【作者】 张凯；

【导师】 胡文彬； 刘河洲；

【作者基本信息】 上海交通大学 ， 材料学， 2008， 博士

【摘要】 白光LED具有能耗少、无污染等优点,被喻为第四代照明光源,具有广阔的应用前景。铈掺杂钇铝石榴石（ Ce-doped Yttrium Aluminum Garnet, YAG:Ce ）是制备白光LED的首选荧光粉。湿化学方法与固相法相比,降低了煅烧温度、缩短了煅烧时间。共沉淀法与其他湿化学方法相比,成本低廉、设备简单、易于大规模生产。因此,共沉淀法被广泛应用于制备荧光粉。但前人的研究主要集中在对荧光粉发光性能的分析,未系统探究沉淀工艺对荧光粉理化性能,特别是发光性能的影响。同时,为了满足不同照明环境要求,需要生产不同色坐标和色温的光源,这需要调节YAG:Ce荧光粉的发射波长,而稀土掺杂是一种有效的改变YAG:Ce发射波长的方法。表面包覆是一种常用的荧光粉后续处理工艺,包覆使荧光粉和外界隔离,可以大大改善荧光粉表面结构、有效降低表面缺陷,改进荧光粉的发光性能。本论文工作以共沉淀法制备YAG:Ce荧光粉为目标,研究了沉淀工艺条件对荧光粉理化性能的影响,探讨了稀土掺杂对荧光粉结构和发光性能的影响,并对制备的荧光粉体进行了包覆处理。主要研究内容及结论如下:（1）研究了沉淀剂对前驱体制备的影响,结果表明:碳酸氢铵作沉淀剂制备的前驱体分散性好,煅烧过程中直接转变为YAG相,没有杂相生成。讨论了沉淀剂浓度、沉淀和陈化温度以及陈化时间对化学计量比的影响,所得最佳工艺参数为:碳酸氢铵与总阳离子摩尔比>4,沉淀和陈化在室温下进行,沉淀结束后陈化8小时以上。分析了团聚产生的原因以及控制团聚的工艺措施。结果表明,在乙醇和水的混合溶液中进行沉淀,并在沉淀时加入适量的高分子表面活性剂,采用共沸蒸馏对粉体进行干燥,能改善粉体的分散性。（2）煅烧过程中保持铈为三价对提高粉体发光性能非常重要。因此,对还原方式进行了分析,研究认为,先在空气中合成YAG结构,然后在还原气氛中煅烧,能使铈充分还原。粉体荧光衰减过程由于受到表面效应影响,含有两个指数项,长指数项代表了体相Ce³⁺的衰减,短指数项反映了表面Ce³⁺的衰减。研究了不同助熔剂对荧光粉性能的影响。结果表明,BaF₂是最合适的助熔剂,最佳添加量为助熔剂与前驱体质量比1/10。对煅烧中的脱水、脱碳、晶化和晶粒生长过程进行了动力学分析,结果表明:脱水过程反应机理遵循Jander方程,对应的活化能为119.3 kJ/mol,Arrhenius频率因子自然对数lnA=24.6 s^-1;脱碳过程对应的反应机理按Avrami-Erofeev方程（n=3）,对应的活化能为222.59 kJ/mol,lnA=24.7 s^-1。晶化过程的表观晶化活化能为1115.18 kJ/mol。受扩散控制的晶粒生长活化能为226.06 kJ/mol。（3）研究了环境温度、镧系元素共掺杂对YAG:Ce粉体性能的影响。结果表明, YAG:Ce粉体的发光强度随着环境温度升高而减弱。当温度高于150K,开始出现明显的发光热猝灭。YAG:Ce衰减长时间项随着温度升高减小;衰减短时间项,随着温度的升高呈下降趋势,由于受到表面效应的影响中间出现小幅阶跃。Ce 1mol%取代Y时,发光强度达到最大,进一步增加铈的掺杂量导致浓度猝灭的发生。当Gd³⁺取代Y³⁺超过50 mol%时,导致GdAlO₃相的出现。当La³⁺取代Y³⁺超过20 mol%时,YAG结构完全消失,只存在LaAlO₃相。随着Gd、La掺杂量的增加,发射波长向长波长移动,发射光谱中红色成分增加,蓝色成分减少。（4）研究了溶胶凝胶包覆Al₂O₃和固相法包覆SiO₂。研究表明,溶胶凝胶法包覆Al₂O₃最佳工艺为:温度90℃以上;选择HNO₃作胶溶剂,最佳浓度为0.025mol/L。两种包覆都没有改变荧光粉的发射波长。Al₂O₃包覆使荧光粉发射强度略有下降,而SiO₂包覆使荧光粉发射强度一定程度上得到提高。本文的主要创新之处在于:以阳离子共沉淀为目标,提出了合理的沉淀工艺参数。探讨了环境温度对荧光粉发光性能的影响。研究表明,随着环境温度的升高,表面Ce³⁺的衰减时间呈下降趋势,由于受到表面效应的影响中间出现小幅的阶跃。Gd、La掺杂使发射波长向长波长移动,发射光谱中红色成分增加,蓝色成分减少。用室温固相法进行了SiO₂包覆,SiO₂包覆荧光粉的发射强度一定程度上得到提高。上述研究为YAG:Ce荧光粉体的合成作了有意义的尝试和探讨,并为进一步的研究提供了理论依据。更多还原

【Abstract】 White light emitting diodes （WLEDs） have the advantage of less energy consumption, no pollution, etc. WLEDs are called the forth generation lighting and have wide application prospect. Trivalent cerium activated yttrium aluminum garnet （YAG:Ce） is the primary phosphor for WLEDs. Recently, wet chemical method attracts more attention for preparation of phosphors, which has the advantage of reducing calcination temperature and shortening calcination time. Compared with other wet chemical methods, co-precipitation method has advantage of low cost, simple equipment and large scale production. Therefore, co-precipitation is a widely used method for preparation of phosphors. Previous research mostly focuses on luminescent properties of phosphor. The effect of Co-precipitation process on the physical, chemical and luminescent properties of phosphor is hardly systematically explored. For different lighting conditions, the phosphors owning different color coordinate and color temperature must be prepared. So, emission wavelength of YAG:Ce should be changed. Rare earth doping is an effective way to change emission wavelength. Coating process is an effective subsequent process. Film isolates phosphor from outside, eliminates the surface defects and improves the physical, chemical and luminescent properties of phosphor.This paper aims to develop YAG:Ce phosphor by co-precipitation and systematically researches effect of process conditions on the physical and chemical properties. The doping effect of rare earths on the physical, chemical and luminescent properties is explored. Meanwhile, coating treatment is applied to improve phosphor surface. Specific contents and main conclusions are shown as follows:（1） The influence of precipitant on the properties of YAG:Ce phosphor was investigated. The precursor has good dispersity, when selecting ammonia hydrocarbonate （AHC） as precipitant. The precursor directly transfers to YAG phase without appearance of other phases. Effect of concentration of precipitant, precipitation and aging temperature, aging time on the stoichiometry is discussed. The optimal process parameters are suggested. A molar concentration ratio between AHC and total cation is more than 4. Precipitation and aging temperature is room temperature （RT）. Aging time is beyond 8h. The generation and control of agglomeration is analyzed. Ethanol–aqueous mixed solvents, adding polymeric surfactant and adopting heterogeneous azeortrope distillation （HAD） for drying precursor are constructive methods to reduce agglomeration.（2） Keeping Ce trivalence is a key circs during calcination. First, calcine the precursor in air. Then reduce the phosphor in reducing atmosphere. This procedure can reduce quadrivalent Ce sufficiently. The size of precursor comes into micron-size, when calcined above 1400℃. The luminescent decay contains two components, because of size effect. The long term reflects body Ce³⁺ luminescence, while the short term indicates surface Ce³⁺ luminescence. The most appropriate flux is BaF₂. The optimal added amount of BaF2 is 1/10 （mass ratios between fluxes and precursor）. Systematic kinetic study is conducted on de-hydration, de-carbon, crystallization and crystal growth. The de-hydration fits the Jader function. The kinetics activity energy is 119.3 kJ/mol. Natural logarithm of Arrhenius frequency factor is lnA=24.6 s^-1. The de-carbon accords with Avrami-Erofeev function （n=3）. The kinetics activity energy is 222.59 kJ/mol. Natural logarithm of Arrhenius frequency factor is lnA=24.7 s^-1. The apparent activity energy of crystallization is 1115.18 kJ/mol. Crystal growth coincides with the ninth order kinetic equation D 9 = kt = k 0 t exp（ ? E / RT）. The activity energy for crystal growth is 226.06 kJ/mol.（3） The effect of environmental temperature and lanthanide elements doping on the properties of YAG:Ce is investigated. The luminescence intensity decreases with the increase of environmental temperature, which is caused by the increase of the non-radiative relaxation rate. When environmental temperature increases above 150K, thermal quench effect is observed. The long decay component decreases with the increase of environmental temperature. While the short decay component shows intricate behavior, which results from effect of surface. The maximum emission intensity is obtained for 1mol% Ce additions. Concentration quench effect is observed when Ce amount increases beyond this limit. While Gd³⁺ doping concentration beyond 50 mol%, GdAlO₃ phase appears. When more than 20 mol% Y³⁺ is replaced by La³⁺, YAG phase completely disappears and LaAlO₃ phase emerges. As the doping concentration of Gd³⁺and La³⁺ increases, the maximum emission band red shifts, while the emission intensity decreases. As increment of Gd and La doping, the red content in the emission spectra increases, while blue content decreases.（4） Coating phosphor with Al by the sol-gel route and with Si by the solid-state reaction at room temperature was studied. The optimal parameters for sol-gel route are temperature above 90℃, HNO₃ as peptizer, concentration of HNO₃ 0.025mol/L. Al and Si coating treatment not change emission wavelength. Al coating slightly decreases the luminescent intensity. While, Si coating little increases the intensity.In this paper, the optimal parameters of co-precipitation are developed aimed at al cations coprecipitation. The effect of environmental temperature on the luminescent properties of YAG:Ce is investigated. The decay time of surface Ce³⁺ shows complex behavior, because of surface effect. The wavelength shows red shift, when YAG:Ce doped with Gd and La. The red content increases in the emission spectra of Gd and La doped YAG:Ce. Si coating using the solid-state reaction at room temperature makes the luminescent intensity slightly increases. These results provide instructive attempts of synthesizing YAG:Ce phosphor, and enrich the theoretical knowledge for further research.更多还原