Synthesis And Luminescent Performance Research On Ca₁₂Al₁₄O₃₂F₂ As The Phosphors Host For White LEDS

White Light emitting diodes（WLEDs） areregarded as a new typelighting device, because of their small volume, excellent longevity, energy conservation and environmental protection. The color rendering index of white light which produced by combining a LED chip with phosphers has a greater improvement than the other methods, while the main factor is phosphors. Oxyfluorides have good chemical stability, low probability of non-radiative relaxation, broad band gap, and they are able to withstand bombardment of high energy electron. So the rare-earth doped oxyfluorides have an effective improvement for the luminous efficiency of phosphors. Aa a kind of oxyfluorides, Ca₁₂Al₁₄O₃₂F₂ has many advantages as follows:（1） broad range of pervious to light from 250 nm to 800 nm;（2） lower index of refraction, good chemical stability and mechanical strength;（3） melting point is 1465 ℃,good thermostability and low coefficient of thermal expansion;（4） it has a sensitized effect on the luminescence of rare earth ions, at the same time, it can absorb energy and pass the energy to the rare earth ions, in which the energy loss is very few;（5）Under ultraviolet excitation,it has a stable luminous efficiency.In this work, we synthesized a series of phosphors used Ca₁₂Al₁₄O₃₂F₂ as hosts suitable for WLEDs by sol- gel method, high temperature solid-state method and combustion method. Techniques such as X-ray powder diffraction（XRD）, X-ray photoelectron spectroscopy（XPS）, transmission electron microscope（TEM）, photoluminescence spectra（PL） and fluorescence lifetime were used to characterize the phosphors, and the crystal structure, element, morphological structure and luminescence properties of these phosphors were studied. Furthermore, the effect of factors, such as reaction temperature, soaking time, activators Eu³⁺ and Tb³⁺ and co-activator Eu²⁺,on the luminescent properties of these phosphors were also studied.The main content is as follows:（1） A series of phosphorsCa₁₂Al₁₄O₃₂F₂:x Eu³⁺were prepared by the sol-gel method with CF3 COOH as fluorine source at the first time. Under 233 nm excitation, the emission spectra for Eu³⁺ ions in Ca₁₂Al₁₄O₃₂F₂ phosphors give emission peaks at 588 and 612 nm, these peaks have been identified as 5D0 â†' 7F0 and 5D0 â†' 7F2 transition. The as-obtained Ca₁₂Al₁₄O₃₂F₂: x Eu³⁺ consists of nanoparticals with sizes of about 40-100 nm, which shows that the as-synthesized phosphors are nanoparticles.（2） A series of luminescent emission-tunable phosphors Ca₁₂Al₁₄O₃₂F₂:Tb³⁺, Eu²⁺, Eu³⁺ have been synthesized by a high temperature solid-state reaction ina reductive atmosphere（N2:H2 = 9:1）.When Tb³⁺ ions are codoped into Ca11.52Al14O32F2:0.48 Eu phosphors, the emission intensities of Eu²⁺ and Eu³⁺ increase with increase in Tb content under 254 nm excitation, and the decay times of Eu³⁺and Eu²⁺ emission increase. These results can be ascribed to ene rgy transfer from Tb³⁺ to Eu²⁺ and Tb³⁺ to Eu³⁺. The energy transfer critical distance for Eu and Tb in Ca₁₂Al₁₄O₃₂F₂ was calculated to be 7.88 ?. The emission color of the prepared phosphors can be tuned appropriately from pale blue to white by adjudting the proportion of doped ions.（3） A series of luminescent emission-tunable phosphors Ca₁₂Al₁₄O₃₂F₂:x Eu have been synthesized by combustion method. Due to the coexistence of Eu³⁺and Eu²⁺, excitation wavelength of Ca₁₂Al₁₄O₃₂F₂:x Eu phosphors contains all of the near ultraviolet region. When excitation wavelength increases from 230 nm to 330 nm, the red emission from 580 nm to 620 nm rises first and then falls to almost disappeared, at the same time, the blue emission from 400 nm to 500 nm increases gradually.When excitation wavelength increases from 330 nm to 392 nm, the blue emission decreases gradually. The emission color of the prepared phosphors can be tuned appropriately from orange red to blue by adjudting the proportion of Eu²⁺ and Eu³⁺.