THE ELECTRORHEOLOGY OF BARIUM-TITANATE SUSPENSIONS

Barium titanate/insulating oil suspensions were investigated to determ ine the dielectric polarization mechanisms that govern their electrorh eological (ER) behavior. The dynamic yield stress of 19.3 vol % suspen sions exhibited Maxwell-Wagner-like frequency dependence, with the dyn amic yield stress increasing significantly with ac electric field freq uency, as expected for suspensions composed of particles with large di electric constants. The dynamic yield stress at a given frequency was proportional to E-m, where E is the applied electric field strength an d the exponent in increased with frequency. For electric field strengt hs of 2 kV/mm at 1 kHz, dynamic yield stresses were approximately 500 Pa. Rheological experiments in which particle surface and oil conducti vities were varied suggest that the dielectric relaxation of these sus pensions is controlled by the particles' bulk conductivity. The dielec tric relaxation depended strongly on field strength, becoming broader with increasing field strength. Harmonic analysis of the current passi ng through the suspensions verified that the nonlinear contribution to the apparent suspension conductivity increased with E and decreased w ith frequency. The current harmonics for barium titanate/dry air suspe nsions were similar to those of the oil based suspensions, suggesting that nonlinear conduction may arise from field-enhanced dissociation o f surface groups, as opposed to field-enhanced dissociation of ion pai rs within the continuous phase. (C) 1998 The Society of Rheology.