Page - 176 - in Advanced Chemical Kinetics

two classes of reactions. This is also true for the gasless systems investigated. This method providesawindowintodetermining themechanismand,due to thenatureof theexperiment, allows for control over the experimental conditions to thedegree that individual steps in the reactioncanbe isolated. 7.Modern insituhigh-speedhigh-resolutionmethods There currentlyexist anumberof techniques to study in situ reactionson the timeand length scales that occurduring SHS reactions; these techniques are incredibly valuable todetermine the reactionmechanisms. Themostwidespread technique is time-resolved X-ray diffraction (TRXRD),andisusedtodeterminethephasesthatarepresentduringthereaction. Itallowsfor information on the phases present at every stage of the reaction, depending on the time resolution. The lower the time resolution, the more information that can be attained. Dependingon the specific setup,whether synchrotronor laboratory-scale based, time resolu- tions ranging from 10 6 to 10 2 s are reasonable, with the absolute limit being continually improved with improved synchrotron and detector technology. There has been significant work done with SHS systems due to their solid nature, which is simple to use in TRXRD systems. It is possible tomeasure solid solution formations, intermediatephases, anymelting processes, and the general reaction progress. Through these data it should be possible to extract kinetic data on all reaction stages basedon the growth rates of the peaks for the new phase formationcoupledwith thedecompositionofpeaks fromthepreviousphase,however, therearecurrentlynoestablishedmodels illustrating this. There have been awide variety of experiments conducted on SHS systems by a number of differentgroups.For intermetallic systems,groupshavestudied theNi–Al [9, 110–116],Fe–Al [111, 117–121], Nb–Al [122–124], and numerous other systems [110, 125–127]. Additionally, manygroupshaveexaminedotherSHSbasedsystems, suchas carbides, includingTi–C[110, 128, 129], Ta–C [129, 130], and other carbides and cermets [129, 131–134], nitrides [135, 136], oxides [137–139], silicides, includingFe–Si [140,141],Mo–Si [119,123,142,143]andTi–Si [144, 145], amongavarietyofother systems [113, 129, 146–155]. Inaddition toTRXRD, there isavarietyofother, less common,but still veryuseful techniques available. For example, high-speed X-ray phase-contrast imaging [10] utilizes a synchrotron sourcecoupledwiththefact thatdifferentphasesabsorbX-raysdifferently todeterminewhich phase transformations occur during reaction, essentially high-speed X-ray phase contrast imaging.This techniquewas illustratedon theW-Si systemat theAdvancedProtonSource in ArgonneNationalLaboratory.Thismethodallowedfordirect imagingof irreversiblereactions in theW-Si reactivesystemat frameratesupto36,000 framespersecondwitha4-μsexposure timeandspatial resolutionof10μm.Anotheradvanced technique ishigh-speed transmission electron microscopy (HSTEM) [11], which utilizes all abilities of conventional TEM, but at nanosecond time scales. This allows fordirect observationof both the structural changes and crystal structure during the reaction with unprecedented resolution, as shown in Figure 5. Specifically, a high-time resolution dynamic transmission electronmicroscopy (DTEM)was developed in Lawrence Livermore National Laboratory (USA) and captures the material Advanced Chemical Kinetics176