bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of reflected neutrons of wall on waveform of burst reactors
Authors: Gao Hui  , Song Ling-Li  , Li Bing  ,   Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China
Published: December 2020

The reflected neutrons from the wall of the reactor have a significant effect on the waveform of the fast burst reactor. The leakage neutrons from the reactor core have a certain probability that they will come back. Their return time displays a continuous distribution because of the difference in energy among the reflected neutrons. In the stable state, the influence of the reflected neutrons is not obvious. However, in a prompt state, it is obvious because the reflected neutrons are not synchronized with the neutrons in the reactor core, which leads to some strange phenomena in experiment. For example, in the process of erupting a fission burst in a metal reactor, the number of neutrons in core increases very rapidly, while the return time of reflected neutrons lags behind, which causes the falling edge to slow down. The two-region kinetic model, which divides the reactor core into a fission region and a reflected region, is generally used to study the reflected reactor. The traditional two-region kinetic model only takes into account the interaction probability between the two regions but the time property of the interaction is not considered at all. Therefore, the traditional two-region model can well describe the stable state process rather than the prompt one. In the early stage, the delayed neutron approximation method was used to study the reflected neutron problem of metal burst reactors. Although some parameters were obtained to be in accordance with the experimental results, there existed a significant difference in behavior between delayed neutrons and reflected neutrons. In this paper, we present a time-dependent two-region model which can effectively describe the behavior of the reflected neutrons in both stable and prompt states. Firstly, we use the Monte-Carlo method to calculate the returning behavior of one leakage neutron from the reactor core. The equivalent eigen source is obtained by solving the kinetic equation with the Monte-Carlo calculating result. This source, including time information, causes the same effect as that of one leakage neutron in the reactor. Secondly, we establish the kinetic equation with reflection effect by introducing the eigen source. In short, the reflected neutrons are treated as an equivalent neutron source. The waveform acquired through solving the equation is consistent with the experiment data of CFBR-Ⅱ, which reasonably describes the experimental phenomenon of falling edge slow-down and plateau power increase.
Research progress of micromagnetic magnetic skyrmions and applications
Authors: Jin Chen-Dong  , Song Cheng-Kun  , Wang Jin-Shuai  , Wang Jian-Bo  , Liu Qing-Fang  ,   Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China;,   Key Laboratory for Special Function Materials and Structural Design of the Ministry of the Education, Lanzhou University, Lanzhou 730000, China
Published: December 2020

Magnetic skyrmion, as a quasi-particle, with topologically protected property has received wide attention. In this article, We first review the existence conditions and transport characteristics of magnetic skyrmions theoretically, then view recent micromagnetic simulation researches on creation and controlling as well as the device design, which includes racetrack memories, spin transfer nano-oscillators, transistors and logic gates. We hope this paper can provide a reference for the applications of magnetic skyrmions in the future.
Improved hybrid plasmonic microcavity laser
Authors: Dong Wei  , Wang Zhi-Bin  ,   Institute of Electrical Engineering, Yanshan University, Qinhuangdao 066004, China
Published: December 2020

In this paper, an improved hybrid surface plasmon nanolaser with a gain medium ridge and a layer of air gap is proposed. In order to achieve low propagation loss and sub-wavelength field confinement, a triangular air gap and a 50 nm microcavity end face silver mirror are adopted in this structure, and the combination of this particular triangular structure and silver mirror effectively improves the performance of nano-laser. In this paper, we numerically simulate the waveguide by using the finite-element method. The COMSOL multiphysics software is a superior numerical simulation software to simulate the real physical phenomena based on the finite element method. On the basic of the COMSOL multiphysics software, a two-dimensional cross-section model and a three-dimensional model are built, the transmission performance and microcavity performance of the improved structure are analyzed in detail at a working wavelength of 1550 nm. Some quantities including the electric field distribution, transmission length, normalized mode field area, average energy density, foundation modal volume, quality factor of the structure, threshold gain, quality factor, effective modal volume, and Purcell factor are considered here which are dependent on the dielectric constant and geometrical parameters. The results indicate that on a two-dimensional scale, the contradiction between transmission loss and transmission distance can be effectively solved by the guidance of Fom value, and the IHPM laser structure with optimal transmission characteristics is obtained under the guidance of quality factor and foundation modal volume. A deep sub-wavelength constraint on light is achieved:the propagation length of the electromagnetic mode reaches a millimeter level and the longest distance can reach 1.29 mm. When testing the microcavity performance of the laser separately on a two-dimensional scale and three-dimensional scale, the high quality factor, low gain threshold, ultra-small effective mode volume of 0.001092 μm3 and ultra-high Purcell factor of 8.29×105 are obtained by adjusting the structural parameters and plating a 50 nm-thick silver layer on the end face of the laser microcavity. Compared with the previous structure without air gaps, the designed structure has a low laser lasing threshold and strong micro-cavity local capability when these two structural parameters are unified. The designed hybrid surface plasmon nanolaser may serve as a fundamental building block for various functional photonic components and can have applications such as in sensing, nanofocusing, and nanolasing.
Shape controlled growth for type Ib large diamond crystals
Authors: Wang Jun-Zhuo  , Li Shang-Sheng  , Su Tai-Chao  , Hu Mei-Hua  , Hu Qiang  , Wu Yu-Min  , Wang Jian-Kang  , Han Fei  , Yu Kun-Peng  , Gao Guang-Jin  , Guo Ming-Ming  , Jia Xiao-Peng  , Ma Hong-An  , Xiao Hong-Yu  ,   School of Materials Science and Engineering, Henan Polytechnic University, Cultivating Base for Key Laboratory of Environment-Friendly Inorganic Materials in University of Henan Province, Jiaozuo 454000, China;,   School of Physics and Electronic Information Engineering, Henan Polytechnic University, Jiaozuo 454000, China;,   State Key Laboratory of Superhard Materials, Jilin University, Changchun 130022, China;,   Department of Mathematics and Physics, Luoyang Institute of Science and Technology, Luoyang 471023, China
Published: December 2020

The shape controlled growth of diamond is beneficial to its subsequent processing. The shape controlled growth for abrasive grade diamond, whose particle size is less than 1 mm, has been studied extensively, while the shape controlled growth of large diamond crystals, which have important commercial and scientific applications, has not been investigated in detail. Therefore, it is necessary to do further researches. In this study, we synthesize large type Ib diamond crystals and investigate their growth shapes at pressures of 5.3-5.9 GPa and temperatures of 1200-1370℃, by using Fe64Ni36 alloy as the catalyst and (100) or (111) face of seed as growth face. Experimental results show that for the diamond crystals grown along the (100) face, the crystal shapes presents plate shape at 1206-1215℃, tower shape at 1216-1260℃, and tower steeple shape at 1261-1360℃; in sequence while for those grown along the (111) face, the crystal shape is of tower at 1233-1238℃ and becomes plate at 1239-1364℃. The ratio of height to diameter, which can provide a standard to quantify the shape of a diamond, is used to describe the crystal shape in detail. For large diamond crystals growing along the (100) face, under a high pressure of 5.6 GPa, the ratio of height to diameter increases with temperature increasing but the ratio of height to diameter, when growing along the (111) face, decreases. The shape distributions of large diamond crystals in the V-shaped region can be determined in the experiments of large diamond crystal synthesis at different temperatures (1200-1370℃) and pressures (5.3 GPa, 5.6 GPa, 5.9 GPa). The lower limit temperature of large diamond crystal growing along the (111) face in the V-shape region is obviously higher than that growing along the (100) face, but the difference between the higher limit temperatures for growing along these two faces is not obvious. The difference between the lower temperature limits of large diamond crystals growing along the (100) and (111) face can be explained by the different energies of the crystal surface and diamond/graphite equilibrium line in the phase diagram of carbon/alloy. Therefore, it has been realized that the shapes for type Ib large diamond crystals are controlled.
Numerical study of droplet impact on the inner surface of a cylinder
Authors: Li Yu-Jie  , Huang Jun-Jie  , Xiao Xu-Bin  ,   College of Aerospace Engineering, Chongqing University, Chongqing 400044, China;,   Chongqing Key Laboratory of Heterogeneous Material Mechanics(Chongqing University), Chongqing 400044, China;,   State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing 400044, China
Published: December 2020

Droplet impact on a solid surface is ubiquitous in daily life and various engineering fields such as ink-jet printing and surface coating. Most of existing studies focused on the droplet impact on flat or convex surface whereas the droplet impact on a concave surface has been less investigated. The purpose of this paper is to investigate the dynamic process of droplet impact on the inner surface of a cylinder numerically by using the phase-field-based lattice Boltzmann method. This method combines the finite-difference solution of the Cahn-Hilliard equation to capture the interface dynamics and the lattice Boltzmann method for the hydrodynamics of the flow. Besides, a recently proposed method is employed to deal with the wetting boundary condition on the curved wall. The method is first verified through the study of the equilibrium contact angle of a droplet on the inner surface of a cylinder and the droplet impact on a thin film, for which good agreement is obtained with theoretical results or other numerical solutions in the literature. Then, different droplet impact velocity, initial height of the droplet, surface wettability and radius of the cylinder are considered for the main problem and their effects on the evolution of the droplet shape are investigated. The physical properties of the droplet including the density and viscosity are also varied to assess their effects on the impact outcome. It is found that the impact Weber number, the liquid/gas density and dynamic viscosity ratios, the wettability of the inner surface of the cylinder, and the radius of the cylinder may have significant effects on the deformation and spreading of the droplet. At low Weber numbers, when the density and dynamic viscosity ratios are sufficiently high, their variations have little effect on the droplet impact process. At high Weber numbers, changes of these two ratios have more noticeable effects. When the Weber number is high enough, droplet splashing appears. When the density and dynamic viscosity ratios are high, the initial height of the droplet only has a minor effect on the impact results. The increment of the cylinder radius not only increases the maximum spreading radius but also enlarges the oscillation period of the droplet after its impact. Rebound of the droplet may be observed when the contact angle of the inner surface of the cylinder is large enough. Besides, the gravity force is found to suppress the oscillation of the droplet on the cylinder's inner surface. This work may broaden our understanding of the droplet impact on curved surfaces.
Cusped field thruster using different propellants
Authors: Liu Hui  , Jiang Wen-Jia  , Ning Zhong-Xi  , Cui Kai  , Zeng Ming  , Cao Xi-Feng  , Yu Da-Ren  ,   Institute of Advanced Power, School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
Published: December 2020

Cusped field thruster is a new kind of thruster which confines plasma by magnetic mirror effect to produce thrust. It is characterized by long lifespan and adjustable thrust in a large range, which makes it have great potential applications in drag free satellites and commercial space satellites. It was put forward first by THALES Electron Devices in Germany and sponsored from European Space Agency. There are several institutions are engaged in the research of this thruster, including Massachusetts Institute of Technology, Stanford University and Technische Universiteit Delft. Now the test experiments on the cusped field thruster using Xe, Kr and Ar are being carried out in the laboratory of plasma propulsion of Harbin Institute of Technology to ascertain the ionization regulations of different propellants under the high voltage and strong magnetic field conditions. On this basis, it is significant to know the mechanism about how the performances change with propellant and provide the foundation for the cusped field thruster using different propellants. In this paper, the principle and design process of this thruster are presented. Then it can be found that the thruster can be ignited easily by using Xe compared with by using Kr and Ar under the same volume flux, which is caused by their differences in ionization energy and ionization section. Experiments show that the cusped field thruster can be ignited under 200 V while it cannot be ignited by using Kr and Ar even under 1000 V under the same volume flux. Then the performances of cusped field thruster using three propellants are tested. It can be found that there are obvious differences in anode current, thrust, efficiency and impulse using three propellants under the same conditions. The diagnosing of plume using Faraday probe shows that the propellant utilization causes the difference in performance which is related to ionization process. The experiments show that the utilization rate of Xe is over 90 percent, while the utilization rate of Kr is less than 60 percent and the utilization rate of Ar is less than 20 percent. The obvious difference in ionization voltage can reflect the difference in performance. The experimental results under the same flux show that the utilization rates of Kr and Ar can be improved by increasing flow density and reducing the collision free path between atoms. Experiments show that the peak utilization rate of Ar can be improved to 50 percent approximately. In the aspect of plume structure, the results of Faraday probe show that the hollow plume can be observed and the angle linked with peak ion current density decreases with atom mass decreasing.
Research progress of bicontinuous interfacially jammed emulsion gel (Bijel)
Authors: Li Tao  , Chen Ke  , Jure Dobnikar  ,   Key Laboratory of Soft Matter Physics, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

In 2005, a bicontinuous arrangement of domains was explored by large-scale computer simulations. In a binary liquid host, the behaviors of neutrally wetting particles were simulated following an instantaneous quench into the demixed region. As the two mutually immiscible liquids phase separate, particles can be swept up by the freshly created interface and jam together as the domains coarsen, forming a particle-stabilized interface between two continuous liquid phases. This type of material is known as “bicontinuous interfacially jammed emulsion gel” (Bijel), and has been demonstrated experimentally using water-lutidine mixture in 2007. It is believed that Bijels have rich potential applications in diverse areas including healthcare, food, energy and reaction engineering due to their unique structural, mechanical and transport properties.As a new class of soft materials, Bijels have received great attention in recent years, and have been developed by using different liquids and non-spherical particles. However, a wide gap remains between the experimental systems and the industrial applications. This short review will critically assess current progress of Bijels and relevant studies including the attempts and challenges to use them in industry; the creation of Bijels by direct mixing at room temperature will be highlighted specifically.Chapter 1 presents the theoretical background. For binary-liquid systems containing dispersed colloidal particles, arrested composites can be created via the stabilization of convoluted fluid-fluid interfaces. Based on this, different morphologies of Pickering emulsions would be obtained. Chapter 2 first focuses on some complex emulsions, including Janus droplets and multiple emulsions, and then induces the bi-continuous structures. Such structures were originally formed through spinodal decomposition, which catches the phase demixing of an initially single-phase liquid mixture containing a colloidal suspension, and normally needs to control the temperature carefully. In Chapter 3, the mechanism of spinodal decomposition is presented. Chapter 4 shows some recent research progress of Bijels, including the studies with different liquid systems, nonspherical particles and some chemical property measurements. This chapter also summarizes the challenges in using Bijels in industry. In Chapter 5, a new method of creating Bijels by direct mixing at room temperature is demonstrated. This method simply needs high viscosity liquids, nanoparticles and a surfactant; it not only bridges the gap between conventional Bijel production (see Chapter 3) and that of particle stabilized bicontinuous structures using bulk polymers, but also bypasses the careful particle modification and phase separation steps for conventional Bijels. In Chapter 6 some conclusions are drawn and a general outlook is also provided.
Biological effects on cells in strong static magnetic field
Authors: Tian Xiao-Fei  , Zhang Xin  ,   High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China;,   University of Science and Technology of China, Hefei 230036, China
Published: December 2020

With the development of technology and the widespread use of high static magnetic fields (SMFs) in medical diagnosis, such as MRI (magnetic resonance imaging) in hospitals, patients have more and more chances to encounter high SMFs (higher than 1 T), which invokes increasing public concerns about human health. However, due to the experimental limitations, there are very few studies of high SMFs (above 1 T) on animals and human bodies. In contrast, cell, as a basic unit of various organisms, is the primary research target for most researches of the biological effects under the action of magnetic fields. However, due to the differences in magnetic field parameter, exposure condition and cell type, there are diverse experimental outcomes reported by individual studies in the literature. Here in this review, we summarize the results about the cellular effects under SMFs above 1 T, including changes of cell orientation, cell proliferation, microtubule and mitotic spindle orientation, DNA and cell cycle. Moreover, we also compare and analyze the factors that could cause these experimental variations, including the differential effects of high SMFs on cell type, such as cancer and non-cancer cells, as well as magnetic field intensity-induced experimental variations. The most well studied cellular effects are SMF-induced cell and polymer orientation changes, and the cellular composition is a key factor that determines the exact orientation of a cell in an SMF. For example, the normal red blood cell is aligned parallelly to the SMF direction, but the whole bull sperm is aligned perpendicularly to the SMF direction. Among the magnetic field parameters, the magnetic field intensity is especially critical. The red blood cells can only be partially aligned by 1 T SMF, but an 8 T SMF could align the red blood cells 100% along the magnetic direction. Overall, the biological research of high SMFs above 1 T, especially above 10 T, is still at an initial stage. Biological experiments in high SMFs above 20 T are especially lacking. This review could help provide some biological bases for future high SMF investigations, which is important not only for the basic understanding of the biological effects of high SMFs, but also for the applications of high SMFs in medicine, such as high field MRI.
Effects of oxide isolation layer on magnetic properties of L10 FePt film grown on Si substrate
Authors: Li Dan  , Li Guo-Qing  ,   School of Physical Science and Technology, Southwest University, Chongqing 400715, China
Published: December 2020

Magnetic force microscope (MFM) is a powerful tool to subtly detect the stray field distribution of magnetic film or particles on a sub-micrometer scale. Due to its huge uniaxial magnetocrystalline anisotropy (Ku~7107 erg cm-3) and high Currie temperature (TC~500℃), FePt alloy in an L10 phase is expected to be coated on the MFM tip to display high coercive force (Hc) and to improve the magnetic stability and MFM resolution. A grain size of~3 nm will be enough to overcome the super paramagnetism. However, the growing fresh FePt films must experience a high temperature annealing (exceeding 700℃) in order to transform their structures thoroughly from a soft A1 phase into the desired hard L10 phase. This brings the risk of diffusion between FePt coating layer and the underneath Si cantilever. Several admixtures have been attempted by other researchers to obtain granular films with FePt grains separated by oxides, with the purpose to prevent the diffusion from happening between FePt and Si. But apparently, it will be very difficult to fabricate a separated FePt grain exactly on the top of MFM tip. This is a critical factor to affect the MFM resolution. And discussion about the influence of the interface diffusion is avoided in most of published papers. Alternatively, some oxide isolation layers with higher melting temperature can be useful for separating the top FePt film from the bottom Si crystal. In this paper, MgO and SiO2 are selected as isolation layers, deposited by magnetron sputtering. Subsequently, the FePt films are deposited at 400℃ and annealed at different temperatures (500℃ to 800℃) for 2 h. The experimental results indicate that the diffusion between FePt and Si substrate always occurs in the absence of any isolation layer, leading to a reluctant maximum Hc of~5 kOe for 50 nm FePt film. However, the coercive force could remarkably exceed 10 kOe if an isolation layer is used. In the case of MgO, a maximum Hc of~12.4 kOe for 50 nm FePt could be stably measured. However, the annealing temperature must be lower than 600℃ to hold back the occurrence of brittle cracks in isolation layer. Because of the smaller lattice mismatch and expansion coefficient difference between SiO2 isolation layer and Si substrate, the highest annealing temperature could exceed 800℃ when replacing MgO with SiO2. The Hc of FePt film could be adjusted in a range from~5 kOe to~15 kOe by changing the annealing temperature. These findings greatly benefit the fabrication of FePt-based MFM tips with high Hc. And it is expected to be able to effectively enhance the resolution of MFM image.
Resistive switching characteristics and resistive switching mechanism of Au/TiO2/FTO memristor
Authors: Yu Zhi-Qiang  , Liu Min-Li  , Lang Jian-Xun  , Qian Kai  , Zhang Chang-Hua  ,   Department of Electrical Engineering, Hubei University for Nationalities, Enshi 445000, China;,   School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China
Published: December 2020

Resistance random access memory is regarded as one of the most promising candidates for the future nonvolatile memory applications due to its good endurance, high storage density, fast erase speed and low power consumption. As one of the most important transition-metal oxides, the anatase TiO2 has received intense attention due to its inexpensive cost, strong optical absorption, favorable band edge positions and superior chemical stability. In the last decade, the nanometer-sized TiO2 has been shown to exhibit a wide range of electrical and optical properties, such as nanoscale electronics and optoelectronics, which rely mainly on the unique size and shape. Recently, various anatase TiO2 based devices such as the anatase TiO2 nanotube based memristor and the anatase TiO2 nano-film based memristor have been intensively studied due to their nonvolatile resistive switching performances. Furthermore, many conduction mechanisms have been used to elucidate the resistive switching behaviors of the anatase TiO2 based devices. However, the direct growth of anatase TiO2 nanowire arrays (NWAs) on the FTO substrate is still a challenge since there exists a large lattice mismatch of about 19% between the anatase TiO2 NWAs and the FTO substrate. Moreover, the Au/TiO2/FTO based device has not been reported and the resistive switching mechanism of the anatase TiO2 NWAs based memristor is still unclear. In this work, the anatase TiO2 NWAs with (101) preferred orientation are successfully grown on the FTO substrate by a facile one-step hydrothermal process. The resistive switching characteristics and resistive switching mechanism of the as-fabricated Au/TiO2/FTO memristor are investigated systemically. The result indicates that the Au/TiO2/FTO memristor exhibits nonvolatile bipolar resistive switching behavior. Meanwhile, the resistance ratio between high resistance state and low resistance state exceeds 20 at 0.1 V, which can be maintained over 103 s without significant degradation. In addition, the conduction mechanism of the low resistance state is governed by the ohmic conduction mechanism, while the trap-controlled space charge limited current conduction mechanism dominates the high resistance state. The resistive switching model of the Au/TiO2/FTO memristor is developed, and the resistive switching mechanism could be attributed to the formation and rupture of the conductive filaments relating to the localized oxygen vacancies. It demonstrates that the Au/TiO2/FTO memristor may be a potential candidate for the future nonvolatile memory applications.
Axial multi-particle trapping and real-time direct observation
Authors: Wang Yue  , Liang Yan-Sheng  , Yan Shao-Hui  , Cao Zhi-Liang  , Cai Ya-Nan  , Zhang Yan  , Yao Bao-Li  , Lei Ming  ,   State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China;,   University of Chinese Academy of Sciences, Beijing 100049, China
Published: December 2020

The optical tweezers with the special advantages of non-mechanical contact and the accurate measurement of positions of particles, are a powerful manipulating tool in numerous applications such as in colloidal physics and life science. However, the standard optical tweezers system uses a single objective lens for both trapping and imaging. As a result, the trapping and imaging regions are confined to the volume near the focal plane of the objective lens, making it difficult to track the trapped particles arranged in the axial direction. Therefore, multiple trapping along axial direction remains a challenge. The three-dimensional imaging technology can realize the monitoring of the axial plane, but neither the laser scanning microscopy nor the wide-field imaging technology can meet the requirement of the real-time imaging. To address this issue, we propose a modified axial-plane Gerchberg-Saxton (GS) iterative algorithm based on the Fourier transform in the axial plane. Compared with the direct algorithm such as the Fresnel lens method, the modified axial-plane GS iterative algorithm has a higher modulation efficiency, and the generated axial distribution has a sharper intensity. In theory, the traps generated each have an ideal Gaussian intensity distribution independently, which is proved by the simulation of reconstructed field. With such an iterative algorithm, we can directly create multiple point-trap array arranged along the axial direction. We also develop an axial-imaging scheme. In this scheme, the particles are trapped and a right-angled silver-coated 45 reflector is used to realize axial-plane imaging. The scheme is verified by imaging silica particles in an axial plane and a lateral plane simultaneously. Furthermore, we combine the axial-plane imaging technique with holographic optical tweezers, and demonstrate the simultaneous optical trapping in 22 trap array and the monitoring of multiple silica particles in the axial plane. The trap stiffness of traps array in axial plane is calibrated by measuring the Brownian motion of the trapped particles in the axial trap array with digital video microscopy. The proposed technique provides a new perspective for optical micromanipulation, and enriches the functionality of optical micromanipulation technology, and thus it will have many applications in biological and physical research.
Optimization of surface morphology with micro meter size for suppressing secondary electron emission
Authors: Hu Jing  , Cao Meng  , Li Yong-Dong  , Lin Shu  , Xia Ning  ,   Key Laboratory for Physical Electrons and Devices of the Ministry of Education, Department of Electronic Science and Technology, School of Electronic and Information Engineering, Xi'an Jiaotong University, Xi'an 710049, China
Published: December 2020

Suppression of the secondary electron (SE) multipactor is a key issue for improving the performance of high power microwave devices and particle accelerators. The decrease of the SE emission yield (SEY) by using certain surface morphology is one of the effective methods. To optimize the surface morphology, we simulate the SE emissions of different surface structures by using the Monte Carlo method. The effects of geometric parameters, such as duty ratio of area, depth-to-height ratio, pattern and its arrangement on SEY are investigated. For surface morphology with patterns of square, round and triangle, and for both convex and concave structures, the corresponding values of SEY first decrease and then become steady with the increase of duty ratio of area and depth-to-height ratio. For convex structures, the values of SEY are different for different pattern shapes, in which triangle pattern has the smallest SEY. However, the value of SEY is nearly independent of arrangement of pattern. For concave structures, on the other hand, the value of SEY is scarcely different for different patterns or different arrangements. In general, a convex structure has a better suppression effect than a concave structure if other geometric parameters are identical. The shading effect from side wall of structure is found to be the intrinsic reason of the suppression effect.
Analysis of magnetohydrodynamic drag character for hypersonic vehicles
Authors: Yao Xiao  , Liu Wei-Qiang  , Tan Jian-Guo  ,   College of Aerospace Science, National University of Defense Technology, Changsha 410073, China
Published: December 2020

In hypersonic flight, a very high temperature area can form ahead of the nose of aerocraft due to the shock aerodynamic heating, which leads to air weakly ionized. Many researchers have demonstrated that it is effective to control flow by utilizing the interaction between weakly ionized air and a magnetic field. Most of previous researches focus on magnetohydrodynamic (MHD) heat shield, because the Lorentz force can increase the shock stand-off distance, further reduce convective heat flux. In this study, the MHD force effect is mainly considered, and the MHD drag characters under different types of magnetic field are discussed.The numerical simulation of hypersonic hemispherical flow field with external magnetic field is carried out by using a low magnetic-Reynolds MHD model. Three kinds of simple ideal magnetic fields (axial, radial and circle uniformly distributed magnetic field) are compared and analyzed. The influence and mechanism of the structure of the flow field, the aerodynamic drag and the Lorentz resistance of different magnetic fields are analyzed. It is found that under the radial ‘extrusion’ effect of the axial magnetic field, the shock wave shape is protruded and a ‘saturation phenomenon’ of pressure exists on the wall; the radial magnetic field has the axial ‘extrusion’ effect, the larger magnetic field intensity will lead to the formation of the high temperature area of the shoulder, and the induced electric field in the circle magnetic field leads to the poor effect of increasing resistance. Then the flow fields of two special magnetic fields (dipole magnetic field and solenoid magnetic field) are compared, and the radial ‘dilatation’ effect is found to be different from the ideal magnetic field. Compared with the Lorentz force under the different magnetic fields, the Lorentz force in the radial magnetic field is found to be concentrated in the high temperature area of the shoulder, and the Lorentz force is generally small under the circle magnetic field. The direction near the standing point will have an adverse effect, i.e., the resistance increases. In the specially distributed magnetic field, the direction of Lorentz force near the shoulder is approximately parallel to that of the shoulder, while the direction near the standing point is approximately perpendicular to the axis. Compared with the dipole magnetic field, the solenoid magnetic field with high Lorentz force region is close to the shoulder, so it will have good resistance enhancement effect. The influence of the dipole magnetic field on the wall pressure is weak. The effect of increasing resistance, caused by the magnetic field induced electric field, evolves from weak to strong in the following sequence:radial magnetic field, solenoid magnetic field, axial magnetic field, dipole magnetic field and circle magnetic field.
Organic color photodetectors based on tri-phase bulk heterojunction with wide sectrum and photoelectronic mltiplication
Authors: An Tao  , Tu Chuan-Bao  , Gong Wei  ,   Department of Electronic Engineering, Xi'an University of Technology, Xi'an 710048, China
Published: December 2020

In order to obtain highly sensitive broadband organic photodetectors (OPDs) used for image sensors with the stable ability to detect three primary colors (RGB), in this paper, the spectral broadening of organic active layer based on tri-phase bulk heterojunction formed by P3HT:PCBM doped with narrow band material PBDT-TT-F which absorbs red light is investigated. The influences of PBDT-TT-F doping ratio on the morphology of active layer film and detector photoelectric properties are further analyzed. Finally, the operating mechanism of trap-assisted photoelectronic multiplication is discussed. On this basis, the detector with 350-750 nm wide spectrum is obtained where the optimum mixing ratio of P3HT:PCBM:PBDT-TT-F is 12:8:3. At a small reverse bias of 1 V, the values of responsivity and external quantum efficiency of the photodetector can reach 470, 381, 450 mA/W and 93%, 89%, 121% respectively under the illumination of three primary colors and its normalized detectivity to the RGB is close to 1012 Jones. Additionally, the maximum relative difference between each parameter and its average value is lower than 20%; the bandwidths are 5, 8, and 8 kHz respectively, which reach the imaging requirements for image sensors. The experimental results show that not only the absorption spectra of the active layer can be broadened but also the carriers collection efficiency of respective electrodes can be well maintained by adding a small quantity of spectral broadening material while keeping the microstructure of the original binary bulk heterojunction. Utilizing the reasonable combination of materials to form electron traps, photoelectronic multiplication can be realized by trap-assisted hole tunneling injection from the Al cathode into active layer, and thus improving the normalized detectivity. Moreover, in order to detect different light intensities, the hole injection barrier width should be controlled by the corresponding light intensity. The resulting OPD shows a good liner response to all three primary colors when light intensity increases from 0.1 to 10 mW/cm2. By adjusting the mixing ratio of the tri-phase materials, the stable ability to detect the primary color can be achieved. The present study paves the way for high responsivity broadband OPDs based on tri-phase bulk heterojunction.
Collin variational study of charged conductors' energy in plasma environment
Authors: Tan Kang-Bo  , Lu Hong-Min  , Su Tao  ,   National Key Laboratory of Antennas and Microwave Technology, Xidian University, Xi'an 710071, China
Published: December 2020

Plasma, as a special state of matter, has an effect on its inner conductors. Practically in the plasma environment, the effect may induce surface to charge and discharge, and may degrade the performance of spacecraft. Therefore, this effect needs to be further studied in the electromagnetic compatibility. The energy in a conductors' system is a key factor of the effect, which can also be used to depict the system consisting of relevant conductors and plasma environment. In order to investigate the essence of the system, the variational method is adopted. So with considering the electromagnetic compatibility and protection of this system, the energy of related conductors should be estimated by the theoretical method in the plasma environment. In the stochastic movement, electrons are faster than the irons. Therefore, the negative energy is cumulated. Considering the definition of capacitance, the system energy can be represented by the conductor capacitance and charging potential. Meanwhile, from the plasma kinetic theory, the potential can be obtained in the steady state. Thus, the relations among electromagnetic parameters of conductors, environmental features of plasma, and systematic energy are established, from which the corresponding Collin principle is also investigated. The principle indicates the system essence in the complex electromagnetic environment. In order to illustrate the utility of the variational principle, a simple cubic model is theoretically analyzed directly. From the typical instance, the relation between the geometric dimension and electric energy is illustrated, which is in consistence with the results in the early literature. The relation between secondary electrons and systematic energy is also analyzed. Starting from these theoretical investigations, in order to estimate the complicated structures, the analysis needs to be generalized further. With the assistance of discrete technology, the numerical method is established for analyzing the system energy of the complex conductive system in plasma environment. The generalized method is based on the equation with integral operator, in the calculation of which the method of moment is practically employed. As an application, the estimated energy of cube in plasma environment is compared with the theoretical estimation and the numerical estimation, which are in good agreement with each other. And then a composited structure is numerically analyzed. Obviously, the vairational analysis is beneficial to investigating the physical and principal regulation for conductors in the plasma environment, and the generalized method has wide potential applications in controlling the energy of complex charged conductors, electromagnetic protection, compatibility engineering in plasma environment, etc.
Magnetic domain chirality and tuning of skyrmion topology
Authors: Xu Gui-Zhou  , Xu Zhan  , Ding Bei  , Hou Zhi-Peng  , Wang Wen-Hong  , Xu Feng  ,   School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China;,   State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Published: December 2020

Owing to the topologically protected properties, magnetic skyrmions possess high stability and small critical driving current, thus making them potentially applied to future racetrack memory devices. Skyrmions have been identified in several material systems. One large class contains the centrosymmetric materials, where skyrmions emerge as the competition between perpendicular magnetic anisotropy and magnetic dipolar interactions. The recently reported skyrmion host includes La-Sr-Mn-O, hexagonal MnNiGa, Fe3Sn2, etc. In these systems, due to the isotropic characteristic of the dipolar interaction, magnetic bubble can exhibit various topologies and helicities. The common types of bubbles existing in the materials are the trivial one with n=0 (n is the topological charge) and the non-trivial one with n=1, and the latter is taken to be equivalent to magnetic skyrmion. In this article, we investigate the formation of skyrmions under various magnetic parameters and the role of stripe domain chairity in tuning the bubble topology. The main method we use here is micromagnetic simulation with the Object Oriented MicroMagnetic Framework (OOMMF) code. Also some recent experimental results on MnNiGa and Fe3Sn2 are exhibited and compared with the simulation prediction. Under a fixed magnetization (Ms), by tuning the exchange constant A and magnetic anisotropy Ku, we find that the domains can evolve into a bubble state under a moderate anisotropy value, and to some extent, large anisotropy favors the formation of n=1 topological skyrmion. In the case of the stripe domains, it is found that different initial configuration can lead to different domain wall charity and further change the process of skyrmion formation. When the magnetization in the domain wall orients in the same direction, n=0 bubble will form upon applying magnetic field. While the magnetization in the wall orients alternatively up and down, a topological skyrmion is directly formed. In the stripe domains with inversed 180 Bloch wall, in-plane magnetization dominates and no bubble or skyrmion can form. In addition, the tilt of the magnetic field and uniaxial anisotropy can also change the morphology and topology of the skyrmions, which has been verified in our experiments. According to the above results, we propose to tune the topology of skyrmions in centrosymmetric material through adjusting the ground magnetic state, magnetic anisotropy and in-plane components, which can be realized by element doping at different sites and appropriately designing the sample.
Tuning the electronic and magnetic property of semihydrogenated graphene and monolayer boron nitride heterostructure
Authors: Gao Tan-Hua  , Zheng Fu-Chang  , Wang Xiao-Chun  ,   School of Electromechanical Engineering, Wuyi University, Wuyishan 354300, China;,   Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, China
Published: December 2020

The structural stability, electronic and magnetic properties of semihydrogenated graphene and monolayer boron nitride (H-Gra@BN) composite system are studied by the first principles calculation. First, for the six possible stacked configurations of H-Gra@BN in three kinds of magnetic coupling manners, including the nonmagnetic, ferromagnetic and antiferromagnetic, the geometry optimization structures are calculated. The formation energies (Ef) are -28, -37, -40, -35, -28, and -34 meV/atom for AA-B, AA-N, AB-B, AB-B-H, AB-N and AB-N-H configurations of H-Gra@BN, respectively. The details of the six H-Gra@BN configurations are presented. The results show that the AB-B configuration of H-Gra@BN system is most stable with the largest formation energy in the six configurations. Its thickness is the smallest in all six configurations. The formation energies of all configurations are very close to each other and show that the combination of the interlayer between layers is very weak, The interaction between H-Gra and monolayer BN is van der Waals binding. Second, the band structure, total density of states (TDOS), partial density of states and polarization charge density of the most stable H-Gra@BN system are systematically analyzed. This material is ferromagnetic semiconductor. The band gaps for majority and minority spin electrons are 3.097 eV and 1.798 eV, respectively. Each physical cell has an about 1 μB magnetic moment, which is mainly derived from the contribution of the unhydrogenated C2 atom. Furthermore, while the pressure is applied along the z direction, we analyze the TDOS and band structure of H-Gra@BN system, and find that when the z axis strain is more than -10.48% (Δh=-0.45 Å), the valence band maximum of minority spin moves down. The conduction band minimum of minority spin moves from the high symmetry Γ position into a position between Γ and K. The electronic properties of the most stable H-Gra@BN system change from magnetic semiconductor into half metal. When the strain is increased by more than -11.65% (Δh=-0.5 Å), the most stable H-Gra@BN changes into a nonmagnetic metal. To analyze the effect caused by different strains, we analyze the difference in three-dimensional charge density, and find that with the decrease of the layer spacing, the interlayer interaction gradually increases and shows the obvious covalent bond characteristics. This paper predicts a new type of two-dimensional material of which the electronic and magnetic properties can be easily tuned by pressure, and it is expected to be used in nano-devices and serve as an intelligent building material.