Download Electrosprayed Heavy Ion and Nanodrop Beams for Surface

Electrosprayed Heavy Ion and Nanodrop Beams for Surface
Engineering and Electrical Propulsion
Summary of research for the period 09/30/2011-09/01/2012
In this project we investigate the generation, interaction and energetic impact of electrosprayed
nanodroplets and molecular ions, covering continuously the projectile size range from 1 nm up
to hundreds of nm. The 1-20 nm size range is of great interest to achieve variable specific
impulse (Isp) from 100 s to >1000 s in drop-based electrical propulsion, while pure molecular
ion emission enables higher Isp. The resulting knowledge is directly applicable to electrospray
thrusters/electric propulsion (achievement of improved and variable Isp, maximization of thrust,
power density and propulsive efficiency, quantification of the lifetime of electrospray thrusters),
and will lead to new opportunities in other fields such as MEMS and IC fabrication (broad-beam
and focused-beam nanodroplet and ion sources for high speed beam milling and
microfabrication, reactive nanodroplet and ion etching, polishing of large and curved mirrors),
surface processing (patterning of crystalline surfaces with amorphous layers, patterning of a
textured surface with controllable roughness, strengthening of materials for increased thruster
life, microscopy), and secondary ion mass spectrometry (SIMS) of organic surfaces. The goals
of this project are to gain a detailed, first-principles understanding of the production of
nanoprojectiles, the extraction of their beams, their interaction with surfaces, and the
investigation of the propulsive and surface processing applications outlined above.
Electrospray-based propulsion has been supported by the AFOSR over recent years. Small drops
and ions having high charge to mass ratio, can be electrostatically accelerated to very high
Isp, but produce little thrust for given power. Larger drops having smaller  produce larger
thrust, though at lower Isp. Controlling  hence provides the freedom to tune Isp for a given
single thruster-propellant combination. It is therefore possible to compromise between the desire
to consume little fuel at the expense of a slow acceleration, and the occasional need to achieve
high thrust at the expense of a less efficient use of propellant. In either case controlling the
production of nanodrops and ions is at the heart of developing improved electrospray
propellants, and studying this problem is an important thrust of the proposed research. Another
attractive strategy for operating at both high thrust/low Isp and low thrust/high Isp is by
employing liquid propellants that can be both used as chemical monopropellants and
electrosprayed into charged particles (dual chemical/electrical propulsion). This project studies
both strategies for operating electrospray thrusters in very different Isp regimes.
There is limited information available on how the particles in electrospray beams (beams made
of either charged droplets, ions or a combination of both) interact with each other and with
thruster surfaces. This knowledge is key for designing the geometry of extracting electrodes in
electrospray thrusters, maximizing thrust density while minimizing particle impingement, and to
understand and predict the damage on spacecraft surfaces caused by energetic particle impact.
An important fraction of our work is devoted to this line of research.
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The operation of electrospray thrusters in the purely ionic regime relies on a good understanding
of fundamental problems such as the field emission of ions, and the fragmentation of ions in
flight which may reduce thrusting efficiency and cause surface contamination. This project is
investigating both problems.
To tackle the multidisciplinary problems object of this program we have assembled a team of
experts on electrospray atomization and colloid thruster technology, materials and surface
science, molecular dynamics and multiscale modeling, and surface spectrometry. The key
investigators and their main expertise related to this project are Prof. Gamero (project lead,
nanodroplet beams, UCI), Prof. Paulo Lozano (molecular ion beams, MIT), Prof. Fernández de
la Mora (electrospray atomization, Yale University), Prof Daniel Mumm (materials science,
UCI), Prof. Markus J. Buehler (molecular dynamics and multiscale modeling, MIT), Prof.
Alessandro Gomez (colloid thrusters, Yale University), and Dr. Jian-Guo Zheng (surface
spectrometry, UCI). Below we describe the research activities carried out during the first year of
the program, and how they will be continued into the second year.
1. Experimental characterization and modeling of electrospray beams.
The goal of this line of research is to develop a modeling tool for reproducing the geometry of
an electrospray beam as a function of beam current, extracting electrode geometry and electrode
potentials. This model could be used, for example, to determine the operational range (i.e. beam
flow rates and extraction potentials) of a given thruster geometry; or for the optimal design of
the thruster geometry.
In parallel to the modeling effort, it is essential to experimentally characterize electrospray
beams emitted by a source of known geometry for the following three reasons: a) it gives
insights on how to construct the model; b) it provides parameters of the beam such as the
particles’ specific charge distribution, initial velocity and breakup position, etc. which are inputs
to the model; and c) it makes it possible to compare experimental and model results, and hence
validate the model.
The model is based on the integration of particle trajectories defining the surface envelopes
containing a fraction of the beam particles. The particles in the beam are divided into discrete
groups of identical particles (same specific charge and diameter), and carrying a fraction of the
total beam current. To make this partition knowledge of the particles’ specific charge
distribution is required; in our case we measure this distribution using time of flight and
retarding potential spectrometry. The envelope of each particle group is then computed by
integrating the equation of motion of a particle flying along the envelope. The electric field
acting on this particle is the superposition of the electric field induced by the electrodes (and
computed by a commercial program solving Laplace equation), and the electric field induced by
the sum of the volumetric charge densities contained by each envelop. A detailed description of
this model can be found in M. Gamero-Castaño, Journal of Fluid Mechanics, 604, 339-368
(2008). The main novelties of the new model are: 1) we now compute the space charge field by
solving Poisson’s equation for the volumetric charge distribution associated with the envelopes,
rather than using an approximate analytical field. This change improves the accuracy of the
electric field and therefore the computed structure of the beam; 2) we now construct the model
for a beam typical of electrospray thrusters (in particular for a beam of the ionic liquid EMI-Im),
rather than for a beam of large droplets with low specific charge and Isp.
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Figure 1.a) shows the experimental setup used to characterize the EMI-Im beams. Two time of
flight collectors and a retarding potential analyzers mounted on an XYZ stage are used to
characterize the beams, electrosprayed from a single emitter/extractor source. The temperature
of the emitter can be varied and controlled with an electrical heater, a thermocouple and a
commercial digital controller. With this arrangement we measure beam profiles and particle
composition inside the beam as a function of position, mass flow rate and working temperature.
Figure 1.b) shows several beam profiles, at different emitter temperatures. The profiles are
recorded by the retarding potential analyzer operated without a potential barrier, and moving
along a plane perpendicular to the beam axis located 0.102 m downstream from the emission
point. Note that the profiles are rather axisymmetric, due to the accurate construction of the
electrospray source. The retarding potential and time of flight of particles along any point in the
profiles can be determined with the RPA and small collector TOF detector shown in Fig. 1.a).
Figure 1.c) shows time of flight waves for the whole beam, recorded with the large collector
TOF detector. These curves yield the fractions of ions and droplets making the beam, and
properly integrated provide the beam mass flow rate, thrust and Isp.
a)
b)
c)
Figure 1. a) Experimental setup for characterizing EMI-Im beams; b) beam profiles at different working temeperatures and beam
currents; c) time of flight waves for several beam currents at 21C
Figure 2.a) shows the geometry of emitter and extractor electrodes used in the model (and in the
experiments), together with particle trajectories obtained with the beam model. In this
simulation the beam was divided into 80 particle groups, yielding 80 envelopes. The figure
shows some of the envelopes, including the most outer one. Once the envelopes are computed,
and since each one carries a known fraction of the beam current, the current carried by the beam
between the axis and a given polar angle can be computed. This function (experimental and
model) is plotted in Fig. 2.b). The model reproduces rather well the overall shape and the
angular extend of the accumulated current function. The total current is smaller in the model
because we track droplets and not ions (which in this case carry 17% of the total beam current).
However we know from the experimental characterization that the ion population is located in
the core of the beam, while the droplets appear throughout the angular range of the beam. Thus
the maximum polar angle at which droplets are observed is the maximum angle of the beam;
they are seen to coincide in Fig. 2.b).
At this point of the program the beam model for electrosprays of droplets is completed. Moving
forward into the second year the model will be used for:
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1) The determination of optimum thruster electrode geometries, i.e. designs that
maximize thrust density while avoiding beam impingement. The model will be
used to guide the design of single emitter sources, which then will be
microfabricated and tested in the lab.
2) An extension of the model for ionic beams. This will be a collaborative effort
between the UCI group of Prof. Gamero (beam model expertise) and the MIT
group of Prof. Lozano (expertise on Ionic Liquid Ion Sources for electrospray
thrusters).
a)
b)
Figure 2. a) Electrospray source geometry and particle trajectories obtained with the model; b) comparison between experimental
and model beam accumulated current vs. polar angle functions
2. Characterization of thruster surfaces bombarded by nanodroplets and
modeling of nanodroplet energetic impact.
We are investigating the impact of energetic nanodroplets on a variety of surfaces, with the goal
of understanding the erosion patterns on the surfaces of spacecrafts operating electrospray
thrusters. We have characterized the phenomenology of nanodroplet bombardment on silicon,
aluminum, nickel and tungsten. This is a collaborative effort between the UCI groups of Prof.
Gamero, Prof. Munn and Dr. Zheng.
Figure 3.a) shows pictures and AFM profiles of a single-crystal Si surface bombarded at
increasing acceleration voltage. The droplets’ size and velocity are comparable to those in
electrospray thrusters. Silicon is a relevant material because it is the substrate of choice for most
micromachined devices, including MEMS electrospray thrusters. The sputtering yields
measured in these experiments are of order one (one Si atom ejected per molecule of
propellant), which combined with the high beam molecular throughput of electrospray thrusters,
will cause unacceptable erosion rates on thruster and spacecraft surfaces if the particle
trajectories in the beam are not carefully designed. The erosion pattern in silicon at intermediate
acceleration voltages is dominated by craters with diameters as large as a few microns. If left
untreated, such erosion on optical surfaces would be a concern. Figure 3.b) shows a crosssectional TEM micrograph of a Si surface bombarded by nanodroplets. The bombarded surface
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develops a damaged layer of some 20 nm, with undulations and typical roughness comparable to
the size of the droplets. This layer is amorphous, and contains a significant fraction of oxygen.
At this point we do not know if the oxygen is implanted during the impact or after exposure of
the surface to atmospheric conditions.
Pt protective
layer
c-Si
bombarded
surface
with amorphous
layer
a)
b)
Figure 3. a) Single-crystal Si wafers bombarded by nanodroplets at increasing acceleration voltage; b) cross-sectional TEM image
of a silicon surface bombarded by nanodroplets
Figure 4.a) shows cross-sectional TEM micrographs of a single crystal nickel target bombarded
by nanodroplets. In this case we do not observe an amorphous layer on the surface of the
bombarded target, but the fringe pattern is indicative of a polycrystalline morphology.
Molecular dynamics simulations of the impact on Si (described below) show that the
temperature of the area surrounding the impact increases substantially, likely exceeding the
melting point of nickel. The simulations also show that the subsequent quench of the melt is
extremely fast, and one possible reason behind the observed polycrystalline morphology of the
surface is that the fast recrystallization prevents the homogeneous growth of a single crystal.
Figure 4.b) shows similar TEM micrographs for a single-crystal aluminum target. The damaged
area is an amorphous oxide layer of approximately 40 nm. We are currently investigating
whether the oxidation occurs during the impact, or after exposure of the sample to atmospheric
conditions.
a)
b)
Figure 4. a) cross-sectional TEM micrographs of a single crystal nickel target bombarded by nanodroplets ; b) cross-sectional TEM
micrograph of a single crystal aluminum target bombarded by nanodroplets
Besides the characterization of bombarded surfaces outlined above, we are interested in
understanding the physics of the impact. To that effect we have devoted a considerable effort in
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the simulation of the impact of a nanodroplet on single-crystal Si via molecular dynamics. We
have first considered the case of an individual impact by a droplet 10 nm in diameter, at an
impact velocity of 6.4 km/s and an acceleration voltage of 24 kV. The density of the nanodroplet
is that of EMI-Im, and is made of 1224 spheres with the EMI-Im molecule mass. The target is a
30.55 nm x 48.88 nm × 48.88 nm slab. Figure 5.a) shows the temperature field in the target at 0
ps, 1 ps, 5 ps, 10 ps, 20 ps, and 70 ps from the time of impact. Upon impact a substantial amount
of energy is dissipated in the contact region, raising its temperature above several thousand
degrees (see 5 ps frame). A layer surrounding the crater remains above the normal melting point
for over 10 ps, before cooling down and equilibrating with its surroundings. Figure 5.b) shows
the pressure field at 3.5 ps. The area at the bottom of the crater is decelerating the projectile and
therefore is highly compressed, while a nearly spherical elastic wave moves away from the point
of impact. The jump in temperature across the shock wave averages 9 K, a marginal increase
consistent with the nearly isentropic nature of shock compression.
a)
b)
Figure 5. a) temperature field in a silicon target following the impact of a nanodropet; b) pressure field at 3.5 ps from the time of
impact. F. Saiz and M. Gamero-Castaño, Journal of Applied Physics, in print
Figure 6.a) shows the evolution of the total thermal, translational and potential energies of the
slab, and the total kinetic energy of the projectile, ETh(t), ECM(t), EP(t) and EK(t). The projectile
molecules lose 90% of their initial kinetic energy within 3 ps. By this time 12%, 18% and 60%
of the projectile’s energy has been transferred to the slab in the form of translational, thermal
and potential energies respectively. After peaking at 12 ps, the thermal energy in the slab is
gradually lost by heat conduction with the surrounding thermal bath, while the potential energy
asymptotes to a constant fraction of 24%. This potential energy excess is mostly distributed in
the region near the impact, where the atomic arrangement is highly disordered by the end of the
simulation. Figure 6.b) shows the evolution of the temperature, pressure, coordination number
and melting point in a control volume below the surface of impact. Upon impact the pressure
and temperature start to increase, and rapidly plateau around 5.2 GPa and 350 K at 1.8 ps. At
about 2.2 ps the temperature exceeds the melting point, and the control volume starts to undergo
a solid to liquid phase transition. This is confirmed but the coordination number fluctuating
around a value of 6. After 8 ps the temperature decays with cooling rates as high as 6.8x1013 K/s
at the normal melting point, and 3.7x1013 K/s at the glass transition temperature of 1060 K.
These cooling rates exceed by over four orders of magnitude the typical value of 10 9 K/s needed
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to prevent the regrowth of the crystalline phase, and the previously melted material solidifies as
an amorphous phase.
a)
b)
Figure 6. a) Conversion of the projectile’s kinetic energy into thermal and potential energy in the target ; b) evolution of the
temperture, pressure, coordination number and melting point in a control volume below the impact surface
During the second year of the program we will continue investigating these problems. In
particular we would like to determine whether the appearance of oxygen in some of the
bombarded surfaces is associated with oxidation during the bombardment or after exposure of
the sample to atmospheric conditions; and model the effect of multiple nanodroplet impacts.
3. Fundamental study of the generation of nanoparticles and evaluation of
chemical monopropellants for use in electrospray thrusters.
This line of research, a collaborative effort between the groups of Prof. Gamero and Prof.
Fernández de la Mora, will be pursued in the second year of the program. So far we have
identified and purchased an exothermic propellant (the ionic liquid ethylammonium nitrate), and
established a collaboration with AFRL-Edwards scientists (Dr. Adam Brand and Dr. Tommy
Hawkins) working on energetic ionic liquids to test some of their propellants. We have already
received an AFRL propellant sample, AF-M1028A. This propellant and the commercial
ethylammonium nitrate will be electrosprayed in the near future to evaluate their suitability for
electric propulsion and surface engineering.
4. Fundamental research on ionic liquid ion sources.
This line of research is being executed by the MIT group lead by Prof. Lozano. Molecular
dynamics simulations are being used to explore electrospray thrusters on a scale that cannot be
seen with experiments. A complete thruster cannot be modeled at the atomic scale because of
large computational expense, but emissions from smaller droplets of EMI-BF4 have been
simulated. The largest simulation so far has involved 117,912 atoms - a significant
improvement from previous work that only used 3,000. This new droplet has dimensions of
approximately 100 A, which is important to capture the majority of the effects of long-range
Coulomb interactions.
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A number of interesting emission processes have been observed with molecular dynamics
simulations. Figure 7 illustrates a typical simulation. These provide possible explanations for
some aspects of experimental data and improve general understanding of electrospray emission.
Two particularly interesting examples are mentioned below, and illustrated in Fig. 8:

Neutral Fragmentation /Complete Solvated Ion Fragmentation. It had previously
been assumed that solvated ions would fragment into neutrals and ions of lower
solvation degree. However, it has now been seen that neutrals sometimes further
fragment into individual ions and solvated ions sometimes completely fragment into
monomers - neutral fragmentation effectively occurs at the same time as fragmentation
of the complete solvated ion. This results in increased thrust and specific impulse over
models that do not consider neutral fragmentation.

Bond Stretching. A wide spread in emitted ion energies has been observed with
possible ion energies varying by a few eV. The highest energy ions appear to be
produced by bond stretching. While some ions are emitted effectively directly from the
surface, others start to be emitted as long chains. These chains are gradually stretched by
the electric field because they are still attracted by the droplet in the opposite direction.
They are occasionally emitted as complete solvated ions, but often snap if one end
remains firmly attracted to the liquid surface and the strain becomes too high. In this
case, the lower degree solvated ions comprising the remnants of the chain are emitted at
relatively high temperatures because of their stretched bonds.
Figure 7. Steps followed in our molecular dynamics simulation to capture the process of ion emission
Ion Fragmentation Studies
Large droplets can be used to identify typical energies and configurations of emitted ions, but
the simulations take 2-3 weeks to run and hence cannot be repeated a large number of times.
Studies on ion fragmentation were performed by sampling individual solvated ions at selected
temperatures. Sampling particular energies would be more rigorous at this scale, as there was
some variation (around ±1eV) in energies for ions sampled at the same temperature, but the
selected temperatures were sufficiently widely spaced for each to represent a separate region of
energy values.
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It was found that the emission energy was as important as electric field strength in determining
the likelihood of fragmentation; this is because an electric field cannot break a solvated ion apart
unless the individual ions have undergone an oscillation (resulting from high internal energy) to
temporarily increase the spacing between them; this increased distance can reduce the Coulomb
force to be weaker than the external electric field, which is then able to fragment the solvated
ion.
Relevance to Propellant Selection
Fragmentation is heavily dependent on initial energy, but the relationship varies with the
complexity of the ion. In particular, a more complex ion (i.e. an ion with more atoms and hence
more degrees of freedom) distributes the energy in a larger number of modes and hence more
energy is required to achieve a sufficiently large ion spacing for fragmentation to occur in the
presence of an external electric field.
With this in mind, experimental results for three ionic liquids were compared: EMI-I, EMI-BF4,
and EMI-Im. The expected relationship between increasing complexity and decreasing
fragmentation was clear and substantial. Molecular dynamics studies comparing EMI-BF4 and
BMI-PF6 also demonstrated this relationship.
This assumes that the ions are emitted with similar energy distributions in all liquids; this
remains to be confirmed and molecular dynamics simulations for this purpose are currently in
progress, but it appears to be a reasonable assumption in light of past experimental data
demonstrating similar (at least to order of magnitude) energy deficits during emission of
different ionic liquids.
Ion beam polydispersity is the only significant cause of inefficiency in electrospray thrusters;
ion fragmentation is a major contributor to this and so finding a means to mitigate it (i.e. by
using a more complex ionic liquid) is significant.
Experimental Work
Experimental work is in progress towards improving the quality of RPA and TOF
measurements by shaping a large spherical detector to capture almost the emitted beam while
maintaining a constant distance of travel (regardless of deflection angle) and ensuring that the
stopping potential acts on the full kinetic energy of the ions and not just the horizontal
component. The hardware for this purpose has been partly assembled - the collector itself has
been built, but more grids need to be added.
New experimental data was gathered on the impact of increasing temperature on EMI-BF4
emissions. It was found that the current increased rapidly with temperature, roughly doubling
between 25C and 99C. This was to be expected, as an increase in temperature of an ionic liquid
has previously been widely reported to result in an increase in conductivity and decrease in
viscosity. However, the increased current in this new data was accompanied by higher than
expected fragmentation, particularly in the acceleration region - the current doubled while
acceleration region fragmentation increased by a factor of approximately 3.3. This was
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attributed to an increase in the current dimer fraction - most of the increased current resulting
from increasing the temperature was composed of dimers rather than monomers.
Solid Interactions
A key aspect of future work with molecular dynamics simulations will be in understanding ionic
liquid interactions with surfaces under a high applied potential. This is essential for modeling
electrochemical interactions and thruster degradation, which will determine lifetime. Initial
prototype simulations have been constructed, but there are no results yet.
Figure 8. Neutral fragementation and bond stretching reproduced via molecular dynamics
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