Interphase control of boron nitride/epoxy composites for · PDF file · 2011-01-04Interphase control of boron nitride/epoxy composites for high thermal conductivity Korea-Australia

Korea-Australia Rheology Journal December 2010 Vol. 22, No. 4 259

Korea-Australia Rheology JournalVol. 22, No. 4, December 2010 pp. 259-264

Interphase control of boron nitride/epoxy composites for high thermal conductivity

Jung-Pyo Hong1, Sung-Woon Yoon

1, Tae-Sun Hwang

1, Young-kwan Lee

2, Sung-Ho Won

3 and Jae-Do Nam

4,*1Department of Polymer Science and Engineering, Sungkyunkwan University, 300 Chunchun-dong,

Jangan-gu, Suwon 440-746, South Korea2Department of Applied Chemistry, Sungkyunkwan University, 300 Chunchun-dong,

Jangan-gu, Suwon 440-746, South Korea3Department of Applied Chemistry, Dongyang Technical college 62-160 Kochuk-dong, Seoul 152-714, Korea

4Department of Energy Science, Sungkyunkwan University, 300 Chunchun-dong, Jangan-gu, Suwon 440-746, South Korea

(Received April 8, 2010; final revision received July 26, 2010; accepted July 29, 2010)

Abstract

The effects of the surface treatment of boron nitride (BN) particles on the thermal conductivity of BN/epoxycomposite systems was investigated. By coating an amino silane compatibilizer on the BN surface, theinterfacial space could be decreased so as to minimize the phonon scattering and thermal-interface resis-tance. When an excessive amount of silane compatibilizer was present at the BN/epoxy interphase bound-ary, it acted as a thermal insulation layer, resulting in the reduction of the thermal conductivity. Accordingly,the thermal conductivity was maximized when the optimal amount of silane compatibilizer was used, whichwas associated with the specific surface area or the size of the incorporated BN particles. In the case of theBN particles, whose specific surface areas were 14.3 m2/g and 11 m2/g (average particle size: 1 µm and5 µm, respectively), the highest thermal conductivity was observed at 3.0 wt% and 2.5 wt% of the silanecompatibilizer, respectively. By converting the particle size and specific surface area into the shape factor,the optimal amount of amino silane required to maximize the thermal conductivity was discussed in relationwith the interphase structure and thermal resistance.

Keywords : thermal conductivity, silane treatment, boron nitride

1. Introduction

Boron nitride (BN) has a similar plate structure to graphite

and thus superior thermal conductivity, but it is a noncon-

ductor of electricity(Yung and Liem, 2007). BN has been stud-

ied extensively as a filler to improve the thermal conductivity

of a variety of polymer-based composite systems, most often

in epoxy composite systems, which are applied to the pack-

aging of electronic parts such as PCBs and EMCs(Lee et al.,

2006; Nagai and Lai, 1997; Yung et al., 2006).

On the other hand, heat transfer is restricted by phonon-

scattering at the matrix-filler interface(Berman,1973). As

the loading content of the filler is increased, the polymer-

filler interfacial area increases, which subsequently

increases the phonon-scattering. It was found that, in most

cases, phonon scattering reduces the amount of heat trans-

fer in composite material systems(Ishida and Rimdusit,

1998). Consequently, if the interfacial adhesion is not good

and, thus, the extent of phonon scattering is significant in

composites, the highly-conductive fillers may decrease the

thermal conductivity with increasing filler-loading content.

In addition, when the particles have different sizes, the

numbers of interfaces along the path through which heat is

transferred varies; therefore, composite materials with the

same filler content have different values of the thermal

conductivity(Nagai and Lai, 1997; Yung et al., 2007; Hsieh

and Chung, 2006). In short, composite materials have dif-

ferent thermal conductivities depending on the size or load-

ing content of the particles, characteristics of the interfaces,

or relative thermal conductivities of the particles and resin.

The surface treatment of fillers is widely known to be an

important factor for reducing the thermal resistance of the

interfaces between the polymer and filler. Surface modi-

fication using silane in particular is the most widely known

method. Based on the results of research on the surface

treatment of particles using silanes, it is known that surface

treatment can indeed play an important role in reducing the

thermal resistance between the filler and polymer depend-

ing on the amount of silane; note, however, that the thermal

conductivity may be reduced when an excessive amount of

silane is used (Hsieh and Chung, 2006; Xu and Chung,

2000; Xie et al., 2004). In other words, using silane may

initially increase the thermal conductivity, since it lowers*Corresponding author: [email protected]© 2010 by The Korean Society of Rheology

Jung-Pyo Hong, Sung-Woon Yoon, Tae-Sun Hwang, Young-kwan Lee, Sung-Ho Won and Jae-Do Nam

260 Korea-Australia Rheology Journal

the interface resistance, but an excessive amount may actu-

ally reduce the thermal conductivity. This suggests that

there must be an optimum amount of silane to be used.

Estimating and fully understanding this optimum amount

of silane to secure the maximum thermal conductivity are

very important in designing composite materials with guar-

anteed high thermal conductivity.

In this research, the change of the thermal conductivity

depending on the amount of amino silane was studied

using an epoxy composite material and BN fillers with a

certain specific surface area; the optimum amount of

amino silane for obtaining the maximum thermal conduc-

tivity was then monitored for each filler system. Further-

more, the relation between the amount of silane coating

and the specific surface area and size of the particles was

analyzed using the shape factor.

2. Experiment

2.1. MaterialsIn this research, BN with an average size of 5 µm pur-

chased from DENKA (Japan) and BN with an average size

of 1 µm purchased from SINTEC (England) were used.

The detailed characteristics of the filler are shown in Table 1.

Fig. 1(a) shows a scanning electron microscope image of

the particles with an average size of 1 µm used in the

experiment.

The epoxy resin was YD-128, a liquid-type standard

epoxy resin derived from bisphenol A diglycidyl ether

(DGEBA) supplied by Kukdo Chemical. The hardener,

methyl tetrahydrophthalic anhydride (MTHPA), was HN-

2200, also provided by Kukdo Chemical. The catalyst was

1-methylimidazole (1-MI) and the surface modifier was 3-

aminopropyl-triethoxy silane (aminosilane); both were pur-

chased from Aldrich.

2.2. Preparing samples Moderate amounts of filler and amino silane were added

to acetone. After more than three hours of agitation at

ambient temperature, the filler, whose surface was coated

with silane, was obtained using a filter. The obtained filler

was then placed in a vacuum oven at 120oC for two hours

to dry(Hsieh and Chung, 2006). YD-128, MTHPA, and 1-

MI were placed in a moderately-sized container and stirred

well. The stirred resin was mixed with the surface-coated

fillers evenly. The filler content was adjusted to 60 vol%.

The mixture was then put into a 1 millimeter thick mold

and a pressure of 3,000 psi was applied for 4 hours at 80oC

and then for 2 hours at 145oC. Figs. 1(b) and 1(c) show the

BN/epoxy mixture and cured specimens with a coin shape,

respectively.

2.3. Analysis of properties The scanning electron microscope was an FEI Helios

600 NanoLab. The specific surface area was measured

using a Micromeritics Instruments Corporation ASAP

2020 (USA). The thermal diffusivity and specific heat

were obtained using a Netzsch Nanoflash 447. The mea-

surements were performed twice and the average was

taken to calculate the thermal conductivity. The density

was measured using Archimedes’ principle. The coin-

shaped sample that was used had a thickness of

1±0.5 millimeters and diameter of 12.7 millimeters.

3. Results and Discussion

In the case of aluminum nitride (AIN), which is fre-

quently used to increase the thermal conductivity of com-

posite materials, the surface reacts sensitively to moisture

in the air. The surface of AlN is converted into Al(OH)3 as

a result of the reaction with moisture in the air, and ammo-

nia gas is produced as a byproduct(Kameshima et al.,

1998). The surface roughness of AlN increases due to the

reaction with moisture; contact with moisture over an

extended period of time may reduce the thermal conduc-

tivity of AIN, because its surface will have been covered

with coarse lumps created as a result of hydrolysis(Li et

Table 1. Fillers properties

Filler B1 B5

Commercial grade MBN HGP

Chemical formula BN BN

Tap Density (g/cm3) 0.3 0.4

Particle

Size(µm)

D10 - 1.9

D50 1.0 5.0

D90 - 10.6

Particle thickness 0.1 0.2

Specific surface area (m2/g) 14.4 11.0

Oxygen Content (wt %) 5.5 1.0

Fig. 1. The SEM image of (a) B1 particle, (b) its mixture with

resin at 60 vol%, and (c) cured composite specimen of

60 vol% of B1 particle.



al., 2005). As with AIN, BN also reacts with moisture sen-

sitively to form –OH groups on the surface. Therefore, in

order to ensure its high thermal conductivity, BN should

not come into contact with moisture for an extended period

of time.

According to a research report, surface treatment using

silane is very effective in protecting inorganic particles

against moisture. Furthermore, surface treatment using

silane in the case of a composite material is highly rec-

ommended to maximize the thermal conductivity, because

the coupling agent allows the the matrix to be connected

with the filler through the various functional groups of

silane, which can minimize the interfacial gap or space(Xu

and Chung, 2000). Among the various silane chemicals,

amino silane gives higher thermal conductivity than gly-

cidyl silane in epoxy and amino systems(Xu et al., 2001).

Scheme 1 shows the reaction mechanism of amino silane

to form silane layers on the surface of BN particles. The

BN particles are produced through the thermal hydrolysis

reduction process and, thus, –OH groups are present on its

surface by nature(Li et al., 2005; Hub ek and Ueki,

1996). As shown in Scheme 1(a), the alkoxy groups (-OR)

of silane react with the hydroxy groups (-OH) on the sur-

face of the BN particles to form Si-O bonds through

silanization reactions. In addition, as shown in Schemes

1(b) and (c), some of the -OR groups in the silane are con-

verted to Si-OH by the alcoholysis reaction. These –OH

and -NH2 groups in the silane form hydrogen bonds with

other silane molecules as well as –OH groups in the BN

surface to form a robust aminosilane layer (see Scheme

1(d))(Fiorilli et al., 2008). Finally, the -NH2 groups in the

aminosilane layer would react with the epoxide groups in

DGEBA to give chemical bondings eventually connecting

BN and the epoxy matrix, as shown in Scheme 1(e).

The aminosilane layer formed between the BN surface

and epoxy matrix eventually determines the thermal resis-

tance through the composites. If the amount of aminosilane

is not sufficient to wet the BN surface, the air gap would

insulate it, resulting in high thermal resistance. On the

other hand, if the amount of aminosilane is excessive,

forming a thick coating on the BN surface, the thermal

resistance would become high, due to the coating thick-

ness. In this sense, Fig. 2 shows four different cases cor-

responding to different features of the aminosilane layers,

giving different thermal resistances or thermal conductiv-

ity. Fig. 2(a) shows the case where the aminosilane is

absent and an air gap is developed to give an interfacial

thermal resistance mostly due to the air gap( )Ra

Rair

a=

Scheme 1. Schematic representation of silane reaction at the BN surface.



stemming from the phonon-scattering. The conductivity of

air is fairly low at 0.026 W/mK as compared with epoxy

(0.165 W/mK) or BN (60 W/mK) and, thus, may well

be very high. When the BN particles are treated with an

amount of aminosilane which is not sufficient to fill the

interfacial gap, as represented in Fig. 2(b), the thermal

resistance of the total interfaces becomes the sum of the

thermal resistances of the air gap ( ) and aminosilane

layer ( ), viz. . In this case, the phonon scat-

tering taking place in the air gap could be reduced by the

silane layers and, thus, the thermal conductivity may well

be slightly improved or similar to that of the air gap, i.e.,

Ra ≥ Rb. Fig. 2(c) shows the case where the air gap is com-

pletely filled, without excessive aminosilane existing at the

interface, which may well give the lowest thermal resis-

tance ( ). Subsequently, when the amount of ami-

nosilane becomes excessive at the interface, as shown in

Fig. 2(d), the thermal resistance would increase with

increasing thickness of the aminosilane layer, viz. Rd = Rd

S.

Comparing the cases in Fig. 2(c) and (d), it can be realized

that Rc < Rd, in the case where the aminosilane layers act

as a thermal barrier to give a thermal resistance which

increases with increasing thickness(Xu and Chung, 2000).

In the case of BN, the maximum thermal conductivity

was reportedly obtained with 2.4 wt% of silane when the

particle size was in the 5~11 µm range(Xu and Chung,

2000). Similarly, the thermal conductivity values vary with

the amount of silane coupling agents in AlN composite

systems. For example, when the average AlN particle size

was 6.3 µm with a specific surface area of 0.6 m2/g, the

maximum thermal conductivity was obtained at 1.0 wt% of

silane(Hsieh and Chung, 2006). Figs. 3 (a) and (b) show

the thermal conductivities of the BN composite systems at

60 vol% of BN particles plotted as the amount of ami-

nosilane coupling agent for BN particles with sizes of

1 µm (specified as B1) and 5 µm (specified as B5), respec-

tively. The B1 composite specimen in Fig. 3(a) has an

average particle size and specific surface area of 1 µm and

14.3 m2/g, whereas those of B5 in Fig. 3(b) are 5 µm and

11 m2/g, respectively. As can be seen, the maximum ther-

mal conductivities of B1 and B5 appear at 3.0 and 2.5 wt%

of aminosilane, respectively. Since the specific surface area

(surface area per unit weight) depends on the particle size,

the coating thickness of aminosilane may well be depen-

dent on the particle size, resulting in different thermal con-

ductivities. Thus, effective thickness of aminosilane can be

calculated and expressed as:

(1)

where ρ is density of aminosilane. Based on Fig 3, cal-

culating eqn. (1) using 3 and 2.5 wt% of aminosilane at the

highest thermal conductivity of B1 and B5 composite, the

optimal thickness of aminosilane layer is 2.2~2.4 nm, and

which may well give the lowest thermal resistance. Also,

Rair

a

Rair

b

Rs

bRb

Rair

bRs

b+=

Rc

Rs

c=

effective thickness

amount of aminosilane1

ρ---×

the specific surface area of filler------------------------------------------------------------------------------------=

Fig. 2. Schematic of silane thickness variation at the interphase between epoxy resin and BN surface representing (a) no silane treatment,

(b) lack of silane, (c) optimal amount of silane, and (d) excessive amount of silane.



aminosilane layer thickness becomes 3.6~3.8 nm using 5

and 4 wt% of aminosilane at the lowest thermal conduc-

tivity, which may become thermal insulator.

The effect of the particle shape on the thermal conduc-

tivity has been analyzed using the shape factor, which may

quantify the particle packing with its size and thick-

ness(Nielsen, L. E., 1974). Since the specific surface area

depends on the size of the particles and the thermal con-

ductivity depends on the thickness of the aminosilane layer,

the shape factor (A) may be defined as follows to represent

the thermal conductivity of our composite systems:

(2)

where D is the average particle size and L is the thickness

of the particles. In the case of plate-type particles, the

shape factor largely depends on the aspect ratio, which

itself is affected by the particle size(Progelhof et al., 1976;

Droval et al., 2006). The smaller the specific surface area

is, the greater the shape factor becomes, corresponding to

the fact that the specific surface area is inversely propor-

tional to the particle size. In the case of B1, the shape fac-

tor A is 5.47, whereas that of B5 is 8.66, corresponding to

the specific surface areas of B1 and B5 of 14.3 m2/g and

11 m2/g, respectively. Accordingly, as the shape factor

decreases, the specific surface area increases and, conse-

quently, the amount of aminosilane coating on the BN par-

ticles increases. As a result, the optimal amount of

aminosilane which maximizes the thermal conductivity

becomes 3 wt% for B1 and 2.5 wt% for B5, as also rep-

resented by their shape factors.

4. Conclusion

The thermal conductivity of BN particle/epoxy compos-

ite systems was investigated in order to identify the optimal

amount of an aminosilane compatibilizer. The aminosilane

coupling agent was expected to reduce the gap between the

polymer and the surface of the BN filler, thereby improv-

ing the thermal conductivity of the composite material. If

the thickness of the silane layer was in excess of a certain

value, the thermal conductivity of the composite material

decreased, due to the thermal resistance of the silane layer.

The specific surface area was converted into the shape fac-

tor using the filler size and thickness, and the shape factor

successfully represented the maximum thermal conduc-

tivity of the composite systems.

Acknowledgments

This work was supported by the WCU program (R31-

2008-000-10029-0) through the Korea Science and Engi-

neering Foundation funded by the Ministry of Education,

Science and Technology. We also appreciate the project

and equipment support from Gyeonggi Province through

the GRRC program in Sungkyunkwan University(GRRC-

SKK-2009-B06).

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Interphase control of boron nitride/epoxy composites for · PDF file · 2011-01-04Interphase control of boron nitride/epoxy composites for high thermal conductivity Korea-Australia