Super-reactive nanoenergetic gas generators based on periodate salts.

Composite energetic materials are simple mixtures of the fuel and oxidizer (e.g., thermite). Although composite energetic materials usually have much higher energy density than monomolecular energetic materials such as 2,4,6-trinitrotoluene (TNT), nitrocellulose, cyclotrimethylenetrinitramine (RDX) etc., they suffer from slow rates of energy release, limited by the mass transfer rate between reactants. In large part, the idea of nanoenergetics is to promote intimate mixing between the fuel and oxidizer by decreasing the length scale. This relatively new class of energetic materials has been a topic of extensive research and has been investigated for applications involving gas generators, initiators, propellants, and explosives as well as propulsive power in micro-/nanoelectromechanical systems (MEMS/NEMS). In the most widely studied nanoenergetic formulations, nanoaluminum (aluminum nanoparticles) is employed as the fuel because of its high reaction enthalpy and ready availability, and metal oxide nanoparticles serve as oxidizers (e.g. Fe2O3, CuO, MoO3). [2,3] More recently, some other oxidizers, including KMnO4, [4a] I2O5, [4b] NaClO4, [4c,d] have been introduced into nanoenergetic formulations for their high oxygen content and strong oxidizing nature. These strong oxidizers also display very promising gas-generating behavior, however, most of them have a reduced shelf life compared to metal oxide nanoparticles, for reasons of light sensitivity or hygroscopicity. Recently, efforts have been made to encapsulate perchlorate salt nanoparticles with less reactive metal oxide layers as a moisture barrier. However, perchlorate salts, particularly potassium perchlorate (KClO4), have raised environmental and public health concerns during manufacture, transport, and applications, and have been targeted for elimination from many traditional pyrotechnic formulations. In a recent report, Moretti et al. introduced periodate salts as an alternative to perchlorate salts as pyrotechnic oxidizers because of their low toxicity and hygroscopicities. Their results show that periodate salt based formulations have good performance in illumination applications. The fabrication of periodate salt nanoparticles and their applications as gas generators, however, remains a challenge. Herein, we present an example of gas generators based on periodate salts as oxidizers in nanoenergetic formulations. A simple yet versatile aerosol spray drying approach was developed to produce periodate salt nanoparticles. The aerosol spray drying method is a promising method for the production of salt oxidizer nanoparticles with a high oxygen content and for the fabrication of salt nanoparticles that are not accessible by wet-chemistry methods. The prepared periodate salt nanoparticles were then tested as oxidizers in nanoenergetic formulations with nanoaluminum as the fuel. These periodate salt nanoparticles exhibit superior reactivity when evaluated as the oxidizers in nanoenergetic formulations, producing the highest reported gas pressure pulses. Fast heating scanning electron microscopy and temperature-jump mass spectrometry techniques were employed to probe the initiation/reaction mechanisms and provided direct evidence that gas phase oxygen release is responsible for the initiation of the periodate nanoenergetic formulations. The general pathway of preparing periodate salt nanoparticles by using an aerosol spray drying method is illustrated in Figure 1a (for details see the Supporting Information).