Study On Preparation And Luminescence Mechanism Of Ca₃Al₂O₆ Red Phosphors Activated By Rare Earth

The alkaline earth aluminates luminescent materials activated by rare earth ions have good prospects in the field of new-generation illuminant, due to their high quantum efficiency in the visible region. M3Al2O6:Eu3+ (M=Ca, Sr, Ba) is a new type of alkaline earth aluminates red phosphors. The absorption (7F0,1→5L6) of Eu3+ is in the near ultraviolet region, the strongest emission(5D0→7F1,2) is in the visible red region,M3Al2O6:Eu3+may be used in the white LED as red phosphors. There is little literature report about Ca3Al2O6:Eu3+(C3A:Eu3+), because the single phase is difficult to get, and the research of luminescence mechanism is also absent. The valence change of rare earth is an important factor to change the functions of materials; some features of luminescent materials are achieved by the change of valence of rare earth.In this paper, C3A:Eu3+ phosphors were prepared via solution combustion synthesis at low temperature using a fuel mixture of urea andβ-alanine. Effect of temperature on crystal structure and luminescence properties of C3A:Eu3+was studied. The effect of Eu3+ doped concentration, flux H3BO3, sensitizers Bi3+ and charge compensation Li+, Na+, K+ on C3A:Eu3+ luminescence properties and their enhanced luminescence mechanism were also studied. Finally, through the high-temperature carbon-reduction method, C3A:Eu2+,Nd3+ was prepared. Reduction temperature of the right to get C3A:Eu2+,Nd3+ and its luminescence properties were studied. The main results are as follows:(1) In the process of preparation of C3A:Eu3+ with the solution combustion synthesis, the gas and water vapor release as the fast response, which make the product showing porous structure. In the range of 250～600℃, the crystallization properties of the products are getting better and the luminescence intensity of C3A:Eu3+ increase gradually. The impact of temperature on the emission intensity decrease, when the temperature is higher than 500℃. The luminescent color of C3A:Eu3+ is very close to the NTSC. C3A:Eu3+ can emit red fluorescence under UV excitation at 394nm, which is expected to apply as the red phosphors in the white LED. (2) The emission intensity increases gradually with the addition of Eu3+ up to the quenching concentration 6%, the quenching mechanism is considered to be the exchange interaction. With the increasing of Eu3+ doped concentration the lattice parameters decreases together with a blue shift of the Raman peak of the symmetric stretching vibration of [O-Al-O]. The intensity ratio of 5D0→7F2 to 5D0→7F1 increases following the enhanced Eu3+ doped concentration, which indicates that the Eu3+ ions occupy more inversion symmetry sites of Ca2+ in C3A:Eu3+.(3) The flux H3BO3 can not reduce the temperature of combustion reaction and not increase the concentration of Eu3+ in the lattice, but B2O3 and H3BO3 can change the contact state, reduce the atom and ion diffusion mass transfer resistance during combustion. The best doped concentration of H3BO3 is 10%, after which, the hardness result of the emission intensity decreases due to the formation of trace impurities. Codoped with Bi3+ can enhance the emission intensity of C3A:Eu3+. The 1S0→3P2 absorption of Bi3+ is close to charge transfer absorption of Eu3+, the energy absorbed by charge transfer can transfer to the Eu3+ with resonance process, which enhance the emission intensity of C3A:Eu3+. Resonance transfer is the Bi3+ sensitization C3A:Eu3+ luminescence energy transfer process of the main form, but also there is resorption. Li+, Na+, K+ codoped C3A:Eu3+ can balance the equivalent produced by charge imbalance. The ionic radius of K+ is difference to Ca2+ markedly, the crystal field near the center of symmetry of the largest and most significant increase luminous intensity, Li+ followed, Na+ is the weakest.(4) C3A will be created again when annealed at high temperature, but that need a suitable high temperature. C3A:Eu2+,Nd3+ phosphors were prepared at 1350℃through high temperature carbon reduction of C3A:Eu3+,Nd3+ precursor prepared with combustion method. The luminescencent color of C3A:Eu2+,Nd3+ is difference with C3A:Eu3+, but all of them are similar to the NTSC.