|Ph.D Student||Roman Yasinov|
|Subject||Piezoelectric Nano-Composite Thin Films|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Berger Shlomo|
|Full Thesis text|
Dense arrays of crystalline polar piezoelectric nanorods, with a diameter of several tens of nanometers, have great importance for future nano-electromechanical devices intended for sensing and actuation. The production of such arrays requires a strict control over the uniformity of crystal size and crystallographic orientation on that scale. The piezoelectric properties of such nanorods are expected to be dependent on the crystal size with respect to the possible instability of the polar phase.
This research was aimed at studying the possibility to prepare dense arrays of highly oriented piezoelectric crystalline nanorods and forming a novel nano-composite thin film by growing piezoelectric crystals inside porous anodic alumina templates via precipitation from a supersaturated solution. The templates have thin film geometry and are composed of a dense array of high aspect ratio cylindrical pores, with a sub-micron length and a tunable average diameter of 14-84 nm, uniformly aligned with the cylinder axis oriented normally to the film plane. The main focus of this work is on the crystallographic orientation of the grown crystals and the possibility to obtain a high degree of preferred orientation with respect to the pore’s longitudinal axis (uniaxial texture).
Crystal growth inside the pores was investigated with respect to three different types of piezoelectric crystals: Rochelle Salt (RS) - a uniaxial ferroelectric, Potassium Iodate (KIO3) - a ferroelectric with complex polarization and β-Glycine - a uniaxial polar crystal. The main tool for the investigation of crystal phase and orientation was X-ray diffraction.
The results indicate the ability to obtain high degree of preferred orientation: up to about 85% for RS, 50% for KIO3 and above 70% for β-glycine. The orientation type (the crystallographic plane parallel to the film plane) is (100) for RS, (1-11) for KIO3 and (010) for β-Glycine. Comparing these results with crystal growth on top of a flat surface (dense anodic aluminum oxide) shows that the effect of the porous structure lies in enhancing the degree of preferred orientation by about 20%.
Dielectric properties of the prepared nano-composite films based on KIO3 and RS are reported for different investigated average pore diameters. For the case of KIO3 these indicate the existence of the polar phase up to the smallest investigated average pore diameter of 14 nm. The piezoelectric response of such nano-composite thin films (based on KIO3) is reported for the first time, with values of the effective transverse piezoelectric coefficient, e31,f , ranging between about 0.1-0.5 C/m2.