Name: nanothermite with meringue-like morphology: from loose powder to ultra-porous objects
Uploaded: 2017-12-24T13:00:00
Description: Watch this Scientific Journal Video about Nanothermite with Meringue-like Morphology: From Loose Powder to Ultra-porous Objects at JoVE.com

The authors would like to thank the photographers of ISL, Yves Suma and Yannick Boehrer, for the photos of samples and for the observation by high speed video of the synthesis and the combustion of nanothermite foams. They also would like to express their gratitude to their colleague Dr. Vincent Pichot from NS3E laboratory for the characterization of the materials by X-ray diffraction.

CAUTION: Perform all reactions described in this article in an explosion-proven chamber with an armored window that allows both visual inspection and observation of the foaming/combustion processes by high speed video. Take care regarding the experimental risk arising from the potential ignition of aluminothermic compositions and the hydrogen explosion in the air. For this reason, always work in an explosion-proven chamber equipped with appropriate exhaust ventilation. Remember that experiments on energetic materials mus...

<ol>
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The mixing process of nanopowders with acid and the closing of the explosion chamber must be performed quickly, for safety reasons. The reaction delay may vary to some extent (1 - 10 min), depending on experimental conditions. It is shortened when room temperature is too high or in the presence of external heating sources like a spotlight, which can cause early activation of the foaming reaction. Conversely, it is increased when room temperature is low. In the case of too much foaming delay (&#62; 15 min), the reaction c...

This article reports on a method for transforming nano-sized aluminothermic mixtures (Al/WO3) from a loose powder state to foams1. Nanothermites are fast burning energetic compositions, which are most frequently prepared by the physical mixing of a metallic oxide/salt with a reducing metal, in the form of nanopowders2. The most representative oxides used to prepare nanothermites are Cr2O33,4, Fe2O35, MnO26, WO37, MoO38, CuO9 and Bi2O310,11, while the metallic salts used are perchlorates12,13, iodates14,15, periodates16, sulfates17 or persulfates18. Aluminum nanopowder is the best choice as fuel for nanothermites owing to their numerous desirable properties, such as a high oxidation heat (10 - 25 kJ/g)19, fast reaction kinetics20, low toxicity21, and a fair degree of stability once it has been accurately passivated22.
In Al-based nanothermites, the flame front propagates at high velocities (0.1 - 2.5 km/s), but this cannot, however, be considered as detonation23. The reaction mechanism is actually driven by the convection of hot gases in the porosity of unreacted material. In other words, the porosity is essential to the fast burning of nanothermites. However, loose nanothermite powder is not stable from a physical standpoint. They are compacted by shocks or vibrations, and their densest component (generally the oxide) progressively separates from the composition by the effect of gravity. The stabilization of nanothermite porosity is a crucial challenge for their integration in future pyrotechnic systems.
The main advantage of the preparation process described herein is to give highly porous, solid, nanothermite monoliths, which can be shaped by molding the paste from which they form. Additionally, nanothermite foams are quite insensitive to shock, friction and electrostatic discharge compared to nanothermite loose powders. This insensitivity makes them particularly safe to handle and machine, for instance by sawing or drilling.
When loose nanothermite powders are pressed or pelletized, their porosity decreases and objects are formed. The cohesion of such materials originates from the surface forces, which are responsible for the aggregation of nanoparticles. The mechanical strength of nanothermite pellets can be improved in the presence of carbon nano-fibers, which act as a framework to reinforce these objects24. Unfortunately, pressing strongly decreases the reactivity of nanothermites. According to Prentice et al., the pressing of nano-Al/nano-WO3 compositions induces a collapse of their reaction velocity by two orders of magnitude7. In conclusion, contrary to most explosives, nanothermites cannot be shaped by pressing.
To date, very few methods for structuring nanothermites have been reported in scientific literature dealing with nanothermites. Nanothermites can be deposited on substrates, either from the powders of their components dispersed into a liquid medium by electrophoresis25, or by the sputtering of their components in successive layers26. Both approaches lead to dense deposits, which are less reactive than loose powders and tend to delaminate from the substrate on which they are prepared.
The preparation of &#34;three-dimensional&#34; objects composed of nanothermite was proposed by Tillotson et al.5, who used the sol-gel synthesis developed by Gash et al. that consists of gelling solutions of metallic salts by epoxides27. Nanothermite monoliths are prepared by dispersing Al nanopowder in the sol, before gelling. The gels are subsequently dried either in a heat chamber to produce xerogels or by a complex process involving the use of supercritical CO2 to obtain aerogels. Nanothermite aerogels not only have strong reactivity but can also be machined owing to their excellent mechanical properties. In addition, the sol-gel process allows one to synthesize micro- and mesoporous materials with an unrivalled degree of homogeneity between the fuel (Al) and the oxide in the mix. Despite these interesting features, the use of the sol-gel process is limited by: (i) the complexity of the batch synthesis, which depends on numerous parameters; (ii) the unavoidable presence of synthesis byproducts (impurities) in the final material, and (iii) the very long time needed by the different steps of the process.
Combustible mats of nanothermite were prepared by the electrospinning of nitrocellulose (binder) from solutions charged with Al and CuO nanoparticles28. These nanothermite felts are composed of fibers with sub-micrometer scale diameters, which are a priori non-porous. In these materials, the porosity is defined by the entanglement of fibers. The samples of nanothermite mats burn slowly (0.06 - 1.06 m/s) compared to pure nano-sized Al/CuO mixtures in a loose powder state, in which the flame front propagates at a velocity of several hundred m/s29. Finally, the use of nitrocellulose as a binder for nanothermites is not ideal, because it considerably increases their thermal sensitivity and alters their long-term chemical stability.
Membranes of nanothermites were prepared by Yang et al. from complex hierarchical MnO2/SnO2 heterostructures mixed with Al nanoparticles6. In these materials, the oxide phase has a very specific morphology, in which MnO2 nano-wires are covered by SnO2 branches. Because of its very particular structure, the oxide not only traps Al nanoparticles, but also ensures the mechanical resistance of the membrane. The preparation process of MnO2/SnO2/Al membranes is very simple; it consists of filtering the nanothermite contained in the liquid in which it has been prepared, using the filtration cake as a membrane.
To summarize, the only nanothermite objects mentioned in the scientific literature are deposits on substrates, aerogels, or mats. The idea of preparing nanothermites in the form of solid foams opens new horizons for the integration of these energetic materials into functional pyrotechnics systems. The foaming process reported in this article is simple to perform and can be virtually applied to any nanothermite prepared from aluminum nanopowder. The foaming agent is orthophosphoric acid (H3PO4), a common, inexpensive and non-toxic chemical, which reacts with nano-Al to give the cement (AlPO4) and the gases (H2, H2O vapor) that create the porosity of the material1. Aluminum phosphate is particularly stable at high temperatures, contrary to organic binders such as energetic polymers (nitrocellulose). However, AlPO4 behaves as an oxidizer towards nano-Al at high temperature, according to the concept of &#34;negative explosives&#34; proposed by Shimizu30.

<table>
	<tbody>
		<tr>
			<td>Aluminum nanopowder</td>
			<td>Intrinsiq Materials</td>
			<td>-</td>
			<td>nanopowder, &#8776; 100 nm particle size Al QNA891</td>
		</tr>
		<tr>
			<td>Tungsten(VI) oxide</td>
			<td>Sigma-Aldrich</td>
			<td>550086-25G</td>
			<td>nanopowder, &#60;100 nm particle size (TEM) Lot# MKBR9903V</td>
		</tr>
		<tr>
			<td>Orthophosphoric Acid</td>
			<td>Fisher Scientific</td>
			<td>-</td>
			<td>85% solution</td>
		</tr>
		<tr>
			<td>polyethylene Pasteur pipette 3 mL</td>
			<td>Th. Geyer</td>
			<td>7691062</td>
			<td>LABSOLUTE Pasteur pipettes made of polyethylene (PE) graduation 0,50 ml, Length 145 mm</td>
		</tr>
		<tr>
			<td>polyethylene Pasteur pipette 1 mL</td>
			<td>Th. Geyer</td>
			<td>7691063</td>
			<td>LABSOLUTE Pasteur pipettes made of polyethylene (PE) graduation 0,25 ml, Length 150 mm</td>
		</tr>
		<tr>
			<td>Test tube shaker Reax Control</td>
			<td>Heidolph</td>
			<td>541-11000-00</td>
			<td>Vortex mixer with strong 5 mm vibration orbit yields</td>
		</tr>
	</tbody>
</table>

nanothermite with meringue-like morphology: from loose powder to ultra-porous objects

The goal of the protocol described in this article is to prepare aluminothermic compositions (nanothermites) in the form of porous, monolithic objects. Nanothermites are combustible materials made up of inorganic fuel and an oxidizer. In nanothermite foams, aluminum is the fuel and aluminum phosphate and tungsten trioxide are the oxidizing moieties. The highest flame propagation velocities (FPVs) in nanothermites are observed in loose powders and FPVs are strongly decreased by pelletizing nanothermite powders. From a physical standpoint, nanothermite loose powders are metastable systems. Their properties can be altered by unintentional compaction induced by shocks or vibrations or by the segregation of particles over time by settling phenomena, which originates from the density differences of their components. Moving from a powder to an object is the challenge that must be overcome to integrate nanothermites in pyrotechnic systems. Nanothermite objects must have both a high open porosity and good mechanical strength. Nanothermite foams meet both of these criteria, and they are prepared by dispersing a nano-sized aluminothermic mixture (Al/WO3) in orthophosphoric acid. The reaction of aluminum with the acid solution gives the AlPO4 &#34;cement&#34; in which Al and WO3 nanoparticles are embedded. In nanothermite foams, aluminum phosphate plays the dual role of binder and oxidizer. This method can be used with tungsten trioxide, which is not altered by the preparation process. It could probably be extended to some oxides, which are commonly used for the preparation of high performance nanothermites. The WO3-based nanothermite foams described in this article are particularly insensitive to impact and friction, which makes them far safer to handle than loose Al/WO3 powder. The fast combustion of these materials has interesting applications in pyrotechnic igniters. Their use in detonators as primers would require the incorporation of a secondary explosive in their composition.

The aluminophosphate matrix contains crystallized aluminum (Al) and aluminum phosphate (AlPO4). The presence of these phases was confirmed by X-ray diffraction (Figure 1). Additionally, gravimetric experiments have shown that this material also contains a non-crystalline part, which is amorphous alumina. In these materials, aluminum phosphate behaves both as binder and oxidizer. The oxidizing properties of AlPO4 were evidenced by measuri...

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Nanothermite with Meringue-like Morphology: From Loose Powder to Ultra-porous Objects

This manuscript describes the synthesis of combustible aluminophosphate matrices by the reaction of orthophosphoric acid (H3PO4) with aluminum nanopowder. When this reaction is carried out with excess aluminum in the presence of tungsten trioxide nanopowder, it leads to a solid, porous nanothermite foam.

The goal of the protocol described in this article is to prepare aluminothermic compositions (nanothermites) in the form of porous, monolithic objects. ...

The overall goal of this procedure is to describe the preparation of porous and solid aluminophosphate matrices or nanothermite foams by the reaction of aluminum nanopowder with orthophosphoric acid. This method can help answer key questions in the nanothermite field, such as how to stabilize a nanothermite loose powder into a solid, porous, monolithic, reactive object. The main advantage of this technique is that nanothermites are transformed into solid objects, which are easy to handle and do not contain free nanoparticles.Demonstrating the procedure will be Cedric Martin, a post-doc from my laboratory. To begin this procedure, first weigh out 3.00 grams of aluminum nanopowder. Using a three milliliter polyethylene Pasteur pipette, add 4.00 grams of an 85%commercial solution of orthophosphoric acid dropwise to a 150 milliliter beaker.Use a one milliliter polyethylene Pasteur pipette to add between zero and two milliliters of deionized water and slowly rotate the beaker by hand to homogenize the solution. Next, transfer the beaker to the explosion chamber. Add the pre-weighed aluminum nanopowder to the beaker.Use a stainless steel spatula to quickly mix the solution. Immediately close the explosion chamber. Then wait for the foaming reaction to occur.Once the reaction is complete, wait an additional 10 minutes for the aluminophosphate matrix to cool. After this, use a laboratory bow tong to remove the beaker from the explosion chamber. Carefully break the sample, which adheres to the beaker wall, to recover it.To begin, add 3.00 grams of aluminum nanopowder and 3.45 grams of tungsten trioxide nanopowder to a 100 milliliter round-bottom flask. Using a vortex mixer, mix the nanopowders at 2, 500 rpm. Put on an electrostatic discharge wrist strap, and ensure that you are properly grounded.Next, use a stainless steel spatula to gently stir the mixture. Then use the vortex mixer to further homogenize the nanothermite mixture at 2, 500 rpm. After this, use a three milliliter polyethylene Pasteur pipette to add 4.00 grams of orthophosphoric acid dropwise to a 150 milliliter beaker.Using a one milliliter polyethylene Pasteur pipette, add between zero and two milliliters of deionized water. Slowly rotate the beaker by hand to homogenize the solution. Then transfer the beaker to the explosion chamber.Add the prepared nanothermite mixture to the beaker. Next, use a stainless steel spatula to quickly mix the solution. After this, immediately close the explosion chamber.Once the foaming reaction is complete, wait an additional 10 minutes for the nanothermite foam to cool. Using a laboratory bow tong, remove the beaker from the explosion chamber. Carefully break the sample, which adheres to the beaker wall, to recover it.To begin, place either the prepared aluminophosphate matrix or the prepared nanothermite foam into the explosion chamber. Place a pyrotechnic igniter close to the chosen sample. Close the explosion chamber.After this, connect the igniter to a secure electronic device. Next, fire the pyrotechnic chain. Using an ultrafast camera operating at a speed between 10, 000 to 30, 000 frames per second, observe the combustion through the armored window.In this procedure, combustible aluminophosphate matrices are synthesized by the reaction between orthophosphoric acid and aluminum nanopowder. X-ray diffraction analysis confirms the presence of both crystallized aluminum and aluminum phosphate in the synthesized aluminophosphate matrix. Thermocouple data reveals that the temperature of each nanothermite paste during the foaming reaction is dependent on the concentration of orthophosphoric acid.The higher the concentration, the faster and higher the temperature will rise. In other words, diluting the acid solution with water decelerates the rise in temperature of the reaction medium. However, drying orthophosphoric acid with a strong desiccant such as phosphorus anhydride is not advised.Without water, the paste undergoes rapid heating, which can ignite of the energetic foam and cause a hydrogen explosion in the air. Nanothermite foams produced from these diluted solutions have greater mechanical strength but expand less. X-ray diffraction analysis reveals that these foams contain crystallized aluminum, aluminum phosphate, and tungsten trioxide, although tungsten trioxide does not chemically interact with the foaming reaction.While attempting this procedure, it's important to remember that aluminophosphate and nanothermite forms are energetic materials which burn violently when they are ignited. After its development, this technique paved the way for research in the field of energetic materials to explore the integration of nanothermite in pyrotechnic systems. Don't forget that working with energetic nanomaterials can be extremely hazardous, and precautions such as wearing appropriate protections and operating in pyrotechnic installations should always be taken while you're performing this procedure.

Research

JoVE Journal

Chemistry

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