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
Blind source separation and estimation of the number of sources usually demand that the number of sensors should be greater than or equal to that of the sources, which, however, is very difficult to satisfy for the co...Blind source separation and estimation of the number of sources usually demand that the number of sensors should be greater than or equal to that of the sources, which, however, is very difficult to satisfy for the complex systems. A new estimating method based on power spectral density (PSD) is presented. When the relation between the number of sensors and that of sources is unknown, the PSD matrix is first obtained by the ratio of PSD of the observation signals, and then the bound of the number of correlated sources with common frequencies can be estimated by comparing every column vector of PSD matrix. The effectiveness of the proposed method is verified by theoretical analysis and experiments, and the influence of noise on the estimation of number of source is simulated.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
A consequence of nonlinearities is a multi-harmonic response via a monoharmonic excitation.A similar phenomenon also exists in random vibration.The power spectral density(PSD)analysis of random vibration for nonlinear...A consequence of nonlinearities is a multi-harmonic response via a monoharmonic excitation.A similar phenomenon also exists in random vibration.The power spectral density(PSD)analysis of random vibration for nonlinear systems is studied in this paper.The analytical formulation of output PSD subject to the zero-mean Gaussian random load is deduced by using the Volterra series expansion and the conception of generalized frequency response function(GFRF).For a class of nonlinear systems,the growing exponential method is used to determine the first 3 rd-order GFRFs.The proposed approach is used to achieve the nonlinear system’s output PSD under a narrow-band stationary random input.The relationship between the peak of PSD and the parameters of the nonlinear system is discussed.By using the proposed method,the nonlinear characteristics of multi-band output via single-band input can be well predicted.The results reveal that changing nonlinear system parameters gives a one-of-a-kind change of the system’s output PSD.This paper provides a method for the research of random vibration prediction and control in real-world nonlinear systems.展开更多
To ensure the performance of the optical system, the machining accuracy of lens with long focal lengths is required to ensure the image quality. A new method for lens transmission wavefront power spectral density (PSD...To ensure the performance of the optical system, the machining accuracy of lens with long focal lengths is required to ensure the image quality. A new method for lens transmission wavefront power spectral density (PSD) in mid-frequency domain measurement using binary phase computer-generated hologram (CGH) is presented. This technique is widely applicable and is particularly useful for measuring large-size lenses with long focal lengths. A comparison experiment of the CGH measurement with results from a Fizeau sphere interferometry method is carried out to verify the accuracy and convenience of the measurement. Furthermore, measurement uncertainty due to CGH fabrication process is analysed. Analysis of the CGH test showed the overall accuracy of less than 1 nm RMS for a sphere lens with over 30 m focal length and Φ410 mm clear aperture. CGH can provide reference spheres with high precision, in the meantime greatly shorten air space, thus reducing the effect of vibration and air turbulence, therefore is of great importance for lens transmission wavefront PSD measurement. The realization of high precision, high efficiency and nondestructive testing of long focal-lens wavefront PSD ensure the ultra-precision and certainty level of machining, hence improving the comprehensive performance of the optical system.展开更多
Diabetes is a widespread and serious disease and noninvasive measurement has been in high demand.To address this problem,a power spectral density-based method was offered for determining glucose sensitive sub-bands in...Diabetes is a widespread and serious disease and noninvasive measurement has been in high demand.To address this problem,a power spectral density-based method was offered for determining glucose sensitive sub-bands in the nearinfrared(NIR)spectrum.The experiments were conducted using phantoms of different optical properties in-vitro conditions.The optical bands 1200–1300 nm and 2100–2200 nm were found feasible for measuring blood glucose.After that,a photoplethysmography(PPG)-based low cost and portable optical system was designed.It has six di®erent NIR wavelength LEDs for illumination and an InGaAs photodiode for detection.Optical density values were calculated through the system and used as independent variables for multiple linear regression analysis.The results of blood glucose levels for 24 known healthy subjects showed that the optical system prediction was nearly 80%in the A zone and 20%in the B zone according to the Clarke Error Grid analysis.It was shown that a promising easyuse,continuous,and compact optical system had been designed.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
Periodical macro stripes have been found on diamond film prepared on a Si substrate by the multifilament chemical vapor deposition (CVD) method when the filament substrate distance is less than 2 mm. The properties ...Periodical macro stripes have been found on diamond film prepared on a Si substrate by the multifilament chemical vapor deposition (CVD) method when the filament substrate distance is less than 2 mm. The properties of the stripe on the film were characterized qualitatively by a scanning electron microscope (SEM) and Raman scattering spectrometer. The measurement results show that this stripe corresponds to the different kinds of the microstructure and thickness of the film. Through calculation of the thermal radiation energy density distribution on the surface of substrate, this phenomenon can be explained successfully.展开更多
To study the role of autonomic nervous system in the period of developing syncope induced by head-up tilt test(HUT), we analysed the changes of heart rate power spectral density(HRPSD) in 50 patients with unexplained ...To study the role of autonomic nervous system in the period of developing syncope induced by head-up tilt test(HUT), we analysed the changes of heart rate power spectral density(HRPSD) in 50 patients with unexplained syncope, including 15 positive patients (Group 1) and 35 negative patients(Group 2), and 15 negative healthy persons(Group 3) in 5 minute periods before and after tilting and 5 minutes before the end of test. HRPSD and their changes in total(T), very low-frequence(VLF), low-frequence(LF), high-frequence(HF) and the ratio of low/high frequence(LF/HF) were similar (P>0.05) 5 minutes before and after tilting among three groups. Five minutes before the end of test, Group 1 had obvious increase of T, VLF, LF and LF/HF while Group 2 and 3 had not such significant changes. There was significant difference(P<0.01) compared Group 1 with Group 2, 3. The results showed that the abnormal regulatory function of autonomic nervous system played an important role in the mechanism of symcope induced by HUT, the positive group had abnormal increase of sympathetic tone and imbalance of sympathetic/parasympathetic neural tone before syncope appeared.展开更多
Nano Technology is the branch of technology that deals with dimensions and tolerances in terms of nanometers. In this paper, the electrical characteristics analysis is determined for the Nano-GaN HEMT and Micro-GaN HE...Nano Technology is the branch of technology that deals with dimensions and tolerances in terms of nanometers. In this paper, the electrical characteristics analysis is determined for the Nano-GaN HEMT and Micro-GaN HEMT and also power spectrum density is determined for GaN Nano-HEMT by reducing the gate length Lg in nm range. The GaN Nano HEMT is producing high current comparing to Micro GaN HEMT. Accuracy of the proposed analytical model results is verified with simulation results.展开更多
We report on the room-temperature cascade laser (QCL) at λ -4.7μm. cw operation of a surface grating Both grating design and material distributed feedback (DFB) quantum optimization are used to decrease the thre...We report on the room-temperature cascade laser (QCL) at λ -4.7μm. cw operation of a surface grating Both grating design and material distributed feedback (DFB) quantum optimization are used to decrease the threshold current density and to increase the output power. For a high-reflectivity-coated 13-μm-wide and 4- mm-long laser, high wall-plug efficiency of 6% is obtained at 20℃ from a single facet producing over I W of ew output power. The threshold current density of DFB QCL is as low as 1.13kA/cm^2 at 10℃ and 1.34kA/cm2 at 30℃ in cw mode. Stable single-mode emission with a side-mode suppression ratio of about 30 dB is observed in tile working temperature range of 20-50℃.展开更多
The sluggish kinetics of the electrochemical oxygen reduction reaction(ORR)in intermediatetemperature solid oxide fuel cells(IT-SOFCs)greatly limits the overall cell performance.In this study,an efficient and durable ...The sluggish kinetics of the electrochemical oxygen reduction reaction(ORR)in intermediatetemperature solid oxide fuel cells(IT-SOFCs)greatly limits the overall cell performance.In this study,an efficient and durable cathode material for IT-SOFCs is designed based on density functional theory(DFT)calculations by co-doping with Nb and Ta the B-site of the SrFeO_(3-δ)perovskite oxide.The DFT calculations suggest that Nb/Ta co-doping can regulate the energy band of the parent SrFeO_(3-δ)and help electron transfer.In symmetrical cells,such cathode with a SrFe_(0.8)Nb_(0.1)Ta_(0.1)O_(3-δ)(SFNT)detailed formula achieves a low cathode polarization resistance of 0.147Ωcm^(2) at 650℃.Electron spin resonance(ESR)and X-ray photoelectron spectroscopy(XPS)analysis confirm that the co-doping of Nb/Ta in SrFeO_(3-δ)B-site increases the balanced concentration of oxygen vacancies,enhancing the electrochemical performance when compared to 20 mol%Nb single-doped perovskite oxide.The cathode button cell with NiSDC|SDC|SFNT configuration achieves an outstanding peak power density of 1.3 W cm^(-2)at 650℃.Moreover,the button cell shows durability for 110 h under 0.65 V at 600℃ using wet H_(2) as fuel.展开更多
Triboelectric nanogenerators(TENGs)have emerged as a promising technology to harvest electrical energy from natural motions such as human movement,wind,and water flow.Although TENGs show significant potential in small...Triboelectric nanogenerators(TENGs)have emerged as a promising technology to harvest electrical energy from natural motions such as human movement,wind,and water flow.Although TENGs show significant potential in small-scale applications,developing large-scale TENGs capable of generating high power remains a significant challenge.Several factors that can affect the performance of large-scale TENGs are being investigated to overcome this challenge,including the size and configuration of dielectric materials.This study optimizes dielectrics regarding surface area,thickness,and multicell configuration to improve harvested electrical power density in large-scale TENGs.In the studies,glass fiber was used as the positive dielectric,and multipurpose white silicone was used as the negative dielectric because of their high tribo-potential,durability,and easy accessibility.In the size optimization phase,dielectric thicknesses and surface areas that provide the maximum power density were determined.Subsequently,horizontal and vertical multicell configurations were examined to efficiently integrate size-optimized dielectrics.The results reveal that large-scale TENGs with vertical multicell configurations can achieve high and usable energy density for electronics.The findings provide valuable insight into the development of large-scale TENGs with advanced power generation capabilities.展开更多
Lithium-ion batteries(LIBs)with the“double-high”characteristics of high energy density and high power density are in urgent demand for facilitating the development of advanced portable electronics.However,the lithiu...Lithium-ion batteries(LIBs)with the“double-high”characteristics of high energy density and high power density are in urgent demand for facilitating the development of advanced portable electronics.However,the lithium ion(Li+)-storage performance of the most commercialized lithium cobalt oxide(LiCoO_(2),LCO)cathodes is still far from satisfactory in terms of high-voltage and fast-charging capabilities for reaching the double-high target.Herein,we systematically summarize and discuss high-voltage and fast-charging LCO cathodes,covering in depth the key fundamental challenges,latest advancements in modification strategies,and future perspectives in this field.Comprehensive and elaborated discussions are first presented on key fundamental challenges related to structural degradation,interfacial instability,the inhomogeneity reactions,and sluggish interfacial kinetics.We provide an instructive summary of deep insights into promising modification strategies and underlying mechanisms,categorized into element doping(Li-site,cobalt-/oxygen-site,and multi-site doping)for improved Li+diffusivity and bulkstructure stability;surface coating(dielectrics,ionic/electronic conductors,and their combination)for surface stability and conductivity;nanosizing;combinations of these strategies;and other strategies(i.e.,optimization of the electrolyte,binder,tortuosity of electrodes,charging protocols,and prelithiation methods).Finally,forward-looking perspectives and promising directions are sketched out and insightfully elucidated,providing constructive suggestions and instructions for designing and realizing high-voltage and fast-charging LCO cathodes for next-generation double-high LIBs.展开更多
Exploring high ion/electron conductive olivine-type transition metal phosphates is of vital significance to broaden their applicability in rapid-charging devices.Herein,we report an interface engineered Li Fe0.5Mn0.5P...Exploring high ion/electron conductive olivine-type transition metal phosphates is of vital significance to broaden their applicability in rapid-charging devices.Herein,we report an interface engineered Li Fe0.5Mn0.5PO4/rGO@C cathode material by the synergistic effects of r GO and polydopamine-derived N-doped carbon.The well-distributed Li Fe0.5Mn0.5PO4nanoparticles are tightly anchored on r GO nanosheet benefited by the coating of N-doped carbon layer.The design of such an architecture can effectively suppress the agglomeration of nanoparticles with a shortened Li+transfer path.Meantime,the high-speed conducting network has been constructed by r GO and N-doped carbon,which exhibits the face-to-face contact with Li Fe0.5Mn0.5PO4nanoparticles,guaranteeing the rapid electron transfer.These profits endow the Li Fe0.5Mn0.5PO4/rGO@C hybrids with a fast charge-discharge ability,e.g.a high reversible capacity of 105 m Ah·g^-1at 10 C,much higher than that of the Li Fe0.5Mn0.5PO4@C nanoparticles(46 mA·h·g^-1).Furthermore,a 90.8%capacity retention can be obtained even after cycling 500 times at 2 C.This work gives a new avenue to fabricate transition metal phosphate with superior electrochemical performance for high-power Li-ion batteries.展开更多
Due to their rapid power delivery,fast charging,and long cycle life,supercapacitors have become an important energy storage technology recently.However,to meet the continuously increasing demands in the fields of port...Due to their rapid power delivery,fast charging,and long cycle life,supercapacitors have become an important energy storage technology recently.However,to meet the continuously increasing demands in the fields of portable electronics,transportation,and future robotic technologies,supercapacitors with higher energy densities without sacrificing high power densities and cycle stabilities are still challenged.Transition metal compounds(TMCs)possessing high theoretical capacitance are always used as electrode materials to improve the energy densities of supercapacitors.However,the power densities and cycle lives of such TMCs-based electrodes are still inferior due to their low intrinsic conductivity and large volume expansion during the charge/discharge process,which greatly impede their large-scale applications.Most recently,the ideal integrating of TMCs and conductive carbon skeletons is considered as an effective solution to solve the above challenges.Herein,we summarize the recent developments of TMCs/carbon hybrid electrodes which exhibit both high energy/power densities from the aspects of structural design strategies,including conductive carbon skeleton,interface engineering,and electronic structure.Furthermore,the remaining challenges and future perspectives are also highlighted so as to provide strategies for the high energy/power TMCs/carbon-based supercapacitors.展开更多
Electrode material based on a novel core–shell structure consisting of NiCoS(NCS) solid fiber core and Mn S(MS) sheet shell(NCS@MS) in situ grown on carbon cloth(CC) has been successfully prepared by a simple...Electrode material based on a novel core–shell structure consisting of NiCoS(NCS) solid fiber core and Mn S(MS) sheet shell(NCS@MS) in situ grown on carbon cloth(CC) has been successfully prepared by a simple sulfurization-assisted hydrothermal method for high performance supercapacitor. The synthesized NiCoS@Mn S/CC electrode shows high capacitance of 1908.3 F gat a current density of 0.5 A gwhich is higher than those of NiCoSand Mn S at the same current density. A flexible all-solid-state asymmetric supercapacitor(ASC) is constructed by using NiCoS@Mn S/CC as positive electrode, active carbon/CC as negative electrode and KOH/poly(vinyl alcohol)(PVA) as electrolyte. The optimized ASC shows a maximum energy density of 23.3 Wh kgat 1 A g, a maximum power density of about7.5 kw kgat 10 A gand remarkable cycling stability. After 9000 cycles, the ASC still exhibited67.8% retention rate and largely unchanged charge/discharge curves. The excellent electrochemical properties are resulted from the novel core–shell structure of the NiCoS@Mn S/CC electrode, which possesses both high surface area for Faraday redox reaction and superior kinetics of charge transport. The NiCoS@Mn S/CC electrode shows a promising potential for energy storage applications in the future.展开更多
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This project is supported by National Natural Science Foundation of China(No.50675076).
文摘Blind source separation and estimation of the number of sources usually demand that the number of sensors should be greater than or equal to that of the sources, which, however, is very difficult to satisfy for the complex systems. A new estimating method based on power spectral density (PSD) is presented. When the relation between the number of sensors and that of sources is unknown, the PSD matrix is first obtained by the ratio of PSD of the observation signals, and then the bound of the number of correlated sources with common frequencies can be estimated by comparing every column vector of PSD matrix. The effectiveness of the proposed method is verified by theoretical analysis and experiments, and the influence of noise on the estimation of number of source is simulated.
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.
基金the National Natural Science Foundation of China(Nos.11772084 and U1906233)the National High Technology Research and Development Program of China(No.2017YFC0307203)the Key Technology Research and Development Program of Shandong Province of China(No.2019JZZY010801)。
文摘A consequence of nonlinearities is a multi-harmonic response via a monoharmonic excitation.A similar phenomenon also exists in random vibration.The power spectral density(PSD)analysis of random vibration for nonlinear systems is studied in this paper.The analytical formulation of output PSD subject to the zero-mean Gaussian random load is deduced by using the Volterra series expansion and the conception of generalized frequency response function(GFRF).For a class of nonlinear systems,the growing exponential method is used to determine the first 3 rd-order GFRFs.The proposed approach is used to achieve the nonlinear system’s output PSD under a narrow-band stationary random input.The relationship between the peak of PSD and the parameters of the nonlinear system is discussed.By using the proposed method,the nonlinear characteristics of multi-band output via single-band input can be well predicted.The results reveal that changing nonlinear system parameters gives a one-of-a-kind change of the system’s output PSD.This paper provides a method for the research of random vibration prediction and control in real-world nonlinear systems.
文摘To ensure the performance of the optical system, the machining accuracy of lens with long focal lengths is required to ensure the image quality. A new method for lens transmission wavefront power spectral density (PSD) in mid-frequency domain measurement using binary phase computer-generated hologram (CGH) is presented. This technique is widely applicable and is particularly useful for measuring large-size lenses with long focal lengths. A comparison experiment of the CGH measurement with results from a Fizeau sphere interferometry method is carried out to verify the accuracy and convenience of the measurement. Furthermore, measurement uncertainty due to CGH fabrication process is analysed. Analysis of the CGH test showed the overall accuracy of less than 1 nm RMS for a sphere lens with over 30 m focal length and Φ410 mm clear aperture. CGH can provide reference spheres with high precision, in the meantime greatly shorten air space, thus reducing the effect of vibration and air turbulence, therefore is of great importance for lens transmission wavefront PSD measurement. The realization of high precision, high efficiency and nondestructive testing of long focal-lens wavefront PSD ensure the ultra-precision and certainty level of machining, hence improving the comprehensive performance of the optical system.
基金The Scientic and Technological Research Council of Turkey(TUBITAK)under Grant No.113E610.
文摘Diabetes is a widespread and serious disease and noninvasive measurement has been in high demand.To address this problem,a power spectral density-based method was offered for determining glucose sensitive sub-bands in the nearinfrared(NIR)spectrum.The experiments were conducted using phantoms of different optical properties in-vitro conditions.The optical bands 1200–1300 nm and 2100–2200 nm were found feasible for measuring blood glucose.After that,a photoplethysmography(PPG)-based low cost and portable optical system was designed.It has six di®erent NIR wavelength LEDs for illumination and an InGaAs photodiode for detection.Optical density values were calculated through the system and used as independent variables for multiple linear regression analysis.The results of blood glucose levels for 24 known healthy subjects showed that the optical system prediction was nearly 80%in the A zone and 20%in the B zone according to the Clarke Error Grid analysis.It was shown that a promising easyuse,continuous,and compact optical system had been designed.
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Periodical macro stripes have been found on diamond film prepared on a Si substrate by the multifilament chemical vapor deposition (CVD) method when the filament substrate distance is less than 2 mm. The properties of the stripe on the film were characterized qualitatively by a scanning electron microscope (SEM) and Raman scattering spectrometer. The measurement results show that this stripe corresponds to the different kinds of the microstructure and thickness of the film. Through calculation of the thermal radiation energy density distribution on the surface of substrate, this phenomenon can be explained successfully.
文摘To study the role of autonomic nervous system in the period of developing syncope induced by head-up tilt test(HUT), we analysed the changes of heart rate power spectral density(HRPSD) in 50 patients with unexplained syncope, including 15 positive patients (Group 1) and 35 negative patients(Group 2), and 15 negative healthy persons(Group 3) in 5 minute periods before and after tilting and 5 minutes before the end of test. HRPSD and their changes in total(T), very low-frequence(VLF), low-frequence(LF), high-frequence(HF) and the ratio of low/high frequence(LF/HF) were similar (P>0.05) 5 minutes before and after tilting among three groups. Five minutes before the end of test, Group 1 had obvious increase of T, VLF, LF and LF/HF while Group 2 and 3 had not such significant changes. There was significant difference(P<0.01) compared Group 1 with Group 2, 3. The results showed that the abnormal regulatory function of autonomic nervous system played an important role in the mechanism of symcope induced by HUT, the positive group had abnormal increase of sympathetic tone and imbalance of sympathetic/parasympathetic neural tone before syncope appeared.
文摘Nano Technology is the branch of technology that deals with dimensions and tolerances in terms of nanometers. In this paper, the electrical characteristics analysis is determined for the Nano-GaN HEMT and Micro-GaN HEMT and also power spectrum density is determined for GaN Nano-HEMT by reducing the gate length Lg in nm range. The GaN Nano HEMT is producing high current comparing to Micro GaN HEMT. Accuracy of the proposed analytical model results is verified with simulation results.
基金Supported by the National Basic Research Program of China under Grant Nos 2013CB632801 and 2013CB632803the National Natural Science Foundation of China under Grant Nos 61435014,61306058 and 61274094the Beijing Natural Science Foundation under Grant No 4144086
文摘We report on the room-temperature cascade laser (QCL) at λ -4.7μm. cw operation of a surface grating Both grating design and material distributed feedback (DFB) quantum optimization are used to decrease the threshold current density and to increase the output power. For a high-reflectivity-coated 13-μm-wide and 4- mm-long laser, high wall-plug efficiency of 6% is obtained at 20℃ from a single facet producing over I W of ew output power. The threshold current density of DFB QCL is as low as 1.13kA/cm^2 at 10℃ and 1.34kA/cm2 at 30℃ in cw mode. Stable single-mode emission with a side-mode suppression ratio of about 30 dB is observed in tile working temperature range of 20-50℃.
基金supported by the National Natural Science Foundation of China (51702039)。
文摘The sluggish kinetics of the electrochemical oxygen reduction reaction(ORR)in intermediatetemperature solid oxide fuel cells(IT-SOFCs)greatly limits the overall cell performance.In this study,an efficient and durable cathode material for IT-SOFCs is designed based on density functional theory(DFT)calculations by co-doping with Nb and Ta the B-site of the SrFeO_(3-δ)perovskite oxide.The DFT calculations suggest that Nb/Ta co-doping can regulate the energy band of the parent SrFeO_(3-δ)and help electron transfer.In symmetrical cells,such cathode with a SrFe_(0.8)Nb_(0.1)Ta_(0.1)O_(3-δ)(SFNT)detailed formula achieves a low cathode polarization resistance of 0.147Ωcm^(2) at 650℃.Electron spin resonance(ESR)and X-ray photoelectron spectroscopy(XPS)analysis confirm that the co-doping of Nb/Ta in SrFeO_(3-δ)B-site increases the balanced concentration of oxygen vacancies,enhancing the electrochemical performance when compared to 20 mol%Nb single-doped perovskite oxide.The cathode button cell with NiSDC|SDC|SFNT configuration achieves an outstanding peak power density of 1.3 W cm^(-2)at 650℃.Moreover,the button cell shows durability for 110 h under 0.65 V at 600℃ using wet H_(2) as fuel.
基金supported by the Scientific and Technological Research Council of Turkey(TUBITAK)under project number 121M608.
文摘Triboelectric nanogenerators(TENGs)have emerged as a promising technology to harvest electrical energy from natural motions such as human movement,wind,and water flow.Although TENGs show significant potential in small-scale applications,developing large-scale TENGs capable of generating high power remains a significant challenge.Several factors that can affect the performance of large-scale TENGs are being investigated to overcome this challenge,including the size and configuration of dielectric materials.This study optimizes dielectrics regarding surface area,thickness,and multicell configuration to improve harvested electrical power density in large-scale TENGs.In the studies,glass fiber was used as the positive dielectric,and multipurpose white silicone was used as the negative dielectric because of their high tribo-potential,durability,and easy accessibility.In the size optimization phase,dielectric thicknesses and surface areas that provide the maximum power density were determined.Subsequently,horizontal and vertical multicell configurations were examined to efficiently integrate size-optimized dielectrics.The results reveal that large-scale TENGs with vertical multicell configurations can achieve high and usable energy density for electronics.The findings provide valuable insight into the development of large-scale TENGs with advanced power generation capabilities.
基金supported by the National Key Research and Development Program of China(2022YFA1504100)the National Natural Science Foundation of China(22125903,51872283,and 22005298)+4 种基金Dalian Innovation Support Plan for High Level Talents(2019RT09)Dalian National Laboratory For Clean Energy(DNL),Chinese Academy of Sciences(CAS),DNL Cooperation Fund,CAS(DNL202016 and DNL202019)Dalian Institute of Chemical Physics(DICP I2020032)Exploratory Research Project of Yanchang Petroleum International Limited and DICP(yc-hw-2022ky-01)the Joint Fund of the Yulin University and the Dalian National Laboratory for Clean Energy(YLU-DNL Fund 2021002 and 2021009).
文摘Lithium-ion batteries(LIBs)with the“double-high”characteristics of high energy density and high power density are in urgent demand for facilitating the development of advanced portable electronics.However,the lithium ion(Li+)-storage performance of the most commercialized lithium cobalt oxide(LiCoO_(2),LCO)cathodes is still far from satisfactory in terms of high-voltage and fast-charging capabilities for reaching the double-high target.Herein,we systematically summarize and discuss high-voltage and fast-charging LCO cathodes,covering in depth the key fundamental challenges,latest advancements in modification strategies,and future perspectives in this field.Comprehensive and elaborated discussions are first presented on key fundamental challenges related to structural degradation,interfacial instability,the inhomogeneity reactions,and sluggish interfacial kinetics.We provide an instructive summary of deep insights into promising modification strategies and underlying mechanisms,categorized into element doping(Li-site,cobalt-/oxygen-site,and multi-site doping)for improved Li+diffusivity and bulkstructure stability;surface coating(dielectrics,ionic/electronic conductors,and their combination)for surface stability and conductivity;nanosizing;combinations of these strategies;and other strategies(i.e.,optimization of the electrolyte,binder,tortuosity of electrodes,charging protocols,and prelithiation methods).Finally,forward-looking perspectives and promising directions are sketched out and insightfully elucidated,providing constructive suggestions and instructions for designing and realizing high-voltage and fast-charging LCO cathodes for next-generation double-high LIBs.
基金supported by the National Natural Science Foundation of China(21975074,91534202,and 91834301)the Shanghai Scientific and Technological Innovation Project(18JC1410500)the Fundamental Research Funds for the Central Universities(222201718002)。
文摘Exploring high ion/electron conductive olivine-type transition metal phosphates is of vital significance to broaden their applicability in rapid-charging devices.Herein,we report an interface engineered Li Fe0.5Mn0.5PO4/rGO@C cathode material by the synergistic effects of r GO and polydopamine-derived N-doped carbon.The well-distributed Li Fe0.5Mn0.5PO4nanoparticles are tightly anchored on r GO nanosheet benefited by the coating of N-doped carbon layer.The design of such an architecture can effectively suppress the agglomeration of nanoparticles with a shortened Li+transfer path.Meantime,the high-speed conducting network has been constructed by r GO and N-doped carbon,which exhibits the face-to-face contact with Li Fe0.5Mn0.5PO4nanoparticles,guaranteeing the rapid electron transfer.These profits endow the Li Fe0.5Mn0.5PO4/rGO@C hybrids with a fast charge-discharge ability,e.g.a high reversible capacity of 105 m Ah·g^-1at 10 C,much higher than that of the Li Fe0.5Mn0.5PO4@C nanoparticles(46 mA·h·g^-1).Furthermore,a 90.8%capacity retention can be obtained even after cycling 500 times at 2 C.This work gives a new avenue to fabricate transition metal phosphate with superior electrochemical performance for high-power Li-ion batteries.
基金This work was supported by the National Natural Science Foundation of China(Nos.51972342,and 51872056)Taishan Scholar Project of Shandong Province(ts20190922)+3 种基金Key Basic Research Project of Natural Science Foundation of Shandong Province(ZR2019ZD51)Project funded by China Postdoctoral Science Foundation(2019TQ0353 and 2020M672165)Fundamental Research Funds for the Central Universities(20CX06024A)Shandong Provincial Natural Science Foundation,China(ZR201911040344).
文摘Due to their rapid power delivery,fast charging,and long cycle life,supercapacitors have become an important energy storage technology recently.However,to meet the continuously increasing demands in the fields of portable electronics,transportation,and future robotic technologies,supercapacitors with higher energy densities without sacrificing high power densities and cycle stabilities are still challenged.Transition metal compounds(TMCs)possessing high theoretical capacitance are always used as electrode materials to improve the energy densities of supercapacitors.However,the power densities and cycle lives of such TMCs-based electrodes are still inferior due to their low intrinsic conductivity and large volume expansion during the charge/discharge process,which greatly impede their large-scale applications.Most recently,the ideal integrating of TMCs and conductive carbon skeletons is considered as an effective solution to solve the above challenges.Herein,we summarize the recent developments of TMCs/carbon hybrid electrodes which exhibit both high energy/power densities from the aspects of structural design strategies,including conductive carbon skeleton,interface engineering,and electronic structure.Furthermore,the remaining challenges and future perspectives are also highlighted so as to provide strategies for the high energy/power TMCs/carbon-based supercapacitors.
基金supported by the Grant-in-Aid for Scientific Research (KAKENHI) program, Japan (C, Grant Number 15K05597)Takahashi Industrial and Economic Research Foundation (Takahashi Grant Number 06-003-154)
文摘Electrode material based on a novel core–shell structure consisting of NiCoS(NCS) solid fiber core and Mn S(MS) sheet shell(NCS@MS) in situ grown on carbon cloth(CC) has been successfully prepared by a simple sulfurization-assisted hydrothermal method for high performance supercapacitor. The synthesized NiCoS@Mn S/CC electrode shows high capacitance of 1908.3 F gat a current density of 0.5 A gwhich is higher than those of NiCoSand Mn S at the same current density. A flexible all-solid-state asymmetric supercapacitor(ASC) is constructed by using NiCoS@Mn S/CC as positive electrode, active carbon/CC as negative electrode and KOH/poly(vinyl alcohol)(PVA) as electrolyte. The optimized ASC shows a maximum energy density of 23.3 Wh kgat 1 A g, a maximum power density of about7.5 kw kgat 10 A gand remarkable cycling stability. After 9000 cycles, the ASC still exhibited67.8% retention rate and largely unchanged charge/discharge curves. The excellent electrochemical properties are resulted from the novel core–shell structure of the NiCoS@Mn S/CC electrode, which possesses both high surface area for Faraday redox reaction and superior kinetics of charge transport. The NiCoS@Mn S/CC electrode shows a promising potential for energy storage applications in the future.