Power Density Range Research Articles

Enhanced electrochemical energy storage performance of reduced graphene oxide by incorporating oxygen-rich in-plane pores

To effectively enhance the electrochemical energy storage performance of graphene-based materials, oxygen-rich in-plane pores were incorporated into graphene backbones through sonochemical etching of graphene oxide (GO) and sequential chemical reduction treatment. The etching of GO produced a large amount of in-plane nanoscale pores with a high content of oxygen atoms. The hydrazine reduction treatment of etched GO (EGO) resulted in a porous basal plane with good electrical conductivity and preserved electrochemically active oxygen atoms. It is found that good conductivity and in-plane nanoscale pores of reduced EGO (REGO) with enriched electrochemically active oxygen groups synergistically reinforce the electric double-layer capacitance (EDLC) and the pseudocapacitance performance over a wide power density range. Electrodes using REGO demonstrated a maximum energy density of 47 W h kg−1, which is almost three times that of reduced graphene oxide (RGO) without etching treatment, while simultaneously displaying a high power density of up to 100 kW kg−1 with considerable energy density.

Enhanced electrical capacitance of tetraethyl orthosilicate-derived porous carbon nanofibers produced via electrospinning

Porous carbon nanofibers with SiOC/SiOSi group (Si/ACNF) mats are produced by incorporating tetraethyl orthosilicate (TEOS) into polyacrylonitrile (PAN) via electrospinning/activation, and their electrochemical properties as a supercapacitor electrode are investigated. The Si/ACNF exhibits a high surface area of up to 1386.91m2g−1 as result of the ultramicropore size distributions centered at approximately 0.8nm and mesopore volume fraction up to 38%. The gravimetric capacitance of the Si/ACNF webs in an organic electrolyte is 91.95Fg−1, and the energy densities are 59.72–88.23Whkg−1 over the power density range of 1000–20,000Wkg−1. The electrochemical capacitance of a Si/ACNF electrode is closely related to the pore size distribution, surface area, and ionic accessibility of the electrode material, providing a large charge capacity and high rate capability.

A continuous electromagnetic radiation exposure affected the expressions of Clock and fvii genes in mice

The exposure to electromagnetic radiation (EMR) would increase the risk of cardiovascular diseases. Central nervous system, especially the suprachiasmatic nucleus in hypothalamus, is a sensitive target of EMR. Clock is a main circadian gene which plays an important role in the circadian system, while factor VII (FVII) plays a central role in the coagulation cascade in the cardiovascular system. To investigate the effect of a continuous EMR exposure on the circadian and cardiovascular systems, we had mature Institute of Cancer Research (ICR) mice exposed to EMR of different power densities. The intake of food and water, the physiological signs, and the motor activity were monitored to have no significant difference. And their expressions of Clock and fvii genes were detected. Our study showed that a series of EMR exposure to ICR mice would have some key gene expression changed without visible changes in physiological signs and activities. We assume that Clock gene could only regulate fvii gene of some specific organ when the exposure to EMR in a certain range of power density.

Femtosecond laser ablation of aluminum in vacuum and air at high laser intensity

In this study, the ablation of aluminum by a near-infrared femtosecond laser pulse (800nm, 100fs) at different intensity is investigated by a two-dimensional hydrodynamic model. The ablation rates are compared between the cases in vacuum and in air over a wide range of laser power density. It has been reported before that at low (<1013W/cm2) and moderate laser intensity (1013–1014W/cm2), two different ablation regimes exist, and the ablation depth per pulse is dependent on the optical penetration depth and electron heat penetration depth, respectively. By considering both collisional and collisionless absorptions, the model in this study predicts the third ablation regime with a much higher ablation rate increase with respect to laser intensity in the high intensity range (>1014W/cm2) in vacuum, which shows good agreement with the experimental data. This phenomenon is attributed to the change of dominant absorption mechanism from collisional to collisionless absorption. For the case in air, the ablation depth increases slowly with the laser intensity in the high intensity regime, and is much smaller than that in vacuum. It is revealed that this is due to the strong early plasma-laser interaction in air.

Carrier Density Dependent Localization and Consequences for Efficiency Droop in InGaN/GaN Quantum Well Structures

We report on the observation of a reduction in the depth of the S-shape in the temperature dependence of the photoluminescence peak energy with increasing excitation power density. Over the range of excitation power density where the depth of the S-shape is reduced, we also observe a reduction in the integrated photoluminescence intensity per unit excitation power, i.e., efficiency droop. Hence, the onset of efficiency droop occurs at the same carrier density as the onset of carrier delocalization. We correlate these experimental results with the predictions of a theoretical model of the effects of carrier localization due to local variations in the concentration of the randomly distributed In atoms on the optical properties of InGaN/GaN quantum wells. On the basis of this comparison of theory with experiment we attribute the reduction in the S-shape temperature dependence to the saturation of the available localized states. We propose that this saturation of the localized states is a contributory factor to efficiency droop whereby nonlocalized carriers recombine non-radiatively.

Molecular Dynamics Study of Laser and Plasma Nitriding of Titanium

The molecular dynamics (MD) method is successfully applied to simulate the nitridation of titanium by the mixing technology with laser and plasma. Based on the simulation, the influence of the processing parameters, such as the laser power density and the scanning velocity on the effective thickness of the nitride layer, was investigated. It was found that, for each scanning velocity, there is a laser power density range within which the higher laser power density has the beneficial effect for nitriding treatment. Comparing the simulation and experimental results shows that the calculated results are in good qualitative agreement with the experimental results.

A survey of power density of light-curing units used in private dental offices in Changchun City, China

This study investigated power density and relevant information related to light-curing units used in private dental offices in Changchun City, China. The power density of 196 light-curing units used in private dental offices in Changchun City was measured using a simple random sampling method. Relevant information included the brand, type, years of operation, frequency of use, model numbers and types of light guide, resin buildup on the light guides, damage caused by the light guides, required maintenance of the curing lights, and ratio of the unit and chair number. There were 132 quartz tungsten halogen (QTH) units and 64 light-emitting diode units. The power density range was defined as 0-1,730mW/cm(2). The mean power density was 453.1mW/cm(2). The mean years of operation of the light-curing units were 3.96. The majority of dentists never tested the power density of the light-curing units and a considerable number of light guide surfaces showed resin buildup and damage. In Changchun City, the majority of light-curing units were QTH. Some units needed to be replaced due to aging. The majority of dentists were not aware that the light-curing units require periodic testing and maintenance. The data herein indicate the importance of periodic testing of the power density of light-curing units and timely replacement of the components and then guarantee the quality of medical services and their benefits to patients.

Influence of laser pulse parameters on characteristics of a source of multicharged metal ions based on laser-induced medium-power spark discharge

The dynamics of laser-induced vacuum spark discharge with storage energy not exceeding 25 J has been studied in a broad range of laser pulse energies and power densities. It is shown that, using discharge-initiating laser pulse of nanosecond duration, it is possible to obtain stable single pinching of cathode jet plasma at a storage voltage above 10 kV. Plasma pinching is accompanied by the generation of a beam of multicharged ions of the cathode material (aluminum) up to Al8+. The maximum energies of ions obey the scaling relation Emax = 5ZeU0 that has been obtained previously for a low-voltage discharge. An increase in the laser pulse energy leads to growth of the average beam charge and a sharp decrease in the ion energy.

Dependence of the shapes of nonzero-field level-crossing signals in rubidium atoms on the laser frequency and power density

We studied magneto-optical resonances caused by excited-state level crossings in a nonzero magnetic field. Experimental measurements were performed on the transitions of the $D_2$ line of rubidium. These measured signals were described by a theoretical model that takes into account all neighboring hyperfine transitions, the mixing of magnetic sublevels in an external magnetic field, the coherence properties of the exciting laser radiation, and the Doppler effect. Good agreement between the experimental measurements and the theoretical model could be achieved over a wide range of laser power densities. We further showed that the contrasts of the level-crossing peaks can be sensitive to changes in the frequency of the exciting laser radiation as small as several tens of megahertz when the hyperfine splitting of the exciting state is larger than the Doppler broadening.

EXPERIMENTAL RESEARCH ON LASER SHOCK WAVE LOADING MECHANISM OF 2024 ALUMINUM ALLOY SHEET

aluminum alloy sheets with three different thicknesses were shocked by means of a single laser shock processing. Static and dynamic strain signals from the strain gauges on the back of the target were measured by STSS-1 stress test module. Residual stresses of target surfaces were measured by XRD. The loading model by the laser shock waves in 2024 aluminium alloy sheets was established, the stress wave propagations and stress wave structures inside the targets were described, and the reasons of generating residual tensile stress in the impact areas were explained. The results indicate that laser shock waves reflected and transmitted have different properties and intensities at the different places of target surfaces. When the laser power density is far beyond the best power density range of 2024 aluminum alloy and the target material is thin enough, the tensile residual stresses are generated.

Solution blowing of ZnO nanoflake-encapsulated carbon nanofibers as electrodes for supercapacitors

A facile spinning-based strategy is developed to fabricate zinc oxide nanoflake-encapsulated carbon nanofibers (ZnO/CNFs) as electrodes for supercapacitors. The zinc oxide/carbon nanofiber mats were solution blown with zinc acetate (Zn(Ac)2) and polyacrylonitrile (PAN) as the metal and carbon precursor to get Zn(Ac)2 core-enriched precursor nanofibers. After annealing under nitrogen, the precursor nanofibers were converted to ZnO/CNFs with ZnO nanoflakes encapsulated in the core of carbon nanofibers. In the constructed architecture, carbon nanofibers can avoid the direct exposure of ZnO to the electrolyte and preserve the structural and interfacial stabilization of ZnO nanoflakes. Meanwhile, the flexible entangled carbon nanofibers can accommodate temperate porosities thus providing the pore channel for electrolyte ions and maintain the structural and electrical integrity of the ZnO/CNF electrode during the charge–discharge processes. By loading different contents of ZnO, the microstructures of CNFs were changed, and the textural parameters significantly affected their electrochemical properties as electrodes. As a result, the ZnO/CNF electrodes exhibit high specific capacitance (216.3, 212.7, 208.8 and 172.5 F g−1 at 1, 5, 10, and 50 A g−1, respectively) and extremely excellent cycling performance at high current density (only 5.41% capacitance loss after 2000 cycles at a high rate of 10 A g−1), with promising energy densities of 29.76 kW h kg−1, over a power density range of 2.5–30 kW kg−1. The ZnO/CNFs simultaneously exhibit excellent capacity retention. These encouraging results indicate great potential applications of ZnO/CNFs in developing energy storage devices with high energy and power densities.

Architectural design of hierarchically ordered porous carbons for high-rate electrochemical capacitors

The rate capability of carbon-based electrochemical capacitors (ECs) is an important issue for applications and the three-dimensional (3D) hierarchically ordered porous carbon (HOPC) can reduce the diffusion length to improve the rate performance by the unique architectural design. In this study, a modified dual-templating strategy for architectural design of 3D HOPC as a promising electrode material for high-rate EC applications was developed by using polystyrene and Pluronic F127 as macro- and meso-porous templates, respectively. Tetraethyl orthosilicate was also added as the silica precursor, and then the sol–gel-derived SiO2 was removed by sodium hydroxide to form the second mesopore in HOPC (HOPC-s) for enhancing the accessible surface areas and pore structures. The HOPC-s formed by the templating strategy is composed of highly ordered macropores, macroporous windows, bimodal mesopores, and micropores, resulting in high specific surface area (1112 m2 g−1), high total pore volume (1.18 cm3 g−1) and an easily accessible environment for fast electrolyte ion transport. The specific capacitance of the HOPC-s electrodes can reach 316 F g−1 at 25 mV s−1 and maintain excellent capacitive retention at a high scan rate of 1000 mV s−1 when compared with that of ordered mesoporous carbon (OMC-s). Electrochemical impedance spectroscopy fitting shows that the pore electrolyte resistance of HOPC-s is 3 times lower than that of OMC-s, which is attributed to the hierarchical macroporous structures and short mesoporous channels. In the symmetric capacitor test, the HOPC-s also shows excellent power capability, and the energy density of 4–10 W h kg−1 can be maintained over the power density range of 1–14 kW kg−1. In addition, the capacitance of HOPC-s in the polycarbonate-containing ionic liquid shows 80% retention at a scan rate of 500 mV s−1, indicating that the unique hierarchical structure can provide efficient ion-buffering capacity for high-rate performance. These exceptional electrochemical performances clearly demonstrate that 3D HOPC-s is a superior material to solve the poor ion transport limitation, which can open an avenue to fabricate high-rate ECs with high power and energy densities for energy storage.

Boron-nitrogen functional groups on porous nanocarbon fibers for electrochemical supercapacitors

Boron- and nitrogen-containing porous carbon nanofibers (BN-CNFs) were prepared through one-step electrospinning using boric acid (H3BO3) and urea as boron and nitrogen precursors and a subsequent thermal treatment. The BN-CNFs showed higher capacitance and energy/power density values than the control sample of CNFs in aqueous electrolyte; the large surface area of 559m2g−1 and the NH, COOH, and [O]BN functional groups facilitated these improvements. The specific capacitance of the supercapacitor was 180Fg−1 at 1mAcm−2, and the energy density was 17.2–23.5Whkg−1 in the power density range of 400–10,000Wkg−1 in an aqueous electrolyte.

Enhanced electrical capacitance of porous carbon nanofibers derived from polyacrylonitrile and boron trioxide

Carbon nanofibers (CNFs) containing boron and nitrogen are prepared from polyacrylonitrile and boron trioxide (B2O3) by using simple electrospinning. The B2O3 introduction into a PAN solution causes a porous structure with stabilized [O]BN functional groups to develop in the processes of stabilization and carbonization. The pore structure and the functional groups such as B atoms and [O]BN introduce synergistic effects by not only increasing the power density but also the energy density, as shown by the results. The energy storage capabilities of the electrode prepared from 20wt% B2O3 added to the PAN solution are as follows: a capacitance of 184.0Fg−1 and an energy density of 18.7–25.2Whkg−1 in the respective power density range of 400–10,000Wkg−1 in 6M KOH electrolyte. Hence, these CNFs exhibit a very promising potential as electrode materials for electrical double-layer capacitors due to their unique microstructure and proper proportion of heteroatoms.

Analysis of tungsten laser produced plasmas in the extreme ultraviolet (EUV) spectral region

Tungsten will be used as a wall material in ITER and therefore will be present as an intrinsic plasma impurity with the resulting emission having the potential to be used as a plasma diagnostic. We have recorded spectra of tungsten laser produced plasmas in the 1–7 nm region using Nd:YAG lasers operating at a range of power densities. We have analysed these spectra, giving special attention to the unresolved transition arrays in the 3 nm region that appear at the highest laser power densities. We compare our results to those from previous work and also use new atomic structure calculations to identify a number of new features.

Near-Infrared Detecting Array for High Energy Laser Beam Diagnostics

A near-infrared detecting array is developed combining photoelectric and calorimetric method together to measure CW beam profile of high energy lasers. Detailed structure and functional modules of this system are described. The system is widely sensitive to the wavelength range of 0.9~1.7μm and the power density range of W/cm2～kW/cm2. It has an effective detecting area of 22cm×22cm with a spatial resolution of 1.1cm and a temporal resolution of 20ms. Experiments show that the uncertainty of the total energy measurement is 10% and the uncertainty of the power density distribution measurement is less than 15%.The measuring system has been successfully applied in the measurement for the far-field beam of high energy lasers.

Electrospun vanadium pentoxide/carbon nanofiber composites for supercapacitor electrodes

The vanadium pentoxide (V2O5)/carbon nanofiber composites (CNFCs) were prepared from polyacrylonitrile/V2O5 in N,N-dimethylformamide by a simple electrospinning method, and their electrochemical properties as supercapacitor electrodes were investigated. Different loadings of V2O5, the microstructures of the CNFCs (e.g., nanometer-size diameters, high specific surface areas, narrow pore size distributions, and tunable porosities) were changed, and the textural parameters significantly affected the electrochemical properties of the composites. The CNFC capacitors delivered the high specific capacitances of 150.0Fg−1 for the CNFCs in an aqueous, with promising energy densities of 18.8Whkg−1, over a power density range of 400–20,000Wkg−1. The CNFCs simultaneously exhibited excellent capacity retention.

Freestanding CVD boron doped diamond single crystals: A substrate for vertical power electronic devices?

AbstractThe development of ‘all‐diamond’ devices for power electronics is attracting more and more interest as judged by the recent increase in the number of publications on the subject. Nevertheless most devices reported in the literature used coplanar or pseudo‐vertical geometries which, although promising in term of breakdown voltage, have still a relatively high on‐state resistance. This could be related to current crowding due the low cross‐section p+ layer. Vertical configuration, which requires thick heavily doped substrates, is a possible alternative usually used in conventional semiconductors. In this study, chemical vapour deposition (CVD) diamond growth conditions allowing heavy boron doping over an important thickness are discussed. It was found that there is an optimal range of microwave power density (MWPD) for which reasonable doping efficiencies and growth rates can be obtained leading to hundreds of micrometers thick crystals with a doping level higher than 1020 cm−3. The crystal morphology was predicted thanks to a 3D geometrical model and a small addition of oxygen to the gas phase was efficient to avoid the appearance of undesirable crystals faces and keep the crystal integrity. Freestanding boron‐doped diamond single crystals were eventually grown and characterized by secondary ion mass spectrometry (SIMS), Fourier transformed InfraRed (FTIR) spectroscopy, Raman spectroscopy, high resolution X‐ray diffraction (HRXRD) and four‐point probe measurements. The high quality of the synthetic crystals was confirmed exhibiting electrical resistivities as low as 0.26 Ω cm illustrating that this material is suitable for the development of vertical power electronic devices.

Measurements of spatially resolved electron number densities and modes temperatures using optical emission spectroscopy of atmospheric pressure microplasma jet

The electrical gas discharge parameters of direct-current non-thermal microplasma jet in Ar-2%H2 flow at open atmospheric air was investigated by using spatially resolved optical emission spectroscopy (OES). The jet was confined from microhollow of tungsten-carbide (∼500 μm inner diameter) to a molybdenum foil. Despite its small volume, the atmospheric pressure microplasma jet provides a range of power densities, from low to ∼1012 W m−3 generated either in rare gases or in molecular gases. A high resolution spectrometer (Jobin-Yvon, Czerny-Turner model THR1000, resolution of 0.001 nm, with focal length of 1.0 m and numerical aperture of 0.13 − f/7.5) was used to allow registration of OH (A2Σ+, ν = 0 → X2Π, ν′ = 0) rotational bands at 306.357 nm, Ar I 603.213 nm line and N2 (C3Πu, ν = 0 → B3Πg, ν′ = 0) second positive system with the band head at 337.13 nm in order to estimate the rotational temperature from the cathode sheath of the plasma jet to the anode. For currents ranging from 20 to 100 mA and for a particular excited levels, the excitation temperature was measured in the negative glow region either from a Boltzmann plot of Ar I 4p–4s and 5p–4s transitions of excited argon or using the Mo I (from 440 to 450 nm) two-lines method of excited Mo atoms sputtered from the cathode surface, giving 24 000 K (100 mA at 100 μm) and 7000 K (20 mA at 500 μm from the cathode). From the N2 (C3Πu, ν = 0 → B3Πg, ν′ = 0) rotational transition the rotational temperature along the positive column was estimated. The vibrational temperature of the bulk plasma (1400 to 4500 K) was estimated for a current varying from 20 to 120 mA using the N2 second positive system with Δν = −2. Using the broadening of Hβ Balmer line it was possible to estimate the electron number density of the negative glow (1014 to 1015 cm−3) as a function of the current.

Low-Power Wireless Power Delivery

This paper addresses design and implementation of integrated rectifier-antennas (rectennas) for wireless powering at low incident power densities, from 25 to 200 μW/cm2. Source-pull nonlinear measurement of the rectifying devices is compared to harmonic-balance simulations. Optimal diode RF and dc impedances for most efficient rectification, as a function of input power, are obtained. This allows optimized antenna design, which can eliminate or simplify matching networks and improve overall efficiency. As an example of the design methodology, Schottky diodes were characterized at 1.96 GHz and an antenna is matched to the optimal complex impedance for the most efficient rectifier. For incident power density range of interest, the optimal impedance is 137 + j149 Ω, with an RF to dc conversion efficiency of the rectifying circuit alone of 63% and total rectenna efficiency of 54%.