ZrO2/epoxy nanocomposite for LED encapsulation

ZrO2/epoxy nanocomposite has been prepared and tested as encapsulant in several types of LED package. The composite was prepared by the blending of cycloaliphatic epoxy/anhydride resin with silane-modified ZrO2 fillers in ethyl acetate solvent, which was removed before curing. The maximum filler loading achieved for a workable encapsulant was about 20% by volume. This led to an increase of 0.04 on the refractive index. A high-power LED package encapsulated with this composite showed more than 10% increase of the light output compared to that with the pure epoxy. The enhancement was more than that predicted by the reduction of internal total reflection at the chip/encapsulant interface. The extra enhancement was traced back to the unusually high scattering and a better heat dissipation. The scattering was found to be wavelength independent, thus more consistent with the Mie theory than the Rayleigh one. The thermal resistance of the composite was measured directly in a LED package and found to be 16% lower than that of a commercial silicone encapsulant. Unfortunately, no improvement on the durability was observed with the addition of inorganic fillers. The temperature-humidity-bias life-time of the packages encapsulated with our composite was practically the same as that with the pure cycloaliphatic epoxy. Therefore, the durability of the composite is largely determined by the matrix itself.