Thermally conducting aluminum nitride polymer-matrix composites

doi:10.1016/S1359-835X(01)00023-9

Composites Part A: Applied Science and Manufacturing

Volume 32, Issue 12, December 2001, Pages 1749-1757

https://doi.org/10.1016/S1359-835X(01)00023-9 Get rights and content

Abstract

Thermally conducting, but electrically insulating, polymer-matrix composites that exhibit low values of the dielectric constant and the coefficient of thermal expansion (CTE) are needed for electronic packaging. For developing such composites, this work used aluminum nitride whiskers (and/or particles) and/or silicon carbide whiskers as fillers(s) and polyvinylidene fluoride (PVDF) or epoxy as matrix. The highest thermal conductivity of 11.5 W/(m K) was attained by using PVDF, AlN whiskers and AlN particles (7 μm), such that the total filler volume fraction was 60% and the AlN whisker–particle ratio was 1:25.7. When AlN particles were used as the sole filler, the thermal conductivity was highest for the largest AlN particle size (115 μm), but the porosity increased with increasing AlN particle size. The thermal conductivity of AlN particle epoxy-matrix composite was increased by up to 97% by silane surface treatment of the particles prior to composite fabrication. The increase in thermal conductivity is due to decrease in the filler–matrix thermal contact resistance through the improvement of the interface between matrix and particles. At 60 vol.% silane-treated AlN particles only, the thermal conductivity of epoxy-matrix composite reached 11.0 W/(m K). The dielectric constant was quite high (up to 10 at 2 MHz) for the PVDF composites. The change of the filler from AlN to SiC greatly increased the dielectric constant. Combined use of whiskers and particles in an appropriate ratio gave composites with higher thermal conductivity and low CTE than the use of whiskers alone or particles alone. However, AlN addition caused the tensile strength, modulus and ductility to decrease from the values of the neat polymer, and caused degradation after water immersion.

Introduction

Thermally conducting, but electrically insulating, polymer-matrix composites are increasingly important for electronic packaging because the heat dissipation ability limits the reliability, performance and miniaturization of electronics. In addition to high thermal conductivity and high electrical resistivity, a low dielectric constant is needed for fast signal propagation and a low coefficient of thermal expansion (CTE) is needed for resistance to thermal fatigue. Applications include encapsulations, die (chip) attach, thermal grease, thermal interface material and electric cable insulation.

In order to provide thermally conducting but electrically insulating polymer-matrix composites, fillers (such as diamond, boron nitride, aluminum nitride, silicon carbide and alumina) which are thermally conducting but electrically insulating are used [1], [2], [3], [4], [5].

It is known that the transport of heat in nonmetals occurs by phonons or lattice vibrations. The thermal resistance is caused by various types of phonon scattering processes, e.g. phonon–phonon scattering, boundary scattering and defect or impurity scattering [6]. In order to maximize the thermal conductivity, these phonon scattering processes must be minimized. Phonons travel at the speed of sound. The scattering of phonons in composite materials is mainly due to the interfacial thermal barriers resulting from acoustic mismatch and flaws associated with the filler–matrix interface [4], [7], [8]. For a given filler composition and volume fraction, there are methods of increasing the thermal conductivity of the composite, namely (i) forming conductive networks through appropriate packing of the filler in the matrix, (ii) decreasing the amount of thermally resistant junctions involving a polymer layer between adjacent filler units by using large filler units with little or no defects and (iii) decreasing the thermal contact resistance at the filler–matrix interface by minimizing the interfacial flaws. The purpose of this study is to develop highly thermally conductive composite materials through (i) the use of aluminum nitride particles and whiskers of different sizes and aspect ratios in order to enhance the formation of a thermally conductive network and (ii) the improvement of the interface between the filler and matrix by surface treatment of the filler.

This work is focused on aluminum nitride due to its combination of high thermal conductivity, low dielectric constant and low cost. Previous work on aluminum nitride (AlN) polymer-matrix composites was limited to the use of epoxy [1] and polyimide [2] as the matrices and AlN particles as the filler [1], [2]. Previous work [2] has also included hybrid composites involving AlN particles and SiC whiskers as fillers. The hybrid composites with both AlN particles and SiC whiskers exhibit higher thermal conductivity than those of the composites with SiC whiskers or AlN particles as the sole filler, but the presence of SiC causes the dielectric constant to be undesirably high [2].

In this work, we have investigated the use of AlN in the form of AlN whiskers, with and without the presence of AlN particles. In addition, the effects of the AlN whisker–particle ratio and the AlN particle size were investigated. Furthermore, we have investigated the use of polyvinylidene fluoride (PVDF) as the matrix and found that PVDF gave composites with high thermal conductivity. By the use of AlN whiskers and particles, and PVDF as the matrix, a thermal conductivity up to 11.5 W/(m K) was attained.

This paper also addresses the third method described above for increasing the thermal conductivity, because this method has received little attention. The value of the third method stems from the tendency for gaps or other flaws to occur at the filler–matrix interface due to the insufficient affinity between filler and matrix. Such interfacial flaws cause a high thermal resistance at the interface, thus reducing the thermal conductivity of the composite. Ref. [3] mentioned the importance of surface modification to the thermal conductivity of BN epoxy-matrix composites, but the method of modification was proprietary and was not disclosed. This paper uses surface treatment of the filler to improve the affinity between filler and matrix, thereby significantly increasing the thermal conductivity of the composite. In particular, this paper uses silane coupling agents for surface treatment. Silane acts as a bridge to connect the ceramic filler and the polymer matrix, because it has two different chemical structures at the two ends of the molecule. One end is chemically reactive with the polymer; the other end is chemically reactive with the surface of the ceramic filler. The surface treatment of AlN may have an additional function; the coating on the surface may protect AlN from the attack of water.

Section snippets

Materials

The AlN particles (AlN_p), AlN whiskers (AlN_w) and SiC whiskers (SiC) were obtained from Advanced Refractory Technologies (Buffalo, NY). Both AlN_p and AlN_w were made by direct nitridation. The AlN particles used were of six size classes, with average particle sizes shown in Table 1. The AlN whiskers were of two types, labeled I and II, which mainly differed in the whisker content (the balance being particulate) and the particle size of the particulate portion, as shown in Table 1. The mean

Results and discussion

Table 3 gives the thermal conductivity of PVDF-matrix composites with various types of AlN_p at various volume fractions. For a given AlN_p type, the thermal conductivity increased with increasing AlN_p volume fraction, except that the thermal conductivity decreased when the AlN_p volume fraction was increased from 70 to 73% (due to increase in porosity). At a fixed filler volume fraction of 55%, AlN_p (E) gave a composite of higher thermal conductivity than any other type of AlN_p, due to its

Conclusions

1.
The thermal conductivity increases with increasing filler volume fraction regardless of the filler type among the various types of AlN_p at a fixed volume fraction, the type with the greatest mean particle size (115 μm) gives composites with the highest thermal conductivity. However, the porosity of the composites increases with increasing AlN_p particle size.
2.
The combination of AlN_w (II) and AlN_p (M) as fillers in an appropriate ratio gives the highest thermal conductivity. By using the AlN_w (II)

Acknowledgements

This work was supported by New York State Energy Research and Development Authority, Advanced Refractory Technologies Inc. (Buffalo, NY), and the Defense Advanced Research Projects Agency (USA).

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Thermally conducting aluminum nitride polymer-matrix composites

Abstract

Introduction

Section snippets

Materials

Results and discussion

Conclusions

Acknowledgements

Thermochim Acta

Int J Heat Mass Transfer

J Electron Mater

J Composite Mater