Research On The Removal Of Nonmetallic Impurities O, N, H In Rare Earth Metals Gd, Tb, And Dy With Solid State External Gettering Method

This dissertation is focused on the optimum technique parameters and mechanism to purify rare earth metals by external gettering method. The purification effect of this method were studied by varying annealing time, annealing temperature, system vacuum, sample sizes, sorts and quality of getters. The main research contents and results are summarized as follows:The reaction Gibbs free energies when some general getters(Ca, Y, Ti, Zr) react with rare earth oxides(Gd2O3. Tb2O3. Dy2O3) are calculated respectively. The results indicate that only Ca and Y can be used to deoxidize Gd, Tb, Dy. Two experimental schemes are established according to the thermodynamic calculation results: isothermal degassing scheme and anisothermal degassing scheme.Isothermal degassing treatment is conducted to Gd, Tb, Dy respectively with Ca and Y as getters. After isothermally annealing with Ca at700℃for24h, the lowest oxygen content of800ppm in Gd is obtained. The disadvantages of Ca as getter lie in its unstable deoxidation effect, and that it can not be used to remove nitrogen in Gd. When purifying Gd, Tb, Dy with Y, the oxygen, nitrogen, and hydrogen contents in them can be reduced to below20ppm,45ppm and10ppm, respectively. To remove nonmetallic impurities in rare earth metals adequately, annealing temperature shall be above900℃, and annealing time have to be determined based on the thickness of samples. To remove oxygen and nitrogen in Tb and Dy to as low level as that in Gd, the initial impurity contents in Y required for Tb and Dy are much lower than that for Gd. Improving vacuum in the sealed system has almost no effect on degassing effect.Anisothermal degassing treatment is conducted to Gd, Tb, Dy respectively with Ca and Y as degassers. The temperatures in the rare earth metal(Gd, Tb, Dy) ends are900℃and1000℃, with a stable temperature gradient of150-200℃to Ca ends. After annealing, only oxygen in Gd is removed, while oxygen and nitrogen in1h increases instead. When annealing with Y, temperatures in rare earth metal ends (?) from900-1200℃, with a stable temperature gradient of150-200℃to Y ends. After annealing for longer than12h, the oxygen content in Gd can be decreased by400ppm at most. The deoxidation requirement for Tb is more strict than that for Gd. Nitrogen in two rare earth metals can not be removed under current experimental condition in Anisothermal degassing scheme.After annealing with getters, nonmetallic impurity contents in the rare earth metals and getters redistribute. Based on the distribution coefficients changing with temperature, the reaction process of degassing can be evaluated, and the required impurity contents in getters to purify rare earth metals can be determined before operation.The degassing models of external gettering to purify rare earth metals are built, and the degassing process is illustrated. The reason that the deoxidation effect of Ca is unstable is experimentally explained. Calcium atoms diffuse into the surface of Gd, and a layer of calcia forms in the surface of Gd samples, which restrains the external diffusion of oxygen atoms. Co-diffusion phenomenon is not found between the interface of rare earth metals and Y.