Evidence and modeling of heterogeneous reactions of low temperature ammonium perchlorate decomposition

Combustion and Flame 200 (2019) 316–324

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Evidence and modeling of heterogeneous reactions of low temperature ammonium perchlorate decomposition Erik D. Tolmachoff∗, Jonathan T. Essel Naval Air Warfare Center 1 Administration Circle China Lake, CA 93555, USA

a r t i c l e

i n f o

Article history: Received 3 August 2018 Revised 8 October 2018 Accepted 28 November 2018

Keywords: Ammonium perchlorate Heterogeneous reaction Decomposition Thermogravimetric analysis (TGA) Thermal runaway

a b s t r a c t To date, research on the low temperature (< 240 °C) decomposition of ammonium perchlorate (AP) has typically presented decomposition data either as mass loss curves or Avrami equation fits. Neither approach measures true decomposition reaction rates, which should be a fundamental and early consideration in understanding the chemistry underlying AP decomposition. In addition, concern for low temperature AP decomposition usually occurs in confined environments where unvented product gases may react in a heterogeneous manner with the pristine AP material. In this work, we show evidence that decomposition gas products play a role in promoting the decomposition of solid AP, which suggests that heterogeneous reactions are a major driving force behind these decomposition reactions. A simple heterogeneous chemistry model is proposed to describe the growth and evolution of pores as AP decomposes. The model qualitatively predicts a size dependence on maximum decomposition rates with respect to particle size using kinetic information derived from literature. The experimental data and model provide some insight into decades-old questions surrounding the unique decomposition behavior of AP. An understanding of the role that heterogeneous reactions play in AP decomposition may be used to develop gas scavenging strategies that can protect AP-based propellants from thermal threats. Published by Elsevier Inc. on behalf of The Combustion Institute.

1. Introduction Ammonium perchlorate (AP) is the most abundant ingredient in solid composite rocket propellants. AP has many advantages from an ideal density, favorable oxygen balance, and tailorable burn rates that make it a preferred ingredient compared to other solid particle oxidizers. While AP is used as an oxidizer, it has enough fuel components in its nitrogen, hydrogen, and chlorine atoms to be used as a monopropellant. Therefore, ammonium perchlorate has the ability to react exothermically without any extra fuel or oxygen, rendering it a potential explosive hazard under certain conditions. The 1988 PEPCON explosion [1] highlights the potential for AP to react violently and explode and illustrates the need for explosion mitigation and protection in AP-based solid rocket motors. AP is thermodynamically unstable, though reactions are slow at low temperatures. At temperatures around 150 °C, noticeable decomposition occurs. In much of the literature, and for this work, low temperature decomposition is considered to be < 240 °C, the temperature at which AP experiences a change from orthorhombic to cubic phase. Since the 1950s, it has been known that low

∗

Corresponding author. E-mail address: [email protected] (E.D. Tolmachoff).

https://doi.org/10.1016/j.combustflame.2018.11.030 0010-2180/Published by Elsevier Inc. on behalf of The Combustion Institute.

temperature decomposition of orthorhombic AP results in porous particles. The chemical composition of the particle remains unchanged; however, the remaining material contains pores up to a few microns in size [2–9]. Decomposition drastically slows when the particles have lost approximately 30–40% of their original mass [3–16]. It should be noted that mass loss during low temperature decomposition is size-dependent and that fine (a few μm) orthorhombic AP does not experience the same pore-forming phenomenon behavior as large orthorhombic AP; fine orthorhombic AP experiences minimal decomposition at low temperatures and low confinement [6–8]. Thermal decomposition of cubic AP (> 240 °C, high temperature decomposition), results in complete decomposition. It is emphasized that this work focuses on the low temperature decomposition of coarse (> 38 μm) orthorhombic AP. The evolution of these pores is also phenomenologically described in the literature. Pores initially appear to form a small distance from the surface of AP crystals. The growth of the pore network proceeds in an anisotropic manner with a pore front sweeping through the volume of the crystal in the [010] direction at an approximately linear rate of a few nanometers per second in the low temperature decomposition range. Pores grow in size— along other crystalline directions—much more slowly than they are generated at the pore front. The driving force behind pore formation, growth and the cessation of growth are not well understood,

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Table 1 Median size and bulk density of sieved AP samples. Particle size (μm)

Median size (μm)

Bulk density (g/ml)

>425 180–212 88–124 38–63

480 200 100 45

1.15 1.32 1.28 1.14

though pore growth is often attributed to crystalline defects and the drastic drop in decomposition rate is often attributed to the exhaustion of these defect nuclei [13–20]. For a recent work examining the various theories of pore evolution from the perspective of percolation theory, see [21]. Commonly used thermal runaway experiments, in which AP is heated in a confined vessel, as well as sealed-pan differential scanning calorimetry tests e.g., Erikson [22] shows that confining the gaseous AP decomposition products does in fact lead to a rapid increase in reaction rates and ultimately an explosion not observed with vented AP. Although such tests are often performed under temperature ramping conditions, and the effects of external heating, complex chemistry, and self-heating cannot be precisely decoupled, the fact that gas confinement during pore forming (surface area increasing) reactions leads to thermal runaway suggests that heterogeneous reactions may increase decomposition rates. In addition, researchers in the past have pre-treated or introduced AP to various catalysts and compounds that may be present during AP decomposition and have observed changes in reaction rates, suggesting there may be a heterogeneous component to decomposition [2–4,14,23,24]. In particular, perchloric acid [2,24] enhanced the reaction rates and ammonia was shown to decrease or increase decomposition rates under certain conditions [2,24]. A more careful study is needed to determine how these heterogeneous reactions affect low temperature reaction rates, extent of decomposition, and which product gases affect AP decomposition the most. In this work, experiments are carefully conducted so that the atmosphere surrounding the decomposing particles is altered. Small amounts of sample are decomposed in well-controlled isothermal ovens to minimize self-heating. It is shown that by removing certain gaseous products or diluting the atmosphere surrounding the particles, decomposition rates can be reduced or enhanced. A simplified heterogeneous chemistry model, based on the assumption of a steady state concentration of gaseous species surrounding the particles is proposed. Using simple kinetic data from literature, the model qualitatively describes the size dependence of AP decomposition reactions. 2. Experimental methods Ammonium perchlorate (AMPAC, Cedar City, Utah) was triple sieved into four size classes: 38-63 μm, 88-124 μm, 180–212 μm, and > 425 μm. Particle sizes and bulk powder density were measured (Table 1). The sorbents used to selectively adsorb decomposition product gases were 3 A˚ molecular sieve (Interra, Park Ridge, Illinois), 4 A˚ molecular sieve (Interra, Park Ridge, Illinois), activated alumina (BASF, F-200) and carbon activated with phosphoric acid (Chemsorb 1425, Molecular Products, Boulder Colorado). These four sorbents were chosen because they had distinct differences in the pore sizes and surface chemistry, which should make them effective in adsorbing different types of AP product molecules. In one series of experiments, AP (200 μm nominally, not size separated) was decomposed in a modified combination batch reactor/separator. The reactor/separator apparatus consists of two glass vessels (Ace Glass) each in its own temperature controlled oven. The temperature of the reactor vessel is held at 210 °C and the temperature of the separator vessel is held at 80 °C. The vessels

Fig. 1. Schematic of the reactor/separator. The reactor oven is set to 210 °C and the separator oven is set to 80 °C. Plumbing connecting the ovens is heated to 100 °C to prevent moisture condensation.

are connected to a diaphragm pump, which provides a continuous flow of gas between the vessels. Plumbing connecting the vessels and the pump are wrapped with a temperature controlled heating tape set to 100 °C in order to prevent condensation in the lines. The fittings on all of the glassware and the pipes as well as the diaphragm in the pump are made of inert fluoropolymer in order to prevent reactions of the apparatus with the gas it contains. AP and sorbent samples (∼500 mg) were placed is small glass containers (80 mm H, 25 mm ID) at the bottom of their respective vessels; fluoropolymer tubing was inserted approximately 70 mm into the sample container in order to recirculate the atmosphere in sample container. This reactor separator experiment is run for three hours. Although the experiment was enclosed in a safe box, great care was taken to limit high pressures in the apparatus by charging the ∼500 ml reactor with 500 ± 10 mg of AP at most. Maximum pressures recorded with a digital pressure gauge (Omega PX409) were 5.1 psig. A depiction of the reactor/separator is shown in Fig. 1. After the experiment is finished, all heaters are turned off in order to slow reactions and the recirculation pump is disconnected in order stop the exchange of gasses between the reactor and separation chambers. The partially decomposed AP was weighed in order to determine the extent of its reaction. Containers of used sorbent were sealed so that adsorbed products could be analyzed. Analysis of used sorbents from the reactor/separator tests was carried out in a TGA/MS (Discovery model 5500). Samples of used sorbents were heated on a platinum weighing pan at a ramp rate of 50 °C per minute to a temperature of 500 °C and held at that temperature for a period of 20 min. In a second series of experiments, the TGA was used to analyze the decomposition rates of AP as a function of size and temperature. The mass loss experiments performed on AP were carried out by placing ∼30 ± 3 mg of size-separated AP in an aluminum pan (Tzero, TA instruments, 5.4 mm diameter, 2.7 mm height) designed for differential scanning calorimetry. Samples were heated at a ramp rate of 20 °C per minute followed by an isothermal hold at temperatures between 200 and 230 °C for a period of time sufficient to reach the deceleratory period of decomposition, the time at which decomposition slows, typically coinciding with a mass loss of 30–40%. Except for a few tests, the sample pan was covered with a lid but was not sealed (otherwise there would be no measurable mass loss and there could be a potential explosion). Covering the sample with the lid also served to contain any particle fragments which may result from the occasional fracture of AP particles.

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Fig. 2. The extent of mass loss during low temperature decomposition of 200 μm AP at 210 °C over a three-hour period. By selectively removing intermediates it is possible to alter the extent of decomposition. This behavior suggests that the removal of certain decomposition products can effect change in overall decomposition rates of AP.

3. Results and discussion The reactor/separator experiments showed that it is indeed possible to alter the rate of decomposition by changing the atmosphere under which AP decomposes. Left to decompose without the applications of sorbents in the reactor/separator, the 200 μm AP lost approximately 16% of its mass over a three-hour period at 210 °C. When 3 A˚ and 4 A˚ mol sieves and activated alumina were incorporated as selective sorbents, decomposition rates increased. Using the Chemsorb 1425 activated carbon as a decomposition product sorbent significantly reduced AP decomposition (Fig. 2). Though simple in design, this series of experiments performed in the reactor/separator illustrate that the atmosphere in which AP decomposes clearly plays a role in decomposition rates. This behavior is evidence of a heterogeneous reaction between decomposition products and solid AP. It is helpful to understand some basics of the chemistry of low temperature AP decomposition. AP exists in its orthorhombic form below 240 °C and is thermodynamically unstable, although decomposition is slow. Equilibrium calculations [25] performed at constant temperature and pressure P = 1 bar are shown in Fig. 3. For the range of temperatures and pressure of interest in this work, (200–230 °C, ∼1 bar) equilibrium calculations predict the product distributions to be well approximated by the global reaction:

2NH4 ClO4 -> N2 + 2O2 + 4H2 O + Cl2

Hrxn = ∼−189 kJ/mol (T = 200-230 °C).

(1 )

Whereas at high temperatures, > 800 °C, the global reaction is better approximated by:

4NH4 ClO4 -> 6H2 O + 5O2 + 4HCl + 2N2

Hrxn = ∼−162 kJ/mol (T = 200-230 °C).

(2 )

Importantly, both approximate reaction schemes are exothermic, a fact often overlooked in low temperature AP decomposition literature. Mass spectroscopy data of decomposing AP as well as analysis of used sorbents from the reactor/separator experiments show very little evidence of Cl2 . It is concluded that over the

Fig. 3. Equilibrium calculations at constant temperature, pressure = 1 bar. Low temperatures favor the formation of Cl2 and H2 O whereas at higher temperatures, the Cl and much of the H atoms form HCl. The presence of NOx and other trace species is also predicted in small quantities by equilibrium calculations but not shown in this figure for clarity.

timescales relevant to these experiments, the global decomposition reaction likely better resembles that of reaction (2 ) rather than reaction (1 ). In-situ mass spectroscopy could not be performed in the reactor/separator test setup, so used samples of sorbent were subjected to mass spectroscopy analysis afterwards. Each sorbent showed a significant signal for H2 O and small signals for NO and N2 O, which are expected to be present in small amounts. Used activated alumina released very small amounts of Cl2 . Activated carbon was the only sorbent that released significant amounts (relative basis) of chlorinated species in the form of HCl (Fig. 4). Because of the fact that activated carbon was the only sorbent that significantly retarded decomposition and the only sorbent that captured significant amounts of chlorinated species, it seems likely that chlorinated species play an important role in increasing the rate of surface reactions underlying low temperature decomposition, in agreement with [2,24]. Alternatively, the effective removal of water and coinciding increase in decomposition rates seen with the other sorbents may be a sign that moisture can retard surface reaction rates, also in agreement with [24]. While the chlorine detected in the analysis of these used sorbents was almost exclusively in the form of HCl, it cannot be concluded that HCl is responsible for driving decomposition reactions. Recent in situ investigations of decomposition products show evidence of a variety of short-lived chlorinated compounds resulting from the decomposition of perchloric acid [26,27]. It is possible that the sorbent captures and reacts with another chlorinated species, say Cl2 , forming and binding HCl on the sorbent surface. Indeed, the literature is in disagreement as to whether low temperature decomposition reactions favor the formation of primarily HCl or Cl2 . Both chlorinated compounds are often present in relative abundance in experiments [2,6–8,12,15,26–30]. The fact that products of low temperature AP decomposition do not tend toward equilibrium in much of the literature is more evidence of the difficulty in understanding the reactions underlying AP decomposition. Historically, most AP decomposition kinetic data have been analyzed in an Avrami framework or, more simply by the plotting of mass loss curves as delivered by TGA instruments. These are not true reaction rates in the sense that these analyses do not directly measure change in reactant per time. The experimental setup in this work, which uses a loose-lidded DSC pan in a

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Fig. 4. TGA/MS data of sorbents. The vertical axis of the MS data is ion current and shows that for all sorbents tested here, H2 O is the most abundant adsorbed species. Except for activated carbon, which adsorbs significant amounts of HCl, the concentration of other adsorbed species is an order of magnitude lower than adsorbed water. The data “mass 44” is a combination of N2 O and CO signal from air, which is drawn into the instrument in small amounts.

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Fig. 5. Schematic of the loose-lidded DSC reactor vessel with approximate dimensions. Although the lid is flared and designed to form a compression seal with the pan, in these experiments, the lid is not truly hermetically sealed, allowing for gas exchange between the pan and the furnace atmosphere through microscopic gaps between the lid and pan.

carefully-controlled temperature environment is similar in concept to a constant pressure zero-dimensional reactor. As such, and with measured particle bulk density and mass loss rates, it is possible to arrive at time-dependent global reaction rates in terms of change in molar concentration per unit time. Obtaining true and precise reaction rates of heterogeneous reactions is not directly possible here because both the concentrations of active surface area and gaseous decomposition products vary over time within the loose-lidded reaction vessel. Initially, the vessel contains ambient air and AP. Inside the TGA, the vessel is surrounded with argon carrier gas. Initially the particle begins to decompose slowly, generating product gases. These generated gases displace the atmosphere initially present in the reaction vessel. The loose-lidded nature of this reaction vessel serves to maintain a relatively steady concentration of gas reactants inside the reaction chamber over the course of the experiment. The flared lid sits atop approximately 1.5 mm of AP sample. Since the flared lid is not hermetically sealed to the pan, gases enter/escape the pan over the approximately 1.2 mm distance between the sample and the top of the lid (Fig. 5). The flux of argon entering the pan is estimated by Fick’s law:

JAr = D

C∞ − C0 x

Fig. 6. Reaction rates mg/min of 88–124 μm particles with respect to time and conversion fraction. Decomposition is faster when the chamber has a lid on compared to when there is no lid and the ambient gases can dilute the atmosphere in contact with the particles.

(1)

where D is the diffusion coefficient, taken to be an approximate 0.5 cm2 /s, C∞ is the concentration of argon in the furnace, calculated by the ideal gas law to be 24.2 mol/m3 at 230 °C, C0 is the argon concentration in the reaction vessel, taken to be zero and ࢞x is the distance the gas would travel from the entrance of the lid to the reacting AP, 1.2 mm. The diffusive flux of argon into the vessel under these conditions is 1.0 mol/m2 /s. Conservatively estimating the area of the gap between the lid and the pan to be 10−8 m2 yields an upper limit of argon flux into the vessel to be 10−8 mol/s. Assuming that each mole of AP decomposes into four moles of gas (i.e., reaction (1 ) and a mass loss rate of just 0.02 mg/minute (a low rate for these experiments), yields a flux of 1.1 × 10−8 mol/s of products out of the vessel. Therefore, over most of the course of the reaction, decomposition rates are high enough that the convective flux of products out of the vessel is expected to be larger than the diffusive flux of the external atmosphere into the vessel, which serves to maintain a relatively high and steady concentration of gaseous products. To further illustrate the effects of altering the atmosphere in contact with decomposing particles, TGA testing was performed on select samples to show the effects that the loose-lidded DSC pan configuration has on maintaining a steady state atmosphere within the tiny reaction vessel. Samples decomposed with the lid in place are expected to have a relatively constant gas phase composition during the majority of the reaction whereas the samples decomposed with no lid present are expected to be in contact with an atmosphere that is highly diluted by the

diffusive flux of argon from the TGA furnace to the sample surface. The gradual loss of mass is evidence of an imperfect seal for the loose-lidded vessels. Slight overpressures in the vessel can exist. Though it is not possible to measure pressure in the loose-lidded vessel, they are thought to be small since the unsealed flared lid can be lifted and reseated under overpressure conditions. Figure 6 shows how decomposition rates decrease when the atmosphere surrounding the particle is diluted with ambient argon. Given the steady state assumption for the gas phase concentrations and some phenomenological behavior of AP decomposition, we present a rudimentary model of the decomposition process. As discussed previously, the decomposition process is anisotropic. A porous front consisting of small nascent pores appears to grow from the particle surface radially inward at a rate far greater than individual pores grow in size. The anisotropic pore growth is evidence that there are at least two important rate-limiting factors in AP low temperature decomposition; if there were only a single rate limiting step, decomposition would more closely resemble a shrinking core. Here the two reaction rates are denoted kfront , describing the rate at which the nucleating porous front sweeps from the particle surface to its center and kpore , which is the rate that describing the slow growth of the pores. Decomposition of AP slows dramatically when the particles reach a total porosity > ϕ = 0.3–0.4. The reason for slowed decomposition is often attributed to exhaustion of pore nucleation sites. However, for surface reaction-driven pore formation, reactions may cease due to

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the fact that the porous particle has surpassed its critical percolation threshold (typically in the range of ϕ = 0.1–0.3 for most geometries) [21] rendering the particle a network of connected pores with surfaces oriented in a slow-reacting direction. It is possible to show that for anisotropic decomposition, overall decomposition rate maximums are size dependent. The model tracks the population of pores as:

N pore = Ngen −

1 Nover 2

(2)

where Ngen is the concentration of pores generated and Nover is the concentration of overlapping pores. Ngen is described by:

dNgen = 4π r 2 Nv k f ront dt

(3)

where Nv is the number of pore nuclei per volume, the particle volume is modeled as a sphere, r is the position in the particle, dr

f ront and k f ront = dt (m/s) is a modified reaction rate which is of the order a few nanometers per second. Note that while reaction rate units reported in this way are not typical, they can be thought of as a typical surface reaction with units (mol/cm2 /s), divided by molar density. The probability of pores overlapping is described by the Poisson distribution:

ϕ = 1 − exp(−NporeVpore )

(4)

where Vpore is the pore volume. Differentiating the Poisson distribution provides an expression for the rate of pore elimination through overlap

dN pore dr pore dNover = dt dr pore dt where rpore is the radius of a pore and

(5) dN pore dr pore

is derived from

(4). Pores are treated as cylinders, which intrinsically allows for anisotropic growth, and initially assigned a nominal size. The evolution of pore volume is found by:

Vpore = π l (r pore0 + k pore t p )2

(6)

dr pore dt

where k pore = is the modified reaction rate of pore growth (m/s), which is much lower than the rate of the pore front, tp is the age of the pore, taken to be zero at the time when the front passes the location where the pore is located, l is the length of the pore assumed to be constant and rpore0 is the initial pore radius. The model is discretized in time and space. At each location, the local porosity is integrated in time until the porous fraction reaches a critical limit, ϕ crit , beyond which no further reaction occurs and Vpore reaches a terminal value. Overall reaction progress is defined as:

ϕ=

1 Vsol id

particl e



R

N poreVpore dr

(7)

o

where R is the radius of the particle. Decomposition reaction rates are defined as:

rate =

dϕ ρ dt sample

(8)

where ρ sample is the measured density of the bulk powder shown in Table 1. The model oversimplifies pore surface area and volume by assuming that overlapping pores become a single pore and treating potentially complex reactions as having constant rates over the duration of the reaction. Although it treats particles as monodisperse spheres, lacks in rigorous topological treatment, and uses constant reaction rates, the model shows that the anisotropic nature of the heterogeneous decomposition leads to size dependent decomposition rates. For sufficiently high mismatches between kfront and kpore the model predicts that particles of intermediate size decomposed

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faster than very large or very small particles, consistent with the experimental data below and in agreement with [3]. kfront is taken from the literature [10,12] to be 3 and 20 nm/s for T = 200 and 230 °C, respectively. As explained later, kpore is taken to be 0.015 and 0.133 nm/s (200 and 150 times smaller than kfront ) for T = 200 and 230 °C. Nv , l, and rpore0 are taken to be 1018 m−3 , 10−6 m and 10−9 m, respectively. ϕ crit is assumed to be 0.35, in general agreement with experimental results here and throughout the literature. Figure 7 shows modeled decomposition rates (mg/ml/min) with respect to overall conversion and time. As shown in experimental data later, the model qualitatively predicts that decomposition rates are size independent at low and high conversion and show a significant size dependence at intermediate conversion values. Pore diameters are typically around 1 μm [24] and the pore network extends through the particle, which may be hundreds of microns, therefore it is reasonable to estimate that kpore is approximately two orders of magnitude smaller than kfront . Experiments measuring kfront [10,12] show an activation energy of ∼ 30 kcal/mol and experiments designed to measure overall decomposition rates, which must be mainly determined by pore growth rates, show an activation energy of ∼ 35 kcal/mol [11,23,24]. Simple desorption reactions with activation energies of these magnitudes may explain both the anisotropic nature of AP decomposition as well as the approximate reaction rates for kfront and kpore . Assuming a first order desorption type reaction, the modified reaction rate can be described by:

k f ront,pore =

A

ρmolar

 −E 

exp

A

RT

[C ∗ ]

(9)

where A is typically 1013 –1015 s−1 , [C∗ ] is the concentration of adsorbed reactant, typically ∼10−9 mol/cm2 [31], and ρ molar is the molar density of AP, ∼0.017 mol/cm3 . Assuming A is1014 s−1 and using activation energies from the literature, one arrives at values for kfront and kpore of 5 and 0.04 nm/s, respectively for T = 230 °C. The ratio of kfront to kpore varies from approximately 150–200 over the temperature range examined here. While the reactor/separator experiments show evidence of heterogeneous reactions and reaction rates can be plausibly explained by simple surface chemistry theory, it cannot be concluded that kfront and kpore are both limited by heterogeneous reactions or that Eq. (9) accurately describes the surface chemistry underlying AP decomposition. Using the heterogeneous reaction model above, the reaction history can now be pieced together for the decomposition of AP in these loose-lidded reaction vessels. Initially, the vessel has low concentrations of AP surface area and small concentrations of gas reactants. Once pores begin to develop, the reaction rate soon becomes high enough that the convective flux of gas out of the reactor are greater than the diffusive flux of carrier gas from the TGA into the reactor vessel. Because the reactor operates at ambient pressure and a relatively large volume of gas escapes the looselidded vessel, the concentration of gas decomposition reactants— though its makeup unknown—remains relatively constant (steady state gas concentration assumption) over most of the mass loss event. Due to the unequal rates of pore formation and growth, total decomposition can show a size dependence even under conditions of constant pore formation and growth rates. The comparatively fast anisotropic pore-forming front sweeps small particles in a relatively short period of time and large particles in a longer period of time. Meanwhile pores grow in size at a slower rate. For small particles, the pore-forming front nearly completely sweeps the volume of the particle before pore growth near the surface of the particle ceases. For large particles, early formed pores near the surface cease growing before the pore forming front sweeps the volume of the particle. Medium size particles show maximal decomposition rates due to the fact that both pore growing and pore

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Fig. 7. Modeled decomposition rates with respect to conversion and time for T = 200 (top) and 230 °C (bottom). Particle sizes of 50, 100, 200, and 500 μm are chosen for the purpose of comparisons to experimental data. Conversion rates are size independent at low and high conversion. At intermediate conversions, 100 μm particles are predicted to decompose fastest.

forming reactions occur simultaneously throughout a large portion of the particle. This sequence of events can qualitatively describe the reaction rates of AP in the TGA experiments presented here. Temperatures in the TGA are ramped to a constant level with essentially no mass loss for a period of time. Then rates of decomposition increase to a maximum, occurring around 10–20% mass loss, then once again decrease. Figure 8 shows the measured global reaction rates of four sizes of AP particles at 200 °C. At all temperatures peak reaction rates are highest for the particle sizes 88–124 followed by 180–212, 38–63 and > 425 μm. A similar trend with respect to reaction rates and particle sizes was shown in [3]. The order of peak reaction rates is in qualitative agreement with the simple model proposed for an anisotropic heterogeneous reactor with constant gas concentration. At low conversion, measured reaction rates are similar for all particle sizes at a given temperature as a function of conversion (< 0.02). Similarly, at high conversions (> 0.35) decomposition rates once again approach a size independent behavior (Fig. 9). At intermediate conversions, reaction rates vary in a pronounced way depending on particle size, as expected from the model for anisotropic decomposition presented earlier. The degree

Fig. 8. Global decomposition rates of AP decomposition of four size classes of AP at 200 °C. There is no measurable mass loss for approximately 150 min. Reaction rates then rise quickly and eventually fall again. At long times reaction rates are identical regardless of initial particle size.

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Fig. 9. Measured reaction rates as a function of particle size and temperature with respect to conversion.

to which size dependence affects decomposition rates increases at higher temperatures. It is noted that because the decomposition reactions are exothermic, under conditions of fast reaction, the internal temperature of the reaction vessel may be slightly higher than that of the well-controlled furnace, which would further increase decomposition rates of the fastest reacting samples. 4. Conclusions In this work, we have shown that it is possible to alter the rate of low temperature decomposition of AP by altering the atmosphere in which AP decomposes. Evidence from a series of experiments designed to selectively adsorb gaseous decomposition products suggests that chlorinated species play a role in promoting decomposition while moisture may retard surface reactions. Results from TGA analysis of decomposing AP further illustrates the effect that decomposition products have on increasing overall decomposition rates of AP. Importantly, decomposition data is presented in reaction rate units rather than as mass loss curves, which is a necessary first step in gaining a true understanding of the kinetics of the underlying reactions. A simple heterogeneous chemistry model was proposed to describe the growth and evolution of pores as AP decomposes. Although simple, the model qualitatively predicts a size dependence on maximum decompo-

sition rates with respect to size using kinetic data derived from literature. While decomposition may not reach equilibrium for the time scales of these experiments, the decomposition reactions are exothermic and therefore self-heating, a fact typically overlooked in low temperature AP decomposition research. This work illustrates the difficulty of measuring heterogeneous kinetics and interrogating complex pore formation under conditions in which the atmosphere surrounding the reacting particles is well characterized and temperatures are truly isothermal. Going forward, a more detailed understanding of pore growth rates and topology and a clearer understanding of the role that specific products, such as chlorinated species and moisture play in heterogeneous reactions are needed. Evidence of heterogeneous reactions promoting/retarding AP decomposition shows promise to develop gas scavenging strategies that will protect AP from thermal threats.

Acknowledgments The authors thank the NAVAIR ILIR program, managed by ONR and administered at China Lake by Dr. Lee Cambrea. We also thank Dr. Gray Slough for helpful discussions and instruction on TGA and MS use and analysis.

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References [1] O.O. Ibitayo, A. Mushkatel, K.D Pijawka, Social and political amplification of technological hazards: The case of the PEPCON explosion, J. Hazard. Mater. 114 (2004) 15–25. [2] L.L. Bircumshaw, B.H Newman, The thermal decomposition of ammonium perchlorate. I. Introduction, experimental analysis of gaseous products, and thermal decomposition experiments, Proc. R. Soc. A 227 (1954) 115–132. [3] L.L. Bircumshaw, B.H. Newman, The thermal decomposition of ammonium perchlorate, II. The kinetics of the decomposition, the effect of particle size, and discussion of results, Proc. R. Soc 227 (1955) 228–241. [4] L.L. Bircumshaw, T.R Phillips, The kinetics of thermal decomposition of ammonium perchlorate, J. Chem. Soc 12 (1957) 4741–4747. [5] A. Galwey, P Jacobs, High-temperature thermal decomposition of ammonium perchlorate, J. Chem. Soc (1959) 837–844. [6] R. Behrens, L. Minier, The thermal decomposition behavior of ammonium perchlorate and an ammonium-perchlorate-based composite propellant, 33rd JANNAF Combustion Meeting, Monterey, CA (1996), pp. 1–19. CPIA Publication No. 653 (2). [7] L. Minier, R. Behrens, Thermal decomposition characteristics of orthorhombic ammonium perchlorate (o-AP), 17th JANNAF Propulsion Systems Hazards Subcommittee Meeting, 1, Tucson, AZ (1998), pp. 57–72. CPIA Publication No. 681. [8] J.J. Kay, D Wiese-Smith, A. Highley, S. Maharrey, Role of internal defects in promoting thermal decomposition of ammonium perchlorate, JANNAF 45th CS, 33rd APS, 33rd EPSS and 27th PSHS Joint Subcommittee Meeting, Monterey, CA (2012) December 3–7 CPIAC JSC CD-70. [9] K.J. Kraeutle, A.I. Atwood, P.O. Curran, The partial decomposition and sublimation of propellant grade ammonium perchlorate: Effect of temperature, time, and particle size, JANNAF 35th CS, 35th APS, and 17th PSHS Joint Subcommittee Meeting, 1, Tucson, AZ (1998), pp. 45–56. CPIA Publication No. 681. [10] K.J. Kraeutle, The thermal decomposition of orthorhombic ammonium perchlorate single crystals, J. Phys. Chem 74 (1970) 1350–1356. [11] P.W. Jacobs, H.M Whitehead, Decomposition and combustion of ammonium perchlorate, Chem. Rev 69 (1969) 551–590. [12] B. Longuet, P. Gillard, Experimental investigation on the heterogeneous kinetic process of the low thermal decomposition of ammonium perchlorate particles, Propell. Explos. Pyrotech 34 (2009) 59–71. [13] A.V. Rayevskiy, G.B Manelis, Development of reaction centers with thermal decomposition of orthorhombic ammonium perchlorate and the role of dislocations in this process, Goreniye i Vzryv, Izd vo Nauka (1972) 748–751. DTIC ADA004407. [14] V.V Boldyrev, Thermal decomposition of ammonium perchlorate, Thermochim. Acta 443 (2006) 1–36.

[15] A.G. Keenan, R.F Siegmund, Thermal decomposition of ammonium perchlorate, Q. Rev. Chem. Soc. 23 (1969) 430–452. [16] P.J. Herley, P.W.M. Jacobs, P.W Levy, A photomicrographic and electron microscopy study of nucleation in ammonium perchlorate, R. Soc. Lond. A 318 (1970) 197–211. [17] P.J. Herley, P.W.M. Jacobs, P.W Levy, Dislocations in ammonium perchlorate, J. Chem. Soc. A 0 (1971) 434–440. [18] J.O. Williams, J.M. Thomas, Y.P. Savintsev, V.V Boldyrev, Dislocations in orthorhombic ammonium perchlorate, J. Chem. Soc. A 0 (1971) 1757–1760. [19] T.D. Hedman, M.L Gross, On the thermal stability of partially decomposed ammonium perchlorate, Propell. Explos. Pyrotech. 41 (2016) 254–259. [20] J. Kalman, T. Hedman, B. Varghese, G Dagliyan, Nano-computed tomographic measurements of partially decomposed ammonium perchlorate particles, Propell. Explos. Pyrotech 42 (2017) 1–7. [21] W.W. Erikson, A Monte Carlo percolation method applied to the decomposition of ammonium perchlorate, 29th JANNAF PSHS Meeting, Newport News, VA (2016). [22] T.L Boggs, The hazards of solid propellant combustion, Int. J. Energ. Mater. Chem. Propul. 4 (1997) 233–267. [23] S.H. Inami, W.A. Rosser Jr., H. Wise, Heat-release kinetics of ammonium perchlorate in the presence of catalysts and fuel, Combust. Flame 12 (1968) 41–44. [24] E.F. Khairetdinov, V.V. Boldyrev, The mechanism of the low-temperature decomposition of NH4ClO4, Thermochim. Acta 41 (1980) 63–86. [25] McBride, B.J., Gordon, S. Computer program for calculating and fitting thermodynamic functions. NASA RP-1271, NASA, 1992. [26] L. Mallick, S. Kumar, A Chowdhury, Thermal decomposition of ammonium perchlorate—A TGA–FTIR–MS study: Part I, Thermochim. Acta 610 (2015) 57–68. [27] S. Góbi, L Zhao, B. Xu, U. Ablikim, M. Ahmed, R.I. Kaiser, A vacuum ultraviolet photoionization study on the thermal decomposition of ammonium perchlorate, Chem. Phys. Lett. 691 (2018) 250–257. [28] L. Mallick, S. Kumar, A Chowdhury, Thermal decomposition of ammonium perchlorate—A TGA–FTIR–MS study: Part II, Thermochim. Acta 653 (2017) 83–96. [29] W.A. Rosser, S.H. Inami, H Wise, Thermal decomposition of ammonium perchlorate, Combust. Flame 12 (1968) 427–435. [30] L. Minier, R Behrens, Thermal decomposition characteristics of orthorhombic ammonium perchlorate (o-AP) and an o-AP/HTPB/Al-based propellant, 36th JANNAF Combustion and 18th Propulsion Systems Hazards Subcommittee Meetings, 691 (1999), pp. 626–636. [31] I. Chorkendorff, Concepts of modern catalysis and kinetics, Wiley, 2003.