Hypergolic ionic liquids to mill, suspend, and ignite boron nanoparticles

Article abstract of DOI:10.1039/c2cc30957b

Boron nanoparticles prepared by milling in the presence of a hypergolic energetic ionic liquid (EIL) are suspendable in the EIL and the EIL retains hypergolicity leading to the ignition of the boron. This approach allows for incorporation of a variety of nanoscale additives to improve EIL properties, such as energetic density and heat of combustion, while providing stability and safe handling of the nanomaterials. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc30957b

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COMMUNICATION
Hypergolic ionic liquids to mill, suspend, and ignite boron
nanoparticleswz
Parker D. McCrary,a Preston A. Beasley,a O. Andreea Cojocaru,a Stefan Schneider,b
Tommy W. Hawkins,*b Jesus Paulo L. Perez,c Brandon W. McMahon,c Mark Pfeil,d
Jerry A. Boatz,b Scott L. Anderson,*c Steven F. Son*d and Robin D. Rogers*a
Received 9th February 2012, Accepted 27th February 2012
DOI: 10.1039/c2cc30957b
Boron nanoparticles prepared by milling in the presence of a
hypergolic energetic ionic liquid (EIL) are suspendable in the
EIL and the EIL retains hypergolicity leading to the ignition of
the boron. This approach allows for incorporation of a variety of
nanoscale additives to improve EIL properties, such as energetic
density and heat of combustion, while providing stability and
safe handling of the nanomaterials.
and hydrocarbon-dispersible, nanoparticulate B can be prepared
by milling micron-sized B with a ligand to create ligand-protected
B nanoparticles. For example, oleic acid was utilized as a ligand to
create unoxidized B nanoparticles (60 nm in diameter) that are
easily dispersible in petroleum based jet fuels.15,16 Here, we report
the use of the IL 1-methyl-4-amino-1,2,4-triazolium dicyanamide
([MAT][DCA]) as milling agent for B. [MAT][DCA] was chosen
based both on the hypergolic nature of this IL and the likely
amine-B surface interactions, which we hypothesized would form.
Following the protocols developed by Anderson et al., B with
an average diameter of 2 mm was ball-milled using a tungsten
carbide milling jar and 1/8’’ diameter spherical balls to create B
nanoparticles (o20 nm in diameter).15,16 Boron (2 g) was added to
the ball milling apparatus and dry milled, followed by additional
milling with either no ligand, a combination of oleic acid and oleyl
amine (1.5 mL, 1 : 1 v/v), or [MAT][DCA] (1.5 mL). Acetonitrile
was then added for the final milling as a co-solvent to help reduce
the viscosity and easily transfer the nanoparticles.
Energetic ionic liquids (EILs, salts which melt below 100 1C with
potential as energetic materials)1 have been reported as hypergolic,
indicating they spontaneously ignite on contact with a variety of
oxidizers,2–4 but many challenges still remain to their practical use
such as low density5 and relatively low heats of combustion6 when
compared to the current state of the art hypergols, such as
hydrazine.7 One approach that can be taken to improve EIL
performance is to introduce an additive which does not interfere
with the desired IL traits such as low or negligible vapor pressure.
Ionic liquids are already known as solvent media to synthesize and
stabilize nanoscale additives, such as Pt, Ir, and Pd;8–11 however,
we are interested in the ability of an IL to passivate the surface of
nanoparticles while providing a stable suspension which could lead
to higher energy density EILs.
The resulting acetonitrile suspensions were stable to air and
these samples were manipulated on the benchtop. The solvent
was removed by rotoevaporation followed by heating and
stirring under high vacuum. The samples were taken into a
drybox where they were stored in an Ar atmosphere until used.
The suspendability and stability of the milled B particles were
investigated by preparing mixtures with the IL 1-butyl-3-methyl-
imidazolium dicyanamide ([BMIM][DCA]). This hypergolic IL
was chosen for the initial studies in determining the appropriate
loadings and handling conditions due to its easier preparation,2
characterization,5,6 and availability. [BMIM][DCA] was freeze
thawed to remove any dissolved gases or water by placing the vial
in a N2(l) bath while under high vacuum and subsequently allowed
to warm, forcing out any trapped gases.
Boron (B) is widely studied for its use as an energetic additive
in both micro12 and nano13 sizes as a result of its high heat of
combustion; however, because it is normally coated by a
passivating oxide layer, it requires temperatures over 1500 1C
to ignite.14 Anderson et al. have demonstrated that air-stable
a Center for Green Manufacturing and Department of Chemistry The
University of Alabama, Tuscaloosa, AL 35487, USA.
E-mail: rdrogers@as.ua.edu
b Space and Missile Propulsion Division, Propulsion Directorate, Air
Force Research Laboratory, AFRL/RZSP, 10 E Saturn Boulevard,
Edwards AFB, CA 93524, USA.
Compositions of 0.2% to 0.7% w/w B from each of the three
milled samples were prepared by diluting the weighed B samples
with neat [BMIM][DCA] to prepare 1–2 mL samples. Initially a
clear IL phase with aggregated B particles resting on the bottom
was observed in each case. The vials were then removed from the
drybox and vortex mixed and stirred, but without dispersion.
Each of the samples was ultimately dispersed by using a
Branson 5510 bath sonicator. The vials were sonicated for
consecutive 99 min cycles until no particles were visible, typically
at least eight cycles. In each case black colloids formed with very
E-mail: Tommy.Hawkins@edwards.af.mil
c Department of Chemistry, The University of Utah, 315 S. 1400 E.,
Rm 2020, Salt Lake City, UT 84112, USA.
E-mail: anderson@chem.utah.edu
d School of Mechanical Engineering, Purdue University, Zucrow
Laboratories, 500 Allison Road, West Lafayette, IN 47907, USA.
E-mail: sson@purdue.edu
w This article is part of the ChemComm ‘Ionic liquids’ web themed
issue.
z Electronic supplementary information (ESI) available: Characteri-
zation, Preparation, and Data. See DOI: 10.1039/c2cc30957b
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4311–4313 4311

weak yellow-orange hues around the edges. These suspensions did
settle out over time (see below), however, each sample (all milling
conditions and loadings) was easily resuspended upon vortex
mixing and sonication (fewer cycles required). All colloids exhibited
the same stability whether freshly prepared or redispersed.
Interesting differences in the stabilities of the particles milled
under different conditions were noted. Boron milled with no
surfactant gave the least stable colloids with aggregation in less
than 24 h. Boron particles milled with the surfactant mixture were
slightly more stable, lasting 24 h, however, the compositions with B
milled in [MAT][DCA] had the greatest stability with colloids
lasting 48 h. In all cases, higher B loadings (0.5–0.7%) were more
prone to aggregation, but little difference in stability was noted for
either the 0.2% or the 0.33% loadings. Boron nanoparticles from
the three milling conditions were next loaded to 0.33 wt% in
[MAT][DCA] utilizing the methods described above (Fig. 1).
Thermal gravimetric analyses were performed on the colloidal
nanofluids in [BMIM][DCA] and [MAT][DCA]. The addition of B
nanoparticles from any preparation did not have an effect on the
decomposition temperature or the characteristic decomposition
profile of either IL (see ESIz). When these tests were conducted
with an air purge, mass gain was noted corresponding to the
oxidation of the B above 400 1C.
Table 1 Hypergolic and stability properties of nanoparticle dispersions
Colloidal Ignition Ignition
stability delaya duration
Compound
(h)
(ms)
(ms)
Notes
[BMIM][DCA]
One Flame –
Medium
N/A
44(2)bc 106(6)
One Flame –
Very weak
Two Flames –
Medium
Three Flames –
0.33% B no ligand
0.33% B surfactant
o24
44(3)
43(3)
108(41)
110(50)
24
0.33% B [MAT][DCA] 48
[MAT][DCA]
44(3)
130(31) Medium and
High
One Flame –
77(18)
N/A
37(6)
43(9)
Medium
One/Two
Flames – Very
weak
0.33% B no ligand
0.33% B surfactant
o24
52(12)
Combustion microcalorimetry was used to determine the heats
of combustion of neat [BMIM][DCA] and the colloids using a
Parr (series 1425) semimicro oxygen bomb calorimeter (see ESIz).
The samples with B exhibited lower heats of combustion
compared to the neat IL and to the values expected when
calculated using the B contributions indicating that the energies
and temperatures were insufficient to combust B.
One Flame -
Medium
One Flame –
Very Strong
24
45(11) 56(7)
45(14) 43(4)
0.33% B [MAT][DCA] 48
a
As measured from the time the drop hits the oxidizer until the first
b
sign of a flame. 47 ms has been reported in the literature for this IL.2
c
Numbers in paranthesis denote calculated standard deviations.
Hypergolic drop tests were then conducted to determine the
effects, if any, the B nanoparticles might have on ignition delay
and other ignition characteristics. The hypergolic apparatus and
tests described by Schneider et al.2 were employed. A droplet
(10 mL) of the IL was added to a vial, via a Hamiltonian syringe,
containing the oxidizer in large excess (500 mL of 98% white
fuming nitric acid (WFNA)) at 23 1C to ensure complete ignition of
the fuel. The ignition delay and flame duration times were
determined using a Redlake MotionPro HS-4 high speed camera
at 1000 frames per second. Each experiment was repeated for three
ignitions and the values for ignition delay and ignition duration
were averaged. The results are presented in Table 1.
milled in the oleic acid/olelyl amine surfactant mixture led to a
complex flame behavior reproducibly exhibiting two flames
which lengthened the overall time of combustion. The samples
loaded with B milled in [MAT][DCA] gave the most complicated
behavior with three flames (Fig. 2).
It is clear from comparing the runs with neat [BMIM][DCA]
with those loaded with B nanoparticles (e.g., Fig. 2c–g) that
addition of the B nanoparticles did not enhance the reaction.
There is also no evidence that the B ignited in these experiments.
Additionally, such complicated behavior and multiple flames
would provide extremely poor performance.
The incorporation of 0.33 wt% B from any of the three
preparations did not have any effect on the observed ignition delay
of either IL tested (Table 1) within the accuracy of determination.
However, as discussed below, differences in the number, intensity,
and duration of the resulting flames were clearly different depending
on which capping agent was utilized in the B milling step.
The hypergolic behavior of [BMIM][DCA] can be contrasted to
that of [MAT][DCA] where significant differences suggest a more
robust hypergolic system. Incorporation of B nanoparticles milled
with no ligand to [MAT][DCA] shortened the duration of the burn
and led to very weak flames. Adding the B milled with the oleic
acid/oleyl amine surfactant mixture also led to a shorter burn, but
flame intensity observed was only slightly reduced.
Neat [BMIM][DCA] exhibited a single medium intensity flame
(See ESIz) as also reported in the literature.2,17 Incorporation of the
B milled with no ligand significantly reduced hypergolic perfor-
mance leading to smaller flames with observable residual mass
deposited on the sides of the vial after ignition. Utilizing B particles
Adding B milled in [MAT][DCA] led to significant enhance-
ments in performance (Fig. 3). When the 0.33% B loaded
[MAT][DCA] was dropped into WFNA, an extremely powerful
and intense single flame appeared with a very short burn
indicating a very quick and powerful ignition. These drop tests
led to the largest and most intense flames of any of the tested
materials. The extremely bright, white flames qualitatively
indicated B ignition.13,14
Fig. 1 (a) [MAT][DCA]-milled B, (b) [MAT][DCA], and (c) 0.33% B.
4312 Chem. Commun., 2012, 48, 4311–4313
c
This journal is The Royal Society of Chemistry 2012

Fig. 3 [MAT][DCA] neat (a and b) and loaded with 0.33 % B milled
in [MAT][DCA] (c and d) with 98 % WFNA: (a and c) first sign of
ignition and (b and d) burn ongoing. The resolution was lowered to fit
the larger flames: vial sizes in all runs are identical.
Fig. 2 [BMIM][DCA] and 98% WFNA with 0.33% B milled in
[MAT][DCA]: (a) drop hitting, (b) first sign of ignition, (c) flame from
first ignition, (d) end of first flame, (e) second flame, (f) second flame
diminishes, (g) third flame starts, (h) last indication of burn.
Awards No. F49550-10-1-0521 (UA), FA9300-06-C-0023
(AFRL), FA9550-08-1-0400 (UT), US Department of Education
(UA: GAANN P200A100190), and UT Research Foundation
(Grant 51003387). We would also like to thank Prof. A. K.
Agrawal (UA) for access to the high speed camera.
FT-IR data (see ESIz) suggests at least one partial explanation
for the differences in the hypergolicity after adding B to the two
ILs. The data indicate that the hypergolic anion is interacting
with the B surface in [BMIM][DCA], but not in [MAT][DCA]
with its coordinating cation. Studies to confirm the exact nature
of the IL/B surface interactions are currently in progress.
Production of B nanoparticles by milling B in the presence of the
hypergolic IL [MAT][DCA] leads to more stable dispersions of
these nanoparticles in [BMIM][DCA] and [MAT][DCA]. The
presence of these nanoparticles does not change the hypergolic
ignition delay for these ILs, however, when added to
[BMIM][DCA] there is little enhancement of the hypergolic
burn and instead a complicated burn pattern emerges. By
contrast, when B nanoparticles are milled in [MAT][DCA] and
then dispersed in this IL, a single, shorter, much more intense
burn is observed suggesting ignition of the B. These differences
could be the result of the differing coordinative abilities of the
two ILs to B, where, for example [BMIM][DCA] can coordinate
B via the anion, while [MAT][DCA] can coordinate B with the
cation, the anion or both.
Notes and references
1 M. Smiglak, A. Metlen and R. D. Rogers, Acc. Chem. Res., 2007,
40, 1182.
2 S. Schneider, T. Hawkins, M. Rosander, G. Vaghjiani,
S. Chambreau and G. W. Drake, Energy Fuels, 2008, 22, 2871.
3 L. He, G. Tao, D. A. Parrish and J. M. Shreeve, Chem.–Eur. J.,
2010, 16, 5736.
4 S. Schneider, T. Hawkins, Y. Ahmed, M. Rosander, L. Hudgens
and J. Mills, Angew. Chem., Int. Ed., 2011, 50, 5886.
5 L. G. Sanchez, J. R. Espel, F. Onink, G. W. Meindersma and
A. B. de Haan, J. Chem. Eng. Data, 2008, 54, 2803.
6 V. N. Emel’yanenko, S. P. Verevkin and A. Heintz, J. Am. Chem.
Soc., 2007, 129, 3930.
7 J. D. Clark, Ignition: An Informal History of Liquid Rocket
Propellants, Rutgers University Press, New Brunswick, NJ, 1972.
8 J. Dupont, G. S. Fonseca, A. P. Umpierre, P. F. P. Fichtner and
S. R. Teixeira, J. Am. Chem. Soc., 2002, 124, 4228.
9 A. S. Pensado and A. A. H. Padua, Angew. Chem., Int. Ed., 2011,
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10 J. C. Rubim, F. A. Trindade, M. A. Gelesky, R. F. Aroca and
J. Dupont, J. Phys. Chem. C, 2008, 112, 19670.
11 N. J. Bridges, A. E. Visser and E. B. Fox, Energy Fuels, 2011, 25, 4852.
12 A. Macek and J. M. Semple, Combust. Sci. Technol., 1969, 1, 181.
13 G. Young, K. Sullivan, M. R. Zachariah and K. Yu, Combust.
Flame, 2009, 156, 322.
14 D. Dreizin, D. G. Keil and W. Felder, Combust. Flame, 1999, 119, 272.
15 B. Van Devener, J. P. L. Perez and S. L. Anderson, J. Mater. Res.,
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16 B. Van Devener, J. P. L. Perez, J. Jankovich and S. L. Anderson,
Energy Fuels, 2009, 23, 6111.
Taken as a whole, the results reported here suggest EILs can be
designed to provide unique stabilizing environments for reactive
nanoparticles and that the nanoparticles can provide unique
enhancements of the EIL properties such as increased density or
hypergolic performance. Future work from our Groups will focus
on understanding the surface interactions of the ILs and nano-
particles in an effort to find suitable IL/nanoparticle combinations
that will provide infinitely suspendable nanofluids with appropriate
properties needed for new energetic materials.
This material is based upon work supported by the
Air Force Office of Scientific Research under AFOSR
17 Y. Zhang, H. Gao, Y. Joo and J. M. Shreeve, Angew. Chem., Int.
Ed., 2011, 50, 9554.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 4311–4313 4313