**4.3 Sintering behavior and TCO microstructure**

Yanai and Nakamura (Yanai & Nakamura, 2003) found that SnO2 aggregates in the powder could negatively influence the sintering of ITO and could consequently result in higher porosity of the sintered body. In this case the walls of the pores are covered by SnO2 segregations. In case of finely distributed SnO2 segregations decreasing porosity is achieved. And finally in (Hsueh & Jonghe, 1984) it was agreed upon that inhomogeneous sintering takes place in case of heterogeneous density and stress distribution. The experimental evidence of these facts was given by measurements of heterogeneous stress distribution along the grain boundaries causing gradients of the chemical potential and acting as a driving force of the material transport.

Investigations of the microstructure of target materials processed at different processing conditions have shown that finely distributed metallic phases led to improved densification and significant recrystallization of the matrix during hot isostatic pressing.

This means that the liquid metal phase could further enhance the densification although under normal operation condition the wetting of indium oxide and tin oxide by the metallic phases of indium and tin is not observed (Schlott, et al., 1995).

The recrystallization leads to higher densities and and an essantial increase of fracture toughness (Falk, 2007, Falk, 2009) . This is unexpected too, since for ceramic materials a

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

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

Yanai and Nakamura (Yanai & Nakamura, 2003) found that SnO2 aggregates in the powder could negatively influence the sintering of ITO and could consequently result in higher porosity of the sintered body. In this case the walls of the pores are covered by SnO2 segregations. In case of finely distributed SnO2 segregations decreasing porosity is achieved. And finally in (Hsueh & Jonghe, 1984) it was agreed upon that inhomogeneous sintering takes place in case of heterogeneous density and stress distribution. The experimental evidence of these facts was given by measurements of heterogeneous stress distribution along the grain boundaries causing gradients of the chemical potential and acting as a

Investigations of the microstructure of target materials processed at different processing conditions have shown that finely distributed metallic phases led to improved densification

This means that the liquid metal phase could further enhance the densification although under normal operation condition the wetting of indium oxide and tin oxide by the metallic

The recrystallization leads to higher densities and and an essantial increase of fracture toughness (Falk, 2007, Falk, 2009) . This is unexpected too, since for ceramic materials a

and significant recrystallization of the matrix during hot isostatic pressing.

phases of indium and tin is not observed (Schlott, et al., 1995).

**4.2.4 Sintering with additives** 

cause the formation of nodules.

driving force of the material transport.

**4.3 Sintering behavior and TCO microstructure** 

al., 1997).

reduction of grain size leads to improved mechanical properties. Parallel to these findings it was also oberserved that the morphology of partially reduced powders have a major impact on densification and mechanical stability (Schlott, et al., 1995).

Studies of the fracture surface have shown that the intergranular bonding was much higher in recrystallized microstructures. Hereby the metallic phases take over a crack arresting function.

Good quality targets are attained at DOD values in the area of 0.02 to 0.2. If the degree of oxygen deficiency is too low metallic phases are rare and the positive influence of these phases on densification and mechanical strength is neglectable.

If the degree of oxygen deficiency is too high large area metal segregations act as microstructural failures and cause decreasing mechanical strength and worse densification (Schlott, et al., 1995). These authors processed targets of two different powders incoporting metal segregations of mean diameter between 1 µm to 10 µm and < 200 µm. Grain growth effects as a function of the seggregation size was detected and it was conclued that grain growth was especially pronounced in the case of large seggregated microstructures and consequently the maximum sputtering efficiency could not be achieved due to temporarily devating arc discharge at the surface of the target material.

In case of slightly reduced target materials especially tin segregation have been observed and in the event of substantially reduced targets the metallic phases are indentified as InSn alloys. The In-Sn eutectic phase with 48.3 at.-% Sn is melting at 120 °C according to (Shunk, 1969).
