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Author: Samuel Chuah 21642303
CIV 4210: Final Year Project
Fabrication and Characterization of Carbon Nanotube Epoxy Nanocomposites:
Effect of the Geometry of Carbon Nanotubes
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Executive Summary The final year project encompasses two fundamental components in research. Literature review and experiments were conducted to study the properties, problems and potential of carbon nanotubes (CNT). A lot of interest is generated in this material because it displays exceptional mechanical, thermal and electrical properties. However, agglomeration is the biggest problem that limits the mechanical properties of CNT. To overcome the string intermolecular forces, dispersion during fabrication is necessary to enhance the mechanical properties. Chemical and physical dispersion can be performed to achieve this goal.
The conference paper was produced to investigate the effect of carbon nanotube (CNT) geometry on quality of CNT dispersion in solvent media. Results show that the CNT diameter has a significant effect on quality of its dispersion in matrix. It demonstrates that, Bigger the CNT diameter, better the CNT dispersion in media. It is because, the bigger diameter leads to less interaction energy in CNT bundle and make it easier to exfoliate CNT from bundle. In contrast, CNT length has not significant influence on quality of CNT dispersion in matrix. It is because, although long CNTs entangle each other more than short CNTs, entanglement is not dominant reason and with constant diameter and weight quantity, short or long CNT bundle have the same interaction energy in bundle. Accordingly, with constant dispersion energy both of them have almost equal dispersion quality in matrix. We hope that this study will provide insight into further understanding of the intricacies of dispersing CNTs in media.
Acknowledgements I would like to thank my supervisor, Asghar who taught me a lot on CNTs as well as my lecturer, Dr. Wen Hui Duan who patiently guided me throughout the year. I would also like to take this opportunity to thank my family for their unending support.
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Table of Contents Executive Summary ...................................................................................................................................... ii
Acknowledgements ...................................................................................................................................... ii
Introduction ................................................................................................................................................. 1
Problem Statement ................................................................................................................................. 1
Aim ........................................................................................................................................................... 2
Outline ..................................................................................................................................................... 2
Literature review .......................................................................................................................................... 3
General .................................................................................................................................................... 3
a. Synthesis of carbon nanotube ...................................................................................................... 3
b. Properties of carbon nanotube .................................................................................................... 3
c. Mechanics of carbon nanotube ................................................................................................... 4
d. Characteristics of epoxy ............................................................................................................... 5
e. Epoxy-‐Carbon Nanotube Composite Characteristics ................................................................... 5
f. Mechanical Properties ................................................................................................................. 5
g. Themo mechanical Properties ..................................................................................................... 8
Specific Topic: Dispersion ........................................................................................................................ 9
a. Fabrication methods .................................................................................................................... 9
b. Polymers to disperse CNT .......................................................................................................... 10
c. Uv-‐Vis to monitor dispersion of CNT .......................................................................................... 10
Experimental .............................................................................................................................................. 11
Experiment 1: Procedure ....................................................................................................................... 11
Experiment 1: Results and discussion .................................................................................................... 12
a. UV–vis spectra of MWCNTs–BYK9076 solutions ........................................................................ 12
a. Effect of CNT diameter on dispersion ........................................................................................ 14
b. Effect of CNT length on CNT dispersion ..................................................................................... 15
Experiment 2: Procedure ....................................................................................................................... 16
Experiment 2: Results and discussion .................................................................................................... 17
a. High speed shear mixing ................................................................................................................ 17
b. Ultrasonication: .............................................................................................................................. 18
c. Amount of CNT ............................................................................................................................... 19
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Introduction Carbon nanotube, which is also known as CNT is referred to the small, nano-‐sized cylindrical tubes composed of sheets of carbon atoms which was discovered by Iijima in 1991 (S. Iijima 1991). At present, CNTs are hailed as the building blocks of nanotechnology with possible applications in the near future. This bold statement arises from the exceptional mechanical, thermal and electrical properties which generate interest among researchers and the society alike (Montazeri, Montazeri et al. 2011). CNT holds the promise of delivering superior composite materials(Sun, Warren et al. 2008), electronic appliances(Zhu, Peng et al. 2004), lightweight products in the sports and transportation industries(Yuanxin, Pervin et al. 2007). In relation to the potential application in the construction industry, CNT mechanical properties such as the high elastic modulus, tensile strength, flexural strength and hardness are the focus of attention because of its immense potential as a reinforcement (Young Seok and Jae Ryoun 2005; Zheng, Zhang et al. 2006).
Problem Statement These rolled graphite sheets face a major obstacle, namely the tendency to agglomerate and entangle. Factors contributing to this agglomeration phenomenon include the atomically smooth surfaces, flexible CNT and the high aspect ratio (Fukui, Taninaka et al. 2007). Moreover, CNTs have small diameters that tend to form bundle structures due to their substantial van der Waals interaction. There is significant dependence of the thermal, rheological, and mechanical properties of the CNT nanocomposites on the concentration and dispersion state of CNT. Literature shows CNT-‐epoxy nanocomposites have either weaker or just a little bit higher mechanical properties compare to that of pure epoxy (Wladyka-‐Przybylak, Wesolek et al. 2011; Loos, Yang et al. 2012). CNT poor dispersion and weak CNT-‐matrix interaction are being generally described as the cause for this lack of enhancement. Therefore, good dispersion is necessary to realize the full potential of the CNT mechanical properties. Different methods have been investigated to efficiently disperse the CNT such as high speed shear mixing, calendaring, ultrasonication, use of solvent and surfactant (Rana, Alagirusamy et al. 2009). If CNT well dispersed, the potential filler-‐matrix interface area is huge, and a perfect control of the interfacial interaction is crucial for obtaining optimal properties (Vaisman, Wagner et al. 2006).
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Aim The goals of the final year project are listed as follows:
1. Study the characteristics of CNT to realize its full potential
2. Understand the role of carbon nanotube geometry on efficient dispersion
3. Harnessing the superior properties of nanocarbon in the construction industry
The experiment was performed to investigate the effects of CNT diameter and length on CNT dispersion and understand the role of carbon nanotube geometry on efficient dispersion.
Outline This report provides a holistic literature review concerning the synthesis, fabrication and properties of carbon nanotube to gain in-‐depth background knowledge on the research topic. The specific topic for the conference paper is titled “Carbon nanotube dispersion in solvent media: Effect of carbon nanotube geometry”. The next section describes the experiment conducted during this semester with corresponding results and discussion. The findings are then compiled into a conference paper.
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Literature review The literature review covers two aspects, namely a general knowledge on CNT followed by a literature review on the specific topic of CNT dispersion.
General
a. Synthesis of carbon nanotube During the early 90s, carbon nanotubes were first synthesised by arc-‐evaporation. Similar to electrolysis, the process requires two pure graphite electrodes and a power supply but there is no electrolyte. Instead, the chamber is filled with inert gas such as argon or helium as the graphite anode is vapourised and deposited on the cathode. The carbon vapour condenses on the cathode to form deposits of nanotubes (Jones, et. al., 1996). The set-‐up is in Figure 1.
Figure 1 Schematic diagram of the modified arc evaporation apparatus (Coll, et. al., 1992)
Nevertheless, other methods can be employed to obtain carbon nanotubes. Those methods include laser ablation, gas phase catalytic growth from carbon monoxide and chemical vapour deposition form hydrocarbons (Nikolaev et. al., 1999).
b. Properties of carbon nanotube Carbon nanotube is touted as the construction material of the future because of their high strength-‐to-‐weight ratio. Pipes et. al investigated the relationship between chiral integers and density, while establishing the density of carbon nanotube at 1.4g/cm3 for single-‐walled carbon nanotube and a maximum of 2.1 g/cm3 (2003). As a comparison, steel has a density of 7.84g/cm3, making carbon nanotube an interesting proposition especially in terms of material mobility at site.
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The strength and stiffness of carbon nanotube are best described by their high tensile strength and elastic modulus respectively. A background in quantum chemistry is required to explain the unique mechanical property of carbon nanotube. The chemical bonding within nanotubes consists of sp2 bonds, similar to those of graphite. These bonds or rehybridisations, which are stronger than the sp3 bonds found in alkanes enable nanotubes to possess superior strength (Ebbesen, 1997).
In addition, carbon nanotubes are good thermal conductors. Kim, et. al. demonstrated that the room-‐temperature thermal conductivity over 200 W/m K for bulk samples of single-‐walled nanotubes whereas 3000 W/m K for individual multiwalled nanotubes (2001). Additions of nanotubes to epoxy resin can double the thermal conductivity for a loading of only 1%, showing that nanotube composite materials may be useful for thermal management applications (Hone, 2004).
c. Mechanics of carbon nanotube The behaviour of carbon nanotube in response to loading is the next focus of this study. This section also demonstrates the two varieties of atomic structure which differ in vector notation as shown in Figure 2. Chirality is a vector that describes the non-‐identical plane. This characteristic dictates the electrical conductivity and torsional resistance of the specific shell of the carbon nanotube.
Figure 2 Illustrations of the atomic structure of (a) an arm chair, (b) a ziz-‐zag nanotube and (c) Chiral vector. (Makar & Beaudoin, 2003)
Generally, carbon nanotubes can be synthesised as single walled (SWNT) or multi walled nanotubes (MWNT). The defects in SWNTs can be a point of weakness while MWNTs contains many layers that can compensate defects present at any given layer (Harris, 2009). Moreover,
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SWNTs are susceptible to elastic bucking under high pressure whereas MWNTs have weak van der Waals forces but negligible contribution to both the tensile and shear stiffness (Thostenson et. al., 2001).
d. Characteristics of epoxy Epoxy is a thermosetting polymer resulting from the chemical reaction when the resin and hardener are mixed in equal proportions. Unlike thermoplastic materials, epoxy is hard, rigid but brittle. In the epoxy-‐carbon nanotube composite specifically, the epoxy plays a dual role of being an adhesive resin and a structural matrix.
The physical appearance is always best described in terms of the chemistry and molecular interactions. An epoxide group is mixed with bisphenol to produce an epoxy resin. The amine groups react with the epoxide group to form a covalent bond reinforced with dense cross-‐links arising from the reaction of the NH group and epoxide group. Subsequently, the resulting polymer is a thermoset exhibiting high rigidity and strength (Jin, Qipeng et al. 2010).
e. Epoxy-‐Carbon Nanotube Composite Characteristics Nanocomposites are engineering materials made up of carbon nanotube core embedded in an epoxy resin. Composites are vital engineering materials because the composite utilises the high strength of the carbon fibre while the epoxy matrix serves to protect the reinforcement in order to produce a composite with better properties better than its individual materials (Philip & Bolton, 2002). However, the strength of the new material depends on the direction of the load due to material anisotropy.
The application of epoxy matrix reinforced with carbon nanotube into the construction industry is still premature at this stage due to several shortcomings. These challenges include poorly dispersed multiwalled carbon nanotube, aligment problems and weak interfacial bonding in the epoxy matrix (Kathi et. al., 2009). These problems are a direct result of the chemically inert nature of the carbon nanotube. Inevitably, carbon nanotubes are supplied as heavily entangled bundles which results in agglomeration issues.
Therefore, several techniques are available to produce a successful composite. The experiments performed involve manipulating the temperature by pre-‐heating and the application of sonication whereby the samples are prepared during the fabrication phase.
f. Mechanical Properties Firstly, the density and hardness of the nanocomposite can be measured easily to obtain a general idea of the mechanical properties. According to Le, et. al., the Vickers hardness is 8.5 at an optimum CNT content of 1.5-‐2% weight (undated). Zheng et.al. measured the density of the MWNT which lies around 1.26kg/m3. The bending strength is recorded within a range of 30 to 70 MPa depending on the method of treatment. Meanwhile, the flexural modulus is
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significantly higher at 1500 to 2400 MPa. Figures 1 to 3 provides a graphical representation of the mechanical properties based on Zheng et. al.’s experiments (2005).
Figure 3 Correlation between MWNT content with density
Figure 4 Correlation between MWNT content with bending strength
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Figure 5 Correlation between MWNT content with bending modulus
The Young’s modulus is the main mechanical characteristic of interest since it is related to stiffness as well as describing the correlation between stress and strain of the nanocomposite. Experimental results suggest the elastic modulus is 1 GPa and capable of reaching up to 1.29 GPa as the carbon nanotibe fibre is added to 2% by weight (Sun, et. al., 2011). Certainly, carbon nanotubes embedded in an epoxy matrix displays superior properties provided several problems are mitiated.
Composite structural properties rely on the characteristics of the individual components. Besides the compatibility of its component materials, the interfacial adhesion between the carbon nanotube and the matrix dictates the mechanical properties of the composite. Effective load transfer between the carbon nanotube reinforcement and the epoxy matrix is crucial in producing a strong and superior composite. Otherwise, the composite mechanical properties will only be slightly stronger than an ordinary weak pure epoxy. Research by Lau et. al. suggested that the mechanical properties of the composite is inferior to pure epoxy when excessive di-‐methylformamide is used in treating the carbon nanotube (2005). In addition, the pull out of carbon nanotube reinforcement phenomenon is associated to the weak interface between the two materials.
Another major problem is the dispersion of carbon nanotube in the matrix. The weak Van der Waals forces of attraction between the carbon nanotube graphene layers will deteriorate the properties and ductility of the matrix (Bai, 2003). The carbon nanotube reinforcement has a small diameter which promotes adhesion with the epoxy matrix and desirable as an interface for stress transfer. However, the downside of this large total surface area is strong attractive forces between the carbon nanotube fibres are induced (Gojny, et. al., 2005).
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g. Themo mechanical Properties Temperature plays a significant role in influencing the mechanical properties of the nanocomposite. The inclusion of carbon nanotube fibres will enhance the glass transition, melting and thermal decomposition temperatures of the composite. For instance, the addition of 1% by weight of carbon nanotube raises the glass transition temperature from 63 to 88⁰C. On top of that, the thermal conductivity is improved by 70% (Xiao-‐Lin, Yiu-‐Wing et al. 2005).
The storage modulus G’ and tanδ curves of the CNT/epoxy nanocomposite are plotted based on the results of the DMA analysis carried out by Yan, Ming et.al. in figure 4 (2008). Tanδ behaves as an indicator if the relative importance of both viscous and elastic behaviours of materials such that tanδ < 1 tends to possess elastic behaviour and acts like a solid whereas tanδ > 1 shows viscosity and liquid-‐like (Jeefferie, et.al., undated). In simpler terms, the glass transition temperatures can be estimated from the peaks of the tanδ curve versus CNT content.
Figure 6 Storage Modulus and tanδ as a function of temperature.
The epoxy based nanocomposites are popular and attractive research topic but the dilemma lies in the dispersion of CNTs and interfacial bond between CNTs and epoxy. There are multiple ways to counteract those problems. The solution is to add functionalised group to the surface of CNTS. The use of a nonionic surfactant is proposed to treat CNT surface for nanocomposite fabrication, which can act as a bridge between CNTs and epoxy matrix without disturbing CNT structure or introducing defects. Another method to improve interfacial adhesion is by mechanical means such as using vibratory methods such as sonication. Once the CNTs are will dispersed, those epoxy based composites will fulfill its potential of exhibiting excellent mechanical, electrical and thermal properties.
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Specific Topic: Dispersion
a. Fabrication methods Regarding the original prediction, the carbon nanotube and epoxy composite should possess excellent mechanical properties and thermal resistance to apply as an effective structural member. The application of epoxy matrix reinforced with carbon nanotube into the construction industry is still premature at this stage due to several shortcomings. These challenges that are well documented laments the unexpected result of an ineffective composite as a result of poorly dispersed multiwalled carbon nanotube and weak interfacial bonding in the epoxy matrix (Amal and Mahmoud 2007).
Despite the aforementioned shortcomings, there are several available techniques to amend the properties of epoxy resin. The rule of thumb centres on a homogenous distribution of epoxy resin and increase the interfacial friction with epoxy matrix. The first method is the direct dispersion, commonly known as mechanical method. Since the composite is in nano scale, devices like the ultrasonicator in a bath or probe sonicator, high shear mixing in a solvent, calendaring and ball milling can be used as a combination in series or parallel. These tools are able to disentangle CNTs from each other by means of vibratory energy or shear force. Although this technique successfully separate the fibres from each other, a substantial amount of energy input is required besides resulting in damage and breakage of CNTs into smaller lengths (Sohel and Mangala, 2009).
Chemical methods create surface functionalities in CNTs to promote the intermolecular dispersion by improving the chemical compatibility or interactions with a polymer or solvent. Functionalities refer to the creation of functional group on the CNT surface to encourage interfacial interactions (Young Seok and Jae Ryoun 2005). Two pertinent issues to worry about when chemical methods are used is the aggressive nature of treatment and unexpected interfacial bonding results. The most effective chemical method requires concentrated acids in the oxidation process. Then again, the corrosiveness of acids generates structural defects by deteriorating the intrinsic properties of CNTs, creating defects and reduces the aspect ratios of CNT which result in degraded mechanical properties. Replacing acids with milder functionalisation processes such as UV/ozone treatment or plasmas followed by amine, silane or fluorine treatments limits the active sites on the CNT surface, leading to a low efficiency of functionalisation. Milder treatment also means the dispersibility of CNTs in the composite is marginally altered.
Recently, amino functionalisation is devised to improve the dispersion and interfacial adhesion of CNTs with polymer resins. Demonstrations suggest strong correlations between amino-‐functionalisation, dispersion, wettability, interfacial interaction and re-‐agglomeration behaviour of CNTs and the corresponding mechanical and thermo-‐mechanical properties of
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nanocomposites (Peng-‐Cheng, Shan-‐Yin et al. 2010). To sum up, several targets can be met by the synthesis of amino functionalisation. The uniform dispersion of agglomerated CNTs in the epoxy resin are stabilised and dispersed CNTs under high temperature applied for curing can be achieved to prevent re-‐agglomeration.
Application of surfactants and polymer coatings provides an interesting prospect to disperse the CNT fibres. Surfactant treatment is widely considered as the best choice of CNT dispersion because the physical adsorption seldom damages the CNT structure, nor disrupts the π-‐bond of CNTs and thus, the electrical properties are not perturbed (Sun, Nicolosi et al. 2008). Other novel method in progress worth mentioning to cure the epoxy is by exposing the epoxy to gamma radiation and electron beam in order to improve the thermal stability and yield strength (Nho, Kang et al. 2004). In essence, continuous efforts are required to explore various treatment methods besides improving the current treatment practices to make the CNT and epoxy a successful composite.
b. Polymers to disperse CNT The literature review was conducted during the mid-‐semester break. The findings of the literature review were presented in point form in a presentable manner. Refer to Appendix A for the information obtained.
c. Uv-‐Vis to monitor dispersion of CNT The literature review was conducted during the beginning of semester 2. The findings of the literature review were presented in point form in a presentable manner. Refer to Appendix B for the information obtained.
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Experimental Several experiments were performed throughout the year. However, out of the four experiments performed, only one is selected to be presented in a conference paper. The first experiment, Experiment 1 describes the effect of carbon nanotube geometry on dispersion. Meanwhile, Experiment 2 explains the effect of ultrasonication and amount of CNT on the composite mechanical properties.
Experiment 1: Procedure Experiment 1 investigates the effect of CNT geometry on dispersion. In this study, the multi walled carbon nanotubes (MWCNT) used was supplied by NTP Company. Properties of used CNTs are tabulated in Table 1 and SEM images of CNTs are illustrated in Figure 7. Dispersing agent was BYK9076, an Alkylammonium salt of a high molecular weight copolymer which was kindly offered by Nuplex Resins Company. The solvent was ethanol with 99% purity from Grale Scientific.
Table 1: CNTs data provided by manufacturer
S-1020 L-1020 L-2040 L-4060 Main rang of diameter (nm) 10-20 10-20 20-40 40-60
Length (μm) 1-2 5-15 5-15 5-15 Purity (%) ≥ 95 ≥ 95 ≥ 95 ≥ 95 Ash (wt %) ≤ 0.2 ≤ 0.2 ≤ 0.2 ≤ 0.2 Special surface area (m2/g) 40-300 40-300 40-300 40-300
Amorphous carbon (%) < 3 < 3 < 3 < 3
Figure 7: SEM image of used MWCNT in this research A: S1020, B: L1020, C: L2040, D: L4060
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All solutions were prepared by mixing 315.8 mg CNT with 40 ml ethanol and 79 mg surfactant in a beaker. Thereafter the resulting solution was sonicated about 1 hour by 100 KJs dispersion energy. All sonication processes were carried out with a horn sonicator (VCX 500W) with a cylindrical tip (19 mm end cap diameter). The output power was fixed at around 25W. To prevent rising the mixture temperature the beaker of solution was placed in a water-‐ice bath during sonication.
UV–vis measurements were carried out on a DR 5000 Spectrophotometer with a wavelength range of 190 to 1100 nm and Wavelength accuracy of ± 1 nm in Wavelength Range 200–900 nm. Samples were taken after the sonication process, diluted by a factor of 35, resulting in a CNT content of 0.15 mg, and measured in the UV–Vis spectrometer. All absorbance intensities are used after baseline subtraction. The ethanol-‐surfactant solution was used to get the baseline in corresponding measurements. For each test 3 samples were tested and the average of results was represented. Scanning electron microscopy (SEM) morphology was studied by using a JEOL 7001F field emission SEM operating at 5 kV. To prepare SEM sample a drop of CNT dispersed in solution was deposited on a silicon substrate, dried, and coated with 1 nm thickness Pt.
Experiment 1: Results and discussion
a. UV–vis spectra of MWCNTs–BYK9076 solutions UV-‐vis spectroscopy correlates intensity of absorption of UV-‐visible radiation to the amount of substance present in a solution. Individualized CNTs are active and show characteristics bands in the UV region. Therefore measured absorbance at specific wavelength can be related to their degree of exfoliation(Grossiord, Loos et al. 2007). Bundled CNTs are hardly active in the wavelength region between 200 and 900 nm which is most probably because of carrier are tunnelling between the nanotubes(Grossiord, Loos et al. 2007). Thus, UV-‐vis is an ideal method to monitor the dispersion of CNT in the organic and inorganic solvent. However, this relationship is only true in a dilute sample. Dilution decreases the concentration of CNT so that the light will not be completely blocked off by the suspension. The spectrophotometer has a light source emitting light covering the entire visible spectrum and the near ultraviolet, covering a range of 200nm to 800nm. Monochromatic light is passed through the sample. The incident light is reduced in intensity due to absorption, reflection, transmittance, interference and scattering of light.
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Figure 8: Normalized UV-‐vis adsorption spectrum of L-‐4060 CNT in ethanol (43 µg ml-‐1) at power of 25 W.
Figure 8 illustrates a normalized UV-‐vis spectrum of L-‐4060 CNT in ethanol (43 µg ml-‐1) at continuous power of 25 W. As can be seen, the evaluation of the degree of dispersion of CNTs in ethanol can be achieved by recording the UV-‐vis spectra of the solution. It can be clearly seen that the sample shows a peak at about 260nm, confirming the presence of successfully dispersed CNTs. At this point, it demonstrates the strong absorption by dispersed CNT. Experiments conducted by (Yu, Grossiord et al. 2007) also reported that the maximum absorbance occurs at the same wavelength of 260nm. Absorbance decreases steadily due to scattering in the lower wavelength range after peak absorbance at about 260 nm. In this case, Rayleigh scattering occurs because the size of CNTs is small compared to the radiation wavelength.
It is worth pointing out that CNTs can be effectively dispersed in ethanol solution by π-‐stacking interaction. The ethanol is selected over many other solvents to homogeneously disperse the CNT due to some factors. Mainly, the ethanol solvent does not interfere with the absorption during the UV-‐vis test. Moreover, dispersion can be attained without degrading or destroying the CNTs, unlike acid treatment.
It was reported in the literature (Grossiord, Loos et al. 2007; Yu, Grossiord et al. 2007) that, during the sonication process the relative evolution of the spectrum underneath area is proportional to the relative value of absorbance at a specific wavelength; Therefore, it was decided to specify the absorbance maximum about 260 nm and to plot this value as a function of the total sonication energy provided to disperse CNT in solution.
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a. Effect of CNT diameter on dispersion To investigate the effect of CNT diameter on quality of CNT dispersion in media three CNTs with different diameter were tested. CNTs diameter ranges were 10-‐20 nm, 20-‐40 nm and 40-‐60 nm. All CNTs had equal length range of 5-‐15 µm. More information of CNTs is tabulated in table 1. CNTs were dispersed in BYK9076-‐ethanol solution as explained in experimental section and as prepared solutions were used for UV-‐vis analysis.
Figure 9 shows that the smallest CNT diameter of 10 to 20 nm has the lowest absorbance which is about 0.18. However, the biggest CNT diameter of 40 to 60 nm has the highest absorbance of around 0.55 which is 3-‐fold of that of 10 to 20 nm. This can be interpreted as a CNT with small diameter is poorly dispersed. Smaller diameter of CNT corresponds to a large surface area. This means CNT with smaller diameter requires more surfactant molecules to achieve stable dispersion compared to that of CNT with larger diameter. The difficulty in dispersing the CNT with small diameter arises from the stronger van der Waals attraction. In addition, more energy is necessary to overcome the CNT interaction energy of bundled CNT with smaller diameter compared to that of Sample with the larger diameter in this experiment. Therefore, with the constant concentration of surfactant and energy input for CNT dispersion, CNT with larger diameter can be easily dispersed compared to CNT with smaller diameter.
Figure 9: Effect of CNT diameter on CNT dispersion in ethanol solvent. Normalized height of the UV-‐vis spectra peak located around 260 nm wavelengths for 3 different examined CNT diameters, bigger the diameter better the dispersion. Top left: Evolution of the colour of 1% wt CNT 0.25% wt BYK9076 in Ethanol as a function of the CNT diameter. A: 10-‐20 nm, B: 20-‐40 nm and C: 40-‐60 nm (solutions are diluted by a factor of 35).
This statement is in agreement with visual observation of dispersed CNT solution in top left corner of Figure 3. As can be seen, the sample A which is 10-‐20 nm CNT solution has the lightest colour and the sample C with 40-‐60 nm CNT has the darkest colour. It means that 40-‐60 nm CNT solution has more suspended CNT nanoparticle in solution compared to that of 10-‐20 nm solution.
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b. Effect of CNT length on CNT dispersion The influence of CNT length on efficient dispersion of CNT has been investigated in this section. To do this, two CNTs with different lengths but the same diameter were chosen. CNTs were S-‐1020 and L-‐1020 with 1-‐2 µm and 5-‐15 µm length respectively and 10-‐20 nm diameters. More information of CNTs is provided in table 1. Figure 4 illustrates normalized height of the UV-‐vis spectra peak located around 260 nm wavelength for 2 different examined lengths, and also evolution of the colour of 1% wt CNT 0.25% wt BYK9076 in Ethanol as a function of the CNT length. As can be seen, both 1-‐2 µm and 5-‐15 µm length CNTs have almost the same absorbance value. It testifies that CNT length has not significant effect on quality of CNT dispersion in matrix.
Figure 10: Effect of CNT length on CNT dispersion in ethanol solvent. Normalized height of the UV-‐vis spectra peak located around 260 nm wavelength for 2 different examined lengths, Top right: Evolution of the colour of 1% wt CNT 0.25% wt BYK9076 in Ethanol as a function of the CNT length. Left: 1-‐2 µm, right: 5-‐15 µm (solutions are diluted by a factor of 35).
Theoretically, a shorter CNT would be more easily dispersed than longer CNT. A longer CNT will provide a larger area for entanglement. However, the dispersion results in Figure 10 shows that the CNT entanglement is negligible and the surface energy is dominant obstacle for efficient dispersion. CNT length has a negligible influence on CNT dispersion compared to diameter. It is because; the total Surface area to volume ratio is independent from CNT length but proportion to inverse CNT radius. Therefore, for a constant CNT content, CNT length variation has not significant effect on its dispersion efficiency.
Equivalent amounts of short or long CNT with the same diameter have equal surface area. In the other hand, the equal amounts of short and long CNT with constant diameter have the
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same interaction energy in bundle. Accordingly, both samples need the same value of dispersion energy to break down the CNT bundle. Therefore, as Figure 10 demonstrate, with the same provided dispersion energy and surfactant short and long CNT have almost the same quality of dispersion in solvent. However, the point here is that although CNT length has not significant effect on CNT dispersion, it influences the aspect ratio of CNT. Since CNT aspect ratio plays a key role on reinforcement role of CNT in CNT nanocomposites it is still essential to use long CNT rather than short CNT.
Experiment 2: Procedure Three additional experiments were performed to investigate the factors influencing the strength of the CNT-‐epoxy composite. The variables to be investigated are the effect of shear mixing, ultrasonication and amount of CNT (measured in terms of percent mass) on the mechanical properties. These mechanical properties include Young’s Modulus and tensile strength.
Pure epoxy samples (Epoxy: Araldite 2011) were prepared to analyse its properties as a resin. Since the weakness of a composite lies in its adhesive layer, the importance of investigating the behavior of pure epoxy to the action of load and temperature cannot be underestimated. In addition, MWCNTs with large diameter within the range of 40 to 60 nm are embedded in the epoxy matrix. On top of that, the scanning electron microscope (SEM) will be utilized to study the fracture surface.
Firstly, either pure epoxy or CNT composite is placed into a mould with the dimensions stipulated in Table 2 to obtain a dog-‐bone shaped sample. This is in conformance with the ASTM International Standards and the dimensions are stated in Table 2. Twenty-‐four similar samples were prepared to perform several tests later. The samples underwent high speed shear mixer and ultrasonication at 2000rpm and 3500rpm in shear mixer as well as 15 and 30 minutes in the ultrasonicator respectively. The sample was left to cure. Table 3 mentions the type of CNT fibre used.
Table 2 Dimension of sample Dimension Length (mm) Thickness 3 Width 12.7 Length 100 Span 48-‐50
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Table 3: CNTs data provided by manufacturer S-1020 L-1020 L-2040 L-4060
Main rang of diameter (nm) 10-20 10-20 20-40 40-60
Length (μm) 1-2 5-15 5-15 5-15 Purity (%) ≥ 95 ≥ 95 ≥ 95 ≥ 95 Ash (wt %) ≤ 0.2 ≤ 0.2 ≤ 0.2 ≤ 0.2
Special surface area (m2/g) 40-300 40-300 40-300 40-300
Amorphous carbon (%) < 3 < 3 < 3 < 3
Upon completion of the fabrication process, the samples are ready to be tested to failure, also known as destructive tests. To investigate the mechanical properties of the composite, the tensile is performed whereby the ASTM D 638-‐10 standard will be used as a guide. The Bluehill software will be utilised to calibrate the strain experienced by the sample when it is loaded to failure. The fracture surface is then observed by SEM at room temperature and cold freeze by liquid nitrogen.
Experiment 2: Results and discussion
a. High speed shear mixing Fluid undergoes shear when one area of fluid travels with a different velocity relative to an adjacent area. In a high shear mixer the tip velocity, or speed of the fluid at the outside diameter of the rotor, will be higher than the velocity at the centre of the rotor, and it is this velocity difference that creates shear. This shear can be used to load filler such as nano particle in matrix. Figures 5 to 7 show the comparison between stress-‐strain curves of pure epoxy with CNT-‐epoxy composite using high shear mixer.
Figure 11 Tensile stress-‐strain curve of CNT-‐Epoxy fabricated by shear mixing method (left) and Effect of tensile speed on tensile curve (right)
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This method couldn’t produce efficient CNT-‐Epoxy composite due to the limitation of mixing speed. The pure epoxy demonstrates a ductile behaviour as a result of the usage of an overage hardener. The composite has a marginally higher Young’s Modulus than pure epoxy which may be the direct result of insufficient mixing speed. Therefore, shear mixing marginally improves the CNT elastic modulus of CNT.
b. Ultrasonication: Ultrasonication generates alternating low-‐pressure and high-‐pressure waves in liquids, enabling the formation and violent collapse of small vacuum bubbles. The cavitation phenomenon causes high speed impinging liquid jets and strong hydrodynamic shear-‐forces. The induced effects are used for the deagglomeration and milling of micrometre and nanometre-‐size materials in matrix. Figure 8 compares the stress-‐strain curves of pure epoxy with CNT-‐epoxy composite using ultrasonication.
Figure 8: Effect of sonication energy on tensile stress-‐strain curve
Table 4 Effect of sonication energy on mechanical properties of CNT-‐Epoxy composite
Dispersion Energy (KJ)
Tensile Stress Mean (Mpa)
Ultimate Strain Mean (%)
Elastic Modulus Mean (Mpa)
60 37.91 13.52 1984.22 90 39.33 10.86 2031.18 120 38.17 11.26 1982.65 180 35.35 7.87 1897.09
Generally, all tensile stress-‐strain curves show that the stress increases with strain till a peak value is reached before the stress decreases slightly until it reaches a plateau as the load is
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continually applied. Similarly, the relationship also applies to the dispersion energy versus elastic modulus. The optimal dispersion energy is 60 kJ whereby the composite elastic modulus is the maximum. At this stage, energy is applied mechanically to physically disperse the nanotubes from its agglomerated bundles. In other words, larger dispersion energy corresponds to a higher rate of dispersion. However, excessive application of sonication will result in damage and breakage of CNTs into smaller lengths (Rana, Alagirusamy et al. 2009). Clearly, degradation in tensile stress and elastic modulus is observed when the dispersion energy is greater than 60 kJ.
c. Amount of CNT The tensile test is performed on six types of composite with varying CNT content at 0, 0.1%, 0.3%, 0.5%, 1% and 1.5%. The stress-‐strain curves of all six cases are summarized in Figure 9 while their corresponding mechanical property values are stated in Table 4.
Figure 9: Effect of CNT percentage on tensile stress-‐strain curve
Table 4: Effect of CNT percentage on mechanical properties of CNT-‐Epoxy composite
CNT percentage (%)
Tensile Stress Mean (Mpa)
Ultimate Strain Mean (%)
Elastic Modulus Mean (Mpa)
0.0 35.25 13.23 1737.66 0.1 38.17 11.26 1982.65 0.3 35.9 4.74 2001.67 0.5 35.15 6.53 1853.66 1.0 30.1 2.4 1734.21 1.5 31.18 3.46 1878
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The mechanical properties of the composite improve as the amount of CNT embedded in the matrix increases. The optimal percentage of CNT is 0.3% in this experiment and the results are supported by Yuanxin et. al. (2007). The apparent decrease in mechanical properties is best described in terms of the molecular interactions between the CNT. The weak Van der Waals forces of attraction between the carbon nanotube graphene layers will deteriorate the properties and ductility of the matrix (Bai 2003). The carbon nanotube reinforcement has a small diameter which promotes adhesion with the epoxy matrix and desirable as an interface for stress transfer. However, the downside of this large total surface area is strong attractive forces between the carbon nanotube fibres are induced (Gojny, Wichmann et al. 2005). When the amount of CNT is increased, more CNT fibres are present to form bundles via molecular attraction. This reagglomeration scenario is not ideal as the CNT can experience a failure mode called pull-‐out of fibres. Gojny et. al. also experienced the same phenomenon and concluded that the discrepancy is a result of the variation in quality of dispersion in all nanocomposite samples after performing sonication (2005). The upper limit to the addition of CNT is 4% because the nanotube content would be saturated and this leads to a significant increase in viscosity that causes void defects in the composite (Zhu, Peng et al. 2004).
Conclusion In the beginning of the year, a comprehensive literature review and experiments were conducted to study the properties, problems and potential of carbon nanotubes (CNT). Despite its exceptional mechanical, thermal and electrical properties, agglomeration is the biggest problem that limits the mechanical properties of CNT. To overcome the string intermolecular forces, dispersion during fabrication is necessary to enhance the mechanical properties. Experiments using chemical and physical dispersion were performed to achieve this goal. Several experiments were conducted to investigate the possible factors that may improve dispersion in CNT and ultimately improve its mechanical properties.
The second semester is focused on the effect of CNT geometry on efficiency of CNT dispersion in media. Results show that CNT diameter has significant influence on quality of CNT dispersion in media, the bigger diameter the better CNT exfoliation in matrix. In contrast, CNT length has insignificant influence on quality of CNT dispersion in matrix. In mathematical terms, the total Surface area to volume ratio is independent from CNT length but proportional to inverse CNT radius. Therefore, for a constant CNT content, CNT length variation converse to radius variation has no significant effect on its dispersion efficiency.
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Appendix A: Conference Paper
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Appendix B: Project Management Statement The project was conducted in a continuous manner beginning from the first week of the first semester until the end of September whereby the conference paper was completed. This was managed effectively by prioritisation of tasks with the guidance of the lecturer, Dr. Wen Hui Duan. The literature review was performed individually with the supervisor providing a set of compulsory reading materials to be reported on a weekly basis. Software such as JabRef, Lyx and CTex were used to present the findings in a systematic and organised form. On the other hand, I assisted the supervisor to perform the laboratory experiments. In the first semester, the first two weeks were utilised to familiarise myself with the topic at hand. The next 3 weeks (Week 3 to week 6) were spent at the laboratory performing experiments. During that time period, a total of about 10 hours were spent at the laboratory. Tensile tests and fabrication of CNT epoxy specimens were carried out. A poster presentation was delivered Week 4 to understand the requirements of the task and begin scoping the project. In addition, the literature review regarding the general topic of the “Properties, Problems and Potential of CNT” took place in a continuous manner till the end of the semester. At the end of the semester in week 12, a preliminary report was submitted to monitor the progress. The holidays were well-‐spent as I channelled my time and energy on the final year project. During the mid-‐semester break, duration of four weeks was set aside to identify the specific topic for the final year project and conference paper submission. As a result, the topic of “Fabrication and Characterisation of CNT Epoxy Nanocomposites: Effect of the Geometry of Carbon Nanotubes” was selected. As shown in Appendix A and B, the mid-‐term break was used to present the findings in a presentable manner. During the second semester, the first three weeks was used to perform experiments at the laboratory to investigate the effect of geometry on CNT. About 12 hours were spent at the laboratory to obtain the results. The remaining time until the end of September was dedicated to write a conference paper as shown in Appendix C. Based on the compiled notes gathered throughout the semester, the literature review findings were applied as background knowledge and references to help write the conference paper. Overall, the unit certainly helped me to juggle and manage my time for research and coursework. In addition, the time used during the mid-‐term and mid semester breaks allowed ne to use my time in a much more effective manner. As a result, the conference paper managed to be produced ahead of schedule. Moreover, a full day (11 hours) per week was set aside to familiarise myself with the topic by reading the conference papers in the database.
