Low Temperature Sintering of Nanocrystalline Zinc Oxide: Effect of Heating Rate Achieved by Field Assisted Sintering/Spark Plasma Sintering

Abstract

Using Field Assisted Sintering Technique/Spark Plasma Sintering the effect of heating rate on the sintering of zinc oxide at a temperature of 400°C has been investigated. For the highest heating rate of 100°C/min, relative density larger than 95% was achieved whereas at low heating rates only little shrinkage occurred. Hardness measurements, Transmission Electron Microscopy, and impedance spectroscopy revealed clear differences between heating rates. It was found that residual water is responsible for this behavior, enhancing particle rearrangement and diffusion kinetics.

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

“…However, no faceting or abnormal grain growth is observed for NG20 sintered under dry conditions. In contrast, a raised fraction of faceted grain boundaries was recently observed for sintering of nanocrystalline ZnO in presence of water [60], which is discussed in context of occurrence of abnormal grain growth [61,62]. The formation of faceting is probably due to enhancement of surface diffusion by increased water vapor pressure.…”

Field‐Assisted Sintering Technology/Spark Plasma Sintering: Mechanisms, Materials, and Technology Developments

“…In general, the reduction in surface energy of the particles resulting from the adsorption of specific species of gas, water, or organics should be considered, [77] because this modifies the thermodynamical driving force to surface reduction and sintering. Schwarz et al [49] observed not only an increased densification rate for nanocrystalline zinc oxide with moisture in FAST/SPS but also a remarkable difference in final density at a lowered sintering temperature. The presence of water vapor in the sintering atmosphere is known to enhance surface diffusion of magnesia doped alumina, [65] titania, [78] magnesia, [79][80][81][82] or zinc oxide [64] due to chemical interaction of water with the surface of the oxide and surface diffusion of hydroxyl ions and protons.…”

“…[48] But in some cases (and possibly due to adsorbed water) higher heating rate can lead to higher final density, as shown for nanocrystalline zinc oxide and hydroxyapatite. [49,50] The fast densification by FAST/SPS can also reduce the interaction between the sintering material and graphite tool. An example is the full densification of boron suboxide (B 6 O) showing very high-hardness values of up to 45 GPa achieved in graphite dies by FAST/SPS whereas the synthesis in HP requires expensive hexagonal BN dies.…”

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

“…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 × T m at an external pressure of 50 MPa [37], i.e. 400 °C, which is well above the temperature found in the environmental chamber.…”