Spectral Tuning Of Phase-Mixed CaO-SiO₂Phosphors Applied For White LED

To approach the solutions associated with the problems in phosphor blending andwhite-light quality in the present color-mix combination of blue InGaNchip/phosphor-converted white light-emitting-diodes （pc-wLED）, a variety ofphase-mixed phosphors in both powder and glassceramics forms involved inCaO-SiO2system were investigated. A green-emitting phosphor Ca₂SiO₄and ared-emitting phosphor Ca₃Si₂O₇activated by europium were examined in detail,respectively. The relations of structure, photoluminescence and color-mix involvedwere investigated experimentally. The main achievements made in this work include:1. The electronic structure and optical properties of Ca₃Si₂O₇and Ca₂SiO₄werecalculated in terms of the First Principle Calculation method. The calculatedresults showed both of host compounds exhibit in direct band gap while theformer is smaller than the later.2. A relationship between atomic structure of host and characteristicphotoluminescence properties of green-emitting phosphor Ca₂SiO₄: Eu²⁺and thatof the orange-red-emitting phosphor Ca₃Si₂O₇: Eu²⁺were investigated.Integrating the information of calculated results of band gaps, XRD andphotoluminescence spectra, the calculated lower energy edge position from VanUitert formula, Pauling bond valence and covalent bonding theory as well, wedraw the conclusion that Eu²⁺substitutes for Ca²⁺at the sites in8-coordination inCa₂SiO₄lattice to emit a505nm-peaked broad band green light, while that in7-coordination to give out600nm-peaked reddish orange band emission.3. The orange-red-emitting Ca₃Si₂O₇: Eu²⁺in pure phase was particularlyinvestigated by the addition of flux NH4Cl in an amount of3wt. percent at1300°C. It was found that the co-doping of Ce3+ions has an enhancement effecton the luminescent intensity of Eu²⁺in a resonance energy transfer. A suitabledoping concentration of Eu²⁺was optimized to be around2.5wt. percent. Thefeatured photoluminescence spectral properties indicate that this phosphor ispotential to be used to lower the color temperature of white-light as combinedwith blue InGaN chips.4. A strategy was proposed further for Ca₃Si₂O₇-Ca₂SiO₄phopshor system that a color-mix combination of simultaneously green-emitting from Ca₂SiO₄:Eu²⁺andorange-red-emitting from Ca₃Si₂O₇:Eu²⁺can be achieved through adjusting thephase-mixed composition by changing the flux amount of NH4Cl. The phosphorceramics can be pumped effectively by400nm blue and violet light to emit thedual light simultaneously.5. A new variety of glass ceramics phosphor that contains crystallite-mixed phaseswith both green and the orange-red emissions was explored primarily. It wasfound that in the designed heating process, starting with raw material in thenominal composition of (Ca_0.99Eu_0.01)3Si₂O₇, a direct glass ceramics with Ca₂SiO₄:Eu²⁺crystal was obtained through melt-quenching method in terms of asolid-liquid （SL） two-phase region of CaO-SiO2binary phase diagram. The meansize of spherical β-Ca₂SiO₄crystal in microsize and different luminescenceproperties were found to be varied with the holding time in SL two-phase region.Based on scattering theory, a trade-off of the luminescence and optical propertiesare discussed by closely associating with refractive index, size and morphology ofluminous crystal phase in glass.The results of structure-property relation we obtained above could be ofscientifical significance in developing some new phosphors for pc-wLED withhigher color rendering index, while those phase-mixed phosphors in both powderand glass ceramics forms are of potential in the application of WLED packagingprocess with blending–free color-mix operation as compared to the present powdermix with multicomponent phosphors in microsize.