Seite - 170 - in Advanced Chemical Kinetics

transfer andchemical reactions, it is necessary to consider the specifics of heat transfermech- anisms [5].Typically, theactivationparametersareobtainedafterexperiments considering the dependences of timevs. temperature (for isothermalmeasurements), temperature vs. heating rate (for integral and incrementalmethodswith linearheating rates), or fromreaction ratevs. temperature.Consideringtheabove limitationsandcomplications,only theeffectiveorappar- entactivationenergycantrulybeconsidered,as it includesboththeintrinsickineticsaswellas processesofheatandmass transport. 2.Techniques forstudyingSHSkinetics Thetaskofaccuratelydeterminingkineticsbecomesevenmorecomplicatedwhenaccountingfor theextremelyhightemperaturesofSHSprocesses(>1800K)andrapidheatingrates(103–105K/s). Such parameters are essentially impossible to achieve using conventional approaches formea- surementofkineticsparameters.WhilestandardnonisothermalTGA/DTA-basedapproaches [6] are still used to evaluate thekinetics of SHS reactions, several uniquemethods such as electro- thermal explosion (ETE) [7] andelectrothermography (ET) [8]were specificallydeveloped to fit the experimental conditions of SHS reactions.Moreover, recently a variety of advanced in situ diagnostics, including time-resolved X-ray diffraction (TRXRD) [9], high-speed X-ray phase- contrast imaging [10], and high speed transmission electron microscopy (HSTEM) [11] were modifiedtoobtainthekineticsofphasetransformationsduringSHSreactions. 3.Electrothermalexplosion The ETEmethodwas developed in 1977 to study the rapid, high-temperature kinetics that occur in SHS systems [7]. It relies on rapid, uniformpreheating of the sample until adiabatic thermalexplosionoccurs.Arepresentationofa typicalETEsetupisshowninFigure1.Briefly, the sample is clamped between twometallic electrodeswith sufficient clamping pressure to ensureadequatecontact.Thepower is then initiated, leading topreheatingof thesampleuntil a set Toff point. After initiation, the resulting time-temperature profiles are simultaneously collectedacrossanumberofhigh-speedphotodiodes. In the commonlyusedETA-100 system (Aloft, Inc., Berkley,CA), there are 16photodiodespresent,with 1mminbetween them; this corresponds to 0.5 mm spatial resolution. The photodiodes have a temporal resolution of 10 5 sandareaccuratewithin900–3000K.Once thesample isheatedto theselectedToffpoint, the equipment heating is halted, with the consequent rate of self-heating determined solely by the chemical reaction rate. Due to the experimental conditions, i.e., the rapid initial preheating, the reaction occurs in the adiabaticmode.Once thermal ignition occurs, analysis of the time-temperatureprofileenablesextractionof thekineticparameters (seedetails in [12]). This technique can be used to study the kinetics at temperatures much higher than can be achieved inother experiments.However, it is often limited in the systems that canbe studied due to thestringentheatingconditions causedby Joulepreheating. ETE has been used for different gasless SHS systems. These studies have provided valuable kinetic data in extremely high-temperature ranges that are essentially inaccessible by other Advanced Chemical Kinetics170