Study On A Novel Alkaline Metal Phosphosilicate Phosphor For Near - UV Excited White

White light-emitting diode (wLED) is a novel light source. Compared to incandescent, fluorescent, and halogen lighting solutions, wLED presents competitive advantages in terms of its small size, long life, energy saving and environment-friendly. It is regarded as the fourth generation of light source. Currently, there are two different approaches which can be used for generating white light based on LEDs:(1) by mixing reds, greens, and blues, i.e. red-green-blue (RGB) LEDs,(2) LED pumps and a phosphor blend as color conversion (pc-wLED). The latter approach, i.e pc-wLED, is more advantageous in terms of operating easiness, cost, reliable, maintenance and security. Two alternative pc-wLEDs are being developed. The first approach consists of a blue emitting LED and a phosphor blend capable of emitting in the green red region of the visible spectrum. Today’s commercial pc-WLEDs normally use a450-470nm blue GaN LED chip covered by a yellowish phosphor coating, which is usually made of YAG:Ce. However, pc-WLEDs made by means of blue-LED+YAG:Ce yellow phosphors suffer some weaknesses, such as poor color rendering index and low stability of color temperature. Since the white light is generated by the combination of blue light emitted by an LED chip and yellow light emitted by YAG:Ce phosphors, deterioration of the chip or YAG:Ce phosphors would cause some significant color changes. The second pc-wLED system uses a near-UV (nUV) LED chip coated with white light-emitting single-phased phosphors or RGB tri-color phosphors. Since the visible color depends entirely on the phosphor blend to generate white light, white UV-LEDs are optically much more stable. It is considered that white UV-LEDs might be the direction of SSL development and hence attracted increasing attentions. Development of novel phosphors for nUV-wLED is of great significance.In this thesis, a novel type of alkaline earth phosphosilicate phosphors M5(PO4)2(SiO4)(M=Ba, Sr, Ca)singly doped with Eu2+and codoped with Eu2+and Mn2+used for nUV w-LED were studied systematically.The phosphor samples were prepared through high temperature solid-state reactions. By using Photoluminescence excitation and emission spectroscopy(PLE and PL), Thermo-analysis (TG-DTG-DSC), X-ray diffraction (XRD), ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS), and scanning electron microscopy (SEM) methods, effects of the raw materials employed, reaction conditions on the structure, morphology, physical properties as well as on the PL and PLE of phosphors were studied. The scheme of solid phase reactions was discussed. The effects of alkaline earth ions on the structure and luminescent properties of M5-x(PO4)2(SiO4):Eux(M=Ba,Sr,Ca) were also studied. Luminescent properties of (Ba,Ca)5-x-y(PO4)2(SiO4):EuxMny phosphor, single phase white light emission phosphor, were studied. The efficiency and mechanism of energy transfer from Eu2+to Mn2+were also discussed. Main results are as follows:The Ba5(PO4)2(SiO4):Eu phosphors were successfully synthesized by high temperature solid-state reactions. The effect of reaction conditions (such as types and amount of flux, calcination temperature), the concentration of Eu2+, partial replacement of Ba2+with other divalent ions (Sr2+, Ca2+, Zn2+) on the luminescence properties and crystal structure were studied. The results showed that using BaCO3, SiO2, Eu2O3,(NH4)2HPO4(with3%excess) as the raw materials,3%NH4HF2as flux, pure phase phosphors with BaioF2(P04)6structure could be obtained after calcination at1400℃for5h. The phosphors could be excited by UV to blue light (300-450nm) with a band emission peaking at510nm. The emission intensity increased with increasing of the doping concentration of Eu2+and showed a maximum at Eu2+concentration of0.04. It decreased with increasing Eu2+concentration above0.04due to the concentration quenching. When partially substituting Ba2+by Sr2+, the phosphor lattice shrinked. When the Sr2+concentration was greater than3, the host lattice structure transformed from BaioF2(P04)6to Sr5(PO4)2(SiO4). Meanwhile, the emission peak wavelength was gradually red shifted with the substituting Sr2+for Ba2+. When the concentration of Sr2+was4, it shifted to550nm accompanied by significantly reduced emission intensity. As the radius of Ca2+or Zn2+were greatly less than that of Ba2+, when substituting concentration of Ca2+or Zn2+was larger than0.5, the emission intensity weakened sharply. The XRD showed that the large lattice distortion and a large number of impurity phase formation occurred. Hence,the optimum mole fractions that fur substitution Ba2+with Ca2+or Zn2+should be between0.25-0.5.The effects of Eu2+concentration, strontium and phosphorus sources on the structure, morphology and luminescence of Sr-5(PO4)2(SiO4):Eu phosphors were studied. The phosphor is excitable by near UV light with the emission peak wavelength at ca.550nm. The emission peak red shifted with increasing the concentration of Eu2+in the cases studied. The emission intensity showed a maximum at Eu2+concentration of0.08. The investigation of the influence of raw materials showd that SrO generated in-situ by decomposition of nitrate or carbonate during solid-state reactions showed higher reactivity than that of reagent SrO directly added, the phase purity of the apatite phosphors prepared from the strontium salts was better than that using SrO. The phosphors prepared from the strontium salts also exhibited smaller particle size and less aggregation and hence higher PLE and PL intensity than that using SrO. Due to both the volatility of (NH4)2HPO4and low reactivity of SiO2, it was hard to prepare pure phase phosphosilicate phosphor with a stoichiometric (NH4)2HPO4as the phosphorus source. Minor Sr2Si04impurity phase was detected in the phosphors. The phase purity of the phosphor was improved with the addition of excess (NH4)2HPO4. When (NH4)2HPO4with5%excess was used as the phosphorus source, the impurity phase was not detected. On the other hand, when stoichiometric SrHPO4was used as the phosphorus source, the pure phase phosphosilicate phosphor was obtained. The phosphor showed higher PL and PLE intensity than that prepared from (NH4)2HPO4phosphorus source.The effect of Eu2+concentration on the luminescence properties and crystal structure of Ca5(PO4)2(SiO4):Eu phosphor was studied. The phosphors could be excited by ultraviolet light, and the emission peak wavelength was about500nm. With increasing the doping concentration of Eu2+, the emission wavelength was red shifted.The optimal concentration of Eu2+was0.06.The (Ba,Ca)5-x-y(PO4)2(SiO4):EuxMny phosphors were synthesized by high temperature solid-state reaction. The effects of Ca2+concentration on the luminescence properties and crystal structure of Ba4.9-xCax (P04)2(SiO4):Eu0.04, Mn0.06were studied. Results showed that when the concentration of Ca2+was0.5, phosphor exhibited the best performance which demonstrated both green and red emissions with tunable green and red emission intensity under nUV excitation. Through the study of excitation and emission spectra and decay of Eu2+in the presence and the absence of Mn2+, mechanism of Eu2+-Mn2+energy transfer was discussed. The investigation confirmed that Eu2+â†'Mn2+energy transfer was through a dipole-quadrupole interaction mechanism.