Wallace Arden Smith

The role of piezocomposites in ultrasonic transducers

Combining a piezoelectric ceramic and a passive polymer to form a piezocomposite allows the transducer engineer to design new piezoelectrics that offer substantial advantages over the conventional piezoelectric ceramics and polymers. The rod composite geometry provides materials with enhanced electromechanical coupling and with acoustic impedance close to that of tissue; these factors yield transducers for medical ultrasonic imaging with high sensitivity and compact impulse response. The dice-and-fill technique produces piezocomposites that can be readily formed into complex shapes to facilitate focusing the ultrasonic beam. Proper design of the rod spacing yields materials that exhibit low crosstalk between array elements formed by patterning the electrode alone, without cutting between the elements. In this way, curved annular arrays have been made that provide high-quality clinical images of substantial diagnostic value to physicians. Included is an extensive bibliography of papers documenting the role of piezocomposites in ultrasonic imaging transducers.<<ETX>>

Design of piezocomposites for ultrasonic transducers

Abstract 1–3 piezoelectric-rod/passive-matrix composites offer advantages over the conventional piezoceramics and piezopolymers for the pulse-echo transducers used in medical ultrasonic imaging. Their benefits include high electromechanical coupling, acoustic impedance close to that of tissue, a wide range of dielectric constants, low dielectric and mechanical losses, an adjustable sound speed, low coupling to spurious oscillations, ease of subdividing into acoustically isolated array elements, and formability into complex curved shapes. Not all benefits are achieved simultaneously. In designing a material for a specific application, the material engineer can choose the piezoceramic, the passive matrix, their relative proportions and the spatial scale of the composite. We delineate the trade-offs in designing piezocomposites which enhance the performance of present ultrasonic transducers as well as make new transducer designs feasible.

New opportunities in ultrasonic transducers emerging from innovations in piezoelectric materials (Invited Paper)

Piezoelectric materials lie at the heart of ultrasonic transducers. These materials convert electrical energy into mechanical form when generating an interrogating acoustic pulse and convert mechanical energy into an electric signal when detecting its echoes. This paper first surveys the piezoelectric materials in current use: piezoceramics, such as barium titanate, lead zirconate titanate, and modified lead titanate; piezopolymers, such as polyvinylidene difluoride and its copolymer with trifluroethylene; and piezocomposites, consisting of piezoceramic rods in a passive polymer matrix. Each material system has properties which commend them for use in the present single element transducers, annular arrays, sequenced linear arrays, and steered phased arrays. Looking to the future, new transducer possibilities are opening up due to recent piezoelectric material developments, such as, for example, synthesis techniques for fine-grained high-density piezoceramics, electrostrictive relaxor ferroelectric ceramics, novel piezoceramic forming methods, piezoceramic fiber synthesis, piezoceramic/metal multilayer structures, composite acoustoelectric materials, ferroelectric thin film growth and processing, and new piezopolymers. These innovations lead to fabrication of conventional transducers at high frequencies, fine-scale piezocomposites, 11/2-D and 2-D arrays, small intravascular transducers, as well as provide opportunities for new ultrasonic imaging techniques, using pitch-catch and non-resonant traveling wave transducers.

Optimizing electromechanical coupling in piezocomposites using polymers with negative Poisson's ratio

The invention of methods to make new materials with an engineered microstructure that yields a negative Poisson ratio opens a new avenue to optimize the performance of piezoelectric-rod/polymer-matrix composites. Such negative Poissons ratio materials can be used as the passive phase in the composite to redirect the external stress acting on the piezocomposite so that the resulting stress bearing on the piezoceramic rods produces a maximal piezoelectric response. To project the properties of such a piezocomposite, a simple physical model that assumes constant strain along the rods and constant stress in the perpendicular plane is used. Interesting opportunities to improve the performance of devices made from 1-3 piezocomposites are seen in results calculated for the thickness-mode electromechanical coupling constant relevant to pulse-echo ultrasonic applications and for the hydrostatic electromechanical coupling constant relevant to passive hydrophones.<<ETX>>

Piezocomposite Materials for Acoustical Imaging Transducers

This overview describes the application of composite piezoelectric materials in acoustical imaging transducers. Attention is focused on one composite structure which has found particularly fruitful applications in acoustical imaging—the 1–3 piezocomposite structure consisting of long thin piezoceramic rods held parallel to each other by a polymer matrix. These piezocomposites may be viewed as ‘new’ materials with a set of ‘effective’ homogeneous materials properties whenever the spatial scale of the constituents in the composite structure is smaller than the wavelength of sound. From knowledge of the sub-wavelength ‘microstructure,’ we obtain an intuitive understanding of the origin of the piezocomposite’s properties, and see how the material properties can be tuned to specific transducer application needs. Commercial applications in medical ultrasonic diagnostics, non-destructive testing, and undersea imaging illustrate the practical exploitation of 1-3 piezocomposites.

Limits to the enhancement of piezoelectric transducers achievable by materials engineering

The case of a piezoelectric material with infinity m axial symmetry (applicable to piezoceramics and most piezocomposites) is analyzed, and formulae for the resulting structures are derived. In terms of transducer design, the constraints on the elastic and dielectric constants provide no blockage to achieving acoustic impedance matching to the medium and electric impedance matching to the electronics. Electromechanical coupling is shown to be limited by the materials maximal coupling factors. A simple formula is derived which facilitates evaluating the maximal electromechanical coupling factor when the electric field is applied along the polar axis from the planar coupling factor and the thickness coupling factor, which can be readily evaluated from the lateral and thickness resonances of a disk.<<ETX>>

Evaluation of the properties of 1-3 piezocomposites of a new lead titanate in epoxy resins

Abstract By using a new calcium-modified lead titanate ceramic with a near-zero planar coupling coefficient, a series of 1-3 piezocomposite samples was fabricated with a dice-and-fill technique. The ceramic rods were approximately 0.10 mm in size, and the percent of ceramic loading varied from 10 to 30%. Two epoxy resins with different glass transition temperatures and moduli were used. The dielectric properties and the piezoelectric dh and gh coefficients of the composites were measured as a function of pressure and temperature and were found to exhibit little variation, but 10-25% lower than theoretical predictions. A prototype hydrophone made from one of the piezocomposite samples was tested to show a constant free-field voltage sensitivity of -201 dB re V/μPa from 100 Hz to 6 kHz.

Suppression of Noise to Extract the Intrinsic Frequency Variation from an Ultrasonic Echo

We suppress the effects of noise and extract the intrinsic frequency variation from an ultrasonic echo using a singular value decomposition of the Wigner distribution of the signal. This method performs a non-linear filtering on the echo signal to reduce greatly the interference effects between overlapping echoes - correlated noise, as well as uncorrelated electronic noise. We can then evaluate the local frequency both accurately (i.e. without bias by noise) and efliciently (i.e. with a small data set). This analysis is carried out on the Wigner time-frequency function calculated from a segment of echo data. With the help of the singular value decomposition of thc Wigner distribution, we reject echo segments corrupted by interference and noise. In this way, correlated interference is suppressed much more efficiently than by averaging. We illustrate this approach on the traditional problem of estimating the attenuation slope from the downward shift in mean frequency with depth. Our analysis of simulated echoes shows that accurate estimates can be obtained from a single A-line. Moreover, we obtain the correct answer with two orders of magnitude less data than the conventional Fourier approach. The resulting savings in data acquisition time and computation time are substantial.

Piezoelectric properties of 1-3 composites of a calcium-modified lead titanate in epoxy resins

A series of 1-3 piezocomposite samples was fabricated by using lead titanate ceramic with a dice-and-fill technique. The ceramic rods were approximately 0.10 mm in size, and the ceramic volume fraction varied from 10% to 30%. Two different epoxy resins were used. The piezoelectric d/sub h/ and g/sub h/ coefficients of the composites were found to be stable with pressure from ambient to 20 MPa. The temperature dependence of the composite properties was similar to that of the solid ceramic. The free-field voltage sensitivity of a prototype hydrophone made from the composite was calibrated and shown to be constant up to 6 kHz.<<ETX>>

First principles calculations for ferroelectrics — a vision

The potential for using first principles calculations for ferroelectrics is delineated. Significant insights into the microscopic origins of ferroelectricity are expected. In terms of practical mat...