**4.2.3 Sintering without additives**

600 Sintering of Ceramics – New Emerging Techniques

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

a)

b) 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).

the surface area.

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

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% related to the theoretical density was observed.

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 compared to intragranular pores.

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 ITO densification.

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

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 sintering densities, probably due to inhomogeneous temperature distribution.

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., 2002).

Sintering of Transparent Conductive Oxides:

function.

1969).

on densification and mechanical stability (Schlott, et al., 1995).

phases on densification and mechanical strength is neglectable.

devating arc discharge at the surface of the target material.

**4.4 Sintering behavior and electrical conductivity** 

oxygen compared to indium.

From Oxide Ceramic Powders to Advanced Optoelectronic Materials 603

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

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

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

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

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,

The electrical conductivity of both pure indium oxide and pure tin oxide is a result of stoichiometry disturbance due to formation of oxygen vacancies. In case of In2O3 this structure can be described by the complex In2O3-x(Vo)xe′2x (Mayr, 1998). By tin doping, having a higher valence number compared to indium, negative charge carriers are incorporated into the lattice contributing to an additional increase of electrical conductivity. Freeman and co-workers (Freeman, et al., 2000) have calculated the theoretical distribution of energy bands in tin doped and undoped In2O3. In the case of tin doped indium oxide the s-band in the lower section of the conduction band is broadened. Consequently a high mobility of electrons is achieved explaining the high electrical conductivity. Further

In general the electrical conductivity of Sn doped ITO is lower than theoretically predicted. It was observed that high Sn doping concentrations even reduce the electrical conductivity. The In4Sn3O12 phase mentioned before is not or slightly electrically conductive (Nadaud, et al., 1998). This fact can be explained by the inactivity of Sn-cations located in M1-positions due to the ternary coordiation of the surrounding oxygen ions. This is similar to the structure of SnO2 which is neutral. Enventhough In and Sn are located in close vicinity of the periodic system of elements the additional electron of the Sn atom causes a higher affinity to

investigation of electronic ITO structure was elaborated in (Odaka, et al., 2001).
