Development of in situ Mechanical Characterization Platforms for 1-D/2-D Micro/Nanostructures


Student thesis: Doctoral Thesis

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Awarding Institution
Award date4 Sep 2017


One-dimensional (1-D) micro/nanostructures, such as metallic, semiconductor and polymer micro/nanowires (NWs) and carbon nanotubes (CNTs), or two-dimensional (2-D) micro/nanostructures, such as graphene thin films, are not just simple miniaturization in size of their bulk-scale counter parts. Their physical and chemical properties are different from, usually superior to bulk materials, such as the enhanced mechanical strength, relative low melting point and superior magnetic property. These “size effects” make them great candidates as the building blocks for miniature electronics and electro-mechanical systems in numerous emerging engineering applications, such as flexible/wearable electronics, bio-integrated electronic sensors/detectors. However, the ability to achieve the full potential of aforementioned new technologies in these fascinating applications is ultimately limited by how these micro/nanostructures will behave at relevant length scales, in particular, their mechanical performance and reliability. Despite that substantial research efforts have been recently made on the mechanical characterizations of various micro/nanomaterials under static loading, there still exists little studied area in this field, such as, fracture behavior of microwire under torsion loading, fatigue behavior of 1-D nanomaterials under different loading modes, and in-plane fracture mechanism and fatigue behavior of 2-D nanomaterials.
In this thesis work, after investigating the mechanical property of the metallic glass microwire through tensile test, a micro robotic mechanical torsion testing system was developed and the in situ SEM torsion tests of two kinds of metallic glass (MG) microwires were successfully implemented based on it. Through the postmortem fractographic analysis, it can be revealed that the revolved vein-pattern microstructure on La-based microwire upon torsion loading could be mainly attributed to the localized adiabatic work accumulated at a very large elastic strain confined within the microscale sample volume, followed by a localized high temperature rise up to ~1000 K at the fracture surface through localized energy dissipation. While for the Co-based MG microwire, the fracture surface indicated a different fracture mechanism, which shows the coexisting of homogeneous plasticity and inhomogeneous plasticity (shear banding).
Compare to microwires, 1-D nanostrcutrues such nanowires are more interesting due to their much-increased surface-to-volume ratio and the resulted unconventional physical and mechanical properties. Despite significant progresses made in both theoretical and experimental aspects, dynamical characterization of 1-D nanomaterials, which were often very costly and time consuming, remains extremely challenging and less reported. Firstly, we have demonstrated the first quantitative in situ low-cycle tensile fatigue testing of individual nanowires inside a high-resolution scanning electron microscope (SEM), based on the nanoindenter-assisted “push-to-pull” dynamic tensile straining mechanism and revealed the low-cycle fatigue behavior of pristine single crystalline nickel (Ni) nanowires. With the aim of reducing the time of fatigue study, a novel high-cycle tensile/torsion straining micromachine, based on the digital micromirror device (DMD), has been developed for the high-cycle tensile/torsion fatigue study on various one-dimensional (1-D) nanostructures, such as metallic and semiconductor nanowires.
​Recently, 2-D nanomaterials, represented by graphene, have raised as the most promising candidates for the future nanoelectronics applications, because of their scalability and interesting properties. However, most of the previous researches focused on the functional properties and applications, with less attentions put onto their mechanical properties and stability. On the existing studies of their mechanical properties, major efforts were carried out in the theoretical calculations and simulations, with a few experimental works focus on local mechanical properties by indentation type experiments. There is a lack of global mechanical characterizations of whole pieces of 2-D materials. In the final part of the thesis work, the in-plane mechanical properties of MoS2 membranes and graphene were investigated by in situ tensile mechanical testing inside scanning electron microscope (SEM). The fact that the ultimate strengths of both kinds of thin film materials would decrease as the thickness increasing has been verified. We also have developed a 2-D nanomaterials fatigue testing platform based on a piezoelectric MEMS and demonstrated the platform by studying the fatigue behavior of graphene.