As an application of electrospray, electrospray thrusters are preferred to work in a steady electrohydrodynamic mode to output high-precision and adjustable thrust through controlling voltages and flow rates. However, voltages and flow rates can only be modified in a small range to avoid mode transiting, thereby limiting the thrust range. To increase the thrust, adding the number of emitters or more grids for beam acceleration is considered, but it inevitably increases mechanical complexities and risks of component failure. In this paper, electrosprays are investigated by analyzing the beam current over the time and frequency domains. Results suggest that the steady cone-jet transfers into an oscillating cone-jet with a frequency of several kHz as the flow rate increases. In the oscillating cone-jet mode, beam currents and flow rates still follow the power-law relation, which is derived for the steady cone-jet. The finding suggests that setting a desired thrust over the oscillating cone-jet mode is feasible, whose maximum thrust allowed is 2.3 times the one in the steady cone-jet mode. The calculated thrust noises of the oscillating cone-jet mode are lower than 0.1 μN/Hz0.5 in the millihertz band, which meets the requirement of most drag-free satellites. The oscillation frequency is also adjustable by changing voltages or flow rates, offering another control parameter for electrospray devices. Compared to other instable modes, the oscillating cone-jet mode demonstrates superior low-noise output and controllability, thereby making it another suitable operational mode for high-precision electrospray thrusters in addition to the steady cone-jet mode.
Self-powering electronics by harvesting mechanical energy has been widely studied, but most self-powering processes require a long time in the energy harvesting procedure, resulting in low efficiency or even system failure in some specific applications such as instantaneous sensor signal acquisition and transmission. In order to achieve efficient self-powered sensing, we design and construct an instantaneous self-powered sensing system, which puts heavy requirements on generator’s power and power management circuit. Theoretical analysis and experimental results over two types of generators prove that the planar-structured rotary triboelectric nanogenerator possesses many advantages over electromagnetic generator for the circumstances of instantaneous self-powering. In addition, an instantaneous driving mode power management circuit is also introduced showing advanced performance for the instantaneous self-powering sensing system. As a proof-of-concept, an integrated instantaneous self-powered sensing system is demonstrated based on Radio-Frequency transmission. This work demonstrates the potential of instantaneous self-powered sensing systems to be used in a wide range of applications such as smart home, environment monitoring, and security surveillance.
Electrospray technology is widely used in many technological areas. The beam current of electrospray is an important parameter since it directly associates with the electrohydrodynamic behavior of the cone jet and can be precisely measured. Although how the beam current changes with other variables has been theoretically and experimentally researched, the accurate prediction of the current is still difficult. Particularly, for liquids with high electrical conductivity, Ohmic conduction is a major component of the beam current, but it is ignored in many theoretical models. In this study, the beam current components are investigated via numerical simulation developed based on hydrodynamics and electrostatics equations. Consideration of both convection and conduction currents of the cone jet affords a more accurate calculation of the total beam current. Moreover, an interpolation method is employed to solve the charge "escape" problem, providing a more accurate calculation of charges as well as the currents. The results of the numerical model are validated against experimental results, showing good agreement regarding the meniscus shape and droplet diameters. For a highly conductive ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (EMI-Im), the simulated beam current also shows good agreement with the experimental data, with a maximum error of 13%. Using the improved simulation model, temperature-induced beam current fluctuations are investigated to understand how an electrospray thruster behaves with temperature variations.
The gate-type carbon nanotubes (CNTs) cathodes exhibit advantages in long-term stable emission owing to the uniformity of electrical field on the CNTs, but the gate inevitably reduces the transmittance of electron beam (E-beam), posing challenges for system stabilities. In this work, we introduce an E-beam focusing technique using the self-charging SiNx/Au/Si gate. The surface potential of SiNx is measured to be approximately -60 V quickly after the cathode turning-on, the negative potential can be maintained as the emission goes on. The charged surface generates rebounding electrostatic forces on the following electrons, significantly focusing the E-beam on the center of gate hole and allowing them to pass through with minimal interceptions. An average transmittance of 96.17% is observed during a 550-hour prototype test. We believe that this high-efficiency, long-term stable cathode has great potential for using in neutralizers, solar sails and tethered satellites in the long-term space missions.
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