Reaction Kinetics and Combustion Dynamics of I₄O₉ and Aluminum Mixtures

Figure 1a shows the endothermic and exothermic behavior of the initial states for all six oxidizers examined from DSC analysis and Figure 1b shows the corresponding mass loss from TGA analysis. It is noted that all oxidizers when heated beyond the dissociation temperature of I₂O₅ (350 °C) lose 100% mass but the I₄O₉ sample releases I₂ gas instead of water liberated by dehydration of hydrated species at temperatures below the dissociation temperature of I₂O₅. The abundance of I₂ gas released from the I₄O₉ samples is specified in Figure 1b at over 7 wt.%. Figure 1a shows that the only iodine compounds that produce exothermic behavior is I₄O₉ and I₂O₅ formed by decomposition of I₄O₉, and the exotherm corresponds to an onset temperature of about 180 °C for decomposition into I₂O₅. Figure 1b also shows that the iodine compound with the greatest overall iodine gas generation is I₄O₉.

There are no endotherms for commercial and amorphous I₂O₅ samples between 110 °C and 210 °C. This shows that these samples are purely I₂O₅ with no iodic acids. The nano I₂O₅ and I₄O₉ have exotherms with onset temperature at 150 °C. This shows those samples contain I₄O₉⁶. The nano I₂O₅ is thermally treated I₄O₉. The small exotherm at 150 °C shows that there is some residual I₄O₉ in the nano I₂O₅ sample. Using the TGA data from Figure 1b, the residual I₄O₉ in the sample is less than 15 wt.%. The HIO₃ dehydrate has a single endotherm starting at 130 °C and indicates this sample is crystalline HIO₃. The commercial HIO₃ sample has two distinct endotherms starting at 160 °C and 210 °C. An endotherm with an onset temperature of 160 °C is outside of the range of HIO₃ dehydration in the CRC handbook¹⁶. However, TGA results show a mass loss of 2.47 wt.% over this range indicating it is the dehydration of a hydrated species.

Figure 2a shows the heat flow behavior from DSC analysis of four Al and iodine oxide mixtures. The Al + I₄O₉ dry mixture has an exotherm at 180 °C indicating the oxidizer is still I₄O₉ with an increased onset temperature. The Al + I₄O₉ and Al + nano I₂O₅ samples are almost identical. Both of these mixtures were processed using isopropanol as the carrier fluid to aid intermixing and identical thermal behavior seen in Figure 2a indicating that mixing powders in isopropanol transformed I₄O₉ into I₂O₅.

Figure 2b shows I₄O₉, I₄O₉ exposed to 20% RH for 4 hours, and nano I₂O₅ exposed to 20% RH for 4 hours. After 4 hours exposure to 20% RH, I₄O₉ is identical to nano I₂O₅ and there are endotherms at 110 °C and 210 °C showing the samples are partially composed of HIO₃ and HI₃O₈. This may be explained by the I₄O₉ "dry" synthesis method which is then heated and transformed into I₂O_5. The crystal structure of I₄O₉ is not known, but because hydration is observed (Figure 2b bottom two curves with endotherms shaded), the low density of the powder (i.e., fluffy, highly porous nature), and the lack of a reported crystalline structure, an amorphous structure is assumed. Nano I₂O₅ is formed by thermal decomposition of amorphous I₄O₉, according to Eq. (1) such that I₂ is released from amorphous I₄O₉ leaving amorphous I₂O₅. Crystal structure formation of iodine(V) oxides is catalyzed by water. Since the synthesis method is dry, there is no water to catalyze crystal formation. The first step required for absorption of water is to raise the relative humidity to disrupt the bonds between molecules. Without this crystal structure (that wants to release the weakly bonded water) the dynamic equilibrium is shifted so I₂O₅ will absorb any available water. As soon as water is absorbed by I₂O₅, formation of HIO₃ molecules begin. HIO₃ has a hydrogen that attracts oxygen in amorphous I₂O₅ and creates an HI₃O₈ crystal structure. Water is still the catalyst in crystal structure formation. The synthesis method of I₂O₅ and I₄O₉ which form an amorphous structure instead of a crystalline structure is the reason water absorption is seen at lower RH (i.e., 20%) than what was required to start absorption in previous studies (i.e., 70-80% RH)^5,17. In summary, the iodine compounds' amorphous structure may promote absorption of hydrated species and formation of iodic acids.

In Figure 2a the main exotherms around 500 °C are all similar. In the Al + I₄O₉ dry mix, the exotherm at about 180 °C indicates phase transition from I₄O₉ to I₂O₅. Also, all mixtures have a pre-ignition reaction (PIR) between 300 – 400 °C, but Al + I₄O₉ and Al + nano I₂O₅ have a PIR with lower onset temperature and greater magnitude but also show unique endotherms followed by exotherms at 210 °C. These samples were processed in isopropanol and the behavior at 210 °C indicates these samples are partially HI₃O₈. The exotherm may be reaction between HI₃O₈ and I₄O₉ because HI₃O₈ is dissociating at nearly the same temperature as I₄O₉ decomposition. These reactions may help promote greater exothermicity in the PIR and catalyze earlier onset of the PIR. Osborne and Pantoya ²⁰ first showed an exothermic reaction preceding the main exothermic reaction in Al combustion and coined this a PIR. Their analysis indicated the PIR reaction was between the alumina shell surrounding an aluminum core particle with fluorine from decomposing fluoropolymer^19,20. Farley et al.⁴ then extended observations of the PIR to aluminum-alumina core-shell particles reacting with iodine based oxidizers. The Al + commercial I₂O₅ sample has an endotherm at 210 °C indicating HI₃O₈ presence and a mild PIR exotherm with delayed onset temperature. Mulamba et al.²¹ also showed that the PIR onset temperature is concentration dependent.

Table 1 shows measured flame speed results for Al mixed with the indicated oxidizer as a dry mixture and also mixed using isopropanol as the processing carrier fluid. Only the first three samples were tested after being mixed in isopropanol and the Al + commercial HIO₃ dry mix either did not ignite or did not sustain reaction long enough to obtain measureable results. The percent uncertainty is determined based on repeatability of up to three separate experiments for each mixture. Bulk density is determined as a function of mixture powder mass and volume of the tube.

When interpreting reactivity with flame speed measurements, there are many factors that influence results such as homogeneity of the mixture, particle size, and bulk density. Mixture homogeneity can be optimized using a carrier fluid to aid intermixing of the reactants. The I₄O₉ examined in this study is more stable than previously studied I₄O₉ samples and did not appear to decompose into I₂O₅ or form hydrated species seen in Wikjord et al.⁶ (as seen in Figure 1a with only an exotherm corresponding to I₄O₉ decomposition). However, the only way to observe the combustion performance of Al + I₄O₉ is by dry mixing with as little exposure to the atmosphere as possible in order to maintain the integrity of I₄O₉. Additionally, the flame speed measurements of Al with the oxidizers in different states allowed us to narrow the effects that cause variance in flame speed results and reveal differences attributed specifically to particular iodine compounds. These comparisons will be discussed below. Overall Table 1 shows that I₄O₉ significantly improves reactivity when compared with other iodine compounds.

Mixing in a carrier fluid provides improved distribution of fuel and oxidizer particles that increases mixture homogeneity and reactivity. This is seen in the difference in flame speeds in Table 1 for the dry and isopropanol mixed Al + nano I₂O₅ and Al + commercial I₂O₅ samples where the flame speeds increased by 1.07 and 3.34 times, respectively. Mixture homogeneity implicit from the measured flame speed is only slightly improved for the Al + nano I₂O₅ mixture, whereas the Al + micron scale commercial I₂O₅ exhibits a three times increase in flame speed when the carrier fluid aids intermixing. Clearly particle size and carrier fluid contribute to measured flame speed. The homogeneity effects can also be seen by the uncertainty between flame speeds. The samples that are mixed in isopropanol and the samples with smaller particles have less uncertainty in measured flame speeds. This small uncertainty is also seen in all of the amorphous samples, which suggests that an amorphous structure facilitates improved homogeneity in dry mixing. It is further noted that each sample was sieved before dry mixing to break up agglomerates and help eliminate uncertainty caused by poor homogeneity.

The HIO₃ molecules have a hydrogen end cap, which is electropositive, and an oxygen end cap, which is electronegative and causes an attraction between the ends of individual HIO₃ molecules²². Because of this attraction, during sieving the HIO₃ particles instantly agglomerated before the Al can be mixed. This caused extremely poor homogeneity and is the reason Al + commercial HIO₃ samples could not sustain the reaction. The Al + HIO₃ dehydrate sample had water available to catalyze crystal structure formation (Figure 1a), which reduced, but did not eliminate this effect.

Energy propagation is dependent on the bulk density of the reactant mixture. The density will change based on the concentration of the reactants, so the bulk density of the mixture is usually reported as the percentage of the theoretical maximum density (TMD). The %TMD is calculated using a weighted average of the concentration and the densities of the reactants and accounts for the density of the actual sample according to its mass and volume occupied. In this way, bulk density in terms of %TMD represents the amount of solid space occupied by the volume (i.e., 60% TMD is equivalent to 40% air voids and 60% solids). Low %TMD usually result in higher flame speeds than high %TMD powders. The higher concentration of air voids with lower %TMD provide convective pathways to enhance flame speeds. For this reason, the flame speeds reported in Table 1 are not comparable as a function of mixture, because each was prepared at a discretely different bulk density.

Two conclusions can be drawn from Table 1: (1) I₄O₉ cannot be processed in isopropanol because it changes into I₂O₅ and thus alters its reactivity; and (2) I₄O₉ is more reactive than I₂O₅ because at higher and lower bulk densities (i.e., 11% TMD compared with 8 or 17% TMD), I₄O₉ demonstrates increased reactivity. This finding suggests that I₄O₉ would be advantageous for reactive applications if it could be passivated to improve stability.

Through reactivity and thermal analysis, results show I₄O₉ can be more reactive than other forms of iodine(V) oxides when combined with aluminum (Al) powder. The I₄O₉ sample used here was synthesized using a 'dry' method that combines elemental oxygen and iodine such that hydrated species are not introduced at any point during synthesis. For this reason, the I₄O₉ sample is initially devoid of iodic acids and produces a large exotherm at a low temperature (i.e., 180 °C) corresponding to its decomposition into I₂O₅. The nano-scale I₂O₅ particles that are created by thermal decomposition of I₄O₉ are likely amorphous and produce flame speeds over 1,000 m/sec when combined with Al powder (Table 1). The Al + I₄O₉ reaction produces flame speeds over 1,500 m/sec. This is the first study to explore the potential of I₄O₉ as an alternative to I₂O₅ for energy generation technologies, especially motivated by high iodine gas generation.

Figure 1. DSC Analysis of Heat Flow / TGA Analysis of Mass Loss. a) Heat flow behavior from DSC analysis of six oxidizers and shows different states of iodine(V) oxides used in the range of iodic acid dehydration. b) Mass loss from corresponding TG analysis.

Figure 2. DSC Analysis of Heat flow. a) Heat flow behavior from DSC analysis for Al + I₄O₉ dry mix and Al + I₄O₉, Al + nano I₂O₅ and Al + commercial I₂O₅ mixed in isopropanol. Temperature range includes iodic acid dehydration and main reaction temperature range. b) I₄O₉ initially and I₄O₉ exposed to 20% RH for 4 hours. Also, the I₂O₅ was exposed to 20% RH for 4 hours.

Table 1. Flame Speed Results. Flame speed results for Al + oxidizer indicated in first column. NM indicates not measurable. * Indicates I₄O₉ was decomposed into I₂O₅ during mixing.