Shim coil design for Halbach magnet by equivalent magnetic dipole method
Low-field nuclear magnetic resonance magnet (2 MHz) is required for rock core analysis. However, due to its low field strength, it is hard to achieve a high uniform B0 field only by using the passive shimming. Therefore, active shimming is necessarily used to further improve uniformity for Halbach magnet. In this work, an equivalent magnetic dipole method is presented for designing shim coils. The minimization of the coil power dissipation is considered as an optimal object to minimize coil heating effect, and the deviation from the target field is selected as a penalty function term. The lsqnonlin optimization toolbox of MATLAB is used to solve the optimization problem. Eight shim coils are obtained in accordance with the contour of the stream function. We simulate each shim coil by ANSYS Maxwell software to verify the validity of the designed coils. Measurement results of the field distribution of these coils are consistent with those of the target fields. The uniformity of the B0 field is improved from 114.2 ppm to 26.9 ppm after using these shim coils.
Design of multi-band metasurface antenna array with low RCS performance
In this paper, a multi-band metasurface (MS) antenna array with low radar cross section (RCS) performance is proposed and measured. Firstly, a 4×4 antenna array is composed of four 2×2 Jerusalem cross structure antenna arrays working at different frequency bands, which is aimed at enhancing the bandwidth effectively. Then, each antenna can be seen as a unit of MS in spite of adding the feeding structure. Based on phase cancellation principle, the MS is arranged into a chessboard configuration in order to realize wideband RCS reduction. Thus, excellent radiation and scattering characteristics are obtained simultaneously. Simulated and measured results indicate that this work provides a novel method to achieve bandwidth expansion as well as wideband RCS reduction of the antenna array.
Nonparaxial propagation properties of the chirped Airy Gaussian vortex beams in uniaxial crystals orthogonal to the optical axis
Speckle reduction by selective spatial-domain mask in digital holography
An improved method of using a selective spatial-domain mask to reduce speckle noise in digital holography is proposed. The sub-holograms are obtained from the original hologram filtered by the binary masks including a shifting aperture for being reconstructed. Normally, the speckle patterns of these sub-reconstructed images are different. The speckle intensity of the final reconstructed image is suppressed by averaging the favorable sub-reconstructed images which are selected based on the most optimal pixel intensity sub-range in the sub-holograms. Compared with the conventional spatial-domain mask method, the proposed method not only reduces the speckle noise more effectively with fewer sub-reconstructed images, but also reduces the redundant information used in the reconstruction process.
Tunneling exits of H2+ in strong laser fields
Different from atoms, the multicenter of the Coulombic potentials in molecules makes the tunneling ionization complex, and the electron tunnels out the laser-dressed Coulomb potential with a complex structure. We study tunneling exits of H2+ at large internuclear distance in strong laser fields by numerically simulating the time-dependent Schrödinger equation plus a classical backward propagation of the ionized wave packet. This study strengthens the understanding of molecular tunneling ionization in strong laser fields.
Generation of few-cycle radially-polarized infrared pulses in a gas-filled hollow-core fiber
We perform a numerical study for temporally compressing radially-polarized (RP) infrared pulses in a gas-filled hollow-core fiber (HCF). The dynamic transmission and nonlinear compression of RP pulses centered at wavelengths of 0.8 μ, 1.8 μ, 3.1 μ, and 5.0 μ in HCFs are simulated. By comparing the propagation of pulses with the same optical cycles and intensity, we find that under proper conditions these pulses can be compressed down to 2-3 cycles. In the transverse direction, the spatiotemporal beam profile ameliorates from 0.8-μ to 1.8-μ and 3.1-μ pulses before the appearance of high-order dispersion. These results show an alternative method of scaling generation for delivering RP infrared pulses in gas-filled HCFs, which can obtain energetic few-cycle pulses, and will be beneficial for relevant researches in the infrared scope.
Scanning the energy dissipation process of energetic materials based on excited state relaxation and vibration-vibration coupling
The energy dissipation mechanism of energetic materials (EMs) is very important for keeping safety. We choose nitrobenzene as a model of EM and employ transient absorption (TA) spectroscopy and time-resolved coherent anti-stokes Raman scattering (CARS) to clarify its energy dissipation mechanism. The TA data confirms that the excited nitrobenzene spends about 16 ps finishing the twist intramolecular charge transfer from benzene to nitro group, and dissipates its energy through the rapid vibration relaxation in the initial excited state. And then the dynamics of vibrational modes (VMs) in the ground state of nitrobenzene, which are located at 682 cm-1 (v1), 854 cm-1 (v2), 1006 cm-1 (v3), and 1023 cm-1 (v4), is scanned by CARS. It exhibits that the excess energy of nitrobenzene on the ground state would further dissipate through intramolecular vibrational redistribution based on the vibrational cooling of v1 and v2 modes, v1 and v4 modes, and v3 and v4 modes. Moreover, the vibration-vibration coupling depends not only on the energy levels of VMs, but also on the spatial position of chemical bonds relative to the VM.
Study on polarization properties of graphene coated D-shaped fiber
The optical properties of graphene coated D-shaped single mode fiber and photonic crystal fiber are numerically analyzed. Enhancement of the graphene-light interaction is found in graphene coated D-shaped photonic crystal fiber, which introduces a tunable polarization of the D-shaped fiber by changing the chemical potential of the coated graphene. An optimal polarizer model is demonstrated with the extinction ratio of 66.26 dB/mm and the insertion loss of 9.4 dB/mm. The modulator extinction ratios of the TE mode and TM mode are 11.5 dB and 5 dB, respectively, with a device length of 100 μ m. This paper provides a theoretical reference for the optical property research of the graphene fiber.
Total ionizing dose effects in pinned photodiode complementary metal-oxide-semiconductor transistor active pixel sensor
A pinned photodiode complementary metal-oxide-semiconductor transistor (CMOS) active pixel sensor is exposed to 60Co to evaluate the performance for space applications. The sample is irradiated with a dose rate of 50 rad (SiO2)/s and a total dose of 100 krad (SiO2), and the photodiode is kept unbiased. The degradation of dark current, full well capacity, and quantum efficiency induced by the total ionizing dose damage effect are investigated. It is found that the dark current increases mainly from the shallow trench isolation (STI) surrounding the pinned photodiode. Further results suggests that the decreasing of full well capacity due to the increase in the density, is induced by the total ionizing dose (TID) effect, of the trap interface, which also leads to the degradation of quantum efficiency at shorter wavelengths.
Optical power limiting of ultrashort hyper-Gaussian pulses in cascade three-level system
Propagation of strong femtosecond hyper-Gaussian pulses in a cascade three-level molecular system is studied by solving numerically the Maxwell-Bloch equations by the iterative predictor-corrector finite-difference time-domain method. Optical power limiting behavior induced by strong nonlinear two-photon absorption is observed for different orders of the femtosecond hyper-Gaussian pulses. Pulses of a higher order temporal profile are found to have a wider power range of optical limiting but a larger output saturation intensity. Both the output saturation value and the damage threshold of optical power limiting decrease with pulse duration increasing. The decrease of the pulse area along the pulse propagation is much slower than that obtained from the two-photon area theorem due to invalidity of the slowly varying amplitude approximation and the monochromatic field hypothesis.
Theoretical analysis of optical force density distribution inside subwavelength-diameter optical fibers
We investigate the microscopic optical force density distributions respectively inside a subwavelength-diameter (SD) fiber with flat endface and inside one with oblique endface by using a finite-difference time-domain (FDTD) method. Optical force density distributions at the fiber endfaces can now be readily available. The complete knowledge of optical force density distributions not only reveal features regarding the microscopic near-field optomechanical interaction, but also provide straightforward explanations for the sideway deflections and other mechanical motions. Our results can provide a useful reference for better understanding the mechanical influence when light transports in a microscale or nanoscale structure and for developing future highly-sensitive optomechanical devices.
Radiation from finite cylindrical shell with irregular-shaped acoustic enclosure
In practical situations, large machinery is usually placed in an underwater vessel and changes the acoustic enclosure shape into an irregular one. The existence of machinery causes the difficulties in expressing sound transmission and radiation analytically. In this study, the sound radiation of a cylindrical shell excited by an internal acoustic source is modeled and analyzed. The cylindrical shell contains a machine modeled as a rectangular object, which is attached to a shell with a spring-mass system. The acoustic field of the cavity is computed by the integro-modal approach. The effect of object size on the coupling between acoustic mode and structural mode is investigated. The relationship between object volume and sound radiation is also studied. Numerical results show that the existence of objects inside vessels leads to a more effective coupling between the structure and acoustic enclosure than the existence of no objects in a regular-shaped cavity (i.e. empty vessel).
Simulation research on effect of magnetic nanoparticles on physical process of magneto-acoustic tomography with magnetic induction
Magneto-acoustic tomography with magnetic induction (MAT-MI) is a multiphysics coupled imaging technique that is combined with electrical impedance tomography and ultrasound imaging. In order to study the influence of adding magnetic nanoparticles as a contrast agent for MAT-MI on its physical process, firstly, we analyze and compare the electromagnetic and acoustical properties of MAT-MI theoretically before and after adding magnetic nanoparticles, and then construct a two-dimensional (2D) planar model. Under the guidance of space-time separation theory, we determine the reasonable simulation conditions and solve the electromagnetic field and sound field physical processes in the two modes by using the finite element method. The magnetic flux density, sound pressure distribution, and related one-dimensional (1D), 2D, and three-dimensional(3D) images are obtained. Finally, we make a qualitative and quantitative analysis based on the theoretical and simulation results. The research results show that the peak time of the time item separated from the sound source has a corresponding relationship with the peak time of the sound pressure signal. At this moment, MAMPT-MI produces larger sound pressure signals, and the sound pressure distribution of the MAMPT-MI is more uniform, which facilitates the detection and completion of sound source reconstruction. The research results may lay the foundation for the MAT-MI of magnetically responsive nanoparticle in subsequent experiments and even clinical applications.
Influence of particle packing structure on sound velocity
The anisotropy in the particle systems of different packing structures affects the sound velocity. The acoustic propagation process in four kinds of packing structures (denoted as S45, H60, S90, and D) of two-dimensional granular system is simulated by the discrete element method. The velocity vtof obtained by the time of flight method and the velocity vc obtained from the stiffness tensor of the system are compared. Different sound velocities reflect various packing structures and force distributions within the system. The compression wave velocities of H60 and S90 are nearly the same, and transmit faster than that of D packing structure, while the sound velocity of S45 is the smallest. The shear wave velocities of S45 and H60 are nearly the same, and transmit faster than that of D packing structure. The compression wave velocity is sensitive to the volume fraction of the structure, however, the shear wave velocity is more sensitive to the geometrical structure itself. As the normal stress p is larger than 1 MPa, vtof and vc are almost equal, and the stiffness tensors of various structures explain the difference of sound velocities. When the normal stress is less than 1 MPa, with the coordination number unchanged, the law vtof∝ p1/4 still exists. This demonstrates that apart from different power laws between force and deformation as well as the change of the coordination number under different stresses, there are other complicated causes of vtof∝ p1/4, and an explanation of the deviation from vtof∝ p1/6 is given from the perspective of dissipation.
Coherent structures over riblets in turbulent boundary layer studied by combining time-resolved particle image velocimetry (TRPIV), proper orthogonal decomposition (POD), and finite-time Lyapunov exponent (FTLE)
Time-resolved particle image velocimetry (TRPIV) experiments are performed to investigate the coherent structure's performance of riblets in a turbulent boundary layer (TBL) at a friction Reynolds number of 185. To visualize the energetic large-scale coherent structures (CSs) over a smooth surface and riblets, the proper orthogonal decomposition (POD) and finite-time Lyapunov exponent (FTLE) are used to identify the CSs in the TBL. Spatial-temporal correlation is implemented to obtain the characters and transport properties of typical CSs in the FTLE fields. The results demonstrate that the generic flow structures, such as hairpin-like vortices, are also observed in the boundary layer flow over the riblets, consistent with its smooth counterpart. Low-order POD modes are more sensitive to the riblets in comparison with the high-order ones, and the wall-normal movement of the most energy-containing structures are suppressed over riblets. The spatial correlation analysis of the FTLE fields indicates that the evolution process of the hairpin vortex over riblets are inhibited. An apparent decrease of the convection velocity over riblets is noted, which is believed to reduce the ejection/sweep motions associated with high shear stress from the viscous sublayer. These reductions exhibit inhibition of momentum transfer among the structures near the wall in the TBL flows.
Numerical study of heat-transfer in two-and quasi-two-dimensional Rayleigh-Bénard convection
A detailed comparative numerical study between the two-dimensional (2D) and quasi-two-dimensional (quasi-2D) turbulent Rayleigh-Bénard (RB) convection on flow state, heat transfer, and thermal dissipation rate (TDR) is made. The Rayleigh number (Ra) in our simulations ranges up to 5×1010 and Prandtl number (Pr) is fixed to be 0.7. Our simulations are conducted on the Tianhe-2 supercomputer. We use an in-house code with high parallelization efficiency, based on the extended PDM-DNS scheme. The comparison shows that after a certain Ra, plumes with round shape, which is called the “temperature islands”, develop and gradually dominate the flow field in the 2D case. On the other hand, in quasi-2D cases, plumes remain mushroom-like. This difference in morphology becomes more significant as Ra increases, as with the motion of plumes near the top and bottom plates. The exponents of the power-law relation between the Nusselt number (Nu) and Ra are 0.3 for both two cases, and the fitting pre-factors are 0.099 and 0.133 for 2D and quasi-2D respectively, indicating a clear difference in magnitude of the heat transfer rate between two cases. To understand this difference in the magnitude of Nu, we compare the vertical profile of the horizontally averaged TDR for both two cases. It is found that the profiles of both cases are nearly the same in the bulk, but they vary near boundaries. Comparing the bifurcation height zb with the thermal boundary layer thickness δθ, it shows that zb < δθ(3D) < δθ(2D) and all three heights obey a universal power-law relation z~Ra-0.30. In order to quantify the difference further, we separate the domain by zb, i.e., define the area between two zb (near top and bottom plates respectively) as the “mid region” and the rest as the “side region”, and integrate TDR in corresponding regions. By comparing the integral it is found that most of the difference in TDR between two cases, which is connected to the heat transfer rate, occurs within the thermal boundary layers. We also compare the ratio of contributions to total heat transfer in BL-bulk separation and side-mid separation.
Acoustic characteristics of pulse detonation engine with ellipsoidal reflector
Acoustic characteristics of a pulse detonation engine (PDE) with and without an ellipsoidal reflector are numerically and experimentally investigated. A two-dimensional (2D) non-splitting unstructured triangular mesh Euler solver based on the space-time conservation element and solution element (CE/SE) method is employed to simulate the flow field of a PDE. The numerical results clearly demonstrate the external flow field of the PDE. The effect of an ellipsoidal reflector on the flow field characteristic near the PDE exit is investigated. The formation process of reflected shock wave and reflected jet shock are reported in detail. An acoustic measurement system is established for the PDE acoustic testing. The experimental results show that the ellipsoidal reflector changes the sound waveform and directivity of PDE sound. The reflected shock wave and reflected jet shock result in two more positive pressure peaks in the sound waveform. The ellipsoidal reflector changes the directivity of PDE sound from 20° to 0°. It is found that the peak sound pressure level (PSPL) and overall sound pressure level (OASPL) each obtain an increment when the PDE is installed with a reflector. The maximum relative increase ratio of PSPL and OASPL are obtained at the focus point F2, whose values are 6.1% and 6.84% respectively. The results of the duration of the PDE sound indicate that the reflecting and focusing wave generated by the reflector result in the increment of A duration and B duration before and near focus point F2. Results show that the ellipsoidal reflector has a great influence on the acoustic characteristic of PDE sound. The research is helpful for understanding the influence of an ellipsoidal reflector on the formation and propagation process of PDE sound.
Ion population fraction calculations using improved screened hydrogenic model with l-splitting
Ion population fraction (IPF) calculations are very important to understand the radiative spectrum emitted from the hot dense matter. IPF calculations require detailed knowledge of all the ions and correlation interactions between the electrons of an ion which are present in a plasma environment. The average atom models, e.g., screened hydrogenic model with l-splitting (SHML), now have the capabilities for such calculations and are becoming more popular for in line plasma calculations. In our previous work[Ali A, Shabbir Naz G, Shahzad M S, Kouser R, Rehman A and Nasim M H 2018 High Energy Density Phys. 26 48], we have improved the continuum lowering model and included the exchange and correlation effects in SHML. This study presents the calculation of IPF using classical theory of fluctuation for our improved screened hydrogenic model with l-splitting (I-SHML) under local thermodynamic equilibrium conditions for iron and aluminum plasma over a wide range of densities and temperatures. We have compared our results with other models and have found a very good agreement among them.
Dense pair plasma generation and its modulation dynamics in counter-propagating laser field
With two-dimensional quantum electrodynamics (QED) particle-in-cell simulations, a dense electron-positron (e-e+) pair generation from laser-solid interactions is demonstrated. When the interaction of two linearly polarized laser pulses with a thin target enters into the relativistic transparency regime, a stable standing wave (SW) field can be formed by the overlap of the two counter-propagating laser pulses directly. The present study aims to clarify the effects of the SW field on the dynamics of e-e+ pair plasmas. Our results indicate that under the combined effect of the SW field and radiation reaction (RR) effect, the created e-e+ pairs can be trapped into the electric field nodes when the field strength is strong. The trapping effect contributes to the generation of γAV ≥ 400 and ultra-dense pair plasmas in the two-side irradiation scheme. Despite different laser intensities, these pair plasmas have a Maxwellian spectral distribution with a peak energy of 150 MeV. Besides, the periodical modulation of the average energy, spatial, phase-space, and angular patterns of the e-e+ pair plasmas can be triggered. In the angular patterns, as long as the SW field exists, pair plasmas can be pinched along the laser polarization direction. These results may offer a better understanding of the laser-solid interactions in the experiments when 10-PW laser facilities come into operation in the future.
Tunneling dynamics of bosons in the diamond lattice chain
We analyze the effect of tilting and artificial magnetic flux, on the energy bands structure for the system and the corresponding tunneling dynamics for bosons with various initial configurations in the diamond lattice chain, where intriguing and significant phenomena occur, including Landau-Zener tunneling, Bloch oscillations, and localization phenomenon. Both vertical tilting and artificial magnetic flux may alter the structure of energy levels (dispersion structure or flat band), and enforce the occurrence of Landau-Zener tunneling, which scans the whole of the Bloch bands. We find that, transitions among Landau-Zener tunneling, Bloch oscillations, and localization phenomenon, are not only closely related to the energy bands structure, but also depends on the initial configuration of bosons in the diamond lattice chain. As a consequence, Landau-Zener tunneling, Bloch oscillations, and localization phenonmenon of bosons always counteract and are complementary with each other in the diamond lattice chain.
Small amplitude double layers in an electronegative dusty plasma with q-distributed electrons
The small amplitude dust ion-acoustic double layers in a collisionless four-component unmagnetized dusty plasma system containing nonextensive electrons, inertial negative ions, Maxwellian positive ions, and negatively charged static dust grains are investigated theoretically. Using the pseudo-potential approach and reductive perturbation method, an energy integral equation for the system has been derived and its solution in the form of double layers is obtained. The results appear that the existence regime of the double layer is very sensitive to the plasma parameters, e.g., electron nonextensivity, negative-to-positive ion number density ratio etc. It has been observed that for the selected set of parameters, the system supports rarefactive, (compressive) double layers depending upon the degree of nonextensivity of electrons.
UAV flight test of plasma slats and ailerons with microsecond dielectric barrier discharge
Plasma flow control (PFC) is a promising active flow control method with its unique advantages including the absence of moving components, fast response, easy implementation, and stable operation. The effectiveness of plasma flow control by microsecond dielectric barrier discharge (μs-DBD), and by nanosecond dielectric barrier discharge (NS-DBD) are compared through the wind tunnel tests, showing a similar performance between μs-DBD and NS-DBD. Furthermore, the μs-DBD is implemented on an unmanned aerial vehicle (UAV), which is a scaled model of a newly developed amphibious plane. The wingspan of the model is 2.87m, and the airspeed is no less than 30m/s. The flight data, static pressure data, and Tufts images are recorded and analyzed in detail. Results of the flight test show that the μs-DBD works well on board without affecting the normal operation of the UAV model. When the actuators are turned on, the stall angle and maximum lift coefficient can be improved by 1.3° and 10.4%, and the static pressure at the leading edge of the wing can be reduced effectively in a proper range of angle of attack, which shows the ability of μs-DBD to act as plasma slats. The rolling moment produced by left-side μs-DBD actuation is greater than that produced by the maximum deflection of ailerons, which indicates the potential of μs-DBD to act as plasma ailerons. The results verify the feasibility and efficacy of μs-DBD plasma flow control in a real flight and lay the foundation for the full-sized airplane application.
On the dielectric response function and dispersion relation in strongly coupled magnetized dusty plasmas
Using the generalized viscoelastic fluid model, we derive the dielectric response function in a strongly coupled dusty magnetoplasma which reveals two different dust acoustic (DA) wave modes in the hydrodynamic and kinetic limits. The effects of the strong interaction of dust grains and the external magnetic on these DA modes, as well as on the shear wave are examined. It is found that both the real and imaginary parts of DA waves are significantly modified in strongly coupled dusty magnetoplasmas. The implications of our results to space and laboratory dusty plasmas are briefly discussed.
Nonlinear ion-acoustic solitary waves in an electron-positron-ion plasma with relativistic positron beam
The propagation characteristics of nonlinear ion-acoustic (IA) solitary waves (SWs) are studied in thermal electron-positron-ion plasma considering the effect of relativistic positron beam. Starting from a set of fluid equations and using the reductive perturbation technique, we derive a Korteweg-de Vries (KdV) equation which governs the evolution of weakly nonlinear IA SWs in relativistic beam driven plasmas. The properties of the IA soliton are studied, and it is shown that the presence of relativistic positron beam significantly modifies the characteristics of IA solitons.
Practical 2.45-GHz microwave-driven Cs-free H- ion source developed at Peking University
A practical 2.45-GHz microwave-driven Cs-free H- source was improved based on the experimental H- source at Peking University (PKU). Several structural improvements were implemented to meet the practical requirements of Xi'an Proton Application Facility (XiPaf). Firstly, the plasma chamber size was optimized to enhance the plasma intensity and stability. Secondly, the filter magnetic field and electron deflecting magnetic field were enhanced to reduce co-extracted electrons. Thirdly, a new two-electrode extraction system with farther electrode gap and enhanced water cooling ability to diminish spark and sputter during beam extraction was applied. At last, the direct H- current measuring method was adopted by the arrangement of a new pair of bending magnets before Faraday cup (FC) to remove residual electrons. With these improvements, electron cyclotron resonance (ECR) magnetic field optimization experiments and operation parameter variation experiments were carried out on the H- ion source and a maximum 8.5-mA pure H- beam was extracted at 50 kV with the time structure of 100 Hz/0.3 ms. The root-mean-square (RMS) emittance of the beam is 0.25π·mm·mrad. This improved H- source and extraction system were maintenance-free for more than 200 hours in operation.
Experimental and numerical investigation of a Hall thruster with a chamfered channel wall
A discharge channel with a chamfered wall not only has application in the design of modern Hall thrusters, but also exists where the channel wall is eroded, and so is a common status for these units. In this paper, the laws and mechanisms that govern the effect of the chamfered wall on the performance of a Hall thruster are investigated. By applying both experimental measurement and particle-in-cell simulation, it is determined that there is a moderate chamfer angle that can further improve the optimal performance obtained with a straight channel. This is because the chamfering of the wall near the channel exit can enhance ion acceleration and effectively reduce ion recombination on the wall, which is favorable to the promotion of the thrust and efficiency. However, the chamfer angle should not be too large; otherwise, both the density of the propellant gas and the distribution of the plasma potential in the channel are influenced, which is undesirable for efficient propellant utilization and beam concentration. Therefore, it is suggested that the chamfer shape of the channel wall is an important factor that must be carefully considered in the design of Hall thrusters.
Effects of Al component content on optoelectronic properties of AlxGa1-xN
Using density functional theory, the electronic structures, lattice constants, formation energies, and optical properties of AlxGa1-xN are determined with Al component content x in a range from 0 to 1. As x increases, the lattice constants decrease in e-exponential form, and the band gap increases with a band bending parameter b=0.3954. The N-Al interaction force in the (0001) direction is greater than the N-Ga interaction force, while the N-Al interaction force is less than the N-Ga interaction force in the (1010) direction. The formation energies under different Al component content are negative and increase with Al component content increasing. The static dielectric function decreases, the absorption edge has a blue shift, and the energy loss spectrum moves to high energy with the Al component content increasing.
Unusual softening behavior of yield strength in niobium at high pressures
In situ synchrotron angle-dispersive x-ray diffraction experiments on niobium powders have been conducted at pressures up to 61 GPa and room temperature using the diamond anvil cell technique. From the full width at half maximum of the measured diffraction lines, the yield strength was derived with the line-width analysis theory. The niobium powder sample was found to be compressed more packed firstly and then yielded at ~14 GPa-18 GPa. Following an initial increase in the yield strength with pressure, an obvious decrease was observed occurring at ~42 GPa-47 GPa accompanying with a typical pressure dependence above 47 GPa. The experimentally observed anomalous softening of the yield strength in niobium surprisingly follows the trend of the predicted unusual softening in the shear modulus by the recent theoretical investigations. The possible mechanisms, applicable to interpret the yield strength softening of materials at high pressure, were also discussed in detail.
The affection on the nature of percolation by concentration of pentagon-heptagon defects in graphene lattice
In this paper the percolation behavior with a specific concentration of the defects was discussed on the two-dimensional graphene lattice. The percolation threshold is determined by a numerical method with a high degree of accuracy. This method is also suitable for locating the percolation critical point on other crystalline structures. Through investigating the evolution of the largest cluster size and the cluster sizes distribution, we find that under various lattice sizes and concentrations of pentagon-heptagon defects there is no apparent change for the percolation properties in graphene lattice.
Study on the phase transition of the fractal scale-free networks
Based on the Ising spin, the phase transition on fractal scale-free networks with tree-like skeletons is studied, where the loops are generated by local links. The degree distribution of the tree-like skeleton satisfies the power-law form P(k)~k-δ. It is found that when δ ≥ 3, the renormalized scale-free network will have the same degree distribution as the original network. For a special case of δ=4.5, a ferromagnetic to paramagnetic transition is found and the critical temperature is determined by the box-covering renormalization method. By keeping the structure of the fractal scale-free network constant, the numerical relationship between the critical temperature and the network size is found, which is the form of power law.
Elastocaloric effect and mechanical behavior for NiTi shape memory alloys
The NiTi shape memory alloy exhibits excellent superelastic property and elastocaloric effect. The large temperature change (ΔT) value of 30 K upon loading and -19 K upon unloading are obtained at room temperature, which are higher than those of the other NiTi-based materials and among the highest values reported in the elastocaloric materials. The asymmetry of the measured ΔT values between the loading and unloading process is ascribed to the friction dissipation. The large temperature change originates from the large entropy change during the stress-induced martensite transformation (MT) and the reverse MT. A large coefficient-of-performance of the material is obtained to be 11.7 at ε=1%, which decreases with increasing the applied strain. These results are very attractive in the present solid-state cooling, which potentially could replace the vapor compression refrigeration technologies.
Adsorption and diffusion of F2 molecules on pristine graphene
The adsorption and diffusion of F2 molecules on pristine graphene are studied by using first-principles calculations. For the diffusion of F2 from molecular state in gas phase to the dissociative adsorption state on graphene surface, a kinetic barrier is identified, which explains the inertness of graphene in molecular F2 at room temperature, and its reactivity with F2 at higher temperatures. Study of the diffusion of F2 molecules on graphene surface determines the energy barrier along the optimal diffusion pathway, which conduces to the understanding of the high stability of fluorographene.
Evolutionary algorithm for optimization of multilayer coatings
In this paper, a new evolutionary algorithm, the well-known imperialist competition algorithm, is proposed for optimizing the optical thin-films. In this method, the process is modeled of the competition between countries as imperialists and their colonizing of others as colonies. This algorithm could be an appropriate alternative to some of the more popular algorithms for optimizing the optical thin-films for good performance. The polarizer and edge filter for example are designed by using the imperialist competition algorithm method and the results are compared with those from two optimization high-performance methods:the genetic algorithm and differential evolutionary algorithm. Based on these results, the performance of the imperialist competition algorithm method shows that this algorithm is not sensitive to the change of its parameters and it can be an important advantage for quickly achieving a global optimal point. On the other hand the results show a better ratio of P-polarization transmittance to S-polarization transmittance in the design of a 1540-nm polarizer, which is more appropriate than the results from the other two methods. In the second design, an edge filter with a lower number of layers and more uniform bandpass spectrum than the counterparts of those methods is obtained. These results indicate that the imperialist competition algorithm is a robust method for optical thin-film designs.
Effect of Mn doping on mechanical properties and electronic structure of WCoB ternary boride by first-principles calculations
The first-principles calculations are performed to investigate the structural, mechanical property, hardness, and electronic structure of WCoB with 0, 8.33, 16.67, 25, and 33.33 at.% Mn doping content and W2CoB2 with 0, 10, and 20 at.% Mn doping content. The cohesive energy and formation energy indicate that all the structures can retain good structural stability. According to the calculated elastic constants, Mn is responsible for the increase of ductility and Poisson's ratio and the decrease of Young's modulus, shear modulus, and bulk modulus. By using the population analysis and mechanical properties, the hardness is characterized through using the five hardness models and is found to decrease with the Mn doping content increasing. The calculated electronic structure indicates that the formation of a B-Mn covalent bond and a W-Mn metallic bond contribute to the decreasing of the mechanical properties.
First principles study of stability, mechanical, and electronic properties of chromium silicides
Through the first principles calculations, the chemical stability, mechanical, and electronic properties of chromium silicides are predicted. Estimating enthalpies and binding energies, density state density and electron density distribution are combined to analyse the thermodynamic stability and physical properties of chrome-silicon binary compounds. The chromium silicide includes Cr3Si, Cr5Si3, CrSi, and CrSi2. The chemical stability and the information about electronic structure, mechanical properties, Debye temperature, and anisotropy properties are obtained by density functional theory and Debye quasi-harmonic approximation. Meanwhile, the calculation of elastic modulus shows that Cr3Si has the highest body modulus value (251 GPa) and CrSi2 possesses the highest shear modulus (169.5 GPa) and Young's modulus (394.9 GPa). In addition, the Debye temperature and the speed of sound of these Cr-Si compounds are also calculated. Since the calculated bulk modulus is different from Young's modulus anisotropy index, and also different from Young's modulus of a three-dimensional surface shape, the different mechanical anisotropies of all the compounds are obtained.
Structural, Mössbauer spectroscopy, magnetic properties, and thermal measurements of Y3-xDyxFe5O12
Yttrium iron garnet powder samples (Y3-xDyxFe5O12), where part of yttrium ions are substituted by dysprosium ions with different concentrations are prepared by the solid state reaction method. The properties of the prepared samples are examined by different methods such as x-ray diffraction (XRD), Mössbauer spectroscopy, macroscopic magnetization measurements, and thermal measurements. The XRD measurements show that all the samples reveal the presence of a single garnet phase with a BCC structure. Room temperature Mössbauer spectra indicate that iron ions occupy three magnetic sites, i.e., two octahedral sites and one tetrahedral site. The saturation magnetization and the initial magnetic susceptibility decrease with the increase of Dy3+ substitution. The Curie temperature obtained from the thermal measurements seems to be independent of Dy3+ substitution.
Ferromagnetism and magnetostructural coupling in V-doped MnNiGe alloys
The magnetostructural coupling between magnetic and structure transitions plays an important role in the multifunctional applications of magentocaloric materials. In this work, ferromagnetism and magnetostructural transformation are achieved in nonmagnetic V-doped MnNiGe alloys. With simultaneously reducing the transformation temperature and converting antiferromagnetic martensite to ferromagnetic state, the magnetostructural transformation between ferromagnetic orthorhombic phase and paramagnetic hexagonal phase is established in a temperature region as large as 130 K. The magnetic-field-induced magnetostructural transformation is accompanied by considerable magnetocaloric effect.
Electro-optical properties and (E, T) phase diagram of fluorinated chiral smectic liquid crystals
Fluorinated smectic liquid crystals each with a biphenyl benzoate rigid core are investigated. Molecular structures of the studied compounds have difference only in fluorine position and the length of the carbon chain. Dielectric relaxation study and electro-optical measurements are carried out with the classical SSFLC geometry. The field-induced phase transitions are studied and the (E,T) phase diagram is established.
Low-temperature green synthesis of boron carbide using aloe vera
The unique structural and physical properties of boron carbide, which make it suitable for a wide range of applications, demands the development of low-cost and green synthesis method. In the present work, the commonly available leaves of aloe vera are hydrothermally treated to form the carbon precursor for the synthesis of boron carbide. The morphological characterization reveals the porous nature of the precursor turning into a tubular structure upon boron carbide formation. The structural characterization by x-ray diffraction and other spectroscopic techniques such as Fourier transform infrared, Raman, photoluminescence and uv-visible near-infrared spectroscopy confirm the formation of boron carbide. The thermogravimetric analysis of the sample is found to exhibit good thermal stability above 500℃. When the sample is annealed to 600℃, boron carbide with phase purity is obtained, which is confirmed through XRD and FTIR analyses. The optical emission properties of the sample are studied through CIE plot and power spectrum. Compared with other natural precursors for boron carbide, the aloe vera is found to give a good yield above 50%.
Improvement of electro-optic performances in white organic light emitting diodes with color stability by buffer layer and multiple dopants structure
A series of white phosphorescent OLED devices with buffer layer and multiple dopant structure is investigated in order to obtain better electro-optic performances and color stability. The color coordinate and color stability are related to the location of multiple dopants layer, and the optimized location can compensate for the change of the blue emission intensity under a high voltage and stabilize the spectrum. The electro-optic performances and color stability can be further improved by changing the composition and thickness of the buffer layer between the emitting layer and the electron transport layer. In device B2, the distance from multiple dopant layer to buffer layer is 2 nm and the thickness of buffer layer is 5 nm, the maximum luminance, current density, and power efficiency can reach 9091 cd/m2, 364.5 mA/cm2, and 26.74 lm/W, respectively. The variation of international commission on the illumination (CIE) coordinate of device B2 with voltage increasing from 4 V to 7 V is only (0.006, 0.004).
Charge compensation and capacity fading in LiCoO2 at high voltage investigated by soft x-ray absorption spectroscopy Hot!
In order to obtain an in-depth insight into the mechanism of charge compensation and capacity fading in LiCoO2, the evolution of electronic structure of LiCoO2 at different cutoff voltages and after different cycles are studied by soft x-ray absorption spectroscopy in total electron (TEY) and fluorescence (TFY) detection modes, which provide surface and bulk information, respectively. The spectra of Co L2,3-edge indicate that Co contributes to charge compensation below 4.4 V. Combining with the spectra of O K-edge, it manifests that only O contributes to electron compensation above 4.4 V with the formation of local O 2p holes both on the surface and in the bulk, where the surficial O evolves more remarkably. The evolution of the O 2p holes gives an explanation to the origin of O2- or even O2. A comparison between the TEY and TFY of O K-edge spectra of LiCoO2 cycled in a range from 3 V to 4.6 V indicates both the structural change in the bulk and aggregation of lithium salts on the electrode surface are responsible for the capacity fading. However, the latter is found to play a more important role after many cycles.
Efficient thermal analysis method for large scale compound semiconductor integrated circuits based on heterojunction bipolar transistor
In this paper, an efficient thermal analysis method is presented for large scale compound semiconductor integrated circuits based on a heterojunction bipolar transistor with considering the change of thermal conductivity with temperature. The influence caused by the thermal conductivity can be equivalent to the increment of the local temperature surrounding the individual device. The junction temperature for each device can be efficiently calculated by the combination of the semi-analytic temperature distribution function and the iteration of local temperature with high accuracy, providing a temperature distribution for a full chip. Applying this method to the InP frequency divider chip and the GaAs analog to digital converter chip, the computational results well agree with the results from the simulator COMSOL and the infrared thermal imager respectively. The proposed method can also be applied to thermal analysis in various kinds of semiconductor integrated circuits.
Performance of n-type silicon/silver composite anode material in lithium ion batteries: A study on effect of work function matching degree
In this paper, two types of silicon (Si) particles ball-milled from n-type Si wafers, respectively, with resistivity values of 1 Ω·cm and 0.001 Ω·cm are deposited with silver (Ag). The Ag-deposited n-type 1-Ω·cm Si particles (n1-Ag) and Ag-deposited n-type 0.001-Ω·cm Si particles (n0.001-Ag) are separately used as an anode material to assemble coin cells, of which the electrochemical performances are investigated. For the matching of work function between n-type 1-Ω·cm Si (n1) and Ag, n1-Ag shows discharge specific capacity of up to 683 mAh·g-1 at a current density of 8.4 A·g-1, which is 40% higher than that of n0.001-Ag. Furthermore, the resistivity of n1-Ag is lower than half that of n0.001-Ag. Due to the mismatch of work function between n-type 0.001-Ω·cm Si (n0.001) and Ag, the discharge specific capacity of n0.001-Ag is 250.2 mAh·g-1 lower than that of n1-Ag after 100 cycles.
Synergistic effect of total ionizing dose on single event effect induced by pulsed laser microbeam on SiGe heterojunction bipolar transistor
The synergistic effect of total ionizing dose (TID) on single event effect (SEE) in SiGe heterojunction bipolar transistor (HBT) is investigated in a series of experiments. The SiGe HBTs after being exposed to 60Co γ irradiation are struck by pulsed laser to simulate SEE. The SEE transient currents and collected charges of the un-irradiated device are compared with those of the devices which are irradiated at high and low dose rate with various biases. The results show that the SEE damage to un-irradiated device is more serious than that to irradiated SiGe HBT at a low applied voltage of laser test. In addition, the γ irradiations at forward and all-grounded bias have an obvious influence on SEE in the SiGe HBT, but the synergistic effect after cutting off the γ irradiation is not significant. The influence of positive oxide-trap charges induced by TID on the distortion of electric field in SEE is the major factor of the synergistic effect. Moreover, the recombination of interface traps also plays a role in charge collection.
Shortening turn-on delay of SiC light triggered thyristor by 7-shaped thin n-base doping profile
A new 4H-SiC light triggered thyristor (LTT) with 7-shaped thin n-base doping profile is proposed and simulated using a two-dimensional numerical method. In this new structure, the bottom region of the thin n-base has a graded doping profile to induce an accelerating electric field and compensate for the shortcoming of the double-layer thin n-base structure in transmitting injected holes. In addition, the accelerating electric field can also speed up the transmission of photon-generated carriers during light triggering. As a result, the current gain of the top pnp transistor of the SiC LTT is further increased. According to the TCAD simulations, the turn-on delay time of the SiC LTT decreases by about 91.5% compared with that of previous double-layer thin n-base SiC LTT. The minimum turn-on delay time of the SiC LTT is only 828 ns, when triggered by 100 mW/cm2 ultraviolet light. Meanwhile, there is only a slight degradation in the forward blocking characteristic.
Phase transition and charge transport through a triple dot device beyond the Kondo regime
Semiconductor quantum dot structure provides a promising basis for quantum information processing, within which to reveal the quantum phase and charge transport is one of the most important issues. In this paper, by means of the numerical renormalization group technique, we study the quantum phase transition and the charge transport for a parallel triple dot device in the strongly correlated limit, focusing on the effect of inter-dot hopping t beyond the Kondo regime. We find the quantum behaviors depend closely on the initial electron number on the dots, and the present model may map to single, double, and side-coupled impurity models in different parameter spaces. An orbital spin-1/2 Kondo effect between the conduction leads and the bonding orbital, and several magnetic-frustration phases are demonstrated when t is adjusted to different regimes. To understand these phenomena, a canonical transformation of the energy levels is given, and important physical quantities with respect to increasing t and necessary theoretical discussions are shown.
Effects of edge hydrogenation and Si doping on spin-dependent electronic transport properties of armchair boron-phosphorous nanoribbons
In this article, the spin-dependent electronic and transport properties of the armchair boron-phosphorous nanoribbons (ABPNRs) are mainly studied by using the non-equilibrium Green function method combined with the spin-polarized density function theory. Our calculated electronic structures indicate that the edge hydrogenated ABPNRs exhibit a ferromagnetic bipolar magnetic semiconductor property, and that the Si atom doping can make ABPNRs convert into up-spin dominated half metal. The spin-resolved transport property results show that the doped devices can realize 100% spin-filtering function, and that the interesting negative differential resistance phenomenon can be observed. Our calculations suggest that the ABPNRs can be constructed as a spin heterojunction by introducing Si doping partially, and it would be used as a spin-diode for nano-spintronics in future.
Controllability of heterogeneous interdependent group systems under undirected and directed topology
The controllability problem of heterogeneous interdependent group systems with undirected and directed topology is investigated in this paper. First, the interdependent model of the heterogeneous system is set up according to the difference of individual characteristics. An extended distributed protocol with the external sliding-mode control is designed, under which it is shown that a heterogeneous interdependent group system is controllable when the corresponding communication topology is controllable. Then, using the network eigenvalue method, the driving individuals are determined for a heterogeneous system with undirected topology. Under directed topology, the maximum match method is utilized to confirm the driving individuals. Some sufficient and necessary conditions are presented to assure that the heterogeneous interdependent group system is structurally controllable. Via theoretical analysis, the controllability of heterogeneous interdependent systems is related to the interdependent manner and the structure of the heterogeneous system. Numerical simulations are provided to demonstrate the effectiveness of the theoretical results.
Synchronization performance in time-delayed random networks induced by diversity in system parameter
Synchronization rhythm and oscillating in biological systems can give clues to understanding the cooperation and competition between cells under appropriate biological and physical conditions. As a result, the network setting is appreciated to detect the stability and transition of collective behaviors in a network with different connection types. In this paper, the synchronization performance in time-delayed excitable homogeneous random networks (EHRNs) induced by diversity in system parameters is investigated by calculating the synchronization parameter and plotting the spatiotemporal evolution pattern, and distinct impacts induced by parameter-diversity are detected by setting different time delays. It is found that diversity has no distinct effect on the synchronization performance in EHRNs with small time delay being considered. When time delay is increased greatly, the synchronization performance of EHRN degenerates remarkably as diversity is increased. Surprisingly, by setting a moderate time delay, appropriate parameter-diversity can promote the synchronization performance in EHRNs, and can induce the synchronization transition from the asynchronous state to the weak synchronization. Moreover, the bistability phenomenon, which contains the states of asynchronous state and weak synchronization, is observed. Particularly, it is confirmed that the parameter-diversity promoted synchronization performance in time-delayed EHRN is manifested in the enhancement of the synchronization performance of individual oscillation and the increase of the number of synchronization transitions from the asynchronous state to the weak synchronization. Finally, we have revealed that this kind of parameter-diversity promoted synchronization performance is a robust phenomenon.