The effects of grain and particle size on the microwave dielectric properties of ferroelectric barium titanate

The use of ferroelectric ceramics and thin-films in microwave devices requires they possess frequency stable, low-loss dielectric properties. At microwave frequencies, ferroelectric polycrystalline ceramic materials exhibit a large dielectric relaxation, characterized by a decrease in the dielectric constant and a peak in the dielectric loss. 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, consequently, the relaxation phenomenon is intimately linked to the domain state of the ferroelectric. The domain state of a ferroelectric is, in part, dependent upon its microstructure. In this work, the microwave dielectric properties of ferroelectric barium titanate were measured as a function of grain and particle size. Polycrystalline ceramic ferroelectric BaTiO{dollar}sb3{dollar} (having grain sizes of 14.4, 2.14, and 0.26 {dollar}mu{dollar}m) and BaTiO{dollar}sb3{dollar} powder-polymer matrix composites (possessing average particle sizes of 1.33 {dollar}mu{dollar}m, 0.19 {dollar}mu{dollar}m, and {dollar}sim{dollar}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 a ceramic. Characterization studies were performed to determine the effects of grain size and particle size on the crystal and domain structures. Microwave dielectric measurements through 6 GHz were carried out using various techniques (lumped impedance, cavity perturbation and dielectric post resonance). All samples exhibited evidence of relaxation or resonance in their dielectric spectra. Except for the 0.26 {dollar}mu{dollar}m ceramics and the 66 nm composites, all other samples exhibited relaxation in their dielectric spectra. The 0.26 {dollar}mu{dollar}m ceramic and 66 nm composite exhibited evidence of resonance in their dielectric spectra. This work clearly showed the potential to tune the microwave properties of ferroelectrics through control of 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.