A protocol for measuring flame speeds of a reactive mixture composed of tetraiodine nonoxide (I4O9) and aluminum (Al) is presented. A method for resolving reaction kinetics using differential scanning calorimetry (DSC) is also presented. It was found that I4O9 is 150% more reactive than other iodine(V) oxides.
Tetraiodine nonoxide (I4O9) has been synthesized using a dry approach that combines elemental oxygen and iodine without the introduction of hydrated species. The synthesis approach inhibits the topochemical effect promoting rapid hydration when exposed to the relative humidity of ambient air. This stable, amorphous, nano-particle material was analyzed using differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) and showed an exothermic energy release at low temperature (i.e., 180 °C) for the transformation of I4O9 into I2O5. This additional exothermic energy release contributes to an increase in overall reactivity of I4O9 when dry mixed with nano-aluminum (Al) powder, resulting in a minimum of 150% increase in flame speed compared to Al + I2O5. This study shows that as an oxidizer, I4O9 has more reactive potential than other forms of iodine(V) oxide when combined with Al, especially if I4O9 can be passivated to inhibit absorption of water from its surrounding environment.
There are many iodine oxide compounds (e.g., HIO3, HI3O8, I2O5, I4O9) but the one most commonly studied for reaction with aluminum (Al) is diiodide pentoxide, I2O51–16. There are reasons for favoring I2O5 for combustion with Al: (1) I2O5 has an oxidation state of five which makes it a strong oxidizer for combustion applications; (2) I2O5 is semi-stable, depending on atmospheric conditions, and easily handled in powder form; and (3) I2O5 is relatively easy to produce and readily available.
Other forms of iodine oxide that have been studied are HIO3, HI3O8, and I4O9. When heated to low temperatures (i.e., 180 °C), I4O9 thermally decomposes into I2O56 as shown in Equation (1) and the decomposition reaction is exothermic.
(1) 5I4O9→9I2O5+I2
If I4O9 could be used in place of I2O5, the total energy liberated from reaction could increase due to the exothermic decomposition of I4O9 at temperatures below the Al ignition and reaction temperatures (i.e., < 660 °C) and below the dissociation temperature of I2O5 (i.e., about 350 °C). Also, I4O9 generates 8 wt.% more I2 gas when compared to I2O5 that could be used to neutralize biological agents. However, I4O9 has a higher molecular weight when compared to I2O5 and it is unknown if more energy is released per mass or per volume when using I4O9 compared to I2O5. Using I4O9 could provide a way to transport large quantities of solid iodine and upon ignition, release gaseous iodine. But, I4O9 powder is usually unstable. In fact, Wikjord et al.6 showed that over very short times I4O9 decomposed into I2O5 even with limited exposure to the atmosphere. This instability limits the usefulness of I4O9 as an oxidizer in combustion applications.
Iodic acids, such as HIO3 and HI3O8, form when I2O5 is exposed to water either from the relative humidity (RH) of the atmosphere or from immersion in a fluid1,3. For combustion applications, I2O5 is usually preferred over the hydrated iodic acids because evaporation of water upon combustion absorbs energy and reduces the overall heat produced. Despite the endothermic nature of this phase change, Smith et al.3 showed rapid evaporation of water during combustion of Al with I2O5 comprised partially of iodic acids produced significant gas generation that increased convective energy transport and produced higher flame speeds than Al + I2O5 alone. Specifically, mixtures with higher concentrations of iodic acids had up to 300% higher flame speeds than mixtures with lower concentrations of iodic acids. 3
The rate of absorption from atmospheric water is dependent on RH. There is a RH threshold where absorption begins and dependent on the hydration state2. Little et al. showed a RH threshold of 70% for HIO3 and a RH threshold of 40% for HI3O82. From this, it is assumed that the RH threshold increases with increasing hydration states. Because of its deliquescent properties, most studies that use I2O5 as an oxidizer are actually using I2O5 with significant concentrations of iodic acid2,5,7,17. However, the initial concentrations of the samples can be controlled by heating the samples above 210 °C until all of the hydrated species have dehydrated. This follows the hydration and dehydration mechanism of I2O5 shown in Selte et al.1 in Equation (2).
(2a) Hydration: 3I2O5+3H2O→2HI3O8+2H2O→6HIO3
(2b) Dehydration: 6HIO3→2HI3O8+2H2O→ 3I2O5+3H2O
The first step in dehydration happens at 110 °C when HIO3 dehydrates into HI3O8. The second step of dehydration happens at 210 °C when HI3O8 dehydrates into I2O5. Because the initial concentration of commercially available I2O5 is composed mostly of iodic acids, the absorption characteristics of pure I2O5 have not been studied thoroughly. It is assumed that the RH threshold and absorption rates are dependent on physical properties (i.e., particle size, crystal structure) along with initial hydration state and that an iodine oxide that is amorphous may have a lower RH threshold and increased hydration rates. Isolating iodine oxides from atmospheric water is needed to control the initial state of iodine oxide compounds. One method of isolating I2O5 from the atmosphere is blocking water absorption with coatings. For example, Little et al. was able to reduce the absorption rate and total amount of hydrated iodine oxides by sputter coating samples with Au/Pd2. Feng et al.8 passivated the surface of I2O5 particles with an Fe2O3 coating which prevented water absorption over long durations of exposure to ambient atmosphere. A similar approach could be applied to help stabilize I4O9.
Another way to improve the stability of I4O9 may be new approaches for its synthesis. If the material could be synthesized in a way that prevents introduction of hydrated species, then the topochemical effect that catalyzes water absorption could be inhibited thereby stabilizing the oxidizer. The I4O9 examined here was synthesized using a 'dry' process that does not introduce hydrated species and enables analysis of a more stable form of I4O9 powder. Our objective is to analyze the fundamental kinetics associated with I4O9 decomposition and reaction with Al as well as the basic energy propagation behavior of the Al + I4O9 reaction. Reaction kinetics are analyzed using thermal equilibrium diagnostics including differential scanning calorimetry and thermal gravimetric analysis (DSC-TGA). Energy propagation is analyzed using high speed imaging of reaction propagation through a powder mixture upon ignition in a transparent tube. Development of synthesis methods to produce I4O9 and methods to stabilize I4O9 have been slow in comparison to other forms of iodine oxide. A goal of this study is to show that the energy and gas liberated from reactions involving I4O9 are greater than reactions involving other iodine oxides. In this way, future research on synthesis and characterization of I4O9 may be beneficial for many applications.
The I4O9 powder studied here was synthesized using a "dry" approach to form I4O9 by combining elemental iodine and oxygen. This sample is referred to as I4O9. Nano-particle I2O5 was also synthesized for this study. Specifically, a portion of I4O9 was heated past the dissociation temperature of I4O9 (i.e., 180 °C) but under the dissociation temperature of I2O5 (400 °C). This process results in particles with a diameter between 200-400 nm. This sample is referred to as nano I2O5. Particle size measurements were obtained by TEM which requires the sample to be in a vacuum. However, I4O9 dissociates into I2O5 in a vacuum, so dimensions of I4O9 were not obtained directly. Because the nano I2O5 particle diameters are between 200-400 nm and synthesized by heating the I4O9 sample, it is assumed that I4O9 has similar diameters.
A more common approach to synthesizing I2O5 is by thermal dehydration of iodic acid to form I2O51,2,8 and material made using this process is commercially available. The commercial I2O5 is received as coarse crystals and can have different concentrations of iodic acids depending on storage and handling conditions. To ensure samples are pure I2O5, the samples are dehydrated prior to use as explained in step 2.1.1.3. The diameter of the particles in this sample are between 1-5µm. This sample is referred to as commercial I2O5.
The amorphous I2O5 sample is made from this saturated IO3 solution. When I2O5 is mixed with water, a solution of IO3 is created. This is done in step 2.1.2 and these steps will leave a saturated IO3 solution. Water catalyzes the formation of crystals in iodic acids. To form amorphous I2O5 the temperature must be above the dehydration temperature of HI3O8 and heated at a rate that will not allow a crystal structure to form, this is done in step 1.2. The concentration of IO3 in solution will determine the amount of amorphous I2O5 created during dehydration. These samples should turn red after dehydration indicating the sample is an amorphous form of I2O5. This sample is referred to as amorphous I2O5. Also, XRD analysis (not included) was preformed and confirmed the amorphous structure of the I4O9 and amorphous I2O5 samples.
When in solution, HIO3 will release excess water and create a crystal structure. The time needed to evaporate the excess water is dependent on the size of the beaker, RH, and concentration of the IO3 solution. In our lab at 20% RH mixing in the manner discussed above, 3-5 days were required to evaporate excess water from the samples. The solution will turn into a solid clear crystal. This process is shown in step 2.1.3 and the sample referred to as HIO3 dehydrate. Iodic acid will be referred to as commercial HIO3.
When exposed to solution or atmospheric water, iodine(V) oxide undergoes chemical reactions that change the composition of the final product. To mitigate this transformation, all six oxides are also mixed with Al without solution.
Thermal analysis using DSC-TGA was calibrated in an argon atmosphere using samples with known onset temperatures and mass losses. A flame tube apparatus known as a Bockmon Tube23 is used to measure flame speeds. Flame speed experiments are sensitive to the bulk density of the mixture. Pantoya et al. showed that for nano-Al based thermites, increasing bulk density can suppress the Al reaction mechanism and reduced the role of convective energy transport thereby retarding flame speed24. For this reason, experiments performed for different mixtures are usually designed to keep a constant bulk density. However, the physical and chemical properties of the oxidizers examined here vary dramatically such that it was not possible to obtain consistent bulk density with all six dry mixtures. Because of this, multiple iodine oxides with different physical and chemical properties are tested to provide a basis of comparison which includes differences in %TMD, crystal structure, and hydration states. After the powder has been placed inside the tubes and measured, hot wire is used to remotely ignite the mixture.
After the flame tubes are prepared with powder mixture, flame speeds are measured in a combustion chamber using a high-speed camera. The frame rate of the camera can be increased by lowering the resolution. Reducing the resolution to increase frame rate will produce less error than higher resolution at a slower frame rate. This is why, in step 4.2.2, the lowest resolution that can still image the entire flame tube is used, this will increase the maximum frames per second the camera can record without losing information. For our chamber, a resolution of 256 x 86 was used which allowed the camera to record at 300,000 fps.
Quantifying flame speeds in highly reactive mixtures is inherently difficult because of the large number of variables that can influence reactivity (i.e., mixture homogeneity, particle size, density, propagation direction, propagation velocity, etc.). By using a quartz tube with an inside diameter less than 4 mm in combination with a high speed camera with neutral density filters, the direction of propagation is controlled (i.e., 1-D) and the amount of light that is received by the camera can be reduced to a minimum threshold such that the leading edge of light emitted by the reaction can be seen and measured clearly. This measurement assumes that the progression of this low light level is at the same rate as the reaction front. For this reason, photodiodes may not be as accurate for tracking the reaction propagation because the high light intensity of emission may cause light to travel and saturate the sensors faster than the reaction. Also, the first 1-2 cm of tube length is considered an entrance region, or region of unsteady or accelerating propagation. Linear measurements of distance as a function of time must be taken beyond this entry region to determine steady state flame speed.
The DSC/TGA is a thermal equilibrium analysis that shows detailed reaction kinetics that cannot be observed in highly reactive materials (i.e., cannot be observed under non thermal equilibrium conditions). The combination of DSC/TGA analysis and flame speeds give specific information on differences in the reaction kinetics that may have implications for differences in the flame speed results. Because of this, the combination of these two measurement methods is a powerful tool for understanding and controlling highly reactive materials.
The authors have nothing to disclose.
The authors Smith and Pantoya are grateful for partial support from DTRA under award HDTRA1-15-1-0029; and, ARO (and Dr. Ralph Anthenien) under award W911NF-14-1-0250 and equipment grant W911NF-14-10417. The authors J. Parkey and M. Kesmez are grateful for support from DTRA under award HDTRA1-15-P-0037. Thank you to Dr. Douglas Allen Dalton for helpful discussion.
Iodine pentoxide (Commercial I2O5) | Sigma Aldrich | 229709 | Commercial I2O5 |
Iodic Acid (Commercial HIO3) | Alfa Aesar | A11925 | Comercial HIO3 |
Tetraiodine nonoxide (I4O9) | Lynntech Inc | synthesized using a dry process | |
Water | Local distilled water run through micron filter | ||
80 nm Aluminum | NovaCentrix | AL-80-P | Nano Aluminum |
Differential Scanning Calorimeter with Thermal Gravimetric Analyzer (DSC-TGA) | Netzsch | STA-449 | Equilibrium analysis of heat flow and mass loss |
Sonic Wand | Misonix | Sonicator 3000 | Provides ultrasonic waves to aid intermixing of reactant powders |
Phantom High Speed Camera | Vision Research | Phantom 2512 | High speed camera for visualzing flame front |
Mass Balance | Ohaus | Ohaus Explorer | Weigh powders with a 0.1mg resolution up to a 110 g capacity |