L.F. Brown

Design considerations for piezoelectric polymer ultrasound transducers

Much work has been published on the design of ultrasound transducers using piezoelectric ceramics, but a great deal of this work does not apply when using the piezoelectric polymers because of their unique electrical and mechanical properties. The purpose of this paper is to review and present new insight into seven important considerations for the design of active piezoelectric polymer ultrasound transducers: piezoelectric polymer materials selection, transducer construction and packaging requirements, materials characterization and modeling, film thickness and active area design, electroding selection, backing material design, and front protection/matching layer design. Besides reviewing these design considerations, this paper also presents new insight into the design of active piezoelectric polymer ultrasonic transducers. The design and fabrication of an immersible ultrasonic transducer, which has no adhesive layer between the active element and backing layer, is included. The transducer features direct deposition of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer onto an insulated aluminum backing substrate. Pulse-echo tests indicated a minimum insertion loss of 37 dB and -6 dB bandwidth of 9.8 to 22 MHz (71%). The use of polymer wear-protection/quarter-wave matching layers is also discussed. Test results on a P(VDF-TrFE) transducer showed that a Mylar/sup TM/ front layer provided a slight increase in pulse-echo amplitude of 15% (or 1.2 dB) and an increase in -6 dB pulse-echo fractional bandwidth from 86 to 95%. Theoretical derivations are reported for optimizing the active area of the piezoelectric polymer element for maximum power transfer at resonance. These derivations are extended to the special case for a low profile (i.e., thin) shielded transducer. A method for modeling the non-linear loading effects of a commercial pulser-receiver is also included.

Spin-cast P(VDF-TrFE) films for high performance medical ultrasound transducers

Ferroelectric polymers are used in many special medical and industrial ultrasonic transducer applications. In most of these applications a free-standing metallized (i.e., patterned) film is adhered to a substrate material which is either reflective (i.e., high acoustic impedance) or acoustically matched in impedance but highly lossy. The adhesive layer can have adverse effects on ultrasonic transducer performance, especially at high frequencies. This work focussed on the direct formation of high performance ferroelectric copolymer films onto substrate materials suitable for fabricating medical ultrasound transducers. P(VDF-TrFE) copolymer resin was dissolved into methylethyl-ketone and subsequently spin-coated onto various ultrasound transducer substrate materials. After deposition, the 6-25 /spl mu/m thick films were cured, crystallized and poled on the substrates at fields of 100-120 V//spl mu/m. Several AC and DC poling techniques were used. The films were then characterized for their high frequency properties of interest for medical ultrasound, including their clamped dielectric properties, electromechanical coupling coefficient (k/sub t/), and mechanical quality factor (Q/sub m/). The results yielded k/sub t/ values as high as 0.30 and Q/sub m/ values in excess of 24. Ultrasonic tests confirmed that high performance broadband ultrasound transducers can be fabricated with ferroelectric polymers without the need for adhesive layers.

Real-time estimation of the ECG-derived respiration (EDR) signal using a new algorithm for baseline wander noise removal

Numerous methods have been reported for deriving respiratory information such as respiratory rate from the electrocardiogram (ECG). In this paper the authors present a real-time algorithm for estimation and removal of baseline wander (BW) noise and obtaining the ECG-derived respiration (EDR) signal for estimation of a patient’s respiratory rate. This algorithm utilizes a real-time “T-P knot” baseline wander removal technique which is based on the repetitive backward subtraction of the estimated baseline from the ECG signal. The estimated baseline is interpolated from the ECG signal at midpoints between each detected R-wave. As each segment of the estimated baseline signal is subtracted from the ECG, a “flattened” ECG signal is produced for which the amplitude of each R-wave is analyzed. The respiration signal is estimated from the amplitude modulation of R-waves caused by breathing. Testing of the algorithm was conducted in a pseudo real-time environment using MATLABTM, and test results are presented for simultaneously recorded ECG and respiration recordings from the PhysioNet/PhysioBank Fantasia database. Test data from patients were chosen with particularly large baseline wander components to ensure the reliability of the algorithm under adverse ECG recording conditions. The algorithm yielded EDR signals with a respiration rate of 4.4 breaths/min. for Fantasia patient record f2y10 and 10.1 breaths/min. for Fantasia patient record f2y06. These were in good agreement with the simultaneously recorded respiration data provided in the Fantasia database thus confirming the efficacy of the algorithm.

Ferroelectric nylon materials and their feasibility for ultrasound transducers

Measurements of the high frequency electrical and acoustic properties of oriented ferroelectric nylon-7 and nylon-11 are reported for the frequency range of 10 to 50 MHz and temperature range of 20 to 160/spl deg/C. The dielectric properties of nylon films were measured over a broadband frequency range including the fundamental halfwave resonance frequency for each sample, and a curve-fitting resonance technique was used to determine the properties of interest for ultrasound transducer design. Despite the high remanent polarization measured in the nylon films (1.25 mC/cm/sup 2/), the measured electromechanical coupling constant (k/sub t/=0.11), and mechanical quality factor (Q/sub m/=8) were lower than typically seen for PVDF and P(VDF-TrFE) copolymer. Poor thickness uniformity of the nylon film samples is suspected of dampening the features of the dielectric spectra near resonance, resulting in lower measured values for k/sub t/ and Q/sub m/. Comparisons of both simulated and actual measured pulse-echo performance showed the nylon materials to be competitive with PVDF for ultrasonic transducer performance. Actual pulse-echo tests with nylon-7 yielded a -6 dB fractional bandwidth of 144%. Since the ferroelectric nylons are known to be hydrophilic, tests were conducted to quantitatively determine the effects of water absorption on the ultrasonic performance of nylon-7 transducers. The results showed the water exposure slightly reduces the pulse-echo amplitude response and resonant frequency of a nylon-7 transducer. The water absorption effects were shown to be fully reversible upon drying the nylon film.

New developments in piezoelectric polymer ultrasound transducers and transducer systems

Piezoelectric polymers, such as PVDF and its copolymers, are finding increased use for ultrasound transducers in both medical and nondestructive testing applications. Because of their inherent properties of high compliance, low acoustic impedance, availability in large areas, and broadband acoustic performance, they are particularly useful in medical applications that require miniature transducers for high-frequency/high-resolution and low ultrasonic penetration, such as invasive medical imaging. The technology also provides great utility in nondestructive testing applications which require low-profile ultrasonic inspection of fiber-composite structures, especially for non-planar surfaces. This paper reviews some recent developments in piezoelectric polymer ultrasound transducer technology in both the medical and nondestructive testing (NDT) application areas. The presentation covers recent developments in invasive medical ultrasound transducers and transducer systems that employ the piezoelectric polymer technology for such applications as intraluminal imaging of the coronary arteries. A discussion of piezoelectric polymer ultrasound sensor arrays for NDT of fiber composite structures is also included.

The effects of material selection for backing and wear protection/quarter-wave matching of piezoelectric polymer ultrasound transducers

Selection of the proper backing material for a piezoelectric polymer ultrasonic transducer demands careful design consideration. Many ultrasonic transducers also require a front layer to protect the otherwise exposed electroding from the environment. By carefully choosing the right front layer material, one can also improve acoustic performance with the use of a quarter-wave matching layer. However, there are few practical materials which can be effectively used for quarter-wave matching polymer transducers. This paper describes the practical selection of materials for the design of the backing substrate and front wear/matching layers for piezoelectric polymer ultrasound transducers.

High vinylidene-fluoride content P(VDF-TrFE) films for ultrasound transducers

A significant limitation for commercially available (non-voided) PVDF is its permanent degradation of piezoelectric properties when exposed to temperatures above approximately 65/spl deg/C, and nearly complete loss of piezoelectric properties when exposed to temperatures above approximately 100/spl deg/C. In addition, most commercially available P(VDF-TrFE) films have temperature limitations that rule out their use in transducers which require high temperature use or exposure (e.g., conventional medical autoclave sterilization). The purpose of this work was to investigate high-Curie temperature unoriented P(VDF-TrFE) films for their viability in ultrasound transducer applications.

High frequency dielectric and electromechanical properties of ferroelectric nylon 11

Abstract Measurements of the high frequency dielectric and thickness-mode electromechanical properties of ferroelectric nylon 11 are reported. Two sets of nylon 11 film samples were produced. The first set was melt-quenched and poled, while the second set was melt-quenched and then cold-drawn at room temperature before poling, resulting in a three-dimensionally ordered sample. Gold electrodes, 10 mm by 10 mm, were evaporated onto opposing sample surfaces. The remanent polarization of the unoriented sample measured 27 mC/m2 while that of the oriented sample measured 52 mC/m2. Samples of each film were placed in a programmable environmental chamber while connected to a Hewlett-Packard 4195A Network/Impedance Analyzer via a custom remote fixture. Broadband measurements were made of the clamped capacitance and dissipation factor, thickness-mode electromechanical coupling coefficient kt , mechanical quality factor Qm , and longitudinal sound velocity v 1 for each sample over a temperature range of 20°–160°C. C...

Custom PVDF transducers for pulse-echo testing of solid rocket motors for detection of propellant-to-boot-liner unbonds

Solid rocket motor segments (SRMs) typically consist of a rigid cylindrical shell, an elastomeric insulation liner, and a core of solid propellant. A rubber boot-liner is often included to relieve stresses between the propellant and insulation liner, produced near the ends of the SRM during the cure of the solid propellant. In the manufacture of the SRM, good bond integrity is required between the boot liner and propellant to prevent detrimental effects on motor performance, thus, a reliable NDE technique is required to detect possible unbond regions. One SRM manufacturer’s approach was a radiographic method consisting of analyzing overlapping tangential x-rays of the bond line. The SRMs had to be oriented horizontally in a special fixture.

Investigation of the dielectric and piezoelectric properties of PMN and PMN-PT materials for ultrasonic transducer applications

The dielectric and piezoelectric properties of lead magnesium niobate, Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PMN), and the solid solution systems with lead titanate, PbTiO/sub 3/ (PT), are of importance for investigating the viability of these relaxor ferroelectric materials for sensor/actuator applications. Fundamental investigations of the relationship between the synthesis and processing, atomic-level chemical structure, and the electrical properties of PMN and PMN/PT have been initiated. This presentation summarizes the results of recent investigations of the dielectric and piezoelectric properties of (1-x)PMN-xPT across a compositional range which includes the morphotropic phase boundary (0/spl les/x/spl les/0.5). Measurements include the dielectric constant (K), dielectric loss tangent (tan/spl delta//sub e/), thickness-mode piezoelectric strain constant (d/sub 33/), longitudinal sound velocity (v/sub 1/), mechanical quality factor (Q/sub m/), and the thickness-mode electromechanical coupling factor (k/sub t/) as functions of both frequency and temperature. These results are being used with various spectroscopic studies including powder XRD, solid state NMR and EPR spectroscopy, to further investigate the atomic level information of PMN and PMN-PT. The viability of these materials for electrostrictive ultrasonic transducers is being further investigated.