| Light emitting diode(LED),as a new type of high efficiency solid-state light source has many advantages,such as long life time,economic and environmental protection,green energy saving and so on.It has huge implications in economic and social development.However,the actual life of LED is still low,which seriously restricts the promotion of LED applications.Studies found that high junction temperature is the key factor in reducing the life time of LED.The main factors that affecting the junction temperature are the light transmittance and UV-aging of the top encapsulation materials and the thermal conductivity of the adhesive layer.In order to improve the thermal conductivity of the adhesive layer and enhance the transmittance and anti-UV aging ability of the top encapsulation materials to reduce the junction temperature of LED,the following three aspects were studied in this paper:(1)Effect of adhesive layer on junction temperature: BN and Al2O3 particles were surface modified by KH-550,BN/EP composites and Al2O3/EP composites were then prepared,and were used as the adhesive layer of LED.Experimental results showed that the junction temperature of LED decreases with the increasing thermal conductivity of the adhesive layer composites and increases with the increasing thickness of the adhesive layer.Finally,the optimum adhesive layer thickness is 0.1mm.When the content of surface modified BN was 60 wt%,the thermal conductivity of BN/EP composites was the highest,when it was used as the adhesive layer of 1W LED,the experimental junction temperature was the lowest,which could drop to 75.2?C,that was 27.6?C lower than the pure epoxy resin.While the minimum experimental junction temperature of the Al2O3/EP composites was 78.2?C.The experimental junction temperature of LED is similar to the results of the COMSOL simulation.(2)Electric field aligned MgO/EP composites to improve the transmittance: magnesium oxide(MgO)was introduced to the epoxy resin,and an DC external electric field is also applied during the curing process,which tent to align the MgO particles and epoxy molecular to construct the photon propagation channels,thus decreases light scattering and enhance the light diffraction.As a result,the transmittance of the composites was greatly enhanced.When the concentration of MgO was 0.02 wt%,the transmittance of electric field aligned MgO/EP composite was increased to 93.8%,which is 1.09 times that of the EP,1.02 times that of the SI(GN-502L)and 1.04 times that of the SI/PC double-layered material.Moreover,phonon conducting channels were also constructed in the aligned composites,the thermal conductivity of the electric field aligned 0.02 wt% MgO/EP composite was increased to 0.353 W/m·K,which is 1.7 times that of the EP.(3)Light-stabilizers modified epoxy composites to improve the anti-UV aging ability: inorganic UV shielding light-stabilizer nano-ZnO was selected,and combined with common organic light-stabilizers HS-508,UV-329 and UV-P to modify the LED encapsulation epoxy resin.It was found that all the selected light-stabilizers have great anti-UV aging effect on epoxy resin,and nano-ZnO has the best anti-UV aging effect.The hindered amine light-stabilizer HS-508 has a synergistic effect with the UV absorbers UV-329 and UV-P.When 0.1 wt% HS-508 was combination used with 0.1 wt% UV-329,the anti-UV aging effect of the epoxy composites is the best.Through these three aspects research,the junction temperature of LED could be greatly reduced. |
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