Graded KNN-Based Lead-Free Ultrasonic Transducer for Wide-Temperature Nondestructive Testing Imaging

To advance the development of potassium sodium niobate (KNN)-based materials, devices, and their associated applications, this study introduces a collaborative approach to the optimization of materials and devices for wide-temperature, high-resolution nondestructive testing (NDT) imaging. This is achieved through the integration of graded lead-free KNN-based ceramics and wide-temperature-focused ultrasonic transducers. The graded KNN ceramics, featuring a coexistence of orthorhombic-tetragonal, rhombohedral-orthorhombic-tetragonal, and rhombohedral-tetragonal (O-T/R-O-T/R-T) crystalline phases, were engineered to enhance both thermal stability and piezoelectric performance. The <i>in</i>-<i>situ</i> deviation of the piezoelectric constant for graded KNN-based ceramics remains below 3.5% at 140 °C. Ultrasonic transducers capable of operating across a wide temperature range were designed and fabricated by using these graded KNN ceramics. The transducers demonstrated robust thermal stability and high-resolution imaging performance within a temperature range of 30 to 120 °C. C-scan images obtained across this temperature spectrum further illustrated their potential for NDT applications in diverse thermal environments. This study marks a significant breakthrough in the field of wide-temperature KNN-based ultrasonic transducers, as it indicates the first successful demonstration of imaging capabilities at temperatures exceeding 100 °C.