Development of a Nanoscale Material Testing System and in situ SEM/TEM Study of the Mechanical Behavior of NanostructuresDr. Yong Zhu, Ph.D., Northwestern University, 2005.
Major Professor: Dr. Horacio D. Espinosa.
This dissertation outlines the development of a novel nanoscale material testing system based on microelectromechanical system (MEMS) for in-situ electron microscopy testing of nanostructures. The testing system provides continuous observation of specimen deformation and failure at sub-nanometer resolution, while simultaneously measuring the applied load electronically with nano-Newton resolution. The device achieved resolutions of 0.05 fF in capacitance, 1 nm in displacement and 12 nN in load.
Design of the actuator and the load sensor was the important first step. Analytical and finite element analyses were employed to predict the actuator displacement and possible temperature influence. Particular emphasis was placed on the appropriate selection of actuator and load sensor stiffness as a function of the sample geometry and expected material behavior. Capacitance measurement with sub-femto Farad resolution was the key to the performance of the testing system. Special care was taken in selecting the measurement circuitry, finding the capacitance sensing chip, and performing the system integration to achieve the desired resolution.
The MEMS-based material testing systems have been employed in testing freestanding polysilicon films and palladium nanowires in-situ in the scanning electron microscopy (SEM), and carbon nanotubes in-situ in the transmission electron microscopy (TEM). The polysilicon specimens exhibit nearly the same Young’s modulus of 1555 GPa and failure strengths ranging from 0.7 GPa to 1.42 GPa, which are consistent with the values reported for MUMPs Poly-Si thin films. For palladium nanowires, considerable Young’s modulus reduction (~20% lower than its bulk counterpart) and extremely high strength has been observed. For multi-walled carbon nanotubes tested in-situ in TEM, “inter-layer load transfer” failure mechanism was found other than the “sword-in-sheath” mechanism. This is probably due to the inter-layer defects introduced by the TEM imaging.