The digital transformation in Cameroon presents critical cybersecurity challenges that demand immediate attention and strategic intervention. This comprehensive analysis examines the evolving cybersecurity landscape i...The digital transformation in Cameroon presents critical cybersecurity challenges that demand immediate attention and strategic intervention. This comprehensive analysis examines the evolving cybersecurity landscape in Cameroon from 2020 to 2023, during which cyber-attacks increased by 156% and financial losses from digital fraud exceeded $45 million. This research identifies significant vulnerabilities in Cameroon’s cybersecurity ecosystem through a rigorous assessment of national infrastructure, policy frameworks, and institutional capacities. Recent data indicates that while digital service adoption has grown exponentially, with internet penetration reaching 35.2% in 2023, cybersecurity measures have lagged significantly behind international standards. This analysis draws on comprehensive data from multiple sectors, including financial services, government institutions, and telecommunications, incorporating findings from the National Cybersecurity Assessment Program and the Digital Infrastructure Security Report. The research reveals that 73% of organizations lack dedicated security teams, while response times to cyber incidents average 72 hours—three times than the global standard. Based on these findings, this paper proposes evidence-based solutions for enhancing digital resilience, including policy modernization, capacity-building initiatives, and technical infrastructure development. The recommendations encompass short-term tactical responses, medium-term strategic improvements, and long-term structural changes, providing a comprehensive roadmap for strengthening Cameroon’s national cybersecurity frameworks.展开更多
By measuring scanning tunneling spectroscopy on some large Bi islands deposited on FeTe_(0.55)Se_(0.45)superconductors,we observe clear in-gap edge states with double peaks at about±1.0 me V on the spectra measur...By measuring scanning tunneling spectroscopy on some large Bi islands deposited on FeTe_(0.55)Se_(0.45)superconductors,we observe clear in-gap edge states with double peaks at about±1.0 me V on the spectra measured near the perimeter of the islands.This feature is very different from the single zero-energy peak observed on some other small Bi islands.The edge states spread towards the inner side of the islands over a width of 2-3 nm.The two edge-state peaks at positive and negative energies move to higher values with the increase of the magnetic field,and they disappear near the transition temperature of FeTe_(0.55)Se_(0.45).Meanwhile,enhanced superconducting gaps are observed in the central regions of these Bi islands,which may be induced by the enhanced pair potential of the topological surface state.Our observations provide a valuable message for the edge state and the proximity-induced superconductivity on specific Bi islands grown on FeTe_(0.55)Se_(0.45)substrate.展开更多
A significant challenge in developing block copolymer photonic crystals is constructing low-symmetric ordered phases,which are essential for achieving a complete photonic band gap.Here,we propose a promising strategy ...A significant challenge in developing block copolymer photonic crystals is constructing low-symmetric ordered phases,which are essential for achieving a complete photonic band gap.Here,we propose a promising strategy to create low-symmetric ordered morphologies by incorporating shape-anisotropic rod-like side chains into block copolymers.Using dissipative particle dynamics simulations,we demonstrate that block copolymers with longer rod-like side chains can self-assemble into a hexagonally packed columnar phase characterized by a low-symmetric rectangular cross-section.Photonic band structure calculations reveal that this low-symmetric columnar phase can exhibit a complete photonic band gap,with the gap size dependent on the aspect ratio of the rectangular cross-sections of the columns.Our findings suggest an effective approach to constructing low-symmetric photonic crystals through the self-assembly of block copolymers with shape-anisotropic segments.展开更多
By comprehensively analyzing the current status of education development and the achievements of economic development in China,this paper reveals the synergistic effect between the overall improvement of education lev...By comprehensively analyzing the current status of education development and the achievements of economic development in China,this paper reveals the synergistic effect between the overall improvement of education levels and economic growth.The paper points out that although the increase in educational investment has provided talent support for economic development,the imbalance in regional education development still exists.Furthermore,the paper explores the two-way promotion mechanism between education and economic development,and proposes a series of strategies,including balanced layout,targeted support,and innovation-driven development,to achieve the coordinated and balanced development of regional education and narrow regional gaps.These strategies aim to optimize the allocation of educational resources,improve the quality of education,and promote the all-round progress of the economy and society.展开更多
To analyze the band gap characteristics of phononic crystals,a two-dimensional phononic crystal plate model with an elastic foundation was first established.The plane wave expansion method was used to compute the disp...To analyze the band gap characteristics of phononic crystals,a two-dimensional phononic crystal plate model with an elastic foundation was first established.The plane wave expansion method was used to compute the dispersion curves of this phononic crystal model,and the results were compared with those from the finite element method to verify their accuracy.Subsequently,a parameter study explored the effects of the elastic foundation coeffi-cient and coverage ratio on the band gap.The results indicate that as the coverage ratio of the elastic foundation increases,the band gap significantly expands,reaching its maximum value at 100%coverage.Additionally,as the elastic foundation stiffness increases,the band gap gradually widens and converges toward fixed boundary conditions.The study also investigated the band gap of phononic crystal plates with defects,finding that the vibrational energy concentrates at the defect unit cell.Furthermore,the defect band frequency can be effectively modulated by adjusting the coefficient of the elastic foundation,providing a theoretical basis for achieving efficient energy conversion.展开更多
The seismic performance of rubber concrete-layered periodic foundations are significantly influenced by their design,in which the band gaps play a paramount role.Aiming at providing better designs for these foundation...The seismic performance of rubber concrete-layered periodic foundations are significantly influenced by their design,in which the band gaps play a paramount role.Aiming at providing better designs for these foundations,this study first proposes and validates the analytical formulas to approximate the bounds of the first few band gaps.In addition,the mapping relations linking the frequencies of different band gaps are presented.Furthermore,an optimal design method for these foundations is developed,which is validated through an engineering example.It is demonstrated that ensuring the superstructure’s resonance zones are completely covered by the corresponding periodic foundation’s band gaps can achieve satisfactory vibration attenuation effects,which is a good strategy for the design of rubber concrete layered periodic foundations.展开更多
The outlet flow fields of a low-speed repeating-stage compressor with bowed stator stages are measured with five-hole probe under the near stall condition when the rotor/stator axial gap varies. The performances of th...The outlet flow fields of a low-speed repeating-stage compressor with bowed stator stages are measured with five-hole probe under the near stall condition when the rotor/stator axial gap varies. The performances of the straight stator stages are investigated and compared to those of the bowed stator stages. The results show that using bowed stator stages could alleviate the flow separation at both upper and low corners of the suction surface and the endwalls, and decrease the losses along the flow passage as well as the outlet flow angle. As the rotor/stator axial gap decreases, although the diffusion capacity of the compressor increases obviously, the outlet flow field in the straight stator stages deteriorates quickly. By contrast, little changes occur in the bowed stator stages, indicating that as the rotor/stator axial gap decreases, improved performance is achieved in the bowed stator stages.展开更多
China is in a dominant position in apple production globally with both the largest apple growing area and the largest export of fresh apple fruits. However, the annual productivity of China＇s apple is significantly l...China is in a dominant position in apple production globally with both the largest apple growing area and the largest export of fresh apple fruits. However, the annual productivity of China＇s apple is significantly lower than that of other dominant apple producing countries. In addition, apple production is based on excessive application of chemical fertilizers and the nutrient use efficiency （especially nitrogen） is therefore low and the nutrient emissions to the environment are high. Apple production in China is considerably contributes to farmers＇ incomes and is important as export product. There is an urgent need to enhance apple productivity and improve nutrient use efficiencies in intensive apple production systems in the country. These can be attained by improved understanding of production potential, yield gaps, nutrient use and best management in apple orchards. To the end, priorities in research on apple production systems and required political support are described which may lead to more sustainable and environmental-friendly intensification of apple production in China.展开更多
Electrochemical water splitting represents one of the most promising technologies to produce green hydrogen,which can help to realize the goal of achieving carbon neutrality.While substantial efforts on a laboratory s...Electrochemical water splitting represents one of the most promising technologies to produce green hydrogen,which can help to realize the goal of achieving carbon neutrality.While substantial efforts on a laboratory scale have been made for understanding fundamental catalysis and developing high-performance electrocatalysts for the two half-reactions involved in water electrocatalysis,much less attention has been paid to doing relevant research on a larger scale.For example,few such researches have been done on an industrial scale.Herein,we review the very recent endeavors to bridge the gaps between fundamental research and industrial applications for water electrolysis.We begin by introducing the fundamentals of electrochemical water splitting and then present comparisons of testing protocol,figure of merit,catalyst of interest,and manufacturing cost for laboratory and industry-based water-electrolysis research.Special attention is paid to tracking the surface reconstruction process and identifying real catalytic species under different testing conditions,which highlight the significant distinctions of corresponding electrochemical reconstruction mechanisms.Advances in catalyst designs for industry-relevant water electrolysis are also summarized,which reveal the progress of moving the practical applications forward and accelerating synergies between material science and engineering.Perspectives and challenges of electrocatalyst design strategies are proposed finally to further bridge the gaps between lab-scale research and large-scale electrocatalysis applications.展开更多
The complete flexural vibration band gaps are studied in the thin plates with two-dimensional binary locally resonant structures, i.e. the composite plate consisting of soft rubber cylindrical inclusions periodically ...The complete flexural vibration band gaps are studied in the thin plates with two-dimensional binary locally resonant structures, i.e. the composite plate consisting of soft rubber cylindrical inclusions periodically placed in a host material. Numerical simulations show that the low-frequency gaps of flexural wave exist in the thin plates. The width of the first gap decreases monotonically as the matrix density increases, The frequency response of the finite periodic thin plates is simulated by the finite element method, which provides attenuations of over 20dB in the frequency range of the band gaps. The findings will be significant in the application of phononic crystals.展开更多
Periodic arrays of resonant shunted piezoelectric patches are employed to control the wave propagation in a two-dimensional （2D） acoustic metamaterial. The performance is characterized by the finite element method. ...Periodic arrays of resonant shunted piezoelectric patches are employed to control the wave propagation in a two-dimensional （2D） acoustic metamaterial. The performance is characterized by the finite element method. More importantly, we propose an approach to solving the conventional issue of the nonlinear eigenvalue problem, and give a convenient solution to the dispersion properties of 2D metamaterials with periodic arrays of resonant shunts in this article. Based on this modeling method, the dispersion relations of a 2D metamaterial with periodic arrays of resonant shunted piezos are calculated. The results show that the internal resonances of the shunting system split the dispersion curves, thereby forming a locally resonant band gap. However, unlike the conventional locally resonant gap, the vibrations in this locally resonant gap are unable to be completely localized in oscillators consisting of shunting inductors and piezo-patches.展开更多
A tubular moving-magnet linear oscillating motor（TMMLOM） has merits of high efficiency and excellent dynamic capability. To enhance the thrust performance, quasi-Halbach permanent magnet（PM） arrays are arranged on...A tubular moving-magnet linear oscillating motor（TMMLOM） has merits of high efficiency and excellent dynamic capability. To enhance the thrust performance, quasi-Halbach permanent magnet（PM） arrays are arranged on its mover in the application of a linear electro-hydrostatic actuator in more electric aircraft. The arrays are assembled by several individual segments, which lead to gaps between them inevitably. To investigate the effects of the gaps on the radial magnetic flux density and the machine thrust in this paper, an analytical model is built considering both axial and radial gaps. The model is validated by finite element simulations and experimental results.Distributions of the magnetic flux are described in condition of different sizes of radial and axial gaps. Besides, the output force is also discussed in normal and end windings. Finally, the model has demonstrated that both kinds of gaps have a negative effect on the thrust, and the linear motor is more sensitive to radial ones.展开更多
Pairing in the cuprate high-temperature superconductors and its origin remain among the most enduring mysteries in condensed matter physics. With cross-sectional scanning tunneling microscopy/spectroscopy, we clearly ...Pairing in the cuprate high-temperature superconductors and its origin remain among the most enduring mysteries in condensed matter physics. With cross-sectional scanning tunneling microscopy/spectroscopy, we clearly reveal the spatial-dependence or inhomogeneity of the superconducting gap structure of Bi2Sr2CaCu2O8＋δ （Bi2212） and YBa2Cu3O7-x （YBCO） along their c-axes on a scale shorter than the interlayer spacing. By tunneling into the （100） plane of a Bi2212 single crystal and a YBCO film, we observe both U-shaped tunneling spectra with extended fiat zero-conductance bottoms, and V-shaped gap structures, in different regions of each sample. On the YBCO film, tunneling into a （110） surface only reveals a U-shaped gap without any zero-bias peak. Our analysis suggests that the U-shaped gap is likely a nodeless superconducting gap. The V-shaped gap has a very small amplitude, and is likely proximity-induced by regions having the larger U-shaped gap.展开更多
A new model for producing band gaps for flexural elastic wave propagation in a periodic microbeam structure is developed using an extended transfer matrix method and a non-classical Bernoulli–Euler beam model that in...A new model for producing band gaps for flexural elastic wave propagation in a periodic microbeam structure is developed using an extended transfer matrix method and a non-classical Bernoulli–Euler beam model that incorporates the strain gradient,couple stress and velocity gradient effects.The band gaps predicted by the new model depend on the three microstructure-dependent material parameters of each constituent material,the beam thickness,the unit cell length and the volume fraction.A parametric study is conducted to quantitatively illustrate these factors.The numerical results reveal that the first band gap frequency range increases with the increases of the three microstructure-dependent material parameters,respectively.In addition,the band gap size predicted by the current model is always larger than that predicted by the classical model,and the difference is large for very thin beams.Furthermore,both the unit cell length and volume fraction have significant effects on the band gap.展开更多
The understanding of electrical breakdown in atmospheric air across micrometer gaps is critically important for the insulation design of micro ＆ nano electronic devices. In this paper, planar aluminum electrodes with...The understanding of electrical breakdown in atmospheric air across micrometer gaps is critically important for the insulation design of micro ＆ nano electronic devices. In this paper, planar aluminum electrodes with gaps ranging from 2μm to 40 #m were fabricated by microelectromechanical system technology. The influence factors including gap width and surface dielectric states were experimentally investigated using the home-built test and measurement system. Results showed that for SiO2 layers the current sustained at 2-3 nA during most of the pre-breakdown period, and then rose rapidly to 10-30 nA just before breakdown due to field electron emission, followed by the breakdown. The breakdown voltage curves demonstrated three stages： （1） a constantly decreasing region （the gap width d 〈5 μm）, where the field emission effect played an important role just near breakdown, supplying enough initial electrons for the breakdown process; （2） a plateau region with a near constant breakdown potential （5 μm〈 d 〈10 μm）; （3） a region for large gaps that adhered to Paschen＇s curve （d 〉10μm）. And the surface dielectric states including the surface resistivity and secondary electron yield were verified to be related to the propagation of discharge due to the interaction between initial electrons and dielectrics.展开更多
文摘The digital transformation in Cameroon presents critical cybersecurity challenges that demand immediate attention and strategic intervention. This comprehensive analysis examines the evolving cybersecurity landscape in Cameroon from 2020 to 2023, during which cyber-attacks increased by 156% and financial losses from digital fraud exceeded $45 million. This research identifies significant vulnerabilities in Cameroon’s cybersecurity ecosystem through a rigorous assessment of national infrastructure, policy frameworks, and institutional capacities. Recent data indicates that while digital service adoption has grown exponentially, with internet penetration reaching 35.2% in 2023, cybersecurity measures have lagged significantly behind international standards. This analysis draws on comprehensive data from multiple sectors, including financial services, government institutions, and telecommunications, incorporating findings from the National Cybersecurity Assessment Program and the Digital Infrastructure Security Report. The research reveals that 73% of organizations lack dedicated security teams, while response times to cyber incidents average 72 hours—three times than the global standard. Based on these findings, this paper proposes evidence-based solutions for enhancing digital resilience, including policy modernization, capacity-building initiatives, and technical infrastructure development. The recommendations encompass short-term tactical responses, medium-term strategic improvements, and long-term structural changes, providing a comprehensive roadmap for strengthening Cameroon’s national cybersecurity frameworks.
基金supported by the National Key R&D Program of China(Grant Nos.2022YFA1403201 and 2024YFA1408104)the National Natural Science Foundation of China(Grant Nos.11927809 and 12434004)the Natural Science Foundation of Jiangsu Province(Grant No.BK20233001)。
文摘By measuring scanning tunneling spectroscopy on some large Bi islands deposited on FeTe_(0.55)Se_(0.45)superconductors,we observe clear in-gap edge states with double peaks at about±1.0 me V on the spectra measured near the perimeter of the islands.This feature is very different from the single zero-energy peak observed on some other small Bi islands.The edge states spread towards the inner side of the islands over a width of 2-3 nm.The two edge-state peaks at positive and negative energies move to higher values with the increase of the magnetic field,and they disappear near the transition temperature of FeTe_(0.55)Se_(0.45).Meanwhile,enhanced superconducting gaps are observed in the central regions of these Bi islands,which may be induced by the enhanced pair potential of the topological surface state.Our observations provide a valuable message for the edge state and the proximity-induced superconductivity on specific Bi islands grown on FeTe_(0.55)Se_(0.45)substrate.
基金financially supported by the National Key R&D Program of China(No.2022YFB3707300)the National Natural Science Foundation of China(Nos.22133002,22373089)the support from the Excellent Youth Foundation of Henan Scientific Committee(No.242300421032).
文摘A significant challenge in developing block copolymer photonic crystals is constructing low-symmetric ordered phases,which are essential for achieving a complete photonic band gap.Here,we propose a promising strategy to create low-symmetric ordered morphologies by incorporating shape-anisotropic rod-like side chains into block copolymers.Using dissipative particle dynamics simulations,we demonstrate that block copolymers with longer rod-like side chains can self-assemble into a hexagonally packed columnar phase characterized by a low-symmetric rectangular cross-section.Photonic band structure calculations reveal that this low-symmetric columnar phase can exhibit a complete photonic band gap,with the gap size dependent on the aspect ratio of the rectangular cross-sections of the columns.Our findings suggest an effective approach to constructing low-symmetric photonic crystals through the self-assembly of block copolymers with shape-anisotropic segments.
文摘By comprehensively analyzing the current status of education development and the achievements of economic development in China,this paper reveals the synergistic effect between the overall improvement of education levels and economic growth.The paper points out that although the increase in educational investment has provided talent support for economic development,the imbalance in regional education development still exists.Furthermore,the paper explores the two-way promotion mechanism between education and economic development,and proposes a series of strategies,including balanced layout,targeted support,and innovation-driven development,to achieve the coordinated and balanced development of regional education and narrow regional gaps.These strategies aim to optimize the allocation of educational resources,improve the quality of education,and promote the all-round progress of the economy and society.
基金The National Natural Science Foundation of China(No.12002086)。
文摘To analyze the band gap characteristics of phononic crystals,a two-dimensional phononic crystal plate model with an elastic foundation was first established.The plane wave expansion method was used to compute the dispersion curves of this phononic crystal model,and the results were compared with those from the finite element method to verify their accuracy.Subsequently,a parameter study explored the effects of the elastic foundation coeffi-cient and coverage ratio on the band gap.The results indicate that as the coverage ratio of the elastic foundation increases,the band gap significantly expands,reaching its maximum value at 100%coverage.Additionally,as the elastic foundation stiffness increases,the band gap gradually widens and converges toward fixed boundary conditions.The study also investigated the band gap of phononic crystal plates with defects,finding that the vibrational energy concentrates at the defect unit cell.Furthermore,the defect band frequency can be effectively modulated by adjusting the coefficient of the elastic foundation,providing a theoretical basis for achieving efficient energy conversion.
基金National Natural Science Foundation of China under Grant Nos.52078395 and 52178301the Open Projects Foundation of the State Key Laboratory for Health and Safety of Bridge Structures under Grant No.BHSKL19-07-GF+1 种基金the Dawn Program of Knowledge Innovation Project from the Bureau of Science and Technology of Wuhan Municipality under Grant No.2022010801020357the Science Research Foundation of Wuhan Institute of Technology under Grant No.K2021030。
文摘The seismic performance of rubber concrete-layered periodic foundations are significantly influenced by their design,in which the band gaps play a paramount role.Aiming at providing better designs for these foundations,this study first proposes and validates the analytical formulas to approximate the bounds of the first few band gaps.In addition,the mapping relations linking the frequencies of different band gaps are presented.Furthermore,an optimal design method for these foundations is developed,which is validated through an engineering example.It is demonstrated that ensuring the superstructure’s resonance zones are completely covered by the corresponding periodic foundation’s band gaps can achieve satisfactory vibration attenuation effects,which is a good strategy for the design of rubber concrete layered periodic foundations.
基金National Natural Science Foundation of China (50646021)Chinese Specialized Research Fund for the Doctoral Pro-gram of Higher Education (20060213007)
文摘The outlet flow fields of a low-speed repeating-stage compressor with bowed stator stages are measured with five-hole probe under the near stall condition when the rotor/stator axial gap varies. The performances of the straight stator stages are investigated and compared to those of the bowed stator stages. The results show that using bowed stator stages could alleviate the flow separation at both upper and low corners of the suction surface and the endwalls, and decrease the losses along the flow passage as well as the outlet flow angle. As the rotor/stator axial gap decreases, although the diffusion capacity of the compressor increases obviously, the outlet flow field in the straight stator stages deteriorates quickly. By contrast, little changes occur in the bowed stator stages, indicating that as the rotor/stator axial gap decreases, improved performance is achieved in the bowed stator stages.
基金the project "Cash Crops Research Network of China" of the Center for Resources, Environment and Food Security, China Agricultural UniversityProfessor Oene Oenema from Alterra Wageningnen University, the Netherlands, for his financial support of the research
文摘China is in a dominant position in apple production globally with both the largest apple growing area and the largest export of fresh apple fruits. However, the annual productivity of China＇s apple is significantly lower than that of other dominant apple producing countries. In addition, apple production is based on excessive application of chemical fertilizers and the nutrient use efficiency （especially nitrogen） is therefore low and the nutrient emissions to the environment are high. Apple production in China is considerably contributes to farmers＇ incomes and is important as export product. There is an urgent need to enhance apple productivity and improve nutrient use efficiencies in intensive apple production systems in the country. These can be attained by improved understanding of production potential, yield gaps, nutrient use and best management in apple orchards. To the end, priorities in research on apple production systems and required political support are described which may lead to more sustainable and environmental-friendly intensification of apple production in China.
基金supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions(PAPD)supported by National R&D Program through the National Research Foundation of Korea(NRF),grant number 2021M3H4A1A01079300the Korea Research Institute of Chemical Technology Core Research Program funded by the Korea Research Council for Industrial Science and Technology,grant number KS2222-10.
文摘Electrochemical water splitting represents one of the most promising technologies to produce green hydrogen,which can help to realize the goal of achieving carbon neutrality.While substantial efforts on a laboratory scale have been made for understanding fundamental catalysis and developing high-performance electrocatalysts for the two half-reactions involved in water electrocatalysis,much less attention has been paid to doing relevant research on a larger scale.For example,few such researches have been done on an industrial scale.Herein,we review the very recent endeavors to bridge the gaps between fundamental research and industrial applications for water electrolysis.We begin by introducing the fundamentals of electrochemical water splitting and then present comparisons of testing protocol,figure of merit,catalyst of interest,and manufacturing cost for laboratory and industry-based water-electrolysis research.Special attention is paid to tracking the surface reconstruction process and identifying real catalytic species under different testing conditions,which highlight the significant distinctions of corresponding electrochemical reconstruction mechanisms.Advances in catalyst designs for industry-relevant water electrolysis are also summarized,which reveal the progress of moving the practical applications forward and accelerating synergies between material science and engineering.Perspectives and challenges of electrocatalyst design strategies are proposed finally to further bridge the gaps between lab-scale research and large-scale electrocatalysis applications.
基金Project supported by the State Key Development Program for Basic Research of China （Grant No 51307） and the National Natural Science Foundation of China （Grant No 50575222）.
文摘The complete flexural vibration band gaps are studied in the thin plates with two-dimensional binary locally resonant structures, i.e. the composite plate consisting of soft rubber cylindrical inclusions periodically placed in a host material. Numerical simulations show that the low-frequency gaps of flexural wave exist in the thin plates. The width of the first gap decreases monotonically as the matrix density increases, The frequency response of the finite periodic thin plates is simulated by the finite element method, which provides attenuations of over 20dB in the frequency range of the band gaps. The findings will be significant in the application of phononic crystals.
基金the National Natural Science Foundation of China(Grant Nos.50905182 and 51175501)
文摘Periodic arrays of resonant shunted piezoelectric patches are employed to control the wave propagation in a two-dimensional （2D） acoustic metamaterial. The performance is characterized by the finite element method. More importantly, we propose an approach to solving the conventional issue of the nonlinear eigenvalue problem, and give a convenient solution to the dispersion properties of 2D metamaterials with periodic arrays of resonant shunts in this article. Based on this modeling method, the dispersion relations of a 2D metamaterial with periodic arrays of resonant shunted piezos are calculated. The results show that the internal resonances of the shunting system split the dispersion curves, thereby forming a locally resonant band gap. However, unlike the conventional locally resonant gap, the vibrations in this locally resonant gap are unable to be completely localized in oscillators consisting of shunting inductors and piezo-patches.
基金supports of National Basic Research Program of China(973 Program)(No.2014CB046402)National Natural Science Foundation of China(Nos.51620105010,51575019,51675019,51505015)111 Program of China
文摘A tubular moving-magnet linear oscillating motor（TMMLOM） has merits of high efficiency and excellent dynamic capability. To enhance the thrust performance, quasi-Halbach permanent magnet（PM） arrays are arranged on its mover in the application of a linear electro-hydrostatic actuator in more electric aircraft. The arrays are assembled by several individual segments, which lead to gaps between them inevitably. To investigate the effects of the gaps on the radial magnetic flux density and the machine thrust in this paper, an analytical model is built considering both axial and radial gaps. The model is validated by finite element simulations and experimental results.Distributions of the magnetic flux are described in condition of different sizes of radial and axial gaps. Besides, the output force is also discussed in normal and end windings. Finally, the model has demonstrated that both kinds of gaps have a negative effect on the thrust, and the linear motor is more sensitive to radial ones.
基金Supported by the National Natural Science Foundation of Chinathe National Key Research and Development Program of China under Grant No 2016YFA0300203
文摘Pairing in the cuprate high-temperature superconductors and its origin remain among the most enduring mysteries in condensed matter physics. With cross-sectional scanning tunneling microscopy/spectroscopy, we clearly reveal the spatial-dependence or inhomogeneity of the superconducting gap structure of Bi2Sr2CaCu2O8＋δ （Bi2212） and YBa2Cu3O7-x （YBCO） along their c-axes on a scale shorter than the interlayer spacing. By tunneling into the （100） plane of a Bi2212 single crystal and a YBCO film, we observe both U-shaped tunneling spectra with extended fiat zero-conductance bottoms, and V-shaped gap structures, in different regions of each sample. On the YBCO film, tunneling into a （110） surface only reveals a U-shaped gap without any zero-bias peak. Our analysis suggests that the U-shaped gap is likely a nodeless superconducting gap. The V-shaped gap has a very small amplitude, and is likely proximity-induced by regions having the larger U-shaped gap.
基金The work reported here is funded by the National Natural Science Foundation of China[grant numbers 12002086,11872149 and 11472079]the Fundamental Research Funds for the Central Universities[grant number 2242020R10027].These supports are gratefully acknowledged.
文摘A new model for producing band gaps for flexural elastic wave propagation in a periodic microbeam structure is developed using an extended transfer matrix method and a non-classical Bernoulli–Euler beam model that incorporates the strain gradient,couple stress and velocity gradient effects.The band gaps predicted by the new model depend on the three microstructure-dependent material parameters of each constituent material,the beam thickness,the unit cell length and the volume fraction.A parametric study is conducted to quantitatively illustrate these factors.The numerical results reveal that the first band gap frequency range increases with the increases of the three microstructure-dependent material parameters,respectively.In addition,the band gap size predicted by the current model is always larger than that predicted by the classical model,and the difference is large for very thin beams.Furthermore,both the unit cell length and volume fraction have significant effects on the band gap.
基金supported by Research Funds of State Key Laboratory of Electrical Insulation and Power Equipment (Xi'an Jiaotong University) of China (No.EIPE14107)
文摘The understanding of electrical breakdown in atmospheric air across micrometer gaps is critically important for the insulation design of micro ＆ nano electronic devices. In this paper, planar aluminum electrodes with gaps ranging from 2μm to 40 #m were fabricated by microelectromechanical system technology. The influence factors including gap width and surface dielectric states were experimentally investigated using the home-built test and measurement system. Results showed that for SiO2 layers the current sustained at 2-3 nA during most of the pre-breakdown period, and then rose rapidly to 10-30 nA just before breakdown due to field electron emission, followed by the breakdown. The breakdown voltage curves demonstrated three stages： （1） a constantly decreasing region （the gap width d 〈5 μm）, where the field emission effect played an important role just near breakdown, supplying enough initial electrons for the breakdown process; （2） a plateau region with a near constant breakdown potential （5 μm〈 d 〈10 μm）; （3） a region for large gaps that adhered to Paschen＇s curve （d 〉10μm）. And the surface dielectric states including the surface resistivity and secondary electron yield were verified to be related to the propagation of discharge due to the interaction between initial electrons and dielectrics.
