4. Morphology, optical and acoustic characterization of sintered ZnO-SnO2 system

The influence of mechanical activation on the morphology, optical and thermal properties of the solid-state synthesized Zn2SnO4 was investigated by the scanning electron microscopy, 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

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

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

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].

4. Morphology, optical and acoustic characterization of sintered ZnO-SnO2

The influence of mechanical activation on the morphology, optical and thermal properties of the solid-state synthesized Zn2SnO4 was investigated by the scanning electron microscopy,

stannate synthesis.

at 900C for 2 hours.

86 Recent Advances in Porous Ceramics

system

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.

during the synthesis procedure. The thermal characterization of these materials is very important. Photoacoustic spectroscopy was used to determine the thermal and transport properties of 40, 80, and 160 min activated and non-isothermally sintered up to 1200C samples (heating rate of 5C/min) [28] and the 10, 40, 80 and 160 min activated and isothermally sintered at 1300C for 2 hours samples [34]. The experimental photoacoustic phase and amplitude spectra were recorded as a function of the chopped frequency of the laser beam (red laser with power of 25 mW, λ = 632 nm) in a thermal-transmission detection configuration and analyzed by theoretical Rosencwaig-Gersho thermal-piston model [35], which enable determination of the materials thermal properties including thermal diffusivity, diffusion coefficient of the minority free carriers and optical absorption coefficient.

and 160 min), pressing under 980 MPa in 10 mm diameter pellets, and isothermal sintering at 1300C, for 2 hours. They were used for selecting the optimal conditions for the preparation of Bi-doped ZnO-SnO2 samples (the second series samples). The infrared spectra of the first series samples were used also as the reference spectra to compare with infrared spectra's of samples of the second series. XRD analysis of the first series samples confirmed the presence of only Zn2SnO4 spinel phase [36, 37]. The density measurements, however, show that the slightly higher density was obtained for the starting powder mixture activated for 10 min (Table 2). Comparing the obtained relative densities of the sintered samples at temperatures from 900 to 1200C (Table 1, where the values were in the range from 52.8–69.2%), and the ones in Table 2, it is easy to conclude that by increasing the sintering temperature the relative density of the ZnO-SnO2 ceramic material is increased. Because of the slightly higher densities obtain for the 10 min activated sample; this parameter of synthesis (10 min of activation) was applied in the preparation of the samples from the second series. Samples of the second series were then prepared with the same starting powder mixtures, with the molar ratio of ZnO:SnO2 = 2:1, but with the addition of 0.5, 1.0 and 1.5 mol.% of Bi2O3 (samples marked as ZSO-0.5, ZSO-1, and ZSO-1.5, respectively), mechanical activation for 10 min, pressing under 980 MPa and isothermal sintering at 1300C for 2 hours. The highest relative density was obtained for the 1.0 mol.% of Bi2O3 (~92%) (Figure 10a), while the increasing concentration of Bi2O3 in the system (1.5 mol.%) lead to the decrease of the relative density (~87) probably because of the further grain growth and problematic packing of the bigger particles that are now more present in the system (Figure 10b). These conclusions are confirmed by the scanning electron microscopy

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measurements shown in Figure 10.

sintering for the Bi2O3 doped ZnO-SnO2 samples.

Samples of the first series rS/r<sup>T</sup> (%) Samples of the second series r<sup>0</sup> (g/cm<sup>3</sup>

Figure 10. SEM micrographs of (a) 1 and (b) 1.5 mol.% Bi2O3 doped ZnO-SnO2 ceramics.

ZSO-10 75.9 ZSO 3.92 62.96 5.25 84.44 ZSO-40 75.7 ZSO-0.5 4.12 65.79 5.63 89.93 ZSO-80 75.5 ZSO-1.0 4.12 65.19 5.81 92.21 ZSO-160 75.3 ZSO-1.5 4.22 66.51 5.49 86.58

Table 2. The sintered relative densities of the samples from the first series, and relative densities before and after the

) r0/r<sup>T</sup> (%) r<sup>S</sup> (g/cm<sup>3</sup>

) rS/r<sup>T</sup> (%)

The differences in thermal-electrical characteristics that were obtained again indicated that the changes in the material are induced by the differences in the processing routes of the powders before sintering. With the increase of the activation time and the formation of a single phase zinc stannate the thermal diffusivity value increases. The thermal diffusivity of zinc stannate material obtained by non-isothermal sintering route (0.21 <sup>10</sup><sup>7</sup> , 1.80 <sup>10</sup><sup>7</sup> , and 10.06 <sup>10</sup><sup>7</sup> m2 <sup>s</sup> 1 , for the ZSO-40, ZSO-80, and ZSO-160 samples, respectively) are to our best knowledge the first time ever measured values of this kind [28]. The photoacoustic analysis for differently activated and isothermally sintered ceramic samples shown differences in photoacoustic spectra, especially specific are the results obtained for the ZSO-10 sample. The frequency dependence photoacoustic phase has an explicit minimum for the samples prepared with higher activation times [34]. In the amplitude diagrams, at critical frequencies where phase diagram has the explicit minimum, a knee like a change of the curve rate occurs. It is obvious that at those frequencies in the samples activated longer than 10 min, the significant changes in the material properties appear. Those samples act as thermally thick at frequencies higher than the critical, and thermally thin at the frequencies lower than critical. In the area of lower frequencies, the dominant role in the generation of the photoacoustic signal has the thermal diffusivity and optical absorption coefficient. In the frequency range when the samples are thermally thick, the intensity and phase of the photoacoustic signal depend primarily upon the electrical transport properties of the investigated samples. The increase in the thermal diffusivity value with the increase of the activation time is confirmed in these samples as well which again points to the fact that higher activation times are responsible for the formation of porous, defect and lowdensity microstructure. This confirms, once more, that the grain growth of the spinel phase slows down the densification process, and together with the agglomerates formed during the mechanical activation, causes the appearance of a highly porous microstructure [34].

## 5. Liquid phase sintering of the ZnO-SnO2 system

The effect of small amounts of bismuth oxide (Bi2O3) on the microstructure, optical, structural and electrical properties of the spinel-type ZnO-SnO2 (zinc-tin-oxide) ceramics was investigated [36–39] to complement the above-shown research on the zinc-tin-oxide porous ceramics. Two series of samples were made for this purpose. Samples of the first series were used as the reference and were prepared by mechanical activation in different time intervals (10, 40, 80, and 160 min), pressing under 980 MPa in 10 mm diameter pellets, and isothermal sintering at 1300C, for 2 hours. They were used for selecting the optimal conditions for the preparation of Bi-doped ZnO-SnO2 samples (the second series samples). The infrared spectra of the first series samples were used also as the reference spectra to compare with infrared spectra's of samples of the second series. XRD analysis of the first series samples confirmed the presence of only Zn2SnO4 spinel phase [36, 37]. The density measurements, however, show that the slightly higher density was obtained for the starting powder mixture activated for 10 min (Table 2). Comparing the obtained relative densities of the sintered samples at temperatures from 900 to 1200C (Table 1, where the values were in the range from 52.8–69.2%), and the ones in Table 2, it is easy to conclude that by increasing the sintering temperature the relative density of the ZnO-SnO2 ceramic material is increased. Because of the slightly higher densities obtain for the 10 min activated sample; this parameter of synthesis (10 min of activation) was applied in the preparation of the samples from the second series. Samples of the second series were then prepared with the same starting powder mixtures, with the molar ratio of ZnO:SnO2 = 2:1, but with the addition of 0.5, 1.0 and 1.5 mol.% of Bi2O3 (samples marked as ZSO-0.5, ZSO-1, and ZSO-1.5, respectively), mechanical activation for 10 min, pressing under 980 MPa and isothermal sintering at 1300C for 2 hours. The highest relative density was obtained for the 1.0 mol.% of Bi2O3 (~92%) (Figure 10a), while the increasing concentration of Bi2O3 in the system (1.5 mol.%) lead to the decrease of the relative density (~87) probably because of the further grain growth and problematic packing of the bigger particles that are now more present in the system (Figure 10b). These conclusions are confirmed by the scanning electron microscopy measurements shown in Figure 10.

during the synthesis procedure. The thermal characterization of these materials is very important. Photoacoustic spectroscopy was used to determine the thermal and transport properties of 40, 80, and 160 min activated and non-isothermally sintered up to 1200C samples (heating rate of 5C/min) [28] and the 10, 40, 80 and 160 min activated and isothermally sintered at 1300C for 2 hours samples [34]. The experimental photoacoustic phase and amplitude spectra were recorded as a function of the chopped frequency of the laser beam (red laser with power of 25 mW, λ = 632 nm) in a thermal-transmission detection configuration and analyzed by theoretical Rosencwaig-Gersho thermal-piston model [35], which enable determination of the materials thermal properties including thermal diffusivity, diffusion coefficient of the minority

The differences in thermal-electrical characteristics that were obtained again indicated that the changes in the material are induced by the differences in the processing routes of the powders before sintering. With the increase of the activation time and the formation of a single phase zinc stannate the thermal diffusivity value increases. The thermal diffusivity of zinc stannate material

for the ZSO-40, ZSO-80, and ZSO-160 samples, respectively) are to our best knowledge the first time ever measured values of this kind [28]. The photoacoustic analysis for differently activated and isothermally sintered ceramic samples shown differences in photoacoustic spectra, especially specific are the results obtained for the ZSO-10 sample. The frequency dependence photoacoustic phase has an explicit minimum for the samples prepared with higher activation times [34]. In the amplitude diagrams, at critical frequencies where phase diagram has the explicit minimum, a knee like a change of the curve rate occurs. It is obvious that at those frequencies in the samples activated longer than 10 min, the significant changes in the material properties appear. Those samples act as thermally thick at frequencies higher than the critical, and thermally thin at the frequencies lower than critical. In the area of lower frequencies, the dominant role in the generation of the photoacoustic signal has the thermal diffusivity and optical absorption coefficient. In the frequency range when the samples are thermally thick, the intensity and phase of the photoacoustic signal depend primarily upon the electrical transport properties of the investigated samples. The increase in the thermal diffusivity value with the increase of the activation time is confirmed in these samples as well which again points to the fact that higher activation times are responsible for the formation of porous, defect and lowdensity microstructure. This confirms, once more, that the grain growth of the spinel phase slows down the densification process, and together with the agglomerates formed during the mechan-

ical activation, causes the appearance of a highly porous microstructure [34].

The effect of small amounts of bismuth oxide (Bi2O3) on the microstructure, optical, structural and electrical properties of the spinel-type ZnO-SnO2 (zinc-tin-oxide) ceramics was investigated [36–39] to complement the above-shown research on the zinc-tin-oxide porous ceramics. Two series of samples were made for this purpose. Samples of the first series were used as the reference and were prepared by mechanical activation in different time intervals (10, 40, 80,

5. Liquid phase sintering of the ZnO-SnO2 system

, 1.80 <sup>10</sup><sup>7</sup> , and 10.06 <sup>10</sup><sup>7</sup> m2 <sup>s</sup>

1 ,

free carriers and optical absorption coefficient.

88 Recent Advances in Porous Ceramics

obtained by non-isothermal sintering route (0.21 <sup>10</sup><sup>7</sup>


Table 2. The sintered relative densities of the samples from the first series, and relative densities before and after the sintering for the Bi2O3 doped ZnO-SnO2 samples.

Figure 10. SEM micrographs of (a) 1 and (b) 1.5 mol.% Bi2O3 doped ZnO-SnO2 ceramics.

Addition of the small amounts of Bi2O3 to the ZnO-SnO2 system creates conditions for the liquid phase sintering and enhances the densification process. The obtained structure and morphology was examined using XRD and SEM-EDS [36–39]. FTIR and impedance spectroscopy were used to investigate the optical and electrical properties [38, 39]. Bi2O3 doping creates the conditions for the liquid phase sintering. Bismuth oxide forms Bi2Sn2O7 pyrochlore phase with SnO2 and in the presence of ZnO leads to the formation of Zn2SnO4 spinel and Bi2O3 liquid phase between 1000 and 1100�C, according to the following reaction [36]: Bi2Sn2O7 + 4ZnO ! 2Zn2SnO4 + Bi2O3(l). This brings a new dynamic into the ZnO-SnO2 sintering mechanism. The base system mixture (2ZnO-SnO2) grains are completely surrounded by the thin film of liquid Bi2O3, which directly influence the densification, grain growth and the solid-state reaction between the ZnO and SnO2. No XRD proof of Bi2O3 was found in the sintered samples, either because the amount of Bi2O3 was under the XRD detection limit, or such high temperature of sintering (1300�C) caused the Bi2O3 to evaporate (evaporation of Bi2O3 starts at 825�C) [37]. The addition of Bi2O3 stimulates the ion substitution between Sn4+ and Zn2+ which results in ZnO/SnO2 solid solution formation with rather limited regions of pure Zn2SnO4 [36, 38] (Figure 11). Probably the diffusionevaporation mechanisms are responsible for the reaction between the ZnO and SnO2 in this case of this material sintered at such high temperatures, and not just the ordinary diffusion processes that happen in the usual solid-state chemical reactions. A limited evaporation of ZnO is also inevitable [37]. The ZnO evaporation opens up larger pores in places where the SnO2 was not available for the reaction due to the incomplete mixing. The zinc oxide condenses unreacted on the walls of the cavities upon cooling. The residual SnO2 appears to balance the evaporated ZnO. As said previously, the Zn2SnO4 has a cubic inverse spinel (AB2O4) structure so the A and B sites can substitute for each other during sintering based on the following reaction: AB2O4 ! (AB) BO4. Even though the valance of the Sn4+ is higher than of the Zn2+ ion the substitution is mutual [38] and according to the obtained results [36–39] the Bi-doping strongly promoted this substitution and contribute to the Zn2SnO4-SnO2 solid solution formation, along with the larger regions of pure spinel Zn2SnO4 (Figure 11) and smaller areas of residual SnO2. In Bi-doped ZnO-SnO2 system, the liquid phase assisted sintering mechanism is obviously responsible for these dramatic microstructural changes and the deviation from the expected formation of single phase cubic spinel Zn2SnO4. The enhancement of the densification process and higher bulk

relative densities are expected. The influence of the addition of Bi2O3 on the densification process was investigated by monitoring the change in the relative density vs. the molar percentage of the Bi2O3 added, and it shows an increase in relative density of sintered samples (~ 92%) with the addition of Bi2O3 up to 1.0 mol%, while further addition of 1, 5 mol% Bi2O3 leads to a decrease in relative density (~ 87%) [37]. This is probably due to further grains growth and packaging problem of much larger particles. The general conclusion is that the optimal amount of Bi2O3 under applied sintering conditions (1300C, 2 hours) for achieving the best possible densification

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The agglomeration and high porosity present in the starting mixtures are also retained in the sintered samples (porosity ~25%) (Table 2). The general microstructure of the sintered samples of the second series is characterized by the denser sintered areas compared to the reference sample (Figures 9 and 10). The "matrix" consists more of the non-stoichiometric zinc stannate, Zn2xSn1+xO4 and represent some kind of a solid-solution, where dark-gray regions are closer to the composition of pure Zn2SnO4, while smaller areas are composed of the residual SnO2

The Bi-addition to the ZnO-SnO2 system reflected on its optical properties as well. Although, the FTIR spectra [38] were similar to the previously obtained for Zn2SnO4 spinel [30, 32, 33] and were characterized by two extra oscillators (eight in total) attributed to the cation disorder in the crystal lattice induced by preparation procedure, compared to the six predicted by group theory analysis that belongs to the zinc stannate structure, it showed also the additional

The impedance spectroscopy measurements provided evidence of the intrinsic features of the grain-boundary phenomenon in this kind of ceramic material and point to its possible application in the nonohmic devices [39]. Impedance diagrams of SnO2-Zn2SnO4 ceramics showed Cole-Cole type behavior, where complex impedance data resulted in the semi-circles with the high degree of overlapping which is consistent with the reported semiconductor properties of

The optimal conditions for the best zinc stannate synthesis by reaction sintering of the mechanically activated ZnO and SnO2 powder mixtures in a high-energy planetary ball mill, in the time intervals from 0 to 160 min, were obtained. The mechanochemical activation of 40 min and more creates the conditions for the beginning of the Zn2SnO4 formation. The pure stannate phase was obtained in the sample activated for 160 min and sintered at 1200 C and higher. A joint structural feature of all the polycrystalline sintered bulk samples is a large open porosity. Formal analysis of the sintering kinetics using Lanel parameter confirmed that mechanical activation affects the sinterability only for the activation less than 40 min, while longer milling intervals have no more effect on the obtained Zn2SnO4 large porosity. In the materials characterization, it was shown that thermal diffusivity increases with the activation

, and the most

, which position was shifted to higher frequency in

whose polygonal grains are like inserted into the spinal structure (Figure 10b) [36–38].

three more peaks that belong to SnO2 phase (ωTO) found at 244 and 288 cm<sup>1</sup>

intensive bulk mode of SnO2 at 613 cm<sup>1</sup>

ZSO-1 sample [36, 38].

this kind of ceramic material.

6. Summary

is 1.0 mol%.

Figure 11. The ZSO-1 sample image taken with an optical microscope. The dark-gray area belongs to the pure Zn2SnO4 phase and it is surrounded by the "matrix" region composed of Zn2SnO4-SnO2 solid solution (noticeable recording points 6 and 7, marks the positions where the additional IR and EDS analysis were performed and shown elsewhere [36, 38]).

relative densities are expected. The influence of the addition of Bi2O3 on the densification process was investigated by monitoring the change in the relative density vs. the molar percentage of the Bi2O3 added, and it shows an increase in relative density of sintered samples (~ 92%) with the addition of Bi2O3 up to 1.0 mol%, while further addition of 1, 5 mol% Bi2O3 leads to a decrease in relative density (~ 87%) [37]. This is probably due to further grains growth and packaging problem of much larger particles. The general conclusion is that the optimal amount of Bi2O3 under applied sintering conditions (1300C, 2 hours) for achieving the best possible densification is 1.0 mol%.

The agglomeration and high porosity present in the starting mixtures are also retained in the sintered samples (porosity ~25%) (Table 2). The general microstructure of the sintered samples of the second series is characterized by the denser sintered areas compared to the reference sample (Figures 9 and 10). The "matrix" consists more of the non-stoichiometric zinc stannate, Zn2xSn1+xO4 and represent some kind of a solid-solution, where dark-gray regions are closer to the composition of pure Zn2SnO4, while smaller areas are composed of the residual SnO2 whose polygonal grains are like inserted into the spinal structure (Figure 10b) [36–38].

The Bi-addition to the ZnO-SnO2 system reflected on its optical properties as well. Although, the FTIR spectra [38] were similar to the previously obtained for Zn2SnO4 spinel [30, 32, 33] and were characterized by two extra oscillators (eight in total) attributed to the cation disorder in the crystal lattice induced by preparation procedure, compared to the six predicted by group theory analysis that belongs to the zinc stannate structure, it showed also the additional three more peaks that belong to SnO2 phase (ωTO) found at 244 and 288 cm<sup>1</sup> , and the most intensive bulk mode of SnO2 at 613 cm<sup>1</sup> , which position was shifted to higher frequency in ZSO-1 sample [36, 38].

The impedance spectroscopy measurements provided evidence of the intrinsic features of the grain-boundary phenomenon in this kind of ceramic material and point to its possible application in the nonohmic devices [39]. Impedance diagrams of SnO2-Zn2SnO4 ceramics showed Cole-Cole type behavior, where complex impedance data resulted in the semi-circles with the high degree of overlapping which is consistent with the reported semiconductor properties of this kind of ceramic material.

## 6. Summary

Addition of the small amounts of Bi2O3 to the ZnO-SnO2 system creates conditions for the liquid phase sintering and enhances the densification process. The obtained structure and morphology was examined using XRD and SEM-EDS [36–39]. FTIR and impedance spectroscopy were used to investigate the optical and electrical properties [38, 39]. Bi2O3 doping creates the conditions for the liquid phase sintering. Bismuth oxide forms Bi2Sn2O7 pyrochlore phase with SnO2 and in the presence of ZnO leads to the formation of Zn2SnO4 spinel and Bi2O3 liquid phase between 1000 and 1100�C, according to the following reaction [36]: Bi2Sn2O7 + 4ZnO ! 2Zn2SnO4 + Bi2O3(l). This brings a new dynamic into the ZnO-SnO2 sintering mechanism. The base system mixture (2ZnO-SnO2) grains are completely surrounded by the thin film of liquid Bi2O3, which directly influence the densification, grain growth and the solid-state reaction between the ZnO and SnO2. No XRD proof of Bi2O3 was found in the sintered samples, either because the amount of Bi2O3 was under the XRD detection limit, or such high temperature of sintering (1300�C) caused the Bi2O3 to evaporate (evaporation of Bi2O3 starts at 825�C) [37]. The addition of Bi2O3 stimulates the ion substitution between Sn4+ and Zn2+ which results in ZnO/SnO2 solid solution formation with rather limited regions of pure Zn2SnO4 [36, 38] (Figure 11). Probably the diffusionevaporation mechanisms are responsible for the reaction between the ZnO and SnO2 in this case of this material sintered at such high temperatures, and not just the ordinary diffusion processes that happen in the usual solid-state chemical reactions. A limited evaporation of ZnO is also inevitable [37]. The ZnO evaporation opens up larger pores in places where the SnO2 was not available for the reaction due to the incomplete mixing. The zinc oxide condenses unreacted on the walls of the cavities upon cooling. The residual SnO2 appears to balance the evaporated ZnO. As said previously, the Zn2SnO4 has a cubic inverse spinel (AB2O4) structure so the A and B sites can substitute for each other during sintering based on the following reaction: AB2O4 ! (AB) BO4. Even though the valance of the Sn4+ is higher than of the Zn2+ ion the substitution is mutual [38] and according to the obtained results [36–39] the Bi-doping strongly promoted this substitution and contribute to the Zn2SnO4-SnO2 solid solution formation, along with the larger regions of pure spinel Zn2SnO4 (Figure 11) and smaller areas of residual SnO2. In Bi-doped ZnO-SnO2 system, the liquid phase assisted sintering mechanism is obviously responsible for these dramatic microstructural changes and the deviation from the expected formation of single phase cubic spinel Zn2SnO4. The enhancement of the densification process and higher bulk

90 Recent Advances in Porous Ceramics

Figure 11. The ZSO-1 sample image taken with an optical microscope. The dark-gray area belongs to the pure Zn2SnO4 phase and it is surrounded by the "matrix" region composed of Zn2SnO4-SnO2 solid solution (noticeable recording points 6 and 7, marks the positions where the additional IR and EDS analysis were performed and shown elsewhere [36, 38]).

The optimal conditions for the best zinc stannate synthesis by reaction sintering of the mechanically activated ZnO and SnO2 powder mixtures in a high-energy planetary ball mill, in the time intervals from 0 to 160 min, were obtained. The mechanochemical activation of 40 min and more creates the conditions for the beginning of the Zn2SnO4 formation. The pure stannate phase was obtained in the sample activated for 160 min and sintered at 1200 C and higher. A joint structural feature of all the polycrystalline sintered bulk samples is a large open porosity. Formal analysis of the sintering kinetics using Lanel parameter confirmed that mechanical activation affects the sinterability only for the activation less than 40 min, while longer milling intervals have no more effect on the obtained Zn2SnO4 large porosity. In the materials characterization, it was shown that thermal diffusivity increases with the activation time and progression of the zinc stannate formation. SEM and FTIR results agree well in the conclusion that longer times of mechanical activation lead to increased porosity and defects. The FTIR spectra were numerically analyzed and oscillator parameters were calculated. Two more oscillators were observed compared to six predicted by the group theory for the single crystal Zn2SnO4, as a result of the synthesis procedure. The obtained defect structure of the Bidoped ZnO-SnO2 system is a direct consequence of the structural changes in all the hierarchy levels induced by the liquid phase sintering mechanism, which strongly influences the optical and electrical properties of the obtained material as well. By selecting the conditions and the ways of the sample preparation during the process of mechanical activation and sintering, it is possible to alter the microstructure, phase composition, optical and electrical properties of the resulting zinc-tin-oxide ceramics to fit the best the desired applications.

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