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New method of insertion of hydrogen in R2Fe16Ti alloys with RY and Nd
Journal of Alloys and Compounds 322 (2001) 211–213
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New method of insertion of hydrogen in R 2 Fe 16 Ti alloys with R=Y and Nd b ´ M. Ellouze a , *, Ph. l’Heritier , A. Cheikh-Rouhou a , J.C. Joubert b a
LPMS, Faculte´ des Sciences de Sfax, Route de Soukra Km 3,5, B.P. 802, 3018 Sfax, Tunisie b ` , France LMGP ( UMR 5628 CNRS) ENSPG, B.P. 46, 38402 Saint Martin d’ Heres Received 7 November 2000; accepted 4 December 2000
Abstract A new method of inserting hydrogen in R 2 Fe 16 Ti compounds with R=Y and Nd was employed with success. The insertion of hydrogen in the structure increases the lattice parameters and, in turn, the unit cell volume. The changes in lattice constants are in good agreement with those obtained by the classic hydrogenation method. The Curie temperature as well as the saturation magnetization increase after insertion of hydrogen. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Rare earch compounds; Hydrogen absorbing materials; Chemical synthesis; Magnetic measurements
1. Introduction Intermetallic alloys have attracted a great interest in the fundamental area (crystallographic and magnetism) as well as in the applications area (materials for permanent magnets). The R 2 Fe 17 compounds (where R is rare earth element) crystallize in the Th 2 Ni 17 type symmetry (hexa¯ [2]). gonal, P6 3 /mmc) [1] or Th 2 Zn 17 (rhombohedral, R3m These structures derive from the hexagonal structure of CaCu 5 . Many studies have focused on the insertion of light elements such as hydrogen [3,4], carbon [5] and / or nitrogen [6,7] as are interstitial element in R 2 Fe 17 compounds. Coey et al. [7] have shown that the insertion of nitrogen in the R 2 Fe 17 compounds is possible. Isnard [8] has shown that interstitial elements such as hydrogen, nitrogen and / or carbon increase the magnetic properties, especially the Curie temperature. The insertion of light elements such as hydrogen, nitrogen and carbon in this type of alloy has been undertaken, until now, under a static gas pressure and with thermal activation. This method led generally to the appearance of free a-iron as a second phase and / or sometimes to the destruction of the structure. *Corresponding author. E-mail address: [email protected] (M. Ellouze).
In this work, we report on a new method of insertion of hydrogen in pseudobinary alloys and we compare our results to those obtained by the classic method.
2. Experimental aspects The R 2 Fe 16 Ti compounds with R=Y and Nd have been prepared from constituents the purity of which is better than 99% by fusion in an induction furnace. The alloys thus obtained have been given a thermal homogenization treatment. These annealings have been realized in a quartz tube at 11508C for 20 days. Then the material was quenched in water. The phase purity was checked by X-ray diffraction (XRD) using a SIEMENS diffractometer and iron lk(a ) 5 ˚ radiation. The magnetization was measured using a 1936 A vibrating sample magnetometer with an applied field up to 8 T. The Curie temperature was obtained from thermomagnetic analysis (TMA) using a home-made Faraday-type balance. The phase purity and the grain boundaries were investigated by scanning electron microscopy. The insertion of hydrogen in the compounds has been undertaken on the one hand, by the classic method and on the other hand by using an aqueous solution of NaBH 4 . The hydride of Y 2 Fe 16 Ti has been obtained under a
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01501-2
212
M. Ellouze et al. / Journal of Alloys and Compounds 322 (2001) 211 – 213
Fig. 1. The different stages of hydrogen insertion by using NaBH 4 .
pressure of 13 MPa at the temperature of 3008C during 20 h. The insertion of a light element such as hydrogen in the Nd 2 Fe 16 Ti compound has been realized under a hydrogen pressure of 12 MPa at a temperature of 3508C during 24 h. The obtained alloys were saturated in hydrogen. The moderated temperature at which this has been realized does not lead to the following decomposition: R 2 Fe 17 Hd 1 (2 1 x 2 d / 2)H 2 → 2 RH ( 21x) 1 17 Fe(a ) This reaction proceeds generally in the temperature range between 500 and 6008C [9]. Being basic, the concentrated aqueous solution of NaBH 4 reduces the risks of corrosion of alloys to a minimum. On the other hand, according to works of Murphy et al. [10], NaBH 4 can be used as a source of hydrogen at room temperature. According to Murphy et al. [10] as well as Kramp et al. [11], this method of insertion of hydrogen at room temperature has been used successfully on a certain number of the alloys, such as LaNi 5 , Nd 2 Fe 14 B, CaNi 5 and MnNi 5. The reaction scheme that was proposed is the following [10,11]: 2 M 1 BH 2 4 1 3H 2 O → M 2 H 1 H 2 BO 3 1 3.5H 2
where M and M2H are the alloy and its hydride, respectively. Fig. 1 represents the different stages that lead to the insertion of hydrogen in such compounds. This method consists therefore in mixing in distilled water the powder of the compound as well as a small quantity of NaBH 4 . After 24 h of reaction the obtained compound was rinsed with alcohol and then dried. The difference of mass before and after hydrogenation of the alloy allows us to determine approximately the number of inserted hydrogen atoms.
Fig. 2. Zoom of X-ray diffraction of Nd 2 Fe 16 Ti and Nd 2 Fe 16 TiH y .
Fig. 2 represents a selected part of the X-ray diffraction patterns of the powder of the compound Nd 2 Fe 16 Ti and its hydride obtained by reaction with NaBH 4 . This figure shows that hydrogen has been inserted in the structure by this method since the peaks have undergone a displacement to lower angles. The refinement of the lattice parameters indicates that these parameters increase after insertion of hydrogen. The obtained parameters are similar for the two hydrogenation methods (Table 1). The increase of the volume is about 2.47% for the Y 2 Fe 16 Ti compound and it is 3.9% for the Nd 2 Fe 16 Ti compound. Based on scanning electron microscopy, Fig. 3 shows granular cracks which confirms that hydrogen has been introduced into the structure. The grain size is less than 50 mm. Thermomagnetic analyses shows that the hydrides exhibit only one type of transition which is ferromagnetic– paramagnetic, the Curie temperatures of the hydrides being higher than in the parent alloys. The Curie temperatures of the hydrides obtained by reaction with NaBH 4 are almost equal to those obtained by solid–gas reaction, as seen in Fig. 4. The insertion of hydrogen by both methods is beneficial since it increases the Curie temperature about 100 K. The field dependence of the magnetization has been measured at room temperature for the parent alloys as well Table 1 Lattice constants obtained before and after insertion of hydrogen
3. Results and discussions X-ray diffraction of the hydrides of the pseudobinary alloys shows that they basically preserve the structural features of the parent alloys. Indeed, the Y 2 Fe 16 TiH y compound crystallizes in the hexagonal structure, while the Nd 2 Fe 16 TiH y compound preserves the rhombohedral structure.
Compounds
˚ a (A)
˚ c (A)
˚ 3) V (A
Y 2 Fe 16 Ti Y 2 Fe 16 TiH y a Y 2 Fe 16 TiH y b Nd 2 Fe 16 Ti Nd 2 Fe 16 TiH y a Nd 2 Fe 16 TiH y b
8.4810 8.5705 8.5682 8.5987 8.7302 8.7289
8.3740 8.4015 8.4075 12.509 12.605 12.613
521.62 534.44 534.50 800.97 832.00 832.3
a b
Hydrides obtained by the classic method. Hydrides obtained by reaction with NaBH 4 .
y (H / f.u.)
DV/V (%)
4.6 4.5
2.46 2.47
4.8 4.9
3.87 3.91
M. Ellouze et al. / Journal of Alloys and Compounds 322 (2001) 211 – 213
213
Table 2 Curie temperatures and saturation magnetization obtained before and after insertion of hydrogen Compounds
T c (K)
Ms (emu / g)@300 K
Y 2 Fe 16 Ti Y 2 Fe 16 TiH y a Y 2 Fe 16 TiH y b Nd 2 Fe 16 Ti Nd 2 Fe 16 TiH y Nd 2 Fe 16 TiH y
400 493 490 390 503 500
105.7 127.4 125.3 117.5 135.5 136.8
a b
Fig. 3. Scanning electron microscopy photo of the Nd 2 Fe 16 TiH y after hydrogen insertion.
a b
Hydrides obtained by the classic method. Hydrides obtained by reaction with NaBH 4 .
as for their hydrides (Fig. 5). The saturation magnetization (Ms ) increases after insertion of hydrogen and we obtain practically the same value for the two hydrides of the same parent compound. The Curie temperatures as well as the saturation magnetization at 300 K of the alloys before and after insertion of hydrogen are summarized in Table 2.
4. Conclusion We have successfully obtained hydrides by using a solution of NaBH 4 . Crystallographic and magnetic properties of these hydrides are similar with those obtained by the classic gas–solid method. This new method has several advantages. On the one hand, it proceeds at room temperature and does not require thermal activation. On the other hand, contrary to the classic method, we can avoid free a-iron as a secondary phase and / or the temperature induced decomposition of the ternary hydride.
References Fig. 4. Thermomagnetic analyses of the hydrides of Y 2 Fe 16 Ti obtained with the two methods.
Fig. 5. Magnetization versus applied field at 300 K of Nd 2 Fe 16 Ti and Nd 2 Fe 16 TiH y .
[1] J.V. Florio, N.C. Baenziger, R.E. Rundle, Acta Crystallogr. 9 (1956) 367. [2] E.S. Makarov, S.P. Vinogradov, Sov. Phys.-Crystallogr. 1 (1956) 634. [3] B. Rupp, A. Resnik, D. Shaltiel, P. Rogl, J. Mater. Sci. 23 (1988) 2133. [4] W.X. Zhong, K. Donnelly, J.M.D. Coey, B. Chevallier, J. Etourneau, T. Berlureau, J. Mater. Sci. 23 (1988) 329. [5] X.P. Zhong, R.J. Radwanski, F.R. de Boer, T.H. Jacobs, K.J.H. Buschow, J. Magn. Magn. Mater. 86 (1990) 333. [6] J.M.D. Coey, H. Sun, J. Magn. Magn. Mater. 87 (1990) L251. [7] J.M.D. Coey, J.F. Lawler, H. Sun, J.E.M. Allan, J. Appl. Phys. 69 (5) (1991) 300. [8] O. Isnard, Thesis of the Universite´ Joseph Fourier (Grenoble), (1993). [9] C.N. Chritodoulou, T.Tu. Keshita, J. Alloys Comp. 194 (1993) 113. [10] D.W. Murphy, S.M. Zahurak, B. Vyas, M. Thomas, M.E. Badding, W.C. Fang, Chem. Mater. 5 (1993) 767. [11] S. Kramp, M. Febri, J.C. Joubert, J. Solid State Chem. 133 (1997) 145.
L
www.elsevier.com / locate / jallcom
New method of insertion of hydrogen in R 2 Fe 16 Ti alloys with R=Y and Nd b ´ M. Ellouze a , *, Ph. l’Heritier , A. Cheikh-Rouhou a , J.C. Joubert b a
LPMS, Faculte´ des Sciences de Sfax, Route de Soukra Km 3,5, B.P. 802, 3018 Sfax, Tunisie b ` , France LMGP ( UMR 5628 CNRS) ENSPG, B.P. 46, 38402 Saint Martin d’ Heres Received 7 November 2000; accepted 4 December 2000
Abstract A new method of inserting hydrogen in R 2 Fe 16 Ti compounds with R=Y and Nd was employed with success. The insertion of hydrogen in the structure increases the lattice parameters and, in turn, the unit cell volume. The changes in lattice constants are in good agreement with those obtained by the classic hydrogenation method. The Curie temperature as well as the saturation magnetization increase after insertion of hydrogen. 2001 Elsevier Science B.V. All rights reserved. Keywords: Transition metal compounds; Rare earch compounds; Hydrogen absorbing materials; Chemical synthesis; Magnetic measurements
1. Introduction Intermetallic alloys have attracted a great interest in the fundamental area (crystallographic and magnetism) as well as in the applications area (materials for permanent magnets). The R 2 Fe 17 compounds (where R is rare earth element) crystallize in the Th 2 Ni 17 type symmetry (hexa¯ [2]). gonal, P6 3 /mmc) [1] or Th 2 Zn 17 (rhombohedral, R3m These structures derive from the hexagonal structure of CaCu 5 . Many studies have focused on the insertion of light elements such as hydrogen [3,4], carbon [5] and / or nitrogen [6,7] as are interstitial element in R 2 Fe 17 compounds. Coey et al. [7] have shown that the insertion of nitrogen in the R 2 Fe 17 compounds is possible. Isnard [8] has shown that interstitial elements such as hydrogen, nitrogen and / or carbon increase the magnetic properties, especially the Curie temperature. The insertion of light elements such as hydrogen, nitrogen and carbon in this type of alloy has been undertaken, until now, under a static gas pressure and with thermal activation. This method led generally to the appearance of free a-iron as a second phase and / or sometimes to the destruction of the structure. *Corresponding author. E-mail address: [email protected] (M. Ellouze).
In this work, we report on a new method of insertion of hydrogen in pseudobinary alloys and we compare our results to those obtained by the classic method.
2. Experimental aspects The R 2 Fe 16 Ti compounds with R=Y and Nd have been prepared from constituents the purity of which is better than 99% by fusion in an induction furnace. The alloys thus obtained have been given a thermal homogenization treatment. These annealings have been realized in a quartz tube at 11508C for 20 days. Then the material was quenched in water. The phase purity was checked by X-ray diffraction (XRD) using a SIEMENS diffractometer and iron lk(a ) 5 ˚ radiation. The magnetization was measured using a 1936 A vibrating sample magnetometer with an applied field up to 8 T. The Curie temperature was obtained from thermomagnetic analysis (TMA) using a home-made Faraday-type balance. The phase purity and the grain boundaries were investigated by scanning electron microscopy. The insertion of hydrogen in the compounds has been undertaken on the one hand, by the classic method and on the other hand by using an aqueous solution of NaBH 4 . The hydride of Y 2 Fe 16 Ti has been obtained under a
0925-8388 / 01 / $ – see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 00 )01501-2
212
M. Ellouze et al. / Journal of Alloys and Compounds 322 (2001) 211 – 213
Fig. 1. The different stages of hydrogen insertion by using NaBH 4 .
pressure of 13 MPa at the temperature of 3008C during 20 h. The insertion of a light element such as hydrogen in the Nd 2 Fe 16 Ti compound has been realized under a hydrogen pressure of 12 MPa at a temperature of 3508C during 24 h. The obtained alloys were saturated in hydrogen. The moderated temperature at which this has been realized does not lead to the following decomposition: R 2 Fe 17 Hd 1 (2 1 x 2 d / 2)H 2 → 2 RH ( 21x) 1 17 Fe(a ) This reaction proceeds generally in the temperature range between 500 and 6008C [9]. Being basic, the concentrated aqueous solution of NaBH 4 reduces the risks of corrosion of alloys to a minimum. On the other hand, according to works of Murphy et al. [10], NaBH 4 can be used as a source of hydrogen at room temperature. According to Murphy et al. [10] as well as Kramp et al. [11], this method of insertion of hydrogen at room temperature has been used successfully on a certain number of the alloys, such as LaNi 5 , Nd 2 Fe 14 B, CaNi 5 and MnNi 5. The reaction scheme that was proposed is the following [10,11]: 2 M 1 BH 2 4 1 3H 2 O → M 2 H 1 H 2 BO 3 1 3.5H 2
where M and M2H are the alloy and its hydride, respectively. Fig. 1 represents the different stages that lead to the insertion of hydrogen in such compounds. This method consists therefore in mixing in distilled water the powder of the compound as well as a small quantity of NaBH 4 . After 24 h of reaction the obtained compound was rinsed with alcohol and then dried. The difference of mass before and after hydrogenation of the alloy allows us to determine approximately the number of inserted hydrogen atoms.
Fig. 2. Zoom of X-ray diffraction of Nd 2 Fe 16 Ti and Nd 2 Fe 16 TiH y .
Fig. 2 represents a selected part of the X-ray diffraction patterns of the powder of the compound Nd 2 Fe 16 Ti and its hydride obtained by reaction with NaBH 4 . This figure shows that hydrogen has been inserted in the structure by this method since the peaks have undergone a displacement to lower angles. The refinement of the lattice parameters indicates that these parameters increase after insertion of hydrogen. The obtained parameters are similar for the two hydrogenation methods (Table 1). The increase of the volume is about 2.47% for the Y 2 Fe 16 Ti compound and it is 3.9% for the Nd 2 Fe 16 Ti compound. Based on scanning electron microscopy, Fig. 3 shows granular cracks which confirms that hydrogen has been introduced into the structure. The grain size is less than 50 mm. Thermomagnetic analyses shows that the hydrides exhibit only one type of transition which is ferromagnetic– paramagnetic, the Curie temperatures of the hydrides being higher than in the parent alloys. The Curie temperatures of the hydrides obtained by reaction with NaBH 4 are almost equal to those obtained by solid–gas reaction, as seen in Fig. 4. The insertion of hydrogen by both methods is beneficial since it increases the Curie temperature about 100 K. The field dependence of the magnetization has been measured at room temperature for the parent alloys as well Table 1 Lattice constants obtained before and after insertion of hydrogen
3. Results and discussions X-ray diffraction of the hydrides of the pseudobinary alloys shows that they basically preserve the structural features of the parent alloys. Indeed, the Y 2 Fe 16 TiH y compound crystallizes in the hexagonal structure, while the Nd 2 Fe 16 TiH y compound preserves the rhombohedral structure.
Compounds
˚ a (A)
˚ c (A)
˚ 3) V (A
Y 2 Fe 16 Ti Y 2 Fe 16 TiH y a Y 2 Fe 16 TiH y b Nd 2 Fe 16 Ti Nd 2 Fe 16 TiH y a Nd 2 Fe 16 TiH y b
8.4810 8.5705 8.5682 8.5987 8.7302 8.7289
8.3740 8.4015 8.4075 12.509 12.605 12.613
521.62 534.44 534.50 800.97 832.00 832.3
a b
Hydrides obtained by the classic method. Hydrides obtained by reaction with NaBH 4 .
y (H / f.u.)
DV/V (%)
4.6 4.5
2.46 2.47
4.8 4.9
3.87 3.91
M. Ellouze et al. / Journal of Alloys and Compounds 322 (2001) 211 – 213
213
Table 2 Curie temperatures and saturation magnetization obtained before and after insertion of hydrogen Compounds
T c (K)
Ms (emu / g)@300 K
Y 2 Fe 16 Ti Y 2 Fe 16 TiH y a Y 2 Fe 16 TiH y b Nd 2 Fe 16 Ti Nd 2 Fe 16 TiH y Nd 2 Fe 16 TiH y
400 493 490 390 503 500
105.7 127.4 125.3 117.5 135.5 136.8
a b
Fig. 3. Scanning electron microscopy photo of the Nd 2 Fe 16 TiH y after hydrogen insertion.
a b
Hydrides obtained by the classic method. Hydrides obtained by reaction with NaBH 4 .
as for their hydrides (Fig. 5). The saturation magnetization (Ms ) increases after insertion of hydrogen and we obtain practically the same value for the two hydrides of the same parent compound. The Curie temperatures as well as the saturation magnetization at 300 K of the alloys before and after insertion of hydrogen are summarized in Table 2.
4. Conclusion We have successfully obtained hydrides by using a solution of NaBH 4 . Crystallographic and magnetic properties of these hydrides are similar with those obtained by the classic gas–solid method. This new method has several advantages. On the one hand, it proceeds at room temperature and does not require thermal activation. On the other hand, contrary to the classic method, we can avoid free a-iron as a secondary phase and / or the temperature induced decomposition of the ternary hydride.
References Fig. 4. Thermomagnetic analyses of the hydrides of Y 2 Fe 16 Ti obtained with the two methods.
Fig. 5. Magnetization versus applied field at 300 K of Nd 2 Fe 16 Ti and Nd 2 Fe 16 TiH y .
[1] J.V. Florio, N.C. Baenziger, R.E. Rundle, Acta Crystallogr. 9 (1956) 367. [2] E.S. Makarov, S.P. Vinogradov, Sov. Phys.-Crystallogr. 1 (1956) 634. [3] B. Rupp, A. Resnik, D. Shaltiel, P. Rogl, J. Mater. Sci. 23 (1988) 2133. [4] W.X. Zhong, K. Donnelly, J.M.D. Coey, B. Chevallier, J. Etourneau, T. Berlureau, J. Mater. Sci. 23 (1988) 329. [5] X.P. Zhong, R.J. Radwanski, F.R. de Boer, T.H. Jacobs, K.J.H. Buschow, J. Magn. Magn. Mater. 86 (1990) 333. [6] J.M.D. Coey, H. Sun, J. Magn. Magn. Mater. 87 (1990) L251. [7] J.M.D. Coey, J.F. Lawler, H. Sun, J.E.M. Allan, J. Appl. Phys. 69 (5) (1991) 300. [8] O. Isnard, Thesis of the Universite´ Joseph Fourier (Grenoble), (1993). [9] C.N. Chritodoulou, T.Tu. Keshita, J. Alloys Comp. 194 (1993) 113. [10] D.W. Murphy, S.M. Zahurak, B. Vyas, M. Thomas, M.E. Badding, W.C. Fang, Chem. Mater. 5 (1993) 767. [11] S. Kramp, M. Febri, J.C. Joubert, J. Solid State Chem. 133 (1997) 145.