Preparation and Reactivity of Gasless Nanostructured Energetic Materials

The overall goal of the following experiment is to prepare and characterize highly reactive gas, less nano structured energetic materials. This is achieved by high energy ball milling of nickel and aluminum powder mixtures to enhance their reactivity through formation of nickel aluminum composite particles with fresh oxygen free intimate surface contact between reactants. Next, high speed infrared imaging is used to determine combustion characteristics such as front propagation velocity and ignition temperature of the mechanically fabricated energetic materials.

Then field emission, SEM with a focused ion beam is used to analyze the microstructure of the energetic materials. This slice and view technique characterizes the microstructure of nano structured gas less energetic materials. The results of the SEM and high speed imaging showed that higher energy ball milling can be used to tailor micro structural and ignition characteristics of nano structured energetic materials.

The main advantages of this technique over existing mixing methods like ultrasonic mixing self-assembly approaches, so gels vapor deposition is that it enables preparation of energetic materials in a scalable and environmentally friendly manner. This method can help answer key questions in the announcement of reactivity of castless heterogeneous systems through the tuning of the material microstructure. This method can provide insight into the preparation and characterization of energetic materials based on gas, less reactive systems.

It can also be applied to other systems such as termites.Thermites. For this protocol, prepare a 35 gram mixture of nickel and aluminum in a one-to-one molar ratio. To define the intensity of the powder and milling interactions, use a five to one ball to powder charge ratio.

So for 35 grams of powder, prepare 175 grams of 10 millimeter balls made of the same material as the milling jar to be used. Then add the balls and powder to the jar. The jar must be harder than the powders being added.

Steel zirconium oxide, or tungsten carbide jars will all function adequately for this system. Then seal the jar and purge out the air by pumping in Argonne gas fill and purge the jar four times with Argonne, and then fill the jar with enough Argonne to be slightly above atmospheric pressure with the jar prepared, loaded into the planetary ball mill and set the RPM to six 50 for the jar and to 1400 for the internal rotation of the sun wheel. Sometimes the internal rotation can be varied to regulate the microstructure of the composite particles.

Now run the system for 15 minutes. Do not run the system for more than the critical time, which in this case is 17 minutes. Otherwise, unwanted reactions will occur once the jar has cooled to room temperature while wearing protective garments, vent the gas under a fume hood by removing the lid.

Let the jar air out for five minutes before collecting the powder to prevent spontaneous reactions. When collecting the powder from the jar, do not use a metallic spatula using thieves, sort the powder into various sizes. Use the particles between 20 and 53 micrometers for the remainder of this protocol, now press the Sieved powders into a pellet.

Using a uni axial press set to 1100 kilograms on a five millimeter stainless steel press die. Use a two minute dwell time and then take the physical measurements of the pellet provided the pellet is large enough. Set it up for combustion testing on a graphite plate with a tungsten wire attached to a transformer.

For this test, utilize a high speed infrared camera. To accurately measure the combustion wave velocity, focus the camera on the pellet from a perpendicular angle and start the recording. Then slowly turn on the transformer to heat the wire, which will initiate the combustion reaction.

Making a frame by frame analysis of the data plot the position of the reaction front propagation against time to get the average combustion velocity. To determine the combustion velocity, divide the sample height by the time it takes to completely propagate. Then plot the temperature changes at the middle area of the sample.

To define the ignition characteristics, make a thin disc from the sead powder and put the disc on a hot plate that is set to the desired temperature, such as 800 kelvin. When the hot plate settles at its temperature, focus the high speed camera on the pellet and start recording. The view of the camera should show the contact point between the pellet and the plate so that the first moment at which the temperature point on the pellet exceeds that of the plate is recorded in the analysis.

Define this as the reaction initiation point. To determine the ignition temperature plot, the temperature of the ignition point of the particle. The ignition temperature is the inflection point at which the temperature profile switches to that of a thermal explosion.

After preparing the sample of suspended particles for the scanning electron microscope, prepare it with a five minute plasma cleaning and then focus the electron beam on a single particle. Link the Z height to the working distance, and then raise the sample to the eccentric height. Now use the electron beam with the gas injection needle to deposit an initial 70 nanometer thick layer of platinum onto the sample.

This protects it from degradation from the gallium ion beam or I beam. Next, tilt the sample to 52 degrees and turn on the I beam. Set the I beam to five kilovolts and 0.28 nano amps.

Then utilize the gas injection needle to deposit another 0.5 micrometers of platinum onto the sample. After cutting fiduciary marks, use a program to slice the particle with the I-beam first open file, and then fill in the image save location to choose where the data will be stored. Next from the slice tab, make the following changes.

Set the width, length, and depth to mill through the entire particle. Then set the number of slices and the number of slices per image. Under the utilities tab, select the suggest currents option so that the program will select the appropriate beam current to mill the sample without damaging it.

Now, click show. The software then provides a visual of the milling grid. Double check the plan and then start milling the particle.

After each slice is made, take a high quality electron beam image of the slice for reconstruction. The parameters for the ebeam are found under setup and ebeam image scan parameters. These settings will give the grid select resolution and dwell time.

The higher the dwell time, the more time it will take to collect the image using the described procedure. Homogenous nano composite particles were compared to initial mixtures. The surface area between the reactants and composite particles significantly increased compared to that of the initial mixture.

After the high energy ball milling or HEBM, each component was incorporated into another component's matrix. In most cases, the obtained nano structured energetic composites are fully dense with high contact area between reactants. High energy ball milling effectively removed the protective oxide layer on the initial aluminum particles.

A dark field image of TEM analysis coupled with energy dispersive, x-ray spectroscopy of nickel aluminum composite particles clearly indicates that the new boundaries are oxygen free. HEBM enables the regulation of particle size by changing the rotation ratio of the sun wheel and milling jar abbreviated as K.In this mixture, the K is less than 1.5, and sliding of the balls and powder was observed. However, when K was increased to between 1.85 and 1.5, intense collisions of balls took place.

Such different HEBM regimes significantly influence the sizes of particles. Coarse particles are formed in the sliding regime, while much finer particles are prepared by the collision regime After its development. This technique paved the way for researchers in the field of energetic nanomaterials to explore the relationships between the microstructure and reactivity of high energy density composites.

After watching this video, you should have a good understanding of how to prepare energetic nano composites by high energy pole milling and determine their ignition and combustion characteristics. Don't forget that energetic nano materials can be extremely hazardous, and precautions such as personal protective equipment should always be taken while performing this procedure.