Preparation And Thermal Stability Of Nano The Laf <sub> 3 </ Sub>, The Conductive Properties

The object of this dissertation is to study the conductivity property of nano-crystalline LaF₃. The study work including preparation and characterization of nano-crystalline LaF₃ powder, preparations of nano-crystalline LaF₃ bulk material, thermo-stability of nano-crystalline LaF₃, conductivity and dielectric constant of nano-crystalline LaF₃.In this dissertation, nano-crystalline LaF₃ powder was prepared with the method of direct precipitation from aqueous solution at room temperature and atmosphere. The resulted powder was characterized with Transmission Electronic Microscope (TEM) and X-ray Diffraction (XRD). The particles of nano-crystalline LaF₃ were spherical with diameter of 15nm²5nm and narrow size distribution. The powder of nano-crystalline LaF₃ is of tysonite structure with high purity. No impurity phase was found in XRD patterns. Other fluorides including NaF, BaF₂, and CeF₃ were also prepared with the same method and characterized TEM and XRD. Solidification of nano- crystalline LaF₃ bulk material was carried out at room temperature under high pressure (1Gpa) and high vacuum (10^-4Pa). The relative density of the prepared nano-LaF₃ bulk material was 91 %.Based on theoretic analysis and the experimental data of NaF, BaF₂, CeF₃, it was pointed out in this dissertation that solubility of resulted precipitate is the key factor affect particle size while the method of direct precipitation from aqueous solution is applied. Other factors including reactant concentration, super-saturation, ageing time, and solvent system were also studied and analyzed.The thermo-stability and grain growth of nano-crystalline LaF₃ were studied with Differential scanning calorimetry (DSC) and Thermo-gravimetry (TG) within room temperature to 1000℃. The activation energy for grain growth of nano- LaF₃ bulk material was calculated to be 2.01 eV based on DSC test data. Nano- crystalline LaF₃ bulk material was also annealed at different temperature in vacuum (10^-4Pa) and atmosphere to studied grain growth behaviors. While annealed in vacuum, grain sizeof nano-crystalline LaF₃ grew quickly over 600 °C. While annealed in the atmosphere, nano-crystalline LaF₃ was oxidized over 500 °C.The ionic conductivity of nano-crystalline LaF₃ bulk material was studied with complex impedance spectra at room temperature. The ionic conductivity of nano-LaF₃ bulk material (lO^Scm1) at room temperature was significantly increased compared to that of single crystal LaF₃. A special phenomenon was observed for the first time that the ionic conductivity increased gradually with AC scanning times. Possible mechanism for the increase of ionic conductivity was discussed based on the microstructure of grain boundary.The temperature dependence of ionic conductivity in nano-crystalline LaF₃ materials was studied with AC (alternating current) complex impedance plots measured at rising temperature. The ionic conductivity of nano-crystalline LaF₃ materials is 1.2 - 0.4 magnitude higher than that of LaF₃ single crystal in the temperature range from room temperature to 400 °C. The conduction activation energy of nano-crystalline LaF₃ materials is 0.34eV. The reason for this phenomenon was illuminated based on the grain boundary structure and sub-lattice of nano-crystalline LaF₃ related to ionic conductivity. Dielectric constant of nano-crystalline LaF₃ decreased with rising frequency and exhibited maximum value with rising temperature. The reason for this phenomenon was illuminated based on the reaction of different polarizations of nano-crystalline LaF₃ to frequency and temperature.