Dynamic Pressure-measuring Thermal Analysis Technique And Applications

The thermal stability and compatibility of energetic materials are directly related to the reliability of weapons, ammunition, space propulsion and delivery systems during their uses and storages, and the safety of relevant personnel and property. Therefore, a scientific and reliable thermal analysis technique is urgently needed to achieve the accurate evaluation of the thermal stability of energetic materials. In this paper, the Dynamic Pressure-measuring Thermal Analysis technique was developed and improved, and the method available for investigations of the thermal decomposition regularities and thermal performances of many types of energetic materials was also explored. The main work and achievements are summarized as follows:1) DPTA technique and method was developed based on the built-in pressure and temperature mini-sensors. After the correction via the standard water-loss of CaC2O4·H2O, it achieved the dynamic, real-time, direct measurement of pressure and temperature in a vacuum system. The pressure range is 0~70 kPa with the accuracy of ± 0.01 kPa, and the temperature range of 0~200 oC with the accuracy of ± 0.1 oC. The loading density is 5.0×10-4~1.0×10-1 g·cm-3. The data of decomposition, kinetics and mechanisms are calculated by the data-processing softwares. DPTA is very suitable for the initial thermal decomposition and accelerated aging measurement of energetic materials, and also provides a reliable technique for their stability and compatibility evaluation.2) DPTA was used to achieve the study of the thermal decomposition regularities of the volatile low-melting explosives, TNT, DNAN and DNTF. Through the combination of DPTA and isothermal TG methods, the temperature dependences of vapor pressures described by the Clausius–Clapeyron equations were determined. After deduction of the vapor pressure, the net gas volume from thermal decomposition was obtained, and the kinetic parameters and decomposition mechanisms were also calculated. DNAN has the best thermal stability, and DNTF is most sensitive to heat. Through the combination of DPTA and mass spectrometer, the gaseous products were analyzed. The gases from the decompositions of TNT and DNAN were H2 O, CO, N2, NO, CH2 O, N2 O, and CO2, while O2, CH3 OH, NO2 were released only at high temperatures. DNTF decomposed and released CO, N2, NO, N2 O, CO2, and NO2, but O2 at higher than 100 oC. Based on the experiments of DPTA and MS, their thermal decomposition mechanisms were theoretically calculated using Gaussian programs. The molten-decomposition of the low-melting explosives mainly took place in the gas phase. They released NO2 through the C-NO2 breaking, NO and CO through the-NO2 rearrangement, and had an equilibrium reaction of CO + NO2 ? CO2 + NO. Moreover, TNT and DNAN also generated H2 O and CH2 O through the α-H transfer and isomerization.3) The method of study the thermal decomposition regularities of the mixing energetic materials was established by DPTA technique. The effects of the additives and their contents on the thermal properties were explored. The gas-evolved volumes of five CMDB propellents with 20~60 % RDX were all less than 0.5 mL·g-1, indicating their good thermal stability. The dependence of stability on RDX content showed a parabolic curve. As the RDX content increased from 20 % to 50 %, the stability increased. But more than 50 % of RDX, the reactivity increased and the stability decreased. It was because that the interaction between RDX and double-base materials caused the accelerating autocatalysis. The stabilities of four aluminized explosives increased first and decreased afterwards with increasing Al content from 10 % to 40 %, and the explosive with 30 % Al had the best stability. A small amount of Al powder like an inert layer prevented the explosives from the decomposition. However, when Al was more than 30 %, the interaction of Al and RDX strengthened, forming the adsorption-catalysis center. The autocatalytic decomposition occurred and therefore the stability of explosives reduced. The stabilities of four RDX/Al/AP mixing explosives were in inverse proportion to the AP content, and the explosive with 10 % AP had the best stability. AP played a stronger catalysis on the thermal decomposition of RDX-based mixing explosives than Al. Because of the dissociation and strong oxidizability, AP improved the oxygen balance and catalyzed the thermal decomposition of RDX.4) The thermal decomposition regularities of the micro- and nano refined materials were studied using DPTA technique. The refined particle size effects on the thermal properties of energetic materials were obtained. As the particle sizes of TATB and HNS decreased into nano-size, the thermal sensitivity was significantly increased but the good thermal stability was still maintained. The pre-exponential factor A can be used to explain the reactivity of nanomaterials during the DPTA decomposition, that is, the larger value of A indicated the greater effective collision probability of the reactive molecules and the higher reactivity. For the multi-nitrophenol alkali metal salts, KPA, RbPA, CsPA and K2 TNR, Rb2 TNR, Cs2 TNR, their thermal sensitivity increased with decreasing particle size. As the particle size reduced, the specific surface area and surface energy increased, leading to the increasing reactive sites and reaction probability. The kinetic compensation effects between Ea and A indicated that the DPTA initial decomposition mechanisms were not affected by the refined grain size. TATB and HNS conformed to the "nitrobenzene" decomposition mechanism of the α-H attracting nitro-oxygen atom and N-O bond breaking, and the multi-nitrophenol salts followed the-NO2 â†'-ONO isomerization mechanism.5) The graphene-modified refined KPA, nanodiamond-modified RDX, polycrystal diamond-modified RDX composites were prepared and investigated by DPTA, exploring the effects of the modified materials on the thermal properties of the main energetic materials. Graphene has excellent thermal and electrical conductivity. As a modified material, it improved the thermal stability and reduced the thermal sensitivity of the refined KPA. The modified ultrafine diamonds catalyzed the thermal decomposition of RDX, and the highest catalytic efficiency corresponded to an optimal modification amount. When the amount of DND less than 1/5 or DPD less than 1/7, the diamond modification increased the surface activity of RDX and catalyzed the thermal decomposition. However, the excess modification hindered the thermal decomposition. The isoconversional kinetics proved that the thermal decomposition of the composite included an accelerated reaction stage which was caused by the autocatalysis of decomposition products and the modified catalysis of refined diamonds.