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25SHOCK-WAVE TREATMENT OF REACTIVE POWDERS
I. V. Saikov*", S. G. Vadchenko", I. D. Kovalev", and M. I. Alymov"
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of
Sciences, Chernogolovka, Moscow, 142432 Russia
*e-mail: revan.84@mail.ru
DOI: 10.24411/9999-0014A-2019-10142
At the moment, there is an increased interest in research aimed at identifying the main laws of the interaction of metals with a polytetrafluoroethylene (PTFE) polymer matrix under shock-wave loading of powder mixtures. Mixtures of PTFE with metals are widely used in pyrotechnics due to the high temperatures developed during their combustion [1-7]. At the same time, in some cases, a high density of energy-releasing composites is needed, which can be achieved using high-density metals such as tungsten as a filler [8-10]. Unlike well-studied reactions of PTFE with aluminum and magnesium, there is practically no information about the kinetics of the reaction of interaction with other metals, ignition, combustion, and detonation of mixtures with PTFE in open sources. Analysis of literature data shows that the main direction in the development of reaction materials is the search for activating additives (including aluminum and titanium, referred to in this work) and the definition of criteria for reliable initiation. In this work, an important aspect is additionally taken into account concerning not only the fact of shock-wave initiation, but also the features of the exothermic reaction (the ratio of liquid, solid and gaseous products), which strongly affect the characteristics of the reactive material.
The research aims to identify the main patterns of interaction of metal powders with a polymer matrix during combustion and shock-wave loading of the reactive materials [11, 12]. The features of ignition and gas evolution, as well as the conditions of shock-wave initiation were investigated. It is shown that the structure of the formed products and the amount of the melt depend on the heating rate, and in the samples pressed from mechanically activated powders, the amount of melt is less than from non-activated ones. Experiments have shown that compositions that form a large volume of gaseous products while simultaneously forming a melt are completely sprayed (W-Ti-PTFE), or form a highly porous structure. The heating rate has little effect on the ignition temperature, but it has a strong effect on the amount of melt produced: as the heating rate increases, the amount of the liquid phase increases. The high degree of homogeneity of the mixture, achieved by mechanical activation, also does not contribute to the consolidation of particles of combustion products. Since tungsten and aluminum practically do not contain impurity gases, gas evolution is mainly due to the destruction and gasification of the PTFE flowing in the combustion wave front. It is characteristic that the burning rate decreases with increasing sample density. This is due to the fact that with an increase in the density, the thermal conductivity of the sample increases, the destruction of the PTFE begins in the heating zone at a temperature of about 300°C and the released gases loosen the sample. This leads to instability of burning, which is observed in the form of a sequence of depressions and inflammations. The same factors lead to an anomalous dependence of the burning rate on the calculated adiabatic temperature - compositions with a higher combustion temperature burn at a lower speed. It is shown that there are two limiting values of the liquid/gaseous and liquid/solid product ratios, which determine the preservation of the product with a porous structure, or its expansion and deposition on the reactor parts. Saving the sample shape is impossible both in the presence of high concentrations of the liquid and gas phases in the combustion zone (composition 72W-18PTFE-10Al), and with a low
XV International Symposium on Self-Propagating High-Temperature Synthesis
concentration of the liquid phase and large gas evolution (composition 64W-16PTFE-20A1). In the first case, the gas not only loosens the heating zone, but also breaks the melt in the combustion zone. In the second case, the melt is not enough for sintering and the formation of a frame of the resulting products, and they are sprayed. In this case, the rate of gas evolution is not a defining characteristic. So the composition of 72W-18PTFE-10Al has a burning rate and gas evolution is less than that of other compositions, but at the same time there is a destruction of the sample and the spraying of products. Experiments on determination of gas evolution and burning rate of the reaction materials showed that with an increase in the concentration of aluminum in the mixture from 5 to 10 wt % the combustion mode, the structure of the resulting products, and the completeness of combustion change dramatically. Therefore, for practical use, the optimal concentration of aluminum, taking into account the scale factor, is about 5 wt % [12]. The essential role of "activating" additives (Al, Ti, B) in initiating, passing a chemical reaction in a powder mixture and the final phase formation was experimentally shown. Tungsten-based systems (W-Al-PTFE, W-Ti-PTFE, W-Hf -PTFE) proved to be difficult to initiate under shock-wave loading. In this case, the additive metal (Al, Hf) reacted quite actively with fluorine from Teflon to form fluorides AlF3 and HfF4. The most fully react are Ni-Al, Ni-Al-PTFE MA, Ti-B-PTFE, Hf-B-PTFE. It is important to note that the two systems Ti-B-PTFE and Hf-B-PTFE are most advantageous in terms of energy release, their adiabatic combustion temperatures are 2920°C and 3280°C, respectively. Thus, the systems based on metals (titanium and hafnium) with additions of boron and PTFE are the most promising from the point of view of the attained synthesis temperature, initiation by shock-wave action, and completeness of the reaction passage for the application of the reactive material.
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