Abstract
Wider operational bandwidth is an important requirement of an ultrasound transducer across many applications. In nature, it can be observed that several hearing organs possess a broad operating bandwidth by having a varying length scales structure. Moreover, conventional 1-3 piezoelectric composite transducers have been widely recognized for their wider bandwidth over their piezoelectric ceramic counterparts. In this paper, a novel 1-3 piezoelectric composite design using a fractal geometry, known as the Sierpinski Gasket (SG), is proposed in order to explore the potential of further extending the operational bandwidth and sensitivity of the transducer. Two equivalent 1-3 piezocomposite designs are compared to this end, one with a conventional periodic parallelepiped-shaped pillar structure and one with the SG fractal geometry, both theoretically, using a finite-element analysis package, and experimentally. The transmit voltage response and open-circuit voltage response are used to illustrate bandwidth improvement from the fractal composite design. Following the simulation results, a 580-kHz single-element transducer, utilizing the proposed SG fractal microstructure, is fabricated using a pillar placement methodology. The performance of the prototyped device is characterized and compared with a conventional 1-3 composite design, as well as with a commercial ultrasound transducer. In the one-way transmission mode, a bandwidth improvement of 27.2% and sensitivity enhancement of 3.8 dB can be found with the SG fractal design compared to an equivalent conventional composite design and up 105.1% bandwidth improvement when compared to the commercial transducer. In the one-way reception mode, the bandwidth improvement for the SG fractal design is 2.5% and 32.9% when compared to the conventional and commercial transducers, respectively.
Highlights
T HE concept of a “piezoelectric composite” ultrasound transducer is well-established [1]–[3]; such ultrasound transducers comprise an active piezoelectric phaseManuscript received June 15, 2018; accepted October 2, 2018
This paper describes the implementation of the Sierpinski gasket (SG) fractal geometry as the structure of a piezocomposite design in order to improve the transducer operational bandwidth
The simulation results showed that when applying the SG fractal geometry at fractal generation levels greater than level III; a wider bandwidth can be achieved in both transmission and reception modes compared to an equivalent conventional design
Summary
T HE concept of a “piezoelectric composite” ultrasound transducer is well-established [1]–[3]; such ultrasound transducers comprise an active piezoelectric phase. Transducer designs to enhance the operating frequency of the device for air-coupled nondestructive evaluation, the dual thickness piezocomposite and conical piezocomposite design Both designs achieved a bandwidth enhancement successfully by having a varied thickness dimension to introduce multiple thickness mode resonances into one piezocomposite design. A self-similar fractal geometry known as the Sierpinski gasket (SG), shown, will be adopted as the structure of a piezocomposite design in order to explore improvements in the bandwidth of the 1–3 composite configuration transducer This concept of engineered transducers comprised multiple length scales has been developed mathematically [24]–[26], and these analytical models indicate that by having elements with varying length scales in the piezoelectric transducer design, the device may possess a wider operational bandwidth or a higher sensitivity compared to a conventional device. The thickness coupling efficiency can be increased, leading to a potential improvement in the device sensitivity
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