the potential of carbon nanotubes production as …...however, synthesis methods of both carbon...

Minia Journal of Engineering and Technology MJET Volume 30, No. 1, January 2011

Corresponding Author: Dr. Isam Yassin Qudsieh, Department of Chemical Engineering , College of Engineering, Jazan University, P. O. Box 706, Jazan 45142, SAUDI ARABIA, Tel: 966-7-3215077 Ext:1017 (Head office), Fax: 966-7-3232900, e mail: [email protected], [email protected]

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THE POTENTIAL OF CARBON NANOTUBES PRODUCTION AS A FILLER ELEMENT IN

POLYMER NANOCOMPOSITE: A REVIEW ARTICLE

Isam Y. Qudsieh Nanotechnology Research Unit (NTRU), Department of Chemical Engineering, College of

Engineering, Jazan University, P. O. Box 706, 45142 Jazan, Saudi Arabia. E-mail: [email protected]

ABSTRACT: The properties of the materials such as microstructure, atomic structure, composition, mechanical properties determine the performance of the materials that are controlled by thermodynamic of the reaction as well as the kinetics of the reaction steps. Nanocomposite such as carbon nanotubes/clay polymer nanocomposite is a better choice to the most commonly adopted conventional composite with lot of defects such as brittleness, flexural strength, and low elongation due to the presence of carbon nanotubes in the nanocomposite matrix. Carbon nanotubes are noted for high mechanical resilience such as tensile strength and young’s modulus in the axial direction; they are equally soft and elastic in the radial direction and as such have been receiving attention in the formation of polymer nanocomposite. However, synthesis methods of both carbon nanotubes and polymer nanocomposite have great influence on their performance and structural stability respectively. In this study, some known synthesis and preparation methods, the effect of carbon nanotubes and effect of various fillers on nanocomposite on the conventional composite are reviewed. Identified drawbacks are discussed and suggestions are made for further improvement in the synthesis and production of carbon nanotubes and its application to polymer nanocomposite. Finally, a ternary nanocomposite is proposed using carbon nanotubes as one of the fillers to cater for the draw backs commonly found in the conventional polymer nanocomposite. KEYWORDS: Polymer, Nanocomposite, Carbon nanotubes, Filler element, Mechanical Properties.

CONTENTS

INTRODUCTION NANUOSTRCTURE MATERIALS

Allotropes of Carbon CARBON NANOTUBES

Single-Wall and Multi-Wall Carbon nanotubes Chirality in Carbon nanotubes Properties of Carbon nanotubes Potential Application of Carbon nanotubes Carbon nanotubes Synthesis Methods

POLYMER COMPOSITE Polymer fibre Nanocomposite Polymer Clay Nanocomposite Polymer Carbon Nanocomposite Ternary Polymer Nanocomposite

GENERAL CONCLUSIONS ACKNOWLEDGEMENT REFERENCES

ABBREVIATION

CNTs: Carbon Nanotubes SWCNTs: Single Walled Carbon

Nanotubes MWCNTs: Multiple Walled Carbon

Nanotubes F-CNTs: Functionalized Carbon

Nanotubes MMT: Montmorilonite clay PEO: Polyethylene Oxide BNNTs: Boron Nitride Nanotubes


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LaB6 Lanthanum Hexaboride CVD: Chemical Vapor Deposition PE-CVD: Plasma Enhanced Chemical

Vapor Deposition SEM: Scanning Electron

Microscope TEM: Transmission Electron

Microscope XRD: X-Ray Diffraction n & m Chiral Vector UO Operating Fluidization

Velocity Umf Minimum Fluidization Velocity

INTRODUCTION

The evolution of nanostructured materials in the last few decades has brought a rapid development in composite formation technology. Attention has shifted to tiny but fascinating nanostructured materials called Carbon nanotubes (CNTs) because they are considered as ideal reinforcing fillers in a wide range of composite systems [1]. An effort to improve on physical and mechanical properties of various materials like metals, nonmetals, earth metals and polymers led to the development of what are now called Composite Technology. The application of any material in nanoscale to enhance certain properties on other materials like polymer leads to the production of nanocomposite. Presently, large quantities of nanostructured material are available due to the advancement in nanoscience and nanotechnology [2]. Nanoscience and nanotechnology primarily deal with the synthesis, characterization, exploration and exploitation of nanostructured materials. These materials are characterized by at least one dimension in the nanometer (1 nm = 10-9m) range [3]. This length scale could be a particle diameter, grain size, layer thickness, or width of a conducting line on an electric chip. Increase in demand for polymers due to their wide industrial applications has lead to large interest in polymer composites in order to enhance their properties for service situations where pure polymers are not sustainable. Any material that combines one or more separate components in order to obtain the best properties of each component is called composite. If such materials present in nanoscale, they are called nanocomposite. Four classes of composites are identified and these include: polymer matrix, metal matrix, carbon matrix and ceramic matrix. Hitherto, fibres, obtained from agricultural materials and some inorganic materials like clay are used as fillers in properties enhancement for

polymers. The assessments of natural fillers show impressive result due to their intrinsic advantages over synthetic fibres such as glass. These advantages include low cost, low density, renewability, favorable values of specific strength and specific modulus, excellent chemical resistance and significant processing benefit which required minor changes to equipment [4]. However, cellulose fibre has little use as fillers in thermoplastic compared to the inorganic fillers like tack, clay, mica, and glass fibre despite its intrinsic advantages [5]. This is because when cellulose fibre is compounded with thermoplastic polymer, the main problem encountered is the poor interfacial adhesion between the hydrophobic polymer and hydrophilic filler. Currently, attention has shifted towards fabrication of nanocomposites with the use of carbon nanotubes (CNTs) as reinforcing agent in polymer materials to harness the exceptional intrinsic properties such as large Young Modulus and flexural strength of CNTs. In particular, polymer carbon nanocomposite show great potential for electronic device application, such as organic field emitting displays, photovoltaic cells, highly sensitive strain sensors and electromagnetic interference materials [6]. However, research work in this area is still like a drop in the ocean, therefore in this paper, attempt is made to review some existing CNTs synthetic methods, assess some reported efforts at reinforcing polymers with different types of filler elements both at the micro and nano level. Finally, some drawbacks are identified and suggestions are made towards achieving quality products at minimal cost. NANOSTRUCTURE MATERIALS Nanostructures constitute a bridge between molecules and infinite bulk system. Individual nanostructure includes; Clusters, Quantum dots, Nanaocrystals, Nanowires, Nanotubes, and Nanofibres while collections of nanostructures involved arrays and assemblies of the individual nanostructure [7,8]. Carbon nanostructures are becoming commercially important with interest growing rapidly since the discovery of buckminsterfullerene, carbon nanotubes, and carbon nanofibres over a decade ago [9]. Carbon nanotubes [CNTs] and carbon nanofibres (CNFs) are among materials in the first rank of nanotechnology revolution. Some of the important features of these structures are their unique mechanical, electronic and magnetic properties which have caused them to be widely studied [10,11]. High Young’s modulus and tensile strength of CNTs which rival that of diamond (1TPa~200GPa) respectively enable this material to be used as a reinforcement in composite structures [12]. The main


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areas of potential application of CNTs and CNFs as a reinforcement materials include-: Conducting nanowires, field emitters, (individual nanotubes field emitters and large area flat panel displays), nanotools (tips for scanning, tunneling, atomic force, magnetic resonance force and scanning near field optical, chemical/biological force microscope tips, nanomanipulators and nanotweezers) [13]. Researchers are currently at the threshold of a revolution in the way in which materials and products are created. How this revolution will develop and how great will be the opportunities that nanostructuring can yield and progress, will depend upon the ways in which a number of challenges are met. Allotropes of Carbon Carbon exhibits three allotropic forms in the solid phase. These include diamond, graphite and buckminsterfullerene or otherwise called fullerene. Diamond has a crystalline structure where each carbon atom (SP3 hybridized) is bonded to four others in a tetrahedral arrangement [14]. This crystalline network gives the diamond its exceptional intrinsic properties like hardness, heat conduction, electrical insulation and optical transparency. Graphite is made by layered planar sheets of SP2 hybridized carbon atoms bonded together in a hexagonal network [14]. The different geometry of the chemical bonds makes it slippery, opaque and electrical conductive. The third allotrope of carbon is the buckminsterfullerenes which consist of a family of cylindrical molecules with all the carbon atoms SP2 hybridized. Buckminsterfullerenes gave birth to what is now called carbon nanotubes.

Figure 1: The Three Allotropes of Carbon [18]: Retrieved from http://smalley.rice.edu/smalley.cfm?doc_id=4866 on 12 June, 2008.

CARBON NANOTUBES Carbon nanotubes are cylindrical molecules with a diameter of as little as 1-nanometer and a length up to many micrometers [15]. They consist of only carbon atoms, and can essentially be thought of as a single

layer of graphite (Figure.2a) that has been wrapped into a cylinder (Figure. 2b).

Figure 2: Single Wall Carbon Nanotubes Before (a) and After Wrapping (b), Cees Dekker [15] The discovery of carbon nanotubes in 1991 as a minor by-product of fullerene synthesis [17,18] revealed that there are two types of carbon nanotubes namely Single-wall and Multi-wall carbon nanotubes. Single-Wall and Multi-Wall Carbon nanotubes CNTs can either be single-walled (SWCNTs) or multi-walled carbon nanotubes (MWCNTs), where MWCNTs are made of concentric SWCNTs. They are slightly pyramidalized by curvature from the pure SP2 hybridization of graphene, towards the diamond-like SP3 [19]. Infinitely long in principle, a perfect tube is capped at both ends by hemi-fullerenes, leaving no dangling bonds. A single wall carbon nanotube (SWCNT) Fig. 2b is one of such cylinder, while multiwall carbon nanotubes (MWCNTs) Fig. 3 consist of nested cylinders whose successive radii differ by roughly the interlayer spacing of graphite.

Figure 3: Multiple-Wall Carbon Nanotubes Wikipedia [32]

The minimum diameter of a stable free standing SWCNT is limited by curvature-induced strain to ~0.4 nanometer [20]. Multi-wall nanotubes may have outer shells > 30nm in diameter, with varying numbers of shells, affording a range of empty core diameters. Length up to 3mm has been reported. Multiwall tubes have two advantages over their single-wall cousins. The multishell structure is stiffer than single-wall especially in compression [21]. Nanotubes


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are distinguished from less-perfect quasi-one-dimensional carbon materials by their well developed parallel wall structure. It should be noted that other elements can also be made into nanotubes, so one often encounters the term (SWCNTs) to distinguish them from non-carbon nanotubes such as Boron Nanotube (BN), Boron Carbon Nanotube (BCN) and metal dichalcogenides [22]. Chirality in Carbon-nanotube Carbon nanotubes are allotropes of carbon and hence they exist in different forms of chirality ranging from armchar to chiral and zigzag. Difference in CNTs chirality gives different electrical and optical properties and this also depends on the chiral vector (m,n). The (5,5) armchair nanotubes are metallic for symmetry reasons while the (7,1) chiral tubes display a small gap due to curvature effect and is metallic at room temperature and the (8,0) zigzag type is a layer – gap semiconductor [65] as shown in Fig. 4.

Figure 4: Atomic Structure of CNTs Chiralities (Adapted from Saavedra M.S. [23])

Properties of carbon Nanotubes Carbon nanotubes are the strongest and stiffest materials on earth, in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp2

bonds formed between the individual carbon atoms. The various properties exhibited by carbon nanotubes are traceable to their structural affiliation to graphite. Hence, they have varying properties like mechanical, electrical and thermal properties. Thermal properties Prior to carbon nanotubes, diamond was the best thermal conductor known. For heat flowing along their axes, carbon nanotubes have now been shown to have a thermal conductivity of about 3000 Watts per meter-degree Kelvin [24]. In comparison, the thermal conductivity of diamond is between about 900 and 2300 W/m-K, while copper is only about 400 W/m-K. The thermal properties of carbon nanotubes are directly related to their unique structure and small size [5]. These have caused them to be used as material for the study of low-dimensional phonon physics, and for

thermal management, both on the macro- and micro scale [25]. Lasjaunias et al. [26] reported that at temperature above 1 K and below room temperature there is a linear dependence of specific heat and thermal conductivity on temperature. The carbon-nanotube's thermal conductivity is very large along its axis because vibrations of the carbon atoms propagate easily down the tube and in the direction transverse to its axis (60). And because of this CNTs are used to conduct heat away from electronic materials. Electrical properties Carbon nanotubes are quasi-one-dimensional molecular structures with semiconducting or metallic properties that make them promising material for future electrical and electronic applications [27]. The electrical and electronic behavior of carbon nanotubes depends on their chirality. Hence, because of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given chiral vector (n,m) nanotubes, if n−m is a multiple of 3, then the nanotube is metallic, otherwise the nanotube is a semiconductor. Thus for all armchair the chiral vector n=m and hence the nanotubes are metallic, and nanotubes zig-zag with chiral vectors (5, 0), (6, 4) and (9, 1) etc. are semi-conducting [65]. In theory, metallic nanotubes can have an electrical current density more than 1,000 times greater than metals such as silver and copper. Mechanical properties The strength of the carbon-carbon bond gives rise to the extreme interest in the mechanical properties of nanotubes. Theoretically, CNTs is stiffer and stronger than any known substance. Simulations [28,29] demonstrate a remarkable “bend, don’t break’ response of individual SWCNT to large transverse deformations. Young‘s Modulus of a cantilevered individual MWCNT was measured as 1.0 to 1.8 TPa from the amplitude of thermally driven vibrations observed in the TEM 928. Both the modulus and strength are highly dependent on the nanotube growth method and subsequent processing method, depending on the variables and uncontrolled defects [30]. This method was faulted by Krishnan et. al. [31] when he obtained an average of 1.25 TPa for isolated SWCNTs.


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Table.1 Comparison of Mechanical Properties of Various Type of CNT : Adapted From Wikipedia [32]

Material Young

Modulus (TPa)

Tensile Strength (GPa)

Elongation at Break

(%) SWCNT ~1(1-5) 13-53E 16 Armchair SWCNT 0.94T 126.2T 23.1

Zig-zag SWCNT 0.94T 94.5T 15.6-17.5

Chiral SWCNT 0.92 - -

MWCNT 0.8-0.9E 150 - E: Experimental Observation T: Theoretical Prediction Potential Applications of Carbon Nanotubes There are countless potential applications of carbon nanotubes, depending on which combination of novel properties one is interested in exploiting. Carbon nanotubes have found its potential applications in Biomedical and Drugs delivery, Electrical and Electronics, Composite technology transport and aviation industries [15, 27, 36, 37]. Drugs delivery Recently, CNT have generated great interest in Biology [33], where suitably modified CNT can serve as vaccine delivery systems [34] or protein transporters [35]. The ability of functionalized carbon nanotube (f-CNT) to penetrate into the cells offers the potential of using f-CNT as vehicles for the delivery of small drug molecules [36,37]. Murakami et all [38], loaded single walled carbon nanotubes with dexamethason and studied the binding and release of drug. They observed that drugs could be adsorbed in large amounts into oxidized nanohorns and maintain its biological integrity after been liberated. In view of these results, f-CNT represents a new, emerging class of delivery systems for the transport and translocation of drug molecules into different part of mammalian cells. Bone Grafts Bone grafting is a surgical procedure by which new bone or a replacement material is placed into spaces between or around broken bone (fractures) or holes in bone (defects) to aid in healing [32] It is primarily used to repair bone fractures that are extremely complex, pose a significant risk to the patient, or fail to heal properly. Researchers at the University of California, Riverside (USA), have found that single-walled carbon nanotubes could be used as an artificial scaffold for the growth of bone tissue. Carbon nanotubes are found to be ideal for this application as they have a high strength, high flexibility and are of low density. In the future,

doctors could treat broken bones by injecting a nanotube solution into the fracture to enhance the healing process [40]. Composite materials Carbon nanotubes are receiving a good deal of attention in material reinforcement due to its special intrinsic properties. It has been shown that carbon nanotubes could behave as the ultimate one-dimensional material with remarkable mechanical properties [40]. Endo et al. [41] in his work reported that the density-based modulus and strength of highly crystalline SWCNTs are 19 and -56 times that of steel. Therefore, carbon nanotubes (single and multi-walled) have been studied intensively as fillers in various matrices, especially polymers [42]. Zyvex Corp. and NASA (USA) developed ultra high strength, low-weight composites for aerospace applications. These composites contain carbon nanotube (SW and MW) reinforcements [43]. Composite materials reinforced with carbon or graphite fibres are often used in sporting goods, high performance air or space craft, and other applications where stiffness and lightweight are required. Electrical and electronics The conductive property of carbon nanotubes due to its diameter and chirality is exploited in electrical and electronics application. Hence, CNT have found application in manufacturing of molecular electronics, electronics panel and other electronics devices. There has been a lot of researches conducted on the electrical properties and applications of CNTs [44,45] and Endo et. al. [41] due to the interest in using CNTs in nano-scale electronics devices. Depending on their structure, carbon nanotubes can be almost perfect one-dimensional conductors in which various phenomena such as single electron charging, resonant tunneling through discrete energy levels and proximity-induced superconductivity have been observed at low temperature [71]. But at high temperature, CNTs can be almost perfect one-dimensional luttinger liquid. (A liquid where the energy state of its electron is strongly affected by weak coulomb interactions) In addition, CNTs can be used as junctions between metal-semiconductor, semiconductor-semiconductor and metal-metal since the electrical properties of CNTs are dependent on the structure [71]. Carbon Nanotubes Synthesis Methods There is a remarkable growth in the field of nano-science and nanotechnology in the last decade because of the success in the synthesis of nonmaterial in conjunction with the advent of tools for characterization and manipulation. The synthesis of nano-materials include: control of size, shape, and structure by


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manipulation of some physical parameters like temperature, flow rate, carbon-source and catalyst. The broad classification of carbon nanotubes synthesis methods are as shown in Table 2. Generally, nano-particle synthesis falls into two groups. These are: Sublimation of graphite and decomposition of carbon- containing compounds [46,47]. The first group of processes is associated with high temperatures (up to 4000oC). This can be obtained in electric arcs by the process of laser ablation, by focused solar radiation, [1,3,48,49] or by resistive heating of graphite (17). The second group of methods has its own variations: Pyrolysis of gases (Chemical Vapour Deposition (CVD), Solids e.g. (Pyrolysis of polymers), Aqueous solutions (hydrothermal synthesis) [50,51], organic solution of supercritical toluene) [52]. There is handful of process that combines aspect of one or more of these broad categories of processes.

Arc Discharge The arc process is remarkable for the lager number of versions, among which some versions allow the realization of a semicontinous process in automated facilities. Arching process in liquid nitrogen, [53,54] in water [53] and in aqueous solution [55,56] has also been developed. Conventionally, arc discharge is a method in which the reactants are used as electrodes and vaporized between the two electrodes by electric energy [10].

Figure 5: Illustration of an arc-discharge apparatus for the production of CNTs (Adapted from Yoshinori et. al. [57].

Figure 5 is a schematic diagram of an arc-discharge set-up. The basic configurations of the set-up consist of a vacuum chamber, gas flow controls, and two electrodes with a DC power supply. Depending on the

requirements of specific experiments, the ambient condition inside the chamber can be either inert gas

Table 2 Synthesis and Methods of Characterization of Nanomaterials: Source: Yury G. [61]

Scal

e (a

ppro

xim

atel

y.)

Synt

hetic

M

etho

ds

Stru

ctur

al T

ools

Theo

ry a

nd

Sim

ulat

ion

0.1- 10nm

Covalent Synthesis

Vibrational, Spectroscopy,

NMR, Diffraction methods, Scanning

Probe Microscopies

Electronic Structure, Molecular Dynamics, Transport

<1-100nm Self-

assembly techniques

SEM, TEM, SPM

Molecular dynamics

and mechanics

100nm-1microns Processing SEM, TEM

Coarse-grained

models for electronic

interactions, vibronic effects,

transport. (e.g., helium, argon) or some reactive gases, for instance, nitrogen gas in the case of boron nitride nanotubes (BNNTs). During the growth, the pressure inside the chamber is usually a few hundred torr [58-60]. The reactant materials are compressed or shaped into rods, which are used as electrodes. To synthesize carbon nanotubes, the electrodes used are graphite rods, while for the growth of BNNTs, the situation is more complicated. But in both cases, the metal particles from the anode were found in the samples and they accidentally play the role of catalyst. However, unlike the case of CNTs, in which the arc-discharge is already a routine method for high yield synthesis, the yield of BNNTs, are very low through the use of this method. Arc discharge method was the first technique in successful synthesis of CNTs and BNNTs, but the complication in the scale up process keep it as a laboratory technique [61]. Arc Melting Arc melting is similar to the conventional arc-discharge method but with a simpler setup configuration and experimental procedure (10). Instead of shaping the reactants into the rod like electrodes, the reactant powder is put in a cupper mold (anode), as shown in


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Figure 6.The cathode is tungsten gun. When the voltage and current (e.g., 200V, 125A) are applied between the electrodes, an arc is generated and the powders on the mold are melted [62,63]. Together with the contribution from the ambient gas, which may be one of the reactants, the evaporated clusters from the melted sources form an ionized gas. The growth of nanostructures can then occurs in the ionized gas by forming quasi-liquid particles first. The presence of a catalyst especially transition metal borides exemplified by Lanthanum hexaboride (LaB6) are necessary in this method [64]. Arc melting method is also called plasma-jet evaporation [62,63]. The schematic diagram to show the comparison between arc discharge and arc melting are shown in Figure 6.

Figure 6: Comparison between Arc Discharge (a) and

Arc Melting (b) [Adapted From Yury, G. [61] Laser Assisted Method Laser assisted method is a system whereby photonic energy is converted to heat energy and this heat in turn is used to evaporate the starting materials into ions gas instantly. Guo et. al. [69] were the first to synthesize

CNTs by laser ablation method [55] .More specifically, SWCNTs were synthesized by the laser vaporization of a mixture of carbon (graphite) and transition metals located on a target. Nowadays, the outputs of some advanced lasers have a very high energy density in either a continuous wave mode or at kHz-repetition rates. For example at 10.6microns, the output power of CO2 laser-emitting infra red radiation can reach as high as 1kW, and can be further focused to a spot of several millimeters. When the light, either continuous beam or discrete pulses, is focused on the target of the source materials with such small spot size, the energy provided by the incident light elevates the temperature of the irradiated zone to several thousands Kelvin within a very short period of time. If the temperature is above the sublimation temperature of the target material, local explosion may occur and the source materials may effuse from the surface, thus the ion gas of the atomic scale reactants is generated. This process is called laser ablation [66,67]. On the other hand, if the target temperature is below the sublimation point under the laser radiation, the radiation then only increases the temperature of the target without obvious ejection of materials from it. This process is referred to as laser heating [68]. Both of these processes have been attempted for the fabrication of CNTs in recent years. The schematic diagram of the laser ablation set up is shown in Figure 7.

Figure 7: Laser ablation apparatus: Adapted from Guo et. al. [69] As shown in Figure. 7, air cool metallic trap and filler are used to collect the ablation products, while the products of laser heating are obviously located on the surface and around the irradiated zone of the target. Additional energy is generated by the tube furnace to maintain the temperature of the target [70]. A diamond anvil cell under high N2 pressure (2GPa) was also used for this purpose in laser heating process [70]. Catalyst atom plays a major role in the mechanism of growth of SWCNTs. But currently there are no reports of MWCNT synthesis by the laser ablation technique but

b

a


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only possible by arc-discharge in the presence of pure graphite [71]. Chemical Vapor Deposition Methods. Chemical Vapor Deposition (CVD) is a versatile process suitable for the manufacturing of coatings, powders, fibres, and monolithic components. The concept of decomposing hydrocarbons over a catalyst, referred to as Chemical Vapour Deposition is not new in the field of science and technology. CVD has found its applications in the production of so many materials like carbon fibre, most metals, many non-metallic elements such as carbon and silicon as well as a large number of compounds including carbides, nitrides, oxides, intermetallics, and many others. This technology is now an essential factor in the manufacturing of both MWCNTs and SWCNTs, semiconductors and other electronic components. Chemical vapour deposition can be defined as the deposition of a solid on a heated surface from a chemical reaction in the vapour phase. It belongs to the class of vapour transfer process which is atomistic in nature, that is, the deposition species are atoms or molecules or a combination of these [72]. In the chemical vapour deposition method, different hydrocarbons such as benzene (C6H6) pentane (C5H12 ) , acetylene (C2H2 ), methane (CH4) and carbon monoxide are decompose over different metals (Fe, Co, Ni ) at temperature between 500 and 1200oC. These methods were used for a long time for the synthesis of carbon fibres and nano fibers [73-78] but there were no indications that it could also be used for the synthesis of carbon nanotubes until Yacaman et all. [79] reported this method the first time for the production of nanotubes.The CVD method deposit hydrocarbon molecules on top of heated catalyst material and the metal catalysts dissociate the hydrocarbon molecules and then CNTs will form in the reaction chamber. CVD apparatus consist of several basic components which include:

• Gas delivery system – For the supply of precursors to the reactor chamber

• Reactor chamber – Chamber within which deposition takes place

• Substrate loading mechanism – A system for introducing and removing substrates, mandrels etc.

• Energy source – Provide the energy/heat that is required to get the precursors to react/decompose.

• Vacuum system – A system for removal of all other gaseous species other than those required for the reaction/deposition.

• Exhaust system – System for removal of volatile by-products from the reaction chamber.

• Exhaust treatment systems – In some instances, exhaust gases may not be suitable for release into the atmosphere and may require treatment or conversion to safe/harmless compounds.

• Process control equipment – Gauges, controls etc to monitor process parameters such as pressure, temperature and time. Alarms and safety devices would also be included in this category. Figure 8 shows the basic apparatus of the CVD process.

Figure 8: Schematic Diagram of a Classical Reactor (Adapted from Popov [71])

The CVD method produces both single-wall and multi-wall nanotubes and the process uses hydrocarbons as the carbon source. The hydrocarbon flows through the quartz tube with inert gas at a specified temperature [80]. The energy source is used to crack the molecule into reactive atomic carbon. Then, the carbon diffuses towards the substrate which is heated and coated with catalyst (usually first row transition metals where it will bind). Carbon nanotubes will be formed if proper parameters like excellent alignment, appropriate catalyst, flow rate and temperature are maintained. The appropriate metal catalyst can also be used for preferential growth of any type of nanotubes. Various techniques developed under CVD includes: Vapour phase growth, Method using substrate catalyst, Plasma enhanced chemical vapour deposition and Fluidized-bed CVD method etc [61].


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Vapor phase growth Most synthesis methods of carbon nanotubes are carried out by deposition of catalytic metals on a substrate using conventional gas, such as C2H2, CH4, C2H4, C2H6. However, vapor phase growth is a synthesis method of carbon nanotubes, directly supplying reaction gas and catalytic metal in the chamber without a substrate [81]. Vapour phase growth is one of the synthesis methods which have been used to produce carbon nanotubes [75,76,82] and vapour growing carbon fibre [62] directly by supply reaction gas and catalytic metal in the chamber without a substrate. This method is otherwise called floating catalyst chemical vapour deposition simply because the catalyst is introduced in the flowing gas stream e.g. in form of volatile organometallic molecules like ferrocene nickelocene. This method has been suggested as a good method for mass production of carbon nanostructure materials and fibre [83] as shown in Fig. 9.

Figure 9: Schematic diagram of vapor phase growth apparatus (Adapted from www.iljinnanotech.co.kr, [83]).

The flow controller for the gas flow rate is placed in one corner and a ceramic boat for catalytic metal powder in the chamber. Two furnaces are placed in the reaction chamber. In the first furnace, vaporization of catalytic carbon is maintained at a relatively low temperature while higher temperature is maintained in the second furnace where the synthesis occurs. The hydrocarbon gas in the first furnace will not decompose, while the low temperature at this furnace will be supplied to vaporize the catalysis. During the vaporization of the catalyst, fine catalytic particles will form in the low temperature furnace and migrate to the high temperature compartment. When these particles reach the second furnace, the liberated carbon atoms are absorbed and diffused on the surface of the catalytic metal particles [84,85] and the carbon nanotubes will form in the reaction chamber. Plasma enhanced chemical vapor deposition Plasma enhanced chemical vapour deposition (PE-CVD) is another methods that operates by generation of a glow discharge in a reaction furnace by a high frequency voltage applied to both electrodes. Figure 10 shows a schematic diagram of a typical plasma CVD apparatus with a parallel plate electrode structure. A substrate is placed on the grounded electrode. In order to form a uniform film, the reaction gas is supplied from the opposite plate. Catalytic metal, such as Fe, Ni, and Co are used on a Si, SiO2, or glass substrate using

thermal CVD or sputtering. The deposited metal on the substrate can be etched using H2 gas. After nanoscopic fine metal particles are formed, carbon nanotubes will be grown on the metal particles on the substrate by glow discharge generated from high frequency power. A carbon containing reaction gas, such as C2H2, CH4, C2H4, C2H6, CO is supplied to the chamber during the discharge [86]. Ni seems to be the most suitable pure-metal catalyst for the growth of aligned MWCNTs [55]. The diameter of the nanotubes produced is approximately 15 nm. The highest yield of carbon nanotubes achieved was about 50% and was obtained at a relatively low temperature (below 330oC).

Figure 10: Schematic Diagram of a Plasma Enhance CVD: (Adapted from www.iljinanotech.co.kr, [83]).

Fluidized-bed CVD In order to avoid obstruction of the carbon materials deposited and damage to the fixed bed reactor walls, the use of the fluidized bed reactor has been proposed for large scale production of CNTs. But this kind of reactor has not been reported on carbon nanotube formation until 2001 and 2002 when Liu et al. [87] and Wang et al. [88] respectively reported the fluidized bed synthesis of MWCNTs using bulk catalyst. Leu et al. [87] developed a technique to synthesize CNTs by decomposing acetylene over a pre-reduced LaCoO3 catalyst in a fluidized bed. Typically, catalysts such as Fe, Co, or Ni are directly used for the synthesis of CNTs. However, Leu et al. [87] reported that a pre-reduced LaCoO3 is an effective catalyst to produce CNTs with a narrow diameter distribution [89]. However, the results 96% purity CNTs can not be substantiated under fluidized conditions. Corrias et al. [90] properly investigated the production of MWCNTs by fluidized bed CVD using an iron supported catalyst (2.5% Fe/Al2O3 w/w). A fluidized bed of 5.3 cm in diameter and 1 m in height was used for this study. The column also had a 10 cm diameter expansion zone to reduce particle entrainment. A 1.2% porosity stainless steel perforated plate evenly distributed the feed gas


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consisting of ethylene, hydrogen and nitrogen. The minimum fluidization velocity of the 100 µm Al2O3 support particles was found to be 0.14 cm/s at the operating temperature of 650oC. The procedure used by Corrias et. al. [90] is as follows: first, a known mass of catalyst particles was loaded into the column and the bed was fluidized with a flow of nitrogen and hydrogen. The entire mixture was then heated at the same time. The moment the required temperature was reached and stabilized, the nitrogen flow rate was reduced and ethylene is simultaneously introduced into the reactor. Corrias et al. [90] ran the experiments for time spans ranging from 10 to 120 minutes at various ratios of the initial fluidization velocity Uo/Umf (where Uo is the operating fluidization velocity and Umf is the minimum fluidization velocity corresponding to the support particles). It must be noted that the Uo/Umf ratio is referred to as the initial fluidization ratio because Umf of the bed of particles will change with time as the nanotubes grow. The two fluidization ratios used were Uo/Umf = 7.14 and Uo/Umf = 12.3. Corrias et al. [90] reported high carbon yields greater than 75% with the 120 minutes runs. Additionally, a carbon nanotubes deposition rate of up to 0.3 g/min was obtained. The application of fluidized CVD was also reported by Nig et all [91] who synthesized MWCNTs for hydrogen storage. They had a very good yield of MWCNTs with high purity.

Figure 11: Schematic Diagram of Fluidized Chemical Vapor Deposition Adapted from Mauron Ph. [49].

POLYMER COMPOSITE

One of the main important area of application of CNTs is in the polymer composite formation due to their ability to impact their intrinsic properties to the polymer matrices. It has been shown that CNTs could behave as the ultimate one-dimensional material with remarkable mechanical properties [92]. Therefore, CNTs, (single and multi-walled) have been receiving intensive study

as property enhancers in various matrices, especially polymers [93, 94]. Currently, attention has shifted towards fabrication of nanocomposites with the use of carbon nanotubes (CNTs) as reinforcing agent in polymer materials or conventional composite, to harness the exceptional intrinsic properties such as large Young`s Modulus and flexural strength of CNTs [92]. However, the broad diversity of molecular structures in conventional polymers that makes them nonsustainable in some applications especially in the area where material of a very high strength and large Young`s modulus is required lead to combination of materials to give the desired properties. Polymer composite therefore, is any plastic that contains any of the reinforcing elements or fillers to give the required properties. The conventional Polymer composite includes: Polymer clay composite, polymer fibre composite and polymer carbon composite. The idea of carrying on manipulation at smaller scales gave birth to nanocomposites which are those matrices reinforced with nanostructured materials. Polymer fiber nanocomposite Composite of polymers reinforced with natural fibres have received attention and there are wealth of available literatures in this area. Natural fibre such as sisal, flax, jute and wood fibres posses good reinforcing capability when properly compounded with polymers [95, 97]. These natural fibres reinforced composites find a wide array of applications in the building and construction and automobile industry. Extensive works have been done on the production and application of polymer fibres composite: Mohammed et. al. [96] studied the use of empty palm fruit bunches (EFB) as a filler element in fibre polypropylene composite. Their work revealed that the flexural and impact properties of the produced composite were enhanced by adding maleated polypropylene. In another work, Mohamed et. al. [96] observed that rice husk (fiber) increased flexural modulus but decreased the flexural and impact strength of polypropylene composite. Also life cycle assessment of wood-fibre-reinforced polypropylene-composite and its environment impact assessment were studied by Xunxu et. al. [98]. The study showed that the life cycle assessment of the locally manufactured composite in New Zealand compared favorably with that imported from Australia. Moreover, the results showed that in the composite, the polypropylene content is the dominant factor on environmental damages and not the fibre. The properties of plant fiber composites depend strongly on the type of fiber and also on the type of matrix, the fiber-matrix combination and the manufacturing process [100]. Table 3 shows the mechanical properties of some

Gas mixture of 42 sccm C2H2 & 468 sccm Ar


11

natural fibres and the graph (Figure 12) compares the stress-strain relation of deferent type of plant fibre materials in a bending test. The differences in the deformation behavior of the materials are significant [99].

Table 3: Mechanical Properties of Some Natural Fibres Source: Joseph [99]

Fiber Tensile Strength (MPa)

Elongation at Break

(%)

Young’s Modulus

(MPa) Cotton Wool Silk Flax Jute Sisal

Ramine

264-654 20-174 252-528 300-900 342-672 444-552 348-816

3.0-7.0 25.0-35.0 20.0-25.0

2.7-3.2 1.7-1.8 2.0-2.5 3.6-3.8

4,980-10,920 2,340-3,420

7,320-11,220 24,000 43,800

- 53,400

Figure 12: Stress-Strain Diagrams of Different Plant Fiber Materials (Thomas G.S) [101] Polymer clay nanocomposite Polymer – clay nanocomposites are among the most successful nanotechnological materials today because clay is environmentally friendly and can simultaneously improve material properties without significant tradeoffs. Recent efforts have focused upon polymer-layered silica nanocomposites and other polymer clay composite [9]. These materials have improved mechanical properties without the large loading required by traditional particulate fillers [102]. Increased mechanical stability in polymer-clay nanocomposite also contributes to an increased heat deflection temperature. Traditional polymer composites often have a marked reduction in optical clarity; however, nanoparticles cause little scattering in the optical spectrum and very little ultra violet scattering. Although flame retardant additives to polymer typically reduce

the latter mechanical properties and hence such polymer-clay nanocomposites have enhanced barrier and mechanical properties and less flammable [103]. Organically modified layered silicates have been widely studied for the past decade as property enhancers for polymeric materials [104]. Various studies have reported improvements in mechanical [105-107], thermal [108,109], flammability [108,109], and barrier [110,111] properties of thermoplastic by addition of organically modified layered silicates to polymer matrices. Fakhru’l-Razi et. al. [112] also reported that polystyrene-clay nanocomposite produced in their work have higher dynamic modulus than pure polystyrene. This claim on polymer-clay nanocomposite was further supported by Chen [110] who reported that a two-fold increase in the tensile strength and a three-fold increase in the elongation were found for 1% BZD-montmorillonite /polyurethane as compared to that of pure polyurethane. But both 1% RCOOH-montmorillonite/polyurethane and 1% BZD-montmorillonite/polyurethane exhibited lower water absorption properties than polyurethane [110]. On a general note, modified organoclay have the potential of improving the mechanical properties of the matrix polymer substantially even with a low level of filler loading in the range 1-5 wt% [99]. Polymer nanocomposite In recent years, polymer carbon nanocomposites have been the subject of highly intensive research due to the failure of inorganic and fibre fillers in the areas where materials of a very high mechanical properties (Young`s modulus, tensile strength, flexural strength and flexural modulus) are required. Polymer nanocomposites are now becoming commercially important with interest growing rapidly on carbon nanotubes (CNTs) and carbon nanofibres (CNFs) as reinforcing elements [100]. However, CNTs attracts more attention for this application because of its unique mechanical, electronic and magnetic properties and these have caused them to be widely studied [113,114]. They are probably the strongest substances that ever exist with a tensile strength greater than steel, but only one sixth the weight of steel [115]. This fantastic property of mechanical strength allows this material to be used as filler for reinforcement in both micro and nano scale. Carbon polymer nanocomposite is now receiving a global attention in its application and production. Effect of multi-wall CNTs on the mechanical properties of natural rubber was investigated [112] and the results showed that, by increasing the amount of CNTs in the rubber matrix, the ductility of the composite decreased and become stronger and tougher but at the same time more brittle. These results were further substantiated by


12

Yoong et. al. [116] who reported that mechanical properties and thermal conductivity of the fabricated rubber composite increases with addition of CNTs or Boron Nitride but the tensile strength at break of the added nanotube is twice as high as those of neat rubber sheet and boron added rubber. This is because the nano sized effect of fibrous carbon (approximately 100nm), indicating highly increased interface area between filler and rubber when compared to boron nitride [113]. A similar result was reported on mechanical properties [117-119] of polypropylene carbon nanocomposites. Ternary Polymer Nanocomposite Most conventional composites are binary composite because the entire matrix is made up of two materials. Classical examples are polymer-fibre nanocomposite and polymer clay nanocomposite. Polymer composites reinforced with particulates, fibres and inorganic fillers have been widely studied [110-112,118] and a lot of drawbacks are reported such as brittleness, reduction in elongation etc. However, recent advances in polymer nanocomposites have inspired effort to form a composite that will combine more than one filler (ternary composite) in order to address the problems of loosing one property to gain the another. For example, both thermal and mechanical properties were enhanced by filling of the clay and CNTs in polymer layered silicate carbon nanotube nanocomposite [119]. Similarly, a synergetic effect was also observed when organomodified clay and CNTs were added simultaneously: the thermal and flame retardant properties of ethylene-vinyl acetate matrices were enhanced without any effect on the mechanical properties of the composite.

Applications of polymer nanocomposite However, due to the anticorrosion effect and high mechanical properties combined with lighter construction weight of polymer composite, for example polypropylene nanocomposite, they have found applications in different areas as shown in table 4.

Table 4: Applications of Polypropylene Composite

Daniel et. al. [120] Industry Applications

Electrical/ Electronics

Insulation, Support, boxes

Transport Road: Dash board, Furniture and Body Components. Rail: Wagons and Doors

Aviation Radomes, Helicopter blade, Brake disk, and furnitures

Biomedical Artificial organs, Bone implantation, density and drugs delivery

Biotechnology Reactor components e.g vessels, paddles, impeller and pipes

Packaging Plastic bottles, kegs etc

Manufacturing process There are basically three methods of production in polymer composite formation. Other methods either combine two or the whole methods together to achieve the required products. The three methods include: In-situ Polymerization Method, Solvent Blending Method, and Melt Mixing Method. In-situ intercalation polymerization In-situ intercalative polymerization was the first method used to synthesize polymer-clay nanocomposites [121]. This method is a promising approach for a more homogeneous distribution due to the close contact of polymer and filler during synthesis [122]. It often gives better filler dispersion, especially at higher filler contents than melt compounding [123]. Currently, it is the conventional process used to synthesize thermoset clay nanocomposites. In case of thermosets such as epoxies or unsaturated polyesters, a curing agent or a peroxide is added to initiate the polymerization but in case of thermoplastic like polypropylene, the polymerization can be initiated either by addition of curing agent or by increasing the temperature [124]. The first step is swelling of the organoclay in the monomer matrix. This stage requires a certain amount of time, which depends on the polarity of the monomer molecules, the surface treatment of the organoclay, and the swelling temperature. This is followed by reaction initiation depending on the type of matrix. During the swelling phase, the high surface energy of the clay attracts polar monomer molecules so that they diffuse between the clay layers. When certain equilibrium is reached, the diffusion stop and the clay swell in the monomer to a certain extent corresponding to a perpendicular orientation of the alkylamonium ions [124]. The alkylammonium ions adopt a perpendicular orientation in order to optimize solvation interactions with the monomer. The polymerization reaction leads to delamination of the clay. The driving force of the in-situ polymerization method is linked to the polarity of the monomer molecules. When the polymerization is initiated, the monomer reacts with the curing agent. This reaction lowers the overall polarity of the intercalated molecules and displaces the thermodynamic equilibrium so that more polar molecules are driven between the


13

Monomer

Nanomatirial

Swelling

Curing Agent

Polymerization

Nanocomposite

Solvent Evaporation

Nanocomposite

Nonmaterial

Solvent

Swelling Intercalation

Polymer Solution

clay layers. As this mechanism occurs, the organic molecule can eventually delaminate the clay. Polymer-clay nanocomposites based on polyurethane [125] and polyethylene terephthalate [126] have been synthesized by this method. Similarly, the same method has also been used for the preparation of polystyrene based nanocomposites [127]. The resulting composite were isolated by precipitation of the colloidal suspension in methanol, filtered off, and dried. In this way intercalated polystyrene /montmorillonite nanocomposites were produced. The extent of intercalation depends completely upon the nature of solvent used. Although, the polystyrene is well intercalated, but the only drawback in this procedure is that the micromolecule produced is not pure polystyrene, rather a copolymer between styrene and vinylbenzyltrimethl-ammonium cations.

Figure 13: Process Flow for Insitu Polymerization Method

Solvent casting method The procedure in this method is similar to the in-situ polymerization method described above except that solvent is used here as a medium of swelling for the organoclay. Different steps involved are shown in figure 14. The process involves dispersion of organoclay in an organic solvent like toluene or N-dimethylformamide. Alkylamonium treated clays swell considerably in polar organic solvents, forming gel structures. Then the polymer dissolved in the respective solvent, will be added to the clay solution and intercalates between the clay layers. The last step consists of the removal of the solvent by evaporation usually under vacuum. This method has received great attention from various researchers. Choi [128] prepared

Figure 14: Process Flow Chart for Solvent Casting Method

PEO/MMT nanocomposites by a solvent casting method using chloroform as a co-solvent. X-ray diffraction analysis and TEM observations established the intercalated structure of these nanocomposites. Other researchers that used similar method include Hyun [129] and Lim [130]. The major advantage of this method is that it offers the possibilities to synthesize intercalated nanocomposites based on polymers with low or even no polarity. However, this method is difficult to apply in industry due to the problems associated with the use of large quantity of solvents.

Melt mixing or blend intercalation method

This method has become the most industrially feasible and versatile technique of polymer clay nanocomposites productions due to its compatibility with current industrial processes, such as extrusion and injection molding, the absence of organic solvents and its suitability with most commodity polymers [131].The process involves disorbtion of a relatively large number of solvent molecules from the host to accommodate the incoming polymer chains. The desorbed solvent molecules gain one translational degree of freedom, and


14

the resulting entropy gain compensates for the decrease in conformational entropy of the confined polymer chains. The major advantages of blend intercalation method over solvent intercalation are their specificity for the polymer leading to a new hybrid that was previously inaccessible and that the absence of solvent makes it an environmentally-friendly and hence economically favorable for industries. This process is otherwise called melt mixing. The flow chart is shown in Fig. 15. As shown in Figure 15, the polymer chains experience a dramatic loss of conformational entropy during the intercalation. The proposed driving force for this mechanism is the important enthalpy contribution of the polymer/organoclay interactions during the blending and annealing steps. Polystyrene was the first polymer used for the preparation of nanocomposites using the melt intercalation technique with alkylamonium cation modified montmorillonite [132]. Vassiliou [132] also employed this method in nanocomposite of isotactic polypropylene with carbon nanoparticle. Other researchers who have used this method include Kim [133] and Fornes [106].

Figure 15: Flow Chart for Melt Intercalation Method

GENERAL CONCLUSIONS AND REMARKS Judging from all the established facts reviewed in this study the following remarks are made: Carbon Nanotubes Synthesis Methods: It is obvious that all the above synthesis methods have both advantages and disadvantages but the issue is to select the best method that is suitable for commercial purpose.

Arc-discharge method is very simple and inexpensive with a yield less than 75% maximum [65]. However, the products (crude) always required more expensive purification technique and the equipment can not be scaled up due to a very high operating temperature. Laser ablation produces relatively high purity CNTs with a yield less than 75% at a lower operating temperature. Apart from scale up problems which is paramount to arc-discharge and laser ablation methods, the latter can not be used in the production of MWCNTs and moreover little purification is also necessary. In conclusion, since large scale production of CNTs is required, most industries currently prefer CVD method with preference for fluidize bed technique due to its simplicity, low cost, low temperature, high purity, high yield (>75%) and possibility of scale up.

Effect of Filler Elements: Generally, shortenings organic and inorganic filler elements in polymer composites shows that improvement on composite materials using natural fibres or clay usually require high level of fillings sometimes up to 50% and with this, difficulty exist to get the homogenous dispersion of the filler in the polymer matrices. Most often this limits the performance of the recovered composite materials [96]. However, CNTs offer the best enhancement in the polymer nanocomposite but the current production technique is rather too expensive. In the light of the above stated facts, it is concluded that CNTs should be used as a secondary filler element to reduce all the shortcomings found in the conventional composites.

Nanocomposite Manufacturing Processes: Three methods reviewed in this study include Insitu Intercalation Method, Solvent Mixing Method and Melt Mixing Method. Though the Insitu intercalation method gives better filler dispersion [123] and allows close contact of polymer and the filler during synthesis [122], it involves chemical reaction which requires a curing agent as initiator and solvent, and at the end, the micromolecule produced is not pure polymer, rather a copolymer is formed which in turn reduces the potential of the polymer composite. Solvent mixing is quite good as it does not require a high temperature. However, this method is difficult to apply in industry due to the problems associated with the use of large quantity of solvents. Melt mixing method has become the most industrially feasible and versatile technique of polymer clay nanocomposites synthesis due to its compatibility with current industrial processes, absence of organic solvent that makes it to be environmental friendly and its suitability with most commodity polymers [131].It is

Polymer

Melt mixing

Annealing

Nanocomposite

Nanomaterial


15

therefore recommended for use in the preparation of nanocomposites.

In conclusion, Carbon nanotubes are recommended for use as filler element in polymer nanocomposite in order to address all the aforementioned drawbacks found in conventional binary composites. ACKNOWLEDGEMENT

The author is grateful to Research Management Center, International Islamic University Malaysia for funding this project. REFERENCES

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the potential of carbon nanotubes production as …...however, synthesis methods of both carbon...

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