Self-propagating reaction

Flanumbility limits (or explosion limits) for a flammable gas define tlie concentration range of a gas-air ini. ture witliin wliich an ignition source can start a self-propagating reaction. Tlie minimmn and maximmn fuel concentrations in air tliat will produce a self-sustaining reaction mider given conditions are called tlie lower Jlammability limit (LFL) and tlie upper Jlammability limit (UFL). (The abbreviations LEL and UEL, for lower and upper explosivity limits, are sometimes used.) The flanunability limits are functions of... 

When rx r2 0, self-propagation reactions are not taking place, M will add to M2 and M will add to M,. The formation of an alternating copolymers is expected. 

When rx> 1 and r2 < 1, the self-propagation reaction of type 11 and cross propagation reaction of type 21 are more probable. The copolymer formed will be richer in Mx. Similarly if r2 > 1 and r < 1, copolymer will be richer in M2. 

Fast reactions between pure metal powders (A1 + Ni, A1 + Ti and A1 + Ni + Ti) have been studied by Javel etal. (1997) by using time-resolved X-ray diffraction with the help of synchrotron radiation. The sample (20 X 10 X 2 to 3 mm3 was prepared under purified argon by cold pressing the metal powders mixed in the required proportion. It was then placed in a reaction chamber kept under He gas. A mylar window allows the incident and diffracted X-ray beams to pass in and out. Two small heating devices (tungsten coils on alumina supports) were included. The first one was used to keep the sample at a uniform temperature before ignition and the second one to start the self-propagating reaction at one end of the sample. X-rays irradiated the centre of the specimen. 

Figure 6.16. Metathesis synthesis of AIN. Temperature trends in the self-propagating reactions (Janes et al. 2003). (a) A1C13 + 0.5Ca3N2 (in an insulating ceramic cap) (b) A1C13 + 0.5Ca3N2 (in a steel vessel) (c) A1C13 + Li3N (d) 0.5A12S3 + Li3N.

For ignition to occur, a portion of the mixture must be heated to its ignition temperature, which is defined as the minimum temperature required for the initiation of a self-propagating reaction. Upon ignition, the reaction then proceeds on its own, in the absence of any additional energy input. 

In an earlier study the min concns of H202 in a vapor mixt below which powerful external ignition cannot initiate a self-propagating reaction were determined over the pressure ranges of 1 to 6.5 atm. These min concns were found to be 25.6 mole% at 1 atm (or a little lower than above) and 20.7 mole% in the 2-6 atm region (Ref 13)... 

According to the tests conducted at PicArsn, aliophany 1 azide is practically non-hygroscopic (gain in wt at 90% RH and RT 0.19% in 24 hrs). The 100° thermal stability test showed that die azide is appreciably unstable on prolonged heating (loss of wt 5.98% in 24 hrs and 7.15% in 48 hrs). The impact test with a 2 kg wt showed that the azide is insensitive. Contact with a hot wire did not cause the material to inflame to a self-propagating reaction (Ref 3i p 35)... 

S.Gordon, JPhysChem 60, 867-7l(1956)(Upon heating intimate mixts of-finely divided alkali nitrates with powdered Mg at temps greater than 480°, ignition ensues, accompanied by a highly exothermal self-propagating reaction) 21)D.W, Jensen, NAVORD Rept 6221(1958)(Pyrotechnic ammunition compositions and physical data)... 

Sata, N Sanada, N., Hirano, T., and Niino, M., Fabrication of a functionally gradient material by using a self-propagating reaction process. In Combustion and Plasma Synthesis of High-Temperature Materials (Z. A. Munir and J. B. Holt, eds.). VCH Publishers, New York, 1990b, p. 195. 

LiBFi LijN 1770-2100K Self-propagating reaction 28... 

---