Extended Abstract deposited in White Rose Online Repositor

Ahmad, R

Raza, MA

Stirling, C

Westwood, AVK

White Rose Research Online

This is a repository copy of Comparison of thermal interfacial performance of carbon nanofiller-based polymer composites.White Rose Research Online URL for this paper:http://eprints.whiterose.ac.uk/83247/Version: Accepted VersionProceedings Paper:Raza, MA, Westwood, AVK, Stirling, C et al. (1 more author) (2013) Comparison of thermalinterfacial performance of carbon nanofiller-based polymer composites. In: Carbon 2013. Carbon 2013, 14-19 July 2013, Rio de Janeiro, Brazil. . eprints@whiterose.ac.ukhttps://eprints.whiterose.ac.uk/Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version - refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher’s website. Takedown If you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request. Comparison of Thermal Interfacial Performance of Carbon Nanofiller-Based Polymer Composites Mohsin Ali Raza*1, Aidan Westwood2, Chris Stirling3, Rafiq Ahmad1 1College of Engineering and Emerging Technologies, Univers ty of the Punjab, Lahore, Pakistan 2Institute for Materials Research, SPEME, University of Leeds, LS2 9JT, UK 3Morgan AM&T, Swansea, SA6 8PP, UK *Corresponding author email: mohsinengr@yahoo.com  Introduction Thermal interface materials (TIMs) are essential components of microelectronics as they improve interfacial contacts between the microchips and heat sinks, thus ensuring sufficient electronic cooling [1]. Thermal interface adhesives are polymer-based composites which improve contacts between the mating surfaces, offer good thermal conductivity and also bind mating surfaces to improve mechanical integrity of the electronics packaging [2]. High thermal conductivity and low thermal contact resistance are desirable characteristics of TIMs [3]. Carbon nanofillers such as graphite nanoplatelets (GNP), carbon nanotubes and carbon nanofibres are being extensively researched as fillers for polymer composites due to their very high thermal conductivity [4, 5]. On the other hand, carbon black (CB)-based thermal pastes have been reported to offer very low thermal contact resistances [6]. Researchers have reported the potential of carbon nanofiller-based polymer composites for thermal interface applications due to their high thermal conductivity [4, 7]. However, high thermal conductivity alone cannot guarantee good TIM performance. The performance of TIMs mainly depends on wt.%, size, shape and orientation of the fillers and on the adhesion, wettability and spreadability of the resulting polymer composite dispersions, which improves thermal contacts between the mating surfaces [8]. The present work reports comparison of thermal interfacial performance of carbon nanofiller-based polymer composite adhesives, measured according to an ASTM standard, D5470, that mimics the conditions prevailing in electronics packages.   Experimental GNP/rubbery epoxy, GNP/glassy epoxy, CB/rubbery epoxy, CB/silicone and CB/GNP/rubbery epoxy hybrid composite dispersions were produced by mechanical mixing. The details of the production and thermal and mechanical characterisation of these composites have been reported in [9-11]. These composites were tested, with cured matrix, as adhesives according to ASTM standard (ASTM D5470) on a thermal contact resistance measurement rig. The details of the rig and the testing procedure have been demonstrated previously  [12]. Briefly, these composite pastes (uncured) were sandwiched between the copper substrates and cured at 125 ºC for 3 h and then placed in the rig for the measurement of thermal interfacial transport properties. The effect of GNP, hybrid combination of CB and GNP, types of polymer matrix and CB on the thermal interfacial performance of the composites was studied and is reported here. The effect of applied pressure, temperature and surface roughness of the substrate on the thermal interfacial performance of these composites is also reported.    Results and Discussion The thermal contact resistance of GNP/rubbery epoxy composite (containing 25 wt.% GNPs with average lateral width of 5 µm) measured on smooth and rough surfaces and the thermal contact resistance of GNP/glassy epoxy composite measured on smooth surfaces, each as a function of coating thickness are presented in Fig. 1 and Fig. 2, respectively.                      Figure. 1. Thermal contact resistance of 25 wt.% GNP/rubbery epoxy (RE) composite (cured between copper substrates) as a function of coating thickness measured between (a) smooth surfaces (b) rough surfaces at 25 ºC and under a pressure of 0.032 MPa.                 Figure. 2. Thermal contact resistance of 25 wt.% GNP/glassy epoxy (GE) composite (cured between copper substrates) as a function of coating thickness measured between smooth surfaces at 25 and 42 ºC and under a pressure of 0.032 MPa.  The thermal contact resistance of both GNP/rubbery epoxy and GNP/glassy epoxy at a coating thickness of ~150 µm is approximately 1 x 10-4 m2/K.W. The glassy epoxy-based composite, due to its high crosslinking, forms much stronger bonds with the copper substrates compared to the rubbery epoxy (lightly cross-linked). Despite this difference in bonding strength, the thermal transport behaviour of the two composites is similar. However, the GNP/glassy epoxy dispersions could not be applied as thin bond lines due to their very high viscosity compared to GNP/rubbery epoxy [9]. The GNP/rubbery epoxy could give a thermal contact resistance as low as ~ 0.2 x 10-4 m2.K/W at bond line thickness of 25 µm. Fig. 1 also shows that the thermal contact resistance of GNP/rubbery epoxy is much lower on rough surface than smooth surface [11] at bond line thicknesses < ca. 150 m .   The thermal contact resistance of CB/rubbery epoxy is presented in Fig. 3.   Figure. 3. Thermal contact resistance of 6 wt.% CB/rubbery epoxy (RE) composite (cured between copper substrates) as a function of coating thickness measured between (a) smooth copper surfaces (Ra= 0.03 µm)  & (b) rough surfaces (Ra= 0.45 µm) at ~30 °C and under a pressure of 0.032 MPa.  The CB/rubbery epoxy composites can be applied as thin bond lines of ~15 µm. Despite this very low bond line thickness, the thermal transport performance of CB/rubbery epoxy coating is much inferior to that of GNP/rubbery epoxy composite, attributed to ~4 times lower thermal conductivity of the former than the latter. However, the thermal contact resistance of CB/silicone composite (1.18 x 10-4 m2.K/W) as an adhesive was 2x higher than for CB/rubbery epoxy (6.2 x 10-5 m2.K/W) composite at equivalent bond line thickness of 20 µm. Perhaps, the more highly adhesive nature of rubbery epoxy composite on the copper surface contributed to its enhanced thermal interfacial transport.  The thermal contact resistance of 4 wt.% CB/12 wt.% GNP/rubbery epoxy hybrid composite measured between smooth and rough copper surfaces is presented in Fig. 4. The hybrid composite coating displays higher thermal contact resistance than GNP/rubbery epoxy composite coating at equivalent bond line thickness, suggesting no significant benefit for addition of CB in terms of the thermal interfacial performance of the GNP/rubbery epoxy composite adhesive coating. Conversely, the data also suggest that the addition of GNPs into CB/rubbery epoxy composite improves the performance of CB/rubbery epoxy composite. Thus, addition of more thermally conducting filler plays an important role in improving thermal transport performance of adhesives at the interfaces. The thermal contact resistance of commercial 65 wt.% BN/silicone based TIM (EPM 2490, a product of Nusil) was 27 % higher than that of 25 wt.% GNP/rubbery epoxy. The thermal contact resistance of adhesives was not affected by the application of pressure in the range of 0.032-0.1 MPa, suggesting that thermal interface adhesives can give better longer term performance without risks of leakage compared to commercial thermal pastes.  Conclusions The thermal interfacial performance of various carbon nanofiller-based polymer composites was studied to explore their potential as thermal interface adhesives for electronics thermal management. The comparative study suggests that GNPs offer potential as fillers for enhancing the thermal interfacial performance of polymer composite adhesives and that thermal interfacial performance of the adhesives depends on having a good combination of their thermal conductivity and their interfacial substrate contact resistance.    Figure. 4. Total thermal contact resistance versus coating thickness of 6 wt.% CB/12 wt.% GNP-5/rubbery epoxy hybrid composite (produced by mechanical mixing) measured between (a) smooth copper surfaces   & (b) rough surfaces at ~30 °C and under a pressure of 0.032 MPa.  Acknowledgements The work was carried out during M.A.Raza’s PhD studies at University of Leeds. The authors would like to thank EPSRC and Morgan Electrical Carbon for sponsoring M.A.Raza’s PhD studies.  References [1] Chung DDL. Thermal Interface Materials. Journal of Materials Engineering and Performance. 2001;10:56-9. [2] Kohli P, Sobczak M, Bowin J, Matthews M. Advanced Thermal Interface Materials for Enhanced Flip Chip BGA. Electronic Components and Technology Conference. 2001;51:564-70. [3] Lin C, Chung DDL. Graphite nanoplatelet pastes vs. carbon black pastes as thermal interface materials. Carbon 2009;47:2 5-305. [4] Yu A, Ramesh P, Itkis ME, Bekyarova E, Haddon RC. Graphite Nanoplatelet−Epoxy Composite Thermal Interface Materials. J Phys Chem C. 2007;111:7565-9. [5] Shahil KMF, Balandin AA. Thermal properties of graphene and multilayer graphene: Applications in thermal interface materials. Solid State Communications.152(15):1331-40. [6] Leong C-K, Chung DDL. Carbon black dispersions as thermal pastes that surpass solder in providing high thermal conta t conductance. Carbon. 2003;41(13):2459-69. [7] Yu A, Itkis ME, Bekyarova E, Haddon RC. Effect of single-walled carbon nanotube purity on the thermal conductivity of carbon nanotube-based composites. APPLIED PHYSICS LETTERS 2006;89(133102):1-3. [8] Sarvar F, Whalley DC, Conway PP. Thermal Interface Materials - A Review of the State of the Art. IEEE ,2006 Electronics Systemintegration Technology Conference Dresden,Germany. 2006:1292-302. [9] Raza MA, Westwood AVK, Stirling C. Effect of processing technique on the transport and mechanical properties of graphite nanoplatelet/rubbery epoxy composites for thermal interfac  applications. Materials Chemistry and Physics. 2012;132(1):63-73. [10] Ali Raza M, Westwood A, Stirling C, Brydson R, Hondow N. Effect of nanosized carbon black on the morphology, transport, and mechanical properties of rubbery epoxy and silicone composites. Journal of Applied Polymer Science. 2012;126(2):641-52. [11] Raza M, Westwood A, Stirling C. Carbon black/graphite nanoplatelet/rubbery epoxy hybrid composites for thermal interface applications. Journal of Materials Science. 2012;47(2):1059-70. [12] Raza MA, Westwood AVK, Brown AP, Stirling C. Performance of graphite nanoplatelet/silicone composites as thermal interface adhesives. Journal of Materials Science: Materials in Electronics. 2012;23(10):1855-63.  

Comparison of thermal interfacial performance of carbon nanofiller-based polymer composites

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Comparison of thermal interfacial performance of carbon nanofiller-based polymer composites

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