Thermal Expansion Behaviors Of Typical Layered Packing Explosives

Energy and safety are the two most important properties of explosives, which are effected by crystal packing directly. In recent years, many researchers have proofed that layered packing explosives possess not only high energy but also good safety, so the layered packing explosives have broad application prospects. Thermal expansion as one of the most common performance for materials, it will affect the application of materials. Explosives are influenced by environmental factors such as temperature significantly. Obtain the accurate data of thermal expansion properties of explosive are significant for the research of charge structure design, mechanical properties of simulation, the assessment of environmental adaptability and stored aging property.The main research works of this paper are:by means of in-situ X-ray powder diffraction and Retiveld structure refinement, the coefficients of thermal expansion of four layered packing explosives (LLM-105, TATB, FOX-7 and NQ) are obtained. The :emperature-dependent FT-IR spectroscopy was used to study the changes of the crystals structure of layered packing explosives under thermal stimulation, combing with the molecular packing style in crystal cell, we research the cause of thermal expansion characteristics of the four kinds of layered packing explosives. In addition, by mean of temperature-dependent FT-IR spectroscopy, the absorption bands of infrared of LLM-105 and FOX-7 were assigned in this paper. The primary results were as follows:(1)The coefficients of thermal expansion of LLM-105 were obtained by the means of in-situ X-ray powder diffraction and Retiveld structure refinement. The results show that the thermal expansion has obvious anisotropy, and the cell parameters change with temperature yield a second order polynomial, which means the second-order thermal expansion was happened. The results of temperature-dependent IR spectra indicated that the intermolecular hydrogen bonds network relaxed at the temperature-rise period, which cause the second-order thermal expansion.The change of IR spectra of LLM-105 with heating was investigated by temperature-dependent FT-IR spectroscopy. By statistics the peak positions of different vibration modes at different temperatures, results show that the intermolecular hydrogen bonding of LLM-105 is significantly weakened with the increasing temperature; the amino groups and nitro groups have participated in the formation of hydrogen bonds, and their peak positions shift 4 cm-1 at least. The pyrazine ring and C-N bond have not participated in the formation of hydrogen bonds, their peak positions shift 1 cm-1.(2)When the crystal of TATB suffer the thermal stimulation, the hydrogen bond network on a-b plane of crystal will relaxed, which make the a- and b-axis take place the second order thermal expansion. Because there is not hydrogen bond on the c-axis, so the thermal expansion coefficient of c-axis is constant. The results of emperature-dependent IR spectra indicated that the absorption peak of-NH2 blue shift about 15cm-1 during the temperature rose from 50℃ to 200℃, which means the hydrogen bond network in the crystal of TATB elaxed significantly in the heating process.(3)The a-,b-, and c-axis will take place the second order thermal expansion, which are due to the hydrogen bond network relaxed when the crystal of FOX-7 suffer the thermal stimulation. In addition, FOX-7 :ake place phase transition in the heating process, the results of in-situ XRD show that:the first phase transition (αâ†'β) happen at 113℃, the second phase transition (βâ†'γ) happen at 173℃; the incomplete phase transition (γâ†'α(+γ)) happen at 50～75℃ during the process of cooling. The restructuring of hydrogen bond network take place along with the phase transition. The absorption peaks of FOX-7 are assigned by means of temperature-dependent FT-IR spectroscopy, then combining with structure changes before and after the phase transition to identify α-FOX-7 and β-FOX-7.(4)The a-,b-, and c-axis will take place the second order thermal expansion, which are due to the hydrogen bond network relaxed when the crystal of NQ suffer the thermal stimulation, the sliding of layers driving by the directionality of some hydrogen bonds leaded to negative thermal expansion happened on b-axis.(5) Contrast of the average thermal expansion coefficients of four kinds of layered packing explosives, the results show that the degree of anisotropy thermal expansion are affect by the molecule packing style in crystal.