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Research on the removal of impurity elements during ultra-high purification process of terbium
Vacuum 125 (2016) 21e25
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
Research on the removal of impurity elements during ultra-high purification process of terbium Guoling Li a, c, Li Li a, Ruiying Miao b, Wenhuai Tian c, Shihong Yan b, **, Xingguo Li a, * a Beijing National Laboratory of Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China b General Research Institute for Nonferrous Metals, Grirem Advanced Materials Co. Ltd., Beijing 100088, PR China c Department of Materials Physics and Chemistry, University of Science and Technology Beijing, NO. 30, Xueyuan Road, Beijing 100083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2015 Received in revised form 26 November 2015 Accepted 4 December 2015 Available online 11 December 2015
Ultra-high purification of terbium (Tb) by vacuum distillation and external getter was investigated. More than 20 kinds of impurity elements were analyzed by glow discharge mass spectrometry (GDMS) and inert gas analysis (IGA). The analysis results show that total impurities decrease from 1114 ppm to 63 ppm. High volatile metallic impurities such as Mg, Ca, Mn, Zn, and Tm can be reduced effectively after distillation. The oxygen (O) concentration can be decreased to 1.8 ppm after deoxidation process, which currently is one of the best results in the contemporary studies. The chemical driving force for deoxidation has been calculated in this paper. The different deoxidation results of Tb metal with various purity have been discussed in detail using the secondary ion mass spectroscopy (SIMS). Finally, a ultra-high purity Tb with low concentration of O can be obtained through vacuum distillation and external getter method. © 2015 Published by Elsevier Ltd.
Keywords: Rare earth alloys and compounds Purification of terbium Vacuum distillation External deoxidation method Thermodynamic modeling
1. Introduction Terbium (Tb) is a heavy rare earth element whose compounds have been applied extensively in biomedical materials, magnetic materials, fluorescent films and luminescent materials [1e5]. The performance of such compounds is strongly depended on the purity of raw materials [6]. It has been reported that trace impurities at ppm or sub-ppm level, especially the interstitial impurity oxygen (O), can remarkably affect their physical and chemical properties [7]. However, it is an extremely difficult and complex problem to get rare earth metals of ultra-high purification. Pioneers have done a lot of work on this. Z. Zhang et al. [8,9] prepared praseodymium and yttrium metal by distillation under a high vacuum condition. The purification of the metals was improved to 99.99%, while the oxygen contents were both nearly 100 ppm. K. Mimura et al. [10] purified the lanthanum and cerium by plasma arc zone melting to about 99.83% and 99.87%, respectively, which was not really high. The same problem was that the oxygen impurity still remained
* Corresponding author.. ** Corresponding author. E-mail addresses: [email protected] (S. Yan), [email protected] (X. Li). http://dx.doi.org/10.1016/j.vacuum.2015.12.003 0042-207X/© 2015 Published by Elsevier Ltd.
above 100 ppm. J.W. Bae [11] purified hafnium from 99.47% to 99.49%, and K. Mimura [12] purified titanium from 99.96% to 99.99%, yet both of them did not mention the results of the oxygen impurity. Tb has extremely high chemical reactivity, and hence considerably rigorous equipment are required to preserve high purity, which makes the preparation of ultra-high pure Tb still a great challenge [13,14]. The study of purification of Tb by using solid state electro transport processing was reported [15], while there was no detailed evaluation of overall purity of Tb using glow discharge mass spectrometry (GDMS) and interstitial gas analysis (IGA). Z.A.Li has prepared high-purity Tb by using distillation, however, few purification mechanism was evidently presented [16]. For the oxygen impurity, external gettering technology was ever experimentally applied to Tb metals by Kamihira et al. [14]. However, the final content of oxygen was still above 1000 ppm. Hirota successfully decreased the O content to 20 ppm using electrochemical method after a long experimental time [17]. Gas gettering method could reduce the O content below 10 ppm [18]. It can be concluded from the previous research that, the ultra-high purification of Tb requires the refined calculations and accurate operation steps. Rare-earth metals has strong affinity with O element, making O the most difficult impurity to remove [19]. It is very rare to reduce O
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content to under 10 ppm, extend processing time seems to be ineffective. The reasons obstructing the deoxidation process still have not been reported clearly so far. In this paper, we investigate, the removing of impurities in Tb during distillation followed by calcium purification. Various instrumental techniques have been developed in response to specific analysis requirements of ultra-trace analysis. The underlying mechanisms associated with the refining process have been demonstrated. In addition, SIMS analysis provides impurity elemental mapping which makes it an exceptionally powerful technique to elucidate the interactions between metallic and oxygen impurities.
2. Experiments 2.1. Distillation process The vacuum distillation system was consisted of an evaporator and a condenser as shown in Fig. 1. The evaporator was made up of cylindrical tungsten crucible in the dimensions of 115 mm 100 mm 220 mm. The condenser was made up of tantalum sheet comprising of water cooled pipes to collect the distillate. The chamber was evacuated for about 2e3 h at 200e300 C to remove the moisture. A dynamic vacuum level of approximately 1.5 104 Pa was maintained in the chamber. Then the chamber was heated, and the material started evaporating and condensing at the bottom of cooled tantalum sheet placed on top, leaving high vapor pressure impurities at the top of the condenser and low vapor pressure impurities at the bottom of the crucible. The raw material of Tb used in the paper was made from Tb4O7 with following steps. Terbium oxide was firstly fluorinated (TbF3), and then reduced in a vacuum induction furnace by calcium (>99.9% purity). To remove the redundant calcium and obtain high purity Tb (>99%), the reduced Tb should be melt again in the vacuum induction furnace. The GDMS analysis of the starting material is given in Table 1. The distillation experiment was conducted in two steps. In the first step, the metal was melt at a comparatively lower temperature of 1200e1400 C to remove higher vapor pressure impurities than Tb. A soaking temperature of 1580e1650 C was maintained for the second phase to collect the distilled Tb. The condensation temperature was about 800e900 C. The distillation process maintained 18e20 h. The initial weight of Tb loaded into the crucible is about 2 kg, and the final yield after the distillation is near 95%.
Table 1 GDMS analysis of Tb. Impurity element
Raw material (ppm)
Distillation material (ppm)
Mg Al Si Ca Ti V Cr Mn Fe Co Ni Cu Zn Y La Ce Nd Sm Eu Gd Dy Ho Tm Others Sum Purity (mass%)
126 138 100 100 21 13 54 14 98 10 42 107 10 28 36 6 32 10 10 77 37 30 10 <5 <1114 >2N
<0.05 2.1 1.9 <0.1 2 0.01 0.01 <0.01 23 0.05 8.3 3.5 <0.5 6 6.8 2.8 0.07 0.01 <0.01 1.5 0.08 0.09 0.01 <5 <63 >4N
2.2. Deoxidation process High purity Tb ingot was cut into F5 mm L50 mm columns, and each specimen weights approximately 8e9 g. Impurities on the surface of these columns during handling were removed mechanically. After that, the specimens were rinsed with distilled water, ethanol and acetone, respectively. Then, they were dried for later use. Commercial granular Ca (280 ppm oxygen) were used as the external getter. The samples full filled with granular Ca in a graphite crucible (F22 mm L70 mm), were placed into the vacuum sintering furnace (Fig. 1) which was maintained at 1.25 105 Pa. Sufficient quantities of granular Ca (~12.5 g) were added so that the molten calcium could cover the whole Tb specimens. The furnace temperature was raised to a final depositing temperature of 500e1100 C at a rate of 10 C/min and hold for 30e180 min. The furnace maintaining the vacuum was then cooled to ambient temperature at natural rate.
Fig. 1. Schematic diagram of the vacuum distillation system (a), the calcium coating sample (b) and the vacuum annealing process (c).
G. Li et al. / Vacuum 125 (2016) 21e25
23
Metal impurities were analyzed by glow discharge mass spectrometry (GDMS-VG9000). The O contents were analyzed by an inert gas fusion-infrared absorption analysis (IGA-TCH-600). The average concentration of several pieces was taken as the analytical value at a given experimental condition. The morphology of the samples were studied by scanning electron microscopy (SEMS4800). Secondary Ion Mass Spectrometry (SIMS) was used to map the impurity elements inside the basic metal [20].
4NeTb, this final oxygen concentration can drop to 7.1 ppm by the same refining process. Thus, it is proved that the external deoxidation method is useful for Tb. Fig. 3 further shows the relationship between oxygen concentrations and the annealing time at 600 C. Concentrations of oxygen decrease clearly with extension of time. As the holding time reached 180 min, the oxygen contents remain steady. The final oxygen contents in 2NeTb and 4NeTb are 15.3 and 1.8 ppm respectively. It is concluded that an annealing duration of 180 min is sufficient for deoxidization of Tb.
3. Results and discussions
3.3. Thermodynamic mechanisms
3.1. Metallic impurities
Tb samples were embedded in a crucible containing granular Ca and subsequently annealed at different temperatures. Fig. 2 shows the dependence of oxygen concentration under different annealing temperatures. All the samples were annealed for 60 min. The oxygen concentrations of starting terbium (2NeTb) and distillation terbium (4NeTb) were 489.7 and 334.7 ppm, respectively. Granular Ca lead to a distinct phenomenon of deoxidation after the annealing process, which is shown in Fig. 2. However, there is an increasing tendency of oxygen concentrations with the increase of annealing temperature. It can be found that excellent deoxidation is obtained at 600 C annealing. The oxygen concentration of 2NeTb can decrease to 14.8 ppm after annealing at 600 C for 60min. For
To study the deoxidation mechanisms systematically, we further conducted a series of relevant research experiments. Fig. 4 shows the cross-section of Tb annealed with granular Ca at 600 C for 180min. As we know 600 C is below the melting point of Ca, so granular Ca can stay in a micro-molten state at this temperature. It can be seen from the SEM image that there is a typical region of Ca layer covered on the solid surface smoothly, the thickness of which is about 200 mm. The specific Ca layer is considered to be the chemical reaction zone during the purification process. A concentration map by EDX in Fig. 4 shows non-interdiffusion between the deposited layer and the basic metal. That is to say, the reaction zone, as well as the coated film, can be easily removed by physical etching. The thermodynamic data of solid Tb to the high temperatures has been calculated in order to estimate the Gibbs free energies of different reactions for Tb. The equations are presented in (1)e(3) [13,17]. These equations indicate the oxidation reactions about Tb and Ca, respectively, and the respective DG with temperature. Fig. 5 shows the changes in DG of them as a function of temperature. Both pure Tb and Ca can be easily oxidized at elevated temperatures. Oxidation of Ca is evaluated to be more favorable due to the lower DG. Ca has a higher chemical activity than Tb, and performs as the reducing agent during the deoxidation process. The deposited Ca layer contributes to the interaction process with oxygen on the solid metal surface, and the oxygen concentration of the surface can be reduced rapidly by reaction (3). Furthermore, due to the concentration gradient, the interstitial oxygen atoms in the TbeO solid solution can move to the surface, greatly promoting the migration of impurity atoms. In this process, thermodynamic reactions are believed to be the rate determining
Fig. 2. Concentration change of oxygen in two sets of metal as a function of temperature.
Fig. 3. Oxygen concentrations of 2NeTb and 4NeTb with different annealing time at 600 C.
2.3. Characterization
Table 1 shows the concentration of more than 20 kinds of impurities in Tb before and after purification. The total contents are decreased from <1114 ppm to <63 ppm. We can observe a significant reduction of the concentrations of major impurities such as Mg, Ca, Cr, Mn, Zn, Dy, Ho. These elements with a high vapor pressure retain almost <1 ppm after the distillation process. Impurities having a vapor pressure close to Tb, like Fe, Ni, Cu, and Ti, seem to be difficult to remove through the whole process. For impurities La, Ce and Y with lower vapor pressure than Tb material, their content changes are also obvious. The contents of heavy rareearth elements impurities such as Eu, Sm, Gd and Dy, are all under 2 ppm. 3.2. Removal of oxygen
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G. Li et al. / Vacuum 125 (2016) 21e25
Fig. 4. SEM micrograph of Tb covered with Ca.
Fig. 6. Plane of denudation in 2NeTb by using SIMS and the accordance EDS result.
contents (Al: 138 ppm, Mg: 126 ppm, shown in Table 1). The EDS data can powerfully elucidate that the initial metallic impurities can pin the oxygen inside the metal and obstruct the removal process. In other words, only after the metallic impurities reduce to an ultra-low level, can the ultra-high purity “oxygen free” terbium metal be produced. 4. Conclusion
Fig. 5. Free energy changes with temperature for reaction of Tb and Ca with O.
step. On the other hand, Ca is mainly distributed on the surface of Tb as indicated in Fig. 4. The coated film can be removed mechanically in the glove box filled with argon, without bringing any recontamination to the basic metal. We detected the Ca concentrations in the high-purity terbium by inductively coupled plasma-atomic emission spectrometer, and the final content was below 1 ppm. 1/2O2 (g) ¼ O (Tb, 1mass %) DG (kJ/mol) ¼ 591 þ 0.0789 T (1) 2/3Tb (s) þ 1/2O2 (g) ¼ 1/3Tb2O3 (s) DG (kJ/ mol) ¼ 616 þ 0.0905 T
(2)
Ca þ 1/2O2 (g) ¼ CaO (s) DG (kJ/mol) ¼ 637 þ 0.1060 T
(3)
A two-step approach to produce high purity Tb was employed. More than 20 kinds of impurity elements of raw Tb (2N purity) had been reduced, by vacuum distillation process, to the extent of achieving 4NeTb. Oxygen as the major non-metallic impurity, which was highly difficult and energy-consuming to be removed, could be effectively reduced by the external deoxidation method. By the vacuum distillation process, followed by annealing at 600 C, we successfully obtained ultra-high purity Tb (>99.99%) with substantially reduced non-metallic impurities. The final oxygen concentration is 1.8 ppm, which is one of the best results in the contemporary studies reported so far. Ca, which performed as the effective deoxidizing metal, is the crucial factor. The SIMS powerful technique illustrates that the main obstructive factor is the pinning force caused by the residual metallic impurities. The method proposed here may shed light on the future research on removing impurities in other RE metals, such as Gd, Dy and etc., which share similar properties and hence difficulties with Tb in purification. Acknowledgments The authors acknowledge MOST of China (No. 2012CBA01207) and NSFC (No. U1201241, 11375020 and 51431001). References
3.4. Obstructive factors It can be seen from Figs. 2 and 3 that the external gettering method displays perfect deoxidation capability, while the final oxygen concentration of 2NeTb is higher than the 4NeTb sample. The initial metallic impurities can seriously affect the deoxidation process. Fig. 6 shows the plane of denudation in 2NeTb after deoxidation by using SIMS. Luckily, we clearly observed a parcel of AleMgeO alloy inside the Tb sample, in spite of the low impurity
[1] L.J. Sun, P.F. Ni, D.G. Guo, C.Q. Fang, J. Wang, F. Yang, X.F. Huang, Y.Z. Hao, H. Zhu, K.W. Xu, J. Mater. Sci. Technol. 28 (2012) 773e778. [2] M. Daszkiewicz, YuO. Pashynska, O.V. Marchuk, L.D. Gulay, D. Kaczorowski, J. Alloys Compd. 616 (2014) 243e249. [3] Q. Zhu, M. Xiong, J. Li, W. Liu, Z. Wang, X. Li, X. Sun, RSC Adv. 5 (2015) 36122e36128. _ [4] L. Zur, M. Sołtys, J. Pisarska, W.A. Pisarski, J. Alloys Compd. 578 (2013) 512e516. [5] F.A. Rafiqi, K. Majid, Synth. Met. 202 (2015) 147e156. [6] Minoru Isshiki, Kouji Mimura, Masahito Uchikoshi, Thin Solid Films 519 (2011) 8451e8455. [7] Y. Waseda, M. Isshiki (Eds.), Purification Process and Characterization of Ultra
G. Li et al. / Vacuum 125 (2016) 21e25 High Purity Metals, Springer, London, 2002. [8] Z.Q. Zhang, Z.Q. Wang, D.H. Chen, R.Y. Miao, Q. Zhu, X.W. Zhang, L. Zhou, Z.A. Li, Vacuum 102 (2014) 67e71. [9] Z.Q. Zhang, Z.Q. Wang, R.Y. Miao, Q. Zhu, D.H. Chen, X.W. Zhang, L. Zhou, Z.A. Li, S.H. Yan, Vacuum 107 (2014) 77e82. [10] K. Mimura, T. Sato, M. Isshiki, J. Mater. Sci. 43 (2008) 2721e2730. [11] J.W. Bae, J.W. Lim, K. Mimura, M. Isshiki, Mater. Lett. 60 (2006) 2604e2605. [12] K. Mimura, J.W. Lim, J.M. Oh, G.S. Choi, S.W. Cho, M. Uchikoshi, M. Isshiki, Mater. Lett. 64 (2010) 411e414. [13] T.H. Okabe, K. Hirota, E. Kasai, F. Saito, Y. Waseda, K.T. Jacob, J. Alloys Compd. 279 (1998) 184e191. [14] K. Kamihira, R. Hasegawa, O. Ogawa, Mater. Trans. JIM 34 (1993) 243e247.
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[15] R.G. Jordan, D.W. Jones, J. Less Commmon Met. 42 (1975) 101e110. [16] Z.A. LI, W. Zhang, J. Xu, B. Zhao, F. Xiao, Y. Zhang, China Rare Earths 23 (2002) 36e39. [17] K. Hirota, T.H. Okabe, F. Saito, Y. Waseda, K.T. Jacob, J. Alloys Compd. 282 (1999) 101e108. [18] G.L. Li, L. Li, C. Yang, W.H. Tian, X.G. Li, Int. J. Hydrog. Energy 40 (2015) 7943e7948. [19] X.Y. Song, J.X. Zhang, M. Yue, E.D. Li, H. Zeng, N.D. Lu, M.L. Zhou, T.Y. Zuo, Adv. Mater. 18 (2006) 1210e1215. [20] S.S. Yardley, K.L. Moore, N. Ni, J.F. Wei, S. Lyon, M. Preuss, S.L. Perez, C.R.M. Grovenor, J. Nucl. Mater. 443 (2013) 436e443.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Research on the removal of impurity elements during ultra-high purification process of terbium Guoling Li a, c, Li Li a, Ruiying Miao b, Wenhuai Tian c, Shihong Yan b, **, Xingguo Li a, * a Beijing National Laboratory of Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China b General Research Institute for Nonferrous Metals, Grirem Advanced Materials Co. Ltd., Beijing 100088, PR China c Department of Materials Physics and Chemistry, University of Science and Technology Beijing, NO. 30, Xueyuan Road, Beijing 100083, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2015 Received in revised form 26 November 2015 Accepted 4 December 2015 Available online 11 December 2015
Ultra-high purification of terbium (Tb) by vacuum distillation and external getter was investigated. More than 20 kinds of impurity elements were analyzed by glow discharge mass spectrometry (GDMS) and inert gas analysis (IGA). The analysis results show that total impurities decrease from 1114 ppm to 63 ppm. High volatile metallic impurities such as Mg, Ca, Mn, Zn, and Tm can be reduced effectively after distillation. The oxygen (O) concentration can be decreased to 1.8 ppm after deoxidation process, which currently is one of the best results in the contemporary studies. The chemical driving force for deoxidation has been calculated in this paper. The different deoxidation results of Tb metal with various purity have been discussed in detail using the secondary ion mass spectroscopy (SIMS). Finally, a ultra-high purity Tb with low concentration of O can be obtained through vacuum distillation and external getter method. © 2015 Published by Elsevier Ltd.
Keywords: Rare earth alloys and compounds Purification of terbium Vacuum distillation External deoxidation method Thermodynamic modeling
1. Introduction Terbium (Tb) is a heavy rare earth element whose compounds have been applied extensively in biomedical materials, magnetic materials, fluorescent films and luminescent materials [1e5]. The performance of such compounds is strongly depended on the purity of raw materials [6]. It has been reported that trace impurities at ppm or sub-ppm level, especially the interstitial impurity oxygen (O), can remarkably affect their physical and chemical properties [7]. However, it is an extremely difficult and complex problem to get rare earth metals of ultra-high purification. Pioneers have done a lot of work on this. Z. Zhang et al. [8,9] prepared praseodymium and yttrium metal by distillation under a high vacuum condition. The purification of the metals was improved to 99.99%, while the oxygen contents were both nearly 100 ppm. K. Mimura et al. [10] purified the lanthanum and cerium by plasma arc zone melting to about 99.83% and 99.87%, respectively, which was not really high. The same problem was that the oxygen impurity still remained
* Corresponding author.. ** Corresponding author. E-mail addresses: [email protected] (S. Yan), [email protected] (X. Li). http://dx.doi.org/10.1016/j.vacuum.2015.12.003 0042-207X/© 2015 Published by Elsevier Ltd.
above 100 ppm. J.W. Bae [11] purified hafnium from 99.47% to 99.49%, and K. Mimura [12] purified titanium from 99.96% to 99.99%, yet both of them did not mention the results of the oxygen impurity. Tb has extremely high chemical reactivity, and hence considerably rigorous equipment are required to preserve high purity, which makes the preparation of ultra-high pure Tb still a great challenge [13,14]. The study of purification of Tb by using solid state electro transport processing was reported [15], while there was no detailed evaluation of overall purity of Tb using glow discharge mass spectrometry (GDMS) and interstitial gas analysis (IGA). Z.A.Li has prepared high-purity Tb by using distillation, however, few purification mechanism was evidently presented [16]. For the oxygen impurity, external gettering technology was ever experimentally applied to Tb metals by Kamihira et al. [14]. However, the final content of oxygen was still above 1000 ppm. Hirota successfully decreased the O content to 20 ppm using electrochemical method after a long experimental time [17]. Gas gettering method could reduce the O content below 10 ppm [18]. It can be concluded from the previous research that, the ultra-high purification of Tb requires the refined calculations and accurate operation steps. Rare-earth metals has strong affinity with O element, making O the most difficult impurity to remove [19]. It is very rare to reduce O
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G. Li et al. / Vacuum 125 (2016) 21e25
content to under 10 ppm, extend processing time seems to be ineffective. The reasons obstructing the deoxidation process still have not been reported clearly so far. In this paper, we investigate, the removing of impurities in Tb during distillation followed by calcium purification. Various instrumental techniques have been developed in response to specific analysis requirements of ultra-trace analysis. The underlying mechanisms associated with the refining process have been demonstrated. In addition, SIMS analysis provides impurity elemental mapping which makes it an exceptionally powerful technique to elucidate the interactions between metallic and oxygen impurities.
2. Experiments 2.1. Distillation process The vacuum distillation system was consisted of an evaporator and a condenser as shown in Fig. 1. The evaporator was made up of cylindrical tungsten crucible in the dimensions of 115 mm 100 mm 220 mm. The condenser was made up of tantalum sheet comprising of water cooled pipes to collect the distillate. The chamber was evacuated for about 2e3 h at 200e300 C to remove the moisture. A dynamic vacuum level of approximately 1.5 104 Pa was maintained in the chamber. Then the chamber was heated, and the material started evaporating and condensing at the bottom of cooled tantalum sheet placed on top, leaving high vapor pressure impurities at the top of the condenser and low vapor pressure impurities at the bottom of the crucible. The raw material of Tb used in the paper was made from Tb4O7 with following steps. Terbium oxide was firstly fluorinated (TbF3), and then reduced in a vacuum induction furnace by calcium (>99.9% purity). To remove the redundant calcium and obtain high purity Tb (>99%), the reduced Tb should be melt again in the vacuum induction furnace. The GDMS analysis of the starting material is given in Table 1. The distillation experiment was conducted in two steps. In the first step, the metal was melt at a comparatively lower temperature of 1200e1400 C to remove higher vapor pressure impurities than Tb. A soaking temperature of 1580e1650 C was maintained for the second phase to collect the distilled Tb. The condensation temperature was about 800e900 C. The distillation process maintained 18e20 h. The initial weight of Tb loaded into the crucible is about 2 kg, and the final yield after the distillation is near 95%.
Table 1 GDMS analysis of Tb. Impurity element
Raw material (ppm)
Distillation material (ppm)
Mg Al Si Ca Ti V Cr Mn Fe Co Ni Cu Zn Y La Ce Nd Sm Eu Gd Dy Ho Tm Others Sum Purity (mass%)
126 138 100 100 21 13 54 14 98 10 42 107 10 28 36 6 32 10 10 77 37 30 10 <5 <1114 >2N
<0.05 2.1 1.9 <0.1 2 0.01 0.01 <0.01 23 0.05 8.3 3.5 <0.5 6 6.8 2.8 0.07 0.01 <0.01 1.5 0.08 0.09 0.01 <5 <63 >4N
2.2. Deoxidation process High purity Tb ingot was cut into F5 mm L50 mm columns, and each specimen weights approximately 8e9 g. Impurities on the surface of these columns during handling were removed mechanically. After that, the specimens were rinsed with distilled water, ethanol and acetone, respectively. Then, they were dried for later use. Commercial granular Ca (280 ppm oxygen) were used as the external getter. The samples full filled with granular Ca in a graphite crucible (F22 mm L70 mm), were placed into the vacuum sintering furnace (Fig. 1) which was maintained at 1.25 105 Pa. Sufficient quantities of granular Ca (~12.5 g) were added so that the molten calcium could cover the whole Tb specimens. The furnace temperature was raised to a final depositing temperature of 500e1100 C at a rate of 10 C/min and hold for 30e180 min. The furnace maintaining the vacuum was then cooled to ambient temperature at natural rate.
Fig. 1. Schematic diagram of the vacuum distillation system (a), the calcium coating sample (b) and the vacuum annealing process (c).
G. Li et al. / Vacuum 125 (2016) 21e25
23
Metal impurities were analyzed by glow discharge mass spectrometry (GDMS-VG9000). The O contents were analyzed by an inert gas fusion-infrared absorption analysis (IGA-TCH-600). The average concentration of several pieces was taken as the analytical value at a given experimental condition. The morphology of the samples were studied by scanning electron microscopy (SEMS4800). Secondary Ion Mass Spectrometry (SIMS) was used to map the impurity elements inside the basic metal [20].
4NeTb, this final oxygen concentration can drop to 7.1 ppm by the same refining process. Thus, it is proved that the external deoxidation method is useful for Tb. Fig. 3 further shows the relationship between oxygen concentrations and the annealing time at 600 C. Concentrations of oxygen decrease clearly with extension of time. As the holding time reached 180 min, the oxygen contents remain steady. The final oxygen contents in 2NeTb and 4NeTb are 15.3 and 1.8 ppm respectively. It is concluded that an annealing duration of 180 min is sufficient for deoxidization of Tb.
3. Results and discussions
3.3. Thermodynamic mechanisms
3.1. Metallic impurities
Tb samples were embedded in a crucible containing granular Ca and subsequently annealed at different temperatures. Fig. 2 shows the dependence of oxygen concentration under different annealing temperatures. All the samples were annealed for 60 min. The oxygen concentrations of starting terbium (2NeTb) and distillation terbium (4NeTb) were 489.7 and 334.7 ppm, respectively. Granular Ca lead to a distinct phenomenon of deoxidation after the annealing process, which is shown in Fig. 2. However, there is an increasing tendency of oxygen concentrations with the increase of annealing temperature. It can be found that excellent deoxidation is obtained at 600 C annealing. The oxygen concentration of 2NeTb can decrease to 14.8 ppm after annealing at 600 C for 60min. For
To study the deoxidation mechanisms systematically, we further conducted a series of relevant research experiments. Fig. 4 shows the cross-section of Tb annealed with granular Ca at 600 C for 180min. As we know 600 C is below the melting point of Ca, so granular Ca can stay in a micro-molten state at this temperature. It can be seen from the SEM image that there is a typical region of Ca layer covered on the solid surface smoothly, the thickness of which is about 200 mm. The specific Ca layer is considered to be the chemical reaction zone during the purification process. A concentration map by EDX in Fig. 4 shows non-interdiffusion between the deposited layer and the basic metal. That is to say, the reaction zone, as well as the coated film, can be easily removed by physical etching. The thermodynamic data of solid Tb to the high temperatures has been calculated in order to estimate the Gibbs free energies of different reactions for Tb. The equations are presented in (1)e(3) [13,17]. These equations indicate the oxidation reactions about Tb and Ca, respectively, and the respective DG with temperature. Fig. 5 shows the changes in DG of them as a function of temperature. Both pure Tb and Ca can be easily oxidized at elevated temperatures. Oxidation of Ca is evaluated to be more favorable due to the lower DG. Ca has a higher chemical activity than Tb, and performs as the reducing agent during the deoxidation process. The deposited Ca layer contributes to the interaction process with oxygen on the solid metal surface, and the oxygen concentration of the surface can be reduced rapidly by reaction (3). Furthermore, due to the concentration gradient, the interstitial oxygen atoms in the TbeO solid solution can move to the surface, greatly promoting the migration of impurity atoms. In this process, thermodynamic reactions are believed to be the rate determining
Fig. 2. Concentration change of oxygen in two sets of metal as a function of temperature.
Fig. 3. Oxygen concentrations of 2NeTb and 4NeTb with different annealing time at 600 C.
2.3. Characterization
Table 1 shows the concentration of more than 20 kinds of impurities in Tb before and after purification. The total contents are decreased from <1114 ppm to <63 ppm. We can observe a significant reduction of the concentrations of major impurities such as Mg, Ca, Cr, Mn, Zn, Dy, Ho. These elements with a high vapor pressure retain almost <1 ppm after the distillation process. Impurities having a vapor pressure close to Tb, like Fe, Ni, Cu, and Ti, seem to be difficult to remove through the whole process. For impurities La, Ce and Y with lower vapor pressure than Tb material, their content changes are also obvious. The contents of heavy rareearth elements impurities such as Eu, Sm, Gd and Dy, are all under 2 ppm. 3.2. Removal of oxygen
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Fig. 4. SEM micrograph of Tb covered with Ca.
Fig. 6. Plane of denudation in 2NeTb by using SIMS and the accordance EDS result.
contents (Al: 138 ppm, Mg: 126 ppm, shown in Table 1). The EDS data can powerfully elucidate that the initial metallic impurities can pin the oxygen inside the metal and obstruct the removal process. In other words, only after the metallic impurities reduce to an ultra-low level, can the ultra-high purity “oxygen free” terbium metal be produced. 4. Conclusion
Fig. 5. Free energy changes with temperature for reaction of Tb and Ca with O.
step. On the other hand, Ca is mainly distributed on the surface of Tb as indicated in Fig. 4. The coated film can be removed mechanically in the glove box filled with argon, without bringing any recontamination to the basic metal. We detected the Ca concentrations in the high-purity terbium by inductively coupled plasma-atomic emission spectrometer, and the final content was below 1 ppm. 1/2O2 (g) ¼ O (Tb, 1mass %) DG (kJ/mol) ¼ 591 þ 0.0789 T (1) 2/3Tb (s) þ 1/2O2 (g) ¼ 1/3Tb2O3 (s) DG (kJ/ mol) ¼ 616 þ 0.0905 T
(2)
Ca þ 1/2O2 (g) ¼ CaO (s) DG (kJ/mol) ¼ 637 þ 0.1060 T
(3)
A two-step approach to produce high purity Tb was employed. More than 20 kinds of impurity elements of raw Tb (2N purity) had been reduced, by vacuum distillation process, to the extent of achieving 4NeTb. Oxygen as the major non-metallic impurity, which was highly difficult and energy-consuming to be removed, could be effectively reduced by the external deoxidation method. By the vacuum distillation process, followed by annealing at 600 C, we successfully obtained ultra-high purity Tb (>99.99%) with substantially reduced non-metallic impurities. The final oxygen concentration is 1.8 ppm, which is one of the best results in the contemporary studies reported so far. Ca, which performed as the effective deoxidizing metal, is the crucial factor. The SIMS powerful technique illustrates that the main obstructive factor is the pinning force caused by the residual metallic impurities. The method proposed here may shed light on the future research on removing impurities in other RE metals, such as Gd, Dy and etc., which share similar properties and hence difficulties with Tb in purification. Acknowledgments The authors acknowledge MOST of China (No. 2012CBA01207) and NSFC (No. U1201241, 11375020 and 51431001). References
3.4. Obstructive factors It can be seen from Figs. 2 and 3 that the external gettering method displays perfect deoxidation capability, while the final oxygen concentration of 2NeTb is higher than the 4NeTb sample. The initial metallic impurities can seriously affect the deoxidation process. Fig. 6 shows the plane of denudation in 2NeTb after deoxidation by using SIMS. Luckily, we clearly observed a parcel of AleMgeO alloy inside the Tb sample, in spite of the low impurity
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