Seung Eek Eagle Park

Enhanced piezoelectric property of barium titanate single crystals with engineered domain configurations

Piezoelectric properties of barium titanate single crystals were investigated at room temperature as a function of crystallographic orientation. When a unipolar electric field was applied along [001], its strain vs electric-field curve showed a large hysteresis, and finally barium titanate crystal became to single-domain state with piezoelectric constant d33 of 125 pC/N over 20 kV/cm. On the other hand, electric-field exposure below 6 kV/cm along [111] resulted in a high d33 of 203 pC/N and a hysteresis-free strain vs electric-field behavior, which suggested the formation of an engineered domain configuration in a tetragonal barium titanate crystal. Moreover, when an electric field over 6 kV/cm was applied along [111], two discontinuous changes were observed in its strain vs electric-field curve. In situ domain observation and Raman measurement under an electric field suggested an electric-field-induced phase transition from tetragonal to monoclinic at around 10 kV/cm, and that from monoclinic to rhombohedral at around 30 kV/cm. Moreover, in a monoclinic barium titanate crystal, electric-field exposure along [111] resulted in the formation of another new engineered domain configuration with d33 of 295 pC/N.

The Role of Processing Variables in the Flux Growth of Lead Zinc Niobate-Lead Titanate Relaxor Ferroelectric Single Crystals

Relaxor ferroelectric single crystals of Pb(Zn1/3Nb2/3)O3–PbTiO3 (PZN-PT) are of interest as high performance transducers due to their very large piezoelectric coupling and dielectric properties. A high temperature flux solution method was used to grow (1- x)PZN-( x)PT, where x= 0.0, 0.1 and 0.15 single crystals. Processing conditions were optimized to increase the size and yield of the perovskite crystals, including variation of the flux to composition ratio, cooling rate, soak time and soak temperature. The crystals varied in size from 0.01 cm to 1.5 cm on an edge, and in color from opaque to brown due to the changes in processing conditions. The crystals were characterized by XRD, dielectric constant and dielectric loss measurements. As the content of PT increased the transition from the paraelectric to the ferroelectric phase approached first order behavior and the crystal structure transformed from rhombohedral to tetragonal. This structure transition caused the lattice constant along the c-axis to elongate as the c/a ratio increased. At room temperature, the dielectric constants for the PZN-PT compositions along the [111] or [001] axes were as great as 5000 and the dielectric losses were as low as 0.01.

The Effect of Growth Conditions on the Dielectric Properties of Pb(Zn1/3Nb2/3)O3 Single Crystals

The effect of growth conditions on the dielectric properties and ferroelectric transition of perovskite Pb(Zn1/3Nb2/3)O3 single crystals was examined. Crystals were grown by the flux technique using PbO as a self flux. Increased cooling rate and soaking temperature resulted in a decreased Zn/Nb ratio, and corresponding increased temperature of the maximum dielectric constant (Tmax ), and decreased room temperature values of the dielectric constant and loss. Different crystal colors and quality were also associated with the various growing conditions. Although dielectric properties, domain stability after poling, and associated piezoelectric properties are a function of Tmax , it was shown that crystal quality was critical for the observed variation in these properties.

Relaxor-based ferroelectric single crystals for electromechanical actuators

The piezoelectric properties of relaxor based ferroelectric single crystals, such as Pb(Zn1/3Nb2/3)O3 - PbTiO3 (PZN-PT) and Pb(Mg1/3Nb2/3)O3 - PbTiO3 (PMN- PT) were investigated for electromechanical actuators. In contrast to polycrystalline materials such as Pb(Zr,Ti)O3 (PZTs), morphotropic phase boundary compositions were not essential for high piezoelectric strain. Piezoelectric coefficients (d33s) > 2500 pC/N and subsequent strain levels up to > 0.6% with minimal hysteresis were observed. Crystallographically, high strains are achieved for <001> oriented rhombohedral crystals, though <111> is the polar direction. Ultrahigh strain levels up to 1.7%, an order of magnitude larger than those available from conventional piezoelectric and electrostrictive ceramics could be achieved, being related to an E-field induced phase transformation. Strain vs. E-field behavior under external stress was also much superior to that of conventional piezoelectric ceramics. High electromechanical coupling (k33) > 90% and low dielectric loss <1%, along with large strain make these crystals promising candidates for high performance solid state actuators.

Relaxor-based single-crystal materials for ultrasonic transducer applications

Relaxor ferroelectric single crystals of Pb(Zn1/3Nb2/3)O3 (PZN), Pb(Mg1/3Nb2/3)O3 (PMN) and their solid solutions with normal ferroelectric PbTiO3 (PT) were investigated for ultrasonic transducer applications. Crystals offer adjustable properties not only by compositional tailoring but also by domain state engineering associated with different crystallographic orientation, which is not achievable in polycrystalline materials. Longitudinal coupling coefficients (k33) as high as 94% and dielectric constants (K3T) in the range of 3500 - 6000 were achieved with low dielectric loss (less than 1%) using <001> oriented rhombohedral crystals of (1-x)PZN-xPT and (1-y)PMN-yPT, where x less than 0.09 and y less than 0.35. Dicing direction as well as poling direction were critical for high coupling under laterally clamped condition. Dicing parallel to the (001) yields 90% of laterally clamped coupling (kbar) out of 94% longitudinal coupling (k33) for PZN-8%PT. On the other hand, samples diced parallel to (110) exhibited no dominant mode present. Thickness coupling (kT) as high as 64% and low dielectric constant (K3T) less than 600 with low loss (less than 1%) could be achieved using tetragonal crystals of (1-x)PZN-xPT and (1-y)PMN-yPT, where x greater than 0.1 and y greater than 0.4. The performance gains associated with these ultra-high coupling coefficients and range of dielectric constants are evident in relation to broader bandwidth and electrical impedance matching. Specifically, rhombohedral crystals offer the possibility of extremely broad bandwidth devices for transducer arrays and tetragonal crystals for single element transducers. Transducer simulation was performed using the KLM model. The pulse/echo response simulated a 124% bandwidth subdiced array element with a center frequency of 10 MHz. An optimized array design of the same geometry constructed of PZT 5H displays an 87% bandwidth.

Innovations in piezoelectric materials for ultrasound transducers

Piezoelectric materials lie at the heart of ultrasonic transducers. For transducers used in medical imaging (3¿7 MHz), PZT-5H ceramics offer high electromechanical coupling (k33 ¿ 75%), resulting in good bandwidth and sensitivity. As transducer arrays become smaller with increasing frequency, the development of high permittivity ( ¿RT > 7,000 vs. 3,400 for PZT-5H), piezoelectrics based on polycrystalline PMN-PT, provide improved electrical impedance matching. Advanced medical diagnostic techniques, including contrast and harmonic imaging, have taken advantage of the recent development in single crystal Relaxor-PTs that offer coupling k33¿s > 90% and subsequently, significant increases in bandwidth. For small animal, ophthalmology and cellular imaging, higher resolution is demanded, thus requiring transducers operational in the range of 20¿100 MHz. Advancements in ceramic processing include pore-free and fine-grain (¿1 micron) piezoelectric ceramics of PT and PZT, being an ¿enabling¿ technology, allowing the fabrication of high frequency single element and annular arrays. Innovations in the fabrication of high frequency arrays (¿ 30 MHz) include tape casting and sol-gel molding techniques. Of particular significance, DRIE (deep reaction ion etching), has demonstrated the ability to mill out ultrafine features, allowing 1¿3 crystal-polymer composites operational at frequencies ¿ 60 MHz, far beyond that achieved by current state-of-the-art dicing.

Comparison of actuator properties for piezoelectric and electrostrictive materials

The field induced strain has been measured for a broad variety of piezoelectric and electrostrictive actuator materials. These measurements have been made under AC drive conditions with variations in DC bias, peak to peak voltage, and prestress. Data for three types of PMN-PT electrostrictors, hard and soft piezoelectric ceramics, and PZN-PT single crystal have been collected. For smart structures applications fine grain Type II ceramic and PZN- PT single crystals were found to have the best combination of moderate to high strain, low to moderate hysteresis, and resistance to stress depoling. Electrostrictive ceramics used for high frequency transducers were found to exhibit some stress induced domain reorientation effects that depended on drive conditions and operating temperature. These effects became more pronounced for electrostrictors with high lead titanate content. Epoxy bonded stacks have been constructed form some of the materials to determine the merits of materials properties for actuator performance. This work has shown that fine grain Type II ceramics have many advantages for high authority stack actuators including high strain energy density and lifetimes > 109 cycles at 100 percent rated peak-to-peak voltage.

Crystallographic engineering in high-performance piezoelectric crystals

Crystallographic engineering, a concept to utilize crystal anisotropy as well as an engineered domain configuration, resulted in significant enhancement in piezoelectric activity for normal ferroelectric BaTiO3 crystals. Electromechanical couplings (k33) approximately 85 percent and piezoelectric coefficients (d33) as high as 500 pC/N, higher or comparable to those of lead based ceramics such as PZT and significantly larger than those of tetragonal BaTiO3 crystals, were detected from crystallographically engineered orthorhombic BaTiO3 crystals. Orthorhombic BaTiO3 phase could be stabilized by Zr-doping at room temperature and enhanced electromechanical coupling (k33) approximately 75 percent was detected also by using crystallographic engineering. Macroscopic symmetry was suggested for <001> poled rhombohedral (3m) and orthorhombic (2mm) crystals, based on the engineered domain configuration.

1-3 single-crystal composites for ultrasonic transducer arrays

16801)ABSTRACTThe development of single crystal relaxor-PT piezoelectrics is an exciting advance in ultrasoundtransducer technology. The high electromechanical coupling coefficients and variable dielectric constant couldbe used to significantly enhance bandwidth and sensitivity of array transducers. In this study 1-3 composites ofsingle crystal material were engineered and applied to an array design. Both predicted and actual performancesare reported and compared to array designs using PZT-5H based 1-3 composite material.In order to take advantage of the performance enhancement of single crystal materials a 1-3 compositeconnectivity was selected. Two techniques were used to tile together small pieces of crystal into a largercomposite plate suitable for array applications. A coupling coefficient of .81 and an acoustic impedance of 15.6Mrayls were obtained using a 58% volume fraction of single crystal. A comparable PZT composite displayed acoupling coefficient of .66 and an acoustic impedance of 1 7.0 Mrayls. A sufficiently fine spatial scale resulted inlateral resonances being well above the thickness mode resonance for both materials.One-dimensional modeling using the Redwood equivalant circuit in PSpice was used to investigatematching and backing. An optimization scheme resulted in a modeled bandwidth of 134% and a -6 dB pulselength of only one cycle using a single front matching layer and a castable backing. Eight element arrays ofsingle crystal and PZT-5H composite were constructed to verify the theoretical results.KEYWORDS: Transducers, Single Crystal, Arrays, Piezoelectric Composites

Recent advances in piezoelectric materials

Recent developments in piezoelectric materials include submicron grain size ceramics and single crystals. Pb(Zr,Ti)O3 (PZT) ceramics with submicron grain sizes (approximately 0.5 micrometer) have been produced with properties comparable to conventional, coarse grained (approximately 3 to 5 micrometer) ceramics. The fine grain ceramics exhibit improved machinability over conventional materials. Ultrasonic transducer arrays with post widths less than 15 micrometers have been fabricated as well as thin plates with thicknesses as low as 10 micrometers. The yields and performance of such operations are expected to be much greater with fine grain ceramic than with conventional material. The single crystal piezoelectrics developed offer field induced strain an order of magnitude higher than what can be achieved in piezoelectric ceramic actuators (greater than 1%). Furthermore, the strain electric field hysteresis and dielectric losses are very low for these materials and electromechanical coupling factors (k33) are greater than 90%. Applications which may benefit from the recent developments include smart materials and structures and MEMS.