Rheological and dielectric studies of aggregation of barium titanate particles suspended in polydimethylsiloxane

Viscoelastic and rheo-dielectric properties were investigated for suspensions composed of barium titanate (BaTiO3) particles having an average size of 0.3 mu m and polydimethylsiloxane (PDMS). Steady shear viscosity decreased with increasing shear rate. Stepwise increase of shear rate resulted in a thixotropic decrease of the viscosity due to reorganization of aggregates of the BaTiO3 particles in the shear held. The transient change occurred through the fast and slow processes with the rime constants of 5-30 and ca 100 s, respectively. Yield stress and plasticity were also observed for suspensions of high BaTiO3 content. Since BaTiO3 particles possess a strong dipole moment due to spontaneous polarization, the suspensions exhibited dielectric dispersion due to reorientation of the particles. The dielectric constant decreased with increasing viscosity of PDMS due to the increase of the dielectric relaxation rime in the viscous medium. In order to investigate the aggregation of the particles in a shear field, we also carried out rheo-dielectric measurements in shear fields. The dielectric constant. in a shear field decreased with increasing shear rate. After sudden stop of shear flow, a recovery of the dielectric constant was observed and was resolved into the fast and slow processes. The characteristic time for the fast process was close to the relaxation time for the thixotropy. Those results were explained by assuming chain-like aggregates of BaTi03 particles formed by the dipole-dipole interactions. (C) 2000 Elsevier Science Ltd. All rights reserved.