Optimization Of Aluminate Phosphors For Novel Lighting And Display Devices

With the appearance of novel kinds of displays and lighting devices, such as plasma display panel (PDP), light emitting diodes (LED), field emission display (FED), inorganic solid-state fluorescent materials are still very promising and therefore in the focus of current research activities. The phosphors should meet the requirement of the different devices. For example, thanks to its many advantages, including large screen, high definition, light weight and thin wall, plasma display panels (PDPs) are promising for large screen size displays. The phosphors for PDPs should have good color purity, high luminescent efficiency under vacuum ultraviolet (VUV) excitation, moderate decay time and high thermal stability. They still have high requirement on the particle size, size distribution and particle morphology. White light-emitting diodes (pc-WLEDs) are emerging as an indispensable solid-state light source for the forth generation lighting industry and display systems due to their unique properties including small volume, low heat radiation, energy savings, long persistence, short response time and environment-friendliness. The white light is often produced by the combination of phosphors and light emitting diodes. The phosphors should be efficiently excitated by near ultraviolet or blue lights and have high thermal quenching temperature.Aluminate-based phosphors have been widely used, but most of them could not be directly used in the new lighting and display devices. For example, the most widely used blue BaMgAl10O17:Eu²⁺(BAM) phosphors suffer from bad stability under thermal treatment or vacuum ultraviolet excitation. The PDP green phosphor candidate BaAl₁₀O₁₇:Mn²⁺(BHA) phosphors have low luminous efficiency and long decay time. To meet the requirements of PDPs and LEDs, we tried to optimize aluminate phosphors by substituting Si-N bonds for some Al-O bonds in aluminate phosphors. Systematical research has been done on the synthesis methods, crystal structure, photoluminescence properties, stabilities and optimization mechanisms of the aluminate phosphors before and after Si-N doping. After in-depth study on their luminescent properties, the aluminate phosphors are expected to find wide applications in novel lighting and display devices including PDP and LED.Chapter 1 starts with a brief description of the lighting history, followed by some related topics, such as solid state luminescent material, the optical properties of rare earth ions, the pricinples, the applications of white light LEDs and PDPs and some conventional phosphors. Then we especially reviewed the crystal structure, photoluminescence, advantages and disadvantages of aluminate phosphors. Finally, the main ideas of this doctoral dissertation were proposed. Chapter 2 is the experimental procedure, including the starting materials, synthesis equipments, the process synthesis, characterization methods, et al.In chapter 3, the Eu²⁺ doped BaMgAl10O17 (BAM) blue phosphors were prepared by high temperature solid state reaction method. The stability and optical properties of the obtained phosphors were studied. The results are shown as following: In the UV-visible spectrum, the excitation bands between 220 nm and 450 nm are orginted from the 4f-5d electronic transition of Eu²⁺ ions. In the vacuum ultraviolet spectrum, the energy was adsorbed by the BAM crystal lattice and transformed to Eu²⁺ ions, two excitation bands could be observed, which corresponding to the spinel crystal structure layers and mirror layers. The emission spectrum under either UV-visible or VUV lights consists of a broad emission band ranging from 400 nm to 600 nm with maximum at about 450 nm, which is characteristic of the 4f⁶5d¹?4f⁷ transition of Eu²⁺.In chapter 4, first of all, a small amount of Si-N has been introduced into the the BAM crystal lattice through high temperature solid state reaction. A small amount of doping did not affect the crystal structure, only slightly reduced cell parameters. Excitation-emission bands of BAM phosphors did not change after the incorporation of Si-N bonds, except a small redshift of emission bands under the excitation of either VUV excitation or UV-visible excitation. It should be emphasized that Si-N incorporated BAM (SiN-BAM) phosphors have significantly increased photoluminescence intensities compared with that of pure BAM phosphors before and after heat treatment in air at 600oC. According the XANES spectra et al, the improvements were due to the lower defect density for as-received SiN-BAM phosphors and due to the stability of Eu²⁺ ions for thermal treated SiN-BAM phosphors. However, we found that the Si-N concentration was very small by the high temperature solid state reaction. If the formula of SiN-BAM is BaMgAl_10-xSi_xO_17-xN_x:Eu²⁺, then the optimized photoluminescence properties and thermal stability were achieved when x equaled to 0.03. This suggests that the substitution occurs only in the surface of the BAM phosphors due to the thermodynamic and kinetic effects, so we tried high-energy ball milling to achieve high Si-N incorporation. Mechanical milling mostly transformed the starting powder mixture into an amorphous phase and homogeneity of elements at atomic level was achieved in amorphous phase. Thus, the optimized photoluminescence properties and thermal stability with a better long-term stability were achieved when x equaled to 0.25, Finally, a theoretical calculation has been done to search the preferential site for Si-N doping and Mg²⁺ ions. Mg²⁺ ions prefer to occupy the tetrahedral Al position of the spinel layer, and locate in different spinel layer of one cell. Si tends to occupy the tetrahedral Al position and link to the O in the conductive layer, N tends to link to the Si and lie at the edge of the spinel layer. The coordination distance between the metal ions and N in the most stable SiN-BAM crystal decreased, indicating that the N could effectively inhibit the movement of metallic ions in the mirror layer. This could explain that why the Eu²⁺ ions were stabilized with the small amount of Si-N incorporation.In chapter 5, the BaAl₁₀O₁₇:Mn²⁺ green phosphors were prepared by a high temperature solid state reaction method. Under UV-Visible excitation, each excitation spectrum consists of several narrow bands ranged from 270 to 515 nm. These bands are due to the spin-forbidden transitions in the 3d5 electron configuration of the Mn²⁺ ions. According to the Orgel diagram for divalent manganese, these bands peaking at about 505, 453, 427, 386, 361, and 280 nm are attributed to the transitions of ⁶A₁?⁴T₁ (⁴G), ⁶A₁?⁴T₂ (⁴G), ⁶A₁?⁴E, ⁴A₁(⁴G), ⁶A₁?⁴T₂ (⁴D), ⁶A₁?⁴E(⁴D), and ⁶A₁?⁴A₂(⁴F), respectively. Each emission spectrum consists of a broad green emission band ranging from 480 nm to 620 nm with maximum at about 515⁵18 nm, which is characteristic of the transition from the lowest excited state to the ground state of the Mn²⁺ ions, i.e., the ⁴T₁(⁴G)?⁶A₁(⁶S) transition. The emission of Mn²⁺ ions is related to the coordinating environment of Mn²⁺ ions. They show green emission in tetrahedral sites, but red emission in octahedral sites, incating Mn²⁺ in BHA locate in tetrahedral sites. Under VUV excitation, BaAl₁₀O₁₇ crystal lattice could efficiently adsorb energy and transfer to to Mn²⁺, and emit a bright green light.In Chapter 6, Si-N doped BHA (SiN-BHA) was obtained by high energy ball milling and solid state reaction, and the concentration of Si-N substitution could effectively increase by high-energy ball milling. The luminescence properties of Mn²⁺ were studied in terms of the Si-N incorporation. The emission spectra are dominated by a green emission band at about 517 nm, which is assigned to the transition from the lowest excited state to the ground state (⁴T₁(⁴G)?⁶A₁(⁶S)) of the Mn²⁺ ions. Each excitation spectrum consists of several narrow bands in the UV and visible regions, the two strong peaks of 453 and 427nm were attributed to the electronic transition from the ground state to the excited state ( ⁶A₁?⁴T₂ (⁴G)?⁶A₁?⁴E, ⁴A₁(⁴G) ) of the Mn²⁺ ions. The emission intensity of SiN-BHA phosphor is higher than that of the BHA phosphor and the emission wavelength had a little red shift. Mn²⁺ in SiN-BHA is tetrahedrally coordinated with mixed O^2- and N^3- ions. Because nitrogen has a lower electronegativity (?(N)?3.04) than oxygen (?(O)?3.44), according to the electron diffusion effect, the covalence of Mn²⁺ in BaAl_12-xSi_xO_19-xN_x:Mn²⁺ was greater than that in BHA, and the crystal field energy was stronger than BHA, so the luminous performance of Si-N doped BHA was effectively improved.In chapter 7, a short conclusion has been made.