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2016 Nanocarbon Foam/Polymer Composite Muhamad Shahrizan Jamal
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FLORIDA STATE UNIVERSITY
COLLEGE OF ENGINEERING
NANOCARBON FOAM/POLYMER COMPOSITE
By
MUHAMAD SHAHRIZAN JAMAL
A Thesis submitted to the Department of Industrial and Manufacturing Engineering in partial fulfillment of the requirements for the degree of Master of Science
2016
Muhamad Shahrizan Jamal defended this thesis on June 7, 2016.
The members of the supervisory committee were:
Mei Zhang
Professor Directing Thesis
Okenwa Okoli
Committee Member
Abhishek Shrivastava
Committee Member
The Graduate School has verified and approved the above-named committee members, and certifies that the thesis has been approved in accordance with university requirements.
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ACKNOWLEDGMENTS
I would first like to thank my thesis advisor Dr. Mei Zhang for the continuous support of my graduate study. The door to her office was always open whenever I had a question about my research. Her guidance helped me in all the time of research and writing of this thesis
Next, I would to thank my thesis committee members: Dr. Okenwa Okoli and Dr.
Abshishek Shrivastava, for their astute comments and suggestions in helping me complete my research. Their suggestions helped me to gain a different perspective of my research.
I thank my fellow coworkers at the High-Performance Materials Institute for the exciting discussions, continuous help and fun that we have had working together in the past two years.
Last and most importantly, I would like to thank my family and friends for their moral support throughout my graduate studies and my life in general.
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TABLE OF CONTENTS
List of Tables ...... v List of Figures ...... vi List of Symbols ...... ix Abstract ...... x
1. INTRODUCTION ...... 1
2. PROBLEM STATEMENT AND RESEARCH OBJECTIVES ...... 4
2.1 Problem Statement ...... 4 2.2 Research Objectives ...... 5
3. LITERATURE REVIEW...... 7
3.1 Carbon Foam ...... 7 3.2 Composite Foam ...... 13 3.3 Summary ...... 17
4. FABRICATION PROCESS ...... 18
4.1 Nanocarbon Foam ...... 18 4.2 Nanocarbon Foam/Polymer Composite ...... 20
5. RESULTS AND DISCUSSIONS ...... 24
5.1 Nanocarbon Foam ...... 24 5.2 Nanocarbon Foam/PMMA Composite ...... 30 5.3 Nanocarbon Foam/SEBS Composite ...... 46
6. CONCLUSION ...... 51
References ...... 52 Biographical Sketch ...... 57
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LIST OF TABLES
Table 1: Summary of reported strength by various authors in literature ...... 17
Table 2: Resistance of composite foams with varying density ...... 44
Table 3: Resistivity of composite foams with varying density ...... 45
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LIST OF FIGURES Figure 1: Traditional honeycomb structure ...... 1
Figure 2: Graphical illustration on the difference between open cell foam and closed cell foam .. 2
Figure 3: Ashby plot of strength against density of various materials available to date. The unattainable material space is defined by Hashin bound. Adapted from Youssef.[15] ...... 4
Figure 4: Stress-strain curves and Transmission Electron Microscopy (TEM)images for foams made by Bradford et.al with varying post growth treatment time [18] ...... 8
Figure 5: Stress strain curve of multi cycle compression and the SEM image of the ultra- lightweight foam by Sun et.al [21] ...... 9
Figure 6: (a – c)SEM images of foam fabricated using dichlorobenzene supply rate of 0.1 mL/min, 0.3 mL/min and 0.7 mL/min. (d) Different types of welded junction within the foam [22] ...... 10
Figure 7: The aerogel being compressed up to 90% strain and the resulting stress strain curve [25] ...... 12
Figure 8: Graphical illustration of the infiltration process in foam composite fabrication ...... 14
Figure 9: PAN oxidation process during the heat treatment at 300oC in air...... 18
Figure 10: Graphical summary of the nanocarbon foam fabrication process ...... 19
Figure 11: Graphical summary of the composite foam fabrication process ...... 21
Figure 12: Foam infiltration process ...... 22
Figure 13: Rotational drying to ensure uniformity in polymer infiltration ...... 23
Figure 14: A) Illustration on how the CNTs are held together at the joint by PAN in the nanocarbon foam and how polymer coats the CNTs in the composite foam B) Image of two nanocarbon foam stacked on top of one another ...... 23
Figure 15: SEM images at different magnification level showing the structure of the nanocarbon foam. The magnification levels are: A) X350 B) X2000 C) X5000 D) X10000 ...... 24
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Figure 16: Stress-strain curve of the nanocarbon foam with a density of 26.42 mg/cm3 at different strain ...... 26
Figure 17: The strength of the nanocarbon foam as compared to other materials ...... 26
Figure 18: The fatigue test of the nanocarbon foam after multiple consecutive compression to 70% strain...... 27
Figure 19: The heating and cooling between room temperature and 130°C of a curved nanocarbon foam to demonstrate its heat transfer effectiveness...... 28
Figure 20: Temperature change of nanocarbon foams with varying thickness when placed on surface heated to 200°C ...... 29
Figure 21: SEM images showing the structure of the foam before and after polymer infiltration 30
Figure 22: SEM images showing polymer concentration at different location within the composite foam ...... 31
Figure 23 : Stress-strain curve for composite foam made from 1 wt% polymer concentration which has a density of 41 mg/cm3 ...... 32
Figure 24 : Stress-strain curve for composite foam made from 3 wt% polymer concentration which has a density of 60 mg/cm3 ...... 33
Figure 25: Stress-strain curve for composite foam made from 5 wt% polymer concentration which has a density of 88 mg/cm3 ...... 33
Figure 26: Stress-strain curve for composite foam made from 10 wt% polymer concentration which has a density of 118 mg/cm3 ...... 34
Figure 27: Stress-strain curve for composite foam made from 15 wt% polymer concentration which has a density of 206 mg/cm3 ...... 34
Figure 28: Stress-strain curve for composite foam made from 20 wt% polymer concentration which has a density of 332 mg/cm3 ...... 35 Figure 29: Strength at 50% strain versus density plot of multiple replications of the composite foam at different density ...... 36
Figure 30: Strength of nanocarbon foam and composite foams at varying density and strain. .... 36
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Figure 31: Specific strength at 50% strain versus density plot for the composite foam ...... 38
Figure 32: Specific strength at 70% strain versus density plot for the composite foam ...... 38
Figure 33: Strength at 50% strain versus density for composite samples against other available materials ...... 39
Figure 34: DSC analysis for PMMA polymer to determine its glass transition temperature ...... 40
Figure 35: TGA analysis of PMMA polymer to determine its decomposition temperature ...... 40
Figure 36: Rigidity of the composite foam at different temperature. At room temperature, the composite foam is able to withstand the mass of 900 g while at 30°C but undergoes recoverable deformation at temperature beyond the Tg of PMMA...... 41
Figure 37: Stress-strain curve for compression tests on composite foam made from 5 wt% polymer concentration at 140°C and nanocarbon foam at room temperature and at 140°C ...... 42
Figure 38: Compression test on composite foam with density of 332mg/cm3 after being healed with the use of heat ...... 42
Figure 39 : Graphical illustration of electrical testing ...... 43
Figure 40 : Conductivity of nanocarbon foams and composite foams at varying density ...... 46
Figure 41: Stress-strain curve for composite foam made from 20 wt% SEBS concentration which has a density of 172 mg/cm3 ...... 47
Figure 42: Strength at 50% strain versus density plot of multiple replications of the SEBS composite foam at different density ...... 48
Figure 43: Stress-strain curve of SEBS polymer which has a density of 1231 mg/cm3 ...... 49
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LIST OF SYMBOLS
σ Stress/Strength F Force A Area ε Strain L Length
Lo Initial Length ρ Density m Mass V Volume SS Specific Strength R Resistance r Resistivity κ Conductivity Ec Elastic Modulus of Composite Ef Elastic Modulus of Fiber Em Elastic Modulus of Matrix f Fiber Volume Fraction
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ABSTRACT
Weight of a material and the system formed by such material is a critical factor for many applications. Traditionally, engineering designed porous structures, typically honeycomb structures, have been utilized for weight critical applications. The goal of this thesis work is to utilize the material with the lightest weight to fabricate a new type of foam that is not only lightweight and strong, but also electrically as well as thermally conductive and with tunable elasticity. A carbon nanotube (CNT) based nanocarbon foam was fabricated by using poly
(methyl methacrylate) spheres as a template to create engineered pores. The junctions between the CNTs are secured using nanocarbon via the oxidation and carbonization of polyacrylonitrile.
The resulting low density foam exhibits robustness in structure, high elasticity, thermal stability, corrosion resistance and is also electrically as well as thermally conductive. The strength of the foam is further boosted with the infiltration of PMMA polymer. The resulting composite foam is still porous and has higher mechanical strength. The electrical conductivity of the composite foam is not affected despite the presence of PMMA.
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CHAPTER 1
INTRODUCTION
The increasing demand for lightweight and strong material especially for applications in which weight is a critical factor such as aerospace and automotive has resulted in the invention of various new materials and structures. One of those inventions is the honeycomb structure which is made up of parallel and prismatic cells.[1] It has been traditionally used in the sandwich panel construction due to its lightweight and high strength.[2] As recent as last year, NASA was seeking a new material to replace the traditional honeycomb structure (shown in Figure 1) for its future missions.[3] One of the structures that has attracted interest and has great potential to replace the honeycomb structure is the porous aerogel or foam structure.
Figure 1: Traditional honeycomb structure
Solid foam is a material which has gaseous voids that is enclosed by matrix with a higher density. Foam can be classified into two types which are rigid and flexible depending on its
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structure and morphology.[4] Besides that, foam can also be categorized as open cell or closed cell. Closed cell foam has pores which are completely surrounded by the walls which the foam is made of resulting in no gas or liquid transport from one cell to another. On the contrary, the walls of open cell foams are incomplete. These differences are illustrated in graphical form in
Figure 2. In general, all foams exhibit attributes such as lightweight, highly porous and large surface area. Some are also good thermal and/or electrical conductor. Due to this, foam not only fits the bill for various aviation and automotive applications but it is also suitable for many industrial applications such as packaging, buoyancy, thermal insulation, acoustic attenuation, vibration damping, absorbent and energy storage.[5, 6] Investigators have developed various foams from different kind of materials such as metal, polymer and ceramic. [4, 7, 8] On the other hand, nano foam has also been fabricated by the means of graphene and carbon nanotubes
(CNT).[5, 9]
Figure 2: Graphical illustration on the difference between open cell foam and closed cell foam
CNT, which is one of the allotropes of carbon, has been subjected to an extensive research ever since it was re-discovered by IIjima in 1991.[10] This is due to its superior mechanical, physical, thermal and electrical properties resulting from its symmetrical structure.[11] The mechanical properties of CNT, specifically the strength and stiffness are expected to be higher than that of an ideal carbon fiber, which has the impeccable orientation of
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a perfect graphene layers in the direction of the fiber axis.[12] CNT can be divided into two types; single wall carbon nanotube (SWCNT) and multiwalled carbon nanotube (MWCNT). The tensile strength of SWCNT and MWCNT has been reported to be around 50 GPa to 500 GPa and
10 to 60 GPa respectively while the modulus is in the region of 1.5 TPa for SWCNT and 1 TPa for MWCNT.[13] CNT is highly anisotropic which means the properties are directionally dependent.[14] It is strong in its axial direction due to the carbon-carbon double bond (C=C) and weaker in the radial direction. Besides having excellent mechanical properties, CNT is also lightweight which makes it desirable for lightweight application.
In this work, CNT based nanocarbon foam has been fabricated by utilizing poly (methyl methacrylate) (PMMA) microspheres as template to create the micro-scale pores.
Polyacrylonitrile (PAN) is used to hold the CNTs together by forming crosslinks between the tubes. The nanocarbon foam is lightweight, conductive, highly porous and also exceptionally elastic. To further improve the compressive strength of the nanocarbon foam without increasing the weight considerably, the walls of the foam was reinforced by the means of thermoplastic polymer infiltration while keeping the porous structure of the nanocarbon foam intact. In this work, PMMA was used for this purpose due to its high strength and low density. The resulting nanocarbon foam/polymer composite exhibits improved mechanical performance while being lightweight and conductive.
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CHAPTER 2
PROBLEM STATEMENT AND RESEARCH OBJECTIVES
2.1. Problem Statement
Throughout the years, various materials have been developed ranging from low density to high density spanning a wide range of strength. These materials density and strength properties can be summarized in the Ashby Plot[15, 16] shown in Figure 3.
Room For Improvement
Figure 3: Ashby plot of strength against density of various materials available to date. The unattainable material space is defined by Hashin bound. Adapted from Youssef.[15]
The continuous and persistent yearning for a better material that eclipses the existing materials in terms or properties and performance to fulfill the demand of various industries such as aerospace and automotive has result in an extensive research to develop the next high performance material. Various investigations have been carried out by researchers all over the
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world to develop a material that fulfills the requirement of having a high strength while keeping the weight low. However, as can be seen in the Ashby Plot, there still exist a large gap in literature, especially in the region of lower densities (yellow region) which is still left undiscovered today.
2.2. Research Objectives
The goal of this research is to explore a three-dimensional material that can be mass produced cheaply and exhibits the following characteristics:
1. Light weight
2. High specific strength
3. Electrically conductive
The definition of lightweight varies from literature to literature. To avoid ambiguities, in this literature, lightweight is defined as having a density of less than 400 mg/cm3. In order to achieve this, a porous structure fabricated using CNT is proposed. Porous structure is able to retain the properties of a solid structure but at the same time being lightweight and cheaper due to the less amount of materials used. In addition to that, utilizing CNT also help this cause.
As discussed earlier CNT has exceptional properties. Besides being light weight, CNT also has excellent mechanical properties and is also a good electrical conductor. To further boost the strength of the foam, polymers will be used as reinforcement to the CNT based foam. The addition of polymers will not disrupt the already present network of CNT but instead will compliment it by making it more rigid. As a result, the conductivity of the foam will be preserved. In addition to this, the materials are also readily available in the current market.
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The objectives of this work are to:
1. Fabricate a light weight, strong, conductive and porous nanocarbon foam/polymer
composite.
2. Study the effect of polymer concentration in the composite on the mechanical
performance of the composite foam.
3. Understand the structure-property relationship of the nanocarbon foam/polymer
composite.
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CHAPTER 3
LITERATURE REVIEW
3.1. Carbon Foam
Large number of publications has been reported in the fabrication of foam or aerogel structures using different materials such as metal, polymer and ceramics. A specific portion of this massive knowledge base, which is carbon based foam, will be reviewed in this section.
Numerous methods and techniques have been employed to fabricate different carbon foam with varying properties. Here, the different fabrication techniques and the resulting foams’ structure and properties will be reviewed and summarized.
A team of researchers led by Worsley fabricated a low density, stiff and conductive CNT foam. The foam was made by using carbon based nanoparticles to lock the single walled CNT together. Organic sol-gel was used to incur polymerization between the junctions of CNT. These organic binders are then converted to carbon through pyrolysis. The reported density of the foam varies from 10 mg/cm3 to 100 mg/cm3 depending on the amount of materials used. The author reports the stiffness modulus of the material ranging from 1 MPa to 10 MPa. However, the strength of the material was not reported in this literature. The foam exhibits elastic behavior up to 80% strain.[17]
Bradford et.al transformed a CNT arrays into a strong and flexible CNT foam via post growth chemical vapor deposition (CVD). The fabrication of the CNT arrays were made via nano-particle catalyzed synthesis on a silicon wafer using Al2O3 as the buffer and thick iron as the catalyst in a tube furnace. The temperature was ramped and kept at 750oC for 25 minutes while the flow of the growth gases; argon (Ar), hydrogen (H2) and ethylene (C2H4); were 7
regulated.
Then, the growth gases were replaced with just ethylene for 30 seconds. This step increases the buildup of amorphous carbon which in turn inhibits the CNT growth within 30 seconds. The growth gases flow was resumed to ensure the CNT growth is stopped and the furnace was cooled to room temperature under argon environment. Post CVD treatment was carried out at different time span and the resulting foam exhibits flexibility and also high strength. Depending on the post growth treatment time, the resulting density and strength varies.
A foam with post growth time of 155 minutes has a density of 179 mg/cm3. The strength at a compression strain of 90% was reported to be 4.3 MPa.[18]
Figure 4: Stress-strain curves and Transmission Electron Microscopy (TEM)images for foams made by Bradford et.al with varying post growth treatment time [18]
CNT foam with tunable size and shape were fabricated by Liu et.al through the low temperature chemical fusion method using dextrose and citric acid.[19] Ammonium carbonate which act as the foam forming agent were added to the mechanically crushed mixture of CNT, dextrose and citric acid which were then mixed mechanically to obtain a homogenous solid
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mixture. During this process, dextrose act as a carbon source while the carboxyl groups in citric acid react with the hydroxyl groups in dextrose. Heat treatment results in the decomposition of ammonium carbonate and also the citric acid and dextrose to create the porosity within the structure. The foam strength was reported to be 1.39 MPa but the strain at which the stress value was obtained was not reported in this literature. The foam also has a high accessible surface area which makes it a good candidate for organic absorbers in waste management treatment.
A group led by Sun et.al fabricated a lightweight foam with a very low density of 56 mg/cm3. The foam was created by freeze drying CNT in an aqueous solution with giant graphene oxide (GGO). Hydrazine vapor was introduced to convert the GGO into graphene through chemical reduction. The mechanical testing result shows the foam being extremely flexible. The stress value reported at 50% stain is in the region close to 7 kPa. Even though the strength is rather low in general, the specific strength of the foam is high given its very low density.[20]
Figure 5: Stress strain curve of multi cycle compression and the SEM image of the ultra- lightweight foam by Sun et.al [21] Lin et.al demonstrated the fabrication of CNT foam using nanoscale welding process.[21]
Precisely, CVD was used to synthesize this structure with ferrocene acting as the catalyst while
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dichlorobenzene and methane act as the carbon source. During the CVD process, amorphous carbon is deposited in situ generating amorphous carbon (AC) - CNT core shell structures and welding at inter-CNT junctions. The resulting foam has a significantly higher density as compared to other foams. The density of the foam was reported at 210 mg/cm3 and has a compressive strength close to 13.5 MPa.
Figure 6: (a – c)SEM images of foam fabricated using dichlorobenzene supply rate of 0.1 mL/min, 0.3 mL/min and 0.7 mL/min. (d) Different types of welded junction within the foam [22]
An aerogel made from graphene and CNT were prepared by a group led by Zhang.
Hydrothermal reduction of graphene oxide and CNTs in the presence of ferrous ions (FeSO4 solution) was used in the preparation of this aerogel. The aerogel contains micron-sized pores within the network of carbon and also the α-FeOOH nano rods within the matrix. The density of this aerogel was reported to be ranging from 11 mg/cm3 to 87 mg/cm3 depending on the amount of ferrous ions used during the fabrication process. Mechanical testing shows that the aerogel is not rigid. However, they are brittle and break at a certain strain. This is due to presence of rigid
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α-FeOOH which decreases the ductility of the graphene-CNT aerogel. The compressive strength of the aerogel was reported to be slightly less than 0.3 MPa for the aerogel with a density of 87 mg/cm3 at a strain of 50%.[22]
Sui et.al also fabricated a graphene-CNT aerogel but using a different technique. In this case, super critical drying was employed. Two types of CNT were used in this work; pristine
MWCNT and acid treated MWCNT. In a typical simplified fabrication methodology, MWCNT and graphene oxide (GO) suspension are mixed in distilled water and sonicated. Ascorbic acid and hydrochloric acid were then added to the mixture with the latter helping to reduce the reaction time between GO and ascorbic acid. The mixture is the heated to form the aerogel precursors before being super critically dried with CO2 to form the graphene-CNT aerogel. The density of the hybrid aerogel and the acid treated hybrid aerogel was reported to be 54.4 mg/cm3 and 31.8 mg/cm3 respectively. Compression test were carried out to determine the mechanical properties of the aerogel. The result shows that the acid treated aerogel has a compressive strength of 0.3 MPa while the regular aerogel has a compressive strength of close to 0.8
MPa.[23]
An ultralight weight graphene aerogel which has an average density of 4 mg/cm3 was synthesized by Hu et.al. The foam was made by functionalizing the colloidal graphene oxide using a weak reducing agent, ethylene diamine at an elevated temperature. The mixture is then freeze dried to produce the functionalized graphene hydrogel which was subsequently exposed to microwave irradiation in an inert environment to produce the final ultralight graphene aerogel.
The strength of the aerogel was reported to be 20 kPa at a stain level of 90%. The structure shows good recoverability up to a strain of 90%.[24]
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Figure 7: The aerogel being compressed up to 90% strain and the resulting stress strain curve [25]
Qiu et.al fabricated a graphene-based cellular monolith structure imitating the hierarchal structure of a cork. The fabrication process is similar to the one synthesized by Hu et.al except that instead of microwave irradiation, dialysis in water was used to remove the soluble species.
The structure has a density of 5.1 mg/cm3 and strength value close to 20 kPa at a strain of 80%.
Similar to other foam structures discussed, this structure is also very elastic.[25]
On the other hand, Zhang et.al shows that graphene aerogel which is conductive and mechanically strong can be synthesized using both supercritical drying and freeze drying of hydrogel precursor derived from the reduction of graphene oxide and ascorbic acid. A variation of aerogel exhibiting different densities were fabricated. The highest strength reported is 0.66
MPa for the aerogel with a bulk density of 96.1 mg/cm3.[26]
A high strength compact graphene macrostructure were developed by Bi et.al via a pH mediated hydrothermal reduction coupled with the moulding method. The reported compressive strength of this structure is 381 MPa but the strain at which the stress value is obtained is
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unknown. Although the strength value is high, the density of this compact graphene is also high at 1600 mg/cm3. [27]
3.2. Composite Foam
The first CNT/polymer composite was reported by Ajayan et.al whereby a mechanical mixing technique was used to randomly disperse the CNT into a liquid epoxide-base resin.[28]
Thereafter, various other works pertinent to CNT/polymer composite have been reported. In general, these works can be categorized into two parts; mixing and infiltration. Mixing can be further subdivided into different processes which are solution process, melt blending and in-situ polymerization of the monomers.[29]
In the solution process, polymers and CNT are dispersed in a solvent before being formed into the desired shape. One of the earliest polymer-CNT composite foam was fabricated by Yang et.al using this process. In their work, the CNT was dispersed in a polystyrene (PS) /toluene solution which contains 5% foaming agent.[30] The solution is sprayed and heat treated before being hot pressed resulting in the release of nitrogen gas within the composite due to the decomposition of the foaming agent. These bubbles eventually create the CNT/PS composite.
The composite foam, which has a density of 560 mg/cm3, shows great potential as an electromagnetic interference (EMI) shielding.[31] However, the mechanical properties of the foam were not reported in this work. Similar approach using other different polymers such as
PMMA, nafion and polypropylene has also been reported.[6, 32-34]
The melt blending process is used only with thermoplastic polymers. In this process,
CNT is added and dispersed in the molten form of the thermoplastic polymer. One of the major challenges of this process is achieving CNT uniformity due to the high viscosity of the molten
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polymer. Park et.al demonstrated this process using ethylene vinyl acetate copolymer (EVA) with MWCNT. The MWCNT is added to the molten EVA before being mixed in a two-roll mill while the chemical blowing agent and crosslinking agent were added. The mixture is the put into a mold before being foamed using the compression molding method.[35] Many other
CNT/polymer has been fabricated using the melt blending process. Some of the polymers that has been reported are polycaprolacton, PMMA, polyethylene, polypropylene and poly[styrene-b-
(ethyleneco-butylene)-b-styrene] to list a few. [36-40] The emphasis of the composite foams fabricated in this manner is more towards applications as transparent conductive coatings, electrostatic dissipation, electrostatic painting and electromagnetic interference shielding.[41] As a result the compressive strengths are not reported.
Figure 8: Graphical illustration of the infiltration process in foam composite fabrication
In situ polymerization is favorable for thermoset polymer but this method has also been used extensively for thermoplastic. This method allows for uniform distribution of CNT due to the low viscosity of the monomers.[4] Lee et.al used this process to fabricate a carbon nanofiber
(CNF)/MWCNT/polystyrene composite. This is prepared by adding the CNF/MWCNT to the PS monomer and azobisisobutyronitrile (AIBN) which act as polymerization initiator. After the completion of polymerization, batch foaming process was used to foam the mixture using supercritical carbon dioxide as the blowing agent. The system were heated and pressurized for a
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day before being rapidly cooled and depressurized to fix the foam cells.[42] The same method was utilized by Kong et.al to fabricate a MWCNT/polyaniline (PANI).[43]
On the other hand, the infiltration process requires a CNT scaffold which allows the polymer to fil the spaces within the scaffold as illustrated in Figure 8. To date, buckypaper and foams has been used comprehensively for this purpose.[28] A group led by Gui developed an epoxy/CNT composite using a CNT foam fabricated through chemical vapor deposition (CVD) as the scaffold. Epoxy resin is used to fill all the spaces within the CNT foam resulting in a structure that is dominated by epoxy in terms of weight. The resulting composite has a compressive strength of 3 MPa at a strain rate of 7%. The density is not explicitly expressed in the literature but calculation estimates the density to range from 800 mg/cm3 to 4000 mg/cm3.[44] Similar apporaoch was implemented by Li et.al.[45] However, the weight percentage of CNT in this method is very low resulting in a composite with high density.
Gui and co-workers used polydimethylsiloxane (PDMS) to create the composite.
Droplets of PDMS resin containing harderner were added to the CNT scaffold drop by drop. It was carried out until saturation is achieved whereby all the pores are filled with PDMS. The system were then cured to obtain the PDMS/CNT foam composite.[46] A team led by Worsley used the same polymer but instead of adding droplets of PDMS, they immerse the foam into the
PDMS resin before curing the system.[47] The resulting composite show increased stiffness as opposed to the neat foam.
Apart from CNT foam, graphene foam composite has also been widely studied by many researchers. A group led by Feng produced a graphene oxide – epoxy composite aerogel via freeze drying method.[48] Water soluble epoxy was added into the graphene oxide solution. The
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mixture then undergoes freeze drying before being cured. The resulting composite exhibits a very low density at 90 mg/cm3 and a compressive strength of 0.231 MPa at a strain of 75%.
Similar method was employed by using polyvinyl alcohol (PVA) by Zhang et.al and the resulting composite which has a density of 102.89 mg/cm3 exhibits tensile strength of 3.48 MPa at failure.[49]
Fan et.al fabricated a graphene aerogel - PMMA composite using the in situ bulk polymerization.[50] The graphene aerogel was prepared using the Hummers Method followed by the chemical reduction using L-ascorbic acid. The aerogel is then made into a graphene aerogel –
PMMA composite via in-situ bulk polymerization. The density and mechanical properties of these composite aerogel is not reported in the literature. Similar techniques were employed by other research groups to create graphene aerogel – PMMA composite.[50-53] The composites exhibit improved thermal and electrical properties compared to the graphene aerogel by itself.
However, the mechanical properties were not reported.
Tang and co-workers fabricated an epoxy/graphene aerogel composite using the vacuum impregnation process. The aerogel was prepared by in situ reduction-assembly method whereby paraphenylene diamine (PPD) was utilized as the reducing agent and functionalization agent of the graphene oxide. At a 0.95 epoxy volume percentage, the strength of the foam was increased from 0.146 MPa to 0.75 MPa. This is also accompanied by a large increase in density due to the high volume percentage of the epoxy. However, the density value is not reported explicitly in this literature.[54]
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3.3. Summary
Even though there are a lot of aerogel-polymer composite reported in the literature, the applications of these reports are mostly for electromechanical interference (EMI) shielding, sensors, actuators and electrodes. However, there are limited numbers of literatures or studies that is directed towards developing CNT based foam or aerogel as a light weight and strong material. This goes to show that, this idea of developing light weight and strong material using foam and aerogel is novel and still unexplored. Table 1 summarizes the mechanical properties of selected foams that have been discussed. As can be seen, the specific strength of the current foams that have been reported is not very high compared to the currently available materials. The only exception here is the in-situ welded foam fabricated by Lin et.al. The drawback for this foam on the other hand is the fact that it was grown using CVD and as a result will not be suitable for large sized fabrication. Thus, this method is not feasible for real world application.
Table 1: Summary of reported strength by various authors in literature
Strain at Compressive Specific Density Reported Type of Foam Strength Strength Author (mg/cm3) Strength (kPa/(mg/cm3)) (MPa) (%) CNT 179 4.3 90 24.02 Bradford [18] CNT N/A 1.39 Max N/A Liu [19] Carbon 5.6 0.007 50 0.13 Sun [20] CNT 210 13.5 50 64.29 Lin [21] Graphene 4 0.02 90 5 Hu [24] Graphene 5.1 0.02 80 3.92 Qiu 25] Graphene 96.1 0.66 40 6.87 Zhang [26] Graphene 1600 361 Max 238.13 Bi [27] Graphene/CNT 87 0.3 47 3.45 Zhang [22] CNT/Graphene 54.4 0.8 50 14.71 Sui [23] CNT/Epoxy 4000 3 7 0.75 Gui [44] Graphene/Epoxy N/A 0.75 10 N/A Tang [54]
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CHAPTER 4
FABRICATION PROCESS
4.1. Nanocarbon Foam
The CNT foam is fabricated by using PMMA microspheres as template to create the porous structure. Specifically, PMMA spheres are added together with the CNT during the fabrication of the foam. After that, the spheres will be removed leaving spherical pores within the foam. PAN is used to further reinforce the wall by locking the CNT in place and preventing them from sliding.
The solid three-dimensional structure is subjected to two different heat treatments to form the final foam. The first heat treatment is at 300oC in air. During this process, the PMMA spheres are evaporated leaving empty spaces behind. This allows PAN to undergo oxidation by releasing hydrogen and adding less volatile oxygen atoms. As a result a ladder structure is formed as can be seen in Figure 9.
Figure 9: PAN oxidation process during the heat treatment at 300oC in air
This is followed by another heat treatment at a higher temperature of 1200oC in inert environment. In this case, nitrogen gas is used to create the inert environment. During this step,
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the carbonization process takes place whereby the oxygen and nitrogen are removed leaving only carbon atoms behind.
The following graphic illustrates the fabrication process of the foam.
Figure 10: Graphical summary of the nanocarbon foam fabrication process
The detailed fabrication process is described below.
1. CNT is weighed and added into a beaker.
2. The required amount of PAN which was already dissolved in dimethylformamide
(DMF) to form a 1 wt% concentration solution is added into the beaker using a pipette.
3. Isopropyl alcohol (IPA) is added to the mixture to act as a medium for sonication.
4. The beaker is immersed into a bowl filled with iced water to keep the mixture from
overheating during sonication.
5. The mixture is sonicated using the Qsonica Q500 Sonicator for a total time of 20
minutes.
6. The mixture is taken out from the bowl and placed into a bath sonicator to ensure the
CNT is fully dispersed.
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7. PMMA is weighed and added into the mixture and the mixture is sonicated for 10
minutes for the PMMA to be uniformly distributed in the mixture.
8. A vacuum filtration system is set up as shown in step 3 of Figure 10.
9. The mixture is poured into the filter and the vacuum valve is slowly turned on.
10. After most of the IPA has been removed, the vacuum valve is turned off and the filter
paper holding the sample is removed and covered up by another piece of filter paper.
11. The samples are left to dry overnight.
12. The sample is heated at 300oC in air for 3 hours using the MTI Corporation GSL-1700X
furnace.
13. The sample is heated at 1200oC in inert environment for an hour using the same furnace.
4.2. Nanocarbon Foam/Polymer Composite
The nanocarbon foam has a very low density. However, the strength is not as high as expected. These will be further discussed in Chapter 5. To further boost the strength of the foam, the walls of the foam are reinforced with the infiltration of polymer. The addition of polymer will act as a stiff coating around the CNTs which prevents the CNTs from buckling when subjected to external force. The density of the foam is expected to increase but the notion is to increase the strength of the foam more than the increase in weight.
The amount of polymer being infiltrated into the foam and ultimately the density of the composite foam is controlled by using polymer solution with different concentration. The polymer is transported into the foam by the solvent through capillarity or capillary forces.
Getting a uniform distribution of polymer can be a challenge especially with higher concentration polymer solution due to the high viscosity.
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The polymer used thus far in the research is PMMA. PMMA is chosen because it possesses good mechanical properties in terms of strength. The ultimate tensile strength of
PMMA typically ranges from 47MPa to 70MPa [57] On top of that, it also has a relatively low density as compared to other polymers at around 1150mg/cm3 to 1190mg/cm3. Besides that, the cost of PMMA is also low making it ideal for mass production.
The following graphic provides an overview of the nanocarbon foam/polymer composite foam fabrication process.
Figure 11: Graphical summary of the composite foam fabrication process
The fabrication of the nanocarbon foam/polymer composite is described in detail here.
1. The required amount of the polymer is weighed and added into a glass jar.
2. The required amount of solvent is added into the glass jar to form a 5 wt%, 10 wt%, 15
wt% and 20 wt% polymer concentration.
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3. The mixture is stirred continuously using a magnetic stir bar at different temperature
dependent upon the polymer and solvent used.
4. The foam is cut into a 5 mm x 5 mm size sample using the Universal Versalaser VLS
2.30 laser machine.
5. The sample is immersed into the polymer solution for an hour to coat polymers around
the CNTs.
Figure 12: Foam infiltration process
6. The sample is removed from the solution and the surface is cleaned using a dry tissue
paper.
7. The composite sample is left to dry for 3 hours inside a tube which is attached to the shaft
of a motor rotating at speed of 20 rpm to ensure uniform distribution of polymer and
prevent the polymer from settling down due to gravity.
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Figure 13: Rotational drying to ensure uniformity in polymer infiltration
8. The composite sample is heated using Thermo Scientific Lab-Line Vacuum oven in a
vacuum environment to a temperature below the boiling point of the solvent for a time
period dependent upon the solvent used.
Nano Carbon Foam Composite Foam
Figure 14: A) Illustration on how the CNTs are held together at the joint by PAN in the nanocarbon foam and how polymer coats the CNTs in the composite foam B) Image of nanocarbon foam cut into various size and shapes.
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CHAPTER 5
RESULTS AND DISCUSSIONS
5.1. Nanocarbon Foam
The structure and morphology of the nanocarbon foam was characterized with scanning electron microscopy (SEM). The SEM images at different magnification are shown in Figure 15.
Figure 15: SEM images at different magnification level showing the structure of the nanocarbon foam. The magnification levels are: A) X350 B) X2000 C) X5000 D) X10000
As can be seen in the SEM images, the foam is extremely porous. The pores in the foam were created by the PMMA spheres which have been burnt off resulting in the porous structure.
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The size of the pores ranges from 8 to 11 micrometer. This is the size of the PMMA sphere used in the fabrication of the foam. The size of the pores can be engineered to be larger or smaller by using PMMA spheres of a different size.
Mechanical test was carried out on the nanocarbon foam using the Shimadzu AGS-J mechanical test machine. A load cell with a maximum capacity of 500N was used in the testing.
The foam is compressed initially to 60% strain. After that, compression test at strain ranging from 10% to 70% with increment of 20% is performed on the foam sample. The stress, strain and density are calculated using the following formulas.