Agenda
13:15–14:45 - Session 7: Mechanical Properties of Nanostructures
Professor Rui Huang (University of Texas), Chair
-
13:15–13:45
Superplasticity and mechanics of carbon nanotubes
Boris I. Yakobson, F. Ding, J. Kun, Y. Lin
ME&MS Dept., Rice University, Houston, TX 77006
Strength limits and mechanism of yield to deformation in carbon (C) and boron nitride (BN) nanotubes has been of great interest over the last decade. Advances in atomistic modeling have made possible quantitative mapping of these mechanical properties. Recent series of experimental observations of surprisingly plastic response to tension (e.g., up to threefold elongation!) can partially be explained by the early theory of the 5/7 dislocation glide. However important details like significant loss of mass through concurrent sublimation and also too high rates of plastic deformation at the temperatures in some cases much lower than sublimation, pose significant further problem for theory. We present complete kinetic description, based on calculation of energy and barriers for the elementary processes, detailed structural evolution, and phenomenological plasticity equations. Notably, it explains how the perfection of the lattice can be preserved, and how the radiation process serves as amplifier for the plastic yield rate.
[1] B.I. Yakobson and R.E. Smalley, American Scientist 85, 324-337 (1997).
[2] T. Dumitrica¸ M. Hua, and B.I. Yakobson, Proc. Natl. Acad. Sci. 103, 6105-6109 (2006).
[3] F. Ding, K. Jiao, M. Wu, and B.I. Yakobson, Phys. Rev. Lett., accepted (2007).
-
13:45–14:05
The Development of Nano-Mechanics and Its Applications for Characterizing the Mechanical Property of Nano-Structures
Yeau-Ren Jeng
Department of Mechanical Engineering, National Chung Cheng University, Chia-Yi, Taiwan
This presentation will introduce various computational algorithms for nano-mechanics including molecular dynamics and energy minimization. These computational approaches were used for nanoindentation technique to characterize the mechanical property of nanostructures. Applications including mechanical property characterization of nano-scale thin films, nanotubes and bio-materials will be discussed.
Nanoindentation has evolved to be a powerful means of characterizing mechanical properties through analysis of the load-depth curve obtained during nano-scale indentation. This presentation describes the nanoindentation using the depth-sensing technique for the characterization of the nano-structure. A molecular statics approach that is computationally efficient and can better reflect the physical reality during simulation will be introduced. With this approach, the mechanism of the elasto-plastic deformation can be examined more easily without the fluctuant motion of atoms. The simulation results show that the microscopic plastic deformation in the nano-scale thin film during indentation is caused by the instability of the crystalline structure. Homogeneous dislocation nucleation, glide and reactions are observed by virtue of slip vector analysis.
The validity of strain-gradient theory and applicability of the Oliver-Pharr method representing elastic-plastic continuum contact model are also examined. Moreover, the applications of nano mechanics and nanoindentation technique to the investigation of nanotubes, nanowires and bio-materials will be discussed.
-
14:05–14:25
Investigation of Chromium Carbide/Alumina Nano-composite Prepared via MOCVD in Fluidized Bed
Hao-Tung Lin , Jow-Lay Huang*,
Department of Materials Science and Engineering, National Cheng-Kung University, Tainan 701, Taiwan, ROC
Nanoscaled Cr2O3 powder with an average particle size of 20~40 nm, coated on alumina particles, has been produced by means of chemical vapor deposition (CVD) in a fluidized chamber, using the pyrolysis of Cr(CO)6 precursor. The precursor decomposed and formed the mixture of CrC1-x, Cr2O3, and free carbon on the surface of the Al2O3 particles when paralyzed in fluidized bed. Amorphous and crystalline Cr2O3 particles were obtained when the temperature of the pyrolysis were 300oC and 400 oC, respectively. The particles of decomposed Cr(CO)6 treated at temperatures from 700 oC to 1000 oC in the graphite furnace in a vacuum were not carbonized until at 1150 oC. Amorphous Cr2O3 powder were carbonized and transformed into Cr3C2, while the crystalline Cr2O3 was transformed into a mixture of Cr7C3 and Cr3C2 under same thermal treatment. As Cr2O3 react with Al2O3 to form a solid solution and also it react with carbon to transform into chromium carbide, solid solution (Cr2O3 / Al2O3), Cr3C2, and Cr7C3 were formed in this study. Combination of solid solution and nanosized 2nd phase reinforced were used for strengthening Al2O3 base composites. From the observation in the microstructure of these high pressed specimens, the nanosized Cr-carbide particles disperse uniformly on the Al2O3 grain boundaries and it can strengthen Al2O3 matrix and suppress its grain growth. Comparing with the monolithic alumina, the mechanical properties of these nano-composites such as flexible strength and toughness significantly improved.
-
14:25–14:45
The Tensile Properties of Single-wall Carbon Nanotubes
Weiqiang Ding1, Mingyuan Huang2, James Hone2, Rodney S. Ruoff3
1Department of Mechanical and Aeronautical Engineering, Clarkson University, Potsdam, NY, 13676-5725
2Department of Mechanical Engineering, Columbia University, New York, NY, 10027
3Department of Mechanical Engineering, Northwestern University, Evanston, IL, 60201
Nanoscale tensile tests were performed on individual single-wall carbon nanotubes (SWCNTs) with a custom-made nanomanipulator inside the vacuum chamber of a scanning electron microscope (SEM). Freely-suspended individual SWCNTs were clamped to an atomic force microscope (AFM) cantilever probe with the electron beam induced deposition (EBID) method, and were loaded in tension until fracture. The AFM cantilever served as the force-sensing element. The applied tensile load was increased in discrete steps, and an SEM image was taken at each step. From image analysis the cantilever deflection and the nanotube elongation were obtained, and the tensile load and the strain were calculated. The diameter and chirality of each SWCNT were characterized with a Rayleigh scattering method prior to the test, and the tensile stress in the nanotube was calculated. A stress/strain curve was thus obtained, from which the fracture strength, failure strain, and elastic modulus of each SWCNT were obtained. In 9 samples the SWCNTs after loading to some value, slipped at the clamp on the AFM tip—a rather interesting result, but not allowing for study of fracture of these samples; the measured maximum tensile stress is only a lower bound of the true fracture strength. For the 13 SWCNTs tested that could be fractured failure always occurred at the EBID clamps, possibly due to stress concentration at the clamps or perhaps due to damage that might occur during EBID clamp formation; further study of these issues is indicated. The measured fracture strength values were 20 - 60 GPa.
