂ films. Besides, the microstructure by high resolution transmission electrical microscopy (HRTEM) was also investigated.     

Toward self-assembled ferroelectric random access memories: hard-wired switching capacitor arrays with almost Tb/in.(2) densities

We report on the successful fabrication of arrays of switchable nanocapacitors made by harnessing the self-assembly of materials. The structures are composed of arrays of 20-40 nm diameter Pt nanowires, spaced 50-100 nm apart, electrodeposited through nanoporous alumina onto a thin film lower electrode on a silicon wafer. A thin film ferroelectric (both barium titanate (BTO) and lead zirconium titanate (PZT)) has been deposited on top of the nanowire array, followed by the deposition of thin film upper electrodes. The PZT nanocapacitors exhibit hysteresis loops with substantial remnant polarizations, while although the switching performance was inferior, the low-field characteristics of the BTO nanocapacitors show dielectric behavior comparable to conventional thin film heterostructures. While registration is not sufficient for commercial RAM production, this is nevertheless an embryonic form of the highest density hard-wired FRAM capacitor array reported to date and compares favorably with atomic force microscopy read-write densities.     

Metallopolymer capacitor in "one pot" by self-directed UV-assisted process

Silver metalized methacrylate films are prepared by single-step UV curing process with good conductivity on both sides. The major component of the composite is Bisphenol A ethoxylate dimethacrylate, which can be photopolymerized by a photoreactive initiator under UV light. Under the same conditions of UV irradiation, silver ions are deposited as metal nanoparticles while the pyrrole is oxidized to polypyrrole. The migration of silver ions and pyrrole toward both surfaces during polymerization leads to the formation of a metallo-polymer capacitor. The composite films are characterized by SEM-EDX and electrical measurements for possible applications as capacitors in flexible and/or nonplanar electronics.     

Tolerances in microlens fabrication by multilevel etching and mass-transport smoothing

A method is developed for the quantitative assessment of microlens deficiencies caused by inaccuracies in the multistep mesa fabrication or by incomplete mass-transport smoothing. Analytical expressions are obtained and tolerable imperfections are deduced for the practical effort-saving fabrication of highperformance micro-optical elements.     

An examination of exposure control and content balancing restrictions on item selection in CATs using the partial credit model

The purpose of the present investigation was to systematically examine the effectiveness of the Sympson-Hetter technique and rotated content balancing relative to no exposure control and no content rotation conditions in a computerized adaptive testing system (CAT) based on the partial credit model. A series of simulated fixed and variable length CATs were run using two data sets generated to multiple content areas for three sizes of item pools. The 2 (exposure control) X 2 (content rotation) X 2 (test length) X 3 (item pool size) X 2 (data sets) yielded a total of 48 conditions. Results show that while both procedures can be used with no deleterious effect on measurement precision, the gains in exposure control, pool utilization, and item overlap appear quite modest. Difficulties involved with setting the exposure control parameters in small item pools make questionable the utility of the Sympson-Hetter technique with similar item pools.     

Flow transitions and pattern selection of the rayleigh-bénard problem in rectangular enclosures

In this paper, research efforts in the broad area of flow transitions of Rayleigh-Bénard convection in rectangular enclosures with sidewalls are reviewed. Numerical studies are given primary emphasis. However, experimental works that are relevant are described. Our current physical understanding of the transition phenomena as occurring in the Rayleigh-Bénard problem is critically reviewed. Two broad categories of transition are discussed. In the former, the transitions are temporal in nature, and mostly confined to small enclosures. In the latter, transitions are a result of change in spatial patterns. This phenomenon, known as pattern selection, is looked into for both small and intermediate enclosures.     

Chiral switching by spontaneous conformational change in adsorbed organic molecules

Self-assembly of adsorbed organic molecules is a promising route towards functional surface nano-architectures, and our understanding of associated dynamic processes has been significantly advanced by several scanning tunnelling microscopy (STM) investigations. Intramolecular degrees of freedom are widely accepted to influence ordering of complex adsorbates, but although molecular conformation has been identified and even manipulated by STM, the detailed dynamics of spontaneous conformational change in adsorbed molecules has hitherto not been addressed. Molecular surface structures often show important stereochemical effects as, aside from truly chiral molecules, a large class of so-called prochiral molecules become chiral once confined on a surface with an associated loss of symmetry. Here, we investigate a model system in which adsorbed molecules surprisingly switch between enantiomeric forms as they undergo thermally induced conformational changes. The associated kinetic parameters are quantified from time-resolved STM data whereas mechanistic insight is obtained from theoretical modelling. The chiral switching is demonstrated to enable an efficient channel towards formation of extended homochiral surface domains. Our results imply that appropriate prochiral molecules may be induced (for example, by seeding) to assume only one enantiomeric form in surface assemblies, which is of relevance for chiral amplification and asymmetric heterogenous catalysis.