|Ph.D Student||Nitzani Michal|
|Subject||Nano-Structured Composite Pyroelectric Thin Films|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Berger Shlomo|
|Full Thesis text|
Pyroelectric materials are polar dielectric crystals which belong to the group of functional electroceramics. These unique materials are used in many applications based on their ability to detect infrared radiation. The use of such materials in advanced applications requires both the technology for miniaturization and a deep understanding of the effect of down scaling on the physical mechanisms involved. In this work we deal with several aspects of size effect arising when approaching the nanometric scale; among those, the effect of nanometric confinement on crystal growth and the thermodynamic stability of the polar pyroelectric phase.
We created a new type of a pyroelectric nanocomposite thin film with preferred crystallographic orientation and alignment of the polar axis of pyroelectric triglycine sulfate (TGS)vertically to film plane. TGS is grown as nanometric rods within the porous matrix of anodized aluminum oxide (AAO). The films consist of a high density array of cylindrical nanometric pores having diameter varying in the range 10nm-100nm (1011 pores/cm2) aligned with the cylinder axis perpendicular to the film plane.
In the study of crystal growth under the confinement of cylindrical nanopores, it was found that the level of supersaturation required for an oriented crystals growth inside the pores decreases as the pore diameter decreases. The level of supersaturation is the driving force for crystal growth and therefore it was concluded that the narrow pores somehow promote the growth by an effect of surface/solution interaction.
Two models are suggested for the growth mechanism of TGS crystals with a preferred orientation of the polar direction. The first model consists of thermodynamic considerations regarding the determination of preferred orientation by an oriented nucleus. The second model is based on kinetic considerations and based on a high growth rate of the polar planes.
The pyroelectric nanocomposite thin films are characterized by a pyroelectric coefficient of ~4mC/m2K and the voltage figure of merit is calculated 40-70kV/mK. A clear pyroelectric current response was measured against a calibrated source of black body radiation in the spectral band of 8-12 mm at 1Hz, the responsivity under these conditions is calculated 2E-4 mA/W.
The thermodynamic stability of the polar phase of TGS nanocrystals was evaluated compared to bulk TGS (49°C). It was found that the Curie temperature (Tc) decrease with reducing pore diameter and was shifted to 42°C?1°C in the nanocomposite films having an average pore diameter of 14nm.