Accepted
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Abstract +
When high-energy heavy ions beam is incident into solid material, the energy deposition density along the ions flight path can change the macroscopic target temperature and pressure, and may create new material defects under such high-pressure and high-density conditions. To accurately control the extreme state in material generated by heavy ions beam, it is necessary to conduct detailed research on the energy deposition density of ions and figure out the new potential defects in matter. This paper reports the new experiment at the HIRFL-CSR at Lanzhou, where 264 MeV/u Xe36+ions beams are extracted to irradiate a LiF crystal target. The emission spectrum of the LiF was measured in-situ. Moreover, the changes in crystal color along ions path are observed (shown as Fig.1), and XRD (X-ray Diffraction) as well as XPS(X-ray photoelectron spectroscopy) are applied to predict the potential new phases at different positions of crystal through the target dissociation method. It is apparent that in No.3- front(the red line）a new phase around 52.6 degree is found in XRD result, which is believed as LiF3 (LiF+F2) structural phase and appear in the Bragg peak region of Xe ions in LiF. Furthermore, to verify this result, a similar experiment was done by using 430MeV/u 84Kr26+ions beam, and the stacked layered LiF target was analyzed after the irradiation. XPS result shows more complex defects aggregates in the Bragg peak region of Kr ions in LiF at room temperature. In previous study, such complex defects all generated under high temperature conditions. We figure out that these complex defects can be produced around the Bragg peak region of ions in LiF at room temperature, where a temporally high temperature and high pressure condition could be generated. This paper can provide some experimental evidences and references for the target material modification in high-energy density physics research driven by heavy ions beam.
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Perovskite is a material with excellent photovoltaic properties, and the efficiency of perovskite solar cells has increased rapidly in recent years. By utilizing the adjustable bandgap characteristics of perovskite materials, wide-bandgap perovskite solar cells can be combined with narrow-bandgap solar cells to prepare tandem solar cells. Tandem devices can improve the utilization of the solar spectra and achieve higher power conversion efficiency. An important prerequisite for preparing efficient photovoltaic devices is to fabricate high-quality perovskite active layers. Antisolvent-assisted spin-coating is currently a commonly used method for preparing high-quality perovskite films in the laboratory. However, the low solubility of inorganic cesium and bromine salts in the preparation of wide-bandgap perovskite thin films leads to a fast crystallization rate, poor crystallization quality and a large number of defects, seriously reducing the photovoltaic performance of the devices. In addition, the antisolvent has a narrow working window, which is not conducive to the preparation of large-area perovskite films. In this work, a mild gas quenching process was used to assist the spin-coating method for the preparation of wide-bandgap perovskite films, and propylamine hydrochloride was introduced as an additive to improve the crystallization quality and uniformity of large-area preparation of perovskite films. The interaction between the propylamine cation and the perovskite component produced a two-dimensional perovskite phase. The perovskite component used two-dimensional phases as growth templates to reduce the formation energy of α-phase perovskite, which favored uniform nucleation and preferred orientation growth of perovskite, increasing grain size and reducing grain boundaries within the film. The improvement of the crystalline quality of the perovskite film reduced the defect density inside the film and suppressed the non-radiative recombination of the photogenerated carriers. The perovskite solar cell with a bandgap of 1.68 eV prepared using this strategy achieved a power conversion efficiency of 21.48%. In addition, the 8×8 cm2 wide-bandgap perovskite films prepared by this method exhibited good uniformity. This work provides a strategy for the process development of efficient and large-area perovskite photovoltaic devices.
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How to characterize thermodynamic non-equilibrium characteristics of flow field accurately and reliably is the key to solve the thermal and chemical non-equilibrium problem, which is one of the most basic scientific problems in hypersonic aerodynamcis. Based on the principles of Coherent Anti-Stokes Raman Scattering (CARS) and Modified Exponential Gap (MEG) Raman linewidth model, a CARS spectral computation and vib-rotational temperature inversion program is proposed for characterizing the thermodynamic non-equilibrium properties of high-temperature gas flow field. The influence of vibrational and rotational temperatures on Raman linewidth and CARS spectral characteristics is studied theoretically. A CARS system is built and the corresponding accuracy over a wide temperature range is verified in a static environment that is established using a high-temperature tube furnace and a McKenna burner. The result show that the average relative deviation of the vibration temperature Tv and rotational temperature Tr from the equilibrium temperature Teq are 4.28% and 3.34% respectively in the range of 1000K to 2300K, and the corresponding average repeatability are 1.95% and 3.03% respectively. These results indicate that the vibrational and rotational temperatures obtained by the non-equilibrium program are in good agreement with those obtained by the thermal equilibrium program. Finally, a non-equilibrium microwave plasma flow is built and its vibrational and rotational temperatures are measured using the developed program. The result show that the microwave plasma is in thermodynamic non-equilibrium, and the vibrational temperature and rotational temperature are proportional to microwave power, while the thermodynamic non-equilibrium degree exhibits the opposite trend. With microwave power increasing from 80W to 180W, the vibrational temperature of plasma increases from 2201 ±43 K to 2452 ±56 K, the rotational temperature increases from 382 ±20 K to 535 ±49 K. The principal reasons are that, the increase in microwave power leads to an increase in electron number density, and neutral particles obtain energy through collision with electrons, resulting in an increase in vibrational temperature, rotational temperature, and translational temperature. The thermodynamic non-equilibrium degree decreases from 0.83 to 0.78 with the microwave power increasing is due to the V-T relaxation rate increasing. The molecules in the vibrational excited states lose energy through collision with ground state molecules (i.e. V-T relaxation process), resulting in vibrational energy being converted into translational energy. For N2 molecules, the V-T relaxation rate is directly proportional to the temperature, which leads to a decrease in the difference between vibrational and rotational temperatures with increasing microwave power, leading to a decrease in non-equilibrium degree with increasing microwave power.
, , Received Date: 2024-03-26
Abstract +
The quantum restriction effect of charge carriers in two-dimensional materials can significantly improve their power factors. MXene, as a new type of two-dimensional double transition metal material, has attracted extensive attention due to thermoelectric properties, and higher controllability than single transition metal MXene, which has potential applications in thermoelectric devices. In this work, new two-dimensional monolayer double transition metal MXene, i.e. TiZrCO2 and VYCO2, are designed and their stabilities, electronic and thermoelectric properties are studied by the first principles and Boltzmann transport theory. It has been shown that both are indirect bandgap semiconductors with mechanical, thermodynamic and kinetic stability, and their thermoelectric properties (Seebeck coefficients, electrical and electronic thermal conductivities and lattice thermal conductivities) in a temperature range from 300 K to 900 K are studied. For the optimal carrier concentration at 300 K, the p-type TiZrCO2 power factor is 11.40 mW/(m·K2), much higher than that of n-type one, and the VYCO2 power factor of p-type (2.80 mW/(m·K2)) and n-type (2.20 mW/(m·K2)) are similar to each other. At 300 K, TiZrCO2 and VYCO2 have low lattice thermal conductivities of 5.08 W/(m·K) and 3.22 W/(m·K), respectively, and the contributions of optical phonon to the lattice thermal conductivity are both about 30%, i.e. 2.14 W/(m·K) and 1.09 W/(m·K) at 900 K, respectively. At the same time, it is found that at 300 K, when the material size of TiZrCO2 and VYCO2 re within 12.84 nm and 5.47 nm respectively, their lattice thermal conductivities are almost unchanged, and can be adjusted by adjusting the compositions. At 900 K, the thermoelectric value of p-type TiZrCO2 and VYCO2 reach 1.83 and 0.93, respectively, which are better than those of n-type, 0.23 and 0.84. The double transition metals MXene TiZrCO2 and VYCO2 have better thermoelectric properties than the single transition metal MXene(such as Sc2C(OH)2, ZT = 0.5), and have the potential applications in new thermoelectric materials with excellent comprehensive properties. A set of calculation methods used in this paper can also provide some reference for exploring the thermoelectric properties of a new double transition metal element MXene.
, , Received Date: 2024-02-28
Abstract +
In general, more attention is paid to how to improve the characteristic parameters of plasma in plasma applications. However, in some cases, it is necessary to produce plasma with low-electron density, such as in the laboratory simulation of ionospheric plasma in space science. In this study, a low-density plasma is generated by electron beams passing through a silicon nitride transmission window under low pressure condition. The transmission properties of electron beam passing through silicon nitride films are investigated by Monte Carlo simulation, and the plasma feature is studied by a planar Langmuir probe and a digital camera. It is found that the plasma exhibits a conical structure with its apex located at the transmission window. At a constant pressure, the cone angle of conical plasma decreases with the electron energy increasing. This is qualitatively consistent with the Monte Carlo simulation result. The frequency of electron-neutral collisions increases as the working pressure rising, which leads the plasma cone angle to increase. When the beam current is reduced from 10 μA to 0.5 μA at 40 keV, the electron density decreases, in a range between 105 and 106 cm–3, while the electron temperature does not change significantly but approaches 1 eV. It can be inferred that the electron density decreases with the distance z from the transmission window in the incident direction of the electron beam. A low-density plasma of less than 105 cm–3 can be obtained further away from the transmission window.
, , Received Date: 2024-02-02
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Digitally encoded hypersurfaces show great potential in the field of electromagne-tic wave modulation. Currently, digitally encoded hypersurfaces in the terahertz band are mainly classified into two types: structure-encoded and controllable material-encoded. Once a structure-encoded hypersurface is fabricated, its function is fixed, which makes it difficult to adapt to changing application requirements. In contrast, the controllable material-encoded hypersurfaces can achieve dynamic regulation and multifunctional switching of terahertz beams by changing the external excitation, which shows good reconfigurability. To address this challenge, a Dirac semimetal-based encoded hypersurface is proposed in this paper. The Fermi energy level of the Dirac semimetal is varied by changing the bias voltage, which in turn dynamically adjusts its relative permittivity to obtain the coded unit. Besides, the traditional gradient-phase method encodes arrays by periodically arranging the cell structure, but there are limitations in the flexibility and accuracy of beam modulation. In order to break through these limitations, this paper employs a genetic algorithm for the inverse design of hypersurface coding arrays, which effectively improves the initiative and flexibility of beam modulation. In this paper, a three-layer terahertz-encoded hypersurface unit with a “back” structure composed of Dirac semimetallic materials is firstly designed, and the Dirac semimetallic dielectric constant is dynamically adjusted by using an applied bias voltage, so that the hypersurface unit is at 1.95 THz when the Fermi energy levels are 0.01 eV, 0.05 eV, 0.09 eV, and 0.55 eV can achieve 2bit coding. The results show that, for beam configuration, single-beam and multi-beam (two-beam to five-beam) modulation can be achieved at 1.95 THz within 40° pitch angle and 360° azimuth angle; for vortex beam generation, single-vortex beams with ±1 and ±2 topological charges can be generated, with mode purity exceeding 60%, and single-vortex, double-vortex and triple-vortex beams in pitch angle and 360° azimuth angle can be realised with the vortex-phase convolution. In terms of RCS reduction, in the frequency range of 1.72–2.51 THz, the hypersurface is able to achieve more than 10 dB of RCS reduction, especially in the frequency range of 1.82 THz, the maximum reduction value is up to 27.5 dB. achieves the diversity of functions, but also has a high degree of reconfigurability to meet the needs of complex application scenarios.
, , Received Date: 2024-02-02
Abstract +
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Water-side oxidative corrosion of zirconium alloys is a critical concern in the design of cladding materials for nuclear fuel rods in pressurised water reactors (PWRs), and their corrosion resistance is one of the main factors limiting service life. At present, Zr-Sn-Nb system alloys are still the main development direction of advanced zirconium alloys. Sn and Nb can exhibit a variety of valence states in the oxide film of the cladding and significantly affect the stability of ZrO2. However, the influence mechanism of Sn and Nb on the fraction of t-ZrO2 and the t-m phase transition is unclear. In this paper, the lattice properties, formation enthalpies, and oxygen vacancy formation energies of ZrO2 under the doping conditions of Sn and Nb with different valence states are calculated based on the first-principles, and the influence mechanism of Sn and Nb on the stability of ZrO2 is revealed from the atomic scale. The results show that there is a significant difference in the effects of Sn and Nb, as well as low-valent and high-valent elements. Sn2+ and Nb3+ cause significant lattice swelling distortion, Nb5+ causes lattice shrinkage which contributed to the reduction of stresses within the film, and Sn4+ causes slight lattice swelling. The low-valent elements all make ZrO2 less stable and are unfavourable to the stability of t-ZrO2 relative to m-ZrO2. The high-valent Nb5+、Sn4+ promote the relative stability of t-ZrO2 and thus inhibit the t-m phase transition, with Nb5+ having a significant effect and Sn4+ having a weak effect. The relative stability of t-ZrO2 increases with pressure in the range of 0-3.5 GPa. Compared with high-valent elements, low-valent elements are more favourable to introduce oxygen vacancies in t-ZrO2, thus stabilising the interfacial t-ZrO2 and enhancing the corrosion resistance of the cladding. By investigating the electronic structure, it is found that the oxygen vacancy formation energy is positively correlated with the magnitude of charge transfer (or degree of electron localisation) between the alloying element ion and the oxygen vacancy. These results contribute to composition optimisation and structural design for corrosion resistance of zirconium alloys.
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Abstract +
Friction occurs in various systems from the nanoscale to the geophysical scale and plays a crucial role. The microscopic mechanism of friction and the origin of the dynamic ordering in interacting particle systems are still controversial. Using Langevin simulations, we study the friction of two-dimensional colloids on the substrate with randomly distributed point-like pinning centers. We consider three different model colloidal systems, and in each system the colloidal particles interact with each other through repulsive interactions that have two different force ranges, as shown in Fig-1. We find two maximum static friction forces (the first maximum static friction fc1d and the second maximum static friction fc2d). The interference between short-range repulsive interactions with similar force ranges in model 3 colloidal system can lead to significantly increased repulsion between particles near pinning centers, resulting in a decrease in fc1d and an enhanced orderly movement along the direction of external driving forces above fc2d. The results provide guide for revealing the mechanisms of friction in the colloidal particles with interactions that have different force ranges.
, , Received Date: 2024-05-26
Abstract +
We study the resonance exchanges of two chiral Majorana fermions in two distinct systems theoretically in this work: one is an isolated Majorana zero mode interacting with complexes formed by two chiral Majorana fermions and a Majorana zero mode, and the other involves isolated quantum dots that are coupled to a system composed of Majorana fermions and a quantum dot. Our research results reveal that both of these coupled systems can facilitate the effective transmissions of the two chiral Majorana fermions as $ {\gamma _1} \to - {\gamma _2} $ and $ {\gamma _2} \to - {\gamma _1} $ , and the resonant tunneling effects in the two systems are equivalent. Therefore, quantum dots can replace Majorana zero modes to achieve resonant tunneling. In order to observe the resonance exchange of two chiral Majorana fermions with the two quantum dots, a circuit based on anomalous quantum Hall insulator proximity-coupled with s-wave superconductor is proposed as shown in figure. The numerical results indicate that the resonant exchange of chiral Majorana fermions can be modulated by the coupling strength between the two quantum dots, and it is particularly noteworthy that the tunneling process is independent of the superconducting phase. If one of the chiral Majorana fermions undergoes resonance coupling with another quantum dot or Majorana zero mode, an additional negative sign is obtained, leading to $ - {\gamma _2} \to {\gamma _1} $ . After experiencing two resonance exchange processes, the final result is $ {\gamma _1} \to {\gamma _2} $ and $ {\gamma _2} \to - {\gamma _1} $ , which implies the realization of non-Abelian braiding operations. Our conclusion is that the modulation of coupling strength between two quantum dots can be used to achieve the switch of Majorana fermions braiding-like operation, which is independent of superconducting phase. Therefore, the designed scheme provides a new way for adjusting the braiding-like operation of Majorana fermions. These findings may have potential applications in the realization of topological quantum computers.
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Local-resonance bandgap and Bragg bandgap can coexist in a metamaterial beam, and their coupling effect can be used to realize ultra-wide bandgap, which has great application potential in the field of wide-band vibration reduction. Previous studies usually consider the single-order coupling of local-resonance and Bragg bandgaps in metamaterial beams with a single array of local resonators, so that only a single-order ultra-wide coupling bandgap can be achieved, which cannot meet the needs of wide-band vibration reduction of double/multiple target frequency bands. In this paper, metamaterial beams with double arrays of local resonators are considered, and the regulation design and analysis of double-order coupling of local-resonance and Bragg bandgaps are carried out based on an analytical model of bending wave dispersion relation. Moreover, the vibration reduction characteristics of the double-frequency-resonator metamaterial beams with double-order coupling bandgaps are studied by using spectral element method and the finite element method. The main conclusions are as follows
1) A design method for realizing double-order coupling wide bandgap in a metamaterial beam with double arrays of local resonators is proposed. By using this method, the resonance frequencies of the local resonators can be quickly designed under the conditions of given host beam parameters, lattice constant and added mass ratio of the local resonators.
2) The double-order coupling bandgaps in a metamaterial beam carrying double arrays of local resonators are compared with the single-order coupling bandgaps in metamaterial beams with a single array of local resonators. It is found that, through proper design, the total normalized width of the double-order coupling bandgaps can be much broader than that of the single-order coupling bandgaps, so the double-order coupling bandgaps are more beneficial to wide-band vibration reduction.
3) It is found that for a given total added mass ratio of the double arrays of local resonators, it is necessary to optimize the mass distribution ratio of the double resonators to achieve a maximization of the total normalized width of double-order coupling bandgaps. An approximate formula for designing the optimal mass distribution ratio of the double resonators is further established.
4) The spectral element method is used to study the vibration reduction characteristics of the metamaterial beams carrying double arrays of local resonators designed based on double-order bandgap coupling. The accuracy of the spectral element method is verified by comparing with the finite element method. The results show that significant vibration reduction can be achieved in two wide frequency bands corresponding to the double-order coupling bandgaps. The influence of number of unit cells and resonator damping on the vibration reduction characteristics of the metamaterial beam is further analyzed. It is shown that the increase of number of unit cells can enhance the reduction performance in the bandgaps, and the increase of resonator damping can effectively broaden the vibration reduction frequency band.
1) A design method for realizing double-order coupling wide bandgap in a metamaterial beam with double arrays of local resonators is proposed. By using this method, the resonance frequencies of the local resonators can be quickly designed under the conditions of given host beam parameters, lattice constant and added mass ratio of the local resonators.
2) The double-order coupling bandgaps in a metamaterial beam carrying double arrays of local resonators are compared with the single-order coupling bandgaps in metamaterial beams with a single array of local resonators. It is found that, through proper design, the total normalized width of the double-order coupling bandgaps can be much broader than that of the single-order coupling bandgaps, so the double-order coupling bandgaps are more beneficial to wide-band vibration reduction.
3) It is found that for a given total added mass ratio of the double arrays of local resonators, it is necessary to optimize the mass distribution ratio of the double resonators to achieve a maximization of the total normalized width of double-order coupling bandgaps. An approximate formula for designing the optimal mass distribution ratio of the double resonators is further established.
4) The spectral element method is used to study the vibration reduction characteristics of the metamaterial beams carrying double arrays of local resonators designed based on double-order bandgap coupling. The accuracy of the spectral element method is verified by comparing with the finite element method. The results show that significant vibration reduction can be achieved in two wide frequency bands corresponding to the double-order coupling bandgaps. The influence of number of unit cells and resonator damping on the vibration reduction characteristics of the metamaterial beam is further analyzed. It is shown that the increase of number of unit cells can enhance the reduction performance in the bandgaps, and the increase of resonator damping can effectively broaden the vibration reduction frequency band.
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With decreasing size of high-performance electronic devices (down to nanoscale), and the accompanying problem of heat dissipation becomes a big issue owing to its extremely high heat generation density. To tackle the ever-demanding heat dissipation requirement, intensive work has being carried out to develop techniques for chip-level cooling. Among the techniques reported in open literatures, liquid cooling appears to be a good candidate for cooling high-performance electronic devices. However, the solid-liquid interfacial thermal resistance cannot be ignored in the heat transfer process as the device size shrinks to the sub-microscale or nanoscale. Usually, the interfacial thermal transport can be enhanced by using nanostructures on the solid surface because of the confinement effect of the fluid molecules filling up the nano-grooves and the increase of the solid-liquid interfacial contact area. However, in the case of weak interfacial couplings, the fluid molecules cannot get into the nano-grooves and the interfacial thermal transport is suppressed. In the present paper, the heat transfer system between two parallel metal plates filled with deionized water is investigated by molecular dynamics simulation. Electronic charges are inflicted in the upper and lower plates to generate a uniform electric field which is perpendicular to the surface, and three types of nanostructures with varying size are constructed to the lower plate. It is found that the wetting state at the solid-liquid interface changes from Cassie to Wenzel states with increasing strength of the electric field. Owing to the transition from the dewetting to wetting state (from Wenzel to Cassie wetting state), the Kapitza length can be degraded and the solid-liquid interfacial heat transfer can be enhanced. The mechanism of the enhanced hart transfer is discussed based on the calculation of the number density distribution of the water molecules in between the two plates. As the charge is further increased, electrofreezing appears, and a solid hydrogen bonding network is formed in the system, resulting in an increase in thermal conductivity to 1.2 W/(m·K) while the thermal conductivity remains almost constant as the electric charge continues to increase.
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A cylindrical electrode is approximated as a long cylinder in most existing models in which generalized plane strain condition/plane strain is used. Based on the theory of elasticity, analytical expressions are derived for concentration distribution and stress component in a finite-length cylindrical electrode under galvanostatic operation. Applying the superposition theorem, the Li-concentration is a sum of the concentration due to axial diffusion and the concentration due to lateral diffusion, and separation of variable method are used to solve diffusion equations separately. Employing Boussinesq-Papkovich function, the stress component distributions which are generalized for a linear combination products of the Fourier-Bessel series of exponential type are derived. The spatiotemporal of distribution of concentration and diffusion-induced stresses are calculated in a cylindrical electrode with traction-free condition. The results are compared with a simulation results calculated with a finite element software. For the concentration distribution, the numerical result and simulation result are almost identical. For the stress component, no significant difference exists between the two results, the largest relative difference for radial stress of ~4% is found at center and SOC=17.9%. The radial stress decreases with an increasing radial position, decrease to zero at the surface which is consistent with the boundary condition. The hoop stress is tensile around the center of electrode, turn to compressive near surface. Since the tensile hoop stress is responsible for crack initiation, this suggests cracks is first to found at the center when plastic deformation is negligible. The stress component with different length to radius ratios is calculated. It is found that the stress due to lateral diffusion increases with an increase of length to radius ratios, while the stress due to axial diffusion decreases. This is because that the lateral diffusion has a greater influences on Li-concentration distribution in a cylinder electrode with increasing length to radius ratio.
Abstract +
In active matter, the effective force between passive objects is crucial for their structure and dynamics, which is fundamental to understand the complex behaviors within active systems. Unlike equilibrium states, factors such as the surface configuration, size, and confinement strength significantly influence the effective forces between passive particles. Previous studies have shown that the shape of passive particles affects the aggregation of active particles, leading to different forces experienced by passive particles with different shapes. However, recently, Ning et al. discovered that a long-range attractive force between passive platelike particles, caused by the bacterial flow field instead of the direct bacterium-plate collisions in active bacterial suspensions. This raises an intriguing question: how does hydrodynamics differently affect the forces on passive particles of different shapes?
In this work, we investigated the effective forces exerted on passive spherical and platelike particles immersed in bacterial suspensions by optical-tweezers experiments. The effective force between passive particles can be calculated by the formula, Feff=k<△d>/2, where <△d> represent the difference of the distance between passive particles in the bacterial bath compared to the solution without bacteria, k is the effective stiffness of optical traps.Feff＞0 indicates a repulsive force between passive particles, and Feff＜0 represents an effective attractive force between passive particles. Our results demonstrate that the passive spherical particles experience short-range repulsion, while platelike particles exhibit long-range attraction. This highlights the substantial impact of particle shape on their effective forces.
The forces on passive particles are primarily attributed to two factors: direct bacterium-particle collisions and the bacterial flow field. Analysis of the bacterial concentration and orientation distribution around passive particles reveals that for spherical particles, the bacterial concentration is higher between particles than outside the particles, yet there is little difference in the orientation order of bacteria between inside and outside the particles. This suggests that the effective repulsion between spherical particles is mainly due to the direct bacterial collisions. Conversely, for platelike particles, the long-range attraction is primarily influenced by the bacterial flow field rather than direct collisions, which is evidenced by the higher bacterial density and orientation order inside the two plates compared to outside that. This study provides strong evidence that the effective force between passive particles is shape-dependent in active bath, and offers new insights into controlling active-directed assembly.
In this work, we investigated the effective forces exerted on passive spherical and platelike particles immersed in bacterial suspensions by optical-tweezers experiments. The effective force between passive particles can be calculated by the formula, Feff=k<△d>/2, where <△d> represent the difference of the distance between passive particles in the bacterial bath compared to the solution without bacteria, k is the effective stiffness of optical traps.Feff＞0 indicates a repulsive force between passive particles, and Feff＜0 represents an effective attractive force between passive particles. Our results demonstrate that the passive spherical particles experience short-range repulsion, while platelike particles exhibit long-range attraction. This highlights the substantial impact of particle shape on their effective forces.
The forces on passive particles are primarily attributed to two factors: direct bacterium-particle collisions and the bacterial flow field. Analysis of the bacterial concentration and orientation distribution around passive particles reveals that for spherical particles, the bacterial concentration is higher between particles than outside the particles, yet there is little difference in the orientation order of bacteria between inside and outside the particles. This suggests that the effective repulsion between spherical particles is mainly due to the direct bacterial collisions. Conversely, for platelike particles, the long-range attraction is primarily influenced by the bacterial flow field rather than direct collisions, which is evidenced by the higher bacterial density and orientation order inside the two plates compared to outside that. This study provides strong evidence that the effective force between passive particles is shape-dependent in active bath, and offers new insights into controlling active-directed assembly.
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Abstract +
Based on first-principles combined Monte Carlo method, the effect of vacancy defect on secondary electron characteristics of Al2O3 are studied in this work. The density functional theory (DFT) calculation results show that the band structure changes when the vacancy defect exists. The existence of Al vacancy defect leads to the narrowing of band gap, which decreases from 5.88 eV to 5.28 eV. At the same time, the Fermi level decreases below the energy of the valence band maximum. Besides, the elastic and inelastic mean free paths of electrons in different crystal structures are also obtained. Compared with Al2O3 without defect and Al2O3 with Al vacancy defect, the inelastic mean free path of electrons in Al2O3 with O vacancy defect is the largest. When the energy of electrons is smaller than 50 eV, the inelastic mean free path of electrons in Al2O3 without defect is larger than that of Al2O3 with Al vacancy defect. The elastic mean free paths of electrons slightly increases when the vacancy defect exists, and that of Al2O3 with Al vacancy defect is the largest. To investigate the secondary electron emission characteristics with different ratio of vacancy defect, an optimized Monte Carlo algorithm is proposed. When the ratio of O vacancy defect and Al vacancy defect increase, the simulation results show that the maximum value of secondary electron yield (SEY) decreases with the increase of the ratio of vacancy defect. The existence of O vacancy defect increases the probability of inelastic scattering of electrons, so electrons are difficult to emit from the surface. As a result, compared with Al vacancy defect, the SEY of Al2O3 decreases more with the same ratio of O vacancy defect.
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