In this paper low voltage single crystal actuators were investigated using thin PMN-PT plates for applications requiring low voltage, large strain, low profile and/or actuation at cryogenic temperatures. Firstly, single crystal thickness effect on piezoelectric properties was studied by investigating the relationship between electromechanical coupling coefficient of PMN-PT crystals and the crystal thickness. It was found that electromechanical coupling coefficient (kt) of 50 μm, 75 μm and 100 μm PMN-PT single crystal thin plates are 0.5, 0.51, and 0.55, respectively, which are slightly lower than that of bulk single crystal (0.6). A couple of single crystal actuators were then assembled using crystal plates with thickness of 150-200 μm. These actuators were characterized by measuring strain vs. electric field at room temperature and cryogenic temperatures. A 3 mm x 3 mm x 19 mm single crystal stack actuator showed a 21 μm stroke at room temperature under 150 V, and a 10 μm stroke at 60 K under 200 V. A 5 mm x 5 mm x 12 mm single crystal actuator showed 13.5 μm stroke at room temperature under 150 V, and 6 μm stroke at 77 K under 150 V. These low voltage actuators hold promising for space precise positioning and adaptive structures and cryogenic SEM, SPM and STM applications.
Transducers incorporating single crystal piezoelectric Pb(Mg1/3Nb2/3)x-1TixO3 (PMN-PT) exhibit significant advantages over ceramic piezoelectrics such as PZT, including both high electromechanical coupling (k33 > 90%) and piezoelectric coefficients (d33 > 2000 pC/N). Conventional <001> orientation gives inherently larger bandwidth and output power than PZT ceramics, however, the anisotropy of the crystal also allows for tailoring of the performance by orienting the crystal along different crystallographic axes. This attribute combined with composition refinements can be used to improve thermal or mechanical stability, which is important in high power, high duty cycle sonar applications.
By utilizing the "31" resonance mode, the high power performance of PMN-PT can be improved over traditional "33" mode single crystal transducers, due to an improved aspect ratio. Utilizing novel geometries, effective piezoelectric constants of -600 pC/N to -1200 pC/N have been measured. The phase transition point induced by temperature, pre-stress or field is close to that in the "33" mode, and since the prestress is applied perpendicular to the poling direction in "31" mode elements, they exhibit lower loss and can therefore be driven harder.
The high power characteristics of tonpilz transducers can also be affected by the composition of the PMN-PT crystal. TRS modified the composition of PMN-PT to improve the thermal stability of the material, while keeping the loss as low as possible. Three dimensional modeling shows that the useable bandwidth of these novel compositions nearly equals that of conventional PMN-PT. A decrease in the source level of up to 6 dB was calculated, which can be compensated for by the higher drive voltages possible.
An electroactive polymer (EAP)-ceramic hybrid actuation system (HYBAS) was developed recently at NASA Langley Research Center. This paper focuses on the effect of the bending stiffness of the EAP component on the performance of a HYBAS, in which the actuation of the EAP element can match the theoretical prediction at various length/thickness ratios for a constant elastic modulus of the EAP component. The effects on the bending stiffness of the elastic modulus and length/thickness ratio of the EAP component were studied. A critical bending stiffness to keep the actuation of the EAP element suitable for a rigid beam theory-based modeling was found for electron irradiated P(VDF-TrFE) copolymer. For example, the agreement of experimental data and theoretical modeling for a HYBAS with the length/thickness ratio of EAP element at 375 times is demonstrated. However, the beam based theoretical modeling becomes invalid (i.e., the profile of the HYBAS movement does not follow the prediction of theoretical modeling) when the bending stiffness is lower than a critical value.
(1-x)BiScO3-xPbTiO3 (BSPT) polycrystalline material with a morphotropic phase boundary (MPB) composition (x=0.64) exhibits a high Curie temperature (TC) about 450°C and good piezoelectric properties with d33 values around 460pC/N. Manganese (Mn) modified BSPT was utilized in order to increase the electric resistivity and RC time constant. At 450°C, BSPT66-Mn ceramic exhibited a resistivity of 3x107 Ohm.cm and RC value of 0.08s, respectively, significantly higher than the values for undoped BSPT and commercial PZT5 materials. The manganese additive shifts TC of BSPT materials to lower temperatures, which were found to be 442°C and 462°C for modified BSPT64 and BSPT66, respectively. The piezoelectric behavior for the modified BSPT material was found to deteriorate slightly owing to the hardening effect of manganese, but showed superior temperature stability and enhanced resistivity. The detailed temperature dependent properties were studied in this work and compared to commercial PZT5 materials. The complete set of materials constants, including the elastic sij, cij, piezoelectric dij, eij, gij, hij, dielectric and electromechanical kij values were determined using resonance technique and derived from the experimental data.
TRS is developing new transducers based on single crystal piezoelectric materials such as Pb(Mg1/3Nb2/3)x-1TixO3 (PMN-PT). Single crystal piezoelectrics such as PMN-PT exhibit very high piezoelectric coefficients (d33 ~ 1800 to >2000 pC/N) and electromechanical coupling factors (k33 > 0.9), respectively, which may be exploited for improving the performance of broad bandwidth and high frequency sonar. Apart from basic performance, much research has been done on reducing the size and increasing the output power of tonpilz transducers for sonar applications. Results are presented from two different studies.
"33" mode single crystal tonpilz transducers have reduced stack lengths due to their low elastic stiffness relative to PZTs, however, this produces non-ideal aspect ratios due to large lateral dimensions. Alternative "31" resonance mode tonpilz elements are proposed to improve performance over these "33" designs. d32 values as high as 1600 pC/N have been observed, and since prestress is applied perpendicular to the poling direction, "31" mode Tonpilz elements exhibit lower loss and higher reliability than "33" mode designs.
Planar high power tonpilz arrays are the optimum way to obtain the required acoustic pressure and bandwidth for small footprint, high power sensors. An important issue for these sensors is temperature and prestress stability, since fluctuations in tonpilz properties affects power delivery and sensing electronic design. TRS used the approach of modifying the composition of PMN-PT to improve the temperature dependence of properties of the material. Results show up to a 50% decrease in temperature change while losing minimal source level.
TRS is developing new actuators based on single crystal piezoelectric materials such as Pb(Zn1/3Nb2/3)1-xTixO3 (PZN-PT) and Pb(Mg1/3Nb2/3)x-1TixO3 (PMN-PT) which exhibit very high piezoelectric coefficients (d33 = 1800-2200 pC/N) and electromechanical coupling factors (k33 > 0.9), respectively, for a variety of applications, including active vibration damping, active flow control, high precision positioning, ultrasonic motors, deformable mirrors, and adaptive optics. The d32 cut crystal plate actuators showed d32 ~ -1600 pC/N, inter-digital electroded (IDE) plate actuators showed effective d33 ~ 1100 pC/N. Single crystal stack actuators with stroke of 10 μm-100 μm were developed and tested at both room temperature and cryogenic temperatures. Flextensional single crystal piezoelectric actuators with either stack driver or plate driver were developed with stroke 70 μm - > 250 μm. For large stroke cryogenic actuation (> 1mm), a single crystal piezomotor was developed and tested at temperature of 77 K-300K and stroke of > 10mm and step resolution of 20 nm were achieved. In order to demonstrate the significance of developed single crystal actuators, modeling on single crystal piezoelectric deformable mirrors and helicopter flap control using single crystal actuators were conducted and the modeling results show that more than 20 wavelength wavefront error could be corrected by using the single crystal deformable mirrors and +/- 5.8 ° flap deflection will be obtained for a 36" flap using single crystal stack actuators.
High frequency sonar is becoming ever more important to the Navy through expanded use of unmanned underwater vehicles (UUV). Proposed missions for many UUV's involve shallow water operation where broad bandwidth is required making these applications ideal candidates to use single crystal piezoelectrics. In addition, many UUV sonar systems have commercial uses including oceanographic research, oil and mineral prospecting, salvage, and undersea equipment inspection and maintenance. The properties of single crystal piezoelectrics were exploited for broad bandwidth, high frequency sonar. Crystal sonar investigations based on Tonpilz transducers utilizing the '33' resonance mode have shown limitations on bandwidth due to less than ideal resonator aspect ratio. This is a result of the crystals' low elastic stiffness, which leads to short resonators with large lateral dimensions. To address this issue an alternative design was proposed utilizing the '32' resonance mode with the resonating length oriented along a special crystallographic cut. 'Crystals with this orientation are known to have high properties; d32 values as high as 1600 pC/N have been observed. Since prestress for such a design is applied perpendicular to the poling direction, '32' mode Tonpilz elements exhibit lower loss and higher reliability than '33' mode designs. The feasibility of such '32' mode Tonpilz resonators will be presented as determined through property measurements and finite element analysis. The targeted application for this work is broadband (>100%), high frequency (45 kHz) synthetic aperture arrays for unmanned underwater vehicles.
TRS single crystal plates with special crystal orientations and dimensions of 10×5×0.5mm were prepared, and the in-plane strain-electric field behavior was measured using a modified Sawyer-Tower circuit with an LVDT. A d32 coefficient as high as -1600 pm/V was observed which is 60-75% higher than d31 in the conventional cut crystal. The increased performance of this cut can be directly applied to bending mode actuators and other devices that utilize the d31 mode. The strain response was both linear and non-hysteretic up to 15 kV/cm. Large stroke and highly directional strain was also achieved from a quasi-d33 mode single crystal plate with interdigital electrodes (IDE). These were prepared with plate thickness of 0.2 mm, and an effective d33>1000 pC/N was obtained under 10 KV/cm driving field. Both types of plate actuators show at least 5 times larger piezoelectric coefficient than the d31 of PZT materials. Amplified out-of-plane stroke could be easily achieved by integrating the developed thin plate actuators into unimorph, bimorph, or moonie structures.
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.
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, 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.