【作者】 王鹏；

【导师】 刘冰冰；

【作者基本信息】 吉林大学 ， 凝聚态物理， 2009， 硕士

【摘要】 CeF₃作为一种典型稀土氟化物材料,因其优异的光学性质及在光电子纳米器件上的应用而备受关注。目前对于CeF₃纳米材料的合成,物性研究还只是处于起步阶段,尤其是制备具有更加规则形貌的CeF₃纳米片,仍是一项具有挑战性的课题。碳管包覆复合纳米线中,纳米线因其外部碳管的量子限域效应而表现出不同于其体材料甚至普通形态纳米材料的特殊性质,成为了近期的研究热点。高压是深入认识材料结构和性质的有效途径,为了探求高密度CeF₃结构,人们已经开展关于CeF₃高压结构相变理论模拟与实验,但是CeF₃纳米材料的高压相变研究仍属空白。本论文主要围绕于CeF₃纳米片及碳管包覆CeF₃纳米线的实验制备研究,并对CeF₃纳米片进行了在位高压相变研究。利用水热合成与超声震荡辅助处理的实验方法首次得到了具有规则六角形貌的六角（P63/mcm）结构CeF₃单晶纳米片。系统研究了反应物之间相对摩尔比例对产物的影响,得到最佳实验条件。分析了各反应物在制备过程中所起到的作用。利用XRD、TEM、SEM对反应产物进行了表征。利用金刚石对顶砧高压技术及同步辐射分析技术,首次研究了高压下CeF₃纳米片结构相变过程。发现CeF₃纳米片的相变压力明显低于体材料,且其相变过程具有明显的阶段性。相变历经cubic亚稳相,这可能是CeF₃纳米片相变压力的降低的的主要原因。CeF₃纳米片相变过程所呈现的阶段性与CeF₃纳米片的准二维结构相关,是由高压下二维纳米片状材料中各向异性的压力效应所导致的。对于电弧法制备碳纳米管包覆CeF₃纳米线（CeF₃NWs@CNT）实验条件进行了探索。系统的研究了气氛压力对实验结果的影响,发现当气氛压力达到8.2x104Pa时产物的中CeF₃NWs@CNT含量达到最大。对于提高实验产物的纯度提出了相应建议。初步讨论了实验的基本原理。更多还原

【Abstract】 Lanthanide fluoride crystals have become the focus of recent investigations for their outstanding luminescent characteristics and potential applications in optical devices. As typical lanthanide fluorides crystal, Cerium fluoride （CeF₃） has been considered as one of the most promising scintillators of the next generation because of its high density, fast response and high-radiation resistance. Stimulated by both the promising applications and the interesting properties, great efforts have been devoted to the synthesis of CeF₃ nanostructures. For CeF₃ nanoplate previously, all the reports show that the obtained nanoplates were only round shaped with hexagonal （P63/mcm） structure synthesized via various methods. So it is still a challenge to prepare high crystallinity CeF₃ nanoplates with other regular shapes.High pressure technique provides us a very powerful tool to study the relations between the structure and physical properties and many new structures have been found under high pressure. High density is one of the most important parameters of conventional scintillator. With expectation to find higher density phase, several investigations have been focused on the high pressure phase transitions of bulk lanthanide fluorides crystals. However, there is no direct structural study on CeF₃ nanoplates, especially determinative X-ray diffraction data under high pressure. So it is important to study the phase transition and compressibility of CeF₃ nanoplates and the size effect on the phase transition process under higher pressure.The single crystalline CeF₃ nanoplates with regular hexagonal shape have been successfully synthesized by a ultrasound vibration assisted hydrothermal process for the first time. In the experiment , we chose to treat the mixing solution of CeCl3 and trisodium citrate with ultrasound irradiation（40 KHz）. The mixing solution of CeCl3 , trisodium citrate and NaF was transferred into a Teflon bottle held in a stainless steel autoclave, sealed, and maintained at 180°C for 24 h. After centrifugation, washing with deionized water, and then dried in air we got final product. We set the molar ratio of the CeCl3, trisodium citrate and NaF at 1:2:6.25（0.4mmol, 0.4mmol, 2.5mmol）. The phase structure of the as-prepared sample were investigated by XRD. All of the diffraction peaks can be identified to a pure hexagonal phase [space group : P63/mcm （193）] . From the ED patterns, the CeF₃ nanoplates is shown to be single crystal with high crystalline quality. SEM and TEM observations show that product is congeries of hexagonal CeF₃ nanoplates with an average diameter of 110 nm and thickness of 17 nm which is obviously different from the round nanoplates reported in study of Li et al. These results show that the ultrasound vibration assisted hydrothermal method is possible tool to synthesize nanomaterials with regular shapes and controllable size.The ADXD measurements were carried out at 4W2 High-Pressure Station of Beijing Synchrotron Radiation Facility （BSRF）. High-pressure was generated by diamond anvil cell （DAC）. CeF₃ powders were mounted in a 140-μm-diameter hole of the T301 stainless-steel gasket with thickness of 70μm. 4:1 methanol-ethanol mixture was chosen as the pressure-transmitting medium. The pressure was calibrated by the pressure dependent shift of the R1 ruby fluorescence line. The geometry correction for the radial integration of the two-dimensional data and the transformation into standard one-dimensional powder patterns were performed using Fit2d. The ADXD patterns of CeF₃ nanoplates showed that when the pressure increasing to 6.8 GPa two new peaks appeared which can be identified as （040）, （222） peaks of orthorhombic （Cmma） structure. At 19.3 GPa another prominent change is that （021） ,（201）, （220） and （002） peaks of orthorhombic structure appeared and the disappearance of （112） peak of hexagonal （P63/mcm） structure was observed. As the pressure reached 24GPa, all of diffraction peaks of the initial phase had disappeared which means that the CeF₃ nanoplates of hexagonal （P63/mcm） structure would completely transformed to the orthorhombic （Cmma） structure. During the transformation of lanthanide fluorides from hexagonal structure to orthorhombic structure, it have been guess that the structure of the high pressure phase is adistorted variant of the structure of a hypothetical cubic phase which is metastable under normal conditions and （040）, （400）, （222） peaks of orthorhombic structure is distorted from （220） peak of cubic phase. So we proposed a preferential phase transformation from （220） plane in the cubic structure to （040）, （400）, （222） plane of orthorhombic structure with compressing the （001） plane of initial hexagonal structure up to 6.8 GPa and the phase transition detailes accorded with conditions of phase transition pressure reduction with size decreasing in nanomaterials mentioned by Wang very well. In summary, the two parts of hexagonal-orthorhombic phase transition process should be attribute to size effect and anisotropy pressure-induced behaviors of 2D CeF₃ naoplates with high aspect ratio.The synthesis of CeF₃NWs@CNT was conducted in an arc discharge reactor in helium. A high-purity graphite tube filled with a mixture of polyetherimide （PEI） and CeF₃ powders in a weight ratio of 3:7 was used as the consuming anode while the cathode was a high-purity graphite rod. The arc discharge was conducted with a direct current of 70-90 A and voltage of 20-30 V. We found that when the He pressure was lower than 2.8x104Pa most of the product was carbon nanotubes and with He pressure increasing nanoparticles appeared in the product and there was no CeF₃NWs@CNT could be observed. As the He pressure up to 4.2x104Pa, CeF₃NWs@CNT came into observation and the content increase with the He pressure increasing. At 8.2x104Pa the content of CeF₃NWs@CNT obviously increased but the CNTs and amorphous carbon still exist in the product. In the experiment process, we found that the CNTs and amorphous carbon were mainly produce by graphite anode. So if we can find a polymer with good conductivity and high melting temperature as the anode, the outgrowth will decrease and the CeF₃NWs@CNT could be separated easily. The exploration of experimental conditions provides important detailed experiment data for synthesis of CeF₃NWs@CNT which is a great support for the primary exploration of the growth mechanism.更多还原