Flexoelectric barium strontium titanate (BST) hydrophones

Flexoelectric hydrophones offer the possibility of reasonable sensitivity in lead-free systems. In this work, a dense barium strontium titanate ceramic with a Ba:Sr ratio of 70:30 and an effective flexoelectric coefficient, μ~12, of 105.6 ± 0.6 μC/m at room temperature was utilized in a prototype three-point bending hydrophone with dimensions of 77 × 10 × 0.67 mm3. Tap testing of this hydrophone with a calibrated acoustic hammer showed a resonant frequency of 250 Hz and a maximum sensitivity of 80 pC/N. Finite element analysis (FEA) was employed to verify the experimental results and to refine the predictive modeling capability. FEA was used to predict the resonant frequency of devices based on geometry, boundary conditions, and material properties. The error in resonant frequency between the FEA model and the experiment was 7.6%. FEA also enables calculations of the strain gradient produced in a material, allowing the numerical analysis of the element's expected flexoelectric output. Using this technique, single and three bending point hydrophone designs were compared. The results showed a 43% increase in charge output in the three bending point design vs the single bending point design despite an average strain decrease of 48% in each electrode pair. This design would lower the voltage output by 48% in a voltage-based design unless the voltages could be added in series. FEA studies also found the greatest flexoelectric output at low frequencies with improved high frequency output using larger electrode areas.