Vol. 63, No. 5 (2014)
2014, 63 (5): 050201. doi: 10.7498/aps.63.050201
In this paper, the conformal invariance and Mei symmetry for a generalized Hamilton system under infinitesimal transformations are discussed in details. A necessary and sufficient condition for conformal invariance of systems to be Mei symmetry is given. We get the Mei conserved quantities of the conformal invariance. Finally, an example is given to illustrate the application of the result.
Particles discrete element method based on manifold cover for macro-mesoscopic fracture of rock mass
2014, 63 (5): 050202. doi: 10.7498/aps.63.050202
The rock mass can be assumed to be homogeneous material from the macroscopic view, but it is a heterogeneous material at a mesoscopic scale and its physico-mechanical properties are discontinuous in space. Therefore, it is necessary to research the generation and development of pre-peak micro cracks in rocks and frictional characteristics between post-peak mineral particles from macro-meso and multi-scale perspectives to know the substantive characteristics of rock failure and instability. This paper, based on the manifold cover concept, proposes a new discrete element numerical method (i.e. Manifold Particle Discrete) combining with the particle contact model, so as to introduce the concept of stress boundary. This method can be applied to the entire process of analyzing rock failure. By analyzing the manifold cover and ball particle model, this paper constitutes the ball unit cover function of three-dimensional manifold cover, establishes tetrahedron units, obtains the equilibrium equation and compatible equation of the MPD model. It also verifies the accuracy of the proposed numerical method and the feasibility of rock failure analysis through numerical examples.
2014, 63 (5): 050203. doi: 10.7498/aps.63.050203
Gyrotrons are the most promising microwave source devices that can be used in the International Thermonuclear Experimental Reactor, but there are many difficulties to be solved in study and design of gyrotrons to meet the requirements. In this paper, the beam-wave interactions of a 170 GHz megawatt-level smooth-wall coaxial gyrotron are studied numerically. In order to attain high efficiency and stable radiation, TE31,12 mode that lies in a relative sparse spectrum is selected as the operating mode, and the beam-wave coupling coefficient and start oscillation current are calculated by a set of source codes developed by Matlab. Taking into account the electronic velocity spread and cavity wall resistivity, and based on a single-mode approximation, the optimization design and simulation of beam-wave interaction of a 170 GHz megawatt smooth-wall coaxial gyrotron have been fulfilled. The relationships between efficiency and magnetic field, and the voltage, current, taper angle of insert, and other parameters are presented. Results show that the voltage and magnetic field have great influence on efficiency; however, the current and velocity spread do change slightly, thus reduce the requirements of electron gun design. In addition, the optimized taper angle of insert and coaxial cavity geometry parameters can improve the efficiency, reduce the impact of velocity spread on efficiency, and can achieve an electronic efficiency around 50% and an output power 1.7 MW.
2014, 63 (5): 050501. doi: 10.7498/aps.63.050501
Most dynamical models with continuous opinion lack the considerations of trust between individuals and different weights of opinions. This article extends the Hegselmann-Krause model by introducing the trust between individuals and the similarity between opinions, and extending the hypothesis of bounded confidence to bounded influence. Finally, the model of opinion updating with weights is proposed. We have studied the formation, evolution of opinions in a social group and consensus-building process under the influence of a few of narrow-minded and authoritative individuals. Simulation results show that decreases of the difference between the initial opinions of two types of individuals and the midpoint of distribution range (0.5), or increases of the influence thresholds of narrow-minded individuals and the trust degree of authoritative individuals would form larger and fewer opinion clusters. As the influence thresholds of narrow-minded individuals increases, the opinion of maximum cluster concentrates at near 0.5, but the trust degree of authoritative individuals will increases to make the opinion of maximum cluster constantly approach the final opinion of authoritative individuals. These results can reflect and explain the realistic social phenomenon with narrow-minded individuals and authoritative individuals to a certain extent.
2014, 63 (5): 050502. doi: 10.7498/aps.63.050502
Chaotic signal is essentially a nonlinear and non-Gaussian signal, which involves signal quantization when used in wireless sensor networks (WSNs). It makes the blind source separation of chaotic signal in WSNs more difficult to address. To solve the problem, we propose a new source separation algorithm based on cubature Kalman particle filter (CPF) in this paper. First the probability density function of the observed signal is derived and the optimal quantization is used; this can achieve the optimal quantization of signal under the limited budget of quantization bits. After that, the algorithm uses cubature Kalman filter (CKF) to generate the important proposal distribution of the particle filter (PF), integrating the latest observation and improving the approximation to the system posterior distribution, which will improve the performance of the signal separation. Simulation results show that the algorithm can separate mixed chaotic signal effectively, it is superior over the unscented Kalman particle filter (UPF) counterpart in accuracy and computation overhead. The running time is 88.77% compared to the UPF counterpart.
Effect of coupling modes and initial structures on the synchronization of a ring network with fractional order bistable oscillators
2014, 63 (5): 050503. doi: 10.7498/aps.63.050503
A ring network with fractional-order bistable oscillators is proposed, and the relationship between synchronization and parameters, such as coupling modes and the initial structural conditions, etc., is investigated. Based on the bistable characteristics of P-R oscillator, the effects of the coupling strength and the structures in initial conditions on the dynamic behaviors of the ring network are investigated by analyzing the largest conditional Lyapunov exponents, the largest Lyapunov exponents and the bifurcation diagrams, etc. Further investigation reveals that the ring network can be controlled to form chaotic synchronization, chaotic non-synchronization, synchronous amplitude death, synchronous non-amplitude death, etc. by changing the initial conditions and the coupling strength. Furthermore, the contours of the largest conditional Lyapunov exponents and the largest Lyapunov exponents also show how the dynamic behaviors of the network are influenced by the competition between couplings along directions of y and z, strongly relies on the initial structural conditions of network.
2014, 63 (5): 050504. doi: 10.7498/aps.63.050504
Chaotic oscillation in a power system is very harmful for the large-scale interconnected power grid. Here, the basic dynamic properties of the interconnected power system are investigated under the disturbance of electromagnetic power through the Lyapunov exponent spectrum, bifurcation diagram, phase plane, and power spectrum. The sensitivity to the disturbance of electromagnetic power is also studied. Meanwhile, from the sliding mode variable structure control and relay characteristic function, a new method to control the chaotic oscillation is presented, which can quickly and smoothly make the system reach the expected target. Simulation results show that the method can not only shorten the control time, but also reduce the possibility of the impulse response from the system, due to the large parameters. It can inhibit the high-frequency buffeting effectively and has strong robustness.
A least square support vector machine prediction algorithm for chaotic time series based on the iterative error correction
2014, 63 (5): 050505. doi: 10.7498/aps.63.050505
This paper analyzes the error characteristic of traditional support vector machine prediction algorithm, where the error series are smooth and regular. This is because a single prediction model is incapable of fitting chaotic system mapping function and omitting some deterministic component. On this basis, a prediction algorithm that consists of an iterative error correction and a least square support vector machine (LSSVM) is proposed. The algorithm creats multiple predictive models via the method of iterative error correction to approximate the chaotic system mapping function and obtain significant improvements of predictive performance. In addition, the optimal parameters of the prediction model are automatically obtained from the pattern search algorithm which is simple and effective. Experiment conducted on Lorenz time series and MackeyGlass time series indicates that the proposed algorithm has a much better performance than that recorded in the literature.
2014, 63 (5): 050506. doi: 10.7498/aps.63.050506
Automatic detection and classification of epileptic EEG signals have been a significance method for the clinical diagnosis and treatment of epilepsy. The recurrence quantification analysis (RQA) based on the recurrence plot could visualize the recurrence behaviors of dynamical systems from the nonlinear time series and analysis of the recurrence properties. This paper presents a new feature extraction method for epileptic EEG signals based on the recurrence quantification analysis. Experimental results show that the seizure detection directly based on recurrence quantification analysis features has a higher detection performance; especially the classification accuracy based on the deterministic feature can be up to 90.25%. This paper also combines the RQA features with the variation coefficient and fluctuation index, and then puts the feature vectors into a support vector machine (SVM) to automatically detect the epileptic EEG from EEG recordings. Experimental results shows that the proposed methods could achieve a great classification accuracy of 99%.
2014, 63 (5): 050507. doi: 10.7498/aps.63.050507
Recently, many chaotic maps-based authenticated key agreement protocols using smart cards have been proposed. Unfortunately, tamper-resistant card readers make these protocols costly and unpractical. In addition, the digital signature scheme based on chaotic maps requires high computational resources. There exists security problem in publishing public keys which depend on signature schemes. In this paper, we will present a novel authenticated key protocol without smart cards while using extended Chebyshev maps. The proposed protocol eliminates the process of publishing the public key. Security and performance analysis show that our protocol can resist various attacks and yet is reasonably efficient. Therefore, our protocol is more suitable for practical applications.
ATOMIC AND MOLECULAR PHYSICS
2014, 63 (5): 053101. doi: 10.7498/aps.63.053101
The programming speed of charge trapping memories (CTM) with different defects were studied based on the first principle and VASP package. The defects include threefold oxygen vacancy (VO3), fourfold oxygen vacancy (VO4), hafnium vacancy (VHf), and interstitial oxygen (IO). Trapping energy, energy band offset, and the trapping density were calculated and compared. Results show that VO3, VO4 only trap holes, VHf only trap electrons, and IO trap electrons and holes; the most important is the trapping energy which is greater in VHf. It is the best for trapping charges; because the charge tunneling into trapping layer is easy in VHf. It can also reduce the tunneling time. Finally, the trapping densities were compared with each other: VHf's trapping density is greater than other defects, i.e. charges can be trapped easier than by other defects. All of these show that VHf is the best one for reducing programming time. This paper will provide a theoretical guidance for increasing the programming speed ofCTM.
2014, 63 (5): 053102. doi: 10.7498/aps.63.053102
We have performed geometry optimizations of crystal structure and contrasted the calculated results of band structure, density of states, and permittivity of 3C-SiC for four kinds of doped supercell models: undoped, Al-doped, N-doped and Al-N codoped ones, by using the first principles plane wave ultrasoft pseudopotential method based on the density functional theory. Results show that Al doping increases the lattice constant of 3C-SiC, while N doping has little effect on the SiC lattice. The Fermi energy level introduced into valence band and the band gap is slightly widened through Al doping for 3C-SiC, and the SiC becomes a p-type semiconductor. Both the conduction band and the valence band of N-doped SiC move toward low energy side, and its band gap is slightly reduced. Intrinsic 3C-SiC has shown poor dielectric loss properties in the microwave range, but the dielectric property can be improved significantly through the Al doping or N doping, especially the former. The microwave dielectric loss performance of 3C-SiC doped with Al and N in the range of 8.2–12.4 GHz declined sharply, which validates the results of experiments. We finally analyzed and discussed the reason for the decrease of permittivity.
2014, 63 (5): 053103. doi: 10.7498/aps.63.053103
The influences of doping whit congeners on the band topology in half Heusler-type of NaAlGe alloys are investigated using the first-principles calculations. It is found that the Na1-xCsxAlGe alloys with a normal band order are converted into topological nontrivial phases when x is up to 0.125. We argue that it is the degree of hybridization between Al and Ge determine the band order at the Fermi level. The Na or Cs only plays a role of the valence electron contributor and influences the lattice parameter.
2014, 63 (5): 053104. doi: 10.7498/aps.63.053104
For the computation of excited states, the traditional solutions of the Schredinger equation, using higher roots of a secular equation in a finite N-dimensional function space, by the Hylleraas-Undheim and MacDonald (HUM) theorem, we found that it has several restrictions which render it of lower quality, relative to the lowest root if the latter is good enough. In order to avoid the variational restrictions, based on HUM, we propose a new variational function and prove that the trial wave function has a local minimum in the eigenstates, which allows to approach eigenstates unlimitedly by variation. In this paper, under the configuration interaction (CI), we write a set of calculation programs by using generalized laguerre type orbitals (GLTO) to get the approximate wave function of different states, which is base on the HUM or the new variational function. By using the above program we get the approximate wave function for 1S (e), 1P (o) state of helium atoms (He) through the different theorems, the energy value and radial expectation value of related states. By comparing with the best results in the literature, the theoretical calculations show the HUM's defects and the new variational function's superiority, and we further give the direction of improving the accuracy of excited states.
We have investigated surface morphology and visible light emission from slow ions Kr15+, 17+ colliding with GaAs (100). The surface disorder of GaAs films mainly depends on the charge state of incident ions. The two spectral lines of target atom Ga belong to transitions of GaⅠ 4p 2P1/2o5s 2S1/2 at 403.2 nm and 4p 2P3/2o5s 2S1/2 at 417.0 nm. Light emissions of target species depend on the energy of the incident ions deposited on the target surface atoms. During the neutralization process, the four spectral lines of Kr+ respectively can be attributed to the transitions of Kr Ⅱ 4d 4F7/25p 2D5/2o at 410.0 nm, 5s 2P3/25p 4S3/2o at 430.4 nm, 5p 4D3/2o4d 2D3/2 at 434.0 nm and Kr Ⅱ 4d 4D1/25p 2S1/2o at 486.0 nm. They are induced by cascade de-excitation after many electrons of the conductions band of the solid surface captured in highly excited states of the incident ion. Intensities of these six spectral lines from incident ions Kr17+ are obviously larger than Kr15+'s.
Optimization of the light-induced-fluorescence signals of single atoms and efficient loading of single atoms into a magneto-optical trap
2014, 63 (5): 053202. doi: 10.7498/aps.63.053202
In our experiment, firstly, we carry out the loading of single atoms in the magneto-optical trap (MOT) by increasing the quadrupole magnetic field gradient, improving the background vacuum, and reducing the diameters of the cooling and trapping laser beams. Secondly, we get the single atomic fluorescence signal of a high signal-to-background ratio in the MOT by means of reducing the detuning of cooling light and increasing its intensity appropriately, and using the polarization spectroscopy locking technique to suppress the fluctuations of cooling laser. In addition, with the real-time feedback on quadrupole magnetic field gradient, we demonstrate a probability of loading single atoms in the MOT as high as 98%. We also measure the statistical properties of the single atomic fluorescence which is excited by continuous light in the MOT; the measured second-order correlation is g(2)( = 0) = 0.09.
ELECTROMAGENTISM, OPTICS, ACOUSTICS, HEAT TRANSFER, CLASSICAL MECHANICS, AND FLUID DYNAMICS
Study on wide-band scattering from rectangular cross-section above rough land surface with exponential type distribution using FDTD
2014, 63 (5): 054101. doi: 10.7498/aps.63.054101
The dielectric property of the actual land surface is expressed using the four-component model of dielectric constant of soil, the actual rough land surface is simulated with model of exponential type distribution rough surface and Monte Carlo method, the wide-band electromagnetic scattering from column with rectangular cross-section above rough land surface under Gaussian differential pulse irradiation is studied using FDTD, the curves of frequency distribution of back composite scattering coefficient is obtained, the back composite scattering coefficient varying with the root-mean-square of height fluctuation, the correlation length of rough land surface, the moisture capacity of soil, the geometric parameter of the target, such as height, size, tilt and dielectric parameter of the target is calculated in details, the characteristics of the wide-band composite scattering from object above rough land surface with exponential type distribution are obtained.
2014, 63 (5): 054201. doi: 10.7498/aps.63.054201
We proposed a facile approach for rapid and maskless production of zinc sulfide microstructure used as antireflective and hydrophobic surface by direct laser interference ablation. This surface could improve the infrared transmittance from 75% to 92%; and the problem of bad film adhesion, corrosion resistance, and the thermal expansion mismatch in the antireflective coating membrane were solved. By this method a quick and simple fabrication, and a large-area production as well as the hydrophobic properties with a contact angle of 145° can be realized. Hence this approach can have future broad applications in areas of optoelectronics, solar energy, aerospace, and infrared guidance etc.
2014, 63 (5): 054202. doi: 10.7498/aps.63.054202
Pulse compression is an important property of stimulated Brillouin scattering (SBS), and the SBS pulse duration becomes smaller with the increase of pump energy. An unusual pulse compression was investigated of the stimulated Brillouin scattering in water, it was found that the SBS pulse duration becomes larger as the pump energy increases. The pulse duration of SBS alters differently with the change of pump energy in strong focusing and weak focusing. Numerical simulation of pump light transmission in water cell has been made to explain the unusual pulse compression phenomena. Different real gain lengths in strong and weak focusing make different SBS pulse compression.
2014, 63 (5): 054203. doi: 10.7498/aps.63.054203
Self-diffraction effect has very important applications in the field of femtosecond laser, such as to improve the temporal contrast of femtosecond pulses, to be used as the reference pulse of self-reference spectral interferometry, and as the signal of self-diffraction frequency-resolved optical gating, etc. However, in the self-diffraction signal of femtosecond pulses with a broadband spectrum angular dispersion exists obviously, which will limit the application of self-diffraction effect. In this paper, the self-diffraction signal angular dispersion of femtosecond pulses is obviously reduced, when a right-angle prism is located in the light path of an incident beam with an incident angle of 23° to the prism. The experimental result provides a useful reference for the application of the self-diffraction effect to the research of femtosecond laser pulse in the future.
2014, 63 (5): 054204. doi: 10.7498/aps.63.054204
The 266 nm continuous-wave coherent radiation is generated by the second harmonic generation of a β-BBO crystal placed inside an external enhancement cavity. Its fundamental beam is a 532 nm laser from commercial production of Coherent Company. Impact of focusing shape of the crystal on the conversion efficiency is simulated detailedly. Theoretical and experimental results show that the elliptical focusing can decrease the walk-off effect and improve conversion efficiency of the second harmonic generation (SHG). We obtain experimentally the 266 nm radiation output of 180 mW with elliptical focusing in a BBO crystal using bowtie cavity when the fundamental wave input 1 W . The SHG conversion efficiency reaches 18%.
High power red to mid-infrared laser source from intracavity sum frequency optical parametric oscillator pumped by femtosecond fiber laser
2014, 63 (5): 054205. doi: 10.7498/aps.63.054205
A femtosecond laser source tuned from red to mid-infrared is demonstrated. It is based on intracavity sum frequency generation of a MgO-doped periodically poled LiNbO3 optical parametric oscillator synchronously pumped by mode-locked Yb large-mode-area photonic crystal fiber, which has high average power and high repetition rate. The optical parametric oscillator has a wide spectral tuning range from 1450–2200 nm (for the signal) and 2250–4000 nm (for the idler) while the wavelength of the pump is 1040 nm. In the experiment, the output power of 374 mW at 1502 nm is achieved when the pump power is 2 W and the slope efficiency is 18.7%. In addition, 166 mW idler at 3.4 μm are achieved. By using a β-BaB2O4 for intracavity sum frequency generation, the femtosecond pulse over 610–668 nm is obtained. A 694 mW average output power of sum frequency generation is achieved for 4.1 W pump, representing 16.9% conversion efficiency at 615 nm.
Introduction of two-dimensional defects in inverse opal films by means of planar lithography and sol-gel co-assembly methods
2014, 63 (5): 054206. doi: 10.7498/aps.63.054206
By applying planar lithography and sol-gel co-assembly methods, the designed two-dimensional defects were introduced in inverse opal films. Composite colloidal crystal films which can turn into inverse opal films after calcination were fabricated by sol-gel co-assembly method. Photoresist patterns produced by planer lithograph either on silicon or composite colloidal film/silicon substrate were used as the sacrificial structure to form the designed defects. To form the embedded defects in inverse opal films, a layer of composite colloidal crystal film was assembled on the photoresist patterns. After calcination, both PS spheres and photoresist were removed, and the designed defects were introduced in the inverse opal films. Influence of the photoresist patterns on self-assembled PS particles was also characterized.
Phase singularities of an ultrasonic speckle field back-scattered from an underwater Gaussian interface
2014, 63 (5): 054301. doi: 10.7498/aps.63.054301
In order to describe the characteristics of the ultrasonic speckle field accurately, a computation-simulated generation of ultrasonic speckle field is proposed. Using the Gaussian correlation random surfaces that are generated by the simulation, the simulation of the speckle field produced by this kind of surfaces in the far field, the sound intensity distribution, and phase distribution can also be obtained. To compare the difference between speckle field from experiment and that by the simulation, the ultrasonic speckle field receiving experimental system is established, taking the same parameters to calculate and simulate, the experimental speckle field is obtained. By contrast, phase singularity exists in the simulated speckle field and the phase distribution is similar to the swirl around singular points, The intensity distribution of the simulated speckle field and that obtained experimentally are similar, but the former has a bigger strength, more highlight speckles carrying useful information, dark speckle being less, so that it is more conducive to the research and analysis.
Research on prediction and methods of evaluating sound exposure from a mixture of multiple single sources
2014, 63 (5): 054302. doi: 10.7498/aps.63.054302
In this paper, the prediction and the way for calculating the total exposure level (to be denoted by LTotal) from a mixture of multiple single sources are proposed, on the premise that each single exposure level from every component single source (to be denoted by Li, where i denotes the number of single sources and i=1, 2, , K) is known. Firstly, a novel method for sound exposure level evaluation, based on a short-term exposure level in the duration of the sound event, is proposed. Using this method, each single exposure level obtained from all the single sound samples and the total exposure level obtained from every artificially combined sound samples are evaluated. Then, we lay special stress on analyzing the quantitative relationships between LTotal and Li (i=1, 2, , K) measured in three types of sound exposure indicators. All analytical results indicate that our anticipative gain of the total exposure level LTotal from a mixture of multiple single sources can be predicted on the premise that each single exposure level from every componential single source is known. To modify the predicted results, we just need to know the number of specific independent components K and the range of the single exposure levels .
Design of multistatic sonar space-time code detection signal and time reversal copy-correlation detection technology
2014, 63 (5): 054303. doi: 10.7498/aps.63.054303
A space-time code (STC) detection signal is designed for multistatic sonar to distinguish the multiple access echoes in the shallow water, and a time reversal (TR) copy-correlation detection method is presented for solving the problem of multi-path sub-channel differences when the vertical array is oblique. STC detection signal has a good orthogonality through modulating with pseudo-random signals, can distinguish multiple access target echoes while combating with sub-channel attenuation. The vertical array affected by current is oblique, its multi-path sub-channel differences causing signals from different sub-channels can not focus on the receiving end, so the array gain decreases. Meanwhile, it causes resolution to decline and the error rate of signal judgment to rise. Channel training signal is designed to estimate the sub-channel multipath environment, and the best matched detection signal for sub-channel differences is got through virtual time reversal mirror (VTRM) with the estimated channels and the received signal being detected by TR copy-correlation. Simulation results of tristatic sonar (two transmitters and one receiver) show that the SPC detection signal and TR copy-correlation can meet the requirements of multistatic sonar detection and distinguish the multiple access echoes in low SIR and complex channel environment. The best matched detection signal is got through VTRM (virtual time reversal mirror) with the estimated channels, and the received signal is detected by TR copy-correlation. Simulation results of tristatic sonar (two transmitters and one receiver) show that the STC detection signal and TR copy-correlation can meet the requirements of multistatic sonar detection and distinguish the multiple access echoes in low SIR environment.
2014, 63 (5): 054304. doi: 10.7498/aps.63.054304
When in WSNs sensors receive different noise intensities or the wireless transmission channel has the shadow fading effect, the association metrics estimation method for range ratios of arrival (RROA) and the passive source localization algorithm based on RROA are studied. Firstly, the eigenvector decomposition (EVD) approach is used to estimate the RROA association metrics. The noise intensity received by each sensor can be estimated by performing EVD on the covariance matrix of the received signal. Secondly, by rotating the array reference point at each of the array sensors, a number of covariance matrices are constructed and the EVD approach can be used to cancel the shadow fading effect. Thus RROA association metrics can be estimated reliably. Finally, the weighted-least-squares (WLS) algorithm based on the RROA association metrics is proposed. The proposed approach is robust to channel shadow fading effect and different noise intensities received.
Research and design of thermal concentrator with arbitrary shape based on transformation thermodynamics
2014, 63 (5): 054401. doi: 10.7498/aps.63.054401
How to control and manipulate the heat flow in a flexible way is a hotspot of current research. According to transformation thermodynamics method, material parameter expressions for thermal concentrator with an arbitrary cross section are derived, and thermal concentrator with special symmetrical profiles, such as circle, ellipse, and pentagon, as well as thermal concentrator with arbitrary conformal/non-conformal cross section are designed on this basis. Full wave simulation results show that these thermal concentrators can bend the isotherm and heat flux towards their compressive regions, making the heat diffuses faster on the side near heat source and slower on the opposite side. Heat concentrated into a tiny region may have potential applications in thermal engineering. In addition, research on layered realization method of cylindrical thermal concentrator is conducted. Results reveal that the thermal concentrator can be realized through alternatively filling isotropic materials into layers along the angular direction. This work has a guiding significance for the design and manufacturing of thermal concentrator.
Lattice Boltzmann simulation of slip flow and drag reduction characteristics of hydrophobic surfaces
2014, 63 (5): 054701. doi: 10.7498/aps.63.054701
The relationship between the surface wettability and the interaction strength of liquid-wall particles is first investigated using a lattice Boltzmann method, and then the liquid flow over hydrophobic surfaces is simulated in a microchannel. Effect of surface wettability on the slip flow and drag reduction characteristics of hydrophobic surfaces is obtained. Existence of the apparent slip on hydrophobic surfaces is confirmed and its mechanism is revealed. Simulation results show that the hydrophobicity induces a low density layer near the wall of hydrophobic surfaces and the apparent slip is observed on the low density layer. It is shown that the apparent slip is a direct cause of hydrophobic surfaces' drag reduction effect. Thus the drag reduction effect increases with increasing slip length. For a specific fluid system, the slip length is an inherent property of the hydrophobic surfaces and is a single function of the surface wettability. The slip length does not change with the external flow property.
Vortex structures in turbulent channel flow modulated by a steadily distributed spanwise Lorentz force
2014, 63 (5): 054702. doi: 10.7498/aps.63.054702
Turbulence control and drag reduction in a channel flow by using a steadily distributed spanwise Lorentz force are investigated numerically via a direct numerical simulation (DNS). The characteristics of controlled flow fields and vortex structures are described. Meanwhile, the mechanisms of turbulence suppression and drag reduction by the Lorentz force are also discussed. Calculated results indicate that: (1) The shear layers with a arge gradient of spanwise velocity are created in the laminar boundary layer induced by the spanwise Lorentz force, where the streamwise vortices are easily generated by perturbations. (2) Under the action of the distributed Lorentz force with proper control parameters, only periodically well-organized streamwise vortices are observed in the near-wall region of the turbulent channel flow. (3) After controlling, the averaged lift height of inclined streamwise vortices is reduced significantly as compared with the uncontrolled turbulence flow, resulting in the reduction of the burst strength and subsequent drag reduction on the wall.
Theoretical and experimental investigations on aero-optical effect at the second stage of the compressible mixing layer
2014, 63 (5): 054703. doi: 10.7498/aps.63.054703
Using dimensional analysis and experimental technique, the aero-optical effect at the second stage of a compressible mixing layer has been investigated. The theoretical analysis focuses on the relation between two-dimensional large-scale structure of a mixing layer and its corresponding aero-optical effects. Results show that the time-averaged bore-sight error (BSE) has nonlinear variability with the convective Mach number (Mc). Moreover, we also present the relation among time-averaged BSE and turbulent kinetic energy, Reynolds shear stress at the interface between the mixing layer and free stream. Experimental results from a thin beam light passing through the compressible mixing layer verifly the theoretical result between time-averaged BSE and Mc.
2014, 63 (5): 054704. doi: 10.7498/aps.63.054704
Vortices play a crucial role in fluid dynamics, which is closely related to fluid diffusion mixing, force, heat, and noise. Five widely-used vortex identification criteria, i.e. the -criterion, Q-criterion, -criterion, ci-criterion, and 2-criterion are analyzed, and four of them are compared with each other based on the velocity-gradient-tensor decomposition method. A new quadruple decomposition method (QDM) is introduced for the first time, so far as we know, to decompose fluid motions into four fundamental components: dilatation, axial deformation along the principal axes of the strain-range sensor, planar motion, and pure shearing. This method helps make the kinematic implications of the four vortex identification criteria more clear. It is found that the mean rotation of fluid elements always contains the pure shearing motion. Non-zero mean rotation does not guarantee the existence of the spiraling streamlines, e.g. in a typically parallel shear flow. A positive Q value indicates the strength of the pure rotation of a fluid element in the 2D complex eigenvalue plane on top of the axial deformation, which however is a sufficient but not a necessary condition for the existence of pure rotation. The -criterion can correctly tell the existence of pure rotation, but cannot accurately determine its strength. The ci-value represents the absolute strength of the pure rotation, which is the combined effect of the canonical rotation in the complex eigenvector plane and the pure shearing. The proposed QDM enables us to achieve a deeper understanding of vortices and motions in fluid dynamics.
2014, 63 (5): 054705. doi: 10.7498/aps.63.054705
Some numerical studies were carried out on micrometer-sized rising bubble bouncing near a rigid boundary. Taking surface tension into consideration, the bubble motion could be solved using the potential flow theory. A correction should be made in Bernoulli equation because the pressure gradient was caused by the viscosity between the bubble and the wall. The numerical result agree well with the experimental data. Based on the fundamental phenomenon, we have studied the influence of characteristic parameter on bubble bouncing behavior, and the balanced shape due to the action of the wall. With the increase of the rising distance of the bubble, the distance of the bubble bouncing downward and the period of bouncing would increase. However, they would not change obviously when the rising distance is large enough. Surface tension has great effect on the dynamic behavior of the bubble. The bouncing period decreases when surface tension becomes large, but the bouncing distance will have an increase before it gradually decreases. Finally, the balanced shape of the bubble due to the wall effect can be mainly controlled by buoyance parameter and the Weber number.
2014, 63 (5): 054706. doi: 10.7498/aps.63.054706
The study of far-field pressure wavelet of air-gun bubble is the basic research of air-gun sources. In this paper, a boundary element bubble model is established to simulate the dynamics of bubble, and an energy equivalent method is put forward to solve the bottleneck of bubble jet. In our simplified model, the bubble model takes into consideration the thermodynamic factors such as heat transfer and gas release, and the numerical results agree with the data of Nucleus software well. Based on the established model, the influence of heat transfer coefficient, air-gun firing time, and specific heat at constant volume on the pressure wavelet are studied. Finally, the three-dimensional bubble merging numerical method is put forward, which is an expansion of the axisymmetric model. Thus, the preliminary study of coherent air-gun is conducted, especially the influence of the space between two bubbles on the wavelet. This paper provides a reference for the research of air-gun.
2014, 63 (5): 054707. doi: 10.7498/aps.63.054707
Laser-induced bubble was generated by YAG laser bombarding the metal target in the water at different venturi inlet pressures. The movement of laser-induced bubble in venturi was recorded by a high-speed video system. Characteristics of flow field in venturi and bubble collapse were analyzed by computational fluid dynamics. Results show that the process of laser-bubble moving in venturi can be classified into four periods: generation, extrusion, initial collapse, and collapse period. Bubble collapse depends on the state of flow field. Bubble does not collapse when the flow is in laminar state. However, bubble collapses when the flow is in turbulent state. The collapse phenomenon becomes prominent with increasing turbulence intensity. The bubble collapse at a fixed point can be realized in the laser-bubble generation and movement system.
2014, 63 (5): 054708. doi: 10.7498/aps.63.054708
This article studies drag reduction rule and gas restoration and retention of hydrophobic surfaces numerically when taking into consideration the surface tension effect, the microstructure here is chessboard-like and the Reynolds number varies from 3,000 to 30,000. Results show that gas restoration and retention keep well, and a maximum drag reduction rate of approximately 30% has been gained at small Reynolds number(Re<15000). When Re is too large, water will swarm into microstructures, and keeping a good gas-liquid interface becomes difficult. Meanwhile, drag reduction rate remains variable and hydrophobic surfaces do not reduce drag. Through mechanical analysis we find that the influence of shear stress is weakened due to surface tension effect, thus the gas in microstructures can be effectively stored at low flow speed and drag is reduced.
PHYSICS OF GASES, PLASMAS, AND ELECTRIC DISCHARGES
2014, 63 (5): 055201. doi: 10.7498/aps.63.055201
The planar wire array Z pinch experiments with insulated aluminum wires coated with 2 μm polyimide were carried out on "QiangGuang-I" facility (100ns, 1.3 MA) and compared to those with ordinary aluminum wires. Experimental results indicate that surface insulation can affect the implosion dynamics and radiation character of Z pinches. The resistive heating energy and imploding mass may be increased by exploiting the effect of surface insulation. In the surface-insulated wire array Z pinch experiments the multi-peak phenomena were considerably suppressed, whereas the improvement on the X-ray yield was not observed.
CONDENSED MATTER: STRUCTURAL, MECHANICAL, AND THERMAL PROPERTIES
2014, 63 (5): 056101. doi: 10.7498/aps.63.056101
Motion and friction of carbon nanotubes with vacancy defects or Stone-Thower-Wales (STW) defects on them are investigated in commensurate states and incommensurate states by molecular dynamics simulation. Results show that defects lead to incommensurate state in part of interfaces, thus decreasing the friction. More amount of STW defects would cause larger distortion of carbon nanotube, smaller lateral force amplitude, more local incommensurate state of interfaces and smaller friction. The friction of carbon nanotube with vacancy defects is obviously larger than carbon nanotube with STW defects. The reason is that the carbon nanotube with vacancy defects will change its motion in later period of motion, which can increase energy dissipation. Defects barely have influence on the friction of carbon nanotubes in incommensurate state because interfaces are all in incommensurate state whether they are having defects or not.
2014, 63 (5): 056102. doi: 10.7498/aps.63.056102
A study of ionizing radiation effects is presented for CMOS active pixel sensors manufactured in a 0.5-μm CMOS (complementary metal oxide semiconductor)by n-well technology. The basic mechanisms that may cause failure are also presented. After exposure in γ-rays, the most sensitive parts to radiation-dark signals and dark signal non-uniformity are discussed, i.e. the physical mechanism of the degradation by irradiation. One can see from the experiment that the mean dark signals are dramatically increased with total dose for both operated and static devices. Static device seems more affected by irradiation than operated device. We find that most part of the total dark signal in a pixel comes from the depletion of the photodiode edge at the surface and the rest part is caused by the leakage of the source region of the reset transistor. Dark signal non-uniformity follows the dark current evolution with total dose. Further study of photodiode and LOCOS (local oxidation of silicon) isolation behaviors under irradiation should be done so as to correctly use this qualification techniques on MOS sensors manufactured in CMOS n-well technology process.
2014, 63 (5): 056801. doi: 10.7498/aps.63.056801
Boron-doped zinc oxide (BZO) films with a natural pyramid-textured surface grown by metal organic chemical vapor deposition (MOCVD) have large light trapping effect in thin film silicon solar cells when used as front contact electrodes. However, the surface topography of traditional BZO films is so sharp as to damage the quality of the subsequent silicon thin film materials and to reduce the photovoltaic conversion efficiency of the solar cells. In this work, an ultra-thin In2O3:Sn(ITO)film (～ 4 nm) is used as the interlayer in the sandwiched structure of the multilayer films, i.e. glass/bottom BZO layer /ITO interlayer/top BZO layer. The surface properties can be improved through modulating the thickness of the top BZO layer. Appropriate thickness of top BZO layer and ITO interlayer are helpful for obtaining the cauliflower-like surface morphology and thus the sharp structure becomes relatively gentle, but the surface morphology still keeps a pyramid feature when depositing thicker top BZO layer. The relatively gentle surface morphology could promote crystallization quality ofc-Si:H thin film materials and reduce cracks in intrinsic layer and TCO/P-Si interface defects. Finally, this new sandwiched structure of multilayer ZnO films is applied in c-Si:H p-i-n thin film solar cells. Compared with traditional BZO films, the quantum efficiency (QE) of solar cells with a sandwiched structure of ZnO increases by about 10%, and both the open-circuit voltage (Voc) and short-circuit current density (Jsc) may increase and thus improve the solar cell efficiency.
2014, 63 (5): 056802. doi: 10.7498/aps.63.056802
Based on the Landau-Khalatnikov equation of motion, the switching dynamic behaviors, (including the average polarization, switching time, switching current and coercive field) of a ferroelectric bilayer film with a surface transition layer within each constituent thin film and a ferroelectric interfacial coupling between two thin films have been investigated. Results reveal that there is a competitive mechanism in the bilayer film, the action of surface transition layer and the interfacial coupling. The abnormal behavior is discovered in the polarization reversal process of the bilayer film, which can be attributed to the competition between the surface transition layer and the interfacial coupling. The combined action of surface transition layer and interfacial coupling plays a decisive role on the dynamic properties of a ferroelectric bilayer film.
CONDENSED MATTER: ELECTRONIC STRUCTURE, ELECTRICAL, MAGNETIC, AND OPTICAL PROPERTIES
2014, 63 (5): 057101. doi: 10.7498/aps.63.057101
At present, although there is some studies about the theoretical calculation studies of Zn1-xTMxO1-yNy(TM=Al, Ga, In) p-type doped have been reported. But, they are random doping and without considering the asymmetry of ZnO preferred orientation to doping. Therefore, Six different supercell models Zn1-xTMxO1-yNy (TM = Al, Ga, In. x = 0.0625, y = 0.125) which proportion is TM:N = 1:2 and preferred orientation to co-doped have been constructed based on the first-principles plane wave ultra-soft pseudo potential method of density function theory, in this study.Then calculate the geometric optimization, State density distribution and Band structure distribution for all models, respectively. Results indicate that with the condition of heavily doped and preferred orientation to co-doped, in the same kind of preferred orientation co-doping systems, the electrical conductivity of the system which TM-N bond along the c-axis direction is greater than it perpendicular to the c-axis. In the different kinds co-doping ZnO systems which TM-N bond along the c-axis direction, The co-doping systems of In-N bond along the c-axis direction has the strongest conductivity and the lowest ionization energy and the largest Bohr radius. It is more favorable for electrical conductivity of p-type ZnO. This study can be a theoretical guidance for improve the electrical conductivity of which design and preparation TM:N=1:2 ratio preferred orientation co-doping ZnO systems.
2014, 63 (5): 057102. doi: 10.7498/aps.63.057102
The conductivity of W-doped β-Ga2O3 is investigated by using the ultra-soft pseudopotential (USP) approach of the plane-wave based upon density functional theory. The optimized structural parameters, total electron density of states, and energy band structures of β-Ga2(1-x)W2xO3 (x=0, 0.0625, 0.125) are calculated. It is found that the volumes are slightly increased and the total energies are going up in the Ga2(1-x)W2xO3 system with increasing W-doping concentration, which causes the system instability. When the W concentration is smaller, the calculated conductivity and electronic mobility are higher, but when the W concentration is increased, the average electron effective mass becomes bigger and the energy gap becomes narrower. The results are consistent with experimental data.
First-principle study on the effect of high Pr doping on the optical band gap and absorption spectra of TiO2
2014, 63 (5): 057103. doi: 10.7498/aps.63.057103
Nowadays, the studies on band gap and absorption spectrum of TiO2 doped with Pr lead to opposite conclusions. Two experimental results about red-shift and blue-shift are reported in the literature. We have set up models for pure TiO2 and different doping concentrations of Pr-doped TiO2 to calculate the electronic structure and absorption spectrum based on the first-principle plane-wave ultrasoft pseudopotential in terms of the density functional theory (DFT) to slove the above problem. Results indicate that under the condition of heavy doping Pr, compared with the pure TiO2, as the Pr concentration increases the atomic charge of the doped system reduces and the total energy of the doped system becomes higher, and its formation energy will be greater. This makes the stability decline, the band gap narrowed, the absorption spectrum red-shift, and the absorption strength more significant. The results of the calculation is in agreement with the experimental data.
Thermoelectric properties of chalcopyrite Cu3Ga5Te9 with Sb non-isoelectronic substitution for Cu and Te
2014, 63 (5): 057201. doi: 10.7498/aps.63.057201
Thermoelectric materials, which allow the conversion between heat and electricity, can be directly applied in the fields of cooling and power generation. Here we report an effective approach: non-isoelectronic substitution of Sb for Cu and Te in Cu3Ga5Te9 to increase the Seebeck coefficient and electrical conductivity. This improvement is attributed to the enhancement in carrier concentration n and effective mass as well as the conservation of the carrier mobility μ. The enhancement of the carrier concentration is caused by the hole doping effect due to the drop of the Fermi level into the valence band when Sb occupies the Te lattice sites, and also due to the increase of the copper vacancy (V-1Cu) concentration when Cu content decreases. In addition, the non-isoelectronic substitution can yield extra crystal structure defects. These defects, which are represented by the alterations of anion (Te2-) position displacement (u) and tetragonal deformation (η), directly govern the lattice thermal conductivity (κL) on an atomic scale. The maximum ZT value is 0.6 at 766 K with proper Sb substitution, which is about 25% higher than that of Cu3Ga5Te9. Therefore, we are able to effectively manipulate the electrical and thermal properties through proper selections of the substituting / substituted elements and their quantities, and prove that the non-isoelectronic substitution approach in the chalcopyrite semiconductors is an effective way to improve the thermoelectric performance.
Effect of temperature and external magnetic field on the structure of electronic state of the Si-uniformlly-doped GaAs quantum well
2014, 63 (5): 057301. doi: 10.7498/aps.63.057301
By solving the Schrödinger equation and the Poisson equation self-consistently, we have calculated the electronic structure for the Si-uniformally-doped GaAs/AlGaAs quantum well system at T=273 K and B=25 T in the effective mass approximation. We also studied the influence of the temperature and the external magnetic field on the subband energies, eigen-envelope functions, self-consistent potential, density distribution of the electrons, and the Fermi energy. It is found that at the given magnetic field B≠0, with the increase of temperature, the subband energies increase monotonically, the Fermi energy decreases monotonically, the width of the self-consistent potential well decreases, the depth of the well increases, the distribution of the electron density becomes wider, and the peak value is reduced. At the given temperature, with the increase of the strength of the magnetic field, the subband energies and Fermi energy increase monotonically, the depth of the self-consistent potential well is reduced, the peak of the electron density distribution becomes higher, and concentrated around the center of the well.
2014, 63 (5): 057302. doi: 10.7498/aps.63.057302
In order to optimize the surface electric field of AlGaN/GaN high electron mobility transistors (HEMTs), a novel AlGaN/GaN HEMT has been grown with a step AlGaN layer, made for the first time as far as we know, to improve the breakdown voltage. The discipline of the 2DEG concentration varying with the thickness of the AlGaN epitaxy layer has been applied to the new AlGaN/GaN HEMTs with AlGaN/GaN heterostructure. By thinning the AlGaN layer near the gate edge, the 2DEG concentration in the channel is made to form the low concentration region near the gate edge. New electric field peak has appeared at the corner of the step AlGaN layer. The high electric field has been decreased effectively due to the emergence of new electric field peak; this optimizes the surface electric field of the new AlGaN/GaN HEMTs. Then the breakdown voltage is improved to 640 V in the new AlGaN/GaN HEMTs with the step AlGaN layer as compared with 446 V for the conventional structure. In order to let the breakdown curve consistent with the test results, a certain concentration of the acceptor-like traps is added to the GaN buffer to capture the leaking current coming from the source electrode. Simulation results verify the causes for doping acceptor type ions to the GaN buffer, given by foreign researchers. The breakdown curves have been obtained which are consistent with the test results in this paper.
Nuclear magnetic resonance experimental study on the characteristics of pore-size distribution in muck under several typical loading cases
2014, 63 (5): 057401. doi: 10.7498/aps.63.057401
Several typical loads are applied to muck specimens to simulate muck ground treatment conditions. Then tests of pore structural distribution of the soil are performed by the method of nuclear magnetic resonance (NMR), for seeking variation of their internal structure under the typical load level and rate, and thus to further determine the muck ground reinforcement mechanism and responses in micro-structure aspect. The results suggest that: (1) the max-pore decreases under a certain load, and the reduction increases with load level; nevertheless, when the impact load reaches a considerable level, both the max-pore and small-pore decrease; (2) the confining stiffness effect of constraint samples induces a decrease in the proportion of large-pore; (3) at a certain load level (680 kPa or less), the loading rate is the key factor determining the relative proportion of maximum void ratio: smaller lading rate will make the void ratio increase, larger lading rate will make the void ratio reduced, and its boundary value is between > 0.8 MPa/s and ≤ 1.6 MPa/s; (4) with a certain load level and rate, an the number of impact times increses, namely as the total energy is enhanced, the large-pore as well as the max-pore considerably decreases; however, this effect will be reduced when the interval time is short and as the number of impact times increases further. i.e. the large-pore effectively decreases. There exists a suitable value of the number of impact times for the muck specimens. These results reflect the rule of different loading effects in the micro of micro fine structue and provide a basis for the design and construction optimization for muck ground improvement.
Effect of magnetic capacitance in the Fe3O4 nanopartides and polydimethylsiloxane composite material
2014, 63 (5): 057501. doi: 10.7498/aps.63.057501
In this paper, a parallel plate structure for the magnetic capacitor applications is presented, which consists of hybrid materials of Fe3O4 nanoparticles with polydimethylsiloxane (PDMS) as the dielectric medium. By changing the nanoparticle sizes and concentrations in PDMS, the magnetic-capacitance effect of the designed structure is investigated, and some key factors which may affect the performances are studied. It can be concluded from the results that a clear magnetic-capacitance coupling effect is observed by putting the designed Fe3O4 nanoparticles and PDMS hybrid material in or out of a magnetic field. Meanwhile, as we increase the concentration of the nanoparticles, an increase of capacitance variation may be observed. If the nanoparticle sizes are bigger than the critical dimension of the super-paramagnetic effect, the capacitance variations is increased as the nanoparticle size increases.
2014, 63 (5): 057801. doi: 10.7498/aps.63.057801
We report the photoluminescence of monocrystalline silicon irradiated by femtosecond pulsed laser in different environments (deionized water and air) and energy density conditions. The field emission scanning electron microscope (FESEM) measurement results show the formation of completely different morphologies on silicon surface in different environments. A stripe-like microstructure on the silicon surface in air is formed in contrast to the smaller and coral-like microstructure generated in the deionized water. By using the energy dispersive spectroscopy (EDS) we find that silicon and oxygen is the main elemental composition on femtosecond laser-induced silicon surface, and the content of oxygen on the sample surface formed in the deionized water is nearly four times larger than that in air. The Si-Si bond (610 cm-1) and Si-O-Si bond vibrations (1105 cm-1) are detected mainly in the Fourier transform infrared transmission spectrum (FT-IR). The photoluminescence (PL) spectroscopy measurement results show that visible blue luminescence is observed both from the silicon ablated in the deionized water and in air, while the shape and position of the emitted luminescence peak are substantially the same. However, the luminescence intensity of silicon etched in the deionized water is close to 3 times stronger than that in air when the photoluminescence is excited at respective most suitable excitation wavelength. A more interesting phenomenon is that the position and shape of the photoluminescence peak in the visible range are basically not changed. The studies confirm that oxygen plays an important role in photoluminescence enhancement. Photoluminescence may be mainly generated by the formation of oxygen defects SiOx and the content of low oxide SiOx (x<2) determines the luminous intensity level.
2014, 63 (5): 057802. doi: 10.7498/aps.63.057802
Spherical silver (Ag) nanoparticles, triangular Ag nanoprisms, and triangular gold (Au) nanoparticles were separately deposited on the Eu(TTFA)3-doped Su8 films. Compared with the fluorescence of films without nanoparticles, the fluorescence of the doped films was greatly enhanced by the two kinds of the Ag nanoparticles, the enhancement is greater for the spherical Ag nanoparticles than that for the triangular Ag nanoprisms. The fluorescence was quenched for the film with Au nanoparticles. Results show that the nanoparticles can both enhance and quench the fluorescence of the films, which depend mainly on the local surface plasmon resonance (LSPR) of the nanoparticles. When the LSPR of nanoparticles overlaps with the excitation or the emission spectrum of the films, the fluorescence would be enhanced. Otherwise the fluorescence of the films would be quenched.
2014, 63 (5): 057803. doi: 10.7498/aps.63.057803
The terahertz (THz) radiation absorption of graphene layers in a pseudomagnetic field, prepared on top of a one-dimensional photonic crystal (1DPC), is investigated theoretically. Discrete Landau levels can be found in graphene in a pseudomagnetic field. Strong THz transitions may be found between the discrete Landau levels. The THz absorption of graphene can also be tuned by varying either pseudomagnetic field or the distance between the graphene and the 1DPC.
2014, 63 (5): 057901. doi: 10.7498/aps.63.057901
THz technology has attracted great attention for decades of years. Among the wide research areas of THz technologies, vacuum electron terahertz radiation sources have obvious advantages in high power region. For the THz vacuum electro devices (VED), high current density electron beams with small dimensions are required. Nanosized scandia doped dispenser (SDD) cathodes have the capability to operate stably at pulsed current densities of over 100 A/ cm2 at 950 ℃ so it becomes the most promising cathodes to meet the requirements for THz VEDs. In this paper, we report a new approach for developing miniaturized electron beam sources on normal SDD cathodes. An electron beam of 400 μm in diameter has been directly generated on an SDD cathode by deposition of a Zr/W double-layer anti-emission film and followed by a focused ion beam (FIB) milling. Results indicate that the electron beam is able to provide a space charge limited (SCL) current density of over 50 A/cm2 at the operating temperature of 950 ℃ with proper laminarity and works stably for more than 1000 hours. The beam emission characteristics and the function of the anti-emission film have been discussed and related to the surface analysis results. The approach opens a new way for producing high emission mini-electron sources to satisfy the requirment of THz VEDs.
INTERDISCIPLINARY PHYSICS AND RELATED AREAS OF SCIENCE AND TECHNOLOGY
2014, 63 (5): 058101. doi: 10.7498/aps.63.058101
Three metallic glasses Zr41.25Ti13.75Ni10Cu12.5Be22.5, Ce68Al10Cu20Co2 and Fe41Co7Cr15Mo14Y2C15B6(all in at.%), were compressed experimentally, and the fractured samples were investigated by scanning electron microscope. It was found that they exhibit different fracture modes. In three-point bending tests of the three pre-notched metallic glass samples, a digital speckle technique is adopted to in-situ observe the elastic strain field evolution in front of the crack tip. Accroding to the yield criterion of metallic glasses, the mechanism of different fracture processes in metallic glasses with various fracture toughnesses are elucidated.
2014, 63 (5): 058401. doi: 10.7498/aps.63.058401
An advanced method of signal decomposition-advanced adaptive chirplet decomposition(AACD) is presented to separate the radar fuze mixed signal in single-channel. The method adds curvature factor to the adaptive Gauss chirplet decomposition (AGCD), thus it can be used to decompose nonlinear time varying components. It uses the fast algorithm and trust region method to optimize arithmetic process, then all the parameters can be solved accurately and quickly, and the mixed signal's time-frequency distribution without cross terms is obtained. In the simulation, the advanced method is used to improve the time-frequency distribution of complex mixed signals, then it can separate all the components and calculate the mixing coefficients. Results indicate that the proposed method has higher effectiveness and strong performance of anti-noise
Design and evaluation of a pre-traveling wave deflector magnetic solenoid lens focused streak image tube
2014, 63 (5): 058501. doi: 10.7498/aps.63.058501
Owing to the excellent feature of ultrahigh temporal resolution, the streak image tube has been the indispensable scientific instrument for the application to measurement of ultrafast phenomena on subpicosecond time scale. In this paper, a new femtosecond streak image tube with both high temporal and spatial resolution is designed. To improve its integrated performance, a pair of traveling wave deflectors is located before a magnetic solenoid lens in the tube, which minimizes the electron transit time from anode to deflection plates and therefore reduces time dispersion and space charge effects. Using CST microwave studio finite element method, bandwidth and deflection sensitivity of traveling wave deflector have been studied numerically. The result shows that the wide bandwidth of the traveling wave deflectors designed is beneficial to slow down the electrical deflecting pulse to match the beam velocity in a wide frequency range, which can improve the deflection sensitivity. After tracing the trajectory of electrons and calculating the modulation transform function, we get temporal resolution of 220 fs and spatial resolution above 100 lp/mm. Based on the aberration theory, the aberration formulas of electron trajectory are deduced, which systematically evaluate the imaging quality of streak image tube with modulation transfer function. Finally, static experiment is carried out, and the result shows that the static spatial resolution is better than 35 lp/mm.
2014, 63 (5): 058702. doi: 10.7498/aps.63.058702
High-attenuation objects like metals will result in metal artifacts in computed tomography images. Compared with single metallic object, artifacts due to multiple and large-scaled metallic objects is more complicated in representation and have much worse effects on reconstructed image. State-of-the-art metal artifacts reduction for multiple metal objects based on interpolation method cannot solve the beam hardening inside the metals, and can easily make mistakes in segmentation and interpolation. Aiming at reduction of multiple metallic objects, this paper simulates the production of the artifacts and proposes a metal artifacts reduction method based on projections correction. In this method, metal regions are firstly segmented directly from projection domain, and then a correction model is established for projections in metal regions. Finally, correction is made by adjusting parameters of the model. The optimal solution of the parameters is achieved by NM-simplex method that makes the gray entropy of the reconstructed image minimum. The simulation results and obtained data show that the present method significantly improves metal artifact due to multiple metallic objects and provides a better image quality than that obtained using interpolation.
2014, 63 (5): 058703. doi: 10.7498/aps.63.058703
A multi-chamber heart magnetic field model with two atria and two ventricles, boundaries of which were picked up from a magnetic resonance imaging, was established based on the boundary element method (BEM). Moreover, the model-based 36-channel cardiac magnetic field data and magnetic field maps at a specific time were analyzed. We also studied the heart electrical activity during ST-T segment from patients with complete right bundle branch block (CRBBB) and complete left bundle branch block (CLBBB) by the model, respectively. Results show that the model-based magnetic field map generated by the electrical excitation with a moving single current dipole in single bundle branch is similar to the magnetocardiogram (MCG) of the CRBBB/CLBBB patient acquired using a superconducting quantum interference device (SQUID) in cardiac repolarization. It demonstrates that the multi-chamber heart BEM model can be used to study cardiac magnetic inverse problem of CLBBB/CRBBB patient. In addition, two evaluation criteria are given as follows: the ratio of the maximum on the magnetic field strength measurement plane in the multi-chamber model to that in the single-chamber model; and the ratio of root mean squares of the magnetic field strength at the 36 measurement points of the two models. This result indicates that the magnetic field maps generated by the multi-chamber heart model are close to the measured MCG maps. In this model, the strength and topography of the magnetic field lie in the conductivity parameters of cardiac tissues, the position and the number of the equivalent current dipoles.
2014, 63 (5): 058701. doi: 10.7498/aps.63.058701
This paper mainly reviews the progress of methods, and research development in the field of computed tomography(CT) with incomplete projection data, at home and abroad, on limited angle projection data reconstruction with the detector fully covered, and truncated data reconstruction with the detector partially covered. Firstly, the discrete model iterative reconstruction algorithm and the compressed sensing(CS) sampling reconstruction algorithm are discussed for the sparsely uniform and limited-view angle sampling in the case that the detector fully covers the detected object. Secondly, the reconstruction algorithm of back-projection filtration(BPF) for helical cone beam and improved ones for cone beam FDK are discussed in the case that the detector could not fully cover the detected object. This paper could provide the researchers in CT reconstruction field the criticism of methods and summaries. Furthermore, it also points out current focus of the study and the research direction in the future.
GEOPHYSICS, ASTRONOMY, AND ASTROPHYSICS
The distribution of large-scale drought/flood of summer in China under different configurations of monsoon and polar vortex
2014, 63 (5): 059201. doi: 10.7498/aps.63.059201
From the view of warm and cold system configuration, an East Asian summer monsoon (EASM) index and the Asian polar vortex area (APVA) index in July were selected, and their climatic characteristics and relationships with the distribution of summer rainfall over China were analyzed. On this basis, the relationships between the EASM and APVA in July from 1951 to 2010 were divided into four types: type A, stronger monsoon and bigger polar vortex area; type B, stronger monsoon and smaller polar vortex; type C, weaker monsoon and bigger polar vortex; type D, weaker monsoon and smaller polar vortex. Based on the method of composite analysis, we found that on the seasonal scale, large-scale distributions of summer droughts and floods over China showed multi-faceted characteristics in different configurations of the EASM and the APVA in July. In A type years, the summer rainfall of China was overall less than normal, the region between eastern parts of Northwest and the western parts of North China, and the eastern regions of Northeast suffered from flood, while Yangtze River valley suffered from drought; in B type years, it had flood in South and drought in North, and the precipitation in the south/north of northern regions in the Yangtze River valley was more/less than normal; in C type years, North China was flooded and South of China suffered from drought, while the precipitation in the northern parts of the Huaihe River valley was more than normal; and in the middle and lower reaches of Yangtze River and to its south the precipitation was less than normal; in D type years, the overall summer rainfall was more than normal, with the precipitation core lying in the middle and lower reaches of Yangtze River. Thus, it can be seen that the distribution of summer rainfall is not only related to the EASM, but also to the PAVA in July. Under different configurations of the both, the precipitation and its distribution display obvious difference and regularity, which, to some extent, have effects on predicting the total trend of summer rainfall. Besides, by discussing the anomaly characteristics of atmospheric circulation due to different configurations and by analyzing the influences of different meteorological factors on the relationship between the monsoon and the polar vortex, we can conclude that the difference and regularity of summer rainfall directly depend on the configurations of circulation. Among them, the western Pacific subtropical high and the blocking situation in the mid and high latitudes are the overriding factors.