3. Mechanism and kinetics of sintering, and the porosity of consolidated ZnO-SnO2 system

In order to investigate the influence of the mechanical activation on ZnO-SnO2 systems densification, the green bodies of the different activated ZnO-SnO2 powder mixtures were prepared by uniaxial pressing with different pressures (250, 50, 150, 200, 200 MPa was applied for ZSO-00, ZSO-10, ZSO-40, ZSO-80, and ZSO-160, respectively) in accordance with the findings in Figure 4, so the starting sintering density would be the same (3.769 g/cm3 ) [27, 28] for all the investigated samples. Non-isothermal sintering kinetics was determined by monitoring the relative shrinkage of the green body by sensitive dilatometer, in the air up to 1200C, with three different heating rates (5C/min in Figure 5a, 10C/min in Figure 5b, and 20C/min in Figure 5c). All the results are shown in Figure 5.

The dilatometric results point to 800C as the temperature when the densification process starts in ZSO-40, ZSO-80, and ZSO-160 samples. The dilatometric behavior of ZSO-00 and ZSO-10 is somehow different. The initial expansion in the ZSO-00 and ZSO-10 samples starts around 1000C and continues in shrinkage. It is very important to determine the origin of this

Figure 5. The dilatometric curves of differently activated ZnO-SnO2 samples obtained by non-isothermal sintering in air, at different heating rates: (a) 5C/min, (b) 10C/min, and (c) 20C/min, and up to 1200C [28–30]. The inset table shows the minimum and maximum temperatures where the expansion and the shrinkage start, respectively, for the ZSO-00 and ZSO-10 samples.

activated powder mixture to have the green body with the same density. It is easy to spot on the so-called, "the same density line" in Figure 4, that shows the values of the pressure that have to be applied in order to obtain the green body that has the same density (60% of the theoretical

3. Mechanism and kinetics of sintering, and the porosity of consolidated

In order to investigate the influence of the mechanical activation on ZnO-SnO2 systems densification, the green bodies of the different activated ZnO-SnO2 powder mixtures were prepared by uniaxial pressing with different pressures (250, 50, 150, 200, 200 MPa was applied for ZSO-00, ZSO-10, ZSO-40, ZSO-80, and ZSO-160, respectively) in accordance with the findings in

investigated samples. Non-isothermal sintering kinetics was determined by monitoring the relative shrinkage of the green body by sensitive dilatometer, in the air up to 1200C, with three different heating rates (5C/min in Figure 5a, 10C/min in Figure 5b, and 20C/min in

The dilatometric results point to 800C as the temperature when the densification process starts in ZSO-40, ZSO-80, and ZSO-160 samples. The dilatometric behavior of ZSO-00 and ZSO-10 is somehow different. The initial expansion in the ZSO-00 and ZSO-10 samples starts around 1000C and continues in shrinkage. It is very important to determine the origin of this

) [27, 28] for all the

Figure 4, so the starting sintering density would be the same (3.769 g/cm3

Figure 5c). All the results are shown in Figure 5.

density), no matter how activated ZnO-SnO2 powder mixture is.

Figure 4. The pressing pressure vs. green density for the ZnO-SnO2 system [27].

ZnO-SnO2 system

82 Recent Advances in Porous Ceramics

deviation and its impact on the process of densification and further evolution of the material microstructure. This expansion cannot be explained just by the simple differences in the molar volumes of the samples because the theoretical density of zinc stannate spinel (6.42 g/cm3 ) is very close to the theoretical density of the equimolar mixture of ZnO and SnO2 (6.24 g/cm3 ). Several mechanisms are described in the literature to explain phenomena like this, that is, the expansion of the powders during the solid-state reactions. In the oxide systems as ZnO-SnO2, it is explained by to the separation of the particles when the reaction product is formed. The chemical reaction starts at 1026C (when the heating speed is 5C/min), 1044C (when the heating speed is 10C/min) and 1060C (when the heating speed is 20C/min) in the ZSO-00 sample, and at 916C (heating speed, 5C/min), 945C (heating speed, 10C/min) and 957C (heating speed, 20C/min) for the ZSO-10 sample (Inset Table in Figure 5). During the chemical reaction, the product, zinc stannate, causes the expansion of the ZSO-00 and ZSO-10 samples because the starting powder grains are being separated in the way that is illustrated in Figure 6.

The reaction sintering is a process during which the chemical reaction and the densification happen simultaneously. The temperature of the reaction sintering beginning is obviously lower for the longer mechanically activated ZnO-SnO2 powder mixtures and higher when higher heating speeds were used during the thermal treatment. Some believe that the most important fact for the reaction sintering process is a defect degree of the formed microstructure during the chemical reaction. For the systems where the chemical reaction does not induce the

The isothermal sintering was performed at 900 to 1200 C (Figure 7a) for different time intervals (30–120 min) (Figure 7b). The relative densities were determined and used to calculate the obtained ceramics porosities (Table 1). The isothermal sintering is usually analyzed by investigating the shrinkage degree, which is described by the so-called Lanel parameter. This parameter connects the after sintering Π<sup>S</sup> and starting porosity Πo, that is, the green r0, sintered rS, and theoretical r<sup>T</sup> density of the investigated material: L = 1-(ΠS/ Π0)=(r<sup>S</sup> – r0)/(r<sup>T</sup> – r0). The joint structural characteristic of all the sintered samples is a large open porosity, ~40% and inhomogeneity which is a consequence of the powders "history" (the presence of the agglomerates and aggregates) (Figure 2), porosity in the starting powder mixtures and the solid-state reaction with the spinel formation (Figure 6). The formal sintering kinetic analysis by Lanel parameter (Figure 8) confirmed that mechanical activation of the starting powder mixtures influences the sinterability, but having all the results in mind, longer activation than 40 min is not necessary because the

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Figure 7. The change of the relative density vs. a) Temperature of the isothermal sintering and b) time of the mechanical

Sample Π<sup>0</sup> = 1-r0/r<sup>T</sup> (%) Π<sup>S</sup> = 1-rS/r<sup>T</sup> (%) Sample Π<sup>0</sup> = 1-r0/r<sup>T</sup> (%) Π<sup>S</sup> = 1-rS/r<sup>T</sup> (%)

ZSO-00 48.6 45.3 ZSO-00 48.6 38.6 ZSO-10 41.9 46.4 ZSO-10 43.1 37.7 ZSO-40 45.3 44.4 ZSO-40 44.7 41.1 ZSO-80 45.8 44.2 ZSO-80 46.1 42.8 ZSO-160 46.4 46.1 ZSO-160 46.4 46.6

ZSO-00 46.9 45.3 ZSO-00 48.4 31.3 ZSO-10 43.8 41.7 ZSO-10 42.8 30.8 ZSO-40 45.3 43.5 ZSO-40 45.3 39.7 ZSO-80 45.6 43.5 ZSO-80 46.4 40.3 ZSO-160 45.6 45.1 ZSO-160 46.6 47.2

Table 1. Porosity (%) for the green and isothermally sintered ZnO-SnO2 samples.

TSINTER. = 900 C TSINTER. = 1100 C

TSINTER. = 1000 C TSINTER. = 1200 C

activation, of the ZnO-SnO2 system.

Figure 6. The schematic illustration of the solid-state reaction in the ZnO-SnO2 system.

huge microstructure changes, obtaining higher densities and controlled grain growth does not depend on the fact whether the chemical reaction begins before or after the densification process. However, if the volume change during the reaction sintering is large, the densification should occur before the reaction if the high density and controlled grain growth are desired. The results obtained for ZSO-00 and ZSO-10 samples imply that the reaction between the ZnO and SnO2 and formation of the Zn2SnO4 spinel in these samples begins before the process of sintering and it is accompanied by a very large expansion of the samples. In samples activated for 40 min and more, the shrinkage is dominant. The XRD results confirmed that in these samples zinc stannate spinel is a major phase so the process of densification prevails. The slope of the dilatometer curves of ZSO-40, ZSO-80, and ZSO-160 samples point to the possibility of the spinel phase formation in the shape of the agglomerates that progress by the increased activation time because the agglomeration inhibits the densification. The calculated green and sintered densities were 3.74 and 4.14 g/cm<sup>3</sup> (ZSO-40), 3.78 and 4.12 g/cm3 (ZSO-80) and 3.79 and 3.86 g/cm<sup>3</sup> (ZSO-160), respectively. The densification is the highest for the sample activated for 40 min and the lowest for the longest activated sample (ZSO-160). The densities of all the samples increase during sintering but the highest value is obtained for the ZSO-40 sample. Hence, the relative shrinkage during sintering is primarily dependent on a distribution of the starting particles, their consolidation, and activity. The densities of all the sintered samples are lower than 70% of the theoretical density of zinc stannate. It is obvious that this structure is difficult to sinter. The porosity present in starting powder mixtures is preserved in the sintered samples as well. The starting powders are mainly made of the agglomerates, which are the reason why the packing of the particles during consolidation step is not ideal, and then it is so difficult to reach the high densification degree during sintering. This, together with the formation of the agglomerated spinel, is the main reason for the slow sintering process in this system. The moving force of the reaction sintering is the low free energy of the system as a result of the spinel phase formation through the diffusion mechanisms. The additional force is the high surface free energy induced by the process of mechanical activation of the starting powders, but only after the system gains the chemical balance.

The isothermal sintering was performed at 900 to 1200 C (Figure 7a) for different time intervals (30–120 min) (Figure 7b). The relative densities were determined and used to calculate the obtained ceramics porosities (Table 1). The isothermal sintering is usually analyzed by investigating the shrinkage degree, which is described by the so-called Lanel parameter. This parameter connects the after sintering Π<sup>S</sup> and starting porosity Πo, that is, the green r0, sintered rS, and theoretical r<sup>T</sup> density of the investigated material: L = 1-(ΠS/ Π0)=(r<sup>S</sup> – r0)/(r<sup>T</sup> – r0). The joint structural characteristic of all the sintered samples is a large open porosity, ~40% and inhomogeneity which is a consequence of the powders "history" (the presence of the agglomerates and aggregates) (Figure 2), porosity in the starting powder mixtures and the solid-state reaction with the spinel formation (Figure 6). The formal sintering kinetic analysis by Lanel parameter (Figure 8) confirmed that mechanical activation of the starting powder mixtures influences the sinterability, but having all the results in mind, longer activation than 40 min is not necessary because the

huge microstructure changes, obtaining higher densities and controlled grain growth does not depend on the fact whether the chemical reaction begins before or after the densification process. However, if the volume change during the reaction sintering is large, the densification should occur before the reaction if the high density and controlled grain growth are desired. The results obtained for ZSO-00 and ZSO-10 samples imply that the reaction between the ZnO and SnO2 and formation of the Zn2SnO4 spinel in these samples begins before the process of sintering and it is accompanied by a very large expansion of the samples. In samples activated for 40 min and more, the shrinkage is dominant. The XRD results confirmed that in these samples zinc stannate spinel is a major phase so the process of densification prevails. The slope of the dilatometer curves of ZSO-40, ZSO-80, and ZSO-160 samples point to the possibility of the spinel phase formation in the shape of the agglomerates that progress by the increased activation time because the agglomeration inhibits the densification. The calculated green and sintered densities were 3.74 and 4.14 g/cm<sup>3</sup> (ZSO-40), 3.78 and 4.12 g/cm3 (ZSO-80) and 3.79 and 3.86 g/cm<sup>3</sup> (ZSO-160), respectively. The densification is the highest for the sample activated for 40 min and the lowest for the longest activated sample (ZSO-160). The densities of all the samples increase during sintering but the highest value is obtained for the ZSO-40 sample. Hence, the relative shrinkage during sintering is primarily dependent on a distribution of the starting particles, their consolidation, and activity. The densities of all the sintered samples are lower than 70% of the theoretical density of zinc stannate. It is obvious that this structure is difficult to sinter. The porosity present in starting powder mixtures is preserved in the sintered samples as well. The starting powders are mainly made of the agglomerates, which are the reason why the packing of the particles during consolidation step is not ideal, and then it is so difficult to reach the high densification degree during sintering. This, together with the formation of the agglomerated spinel, is the main reason for the slow sintering process in this system. The moving force of the reaction sintering is the low free energy of the system as a result of the spinel phase formation through the diffusion mechanisms. The additional force is the high surface free energy induced by the process of mechanical activation of the starting

Figure 6. The schematic illustration of the solid-state reaction in the ZnO-SnO2 system.

84 Recent Advances in Porous Ceramics

powders, but only after the system gains the chemical balance.

Figure 7. The change of the relative density vs. a) Temperature of the isothermal sintering and b) time of the mechanical activation, of the ZnO-SnO2 system.


Table 1. Porosity (%) for the green and isothermally sintered ZnO-SnO2 samples.

room temperature far infrared and photoacoustic spectroscopy [28, 32–34]. The reflectivity of near normal incidence light in the range between 100 and 1400 cm<sup>1</sup> as a function of the wave number showed the existence of eight ionic oscillators for all the investigated samples. The intensity of the reflectivity peaks was the highest for the sample activated for 10 min and gradually decreased with longer mechanical activation (40, 80 and 160 min, respectively). As confirmed by microstructural analysis longer times of activation lead to the increase of porosity and defects (Figure 2). The specificity of the obtained results is the two extra oscillators from the six that are expected to show for the known Wyckoff sites for zinc stannate structure and calculated by nuclear site group analysis, which is obviously the result of mechanical activation and sintering [32, 33]. It is known that the mechanical activation is responsible for the formation of the defects and after sintering a structure that contains pores, aggregates and intergranular material besides crystalline grains [32] (Figure 9a–d). As shown previously, the activation of 10 min brings the significant refinement in the crystallite size of the initial oxides, while the beginning of the spinel zinc stannate phase formation starts after 40 min of activation [21, 30]. The agglomeration and high porosity is a feature of all the mechanically activated samples, and it increases with the longer activation times and remains also after the sintering process (Figure 9). The ceramic materials with a large open porosity like zinc stannate obtained by the reaction sintering processing described in this review paper are convenient for the application in the humidity sensors. Atmospheric water can be absorbed on the grain surfaces inside pores or condensed in the small channels and pores [28]. Humidity sensor has to have features like high sensitivity, reversibility, fast response, the broad range of moisture selectivity, chemical, and thermal stability, which depends on the microstructure formed

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Figure 9. SEM micrographs of ZnO-SnO2 ceramics prepared by mechanical activation for, (a) 10, (b) 40, (c) 80 and (d) 160

min and isothermal sintering at 1300C for 2 hours.

Figure 8. The Lanel parameter vs. time of sintering for the ZnO-SnO2 samples activated 40, 80, and 160 min and sintered at 900C for 2 hours.

effects of longer mechanical activation have a very little impact on the sinterability improvement. The mechanical activation longer than 40 min, on the other hand, is needed for more efficient zinc stannate synthesis.

The behavior of the differently activated samples of ZnO-SnO2 system during the heat treatment was complemented with differential thermal analysis (DTA). From the shape of the exothermal peak of the nonactivated ZnO-SnO2 powder mixture, it is obvious that starting from 400C, the serious of exothermic, poorly separated changes (processes) occur. These processes lead to the formation of zinc stannate phase, which is confirmed by the XRD analysis [29, 30]. The exothermal effect in ZSO-10 is a consequence of the two processes: formation of the zinc stannate at lower temperatures and spinel crystal growth at higher temperatures.

The XRD analysis confirms the existence of the spinel peaks in ZSO-40, ZSO-80, and ZSO-160 samples, so the exothermal effect in these samples is only a result of the spinel growth during the thermal treatment [30]. With the higher activation times used in the preparation of the starting powder mixtures, the temperature of the exothermal peak is lowering (ZSO-00: 1135, ZSO-10: 1031, ZSO-40: 941, ZSO-80: 892, and ZSO-160: 849C). The specific reaction enthalpy values were 1.69 (ZSO-10), 3.10 kJ/g (ZSO-40), and for the ZSO-80 and ZSO-160 samples, had the same value of 3.69 kJ/g, so the conclusion could be that the good condition for mechanochemical synthesis of the zinc stannate spinel is obtained already with 80 min of mechanical activation. The obtained results imply also the possibility of zinc stannate solid-state synthesis already at 900C, which is a much lower temperature of Zn2SnO4 synthesis than previously found (1280C) [31].
