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Raman Spectroscopy
This is a spectroscopic technique which measures vibrational, rotational and low-frequency modes in the system or sample by using inelastic scattering of photons resulting from interaction of laser (in the visible, infrared or near ultraviolet region) with sample or system. In a typicalRaman spectroscopy measurement, a sample is incident with a monochromatic laser beam and then light from the illuminated spot is collected with a lens and sent through a monochromator whereby wavelengths closer to that of the laser are filtered out while the rest is collected into a detector. Typically, a monochromatic laser beam is incident on a molecule and interacts with the electron cloud and bonds of the molecule and as a result a laser photon excites the molecule from the ground to the virtual energy state. This is called spontaneous Raman effect. This molecule will emit a photon and returns to a different vibrational or rotational state during its relaxation.
The difference in energy between the new vibrational or rotational state and the ground state result in the shift in the frequency of the emitted photon relative to the laser excitation wavelength. This shift is called Raman shift wave number and is characteristic of the sample or system under study and serves as its finger print of the sample. This technique has been used extensively in carbon science to characterize different types of materials such as graphite (Tuinstra and Koenig, 1970, Nakamizo et al., 1974a, Wang et al., 1990), diamond (Prawer and Nemanich, 2004, Ferrari and Robertson, 2004), carbon nanotubes (Dresselhaus et al., 2004, Hodkiewicz, 2013), fullerenes (Schettino et al., 2002), grapheme (Dresselhaus et al., 2010a, Dresselhaus et al., 2010b) and others. The Raman spectra of different carbon materials are shown in Figure 4.3 and Figure 4.4below. Figure 4.3shows the Raman spectra of diamond and graphite on silicon substrate, the diamond spectra exhibit a single band around 1332 cm-1due to vibration of the C-C single bonds in the tetrahedral crystal structure of diamond. The graphite on silicon substrate spectrum shows bands around 520 and 950 cm-1 characteristic of silicon, a 1st order band around 1582 cm-1 (called G band) attributtable to the vibration of the C-C double bonds (sp2) and a 2nd order peak around 2650 cm-1. In Figure 4.4the spectra of graphene, highly oriented pyrolytic graphite (HOPG), single-walled carbon nanotubes (SWNT) and amorphouscarbon are shown; due to their different vibrational modes or Raman signatures these materials can be distinguished with ease. Graphene comprise two main bands around 1580 and 2650 cm-1 due to C-C double vibrations (sp2); the latter is the second order peak also called G’ band. The HOPG has a similar Raman signature as the graphene except that it’s G’ band is of low intensity.
On the other hand SWNT possess additional peaks around 250 cm-1 and 1340 cm-1 due to radial breathing modes (RBM) of the tubes and disorder induced D-band, respectively. The RBM is very important in calculating the diameter of the SWNT. The amorphous carbon spectrum consist of mainly two bands, i.e. the D- and the G-bands, which are characteristic of C-C single (sp3) and double (sp2) vibrations, respectively. In amorphous carbon, the D-peak is more intense than the G-peak showing high degree of disorder or high fraction of sp3 carbon atoms. This illustrates the power of Raman spectroscopy as a characterization tool.
The advantages of using this technique are it is non-destructive and requires no or little sample preparation. It can also be utilized to determine parameters such as crystallite size and perfection, stress evaluation and structural defects.
Optical Microscopy (OM)
Optical microscopy is a technique which uses visible light together with a system of lenses and cameras to magnify the image of small specimens. An example of a modern optical microscope is shown in Figure 4.5 below. It typically consists of structural components such as an eyepiece (1) used for bringing the sample image under view into focus to the eye; an objective turret or revolver (2) which is used to hold and change between different objective lenses (3). Objective lenses are usually cylindrical and contain a glass single or multi-element lens. They arecharacterized by mainly two parameters, magnification and numerical aperture; the former ranges from 5× to 100× while the latter ranges from 0.14 to 0.7. Objectives having higher magnification normally possess higher numerical aperture and shorter focal lengths.
Some optical microscopes use oil-immersion or water-immersion objectives so as to improve resolution at higher magnification. This is because the refractive index of the oil or water is higher than that of air and therefore allows the objective to have larger numerical aperture (> 1)so that the light is transmitted from the specimen to the outer face of the objective lens with minimal refraction. This allows for detection of smallest microstructural details in the sample.
The adjustment knobs (fine knob (4) and coarse knob (5)) are used to move the stage (6) up and down, this operation helps to fine tune the clarity of the sample image under view. Below the objective is a stage which serves as a support platform for the specimen under view. The stage also possesses a hole at its centre to which light passes through to illuminate the sample. The light source (7) is commonly made of a halogen lamp although LEDs and lasers could also be utilized. The light passes through the diaphragm and condenser (8) which is effectively a lens designed to focus the illuminated light on to the sample. The condenser may include features such as filters to control the quality and intensity of the incident light (Richardson, 1971, Haynes, 1927-, Murphy, 2001). Illumination techniques such as cross-polarized light, dark field and phase contrast and others can be utilized to improve contrast of the image from the sample.
CHAPTER 1: INTRODUCTION
1.1. Background
1.2. Aims and objectives of the project
References
CHAPTER 2: REVIEW ON GRAPHITE
2.1 Crystalline forms of carbon
2.2 Graphite manufactur
2.3 The graphitisation process..
2.2.1 Graphitising carbons .
2.2.2 Non-graphitising carbons..
2.3 Factors affecting graphitisation
2.3.1 Mesophase formation
2.3.2 Fusion during carbonisation
2.3.3 Cross-linking
2.3.4 Catalytic graphitisation
2.4 Graphite properties applicable to nuclear indust
CHAPTER 3: PHENOLIC RESINS .
Introduction
3.1 Raw materials for phenolic resins production.
3.2 Phenol/Formaldehyde reaction
3.3 Use of phenolic resins as binders
CHAPTER 4: THEORETICAL BACKGROUND ON TECHNIQUES
4.1. Laser diffraction
4.2. X-ray diffraction (XRD
4.3. Raman Spectroscopy.
4.4. Optical Microscopy (OM)
4.5. Scanning Electron Microscopy (SEM)
4.6. Pycnomet
4.7. Thermogravimetric analysis.
4.8. Thermomechanical analysis .
4.9. Xenon Flash Photolysis .
4.10. Four-point bending
CHAPTER 5: EXPERIMENTAL
CHAPTER 6: RESULTS AND DISCUSSION-CHARACTERISATION..
CHAPTER 7: RESULTS AND DISCUSSION-PROPERTIES
CHAPTER 8: CONCLUSIONS AND FUTURE WORK
