Development of surface acoustic wave devices as micro/nano actuators.
thesisposted on 17.02.2017, 02:14 by Tjeung, Ricky Theodore
Despite comprehensive reports on the utilization of SAW (surface acoustic wave) devices as micro/nano actuators, there are still unexplored potentials to be pursued. Most current SAW actuators utilize 128˚ Y-X cut lithium niobate (LiNbO3) piezoelectric substrates due to the high electromechanical coupling coefficient in this crystal direction. However, these substrates have a major disadvantage of being highly anisotropic, the piezoelectric coupling is therefore exceptionally dependent on the propagation direction and cut. Utilization of LiNbO3-based SAW devices as linear microactuators has been extensively reported; however, work on extending the work to rotate a solid object has been limited. At the same time, SAW based microfluidics applications has been gaining interest in the past decade; however, extension of this work, using piezoelectric materials other than LiNbO¬3 has rarely been reported. Hence, there is a need to advance this field further. This thesis proposes the use of alternative—isotropic—piezoelectric materials for SAW actuators. The materials selected for this purpose are lead zirconate titanate (PZT) and a hybrid of zinc oxide (ZnO) and sapphire (Al2O3) material. PZT and ZnO are common piezoelectric materials, but have not been used widely as SAW actuators. They are selected due to their isotropic piezoelectric properties in the plane of the substrate, which is essential for generating multi-axis actuation on the plane of the substrate The hybrid ZnO/Al2O3 material is selected to exploit the collective benefits of these materials for the development of a high performance SAW device. A higher order harmonic wave, the Sezawa wave, can be induced in a stratified material. This mode of SAW propagation is generally perceived to have superior performance compared to the Rayleigh mode. The foremost aim of this thesis is to develop SAW based actuators for both solid and liquid objects: (1) the utilization of SAW generated on PZT material to drive a rotary motor, and (2) the utilization of a hybrid ZnO/Al2O3 material for microfluidics applications. The first aim of the thesis is to develop a high speed SAW actuated motor prototype with the capability of rotating about an arbitrary axis. The following work was performed to achieve this goal: the fabrication and characterization of SAW devices made on the PZT substrate (the stator), the development of the SAW rotary motor operating with and without an external preload, and performance analysis of the motor with two different hole size configurations, specifically 0.25 and 0.45 mm. The main contribution of this work is an operational SAW actuated rotary motor with an arbitrarily chosen rotational axis. The motor has a simpler configuration and exhibits superior performance compared to SAW rotary motors reported in literature. The second aim of the project is to develop a SAW device exploiting the beneficial properties of a hybrid material for microfluidics applications, specifically SAW devices made on a ZnO/Al2O3 stratified material. The following work was performed to achieve this goal: the fabrication and characterization of the performances of SAW devices made on ZnO/Al2O3 hybrid material, verification and performance characterization of the generated Sezawa wave, and the utilization of the SAW device for microfluidics applications. The key finding of this work is that the wave vibration speed is the most influential parameter for acoustic streaming. As generally perceived, the Sezawa mode demonstrates better performance (it induces higher acoustic streaming speed) because it has higher electromechanical coupling (validated by the network analyzer data) and hence higher vibration speed for a given input power (validated by the LDV data). The main contribution of this work is the development of SAW devices based on a ZnO/Al2O3 stratified material for microfluidics application. The devices performances are comparable to those based on the more common LiNbO3 material; in addition, utilization of the Sezawa mode offers superior performances.