Preparation Of Alkaline Earth Fluoride Powders Of Different Particle Size Of Nano-and Thermal Stability

The wet chemical methods of microemulsion, aqueous precipitation, double jet and water/ethanol mixed solvents were applied to synthesize nanocrystalline of alkaline fluorides including BaF₂, CaF₂ and SrF₂. The object of this dissertation is to obtain nanocrystalline of alkaline fluorides with controllable particle size, study the thermo-stability, and provide fundamental data for the research of properties and application of nanocrystalline alkaline fluorides powders and bulk materials.Firstly, Cetyltrimethylammonium bromide(CTAB)/butanol/heptane/water microemulsion system was applied to synthesize BaF₂ nanoparticles. The influences of the mass ratio of butanol to CTAB, the content of heptane and water, and the species of solute in water phase on the stability of microemulsion system and resultant BaF₂ particle size and size distribution were investigated. It was found that, the mass ratio 1 : 1 of butanol to CTAB favor to form stable microemulsion, and about 10nm BaF₂ particles with well monodispersed and ball-shaped were synthesized by reaction of stable Ba(NO₃)₂ microemulsion and NH₄F microemulsion, the BaF₂ particle size and distribution decreases with the mass ratio of CTAB to water increasing.Although it is easy to control the particle size of fluorides while synthesized with the method of microemulsion, the productivity of the method is quite low and cost is relatively high. A high productive and low cost method — aqueous precipitation was applied to syntheses nanocrystalline BaF₂. Based on the dependence of precipitate particle size and size distribution on the supersaturation, different reactant concentration and mixed manner were utilized to change supersaturation, and the precipitation in the vicinity of the critical supersaturation was also investigated. The results indicated that particles were easy to grow into submicro-crystalline, as the supersaturation was quite low while the reactants were added in droplet mode. In rapidly mixing mode, the particle size of resultants was much smaller as the supersaturation was relatively high. The nano-BaF₂ powders with the size of about 10nm were produced when the reactions happened in the vicinity of the critical supersaturation.A device for double jet reaction was designed and the influence of ultrahigh supersaturation on the particle size and size distribution was study so as to increase the supersaturation and uniformity of precipitation. The influences of surfactants and the concentration of NH4F solution were also studied. The results indicated that the method of double jet could provide very high supersaturation and large numbers of ultrafine particlesformed at the beginning of precipitation. But the ultrafine particles were quite unstable and tend to grow into large particles, seem that they had not been affect by early supersaturation. It can be deduced to be an intrinsic stable state of aqueous precipitation. The concentration variety of NH4F solution and the application surfactants had little influence on the particle size and distribution. It can be deduced that further increase of supersaturation is not a effective method to obtain smaller particles and narrower distribution.A low dielectric constant system of water/ethanol mixed solvents was applied to synthesize nano-BaF2 powders so as to investigate the dependence of the particle size of the precipitates on the solubility. The average particle size 70nm, 61nm, 52nm, 43nm and 33nm of nano-BaF2 powders were obtained separately in pure water solvent and mixed solvents with the volume ratio of water to ethanol 3 I 1,1 I 1,3 I 5 and 1 : 4. The results indicated that the particle size of nano-BaF2 powders decreases with increasing the volume ratio of ethanol to water of the mixed solvents, and the dispersion and the morphology are better together. The relationship between the reciprocal of BaF2 particle size and the reciprocal of dielectric constant of mixed solvents is linear. This work showed the particle size can be successfully controlled by the mixed solvents.Ethanol/Water mixed solvents were also used to prepare nano-CaF2 powders with different particle size. CaF2 powders well dispersed , narrow size distribution, with the particle size 242nm, 78.7nm, 51nm, 31nm, 22.7nm and 14nm were obtained by combining the aqueous solvents with low reactant supersaturation and mixed solvents. The results showed that relatively bigger particles can be synthesized in the condition of low reactant supersaturation, small particles can be produced in mixed solvents. Precipitates with average size from micrometer to several nanometer region can be controllably synthesized by combining the low supersaturation manner and mixed solvent manner.Nano-SrF2 powders were synthesized by precipitation in the ethanol/water mixed solvents. Well dispersed Nano-SrF2 powders with the average size 32nm, 18.6nm, 23.2nm, 16nm, and 14.9nm were obtained in pure water solvent and mixed solvents with the volume ratio of water to ethanol 3 : 1, 1 : 1, 3 .' 5 and 1 ! 4, respectively.Finally, the thermal stability of Nano-BaF2 powders with different particle size was investigated by Differential scanning calorimetry (DSC). By Kissinger equation, the activation energy of BaF2 nanopowder with particle size 70nm, 52nm, 43nm and 33nm is 187.8KJ/mol or 1.947eV, 137.1KJ/mol or 1.422eV, 132.2KJ/mol or 1.371eV, 111.167KJ/mol or 1.153eV, respectively. An interesting result was showed that the activation energydecreased linearly to the particle size of nanocrystalline BaF2 powders decreasing, which can be explained in terms of the larger grains grow at the expense of the smaller ones in the grain growth.