NORA Manufacturing Sector Strategic Goals
927ZJEXa - Explosivity and Flammability of Carbon Nanotubes (NTRC)Start Date: 10/1/2009
End Date: 9/30/2013
Principal Investigator (PI)Name: Leonid Turkevich
Funded By: NIOSH
Primary Goals Addressed10.09.0
Secondary Goal Addressed
Attributed to Manufacturing
NIOSH is undertaking an initial assessment of the explosion hazards of carbonaceous nanoparticles—in particular, carbon nanotubes. Explosive dust clouds can be generated from most organic materials. Dust explosions involving 100–102 ?m particles have been extensively studied. Ignition sensitivity and explosion violence of a dust cloud depend strongly on the particle size, primarily via specific surface area. The violence of the dust explosion and the ease of ignition generally increase as the particle size decreases. Particle explosive characteristics for carbon nanotubes will be related to material oxidation parameters.
This project will measure explosion parameters of several identified carbon nanomaterials: carbon nanofibers (CNFs), multiwalled carbon nanotubes (MWCNTs), single-walled carbon nanotubes (SWCNTs). Such measurements have not been previously made. Explosion experiments will be carried out at PRL in a 20L chamber that has been utilized extensively to characterize the explosion characteristics of coal dust. These explosion parameter measurements will be benchmarked against other carbon particulate systems (e.g. carbon black) and correlated with specific surface area. Parameters to be measured include MIE, LEL, maximum explosion pressure (Pmax), violence explosion index (Kst = dP/dt V1/3). In parallel, we will monitor the flammability and onset of combustion of these materials via controlled atmosphere temperature studies at DART. Such studies have been useful in characterizing diesel particulates.
SWCNTs are susceptible to photoignition [Ajayan et al., 2002], which phenomenon poses a novel hazard. We aim to quantify the response to intensity and frequency of radiation. Ajayan et al. speculate that the initial explosion is generated by heating of the internal oxygen, producing a shock wave, and then initiating combustion. The explosion occurs because the black carbon nanotubes efficiently absorb visible light yet cannot sufficiently dissipate the converted heat. However, this mechanism should also obtain for MWCNTs, which do not photoignite.
These de novo measurements will represent a significant step towards understanding the potential explosivity and flammability of these materials, which understanding is needed if adequate precautions are to be taken in order to safely manufacture and process high volumes of these materials. The goals of this project are limited, with high likelihood of success.
Literature citations of the publications produced by this project will substantiate the project's usefulness to the scientific community. Application of the research findings by other government agencies will be substantiated by discussion of the results in policy documents. Explosivity hazards discovered by this project will be communicated to material vendors and to external testing laboratories.
Under certain conditions, engineered nanomaterials may pose a dust explosion hazard and spontaneously ignite when exposed to air, due to their large surface area. Little is known about the dust explosion hazard for the general class of nanoparticles (carbon, metal and metal oxide).
Explosive dust clouds can be generated from most organic materials. Dust explosions involving 100–102 ?m particles have been extensively studied [Eckhoff 2003]. Ignition sensitivity and explosion violence of a dust cloud depend strongly on the particle size, primarily via specific surface area. The violence of the dust explosion and the ease of ignition generally increase as the particle size decreases, although for many dusts (e.g. coals, flour, methylcellulose, polyethylene), these trends plateau for particle sizes < 10 ?m.
This project is limited to examining the explosivity and flammability of fibrous carbon nanoparticles.
NIOSH is undertaking an initial assessment of the explosion hazards of carbonaceous nanoparticles—in particular, carbon nanotubes. Carbon nanotube explosivity will be related to ease of aerosolisation (‘dustiness’) and to material oxidation and thermal decomposition characteristics, as measured by thermogravimetric analysis (TGA).
The lower explosion limit (LEL) is the lowest dust concentration at which an ignition can be achieved. The LEL also exhibits a plateau with particle size (typically at 20-50 ?m for coals). The minimum ignition energy (MIE), the minimum energy required to ignite the dust cloud, also strongly depends on particle size, with no obvious plateau at small particle sizes; typical dust cloud MIE are several orders of magnitude larger than MIE for hydrocarbon/air mixtures.
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