Abstract
The considerable decrease in temperature and time makes FLASH sintering a more sustainable alternative for materials processing. FLASH also becomes relevant if volatile elements are part of the material to be processed, as in alkali-based piezoelectrics like the promising lead-free K0.5Na0.5NbO3 (KNN). Due to the volatile nature of K and Na, KNN is difficult to process by conventional sintering. Although some studies have been undertaken, much remains to be understood to properly engineer the FLASH sintering process of KNN. In this work, the effect of FLASH temperature, TF, is studied as a function of the particle size and impurity content of KNN powders. Differences are demonstrated: while the particle size and impurity degree markedly influence TF, they do not significantly affect the densification and grain growth processes. The conductivity of KNN FLASH-sintered ceramics and KNN single crystals (SCs) is compared to elucidate the role of particles’ surface conduction. When particles’ surfaces are not present, as in the case of SCs, the FLASH process requires higher temperatures and conductivity values. These results have implications in understanding FLASH sintering towards a more sustainable processing of lead-free piezoelectrics.
Highlights
FLASH sintering is a powerful technique to sinter ceramics at low temperature in a very short period of time [1]
Ceramics were obtained by FLASH sintering 99.9% ball milling (BM) powders in Isothermal Conditions (I.C.), as reported before [7], at an isothermal furnace temperature of 900 ◦ C, with 300 V/cm applied after a 30 min dwell, a current limit of 20 mA/mm2, and a holding time of 60 s
It is concluded that while the densification and grain growth processes of FLASH-sintered K0.5 Na0.5 NbO3 (KNN) ceramics are not affected by the particle size and impurity content of precursor powders, TF is
Summary
FLASH sintering is a powerful technique to sinter ceramics at low temperature in a very short period of time [1] This occurs by the application of an electric field to a green compact. Rather than applying an electric field to increase the current through the sample, a controlled current rate process produces denser and more uniform ceramics [4,5]. This approach makes it difficult to fully understand the role of the conductivity during stage I of FLASH sintering. The KNN FLASH process is studied using KNN powders with different purities and particle size distributions to establish the relation between TF and the particle size and impurity content
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