**4.2.1 Thermal vacuum degassing**

598 Sintering of Ceramics – New Emerging Techniques

the discontinuous oxygen release. The related phases exhibit similar chemical composition and resemble the CaF2 structure. The phases are devided in coherent domaines and the surface energy could have a major impact on the *Gibb's free energy.* This means that chemical hysteresis appears and the oxygen concentration can not be determined explicitly as a function of the temperature and the oxygen partial pressure (Otsuka-Matsua-Yao, et al.,

But there is evidence by experimental investigations that several microphases could act as barriers thereby preventing the oxygen release. These microstructures could originate from extended defects, these are sheared structures or domaines, which diverge from each other in therms of chemical compositions on a very small scale. However the exact analysis and qualitiative and quantitative identification of these structures by X-ray diffraction is not possible since they are charcterized by the structure of the rare earth elements, as for example In4SnO8, In4Sn2O10, In2O3 etc. The oxygen release could therefore depend on the interfacial energy between these microphases. In sputtering processes partially reduced indium-tin-oxide target materials are preferred in order to achieve thin film characteristics of minimum specific electric resistance and maximum sputtering efficiencies. Partially reduced targets reveal

The reduction of the powder or sintered body can be achieved by several methods, i.e. sintering in vacuum or reducing atmospheres (H2, CO, H2-Ar oder H2-N2). Alternatively the specimen can be hot pressed in graphite moulds or carbon releasing materials or the carbon releasing materials can be doped with the specimen (Weigert, et al., 1992). However under these reducing conditions the caculated oxygenn stoichiometry can not be accurately controlled. Target materials with reproducible and uniform degree of oxygen deficiency are preferred, since they guarantee superior sputtering efficiencies and reduced operation and

Furthermore the adding of reducing agents and ingredients causes normally the formation of metal particles of different sizes considerably larger than 50 µm (Schlott, et al., 1995). The microstructure appears spotty and targets processed by the powder qualities reveal lower fracture toughness characteristics. From this reason the target specimen shall be reduced below 1000 °C and preferably even below 800 °C. The subsequent densification of the powder and/or the sintered body is realized usually by hot isostatic pressing at temperatures above the melting point of the metal indium-tin-phases (Schlott, et al., 1995).

The hydrophilic characteristics of In2O3 should be noted. The water uptake from the surrounding atmosphere leads to the transformation of In2O3 to In(OH)3. The water

Ceramic ITO target materials show low thermal conductivities compared to the metallic target materials and are therefore very susceptible to thermal stresses resulting form nonuniform heating during the sputtering process. It is therefore preferred to increase the thermal conductivity within certain limits in order to guarantee improved thermal shock

Based on those optimized target materials the sputtering process is able to be performed at higher specific powder densities without resulting in target desintegration due to formation

absorption capacity is increasing with increased specific surface area (Lippens, 1996).

improved electrical and thermal characteristics as well as optimized densification.

1997).

coating times (Weigert, et al., 1992).

resistance characteristics (Schlott, et al., 2001).

of thermal gradients.

Water absorbed in the green body has to be thermally desorbed in order to avoid crack formation and specimen desintegration after sintering and subsequent cooling. This is especially true for capsuled and hot isostatically pressed samples. The gas formation during sintering has to be prevented by preheating of green bodies within controlled degassing atmospheres as for example N2 or Ar (Lippens, 1996). The thermal degassing process purifies the grain boundaries and is resulting in a reduction of specific surface area. Hereby the sintering activity is reduced and the densification has to be achieved by pressure assisted sintering techniques (Schlott, et al., 2001).

As referred to the previous section, thermal vacuum degassing exerts a strong influ-ence on the dgree of oxygen deficiency. Other studies (Falk, 2008) came to the result that vauum degassing and combined hot isostatic pressure sintering of capsuled ITO resulted in optimized sintering densities if the vacuum degassing time and temperature is correlated with a defect free micro structure, adopted concentration of free metal species as well as suitable degrees of oxygen deficiency (see Figure 8).
