By nature, epoxies are inherently thermally insulating, typically falling in the range of 0.25 W/mK. Below are some examples of materials that adhesive formulators use to raise the thermal conductivity of epoxy, polyurethane and silicone systems when creating thermal transfer products.
1. Alumina - Traditional Thermally Conductive Epoxy Filler
Alumina, otherwise known as aluminum oxide, offers thermal conductivity, electrical resistivity, dimensional stability, and toughness. Alumina provides fantastic thermal properties, including high thermal conductivity and thermal shock resistance. Alumina is most commonly used for thermally conductive potting epoxies. Epoxies filled with significant amounts of alumina have been shown to exibit thermal conductivity as high as 2.2 W/mK.
2. Hydrated Alumina - Flame Retardancy
Hydrated alumina is also known as aluminum hydroxide. Not only is hydrated alumina exceptionally thermally conductive, but at high temperatures (around 180°C) hydrated alumina decomposes, absorbing a significant amount of heat and releasing water vapor. This makes aluminum hydroxide a great additive for flame retardant systems.
3. Boron Nitride - High Performance Applications
A pure substance boron nitride exhibits thermal conductivity of up to 600 W/mK within the basal plane. Epoxies filled with boron nitride can exhibit levels of thermal conductivity as high as 4 W/mK. Furthermore, boron nitride is a strong electrical insulator which makes it great for applications where electrical insulation and heat transfer are both priorities. Pure boron nitride has excellent thermal and chemical stability.
4. Diamond - Ultra-High Performance Properties
Diamond is an exceptional heat transfer filler material. As a pure substance, diamond exhibits extremely high thermal conductivity of 2,300 W/mK. Diamond, like boron nitride, is highly electrically resistant (1x 10^12 ohm m), making it a great additive for applications that require electrical insulation and thermal conductivity. Due to the cost of diamond material, this filler is reserved for super-high performance applications.
While a number of thermally conductive epoxy fillers exist, alumina, hydrated alumina, boron nitride, and diamond are among the highest quality of materials which adhesive formulators have found to increase the thermal conductivity of polymer adhesives.