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We study the suppression of time-dependent inelastic deformation due to creep, stress relaxation, and microstructural evolution in multilayer thin film microstructures by the use of alumina nanocoatings realized by atomic layer deposition (ALD). Gold (0.5 μm thick)/polysilicon (1.5 or 3.5 μm thick) beam and plate microstructures were fabricated using surface micromachining. The microstructures were then coated on each side with a 40-nm-thick amorphous Al2O3 layer by ALD. The beam and plate microstructures were initially thermal cycled between room temperature and 190 °C to partially stabilize the gold microstructure. After the initial thermal cycles, the microstructures were cooled from 190 to 120 °C and held at 120 °C for about 700 h (4 weeks). We measured, using an interferometric microscope with a custom-built temperature chamber, full-field deformed shapes (and from these determined the average curvatures in the x and y directions) of the microstructures during the initial thermal cycles, during the c...

We study the viscoelectroelastic behavior of heterogeneous piezoelectric solids, focusing on the connection between heterogeneity and coupled mechanical and electrical relaxations. Our approach is based on the existence of a correspondence between quasistatic viscoelectroelasticity and static piezoelectricity when linear constitutive response exists. We couple this correspondence principle with micromechanics models to predict the overall behavior of heterogeneous piezoelectric solids in terms of microstructural details. We devote specific attention to a class of two-phase materials consisting of a lossless piezoelectric phase embedded in a lossy (mechanically and electrically) matrix and obtain closed form expressions for the effective complex electroelastic moduli. Numerical results are presented and discussed, and qualitative agreement with experiment is observed.

Ogi H., Dunn M., Takashima K., et al. "Elastic properties of a unidirectional SiCf/Ti composite: Acoustic-resonance measurements and micromechanics predictions", Journal of Applied Physics, 87(6), 2769-2774 (2000) https://doi.org/10.1063/1.372254., Using the mode-selective contactless resonance ultrasound spectroscopy, we measured the complete elastic-stiffness tensor Cij and the corresponding internal-friction tensor Q-1ij of a silicon-carbide fiber unidirectionally reinforced Ti-6Al-4V-matrix composite. This material shows orthorhombic symmetry and possesses nine independent Cij and Q-1ij. We independently excited and detected an unique vibration-mode group among the eight vibration groups of a orthorhombic rectangular parallelepiped. This provides a great advantage in identifying the vibration modes of the observed resonance peaks, whose frequencies yield the Cij by solving an inverse problem. To verify the resultant Cij, we also determined Cij by point-contact resonance ultrasound spectroscopy and pulse-echo methods. Then, using the composite Cij, we inversely calculated the transverse-isotropic fiber elastic stiffnesses with micromechanics theory. With the fiber Cij and the isotropic matrix Cij, we reconstructed the composite Cij as a function of fiber-volume fraction. © 2000 American Institute of Physics.

Conditions are given for the existence of exact solutions for the effective thermal and electroelastic moduli of polycrystals exhibiting fiber texture. The exact solutions are then developed through the consideration of a class of uniform fields in a polycrystal acted upon by various external thermal and electroelastic loadings. These exact results are verified by direct orientational averaging that shows the coincidence of the Voigt and Reuss bounds on the overall moduli.

We designed and fabricated a series of micromechanical test structures for microtensile testing by anisotropically etching epitaxial silicon. Specimens were fabricated to study Young’s moduli, the uniaxial tensile strength, and the strength of T-structures which are tensile bars with an abrupt reduced cross section that have a 90° corner at the point of reduction. They are a generic mimic of actual transitions that occur in micromechanical structures due to anisotropic etching. The test structures were loaded in uniaxial tension in a piezoactuated microtensile test apparatus. The applied force and crosshead displacement were recorded and displacements in the specimen gage section were directly measured using a speckle interferometry technique. During tensile loading of the T-structures, fracture always initiates at the sharp 90° corners. This results in an interesting apparent strength scaling where the nominal strength of the structures increases as their width decreases. In order to understand the fract...

A theoretical approach is proposed to estimate the effects of grain shape anisotropy on the overall response of polycrystalline piezoelectric ceramics. The self‐consistent approach is applicable to a polycrystal with arbitrary grain shapes, orientations, and anisotropy, but emphasis is placed on the effective bulk and shear moduli and the dielectric constant of a polycrystal consisting of randomly oriented piezoelectric grains. Because typical piezoelectric ceramics often contain substantial porosity, the effects of randomly oriented pores of various shapes on the overall electroelastic properties are considered. Analytical predictions are compared to simplified predictions which ignore the effects of piezoelectricity at the grain level and the effects of grain shape. Analytical predictions are in good agreement with measurements for unpoled, porous BaTiO3 polycrystals. A FORTRAN computer program developed to implement the theory is available upon request from the author.

We examine thermal expansion of textured polycrystalline aggregates and focus on polycrystals containing hexagonal monocrystals. Our analysis is based on the coupling of the Voigt–Reuss–Hill averaging procedures for elastic constants, Roe’s [R. J. Roe, J. Appl. Phys. 36, 2024 (1965)] framework for the mathematical description of the crystallite orientation‐distribution function (ODF), and the exact connections of Hashin [Z. Hashin, J. Mech. Phys. Solids 32, 149 (1984)] and Schulgasser [K. Schulgasser, J. Mech. Phys. Solids 35, 35 (1987)] between the thermal‐expansion coefficients of a polycrystalline aggregate and the corresponding polycrystal elastic constants, monocrystal elastic constants, and monocrystal thermal‐expansion coefficients. The results are simple expressions for the polycrystal thermal‐expansion coefficient in terms of the monocrystal elastic constants, thermal‐expansion coefficients, and the coefficients in the expansion of the crystallite ODF in a series of generalized spherical harmonic...

The effective thermal‐expansion and pyroelectric coefficients of two‐phase coupled electroelastic composite materials are obtained by coupling various micromechanics theories with exact relations connecting their effective thermal and electroelastic moduli. Results are obtained for composites reinforced by ellipsoidal piezoelectric inhomogeneities and thus are applicable to a wide range of microstructural geometry including lamina, spherical particle, and continuous fiber reinforcement. The analytical framework is developed in a matrix formulation in which the electroelastic moduli are conveniently represented by a 9×9 matrix and the thermal‐expansion and pyroelectric coefficients by a 9×1 column vector. Numerical results are presented for some typical composite microstructures which show the interesting behavior of these materials.

An analytical model is developed to predict the effective thermal conductivity of multiphase composites reinforced with coated fillers. The coated fillers are modeled as confocal spheroids, thus enabling the simulation of a wide range of reinforcement geometry ranging from thin flake to continuous fiber. The orientation of the coated fillers is described by a density distribution function which can simulate completely random, in‐plane random, and aligned distributions as special cases. The analytical approach appears to be the only one in the literature that renders closed‐form predictions of the effective thermal conductivity of composites with misoriented coated reinforcement and thus recourse to a numerical scheme is not required. The analytical approach is specialized to consider the effects of a thermal resistance at the filler‐matrix interface of a composite. Results of the proposed model are compared with those obtained by self‐consistent and differential estimates [Y. Benveniste and T. Miloh, J. Appl. Phys. 69, 1337 (1991)] for the effective thermal conductivity of a composite reinforced by completely random coated short fibers. The proposed model is in close agreement with the other two models for low conductivity coatings, but differs appreciably for high conductivity coatings. Finally, good agreement is shown to exist between analytical predictions and experimental results obtained from the literature for a diamond particle/zinc‐sulfide matrix composite with an interfacial thermal resistance.

Metal oxide thin film coatings produced by atomic layer deposition have been shown to be an effective permeation barrier. The primary failure mode of such coatings under tensile loads is the propagation of channel cracks that penetrate vertically into the coating films. Recently, multi-layer structures that combine the metal oxide material with relatively soft polymeric layers produced by molecular layer deposition have been proposed to create composite thin films with desired properties, including potentially enhanced resistance to fracture. In this paper, we study the effects of layer geometry and material properties on the critical strain for channel crack propagation in the multi-layer composite films. Using finite element simulations and a thin-film fracture mechanics formalism, we show that if the fracture energy of the polymeric layer is lower than that of the metal oxide layer, the channel crack tends to penetrate through the entire composite film, and dividing the metal oxide and polymeric materials into thinner layers leads to a smaller critical strain. However, if the fracture energy of the polymeric material is high so that cracks only run through the metal oxide layers, more layers can result in a larger critical strain. For intermediate fracture energy of the polymer material, we developed a design map that identifies the optimal structure for given fracture energies and thicknesses of the metal oxide and polymeric layers. These results can facilitate the design of mechanically robust permeation barriers, an important component for the development of flexible electronics.

We demonstrate the ability to use photoactivated stress relaxation to improve flaw tolerance in network polymers. Unlike most self-healing polymers, which effectively close flaws by locally introducing healing agents (such as uncured resins), here light is used to relax elevated stresses around a flaw before it reaches a critical state, which reduces the threat that the flaw poses to the structural integrity of the material. In this study, we fabricate specimens with well-defined flaws and uniaxially stretch them to failure. By irradiating the specimens with UV light (365 nm) before failure, the nominal strain at failure is increased by 70% and the corresponding nominal stress is increased by 30% compared with nonirradiated specimens. To better understand the phenomena that occur at the multiaxial stress state at the flaw, we model the photomechanics using a recently developed finite element approach that accurately describes the light propagation, photochemistry, radical-induced network evolution, and the mechanical behavior of the material. Model predictions agree well with the experimental results and elucidate the role that photoinduced stress relaxation has on improving flaw tolerance.

We study the nanomechanics of graphene and other ultrathin membranes adhered to substrates with open cavities by van der Waals forces, and subjected to mechanical loads (concentrated or pressure) that cause stretching, decohesion, and/or sliding. We develop analytical models to describe equilibrium configurations similar to those observed in recent nanomechanical experiments, as well as various deformation regimes that occur upon mechanical loading. We apply the analytical models to single layers of graphene and carry out atomistic simulations of graphene adhered to a substrate to validate the models and illustrate the operative phenomena. We obtain excellent agreement between theory and atomistic simulations and identify the influence of van der Waals adhesion energy, membrane elasticity, geometry, and loading on membrane decohesion from and/or sliding along a substrate.

films was determined to be KIC= 1.89 0.10 and 0.17 0.02 MPa m 0.5 , respectively. From measurements of the saturated crack density, the critical interfacial shear stress was estimated to be c = 39.5 8.3 and 66.6 6.1 MPa, respectively. The toughness of nanometer-scale alumina was comparable to that of alumina thin films grown using other techniques, whereas alucone was quite brittle. The use of alucone as a spacer layer between alumina films was not found to increase the critical strain at fracture for the composite films. This performance is attributed to the low toughness of alucone. The experimental results were supported by companion simulations using fracture mechanics formalism for multilayer films. To aid future development, the modeling method was used to study the increase in the toughness and elastic modulus of the spacer layer required to render improved critical strain at fracture. These results may be applied to a broad variety of multilayer material systems composed of ceramic and spacer layers to yield robust coatings for use in chemical barrier and other applications. © 2009 American Institute of Physics. DOI: 10.1063/1.3124642

The coupling of optical and mechanical energy has recently been demonstrated in a class of liquid crystal elastomers that contain light-sensitive molecules. Their molecular structure consists of stiff, ordered, rod-like molecules (the liquid crystal feature) connected to long cross-linked molecular chains (the elastomer feature). This connection leads to a coupling between the orientational order of the rods and mechanical deformation of the network. Irradiation with light at appropriate wavelengths leads to isomerization of the stiff rod-like molecules, resulting in bending (straightening) of the molecules which dilutes (enhances) the orientational order, thus causing macroscopic deformation. This deformation, photostrain, can be as much as hundreds of percent. These materials are typically fabricated as thin monodomain or polydomain films. If uniformly irradiated through the thickness, they undergo anisotropic incompressible deformation in the film plane. If the light intensity varies through the thickn...

Particulates can strongly influence interfacial adhesion between rough surfaces by changing their average separation. In a cantilever beam adhesion test structure, a compressive zone exists just beyond the crack tip, which may act to deform such particles. To explore this phenomenon quantitatively, we compared finite element method calculations of the interface to load-displacement experiments of individual particles. Below a certain threshold density, we show that the stress distribution at the interface is sufficient to deform individual particles. In this regime, the adhesion is controlled by the intrinsic surface roughness and under dry conditions is mainly due to van der Waals forces across extensive noncontacting areas. Above this threshold density, however, the particles introduce a topography that is more significant than the intrinsic surface roughness. As a result, the interfacial separation is governed by the particle size and the adhesion is lower but stochastic in nature. We demonstrate that the particles on the micromachined surfaces are silicon carbide (SiC). The cantilever test structures were fabricated using standard surface micromachining techniques, which consisted of depositing, patterning, and etching two polycrystalline silicon (polysilicon) layers separated by a tetraethylorthosilicate (TEOS) sacrificial oxide layer. High temperature annealing in the fabrication process allows residual carbon in the TEOS sacrificial oxide layer to migrate to the polysilicon surface and form the SiC particles.

We study phonon dispersion in layered prismatic nanowires that can have an arbitrary cross-sectional geometry. We calculate phonon dispersion using a hybrid analytical/numerical approach that models propagation along the wire analytically, but deformation in the plane of the cross section using the finite element method. In our results we consider only two layers, but the calculational approach is applicable to multiple layers.