(Nanowerk Spotlight) Recent studies have found that nanomaterials – in this case dusts and powders having nanosize particles – exhibit an explosion severity which is not disproportionate to micrometer-sized materials, but the likelihood of explosion is quite high due to very low ignition energies and temperatures.
The review's authors note that research is required to gain a better understanding of nanomaterial ignition behavior, in particular, the effects of agglomeration on ignition and subsequent explosion development.
For their review, the authors looked at three measures of the likelihood of explosion occurrence: minimum explosible concentration of a dust; minimum ignition energy of a dust cloud; and minimum ignition temperature of a dust cloud.
In general as particle size decreases – and the specific surface area increases – it has been found that explosion severity will increase. Following this logic, it would be expected that nanomaterials would exhibit very high explosibility.
However, there are two physical processes that are believed would reduce explosion severity with nanosized particles: limited dispersibility and high coagulation rates, resulting in dusts where the effective size of particles will be greater than the particles' primary nanometer size.
The reason for this is that the dispersion of fine, cohesive powders into a cloud of individual particles is not possible without significant stresses to break interparticle bonds of agglomerates. After the incomplete dispersion, agglomerates will continue to form as a result of collision between particles.
While the minimum explosion concentration appears not to change significantly with reduced particle size, minimum ignition energy does. Experimentation with metallic nanopowders has shown that they can explode with energies less than 1 mJ, which is the lowest energy that can be tested using a MIKE3 apparatus (a common test apparatus for minimum ignition energy values).
MIKE3 test apparatus. (image taken from "Fire and explosion properties of nanopowders", Research Report RR782, UK Health and Safety Executive)
The reviewers write that this low MIE puts these nanopowders at a higher ignition risk than similar micrometer-sized dusts. The nanopowders could ignite as a result of electrostatic spark, collision or mechanical friction, and precautions should be taken to prevent such events.
Looking at the third measure of likelihood of explosion occurrence, minimum ignition temperature, researchers have found that it decreases with decreasing particle size – thus increasing the likelihood of explosions with nanosized particles over larger particles.
For instance, nanosized aluminum has been found to ignite at a rather low ignition temperature of approximately 900 K, which is below aluminum's melting point, as a result of the oxidation of the aluminum. By comparing the ignition of thermites prepared with nano- and micrometer-sized particles, researchers found that micron powder ignited at 610°C, while the nanopowders already ignited at 100°C.
The fact that nanomaterials have different properties than their respective micrometer-sized counterparts – due to very high specific surface areas and high reactivities – results in lower ignition and melting temperatures and faster burning rates. For aluminum nanoparticles, these changes become more significant at a particle size less than 10 nm.
The review points out that changes in the oxide shell at this size range, which are often overlooked, may also have an impact on particle combustion and ignition. Both the particle fuel and oxide size affect reaction, and decreasing either increases the reaction rate.
It is important to note that the combustion reaction of microsized particles is controlled by diffusion, whereas for nanosized particles the reaction is kinetically controlled. That means that the severity of nanomaterial explosions will not be controlled by the particle size but rather by the combustion of the pyrolysis gas/air mixture. For most organic materials, this transition from diffusion to kinetically controlled reactions occurs at approximately 10 µm.
The review concludes that nanomaterials present a dust explosion hazard, with metallic nanoparticles being particularly reactive. Nanomaterials have been shown to display lower ignition energy and temperature requirements than larger particles. Due to this high sensitivity, explosion hazards may exist for many processes including, but not limited to, mixing, grinding, drilling, sanding, and cleaning.