**3.1 ITO powder synthesis**

594 Sintering of Ceramics – New Emerging Techniques

Oxo ↔ (1/2O2)g + V••o + 2e′ (13)

Table 3. Overview of reactions on formation and self-compensation of vacancies as well as

Nadaud and co-workers investiated oxygen concentration of bulk-ITO by neutron diffraction and Rietveld analysis (Nadaud, et al., 1998). After sintering at 1400 °C in reducing nitrogen atmospheres stoiciometric oxygen concentrations were detected for both undoped and 6 at.-% Sn doped In2O3. On the other hand sintering in oxygen atmospheres resulted in bulk oxygen excess of about 3 %. This could be explained by the stoiciometry of

oxygen complexes being difficult to get reduced in the intermediate temperature regime. The same authors investigated these large complexes by Mössbauer, EXAFS and neutron

ITO Mössbauer- and EXAFS data von ITO reveal a relaxation of the Sn-O shell similar to the observed relaxation in Sn rhich In4Sn3O12 (Nadaud, et al., 1998). The analytical data allow to extend the explanation of inefficacy doping above 6 at.-% Sn to the effect that Sn atoms are

The scientific community is controversial discussing the precipitation of the rhomboedric phase In4Sn3O12 at Sn concentrations exceeding 6 at.-% Sn. The In4Sn3O12 structure has been analysed by neutron diffraction experiments and it was ascertaiend that the structure is similar to the Fluorite structure and that a sixfold occupation of M1-sites by Sn cations and sevenfold occupation of M2-sites by In- and Sn cations occurs (Nadaud, et al., 1998). The following figure shows the unit cell of the rhomboedric In4Sn3O12 phase. The lattice constant

InO''i)x complexes. Another explanation is the formation of large Sn-based

formation of donators of the system In2O3:Sn according to Kröger-Vink notation.

InOi′′)x ↔ 2Sn•

InOi′′)x + In2O3 (11)

In + (1/2O2)g + 2e′ (12)

In + 2e′ + In2O3 + (1/2O2)g (14)

system In2O3:Sn vacancy formation &

the neutral (2Sn•

diffraction.

donator formation (2Sn•

self-compensation 2InInx + 2SnO2 ↔ (2Sn•

2InInx + 2SnO2 ↔ 2Sn•

incorporated at cation sites where they are inactive.

is 6.2071 Å and the mismatch angle ø is 99.29°.

Fig. 6. Unit cell of the In4Sn3O12 phase.

In most cases indium-tin-oxide powders are synthesized by hydrothermal processes. Gel formation is based on the co-precipitation of InCl3⋅H2O and SnCl2 educts (Udawatte & Yanagisawa, 2000). The microcrystalline powder were homogeneous and reveal the composition of tin doped indium oxide [In(OH)3:Sn] and tin doped indium hydroxide [InOOH:Sn]. Calcination of In(OH)3:Sn at 300 °C resulted in cubic tin doped indium oxide [In2O3:Sn]. At calcination temperatures above 500 °C the InOOH:Sn phase is transformed to a solid solution of the formula (In2Sn1-xO5-y). Both powders have been calcinated in air atmosphere. It was found by Mössbauer analysis that the Sn4+-ion coordination number is 8 because each tin atom has 8 neighbouring oxygen atoms which bear opposite charge. A similar synthesis scheme is presented in (Yanagisawa, et al., 2000). In this case indium-tinoxide has been synthesized from a In-Sn-hydrogel. The hydrothermal treatment of the gel at 300 °C resulted in the formation of InOOH:Sn with mean particle sizes in the range of 80 nm. The subsequent calcination of the product at different calcination temperatures led to different microstructural vacancy configurations of indium tin oxide solid solutions. Calcination at 700 °C lead to a powder with primary particles sizes in the range of about 160 nm. Another approach is the processing of aqueous In(NO3)3⋅H2O solutions, subsequent heating and calcination at 500 °C in order to achieve nanosized ITO powders (Sorescu, et al., 2004). The solution of metallic tin and indium in HCl is precipitated by [NH4OH] (Nam, et al., 2001) and the resulting indium-tin-hydroxide gel is dried, grinded and calcinated at 600 °C.

The processing of tin doped indium oxide crystalites from a direct indium-tin smelting enriched with oxygen is described in (Frank, et al., 1976). The hetero-geneous nucleation process was initiated by small In2O3 crystalites. Different compositions and concentration ranges of the smelting process resulted in different tin doping concentrations. Indium tin oxide synthesis by a chemical transport process is reported in (Werner, et al., 1996). Starting from metallic indium and tin and dissolution of the educts in nitric acid the solution is dried and calcinated at 900 °C to achieve In2O3 and SnO2. The mixture of both oxides were doped with transport media iodine, or sulfur, or chlorine. Indium oxide crystals doped with 8.2 mole-% tin and tin oxide crystals doped with 2.4 mole-% indium were attained. These powders have been characterized and the synthesis reactions based on chlorine transport media have been thermodynamically modeled (Patzke, et al., 2000).

In case the seperate oxides are provided as educt materials for the synthesis reaction, indium doped tin oxide, as reported in (Nadaud, et al., 1994) could be processed. In this case In2O3 und SnO2 powder (purity of 99,99 wt.-%) have been mixed in ethanol and calcinated at

Sintering of Transparent Conductive Oxides:

processing chains is still missing.

**4. Sintering of TCO materials**

powder synthesis.

layers (Weigert, et al., 1992).

**4.1 Sintering and Degree of Oxygen Deficiency (DOD)** 

degree of oxygen deficiency is defined as follows:

From Oxide Ceramic Powders to Advanced Optoelectronic Materials 597

success. AZO particles synthesized by co-precipitation (Nayak, et al., 2007, Aimable, et al., 2010, Shui, et al., 2009) and solvothermal decomposition (Thu & Maenosono, 2010) are characterized by wide particle size distributions and heterogeneous Al doping concentrations of the microstructure. Hydrothermally (Piticescu, et al., 2006) and sol-gel based processing (Chen, et al., 2008) of AZO particles resulted in particles of high agglomeration degree and mean particles diameters considerably higher than 30 nm and undefined redispersing behaviour. The sintering and densification kinetics of Al:ZnO green bodies is slowed down with increasing Al doping concentrations and the grain growth effects are significantly reduced (Han, et al., 2001, Hynes, et al., 2002). Information about processability of synthesized AZO particles within technical and functional ceramic

The degree of oxygen deficiency (DOD) is referred to oxygen deficient in relation to oxygen concentration of the fully oxidized powder (Schlott, et al., 1995). In (Weigert, et al., 1992) the

Here a is the oxygen concentration of stoiciometric oxidic compound, b is the oxygen concentration of the partially reduced oxidic compound and c is the oxygen concentration of the completely reduced metallic compound. The degree of oxygen deficiency is closely related to the oxygen vacancies contributing in parts to the electrical conductivity of ITO. Oxygen release is caused by thermolysis of the ITO powder and oxygen uptake is observed during cooling. The arrangement of oxygen vacancies is likely to be closely connected to the

Thermal treatment in air leads to light-green colouring of the powder and thermal treatment in reducing atmospheres results in black colouring of the powder. Sputtering is causing a yellow colour of the target (Otsuka-Matsua-Yao, et al., 1997). After sintering the powder can also change colour to dark green or black(Udawatte, et al., 2000). The thermal history of the powder is of significant importance since the powder characteristics are closely correlated with the target quality having a profound impact on the characteristics of the sputtered thin

Evidence proves that oxygen release and uptake of ITO follows a chemical hysteresis (Otsuka-Matsua-Yao, et al., 1997). In repeated heating and cooling cycles different quantities of oxygen are incorporated or released. According to the findings of experimental sintering investigations (Otsuka-Matsua-Yao, et al., 1997) the quantities of released oxygen below 773 K was marginal. A preliminar thermolysis at 1073 K was needed to release carbon monoxide absorbed on the surface. The authors came to the result, that the thermodynamic oxygen characteristics of ITO samples were changed during heating and cooling cycles. Hereby the total oxygen concentration is reduced during each cycle and the kinetics of oxygen release is slowed down cycle by cycle. The initiation temperaure of oxygen release was observed to increase on every iteration of heating and cooling cycle. It was assumed by the authors that there are several phases and several phase transformations inbetween these cycles causing

DOD = (a-b) /(a-c) (15)

1380 °C for theree hours. Colouration from yellow to green and XRD peaks of indium-tinoxide without exception proved the complete transforma-tion to indium tin oxide.

An alternative ITO synthesis process is given in (Stenger, et al., 1999). Here molten metallic indium tin alloy has been reacted with oxygen in a plasma arc furnace. The product is subsequently quenched by a gas stream at cooling rates between 105 K/s to 108 K/s to a final temperature between +50 °C and +400 °C. Thereby mixed indium tin oxide powders were produced with specific surface area of 3 m2/g at maximum. As the powder tends to formation of large agglomerates the size of different ramifications has been analysed by electron microscopy and documented as primary particle size being in the range of 0.03 µm up to 1.0 µm. Hot melt spraying of indium tin melts in oxydizing atmospheres and subsequent quenching is also suitable to process indium tin oxide powders. The agglomerated particle are porous and reveal particle sizes up to 170 µm (see Figure 4).

Compared to this the commercial powder ZY-ITO (Dow Chemical Co. Ltd.) is composed of a mixture of 90 wt.-% In2O3 and 10 wt.-% SnO2, with a specific surface area of 32.0 m2/g and a mean primary particle size in the range of 3.4 µm.

### **3.2 AZO powder synthesis**

Although various studies of synthesis, characterization and application of Al:ZnO thin layer systems are given in the literature (Stanciu, et al., 2001, Selmi, et al., 2008), there are only a few investigations referring to synthesis and processing of nanosized Al:ZnO particles. Flame spray pyrolysis of liquid Al- ad Zn precursors led to uncontrolled Al:ZnO stoichiometries and segregations of the spinel and wurzite phase combined with evaporation of the metallic Zn phase (Kim, 2008). Vapour condensation (Strachowski, et al., 2007) and spray pyrolysis (Hsiao, et al., 2007) as well as low-pressure spray pyrolysis (Hidayat, et al., 2008) resulted in highly agglomerated particles exhibiting poor redispersing characteristics.

Fig. 7. Indium tin oxide powder produced by hot melt spraying and subsequent quenching: Pore volume of the powder as a function of particle size.

The processing of suitable AZO particles for optoelectronic applications is therefore still challenging. Several alternative processing routes have been elaborated with limited success. AZO particles synthesized by co-precipitation (Nayak, et al., 2007, Aimable, et al., 2010, Shui, et al., 2009) and solvothermal decomposition (Thu & Maenosono, 2010) are characterized by wide particle size distributions and heterogeneous Al doping concentrations of the microstructure. Hydrothermally (Piticescu, et al., 2006) and sol-gel based processing (Chen, et al., 2008) of AZO particles resulted in particles of high agglomeration degree and mean particles diameters considerably higher than 30 nm and undefined redispersing behaviour. The sintering and densification kinetics of Al:ZnO green bodies is slowed down with increasing Al doping concentrations and the grain growth effects are significantly reduced (Han, et al., 2001, Hynes, et al., 2002). Information about processability of synthesized AZO particles within technical and functional ceramic processing chains is still missing.
