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Title: Carbon nanofiller-based composites for thermal interface applications
Author: Raza, Mohsin Ali
ISNI:       0000 0004 2742 5957
Awarding Body: University of Leeds
Current Institution: University of Leeds
Date of Award: 2012
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Carbon nanofillers such as graphite nanoplatelets (GNPs) and vapour grown carbon nanofibres (VGCNFs) have enormous potential for developing thermal interface materials (TIMs), mainly due to their high thermal conductivity. In this project GNPs, VGCNFs and carbon black (CB) fillers were dispersed in the compliant polymer matrices, rubbery epoxy and silicone, to form composites. Mechanical mixing, dual asymmetric centrifuge speed mixing, three-roll milling or combined sonication and solvent mixing were used to produce composites. The effects of processing technique, wt.% offiller(s), particle size and silane-functionalisation of fillers on the properties of composites were studied. Composites were characterised mainly in terms of morphology, texture, thermal conductivity, electrical conductivity and mechanical properties. The interfacial thermal transport performance of carbon nanofiller/polymer composites was studied using a steady state method, with a view to their use as 'thermal interface adhesives and thermal pads. Roll milling was found to be the best method for producing composites with superior transport properties. GNP/rubbery epoxy and GNP/silicone composites produced by roll milling have thermal conductivities in the range of 1-3 W/m.K with 8-25 wt.% GNP. The thermal conductivities of the composites increase with increasing GNP loading and particle size but slightly decrease with silane-functionalisation of GNPs. Composites produced using GNPs ) synthesised (in-ho~se) via graphite, oxidation and thermal exfoliation offered improved transport properties compared to corresponding composites produced with commercial GNPs. Development of good interconnects between carbon nanofillers was found to be vital for producing composites with improved transport properties. GNP/silicone composites are more compliant materials than GNP/rubbery epoxy composites. VGCNF/rubbery epoxy composites have thermal conductivities in the range of 0.2-1.8 W/m.K with 2-40 wt.% VGCNF. VGCNFs increase the compressive strength of both rubbery epoxy and silicone without compromising their compliant nature. The thermal conductivity of CB/polymer composites reached ~O.2-0.3 W/m.K with 8-36 wt.% CB, depending upon the CB used. CB incorporation improved dispersion of GNPs in hybrid CB/GNP/rubbery epoxy composites and produced a thermal paste-type morphology. Similarly, VGCNFs improved the dispersion of GNPs in GNPNGCNF/rubbery epoxy hybrid composites but reduced the density of interconnects between GNPs. GNP/rubbery epoxy and VGCNF/rubbery epoxy composites offered the best performance as thermal interface adhesives compared to CB/rubbery epoxy and commercial thermal interface adhesive. The thermal contact resistance of the adhesives depends on their viscosity/conformability, bond line thickness, filler particle size, surface roughness of the substrate and thermal conductivity.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available