The effect of grain and particle size on the microwave properties of barium titanate (BaTiO3)

The use of ferroelectric ceramics and thin films in microwave devices requires that they possess frequency-stable, low-loss dielectric properties. At microwave frequencies, ferroelectric polycrystalline ceramic materials typically exhibit a large dielectric relaxation, characterized by a decrease in the relative permittivity (εr) and a peak in the dielectric loss (tan δ). Mechanisms attributed to the relaxation phenomenon include piezoelectric resonance of grains and domains, inertia to domain wall movement, and the emission of gigahertz shear waves from ferroelastic domain walls. As a result, the relaxation phenomenon appears to be intimately linked to the domain state of the ferroelectric. The domain state of a ferroelectric is, in part, dependent upon its microstructure. In this study, the microwave dielectric properties of ferroelectric barium titanate were measured as a function of grain and particle size. Polycrystalline ceramic ferroelectric BaTiO3 (having average grain sizes of 14.4, 2.14, and 0.26 μm) and BaTiO3 powder-polymer matrix composites (possessing average particle sizes of 1.33 μm, 0.19 μm, and ∼66 nm) were employed. The composite samples were used to decouple resonances between adjacent grains as well as reduce the three dimensional clamping experienced by grains in ceramic. Characterization studies were performed to determine the effects of grain size and particle size on the crystal structure and degree of tetragonality. Microwave dielectric measurements through 6 GHz were carried out using lumped impedance, cavity perturbation, and post resonance experiments. All samples exhibited evidence of relaxation or resonance phenomena in their dielectric spectra. Except for the 0.26 μm grain size ceramics and the 66 nm particle size composites, all other samples exhibited relaxation in their dielectric spectra. The 0.26 μm ceramic and 66 nm composite showed evidence of resonance in their dielectric spectra. This work clearly shows the potential to tune the microwave properties of ferroelectrics through control of grain/particle size and the domain state. In general, relaxation frequencies increased and loss tangents decreased with decreasing grain/particle size. The relaxation mechanisms were identified and correlated with the material characterization results and theoretical models. Relaxation frequencies were generally governed by the smallest resonant width, i.e., the domain width.