Jinjiang Xu

Crystal structure transformation and step-by-step thermal decomposition behavior of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate

The crystal structure transformation and step-by-step thermal decomposition behavior of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate (TKX-50) under thermal stimulation were studied and the whole process included thermal expansion, primary decomposition and secondary decomposition. The thermal expansion and primary decomposition of TKX-50 were studied using in situ powder X-ray diffraction (in situ XRD) together with Rietveld refinement, by which the crystal structure transformation process can be accurately traced. The results showed that the thermal expansion of TKX-50 was anisotropic. In particular, the a-axis exhibited a negative thermal expansion (NTE) that may be attributed to the distortion of the six-membered ring, which results in H-transfer between the cation and dianion. The crystal structure of the intermediate product after primary decomposition was also obtained. The secondary decomposition process was analysed using thermogravimetric-differential scanning calorimetry (TG-DSC) due to the safety risk of TKX-50 analyzed by in situ XRD. The crystal structure transformation process from TKX-50 to the intermediate product under heat stimulation was deduced. Meanwhile, the morphological change of the whole process was obtained using hot stage microscopy (HSM). Combined with TG-FTIR technology, the gaseous decomposition products at each step were analysed and the thermal decomposition mechanism of TKX-50 was proposed. This study further reveals the thermal decomposition behavior of TKX-50 and is helpful for better understanding the thermal decomposition mechanism.

Investigation on the Thermal Expansion of Four Polymorphs of Crystalline CL-20

The thermal expansion behaviors of α-CL-20 · 1/2H2O, anhydrous α-, β-, ε-, and γ-CL-20 crystals have been investigated by means of variable-temperature X-ray powder diffraction (XRD) together with Rietveld refinement. The results show that hexanitrohexaazaisowurtane (CL-20) with four polymorphs exhibits linear thermal expansion. The ε phase performs approximately isotropic expansion in the temperature range of 30 to 130°C, but α, β, and γ phases exhibit anisotropic expansion in the temperature ranges of 30 to 130°C, 30 to 120°C, and 30 to 180°C, respectively. The different expansion behaviors are due to the different structures of the four polymorphs. The different thermal expansion behaviors of α-CL-20 · 1/2H2O and anhydrous α are revealed in this work. The a-axis expansion of α-CL-20 · 1/2H2O exhibits a switch from positive thermal expansion (PTE) to negative thermal expansion (NTE) at 90°C, whereas the a-axis of anhydrous α is resilient to PTE. The cause is the loss of the structural water. Moreover, it is easily found that the b-axis of the γ phase shows a constriction that may be attributed to the distortion of the six-membered ring.

Three-dimensional hierarchical 2,2,4,4,6,6-hexanitrostilbene crystalline clusters prepared by controllable supramolecular assembly and deaggregation process

A facile chemical template method named supramolecular assembly and deaggregation was used to prepare three-dimensional (3D) hierarchical 2,2,4,4,6,6-hexanitrostilbene (HNS) crystalline clusters. Dioxane was used as the guest molecule for guidance in the assembly process. The 3D hierarchical HNS crystalline clusters possess multiple distributions of particle size and pore size, which results in significantly different safety performance compared to raw HNS. Furthermore, the formation mechanism of these unique structures was proposed based on the experimental results. This work provides a convenient method to prepare hierarchical energetic materials with porous micro/nano structures and other hierarchical functional materials such as catalysts, adsorbents, micro reactors and sensors.

The Temperature-Dependent Thermal Expansion of 2,6-Diamino-3,5-dinitropyrazine-1-oxide Effected by Hydrogen Bond Network Relaxation

The temperature-dependent thermal expansion of 2,6-diamino-3,5-dinitropyrazine-1-oxide (LLM-105) was investigated by using powder X-ray diffraction (PXRD) together with Rietveld refinement to estimate the dimension at a crystal lattice level. In the temperature range of 30–200°C, the coefficient of thermal expansion (CTE) of LLM-105 is temperature dependent, which is different from other explosives, such as hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), 2,2′,4,4′,6,6′-hexanitrostilbene (HNS) and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), with constant CTEs. The results of temperature-dependent infrared (IR) spectra indicated that the intermolecular hydrogen bond network relaxes with increasing temperature, which results in temperature-dependent thermal expansion. In this work, more accurate CTEs for LLM-105 crystals are obtained and the effects of the hydrogen bond network on the thermal expansion are further clarified. These results are beneficial to the design of materials with structural peculiarities and as-expected thermal expansion to satisfy different application requirements.