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8XV International Symposium on Self-Propagating High-Temperature Synthesis
MA SHS AS A KINETIC PHENOMENON N. Lyakhov", T. Grigorieva", and M. Korchagin"
aInstitute of Solid State Chemistry and Mechanochemistry, Novosibirsk, Russia DOI: 10.24411/9999-0014A-2019-10089
Mechanical activation is a rather complex process, which is not easy to investigate and to describe correctly. It depends on the type of mills applied, the milling procedure and regime and of course on mechanical properties of solid substances, which are the subject of mechanical activation (MA). The result of milling is a particle size distribution shifted to smaller dimensions. The result of MA we are waiting for, is an increased reactivity of solid reagents. Evidently, we never can "detach" the MA, if it exists, from a simple comminution that result in any milling technology. Let us note that in case of MA SHS we consider the MA as pretreatment of a (potentially) reactive mixture of reagents, sometimes very short, which do not change the chemical composition of starting materials involved in combustion, if any. In addition, we have to take into account that mechanical properties of mixtures under milling are very far from individual phases. That means the result of a separate MA is never equal to the preliminary treatment of reagent's mixtures.
However, from kinetic point of view the mechanism of MA, which we understand as a highly increased reactivity, looks even more complicated! For reactions of the type A solid + B solid = (AB) solid, it is always assumed that a limiting stage is the diffusion of one, or both components through the newly formed interphase layer of (AB) product. There is no other way to activate this process but to continue the MA until the reaction is finished. We have plenty of such solid + solid reactions, which we call mechanochemical.
Nevertheless, in many cases of SHS (exothermic) reactions we observe a significant increase of reactivity both for the ignition and for the combustion itself after preliminary MA. Same conclusions we can make from the so-called "thermal explosion" mode, when the heat evaluation gaps shift to lower temperatures with the increased time of preliminary activation. It is to note that even for simple systems like Ni + Al or Ti + Al the phase composition of reaction product do not correspond to the initial stoichiometry until some critical time of preliminary MA is reached (multiphase reactions). That means the mechanical activation results in some unusual processes before the reaction starts. Of course, there are mixing and comminution due to plastic deformation of one or both reagents, but not only. Mechanical activation of immiscible couples of metals, at least, shows that the more plastic metal can disappear from X-ray diffraction detection after few minutes of MA. Such is the case of Bi + Fe, where no new product appears while Bi reflections totally disappear because of MA.
Though we cannot eliminate diffusion as a rate determining process for solid + solid reactions, some other (coupled) phenomena should be taken into consideration for SHS reactions, such as heat evaluation, wetting and spreading of reagents over the fresh surface generated by milling, and the particle size. The overall model of MA SHS should incorporate all processes that provide formation of new and powerful diffusion sours which is proportional to the large contact surface formed at the very beginning of preliminary MA. Our presentation is an attempt to analyze the most valuable MA SHS experiments in order to formulate non-contradictory interpretation of this very interesting kinetic phenomenon.
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