Reduction In Power Density Research Articles

Reverse electrodialysis with saline waters and concentrated brines: A laboratory investigation towards technology scale-up

The use of concentrated brines and brackish water as feed solutions in reverse electrodialysis represents a valuable alternative to the use of river/sea water, allowing the enhancement of power output through the increase of driving force and reduction of internal stack resistance. Apart from a number of theoretical works, very few experimental investigations have been performed so far to explore this possibility.In the present work, two RED units of different size were tested using artificial saline solutions. The effects of feed concentration, temperature and flowrate on process performance parameters were analysed, adopting two different sets of membranes. These experiments allowed to identify the most favourable conditions for maximising the power output within the presently investigated range, i.e. 0.1M NaCl as diluate and 5M NaCl as concentrate at 40°C. Under these conditions a power density equal to 12W/m2cell_pair was reached, among the highest so far reported in the literature. Increasing the unit size a slight reduction in power density was observed. These results indicate new directions for a successful scale-up and development of the Reverse Electrodialysis technology.

Effect of Polytetrafluoroethylene on Ultra-Low Platinum Loaded Electrospun/Electrosprayed Electrodes in Proton Exchange Membrane Fuel Cells

In this study, catalyst layers (CLs) were fabricated using a simultaneous electrospinning/electrospraying (E/E) technique to produce unique nanofiber/nanoparticle membrane electrolyte assemblies (E/E MEAs) evidenced by scanning electron microscopy. Specifically, the effect of polytetrafluoroethylene (PTFE) in these E/E MEAs on polymer electrolyte membrane (PEM) fuel cell performance was evaluated. E/E MEAs result in high fuel cell performance at ultra-low platinum (Pt) loadings with higher electrochemical surface areas as evidenced by cyclic voltammetry experiments. Without PTFE, an E/E MEA operated at 172kPa (25 psi) back pressure results in a maximum power density of 1.090W/cm2 (H2/O2) and 0.647W/cm2 (H2/air) with only 0.112mgPt/cm2 total Pt MEA loading. Introducing PTFE (at only 1 wt%) to the electrospinning process results in an E/E MEA operated at the same back pressure (172kPa (25 psi)) with an even higher maximum power density of 1.240W/cm2 (H2/O2) and 0.725W/cm2 (H2/air) at a lower total Pt MEA loading of 0.094mgPt/cm2. This corresponds to a significant reduction in Pt loading (16% of control) with only a modest reduction in power density (∼86-87% of control), where the control MEA was produced using a conventional coating method and resulted in maximum power density of 1.420W/cm2 (H2/O2) and 0.839W/cm2 (H2/air) at a Pt MEA loading of 0.570mgPt/cm2 (172kPa (25 psi)). An excellent total MEA platinum utilization of 0.076gPt/kW (∼13.2kW/gPt) was achieved with the E/E MEA with PTFE at only a 0.094mgPt/cm2 total Pt MEA loading. The improvement in E/E MEA with PTFE was a result of increased hydrophobicity of the nanofibers evidenced by contact angle measurements and improved PEM fuel cell performance at higher limiting current density in the mass transport region.

Ultra-low platinum loadings in polymer electrolyte membrane fuel cell electrodes fabricated via simultaneous electrospinning/electrospraying method

In this study, membrane electrode assemblies (MEAs) were fabricated using a simultaneous electrospinning/electrospraying (E/E) technique to produce a unique nanoparticle/nanofiber cathode catalyst layer morphology evidenced by scanning electron microscopy. H2/O2 and H2/air polymer electrolyte membrane fuel cell performance was evaluated for E/E MEAs at ultra-low Pt cathode loadings of 0.052 and 0.022 mgPt cm−2, where maximum power densities of 1.090 and 0.936 W cm−2 (H2/O2) and 0.656 and 0.625 W cm−2 (H2/air) were achieved at these two Pt loadings, respectively. This was compared to a conventional control MEA at a 0.42 mgPt cm−2 cathode catalyst loading with maximum power densities of 1.420 and 0.839 W cm−2 for H2/O2 and H2/air, respectively. These results correspond to a significant reduction in Pt loading (5–12% of control) at only a modest reduction in power density (∼66–78% of control) for the E/E electrodes. Excellent platinum utilization in the cathode of 0.024 gPt kW−1 (∼42 kW gPt−1) was achieved for the E/E electrode at 0.022 mgPt cm−2 cathode loading. Cyclic voltammetry results show an electrochemical surface area higher in the E/E electrodes compared to the control, which provides a rationale for the excellent platinum utilization results, where the E/E morphology results in more triple phase boundaries with more accessible Pt in the electrode.

Study of Hetero-Tunneling Gfet with an Ultra-Shallow Pocket Junction

Because the subthreshold swing (SS) of a MOSFET is larger than 60mV/dec at room temperature, its threshold voltage cannot be very small. This limits the scaling of power supply voltage (VDD) and the reduction of power density. Tunnel field effect transistor (TFET) can overcome this fundamental limitation, which mechanism is based in the band-to-band tunneling (BTBT). However, the drive current in TFETs is significantly lower than in MOSFETs owing to tunneling resistance which arises due to carrier tunneling. Achieving large drive current at low VDDhas been a great challenge. Green transistor (gFET) is a novel structure TFET with a heavily doped pocket in the gate-source overlap region. The pocket junction can provide large and uniform field over a substantial area, which can offer larger drive current and steeper subthreshold swing as compared to conventional tunneling transistors. And, when the pocket junction depth becomes shallower, the drive current will be improved significantly. But study reveal that gFET is only suitable for orepation at 0.5V. To further reduce Vdd, even smaller bandgap material may be employed.In this study, a hetero-tunneling gFET with an ultra-shallow pocket junction has been developed, as shown in Fig.1. The gFET was fabricated in a 8inch (100) silicon (Si) substrate, as shown in Fig.2. After LOCOS formation, silicon-germanium (Si1-xGex) and Si cap layer were grown on Si substrate using selective epitaxy. After source implantation, an ultra-shallower pocket is formed by using of the Ge preamorphization implantation (PAI), ultra low energy implantation and spike annealing. Si1-xGex and Si have different lattice constant. Biaxially strained will be formed in the Si1-xGex/Si heterostructures. The biaxial strain will shift the conduction and valance band energy levels of semiconductors and obtain ultra low effective tunneling bandgaps, which is helpful to get smaller tunneling length and barrier height. The same current can be obtained at lower gate voltage and drain voltage. The hetero-tunneling gFET integrated with the ultra-shallow pocket junction exhibits good electrical characteristics, such as smaller subthreshold swing, higher Ion/Ioff ratio and so on.

Enhancing SOFC-Stack Performance by Model-Based Adaptation of Cathode Gas Transport Conditions

High performance anode–supported cells (ASC), contacted in a planar stack by metallic interconnectors (MIC), undergo a significant power density reduction. Performance limiting factors originating from the MIC-design were identified by detailed experimental data analysis and via a straightforward FEM simulation. Conclusively, gas diffusion polarisation contributes close to the sum of all ohmic losses to the overall polarization, both controlling stack performance. Based on this knowledge, a multiphysic-FEM-model was developed, considering coupled ohmic, gas diffusion and nonlinear polarization losses in the electrodes and ohmic losses in the electrolyte. Consequential ASC-performance depends over a broad range of operating conditions on i) an optimal MIC-design and ii) a well-chosen cathode thickness increases the overall power output.

Nanostructured Pd barrier for low methanol crossover DMFC

This work reports the successful fabrication of Pd nanostructured proton conducting-fluid blocking barriers on standard perfluorosulfonic acid membranes, able to reduce considerably methanol crossover in Direct Methanol Fuel Cells. The barrier nanostructure and grafting to the underlying membrane was optimized by fine-tuning the deposition conditions. A reduction in methanol crossover up to 50% was achieved, increasing significantly the fuel cell efficiency, admitting a slight power density reduction with respect to a reference cell. The developed barriers hindered methanol transport through the membrane as well as water transport, therefore membrane dehydration can be observed at low current density, if operating conditions are not suitably modified.

A 0.25 V 460 nW Asynchronous Neural Signal Processor With Inherent Leakage Suppression

Further power and energy reductions via technology and voltage scaling have become extremely difficult due to leakage and variability issues. In this paper, we present a robust and energy-efficient computation architecture exploiting an asynchronous timing strategy to dynamically minimize leakage and to self-adapt to process variations and different operating conditions. Based on a logic topology with built-in leakage suppression, the prototype asynchronous neural signal processor demonstrates robust sub-threshold operation down to 0.25 V, while consuming only 460 nW in 0.03 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">${\rm mm}^{2}$</tex></formula> in a 65 nm CMOS technology. These results represent a 4.4 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\times$</tex> </formula> reduction in power, a 3.7 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\times$</tex> </formula> reduction in energy and a 2.2 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\times$</tex></formula> reduction in power density, when compared to the state-of-the-art processors.

Scale up considerations for sediment microbial fuel cells

Scale-up of sediment microbial fuel cells (SMFCs) is important to generating practical levels of power for undersea devices. Sustained operation of many sensors and communications systems require power in the range of 0.6 mW to 20 W. Small scale SMFC systems evaluated primarily in the laboratory indicate power densities for typical graphite plate anodes on the order of 10–50 mW m−2. However, previous work also suggests that SMFC power production may not scale directly with size. Here, we describe a combination of lab and field studies to evaluate scale up for carbon fabric anodes with a projected surface area ranging from 25 cm2 to 12 m2. The results indicate that power generation scales almost linearly with anode size up to about 1–2 m2 of projected surface area. Our model suggests that anodes larger than this can experience significant reduction in power density, confirming laboratory observations. These results suggest that the majority of losses along the anode surface occur closest to the electronics, where the amount of current passing along an anode is the greatest. A multi-anode approach is discussed for SMFCs, suggesting that scale-up can be achieved using segmented anode arrays.

A comparison of direct methanol fuel cell degradation under different modes of operation

The degradation test of direct methanol fuel cells (DMFCs) under on–off operation with variable load profile, known as a variable load on-off operation (VLOO) mode, was compared to continuous operation with a constant load, known as a constant load continuous operation (CLCO) mode. It was found that the performance loss in the VLOO mode was higher than that under the CLCO mode in terms of open circuit voltage drop (3.6 times higher) and the maximum power density reduction rate (1.23 times greater). Under both operation modes, the anode catalyst was deteriorated by Ru dissolution. However, the faster decay of cell performance under VLOO mode was mainly caused by the increase of cathode polarization due to higher methanol crossover and the decrease of Pt activity to the oxygen reduction reaction induced by Ru deposition.

Power density of various light curing units through resin inlays with modified layer thickness

ObjectivesThe purpose of this study was to enhance curing light penetration through resin inlays by modifying the thicknesses of the dentin, enamel, and translucent layers.Materials and MethodsTo investigate the layer dominantly affecting the power density of light curing units, resin wafers of each layer with 0.5 mm thickness were prepared and power density through resin wafers was measured with a dental radiometer (Cure Rite, Kerr). The dentin layer, which had the dominant effect on power density reduction, was decreased in thickness from 0.5 to 0.1 mm while thickness of the enamel layer was kept unchanged at 0.5 mm and thickness of the translucent layer was increased from 0.5 to 0.9 mm and vice versa, in order to maintain the total thickness of 1.5 mm of the resin inlay. Power density of various light curing units through resin inlays was measured.ResultsPower density measured through 0.5 mm resin wafers decreased more significantly with the dentin layer than with the enamel and translucent layers (p < 0.05). Power density through 1.5 mm resin inlays increased when the dentin layer thickness was reduced and the enamel or translucent layer thickness was increased. The highest power density was recorded with dentin layer thickness of 0.1 mm and increased translucent layer thickness in all light curing units.ConclusionsTo enhance the power density through resin inlays, reducing the dentin layer thickness and increasing the translucent layer thickness would be recommendable when fabricating resin inlays.

A Single Layer Glucose Fuel Cell Intended as Power Supplying Coating for Medical Implants

AbstractWe present a novel type of abiotically catalysed implantable glucose fuel cell with anode and cathode placed side by side, using a Raney‐platinum glucose oxidation anode with high tolerance towards oxygen. In contrast to conventional assembly designs used for implantable glucose fuel cells, no permeable cathode mounted in front of the anode to effect oxygen depletion is required. At 2.2 ± 0.1 μW cm–2 the single layer fuel cell exhibits only half the maximum power density of the conventional fuel cell, which solely stems from the doubled total fuel cell area demand. Nevertheless, the novel single layer design is advantageous in terms of simplified fabrication and reduced overall thickness, facilitating implementation of the fuel cell as a power supplying coating directly on the surface of medical implants. Furthermore, the single layer design offers an attractive possibility to diminish the reduction of power density by limited oxygen mass transfer to the cathode by increasing the cathode to anode area proportion. With doubled cathode area proportion a by 36% higher power density can be reached. To calculate the optimum cathode to anode area proportions, we introduced an analytical model based on the experimentally determined polarisation resistances of the individual electrodes.

Electrochemical Performance and Stability of the Cathode for Solid Oxide Fuel Cells: III. Role of Volatile Boron Species on LSM/YSZ and LSCF

Boron oxide is a key component to tailor the softening temperature and viscosity of the sealing glass for solid oxide fuel cells (SOFCs). The primary concern regarding the use of boron-containing sealing glasses is the volatility of boron species, which possibly results in cathode degradation. In this paper, we report the role of volatile boron species on the electrochemical performance of LSM/yttria-stabilized zirconia (YSZ) and LSCF cathodes at various SOFC operation temperatures. The transport rate of boron, was measured at with air saturated with moisture. A reduction in power density was observed in the cells with the LSM/YSZ cathodes after the introduction of boron source to the cathode air stream. A partial recovery of the power density was observed after the boron source was removed. Results from post-test secondary-ion mass spectroscopy (SIMS) analysis showed that the partial recovery in the power density correlated with the partial removal of the deposited boron by the clean air stream. The presence of boron was also observed in the LSCF cathodes by SIMS analysis; however, the effect of boron on the electrochemical performance of the LSCF cathode was negligible. The coverage of triple phase boundaries in LSM/YSZ was postulated as the cause for the observed reduction in the electrochemical performance.

Tunneling MOSFETs Based on III-V Staggered Heterojunctions

AbstractA critical problem for the progression of CMOS electronics to the nanoscale is the reduction of power density, while at the same time preserving high speed performance. One of the most promising approaches is to aggressively reduce the power supply voltage by using a novel device, the tunneling MOSFET (TMOSFET), which is a MOSFET that operates by tunnel-injection of carriers from source to channel, rather than by conventional thermionic emission. TMOSFETs benefit from steep (sub-60mV/dec) gate turn-on characteristics. In this paper we show that TMOSFET designs based on staggered heterojunctions are particularly promising, since the choice of materials for the injector (source) and channel allows optimization of the tunneling probability at the heterojunction. Analysis and simulation of MOSFETs based on the GaAlSb / InGaAs material system are presented. The energy offset between the valence band of the injector and the conduction band of the channel at the heterojunction can be tailored over a wide range, from negative values (“offset” band lineup) to values in excess of 1eV. We find by simulation that for optimal values of effective heterojunction bandgap near 0.2eV, the resulting MOSFETs are capable of delivering &gt;0.5mA/mm while maintaining on-off ratio greater than four orders of magnitude over voltage swing of 0.3V. We also discuss a variety of materials-related challenges that must be overcome to realize the predicted performance. Among these are the need to provide near ideal heterojunctions between the materials, employ high K dielectrics with very low interface state density, and achieve good alignment between the gate and the heterojunction. Different configurations for the tunneling MOSFETs are presented.

Complaint-adaptive power density optimization as a tool for HTP-guided steering in deep hyperthermia treatment of pelvic tumors

For an efficient clinical use of HTP (hyperthermia treatment planning), optimization methods are needed. In this study, a complaint-adaptive PD (power density) optimization as a tool for HTP-guided steering in deep hyperthermia of pelvic tumors is developed and tested. PD distribution in patients is predicted using FE-models. Two goal functions, Opt1 and Opt2, are applied to optimize PD distributions. Optimization consists of three steps: initial optimization, adaptive optimization after a first complaint and increasing the weight of a region after recurring complaints. Opt1 initially considers only target PD whereas Opt2 also takes into account hot spots. After patient complaints though, both limit PD in a region. Opt1 and Opt2 are evaluated in a phantom test, using patient models and during hyperthermia treatment. The phantom test and a sensitivity study in ten patient models, show that HTP-guided steering is most effective in peripheral complaint regions. Clinical evaluation in two groups of five patients shows that time between complaints is longer using Opt2 (p = 0.007). However, this does not lead to significantly different temperatures (T50s of 40.3 (Opt1) versus 40.1 °C (Opt2) (p = 0.898)). HTP-guided steering is feasible in terms of PD reduction in complaint regions and in time consumption. Opt2 is preferable in future use, because of better complaint reduction and control.

Citalopram: differential sleep/wake and EEG power spectrum effects after single dose and chronic administration

The sleep/wake effects of the selective serotonin re-uptake inhibitor citalopram were studied in both a single-dose study with three dose levels (0.5, 2.0 and 5.0 mg/kg), and a 5-week chronic administration study (15 mg/kg/24 h). Single doses of citalopram resulted in a dose-dependent inhibition of rapid eye movement (REM) sleep. After chronic citalopram treatment there was a sustained REM sleep inhibition. Single doses of citalopram resulted in only minor changes in non-REM (NREM) sleep as well as in NREM EEG power spectral density. Chronic administration resulted in a major shift from SWS-2 to SWS-1. The observed corresponding changes in EEG power density were regional. A 30 to 40 percent reduction of power density in the 0.5–15 Hz range in the fronto-parietal EEG derivation was seen for the whole 8-h registration period. In the fronto-frontal EEG derivation only minor changes were seen. A decreasing trend in NREM sleep power density between 0.5 and 7 Hz, usually seen during the course of the light period, was not observed in the chronic condition, but was seen in control and single-dose condition, suggesting altered diurnal distribution of slow wave activity in the chronic condition. The data indicate that acute and chronic administration of citalopram shows clear differences in sleep effect, which may be caused by alteration of serotonergic transmission, and may be related to the antidepressant effect

Long lifetime operation of an ArF-excimer laser

An experimental investigation to improve the lifetime of a discharge-excited ArF-excimer laser is presented. The three dominant factors restricting its lifetime are CF4 generation in the laser gas, color-center formation in the optics and input power density reduction due to electrode ablation. Copper electrodes were superior to nickel electrodes in regard to electrode ablation. A gas lifetime of more than 109 shots (about one month at 400 Hz) is shown for an ArF-excimer laser with a liquid-nitrogen trap and high-temperature zirconium alloy trap.

Sleep and EEG power spectrum effects of the 5-HT 1A antagonist NAN-190 alone and in combination with citalopram

The sleep and waking and EEG power spectrum effects of the putative 5-HT 1A antagonist NAN-190 (0.5 mg/kg, i.p.) were studied alone and in co-administration with the selective serotonin re-uptake inhibitor citalopram (5.0 mg/kg, i.p.) in the rat. Citalopram, as in a prior dose-response study, reduced REM sleep. In addition, a slight increase in NREM sleep was observed. Citalopram reduced NREM fronto-parietal (FP) EEG power density in the 5–20 Hz range. When administered alone, NAN-190 suppressed REM sleep in the first 2 h, and reduced SWS-2 in the first 4 h after administration. NAN-190 also suppressed selectively NREM sleep slow-wave activity in both fronto-frontal (FF) and FP EEG power spectrum. When administrated in combination with citalopram, an attenuation of the power density reduction in the 7–15 Hz range in the FF EEG of citalopram alone, was observed. However, the EEG power spectral density and REM sleep suppressive effects of NAN-190 were both augmented. The results are compatible with the notion that serotonin is involved in the modulation of the slow wave activity in the EEG during NREM sleep. The results are cordant with other data suggesting that postsynaptic 5-HT 1A stimulation might increase slow wave activity in the NREM EEG, and that serotonergic stimulation of other receptor subtypes (possibly 5-HT 2) may decrease slow wave activity in the NREM EEG.

Changes in sleep and sleep electroencephalogram during pregnancy.

The impairment of sleep quality is a common complaint during pregnancy. To investigate the changes in sleep in the course of pregnancy, the sleep electroencephalogram (EEG) was recorded and analyzed in nine healthy women on 2 consecutive nights during each trimester of pregnancy. Waking after sleep onset increased from the second (TR2) to the third (TR3) trimester, whereas rapid eye movement (REM) sleep decreased from the first trimester (TR1) to TR2. Spectral analysis of the EEG in nonrapid eye movement (NREM) sleep revealed a progressive reduction of power density in the course of pregnancy. In comparison to TR1, the values in TR2 were significantly lower in the 10.25-11.0-Hz and 14.25-17.0-Hz bands. In TR3, the significant reduction extended over the ranges of 1.25-12.0 Hz and 13.25-16.0 Hz. The largest decrease (30%) occurred in the 14.25-15.0-Hz band. In REM sleep, the spindle frequency range was not affected, and a minor reduction of power density in some frequency bins below 12 Hz was present only in TR3. The study documents major alterations of the sleep EEG that are not evident from the sleep scores and that may be associated with the characteristic hormonal changes occurring during pregnancy.

Safety characteristics of the conically sculpted Nd:YAG laser fiber for operative laparoscopy.

The objective of this study was to define the effects and safety margins of available Nd:YAG laser fibers with flat and conically sculpted tips for operative laparoscopy. An in vitro agar base model was designed to define the effect of the conically sculpted fiber at distances of 5 and 10 mm beyond the fiber tip, and an in vivo rat model was used to assess similar effects in the contact mode, as well as at 1, 3, 5, and 10 mm beyond the tip. The beam divergence emitted through conically sculpted tips demonstrated significant reduction in power density at 3-5 mm beyond the tip, which is suggestive of minimal risk of undesired effects. Threshold energy levels required for thermal effects are lower for darker organs than for light-colored objects. The Nd:YAG laser cutting effect is satisfactory when the conically sculpted tips are in contact with soft tissue, and subjacent organs are relatively more protected when compared to the flat tips. The potential advantages of this tool over other laser and conventional accessories for operative laparoscopy has yet to be determined.

Ray-tracing analysis of ICRH power-deposition profiles in non-circular large Tokamaks

Using self-consistent initial conditions obtained from the full-wave solution of the field radiated by an ion-cyclotron resonance heating (ICRH) antenna, the ray-tracing technique is applied to generate r.f. power-deposition profiles in non-circular large Tokamaks such as JET and NET/INTOR. An analytic expression has been used to simulate the shape of the flux surfaces, which fit reasonably well to the numerical solutions of the Grad-Shafranov equation describing the Tokamak equilibrium, by adjusting three free parameters: the flux-surface shift Delta , the elongation ratio kappa and the triangularity parameter delta . For an elliptic INTOR plasma, it is found that the focussing of rays is much reduced and when the absorption layer is located in the center, the r.f. power density figures are lower approximately by a factor of 1.9 compared to that obtained in an equivalent circular case. This reduction in power density is not so significant when the power is deposited off-center, as demonstrated by an example treated for the JET D-shaped plasma.