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Microscopic phase-field simulation for precipitation process of Ni60Al20V20 medium entropy alloy
Yang Yi-Bo, Zhao Yu-Hong, Tian Xiao-Lin, Hou Hua
2020, 69 (14): 140201.
Abstract +
$ {\gamma }' $ phase with L12 structure and of $ \theta $ phase with DO22 structure.The two ordered phases constitute a pseudo binary system. The L10 phase precipitates at the same time as DO22, and the L10 phase gradually transforms into the L12-γ′ phase, while the traditional Ni75Al7.5V17.5 alloy first precipitates L10 phase, and then the DO22 phase precipitates at the boundary of anti-phase domain of L12 phase. In the transition from L10 to L12, α position of fcc lattice is occupied by Ni atom, and the β position is occupied by Al atom and V atom. The congruent ordering of atoms results in the formation of θ single-phase ordered domain of DO22 structure, followed by spinodal decomposition; the non-classical nucleation of L10 structure gradually transforms into L12-γ′ phase and spinodal decomposition. The interaction potential between the first-nearest-neighbor atoms of Ni-Al increases linearly with temperature, and increases gradually with the increase of long range order parameters. The incubation period of Ni60Al20V20 medium entropy alloy lengthens with temperature increasing. This study can be applied to the design of Ni-Al-V medium entropy alloy.">Medium entropy alloys have attracted much attention because of their excellent physical and chemical properties. Nano-scaled L12 structure ordered phase plays an important role in strengthening the mechanical properties of medium entropy alloys, and its local atomic arrangement plays a decisive role in yield strength of medium entropy alloys. In this paper, the microscopic mechanism of the precipitation process of Ni60Al20V20 medium entropy alloy is studied by using the micro diffusion phase field dynamics model, in which the probability of atoms to occupy the lattice position is taken as a field variable to describe the configuration of atoms and the morphology of precipitates. In this model, the shape and concentration of precipitate phase, the position and appearance of new phase cannot be set in advance. Combined with the inversion algorithm, the precipitation mechanism of ordered phases of γ' (L12-Ni3Al) and θ (DO22-Ni3V) is discussed by analyzing the evolution of atomic images, the change of order parameters and volume fraction. The result shows that two kinds of ordered phases are precipitated in the kinetical process of disordered phase ordering into Ni60Al20V20 medium entropy alloys, which are of $ {\gamma }' $ phase with L12 structure and of $ \theta $ phase with DO22 structure.The two ordered phases constitute a pseudo binary system. The L10 phase precipitates at the same time as DO22, and the L10 phase gradually transforms into the L12-γ′ phase, while the traditional Ni75Al7.5V17.5 alloy first precipitates L10 phase, and then the DO22 phase precipitates at the boundary of anti-phase domain of L12 phase. In the transition from L10 to L12, α position of fcc lattice is occupied by Ni atom, and the β position is occupied by Al atom and V atom. The congruent ordering of atoms results in the formation of θ single-phase ordered domain of DO22 structure, followed by spinodal decomposition; the non-classical nucleation of L10 structure gradually transforms into L12-γ′ phase and spinodal decomposition. The interaction potential between the first-nearest-neighbor atoms of Ni-Al increases linearly with temperature, and increases gradually with the increase of long range order parameters. The incubation period of Ni60Al20V20 medium entropy alloy lengthens with temperature increasing. This study can be applied to the design of Ni-Al-V medium entropy alloy.
Formation and suppression of nonthermal statistics in peridically driven quantum Ising models
Jiang Lu-Bing, Li Ning-Xuan, Ji Kai
2020, 69 (14): 140501.
Abstract +
In classic statistical physics, an isolated system corresponds to a constant energy shell in the phase space, which can be described by the microcanonical ensemble. While, for an isolated quantum system, the conventional treatment is to subject the system to a narrow energy window in the Hilbert space instead of the energy shell in classical phase space, and then confine the participating eigen states of system wave function in the narrow window, so that the microcanonical ensemble can be recovered in the framework of quantum mechanics. Apart from the traditional theory, there is a more self-consistent description for the isolated quantum system, that is, the quantum microcanonical (QMC) ensemble. The QMC ensemble abandons the narrow energy window assumption, and allows all the eigen states to contribute to the system wave function on condition that the system average energy is fixed at a given value. At the same time, the total occupation probability of these eigen states is conserved to unity. The most probable probability distribution obtained in the Hilbert space for an isolated quantum system according to the constraints specified above is called the QMC statistics. There is a clear difference between the QMC distribution and the traditional Gibbs distribution having an exponential form. Through the external periodic drives, an isolated quantum system may produce the QMC distribution, which is a consequence of the interplay between internal origins and external drives. In this paper, we investigate the conditions for the formation and suppression of QMC distribution by using the exact diagonalization method based on the one-dimensional Ising model. We start with the one-dimensional Ising model and focus on three different cases of periodic drives: systems under vertical (along the z axis), horizontal (along the x axis), horizontal magnetic field together with random internal (along the y axis) magnetic field. For all these three cases, the external magnetic fields are set to be ordinary rectangular pulses and the Gibbs distributions are taken as the initial states. We then study the evolutions and their asymptotic tendencies to the QMC distributions of the eigen state occupation probability under the effect of external periodic magnetic field. The results show that under the vertical magnetic field, the eigen state occupation probability does not change, and the system cannot produce the QMC distribution; under the horizontal magnetic field, the system tends to display a QMC distribution, but only partly; under horizontal and random internal magnetic fields at the same time, the transition to QMC distribution can be fully realized, and finally the system is almost completely thermalized. In order to clarify the different behaviors of the Ising model in the three cases, we also calculate the information entropy of the eigen state of Floquet operator in the eigen representation of the unperturbed Hamiltonian. We find that as the information entropy of the Floquet eigen state increases, the convergence to the QMC distribution in the Hilbert space is improved. We also notice that the mechanism for the emergence of QMC distribution is closely related to the thermalization effect of the isolated quantum system. Our analyses show that when the magnetic field is vertical, it cannot trigger the thermalization of the system. When the magnetic field is horizontal, the system becomes partly, but not completely, thermalized. When we add a horizontal periodic magnetic field and a random internal magnetic field at the same time, the system can be completely thermalized to infinite temperature. Thus, the asymptotic behavior towards the QMC statistics is a reflection of the fact that the isolated quantum system is thermalizable under periodic drives.
Analysis of magnetic force and potential energy function of multi-stable cantilever beam with two magnets
Sun Shuai-Ling, Leng Yong-Gang, Zhang Yu-Yang, Su Xu-Kun, Fan Sheng-Bo
2020, 69 (14): 140502.
Abstract +
Multi-stable structures are deformable structures that can have large deformations under external excitation. Generally, multi-stable structures have at least two stable points and can jump from one to another. Because multi-stable structures have excellent nonlinear characteristics, they are widely used in many fields. In the field of energy harvesting, multi-stable structures are often obtained by means of cantilever beams. This is because the cantilever beam is simple to make, low in stiffness, and high in sensitivity, and can undergo large deformations under small excitation forces. Besides, by simply sticking magnets on its free end and its outside, various kinds of multi-stable characteristics can be constructed, such as bi-stable characteristics, tri-stable characteristics, quad-stable characteristics, etc. Furthermore, the cantilever beam and the magnet at its end can generally be simplified into an equivalent mass-spring-damping mechanical model, which is convenient for the analysis of system potential function and dynamics.In recent years, many vibration energy harvesters proposed by researchers have adopted the conventional multi-stable cantilever beams, which involve many bi-stable cantilever beams and tri-stable cantilever beams. However, if the cantilever beams need to introduce more stable points, the number of magnets required will also increase accordingly. As a result, the adjustable parameters are continuously increasing, which increases the complexity of structural optimization and the tediousness of dynamic analysis. In order to make up for the shortcomings of conventional multi-stable cantilever beams, in this paper we present a multi-stable cantilever beam with only two magnets, a ring magnet and a rectangular magnet. By changing the size of the rectangular magnet and the distance between the two magnets, this cantilever beam can have mono-stable, bi-stable, tri-stable or quad-stable characteristics. This multi-stable cantilever beam greatly simplifies the complexity of the system design, dynamic analysis, debugging and installation, and provides new ideas and technical methods for the design and application of the vibration energy harvester realized by the multi-stable cantilever beam.In this paper, firstly, the magnetizing current method is used to analyze the magnetic induction intensity of the ring magnet at any point in the three-dimensional coordinate system, and the simulation and experimental results prove its correctness. Secondly, two methods of calculating the position of the rectangular magnet at the free end of the cantilever beam are compared. Thirdly, the magnetic force between the ring magnet and the rectangular magnet is calculated and verified in experiment. Fourthly, the system potential functions under different structural parameters are analyzed and it is found that the change of the number of the stable points of the system is caused by the change of the magnetic force between the two magnets. Finally, the correctness of the number of stable points of the system under different parameters is verified in experiment and by dynamic simulations.
Phase field crystal simulation of effect of misorientation angle on low-angle asymmetric tilt grain boundary dislocation motion
Qi Ke-Wu, Zhao Yu-Hong, Tian Xiao-Lin, Peng Dun-Wei, Sun Yuan-Yang, Hou Hua
2020, 69 (14): 140504.
Abstract +
Grain boundary affects the microstructure of metal material, and thus further its macroscopic properties. As is well known, under the action of applied stress, the grain boundary migrates. The structures and arrangements of grain boundary dislocations at different misorientation angles are very different, which affects the macrophysical and chemical properties of metal crystal. Therefore, it is of great theoretical and practical significance to study the dislocation structure and reaction mechanism of grain boundary under different misorientations for further studying the material properties.The phase field crystal method is used to simulate the low-angle asymmetric tilt grain boundary structure and dislocation motion on a nanoscale. From the perspective of the change of the position of the grain boundary dislocation motion under the applied stress and the change of the free energy of the crystal system, the influences of the misorientation angle on the low-angle asymmetric tilt grain boundary structure and the motion of the grain boundary dislocation are analyzed. The results show that the types of dislocation pairs of low-angle asymmetric tilt grain boundaries at different misorientation angles are the same. With the increase of misorientation angle, the grain boundary dislocation pairs increase, and n1n2 and n4n5 type dislocation pairs are more easily formed at the grain boundaries. Under the action of applied stress, the initial movement states of the grain boundary dislocation pairs at different misorientation angles are all climbing along the grain boundaries. As the system energy accumulates, the larger the misorientation angle is, the more the number of decomposed grain boundary dislocation pairs decomposed will be, and only in the dislocation pairs of n1n2 and n4n5 type there occurs decomposition reaction. There are four stages in the free energy curve of the low-angle asymmetric tilt grain boundary system at different misorientation angles, which correspond to the dislocation pairs climbing, dislocation pairs sliding and decomposition, dislocation pairs reaction to form single crystal, and the free energy rising process of the system. Further research shows that as the misorientation angle increases, the time for the single crystal system formed by the dislocation of grain boundary pairs to annihilate is required to be long.
Helicon wave damping coefficient of Chinese fusion engineering testing reactor plasma
Li Xin-Xia, Li Guo-Zhuang, Liu Hong-Bo
2020, 69 (14): 145201.
Abstract +
$ Z(\xi )$ is performed and a numerical solution of $ Z(\xi )$ is obtained. As the consequence, the dependence of helicon wave damping factor G on the plasma parameters and that on the wave properties are both achieved. The results show that an off-axis power deposition of the wave along the device radius can be achieved under the condition of plasma discharge on CFETR tokamak. Moreover, by calculating the ratio of the electron Alfven damping in the ion cyclotron range of frequencies to the electron Landau damping, we find that the electron Alfven damping is dominant at lower wave frequencies. With the wave frequency increasing, the electron Alfven damping remains unchanged while the Landau damping increases rapidly. With the discharge parameters of CTETR hybrid mode, the electron Landau damping proves to be dominant. Moreover, the off-axis power deposition and current drive profiles are produced. It is shown that the helicon wave damping factor increases with wave frequency increasing and it is closely related to the parallel refractive index of the injected wave spectrum, the plasma density, and plasma temperature. Significant off-axis power deposition and current drive are shown in CTETR hybrid mode operation, and the current drive efficiency reaches 50 kA/MW for helicon wave with a frequency of 800 MHz. Numerical simulation performed on the GENRAY/CQL3d shows a good consistence with the experimental results.">The Chinese fusion engineering testing reactor (CFETR), complementing the ITER facility, is aimed at building up the science and technology base for the prototype of fusion power plant (PFPP). Based on the dispersion relation of fast wave, the analysis of the plasma dispersion function $ Z(\xi )$ is performed and a numerical solution of $ Z(\xi )$ is obtained. As the consequence, the dependence of helicon wave damping factor G on the plasma parameters and that on the wave properties are both achieved. The results show that an off-axis power deposition of the wave along the device radius can be achieved under the condition of plasma discharge on CFETR tokamak. Moreover, by calculating the ratio of the electron Alfven damping in the ion cyclotron range of frequencies to the electron Landau damping, we find that the electron Alfven damping is dominant at lower wave frequencies. With the wave frequency increasing, the electron Alfven damping remains unchanged while the Landau damping increases rapidly. With the discharge parameters of CTETR hybrid mode, the electron Landau damping proves to be dominant. Moreover, the off-axis power deposition and current drive profiles are produced. It is shown that the helicon wave damping factor increases with wave frequency increasing and it is closely related to the parallel refractive index of the injected wave spectrum, the plasma density, and plasma temperature. Significant off-axis power deposition and current drive are shown in CTETR hybrid mode operation, and the current drive efficiency reaches 50 kA/MW for helicon wave with a frequency of 800 MHz. Numerical simulation performed on the GENRAY/CQL3d shows a good consistence with the experimental results.
Application of Ge50Te50/Zn15Sb85 nanocomposite multilayer films in high thermal stability and low power phase change memory
Zhu Xiao-Qin, Hu Yi-Feng
2020, 69 (14): 146101.
Abstract +
The Ge50Te50/Zn15Sb85 nanocomposite multilayer films are prepared by the magnetron sputtering. The variation of resistance with temperature and with crystallization activation energy is studied. The multilayer structure of the section before and after the crystallization for Ge50Te50/Zn15Sb85 nanocomposite multilayer film is compared by transmission electron microscope. The phase change memory device based on [GT(7nm)/ZS(3nm)]5 is manufactured, and the electrical performance is measured. The fast switching speed, low operating power consumption, and good cycling performance are achieved for Ge50Te50/Zn15Sb85. Ge50Te50/Zn15Sb85, which is a kind of nanocomposite multilayer film, a promising phase change storage material with high thermal stability and low power consumption.
Grain and grain boundary characteristics and phase transition of ZnS nanocrystallines under pressure
Wang Chun-Jie, Wang Yue, Gao Chun-Xiao
2020, 69 (14): 147202.
Abstract +
In this paper, the grain and grain boundary characteristics and mechanisms of phase transition (from wurtzite to zinc-blende to rock-salt phase structure) of ZnS nanocrystallines are investigated via in situ impedance measurement under pressure up to 29.8 GPa. It should be noted that there are two semiarcs can be found from the modulus plots of ZnS under different pressures. The semiarc in high frequency region represents the grain characteristic, and another one in low frequency region refers to the grain boundary characteristic. The former decreases gradually with pressure increasing and the latter shows an opposite trend. This fact indicates that the effect of grain characteristic becomes weaker and weaker, and the role of grain boundary characteristic is just on the contrary. The grain resistance and grain boundary resistance of ZnS nanocrystalline are also studied. In the low pressure region, both resistances increase with different increment rate with pressure increasing, which can be attributed to the enhanced ability of trap charge carriers due to the small size effect of nanoparticles. In addition, two discontinuous points (about 11 and 15 GPa) can be observed in both resistance curves, corresponding to the points of phase transition from wurtzite to zinc-blende to rock-salt phase structure. With pressure increasing, both resistances decrease gradually until 21 GPa, and this point corresponds to the end of transition from zinc-blende to rock-salt phase structure. Their consequent variations are different, grain boundary resistance gradually decreases with the pressure increasing, while the grain resistance is almost a constant. Additionally, the relaxation frequency, as an intrinsic characteristic, is not affected by the geometrical parameters. According to the linear relation between the grain boundary relaxation frequency and pressure in the pressure range of phase transformation, the mechanism of structure transition from wurtzite to zinc-blende to rock-salt phase structure is also discussed in detail. Based on the investigations, the in situ impedance spectroscopy can not only be used to accurately measure the grain and grain boundary characteristics, but also provide information for studying the phase transformation under pressure.
Optimization of optical control of nitrogen vacancy centers in solid diamond
Feng Yuan-Yao, Li Zhong-Hao, Zhang Yang, Cui Ling-Xiao, Guo Qi, Guo Hao, Wen Huan-Fei, Liu Wen-Yao, Tang Jun, Liu Jun
2020, 69 (14): 147601.
Abstract +
The nitrogen-vacancy (NV) centers in diamond have the advantages of stable triaxial structure, ultra-long electron spin coherence time and simple optical readout at room temperature. A nitrogen atom in the diamond crystal replaces a carbon atom and a vacancy is generated at the adjacent position, forming a point defect in the C3v space group structure. Its ground state and excited state are both spin triplet states. It is the key to achieving efficient preparation of optical initial state and extracting NV color center’s information in the researches of highly sensitive sensing magnetic detection, temperature detection, biological imaging, quantum computing, etc. However, there was no systematic study on relevant parameters of laser for high-concentration NV color center’s samples in previous experimental studies. Based on a high concentration diamond NV ensemble, we use pulsed optical detection magnetic resonance (ODMR) technology to systematically study the relationship among laser initial polarization time, information reading time and laser power, and the influence of laser incident polarization angle on the accuracy of sensing information. The effects of various laser parameters on the NV1 peak of ODMR on the [111] axis of the NVs of diamond are also investigated. The contrast of ODMR increases firstly with a sigmoid function and then decreases with an e-exponential function as the information reading time increases. The incident polarization angle of the laser is sinusoidal, with a period of 90°. According to the above experimental results, we finally choose the appropriate experimental parameters at 45.8 W/cm2 (300 μs of polarization, 700 ns, reading time, laser incident angle is 220°) for ODMR test. Compared with previous experimental parameters (polarization time was 50 us, read the time of 3000 ns, laser incident angle was 250°), the experimental results show that the contrast of ODMR increases from 2.1% to 4.6%, and the typical magnetic sensitivity is improved from 21.6 nT/Hz1/2 to 5.6 nT/Hz1/2. The optimization of the optical control of NVs in solid diamond is realized. The above results provide an effective support for the detection of high-sensitivity manipulation sensing based on high-concentration NV ensemble.
Analytical strong-stretching theory of polyelectrolyte brushes loaded with charged nanoparticles
Qu Li-Jian
2020, 69 (14): 148201.
Abstract +
$H \sim (Z\varPhi)^{-1/3}$, where $\varPhi$ is the nanoparticle volume fraction. When the nanoparticle charge Z is large enough, nanoparticles are mainly distributed outside the brush and the brush thickness is scaled by $H \sim (Z\varPhi)^{-1}$. In the former case, the Coulombic repulsion between the grafted polyelectrolyte chains is screened by the counterions and the nanoparticles, and the brush behavior is determined by the balance between the chain elasticity and the osmotic pressure of the counterions and the nanoparticles. In the latter case, the electrostatic screening is executed by the counterions, and the chain elasticity is balanced by the osmotic pressure of the counterions. The two regimes are divided into subregimes which are dominated respectively by electrostatic or non-electrostatic interaction. The effects of size polydispersity of the nanoparticles are also investigated. It is found that the behaviors of the grafted polyelectrolyte chains are mainly determined by the ratio between the first two moments of the nanoparticle size distribution function. The polyelectrolyte brush is compressed more by the polydispere nanoparticles than by the monodisperse ones. Possible improvement in the present theory is discussed in the conclusion section.">Nanoparticles can be used to tune the properties of polyelectrolyte brushes, and polyelectrolyte brushes can be used to control the interaction between nanoparticles and substrates. In the present paper, we investigate the polyelectrolyte brushes immersed in a nanoparticle solution within the analytical strong-stretching theoretical framework. The theoretical model does not take the excluded volume interaction between any two components into account. When there is no nanoparticle loaded, the polyelectrolyte brush is assumed to be an osmotic brush. Local electroneutral approximation is assumed to be still valid after the nanoparticles have been loaded. The loaded nanoparticles are not big enough to deform the grafted polyelectrolyte chains laterally. Analytical formulae for density profiles of each component and brush thickness are derived. The loaded nanoparticles always compress the polyelectrolyte brush. By analyzing the limiting case, a scaling-type diagram for behaviors of the nanoparticle-loading polyelectrolyte brush is constructed. Two characteristic nanoparticle controlling regimes are shown. When the charge of the nanoparticle, Z, is not very large, charged nanoparticles penetrate into the brush and the brush thickness is scaled by $H \sim (Z\varPhi)^{-1/3}$, where $\varPhi$ is the nanoparticle volume fraction. When the nanoparticle charge Z is large enough, nanoparticles are mainly distributed outside the brush and the brush thickness is scaled by $H \sim (Z\varPhi)^{-1}$. In the former case, the Coulombic repulsion between the grafted polyelectrolyte chains is screened by the counterions and the nanoparticles, and the brush behavior is determined by the balance between the chain elasticity and the osmotic pressure of the counterions and the nanoparticles. In the latter case, the electrostatic screening is executed by the counterions, and the chain elasticity is balanced by the osmotic pressure of the counterions. The two regimes are divided into subregimes which are dominated respectively by electrostatic or non-electrostatic interaction. The effects of size polydispersity of the nanoparticles are also investigated. It is found that the behaviors of the grafted polyelectrolyte chains are mainly determined by the ratio between the first two moments of the nanoparticle size distribution function. The polyelectrolyte brush is compressed more by the polydispere nanoparticles than by the monodisperse ones. Possible improvement in the present theory is discussed in the conclusion section.