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Acceptor doping is the standard route to raising the mechanical quality factor $Q_m$ of lead-free piezoceramics for high-power and resonant applications, but in solid-state processed ceramics the same additive also changes sintering behaviour, so gains in $Q_m$ are often confounded with changes in porosity. We separate these two effects in BNT-BT-KNN ceramics modified with 0 to 0.5 wt% MgO and prepared by conventional solid-state reaction.
X-ray diffraction with Rietveld refinement confirms a perovskite structure for all compositions, and SEM shows grain sizes of about 1.8 to 2.0 µm. Relative density peaks at 0.3 wt% MgO, whereas $Q_m$, determined by resonance and anti-resonance impedance spectroscopy following IEEE 176, rises monotonically from 67 for the undoped ceramic to 350 at 0.5 wt%. Because densification and hardening peak at different compositions, the five-fold rise in $Q_m$ cannot be attributed to reduced porosity and is consistent with intrinsic acceptor hardening by Mg on the B site. The planar coupling factor stays close to 0.2 across the series, while $d_{33}$ falls from 200 to 110 pC/N and the voltage coefficient $g_{33}$ falls from 29.7 to 14.1 mV·m/N. The undoped ceramic’s $g_{33}$ exceeds those of commercial PZT-5A and PZT-5H. Compositions above 0.5 wt% showed dielectric breakdown below 20 kV/cm and could not be poled.
The series therefore offers a tunable trade-off between voltage output and mechanical quality factor within a single lead-free system: low MgO contents suit sensing and energy harvesting, and higher contents suit resonant actuation and impedance-based structural health monitoring. Dielectric and ferroelectric characterisation are discussed in relation to the proposed defect-dipole mechanism.
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| Figure caption | Piezoelectric coefficient d33 and mechanical quality factor Qm of 0.91BNT-0.05BT-0.04KNN ceramics as a function of MgO content: acceptor hardening raises Qm five-fold while d33 falls. |