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Preparation And Optical Properties Of Transparent Epoxy Nanocomposites As Encapsulating Materials For LED Chips

Posted on:2008-10-25Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y Q LiFull Text:PDF
GTID:1101360215476737Subject:Physical chemistry
Abstract/Summary:PDF Full Text Request
Since the first demonstration of the high-brightness blue light-emitting diode (LED) and subsequently the white LED, interests in using solid-state light sources for general illumination have been rapidly growing. Transparent epoxy resins have been most frequently employed in standard LED technology as packaging materials because of their excellent properties. With the rapid development of LED, the emission spectrum wavelength is becoming shorter and light output intensity is rapidly increasing. At the same time, discoloration of epoxy resins as encapsulating materials and maintenance of light output intensity have become the major issues. In this thesis, many efforts have been made to (1) improve UV light shielding efficiency and photo-stabilization properties of epoxy resins as encapsulating materials for LED chips by incorporating both organic and inorganic UV-absorbents into pure transparent epoxy resins and (2) to make ZnO-QD/epoxy and ZnO-QD/SiO2/epoxy nanocomposites for the first time as encapsulating materials for LED chips in which ZnO-QD will function as phosphor powders but avoid light scattering between the phosphors and epoxy resins. The major results are presented as follows:(1) First, zinc oxide (ZnO) precursor was synthesized via the homogeneous precipitation method and ZnO nanoparticles were then made by calcination of the precursor at different temperature. Transparent ZnO/epoxy nanocomposites were subsequently prepared from transparent epoxy (EP-400) and as–prepared ZnO nanoparticles via in-situ polymerization. The nanocomposite containing a very low content (0.07% in weight) of ZnO nanoparticles with an average particle size of 26.7 nm after calcination at 350℃possessed the most optimal optical properties, namely high visible light transparency and high UV light shielding efficiency.(2) A novel solid titania precursor, with a general chemical formula of TiSO4(AcAc)2-n(OH)n·mH2O (n<2 and m<2), were prepared using a simple synthetic method. The novel titania precursor possesses high stability against hydrolysis and can be very conveniently employed for preparation of fine titania particles with no need of any solvent. Well dispersed TiO2 nanoparticles with an average size of about 20 nm were prepared by directly annealing the solid titania precursor at 600 oC for 2 hours. TiO2/epoxy nanocomposites that were subsequently prepared via in-situ polymerization showed high shielding efficiency for both visible and UV light.(3) A strategy for facile preparation of highly transparent polymer nanocomposites in terms of the filler-matrix refractive index (RI) matching principle was presented. Silica-titania (S-T) core-shell composite nanoparticles with tunable refractive index (RI) were first synthesized and the S-T/epoxy nanocomposites were then prepared by incorporation of the as-obtained S-T composite nanoparticles into a transparent epoxy matrix. The nanocomposite containing 1 wt% S-T with the shell-core weight ratio of 36.5% possessed the most optimal optical properties, namely high visible light transparency, 87% transmittance at 800nm, and high UV light shielding efficiency.(4) The effects of organic and inorganic photo-stabilizers on the optical transmittance and photo-stabilization properties of epoxy resins as encapsulating materials for LED lamps have been investigated. The results indicated that all the selected photo-stabilizers have excellent compatibility with epoxy. It was shown that ZnO possesses the best photo-stabilization effect and the lifetime of LED has been improved by 76%. Moreover, the simultaneous usage of organic photo-stabilizers of HALS with UV-absorbers showed a synergetic effect on the lifetime of LED lamps and the lifetime of LED encapsulated with the optimally modified epoxy resin has been improved by 170% compared to the LED lamp encapsulated with the pure transparent epoxy resin (EP-400). (5) ZnO quantum dots (ZnO-QD) with about 3 nm particle size were synthesized via the modified Spanhel's method and then transparent ZnO-QD/epoxy nanocomposites were subsequently prepared via in-situ polymerization. The ZnO-QD/epoxy nanocomposites showed excellent visible light transmittance when the filler content was below 4 wt%. At the same time, a broad fluorescent emission peak at 450 nm was observed and the intensity of the fluorescent peak increased monotonically with the increase of filler content.(6) ZnO-QD/silica (Z-S) composite nanoparticles were synthesized by hydrolyzing TEOS within the ZnO-QD-containing ethanol solution. Z-S composite nanoparticles with the best fluorescence were obtained by calcination at 300 oC for 2h. Moreover, preparation of Z-S composite nanoparticles that possessed excellent phosphorescence properties was reported for the first time by calcinations of Z-S at 500 oC for 2h. The phosphorescence lifetime of Z-S was about 2 seconds. Then, the transparent Z-S/epoxy nanocomposites were successfully prepared in terms of matrix-filler RI matching method and the phosphorescence lifetime of the Z-S/epoxy nanocomposites was about 4 seconds. Furthermore, Z-S/epoxy nanocompsites were employed to encapsulate UV-LED chips (370 nm) and the resulting LED lamp encapsulated with Z-S/epoxy nanocomposites emitted light-green-like white light, showing that the Z-S/epoxy nanocomposite have the great potential to be applied as encapsulating materials for LEDchips while Z-S nanocomposite particles would be excellent substitutes for traditional phosphor powders widely used in white LED.
Keywords/Search Tags:LED, encapsulation, epoxy, nanocomposite, photostabilization, ZnO, TiO2, SiO2, refractive index, quantum dot, fluorescence, phosphorescence
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