Regulating the spherulitic evolution of 3-nitro-1,2,4-triazol-5-one via controlled supersaturation in cooling crystallization: growth mechanism and morphological consequences

Compared with jagged and rod-like raw products, the spherulitic 3-nitro-1,2,4-triazol-5-one (NTO) crystalline powder is generally superior in the formulation. However, the growth mechanism and corresponding influential factors for NTO spherulites remain to be deeply explored. Herein, the morphological evolutions of NTO crystals in alcohol solvents under a range of maximum supersaturation (S-max) are systematically investigated. It was found that differences in solvent-crystal interface interactions determine the primary morphology of the growing crystals while the magnitude of S-max determines the subsequent polycrystalline evolution. The cooling rate, initial concentration and stirring rate all could affect the crystal morphology by changing the S-max. It is demonstrated that there exists a critical S-max range (1.75-1.88) above which the formation of spherulites is kinetically favorable. Face indexing tests suggest that the (0 0 1) plane of NTO is mostly prone to twinning at low supersaturation, which is because that the molecular layers on the (0 0 1) surface are prone to twisting and the energy difference before and after is only 8.77 kJ mol(-1). Then, the interfacial growth theory is employed to explain how a growing crystal plane develops from smooth to rough with non-crystallographic sub-individuals as the crystallization driving force increases. The change of morphology has a positive consequence in improving the safety performance of NTO crystalline powder under drop-weight impact. These findings would forward the understanding of the spherulitic crystallization process for energetic crystals.