BaTiO3: from nanopowders to dense nanocrystalline ceramics

Ferrolectric ceramics with perovskite structure, like BaTiO3 (BT), are extensively used in the electronic industry for the fabrication of multilayer ceramic capacitors, piezoelectric transducers and ferroelectric memories. High-quality nanopowders with narrow particle size distribution beside suitable sintering techniques that minimise the grain growth are required to produce miniaturised and integrated devices with grain size below a few hundred of nanometres. However, polarisation in ferroelectrics is strongly affected by the physical size of the system and suppression of the ferroelectic behaviour is predicted below a critical size. Whilst the size effect was extensively investigated in powders and thin films, the behaviour of ceramics is far less known. Ultra-fine, non-agglomerated BT powders with narrow particle size distribution were prepared by precipitation from aqueous solutions of TiCl4 and BaCl2 at 90degreesC and pH approximate to 14. Preparation of 30, 50 and 100 nm dense, bulk nanocrystalline ceramics was carried out by spark plasma sintering. Crystal structure and phase transitions were studied by X-ray diffraction and differential scanning calorimetry. Permittivity measurements were also carried out. A progressive reduction of the tetragonal distortion of the polar phase with decreasing grain size was observed. The heat of transition from tetragonal to cubic phase accordingly decreases. AFM piezoresponse investigations show that nanoceramic BT with grain size of 50nm is still ferroelectric, as proved by the local hysteresis obtained in various regions. From the experimental data the critical size for disappearance of ferroelectricity in BaTiO3 has been evaluated in the range 10-30 nm.