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

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Summary

INTRODUCTION

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

SIERPINSKI GASKET GEOMETRY
MODELING
SG Fractal Composite at Fractal Generation Level IV
FRACTAL COMPOSITE TRANSDUCER FABRICATION
EXPERIMENTAL VALIDATION
Transmission Response Measurement
Reception Response Measurement
Findings
DISCUSSION AND CONCLUSION
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