Seite - 179 - in Advanced Chemical Kinetics

Mukasyan [20] further studied these effectson thekinetics in theNi–Alsystemusing theETE approach.Theyshowed,byuseof theabove3Dreconstructiontechniques, that it ispossible to control theactivationenergybymodificationof thecontact surfaceareabetweenthereactants. Effectively, they lowered the effective activation energy from 79 to 137 kJ/mol, which corresponded to a change in the contact surface area/volume ratio between 0.0120 and 0.0032 nm 1, respectively; this relationship is shown in Figure 7. Additionally, it was suggested that, for SHS systems, the apparent activation energy is affected primarily by the contributions between the diffusion and intrinsic reaction activation energies. Additionally, theyofferedanexplanationfortherelationship,relatingtothedifferenceincontributionbetween thediffusive activation energies (volume, grain-boundary, and surface) in conjunctionwith the intrinsic activation energy. This suggests that the measured and reported activation energies presented in literature are effective, or apparent, activation energies that dependhighly on the structureandexperimentalconditions. 9.Modificationof thereactionmechanismdependingonthe experimental conditions Inaveryearlystudyonthe reactionmechanisms,Philpot etal. examined theeffectofvarying factorsonthereactionrate [35]. Inonestudy, theysystematicallyvariedthealuminumconcen- tration, the heating rate, and thenickel particle size. Briefly, they showed that, dependingon the applied heating rate, twodifferentmechanisms could be initiated. The first, when using slowerheatingrates,wasrelatedto themeltingof thealuminummetal, followedbyspreading over thenickelparticles.For their studies, theysawtwodefinitepeaksrelating to thereaction. Figure7. Dependenceof effectiveactivationenergyof the reactionasa functionspecific contact surfaceareabetweenNi andAlphases.Adapted fromShuckandMukasyan [20]. Kinetics of Heterogeneous Self-Propagating High-Temperature Reactions http://dx.doi.org/10.5772/intechopen.70560 179