Stephen T. Montgomery

The effects of nonhydrostatic compression and applied electric field on the electromechanical behavior of poled lead zirconate titanate 95/5-2Nb ceramic during the ferroelectric to antiferroelectric polymorphic transformation

We conducted hydrostatic compression and constant-stress-difference experiments, with and without an applied electric field, on poled, niobium-doped lead zirconate titanate ceramic. The objective was to quantify the effects of nonhydrostatic stress and electric field bias on electromechanical behavior of the ceramic during the ferroelectric, rhombohedral {r_arrow} antiferroelectric, orthorhombic phase transformation. Increasing stress difference (shear stress) decreases the mean stress at which the transformation occurs. Increasing shear stress also retards the rate of transformation, causing reductions in both the rate of charge release and peak voltage attained during depoling. Application of the electric field bias slightly increases the transformation pressure for poled ceramic. Previously, we showed that under nonhydrostatic stress, the transformation took place in {ital unpoled} ceramic when the maximum compressive stress equalled the hydrostatic pressure at which the transformation would otherwise occur. This simple stress criterion does not apply to poled ceramic. However, poled material has a preferred crystallographic orientation and mechanical anisotropy, whereas unpoled ceramic is isotropic. We present a qualitative model for the transformation under nonhydrostatic stress-related to that anisotropy, which resolves these seemingly disparate observations. {copyright} {ital 1999 Materials Research Society.}

Compositional effects on the shock-compression response of alumina-filled epoxy

Alumina-filled epoxies are composites having constituents with highly dissimilar mechanical properties, resulting in complex behavior during shock compression and release. A previous study examined the shock properties of a particular composition in some detail. In the current study, the effects of compositional variations on shock properties were examined. Planar-impact experiments producing states of nearly equal strain were conducted to investigate the effects of changes in the size and shape of alumina particles, and in the total volume fraction of alumina. Laser interferometry and wave timing were used to obtain transmitted wave profiles, Hugoniot states, and release wave velocities. In addition, wave profiles and velocities were obtained in “thin-pulse” experiments that examined the combined effects of compression and release properties in different compositions. Changes in the size and shape of alumina particles were found to have little effect except in the viscous spreading of wave profiles durin...

Uniaxial compression experiments on lead zirconate titanate 95/5-2Nb ceramic: Evidence for an orientation-dependent, ''maximum compressive stress'' criterion for onset of the ferroelectric to antiferroelectric polymorphic transformation

Some time ago we presented evidence that, under nonhydrostatic loading, the F{sub R1} {r_arrow} A{sub O} polymorphic transformation of unpoled PZT 95/5-2Nb (PNZT) ceramic began when the maximum compressive stress equaled the hydro-static pressure at which the transformation otherwise took place. Recently we showed that this simple criterion did not apply to nonhydrostatically compressed, poled ceramic. However, unpoled ceramic is isotropic, whereas poled ceramic has a preferred crystallographic orientation and is mechanically anisotropic. If we further assume that the transformation depends not only on the magnitude of the compressive stress, but also its orientation relative to some feature(s) of PNZTs crystallography, then these disparate results can be qualitatively resolved. It has long been known that this transformation can be triggered in uniaxial compression. Our modified hypothesis makes two predictions for transformation of unpoled polycrystals under uniaxial stress: (i) the transformation should begin when the maximum compressive stress, {sigma}{sub 1}, equals the hydrostatic pressure for transformation, and (ii) a steadily increasing axial stress should be required to drive the transformation.

Dynamic behavior of tungsten carbide and alumina filled epoxy composites

The dynamic behavior of a tungsten carbide filled epoxy composite is studied under planar loading conditions. Planar impact experiments were conducted to determine the shock and wave propagation characteristics of the material. Its stress-strain response is very close to a similar alumina filled epoxy studied previously, suggesting that the response of the composite is dominated by the compliant matrix material. Wave propagation characteristics are also similar for the two materials. Magnetically driven ramp loading experiments were conducted to obtain a continuous loading response which is similar to that obtained under shock loading. Spatially resolved interferometry was fielded on one experiment to provide a quantitative measure of the variability inherent in the response of this heterogeneous material. Complementing the experiments, a two-dimensional mesoscale model in which the individual constituents of the composite are resolved was used to simulate its behavior. Agreement of the predicted shock an...

The deviatoric response of an alumina filled epoxy composite during shock loading

The deviatoric response of a particulate alumina-epoxy composite to shock loading has been investigated using manganin stress gauges sensitive to the lateral component of stress. Results show that the lateral stress and thus the shear strength are near constant behind the shock front, indicating that the presence of alumina has a diluting response of the epoxy resin. The shear strength has been observed to increase with increasing shock stress, in agreement with comparisons between the measured shock stress and the calculated hydrodynamic pressure. Finally, the Hugoniot elastic limit of this material has been estimated at ∼1.6GPa by the intersection between the elastic and inelastic shear strengths.

Dielectric Properties of PZT 95/5 during Shock Compression under High Electric Fields

Shock‐induced depoling of the ferroelectric ceramic PZT 95/5 is utilized in pulsed power devices. High electric fields are generated within a normally poled ceramic element when the depoling current is passed through a large load resistor. Under these conditions, a portion of the depoling current is retained on the element electrodes to account for capacitance. This effect is governed by the dielectric properties of both unshocked and shocked PZT 95/5 as the field develops during shock transit. Previous studies proposed either constant or relatively simple relaxing behavior for dielectric properties on either side of the shock front. However, interpretation of the corresponding experiments was complicated by possible field effects on depoling kinetics. Recent studies have used different experimental configurations to better isolate the dielectric behavior. Multiple PZT 95/5 elements are displaced both parallel and perpendicular to the direction of shock motion to allow for sequential charging of unshocked...

Simulation of the Effects of Shock Stress and Electrical Field Strength on Shock‐Induced Depoling of Normally Poled PZT 95/5

Shock‐induced depoling of the ferroelectric ceramic PZT 95/5 is utilized in pulsed power applications. Experiments to examine the shock response of normally poled PZT 95/5 under uniaxial strain conditions show that depoling kinetics, as reflected in current generation through an external circuit, is inhibited by both decreasing the shock pressure and increasing the electric field within the ceramic. A model to describe the response of the ferroelectric ceramic has been developed and implemented into simulation codes. Measured currents in the external circuit and transmitted waveforms at a window interface have been compared with results from simulations for experiments with shock pressures varying from 0.6 to 4.6 GPa and electric fields varying from 0.3 to 37 kV/cm.

The effects of shock stress and field strength on shock-induced depoling of normally poled PZT 95/5

Shock-induced depoling of the ferroelectric ceramic PZT 95/5 is utilized in a number of pulsed power devices. Several experimental and theoretical efforts are in progress in order to improve numerical simulations of these devices. In this study we have examined the shock response of normally poled PZT 95/5 under uniaxial strain conditions. On each experiment the current produced in an external circuit and the transmitted waveform at a window interface were recorded. The peak electrical field generated within the PZT sample was varied through the choice of external circuit resistance. Shock pressures were varied from 0.6 to 4.6 GPa, and peak electrical fields were varied from 0.2 to 37 kV/cm. For a 2.4 GPa shock and the lowest peak field, a nearly constant current governed simply by the remanent polarization and the shock velocity was recorded. Both decreasing the shock pressure and increasing the electrical field resulted in reduced current generation, indicating a retardation of the depoling kinetics.

Initial Temperature Effects on the Dielectric Properties of PZT 95/5 During Shock Compression.

A strong electric field can be generated when the shock‐induced depoling current from a normally poled PZT 95/5 sample is passed through a large resistive load. The portion of total depoling current that is retained on the sample electrodes to account for capacitance is governed by the dynamic dielectric properties of both unshocked and shocked material. Early studies used measured load currents from single samples to assess models for dielectric response. In more recent studies, we used shock‐driven circuits in which multiple PZT 95/5 elements were displaced both parallel and perpendicular to the shock motion. This allowed both load and charging currents to be measured for individual elements that are subjected to shock compression and release at different times. In the present study, these techniques have been utilized to examine dielectric properties in PZT 95/5 samples at initial temperatures from −56 to 74 °C. Significant changes in permittivity with temperature are observed in both unshocked and sho...

Dynamic electromechanical characterization of axially poled PZT 95/5

We are conducting a comprehensive experimental study of the electromechanical behavior of poled PZT 95/5 (lead zirconate titanate). As part of this study, eight plane-wave tests have been conducted on axially poled PZT 95/5 at stress levels ranging from 0.9 to 4.6 GPa, using VISAR and electrical diagnostics. Observed wave velocities were slightly decreased from ultrasonic values, by contrast with unpoled samples. Compression waveforms show a step at 0.6 GPa more marked than for normally poled or unpoled samples; this may correspond to a poling effect on the ferroelectric/antiferroelectric transition. A similar step is observed on release. The released charge upon loading to 0.9 GPa is consistent with nearly complete depoling. Loading to higher stresses gave lower currents (factor of 10), suggesting shock-induced conductivity or electrical breakdown.