Dry Coupling of Ultrasonic Transducer Components for High Temperature Applications

Abstract

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.

High Temperature Ultrasonic Transducers: A Review

“…In order to avoid this problem Bhadwal et al proposed the transducer in which a spring is used to keep a constant pressure between transducer elements in a wide temperature range. The acoustic contact is obtained due to use of soft foils coupling all parts of the spring-loaded transducer at the pressure of only 25 MPa [ 32 ]. Such coupling enabled getting a good signal to noise ratio.…”

“…Therefore, between bonded surfaces a thin ductile metallic foil is inserted that partially compensates stresses caused by variations of temperature due to different thermal expansion coefficients. For such a purpose, Bhadwal et al proposed to use an annealed 50 mm thickness silver foil [ 32 ]. We have proposed to use a soft gold foil with a purity of 99.99% with a thickness 5–20 μm, which is more expensive than the silver foil, but it is more ductile [ 124 , 125 ].…”

“…The protector was made of stainless-steel 321. This transducer was confirmed to work up to 800 °C when the temperature was increased with the speed 4 °C/min [ 32 ].…”

Sensors for Ultrasonic Nondestructive Testing (NDT) in Harsh Environments

“…The second paper by Bhadwal et al [2] investigates the problem of coupling together the various components of an ultrasonic transducer that would undergo excursions to temperatures as high as 700 • C. Due to the differential thermal expansion of the piezoelement, backing layer, and wear plate, solid coupling techniques such as brazing would lead to cracking or even total transducer failure. The authors evaluated the feasibility of using dry coupling, but at far lower clamping loads than the 200-500 MPA interfacial pressure often quoted for high dry coupling efficiency.…”

High-Altitude Balloon-Based Sensor System Design and Implementation

“…The temperature in the near-space can be very low from −50 • C to −70 • C, and it is important to measure the environment temperature around the payloads and other equipment [17]. The temperature sensor system designed in this paper adopts PT100 as a temperature sensor whose resistance changed with temperature.…”