K. H. (Koko) Lam - The Hong Kong Polytechnic University © All rights reserved.
Smart materials are kind of materials that have intrinsic and/or extrinsic capabilities, to respond to stimuli and environmental changes and activate their functions according to these changes. As known, there are many types of smart materials including piezoelectric, pyroelectric, magnetostrictive, magnetorheological, electrostrictive, and so on. In the past 10 years, I did the research on all these kinds of smart materials.
In the piezoelectric community, because of the environmental concern, all materials are divided into two main types: lead-based and lead-free. Lead-based single crystals have attracted much interest because of their outstanding piezoelectric performance. Their performance is ideal for being actuator and transducer elements. Nevertheless, they are fragile and expensive. Compared to the single crystals, piezoelectric ceramics are the most popular materials used in engineering applications. It is because they have stable and good piezoelectric performance. Similarly, the development of lead-free ceramics is also a hot research area in recent years. In the past, we mainly studied the electromechanical applications of PMN-PT and PIN-PMN-PT single crystals. For the ceramics, both lead-based (PZT-based, PMN-PT) and lead-free (BNT-based, KNN-based) systems were extensively studied and employed for various applications.
Besides the single-phase material, piezoelectric composites have also been developed and extensively used in engineering applications. Among various types of composites, piezoelectric ceramic / polymer 1-3 composites consist of piezoelectric rods embedded in a passive polymer matrix have been widely employed for transducer applications. With the 1-3 connectivity, a large thickness mode electromechanical coupling coefficient can be obtained. Besides, the 1-3 connectivity maintains the high piezoelectric characteristics of active ceramic phase and offers low acoustic impedance because of the incorporation of the passive phase. Since the 1-3 composites are found to have superior properties compared to the single-phase material, the composites have been studied using different materials for specific applications: polymer-based composites were developed for ultrasonic transducer applications, cement-based composites were developed for civil engineering applications, and Terfenol-D epoxy-based composites were also developed for magnetoelectric application. Besides the traditional dice-and-fill technique, the pick-and-place technique was also adopted to develop high-frequency composites and low-ceramic-volume fraction composites.
Lam, K.H.*, et al. Journal of Electroceramics, Vol.21(1-4), pp.724-728 (2008)
Lam, K.H.*, et al. Applied Physics A: Materials Science and Processing, Vol.81, pp.1451-1454 (2005)
0-3 connectivity is the other famous composite configuration. The 0-3 composites consist of piezoelectric ceramic powder embedded in an active polymer matrix. Because of the totally different piezoelectric nature of ceramic and polymer, the 0-3 composite properties can be tailored by using different poling conditions. Combining the high voltage coefficient of active polymer matrix and high strain coefficient of active ceramic inclusion, the 0-3 composites are ideal for sensor elements.