ZnO and BaTiO3 powder compaction – a physicochemical characterization study

Ceramic production is typically associated with high sintering temperatures, requires a lot of energy, and results in high CO2 emission loads. Recent advances in cold sintering [1,2] have shown that compaction of nanoparticle powders in the presence of a transient chemical liquid phase give rise to significant densification behaviour. The underlying physicochemical processes occur at moderate sintering temperatures that are significantly below those of conventional sintering approaches. Until now the role of reactive paramagnetic point defects and oxygen radicals that emerge during compaction have not been addressed.
In this study we investigated on zinc oxide (ZnO) and barium titanate (BTO) nanoparticle powders the compaction induced changes in paramagnetic defects and oxygen radicals using electron paramagnetic resonance spectroscopy (EPR).
EPR measurements revealed characteristic extrinsic and intrinsic point defects as well as oxygen radicals. In the case of ZnO a depletion of the intrinsic defects with a g factor of 1.96 can be observed as a result of compaction. This defect annihilation is attributed to local strain effects that emerge upon compaction and propagate through particle-particle interfaces. This strain leads to changes in the local electric field in the particles and impacts the redistribution of the unpaired electron. As the phenomenon strongly depends on the particle size, we propose a model that involves the different responses of particle volume and shell region to the effect.
BTO nanoparticle powder compaction, on the other hand, leads to a depletion of paramagnetic Ti3+ polaron states and to the formation of surface adsorbed oxygen radicals. We will discuss the formation of these defects in comparison to the photoexcitation [3] of related powders.