Raza, M

Stirling, C

Westwood, AVK

White Rose Research Online

Boron nitride/vapour grown carbon nanofibre/rubbery epoxy-based hybrid composites for thermal interface applications

This is a repository copy of Boron nitride/vapour grown carbon nanofibre/rubbery epoxy-based hybrid composites for thermal interface applications.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/84149/Version: Accepted VersionConference or Workshop Item:Raza, M, Westwood, AVK and Stirling, C (2015) Boron nitride/vapour grown carbon nanofibre/rubbery epoxy-based hybrid composites for thermal interface applications. In: 4th International Conference on Mulifunctional, Hybrid and Nanomaterials, 08-14 Mar 2015, Barcelona, Spain. (Unpublished) eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Items deposited in White Rose Research Online are protected by copyright, with all rights reserved unless indicated otherwise. They may be downloaded and/or printed for private study, or other acts as permitted by national copyright laws. The publisher or other rights holders may allow further reproduction and re-use of the full text version. 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Boron Nitride/Vapour Grown Carbon Nanofibre/Rubbery Epoxy-based Hybrid Composites For Thermal Interface Applications  Mohsin Ali Raza1, Aidan Westwood2, Chris Stirling3 and Rafiq Ahmad1 1Department of Metallurgy and Materials Engineering, CEET University of the Punjab, Lahore, Pakistan 2Institute for Materials Research, SCAPE, University of Leeds, LS2 9JT, UK 3Morgan AM&T, Swansea, SA6 8PP, UK (current address: Haydale Ltd., Ammanford, SA18  3BL, UK) *Corresponding author email: mohsin.ceet@pu.edu.pk  Background Thermal interface materials (TIMs) are vital for microelectronics packaging as they are responsible for improving interfacial contacts between the microchips and heat sinks, thus ensuring sufficient heat removal from chips [1]. Boron nitride (BN) is the most popular filler material for TIMs mainly due to its high thermal conductivity (300 W/m.K) and electrical resistivity. Recently, carbon nanofillers such as graphite nanoplatelets, carbon nanotubes and carbon nanofibres have been widely researched as fillers to produce polymer based composites due to their very high thermal conductivity [2, 3]. One solution to further improve the conducting and mechanical properties of carbon nanofiller based composites is to produce hybrid nanocomposites.  Aims of the study The present work reports novel hybrid rubbery epoxy composites produced by dispersing inorganic filler, BN, and carbon nanofiller, vapour grown carbon nanofibres (VGCNF), with the aim of producing much better TIMs than conventional ones.  Experimental Materials Boron nitride (particle size 5-10 μm), VGCNF (diameters = 70-200 nm and lengths = 50-100 μm) were used as filler materials.  Rubbery epoxy was used as matrix.  Composite Fabrication The fillers were dispersed in epoxy resin by 3 roll mill. Composites dispersions were cured at 120 °C for 3 hr. Composites were produced with BN content fixed at 6 wt.% while VGCNF content was varied from 8 to 12 wt.%.   Characterisation The morphology, thermal conductivity, electrical conductivity and compression properties of the resulting composites were studied. The interfacial thermal transport performances of selected hybrid composite coatings were studied as thermal interface adhesives according to ASTM D5470 (Fig. 1) [4] Fig. 1: Air gaps between chip (top, left) and TIM I and TIM 2 placed in the die (bottom  left), Schematic of Thermal Contact Resistance Measurement Rig (right)  The total thermal resistance (RT) of the coating joining the copper cylinders is proportional to the thickness of the coating .The total thermal resistance of the system shown in Fig. 1 is described as,   RT = Rc + Rint1+ Rint2 where Rc is the thermal resistance of the coating, and Rint1 and Rint2 are the geometric interface contact resistances between the copper cylinder A and the coating and cylinder B and the coating, respectively. Fig. 2 (a) SEM images of (a) 6 wt.% BN/8 wt.% VGCNF/rubbery epoxy (b & c) 6 wt.% BN/12 wt.% VGCNF/rubbery epoxy composites produced by roll milling.  SEM images of BN/VGCNF/Rubbery Epoxy Hybrid Composites  BN VGCNF Thermal Conductivity and Thermal Contact resistance  of BN/VGCNF/Rubbery Epoxy Hybrid Composites  Material Bond line thickness (µm) Thermal contact resistance ( m2.K/W) Commercial TIM (Matrix II) paste 18 4.2 × 10-6 45 wt.% BN/rubbery epoxy (RE) composite 18 2.28 × 10-5 6 wt. % BN/8 wt. % VGCNF/RE composite 18 3.38 × 10-5 15 wt.% VGCNF/rubbery epoxy 18 1.93 × 10-5 45 wt.% BN/rubbery epoxy composite 95 8.82 × 10-5 65 wt.% BN/silicone (EPM 2490) (Commercial TIM adhesive) 95 1.36 × 10-4 6 wt. % BN/8 wt. % VGCNF/RE composite 95 1.39 × 10-4 Fig. 3.  Thermal conductivity of composites (left) and Thermal contact resistance of BN/VGCNF/epoxy composite coatings (right)  Results Table 1. Comparison of Thermal Contact Resistance of BN/VGCNF/Rubbery Epoxy Composites with Commercial TIMs  Conclusions  SEM images show that both VGCNFs and BN are uniformly dispersed in the rubbery epoxy matrix.  VGCNFs improved BN dispersion in the matrix but reduced the number of interconnects between BN plates.   The hybrid combination of BN and VGCNF did not significantly  increase the thermal conductivity of the composites .  The incorporation of BN decreases the electrical conductivities of hybrid composites significantly compared with the corresponding BN-free composite. The electrical conductivity data suggest that inclusions of BN in the conducting networks of VGCNFs add significant resistance to the electron transport and reduce the efficiency of the conducting networks.   The data suggest that the combination of BN and VGCNF (at the loadings studied in this work) neither improves the mechanical properties of the hybrid composites nor deteriorates them.  The thermal contact resistance of 6 wt.% BN/12 wt.% VGCNF/rubbery epoxy coating is still 1.4× higher than that of the 15 wt.% VGCNF/rubbery epoxy composite coating. This again suggests that inclusion of BN in VGCNF-based coatings deteriorates their performance as thermal interface adhesives and that the non-hybrid coatings can perform much better than hybrid coatings.  The thermal contact resistance of hybrid composite is almost  similar to that of commercial BN-based adhesive but is achieved at 50 wt.% lower loading  of filler.   The incorporation of VGCNF in BN/Rubbery epoxy composite coating however can easily disssipate electrostatic charges. Acknowledgement The authors would like to thank University of the Punjab, Lahore, Pakistan for providing funding for presentation of this paper in the conference. The authors would also like to thank University of Leeds, UK for sponsoring M.A.Raza’s PhD studies. References [1] Chung DDL., Journal of Materials Engineering and Performance. 2001;10:56-9. [2] Yu A, J Phys Chem C. 2007;111:7565-9 [3] Shahil KMF, Balandin AA, Solid State Communications.152(15):1331-40. [4] Raza MA, Journal of Materials Science: Materials in Electronics. 2012;23(10):1855-63. 

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Boron nitride/vapour grown carbon nanofibre/rubbery epoxy-based hybrid composites for thermal interface applications

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