Xiaodong Li's blog

We have developed a technique, described in Advanced Materials , which combines hydrogen passivation with ultrasonication to successfully disperse multiwalled carbon nanotubes (MWCNTs) in both absolute ethanol solution and epoxy resin, overcoming one of the greatest challenges in the application of nanotubes.

We report the observation of remarkable electrical self-healing in mechanically damaged ZnO nanobelts. Nanoindentation into intrinsically defect-free ZnO nanobelts induces deformation and crack damage, causing a dramatic electrical signal decrease. Two self-healing regimes in the nanoindented ZnO nanobelts are revealed. The physical mechanism for the observed phenomena is analyzed in terms of the nanoindentation-induced dislocations, the short-range atomic diffusion in nanostructures, and the local heating of the dislocation zone in the electrical measurement. For details, please see

Can we map the eastic modulus of a and c domians? Can we mechanically switch the domains and let them function as nanoactuators and sensors?

You are cordially invited to submit an abstract to the symposium on “Emerging Methods To Understand Mechanical Behavior” at 2008 TMS annual meeting, New Orleans, LA, March 9-13, 2008.  

Welcome to the May 2007 issue. This issue focuses on experimental nanomechanics of nanobuilding blocks. The extremely small dimensions of nanobuilding blocks (for instance, nanoparticles, nanotubes, and nanowires) have imposed great challenges to many existing instruments, methodologies, and even theories.  In this issue, we will discuss – (1) experimental techniques and (2) size-effects. 

Indentation is widely used to measure material mechanical properties such as hardness, elastic modulus, and fracture toughness (for brittle materials). Can one use indentation to extract material elastoplastic properties directly from the measured force-displacement curves? Or simply, is it possible to obtain material stress-strain curves from the corresponding indentation load-displacement curves? In an upcoming paper in JMPS titled "On the uniqueness of measuring elastoplastic properties from indentation: The indistinguishable mystical materials," Xi Chen and colleagues at Columbia University and National Defense Academy, Japan show the existence of "mystical materials", which have distinct elastoplastic properties yet they yield almost identical indentation behaviors, even when the indenter angle is varied in a large range. These mystical materials are, therefore, indistinguishable by many existing indentation analyses unless extreme (and often impractical) indenter angles are used. The authors have established explicit procedures of deriving these mystical materials. In many cases, for a given indenter angle range, a material would have infinite numbers of mystical siblings, and the existence maps of the mystical materials are also obtained. Furthermore, they propose two alternative techniques to effectively distinguish these mystical materials. The study in this paper addresses the important question of the uniqueness of indentation test, as well as providing useful guidelines to properly use the indentation technique to measure material elastoplastic properties.

It is well known that metals are hardened by deformation and soften by annealing. How about nanostructured metals? Can we reply on conventional metal-working lore?

Although nanostructures, such as nanoparticles, nanotubes, nanowires, nanobelts, and nanometer thick films, nanostructured materials and nanocomposites have been synthesized and fabricated by various techniques, their mechanical properties have not been well explored. These nanostructures are being used as structural and functional building blocks to construct micro/nanodevices. Some nanostructured materials exhibit the breakdown of Hall-Petch behavior. The failure of conventional reinforcing models has been found in nanocomposites. The extremely small dimensions of nanomaterials and micro/nanodevices impose tremendous challenges to many existing experimental techniques and modeling tools. An in-depth understanding of mechanics at the nanoscale is greatly needed. Development of mechanical testing, and manipulation instruments and techniques, is also a technological necessity. This symposium will focus on research on mechanical properties of nanostructures, nanostructured materials and nanocomposites, and reliability testing of micro/nanodevices.

Professor Rodney Ruoff and colleagues at Northwestern University and Purdue University have developed a process that promises to lead to the creation of a new class of composite materials - graphene-based materials. They reported the results of their research in Nature, 442 (2006) 282-286. This team has overcome the difficulties of yielding a uniform distribution of graphene-based sheets in a polymer matrix. Such composites can be readily processed using standard industrial technologies such as moulding and hot-pressing. The technique should be applicable to a wide variety of polymers. The graphene composites may compete with carbon nanotube-based materials in terms of mechanical properties. This new class of composites may stimulate the applied mechanics community to study the fundamental reinforcing mechanisms of graphene sheets from both experimental and theoretical approaches.