Burning Characteristics of Pyrotechnic Time-Delay CompositionSubjected to Moisture and Heat
Kanagaraj Gnanaprakash∗ and Jack J. Yoh†
Seoul National University, Seoul 08826, Republic of Korea
https://doi.org/10.2514/1.B38215
The combustion of pyrotechnic delay composition based onmetallic fuel and perchlorate oxidizer involves complex
reactionmechanisms and condensed phase heat release. Studies on the effect of aging on ignition/burning properties,
and understanding of such multicomponent pyrotechnic delay mixtures are quite limited. The present work focuses
on investigating the reaction kinetics, ignition, and combustion behavior of a zirconium/nickel (Zr/Ni)-alloy-based
pyrotechnic delaymixture. Comparisons are presented between pristine and hygrothermally aged samples subjected
to accelerated aging conditions under a constant temperature of 71°C and 95% relative humidity for 2 weeks.
Activation energy versus reaction progress, ignition delay time as a function of maximumwire temperature, reaction
zone temperature profile, and burning rate with respect to pressure are obtained from various experimental
procedures. Results indicate that aging increases the ignition delay time and temperature rise time significantly,
and a substantial decrement in the heat of reaction and burning rates along with incomplete combustion are evident.
I. Introduction
P YROTECHNIC delay compositions are commonly used intime-controlled energetic devices, and it necessitates precise
time delay between two ignition events with high repeatability andreliability for military and civil applications [1,2]. Some of theimportant characteristics of these delay compositions are gaslesscombustion behavior, low ignition sensitivity to friction and electro-static discharge, long-term storage stability, and wide range of burn-ing rates or combustion velocities [3–5]. Typically, these materialsare composed of metallic fuels, perchlorate/chromate-based oxidiz-ers, and some modifiers/diluents. Combustion of such delay materi-als involves complex physical and chemical processes, multiplereaction mechanisms, and condensed phase heat release. Reactionmechanisms in these mixtures mostly include solid–solid reactionsand solid–liquid reactions involving molten component, and somesolid–gas reactions [5,6].Many studies have reported on combustion velocities and reaction
mechanisms of extensively used delay composition in various fields,which is composed of tungsten, potassium perchlorate (KClO4),barium chromate (BaCrO4), and diatomaceous earth [3,7,8]. Shacharand Gany [3] have studied the effect of tungsten particle size and itscontent on the burning rate and effective activation energy of differentdelay mixtures. Lu et al. [9] have reported that the thermal conduc-tivity of themixturewith highmetal content plays a significant role incausing an increase in burning rates. In another study, thermalanalysis is performed to investigate various reaction characteristicsin these compositions [10], using differential scanning calorimetry(DSC) and thermogravimetric analysis (TGA).Similarly, investigations on delay compositions based on zirco-
nium (Zr) as fuel and KClO4 as oxidant have been reported in theliterature [9,11]. Kang et al. [11] have compared the surface mor-phology, light radiation properties, and flame evolution character-istics of Zr∕KClO4 mechanical mixtures with their correspondingcomposites, for various contents ofKClO4 and particle sizes of Zr. Astudy on the oxidation behavior of alloys based on Zr and nickel (Ni),with different Zr contents, as individual metal particles in air has
illustrated that Zr is selectively oxidized in several combinations, inaddition to cracks observed on the surface of these particles [10].However, the combustion characteristics of pyrotechnic delay com-positions containing Zr/Ni alloy as metallic fuel have not beeninvestigated in the past.More often these pyrotechnic delay materials are subjected to
prolonged storage under different conditions of temperature, humid-ity, stress, pressure, etc. This aging process induces gradual degra-dation in their performance and could cause problems such asdecreased heat release, inconsistent burning rates, lower reactionzone temperature, reduced ignition characteristics and sensitivity,increased unsteady burning behavior, and diminished reaction rate,which eventually result in the failure of energetic devices. Manyinvestigations have studied the aging process of various pyrotechnicssubjected to accelerated aging of temperature and humidity toimprove upon their shelf-life [2,12–17]. It has been broadly reportedthat prolonged storage of such materials causes a decrease in overallheat of reaction and an increase in activation energy. Specifically, thethermochemical characterization of Zr∕Fe2O3 described that thepre-oxidation of metallic fuel due to aging is an important factor indegrading their performance [16]. The aging study on Zr∕KClO4
pyrotechnic delay composition has theoretically shown that the flametemperature decreases due to the pre-oxidation of metal fuels [2],which is significantly enhanced in the presence of moisture. Theseobservations are mainly based on thermodynamic calculations usingthe Thermo-Calc software. But this decrease in flame temperature isonly marginal and the maximum value is significantly high for mostmaterials.Despite the available literature on different pyrotechnic delay
materials, a complete understanding of their reaction/combustionmechanism is still limited. Moreover, this mechanism is furthercomplicated due to prior decomposition/oxidation of condensedphase reactives, when subjected to the aging process. In past studies,the aging effect has been investigated mainly from the aspect ofthermochemical characterization to determine the thermodynamicand kinetic parameters of these materials [2,14,17]. However, theeffect of aging on the ignition and combustion properties of pyro-technic delay compositions is lacking. Specifically, its influence onthe ignition delay time and the burning rate dependence on pressurehave not been previously examined.The present study focuses on experimentally investigating the
impact of accelerated hygrothermal aging on the burning behaviorof Zr/Ni-alloy-based pyrotechnic delay, whereas previous work byHan et al. [17] primarily studied the thermochemical and physicalbehavior of the material. This delay composition used in time-controlled energetic devices is based on Zr/Ni alloy, BaCrO4, andKClO4 oxidizer. The mixture as powder and pressed pellets are agedunder constant temperature and relative humidity (RH) over specific
time. Reaction kinetics, ignition delay time, reaction zone temper-ature profile, and burning rates of hygrothermally aged Zr/Ni delaysamples are compared with pristine ones in order to elucidate theircorresponding reaction/combustion mechanisms.
II. Experimental Details
A. Ingredients and Samples
All samples are delivered by Hanwha Corporation, Ltd., and thecurrent formulation is selected to achieve a desired overall burn time,as per the requirements. The pyrotechnic delay compositionconsidered in the present study consists of Zr/Ni alloy as metallicfuel, KClO4 as oxidant, BaCrO4 as modifier, and some amount ofbinder. The compositional ratio for Zr-Ni∕KClO4∕BaCrO4 delay is8% Zr7Ni3 amorphous alloy, 25% Zr3Ni7 amorphous alloy, 15%KClO4, 52% BaCrO4, and 1% binder, by mass [13,17]. Averageparticle sizes for all solid ingredients are maintained to be ∼45 μm,except forBaCrO4 (∼10 μm). For better readability, this pyrotechnicmixture will be referred to as Zr/Ni delay further in this paper.These ingredients are mechanically mixed and used in powder
form for the thermal analysis, x-ray diffraction (XRD), reaction zonetemperature, and hot wire ignition experiments. Further, thesemechanical mixtures are pressed in a hydraulic press into pelletsfor using in burning rate tests [18]. This pressed pellet, which is incylindrical form of diameter 20 mm and length 20 mm, is used tomeasure its actual bulk density. The theoretical maximum density(TMD) is calculated using the mass fraction and density of eachingredient in the mixture. The TMD and actual measured density ofthismixture are 4.59 and 3.13 g∕cm3 (0.68 TMD), respectively. Bothmechanical mixtures and pressed pellets are categorized into pristineand hygrothermally aged. The aged samples are subjected to accel-erated aging at constant temperature of 71°C and 95% RH for 2weeks. Generally, in most aging studies, samples are subjected toaccelerated aging under extreme conditions of elevated temperatureand high RH conditions to mimic the natural aging process thatoccurs over an extended time under realistic storage environments.The Van’t Hoff equation is used to predict the equivalent naturalstorage period of these samples under room temperature by using theaccelerated aging period and conditions [15]. Based on this, thenatural aging period at ambient temperature of 25°C and accelerationfactor of 3.5 is determined to be 12.2 years for 2-week-aged samples.This is a reasonable estimate for the aging period, given that suchpyrotechnic materials are expected to have high reliability and longshelf life.
B. Techniques
Thermal analysis is performed using Mettler Toledo DSC 3� andTGA 2 facilities. Mechanical mixtures of 2–3 mg are placed inside a40 μL aluminum crucible that is sealed using a lid having a 0.5-mm-diam perforation. Experiments are performed under 40 mL∕min ofnitrogen flow inside the furnace in the temperature range of30–650°C [15,16]. Meanwhile, TGA experiments are performedwith a standard 70 μL alumina open pan with a sample weight of∼4 mg in the same temperature range. The heating rate is varied as 2,4, 8, and 10°C/min. All experimental conditions and proceduresfollow the International Confederation for Thermal Analysis andCalorimetry Kinetics Committee’s recommendations [19]. Further,the heat of reaction for all tested samples is calculated by integratingthe area under a peak in DSC curves using the sigmoidal tangentialbaseline type in Advanced Thermo Kinetic Analysis soft-ware (AKTS).Chemical kinetic parameters, the activation energy Eα and pre-
exponential factor Aα, are extracted from DSC curves obtained atvarious heating rates by employing the Friedman differential iso-conversional method [20]. Under nonisothermal conditions, theproduct of the heating rate �β � dT∕dt� and the reaction rate isdefined in Eq. (1).
βdα
dT� Aαf�α� exp
−Eα
RT(1)
Taking the logarithm of Eq. (1) gives
ln�βdα
dT
�� ln �f�α�Aα� −
Eα
RT(2)
where R; T; α, and f�α� correspond to the universal gas constant,temperature, reaction progress/product mass fraction, and a functionthat governs the dependence of the rate on reaction progress, respec-tively. In addition,Eα and Aα can be determined from the slope and yintercept, respectively, of a linear equation (y � ax� b), and�dα∕dt� � β�dα∕dT�, where x is 1∕T and y is ln �dα∕dt�.XRD analysis is performed using a powder x-ray diffractometer
(Rigaku, SmartLab model). This equipment uses an Ultra250 detec-tor andCu targetwith a tubevoltage of 3 kV. The ground fine powdersof actual Zr/Ni delay mixtures before combustion and product resi-dues from strand burner tests are analyzed in XRD. Approximately,4 mg of samples is used for this analysis. The diffraction peakintensities are obtained in the 2θ range of 10–90°, and the resultsare examined in PDXLV2.8 software using Crystallography OpenDatabase [21].Hot wire ignition experiments are performed with mechanical
mixtures to determine the ignition delay time as a function of maxi-mum wire temperature under atmospheric conditions. Ignition isachieved through Joule heating [22], by supplying heat energy tosamples using a nichromewire, with a length of 100mmand diameterof 0.3 mm and a DC power supply. This hot wire attains a maximumtemperature depending on the electrical energy supplied to it. Thevoltage is maintained at 30 Vand the current is varied in the range of3.0–5.50 A (0.5 A interval) in the DC power source that is connectedto the nichrome wire, in order to attain specific maximum wiretemperatures. This temperature can be calculated based on the energybalance at the wire. The energy stored in the wire is equated to thedifference between the electrical energy supplied to it and any heattransfer that happens due to conduction, convection, or radiation.Assuming that there is no heat loss and using known parameters ofwire’s emissivity, resistivity, length, diameter, and its convective heattransfer coefficient, this maximum wire temperature is determined.Hot wire ignition technique has been reported commonly in the
recent literature to measure the ignition delay time of different solidfuels [23–26]. Powder samples of ∼50 mg are packed inside analumina crucible, and the nichrome wire is placed in contactwith the mixture at the top. The ignition process is captured usinga high-speed camera (Phantom V711, with a resolution of
800 × 600 pixels2) fitted with a 105 mm Nikkor macro lens, at aframing rate of 250 fps and exposure time of∼30 μs. The high-speedcamera and ignition source are triggered externally with 5 V pulse toensure synchronization between the image acquisition and wireheating. The ignition delay time is obtained based on the differencebetween the onset of trigger source and first light emission from thesample burning [27]. Note that the time taken by the nichromewire toreach its maximum temperature is inherently included in the ignitiondelay time, which varies depending upon the applied electricalpower. The uncertainty in measuring this ignition delay time is�10 ms, and more than 90% of data are repeated at least twice forall cases.The reaction zone temperature profile is obtained using a combus-
tion boat-type configuration similar to previous studies [4,28]. About100–130 mg of loose powder is packed inside this cylindrical con-figuration, which is open on the top side and has a small port at thecenter of the bottom side for inserting a K-type thermocouple(0.6 mm). The thermocouple is inserted ∼2.0 mm into the powderfrom the bottom. Ignition is achieved by blowing a butane flame torchonto the top surface of the sample for a few seconds. Temperaturedata are collected using an NI module 9123 connected to NI DAQ9174 at the sampling rate of 100 Hz, and LabVIEW software.Experiments have been repeated three times for both pristine andaged samples, and an average temperature profile along with thedeviation not exceeding�5% is presented for discussion.Awindowed strand burner developed previously at our lab is used
for determining the burning rates of pressed pellets as a function ofpressure, in the range of 1–40 bar under nitrogen ambience, through
2 Article in Advance / GNANAPRAKASH ANDYOH
Dow
nloa
ded
by S
EO
UL
NA
TIO
NA
L U
NIV
ER
SIT
Y o
n M
ay 3
1, 2
021
| http
://ar
c.ai
aa.o
rg |
DO
I: 1
0.25
14/1
.B38
215
combustion photography. The test procedure and details of theexperimental setup arementioned elsewhere [18]. Pressed cylindricalpellets are cut into samples of rectangular cross section 7 × 4 mm2
and length 10 mm. The combustion process is captured using theabove-mentioned high-speed imaging setup at a framing rate of100 fps and exposure time of ∼50 μs. A custom MATLAB code isused to locate the leading reaction zone locations in subsequentburning images, which is then plotted against the framing timeinterval in order to determine the burning rate from the slope of thestraight line. Most burning rate data at each pressure are repeated atleast once and the error involved in thismeasurement is�3% for eachdata point.
III. Results and Discussion
A. Reaction Kinetics
DSC tests for pristine and 2-week hygrothermally aged Zr/Nidelay samples are performed at various heating rates of 2, 4, 8, and10°C/min. DSC/TGA curves obtained at 10°C/min for pristine delaysample are shown separately in Supplemental Fig. S1a, in the temper-ature range of 250–600°C. The reaction mechanism of pristinesample is composed of one endothermic and two exothermic peaks,as reported previously [17]. The first significant exothermic peak isnoticed at∼420°C, and the second relatively small one is observed at∼530°C (Fig. S1a). The major primary reaction is caused by theoxidation of metallic alloy with KClO4. The onset of this primaryreaction occurs at ∼360°C, which is significantly lower than themelting temperature of KClO4. This has been ascribed to variousaspects, such as solid-state reactions, interdiffusion of reactive com-ponents, local hotspot regions, and formation of cracks and distor-tions [29–31]. There is a slight mass increase (∼1%) during thisprimary reaction in the TGA curve, which is due to the involvementof sparsely available oxygen in this reaction, present in the inert purgegas [17]. A mass decrease (∼4%) observed at temperature >470°Ccorresponds to the decomposition of the unreacted KClO4 from theprimary oxidation reaction, thus causing the second exothermic peak.The thermal behavior of these delay samples over different heatingrates is presented in Supplemental Figs. S1 and S2.The primary reaction with this delay composition results in
BaZrO3, ZrO2, NiO, and KCl as major products. The decompositionreaction ofKClO4 producesKClO3 andO2. Based on this, the overallreactionwith this pyrotechnic compositionwould involve the follow-ing proposed scheme. These reaction products will be confirmedusing XRD analysis in the next subsection.
KClO4 → KClO3 � 1∕2O2 (3)
2Zr∕Ni� KClO3 � 2BaCrO4 → 2ZrO2 � 2NiO� 2BaO
� Cr2O3 � KCl (4)
ZrO2 � BaO → BaZrO3 (5)
Samples subjected to hygrothermal aging exhibit a degradation inboth exothermic peaks relative to pristine ones (Fig. S1b). Thisaffirms that the presence of moisture during aging influences theoxidation and decomposition process of themetallic fuel and oxidant,respectively, as reported in previous studies from our group [14–17].The heat of reaction from the primary oxidation reaction of Zr/Nidelay is decreased to 66.9 J∕g from 88.2 J∕g due to aging, whereasthe decomposition reaction is reduced to 19.6 J∕g from 31.8 J∕g.Heat-of-reaction values obtained at various heating rates for allsamples considered in the present study are listed in SupplementalTable S1.Adecrement in the heat of reaction for hygrothermally agedsamples is observed consistently for both reactions.Reaction kinetic parameters for both samples are determined from
DSC curves by employing the Friedman isoconversional method inthe AKTS kinetic software package. The reaction rates calculated atvarious heating rates over the temperature range, corresponding tothe primary oxidation reaction (first peak), for pristine andhygrothermally aged samples are shown in Figs. 1a and 1b,respectively. A typical increase in the reaction rate with the heatingrate is observed for both cases. Notice a high peak and narrow widthfor the 2-week-aged sample, because the actual fuel and oxidizercontents are reduced and the oxidation reaction gets completed in ashort time. For this primary reaction, the effective activation energyEα obtained as a function of the instantaneous state of reactionprogress α is shown in Fig. 2, over the range of 0.1 ≪ 0.9α. Thelowest and highest Eα values for the pristine sample occur atα � 0.25 and α � 0.8, respectively. However, at α � 0.75, thelowestEα value for the aged sample is noticed. The average activationenergy for the pristine delay sample is ∼170 kJ∕mol, whereas it is∼155 kJ∕mol for the aged case. The extent of reaction progressand ln �dα∕dt� curves for both samples at various heating rates arepresented in Supplemental Fig. S3.Overall, a decrement in theEα value is noticed for the aged sample
compared with the pristine one in the entire range of reactionprogress. A similar observation has been reported previously fortitanium-hydride-based pyrotechnic compositions, wherein thereaction corresponding to the second peak indicated a decrement inthe activation energy in the full α range for hygrothermally agedsamples [14]. This is attributed to the presence of large content ofKClO3, caused by moisture-induced decomposition of KClO4 inaged samples. Because the former undergoes melting at significantlylower temperatures than the latter, the oxidation reaction is initiatedrelatively easily, thus resulting in low Eα values. On the other hand,the presence of highly reactive decomposition products would causeprior oxidation of metallic fuels as well, which reduces the overallheat of reaction in aged samples, as seen in Fig. S1b. So, there exists acompeting effect for decomposed oxidants between a decrease in theheat release against an increase in the reactivity of the mixture,i.e., decrease in the activation energy. Furthermore, the peak reaction
Fig. 1 Kinetic parameters of primary oxidation reaction in Zr/Ni pyrotechnic delay obtained at various heating rates. Reaction rate for a) pristinesamples and b) 2-week hygrothermally aged samples.
temperature for aged samples is lower compared with pristine ones,
which implies that the reaction rate constant is high with the former.
B. XRD Results
XRD analysis is performed on the actual Zr/Ni delay mixture
before combustion for both pristine and hygrothermally aged sam-
ples. These patterns are shown in Fig. 3, in which the data for pristine
case are translated in the vertical direction for clarity. The compo-
nents detected include Zr/Ni alloy, BaCrO4, and KClO4, which are
major ingredients of this delay composition. As expected, a small
amount of ZrO2 is noticed as well even in the pristine sample due to
the presence of an outer oxide layer in the metallic fuel. Apart from
similar peaks with the pristine sample at different 2θ angles, signifi-cant presence of ZrO2 is further observed at 2θ of 35, 40, and 42° inthe hygrothermally aged case. This affirms that prior oxidation of the
metallic fuel has occurred due to the aging process, as reported in
many previous studies [13–17].In addition, XRD is also performed on combustion product resi-
dues from strand burner tests at 2 MPa for both types of samples and
presented in Fig. 4, wherein the data of the hygrothermally aged one
are translated vertically. The main diffraction peaks for products
residues are composed of BaZrO3, ZrO2, NiO, KCl, and some
unreacted Ni and Ba. This corroborates with the reaction scheme
proposed earlier for this Zr/Ni delay composition in the previous
subsection. Furthermore, the presence of some amount of unreacted
Ni and Ba in the residue suggests that incomplete combustion has
occurred with these mixtures. Qualitatively, this is noticed more with
the hygrothermally aged case at 2θ of 28, 80, and 88° than in the
pristine sample. Also, the presence of KCl is reduced in the former
compared with the latter. This implies that a large degree of incom-plete combustion and oxidant decomposition is plausible with this
pyrotechnic delay when subjected to the aging process.
C. Ignition Delay Time
Ignition delay time is the time taken to show first light emission
during the onset of combustion after switching on the electrical powersupply. This power is varied to obtain the go/no-go ignition boundary
as a function of maximum wire temperature [26]. Ignition times
obtained at variouswire temperatures for pristine and hygrothermally
aged Zr/Ni delay samples are shown in Fig. 5. A separate ignition
experiment is performed, in order to examine the influence ofKClO4
on the ignition behavior of pristine samples. It is observed thatignition occurs only when KClO4 is present in the mixture, even
though oxygen is available in the form of BaCrO4.Pristine Zr/Ni delay samples attain ignition only at a wire temper-
ature of ∼1000°C with delay time of ∼2.0 s and exhibit a decrement
in this time as electrical energy supplied to the wire is increased,i.e., at high wire temperatures. Ignition delay time of ∼1.0 s is
observed for the maximum possible wire temperature (∼1450°C)with these samples. Such a decreasing trend of the ignition time with
respect to the energy flux has been reported in the past for composite
solid propellants and other pyrotechnic compositions in laser ignition
as well as hot wire ignition studies [23,27,32,33]. Note that high
Fig. 2 Comparison of Eα − α relationship curves for Zr/Ni delaysamples.
Fig. 3 XRDpatterns of actual Zr/Ni delay composition (before combus-tion).
Fig. 4 XRD patterns for combustion product residues of Zr/Ni delaycomposition at 20 bar.
Fig. 5 Ignition delay time versus maximumwire temperature curves ofZr/Ni pyrotechnic delay samples.
4 Article in Advance / GNANAPRAKASH ANDYOH
Dow
nloa
ded
by S
EO
UL
NA
TIO
NA
L U
NIV
ER
SIT
Y o
n M
ay 3
1, 2
021
| http
://ar
c.ai
aa.o
rg |
DO
I: 1
0.25
14/1
.B38
215
electrical power represents that the wire is heated up quickly to reachits maximum temperature, which essentially causes fast heat con-duction rates into these mechanical mixtures, thus resulting in lowignition delay time. This process is dependent on time and one cannotdetermine the exact temperature at which these samples achieveignition. Eventually, such process occurs in most ignition experi-ments, irrespective of the power source (either laser or hot wireignition). Typically, in laser ignition studies, it is quite common torepresent the ignition delay time as a function of laser energy flux,which is similar to electrical energy per unit area in the present study.Because the contact area between the wire and powder samplescannot be precisely maintained the same in all these tests, it is quitedifficult to define the electrical energy flux as a varying parameter,and therefore the maximum wire temperature is considered inthis study.It is known that most pyrotechnic delay compositions undergo
primarily condensed phase reactions before proceeding to solid–gasreactions. The reaction process of Zr/Ni delay mixture involvesdecomposition/melting of KClO4 (∼600°C), which is well belowthe minimum ignition threshold of these samples. This implies thatthere exist solid–liquid reactions during the ignition process [26].Further, it has been reported that the presence of BaCrO4 in typicalpyrotechnic delay mixtures influence their combustion velocity sig-nificantly [4,8]. However, besides the participation in barium zircon-ate (BaZrO3) formation reaction, it does not play an important roleespecially during initiation of the overall reaction process, as men-tioned before, in the present study. Note that large amounts ofBaCrO4 in the mixture act as heat sink and reduce the available netheat release from the oxidation process, which is a common designaspectwithmost pyrotechnic delaymaterials in order to attain desiredburn time in energetic devices.In case of hygrothermally aged samples, the minimum ignition
threshold is noticed at marginally higher wire temperature(∼1050°C) relative to that of pristine ones. However, the ignitiondelay time is greater by 250% (∼5.0 s) in the former compared withthe latter. At 1450°C, this disparity in the ignition time is reduced to∼100% between these samples. A long ignition delay time and highignition threshold with hygrothermally aged samples are mainly dueto the reduced active metal content, thicker metal oxide layer, anddecreased thermal conductivity of the mixture. This would causesignificant reduction in the burning rates of aged samples as will beshown in the next subsection.
D. Combustion Behavior
The reaction zone temperature profiles of Zr/Ni delay loose pow-ders measured experimentally are shown in Fig. 6. A peak in thetemperature profile is noticed at ∼550°C for both pristine andhygrothermally aged samples, which indicates that the maximumcombustion temperature is not significantly affected by the agingprocess, contrary to the theoretical trend reported for agedZr∕KClO4
pyrotechnic materials [2]. Because the current experiments involvesmall scales and high heat losses, adiabatic conditions cannot be
considered, and therefore the observed disparity. In addition, thepresence of BaCrO4 and the metallic fuel in alloy form in the presentpyrotechnic material could cause such negligible effect on the peakreaction zone temperature. On the other hand, this could be caused bythe uncertainty involved particularly with this measurement. How-ever, the time taken to reach the peak temperature from the onset oftemperature rise ismarginally higher for hygrothermally aged sample(∼4.5 s) compared with the pristine one (∼4.0 s). The temperaturerise time Tr is related to the average thermal diffusivity of themechanical mixture. A large rise time corresponds to small thermaldiffusivity of these samples, i.e., slow heat conduction into unburntregions of the sample. This implies that the regression rate would bereduced due to hygrothermal aging for this delay.Pressed pellets shaped into rectangular cross section are used to
obtain the burning rates of pyrotechnic samples as a function ofpressure in a windowed strand burner, fabricated previously at ourlab [18]. The pressure range is considered from 1 to 40 bar, based onactual conditions experienced by this pyrotechnic delay compositionunder realistic conditions. Ignition is achieved using hot nichromewire placed at the top surface of pellet samples. First, we will look atthe qualitative features of the pellet burning under pressurized con-ditions and elucidate the combustion behavior of these samples.Next, the actual regression/burning rates deduced from subsequentimages in the videowill be discussed alongwith the effect of pressureand aging on the combustion mechanism of this delay composition.Subsequent instantaneous snapshots of Zr/Ni delay pellets are
depicted in Fig. 7 for both pristine and 2-week hygrothermally agedsamples burning at a pressure of40bar. The corresponding timestampsof these burning images are indicated in each of them. Ignition istypically achieved at the center of pellet and spreads across the crosssection to establish a reaction zone along the planar surface of thepellet.After reaching a steady-state condition, this planar reaction zonepropagates in the downward direction. High luminous regions areobserved in the condensed phase that separate the unburnt region fromthe reaction zone in pristine samples (Fig. 7a). Product gases emanatemildly from the sample regressing surface in normal directions only atthe location of reaction zone. In addition, note that product residuesthat remain after the combustion process retain almost the initial shapeof the pellet, sometimes with expanded cross section as well. Theabove observations suggest that the reaction between metallic fuelalloy and perchlorate oxidant occurs primarily in the condensed phase,and there are no significant (or onlyminimal) reactions in the gas phasewith this delay composition. Because the KClO4 component exists inmolten state before ignition as mentioned before (Fig. 5), solid–liquidreactions are predominant with these samples. Thus, conductive burn-ing governs the reaction propagation wherein conduction is the pri-mary heat transfer mechanism that controls the burning rate of thesesamples. In addition, it is observed from these images that the rate ofregression is not significantly increased at high pressures. This istranslated into onlymarginal increment in burning rateswhen pressureis increased, as will be seen later.In case of hygrothermally aged samples (Fig. 7b), combustion
behavior is dictated by conductive burning similar to that of pristineones. However, there is a decrease in the intensity of reaction propa-gation regions, and an increase in gas generation of reaction productswith the former relative to the latter. This is due to the vaporization ofprior decomposition products (caused by aging) of perchlorateoxidant during burning, which also induces dispersed thin luminouszones on the pellet surface. It can be postulated that these regions arecaused by hot gas permeation along cracks/interfaces formed onpellets due to an increase in the product gas generation. But it wouldbe highly challenging to quantify or substantiate the amount of gasgenerated and variations in the product composition with respect tothe initial composition for such multicomponent pyrotechnicmaterials, because they involve multiple/complex reaction mecha-nisms and cannot be easily computed. Moreover, aging could causeoscillatory/incomplete burning with these samples under certainconditions in which either fuel and/or oxidant are unavailable toconstitute the reaction zone, similar to that elucidated byMiklaszew-ski et al. [28]. In the present study, the oscillatory burning behaviorwith hygrothermally aged Zr/Ni delay sample is observed at 6.80 and
Fig. 6 Measured combustion zone temperature profiles of Zr/Ni pyro-technic delay samples.
Article in Advance / GNANAPRAKASH ANDYOH 5
Dow
nloa
ded
by S
EO
UL
NA
TIO
NA
L U
NIV
ER
SIT
Y o
n M
ay 3
1, 2
021
| http
://ar
c.ai
aa.o
rg |
DO
I: 1
0.25
14/1
.B38
215
7.90 s, wherein uneven propagation of the reaction zone surface isseen in the front side of pellets. This is reflected as large standarddeviation in each burning rate data, as will be shown next. Thus, thecombustion stability of Zr/Ni delay composition is significantlyinfluenced as well by the aging process.The burning rate dependence on pressure and aging of Zr/Ni delay
pellets are shown in Fig. 8. In the pressure range of 1–40 bar, burningrates of these samples vary in the range of 0.85–1.35 mm∕s. A linearincrease in the burning rate is observed with pristine delay whenpressure is increased. However, this increment is only marginal andnot as significant as in the case of composite solid propellants, whichundergoes predominantly gas phase combustion, rather thancondensed phase reactions as with most pyrotechnic delay mixturesin general. Burning rate versus pressure plots followVielle’s burn rateequation, r � aPn. A power law fit to these data points results in thepressure exponentn of 0.11 for pristine delay pellets. A low n impliesthat this composition offers high stability to burning rate fluctuationseven for considerable variations in pressure, mainly due to itsdominant condensed phase reactions.Burning characteristics of any multicomponent solid fuels depend
onmany significant parameters such as overall composition, fuel andoxidizer particle size, bulk thermal conductivity and heat of reactionof themixture, reaction rate progress, reaction zone temperature, heattransfer mechanism, and ambient temperature and pressure [3,4].Note that a large amount of BaCrO4� 52%) is present in the Zr/Nidelay, which acts as a heat sink and absorbs some heat from the
primary oxidation reaction, as mentioned before. Similarly, Zr/Niparticles present in the form ofmetallic alloy reduces the heat transferrate into the solid pellet. Further, a relatively low heat release from theprimary oxidation reaction between fuel and perchlorate oxidant isobserved as well (Fig. S1). All these above effects cause relativelylow burning rates for this delay when compared with other metalbased pyrotechnic materials [3,7].Such slow burning rates further affirm that the primary rate con-
trolling mechanism is governed by the conductive burning andmajorheat transfer occurs through conductionwith onlyminor impact fromconvection for this delay composition, as presented in precedingsections. Further, the particle size (∼45 μm) of most ingredients usedin the present study is in the range to cause conductively dominantheat transfer, as reported previously for similar size ranges ofAl/Zr-MoO3 thermite mixtures [24].When subjected to hygrothermally aging, the pressure exponent
varies only slightly with this delay (n � 0.12), whereas the burningrate increases marginally with pressure similar to pristine samples.More importantly, a substantial reduction in the burning rates ofhygrothermally aged samples is clearly evident when compared withthe latter, over the entire pressure range. A decrement in the range of15–35% is observed at each pressure level. Moreover, with the hygro-
thermally agedZr/Ni delay, a large scatter in both burning rates (lowR2
value) and standard deviation (error bar) with each data point areobserved, implying that aging influences unstable burning behavioras well, corroboratedwith burning images previously shown (Fig. 7b).Typically, such reduction in burning rates is common among solidpropellants subjected to long-term storage [34–36]. Although a similareffect of aging on burning rates of pyrotechnic delay compositionscould have been anticipated, the quantitative extent of the decrement inburning rates would not have been evident without the present results.It is reported in many investigations that prolonged storage of
delay samples under accelerated aging conditions of temperatureand RH induces pre-oxidation of the metallic fuel and prior decom-position of the oxidant to some extent [12,15–17]. Note that thermalconductivity plays a significant role in determining the burning rateof pyrotechnic delay materials as well. Because bulk thermal con-ductivity of metallic oxides (ZrO2 and NiO) is generally lower thantheir correspondingmetal (Zr andNi), the presence of relatively largeamount of oxide results in low average thermal conductivity of agedsamples. Consequently, there is a reduced net conductive heat trans-fer into pressed pellets as burning progresses, thus causing a reduc-tion in the burning rate with aged samples. Furthermore, in agedsamples, the metal oxide layer would be thicker than in pristinesamples. Because the rate of reaction zone propagation is determined
Fig. 7 Burning snapshots at different time intervals of Zr/Ni pyrotechnic delay pellets at 40 bar: a) pristine and b) 2-week hygrothermally aged.
Fig. 8 Burning rate versus pressure plots of Zr/Ni pyrotechnic delaysamples.
by net heat feedback and diffusion of reactants through this oxidelayer, it can be inferred that a thick oxide layer induces slow diffusionof reactants as well. Moreover, such mechanisms of reduced net heattransfer and slow reactant diffusion are observed throughout thecombustion process along the entire cross section of the pellet. Theseattributes imply that aging influences the combustion behavior ofpressed pellets in bulk manner and not merely along the outer edges,as a surface phenomenon. All the aforementioned aspects wouldinduce a significant reduction in the burning rate and heat of reaction,an increment in the ignition delay time, and unstable burning behav-ior with most aged pyrotechnic delay materials, dominated by con-densed phase reactions, irrespective of the type of metallic fuel.
IV. Conclusions
In the present study, a pyrotechnic delay composition (Zr/Ni alloyas metallic fuel, KClO4 as oxidant, and BaCrO4 as modifier) wasexperimentally investigated for the effect of hygrothermal aging onthe ignition and combustion behavior. Bothmechanical mixtures andpressed pellets were subjected to accelerated aging conditions at aconstant temperature of 71°C and 95% RH for 2 weeks. DSC/TGAanalysis, hotwire ignition experiments, and combustion photographywere adopted to obtain the heat release, reaction kinetics, ignitiondelay time, reaction zone temperature profile, and burning rate of thisdelay mixture. Combustion mechanism of this delay compositionwas predominantly governed by solid–liquid reactions during theinitial stage, followed by solid–gas reactions through conductive heattransfer. This mechanism was affected by the humidity-inducedaging process, which resulted in the decrement of heat release fromthe primary oxidation reaction and an increase in the reactivity of themixture. Such behavior was observed due to the pre-oxidation of themetallic fuel and presence of more reactive prior decompositionproducts in hygrothermally aged samples in comparison to pristineones. Thus, there existed a competing effect between the heat releaseand reactivity that influenced the overall reaction propagation of suchaged delay samples. Further, a large ignition delay time and combus-tion temperature rise time were noticed with hygrothermally agedsamples. More importantly, a significant reduction of 15–35% inburning rates was clearly observed in the entire pressure range due tohygrothermal aging, in addition to the unstable burning behavior.These results suggest that there was a reduced net conductive heattransfer andmass diffusion of reactants, which induced this degradedperformance with aged pyrotechnic time-delay materials.
Acknowledgments
This work was supported by the Brain Korea 21 plus program,HanwhaCorp. (No.HanwhaSNU-2018), andNextGeneration SpacePropulsion Research Center (No. NRF-2013R1A5A1073861) con-tracted through Institute of AdvancedAerospace Technology (IAAT)and Institute of Engineering Research (IOER) at Seoul NationalUniversity.
References
[1] Focke,W.W., Tichapondwa, S.M.,Montgomery,Y.C.,Grobler, J.M., andKalombo, M. L., “Review of Gasless Pyrotechnic Time Delays,” Propel-lants, Explosives and Pyrotechnics, Vol. 44, No. 1, 2019, pp. 55–93.https://doi.org/10.1002/prep.201700311
[2] Kim, K. M., Choi, S. I., Eom, K. H., Ahn, G. H., Paik, J. G., Ryu, B. T.,Kim, Y. H., andWon, Y. S., “Thermodynamic Analysis on the Aging ofTHPP, ZPP and BKNO3 Explosive Charges in PMDs,” Energies,Vol. 12, No. 2, 2019, Paper 209.https://doi.org/10.3390/en12020209
[3] Shachar, E., andGany,A., “Investigation of Slow-PropagationTungstenDelay Mixtures,” Propellants, Explosives and Pyrotechnics, Vol. 22,No. 4, 1997, pp. 207–211.https://doi.org/10.1002/prep.19970220405
[4] Koenig, J. T., Shaw, A. P., Poret, J. C., and Groven, L. J., “StrontiumMolybdate as a Replacement for Barium Chromate in the TraditionalTungsten Time Delay: An Exploration of Reaction Mechanisms forDesign of PyrotechnicDelays,” Journal of EnergeticMaterials, Vol. 38,No. 1, 2020, pp. 20–34.https://doi.org/10.1080/07370652.2019.1657205
[5] Tribelhorn, M. J., Venables, D. S., and Brown, M. E., “Combustion ofSome Zinc-Fuelled Binary Pyrotechnic Systems,” Thermochimca Acta,Vol. 256, No. 2, 1995, pp. 309–324.https://doi.org/10.1016/0040-6031(95)02248-Z
[6] Kang, X., Zhang, J., Zhang, Q., Du, K., and Tang, Y., “Studies onIgnition and Afterburning Processes of KClO4∕Mg Pyrotechnics
Heated in Air,” Journal of Thermal Analysis and Calorimetry, Vol. 109,
2012, pp. 1333–1340.
https://doi.org/10.1007/s10973-011-1991-x[7] Lu, K. T., Yang, C. C., and Ko, Y. H., “Investigation of the Burning
Properties of Slow-Propagation Tungsten Type Delay Compositions,” Pro-pellants, Explosives and Pyrotechnics, Vol. 33, No. 3, 2008, pp. 219–226.https://doi.org/10.1002/prep.200800219
[8] Lu, K. T., and Yang, C. C., “Thermal Analysis Studies on the Slow-Propagation Tungsten Type Delay Composition System,” Propellants,
Explosives and Pyrotechnics, Vol. 33, No. 5, 2008, pp. 403–410.
https://doi.org/10.1002/prep.200700287[9] Pourmortazavi, S. M., Hosseini, S. G., Hajimirsadeghi, S. S., and
Alamdari, R. F., “Investigation on Thermal Analysis of Binary
Zirconium/Oxidant Pyrotechnic Systems,” Combustion Science and
Technology, Vol. 180, No. 12, 2008, pp. 2093–2102.
https://doi.org/10.1080/00102200802466014[10] Asami, K. K., Kimura, H.M., Hashimoto, K., andMasumoto, T., “High
Temperature Oxidation Behavior of Amorphous Zr-Ni Alloys in Air,”
Materials Transactions, Vol. 36, No. 7, 1995, pp. 988–994.
eter KClO4/Zr Energetic Composite Particles with Enhanced LightRadiation,” Materials, Vol. 12, 2019, Paper 199.https://doi.org/10.3390/ma12020199
[12] Lee, J., Kim, T., Ryu, S. U., Choi, K., Ahn, G. H., Paik, J. G., Ryu, B.,Park, T., and Won, Y. S., “Study on the Aging Mechanism of Boron
Potassium Nitrate (BKNO3) for Sustainable Efficiency in Pyrotechnic
Mechanical Devices,” Scientific Reports, Vol. 8, 2018, Paper 11745.
https://doi.org/10.1038/s41598-018-29412-8[13] Ryu, J. H., Yang, J. H., and Yoh, J. J., “Novel Utilization of the
Molecular Band Signal in Metal Oxides: Understanding the Aging
Process of Pyrotechnic Substances by Using Laser Induced Plasma
Emissions,” Optics Express, Vol. 9, No. 2, 2019, pp. 410–422.
https://doi.org/10.1364/OME.9.000410[14] Oh, J., Ambekar, A., andYoh, J. J., “TheHygrothermal Aging Effects of
TitaniumHydride Potassium Perchlorate for Pyrotechnic Combustion,”
Thermochimca Acta, Vol. 665, No. 1, 2018, pp. 102–110.
https://doi.org/10.1016/j.tca.2018.05.019[15] Oh, J., Jang, S., and Yoh, J. J., “Towards Understanding the Effects of
Heat andHumidity onAgeing of aNASAStandardPyrotechnic Igniter,”Scientific Reports, Vol. 9, No. 1, 2019, Paper 10203.https://doi.org/10.1038/s41598-019-46608-8
[16] Han, B., Kim, Y., Jang, S., Yoo, J., and Yoh, J. J., “ThermochemicalCharacterization ofZr∕Fe2O3 PyrotechnicMixtureUnderNatural AgingConditions,” Journal of Applied Physics, Vol. 126, 2019, pp. 105–113.https://doi.org/10.1063/1.5096803
[17] Han, B., Gnanaprakash, K., Park, Y., and Yoh, J. J., “Understanding theEffects of Hygrothermal Aging on Thermo-Chemical Behaviour of
Zr-Ni Based Pyrotechnic Delay Composition,” Fuel, Vol. 281,
Dec. 2020, Paper 118776.
https://doi.org/10.1016/j.fuel.2020.118776[18] Ambekar, A., Kim, M., and Yoh, J. J., “Characterization of Display
Pyrotechnic Propellants: Burning Rate,” Applied Thermal Engineering,Vol. 121, July 2017, pp. 761–767.https://doi.org/10.1016/j.applthermaleng.2017.04.097
[19] Vyazovkin, S., Burnham, A. K., Criado, J. M., Pérez-Maqueda, L. A.,Popescu, C., and Sbirrazzuoli, N., “ICTAC Kinetics Committee Recom-
mendations for Performing Kinetic Computations on Thermal Analysis
Data,” Thermochimca Acta, Vol. 520, Nos. 1–2, 2011, pp. 1–19.
https://doi.org/10.1016/j.tca.2011.03.034[20] Friedman, H. L., “Kinetics of Thermal Degradation of Char-Forming
Plastics from Thermogravimetry: Application to a Phenolic Plastic,”Journal of Polymer Science, Part C: Polymer Symposium, Vol. 6, No. 1,1964, pp. 183–195.https://doi.org/10.1002/polc.5070060121
[21] Grazulis, S., Chateigner, D.,Downs,R. T., Yokochi,A. F. T., Quiros,M.,Lutterotti, L., Manakova, E., Butkus, J., Moeck, P., and Le Bail, A.,“Crystallography Open Database—An Open-Access Collection ofCrystal Structures,” Journal of Applied Crystallography, Vol. 42,2009, pp. 726–729.https://doi.org/10.1107/S0021889809016690
[22] Jian, G. Q., Chowdhury, S., Sullivan, K., and Zachariah, M. R., “Nano-thermite Reactions: Is Gas Phase Oxygen Generation from the Oxygen
Carrier an Essential Prerequisite to Ignition,” Combustion and Flame,Vol. 160, No. 2, 2013, pp. 432–437.https://doi.org/10.1016/j.combustflame.2012.09.009
[23] Valluri, S. K., Ravi, K. K., Schoenitz, M., and Dreizin, E. L., “Effect ofBoron Content in B·BiF3 and B·Bi Composites on Their Ignition andCombustion,” Combustion and Flame, Vol. 215, May 2020, pp. 78–85.https://doi.org/10.1016/j.combustflame.2020.01.026
[24] Woodruff, C., Wainwright, E. R., Bhattacharia, S., Lakshman, S. V.,Weihs, T. P., and Pantoya, M. L., “Thermite Reactivity with Ball MilledAluminum-ZirconiumFuelParticles,”CombustionandFlame,Vol. 211,Jan. 2020, pp. 195–201.https://doi.org/10.1016/j.combustflame.2019.09.028
[25] Zhao,W.,Wang, X.,Wang, H.,Wu, T., Kline, D. J., Rehwoldt,M., Ren,H., andZachariah,M.R., “TitaniumEnhanced Ignition andCombustionof Al∕I2O5 Mesoparticle Composites,” Combustion and Flame,Vol. 212, Feb. 2020, pp. 245–251.https://doi.org/10.1016/j.combustflame.2019.04.049
[26] Gnanaprakash, K., Han, B., and Yoh, J. J., “Ignition and CombustionBehaviour of Zirconium-Based Pyrotechnic Igniters and PyrotechnicDelays Under Aging,” Proceedings of Combustion Institute, Vol. 38,No. 3, 2021, pp. 4373–4381.
[27] Ulas, A., and Kuo, K. K., “Effect of Aging on Ignition Delay Times of aComposite Solid Propellant Under CO2 Laser Heating,” Combustion
Science and Technology, Vol. 127, Nos. 1–6, 1997, pp. 319–331.https://doi.org/10.1080/00102209708935699
[28] Miklaszewski, E. J., Shaw, A. P., Poret, J. C., Son, S. F., and Groven, L.J., “Ti/C-3Ni/Al as a Replacement Time Delay Composition,” Propel-lants, Explosives and Pyrotechnics, Vol. 39, No. 1, 2014, pp. 138–47.https://doi.org/10.1002/prep.201300099
[29] Sorensen, D. N., Quebral, A. P., Baroody, E. E., and Sanborn, W. B.,“Investigation of the Thermal Degradation of the Aged PyrotechnicTitaniumHydride/Potassium Perchlorate,” Journal of Thermal Analysisand Calorimetry, Vol. 85, 2006, pp. 151–156.https://doi.org/10.1007/s10973-005-7365-5
[30] Fathollahi, M., and Behnejad, H., “A Comparative Study of ThermalBehaviours and Kinetics Analysis of the Pyrotechnic Compositions
Containing Mg and Al,” Journal of Thermal Analysis and Calorimetry,Vol. 120, 2015, pp. 1483–1492.https://doi.org/10.1007/s10973-015-4433-3
[31] Shamsipur, M., Pourmortazavi, S. M., and Hajimirsadeghi, S. S., “AnInvestigation on Decomposition Kinetics and Thermal Properties ofCopper-Fueled Pyrotechnic Compositions,” Combustion Science and
Technology, Vol. 183, No. 6, 2011, pp. 575–587.https://doi.org/10.1080/00102202.2010.523032
[32] Rubio, M. A., Gunduz, I. E., Groven, L. J., Sippel, T. R., Han, C. W.,Unocic, R. R., Ortalan,V., and Son, S. F., “Microexplosions and IgnitionDynamics in Engineered Aluminum/Polymer Fuel Particles,” Combus-tion and Flame, Vol. 176, Feb. 2017, pp. 162–171.https://doi.org/10.1016/j.combustflame.2016.10.008
[33] Belal, H.,Han, C.W.,Gunduz, I. E., Ortalan,V., and Son, S. F., “Ignitionand Combustion Behavior of Mechanically Activated Al–Mg Particlesin Composite Solid Propellants,” Combustion and Flame, Vol. 194,Aug. 2018, pp. 410–418.https://doi.org/10.1016/j.combustflame.2018.04.010
[34] Thomas, J. C., Sammet, T. E., Dillier, C. A. M., Demko, A. R.,Rodriguez, F. A., and Petersen, E. L., “Aging Effects on the Burningrates of Composite Solid Propellants with Nano-Additives,” Journal ofPropulsion and Power, Vol. 35, No. 2, 2019, pp. 342–351.https://doi.org/10.2514/1.B37189
[35] Kadiresh, P., and Sridhar, B. T. N., “Experimental Study on BallisticBehaviour of an Aluminised AP/HTPB Propellant During AcceleratedAging,” Journal of Thermal Analysis and Calorimetry, Vol. 100, 2010,pp. 331–335.https://doi.org/10.1007/s10973-009-0569-3
[36] McDonald, B. A., Rice, J. R., and Kirkham, M. W., “Humidity InducedBurning Rate Degradation of an Iron Oxide Catalyzed AmmoniumPerchlorate/HTPB Composite Propellant,” Combustion and Flame,Vol. 161, Jan. 2014, pp. 363–369.https://doi.org/10.1016/j.combustflame.2013.08.014
