**4.2.2 Pre-sintering**

Based on the findings of Son (Son & Kim, 1998) it was shown that pressure assisted presintering of In2O3 at 5 MPa could increase the densification rate significantly. Dilatometer experiments have proven that maximum densification rates were achieved at 1130 °C and the pressure was increased for another 5 MPa at this temperature. Sintering in air (1 atm) at 1500 °C, however, only leads to a desification of 76 % of the theoretical density.

In (Son & Kim, 1998) it was found, that the agglomerated green structure was transformed in a homogeneous polycrystalline microstructure at temperatures above 1070 °C and the agglomerates showes increased densification rates compared to the surrounding matrix leading to pore formation, so called interagglomerate pores, inbetween the matrix and the

Sintering of Transparent Conductive Oxides:

related to the theoretical density was observed.

compared to intragranular pores.

ITO densification.

2002).

**4.2.3 Sintering without additives** 

From Oxide Ceramic Powders to Advanced Optoelectronic Materials 601

In (Stenger, et al., 1999) the avoidance of binders and/or dry pressing agents is proposd to prevent any contamination of the received ITO powder. Futhermore it is suggested to avoid the evaporation of gaseous species during pyrolysis of additives since these proceses are

In (Udawatte, et al., 2000) the group reported about additive-free sintering of ITO powder compacts in air atmospheres. Starting from hydrothermally prepared and at 500 °C calcinated precursors ITO powders of the composition In2Sn1-xO5-y were attained. The authors have shown previously that pre-sintering of ITO and significant densification is achieved when the In2Sn1-xO5 phase is transformed to cubic In2-ySnyO3 in the temperature range between 1000 °C and 1200 °C. Sintering necks were observed in this temperature range and by excceding the sintering temperature above 1250 °C sigificant grain growth was initiated. At 1300 °C a uniform grain size of 2 µm up to 3 µm and a sintering density of 65%

The maximum densification was achieved at 1450 °C correlated with a mean grain size of about 7 µm. In this case triple grain boundary pores arised more and more frequently

The conclusion of these experimental investigations were that the sintering is activated mainly in the temperature range between 1300 °C and 1400 °C. The maximum density of about 92 % of the theoretical density was achieved after sintering at 1450 °C for three hours. Furthermore it was concluded that tin doping results in higher densification rates. In (Udawatte, et al., 2000) it is mentioned that tetragonal SnO2 phase formation counteracts

In (Udawatte & Yanagisawa, 2001) small dry pressed powder compacts (diameter 10 mm, thickness 1.5 mm) have been sintered at 1400 °C for three hours. Taking the theoretical density of 7.106 g/cm3 as a basis a maximim density of 99.3 % of the theoretical density was

Compared to conventional sintering elevated densities have been achieved by "spark plasma sintering" (SPS) (Takeuchi, et al., 2002). At a reduced dwell time of 5 minutes and high heating rates up to 50 K/min the SPS experiments resulted in considerably low

The sintering of cubic and rhomboedric nanosized ITO powders with mean particle sizes in the range of 50 to 100 nm were sintered up to 900 °C (Kim, et al., 2002) where the cubic phase was transformed. This transformation should theoretically results in a volume expansion of 2.1 % which was not observed since grain growth and pore formation were initiated. It was very complicated to eliminate these pores by subsequent sintering at elevated temperatures. The phase transformation promoted the active diffusion of atoms resulting in inhomogeneous grain growth with intragranular pore formation. It is therefore recommended to prevent phase transformation during sintering in order to achieve higher densification rates and more homogeneous microstructures (Kim, et al.,

achieved. The powder used had a mean particle size of about 80 nm.

sintering densities, probably due to inhomogeneous temperature distribution.

likely to reduce efficient pressure build up during hot isostatic pressing.

densified microstructure regimes. These pore formations could be effectively prevented by pressure assisted sintering and rearrangement of densified areas and by moving the pores to the surface area.

Fig. 8. Oxygen release and oxygen capturing and attained microstructures during vacuum degassing at 475 °C and 800 °C as a function of vacuum degassing time (a), HIP densities, free total metal content and degree of oxygen deficiency (DOD) as a function of vaccum degassing temperature of ITO sintered bodies (b) according to (Andersson, et al. 2005).
