Finely powdered iron metal particles are pyrophoric as they can ignite and burn spontaneously upon direct contact with the air. The reaction is highly exothermic and self-sustainable, in which the iron particles react naturally with the oxygen in the atmosphere to form iron oxides, which is otherwise commonly known as the “rusting of steels”. The reactivity of the iron metal particles in the air has, however, been found to be largely a function of its particle sizes and the corresponding surface areas available to react with the oxygen in the atmosphere. Another important aspect of very fine iron particles, as with any nanosized metal powders, is its natural tendency to aggregate or sinter at elevated temperature to form larger particles resulting in dramatic loss of its pyrophoric properties.
Therefore, significant efforts have been made over past decades to segregate and stabilize nanosized iron particles on various substrates with large surface areas, such as metallic foils, non-combustible fiber meshes, activated carbons, zeolites, and, most recently, carbon nanotubes, etc. The resulting pyrophoric materials, in various forms, have been an important subject in a wide array of applications such as cathode materials for fuel cells, active agent in chemical sensing device, and catalysts for ammonia synthesis, liquid-phase hydrogenation reactions for fine chemicals, and groundwater remediation, etc. Once passivated slightly in controlled environments, certain pyrophoric iron materials have found applications as pigments for magnetic tapes and in medical practice for its bacteriostatic properties. In military, pyrophoric iron materials have long been considered as the primary pyrotechnic charges in pyrophoric penetrator, ammunition training round markers, and infrared aerial decoy devices against heat-seeking missiles.
Pyrophoric iron particles found on various substrates are well documented, but most of those substrates reported in the literature do not actually provide the kind of confinement down to nanometer scales to prevent aggregation and sintering of fine particles at elevated temperature, as is the case with metallic foils and fabric materials. The pyrophoric iron composite materials thus obtained are normally dusty in nature, and the loading of active iron ingredients and the thermal activation are considered extremely delicate processes. In the cases of substrates with highly porous structures, such as zeolites or activated carbons, the transportation of the precursor iron ingredients into the existing narrow channels, less than 0.7 nm or around 1.0 nm of zeolites or activated carbons respectively, is severely hindered, so is its interaction with oxygen in air as pyrophoric materials. For instance, Gash et al, U.S. Patent Application Publication No. 2010/0139823, discloses methods for creating pyrophoric materials by first heating a carbon foam to create micropores, then depositing liquid solution containing metal ions into the micropores. This results in overall lower loading of pyrophoric iron particles and limited reactivity, and this class of materials is reported mostly as catalysts rather than pyrophoric materials.
The present invention provides an alternative approach to make pyrophoric foam materials by disclosing a simple in-situ or one-pot process for making pyrophoric foam materials in which the precursor chemicals with potential to form pyrophoric metal particles and porous carbon materials, respectively, are uniformly mixed, with or without a solvent, to produce a paste, molded or casted into any desirable geometry. Further thermal treatments results in the formation of the pyrophoric foams with pyrophoric metal particles uniformly distributed in a highly microporous carbon matrix which are also pyrophoric in nature.