LLNL scientists are working to develop methods to optically ignite and initiate energetic materials. Optical ignition is highly desirable because: (1) the optical signal is immune to electromagnetic interference and varying conditions of pressure and temperature, and (2) the pulse delivery is not dependent on materials that might degrade over time. Ignition in an energetic materials target can be achieved by applying a thermal stimulus that causes local burning at a particular site. Initiation is achieved once the local burning accelerates into a violent reaction with the formation of a shock front and the rapid build-up to detonation.
The scientists' work demonstrates for the first time that an ignition and an initiation processes do occur for energetic materials in lax contact with carbon nanotubes that are prone to opto-thermal activity via a conventional flashbulb. The absorbing particles in the nanotubes are the natural hot sites of reactions, the counterparts of local zones of high temperatures known as hot spots, whose growth and interaction is essential for ignition and initiation of energetic materials.
|Figure 1. Burning of a 10 mg sample of SWNT placed on top of 10 mg of PETN upon irradiation with a common camera flashlight. Samples were placed 3 cm from the light source.|
The recent discoveries of ignition in single-walled carbon nanotubes (SWNTs) and polyaniline nanofibers after exposure to an ordinary photographic flash prompted us to explore a possible further instigation of energetic materials. Using a common electronic strobe of flash photography, we flashed several samples with various combinations of the explosive PETN (pentaerythritol tetranitrate) and commercially obtained SWNTs.
Ignition and burning proceeded with bright illumination and glow (see Figure 1), compared to a control sample of only SWNTs. Based on weight measurements, PETN was completely burned in all experiments, while the SWNTs lost about a third of its mass during the burning process. Further experiments demonstrated the transition from deflagration to actual detonation via the same procedure of optical triggering of the explosive K-6 (see Figure 2).
These experiments demonstrated for the first time the possibility of ignition
and initiation of energetic materials and nanotubes mixtures via conventional
optical triggering. Our results have important implications: whereas
optical initiation of energetic materials can be achieved with laser power
of GW/cm2 on materials samples of 0.5 mm in size, our reported results established
optical initiation with a light source of only several W/cm2 on a much larger
target area of materials. The implication is that energetic materials mixed
with optically active nanotubes have greater suitability for applications of
optical triggering devices: they represent new ideal candidates for safety
apparatus such as the firing of bolts on space shuttles rockets and aircraft
|Figure 2. (a) Assembly of copper cylinder and funnel containing K-6 explosive prior to addition of SWNT and irradiation with a flashbulb. We also measured the shock wave arrival time. This experiment was conducted in a specialty explosive tank at the High Explosives Application Facility (HEAF) at LLNL. (b) Assembly after detonation showing a destructed copper cylinder, funnel, and witness plate.|
Carbon nanotubes are not unique for opto-thermal activity. We are currently investigating the possibility to initiate pyrotechnic mixtures and deflagrations of energetic materials by the incorporation of gold-coated nanoshells that possess tuned resonant absorption properties at specific wavelengths. Nanoshells of silica, ~60 nanometers in diameters with a gold coating of 20 nanometers over the surface, provide a specific peak absorption of laser light (see Figure 3, right), which might make tailored remote optical triggering of energetic materials a reality in the near future.
Related Publications, Presentations, and Press
Manaa, M. R., A. R. Mitchell, R. G. Garza, P. F. Pagoria, and B. E. Watkins, “Flash Ignition and Initiation of Explosives-Nanotubes Mixture,” Journal of the American Chemical Society 127, 13786 (2005), UCRL-JRNL-212655.
"Optical Ignition of Nanotubes-Doped Energetic Materials," First Energy Nanotechnology International Conference. Cambridge, Massachusetts, USA, June, 2006.
"Explosive Tempers," Science News, October 22, 2005.
Science in Support of National Objectives
Optical ignition and initiation of energetic materials could thus far be only accomplished through lasers, with specific characteristics of high power, pulse length, wavelength, and a small target area. The radiation beam is usually either directly incident on the target explosive or on explosive containing absorbing particles placed within the matrix to create hot sites of reaction. This process greatly inhibits envisioned applications, due to the required high power laser source and the limited size of targeted area. Instead, formulations of energetic materials with opto-thermally active nano-materials are new ideal candidates for remote optical triggering of safety apparatus and for controlled burning of explosives as energy actuators, via the application of conventional, low power light source.