Impact of crystal structure and microstructure on electrical properties of Ho doped lead-free BCST piezoceramics

Abstract

Conventional solid state sintering method was used to synthesize lead-free (Ba0.91Ca0.09Sn0.07Ti0.93)O3-xHo2O3 (x = 0, 1.2,1.4,1.6,1.8 and 2.0 mol%) ceramics. The influence on electrical properties of the system as a result of the structural and microstructural changes introduced by the incorporation of rare earth Ho3+ ions has been investigated. The X-ray diffraction analysis reveals that Ho3+ ions completely diffuse into the (Ba0.91Ca0.09Sn0.07Ti0.93)O3 lattice to form a homogeneous solid solution with a pure perovskite structure having tetragonal symmetry. Evidence of Ho3+ substituting Ti4+ via the oxygen vacancy compensation mechanism exists in the range of 0–1.6 mol % Ho content, while the self-compensation mode is the preferred mechanism beyond 1.6 mol %. The average grain size exhibits a drastic reduction from 16 μm to 0.7 μm as the Ho content increases from 0 to 1.6 mol%, followed by a slight increase at higher Ho concentration. It suggests that addition of Ho3+ inhibits grain growth in the ceramics. In the composition range studied, increasing Ho3+ content produces a gradual decrease in the relative density from 93% to 81%, room temperature dielectric constant (εrt) from 3997 to 807, electromechanical coupling factor (kp) from 0.23 to 0.06, and piezoelectric charge constant (d33) from 102 to 38 pC/N. This degradation in the properties is attributed to the crystalline and microstructural changes driven by the increasing presence of Ho content in the ceramics.