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Viscoelasticity of glass-forming materials: What about inorganic sealing glasses?

Conference Proceedings of the Society for Experimental Mechanics Series

Chambers, Robert S.; Stavig, Mark E.; Tandon, Rajan T.

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.

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TYPE Conference Poster YEAR 2016
ScopusOSTI

Characterization and calibration of a viscoelastic simplified potential energy clock model for inorganic glasses

Journal of Non-Crystalline Solids

Chambers, Robert S.; Tandon, Rajan T.; Stavig, Mark E.

To analyze the stresses and strains generated during the solidification of glass-forming materials, stress and volume relaxation must be predicted accurately. Although the modeling attributes required to depict physical aging in organic glassy thermosets strongly resemble the structural relaxation in inorganic glasses, the historical modeling approaches have been distinctly different. To determine whether a common constitutive framework can be applied to both classes of materials, the nonlinear viscoelastic simplified potential energy clock (SPEC) model, developed originally for glassy thermosets, was calibrated for the Schott 8061 inorganic glass and used to analyze a number of tests. A practical methodology for material characterization and model calibration is discussed, and the structural relaxation mechanism is interpreted in the context of SPEC model constitutive equations. SPEC predictions compared to inorganic glass data collected from thermal strain measurements and creep tests demonstrate the ability to achieve engineering accuracy and make the SPEC model feasible for engineering applications involving a much broader class of glassy materials.

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TYPE Journal Article YEAR 2016
ScopusOSTIDOI

The Compelling Case for Indentation as a Functional Exploratory and Characterization Tool

Journal of the American Ceramic Society

Marshall, David B.; Cook, Robert F.; Padture, Nitin P.; Oyen, Michelle L.; Pajares, Antonia; Bradby, Jodie E.; Reimanis, Ivar E.; Tandon, Rajan T.; Page, Trevor F.; Pharr, George M.; Lawn, Brian R.

The utility of indentation testing for characterizing a wide range of mechanical properties of brittle materials is highlighted in light of recent articles questioning its validity, specifically in relation to the measurement of toughness. Contrary to assertion by some critics, indentation fracture theory is fundamentally founded in Griffith-Irwin fracture mechanics, based on model crack systems evolving within inhomogeneous but well-documented elastic and elastic-plastic contact stress fields. Notwithstanding some numerical uncertainty in associated stress intensity factor relations, the technique remains an unrivalled quick, convenient and economical means for comparative, site-specific toughness evaluation. Most importantly, indentation patterns are unique fingerprints of mechanical behavior and thereby afford a powerful functional tool for exploring the richness of material diversity. At the same time, it is cautioned that unconditional usage without due attention to the conformation of the indentation patterns can lead to overstated toughness values. Limitations of an alternative, more engineering approach to fracture evaluation, that of propagating a precrack through a "standard" machined specimen, are also outlined. Misconceptions in the critical literature concerning the fundamental nature of crack equilibrium and stability within contact and other inhomogeneous stress fields are discussed.

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TYPE Journal Article YEAR 2015
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Large motion high cycle high speed optical fibers for space based applications

Stromberg, Peter G.; Tandon, Rajan T.; Gibson, Cory S.; Reedlunn, Benjamin R.; Rasberry, Roger D.; Rohr, Garth R.

Future remote sensing applications will require higher resolution and therefore higher data rates (up to perhaps 100 gigabits per second) while achieving lower mass and cost. A current limitation to the design space is high speed high bandwidth data does not cross movable gimbals because of cabling issues. This requires the detectors to be off gimbal. The ability to get data across the gimbal would open up efficiencies in designs where the detectors and the electronics can be placed anywhere on the system. Fiber optic cables provide light weight high speed high bandwidth connections. Current options are limited to 20,000 cycles as opposed to the 1,000,000 cycles needed for future space based applications. To extend this to the million+ regime, requires a thorough understanding of the failure mechanisms and the materials, proper selection of materials (e.g., glass and jacket material) allowable geometry changes to the cable, radiation hardness, etc.

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TYPE SAND Report YEAR 2014
OSTIDOI

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

Chambers, Robert S.; Emery, John M.; Tandon, Rajan T.; Antoun, Bonnie R.; Stavig, Mark E.; Newton, Clay S.; Gibson, Cory S.; Bencoe, Denise N.

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.

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TYPE SAND Report YEAR 2014
OSTIDOI

Characterization & Modeling of Materials in Glass-To-Metal Seals: Part I

Chambers, Robert S.; Emery, John M.; Tandon, Rajan T.; Antoun, Bonnie R.; Stavig, Mark E.; Newton, Clay S.

To support higher fidelity modeling of residual stresses in glass-to-metal (GTM) seals and to demonstrate the accuracy of finite element analysis predictions, characterization and validation data have been collected for Sandia’s commonly used compression seal materials. The temperature dependence of the storage moduli, the shear relaxation modulus master curve and structural relaxation of the Schott 8061 glass were measured and stress-strain curves were generated for SS304L VAR in small strain regimes typical of GTM seal applications spanning temperatures from 20 to 500 C. Material models were calibrated and finite element predictions are being compared to measured data to assess the accuracy of predictions.

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TYPE SAND Report YEAR 2014
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Strength and reliability estimation of a low temperature co-fired ceramic (LTCC) with and without metallic features

44th International Symposium on Microelectronics 2011, IMAPS 2011

Tandon, Rajan T.; Newton, Clay S.

The use of Low Temperature Co-Fired Ceramics (LTCC) is a very attractive material option for advanced packaging. For applications, a variety of features are printed in the base material: thermal and electrical vias, resistors, solder pads to name a few. Most of these features have materials that are thermally and elastically mismatched from the LTCC, producing a localized residual stress. These stresses impact the strength and reliability of the LTCC package. Here we present results and analysis for the strength and reliability assessment of an LTCC (DupontTM 951) with and without Au vias. The reliability of the ceramic material is assessed from the perspective of its susceptibility to sub-critical crack growth (SCG). Metallic vias can significantly lower the strength of the LTCC, however, their presence does not change the measured susceptibility of the material to SCG. Using our experimental data, and empirical descriptions of SCG laws, safe design life for LTCC packages under a particular stress state is estimated.

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TYPE Conference YEAR 2011
ScopusOSTI

Filled glass composites for sealing of solid oxide fuel cells

Garino, Terry J.; Tandon, Rajan T.

Glasses filled with ceramic or metallic powders have been developed for use as seals for solid oxide fuel cells (SOFC's) as part of the U.S. Department of Energy's Solid State Energy Conversion Alliance (SECA) Program. The composites of glass (alkaline earth-alumina-borate) and powders ({approx}20 vol% of yttria-stabilized zirconia or silver) were shown to form seals with SOFC materials at or below 900 C. The type and amount of powder were adjusted to optimize thermal expansion to match the SOFC materials and viscosity. Wetting studies indicated good wetting was achieved on the micro-scale and reaction studies indicated that the degree of reaction between the filled glasses and SOFC materials, including spinel-coated 441 stainless steel, at 750 C is acceptable. A test rig was developed for measuring strengths of seals cycled between room temperature and typical SOFC operating temperatures. Our measurements showed that many of the 410 SS to 410 SS seals, made using silver-filled glass composites, were hermetic at 0.2 MPa (2 atm.) of pressure and that seals that leaked could be resealed by briefly heating them to 900 C. Seal strength measurements at elevated temperature (up to 950 C), measured using a second apparatus that we developed, indicated that seals maintained 0.02 MPa (0.2 atm.) overpressures for 30 min at 750 C with no leakage. Finally, the volatility of the borate component of sealing glasses under SOFC operational conditions was studied using weight loss measurements and found by extrapolation to be less than 5% for the projected SOFC lifetime.

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TYPE SAND Report YEAR 2009
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Effect of incorporation of cylindrical inhomogeneities on the strength distributions of brittle materials

Journal of the American Ceramic Society

Tandon, Rajan T.

Cylindrical inhomogeneities are often deliberately incorporated into engineering ceramics (e.g., fibers, vias, electrical feedthroughs). The thermal expansion mismatch between the matrix and inhomogeneity creates a state of localized stress. We show that for radial cracks around such inhomogeneities, there may be conditions of crack stability even in the presence of an external, destabilizing field. This stability, and the nature of the stress intensity factor due to local stresses, modifies the strength distribution of the matrix. A fracture-mechanics approach allows the prediction of the new strength distributions. As an illustration of this approach, calculations for commonly used ceramic-metal inhomogeneity material pairs are discussed. Depending on the inhomogeneity/flaw size ratio, the new strength distributions can have lower or higher strength variability than the matrix. If the inhomogeneity radius (R) is chosen such that a majority of the cracks in the matrix are >0.25R, the material will have the highest possible strength and reduced variability. © 2008 The American Ceramic Society.

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TYPE Conference YEAR 2008
ScopusOSTI

Advanced modeling and simulation to design and manufacture high performance and reliable advanced microelectronics and microsystems

Ewsuk, Kevin G.; Hinklin, Thomas R.; Neilsen, Michael K.; Tandon, Rajan T.; Arguello, Jose G.; Dempsey, James F.; Holcomb, David J.

An interdisciplinary team of scientists and engineers having broad expertise in materials processing and properties, materials characterization, and computational mechanics was assembled to develop science-based modeling/simulation technology to design and reproducibly manufacture high performance and reliable, complex microelectronics and microsystems. The team's efforts focused on defining and developing a science-based infrastructure to enable predictive compaction, sintering, stress, and thermomechanical modeling in ''real systems'', including: (1) developing techniques to and determining materials properties and constitutive behavior required for modeling; (2) developing new, improved/updated models and modeling capabilities, (3) ensuring that models are representative of the physical phenomena being simulated; and (4) assessing existing modeling capabilities to identify advances necessary to facilitate the practical application of Sandia's predictive modeling technology.

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TYPE SAND Report YEAR 2007
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Measuring residual stress in glasses and ceramics using instrumented indentation

Proposed for publication in the Journal of Materials Research.

Buchheit, Thomas E.; Tandon, Rajan T.

Instrumented indentation has yielded mixed results when used to measure surface residual stresses in metal films. Relative to metals, many glasses and ceramics have a low modulus-to-yield strength (E/sy) ratio. The advantage of this characteristic for measuring residual stress using instrumented indentation is demonstrated by a series of comparative spherical and conical tip finite element simulations. Two cases are considered: (i) a material with E/s{sub y} = 24-similar to glass and (ii) a material with E/s{sub y} = 120-similar to metal films. In both cases, compressive residual stress shifts the simulated load-displacement response toward increasing hardness, irrespective of tip geometry. This shift is shown to be entirely due to pile up for the ''metal'' case, but primarily due to the direct influence of the residual stress for the ''glass'' case. Hardness changes and load-displacement curve shifts are explained by using the spherical cavity model. Supporting experimental results on stressed glasses are provided.

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TYPE Journal Article YEAR 2007
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Ultra high temperature ceramics for hypersonic vehicle applications

Loehman, Ronald E.; Corral, Erica L.; Kotula, Paul G.; Tandon, Rajan T.

HfB{sub 2} and ZrB{sub 2} are of interest for thermal protection materials because of favorable thermal stability, mechanical properties, and oxidation resistance. We have made dense diboride ceramics with 2 to 20 % SiC by hot pressing at 2000 C and 5000 psi. High-resolution transmission electron microscopy (TEM) shows very thin grain boundary phases that suggest liquid phase sintering. Fracture toughness measurements give RT values of 4 to 6 MPam{sup 1/2}. Four-pt flexure strengths measured in air up to 1450 C were as high as 450-500 MPa. Thermal diffusivities were measured to 2000 C for ZrB{sub 2} and HfB{sub 2} ceramics with SiC contents from 2 to 20%. Thermal conductivities were calculated from thermal diffusivities and measured heat capacities. Thermal diffusivities were modeled using different two-phase composite models. These materials exhibit excellent high temperature properties and are attractive for further development for thermal protection systems.

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TYPE SAND Report YEAR 2006
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