Strong lead-free bioinspired piezoceramics for durable energy transducers

Technical Summary: Strong Lead-Free Bioinspired Piezoceramics for Durable Energy Transducers

Problem Statement
Polycrystalline piezoceramics are essential for next-generation sustainable energy transducers and electromechanical systems. However, a critical limitation of both lead-based and lead-free piezoceramics (such as Bi₀.₅Na₀.₅TiO₃ or BNT) is their intrinsic mechanical weakness. Low strength and fracture toughness compromise material reliability, constrain operational load thresholds, and exacerbate electromechanical fatigue and dielectric breakdown under cyclic loading. Previous strategies to enhance mechanical properties, such as adding second-phase particles or whiskers, have typically resulted in a trade-off where mechanical gains come at the expense of piezoelectric performance. Furthermore, while natural materials like nacre exhibit exceptional toughness through "brick-and-mortar" microstructures, transferring this design to piezoceramics is challenging because the "mortar" phase in nacre is typically weaker than the "bricks," a condition that does not align with the low toughness values of most piezoceramic compositions.

Methodology
The authors propose a bioinspired solution to simultaneously enhance mechanical durability and maintain high piezoelectric performance in lead-free BNT ceramics. The methodology involves:

1. Material Synthesis: Synthesis of platelet-like BNT "bricks" via molten salt synthesis and topochemical microcrystal conversion. These platelets possess a high degree of <001>ₚc (pseudo-cubic) texture.
2. Functionalization: The BNT platelets are functionalized with superparamagnetic iron oxide nanoparticles (SPIONS) to enable magnetic alignment and coated with colloidal silica to act as the "mortar" phase.
3. Processing: A low-intensity rotating magnetic field is used to align the functionalized platelets in an ethanol suspension, followed by sedimentation and drying. The green bodies are then consolidated using Spark Plasma Sintering (SPS) with a fast heating rate.
4. Composition: The study varies the volume fraction of silica mortar (0, 1, 3, and 5 vol%) to optimize the microstructure. Reference samples include equiaxed BNT and BNT with silica but without magnetic alignment.
5. Characterization: Comprehensive analysis includes XRD, EBSD, SEM, TEM, and EDX for microstructural and crystallographic analysis. Mechanical properties (flexural strength, fracture toughness, Young's modulus) are measured via three-point bending and single-edge-notch bending (SENB). Functional properties (polarization, strain, piezoelectric coefficients) are evaluated through P-E and S-E loops, while durability is assessed via bipolar fatigue cycling and energy harvesting tests.

Key Results

• Microstructure: The process successfully yields a textured brick-and-mortar (B&M) microstructure. The addition of silica prevents the grains from becoming equiaxed during sintering, preserving the anisotropic platelet morphology. The silica forms pockets at the edges of the bricks, creating a distinct interface.
• Mechanical Performance: The B&M BNT composites exhibit a 2- to 3-fold increase in flexural strength (reaching up to 294 MPa) and a 1.6- to 2-fold increase in fracture toughness compared to conventional equiaxed BNT. These values are comparable to structural ceramics like alumina.
• Piezoelectric Performance: Crucially, the mechanical improvements do not sacrifice piezoelectric performance. The textured B&M samples maintain high piezoelectric charge coefficients (d₃₃), with some compositions showing a 25–30% increase over the reference sample. The materials also exhibit high voltage coefficients (g₃₃) and figures of merit (FoM).
• Durability and Fatigue: The mechanical enhancements translate directly into superior durability. The B&M samples show a 10- to 15-fold increase in ferroelectric fatigue lifetime. In energy harvesting tests, the B&M cantilevers generated 46% higher voltage output and demonstrated significantly slower degradation (30% voltage drop vs. 50% for reference) after 180,000 cycles.
• Mechanism: The improvements are attributed to anisotropic residual stress fields generated by the thermal expansion mismatch between the BNT bricks and the silica mortar. Raman spectroscopy and XRD confirm that the silica pockets are under compression, placing the adjacent BNT bricks under tension. This anisotropic stress field delays crack initiation and modifies the stress intensity field, enhancing toughness without the stable crack growth mechanisms seen in traditional nacre-like composites.

Significance and Claims
The paper claims that this work establishes a broadly applicable strategy for designing high-performance, resilient functional materials. By utilizing a bioinspired brick-and-mortar architecture combined with residual stress engineering, the authors have overcome the traditional trade-off between mechanical strength and piezoelectric functionality.

• Prototype Design: The B&M BNT serves as a prototype for bioinspired piezoceramics, demonstrating that microstructural design can decouple mechanical and functional properties.
• Broad Applicability: Since the enhancement mechanism relies on microstructural engineering and the use of silica (which has a low coefficient of thermal expansion), the authors assert this strategy can be extended to other piezoelectric systems that can be synthesized as monocrystalline platelets, including BiT, BaTiO₃, and BNT-BaTiO₃ systems.
• Practical Impact: The research provides a pathway to developing durable energy transducers and sensors capable of withstanding high-stress environments and long-term cyclic loading, addressing a critical bottleneck in the deployment of lead-free piezoelectric technologies.