Specific properties of fine SnO2 powders connected with surface segregation

The effect of surface segregation in Sb- and In-doped SnO2 fine-grained powders has been analyzed in comparison with single-crystalline samples. The kinetics and thermodynamics of the Sb and In segregation processes were studied as a function of annealing temperature by X-ray photoelectron spectroscopy (XPS) after annealing in an oxygen-containing atmosphere. Significant differences between diffusion and segregation were revealed for doped powders and single crystals, obviously because of simultaneous diffusion and particle-growth processes proceeding during annealing of powders. For doped single crystals the thermodynamic equilibrium is approached after 24 h annealing above 850 degrees C and at 1000 degrees C for Sb and In, respectively. Higher effective activation energies of diffusion are observed for doped powders and the thermodynamic equilibrium is not achieved under technologically relevant annealing conditions. On the basis of dopant profile measurements anomalies in the electrical resistivity at 300 degrees C of Sb-doped SnO2 powders annealed at 700 and 900 degrees C were attributed to an Sb-depleted zone formed beneath the segregated surface during the kinetic regime. To achieve optimum resistivity behavior for commercial application, inhomogeneous doping of powders must be avoided by appropriate preparation steps.