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Nano- and submicron-sized crystals are too small to contain inclusions and are, therefore, expected to have a higher internal quality compared to conventionally sized particles (several tens to hundreds of microns). Using electrospray crystallization, nano- and submicron-sized crystals can be easily produced. With the aid of electrospray crystallization, a mist of ultrafine solution droplets is generated and subsequent solvent evaporation leads to crystallization of submicron-sized crystals. Using cyclotrimethylene trinitramine (RDX) solutions in acetone, the conditions for a stable and continuous jet were established. At relatively small nozzle diameters and relatively low potential differences, hollow spheres of RDX crystals were observed. At a higher nozzle diameter and potential difference and in the region of a continuous jet, RDX crystals with an average size of around 400 nm could be produced. In order to test the quality of the submicron-sized energetic material, impact and friction sensitivity tests were carried out. The test results indicate that the submicron-sized product had reduced friction sensitivity, indicating a higher internal quality of the crystalline product.
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Research Article
Electrospray Crystallization for High-Quality
Submicron-Sized Crystals
Nano- and submicron-sized crystals are too small to contain inclusions and are,
therefore, expected to have a higher internal quality compared to conventionally
sized particles (several tens to hundreds of microns). Using electrospray crystalli-
zation, nano- and submicron-sized crystals can be easily produced. With the aid
of electrospray crystallization, a mist of ultrafine solution droplets is generated
and subsequent solvent evaporation leads to crystallization of submicron-sized
crystals. Using cyclotrimethylene trinitramine (RDX) solutions in acetone, the
conditions for a stable and continuous jet were established. At relatively small
nozzle diameters and relatively low potential differences, hollow spheres of RDX
crystals were observed. At a higher nozzle diameter and potential difference and
in the region of a continuous jet, RDX crystals with an average size of around
400 nm could be produced. In order to test the quality of the submicron-sized
energetic material, impact and friction sensitivity tests were carried out. The test
results indicate that the submicron-sized product had reduced friction sensitivity,
indicating a higher internal quality of the crystalline product.
Keywords: Crystallization, Cyclotrimethylene trinitramine, Electrospray crystallization,
Received: December 09, 2010; revised: January 20, 2011; accepted: January 21, 2011
DOI: 10.1002/ceat.201000538
1 Introduction
The product quality of crystalline materials is determined by
the crystallization process applied to produce these materials.
The crystalline product quality is in turn determined by the
crystal size distribution, kind of solid state, morphology, pur-
ity, and internal defects. There are several methods to optimize
the crystallization conditions in conventional crystallization
processes in order to arrive at a certain quality of the crystal-
line product. Using a specific geometry with low cooling rate
can end up in producing 1-mm crystals with narrow crystal
size distribution [1].
Inclusions, dislocations, and point, line, or surface defects
are internal defects [1]. For energetic materials it is thought
that the initiation of an explosion can be caused by an unin-
tentional shock wave hitting a defect upon its propagation
through a crystal. It has been demonstrated that the higher the
product quality of a solid energetic material, the less sensitive a
plastic-bonded explosive containing this energetic material
becomes [2]. A reduced sensitivity means that the detonation
threshold is higher which would prevent unintentional detona-
tions, therefore, these explosives are safer to handle. For the
energetic material cyclotrimethylene trinitramine (RDX), for
instance, a reduced cooling rate in batch cooling crystalliza-
tions resulted in RDX crystals with decreased inclusion con-
tent [3]. Therefore, optimizing conventional crystallization
conditions is a good approach to achieve lower internal defect
densities and thus less sensitive energetic materials.
Another approach is to greatly reduce the crystal size. Since
inclusions are relatively large, submicron-sized crystals are
believed to contain a smaller amount of inclusions. Crystal
defects like dislocations usually originate from inclusions and,
therefore, a much lower overall defect content is expected in
nano- and submicron-sized crystals. Due to the relation of
sensitivity and defect content of energetic compounds, the
product quality can be easily quantified with the help of sensi-
tivity tests.
In this study, an electrospray crystallization method for pro-
ducing submicron-sized crystals of the energetic compound
RDX is presented. The method combines electrospraying of an
undersaturated solution and solvent evaporation from the cre-
ated droplets. The extent to which submicron particles can be
produced with electrospray crystallization as well as their sen-
sitivity performance as energetic material were investigated. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2011,34, No. 4, 624–630
Norbert Radacsi
Andrzej I. Stankiewicz
Yves L. M. Creyghton
Antoine E. D. M. van der
Joop H. ter Horst
Delft University of Technology,
Process and Energy
Laboratory, Intensified
Reaction and Separation
Systems, Delft,
The Netherlands.
TNO Defence, Security and
Safety, Rijswijk, The
Correspondence: N. Radacsi (, Delft University of
Technology, Process and Energy Laboratory, Intensified Reaction and
Separation Systems, Leeghwaterstraat 44, NL-2628 CA Delft, The
2 Electrospray Crystallization
There are several techniques for producing nanoparticles. Con-
ventional methods like milling or pyrolysis and advanced
methods like using thermal plasma [4], rapid expansion of
supercritical solutions [5], or spray drying [6] are often used
for nanoparticle production. Electrospray crystallization is one
of the advanced methods to achieve nano- and submicron-
sized crystals [7]. A constant potential difference is applied
between a grounded plate and a metal capillary (the nozzle)
through which a conductive solution is pumped. If the poten-
tial difference is sufficiently high, electrostatic forces overcome
the surface tension and any flow arriving at the tip of the noz-
zle is directly emitted as a jet of liquid droplets from a Taylor-
cone formed at the nozzle [8]. Due to the potential difference
and the use of a conducting solution the surface of the droplets
is charged. The surface charge density is mainly determined by
the potential difference.
The jet contains small droplets which are accelerated to-
wards the grounded plate by the electric field. Upon using a
sufficiently volatile solvent such as acetone, solvent evapora-
tion occurs which increases the surface charge because of the
decrease in droplet volume. As the surface charge reaches a
critical value (Rayleigh limit [9]), electrostatic forces overcome
the surface tension, and the droplet disrupts into smaller drop-
lets to reduce the surface charge density by creating more sur-
face area. This disruption process is called Coulomb fission
The solute concentration increases as the solvent evaporates
from the droplet. At some point during this process of droplet
evaporation and disruption, the driving force for crystalliza-
tion becomes sufficiently large for crystal nucleation and
growth to occur. It is assumed that crystallization is confined
to the volume of the droplet [11]. Therefore, if the droplets are
sufficiently small, typically one crystal per droplet is formed.
Aggregation of droplets is prevented because the droplets have
surface charges of equal sign. Thus, submicron or even nano-
sized crystals can be formed. These submicron crystals accu-
mulate at the grounded surface loosing their surface charge.
Fig. 1 shows the scheme of the electrospray crystallization pro-
cess and the investigated process parameters (nozzle diameter
d, flow rate u, potential difference DU, solution concentration
c, and working distance D).
3 Experimental
3.1 Materials
Class 2 RDX with an average size of around 400 lm was pur-
chased from Chemring Nobel A.S., Norway. For the electro-
spray crystallization different solution concentrations were
prepared with 99.8 % acetone obtained from Merck. Care was
taken to choose materials in the device that withstand the
exposure to acetone.
3.2 Solubility and Crystal Growth Measurements
The Crystal16 developed by Avantium Technologies BV was
used to determine the RDX solubility in acetone. With Crys-
tal16, cloud points and clear points of sixteen 1-mL solution
aliquots can be measured in parallel and automatically, based
on turbidity. The temperature at the point where the suspen-
sion becomes a clear solution (clear point) upon slow heating
(0.3 °C min
) was taken as the saturation temperature of the
measured sample, of which the composition was established
beforehand. The cloud point refers to the point at which solid
material first appears upon a decrease of temperature.
The crystal growth rate was studied in a stirred cooling crys-
tallization process. For this, the Crystalline Particle Viewer,
also developed by Avantium Technologies BV, was used to
visualize the suspension during cooling crystallization of RDX
with an on-board camera. This multiple 3-mL reactor setup
combines turbidity measurements with four independent real-
time particle visualization modules. This device is capable to
take images every second, and its software analyzes and calcu-
lates particle size, distribution, and shape. The crystalline soft-
ware calculates the median crystal size for every image which
was taken every second. Each minute an average crystal size
was determined from these images. The average crystal growth
rate during the crystallization process can then be
estimated by dividing the size increase and the
time needed to obtain that size increase. Suspen-
sions (3 mL) were prepared of RDX in acetone.
The samples were heated above their saturation
temperature in order to obtain a clear solution.
Then they were cooled down to –15 °C using a
cooling rate of 0.5 °C min
3.3 Single-Nozzle Electrospray
Crystallization Setup
The electrospray crystallization setup (see Fig. 2)
consisted of an Aitecs SEP-10S Syringe Pump with
a 50-mL plastic syringe. A Wallis ±10 kV DC power
supply was used to provide the potential difference
DUbetween the tip of the nozzle and the grounded
Chem. Eng. Technol. 2011,34, No. 4, 624–630 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 1. Process scheme of electrospray crystallization. Varied operation condi-
tions: nozzle diameter (d), flow rate (f), concentration (c), potential difference
(DU), and working distance (D). Solvent evaporation, Coulomb fission, and crys-
tallization occur during the process.
Electrospray crystallization 625
plate which were separated at a working distance D. The pump
and a Festo 6x1 pneumatic tube were used to transport the
solution with concentration cto the nozzle with a certain flow
rate u. Seven different nozzles (EFD, USA) were used for the
experiments. All of them had the same length (25.4 mm) but
varying inner diameters d(0.1524, 0.254, 0.3302, 0.4064,
0.508, 0.5842, or 1.3716 mm, referred to in the paper as, re-
spectively, the 0.15, 0.25, 0.33, 0.4, 0.51, 0.58mm nozzle for
convenience). The experiments were performed at room tem-
3.4 Multiple-Nozzle Electrospray Crystallization
Multiple nozzles have the advantage that higher production
rates can be achieved. An eight-nozzle system was used with a
Meredos TL-EAD peristaltic pump in order to have an
equal distribution of the solution flow over the nozzles
(Fig. 2). The rest of the setup was similar to the single noz-
zle setup. The experiments were performed at room tem-
3.5 Characterization Tests
For morphological and size characterization of the crystal-
line samples a Philips XL30 FEG scanning electron micro-
scope was applied. Typical instrument settings of the elec-
tron beam were 2 kV, spot size 3, and working distance
10 mm. Samples were investigated as is, without any
further treatment. Only low electron beam energies were
used since energies higher than 2 kV at higher magnifica-
tions resulted in decomposition of the energetic materials.
The BAM Fallhammer and Friction Apparatus tests are
the recommended test methods in the UN recommenda-
tions for the transport of dangerous goods [12]. For the
impact sensitivity test a BAM Fallhammer device was used
(according to the international guidelines EC A.14 and UN
3(a) (ii)) [12]. The temperature during the test was 23 °C.
The volume of the submicron-RDX sample was approxi-
mately 40 mm
and the drop weight was 5 kg. During the
test, the drop height was decreased from 50 cm until initia-
tion of the sample did not occur for six times in series.
The sensitivity to friction was determined by a BAM
Friction Apparatus (according to the international guide-
lines EC A.14 and UN 3(b) (i) [12]. The sample is placed
on a roughened porcelain plate which is rigidly attached to
the sliding carriage of the friction apparatus. A cylindrical
porcelain peg is placed on top of the sample. The porcelain
plate can move forward and backward under the porcelain
peg. The smallest load of the peg, which causes deflagra-
tion, crackling, or explosion of the test sample at least once
in six consecutive trials, is the outcome of the friction sen-
sitivity test. The quantity of the test sample is 10 mm
4 Results and Discussion
4.1 Solubility and Conventional Growth Rate
Fig. 3 shows the solubility line of RDX in acetone determined
from the saturation temperature measurements carried out
with Crystal16. At 45 °C the solubility for RDX in acetone is
111 mg mL
. The RDX solubility in acetone increases with
Cooling crystallization experiments were performed with
the Crystalline Particle Viewer setup. During cooling crystalli-
zation, the solution was monitored in situ by measuring the
transmission of light through the solution and by taking im-
ages simultaneously. The evolving crystal size was monitored
after a sufficient amount of crystals was present. Since the res-
olution of the on-board camera is sufficiently high (5–10 lm/
pixel) to identify crystals above 30 lm size, only these were
taken into account. Fig. 4 presents the average size of the RDX
crystals as a function of time. Initially the size seems to be con- © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2011,34, No. 4, 624–630
Figure 2. Single-nozzle electrospray crystallization setup on the left and
the eight-nozzle experimental setup on the right showing the capillary
nozzle (A), high-voltage connectors (B), grounded plate of aluminum
foil on which the particles are collected (C), the nozzle holders (D), and
the Meredos TL-EAD peristaltic pump (E).
Figure 3. Solubility of RDX in acetone as a function of temperature T.
626 N. Radacsi et al.
stant slig htly above 30 lm. After around 10 min, the size seems
to increase roughly linearly with time, leading to an average
growth rate of about 0.061 lms
for RDX in acetone (see
Fig. 4). These values are typical for the crystal growth of or-
ganic compounds from solutions
4.2 Single-Nozzle Electrospray Crystallization
Operation Conditions
For RDX, acetone is a favorable solvent to use since RDX crys-
tals from acetone have a compact shape [13]. Moreover, ace-
tone is a (slightly) conductive solvent which allows to electro-
spray the solution. Furthermore, acetone has a high vapor
pressure, resulting in a relatively high evaporation rate during
electrospray crystallization.
The first goal was to find the process conditions leading to
the formation of a continuous jet in the single nozzle setup. A
continuous jet is established when there is a stationary droplet
emission from the nozzle without any interruption. Then, the
effect of the five process variables was studied: the flow rate u,
potential difference DU, working distance D, nozzle diameter
d, and the solution concentration c.
Using a solution with a concentration of 20.8mg mL
a continuous jet was observed at a potential difference of
+4.8 kV, a nozzle diameter of 0.58mm, a working distance of
25 mm, and a flow rate of 2.8 mL h
. This is taken as our refer-
ence experiment for studying the operation conditions as well
as the crystalline product characteristics.
4.2.1 Flow Rate
A continuous jet was observed at flow rates between 1 and
and otherwise equal settings compared to the stan-
dard experiment. At flow rates below 1 mL h
, the solution
was only intermittently sprayed from the nozzle. Apparently
the flow rate then is too small to achieve a steady-state electro-
spray of droplets from the tip. Flow rates higher than 5 mL h
resulted in formation of an unwanted continuous flow of solu-
tion from the tip without jet formation: The electrospraying
cannot keep up with the accumulation of solution at the tip
due to the relatively high flow rate.
4.2.2 Potential Difference
A continuous jet is obtained when the potential difference is
varied between 3.8 and 4.9 kV while using the standard settings
for the other variables. Applying a potential difference above
4.9 kV resulted in formation of multiple jets. Three jets were
observed when using a potential difference of 6.2 kV with an
RDX solution of 20.8 mg mL
, a working distance of 25 mm,
and a flow rate of 2.8 mL h
. Multiple jets are caused by the
formation of multiple Taylor cones. These multiple Taylor
cones form because of the extremely high charge density at the
nozzle tip.
Fig. 5 shows the minimum and maximum potential differ-
ence for obtaining a continuous jet for different working dis-
tances using an RDX solution concentration of 20.8 mg mL
a flow rate of 2.8 mL h
, and a nozzle diameter of 0.58 mm.
The area between the two lines indicates the region for obtain-
ing a continuous jet.
4.2.3 Working Distance
Fig. 5 also shows that at larger working distances a higher
potential difference is needed to obtain a continuous jet. The
working distance between the nozzle tip and the grounded
plate was varied between 5 and 70 mm. Working distances
smaller than 5 mm were not tested since unwanted sparks
might occur. At working distances smaller than 10 mm solvent
evaporation of the droplets travelling from the tip to the
grounded plate was only partial, and the remaining acetone
evaporated on the grounded plate. This is unwanted since
crystallization then partly occurs at the grounded plate. At
working distances above 35 mm, the developed jet was contin-
Chem. Eng. Technol. 2011,34, No. 4, 624–630 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 4. Left: RDX crystals during cooling crystallization from acetone. Right: median crystal size (D
) as a function of time. The cooling
rate during the experiment was set to 0.5 °Cmin
Electrospray crystallization 627
uous but the droplets were not homogeneously distributed in
time and space within the jet, i.e., it was observed that after a
time interval of several seconds a part of the flow was emitted
in relatively large droplets from the nozzle tip which travelled
at the outer side of the jet to the ground plate. These droplets
often partly evaporated on the collector plate.
4.2.4 Nozzle diameter
The minimum and maximum potential difference for the for-
mation of a single continuous jet were determined for seven
different nozzle diameters (Fig. 6). For the concentration and
flow rate the standard values were used, while 35 mm was cho-
sen for the working distance since that was the upper limit
below which no dripping occurred. A larger nozzle diameter
demanded a higher potential difference in order to obtain a
continuous jet.
4.2.5 Concentration
While using the standard settings for the other variables, de-
creasing the RDX concentration did not affect the continuous
jet formation: For concentrations between 2 and 20.8 mg mL
a continuous jet was formed. However, increasing the concen-
tration led to unwanted crystallization at the nozzle tip. This
eventually led to blocking of the nozzle. Using a concentration
of 24.9 mg mL
, crystals were growing on the nozzle tip
which blocked the flow within 15 min. Using a solution
close to the supersaturated concentration
(41.6 mg mL
RDX) caused the blocking
to occur within 40 s.
4.3 Product Characterization
For checking the product size and shape,
scanning electron microscopy was used.
The resulting submicron-sized crystals of
the standard experiment are shown in
Fig. 7. The produced crystals have a spher-
ical shape and a size ranging from 200 to
600 nm. No extensive agglomeration was
observed which indicates that under these
conditions droplets are not aggregating
and the small droplet volume causes the
nucleation of only a single RDX crystal per
droplet. This single crystal per droplet then
consumes the dissolved RDX by growth
upon further evaporation of the acetone in
the droplet. A crystal size of 400 nm indi-
cates that the solution droplet from which
the crystal is formed is around 1.77 lm.
These experiments show that electrospray
crystallization can be used to obtain sub-
micron-sized crystals of RDX.
4.3.1 Hollow-Sphere Agglomerates
When the potential difference was decreased to below 4.5 kV
or lower compared to the standard experiment, a considerable
amount of hollow spheres of multiple agglomerated RDX crys-
tals were observed in the crystalline material (Fig. 8). These
hollow spheres probably develop due to the combination of a © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2011,34, No. 4, 624–630
Figure 5. Potential difference DUand working distance Dto obtain a stable and continu-
ous single jet using a c= 20.8 mg mL
RDX solution in acetone, a nozzle diameter of
d= 0.4 mm, and a flow rate of u= 2.8 mL h
. Upper line: potential difference above which
multiple jets were obtained; lower line: potential difference below which intermittently
sprays were emitted from the nozzle tip. Area between the lines: region for continuous
jet formation. Black point: conditions of the reference experiment.
Figure 6. Minimum and maximum potential differences DUfor
obtaining a single continuous jet as a function of the nozzle
diameter dat a working distance D= 35 mm, concentration
c= 20.8 mg mL
, and flow rate u= 2.8 mL h
. In the region be-
tween the lines a continuous jet was formed.
628 N. Radacsi et al.
relatively low potential difference and a relatively high RDX
concentration. Under these conditions, relatively large droplets
might form due to a decreased surface charge, less Coulomb
fission, and more aggregation of droplets. Due to the large
concentration, these droplets become supersaturated relatively
soon after being sprayed from the nozzle. Then multiple crys-
tals can form at the surface of the droplet which develop in a
shell of connected elongated crystals of which the internal is
empty. An alternative explanation for forming similar particles
can be found elsewhere [14].
At an RDX concentration c= 20.8 mg mL
, these hollow-
sphere agglomerates could be prevented by using a combina-
tion of a high potential difference (DU+4.5 kV) and a large
nozzle diameter (d0.58 mm). These boundar y values for
nozzle diameter and potential difference decrease when lower
concentrations are used. This indicates that also decreasing the
solution concentration can prevent the presence of these hol-
low-sphere agglomerates in the product.
In case the potential difference is higher than +4.5 kV, the
hollow spheres were not observed, also above the potential dif-
ference threshold for a continuous single jet. At higher poten-
tial differences the charge density on the droplet surface is larg-
er which might result in smaller droplet sizes after Coulomb
fission. In smaller droplets the chance of forming only one
crystal is higher. Moreover, at higher surface charges the aggre-
gation of droplets is decreased.
With 20.8 mg mL
concentration, hollow spheres and ag-
glomerates can be observed in case the potential difference is
rather low. In Fig. 5 it can be seen that the operation window
for potential difference is around 1 kV for obtaining a continu-
ous jet. According to our measurements with this relatively
high concentration, hollow spheres can be observed in the low-
er region of this window.
The formation of these hollow-sphere agglomerates makes
the operation window for submicron-sized RDX production
without agglomerates produced from a continuous jet rather
small. To obtain submicron-RDX crystals from a continuous
jet, a low concentration (c20.8 mg mL
) and a high potential
difference (DU+4.5 kV) are recommended with a nozzle
diameter of d0.58 mm at a working distance of D35 mm
and a flow rate of 2.8 mL h
4.3.2 Crystal Growth Rate during Electrospray Crystallization
The average growth rate in the electrospray crystallization pro-
cess was calculated by dividing the product crystal size
(400 nm) with the estimated time the solvent needed for evap-
oration. From the experiments with different working dis-
tances it was observed that working distances below 10 mm
resulted in acetone evaporation partly on the collector plate.
Therefore, 10 mm was chosen as the working distance for cal-
culating the time for solvent evaporation. The actual time of
growth of the nanoparticle is smaller since the nanoparticle is
formed somewhere along the path from nozzle to plate. In
order to calculate the time, the droplet velocity is also needed.
At a voltage of 5 kV in an electrospraying process the droplet
velocity of acetone was reported to be 10.8 m s
[15]. The time
the droplet is travelling was thus estimated to be 1.08 ms.
Thus, the estimated growth rate was 370 lms
. Compared
with cooling crystallization, electrospray crystallization results
in at least 6000 times higher growth rates which seems very
4.4 Multiple-Nozzle Electrospray Crystallization:
Product Sensitivity
To check the expected product quality improvement, impact
and friction sensitivity tests were performed. For these tests, at
least 1 g of material was needed and, therefore, the multiple
nozzle setup was used to increase the production rate. The
production rate with the eig ht-nozzle setup was 84 mg h
using 24.9 mg mL
RDX at a flow rate of 1.8 mL h
Chem. Eng. Technol. 2011,34, No. 4, 624–630 © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 7. 200–600 nm sized RDX crystals from the standard elec-
trospray crystallization experiment using the 0.58-mm nozzle at
a potential difference of +4.8 kV, a working distance of 25 mm, a
solution concentration of 20.8 mg mL
, and a flow rate of
2.8 mL h
Figure 8. Hollow spheres of RDX crystals produced with the
0.58-mm nozzle at a potential difference of 4.2 kV, 25 mm work-
ing distance, a solution concentration of 20.8 mg mL
, and a
flow rate of 2.8 mL h
. Typically the size of these spheres was
around 4 lm.
Electrospray crystallization 629
With the produced samples, impact sensitivity tests were
carried out. When the drop height was set at 15 cm w ith a 5-kg
drop weight (7.5 J impact energy), there was no ignition using
the produced submicron-sized RDX. Ignition only occurred
when the height was set to 20 cm (10 J). The conventional
RDX needs 7.5 J energy for ignition [12] which is not signifi-
cantly different from that measured for the submicron-sized
Friction sensitivity tests were performed as well with nano-
crystals of RDX. Tests were carried out first at the highest
possible load (360 N) to check the friction sensitivit y of the
400-nm RDX. For this energetic material even with this high
load no ignition occurred. Conventional RDX needs 120 N
load for ignition [12]. Apparently, the friction sensitivity is
much lower for the submicron-sized energetic particles than
for the conventionally sized explosive crystals. These sensitivity
tests might indicate that the submicron-sized crystals contain
less defects and, therefore, have a higher internal quality.
5 Conclusions
Submicron-sized crystals of RDX can be produced using elec-
trospray crystallization. The sensitivity of samples having an
average size of around 400 nm for RDX was tested. The sam-
ples were remarkably insensitive to friction stimuli while an
insignificant difference for the impact sensitivity was observed.
The operation window to establish a continuous jet and
produce submicron-sized crystals is relatively narrow, but
experimentally feasible. It is being worked on to optimize this
process to higher yields and improved process control using a
new process design.
We would like to thank Leon Wassink for performing some ex-
periments, Willem Duvalois for the SEM analysis, and Ilse
Tuinman for providing the peristaltic pump.
The authors have declared no conflict of interest.
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Table 1. Impact and friction sensitivity of 400-nm RDX compared
with conventionally sized RDX.
Impact sensitivity [J] Friction sensitivity [N]
Conventional RDX 7.5 120
Submicron-sized RDX 10 > 360
630 N. Radacsi et al.
... Nevertheless, energetic materials have only been processed by EHDA for one decade. At the end of the 2010s, nanofibers of nitrocellulose (NC) were first prepared by electrospinning [3], while nanocrystals of hexogen (RDX) were successfully produced by electrospray crystallization [4]. Then, efforts to control the reactivity and output performance of EMs with designed microstructures by EHDA became active, and a considerable number of academic contributions have been made by researchers worldwide with multidimensional impacts. ...
... Nanoparticles of RDX, HMX, LLM-105 and CL-20 have been fabricated by this procedure, as summarized in Table 1. Radacsi [4] fabricated submicron single crystals of RDX (~400 nm) with lower impact sensitivity (10 J) and friction sensitivity (>360 N) by electrospray, as shown in Figure 2b. However, when the applied voltage was decreased to 4.5 kV or lower, a certain content of hollow spheres (~4 µm) composed of multiple crystals appeared due to the lower surface charge and less Coulomb fission. ...
... The calculated crystal growth rate in the electrospray crystallization process within an ultrashort time (~1.08 ms) is nearly 6000 times higher than that of cooling crystallization [4]. Therefore, the crystal phase of the obtained particles may be different from that of the raw materials due to insufficient crystal growth. ...
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Constructing ingenious microstructures, such as core–shell, laminate, microcapsule and porous microstructures, is an efficient strategy for tuning the combustion behaviors and thermal stability of energetic materials (EMs). Electrohydrodynamic atomization (EHDA), which includes electrospray and electrospinning, is a facile and versatile technique that can be used to process bulk materials into particles, fibers, films and three-dimensional (3D) structures with nanoscale feature sizes. However, the application of EHDA in preparing EMs is still in its initial development. This review summarizes the progress of research on EMs prepared by EHDA over the last decade. The morphology and internal structure of the produced materials can be easily altered by varying the operation and precursor parameters. The prepared EMs composed of zero-dimensional (0D) particles, one-dimensional (1D) fibers and two-dimensional (2D) films possess precise microstructures with large surface areas, uniformly dispersed components and narrow size distributions and show superior energy release rates and combustion performances. We also explore the reasons why the fabrication of 3D EM structures by EHDA is still lacking. Finally, we discuss development challenges that impede this field from moving out of the laboratory and into practical application.
... The electrospray technique (also called electrospray ionization deposition) is one of the most efficient ways for the preparation of nanospheres and nanoparticles as it is a simple and inexpensive method [1][2][3]. Electrosprayed nanoparticles are often used for pharmaceutical, biological or medicinal applications [4][5][6]. For example, electrospray can be used to fabricate nanoparticles loaded with drugs for nanoparticle drug delivery or loaded with cell growth factors for tissue engineering [4,7,8]. ...
... A typical laboratory electrospinning setup can be capable of both the electrospray and electrospinning modes. In general, the setup is made of 4 main parts ( Fig. 1): (i) a syringe, which is placed inside a syringe pump for continuous solution flow; (ii) a metallic nozzle; (iii) a high voltage power supply (which is connected to the nozzle); (iv) and a collector (which is conductive to attract the charged nanoparticles/nanofibers, and is placed opposite to the high voltage electrode) [3,22]. ...
... In both cases, the liquid that is ejected from the nozzle forms a specific cone geometry, called Taylor cone [23]. In the electrospray mode, highly charged droplets are ejected as a form of a jet from the Taylor cone, and upon solvent evaporation, solid nanoparticles can be collected [3]. While in electrospinning mode, continuous fibers are emitted from the Taylor cone, and the nanofibers solidify after the complete solvent evaporation [24]. ...
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Here, we report on the inexpensive fabrication of an electrospray/electrospinning setup by fused deposition modelling (FDM) 3D printing and provide the files and parameters needed to print this versatile device. Both electrospray and electrospinning technologies are widely used for pharmaceutical, healthcare and bioengineering applications. The setup was designed to be modular, thus its parts can be exchanged easily. The design provides a safe setup, ensuring that the users are not exposed to the high voltage parts of the setup. PLA, PVA, and a thermoplastic elastomer filament were used for the 3D printing. The filament cost was $100 USD and the rig was printed in 6 days. An Ultimaker 3 FDM 3D printer was used with dual print heads, and the PVA was used as a water-soluble support structure. The end part of the setup had several gas channels, allowing a uniform gas flowing against the direction of the nanoparticles/nanofibers, enhancing the drying process by enhancing the evaporation rate. The setup was tested in both electrospray and electrospinning modes successfully. Both the .sldprt and .stl files are provided for free download.
... ES is considered a part of EHDA [17,19]. Generally, ES consists of components such as a high voltage source, a syringe pump (SP), a spray head, and a collector [12,20,21]. ES proceeds by forcing a liquid through a capillary to a collector under a potential drop, on the order of kilovolts (kV), which is applied between the capillary and the collector [22]. ...
... When the polymer solution flows through the capillary, the droplet is highly electrified and distorted into a conical shape commonly known as a Taylor cone (Figure 1). After the solvents are evaporated during the course of the jet spray, the generated electrospun polymeric materials are collected on or in a grounded collector [21]. Here, in the ES technique, the main aspect consists on the deformation of the liquid flow at the emitter output, in the case of the needle tip, acquiring a conical structure (Taylor cone), due to EHDA interactions, as shown in Figure 1 [17,23,24]. ...
... The principles of ES can be grounded into the theory of charged drops, which in its concept, states that if an electric field is applied to a drop, the electric charge generates an electrostatic force on the drop (a Coulomb force), that competes with the cohesion force in the drop. Hence, when the Coulomb force overcomes the cohesive force of a drop, it results in the release of surface tension and particle formation [21,25]. Nevertheless, there is a limit, called the Rayleigh limit (L R ), which exists to determine when the drop breaks. ...
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Microparticles (MPs) with controlled morphologies and sizes have been investigated by several researchers due to their importance in pharmaceutical, ceramic, cosmetic, and food industries to just name a few. In particular, the electrospray (ES) technique has been shown to be a viable alternative for the development of single particles with different dimensions, multiple layers, and varied morphologies. In order to adjust these properties, it is necessary to optimize different experimental parameters, such as polymer solvent, voltage, flow rate (FR), type of collectors, and distance between the collector and needle tip, which will all be highlighted in this review. Moreover, the influence and contributions of each of these parameters on the design and fabrication of polymeric MPs are described. In addition, the most common configurations of ES systems for this purpose are discussed, for instance, the main configuration of an ES system with monoaxial, coaxial, triaxial, and multi-capillary delivery. Finally, the main types of collectors employed, types of synthesized MPs and their applications specifically in the pharmaceutical and biomedical fields will be emphasized. To date, ES is a promising and versatile technology with numerous excellent applications in the pharmaceutical and biomaterials field and such MPs generated should be employed for the improved treatment of cancer, healing of bone, and other persistent medical problems.
... Subsequent solvent evaporation leads to crystallization of submicron-sized crystals. 28 For example, hollow spherical RDX crystals were obtained at relatively small nozzle diameters and relatively low potential differences, whereas larger nozzles and higher potential differences approaching a continuous jet delivered products with larger average particle sizes. The RDX nanoparticles (500 nm) with a narrow particle size distribution obtained by this method 29 are less sensitive due to fewer inclusions and crystal defects. ...
... (a), Electrospray crystallization process diagram.28 (b), Schematic representation of the RESS setup. ...
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The safety properties and desirable detonation performance of energetic materials (EMs) are mutually exclusive, therefore, various strategies including the coating, doping, crystallization, and co-crystallization, are applied to achieve high-energy insensitive explosives with well-balanced energy and safety level. Among these strategies, the crystallization is the most commonly method owing to its low cost and facile process, through which the tuning of the particle size and morphology, adjust sensitivity of EMs by tailoring the processes conditions. As the control of the crystal particle size is difficult, the ultrasound and electrospray are introduced, and by use of the spray drying or spray-assisted electrospray methods, the spherical RDX, HMX, and CL-20 crystals with less defects is obtained. Moreover, the perfect spherical crystals are gained without agglomeration through employing polymeric additives in the crystallization process. In general, the crystallization with spray drying, electrospray, and ultrasound-assisted solvent/antisolvent are the optimal crystals preparation methods. The nano-crystals with narrow particle size distribution are less sensitive to external stimuli than irregular microcrystals, and defects are associated with hot spots, the safety and energy performance of EMs could be well balanced by crystallization.
... Process parameters including nozzle diameter, electrical potential, flow rate, working distance, and the drug solution properties (solvent conductivity, solute concentration, and the surface tension of the solution) are the factors affecting the formation of a stable and continuous cone-jet mode (Sverdlov Arzi and Sosnik, 2018). In general, within the standard range, lower nozzle diameter, higher electrical potential, higher flow rate, and lower distance between the collector and the nozzle tip can result in smaller droplets, which is more likely to generate a stable cone jet mode with more monodisperse particles produced (Nguyen et al., 2016;Radacsi et al., 2011). The conductivity of the drug solution has to meet the standard range to produce sufficient current while preventing an unstable jet process (Ijsebaert et al., 2001). ...
... Ionization of montelukast in the solvent can enhance the electroconductivity of acetone (more than 10 − 6 S.m − 1 ) which leads to an unstable spray process associated with the coalescence of droplets (Ijsebaert et al., 2001). Formation of drug monolayer on the surface of the collector lowers its conductivity and, despite the reduction of solvent surface free energy as a function of montelukast surface activity, the electrical forces between the nozzle and collector are not yet enough to overcome the surface tension of the solution Radacsi et al., 2011). While the lower electroconductivity of dichloromethane (a slightly conductive solvent) compared to acetone effectively disables this phenomenon. ...
In the current study electrospraying methodology was used for particle engineering of montelukast and budesonide to prepare a combined inhalable dry powder formulation applicable as a smart regimen in asthma treatment. For this, electrospraying was carried out using different solvents and drug concentrations. No carrier was added for the formulation of montelukast-budesonide combination as montelukast played the role of both active ingredient and carrier. Scanning electron microscopy, particle size analysis, gas chromatography, powder X-ray diffraction, Fourier transform infrared spectroscopy, and differential scanning calorimetry were used to evaluate the physicochemical properties of the produced drug particles. In vitro drug deposition pattern was assessed using next generation impactor, and the dissolution profile of the selected formulations was characterized via modified diffusion franz cell method. The FPF value for the co-electrosprayed carrier free formulation of montelukast-budesonide was 38% with a significantly enhanced dissolution rate for budesonide compared to the budesonide alone formulations. The pharmacological effects of hypothesized combined formulation was assessed by measuring its power to inhibit the production of reactive oxygen species in human normal lung cells. The results showed that the combination of montelukast and budesonide can exert a synergistic effect. The findings in the current study emphasize that using montelukast as a carrier for budesonide not only has greatly improved the aerosolization behavior and dissolution rate of budesonide but also has resulted in synergistic pharmacological effects, indicating the suitability of this combination as an anti-asthmatic therapeutic.
... According to Radacsi et al. (2011), multiple Taylor cones can be obtained via the application of a high voltage (6.2 kV). Based on these results, it is considered that separating the needles of the needle electrodes from each other is useful for the stable formation of multiple Taylor cones. ...
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As a simplistic concept, needle electrodes to generate atomization in the liquid phase were increased for forming multiple Taylor cones. Numerical simulations using a Poisson equation were performed to optimize the distance between the needle electrodes. The simulation results revealed that the distance between the needle electrodes affected the formation of the Taylor cone, and that the stabilization of the Taylor cone required a distance of over 30 mm between electrodes. The experimental results similarly showed the stabilization of Taylor cones by a distance of 30 mm between electrodes. Multiple Taylor cones stabilized via electrostatic atomization were applied to produce encapsulated oil powders. The increase in the number of electrodes enabled the mass production of encapsulated oil powders via nozzleless electrostatic atomization. This technology may allow the formation of oil powders with functional particles. Practical Applications Electrostatic atomization in the liquid phase is an attractive process for obtaining spherical particles; however, this technique affords a low level of production of the generated particles. The present study aimed to develop a device for the mass production of encapsulated oil powders using nozzleless electrostatic atomization. Numerical simulations were used to design the device for electrostatic atomization for mass production. Multiple Taylor cones may allow the mass production of encapsulated oil powders.
... It was revealed that the friction and spark sensitivity were remarkably reduced with the decreasing of the crystal size. The reduction in the friction sensitivity is in line with theoretical postulates and similar results are reported in the literature also for HMX [32] and RDX [33] while the crystal entered into the nano domain. It may be attributed to the fact that decrease in the size reduces the internal defects but may lead to enhancement in surface imperfections. ...
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Nanotechnology has played an influential role in improving the energetic content without subsiding the performance of high energy materials in the current era. In this work, HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) nanoparticles were prepared by sonochemically assisted solvent-antisolvent spray technique focussing the reduction in its size so as to improve its energetic properties. In order to fabricate nano HMX various parameters such as different solvents and temperature were investigated. Sonication is one of the strategies recently explored in this regard; so time dependent study of sonication using probe sonicator was performed. It has been postulated that bubble formed during sonication when collapses generate high temperature and many nucleation sites which leads to the formation of uniform spherical particles with small size and fast transition phase. XRD studies depicted phase transformation from α to β as a result of sonication. The TEM images revealed that the rise in the sonication time resulted into decrease in the particle size from 300 to 10 nm. Differential scanning calorimetry (DSC) was employed to determine the heat release of the samples and enhancement in the heat release with the decrease in the particle size. A decrease in the spark sensitivity was observed from 2J (regular HMX) to 50 mJ (nano HMX). Keywords: Sonication, Spray technique, Nano HMX, Probe sonicator, Impact sensitivity
An accurate monitoring of glucose level can facilitate appropriate treatment. An electrochemical sensor consisting of a screen-printed carbon electrode (SPCE) was enhanced by the addition of electrosprayed gold nanoparticles and electrospun polyacrylic acid/polyacrylonitrile nanofibers (PAA/PAN/AuNPs) for the non-enzymatic determination of glucose. The volume ratio of PAA:PAN of 6:4 was used due to the highest conductivity value of 1.64 × 10−3 ± 1.11 × 10−4 S cm−1. Addition of electrosprayed gold nanoparticles onto the electrospun PAA/PAN increased the conductivity up to 2.65 × 10–3 ± 2.97 × 10–4 S cm−1. The electrochemical behavior of SPCE before and after modification was studied by electrochemical impedance spectroscopy and cyclic voltammetry. To achieve the optimal electrocatalytic oxidation of glucose, pH, deposition potential, and deposition time of differential pulse voltammetry were utilized. Under the optimal conditions, the sensor had a detection limit of 0.756 mM with sensitivity of 1.043 µA cm−2 mM−1 for linear range of 2 mM to 16 mM.Graphic abstract
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In order to decrease the sensitivity and broaden the application of pentaerythritol tetranitrate (PETN), a novel energetic co-crystal composed of PETN and dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) with high energy and low sensitivity was successfully prepared through the solvent/non-solvent method. The morphology and structure of the as-prepared co-crystal were characterized by scanning electron microscopy (SEM), X-ray diffraction spectroscopy (XRD), X-ray photoelectron spectrometry (XPS), fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy and high performance liquid chromatography (HPLC). The thermal decomposition properties were also analyzed by simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC). The safety performance was judged by mechanical sensitivity tests. The SEM results revealed that the prepared new material was homogeneous with a mean granularity of 1 μm and the morphology was distinct from raw PETN and TKX-50. The XRD analysis indicated that a new crystalline formation appeared in the co-crystal which was quite different from the raw materials and their mixture. The XPS analysis showed peak shifts of C, N, O elements in the co-crystal. The FTIR spectra and Raman spectra suggested that hydrogen bond interactions existed between PETN and TKX-50 molecules. The molar ratio of PETN and TKX-50 was 1:1 determined by HPLC. There were two thermal decomposition peaks (194.1 °C and 261.3 °C) for the co-crystal at 20 °C min ⁻¹ , while the raw materials and mixture had only one. Besides, the activation energy of the co-crystal increased compared to the raw materials, indicating better thermal stability of the co-crystal. The impact sensitivity and friction sensitivity of the PETN/TKX-50 co-crystal were reduced compared to raw PETN, and were even better than for 1,3,5-trimethylene trinitramine (RDX). The results showed a prospective application of the prepared PETN/TKX-50 co-crystal in the future.
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Flexible carbon fiber fabrics of 1.7 μm diameter fiber were prepared by electrospray deposition (ESD) from phenolic resin/poly(vinyl butyral) and successive curing and carbonization. In the present study, phenolic resin (thermostable 3-dimensional cross-linked polymer) was used for ESD and successive carbonization. The bead-free fiber was obtained by ESD from a phenolic resin solution with concentration above 55 wt %. The as-deposited fabrics, however, were very brittle. The addition of poly (vinyl butyral) (Mw=110,000) improved the dimensional stability of the as-deposited fabrics. After carbonization of the as-deposited fabrics, the flexible carbon fiber fabrics were obtained. The carbon fiber fabrics showed relatively high electrical conductivity and a large specific surface area (BET surface area = 495 m2/g). This indicates that the flexible carbon fiber fabrics are applicable not only for adsorbent and composite materials but also for materials for electronic device use.
A solvent can have a large influence on the crystal morphology. A molecular modeling technique based on the adsorption of solvent molecules on a crystal surface has been used to predict the influence of a solvent on the morphology of RDX. The predicted morphology has been compared with the experimentally found morphology of RDX crystallized from several solvents by cooling crystallization.
Electrospraying is a method of liquid atomisation by electrical forces. Droplets produced by electrospraying are highly charged, that prevents their coagulation, and promotes self-dispersion. Droplets can be extremely small, in special cases down to nanometers, and the charge and size of the droplets can be controlled to some extent by voltage and flow rate. Electrospraying is applied in many industrial processes such as painting, microencapsulation, electroemulsification, fine powder production, or micro- and nanothin film deposition prepared from solutions or colloidal suspensions. Recently, the electrospraying entered in microfluidic devices and nanotechnology. Spraying solutions or suspensions allows production of fine particles, down to nanometer size, which can be used in industrial processes or for research purposes. The paper reviews electrostatic methods for fine particles production.
The deposition of La0.7Ca0.3CrO3-delta (LCC) thin films was studied in detail by electrostatic spray deposition, considering the functions of deposition temperature, substrate material, and solvent composition. The microstructures of LCC films, which varied from porous reticulated models and cagelike particles to interconnect nanowire structures, can be effectively assembled. With increasing substrate temperature, the porosity decreases considerably; the pore and pore-wall sizes both became smaller. When changing the substrate material from nickel, aluminum, and alumina substrates to copper substrate, the microstructure of LCC film converted from the porous reticulated model to the cagelike particle model, as determined by the bigger contact angle on the copper substrate than that on other substrates. A qualitative mechanism involved in the formation of various microstructures is presented. Moreover, by changing the solvent composition, the layer morphology may also be significantly changed from the porous reticulated model to interconnect nanowire structure. (c) 2007 The Electrochemical Society.
Liquid inclusion phenomenon of cyclotrimethylene trinitramine (RDX) crystals commercially produced (Hanwha-RDX as a conventional RDX and I-RDX as a reduced sensitivity RDX (RS-RDX)) and recrystallized by a cooling method was quantitatively analyzed by using matching refractive index/digital image analysis, scanning electron microscopy, X-ray diffraction, and gas chromatography. Quantitative estimation of inclusion of RDX crystals by area distribution was made using image analysis and average values of Hanwha-RDX and I-RDX were 9.3% and 5%, respectively, and that of RDX recrystallized from γ-butyrolactone with a controlled cooling policy was 3.5−3.8% with initial crystallization temperatures smaller than 90 °C. It was found that the solvent content of RDX crystals from γ-butyrolactone by a linear cooling from 90 to 20 °C was about 1686−3140 ppm (cooling rates: 0.5, 1, 1.5, and 2 °C/min) and was reduced to 970.7 ppm by controlled cooling. Liquid inclusions in RDX crystals produced from γ-butyrolactone were found to increase rapidly at the initial stage of crystallization. These results suggest that reduction of the amount of liquid inclusion in RDX is possible by a careful controlled cooling strategy at nucleation and the initial step of crystal growth.
The product quality of energetic materials is predominantly determined by the crystallization process applied to produce these materials. It has been demonstrated in the past that the higher the product quality of the solid energetic ingredients, the less sensitive a plastic bonded explosive containing these energetic materials becomes. The application of submicron or nanometric energetic materials is generally considered to further decrease the sensitiveness of explosives. In order to assess the product quality of energetic materials, a range of analytical techniques is available. Recent attempts within the Reduced-sensitivity RDX Round Robin (R4) have provided the EM community a better insight into these analytical techniques and in some cases a correlation between product quality and shock initiation of plastic bonded explosives containing (RS-)RDX was identified, which would provide a possibility to discriminate between conventional and reduced sensitivity grades.
Nanoscale crystals of cyclotrimethylenetrinitramine (RDX) were produced by Rapid Expansion of Supercritical Solutions (RESS). The experiments were performed by expansion of supercritical solutions of RDX in carbon dioxide through sapphire nozzles (ID: 100 and 150  μm) at pressures of 15.0–29.5 MPa and temperatures of 343–348 K. Recrystallized particles were characterized by Field Emission Scanning Electron Microscopy (FESEM), X-ray diffraction, HPLC and melting point analysis. The process produced particles with a mean size in the range 110–220 nm and a narrow size distribution. The product was crystalline as determined by X-ray diffraction. The effect of process conditions (T, P, nozzle diameter) on the crystal size distribution was determined.
We describe a process in which nanosize particles with u narrow size distribution are generated by expanding a thermal plasma carrying vapor-phase precursors through a nozzle. The plasma temperature and velocity profiles are characterized by enthalpy probe measurements. by calorimetric energy balances. and by a model of the nozzle flow. Aerosol samples are extracted from the flow downstream of the nozzle by means of a capillary probe interfaced to a two-stage ejection diluter. The diluted aerosol is directed to a scanning electrical mobility spectrometer (SEMS) which provides on-line size distributions down to particle diameters of 4 nmt. We have generated silicon, carbon, and silicon carbide particles with number mean diameters of about 10 not or less, and we have obtained some correlations between the product and the operating conditions. Inspection of the size distributions obtained in the experiments, together with the modeling results, suggests that under our conditions silicon carbide formation is initiated by nucleation of extremely small silicon particles from supersaturated silicon vapor, followed by chemical reactions at the particle surfaces involving carbon-containing species from the gas phase.
This paper presents the characteristics of two-nozzle electrospray deposition with a capillary–extractor–substrate configuration. A three-dimensional Lagrangian model was developed to simulate the spray evolution and deposition patterns. The droplet size distribution was not affected by the extractor–substrate voltage; however, spray evolution was found to be influenced considerably by the extractor–substrate voltage. Smaller particles are deposited densely in the outer region of the spray, whereas larger particles are deposited in the core region of the spray due to the size segregation effect. The normalized thickness in the intervening region of adjacent sprays is thinner than in the core region of sprays and gradually decreases as the extractor–substrate voltage increases. The normalized surface number density of deposited particles in the intervening region of adjacent sprays also decreased with the increasing extractor–substrate voltage, whereas the surface number density of them is larger than that in the core regions of sprays at an extractor–substrate voltage below 3 kV. The deposition patterns using a diluted PSL solution and spray visualization were in good agreement with the simulation results. It is suggested that the electrical self-dispersion effect and electrical repulsive interaction between adjacent sprays play an important role in determining the characteristics of the spray evolution and deposition patterns for multiplexed electrospray deposition.