Fabrication of Damage-Tolerant PZT Thick Films for Ultrasonic Transducers
Open Access
- Author:
- Sherbondy, Rachel L
- Area of Honors:
- Materials Science and Engineering
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Dr. Susan E Trolier Mckinstry, Thesis Supervisor
Dr. Robert Allen Kimel, Thesis Honors Advisor - Keywords:
- PZT
ultrasonic transducer
piezoelectric
sol gel
corona poling
corrosion - Abstract:
- Lead zirconate titanate (PZT) thick films were synthesized to be used in ultrasonic transducers for pipeline corrosion monitoring. For this application, the films required a strong piezoelectric response, uniformity for manufacturing reproducibility, and tolerance to mechanical damage. Fabrication began with the synthesis of a PZT sol gel, to which PZT powder was added to create a slurry. The sol gel and the slurry were characterized to determine aging behavior as well as thermal processing necessary to produce high quality films. Studies were also performed on the process of mixing the slurry, where it was shown that planetary mixing could produce smaller average particle sizes much faster than ball milling, but that the powder tends to agglomerate. The films were deposited by spraying the slurry through an airbrush. Dripping was noted on the elements after spraying, producing substantial variations in the deposit thickness. To mitigate the effect of variation, deposition parameters were tested for the slurry, with the goal of creating uniform films. Decreasing the time of the cylinder in the spray zone and increasing the airflow through the deposition chamber resulted in faster-drying films and reduced thickness variation from approximately 34% to 14%, which showed progress toward the target goal of 5%. The microstructure and dielectric properties were evaluated for the completed films. The dielectric constant of the film was 150±15, which is an order of magnitude smaller than those of bulk PZT, which suggested that the films behaved as composites of PZT and air. The volume fraction of this included porosity was estimated to be approximately 35±1%. Cross-sections of the films were analyzed, and the microstructure validated the model of the system as a 3-3 composite of PZT and air. Measurements of polarization as a function of electric field showed that the films retained ferroelectric character despite the lower connectivity of the PZT compared to bulk. Films were also fabricated using an alternate processing route in which methanol was used to suspend the slurry rather than sol gel. The samples produced by this method had a very chalky texture which led to poor contact to electrodes and therefore an extremely low measurement of dielectric constant of 2.1±0.1 (probably caused by poor adherence of the electrodes to the piezoelectric). Polarization as a function of electric field curves that did not exhibit ferroelectric switching behavior due to the poor contacts, so this method was dismissed as an alternate pathway for fabrication. The corona poling method was used to pole the completed films. Parameters including the time of poling and the applied voltage divided by the distance between the corona point and the sample were tested to determine their impact on the measured d33 coefficient, and values were chosen to reproducibly yield the saturation polarization with maximum efficiency. Then an automated system was trialed using 4 corona points, and it was shown that an array of 84 samples can be poled in 20 minutes, whereas single elements required 15 minutes in the individual poling system. Studies were performed to determine the effect of heating the sample to the crystallization temperature of 600°C during processing. To exaggerate these effects, sprayed films were exposed to this temperature for 1 week, and it was observed that PZT grains near the substrate were engulfed as the steel substrate oxidized, effectively reducing the thickness of the PZT layer. The powder to sol gel ratio was increased. It was shown that increasing the powder to sol gel ratio by 40% compared to the standard value increased the remanent polarization from 1.2±0.1 μC/cm2 to 1.7±0.2 μC/cm2. The microstructure for these films was investigated, and the films with a higher powder to sol gel ratio had decreased porosity of approximately 19±2% compared to 26±4% for films made using the usual ratio.