The viability for dry coupling of piezoelectric ultrasonic transducer components was investigated, using a thin foil of annealed silver as a filler material/coupling agent at each component interface. Criteria used for room temperature evaluation were centered on signal-to-noise ratio (SNR) and echo bandwidth, for a Li-Nb based transducer operating in pulse-echo mode. A normal clamping stress of only 25 MPa, applied repeatedly over three loading cycles on a precisely-aligned transducer stack, was sufficient to yield backwall echoes with a SNR greater than 25 dB, and a 3 dB bandwidth of approximately 65%. This compares to a SNR of 32 dB and a 3 dB bandwidth of 65%, achievable when all transducer interfaces were coupled with ultrasonic gel. The respective roles of a soft filler material, alignment of transducer components, cyclic clamping, component roughness, and component flatness were evaluated in achieving this high efficiency dry coupling, with transducer clamping forces far lower than previously reported. Preliminary high temperature tests indicate that this coupling method is suitable for high temperature and achieves signal quality comparable to that at room temperature with ultrasonic gel.

A novel approach to designing the encapsulation for a high-temperature ultrasonic transducer to be capable of continuous operation over a temperature range of 25 to 650 °C is proposed. The transducer’s active element is a heavily damped lithium niobate disc of 3 MHz resonance frequency, operating in pulse-echo mode. The initial encapsulation design is developed based on the geometrical, thermal, mechanical, electrical, and ultrasonic requirements. Two finite element modeling systems are developed to analyze the thermal-induced stresses in the transducer at low and high temperatures as well as its ultrasonic performance. The simulation results are used to optimize the design before manufacturing a transducer prototype. The prototype is tested at room and elevated temperatures to verify performance.