Influence of Zr doping on solubility of Xe in UO2: A first-principle study
As a major fuel of the light-water reactors, UO2 has excellent properties such as high melting point, good radiation resistance, corrosion resistance, compatibility with cladding materials, and strong ability to tolerate fission gas. The Zr atoms are inevitably introduced into UO2 lattice during the operation of a nuclear reactor, which can affect the solubility of Xe in the UO2. In this paper, we calculate the formation energy of vacancy defect and the binding energy of Xe in vacancy of Zr doped UO2.
The calculations presented here are based on density functional first-principle and projector augmented-wave method. A plane-wave basis set with a cutoff energy of 400 eV is used. The generalized gradient approximation refined by Perdew, Burke and Ernxerhof is employed for determining the exchange and correlation energy. Hubbard U term is used for considering the f-electron localization. Brillouin zone is set to be within 5×5×5 k point mesh generated by the Monkhorst-Pack scheme. The self-consistent convergence of total energy is 1×10-4 eV/atom. The calculations are performed in a 2×2×2 supercell.
In order to verify the calculating process, the formation energies of U and O point defects are compared with those in the literature. Then the influence of Zr doping in the UO2 on the solubility of Xe in the UO2 is studied. The results show that the ability to form the vacancy defects is different in the U-rich and O-rich environment of UO2. The vacancy defects in UO2 are more likely to form in O-rich UO2. The Zr doping will lead to the increasing of the formation energies of defects in both cases.
The Zr doping will also change the binding energy of Xe in void. For all the systems studied, only the binding energy of Xe adsorbed to the void consisting of four point defects increases, while the rest decrease. The solution energy, equaling the sum of the binding energy of Xe and the vacancy formation energy, will increase after doping Zr, because the decrement in binding energy is generally less than the increment in vacancy formation energy.
In summary, the presence of Zr will weaken the solubility of Xe in UO2, which is mainly due to the hindering of vacancy defects from forming. This result has a certain value in studying the dissolution of fission product Xe after a small amount of Zr has entered into the UO2 fuel in nuclear reactor.
Softening of sound velocity and Hugoniot parameter measurement for shocked bismuth in the solid-liquid mixing pressure zone
Polymorphic phase transformation and melting under shock wave loading are important for studying the material dynamic mechanical behavior and equation of state in condensed matter physics. In this paper, the accurate Hugoniot parameter and sound velocity of shocked pure bismuth (Bi) in a pressure range of 17.3-28.3 GPa are obtained by using flyer impact method and rarefaction overtaking technique, respectively, and the sound velocity softening trend in shock-induced melting zone and the melting kinetics of Bi are then analyzed. In each experiment, six Bi samples with different thickness values are affected by oxygen-free-high-conducticity copper flyer fired through power gun. Shock wave velocity and particle velocity in Bi are experimentally determined through measuring the impact velocity and shock wave time in the thickest sample by photon Doppler velocimetry (PDV) technique. The velocity profiles on each interface between Bi and lithium fluoride (LiF) window are measured by displacement interferometer system of any reflector (DISAR), and then the sound velocity of shocked Bi is determined using the rarefaction overtaking method. The analyses of our results show that the softening of sound velocity of Bi approximatively satisfies the linear relation of Cs=3.682-0.015 p in the solid-liquid coexistence zone, and the pressure zone of the solid-liquid coexistence phase is further affirmed to be in a range of 18-27.4 GPa. Additionally, the obtained Hugoniot data for Bi in this paper supply a gap in the pressure zone of solid-liquid mixing phase. The quadratic equation with the expression of Ds=0.401+ 3.879 up-0.876 up2 can better demonstrate the relation between shock wave velocity and particle velocity than a linear one when the particle velocity lies in a range of 0.5-1.0 km/s, and this non-linear property maybe has a relationship with the shock-induced melting of Bi. Finally, our wave profile measurement of the Bi/LiF interface shows peculiar ramp characteristics in the expected velocity plateau zone in the pressure zone of solid-liquid coexistence phase, which may be associated with both the nonhomogeneous melting kinetics and the long time scale of melting for bismuth.
Dendrite growth and Vickers microhardness of Co7Mo6 intermetallic compound under large undercooling condition
The dendritic growth process and Vickers microhardness enhancement of primary Co7Mo6 phase in undercooled liquid Co-50%Mo hypereutectic alloy are systematically investigated by using electromagnetic levitation and drop tube. It is found that the rapid solidification microstructures are mainly characterized by primary Co7Mo6 dendrites plus interdendritic (Co7Mo6+αCo) eutectic irrespective of experimental conditions. In electromagnetic levitation experiment, the obtained maximum undercooling reaches 203 K (0.12TL). With the rise in bulk undercooling, primary Co7Mo6 dendrite growth velocity monotonically increases according to a power function and reaches 22.5 mm·-1 at the highest undercooling. The secondary dendrite spacing decreases from 45.8 to 13.6 μm, while Co content in primary dendrites shows an increasing trend. This indicates that an evident grain refinement and solute trapping take place for primary Co7Mo6 dendrites during rapid solidification. The dependence of Vickers microhardness on Co content follows an exponential function. Moreover, the variation of Vickers microhardness with the grain size also satisfies an exponential relationship. In addition, Lipton-Kurz-Trivedi/Boettinger-Coriel-Trivedi model is used to analyze the growth kinetics of primary Co7Mo6 dendrites. In the experimental undercooling range, the growth process of primary Co7Mo6 dendrites is controlled mainly by solute diffusion and they grow sluggishly. Under free fall condition, liquid Co-50%Mo alloy is subdivided into many droplets inside a drop tube and their diameters range from 1379 to 139 μm. With alloy droplet size decreasing, both droplet undercooling and cooling rate increase rapidly. In a large droplet-diameter regime above 392 μm, primary Co7Mo6 phase displays faceted-growth characteristics. Furthermore, primary Co7Mo6 dendrites are refined greatly and their solute solubility is significantly extended as droplet size becomes smaller. Once the alloy droplet diameter decreases to a value below this threshold value, the faceted-growth characteristics start to disappear gradually, which is accompanied with a conspicuous grain refinement and a solute solubility extension. Both the solute solubility enhancement and grain size refinement contribute significantly to the exponential improvement in microhardness if primary Co7Mo6 phase grows in a faceted way. Otherwise, the solute solubility enhancement and grain size refinement result in the linear increase of Vickers microhardness. Theoretical analyses demonstrate that the primary phase microhardness is strongly dependent on its solute content and morphology characteristic.
Electronic structure and optical properties of Cu-O co-doped AlN
The geometry parameters, band structure, electronic density of states, and optical properties of AlN before and after being co-doped by Cu and O are investigated by the ultra-soft pseudo-potential plane wave based the density functional theory. The results show that the lattice volume increases and the total energy of the system decreases after doping. The Cu doping system makes Cu 3d electrons hybridize with its nearest neighbor N 2p electrons strongly. In the Cu-O co-doped system, Cu and O attract each other to overcome the repelling of acceptor Cu atoms, thereby increasing the doping concentration of Cu atoms and the stability of the system. Dielectric function calculation results show that Cu-O co-doping can improve the optical transition characteristics in low energy area of AlN electrons, and thus enhancing the optical transition of electrons in visible area. The complex refractive index calculation results indicate that Cu-O co-doped system increases the absorption of low frequency electromagnetic wave.
Fabrication and electrical transport characteristics of the polycrystalline Ca12Al14O33 electride
The[Ca24Al28O64]4+:4e- (C12A7:e-) electride composed of densely packed, subnanometer-sized cages. This unique structure makes it possess distinctive applications in fields of electronic emission, superconductor, electrochemical reaction. In this paper, we explore a new method to prepare the bulk of C12A7:e- electride. The following areare systematically studied in this work. 1) the condition of preparing bulk of C12A7:e- electride by solid reaction combining spark plasma sintering and reduction with Ti particles at high temperature, CaCO3 and Al2O3 powders are used as raw materials; 2) the first principle calculations of band structure and density of states of the C12A7:e- electride; 3) the analysis of the electrical transport properties of the C12A7:e- electride. The bulk of C12A7:e- electride is successfully prepared by this method, so the results show that the bulk of C12A7:e- electrode with the electron concentration 1018-1020 cm-3 is synthesized at 1100 ℃ and a vacuum pressure of 10-5 Pa for 10-30 h. In the process of Ti reduction, Ti particles become evaporated and deposit on the surface of C12A7, the free O2- atom in the cages diffuse to the sample surface, the Ti vapor reacts with the O2-, forming a loose TiO_x layer. In order to maintain electrical neutrality, the electrons of the free O2- atom leave from the cages, forming the C12A7:e- electride. In addition, the loose TiO_x layer also provides a channel for the diffusion of the O2- atoms in the cage, ensuring the continuation of the reduction reaction. The calculated band structure and density of states of the bulk C12A7:e- electride show that when electrons replace the O2- atoms in the cage, the Fermi level of C12A7:e- crosses over the cage conduction band (CCB). Thus the free movement of the electron is the main reason for the insulator C12A7 to convert into conductor C12A7:e-. At the same time the electrons near the Fermi level in the cages are easy to jump from the CCB to the frame conduction band (FCB). Combination of the above experimental results suggests that the electrons in cages are easier to escape to vacuum under the action of electric field or thermal field, which is the main reason for low work function of C12A7:e-. This way provides an new approach to the realization of the insulator C12A7 converting into C12A7:e- electride. And the C12A7:e- is a good electronic emission material due to low work function, low working temperature, and highly anti-poisoning ability, so this method of preparing bulk C12A7:e- electride provides a good new way to synthesize a new electronic emission material.
Magnetotransport in antidot arrays of three-dimensional topological insulators
Three-dimensional topological insulators are a new kind of quantum matter featured with gapless Dirac-like energy-dispersive surface states in the insulating bulk band gaps. However, in experiment, it is difficult to study quantum interference effect of surface states due to considerable contribution from bulk carriers in thick bulk material. To suppress such a bulk state contribution, nanostructures, such as ultra-thin films, nanowires and nanoribbons, have been employed in the study of quantum interference effects of the surface states. Here, we report on a magnetotransport measurement study of nanoscaled antidot array devices made from three-dimensional topological insulator Bi2Se3 thin films. The antidot arrays with hundreds of nanometers in diameter and edge-to-edge distance are fabricated in the thin films by utilizing the focused-ion beam technique, and the magnetotransport properties of the fabricated devices are measured at low temperatures. The results of the magnetotransport measurements for three representative devices, denoted as Dev-1 (with no antidot array fabricated), Dev-2 (with an antidot array of a relatively large period), and Dev-3 (with an antidot array of a relatively small period), are reported in this work. Weak anti-localization indicated by a sharp peak of conductivity at zero magnetic field is observed in all the three devices. Through theoretical fitting to the measurement data, the transport parameters in the three devices, such as spin-orbit coupling length Lso, phase coherence length Lφ, and the number of conduction channels α, are extracted. The extracted Lso value is tens of nanometers, which is consistent with the presence of the strong spin-orbit interaction in the Bi2Se3 thin film. The extracted Lφ value is hundreds of nanometers and increases exponentially with temperature decreasing. It is found that the magnetotransports in Dev-1 and Dev-2 are well characterized by the coherent transport through a single conduction channel. For Dev-3, the magnetotransport at low temperatures is described by the coherent transport through two independent conduction channels, while at elevated temperatures the magnetotransport is dominantly described by the transport through one single conduction channel. Unlike the case where the transport occurs dominantly through a single conduction channel, the transport through two independent conduction channels in Dev-3 implies that at least one surface channel is present in the device.
Floating gate effect in amorphous InGaZnO thin-film transistor
In recent years, considerable attention has been paid to amorphous indium gallium-zinc-oxide (a-IGZO) thin film transistors (TFTs) for high performance flat panel display, such as liquid-crystal displays (LCDs), active-matrix organic light-emitting diode (AMOLED) display and flexible display. This is because IGZO TFTs are more suitable for pixels and circuit integrations on display panel than the conventional silicon-based devices. The merits of IGZO TFT technology include high mobility, decent reliability, low manufacturing cost, and excellent uniformity over large fabrication area. However, it was reported that the electrical characteristics of IGZO TFT are susceptible to shift after electrical aging measurement under illumination, which is caused by the activation of trapped electrons from sub-gap states to conducting states. Therefore, it is necessary to introduce light shielding layer to suppress the electrical characteristic shift under illumination aging measurements. Lim et al. demonstrated the characteristics of IGZO TFT with additional light shielding metal layer, and proved that the threshold voltage of TFT can be tuned linearly by adjusting the biasing voltage of the light shielding metal. Taking advantage of this tunable threshold voltage, AMOLED pixel circuit with a threshold voltage shift compensation function can be implemented. However, drawback of this method lies in the adding of additional biasing line, which increases the circuit area and restricts the integration of high-resolution pixel circuits. Thus, Zan et al. proposed adopting floating (unbiased) light shielding metal layer to improve the characteristics of device. However, Zeng et al. demonstrated the abnormal output characteristics of the IGZO TFT, as it cannot be saturated due to the introduction of floating light shielding metal layer. It seems that the IGZO TFT with floating metal is different from the conventional double-gate or single gate structure. To date, the current conducting mechanism of IGZO TFT with floating metal has not been discussed yet. In this paper, the distribution of electrical potential in the IGZO TFT with a cross sectional view is thoroughly analyzed. It is confirmed that the abnormal output characteristic of IGZO TFT is caused by the capacitive coupling between the floating gate and the drain electrode of the transistor. On the basis of the voltage distribution relationship between the equivalent capacitances, a threshold-voltage-dependent current-voltage model is proposed. The simulated results by technology computer-aided design tool and those by the proposed model are in good agreement with each other. Therefore, the mechanism of floating gate effect for IGZO TFT is comprehensively demonstrated. The illustrated conducting mechanism and the proposed current-voltage model are helpful in developing the device and process of IGZO TFT with novel structure.
A method of measuring micro-displacement based on spin magnetic resonance effect of diamond color center
As one of the excellent piezoelectric materials, piezoelectric ceramic has been widely used to develop a highly precise displacement measurement system, which is the key part of the scanning probe system of the high-precision measuring instrument.Based on the high-precision scanning probe system, the micro/nano structures can be easily and accurately detected by the instrument system.However, due to the limitations caused by the character of hysteresis and nonlinearity, it is difficult to further improve the precision of highly precise displacement measurement system.In this work, we present a novel method to develop the highly precise displacement measurement system based on the quantum spin effect.The nitrogen vacancy (NV) color center of single crystal diamond as a sensitive element senses the change of the micro-displacement.Based on the electron spin magnetic resonance effect of diamond nitrogen vacancy color center, the variation of the magnetic field generated from the magnetic steel can be detected with high precision by the electron spin.The relative relation between the displacement and the magnetic gradient field can be used to establish the correlation model between the displacement and the electron spin resonance peak.In the experiment, a corresponding micro-displacement measurement system is established based on the cylindrical permanent magnet, according to the correlation model between the electron spin resonance effect and micro-displacement.The linear region of magnetic field gradient is designed to detect the micro-displacement.Firstly, the intensity distribution of magnetic field gradient is measured by the gauss meter.As the measurement results show, the gradient value is -7.77 Gauss/mm along the core axis of cylindrical permanent magnet, and the intensity of magnetic field gradient distribution region is linear in the millimeter range.Meanwhile, the electron spin magnetic resonance peak of diamond nitrogen vacancy color center is achieved by the optically detected magnetic resonance technology.The electron spin magnetic resonance peak is approximately 2.79 MHz/Gauss in the magnetic field achieved by the fluorescence spectrum of diamond nitrogen vacancy color center, attributed to the relation model between Zeeman splitting effect and magnetic field.
In the experiment, the electron spin magnetic resonance signal of diamond nitrogen vacancy color center is lockedin by the demodulation method to achieve the change of micro-displacement.As the results show, the sensitivity is about 16.67 V/mm at the corresponding demodulation frequency of 3000.56 MHz.By the calculation, the resolution of micro-displacement measurement system is about 60 nm based on our method.It proves out a high precision and well reliability method to detect the micro-displacement.By the further theoretical calculation, based on the electron spin effect, the detection resolution of our method can be enhanced up to sub-nanometer scale by reducing the distance between the NV color center and the magnet.It presents a new research direction and field for the micro-displacement detection system.
Investigation on β voltaic effect based on one-dimensional TiO2 nanotube array thin film
This work is to develop a high-reliability long-life high-conversion-efficiency radio-isotope microbattery in order to meet power requirements of micro-electromechanical systems, micro-sensors, micro-actuators, wireless sensing net, and other electron devices working in harsh circumstances, such as polar, desert, subsea, outer surface, etc. Compared with traditional dry batteries, chemical batteries, fuel cells and solar cells, the radioactive isotope batteries have long service life, higher energy density, strong adaptability to environment, good work stability, no maintenance, and miniaturized size, etc. These advantages make the β voltaic battery an attractive alternative. In this paper we present a β voltaic battery with enhanced β voltaic effect by using a wide-bandgap semiconductor TiO2 nanotube array thin film. An electrochemical anodic oxidation method is used to prepare the vertically oriented and highly ordered TiO2 nanotube array film on Ti plate. Electrolyte solution consists of ammonium fluoride, ethylene glycol, and deionized water. The structure (TiO2 nanotube array with diameter about 80-100 nm, wall thickness about 15-25 nm, and length 9 μm) is characterized by field emission scanning electron microscope. The microstructure of the TiO2 nanotube array is characterized using X-ray diffraction. The effects of annealing condition on optical and electrical properties are studied. The electrical property is characterized by Keithley model 2450 source meter semiconductor characterization system in dark at room temperature. The β voltaic batteries are assembled as a sandwiched structure (63Ni/TiO2 nanotube arrays film/Ti) using a radioisotope 63Ni plate and TiO2 nanotube array films. The experimental results show that the black TiO2 nanotube array film annealed at 450 ℃ in argon atmosphere could creates high visible-ultraviolet absorption due to a great many of oxygen vacancy defects generated in TiO2 nanotube array film. The oxygen vacancy signals are found by electron spin resonance. Compared with the planar structure, the nano-porous array structure has strong absorption to β particles:most of the β particles enter into the pores and are reflected or absorbed by the surface of the tube walls. With a 10 mCi 63Ni radiation source, the β voltaic battery using black TiO2 nanotube array film can generate an open-circuited voltage of 1.02 V, a short-circuited current of 75.52 nA, and a maximum effective conversion efficiency of 22.48%.
Surface electronic structures and emission property of single crystal LaB6 typical surfaces
The electron emission properties of lanthanum hexaboride (LaB6) have received much attention because its low work function, low volatility, high brightness, thermal stability and high mechanical strength. However, single crystal LaB6 is an ideal thermionic emission and field emission cathode material, its different crystal surfaces exhibit different emission properties. So far the physical factors of the emission properties of different crystal surfaces of LaB6 single crystal have been rarely reported. In this paper, the density function theory based first-principles calculations are used to analyze the electron density differences, band structures and densities of states of the typical LaB6 (100), (110), (111), (210), (211) and (310) surfaces, and the thermionic emission properties of the high-quality single crystal LaB6 typical surfaces are tested. The theoretical calculation results show that single crystal LaB6 has metal properties, electron emission characteristics and anisotropy of emission performance which are mainly caused by different crystal structures and electronic structures of LaB6 typical surfaces. The densities of La atoms in different surfaces of LaB6 single crystal are different, and a high density of La atoms in a surface is beneficial to its emission performance. The difference between relative positions for the Fermi level of different surfaces has different effect on their emission performance, and a surface with high position of Fermi level against the bottom of conduction band could have small work function and good emission performance. In addition, a surface structure of single crystal LaB6 has a large density of states and a high number of distributions of conduction band near the Fermi level, which are in favor of its electron emission. The (100) surface of single crystal LaB6 with the highest density of La atoms and electronic structure in favor of electron emission could have optimal electron emission performance compared with the remaining crystal surfaces. Thermionic emission test results show that maximum emission current densities of the (100), (110), (111), (210), (211) and (310) surfaces are 42.4, 36.4, 18.4, 32.5, 30.5 and 32.2 A/cm2 at the cathode temperature 1773 K and the voltage 1 kV. The (100) surface of LaB6 single crystal has a maximum emission current density under the same test condition, meaning that this surface has a smallest work function and best emission property compared with the other crystal surface. The thermionic emission test results show that the actual performances are basically accordant with the calculated results, demonstrating that the first principle calculation could provide a good theoretical guidance for studying the electron emission properties of rare earth hexaborides (REB6) and other cathode materials.
Temperature dependence of spherical micelles of PS3000-b-PAA5000 studied by in-situ small angle X-ray scattering
Amphiphilic block copolymer has a character that it spontaneously self-assembles into various micellar morphologies when dissolved in selective solvents with different proportions. Amphiphilic block copolymer has wide potential applications in drug delivery such as the targeting delivery, controlled release, molecular recognition, etc. Poly (styrene)-block-poly (acrylic acid) (PS-b-PAA) is a representative amphiphilic block copolymer whose self-assembly in the selective solvents has been widely studied during the past years. Micellar morphology of PS-b-PAA sensitive to temperature, and temperature effect of PS-b-PAA are of great importance for the drug delivery. However, the micellar morphologies of PS-b-PAA have been investigated mainly at the room temperature so far. The understanding is still limited to micellar morphology of PS-b-PAA in the varying temperature processes. In the present work, an investigation of the relationship between micellar morphology of PS-b-PAA and the temperature is conducted by using in-situ small-angle X-ray scattering (in-situ SAXS). The SAXS experiments are performed on the BL19U2 beamline of Shanghai Synchrotron Radiation Facility. The energy is selected to be 10 keV and the wave length is 0.1033 nm. The two-dimensional (2D) SAXS patterns are recorded by Pilatus 1 M with a pixel size of 172 μm×172 μm. A sample-to-detector distance of 5340 mm is chosen, giving access to a range of scattering vectors q of 0.11-0.89 nm-1. The temperatures of the specimens are monitored by using a Linkam thermal stage THMS600 (Linkam Scientific Instruments). One-dimensional (1D) integrated intensity curves are obtained from the 2D SAXS patterns by employing the Fit2D software. The PS-b-PAAs (PS:PAA=3000:5000) is purchased from Sigma-Aldrich Inc and used directly (without any treatment prior to experiment). The PS-b-PAA is dissolved in solvents of N, N-Dimethylformamide and H2O with various proportions. The concentration of solution of PS-b-PAA is 10 mg/mL. The experiments show that the sizes of micelle particles in PS3000-b-PAA5000 solution are grown with water content increasing, and double scattering peaks (qpeak1=0.418 nm-1, qpeak1=0.456 nm-1) appear for the solution with 10% water. A temperature-dependent change of SAXS intensity is demonstrated by in-situ SAXS. The intensities of peak 1 and peak 2 vary in a contrary waywith the sample's warming up, but the positions of the peaks are independent of temperature. The double peaks in SAXS profiles suggest that the size of micelle particles in the solution is not homogeneous but the micelles with two close sizes coexist. It is interesting that the number of two-sized particles changes at the same rate in the heating process although there is a significant difference between the initial number and the final number of micelles.
Node importance idenfication for temporal network based on inter-layer similarity
Measuring node centrality is important for a wealth of applications, such as influential people identification, information promotion and traffic congestion prevention. Although there are many researches of node centrality proved, most of them have assumed that networks are static. However, many networks in our real life are dynamic, and the edges will appear or disappear over time. Temporal network could describe the interaction order and relationship among network nodes more accurately. It is of more important theoretical and more practical significance to construct proper temporal network model and identify vital nodes. In this paper, by taking into account the coupling strength between different network layers, we present a method, namely similarity-based supra-adjacency matrix (SSAM) method, to represent temporal network and further measure node importance. For a temporal network with N nodes and T layers, the SSAM is a matrix of size NT×NT with a collection of both intra-layer relationship and inter-layer relationship. We restrict our attention to inter-layer coupling. Regarding the traditional method of measuring the node similarity of nearest-neighbor layers as one constant value, the neighbor topological overlap information is used to measure the node similarity for the nearest-neighbor layers, which ensures that the couplings of different nodes of inter-layer relationship are different. We then compute the node importance for temporal network based on eigenvector centrality, the dominant eigenvector of similarity-based supra-adjacency matrix, which indicates not only the node i's importance in layer t but also the changing trajectory of the node i's importance across the time. To evaluate the ranking effect of node importance obtained by eigenvector-based centrality, we also study the network robustness and calculate the difference of temporal global efficiency with node deletion approach in this work. In order to compare with the traditional method, we measure the node ranking effect of different time layers by the Kendall rank correlation coefficient of eigenvector centrality and the difference of temporal global efficiency. According to the empirical results on the workspace and Enrons datasets for both SSAM method and tradition method, the SSAM method with neighbor topological overlap information, which takes into account the inter-layer similarity, can effectively avoid overestimating or underestimating the importance of nodes compared with traditional method with one constant value. Furthermore, the experiments for the two datasets show that the average Kendall's τ could be improved by 17.72% and 12.44% for each layer network, which indicates that the node similarity for different layers is significant to construct temporal network and measure the node importance in temporal network.
Plume neutralization mechanism for ultrasonically aided electrospray thruster
The ultrasonically aided electrospray thrusters (UAET) are used mainly on micro-satellites (with mass less than 10 kg). In this work, numerical simulation studies of the UAET plume field are conducted to investigate the following two problems encountered during operational tests:the avertence angle of thrust direction, which exists between the design and test outcome, and the lower energy efficiency than the established theoretical value. In order to precisely model the special physical process of the UAET plume neutralization, we develop a new hybrid model named the neutralization of electrons and charged droplets for the plume fluid field to capture the neutralization process of electrons and positively charged droplets. This model describes the dynamical movement of particles, the collision between electrons and droplets, the breakage and coalescence of the droplets, and the flow and heat transfer between the droplets and background gas. To show the feasibility and accuracy of the model, experimental tests involving thrust measurements and high-speed photography of the plume are conducted. The comparison between the test and simulation results under the same study conditions shows that the average error of this model is about 20%, and both the test and calculation exhibit a consistent trend in the various study cases. According to this model, we simulate the plume fluid field of UAET (with 2-W discharge power and 2-mA current) and identify the distribution characteristics of several parameters, including the droplet number density, charge density and the droplet volume, as well as the energy consumption categories that occur. Our model can successfully demonstrate the internal mechanisms that cause the two problems identified above. Our work will provide support for future studies of optimal design.
Path-entanglement microwave signals detecting method based on entanglement witness
In recent years, the great progress of studying the quantum entanglement has been made. In the field of optics, the great success has been achieved in quantum entanglement theory and technology. Then researchers concentrate on the microwave frequency band whose frequency is lower than that of optical frequency band. The signal in the microwave frequency band has a longer wavelength, and it has the diffraction capability that the optical signal does not possess. Furthermore, it can spread further in complex environments. Now it is possible to experimentally produce squeezed state of microwave signals and spatially separated path-entangled microwave signals. It is an important issue to judge whether the microwave signals received through dual paths are in entanglement state. In this paper, we firstly introduce the method of using squeezed state of microwave and microwave beam splitter to prepare path-entangled microwave signals. Then we use entanglement witness method to detect entanglement. Through constructing the entanglement witness operator in path-entangled microwave signals, the entanglement of path-entangled microwave signals can be effectively detected. We decompose the expression of the continuous variables path-entangled microwave signals into a large number of 2 ⊗ 2 entangled superposition states, deduce an entangled witness operator of path-entangled microwave signals based on the principle of partial transpose criterion and entanglement witnessing, and prove that the entangled witness can be used to detect the path-entangled microwave signals. Finally, we propose a physical verification of path-entangled microwave signal entanglement. The verification can be realized as follows:firstly, we reverse the phase of a received quantum-state microwave signal by utilizing continuous variable controlled phase gate in a range of 0-π, then we send two microwave signals into the two input ports of the microwave beam splitter, and we operate coincidence counting of microwave photons on the two output ports after entanglement microwave signals have passed through the microwave splitter. By analyzing the results of the whole process, we have the following conclusions:if the coincidence rate of two input signals is higher than that of non-entangled microwave signals under the same power, signals can be counted as entanglement. The proposed method can detect the entangled microwave signals more efficiently than the conventional methods, such as quantum state reconstruction, and thus reduce the detection and computational complexity. The entanglement of the two microwave quantum state signals can be observed directly by using this method. This paper provides a new idea for detecting the path-entangled microwave signals.
Quadrature squeezing of the system consisting of nitrogen-vacancy centers in diamond coupled to cavity field and mechanical resonator
With the great improvement of nanotechnology, it is now possible to fabricate mechanical resonator with dimension on a micro and even nanometer scale.Because of its high vibration frequency, quality factor, very small mass, and low intrinsic dissipation, nanomechanical resonator has important applications in the field of high-precision displacement detection, force detection, mass measurement, and accurate quantum computation.Mechanical resonator is also a promising candidate for observing quantum effects in macroscopic objects.By coupling nanomechanical resonator to other solid-state system such as optical cavity, microwave cavity, nitrogen-vacancy center (NV center) and superconducting qubits, researchers have successfully cooled the mechanical resonator to its quantum ground state, which paves the way for observing nonclassical states in resonator such as superposition state and Fock state.On the other hand, the nitrogenvacancy center in diamond has attracted more and more attention because of its advantages of long coherence time at room temperature, the ability to implement initialization and readout, and microwave control.Moreover, these NV centers can be used to detect weak magnetic field and electric field at room temperature.By using both laser field and microwave field, one can implement the manipulation, storage, and readout of the quantum information.In addition, because NV centers couple to both optical field and microwave field, they can also be used as a quantum interface between optical system and solid-state system.This provides a promising platform to study novel quantum phenomena based on NV centers separated by long distances.The nitrogen-vacancy center in diamond coupled to nanomechanical resonator can be used in precision measurement and quantum information processing, which has become a hot research topic.In this paper, we study the dynamics of quadrature squeezing of the phonon field in the system consisting of nitrogen-vacancy centers in diamond coupled to both cavity field and mechanical resonator.The effects of initial state of nitrogen-vacancy center and the coupling strength between nitrogen-vacancy center and mechanical resonator on the quadrature squeezing of the phonon field are analyzed.It is shown that the phonon field squeezed state with longtime and high-degree can be generated.The physical reason is that the mechanical resonator has the largest coherence.Moreover, the non-classical property of quadrature squeezing of mechanical resonator can be achieved by manipulating the initial state of nitrogen-vacancy center and magnetic field gradient.The proposal may provide a theoretical way to control and manipulate the quadrature squeezing of the phonon field.The results obtained here may have great significance and applications in the field of quantum information processing and precision measurement.
Lattice scattering in n-type Ge-on-Si based on the unique dual-valley transitions
Silicon-based light emitting materials and devices with high efficiency are inarguably the most challenging elements in silicon (Si) photonics. Band-gap engineering approaches, including tensile strain and n-type doping, utilized for tuning germanium (Ge) to an optical gain medium have the potential for realizing monolithic optoelectronic integrated circuit. While previous experimental research has greatly contributed to optical gain and lasing of Ge direct-gap, many efforts were made to reduce lasing threshold, including the understanding of high efficiency luminescence mechanism with tensile strain and n-type doping in Ge. This paper focuses on the theoretical analysis of lattice scattering in n-type Ge-on-Si material based on its unique dual-valley transition for further improving the efficiency luminescence of Ge direct-gap laser. Lattice scattering of carriers, including inter-valley and intra-valley scattering, influence the electron distribution between the direct Γ valley and indirect L valleys in the conduction of n-type Ge-on-Si material. This behavior can be described by theoretical model of quantum mechanics such as perturbation theory. In this paper, the lattice scatterings of intra-valley scattering in Γ valley and L valleys, and of inter-valley scattering between the direct Γ valley and L valleys in the n-type Ge-on-Si materials are exhibited based on its unique dual-valley transition by perturbation theory. The calculated average scattering times for phonon scattering in the cases of Γ valley and L valleys, and for inter-valley optical phonon scattering between Γ valley and L valleys are in agreement with experimental results, which are of significance for understanding the lattice scattering mechanism in the n-type Ge-on-Si material. The numerical calculations show that the disadvantaged inter-valley scattering of electrons from the direct Γ valley to indirect L valleys reduces the electrons dwelling in the direct Γ valley slightly with n-type doping concentration, while the strong inter-valley scattering from the indirect L valleys to indirect Γ valleys increases electrons dwelling in the direct Γ valley with n-type doping concentration. The competition between the two factors leads to an increasing electrons dwelling in the direct Γ valley with n-type doping in a range from 1017 cm-3 to 1019 cm-3. That the electrons in the indirect L valleys are transited into the direct Γ valley by absorbing inter-valley optical phonon modes is one of the effective ways to enhance the efficiency luminescence of Ge direct-gap laser. The results indicate that a low-threshold Ge-on-Si laser can be further improved by engineering the inter-valley scattering for enhancing the electrons dwelling in the Γ valley.
Performance analysis of a refrigerator operating between an infinite-sized hot reservoir and a finite-sized cold one within linear irreversible thermodynamics
How to optimize the performances of heat devices operating between finite-sized heat sources and sinks has become a very important issue in the field of finite-time thermodynamics. In this paper, a physical model of the refrigerator operating between an infinite-sized hot reservoir and a finite-sized cold one is proposed, and by using the principles of finite-time thermodynamics and the theory of linear irreversible thermodynamics, we present the analytical expressions of the input power and the coefficient of performance (COP) under the tight-coupling condition, and analyze the performance characteristics of the refrigerator in detail. When the temperature of the cold reservoir is changed with fixing the environment temperature (the temperature of the hot reservoir), it is found that there does not exist a well-defined optimal relation between the input power and a duration time τ of the refrigerating process, which is a remarkable difference from the working process of a heat engine operating between a finite-sized hot reservoir and an infinite-sized cold one. We further find that the COP exhibits the monotonically decreasing trend with the increase of the input power, but the increase of the exergy leads to the enhancement of the COP. This feature can be understood as follows:when P is small, this means that the duration time τ is large, thus the refrigerating process approaches to the quasistatic operation, which induces the large COP. In particular, when P→0, the COP ε→εmax. The increase of P implies the reduction of τ, thus the refrigerating process keeps away from the quasistatic process and approaches to the actual irreversible process, which causes the COP to decrease. On the contrary, ε shows the increasing trend with the increase of the exergy E. This is because the increase of E means the enhancement of τ at fixing the input power P, which corresponds to a slow refrigerating process. As a result, E exhibits the increasing behaviors due to the emergence of the quasistatic process. From the above analyses, we can find that an appropriate proposal to optimize the refrigerating performance of heat devices should be based on the actual parameters and the real external environment, thus it is possible to obtain the optimal refrigerating objective at the expense of the suitable input power. These results are not only helpful in the in-depth understanding of the refrigerator operating between an infinite-sized hot reservoir and a finite-sized cold one, but also of great engineering interest in designing realistic heat deices. Our method can also generalize the investigation of heat pumps. In addition, when the tight-coupling condition is false due to the breaking of time-reversal symmetry, there needs to be further consideration about it from the angle of physics.
A method of reducing false signature in spatially-modulated snapshot imaging polarimeter
Spatially-modulated snapshot imaging polarimeter can encode four Stokes parameters (S0, S1, S2 and S3) into a single interferogram and allow the instantaneous measurement of polarization from a single snapshot.However, the reconstructed polarization information contains aliasing signal, and the reconstructed intensity images suffer low spatial resolution because of the crosstalk between high frequency components of the image and frequency domain filtering for the polarization channels.In this paper, we propose an image superposition and subtraction method to mitigate the aliasing problem and to recover the image resolution.The two interferograms acquired from two snapshot measurements are superposed to obtain the intensity image (S0 component) of an object without the polarization components because the phases of the polarization components in the two interferograms are opposite.In comparison with the intensity of each of the original interferograms, the intensity of S0 component increases twice and its spatial resolution improves up to a maximum value offered by the instrument.Then a subtraction between the two interferograms is performed to derive the pure interference fringes while the intensity image vanishes.The intensity of the pure interference fringes also increases twice compared with that of each original interferogram because phases of the interference terms in original interferograms are opposite.The polarization images (S1, S2 and S3 components) can be reconstructed from the pure interference fringes, and do not include crosstalk signals between the high frequency components of the intensity image. The theoretical basis of the method is presented through a detailed analysis.Its feasibility is verified by both computer simulation and experiment.The simulation results show that the otherness and the structural similarity index between the input and reconstructed intensity images is zero and 1, respectively, indicating a perfect reconstruction of S0.The results also make it clear that the pure interference fringes do not include any component of intensity image, and thus the reconstructed polarization information does not contain any crosstalk signals.Moreover, the experimental results are in accordance with the theoretical expectation and the computer simulations.This research provides a novel means for spatially-modulated snapshot imaging polarization technology to obtain full-resolution object images and high-quality reconstructed polarization information.
Potential energy curves and transition properties for the ground and excited states of BH+ cation
BH+ cation is one of the candidates for laser cooling. The potential energy curves (PECs) for nine electronic states (X2Σ+, A2Π, B2Σ+, a4Π, b4Σ+, 32Σ+, 22Π, 32Π, 42Σ+) relating to the B+(1Sg)+H(2Sg), B+(3Pu)+H(2Sg), B(2Pu)+H+(1Sg), and B+(1Pu)+H(2Sg) dissociation channels of BH+ cation are obtained using highly accurate multi-reference configuration interaction (MRCI) plus Davidson correction. All-electron basis sets AV5Z-DK for H and ACV5Z-DK for B are used in PEC calculations for the Λ-i-S states of BH+ cation, respectively. In complete active space self-consistent field (CASSCF) calculation, H(1s2s2p3s3p) and B(2s2p) are chosen as active orbitals, B(1s) is the closed shell; in the MRCI calculation, the core-valence (CV) correction is considered, i.e., B(1s) shell is used for CV correlation. Spin-orbit coupling effects are considered with Breit-Pauli operators. Spectroscopic constants are fitted using the Murrell-Sorbie function. Spectroscopic constants for the X2Σ+, A2Π, and B2Σ+ states are in excellent agreement with the available experimental data; spectroscopic constants for the b4Σ+, 32Σ+, 32Π, and 42Σ+ states are reported. Two potential wells for the 32Π and 42Σ+ states are found. The maximum fitting error of all electronic states is only 3.407 cm-1. In addition, PECs for the A2Π and B2Σ+ states are crossed at about 2.7 Å. Then, the transition dipole moments (TDMs) for the A2Π→ X2Σ+, B2Σ+→X2Σ+, 32Σ+→X2Σ+, B2Σ+→ A2Π, 32Π→ X2Σ+ and b4Σ+→ a4Π transitions are also obtained. The strength for the B2Σ+→ A2Π transition is very weak. Based on the accurate PECs and TDMs, the Franck-Condon factors and spontaneous radiative lifetimes are calculated. A strongly diagonal Franck-Condon factor (f00) for the A2Π→X2Σ+ transition is obtained, which equals 0.9414. Spontaneous radiative lifetime for the A2Π and B2Σ+ states is also predicted. i.e., τ (A2Π)=239.2 ns and τ(B2Σ+)=431.2 ns. When SOC effect is considered, the A2Π1/2 and B2Σ1/2+ states avoid crossing in the Franck-Condon region (R is about 2.7 Å). Calculated f00 for the A2Π1/2→ X2Σ1/2+ transition is 0.9430; spontaneous radiative lifetime for the A2Π1/2 is 239.0 ns. Our calculated results indicate that the influence for laser cooling BH+ cation via the crossing between B2Σ+ and A2Π states can be ignored.
Three-body fragmentation dynamics of OCS3+ induced by intermediate energy Ne4+ ion impact
The fragmentation experiment of OCS3+ induced by 56 keV/u Ne4+ ions is performed using reaction microscope, and the corresponding dissociation dynamics is investigated. By detecting the three fragment ions in coincidence, the three-dimensional (3D) momenta of all ions and the corresponding kinetic energy release (KER) distributions are reconstructed. It is found that a peak maximum of the KER distribution is locates at about 25 eV, and a shoulder structure appears around 18 eV. This result is consistent with previous heavy ion experimental results with different perturbation strengths. Taking into account that the KER distribution is related to the initial state population of the OCS3+ parent ions, it can be concluded that the perturbation strength is not a decisive parameter leading to the initial state population of OCS3+ ions. We also reconstruct the Newton diagram and Dalitz plot for the three-body fragmentation of OCS3+ ion, from which the sequential dissociation is distinguished from nonsequential dissociation clearly. By analyzing the kinetic energy of ions from each fragmentation process, we find that the KER peak at 25 eV corresponds to nonsequential dissociation process, but the shoulder at 18 eV arises from both sequential and nonsequential dissociation processes. This phenomenon suggests that the parent OCS3+ ions in ground state and low excitation states tend to fragment through sequential dissociation, while those in high excitation states tend to fragment through nosequential dissociation. Furthermore, we reconstruct the KER distributions in the second fragmentation step of sequential dissociation, whose peak maximum is at 6.2 eV, corresponding to X3Π, 1Σ+ and 1Π metastable states of CO2+ ion. A similar KER distribution is obtained for the second fragmentation step of the OCS4+ ion. By comparing our experimental results with previous ones, we conclude that the origin of sequential dissociation process is the existence of metastable state, and the reconstructed KER in the second step reflects the initial state information about the metastable state.
Experimental investigation on diagnosing effective atomic density in gas-type target by using proton energy loss
The investigations of interaction processes between ion beams and gas and between ion beams and plasma play important roles in atomic physics, astrophysics, high energy density physics, and inertial confinement fusion.The atomic density of target is one of the key experimental parameters which may determine the interaction mechanism and experimental results.How to precisely diagnose the atomic density of target in different matter states, like gas phase and plasma phase, is challenging work on the experiments in laboratory.Conventionally the vacuum gauges are used to measure the pressure inside the gas target, but the accuracy is limited for a complex target system and they can hardly work in a strong radiation surrounding, especially in plasma where the high temperature can physically damage the gauges.Therefore we propose a new method to measure the atomic densities for both gas target and plasma target based on the heavy ion beam accelerator facility at the Institute of Modern Physics, Chinese Academy of Sciences.In our experiment the protons are extracted from an electron cyclotron resonance ion source (ECRIS) and accelerated to 100 keV then transmitted to the target.A two-stage differential pumping system is constructed to keep 10-7 mbar order of magnitude in beam line when the gas is filled into the target area where the pressure could increase to higher than 1 mbar.A 45° dipole magnet is used to bend the protons which have passed through the gas.The energy is determined by the different positions of protons at the position-sensitive detector which is placed at the end of magnet.Consequently the energy losses of protons at different pressures are obtained.There have been proposed many theories for calculating the energy loss of protons in gas, and we chose the very popular code named SRIM to simulate the experimental case. Finally the effective linear atomic density of target along the ion beam trajectory in the target area is obtained.For comparison, the conventional vacuum gauges (one is the hot cathode gauge-IonIVac ITR 90 and the other is capacitance diaphragm gauge-Varian CDG-500) are simultaneously used in the experiment.The results show that the recalibrated effective pressure obtained by the energy loss is close to the pressure measured by Varian CDG-500 but much lower than the pressure from IonIVac ITR 90.Only after the detection efficiency correction, could the corrected results of IonIVac ITR 90 be coincident with the effective pressure obtained according to energy loss.Moreover we find that the effective atomic density determined by the protons energy loss shows that these advantages over the conventional gauges are not only the high accuracy and reliability but also the in-situ measurement, high temporal resolution and the ability to work in the complex radiation and hot plasma environment.These properties may play a great role in the experimental researches and relevant topics.
Beat frequency error rectifying in multi-beam laser coherent remote tmaging
Coherent imaging with a multi-beam laser is considered as a key technique in ground based imaging. The image quality is directly determined by stability and consistency of each beam in transmitter. Although the stabilities of laser frequency and the drifting compensation methods have been studied previously, they mostly focused on the laser source. In most cases, especially in large transmitter array, however, transmitted beams are always disturbed by different influential factors, such as frequency drift induced by acoustic-optical modulation (AOM) and high power driven amplification. Therefore this kind of frequency drifting needs further rectification. Aiming at this problem, in this paper we propose two new methods called dynamic demodulation and dependence range demodulation. Firstly, the dynamic demodulation takes the whole drifting frequency drift as a changing procedure. It is believed that the beat frequency drifted at any position still carries the target information, so the system demodulates the signal at that drifted position. According to this method, the response speed of the demodulation system should be very high. But in a real system this acquisition is too high to be satisfied. It cannot work as quickly as expected. In computer simulation some slow varying drifts are induced at the beat frequency and the variation is distributed only in three parts of spatial frequency of transmitter interfering array. Simulation results show that this method may well compensate for slow drifting beat frequency. While its response speed is often limited by hardware system. On the other hand, for the dependence range demodulation, the beat drifting range is considered as a useful district, in which all the beat energy is added and demodulated at a preset position. An experiment is carried out to verify this method. The result demonstrates that it can well restrict the beat frequency drift within 100 Hz, which often happens in the procedure of AOM and driving amplification. Besides the laboratory setup research, the field experiments in 200 m and 1.5 km range are also carried out. The dependence range demodulation is proved to be well performed as well. The resolution of the 25 cm simulated target in 1.5 km reaches 0.008 rad. In the consideration of real system, the imaging range is further expanded and the amplifier power is stronger. The field experiments reveal that this demodulation method is applicable in such a condition. Therefore the research in this article provides some new techniques for the remote high resolution imaging in multi-beam laser interfering imaging.
Adaptive imaging by incoherent digital holography based on phase change
Fresnel incoherent correlation holography (FINCH) is a unique scanning-free three-dimensional imaging technique which enables holograms to be created from incoherent light illumination. However, the image quality is inclined to be destroyed by various optical aberrations, in the practical application of microscopic imaging. In order to solve this problem, some kinds of adaptive optics are combined with imaging technologies to detect the distorted wavefront and compensate for the aberrations. Phase diversity is an image-based adaptive optics method where two intensity images with a certain phase difference are used for wavefront sensing. In this paper, we develop an adaptive imaging technique by Fresnel incoherent digital holography combined with phase diversity (PD-FINCH). Two recorded phase-shift holograms are applied to wavefront sensing, and the phase of aberration is further extracted by phase diversity reconstruction algorithm. The compensation phase is uploaded on SLM in turn, thus the aberrations are corrected while recording holograms. Both the simulation and experimental results verify the validity of phase diversity in FINCH. All the results show the improvement of reconstructed image quality after wavefront aberration compensation.
Unconventional phonon blockade in multimode optomechanical system
In this paper, we proposed to observe a phonon blockade in multimode optomechanical system. The multimode optomechanical system is consisting of one mechanical mode driven by a weakly mechanical field and two optical modes driven by two optical fields (a weak one and a strong one). Under the interaction of the strong optical driving field, the multimode optomechanical system can be reduced to a much simple model for a mechanical mode linearly coupled to an optical mode with Kerr nonlinearity. Our calculations show that strong phonon antibunching effects can be observed even with weak optomechanical coupling. This counter-intuitive phenomenon, i.e., unconventional phonon blockade, results from the destructive interference between different paths for two-phonon excitation and the optimal conditions for unconventional phonon blockade are obtained analytically. Moreover, the statistical properties of the phonons can be controlled by regulating the strength ratio and the relative phase between the weakly driving fields, and this provides us an effective way to realize tunable single-phonon sources. Finally, we show that the thermal phonons have a detrimental impact on the unconventional phonon blockade and a proper increase of the strengths of the weakly driving fields can be helpful to overcome the detrimental impact induced by the thermal phonons.
Laser emission characteristics of the capillary of dye-doped liquid crystal
The dye doped liquid crystal filling tunable laser has been widely adopted in many areas, such as optical communication, sensor and medical imaging with a low cost. The temperature-sensitive refractive indice of liquid crystal makes it a filling material suitable for being used in the capillary. The existing studies have introduced the liquid crystal filled with capillary, which has the complicated craft and big cost. As is well known, the capillary has the advantages of the easy preparation and low cost, but the liquid crystal filled capillary based dye doped liquid crystal filling tunable laser is rarely studied.
Dye-doped cholesteric liquid crystal (CLC) based tunable laser has many advantages such as small-size, low-threshold, high-efficiency, wide-tunability with wavelength varying from ultraviolet to infrared. So It shows great promise in applications of single-chip experiment, biological identification and sensor. To develop high-efficiency dye-doped CLC tunable lasers for different potential applications, it is crucial to explore their emission performances in three laser emission modes:distributed feedback (DFB), whispering gallery modes (WGMs) and random laser (RL). We theoretically propose and experimentally demonstrate the characteristics of laser emission based on dye-doped CLC in capillary tubes which are treated with the photo-alignment PI films. Firstly, we prepare capillary tubes filled with dye-doped CLC with three inner diameters of 100 μm, 200 μm and 300 μm. By using a double-frequency Nd:YAG 532 nm laser as a pump source, the emission spectra, energy thresholds and temperature dependent tunabilities in the cases with and without PI films are analyzed, respectively. It is clearly shown that dye-doped CLC in the capillary with the PI films generate DFB-mode lasing and WGMs lasing. Experimental results show that the capillaries with thinner-inner diameters and PI films have lower emission threshold energies than without PI films, the former threshold can be reduced to as low as 4.5 μJ · mm-2. Meanwhile, with temperature increasing, the DFB wavelength is blue-shifted, resulting in a central wavelength tuning range of 5.9 nm. Then high performance WGM with an FSR of 1.05 nm is created when the temperature is increased up to as high as 43 ℃. It can be found that the laser emission with photo-alignment PI films shows an optimum RL mode with less laser emission peaks than the laser emission without photo-alignment PI films. In this work we propose and demonstrate that a capillary based dye-doped CLC tunable laser with photo-alignment PI films can easily work with three emissions:DFB-mode, WGMs or RL by changing optical field and the applied temperature. The above research results provide valuable clues and methods to develop high-quality dye-doped CLC based tunable laser, filter, optical switch and sensor.
Numerical experiment studies of clogging during the discharge of granular matter in a three-dimensional hopper
For a granular flow in hopper in engineering and experimental applications, it is necessary to guarantee the discharge continuously and steadily. The clogging will easily happen if the outlet size is small enough via formation of the arch above the outlet. The clogging phenomenon is also important for studying traffic or evacuation problems. In previous numerical and experimental study, to expedite the experiments or simulations, the perturbations, such as a jet of pressurized air or the vibration of the wall of the hopper, were induced to break the clogging and restart the flow. But these perturbations are hardly normalized and described in modeling the process. In this paper, we present a series of numerical experiments of clogging in the discharge of particles from a three-dimensional hopper through a circular opening. We employ our discrete element method simulation code for large scale dense granular flow based on the graphic processing unit to expedite this simulation. In contrast to pervious studies, here we study the first clogging after opening the outlet of hopper, thus the above perturbations are avoided. From simulating granular flow in hopper in a wide range of outlet size and cone angle, we obtain the size of distribution of avalanche, which is defined as the number of particles that fall through the opening from the outlet opening to the first clogging. The effects of the outlet size and cone angle of hopper on avalanche size are investigated and discussed. The results show that the previous conclusion of the distribution of possibility of avalanche size is also valid in this study. There is a peak in the distribution of possibility of avalanche size, and the distribution can be divided into two regions, which can be fitted with a power-law and an exponential function respectively. The exponential part can be explained by a possibility model which is suggested by Janda et al. From the fitting we find that it has a critical value for the outlet size above which no clogging will occur and the value in this work (4.75d) is slightly lower than in Zuriguel et al.'s experiment (4.94d). Moreover, there is also a critical value for the cone angle of hopper, which supports the inference in previous study and the value in this paper (77°) is closed to the predicted one (75°) in To et al.'s work.
A thermodynamic model of grain-grain contact force
The starting premise of any soft discrete element method simulation, widely used in granular physics and granular mechanics, is the modelling of grain-grain contact force. Most of models often used in the literature including the famous ones by Hertz-Mindlin and Luding, do not present the algorigthy of total elastic potential, or the rate of dissipation which is mainly due to the partially frictional character of the forces. This renders the question of thermodynamic consistency unsettled. A model that possesses explicit expressions for both is proposed here. It is conceptually closely related to the continuum-mechanical theory of granular solid hydrodynarmics (GSH). This theory contains expressions for the total elastic potential and the thermal energy, it accounts for energy conservation and the positivity of entropy production, and it clarifies the equilibrium properties of granular media. All these are lacking (or hidden) in the contact models widely used in the literature. A preliminary calculation shows that the restitution coefficient varies with the impact velocity, which is an added bonus, and demonstrates the model's increased realism. For simplicity, the equations presented in this work are limited to the 2D-case and neglect granular rotations. Nevertheless, the generalization to the 3D-case and the inclusion of granular rotations are carefully discussed, clarifying how to treat rolling and the torsional forces in a thermodynamically consistent fashion. A key point of the present approach, and the major difference to other force models, is the fact that, starting from the characteristic thermodynamic potential, we employ the Onsager reciprocity relation to set up the transport coefficients. The contact forces (usually postulated) are then derived from them. This difference is both conceptually and methodologically relevant. We discussed in detail off-diagonal transport coefficients, especially the so called “gear ratio” that is particular to granular matter. It reflects the difference between the elastic and the total strain, and is closely related to the slip movement of contact surface, which occur during shear, rolling and torsional deformations. It is relevant to both the macroscopic GSH scales, and the mesoscopic granular scale.
Three-dimensional PIC/MCC numerical study on the initial process of streamer discharge in a needle-plate electrode in atmospheric air
Streamer, which usually appears at the initial stage of atmospheric pressure air discharge, acts as a precursor of lightning. It also occurs as large discharges (called sprites) in upper atmosphere, far above the thundercloud. The streamer discharge has many potential applications in industry, such as gas or water cleaning, ozone generation, assisted combustion, etc. The streamer discharge is difficult to investigate both experimentally and computationally, because of its non-linear and multi-scale characteristics. Various studies on streamer discharge have been carried out, and some progress has been made. However, some things remain to be further understood, i.e., the law of particles motion and the factors influencing streamer discharge. In this paper, we use a pre-established three-dimensional (3D) particle model (PIC/MCC) to study streamer discharge with a needle-plate electrode in air. To simplify the condition, we only use nitrogen-oxygen mixture to represent dry air, regardless of other components such as CO2, H2O gases, etc. In this model, we take photoionization, attachment and detachment processes into account. The adaptive mesh refinement and adaptive particle weight techniques are used in the code. In order to facilitate the simulation, we artificially put a Gaussian seed right on the top of the needle electrode. We adjust some computational parameters to analyze how the streamer discharge starts and evolves from the needle electrode. Many factors can influence streamer discharge during its evolution, from among which we choose three important parameters:voltage amplitude, gas component, and the radius of curvature of the needle electrode tip, to study the generation and evolution of streamer discharge, and focus on inception cloud, streamer branches, and electric fields. The simulation results show that the radius of inception cloud increases with the increase of voltage amplitude, and the diameter of steamer channel and the number of branches also increase with voltage increasing. We choose 4 kV as a proper simulation voltage for next two parts of simulations. By comparing the results obtained in the cases of different gas components (pure oxygen and different ratios of nitrogen-oxygen mixtures), we discover that the nitrogen-oxygen mixture ratio significantly affects the total number of streamer branches. With 0.1% oxygen, discharge grows irregularly with small protrusions on streamers. In the pure oxygen case, streamer seems to have much more thin branches than in other cases. Needle geometry directly changes the inception cloud of the streamer and its morphology, especially when the tip becomes blunter. In this circumstance, electric field strength around the electrode decreases, and inception cloud can be barely seen. Instead, a single-channel streamer discharge develops right toward the plate electrode, later this single-channel streamer splits into branches.
Characteristics of meso-pressure six-phase alternative current arc discharge plasma jet: Experiments
During the re-entry process of a supersonic vehicle in near space, the interaction between the flying vehicle and surrounding air is violent due to the hypersonic relative speed.As a consequence, the shock-heated air in the vicinity of the vehicle surface is ionized.Thus, the formed plasma layer operates in strong collision, non-uniform and nonequilibrium states.One of the serious system operation problems resulting from this non-equilibrium plasma layer is the so-called communication blackout.Physical simulation of the near-space plasma environment in laboratory based on various plasma sources is a much lower cost method than the in-situ measurements in the vehicle re-entry process.In this paper, based on the ideas for designing the dual jet direct current arc plasma and the muti-phase alternating current discharge plasma, a physical design on the multi-phase alternating discharge apparatus is proposed for generating a large volume plasma arc-jet.And a multi-phase gas discharge plasma experimental platform-2015(MPX-2015) is established with the image recording/processing, electrical and optical emission spectroscopy measurement system in this laboratory. The preliminary experimental observations show that under a typical operating condition with a 500 Pa background pressure, a large volume plasma jet with a maximum diameter of 14.0 cm and a maximum length of 60.0 cm is obtained on this newly developed platform.The influences of the gas flow rate, the chamber pressure, the electrode gap spacing and the arc current on the characteristics of the plasma free jet and impinging jet are also studied.The experimental results show that within the parameter ranges studied in this paper, the chamber pressure has a very significant influence on the size of the plasma jet, i.e., both the diameter and length of the plasma free jet increase with chamber pressure decreasing, and a similar variation trend is also observed for the thickness and length of the plasma layer surrounding a bluff body.In addition, the size of the plasma layer also increases with the increase of the plasma working gas flowrate and the discharge current.These results are helpful in the more in-depth investigating of the aerodynamic heat effect and blackout issue of the re-entry process of supersonic vehicle in near space in future.In the future research, we will modify the structures of the plasma generators in order to obtain supersonic plasma arc-jets, and study both the quasi-steady and transient characteristics of the arc plasmas, as well as the strong interactions among the plasma jet, the surrounding air and the solid bluff body.
Progress of laser-driven quasi-isentropic compression study performed on SHENGUANG III prototype laser facility
The equation of state for solid at extreme pressure and relatively low temperature is an important topic in the study of astrophysics and fundamental physics of condensed matter. Direct laser-driven quasi-isentropic compression is a powerful method to achieve such extreme states which have been developed in recent years. A lot of researches have been done in Research Center of Laser Fusion in China since 2012, which are introduced in this article. The researches include an analytical isentropic compression model, a developed characteristic method, techniques for target manufacture, and experiments performed on SHENGUANG Ⅲ prototype laser facility. The analytical isentropic compression model for condensed matter is obtained based on hydrodynamic equations and a Murnaghan-form state equation. Using the analytical model, important parameters, such as maximum shockless region width, material properties, pressure pulse profile, and pressure pulse duration can be properly allocated or chosen, which is convenient for experimental estimation and design. The characteristic method is developed based on a Murnaghan-form isentropic equation and characteristics, which can be used for experimental design, simulation, and experimental data processing. Based on the above researches, several rounds of experiments have been performed to obtain better isentropic effect by upgrading the target configurations. Five kinds of target configurations have been used up to now, which are three-step aluminum target, CH-coated planar aluminum target, CH-coated three-step aluminum target, planar aluminum target with Au blocking layer, and three-step aluminum target with Au blocking layer. The rear surface of three-step aluminum target is found to be destroyed when the loading pressure rises up to 194 GPa, and weak shock appears in CH-coated planar aluminum target and CH-coated three-step aluminum target. Besides, velocity interferometer system for any reflector (VISAR) fingers are found to decrease when the pressure rises up to about 400 GPa and disappears at 645 GPa. By reducing laser intensity, the whole interface velocities on three steps are obtained in the CH-coated three-step aluminum target and a stress-density curve is calculated. In order to eliminate the weak shock, the target configurations are upgraded by changing the ablation layer and putting a gold blocking layer after it. The experimental results show that the weak shock is eliminated and much clearer VISAR fingers are obtained when pressure rises to as high as 570 GPa.