YAGG:Ce transparent ceramics with high luminous efficiency for solid-state lighting application

A series of Y2.985Al5–xGaxO12:0.015Ce(YAGG:Ce,x=0,1,2,3,4,5)transparent ceramics were prepared via a solid-state reaction method.Two-step sintering technique was proved to be an effective approach to prepare functional ceramics with high Ga concentration,and Y3Ga5O12(YGG)transparent ceramic was successfully prepared for the first time.According to the variation of Al/Ga ratio,regulation of band structure and luminescence properties of YAGG:Ce transparent ceramics were effectively investigated.When Ga substitutes Al sites,the tetrahedral site is more favorable compared to the octahedral site for Ga to occupy according to the first-principle calculation.A continuous blue shift of the emission from 565 to 515 nm was achieved as Ga was gradually introduced into Y3Al5O12:Ce matrix.High quality green light was obtained by coupling the YAGG:Ce ceramics with commercial blue InGaN chips.Transparent luminescence ceramics accomplished in this work can be quite prospective for high power LED application.

Strategies to approach high performance in Cr^(3+)-doped phosphors for high-power NIR-LED light sources

Broadband near-infrared(NIR)-emitting phosphors are key for next-generation smart NIR light sources based on blue LEDs.To achieve excellent NIR phosphors,we propose a strategy of enhancing the crystallinity,modifying the micromorphology,and maintaining the valence state of Cr^(3+) in Ca_(3)Sc_(2)Si_(3)O_(12) garnet(CSSG).By adding fluxes and sintering in a reducing atmosphere,the internal quantum efficiency(IQE)is greatly enhanced to 92.3%.The optimized CSSG:6% Cr^(3+) exhibits excellent thermal stability.At 150℃,97.4% of the NIR emission at room temperature can be maintained.The fabricated NIR-LED device emits a high optical power of 109.9mW at 520 mA.The performances of both the achieved phosphor and the NIR-LED are almost the best results until now.The mechanism for the optimization is investigated.An application of the NIR-LED light source is demonstrated.

Thermoelectric properties of In-Hg co-doping in SnTe:Energy band engineering

Synergistic effect of band convergence and resonant level could be manipulated in SnTe by co-doping In and Hg,leading to a potential thermoelectric performance enhancement in a much wider temperature range.In this work we carefully investigated thermoelectric properties of the In-Hg co-doped SnTe,synthesized by a hot pressing method.With this co-doping the Seebeck coefficients of the co-doped samples were greatly improved(over 50 mVK^(-1))at the room temperature.Although power factors of the In-Hg co-doped SnTe were also able to be optimized,the peak ZT(0.9 at 850 K in Sn_(0.98)Bi_(0.02)Te-1%HgInTe_(2)),however,is not high enough when comparing to other co-doped SnTe systems.This may be caused by the relatively high lattice thermal conductivity.An apparent competition between band convergence doping and resonant level doping was observed in our experiment.The results suggest that band engineering via co-doping should be further understood in order to optimize the thermoelectric properties inside the material system.

Growth and Optical Spectra of Tb^(3+)/Eu^(3+) Co-doped Cubic NaYF_4 Single Crystal for White Light Emitting Diode

High quality Tb^3＋/Eu^3＋ co-doped cubic NaYF 4 single crystal in the size of Φ1.0 cm×6.6 cm was grown by a modified Bridgman method using KF as assistant flux for NaF-YF 3 system under the condition of completely closed Pt crucible.A white light emission from the combination of the violet-blue,blue,green,orange,and red lights with chromaticity coordinates of x = 0.3107,y = 0.3274,correlated color temperature of T c = 6637 K,color rendering index of R a = 83,and color quality scale of Q a = 82 could be obtained from 1.51 mol%Tb^3＋ and 1.42 mol%Eu^3＋ co-doped cubic NaYF 4 single crystal when being excited by a 369 nm light.This indicates that Tb^3＋/Eu^3＋）co-doped cubic NaYF 4 single crystal has a potential application in white light emitting diode excited by ultraviolet light.

Optical Spectra and Gain Properties of Ho^(3+)/Pr^(3+)Co-doped LiYF_4 Crystal

The LiYF4 single crystals singly doped Ho3＋ and co-doped Ho3＋, Pr3＋ ions were grown by a modified Bridgman method. The Judd-Ofelt strength parameters （Ω2, Ω4, Ω6） of No3＋ were calculated according to the absorption spectra and the Judd-Ofelt theory, by which the radiative transition probabilities （A）, fluorescence branching ratios （β） and radiative lifetime （τ rad） were obtained. The radiative lifetimes of 5/6 and 5/7 levels in Ho3＋ （1 mol%）：LiYF4 are 10.89 and 20.19 ms, respectively, while 9.77 and 18.50 ms in Ho3＋/pr3＋ doped crystals. Hence, the τ rad of 5/7 level decreases significantly by introduction of Pr3＋ into Ho3＋：LiYF4 crystal which is beneficial to the emission of 2.9 μm. The maximum emission cross section of Ho3＋：LiYF4 crystal located at 2.05 μm calculated by McCumber theory is 0.51 ×10-20 cm2 which is compared with other crystals. The maximum emission cross section at 2948 nm in Ho3＋/pr3＋ co-doped LiYF4 crystal obtained by Fuchtbauer- Ladenburg theory is 0.68 × 10-20 cm2, and is larger than the value of 0.53 × 10-20 cm2 in Ho3＋ singly doped LiYF4 crystal. Based on the absorption and emission cross section spectra, the gain cross section spectra were calculated. In the Ho3- ions singly doped LiYF4 crystal, the gain cross sections for 2.05 μm infrared emission becomes positive once the population inversion level reaches 30%. It means that the pump threshold for obtaining 2.05 μm laser is probably lower which is an advantage for Ho3＋-doped LiYF4 2.05 μm infrared lasers. The calculated gain cross section for 2.9 μm mid-infrared emission does not become positive until the population inversion level reaches 40% in Ho3＋/pr3＋：LiYF4 crystal, but 50% in Ho3＋ singly doped LiYF4 crystal, indicating that a low pumping threshold is achieved for the H03＋：5/6 → 5/7 laser operation with the introduction of Pr3＋ ions. It was also demonstrated that Pr3＋ ion can deplete rapidly the lower laser Ho3＋：5/7 level and has influence on the Ho3＋：5/6 level. The Ho3＋/pr3＋：LiYF4 crystal may be a potential media for 2.9 μm mid-infrared laser.