Power Density Range Research Articles

Electrically Driven Interfacial Evaporation for High-Efficiency Steam Generation and Sterilization.

Electrically driven steam generation is a critical process for many heating-related applications such as sterilization and food processing. Current systems, which rely on heating up the bulk water to generate steam, face the dilemma in achieving a large evaporation flux and fast thermal response. Herein, we report a self-floating electrically driven interfacial evaporator for fast high-efficiency steam generation independent of the amount of loaded bulk water in the system. Through localized heating of the wicked water at the air–water interface, the evaporator has achieved an electrical-to-steam energy conversion efficiency of ∼90% at a heating power density of 10 kW/m2 and a fast thermal response of 20 s. The interfacial evaporation design not only achieves a high evaporation efficiency within a broad range of heating power densities by using different wicking materials, but also enables attaining a high evaporation temperature under low heating power densities by tuning the ratio of the vapor outlet area and the evaporation surface area. By integrating an interfacial evaporator within a sanitizer, the resultant system has demonstrated a faster steam temperature rise and superior steam sterilization performance than the commercial bulk heating-based approach.

Template-assisted synthesized MoS2/polyaniline hollow microsphere electrode for high performance supercapacitors

Herein we report a template-assisted method to synthesize molybdenum disulfide/polyaniline (MoS2/PANI) hollow microspheres with excellent electrochemical performances for supercapacitors applications. The hollow MoS2 microspheres are assembled by layered MoS2 nanosheets, which provide substantial ion channels and large surface area for electrolyte ion transport. PANI is grown on the surface of the hollow MoS2 microspheres by oxidative polymerization with different mass. The MoS2/PANI-1:2 hollow microspheres electrode possesses the maximum specific capacitance of 364 F/g at a scan rate of 5 mV/s. In addition, the specific capacitance achieves 299 F/g with a current density of 10 A/g and 84.3% of the specific capacitance is retained after 8000 cycles at 10 A/g. The energy density can be as high as 32 Wh/kg at a power density of 320 W/kg. Even at a very high power density of 8320 W/kg, the energy density still maintains 23.1 Wh/kg, showing a large power density range for supercapacitor applications. We assemble a symmetric supercapacitor by using two identical MoS2/PANI microspheres electrodes, and it shows high specific capacitances of 231, 139 F/g at 0.2 and 0.5 A/g, respectively. Our results suggest that the MoS2/PANI hollow microspheres electrode have good cycle stability and significant potential for supercapacitor applications.

Facile preparation and capacitive properties of low-cost carbon nanofibers with ZnO derived from lignin and pitch as supercapacitor electrodes

To develop economical and high-performance supercapacitor electrodes in an aqueous electrolyte, polyacrylonitrile (PAN)/pitch/lignin-based carbon nanofibers (CNFs) with ZnO (PPL-Zn) are successfully fabricated by one-step electrospinning of PAN, pitch, lignin, and zinc acetate. The oxygen-rich lignin in the PPL-Zn electrodes induces porosity on the fiber surface by eliminating many functional groups such carboxylic, carbonyl, hydroxyl and ketone and organic moieties in the simple heat treatment without any activation agent/activation process. Moreover, the many aromatic components, high carbon content, and low glass transition temperature of lignin are excellent precursors to CNF composites. In addition to lignin, pitch is used as a precursor to CNF to reduce the cost and increase the carbon yield and electrical conductivity of the CNFs. These characteristics afford PPL-Zn(5) (5 wt% of zinc acetate relative to PAN and lignin) with good capacitive behavior for application as supercapacitor electrodes with a maximum specific capacitance of 165 Fg-1 at 1 mAcm−2 with a capacitance retention of 88%, high energy densities of 22–18 Whkg−1 in the power density range of 400 to 10,000 Wkg-1, and excellent cycling performance reduced by only ∼6% after 3000 cycles of charge/discharge due to the synergistic effect of the electrical double layer and the pseudocapacitive effect.

Controlling Red Color–Based Multicolor Upconversion through Selective Photon Blocking

AbstractPhoton upconversion multiplexing has attracted increasing interest in recent years; however, realizing the red color–based multicolor‐tunable output in upconversion nanoparticles (UCNPs) at a fixed composition remains a huge challenge. Here, a novel and versatile approach to fine‐control upconversion luminescence (UCL) colors from UCNPs through selectively confining specific excitation energy by the photon blocking effect is reported. Four types of dual‐color switchable UCNPs capable of emitting red‐blue and red‐green emissions are successfully designed following this strategy, and their UCL performance shows a multiwavelength (808/980/1550 nm) excitable feature that is well sustained in a wide range of excitation power density. The use of the photon blocking effect further enables the dynamically switchable red‐green‐blue UCL with 808/980 nm excitations. These findings provide a general method to achieve multicolor‐tunable UCL at a single nanoparticle level. Moreover, the UCNPs with red‐based multicolor emissions in this work enriches the upconversion system and should have potential applications in display, anti‐counterfeiting, and bioimaging.

Rational Surface Tailoring Oxygen Functional Groups on Carbon Spheres for Capacitive Mechanistic Study

Porous carbons represent a typical class of electrode materials for electric double-layer capacitors. However, less attention has been focused on the study of the capacitive mechanism of electrochemically active surface oxygen groups rooted in porous carbons. Herein, the degree and variety of oxygen surface groups of HNO3-modified samples (N-CS) are finely tailored by a mild hydrothermal oxidation (0.0-3.0 mol L-1), while the micro-meso-macroporous structures are efficiently preserved from the original sample. Thus, N-CS is a suitable carrier for separately discussing the contribution of oxygen functional groups to the electrochemical property. The optimized N-CS shows a high capacitance of 279.4 F g-1 at 1 A g-1, exceeding 52.8% of pristine carbon sphere (CS) (182.8 F g-1 at 1 A g-1) in KOH electrolyte. On further deconvoluting the redox peaks of cyclic voltammetry curves, we find that the pseudocapacitance not only associates with the surface-controlled faradic reaction at high scan rate but also dramatically stems from the diffusion-controlled capacitance through potassium and hydroxyl ion insertion/deinsertion into the underutilized micropores at low scan rate. The assembled supercapacitor based on N-CS presents a stable energy density of 5 Wh kg-1 over a wide range of power density of 250-5000 W kg-1, which is higher than 0.0N-CS in KOH electrolyte. In TEABF4 electrolyte, the N-CS supercapacitor has an energy density of 26.9 Wh kg-1 at the power density of 1350 W kg-1 and exhibits excellent cycling stability with a capacitance retention of 93.2% at 2 A g-1 after 10 000 cycles. These results demonstrate that surface oxygen groups alter the capacitive mechanism and contribution of porous carbons.

Influence of weld parameters on weld regimes and vaporization rate in electron beam welding of Ti6Al4V alloy

Electron beam welding (EBW) is the most preferable and efficient joining technique to weld titanium alloys, since it provides a vacuum atmosphere and eliminates the use of shielding gas during the welding process. The present work is focused on understanding the electron beam weld parameters which controls the weld limits, microstructural and mechanical properties and vaporization rate of metal elements in the Ti6Al4V alloy. The influence of power density on characteristics of electron beam welded Ti6Al4V alloy at constant linear energy conditions is examined. The welding experiments were performed to evaluate the weld geometry, weld regimes, microstructural properties and microhardness characteristics using fundamental parameters such as power density and interaction energy per unit volume (IE). Moreover, the vaporization heat transfer model of the EBW process is developed by employing Fourier’s law of heat conduction to estimate the vaporization rate of Ti6Al4V alloying elements. The rate of vaporization of major alloying elements of Ti6Al4V is analysed with respect to power density. Based upon the results obtained, it is witnessed that the weld bead geometry, weld limits and grain size are controlled by power density for constant linear energy conditions. Also, the solidified microstructure in the weld zone at different power density significantly effects the final strength of the weld joint. The conduction and keyhole weld limits are identified for power density values below 0.31 W cm−2 and above 2.12 W cm−2, respectively. The transition mode welding in the Ti6Al4V alloy is identified by a nominal increase in aspect ratio for a wide range of power density and IE values. The results also established that the size of β grain in the fusion zone reduces with increase in power density at constant linear energy condition. Also, the average α1 martensitic grain size in the weld zone is determined to be higher in keyhole welds (128 µm) relative to conduction welds (48 µm) on account of higher solidification rate. In effect, the solidified martensitic phases influence the microhardness distribution proportionately across the weld sample of Ti6Al4V alloy. Vaporization of metal elements of the Ti6Al4V alloy is induced in few milliseconds and evaporation rate of ‘Al’ element is least followed by ‘Ti’ and ‘V’ elements. It is also observed that by reducing the power density from 2.54 to 2.12 W cm−2, keyhole mode welding can be acquired and vaporization rate of ‘Al’ is reduced by 20.5%.

Effective Upconversion in Alkaline Earth Fluorides Activated by Yb3+–Ho3+ Ions

A study is performed of optical spectra (absorption, excitation, luminescence, energy yield) associated with the upconversion of infrared excitation of 980 nm into visible radiation in crystals of alkaline–earth fluorides CaF2, SrF2, and BaF2, co-doped with YbF3 (0.01–10 mol %) and HoF3 (0.01–0.3 mol %). In the 0.1–10 W cm−2 range of excitation power densities, the intensity of the upconversion bands at 542, 650, and 752 nm grows as the square of the power. A quadratic dependence of the intensity of upconversion on the concentration of Yb3+ (0.03–3 mol %) and a weak dependence on the Ho3+ concentration (0.01–0.3 mol %) with a maximum at 0.1–0.15 mol % are observed. It is assumed that the upconversion is due to consecutive energy transfer from two closely spaced excited ytterbium ions to the holmium ion.

Mechanical behavior and fracture characteristics of simultaneously tensile loaded and laser heated Ti6Al4V alloy

The mechanical behavior and fracture characteristics of simultaneously tensile loaded and laser heated Ti6Al4V alloy are investigated. Aiming at a comprehensive damage evolution and failure properties, a wide range of laser power densities and preload levels are considered. Stress-strain flow curve, temperature evolution, deformation degree and type, and corresponding mechanisms are explored in detail. The presented results demonstrate that temperature has a significant effect on the flow behavior of the material. The strength of the material and resistance to plastic flow decreased rapidly with the increase of laser power density. The remarkable reduction in failure time with the increase of either laser power density or preload is found. However, for higher preloaded states, there is the relatively smaller impact of laser power density on failure time. The metallographic analysis revealed that fracture characterizing features for Ti6Al4V alloy mainly comprises on micro-void nucleation, growth, and coalescence. Most of the fracture surfaces exhibited dimples and veins which indicates a ductile failure mode. With the increase of laser power density, dimples rupture is the predominant failure mode. While with the increase of preloading value, relatively more in-depth deformation effects and melting features were observed. On a macroscopic level, specimens experienced fracture after following the necking phenomenon. Temperature evolution history during the specimen exposure to combined tensile load and laser heating is providing a variety of information like phase change and melting etc. In addition, temperature profiles depict superheating of exposed surfaces to a temperature level notably beyond the melting temperature of the Ti6Al4V material.

Design of an Axial-Flux PM-Assisted Claw-Pole Generator Based on an Equivalent Magnetic Circuit Model

An axial-flux permanent-magnet (PM)-assisted claw-pole generator (APCG) is designed based on an equivalent magnetic circuit model (EMCM). It is designed for generating power in vehicle with the merits of high power density, no brush, and wide airgap flux density modulation range. This paper mainly focuses on establishing the EMCM of the APCG and designing a set of optimal structure parameters. First, the topology and operation principle of the APCG are introduced in detail, including material, fabrication, and magnetic flux distribution. Second, the magnetic circuits in claw poles are simplified and the relationships among the key parameters are discussed. Finally, based on the simplified model and the relationships among the key parameters, the EMCMs of current and PM excitation are established, respectively. According to the EMCM, a scheme of a 4 kW APCG is carried out. Comprehensive investigations demonstrate that the EMCM is time-saving and relatively accurate when compared with a finite element method.

Mesoporous niobium pentoxide/carbon composite electrodes for sodium-ion capacitors

Sodium-ion capacitors with unique characteristics such as higher energy density than electrical double-layer capacitors, higher power density than rechargeable batteries, and abundant sodium resources represent current research trend in developing large-scale electrical energy storage technology. One of the key challenges presently facing the development of this technology is the imbalanced kinetics between the sluggish Faradaic sodium insertion in the anode and the fast capacitive ion adsorption on the cathode. Here we demonstrate the sol-gel synthesis of a novel, high-rate, stable composite anode material for sodium-ion capacitors (NICs). The composite consisted of Nb2O5 nanoparticles embedded in a carbon matrix (denoted by m-Nb2O5/C). Sodium-ion capacitors employing the m-Nb2O5/C anode and a commercial activated carbon as the cathode showed an admirable performance, delivering high energy densities in a wide range of power densities (73 Wh kg−1@250 W kg-1 and 16.8 Wh kg−1@20 kW kg−1). These favourable cell characteristics are attributed to the properties of the m-Nb2O5/C anode: the mesoporous structure that facilitates electron and ion transport, the presence of the niobium carbide interlayer between the Nb2O5 nanoparticles and the surrounding graphitic carbon that additionally improves the electron conductivity, and the predominant capacitive charge storage mechanism.

The influence of spatial inhomogeneity of pulsed capillary discharge on the gas dynamics of multicomponent plasma

The results of experimental research of a pulsed capillary discharge with an evaporating wall in the power density range of qs = 10 − 100 kW/cm2 obtained by using independent methods of diagnostics, including optical spectroscopy, the ballistic pendulum method, and direct measurement of pressure inside the capillary are presented. The analysis of experimental results, carried out with the help of a two-zone model of a capillary discharge, revealed the decisive influence of the discharge spatial inhomogeneity on the gas dynamics of a multicomponent plasma, which turns out to be substantially different in the high-temperature paraxial and low-temperature peripheral discharge zones. It is shown that the gas dynamics of the paraxial discharge zone corresponds to the regime of the ideal expendable nozzle with good accuracy, and the paraxial zone itself provides the main contribution to the creation of the jet momentum, in spite of the fact that more than 80% of the evaporated substance is carried away from the capillary through the peripheral zone. It is shown that high losses in the plasma jet stagnation pressure, reaching up to 80% relative to the pressure measured inside the capillary, belong to the peripheral discharge zone, in which the density and velocity, respectively, are two orders of magnitude higher and an order of magnitude lower than the corresponding values in the paraxial zone.

Noninvasive real-time assessment of riboflavin consumption in standard and accelerated corneal crosslinking

To estimate the noninvasive riboflavin concentration in the corneal stroma using a new ultraviolet-A (UVA) theranostic device for corneal crosslinking (CXL). Vision Engineering Italy srl, Rome, Italy. Experimental study. Fourteen human donor corneas were treated according to conventional (UVA irradiance of 3mW/cm2 for 30minutes) and rapid (10mW/cm2 for 9minutes) riboflavin-UVA CXL protocols using a theranostic UVA device. Five additional samples were treated by 0.5mW/cm2 for 9minutes and used as positive controls to determine riboflavin photodegradation under near ambient lighting conditions. A 20% dextran-enriched 0.1% riboflavin solution was used in all cases. The device consisted of a UVA light source; a red-green-blue camera, which acquires the fluorescence images of the cornea during treatment; and a single-board computer for managing the overall operations and the raw data processing. Preirradiation stromal soaking for 30minutes achieved highly consistent intrastromal riboflavin concentration in all tissues (mean 0.015%±0.003% [SD]). There were no differences in thekinetics curves of riboflavin consumption between the 2 UVA irradiation protocols; the intrastromal riboflavin concentration decreased exponentially, with a mean constant energy rate of 2.8±0.2J/cm2. In the control group, the intrastromal riboflavin concentration decreased quasilinearly. The theranostic device provided estimates of the intrastromal concentration of riboflavin noninvasively during treatment. In the 3 to 10mW/cm2 range of power densities, the consumption of riboflavin in the stroma by UVA irradiation was only energy dependent in accordance with the Bunsen-Roscoe law.

2D WSe2/MoS2 van der Waals heterojunction photodiode for visible-near infrared broadband detection

Two dimensional (2D) layered van der Waals (vdW) atomic crystals are an important class of emerging materials due to their unique physical properties. In particular, the nature of dangling–bond–free surfaces in 2D vdW materials enables the formation of heterojunctions without the constraint of atomic lattice match. Here, we report on a 2D WSe2/MoS2 multilayer van der Waals heterojunction PN diode and its application for visible-near infrared broadband detection. The WSe2/MoS2 PN diode shows excellent performance with an ideality factor of 1.5 and a high rectification (ON/OFF) ratio of over 106. This PN diode exhibits spectral photo-responses from the ultraviolet (405 nm) region to the near infrared (808 nm) region with obvious photovoltaic behaviors (very clear open circuit voltage and short circuit current). In addition to the static behavior, photocurrent switching behaviors are clearly observed under periodic illuminations at up to 1 KHz. The device shows a linear response within the optical power density range of 10−5 W cm−2 to 1 W cm−2 and a linear dynamic range is estimated to be 123 dB.

Surface erosion of hot Cr target and deposition rates of Cr coatings in high power pulsed magnetron sputtering

High power pulsed magnetron sputtering (HPPMS) of a hot chromium target by using a combined power source is the aim the present research. The combination of HPPMS technique and hot target can be effective way to produce Cr coatings. The investigations were performed by using a power supply with pulse frequency of 500 Hz, duty cycle of 4% in the range of power density averaged over pulse period from 15 to 65 W/cm2. The specific feature of the investigation is that sputtering and sublimation of the target occur simultaneously. The experimental results and the calculations revealed the important role of sublimated Cr atoms concentration near the target in increasing the pulsed and average discharge current and in setting up the self-sputtering mode. The erosion yield of the hot Cr target is determined, and in the investigated power range, it increases from 1.5 to 30 atoms per ion due to sublimation. As a result, the coating deposition rate by HPPMS from hot solid target can be increased 10–30 times in comparison to cooled target sputtering.

Study of the photoluminescence properties of Eu2+-doped Ca(AlxGa1−x)2S4 solid solutions at high excitation intensity and low temperature

The investigation of photoluminescence (PL) properties of Ca(Al[Formula: see text]Ga[Formula: see text]S4:Eu[Formula: see text] solid solutions with [Formula: see text] = 0, 0.1, 0.2 and 0.3 synthesized for the first time was performed in wide temperature and excitation power density ranges. It is shown that PL intensity decreases by only 20–30% for different [Formula: see text] in the temperature range from 10 K to 300 K. The extreme stability of shape and position of the emission spectra was found in the range of excitation power density from [Formula: see text] W/cm2 to [Formula: see text] W/cm2 by nanosecond pulsed radiation. The reversible PL efficiency droop was only above 104 W/cm2 of excitation power density. The PL decay time constant was found to be of about 560 ns to 595 ns for [Formula: see text] ranging between 0.1 and 0.3 at room temperature.

Preparation of titanium carbon nitride films by reactive high power pulsed sputtering Penning discharges

Titanium carbon nitride (TiCN) films are prepared by reactive high power pulsed sputtering (HPPS) Penning discharges at a total pressure of 0.7 Pa and an average power of about 60 W, where the N2 fraction is 5%. The area ratio (RTC) of carbon target to the overall target is increased from 0 to 50%. This system can be regarded as a kind of double HPPS system with simultaneity. The peak value of the instantaneous power ranges between 3 and about 16 kW. It means that the peak power density range corresponds to 0.3–1.3 kWcm−2.The film hardness of 30 GPa and more and the elastic modulus of 200–240 GPa do not strongly depend on RTC at RTC less than 25%, beyond which they decrease to 15.8 GPa and 122 GPa. However, the value of the compressive stress and the friction coefficient gradually decrease with the increase in RTC. X-ray diffraction peak of (111) texture is observed for the films prepared at RTC less than 25%, showing the grain size of about 8–15 nm. On the other hand, the hardness and the value of the stress increase with the increase in the dissipated energy per pulse (Ep) at RTC less than 25%. Chemical compositions and bonding states of films are investigated by X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM) - electron energy loss spectroscopy (EELS). Based on XPS measurements, the relative content of C increases from 2.8 to 65% with the increase in RTC. On the other hand, the relative content of N is close to that of Ti and it decreases from 46 to 17%. The intensity of the peak related to Ti-N bond is largest among all intensities of the peaks related to the binary or ternary compounds including Ti component, whereas the intensity related to sp2 C-C bond increases with RTC. These intensity ratios depend on RTC, but they are not sensitive to Ep.

Coprecipitation Reaction System Synthesis and Lithium-Ion Capacitor Energy Storage Application of the Porous Structural Bimetallic Sulfide CoMoS4 Nanoparticles.

Lithium-ion capacitors (LICs) are noticed as a new-type of energy storage device with both capacitive mechanism and battery mechanism. The LICs own outstanding power density and energy density. In our work, an LIC was constructed by using a simple method to prepare a bimetallic sulfide of CoMoS4 nanoparticles as the anode and a self-made biochar [fructus cannabis’s shells (FCS)] with excellent specific surface area as the cathode. The CoMoS4//FCS LIC demonstrated that the range of energy density is from 10 to 41.9 W h/kg and the range of power density is from 75 to 3000 W/kg in the meantime, and it also demonstrated a remarkable cycling performance with the capacitance retention of 95% after 10 000 cycles of charging–discharging at 1 A/g. The designed CoMoS4//FCS LIC device exhibits a superior electrochemical performance because of the CoMoS4 loose porous structure leading to excellent dynamic performance, which is conducive to the diffusion of electrolyte and lithium ion transport, and good electric double layer performance of biochar with large specific surface area could be achieved. Therefore, this bimetallic sulfide is a promising active material for LICs, which could be applied to electric vehicles in the future.

Calculation for stator loss of high-speed permanent magnet synchronous machine in torque-speed envelope and restraint approach for circulating current in windings

Iron loss and copper loss are the significant parts of electrical loss of machines, which are the major parts particularly under high frequency condition. High-speed permanent magnet synchronous machines (HS-PMSM) have the benefits of high power density, high efficiency and wide speed range. Which causes the calculation for iron loss and copper loss in whole operating range complex. By analyzing the components and influencing factors of iron loss and copper loss in stator, we have deduced the calculation formula of iron loss and copper loss in whole operating range based on the analytical solution and finite element approach (EFA) solution. According to the calculation solution, taking the influence of operating temperature on the iron loss and copper loss into account, we propose a temperature correction factor and establish the calculation method for the iron loss and copper loss with temperature influences. Finally, by the conductor transposition, we restrain the circulating current under high-frequency operating condition.

Influence of boron content on the structure and capacitive properties of electrospun polyacrylonitrile/pitch-based carbon nanofiber composites

Electrospun boron-enriched PAN/pitch-based CNF composites (PPB) are developed by combining B2O3 as the boron source, and pitch and polyacrylonitrile as the carbon precursor, to introduce surface boron heteroatoms and enhance the electrical conductivity in CNF composites. PPB-10 electrodes prepared from 10 wt% B2O3 added to the PAN/pitch solution exhibit a high specific capacitance of 180 Fg−1 at a discharge current density of 1 mAcm−2 and energy density of 22.0–15.8 Whkg−1 in the power density range of 400–10,000 W kg−1 in 6 M KOH aqueous electrolyte, owing to the larger amount of heteroatoms such as boron and the addition of the pitch, thereby inducing fast and reversible surface redox reactions in aqueous electrolyte. Furthermore, the highly mesoporous PPB-10 composite offers 92% capacity retention of the initial current density and a low equivalent series resistance through quick pathways for ion transport and charge diffusion. Hence, these PPB composites exhibit a very promising potential as electrode materials for supercapacitor electrodes due to boron incorporated into the carbon framework, suitable porosity, and good electrical conductivity.

Highly conductive pitch-based carbon nanofiber/MnO2 composites for high-capacitance supercapacitors

PAN/pitch-based carbon nanofiber/MnO2 (PPMn-CNF) composites are fabricated by electrospinning to obtain a new type of electrode material for application in high-capacitance electrical double-layer capacitors (EDLCs). The energy storage capabilities of these PPMn-CNFs reveal a maximum specific capacitance of 188 Fg-1 and a maximum energy density of 20.5–15.8 Whkg−1 in the power density range of 400–10,000 Wkg-1. Furthermore, the PPMn-CNF electrode shows good rate capability without a significant decrease at high rate. The high electrical conductivity of the pitch promotes the accessibility into the micropores and the adsorption efficiency onto the electrode surface of electrolyte ions. Therefore, the high electrical conductivity and large surface area of the PPMn-CNF composites are beneficial for the storage of charge carriers and induce a short course for charge transport, which maximizes the specific capacitance and ensures good capacitive capability.