Processing Energetic Materials with Supercritical Fluid Precipitation Techniques
Open Access
- Author:
- Essel, Jonathan Thomas
- Graduate Program:
- Mechanical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 15, 2012
- Committee Members:
- Kenneth K Kuo, Dissertation Advisor/Co-Advisor
Kenneth K Kuo, Committee Chair/Co-Chair
James Hansell Adair, Committee Member
Richard A Yetter, Committee Member
Stefan Thynell, Committee Member
Adrianus C Van Duin, Committee Member - Keywords:
- energetic materials
supercritical fluids
nanotechnology
RESS process - Abstract:
- Research has shown that nano-sized particles of explosives have a reduced sensitivity to impact and shock. Nano-sized energetic particles have also shown promise in improving the performance of propellants and explosives. Therefore, a method to produce nano-sized explosive particles could be ideal for sensitivity and performance reasons. Supercritical fluid precipitation has been shown to produce nano-sized explosive particles effectively. This research explores the feasibility of processing energetic materials using three different supercritical fluid precipitation techniques. The first technique is called the Rapid Expansion of a Supercritical Solution (RESS). The RESS process dissolves a solute in a supercritical fluid and then rapidly expands the resulting solution through a nozzle to produce small (nano-sized) and uniform particles from a high degree of supersaturation. The second technique is the Rapid Expansion of a Supercritical Solution into a Liquid Solvent (RESOLV) Process. This process is similar to the RESS process except the supercritical solution is expanded into a liquid and dispersant solution to reduce particle agglomeration and to reduce the size of the particles further. The final technique investigated is the Rapid Expansion of a Supercritical Solution with a Nonsolute (RESS-N) process in which the precipitating solute is used to encapsulate or coat a nonsoluble substance by heterogeneous nucleation. This works takes both a theoretical an empirical approach. On the theoretical side, a numerical code that accounts for nucleation and condensation in the RESS process was written in FORTRAN to predict how altering pre-expansion pressures and pre-expansion temperatures in the RESS process could affect the final particle size of the produced RDX. It was determined that pre-expansion temperature had a marginal impact on final particle size but higher pre-expansion pressures were beneficial in forming smaller particles. Also, a software program called STABIL© was used to model nano-sized RDX particle interactions in a water and CO2 suspension that would be created in the RESOLV process. STABIL© showed that polymer coatings are absolutely necessary as particles will agglomerate within minutes if the coatings are not present. In addition, the software program CEA was used to predict how polymer-coated RDX particles produced through the RESOLV process would perform in rocket and combustion applications. For the empirical part of this work, the RESS process was used to produce nano-sized particles of cyclotrimethylenetrinitramine (RDX) and Bis(2,2,2-trinitroethyl)-3,6-diaminotetrazine (BTAT). Nano-sized particles of RDX were produced in the size range from 65±15 nm to 105±35 nm. Slightly smaller particles were observed for higher pre-expansion pressures. The RESS-produced RDX particles were also tested for their impact, friction, and electrostatic discharge (ESD) sensitivity. The RESS-produced RDX showed a dramatic decrease in impact sensitivity showing an increase in the 50% impact height from 17 cm to 44 cm. The friction and ESD sensitivity of the RESS-produced RDX was approximately the same as military grade RDX particles. BTAT particles were also produced using the RESS system. BTAT particles showed a dramatic decrease in particle size with a more optimal round morphology at higher pressures. The RESOLV process was used to produce well dispersed colloidal suspensions of PVP and PEI-coated RDX particles. The RESOLV-produced particles were as small as 30 nm and fairly stable against agglomeration for extended periods of time. Altered decomposition behavior was observed for RESOLV-produced RDX particles through analysis by differential scanning calorimetry and thermal gravimetric analysis. Enough PVP-coated particles were produced with the RESOLV process to send them to the Naval Air Warfare Center-Weapons Division (NAWC-WD) in China Lake, California. The PVP-coated particles showed no large differences in impact (19 cm 50% drop height instead of the standard 17 cm by ERL impact test) or ESD sensitivity (4 no-gos for NAWC ESD test) but showed a significant reduction towards initiation by friction (521 lbs threshold initiation level force instead of the standard 468 lbs by ABL friction test). The RESS-N process was used to coat nano-sized ALEX® aluminum particles with RDX. Coatings were observed on the ALEX® particles by field emission scanning electron microscopy (FE-SEM) and by energy dispersive X-ray spectroscopy (EDS). A method of injecting ALEX® particles into the RDX/CO2 supercritical solution was developed that was effective in coating small batches with RDX.