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Molecular Dynamics Simulations of the Size-dependent Brittle-to-ductile Transition of Silicon Nanowires

Xu, Wenting
http://rave.ohiolink.edu/etdc/view?acc_num=ucin1613751523441622

Abstract Details

2020, PhD, University of Cincinnati, Engineering and Applied Science: Materials Science.
Silicon has been widely used in various electronic and optoelectronic devices for its excellent properties. As temperature increases, silicon changes from brittle to ductile at around 545 &#8451. The brittle-to-ductile transition (BDT) behavior has been observed with silicon nanostructures at room temperature. The molecular dynamics method has been used to investigate the size-dependence in BDT. In the literature, different transition sizes have been reported based on different potential models. Apart from potentials, simulation types (constant energy vs. constant temperature) and boundary conditions can also influence the behaviors of silicon nanostructures. Furthermore, the oxidization is another key factor that can affect the properties of silicon nanostructures. But the role of the oxidization in the BDT of silicon nanostructures has been rarely investigated. Therefore, this dissertation studies the BDT of [110]- oriented single crystalline Si nanowires in atomic level with focus on the influence of three factors: potential models, simulation types and boundary conditions, and the oxidization of silicon nanowires. In Chapters I to II, some backgrounds and literature review on the BDT of silicon and simulation methods are given. Chapter III investigates the impact of three different modified embedded-atom-method (MEAM) potentials, referred to as Baskes, Lee, and Lee-modified, on BDT. Uniaxial tensile tests of silicon nanowires are conducted by considering several key parameters such as size, temperature, and strain rate. The ductile failure probability parameter is introduced to quantify the failure behaviors of nanowires. Overall, the ductile failure probability increases with decreasing size and increasing temperature. The Baskes nanowires exhibit the most ductile failures whereas most Lee-modified nanowires fail in brittle mode. The Lee model shows intermediate levels of ductile failure behavior. Using the same potentials as in Chapter III, Chapter IV investigates the effects boundary conditions (BCs) and simulation types on BDT during the tensile tests. For BCs, either periodic boundary conditions (PBCs) or fixed-end boundary conditions (FBCs) is applied to nanowires. For simulation types, either the constant energy method or the constant temperature method implemented by Langevin thermostat is used in the simulations. The simulations reveal that Young’s modulus and tensile strength exhibit little dependence on the boundary condition and simulation types while the failure strains of the FBCs nanowires are slightly larger than that of PBCs nanowires. The failure behaviors exhibit larger, but limited variations depending on the boundary conditions, but the trend is mixed and not conclusive. Chapter V is focused on the influence of amorphous silica layers on the BDT of silicon nanowires. This is achieved by performing tensile test simulations on oxidized silicon nanowires with charge-optimized many-body (COMB) potential. Silicon nanowires with different degrees of oxidization are studied, including non-oxidized silicon nanowires (pure silicon nanowires), partially oxidized silicon nanowires, and completely oxidized silicon nanowires (pure amorphous silica nanowires). Necking was observed for all silicon nanowires with diameters varying from 2 nm to 7 nm. Amorphous silica layers with different thickness are created on the 2 nm and 3 nm silicon nanowires. The simulations show that the partially oxidized silicon nanowires become weaker and softer as the thickness of amorphous silica layers increases. All silicon core of partially oxidized nanowires fail by necking, while the amorphous silica layers show different failure behaviors according to the thickness – thinner layers deform by necking and thicker layers fracture in brittle mode. All amorphous silica nanowires fail by brittle fracture with a small tensile strength due to the large number of surface defects and voids.
Woo Kyun Kim, Ph.D. (Committee Chair)
Gregory Beaucage, Ph.D. (Committee Member)
Yao Fu, Ph.D. (Committee Member)
Vesselin Shanov, Ph.D. (Committee Member)
105 p.
Materials Science
Silicon nanowires; Brittle-to-ductile transition; Molecular dynamics; Tensile test

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Citations

  • Xu, W. (2020). Molecular Dynamics Simulations of the Size-dependent Brittle-to-ductile Transition of Silicon Nanowires [Doctoral dissertation, University of Cincinnati]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1613751523441622

    APA Style (7th edition)

  • Xu, Wenting. Molecular Dynamics Simulations of the Size-dependent Brittle-to-ductile Transition of Silicon Nanowires. 2020. University of Cincinnati, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ucin1613751523441622.

    MLA Style (8th edition)

  • Xu, Wenting. "Molecular Dynamics Simulations of the Size-dependent Brittle-to-ductile Transition of Silicon Nanowires." Doctoral dissertation, University of Cincinnati, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1613751523441622

    Chicago Manual of Style (17th edition)

ucin1613751523441622
164
© 2020, some rights reserved.Creative Commons License Molecular Dynamics Simulations of the Size-dependent Brittle-to-ductile Transition of Silicon Nanowires by Wenting Xu is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. Based on a work at etd.ohiolink.edu.
This open access ETD is published by University of Cincinnati and OhioLINK.
Release 3.2.12