Carbon nanotubes (CNTS) and graphene are known to possess exceptional mechanical stiffness and strength. The axial Young's modulus along the tube axis or in the graphe plane can be as high as 1-1.2 TPa, as compared to diamond at about 1.2 TPa, ateel at about 200 GPa, and copper at 100 GPa. Their tensile strngth is about 100-200 GPa, as compared to annealed steel at about 700 MPa and annealed copper at about 200 MPa. But these superior properties have not been fully translated into the composite systems. A critical factor that prevent such a full-scale translation is the incomplete load transfer from the matrix to the prestine condition of CNT or graphene surface. Another critical factors involve the agglomeration of CNTS or graphene dispersion and the wavy shapes of these fillers. In this rpoject we focus on how these critical factors could affect the yield strength for the metal-matrix composites and the viscoelastic characteristics of polymeric composites. The objective is to seek for the optimal conditions so that these modern nanocompsoites could deliver the best possible mechancial strength.

In this project we focus on the thermal and electrical conductivity of CNT and graphene based nanocomposites in order to exploit their superior transport properties. The in-plane thermal conductivity of CNTs and graphene can be as high as 3,000-6,000 W/mK, as compared to 400 W/mK for copper. Electrical transport in arm-chaired CNTs or graphene sheets is metallic and it displays a current-carrying capacity of about 1,000 times higher than the copper wire. To fully translate these superior properties into the composite systems, the interface condition again is a critical factor. In thermal transport, the interfacial resistance, also called the Kapitza resistance, mainly arises from the different phonon state in CNTs or graphene and polymer matrix, and in metal composites from the different transport mechanisms (by electrons in metals and by phonons in CNTS or graphene). In electrical conductivity, the interfacial tunneling resistance plays a similarly vital role. As the electrical conductivity of CNTs and graphene can be 10-12 orders of magnitude higher than that of the polymer matrix, the issue of percolation is also critical. In this project, we focus on the effect of Kapitza thermal resistance and on the interfacial tunneling resistance on the overall thermal and electrical conductivity of these modern nanocomposites. The issue of percolation as a function of filler shape and alignment is also been explored.

Ferroelectric crystals possess several fundamental characteristics that make it a unique class of materials. Most important of all are the existence of polarization domains and their ability to reorient under an external electric field and/or mechanical stress. In this project we focus on the development of fundamental principles of micromechanics and nanomechanics that could describe these characteristics on the micro and nano scales, respectively. In micromechanics, our first focus is to establish the Gibbs free energy of the complex system in which the parent and various product domains coexist in rank-1, rank-2, and higher rank microstructures. We then try to derive the thermodynamic driving force and kinetic equations for the evolution of new domain under an external stress and electric field. We will then proceed to examine how the mechanical-electrical coupling affects the nonlinear hysteresis, butter-fly, and the well-shaped strain and polarization relations. In nanomechanics, our focus is to develop a phase-field approach for the study of nano-scaled ferroelectrics, including nano thin films, nanowires and nanodots. We will explore how the stripe domains and vortex structure develop under different configurations and state of applied electromechanical fields. The effects of surface tension and size (film thickness, and diameter of nanowires and nanodots) on the hysteresis characteristics will be examined. At the end, we will explore the feasibility of developing a phase-field based micromechanics model to study the ferroelectric characteristics of these low-dimensional nanostructures.

The objective of this project is to seek for the strongest possible magnetoelectric coupling through the route of multiferroic composites that consist of a ferroelectric and a ferromagnetic phase. This idea is motivated by the fact that single-phase multiferroic materials are rare, and among the existing ones their magnetoelectric coupling is generally weak. On the other hand a multiferroic composite could deliver strong magneto-electric interactions through their commonly shared mechanical characteristics. In this research, we will investigate this potential by a micromechanics and a phase-field model for the bulk and nanostructured multiferroic composites, respectively. We will apply the developed composite models to tune their volume concentrations, phase connectivity, and property contrast, to achieve this goal. The overall polarization of the composite under an external magnetic field, and its overall magnetization under an external electric field, will be determined to assess the magnetoelectric coupling. In this process the evolution of ferroelectric and magnetic domains will also be explored. We intend to consider BaTiO3-CoFe2O4 composites in our prototype calculations, with the 1-3, 2-2, 0-3, and 3-3 connectivity for the bulk, and 1-3 and 2-2 for nanostructured thin films.

The primary objective of this project is to investigate the mechanical strength of nanocrystalline materials in general and under shock loading in particular. A distinct feature of nanocrystalline materials is that considerable amount of atoms exist in the grain boundary regions (GB zone); as such, the contribution of the GB zone to the overall properties of the polycrystal can be quite significant. Three distinct deformation mechanisms usually operate in nanocrystalline materials: dislocation glide inside the grain interior, deformation in the GB zone, and the interfacial grain-boundary sliding. Under shock loading, grain-boundary sliding is greatly suppressed. As the compressive wave travels faster than the speed of sound and there is no time for lateral stress to relax, hydrostatic pressure tends to build up rapidly. At such high pressure, the GB zone becomes very hard due to its pressure dependence. Deformation mechanism in the grain interior also changes from dislocation glide to dislocation generation, and its flow stress builds up sharply. But the adiabatic process will cause the material to soften. So there is a competition between the pressure strengthening and adiabatic softening. In contrast to condition under normal loading in which the maximum strength depends only on the grain size, the shock loading strength will further depend on the shock pressure. In this project, we will search for the combination of critical grain size and critical pressure at which the maximum strength of the materials can be realized.