Hollow porous ADN microspheres fabricated via SFD-based microdroplet freezing templating: significant improvements in safety and combustion performance

Ammonium dinitramide (ADN), as a novel oxidizer characterized by high energy density, low signature, and environmental friendliness, exhibits significant application potential in composite solid propellants. However, its practical utilization is severely constrained by inherent properties such as high sensitivity and hygroscopicity. Existing modification approaches via additives often face the dilemma of lower energy density or lower oxygen balance. In this study, porous hollow ADN microspheres with particle size distributions ranging from 50-100 µm to 100-200 µm were successfully fabricated via a Spray freeze-drying (SFD)-based micro-droplet freezing templating method. This was achieved solely through the regulation of ADN solution concentration and spray process parameters, without the introduction of any additives. Structural characterization revealed that the ADN microspheres, compared to the raw material, exhibited a reduced single-crystal aspect ratio of ∼2:1. The modified ADN microspheres demonstrated significantly improved safety performance, evidenced by a rise in thermal explosion temperature (Tb, kinetically extrapolated from DSC data) to 205.35 °C from 198.15 °C, along with an increased impact sensitivity threshold (H₅₀) to 4.7 J from 3.9 J and a higher friction sensitivity threshold (P₅₀) to 127 N from 89 N. The critical relative humidity (CRH) was also raised to 52% from 50%. For PBT/Al-based composite solid propellants utilizing the 50-100 µm ADN microspheres as the oxidizer, burning rates under ambient pressure and at 3 MPa were measured at 3.66 mm/s and 51.6 mm/s, respectively. These values represent increases of 114% and 161%, respectively, over propellants formulated with raw ADN.