Mechanically activated self-propagating

Gras, Ch., Gaffet, E., Bernard, F., Chariot, F., and Niepce, J.-C., Mechanically activated self-propagating high temperature synthesis (MASH) applied to the MoSi2 and FeSi2 phase formation. 

Mechanically induced self-propagating reaction — ignition of self-propagating high-temperature synthesis (SHS) (see) by means of mechanochemical treatment, preceded with an activation period, that is, ignition time, during which particle and crystallite size reduction, mixing and defect formation take place. 

Solid-phase reactions are usually activated by high-temperature treatment [1-4]. However, the practical efficiency of this process is rather low since the diffusion rate through a product layer is small, no tight contacts between the particles of components are provided, a particle size distribution is non-uniform, etc. Therefore, the search for new methods of performance of solid-phase reactions is carried out. Some new methods are successfully used for these purposes. These methods include, for example, self-propagating high-temperature synthesis [5], shock waves [6,7], mechanical activation of mixtures in grinding apparatus [8-15]. The latter method becomes more widely used at present due to its relative simplicity and availability. 

A mechanism for self-promoted polyaddition with condensation can be formulated in the following general way, where M represents monomer and M an activated species capable of propagating or condensing. 

The mechanism of pitting is self-initiating and self-propagating. Pit initiation can result from a discontinuity in the film, an impurity, different phase, or a scratch on the surface. The active metal immersed in aerated sodium chloride solution dissolves in the pit, and the oxygen moves toward the pit. The positively charged iron cations attract the chloride anions in the pit. The resulting iron chloride hydrolyses and the sequence of the reactions in the pit are as follows ... 

More evidence has been accumulated [see e. g. ref. (55)] to show that the polymerisation yielding high molecular weight polypeptides proceeds in two steps — initial self-accelerated reaction followed by an apparently first order reaction. It seems that the growing species slowly reach their stationary concentration and in this period the reaction appears to be auto-catalytic. In the terms of Bamford s mechanism this behaviour is easily explained by postulating slow initiation and rapid propagation. The initiation results from an attack of an activated monomer on a non-aetivated NCA. The propagation results from a... 

In hydrocarbon solvents it is known that most of the growing chains are associated and it is necessary to enquire what effect this has on the copolymerization mechanism. The reactivity ratios measured from copolymer composition are unaffected because they refer to a common ion-pair. The equilibrium constants for association cancel and the reactivity ratios measured give a true measure of the relative propagation constants of the two monomers. No assessment can be made of the real reactivity of two types of active chain with the same monomer, however. In this case the observed rates are a function of the relative reactivities of the free ion-pairs and also of the relative extents of association. For example in hydrocarbon solvents polystyryllithium reacts with butadiene much more rapidly than does polybutadienyllithium. Until we know the two equilibrium constants for self-association we cannot find out if the increased rate is due to greater intrinsic reactivity or to a higher concentration of free polystyryllithium. In polar solvents or in hydrocarbon solvents in the presence of small amounts of ethers, these difficulties do not arise as self-association is no longer important. 

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