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Title:Mechanical and interfacial properties of carbon nanofibers for polymer nanocomposites
Author(s):Ozkan, Tanil
Advisor(s):Chasiotis, Ioannis; Polycarpou, Andreas A.
Department / Program:Mechanical Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):carbon nanofibers (CNF)
Polymer nanocomposites
Abstract:The mechanical and failure response of vapor grown carbon nanofibers (VGCNFs) has been unexplored despite the large volume of carbon nanofiber composites fabricated today. Three grades of VGCNFs, namely as-fabricated, high temperature heat treated, and graphitized/surface oxidized, with average diameters of 150 nm were tested individually for their tensile strength by a MEMS mechanical testing platform. Their nominal tensile strengths followed Weibull distributions with characteristic strength values between 2.74- 3.34 GPa, which correlated well with the expected effects of heat treatment and oxidative post-processing. These values are the first measurements reported for VGCNFs and are more than 50% smaller than the generally accepted values for the tensile strength of this class of nanofibers. The as-fabricated nanofibers had the smallest Weibull modulus indicating a wide flaw population that was reduced significantly upon heat treatment. The nanofiber fracture surface was that of a stacked truncated cup structure with oblique graphene layers comprising the backbone of VGCNFs. Under uniaxial tension, cleavage of the outer turbostratic layer occurred first, followed by relative slip of the internal oblique graphene layers. The change in the mechanical strength (Weibull strength), and its scatter (Weibull moduli), with heat treatment correlated well with the fiber structure evidence in Transmission Electron Microscopy (TEM) images, which showed graphitization of the outer turbostratic layer and the formation of a new interface with the inner, originally graphitic, layer that was characterized by structural discontinuities that reduced the total load bearing capacity of the nanofibers. In this work, the strength of the carbon nanofiber-polymer matrix interfaces was also quantified for the frrst time by means of novel nanoscale fiber pull-out experiments. The interfacial shear strength averaged 55 MPa revealing that the adhesion and bonding of the heat-treated, non-functionalized carbon nanofibers is quite better than that of non-functionalized carbon fibers (15-28 MPa) and as good as that of functionalized carbon fibers ( 40-65 MPa), which underscores that extrapolations of macroscale interfacial measurements to the nanoscale are not appropriate.
Issue Date:2009
Rights Information:Copyright 2009 Tanil Ozkan
Date Available in IDEALS:2014-03-20
Date Deposited:2009

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