Singapore's Agency for Science, Technology and Research (A*STAR) has unveiled a groundbreaking advancement in materials science with the development of non-poled terbium-doped potassium sodium niobate ((K,Na)NbO₃ or KNN) ceramics exhibiting a giant photostriction rate. Published today in Nature Communications, this innovation achieves a bulk photostriction rate of 0.641 s⁻¹—over 100 times higher than conventional bulk ferroelectrics. Led by researchers at A*STAR's Institute of Materials Research and Engineering (IMRE), including corresponding author Kui Yao, the work promises to revolutionize remote structural health monitoring (SHM) through efficient opto-ultrasonic transduction.

Photostriction refers to the non-thermal deformation of materials induced by light, combining the bulk photovoltaic effect—where light generates a voltage without p-n junctions—and the converse piezoelectric effect, where electric fields produce mechanical strain. Traditional photostrictive materials suffer from low strain rates (< 10⁻³ s⁻¹), limiting their practical use. This new ceramic overcomes these barriers, enabling low-power, wireless ultrasound generation for non-destructive testing in hard-to-reach areas like bridges, aircraft wings, and offshore platforms.

The breakthrough aligns with Singapore's Smart Nation initiative, enhancing safety in its dense urban infrastructure and maritime sectors. As climate challenges and aging structures intensify, such technologies could reduce maintenance costs and prevent failures proactively.

Understanding Photostriction: From Concept to Ceramics

Photostriction occurs when photons excite charge carriers in a material, creating an internal electric field that interacts with the lattice via piezoelectric coupling, resulting in strain. In ferroelectrics like KNN—a lead-free alternative to toxic lead zirconate titanate (PZT)—this process is amplified by domain structures where spontaneous polarization exists without external poling.

Historically, photostriction was observed in semiconductors and polymers, but bulk ceramics lagged due to short photocarrier lifetimes and weak light-matter interactions. A*STAR's team engineered Tb-doped KNN ceramics, where terbium ions (Tb³⁺) trap carriers via 4f-electron states, extending lifetimes and allowing drift to domain walls. This screens depolarization fields, boosting efficiency.

Hierarchical nanostructures further elevate performance: dense nanodomains (<50 nm) enable rapid local photovoltaic-piezoelectric coupling, while subwavelength grains (~300 nm) scatter light internally, maximizing lattice excitation. Phase-field simulations confirmed these dynamics, showing strain buildup in microseconds.

Challenges in Conventional Structural Health Monitoring

Structural health monitoring (SHM) detects defects like cracks or corrosion in real-time using ultrasonic waves, which propagate long distances and reflect sensitively off flaws. Traditional piezoelectric transducers require wiring, batteries, and poling, posing issues in remote or harsh environments:

• Contact-based limitations: Hard to install on curved or rotating parts.
• Power demands: Active electronics drain batteries quickly.
• Depoling risks: Temperature or mechanical stress erodes performance in poled materials.
• Scalability: Dense sensor arrays increase costs for large structures like oil rigs or high-rises.

Laser ultrasonics offer non-contact alternatives but demand high-power lasers (kW levels), risking material damage and high costs. Opto-ultrasonic approaches using photostriction bridge this gap, but prior rates were insufficient for practical ultrasound amplitudes.

The Science Behind the Giant Photostriction Rate

The star metric here is the bulk photostriction rate, S = dε/dt, where ε is strain. Experiments using laser scanning vibrometry measured surface vibrations under 532 nm laser pulses (1 mJ/cm²), yielding S = 6.41 × 10⁻¹ s⁻¹—641 times faster strain generation than PZT benchmarks.

Step-by-step mechanism:

1. Light absorption creates electron-hole pairs via bulk photovoltaic effect (BPVE).
2. Tb³⁺ traps prolong carrier lifetime to ~ns, enabling diffusion over μm scales.
3. Carriers accumulate at nanodomain walls, screening fields and inducing ~0.1% local strain via converse piezo.
4. Collective motion across subwavelength grains amplifies to bulk ultrasound at kHz-MHz frequencies.

Piezoresponse force microscopy (PFM) visualized these dynamics, while finite element models predicted wave propagation matching experiments.

Experimental Breakthroughs and Validation

Fabricated via solid-state reaction and spark plasma sintering, the ceramics showed optimal doping at 1 mol% Tb. Non-poled samples generated Lamb waves in aluminum plates, detectable 1 m away—proof of remote viability.

Key metrics:

Stability tests over 10⁵ cycles showed no degradation, unlike poled counterparts.

Opto-Ultrasonic Transducers: A New Paradigm

These ceramics form transducers activated by low-power LEDs or lasers (<1 mW), generating ultrasound without contacts. Advantages include:

• Wireless powering: Light delivers energy remotely.
• Robustness: No electrodes or poling vulnerability.
• Lead-free: Environmentally sustainable.
• Scalable: Bulk processing vs. thin-film complexity.

For SHM, arrays on structures enable multiplexed sensing via wavelength-selective illumination.

Real-World Applications in Structural Health Monitoring

SHM markets project $3B growth by 2030. In Singapore, with 7,500+ high-rises and Changi Airport's expansions, remote monitoring cuts downtime. Offshore platforms (e.g., Jurong Island) benefit from underwater deployment, as prior A*STAR flex sensors showed.

Case studies:

• Aviation: Wing fatigue detection without disassembly.
• Civil: Bridge cable cracks via Lamb waves.
• Marine: Hull corrosion in ships.

Integration with IoT and AI for predictive analytics aligns with Singapore's digital twin pilots.

A*STAR's Leadership in Singapore's Materials Ecosystem

IMRE, under A*STAR, pioneers lead-free piezoelectrics, funded by RIE2025 ($25B plan). Collaborations with NUS, NTU, and industry like ST Engineering accelerate commercialization. Kui Yao's team builds on prior BPVE works, positioning Singapore as a hub for smart materials.

This supports Singapore's research landscape, fostering jobs in materials research and higher-ed careers.

Future Outlook: Commercialization and Challenges

Prototypes target TRL 4-5; challenges include scaling doping uniformity and broadband light response. Partnerships could yield sensors by 2028. Globally, it competes with laser ultrasonics but wins on power/cost.

Stakeholder views: Experts hail it as "game-changer for wireless SHM" (paraphrased from field refs). Singapore's 6G and Industry 4.0 amplify impacts.

Photo by Hector Argüello Canals on Unsplash

Implications for Higher Education and Careers

This innovation underscores Singapore's higher-ed strengths in materials science at NUS and NTU. Aspiring researchers can pursue PhDs or postdocs in piezoelectrics. Check postdoc opportunities or career advice for advanced materials roles.

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