Power Density Applications Research Articles

Model Based Comparative Analysis of GDC and YSZ Based Solid Oxide Fuel Cells

Designing solid oxide fuel cell (SOFCs) stacks for transportation applications places new challenges on SOFC architecture and operation with respect to power density, dynamic response, and thermal management. Recent developments in SOFCs with either thin-film yttria stabilized zirconia (YSZ) electrolytes (thickness ≤ 5 μm) or highly conductive, gadolinia-doped ceria (GDC) electrolytes (thicknesses ≥10 μm) enable thigh-power-density stack operation at inlet temperatures well below 700ºC with the potential for using compliant seals and lower cost ferrous interconnect materials. Applications, such as hybridized aircraft engines, can use such SOFC stacks if they meet strict requirements with respect to specific power and thermal management. Highly conductive GDC-based electrolytes offer pathways to high power-density SOFCs at operating temperatures below 650ºC, but the rise in mixed ionic electronic conductivity (MIEC) above 650ºC reduces open circuit voltages and thus fuel cell efficiency [1,2]. This necessitates thermal management to maintain operating temperatures and mitigate thermo-mechanical stresses within an SOFC. Thin-film YSZ electrolytes offer lower power densities below 650ºC, but avoid the penalties associated with MIEC behavior at higher temperatures and therefore reduce the challenges associated with thermal management at high power densities. In this work, a model-based approach is used to compare SOFC stack operation with thin-film YSZ electrolytes and with GDC-based electrolytes for operation on hydrogen and reformate fuels at high-power densities relevant for transportation applications.To calibrate models to experimental cell performance, polarization curves of a commercially available YSZ-based cell operating between 600-750ºC and of a high-power-density GDC cell operating between 500-650ºC are used to fit electrochemical model parameters for both electrolytes. Modeling of the YSZ based cell can neglect electronic charge transfer through the membrane electrolyte, but GDC-cell modeling must account for the multiple charge carriers (ceria polarons and oxygen vacancies) in the electrolyte [3]. Electrochemical parameters for ionic conductivities and electrochemical reaction rate parameters for oxide cathodes and nickel-electrolyte-composite anodes are fit to the experimental polarization data over the broad range of operating temperatures. Global rate expressions to account for internal reforming on the active nickel surfaces of anode are adopted from literature [4,5]. The calibrated cell model for each cell type is adopted to a down-the-channel discretized cell simulation to predict heat loss and temperature gradients at high current densities.This presentation compares GDC- and YSZ-based SOFCs in high power-density applications from fundamental and practical perspectives. The open-circuit voltages for GDC-based cells drops with increasing temperature to about 0.85 V/cell at 650°C which is lower than the Nernst open-circuit voltage of YSZ-based cells (≥ 1.05 V/cell) at similar conditions. Both types of cells can sustain power densities around 1.0 W/cm2 with high excess cathode air ratios ≥ 5 to keep temperature gradients near 10°C/cm. These results show that GDC-based cells support higher power densities at lower cell flow inlet temperatures < 600°C, whereas thin-film YSZ cells provide superior performance at flow inlets that drive cell temperatures > 600°C. Higher stack power densities > 1.0 W/cm2 must be supported by stack designs that alleviate high thermal stresses and allow steeper gradients of temperature. A summary of stack design and operational strategies that meet the stringent requirements of high power-density operation will be presented in the light of the modeling resuls..

An Overview of Active Electrode Materials for the Efficient High-Performance Supercapacitor Application

Recent energy research focuses on the efficiency enhancement of supercapacitor devices for multipurpose applications. Several materials have been used as electrode materials to achieve the maximum specific capacitance. The present review article concludes with three different types of materials recently used to enhance the efficiency of supercapacitors. The first type involves carbon-based materials for storage and supercapacitor applications. The carbon materials could be obtained naturally and synthesized manually based on need. The additional advantage of carbon material is these materials can be obtained from natural sources. The second type discusses the recent advances in metal oxide materials for high-performance supercapacitors. The metal oxide materials are involved in different types of attachment through the bi-tri metallic bonding, which enhances the specific capacitance. The third type involves recently advanced materials for high energy and power density application. The power and energy density of the materials is enhanced by the surface modification of the materials. In recent days, the MXene and nanocomposite materials seem to be an appropriate material to increase the power and energy density of the device. The modification and surface treatment of respective materials could enhance the specific capacitance of the material.

High Flashpoint and Eco-Friendly Electrolyte Solvent for Lithium-Ion Batteries

Since Sony launched the commercial lithium-ion cell in 1991, the composition of the liquid electrolytes has changed only slightly. The electrolyte consists of highly flammable solvents and thus poses a safety risk. Solid-state ion conductors, classified as non-combustible and safe, are being researched worldwide. However, they still have a long way to go before being available for commercial cells. As an alternative, this study presents glyceryl tributyrate (GTB) as a flame retardant and eco-friendly solvent for liquid electrolytes for lithium-ion cells. The remarkably high flashpoint (TFP=174°C) and the boiling point (TBP=287°C) of GTB are approximately 150 K higher than that of conventional linear carbonate components, such as ethyl methyl carbonate (EMC) or diethyl carbonate (DEC). The melting point (TMP=−75°C) is more than 100 K lower than that of ethylene carbonate (EC). A life cycle test of graphite/NCM with 1 M LiTFSI dissolved in GTB:EC (85:15 wt) achieved a Coulombic efficiency of above 99.6% and the remaining capacity resulted in 97% after 50 cycles (C/4) of testing. The flashpoint of the created electrolyte is TFP=172°C and, therefore, more than 130 K higher than that of state-of-the-art liquid electrolytes. Furthermore, no thermal runaway was observed during thermal abuse tests. Compared to the reference electrolyte LP40, the conductivity of the GTB-based is reduced, but the electrochemical stability is highly improved. GTB-based electrolytes are considered an interesting alternative for improving the thermal stability and safety of lithium-ion cells, especially in low power-density applications.

Highly conductive Ag-SiNx composite thin film anode engineering for transparent battery

Silicon nitride-based electrodes (SiNx), have been proposed as promising candidates for high-capacity transparent anode electrodes in earlier studies. Unfortunately, they have low electrical conductivity, resulting in some disadvantages such as power density for application in thin film batteries. To improve electrical conductivity, bulk batteries use conductive carbon additives, which also impair transmittance. Thus, we conducted this research to enhance the electrochemical properties by improving the conductivity without compromising the transmittance by uniformly dispersed Ag nanoparticles in SiNx thin film. We obtained an Ag-SiNx composite thin-film anode via continuous composition spread sputtering and investigated the effect of the Ag concentration. The capacity of the thin-film battery raised to 242.8 μAh/cm2·μm with Ag0.236SiO0.7N anode compared to the SiO0.7N anode which had a capacity of 37.2 μAh/cm2·μm. In addition, while SiO0.7N was operated only up to 2 C, Ag0.236SiO0.7N has driven up to 5 C and 10 C, which are high C rates, was also possible. AgxSiO0.7N thin film with less than 0.236 mol content demonstrates an optical transmittance of over 60% in the visible area. These results indicate that improving the electrical conductivity by the addition of an Ag during the silicon nitride deposition can increase its electrochemical performance while maintaining its applicability to transparent batteries.

High energy and power density achieved in Pb0.94La0.04HfO3 antiferroelectric ceramics with multiple phase transition

Antiferroelectrics (AFEs) with high energy density and fast discharge speed are greatly needed to improve the performance of capacitors. In the present work, La-doped PbHfO3 ceramics were fabricated and the antiferroelectricity was investigated. The complicated phase transition behavior at both high electric field and rising temperature in composition Pb0.94La0.04HfO3 was revealed. Multiple phase transition was observed at high field, and when rising the temperature to 110 °C, an intermediate phase transition to another AFE phase was observed. The Pb0.94La0.04HfO3 ceramics have a high recoverable energy density of 10.2 J/cm3 and efficiency of 81.7%. Double P–E loop could be observed even at 180 °C, ensuring the high energy density at high temperature. In addition, a high current density of 2109 A/cm2 and a power density of 369 MW/cm3 could be obtained. This work revealed the antiferroelectricity in La-doped PbHfO3 ceramics and proved their potential for high energy and power density applications.

A Simple Hydrothermal Synthesis of Cadmium Sulfide Wrapped on Graphene Nanocomposite for Supercapacitor Applications.

Supercapacitor with high specific capacity is desirable for various energy storage and high powerdensity applications. Though Graphene has been the preferred material for high current density, nanocomposites have been attempted to increase the specific capacitance. Hydrothermal synthesis of cadmium sulfide/graphene (CdS/G) nanocomposite with CdS nanoparticles anchored/decorated over the graphene sheets is reported. The structural studies reveal the hexagonal phase of the prepared materials. The specific surface area (BET) and porosity is found to increase upon nanocomposite formation. The electrochemical characteristics such as cyclic voltammetry (CV), GCD and EIS of the CdS/G nanocomposite have been investigated. The capacitance of CdS/G nanocomposite almost doubled to 248 Fg-1 indicating the enhanced performance of the nanocomposite system and in addition it also showed excellent cycling stability of 74.8 percent after 1000 cycles. The supercapacitor investigated retained the initial energy density after charge-discharge, at 0.5 A/g for 1000 cycles. The graphene nanosheets increased the specific surface area and interfacial electron transfer of the composite material. It enhances the specific capacitance and cyclic stability of the supercapacitor device.

High-Performance Triangular Miniaturized-LEDs for High Current and Power Density Applications

This work proposes an effective electrode length model and reveals for the first time the relationship between this model and the mesa shape effect. On the basis of this model, we demonstrate high-performance triangular miniaturized-LEDs (mini-LEDs) and benchmark to the conventional square, and circular shapes of the same mesa area. Here, we systematically study the impact of shaping in mini-LEDs both theoretically and experimentally, which is fundamentally different than that of the conventional regular-sized LEDs. We find that, at the current level of 200 mA, the triangular mini-LEDs deliver an enhancement of 36.4% in the optical output power and a decrease of 9.6% for the forward voltage compared to the commonly used square ones, and also an enhancement of 24.6% in the optical output power and a decrease of 14.3% for the forward voltage compared to the circular ones. The superior optical performance is proved to result from longer effective n-electrode length in the case of the triangular mini-LEDs, which suppresses the self-heating effect and thus well preserves the internal quantum efficiency, whereas the light extraction efficiency and the heat dissipation for the triangular shape are not significantly increased for such small mesa sizes, unlike conventional broad-area LEDs. Meanwhile, the reduced voltage is revealed to stem from the decreased n-GaN resistance. Different than conventional LEDs, these findings therefore indicate that the effective n-electrode length matters substantially for the miniaturized-LEDs.

PLZST antiferroelectric ceramics with promising energy storage and discharge performance for high power applications

AbstractCapacitors are widely used as energy storage elements in electric vehicles (EVs) and pulsed power. At present, it is still challenging to develop capacitor dielectrics with good energy storage and discharge performance. In this work, antiferroelectric (AFE) ceramics (Pb0.94La0.04)[(Zr0.6Sn0.4)0.92Ti0.08]O3 with enhanced antiferroelectricity were fabricated by a rolling process. The obtained ceramics have a high recoverable energy density of 5.2 J/cm3 and an extremely high efficiency of 91.2% at 327 kV/cm. The ceramics have good energy storage and discharge performance in the temperature range from −40°C to 100°C due to the existence of AFE phase. An energy density of 3.7 J/cm3 can be released at 200 kV/cm in less than 500 ns and the discharge current keeps stable after 1000 charge‐discharge cycles. By direct short experiment, a current density of 1657 A/cm2, which is the highest result in recently developed AFE ceramics, and a power density of 228 MW/cm3 were achieved. The possibility of using AFEs at low temperature was confirmed. The excellent energy storage and discharge performance prove the great potential of the obtained ceramics in high energy and power density applications.

Investigation of Magnetic Field Immunity and Near Magnetic Field Reduction for the Inductors in High Power Density Design

This paper investigates the magnetic field immunity and near magnetic field reduction for different inductor-winding structures. A double-twisted winding structure, which includes two magnetic cores, is proposed for common mode (CM) inductors. The proposed CM inductor can achieve much lower near magnetic field emission, much better magnetic field immunity against the external magnetic field, and much bigger differential mode (DM) impedance than conventional winding structures. The proposed inductor structures are good for high power-density applications. Theoretical analysis, magnetic three-dimensional simulations and experiments were conducted to validate the developed theory and technique.

Comsol Simulation of Hierarchical Ordered Porous Microstructure Electrode

High specific energy is always desirable in battery systems, especially for electric vehicles. A key towards improving specific energy for a particular combination of anode and cathode active material is to maximize the mass fraction of active material, which means minimizing the fraction of inactive material such as the separator, the binder, electrode additives for improving electronic conductivity, and the current collector. A seemingly simple strategy for maximizing the mass fraction of active materials is to make the electrodes as thick as possible. However, ion transport in thick electrodes can be particularly problematic since salt diffusion into and through the electrode pore space is slow and can easily limit the battery charge / discharge rate. An ideal microstructure is one that allows for direct line of sight or path for the electrode active material access to the electrolyte, for rapid ion-transport throughout the entire electrode thickness. Therefore, a need exists for electrode fabrication methods that create microstructures that maximize electrolyte transport, as is needed to achieve high-rate charging/discharging in electrodes having high mass loading. Optimal microstructures in electrodes help to achieve the ideal balance of active material and electrolyte mass to give high gravimetric and volumetric energy density. As to design a microstructure with the goal of Li ion salt transport facilitation, we synthesized ordered porous microstructure electrodes using combination of freeze cast and tape cast techniques. This method enjoys the combination of conventional tape casting, a main technique in manufacturing Li ion electrodes in industrial scale, and freeze casting. This study simulate two dimensional ordered porous electrode cross-sections and presents the electrochemical performance results of Mo-doped LTO in a electrochemical cell consist of a lithium metal anode, polymer separator, and liquid ethylene carbonate and ethyl methyl carbonate electrolyte. The electrochemical performance of ordered porous microstructure is investigated by varying electronic conductivity, tortuosity, particle size, electrode thickness, and C rate. We show that these engineered porous electrodes considerably reduces the electrolytes mass transport limitation and electrochemical response of electrode is independent of tortuosity, making it possible to synthesize ultra thick electrode for high energy and power density applications. Simulation results show that 10 micron thickness of wall and pore freeze tape cast Mo-doped LTO with electronic conductivity of 1 S/m, tortuosity as high as 10, electrode thickness of 458 micron and mass loading of 30 mg/cm^2 can deliver full capacity even at 2 C-rate.

(Battery Division Technology Award) Insight of Structures and Properties of Cathode Materials for Li-ion Battery

Insight into relationship between Crystal/Interface structure and properties of capacity, stability and rate capability are important for developing advanced Li-ion batteries. Using theoretical calculations combined with experimental in-situ tests, we did extensive studies on the kinetic of Li-ion diffusion for two representative cathode materials: layered Li(NixMnyCoz)O2 (NMC) (x + y + z = 1) and LiFePO4. We not only focus on the bulk kinetics, but also the kinetics across electrode/electrolyte solid-liquid interface and in the electrolytes. For example, we first proposed that "Janus" solid-liquid interface would facilitate the Li-ion transport in battery and introducing some disordering in non-active cathode materials would activate them for Li-ion storage.（Ref. 1） For high energy and power density applications (e.g., EVs), the safety becomes especially important. Using ab initio calculations combined with experiments, we clarified how the thermal stability of NMC materials can be tuned by the most unstable oxygen, which is determined by the local coordination structure unit (LCSU) of oxygen (TM(Ni, Mn, Co)3-O-Li3-x’): each O atom bonds with three of transition metal (TM) from the TM-layer and three to zero of Li from fully discharged to charged states from the Li-layer. Under this model, how the lithium content, valence states of Ni, contents of Ni, Mn, and Co, and Ni/Li disorder to tune the thermal stability of NMC materials by affecting the sites, content, and the release temperature of the most unstable oxygen is proposed. In NMC, insight of Ni/Li disorder has be investigated by theory and experimental e.g. neutron powder diffraction experiments and magnetization measurements. The spins of TM ions construct a two-dimensional triangular networks, which can be considered as a simple case of geometrical frustration. Remarkably, the frustration parameters of these compounds are estimated to be larger than 30, indicating the existence of strongly frustrated magnetic interactions between spins of TM ions, which give rise to lattice instability, the formation of Li/Ni exchange in NMC will help to partially relieve the degeneracy of the frustrated magnetic lattice by forming a stable antiferromagnetic state in hexagonal sublattice with nonmagnetic ions located in centers of the hexagons. Moreover, Li/Ni exchange will introduce 180° superexchange interaction, which further relieves the magnetic frustration through bringing in new exchange paths. Thus, the variation of Li/Ni exchange ratio vs. TM mole fraction in NMC with different compositions can be well understood and predicted in terms of magnetic frustration and superexchange interactions. (Ref. 2) Ref. (a)F. Pan el al., “Kinetics Tuning of Li-ion Diffusion in Layered Li(NixMnyCoz)O2”, J. Am. Chem. Soc., 2015, 137, 8364; (b) "Optimized Temperature Effect of Li-Ion Diffusion with Layer Distance in Li(NixMnyCoz)O2 Cathode Materials for High Performance Li-Ion Battery", Adv. Energy Mater., 2015, 1501309(1-9).(c) "Janus Solid–Liquid Interface Enabling Ultrahigh Charging and Discharging Rate for Advanced Lithium-Ion Batteries", Nano Lett,2015, 15 (9), pp 6102 (d)“Single-particle performances and properties of LiFePO4 nanocrystals for Li-ion batteries” Energy Mater., (Front page ) 2016, 1601894 (e) Nano Lett, 2017, 17, 6018 (f) Nano Energy 2017 37, 90 (g) Nano Lett., 2017, 17 (8), 4934(a) F. Pan el al., “Tuning of Thermal Stability in Layered Li(NixMnyCoz)O2”, J. Am. Chem. Soc., 2016, 138 (40), 13326, (b)“Aligned Li+ Tunnels in Core−Shell Li(NixMnyCoz)O2@LiFePO4 Enhances Its High Voltage Cycling Stability as Li-ion Battery Cathode” Nano Letters 2016, 16 (10), pp 6357; (c) Nano Letters, 2015, 15, 5590; （d）The Role of Super-Exchange Interaction on Tuning of Ni/Li Disordering in Layered Li (NixMnyCoz) O2, J. Phys. Chem. Lett., 2017, 8 (22), 5537; (e) Insight into the origin of Lithium/Nickel ions exchange in layered Li(NixMnyCoz)O2 cathode materials, Nano Energy 49 2018,49,77

Effect of vanadium doping on electrochemical properties of carbon coated nano lithium iron phosphate for high rate applications

Lithium iron phosphate (LiFePO4) is a promising electrode material for the lithium ion battery technology as it has the potential to meet the requirements of the high energy density and power density applications. However, its limitations such as low conductivity and a low diffusion coefficient lead to high impedance due to which its application is restricted. The cationic substitution of vanadium in the native LiFePO4/C is one of the methods to enhance the conductivity. Here the work is focused on obtaining a cathode material with improved electrical performance by doping it with different compositions of vanadium in the lithium site of LiFePO4/C. Nano powders of LiFePO4/C and Li1–x V x FePO4/C (x = 0.05, 0.1, 0.15–0.2) were synthesized from precursors via the solid state vacuum synthesis. The solid solution formation up to Li1–x V x FePO4/C (x = 0.2) without impurity phases is confirmed by the XRD and FTIR results. The particles distribution of synthesized powders was followed by the SEM. Also, dc conductivities are measured on sintered pellets and activation energies are calculated using the Arrhenius equation. The dependence of conductivity and activation energy of LiFePO4/C on variation of vanadium doping is also investigated in this study. The CR2032 cells were fabricated and subjected to cyclic voltammetry studies, charge-discharges at different C-rates and cycle life. The electrochemical impedance spectroscopy was used to obtain the ac impedance and diffusion coefficients, and the effect of doping on these parameters is discussed. It was observed that the room temperature dc conductivity was improved by vanadium doping when compared to LiFePO4/C (2.141 × 10–2 S cm–1) and it was maximum for Li0.85V0.15FePO4/C (22.201 × 10–2 S cm–1). It was also observed that the diffusion coefficient of Li+ in Li0.85V0.15FePO4/C (12.89 × 10–9 cm2 s–1) improved by two orders of magnitude as compared to that of pure LiFePO4 (10–12 cm2 s–1) and carbon coated nano LiFePO4/C (0.626 × 10–11 cm2 s–1). Cells with Li0.85V0.15FePO4/C are able to deliver useful capacity of around 101 mAh g–1 at 10C rate. More than 1000 cycles were achieved with Li0.85V0.15FePO4/C at 20C rate.

Functionalized, hierarchical and ordered mesoporous carbons for high-performance supercapacitors

Two new ordered mesoporous carbons (OMCs) exhibiting hierarchical structures were obtained through a nanocasting route from nitrogen and oxygen-containing precursors without any additional activation post-treatment. The combination of properties, such as high mesopore volumes, synergetic pseudocapacitive effect of nitrogen and oxygen groups, good wettability, low electrical resistance, low tortuosity of the straight channels and high structural order of the carbon arrangement explains the excellent electrochemical performances of both materials as supercapacitor electrodes. They exhibited excellent rate capability and high specific capacitance (∼390 F g−1 at 0.5 mV s−1). Moreover, the studied materials, M900 and P900, showed energy densities of 54.6–19.6 W h kg−1 and 60.3–22.2 W h kg−1, respectively, under power outputs of 0.54–81.3 kW kg−1 and 0.71–92.5 kW kg−1, which make them excellent candidates in high energy and power density applications.

A Four-Phase High Voltage Conversion Ratio Bidirectional DC-DC Converter for Battery Applications

This study presents a four-phase interleaved high voltage conversion ratio bidirectional DC-DC converter circuit based on coupled inductors and switched capacitors, which can eliminate the defects of conventional high voltage conversion ratio bidirectional DC-DC converters in terms of high-voltage/current stress, less efficiency and low-power limitation. Parallel channels are used to reduce current stress at the low-voltage side and series connected switched capacitors are used to enlarge voltage conversion ratio, reduce voltage stress and achieve auto current sharing. This paper proposes the operation principle, feature analysis and optimization design considerations. On this basis the objectives of high voltage conversion ratio, low voltage/current stress, high power density, high efficiency and high-power applications can be achieved. Some experimental results based on a 500 W prototype converter (24 V to 48 V at low-voltage side, 400 V at high-voltage side) are given to verify the theoretical analysis and the effectiveness of the proposed converter.

Improvement in Power Density of Rechargeable Air Battery using Hydrogen Storage Alloy

AbstractA novel class of secondary battery comprising MH and air electrodes was developed for potential uses in high power density and high energy density applications such as electric or hybrid vehicles and power storage units supporting fuel cell and solar power systems. The air electrode consisted of nickel-based gas diffusion electrode using Ir2Bi2O7-zas oxygen evolution and reduction catalyst. Coin-type of cells using alkaline solutions as electrolyte were designed and fabricated, and the charge-discharge behaviors were evaluated with constant current operation. The discharge voltage and power density were improved by using a thin film membrane, in which the electrolyte was impregnated, between the air and MH electrodes, and the maximum power density was comparable to that of commercially available Ni-MH secondary battery. The MH utilization and the current efficiency of a charge-discharge cycle were found to be more than 90%.

Predicting the Thermal Conductivity of Low Temperature Co-fired Ceramic with Embedded Thermal via Arrays using the Rule-of-Mixtures and Interface Area Analytical Methods

Increased power-density applications especially for front-end modules have created the need for adequate thermal management in microwave design. Although low-temperature co-fired ceramics (LTCCs) have a lower thermal resistance than PCB substrates, it is still desirable to lower the resistance further by introducing thermal via arrays. A thermal imaging system using an IR camera was used to measure the top surface temperature over a range of powers with samples that had varying via sizes and densities. The empirical data results from this study were compared with conventional and an interface area method. The conventional method used to compare the experimental results to predicated results is the simple rule-of-mixtures. The fact that the thermal conductivity and diffusivity of the filled via are so much greater than those of the LTCC material means that another method of theoretically calculating the resulting thermal conductivity can be used by studying the thermal path. The ceramic–conductor interface area is also calculated to predict material system conductivity and compared with the conventional method and experiments.

A Study on Oxidized Glassy Carbon sheets for Bipolar Supercapacitor Electrodes

ABSTRACTElectrochemical Double Layer Capacitors (EDLC) for high energy and power density applications, based on glassy carbon (GO) electrodes, are being developed in our laboratory. In the context of this project, GC sheets were oxidized and investigated with Small Angle X-ray Scattering (SAXS), Electrochemical Impedance Spectroscopy (EIS) and Nitrogen Gas Adsorption (BET). During oxidation an active film with open pores is built on the surface of the GC. Upon oxidation, the internal volumetric surface area of the active film decreases, whereas the volumetric electrochemical double layer capacitance increases. We show that this effect is correlated with the opening, the growth and the coalescence of the pores.

Single-stage current-fed DC-DC converter with time-sharing control of output voltage and input current

A single-stage current-fed DC/DC power converter structure and control strategy for simultaneous control of output voltage and current flowing in the input inductor are described. This configuration is suitable for high power-density applications because it results in reduced converter size and power loss. A very fast control-to-output response is obtained, together with stable operation even for large load and reference variations. Static and dynamic converter performances are investigated and design criteria given. Experimental results obtained on a prototype are also reported.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Electrothermal considerations in display applications of light-emitting diodes

Some of the possibilities and problems of high-density integration and packaging of light-emitting diode displays are explored, using a new electrothermal characterization of light-emitting diodes. After a brief discussion of thermal effects in spontaneous light-emitting diodes, the characterization is derived. It reveals a peak in luminance with increasing electric power due to self heating. Experimental results confirm the accuracy of the characterization for gallium arsenide phosphide monolithic displays. Finally, the characterization is used to evaluate some projected high power-density applications. These particularly involve dense diode arrays and the use of higher levels of integration, together with magnification by projection of an image onto a screen, to reduce the cost per element associated with processed chip area. Self heating is found to cause a strong dependence of the luminance on the number of elements powered in a dense array, and to limit severely the achievable magnification even with improved packaging.