Directional and lattice engineering growth of sapphire fibers for ultrasonic ultra-high-temperature sensors

High-temperature sensors for extreme environments are in urgent demand with the rapid development of aerospace, nuclear energy, and advanced manufacturing. Ultrasonic temperature sensors (UTS) are widely used in high-temperature sensing due to their high working temperature that approaches the melting point of the acoustic waveguide. Al2O3 single-crystal fiber (SCF), also called sapphire fiber, is a promising candidate for ultrasonic thermometry due to its ultra-high melting point (-2050 degrees C), high strength and oxidation resistance. Here, we report for the first time, to the best of our knowledge, large length-diameter ratio sapphire fibers with specific orientations (a-axis, m-axis, and c-axis) have been grown by laser-heated pedestal growth method and applied for ultrasonic temperature sensing. Sapphire fibers exhibit significant acoustic anisotropy, and a-oriented sapphire fiber has a lower acoustic velocity and a greater velocity-temperature variation, leading to higher sensitivity. Moreover, the acoustic velocity of sapphire fiber could be further decreased by doping with Cr ions owing to the enhanced lattice disorder and crystal density. As a result, an improved unit sensitivity of 28.98-52.88 ns degrees C(-1)m(-1) and a superior resolution of 1.73-0.95 degrees C have been achieved in the range of 20-1200 degrees C via a-oriented Cr:Al2O3 SCF-UTS. More importantly, the sensor performance is positively correlated with ambient temperature, accompanied with the high working temperature (-1800 degrees C) and excellent stability (-20 h), demonstrating promising prospects for applications in high-temperature thermometry. This work provides a feasible approach for designing a high-performance UTS based on acoustic anisotropy and lattice engineering.