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In this paper, a class of two-delay differential equations with coefficient-dependent delay is studied. The distribution of the roots of the eigenequation is discussed, and conditions for the stability of the internal equilibrium and the existence of Hopf bifurcation are obtained. Additionally, using the normal form method and the central manifold theory, the bifurcation direction and the stability for the periodic solution of Hopf bifurcation are calculated. Finally, the correctness of the theory is verified by numerical simulation. [ABSTRACT FROM AUTHOR]

In steady-state operation of Hall thrusters excited by permanent magnets, the existence of temperature difference between the inner and outer permanent magnets will cause the deviation of the magnetic field configuration from the design value, thus affecting the discharge process. The simulation results show that the ion power loss on the inner wall increases by over 7.5% when the temperature difference between the inner and outer permanent magnets reaches to 140 ℃. The main reason is that the coercivity of the inner permanent magnet decreases more at higher temperature, and the magnetic field lines will incline toward the inner wall, resulting in the increment of the radial component of the electric field pointing to the inner wall. An optimized magnetic field, obtained by moving the inner permanent magnet outward in an appropriate manner, is proposed to alleviate the inner wall erosion. The optimized magnetic field has a near-symmetrical configuration, especially the nearly equal length of intersection lines of the magnetic field lines with the inner and outer walls, which is contributed to reduce the radial component of the electric field and preventing the inclination of the main ionization zone toward the inner wall, so that the peak value of ion power deposition on inner wall can be reduced by more than 50%. Based on the illustrated features and mechanism of wall erosion, an effective and easy-to-implement optimization scheme was proposed in this work to provide a useful reference for the design of long-life Hall thrusters with large height-radius ratio. [ABSTRACT FROM AUTHOR]

Increasing the specific impulse has become one of the main trends in the development of Hall thrusters and may be achieved by increasing the proportion of multiply charged ions in the plume. In this study, the multiple ionization characteristics of Hall thrusters with large height–radius ratios and their effects on the specific impulse and other performance parameters were examined via numerical simulations. The simulation results show that the proportion of multiply charged ions in a Hall thruster with a large height–radius ratio increases by 7.8 percentage points, compared with that of the traditional Hall thruster, and the multiply charged ions can obtain a higher average jet velocity under an accelerating voltage, thereby improving the specific impulse and thrust of the thruster. Further analyses show that a structure with a large height–radius ratio reduces the cooling effect of the channel wall on the electrons, and the increase in electron temperature effectively promotes the occurrence of the multiple ionization process. In addition, the smaller wall area and the larger magnetic field intensity in the near-wall region inhibit the electron near-wall conduction and prevent the potential negative effect of the increase in electron current caused by multiply charged ionization, which ensures a high level of efficiency. It was proved that multiple ionizations contribute positively to the specific impulse and efficiency of Hall thrusters with a large height–radius ratio. This serves as a reference for the high specific impulse design of Hall thrusters with large height–radius ratios and provides a new optimization perspective for improving the specific impulse of Hall thrusters. [ABSTRACT FROM AUTHOR]

Using a permanent magnet to realize a design with a large height-radius ratio can effectively reduce the radial size of the thruster. The experimental results show that a Hall thruster with a large height-radius ratio also has significant advantages in its discharge performance. Taking the 1.35 kW Hall thruster HEP-1350PM, which has a large height-radius ratio, as an example, the thrust, specific impulse, and efficiency of HEP-1350PM are increased by up to 4.3%, 10.4%, and 7%, respectively, as compared to SPT-100, while its weight is only 56% of that of SPT-100. Numerical simulations were performed to understand the discharge characteristics of a Hall thruster with a large height-radius ratio and the internal mechanism of its performance improvement. The results show that the small middle diameter and wide channel structure concentrate the distribution of the neutral gas in the channel, accelerate the decrease of the propellant density along the axial direction, and move the main ionization zone closer to the channel upstream. These factors increase the ionization concentration, ionization rate peak, working medium utilization rate, and voltage utilization rate. They also reduce the ion loss on the wall surface and plume divergence, thereby improving the comprehensive discharge performance. This study verifies the feasibility of a large height-radius ratio in the design of a Hall thruster, and demonstrates that the design would not only be valuable for reducing the volume and weight of the thruster, but also has the advantages of enhancing its discharge performance and prolonging its working life. • Large height-radius ratio design can reduce the volume and weight of the thruster. • A large height-radius ratio can improve the discharge performance of the thruster. • Hall thruster with a large height-radius ratio has a higher acceleration efficiency. [ABSTRACT FROM AUTHOR]

With the development of electric space propulsion and all-electric propulsion technologies, the application range of the Hall effect thruster (HET) has gradually expanded. As an important part of the HET, the magnetic field plays a decisive role in its performance. In the development of versatile HETs, it is necessary to comprehensively and thoroughly understand the effects of the magnetic field on the HET discharge characteristics. In this paper, a HEP-100X thruster with multiple degrees of freedom in terms of magnetic field control is used to study the effects of the peak magnetic field position on the thruster discharge characteristics. The results indicate that the peak magnetic field position can effectively control the main ionization zone position and affect the ionization rate. The thruster has the best comprehensive discharge performance when the peak magnetic field position is located near the channel exit. Further analysis reveals that the performance improvement is mainly due to the reduced ion loss on the wall surface and plume divergence half-angle, and the increased current utilization efficiency and voltage utilization efficiency. The HEP-100X shows the above trends under various flow conditions, and the anode efficiency is the highest when the anode flow is 50 sccm. In this condition, the propellant is sufficiently ionized and the ion loss on the wall surface is small. This research provides the necessary support for a comprehensive understanding of the role and significance of the magnetic field in the thruster discharge process, and has important significance for the design optimization of multifunctional Hall thrusters and subsequent research. [ABSTRACT FROM AUTHOR]

Abstract HET72-VC is a kW-class Hall effect thruster with a variable channel characterized by high thrust and power densities. In this work, an experimental study of the effect of magnetic field configuration on the discharge characteristics of HET72-VC is carried out. By separately adjusting the current through the front and rear coils to change the magnetic field configuration inside the channel, a change in the plume is observed and the discharge performance parameters are measured. Experimental results show that the effects of reducing the front coil current and increasing the rear coil current on the thruster discharge are consistent. The plume gradually changes from a hollow cone with a clear boundary into a solid cone, the thrust increases by about 3%, and the anode efficiency increases by about 2.5%. Theoretical analysis shows that the degree of inclination of the magnetic lines of force plays an important role on the electron conduction and ion acceleration. When the angle between the magnetic lines of force and the channel centerline becomes larger, the discharge performance improves. This study helps in providing a deeper understanding of the working characteristics of this type of thruster, and can provide important guidelines for its structure design and optimization. Highlights • Plume shape is strongly modulated by magnetic field configuration. • Plume shape of a solid cone rather than a hollow cone represents good performance. • Magnetic field configuration impeding electrons strongly yields good performance. [ABSTRACT FROM AUTHOR]

The large height-radius ratio design of the Hall thruster increases the difference of radial distributions of magnetic field and plasma parameters in the channel, so that the optimal discharge effect of the thruster cannot be guaranteed under the symmetrical magnetic field configuration relative to the channel centerline. In this work, the effects of the unsymmetrical magnetic field on discharge characteristics of Hall thruster with large height-radius ratio were studied through experiments and PIC simulations. The results show that the thruster can realize stable discharge in a wide power range under unsymmetrical magnetic fields. Also, the unsymmetrical magnetic field plays a positive role in improving the discharge performance and prolonging the service life of the thruster. The inward-inclined magnetic field can effectively improve the discharge performance of the thruster, and the discharge efficiency can be increased by 1.5% under the rated working condition compared with the symmetrical magnetic field. The advantage in performance is mainly attributed to the improvement of ionization efficiency of propellant. The outward-inclined magnetic field is more conducive to prolong the service life of the thruster, and the peak value of ion power deposition on the inner wall can be reduced by 40% under the rated working condition due to the outward inclination of the ionization zone and the reduction of the radial electric field. However, under unsymmetrical magnetic field, the ionization zone deviates from the channel center, and the high ionization rate zone is more dispersed in distribution, thus leading to the increase in plume divergence angle. This study verifies that the unsymmetrical magnetic field has certain advantages under the structure of large height-radius ratio, which could be used as a guidance for the optimal design of Hall thruster with large height-radius ratio. • Hall thruster can operate in a wide power range under unsymmetrical magnetic fields. • Inward-inclined magnetic field can improve thruster discharge performance. • Outward-inclined magnetic field is conducive to prolong thruster service life. [ABSTRACT FROM AUTHOR]

Thermoelectric materials PbTe–PbSe solid solutions (with composition of PbTe 1−x Se x (x = 0, 0.1, 0.25, 0.5) were synthesized by high pressure method successfully. The power factors of the PbTe 1−x Se x (x = 0, 0.1, 0.25) samples were nearly independent with the composition. The thermal conductivity of PbTe was depressed largely by alloying with Se. An enhanced figure-of-merit ZT ∼0.55 was obtained for x = 0.25 at room temperature. Especially, we found that PbTe 1−x Se x materials exhibited weakly temperature-dependent figure-of-merit values in the range of 300–600 K. [ABSTRACT FROM AUTHOR]

Nanostructured thermoelectric materials PbSe co-doped with Ag and Sb (with the composition of (AgSb) x Pb 1 − 2 x Se) were prepared by high pressure method. It was found that appropriate doping with Ag and Sb could reduce both the electrical resistivity and the phonon thermal conductivity simultaneously, consequently greatly enhancing the thermoelectric performance. An enhanced figure-of-merit of 1.03 has been achieved at 600 K for the sample with x = 0.03, which is about five times higher than that of undoped PbSe. The improved thermoelectric performance in this study may be due to the effect of high pressure and the low phonon thermal conductivity resulting from Ag and Sb as source of dopants. [ABSTRACT FROM AUTHOR]

Highly dispersed olive-like NiS particles were synthesized in a liquid-liquid biphasic system at room temperature, where nickel xanthate in organic solvents (toluene and benzene) and sodium sulfide in water solution were used as nickel and sulfide sources, respectively. NiS particles were formed at the stabilized phase interface. The structures, chemical composition and optical characteristics of the products were investigated by transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and ultraviolet-visible spectroscopy. The organic solvents obviously influenced the morphology of the NiS particles. The olive-like NiS with smooth surface and sharp ends was obtained at benzene/water interface, while spindle-like NiS particles with rough surface and circle ends were formed when using toluene as a solvent. Analogously, chainlike Bi2S3 nanowires were produced at chloroform/water interface. The effect of the experiment parameters including reaction time, solvent and concentration of reactants on the size and morphology of the products was discussed in detail and a possible formation mechanism was suggested. [ABSTRACT FROM AUTHOR]

Abstract: Thermoelectric materials PbSe were successfully prepared by high pressure and high temperature technique (HPHT) and the pressure-dependent thermoelectric properties were studied from 300 to 600K. The measurement results indicate that the electrical conduction type in PbSe can be changed from p- (<2.0GPa) to n-type (>2.0GPa) by application of pressure of about 2.0GPa. And the electrical resistivity can be reduced effectively by pressure. Low thermal conductivity (~0.8W/mK) is obtained for high pressure synthesized PbSe. These results indicate that high pressure provides a viable and controllable way of tuning the thermoelectric properties for PbSe. [Copyright &y& Elsevier]

Micro-nano satellites show considerable potential in the application of low-orbit satellite networks, and low-power Hall effect thrusters (HETs) that provide the impetus for them have also attracted considerable attention worldwide. However, the relatively large surface-to-volume ratio of low-power HETs leads to significant loss of ions on the wall surface. After the channel width is increased or the ionization area is moved outward, ensuring sufficient ionization of the propellant is very difficult, and thus its integrated performance is not high. Thus, in this study, a 100-W permanent magnet HET with an aft-magnetic field is considered as the carrier to investigate the effect of gas supply direction on the thruster discharge characteristics through a simulation and experiments. Results indicate that under the same working conditions, adopting the circumferential direction gas supply mode effectively increases the density of the propellant in the ionization zone (up to 7%), improves the propellant ionization rate (up to 12.7%), and improves integrated discharge performance (the anode efficiency can be increased by up to 5.3%). In addition, the performance optimization effect of this mode is more evident under the working condition of low flow. When combined with an aft-magnetic field, this method is an effective means to ensure high-efficiency discharge of a low-power HET, which can be used as a reference for the miniaturization development of HETs. • The circumferential direction gas supply mode increases the density and ionization rate of propellant. • The performance optimization effect is more evident under the working condition of low flow. • The circumferential direction gas supply mode is an effective means to ensure high-efficiency discharge of low-power HET. [ABSTRACT FROM AUTHOR]

It is shown from the experimental results of existing Hall thrusters that erosion phenomena occur in the magnetic poles when the maximum magnetic field is located in the plume region. The developed Particle-in-Cell model is used in this paper for simulation research on magnetic pole erosion of Hall thrusters. The simulation results show that when the maximum magnetic field is located in the plume region, there is a large potential drop between the channel outlet and the magnetic pole. Therefore, low-energy ions close to the outlet plane are accelerated to bombard the magnetic pole surface, which is an important reason for the magnetic pole erosion. Moreover, the ion flux and energy bombarded on the magnetic pole under two conditions (magnetic pole grounding and floating) are calculated and compared. It is shown from the simulation results that grounding the magnetic pole can reduce the potential drop between the channel outlet and magnetic pole, thereby reducing the ion energy and flux bombarded on the magnetic pole and effectively alleviating the erosion of the magnetic pole. [ABSTRACT FROM AUTHOR]

Besides inferior conversion efficiency, the widespread application of thermoelectric materials currently faces the challenges of the rare and high cost of constituent elements, complex and time-consuming synthesis. Therefore, it is urgent to develop facile synthesized methods and high-performance thermoelectric materials consisting of elements with earth-abundance and low cost. Meeting above partial requirement, lead sulfide (PbS) recently was identified as a potential thermoelectric material to substitute current commercial PbTe. In this paper, a one-step high-pressure method was employed to facile and fast synthesis of PbS. Trace Cl doping tunes the carrier concentration of PbS effectively, which optimizes the electrical transport properties. Further PbSe alloying sharply reduces the thermal conductivity of PbS. Therefore, an enhanced zT ∼1.0 @ 700 K was obtained for PbS doped with 0.3 at% Cl and alloyed with 25 at% PbSe, which is comparable to the commercially applied PbTe. These results indicate that high-pressure combing with Cl-doping and PbSe-alloying provide a facile method and strategy to fabricate high-performance PbS-based thermoelectric materials. • Earth-abundant and low-cost thermoelectric compound PbS was facile and fast synthesized by one-step high pressure method. • Cl doping modulates the carrier concentration of PbS and thus optimizes its electrical properties at high temperature. • The chemical bond of PbS was weaken by PbSe alloying, which induces an enhanced Hall mobility near room temperature. • The thermal conductivity of PbS was suppressed sharply by PbSe alloying. • The enhanced zT max and zT avg achieve to 1.0 and 0.67 for PbS 0.747 Cl 0.003 Se 0.25 and PbS 0.497 Cl 0.003 Se 0.5, respectively [ABSTRACT FROM AUTHOR]

• The compressive strength and dynamic elastic modulus of HPC exposed in salt lake brine increased first and then decreased. • HPC has good long-term corrosion resistance in salt lake brine. • Under the condition of long-term brine erosion, HPC specimens still maintain the characteristics of high strength and brittleness and relatively dense internal structure. • The three-dimensional concrete meso -model, considering the random aggregate shape, can be used to simulate the uniaxial compression mechanical behavior of HPC specimens in a salt lake brine environment. The study investigated the corrosion resistance, corrosion micro-mechanisms, and the uniaxial compression stress–strain constitutive relationship of high-performance concrete (HPC) specimens subjected to long-term brine corrosion in an underground salt lake environment (Xining, China), which possesses a salt content of 180894.5mg/L. A three-dimensional random aggregate concrete meso -model was established considering the mesostructural characteristics of concrete. The mechanical behavior of HPC was assessed using the LS-DYNA software platform. The findings indicated that both the compressive strength and the dynamic elastic modulus of HPC demonstrated a pattern of initial increase, followed by a decrease, with extended brine exposure. Nevertheless, after 10 years of exposure to salt lake brine, the compressive strength of the HPC samples was still considerably greater than their original strength, with minimal evident corrosion damage to the concrete. Physical and chemical corrosion products formed exclusively in the spherical pores created by the air-entraining agent within the shallow layer of HPC, without notable micro-cracks or spalling. The HPC specimens exposed to brine for ten years under uniaxial compression load exhibited typical characteristics of splitting failure. In addition, the results from the mesoscopic numerical simulations aligned well with the experimental findings. The study shows that the three-dimensional random aggregate concrete mesoscopic model can simulate the uniaxial compressive mechanical behavior of HPC specimens under long-term corrosion conditions. [ABSTRACT FROM AUTHOR]

Flaws existing in rock mass are one of the main factors resulting in the instability of rock mass. Epoxy resin is often used to reinforce fractured rock mass. However, few researches focused on mechanical properties of the specimens with a resin-infilled flaw under triaxial compression. Therefore, in this research, epoxy resin was selected as the grouting material, and triaxial compression tests were conducted on the rock-like specimens with a grout-infilled flaw having different geometries. This study draws some new conclusions. The high confining pressure suppresses the generation of tensile cracks, and the failure mode changes from tensile-shear failure to shear failure as the confining pressure increases. Grouting with epoxy resin leads to the improvement of peak strengths of the specimens under triaxial compression. The reinforcement effect of epoxy resin is better for the specimens having a large flaw length and those under a relatively low confining pressure. Grouting with epoxy resin reduces the internal friction angle of the samples but improves their cohesion. This research may provide some useful insights for understanding the mechanical behaviors of grouted rock masses. [ABSTRACT FROM AUTHOR]

Abstract: The interaction of an amino-functionalized ionic liquid, 1-(2-aminoethyl)-3-butylimidazolium bromide ([NH2C2C4im]Br) with two enzymes, pepsin and papain was investigated using fluorescence spectroscopic technique. It is found that [NH2C2C4im]Br has strong ability to quench the intrinsic fluorescence of pepsin and papain. Quenching mechanisms are considered as static quenching for papain and dynamic quenching for pepsin, respectively. The binding constants and the number of binding sites (n) of [NH2C2C4im]Br to papain were calculated at different temperatures. The thermodynamic parameters such as free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS), were calculated by thermodynamic equations. The values of ΔG, ΔH and ΔS suggest that interaction of [NH2C2C4im]Br with the two enzymes is spontaneous. Hydrogen bonding and van der Waals interactions play important roles in the binding process of [NH2C2C4im]Br to papain. However, hydrophobic interaction is the main driving force for the interaction of [NH2C2C4im]Br with pepsin. The results of three-dimensional fluorescence spectra show that [NH2C2C4im]Br has no obvious effects on the polypeptide structures of the two enzymes. Additionally, the [NH2C2C4im]Br-containing system can slightly increase the activities of the two enzymes. [Copyright &y& Elsevier]

The effect of the magnetic-field intensity near an intermediate electrode in the discharge channel of a Hall thruster with a double-peaked configuration, on the thruster discharge and performance is investigated by applying the particle-in-cell Monte Carlo collision model. The result shows that by increasing the magnetic-field intensity near the intermediate electrode while keeping the magnetic-field peak intensity and position unchanged, the current received by the intermediate electrode is gradually reduced, whereas the propellant utilization and thrust are increased. Thus, the specific impulse and efficiency are improved. The efficiency can be increased by more than 10%. Increasing the magnetic-field strength near the intermediate electrode is conducive to improving the performance of the thruster, which provides a reference for the design of a Hall thruster with a double-peaked magnetic field. [ABSTRACT FROM AUTHOR]