|Ph.D Student||Hanna Bishara|
|Subject||Investigation of Sensitive Piezoelectric Responses of|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Berger Shlomo|
|Full Thesis text|
Detection of ultra-small mechanical pressures (few Pascals and sub-Pascals) in an atmospheric environment is of a high technological interest in various applications such as medical diagnosis, gas leakage detection and artificial intelligence. The piezoelectric effect enables detection of a mechanical pressure by a change of the dielectric polarization inside a material. This research studies detection of ultra-small mechanical pressures in an atmospheric environment, using low dielectric permittivity piezoelectric nanocrystals and thin film in combination with flexible metal substrates. Systematic studies are made on Aluminum Nitride (AlN) thin films and Sodium Nitrite (SN) nanocrystals. Moreover, the concept is proved other materials: Lithium Sulfate Monohydrate, Glycine and Alanine. The research work investigates the correlation between the materials, preferred crystallographic orientations, residual strains and the sensitive piezoelectric responses. The sensitive piezoelectric responses were inspected by a self-made experimental setup.
AlN thin films are deposited by using rf reactive sputtering of pure aluminum target in the presence of a nitrogen gas on flexible  oriented polycrystalline aluminum foils. Different preferred crystallographic orientation of the AlN thin film were formed at different deposition temperatures; the sensitive piezoelectric responses of each orientation of the AlN thin films were inspected. A solid correlation between the crystallographic orientation of the AlN, the internal residual stress in the film and the detection sensitivity of applied mechanical pressures was found. This correlation is presented and explained based on atomic bonds mismatch at the AlN film /Al substrate interface. Ultra-high sensitivity to ultra-low pressure (air flow and sound waves) is obtained when AlN thin films have low residual strain, while strain engineering is made by tuning the crystallographic orientations. Presence of strain inside the thin films reduces the sensitivity to ultra-low pressure. In addition, the piezoelectric responses to thermal strain of AlN with different microstructures are investigated.
Nanocrystals are grown by precipitation from a liquid solution inside nano-pores made of amorphous alumina. The nanocrystals were grown with controlled preferred crystallographic orientations and residual strains. The thesis work supplies analysis of crystal growth processes and suggests models for directional crystal growth of each of the materials. All inspected nanocrystals embedded inside pores exhibited a high sensitivity to ultra-low mechanical pressures down to 0.05-3 Pa, depending on material and its microstructure.
The piezoelectric response to ultra-low pressure by SN nanocrystals is deeply studied long different crystallographic orientations. Asymmetric piezoelectric responses were found for statically compressive vs. tensile strained crystals, and explained based on the SN molecular structure and dielectric polarizability. Piezo-capacitive response of SN nanocrystals to ultra-low pressure is reported too.
The concept of ultra-low pressure piezoelectric detection is proved experimentally for Lithium Sulfate Monohydrate, Glycine and Alanine embedded inside the nanopores. The high piezoelectric sensitivity in each case is attributed to the specific crystal structure of it, e.g. hydrogen bonds, shear strains, ionic-covalent bonds. The proven ability of amino acids crystals to detect ultra-low pressures is promising for biocompatible sensitive detectors. A deep understating of the molecular polarization mechanism of a material is vital for analyzing its piezoelectric response to ultra-low pressure.