Preparation And Luminescence Properties Of Molybdate Red Emitting Phosphor For White LEDs

White light-emitting diodes (LEDs), known as the fourth generation solid-state lighting, have many advantages over the existing incandescent and halogen lamps in reliability, energy saving, long life, small size, maintenance and safety, and therefore are gaining lots of attentions. Today, the white LEDs market is dominated by phosphor-converted LEDs (pcLEDs), which brings new prospect to the development of phosphors.To completely replace fluorescent lamp to be a new generation indoor illumination source, it is necessary that the traditional white LED lighting possesses lower costs, higher luminous efficiency and color rendering index and better adjustability in color temperature of white light emitting and so on. There exist some defect in the commercialized white LEDs by using pcLEDs comprising a blue emitting InGaN semiconductor (420-480 nm) coated with yellow phosphor (YAG:Ce), such as electro-optical intensity of blue spectra increases faster than that of yellow light at high current, the current changes can engender spectrum mismatch, consequently result in the changes of color temperature and low color rendering index. The above circumstances are absent in the ultraviolet (UV) and near-UV LEDs systems. Since the conversion of UV and near-UV LEDs has the virtue of relative lower cost, more easily controlling color than the blue LED, excellent color uniformity, good color rendering etc, it is also a development tendency of white LEDs that UV and near-UV LEDs systems supplant the traditional white pcLEDs operating on the basis of blue InGaN dies. The current commercially applicable red phosphor for UV InGaN-based LEDs is Y2O2S:Eu3+ However, the Y2O2S:Eu3+ red phosphor cannot efficiently absorb in near-UV region and its brightness is about eight times less than that of the blue (BaMgAl10O17:Eu2+) and green (ZnS:(Cu+, Al3+)) phosphors. In addition, the lifetime of the Y2O2S:Eu3+is inadequate under near-UV irradiation for its instability. The drawback of red emitting phosphors has been the main obstacle for the development of white LEDs. Therefore, the study on the red phosphors matching to UV (near-UV) LED is of great significance in theory and practice.In this work, the preparation methods and luminescent properties of the existing red emitting phosphors CaMoO4:Eu3+ systems were systematically investigated, and their light-emitting properties were improved by co-doping rare earth ions. Three types of novel molybdate red emitting phosphors excited by near-UV or blue light were synthesized, whose synthesis method and spectral characteristics were researched in detail. The major research contents and appropriate conclusions are as following:1. Synthesis and photoluminescence properties of red emitting phosphors CaMoO4:Eu3+ forwhite LEDsThe technological conditions for preparation of red emitting phosphors CaMoO4:Eu3+ by traditional solid state reaction were investigated. The optimum calcination temperature and the holding time were determined to be 900℃and 3 hours, respectively. And the superlative Eu3+-doped concentration in red phosphors CaMoO4:Eu3+ was concluded to be 25at.%. It was important and positive to improve doping concentration of Eu3+ from 20at.% to 25at.%. Namely, the luminescence intensity increased with the increase of doping concentration of Eu3+ in the molybdenum calcium system. The effect of charge compensator (Li+, Na+, K+) on the luminescence behavior of Eu3+ activated CaMoO4 phosphor was discussed in detail. The relative intensity decreases with the increases of the ionic radii of alkali metal ions, which are as charge compensator (the ionic radii in the order of Li+ Na+> Li+). Taking all factors into account, the best expression was LiEu(WO4)0.5(MoO4)1.5.A series novel red phosphors LiEu1-xYx (WO4)0.5 (MoO4)1.5 (x=0,0.1,0.2,0.3,0.4,0.5,0.6, 0.8) were prepared, and the best doping concentration of Y3+ was determined to be 0.5mol. The series phosphor can be excited by near-UV(396 nm) and blue (466 nm) and emit red light at 615 nm (5D0→7F2 transition of Eu3+), nicely fitting in with the widely applied output wavelengths of ultraviolet or blue LED chips. Our reserach also proves that the Eu3+ occupies the lattice site of noncentrosymmetric environment in the scheelite phases. All the results indicate that the red phosphor is a suitable candidate of red emitting phosphor for the fabrication of white LEDs.In addition, the effects of the flux on the luminescent properties were studied. The results of the XRD patterns indicated that the proper flux was beneficial to the crystallization of the phosphor, and no other phases were formed. Proper addition amount of flux could improve the relative luminosity and decrease the particle size. Experiments confirmed that the phosphor had the optimal comprehensive properties when the addition amount of flux (WAlF3/WH3BO3=1/1) was 1%.A series of novel red phosphor LiEu1-xBix(WO4)0.5(MoO4)1.5 (x=0,0.1,0.2,0.3,0.4,0.5,0.6, 0.8) were synthesized and their luminescence properties were studied in detail. The emission spectra show the most intense peak is located at 615 nm, which corresponds to the 5D0→7F2 transition of Eu3+. The experimental results show that adding appropriate amount of Bi3+ in the LiEu(WO4)0.5(MoO4)1.5 can not only increase the luminescence intensity of phosphor but also improve the color purity. The optimum doping concentration of Bi3+ was found to be 10at.%. For higher concentration of Bi3+, the absorbed energy is nonradiatively dissipated due to the formation of Bi3+ aggregates, which results in an evident reduction of the sensitized effectiveness of Bi3+ ions on Eu3+ ions. The average separation RBi→Eu (in A) of energy transfer is determined to be 22.3,23.7,24.9,26.1,28.1,29.9, and 31.4 for Bi3+ concentration x=0.05,0.1,0.15,0.2, 0.3,0.4 and 0.5, respectively, in LiEu1-xBix(WO4)0.5(MoO4)1.5. The critical concentration xc, at which the luminescence intensity of Eu3+ is half that in the sample in the absence of Bi3+, is 0.45. Therefore, the critical distance (Rc) of energy transfer was calculated to be about 27.1 A.4. A potential red emitting phosphors scheelite-like triple molybdates LiKGd2(MoO4)4:Eu3+ for white LEDs applicationsA series of novel triple molybdates red emitting phosphors LiKGd2-xEux:(MoO4)4 were successfully synthesized through solid state reaction. The optimum condition was calcination at 850℃for 5 hours. A semi-quantitative estimation of the sample was investigated by adiabatic method. The mass fraction of phase was calculated, which proved that the prepared samples were pure phase. A new practical approach was provided to semi-quantitative analysis of phase purity of the sample.Phosphor LiKGd2(MoO4)4:Eu3+ has two strong peaks in the UV (396 nm) and blue (466nm) region, which match to the near-UV LED chip and blue LED chip well, and emit red light at 615 nm. The results show the optimum doped concentration of Eu3+ in LiKGd2(MoO4)4 is 1.1. Comparing with Ca0.80MoO4:Eu3+0.20 and commercial red phosphors Y2O2S:Eu3+, the luminous intensity of LiKGdo.9(Mo04)4:Eu3+1.1 are 1.40 and 3.68 time higher, respectively. The color coordinates values of the phosphor LiKGdo.9(Mo04)4:Eu3+1.1 are closer to the NTSC standard values than those of the traditional commercial red phosphors Y2O2S:Eu3+. Based on Dexter's energy transfer formula of multipolar interaction and Reisfeld's approximation, the energy transfer among Eu3+ ions in the phosphor LiKGd2-xEux(MoO4)4 is governed by electric dipole-dipole interaction. The critical distance of energy transfer among Eu3+ ions was calculated to be Rc=8.24 A according to the distance formula of critical non-radiative energy transfer. All results show that the triple molybdate phosphors LiKGd0.9(MoO4)4:Eu3+1.1 is highly efficient red phosphors for the'UV chip+tri basic color' system or the complement light phosphors in the 'blue chip+yellow phosphor' system.