First-Order Reversal Curve Probing of Spatially Resolved Polarization Switching Dynamics in Ferroelectric Nanocapacitors

Spatially resolved polarization switching In ferroelectric nanocapacitors was studied on the sub-25 nm scale using the first-order reversal curve (FORC) method. The chosen capacitor geometry allows both high-veracity observation of the domain structure and mapping of polarization switching in a uniform field, synergistically combining microstructural observations and probing of uniform-field polarization responses as relevant to device operation. A classical Kolmogorov-Avrami-Ishibashi model has been adapted to the voltage domain, and the individual switching dynamics of the FORC response curves are well approximated by the adapted model. The comparison with microstructures suggests a strong spatial variability of the switching dynamics inside the nanocapacitors.

Optimisation and scaling of interfacial GeO2 layers for high-k gate stacks on germanium and extraction of dielectric constant of GeO2

Germanium is an attractive channel material for MOSFETs because of its higher mobility than silicon. In this paper, GeO2 has been investigated as an interfacial layer for high-kappa gate stacks on germanium. Thermally grown GeO2 layers have been prepared at 550 degrees C to minimise GeO volatilisation. GeO2 growth has been performed in both pure O-2 ambient and O-2 diluted with N-2. GeO2 thickness has been scaled down to approximately 3 nm. MOS capacitors have been fabricated using different GeO2 thicknesses with a standard high-kappa dielectric on top. Electrical properties and thermal stability have been tested up to at least 350 degrees C. The K value of GeO2 was experimentally determined to be 4.5. Interface state densities (D-it) of less than 10(12) CM-2 eV(-1) have been extracted for all devices using the conductance method.

Loading Artificial Magnetic Conductor and Artificial Magnetic Conductor Absorber with Negative Impedance Convertor Elements

The general properties of a frequency selective surface loaded with negative impedance converter (NIC)-based active loads are discussed from a theoretical perspective.The stability problem associated with NIC circuits embedded in artificial magnetic conductor (AMC) and AMC absorber applications is studied using pole-zero analysis. The requirements and constraints for achieving stable operation with enhanced bandwidth using negative capacitance as realized by a floating NIC network are derived. Furthermore, it is shown that it is nearly impossible to simultaneously implement a negative capacitor and a negative inductor to such structures. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:2111–2114, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27019

Voltage Sensing Using an Asynchronous Charge-to-Digital Converter for Energy-Autonomous Environments

In future systems with relatively unreliable and unpredictable energy sources such as harvesters, the system power supply may become non-deterministic. For energy effective operations, Vdd is an important parameter in any meaningful system control mechanism. Reliable and accurate on-chip voltage sensors are therefore indispensible for the power and computation management of such systems. Existing voltage sensing methods are not suitable because they usually require a stable and known reference (voltage, current, time, frequency, etc.), which is difficult to obtain in this environment. This paper describes an autonomous reference-free voltage sensor designed using an asynchronous counter powered by the charge on a capacitor and a small controller. Unlike existing methods, the voltage information is directly generated as a digital code. The sensor, fabricated in the 180 nm technology node, was tested successfully through performing measurements over the voltage range from 1.8 V down to 0.8 V.

A Lead-Free and High-Energy Density Ceramic for Energy Storage Applications

In this work, we demonstrate a very high-energy density and high-temperature stability capacitor based on SrTiO3-substituted BiFeO3 thin films. An energy density of 18.6 J/cm3 at 972 kV/cm is reported. The temperature coefficient of capacitance (TCC) was below 11% from room temperature up to 200°C. These results are of practical importance, because it puts forward a promising novel and environmentally friendly, lead-free material, for high-temperature applications in power electronics up to 200°C. Applications include capacitors for low carbon vehicles, renewable energy technologies, integrated circuits, and for the high-temperature aerospace sector. © 2013 Crown copyright

Switching ferroelectric domain configurations using both electric and magnetic fields in Pb(Zr,Ti)O3–Pb(Fe,Ta)O3 single-crystal lamellae

Thin single-crystal lamellae cut from Pb(Zr,Ti)O3–Pb(Fe,Ta)O3 ceramic samples have been integrated into simple coplanar capacitor devices. The influence of applied electric and magnetic fields on ferroelectric domain configurations has been mapped, using piezoresponse force microscopy. The extent to which magnetic fields alter the ferroelectric domains was found to be strongly history dependent: after switching had been induced by applying electric fields, the susceptibility of the domains to change under a magnetic field (the effective magnetoelectric coupling parameter) was large. Such large, magnetic field-induced changes resulted in a remanent domain state very similar to the remanent state induced by an electric field. Subsequent magnetic field reversal induced more modest ferroelectric switching.

Sequential injection of domain walls into ferroelectrics at different bias voltages: Paving the way for “domain wall memristors”

Simple meso-scale capacitor structures have been made by incorporating thin (300 nm) single crystal lamellae of KTiOPO4 (KTP) between two coplanar Pt electrodes. The influence that either patterned protrusions in the electrodes or focused ion beam milled holes in the KTP have on the nucleation of reverse domains during switching was mapped using piezoresponse force microscopy imaging. The objective was to assess whether or not variations in the magnitude of field enhancement at localised “hot-spots,” caused by such patterning, could be used to both control the exact locations and bias voltages at which nucleation events occurred. It was found that both the patterning of electrodes and the milling of various hole geometries into the KTP could allow controlled sequential injection of domain wall pairs at different bias voltages; this capability could have implications for the design and operation of domain wall electronic devices, such as memristors, in the future.

Synchronous records of pCO2 and Δ14C suggest rapid, ocean-derived pCO2 fluctuations at the onset of Younger Dryas

Just before the onset of the Younger Dryas (YD) cold event, several stomatal proxy-based pCO2 records have shown a sharp increase in atmospheric CO2 concentration (pCO2) of between ca 50 and 100 ppm, followed by a rapid decrease of similar or even larger magnitude. Here we compare one of these records, a high-resolution pCO2 record from southern Sweden, with the IntCal13 record of radiocarbon (Δ14C). The two records show broadly synchronous fluctuations at the YD onset. Specifically, the IntCal13 record documents decreasing Δ14C just before the YD onset when pCO2 peaks, consistent with a source of “old” CO2 from the deep ocean. We propose that this fluctuation occurred due to a major ocean flushing event. The cause of the flushing event remains speculative but could be related to the hypothesis of the glacial ocean as a thermobaric capacitor. We confirm that the earth system can produce such large multi-decadal timescale fluctuations in pCO2 through simulating an artificial ocean flushing event with the GENIE Earth System Model. We suggest that sharp transitions of pCO2 may have remained undetected so far in ice cores due to inter-firn gas exchange and time-averaging. The stomatal proxy record is a powerful complement to the ice core records for the study of rapid climate change.

Pre-charging and DC Fault Ride-Through of Hybri MMC Based HVDC Systems

Compared to half-bridge based MMCs, full-bridge based systems have the advantage of blocking dc fault, but at the expense of increased power semiconductors and power losses. In view of the relationships among ac/dc voltages and currents in full-bridge based MMC with the negative voltage state, this paper provides a detailed analysis on the link between capacitor voltage variation and the maximum modulation index. A hybrid MMC, consisting of mixed half-bridge and full-bridge circuits to combine their respective advantages is investigated in terms of its pre-charging process and transient dc fault ride-through capability. Simulation and experiment results demonstrate the feasibility and validity of the proposed strategy for a full-bridge based MMC and the hybrid MMC.

New modular structure DC-DC converter without electrolytic capacitors for renewable energy applications

Python PSS/E Simulation to Test Efficacy of Proposed PMU Based WAMS and Potential WAMPAC Applications

A PSS/E 32 model of a real section of the Northern Ireland electrical grid was dynamically controlled with Python 2.5. In this manner data from a proposed wide area monitoring system was simulated. The area is of interest as it is a weakly coupled distribution grid with significant distributed generation. The data was used to create an optimization and protection metric that reflected reactive power flow, voltage profile, thermal overload and voltage excursions. Step changes in the metric were introduced upon the operation of special protection systems and voltage excursions. A wide variety of grid conditions were simulated while tap changer positions and switched capacitor banks were iterated through; with the most desirable state returning the lowest optimization and protection metric. The optimized metric was compared against the metric generated from the standard system state returned by PSS/E. Various grid scenarios were explored involving an intact network and compromised networks (line loss) under summer maximum, summer minimum and winter maximum conditions. In each instance the output from the installed distributed generation is varied between 0 MW and 80 MW (120% of installed capacity). It is shown that in grid models the triggering of special protection systems is delayed by between 1 MW and 6 MW (1.5% to 9% of capacity), with 3.5 MW being the average. The optimization and protection metric gives a quantitative value for system health and demonstrates the potential efficacy of wide area monitoring for protection and control.

Morphology dependence of the dielectric properties of epitaxial BaTiO3 films and epitaxial BaTiO3/SrTiO3 multilayers

Epitaxial BaTiO3 films and epitaxial BaTiO3/SrTiO3 multilayers were grown by pulsed laser deposition on vicinal surfaces of (001)-oriented Nb-doped SrTiO3 (SrTiO3:Nb) single-crystal substrates. Atomic force microscopy was used to investigate the surface topography of the deposited films. The morphology of the films, of the BaTiO3/SrTiO3 interfaces, and of the column boundaries was investigated by cross-sectional high-resolution transmission electron microscopy. Measurements of the dielectric properties were performed by comparing BaTiO3 films and BaTiO3/SrTiO3 multilayers of different numbers of individual layers, but equal overall thickness. The dielectric loss saturates for a thickness above 300 nm and linearly decreases with decreasing film thickness below a thickness of 75 nm. At the same thickness of 75 nm, the thickness dependence of the dielectric constant also exhibits a change in the linear slope both for BaTiO3 films and BaTiO3/SrTiO3 multilayers. This behaviour is explained by the change observed in the grain morphology at a thickness of 75 nm. For the thickness dependence of the dielectric constant, two phenomenological models are considered, viz. a 'series-capacitor' model and a 'dead-layer' model.

Microstructure and dielectric properties of epitaxial BaTiO3 films and BaTiO3/SrTiO3 multilayers

Epitaxial BaTiO3 films and BaTiO3/SrTiO3 multilayers were grown by pulsed laser deposition (PLD) on (001)-oriented Nb-doped SrTiO3 (SrTiO3:Nb) substrates. Measurements of the dielectric properties were performed comparing BaTiO3 films and BaTiO3/SrTiO3 multilayers of different number of individual layers, but equal overall thickness. The dielectric loss saturates for a thickness above 300 nm, and linearly decreases with decreasing film thickness below a thickness of 75 nm, and it is independent on the number of multilayers, pointing to some interface effect. The thickness dependence of the dielectric constant of BaTiO3 films and BaTiO3/SrTiO3 multilayers; exhibits a change in the linear slope at a thickness of 75 nm. This behavior is explained by the change observed in the morphology at a thickness of 75 nm. In order to explain the thickness dependence of the dielectric constant, two approaches are considered in this paper, viz. a "series capacitor" model and a "dead layer" model.

2D Monolayer MoS2–Carbon Interoverlapped Superstructure: Engineering Ideal Atomic Interface for Lithium Ion Storage: Engineering Ideal Atomic Interface for Lithium Ion Storage

A novel strategy for the controlled synthesis of 2D MoS<inf>2</inf>/C hybrid nanosheets consisting of the alternative layer-by-layer interoverlapped single-layer MoS<inf>2</inf> and mesoporous carbon (m-C) is demonstrated. Such special hybrid nanosheets with a maximized MoS<inf>2</inf>/m-C interface contact show very good performance for lithium-ion batteries in terms of high reversible capacity, excellent rate capability, and outstanding cycling stability.