Mark E. Stavig

Electric field effect on the rhombohedral–rhombohedral phase transformation in tin modified lead zirconate titanate ceramics

The ferroelectric–to–ferroelectic phase transformation between the high temperature (FERH) and the low temperature (FERL) rhomobohedral phases in lead based perovskite under the dc bias conditions was investigated. Dielectric measurements show that an external electric field stabilizes the FERL phase and moves the phase transformation to a higher temperature. The observation has been further verified by an in situ microfocused x-ray study where an external field can effectively induce the oxygen octahedral tilting in the crystalline lattice and extends the thermal stability region of the FERL phase to a higher temperature. An analysis based on the combination of the Clausius–Clapeyron relationship with lattice dynamic principles suggests that the transformation from FERH to FERL is driven by a short-range interaction in the crystalline lattice. The origin of this short-range interaction is proposed, based on the structural evolution during the phase transformation. Experimental evidence suggests that such...

High temperature polymer dielectrics from the ring opening metathesis polymerization (ROMP)

Recently much research has focused on the development of new polymer dielectric materials to fabricate capacitors for use in the inverters of next generation hybrid electric vehicles (HEV). The capacitors used in HEVs inverters will be required to operate at 150 °C, 600V, and have an energy density of 0.9 J/cm3. Polymer based thin film capacitors are ideal for this application due to their relatively high energy density, low cost, and high dielectric breakdown strength. We have polymerized several ring strained monomers using ring opening metathesis polymerization (ROMP) and have identified a promising polymer system based on N-Phenyl-7-oxanorbornene-5,6-dicarboximide (PhONDI). Several of its copolymers with norbornene have been evaluated for possible use as next generation high temperature polymer dielectrics in thin film capacitors. The copolymers were cast into thin films and Au electrodes were deposited on the polymer film. The electrical properties were evaluated as a function of temperature. The polymer system exhibited very good high temperature dielectric properties and is potentially useful as a high temperature capacitor dielectric.

Time-Dependence of Epoxy Debonding

Test geometries with well-defined stresses at the initiation of adhesive failure (failure in adhesion) were used to examine debonding of epoxies in controlled ramp and creep tests. Little effect of substrate, curative, or filler content was seen in failure initiation for the variations studied. The time-to-fail in creep tests depended sensitively on the applied load. Sinusoidal shear loads were also applied in both single (zero to max) and double-sided (− max to + max) mode. Whereas the single-sided, oscillatory loaded samples failed much later than samples loaded in creep to the same maximum stress, double-sided times-to-fail were similar to those in creep.

Critical tractions for initiating adhesion failure at interfaces in encapsulated components

ABSTRACT Determining the initiation of adhesive failure at a surface buried deep within the bulk of an epoxy is qualitatively different from measuring the propagation of an existing surface crack. Most current tests are shown to be unsuitable for assessing the critical traction at initiation. A new test geometry is presented that initiates failure away from an air interface, produces a slowly varying stress distribution near the initiation site and minimal contributions from thermal residual stresses, and enables tests with mixed modes of loading. This new geometry is used to examine temperature-dependent adhesive failure in tensile, shear, and mixed modes of loading for both smooth and rough surfaces. Some of the experimental results are unexpected. As examples, the critical traction at initiation of adhesive failure is apparently insensitive to surface roughness, and the critical normal traction is independent of temperature while the critical tangential traction tracks the shear yield stress.

Viscoelasticity of Glass-Forming Materials: What About Inorganic Sealing Glasses?

Glass forming materials like polymers exhibit a variety of complex, nonlinear, time-dependent relaxations in volume, enthalpy and stress, all of which affect material performance and aging. Durable product designs rely on the capability to predict accurately how these materials will respond to mechanical loading and temperature regimes over prolonged exposures to operating environments. This cannot be achieved by developing a constitutive framework to fit only one or two types of experiments. Rather, it requires a constitutive formalism that is quantitatively predictive to engineering accuracy for the broad range of observed relaxation behaviors. Moreover, all engineering analyses must be performed from a single set of material model parameters. The rigorous nonlinear viscoelastic Potential Energy Clock (PEC) model and its engineering phenomenological equivalent, the Simplified Potential Energy Clock (SPEC) model, were developed to fulfill such roles and have been applied successfully to thermoplastics and filled and unfilled thermosets. Recent work has provided an opportunity to assess the performance of the SPEC model in predicting the viscoelastic behavior of an inorganic sealing glass. This presentation will overview the history of PEC and SPEC and describe the material characterization, model calibration and validation associated with the high Tg (~460 °C) sealing glass.

Cracking in a Flame-Sprayed Epoxy.

The objective of this work was to understand the cracking of aluminum flame spray on an epoxy thermoset. In the experiments presented here, epoxy cylinders were uniformly coated with flame spray. The cylinders were put into a state of tensile stress by taking them to elevated temperatures and similarly put into a state of compression by taking them down to cold temperatures. Surface cracks on the outside of the cylinders were photographed and compared. The cylinders were cross-sectioned at room temperature to study how the aluminum surface cracks propagate into the epoxy. It was shown that thicker aluminum generates observable surface cracks at a lower temperature than a thinner coating does. The surface cracks cannot be seen at room temperature. However, some of the coating cracks propagate into the substrate and can be seen at room temperature when the cylinder is cross-sectioned. The substrate cracks tend to be deeper with a larger coating thickness. Similarly, cracks are deeper when the substrate with a given thickness is taken to higher temperature. Supplementary examples that contain the addition of a hard inclusion (copper strip) between the aluminum and epoxy substrate at elevated temperatures were discussed, as well as delamination of the aluminum film at cold temperature.

Residual internal stress optimization for EPON 828/DEA thermoset resin using fiber Bragg grating sensors.

Internal residual stresses and overall mechanical properties of thermoset resins are largely dictated by the curing process. It is well understood that fiber Bragg grating (FBG) sensors can be used to evaluate temperature and cure induced strain while embedded during curing. Herein, is an extension of this work whereby we use FBGs as a probe for minimizing the internal residual stress of an unfilled and filled Epon 828/DEA resin. Variables affecting stress including cure cycle, mold (release), and adhesion promoting additives will be discussed and stress measurements from a strain gauge pop-off test will be used as comparison. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

A Process and Environment Aware Sierra/SolidMechanics Cohesive Zone Modeling Capability for Polymer/Solid Interfaces

The performance and reliability of many mechanical and electrical components depend on the integrity of po lymer - to - solid interfaces . Such interfaces are found in adhesively bonded joints, encapsulated or underfilled electronic modules, protective coatings, and laminates. The work described herein was aimed at improving Sandias finite element - based capability to predict interfacial crack growth by 1) using a high fidelity nonlinear viscoelastic material model for the adhesive in fracture simulations, and 2) developing and implementing a novel cohesive zone fracture model that generates a mode - mixity dependent toughness as a natural consequence of its formulation (i.e., generates the observed increase in interfacial toughness wi th increasing crack - tip interfacial shear). Furthermore, molecular dynamics simulations were used to study fundamental material/interfa cial physics so as to develop a fuller understanding of the connection between molecular structure and failure . Also reported are test results that quantify how joint strength and interfacial toughness vary with temperature.

Proposed Testing to Assess the Accuracy of Glass-To-Metal Seal Stress Analyses.

The material characterization tests conducted on 304L VAR stainless steel and Schott 8061 glass have provided higher fidelity data for calibration of material models used in Glass - T o - Metal (GTM) seal analyses. Specifically, a Thermo - Multi - Linear Elastic Plastic ( thermo - MLEP) material model has be en defined for S S304L and the Simplified Potential Energy Clock nonlinear visc oelastic model has been calibrated for the S8061 glass. To assess the accuracy of finite element stress analyses of GTM seals, a suite of tests are proposed to provide data for comparison to mo del predictions.

Materials-Based Process Tolerances for Neutron Generator Encapsulation

Variations in the neutron generator encapsulation process can affect functionality. However, instead of following the historical path in which the effects of process variations are assessed directly through functional tests, this study examines how material properties key to generator functionality correlate with process variations. The results of this type of investigation will be applicable to all generators and can provide insight on the most profitable paths to process and material improvements. Surprisingly, the results at this point imply that the process is quite robust, and many of the current process tolerances are perhaps overly restrictive. The good news lies in the fact that our current process ensures reproducible material properties. The bad new lies in the fact that it would be difficult to solve functional problems by changes in the process.