24 Septembre – Thesis defense - Onuma Santawitee
10 h30 Amphi Jean-Paul Dom - Laboratoiry IMS (Talence campus)
Study of piezoelectric devices based on free-standing printed thick-films. Application to VOCs detection using micro-cantilevers with mesoporous silica
In recent years, screen-printing process associated with sacrificial layer technique have been developed to prepare piezoelectric MEMS (MicroelectroMechanical Systems) based on lead zirconate titanate (PZT PbZrTiO3) thick films. However, material choices and process conditions (such as pastes compositions, printing conditions and thermal treatment) could affect microstructure and consequently electromechanical properties of printed thick films transducers. With PZT thick films deposited on a composite sacrificial layer based on epoxy and SrCO3, a residual porosity within PZT films is observed. Thus, the obtained PZT films have lower piezoelectric properties compared to a bulk commercial PZT. This work points out improvements of the properties of the printed piezoelectric films by appropriate PZT paste preparation and differences in choices of electrode and sacrificial layer materials. With deposition on a polyester sacrificial layer and using PZT paste containing nano-PZT particles and 3 wt% LBCU sintering aid (Li2CO3, Bi2O3, and CuO), free-standing PZT micro-disks showed improved densification after firing at 900°C (≈7.4 g/cm3). Furthermore, Ag/Pd electrodes led to better PZT/electrode interface compared to Au electrodes. As a result, higher values of effective electromechanical coupling coefficient (≈45%) and relative permittivity (≈1200) were obtained. In the second part of this work, the optimized paste was used for processing of a micro-cantilever dedicated to Volatile Organic Compounds (VOCs) detection. The principle of this sensor is based on the measurement of resonant frequency shifts of the vibrating cantilever when exposed to target species. When the cantilever coated with a sensitive coating presented high sorption capacity, these shifts could be negative due to a mass increase. For this application, various geometries of PZT cantilevers were fabricated (from 310.1 mm3 to 620.1 mm3). Then, the cantilever in size of 320.1 mm3 was chosen for sensing application because it gave acceptable values of mass sensitivity (≈27 Hz/µg) and had enough surface area for receptor depositing. Meso-cellular foam silica (MCF-Si) is one type of mesoporous materials which is attractive to be used as receptor (sensitive layer) due to its large pore size and pore volume. Here, it was synthesized by sol-gel method before functionalization by hexamethyldisilazane (HMDS) to be more hydrophobic. HMDS functionalized MCF-si’s surface area, pore size and pore volume were ≈484 m2/g, 6.59 nm and 0.906 cc/g, respectively. Finally, this porous material (≈50 µg) was dropped onto the free-end of the PZT cantilever before detection of VOCs at room temperature. Low responses under vapors of water, ethanol and benzene were noticed whereas the sensor showed a very good sensitivity to toluene vapor. Also, frequency shifts could be positive or negative because of a competition between mass and stiffness effects. These effects depended on cantilever surface (uncoated or coated with sensitive layer) and on the target vapor nature and concentration. With the acceptable values of sensitivity to toluene (≈ 0.24 Hz/ppmV) and the limit of detection (≈25 ppm), the sensor would be benefit to environmental monitoring field.