Epoxy-cement composite packaging for KNN piezoelectric sensors

Durable and high-performance piezoelectric sensors are critical for long-term infrastructure monitoring, especially under harsh mechanical and environmental conditions. The choice of encapsulation materials plays a decisive role in determining their stability, sensitivity, and service life. This study evaluates solvent-based and waterborne epoxy resins, with/without cement, for potassium sodium niobate (KNN)-based sensors. Cement addition in solvent-based epoxy reduced compressive strength and signal stability, yet maintained high sensitivity and good linearity under loading. Waterborne epoxy-cement composites achieved higher strength (≤57.8 MPa) and 2-4 times greater voltage output. However, these composites failed structurally at loads exceeding 600 N, limiting their use in environments with heavy loads. Fatigue tests showed that solvent-based epoxy encapsulation had excellent durability, with only a 4.4% voltage drop after 100,000 loading cycles. This indicates superior robustness for long-term deployment. The results reveal a trade-off between sensitivity and robustness in packaging strategies. Waterborne systems are suitable for low-to-moderate load conditions where sensitivity is prioritized, while solvent-based epoxies offer long-term reliability under repetitive and higher mechanical stresses. These findings provide important material selection guidelines for the development of reliable, lead-free piezoelectric sensors tailored to the specific demands of infrastructure health monitoring applications. • The influence of resin formulation on the performance of KNN-based piezoelectric sensors was comprehensively examined. • Cement-filled solvent epoxy systems offered higher voltage response, at the cost of reduced strength. • Waterborne epoxy–cement systems achieved compressive strengths up to 57.8 MPa with amplified signal output. • Solvent-based encapsulants demonstrated excellent cyclic stability over 100,000 mechanical loadings. • Findings reveal a trade-off between electrical sensitivity and structural endurance, guiding encapsulation design.