DOI: 10.25549/usctheses-c3-213490
title: Development of high frequency composites for ultrasound transducer applications

description: Conventional ultrasound imaging transducers operating in the 2-16 MHz range have a large footprint and inadequate spatial resolution to resolve fine structures in humans and small animals. Therefore, this research is intended to enhance the design and fabrication of high frequency (HF) composite annular arrays to allow for non-invasive, high-resolution imaging of small biological structures. The annular array transducer is a viable alternative to linear and phased arrays because of the capability of dynamic focusing, low electronic channel count, and more affordable fabrication cost. In this work, the design, fabrication, and testing of HF 1-3 composite annular array transducers are presented. ❧ We have developed an annular array using a piezoelectric ceramic with moderately high dielectric permittivity and electro-mechanical coupling coefficient. The design is optimized via computer simulation. Then the annular array is fabricated with the 1-3 composite of ultra fine ceramic posts of 19 μm wide separated by a 6 μm polymer kerf, and an unloaded epoxy backing layer. Eight annuli electrodes are printed on a custom-designed double-sided flexible circuit to connect the composite annuli with 75 Ohm coaxial cables. Finally, an electroplated epoxy matching layer is used as the ground side to enhance the pulse-echo response of the array. Investigation has also begun into a novel solid free form (SFF) fabrication approach using intense light projection. The overarching aim is the development of piezomaterials and composites quickly and inexpensively. ❧ During the array fabrication process a simple and novel method were established to remove the chrome-gold electrode from polymer portions of the 1-3 composite without removing the electroplating over the ceramic posts. Therefore the fabrication process was greatly simplified since photolithography or laser ablation was not needed for electrode patterning. Also because all posts have independent electrodes, the alignment between the composite and flex circuit was not critical to the success of the fabrication process. ❧ The manufactured annular array is a flat-aperture composite 8-element array transducer. The prototype array was tested and results were in a good agreement with those predicted by a circuit-analogous (KLM) model. The average center frequency estimated from the measured pulse-echo responses of array elements was 33.5 MHz and the -6 dB fractional bandwidth was 57 %. The minimum insertion loss recorded was 12.52 dB, and the maximum crosstalk between the nearest neighbor elements was less than -37 dB. Images of a wire phantom and excised porcine eye were obtained to show the capabilities of the array for high frequency ultrasound imaging. ❧ In the process of developing the 1-3 composites, a new technology capable of fabricating piezoelectric composites, and 2D annular ultrasonic arrays with complex geometry was evaluated and studied. Conventional methods for fabricating ultrasound imaging transducers especially for high frequency (&gt;20 MHz) range are expensive, time consuming and limited to relatively simple geometries. The development of an additive digital micro-manufacturing method for piezomaterials and composites based on digital micromirror devices (DMD) to be used in HF ultrasound transducers is very much desired. ❧ Different piezoelectric characteristics of the Piezomaterials developed in this approach have been tested and compared with the bulk case. The Density of the fabricated samples was increased by applying hot uniaxial press and adding PZT composite sol-gel to the photocure solution. The dielectric constant (εᵣ) and the tangent loss (tgθ) of the samples at 1 KHz were found to be around 614 and 0.025, respectively. The measured piezoelectric constant (d₃₃) of the fabricated samples was around 245×10-12 C/N, and the sound velocity about 3140 m/s. The measured electromechanical coupling coefficient (Kt) was around 0.44, and the polarization (Pᵣ) and the coercive field (Ec) were 29 μC/cm² and 95 kV/cm, respectively. ❧ The dielectric constant (εᵣ) and the tangent loss (tgθ) of the fabricated 2D segmented array prototypes at 1 KHz and its piezoelectric constant (d₃₃) were measured as around 675, 0.025, and 247×10⁻¹²C/N respectively. The measured electromechanical coupling coefficient (Kt) was around 0.43, and the sound velocity about 3050 m/s. The polarization (Pᵣ) and the coercive field (Ec) were 21 μC/cm² and 92 kV/cm, respectively.