摘要
Nd3+离子因具有吸收系数大、吸收带较宽、荧光寿命长、荧光分支比大、能量集中及易于实现室温下的激光等特点,使得掺Nd3+离子的激光材料成为当前的研究热点。其中,掺Nd3+离子的透明氟氧化物微晶玻璃因兼具氟化物玻璃的优异光学性能以及氧化物玻璃优良的化学稳定性和热稳定性而引起材料科学工作者的广泛关注。与其它氟化物相比,CaF2晶体对稀土离子有较高的固溶度,且具有透光范围宽(0.125-10μm)、化学稳定性好、及声子能量低等特点,因此,掺Nd3+离子且含CaF2纳米微晶的氟氧化物微晶玻璃是一种比较有发展潜力的激光材料。
本文采用传统熔体冷却技术及晶化热处理工艺制备了掺Nd3+离子的SiO2-Al2O3-CaF2-NaF-Na2O体系透明氟氧化物微晶玻璃。利用差热分析(DSC)、X射线衍射分析(XRD)、扫描电子显微镜(SEM)等检测手段研究了该系统玻璃的形成能力、析晶机理、微观结构。同时,利用荧光光谱、吸收光谱等测试方法和结果,分析了Nd3+离子掺入浓度对材料光谱和激光性能的影响。结果表明：
1. SiO2-Al2O3-CaF2-NaF-CaO系统玻璃的稳定性较好,在第一析晶峰温度附近析晶不明显,CaF2析出较困难,出现表面析晶现象,当温度较高时可能出现钙长石晶相。用Na2O代替CaO后,玻璃的析晶能力提高,第一析晶峰温度附近析出CaF2晶体,Nd3+离子进入微晶生成Ca0.9Nd0.1F2.1相。
2.在SiO2-Al2O3-CaF2-NaF-Na2O系统玻璃中,氟化物含量较高,因此,由[Si04]和[Al04]四面体组成的网络结构中存在较多网络断开点或非桥氧离子。玻璃的析晶活化能随着Nd3+掺入量的增加而逐渐降低,析晶能力增强。在析晶过程中,晶核呈三维方式生长,其长大主要受扩散机制所控制。随着温度的升高,其形核速率不断降低,最终趋向于零形核率。
3.含不同Nd3+浓度的样品,经过同一条件热处理后,随着Nd3+离子浓度的增加,样品中除CaF2外还形成了Ca0.9Nd0.1F2.1固溶体。Nd3+取代Ca2+离子进入晶体中,衍射峰向小角度方向移动。Nd3+离子浓度越大,进入CaF2微晶中的Nd3+离子就越多,晶格畸变越严重,衍射峰偏移越多。Ca0.9Nd0.1F2.1相的生成以及点阵常数的增大证实了Nd3+离子进入CaF2微晶体。
4.在热处理过程中,Nd3+离子进入CaF2晶体中,其局域环境由声子能较高的玻璃基体转变为声子能低的CaF2晶体,使得Nd3+离子发射强度和吸收强度在晶化之后都得到提高。同时,由于CaF2萤石结构中存在多种不同电荷补偿方式,Nd3+在晶格中产生丰富的不对称格位,使其吸收和发射光谱加宽。
5.掺Nd3+离子的SiO2-Al2O3-CaF2-NaF-Na2O系透明微晶玻璃中Nd3+离子的5个主要吸收峰位于511、582、746、803、875nm处；3个主要荧光峰位于888,1055和1327 nm处,分别对应于4F3／2→4I9／2,4F3／2→4I11/2和4F3／2→4I13／2跃迁。
6.随着Nd3+离子浓度的增加,微晶玻璃中CaF2晶体形成立方八面体团簇结构的正方反棱体格位。Nd3+离子浓度增大时,团簇结构中包含的Nd3+离子的个数也会增加,引起浓度猝灭效应,导致受激发射截面不断减小。
The laser materials doped with Nd3+ ions have become the hotspot because Nd3+ ion has large absorption coefficient, wide absorption band, long fluorescence lifetime, very large fluorescence branching ratio, energy compaction and the possibility of lasing at different wavelengths at room temperature. Transparent oxyfluoride glass ceramics doped with Nd3+ ions have attracted widespread interest due to their excellent optical properties like fluoride nanocrystals and good mechanical, chemical properties like oxide glasses. Compared with other fluoride, CaF2 has high solid solubility to rare earth, wide transmission range (0.125-10um),good chemical stability, and lower phonon energy, so the transparent oxyfluoride glass-ceramics containing CaF2 doped with Nd3+ become one kind of very potential laser materials.
In this thesis, the transparent oxyfluoride glass-ceramics of SiO2-Al2O3-CaF2-NaF-Na2O system doped with Nd3+ ions were prepared by conventional melt-quenching method and optimized heat treating regime. The glass-forming ability, crystallization mechanism, microstructure of SiO2-Al2O3-CaF2-NaF-Na2O system glasses were analyzed by differential thermal analyse (DSC), X-ray diffraction (XRD) analysis and scanning electronic microscopy (SEM). The absorption spectra and fluorescent spectra of the glass-ceramics samples were measured. The influences of Nd3+ content on the spectrum properties of glass-ceramics were discussed. The results show that the glasses of SiO2-Al2O3-CaF2-NaF-CaO have good glass-forming ability. Surface crystallization and the crystals of anorthite appeared at the higher temperatures. When CaO is replaced by Na2O, the crystallization ability of the glass is improved. The crystals of CaF2 is obviously detected at the temperature of the crystallization first peak, and the Nd3+ ions incorporate into CaF2 nano-crystals and Ca0.9Nd0.1F2.1 solid solution formed. Obviously, a large amount of network disconnected points or non-bridge oxygen ions exist in the network structure formed by connecting [SiO4] and [SiO4] or [SiO4] and [AlO4] since the molar percentage of NaF and CaF2 are higher than 25% in SiO2- Al2O3- CaF2- NaF-Na2O system glass. Therefore, the crystallization activation energy declines gradually with Nd2O3 content, resulting in the increasing of crystallization ability in this glass system. During the crystallization, the crystalline grains grow along three-dimension directions when the basic glasses are treated at the crystallization temperature, and the crystallization is controlled by diffusion. The increasing of the crystallization leads to the reduce of nucleation rate, ultimately to zero. With the increasing of Nd3+ content, solid solution Ca0.9Nd0.1F2.1 appear besides CaF2 crystals in different Nd3+ containing samples after the same heat treatment, and Ca2+ ions are replaced by Nd3+ ions which make the diffraction peak move to smaller angle. The more Nd3+ ions incorporate into CaF2, the more serious of the crystal lattice distortion, so diffraction peak shift more. Accordingly, with the amount of Nd3+ ions increase, the number of crystals increases after heat treatment. This further approves that Nd3+ ions are good for the crystallization of CaF2. The crystallization of Ca0.9Nd0.1F2.1 and the increase of crystal lattice distortion approve Nd3+ ions incorporate into CaF2 nano-crystals. Because the phonon energy of CaF2 crystal is less than the one glass matrix, the absorption optical density and emission intensity of Nd3+ ions are enhanced. The various charge compensation mechanisms and asymmetric crystal lattices of CaF2 result in a broaden absorption spectra and fluorescent spectra. In SiO2- Al2O3- CaF2-NaF-Na2O glass system, there are five main absorption peaks of Nd3+ at 511、582、746、803 and 875nm, and three main fluorescent peak of Nd3+ at 888,1055 and 1327 nm corresponding to the transition of 4F3/2→4I9/2, 4F3/2→4I11/2和4F3/2→4I13/2, respectively with the concentration of Nd3+ ions increase, CaF2 crystals form square anti-prism sites of cubo-octahedral cluster, causing the concentration quenching effect, resulting in stimulated emission cross-section of the glass ceramics decreases.
引文
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