L-162 Sensitivity of Nanoscale Energetic Materials 2: Fuels, Pyrolants and Propellants

December 2009
Dr Ernst-Christian Koch (Energetic Materials), Dirk Schaffner (Trainee) Technical University of Kaiserslautern, Germany

Nanoscale aluminium powders are more easily ignited than micron sized powders and they are very sensitive to ESD ignition. Coating of aluminium nanoparticles with reactive or inert polymers (PTFE or PE) yields a significant desensitization against ESD ignition and still proves to leave thermal ignition largely unaffected.

Use of nano-size aluminium as fuel in pyrolants, such as thermites, coruscatives, and metal fluorocarbon systems causes an increase in ESD sensitivity and very often an increase in friction sensitivity as well.

Especially hazardous are thermitic systems comprised from a metal oxide such as MoO3, WO3, Fe2O3, CuO, or Bi2O3, and aluminium. The latter oxidizer is particularly hazardous and exhibits ESD-ignition energies in the range of only a few microjoules.  It proves that a large surface area (SSA) oxidizer is more sensitive to static discharges than a coarse material with small SSA.

While thermitic systems can be desensitized with respect to ESD ignition by the addition of fluoropolymers as binders, the addition of nitrocellulose as a binder has an adverse effect on sensitivity and leads to further sensitization.

Coruscative systems based on Ti, Al, and B generally display a higher sensitivity with large SSA nanomaterial.

Thermites based on nanosilicon and micrometric lead (II, IV) oxide display similar ESD sensitivity to composites using micrometric silicon. However, the friction force necessary to ignite thermites based on nanosilicon is two orders of magnitude smaller than with micrometric silicon, and is about the same level as found for PETN (5 N).

Thermites based on both nano-titanium and nano-ferric oxide are extremely sensitive towards ESD (fraction of a microjoule!). However, the least sensitive mixtures (which require four orders of magnitude higher ignition energy than the all-nano case) are obtained with the same nanometric iron oxide and micrometric titanium but still require only 1 mJ to ignite.

MTV modified with Viton®-coated nanoscale aluminium actually becomes less sensitive but there is an example of an odd outlier that results in an increased sensitivity of 5 mJ at high Viton® content and is incomprehensible.

Thermites based on nanometric copper (II) oxide and red phosphorus are highly sensitive towards ESD (200 µJ) and to friction (5 N), whereas comparable systems based on micrometric copper oxide are much less sensitive.

Composites made from nanodiamond and potassium chlorate are not particularly sensitive. Their sensitivity is mainly dominated by the chlorate anion. Thus, both friction and impact sensitivity are similar to saccharose/chlorate mixtures applied in smoke formulations.

The friction and impact sensitivity of an aluminized composite propellant increases upon substitution of nanoscale aluminium for the micronized aluminium.  At a given level of nanosized aluminium the addition of micronsized ferric oxide as a burning rate modifier further increases sensitivity.

The general sensitivity of sol-gel derived materials is less than that of the comparable physical mixtures of oxidizer and fuel.