Radiative Ignition of Pyrotechnics Effect of Wavelength on Ignition Threshold, CHEMIA I PIROTECHNIKA, Chemia i ...

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328
Propellants, Explosives, Pyrotechnics 23, 328±332 (1998)
Radiative Ignition of Pyrotechnics: Effect of Wavelength on Ignition
Threshold
Leo de Yong and F. Lui
Defence Science and Technology Organisation, Weapons Systems Division, Aeronautical and Maritime Research
Laboratory, Melbourne VIC 3001 (Australia)
Strahlungsz È ndung von Pyrotechnica: Die Wirkung der Wellen-
lÈ nge auf die Z È ndschwelle
Unter Verwendung einer Btreitband-Xenonlampe und einer leis-
tungsfÈhigen Laserdiode wurde die ZÈndung verschiedener pyro-
technischer SÈtze unter Einwirkung von Strahlung im Ultraviolett, im
Sichtbaren und im Infrarot untersucht. Bei StrahlenstÈrken von
4,9 W=cm
2
wurde durch UV-Strahlung keiner der S
È
tze gez Èndet,
dagegen reagierten SR112, B=Fe
2
O
3
und Schieûpulver auf sichtbare
Strahlung bei einer niedrigen Strahlenst
È
rke von 8,1 W=cm
2
AmorËage par rayonnement de produits pyrotechniques: effet de
la longueur d'onde sur le seuil d'amorËage
En utilisant une lampe
Á
x
Â
non
Á
large bande et une diode laser
performante, on a Âtudi l'amorËage de diffÂrentes compositions
pyrotechniques sous l'effet d'un rayonnement dans l'ultraviolet, dans
le visible et dans l'infrarouge. Avec des intensitÂsÂnergÂtiques de
4,9 W=cm
2
, aucune des compositions n'a
Â
t
Â
amorc
Â
e. En revanche,
SR112, B=Fe
2
O
3
et la poudr Á canon ont rÂagi Á un rayonnement
visible de faible intensit
 Â
nerg
Â
tique (8,1 W=cm
2
). La plupart des
compositions ont Ât amorcÂes par un rayonnement IR, mais
Mg=NaNO
3
et SR112 n'ont pu Ãtre amorcÂes mÃme avec des inten-
sitÂsÂnergÂtiques atteignant 300 W=cm
2
. On suppose que la diffÂr-
ence de rÂaction des compositions n'est pas en relation directe avec
leur capacit d'absorption de rayonnement, mais dÂpend de nom-
breuses propriÂtÂs physiques, chimiques et optiques des poudres
pyrotechniques. Une augmentation de l'intensit
 Â
nerg
Â
tique r
Â
duit le
temps d'amorËage de la plupart des compositions.
2
nicht gez Èndet werden. Es wird angenommen, daû der Unterschied im
Ansprechverhalten der SÈtze nicht in direkter Beziehung zu ihrer
Strahlenabsorption steht, aber abhÈngig ist von vielen physikalischen,
chemischen und optischen Eigenschaften der pyrotechnischen Pulver.
Zunahme der StrahlenstÈrke reduziert die ZÈndzeit f Èr die meisten
SÈtze.
Summary
cess
(1±9)
. Highly reliable RF=ESD=EMP immune igniters
have been built using miniature lasers, high power laser
diodes or optically pumped laser rods. The attainment of
high power output has also led to the use of lower sensi-
tivity igniter compositions further increasing the safety of
ignition systems. The concept of laser diodes coupled with
®ber optics has also led to novel and versatile igniter
design
(10±15)
. However, although lightweight, high power
laser diodes are still relatively costly.
Whilst many studies into alternative ignition sources for
pyrotechnics have examined the use of IR radiation, none
have examined the effects of visible or ultraviolet (UV)
radiation. Many UV and visible radiation sources are
available at relatively low cost, have high power output and
they may offer cheap alternatives to IR sources for pyro-
technic ignition.
This preliminary study explores the use of broadband
UV, visible and IR radiation as alternative ignition sources
for some common pyrotechnic compositions. It also uses a
high power laser diode as a narrow band comparative
radiation source.
Using a broadband Xenon lamp and a high power laser diode, the
ignition of several pyrotechnic compositions was evaluated using
ultraviolet (UV), visible and infrared (IR) radiation. At irradiance
levels of 4.9 W=cm
2
. Most of the
compositions were ignited with IR radiation but Mg=NaNO
3
and SR
112 could not be ignited at irradiance levels of up to 300 W=cm
2
.Itis
proposed that the difference in the response of the compositions was
not directly related to the radiation absorption characteristics of the
compositions; but was dependant on many physical, chemical and
optical characteristics of the pyrotechnic powders. Increasing the
irradiance reduces the time to ignition for most compositions.
2
1. Introduction
Ignition of relatively simple pyrotechnic systems or
devices is usually initiated by a percussion mechanism
(including stab) or by a friction process. However, more
complex systems (e.g., rocket motors etc.) use electrically
initiated pyrotechnic igniters which are usually low voltage
devices. But the use of low voltage igniters in the modern
military environment has led to the problem of inadvertent
initiation of the igniter due to stray electrostatic discharge or
spurious RF signals. Designing to a 1 amp=1 watt criteria or
shielding the igniter may reduce the problem but it is
usually at the expense of increased cost and=or weight.
Over the last twenty years, the concept of using infrared
(IR) radiation as an ignition source for energetic materials
has been extensively studied with a large degree of suc-
2. Experimental
2.1 Xenon Lamp
A Cermax Xenon LX300 UV light source with a spectral
output as shown in Fig. 1 was used. The broadband power
was measured as 30 W whilst that in the individual UV,
# WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
0721-3115/98/0306±0328 $17.50:50=0
Die
meisten SÈtze wurden durch IR-Strahlung gez Èndet, jedoch konnten
Mg=NaNO
3
und SR112 selbst bei StrahlenstÈrken bis 300 W=cm
none of the compositions were ignited using UV
radiation but SR112, B=Fe
2
O
3
and gunpowder showed reactions to
visible radiation at irradiance levels as low as 8.1 W=cm
Propellants, Explosives, Pyrotechnics 23, 328±332 (1998)
Effect of Wavelength on Ignition Threshold 329
Figure 3. Schematic diagram of the laser diode experimental system.
Figure 1. Spectral output of the Cermax Xenon UV lamp.
diode with a maximum power output of 250 mW at 810 nm
was used. The diode as supplied was pigtailed and, after
splicing to a 100 mm step index optical ®ber, the maximum
output was reduced to 210 mW. Several arrangements of the
®ber and the sample were tested: the ®ber touching the front
face of the pressed composition, the ®ber touching a 0.5 mm
BK7 glass window onto which the composition had been
pressed and the ®ber positioned a small distance away from
the front face of the pressed composition. The most suc-
cessful arrangement was to position the ®ber 0.5 mm from
the front face of the sample. This gave a spot diameter of
approximately 300 mm at the sample. The sample was
irradiated with a 300 ms pulse of radiation and the response
of the sample was recorded with a second optical ®ber
positioned next to the input ®ber. The difference between
the start of the input laser pulse and the response of the
sample was recorded as the time-to-ignition.
visible and IR portions of the lamp output were 4.7 W,
7.8 W and 17.5 W, respectively.
The experimental arrangement used to measure the
response of the pyrotechnic compositions is shown in Fig. 2.
The output of the lamp was focussed onto the surface of the
sample and thermal paper was used to measure the spot
size. The spot diameter was approximately 11 mm and its
pro®le was approximately Gaussian. Three ®lters were used
to separate the light source output into either the UV (UG-
11, 90% transmission at peak wavelength of 340 nm),
visible (BG-40, 98.5% transmission at peak wavelength of
500 nm) or the IR (RG665, 99% transmission at peak
wavelength of 710 nm) portions of the spectrum. The dis-
tance between the sample and the lamp was varied slightly
for each ®lter arrangement to ensure that the maximum
irradiance (minimum spot size) occurred at the sample
surface. Initially, the power output of the lamp was set to
maximum and the sample irradiated for 10 s using a
mechanical shutter to control the exposure time. If the
composition ignited, the exposure time was decreased in
regular steps until ignition did not occur. The time-to-
ignition was recorded as the minimum exposure time to
achieve sample ignition. Due to the variable response of
some of the samples, the recorded time-to-ignition was
sometimes broad.
2.3 Pyrotechnic Compositions
Table 1 lists the pyrotechnic compositions studied. For
the Xenon lamp tests, 30 mg of the composition was pressed
into a metal cup at 16.5 MPa. For the laser diode tests,
1000 mg of composition was pressed into a 5.8 mm dia-
meter perspex holder at 6.7 MPa.
2.4 Composition Spectra
2.2 Laser Diode
UV and visible absorption spectra of each composition
were measured with a Varian Carey 3 Spectrophotometer.
The experimental setup shown in Fig. 3 was used to
measure the response of the compositions to narrow band IR
radiation. An SDL-2430-H2 (Spectra Diode Labs.) laser
Table 1. Details of Pyrotechnic Compositions Studied
Composition
Designation
Proportions (% w=w)
Mg=NaNO
3
Ð
50 : 50
Mg=NaNO
3
=ZnO
Ð
47.5 : 47.5 : 5
Mg=NaNO
3
=ZnO
Ð
45 : 45 : 10
B=Fe
2
O
3
MRL(X)201
25 : 75
B=Fe
2
O
3
=ZnO
Ð
23.5 : 71.5 : 5
B=Fe
2
O
3
=ZnO
Ð
22.5 : 67.5 : 10
Gunpowder
G20
Ð
B=KNO
3
SR 44
30 : 70
B=KNO
3
SR 43
50 : 50
TNC
=KNO
3
SR 112
40 : 60
Si=KNO
3
=SMP
(b)
SR 252
40 : 40 : 20
Figure 2. Schematic diagram of the Xenon lamp experimental
system.
(a) Tetranitrocarbazole
(b) Sulphurless Mealed Powder
(a)
330 Leo de Yong and F. Lui
Propellants, Explosives, Pyrotechnics 23, 328±332 (1998)
Table 2. Times-to-Ignition for Pyrotechnic Compositions Exposed to UV, Visible and IR Radiation
Composition
Time-To-Ignition (ms)
UV Radiation
Visible Radiation
IR Radiation
Total Radiation
Laser Diode
(c)
Mg=NaNO
3
N
(a)
N
N
N
N
Mg=NaNO
3
5% ZnO
N
N
N
N
N
Mg=NaNO
3
10% ZnO
N
N
N
N
N
B=Fe
2
O
3
N
10000
400±500
120±130
7.5
B=Fe
2
O
3
5% ZnO
N
N
200±300
200
-
B=Fe
2
O
3
10% ZnO
N
N
1000
120
-
SR 43
N
N
750±1000
300±400
-
SR 44
N
N
800±900
150±170
60.0
SR 112
N
P
N
600±700
N
SR 252
N
N
900±1500
170±180
-
G 20
N
P
(b)
250±300
90±100
-
(a) N designates no ignition
(b) P designates partial ignition
(c) The time-to-ignition recorded here is that measured at an irradiance of 200 W=cm
2
for comparison purposes
IR absorption spectra of each composition were measured
with a Bruker ISF88 FTIR Spectrophotometer using a KBr
disc.
Irradiation with the IR portion of the output (irradiance
18.5 W=cm
2
) resulted in successful ignition of many of the
compositions with the exception of the Mg=NaNO
3
and SR
112 formulations. IR spectra of the compositions showed
strong absorption for all the compositions with the excep-
tion of B=Fe
2
O
3
, Mg=NaNO
3
and SR 112. Ignition times
ranged from 200 ms (B=Fe
2
O
3
=ZnO) to 1000 ms (SR 43).
Attempts to ignite the compositions using the combined
UV, visible and IR radiation from the lamp (irradiance
31.5 W=cm
2
) resulted in ignition of all compositions except
those based on Mg=NaNO
3
. Times-to-ignition varied from
90 ms (G20) to 700 ms (SR 112).
Ignition with the laser diode was evaluated with a limited
number of compositions. As observed with the Xenon lamp,
the Mg=NaNO
3
and SR 112 compositions could not be
ignited with the laser diode even at the maximum irradiance
of 300 W=cm
2
. However, SR 44 and B=Fe
2
O
3
compositions
were ignited with threshold laser irradiances of approxi-
mately 39.0 W=cm
2
and 50.0 W=cm
2
, respectively. For
these tests, Fig. 5 shows that as the irradiance increases, the
time-to-ignition decreases to a limiting minimum time.
Conversely, as the irradiance decreases, the time-to-ignition
3. Results
The results for the ignition of the pyrotechnic samples
using the Xenon lamp are detailed in Table 2.
None of the compositions could be ignited with the UV
portion of the lamp output (irradiance 4.9 W=cm
2
) even
after exposure for up to 10 s. All the compositions showed
good absorption of UV radiation except SR 252 (Fig. 4) and
although the addition of 5% and 10% ZnO slightly
increased the UV absorption for the Mg=NaNO
3
and
B=Fe
2
O
3
compositions, it did not result in ignition of the
samples.
Attempts to ignite the compositions with visible radiation
(8.1 W=cm
2
) were more successful with complete reaction
of B=Fe
2
O
3
and partial ignition of SR 112 and G 20 being
observed after exposure for 10 s. Most of the compositions
strongly absorbed the visible radiation; Mg=NaNO
3
showed
the weakest overall absorption and SR 112 exhibited strong
absorption in the waveband 400±500 nm but decreasing
weak absorption from 500±700 nm (Fig. 4).
Figure 4. UV and visible absorption spectra for several pyrotechnic
compositions.
Figure 5. Time-to-ignition measured as a function of laser diode
irradiance for B=Fe
2
O
3
composition.
Propellants, Explosives, Pyrotechnics 23, 328±332 (1998)
Effect of Wavelength on Ignition Threshold 331
increases to the limiting irradiance where ignition will not
be achieved.
Ignition with the laser diode was strongly dependant on
the physical arrangement of the sample and the optical ®ber.
Using the B=Fe
2
O
3
composition and with the ®ber or the
glass window in contact with the composition, the values of
the time-to-ignition were very irregular, often varying by
several hundred milliseconds for identical samples. Exam-
ination of the ®ber and the window after many tests showed
that part of the pyrotechnic composition had ignited and
deposited reaction products either on the tip of the ®ber or
on the window. These deposits blocked all or part of the
output of the ®ber and reduced the input power to the rest of
the sample. The effect was random and was overcome by
placing the ®ber a ®xed distance from the front face of the
pressed composition with no window present.
role in the ignition process for this composition. The results
obtained with the laser diode tend to con®rm this observa-
tion as ignition of SR 112 could not be achieved even at IR
irradiances up to 300 W=cm
2
presumably due to the poor
absorption of the IR radiation. However, Table 2 suggests
that for most of the other compositions it is the IR portion of
the radiation that is the predominant factor in the ignition
process. This may simply be due to the greater available
irradiance of the IR radiation compared to the UV and the
visible portions and=or to the relative difference in the
optical properties of the compositions in the various regions
of the spectrum and the ef®ciency of heat generation in the
molecules.
The results also show that for the Xenon lamp tests,
irrespective of the spectral nature of the radiation, increas-
ing the irradiance from 8.1 W=cm
2
to 18.5 W=cm
2
to
31.5 W=cm
2
resulted in a consistent decrease in the time-to-
ignition for all the compositions. Although similar beha-
viour was recorded with the laser diode, Fig. 5 shows that
there are limits that de®ne the minimum or threshold time-
to-ignition and irradiance for each composition.
The Mg=NaNO
3
compositions could not be ignited even
at irradiance up to 300 W=cm
2
suggesting that ignition of
this composition will only be achieved at very high irra-
diances.
The problems with the experimental setup of the laser
diode, optical ®ber, glass window and the sample were
principally due to the nature of the compositions examined.
All the initial work was conducted with B=Fe
2
O
3
which,
when ignited, produces essentially gasless (liquid) reaction
products. As the products cool, the liquids condense and
adhere to the ®ber or the glass window. As noted earlier,
this blocks part of the incoming radiation in an entirely
random manner. It is expected that this type of problem
would not occur with a more traditional ``gassy'' igniter
composition.
4. Discussion
The ignition of these pyrotechnic compositions appears
to be dif®cult to achieve with UV radiation at the irradiance
levels up to 5 W=cm
2
. It is expected that the UV energy will
become delocalised as thermal energy within a system
undergoing non-radiative energy relaxation. As all the
compositions showed good absorption of the UV radiation,
it is likely that either the energy became too delocalised
within interatomic bonds of the oxidant to initiate decom-
position and subsequent reaction with the fuel, or the irra-
diance level was simply too low.
Visible radiation successfully ignited three compositions
but the times-to-ignition were long and sustained combus-
tion was only observed for one composition. There were
differences in the magnitude and the spectral character of
the visible radiation absorbed between those compositions
that ignited and those that did not (Fig. 4), but any simple
relationship between the radiation absorption and ignition is
dif®cult to propose on the limited data presented here.
Exposure to IR radiation from the Xenon lamp resulted in
ignition of all the compositions except those based on
Mg=NaNO
3
and SR 112. The failure of these two compo-
sitions to ignite may be due to their poor IR absorption of
the radiation. However, B=Fe
2
O
3
also showed similar poor
IR radiation absorption yet was successfully ignited.
The ignition of a pyrotechnic composition by UV, visible
or IR radiation is a complex process. It will be dependant on
a combination of the physical=chemical=thermal and optical
properties of the composition such as the temperature of
ignition, thermal diffusivity, heat of reaction, the radiation
absorption coef®cient, and the conversion of the absorbed
radiation into heat by the molecules due to electronic
transitions or vibrational relaxation. Whilst it is not possible
to compare the properties of each composition examined
here (as they were not measured), the results indicate that
the spectral properties of the radiation used are important
for ignition.
For example, the results for SR 112 suggest that it is the
visible portion of the radiation that plays the more important
5. Conclusions
(1) It has been successfully demonstrated that some pyro-
technic compositions may be ignited using visible and
infrared (both broadband and narrow band) radiation.
But, at an irradiance of 4.9 W=cm
2
, none of the com-
positions could be ignited with UV radiation.
(2) The Mg=NaNO
3
and SR 112 compositions could not be
ignited with IR radiation at power densities up to
300 W=cm
2
but SR 112 was partially ignited with
visible radiation at an irradiance as low as 7.7 W=cm
2
.
(3) For most of the pyrotechnic compositions the IR portion
of the radiation was the predominant factor in suc-
cessful ignition being achieved. This may have simply
been due to the greater irradiance of the IR radiation
compared to the UV or greater delocalisation of the
thermal energy with the UV radiation.
332 Leo de Yong and F. Lui
Propellants, Explosives, Pyrotechnics 23, 328±332 (1998)
(4) The spectral content of the radiation is important in
ignition; SR 112 ignited at low visible irradiance levels
but not at high IR irradiance levels.
(5) The difference in the response of the compositions to
either UV, visible or IR radiation was not directly
correlated with the measured absorption spectra. For all
the compositions, the absorption=re¯ection of the
radiation did not appear to be the dominant factor in the
ignition process.
(6) For SR44 and B=Fe
2
O
3
compositions, a threshold
irradiance and threshold time to ignition were observed
when exposed to narrowband IR radiation.
(7) L. de Yong and F. Valenta, ``A Study of the Radiant Ignition of a
Range of Pyrotechnic Materials Using a CO
2
Laser'', Report
MRL-TR-90-20 (1990), Materials Research Laboratory, Mel-
bourne, Australia.
(8) D. Ewick, L. Dosser, S. McComb, and L. Brodsky, ``Feasibility
of a Laser Ignited Pyrotechnic Device'', 13th International Pyro-
technics Seminar, Grand Junction, July 1988.
(9) D. Kramer, E. Spangler, and T. Beckman, ``Laser Ignited
Explosive and Pyrotechnic Components'', American Ceramic
Society Bulletin 22, 78±84 (1993).
(10) J. Petrick, ``Laser Ignition System Design for Pyrotechnic
Applications'', Proceedings of the American Preparedness
Association Annual Meeting±Pyrotechnics Section, Shreveport,
October 1988.
(11) S. Kunz and F. Silas, ``Diode Laser Ignition of High Explosives
and Pyrotechnics'', 13th International Pyrotechnics Seminar,
Grand Junction, July 1988.
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(14) M. Landry and J. Cobbett, ``Laser Ordnance System (LOIS) for
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(15) B. Purdy, M. Fratta, and C. Boucher, ``Laser Ordnance System
for NRL's ARTS Program'', 29th AIAA=SAE=ASME=ASEE
Joint Propulsion Conference, Monterey, June 1993.
6. References
(1) V. Menichelli and L. Yang, ``Sensitivity of Explosives to Laser
Energy'', Report NASA JPL TR 32-1474 (1970), National
Aeronautics and Space Administration, Jet Propulsion Labora-
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(2) H. Oestmark, ``Laser as a Tool in Sensitivity Testing of Explo-
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(3) J. Holy, ``Laser Initiation of TiH
x
=KClO
4
'', 11th International
Pyrotechnics Seminar, Vail, July 1986.
(4) J. Holy and T. Girman, ``The Effect of Pressure on Laser
Initiation of TiH
x
=KClO
4
and Other Pyrotechnics'', 13th Inter-
national Pyrotechnics Seminar, Grand Junction, July 1988.
(5) F. Al-Ramadhan, I. Haq, and M. Chaudri, ``Low Energy Laser
Ignition of Magnesium-Te¯on-Viton Compositions. J. Phys. D:
Appl. Phys. 26, 880±887 (1993).
(6) B. Fetherolf, P. Liiva, W. Hsieh, and K. Kuo, ``CO
2
Laser
Ignition Behaviour of Several Pyrotechnic Mixtures'', 16th
International Pyrotechnics Seminar, J Ènk Èping, June 1991.
Acknowledgements
The authors would like to thank Lance Redman, for help with some
of the experiments, Brian Jones, Kevin Lynch and Peter Collins for
help with the instrumentation and David Hatt for advice and assistance
with the experimental hardware.
(Received April 3, 1997; Ms 22=97)
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