Accepted
, , Received Date: 2020-12-01
Abstract +
The Single Event Effect (SEE) and Total Ionizing Dose effect (TID) of a commercial gallium nitride (GaN) High Electron Mobility Transistor (HEMT) with P-type gate and Cascode structure has been studied using heavy ions and gamma irradiation. The safe operating areas of SEE and parameters degradation induced by TID are given in this paper. Experimental results show that the GaN HEMT with Cascode structure is much more sensitive to SEE and TID than that with P-type gate structure. Under a linear energy transfer (LET) value of 22 MeV.cm2/mg and a cumulative dose of 20 K rad(Si) radiation, the parameters degradation of cascade-type GaN HEMT was observed. Besides, the SEE sensitivity areas and failure mechanisms of cascade-type GaN HEMT were analyzed using layout photographing technology. The extra carriers caused by heavy-ion radiation could tunnel the Schottky barrier formed by gate-metal and AlGaN layer leading to a large gate-drain current in GaN HEMT. Besides, the metal oxide semiconductor field-effect transistor is the possible reason why the cascade-type GaN HEMT is sensitive to TID.
, , Received Date: 2020-06-19
Abstract +
Radiative shock is an important phenomenon both in astrophysics and inertial confinement fusion. In this paper, the radiation properties of the X-ray heated radiatve shock in xenon is studied with simulation method. The radiative shock is described by a one-dimensional, multi-group radiation hydrodynamics model proposed by Zinn. To make computation, the opacity (Fig. 1) and equation-of-state data (Fig. 2) in a wide scope of density and temperature ranges are calculated and put into the model. The reliabilities of the model and the physical parameters of xenon are verified by comparing the temperature and velocity of the radiative shock calculated by the model with that measured by experiment.
The evolution of the radiative shock involves abundant physical processes. The core of the xenon could be heated to more than 100 eV, result in a thermal wave and forms an expanding high-temperature-core. Shortly, the hydrodynamic disturbances reach the thermal wave front and generate a shock. As the thermal wave slows down, the shock gradually be separated from the high-temperature-core and forms a double-step distribution in the temperature profile (Fig. 3).
The time evolution of the effective temperature of the radiative shock shows two maximum and one minimum value (Fig. 4), and the radiation spectrums are often deviated from blackbody spectrum (Fig. 6). By analyzing the radiation (Fig. 7) and absorption (Fig. 8) properties at different positions of the shock, we find that the optical property of the shock is highly dynamic and could generate the above-mentioned radiation characters. When the radiative shock is just formed, the radiation comes from the shock surface and the shock precursor has a significant absorption of the radiation. As the shock temperature falls during expansion, the shock precursor disappears and the radiation inside the shock could come out because of absorption coefficient decrease. When the shock become transparent, the radiation surface reaches the outside edge of the high-temperature-core. Then, the temperature of the high-temperature-core reduces further, making this region also become optical
, , Received Date: 2020-07-10
Abstract +
With the development of microelectronics and the miniaturization of electronic devices, the use of molecular materials to construct various components in electronic circuits has become a most likely development trend. Compared with silicon-based semiconductor components, molecular electronic device has the advantages of small size, high integration, low energy consumption and fast response. In recent years, more and more molecules have been used to design molecular devices such as molecular diodes, molecular switches, molecular field effect transistors and molecular memories. In this paper, sandwich structure devices based on graphene nanoribbon electrodes are constructed. The first-principles calculation method combining density functional theory and non-equilibrium Green's function is adopted to design the molecular devices with functional characteristics. The effects of redox reactions on the electrical transport properties of molecular devices are systematically discussed. The main research contents of this paper are as follows. The switching characteristics of an anthraquinone molecular device based on graphene electrode are studied. The zigzag-edge nanoribbons and armchair-edge graphene nanoribbons are selected as electrodes. Considering the two isomers of anthraquinone (HQ) and anthraquinone (AQ) molecules in the redox reaction, the double electrode molecular junction is constructed. The effects of redox reaction and electrode structure on the switching characteristics of anthraquinone molecular devices are discussed. It is found that the current in the HQ configuration is significantly greater than that in the AQ configuration, regardless of the zigzag-edge graphene electrode or the armchair-edge graphene electrode. That is, under the redox reaction, the anthraquinone molecules show significant switching characteristics. The switching ratio of zigzag-edge graphene electrode is selected to reach a maximum of 3125, and that of armchair-edge graphene electrode is selected to maximum of 1538. In addition, when the armchair-edge graphene is used as an electrode in the HQ configuration, the negative differential resistance is obviously between 0.7 and 0.9 V.
, , Received Date: 2020-06-28
Abstract +
Supercritical CO2 can be used as a heat transfer fluid in a solar receiver, especially for a concentrating solar thermal power tower system. Such applications require better understanding of the heat transfer characteristics of supercritical CO2 in the solar receiver tube in a high temperature region. However, most of the existing experimental and numerical studies of the heat transfer characteristics of supercritical CO2 in tubes near the critical temperature region, and the corresponding heat transfer characteristics in the high temperature region are conducted. In this paper, a three-dimensional steady-state numerical simulation with the standard k-ε turbulent model is established by using ANSYS FLUENT for the flow and heat transfer of supercritical CO2 in a heated circular tube with an inner diameter of 6 mm and a length of 500 mm in the high temperature region. The effects of the fluid temperature (823—1023 K), the flow direction (horizontal, downward and upward), the pressure (7.5—9 MPa), the mass flux (200—500 kg·m–2·s–1) and the heat flux (100—800 kW·m–2) on the convection heat transfer coefficient and Nusselt number are discussed. The results show that the convection heat transfer coefficient increases while Nusselt number decreases nearly linearly with fluid temperature increasing. Both fluid direction and pressure have negligible effects on the convection heat transfer coefficient and Nusselt number. Moreover, the convective heat transfer coefficient and Nusselt number are enhanced greatly with the increasing of mass flux and the decreasing of heat flux, which is more obvious at a higher heat flux. The influences of buoyancy and flow acceleration on the heat transfer characteristics are also investigated. The buoyancy effect can be ignored within the present parameter range. However, the flow acceleration induced by the high heat flux significantly deteriorates the heat transfer preformation. Moreover, eight heat transfer correlations of supercritical fluid in tubes are evaluated and compared with the present numerical data. The comparison indicates that the correlations based on the thermal property modification show better performance in the heat transfer prediction in the high temperature region than those based on the dimensionless number modification. And Nusselt number predicted by the best correlation has a mean absolute relative deviation of 8.1% compared with the present numerical results, with all predicted data points located in the deviation bandwidth of ±20%. The present work can provide a theoretical guidance for the optimal design and safe operation of concentrating solar receivers where supercritical CO2 is used as a heat transfer fluid.
, , Received Date: 2020-07-01
Abstract +
In order to deal with the thermal management problem of high-energy high-repetition rate laser amplifiers, the efficient heat removal in water-cooled Nd: YAG active mirror amplifiers is investigated in detail through numerical modeling and experimental analysis. According to the low Reynolds number k–ε turbulence model, a full fluid-solid conjugate heat transfer model is established to give a comprehensive model of flow and thermal characteristics in three dimensions. The thermal distributions obtained from the model are then used to calculate all mechanical stresses in the laser medium and thermally-induced wavefront distortions. In comparison with the standard k–ε turbulence model, the influences of the near-wall treatments of the above model on the process of fluid flow, convection diffusion and heat conduction, and temperature distributions are analyzed. Meanwhile, the effects of coolant flow rate and pump parameter on the flow field characteristics, temperature and wavefront distributions of the YAG disk are also studied. Numerical simulation results reveal that the temperature distribution of the laser medium is closely related to the viscous effect in the solid-liquid boundary layer. Although the heat deposition distribution of the laser medium is symmetrical, the temperature profile is asymmetrical as a result of the increasing water temperature along the water flow. The maximum temperature rise of the disk is at the outlet end, and the position remains almost unchanged. The front-surface temperature distributions and wavefront profiles of Nd: YAG vary nonlinearly with the coolant flow rates, but linearly with the pump parameter. Model predictions show that when the laser amplifier operates at a repetition rate of 50 Hz, the thermal diffusion of the coolant mainly occurs in a range of 100 μm, and the maximum temperature difference of the coolant reaches up to 10.85 ℃. Correspondingly, the maximum temperature variation over the front-surface active region is less than 4 ℃, with an average temperature of 49.62 ℃, which leads to a total peak-to-valley wave front distortion of 7.27λ. The experimentally measured temperature distributions are in reasonable agreement with numerical simulations. The research results are beneficial to designing and optimizing the high-energy, high-repetition rate water-cooled Nd: YAG active mirror amplifiers.
, , Received Date: 2020-07-01
Abstract +
Electromagnetic diffusion surface can reduce the radar cross section, thus profiting stealth of targets. Terahertz diffusion surface has a wide prospect in the field of next-generation radar and communication, promising to act as a kind of intelligent smart skin. In this paper, utilizing the excellent tunable properties of graphene in the terahertz band, a hybrid structure of graphene and metal which has inverse phase response of reflecting waves is proposed. The reflection phase switches in the mechanism of resonant modes and can be controlled efficiently by the bias voltage. Meanwhile, unlike metal materials, graphene has a non-negligible loss characteristic, which leads the response amplitudes corresponding to the two different switching states to be inconsistent with each other. According to the interference and superposition principle of electromagnetic field, it is not conducive to eliminating the coherent far-field, leading to an unsatisfactory diffusion result. In this paper, we present a "molecular" structure by secondary combination of the above-mentioned reverse phase element states, and take it as the basic element of the diffusion surface. Finally, we use particle swarm optimization to optimize the arrangement of "molecular" structures. The final diffusion surface consists of a combinatorial design of "molecules" rather than randomly distributed reflection units. In addition, molecules designed artificially have similar amplitude responses but different phase responses, which improves the convergence speed and reduces the computation quantity during algorithm evolution. The method of designing molecular structure, described in this paper, is simple, rapid and widely applicable, which effectively improves the amplitude-to-phase modulation ability of graphene metasurface against electromagnetic waves. When diffuse reflection optimization is applied to most of graphene metasurfaces, the method described in this paper can achieve the results that are the same as or even better than the results after a large number of iterations of traditional particle swarm optimization in the most computation-efficient manner. The results show that the dynamic diffusion surface designed by this method has the advantages of fast convergence speed and small far-field peak.
, , Received Date: 2020-06-24
Abstract +
A broadband and high-efficieny bi-layer metasurface is proposed in this paper. The unit cell of the metasurface is formed by symmetrically etching two cross-type metal patches on both sides of a dielectric plate. Furthermore, the two metal patches have a displacement of half a period along the y-axis. By employing the displacement, the transmission bandwidth of the bi-layer metasurface is significantly expanded. In order to obtain a physical insight into bandwidth broadening, a π-type equivalent circuit that presents the electromagnetic coupling between within the bi-layer metasurfaces is successfully extracted to investigate the influence of electromagnetic coupling on transmission performance. The results show that by shifting the metal patches along the y-axis by half a period, the coupling impedance (Z12 or Z21) of bi-layer metasurface can be significantly modified, which further changes the electromagnetic coupling of the bi-layer metasurface. Correspondingly, the impedances Zp and Zs in the π-type circuit are changed to approximately meet the resonant condition of circuit in broadband, resulting in the bandwidth expansion of the proposed device. By using Pancharatnam-Berry phase theory, we redesign the proposed metasurface unit cell into a broadband orbital angular momentum generator. The simulation and measurement results verify that the bi-layer metasurface can convert a left-hand circularly polarized wave into a right-hand circularly polarized wave carrying orbital angular momentum in a frequency range between 11 GHz and 12.8 GHz, demonstrating the performance of device.
, , Received Date: 2020-06-16
Abstract +
, , Received Date: 2020-06-15
Abstract +
4 flow rate in 4H-SiC epilayers is determined by using single-dilution gas line and double-dilution gas line. Then the p+ layer and p++ layer in PiN diode are obtained by aluminum ion implantation at room temperature and 500 ℃ followed by high temperature activation annealing. Finally, 4H-SiC PiN diodes with a Ti, N co-doped buffer layer are fabricated and tested with a forward current density of 100 A/cm2 for 10 min. Comparing with the PiN diodes without a buffer layer and with a buffer layer only doped with high concentration of nitrogen, the forward voltage drop stability of those diodes with a 2 μm-thick Ti, N co-doped buffer layer (Ti: 3.70 × 1015 cm–3 and N: 1.01 × 1019 cm–3) is greatly improved.">"Bipolar degradation" phenomenon has severely impeded the development of 4H-SiC bipolar devices. Their defect mechanism is the expansion of Shockley-type stacking faults from basal plane dislocations under the condition of electron-hole recombination. To suppress the "bipolar degradation" phenomenon, not only do the basal plane dislocations in the 4H-SiC drift layer need eliminating, but also a recombination-enhancing buffer layer is required to prevent the minority carriers of holes from reaching the epilayer/substrate interface where high-density basal plane dislocation segments exist. In this paper, Ti and N co-doped 4H-SiC buffer layers are grown to further shorten the minority carrier lifetime. Firstly, the dependence of Ti doping concentration on TiCl4 flow rate in 4H-SiC epilayers is determined by using single-dilution gas line and double-dilution gas line. Then the p+ layer and p++ layer in PiN diode are obtained by aluminum ion implantation at room temperature and 500 ℃ followed by high temperature activation annealing. Finally, 4H-SiC PiN diodes with a Ti, N co-doped buffer layer are fabricated and tested with a forward current density of 100 A/cm2 for 10 min. Comparing with the PiN diodes without a buffer layer and with a buffer layer only doped with high concentration of nitrogen, the forward voltage drop stability of those diodes with a 2 μm-thick Ti, N co-doped buffer layer (Ti: 3.70 × 1015 cm–3 and N: 1.01 × 1019 cm–3) is greatly improved.
Longitudinal wave analysis of infinite length piezoelectric circular rod based on temperature effect
, , Received Date: 2020-05-21
Abstract +
Piezoelectric elements have been commonly used because of their wide applications in sensors, transducers, and some micro intelligent structures. However, in the fields of aviation, aerospace, and automation, some relevant equipment works in a harsh environment and is susceptible to the temperature change, thereby leading its performances to be greatly affected. Therefore, the problem of nonlinear wave relating to piezoelectric circular rods in different temperature fields is studied by modeling and numerical analysis. Firstly, based on the theory of finite deformation, we take infinite piezoelectric circular rod as a research object and consider the effects of transverse inertia and equivalent Poisson's ratio under the thermoelectric coupling action. Using the Hamilton principle and introducing the Euler equation, the longitudinal wave equation of piezoelectric circular rod is obtained. Secondly, Jacobi elliptic cosine function and Jacobi elliptic sine function expansion method are used to solve the wave equation of the piezoelectric circular rod, and the solitary wave solution and the exact periodic solution of the wave equation are obtained. It is found that the periodic solution can be reduced into a solitary wave solution under certain conditions, and it is proved theoretically that there may be solitary wave stably propagating in a piezoelectric circular rod. Finally, the dispersion curves of different wave velocity ratios and the curves about influences of temperature field on the waveform, amplitude and wave number of the piezoelectric rod are obtained by Matlab. The numerical results show that the wave velocity decreases with the increase of temperature when the wave velocity ratio is constant. Given the temperature is constant, it can be found that with the increase of the ratio, the amplitude of solitary wave gradually increases while the wavelength gradually decreases. In addition, the images obtained show that although temperature change can cause the characteristics of solitary waves to change, the solitary waves are always symmetrical bell shaped waves in the propagation process, reflecting the stability characteristics under the combined action of nonlinear and dispersion effects. Therefore, the variation of temperature field can influence and control some propagation characteristics of solitary waves. Moreover, the wave theory has been widely used in the nondestructive testing of structures and the improving of information transmission quality due to its special stability.
, , Received Date: 2020-05-20
Abstract +
Reduced activation ferritic/martensitic steel is one of the candidate materials for tritium breeding module in the fusion reactor. In order to control the permeability of tritium in an acceptable range, coating with low hydrogen isotope permeability, known as tritium permeation barrier, is usually prepared on the surface of such structural materials. The FeAl/Al2O3 is the first choice of tritium permeation barrier for many countries, because of its fine performance of high permeation reduction factor, corrosion resistance and high-temperature resistance. The surface morphology and microstructure of Fe-Al infiltrated layer have important influence on the quality of Al2O3 coating. In this study, Al coating on the surface of CLAM steel is prepared by electroplating of aluminum from AlCl3-EMIC. Then the Fe-Al infiltrated layer is obtained by diffusion between Al and substrate by annealing. The effects of annealing time and temperature on the microstructure of Fe-Al infiltrated layer are studied by X-Ray diffraction, scanning electron microscope and energy dispersive spectrometer. The results show that 20-μm-thick aluminum coating is obtained on the CLAM steel surface by electroplating. The Al coating is uniform and compact, and the size of its surface columnar grain decreases with electroplating current density increasing. Annealing results show that neither hole nor gap is observed between the Fe-Al infiltrated layer and the substrate. In addition, the infiltrated layer is found to be tightly bound to the substrate with a thickness ranging from 7 μm to 45 μm, depending on the annealing parameters. At the initial stage of annealing, Cr enriched Fe-Al alloy is formed evidently. However, such a Cr enrichment disappears at higher annealing temperature or longer annealing time due to diffusion. The surface of infiltrated layer changes from aluminum-rich phase to aluminum-poor phase, and its thickness increases with annealing time or temperature rising. The temperature dependence of the growth rate of Fe-Al infiltrated layer can be described by Arrhenius equation. At this time, the Arrhenius activation energy of aluminization on CLAM steel is calculated to be 78.48 kJ/mol. At 640 ℃ and 760 ℃, the growth of Fe-Al infiltrated layer is controlled by the grain boundary as well as the volume diffusion. When the reasonable thickness and microstructure of Fe-Al alloy layer are used and annealing time or temperature keeps as low as possible, the optimal annealing temperature and time are 700 ℃/10 h, respectively.
, , Received Date: 2020-12-08
Abstract +
Population migration is an essential medium for the spread of epidemic, which can accelerate localized outbreaks of disease into widespread epidemic. Large scale population movements between different areas increase the risk of cross-infection and bring great challenges to epidemic prevention and control. As COVID-19 can spread rapidly through human-to-human transmission, understanding migration patterns are essential to model the spreading and to evaluate the efficiency of mitigation policies applied to COVID-19. Using nationwide mobile phone data to track population flows throughout China at prefecture-level, this paper applies temporal network analysis to compare topological metrics of population mobility network during two consecutive months before and after the outbreak, i.e., January 1st to February 29th. To detect regions which are closely connected with population movements, we propose a Spatial-Louvain algorithm through adapting a gravity attenuation factor, and yield an improvement of 14\% in modularity compared with the Louvain algorithm. Additionally, we divide the period into four stages, including normal times, Chunyun
, , Received Date: 2020-11-03
Abstract +
GaInAsSb quaternary alloys have attracted much interest in infrared optoelectronic applications due to their advantage of the versatility in obtaining alloys with a large range of energy gaps from 0.296 eV to 0.726 eV when lattice matched to GaSb wafers. However, due to the high intrinsic carrier concentration and Auger recombination, GaInAsSb p-n junctions typically were characterized by high dark current density at room temperature and need to be operated at low temperature to obtain high optoelectronic performance. In this work, a front surface wide-bandgap semiconductor Nano pillar array (NPA) and a high reflective metal back surface reflector (BSR) were designed to modulate optoelectronic performances for GaInAsSb p-n junction. The optical and optoelectronic characteristics were analyzed by finite difference time domain simulating and numerical solving of carrier transport equations, respectively. It shows NPA-BSR structure can excite Mie-type resonance, Wood-Rayleigh anomaly effect and Fabry–Perot resonance, which can be used to trap the light efficiently in an ultrathin GaInAsSb film. Own to these nanophotonic effects, the average light absorption of ~90% can be obtained at 1~2.3μm infrared waveband for 1μm Ga0.84In0.16As0.14Sb0.86. It also shows that the Auger recombination can be suppressed with the decreased thickness, which lead to the increased carrier collection efficiency and the decreased dark current density. Theoretical results show that the carrier collection efficiency of ~99% and dark current density of ~5×10-6 A/cm2 can be obtained for the 1μm Ga0.84In0.16As0.14Sb0.86 p-n junction. Given these unique optoelectronic properties, NPA-BSR nanophotonic structure represent a very promising method to realize the high performance ultrathin GaInAsSb infrared optoelectronic devices.
, , Received Date: 2020-04-30
Abstract +
Confocal laser scanning microscopy (CLSM) is a powerful imaging tool providing high resolution and optical sectioning. In its standard optical configuration, a pair of confocal pinholes is used to reject out-of-focus light. The diffraction limited resolution can be broken by reducing the confocal pinhole size. But this comes at the cost of extremely low signal-to-noise ratio (SNR). The limited SNR problem can be solved by image scanning microscopy (ISM), in which the single-point detector of a regular point-scanning confocal microscopy is substituted with an array detector such as CCD or CMOS, thus the two-fold super-resolution imaging can be achieved by pixel reassignment and deconvolution. However, the practical application of ISM is challenging due to its limited image acquisition speed. Here, we present a hybrid microscopy technique, named multifocal refocusing after scanning using helical phase engineering microscopy (MRESCH), which combines the double-helix point spread function (DH-PSF) engineering with multifocal structured illumination to dramatically improve the image acquisition speed. In the illumination path, sparse multifocal illumination patterns are generated by a digital micromirror device for parallel imaging information acquisition. In the detection path, a phase mask is introduced to modulate the conventional PSF to the DH-PSF, which provides volumetric information, and meanwhile, we also present a digital refocusing strategy for processing the collected raw data to recover the wild-filed image from different sample layers. To demonstrate imaging capabilities of MRESCH, we acquire the images of mitochondria in live HeLa cells and make a detailed comparison with those from the wide-field microscopy. In contrast to the conventional wide-field approach, the MRESCH can expand the imaging depth in a range from –1 μm to 1 μm. Next, we sample the F-actin of bovine pulmonary artery endothelial cells to characterize the lateral resolution of the MRESCH. The results show that the MRESCH has a better resolution capability than the conventional wide-field illumination microscopy. Finally, the proposed image scanning microscopy can record three-dimensional specimen information from a single multi-spot two-dimensional scan, which ensures faster data acquisition and larger field of view than ISM.
Abstract +
Compressed sensing is a revolutionary signal processing technique, which allows the signals of interest to be acquired at a sub-Nyquist rate, meanwhile still permitting the signals from highly incomplete measurements to be reconstructed perfectly. As is well known, the construction of sensing matrix is one of the key technologies to promote compressed sensing from theory to application. Because the Toeplitz sensing matrix can support fast algorithm and corresponds to discrete convolution operation, it has essential research significance. However, the conventional random Toeplitz sensing matrix, due to the uncertainty of its elements, is subject to many limitations in practical applications, such as high memory consumption and difficulty of hardware implementation. To avoid these limitations, we propose a bipolar Toeplitz block-based chaotic sensing matrix (Bi-TpCM) by combining the intrinsic advantages of Toeplitz matrix and bipolar chaotic sequence. Firstly, the generation of bipolar chaotic sequence is introduced and its statistical characteristics are analyzed, showing that the generated bipolar chaotic sequence is an independent and identically distributed Rademacher sequence, which makes it possible to construct the sensing matrix. Secondly, the proposed Bi-TpCM is constructed, and it is proved that Bi-TpCM has almost optimal theoretical guarantees in terms of the coherence, and also satisfies the restricted isometry condition. Finally, the measurement performances on one-dimensional signals and images by using the proposed Bi-TpCM are investigated and compared with those of its counterparts, including random matrix, random Toeplitz matrix, real-valued chaotic matrix, and chaotic circulant sensing matrix. The results show that Bi-TpCM not only has better performance for these testing signals, but also possesses considerable advantages in terms of the memory cost, computational complexity, and hardware realization. In particular, the proposed Bi-TpCM is extremely suitable for the compressed sensing measurement of linear time-invariant (LTI) systems with multiple inputs and single output, such as the joint parameter and time-delay estimation for finite impulse response. Moreover, the construction framework of the proposed Bi-TpCM can be extended to different chaotic systems, such as Logistic or Cat chaotic systems, and it is also possible for the proposed Bi-TpCM to derive the Hankel blocks, additional stacking of blocks, partial circulant blocks sensing matrices. With these block-based sensing architectures, we can more easily implement compressed sensing for various compressed measurement problems of LTI systems.
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