Sintering studies of nano-crystalline zinc oxide

FAST/SPS sintering of nanocrystalline zinc oxide—Part II: Abnormal grain growth, texture and grain anisotropy

“…More interestingly, the comparison of Fig. 2 with the initial green density exhibits that sintering under aqueous condition with a moderate green density of 50%TD provide the best result for retention of nanocrystallinity for ZnO so far, as much higher initial green density of >63%TD resulted into higher grain growth factor [10,37,41] . This example highlights that the reduction of temperature while increasing densifying mechanism is more powerful than grain boundary pinning by pores or pressure.…”

“…However, countless sintering studies of pure ZnO showed that polyhedral grains with quasi globular shape always result from isotropic (spherical) powder particles [4,10,[37][38][39][40][41][42]. This isotropic morphology was found to be independent from sintering process (free sintering, hot pressing, FAST/SPS) or initial particle size (nano to macro scale).…”

“…However, it is noteworthy that a much higher sintering temperature of 800 • C is necessary to achieve nearly full densification in absence of bound water. Typically, sintering temperature of 800-1300 • C are reported for full densification of ZnO [4,10,37,39,42,49]. In sum, an increase in sintering temperature causes excessive coarsening which explains the main difference between both sintering conditions.…”

“…Typically, grain growth is suppressed by the application of an external pressure [4], grain boundary pinning (addition of a second phase/doping), very high heating rates [5][6][7] or two-step sintering [4,8]. Nevertheless, extensive coarsening takes place for sintering of pure ZnO, resulting into either significant residual porosity [9] or large grain size of microns [4,10]. Moreover, grain boundary mobility is decreased by a large amount of small pores, resulting into a pinning force on the boundary.…”

“…Thus, the retention of nanocrystallinity also depends on a high green density and homogeneity of the material [2,3]. In this context it is worth to mention that dry molding of powders is typically restricted to 50-61%TD [4][5][6][7][8][9][10][11][12][13][14], but aqueous casting, e.g., pressure filtration [8], offers much higher green density by benefiting from reduced friction between particles and prevent large agglomerates [2,15]. Moreover, water is typically chemically adsorbed by the oxide surface and forms hydrated interface [16].…”