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FE-SEM micrographs of the clean needle used in (a) the positive-polarity (OMIM) emission test and (b) the negative polarity (BF4) emission test captured prior to emission; (c) after a positive-polarity (OMIM) emission test without beam exposure and (d) after a negative-polarity (BF4) emission test without beam exposure (different tip than (b) and (f); (e) after a positive-polarity (OMIM) electrospray emission test with 10 keV beam exposure and (f) after a negative-polarity (BF4) emission test with 10 keV beam exposure. Note the features that remain on the needle surface.
Source publication
An extreme electric field on the order of 10(10) V m(-1) was applied to the free surface of an ionic liquid to cause electric-field-induced evaporation of molecular ions from the liquid. The point of ion emission was observed in situ using a TEM. The resulting electrospray emission process was observed to create nanoscale high-aspect-ratio dendriti...
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Citations
... 26,27 Hence, the backstreaming of high-energy secondary electrons and ions can damage the molecular structure of ionic liquid. 28 The measurement of temperature dependence of each current will be the subject of further publications. ...
Ionic liquid ion sources are expected to be used in a wide range of applications such as space electric propulsion and focused ion beam micromachining. It is known that the backstreaming of secondary charged species generated by ion beam impacts can cause unexpected temperature rise and chemical changes in ionic liquids. This paper reports on results of heating experiments using a sharp needle emitter wetted with an ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)amide, at temperatures in a range from room temperature to 120 °C. Current measurements show that positive and negative electrospray currents from the heated emitter increased as the temperature increased. Time-of-flight (TOF) mass spectrometric measurements reveal that the beam composition changed significantly with increasing temperature, indicating that charged droplets as well as ions were emitted from the heated emitter. The TOF data show that a significant fraction of the current is due to droplets at higher temperatures. On the basis of the results obtained, the size and charge of the emitted droplets are discussed. The beam is roughly estimated to contain charged droplets with a diameter of around 20 nm at 120 °C.
... The deposited ionic liquid on the extractor was also deformed into a Taylor cone shape owing to the electric field; thus, backspray phenomena were observed (Fig. 16g), which is considered one of the failure modes of the electrospray thruster [60]. A similar but much larger-scale dendritic growth of the ionic liquid was observed when the voltage was held at 2700 V for a while (several seconds), as shown in Fig. 16h [61]. Finally, a discharge occurred between the emitter and the deposited ionic liquid on the extractor, as shown in Fig. 16i. ...
An externally wetted emitter array with longitudinally grooved structures for ionic liquid electrospray thrusters was fabricated to improve ionic liquid transport to the emitter tips. Two grooved emitter shapes with different groove depths were successfully fabricated using microelectromechanical system processing techniques. We evaluated the current–voltage characteristics, measured the mass spectra using time-of-flight (ToF) spectrometry, and conducted in-situ observations using a high-speed microscope. The experimental results of ion emission show that the absolute emission current increases compared with that of our previous emitter without grooves. This tendency is strengthened with deeper grooves. Moreover, the slope of the current–voltage curve for the grooved emitters does not decrease even when high voltages are applied, indicating that the grooved structure improves the ionic liquid transport to the emitter tips. This improvement is attributed to the low hydraulic impedance of the emitter structure. However, deeper grooving also increases the percentage of current intercepted by the extractor electrode, and electrochemical reactions are not avoided at an alternation frequency of 1 Hz. Although the first current–voltage measurement tended to have unstable characteristics, the ToF results indicated that the emission in the center line was in the pure-ion regime, composed mostly of monomer and dimer ions, under all the measured conditions. High-speed microscope observations showed that too much ionic liquid deposited on the extractor causes ion emission from the extractor to the emitter, known as backspray, and implies that no large droplets are emitted for either grooved emitter structure, which is consistent with the ToF results.
... Impingement of electrospray electrodes has been identified as the source of many critical life-limiting phenomena for electrospray thrusters [36,39,40], and is governed by the mass flux of the plume at wide angles that may subtend the electrodes. The ability of previous efforts to predict mass flux from impinging current have been hindered by polydispersity in the plume [41][42][43][44][45]. Furthermore, measurements of electrode impingement current are particularly susceptible to uncertainty from secondary species emission (SSE) from impinged surfaces [46][47][48][49]. Wide-trajecting massive species also constitute a thrust inefficiency, making their quantification even more desirable. ...
High-resolution mass flux measurements of an electrospray plume are reported at high (>30 $$^{\circ }$$ ∘ ) angles, which are relevant to direct impingement of downstream electrodes. Interrogation of the plume edge greatly reduces uncertainty in electrospray device lifetime estimation related to mass flux to electrode surfaces. An angularly-actuated Thermoelectric Quartz Crystal Microbalance (TQCM) provides resolution down to $$\sim$$ ∼ 2 pg cm $$^{-2}$$ - 2 s $$^{-1}$$ - 1 , allowing the highest resolution mass flux measurements of an electrospray plume to be reported herein. In-situ microscopy of the electrospray meniscus revealed changes to the electrode lines-of-sight of approximately 2°–3° due to the increasing meniscus tip height with beam current. Using the TQCM measurements and previous QCM results, a data-driven model is proposed for estimating electrode impingement as a function of beam current and aperture line-of-sight, which quantitatively captures the rapid increase in mass flux at higher beam currents or at low angles. The results show it is possible to guarantee negligible electrode impingement within a specified range of throttle levels.
... The microscopic menisci that form on a passively fed electrospray can also tilt, as well as their staking locations being away from the assumed apex of the underlying emitter structure, leading to plumes that are not necessarily reflecting the symmetry of the constructed emitter. Two examples of asymmetric emission from passively fed electrosprays are shown in figure 2, taken from [11] and [12]. Tilting of the emission cone away from the thruster axis is an important phenomenon for assessing thruster life and performance due to grid impingement and/or off-axis thrust; and, for extreme cases, can lead to rapid failure modes. ...
Tilted electrospray menisci produce tilted plumes, and can cause off-axis thrust and lifetime reduction. A long distance stereo-microscope has been constructed and deployed to image an electrospray meniscus from two directions simultaneously, allowing a 3D reconstruction of a microscopic meniscus, and arbitrary tilt angles to be measured. Azimuthal tilt varies much more than polar tilt, and there appears to be little correlation of tilt angle with geometric features of the thruster; the causes of tilt are still an open question. Early comparisons of the meniscus shape with an EHD model capture the scale and key features of the meniscus, potentially allowing future predictions of emitted species from optical meniscus data.
... Impingement of electrospray electrodes has been identified as the source of many critical life-limiting phenomena for electrospray thrusters [36,39,40], and is governed by the mass flux of the plume at wide angles that may subtend the electrodes. The ability of previous efforts to predict mass flux from impinging current have been hindered by polydispersity in the plume [41][42][43][44][45]. Furthermore, measurements of electrode impingement current are particularly susceptible to uncertainty from secondary species emission (SSE) from impinged surfaces [46][47][48][49]. Wide-trajecting massive species also constitute a thrust inefficiency, making their quantification even more desirable. ...
High-resolution mass flux measurements of an electrospray plume are reported at high (> 30º) angles, which are relevant to direct impingement of downstream electrodes. Interrogation of the plume edge greatly reduces uncertainty in electrospray device lifetime estimation related to mass flux to electrode surfaces. An angularly-actuated Thermoelectric Quartz Crystal Microbalance (TQCM) provides resolution down to ∼2 pg cm−2 s−1 , allowing the highest resolution mass flux measurements of an electrospray plume to be reported herein. In-situ microscopy of the electrospray meniscus revealed changes to the electrode lines-of-sight of approximately 2° to 3° due to the increasing meniscus tip height with beam current. Using the TQCM measurements and previous QCM results, a data-driven model is proposed for estimating electrode impingement as a function of beam current and aperture line-of-sight, which quantitatively captures the rapid increase in mass flux at higher beam currents or at low angles. The results show it is possible to guarantee negligible electrode impingement within a specified range of throttle levels.
... Attempts for a direct observation of these menisci have been performed with electron microscopy with limited success since electrons were shown to produce abnormal solidification of the ionic liquid during the observation process. 8 Consequently, any experimental characterization of these menisci has been relegated to indirect measurements of the total current emitted or plume characterization. The integration of an electrohydrodynamic source model with a discrete plume model presents new opportunities for validation of the meniscus solution. ...
A multi-scale approach to electrospray ion source modeling has been developed. The evolution of a single-emitter electrospray plume in a pure ionic regime is simulated with a combination of electrohydrodynamic fluids and n-body particle modeling. Simulations are performed for the ionic liquid, EMI-BF4, firing in a positive pure-ion mode. The metastable nature of ion clusters is captured using an ion fragmentation model informed by molecular dynamics simulations and experimental data. Results are generated for three operating points (120, 324, and 440 nA) and are used to predict performance relevant properties, such as the divergence angle and the extractor surface impingement rate. Comparisons to experimental data recorded at similar operating points are provided.
... The electrorheological effect states that when an electric field is applied to some solutions, the liquid can quickly convert from a Newtonian fluid to a solid-like viscoelastic body with a very high yield stress in a very short period of time (about 10 −3 s) and then quickly return to a state of very low viscosity after the removal of the electric field. [52,53] In this study, electricity, which acted as a stimulus source, stimulated ions in the lubricating oil to gather them, which made the viscosity of the lubricating layer increase instantaneously and the coefficient of friction change abruptly. The number of excited ions in the ionic liquidlube oil solution could be controlled by changing the current intensity, so that the amplitude of the abrupt change in the coefficient of friction could be adjusted. ...
Intelligent lubricating materials can control the coefficient of friction between interfaces in response to external stimuli. Considering that mechanical transmission parts are mostly used in oil‐based lubrication environments, the lubrication performance and electrical regulation mechanism of poly‐α‐olefin‐50 (PAO50), a lubricating oil, containing 1‐octyl‐3‐methylimidazolium hexafluorophosphate ([OMIm]PF6), an ionic liquid (IL), are investigated in this study. Friction tests on a universal micro‐tribometer under current stimulation show that ionic lubricating oil has an excellent lubrication performance and electrical responsiveness. By changing the current strength, ion content, and the load, the reasons behind the change in the coefficient of friction are investigated. Ion aggregation results in an increase in the viscosity of the lubricating oil film, which leads to an increase in the coefficient of friction. Moreover, the real‐time resistance value of the oil film is obtained through a self‐built voltage acquisition device. The mechanism of the electronic control of friction is verified by analyzing the influence of the change in ion distribution on the resistance of the oil film. There is a correlation between the resistance and viscosity of the oil film. The results of this study may be significant in guiding research on the intelligent control of friction under oil‐based lubrication. Together, poly‐α‐olefin‐50 (PAO50) lubricating oil and ionic liquid exhibit excellent tribological and electrical response properties. Ion aggregation under current stimulation results in a decrease in the resistance of the oil film. The viscosity of the oil film increases, showing an opposite trend to the resistance. This increase can cause a sudden increase in the coefficient of friction.
... The effects of secondary electrons have been alluded to in prior electrospray thruster studies 19,102-106 and briefly hypothesized as a possible life-limiting mechanism for electrosprays. 27,103 In situ electron microscopy by Terhune et al. 103 revealed that ILIS emission sites will form a dendritic solid structure under electron bombardment. Modeling work by Magnusson et al. 51 and experimental work by Klosterman et al. 107 have characterized IIEE yields in conditions relevant to ILIS thruster operation. ...
... The effects of secondary electrons have been alluded to in prior electrospray thruster studies 19,102-106 and briefly hypothesized as a possible life-limiting mechanism for electrosprays. 27,103 In situ electron microscopy by Terhune et al. 103 revealed that ILIS emission sites will form a dendritic solid structure under electron bombardment. Modeling work by Magnusson et al. 51 and experimental work by Klosterman et al. 107 have characterized IIEE yields in conditions relevant to ILIS thruster operation. ...
... 34 Given the lack of electrochemical decomposition processes in liquid metals and the common observation of emission decay among both LMIS and ILIS thrusters, contamination due to backstreaming secondary species may be another mechanism that contributes to emission decay in ILIS thrusters. Precipitate by-products formed due to collision-induced dissociation, rheological changes that ionic liquids undergo when bombarded with high-energy electrons, 103,124 and accumulation of sputtered steel contaminants from the facility 21 likely promote pore clogging, increasing the emitter's hydraulic resistance, thus causing emission decay. Pore clogging due to backstreaming contaminants suggests that space weather may detrimentally impact ILIS thruster flight performance and life. ...
Theoretical, analytical, and experimental investigations of electrospray operation in vacuum facilities show that secondary species emission (SSE) plays a significant role in the behavior of electrospray thrusters during ground testing. A review of SSE mechanisms, along with an analysis of onset thresholds for electrospray thruster conditions, indicates that secondary species (e.g., electrons, anions, cations, etc.) must be carefully considered for accurate measurements and determination of performance and life. Presented models and experiments show that SSE-induced thruster-to-facility coupling can lead to considerable measurement uncertainty but can be effectively mitigated with an appropriate beam target design. The Electrospray SSE Control-volume Analysis for Resolving Ground Operation of Thrusters model is applied to experimental data to analyze SSE behavior. A heat and mass flux analysis of the Air Force Electrospray Thruster Series 2 (AFET-2) shows that SSE-induced Ohmic dissipation can cause performance limitations in ionic liquid ion source thrusters. The presented analytical models show that backstreaming current density contributing to less than 0.1% of measured emitter current density can cause substantial variation in propellant properties. Additionally, backstreaming current density contributing to less than 3% of emitted current can cause the 0.86 𝜇g s−1 neutral loss rate estimated during AFET-2 testing. Arguments are presented to support the notion that glow discharges observed in electrospray thrusters during vacuum operation are a consequence of secondary species backstreaming to the emission site, rather than a process intrinsically caused by ion evaporation. Recommendations for general best practices to minimize the effects of SSE on electrospray thruster operation are provided.
... In addition to propellant accumulation from overspray, other considerations can contribute to reduced lifetime. Flux of electrons towards the emitter due to the positive potential of the emitter, known as electron backstreaming (EBS), can induce electrochemical reactions in the propellant that can result in emitter damage and propellant decomposition near the emission site [16]. Chemical reactions due to the electric double layer can also contribute to propellant decomposition [17,18], leading to growth of undesirable byproducts on, or near, the emission surface. ...
Ionic liquid electrospray thrusters are capable of producing microNewton precision thrust at a high thrust–power ratio but have yet to demonstrate lifetimes that are suitable for most missions. Accumulation of propellant on the extractor and accelerator grids is thought to be the most significant life-limiting mechanism. In this study, we developed a life model to examine the effects of design features, operating conditions, and emission properties on the porous accelerator grid saturation time of a thruster operating in droplet emission mode. Characterizing a range of geometries and operating conditions revealed that modifying grid aperture radius and grid spacing by 3–7% can significantly improve thruster lifetime by 200–400%, though a need for explicit mass flux measurement was highlighted. Tolerance analysis showed that misalignment can result in 20–50% lifetime reduction. In addition, examining the impact of electron backstreaming showed that increasing aperture radius produces a significant increase in backstreaming current compared to changing grid spacing. A study of accelerator grid bias voltages revealed that applying a reasonably strong accelerator grid potential (in the order of a kV) can minimize backstreaming current to negligible levels for a range of geometries.
... ‡ alexd@physics.gu.se § eva.olsson@chalmers.se observation on the atomic scale, from an ionic liquid [20] and 49 from carbon nanotubes for both reshaping purposes and to 50 improve the properties for electron cold-field emission [21,22]. 51 Cold-field emission is another effect induced by high electric 52 fields (around 2 V/nm [23], whereas field evaporation of 53 gold (Au) commences at around 30 V/nm), utilized in, e.g., 54 electron sources [24] and medical applications. ...
While it is well established that elevated temperatures can induce surface roughening of metal surfaces, the effect of a high electric field on the atomic structure at ambient temperature has not been investigated in detail. Here we show with atomic resolution using in situ transmission electron microscopy how intense electric fields induce reversible switching between perfect crystalline and disordered phases of gold surfaces at room temperature. Ab initio molecular dynamics simulations reveal that the mechanism behind the structural change can be attributed to a vanishing energy cost in forming surface defects in high electric fields. Our results demonstrate how surface processes can be directly controlled at the atomic scale by an externally applied electric field, which promotes an effective decoupling of the topmost surface layers from the underlying bulk. This opens up opportunities for development of active nanodevices in, e.g., nanophotonics and field-effect transistor technology as well as fundamental research in materials characterization and of yet unexplored dynamically controlled low-dimensional phases of matter.
