Residual Stress and Domain Switching in Freeze-Cast Porous Barium Titanate

In this paper, we provide the first direct evidence of enhanced domain mobility in porous barium titanate, compared to a dense sample of the same composition, facilitated by a relaxation in intergranular stress. We demonstrate that porosity can act to unclamp domains in ferroelectric ceramics, increasing the extrinsic contribution to bulk piezoelectric properties, and providing a novel route for functional property engineering in the future. Porous barium titanate with highly aligned, anisometric pores and pore volume fraction, vp = 0.52, was fabricated by the freeze casting method; dense barium titanate (relative density, Ρrel = 0.94) was fabricated for comparison. High energy synchrotron X-ray diffraction experiments with in-situ electric field dependent measurements were used to investigate the residual stress, lattice strain and domain switching contributions to electrostrain in the porous and dense barium titanate ceramics. The domain switching fraction was calculated as a function of the applied electric field. Almost twice as many domains were observed to switch in the porous barium titanate compared to the dense material at 3 kV/mm (41.7% and 21.7%, respectively), which we attribute to the significant reduction in the intergranular stress due to the high volume of zero-stiffness pores. Using a micromechanical approach, the residual stress was estimated to be 70 MPa in the dense barium titanate at 3 kV/mm compared to 40 MPa in the porous material under the same electric field, reducing to 30 MPa and 10 MPa, respectively, in the remanent state. Whilst the fraction of domain switching was enhanced, the intrinsic contribution to the piezoelectric properties was slightly reduced in the porous barium titanate compared to the dense ceramic. The findings presented in this paper demonstrate that as well as providing a route to tailor application-specific effective bulk properties, compliant and low permittivity second phases can be used to engineer the behaviour of ferroelectric composites at the lattice and domain level. This broadens significantly the microstructural design space for ferroelectric composites, which are of particular interest for piezo- and pyroelectric sensors and energy harvesters, in the future.