PhD Defence: Per Kristian Bolstad

Per Kristian Bolstad is defending his thesis for the degree philosophiae doctor (PhD) at the University of South-Eastern Norway.

The doctoral work has been carried out at the Faculty of Technology, Natural Sciences and Maritime Sciences in the program Applied Microsystems.

Welcome to follow the trial lecture and the public defence.

Link to doctoral thesis: Evaluation of Metallurgical Bonding for Ultrasound Transducers

Summary

The ultrasound transducer is an essential component in any ultrasound system, as these sensors are responsible for transmitting and receiving sound. An ultrasound transducer should function as designed for many years and endure challenging environments. For a medical ultrasound transducer, this means several hours of everyday use, over years, with rough handling, cleaning, and perhaps also autoclave sterilization. For downhole oil and gas investigations, the transducer will be exposed to the high pressures and temperatures in an oil well.

This research explores new materials and fabrication methods to ensure robustness and reliable operation of ultrasound transduces under harsh conditions. The developed fabrication methods may also contribute to improved reliability and performance of transducers under standard operating conditions.

Common ultrasound transducers use a piezoelectric element to convert between electrical and acoustical energy. The piezoelectric layer is sandwiched between acoustic matching and backing layers to maximize acoustic performance. In most transducers, these layers are glued together using a polymeric adhesive. Decades of experience verify that it functions excellently in many ultrasound transducer applications. Yet, transducers for operation in harsh environments rely on careful selection of materials, and polymer materials are known to degrade at high temperatures.

It has earlier been proposed to replace the polymeric adhesives with a metallic bond. Metals form strong bonds that can tolerate high temperatures, and they also have very good electrical and thermal conductivity. In addition, the high characteristic acoustic impedances of metals are beneficial for the acoustic performance of the transducer. 

The University of South-Eastern Norway has expertise in a bonding technique called solid-liquid interdiffusion (SLID). The technique is known from the electronics industry where it is used to make robust electrical connections. SLID bonding relies on the formation of intermetallic compounds which are stable at temperatures above the processing temperature.

This PhD thesis investigates the binary metal system of gold (Au) and Tin (Sn) for bonding essential layers of ultrasound transducers, especially the piezoelectric ceramic materials. The different metallurgical bonding techniques Au-Sn SLID and Au-Sn soldering were evaluated and compared to bonding with polymeric adhesives. The fabricated transducer prototypes were tested during and after exposure to high temperature and high pressure, and it was demonstrated that the metallurgical bonding technique offered high-temperature stability with high mechanical strength.  

A common challenge related to metallurgical bonding is the formation of voids within the layer, which are pockets containing gas or vacuum. Voids can have a negative influence on the acoustic performance of the transducer and can also affect the mechanical strength. Hence, a reliable estimation of the influence from such voids is essential if metallurgical bonding is to be used in acoustic transducers. To investigate this, a computer simulation model was developed to estimate effective medium parameters of layers containing voids of arbitrary size, concentration, and distribution.