Study of the luminescent properties of Ca-α-SiAlON phosphors synthesized by spark plasma sintering

Abstract

White light emitting diodes (LEDs) as eco-friendly light sources have been extensively investigated due to their potential applications in backlight units for liquid crystal displays (LCD), indoor/outdoor lamps, traffic signals, and general illuminations. For enerating white LEDs, several approaches have been explored to produce the white light including multi chip LEDs and a combination of color converting phosphors and LEDs (so called pc-LEDs). Generally, phosphors converted-LEDs (pc-LEDs) have been commercially adopted due to good performances such as efficiency, single life time, and CRI. Phosphors in this system play an important role in determining the color temperature, chromaticity coordinates, color rendering index, luminous efficiency and life time of phosphor-converted white LEDs. At present, pc-LED with using combination of blue LED chip and yellow phosphor Y3Al5O12:Ce3+ has been the most popular on the market. A deficiency for pc-LED that use only YAG:Ce phosphor is that they are limited to high CCT and lower CRI due to a lack of a red spectral component. Therefore, phosphors used in white LEDs have been considerably developed in order to replace YAG:Ce yellow phosphor. In general, the requirements for phosphors include that the excitation wavelength of phosphors should match well with the emission wavelength of the LED chips (Blue chips: 450~460 nm); the phosphors must have high efficiency at the emission wavelength of the LED chips; and the phosphors have to be stable against high temperature (at least > 150 ˚C), humidity, and strong irradiation of LED chip. Commonly used phosphors for white LEDs have been mainly based on oxides and sulfides such as (Sr,Ca)S:Eu2+, SrGa2S4:Eu2+, and YAG:Ce3+. These sulfide phosphors have low thermal/chemical stability, resulting in a strong temperature dependence of chromaticity and the degradation in efficiency of white LED. While, oxide-based phosphors except YAG:Ce3+ are barely suitable for absorbing visible light efficiently, which make them impossible to combine with UV or blue LEDs to generate white light. Therefore, novel materials with requirements for white LEDs are in great demand. Very recently, nitride and oxynitride based phosphors such as a green (β-SiAlON:Eu2+, and Ba3Si6O12N2:Eu2+), yellow (Ca-α-SiAlON:Eu2+), and red phosphors (M2Si5N8:Eu2+ ; M=Ca, Sr, and Ba, CaAlSiN3:Eu2+) have been studied as a new hot topic for phosphors, which exhibit unusual, interesting luminescence properties when activated by rare-earth ions. Especially, these phosphors importantly emit visible light efficiently under near UV or blue light irradiation and have superior thermal and chemical stability to their oxide and sulfide counterparts, allowing them to be used as down conversion luminescent materials for white light emitting diodes (LEDs). Their luminescence properties are attributed to the strong nephelauxetic effect and large crystal field splitting effect. Among these various phosphors, Ca-α-SiAlON:Eu2+ phosphor has advantage to replace YAG:Ce3+ yellow phosphor as well as makes up for deficiency of red region as a single phosphor converted LEDs. Up to now, Ca-α-SiAlON phosphors have been synthesized by gas pressure sintering (GPS), gas reduction-nitridation (GRN), and carbothermal reduction-nitridation (CRN) methods. In this work, all α-SiAlON powder samples with composition of Cam/2-xSi12-(m+n)Alm+nOnN16-n:Eux were synthesized by using the spark plasma sintering (SPS) method for the first time. Spark plasma sintering is possible to synthesized nitride phosphors relatively at lower temperature for a short time without reducing gas. α-Si3N4, AlN, CaO, and Eu2O3 (CeO2, Yb2O3) were used as starting materials and the synthetic conditions were optimized, varying the sintering temperature, time, amount of dopants, and compositions. The photoluminescent properties of SiAlON doped with Eu2+ were investigated with a relation to temperature, composition, and concentration. Also, to investigate the effect of dopants on luminescent properties and thermal stability within the same crystal structure, the Eu2+, Yb2+ and Ce3+-doped Ca-α-SiAlON were prepared. Especially, thermal quenching is one of the important parameters for phosphors used in white LED. The temperature-dependent luminescent properties were measured, and the results of different behaviors were compared, relatively. In addition, the phosphors synthesized by spark plasma sintering were post-treated by annealing in a reduction atmosphere and washing with acidic solution for improvement of luminescent properties. The post-annealing conditions for all samples were optimized, varying the annealing temperature, time and gas flow. It was confirmed that the post-annealing led to improve crystallinity, increase the amount of dopant with reduced ion state and reduce glass phase in grain boundary, resulting in the enhanced yellow-orange emission without any change in the emission spectrum. Also, it is thought that washing of phosphors with using an acidic solution affected to selectively dissolve non-participated elements in glass phase on luminescent properties. It was confirmed that the removal or crystallization of glass phase in grain boundary bring respectively the improvement of luminescent property.