Effect of chemical modifiers on the kinetic parameters characterizing the electrothermal atomization of chromium

The mechanism of atomization of chromium, in the absence and in the presence of chemical modifiers, in electrothermal graphite atomizers was investigated. This has been accomplished by the determination of the kinetic parameters (atomization energy, Ea, and order of release, x) characterizing its atomization. The tested chemical modifiers are Na2WO4, Mg(NO3)2, Ca(NO3)2, Rh and Pt. The dissipation process of the atomic vapor was also studied. The diffusion parameters are calculated in the absence and in the presence of chemical modifiers. In the absence of chemical modifiers, Cr atoms are produced in the gas phase by thermal desorption of adsorbed atoms, a first-order process. Pyrolysis temperatures (Tpyr)≥1300°C probably promote the formation of chromium carbide (Cr3C2) and therefore the atomization mechanism includes two steps: (i) the thermal decomposition of Cr3C2 and (ii) the thermal desorption of adsorbed Cr(ads). In the presence of Mg(NO3)2 and Ca(NO3)2 modifiers, atomization of Cr proceeds through a two-precursor mechanism, wherein the first step must include the thermal decomposition of its oxide. In the presence of Na2WO4, Rh and Pt, a single-precursor mechanism is observed. This should be a diffusion process through the bulk of the modifier. Platinum and Rh probably trap Cr atoms and interaction of Cr atoms with graphite must be precluded.