POLYMER MATRIX COMPOSITE: THERMALLY CONDUCTIVE GREASE
PREPARATION AND CHARACTERIZATION
A Thesis
Presented to
The Graduate Faculty of the University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Amit Adhikari
August, 2019 POLYMER MATRIX COMPOSITE: THERMALLY CONDUCTIVE GREASE
PREPARATION AND CHARACTERIZATION
Amit Adhikari
Thesis
Approved: Accepted:
1 1 Advisor Department Chair Dr. Jiahua Zhu Dr. Michael Cheung
1 ______Committee member Interim Dean of the college Dr. Rajeev Gupta Dr. Craig Menzemer
1 1 Committee member Dean of the Graduate School Dr. Zhenmeng Peng Dr. Chand Midha
1 04/30/2019
ii ABSTRACT
The next generation electronic devices are expected to be small in size and of magnified capacity. Denser packaging of the active components is important to miniaturize the electronic devices. Denser packaging is feasible only when heat generated by heat sources is quickly and effectively carried away to the heat sink. Next generation electronic devices with high performance microprocessors and integrated circuits along with diminished volume have led to major heat dissipation issue. Heat dissipation helps to control the temperature of the electronic devices at a desired level. Heat is dissipated to the heat sink from heat generator by the process of thermal conduction. Due to irregularities on the surfaces of the heat generator and heat sink, air is entrapped, and the air gap is formed in the path of thermal conduction. Air gap disturbs the thermal conduction as air is a really poor thermal conductor with a thermal conductivity of 0.026 W/mK. at room temperature. Air acts as a thermal barrier preventing the effective heat transfer between the heat source and heat sink. Different kind of thermal interface materials are used to fill up the air gap between the heat generator and the heat sink to improve thermal conduction.
Introduction of thermal interface material can significantly increase the performance of electronic device. In a typical power electronic package, a grease is used as thermal interface material. Thermal conductive paste with high thermal conductivity (much greater than air) fills up all the air gaps between the heat generator and the heat sink to improve
iii the thermal conduction. Development of the thermal conductive paste with low thermal resistance, high thermal conductivity and low electric conductivity is challenging and the most important aspect in today’s electronic industries. In the current study, we have tried to overcome this challenge by developing a thermally conductive grease with low thermal resistance, high thermal conductivity and low electric conductivity.
iv ACKNOWLEDGEMENTS
I would like to express heartfelt gratitude to my advisor Dr. Jiahua Zhu, for being immeasurably supportive during the past two years for the timely completion of my
Master’s degree. His constant encouragement towards completing this thesis and furthering my education has played a vital role in both my academic and professional career. This thesis is a result of his constant motivation and guidance.
Moreover, I would like to thank my committee members: Dr. Rajeev Gupta and Dr.
Zhenmeng Peng for their valuable advice and suggestions. Their time, instructions and ideas were immensely helpful in the betterment of the project since its inception.
Furthermore, I would like to acknowledge all my labmates: Dr. Liwen Mu, Tuo Ji,
Nitin Mehra, Yifan Li, Han Lin, Logan Brisbin for their support, constructive feedbacks and company. The time spent on the lab were some of the fun and rewarding experiences that will always remain close to my heart.
Most importantly, I would like to express my deepest appreciation to my family for being enormously encouraging and helpful. Thank you very much for always being my strength. You all are the reason I am here today and everywhere I will be tomorrow.
Finally, I appreciate the Department of Chemical and Bio-molecular Engineering for providing me the platform to further my education and for the successful completion of my
Master’s degree.
v TABLE OF CONTENTS
Page
LIST OF TABLES ...... ix
LIST OF FIGURES ...... x
CHAPTER
I. INTRODUCTION ...... 1
Motivation ...... 1
Introduction ...... 1
Outline ...... 4
Literature Review ...... 4
Overview ...... 4
Thermal Conductivity ...... 6
Thermal Resistance ...... 6
Electric conductivity ...... 7
Viscosity ...... 7
Stability ...... 7
Outgassing ...... 8
Cost ...... 8
Theoretical Background ...... 8
Base Materials:...... 10
Silicone Oil ...... 10
Epoxy Resin ...... 11
Polybenzoxazine ...... 12
Poly (alpha-olefin) oil ...... 12
vi Fillers ...... 12
Metallic Fillers ...... 13
Ceramic Fillers ...... 14
Carbon-based Fillers ...... 14
Hybrid Fillers ...... 14
Surface Modification of Fillers ...... 15
Commercial Greases ...... 15
Conclusion ...... 18
II. THERMALLY CONDUCTIVE GREASE WITH BN ...... 19
Introduction ...... 19
Experimental ...... 20
Materials ...... 20
Fillers’ Properties ...... 20
Preparation of Thermal Paste ...... 21
Characterization ...... 22
2.3 Results and Discussion ...... 24
Introduction ...... 37
Experimental ...... 37
Materials ...... 37
Fillers’ Properties ...... 38
Preparation of Thermal Paste ...... 39
Characterization ...... 41
Results and Discussion ...... 42
III. CONCLUSION AND FUTURE WORKS ...... 50
Conclusion ...... 50
Future Works ...... 51
vii BIBLIOGRAPHY ...... 53
viii LIST OF TABLES
Table 1. Commercial thermally conductive greases with reported viscosities...... 7 Table 2. Thermal greases containing silicone oil as a base material...... 11 Table 3. Different fillers used for thermally conductive grease...... 13 Table 4. Different thermally conductive greases available in market...... 17 Table 5. Properties of the Epoxy and thermal fillers...... 21 Table 6. List of thermally conductive grease with hybrid fillers...... 22 Table 7. Results from 2D bin packing...... 28 Table 8. Properties of the Epoxy and thermal fillers...... 38 Table 9. List of thermally conductive grease with hybrid fillers...... 40 Table 10. Thermal conductivity of thermal conductive grease with hybrid fillers...... 45
ix LIST OF FIGURES
Figure 1. Surface roughness filled with air, (b) Surface roughness filled with TIM (thermal grease)...... 2 Figure 2. SEM image of (a) BN and (b) graphite...... 24 Figure 3. Effects of BN and G content on the thermal conductivity of thermally conductive grease...... 25 Figure 4. Effect of hybrid fillers on thermal conductivity of thermally conductive grease...... 26 Figure 5. Co-ordinates of graphite and BN...... 27 Figure 6. Image from 2D bin packing of 30 graphite and 15 BN...... 28 Figure 7. Effect of hybrid fillers on thermal conductivity of thermally conductive grease...... 29 Figure 8. Comparison between theoretical model and experimental value...... 31 Figure 9. Comparison of thermal conductivity between theoretical and experimental value...... 33 Figure 10. Effect of G on the electrical conductivity of the thermally conductive grease...... 34 Figure 11. Effect of filler on the viscosity of the thermally conductive grease...... 35 Figure 12. Temperature rampage of EPON 828 and thermally conductive grease...... 36 Figure 13. SEM image of spherical Alumina...... 42 Figure 14. Effects of Ay and AY content on the thermal conductivity of thermally conductive grease...... 44 Figure 15. Effect of hybrid fillers on thermal conductivity of thermally conductive grease...... 47 Figure 16. Effect of hybrid fillers at AY25 on thermal conductivity of thermally conductive grease...... 48 Figure 17. Effect of filler on the viscosity of the thermally conductive grease with alumina filler...... 49
x CHAPTER I
INTRODUCTION
Motivation
The electronic devices manufactured in the recent years are smaller in size with enhanced power, which increases issues related to heat dissipation. The most commonly known and seen electronic devices in today’s market with heat dissipation issue are smart phones. We all know about Samsung having to recall 1 million out of 2.5 million Note 7 manufactured in 2016 because of 35 reported incidents of overheating worldwide. Phones use lithium ion battery packs for their power which contains flammable liquid electrolyte.
If the liquid heats up quickly, the battery is likely to explode. It is well known that even a small increment in operating temperature at the junction of electronic devices results in two times reduction in the life span of devices [1-4]. Hence, it is important and vital to remove unwanted heat from heat source to heat sink with the help of smooth thermal conduction process.
1 Let’s also take the evolvement of iPhone, from 2007 to 2017, as an example. In the last ten years, iPhones have become thinner every year and with superior power. This is only possible when electronic chips or micro-processors inside iPhone are densely packed that ultimately leads to problem with dissipation.
The trend of miniaturizing electronic devices with enhanced power motivated me to develop thermally conductive grease with high thermal conductivity for smooth operation of electronic devices in today’s market.
Introduction
The trend of shrinking electronic devices with enhanced power has increased rapidly. The size of Pentium 4 was decreased to 12.5 mm square from 25.4 mm square of
Pentium 2 die [5], even though Pentium 4 die dissipates power up to 80 W compared to 33
W of Pentium 2 die. The denser packaging of the active components or microelectronics in electronic devices is the most important aspect to miniaturize the devices and key to the development of high-performance systems [6]. The denser packaging is practicable only when unwanted heat is carried away from the heat source to the heat sink quickly and effectively, and heat is carried out by the process of thermal conduction between the heat source and the heat sink [7]. The reliability of electronic devices depends on the operating temperature of the junction. The small increment of operating temperature (in order of 10-
15oC) at the junction of the electronic devices result in two times reduction in the lifespan of the devices [8]. It is crucial to remove the unwanted heat from heat source of electronic devices as quickly and effectively as possible for the smooth operation of electronic devices at a desired temperature [9].
2 The most common method used to spread the unwanted heat out from electronic devices is the attachment of heat sink on the devices [7, 10, 11]. The thermal conduction is determined by the quality of thermal contact between the heat source and the heat sink
[12]. The performance of heat sink to scatter the unwanted heat is limited due to the interfacial resistance arising from non-surface flatness and the irregularities on the surface of both heat source (electronic devices) and heat sink [13]. Due to the irregularities on the surface of heat source and heat sink, air is entrapped in the gap, which later acts as a barrier for thermal conduction due to its low thermal conductivity: 0.026 W/mK [10, 14-20].
One of the common methods used to reduce interfacial resistance between the contact points is the introduction of an additional layer of material as shown in Figure 1.
The additional material is referred to as thermal interface material (TIM), which provides an effective pathway for thermal conduction [10, 21]. TIMs could be thermal fluids, solders, phase change materials and thermal grease (paste). Thermal grease based on polymer (silicone and epoxy) is the most commonly used thermal interface material [22].
Figure 1. (a) Surface roughness filled with air, (b) Surface roughness filled with TIM (thermal grease).
Different kinds of TIMs are developed when polymer matrix or silicone is reinforced with highly thermally conductive and electrically insulating fillers such as boron
3 nitride, aluminum nitride, alumina or silica [23, 24]. Higher thermal conductivity of TIMs means better thermal conduction between the heat source and heat sink. Along with high thermal conductivity, TIMs also should have low Coefficient of Thermal Expansion (CTE) and must be easily deformed by small contact pressure to contact all the irregular areas of mating surfaces [15].
Thermal grease—also known as thermal paste, heat paste, heat sink paste, thermal compound, thermal gel and gray goo—fills the air gap between the mating surfaces more effectively. Thermal grease also exhibits high thermal performance at small contact pressure and can be applied at minimal thickness at the junction. Thermal grease consists of two primary components: polymer base and thermally conductive filler [25]. The kinematic viscosity of polymer base ranges from 112 to 770 mm2/s at 40 oC [26]. Epoxies have been commonly used as the matrix of coatings, adhesives and composites [27].
Epoxies are high-performance thermosetting resins with high mechanical and adhesion properties, thermal stability, high electrical insulation, low cost and ease of processing [28]
[20]. Epoxy resins have very low thermal conductivity—around 0.2 W/mK—that can be further enhanced by adding high thermally conductive fillers [29]. The preferred volume percentage of filler in a thermal grease is about 55 to 78%. Ceramic fillers with high thermal conductivity and nearly electrically insulative—such as boron nitride, aluminum nitride, alumina and silicon carbide—are widely used [30, 31]. Although metallic fillers can yield grease with very high thermal conductivity, the cost associated with manufacturing metallic filler grease is paramount, and they do not provide electric insulation that might result in electric shocks when used on electronic devices.[10, 15, 19,
32, 33]
4 Outline
The objective of this project was to produce thermally conductive grease with high thermal conductivity and low electric conductivity. Two different projects were completed in this study.
In the first project, thermal greases were developed from epoxy (EPON 828) filled with Boron Nitride (BN) at various loadings: up to 60 wt% and EPON 828 filled with BN and Graphite (G) at various loadings up to 38 wt%. The synergistic effect of BN and G on thermal conductivity was studied. Two-dimensional (2D) bin packing technique was utilized to relate with packing density of fillers used. 2D finite bin packing helps to determine the number of stock pieces that provide all the pieces [34]. Filler loading was limited such that the fluidity of paste was not compromised. The electric conductivity and viscosity of thermal greases were also measured. Different theoretical models were studied to validate the experimental data. Maxwell model and Hamilton and Crosser Semi-
Theoretical (H&C) model were used to confirm the data for single filler. Lewis and Nelsen
Semi- Theoretical Model (L&M) and H&C models were used to authenticate the data for hybrid fillers.
In second project, thermal greases were developed from epoxy (EPON 828) filled with hybrid spherical fillers (Al2O3) of different sizes at various loadings: up to 50 wt%.
The synergistic effect of spherical fillers of different sizes were also studied. The main purpose of this study was to predict the thermal conductivity of thermally conductive grease experimentally and develop the thermal conductivity model using ANSYS in future.
Literature Review
Overview
5 Thermal conduction is involved in the use of a heat sink to dissipate unwanted heat from an electronic packaging, heat engines, the operation of a heat exchanger, and in numerous industrial processes that involve heating or cooling [6]. When two solid surfaces
(heat source and heat sink) come in contact, rough edge on each of the surfaces limit the actual contact between the two solids to a very small fraction [6, 7] leaving the air gap between them which acts as a thermal barrier. Effective transfer of heat by conduction between two solid surfaces requires materials of high thermal conductivity—such as thermal grease—to fill all the air gaps and spaces for denser packaging of electronic and its smooth operation. Thermal grease fills all the microscopic imperfections between the heat source and heat sink to improve the conduction of heat between them [8]. Thermal grease is a very high heat conductive paste that is used as an interface material between a heat source and a heat sink which eliminates the interfacial air gap between the heat source and the heat sink. Thermal grease also gives the mechanical strength to the bond between the heat source and the heat sink. Thermal grease is also called thermal paste, heat paste, heat sink paste, thermal compound, thermal gel and grey goo [10].
Paste comprises a mixture of powder and liquid. The preferred volume percentage of powder also known as a filler is about 55 to about 78 volume percentage [11]. Thermal pastes are made up of two primary components: polymer base having a viscosity of 112 to
770 mm2 at 40oC as a matrix, and fillers (metallic or ceramic) [12]. Both metallic fillers and ceramic fillers have their own pros and cons. Although metallic fillers can give high thermal conductivity, the cost associated with manufacturing metallic fillers paste is high.
Metallic fillers do not provide the electrical insulation which can short the electronics when used on the semiconductor chips [11]. Ceramics have gathered high attention in the
6 manufacturing of thermally conductive paste. Ceramics like Aluminum Nitride (AIN),
Boron Nitride (BN), Beryllium Oxide (BeO) do have high thermal conductivity and low coefficient of thermal expansion. They are resistive to corrosion and erosion, and they also provide the electrical insulation. Metallic fillers have several disadvantages about ceramic fillers including high density and susceptibility to oxidation [20]. Considering the cost of the fillers and their pros and cons, ceramics are considered to be excellent fillers. The most commonly used ceramics are Beryllium Oxide, Aluminum Nitride, Aluminum Oxide, Zinc
Oxide and Silicon Oxide. Epoxy resin, Polybenzoxazine, Silicone, Polyester, Vinyl Ester and Phenolic are some polymer bases used in manufacturing thermal conductive paste [15,
24, 35-37].
There are number of factors that must be taken into consideration when developing a thermal grease:
Thermal Conductivity
Thermal conductivity of thermal grease determines its ability to transfer the heat between heat source and heat sink through the interface [38]. The amount of heat dissipated from heat sink determines the thermal performance of any electronics. The ability to dissipate high heat fluxes helps in reduction in die size, cost, weight, and volume [39]. The thermal conductivity of commercially available greases ranges from 0.4 W/mK to 9 W/mK.
Thermal Resistance
The effective total thermal resistance is defined as the sum of thermal resistance due to the thermal conductivity of thermal grease and the contact resistance between the contact surface and thermal grease [1, 20, 40]. The effective total thermal resistance should be as low as possible to maintain the device below its operating temperature [38].
7 Electric conductivity
Electric conductivity of thermal grease cannot be of major issues where conduction of electricity is not of major issue. But, die attachments, encapsulations, substrates used in electronic packaging require thermal greases with high thermal conductivity and electrically insulative.
Viscosity
Viscosity is the major parameter in designing a thermal grease. The viscosity of an ideal thermal grease can range from around 80,000 centipoises to 545,000 centipoises [41].
The viscosity of thermal grease should be high enough to prevent the thermal grease from pumping out from an interface.
Table 1. Commercial thermally conductive greases with reported viscosities.
Thermal Grease Viscosity in Centipoises
TC-5121 85,013
TC-5022 89,160
SC 102 <100,00
SE 4490CV 500,000
340 Heat Sink Compound 542,000
Stability
Electronic devices such as microprocessors are expected to survive for up to ten years. Thermal greases are required to perform consistently throughout the lifespan of the electronic devices.
8 Outgassing
Silicone is the major polymer base for thermal grease and silicone outgas to some degree [42]. Silicone oil produces volatile gasses when exposed to elevated temperatures or low atmospheric pressure. Outgassing of thermal grease can be a concern in aerospace applications where outgassing is accelerated due to reduced pressures.
Cost
The cost of thermal grease is directly proportional to the filler used in it. Thermal greases with metallic fillers have higher thermal conductivity along with the cost compared to the ceramic fillers. Ultimately, performance cost and manufacturability of thermal grease dictate its market value.
Theoretical Background
The total thermal resistance (RTIM) of thermal grease can be expressed as [43]:
퐵퐿푇 푅푇퐼푀 = + 푅푐1 + 푅푐2 (1) 퐾푇퐼푀 where BLT represents the bond line thickness of thermal grease and Rc represents the contact resistance of the thermal grease with the two contacting surfaces [6, 7]. The total thermal resistance should be minimized for better heat transfer between heat source and sink. When no thermally conductive grease is applied between the heat source and heat sink interstitial air is present in between, which increases the total thermal resistance. The total thermal resistance can be minimized by using the thermally conductive grease in the gap between the heat source and heat sink. Reduction of the total thermal resistance (RTIM) can be accomplished by reducing the bond line thickness (BLT), increasing the thermal conductivity of thermal grease (KTIM) and reducing the contact resistance [44].
9 The thermal conductivity of a grease can be enhanced by increasing the loading percentage of the thermally conductive fillers in the polymeric base (mostly silicone oil). The thermal conductivity of the thermal grease can be functionally expressed as [45]:
퐾푇퐼푀 = 푓(푘푓, 푘푚, ∅, 푅푏) (2)
where 푘푓 is the thermal conductivity of filler (like: Aluminum Nitride, Boron Nitride,
Alumina and many more), 푘푚 is the thermal conductivity of polymeric matrix (like: silicone oil, epoxy and many more), ∅ is the volume fraction of the filler and 푅푏 is the contact resistance between the fillers and polymeric matrix. This model matches the data for spherical particles with ∅ up to 35%. The thermal conductivity of grease with higher loading cannot be predicted using this model. Prasher et al. used the modified Bruggeman model to predict the thermal conductivity with higher loading [46]. Modified Bruggeman model includes the effect of interface resistance between the filler and the matrix. The modified Bruggeman model for Kf/Km>>1 is given by:
퐾푇퐼푀 1 = 3(1−훼)(1+2훼) (3) 퐾푚 (1 − ∅)
Where 훼 is the Biot number and is defined as:
푅 퐾 훼 = 푏 푚 (4) 푑
Where d is the diameter of the particle
Bond line thickness depends on the pressure applied in bringing the two contact surfaces together, and it also depends on the particle volume fraction [47]. Prasher et al. proposed
10 the following correlation for BLT after conducting a study on eight different formulations of particle laden polymeric thermal interface materials including thermal grease [46]:
0.166 휏푦 퐵퐿푇 = 1.31 ∗ 10−4 (5) 푃
Where 휏푦the yield stress of thermal grease and P is the applied pressure. This correlation was validated in the range of 25-200 psi application pressures.
Base Materials:
Silicone, Epoxy resin, Polybenzoxazine, Polyester, Vinyl Ester and Phenolic are some polymer bases used in manufacturing thermal conductive paste. According to
Phillip E. Kendall and Ravi K. Sura, a thermally conductive grease contains up to 49.5 weight percentage of carrier oil or base material [43].
Silicone Oil
The thermal conductivity of silicone oil is around 0.165 W/mK. The thermal conductivity of silicone oil can be increased when filled with highly conductive fillers. The thermal conductivity of methyl silicone oil is reported up to 2.5 W/mK when filled with inorganic thermally conductive fillers. The viscosity of silicone oil is around 500 mPa s which provides good fluidity and processing performance of the grease when filled with highly conductive fillers [27].
11 Table 2. Thermal greases containing silicone oil as a base material.
Base Material Thermally Volume Fraction of Thermal
Conductive Filler a Filler Conductivity
(W/mK)
Silicone Oil α- Al2O3 0.47 1.0
Silicone Oil α- Al2O3 0.64 2.2
Silicone Oil SiC 0.47 1.3
Silicone Oil SiC 0.64 2.6
Volume Fraction of a Filler is defined as the real volume of filler divided by total volume of filler and silicone oil.
Yu and et al. studied the thermally conductive silicone grease reinforced with hybrid fillers
(graphene and alumina composite fillers)[25]. They reported the maximum thermal conductivity of the novel grease is 3.45 W/mK at a volume fraction of 63%. Novel grease contains 1 wt. % of graphene and the thermal conductivity of the novel thermal grease increases from 2.70 W/mK (without graphene) to 3.45 W/mK.
Epoxy Resin
The thermal conductivity of epoxy resin ranges from 0.1 to 0.3 W/mK [46]. The thermal conductivity of epoxy resin can be increased when filled with highly conductive fillers. P. Bujard and J.P. Anserment reported the thermal conductivity of 4 W/mK when the epoxy resin was filled with Aluminum Nitride (AIN) particles [8]. An anhydride cured epoxy labeled SA5 with a thermal conductivity of 0.183 at 30oC was used in the system.
12 Two different grades of AIN (Grade E and Grade H) were used in the system. Both Grade
E and Grade H comprises particles of irregular shapes with a mean diameter of 31 micro- meter and 30 micro-meter, respectively. The thermal conductivity increases gradually from
0.2 W/mK to 1.8 W/mK when filler loading increases from 0% to 50%. After 50%, thermal conductivity increases rapidly up to 62%. The maximum thermal conductivity of 4W/mK is achieved at a loading of 62%.
Polybenzoxazine
Polybenzoxazine is a polymerized phenolic system, having a wide range of interesting features as (i) near zero volumetric change upon curing, (ii) low water absorption and (iii) release of no toxic by-product during curing [11]. Hatsuo Ishida and
Sarawut Rimdusit reported the thermal conductivity of 32.5 W/mK of BN filled poly benzoxazine at its maximum loading of 78.5 % by volume [42]. Polybenzoxazine is a bisphenol-A-methylamine based epoxy with a very less viscosity which aids in very well mixing and wetting of filler (BN). The resin used for this sample preparation is a dry powder, and it was dry mixed with BN at desired volume fraction. The sample was heated up to 80oC and mixed by hand for ten minutes to obtain a paste-like material.
Poly (alpha-olefin) oil
Sushhmna Iruvanti and et al. developed a thermal conductive grease with very thermal conductivity of 3.6 W/mK and with a paste viscosity of 15,140 Pa sec [22].
Aluminum and calcined aluminum oxide with a particle size of 0.5-7 micro-meter were dispersed in a poly (alpha-olefin) oil at 72% by volume. This paste is claimed to be used as a thermal paste connection between the electronic component and cooling surface.
Fillers
13 Metallic and ceramic fillers are two types of commonly used fillers in developing the thermal conductive paste. According to Jeffrey Anderson and Philip Kendall, thermal conductive grease consists of at least 49.5 weight percentage of thermally conductive fillers.
Table 3. Different fillers used for thermally conductive grease.
Filler Thermal Conductivity W/mK
Diamond 2400 – 2500
Silicon Carbide 120
Alumina 30-100
Boron Nitride (Hexagonal or Cubic) 30-130
Boron Carbide 17-42
Silica 1.3-1.5
Graphite 25-470
Metallic Fillers
Silver and aluminum are the most common metallic fillers used for thermal grease because of their high thermal conductivity, availability and affordability [31]. Metallic fillers do not provide the electrical insulation which can short the electronics when used on the semiconductor chips [5]. The cost associated with manufacturing the paste with metallic fillers is high.
14 Ceramic Fillers
Ceramic fillers have gathered high attention in manufacturing the thermal grease as some ceramics have high thermal conductivity with a low coefficient of thermal expansion.
Most commonly used ceramic are Aluminum Nitride, Aluminum Oxide, Boron Nitride
Beryllium Oxide, Silicone Oxide and Zinc Oxide [48].
Arun Gowda and et al. developed a thermal grease incorporating spherical boron nitride into silicone resin [12]. The morphology of boron nitride is inherently hexagonal; spherical boron nitride was synthesized by melting boron nitride in microwave-powered, atmospheric pressure nitrogen plasma [21]. Arun Gowda and et al. achieved the maximum thermal conductivity of 5.39 W/mK at a loading of 35 weight percentage [12].
Carbon-based Fillers
Due to promising thermal, mechanical and functional properties carbon fillers have gained high attention in manufacturing the thermal grease. Graphene, Graphite, Carbon
Nano-Tube (CNT), Carbon Nano-Fiber (CNF) are commonly used carbon fillers. The thermal conductivity of carbon fillers ranges from 25-3000 W/mK [7]. Hong and et al. developed a stable and homogenous grease incorporating CNTs (thermal conductivity- 35
W/mK) in poly alpha olefin oil (thermal conductivity 0.17-0.18 W/mK). They used single- wall and multi wall CNTs, and they prepared samples by dispersing 10-12% SWCNTs and
19-20% MWCNTs in the oil. The thermal conductivity appeared saturated around 0.25-
0.27 W/mK.
Hybrid Fillers
The hybrid size fillers lead to dense packaging structure in the base material which leads to the better thermal conductivity. Yu and et al. studied the effect of hybrid fillers.
15 They studied the synergistic effect of graphene and alumina which decrease the thermal boundary resistance and increase the thermal conductivity. The two-dimensional graphene bridges the gap between alumina particles to provide faster and more effective pathways for phonon transfer in thermal grease. The thermal conductivity of the hybrid alumina filler grease was increased from 2.7 W/mK to 3.45 W/mK after addition of 1 weight percentage of graphene in the sample at a loading of 63 volume percentage [25]. Hybrid alumina filler was comprised of large particle and a small particle with fixed volume ration of 85:15.
Surface Modification of Fillers
The surface modification of fillers helps to improve the interface reaction between the base material and filler. It decreases the contact resistance between the base material and filler which provides faster and more effective pathways for phonon transfer in thermal grease. Many organic coated methods such as chemical and physical adsorption, micro-encapsulation technology, etc. have been used for the surface modification of fillers. The thermal conductivity of BN and AN particle epoxy composites was increased by up to 97% by surface treatment of the fillers [49].
Epoxy resin composite filled with surface modified BN was developed [20]. The thermal conductivity of epoxy resin was increased from 0.202 W/mK to 1.052 W/mK with
60 percentage of a mass fraction of modified BN [20]. Surface modification of boron nitride was carried out by KH550, and it was characterized by FTIR (Fourier Transform
Infrared Spectra).
Commercial Greases
16 Different commercially available greases were compared as to thermal resistance, conductivity, composition and cost [4]. High-performance pastes can yield resistances as low as 13 and 33 mm2 K/W, for a BLT of 25 and 100 microns, respectively [4]. The thermal conductivity of commercially available greases ranges from 1 to 7 W/mK.
17 Table 4. Different thermally conductive greases available in market.
Paste Thermal Contact Thermal Conductivity (W/mK) Composition Cost ($/g) Resistance @ ~ 75 Around 75 oC Mfg. Data oC (mm2K/W)
Wacker Silicone P12 13.6 0.54 0.8 Dimethylsiloxane(s) with filler 0.07 (trade secret)
Aavid Thermalloy 19.6 0.4 0.39 Polydimethylsiloxan-e (30%- 0.04 Thermalcote 251 G 60%); zinc oxide 930%-60%)
Arctic Silver 5 7.9 0.94 8.7 Silver,boron nitride,zinc oxide, 2 aluminum oxide, ester oil blend
Thermaxtech Xt-flux- 6.0 0.78 7 Silicone oil, high-purity particle 1.2 GA (info not available)
Dow Corning TC- 8.7 4.0 4.0 Dimethyl,methyldecylsiloxane 0.43 5022 (1%-5%), metal oxides, treated fillers (trade secret)
Shinetsu X-23-7762- 6.3 3.7 4.4 Aluminum (>60%), zinc oxide 0.9 S (<30%), unknown solvent (trade secret)
18 Conclusion
All unwanted heat should be dissipated from heat source for smooth operation of an electronic device. The heat is dissipated from the heat source to heat sink by the means of thermal conduction. Heat dissipation helps to control the temperature of the electronic devices at the desired level. As we know, the electronic devices are being miniaturized with the requirement of high power dissipation and as such, there is going to be a need for thermal grease with high performance, low thermal resistance and long-term stability.
Hybrid fillers lead to dense packaging structure in the base materials for better thermal conductivity. Hybrid fillers bridge the gap between the different size particles which provides better and effective pathway for transfer of phonons. The surface modification of fillers helps to improve the interface reaction between the base material and filler. It decreases the contact resistance between the base material and filler, which provides faster and more effective pathways for phonon transfer in thermal grease.
Although carbon-based fillers can be used to increase the thermal conductivity of thermal grease, achieving a low thermal resistance and low electric conductivity remains a challenge [28].
19 CHAPTER II
THERMALLY CONDUCTIVE GREASE WITH BN
Introduction
In this study, thermal greases were developed from epoxy (EPON 828) filled with
BN at various loadings: up to 60 wt% and EPON 828 filled with BN and G at various loadings up to 38 wt%. The synergistic effect of BN and G on thermal conductivity was studied. Two-dimensional bin packing technique was utilized to relate with packing density of fillers used. 2D finite bin packing helps to determine the number of stock pieces that provide all the pieces [34]. Filler loading was limited such that the fluidity of paste was not compromised. The electric conductivity and viscosity of thermal greases were also measured. Different theoretical models were studied to validate the experimental data.
Maxwell model and Hamilton and Crosser Semi- Theoretical (H&C) model were used to confirm the data for single filler. Lewis and Nelsen Semi- Theoretical Model (L&M) and
H&C models were used to authenticate the data for hybrid fillers.
20 Experimental
Materials
Undiluted clear di-functional bisphenol A/epichlorohydrin derived liquid epoxy resin (EPON 828) provided by Hexion Chemicals was used, as received, as a matrix in this project. EPON 828 is considered to have a magnificent mechanical, adhesive, dielectric and chemical properties.
Selection of the fillers plays an important role in the enhancement of thermal conductivity of thermally conductive grease because of their shape and size. Two different fillers of similar structure (platelet like) and different size were used in the project to enhance thermal conductivity of EPON 828 which allows them to adhere adequately.
PCTP30 BN—with an average particle size of 30 microns provided by CarboTherm—and
3775 graphite—with an average particle size of 44 microns provided by Asbury Carbon— were utilized as fillers to intensify the thermal conductivity of EPON 828. The average particle size of the fillers was verified from SEM images and using image J software.
Fillers’ Properties
BN is a CarboTherm platelet powder that is well suited to the thermoplastic polymer. BN with a platy lamellar structure like graphite, tends to align with the direction of flow in polymer processing, making it best fit for heat spreading applications. Graphite is a natural based element that has been exfoliated, subsequently calendared, and then sized for thermal management applications. Surface Enhanced Flake Graphite has a vast range of applications including conductive seals, mechanical seals, high-purity metal cover carbon, nano-platelet systems, friction systems, coatings, greases and lubricant additive.
21 The properties of the materials including thermal conductivity (λ), average particle diameter (PD), specific surface area (SSA) and molecular weight (Mn) mentioned above are shown in Table 5.
Table 5. Properties of the Epoxy and thermal fillers.
Density λ PD SSA Materials (g/cm3) (W/mK.) (µm) (m2/g)
EPON 828 1.16 0.202 N/A N/A
G 2.26 230 44.00 17.20
BN 2.10 130 30.00 1.00
Preparation of Thermal Paste
The thermal grease with one filler was prepared by first weighing the filler in a 30 mL beaker, and later adding EPON 828 in desired quantity to meet the specific weight ratio. The mixture was left at room temperature for five hours to moisten. The wetting of sample would prevent blowing off of the filler while using the mechanical stirrer for mixing. The sample was then mixed with the help of mechanical mixer (RZR 2021).
Thermal grease was obtained by mechanical mixing of the sample at 1800 RPM for four hours. The thermal grease with hybrid fillers was prepared using the same procedure as above in which larger particle (G) was mixed first. The smaller particle (BN) was added to the mixture after four hours. The sample was then mixed for two more hours to obtain the hybrid thermal grease. Thermally conductive grease with hybrid fillers are listed in Table
6 and they are named as BxGy, where x and y represent the weight percentage of BN and graphite, respectively.
22 Table 6. List of thermally conductive grease with hybrid fillers.
Sample BN Graphite Total vol% of fillers BxGy wt% vol% wt% vol%
B5G33 5.0 2.3 33.0 14.1 16.4
B10G28 10.0 4.6 28.0 11.9 16.5
B15G23 15.0 6.8 23.0 9.8 16.6
B20G18 20.0 9.1 18.0 7.6 16.7
B25G13 25.0 11.4 13.0 5.5 16.9
B15G10 15.0 7.3 10.0 4.5 11.8
B15G15 15.0 7.1 15.0 6.6 13.7
B15G20 15.0 6.9 20.0 8.6 15.5
B15G25 15.0 6.8 25.0 10.5 17.3
B15G30 15.0 6.6 30.0 12.4 19
Characterization
The morphology of BN and graphite was characterized by scanning electron microscopy (SEM, JEOL-7401). The thermal conductivity of samples was measured with a thermal conductivity analyzer (C-Therm TCi, Canada). TCi is based on a modified transient plane source method, and this method is convenient for measuring the thermal conductivity of solid, liquid and powder. A curling-type heating source is situated at the hub of the sensor, and heat is triggered at the hub. Then, triggered heat enters the material
23 through the sensor during which a voltage diminution occurs rapidly at the heating source, and thermal conductivity is estimated from the decreased voltage data. The testing capability of the system is 0-100 W/mK across a wide range of temperature (-50 to 200 oC), and accuracy of the instrument is more than 5%. Special attention was applied while measuring the thermal conductivity of the grease. The thermal conductive grease was left on the top of the sensor for 30 minutes to stabilize before measuring the thermal conductivity. The stabilization technique helped in taking consistent measurement of thermal conductivity. The electrical insulation properties of the thermal greases were characterized by Keithley 2750 using 2-wire resistivity technique. The electric resistivity was measured by using 2-wire resistivity technique, and electric conductivity was calculated. In a 2-wire resistivity technique, only two probes are needed to be manipulated where each contact served as a current and as a voltage probe [50]. The viscosity of the thermal greases was characterized by Bohlin Gemini Rheometer. The rheometer consisted of cone and plate. A cone with 4-degree angle and plate with 40 mm were used for measuring the viscosities of thermal greases. The gap distance between cone and plate was set at 150 μm. A 3 mega pixels infrared thermal imaging camera (FLIR E40: 19,200 pixels,
Instrumary, USA) was used to assess the heat transfer. It took thermal images of the paste during the process of heating. Thermal grease and epoxy were loaded at the center of aluminum plate using 0.8” inner diameter and 1.0” outer diameter O-ring. DC power supply (PWS2721, Tektronix) was used to heat the aluminum plate and constant voltage mode was used in the process of heating, where the voltage was set at 12 V and current was supplied at 0.32 A.
24 2.3 Results and Discussion
Figure 2. SEM image of (a) BN and (b) graphite.
The size and shape of fillers were seen to have an effect on thermal conductivity of grease. Morphology of BN and G were observed by scanning electron microscope. Both
BN and G had platelets like structure as seen in Figure 2. Average particle size of BN and
G was measured from SEM images using image J software. The average particle size of
BN was derived to be 31.2 µm, and average thickness was 2.01 µm. The average particle size of G was 44.9 µm, and average thickness was 1.9 µm. The aspect ratio (A) was calculated using equation (1) [51]. The average aspect ratio of BN and G was calculated to be 15.52 and 23.63, respectively from 25 different readings.
퐴푣푒푟푎푔푒 퐷𝑖푎푚푒푡푒푟 (퐷) 퐴푠푝푒푐푡 푅푎푡𝑖표 (퐴) = (6) 퐴푣푒푟푎푔푒 푇ℎ𝑖푐푘푛푒푠푠 (푇)
The effects of BN and G content on the thermal conductivity of EPON 828 based thermal grease are shown in Figure 3. Thermal conductivity of the thermal grease was directly proportional to the content of filler. The thermal conductivity of 60 wt% BN in
25 EPON 828 was 1.01 W/mK, five times higher than that of native EPON 828 (0.202 W/mK) and the thermal conductivity of 30 wt% graphite in EPON 828 was 1.21 W/mK, six times higher than that of native EPON 828. The mobility of thermal grease decreased rapidly as the filler content increased beyond 60 wt% for BN and 30 wt% for graphite. We could see that the thermal conductivity of thermal grease increased significantly up to 45 wt% of BN and 25 wt% of G. The viscosity of thermal grease increased with increasing wt% of fillers, and it made difficult for better dispersion of the filler in EPON 828 which limited the thermal conductivity path.
1.5 BN G 1.2
0.9
0.6
0.3 Thermal Conductivity (W/mK)
0.0 0 5 10 15 20 25 Filler Loading (vol. %)
Figure 3. Effects of BN and G content on the thermal conductivity of thermally conductive grease.
26 Different thermal greases with hybrid fillers were prepared for changing wt% of
BN and G to obtain the optimal ratio of BN and G. Total wt% of fillers was kept constant at 38 wt% to maintain the mobility of thermal grease. Figure 4 shows the thermal conductivity of thermal grease as a function of weight fraction of the small particle (BN), in which the total weight fraction of hybrid fillers (BN + graphite) is fixed at 38 wt%.
Beyond 38 wt% of hybrid fillers the mobility of thermal grease decreased while mixing and it limited the dispersion of fillers in EPON 828. It could be seen that the thermal conductivity of thermal grease filled with hybrid fillers increased with the increase in weight fraction of small particle up to 15 wt%. The maximum thermal conductivity is 2.21
W/mK at 38 wt% of total fillers out of which 15 wt% is BN, and 23 wt% is G, almost ten times higher than that of native EPON 828. The thermal conductivity of neat EPON 828 increased by 994%. This suggests that the optimum weight ratio of large particles to small particles improves the packaging volume and enhances the thermal conductivity of thermal grease.
Figure 4. Effect of hybrid fillers on thermal conductivity of thermally conductive grease.
27 2D bin packing technique was used to verify the optimal ratio of BN and graphite directly related to the packing density. The co-ordinates of BN and graphite were generated by using AutoCAD utilizing average particle diameter of BN and G as shown in figure 5.
Regular hexagon was drawn inside circle of specific diameter. Circle was divided into six equal parts using “divide” command and lines were drawn to form a hexagon. The ratio of
BN to G was set in such a way that BN was fixed at 15 and G was ranged from 5 to 30, as shown in Figure 6. The bin size was chosen to fit the combination of BN and G minimally.
The waste area of bin was calculated as shown in Table 3 to reflect the packing density of the combination as they are directly proportional to each other. Area of Graphite was calculated to be 1257.5unit squares and area of BN as 584.6unit squares. Graphical image obtained from 2D bin packing of 30 graphite and 15 BN is shown in Figure 6.
Figure 5. Co-ordinates of graphite and BN.
28 Figure 6. Image from 2D bin packing of 30 graphite and 15 BN.
Table 7. Results from 2D bin packing.
Sample Area of Bin Area of BN + Waste area Ratio
BxGy Area of graphite
B15G5 23,870 15,055.9 8,814.4 0.36
B15G10 31,329 21,343.2 9,985.8 0.31
B15G15 40,401 27,630.5 12,770 0.31
B15G20 48,029 33,917.3 14,111.7 0.29
B15G25 58,081 40,205.2 17,875.8 0.30
B15G30 69,696 46,492.6 23,203.4 0.33
29 Table 7 shows that minimal waste area is obtained when number of graphite is in between 20 and 25 fixing BN at 15. Minimal waste area ratio means that fillers are densely packed reflecting higher packing density.
Different thermal greases were prepared by fixing BN at 15 wt% and altering the wt% of graphite after knowing the optimum weight percentage of the small particle (BN) in hybrid fillers. The effects of hybrid fillers with fixed weight percentage of BN at 15 wt% on the thermal conductivity of the EPON 828 based thermal greases are shown in Figure
7.
Figure 7. Effect of hybrid fillers on thermal conductivity of thermally conductive grease.
30 Thermal conductivity models can be used to predict the thermal conductivity of greases containing thermally conductive fillers. Hamilton and Crosser Semi-Theoretical
Model (H&C) was used to predict the thermal conductivity of thermally conductive grease with single filler. This model accounts for integral thermal conductivities, concentration of each integral, aspect ratio, orientation and packing of the filler [52, 53]. Equation 7 was used to predict the thermal conductivity of grease with:
퐾푑 + (푛 − 1) ∗ 퐾푐 − (푛 − 1) ∗ ∅ ∗ (퐾푐 − 퐾푑) 퐾 = 퐾푐 ∗ (7) 퐾푑 + (푛 − 1) ∗ 퐾푐 + ∅ ∗ (퐾푐 − 퐾푑)
Where n is an empirical constant and n is calculated as follow:
3 푛 = (8) 휑
Where 휑 is the sphericity. The sphericity is defined as the surface area of a sphere, with a volume equal to that of the particle, to the surface area of the particles. The parameter n for
BN and G was calculated from equation 8. Different thermally conductive greases were prepared with single filler at very low loading (1 to 15 wt%) and thermal conductivity was measured in the lab. The measured thermal conductivity was used to back calculate n for each grease and average n was calculated. Calculated n was used to predict the thermal conductivity of thermally conductive grease. Figure 8 shows the comparison between theoretical model and experimental value.
31 Figure 8. Comparison between theoretical model and experimental value.
Lewis and Nielsen Semi-Theoretical Model (L&N) was used to predict the thermal conductivity of thermally conductive grease with hybrid filler. This model accounts for integral thermal conductivities, concentration of each integral, aspect ratio, orientation and packing of the filler. Equation 9 was used to predict the thermal conductivity of grease with hybrid filler:
(1 + (퐴 ∗ 퐵 ∗ ∅)𝑖) 퐾 = 푘 ∗ (9) 1 (1 − (퐵 ∗ 휑 ∗ ∅)𝑖) where B is defined in Equation (10):
32 푘 ( 2 − 1) 푘 퐵 = 1 (10) 푘 ( 2 + 퐴) 푘1
k1 is the thermal conductivity of the matrix, k2 is the thermal conductivity of the filler, ∅ is the filler volume fraction. A is the shape and orientation factor, and B is a factor that takes into account the relative thermal conductivity of the two components. 휑 is given by
Equation 11 [52]:
1 − ∅푚 휑 = 1 + 2 ∗ ∅ (11) ∅푚
∅푚 is the maximum packaging fraction of the filler. 휑 relates to the maximum packing fraction and the filler volume fraction.
Parameters A and ∅푚 were taken from the tables listed elsewhere [32]. A was taken as 1.5 for BN considering it a sphere and A for G was supposed to be 15. Maximum packaging fraction of BN was taken to be 74% considering it as a sphere and packing close to hexagonal packing.
푛 2 ∑푖=1(푥푖 − 푦푖) 휀 = 푛 2 (12) ∑푖=1 푥푖
휀 is defined as a standardized lack of fit term and quantifies how the theoretical model compares to the experimental data [53]. 푥푖 is the experimental thermal conductivity measured in the lab and 푦푖 is the thermal conductivity predicted by the model.
After predicating the theoretical thermal conductivity (kth) from Nielsen’s model, correction factor of -1.79 + 1.91*kth was applied to correct the gap during extrapolation from theoretical to experimental model. The correction factor was obtained from linear
33 regression. Figure 9 shows the comparison of thermal conductivity between the theoretical and experimental models. Standardized lack of fit term for thermal grease with hybrid filler was calculated to be 0.11 using the experimental model. A value of 휀 = 0 indicates a perfect fit of the experimental data with the model.
Figure 9. Comparison of thermal conductivity between theoretical and experimental value.
The thermal grease should be electrically insulated for use in electronic devices to prevent electric shock. The electrical insulating properties are conserved in thermal grease by using electrically insulating filler (BN) and polymer matrix (EPON 828). The electric conductivity of the sample with 25 wt. % of G and 15 wt. % of BN was 7.3E -6 S/m. Figure
10 shows that the electric conductivity of thermally conductive grease is directly proportional to the electrically conductive filler (3775 G in this case) loading.
34 Figure 10. Effect of G on the electrical conductivity of the thermally conductive grease.
Thermal grease should exhibit high thermal performance at small contact pressure, and it can be applied at minimal thickness at the junction. The mobility of thermal grease should be preserved so that it can fill all the air gaps between the heat source and the heat sink providing an implicit pathway for thermal conduction. Thermally conductive grease should not separate, run, migrate or bleed from the contact points. The viscosity of most commonly used thermally conductive grease (Arctic Silver 5) was measured to characterize the viscosity of our thermally conductive greases. The viscosity of arctic silver 5 was measured to be 145 PaS. Figure 11 shows that the viscosity of thermally conductive grease is directly proportional to the filler loading.
35 Figure 11. Effect of filler on the viscosity of the thermally conductive grease.
Three mega pixels infrared thermal imaging camera was used to take thermal images of sample during the process of heating. Thermal images of pure epoxy and thermal grease (B15G23) were taken for five minutes at thirty seconds interval. The results suggest rapid heating of thermal grease as compared to pure epoxy, as shown in Figure 12. This proves that thermally conductive grease helps in quicker dissipation of heat from heat source to heat sink. The maximum temperature reached by EPON 828 after five minutes was 66.3 oC and that of thermal grease was 72.4 oC. EPON 828 started to leak out from O- ring after the temperature reaches 51.5 oC while thermally conductive grease remains intact. This shows that the thermal stability of thermally conductive grease is more than that of pure EPON 828.
36 Figure 12. Temperature rampage of EPON 828 and thermally conductive grease.
37 CHAPTER III
THERMALLY CONDUCTIVE GREASE WITH AL2O3
Introduction
In this study, thermal greases were developed from epoxy (EPON 828) filled with different size of spherical alumina at various loadings: up to 50 wt%. The synergetic effect of different size of spherical fillers was studied using experimental method. Filler loading was limited such that the fluidity of paste was not compromised. The electric conductivity and viscosity of thermal greases were also measured. Theoretical model was studied to validate the experimental data. Lewis and Nelsen Semi- Theoretical Model (L&M) and
H&C models were used to authenticate the data for hybrid fillers.
Experimental
Materials
38 Undiluted clear di-functional bisphenol A/epichlorohydrin derived liquid epoxy resin (EPON 828) provided by Hexion Chemicals was used, as received, as a matrix in this project. EPON 828 is considered to have a magnificent mechanical, adhesive, dielectric and chemical properties.
Selection of the fillers play an important role to enhance the thermal conductivity of thermally conductive grease because of their shape and size. Spherical alumina fillers of different size were used in the project to enhance thermal conductivity of EPON 828 which allows them to adhere adequately. Spherical alumina has high thermal conductivity along with high sphericity which results in high followability and high packing density.[54]
AY35-125 and AY75-150 alumina, with an average particle size of 37 and 75 microns respectively were provided by Sanyo Corporation of America. These fillers were utilized to intensify the thermal conductivity of EPON 828.
Fillers’ Properties
Alumina, also called aluminum oxide, Al2O3, a colorless crystalline substance that is used for various applications like fillers for resin/thermal conductive sheets, placing sand for ceramic sintering, blasting, spacer and many more. Spherical alumina varies in different size from 1-80 microns. The properties of the materials including thermal conductivity
(λ), average particle diameter (PD), specific surface area (SSA) and molecular weight (Mn) mentioned above are shown in Table 1.
Table 8. Properties of the Epoxy and thermal fillers.
Density λ PD SSA Materials (g/cm3) (W/mK.) (µm) (m2/g)
39 EPON 828 1.16 0.202 N/A N/A
AY35-125 3.80 20.00 37.00 0.2
AY75-150 3.80 20.00 75.00 0.2
Preparation of Thermal Paste
The thermal grease with one filler was prepared by first weighing the filler in a 30 mL beaker, and later adding EPON 828 in desired quantity to meet the specific weight ratio. The mixture was left at room temperature for five hours to moisten it. The wetting of sample would prevent blowing off of the filler while using the mechanical stirrer for mixing. The sample was then mixed with the help of mechanical mixer (RZR 2021).
Thermal grease was obtained by mechanical mixing of the sample at 1800 RPM for four hours. The thermal grease with hybrid filler was prepared by weighing the AY75-150 filler in a 30 mL beaker, and later adding EPON 828 in desired quantity to meet the specific weight ratio. The mixture was left at room temperature for five hours to moisten it. The wetting of sample would prevent blowing off of the filler while using the mechanical stirrer for mixing. The sample was then mixed with the help of mechanical mixer (RZR 2021).
The sample was mixed for three hours at 1800 RPM. The sample was then mixed for two more hours to obtain the hybrid thermal grease at same speed. Thermally conductive with hybrid fillers are listed in Table 8 and they are named as AyxBYz, where Ay and AY represent AY35-125 and AY75-150, respectively. Where, x and z represent the weight percentage of A and B, respectively.
40 Table 9. List of thermally conductive grease with hybrid fillers.
AY35-125 AY75-150 Sample Total vol% of fillers wt% vol% wt% vol%
Ay5AY5 5.0 1.5 5.0 1.5 3.0
Ay5AY10 5.0 1.5 10.0 2.9 4.4
Ay5AY15 5.0 1.4 15.0 4.3 5.8
Ay5AY20 5.0 1.4 20.0 5.7 7.1
Ay5AY25 5.0 1.4 25.0 7.0 8.4
Ay10AY5 10.0 7.3 10.0 4.5 11.8
Ay10AY10 15.0 7.1 15.0 6.6 13.7
Ay10AY15 15.0 6.9 20.0 8.6 15.5
Ay10AY20 15.0 6.8 25.0 10.5 17.3
Ay10AY25 15.0 6.6 30.0 12.4 19
Ay15AY5 15.0 7.3 10.0 4.5 11.8
Ay15AY10 15.0 7.1 15.0 6.6 13.7
Ay15AY15 15.0 6.9 20.0 8.6 15.5
Ay15AY20 15.0 6.8 25.0 10.5 17.3
Ay15AY25 15.0 6.6 30.0 12.4 19
Ay20AY5 15.0 7.3 10.0 4.5 11.8
Ay20AY10 15.0 7.1 15.0 6.6 13.7
41 Ay20AY15 15.0 6.9 20.0 8.6 15.5
Ay20AY20 15.0 6.8 25.0 10.5 17.3
Ay20AY25 15.0 6.6 30.0 12.4 19
Ay25AY5 15.0 7.3 10.0 4.5 11.8
Ay25AY10 15.0 7.1 15.0 6.6 13.7
Ay25AY15 15.0 6.9 20.0 8.6 15.5
Ay25AY20 15.0 6.8 25.0 10.5 17.3
Ay25AY25 15.0 6.6 30.0 12.4 19
Characterization
The morphology of BN and G was characterized by scanning electron microscopy
(SEM, JEOL-7401). The thermal conductivity of samples was measured with a thermal conductivity analyzer (C-Therm TCi, Canada). TCi is based on a modified transient plane source method, and this method is convenient for measuring the thermal conductivity of solid, liquid and powder. A curling-type heating source is situated at the hub of the sensor, and heat is triggered at the hub. Then, triggered heat enters the material through the sensor during which a voltage diminution occurs rapidly at the heating source, and thermal conductivity is estimated from the decreased voltage data. The testing capability of the system is 0-100 W/mK across a wide range of temperature (-50 to 200 oC), and accuracy of the instrument is better than 5%. Special care was applied while measuring the thermal conductivity of the grease. The thermal conductive grease was left on the top of the sensor for 30 minutes to stabilize before measuring the thermal conductivity. The electrical
42 insulation properties of the thermal greases were characterized by Keithley 2750 using 2- wire resistivity technique. The electric resistivity was measured by using 2-wire resistivity technique, and electric conductivity was calculated. In a 2-wire resistivity technique, only two probes needed to be manipulated. Each contact served as a current and as a voltage probe [50]. Viscosity of the thermal greases was characterized by Bohlin Gemini
Rheometer. The rheometer consisted of cone and plate. A cone with 4-degree angle and plate with 40 mm were used for measuring the viscosities of thermal greases. The gap distance between cone and plate was set at 150 μm.
Results and Discussion
Figure 13. SEM image of spherical Alumina.
The size and shape of fillers were seen to have an effect on thermal conductivity of grease. Morphology of Al2O3 were observed by scanning electron microscope. Al2O3 had sphere like structure as seen in Figure 13. Average particle size of Al2O3 was measured
43 from SEM images using image J software. The average particle size of AY35-125 was derived to be 30.2 µm, and average thickness was 1.01 µm. The average particle size of
AY75-150 was 69.3 µm, and average thickness was 2.51 µm. The aspect ratio (A) was calculated using equation (1) [51]. The average aspect ratio of AY35-125 and AY75-150 was calculated to be 29.90 and 27.60, respectively, from 25 different readings.
퐴푣푒푟푎푔푒 퐷𝑖푎푚푒푡푒푟 (퐷) 퐴푠푝푒푐푡 푅푎푡𝑖표 (퐴) = (13) 퐴푣푒푟푎푔푒 푇ℎ𝑖푐푘푛푒푠푠 (푇)
The effects of different size of Al2O3 content on the thermal conductivity of EPON
828 based thermal grease are shown in Figure 14. Thermal conductivity of the thermal grease was directly proportional to the content of filler. The thermal conductivity of 30 wt% Ay in EPON 828 was 0.37 W/mK, almost two times higher than that of native EPON
828 (0.202 W/mK) and the thermal conductivity of 25 wt% AY in EPON 828 was 0.34
W/mK, one and half times higher than that of native EPON 828. The mobility of thermal grease decreased rapidly as the filler content increased beyond 30 wt% for Ay and 25 wt% for AY. We could see that the thermal conductivity of thermal grease increased significantly up to 30 wt% of Ay and 25 wt% of AY. The viscosity of thermal grease increased with increasing wt% of fillers, and it made difficult for better dispersion of the filler in EPON 828 which limited the thermal conductivity path.
44 Ay 0.40 AY
0.35
0.30
0.25
Thermal Conductivity (W/mK) 0.20
0 2 4 6 8 10 Filler Loading (vol. %)
Figure 14. Effects of Ay and AY content on the thermal conductivity of thermally conductive grease.
Different thermal greases with hybrid fillers were prepared changing wt% of AY and keeping Ay constant to obtain the optimal ratio of Ay and AY. Total wt% of fillers was not exceeded more than 50 wt% to maintain the mobility of thermal grease. Table 9 shows the thermal conductivity of thermal grease as a function of weight fraction of the small particle (Ay), in which the total weight fraction of hybrid fillers (Ay+AY) did not exceed 50%. Beyond 50 wt% of hybrid fillers the mobility of thermal grease decreased while mixing and it limited the dispersion of fillers in EPON 828. It could be seen that the thermal conductivity of thermal grease filled with hybrid fillers increased with the increase in weight fraction of small particle up to 20 wt%. The maximum thermal conductivity is
45 0.46 W/mK at 45 wt% of total fillers out of which 20 wt% is Ay, and 25 wt% is AY, almost two and half times higher than that of native EPON 828. The thermal conductivity of neat
EPON 828 increased by 227%. This suggests that the optimum weight ratio of large to small particles improves packaging volume and enhances the thermal conductivity of thermal grease.
Table 10. Thermal conductivity of thermal conductive grease with hybrid fillers.
Thermal Grease λ
(W/mK.)
Ay5AY5 0.25
Ay5AY10 0.27
Ay5AY15 0.28
Ay5AY20 0.32
Ay5AY25 0.33
Ay10AY5 0.27
Ay10AY10 0.29
Ay10AY15 0.31
Ay10AY20 0.34
Ay10AY25 0.37
Ay15AY5 0.31
Ay15AY10 0.33
Ay15AY15 0.36
Ay15AY20 0.39
46 Ay15AY25 0.4
Ay20AY5 0.41
Ay20AY10 0.42
Ay20AY15 0.44
Ay20AY20 0.45
Ay20AY25 0.46
Ay25AY5 0.37
Ay25AY10 0.35
Ay25AY15 0.39
Ay25AY20 0.4
Ay25AY25 0.4
47 0.46 0.44 0.45 0.41 0.42 0.4
0.2 Thermal Conductivity (W/mK) Conductivity Thermal
0.0 Ay20AY5 Ay20AY10 Ay20AY15 Ay20AY20 Ay20AY25 Thermally Conductive Grease
Figure 15. Effect of hybrid fillers on thermal conductivity of thermally conductive grease.
Different thermal greases were prepared by fixing AY at 25 wt% and altering the wt% of Ay after knowing the optimum weight percentage of the small particle (Ay) in hybrid fillers. The effects of hybrid fillers with fixed weight percentage of AY at 25 wt% on the thermal conductivity of the EPON 828 based thermal greases are shown in Figure
16.
48 0.44 0.42 0.41 0.41 0.4 0.4 0.39
0.2 Thermal Conductivity (W/mK) Conductivity Thermal
0.0 Ay20AY25 Ay21AY25 Ay22AY25 Ay23AY25 Ay24AY25 Thermally Conductive Grease
Figure 16. Effect of hybrid fillers at AY25 on thermal conductivity of thermally conductive grease.
Thermal grease should exhibit high thermal performance at small contact pressure, and it can be applied at minimal thickness at the junction. The mobility of thermal grease should be preserved so that it can fill all the air gaps between the heat source and the heat sink providing an implicit pathway for thermal conduction. Thermally conductive grease should not separate, run, migrate or bleed from the contact points. The viscosity of most commonly used thermally conductive grease (Arctic Silver 5) was measured to characterize the viscosity of our thermally conductive greases. The viscosity of arctic silver 5 was measured to be 145 PaS. Figure 17 shows that the viscosity of thermally conductive grease is directly proportional to the filler loading.
49 200
180
160 155 148 140 133 125 120 116 101 100
80
Viscosity (PaS) Viscosity 60
40
20
0 Ay20AY25Ay21AY25Ay22AY25Ay23AY25Ay24AY25Ay25AY25 Thermally Conductive Grease
Figure 17. Effect of filler on the viscosity of the thermally conductive grease with alumina filler.
50 CHAPTER IV
CONCLUSION AND FUTURE WORKS
Conclusion
The uniqueness of our study lies in its use of a combination of Boron Nitride (BN) and Surface Enhanced Flake Graphite (G) with similar platelet like structure to develop thermally conductive grease. The results show that the low content of BN disperses randomly and has weak interaction between each other, and it applies with the spherical filler. With the increasing content of filler, the formation probability of thermal conductivity path increases, along with the thermal conductivity of thermally conductive grease. The thermal conductivity of thermal grease increased significantly after introducing the hybrid fillers (BN+G) in the system. The thermal conductivity of thermal grease with hybrid fillers of spherical alumina was higher compared to single filler of spherical alumina. This might be because smaller size particles (PCTP 30 BN) filled the tiny gaps left by bigger size particles (3775 G) [25], also explained in Figure 6. Therefore, the interfacial phonon scattering, and thermal resistance reduces effectively, and the thermal conductivity of the grease increases. At lower loading of filler, there is no sufficient
51 interaction between the thermally conductive fillers and there is no significant enhancement in thermal conductivity from high thermal contact resistance [25]. Figure 7 shows that the electric conductivity of thermally conductive grease is directly proportional to the electrically conductive filler (3775 G in this case) loading.
Thermal conductivity of BN and G is eleven times higher than alumina. Maximum thermal conductivity of thermal grease made of BN was around three times higher than made of alumina. Thermal conductivity of thermal grease made of BN and G was around five times higher than made of hybrid spherical filler. This shows that thermal conductivity of thermal grease is directly proportional to the thermal conductivity of fillers. The thermal conductive grease is a semi-solid material and it is challenging to measure the thermal conductive of the grease. So, special care should be applied in stabilizing while measuring the thermal conductivity of the grease.
Thermal grease should exhibit high thermal performance at small contact pressure, and it can be applied at minimal thickness at the junction. The mobility of thermal grease should be preserved so that it can fill the air gaps between the heat source and the heat sink providing an implicit pathway for thermal conduction. The thermal conductive grease obtained through this study follows above mentioned criteria. Hence, there is a need for similar study in larger scale to address the issue of heating in electronic devices.
Future Works
As we all know electronic devices are getting smaller and thinner in every step of innovation. New innovations are not only getting smaller but are becoming more powerful, which requires denser packaging. Denser packaging of electronic chips inside the electronic devices leads to more powerful and smaller electronic devices. Thermal grease
52 with high thermal conductivity and low electric conductivity will be absolute necessity in electronic packaging fields in the near future. This work can be done in other materials with high thermal conductivity and low electric conductivity like Aluminum Nitride,
Beryllium Oxide, Shapal Hi-M Soft, Silicon Carbide and many more. However, one challenge of minimizing the electric properties of high thermally conductive fillers like
Aluminum Nitride and Silicon Carbide will still remain. Similarly, different polymer composites can also be used to control the viscosity of the paste.
On the other hand, we believe simulation software like ANSYS can be used to predict the thermal conductivity of polymer composites [55]. Models capable of predicting the thermal conductivity using simulation software can save us humongous time in experimental work. The current also suggests a need to focus on the development of such models.
53 BIBLIOGRAPHY
1. Song, S.H., et al., Enhanced Thermal Conductivity of Epoxy – Graphene
Composites by Using Non ‐ Oxidized Graphene Flakes with Non ‐ Covalent
Functionalization. Advanced Materials, 2013. 25(5): p. 732-737.
2. Li, Q., et al., Ultrahigh thermal conductivity of assembled aligned multilayer
graphene/epoxy composite. Chemistry of Materials, 2014. 26(15): p. 4459-4465.
3. Ramirez, S., et al., Thermal and magnetic properties of nanostructured densified
ferrimagnetic composites with graphene-graphite fillers. Materials & Design, 2017.
118: p. 75-80.
4. Shemelya, C., et al., Anisotropy of thermal conductivity in 3D printed polymer
matrix composites for space based cube satellites. Additive Manufacturing, 2017.
16: p. 186-196.
5. Gwinn, J.P. and R.L. Webb, Performance and testing of thermal interface materials.
Microelectronics Journal, 2003. 34(3): p. 215-222.
6. Anderson, J. and P. Kendall, Thermally conductive grease. 2005, Google Patents.
7. Balandin, A.A., et al., Superior thermal conductivity of single-layer graphene.
Nano letters, 2008. 8(3): p. 902-907.
8. Bujard, P. and J. Ansermet. Thermally conductive aluminium nitride-filled epoxy
resin (for electronic packaging). in Semiconductor Thermal and Temperature
Measurement Symposium, 1989. SEMI-THERM V., Fifth Annual IEEE. 1989. IEEE.
9. Chung, D., Thermal interface materials. Journal of Materials Engineering and
Performance, 2001. 10(1): p. 56-59.
54 10. Chung, D., Materials for thermal conduction. Applied Thermal Engineering, 2001.
21(16): p. 1593-1605.
11. Ghosh, N., B. Kiskan, and Y. Yagci, Polybenzoxazines—new high performance
thermosetting resins: synthesis and properties. Progress in polymer Science, 2007.
32(11): p. 1344-1391.
12. Gowda, A., et al. Spherical boron nitride fillers for high performance thermal
greases. in 2005 7th Electronic Packaging Technology Conference. 2005. IEEE.
13. Graebner, J.E., Thermal Measurement Techniques. 1997: Marcel Dekker, New
York.
14. Nielsen, L.E., Thermal conductivity of particulate‐filled polymers. Journal of
applied polymer science, 1973. 17(12): p. 3819-3820.
15. Xu, Y., D. Chung, and C. Mroz, Thermally conducting aluminum nitride polymer-
matrix composites. Composites Part A: Applied science and manufacturing, 2001.
32(12): p. 1749-1757.
16. Mamunya, Y.P., et al., Electrical and thermal conductivity of polymers filled with
metal powders. European polymer journal, 2002. 38(9): p. 1887-1897.
17. Wang, Q., W. Gao, and Z. Xie, Highly thermally conductive room‐temperature‐
vulcanized silicone rubber and silicone grease. Journal of applied polymer science,
2003. 89(9): p. 2397-2399.
18. Moisala, A., et al., Thermal and electrical conductivity of single-and multi-walled
carbon nanotube-epoxy composites. Composites science and technology, 2006.
66(10): p. 1285-1288.
55 19. Lee, G.-W., et al., Enhanced thermal conductivity of polymer composites filled with
hybrid filler. Composites Part A: Applied science and manufacturing, 2006. 37(5):
p. 727-734.
20. Gu, J., et al., Thermal conductivity epoxy resin composites filled with boron nitride.
Polymers for Advanced Technologies, 2012. 23(6): p. 1025-1028.
21. Hong, H., et al., Carbon nanotube grease with enhanced thermal and electrical
conductivities. Journal of Nanoparticle Research, 2010. 12(2): p. 529-535.
22. Iruvanti, S., K.S. Olsen, and K.G. Sachdev, Polyester dispersants for high thermal
conductivity paste. 1997, Google Patents.
23. Rimdusit, S. and H. Ishida, Development of new class of electronic packaging
materials based on ternary systems of benzoxazine, epoxy, and phenolic resins.
Polymer, 2000. 41(22): p. 7941-7949.
24. Yu, S., P. Hing, and X. Hu, Thermal conductivity of polystyrene–aluminum nitride
composite. Composites Part A: Applied science and manufacturing, 2002. 33(2): p.
289-292.
25. Yu, W., et al., Exceptionally high thermal conductivity of thermal grease:
synergistic effects of graphene and alumina. International Journal of Thermal
Sciences, 2015. 91: p. 76-82.
26. Ishigaki, T., Thermal conductive grease. 2006, Google Patents.
27. Kong, J., et al., Synergic effect of acrylate liquid rubber and bisphenol A on
toughness of epoxy resins. Polymer Bulletin, 2008. 60(2-3): p. 229-236.
56 28. Gu, J., et al., Preparation and mechanical properties researches of silane coupling
reagent modified β-silicon carbide filled epoxy composites. Polymer Bulletin, 2009.
62(5): p. 689-697.
29. Yung, K. and H. Liem, Enhanced thermal conductivity of boron nitride epoxy‐
matrix composite through multi‐modal particle size mixing. Journal of Applied
Polymer Science, 2007. 106(6): p. 3587-3591.
30. Zhou, W., et al., Effect of the particle size of Al2O3 on the properties of filled heat‐
conductive silicone rubber. Journal of Applied Polymer Science, 2007. 104(2): p.
1312-1318.
31. Sarvar, F., D.C. Whalley, and P.P. Conway. Thermal Interface Materials - A
Review of the State of the Art. in 2006 1st Electronic Systemintegration Technology
Conference. 2006.
32. Weber, E.H., M.L. Clingerman, and J.A. King, Thermally conductive nylon 6, 6
and polycarbonate based resins. II. Modeling. Journal of applied polymer science,
2003. 88(1): p. 123-130.
33. Huang, H., et al., Aligned carbon nanotube composite films for thermal
management. Advanced materials, 2005. 17(13): p. 1652-1656.
34. Martello, S. and D. Vigo, Exact solution of the two-dimensional finite bin packing
problem. Management science, 1998. 44(3): p. 388-399.
35. Progelhof, R., J. Throne, and R. Ruetsch, Methods for predicting the thermal
conductivity of composite systems: a review. Polymer Engineering & Science, 1976.
16(9): p. 615-625.
57 36. Hill, R.F. and P.H. Supancic, Thermal conductivity of platelet‐filled polymer
composites. Journal of the American Ceramic Society, 2002. 85(4): p. 851-857.
37. Leong, C.-K. and D.D.L. Chung, Carbon black dispersions as thermal pastes that
surpass solder in providing high thermal contact conductance. Carbon, 2003.
41(13): p. 2459-2469.
38. Xu, Y., X. Luo, and D. Chung, Sodium silicate based thermal interface material
for high thermal contact conductance. Journal of electronic packaging, 2000.
122(2): p. 128-131.
39. Yu, A., et al., Enhanced thermal conductivity in a hybrid graphite nanoplatelet–
carbon nanotube filler for epoxy composites. Advanced Materials, 2008. 20(24): p.
4740-4744.
40. Tong, X.C., Thermal interface materials in electronic packaging, in Advanced
Materials for Thermal Management of Electronic Packaging. 2011, Springer. p.
305-371.
41. Lee, S., How to select a heat sink. electronics cooling, 1995. 1(1): p. 10-14.
42. Ishida, H. and S. Rimdusit, Very high thermal conductivity obtained by boron
nitride-filled polybenzoxazine. Thermochimica Acta, 1998. 320(1): p. 177-186.
43. Kendall, P.E. and R.K. Sura, Thermally conductive grease. 2007, Google Patents.
44. Agari, Y. and T. Uno, Thermal conductivity of polymer filled with carbon materials:
effect of conductive particle chains on thermal conductivity. Journal of applied
polymer science, 1985. 30(5): p. 2225-2235.
58 45. Bontemps, A., et al. Thermal conductivity measurements in phase change polymer
composites doped with carbon nanotubes. in ICTEA: International Conference on
Thermal Engineering. 2017.
46. Prasher, R., Thermal interface materials: historical perspective, status, and future
directions. Proceedings of the IEEE, 2006. 94(8): p. 1571-1586.
47. Narumanchi, S., et al. Thermal interface materials for power electronics
applications. in Thermal and Thermomechanical Phenomena in Electronic Systems,
2008. ITHERM 2008. 11th Intersociety Conference on. 2008. IEEE.
48. Ishigaki, T., Thermal conductive grease. 2008, Google Patents.
49. Xu, Y. and D. Chung, Increasing the thermal conductivity of boron nitride and
aluminum nitride particle epoxy-matrix composites by particle surface treatments.
Composite Interfaces, 2000. 7(4): p. 243-256.
50. Schroder, D.K., Semiconductor material and device characterization. 2006: John
Wiley & Sons.
51. Ng, H.Y., X. Lu, and S.K. Lau, Thermal conductivity of boron nitride-filled
thermoplastics: Effect of filler characteristics and composite processing conditions.
Polymer Composites, 2005. 26(6): p. 778-790.
52. Nielsen, L.E., The thermal and electrical conductivity of two-phase systems.
Industrial & Engineering chemistry fundamentals, 1974. 13(1): p. 17-20.
53. Hauser, R.A., et al., Thermal conductivity models for single and multiple filler
carbon/liquid crystal polymer composites. Journal of applied polymer science,
2008. 110(5): p. 2914-2923.
59 54. Sim, L.C., et al., Thermal characterization of Al2O3 and ZnO reinforced silicone
rubber as thermal pads for heat dissipation purposes. Thermochimica Acta, 2005.
430(1–2): p. 155-165.
55. Kumlutas, D. and I.H. Tavman, A numerical and experimental study on thermal
conductivity of particle filled polymer composites. Journal of thermoplastic
composite materials, 2006. 19(4): p. 441-455.
60