The synthesis of nano-size powder of complex chemical composition by wet chemical methods often yields agglomerated particles and not enough controlled chemical homogeneity. The letter is due to different chemical reactivity of precursors in solution.
The high energy milling sometimes called mechanochemical synthesis successfully overcomes these problems.
The kinetics and mechanism of formation of perovskite powders by high energy milling are studied. Mechanochemical synthesis is modelled. For the synthesis of NaNbO3 from Na2CO3 and Nb2O5 the critical cumulative energy is between 7-12 KJ/g (1). Mechanism of NaNbO3 formation includes transformation of precursors to amorphous phase and subsequent formation of sodium-niobium carbonato complex and finally perovskite crystallization. The particle size of the NaNbO3 is about 20 nm and independent of ball impact energy.
This might be quite general mechanism, as confirmed in the Pb(Mg1/3Nb2/3)O3 - PbTiO3 (PMN-PT) system (2).
Using high energy milling the powder of lead free piezoelectric (K,Na,Li)(Nb,Ta)O3is prepared. After hot pressing the completely dense ceramics with very high piezoelectric coefficients is obtained.
Structural and functional properties of sintered ceramics of selected compositions of relaxor ferroelectrics PMN-PT, multiferoic Pb(Fe1/3Nb2/3)O3 (PFN) and-PFN- Pb(Mg1/2W1/2)O3 (PFN-PMW) are also discussed. The nano-size powder combined with a suitable densification technique resulted in ceramics with large variation in grain size ranging from nano to micron dimensions. This enables a so-called size effect studies in ferroelectrics. The results demonstrate a high potential of the mechanochemical synthesis as a method yielding chemically homogeneous fine ceramic powders and consequently high performing ceramics with well controlled microstructure.
(1) T. Rojac, M. Kosec, B. Malic, J. Holc, J Eur. Ceram. Soc., 2006, 26, 3711-3716.
(2) D. Kuscer, J. Holc, M. Kosec, J.Am. Ceram. Soc., 2007, 90, 29-53.