Spatially and temporally resolved temperature and shock-speed measurements behinda laser-induced blast wave of energetic nanoparticles
Sukesh Roy, Naibo Jiang, Hans U. Stauffer, Jacob B. Schmidt, Waruna D. Kulatilaka, Terrence R. Meyer,
Christopher E. Bunker, and James R. Gord
Citation: Journal of Applied Physics 113, 184310 (2013); doi: 10.1063/1.4804410
View online: http://dx.doi.org/10.1063/1.4804410
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/113/18?ver=pdfcov
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JOURNAL OF APPLIED PHYSICS 113, 184310 (2013)

Spatially and temporally resolved temperature and shock-speed
measurements behind a laser-induced blast waveof energetic nanoparticles
Sukesh Roy,1,a) Naibo Jiang,1 Hans U. Stauffer,1 Jacob B. Schmidt,1 Waruna D. Kulatilaka,1Terrence R. Meyer,2 Christopher E. Bunker,3 and James R. Gord3
1

Spectral Energies, LLC, 5100 Springfield Street, Dayton, Ohio 45431, USA
Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011, USA
3
Air Force Research Laboratory, Aerospace Systems Directorate, Wright-Patterson Air Force Base,
Ohio 45433, USA
2(Received 13 July 2012; accepted 25 April 2013; published online 10 May 2013)
Spatially and temporally resolved temperature measurements behind an expanding blast wave are
made using picosecond (ps) N2 coherent anti-Stokes Raman scattering (CARS) following laser
flash heating of mixtures containing aluminum nanoparticles embedded in ammonium-nitrateoxidant. Production-front ps-CARS temperatures as high as 3600 6 180 K—obtained for 50-nmdiameter commercially produced aluminum-nanoparticle samples—are observed. Time-resolved
shadowgraph images of the evolving blast waves are also obtained to determine the shock-wave
position and corresponding velocity. These results are compared with near-field blast-wave theory
to extract relative rates of energy release for various particle diameters and passivating-layer
C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4804410]
compositions. V
I. INTRODUCTION

Energetic nanomaterials represent the next frontier inthe development of controllable energy release for use in
explosives and propellants.1 In particular, aluminum and boron exhibit considerable promise as high-energy-density species in the presence of oxidizers,2 and the increasingly
economical availability of aluminum nanoparticles, alongwith a variety of readily tunable physical properties, such as
particle diameter and composition of passivation layer, makealuminum nanoparticles particularly attractive for further
study.1,3–7 When used as an additive in solid-rocket propellants or high explosives, aluminum particles are rapidly
ejected into the gas phase following the initial impulsive
combustion event; these particles then continue to react withoxidants present in that gas-phase mixture produced by the
initial impulsive event.8 The dynamics of aluminum-particle
reactions in such gas-phase environments have been studied
in depth by several groups, and a notable dependence of
these reaction dynamics on particle size has been observed.
In particular, for large aluminum particles (>10–20 lm), it
has been observed that reaction rates are limited by timescales of oxidizer diffusion to the reaction front, whereas fornanometer-size particles these reactions are governed bykinetics of aluminum nanoparticles.9–11 The use of nanoaluminum has also been shown to increase detonation velocities
in some cases.7
Emission spectroscopy from the gas-phase reaction intermediate aluminum oxide (AlO) has been used to gauge
reaction temperatures during gas-phase combustion of aluminum particles;8,9 however, significantly less AlO is observed
during the combustion of sub-micron-diameter particles,12,13a)Author to whom correspondence should be addressed. Electronic mail:sroy@woh.rr.com

0021-8979/2013/113(18)/184310/7/$30.00

compromising its usefulness for making accurate temperature
measurements. Furthermore, because AlO is expected to bepresent only in the highest temperature regions that containreacting species, this approach provides only the peak temperature attained during the combustion process.8 In the absence of significant AlO emission, an alternative multiwavelength pyrometric approach that relies upon the
Planckian gray-body emission from incandescent reacting
particles has been used to estimate the temperature conditions
associated with reacting aluminum-nanoparticle samples.14
Both of these approaches yield an estimated uncertainty of150 K under peak temperature conditions and provide no
spatial resolution and only limited temporal resolution of thereaction environment. Additionally, absorptive approaches
have been used for continuous time-dependent measurementof temperature behind shock waves;15 however, such measurements have not, to date, been applied toward the gas-phaseoxidation of nanoaluminum and its oxidation products following laser flash heating.
We have applied picosecond coherent anti-Stokes
Raman scattering (ps-CARS), which is a well-establishednon-invasive approach to measuring temperature in a reacting
environment with excellent accuracy and precision,16–19 toallow thermometric measurements of the gas-phase species inthermal equilibrium with reacting, energetic nanoparticle
samples following laser flash heating. This point-wise detection approach permits spatially resolved measurements undersingle-laser-shot conditions, and the temporal resolution ofthis approach is limited only by the greater of the durations of
the flash-heating laser [several nanoseconds (ns) in this work]and the sub-ns pulse durations used in the ps-CARS techniques. Furthermore, employing ps-duration lasers allows theconcurrent use of time-resolved shadowgraphy to visualize
the associated shock wave (or “blast wave”) observed uponlaser flash heating of these nanoparticle samples20 and the113, 184310-1

C 2013 AIP Publishing LLC
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associated time-dependent initial energy-release rates of several aluminum-nanoparticle samples embedded in ammoniumnitrate oxidant as a function of nanoparticle size (diameters
ranging from 18 to 80 nm). Thus, these measurements represent a significant advancement in resolving, both spatially andtemporally, the energy-release rates and temperatures encountered by energetic nanomaterials during oxidation in a gasphase environment.
II. EXPERIMENTAL

Three sizes of Al2O3-passivated aluminum nanoparticles
were used during the course of these experiments, including
nominal particle diameters of 18 nm (Skyspring Nanomaterials,Inc.), 50 nm (MTI Corporation), and 80 nm (synthesized at
Edwards Air Force Base). Additionally, oleic-acid-passivatedcore-shell aluminum nanoparticles (30-nm diameter), produced
via sonochemical methods,21–24 were also studied for comparison purposes; these will be referred to below as 30-nm AlOA.
Transmission-electron-microscopy (TEM) images of thesesamples show a broad size distribution of spherical nanoparticles, with average sizes close to the corresponding nominal
diameters. In all experiments described here, solid sample pellets were created using 20%:80% mixtures (by weight) ofaluminum-nanoparticle fuel in ammonium-nitrate (Sigma
Aldrich) oxidizer. Each 5-mm-diameter, circular sample pellet
contained 10 mg of fuel-oxidizer mixture; their preparation is
described elsewhere.24
The experimental configuration used for the ps-CARS
thermometric measurements is depicted schematically in
Fig. 1, and the ps-CARS optical setup has been described indetail previously.17 Briefly, it consists of a ps-duration,pulsed Nd:YAG laser (Ekspla, Model SL230) that operates
at 10 Hz and produces nearly transform-limited pulses [fullwidth-at-half-maximum (FWHM) duration of 150 ps and

J. Appl. Phys. 113, 184310 (2013)

maximum pulse energy of 100 mJ/pulse] at 532 nm; thisbeam is split into pump and probe beams, each with anenergy of 800 lJ/pulse. A fraction of this laser output is also
split off to pump a home-made modeless dye laser (FWHM
bandwidth of 5 nm and FWHM duration of 110 ps) for
generating the Stokes beam at 606 nm with an energy of
3 mJ/pulse. The pump, probe, and Stokes beams are arranged
in a folded-BOXCARS geometry, and the temporal overlap
is adjusted using translation stages in the paths of the pumpand probe beams. The Rayleigh length of the overlapped psCARS beams is estimated to be 0.5 mm. The spatial resolution of this optical setup in the z-direction (normal to thesample surface) is estimated to be 30 lm. The CARS signal
beam is directed into a 1.25-m spectrometer (SPEX 1250M)
that is equipped with a 2400-groove/mm grating, and the resultant dispersed spectrum is recorded using a backilluminated 2048  512-pixel-array CCD camera with fullvertical binning (Andor Technologies Model DU 440BU).
The measured spectral resolution of the CARS detection system is 0.54 cm1, which is similar to the typical spectral
resolution for ns-CARS systems and is sufficient to resolve
the rovibrational transitions under the current reaction conditions. Laser flash heating of the samples was achieved using
the ns-duration 532-nm output from a separate Nd:YAG
laser (10-ns pulse duration) reduced to a 1.5-mm-diameter
spot at the surface of the sample pellet with an estimated fluence of 0.3 J/cm2. By controlling the delay time between the
flash-heating beam and the three time-overlapped ps-CARS
beams as well as the height of the nanoparticle pellet, wehave the ability to obtain the spatiotemporally resolved temperatures of the nitrogen gas in equilibrium with the reacting,energetic nanoparticle samples. Additionally, time-resolved
shadowgraph images following flash heating were obtainedusing the ps-duration dye-laser pulse as a light source and an
intensified CCD camera with a 30-ns exposure gate for
imaging. Each shadowgraph image corresponds to a single
flash-heating event, with a fixed time delay between the
flash-heating laser and the shadowgraph-light-source laser;
the sample pellet was rotated between laser shots to provide
a fresh sample surface for each shadowgraph image. The
uncertainty in the position of the shock wave was found tobe 62%, resulting primarily from the observed width of
the shock wave in the shadowgraph images. The reproducibility of the observed shock-wave positions was determinedby making multiple single-shot measurements at several
time delays; this reproducibility was found to fall within the
62% uncertainty range.
III. RESULTS AND DISCUSSION
A. Temporally resolved shadowgraphy of shockpropagation

FIG. 1. Composite picture depicting the optical configuration used in psCARS temperature measurements following laser flash heating of energetic
nanomaterial samples. A ns-duration flash-heating laser impinges on thesample from above; three incoming ps-duration CARS beams overlap at apoint above the sample surface (along the z-axis dimension), producing afourth outgoing CARS signal beam. Pictorial overlays within the image
include an example shadowgraph of a shock wave and an example reactingplume Mie-scattering image to provide reference.

Time-resolved shadowgraph images were recorded foreach aluminum-nanoparticle sample to track the initial formation and subsequent evolution of the shock wave that follows the laser flash heating of these materials. A sample
series of shadowgraph images is shown in Fig. 2 for 50-nm
aluminum nanoparticles embedded in ammonium nitrate. A
single strong shock boundary is not observed at the earliest[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
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J. Appl. Phys. 113, 184310 (2013)

FIG. 2. Shock-wave evolution following
laser flash heating of samples containing
50-nm aluminum nanoparticles embedded in ammonium nitrate. Full-frame
image height is 105 mm.

time delays (<1000 ns); instead, a diffraction pattern containing light and dark stripes is observed before coalescing
into a sharper leading shock boundary at longer delays. It
should be noted that similar features have been observed in
the early stages of material ejection during laser ablation ofaluminum using intense (40 J/cm2) femtosecond laser
pulses; in that case, the diffraction is attributed to the coolingprocess associated with initially ejected ablated material following rapid overheating (estimated temperature 105 K) of
the material surface.20 In contrast, the initial flash-heatingprocess under low-laser-fluence (0.3 J/cm2) conditions
used here is expected to produce significantly less initialheating of the absorbing aluminum nanoparticle, with temperatures ranging between the melting point (933 K) and theboiling point (2740 K) of aluminum.3,25 The presumed mechanism for reaction of these heated aluminum nanoparticles
then follows a sequence involving the breakdown of the surrounding passivation layer, the initiation of a reaction
sequence between the heated aluminum and the surrounding
solid-oxidant substrate material, and subsequent sustainedsolid-phase exothermic reactions that produce heat and lightand eject aluminum nanoparticles into the surrounding environment to react with oxidants present in the gas-phase mixture.3 At longer time delays, this explosive ejection ofmaterial from the sample surface is observed—as was thecase for all experimental samples studied here—as an
expanding dark plume emanating from the sample surface.
Mie-scattering measurements, as depicted in Fig. 1, provide
much clearer images of this evidence of reactive oxidation
following laser flash heating of the nanoparticles.
The corresponding time-dependent shock-wave position
was observed to change for the different nanoparticle samples,as shown in Fig. 3(a), with the 50-nm commercial aluminumnanoparticle samples exhibiting the most rapid shock-wave
expansion over the first 10 ls. Prior studies concerned withprobing the shock-wave velocities associated with a variety oflaser-initiated expansions, including ablation and plasmaforming processes,20,26–28 have used classical Taylor-Sedov(T-S) blast-wave theory29–31 or modified variations thereof
that account for drag forces associated with the surrounding
medium.26,27 Classical T-S theory assumes that an initial instantaneous input of energy produces a shock wave that isemitted from a point source, which results in a simple expression for the time (t) dependence of the expanding shock-front
radius, r, as follows:
r ¼ CðE0 =q0 Þ1=5 t2=5 ;(1)where E0 is the initial constant-energy input, q0 is the densityof the ambient surrounding gas, and C is a constant related tothe specific-heat ratio, c, of the gas swept up by the shock
wave.32 A more generalized, time-dependent energy-releaserate model has also been developed to describe variableenergy blast waves, assuming a time-dependent energy, E, ofthe formEðtÞ ¼ Atb ;(2)

where A and b are condition-dependent constants;33,34 andb ¼ 0 corresponds to the classical T-S blast-wave-theory
limit. Moreover, classical T-S blast-wave theory is only
strictly valid within a regime in which the mass of the energy
source is negligible compared to the mass of the ambient surrounding gas that is swept up by the expanding shock. This

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J. Appl. Phys. 113, 184310 (2013)

assumed sphere of material that is producing the shock wave.
In this expression, it is assumed that the heat-capacity ratio, c,
associated with the source debris is equal to the surrounding
heat-capacity ratio. It should be noted that in the limit where
the source mass becomes negligible compared to the mass of
air swept up by the expanding shock wave, K1  K2 r3 , andEq. (3) reduces to an expression that is consistent with theconstant-energy-source classical T-S theory expression
shown in Eq. (1), albeit with an additional contribution to thetotal initial energy from energy contained within the source
debris.32,36
For all samples studied here, the observed expanding
shock wave radius was found to exhibit an approximate powerlaw dependence on time, as demonstrated by the observed linearity in the log–log plots depicted in Fig. 3(a). Therefore, eachdata set was fit to a power-law dependence of the formrðtÞ ¼ Ata :(5)

The extracted A and a fitting parameters were then employed
to calculate the time-dependent energy using Eq. (3), with
q0 ¼ 1:20 kg=m3 and c ¼ 1:4 for room-temperature air; thevalues used for the solid-phase components were estimated tobe qs ¼ 1750 kg=m3 and Rs ¼ 1:0  104 m, and it was
assumed that the combustion-reaction heat-capacity ratio wascomparable to that of room-temperature air (i.e., c ¼ 1:4).
Under these conditions, K1  K2 r3 over the observed timescales and corresponding shock-wave radii, and the calculated
released energy over the observed timescale takes the approximate form
FIG. 3. (a) Time-dependent shock-wave position, r, depicted as symbols forfour aluminum nanoparticle samples; curves correspond to power-law leastsquares fits. (b) Corresponding calculated time-dependent energy release forthe four denoted aluminum nanoparticle samples. (c) Comparison of energy
released, normalized for mass fraction of aluminum in each sample. In (b)and (c), symbols are connected by line segments for clarity.

assumption is inherently invalid during the very earliest
stages of shock expansion, and near-field variants of thistheory have been developed to account for the mass contribution of the point source.32,35,36 In particular, for sphericalshock waves expanding in three dimensions, a near-field
description of the relationship between the source energy
and the expanding shock radius takes the form
 2
dr
;
E ¼ ðK1 þ K2 r Þ
dt
3

(3)

where the constants K1 and K2 contain source debris and surrounding gas-phase-medium parameters as follows:32
K1 ¼
K2 ¼

8pqs Rs 3
3ðc þ 1Þ2;8pq0 ð3c  1Þ
3ðc þ 1Þ2 ðc  1Þ

(4a)

:

(4b)

In these expressions, qs denotes the density of the solid
source material, and Rs denotes the initial source radius of anEðtÞ ¼ K2 A5 a2 tð5a2Þ ;(6)which is consistent with the variable-energy blast-wave
model assumption shown in Eq. (2). The corresponding calculated time-dependent energies released by the four studied
nanoparticle samples following flash heating are shown in
Fig. 3(b). We note that although the estimated nature of thesesolid-phase and debris-field quantities results in uncertainty
in the absolute magnitudes of these energy-release values—
particularly under the assumption that the source-debris andambient-gas heat capacities are equivalent—such estimatesallow comparison of the relative magnitudes of these quantities from sample to sample. In particular, it was observedthat the oxide-passivated 50-nm samples exhibited the most
rapid energy release over the observed timescales, whereas
the 18-nm and 80-nm samples exhibited notably lessreleased energy. It is also important to note that the presenceof passivating surface species (alumina in the case of the
18-nm, 50-nm, and 80-nm samples and a combination of alumina and an organic surface-bound species in the case of the
30-nm AlOA samples) results in varying aluminum compositions among the tested samples. To account for the relative
compositions of aluminum available for oxidative reaction inthese samples, we assume that the spherical aluminapassivated species have a uniform 2.5-nm alumina outershell; measurements showed the 30-nm AlOA samples tocontain 40% Al.21 As depicted in Fig. 3(c), when the relative mass fractions of aluminum are taken into account, the[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:
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FIG. 4. Example single-shot N2 ps-CARS spectrum following flash heatingof sample containing 50-nm aluminum nanoparticles. The included theoretical spectrum was calculated using the modified Sandia CARSFT code withan assumed temperature of 3600 K.

30-nm AlOA samples exhibit the highest energy release peraluminum mass fraction among the tested samples.
B. Temporally and spatially resolved temperature
behind shock waves

A typical single-laser-shot nitrogen ps-CARS spectrum
is shown in Fig. 4; this spectrum was acquired for the
50-nm-particle sample at z ¼ 0.5 mm above the sample surface and a delay of 350 nm after flash heating. For this andall ps-CARS measurements, the temperature was evaluatedby least-squares fitting of the experimental spectrum witha theoretical spectrum that was calculated using the
Sandia CARSFT code;37 here, a best-fit temperature of 3600
6 180 K is observed. As noted above, the ability to makepoint-wise measurements using ps-CARS affords, for the
first time in a reacting energetic-materials sample, the ability
to map the spatial temperature distribution as a function oftime. Figure 5 displays an example of such spatially resolved
temperatures measured behind a blast wave that is inducedby the laser flash heating of samples containing 50-nm aluminum nanoparticles. Overlaid within this figure is an example shadowgraph image that was obtained 1.3 ls after flash

FIG. 5. Spatially resolved ps-CARS temperatures measured for 50-nm nanoparticle samples shown with respect to a measured shadowgraph image. All
depicted measurements were made at a delay of 1.3 ls after flash heating. At
this delay, the shock-wave radius is 2.33 mm, and the three temperature
measurements correspond to heights of z ¼ 1.25, 1.5, and 1.75 above thesample surface.

J. Appl. Phys. 113, 184310 (2013)

heating of the sample; the resultant blast-wave radius is2.3 mm at this delay. Single-shot CARS temperature measurements were made at z ¼ 1.25 mm, 1.5 mm, and 1.75 mmabove the surface of the sample pellet at this delay, and themeasured temperatures were found to be 3000 K, 1450 K,
and 1150 K, respectively, with corresponding uncertainties
of 65%.
To address the spatially resolved temperatures observed
behind the shock wave, we invoke a simple model based on
the measured shock velocity. Hendijanifard and Willis havederived straightforward expressions for the temperatures andpressures expected directly behind a low-Mach-number
shock wave based on measured shock-wave speeds by solving the standing normal shock equations of momentum, continuity, energy, and state.38 Based on those expressions andthe measured shock-wave velocity determined for this sample by differentiation of Eq. (5), we calculate the temperature
directly behind the shock wave under these conditions to be
766 K. Unfortunately, it is challenging to use the ps-CARS
approach for measuring temperature immediately behind theshock front because of the significant changes in the gasphase refractive index associated with large density changes;
therefore, a direct comparison between this theoretical value
and experiment cannot be made. However, under the simplifying assumption that the shock-induced temperature at agiven position remains essentially constant following passage of the shock-wave front, it is possible to estimate theposition-sensitive temperature based on the calculated temperature directly behind the shock front at the moment theshock passed a given position. This calculated temperaturedependence is depicted in Fig. 6, assuming the fitted shockwave-position time dependence [Eq. (5)] for the 50-nm aluminum nanoparticle sample; this theoretical curve corresponds to the calculated temperature behind the shock wave
plotted as a function of shock-wave position. At positions
nearer the shock front, this simple theoretical model agrees
reasonably well with the measured temperatures, whereas
the experimentally observed temperatures show positive
deviation from this theoretical value at greater distances
behind the shock wave. Such deviation may be indicative of
the cumulative effects of additional time-dependent heat

FIG. 6. Comparison of observed ps-CARS (symbols) and theoretical (curve)
temperatures for samples containing 50-nm nanoparticles at several distances, z, above the sample at a delay of 1.3 ls after flash heating. Theoreticaltemperature is determined based on the measured shock-wave position as afunction of time.

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FIG. 7. Temperature at reaction-plume front observed for four aluminumnanoparticle samples.

release as the sample-oxidation process evolves, resulting inan additional temperature increase in the ambient gas as time
progresses.
Additionally, we note that a darker area is observed
within the shadowgraph image shown in Fig. 5, indicating
the presence of a reaction plume that contains the solid debris ejected from the sample after the initial explosive reaction occurs. Mie-scattering images associated with this same
sample material under these conditions demonstrate that thereaction plume reaches a distance of z ¼ 1.2–1.3 mm at adelay of 1.3 ls. Unfortunately, we found that the optical density of this ejected material was sufficient to prevent psCARS measurements at distances inside this leading edge,
and we were unable to measure temperature directly within
the expanding debris field.
Finally, to gauge the temperatures encountered by theejected nanoparticle samples as they move into the gas phasefollowing the initial explosive reaction, we used ps-CARS tomeasure the temperatures directly in front of the reaction
plume at several time delays for the four samples. The resultant temperatures are shown in Fig. 7 as a function of the leading edge of the reaction plume. Consistent with the timedependent energy release determined from the shock-wave
velocities, the 50-nm nanoparticle samples were observed toexperience the greatest temperature increases in the surrounding gas-phase environment, and the resulting expanding reaction plume encountered the highest gas-phase temperatures.
All samples were observed to encounter temperatures exceeding the aluminum melting point (933 K), and the 50-nm particle samples encountered temperatures exceeding both thealumina (Al2O3) melting point (2300 K)12 and the boilingpoint of aluminum (2740 K). Under such conditions, it isexpected that the nanoparticles continue to undergo rapid
reaction with gas-phase oxidant species that are present inthis environment, including ambient oxygen as well as thedecomposition products of the initial solid-phase oxidation
reactions between aluminum and ammonium nitrate.
IV. CONCLUSIONS

Nitrogen ps-CARS has been used to provide, for the firsttime, simultaneous temporally and spatially resolved temperature measurements behind an expanding blast wave. These
measurements, made at several times and positions following

J. Appl. Phys. 113, 184310 (2013)

the flash heating of energetic aluminum nanoparticles embedded in ammonium nitrate, allow determination of the range oftemperatures encountered by particles following ejection into
the surrounding gas-phase medium. Temperatures as high as
3600 K were observed at the reaction-plume front when 50nm-diameter commercial aluminum-nanoparticle samples
were used. These temperatures, measured at positions near
the expanding shock-wave-front position that was determinedusing time-resolved shadowgraphy, were shown to be consistent with position-dependent temperatures calculated using
exact low-Mach-number temperature and pressure relationships across a shock wave with a known time-dependent
speed. In contrast, these simple calculations underestimate
the temperatures observed at positions further behind theshock wave, most likely because of additional heat produced
during the continued reactivity of the energetic nanoparticle
samples.
ACKNOWLEDGMENTS

Funding for this research was provided by the Air Force
Research Laboratory under Contract No. FA8650-10-C-2008.
We thank Dr. Paul Hsu of Spectral Energies, LLC and Dr.
William Lewis of the University of Dayton for many helpful
discussions. We also acknowledge Dr. Barbara Harruff for
the TEM image inspections of various nanoparticles andStephen Danczyk for providing the 80-nm nanoparticles.
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