| Zirconium(Zr)is an important strategic material,it has high reactivity,good corrosion resistance,low thermal neutron absorption cross section,good compatibility with nuclear fuel and moderate mechanical propertiesm.So metal Zr is widely used in manufacturing nuclear compoents,the laser damge resistance film,high k gate dietectric materials,amorphous alloys and other cutting-edge products.But metal Zr,due to its own nature,has a very strong affinity and large solubility of oxygen,so it is easily contaminated by oxygen during smelting and processing,thus can not meet the requirements of the above fields.However,the currently used high purity metal preparation technologies represented by electron beam metlting and vacuum arc melting technology can hardly deoxidize Zr ultimately,so high oxygen content has been a problem in the preparation and processing of high purity zirconium.In view of this facts mentioned above,through a large number of literature research,the "external gettering process" based on Ca-CaC12 deoxidation system was proposed to conduct deoxidation of Zr ultimately.Firstly,the thermodyamic properties of Zr-O solid solution was studied.Thermodynamic information such as activity coefficient of oxygen in Zr-O solid solution and formation Gibbs free energy of Zr-O solid solution were determined,thus laying the thermodynamic foundation of deoxidation and making it possible to predict the oxygen content of Zr.On this basis,by introducing an oxygen tropic rare metal(RE)into the system,a thermodynamic model of Ca-RE combined deoxidation was established,thus achieving ultimate deoxidation of Zr.Finally,a set of optimized deoxidation technology conditions were founded by studying the external gettering technology systematically,which provided useful experience for industrial application.The main findings are as follows:(1)The euqilibrium oxygen content of Zr in Ca,CaO sartuated CaC12 molten salt was detemined by chemical equilibrium technique in the temperature range of 1173-1373K.Nb-O solid solution was employed as "oxygen potential sensor" of molten salt to measure aCaO of molten salt with different oxygen potential indirectly.By linear fitting of aCaO and ln[0]Zr,the fO in Zr obeyed Henry's law within the oxygen content range in this study has been proved.Then the ?GZr-O? and p-T-x relationship were deduced through thermodynamic and mathematic methods as follows,(?)The distribution of oxygen between Zr-O and Nb-O solid solution was discussed.The results showed that the distribution coefficient of oxygen had a positive correlation with temperature and did not change with the external oxygen potential.The above conclusions laid a thermodynamic foundation for deoxidation of metal Zr and made the whole deoxidation process theoretically predictable and controllable.(2)The thermodynamic properties of electrochemical deoxidation method were analyzed by using E-pO2-diagram,which pointed out that the generation of Cl2 and polution of carbon were inevitable.According to the literature investigation and thermodynamic analysis,it was suggested that introducing the rare earth metal Y,whose oxygen affinity is stronger than Ca,to form Ca-Y combined deoxidation was a feasible method to realize the ultimate deoxidation of Zr without secondary pollution,and the Ca-Y combined deoxidation model was constructed.The oxygen-chlorine potential diagram was used to analyzed the change of system equilibrium state.It was found that Y has an"anchoring effect" on aCaO in molten salt,which can stabilize the oxygen potential of the system at a very low level.The oxygen potential decreased significantly with increasing the chlorine potential of the system and eventually reached the ultra-low oxygen potential point of Y-YOCl-YCl3-CaCl2 four-phase coexistence,thus achieving the ultimate deoxidation of Zr.By analyzing the activity-oxygen content relationship of Ca-Y combined deoxidation system,it was found that the thermodynamic condition for Ca-Y combined deoxidation is aY2O3<18.9aCaO3 at 1173K.Through experiments under 1173K,the Ca-Y combined deoxidation model was verified.The experimental results showed that after adding metallic Y,the increasing trend of oxygen content of Zr with the increase of oxygen potential of the system was significantly suppressed.Even if the added amount of CaO was its solubility in molten salt,the equilibrium oxygen content of Zr was still stabilized at less than 100 ppm.Adding YC13 to increase the chlorine potential of the system,the oxygen potential control mode of the system will be changed from indirect control mode of Y-Ca to the direct control mode of Y.Due to the difference in kinetics conditions between the two oxygen potential control modes,the reaction time needs to extended to at least 72h to ensure the equilibrium.When four phases coexistence of Y-YOCl-YC13-CaC12 was formed,the oxygen content of Zr was further reduced to merely 34 ppm.By constructing a thermodynamic model of Ca-Y combined deoxidation,the ultimate deoxidation of Zr was achieved whitout secondary pollution.(3)The main factors of external gettering process such as temperature,molten salt composition were studied by single factor experiment.In order to verify the adaptability of this method to different raw materials,metal Zr and Ti specimens were introduced simultaneously.The obtained optimum process conditions were as follows:the deoxidation temperature was 900?,the deoxidation time was 24h,m(CaCl2)/m(Zr+Ti)=8:1,the amount of Ca added was 200%of its solubility in CaCl2 molten salt,the molten salt composition was CaCl2-5wt%KCl,the added amount of metal Y was 200%of the theoretical amount required for completely react with oxygen in specimens to form Y2O3.All the reagents were directly mixed.Under the optimized conditions,the oxygen concentration of Ti and Zr were reduced from 670 ppm and 800 ppm to 31 ppm and 42 ppm respectively.The above results showed that the process also has good adaptability to different kinds of raw materials.The optimized process parameters provided a reference and necessary basic data for the industrial application of the method. |
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