Viscosity and Thermal Conductivity of Alumina Microball/Epoxy Composites

Hui Yu 1, Liangliang Li1

, ', Longhao Qi2

'Department of Materials Science and Engineering, Key Laboratory of Advanced Materials, Tsinghua University, Beijing,100084, People's Republic of China . .

2Department of Materials Science and Engineering, State Key Laboratory of.New C~ramic and Fine Processing, TsmghuaUniversity, Beijing, 100084, People's Republic of China

*Email: liliangliang@mail.tsinghua.edu.cnPhone: 86-10-62797162

Material Properties

Viscosity at 25°C: 11000-14000 ml'a-s

CYD128 Thermal conductivity:r--'0.2 W/(m·K)Halogen free, environmentally friendly

and low cost

Ab03Thermal conductivity: 35 W/(m·K)

CTE: 8.4 ppm/K

Table I Typical properties of alumina and CYD 128 epoxy

the coupling agents were used was discussed in anotherprevious work [6].

Additionally, various papers [7-12] reported that the fillerswith various shapes such as plates, wires and tubes were filledinto a polymer matrix to enhance their thermal conductivity.Branched alumina and alumina nanofiber were synthesizedand added into an epoxy resin [7]. A1N whiskers and particleswere added into PVDF [8]. The effects of copper fillers withdifferent shapes including spheres, plates and fibers on thethermal conductivity of polymer were investigated [9].Exfoliated graphite [10], carbon black [11], carbon fiber andcarbon nanotube [12] were filled into various matricesincluding epoxy, liquid crystal polymer and phenolic resin.However, the relation between viscosity and thermalconductivity and the relation between viscosity and themaximum filling content were seldom investigated. Therefore,in this study, alumina microballs were used as the fillers tostudy their effects on the viscosity and thermal conductivity.

ExperimentalPreparation of Alz03 microball/epoxy composite

Bisphenol-A liquid epoxy was provided by the EpoxyResin Division of Bailing Petrochemical enterprise. QS-1858Y, a high temperature-actuation acid anhydride cur~ng

agent, was provided by Qingda Qi-Shi Company. Aluminamicroballs with average sizes of 32 urn (Ab03-1) and 84 urn(Ab03-II) were chosen to be the fillers. The size distributionand the scanning electron microscopy (SEM) images wereshown in Fig. 1. The basic properties of alumina and epoxywere listed in Table I. The synthetic routine and the curingcurve were the same as those in our previous work [5]. Thesynthetic routine contained three steps: (1) mixing the e~oxy,

curing agent and alumina microballs, (2) ultrasonicallydispersing for 30 mins, and (3) moulding and curing.

AbstractFilling ceramic particles that exhibit high thermal

conductivity into a polymer is an effective way to enhance thethermal conductivity of the polymer. Alumina (Ab03)microballs were chosen as the fillers to confirm whether theirsuper-spherical structure would enlarge the maximum fillingcontent of the filler into the polymer matrix. The impacts ofAb03 microballs on the thermal conductivity of thecomposites and the viscosity of pre-curing suspension wereinvestigated. The viscosity increased slightly when a 20 vol.% of Ab03 microballs were filled, and when the fillingcontent reached 50 vol. %, the viscosity was still relativelylower than that of the epoxy with roughly spherical A1Ngranules. A high thermal conductivity of 2.70 W/(m·K) wasachieved with an Ab03 microball content of 60 vol. %.Keywords :thermalconductivity,alumina microballs, viscosity,composite

IntroductionComposites based on polymer matrices filled with various

kinds of ceramic particles have attracted a lot of attentionrecently because they can promote heat dissipation and meetthe increasing demand for heat dissipation in high powerelectronic components such as high-power Laser, high-brightness LED and IGBT [1-8]. A polyimide/ceramiccomposite with a thermal conductivity of 9.3 W/(m·K)prepared through filling 49% A1N and 21% h-BN intopolyimide matrix was reported by Shoichi Kume et al. [1]. K.C.Yung et al. reported an epoxy-based composite with a highthermal conductivity of 19.0 W/(m·K) by filling multi-sizeBN granules into epoxy matrix [2]. Cheng-Yu Hsieh andShyan-Lung Chung produced an epoxy molding compound(EMC) with a high thermal conductivity of 14 W/(m·K) withthe A1N filler content of 67 vol. % [3].

As-summarized in our previous work [5], obtaining acomposite with an optimally high thermal conductivityrequired (1) a high filling content, (2) relatively large ceramicgranules and multi-scales filler, (3) filling plate or fiberceramic granules instead of spherical ones, and (4) contactbetween matrix and filler in accordance to a designated fillingcontent. Thus in cases where the thermal conductivity alonewere concerned, the most effective approach was to increasethe filling content and the density of the filler in thecomposites as much as possible, and these were mainlydetermined by the viscosity of pre-curing composite mixture.The relation between the viscosity of pre-curing compositemixture (epoxy/A1N) and the maximum filling content when

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Fig. 1. (a) and (b) show the size distribution of Ab03 microballs with average sizes of 32 urn and 84 urn, respectively. (c)and (d) represent the SEM images of Ab03 microballs with average sizes of 32 urn and 84 urn, respectively.

CharacterizationMeasurement of thermal properties

The thermal diffusivity was measured directly by a FourierTransform Thermal Analysis system [13], consisting of afunction synthesizer, a digital lock-in amplifier, a D.C. sourcefor the sensor, a temperature controller, a specimen holder ona hot stage and a personal computer. This system employed athermal analytic method to detect the propagation of thetemperature wave along the thickness of the film specimenunder a temperature scan.

The thermal conductivity was calculated through theequation K=p·a·Cp , where p is the density, a is the thermaldiffusivity and Cp is the heat capacity.Measurement of viscosity

The viscosity of the pre-curing composite (Ab03microball/epoxy) mixture with varying filling content wasmeasured by a Physica MCR300 Modular Compact Rhometerwith a suspension volume of 20 ml.Scanning electron microscopy (SEM)

The natural fracture surfaces of the epoxy-basedcomposite samples were observed by LE01530 FieldEmission Scanning Electron Microscope (FE-SEM).

The size distribution of Ab03 powder was measured by aMalvern Mastersizer 2000 Laser Diffraction Particle SizeAnalyzer.

Fig. 2. The morphology of the fracture surfaces for Ab03microball/epoxy composites with (a) neat epoxy, (b) 20 vol.%Ab03-II, (c) 30 vol.% Ab03-II, (d) 40 vol.% Ab03-II, (e) 50vol.% Ab03-II, (f) 60 vol.% Ab03-II, (g) 40 vol.% Ab03-I,

(h) 50 vol.% Ab03-1. The scale bar is 20 urn.

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Results and discussion1. Microstructure

Fig. 2 showed the morphology of the fractured surfaces forthe Ab03 microball/epoxy composites with various fillercontents. The Ab03 microballs were embedded into the epoxymatrix and a few gaps were observed around these microball

granules. With an increase in the Ab03 microball content,denser composites were obtained, the Ab03 microballsachieved more contacts with each other and easily formed athermal conductive pathway, thus the thermal conductivity ofthe Ab03 microball/epoxy composites was enhanced.

Fig. 3. Viscosity of the pre-curing suspension with different Ab03 microball volume fractions (a) at 40°C and (b) SO°C. (c)and (d) were the amplificatory images for (a) and (b), respectively.

2. ViscosityGenerally, the maximum filling content was determined

partly by the viscosity of the pre-curing composite mixture,while the shape and the size of filler would strongly influencethe viscosity of fluids. Therefore, the viscosities of Ab03microball/epoxy with different sizes were investigated. Fig.3(a) and 3(b) displayed viscosity of the pre-curing suspensionwith different Ab03 microball volume fractions at 40°C andSO°C, respectively. When a 20 vol. % of Ab03 microballswere added into the liquid epoxy, the viscosity increasedslightly. Compared to the viscosity data of the pre-curingcomposite mixture filled with 50 vol. % AIN, in which theshape of as-used AIN particles was roughly spherical, theviscosity of the Ab03 microball/epoxy composite mixturewas much lower. The surface of Ab03 microballs wassmoother than that of roughly spherical AIN particles and thusengendered a higher shear resistance for the movement ofroughly spherical AIN particles in the liquid epoxy. Therefore,the Ab03 microball/epoxy composite mixture displayed alower viscosity for a certain filling content.3. Thermal conductivity of Alz03 microball/epoxy thinfilm sample

Fig. 4 showed the variation of the thermal diffusivity andthermal conductivity of the composites with various filling

content of Ab03 microballs. The thermal diffusivity and thethermal conductivity presented a similar trend with anincrease in Ab03 microball content. The experimental datashowed that the thermal conductivity of the Ab03microball/epoxy composite increased nearly 14.5 times, from0.19 W/(m·K) to 2.77 W/(m·K), while the volume fraction ofAb03 microballs increased from 0% to 60%.

Fig. 4. Thermal diffusivity and thermal conductivity of thinfilm samples as a function of Ab03 microball content.

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ConclusionsAn effective way of enhancing the thermal conductivity of

polymer was achieved by adding ceramic materials with ahigh thermal conductivity into a matrix. When Ab03microballs were used as the filler, a lower viscosity wasobtained due to the lower shear resistance of the smoothcontact between the Ab03 microballs and liquid epoxycompared to that between roughly spherical AIN granules andliquid epoxy. Correspondingly, the maximum filling contentwas enlarged and better thermal conductive performance wasachieved. The thermal conductivity ofbisphenol-A epoxy wasenhanced to the highest value of2.70 W/(m·K) by filling with60 vol. % Ab03 microballs.

However, the presence of defects, including interfaces andgaps between matrix and filler, indicates areas forimprovement in future. Therefore, improving the contactsbetween matrix and filler, decreasing the viscosity andenlarging the maximum filling content will be the focus of ourfuture work through surface modification of the ceramicgranules and other measures.

AcknowledgmentsThe work at Tsinghua University was partly supported by

Tsinghua University Initiative Scientific Research Program,Daikin Industries, Ltd, Japan and the State Key Laboratory ofNew Ceramic and Fine Processing at Tsinghua University.

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