Relation between grain boundary segregation and plane orientation in niobium-doped titanium dioxide.

摘要

Grain Boundary (GB) segregation has been measured in 2mol%Nb-doped TiO2 (rutile) by Auger electron spectroscopy analyses of the two surfaces exposed by intergranular fracture. The GB composition over 166 fracture planes was found to display a strong anisotropy. The orientations of the exposed GB planes on both sides of the fracture surface have been determined by a stereo-pair method, implemented in a scanning electron microscope (SEM), in conjunction with orientation imaging microscopy (OIM). Measurements of composition and orientation on GB fracture surfaces have shown that high levels of segregation occur along (001)-(101) regions of GB plane orientation space, while low levels of segregation occur along (110)-(100) orientations. Furthermore, the matching halves of GB's planes with either high or low segregation also tend to be terminated by planes in the same respective regions of orientation space. This trend was subsequently supported by transmission electron microscopy (TEM) measurements on a high segregation GB, indicates there is an interaction between segregated atoms across a GB.; GB plane frequency distributions have been determined by OIM measurements on 80,700 GB traces in the undoped TiO2, and 81,400 GB traces in the 2mo1%Nb-doped TiO2. By comparing the GB plane frequency distribution with theoretical estimates of surface energy anisotropy of pure rutile, it was possible to show that the frequency of GB planes in undoped TiO2 is inversely correlated with GB energy, as approximated by the sum of the energies of the two GB terminating surfaces. Comparison of GB plane frequency with measurements of surface energy anisotropy obtained from the shapes of twin boundary grooves in 2mol%Nb-doped TiO2 demonstrates that the inverse correlation between GB plane frequency and energy is unmodified by GB segregation. When comparing the composition distribution in orientation space of Nb segregation at the 166 TiO2 GB planes, to the orientation distribution of the GB energy of TiO2, it is concluded that GB segregation is stronger at high energy GB's than at low energy GB's. This result is consistent with expectations from the Gibbs adsorption rationale and a GB segregation model, and indicates that GB energy anisotropy is decreased by GB segregation.