Epoxy composites thermal conductivity enhancement

EPOXY COMPOSITES AND METHODS TO IMPROVE

CONDUCTIVITY: A RESEARCH REVIEW

Composites: Multiphase material with significant proportions of each phase. Combine materials with the objective of getting a more desirable combination of properties Principle of combined action –Mixture gives “averaged” properties

Nanocomposites are a class of materials in which one or more phases with nanoscale dimensions (0-D, 1- D, and 2-D) are embedded in a metal, ceramic, or polymer matrix. The general idea behind the addition of the nanoscale second phase is to create a synergy between the various constituents, such that novel properties capable of meeting or exceeding design expectations can be achieved. The properties of nanocomposites rely on a range of variables, particularly the matrix material, which can exhibit nanoscale dimensions, loading, degree of dispersion, size, shape, and orientation of the nanoscale second phase and interactions between the matrix and the second phase.

Epoxies are one of the most adaptable and widely sold high performance material. Some of the applications of epoxy and its nanocomposites include aerospace, automotive, marine, sports materials, construction, structures, electrical and electronic systems, biomedical devices, thermal management systems, adhesives, paints and coatings, industrial tooling and other general consumer products. Because of its versatile nature, epoxy is replacing many conventional materials, e.g. epoxy based materials have already replaced wood in majority of the boats and various sports goods. Epoxy resins are thermosetting polymers and defined as a molecule containing more than one epoxide groups. The curing process is a chemical reaction in which the epoxide groups in epoxy resin reacts with a hardener (curing agent) to form a highly crosslinked, three-dimensional network. There are wide varieties of curing agents available for epoxy based materials. Depending on the chemical formulation of the hardeners, epoxy resins can be cured at temperatures range from 5 to 150 C. However, epoxy materials with varying engineering applications are often limited by their brittle nature and poor electrical, thermal properties. A simple solution to overcome this problem is to modify the matrix molecular structure or add compatible fillers. For example, incorporation of inorganic nano-fillers has been shown to be a very efficient strategy to increase the performance of the material. Hence, modification of epoxy resins using suitable modifiers such as phosphorus, sulphone, silicone, polyhedral oligomeric silsesquioxanes (POSS) and nanoclay is mandatory.

Preparation process of the epoxy composites

Ceramic Fillers: eg. Aluminium Nitride (AlN), Boron Nitride (BN), Alumina(Al2O3), Silica(SiO2), Silicon Nitride (Si3N4), Silicon Carbide (SiC)

Metallic Fillers: eg. Nickel, Copper, Alluminium, Silver

Carbon Based Materials: eg. Graphite, Graphene, Carbon nanotube(CNT), fullrenes

Conjugates: eg polyimide-modified aluminum nitride fillers in AlN@PI/Epoxy composites , Silver Nanoparticle-Deposited Boron Nitride Nanosheets as Fillers for Polymeric Composites with High Thermal Conductivity, Highly Electrically Conductive Nanocomposites Based on PolymerInfused Graphene Sponges

Conductivity Enhancement of Epoxy

Alumina/Epoxy Conjugates

Carbon based/Ceramics Conducting Fillers

Carbon base loading eg. graphite nanoparticles

Metallic/Carbon/Epoxy Conjugates

Metallic Filler loading

Metallic Conjugates

Ceramic Filler (Boron Nitride)/Epoxy

Metallic/Ceramic Filler (Boron Nitride)/Epoxy Conjugates

Ceramic Filler (Aluminium Nitride )/Epoxy Conjugates

Electrical Conductivity

Electrical conductivity of graphene/PMMA composites as a function of graphene content .Both in-plane and out-plane electrical conductivity increased about 17 % @ 0.5 wt% CNTs loading

Solvent Casting: eg filler solvent and resin solvent mixing before hardening agent application

Melt Processing: eg Extrusion is a popular technique, in which a polymer melt reinforced with CNTs is extruded through a die and drawn under tension before solidification

In-situ Polymerization: eg filler mixing with monomer solvent during polymerization of epoxy

Other Methods: Pultrusion, Hand layup technique, Pulverization etc

Processing Methods

Solvent Casting

Thermal conductivity increases linearly with increased filler fraction for all composite systems irrespective of fillers. Comparing Gr-Ep and SiC-Ep composites, for the same filler fraction, Graphide-Epoxy composites have higher thermal conductivity than SiC-Ep composites. The hybrid composite 20%G20%S-Ep has a thermal conductivity of 0.71 W/mK which is highest among all the composite considered in this study and is an improvement of 136% over a neat epoxy.

Incorporation of Al results in enhancement of thermal conductivity of epoxy resin. With addition of 3.34 vol. % of Al, the thermal conductivity improves by about 6.3 % with respect to neat epoxy resin

Thermal Conductivity of Graphene Nanoparticles (GNP)/Epoxy Composites:Thermal Conductivity upto (10.12 W/mK- 40% vol) is suitable for packaging, superior than conventional fillers.

Conclusion

The carboxylic surface functionalization/coupling of CNTs affects the thermal conductivity of the CNT – polymer NCs. It was found that the thermal conductivity of polymers with the functionalized carbon nanotubes is weakly or positively affected in the case of the multi-wall carbon nanotubes, while it is reduced in the case of the single-wall carbon nanotubes. Improvement of thermal conduction requires larger loading of CNTs than those needed for increasing the electrical conductivity. The longer and larger diameter carbon nanotubes are more efficient for enhancing the thermal conductivity of polymer composites. The observed trends were explained by the fact that while surface functionalization increases the coupling between carbon nanotube and polymer matrix it also leads to formation of defects, which impede the acoustic phonon transport in the single wall carbon nanotubes.

Adding the graphene-coated PMMA balls to the epoxy polymer matrix significantly boosts thermal conductivity. A composite with 1% (by weight) GPMMA balls increased thermal conductivity by ~7-fold, while the incorporation of 1% (by weight) of graphene platelets improved the thermal conductivity of the composite by ~ 3-fold. This shows that the organic PMMA nanoparticles coated with graphene could be employed to promote well-dispersed conditions (with reduced phonon scattering at the

graphene-polymer interfaces).

Future Possibilities1. Demonstrated BNNT growth onto fiber

substrates • SiC • Al2O3 • 3-D preforms 2. Halogen Intercalation Increases Yarn/Fiber Electrical Conductivity 3. Nano Fluids based on Nanoparticles and Base Fluids, suspensions of oxide and metallic nanoparticles can be achieved in common base fluids.