Novel Synthesis of Bulk Nanocarbon (BNC)

Senam Tamakloe

Thesis submitted to the faculty of the Virginia Polytechnic Institute and State University in

partial fulfillment of the requirements for the degree of

Master of Science

in

Materials Science and Engineering

Alex O. Aning, Chair

William T. Reynolds

Carlos T. A. Suchicital

May 20, 2020

Blacksburg, Virginia

Keywords: Lignocellulose, Carbon, Carbonization, Torrefaction, High Energy Ball Milling,

Nanomaterial, Nanocarbon

Copyright 2020, Senam Tamakloe

ii

Novel Synthesis of Bulk Nanocarbon (BNC)

By

Senam Tamakloe

ABSTRACT

Carbonized organic precursors such as wood, shells and some plant seeds are very porous.

They are nanostructured and tend to be hard, but have pure mechanical properties as a result of

their porosities. An attempt was made to carbonize an organic precursor to produce a bulk material

with much less porosity for possible use in structural applications such as reinforcement in metal

and polymer matrices. A bulk nanocarbon (BNC) material was synthesized using high energy ball

milling and the carbonization of corn cob. Corn cob was mechanically milled for up to 20 hours

by applying high energy ball milling to produce the milled powder. The milled powder was cold-

compacted and carbonized at up to 1500°C to fabricate the BNC material. The material revealed

both micro and nano-porosities; the porosities decreased with carbonizing temperature and hold

time. Micropores were mostly closed for samples carbonized above 1300°C, whereas they formed

interconnected network at lower carbonization temperatures. BNC has a young’s modulus of 120

GPa, about ten times that of extruded graphite.

Sam Tama

iii

Novel Synthesis of Bulk Nanocarbon (BNC)

By

Senam Tamakloe

GENERAL AUDIENCE ABSTRACT

Wood, shells, and plant seeds are examples of organic precursors. When organic precursors

are carbonized, they can become very porous, nanostructured, and hard, but deliver pure

mechanical properties because of their porosities. A selected organic precursor was carbonized, in

an attempt, to produce a bulk material with much less porosity for possible use in structural

applications such as reinforcement in metal and polymer matrices. A bulk nanocarbon (BNC)

material was made using high energy ball milling and the carbonization of corn cob (the selected

organic precursor). This bulk material revealed both micro and nano-porosities, and a young’s

modulus of 120 GPa, about ten times that of extruded graphite.

Sam Tama

iv

ACKNOWLEDGEMENTS

I would like to express my gratitude to my academic advisor, Dr. Alex Aning, for his support and

guidance during my graduate studies and M.S. research. His expertise helped me conduct this

research project and write this thesis.

In addition, I would like to thank my thesis committee, Dr. William Reynolds and Dr. Carlos

Suchicital, for their encouragement and thoughtful feedback.

Acknowledgements are given to Dr. Thomas Staley at the Virginia Tech Materials Science and

Engineering Department and Dr. Weinan Leng at the ICTAS Nanoscale Fabrication and

Characterization Laboratory, for training me on various characterization equipment used during

the journey of this work.

My sincere thanks also goes to the following faculty and staff of the College of Engineering and

the Materials Science and Engineering Department at Virginia Tech: Kim Grandstaff, Dr. David

Clark, Renee Cloyd, and Dr. Jack Lesko for providing me with graduate assistantship positions

throughout my graduate studies.

Special thanks to my labmates in Dr. Aning’s research group: Hesham Elmkharram, Manuel

Umanzor, and Parisa Soltanian for the stimulating discussions, great company, encouragement

and insightful advice.

Also, I thank my former advisor from the University of California, Merced, Dr. Lilian Davila. I

am grateful to her for serving as a mentor to me and for enlightening me on materials science-

related research. Her advice during my undergraduate studies was valuable in conducting my

M.S. research.

I would like to express my gratitude to my fellow graduate colleagues and friends for making my

time in Virginia Tech memorable: Katrina Colucci, Janay Frazier, Adrian Davila, and Matthew

Ferby.

Last but not the least, my biggest thanks to my family: my sister, Elinam Tamakloe and my

parents, Yram Tamakloe and Larry Tamakloe, for their constant support and unconditional love.

I could not have done this without you.

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TABLE OF CONTENTS

ABSTRACT ii

GENERAL AUDIENCE ABSTRACT iii

ACKNOWLEDGEMENTS iv

LIST OF FIGURES vii

LIST OF TABLES viii

CHAPTER 1: INTRODUCTION 1

CHAPTER 2: BACKGROUND 3

2.1. Carbon Nanostructured Materials ................................................................ 3

2.1.1. Diamond and Nanodiamond ................................................. 4

2.1.2. Graphite and Graphenes ........................................................ 5

2.1.3. Fullerenes .............................................................................. 6

2.1.4. Carbon Nanotubes ................................................................. 7

2.1.5. Amorphous Carbon ............................................................... 8

2.1.6. Activated Carbon .................................................................. 9

2.2. Lignocellulosic Materials ........................................................................... 10

2.2.1. Cellulose .............................................................................. 11

2.2.2. Hemicellulose ...................................................................... 12

2.2.3. Lignin ................................................................................... 13

2.3. High Energy Ball Milling ........................................................................... 14

2.3.1. Attrition Ball Milling ........................................................... 14

2.3.2. Planetary Ball Milling .......................................................... 15

2.3.3. Vibratory Ball Milling ......................................................... 16

2.4. Powder Compaction .................................................................................... 17

2.5. Thermochemical Conversion ...................................................................... 18

2.5.1. Torrefaction .......................................................................... 18

2.5.2. Carbonization ........................................................................ 20

2.6. Summary of Literature ................................................................................. 20

CHAPTER 3: EXPERIMENTAL PROCEDURE 21

3.1. Material Selection ........................................................................................ 21

3.2. Material Synthesis and Characterization ..................................................... 21

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3.2.1. High Energy Ball Milling ..................................................... 22

3.2.2. Powder Compaction .............................................................. 22

3.2.3. Heating Conditions ............................................................... 22

3.3. Microstructural Analytical Technique ......................................................... 23

3.3.1. X-Ray Powder Diffraction (XRD) Analysis ......................... 23

3.3.2. Raman Spectroscopy ............................................................. 23

3.3.3. Brunauer-Emmett-Teller (BET) Analysis ............................ 23

3.3.4. Microscopy ........................................................................... 24

3.3.5. Nanoindentation Testing ...................................................... 24

CHAPTER 4: RESULTS AND DISCUSSION 25

4.1. Bulk Nanocarbon (BNC) ............................................................................. 25

4.2. Structure of BNC ......................................................................................... 25

4.2.1 Bulk Defects ......................................................................... 28

4.2.1.1. Chemical Analysis .................................................... 30

4.2.2. Raman Spectroscopy Analysis ............................................. 32

4.2.3. X-Ray Powder Diffraction Analysis .................................... 38

4.2.3.1. Effects of Milling Time on Nanostructure ............... 38

4.2.3.2. Effects of Temperature on Nanostructure ................ 40

4.3. Porosity ....................................................................................................... 41

4.3.1. Pore Size Distribution .......................................................... 41

4.3.2. Average Pore Size ................................................................ 42

4.4. Density ......................................................................................................... 42

4.4.1. Effects of Milling Time ........................................................ 42

4.4.2. Effects of Carbonization Temperature ................................. 43

4.4.3. Effects of Hold Time ............................................................ 44

4.5. Mechanical Properties ................................................................................. 45

4.5.1. Young’s Modulus and Flexural Strength ............................. 45

4.5.2. Hardness Test ....................................................................... 45

CHAPTER 5: SUMMARY AND CONCLUSION 48

CHAPTER 6: FUTURE WORK 49

REFERENCES 50

vii

LIST OF FIGURES

Figure 1: Carbon and carbon-based nanomaterials ....................................................................... 1

Figure 2: Crystalline structures of carbon allotropes .................................................................... 3

Figure 3: Schematic diagram of (a) single-walled carbon nanotube capped by a C60 half sphere

and (b) multi-walled carbon nanotube formed by graphene tubes ............................................... 8

Figure 4: Crystal structure of (a) graphite, (b) diamond, and (c) amorphous carbon ................... 9

Figure 5: General flowchart of activated carbon fabrication ........................................................ 9

Figure 6: General flowchart of thermal activation for activated carbon ...................................... 10

Figure 7: Schematic of (a) three major constituents (cellulose, hemicellulose, and lignin) of

lignocellulosic materials and (b) their chemical structures .......................................................... 11

Figure 8: Schematic of an attrition ball mill ................................................................................ 15

Figure 9: Schematic of a planetary ball mill ................................................................................ 16

Figure 10: Schematic of the high energy collision process for vibratory ball mill ...................... 16

Figure 11: Schematic of the powder compaction for uniaxial pressing ....................................... 18

Figure 12: The relationship between the weight loss and heating temperature of lignin, cellulose,

and hemicellulose during the torrefaction process ....................................................................... 19

Figure 13: Heating profile of torrefaction and carbonization processes ...................................... 23

Figure 14: Bulk nanostructured carbon material (a) top, (b) side, and (c) bottom views of the

material ........................................................................................................................................ 25

Figure 15: HRTEM image of the scattered nanocrystalline regions of a BNC sample ............... 26

Figure 16. The HRTEM images of the pyrolysis of (a) sucrose and (b) anthracene ................... 26

Figure 17: TEM diffraction pattern of a BNC sample ................................................................. 27

Figure 18: Optical microscope observations of the BNC porosity. BNC samples carbonized at (a)

1100℃, (b) 1200℃, and (c) 1300℃ ........................................................................................... 29

Figure 19: SEM images of the structural defects showcases (a, b) fractured and (c) polished

surfaces of BNC samples carbonized at 1200℃ with 1 hour hold time and 5 hour mill time .... 30

Figure 20: (a) SEM image and (b) EDS analysis of a BNC polished surface ............................. 31

Figure 21: (a) SEM image and (b) EDS analysis of the bright regions in the polished BNC

material ........................................................................................................................................ 32

Figure 22: Raman spectra of 5 hour milled samples with 1 hour hold time ............................... 33

viii

Figure 23: Raman spectra of (a) diamond, highly ordered pyrolytic graphite (HOPG),

polycrystalline graphite, glassy carbon (GC), diamond-like carbon (DLC) materials along with

(b) C60 fullerene and nanotube ..................................................................................................... 33

Figure 24: The intensity ratios plotted against carbonization temperatures ................................ 34

Figure 25: Raman spectra for 5 hour milled samples carbonized at 1100℃ with 1 hour sintered

hold time ...................................................................................................................................... 35

Figure 26: Raman spectra for 5 hour milled sample carbonized at 1200℃ with 1 hour sintered

hold time ...................................................................................................................................... 36

Figure 27: Raman spectra for 5 hour milled sample carbonized at 1300℃ with 1 hour sintered

hold time ...................................................................................................................................... 36

Figure 28: Raman spectra of carbon nano-onion and nitrogen-doped carbon nano-onion ......... 37

Figure 29: Raman spectra of graphene oxide (GO) and reduced graphene oxide (RGO) ........... 37

Figure 30: X-Ray Diffraction Patterns for the corn cob milled up to 20 hours ........................... 38

Figure 31: X-Ray Diffraction Patterns for BNC milled up to 20 hours and sintered at 1200℃

with a 1 hour hold time ................................................................................................................ 39

Figure 32: XRD patterns of tungsten carbide contamination ...................................................... 40

Figure 33: XRD patterns for BNC milled for 5 hour and heated at different carbonization

temperatures with a 1 hour hold time .......................................................................................... 40

Figure 34: Pore size distribution for BNC samples carbonized at different carbonization

temperatures ................................................................................................................................. 41

Figure 35: The average pore size of the BNC plotted against carbonization temperature .......... 42

Figure 36: The average bulk density of the BNC plotted against milling time ........................... 43

Figure 37: The average bulk density of the BNC plotted against temperature ........................... 44

Figure 38: The average bulk density of the BNC plotted against carbonization holding time .... 45

Figure 39: BNC nanohardness results for the (a) planar surfaces and (b) cross-sectional surfaces

as a function of the contact depth ................................................................................................ 47

LIST OF TABLES

Table 1: Different lignocellulosic materials and their respective polymer composition

percentages ................................................................................................................................... 21

Table 2: Physical and mechanical properties of BNC, Graphite and Silicon Carbide ............... 45

1

CHAPTER 1

INTRODUCTION

Nanomaterials are classified as natural or engineered materials with particle sizes ranging

from 1 to 100 nm [1]. These materials have gained prominence and increased production growth

in a broad range of industrial sectors such as energy, automotive [2], and biomedicine [3]

industries. Carbon-based nanomaterials, in particular, have been a primary research focus in recent

decades [4]. These carbon-based nanomaterials include graphene, fullerenes, carbon nanotubes,

and carbon nanofibers [5], as seen in Figure 1. Nanostructured carbon materials provide new and

enhanced technological advancements for energy storage and conversion devices [6] due to their

refined microstructure and porosity, high mechanical strength, high corrosion resistance, and

excellent thermal and electrical conductivity [7], [8]. The ability to predict and manipulate their

unique properties can add to their value [1].

Figure 1. Carbon and carbon-based nanomaterials [9].

Many synthesis methods are developed for the preparation of engineering carbon-based

nanomaterials. One example involves the pyrolysis of organic precursors performed in an inert

atmosphere. Typically, these methods are applicable to large-scale production but offer limited

control over the carbon nanostructure. Another example relies on “chemical vapor deposition to

synthesize well-defined carbons nanostructured materials [10]. These techniques provide atomic-

scale precision in-control of the carbon nanostructure; however, they are relatively expensive,

require sophisticated equipment, and offer a limited yield. High-energy ball milling is one of the

few most widely used synthesis methods for nanomaterial fabrication [11]. The high-energy ball

milling method mechanically reduces the size of microcrystalline materials to yield nanostructured

2

materials. This technique can be scaled up easily to produce densified bulk nanostructured

materials [11]. Bulk carbon-based nanostructured materials can serve as promising candidates for

supercapacitor electrodes for energy storage and conversion devices [12]. In this study, a synthesis

method for creating a carbon-based nanostructured material in bulk densified form, bulk

nanostructured carbon (BNC), is outlined. The motivation of this research is to synthesize activated

carbon, specifically, in the bulk form to produce the novel BNC.

Lignocellulosic biomass was the selected precursor to produce the BNC material.

Lignocellulosic biomass is classified as agricultural waste (such as corn cob, corn stalk, hardwood,

softwood, and bagasse) with a high cellulose content [13]. Corn is one of the major and widely

produced feed grains, according to the United States Department of Agriculture [14]. Corn cobs

are dense, have relatively uniform sizes, high heating value, and low nitrogen and sulfur contents

compared to many other feedstocks. As an agricultural residue of corn processing, corn cob was

reported to have an annual production of approximately 30 to 40 million metric tons [15]. In terms

of structure, lignocellulosic compounds have a very complex and intricate non-uniform three-

dimensional structure [16]. The challenge to this recalcitrant property is developing an approach

for breaking down this structure. So, cultivating an understanding of the recalcitrant property of

the lignocellulosic compounds and developing a proper treatment method to achieve the

deconstruction of the lignocellulosic compound network is vital.

In this study, high energy ball milling, coupled with heat treatment processes, played an

essential role in BNC development. High energy ball milling was efficient in reducing the particle

sizes of the lignocellulose. While the heat treatment processes showed to facilitate the

depolymerization and carbonization of the lignocellulosic biomass. The milled powder and the

BNC samples were characterized using the following techniques:

Raman Spectroscopy

X-Ray Diffraction (XRD)

Brunauer-Emmett-Teller (BET) Analysis

Optical Microscopy (OM)

Scanning Electron Microscopy (SEM)

Transmission Electron Microscopy (TEM)

Nanoindentation

This study summarizes the synthesis and characterization of the BNC material.

3

CHAPTER 2

BACKGROUND

2.1 Carbon Nanostructured Materials

Carbon is a nonmetallic element made up of six electrons with four of the six electrons in

its valence shell. The carbon electron configuration is 1s2 2s2 2p2 [17]. This electron configuration

provides carbon with a unique set of properties, as seen in many carbon-structured

materials. Examples include the high hardness values seen in diamond, graphene, and carbon

nanotubes along with the high electrical conductivity values seen in graphite and carbon fibers, to

name a few. Diamond, graphite, fullerenes, and carbon nanotubes are examples of the crystalline

allotropic forms of carbon. These carbon materials share the same chemical composition but

possess different physical forms enabling different crystalline structures and properties, as seen in

Figure 2, which illustrates these different crystallographic forms of carbon. Most of the carbon

allotropes are synthesized artificially, apart from diamond and graphite being the only natural

carbon forms. Moreover, carbon nanostructured materials have recently attracted the interest of

researchers, not only for their unique properties but, most importantly, their high performances,

exploited in high-power supercapacitors, for example, are difficult to obtain from conventional

bulk carbon materials. This observation is likely due to the quantum confinement effect

[18],[19]. “By controlling the structure in the nanometer scale along with the bonding nature of

carbon atoms”, the field of carbon structures has been galvanized by the discovery of novel carbon

nanomaterials such as nanodiamonds in 1963 [20], fullerenes (three-dimensional quantum dots) in

1985, one-dimensional carbon nanotubes in 1991, and two-dimensional graphene in 2004

[21]. Consequently, this era of nanocrystalline carbon materials has introduced many commercial

applications such as novel drug delivery, novel fillers, sensors, electronic devices, and materials

for energy and gas storage [2], [3], [22].

Figure 2. Crystalline structures of carbon allotropes [23].

4

2.1.1 Diamond and Nanodiamond

Diamond has a three-dimensional tetrahedral network of covalent bonds, which enables its

electrons to be held tightly. This assembly produces a close-packed crystal structure and offers a

high-density value of 3.5 g/m3. The structure of diamond can also be described as the formation

of sp3 hybridized carbon atoms. Due to the sp3 hybridization and its close-packed structure,

diamond exhibits high hardness values, high melting and boiling points, excellent thermal

conductivity, and high refractive index [24]. These properties promote great industrial

applications, such as cutting and polishing tools. However, due to the high hardness values, this

can serve as a limitation in industrial workability along with its total covalent bond network and

low electron mobility that prevent the conduction of electricity. Diamonds can be classified in two

distinct forms – natural and artificial. Natural diamond is formed in the earth’s mantle under

extreme pressures and temperatures that enable the crystallization of its carbon fragments [24].

Artificial diamonds can be synthesized through various conversion processes of pure non-diamond

substances of carbon.

Diamond is one of the pure allotropic forms of carbon. This discovery was made possible

by a French chemist, Antoine-Laurent Lavoisier, 1772 theories on combustion, and by the 1796

experimentations performed by an English chemist, Smithson Tennant [25]. Tennant confirmed

the diamond composition by heating powdered diamond fused with potassium nitrate in a gold

metal tube conducted in an oxygen environment. Only carbon dioxide was obtained as the resultant

gas product in this experiment [26]. Consequently, Tennant inferred that by burning the same

amount of diamond and charcoal, individually, would release the same amount of carbon dioxide,

respectively.

The discovery of artificial diamond was first announced by GE Research Laboratories. A

group of scientists (Howard Tracy Hall, Francis Bundy, Robert Wentorf, and Herbert Strong)

discovered that diamond could be synthesized from carbon by forging the diamonds out of either

coal, coke, charcoal or graphite. A non-diamond carbon was subjected to high temperatures at

around 1200 to 2000℃ and compressed at high pressures around 95,000 atmospheres. This process

was conducted in the presence of a metallic catalyst such as iron, cobalt, and nickel to free the

carbon bonds [27]. The method for manufacturing larger and purer diamonds has been improved

by Robert Linares and Patrick Doering, who applied chemical vapor deposition to grow the

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5

artificial diamonds substantially as pure as natural diamonds [28]. Since the advent of the synthetic

diamond synthesis, millions of carat diamonds are produced annually for industrial applications.

Nanodiamond is a nanoparticle with the crystalline structure and properties of diamond.

The properties of nanodiamond involve cutting and polishing tools and serving as an additive of

engine oil. Nanodiamonds offer similar properties as diamonds, including high hardness values,

high thermal conductivity, low friction coefficient, and resistance to harsh environments [29].

However, nanodiamond is only artificially synthesized, unlike diamond. Nanodiamonds were first

discovered by “K.V. Volkov, V.V. Danilenko, and V.I. Elin at the All-Russian Scientific Research

Institute of Technical Physics (VNIITF, Snezhinsk) in 1963” [30] as a new class of carbon

nanoparticles. The researchers who played an important role in the discovery of the nanodiamond

synthesis obtained these nanoparticles “accidentally while studying diamond synthesis via shock

compression of nondiamond carbon modifications in blast chambers” [30].

2.1.2 Graphite and Graphenes

Graphite is one of the primary allotropic forms of carbon. Graphite is an assembly of carbon

layers, tightly stacked in an AB sequence, with each carbon atom held by covalent bonds. Weak

van der Walls forces closely link each carbon layer. An sp2-hybridized crystal structure forms the

graphite structure. The graphite structure consists of three out of the four valence electrons of each

carbon atom. By leaving the fourth valence electron as a free electron, this presents graphite with

a unique set of properties not offered by diamond. The physical and mechanical properties of

graphite include impressive electrical and thermal conductivities, high melting point, low density,

and excellent thermal shock and chemical resistance [31]. However, graphite has an anisotropic

structure, meaning that these excellent properties are only present within the carbon layers as

opposed to the perpendicular direction of these layers. As a result of this anisotropy, the carbon

layers can slide from each other very easily, allowing graphite to serve as a lubricant and pencil

material [32]. Due to its excellent electrical conductivity, graphite can serve as a good candidate

for electrochemical electrodes. In addition, due to its high thermal conductivity, graphite is a

beneficial material in heat exchangers for the petroleum, steel, pharmaceutical, metal finishing,

and food industries [33], [34].

Graphite is opaque and has a gray to black color. It is soft and inelastic. Graphite is a

naturally and artificially occurring form of crystalline carbon. Graphite is produced naturally in

metamorphic rocks and offers properties of both metals and nonmetals such as the thermal and

6

electrical conductivities seen in metals and high thermal resistance and lubricity seen in nonmetals,

as mentioned before [35]. Synthetic graphite can be fabricated either by heat-treating non-

graphitic carbon at temperatures above 2000K via chemical vapor deposition, by the

“decomposition of thermally unstable carbides or by the crystallization of metal melts

supersaturated with carbon” [36].

Graphene is a nanomaterial with a two-dimensional carbon structure. Graphene is made of

sp2-hybridized carbons (seen in graphite), creating a hexagonal lattice that consists of all six-

membered rings of carbon. Each graphite carbon layer is denoted as a graphene layer. Studies of

graphene have revealed outstanding physical, chemical, mechanical, and electrical properties. The

list of graphene properties includes ultralightweight, super-thinness, high thermal conductivity,

and high electrical conductivity and mechanical strength [37]. The use of graphene can be

exploited in multiple applications for example drug delivery, nucleic acid delivery, phototherapy

[38], “electrochemical sensors for the determination of hazardous ions” [39], electronic devices,

supercapacitors [40], and transparent conducting films [41] to name a few. Graphene was

discovered in 2004 by two Russian chemists, Andre Geim and Konstantin Novoselov, who

successfully isolated a thin-flake of graphene. Geim and Novoselov used tape to separate a

graphene layer from highly oriented pyrolytic graphite (HOPG). After repeated rounds, the peeled

graphene layer was fixed on a substrate [42]. In recent years, a standard large-scale preparation

method for the synthesis of graphene involves the oxidation of graphite to produce graphite oxide.

Graphite is oxidized using concentrated acids and by applying “thermal exfoliation and reduction

by thermal shock to produce reduced graphene” [43].

2.1.3 Fullerenes

Fullerenes are another subset of carbon nanomaterials. Fullerenes are described as isolable

spherical carbon compounds contained in a sole molecular species. They can be viewed as large

carbon-caged molecules analogous to benzene. Common examples of fullerenes are C60, C70, C78,

and C84 [44]. In 1970, Eiji Osawa, a Japanese chemist, hypothesized the existence of fullerene.

However, it was not until 1985 that Harold Kroto, Richard Smalley, and Robert Curl made the

first observation of the C60 fullerene [45]. C60 are “60 carbon atoms that consist of 12 five-

membered rings and 20 six-membered rings” [46].

Fullerene and its derivatives (endohedral compounds, exohedral compounds, and

heterofullerenes) are widely valued in the field of biomedicine. In the

7

endohedral fullerene compounds, a minimum of one atom or particle is found within the carbon-

cage. In heterofullerenes, a minimum of one “atom is substituted by a heteroatom such as nitrogen,

sulphur, or boron” [42]. The most versatile of the fullerene derivatives are the exohedral fullerene

compounds. They are “molecules formed by a chemical reaction between fullerenes

and different chemical groups” attached to its exterior [47]. The biological, chemical and physical

properties of fullerenes and its derivatives have provided favorable properties. Some of their

properties include their unique molecular architecture coupled with their hydrophobic core, enable

them to carry “drugs and genes for cellular delivery” [48], for example. In addition, their

antioxidant activity serves as a significant advantage due to their capability of localizing “within

the cell to mitochondria and other cell compartment sites, wherein diseased states, the production

of free radicals can take place” [49]. Other potential applications include “anticancer drug delivery

systems using photodynamic therapy, HIV drugs, and cosmetics to slow down the aging of human

skin” [50]. Recently “polymerized fullerenes phases have attracted attention as a result of their

exceptional hardness and their unique electrical and magnetic properties” [42]. In 1990, Wolfgang

Kratschmer and Donald Huffman succeeded in discovering a preparation methodology suited for

manufacturing gram-sized quantities of fullerenes [45].

2.1.4 Carbon Nanotubes

Carbon nanotubes (CNT) have a cylindrical carbon nanostructure with a diameter similar

to a rolled graphene sheet. In 1991, a Japanese physicist, Sumio Iijima, first observed a carbon

nanotube as a by-product of fullerene synthesis via arc discharge [51]. Similar to fullerene, CNT

are comprised of only sp2-hybridized carbons. Due to its cylindrical structure, a CNT can encase

various atoms and molecules in its internal space. Common physical methods used for preparing

CNTs include chemical vapor deposition, laser furnace, and arc discharge techniques

[52]. Nanotubes can be obtained through a highly efficient approach termed the template

carbonization technique, established by Takashi Kyotani et al. This method uses the internal

channels of an aluminum plate (produced by the aluminum anodic oxidation in sulfuric acid) for

the pyrolytic deposition of the CNTs [42]. Following the deposition reaction, “the anodic

aluminum oxide template is washed with hydrogen fluoride (HF) solution”, leaving only

monodispersed carbon nanotubes with uniform thickness, diameter, and length [53]. CNTs can be

produced in a variety of lengths, diameters, state of chirality, and atomic layers [54]; the variation

of these structures allows for different band structures and semiconducting and metallic properties.

8

Nanotubes are highly hydrophobic [55], have high tensile strength, excellent thermal conductivity

and stability, and switchable electronic properties [56]. CNTs can be divided into two categories,

single-walled tubes (SWNTs) and multi-walled tubes (MWNTs). “SWNTs can be viewed as

rolled-up graphene monolayers, while MWNTs consist of several nested graphene cylinders”, as

seen in Figure 3 [57].

Figure 3. Schematic diagram of (a) single-walled carbon nanotube capped by a C60 half sphere

and (b) multi-walled carbon nanotube formed by graphene tubes [57].

2.1.5 Amorphous Carbon

Amorphous carbon is a noncrystalline allotrope of carbon, illustrated in Figure 4c. Its

structure shows segments of short-range crystalline order [58]; however, no long-range crystalline

order can be observed [59]. There is a mixture of sp2 and sp3 hybridized bonds present in the

material with a high cluster of bonds [60]. Since “amorphous carbon is thermodynamically

metastable, the ratio of sp2 and sp3 hybridized bonds is varying” [61]. Consequently, the properties

of amorphous carbon are then variable and can differ depending on its sp2 and sp3 ratios. To

reiterate, an amorphous carbon exhibiting mainly sp3 hybridization (diamond-like characteristics)

or mainly sp2 hybridization (graphite-like characteristics) [60], can cover a large range of

mechanical properties such as high degree of elastic modulus and hardness, high thermal

conductivity and chemical inertness [59]. Due to its properties, amorphous carbon can be used in

various applications such as in the plastic, textile, and health-care industries, along with food

packaging, electrical applications, and gas and water filtering [62]. Amorphous carbon is produced

via sputtering, chemical vapor deposition, physical vapor deposition, or ion irradiation of either

graphite or diamond [61].

(a) (b)

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9

Figure 4. Crystal structure of (a) graphite, (b) diamond, and (c) amorphous carbon [63].

2.1.6 Activated Carbon

Activated carbon is defined as a non-graphitizable carbon [64] fabricated from a carbon-

rich organic precursor, such as wood, nutshells, olive stones, coal, and petroleum coke [58].

Activated carbon can be used in various filtration systems to purify water and vapor. Commonly,

lignocellulosic materials are selected as the organic precursors used for the fabrication of hard,

granular, and porous activated carbons. The organic precursor, when carbonized, produces the

activated carbon. Figures 5 and 6 outlines its procedures. Non-graphitic carbons are various

carbon-rich solids with a “two-dimensional long-range crystalline order in the planar hexagonal

networks but lack any measurable crystallographic order in the third direction (c-direction)” [58].

Some forms of non-graphitic carbon can convert to graphitic carbon (termed graphitizable carbon)

when heat-treated while others cannot (termed non-graphitizable carbon) [65].

Figure 5. General flowchart of activated carbon fabrication [66].

10

Figure 6. General flowchart of thermal activation for activated carbon [64].

2.2 Lignocellulosic Materials

Lignocellulosic materials are organic materials comprised mostly of the “carbohydrate

polymers, cellulose and hemicellulose, and an aromatic polymer, lignin” [67], as illustrated in

Figure 7. Lignocelluloses have a high degradation resistance due to the crystallinity of the

cellulose, the recalcitrance of the hemicellulose, the hydrophobicity of the lignin, and

“encapsulation of the cellulose by the lignin-hemicellulose matrix” [16]. The chemical

composition of these constituents varies for different raw lignocellulosic materials. Irrespective,

in most raw lignocellulosic materials, the cellulose and lignin compounds offer the maximum

carbon content and the minimum oxygen content. A study of various experimental lignocellulosic

precursors indicated, after further elemental analysis, the presence of approximately 95 wt% of

carbon and oxygen with traces of hydrogen, calcium, and ash content [66], [68].

11

Figure 7. Schematic of (a) three major constituents (cellulose, hemicellulose, and lignin) of

lignocellulosic materials [69] and (b) their chemical structures [70].

2.2.1 Cellulose

Cellulose is a primary part of lignocellulosic materials. Cellulose is classified as a plant

cell wall polysaccharide polymer linked collectively as β-1,4-D-glucan. The β-1,4-glucosidic

bonds linearly link the D-anhydroglucopyranose units together via hydrogen bonding to form the

β-1,4-D-glucan [71]. Cellulose has one reducing end (carries an unsubstituted hemiacetal) and one

non-reducing end (carries a free hydroxyl). The carbon atoms in the ring include one carbon atom

attached to two oxygen atoms, which serve as an acetal center along the whole chain except for

when it serves as a hemiacetal center with inherent reducing properties. The two other carbon

atoms in the ring (serve as the hydroxyl substituents) are involved in the interunit

linkage. An additional carbon atom is attached to an oxygen atom, while one other carbon atom

acts as the hydroxymethyl group [71]. Figure 7 presents the chemical structure of the cellulose

compound. In most crystal structures of cellulose, the molecule has a twofold helical

conformation stabilized by intramolecular hydrogen bonds. This cellulose construction explains

the flat ribbon-like molecular structure. The intramolecular hydrogen bonds between two glucosyl

units are relatively robust and exist in all crystalline forms of cellulose. Other hydrogen bonds

between adjacent molecules are believed to be responsible for the aggregations of the molecules

into crystals.

(a) (b)

Lignin

Hemicellulose

Cellulose

12

The semicrystalline polymer structure of the cellulose, with nonuniform repartitions of

crystalline and amorphous areas, in general, has high values of crystallinity. The degree of

crystallinity is believed to be contingent on the amount of native cellulose present in the material.

Native celluloses frequently have higher values of crystallinity compared to artificial celluloses

(viscose, for example). Native cellulose consists of a cellulose polymorph structure termed

cellulose I. The structure of cellulose I consist of two crystalline allomorphs, Iα and Iβ, that can

coexist within the native cellulose I [71]. The “Iα phase is metastable and can be transformed into

the more thermodynamically stable Iβ phase” by heat treatment processes [72]. “Cellulose Iα has

a triclinic one-chain unit cell” assembly, whereas “cellulose Iβ has a monoclinic two-chain unit

cell” assembly, both employing a parallel cellulose chain stacked via “van der Waals interactions

with a progressive shear parallel to the chain axis” and with alternating shear, respectively [73].

Cellulose Iα has only one chain with two glucose residues in the triclinic unit cell, while cellulose

Iβ has two parallel-up cellulose chains, each with two glucose residues in the monoclinic unit cell.

To reiterate, “native cellulose I is known to simultaneously crystallize in a one-chain

triclinic structure Iα and a two-chain modification Iβ; both polymorphs packed in a parallel chain

arrangement” [74] depending on the material. The relative amount of the crystalline allomorphs

and virtual amount of crystalline areas in native cellulose can vary widely with materials.

Knowledge of the formation and the cellulose chains packing are essential for developing a

comprehensive cellulose description [74].

2.2.2 Hemicellulose

Hemicellulose is a major compound of lignocellulosic materials. Hemicellulose is a large

complex group of carbohydrate polymers located in the primary and secondary walls of various

plant groups. As a polysaccharide polymer, hemicellulose has a backbone structure of β-1,4-

linked-D-pyranosyl residues [75]. The structural resemblance between the hemicellulosic and

cellulosic backbones induces a conformational homology that forms the hydrogen bond network

of the hemicelluloses and cellulose microfibrils system. In contrast with cellulose, only containing

one sugar group (D-glucose), hemicellulose is comprised of nine different sugars (D-xylose, D-

glucose, L-arabinose, D-mannose, L-rhamnose, D-galactose, L-fucose, D-galacturonic acid, and

D-glucuronic acid) [76]. The various sugar units for the hemicelluloses are presented with different

substituents and in different proportions [77].

13

Hemicellulose intersects with other cell wall components via secondary forces and covalent

linkages [78]. Hemicellulose forms ester linkages with acetyl units and hydroxycinnamic acids,

covalent bonds (mainly a-benzyl ether linkages) with lignin, and hydrogen bonds with cellulose

[79]. The hemicellulose can possess a large quantity of side-chain substitutions that bind less

tightly to cellulose and are more water-soluble, while ones with the occasional side chains bind

more tightly to cellulose and are less water-soluble [78]. Additionally, due to the divergence in the

degree of polymerization and crystallinity in comparison to cellulose, hemicelluloses have low

values of crystallinity and are abundantly amorphous in their structure. As presented in Figure 7,

where cellulose microfibrils are embedded in the hemicellulosic amorphous matrix [80]. All the

hemicelluloses show structural differences between different species and cell types within plant

groups. The hemicelluloses encompass a significant number of the cell wall, and forms cross-links

with the carbohydrate polymers to generate a rigid matrix explaining the complexity and structural

features of the hemicellulose [74].

2.2.3 Lignin

Lignin is described as a large group of aromatic polymers and “is the only naturally

synthesized polymer with an aromatic backbone” [71]. Lignins are originated “from the oxidative

combinatorial coupling of 4-hydroxyphenylpropanoids”, also known as monolignols [81]. The

three most abundant lignin monolignols are coniferyl, sinapyl, and p-coumaryl, as seen in Figure

7. The monolignol structures differ in the amount of methoxy groups joined to the aromatic ring

[71]. Lignin delivers rigidity, recalcitrance, strength, and hydrophobicity to the secondary cell

walls of a plant. As found in vascular plants, lignin makes up a significant fraction of the total

organic carbon. It can bear the force of gravity, the mechanical stresses, and harmful pressure-

generated transpiration for terminally differentiated cells [82]. Structurally, the lignin polymers’

stiffness and impermeability properties can protect plants against microbial invasion. However,

the structural characteristics and properties of lignin are a couple of the central challenges in the

chemical conversion of lignocellulosic biomass. The covalent linkages between hemicellulose and

lignin are responsible for the formation of the lignin–carbohydrate complexes. This amorphous

crosslinking of the lignin-hemicelluloses network is where cellulose is embedded, leading to water

removal from the cell wall and the hydrophobic composite formation [83]. From the lignin

recalcitrance to its degradation has led to scientific inquiry and further study into its biosynthesis

and structure [83].

14

2.3 High Energy Ball Milling

High energy ball milling is a simple mixing and grinding method that involves the

mechanical reduction of solid materials for synthesizing various classes of nanocrystalline

materials. During the milling process, various mechanical stresses can be generated that impact

the propagation of fracture paths throughout the selected precursor. The interaction between the

grinding elements and selected precursor affects the energy consumption of the particles. In the

end, the particles can differ in terms of shape, particle size, and surface roughness. The following

sections explore the various types of ball mills such as the planetary, attrition, and vibratory.

2.3.1 Attrition Ball Milling

The attrition ball mill (also known as the stirred ball mill) is a powerful mill utilized

in many areas of metallurgical research for ultrafine grinding and mixing. Figure 8 illustrates

a schematic of an attrition ball mill. Attrition ball milling is a straightforward and effective

comminution and homogenization technique. In the attritor, a selected precursor is

comminuted by free moving milling balls that are set in motion by an axial impeller. The

milling balls are nearly spaced equally along the rotating impeller by a given distance to produce

sufficient circulation [84]. Throughout the mill process, the balls are impacted by each other,

prompting the precursor to fracture and yield fine powder. The grinding effect relies on the

stirrer, the stirrer speed, and the chamber geometry. During this attrition process, the starting

material is charged in a “stationary tank with milling media often made from either stainless

steel, chrome steel, tungsten carbide, or alumina” [85]. The milling process can take place by

the stirring action termed agitation, which features a central vertical rotating shaft coupled

with the impeller that can run at speeds ranging from 75 and 500 rpm [85]. This causes the

milling media to exert both impact and shearing forces on the selected precursor.

Additionally, the attrition milling can be performed in the presence of an inert gas such as

argon for metallic powders [85]. To attenuate the temperature increase that can occur during

the “high-energy milling, the milling system is often cooled down by continuous water flow”

applied to the tank’s exterior jacket to prevent agglomeration [86].

15

Figure 8. Schematic of an attrition ball mill [87].

2.3.2 Planetary Ball Milliing

The planetary ball mill is a popular mill used in various scientific research for preparing

and synthesizing materials. The planetary ball mill experiences the planet-like movement of its

milling vials, where the vials are organized on a rotating supporting disk. Figure 9 presents a

schematic of a planetary ball mill. Its milling media can offer considerably high energy during its

process where the selected powder precursor (

16

Figure 9. Schematic of a planetary ball mill [88].

2.3.3 Vibratory Ball Milling

Vibratory ball milling is a widely used technique designed to refine the microstructure and

improve the mechanical properties of various materials. The advantages to this grinding apparatus

involve low maintenance, application of both shear and compression applied stresses, rapid

comminution, and reduction of the particle sizes. The reduction of the particle sizes to form powder

should be engineered to flow smoothly and consistently. A vibratory ball mill consists of a milling

vial (filled with balls and sample powder), which oscillates at a high frequency along several axes,

as illustrated in Figure 10. The agitation of the milling balls during a mill run is complicated. The

rapid collisions with the top and bottom surfaces of the milling vial along with glancing collisions

with the sides [86] happen continuously. Due to the amplitude and high speed of the clamp motion,

the milling balls can oscillate at high velocities reaching 5 m/s to deliver a high-efficiency

comminution process. A wide assortment of materials is used for the manufacturing of milling

vials and media such as hardened steel, agate, tungsten carbide, stainless steel, alumina, silicon

nitride, zirconia, methacrylate, and plastic [86], [85].

Figure 10. Schematic of the high energy collision process for vibratory ball mill [89].

θ1

θ2

θ3 Rotating stainless pot

Rotating disk

Swiveling base

Milling media

17

2.4 Powder Compaction

The process of powder compaction converts engineered materials from a dry powdered

state into a solid shapes [10]. These engineered powdered materials can be transferred into dies

and then compacted under high pressure to produce the solid [10]. The powder is described as a

collection of particles ranging in sizes of nanometers to micrometers. The particles have their own

physical properties, which depend on the chemical composition along with microstructures of each

particle. Usually, the physical properties of each particle are comparable to the bulk solid of the

same composition. However, the powder behaves differently than the bulk solid. The powder,

unlike a bulk solid, can flow under gravity, which enables the powder to be shaped. Figure 11

presents a schematic diagram of a powder forming process. The sample powder flows into a

cylindrical die creating the shape definition, and the mechanical compaction takes place under

uniaxial pressing conditions. Subsequently, shape retention occurs naturally where the compacted

powder part holds its shape generated by the cylindrical die and does not return to its primary loose

state because of the bonds formed between each particle during this mechanical compaction [10].

For certain powders, binders are needed to aid the interparticle bonding for the compacted part

[10].

Powder interactions can lead to the development of clusters – aggregates or agglomerates.

Typically, these clusters are formed in the dry powder caused by either moisture existing in the

powder or van der Waals attraction [10]. However, there are some aggregates that are comprised

of tightly bonded particles that are hard to break [10]. This presents many difficulties when packing

the powder particles for compaction. The goal for most powder compaction processes is to produce

a highly densified compact of particles to fill voids, efficiently. A powder comprised of spherical

particles where these particles pack together to fill up to 70% of the voids is an ideal scenario.

However, in most cases, the compacted sample is frequently porous, so subsequent heat-treatment

steps are advised to achieve a high density compacted part [10].

18

Figure 11. Schematic of the powder compaction for uniaxial pressing [90].

2.5. Thermochemical Conversions

2.5.1 Torrefaction

Torrefaction is a thermal degradation process for raw materials. This process is performed

in an inert or oxygen-limited environment. The organic precursor is heated up at low temperatures

around 200 to 300°C for up to several hours, depending on the selected precursor

[91]. Torrefaction is the pretreatment process, where under these conditions, the corn cob

experiences partial thermal decomposition leading to moisture evaporation and the dilapidation of

the hemicellulose, lignin, cellulose, and fibrous structures [92]. A set of chemical reactions, along

with mass and heat transfers, is transpired during this thermal degradation process. At an increased

temperature of roughly 200°C, hemicellulose begins the degradation process. This process

includes the devolatilization and depolymerization of the hemicellulose [93]. Throughout this

pretreatment process, the extensive decomposition of the hemicellulose produces a rapid loss of

oxygen and hydrogen atoms compared to the carbon atoms in the raw material. The torrefaction

process changes the chemical structure of the raw material to reduce its hydrogen and oxygen

content while increasing its carbon content [94]. A key characteristic of the torrefaction process

is the slow heating rate, unlike the high heating rate typically used in pyrolysis. The

torrefaction heating rate is generally less than 50 °C/min [95]. A higher heating rate, as seen in

the pyrolysis process, would enable the main product to be liquid instead of the

desired torrefied solid product.

At a temperature around 200°C, partial devolatilization and carbonization of the

hemicellulose are activated where hemicellulose breaks down into a char-like solid product and

volatiles. At increased temperatures, around 250–260°C, substantial hemicellulose devolatilization

19

transpires [92]. At the same time, slight decomposition of lignin and cellulose is effectuated but

does not lead to a extensive mass loss. To reiterate, at a temperatures around 100–260°C, the

hemicellulose is the most chemically activated and thermal degradable starting at 200°C. Cellulose

degrades above 275°C, but most of its degradation occurs within a narrow temperature range at

around 270–350°C [94]. In addition, lignin degrades slowly over a more extensive temperature

range of 250–500°C [94]. Usually, the maximum temperature for this process does not exceed

300°C. Torrefaction, above 300°C, can cause significant devolatilization and carbonization of the

polymers leading to fast thermal cracking of cellulose and significant loss of lignin [94]. This

outcome precedes the initiation of tar formation at the temperature range, 300–320°C [94]. The

incentive to utilizing the torrefaction process for this study is to have a raw biomass lose its fibrous

nature and begin the formation of the bulk nanostructured carbon samples without the need for

binders. The overall dilapidation process of raw materials is segmented into four stages: moisture

evaporation, and the decomposition of cellulose, hemicellulose, and lignin [92], where the torrified

biomass can lose up to 50% of its initial mass as outlined in Figure 12.

Figure 12. The relationship between the weight loss and heating temperature of lignin, cellulose,

and hemicellulose during the torrefaction process [94].

2.5.2 Carbonization

Carbonization is a thermochemical conversion process where carbon residues are formed

from organic materials heat-treated in an inert environment. This process eliminates most of the

noncarbon components from a given organic precursor and initiates the formation of an aromatic

skeleton consisting of a carbon network of six-membered, planar rings. Carbonization is an

intricate process where chemical reactions such as condensation, dehydrogenation, isomerization,

and hydrogen transfer can occur concurrently to produce a solid carbon-rich product [58].

20

Carbonization is a widely practiced technique in many environmental industries. Some of

its applications include water and gas purification and carbon sequestration [96]. This

thermochemical conversion process produces activated carbon (char) in an inert environment,

where the biomass is heated to elevated temperatures at a slow heating rate. It can be conducted

over an extended period ranging from hours to days long. Carbonization can happen at different

conditions, but all forms of carbonization have the shared purpose of forming carbon-rich products

such as activated charcoal, biocoke, and biochar. Although carbonization is comparable

to torrefaction in several aspects, the central differences between the two processes are their end-

products and process temperatures. Similarly, torrefaction reduces the hydrogen to carbon (H/C)

and oxygen to carbon (O/C) ratios of the raw biomass, as does carbonization [97]. However,

carbonization aims to dissipate much of the material volatiles while torrefaction does not.

Typically, torrefied solids have a much higher mass yield, the lower carbon content of the residue,

and less pore surface area than compared to the carbonized solids. Both processes yield reduced

hydrogen to carbon (H/C) and oxygen to carbon (O/C) ratios.

2.6 Summary of Literature

In this literature review, various carbon allotropes, powder processing operations, and heat

treatment processes were summarized. The motivation of this research is to create a novel activated

carbon-based nanostructured material in a bulk densified form, bulk nanostructured carbon (BNC).

The synthesis method of this research involved an organic precursor undergoing high energy ball

milling coupled with heat treatment processes, to produce the BNC material. This form of bulk

carbon-based nanostructured material may serve as a promising candidate for supercapacitor

electrodes for energy storage and conversion devices [12] due its high degree of porosity and

structural disorder. As further discussed in this work, various characterization techniques were

employed to explore the effects of temperature and milling time on the microstructure and

properties of this material.

21

CHAPTER 3

EXPERIMENTAL PROCEDURE

3.1 Material Selection

The lignocellulosic material selected for this research was corn cob, supplied by the vendor,

Amazon. Corn cob was the selected precursor for the BNC fabrication due to its high cellulose

content, high heating value, sustainability, uniformity and can be purchased in abundance, most

importantly. The corn cobs were approximately 155 mm in length and 25 mm in width and had an

average mass of 23 g at room temperature. The cellulose, hemicellulose, and lignin content of corn

cob is 45%, 35%, and 15%, respectively, as seen in Table 1. The corn cob was dried at 90°C

overnight in a vacuum oven to reduce the moisture content and crushed in a blender to reduce its

size for the SPEX mill.

Table 1. Different lignocellulosic materials and their respective polymer composition

percentages [98].

3.2 Material Synthesis and Characterization

The crushed corn cob was mechanically milled into for up to 20 hours fine powder. The

mechanically milled corn cob powder was then cold compacted into small cylindrical-shaped

pellets. Afterward, the cold compacted pellets (or green bodies) were torrefied then carbonized up

to 1500°C to produce the BNC material and later characterized. The characterization technique

used to analyze both the powder samples and the BNC materials was X-Ray Powder Diffraction

(XRD) Analysis. Further analysis of the BNC material involved: Raman Spectroscopy, Brunauer-

22

Emmett-Teller (BET) Analysis, Optical Microscopy (OM), Scanning Electron Microscopy (SEM),

Transmission Electron Microscopy (TEM), and nanoindentation testing.

3.2.1 High Energy Ball Milling

High energy ball milling was performed using an 8000 series SPEX mill, as mentioned and

illustrated in Section 2.3.3. A tungsten carbide milling vial and milling ball media were selected

to pulverize the crushed corn cobs and synthesize the nanoparticles. Both milling vial and ball

media were purchased from the McMaster-Carr Supply Company. The charge ratio, defined as the

ratio of the ball media weight to the powder weight, was 5:1, where the balls and powder were

weighed at 32 g and 6.4 g, respectively. Each high energy ball milling run was performed at room

temperature in air. The following milling times were studied: 1, 5, 10, and 20 hour(s).

3.2.2 Powder Compaction

After pulverizing the corn cob, the mechanically milled powder was cold compacted using

a Carver hydraulic press, and 0.75 inch stainless steel die. A milled powder weighed at 2.75 g, was

pressed at a maximum load of 12 metric tons at room temperature for 10 minutes to produce small

cylindrical-shaped pellets. The dimensions of the resultant powder pellets (green bodies) were 20

mm in width and 8 mm in thickness.

3.2.3 Heating Conditions

Following the powder compaction process, the powder pellets were then evenly spaced

longitudinally in a graphite crucible and sintered in a Lindberg tube furnace. The tube furnace was

used to perform the torrefaction and carbonization processes, respectively, in a 3:1 argon and

hydrogen atmospheric environment. The heating rate was 5 °C/min for both processes.

Torrefaction was at 300°C for 2 hours, followed by carbonization at 1100, 1200, and 1300°C for

up to 10 hours The tube furnace was then cooled down from carbonization temperature to room

temperature. A heating profile of this procedure is shown in Figure 13. The purpose of the

torrefaction process as a pretreatment process was primarily used to warm the lignin compound to

act as a binder [99] for the torrefied pellets while carbonization served to significantly increase the

carbon content and reduce the oxygen and hydrogen content.

23

Figure 13. Heating profile of torrefaction and carbonization processes.

3.3 Microstructural Analytical Technique

3.3.1 X-Ray Powder Diffraction (XRD) Analysis

XRD was used on each powder sample and BNC material to collect information on the

crystalline structures. The XRD used in this procedure was a Single Crystal X-ray Diffraction

System (PANalytical). The XRD patterns were collected using the PANalytical X’pert Pro PW

3040 diffractometer generator with a Cu K-alpha radiation source using 45 kV and 40 mA

generator power.

3.3.2 Raman Spectroscopy

Raman spectroscopy was employed to evaluate the crystallinity and structural disorder of

the BNC samples. Raman spectra were collected using a WiTec Alpha 300 Confocal Raman

Imaging spectrometer system equipped with 633 nm and 785 nm NIR (near-infrared) lasers. A

silicon wafer was used for the calibration of the WiTec Raman spectrometer.

3.3.3 Brunauer-Emmett-Teller (BET) Analysis

The BET instrument used in this research was a Quantachrome Instruments Nova 2200e

Series Surface Area and Pore Size Analyzer. Coupled with the nitrogen adsorption-desorption

method, this characterization technique was used to determine the pore size distribution and

evaluate the average pore sizes of the BNC samples. Due to the size limitation of the BET sample

holder, the BNC samples were cut in half, prior to testing, using a Struers Accustom-5 high speed

300ºC

1100-1500ºC

24

saw. The average pore size and pore size distribution data were determined using the Barrett-

Joyner-Halenda (BJH) analysis method.

3.3.4 Microscopy

The JEOL 2100 TEM was used to observe the nanostructured carbon features and collect

diffraction patterns of a BNC sample. HRTEM imaging and analysis was used to determine the

crystallinity and crystallite sizes of the BNC material. SEM used in this study was the LEO Zeiss

1550, commonly known as a high-performance Schottky field-emission SEM, used for a

submicron structural analysis of the BNC samples. While an Olympus transmitted and reflected

light microscope was selected to compare the pore sizes based on different BNC carbonization

temperatures.

3.3.5 Nanoindentation Testing

The Hysitron TriboIndenter was the nanoindenter used to measure the nanohardness of the

top surfaces and cross-sections of the BNC samples. To prepare the BNC samples for testing, the

reflective surfaces of the BNC samples had to be flat and smooth to collect accurate hardness

results. Before testing, the samples were mounted using a mounting press and phenolic resin

powder. The mounted samples were ground using 120, 240, 400, and 600 grit papers to reduce

surface roughness and polished using an 8in UltraPol and MicroCloth polishing cloths with a 3µm

diamond suspension to smoothen the surfaces. Ultrasonic cleaning was required in between

grinding and polishing steps to remove any polishing compound residue. The hardness tests were

all performed at room temperature in air. The load applied to the BNC was 8000 µN with a loading

function of 0–8000 µN (5 s), 8000 µN (2 s) and 8000–0 µN (5 s).

25

CHAPTER 4

RESULTS AND DISCUSSION

4.1 Bulk Nanocarbon (BNC)

Figure 14 showcases the BNC material milled for 5 hours and sintered at 1200°C for 5

hours. The milled powder pellets with dimensions of 20 mm in diameter and 8 mm in thickness

were reduced to 8 mm in diameter and 3 mm in thickness resulting in the BNC material. The

dimensions for this sample are 8 mm in width and 3 mm in thickness. The black appearance,

similar to graphite, suggests an absorption of radiation in the optical region. The bump formations

are shown on the top (Figure 14a), and bottom (Figure 14c) surfaces of the BNC sample are

believed to be gas-driven pores. In comparison, the bottom surface with seemingly flatter and

smoother regions than the top surface is a result of uneven heat distribution and gas flow during

both heating processes. Figure 14c shows an extruded sharp edge caused by the high pressure

exerted during the powder compaction procedure.

Figure 14. Bulk nanostructured carbon material (a) top, (b) side, and (c) bottom views of the

material.

4.2 Structure of BNC

HRTEM imaging and analysis was employed to determine the crystallinity and crystalline

sizes of the BNC material. The data presented in Figures 15 and 17 show HRTEM images of the

nanocrystalline structure and the diffraction pattern for a BNC sample. A scattering of small,

nanocrystalline regions of about 2-3 nm in size can be seen. The scattered particles, seen in Figure

15, are dispersed randomly and densely throughout the carbon-based matrix. This nanocrystalline

material can be viewed as a cluster of aggregated crystalline structures with different shapes, sizes,

and orientations. This observation shows a lack of long-range crystalline order.

(a) (b) (c)

26

Figure 15. HRTEM image of the scattered nanocrystalline regions of a BNC sample.

Figure 16. The HRTEM images of the pyrolysis of (a) sucrose and (b) anthracene [100].

In Figure 16a, the nanostructure of sucrose is pyrolyzed at 1000°C in N2. In Figure 16b,

anthracene is pyrolyzed at 1000°C. The pyrolyzed sucrose structure has a high presence of

scattered nanocrystalline regions. This nanostructure is comparable to the BNC nanostructure. The

HRTEM image of the pyrolyzed sucrose displays the presence of randomly disordered regions

with single carbon layers. The pyrolyzed anthracene shows a high degree of structural alignment.

27

The alignment of tightly packed carbon layers, indicative of graphitization, is not visibly present

in the HRTEM image of the BNC material.

The selected area diffraction pattern (SADP) of BNC nanocrystals, shown in Figure 17,

reveals layer planes similar to graphite and the detection of three distinct diffraction rings.

Figure 17. TEM diffraction pattern of a BNC sample.

These three diffused rings are located at about the same positions, as seen in most graphitic

diffraction patterns. The symmetrical rings confirm a possible isotropic structured nature of the

BNC material. Typically, a broadened and diffused ring pattern is a characteristic that represents

a highly disordered material. This finding complements the previous HRTEM image (shown

in Figure 15), which illustrates the lack of long-range periodicity and the presence of scattered

lattice fringes across the nanostructured carbon assembly. The three detected diffraction rings

have interplanar distances measured at 3.4Å, 1.94Å, and 1.21Å corresponding to the 002, 100, and

110 planes, respectively, when indexed to graphite [101]. The d-spacing and respective miller

28

indices for graphite are: 3.4Å for (002), 2.13Å for (100), 2.03Å for (101), 1.81Å for (102), 1.70Å

for (004), and 1.55Å for (103) [101], [102]. The broad detection rings in the SADP are to be

expected of graphite. However, there are only three detected rings, which is less than graphite

despite these measured interplanar distances comparable to the basal planes in graphite. This

observation may suggest the presence of carbon layers close to that in graphite.

4.2.1 Bulk Defects

The BNC material can be described as a carbon layered, densified bulk material that

contains a high degree of structural defects. Significant traces of microporosity and cracks were

observed throughout the BNC matrix. In Figure 18, the optical microscope observations from

cross-sections of three BNC samples were performed to study the effects of the carbonization

temperatures on the porosity. Bright areas on the optical microscope images correspond to the

BNC surfaces, while the dark areas are pores in the BNC samples. The significant presence of

asymmetrical pores in the images confirm a wide distribution of pores in each sample. There is not

a clear distinction in comparing the pore sizes and shapes of each sample as the carbonization

temperature increases. However, the BNC sample carbonized at 1100℃ has visibly larger-sized

pore clusters.

29

Figure 18. Optical microscope observations of the BNC porosity. BNC samples carbonized at (a)

1100℃, (b) 1200℃, and (c) 1300℃.

a

b

c

30

Other structural defects, such as cracks and coarse carbon layers, can easily be observed in

Figure 19. The figure displays the SEM images of a porous BNC sample carbonized at 1200℃

and held for 1h was studied. The cracked regions are scattered throughout the images. The

fractured surfaces throughout the carbon matrix illustrate the brittle fractures of the material. The

causation and prevention of these structural defects require further study and analysis.

Figure 19. SEM images of the structural defects showcases (a, b) fractured and (c) polished

surfaces of BNC samples carbonized at 1200℃ with 1 hour hold time and 5 hour mill time.

The formation of numerous asymmetrical pores and coarse regions of the material may attribute

to the sensitivity of the BNC mechanical properties.

4.2.1.1 Chemical Analysis

The high presence of carbon regions in the BNC material is confirmed in Figure 20. Figure

20 shows data from an energy-dispersive X-ray (EDS) analysis technique performed in

concurrence with SEM imaging, where the chemical composition of a BNC polished surface is

determined.

(a) (b) (c)

31

Figure 20. (a) SEM image and (b) EDS analysis of a BNC polished surface.

In Figure 21, the bright regions on the SEM, encasing the pores, show traces of non-carbon

elements such as tungsten, silicon, oxygen and calcium. The non-carbon elements are present due

to the milling and polishing stages of the research; tungsten from the tungsten carbide milling

media, silicon from the silicon carbide abrasive disk and oxygen and calcium from the indefinitive

sources.

Inte

nsi

ty

Energy (keV)

(a)

(b)

32

Figure 21. (a) SEM image and (b) EDS analysis of the bright regions in the polished BNC

material.

4.2.2 Raman Spectroscopy Analysis

An additional characterization technique that provides insight into the structural disorder

of the BNC samples is the Raman spectroscopy. Figure 22 presents and compares the Raman

spectra of five different BNC samples with increased carbonization temperatures. The Raman

spectra were used to recognize and study the structure and hybrid bonding of the BNC

samples. The two broad peaks centered at the 1350 and 1580 cm-1 regions represent the D peak

for disordered carbon regions in the samples while the G peak is indicative of the crystalline

carbon regions [103]. For each sample, the D band displays a higher intensity than the G band.

This finding suggests the lack of periodicity and the high order of structural defects in the BNC

Inte

nsi

ty

Energy (keV)

C

C

(a)

(b)

33

material, as revealed in the microscopy and SADP images. The intensity and shape changes of the

D and G peaks are likely associated with the structural changes that originated in the high energy

ball milling process. The structural defects by milling can result in the breakdown of the sp2 bonds

and the generation of sp3 clusters.

Figure 22. Raman spectra of 5 hour milled samples with 1 hour hold time.

The BNC material is a complex system that has sp2 (represents graphite-like structure) as well as

sp3 (represents diamond-like structure) hybrid bonds. In Figure 23, graphite is composed of

entirely sp2 hybridized bonds with high intensity at the 1580 cm-1 region. In contrast, diamond is

composed of entirely sp3 hybridized bonds, as seen in the 1333 cm-1 region.

Figure 23. Raman spectra of (a) diamond, highly ordered pyrolytic graphite (HOPG),

polycrystalline graphite, glassy carbon (GC), diamond-like carbon (DLC) materials along with

(b) C60 fullerene and nanotube [64].

D G

34

The sp2 and sp3 hybridized atoms initiate the D and G peaks in the Raman spectra. The D

peak is caused by the out-of-plane breathing modes of the sp2 hybridized atoms in the aromatic

rings and caused by the clustering of sp3 regions, which promote the structural bond disorder in

carbonaceous materials. The G peak is generated by the in-plane bond stretching of sp2 hybridized

atoms in both the aromatic rings and carbon chains. The G peak can experience a shift when the

clustering of sp3 hybridized atoms in the carbon chains is generated.

Figure 24 presents the determined intensity ratios of the D to G bands (ID/IG) for the BNC

samples plotted against the carbonization temperature. The intensity ratio of the D and G peaks

estimates the sample defects. The intensity ratio (ID/IG) in the Raman spectra is a parameter used

to quantify the degree of disorder in carbon materials. In other words, a higher value of the intensity

ratio equates to a higher degree of disorder [103] in the BNC sample. The high degree of structural

defect is possibly due to the nanocrystalline nature and high microporosity of the BNC materials.

Figure 24. The intensity ratios plotted against carbonization temperatures.

In Figure 24, the intensity ratio values remained flat, with increased carbonization temperature.

High ratio values signify added defects and the increase in size and number of broken sp2 bonds

and sp3 clusters. This increase in the ratio also suggests a low level of graphite formation was

obtained at higher temperatures during the carbonization process. This physical property can serve

as a benefit to the BNC material since recent research findings suggest graphitic-like structures

with high structural disordering can elicit better capacity for lithium-ion batteries [104]. The reason

being that disordered graphitic structures were found to store more lithium than the crystalline

structures.

1

1.25

1.5

1.75

2

1100 1200 1300 1400

I D/I

G

Temperature (℃)

35

Figures 25-27 present the Raman spectra for the 5h milled samples carbonized at various

carbonization temperatures with a 1h hold time. After conducting a comprehensive literature

review, the Raman spectra of the BNC samples showed strong similarities when compared to the

Raman spectra of the glassy carbon (seen in Figure 23), nitrogen-doped carbon nano-onion (seen

in Figure 28), and reduced graphene oxide (seen in Figure 29). The Raman spectra similarities are

possibly due to their shared amorphous structure caused by the high disorder.

Figure 25. Raman spectra for 5 hour milled samples carbonized at 1100℃ with 1 hour sintered

hold time.

36

Figure 26. Raman spectra for 5 hour milled sample carbonized at 1200℃ with 1 hour sintered

hold time.

Figure 27. Raman spectra for 5 hour milled sample carbonized at 1300℃ with 1 hour sintered

hold time.

37

Figure 28. Raman spectra of carbon nano-onion and nitrogen-doped carbon nano-onion [103].

Figure 29. Raman spectra of graphene oxide (GO) and reduced graphene oxide (RGO) [105].

https://www.google.com/url?sa=i&url=https%3A%2F%2Fpubs.rsc.org%2Fen%2Fcontent%2Farticlelanding%2F2014%2Fta%2Fc3ta13688d&psig=AOvVaw01EqKN1A6QeMyKCUWk9CmD&ust=1585193114585000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCICz_drWtOgCFQAAAAAdAAAAABBT

38

4.2.3 X-Ray Powder Diffraction Analysis

4.2.3.1 Effects of Milling Time on Nanostructure

Figure 30. X-Ray Diffraction Patterns for the corn cob milled up to 20 hours.

Figure 30 compares the milling times of the corn cob powder. As the milling time increases,

the central peak shown at around 2θ = 21° broadens. The nonsymmetric shape of this central peak

can suggest an overlap of two peaks for the 1h milled sample. Additionally, the broadness of these

peaks may be indicative of short-range structural order. The broadened peaks may result in the

variations of the d-spacing caused by microstrain and plastic deformation. As the milling time

reaches 10 hours, the presence of a few more peaks begins to emerge at around 2θ = 30°, 38°, and

49° later confirmed to be tungsten peaks.

Figures 31-33 show XRD patterns collected for the BNC samples used to evaluate the

effects of milling time and carbonization temperature. The central peak shown at around 2θ = 24°

has a broadening nature and is indicative of the small crystalline sizes of the BNC samples.

Moreover, the disordered structure can be seen by the broadening band centered at secondary and

tertiary peaks at around 2θ = 43° and 79° attributed to the (100) and (101) planes, respectively.

This XRD pattern suggests a correspondence to the microscopy, SADP, and Raman data. The

average estimated interlayer spacing indicates d002 is 4.11Å for the different milled powder

samples and an average d002 at 3.65Å for the BNC samples carbonized at different temperatures.

39

Figure 31. X-Ray Diffraction Patterns for BNC milled up to 20 hours and sintered at 1200℃

with 1 hour hold time.

Figure 31 shows that after carbonizing the powder compacts, the presence of tungsten

carbide peaks after 10 hours of milling become more apparent. This tungsten carbide

contamination is attributed to the milling media. Peak fitting analysis was performed to confirm a

tungsten carbide contamination. Figure 32 shows the results of the peak fitting analysis to detect

the tungsten carbide peaks seen in Figures 30 and 31.

C (

002)

C (

100)

WC

(100)

WC

(101)

WC

(0

01

)

WC

(110)

WC

(002)

WC

(111)

WC

(200)

WC

(102)

WC

(201)

40

Figure 32. XRD patterns of tungsten carbide contamination.

4.2.3.2. Effects of Carbonization Temperature on Nanostructure

Due to contamination during the milling process, the tungsten carbide ball media were

replaced for new tungsten carbide balls. Figure 33 shows that after changing the ball media and

sintering at a 1 hour hold time, reduces and mitigates the presence of the tungsten carbide peaks. In

addition, the figure reveals that no visible change in the nanostructure is observed as the

carbonization temperature increases.

Figure 33. XRD patterns for BNC milled for 5 hours and heated at different carbonization

temperatures with a 1 hour hold time.

C (

002)

C (

100)

41

4.3 Porosity

The process of synthesizing the BNC material can yield a large network of tiny pores.

During the heating process, the densification may partially fill the open pores with carbon thus

reducing the average pore size and open porosity. The BET analysis coupled with the N2 gas

adsorption-desorption method was performed to determine the average pore sizes and pore size

distributions of the BNC material and to evaluate the effects of carbonization temperatures.

4.3.1 Pore Size Distribution

A pore size distribution of the BNC samples carbonized at different temperatures was

determined by generating a plot of the derivative of the cumulative pore volume-pore radius curve

versus pore radius [106]. Figure 34 shows the results obtained by the Barrett-Joyer-Halenda (BJH)

pore size distribution method from desorption isotherms. The average pore size distribution for

each sample is around 20 nm in diameter. Each BNC sample tested had bimodal and trimodal pore

distributions, with all average pore sizes being in the mesoporous size range (2 to 50 nm).

Additionally, the results indicate that the majority of the nanopores in the BNC samples are below

6 nm in diameter size. The derivative of pore volume (Dv(r)) is at its highest for the 1200℃ BNC

sample, suggesting that the sample has the highest measured volume of open pores while the

1300℃ BNC sample showed the lowest.

Figure 34. Pore size distribution for BNC samples carbonized at different carbonization

temperatures for 1 hour.

42

The sample pore size distribution can also serve as a method for detecting pore agglomeration in

the BNC samples. Typically, agglomerated samples display bimodal and trimodal pore

distributions, indicating the presence of both intra-agglomerate pores and inter-agglomerate pores

[107] contained within the BNC samples.

4.3.2 Average Pore Size

Figure 35 shows the average pore sizes plotted against the carbonization temperature for a

given milling time. With increased temperature, the 1h milled samples experienced a significant

decrease in their pore sizes up until 1200˚C. After further heating, the pore sizes decreased only

gradually. Conversely, the 5h milled samples maintained a steady decrease, while the 20h milled

samples maintained the same pore size values with increased temperature.

Figure 35. The average pore size of the BNC plotted against the carbonization temperature.

4.4 Density

In Figure 36, 37, and 38, the average density of the BNC samples dropped with increased

carbonization temperature. This can be explained by the fact that the