Densification: A Route towards Enhanced Thermal Conductivity of Epoxy Composites

Abstract

When an amorphous polymer is cooled under pressure from above its glass transition temperature to room temperature, and then the pressure is released, this results in a densified state of the glass. This procedure applied to an epoxy composite system filled with boron nitride (BN) particles has been shown to increase the density of the composite, reduce its enthalpy, and, most importantly, significantly enhance its thermal conductivity. An epoxy-BN composite with 58 wt% BN platelets of average size 30 µm has been densified by curing under pressures of up to 2.0 MPa and then cooling the cured sample to room temperature before releasing the pressure. It is found that the thermal conductivity is increased from approximately 3 W/mK for a sample cured at ambient pressure to approximately 7 W/mK; in parallel, the density increases from 1.55 to 1.72 ± 0.01 g/cm3. This densification process is much more effective in enhancing the thermal conductivity than is either simply applying pressure to consolidate the epoxy composite mixture before curing or applying pressure during cure but then removing the pressure before cooling to room temperature; this last procedure results in a thermal conductivity of approximately 5 W/mK. Furthermore, it has been shown that the densification and corresponding effect on the thermal conductivity is reversible; it can be removed by heating above the glass transition temperature and then cooling without pressure and can be reinstated by again heating above the glass transition temperature and then cooling under pressure. This implies that a densified state and an enhanced thermal conductivity can be induced even in a composite prepared without the use of pressure.

Thermally conductive polymer-based composites: fundamentals, progress and flame retardancy/anti-electromagnetic interference design

“…It was also proved that densification is a route to enhance the thermal conductivity of epoxy resin composites by pressing epoxy resin with a loading of 58 wt% thermally conductive BN platelets of 30 mm average size at a pressure of 2.0 MPa. 105 The study found that, by pressing, the density of the composites increased from 1.55 AE 0.01 g cm À3 to 1.72 AE 0.01 g cm À3 , while the thermal conductivity improved from 3 W m À1 K À1 to 7 W m À1 K À1 . An in-plane thermal conductivity of 67.6 W m À1 K À1 for the poly(vinyl alcohol) (PVA) film with a loading of 83 wt% boron nitride nanosheets (BNNS) was attained, 106 which could be ascribed to the ultrahigh loading of BNNS.…”

Remarkable Thermal Conductivity of Epoxy Composites Filled with Boron Nitride and Cured under Pressure

“…Beloshenko and co-workers [43,44] investigated various epoxy resins and found that increasing pressure during cure results in a higher density of the cured epoxy, attributing this effect to the inhibition of voids and a reduction of the free volume. This latter aspect is equivalent to the effects of densification, discussed elsewhere [22], which requires not only that the cure be made under pressure but also that the pressure be maintained while the sample is cooled from the curing temperature. Hwang and Chang [45] used a plunger-type dilatometer to measure the volume shrinkage during cure of an epoxy molding compound (EMC), and observed an increase in the volume shrinkage with increasing pressure in the range from 2 to 10 MPa.…”

“…In the first place, it might intuitively be expected that applying pressure would consolidate the material, improving the matrixfiller interface, and hence enhancing the thermal conductivity. Additionally, though, there is also the possibility of densifying the material, by cooling the epoxy matrix through its glass transition region under pressure [21,22]. In view of the occasionally noted correlation between density and thermal conductivity [22,23], this would imply a further possible route towards increasing the thermal conductivity.…”

“…Additionally, though, there is also the possibility of densifying the material, by cooling the epoxy matrix through its glass transition region under pressure [21,22]. In view of the occasionally noted correlation between density and thermal conductivity [22,23], this would imply a further possible route towards increasing the thermal conductivity. Accordingly, it would be appropriate to summarize here the state of the art as regards the use of pressure during the cure of epoxy-BN composites for high thermal conductivity applications.…”

Highly Thermally Conductive Epoxy Composites with AlN/BN Hybrid Filler as Underfill Encapsulation Material for Electronic Packaging

“…Nonetheless, these advancements have also resulted in the increase in heat flux from electronic components, owing to the increase in heat produced during the device operation [ 4 , 5 , 6 , 7 ]. It is well known from several studies and experience that the stability and lifespan of electronic components are affected directly by the thermal regime during their operation [ 8 , 9 ]. Consequently, it is essential that the heat accumulated in the device is dissipated as fast as possible to maintain the operating temperatures of devices at an admissible level.…”