Strontium silicate Sr2SiO4 undergoes a displacive phase transformation around 85 °C. However, the steady state spectra involved in the process lacks investigation. In this paper, two kinds of Sr2SiO4:Eu2+ with and without Ba ions are synthesized. They are tested for XRD patterns and luminescent spectra at varying temperatures. The results show that Ba ions could effectively suppress the phase transition. For sample without Ba, temperature-dependent XRD patterns confirm the occurrence of phase transition and the emission peak positions at varying temperatures demonstrate a hysteresis behavior. The color of sample heated to 100 °C under UV illumination is distinguishable from that unheated. This structure-sensitive behavior make Sr2SiO4:Eu2+ as potential thermochromic material.
The dependences of light efficiency of radiation (LER) and color-rendering index (CRI) of trichromatic white light-emitting diode (wLED), composed of blue LED die, green/yellow, and red phosphors, on the peak wavelength of each primary were investigated by theoretical calculations, at correlative color temperature (CCT) from 2,700 to 6,500 K. The peak wavelength of InGaN based blue LED chip ranges from 450 to 471 nm, while those of Ca3Sc2Si3O12:Ce3+, b-SiAlON:Eu2+, and Y3Al5O12:Ce3+ based green/yellow phosphors range from 511 to 572 nm, and those of Sr2Si5N8:Eu2+ and CaAlSiN3:Eu2+ red phosphors range from 620 to 650 nm, which cover almost all the practically used, commercially available wave bands until now. Then, based on the results, selection guides of peak wavelengths for blue LED chip and phosphors to obtain tradeoff LER >280 lm·W−1 as well as CRI >80 in all CCTs are proposed. The favorable wave bands of each primary are suggested.
In this study, small- and large-particle-diameter phosphor powders were mixed together (hybrid phosphors) to balance light efficacy and angular color uniformity and pursue optimal results. Phosphor with small-particle-diameter of 4 μm was employed and it was mixed into each large-particle-diameter phosphor of 10, 16, 22, and 26 μm, at mass percentage from 0 % to 50 % with an interval of 10 %, respectively. Remote phosphor package was adopted and overall phosphor concentration was kept constant for better comparison. Moreover, absorption coefficient μ abs, scattering coefficient μ sca and extinction coefficient μ ext of each hybrid phosphors were calculated based on Mie theory to further discuss the experiment results. Results show that, the introduction of small-particle-diameter phosphor to large one can highly improve angular color uniformity while only slightly reduce light efficacy. The optimal performance with angular color uniformity of 91.6 % as well as normalized light efficacy of 95.7 % is achieved in the white light emitting diode with hybrid phosphors consisting of 60 wt% powder of 22 μm and 40 wt% powder of 4 μm.
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