Pure tin dioxide is an n-type semiconductor with tetragonal crystalline structure similar to that of rutile. SnO2 has many uses such as in gas sensors, electrodes for electric glass melting furnaces, electro chromic devices, crystal displays, photo detectors, solar cells and protective coatings. However, the use of tin dioxide ceramics is limited due to poor densification during sintering caused by the dominance of non-densifying mechanisms for mass transport such as surface diffusion or evaporation condensation. These mechanisms promote grain coarsening of SnO2 leading to poor densification of this ceramics.
To explain the predominance of the non-densifying mechanism during the sintering process of SnO2 powder, several models have been proposed. Varela et al. and Kimura et al [8] proposed a mechanism based on an evaporation /condensation process to explain the grain growth process during the sintering of SnO2 powder, in a temperature range of 1000 – 1400 oC.In both work, no macroscopic shrinkage was observed. Therefore, analysis of the SnO2 evaporation process is therefore of fundamental importance. To understand the sintering mechanism, Hoenig et al [9] studied the evaporation process of SnO2 and reported that at high temperatures this oxide showed a peritectic decomposition according to the reaction:
SnO2 --SnO +1/2 O2 (1)
In the temperature range 1200 to 1700 K, the oxygen partial pressure is given by:
logPO2 (atm) = -2.061 x10 -4/T +8.656 (2) where PO2 is the oxygen partial pressure and T the temperature. Considering above equation, in Fig. 2.7, a substantial increase of PO2 is observed for temperature higher than 1200 oC, which suggests a high evaporation rate for these temperatures. Even using ultrafine SnO2 powder, a full density of SnO2 ceramic is still not achieved. The experiments have been conducted by using sol-gel derived SnO2 powders to study the sintering process of ultrafine SnO2 powders, in the temperature range 400 to 1500 oC. The results showed that the mass transport mechanism is controlled by surface diffusion at low temperature from 500 to 1000
oC. Whereas, the mass transport mechanism is controlled by evaporation -condensation at temperature greater 1300 oC.
Fig 2.7 Oxygen partial pressure as a function of temperature [ 9].
Recently, Shi et al. [10] have analyzed the importance of surface diffusion on densification during the sintering process and proposed that surface diffusion is the most probable mass transport mechanism to promote particle coarsening and center approaching between particles or grains.
E. R. Leite et al [11] had used pure SnO2 to study the densification behavior by constant heating rate and isothermal sintering. Pore size distribution measurements, using gas desorption, and grain size and crystallite size measurements of isothermally sintered samples showed no formation of non-densifying microstructures during the sintering process. Fig. 2.8 showed the grain size and crystallite size of SnO2 as a function of temperature. These results are a strong indication that densification was prevented by thermodynamic factors, mainly the high ratio of GB/SV. An explanation, based on the nature of covalent bonding and the balance between attractive and repulsive forces, was proposed to explain the high GB/SV ratio in SnO2.
Fig. 2.8 The grain size (GBET) and crystallite size (GXRD) of SnO2 as a function of temperature [11].
Maitre et al [12] has studied the effect of ZrO2 additions on sintering of SnO2-based ceramics. It concluded that the zirconia additions limited the densification of the SnO2-based materials. This effect can be imputed to the elastic distortions in the SnO2 lattice due to a significant size mismatch between Ze+4 and Co+2 ions. Consequently, the diffusion rate of associate defects is reduced and this induces closed porosity within the bulk in SnO2 grains.
C.R. Foschini et al [13] have used ZnO as densifying agent for SnO2 through oxygen vacancy formation mechanism. They concludes that ZnO promoted SnO2 sinterability, allowing one to obtain samples with final densities of over 90 %. The formation of a single phase solid solution was observed in SnO2 samples with up
SnZnO3 phase precipitated at the grain boundaries, which may act as a barrier for grain mobility, inhibiting densification. Fig. 2.9 illustrates the micrographs of the fractured samples with 0.5 mol % ZnO doping and the formation of precipitates at the grain boundaries in compositions whose ZnO content exceeds 1 mol%.
Fig. 2.9 SEM micrographs of fractured samples with ZnO content of 0.5 mol %, (b) 1.0 mol % and (c) 5 mol % ZnO [13].
R, Muccillo et al [14] has used ac electric field to evaluate the possibility of densification by flash sintering. Similar to what happens after conventional sintering, this compound does not reach significant densification, even though 11
% shrinkage is attained at 900 oC under 80 V cm-1, The amplitude of the electric current generated at the sample during the application of the electric field is found to be a key factor for welding grains and promoting grain growth. Fig. 2.10 shows SEM micrographs near border of SnO2 not flash sintered and flash sintered at 900, 1100 and 1300 oC with 1 A and % A limiting current, respectively. These SEM micrographs show that under flash sintering there is a huge and abnormal grain growth, the grains sticking together but preserving the pore structure that inhibits densification.
Fig.2.10 Scanning electron microscopy micrographs of SnO2before and after exposure to 80 V cm−1at 900 ◦C (a–c), 1100 ◦C (d–f) and
1300◦C (g–i). Limiting current: 1 A (b, e, h); 5 A (c, f, i).
Images taken at the top fracture center of the samples [14].
M. S. Castro et al [15] have investigated different oxide additives on the densification of SnO2 ceramics. They concluded that the Co3O4 and MnO2 enhanced the densification of SnO2 ceramics by increasing the number of oxygen
formed a liquid phase during the sintering process enhancing the sintering rate.
They also found that electrical properties are improved by the additions of all the studied dopants, due to the modifications in the microstructure and in the defect concentration. Fig. 2.11 illustrated the effect of additives on the dielectric property of SnO2 ceramics.
Fig.2.11 Dielectric constant versus frequency curves of SnO2 samples with different additives [15].
F M. Filho et al [16] studied Ta2O5 doped SnO2 varistor systems containing 0.5 mol % ZnO and 0.5 mol % CoO by mixed oxide method. A small amount of Ta2O5 improved the nonlinear properties of the samples greatly. Fig. 23 showed the SEM micrographs for Co, Zn doped SnO2 system. It can be seen that no new apparent phase precipitation at the grain boundaries exist and the samples doped with 0.05 mol % Ta2O5 exhibited the highest grain size. Both samples present a uniform microstructure containing SnO2 grains free of second phases. The relative densities of all samples exceeded 98 % of the theoretical density.
Fig. 2.12 SEM micrographs for SZC system doped with: (a) 0.05 mol % of Ta2O5 and (b) 0.075 mol% Ta [16].
C. Wang et al [17] have developed an rapid atmospheric pressure plasma jet sintering process for nanoporousSnO2 ceramic. An APPJ sintered nanoporous SnO2 revealed properties comparable to those of furnace sintered SnO2. Fig. 2.12 showed SEM images of nanoporous SnO2 that formed channel with bundles of grains. As the APPJ sintering duration increased, the number of fine pores with dimensions of this order of tens of nanometers decreased. They also found that increased in APPJ sintering time, the electrical conductivity increased and then decreased, the slope of the optical absorption edge decreased and then increased, and the band gap decreased and then increased. It concluded that nanoporous SnO2 with large to volume ratio fabricated by this APPJ sintering process is potentially be used for gas sensor or catalysts.
Fig. 2.13 SEM images of furnace-sintered and APPJ sintered nanoporous SnO2 [17].