JOURNAL OF RARE EARTHS, Vol. 32, No. 11, Nov. 2014, P. 1073

Foundation item: Project supported by Major State Basic Research Development Program of China (973 Program: 2012CBA01207) and the National High Technology Research and Development Program of China (863 Program: 2011AA03A409)

* Corresponding author: MIAO Ruiying (E-mail: ruiyingmiao@163.com; Tel.: +86-10-89583403-235)

DOI: 10.1016/S1002-0721(14)60185-X

Impurities especially titanium in the rare earth metal gadolinium— before and after solid state electrotransport

MIAO Ruiying (苗睿瑛)*, ZHANG Xiaowei (张小伟), ZHU Qiong (朱 琼), ZHANG Zhiqi (张志琦), WANG Zhiqiang (王志强), YAN Shihong (颜世宏), CHEN Dehong (陈德宏), ZHOU Lin (周 林), LI Zong’an (李宗安) (National Engineering Research Center for Rare Earth Materials, General Research Institute for Nonferrous Metals, Grirem Advanced Materials Co. Ltd., Bei-jing 100088, China)

Received 23 May 2013; revised 4 August 2014

Abstract: Gadolinium was prepared by conventional procedures of fluorination, reduction, distillation and solid state electrotransport (SSE). The electronegativities of the metals were found to have an important influence on the electrotransport process and result of the impurity element. Meanwhile, titanium particles in the distilled gadolinium as major metallic impurities were studied by high resolution transmission electron microscopy (HRTEM) before and after solid state electrotransport. The results showed that impurities especially titanium transported from anode to cathode during SSE. In the metal before SSE, there were impurities of titanium in strip shape or embedded round shape. After SSE processing, titanium particles in the metal smaller than 50 nm in the cathode, but existed 6 to 10 times bigger in the anode.

Keywords: impurities; titanium; rare earth metal; gadolinium; solid state electrotransport

Gadolinium has no large-scale applications but has a variety of specialized uses, such as applications in the luminescence, electricity, magnetism, nuclear and espe-cially in medicine.

High purity metals are desired when used for magne-tostrictive and magnetic refrigerant materials, and when used for searching their intrinsic or unknown properties. However, the high purity metal cannot be achieved easily with regard to the highly active rare earth metals. During the past decades, preparation of the rare earth metals was introduced in detail by the researchers of the Ames Laboratory[1]. In recent decades, rare earth has been be-coming a hot topic[2–4]. Solid state electrotransport (SSE) had been applied to rare earth metals since 1961[1], and the first material was yttrium. In 1972[5], electrotransport velocities of O, N and C in gadolinium were determined by Peterson and coworkers at Ames Laboratory. During these decades, purifications of all possible rare earth metals by SSE were studied by researchers at Ames Laboratory and University of Birmingham, United Kingdom[1,6–23] and interstitial impurities such as O, N and C were main topics in these papers. This may be at-tributed to the obvious effect of transport. Compared to the interstitial impurities, metallic impurities were less focused on. As for titanium, still less.

In this paper, SSE was studied from a particular per-spective. Firstly, gadolinium was prepared from common

gadolinium oxide (99%), and the final metal was ob-tained by fluorination, reduction, distillation and solid state electrotransport. And after that, the compositions of the metals before and after SSE were characterized by glow discharge mass spectrometry (GDMS). The micro-structures of metallic impurities especially titanium in the metal and the metal itself were characterized by HRTEM, as well.

1 Experimental

1.1 Fluorination

Gd2O3 was fluorinated by continuously mixing with HF gas at 600 °C for 8 h in accordance with the follow-ing formula[24,25]: Gd2O3(s)+3HF(g)=GdF3(s)+3H2O (g) (1) GdF3(s)+H2O(g)=GdOF(s)+2HF (g) (2)

Here, formula (2) is a side reaction. To avoid the side reaction, multiple fluorinations are recommended. If high purity is desired, multiple fluorinations are essential.

1.2 Reduction

The product in the above step was reduced by calcium particles in the carbon tube furnace in the titanium cruci-ble at 1500 °C for half an hour in the argon atmos-phere[26].

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In the process, the product Gd and CaF2 were layered according to the density difference as shown in Table 1. Thus the metal obtained layered in the bottom of the cru-cible and the slag layered in the top. Both of them could be easily separated.

Table 1 Some of the physical constants in the reduction

Material m.p./°C b.p./°C Density

GdF3 1231 2277 7.1

Gd 1313 3273 7.89

Ca 842 1484 1.55

CaF2 1418 2534 3.18

* m.p.: melting point; b.p.: boiling point

1.3 Melting and distillation

The metal obtained was melted in the vacuum tanta-lum furnace at 1800 °C for half an hour, and distilled at 1725 °C for 20 h[17,27]. In the melting process, those vola-tile and high vapor-pressure impurities were eliminated. And the distillation removed most of those non- volatile and low vapor-pressure impurities from the metal.

1.4 Solid state electrotransport (SSE) processing

Solid state electrotransport device was designed and manufactured according to Refs. [28,29]. In this process, ultra high vaccum is essential[10,30]. In this study, the degree of vacuum was 5×10–7 Pa, approximately 4×10–10 Torr.

The specimens used in the electrotransport measure-ment were rods with 11 cm in length and 0.8 cm in di-ameter machined from the distilled metal.

The experiment was carried out in the current density of 450 A/cm2 at 1250 °C, above the temperature of the crystal transformation (1235 °C) of gadolinium from a structure of hexagonal close packing (h.c.p.) to β struc-ture of body-centered cubic (b.c.c.)[31]. This transforma-tion would result in the decrease of the efficiency of space filling and thus in the increasing of the volume for the crystal transformation part.

1.5 Characterization

For chemical analysis, all of the samples obtained were characterized by glow discharge mass spectrometry (GDMS).

High resolution transmission electron microscopy

(HRTEM, Tecnai G2 F30 S-TWIN) coupled with energy dispersed spectroscopy (EDS) was used to acquire the microstructures and morphologies of the metals.

2 Results and discussion

2.1 Photos of the gadolinium rod during the SSE processing

Photos of the gadolinium rod during the solid state electrotransport processing are shown in Fig. 1.

Non-uniform temperature distribution could be seen from Fig. 1(b). During the processing, the highest tem-perature appeared in the lower part of the rod. The ends of the rod are at a lower temperature than the center. The maximum difference can be 260 °C. This difference can make the impurities at the ends migrate more slowly than those in the center[10].

It is known that the electrotransport mobilities and dif-fusion coefficients for solutes such as C, N and O are considerably higher in β (b.c.c.) form than in α (h.c.p.) form, as shown in Fig. 2[31]. Because of the temperature difference of the whole rod, there should be two forms of gadoliniums that exist in our processing rod. From the dynamic-view, the transport mobilities and diffusion co-efficients of the solutes are different in them. From the thermodynamic-view, the structure of the different parts with different forms should be different, as well. In this paper, the rods after SSE displayed different degrees of bending at the brightest part. It might be interpreted that the brightest part of the rod had a structure of b.c.c., which had low efficiency of space filling. Thus the vol-ume of the b.c.c. part would increase, and meanwhile other part of the bar with h.c.p. form would remain un-changed during the process. This function would cause the bending of the whole rod, and even cause break in some serious cases. It is consistent with the results of Spedding and his research team[32]. In his paper, the atomic volume increased sharply when b.c.c. formed. Thermal expansion coefficient is an average of 8.9 at 400°C for gadolinium with same crystal form. The coefficient in c-axis (13.0) is more than twice as much as in the direc-tion of a-axis (6.3).

Fig. 1 Photos of the gadolinium rod during SSE processing

(a) Before electrotransport; (b) During heating process; (c) During thermal insulation

MIAO Ruiying et al., Impurities especially titanium in the rare earth metal gadolinium—before and … 1075

2.2 Metallic impurities in the gadolinium rod

For chemical analysis, the product rod was separated into several parts. The part close to anode was defined as A, and the bending part was defined as B as shown on the right side in Fig. 1.

The electronegativity and concentration of the rare earth metallic impurities in the gadolinium before and after SSE are listed in Table 2; the alkali and alkali earth metallic impurities are listed in Table 3, and the other metallic impurities are listed in Table 4.

Tables 2, 3, and 4 show that SSE was effective not only for the interstitial impurities but for some of the metallic impurities in gadolinium.

There is an interesting phenomenon that if the electro-negativity of the element is higher than that of the rare earth metal, the effect of SSE is more obvious. From Ta-ble 2, the electronegativity of gadolinium is 1.2. For the rare earth elements, their activities are closely after the elements of alkali metal and several alkali earth metals. Therefore, their electronegativity is bigger than those of the elements of alkali and alkali earth metals and smaller than those of almost all other elements. From Table 2, the effect of SSE processing is less obvious on the ele-ments having smaller electronegativity.

At the same time, in Table 3, the effect of SSE proc-essing on the alkali and several alkali earth metals having smaller electronegativity is less obvious. And opposite

Table 2 Electronegativity and concentration (μg/g) of rare earth metallic impurities in the metal before and after SSE

Element La Ce Gd Dy Ho Er Tm Lu

Electronegativity 1.1 1.12 1.2 1.22 1.23 1.24 1.25 1.27

Initial 14 1.3 Matrix 24 17 0.53 1.7 2

Part A 15 2.1 Matrix 30 21 0.83 2.2 9.4

Part B 18 2.1 Matrix 11 8.8 0.26 0.83 1.5

Table 3 Electronegativity and concentration (μg/g) of sev-eral alkali and alkali earth metallic impurities in the metal before and after SSE

Element Li Na K Ca

Electronegativity 0.98 0.93 0.82 1.00

Initial 0.11 0.54 0.08 4.2

Part A 0.02 0.53 0.1 2.3

Part B 0.02 1 <0.05 1.9

Table 4 Electronegativity and concentration (μg/g) of other metallic impurities in the metal before and after SSE

Element Al Ti Cr Mn Fe Nb Mo Ta W Pb

Electronegativity 1.61 1.54 1.66 1.55 1.83 1.6 2.16 1.5 2.36 2.33

Initial 340 1900 17 23 90 2.5 170 ≤110 3.7 5.2

Part A 320 1200 8.5 13 58 1.7 97 ≤ 80 3.7 4.7

Part B 200 500 <0.005 0.01 0.87 0.87 5.7 ≤40 0.91 0.26

effect, such as Na even appeared. It might be interpreted as follows. For the element having smaller electronega-tivity accompanied with the weaker attraction of the nu-cleus to the outer electron, the strong energy of the elec-tric field may result in the ionization of the metallic par-ticle, and thus the diffusion in the direction backward the electrons occurred.

As for other metals, their electronegativities are larger, that is, their nuclei have larger attraction than outer elec-trons. When current passes through, namely, when the electrons flow through directionally, strong driving force to the outer electrons of those metals will drive them go ahead in the same direction as the electrons, and strong attraction between the nucleus and the outer electrons will make the nucleus go ahead with two kinds of the electrons. The larger electronegativity is, the more obvi-ous this tendency is.

With regard to elements of those interstitial impurities, such as C, N and O, with the largest electronegativities except halogen have very small size in comparison with the metallic impurities. These particles may diffuse fast following the interstitialcy mechanism without the elec-tric field in the close-packed structure at high tempera-tures. They will diffuse faster because of their largest electronegativities in the strong electric field. These two effects working together led to the high removal rate of O, which is more than 98% (890 μg/g before SSE, and 10 after SSE).

2.3 Existence form of impurities

Many different small fragments of the separated part A and B were chosen and processed into samples for TEM to find out the existence of impurities in the metal. In or-der to validate the reliability of the results, repeated tests had been conducted several times. 2.3.1 Existence form of impurities in samples before SSE

Fig. 2 shows the different morphologies of titanium in the rod before SSE.

HRTEM images show titanium of strip shaped (2(a) and (b)) or embedded round shaped (2(c)) structure ex-isting in the metal.

In Fig. 2, the titanium particles appeared in gadolinium with strip shaped (about 100 nm×400 nm) or embedded round shaped (about 400 nm in diameter) structure. Fig. 3 displays the EDS results corresponding to Fig. 2. They showed that the white inclusions were mainly composed of titanium. Other elements such as iron, copper, chromium and molybdenum coexisted within them. For Fig. 3(b), the concentration of gadolinium was higher than that of titanium mainly because the inclusion was covered by the substrate. These white inclusions presented different appearances. Fig. 4 shows the mi-cro-morphology of the gadolinium substrate before SSE. From Fig. 4, lots of tiny pits were seen in the metal. EDS

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Fig. 2 TEM micro-morphology of strip shaped (a, b) and embedded round shaped (c) titanium in gadolinium before SSE

Fig. 3 EDS results (a) (corresponding to Fig. 2(a)), (b) and (c) (corresponding to Figs. 2(b) and 2(c)))

Fig. 4 TEM The micro-morphology of the gadolinium substrate

before SSE results showed small amount of Ti, Mo and Cr existing inside. Titanium impurities here should be introduced by the crucible material. Other impurities such as iron and chromium were probably introduced by titanium crucible, too. As for copper, it might be contaminated during sam-ple preparation. 2.3.2 Occurrence of impurities in sample of part A after

SSE HRTEM images of titanium of part A after SSE are

shown in Fig. 5. From Fig. 5, titanium particles appeared some raised

ellipse shape (300 nm×400 nm) and superimposed foot-print shape (small footprint: about 400 nm×600 nm). EDS results (Fig. 6) show that these different inclusions of titanium in part A were mainly composed of titanium,

Fig. 5 Morphology of titanium (a, raised ellipse shape and b,

superimposed footprint shape) of part A after SSE in gadolinium

and small amount of iron, copper, chromium and mo-lybdenum, etc. And there were only minor differences in the content of both structures. The ellipse shaped inclu-sion illustrated in Fig. 5(a) may be evolved by the em-bedded round shape illustrated in Fig. 2(c). The super-imposed footprint shaped inclusion might be evolved by the strip shape illustrated in Fig. 2(a) or (b).

Fig. 7 demonstrates the micro-morphology and the electronic diffraction pattern of the gadolinium substrate of part A.

Fig. 7(a) shows the morphology of the gadolinium substrate of part A. There were small obvious pits inside the metal. Fig. 7(b) gives the electronic diffraction pat-tern of the gadolinium substrate. In this section, the crystal structure was close-packed structure of h.c.p. EDS results indicate a substantial amount of Cr, Fe and Mo inside. 3.3.3 Occurrence of impurities in sample of part B after SSE

The morphology of titanium of part B is illustrated

MIAO Ruiying et al., Impurities especially titanium in the rare earth metal gadolinium—before and … 1077

Fig. 6 EDS results (a) (corresponding to Fig. 4 (a) and (b) (corresponding to Fig. 4(b))

Fig. 7 Micro-morphology (a) and the electronic diffraction pat-

tern (b) of the gadolinium substrate of part A in Fig. 8. There were two small spots less than 50 nm and one strip less than 50 nm in width. Their EDS results are shown in Fig. 9. Fig. 9 shows EDS results of the two white spots. They mainly consist of titanium and gado-linium. And the gadolinium content was higher than that of the inclusions of part A. It may be interpreted as the diameter of the electron beam of EDS was bigger than that of the inclusion itself. As for the strip shaped inclu-sion, it is very similar to the one in the rod before SSE. The possible reason was that this shaped inclusion was a perfect single crystal. Energy for destroying the single crystal is higher than that for polycrystalline. Therefore the transport of single crystal is more difficult. Thus it is very likely that the inclusion has existed there before SSE. During SSE, transport of polycrystalline is preferred.

Fig. 8 Micro-morphology of the titanium (white raised and strip

shaped) of part B in gadolinium

With regard to the titanium particles, they increased in size from 100 nm×400 nm and 400 nm before SSE to 300 nm×400 nm and more than 400 nm×600 nm of part A after SSE, and decreased to less than 50 nm of part B after SSE. This can be attributed to the effect of the SSE. In the process, titanium and other particles migrated with the effect of the electric, temperature and concentration field. The titanium particles of strip shape and embedded round shape acted as the seed crystal. The particles ac-cumulated around the titanium seeds of part A and thus the size increased. At the same time, the size of titanium

Fig. 9 Micro-morphology and EDS results ((a) the upper spot, (b) the lower one) of the titanium (white raised) of part B in gadolinium

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Fig. 10 Micro-morphology (a) and electronic diffraction pattern

(b) of gadolinium substrate of part B decreased greatly. It is implied that titanium transport from anode to cathode during SSE.

The morphology and the electron diffraction pattern of gadolinium substrate are shown in Fig. 10. In Fig. 10, the structure of the substrate is smooth. There were less and tinier dots inside. EDS results show that the content of gadolinium was more than 95%, the rest was copper. These copper particles may be introduced by sample preparation. The diffraction pattern showed that the structure was b.c.c. As mentioned above, this structure

had low efficiency of space filling. It was because of this lower efficiency that led to the faster transport and thus led to the better purity.

As for substrates, impurity particles as solutes ex-isted in the gadolinium solvent before SSE and in part A after SSE. It was totally different in part B, the sur-face became smoother and had not many pits inside. This phenomenon resulted from SSE. It meant that the sample in part B had been purified by SSE proc-essing.

According to the SSE process, a crude model was es-tablished as shown in Fig. 11. The size of inclusion be-came bigger after transportation. At the same time, im-purities in anode may be less and less until a steady-state distribution realized after enough time[11,33]. On the con-trary, impurities in cathode may be more and more. This was a very easy model. And the fact should be far more complicated than this.

In brief, SSE is a complicated and interesting process worthy of our intensive study to give some helpful guid-ance to the whole metal preparation field to thoroughly uncover its mysterious veil.

Fig. 11 Schematic diagram of the transportation of titanium during SSE

3 Conclusions

Rare earth metal gadolinium was prepared by fluorina-tion, reduction and distillation in this work. After that, the metal was processed into rods to carry out solid state electrotransport. The results showed that the part with the highest temperature had the minimal impurities. The comparison of contents for different parts of the rod gave lights upon the realization of the important effect of the electronegativity on migration of impurity particles dur-ing SSE processing. HRTEM results demonstrated that impurities especially titanium migrated from anode to cathode during SSE.

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