Sep 5, 2011
Versatile nano-tensile testing platform available for TEM
Understanding the mechanical behaviour and deformation mechanisms of one-dimensional (1D) nanomaterials at relevant length scales is critical to make full use of their attractive physical properties. Now, thanks to a micro mechanical device (MMD) assisted by an in situ tunnelling electron microscope (TEM) nanoindenter, it is possible to perform uni-axial tensile tests for various materials with different diameters and quantitatively measure the sample's mechanical properties at unprecedented resolution, while also observing the internal structure evolve in detail and in real time.
Earlier attempts at testing 1D nanomaterials have encountered drawbacks such as uncontrollable sample/loading geometry and limited material selectivity. Researchers at Rice University, US, have been trying to establish a universal tensile testing platform for studying different 1D nanomaterials. Their solution, dubbed MMD, was aligned and actuated by a quantitative nanoindenter inside a scanning electron microscope, and can be used to measure the tensile strength of carbon nanotubes, gold and nickel nanowires with different diameters.
In recent work published in the journal Nanotechnology, the team has taken the idea one step further and placed the MMD inside a TEM. The MMD has a footprint of just 2.5 mm × 1.2 mm, which allows it to be inserted into a TEM chamber for in situ mechanical testing. TEM is capable of revealing the internal crystalline (and even atomic) structures of test samples during and after deformation.
The MMD is actuated by a quantitative TEM-nanoindenter (Nanofactory Instruments AB). The unique "push-to-pull" mechanism transforms the indentation loading into uni-axial tensile loading of the specimen, which is pre-clamped on the device. This purely mechanical actuation eliminates the need for complicated electro-mechanical or thermo-mechanical coupling and provides a larger load and displacement capability for a wider range of samples.
The simple design both minimizes the number of error sources and reduces the cost of the device fabrication. Most importantly, the back-side window design allows proper electron beam alignment and transparency for real-time TEM imaging.
Using the device, the team examined the mechanical behaviour of a nickel nanowire, which fractured at an engineering stress of ~1.2 GPa. More interestingly, the dramatic contrast changes within the strained nanowire imaged under bright-field TEM imaging conditions suggested possible local atomic distortions (for example, a dislocation network). The high magnification TEM images of the fracture surfaces verified the brittle fracture mechanism. And in addition, select area diffraction (SAD) analysis – a unique and important capability of TEM – could offer particularly useful insight into the evolution of the crystalline structure.
This preliminary work demonstrates the set-up's strength and potential and the group is now busy exploring the behaviour of a wide variety of one-dimensional nanomaterials.
More information can be found in the journal Nanotechnology.
About the author
This research was led by Dr Jun Lou, an assistant professor of Mechanical Engineering and Materials Science at Rice University, and first author Yang Lu, who recently earned his doctorate in Lou's lab. Dr Lu is currently a postdoctoral associate in the Nanomechanics Lab at Massachusetts Institute of Technology. Co-authors of the study include Cheng Peng and Yogeeswaran Ganesan at the Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas, US; and Jian Yu Huang at the Center of Integrated Nanotechnologies (CINT), Sandia National Laboratories, Albuquerque, New Mexico, US. This work was performed, in part, at the Center for Integrated Nanotechnologies, a US Department of Energy, Office of Basic Energy Sciences user facility at Los Alamos National Laboratory and Sandia National Laboratories.