Ultrasonic Transducers Made From Freeze-Cast Porous Piezoceramics

Current methods for making porous piezoceramics often involve high material waste and limited control over pore structure. Traditional fabrication techniques like dice-and-fill are resource-intensive and not easily scalable. While porous materials offer benefits like better acoustic matching with water and higher voltage sensitivity, their production remains inefficient. Freeze-casting is a promising alternative that reduces material waste and allows for controlled pore orientation. However, prior work has not fully explored how different porosities affect transducer performance. This gap motivated researchers to investigate freeze-cast piezoceramics as a scalable solution for underwater ultrasonic transducers.

The goal of this study was to evaluate freeze-cast porous piezoceramics as a viable alternative to traditional methods for ultrasonic transducers. The researchers aimed to fabricate transducers with varying porosities and assess their acoustic performance. They sought to determine whether freeze-cast materials could match or exceed the performance of dense piezoceramics while reducing material waste. The study focused on how porosity affects resonant modes and acoustic sensitivity. By comparing transducers with different pore fractions, the authors aimed to identify optimal porosity ranges for underwater applications. They also wanted to confirm whether aligned porosity could suppress unwanted resonant behaviors. The motivation for this work was to develop a more efficient and scalable fabrication method for piezoelectric transducers.

The researchers used freeze-casting to create porous lead zirconate titanate (PZT) samples with different porosities. The freeze-casting process involved directional solidification of a slurry to form anisotropic pores. The fabricated samples were then sintered to achieve piezoelectric properties. After fabrication, the samples were tested for their piezoelectric and dielectric characteristics. The transducers were encapsulated in a water-compatible housing for acoustic testing. Off-resonance receive sensitivity and transmit voltage response (TVR) were measured at various frequencies. Electrical impedance measurements were used to detect resonant modes. Beampatterns were analyzed to observe acoustic output directionality.

Transducers made from freeze-cast PZT showed receive sensitivities of approximately [Formula: see text] off resonance. Their transmit voltage responses ranged from [Formula: see text] at 60 kHz to [Formula: see text] at 180 kHz. The most porous transducers (0.51, 0.43, and 0.33 pore fraction) exhibited thickness mode resonance. In contrast, the least porous transducers (0.29 pore fraction and dense benchmark) showed radial mode resonance. Radial mode was identified through electrical impedance peaks and lateral acoustic lobes. The freeze-cast transducers with porosity over 0.30 suppressed radial mode resonance. Their acoustic sensitivities and TVRs were comparable to dense transducers. The aligned porosity structure contributed to improved acoustic performance.

The authors propose that freeze-cast piezoceramics can match the performance of dense transducers while reducing material waste. They suggest that aligned porosity structures suppress undesired radial resonances. The results indicate that porosity levels above 0.30 are beneficial for underwater transducers. The freeze-cast method offers a scalable and low-waste alternative to traditional fabrication. The transducers with higher porosity showed consistent thickness mode resonance. Their acoustic sensitivities and TVRs were comparable to dense benchmarks. The suppression of radial mode resonance is attributed to aligned porosity. The findings support the use of freeze-cast PZT for underwater ultrasonic applications.