Hamid Garmestani

Enhancement of thermal and electrical properties of carbon nanotube polymer composites by magnetic field processing

We show that the thermal and electrical properties of single wall carbon nanotube (CNT)-polymer composites are significantly enhanced by magnetic alignment during processing. The electrical transport properties of the composites are mainly governed by the hopping conduction with localization lengths comparable to bundle diameters. The bundling of nanotubes during the composite processing is an important factor for electrical, and in particular, for thermal transport properties. Better CNT isolation will be needed to reach the theoretical thermal conductivity limit for CNT composites.

Microstructure-sensitive design of a compliant beam

We show that mechanical design can be conducted where consideration of polycrystalline microstructure as a continuous design variable is facilitated by use of a spectral representation space. Design of a compliant fixed-guided beam is used as a case study to illustrate the main tenets of the new approach, called microstructure-sensitive design (MSD). Selection of the mechanical framework for the design (e.g., mechanical constitutive model) dictates the dimensionality of the pertinent representation. Microstructure is considered to be comprised of basic elements that belong to the material set. For the compliant beam problem, these are uni-axial distribution functions. The universe of pertinent microstructures is found to be the convex hull of the material set, and is named the material hull. Design performance, in terms of specified design objectives and constraints, is represented by one or more surfaces (often hyperplanes) of finite dimension that intersect the material hull. Thus, the full range of microstructure, and concomitant design performance, can be exploited for any material class. Optimal placement of the salient iso-property surfaces within the material hull dictates the optimal set of microstructures for the problem. Extensions of MSD to highly constrained design problems of higher dimension is also described.

Ferromagnetism and magnetoresistance of Co–ZnO inhomogeneous magnetic semiconductors

Co–ZnO inhomogeneous magnetic semiconductor thin films were synthesized on the subnanometer scale by sputtering. Room temperature ferromagnetism with high magnetization was found. Large negative magnetoresistance of 11% was found at room temperature, and its value increased with a decrease in temperature up to 36% at 4.8 K. The mechanism for large negative magnetoresistance is discussed.

Statistical continuum theory for large plastic deformation of polycrystalline materials

This paper focuses on the application of statistical continuum mechanics to the prediction of mechanical response of polycrystalline materials and microstructure evolution under large plastic deformations. A statistical continuum mechanics formulation is developed by applying a Greens function solution to the equations of stress equilibrium in an infinite domain. The distribution and morphology of grains (crystals) in polycrystalline materials is represented by a set of correlation functions that are described by the corresponding probability functions. The elastic deformation is neglected and a viscoplastic power law is employed for crystallographic slip in single crystals. In this formulation, two- and three-point probability functions are used. A secant modulus-based formulation is used. The statistical analysis is applied to simulate homogeneous deformation processes under uniaxial tension, uniaxial compression and plane strain compression of an FCC polycrystal. The results are compared to the well-known Taylor upper bound model and discussed in comparison to experimental observations.

Magnetically induced texture development in zinc alloy sheet

Highly textured Zn–1.1%Al sheet was annealed in a direct-current magnetic field of 25.5 MA/m. Depending on the orientation to the field the texture components strengthened, retained their original intensity or disappeared. The results obtained are interpreted in terms of magnetically induced selective grain growth. 2002 Published by Elsevier Science Ltd. on behalf of Acta Materialia Inc.

Adhesion energy in carbon nanotube-polyethylene composite: Effect of chirality

This work presents a study of the adhesion energy between carbon nanotube-polyethylene matrix based on molecular dynamics simulations. Specifically, the study focuses on the influence of carbon nanotube chirality on adhesion energy. It is observed that composites that utilize nanotubes with smaller chiral angles achieve higher adhesion energy, and tend to have smaller diameter and longer cylindrical axes compared to those with larger chiral angles. A zigzag nanotube (zero-chiral angle) undergoes considerable deformation to achieve an equilibrium configuration that has relatively maximum adhesion energy. On the other hand, the armchair nanotube (30° chiral angle) deforms moderately to reach equilibrium with minimal adhesion bonds to the polyethylene matrix.

Effect of magnetic field applied during secondary annealing on texture and grain size of silicon steel

Abstract Temper cold rolled silicon steel samples were annealed with and without an applied magnetic field. The final grain size was the same for both annealing conditions. Application of a magnetic field affected texture development by increasing the strength of the Goss component and decreasing the intensity of the gamma fiber.

Statistical Continuum Theory for Inelastic Behavior of a Two-Phase Medium

Abstract A statistical continuum mechanics formulation is presented to predict the inelastic behavior of a medium consisting of two isotropic phases. The phase distribution and morphology are represented by a two-point probability function. The isotropic behavior of the single phase medium is represented by a power law relationship between the strain rate and the resolved local shear stress. It is assumed that the elastic contribution to deformation is negligible. A Green’s function solution to the equations of stress equilibrium is used to obtain the constitutive law for the heterogeneous medium. This relationship links the local velocity gradient to the macroscopic velocity gradient and local viscoplastic modulus. The statistical continuum theory is introduced into the localization relation to obtain a closed form solution. Using a Taylor series expansion an approximate solution is obtained and compared to the Taylor’s upper-bound for the inelastic effective modulus. The model is applied for the two classical cases of spherical and unidirectional discontinuous fiber-reinforced two-phase media with varying size and orientation.

Evolution of crystal orientation distribution coefficients during plastic deformation

A general methodology is developed to model the texture evolution of polycrystalline materials during mechanical processing. The methodology is based on the conservation of texture volume as proposed by Bunge. One of the interesting features of the formulation is that the texture coefficients can be predicted for each processing path using a sixth rank tensor. The range of validity for this methodology is investigated using a set of input data predicted by a crystal plasiticity model based on Taylor.

Boundary migration in Zn bicrystal induced by a high magnetic field

A bicrystal of Zn with an originally flat 89° 〈1010〉 symmetric tilt boundary was annealed in a magnetic field of 25 T. The boundary migrated under the action of a magnetic driving force in the direction of the grain with higher diamagnetic susceptibility. In addition, the boundary changed its crystallographic orientation, decreasing length and becoming almost perpendicular to the free surfaces. The results were interpreted in terms of magnetically forced grain boundary motion due to the anisotropy of the magnetic susceptibility in Zn. The absolute boundary mobility was measured to be about 5.1×10−9 m4/J s.