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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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FTIR spectra of (a) zinc acetate dihydrate as-received non-compacted powder, (b) as-received non-compacted ZinCox10, (c) ZinCox10 powder compacted 20 h under humid warm condition (85 °C, 140 g m−3 moisture) (d) as-received non-compacted NG20 powder and (e) ZinCox10 powder fired at 800 °C. The spectra are offset for clarity.
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Figure 1: FTIR spectra of (a) zinc acetate dihydrate as-received non-compacted powder, (b) as-received non-compacted ZinCox10, (c) ZinCox10 powder compacted 20 h under humid warm condition (85 °C, 140 g m−3 moisture) (d) as-received non-compacted NG20 powder and (e) ZinCox10 powder fired at 800 °C. The spectra are offset for clarity.

Mentions: The particle size of both commercial powders was investigated prior to compaction study. The high purity powder (NG20) showed a mean particle size of 26 ± 9 and 33 nm for TEM and XRD observation, respectively. The standard powder (ZinCox10) exhibited a mean particle size of 16 ± 9 and 17 nm for TEM and XRD investigation, respectively. Thus, the mean crystallite size given by XRD corresponds to the particle size observed by TEM and is about twice as large for NG20 powder compared to ZinCox10 powder. A previous study [22] proved by TEM and XRD that the ZnO particles are isometric. Further, TEM confirms the presence of isometric particles with polyhedral shape. XRD analysis on both powders confirmed that all Bragg reflexes are attributed to the hexagonal wurtzite phase of ZnO. ZinCox10 powder was studied for its compaction and sintering behavior. A previous investigation [22] confirmed the presence of organic content (zinc acetate) inside the ZinCox10 powder by comparative infrared spectroscopy investigation with acetate dihydrate powder, whereas the coarser NG20 powder showed negligible impurity. The presence of zinc acetate is proven by FTIR analysis, which is of major interest for the explanation of the compaction behavior of the ZnO powder. FTIR spectra in reflectance mode are illustrated in figure 1 for zinc acetate dihydrate, NG20 and ZinCox10 powders. The broad band reaching from 2700 to 3600 cm−1 is present for all ZnO samples except for the sintered ZnO and corresponds to adsorbed water and OH groups [23]. The band at 470 cm−1 is found for all the ZnO specimens and can be attributed to the vibrational stretching mode of Zn–O bounding [24] (figures 1(b)–d)). The two bands around 1420 and 1560 cm−1 correspond to the symmetrical and asymmetrical stretching vibrations of carboxylate group, respectively [23, 25]. These bands are found for zinc acetate dihydrate powder as well as for the as received and the humid warm processed ZinCox10 powder. Furthermore, no acetate is present in the NG20 powder or after sintering of the ZinCox10 powder. Thus, the zinc acetate is only present in the ZinCox10 prior to sintering. It seems conclusive that residual zinc acetate is still present in the ZinCox10 powder, as zinc acetate is typically used as precursor for the synthesis of ZnO [26–30]. Thermogravimetric analysis of the dried ZinCox10 powder reveals a mass loss of 3.3 ± 0.2 wt% resulting from water and organic removal. If the entire mass is correlated to the molar amount of carbon present in acetate salt, approximately 4.16 wt% of zinc acetate should be contained in the ZinCox10 powder. Spitz et al [31] showed that acetate is covering the surface of ZnO particles. The theoretical thickness of a hypothetical continuous zinc acetate layer covering ZnO nanocrystals has been calculated by assuming a TD of 1.74 g cm−3 as well as ideally spherical ZnO nanoparticles with a mean diameter of 17.2 nm and a TD of 5.606 g cm−3. Such a homogeneous coating of zinc acetate is estimated to be 0.37 ± 0.02 nm thick, which is difficult to highlight by TEM for example.


Improved compaction of ZnO nano-powder triggered by the presence of acetate and its effect on sintering
FTIR spectra of (a) zinc acetate dihydrate as-received non-compacted powder, (b) as-received non-compacted ZinCox10, (c) ZinCox10 powder compacted 20 h under humid warm condition (85 °C, 140 g m−3 moisture) (d) as-received non-compacted NG20 powder and (e) ZinCox10 powder fired at 800 °C. The spectra are offset for clarity.
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Related In: Results  -  Collection

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Figure 1: FTIR spectra of (a) zinc acetate dihydrate as-received non-compacted powder, (b) as-received non-compacted ZinCox10, (c) ZinCox10 powder compacted 20 h under humid warm condition (85 °C, 140 g m−3 moisture) (d) as-received non-compacted NG20 powder and (e) ZinCox10 powder fired at 800 °C. The spectra are offset for clarity.
Mentions: The particle size of both commercial powders was investigated prior to compaction study. The high purity powder (NG20) showed a mean particle size of 26 ± 9 and 33 nm for TEM and XRD observation, respectively. The standard powder (ZinCox10) exhibited a mean particle size of 16 ± 9 and 17 nm for TEM and XRD investigation, respectively. Thus, the mean crystallite size given by XRD corresponds to the particle size observed by TEM and is about twice as large for NG20 powder compared to ZinCox10 powder. A previous study [22] proved by TEM and XRD that the ZnO particles are isometric. Further, TEM confirms the presence of isometric particles with polyhedral shape. XRD analysis on both powders confirmed that all Bragg reflexes are attributed to the hexagonal wurtzite phase of ZnO. ZinCox10 powder was studied for its compaction and sintering behavior. A previous investigation [22] confirmed the presence of organic content (zinc acetate) inside the ZinCox10 powder by comparative infrared spectroscopy investigation with acetate dihydrate powder, whereas the coarser NG20 powder showed negligible impurity. The presence of zinc acetate is proven by FTIR analysis, which is of major interest for the explanation of the compaction behavior of the ZnO powder. FTIR spectra in reflectance mode are illustrated in figure 1 for zinc acetate dihydrate, NG20 and ZinCox10 powders. The broad band reaching from 2700 to 3600 cm−1 is present for all ZnO samples except for the sintered ZnO and corresponds to adsorbed water and OH groups [23]. The band at 470 cm−1 is found for all the ZnO specimens and can be attributed to the vibrational stretching mode of Zn–O bounding [24] (figures 1(b)–d)). The two bands around 1420 and 1560 cm−1 correspond to the symmetrical and asymmetrical stretching vibrations of carboxylate group, respectively [23, 25]. These bands are found for zinc acetate dihydrate powder as well as for the as received and the humid warm processed ZinCox10 powder. Furthermore, no acetate is present in the NG20 powder or after sintering of the ZinCox10 powder. Thus, the zinc acetate is only present in the ZinCox10 prior to sintering. It seems conclusive that residual zinc acetate is still present in the ZinCox10 powder, as zinc acetate is typically used as precursor for the synthesis of ZnO [26–30]. Thermogravimetric analysis of the dried ZinCox10 powder reveals a mass loss of 3.3 ± 0.2 wt% resulting from water and organic removal. If the entire mass is correlated to the molar amount of carbon present in acetate salt, approximately 4.16 wt% of zinc acetate should be contained in the ZinCox10 powder. Spitz et al [31] showed that acetate is covering the surface of ZnO particles. The theoretical thickness of a hypothetical continuous zinc acetate layer covering ZnO nanocrystals has been calculated by assuming a TD of 1.74 g cm−3 as well as ideally spherical ZnO nanoparticles with a mean diameter of 17.2 nm and a TD of 5.606 g cm−3. Such a homogeneous coating of zinc acetate is estimated to be 0.37 ± 0.02 nm thick, which is difficult to highlight by TEM for example.

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