The Synthesis And Properties Of AlN Based Phosphors

In the end of20th centrury, LED industry leader of Japan developed a white luminescence device based on high bright GaN LED coated a yellow phosphor YAG: Ce3+. Compared with Incandescent bulb and fluorescent lamp, LED has many advantages, such as high safety (low voltage used), energy saving (reduced by80%compared with incandescent lamp), strong applicability (each LED size:0.2mm-10cm), long life (in100000hours, luminescence degration remains50%), short response time (LED:-ns, incandescent lamp:ms) and Environmental protection (Without harmful mercury). So LED is called the third generation luminescence technology, which is being applied in wide area. Besides, our country has introduced many preferential policies to speed up the development of LED industry. In genery lighting field, how to get white light similar to sunlight attracts people’s attention. Nowdays, the ways to generate white light are:(1) full LED chips structure, which is the combination of red, green, blue three LED chip, forming a luminous matrix;(2) single LED chip. Blue LED is coated by yellow phosphor or red, green phosphor, which can be excited by blue light. Yellow and blue light or red, green, blue light can generate white light. Another way is the use of UV-LED coated by red, green and blue phosphor, which can be excited bu UV ray. Full chips cost too much and they have different luminous efficiency, leading to an improper colour temperature in lighting device. Full chips also make heat accumulation, which leads to a luminescence degration and a shift of CIE color coordinates and colour temperature. Therefore, the potential realization method is a single LED chip combined with phosphors.Traditional phosphors are majorly Sulfide, aluminate, Portland and so on. In recent years, some novel oxynitride based phosphors have been developed, such as red phosphors M2Si5N8:Eu2+(M=Ca, Sr, Ba) and MAISiN3:Eu2+(M=Ca,Sr), green phosphor MSi2O2N2:Eu2+(M=Ca, Sr, Ba)and AlON:Mn2+, Mg2+, yellow phosphor Ca-a-SiAlON:Eu2+and blue phosphor AlN:Eu2+.For oxynitrides, metal ions are located in the O/N network, which make phosphors excellent chemical and thermal stability.5d energy level of rare earth(Eu2+, Ce3+) is bare in the crystal field and will split to a different degree. The charge of N3-is more than O2-and N3-shows stronger covalence. An N-rich crystal environment will cause larger Nephelauxetic effect, which leads to a larger5d split.4f-5d transitions will shift to longer wavelength. The redshift of excitation and emission spectra can be observed. Matched with blue LED, phosphors will exibit yellow or red emission. Meanwhile, the emission wavlength can be adjusted by changing O/N ratio in the matrix. So oxynitride phosphors are ideal Fluorescence conversion materials used in white LED. This paper majorly talks about structure and luminescence of AlN based phosphors (AlN and AlON). The relationship of preparation conditions, crystal structure, and luminous property is researched. The theoretical calculation is adopted to explain the luminescence mechanism.This paper includes8chapters. The introduction as the first chapter starts with a description of the lighting history, part phosphors and mainly focus on the structure and optical property of AlN based phosphors. Chapter2is the experimental procedure, including raw materials, synthesis equipments, and characterization methods.In chapter3, single phase AlN:Eu2+phosphor is obtained by carbothermal reduction method. The effect of reaction temperature, holding time, Eu2+concentration and C content on the structure and optical property of AlN:Eu2+phosphor is systematically researched. The results indicate low synthesis temperature1750℃and holding time8h is the optimal condition for the syethesis of pure and high luminous phosphors, which proves Eu2+can be dissolved into AlN without other ions co-doping. AlN:Eu2+powders with low oxygen content are achived by carbothermal reduction. It seems easy to get pure AlN:Eu2+phosphor because Aluminate is impossible to generate in this process. Under UV excitation, AlN:Eu2+shows blue emission at470nm, which is ascribed to5d-4f transition of Eu2+. The result indicates1750℃for8h is the optimal condition for the synthesis of AlN:Eu2+with high purity and luminescence intensity. The emission can be adjusted from blue to green band by changing C content in the rew materials. The effect of different fluxes on the structure and optical property of AlN:Eu2+is researched, indicating BaF2can effectively promotes the crystallinity of AlN and increases the luminecnce intensity.In chapter4, high pure AlN:Eu2+phosphor with extreamly low oxygen content and no residual carbon is obtained by a novel gas reduction route. Under UV excitation, a green emission at540nm is observed. Compared with the emission in AlN:Eu2+phosphor prepared by carbothermal reduction, it shows a redshift here, which is due to larger5d split caused by nephelauxetic effect of N3-. EDS, HRTEM, ED and EXAFS results proves Eu substitution for Al sites in AlN crystal llatice. Because of the large different radius between Eu2+and Al3+, the solid solubility must be very low and Eu3+unavoidabley coexist in the products. The theoretical calculation result agrees well with the experimental, further proving that Eu2+ions occupy A13+sites.(P)DOS spectra show Eu5d, Eu6p, Al4s and Al4p all contribute to the luminescence of AlN:Eu2+phosphor. Besides, The structure and optical property and Mn location in AlN:Mn2+phosphor are also simply discussed.Chapter5reports the structure and optical property of AlON:Eu2+and AlON:Eu2+, Mg2+phosphors synthesized by solid-state reaction. Under UV excitation, AlON: Eu2+, Mg2+phosphor show very strong blue-green emission at490nm. In single Eu doped AlON. It is found Eu can’t dissolve into AlON crystal lattice. But with the favor of Mg co-doping, the result is inverse, which is possiblely dute to expanded crstal lattice afer Mg doping.In chapter6, AlON:Eu2+, Mg2+phosphor is synthesized by carbothermal reduction method, which shows a character of low synthesis temperature compared with solid-state reaction. Besides, the phosphor synthesized by carbothermal reduction show much higher luminescence intensity than that by solid-state reaction due to the smaller particle size, better purity, laiger Eu2+ratio and more Eu2+content dissolved into AlON crystal lattice. Under the some365nm excitation, AlON:Eu2+, Mg2+phosphor shows better optical property, compared with famous commercial BAM phosphor.In chapter7, single phase AlON:Eu2+, Mg2+phosphor is firstly obtained by high-energy ball mill route. In the process of mechachemical acticvation, decrystallizatoin and homogeneous mixing at an atom scale is gradualy achived in the starting materials. The reaction activation of the powders is greatly improved, resulting pure AlON:Eu2+, Mg2+phosphor under a low synthesis temperature. The solid solubility of Eu2+in Mg-AION increases from0.2%in products prepared by carbothermal reduction to0.5%here, also leading a better luminescence intensity.Chapter8gives an outlook related polyroid in Al2O3-AIN system. Taken Al7O3N5as an example, the result proves Al7O3N5:Eu2+phosphor can be synthesized under high temperature, which is a blue phosphor with a emission460nm under UVexcitation. Furthermore, it is summarized through the full text.