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Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering

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ABSTRACT

The retention of nanocrystallinity in dense ceramic materials is still a challenge, even with the application of external pressure during sintering. The compaction behavior of high purity and acetate enriched zinc oxide (ZnO) nano-powders was investigated. It was found that acetate in combination with water plays a key role during the compaction into green bodies at moderate temperatures. Application of constant pressure resulted in a homogeneous green body with superior packing density (86% of theoretical value) at moderate temperature (85 °C) in the presence of water. In contrast, no improvement in density could be achieved if pure ZnO powder was used. This compaction behavior offers superior packing of the particles, resulting in a high relative density of the consolidated compact with negligible coarsening. Dissolution accompanying creep diffusion based matter transport is suggested to strongly support reorientation of ZnO particles towards densities beyond the theoretical limit for packing of ideal monosized spheres. Finally, the sintering trajectory reveals that grain growth is retarded compared to conventional processing up to 90% of theoretical density. Moreover, nearly no radial shrinkage was observed after sinter-forging for bodies performed with this advanced processing method.

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TEM investigation of ZinCox10 powder compacted 20 h under humid warm condition (85 °C, 140 g m−3 moisture) with (a) microstructure and (b) diffraction pattern.
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Figure 4: TEM investigation of ZinCox10 powder compacted 20 h under humid warm condition (85 °C, 140 g m−3 moisture) with (a) microstructure and (b) diffraction pattern.

Mentions: Figure 3 shows SEM micrographs of fracture surfaces of samples, which were compacted in an environmental chamber under humid warm (85 °C, 140 g m−3 moisture) and dry warm (85 °C, 1 g m−3 moisture) conditions. The sample kept under the humid warm condition shows a regular distribution of particles and only small pores are found. This finding indicates a homogeneous and high packing of the ZnO crystallites (figure 3(a)). The median particle size was estimated from the fracture surface by SEM and it increased to 25 ± 7 nm (growth factor of 1.51 compared to the initial state) under the humid warm condition. In comparison, the SEM micrograph of the dry compacted sample shows a more heterogeneous particle packing and reveals the presence of pores in the scale of 10–100 nm (figure 3(b)). However, under the dry warm condition the mean particle size stays constant, while the absolute density remains constantly low. A former investigation [22] showed that massive coarsening of ZinCox10 particles takes place under the same environmental conditions, if the powder is stored for long time. Thus, preferential crystal growth along c-axis higher than 400% was observed within 24 h of storage. In contrast, the present study shows that particle coarsening is strongly retarded and no preferred crystal growth occurs if the powder is simultaneously compacted under 50 MPa, although the contacts between particles are fostered by pressure. This surprising compaction behavior only occurs for the ZinCox10 powder, but not for the NG20 powder. Therefore, the presence of zinc acetate in the ZnO powder is the key trigger for better compaction. Figure 4 shows TEM images from a sample compacted for 20 h under the humid warm condition (85 °C, 140 g m−3 moisture). The TEM micrograph in figure 4(a) gives an impression of grown ZnO particles. No indication was found for neck formation or grain boundaries between the ZnO nanocrystals by TEM and HRTEM investigation. The median primary crystallite size of 26 ± 8 nm confirms the results from SEM measurements (this represents an increase in crystallite size of 1.61 times). Figure 4(b) shows the TEM diffraction pattern with the intensity as a function of diffraction angle. The position of reflexes confirms the presence of the wurtzite phase and gives no hint for other phases. XRD analysis also gives no evidence for any additional phase except for the zincite phase of ZnO (figure 5). Neither TEM diffraction pattern nor XRD revealed the presence of phases other than wurtzite ZnO modification (figure 4(b)). The absence of possible additional phases by TEM analysis might be explained by a local temperature increase of the sample under the electron beam and zinc acetate might already be removed, as temperatures about 100 °C initiate strong degradation [35].


Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering
TEM investigation of ZinCox10 powder compacted 20 h under humid warm condition (85 °C, 140 g m−3 moisture) with (a) microstructure and (b) diffraction pattern.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC5036464&req=5

Figure 4: TEM investigation of ZinCox10 powder compacted 20 h under humid warm condition (85 °C, 140 g m−3 moisture) with (a) microstructure and (b) diffraction pattern.
Mentions: Figure 3 shows SEM micrographs of fracture surfaces of samples, which were compacted in an environmental chamber under humid warm (85 °C, 140 g m−3 moisture) and dry warm (85 °C, 1 g m−3 moisture) conditions. The sample kept under the humid warm condition shows a regular distribution of particles and only small pores are found. This finding indicates a homogeneous and high packing of the ZnO crystallites (figure 3(a)). The median particle size was estimated from the fracture surface by SEM and it increased to 25 ± 7 nm (growth factor of 1.51 compared to the initial state) under the humid warm condition. In comparison, the SEM micrograph of the dry compacted sample shows a more heterogeneous particle packing and reveals the presence of pores in the scale of 10–100 nm (figure 3(b)). However, under the dry warm condition the mean particle size stays constant, while the absolute density remains constantly low. A former investigation [22] showed that massive coarsening of ZinCox10 particles takes place under the same environmental conditions, if the powder is stored for long time. Thus, preferential crystal growth along c-axis higher than 400% was observed within 24 h of storage. In contrast, the present study shows that particle coarsening is strongly retarded and no preferred crystal growth occurs if the powder is simultaneously compacted under 50 MPa, although the contacts between particles are fostered by pressure. This surprising compaction behavior only occurs for the ZinCox10 powder, but not for the NG20 powder. Therefore, the presence of zinc acetate in the ZnO powder is the key trigger for better compaction. Figure 4 shows TEM images from a sample compacted for 20 h under the humid warm condition (85 °C, 140 g m−3 moisture). The TEM micrograph in figure 4(a) gives an impression of grown ZnO particles. No indication was found for neck formation or grain boundaries between the ZnO nanocrystals by TEM and HRTEM investigation. The median primary crystallite size of 26 ± 8 nm confirms the results from SEM measurements (this represents an increase in crystallite size of 1.61 times). Figure 4(b) shows the TEM diffraction pattern with the intensity as a function of diffraction angle. The position of reflexes confirms the presence of the wurtzite phase and gives no hint for other phases. XRD analysis also gives no evidence for any additional phase except for the zincite phase of ZnO (figure 5). Neither TEM diffraction pattern nor XRD revealed the presence of phases other than wurtzite ZnO modification (figure 4(b)). The absence of possible additional phases by TEM analysis might be explained by a local temperature increase of the sample under the electron beam and zinc acetate might already be removed, as temperatures about 100 °C initiate strong degradation [35].

View Article: PubMed Central - PubMed

ABSTRACT

The retention of nanocrystallinity in dense ceramic materials is still a challenge, even with the application of external pressure during sintering. The compaction behavior of high purity and acetate enriched zinc oxide (ZnO) nano-powders was investigated. It was found that acetate in combination with water plays a key role during the compaction into green bodies at moderate temperatures. Application of constant pressure resulted in a homogeneous green body with superior packing density (86% of theoretical value) at moderate temperature (85 °C) in the presence of water. In contrast, no improvement in density could be achieved if pure ZnO powder was used. This compaction behavior offers superior packing of the particles, resulting in a high relative density of the consolidated compact with negligible coarsening. Dissolution accompanying creep diffusion based matter transport is suggested to strongly support reorientation of ZnO particles towards densities beyond the theoretical limit for packing of ideal monosized spheres. Finally, the sintering trajectory reveals that grain growth is retarded compared to conventional processing up to 90% of theoretical density. Moreover, nearly no radial shrinkage was observed after sinter-forging for bodies performed with this advanced processing method.

No MeSH data available.


Related in: MedlinePlus