Thomas R. Shrout

Characteristics of relaxor-based piezoelectric single crystals for ultrasonic transducers

For ultrasonic transducers, piezoelectric ceramics offer a range of dielectric constants (K/spl sim/1000-5000), large piezoelectric coefficients (d/sub ij//spl sim/200-700 pC/N), and high electromechanical coupling (k/sub t//spl sime/50%, k/sub 33//spl sime/75%). For several decades, the material of choice has been polycrystalline ceramics based on the solid solution Pb(Zr/sub 1-x/B/sub 2x/)O/sub 3/ (PZT), compositionally engineered near the morphotropic phase boundary (MPB). The search for alternative MPB systems has led researchers to revisit relaxor-based materials with the general formula, Pb(B/sub 1/,B/sub 2/)O/sub 3/ (B/sub 1/:Zn/sup 2+/, Mg/sup 2+/, Sc/sup 3+/, Ni/sup 2+/..., B/sub 2/:Nb/sup 5+/ Ta/sup 5+/...). There are some claims of superior dielectric and piezoelectric performance compared to that of PZT materials. However, when the properties are examined relative to transition temperature (T/sub 3/), these differences are not significant. In the single crystal form, however, Relaxor-PT materials, represented by Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PZN-PT), Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-PbTiO/sub 3/ (PMN-PT) have been found to exhibit longitudinal coupling coefficients (k/sub 33/)>90%, thickness coupling (k/sub t/)>83%, dielectric constants ranging from 1000 to 5000 with low dielectric loss <1%, and exceptional piezoelectric coefficients d/sub 33/>2000 pC/N, the later promising for high energy density actuators. For single crystal piezoelectrics to become the next generation material of ultrasonic transducers, further investigation in crystal growth, device fabrication and testing are required.

Piezoceramics for high-frequency (20 to 100 MHz) single-element imaging transducers

The performance of transducers operating at high frequencies is greatly influenced by the properties of the piezoelectric materials used in their fabrication. Selection of an appropriate material for a transducer is based on many factors, including material properties, transducer area, and operating frequency. The properties of a number of piezoceramic materials have been experimentally determined by measuring the electrical impedance of air-loaded resonators whose thickness corresponds to resonance frequencies from 10 to 100 MHz. Materials measured include commercially available compositions of lead zirconate titanate (PZT) with relatively high dielectric constants and a modified lead titanate (PT) composition with a much lower dielectric constant. In addition, materials which have been designed or modified to result in improved properties at high frequencies are studied. Conclusions concerning the influence of the microstructure and composition on the frequency dependence of the material properties are made from the calculated properties and microstructural analysis of each material. Issues which affect transducer performance are discussed in relation to the properties. For transducers larger than about 1 mm in diameter, the use of a lower dielectric constant material is shown to result in a better electrical match between the transducer and a standard 50 /spl Omega/ termination. For transducers whose impedance is close to that of the connecting cables and electrical termination, equivalent circuit model simulations show improved performance without the need for electrical matching networks. Measurements of fabricated transducers show close agreement with the simulations, validating the measurements and showing the performance benefits of electrically matched transducers.

High frequency synthetic ultrasound array incorporating an actuator

Ultrasound imaging at frequencies above 20 MHz relies almost exclusively on single-element transducers. IN order to apply array technology at these frequencies, several practical problems must be solved, including spatial scale and fabrication limitations, low device capacitance, and lack of a hardware beamformer. One method of circumventing these problems is to combine an array, an actuator, and a synthetic aperture software beamformer. The array can use relatively wide elements spaced on a coarse pitch. The actuator is used to move the array in short steps (less than the element pitch), and pulse-echo data is acquired at intermediate sample positions. The synthetic aperture beamformer reconstructs the image from the pulse-echo data. A 50 MHz example is analyzed in detail. Estimates of signal-to-noise reveal performance comparable to a standard phased array; furthermore, the actuated array requires half the number of elements, the elements are 8x wider, and only one channel is required. Simulated three-dimensional point spread functions demonstrate side lobe levels approaching - 40dB and main beam widths of 50 to 100 microns. A 50 MHz piezo-composite array design has been tested which displays experimental bandwidth of 70% while maintaining high sensitivity. Individual composite sub-elements are 18 microns wide. Once this array is integrated with a suitable actuator, it is anticipated that a tractable method of imaging with high frequency arrays will result.

Effect of Grain Size on Actuator Properties of Piezoelectric Ceramics

Properties of piezoelectric ceramics important for actuator applications have been measured as a function of grain size. Fine grain piezoelectrics (≤1 μm) have been found to exhibit improved machinability and increased mechanical strength over conventional materials. Actuators made from fine grain ceramic are, therefore, expected to have improved reliability, higher driving fields, and lower driving voltages (from thinner layers in stacked or co-fired actuators) over devices fabricated from conventional materials. TRS Ceramics in collaboration with the Pennsylvania State Universitys Materials Research Laboratory, has developed fine grain piezoelectric ceramics with minimal or no reduction in piezoactivity. New chemical doping strategies designed to compensate ferroelectric domain clamping effects from grain boundaries have been successful in yielding submicron grain sized ceramics with both low and high field properties equivalent to conventional materials. In the case of Type II ceramics, reduced grain size results in a very stable domain state with respect to both electric field and compressive prestress. Work is in progress to develop both epoxy bonded stack and co-fired actuators from fine grain piezoelectrics.

Ultrasonic transducers using piezoelectric single crystal perovskites

Solid solutions of the relaxor-based Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PZN) and Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PMN) systems with PbTiO/sub 3/ (PT) have been grown in single crystal form. The piezoelectric and dielectric properties of several compositions are reported along various crystallographic directions. The piezoelectric transducer model developed by Kimholtz, Leedom and Matthaei (KLM) was employed to study the behavior of these materials as ultrasonic resonators. Extremely high piezoelectric coupling coefficients (k/sub 33/>94%) and a range of dielectric constants (3000-5000) have been observed in these systems on the rhombohedral side of the morphotropic phase boundary (MPB). Relatively low dielectric constants (/spl sim/1000) and high thickness mode coupling (kt>63%) were observed as typical of tetragonal formulations. The ability to tailor the dielectric and piezoelectric constants with composition and crystal orientation allows the design of very large bandwidth ultrasonic transducers for applications ranging from medical diagnostic imaging to high frequency single element ultrasound backscatter microscopy.

Single-crystal piezoelectrics for advanced transducer and smart structures applications

Single crystal piezoelectrics based on xPb(Zn1/3Nb2/3)O3-(1-x)- PbTiO3 and xPb(Mg1/3Nb2/3)O3-(1- x)PbTiO3 show great promise for dramatically improving the performance of medical ultrasound transducers, sonar transducers, active flow control actuators, high strain energy density stack actuators, and microactuators. Improvements in crystal growth and manufacturing are yielding large numbers of crystals for device performance evaluations. Property variations have been minimized by identifying the sources of variations and designing manufacturing processes to eliminate property-degrading defects from the final components. Crystal size increases and cost reductions have resulted from replacing flux grown PZN-PT with PMN-PT crystals produced by the Bridgman method. Finally, low crystal stiffness has been shown to not be a hindrance in maintaining high properties under compressive prestress or in packaged devices such as epoxy bonded stack actuators.

Single crystal Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3//PbTiO/sub 3/ (PZN/PT) in medical ultrasonic transducers

Solid solutions of Pb(Zn/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PZN) and PbTiO/sub 3/ (PT) have been grown in single crystal form to sizes large enough to permit the fabrication of a 5 MHz phased array. Transducer construction techniques employed differ from standard ceramic arrays. Crystallographic alignment is found to be a significant contributor to piezoelectric performance. The crystal structure and phase diagram play a major role in processing steps requiring heating such as poling and electroding. A complex composite structure is utilized to allow for efficient operation while maintaining structural integrity. The pulse-echo performance and insertion loss of this array are found to be superior to a similarly constructed PZT 5H array.

Characteristics of electromechanical solid state multilayer actuators

In this study we compare the responses of stacked actuators made of soft ferroelectrics to those of less common materials such as electrostrictive and antiferroelectric to ferroelectric phase-switching ceramics. The same measurements were performed on various samples which were identical in size and manufacturing technology. The latest material results on relaxor-based single crystals were considered to give a more comprehensive overview. Additionally, we discussed the clamping effects caused by the actuator structure, which have to be taken into account comparing the stack results to pure material response.

High frequency properties of piezoceramic resonators

Several methods of calculating the material properties of thickness mode resonators from electrical impedance measurements have been studied. A method accurate at frequencies up to 100 MHz has been used to measure the properties of several commercially available PZT compositions over the frequency range of from 5 to 100 MHz. New materials with sub micron grain sizes have also been evaluated and found to have properties less dependent on frequency. A commercial lead titanate material was evaluated and found to have properties comparable to PZT, but with a much lower dielectric constant. The dielectric permittivity of a material is shown to be particularly important in determining transducer performance due to electrical matching.

Crystal growth and ferroelectric related properties of (1-x)Pb(A/sub 1/3/Nb/sub 2/3/)O/sub 3/-xPbTiO/sub 3/ (A=Zn/sup 2+/, Mg/sup 2+/)

Crystals of (1-x)Pb(A/sub 1/3/Nb/sub 2/3/)O/sub 3/-xPbTiO/sub 3/ (A=Zn/sup 2+/, Mg/sup 2+/) were grown by the flux technique. The dielectric and piezoelectric properties as a function of composition and crystal orientation have been evaluated. Both MPB and non-MPB crystals were found to possess high piezoelectric properties. Values of longitudinal electromechanical coupling coefficients (k/sub 33/)>90%, dielectric constants ranging from 3000 to 5000 with low dielectric loss <1% were observed for rhombohedral crystals. Tetragonal crystals with increased PbTiO/sub 3/ content exhibited large thickness coupling coefficients (k/sub T/)>63% with relatively low dielectric constant (/spl sim/1000). Ultrahigh values of piezoelectric coefficients (d/sub 33/)>2000 pC/N were also measured for non-MPB crystals and confirmed by direct E-field strain measurements. These properties are briefly discussed in relation to device performance.