Zn-based thermal charging devices,utilizing the synergistic effect of ion thermoextraction and thermodiffusion,are able to efficiently convert thermal energy into electrical energy and storage in the devices,making th...Zn-based thermal charging devices,utilizing the synergistic effect of ion thermoextraction and thermodiffusion,are able to efficiently convert thermal energy into electrical energy and storage in the devices,making them a highly promising technology for low-grade heat recovery and utilization.However,the low output power density and energy conversion efficiency resulted by the slow diffusion kinetics of Zn^(2+)hinder their development.Herein,we present a highperformance thermal charging cell design using Zn^(2+)/NH_(4)^(+)hybrid ion electrolyte,which not only maintains the high output voltage of the Zn-based thermoelectric system,but also significantly enhances the output power density due to the fast diffusion kinetics of NH_(4)^(+).Based on this strategy,the thermal charging cell displays a high thermopower of 12.5 mV K^(-1)and an excellent normalized power density of 19.6 mW m^(-2)K^(-2)at a temperature difference of 35 K.The Carnot-relative efficiency is as high as 12.74%.Moreover,it can operate continuously for over 72 h when the temperature difference persists,achieving a balance between thermoelectric conversion and output.This work provides a simple and effective strategy for the design of high-performance thermal charging cells for low-grade heat conversion and utilization.展开更多
With the continued advancement of deep electrification across various industries, the demand for higher power density in electric machines is steadily increasing. However, realizing high power density remains a signif...With the continued advancement of deep electrification across various industries, the demand for higher power density in electric machines is steadily increasing. However, realizing high power density remains a significant technical challenge and has become a major bottleneck in machine development. The design of such machines is inherently constrained by the strong coupling among electromagnetic(EM), thermal, and mechanical domains, while systematic analyses of these challenges remain insufficient. This paper clarifies the interdependent relationships among these domains during the machine design process. It reviews key enabling strategies, including machine design based on advanced electromagnetic theory, innovative thermal management techniques, cutting-edge material advancements, and state-of-the-art manufacturing technologies, that collectively enhance the performance and feasibility of high power density machines(HPDMs). The insights provided aim to support the development of nextgeneration machine systems with higher power density, compact size, and robust, sustainable performance across a wide range of industrial and technological applications.展开更多
The output radiation from the 100μm×1μm aperture of a high power Laser Diode (LD) is efficiently coupled into a 50μm multimode optical fiber.The fiber output of the high power LD with high brightness and high ...The output radiation from the 100μm×1μm aperture of a high power Laser Diode (LD) is efficiently coupled into a 50μm multimode optical fiber.The fiber output of the high power LD with high brightness and high power density is achieved.The power density is up to 3 6×104W/cm2 and the coupling efficiency is 70%.The extreme divergence and the astigmatism of high power LDs require the optics with complex lens structures and high performance.A double-curved lens with two crossed cylindrical lenses structured on both sides of the glass substrate is used in the coupling system.展开更多
The model of heat source(MHS) which reflects the thermal interaction between materials and laser during processing determines the accuracy of simulation results. To acquire desirable simulations results, although vari...The model of heat source(MHS) which reflects the thermal interaction between materials and laser during processing determines the accuracy of simulation results. To acquire desirable simulations results, although various modifications of heat sources in the aspect of absorption process of laser by materials have been purposed, the distribution of laser power density(DLPD) in MHS is still modeled theoretically. However, in the actual situations of laser processing, the DLPD is definitely different from the ideal models. So, it is indispensable to build MHS using actual DLPD to improve the accuracy of simulation results. Besides, an automatic modeling method will be benefit to simplify the tedious pre-processing of simulations. This paper presents a modeling method and corresponding algorithm to model heat source using measured DLPD. This algorithm automatically processes original data to get modeling parameters and provides a step MHS combining with absorption models. Simulations and experiments of heat transfer in steel plates irradiated by laser prove the mothed and the step MHS. Moreover, the investigations of laser induced thermal-crack propagation in glass highlight the signification of modeling heat source based on actual DLPD and demonstrate the enormous application of this method in the simulation of laser processing.展开更多
In this letter,high power density AlGaN/GaN high electron-mobility transistors(HEMTs)on a freestanding GaN substrate are reported.An asymmetricΓ-shaped 500-nm gate with a field plate of 650 nm is introduced to improv...In this letter,high power density AlGaN/GaN high electron-mobility transistors(HEMTs)on a freestanding GaN substrate are reported.An asymmetricΓ-shaped 500-nm gate with a field plate of 650 nm is introduced to improve microwave power performance.The breakdown voltage(BV)is increased to more than 200 V for the fabricated device with gate-to-source and gate-to-drain distances of 1.08 and 2.92μm.A record continuous-wave power density of 11.2 W/mm@10 GHz is realized with a drain bias of 70 V.The maximum oscillation frequency(f_(max))and unity current gain cut-off frequency(f_(t))of the AlGaN/GaN HEMTs exceed 30 and 20 GHz,respectively.The results demonstrate the potential of AlGaN/GaN HEMTs on freestanding GaN substrates for microwave power applications.展开更多
The mechanical properties of laser welded joints under impact loadings such as explosion and car crash etc. are critical for the engineering designs. The hardness, static and dynamic mechanical properties of AISI304 a...The mechanical properties of laser welded joints under impact loadings such as explosion and car crash etc. are critical for the engineering designs. The hardness, static and dynamic mechanical properties of AISI304 and AISI316L dissimilar stainless steel welded joints by CO2 laser were experimentally studied. The dynamic strainstress curves at the strain rate around 103 s-1 were obtained by the split Hopkinson tensile bar （SHTB）. The static mechanical properties of the welded joints have little changes with the laser power density and all fracture occurs at 316 L side. However, the strain rate sensitivity has a strong depen- dence on laser power density. The value of strain rate factor decreases with the increase of laser power density. The welded joint which may be applied for the impact loading can be obtained by reducing the laser power density in the case of welding quality assurance.展开更多
In the present study, wind speed data of Jumla, Nepal have been statistically analyzed. For this purpose, the daily averaged wind speed data for 10 year period (2004-2014: 2012 excluded) provided by Department of Hydr...In the present study, wind speed data of Jumla, Nepal have been statistically analyzed. For this purpose, the daily averaged wind speed data for 10 year period (2004-2014: 2012 excluded) provided by Department of Hydrology and Meteorology (DHM) was analyzed to estimate wind power density. Wind speed as high as 18 m/s was recorded at height of 10 m. Annual mean wind speed was ascertained to be decreasing from 7.35 m/s in 2004 to 5.13 m/s in 2014 as a consequence of Global Climate Change. This is a subject of concern looking at government’s plan to harness wind energy. Monthly wind speed plot shows that the fastest wind speed is generally in month of June (Monsoon Season) and slowest in December/January (Winter Season). Results presented Weibull distribution to fit measured probability distribution better than the Rayleigh distribution for whole years in High altitude region of Nepal. Average value of wind power density based on mean and root mean cube seed approaches were 131.31 W/m<sup>2</sup>/year and 184.93 W/m<sup>2</sup>/year respectively indicating that Jumla stands in class III. Weibull distribution shows a good approximation for estimation of power density with maximum error of 3.68% when root mean cube speed is taken as reference.展开更多
Multi-converter approach based on the series and parallel connection topology of modular power converters has been proposed to realize higher power density DC-DC converter. The availability of the proposed approach ha...Multi-converter approach based on the series and parallel connection topology of modular power converters has been proposed to realize higher power density DC-DC converter. The availability of the proposed approach has been verified through the design consideration and the experiment. The design consideration for two DC-DC converters has been carried out by utilizing the power converter exact loss simulator, and the design parameters to maximize their power densities have been extracted taking the trade-off between the conversion efficiency and the power density into account. The prototypes of a 2,400 W, 256-384 V boost chopper using SiC-MOSFETs and a 300 W, 32-48 V GaN-FETs boost chopper have been also developed based on the design. The SiC chopper achieved the efficiency of 97.8% and the power density of 12,8 W/cm3, and the GaN chopper accomplished 98.9% and 18.6 W/cm3 in the experiment. These results show the validity of the design and the availability of the proposed approach. The multi-converter approach enables the cost reduction of the modular power converters, and contributes to realizing the widespread use of power electronics converters in the future 380 V DC distribution system.展开更多
The impact of ultrasonic power density on changes of heavy metals during sludge sonication was inves-tigated. Results showed that ultrasound could release heavy metals from sludge into the supernatant. There existed a...The impact of ultrasonic power density on changes of heavy metals during sludge sonication was inves-tigated. Results showed that ultrasound could release heavy metals from sludge into the supernatant. There existed an effective power density range of 0.8-1.6 W·ml?1 for the release of the total heavy metal; there was little release below 0.8 W·ml?1 and too high power density was adverse to the release. Furthermore, sonication showed selective release of heavy metal from sludge to the supernatant; copper, cadmium and lead were not released by sonication, while arsenic and nickel were released easily and their release ratio could reach 40%. The effective energy range for each heavy metal was also different that 0.8-1.2 W·ml?1 for arsenic, 0.5-1.6 W·ml?1 for nickel, and 0.8-1.6 W·ml?1 for mercury and chrome. The differences among heavy metal release during sonication might be explained by the different distribution of chemical fractions of each metal in sludge. Such selectivity could be used to control heavy metal release during sludge treatment.展开更多
In engineering practice,the output performance of contact separation TENGs(CS-TENGs)increases with the increase of tribo-pair area,which includes increasing the size of single layer CS-TENGs(SCS-TENGs)or the number of...In engineering practice,the output performance of contact separation TENGs(CS-TENGs)increases with the increase of tribo-pair area,which includes increasing the size of single layer CS-TENGs(SCS-TENGs)or the number of units(zigzag TENGs).However,such two strategies show significant differences in output power and power density.In this study,to seek a universal CS-TENG design solution,the output performance of a SCS-TENG and a zigzag TENG(Z-TENG)is systematically compared,including voltage,current,transferred charge,instantaneous power density,and charging power density.The relationship between contact area and output voltages is explored,and the output voltage equation is fitted.The experimental results reveal that SCS-TENGs yield better performance than Z-TENGs in terms of voltage,power,and power density under the same total contact area.Z-TENGs show energy loss during the transfer of mechanical energy,and such loss is aggravated by the increasing number of units.The instantaneous peak power of the SCS-TENG is up to 22 times that of the Z-TENG(45 cm^(2)).Furthermore,the power density of capacitor charging of SCS-TENGs is 131%of that of Z-TENGs,which are relatively close.Z-TENG is a feasible alternative when the working space is limited.展开更多
Direct liquid fuel cells(DLFCs)are proposed to address the problems of high cost and complex storage and transportation of hydrogen in traditional hydrogen-oxygen proton exchange membrane fuel cells.However,present fu...Direct liquid fuel cells(DLFCs)are proposed to address the problems of high cost and complex storage and transportation of hydrogen in traditional hydrogen-oxygen proton exchange membrane fuel cells.However,present fuels of organic small molecules used in DLFCs are restricted to problems of sluggish electrochemical kinetics and easily poisoning of precious metal catalysts.Herein,we demonstrate reduced phosphotungstic acid as a liquid fuel for DLFCs based on its advantages of high chemical and electrochemical stability,high electrochemical activity on common carbon material electrodes,and low permeability through proton exchange membranes.The application of phosphotungstic acid fuel effectively solves the problems of high cost of anode catalysts and serious fuel permeation loss in traditional DLFCs.A phosphotungstic acid fuel cell achieves a peak power density of466 mW cm^(-2)at a cell voltage of 0.42 V and good stability at current densities in the range from 20 to 200 mA cm^(-2).展开更多
With the cvolution of various high powerr-density machines, it beeomes important to optimize the power potential of machines of vastly different topologies with a variety of waveforms of back emf and current. The appr...With the cvolution of various high powerr-density machines, it beeomes important to optimize the power potential of machines of vastly different topologies with a variety of waveforms of back emf and current. The approach of tins paper is based oil the gencral-purpose sizing equations. which permit the optinlization method of machine power density to be applied to the axial-flux toroidal permanent-magnet (AFTPM) machine, and,furthermore, the power-production capabilities of the AFTPM machinc and the wen-known squirrel-cage indution machine are compared.展开更多
Estimation of peak power density in the vicinity of cellular base stations and comparison of theoretical values with exposure limits for public,offers the possibility of knowing the safety distance from those antennas...Estimation of peak power density in the vicinity of cellular base stations and comparison of theoretical values with exposure limits for public,offers the possibility of knowing the safety distance from those antennas.In this paper,we present results of estimations for the peak power density radiated from antennas,by eliminating or not considering reflected waves from different surfaces.This method is used to estimate the peak power density of non ionizing radiation of cellular antennas,installed in 12 regions of Albanian territory by a cellular operator.We have estimated the safety distances from all types of installed antennas,in order to have a clear idea for the safety distances from cellular base station at 900 MHz.展开更多
GaN （gallium nitride） buck-rectifier has been proposed to realize high power density ISOP （input series and output parallel）-IPOS （input parallel and output series） converter-based dc distribution system. The ul...GaN （gallium nitride） buck-rectifier has been proposed to realize high power density ISOP （input series and output parallel）-IPOS （input parallel and output series） converter-based dc distribution system. The ultra-low loss bi-directional switch can be developed by the GaN power device because of the low on-resistance, the high-speed switching behavior and its own device structure. The buck-rectifier using the GaN bi-directional switches has the potential to achieve higher power density than the commonly utilized boost-rectifier. Availability of the GaN-HEMT （high electron mobility transistor） for the buck rectifier has been verified taking the theoretical limit of the on-resistance and the switching loss energy into account. Design consideration for a high power density buck-rectifier has been also conducted and the application effect of the GaN bidirectional switches has been evaluated quantitatively. The ISOP-IPOS converter-based dc （direct current） distribution system takes full advantage of the buck-rectifier and the rectifier using GaN devices contributes to realizing higher power density dc distribution system.展开更多
Lead-free_(0.94)(Bi_(0.5)Na_(0.5))_(0.94)TiO_(3)-0.06BaTiO_(3)/BiFeO_(3)(abbreviated as BNBT/xBFO)multilayer thin film hetero-structures were deposited on Pt(111)/Ti/SiO_(2)/Si substrates using a sol–gel/spin coating...Lead-free_(0.94)(Bi_(0.5)Na_(0.5))_(0.94)TiO_(3)-0.06BaTiO_(3)/BiFeO_(3)(abbreviated as BNBT/xBFO)multilayer thin film hetero-structures were deposited on Pt(111)/Ti/SiO_(2)/Si substrates using a sol–gel/spin coating method.With the increase of the number of BFO layers,the insulativity and the breakdown field strength can be significantly improved because the interfaces can act as obstructers to the development of the electric field.展开更多
Aqueous Mg–air batteries are promising candidates for large-scale energy storage,but their practical application is significantly hindered by water-induced parasitic reactions and the chunk effect.In this study,tauri...Aqueous Mg–air batteries are promising candidates for large-scale energy storage,but their practical application is significantly hindered by water-induced parasitic reactions and the chunk effect.In this study,taurine(Tau)is an effective electrolyte additive for aqueous Mg–air batteries utilizing various magnesium anodes.The non-sacrificial Tau molecules,possessing both donor and acceptor groups,disrupt the inherent hydrogen bond network of H_(2)O and replace solvated H_(2)O in the Mg^(2+)solvation sheath,forming dynamic adsorption on Mg and creating a hydrophobic electric double layer.Consequently,taurine inhibits H_(2)O attack on Mg,promoting uniform Mg stripping.An appropriate amount of Tau not only enhances the discharge activity of the AZ31 anode but also suppresses its self-discharge phenomenon,achieving a surprising effect of simultaneously boosting voltage and anode utilization.Therefore,0.2 M Tau was identified as the optimal electrolyte concentration and successfully incorporated in Mg–air batteries utilizing various routine Mg anodes(such as VW83,VW103,and LA103Z).The results demonstrate that Tau effectively enhances the discharge properties for Mg–air batteries employing various magnesium anodes.展开更多
A mixed valence V_(4)O_(9)with a tunnel structure and room temperature metallic properties was prepared via partially reducing vanadium pentoxide.Its unique tunnel structure and metallic characteristics are beneficial...A mixed valence V_(4)O_(9)with a tunnel structure and room temperature metallic properties was prepared via partially reducing vanadium pentoxide.Its unique tunnel structure and metallic characteristics are beneficial for fast zinc ion and electron diffusion.A Zn//V_(4)O_(9)battery displays a high rate performance of 234.4 mA h g^(-1)at 50C,a large reversible capacity of 420 mA h g^(-1)at 0.5C,and good cycling stability.The intercalation mechanism of Zn^(2+)and H^(+)is proved by in situ XRD,ex situ XPS,and other characterization techniques,along with density functional theory calculations.This work provides a strategy to improve the electrochemical performance of vanadium oxides through valence regulation.展开更多
Iron single-atom catalysts(Fe SACs)show promise for enhancing zinc-air batteries(ZABs),but their applications are hindered by complex preparation,high cost,and inefficient active site utilization.Here,we present a cos...Iron single-atom catalysts(Fe SACs)show promise for enhancing zinc-air batteries(ZABs),but their applications are hindered by complex preparation,high cost,and inefficient active site utilization.Here,we present a cost-effective,scalable,and environmentally benign approach,using pyrolysis of Ketjen black(KB)with an iron o-phenanthroline complex to create Fe-N-C sites(FeNC-KB).The FeNC-KB catalyst demonstrated high numbers of active sites accessibility and superior oxygen reduction reaction(ORR)properties with a half-wave potential of 0.91 V versus reversible hydrogen electrode(RHE).Notably,its kinetic current density at 0.90 V was 5.1 times that of the 20 wt%Pt/C.In-situ Raman spectroscopy combined with theoretical analysis revealed that the Fe-N_(3)sites exhibited a lower reaction energy barrier,accelerating the formation of*OOH,possessing moderate intermediate adsorption energy,thereby enhancing the ORR activity.When used in a ZAB,the FeNC-KB catalyst achieved an excellent power density of 717mWcm^(-2)and maintained stable operation at 300 mA cm^(-2)for 140 h.These results highlight the efficacy of FeNC-KB catalysts in ZABs and provide a strategy for designing highly active and stable SACs.This advancement has a profound influence on the development of high-performance ZABs and the broader field of energy storage technologies.展开更多
To realize a high-power density capacitive coupler for miniaturized rotary application,a parameter design method is proposed in this article.Since a capacitive coupler is composed of metal plates and external capacito...To realize a high-power density capacitive coupler for miniaturized rotary application,a parameter design method is proposed in this article.Since a capacitive coupler is composed of metal plates and external capacitors,limitations include two parts.One is air breakdown voltage limitation of metal plates,and the other one is withstanding voltage limitation of external capacitors.To quantify voltage limitation,voltage stress is analyzed.Then,an optimal design method for external capacitors is proposed,realizing minimum voltage stress and optimal load condition,simultaneously.Using an optimal external capacitor,a parameter design method is proposed to maximize power density.In which,the air breakdown voltage limitation determines the range of resonant frequency.Then,the withstanding voltage limitation determines specific resonant frequency and composition of external capacitors.Experimental results show the proposed method can realize DC-DC efficiency of 91.35% and power density of 423 W/in^(3)under a 1 kW output condition.展开更多
Governed by the second law of thermodynamics,waste heat generation is inevitable and has been a major source of energy loss and environmental concern in human society.Harvesting waste heat into useful energy has thus ...Governed by the second law of thermodynamics,waste heat generation is inevitable and has been a major source of energy loss and environmental concern in human society.Harvesting waste heat into useful energy has thus become a paramount priority,but has remained challenging with efficiency and cost constraints.Thermoelectric generators(TEGs),which convert heat into electricity whenever there is a temperature difference,play a crucial role in waste heat harvesting.However,sustaining the temperature difference for uninterrupted and high-power density electricity generation is a major challenge in TEGs to achieve practical applications due to the thermal equilibrium.Here,we demonstrate a diurnal waste heat harvester by integrating a high-power radiative cooling film as the cool end of TEGs to enable a large and continuous temperature difference.Significant voltage increase from 30.0 mV to 65.7 mV was achieved,leading to a dramatic power density enhancement of 4.8 times from 35.2 mW m^(-2)to 168.6 mW m^(-2).In an open zone,an ultra-high power density of 2.76 W m^(-2)was achieved at a heat source temperature of 80°C,exceeding the performance of state-of-the-art radiatively cooled TEGs.More importantly,a portable and foldable thermal energy harvesting prototype composed of 24 TEGs arranged in an array has been constructed.When attached to a hot object(e.g.a car engine hood),it can output 5 V to charge personal electronics(e.g.a cellphone),making it a promising practical device for harvesting waste heat in a wide range of outdoor applications.展开更多
基金supported by the Leading Edge Technology of Jiangsu Province(BK20222009-X.Z.,BK20202008-X.Z.)Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD)National Undergraduate Innovation Training Program of NUAA(202410287179Y).
文摘Zn-based thermal charging devices,utilizing the synergistic effect of ion thermoextraction and thermodiffusion,are able to efficiently convert thermal energy into electrical energy and storage in the devices,making them a highly promising technology for low-grade heat recovery and utilization.However,the low output power density and energy conversion efficiency resulted by the slow diffusion kinetics of Zn^(2+)hinder their development.Herein,we present a highperformance thermal charging cell design using Zn^(2+)/NH_(4)^(+)hybrid ion electrolyte,which not only maintains the high output voltage of the Zn-based thermoelectric system,but also significantly enhances the output power density due to the fast diffusion kinetics of NH_(4)^(+).Based on this strategy,the thermal charging cell displays a high thermopower of 12.5 mV K^(-1)and an excellent normalized power density of 19.6 mW m^(-2)K^(-2)at a temperature difference of 35 K.The Carnot-relative efficiency is as high as 12.74%.Moreover,it can operate continuously for over 72 h when the temperature difference persists,achieving a balance between thermoelectric conversion and output.This work provides a simple and effective strategy for the design of high-performance thermal charging cells for low-grade heat conversion and utilization.
基金supported in part by the National Key Research and Development Program of China (No. 2020YFA0710500)in part by the National Natural Science Foundation of China (NSFC)(No. 52277066)in part by the State Key Laboratory of Electrical Insulation and Power Equipment Foundation (No. EIPE23131)。
文摘With the continued advancement of deep electrification across various industries, the demand for higher power density in electric machines is steadily increasing. However, realizing high power density remains a significant technical challenge and has become a major bottleneck in machine development. The design of such machines is inherently constrained by the strong coupling among electromagnetic(EM), thermal, and mechanical domains, while systematic analyses of these challenges remain insufficient. This paper clarifies the interdependent relationships among these domains during the machine design process. It reviews key enabling strategies, including machine design based on advanced electromagnetic theory, innovative thermal management techniques, cutting-edge material advancements, and state-of-the-art manufacturing technologies, that collectively enhance the performance and feasibility of high power density machines(HPDMs). The insights provided aim to support the development of nextgeneration machine systems with higher power density, compact size, and robust, sustainable performance across a wide range of industrial and technological applications.
文摘The output radiation from the 100μm×1μm aperture of a high power Laser Diode (LD) is efficiently coupled into a 50μm multimode optical fiber.The fiber output of the high power LD with high brightness and high power density is achieved.The power density is up to 3 6×104W/cm2 and the coupling efficiency is 70%.The extreme divergence and the astigmatism of high power LDs require the optics with complex lens structures and high performance.A double-curved lens with two crossed cylindrical lenses structured on both sides of the glass substrate is used in the coupling system.
基金Project(2021YFF0500200) supported by the National Key R&D Program of ChinaProject(52105437) supported by the National Natural Science Foundation of China+1 种基金Project(202006120184) supported by the Heilongjiang Provincial Postdoctoral Science Foundation,ChinaProject(LBH-Z20054) supported by the China Scholarship Council。
文摘The model of heat source(MHS) which reflects the thermal interaction between materials and laser during processing determines the accuracy of simulation results. To acquire desirable simulations results, although various modifications of heat sources in the aspect of absorption process of laser by materials have been purposed, the distribution of laser power density(DLPD) in MHS is still modeled theoretically. However, in the actual situations of laser processing, the DLPD is definitely different from the ideal models. So, it is indispensable to build MHS using actual DLPD to improve the accuracy of simulation results. Besides, an automatic modeling method will be benefit to simplify the tedious pre-processing of simulations. This paper presents a modeling method and corresponding algorithm to model heat source using measured DLPD. This algorithm automatically processes original data to get modeling parameters and provides a step MHS combining with absorption models. Simulations and experiments of heat transfer in steel plates irradiated by laser prove the mothed and the step MHS. Moreover, the investigations of laser induced thermal-crack propagation in glass highlight the signification of modeling heat source based on actual DLPD and demonstrate the enormous application of this method in the simulation of laser processing.
文摘In this letter,high power density AlGaN/GaN high electron-mobility transistors(HEMTs)on a freestanding GaN substrate are reported.An asymmetricΓ-shaped 500-nm gate with a field plate of 650 nm is introduced to improve microwave power performance.The breakdown voltage(BV)is increased to more than 200 V for the fabricated device with gate-to-source and gate-to-drain distances of 1.08 and 2.92μm.A record continuous-wave power density of 11.2 W/mm@10 GHz is realized with a drain bias of 70 V.The maximum oscillation frequency(f_(max))and unity current gain cut-off frequency(f_(t))of the AlGaN/GaN HEMTs exceed 30 and 20 GHz,respectively.The results demonstrate the potential of AlGaN/GaN HEMTs on freestanding GaN substrates for microwave power applications.
基金supported by the National Natural Science Foundation of China (10832011)
文摘The mechanical properties of laser welded joints under impact loadings such as explosion and car crash etc. are critical for the engineering designs. The hardness, static and dynamic mechanical properties of AISI304 and AISI316L dissimilar stainless steel welded joints by CO2 laser were experimentally studied. The dynamic strainstress curves at the strain rate around 103 s-1 were obtained by the split Hopkinson tensile bar （SHTB）. The static mechanical properties of the welded joints have little changes with the laser power density and all fracture occurs at 316 L side. However, the strain rate sensitivity has a strong depen- dence on laser power density. The value of strain rate factor decreases with the increase of laser power density. The welded joint which may be applied for the impact loading can be obtained by reducing the laser power density in the case of welding quality assurance.
文摘In the present study, wind speed data of Jumla, Nepal have been statistically analyzed. For this purpose, the daily averaged wind speed data for 10 year period (2004-2014: 2012 excluded) provided by Department of Hydrology and Meteorology (DHM) was analyzed to estimate wind power density. Wind speed as high as 18 m/s was recorded at height of 10 m. Annual mean wind speed was ascertained to be decreasing from 7.35 m/s in 2004 to 5.13 m/s in 2014 as a consequence of Global Climate Change. This is a subject of concern looking at government’s plan to harness wind energy. Monthly wind speed plot shows that the fastest wind speed is generally in month of June (Monsoon Season) and slowest in December/January (Winter Season). Results presented Weibull distribution to fit measured probability distribution better than the Rayleigh distribution for whole years in High altitude region of Nepal. Average value of wind power density based on mean and root mean cube seed approaches were 131.31 W/m<sup>2</sup>/year and 184.93 W/m<sup>2</sup>/year respectively indicating that Jumla stands in class III. Weibull distribution shows a good approximation for estimation of power density with maximum error of 3.68% when root mean cube speed is taken as reference.
文摘Multi-converter approach based on the series and parallel connection topology of modular power converters has been proposed to realize higher power density DC-DC converter. The availability of the proposed approach has been verified through the design consideration and the experiment. The design consideration for two DC-DC converters has been carried out by utilizing the power converter exact loss simulator, and the design parameters to maximize their power densities have been extracted taking the trade-off between the conversion efficiency and the power density into account. The prototypes of a 2,400 W, 256-384 V boost chopper using SiC-MOSFETs and a 300 W, 32-48 V GaN-FETs boost chopper have been also developed based on the design. The SiC chopper achieved the efficiency of 97.8% and the power density of 12,8 W/cm3, and the GaN chopper accomplished 98.9% and 18.6 W/cm3 in the experiment. These results show the validity of the design and the availability of the proposed approach. The multi-converter approach enables the cost reduction of the modular power converters, and contributes to realizing the widespread use of power electronics converters in the future 380 V DC distribution system.
基金Supported by the Basic Research Funds in Renmin University of China from the center government(12XNL101)
文摘The impact of ultrasonic power density on changes of heavy metals during sludge sonication was inves-tigated. Results showed that ultrasound could release heavy metals from sludge into the supernatant. There existed an effective power density range of 0.8-1.6 W·ml?1 for the release of the total heavy metal; there was little release below 0.8 W·ml?1 and too high power density was adverse to the release. Furthermore, sonication showed selective release of heavy metal from sludge to the supernatant; copper, cadmium and lead were not released by sonication, while arsenic and nickel were released easily and their release ratio could reach 40%. The effective energy range for each heavy metal was also different that 0.8-1.2 W·ml?1 for arsenic, 0.5-1.6 W·ml?1 for nickel, and 0.8-1.6 W·ml?1 for mercury and chrome. The differences among heavy metal release during sonication might be explained by the different distribution of chemical fractions of each metal in sludge. Such selectivity could be used to control heavy metal release during sludge treatment.
基金funded by National Natural Science Foundation of China(Nos.:62225308 and 62001281)Shanghai Science and Technology Committee(22dz1204300)
文摘In engineering practice,the output performance of contact separation TENGs(CS-TENGs)increases with the increase of tribo-pair area,which includes increasing the size of single layer CS-TENGs(SCS-TENGs)or the number of units(zigzag TENGs).However,such two strategies show significant differences in output power and power density.In this study,to seek a universal CS-TENG design solution,the output performance of a SCS-TENG and a zigzag TENG(Z-TENG)is systematically compared,including voltage,current,transferred charge,instantaneous power density,and charging power density.The relationship between contact area and output voltages is explored,and the output voltage equation is fitted.The experimental results reveal that SCS-TENGs yield better performance than Z-TENGs in terms of voltage,power,and power density under the same total contact area.Z-TENGs show energy loss during the transfer of mechanical energy,and such loss is aggravated by the increasing number of units.The instantaneous peak power of the SCS-TENG is up to 22 times that of the Z-TENG(45 cm^(2)).Furthermore,the power density of capacitor charging of SCS-TENGs is 131%of that of Z-TENGs,which are relatively close.Z-TENG is a feasible alternative when the working space is limited.
基金financialy supported by the National Key R&D Program of China(No.2018YFB1502303)the National Natural Science Foundation of China(No.21722601,U19A2017)China Postdoctoral Science Foundation(No.2019M660389)。
文摘Direct liquid fuel cells(DLFCs)are proposed to address the problems of high cost and complex storage and transportation of hydrogen in traditional hydrogen-oxygen proton exchange membrane fuel cells.However,present fuels of organic small molecules used in DLFCs are restricted to problems of sluggish electrochemical kinetics and easily poisoning of precious metal catalysts.Herein,we demonstrate reduced phosphotungstic acid as a liquid fuel for DLFCs based on its advantages of high chemical and electrochemical stability,high electrochemical activity on common carbon material electrodes,and low permeability through proton exchange membranes.The application of phosphotungstic acid fuel effectively solves the problems of high cost of anode catalysts and serious fuel permeation loss in traditional DLFCs.A phosphotungstic acid fuel cell achieves a peak power density of466 mW cm^(-2)at a cell voltage of 0.42 V and good stability at current densities in the range from 20 to 200 mA cm^(-2).
文摘With the cvolution of various high powerr-density machines, it beeomes important to optimize the power potential of machines of vastly different topologies with a variety of waveforms of back emf and current. The approach of tins paper is based oil the gencral-purpose sizing equations. which permit the optinlization method of machine power density to be applied to the axial-flux toroidal permanent-magnet (AFTPM) machine, and,furthermore, the power-production capabilities of the AFTPM machinc and the wen-known squirrel-cage indution machine are compared.
文摘Estimation of peak power density in the vicinity of cellular base stations and comparison of theoretical values with exposure limits for public,offers the possibility of knowing the safety distance from those antennas.In this paper,we present results of estimations for the peak power density radiated from antennas,by eliminating or not considering reflected waves from different surfaces.This method is used to estimate the peak power density of non ionizing radiation of cellular antennas,installed in 12 regions of Albanian territory by a cellular operator.We have estimated the safety distances from all types of installed antennas,in order to have a clear idea for the safety distances from cellular base station at 900 MHz.
文摘GaN （gallium nitride） buck-rectifier has been proposed to realize high power density ISOP （input series and output parallel）-IPOS （input parallel and output series） converter-based dc distribution system. The ultra-low loss bi-directional switch can be developed by the GaN power device because of the low on-resistance, the high-speed switching behavior and its own device structure. The buck-rectifier using the GaN bi-directional switches has the potential to achieve higher power density than the commonly utilized boost-rectifier. Availability of the GaN-HEMT （high electron mobility transistor） for the buck rectifier has been verified taking the theoretical limit of the on-resistance and the switching loss energy into account. Design consideration for a high power density buck-rectifier has been also conducted and the application effect of the GaN bidirectional switches has been evaluated quantitatively. The ISOP-IPOS converter-based dc （direct current） distribution system takes full advantage of the buck-rectifier and the rectifier using GaN devices contributes to realizing higher power density dc distribution system.
基金support from the National Natural Science Foundation of China under grant no.51332003 and 51372171.
文摘Lead-free_(0.94)(Bi_(0.5)Na_(0.5))_(0.94)TiO_(3)-0.06BaTiO_(3)/BiFeO_(3)(abbreviated as BNBT/xBFO)multilayer thin film hetero-structures were deposited on Pt(111)/Ti/SiO_(2)/Si substrates using a sol–gel/spin coating method.With the increase of the number of BFO layers,the insulativity and the breakdown field strength can be significantly improved because the interfaces can act as obstructers to the development of the electric field.
基金supported by the National Key Research and Development Program of China(grant no.2023YFB3710900 and 2021YFB3701004)the National Natural Science Foundation of China(grant no.51971054,52171055)+1 种基金the China Postdoctoral Science Foundation(2023M743274)the Liaoning Province Science and Technology Plan Project(2023JH2/101700356).
文摘Aqueous Mg–air batteries are promising candidates for large-scale energy storage,but their practical application is significantly hindered by water-induced parasitic reactions and the chunk effect.In this study,taurine(Tau)is an effective electrolyte additive for aqueous Mg–air batteries utilizing various magnesium anodes.The non-sacrificial Tau molecules,possessing both donor and acceptor groups,disrupt the inherent hydrogen bond network of H_(2)O and replace solvated H_(2)O in the Mg^(2+)solvation sheath,forming dynamic adsorption on Mg and creating a hydrophobic electric double layer.Consequently,taurine inhibits H_(2)O attack on Mg,promoting uniform Mg stripping.An appropriate amount of Tau not only enhances the discharge activity of the AZ31 anode but also suppresses its self-discharge phenomenon,achieving a surprising effect of simultaneously boosting voltage and anode utilization.Therefore,0.2 M Tau was identified as the optimal electrolyte concentration and successfully incorporated in Mg–air batteries utilizing various routine Mg anodes(such as VW83,VW103,and LA103Z).The results demonstrate that Tau effectively enhances the discharge properties for Mg–air batteries employing various magnesium anodes.
基金supported by the National Key R&D Program(2019YFA0705600)the National NSFC(21673243,51771094 and 52001170).
文摘A mixed valence V_(4)O_(9)with a tunnel structure and room temperature metallic properties was prepared via partially reducing vanadium pentoxide.Its unique tunnel structure and metallic characteristics are beneficial for fast zinc ion and electron diffusion.A Zn//V_(4)O_(9)battery displays a high rate performance of 234.4 mA h g^(-1)at 50C,a large reversible capacity of 420 mA h g^(-1)at 0.5C,and good cycling stability.The intercalation mechanism of Zn^(2+)and H^(+)is proved by in situ XRD,ex situ XPS,and other characterization techniques,along with density functional theory calculations.This work provides a strategy to improve the electrochemical performance of vanadium oxides through valence regulation.
基金supported by the National Natural Science Foundation of China(grant nos.22225902,22109164,and U22A20436)the National Key Research&Development Program of China(grant nos.2022YFE0115900 and 2021YFA1501500).
文摘Iron single-atom catalysts(Fe SACs)show promise for enhancing zinc-air batteries(ZABs),but their applications are hindered by complex preparation,high cost,and inefficient active site utilization.Here,we present a cost-effective,scalable,and environmentally benign approach,using pyrolysis of Ketjen black(KB)with an iron o-phenanthroline complex to create Fe-N-C sites(FeNC-KB).The FeNC-KB catalyst demonstrated high numbers of active sites accessibility and superior oxygen reduction reaction(ORR)properties with a half-wave potential of 0.91 V versus reversible hydrogen electrode(RHE).Notably,its kinetic current density at 0.90 V was 5.1 times that of the 20 wt%Pt/C.In-situ Raman spectroscopy combined with theoretical analysis revealed that the Fe-N_(3)sites exhibited a lower reaction energy barrier,accelerating the formation of*OOH,possessing moderate intermediate adsorption energy,thereby enhancing the ORR activity.When used in a ZAB,the FeNC-KB catalyst achieved an excellent power density of 717mWcm^(-2)and maintained stable operation at 300 mA cm^(-2)for 140 h.These results highlight the efficacy of FeNC-KB catalysts in ZABs and provide a strategy for designing highly active and stable SACs.This advancement has a profound influence on the development of high-performance ZABs and the broader field of energy storage technologies.
基金supported in part by the National Natural Science Foundation of China under Grant 52277185.
文摘To realize a high-power density capacitive coupler for miniaturized rotary application,a parameter design method is proposed in this article.Since a capacitive coupler is composed of metal plates and external capacitors,limitations include two parts.One is air breakdown voltage limitation of metal plates,and the other one is withstanding voltage limitation of external capacitors.To quantify voltage limitation,voltage stress is analyzed.Then,an optimal design method for external capacitors is proposed,realizing minimum voltage stress and optimal load condition,simultaneously.Using an optimal external capacitor,a parameter design method is proposed to maximize power density.In which,the air breakdown voltage limitation determines the range of resonant frequency.Then,the withstanding voltage limitation determines specific resonant frequency and composition of external capacitors.Experimental results show the proposed method can realize DC-DC efficiency of 91.35% and power density of 423 W/in^(3)under a 1 kW output condition.
基金support from the Key Research and Development Program of Shandong Province(No.2022SFGC0501)Shenzhen Science and Technology Program(International Cooperation Research)(No.GJHZ20240218113407015)+9 种基金Shenzhen Fundamental Research Program(Natural Science Foundation)(No.JCYJ20240813175900001)support from the Australian Research Council through the Discovery Project scheme(Grant No.DP190103186,DP220100603)support through the Future Fellowship scheme(Grant No.FT210100806)Discovery Project scheme(Grant No.DP250100980)Linkage Project scheme(LP210200345)the Industrial Transformation Research Hubs(Grant No.IH240100009)support through the Future Fellowship scheme(Grant No.FT220100559)Linkage Projects(Grant No.LP210100467)support through the Discovery Early Career Researcher Award scheme(DE230100383)support from the Natural Science Foundation of Shandong Province(Grant No.ZR2021ME162).
文摘Governed by the second law of thermodynamics,waste heat generation is inevitable and has been a major source of energy loss and environmental concern in human society.Harvesting waste heat into useful energy has thus become a paramount priority,but has remained challenging with efficiency and cost constraints.Thermoelectric generators(TEGs),which convert heat into electricity whenever there is a temperature difference,play a crucial role in waste heat harvesting.However,sustaining the temperature difference for uninterrupted and high-power density electricity generation is a major challenge in TEGs to achieve practical applications due to the thermal equilibrium.Here,we demonstrate a diurnal waste heat harvester by integrating a high-power radiative cooling film as the cool end of TEGs to enable a large and continuous temperature difference.Significant voltage increase from 30.0 mV to 65.7 mV was achieved,leading to a dramatic power density enhancement of 4.8 times from 35.2 mW m^(-2)to 168.6 mW m^(-2).In an open zone,an ultra-high power density of 2.76 W m^(-2)was achieved at a heat source temperature of 80°C,exceeding the performance of state-of-the-art radiatively cooled TEGs.More importantly,a portable and foldable thermal energy harvesting prototype composed of 24 TEGs arranged in an array has been constructed.When attached to a hot object(e.g.a car engine hood),it can output 5 V to charge personal electronics(e.g.a cellphone),making it a promising practical device for harvesting waste heat in a wide range of outdoor applications.
