Abstract
In a high-power white light emitting diode (LED) package, the phosphor/silicone composite is typically used for photometric and colorimetric conversions, ultimately producing the white light. However, the phosphor/silicone composite is always exposed under harsh environments with high temperature, high blue light irradiation and high moisture when the LED operates. Therefore, its reliability issue has become one of the critical bottlenecks to improve the lifetime of a high-power white LED package. As the curing process and mechanical behavior of phosphor/silicone composite essentially determine its reliability, this paper firstly uses an in situ viscosity monitoring approach combined with Differential Scanning Calorimetry (DSC) and Fourier Transform Infrared Spectroscopy (FTIR) analysis to explain the curing mechanism of a phosphor/silicone composite by taking the effects of temperature and phosphor mass fraction into consideration. Then, the mechanical properties of phosphor/silicone composites aged under a long-term high moisture condition are evaluated by using the tensile test. Meanwhile, the finite element (FE) simulations, the Mori–Tanaka theoretical estimations and the microstructure analysis are applied to investigate the high moisture induced degradation mechanisms. The results show that: (1) the in situ measured isothermal viscosity curves of both pristine silicone and phosphor/silicone composites follow the Arrhenius empirical model, and high temperature and high phosphor mass fraction can increase the curing rate; (2) the hydrosilylation reaction between silicones determines the curing mechanism of phosphor/silicone composite; (3) the tensile test, FE simulation and Mori–Tanaka theoretical prediction results confirm that the Young’s modulus of phosphor/silicone composite increases by gradually adding phosphors; and (4) the Young’s modulus of phosphor/silicone composite increases after the high moisture ageing test, which can be attributed to the oxidation and cross-linking reaction of silicone and the hydrolysis of phosphor powders.
Highlights
A high-power white light emitting diode (LED) has been considered as a new generation of light source widely used in our daily life due to their high luminous efficiency, low energy consumption, long life and no pollution
The curing process was investigated by the Differential Scanning Calorimetry (DSC) analysis, which showed that the addition of a siloxane-type copolymer into epoxy resin could increase the initial curing temperature and peak curing temperature
Luo et al [15] compared the degradation mechanisms of phosphor/silicone composites used in phosphor-converted white LED (pc-WLED) under both high temperature and high humidity conditions
Summary
A high-power white light emitting diode (LED) has been considered as a new generation of light source widely used in our daily life due to their high luminous efficiency, low energy consumption, long life and no pollution. Polymers 2019, 11, 1277 a blue LED chip with yellow phosphors and its white light luminous mechanism is that the blue light generated by the LED chip mixes with the yellow light excited by phosphors [1,2] In this phosphor-converted white LED (pc-WLED) package, phosphors and silicone encapsulant are usually combined as a composite to undertake the functions of both blue light down-conversion and chip protection [3]. When being used in the harsh environment (e.g., high temperature and high moisture conditions) [4], the phosphor/silicone composites will obviously deteriorate especially in case of the insufficient curing process, which can result in a serious lumen degradation and color shift of a pc-WLED package [5,6]. The results show that the hydrolyzation of phosphors and the oxidation of silicone under a high moisture environment could accelerate the degradation of phosphor/silicone composites
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