Harold C. Robinson

Mechanical and thermal transitions in morphotropic PZN-PT and PMN-PT single crystals and their implication for sound projectors

Isothermal compression experiments on multidomain [001] oriented and poled ferroelectric rhombohedral PZN-0.07PT and PMN-0.30PT single crystals revealed elastic instabilities corresponding to zero field ferroelectric-ferroelectric phase transition under mechanical compression. The application of an appropriate dc bias field doubled the stability range of the ferroelectric rhombohedral state under uniaxial compression for both crystals and maintained a linear elastic response. Youngs modulus as derived from the quasistatic, zero field stress-strain linear response agreed well with that derived from small signal resonance for the ferroelectric rhombohedral FR state of both PZN-PT and PMN-PT. Elastic compliances S33 E as determined from high temperature resonance revealed a monotonically decreasing Youngs modulus as a function of temperature in the ferroelectric rhombohedral state with a sudden stiffening near the ferroelectric rhombohedral (FR)-ferroelectric tetragonal (FT) transition. The reversible ferroelectric-ferroelectric transition of morphotropic PZN-PT and PMN-PT single crystals as accessed by mechanical compression is discussed in terms of strain calculations from Devonshires theory, domain unfolding, and morphotropic phase boundary shift with mechanical stress. The mechanically-induced and thermally-induced ferroelectric-ferroelectric transition trajectories are discussed in terms of the Devonshire theory. Implications of these observations for sound projectors are discussed. A single crystal tonpilz projector fabricated into a 16-element array and a segmented cylinder transducer demonstrated the outstanding capabilities of single crystals to achieve compact, broadband, and high-source level projectors when compared to conventional lead zirconate-titanate PZT8 projectors.

Effect of uniaxial stress on the large-signal electromechanical properties of electrostrictive and piezoelectric lead magnesium niobate lead titanate ceramics

The electromechanical performance characteristics of electrostrictive 0.9 Pb(Mg1/3Nb2/3)O3–0.1 PbTiO3 and piezoelectric 0.7 Pb(Mg1/3Nb2/3)O3–0.3 PbTiO3 ceramics have been investigated under uniaxial stress (σ). The results demonstrate that the large-signal electromechanical properties of electrostrictive ceramics are decreased with increasing σ, whereas those of the piezoelectric are increased but accompanied by significantly increased hysteretic losses.

Single-crystal lead magnesium niobate-lead titanate (PMN/PT) as a broadband high power transduction material

Two experimental underwater acoustic projectors, a tonpilz array, and a cylindrical line array, were built with single crystal, lead magnesium niobate/lead titanate, a piezoelectric transduction material possessing a large electromechanical coupling factor (k33 = 0.9). The mechanical quality factor, Q(m), and the effective coupling factor, k(eff), determine the frequency band over which high power can be transmitted; k(eff) cannot be greater than the piezoelectric material value, and so a high material coupling factor is a requisite for broadband operation. Stansfields bandwidth criteria are used to calculate the optimum Q(m) value, Q(opt) approximately 1.2 (1-k(eff)2 1/2/k(eff). The results for the tonpilz projector exhibited k(eff) = 0.730, Q(m) = 1.17 (very near optimal), and a fractional bandwidth of 0.93. For the cylindrical transducer array, k(eff) = 0.867, Q(m) = 0.91 (larger than the optimum value, 0.7), and the bandwidth was 1.16. Although the measured bandwidths were less than optimal, they were accurately predicted by the theory, despite the highly simplified nature of the Van Dyke equivalent circuit, on which the theory is based.

Enhancement of electromechanical coupling coefficient and acoustic power density in conventional “Hard” Pb(Zr1−xTix)O3 ceramics by application of uniaxial stress

Investigations of the polarization versus electric field (P-E) and strain versus electric field (e-E) responses for “hard” Pb(Zr1−xTix)O3 piezoelectric ceramics have been performed under various uniaxial stresses (σ) and ac electrical drive fields. Investigations revealed a significant decrease in the remanent polarization of specimens with increasing σ. Subsequent calculations of the longitudinal electromechanical coupling coefficient (k33) and acoustic power density revealed significant enhancements with increasing σ.

Single crystal cylinder transducers for sonar applications

A segmented cylinder transducer constructed of single crystal lead magnesium niobate‐lead titanate (PMN‐PT) has been under development at NUWC and EDO Corporation for several years. The purpose of this development was to provide an extremely compact, high power broadband source. By virtue of their extraordinary material properties, ferroelectric single crystals are the ideal transduction material for developing such compact broadband systems. This presentation shall review the evolution of the transducer design as well as present the results of a successful in‐water test conducted at NUWC in October of 2003. It shall be shown that design changes intended to eliminate spurious modes limiting the transducer bandwidth first observed in 2002 were successful, resulting in a transducer with a clean frequency response and an effective coupling factor of 0.85. The measured transducer admittance was in nearly exact agreement with theoretical predictions. The NUWC in‐water tests demonstrated that the single crystal...

Stress-Dependent Behavior of d33 and Y in Navy Types III and VI Ceramics E 33

The stress dependence of hard and soft active ceramic properties is important because many submarine sonar transducers are subject to compressive mechanical prestress as well as dynamic stress variations caused by ac electrical excitation. The level of prestress to which a ceramic in a transducer is subjected also depends in part on operational factors, such as the level of ac activation and depth of the submersible. This investigation builds upon prior work (Yang, G., Liu, S-F., Ren, W. and Mukherjee, B.K. 2000. “Uniaxial Stress Dependence of the Piezoelectric Properties of Lead Zirconate Titanate Ceramics,” Smart Structures and Materials 2000: Active Materials: Behavior and Mechanics, Proceedings of SPIE, Vol. 3992, pp. 103-113.) by examining the time dependence and the uniaxial stress dependence of the average, differential and dynamic d33 and YE 33 of Navy Type III (PZT8) and Navy Type VI (PZT5H) lead zirconate titanate ceramics. This research incorporates higher levels of prestress and various mid-level ac stress cycles. Under short-circuit conditions, large and small compressive stresses are applied while measuring dielectric displacement and strain. The piezoelectric coefficient, d33, is evaluated using the direct method as a function of time, prestress level, and ac stress magnitude. The constant-field modulus is calculated from the slope of the corresponding stress-strain curves. Intrinsic and extrinsic contributions to these properties are discussed.

Frequency dependence of PMN-PT ceramics under electrical bias

It is a well-known fact that electrostrictive materials, such as lead magnesium niobate-lead titanate (PMN-PT) ceramics, exhibit significant frequency dispersion in their small signal dielectric constant below their dielectric maximum temperature Tm. The frequency dispersion in several PMN-PT compositions will be examined in this study using two independent measurement methods: dc biased resonance and large signal quasistatic measurements conducted on NUWC Division Newports SDECS. From these measurements, the coupling factor, piezoelectric constant and Youngs modulus are compared as a function of the applied bias and frequency. Both the DC biased and SDECS measurements were performed on the same 3:1 aspect ratio samples. Finite element calculations will show that the error in determining the Youngs modulus and piezoelectric constant from resonance using these samples is less than 5 percent. It will be shown that when frequency dispersion exists it remains even with the application of dc bias, and that the degree of deviation between these quantities increases the further below Tm the temperature drops. It will also be shown that, like the dielectric constant, the coupling factor, piezoelectric constant and Youngs modulus in PMN-PT ceramics above Tm are non-dispersive.

Resonance measurements as predictors of large-signal electrostrictive material performance

The large signal performance of electrostrictive materials, such as lead magnesium niobate-lead titanate (PMN-PT), is of critical importance to sonar transducer and actuator designers. However, obtaining these large signal parameters properly, particularly under compressive prestress, is an expensive and time-consuming enterprise. The complexity of these measurements, therefore, precludes them as a method for quickly and easily screening materials for their potential as high power materials. Traditionally, resonance measurements, which otherwise are relatively simple to perform, have been used for screening purposes, but they suffer from the drawback that the material parameters obtained are at the incorrect frequency and under no prestress. Furthermore, it was unclear what significance the results of resonance measurements for nonlinear materials such as electrostrictors had. It has recently been suggested that dc biased resonance measurements on electrostrictive ceramics would be an accurate predictor of the coupling factor and optimum bias point. In this paper, dc biased resonance measurements on three different PMN-PT formulations, with varying dielectric maximum temperatures, will be analyzed to determine which composition has the highest predicted coupling factor. This prediction will be compared with large signal quasistatic measurements conducted on NAVSEA Division Newports SDECS (Stress-dependent Electromechanical Characterization System). The predictive ability of the resonance measurements will also be analyzed as a function of temperature normalized with respect to the dielectric maximum temperature.

Evaluation of large signal minor loop behavior in PMN-PT ceramics

The stress and polarization behavior of a lanthanum doped PMN-PT was evaluated using unipolar and biased drives of up to 1.3 MV/m under varying temperatures and mechanical stresses. In particular, the behavior of the strain vs. field and polarization vs. field loops for biased minor loops is be compared to that of the full unipolar drives. It is shown that, as the hysteresis of a material increases, the minor loops tend to rotate around the midline of the major loop. This leads to smaller values for the large signal piezoelectric and dielectric constants than would be predicted from the major loop alone. The behavior of several material performance measures, such as the strain energy density, electromechanical coupling factor and dielectric loss factor, are analyzed as functions of the DC bias and AC drive fields. It is shown that the energy density and coupling factor tend to increase with DC bias field but decrease with AC drive. The dielectric loss factor, on the other hand, decreases as either the DC bias or AC drive fields are increased.

Calculations of the elastic compliance and piezoelectric constants of PVDF and P(VDF‐TrFE) crystals using molecular mechanics

The selection and/or design of projector materials for ultrasonic transducers requires knowledge of their elastic, piezoelectric, and dielectric properties. These properties can be measured directly; however, in order to minimize the fabrication and measurement of many different materials, it would be useful to compute these properties from the chemical composition and structure of the material. In molecular mechanics, a force field is used to describe short‐ and long‐range interactions between atoms in a crystal. The most favored state occurs when the total energy, i.e., the sum of the interaction energies, is minimized. This paper presents calculated compliance and piezoelectric constants for β‐phase crystals of polyvinylidene fluoride (PVDF) and its copolymer with trifluoroethylene, P(VDF/TrFE). The constants were derived by minimizing the energy of the unit cell while varying the stress. The model was validated using experimental and theoretical values for the properties of PVDF. The behavior of the c...