Disintegration in high grade titania slags: low temperature oxidation reactions and associated fracture mechanics of pseudobrookite

摘要

Cooling of solidified titania slag in air results in the oxidation of a pseudobrookite phase (M_3O_5 where M = Fe, Ti) to form phases that are associated with the disintegration of the solid slag. Extensive disintegration of the slag at temperatures below 773 K is accompanied by the formation of a disordered MO_x phase, which is a unit cell intergrowth of anatase (TiO_2), and M_3O_5. structural elements. In situ X-ray diffraction measurement and Rietveld refinement of the pseudobrookite unit cell parameters yielded values for spontaneous strain associated with the oxidation of the slag. In the most reactive samples oxidised isothermally at 473 K there is 0-6 percent contraction along the a-axis, a 0-3 percent expansion along the b-axis and the c-axis expands by 1 percent, while still retaining the pseudobrookite structure. Values for the elastic constants and Young's moduli for M_3O_5 were calculated using the GULP crystal optimisation code. These values, together with experimental values for the spontaneous strain tensor and the fracture toughness of M_3O_5 were used to calculate the critical crack size for crack propagation. Crack sizes of 0-9-9 /Mil are sufficiently large to explain the cracking that preceded the extensive formation of MO_x and the subsequent segmentation of the solid slag into very small particle sizes. Lattice energy calculations of the compounds anatase (TiO_2), FeTi_2O., and FeTi_2O_5 with GULP showed that the latter is unstable by 173 kJ moler~1 as compared to a mixture of anatase (TiO_2) and FeTi_5O-and will eventually revert to anatase (TiO_2) and M_3o_5.