The overall goal of this procedure is to establish a method for characterizing reactivity of highly energetic composites. This method can help answer key questions in the combustion field such as how can we make an energetic mixture more reactive? The main advantage of this technique is that it directly and non-intrusively measures the reaction front.
That is, the location of maximum brightness. The implications of this technique extend towards other reactive systems as long as the reaction generates a self-propagating flame front. Generally individuals new to this method will struggle with packing flame tubes.
Packing density and total mass affect flame speeds, so proper technique is required for accurate and consistent data. First, obtain iodic acid, di-iodine pentoxide, trisynthesized tetraiodine nonoxide, and nanoparticle di-iodine pentoxide. Using a mortar and pestle, crush 10 grams of commercial di-iodine pentoxide crystals to a consistent powder.
Then spread the powder in a ceramic crucible. Heat the crucible at 10 degrees celsius per minute to 250 degrees celsius and hold at that temperature for five minutes to remove iodic acid. To prepare a sample of amorphous di-iodine pentoxide, place three grams of the dried di-iodine pentoxide powder in a glass beaker with a magnetic stirrer.
Add three grams of distilled water and stir the mixture for 20 minutes to form an aqueous iodate solution. Heat the solution in an oven to 250 degrees celsius at a rate of 20 degrees per minute and hold at that temperature for 10 minutes to obtain one gram of amorphous di-iodine pentoxide. Prepare another aqueous iodate solution.
Continue stirring the solution in a low humidity environment until excess water has evaporated, precipitating the iodic acid dehydrate. To ensure that all water has been eliminated, place a sample of the solid iodic acid in a differential scanning calorimeter and heat to 250 degrees celsius at 10 degrees per minute. If thermogravimetric analysis shows that the total mass loss in the temperature range below 210 degrees celsius is more than 5%excess water is present, and the sample needs more time to evaporate.
If the mass loss below 210 degrees celsius is significantly less than 5%di-iodine pentoxide is still present and the sample must be dissolved in water and stirred again. To prepare oxidizer fuel mixtures in a carrier fluid, first carefully mix the oxidizer with 80 nanometer aluminum in a beaker to achieve a final weight of two grams. Add 60 milliliters of isopropanol to the beaker.
Sonicate the mixture for two minutes. Pour the sonicated mixture into a glass dish and a fume hood with a 20%relative humidity atmosphere. Allow the solvent to evaporate for 24 hours.
Ground a razor blade with a conductive wire. Using the grounded razor blade, remove the dry mixture from the glass dish. Sieve the mixture into an airtight container.
To prepare dry mixed samples, first sieve and mix 80 nanometer aluminum and oxidizer to prepare a two-gram sample with the fuel oxygen equivalence ratio of one. Seal the mixture in an airtight container and place the container on a vibration table for three minutes to complete the mixing. In a differential scanning calorimeter with a thermogravimetric analyzer, heat alumina crucibles to 1500 degrees celsius for 30 minutes to remove any residues.
Next, weigh the crucibles and record the weights. Place 10 mg of each mixture into a crucible. Place the sample and reference crucibles on the DSC-TGA on the thermocouple, heat each sample at a rate of 10 degrees celsius per minute to 600 degrees in an argon atmosphere to perform the thermal equilibrium analysis.
To begin preparing the flame tubes for the energy propagation analysis cover one end of each quartz tube with electrical tape. Then weigh each tube. Using a small spatula, place each powder sample into tubes, gently tapping the tube on a hard surface to settle the powder after each addition.
For low bulk density samples use a narrow rod to pack down the powder. Tapping the tube after every scoop of powder settles the powder as it is added, making the packing density even throughout the tube. If the powder is unevenly packed the mass of sample in the tube will vary.
Prepare three tubes for each mixture. Add powder to each of the three tubes until the weights are within 5%of each other and the tubes are completely filled. Then, cut a nickel chromium wire into 10 centimeter long segments.
Bend the wires into a v shape. And insert the v of the wire into each tube. Tape the tube shut so that the wire is stationary and the powder is secure.
Prepare a combustion chamber with suitable viewing ports and an appropriate ventilation system routed through a fume hood. Set up a high speed camera perpendicular to the direction of flame propagation. Focus the camera on the sample tube, adjusting the lens and camera placement so the lowest resolution can be used.
Place a ruler in the camera field of view and take a snapshot for distance calibration. Reduce the exposure time and set up neutral density filters so that the flame image will not be overexposed. The amount of light that reaches the detector on the camera has to be reduced so that the leading edge of the reaction front is recorded and not the leading edge of the light.
Connect a voltage generator to the wires via the insulated leads and then seal the chamber. Set the voltage generator to 10 volts and start the high speed camera. Turn on the voltage generator to initiate the reaction.
After the reaction export the video into a program that tracks the position of the flame front as function of time. Plot a linear trend line from the distance and time information and remove data points collected before the reaction achieved steady state. The flame speed is the slope of the steady state trend line.
Four aluminum oxidizer mixtures were analyzed by differential scanning calorimetry. The dry mixture of aluminum and tetraiodine nonoxide showed exothermic behavior at 180 degrees celsius, which matches the exothermic behavior of unmixed tetraiodine nonoxide. The tetraiodine nonoxide and nano di-iodine pentoxide mixtures in carrier fluid showed almost identical heat flow behavior, suggesting that the wet mixing process converted tetraiodine nonoxide to a di-iodine pentoxide species.
Wet mixed di-iodine pentoxide samples showed higher flame speeds than dry mixed, indicating that use of a carrier fluid increased reactivity, significantly increased homogeneity was also observed in the commercial di-iodine pentoxide sample. However, the flame speed of tetraiodine nonoxide was much higher than wet mixed di-iodine pentoxide samples of both higher and lower bulk densities, indicating that tetraiodine nonoxide is more reactive than di-iodine pentoxide. While attempting this procedure, it's important to remember that these mixtures are impact, ESD, and temperature sensitive.
Wear PPE at all times, ground all metal equipment, isolate these mixtures from heat sources, and take care to not drop these mixtures. Following this procedure other methods like DSC-TGA can be performed in order to answer additional questions about how chemical kinetics influence macroscopic flame speeds and reactivity. After watching this video you should have a good understanding of how to measure reactivity of powder energetic materials.
Don't forget that working with energetic materials can be extremely hazardous and precautions such as limiting sample size to milligram quantities is advised.
