2. Mechanochemical activation and consolidation of ZnO-SnO2 powder system

Figure 1. The specific surface area vs. the time of mechanical activation for the ZnO-SnO2 powder system.

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Figure 2. SEM micrographs of ZnO-SnO2 powder mixtures activated, (a) 0, (b) 10, (c) 40, and (d) 80 min.

Finding optimal conditions of mechanochemical activation is the first and foremost important processing step for obtaining the right texture, particle size distribution and chemical reactivity of the starting powders in the solid-state synthesis of the polycrystalline Zn2SnO4 ceramics and the achievement of the wide range of its applications [21]. The mechanochemical activation consists of several processes that are usually divided into four stages: the material destruction, new surface formation, fine grinding and transformation to a completely different material structure. The optimal powder processing is a way to lower the sintering temperature necessary in the further steps of the ceramics production, which is very important fact from the stand point of the cost-effectiveness. When the starting powders of zinc oxide and tin oxide in the 2:1 molar ratio (ZnO:SnO2 = 2:1) are mechanically activated in a planetary ball mill (Fritsch Pulverisette) in time intervals 0–160 min (with 320 rpm, ball to powder mass ratio of 40:1, in zirconium grinding media of a 500 ml vial and 10 mm diameter balls) the two stages of the grinding process occurs (Figure 1). The investigated samples were marked further in the text as ZSO-00, ZSO-10, ZSO-40, ZSO-80, and ZSO-160, and these marks are related to the not-activated, 10, 40, 80, and 160 min activated 2ZnO-SnO2 powder mixtures, respectively. The first stage, up to 10 min of mechanical activation, is the increase in the powders specific surface area (SBET, calculated by BET method from the linear part of N2 adsorption isotherms [21–23]) which is related to the breaking of the powder particles and formation of the new surfaces (Figure 2a and b). The second stage brings the cold-welding and chemical reaction between the starting precursors (ZnO and SnO2 powders), Zinc-Tin-Oxide-Based Porous Ceramics: Structure, Preparation and Properties http://dx.doi.org/10.5772/intechopen.71581 79

Zn2SnO4 in the bulk form is stable in the inverse spinel structure, with a face-centered cubic unit cell (space group Fd3m), where Zn2+ occupy the 8a sites, and both Zn2+ and Sn4+ cations occupy the 16d sites, while O is placed in the 32e sites. Its unit cell parameter has a value of a = 8.6574 Å (JCPDS PDF 24-1470). The spinel-type structures can have big cation disorders in the crystal lattice and certain nonstoichiometry. Nevertheless, the disorders in spinels are not conventional so there is no change in the symmetry. The most general spinel formula is AB2O4, where A is two valent or four valent metal ions, and B are two valent or tree valent metal ions. To this day, there have been synthesized over 200 different types of spinel oxide compounds. Some spinel compounds are known to have the characteristic sensor and catalytic properties, like Zn2SnO4, and exhibit complex disordering phenomena related to the redistribution of cations over (B) and [A] sublattices in their structure. The partly inverse spinels like Zn2SnO4 have four valent ions on octahedral positions [A] and two valent ions in some ration distributed over tetrahedral (B) and octahedral coordination [A]. The zinc stannate compound may then be presented as (Zn2+)[Sn4+Zn2+]O4 to emphasize the site occupancy at the atomic level.

The previous solid-state synthesis investigations established that complete Zn2SnO4 formation needs prolonged mechanical activation of the starting reaction precursors (ZnO and SnO2 powders) and considerable high temperature of sintering (in the range from 1000 to 1280C). The solid-state chemical reaction between the ZnO and SnO2 starts relatively slow at 1000C

2. Mechanochemical activation and consolidation of ZnO-SnO2 powder

Finding optimal conditions of mechanochemical activation is the first and foremost important processing step for obtaining the right texture, particle size distribution and chemical reactivity of the starting powders in the solid-state synthesis of the polycrystalline Zn2SnO4 ceramics and the achievement of the wide range of its applications [21]. The mechanochemical activation consists of several processes that are usually divided into four stages: the material destruction, new surface formation, fine grinding and transformation to a completely different material structure. The optimal powder processing is a way to lower the sintering temperature necessary in the further steps of the ceramics production, which is very important fact from the stand point of the cost-effectiveness. When the starting powders of zinc oxide and tin oxide in the 2:1 molar ratio (ZnO:SnO2 = 2:1) are mechanically activated in a planetary ball mill (Fritsch Pulverisette) in time intervals 0–160 min (with 320 rpm, ball to powder mass ratio of 40:1, in zirconium grinding media of a 500 ml vial and 10 mm diameter balls) the two stages of the grinding process occurs (Figure 1). The investigated samples were marked further in the text as ZSO-00, ZSO-10, ZSO-40, ZSO-80, and ZSO-160, and these marks are related to the not-activated, 10, 40, 80, and 160 min activated 2ZnO-SnO2 powder mixtures, respectively. The first stage, up to 10 min of mechanical activation, is the increase in the powders specific surface area (SBET, calculated by BET method from the linear part of N2 adsorption isotherms [21–23]) which is related to the breaking of the powder particles and formation of the new surfaces (Figure 2a and b). The second stage brings the cold-welding and chemical reaction between the starting precursors (ZnO and SnO2 powders),

while the monophase polycrystalline zinc stannate is formed at 1280C.

system

78 Recent Advances in Porous Ceramics

Figure 1. The specific surface area vs. the time of mechanical activation for the ZnO-SnO2 powder system.

Figure 2. SEM micrographs of ZnO-SnO2 powder mixtures activated, (a) 0, (b) 10, (c) 40, and (d) 80 min.

a formation of a new phase and a decrease in the SBET (Figure 2c and d). The first sign of the spinel zinc stannate formation was noticed after 40 min of mechanical activation (results obtained by X-ray diffraction, XRD [21]). By increasing the activation time the intensities of the XRD peaks of the starting phases ZnO and SnO2, become lower while XRD peaks of the spinel Zn2SnO4 phase dominates. After 160 min of mechanical activation, the Zn2SnO4 becomes a major phase in the system. Together with the progression of the mechanochemical reaction and formation of the zinc stannate spinel a secondary agglomeration occurs as well, which contributes mostly to a continuous decrease of SBET during the second stage of the grinding process (Figure 2c and d). The microstructure of the starting ZnO and SnO2 powder mixture characterizes in the homogeneously distributed particles of two kinds, the smaller ones with a spherical shape that belongs to ZnO and the longer ones with a polygonal shape, which belongs to the SnO2. After 10 min of activation, there is a noticeable decrease in a number of the spherical particles because ZnO is more than six times softer material than SnO2 (microhardness of ZnO and SnO2 is 1.5 and 10 GPa, respectively) so firstly the drastic changes induced by mechanical activation addresses the ZnO (Figure 2).

pores [22–24]. Isotherms have a reversible part (as physical adsorption is namely reverse) at the low relative pressures, when desorption is happening along the same isotherm path as adsorption, and hysteresis loops at higher relative pressures which happen because the desorption is harder when in very porous adsorbents, like in this material system, the condensation occurs inside fine pores and capillaries at lower than equilibrium pressure. The N2 isotherm analysis (pore size distribution by Dollimore-Heal Poresizes [25] and Dubinin-Radushkevich Line [26] methods, an example shown in Figure 3b and c) gave the textural properties of the investigated different activated powder mixtures, and have shown that pores with the biggest total bulk volume were found in the powder mixture activated for 10 min, while lower total porosity (volume) was determined for the 40 min of activation, and the lowest for the other activation times (80 and 160 min) that had a smaller amount of the big pores (mesopores), like ZSO-40. The average mesopore diameters varied in the range from 22 nm (ZSO-10) to 79 nm (ZSO-80). The increase of the activation time, from 80 to 160 min, did

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The second most important step in the ceramics processing is the consolidation of a mechanically activated power mixture by pressing. The microstructure of the sintered pellets (i.e. thermally treated consolidated powder mixtures) extremely depends on the quality of the green body (pressed mechanically activated powder mixture before the sintering process). It is very crucial to the whole technology of the ceramics production to establish the mathematical–physical correlation between the pressure of the compaction (consolidation), and the main macroscopic features of the investigated material (density or porosity), that is, to determine the compressibility of the investigated powders (dependence of the green body density vs. the compaction pressure). These compressibility investigations have actually a very practical nature, that is, to determine the pressing pressure needed for each of the investigated powder mixtures to obtain a specifically desired density. Different mechanochemically activated ZnO-SnO2 powders were pressed under 49–392 MPa into 10 mm pellets. The green body density (green density) was determined for each of the samples by measuring the weight and dimensions of the obtained pellets with an error not bigger than 1% [27]. Figure 4 shows the influence of the consolidation pressure vs. relative green density for differently activated powders

The green densities of the nonactivated ZnO-SnO2 powder mixture obtained under several applied pressures are significantly lower compared to the green densities of the 10, 40, 80 and 160 min activated ZnO-SnO2 powder mixtures. The highest densities are obtained for the lowest activation time (10 min). Because the green densities of the 80 and 160 min activated powder mixtures have really close values, for clarity, the relative green densities of the 160 min activated powder mixture were left out in Figure 4. All dependency curves in Figure 4 have the same shape, while their relationships point out to the general rule that the longtime of mechanical activation and the same applied compaction pressure results in a green density decrease. This is probably a consequence of the formation of a larger number of harder agglomerates (Figure 2) with prolonged activation and is actually a typical compressibility behavior of the powder materials. In other words, longer mechanical activation demands higher pressure force to obtain the green body with approximately same density. For the 80 min (and 160 min) activated ZnO-SnO2 powder system, it is necessary to apply four times higher pressure than for 10 min

not bring further texture-porosity evolution [21].

from the ZnO-SnO2 system.

The shape of the adsorption isotherms (example shown in Figure 3a) of all the investigated samples (different mechanically activated powder mixtures) confirms mono-multi layered adsorption on the clear and stable powder surfaces with a morphology that suffered fragmentation and aggregation during the mechanical activation and is characterized by macropores or even limited number of the micropores [21]. According to the IUPAC classification, these adsorption–desorption isotherms belong to the aggregated particles that form slit-shaped

Figure 3. The nitrogen adsorption–desorption isotherm (a), the pore size distribution (b), and the pore volume distribution by Dubinin Radushevich line method (c), for the 10 min activated ZnO-SnO2 powder mixture.

pores [22–24]. Isotherms have a reversible part (as physical adsorption is namely reverse) at the low relative pressures, when desorption is happening along the same isotherm path as adsorption, and hysteresis loops at higher relative pressures which happen because the desorption is harder when in very porous adsorbents, like in this material system, the condensation occurs inside fine pores and capillaries at lower than equilibrium pressure. The N2 isotherm analysis (pore size distribution by Dollimore-Heal Poresizes [25] and Dubinin-Radushkevich Line [26] methods, an example shown in Figure 3b and c) gave the textural properties of the investigated different activated powder mixtures, and have shown that pores with the biggest total bulk volume were found in the powder mixture activated for 10 min, while lower total porosity (volume) was determined for the 40 min of activation, and the lowest for the other activation times (80 and 160 min) that had a smaller amount of the big pores (mesopores), like ZSO-40. The average mesopore diameters varied in the range from 22 nm (ZSO-10) to 79 nm (ZSO-80). The increase of the activation time, from 80 to 160 min, did not bring further texture-porosity evolution [21].

a formation of a new phase and a decrease in the SBET (Figure 2c and d). The first sign of the spinel zinc stannate formation was noticed after 40 min of mechanical activation (results obtained by X-ray diffraction, XRD [21]). By increasing the activation time the intensities of the XRD peaks of the starting phases ZnO and SnO2, become lower while XRD peaks of the spinel Zn2SnO4 phase dominates. After 160 min of mechanical activation, the Zn2SnO4 becomes a major phase in the system. Together with the progression of the mechanochemical reaction and formation of the zinc stannate spinel a secondary agglomeration occurs as well, which contributes mostly to a continuous decrease of SBET during the second stage of the grinding process (Figure 2c and d). The microstructure of the starting ZnO and SnO2 powder mixture characterizes in the homogeneously distributed particles of two kinds, the smaller ones with a spherical shape that belongs to ZnO and the longer ones with a polygonal shape, which belongs to the SnO2. After 10 min of activation, there is a noticeable decrease in a number of the spherical particles because ZnO is more than six times softer material than SnO2 (microhardness of ZnO and SnO2 is 1.5 and 10 GPa, respectively) so firstly the drastic

The shape of the adsorption isotherms (example shown in Figure 3a) of all the investigated samples (different mechanically activated powder mixtures) confirms mono-multi layered adsorption on the clear and stable powder surfaces with a morphology that suffered fragmentation and aggregation during the mechanical activation and is characterized by macropores or even limited number of the micropores [21]. According to the IUPAC classification, these adsorption–desorption isotherms belong to the aggregated particles that form slit-shaped

Figure 3. The nitrogen adsorption–desorption isotherm (a), the pore size distribution (b), and the pore volume distribu-

tion by Dubinin Radushevich line method (c), for the 10 min activated ZnO-SnO2 powder mixture.

changes induced by mechanical activation addresses the ZnO (Figure 2).

80 Recent Advances in Porous Ceramics

The second most important step in the ceramics processing is the consolidation of a mechanically activated power mixture by pressing. The microstructure of the sintered pellets (i.e. thermally treated consolidated powder mixtures) extremely depends on the quality of the green body (pressed mechanically activated powder mixture before the sintering process). It is very crucial to the whole technology of the ceramics production to establish the mathematical–physical correlation between the pressure of the compaction (consolidation), and the main macroscopic features of the investigated material (density or porosity), that is, to determine the compressibility of the investigated powders (dependence of the green body density vs. the compaction pressure). These compressibility investigations have actually a very practical nature, that is, to determine the pressing pressure needed for each of the investigated powder mixtures to obtain a specifically desired density. Different mechanochemically activated ZnO-SnO2 powders were pressed under 49–392 MPa into 10 mm pellets. The green body density (green density) was determined for each of the samples by measuring the weight and dimensions of the obtained pellets with an error not bigger than 1% [27]. Figure 4 shows the influence of the consolidation pressure vs. relative green density for differently activated powders from the ZnO-SnO2 system.

The green densities of the nonactivated ZnO-SnO2 powder mixture obtained under several applied pressures are significantly lower compared to the green densities of the 10, 40, 80 and 160 min activated ZnO-SnO2 powder mixtures. The highest densities are obtained for the lowest activation time (10 min). Because the green densities of the 80 and 160 min activated powder mixtures have really close values, for clarity, the relative green densities of the 160 min activated powder mixture were left out in Figure 4. All dependency curves in Figure 4 have the same shape, while their relationships point out to the general rule that the longtime of mechanical activation and the same applied compaction pressure results in a green density decrease. This is probably a consequence of the formation of a larger number of harder agglomerates (Figure 2) with prolonged activation and is actually a typical compressibility behavior of the powder materials. In other words, longer mechanical activation demands higher pressure force to obtain the green body with approximately same density. For the 80 min (and 160 min) activated ZnO-SnO2 powder system, it is necessary to apply four times higher pressure than for 10 min

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

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

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

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

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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

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

close to the theoretical density of the equimolar mixture of ZnO and SnO2 (6.24 g/cm3

grains are being separated in the way that is illustrated in Figure 6.

ZSO-10 samples.

) is very

). Several
