Composites Science and Technology 85 (2013) 98–103

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Composites Science and Technology

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High thermal conductive polyvinyl alcohol compositeswith hexagonal boron nitride microplatelets as fillers

0266-3538/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.compscitech.2013.06.010

⇑ Corresponding authors. Tel.: +86 21 52411080, +86 21 52414318; fax: +86 2152413122.

E-mail addresses: xiaohuang@mail.sic.ac.cn (X. Huang), gjzhang@mail.sic.ac.cn(G.-J. Zhang).

Bin-Huan Xie, Xiao Huang ⇑, Guo-Jun Zhang ⇑State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Shanghai 200050, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 March 2013Received in revised form 14 June 2013Accepted 17 June 2013Available online 24 June 2013

Keywords:A. Polymer–matrix composites (PMCs)B. Thermal propertiesC. AnisotropyE. Casting

Organic polymers are generally thermal insulators. There is a huge demand especially from electronicindustries to develop highly thermal conductive polymers. The most common way to increase the ther-mal conductivity of a polymer is to introduce highly thermal conductive inorganic fillers into the polymermatrix. But up to today, high loading of the fillers is required to achieve high thermal conductivity whichsacrifices the lightweight, excellent processability and low cost of polymeric materials. In this work, hex-agonal boron nitride (h-BN) microplatelets are introduced as thermal conductive fillers into polyvinylalcohol (PVA). Due to the unique shape, h-BN microplatelets orient during processing, forming texturedstructure within the polymer matrix, which helps the formation of thermal conductive pathways. Thus,high thermal conductivities can be achieved at very low filler loadings. The thermal conductivity of the h-BN/PVA composite reaches 1.45 W/m K at only 1 wt% (0.6 vol%) loading, while that number rises up to3.92 W/m K at 10 wt% (5.9 vol%) loading. Our parallel experiments show that the shape of h-BN micro-platelet plays an important role. The surface modification on h-BN fillers improves the compatibilitybetween inorganic fillers and organic matrix, leading to better dispersibility and higher degree of orien-tation, thus higher thermal conductivity.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The past decades witness the uncannily accuracy of Moore’s lawthat the packaging density of components in an electrical devicedoubles approximately every two years [1]. Such an increase ofthe packaging density leads to an unwanted yet unavoidable sideeffect – the generation of a greater amount of heat in a smallerspace. Most electrical components can only function normally atrelative low temperatures. Thus, to ensure proper device operation,the unwanted heat must be dissipated [2,3].

Organic polymers have been widely used in modern electronicindustries such as printed circuit boards (PCBs), thermal interfacematerials (TIMs), and device bases/holders [3,4]. But polymersare usually considered as thermal insulators. Most polymers’ ther-mal conductivities are far less than 1 W/m K [5]. This is not a bigissue in earlier days when the components packing density wasnot this high. But today, it becomes one of the bottle-neck prob-lems for the continuous development in electronic industries.

The most effective and promising way to improve polymers’thermal conductivities is by introducing inorganic fillers with high

thermal conductivities into the polymer matrix. A lot of inorganicmaterials such as metals [6,7], carbon materials [8,9], metal oxides[10,11], metal nitrides [12,13] and clay [14] etc. have all been ap-plied. But polymers with satisfying thermal conductivities aremostly achieved at the filler loading greater than 20 wt% (seeTable 1), which sacrifices the lightweight, low cost and excellentprocessability of the polymeric materials.

Among all kinds of fillers, during the past decade, MWCNTs andSWCNTs have been expected to be capable of improving the ther-mal conductivities of polymeric materials effectively at relativelylow loadings. But up to today, the results are rather frustrating.The poor performances of one-dimensional thermal conductivefillers are due to several possible reasons: the high interfacial ther-mal resistance between nanotubes and matrix, high tube–tubecontact resistance, etc. [9,15,16].

The issues brought by one-dimensional fillers may be avoidedby two-dimensional fillers. Due to its extra large in-plane thermalconductivity and inspiration from global research interest,graphene has been received special attention and given a lot ofexpectations as thermal conductive fillers. Recent works on two-dimensional fillers, including graphene and graphite nanoplatelets,have shown some promising results [17–19]. However, carbon-based fillers due to their electric conductivity are not applicablein applications where both good thermal conductivity and elec-tronic insulation are required [3,4,20].

Table 1Thermal conductivities of polymer composites with different fillers.

Fillers Polymer matrix Solids loading Thermal conductivities (W/m K) Refs.

Mercapto-functionalized AlN Epoxy 65 vol% �6 [44]Graphene Epoxy 25 vol% CNTs and 25 vol% GNPs 7.30 [45]Graphene�multilayer graphene nanocomposite Thermal grease 2 vol% 14 [17]Noncovalently functionalized graphene flakes Epoxy 10 wt% 1.53 [19]Silica-coated multi-walled carbon nanotubes Epoxy 1 wt% 0.240 [46]Py-PGMA–graphene nanosheets Epoxy 4 phr (parts per hundreds of resin) 1.91 [47]Graphene Epoxy 12 wt% 0.73 [48]Micro-SiC Epoxy 63 wt% 2.8 [49]MWCNT 6 wt% 0.8MWCNT + micro-SiC 5 wt% MWCNTs + 55 wt% micro-SiC �7Benzenetricarboxylic acid grafted MWCNT Epoxy 5 vol% 0.96 [50]BNNTs PVA 10 wt% 0.54 [26]Micro and nanosized BN Polyimide 30 wt% 1.2 [25]h-BN Polyimide 30 vol% �3 [2]

60 vol% 7Graphite nanoplatelet Epoxy 10 wt% �2 [18]AlN + h-BN Polyimide 49 vol% AlN + 21 vol% h-BN 9.3 [13]BNNTs PMMA 10 wt% 0.51 [27]

24 wt% 3.16BNNTs PVF 10 wt% 0.45 [34]

PVA 3 wt% 0.3Si3N4 Polystrene 40 vol% 3.0 [51]DWCNT Epoxy 0.6 vol% 0.252 [15]BNNSs Polysiloxane 5 vol% 0.13 [52]h-BN microplatelets PVA 0.6 vol% (1 wt%) 1.45 (k)a; 0.15 (\)b This work

5.9 vol% (10 wt%) 3.92 (k)a; 0.44 (\)b

a k: In-plane direction.b \: Cross-plane direction.

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h-BN is a structural analog and isoelectronic counterpart ofgraphite. Like graphite, it possesses many exceptional in-planeproperties including high thermal conductivity [21,22]. Meanwhile,h-BN is also an excellent electrical insulator and has the highestthermal conductivity among most electric insulators. Thus, h-BNis a very promising filler candidate for preparation of high thermalconductive polymeric materials with excellent electric insulation[23,24]. h-BN particles [2,23,25] and nanotubes (BNNTs) [26,27],have been investigated as thermal conductive fillers. Like otherfillers, thermal conductivities greater than 1 W/m K can only beachieved at loadings over 20 wt%.

In this study, we use commercial available h-BN microplateletsas thermal conductive fillers. Due to the platelet-like morphology,the h-BN fillers can be easily oriented during processing, thus lead-ing to anisotropic thermal conductivity of the composites. Becauseit is very easy to handle in the laboratory, polyvinyl alochol (PVA)is chosen as the polymer matrix to simplify the research process[26,28,29]. We show that by using two-dimensional fillers andcontrolled orientation of the fillers, polymer composites with veryhigh thermal conductivities can be prepared at very low filler load-ings. The in-plane thermal conductivity of the PVA composite canreach 1.45 W/m K at only 0.6 vol% h-BN loading. And that valuecan further increase almost to 4 W/m K at 5.9 vol% loading.

2. Experimental

2.1. Materials

h-BN powder (purity >99.5%, �0.6 lm, with perfect platetletmorphology) was purchased from SICCAS, Shanghai, China. Theirthicknesses are in the range of 45–116 nm, and the diameter–thickness ratios are in the range of 4.1–13.7.h-BN II (purity>99.5%, �0.8 lm, with irregular shape) was purchased fromSanxing Co. Ltd., Gongyi, China.

PVA (MW: 1750 ± 50) and 30% hydrogen peroxide (AR grade)were obtained from Sinopharm Chemical Reagent Co. Ltd.,

Shanghai, China, used as received. Deionized water was preparedin-house by Thermo Scientific Easypure II system.

2.2. Surface modification on h-BN

h-BN powder was mixed with 30% hydrogen peroxide at0.1 g/ml. The mixture was sealed in an autoclave and stirredfor 24 h. And then, the mixture was heated up to 100 �C and keptat that temperature for 24 h [30]. After being cooled to roomtemperature, the mixture was filtered and washed by de-ionizedwater three times, and then dried at 50 �C under vacuum. In thetext, the original h-BN is denoted as p-BN while surface-modifiedh-BN is denoted as m-BN.

2.3. Preparation of BN/PVA composites

To prepare the composites, weighed amount of p-BN or m-BNwas first dispersed in an 8 wt% PVA aqueous solution. The mixturewas magnetically stirred at 90 �C for 5 h. Then the BN/PVA compos-ite films were prepared by two different techniques. One is referredto simple casting: the mixture was poured into a Teflon mold andthen the mold was placed in an oven at 50 �C for 24 h. After cool-ing, a composite film with the thickness of about 200 lm was ob-tained. The other technique is referred to so-called tape-casting:the mixture was poured onto a glass panel and spread to a thinlayer using a doctor blade with a tape casting height of 0.4 mmat a speed of 1 cm/s. After the tape casted layer was dried in airat room temperature for 24 h, a flexible film with a thickness ofabout 15 lm was obtained.

2.4. Instrumentation

SEM images were taken by a Hitachi S-4800 scanning electronmicroscope. XRD patterns were recorded via a Rigaku X-ray dif-fractometer (D/Max-2250 V) using Cu Ka radiation. The FT-IR spec-tra were recorded in a Thermo Nicolet NEXUS spectrometerbetween 400 and 4000 cm�1. X-ray photoelectron spectroscopy

100 B.-H. Xie et al. / Composites Science and Technology 85 (2013) 98–103

measurements (XPS) were performed on a Thermo Scientific,ESCALAB 250 equipped with monochromatic Al Ka as radiationsource. Measurements of the cross-plane and in-plane thermalconductivities of the samples (cross-plane: 10 mm � 10 mm,thickness 0.1–2.5 mm; in-plane: diameter 25.4 mm, thickness0.1–0.5 mm) were carried out using the laser-flash diffusivityinstrument (Nano-Flash-Apparatus, LFA 447, NETZSCH) via twodifferent modes. The zeta potential was measured by the Zeta Plus(Brookhaven Instruments Co., New York, NY, USA).

3. Results and discussion

3.1. Surface modification on h-BN

It is known that the properties of composites depend largely notonly on their compositions but also on the interfacial behaviors be-tween two phases. Generally, the interface interactions can betuned by tailoring the surface chemistry of the filler via chemicalreactions or addition of surfactant-like molecules. For example,silanes are often used to improve the compatibility between the fil-ler and the polymer to reduce the interfacial thermal resistance.However, it is also reported that excessive silanes usage actuallylowered the thermal conductivity because the excessive silanescould form a thermal barrier layer. In order to avoid such issue,we use a very simple reaction to modify the surface chemistry ofh-BN particles.

h-BN particles are treated with hydrogen peroxide at 100 �C inan autoclave. FT-IR spectra of p-BN and m-BN are shown in Fig. 1a.Two strong characteristic absorption bands of h-BN can be clearlyidentified. The peak around 1377 cm�1 is the B–N stretching vibra-tion, while the peak at 814 cm�1 can be attributed to the B–N–Bout-of-plane bending [31]. After surface modification, a new

Fig. 1. (a) FT-IR spectra of pristine h-BN (p-BN) and surface-modified h-BN (m-BN). (b) Dimages of (d) p-BN, (e) m-BN, and (f) h-BN II.

absorption band at 2021 cm�1 can be observed in m-BN, which isprobably corresponding to some type of boron and oxygen bonding[32,33], coming from the reaction with hydrogen peroxide. Theobvious increase of the peak at around 3500 cm�1 in m-BN sug-gests more hydroxyl groups on its surface [34]. Zeta potential mea-surements show that after surface modification, zeta potentialdrops from �39.45 mV to �51.50 mV, also indicating significantsurface properties change.

XPS spectra in Fig. 1b shows notable increase at higher bindingenergy (�192.2 eV), which associates with B–O bonding [35]. TheXPS results are totally consistent with FT-IR observations.

XRD patterns of p-BN and m-BN are shown in Fig. 1c, apparentlythe BN crystal structure does not change before and after surfacemodification. SEM pictures reveal that before surface modification(Fig. 1d), the p-BN shows perfect platelet morphology with smoothand intact edges. After surface modification, the edges of manym-BN platelets show layered structure (Fig. 1e). The previouslyperfectly stacked h-BN layers seem to slide to some extent duringthe hydrothermal surface modification process. But such minorchanges should not affect the filler’s performance.

3.2. Orientation of h-BN microplatelets in PVA matrix

SEM and XRD are used to characterize the orientation of h-BNparticles in polymer matrix. The SEMs of the cross section of theh-BN/PVA composites prepared by simple casting and tape-castingare shown in Fig. 2a–d. Obvious orientation of h-BN microplateletscan be observed even in the simple casting technique. Apparently,due to the perfect two-dimensional shapes, the shear forcegenerated from fluid flow in the simple casting process is enoughfor h-BN particles to align along the flow direction. The degree oforientation is further semi-quantified by the mean orientation

etailed XPS spectra of B in p-BN and m-BN. (c) XRD profiles of p-BN and m-BN. SEM

Fig. 2. SEM images of cross section of PVA composites by simple casting with (a)10 wt% p-BN, (b) 10 wt% m-BN and by tape-casting with (c) 10 wt% p-BN and (d)10 wt% m-BN. (e) Mean orientation angles (h) of BN microplatelets in 10 wt% BN/PVA composites (the inset is the schematic of h).

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angles as summarized in Fig. 2e. Obviously, the tape-casting canprovide more shear force to make the h-BN particles further orienttowards blade direction. Considering the size of the filler and vis-cosity of the fluid, relaxation of microplatelets from orientation isneglectable. Such orientation is preserved during drying and solid-ifying process to the final composite material.

In Fig. 2, it can also been noticed that the m-BN particles dis-perse more homogeneously than p-BN particles due to the en-hanced affinity between the m-BN particles and the polymermatrix after surface modification. Small portion of agglomerationof p-BN particles can be seen in the 10 wt% h-BN/PVA composites(Fig. 2a and c). This is one of the key factors that affect the perfor-mance of the fillers. Apparently, if the fillers agglomerate in thepolymer matrix, it requires higher filler content to form the so-called ‘‘thermal conductive pathways’’ [36].

Fig. 3. XRD patterns of PVA composites with 10 wt% of p-BN and m-BN by two methodscomposites with 5 wt%, 10 wt% and 30 wt% of p-BN or m-BN.

XRD patterns of four h-BN/PVA composites are shown in Fig. 3.The peaks at 2h � 27� and 42� result from the diffraction of (002)and (100) planes of h-BN respectively [37]. Comparing to simplecasting technique, the (002) face becomes more predominantand the ratios of I002/I100 (see Fig. 3) are larger in tape casting tech-nique, indicating higher degree of orientation [38]. It also can beseen in Fig. 3 that the ratios of I002/I100 of m-BN fillers are generallylarger than those of p-BN fillers. Surface-modified h-BN microplat-elets are more easily oriented possibly due to the strongerH-bonding interactions with PVA matrix, since there is morehydroxyl on the surface of m-BN as we previously discussed[39–41]. Such difference in the degree of orientation betweenm-BN and p-BN is much more obvious in tape casting technique,as supported by the bigger difference of I002/I100 values in tapecasting technique. These results are as expected and consistentwith SEM observations.

There is one phenomenon which is very interesting and worthymentioning. In Fig. 3, it appears that the d-spacing of the layer dis-tance of h-BN in the h-BN/PVA composites by tape-casting isslightly larger than that of composites by simple casting, and suchchange in m-BN is greater than that in p-BN. Currently the possiblemechanism of this phenomenon is still under study.

3.3. Thermal conductivities of the BN/PVA composites

It is well known that h-BN possesses different in-plane andcross-plane properties. The in-plane thermal conductivity ofh-BN is more than 20 times larger than the cross-plane value[42]. Due to the orientation of h-BN microplatelets in PVA matrix,the composites also exhibit significant anisotropy in thermalconductivities, as shown in Fig. 4. The thermal conductivities with-in the film plane (in-plane), which are parallel to the direction ofshear force, are much higher than those are perpendicular to theplane (cross-plane). Because it is impossible to obtain 100%alignment, the anisotropy of thermal conductivity in the compositeis much less obvious than the filler itself. Tape-casting can makethe h-BN filler orient further. Thus, the in-plane thermal conductiv-ities of composites prepared by tape-casting are higher than thoseof the composites prepared by simple casting.

When looking at the numbers of the thermal conductivities ofthe composites, very high values can be obtained at relative low fil-ler loadings. For the m-BN microplatelets, thermal conductivity of1.45 W/m K is obtained at only 1 wt% (�0.6 vol%) filler loading bytape-casting, and 1.26 W/m K by simple casting. It also shows thatthe further orientation of m-BN by tape-casting leads to the higherin-plane thermal conductivity. Comparing to previously publishedworks, in which generally over 20 wt% filler loadings are required

(inset: schematic of (002) and (100) plane of h-BN particles), and I(002)/I(100) of PVA

Fig. 4. Thermal conductivities of simple casting and tape-casting composites with different solids loading in two directions.

102 B.-H. Xie et al. / Composites Science and Technology 85 (2013) 98–103

to achieve thermal conductivities to be over 1 W/m K, our resultsare quite phenomenal.

In order to achieve high thermal conductivity, formation of effi-cient thermal conductive pathways within the matrix network isone of the key factors [36,43]. It seems that the texturing of h-BNmicroplatelets favors the formation of in-plane thermal conductivepathways, as illustrated in Fig. 5. On the contrary, the chance offorming cross-plane thermal conductive pathways is less as sup-porting by the fact that higher degree of orientation leads to smal-ler cross-plane thermal conductivities (see Fig. 4).

Although it may not be a perfect comparison, h-BN particles withsimilar particle size but without such perfect two-dimensionalstructure (referred to h-BN II), whose SEM picture is shown inFig. 1f, are also applied to prepare PVA composites in the exactlysame way. The 10 wt% surface-modified h-BN II/PVA compositewhich is prepared by tape-casting has a in-plane thermal conduc-tivity of 0.79 W/m K and cross-plane thermal conductivity of0.14 W/m K. The difference between in-plane and cross-plane ther-mal conductivity indicates the fillers are also nicely oriented. Butthe values of thermal conductivities are much lower than theh-BN fillers with perfect two-dimensional shape and are close toother fillers reported previously (see Table 1). Apparently therather unique perfect two-dimensional geometry of the h-BN fill-ers applied plays a very important role in improving the thermalconductivities of the composites. We believe that the more con-trolled and regular shape will help the formation of the texturedstructure and facilitate the thermal conductive pathway formation.

Fig. 5. Texturing of h-BN microplatelets favors the formation of thermal conductivepathways in-plane. The red lines are heat flows. (For interpretation of the referencesto color in this figure legend, the reader is referred to the web version of thisarticle.)

4. Conclusions

Two types of h-BN fillers are introduced into PVA matrix to im-prove the thermal conductivity of the composites. The fillers can beeasily oriented by external shear forces. Due to the graphite-type oflayered structured, the anisotropic fillers result in different in-plane and cross-plane thermal conductivities of the composites.The in-plane thermal conductivity is much higher than the cross-plane thermal conductivity, because the in-plane direction of thefilm is parallel to the (002) plane of h-BN.

High in-plane thermal conductivities of h-BN/PVA compositescan be achieved at very low filler loadings when h-BN microplat-lets with perfect two-dimensional shape are used. Thermal con-ductivity of 1.45 W/m K can be reached at 1 wt% (0.6 vol%)loading while that number can rise up to 3.92 W/m K at 10 wt%(5.9 vol%) loading and 4.41 W/m K at 30 wt% (19.6 vol%) loading.The performance of our h-BN microplatelets are much better thanmost reported fillers including graphene, whose loadings usuallyhave to be over 20 wt% to reach thermal conductivity of 1 W/m K.

By comparison with h-BN particulates without perfect two-dimensional shape, it seems that the perfect two-dimensionalshape of the fillers favors the formation of textured structure andbenefits the formation of the thermal conductive pathways, thushelps to improve the thermal conductivity of the composite.However, we are aware that high thermal conductivity incross-plane direction is more important in most applications.Experiments to control the h-BN microplatelets to alignperpendicular to the composite film are in progress.

Acknowledgements

Financial supports from the Chinese Academy of Science(Hundred Talents Program) and Science and Technology Commissionof Shanghai Municipality (#11ZR1442200) are gratefullyacknowledged.

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