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

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.


SEM images of polished surfaces for pressureless sintered ZnO samples for (a) and (c) humid warm compacted and (b), (d) conventionally processed green bodies. The samples were sintered at 700 °C (a), (b) without dwell and (c), (d) with dwell of 1 h. The determined relative density is labeled for each image in the right upper corner.
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Figure 8: SEM images of polished surfaces for pressureless sintered ZnO samples for (a) and (c) humid warm compacted and (b), (d) conventionally processed green bodies. The samples were sintered at 700 °C (a), (b) without dwell and (c), (d) with dwell of 1 h. The determined relative density is labeled for each image in the right upper corner.

Mentions: The sintering of humid warm compacted ZnO samples seems promising in comparison to conventionally processed samples, as packing density is already very high prior to sintering step. Thus, a modified sintering trajectory is expected which should be shifted towards smaller grain sizes at a given density compared to dry processed green bodies. The densification behavior for freely sintered ZnO is illustrated in figure 7. The relative density is plotted as a function of temperature (during the heating stage) and isothermal time at a maximum temperature of 700 °C. The specimens start with 50 and 85% TD during initial stage of sintering for dry and humid warm conditions, respectively. Besides, the onset of densification with a temperature of 600 °C is 120 °C above the specimen processed under dry condition. Thus, sinterability of humid warm processed ZnO is decreased, as surface energy for humid warm specimen is reduced due to coarsening of particle size by 50% during green body processing. In comparison, particle size remains constant under conventional processing giving a slightly lower initial particle size prior to sintering. Nevertheless, maximum density of 95% was found to be equal for both samples after one hour at 700 °C, although this value was achieved 10 min later for conventionally processed specimen (dry condition) during isothermal dwell time. Hynes et al [9] observed similar final density for pressureless sintering of nanocrystalline ZnO powder at 700 °C. Additionally, Hynes et al reported about abnormal grain growth with grain sizes ranging from 67 to >530 nm, which is in contrast to the present study. The microstructural development of sintered ZinCox10 specimens is illustrated in figure 8. In general, no abnormal grain growth was observed. Figures 8(a) and (b) show microstructures that were obtained by an interrupted heating experiment for specimen processed under humid warm and dry conditions, respectively. The mean grain size with d50 = 101 nm is much smaller for humid warm processed specimen compared to conventionally processed ZnO (d50 = 206 nm), although density is already 6% higher. On the other hand grain size (d50 ∼ 300 nm) and density are similar for both specimens after sintering one hour at 700 °C (figures 8(c) and (d)). This grain size is surprisingly low taking into account that no external pressure was applied during sintering. In comparison, Mazaheri et al [8] hot pressed ZnO with equal initial powder particle size under 50 MPa external pressure and achieved a significant higher grain size (>600 nm) for the same density. The sintering trajectory (grain size versus relative density) is given in figure 9 with values that were obtained by interrupted heating experiments by sintering schedule from figure 7. The grain sizes of sintered specimen with conventionally processed ZnO result into more than two times higher values compared to humid warm processing for relative densities below 90%. The grain size increases rapidly for >90% TD, until similar grain sizes are achieved for both processing routes around 95% TD.


Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering
SEM images of polished surfaces for pressureless sintered ZnO samples for (a) and (c) humid warm compacted and (b), (d) conventionally processed green bodies. The samples were sintered at 700 °C (a), (b) without dwell and (c), (d) with dwell of 1 h. The determined relative density is labeled for each image in the right upper corner.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 8: SEM images of polished surfaces for pressureless sintered ZnO samples for (a) and (c) humid warm compacted and (b), (d) conventionally processed green bodies. The samples were sintered at 700 °C (a), (b) without dwell and (c), (d) with dwell of 1 h. The determined relative density is labeled for each image in the right upper corner.
Mentions: The sintering of humid warm compacted ZnO samples seems promising in comparison to conventionally processed samples, as packing density is already very high prior to sintering step. Thus, a modified sintering trajectory is expected which should be shifted towards smaller grain sizes at a given density compared to dry processed green bodies. The densification behavior for freely sintered ZnO is illustrated in figure 7. The relative density is plotted as a function of temperature (during the heating stage) and isothermal time at a maximum temperature of 700 °C. The specimens start with 50 and 85% TD during initial stage of sintering for dry and humid warm conditions, respectively. Besides, the onset of densification with a temperature of 600 °C is 120 °C above the specimen processed under dry condition. Thus, sinterability of humid warm processed ZnO is decreased, as surface energy for humid warm specimen is reduced due to coarsening of particle size by 50% during green body processing. In comparison, particle size remains constant under conventional processing giving a slightly lower initial particle size prior to sintering. Nevertheless, maximum density of 95% was found to be equal for both samples after one hour at 700 °C, although this value was achieved 10 min later for conventionally processed specimen (dry condition) during isothermal dwell time. Hynes et al [9] observed similar final density for pressureless sintering of nanocrystalline ZnO powder at 700 °C. Additionally, Hynes et al reported about abnormal grain growth with grain sizes ranging from 67 to >530 nm, which is in contrast to the present study. The microstructural development of sintered ZinCox10 specimens is illustrated in figure 8. In general, no abnormal grain growth was observed. Figures 8(a) and (b) show microstructures that were obtained by an interrupted heating experiment for specimen processed under humid warm and dry conditions, respectively. The mean grain size with d50 = 101 nm is much smaller for humid warm processed specimen compared to conventionally processed ZnO (d50 = 206 nm), although density is already 6% higher. On the other hand grain size (d50 ∼ 300 nm) and density are similar for both specimens after sintering one hour at 700 °C (figures 8(c) and (d)). This grain size is surprisingly low taking into account that no external pressure was applied during sintering. In comparison, Mazaheri et al [8] hot pressed ZnO with equal initial powder particle size under 50 MPa external pressure and achieved a significant higher grain size (>600 nm) for the same density. The sintering trajectory (grain size versus relative density) is given in figure 9 with values that were obtained by interrupted heating experiments by sintering schedule from figure 7. The grain sizes of sintered specimen with conventionally processed ZnO result into more than two times higher values compared to humid warm processing for relative densities below 90%. The grain size increases rapidly for >90% TD, until similar grain sizes are achieved for both processing routes around 95% TD.

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.