J. Appl. Phys. 126, 105113 (2019); https://doi.org/10.1063/1.5096803 126, 105113

© 2019 Author(s).

Thermochemical characterization of Zr/Fe2O3 pyrotechnic mixture under naturalaging conditionsCite as: J. Appl. Phys. 126, 105113 (2019); https://doi.org/10.1063/1.5096803Submitted: 20 March 2019 . Accepted: 02 August 2019 . Published Online: 13 September 2019

Byung Heon Han, Yoocheon Kim, Seung-gyo Jang, Jeayong Yoo, and Jack J. Yoh

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Thermochemical characterization of Zr/Fe2O3

pyrotechnic mixture under natural agingconditions

Cite as: J. Appl. Phys. 126, 105113 (2019); doi: 10.1063/1.5096803

View Online Export Citation CrossMarkSubmitted: 20 March 2019 · Accepted: 2 August 2019 ·Published Online: 13 September 2019

Byung Heon Han,1 Yoocheon Kim,1 Seung-gyo Jang,2 Jeayong Yoo,3 and Jack J. Yoh1,a)

AFFILIATIONS

1Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul 08826, South Korea2Agency for Defense Development, Daejeon 34060, South Korea3Hanwha Corporation, Yeosu 59747, South Korea

a)Author to whom correspondence should be addressed: jjyoh@snu.ac.kr

ABSTRACT

This study investigated the aging characteristics of naturally aged primer that uses zirconium (Zr) as a metallic fuel and iron oxide (Fe2O3)as an oxidizer. Naturally aged samples showed a significant reduction in heat of reaction and an increase in activation energy compared tothermally aged samples. The aging processes of a Zr/Fe2O3 pyrotechnic mixture were proposed using X-ray photoelectron spectroscopy.X-ray powder diffraction experiments verified the proposed processes and further confirmed that natural aging promotes the formationof thermally stable monoclinic ZrO2. The oxygen diffusion depth into the Zr surface layer and ZrO2 layer thickness were measured bytransmission electron microscopy–energy-dispersive X-ray spectroscopy and fast Fourier transform. The exposure to seasonal humidityduring natural aging affected both Zr and Fe2O3 and, in turn, made the mixture more difficult-to-ignite, resulting in the following effects:decrease in heat of reaction, formation of reaction products, and crystal structure growth in the direction of reducing reactivity.

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I. INTRODUCTION

Inorganic mixtures containing solid energy sources have beenused for decades in systems that require high energy such as variousenergetic materials and propellants. These solid mixtures requireproper fuel selection and mixing according to their purpose, andstudies on ignition and thermochemical characterization of such ener-getic sources have been performed. Energetic metal powders such aszirconium, aluminum, and titanium are often mixed with oxidizingagents such as potassium perchlorate (KClO4), iron oxide (Fe2O3),and barium chromate (BaCrO4).

1–5 Zirconium (Zr) reacts instantlyand has an exceptional instantaneous energy release rate. Thus, it hasbeen widely used as a fuel to ignite more difficult-to-ignite materials.To adjust the reaction rate and maximize the energy efficiency of Zr,Fe2O3 is added as a desirable oxidizer,5 even though the reactivitydiffers greatly depending on the surface area.

The energetic particles, including Zr, are highly reactive andvery sensitive to their production and storage environments. Thus,different concerns have been reported with these particles, namely,

the decrease in the heat of reaction due to long-term storage,including misfire. Various factors can contribute to the decreasedfunctionality of energetic materials during storage, such as temper-ature, humidity, vibration, and so on.6–9 Numerous studies haveattempted to predict the degree of aging through accelerated agingcycles or thermal aging experiments in moisture-controlled envi-ronments.6,8 However, these studies have not considered naturallyaged samples that are exposed to seasonal humidity conditions.

Generally, the energetic particles are stable at room temper-ature and do not react over time. However, the exposure tomoisture during long-term storage changes their constituents,resulting in an increase of activation energy and a decrease inthe heat of reaction. The activation energy indicates the energyrequired for the reaction to occur, and the heat of reaction is amajor factor that determines the final performance of energeticmaterials. So, an increase in activation energy and decrease inheat of reaction leads to fatal degradation of energetic materials.The degradation process for a commonly used primer, namely,Zr/Fe2O3, is depicted in Fig. 1.

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In order to further understand the aging effects, a Zr/Fe2O3

pyrotechnic mixture is naturally aged for 8 years under seasonalconditions, and its behavior is compared with accelerated agedsamples [for 8 weeks, under thermal conditions at 0% relativehumidity (RH)]. Using these samples, the present study is aimedto investigate the natural aging effect and the main factor of thenatural aging process. Differential scanning calorimetry (DSC) andthermal gravimetry analysis (TGA) are used to extract the reactionkinetics of Zr/Fe2O3 samples. X-ray photoelectron spectroscopy(XPS) and X-ray powder diffraction (XRD) are used to analyzethe changes in the chemical and crystal structure of the samples.In addition, transmission electron microscopy–energy-dispersiveX-ray spectroscopy (TEM-EDS) is used to analyze the Zr surface,which can facilitate the understanding of the aging effect onsamples in a qualitative way. In particular, analysis of surface crosssection of Zr provides important insights into the aging process.

II. EXPERIMENTAL METHODS

A. Sample preparation

The compositions of samples were 41 ± 1wt. % of zirconiumsolid powder, 49 ± 1 wt. % of Fe2O3 solid powder, 10 ± 1 wt. % of SiO2,and 0.5 wt. % of binder. The samples were divided into three groups.

Sample #1 was untreated (nonaged) and served as a reference. Samples#2 to #4 were stored under natural storage conditions for 6–8 years,respectively. In order words, the aging condition changes by seasonand climatic conditions. Sample #5 was subjected to a thermally agedcondition (91 °C for 8 weeks) maintained at 0% RH. A summary andproperties of the prepared samples are shown in Table I.

The accelerated aging with controllable conditions such astime and temperature suggests a reliable way to simulate naturalaging. Therefore, it is possible to predict the correspondingin-service period under the actual storage conditions by utilizingthe accelerated aging conditions. To interpolate the acceleratedaging period with the actual in-service period, the van’t Hoff equa-tion was used. The van’t Hoff equation is an empirical relation forchemically induced aging, and it suggests a way to convert theaccelerated aging temperature-time to room temperature-time,10

tn ¼ tA � F(TA�Tn )

ΔTF =365:25: (1)

Here, tn represents the period of service years at temperature Tnand tA represents the accelerated aged days at temperature TA.Therefore, Tn was 25 °C for mean atmospheric condition, TA was91 °C, and tA was 56 days for accelerated aged conditions. The factorF is a reaction rate change factor per temperature change ΔTF, whichcan be obtained from the Arrhenius equation as follows:

F ¼ exp þ EaR

� ΔTF

T2A

� �: (2)

Temperature change ΔTF was 10 °C, a generally used tempera-ture interval, and the activation energy (Ea) used for calculation isshown in Table I. As a result, the estimated aging period (tn) foraccelerated aged samples corresponds to 149.70 years at 25 °C.These results show that it is a representative aged sample, reflectingonly the temperature condition as an aging factor.

B. Calorimetry and gravimetry analyses

Calorimetry and gravimetry analyses were performed usingMETTLER TOLEDO DSC 3+ and TGA2, respectively. A sampleamount of 2 mg was used, and the experiments were carried outat 4 ml/min of nitrogen flow. The heating rates were 4, 6, 8, and10 °C/min. The amount of the sample, heating rate, and atmosphere

FIG. 1. In the Zr/Fe2O3 pyrotechnic mixture, Zr exists in the form of α-Zr due toits high reactivity with oxygen from the atmosphere. With the increasing extentof natural aging, the moisture is responsible for forming oxide layers such ast-ZrO2 and m-ZrO2 on the surface of Zr, while Fe2O3 is degraded to Fe3O4 andFeO. The final product, m-ZrO2, is the most thermally stable form of ZrO2 havingalpha-monoclinic-oxide layers. Hence, higher activation energy is required for initiat-ing the final product of a naturally aged sample having lowered heat of reaction.

TABLE I. Samples used in this study were divided into three groups. Sample #1 was untreated (nonaged), samples #2 to #4 were naturally aged, and sample #5 was ther-mally aged at 91 °C for 8 weeks under 0% RH. Column 7 shows the activation energy of each sample obtained via ASTM E698-11.11

Type Sample No.

Aging condition

Aging typeActivation energy

(kJ/mol)Temperature Relative humidity (RH) Aging duration

Zr/Fe2O3 #1 … … … Nonaged 114.9#2 Seasonal Seasonal 6 years Natural aged 124.6#3 7 years 131.3#4 8 years 133.3#5 91 °C 0% 8 weeks Thermal aged 118.9

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were set up in accordance with ASTM E698-11.11 The calorimetryand gravimetry results for nonaged samples (sample #1) at 10 °C/minin the temperature range of 30 °C–640 °C are shown in Fig. 2, andthe heat of reaction for all samples is shown in Fig. 3.

C. Chemical composition analysis using XPS

XPS was used to confirm the chemical state of Zr and Fe ele-ments, the major components of Zr/Fe2O3 samples. The sampleamount was 2 mg of powder. The Kratos Electron Spectroscopy forChemical Analysis II (ESCA II) was adopted, which has a resolu-tion of less than 0.48 eV. The XPS results are shown in Fig. 4.

D. Crystal structure analysis using XRD

XRD was used to confirm the crystal behavior of the Zr/Fe2O3

samples. XRD experiments were performed on samples #1, #4, and #5

having the longest aging period for each aging type, and the patternis shown in Fig. 5. The amount of sample used was 4mg. The equip-ment used was a Rigaku High-Resolution X-ray Diffractor(HR-XRD), and the results were matched using the CrystallographyOpen Database (COD).12

E. Surface analysis using TEM-EDS

The surface of Zr of 8-year naturally aged samples was studiedusing TEM-EDS, in order to analyze the crystal change of chemicalspecies in terms of depth from the outermost surface of Zr, and theresults are shown in Fig. 6. The equipment used for the surfaceanalysis with FE-TEM has a resolution of 0.1 nm, and EDS ofJEOL Dual SDD Type was used for the composition analysis. Thegrowth of the structure of each layer was confirmed by fast Fouriertransform (FFT), and the thickness of the oxide layer wasconfirmed by ex situ investigation of line analysis. The line analysisresults are shown in Fig. 7.

III. RESULTS AND DISCUSSION

A. Gravimetric analysis results for untreated samples(sample #1)

TGA results for untreated samples show an overall one-stagemass reduction, followed by a one-stage mass increase (Fig. 2).First, there is a 0.76% loss of total mass up to about 388 °C,followed by an increase in mass of 8.06%. At a temperature rangeof 30 °C–388 °C, the first mass reduction is the evaporation ofmoisture, which causes the sample to dehydrate and decrease itsmass.13,14 The subsequent mass increase reaction between 388 °Cand 587 °C is the region where Zr forms ZrO2, which increases itsmass because of the formation of ZrO2. After 587 °C, there is nosignificant mass change, representing that the reaction is completewithin the experimental temperature range.

If zirconium in the sample undergoes oxidation with oxygenin the atmosphere, the total weight increase would be 14.7% forstoichiometry, since Zr accounts for 42% mass of a total sample.However, there is only 8.06% mass increase in TGA results. Thissuggests that 8.06% is associated with atmospheric oxygen or mois-ture while the rest, namely, 6.64% oxygen, comes from Fe2O3. Bycombining DSC and TGA results, the start and end temperaturesof reactions are clearly verified.

B. Calorimetry analysis results for untreated samples(sample #1) and reaction kinetics

The reaction between Zr and Fe2O3 forms ZrO2 and Fe asfinal products, which is an exothermic reaction satisfying the fol-lowing reaction formula:14–16

2Fe2O3 þ 3Zr ! 3ZrO2 þ 4Fe:

As shown in Fig. 2, the reaction between Zr and Fe2O3 beginswith a weak exothermic reaction, followed by a large exothermicreaction, and this reaction starts at about 388 °C. This correspondsto the temperature at which typical Zr metal starts to reactexothermically14,17 and also where the mass increase is noticed.

FIG. 2. Results of calorimetry and gravimetry analyses performed at 10 °C/minin a nitrogen atmosphere for nonaged samples (sample #1).

FIG. 3. Heat of reaction of each sample. The points represent individually mea-sured heat of reaction values. The lines represent the Lorentz distribution ofenthalpies, and the square boxes represent the average heat of reaction. Adecrease in the heat of reaction of naturally aged samples (sample #2 to #4) isevident, while there is only a small decrease for accelerated aging sample #5.

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The reason for the two-step exothermic reaction is due to theoxidation process of Zr.

Initially, the formation of ZrO2 from Zr undergoes a ɑ-Zrphase, which is formed when Zr is dissolved with less than 35%oxygen. Then, further oxidation leads to a completely oxidizedZrO2 layer forming over the ɑ-Zr layer.18,19 Therefore, ɑ-Zr layer isformed before ZrO2, and its formation reaction occurs at a temper-ature lower than the formation temperature of ZrO2.

20 On thisbasis, a weak exothermic reaction and weak mass increase from388 °C to 478 °C represents the ɑ-Zr formation reaction, and astrong exothermic reaction along with a significant mass increase

after 478 °C represents the ZrO2 formation reaction. According tothe above, Fe2O3 provides oxygen throughout this reaction.

Based on the reaction kinetics, the ɑ-Zr will be formed first,and eventually ZrO2 will be formed after sufficient time. On theother hand, the oxidizing agent (Fe2O3) loses oxygen as the agingprogresses.

C. Calorimetry results for all samples

The calorimetry results are shown in Fig. 3 and Table I. Eachpoint in Fig. 3 represents the heat of reaction value resulting from

FIG. 4. XPS results of Zr/Fe2O3

samples. (a) XPS peak and detection ofFe bond elements, i.e., Fe2O3 (Fe 2p3/2—710.8 eV), Fe3O4 (Fe 2p3/2—709.2 eV), and Fe(CH3C(O)CHC(O)CH3)3. (b) Ratio of Fe2O3 and Fe3O4

bonds in the total Fe bond. (c) XPSpeak and detection of Zr bond peak ele-ments, i.e., ZrO2 (Zr 3d5/2—182 eV)and ZrO2 (Zr 3d3/2—184.5 eV). (d)ZrO2 peak area by integrating bothZrO2 signals.

FIG. 5. (a) XRD patterns of nonaged(sample #1), 8-year naturally aged(sample #4), and 8-week thermallyaged (sample #5) samples matchedusing COD.12 (b) Magnified version ofthe region in (a).

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FIG. 6. Images for 8-year naturallyaged sample (sample #4). (a) TEM. (b)Zirconium EDS. (c) Oxygen EDS. (d)FFT of “A” layer indicating hexagonalZr. (e) FFT of “B” layer indicating hex-agonal Zr3O. (f ) FFT of “C” layer indi-cating monoclinic ZrO2.

FIG. 7. TEM and EDS line analysis results for (a) nonaged (sample #1), (b) 8-year naturally aged (sample #4), and (c) 8-week thermally aged samples (sample #5). Thephase of each layer is confirmed by FFT. Pink represents hexagonal Zr, green represents Zr3O, gray represents ZrO2, and blue represents oxygen layers.

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DSC experiments. The solid lines represent the correspondingLorentz distributions, and the square boxes represent the averageheat of reaction for each sample. The reason for the lack of repeat-ability is due to the SiO2 and the small quantity of sample used.SiO2 is known as an oxidizer as well as a stabilizer for preventingdegradation of Fe2O3 by providing extra space between fueland oxidizers. Enhanced repeatability is expected if more quantityof test samples is used; however, ASEM-E698-11 recommendskeeping it low such that the maximum heat generation is below8mW. Thus, multiple experiments are performed, and both distri-bution and mean values of heat of reaction are provided.

As shown in Fig. 3, the average heat of reaction decreasesas natural aging progresses. On the other hand, there is a slightdecrease in the heat of reaction of thermally aged samples(sample #5), but this effect is negligible compared to naturally agedsamples (samples #2 to #4). In addition, the activation energyobtained by the ASTM E698-18 method of nonaged samples(sample #1) increased by 16% compared to those of naturally agedsamples (samples #2 to #4), and the activation energy of acceleratedaged samples (sample #5) increased by 3% compared to those ofnaturally aged samples (samples #2 to #4). This result suggests thata decrease in the heat of reaction and an increase in the activationenergy are related to the humidity condition because there is noconsiderable decrease in the heat of reaction of thermally agedsamples, which is only affected by temperature during the periodcorresponding to 149.70 years.

The activation energy increase can be explained by the inhibi-tion of heat conduction due to the oxide layer formation or growthon the outer surface,21,22 and a decrease in the heat of reaction canbe explained by the formation of reaction products during naturalaging. Because, there are two possibilities corresponding to thisdecrease with the heat of reaction, namely, decrease of reactant andpresence of moisture. Since the heat value in Fig. 3 is baselinedafter the evaporation of moisture, the decrease of the reactantwould be one of the reasons for the reduction in the heat of reac-tion. Thus, it means that the reaction may have proceeded towardproduct formation.

D. XPS results: Chemical state of the Fe bondand Fe2O3 reduction kinetics

XPS is used to confirm the chemical composition of Zr andFe elements, which are the main components of Zr/Fe2O3. TheFe bond has three chemical compositions, i.e., Fe2O3 with anenergy level of 710.9 eV at the 2p3/2 position, Fe3O4 with anenergy level of 709.2 eV at the 2p3/2 position, and Fe(CH3C(O)CHC(O)CH3)3. The results are shown in Fig. 4(a). The overall Febond signal intensity is highest in nonaged samples (sample #1)and lowest in 8-year naturally aged samples (sample #4), demon-strating that the degree of decomposition is greatest with naturallyaged samples.

In order to compare the Fe-bonding state in each sample,Fe2O3 bond and Fe3O4 bond peaks are integrated and shown inFig. 4(b), with total Fe bond being 100%. In nonaged samples(sample #1), Fe bond is 56% of Fe2O3 and 18% of Fe3O4. In natu-rally aged samples (samples #2 to #4), Fe2O3 decreases as agingprogresses, thus reducing by 11% in 8-year naturally aged samples

compared to nonaged samples. Meanwhile, Fe3O4 in sample #4increases by about 9% compared to nonaged samples. However,the increase/decrease of Fe2O3 and Fe3O4 is not as significant inthe case of thermally aged samples (sample #5) at 0% RH. The con-tents of Fe2O3 and Fe3O4 of 8-week thermally aged samples(sample #5) are about 53% and 20%, respectively. On the otherhand, Fe(CH3C(O)CHC(O)CH3)3 does not significantly change forboth naturally and thermally aged samples.

The low XPS intensity for 8-year naturally aged samples(sample #4) indicates that the oxidant is mostly degraded by naturalaging. In addition, the results for Fe2O3 of thermally aged samples(sample #5) do not show a significant decrease compared to theinitial Fe2O3 content, suggesting that moisture could also affect thedecomposition of Fe2O3.

The Fe2O3 decomposition under moisture condition is heavilyinfluenced by the unstable crystal structure. On the surface ofFe2O3 (hematite), the oxygen-deficient crystal plane (001) has largeadsorption energy to dissociate H2O into OH− and H+. Then, thedissociated hydroxide (OH−) fills the crystal plane (001) and thenforms Fe3O4.

23–26 Meanwhile, hydrogen easily reacts with Fe3O4,forming H2O and Fe, even at ambient temperatures.25 Since Fe hasexcellent reactivity, iron immediately forms FeO by binding withoxygen in the air. The degradation process is shown as follows,which can be verified by the XRD experiment:

Fe–Fe–O3–termination (of Fe2O3)þH2O ! Fe3O4,

Fe3O4 þ 4H2 ! 3Feþ 4H2O,

2FeþO2, atmosphere ! 2FeO:

Since humidity removes the oxygen from the oxidizer(Fe2O3→ FeO or Fe3O4), it affects the energetic materials adverselyby lowering their heat of reaction. Moreover, the above resultsshow that intermediate species (FeO and Fe3O4) are formed bythe influence of moisture rather than the progress of reaction.Thus, natural aging proceeds in the direction of inhibiting chemicalreactions and forming intermediate species.

E. XPS results: Chemical state of the Zr bond and ZrO2

formation kinetics

In the case of Zr bond, only ZrO2 is detected by XPS, andenergy levels of 182.0 eV and 184.5 eV correspond to ZrO2. Theresults are shown in Fig. 4(c). The intensity of ZrO2 in nonagedsamples (sample #1) is similar to that of accelerated aged samples(sample #5), and it is significantly higher than 8-year naturallyaged samples (sample #4) when compared to nonaged samples(sample #1). This means that ZrO2 is produced in a large amountwith naturally aged samples. Figure 4(d) shows the ZrO2 peak inte-grated over the entire sample.

From the results, the ZrO2 formation is prominent in naturallyaged samples, which implies that the effect of moisture dominatesthe formation of ZrO2. Typically, the formation of ZrO2 reducesthe quantity of Zr powder, which is a source of heat generation,thus decreasing the heat of reaction.

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Lyapin et al.27 studied the reactivity of pure Zr metal withoxygen and found that an oxide film is immediately formed inambient conditions, the newly formed layer being ZrO2. Iglesiaset al.28 reported the ZrO2 formation with moisture even at low tem-peratures. These results suggest that the formation of ZrO2 is dueto the production of oxygen ions through dissociation of water inthe atmosphere, and an immediate reaction of Zr with a dissociatedoxygen ion takes place even under low-temperature conditions.Therefore, the reaction between Zr and moisture can be expressedas follows:27,28

Zrþ xH2O ! ZrOx þ xH2(0 , x � 2):

Here, ZrOx (0 < x < 2, where the stoichiometric condition isnot achieved) is a region where zirconium oxide and metal coexists,and a suboxide layer is formed by oxygen diffusion into the zirco-nium surface during the initial stage of ZrO2 formation.18,27

F. XRD pattern

XRD results are shown in Fig. 5. The pattern of Fe2O3

matches well with the COD card of PDF #96-901-5965, and theintensity of Fe2O3 is strong in both 8-year naturally aged samples(sample #4) and 8-week thermally aged samples (sample #5). Thepattern of Zr3O is in good agreement with the COD card PDF#96-152-1834, and Zr3O intensity shows a strong signal in allpattern ranges due to its high content. In particular, differentpeaks between pristine samples and 8-year naturally aged samplesare noticed for ZrO2 (PDF #96-101-0913, circles) and FeO(#96-101-1170, star). ZrO2 and FeO peaks are seen with 8-year nat-urally aged samples as marked notations in Fig. 5(a). In the case ofFe3O4 (PDF #96-900-2330), it is difficult to identify the peakbecause major intensity peaks overlap with ZrO2 and Zr3O.

G. Iron inspection and vibration as an aging factor

In the Fe2O3 aging process described above, XRD datasupport the formation of FeO as a final product. FeO is detectedonly in 8-year naturally aged samples (sample #4). This is becauseFeO is generated when Fe2O3 decomposition is significant due tomoisture. Since Fe3O4 shares diffraction patterns with Zr3O andZrO2, it is difficult to measure in the XRD phase. Additionally, it isknown that Fe3O4 produced by oxygen adsorption from Fe2O3 ishardly observed in XRD because the layer is too thin.25

The detected phase of Fe2O3 is hematite (α-Fe2O3). Hematiterecrystallizes to maghemite (β-Fe2O3) by shear stress caused bycrushing and milling, even at room temperature.27 Since XRDresults confirm that maghemite is not formed, physical factors donot affect the crystal structure in naturally aged samples.

H. Zirconia diffraction and moisture effects on ZrO2

The detected Zr3O is a type of Zr suboxide, which coincideswith previously reported studies related to early form of oxygendiffusion in Zr metals.29,30 Pure Zr is not observed, suggestingthat the Zr surface is mostly Zr3O because of the exposure toatmospheric oxygen and rapid oxidation.27

ZrO2 detected in naturally aged samples is known as mono-clinic zirconia (PDF #96-101-0913). ZrO2 has three crystal struc-tures, i.e., monoclinic, tetragonal, and cubic. The oxidation processof Zr is known to undergo a phase transition from tetragonal tomonoclinic zirconia through the initial α-Zr phase.18,30 However,only the monoclinic structure is observed in the naturally agedsamples, which implies a rapid transition to a monoclinic phase byskipping the tetragonal phase. This can be attributed to the effectof moisture, as it diffuses into tetragonal ZrO2 surface and inducescrystal structure growth, thus accelerating rapid changes fromtetragonal to monoclinic structure.31 Therefore, moisture not onlyforms ZrO2 during the natural aging process but also acceleratescrystal structure change. Both of these decrease the reactivity of Zrbecause the monoclinic structure is the most thermally stable case.

I. TEM-EDS, zirconium surface analysis

Figure 6(a) shows the cross section of ZrO2 in a 8-year natu-rally aged sample (#4). The composition can be divided into zirco-nium and oxygen using EDS, as shown in Figs. 6(b) and 6(c).Totally four different layers are identified in TEM images markedwith letters “A” to “D” from the innermost layer (A layer) to theoutermost layer (D layer), respectively. The crystal images areconfirmed by FFT from the A layer to the C layer, but notconfirmed in the outermost D layer because oxygen diffuses intoan amorphous state at low temperatures, and this does not form acrystal structure.18,27 The FFT images for A to C layers are shownin Figs. 6(d)–6(f ), respectively. The d-spacing values and millerindices (h, k, l) used to determine the crystal structure are shownin Table II; the COD database is used as a reference. The FFTimages confirm that hexagonal Zr is present in the A layer, hexago-nal Zr3O is present in the B layer, and monoclinic ZrO2 is presentin the C layer. These results are in good agreement with XRDresults of naturally aged samples.

J. Comparison of TEM-EDS line analysis resultsof samples #1, #4, and #5

Similar to the above analysis, surfaces of nonaged (sample #1),8-year naturally aged (sample #4), and 8-week thermally aged(sample #5) samples are visualized to compare each other. FFTanalysis is performed on visible layers to confirm the structure ofZr in TEM. The identified Zr phases are shown in the EDS line

TABLE II. Miller indices (h, k, l) and d-spacing confirming the phase and crystalstructure.

Layer d-spacing (nm) (h, k, l) Phase Crystal structure

A 0.1677 (1, 1, 0) Zr Hexagonal0.2556 (0, 0, 2)

B 0.1704 (1, 2, 1) Zr3O Hexagonal0.1398 (2, 2, 0)0.2561 (0, 0, 2)

C 0.3888 (0, 1, 1) ZrO2 Monoclinic0.5079 (1, 0, 0)0.3313 (1, 1, 1)

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analysis results in Fig. 7. Red color represents hexagonal Zr, darkyellow color represents Zr3O, orange color represents ZrO2, andlight blue color represents oxygen layers. In EDS line analysis, it isfound that the oxygen diffusion zone coincides very well withZr3O, ZrO2, and oxygen layers. The crystal structure, depth of eachlayer, and oxygen diffusion depth are summarized in Table III. Theoxygen diffusion depth is 8.8 nm for nonaged samples (sample #1),and oxygen is diffused up to 47.8 nm in 8-year naturally agedsamples (sample #4), which is considerably deeper than pristinesamples. In the oxygen diffusion zone of naturally aged samples(samples #4), m-ZrO2 and oxygen layers are newly formed. On theother hand, m-ZrO2 is not present in thermally aged samples(sample #5), but Zr3O is significantly grown, and a new oxygenlayer is formed compared to nonaged samples (sample #1).

The oxygen layer could be developed by both oxygen and mois-ture in ambient air.31,32 However, the strength of diffusion is higherwith naturally aged samples (sample #4). This can be explained intwo ways. The first reason is due to oxygen anion. In the presence ofmoisture, a water molecule can dissociate producing 2O− with zirco-nium. But, oxygen anion is less generative by atmospheric oxygen.Thus, in the presence of moisture, zirconium is more likely to besurrounded by an oxygen ion.32 The second reason is that the watermolecule acts on tetragonal ZrO2 to lower the surface density, whichmakes oxygen penetration easier.31 Thus, moisture is not only thesource for ZrO2 formation by combining directly with zirconium butalso the cause for the high oxygen penetration depth.

The order of Zr–α-Zr–ZrO2 layer formed in naturally agedsample (sample #4) is the most stable direction of Zr oxidation.18

In addition, the monoclinic structure formed in the outermostlayer of Zr particle is the most thermally stable phase. Therefore,natural aging proceeds in the direction of stabilization of Zr, andthis stable transition leads to the decrease in reactivity of thosesubstances.

IV. CONCLUSIONS

Three kinds of Zr/Fe2O3 pyrotechnic mixture samples wereprepared: pristine, thermally aged, and naturally aged. DSC wasused to confirm the reaction kinetics of nonaged samples, and itwas confirmed that Zr/Fe2O3 reacts according to the followingformula: 2Fe2O3 + 3Zr→ 3ZrO2 + 4Fe. From the result of DSC,

activation energy and heat of reaction were compared. Naturalaging greatly reduced the heat of reaction and increased the activa-tion energy. To confirm the chemical compositional changes inZr/Fe2O3 due to aging, XPS analysis was performed. The reactionproduct ZrO2 was formed as a result of natural aging, and Fe3O4

was also formed, which is an intermediate form of reaction. Thedecomposition process of Fe2O3 and formation process of ZrO2

were proposed in the presence of moisture. As a result, the exis-tence of moisture lowered the sample reactivity. XRD analysisconfirmed the changes in the crystallographic structure ofZr/Fe2O3. FeO and ZrO2 patterns were newly formed in naturallyaged samples, and this observation confirmed the above agingkinetics due to moisture. The formation of m-ZrO2 without t-ZrO2

was a good basis to confirm that moisture was more important tonatural aging. The absence of β-Fe2O3 allowed to exclude othervibration conditions.

From TEM-EDS results, it was confirmed that only m-ZrO2

was formed in naturally aged samples, and oxygen diffusion depthwas found to be greatest in naturally aged samples; these resultswere explained by the role of moisture. The probability of zirco-nium to combine with oxygen and the role of oxygen penetrationincreased with the moisture. Thus, humidity is responsible forforming the oxygen layer as well as increasing the oxygen penetra-tion depth.

Finally, humidity plays three crucial roles in the aging processof the Zr/Fe2O3 pyrotechnic mixture. The first is reduction in ener-getics and increase in activation energy, the second is the formationof reaction intermediates and final products, and the third is thedevelopment of the crystal and laminated structures of most stablephases.

ACKNOWLEDGMENTS

This work was supported by the Hanwha Corp. (No. Hanwha-SNU-2018) through IAAT at the Seoul National University.Additional funding came from the Advanced Research Center(No. NRF-2013R1A5A1073861) contracted through the NextGeneration Space Propulsion Research Center at Seoul NationalUniversity.

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TABLE III. Oxygen diffusion depth and layer structure of nonaged (sample #1),8-year naturally aged (sample #4), and 8-week thermally aged samples (sample #5).

Sample type LayerDepth(nm)

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(a) Nonaged Alpha-Zirconium, Zr3O 8.8 8.8(b) 8-year naturallyaged

Monoclinic Zirconia,m-ZrO2

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J. Appl. Phys. 126, 105113 (2019); doi: 10.1063/1.5096803 126, 105113-9

Published under license by AIP Publishing.