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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.

No MeSH data available.


Schematic of the compaction process for polyhedral particles. Dry pressed particles for (a) as-received non-compacted powder, (b) after coarsening, (c) after reorientation and (d) combination of reorientation and coarsening.
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Figure 6: Schematic of the compaction process for polyhedral particles. Dry pressed particles for (a) as-received non-compacted powder, (b) after coarsening, (c) after reorientation and (d) combination of reorientation and coarsening.

Mentions: On the other hand, a small nanosized layer of acetate would hardly be detected by HRTEM [36]. In general, there are two possibilities for how mass transport is taking part towards consequent reduction of pore volume under compaction without the necessary activation temperature for sintering. The lowest sintering temperature found for sintering of ZnO is reported to be 0.2 × Tm at an external pressure of 50 MPa [37], i.e. 400 °C, which is well above the temperature found in the environmental chamber. However, the threshold temperature for diffusion of zinc interstitials is below an absolute temperature of 130 K [38]. Moreover, this diffusion process was suggested for coarsening of ZnO particles at room temperature [39]. On top of that, acetate ion interaction with the ZnO particle surface should be taken into account. Thus, dissolved acetate ions can result into the formation of surface defects or enhance dissolution of the particle surface, as pH value should be decreased. As a consequence, the dissolution is resulting in an enrichment of dissolved zinc species, surrounding the ZnO particles. Thus, the aqueous phase can be assumed to be enriched with zinc species and hydroxide ions. Such an aqueous layer is enabled to flow between ZnO particle interspaces and precipitate again. Recently it was reported [25, 35] that formation of ZnO occurred by thermal degradation of zinc acetate at temperatures of 85 °C. Figure 6 illustrates the compaction process for polyhedral shaped ZnO particles beginning with dry pressed state (figure 6(a)). The application of higher external pressure breaks agglomerates and leads to reorientation process (figure 6(b)). Aqueous layer with dissolved zinc species is pulled between particle contacts by capillary forces at the same time. Effective contact area can be decreased by the coarsening of particles (figure 6(c)).


Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering
Schematic of the compaction process for polyhedral particles. Dry pressed particles for (a) as-received non-compacted powder, (b) after coarsening, (c) after reorientation and (d) combination of reorientation and coarsening.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 6: Schematic of the compaction process for polyhedral particles. Dry pressed particles for (a) as-received non-compacted powder, (b) after coarsening, (c) after reorientation and (d) combination of reorientation and coarsening.
Mentions: On the other hand, a small nanosized layer of acetate would hardly be detected by HRTEM [36]. In general, there are two possibilities for how mass transport is taking part towards consequent reduction of pore volume under compaction without the necessary activation temperature for sintering. The lowest sintering temperature found for sintering of ZnO is reported to be 0.2 × Tm at an external pressure of 50 MPa [37], i.e. 400 °C, which is well above the temperature found in the environmental chamber. However, the threshold temperature for diffusion of zinc interstitials is below an absolute temperature of 130 K [38]. Moreover, this diffusion process was suggested for coarsening of ZnO particles at room temperature [39]. On top of that, acetate ion interaction with the ZnO particle surface should be taken into account. Thus, dissolved acetate ions can result into the formation of surface defects or enhance dissolution of the particle surface, as pH value should be decreased. As a consequence, the dissolution is resulting in an enrichment of dissolved zinc species, surrounding the ZnO particles. Thus, the aqueous phase can be assumed to be enriched with zinc species and hydroxide ions. Such an aqueous layer is enabled to flow between ZnO particle interspaces and precipitate again. Recently it was reported [25, 35] that formation of ZnO occurred by thermal degradation of zinc acetate at temperatures of 85 °C. Figure 6 illustrates the compaction process for polyhedral shaped ZnO particles beginning with dry pressed state (figure 6(a)). The application of higher external pressure breaks agglomerates and leads to reorientation process (figure 6(b)). Aqueous layer with dissolved zinc species is pulled between particle contacts by capillary forces at the same time. Effective contact area can be decreased by the coarsening of particles (figure 6(c)).

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.