Oxygen-diffusion limited metal combustions in Zr, Ti, and Fe foils: Time- and angle-resolved x-ray diffraction studies

Abstract

The transient phase and chemical transformations of diffusion controlled metal combustions in bulk Zr, Ti, and Fe foils have been investigated, in situ, using novel time- and angle-resolved x-ray diffraction (TARXD). The TARXD employs monochromatic synchrotron x-rays and a fast-rotating diffracted beam chopper resolving the diffraction image temporally in time-resolution of ∼45 μs along the azimuth on a 2D pixel array detector. The metal foil strips (10–25 μm in thickness) are ignited using a pulsed electrical heating with a typical heating rate of ∼106 K/s. The x-ray results indicate that the combustion occurs in molten metals, producing a wide range of stoichiometric solid oxides. It reflects an enhanced oxygen solubility and mobility of molten metals with respect to those of solid metals. However, the initial oxides formed are mainly oxygen-deficient metal oxides of ZrO, TiO, and FeO/Fe3O4 —the lowest suboxides stable at these high temperatures. These transition metal monoxides further react with unreacted molten metals, yielding the secondary products of Zr3O, Ti3O, and Ti2O — but not in FeO/Fe3O4. On the other hand, the higher stoichiometric oxides of ZrO2 and TiO2 are formed in the later time only on the metal surface. These results clearly indicate that the combustion process of metal strips is diffusion limited and strongly depends on the solubility and diffusivity of oxygen into molten metals. The time-resolved diffraction data reveals no evidence for metal oxidation in solids, but a series of temperature-induced polymorphic phase transitions. The dynamic thermal expansibility of Fe measured in the present fast heating experiments is similar to those in static conditions (3.3*10−5/K vs 3.5*10−5/K for α-Fe and 6.5*10−5/K versus 7.0*10−5/K for γ-Fe).