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Characterization of Carbon Fiber Microstructure and the Stress Distribution in Microregions in Cnt/Expoxy Composites with Defects

Author: RenGuiZhi
Tutor: WuZuoLin
School: Donghua University
Course: Nano- fiber and hybrid materials
Keywords: Raman spectroscopy Carbon fiber Defects Stress distribution
CLC: O657.37
Type: Master's thesis
Year: 2014
Downloads: 37
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Abstract

Raman spectroscopy is an effective tool for characterizing the structure in micro-scale. The structure evolution during the preparation process of carbon fibers can be reflected by the Raman spectra evolution. Surface structure of finished carbon fibers can also be examined through the analysis of typical Raman bands of carbon fibers. When supported with a carbon nanobute (CNT) strain sensor, Raman spectroscopy can be used to study the stress distribution in microregions near the defects.The main contents and conclusions are described as follows:(1) The microstructure evolution from cellulose fibers to carbon fibers is studied. The Raman spectra have a remarkable change during the heat process. The characteristic peaks of cellulose fibers are weakened after a dry process at100℃due to the removal of physical hydrogen bonds, and the characteristic peaks disappear absolutely after the pyrolysis temperature reaches150℃, since the cellulose molecule starts to degrade and release CO2and C2H5OH. When the temperature climbs to200℃, there appear embryonic bands of D and G bands, meaning four-carbon residues start to form. During200-1300℃, the relative amounts of all defect bands firstly increase and then decrease, because the structure firstly turns into disordered carbon fragment and then transverts into ordered graphite layer structure of carbon fibers.An exploratory relation schema of the Raman spectrum change and the fiber structure evolution is established preliminarily, providing valuable information for optimizing the production process of carbon fibers.(2) A new Raman method studying the whole surface of fibers is established; single carbon fibers are rotated360°along with continuous Raman scanning, and the Raman spectra of the whole cylindrical surface of two carbon fibers are obtained. Several key Raman parameters along the fiber axis, including II/IG, ID/IG, IA/IG and IDi/IG, are used to describe, qualitatively and quantitatively, the microstructure heterogeneity along the fiber axis and on the whole surface. The results show that there are apparent structure heterogeneity for both kinds of carbon fibers, and the disordered structure tends to distribute along the fiber axial direction. The comprehensive analysis of surface structure of carbon fibers provides theoretical foundation on improving the carbon fiber properties.(3) The microdeformation behaviors in the vicinity of V notch in CNT/expoxy composites are explored. The relationship between CNT Raman shift and the suffered stress in the material is firstly established; the microregion near the notch is scanned by Raman spectrometer with the help of a CNT Raman strain sensor, and the impact of several factors of the notch on the stress distribution, including position (inside the material/at the edge of the material), angle (60°/90°) and external strain value, are investigated. The results show that defects at the edge of the material, compared to the inner ones, exhibit a greater response to the deformation. Simultaneously defects with an acute angle also show positive sensitivity.(4) The relationship between the CNT Raman shift and the stress condition of the material under specific polarization angle is established, and the stress distributions of stress components (σx、σy and τXY) are determined.The stress distributions in microregions of strained composites are clearly demonstrated by using Raman technique. These information helps us to modify the material designs, and to evaluate the safety status of materials. In the future, Raman spectroscopy will be widely applied in microdeformation study on materials.

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CLC: > Mathematical sciences and chemical > Chemistry > Analytical Chemistry > Instrument analysis ( physics and physical chemistry ) > Photochemical analysis ( spectral analysis method) > Raman spectrometry
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