LOW PERCOLATION THRESHOLD IN ELECTRICALLY CONDUCTIVE

ADHESIVES USING COMPLEX DIMENSIONAL FILLERS

A Thesis

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

Of the Requirements for the

Master of Science

Clinton Taubert

December, 2018 LOW PERCOLATION THRESHOLD IN ELECTRICALLY CONDUCTIVE

ADHESIVES USING COMPLEX DIMENSIONAL FILLERS

Clinton Taubert

Thesis

Approved: Accepted:

Advisor Dean of the College Dr. Yu Zhu Dr. Ali Dhinojwala

Faculty Reader Dean of the Graduate School Dr. Mesfin Tsige Dr. Chand Midha

Department Chair Date Dr. Tianbo Li

ii

ABSTRACT

Electrically conductive adhesives (ECAs) have recently become a critical technological area in development behind solar cell packaging for die attachment, solderless interconnects, and heat dissipation. The standard example of an

ECA employs the use of conductive fillers within a polymeric matrix or host to render the final composite conductive. Many fillers exist for use in this role, but commonly silver is chosen for its high electrical and thermal conductivities1. However, silver (especially micro- or nano-structured) remains an expensive commodity, and typical volume fraction loadings in ECA can approach >30%. This is necessary as the theoretical critical volume fraction required for monodisperse spheres in a randomly oriented isotropic system is

~16%2. Electrical conductivity of an ECA is governed by , wherein the necessary fillers that host electrons transfer, via physical connection or tunneling, must reach some critical volume fraction to accommodate probable conductive pathways that would be large enough to be considered isotropic3,4. Aggregation of small particles is unavoidable; thus filler loadings deviate to higher than ideal in real world systems. Such excessive filler loading not only invalidates economic feasibility, but also deteriorates mechanical properties inherent for the host polymer.

iii To mitigate the critical percolation threshold (pc) for volume fraction loading of a filler, combative methods are articulated herein. One approach is to use low-dimensional, high-aspect ratio fillers, such as graphene and carbon nanotubes (CNTs), which have been

6,7 shown to lower pc . Typically, such fillers are more expensive than silver; however, given the low-loading implied to achieve a percolated network, this approach could improve the economic feasibility as an added filler for reducing total filler loading required8. In this work, commercial CNTs are employed as a high-aspect ratio filler for the reduction of silver filler loading in an ECA system. Graphene nanoplatelets are also synthesized and used to demonstrate a route for creating tailored high-aspect ratio, low- dimensional fillers which are effective at generating a percolated network at relatively low loading.

iv A standard method for dispersing CNTs and graphene in a silicone host polymer is deduced and used as a masterbatch to ensure uniform dispersion of the aggregation- susceptible CNT. Utilizing a pre-percolated CNT system, a hybrid silver/CNT system was then generated to achieve enhanced conductivity at lower total loading over pure silver systems. This composite ECA exhibited a conductivity of 54 S/cm at 12 vol.% loading with a CNT loading of only 8 wt.%. A reference system exhibited conductivity at 15.89

S/cm at over 25 vol.% loading of silver. For the graphene system, a method for fully exfoliating an edge-functionalizing graphene from a natural graphite source is explored.

Preliminary test results for this system are explored in the latter half of this thesis.

v DEDICATION

I dedicate this work to my Grandfather. May he rest in peace.

iv ACKNOWLEDGEMENTS

An enormous amount of thanks must be given to Dr. Yu Zhu, first and foremost. His abundance of patience, wisdom, and diligence has served as the cornerstone of what I believe a professor, and a friend, should embody. Dr. Mesfin

Tsige has also served as a pillar of wisdom in my years as a researcher, and I would formally like to thank him for taking his time to read this thesis. I would also like to thank Kun Chen, for his assistance and drive to go above and beyond as a group member. I would then like to thank Mr. Kewei Liu for his advice as both a researcher and a friend, as well as Fang Peng for his guidance on this project. In the last, I would like to give my thanks and appreciation to my group members and friends who have provided immeasurable help in my many years at Akron.

v

TABLE OF CONTENTS

Page

ABSTRACT ...... 3

LIST OF FIGURES ...... 9

CHAPTER I ...... 10

INTRODUCTION ...... 10

1.1 Electrically Conductive Adhesives ...... 10

1.2 Percolation Theory and Critical Percolation Concentration ...... 11

1.3 Carbon Nanotube Fillers for Electrically Conductive Adhesives ...... 12

1.4 Graphene Nanoplatelet Fillers for Electrically Conductive Adhesives ...... 16

CHAPTER II ...... 18

EXPERIMENTAL ...... 18

2.1 Materials ...... 18

2.2 Dispersion of MWCNT/Silver Composite ECA ...... 19

2.2.1 Dispersion of MWCNT in Solvents ...... 19

2.2.2 Preparation of MWCNT Silicone Masterbatch ...... 20

2.2.3 Preparation of Silver Flake as ECA ...... 21

vi 2.2.4 Preparation of MWCNT/Silver Flake as ECA ...... 22

2.3 Edge-Functionalized Graphene Nanoplatelet Synthesis ...... 23

2.3.1 Graphite Expansion ...... 23

2.3.2 Chlorosulfonic Acid Treatment ...... 24

2.3.3 Graphite Intercalation and Subsequent Exfoliation/Edge-Functionalization ...... 25

2.4 Dispersion of Edge-Functionalized Graphene Nanoplatelet ...... 26

2.4.1 Dispersion of Edge-Functionalized Graphene Nanoplatelet in Solvent ...... 26

2.4.2 Preparation of Edge-Functionalized Graphene Nanoplatelet Dispersion as ECA ...... 27

2.5 Raman Spectroscopy ...... 28

2.6 Scanning and Transmission Electron Microscopy Imaging ...... 29

2.7 Casting and Testing of ECA material ...... 29

CHAPTER III ...... 31

RESULT AND DISCUSSION ...... 31

3.1 Characterization of MWCNT ...... 31

3.1.1 Morphology Characterization of MWCNT ...... 31

3.1.2 Morphology Characterization of Silver Flakes ...... 33

3.2 Characterization of Edge-Functionalized Graphene Nanoplatelets ...... 34

3.2.1 Morphology Characterization of Edge-Functionalized Graphene Nanoplatelets ...... 34

vii 3.2.2 Edge-Functionalized Graphene Nanoplatelet Dispersibility Assay ...... 36

3.2.3 Raman Spectroscopy of Edge-Functionalized Graphene Nanoplatelet ...... 37

3.3 Resistivity Testing of ECAs ...... 38

3.3.1 Resistivity Testing of Silver Flake in Silicone ...... 38

3.3.2 Resistivity Testing of MWCNT ...... 39

3.3.3 Resistivity Testing of Double-Component MWCNT with Silver ECA ...... 40

3.3.4 Resistivity Testing of Single-Component EFGNP ECA ...... 41

CONCLUSION AND PROSPECT ...... 43

BIBLIOGRAPHY ...... 44

viii

LIST OF FIGURES

Figure Page

1.1 Schematic of isotropically conductive adhesive with spherical filler...... 12

1.2 Schematic of isotropically conductive adhesive with ellipsoid filler ...... 13

1.3 Depiction of percolation with increasing filler loading...... 15

2.1 Schematic of percolation test vehicles...... 30

3.1 TEM images of the morphology of the carbon nanotubes ...... 32

3.2 TEM images of the morphology of the silver flakes ...... 33

3.3 TEM, SEM, and optical images of morphology of graphene flakes ...... 35

3.4 Dispersibility assay of graphene ...... 36

3.5 Raman spectra of graphen particle ...... 37

3.6 Single-component silver filled ECA resistivity measurement ...... 38

3.7 Single-component MWCNT filled ECA resistivity measurement ...... 39

3.8 Double-component MWCNT/silver filled ECA resistivity measurement ...... 41

3.9 EFGNP compared to MWCNT in ECA resistivity measurement ...... 42

ix

CHAPTER I

INTRODUCTION

1.1 Electrically Conductive Adhesives

Early mentions of such a material that would function to both mechanically attach substrates adhesively, while allowing for electrical conductivity, exist as early as a patent in 19269. Since then, ECAs have seen use in the electronics industry and photovoltaics industry; especially within the last few decades. Many of the advantages that ECAs provide over traditional interconnects, such as soldering materials, are as listed: low processing temperature, conjoining substrates with differing coefficients of thermal expansion, substrate compatibility, smaller feature size, re-workability, and environmental benevolence over lead containing solders10. Current commercial ECAs depend on high-loadings of monodisperse, micro-sized silver particles or flakes to achieve conductive pathway formation. To be competitive with current solder interconnects and reduce impedance-based power loss, commercial ECAs aim at conductivities ranging from 103-104 S/cm. However, given the ideal and non-ideal volume fraction percent loading of a spherical filler, expense is driven up due to silver incorporation requiring >30 vol.% loading. This factor has become an increasingly relevant reason for the lack of widespread adoption in the electronics packaging market.

10 1.2 Percolation Theory and Critical Percolation Concentration

Percolation theory is a branch of statistical physics and mathematics dealing with the probabilistic properties inherent of a randomly distributed media11. Predicting macroscopic behavior of a composite using statistical interactions is the prime motive behind percolation theory. Ideally, an isotropically conductive adhesive (ICA) can be described as a two-component system containing a conductive filler and insulative matrix; these are arbitrarily described without discrete values for their conductivities. In such a simplified case, any conductivity behavior that can be measured is a result of probabilistic interconnections between fillers. Thus, properties like volume fraction, p, can induce macroscopic changes within the composite system. At increasing volume fraction of the filler, the probability of jointed interconnects increases, until short-range coherence lengths, ξ, are formed8. Further increase of volume fraction will eventually be met with a point where ξ→∞. This sharp increase in coherence length is known as the critical

12 percolation concentration, pc, and is often referred to as the onset percolation threshold .

In macroscopic settings, percolation threshold is seen as a sudden increase in the conductivity of a bulk material at a specific filler loading window.

11 Using percolation theory, relevant variables become notable, such as the geometric properties of filler particles. Other factors must be considered to draw conclusions on the effect of a filler, such as the intrinsic conductance of the filler particles, the contact resistance of particle joints and terminal probing load, the insulative nature of the host polymer, the mechanical force or state that the system is in, and any corrosive events on the filler13.

Figure 1.1 Two-Dimensional schematic of an isotropically conductive composite with spherical filler

1.3 Carbon Nanotube Fillers for Electrically Conductive Adhesives

To circumvent the high-volume fraction required to achieve a percolated network, the use of high-aspect ratio fillers have been reported in the past few decades. In 1996,

Celzard et al. explored the fundamentals about the use of disc-like anisometric single- crystal graphite flakes in place of typical spherical fillers13. These filler dimensional

12 schemes are articulated in Figures 1.1 and 1.2. Their conclusion was based on the concept of excluded volume, wherein onset percolation can be treat as being dependent on the unfilled volume that develops from percolating systems that contain large aspect ratio fillers14 . The findings of Celzard et al. showed agreeable correlation between calculated and experimental values of critical percolation concentration that take only the particle morphology into account. They also showed that the calculated critical percolation concentration is strongly dependent on the orientation and size distribution of the fillers as well. This fits well with other findings on the size distribution of spherical fillers8.

Figure 1.2 Two-Dimensional schematic of an isotropically conductive composite with ellipsoid filler

Given the breadth of conductive nanostructures that carbon can obtain, research has proceeded in using materials like CNTs for their high-aspect ratio, being nearly one- dimensional7. Since their discovery, researchers have employed the use of CNTs in a wide area of electronic device-based fields15. CNT structure can be easily defined as a single sheet of graphene (single plane sp2-hybridized carbon) connected at two of its sides to

13 become a cylindrical tube. The condition of the two connected sides, that is any specified offset angle, results in a change in circumference of the final tube given a set arbitrary size of the originating graphene sheet. The offset angle of folding is denoted by the chiral vector, Ch. The chiral vector is often denoted via Ch = na1 + ma2, wherein, if m = 0 a zigzag tube is resultant and if n = m an armchair tube is resultant; all other combinations being referred to as chiral tubes. CNTs can be produced in a multitude of methods in both single-walled (SWCNT), multi-walled (MWCNT), or in other discrete shell layers; with varying chirality, sizes, and defect quality16-18.

Successful employment of CNTs in ECA typically entails sufficient to complete dispersion within a host polymer matrix. Generally, this approach is successful in achieving a low percolation threshold, with weight percent loadings becoming conductive as low as <1 wt.%19-21. However, full dispersion of CNT is a challenging task, as re- aggregation is difficult to control. Curing of a polymer host can help retard reaggregation of CNT with quick controlled increase in viscosity; however, utilizing a viscous resin

(such as siloxane) as the host polymer to be pre-dispersed in, allows greater processing flexibility and can aid in achieving relative homogeneity at sufficiently large filler concentrations (>5 wt.%)22-25. Aside from the dispersion challenges, CNTs typically possess less than ideal conductivities. Given the quality of the obtainable CNTs, the number of walls, size distribution, and sources of contamination, many factors can contribute to a deviation from an ideal conductivity; aside from this, CNTs by themselves do not possess an intrinsic conductivity that is comparable to metal; outside of pristine armchair CNTs26. Contact resistances about MWCNTs, have also been shown to be

14 relatively high both between nanotubes and between nanotube-gold terminal contacts27,

28. Some efforts have been employed to combat these problems, like utilizing silver-coated

CNTs29, CNT-like silver nanowires30, or mechanical host-polymer shrinkage to induce better particle contact31, with varying results. Inspired by the previous work in this area, the strategy taken herein involves the use of employing CNTs as a pre-percolation network. This allows the benefit of the high aspect ratio of CNT with encountering the dispersion limits or low intrinsic conductivity of CNT. If CNT is to be used as a pre- percolated network, sufficiently small additions to a silver ECA system could provide a conductivity benefit without the consequences encountered with single component systems of either CNT or silver.

Figure 1.3 Depiction of percolation with increasing loading content of filler material

15 1.4 Graphene Nanoplatelet Fillers for Electrically Conductive Adhesives

Since their first encounter by Novoselov et al.32, two-dimensional graphene has been a rising topic in the field of nanocomposites33. Graphene is a semimetal that has a small overlap between its valence and conduction bands; becoming a zero-bandgap material.

Like carbon nanotubes, graphene can be produced from a variety of methods, including from naturally abundant graphite sources. In graphite, the sp2-hybridized sheets of graphene exist in van der Waals bound galleries. Comparatively to CNTs, graphene has many similar exemplary properties such as high mechanical stiffness (E ~ 1TPa) 34, 35, high electrical conductivity (σ ~ 106 S/cm)32, 36-37, and high thermal conductivity (~400

W/m●K)38. Graphene, being two-dimensional, has a considerably high-aspect ratio when existing in few-sheet settings. Thus, graphene is an exceptional candidate to lower the percolation threshold concentration of ECAs.

However, many of the same issues that CNTs face are amplified in graphene.

Dispersibility is an immediate challenge as the strong ᴨ-ᴨ interactions between sheets lead difficulty in complete exfoliation of a solid material, and further stabilization against reaggregation in a solvent or polymer host. While growable, bottom-up strategies have long existed for making single sheets of graphene39-44, exfoliation from natural graphite sources produces the highest throughput in quality graphene nanoplatelets (graphene consisting of few sheets).

16 Exfoliation of graphite to produce graphene has led to numerous methods. One route employs the use of intercalating alkali metals between the galleries of graphene to produce a graphite intercalation compound (GIC) which can undergo further reaction to expand and peel apart the graphene sheets within graphite45, 46. Other intercalation methods employing the use of various electrochemical routes, such as strong acids47-53, strong oxidants54, 55, or various other intercalants56-59have since been reported. Expansion using these methods often require heat or a quenching agent to induce a reaction which results in physical separation of the graphene sheets. For example, in the case of strong acid conditions, acid intercalants are vaporized on introduction to high temperature or microwave treatment which physically expands layer spacing47, 49, 52, 53.

Inherently, electrochemical methods may induce some oxidation and subsequent irreversible damage of the graphene basal planes. Basal plane defects may induce a decrease conductivity or breaking of the plane to make smaller sheets. Alternatively, mechanical methods exist to exfoliate graphene such as sonication60, 61or milling62, 63.

While these methods also induce some basal plane damage, often this is seen as unavoidable as high-energy mechanical methods like sonication are typically required to disperse these materials in solvent. Inspired by methods included here, graphene nanoplatelets were synthesized by acid-expansion and subsequent GIC-type exfoliation that allowed for the use of iodoalkanes to generate an edge-functionalized graphene nanoplatelet product. Edge-functionalization has been shown to produce superior dispersability in common laboratory solvents like chloroform.

17 CHAPTER II

EXPERIMENTAL

2.1 Materials

Baytubes® C 150 HP Multi-walled Carbon Nanotubes (>99% C-purity, ~13 nm outer mean diameter, ~4 nm inner mean diameter, >1 μm length, 140-230 kg/m3 bulk density) were kindly provided by Bayer MaterialScience. Synthetic graphite powder (99.9% metal basis, -10 mesh) was purchased from Alfa Aesar. Silver (Ag) Flake (99.95% metal basis,

5-8 μm) was purchased from Inframat® Advanced MaterialsTM. Silicone Elastomer

(Sylgard 184) was purchased from Dow Corning. Sulfric Acid (98%, ACS Certified) was purchased from Fischer Chemical. Nitric acid (70%, ACS reagent) was purchased from

Sigma-Aldrich. Argon gas cylinder was purchased from Praxair. Hydrogen gas cylinder was purchased from Praxair. Sodium sticks (99%, immersed in protective hydrocarbon oil) was purchased from Alfa Aesar. Potassium chunks (98%, immersed in mineral oil) was purchased from Acros Organics. Dimethoxyethane (99%, Extra Dry, AcroSealTM) was purchased from Acros Organics. 1-Iodohexane was purchased from Sigma-Aldrich.

Isopropanol (ACS Grade), Acetone (ACS Certified), Ethyl Acetate (ACS Certified),

Chloroform (ACS Grade), Methanol (ACS Certified), and Hexanes (mixture of isomers) were purchased from purchase from Fischer Chemical. Ethanol (200 proof) was purchased from Decon Labs. 1,2,4-Trichlorobenzene was purchased from Sigma-Aldrich.

18 All Chemicals were directly used without further purification unless mentioned. THF was refluxed with sodium foil and distilled immediately before use.

2.2 Dispersion of MWCNT/Silver Composite ECA

2.2.1 Dispersion of MWCNT in Solvents

To first access the dispersibility of the selected MWCNT in specified solvents. Pre- dispersion of CNTs in a solvent, especially one that is miscible with the host polymer, assists in the effectivity of the final composite. From previous literature, various organic solvents have been used to assist dispersion of CNTs into host polymers; these solvents include toluene64, chloroform65, 66, dimethylformamide (DMF)67, and tetrahydrofuran

(THF)68. These solvents show sufficient levels of dispersion with well-defined characterization within their respected reports. It is necessary to assess the dispersion quality of each individual batch of obtained CNTs as size distribution, level of aggregation, and quality, can all effect the final viability for dispersion. Generally, relative dispersion can be ascertained via a dispersion test, wherein an assay of solvents is used for dispersing CNTs. Assessing the amount of time to observe visible flocculation gives the relative inference of dispersion quality.

19 2.2.2 Preparation of MWCNT Silicone Masterbatch

To maximize batch comparability, a masterbatch approach was employed, following closely to the method developed in literature.24 The as obtained polydimethylsiloxane resin and curing agent was used with the manufacturer-recommended mixing ratio (10:1, base resin to curing agent). Given silicone was chosen for its high-viscosity which aids in preventing reaggregation, and the silicone base resin possessed a higher viscosity while making up the majority of the polymer host matrix, the resin was used as the basis for generating masterbatch material; the curing agent also possesses radical generating species which may cause premature curing upon later sonication, and thus was added later.

To obtain an 8 wt.% (~3 vol.%) dispersion of MWCNT in silicone, 200 mg of

Baytubes® C 150 HP MWCNT were collected, weighed, and placed in a 100 mL beaker with 50 mL of chloroform. This mixture was then homogenized using a Hielscher UP400S

(400 W, 24 kHz) equipped with a 22 mm Sonotrode at 20% amplitude and 50% pulse for 1 hour with intermittent chloroform side washing. Meanwhile, to a 20 mL scintillation vial, 2.5 g of silicone resin was added with 10 mL of chloroform and allowed to stir at room temperature for at least 20 min. The contents of the 20 mL scintillation vial were immediately added to the beaker following sonication-cycle end. The mixture was then allowed to sonicate using the same settings for 1 hour. Then the mixture stirred overnight at 60 °C to reduce solvent level. The beaker was then dried under reduced pressure for 6 hours before being collected. The yield is a black, viscous polymer paste.

20 2.2.3 Preparation of Silver Flake as ECA

To prepare silver flake loading of ECA, a shear-planetary mixing procedure was adopted. A ThinkyUSA ARV-310 shear-planetary centrifugal vacuum mixer is used with a ThinkyUSA 250AD-5S x 5 adaptor which holds 5 5mL Nordson EFD Optimum Series syringe barrels kits (syringe barrel and plunger). Each Nordson syringe barrel is retrofitted with a one-time-use polytetrafluoroethene (PTFE) tape-wrapped wooden plug that was inserted into the tip. This was done to ensure all material is mixed within the syringe barrel and not occluded within the tip. To the retrofitted tips, ~200 mg of silicone base resin was added. Then, an appropriate amount of silicone curing agent was added to obtain a 10:1 ratio of base resin to curing agent. Silver flakes were then added to obtain a select vol. % that ranged from 5-25 vol.%. The ~1-2 mL of hexanes were then added to assist in mixing the two materials. The syringe barrels (without plunger) and adaptor were then mounted in the ARV-310 and cycled at 1500rpm for 20 minutes. Immediately after, a reduced pressure cycling step of 1200 rpm for 10 minutes at 10kPa was performed. The solvent- dry syringe barrel was then fitted with the plunger and PTFE-wrapped wooden plug was removed. The silver-colored polymer paste was then used for casting and testing.

21 2.2.4 Preparation of MWCNT/Silver Flake as ECA

To prepare MWCNT/silver flake composite loaded ECA, a shear-planetary mixing procedure was adopted. A ThinkyUSA ARV-310 shear-planetary centrifugal vacuum mixer is used with a ThinkyUSA 250AD-5S x 5 adaptor which holds 5 5mL Nordson

EFD Optimum Series syringe barrel kits (syringe barrel and plunger). Each Nordson syringe barrel was retrofitted with a one-time-use polytetrafluoroethene (PTFE) tape- wrapped wooden plug that is inserted into the tip. This is done to ensure all material is mixed within the syringe barrel and not occluded within the tip. To the retrofitted tips,

~200mg of 8 wt.% MWCNT in silicone masterbatch was added using a spatula. Then, an appropriate amount of silicone curing agent was added to obtain a 10:1 ratio of base resin to curing agent. Two drops of curing accelerant were then added, this is to insure quick curing at elevated viscosity that results from the loading of the both high-surface area fillers. Silver flakes were then added to obtain a select vol. % that ranged from 1-12 vol.% for the entire material. Then ~1-2mL of hexanes were then added to assist in mixing the two materials. The syringe barrels (without plunger) and adaptor were then mounted in the ARV-310 and cycled at 1500rpm for 20 minutes. Immediately after, a reduced pressure cycling step of 1200 rpm for 10 minutes at 10kPa was performed. The solvent - dry syringe was then fitted with the plunger and PTFE-wrapped wooden plug was removed. The dark silver-colored polymer paste was then used for casting and testing.

22 2.3 Edge-Functionalized Graphene Nanoplatelet Synthesis

2.3.1 Graphite Expansion

In effort to pre-expand graphene galleries and induce more effective later exfoliation via intercalation, an acid expansion method was adapted from previous literature52, 53, 69.

To obtain acid-washed graphite, 10g of natural graphite flakes was collected, weighed, and added to a 500mL sealable container. A mixture of 9:1 sulfuric acid to nitric acid was added to the container and allowed to sit with minimal agitation for 24 hours. Within the first few minutes, the graphite flakes appeared blueish in color. After 24 hours, the color had subsided leaving a yellowish supernatant, possibly due to graphene oxide generation.

The supernatant was carefully decanted into a large water reservoir before being disposed.

The remaining solid was washed and decanted multiple times with deionized water until a pH of ~7 was measured. The acid-washed flakes were collected and dried in a vacuum oven overnight at 60°C under reduced pressure.

Acid-washed graphite at this state is intercalated with acid generating compounds, care must be taken when handling. Portions of the acid-washed graphite were then transferred into a small transfer tube to be hot-loaded into a Thermo Scientific Lindberg

Blue M tube furnace at 750°C in an argon and hydrogen atmosphere. Hydrogen is necessary for this step to reduce any oxidation that may have occurred as a result of the acid washing step. Visible vapor evolved from the tube, and annealing was allowed to proceed until vapor ceased (~20 minutes). Careful consideration is necessary when

23 performing this step as acid-washed graphite will expand nearly 400% its original volume, which can push unreacted material outside of the hot zone. The expanded graphite was then allowed to cool before being removed and collected. The expanded graphite appeared as elaborately worm like and bundled together.

2.3.2 Chlorosulfonic Acid Treatment

To induce particle size disparity and ensure complete expansion, a subsequent chlorosulfonic acid treatment is performed. Because of the wormlike structure and confinement, some processing is necessary to further break the particles into a powder- like form of small size. Chlorosulfonic is notable in its ability to dissolve small amounts of CNT and is similarly disperses and intercalates graphite materials. Chlorosulfonic is a superacid, reacting violently with water, proper ventilation and safety precautions are required. To 100 mg of expanded graphite, 30 mL of Chlorosulfonic was added in a 100 mL sealable glass container and allowed to stir overnight at room temperature.

The container was then chilled at 0°C before being carefully poured, dropwise, over a large reservoir of ice. Once fully quenched, the resulting water/ice with acid-washed expanded graphite is filtered using a Buchner funnel with aspirated reduced-pressure. The powder is washed several times with deionized water before being collected and dried overnight at room temperature with ambient airflow. The resulting acid-washed expanded graphite is smaller in particle sized when compared to originating graphite flakes or expanded graphite material. Portions of the acid-washed expanded graphite were then

24 transferred into a small transfer tube to be hot-loaded into a Thermo Scientific Lindberg

Blue M tube furnace at 650°C in an argon and hydrogen atmosphere for 20 minutes. In this step, volume expansion is not as drastic but is apparent. The expanded graphite was then allowed to cool before being removed and collected. The expanded graphite appeared as a fine powder-like substance.

2.3.3 Graphite Intercalation and Subsequent Exfoliation/Edge-Functionalization

For the final exfoliation step, an electrochemical intercalation approach was employed which uses Sodium-Potassium liquid alloy (NaK). In preparation, NaK intercalant was prepared using 77/23 weight ratio of potassium to sodium to reach the room temperature liquid-alloy eutectic point. This preparation was performed by adding the corresponding weight ratio amounts of sodium and potassium to a vial within an argon glovebox to equal ~1g total weight. The vial was then vortexed until a metallic liquid was formed with no visible solid remaining.

To a 250 mL round-bottom flask, 100 mg of re-expanded graphite was added with magnetic stirbar and the container was then sealed by rubber septum. Then, 35 mL of dry

1,2-dimethoxyethane was added via 20 mL syringe, followed by 0.33 mL of NaK via separate 1 mL syringe. The flask was then septum sealed and removed from the argon glovebox. The flask was then sonicated by a UCE Ultrasonic Bath Cleaner (400 W, 40 kHz) for 5 minutes. After 5 minutes, a blue solution was visible, this is likely cause to the emergence of a dissolved electron solution. The solution was then allowed to stir for 48

25 hours, after which the 1.2 mL of 1-iodohexane. The solution was then allowed to stir for another 24 hours before being quenched with 20 mL of methanol. The material, edge- functionalized graphene nanoplatelets (EFGNR), was then filtered of a 0.45μm PTFE filter and allowed to dry in ambient conditions and room temperature before being weighed.

2.4 Dispersion of Edge-Functionalized Graphene Nanoplatelet

2.4.1 Dispersion of Edge-Functionalized Graphene Nanoplatelet in Solvent

To access the dispersibility of the synthesized EFGNP in silicone, dispersion in a solvent was first performed. Pre-dispersion of EFGNP in a solvent, especially one that is miscible with the host polymer, assists in the effectivity of the final composite. From previous literature, edge-functionalized materials show good dispersibility in chloroform like solvents70. Like CNTs, it is necessary to assess the dispersion quality of each individual batch of synthesized EFGNP as size distribution, level of aggregation, and quality, can all effect the final viability for dispersion. Relative dispersion is again ascertained via a dispersion test, wherein an assay of solvents is used for dispersing

EFGNP. To perform this, 10 mg of synthesized material was added to a 20 mL scintillation vial along with 10 mL of the desired solvent. The mixture was then sonicated using a Branson 450 Digital Sonifier (400 W, 20 kHz) equipped with a taper microtip operating at 20% output for 5 minutes. The vial was then capped and allowed to sit in an undisturbed area overnight.

26 2.4.2 Preparation of Edge-Functionalized Graphene Nanoplatelet Dispersion as

ECA

To obtain an 8 wt.% (~3 vol.%) dispersion of EFGNP in silicone, 100 mg of dry

EFGNP were collected, weighed, and placed in a 100 mL beaker with 50mL of chloroform. This mixture was then homogenized using a Hielscher UP400S (400 W, 24 kHz) using a 22 mm Sonotrode at 20% amplitude and 50% pulse cycle for 1 hour with intermittent side washing with chloroform. Meanwhile, to a 20 mL scintillation vial, 2.5 g of silicone resin was added with 10 mL of chloroform and allowed to stir at room temperature for at least 20 min. The contents of the 20 mL scintillation vial were immediately added to the beaker following sonication-cycle end. The mixture was then allowed to sonicate using the same settings for 1 hour. Then the mixture was allowed to stir overnight at 60 °C to reduce solvent level. The beaker was then placed under reduced pressure for 6 hours before being collected. The masterbatch yield is a jet black, highly viscous polymer paste; starkly contrasting the flowable silicone base resin. This masterbatch could then be diluted with silicone to obtain proper loadings for conductivity measurement.

To prepare EFGNP loading in ECA, a shear-planetary mixing procedure was again adopted. A ThinkyUSA ARV-310 shear-planetary centrifugal vacuum mixer is used with a ThinkyUSA 250AD-5S x 5 adaptor which holds 5 5mL Nordson EFD Optimum Series syringe barrels kits (syringe barrel and plunger). Each Nordson syringe barrel is retrofitted with a one-time-use polytetrafluoroethene (PTFE) tape-wrapped wooden plug that was

27 inserted into the tip, this was done to ensure all material is mixed within the syringe barrel and not occluded within the tip. To the retrofitted tips, ~180 mg of 8 wt.% EFGNP in silicone masterbatch was added using a spatula. An appropriate amount of silicone base resin was then added to obtain the proper dilution of EFGNP in ECA. Then, the curing agent was added to obtain a 10:1 ratio of base resin to curing agent. Then ~1-2 mL of hexanes were then added to assure proper mixing of the two materials. The syringe barrels

(without plunger) and adaptor were then mounted in the ARV-310 and cycled at 1500 rpm for 20 minutes. Immediately after, a reduced pressure cycling step of 1200 rpm for

10 minutes at 10kPa was performed. The solvent-dry syringe was then fitted with the plunger and PTFE-wrapped wooden plug was removed. The jet black colored polymer paste was then used for casting and testing.

2.5 Raman Spectroscopy

Raman Spectroscopy was utilized to detect quality of graphene sheets produced by the synthesis process listed here-in. The machine used was a HORIBA LabRAM HR

Raman Spectroscope. The wavelength of excitation laser is 532 nm.

28 2.6 Scanning and Transmission Electron Microscopy Imaging

SEM was used to observe size, thickness, morphology, and electron diffraction patterns of the synthesized EFGNP. The SEM used was a JEOL JSM-7401F operated at

10kV. The TEM used was a JEOL JSM-1230 Field Emission Scanning Electron

Microscope operated at 120kV.

2.7 Casting and Testing of ECA material

To test ECA material, a dimensionally defined testing platform was required. This platform, known as a percolation testing vehicle, was fabricated using an epoxy glass fiber board with printed gold strips surmounting the surface as shown in Figure 2.1. To cast an

ECA material, two strips of 400 μm thick copper tape was adhered to the surface, perpendicular to the direction of the underlying gold strips. The strips were accurately premeasured to ensure a proper parallel between the strips. ECA material was them delivered to one edge of the board, between the two copper-tape strips, and a razor blade was used as a doctorblade in spreading the ECA uniformly within the copper tape gap.

Excess ECA was then removed from either side of the strip that did not lie within the gap.

The board was then placed in a laboratory oven at 120°C overnight. Once cured, the two copper strips were removed to reveal a singular ECA strip surmounting the gold strips perpendicularly. The dimensions of the strip were measured using calipers (for width) and a thickness gauge (for thickness); along with known gold-strip gap lengths (length), this allowed for a reliable testing box for resistivity and conductivity elucidation.

29 Above relatively high filler volume loading (>10 vol.%) the ECA became both viscous and highly conductive, disqualifying the use of small and thin casting. Thus, a larger dimensionally defined box was fabricated. This vehicle was composed of a glass base slide which was surmounted by four smaller glass rectangles; so mounted that a cuboid shape was developed. The glass pieces were mounted with Kapton, carefully avoiding exposure to the inner fill-space. Two opposing-sided glass strips were covered with copper tape as to include inner-facing exposed copper surfaces to contact the ECA once filled and allow for pads for external testing. The dimensions of the cuboid cell were measured using calipers and the defined thickness of the glass slides (1 mm thick). The

ECA filled space provided enough resistive load to reliably test within the calibration of the multimeter used. The multimeter used for both test vehicles was a Keithley 2000

Series digital multimeter.

Figure 2.1 (a) Schematic of percolation test vehicle, (b) schematic of glass test vehicle,

(c) scheme for measuring resistivity, (d) percolation test vehicle with strip of carbon

filled ECA, (e) glass test vehicle with filled carbon filled ECA.

30 CHAPTER III

RESULT AND DISCUSSION

3.1 Characterization of MWCNT

The morphology of the as obtained multi-walled carbon nanotubes and silver flakes were tested and analyzed. The ECA composed of the 1-dimensional multi-walled carbon nanotubes was tested for resistivity and subsequent conductivity using the appropriate testing vehicle and multimeter setup within three measurements as well as . The ECA composed of the 1-dimensional multi-walled carbon nanotubes with silver flakes was then fabricated and tested for resistivity and subsequent conductivity using the appropriate testing vehicle and multimeter setup within three measurements amongst triplicate batches.

3.1.1 Morphology Characterization of MWCNT

To analyze the morphology of the as obtained multi-wall carbon nanotubes, transmission electron microscopy (TEM) was employed. The machine used for taking

TEM images was a JEOL, JSM-1230, operated at 120 kV. Samples were prepared by adding a slight amount (<1 mg) of the as obtained carbon nanotubes to a vial and dispersing them in an ethanol solution (~5 mL) using ultrasonication (500 W, 24 kHz) for

5min before loading onto a lacey-carbon TEM grid. The carbon nanotubes existed in a

31 variety of diameters that were <20 nm. They were often bundled and tightly interwoven, even with additional sonication to aid dispersion of ethanol-based TEM liquid sample workup. From simple observation, it is clear that significant effort must be given to obtain a fully dispersed carbon content within an ECA material.

Figure 3.1 TEM images of the morphology of the multi-walled carbon nanotube at (a)

25000x magnification, and (b) 50000x magnification.

32 3.1.2 Morphology Characterization of Silver Flakes

To analyze the morphology of the as obtained multi-wall carbon nanotubes, scanning electron microscopy (SEM) was employed. Samples were obtained by sprinkling a small amount of the as obtained silver flakes onto carbon tape mounted to an SEM holder. The machine used for taking SEM images was a JEOL JSM-7401F operated at 10 kV.

Figure 3.2 SEM images of the morphology of the silver flakes at (a) close magnification showing the thinness of the silver flakes, and (b) distant magnification which articulates

the large size of the silver flakes.

33 3.2 Characterization of Edge-Functionalized Graphene Nanoplatelets

The morphology edge-functionalized graphene nanoplatelets were tested and analyzed after synthesis. The ECA composed of the 2-dimensional edge-functionalized graphene nanoplatelets was tested for resistivity and subsequent conductivity was tested using the appropriate testing vehicle and multimeter setup within three measurements.

3.2.1 Morphology Characterization of Edge-Functionalized Graphene Nanoplatelets

To analyze the morphology of the as synthesized EFGNP product, TEM was employed. The machine used for taking TEM images was a JEOL, JSM-1230, operated at 120 kV. Samples were prepared by adding a slight amount (<1 mg) of the as synthesized

EFGNP to a vial and dispersing them in an ethanol solution (~5 mL) using ultrasonication

(500W, 24kHz) for 5min before loading onto a lacey-carbon TEM grid. The expanded graphite was also examined via SEM. Figure 3.3d shows the morphology of the expanded graphite, which is evident as a plate-like material of relatively large flake size, comparable to the silver flakes shown earlier. The final EFGNP product shows stacked morphology when dry, however, when dispersed well in solution, visible single or low sheet number graphene plates are visible, as demonstrated in Figure 3.3e. Selected-area electron diffraction (SAED) was also performed on the sheet to ascertain a relative number of sheets. It is notable that this pattern is typical of very few graphene sheets present through the identified plate71.

34

Figure 3.3 (a) Picture of expanded graphite, with inset showing starting graphite. (b)

Picture of quenched reaction with inset showing the blue solution color following bath sonication of solution. (c) Picture of collected filter with filtered EFGNP. (d) SEM image of expanded graphite at distant magnification. (e) TEM image of EFGNP, showing large,

low sheet number piece of graphene. (f) Selected-area electron diffraction (SAED)

image, showing typical pattern associated with low sheet number graphene.

35 3.2.2 Edge-Functionalized Graphene Nanoplatelet Dispersibility Assay

To properly ascertain the correct solvent in workup of the masterbatch, as well as a general notion of the dispersibility enhancement of the edge-funtionalization process, a dispersibility assay can be performed. The method used was outlined in 2.4.1 of this text.

The chosen solvents were: Hexanes, Chloroform, Chlorobenzene, Acetone, Ethyl

Acetate, Isopropanol, Deionized Water, and Ethanol. The results of this test are shown in

Figure 3.4. Visually, chloroform showed the best results, having no visible sediment after one day with visible sedimentation taking over 4 days to achieve which was readily re- dispersed with gentle agitation.

Figure 3.4 Dispersibility assay of EFGNP in common solvents. Chloroform and

Chlorobenzene showed the best level of dispersion from visual investigation. Inset shows

the same test for MWCNT in chloroform.

36 3.2.3 Raman Spectroscopy of Edge-Functionalized Graphene Nanoplatelet

To characterize the quality of the graphene sheets, Raman spectroscopy was used.

The material, after synthesis, was prepared by dispersing the EFGNP in a concentrated mixture (~5 mg in 1 mL) with ethanol. The mixture was then dropped onto a flat aluminum surface and allowed to evaporate. The Raman used was a HORIBA LabRAM

HR. From the trace gathered, the material can be regarded as a graphene-like, differing from the typical pattern achieved for graphite. The Id/Ig ratio is often used to indicate the number of defects present in a graphene samples. The ratio typically increases upon introduction of more defects but may also be influenced by the ensemble orientation at which the sample is measured in reference to the Raman laser. High defect density may also produce a low ratio should the material transcends a point of being regarded more as amorphous in nature72-75.

Figure 3.5 Raman spectra of EFGNP particle; Id/Ig = 0.6

37 3.3 Resistivity Testing of ECAs

3.3.1 Resistivity Testing of Silver Flake in Silicone

Single-component, silver flake in silicone ECA was fabricated via the method discussed in 2.2.3 and 2.7 of this text. The tested ECA, shown in Figure 3.6, displayed typical high-loading required to achieve sufficient conductivity. Percolation was observed past 5 vol. %, which is lower than that of the spherical silver filler case shown in literature. This may be due to the high-aspect ratio of the silver filler that was selected to be used in conjunction with the other high-aspect ratio, low-dimensional filler.

Figure 3.6 Single-component Ag filled ECA resistivity measurement.

38 3.3.2 Resistivity Testing of MWCNT

Single-component, multi-walled carbon nanotube in silicone ECA was then fabricated via the masterbatch method discussed in 2.2.2 and casting/testing method discussed in 2.7 of this text. The tested ECA, shown in Figure 3.7, was shown in terms of wt. % as opposed to vol. % in convenience of the density of carbon being much lower than that of silver. The low-loading of carbon to form a percolated network is thus displayed. Percolation was observed past 7 wt. % and was subsequently chosen to be used as the pre-percolated network for further ECA testing.

Figure 3.7 Single-component MWCNT filled ECA resistivity measurement.

39 3.3.3 Resistivity Testing of Double-Component MWCNT with Silver ECA

According to the method described in 2.2.4, a double-component ECA system was fabricated and testing in accordance to method 2.7. The MWCNT/silver flake composite

ECA displayed superior performance over the pure silver system, as seen in Figure 3.8.

The composite system reached a conductivity of 0.2 S/cm at 8 vol. % silver flake loading, while the single component silver system required 20 vol. % loading to reach the same order of magnitude conductivity. Furthermore, the viscosity of the polymer was greatly reduced in this case, as 20 vol. % silver flake content is nearing the limit of filler wetting, requiring extra processing time when using planetary mixing, as well as extra effort required when casting to obtain uniform casts. This data is consistent with previous literature results for CNT/Silver loaded systems69. At low silver loading, it is evident that a large boost in conductivity is achieved by having a pre-percolated network. The single- component CNT system shown in Figure 3.6 displayed a conductivity of 1.1 x 10-4 S/cm at 8 wt. % loading of CNT, while ECA from Figure 3.7 shows a conductivity boost by an entire order of magnitude to 1.1 x 10-3 S/cm with the addition of only 1 vol. % silver loading. From this, it can be inferred that the MWCNT is forming a pre-percolated network for which silver to build upon in improving over the low conductivity of the carbon filler. This scheme is promising given the small amount of carbon material require to achieve such pre-percolation.

40

Figure 3.8 Double-component MWCNT/Ag filled ECA resistivity measurement beside

single-component Ag ECA resistivity measurement.

3.3.4 Resistivity Testing of Single-Component EFGNP ECA

The introduction of EFGNP as a carbon filler, offers an added route for lowering aspect ratio. The EFGNP ECA was prepared first via polymer dispersion discussed in

2.4.2 along with casting/testing discussed in 2.7. The goal was to compare the pre- percolation results of 1-dimensional MWCNT with the synthesized 2-dimensional

EFGNP. The result of the EFGNP filler can be seen in Figure 3.9. At low-loading, the

MWCNT has a reasonably higher conductivity, this can be due to the length of the CNTs themselves, but the cause is not clear. However, at higher loading the EFGNP becomes conductively superior. The high-aspect ratio of the graphene, coupled with the larger size disparity about individual particles makes the percolation threshold much more noticeable when compared to the somewhat linear MWCNT. The EFGNP had a conductivity of 2.8

41 x 10-4 S/cm while the MWCNT was 4.3 x 10-5 S/cm at an identical 5 wt. % loading. This notion alone, could be promising if combined in a double-component system, incorporating EFGNP; however, given the large surface area of graphene flakes, it is difficult to generate uniform casting using EFGNP and silver systems. An alternate route could be to use a ternary-component system, using all three fillers explored in this text.

Optimization of a ternary system could generate a highly-conductive, percolated network, at extremely low filler concentration.

Figure 3.9 EFGNP compared to MWCNT in ECA resistivity measurement.

42

CHAPTER IV

CONCLUSION AND PROSPECT

For the low-dimensional carbon filler study of ECA material, it was shown that pre- percolated networks can be achieved at low filler loading of both commercially available

MWCNT and synthesized EFGNP. The double-component system of MWCNT and silver flake showed superior conductivity at sufficiently lower loading of silver. This highlights an economically feasible route to modifying the loading of expensive silver fillers with the addition of small amounts of high-aspect ratio conductive fillers. The double- component silver flake in 8 wt. % pre-loaded MWCNT ECA showed excellent conductivity at only 8 vol. % silver loading with a conductivity of 0.2 S/cm. At 12 vol. % silver loading for the same system, the conductivity reached 55 S/cm. TEM and Raman data supports the synthetic EFGNP route as being efficient in generating low-sheet amount graphene while promoting dispersion in silicone compatible solvents like chloroform. Such a route can be modified or replaced by similar graphene generating methods to further lower the cost of high-aspect ratio fillers. Future work will center on the expansion of low-dimensional fillers in generating ternary systems which benefit from multiple low-dimensional fillers, like MWCNT and EFGNP, and the size disparity between them. Using such a system to form a pre-percolated network may further reduce necessary loading of silver fillers to achieve commercially accept conductivities.

43

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