Reaction kinetics of nanometric aluminum and iodine pentoxide
- First Online:
- Cite this article as:
- Farley, C. & Pantoya, M. J Therm Anal Calorim (2010) 102: 609. doi:10.1007/s10973-010-0915-5
- 158 Views
Owing to increasing threats of biological attacks, new methods for the neutralization of spore-forming bacteria are currently being examined. Thermites may be an effective method to produce high-temperature reactions, and some compositions such as aluminum (Al) and iodine pentoxide (I2O5) also have biocidal properties. This study examines the thermal degradation behavior of I2O5 mixed with micron and nanometer scale aluminum (Al) particles. Differential scanning calorimetry (DSC) and thermogravimetric (TG) analyses were performed in an argon environment on both particle scales revealing a non-reaction for micron Al and a complex multistep reaction for the nanometer scale Al. Results show that upon I2O5 decomposition, iodine ion sorption into the alumina shell passivating Al particles is the rate-controlling step of the Al–I2O5 reaction. This pre-ignition reaction is unique to nano-Al mixtures and attributed to the significantly higher specific surface area of the nanometric Al particles which provide increased sites for I− sorption. A similar pre-ignition reaction had previously been observed with fluoride ions and the alumina shell passivating Al particles.
KeywordsBiocidal ReactionsHalogen decompositionAluminum combustionThermite decomposition
The increase of organized terrorist cells around the world poses a growing threat to the United States and many other countries. For these terrorist cells, chemical and biological weapons make highly effective terror weapons against civilians and weapons of intimidation against soldiers . While large scale chemical weapon production requires a large chemical plant, biological weapons can be produced in basements and hospitals around the world . Of the organisms that could cause enough disease and death to cripple a region, anthrax poses one of the greatest threats . Bioweapon attacks from agents such as anthrax would be difficult to predict, detect, or prevent . Therefore, complete elimination of the bacterial spore while in a storage bunker can effectively prevent great loss of life and psychological trauma induced from undergoing a terror attack. Popular methods for the destruction of spore-forming bacteria such as anthrax involve either ultraviolet radiation  or an oxidation agent such as peroxide . An assault on a bunker storing anthrax containers does not lend itself to a prolonged ultraviolet radiation exposure. Oxidation of anthrax spores is a slow process with necessary exposure times of up to an hour for effective neutralization .
This study examines the kinetics for nanometer and micron scale Al particles reacting with I2O5 in a thermal equilibrium experiment from 25 to 1000 °C and in an inert argon (Ar) environment.
Samples for each of the powders were prepared by suspending the powders in 60 cc of hexane and sonicating the mixture with a Misonix model S3000 for 70 s. In order to prevent damaging the oxide shell passivating the Al particles, the sonicator was programmed to cyclically mix for 10 s, and then to stop, allowing the mixture to cool for 10 s. The solutions were then placed in a glass tray under a fume hood to allow the hexane to evaporate. The powder mixtures were then reclaimed for further experimentation.
The thermal decomposition of each sample was studied with a Netzsch STA 409 differential scanning calorimeter and thermogravimetric analyzer (DSC/TGA). The system was programmed to heat the samples at a rate of 10 °C min−1 from room temperature to 1000 °C. Samples of masses of 7 mg were loaded into the sample crucible, and the DSC column was evacuated to less than 0.01 Pa using a Pfeiffer model TMU 071 P turbo molecular drag pump. The column was then backfilled with an argon atmosphere before a 50 mL min−1 flow of argon was applied to the furnace for the rest of the heating cycle.
Results and discussion
At 550 °C, mass loss in Fig. 2 continues again accompanied by a third endotherm. This second stage of mass loss and related endotherm may correspond to Al2O3 phase changes from amorphous Al2O3 to γ-Al2O3 . The alumina phase change may trigger a release of iodine gas resulting in a subsequent mass loss.
Examination of the TG curve reveals a similar region to Fig. 2 over 400–550 °C implying an interaction between the Al2O3 passivation shell and the iodine gas. Examination of the DSC curve over this temperature range reveals that the endotherm for the iodine release at 400 °C to the alumina phase change at 550 °C is partially masked by an exothermic Al–I–O reaction.
Finally, an endotherm can be seen at 660 °C where unreacted Al melts.
Differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis of I2O5, I2O5/Al, and I2O5/Al2O3 mixtures in argon show a scale-dependent reaction based on the size of the Al powder. While larger scale micron powders show little to no reactivity, nanometer scale-passivated Al powders undergo a complex multistep reaction when mixed with I2O5. The reaction commences upon I2O5 decomposition and is triggered by I− sorption into the Al2O3 passivation shell, further reactions between the iodine and oxygen gas and Al in the solid phase are then observed.
The authors gratefully acknowledge the support from the Defense Threat Reduction Agency (DTRA) on this project, and the interest and encouragement from our program manager, Dr. Suhithi Peiris.