Functionalized Aluminum Nitride for Improving Hydrolysis Resistances of Highly Thermally Conductive Polysiloxane Composites

Abstract

A series of divinylphenyl-acryloyl chloride copolymers (PDVB-co-PACl) is synthesized via atom transfer radical polymerization employing tert-butyl acrylate and divinylbenzene as monomers. PDVB-co-PACl is utilized to graft on the surface of spherical aluminum nitride (AlN) to prepare functionalized AlN (AlN@PDVB-co-PACl). Polymethylhydrosiloxane (PMHS) is then used as the matrix to prepare thermally conductive AlN@PDVB-co-PACl/PMHS composites with AlN@PDVB-co-PACl as fillers through blending and curing. The grafting of PDVB-co-PACl synchronously enhances the hydrolysis resistance of AlN and its interfacial compatibility with PMHS matrix. When the molecular weight of PDVB-co-PACl is 5100 g mol−1 and the grafting density is 0.8 wt%, the composites containing 75 wt% of AlN@PDVB-co-PACl exhibit the optimal comprehensive performance. The thermal conductivity (λ) of the composite is 1.14 W m−1 K−1, which enhances by 20% and 420% compared to the λ of simply physically blended AlN/PMHS composite and pure PMHS, respectively. Meanwhile, AlN@PDVB-co-PACl/PMHS composites display remarkable hydrothermal aging resistance by retaining 99.1% of its λ after soaking in 90 °C deionized water for 80 h, whereas the λ of the blended AlN/PMHS composites decreases sharply to 93.7%.

1. Introduction

With the rapid development of electronic products toward high power and integration, the problems of heat accumulation and heat dissipation become more and more prominent, which puts forward higher requirements on the thermal conductivity, insulation, and heat resistance of the thermal interface materials. Compared with traditional thermal interface materials (such as epoxy resin, polyurethane, etc.), polymethylhydrosiloxane (PMHS) is an ideal matrix for the preparation of thermal interface materials because of its excellent insulating property, aging resistance, ease of molding, and compatibility with silicon substrates. However, it is usually necessary to fill a large number of thermally conductive fillers into PMHS to obtain high thermal conductivity.

Aluminum nitride (AlN) as a material with high thermal conductivity (λ, 170 W m−1 K−1) and excellent electrical insulation, is an ideal filler for the preparation of PMHS composites with high thermal conductivity to be used as thermally conductive polysiloxane sheets, which play an effective role in heat dissipation to ensure the normal operation of the equipment. However, the poor compatibility of AlN with PMHS is not beneficial to heat transfer, which limits the efficient improvement of thermal conductivity for AlN/PMHS composites. In addition, AlN is prone to hydrolysis in humid air, thus reducing its thermal conductivity and affects the stability of electronic devices after a long period of time with a rapid decline in performance.

The existing methods to modify the surface of AlN mainly involve physical coating and chemical grafting. Physical coating is to combine functional substances on AlN surface by physical interaction. Ganesh et al. treated AlN powder by simultaneously using phosphoric acid and aluminum dihydrogen phosphate in ethanol solution. The treated AlN powder was found to remain stable after 72 h in water. However, physical coating suffers from low binding of the surface cladding layer to AlN and poor modification effect. Chemical grafting is to graft small molecules or polymers onto the surface of AlN through chemical reaction with the modified layer firmly bound to AlN. Lin et al. prepared AlN-VTMS/PMHS composites using PMHS as matrix and vinyl trimethoxysilane (VTMS) modified AlN (AlN-VTMS) as thermally conductive fillers. When the mass fraction of AlN-VTMS was 60 wt%, the thermal conductivity (λ) of AlN-VTMS/PMHS composites was 0.31 W m−1 K−1, which was higher than 0.24 W m−1 K−1 for unmodified AlN/PMHS composites.