Steven M. Pilgrim

Brazing perovskite ceramics with silver/copper oxide braze alloys

In this study the feasibility of eutectic braze alloys based on the silver/copper oxide (Ag/CuO) system were investigated for use in joining lead magnesium niobate (PMN) ceramics. PMN was successfully brazed in air at 1050 and 1100°C using Ag/CuO alloys. The brazed samples had an average four-point-bend fracture strength of approximately 40 percent of the average monolithic PMN strength (with an actual value of 19 ± 11 MPa). The fracture strength was relatively constant for all brazing conditions tested. No significant reaction product layer was observed at the silver/PMN interface and the electrical properties of the PMN were changed only slightly by presence of the braze alloy interlayers.

Measurement of the electrostrictive coefficients of modified lead magnesium niobate using neutron powder diffraction

Neutron powder diffraction measurements and a constitutive model were used to determine the bulk longitudinal and transverse electrostrictive coefficients (Qijkl) for polycrystalline samples of the subject material. Effective single-crystal Qijkl were calculated from the powder neutron diffraction data. The resulting values of the three independent Qijkl, as determined from the cubic symmetry of the underlying lattice, are Q3333=2.1×10−2 m4/C2, Q3322=−0.87×10−2 m4/C2, and Q3232=1.2×10−2 m4/C2. Using these effective single-crystal values, a random ensemble average was used to predict the electromechanical performance of the polycrystalline material. Predicted values of bulk Q3333=1.86×10−2 m4/C2 and Q3322=−0.78×10−2 m4/C2 are 89% and 87%, respectively, of the effective single-crystal values for Q3333 and Q3322, while measured values of polycrystalline specimens are only ∼70% of the single-crystal values.

The use of harmonic analysis of the strain response in Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-based ceramics to calculate electrostrictive coefficients

The electromechanical response of ceramics has long been described with Landau Devonshire phenomenology, wherein the strain response is linked to a polynomial expansion in electric field or dielectric displacement. Consequently, the electromechanical response has been modeled with a variety of basis functions. However, these models have failed to accommodate hysteresis and the harmonic response that arises with saturation phenomena. In addition, no quantitative criterion has been used to truncate the expansion. By implementing a discrete Fourier transform in conjunction with Devonshire phenomenology, these three problems can be overcome as demonstrated with a dielectrically aged, lead magnesium niobate relaxor ferroelectric well above its T/sub max/, i.e., operating in the electrostrictive regime.

Determination of nonlinear electromechanical energy conversion and the field-dependent elastic modulus of Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-based electrostrictors

Energy conversion efficiency is a critical parameter for all electromechanical materials. Although excellent techniques are available for linear materials, nonlinearities complicate the determination of conversion efficiency in electrostrictive Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PMN). The field dependence of the elastic modulus presents additional problems. A technique combining pulse-echo ultrasound has been developed to approximate the change in the Youngs modulus with applied field. A 30% decrease was observed at 1 MV/m (from /spl sim/100 to /spl sim/70 GPa). The boundary condition for these measurements was a constant electric field as opposed to constant dielectric displacement. In combination with the results from harmonic analysis, the modulus data may be used to determine the electromechanical conversion efficiency. This has been accomplished using an energy balance criterion for a PMN-based composition (with and without dc bias). The resulting longitudinal coupling values are >0.5 for practical electric fields (<1 MV/m peak).

Composite hydrophone devices coupling piezoelectricity and tensegrity

Abstract The concept of tensegrity as conceived by Buckminster Fuller has been incorporated into a passive hydrophone device. Tensegrity is described as the physical phenomenon that produces a stable geometric structure using solid compressional elements arranged in tandem with flexible tensional cables. In the devices built by the authors, six PZT 5H™ bars acting as compressional elements in the tensegrity structure have been coupled with tensional bands of either polyaramid or carbon fiber. This stable system is then wrapped with an outer layer of either polyaramid or carbon fiber and rubber film to form a sealed device, which is referred to as a piezotensegritive device in this paper. The six bars are arranged in parallel electrical connectivity for all devices described. The resonant frequency for these devices ranged from 19.5 to 20.3 kHz depending on the material used for wrapping the piezoelectric bars. These devices were also tested in a hydrostatic environment to determine the relevant piezoelectric coefficients. For devices wrapped with carbon fiber, dh peaked at ∼6000 pC/N and gh at ∼275 mVm/N. For devices wrapped with polyaramid, dh peaked at ∼2000pC/N and gh at ∼100mVm/N. Sensitivities from –182–195 db ref. 1V/µPa were calculated for these devices.

Modeling of piezoresistively-controlled electromechanical smart material

A finite element model of a proposed smart material was developed. This is an initial investigation into using a piezoresistive sensor in conjunction with a multilayer ceramic actuator and a resistor network as the control circuit. A device model, incorporating piezoresistive composites (with orders of magnitude change in resistivity) and piezoelectric actuators has also been constructed. The model accounts for all relevant material properties and configurations. The software also provides a method of optimizing geometry to obtain desired response. At present the 2-D case predicts a material package with a controllable compliance. This can be extended to a 3-D case consistent with available components.

Buckling the equatorial anion plane: octahedral anion distortions in ferroic perovskites and related systems resolved via neutron diffraction

A framework for describing anion displacements from perfect octahedra in perovskites has been developed for use with neutron diffraction data sets. We describe the distortions as noncoplanar arrangements, or buckling, of oxygen ions in any central plane of the octahedron, ignoring the central cation. Nonplanar distortions of octahedra have been calculated for perovskite structures contained within the Inorganic Crystal Structure Database. We find that antiferroelectric materials have buckling angles larger than ~2° and ferroelectric materials have buckling angles between 0° and 1°. The trend is found as a function of solid solution composition and temperature for common antiferroelectrics. For example, the described method resolves a structural difference between the end members PbTiO3 and PbZrO3, which exhibit ferroelectric and antiferroelectric responses, respectively. This technique is applicable to other structures containing anion octahedra, e.g., pyrochlores, spinels, and tungsten bronzes.

Application of Fourier harmonic analysis to the electromechanical response of an electroactive material

Signal decomposition through Fourier analysis can aid quantification of the electromechanical properties (induced strain and polarization) of electroactive materials. Spectral analysis of the strain and polarization, obtained from the Fourier transform, provides a unique characterization tool that better conveys material response than can be accomplished with polynomial fitting. The derived coefficients can be mapped onto those in the Devonshire phenomenology. The technique is demonstrated by analysis of a lead magnesium niobate relaxor ferroelectric [0.9875(0.935PMN–0.065PT)–0.0125BT or 0.9233PMN–0.06419PT–0.0125BT] operating in the electrostrictive regime. Fourier analysis, applied to a materials response, provides the first quantitative linkage to materials coefficients. A generalized mathematical approach has been derived that equates a Fourier series expression, from the transform of a time-domain electromechanical response, to the basic underlying physics developed in Devonshire theory. Thus, the el...

Coupling and dielectric properties of lead magnesium niobate-based electrostrictors

Lead magnesium niobate/lead titanate solid solutions with lanthanum dopants show substantial promise as electromechanical materials. In this work, the capacitance, dielectric loss, polarization, and transverse strain measurements of such materials are reported. These data are then matched with a model to predict the bulk coupling coefficient. This characterization avoids the problem of explicitly linearizing the electromechanical response of an electrostrictive material.

Spectral analysis of the electromechanical response of an electroactive material by implementation of Fourier decomposition

Implementation of a discrete Fourier transform in conjunction with Devonshire phenomenology allows the electromechanical response of an electroactive material to be fully characterized. The main thrust of this examination is the incorporation of signal decomposition through Fourier analysis to quantify the electro-mechanical and aging properties. Fourier analysis is a well-used technique in the electrical engineering community; however, it has not been applied to materials. The application presented here allows a fresh look at some old phenomena. The spectral representation provides to the underlying physics of the response, i.e., odd harmonics only exist for acentric materials. The ability to create the harmonic spectrum also provides a new way to characterize the effects of aging by revealing subtle changes in the fundamental components that comprise the response. This study shows how Fourier analysis can be applied to a PMN-PT-BT composition to characterize and quantify its electromechanical properties.