**4.2.4 Sintering with additives**

Vojnovich and co-workers (Vojnovich & Bratton, 1975) investigated the influence of imputities in terms of densification rates of In2O3 powders. Combinations of the impurities Si, Ca, Mg, Pb and Fe as well as the doping with Kaolin, Al2O3 and SiO2 have been experimentally studied. It was found that the impurities forming liquid phases due to eutectic reactions even contribute to increased densification rates as for example by doping with Ca, Si and Mg as well as Kaolin. Impurities leading to low melting eutectic phase formations favour liquid phase sintering conditions and resulting in higher density values (Vojnovich & Bratton, 1975). It was found that TiO2 doping causes an increase of sintering density and limits grain growth. From a concentration of 0.5 wt.-% TiO2 as sintering additive the phase In2TiO5 is precipitated at the grain boundary resulting in increased grain boundary diffusion at reduced diffusion activation energies (Nadaud, et al., 1997).

The influence of Si and Zr impurities on ITO sintering was ivestigated in (Nadaud, et al., 1994). These studies have been motivated by the successful ITO-doping with tetravalent Ti (Nadaud, et al., 1997) and the attempt to achieve even better results with alternative tetravalent doping additives. It came out that a sigificant influence of SiO2 on ITO densification was not observed. Zirconia, however, has a negative influence on densification and increase specific electrical resistance of ITO ceramics at room temperature. The approach to incorporate sintering additives in order to increase densification is being questioned in (Schlott, et al., 1996), since it was found that imputities, i.e. TiO2, presented as inclusions or segregations in the microsructure, could cause the formation of nodules.
