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Proceedings of the Combustion Institute 000 (2020) 1–9 www.elsevier.com/locate/proci

Ignition and combustion behavior of zirconium-based

pyrotechnic igniters and pyrotechnic delays under aging

Kanagaraj Gnanaprakash, Byungheon Han, Jack J. Yoh

∗

Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul 08826, South Korea

Received 4 November 2019; accepted 28 June 2020 Available online xxx

Abstract

Pyrotechnic materials often necessitate high reliability and stability to be utilized in energetic devices. However, prolonged storage of these materials degrades their performance in many ways and results in failure of these devices. Only a few studies have focussed on their burning characteristics, and reported limited

understanding on the effect of aging on this behavior of such materials. In this study, ignition and combustion behavior of pyrotechnic materials based on zirconium (Zr) as fuel and potassium perchlorate (KClO 4 ) or iron(III) oxide (Fe 2 O 3 ) as oxidant are investigated, for various aging conditions. Pristine samples are compared with samples subjected to aging under 91 °C (thermally aged) and seasonal changes (naturally aged). The ignition delay time as a function of maximum wire temperature is obtained through high-speed combustion photography for these samples. Surface features and oxide content are analyzed using scanning electron

microscopy and x-ray photoelectron spectroscopy techniques. Results indicate that ignition delay time increases significantly with aging period for both pyrotechnic igniter and pyrotechnic delay samples. However, this time reduces as the maximum wire temperature is increased for all samples. Naturally aged samples exhibit longer ignition delay times and higher metal oxide content when compared to thermally aged ones. Both

pre-oxidation of metallic fuel and prior thermal decomposition of oxidizer play an important role in causing this behavior with aged samples. © 2020 Published by Elsevier Inc. on behalf of The Combustion Institute.

Keywords: Pyrotechnic delay; Aging; Burning characteristics

1. Introduction

Pyrotechnic materials that generate high energy,light, sound, and smoke, upon their exothermicchemical reaction have been utilized in vari-ous civil, aerospace, and military applications[1-3] . These substances typically consists of

∗ Corresponding author. E-mail address: [email protected] (J.J. Yoh).

https://doi.org/10.1016/j.proci.2020.06.340 1540-7489 © 2020 Published by Elsevier Inc. on behalf of The C

Please cite this article as: K. Gnanaprakash, B. Han and J.J. Yohpyrotechnic igniters and pyrotechnic delays under aging, Proc1016/j.proci.2020.06.340

metallic fuels such as zirconium (Zr), aluminum

(Al), and magnesium, and oxidizers such as metal oxides or inorganic perchlorate materials depending upon their applications. Such materials follow combinations of solid–solid reaction, solid–liquid reaction involving molten component, and

reaction between fuel and gaseous oxygen [4 , 5] . Reaction between metallic fuel and metal ox-

ide in igniters, referred to as thermite reaction, is highly exothermic, and could attain temperatures higher than melting point of reactants [6-8] .

ombustion Institute.

, Ignition and combustion behavior of zirconium-based eedings of the Combustion Institute, https://doi.org/10.

http://www.sciencedirect.com

https://doi.org/10.1016/j.proci.2020.06.340

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2 K. Gnanaprakash, B. Han and J.J. Yoh / Proceedings of the Combustion Institute xxx (xxxx) xxx

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imilarly, reaction of metallic fuels with per-hlorate oxidizers are primarily utilized in delayompositions, which demands high reliabilitynd stability with precise burn time at consistenturning rates, for time-controlled energetic devices.owever, if above such materials are subjected

o prolonged storage/aging, their performance isradually degraded. This causes problems such asecrease in heat release, inconsistent burning rates,

ow flame temperature, and reduced reaction rate,hich eventually result in their failure.

Firstly, it is important to understand the burn-ng behavior of non-aged samples in order to in-estigate their aging mechanism. A recent studyy Yukhvid et al. [8] , on combustion of thermitesAl/Zr/iron(III) oxide (Fe 2 O 3 )), has reported thathe ratio of Al and Zr influences the combus-ion feature and composition of final products.he flame propagation of mechanical mixture of l/CuO thermite exhibits three modes (conduc-

ive, oscillating and convective) at different pres-ure regimes [7] . Several studies on pyrotechnic de-ay compositions based on Zr or Al as fuel and/orotassium perchlorate (KClO 4 ) as oxidant has beeneported in the literature [5 , 9 , 10] .

Many investigations have studied the aging pro-ess of pyrotechnics subjected to accelerated agingf temperature and humidity to improve upon theirhelf-life [3 , 11-13] . It is reported that prolongedtorage of such materials results in decrease of verall heat of reaction and increase in activationnergy. Specifically, studies on thermochemicalharacterization and spectroscopic analysis of r/Fe 2 O 3 based igniter indicates that pre-oxidationf metallic fuel is an important factor in degradingheir performance [13 , 14] . This same factor de-reases the flame temperature of Zr/KClO 4 basedelay compositions, as shown by Kim et al. [3] . Inaterials based on Zr and nickel (Ni) alloy, ther-al aging mainly influences Zr, whereas presence

f moisture affects both metals through oxidantecomposition [15] . Thus, two major factors in ag-

ng that cause physical/chemical modifications andnfluence thermodynamic or kinetic behavior of py-otechnic materials are temperature and humidity.

Similarly, a model developed to study the igni-ion of pyrotechnic decoy flares at various initialemperatures shows that the ignition delay timeecreases with an increase in temperature [16] . Ulasnd Kuo [17] have investigated the effect of agingn ignitability of composite solid propellants andeported that aged samples require longer heatingime to reach self-sustained combustion (longer ig-ition time) relative to non-aged samples. Further,

t is elucidated that propellants subjected to humid-ty exhibit degradation in their burning rates, whichs correlative to the level of exposed moisture [18] .

Earlier studies have focused on thermal char-cterization of aged samples to investigate ther-odynamic and kinetic aspects of these materials.lthough some studies have reported the burning

Please cite this article as: K. Gnanaprakash, B. Han and J.J. Yohpyrotechnic igniters and pyrotechnic delays under aging, Proce1016/j.proci.2020.06.340

behavior of non-aged pyrotechnic materials, a com-plete understanding of their ignition/combustioncharacteristics is still limited and the effect of agingon these mechanisms is not completely understood.In the present study, Zr based pyrotechnics sub-jected to thermal and natural aging are investigatedunder atmospheric conditions, relative to pristineones. A pyrotechnic igniter mixture and pyrotech-nic delay composition, utilized concurrently inenergetic devices to generate a desired time delay,are considered independently. High-speed imaging,electron microscopy and x-ray spectroscopy areperformed to study the influence of aging on theignition and burning behavior of these samples.

2. Experimental details

2.1. Samples

Two different pyrotechnic compositions areconsidered in this study, namely igniter and de-lay. The pyrotechnic igniter is a mixture of 41%Zr, 49% Fe 2 O 3 , 10 ± 1% silicon dioxide (SiO 2 ), and0.5% binder, by mass. The pyrotechnic delay con-sists of 8% Zr 7 Ni 3 alloy, 25% Zr 3 Ni 7 alloy, 15%KClO 4 , 52 ± 1% barium chromate (BaCrO 4 ), and1% Rareox binder, by mass [13 , 14] . Additional de-tails of ingredients are presented in the supplemen-tary (Table T1). These ingredients are mechanicallymixed and utilized for measuring their ignition de-lay time and burn time. Samples are categorizedinto three groups as pristine, thermally aged, andnaturally aged. An actual sealed pyrotechnic de-vice containing both pyrotechnic samples (igniterand delay) is subjected to accelerated thermal ag-ing in a hot air oven at 91 °C and 0% relative hu-midity (RH) for 2 and 6 weeks. Total amount of samples differs slightly for each device, but typically∼2 gm of material is used. In naturally aged sam-ples, there is a slender increase in total weight of about 2–4% caused mainly by moisture absorption.These samples are stored under atmospheric condi-tions for 7, 8 and 9 years, and exposed to seasonalmoisture and temperature changes. The acceleratedaging temperature of 91 °C and storage period forthermally aged samples are selected, based on ourprevious studies [13 , 14] , in order to mimic thermalaging in reality over extended time period. Gen-erally, van’t Hoff equation is utilized to predict,based on the accelerated aging temperature and ag-ing time, the equivalent storage period for whichthese samples would have aged under room tem-perature [13] . In addition, such temperatures arereasonable provided no artificial or secondary re-actions are induced in the material.

2.2. Techniques

Experiments on these mechanical mixtures areperformed to observe the combustion behavior, and

, Ignition and combustion behavior of zirconium-based edings of the Combustion Institute, https://doi.org/10.


K. Gnanaprakash, B. Han and J.J. Yoh / Proceedings of the Combustion Institute xxx (xxxx) xxx 3

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Fig. 1. Ignition image sequences of pyrotechnic igniter samples. (a) Pristine at 1250 °C, (b) thermally aged at 1250 °C, (c) naturally aged at 1300 °C.

Fig. 2. Ignition image sequences of pyrotechnic delay samples. (a) Pristine at 1300 °C, (b) thermally aged at 1400 °C, (c) naturally aged at 1350 °C.

determine their ignition delay time and burn time,under atmospheric conditions [19] . In this study, ig-nition is achieved through joule heating using a hotnichrome wire, with length of 100 mm and diameterof 0.3 mm, and a DC power supply. Powder sam-ples of ∼10 or 50 mg are packed inside an aluminacrucible without pressing, and the nichrome wire isplaced in contact with samples at the top. Heat en-ergy supplied to these samples is varied to deter-mine their minimum ignition threshold and the ig-nition delay time as a function of maximum wiretemperature [20] . In the DC power source, the volt-age is maintained at 30 V and current is regulatedfrom 3.0 to 5.50 A (in steps of 0.5 A) to vary theelectrical power, in order to achieve different wiretemperatures. This temperature is calculated basedon the energy balance at the wire. The energy storedin the wire is equal to the difference between electri-cal energy supplied to it and any heat transfer dueto conduction, convection or radiation. Assumingthere is no heat loss, and using known parametersof wire’s emissivity, resistivity, length, diameter andits convective heat transfer coefficient in air, themaximum wire temperature is calculated.

Ignition process is captured using a high-speedcamera (Phantom V711, 800 × 600 pixels 2 resolu-tion) and 105 mm Nikkor macro lens, at a fram-ing rate of 1000 fps and exposure time of 1–10 μs,depending upon the sample. High-speed cameraand ignition source are triggered externally with5 V pulse to ensure synchronization between imageacquisition and wire heating. Therefore, the starttime for image acquisition is exactly the same forall tests. Hence, there is no delay time caused byswitching of electrical power. However, there is adelay for the wire to reach its maximum temper-ature that is quite difficult to quantify due to itstransient nature and strong dependence on elec-trical power, and is inherently included in igni-tion delay time itself. The ignition time is obtainedbased on the difference between onset of triggersource and first light emission from sample burn-ing [17] . This ignition process is examined visuallyfor measuring the ignition time, which is accurateto a few frames in the image sequence. Therefore,the error involved in this measurement is ± 5 ms(significantly less compared to previous studies),and more than 90% of data are repeated at leastonce.

X-ray photoelectron spectroscopy (XPS) is car-ried out using Axis Supra equipment (Kratos Ltd)for pristine and aged samples with ∼2 mg of mix-ture, to detect the presence of metal oxides anddecomposition products of oxidizer. Surface mor-phology of these samples is obtained using a field-emission scanning electron microscope (FE-SEM,JSM-7800F). Elemental mapping is performed us-ing energy dispersive spectroscopy (EDS) in FE-SEM to identify different elements on the surfaceand their composition.


3. Results

3.1. High-speed images of ignition

Subsequent images captured during ignition of pyrotechnic samples under atmospheric conditions are shown in Figs. 1 and 2 , for different aged cases. The extreme left part in the figure shows the event of first light emission during combustion (ignition delay time). Since ignition is achieved through

heat conduction, maximum wire temperature corresponding to first ignition is presumed to be minimum ignition threshold of these samples. Note that high electrical power to the wire implies that its




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aximum temperature is reached quickly, whichxemplifies that heat conduction into these samplesccurs at faster rate, thus causing ignition at shortime. This process is dependent on time and oneannot exactly determine the wire temperature athis instant. Eventually, this process occurs in allases, irrespective of the energy source (either hotire or laser ignition). Therefore, the ignition delay

ime is represented as a function of maximum wireemperature rather than the actual temperature of gnition.

Pristine or non-aged pyrotechnic igniter sam-les, considered as reference mixture, achieve igni-ion at ∼440 ms at wire temperature of ∼1250 °C,nd have burn time of ∼30 ms ( Fig. 1 a). Ignitiontarts from a local hotspot through heat transfer,hich leads to an exothermic reaction between Zrnd Fe 2 O 3 . Subsequently, this heat is spread toearby regions, causing rigorous reactions to oc-ur throughout the mixture. The highly luminousegions represent burning of individual Zr particlesn the presence of Fe 2 O 3 . This implies that Zr ox-dation generates significantly high flame temper-tures, as commonly expected in metal and metalxide reactions [6] .

Similar burning behavior and burn time arebserved with 6 weeks thermally aged pyrotechnic

gniter samples ( Fig. 1 b), however with longergnition delay time, relative to pristine ones. Onhe other hand, this time with 8 years naturallyged samples is 60% and 35% higher than pristinend thermally aged samples, respectively. In addi-ion, these naturally aged samples exhibit highlyntermittent burning behavior ( Fig. 1 c) with signif-cantly greater burn time ( ∼150 ms). Combustionf these samples is particularly restricted, withoutny pronounced ejection of burning Zr particles.urthermore, low luminosity in the reaction zonef naturally aged samples suggests that the finalombustion temperature and heat release are nots high as in pristine ones.

In case of pyrotechnic delay samples, around0 mg of mixture is used since ignition is notchieved with relatively lesser quantity. The burn-ng behavior of pristine delay samples is similar tohat of corresponding igniter samples with consid-rable ejection of burning particles ( Fig. 2 a). How-ver, the ignition time with the former ( ∼0.74 s)s significantly higher than the latter. Note thathe melting temperature of Zr-Ni alloy is consid-rably less than that of Zr metal, which couldctually lead to reactions relatively earlier withhe former than in the latter. Despite this, the ig-ition time for pyrotechnic delay samples is no-iceably large. This implies that large quantity of aCrO 4 present in these samples, acting as a heat

ink, absorbs some energy desired to initiate reac-ion between Zr-Ni particles and KClO 4 . The to-al burn time of pristine delay samples is also con-iderably high ( ∼0.40 s) when compared to igniteramples.


For thermally and naturally aged pyrotech-nic delay samples, this total burn time is almost∼1.00 s, which is substantially high consideringtheir small quantity. Features such as pronouncedluminous combustion and ejection of burningmetal particles are significantly reduced in bothaged samples relative to non-aged ones. However,marginal gas generation is noticeable with agedsamples, relative to pristine ones, following pre-dominate condensed phase reactions. Due to theheating effect, dispersion of particles is observedin intermittent fashion. Ignition delay time and to-tal burn time increase progressively with aging pe-riod for both thermally ( Fig. 2 b) and naturally aged( Fig. 2 c) samples, within each aging category. Theignition time as a function of maximum wire tem-perature will be shown later in Section 3.4 .

3.2. XPS analysis

XPS analysis is performed to illustrate howdifferent aging conditions influence the com-position of various aged samples. These resultsare discussed qualitatively depending upon thebinding energy of different species. Each XPSspectrum obtained from the equipment includesthe deconvoluted curves as well. These curves aredeemed necessary to identify specific species, basedon their peak intensity and chemical or electronicstates, especially in a multicomponent system,since any single element/species could have 1 to20 peaks corresponding to their binding energy.Further, this analysis is primarily performed toascertain the relative content of metal oxides, anddecomposition species of oxidant in different agedsamples, in comparison to pristine ones.

In pyrotechnic igniter samples, Zr metal is de-tected in low binding energy region at 177.7 eV,and sub-oxide forms of Zr and ZrO x (0 < x < 2)are detected respectively at 181.8 eV and 184.1 eV( Fig. 3 a). Zr becomes Zr 0 to Zr 4 + in the oxidationprocess, and shifts to a high binding energy region.The intensity of Zr is relatively prominent in pris-tine samples, and it reduces with both thermallyand naturally aged samples. An important aspectis the presence of ZrO 2 at 186.1 eV only in natu-rally aged samples. This suggests that as Zr is oxi-dized, peaks move to high energy regions, which isin good agreement with previous study on Zr oxi-dation [21] . The oxidizer is detected in the form of Fe 3 O 4 at 709.4 eV and Fe 2 O 3 at 710.8 eV, respec-tively ( Fig. 3 b). Overall intensity decreases slightlywith thermally aged samples and significantly innaturally aged samples. This implies that metal ox-ide decomposition over time is severe under naturalaging conditions.

In pyrotechnic delay samples, Cl peak is noticedat 208.70 eV corresponding to KClO 4 , 205.90 eV toKClO 3 , and 197.80 eV to KCl ( Fig. 3 d). Zr peaksat different binding energy represent ZrO x , and Nipeak at 855.60 eV indicates the presence of NiO




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Fig. 3. XPS data for pyrotechnic igniter (a, b) and pyrotechnic delay (c, d) samples.

( Fig. 3 c). ZrO x and NiO in naturally aged samplesshow greater peak than pristine and thermally agedsamples. With KClO x (0 ≤ x ≤ 4), the peak intensityis considerably reduced in naturally aged samples.Collectively these results show that the peak in-tensity for metal oxide and oxidant decompositionspecies are high with aged samples relative to pris-tine ones, with igniter and delay samples, respec-tively. Also, as aging progresses, the peak of metaloxide shifts to high binding energy region. Thisrepresents that aging promotes oxidation of metal-lic fuels and decomposition of oxidants, which arelargely pronounced in naturally aged samples com-pared to other cases.

3.3. SEM-EDS

Surface morphology of aged samples is ob-served in SEM, and the presence of metal oxideon their surface is quantified using EDS (Fig. S1in supplementary). It is noticed that the elementaloxygen present in both pyrotechnic igniter andpyrotechnic delay samples increases considerablyas aging period is increased, while active metalcontent decreases ( Table 1 ). This affirms previousobservation of increasing metal oxide contentwith aging from XPS analysis ( Fig. 3 ). A trend forincreasing elemental oxygen and reducing activemetal content is observed in the order of pristine <thermally aged < naturally aged samples. SEM im-ages also reveal formation of cracks on the surfaceof metal particles, specifically in naturally aged


pyrotechnic delay samples (Fig. S2 in supplementary). Newly formed cracks on the surface of metal particles would expose fresh metal layer, which gets oxidized more when ambient conditions are highly oxygen rich, thus further thickening the oxide layer.

3.4. Ignition delay time

Ignition delay is the time taken for first light emission after switching on the electrical power, which is varied to obtain go/no-go ignition boundary as a function of maximum wire temperature for different pyrotechnic samples. This boundary for pristine and different aged pyrotechnic igniters are shown in Fig. 4 . Most samples achieve ignition only at temperatures of ∼900 °C, except for 8 years naturally aged samples (1050 °C). Note that this minimum ignition threshold is considerably less than

the melting temperatures of both Zr (1850 °C) and

Fe 2 O 3 (1560 °C). Ignition time decreases with increase in maximum wire temperature for all cases, similar to previous studies [17 , 19] , due to increase in the heat transfer rate to samples. However, this is noticeably higher for aged samples when compared

to non-aged ones at similar temperatures, irrespective of aging conditions.

For thermally aged igniter samples, ignition

time is 40% higher than pristine samples at 900 °C

but this difference is reduced at high temperatures ( Fig. 4 a). Between 2 and 6 weeks thermally aged

samples, the disparity in this time is marginal and

within error band of the data. On the other hand,




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Table 1 Elemental composition of pyrotechnic igniter and pyrotechnic delay samples obtained using SEM-EDS.

Elements O Zr Fe Si Cl K Cr Ni Ba

Pyrotechnic igniter Pristine 31.02 28.66 34.74 5.59 – – – – –Thermally aged 34.02 23.14 34.82 8.03 – – – – –Naturally aged 36.78 27.45 30.07 5.70 – – – – –

Pyrotechnic delay Pristine 17.99 3.57 – – 0.51 0.41 17.76 6.40 53.35 Thermally aged 23.63 2.55 – – 0.12 0.07 18.80 3.59 51.24 Naturally aged 25.44 0.77 – – 0.52 0.40 18.79 0.77 53.30

Fig. 4. Ignition delay time vs. maximum wire temperature for pyrotechnic igniter samples. (a) Thermally aged, (b) naturally aged.

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Fig. 5. Ignition delay time vs. maximum wire temperature of pyrotechnic delay samples. (a) Thermally aged, (b) naturally aged.

ith naturally aged samples, it is 70% greater thanristine samples at 900 °C. In contrast to above,onsiderable disparity is noticed between 7 and years naturally aged samples. The latter doesot achieve ignition until the temperature reaches050 °C and indicates 20% longer ignition timehan the former almost in entire temperature range Fig. 4 b). Overall, an increasing trend for ignitionelay time is clearly noticed as pristine < thermallyged < naturally aged samples, in the same way asetal oxide content from XPS analysis ( Fig. 3 ).

Similar to pyrotechnic igniter samples, the igni-ion time of pyrotechnic delay samples decreasesith an increase in maximum wire temperature

Fig. 5 ), but it is significantly higher for aged sam-les than pristine ones at a particular given temper-


ature. Note that, the minimum ignition thresholdfor pristine pyrotechnic delay samples is ∼1000 °C.Naturally aged samples achieve ignition only atwire temperatures > 1250 °C, which is larger thanthat of thermally aged samples (1150 °C). Ignitiondelay time of samples that are thermally aged is60% more than pristine ( Fig. 5 a) whereas it is 260%for naturally aged at their corresponding minimumignition threshold ( Fig. 5 b). This disparity is pro-gressively reduced as wire temperature is increasedto 1400 °C. The effect of aging on ignition time isalso more dominant in naturally aged pyrotechnicdelay samples. Note that there is always high uncer-tainty in measuring this time especially at minimumthreshold (1300 °C) for ignition (observed typi-cally in previous studies), for both naturally aged




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samples. Hence, a large error band at this tempera-ture (7 years aged) is observed. Further, the dataobtained for these samples are overlapping witheach other in the entire temperature range. Thus,one cannot ascribe any specific disparity in ignitionbehavior between them. However, it can be postu-lated that both aged samples could have reachedtheir maximum possible threshold for moisture ab-sorption, so that their ignition kinetics does notvary significantly. This behavior and other resultswill be collectively discussed next.

3.5. Discussion

The results from DSC and TGA (Fig. S3 in thesupplementary) are considered to elucidate the re-action mechanisms of pristine pyrotechnic igniterand delay samples. Oxidation reaction between Zrand Fe 2 O 3 results in the formation of ZrO 2 and Feas final products, with substantial heat release atflame temperatures of ∼3500 °C [8 , 13] . This is ob-served from high luminosity in combustion imagesof pyrotechnic igniter ( Fig. 1 a). It is known that themelting of one of components increases the proba-bility of ignition to a significant extent in these sam-ples. However, as mentioned before, minimum igni-tion threshold is less than the melting temperatureof both Zr with an oxide layer and Fe 2 O 3 ( Fig. 4 a).This implies that solid phase heterogeneous reac-tions exist. Such behavior is in concurrence withprevious studies where ignition temperature of cer-tain metals (Zr, Fe, Ti) is reported to be lower thantheir melting temperature [22] . Note that SiO 2 isnot considered to involve in this reaction due to itsrelatively small quantity.

Reaction mechanism of pyrotechnic delay mix-ture (Zr-Ni/KClO 4 ) includes the formation reac-tion of metal oxides (ZrO 2 , NiO 2 ) and decompo-sition of oxidizer. It is experimentally observed inthe present study that these mixtures do not achieveignition and sustain burning in the absence of KClO 4 . This suggests that KClO 4 acts as primaryoxidizer, despite large quantity of BaCrO 4 . Recol-lect that minimum ignition threshold ( Fig. 5 a) forpyrotechnic delay is ∼1000 °C, whereas the meltingand/or decomposition of KClO 4 starts at much lowtemperatures (600 °C), forming KClO 3 and KCl asintermediate species [10 , 23] . This implies that igni-tion is initiated by solid-liquid reactions in this mix-ture.

The foregoing discussion concurs that oxidationof Zr involves solid-solid/solid-liquid and solid-gasreactions. Initially, complete condensed phasereaction occurs between Zr and KClO 4 , or withFe 2 O 3 in the case of igniter, before the reaction of Zr and gaseous oxygen, which dominates the over-all reaction later. This gaseous oxygen is primarilyproduced from the decomposition of KClO 4 inpyrotechnic delay [2 , 23] , and solid-solid reactionin pyrotechnic igniter. Similar processes occur in


the oxidation of Zr-Ni alloy as well, however at marginally slower rate than Zr-KClO 4 reaction

kinetics. Since, the reactivity of Zr is higher than

Ni [15] , oxidation of Ni occurs subsequent to the formation of ZrO 2 . With mechanical mixtures, some of gaseous oxygen from condensed phase reactions expand into the atmosphere, thus re- sulting in incomplete reaction and loss of some combustion heat in the system [9] .

Generally, metal particles enclosed by an oxide layer are difficult to ignite. When such particles present in the vicinity of oxidizer are subjected

to aging, oxygen in the ambience or oxidizer pen- etrates further inside and increases the depth of this oxide layer. Such process is significant in the mixture containing KClO 4 [24] . This is clearly seen

in high elemental oxygen contents with aged samples from EDS results. Furthermore, the reduction

in flame luminosity and solid-gas reaction of aged

samples with respect to pristine ones corroborate the above observation ( Figs. 1 and 2 ). Irrespective of the pyrotechnic mixture, aging increases the ignition delay time due to the presence of thick oxide layer on metal particles.

Moreover, this effect is significant in naturally aged samples when compared to thermally aged

ones, since the former is subjected to humidity as well as temperature. Recall that KClO 4 decom- poses at low temperatures, and it also reacts with

water due to vulnerable Cl-O bond, creating an

ionic liquid [12] . Therefore, presence of moisture enhances the decomposition of KClO 4 and offers more oxygen for further oxidation of metallic fuels, particularly in naturally aged delay samples. This is noticed as large peaks with metal oxides and

shallow peaks with oxidant for these samples in

XPS results ( Fig. 3 c and d). Further, this causes defects and minor cracks on the surface of metal particles as seen in SEM images. Such process also

lowers the surface energy and subsequently the reaction kinetics leading to ignition is slowed down

to cause high ignition delay time ( Fig. 5 b). Hence, the ignition time with naturally aged samples is significantly higher than thermally aged ones. On the other hand, such processes are absent in

naturally aged pyrotechnic igniter samples, since moisture may not influence the oxidation of Fe 2 O 3 as much, which is a highly stable metal oxide.

It is evident from the foregoing that effect of aging is predominant in naturally aged samples, in which temperature promotes the oxidation of metallic fuel and moisture enhances the decomposition of oxidizer. These factors (high metal oxide content and oxidant decomposition species) influence the pyrotechnics’ performance in three ways. First, the amount of available active fuel for reacting with oxidizer is reduced, which would

decrease the net heat release achieved. Secondly, since the thermal conductivity of metal oxide is lower than active metal, rate of heat conduction




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nto the solid is significantly diminished as well.n addition, thick metal oxide or hydroxide layern aged samples causes slow diffusion of reactantshrough itself in aged samples relative to pristinenes, thus decreasing the rate of reactions leadingo ignition. All these effects together could causeignificant reduction in burning rates and increasen burn times of these pressed samples, thus de-rading the intended performance of an actualyrotechnic device.

. Conclusion

This study investigates the influence of agingn ignition and combustion behavior of pyrotech-ic materials. Two types of samples are utilized;yrotechnic igniter (Zr/Fe 2 O 3 /SiO 2 ) and pyrotech-ic delay (Zr-Ni alloy/BaCrO 4 /KClO 4 ). Both theseamples are subjected to aging under natural as wells accelerated conditions. Thermally aged samplesre stored at 91 °C for 2 and 6 weeks, whereas nat-rally aged samples are kept under seasonal expo-ure of both heat and humidity for 7, 8 and 9 years.

aximum wire temperature required to achieve ig-ition in these mechanical mixtures is determinedsing hot wire ignition. High-speed combustionhotography is adopted to obtain the ignition de-

ay time and burn time. Furthermore, the presencef metal oxide is confirmed using SEM-EDS andPS analyses.

High-speed combustion images reveal that rig-rous burning and high flame luminosity observedith pristine samples are significantly reduced inged ones, for both pyrotechnic igniter and py-otechnic delay samples. The minimum thresholdor ignition increases with aging period. Ignitionelay time and metal oxide content in aged sam-les are considerably higher than in pristine sam-les. These effects are pronouncedly noticed in nat-rally aged samples due to the influence of bothoisture and temperature. Thermal effect mainly

auses pre-oxidation of metal particles, whereasoisture effect enhances the decomposition of ox-

dizer as well. Both these aspects result in sub-tantially longer ignition time and overall burnime, especially in naturally aged pyrotechnic de-ay mixtures. Thus, pyrotechnic devices containinguch mixtures that require highest standard per-ormance reliability, need to be designed to avoid

oisture exposure in order to improve upon theirifetime and function appropriately even after pro-onged storage.

eclaration of Competing Interest

The authors declare that they have no knownompeting financial interests or personal relation-hips that could have appeared to influence theork reported in this paper.


Acknowledgments

This work was supported by BK21 Plus, Han-wha Corporation (HanwhaSNU-2018) and NextGeneration Space Propulsion Research Center(NRF-2013R1A5A1073861) contracted throughIAAT at Seoul National University.

Supplementary materials

Supplementary material associated with this ar-ticle can be found, in the online version, at doi: 10.1016/j.proci.2020.06.340 .

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